241 10 13229 https://digitalcollections.lakeheadu.ca/files/original/6069db93fb592ecdd4a76f58a9bae2c9.pdf 47805a815831581565df2436ab456bb3 PDF Text Text 47th Annual Meeting Institute on Lake Superior Geology Proceedings Volume 47 Part 1—Program and Abstracts Madison, Wisconsin • May 9—12, 2001 �INSTITUTE ON LAKE SUPERIOR GEOLOGY U 47th Annual Meeting May 9-12, 2001 Madison, Wisconsin Hosted by: University of Wisconsin-Extension Wisconsin Geological and Natural History Survey University of Wisconsin-Madison Department of Geology and Geophysics Proceedings Volume 47 Part 1 - Program and Abstracts �47th Annual Meeting Institute on Lake Superior Geology Volume 47 contains the following parts: Part 1: Program and Abstracts Part 2: Field Trip Guidebook 1- Sedimentologic, Tectonic and Metamorphic History of the Baraboo Interval: New Evidence from Investigations in the Baraboo Range, Wisconsin 2 - Geology, Ore Deposits, and Cultural History of the Upper Mississippi Valley Zinc-Lead District 3 - Economic Geology of the Baraboo and Waterloo Quartzites of Southern Wisconsin Reference to the material in this volume should follow the example below: Medaris, L.G., Jr., 2001, Precambrian geology of S. Wisconsin: A panorama from the Baraboo Range, [abstract]: Institute on Lake Superior Geology Proceedings, 47th Annual Meeting, Madison, WI, 2001, v. 47, Part 1, p. 51. Volume 47 is published by the Institute on Lake Superior Geology and distributed by the Institute Secretary-Treasurer: Mark Jirsa Minnesota Geological Survey 2642 University Avenue St. Paul, MN USA 55114-1057 (612) 627-4780 email: jirsa001tc.umn.edu ILSG webstite http://www.ilsgeology.org/ ISSN 1042-9964 Cover Illustration: Van Hise Rock, Abelmans Gorge, SW1/4, sec. 28, T21N, R5E, Sauk County, Wisconsin from Salisbury, R.D., and W. W Atwood, 1900, The Geography of the Region About Devil's Lake and the Dalles of the Wisconsin: Wisconsin Geological and Natural History Survey Bulletin V. Plate IX. Charles R. Van Hise and Charles K. Leith used this outcrop as a laboratory to demonstrate the fundamental geometric relationship between slaty cleavage and bedding at an outcrop-scale for inferring larger-scale structures. See Field Trip Stop 4, this meeting, Part 2, p. 17. �CONTENTS Proceedings Volume 47 Part 1—Program and Abstracts Editor: Michael G. Mudrey, Jr. iv Institutes on Lake Superior Geology, 1955-2001 Constitution of the Institute on Lake Superior Geology v By-Laws of the Institute on Lake Superior Geology vi An Obituary for Samuel S. Goldich by Bruce R. Doe vii Goldich Medal Guidelines xiv Goldich Medalists XV Goldich Medal Committee xvi Citation for 2001 Goldich Medal Recipient by Gene L. Laberge xvii Eisenbrey Student Travel Awards xviii Student Travel Award Application Form xviii Student Paper Awards xix Student Paper Awards Committee xix xx Membership Criteria Board of Directors Xxi Local Committee XXi Session Chairs XXU Banquet Speaker Xxii Report of the Chair of the 46th Annual Institute Meeting xxiii Program XXiV Xxiv Xxiv Wednesday May 9 Thursday Morning May 10 Thursday Afternoon and Evening May 11 xxv xxvi xxvii Friday Morning May 11 Friday Afternoon May 11 Saturday May 12 XXVfl xxviii List of Poster Presentations Abstracts 1 111 �INSTITUTES ON LAKE SUPERIOR GEOLOGY, 1955-2001 # 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 DATE PLACE 1955 Minneapolis, Minnesota 1956 Houghton, Michigan 1957 East Lansing, Michigan 1958 Duluth, Minnesota 1959 Minneapolis, Minnesota 1960 Madison, Wisconsin 1961 Port Arthur, Ontario 1962 Houghton, Michigan 1963 Duluth, Minnesota 1964 Ishpeming, Michigan 1965 St. Paul, Minnesota 1966 Sault Ste. Marie, Michigan 1967 East Lansing, Michigan 1968 Superior, Wisconsin 1969 Oshkosh, Wisconsin 1970 Thunder Bay, Ontario 1971 Duluth, Minnesota 1972 Houghton, Michigan 1973 Madison, Wisconsin 1974 Sault Ste. Marie, Ontario 1975 Marquette, Michigan 1976 St. Paul, Minnesota 1977 Thunder Bay, Ontario 1978 Milwaukee, Wisconsin 1979 Duluth, Minnesota 1980 Eau Claire, Wisconsin 1981 East Lansing, Michigan 1982 International Falls, Minnesota 1983 Houghton, Michigan 1984 Wausau, Wisconsin 1985 Kenora, Ontario 1986 Wisconsin Rapids, Wisconsin 1987 Wawa, Ontario 1988 Marquette, Michigan 1989 Duluth, Minnesota 1990 Thunder Bay, Ontario 1991 Eau Claire, Wisconsin 1992 Hurley, Wisconsin 1993 Eveleth, Minnesota 1994 Houghton, Michigan 1995 Marathon, Ontario 1996 Cable, Wisconsin 1997 Sudbury, Ontario 1998 Minneapolis, Minnesota 1999 Marquette, Michigan 2000 2001 CHAIRS C.E. Dutton A.K. Sneigrove B.T. Sandefur R.W. Marsden G.M. Schwartz & C. Craddock E.N. Cameron E.G. Pye A.K. Sneigrove H. Lepp A.T. Broderick P.K. Sims & R.K. Hogberg R.W. White W.J. Hinze A.B. Dickas G.L. LaBerge M.W. Bartley & E. Mercy D.M. Davidson J. Kalliokoski M.E. Ostrom P.E. Giblin J.D. Hughes M. Walton M.M. Kehienbeck G. Mursky D.M. Davidson P.E. Myers W.C. Cambray D.L. Southwick T.J. Bornhorst G.L. LaBerge C.E. Blackburn J.K. Greenberg E.D. Frey & R.P. Sage J. S. Kiasner J.C. Green M.M. Kehienbeck P.E. Myers A.B. Dickas D.L. Southwick T.J. Bornhorst M.C. Smyk L.G. Woodruff R.P. Sage, W. Meyer J.D. Miller, M.A. ursa T.J. Bomhorst, R.S. Regis S.A. Kissin, P. Fralick M.G. Mudrey, Jr., B.A. Brown Thunder Bay, Ontario Madison, Wisconsin iv �__________ CONSTITUTION OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY (Last amended by the Board—May 8, 1997) Article I Name The name of the organization shall be the "Institute on Lake Superior Geology'. Article II Objectives The objectives of this organization are: A. To provide a means whereby geologists in the Great Lakes region may exchange ideas and scientific data. B. To promote better understanding of the geology of the Lake Superior region. C. To plan and conduct geological field trips. Article III Status No part of the income of the organization shall insure to the benefit of any member or individual. In the event of dissolution, the assets of the organization shall be distributed (some tax free organization). to (To avoid Federal and State income taxes, the organization should be not only scientific" or "educational, but also "non-profit") Minn. Stat. Anno. 290.01, subd. 4 Minn. Stat. Anno. 290.05(9) 1954 Internal Revenue Code s.501(c)(3) Article IV Membership The membership of the organization shall consist of persons who have registered for an annual meeting within the past three years, and those who indicate interest in being a member according to guidelines approved by the Board of Directors. Article V Meetings The organization shall meet once a year. The place and exact date of each meeting will be designated by the Board of Directors. Article VI Directors The Board of Directors shall consist of the Chair, Secretary-Treasurer, and the last three past Chairs; but if the board should at any time consist of fewer than five persons, by reason of unwillingness or inability of any of the above persons to serve as directors, the vacancies on the board may be filled by the Chair so as to bring the membership of the board to five members. Article VII Officers The officers of this organization shall be a Chair and Secretary-Treasurer. A. The Chair shall be elected each year by the Board of Directors, who shall give due consideration to the wishes of any group that may be promoting the next annual meeting. His/her term of office as Chair will terminate at the close of the annual meeting over which he/she presides, or when his/her successor shall have been appointed. He/she will then serve for a period of three years as a member of the Board of Directors. B. The Secretary-Treasurer shall be elected at the annual meeting. His/her term of office shall be four years, or until his/her successor shall have been appointed. Article VIII Amendments This constitution may be amended by a majority vote (majority of those voting) of the membership of the organization. V �BY-LAWS OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY I. Duties of the Officers and Directors A. It shall be the duty of the Annual Chairman to: 1. Preside at the annual meeting. 2. Appoint all committees needed for the organization of the annual meeting. 3. Assume complete responsibility for the organization and financing of the annual meeting over which he/she presides. B. It shall be the duty of the Secretary-Treasurer to: 1. Keep accurate attendance records of all annual meetings. 2. Keep accurate records of all meetings of, and correspondence between, the Board of Directors. 3. Hold all funds that may accrue as profits from annual meetings or field trips and to make these funds available for the organization and operation of future meetings as required. C. It shall be the duty of the Board of Directors to plan locations of annual meetings and to advise on the organization and financing of all meetings. II. Duties and Extenses A. Regular membership dues of $5.00 or less on an annual basis shall be assessed each member as determined by the Board of Directors.. B. Registration fees for the annual meetings shall be determined by the Chair in consultation with the Board of Directors. The registration fees can include expenses to cover operations outside of the annual meeting as determined by the Board of Directors. It is strongly recommended that registration fees be kept at a minimum to encourage attendance of students. III. Rules of Order The rules contained in Robert's Rules of Order shall govern this organization in all cases to which they are applicable. IV. Amendments These by-laws may be amended by a majority vote (majority of those voting) of the membership of the organization; provided that such modifications shall not conflict with the constitution as presently adopted or subsequently amended. vi �AN OBITUARY FOR SAMUEL S. GOLDICH By Bruce R. Doe Goldschmidt Medal winner in 1983, Samuel Stephen Goldich died 20 December 2000 at his apartment in Applewood, Colorado (a suburb of system in granites. In brief, the theory says that as the Denver), less than a month before his 92nd birthday. Sam, as he was widely known, received early fame with his 1938 paper in the Journal of Geology on rock weathering based on his Ph.D. thesis, an amazing paper that continued to receive citations into the 1990s, more than 50 years later. In short, he deter- mined that the resistance of igneous minerals to weathering was the inverse of the Bowen Reaction Series, that is minerals crystallized at lower temperatures were more resistant to weathering than those crystallized at high temperatures (and pressures). In other words the last minerals to crystallize (e.g., the most resistant was quartz followed by orthoclase, etc.) from a melt were the most resistant to weathering, a sequence that became known as the Goldich Stability Series (for a short discussion of this on the web see the 1996 web site of Pamela Gore at http://www.dc.peachnet.edu/_pgore/ge0logY/geo 10 1/weáther.htm. A few years later in 1941, a second, two-part widely utilized paper was published by pressure on minerals and rocks is released through uplift and erosion, they expand and make lead that is not in the structure accessible to removal by crustal fluids. No biography of Goldich would be complete without mention of Sam's interest in the 3 ,500-myr.-old rocks of the Minnesota River Valley Sandell and Goldich on the trace-element concentrations in igneous rocks, also in the Journal of Geology. This pioneering paper introduced the dithizone colorimetric analytical technique for trace-element determination to earth science and resulted in some of the most precise trace-element data in extent at the time and for decades thereafter. Sam got into an argument with the late Paul Gast which originally appeared to be 3,800 myr. old in his collaborations with Ed Catanzaro and later Tom Stern (see Goldich, Hedge and Stern, 1970). This discovery over which radiometric dating system on biotite led to the search of other areas in the U.S. for would be more susceptible to alteration by weathering -- the K-Ar system or the Rb-Sr system -- and a bet ensued. Paul thought that K-Ar would be more affected by weathering which would open the structure of biotite and let the argon escape which was not bound in the structure. Sam, however, thought that Rb-Sr would be more affected because the Sr that did perancient rocks with the discovery by others of these sorts of rocks in Michigan and Wyoming. He ended his research career with a very important paper on the air abrasion method ofpreparing zircons for U-Th-Pb dating (Goldich and Fischer, 1986) that has become not fit in the structure would be subject to ion exchange. Thus a paper resulted (Goldich and Gast 1966). Incidentally, Goldich won the bet. A very important paper with Mudrey (first presented in abstract form at the Geological Society of America Interior Distinguished Service Award in 1965, and he much used for Pb isotope tracer work as well as zircon dating. Sam received the Department of was a founder of the Institute of Lake superior Geology and received its first Goldich Award in 1980. S.S. Goldich received an AB from the University meeting as Goldich and Mudrey, 1969, with the paper appearing in 1972) using dilatancy to help explain the ofMinnesotain 1929 andeventuallyaPh.D. in 1936. In between, he earned an M.A. from Syracuse University in 1930 (and was to receive their Alexander Winchell Award in Geology in 1977), spent two years as an assistant in geology at the then Missouri discordance of the U-Th-Pb ages in zircons never received the acclaim it deserved because of its original publication in a Russian book, but Doe was later to make use of it in explain the U-Th-Pb whole-rock vii �School of Mines, 1930-1932 (now, University of Missouri at Rolla) where his association with Garrett Muilenburg resulted in his first paper (Muilenberg and Goldich, 1933), and was a fellow at Washington University in St. Louis where he published a paper with Carl Tolman (Tolman and Goldich, 1935). While a graduate student at the University of Minnesota, he was a chemist in the famous Rock Analysis Laboratory. In the period 1936- 1941, Sam rose from instructor to Associate Professor at the Texas Agricultural and Mechanical College which resulted in a number of papers on Texas geology and developed his research interest in iron ore in a paper with Barnes, Goldich, and Romberg (1949) that resulted in the milestone papers with Henry Lepp (Lepp and Goldich, 1959, 1964). He served World War II in the U.S. Geological Survey in exploration and study of laterite and bauxite in a number of unusual locations that eventually resulted in a series of papers during a second tour with the U.S. Geological Survey in 1947 and 1948. There were papers with Hendricks and Nelson in 1946 on a portable differential thermal analysis unit for bauxite exploration, with Bergquist on aluminous latentic soil ofthe Dominican Republic in 1947, and with Bridge in 1948 on the bauxite of Babelthuap Island in the Palau Group of islands. time and became the founding Branch Chief of the famous Branch of Isotope Geology in 1960, now defunct. Initially, the Branch was located at the old National Bureau of Standards site on Connecticut Avenue and Van Ness in Washington. D.C. (Now the site of the University of the District of Columbia). This location also allowed Goldich to make a close association with Bill Shields of the National Bureau of Standards (NBS, now called the National Institute of Standards and Technology) who was involved in using mass spectrometry for redeterniining atomic weights. This association resulted in much upgrading of the instrumentation in the Branch plus the building of new equipment known as Shields Mass Spectrometers and led the late Paul Gast to say that Shields was the most valuable employee in the Branch of Isotope Geology and the U.S. Geological Survey didn't even have to pay him. The core of this Branch came from the old Nucleonics Group that had been headed by Frank Senftle. In addition to Senftle, there were Irving Friedman, Henry Faul, Lorin Stieff, Thomas Stern, and Meyer Rubin, among others. Stieff shortly left to pursue his interest in world peace. Faul and Goldich had a falling out and Faul eventually left for the University of Pennsylvania. Quickly added were Ron Kistler (who was soon to return to the U.S. Geological Survey in Menlo Park, Sam rejoined the University of Minnesota in California), Carl Hedge, Edward Catanzaro, and 1948 and became Professor and Director of the Rock Analysis Laboratory the following year, a position he became a professor at the University of Alberta, Bruce Doe. Catanzaro soon left to join Shields at the NBS. The NBS site had to be abandoned because of their move to Gaithersburg and the Assistant Chief Geology for the Central Region came up with a small building (Bldg. 21) at the former WW II munitions plant in Lakewood, Colorado, known as the Denver Federal Center. John Rosholt was already there, and Irving Friedman in 1963 was the first to move. John Stacey and Mitsunobu Tatsumoto were soon added Harry Gehman who went to work in the oil industry, and Bruce Doe moved there in 1963 followed by and Richard B. Taylor who became Chief of the Branch of Central Mineral Resources in the U.S. Geological Survey. Master's candidates included most of the others in 1964, including Robert Zartman and John Obradovich. During a period of years after this move, Senftle and Stern stayed in Washington, D.C., and joined other groups. Meyer Rubin was to remain there in the Branch with his carbon- 14 operation. A popular way for the Branch of Isotope Geology to acquire researchers was from other branches. was to hold until his departure in 1959 (and was to receive the Minnesota Outstanding Achievement Award in 1985). Notable among his Ph.D. graduate students were Ralph Erickson who was the founding Branch Chief of the U.S. Geological Survey's Geochemical Exploration Branch, Ronald Burwash who Gary Ernst, currently a Professor at Stanford Univer- sity, and Zell Peterman a former Chief of the late Branch of Isotope Geology, U.S. Geological Survey, and a pioneer in strontium isotope geochemistry. During this period and in collaboration with Alfred Zell Peterman, for example, came from a branch Nier of the University of Minnesota, Sam organized called Geochemical Census (and, for example, later in Menlo Park, Marvin Lanphere from the Branch of a potassium-argon facilty in the Department of Alaskan Geology, and Brent Dalrymple from the Geology with Halfdon Baadsgaard (who also later became a professor at the University of Alberta). A Branch of Theoretical Geophysics) [History of the Branch of Isotope Geology after Sam ceased being Branch Chief in 1964 is not covered here.] . number of important papers resulted from this collaboration. Upon leaving the University of Minnesota, Doe, a Post-doctoral Fellow at the Geophysical Laboratory, was hired under an agreement that the Goldich joined the U.S. Geological Survey a third viii �U.S. Geological Survey would acquire a 12-inch Shields solid-source mass spectrometer. After that, however, most equipment was obtained as a result of cooperation with other organizations. Stieff, for example, arranged an investigation of uranium series disequilibrium in soils that freed up money for a second 12-inch Shields mass spectrometer and the stayed on for a fifth year to get a bachelors degree in geological engineering but decided to go on and get a master's degree somewhere else as well. I went in to Sam to tell him this and I was going to ask him not to prejudice others that I would ask to write letters of reference for me. I only got out that I was thinking of going somewhere for a master degree when he interrupted that he thought that was a great idea and he could get me a good deal at Missouri School of Mines. He added that I would never regret it. So I decided to leave it at that, and he was right, I never did regret it. Later when I mentioned I wanted to go on for a Ph.D., he suggested I apply to Caltech and that he would have a place for me in the isotope lab when I flunked out. Well, I did apply and was accepted, but, fortunately, never flunked out. Once I told Dick Taylor about my confusion concerning the unexpected result over the master's degree proposal, and he replied, that Sam was hard on me because he thought I had potential. I had noticed that with certain students of little accomplishment he would talk about building of a clean laboratory for isotopic investigations in Denver. A program with Saudi Arabia freed up money for a 6-inch Shields solid-source mass spectrometer. Money was obtained from the Japan-U.S. Scientific Cooperation Program for an argon mass-spectrometer (for an entertaining account of the argon mass spectrometer, you are referred to Glynu's book "The Road to Jaramillo"). After his tour as Chief of the Branch of Isotope Geology, Goldich was to leave the U.S. Geological Survey again and from 1964-1965 joined Pennsylvania State University as Professor of Geology and Geochemistry and Director of the Mineral Constitution Laboratories, for which he was to hire his former associate Oliver Ingamells. The State University of New York began upgrading their faculty and Sam moved to the State University of New York at Stony Brook as Professor of Geology from 1965 to 1968 at which he oversaw the building of another isotope fishing, hunting, movies, and the like, but that he eventually even wrote papers with some of the people he was hardest on. However, he left an ill will with many which probably accounts for this remarkable scientist not winning more honors than he did. But there were a lot of us that learned to overcome Sam's laboratory and hired Gil Hanson who became a professor there. Restless, he moved to Northern outbursts and to regard him as a friend and wonderful scientist. Illinois University in 1968 as Professor of Geology until his retirement in 1977 as emeritus and where he organized yet a fourth isotope geology laboratory. He was to move to Denver in his retirement and became emeritus at the Colorado School of Mines. Sam was a fellow of the American Geophysical Union, Mineralogical Society of America, and Geological Society of America. Selected Bibliography of Samuel Stephen Goldich Bridge, Josiah, Goldich, Samuel S., Preliminary report on the bauxite deposits of Babeithuap island, Palau group, p. 46, 1948. Cameron, R.L., Goldich, S.S., Hoffman, J.H., Radioactivity age ofrocks from the Windmill islands, Budd coast, Antarctica, Stockholm Contributions in Geology, 6, p. 1-6, 1960. No discussion of Sam would be complete with- out some mention of his famous personality. Although Sam could be very generous, he was prone to giving unsolicited good advice or opinions. This Goldich, Samuel S., A study of rock-weathering, 97 p., 1936. Thesis Doctoral from University of Minnesota, Minneapolis, Minneapolis, MN, United States advice or opinions was often given in a tone that the recipient would take as criticism or, even, condemnation. Perhaps all those who were close to Sam, and even many more distant, experienced this at some time or other. He was prone to allergies which did not improve his disposition. I recall once in a class when he repeatedly asked some question in an increasingly agitated and loud voice punctuated by his blowing his Goldich, Samuel S., Authigenic feldspar in sandstones of southeastern Minnesota, Journal of Sedi- nose as one student after another he called upon Goldich, Samuel S., A study in rock weathering, couldn't answer it. He finally said with a cute smile, "This class sure is stupid when I don't feel good." I Journal of Geology, 46(1), p. 17-58, 1938. recall thinking he didn't like me when I was an Goldich, Samuel Stephen, Bergquist, Harlan Richard, undergraduate at the University of Minnesota. I had Aluminous latentic soil of the Sierra de Bahoruco mentary Petrology, 4 (2), p. 89-95, 1934. ix �area, Dominican Republic, West Indies, U. S. Geological Survey Bulletin, B 0953-C, p. 53-84, 1947. Goldich, Samuel S., Lidiak, Edward G., Hedge, Carl Goldich, Samuel Stephen, Origin and development of aluminous latente and bauxite, Geological Society of America Bulletin, 59 (12, Part 2), p. 1326, 1948. area, Journal of Geophysical Research, 71(22), p. E., Walthall, Frank G., Geochronology of the midcontinent region, United States; [Part] 2, Northern 5389-5408, 1966. Goldich, Samuel S., Muehlberger, William R., Goldich, Samuel Stephen, Bergquist, Harlan Richard, Lidiak, Edward G., Hedge, Carl E., Geochronology of the midcontinent region, United States; [Part] 1, Scope, methods, and principles, Journal ofGeophysical Research, 71(22), p. 5375-5388, 1966. Aluminous lateritic soil of the Republic of Haiti, West Indies, U. S. Geological Survey Bulletin, B 0954-C, p. 63-111, 1948. Goldich, Samuel Stephen, Oslund, Eileen H., Composition of Westerly granite G- 1 and Centerville diabase W-l, Geological Society of America Bulletin, 67(6), p. 811-815, 1956. Goldich, S.S., Geochronology in the Lake Superior region, Inst. Lake Superior Geology, 13th Ann., East Lansing, Mich., 1967, p. 13, 1967. Goldich, S.S., Ingamells, CO., Suhr, N. H., Anderson, D. H., Analyses of silicate rock and mineral standards, Canadian Journal of Earth Sciences, 4(5), p. 747-755, 1967. Goldich, Samuel Stephen, Nier, Alfred Otto C., Problems of the division of Precambrian time, Institute on Lake Superior geology, April 21-22, 1958., p. 11, 1958. Goldich, S.S., Mudrey, M.G., Jr., Dilatancy model Goldich, Samuel Stephen, Nier, Alfred Otto C., for discordant U-Pb zircon ages, Abstracts with Baadsgaard, Halfdon, Three-fold division ofPrecam- Programs - Geological Society of America, Part 7, p. 80, 1969. brian in the Lake Superior region, Transactions American Geophysical Union, 39 (3), p. 516, 1958. Goldich, S.S., Nier, A.O., Washburn, A.L., A (super Goldich, S.S., Hanson, G.N., Geology of the Saganaga-Northern Light Lakes area, Minne- 40) /K (super 40) age of gneiss from McMurdo sota-Ontario, Inst. Lake Superior Geology, 15th Sound, Antarctica, Transactions - American Geophysical Union, 39 (5), p. 956-958, 1958. Ann., 1969, Tech. Sess. Abs., p. 16, 1969. Goldich, S.S., Geochronology in the Lake Superior region, Geochronology of Precambrian stratified rocks--Internat. Conf., Edmonton, Alberta, 1967, Papers, Canadian Journal of Earth Sciences, 5 (3, Part 2), p. 7 15-724, 1968. Goldich, Samuel S., Nier, Alfred 0., Baadsgaard, Halfdon, Hoffman, John H., Krueger, Harold W., The Precambrian geology and geochronology of Minnesota, Bulletin - Minnesota Geological Survey, 193 p., 1961. Goldich, S.S., Geochronology of the Minnesota-Ontario border region, Summary of fieldwork, 1969, Information Circular - Minnesota Geological Survey, 7, p. 30, 1969. Goldich, S.S., Hedge, C.E., Dating of the Precambrian of the Minnesota River valley, Minnesota, Journal of Geophysical Research, 67 (9), p. 3561-3562, 1962. Goldich, S.S., Ages of rocks assigned to the Penokean orogeny in Minnesota, Summary of fieldwork, 1969, Information Circular - Minnesota Geological Survey, 7, p. 31, 1969. Goldich, S.S., Hedge, C.E., Investigations in Rb-Sr dating, Journal of Geophysical Research, 67 (4), p. 1638, 1962. Goldich, S.S., Age of the Precambrian rocks of southwestern Minnesota, Summary of fieldwork, Goldich, S.S., Ingamells, C.O., Comparative determi- nations of potassium and rubidium, Transactions American Geophysical Union, 44(1), p. 109, 1963. 1969, Information Circular - Minnesota Geological Survey, 7, p. 31, 1969. Goldich, S.S., Gast, P. W., Effects of weathering on the Rb-Sr and K-Ar ages of biotite from the Morton Goldich, S.S., Hanson, G.N., Hailford, C.R., Mudrey, Gneiss, Minnesota, Earth and Planetary Science M.G., Jr., Re-interpretation of the structure of the Letters, 1(6), p. 372-375, 1966. Saganaga-Northern Light Lakes area, x �Minnesota-Ontario, Special Paper Society of America, p. 114, 1969. - Goldich, Samuel S., Ages of Precambrian Banded Geological Iron-Formations, Precambrian iron-formations of the world, Economic Geology and the Bulletin of the the Morton and Montevideo gneisses and related Society of Economic 1126-1134, 1973. rocks, southwestern Minnesota: Geological Society of Amenca Bulletin, 81, p. 367 1-3696, 1970. Goldich, S.S., Hedge, C.E., 3 ,800-Myr granitic gneiss Goldich, S.S., Hedge, C.E., and Stern, T.W., Age of Geologists, 68 (7), p. in south-western Minnesota, Nature (London), 252. (5483), p. 467-468, 1974. Goldich, Samuel S., Turek, Andrew, Hanson, Gilbert N., Peterman, Zell E., Correlation of early Precam- brian basins of the Canadian shield, Geological Goldich, S.S., Hedge, C.E., Interpretation ages in Association of Canada-Mineralogical Association of Canada, Joint Annual Meetings, Abstracts of Papers, p. 27, 1970. Minnesota — Reply: Nature, 257 (5528), p. 722-722, 1975. Goldich, S.S., Bodkin, J.L., Fluorine in Cenozoic Goldich, S.S., Lunar and terrestrial ilmenite basalt, Science, 171 (3977), p. 1245-1246, 1971. volcanic rocks of Ross Island and vicinity, Antarctica; a progress report, Bulletin - Dry Valley Drilling Project (DVDP) (6), p. 6-7, 1975. Goldich, S.S., Geochronology in Minnesota, Geology of Minnesota; A Centennial Volume, p. 27-37, 1972. Goldich, S.S., Doe, Bruce R., Delevaux, M.H., Possible further evidence for 3.8 b.y.-old rocks in the Minnesota River valley of southwestern Minnesota, Goldich, S.S., The Penokean orogeny [abstr.], Proceedings and Abstracts - Institute on Lake Superior Geology, Annual Meeting, 1972. Open-File Report - U. S. Geological Survey, OF 75-0065, p. Il, 1975. Goldich, S.S., Treves, S.B., Suhr, N.H., et al., Geochemistry of Cenozoic volcanic-rocks of Ross-Island and vicinity, Antarctica: Journal of Geology, 83 (4), p. 415-435, 1975 Goldich, S.S., Ages of Precambrian iron-formations, Precambrian iron-formation symposium, Abstracts and Field Guides, p. [19], 1972. Goldich, Samuel S., Geochronology and geochemistry, Field Trip Guide Book for Precambrian Migmatitic Terrane of the Minnesota River Valley, Guidebook Series - Minnesota Geological Survey, 5, p. 17-41, 1972. Goldich, S.S., Precision and accuracy in silicate analysis, National Bureau of Standards Special Publication (422), p. 79-89, 1976. Goldich, S.S., Problems in dating old Precambrian rocks, Program with Abstracts - Geological Associa- Goldich, Samuel S., Fallacious isochrons and wrong numbers, North-Central Section, 6th Annual Meeting, tion of Canada; Mineralogical Association of Canada; Canadian Geophysical Union, Joint Annual Meeting, 1, p. 71, 1976. Meeting: Geological Association of Canada, 29th annual meeting; Mineralogical Association of Canada, 21st annual meeting, Edmonton, Alberta, Canada, May 19-21, 1976. Abstracts with Programs - Geological Society of America, 4 (5), p. 322, 1972. Goldich, S.S., Mudrey, M.G.,Jr., Model' rasshireniya dlya ob"yasneniya nesoglasnykh urano-svintsovykh vozrastov v tsirkonakh; Dilatancy model for explain- Goldich,. S.S., Peterman, Z. E., Geology and geochemistry of the Rainy Lake area, Gorton, M. P. (editor), Archean geochemistry, Precambrian Research, 11(3-4), p. 307-327, 1980. Meeting: Archean geochemistry field conference, Ontario and Minnesota, Canada, Aug. 2-17, 1978. ing discordant uranium-lead zircon ages, Ocherki sovremennoy geokhimii i analiticheskoy khimii, p. 415-418, 1972. Goldich, S.S., Hanson, G.N., Hallford, C.R., and Mudrey, M.G., Jr., Early Precambrian rocks in the Saganaga Lake-Northern Light Lake area, Minnesota-Ontario. Part II. Petrogenesis, Doe. B.R. (edi- Goldich, S.S., Wooden, J.L., Geochemistry of the tor) and Smith, D.K. (editor), Studies in Mineralogy Archean rocks in the Morton and Granite Falls areas, and Precambrian Geology, Memoir - Geological southwestern Minnesota, Gorton, M. P. (editor), Archean geochemistry, Precambrian Research, 11 Society of America, l3S,p. 151-178, 1972. (3-4), p. 267-296, 1980. Meeting: Archean geochemxi �istry field conference, Ontario and Minnesota, brian iron formations, Economic Geology and the Bulletin of the Society of Economic Geologists, 59 Canada, Aug. 2-17, 1978. (6), p. 1025-1060, 1964. Goldich, S.S., Wooden, J.L., Origin of the Morton Gneiss, southwestern Minnesota; Part 3, Geochronol- Lepp, Henry, Goldich, Samuel S., Kistler, Ronald W., A Grenville cross section from Port Cartier to Mount Reed, Quebec, Canada, American Journal of ogy, More, G. B. (editor), Hanson, Gilbert N. (editor), Selected studies of Archean gneisses and lower Proterozoic rocks, southern Canadian Shield, Special Paper - Geological Society of America (182), p. 77-94, 1980. ISBN: 0-8137-2182-2. Science, 261 (8), p. 693-7 12, 1963. Lepp, Henry, Goldich, Samuel S., Origin of Precambrian iron formations, Kvenvolden, Keith A. (editor), Goldich, S.S., Wooden, J.L., Ankenbauer, G.A., Jr., Geochemistry and the origin of life, Benchmark Levy, T.M., Suda, R.U., Origin of the Morton Papers in Geology, 14, p. 195-209, 1974. (reprint) Gneiss, southwestern Minnesota; Part 1, Lithology, More, G. B. (editor), Hanson, Gilbert N. (editor), Ludwig, K.R., Zartman, R.E., Goldich, S.S., Gentry, Robert V., Lead retention in zircons; discussion and reply, Science, 223 (4638), p. 835, 1984. Selected studies of Archean gneisses and lower Proterozoic rocks, southern Canadian Shield, Special Paper - Geological Society of America (182), p. 45-56, 1980. ISBN: 0-8137-2182-2. Peterman, Z.E., Goldich, S.S., Hedge, CE., and Yardley, D.H., Geochronology of the Rainy Lake Goldich, S.S., Hedge, C.E., Stern, T.W., Wooden, Region, Minnesota-Ontario, Doe, B.R. (editor) and J.L., Bodkin, J.B., North, R.M., Archean rocks of the Granite Falls area, southwestern Minnesota, More, G. Smith, D.K. (editor), Studies in Mineralogy and Precambrian Geology, Memoir - Geological Society of America, l35,p. 193-215, 1972. B. (editor), Hanson, Gilbert N. (editor), Selected studies of Archean gneisses and lower Proterozoic rocks, southern Canadian Shield, Special Paper - Sandell, Ernest Birger, Goldich, Samuel S., The rarer Geological Society of America (182), p. 19-43, 1980. metallic constituents of some American igneous ISBN: 0-8137-2182-2. rocks, American Mineralogist, 24(12, Part 2), p. 12, 1939. Goldich, Samuel S., Determination of ferrous iron in silicate rocks, Chemical Geology, 42 (1-4), p. Sandell, Ernest Birger, Goldich, Samuel Stephen, The rarer metallic constituents of some American igneous 343-347, 1984. rocks; Part 1, Journal of Geology, 51. (Part 1), p. 99-115, (Part 2), 167-189, 1943. Goldich, S.S., and Fisher, L.B., Air-Abrasion experiments in U-Pb dating of zircon: Chemical Geology, v. 58: (3), p. 195-215, 1986 Shields, W.R., Garner, E.L., Hedge, C.E., Goldich, S.S., Survey of Rb (super 85)/Rb (super 87) ratios in minerals, Journal of Geophysical Research, 68 (8), p. Hanson, G.N., Goldich, S.S., Early Precambrian Rocks in the Saganaga Lake-Northern Light Lake 233 1-2334, 1963. Area, Minnesota-Ontario; Part II, Petrogenesis, Doe. B.R. (editor) and Smith, D.K. (editor), Studies in Mineralogy and Precambrian Geology, Memoir Geological Society of America, 135, p. 179-192, 1972. Shields, W.R., Goldich, S.S., Garner, E.L., Murphy, T. J, Natural variations in the abundance ratio and the atomic weight of copper, Journal of Geophysical Research, 70 (2), p. 479-49 1, 1965. Jenks, William F., Goldich, S.S., Ignimbrites in Stern, T.W., Goldich, S.S., Newell, M.F., Effects of southern Peru, Geological Society of America Bulletin, 65 (12, Part 2), p. 1271, 1954. weathering on the U-Pb ages of zircon from the Morton Gneiss, Minnesota, Earth and Planetary Science Letters, 1(6), p. 369-371, 1966. Lepp, Henry, Goldich, Samuel Stephen, The chemistry and origin of iron formations, Economic Geology and the Bulletin of the Society of Economic Geologists, 54 (7), p. 1348-1349, 1959. Stuckless, John S., Weiblen, Paul W., Goldich, Samuel S., A petrogenetic model for the alkalic rocks from the Ross Island area, Antarctica, Dry Valley Drilling Project (DVDP) Seminar-i, Bulletin - Dry Valley Drilling Project (DVDP), 4, p. 52-53, 1974. Lepp, Henry, Goldich, Samuel S., Origin of Precamxii �Stuckless, J. S., Miesch, A.T., Goldich, S.S., Weiblen, P. W., A Q-mode factor for the petrogen- esis of the volcanic rocks from Ross Island and vicinity, Antarctica, McGinnis, Lyle D. (editor), Dry Valley Drilling Project, Antarctic Research Series, 33, p. 257-280, 1981. ISBN: 0-87590-177-8. Wooden. J.L., Goldich, S.S., Suhr, N.H., Origin of the Morton Gneiss, southwestern Minnesota; Part 2, Geochemistry, More, G. B. (editor), Hanson, Gilbert N. (editor), Selected studies of Archean gneisses and lower Proterozoic rocks, southern Canadian Shield, Special Paper - Geological Society of America (182), p. 57-75, 1980. ISBN: 0-8137-2182-2. Yardley, D.H., Goldich, S.S., Preliminary review of Precambrian shield rocks for potential waste repository, YIOWIISUB-436712, p. (unpaginated), 1975. xlii �G0LDICH MEDAL GUIDELINES (Adopted by the Board of Directors, 1981; amended 1999) Preamble The Institute on Lake Superior Geology was born in 1955, as documented by the fact that the 27th annual meeting was held in 1981. The Institute's continuing objectives are to deal with those aspects of geology that are related geographically to Lake Superior; to encourage the discussion of subjects and sponsoring field trips that will bring together geologists from academia, government surveys, and industry; and to maintain an informal but highly effective mode of operation. During the course of its existence, the membership of the Institute (that is, those geologists who indicate an interest in the objectives of the ILSG by attending) has become aware of the fact that certain of their colleagues have made particularly noteworthy and meritorious contributions to the understanding of Lake Superior geology and mineral deposits. The first award was made by ILSG to Sam Goldich in 1979 for his many contributions to the geology of the region extending over about 50 years. Subsequent medalists and this year's recipient are listed in the table below. Award Guidelines 1) The medal shall be awarded annually by the ILSG Board of Directors to a geologist whose name is associated with a substantial interest in, and contribution to, the geology of the Lake Superior region. 2) The Board of Directors shall appoint the Goldich Medal Committee. The initial appointment will be of three members, one to serve for three years, one for two years, and one for one year. The member with the briefest incumbency shall be chair of the Nominating Committee. After the first year, the Board of Directors shall appoint at each spring meeting one new member who will serve for three years. In his/her third year this member shall be the chair. The Committee membership should reflect the main fields of interest and geographic distribution of ILSG membership. The out-going, senior member of the Board of Directors shall act as liaison between the Board and the Committee for a period of one year. 3) By the end of November, the Goldich Medal Committee shall make its recommendation to the Chair of the Board of Directors, who will then inform the Board of the nominee. 4) The Board of Directors normally will accept the nominee of the Committee, inform the medalist, and have one medal engraved appropriately for presentation at the next meeting of the Institute. 5) It is recommended that the Institute set aside annually from whatever sources, such funds as will be required to support the continuing costs of this award. Nominating Procedures 1) The deadline for nominations is November 1. Nominations shall be taken at any time by the Goldich Medal Committee. Committee members may themselves nominate candidates; however, Board members may not solicit for or support individual nominees. 2) Nominations must be in writing and supported by appropriate documentation such as letters of recommendation, lists of publications, curriculum vita's, and evidence of contributions to Lake Superior geology and to the Institute. 3) Nominations are not restricted to Institute attendees, but are open to anyone who has worked on and contributed to the understanding of Lake Superior geology. xiv �Selection Guidelines 1) Nominees are to be evaluated on the basis of their contributions to Lake Superior geology (sensu lato) including: a) importance of relevant publications; b) promotion of discovery and utilization of natural resources; c) contributions to understanding of the natural history and environment of the region; d) generation of new ideas and concepts; and e) contributions to the training and education of geoscientists and the public. 2) Nominees are to be evaluated on their contributions to the Institute as demonstrated by attendance at Institute meetings, presentation of talks and posters, and service on Institute boards, committees, and field trips. 3) The relative weights given to each of the foregoing criteria must remain flexible and at the discretion of the Committee members. 4) There are several points to be considered by the Goldich Medal Committee: a) An attempt should be made to maintain a balance of medal recipients from each of the three estates—industry, academia, and government. b) It must be noted that industry geoscientists are at a disadvantage in that much of their work in not published. 5) Lake Superior has two sides, one the U.S., and the other Canada. This is undoubtedly one of the Institute's great strengths and should be nurtured by equitable recognition of excellence in both countries. xv �GOLDICH MEDALISTS 1979 Samuel S. Goldich 1990 Kenneth C. Card 1980 not awarded 1991 1981 Carl E. Dutton, Jr. William Hinze 1992 William F. Cannon 1982 Ralph W. Marsden 1993 Donald W. Davis 1983 Burton Boyum 1994 Cedric Iverson 1984 Richard W. Ojakangas 1995 Gene LaBerge 1985 Paul K. Sims 1996 David L. Southwick 1986 G.B.Morey 1997 RonaldP.Sage 1987 Henry H. Halls 1998 Zell Peterman 1988 Walter S. White 1999 Tsu-Ming Han 1989 Jorma Kalliokoski 2000 John C. Green 2001 John S. Kiasner GOLDICH MEDAL COMMITTEE Mark Smyk (2001) Ontario Geological Survey, Thunder Bay Rod Johnson (2002) Rod Johnson and Associates, Negaunee, Michigan Frank Luther (2003) University of Wisconsin, Whitewater James D. Miller, Jr., as out-going senior member of Institute Board of Directors, is liaison between Goldich Medal Committee and the Board through the 2001 meeting. xvi �CITATION FOR JOHN S. KLASNER 2001 G0LDICH AWARD RECIPIENT It is my distinct honor and privilege to present this citation for John Klasner, the 22 recipient of the Goldich Award. John was born and raised in the Upper Peninsula. He received his Bachelor's degree in 1957 from Michigan State. His first professional job was with [NCO in the bush of northern Ontario in the summer of 1957. From 1958 to 1962, John worked for Geophysical Services International reducing seismic data for petroleum exploration in New Mexico, Texas, Wyoming, and then overseas in Libya and Muscat. He returned to Michigan State in 1962 and earned a Master's degree in geophysics in 1964. His thesis, mapping a bedrock valley by gravity methods was sponsored by the Groundwater Branch of the U.S.G.S. For the next five years, John worked for the Standard Oil Company of California as an exploration geophysicist stationed mainly in Anchorage, Alaska. He worked in the Cook Inlet area, and while in Anchorage, he met and married his wife, Gretchen, who also happens to be from the U.P. of Michigan. He worked in California from 1967 to 1969, and was then transferred back to Alaska in 1969 to work on the North Slope oil project. In the fall of 1969, he left Standard Oil to work on a Ph.D. at Michigan Tech., under Jo Kalliokoski. His dissertation was on the structure and metamorphism in the western Marquette range. I believe that John's thesis was the first study to show that the Early Proteozoic rocks are detached from the Archean in the area, suggesting large-scale horizontal tectonics. Upon completion of his Ph.D. in 1972, John joined the faculty at Western Illinois University, where he taught for 27 years. He resumed his contact with the U.S.G.S. spending summers mapping in northern Michigan. John as applied his knowledge of geophysic and structural geology to solving problems in the Precambrian of a number of areas including he Marquette range, the Gogebic range, the Feich trough, several areas in Wisconsin and in the Trans-Hudson Orogeny. He has done a broader range of structural studies in the Lake Superior region than anyone I know. He has a long list of publications (49) resulting from his work, in addition to the teaching and administrative load at an undergraduate university. He received the highest awards offered by Western Illinois University for his teaching and research. An indication of the esteem with which he was held at Western is shown by his being named Director of their Honors Program from 1994-1998. He introduced many students to the mysteries of he Precambrian by leading a field trip to the area every year. Especially important, I think, has been his role of introducing undergraduate students to professional activities by supervising fourteen Senior theses and six Honors theses during his years of teaching at Western. I have had the privilege of working directly with John since the mid-i 980s, doing field work in Wisconsin and northern Michigan. During this time John has been the mentor for a number of NAGGED/USES Summer trainees and volunteers. I have found John to be an exceptionally dedicated teacher of young geologists, as well as being a very competent geologist himself, a very good woodsman, and a pleasant fellow to work with. Therefore, it is with great pleasure that I present the 2001 recipient of the Goldich Award for "Outstanding Contributions To The Geology Of The Lake Superior Region", John S. Klasner. Gene L. LaBerge xvii �_______________________________ _____________ ___________ EISENBREY STUDENT TRAVEL AWARDS The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name "Eisenbrey" was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. 'Ned" Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope as much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The number of awards and value are determined by the annual Chair in consultation with the Secretary-Treasurer. Recipients will be announced at the annual banquet. The following general criteria will be considered by the annual Chair, who is responsible for the selection: 1) The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. 2) Students who are the senior author on either an oral or poster paper will be given favored consideration. 3) It is desirable for two or more students to jointly request travel assistance. 4) In general, priority will be given to those in the Institute region who are farthest away from the meeting location. 5) Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. The form below is optional. Successful applicants will receive their awards during the meeting. enbrey INSTITUTE ONLAIE SUPERIOR GEOLOGY Student Travel Award Application Student Name: Date. Address: DpartmentHead-Tjped EducationalStatns:__________________ Department Head-Si gnature Are you the senior author of an oral or poster paper? YES_ NO___ Will any other students be traveling with you? Who? Statement of need (use additi onal page Efnecessary) Please xviii rdurn to: �STUDENT PAPER AWARDS Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper—last modified by the Board in 1997—follow: 1) The contribution must be demonstrably the work of the student. 2) The student must present the contribution in-person. 3) The Student Paper Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. 4) In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. 5) The total amount of the awards is left to the discretion of the meeting Chair and Secretary-Treasurer, but typically is in the amount of about $300 US. 6) The Secretary-Treasurer maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. 7) The names of award recipients shall be included as part of the annual Chair's report that appears in the next volume of the Institute. Student papers will be noted on the Program. STUDENT PAPER AWARDS COMMITTEE Dave Meineke - Committee Chair Menden Engineering LLC Anne Argast Indiana University - Purdue University Fort Wayne Thomas J. Evans Wisconsin Geological and Natural History Survey Steve Kircher Nicolet Minerals xix �MEMBERSHIP CRITERIA FOR THE INSTITUTE ON LAKE SUPERIOR GEOLOGY Approved May 8, 1997 A. Membership in the Institute on Lake Superior Geology requires either participation in Institute activities, or an indication on a regular basis of interest in the Institute. Those individuals registering for an annual meeting will remain as members for 4 years unless: 1) they indicate no further interest in the Institute by responding negatively to the statement on meeting circulars "Remove my name from the mailing list"; or 2) two successive mailings in different years are returned by the postal service as address unknown. B. Those individuals who have not registered for an annual meeting in the past 4 years must indicate an interest in the Institute by postal, electronic , or verbal correspondence with the Secretary-Treasurer at least once every two years. Such individuals will be removed from the membership if they indicate no further interest in the Institute or two successive mailing in different years are returned by the postal service as address unknown. C. The Secretary-Treasurer will maintain a list of current members. The list will include the date of the beginning of continuous membership, dates of returned mail, dates of last contact (expression of interest), and the date membership expires, barring a change of status initiated by the member. Those individuals who have become members of ILSG by Section B will have an expiration date listed at 2 years from the upcoming meeting. For example, a member who expresses interest in September of 1997 (the next annual meeting is May, 1998) will have an expiration date of May, 2000, unless the member contacts the Secretary-Treasurer or attends an annual meeting. D. "Member for Life" status is granted to individuals who have been (nearly) continuous participants of the ILSG meetings for 15 years, Goldich Medal recipients, or those who have served as meeting chairs. This status will be further maintained unless the individuals indicate no further interest in the Institute, or 4 mailings in different years are returned by the postal service as address unknown, or they are deceased. E. All members will be mailed the First Circular for the Annual Meeting and the ILSG Newsletter. The Chair of the annual meeting may opt to send the first circular to additional individuals. All returned mail should be reported to the Secretary-Treasurer. F. The Secretary-Treasurer can designate any individual who is on the ILSG membership list (mailing list) as of January 1, 1997 as a member for life based on participation in ILSG activities. G: Members are strongly encourage to send address corrections to the Secretary-Treasurer to avoid unintentional lapse of membership. xx �2001 BOARD OF DIRECTORS (Board membership through the close of the meeting year shown) Michael G. Mudrey, Jr., General Chairman (2004) Bruce A. Brown, Co-chair Wisconsin Geological and Natural History Survey Stephen A. Kissin (2003) Lakehead University Theodore J.Bornhorst (2002) Michigan Technological Univensty James D. Miller, Jr. (2001) Goldich Liaison Minnesota Geological Survey Mark A. Jirsa - Executive Secretary (2002) Minnesota Geological Survey 2001 LOCAL PLANNING COMMITTEE Michael G. Mudrey, Jr. - Co-chair Bruce A. Brown - Co-chair Robert H. Dott, Jr. - Program Co-chair L. Gordon Medaris, Jr. - Program Co-chair Kathleen M. Zwettler - Meeting Coordinator Assistance to the local committee was provided by the following individuals from the Wisconsin Geological and Natural History Survey: James M. Robertson Director and State Geologist - Wisconsin Geological and Natural History Survey Virginia Trapino Office Support Mindy James Publication Preparation Susan Hunt Graphic Arts Michael L. Czechanski Program and Technical Assistance xxi �2001 SEssioN CHAIRS (In order of appearance) James M. Robertson - Geologic Overview of Southern Wisconsin Wisconsin Geological and Natural History Survey D.L. Daniels - Geophysical Overview and Earliest Archean Evolution U.S. Geological Survey William F. Cannon - Geology and Hydrogeology ofArsenic in Domestic and Public Water Supplies U.S. Geological Survey Michael D. Lemcke - Geology and Hydrogeology of Arsenic in Domestic and Public Water Supplies Wisconsin Department of Natural Resources Suzanne W. Nicholson - General Geology U.S. Geological Survey Dean Rossell - Developments in Understanding Keweenawan Geology Kennecott Exploration Company D.K. HoIm - Thermo-Tectonic History of 1800 to 1200 Ma post-Penokean to Pre-Keweenawan Rocks Bowling Green State University D.A. Schneider - Thermo- Tectonic History of 1800 to 1200 Ma post-Penokean to Pre-Keweenawan Rocks Syracuse University Eric Jerde - Developments in Understanding Archean Geology and Ore Depo.its Morehead State University 2001 BANQUET SPEAKER Thomas C. Hunt University of Wisconsin - Platteville A Practical Exercise in Metallic Mine Reclamation - Ladysmith, Wisconsin xxii �REPORT ON THE 46TH ANNUAL MEETING OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY Thunder Bay, Ontario The 46tl Annual Meeting ofthe Institute on Lake Superior Geology was held in Thunder Bay, Ontario, May 8-13, 2000, at Lakehead University. The meeting was sponsored by the Department of Geology, Lakehead University, with the assistance of the Ministry of Northern Development and Mines, Thunder Bay office. The meeting was co-chaired by Stephen A. Kissin and Philip W. Fralick. The meeting was attended by approximately 200 geoscientists from the United States and Canada. The meeting consisted of two days of technical sessions, with welcoming reception and annual banquet, and after six field trips before and after the technical sessions. The Proceedings Volume 46 was published in two parts. Part 1 - Program and Abstracts was edited by Stephen A. Kissin. There were 41 published abstracts for the 23 oral and 18 poster presentations given in the technical sessions. Part 2 - Field Trips was edited by Philip W. Fralick. There were six field trips, one before and after the technical sessions; all were described by the respective leaders in Part 2 of the Proceedings. The field trips were as follows: Mesoproterozoic Sibley Group was lead by Philip Fralick and Mark Smyk. This was a two day trip. 1) Lac des Iles Mine was lead by Moe Lavigne. The trip was run before and after the technical sessions. 2) Geoarcheology of the Thunder Bay Area was lead by Brian Phillips, Scott Hamilton, Joe Stewart, Pat 3) Julig, and Bill Ross. Paleoproterozoic Gunflint Formation was lead by Peir Pufahl and Philip Fralick. 4) Quaternaiy Geology, Shebandowan Belt was lead by Andy Bajc. 5) Steeprock-Finlayson-Lumby Belts, a two day trip was lead by Denver Stone, Kirsty Tomlinson, Ray 6) Bernatchez and Philip Fralick. The annual banquet was held in the Residence Dining Room at Lakehead University. The dinner speaker was Bruce Simonson of Oberlin College whose talk "Depositional Settings and Early Diagenesis of Large Precambrian Iron-Formations" was enthusiastically received. The 2000 Goldich Medal was awarded to John C. Green for his contributions to the geology of the North Shore Volcanic Group. The technical sessions included seven invited papers on the Canadian side of Lake Superior: M.M. Kehlenbeck "A Review of Structures in Rocks of Quetico Subprovence and Adjacent Terranes", D.W. Davis "Western Superior Province: Geochronologic Aspects", K. Tomlinson "Western Superior Provence: Sedimentological Aspects", M.J. Lavigne "The Lac des Ties Mine, Northwestern Ontario", P.W. Fralick "Western Superior Province: Proterozoic Sediments", and S.A. Kissin "Vein-type Deposits of the Thunder Bay Area". Student awards were given to Neil Pettigrew (University of Ottawa) for his oral presentation on the Samuels Lake intrusion, Ontario and to K.F. Beaster and J.D. Kohn (University of Wisconsin-Eau Claire) for their poster presentation on the Hinckley Sandstone of Minnesota. The Eisenbrey Student Travel Award was distributed among eight student presenters of oral and poster papers. The Board of Directors of the Institute met for a brief business meeting on May 10. Michael Mudrey of the Wisconsin Geological and Natural History Survey agreed to host the 2001 meeting at Madison, Wisconsin. The term of the Secretary-Treasurer, Mark Jirsa, expired at this meeting. He was nominated for an additional term as Secretary-Treasurer. There being no further nominations, he was acclaimed for an additional term. We wish to thank all those who contributed to the success of the meeting. The field trip leaders who contributed to the guidebook in Part 2 and devoted time and energy to field trips merit our special thanks. The session chairs are thanked for their services as well as the awards committee. We are especially grateful to Mark Smyk and staff of the Ministry of Northern Development and Mines, Sam Spivak, Karen Farrier, Becky Rogola and France Lagroix of the Department of Geology for invaluable assistance. Finally we would like to thank all participants whose attendance made the meeting a financial success. Stephen A. Kissin and Philip W. Fralick Co-Chairs of the 46th Annual ILSG xxiii �CALENDAR OF EVENTS AND PROGRAM WEDNESDAY MAY 9 8:00 am. FIELD TRIP 1: Sedimenlology, Tectonic and Metamorphic History of the Baraboo Interval: New Evidence from Investigations in the Baraboo Range, Wisconsin L.Gordon Medans, Jr. and Robert. H. Dott, Jr. University of Wisconsin - Madison Return of Trip 1 6:00 p.m. 5:00 p.m. - 8:00 p.m. Registration 7:00 p.m. - 10:00 p.m. Welcoming Reception, Cash bar and Poster Setup THURSDAY MAY 10 7:00 a.m. - 9:00 a.m. REGISTRATION 8:10 a.m. INTRODUCTORYREMARKS M.G. Mudrey, Jr. Chairman Wisconsin Geological and Natural History Survey SESSION I: GEOLOGIC OVERVIEW OF SOUTHERN WISCONSIN Session Chair: J.M. Robertson, Director and State Geologist, Wisconsin Geological and Natural History Survey 8:20 a.m. MICKELSON, D.M. and Clayton, L., (University of Wisconsin - Madison; Wisconsin Geological & Natural History Survey) Recent Advances in Understanding the Glacial Record of Wisconsin 9:05 a.m. BYERS, C.E. (University of Wisconsin - Madison) Overview of Paleozoic Geology in Southern Wisconsin 9:50 a.m. MEDARIS, L.G., Jr. (University of Wisconsin - Madison) Precambrian Geology of S. Wisconsin: A Panorama from the Baraboo Range 10:35 a.m. COFFEEBREAK AND POSTER SESSION SESSION II: GEOPHYSICAL OVERVIEW AND EARLIEST ARCHEAN EVOLUTION Session Chair: D. Daniels, US. Geological Survey 10:55 a.m. CHANDLER, Val W. and MUDREY, M.G., JR. Overview of Aeromagnetic Mapping: Minnesota (Chandler) and Wisconsin (Mudrey) 11:20 a.m. CANNON, W.F., Daniels, David L., Snyder, Stephen L., and Nicholson, Suzanne W. A Preliminary Interpretation of New Aeromagnetic and Gravity Data in Wisconsin 11:40 a.m. VALLEY, J.W., Peck, W.H., King, E.M, Graham, C.M., and Wilde, S.A. The Cool Early Earth: Oxygen isotope Evidence for Continental Crust and Oceans on Earth at 4.4 Ga NOON LUNCH BREAK ILSG BOARD MEETING (by invitation) POSTER SESSION xxiv �SESSION III: GEOLOGY AND HYDROGEOLOGY OF ARSENIC IN DOMESTIC AND PUBLIC WATER SUPPLIES Session Chairs: W.F. Cannon (U.S. Geological Survey) M.D. Lemcke (Wisconsin Department of Natural Resources) 1:30 p.m. NORDSTROM, D. Kirk (US. Geological Survey) Overview of Arsenic Occurrences and Processes in Controlling Mobility in Groundwater 2:30 p.m. MUDREY, M.G., Jr., Brown, B.A., Freiberg P.G., and Simo, J.A. Mississippi Valley-Type Mineralization in the Fox River Valley, Eastern Wisconsin 2:45 p.m. GOTKOWITZ, M.B., Schreiber, M.E., and Simo, J.A. Contrasts in the Geologic and Hydrochemical Occurrences of Arsenic contamination of Groundwater in Eastern Wisconsin 3:05 p.m. KANIVETSKY, Roman ilydrogeochemical Modeling of Arsenic in Minnesota Ground Water 3:25 p.m. KOLKER, Allan, Cannon, W.F., Haack, S.K., Westjohn, D.B. and Woodruff, L.G. Hydrogeologic Setting of Elevated Arsenic in Southeastern Michigan 3:45 p.m. COFFEE BREAK AND POSTER SESSION SESSION IV: GENERAL GEOLOGY Session Chair: Suzanne W. Nicholson, US. Geological Survey 4:05 p.m. WOODRUFF, L.G., Attig, J.W., and Cannon, W.F. Geochemistry of Quaternary Deposits in North-central Wisconsin: Geochemical Exploration and Provenance Analysis 4:25 p.m. FAUBLE, Philip, and Lien, Jennifer Some Observations from the Williams Quarry Exposure: Evidence of Debris Flow Deposits in the Parfeys Glen Formation? 4:45 p.m. BOERBOOM, Terrence J. and Jirsa, Mark A. Stratigraphy of the Paleoproterozoic Denham Formation Basalt, Arkose, and Dolomite 6:00 p.m. a Continental Margin Assemblage of ICE BREAKER - MIXER Cash Bar 7:00 p.m. • • • • ANNUAL BANQUET AND AWARD PRESENTATION Announcement of 48th Annual Meeting Location Memorial on Samuel Stephen Goldich 1909 - 2000 2001 Goldich Award Presentation to John Kiasner Banquet Speaker: Thomas Hunt, University of Wisconsin Platteville A Practical Exercise in Metallic Mine Reclamation - Ladysmith, Wisconsin Participants who are not registered for the banquet are welcome to join for the speaker xxv �FRIDAY MAY 11 8:10 a.m. INTRODUCTORYREMARKS M.G. Mudrey, Jr. Chairman Wisconsin Geological and Natural History Survey V: DEVELOPMENTS IN UNDERSTANDING KEWEENA WAN GEOLOGY Session Chair: Dean Rossell, Ken necott Exploration Company SESSION 8:20 a.m. MILLER, James D., Jr. The Duluth Complex: What it Is, What it Ain't, and What We Still Don't Know 8:50 a.m. GREEN, J.C., Davis, D.W., and Schmitz, M.D. Three New Zircon Dates for the Midcontinent Rift, North Shore, Minnesota: More Data, More Questions 9:10 a.m. ROGALA, B., and Fralick, P.W. A Metamorphosed Evaponte Sequence from the Sibley Basin SESSIoN VI: THERMO-TECTONIC HISTORY OF 1800 TO 1200 MA POST-PENOKEAN TO PRE-KEWEENA WAN ROCKS IN THE MIDWEST Session Chairs: D.K. Holm, Kent State University D.A. Schneider, Syracuse University 9:30 a.m. HOLM, D.K., Van Schmus, W.R. and MacNeill, L.C. Age of the Humboldt granite, northern Michigan: Implications for the origin of the Republic metamorphic node 9:50 a.m. VAN SCHMUS, W.R., MacNeill, L.C., Hoim, D.K., and Boerboom, T.J. New U-Pb Ages from Minnesota, Michigan, and Wisconsin: Implications for Late Paleoproterozoic Crustal Stabilization 10:10 a.m. COFFEEBREAK AND POSTER SESSION 10:30 a.m. SCHWEITZER, D., Hoim, D., Van Schmus, W.R. and Boerboom, T. Results of Igneous Thermometry and Barometry on the East-central Minnesota Batholith: Evidence for Post-emplacement Exhumation and Cooling 10:50 a.m. NAYMARK, Alissa, Singer, Brad, and Medaris, L.G., Jr., Recognition of Post-1630 Ma Fluiddriven Metamorphism in Baraboo Interval Quartzites by Means of Laser Probe 40Ar/39Ar Geochronology 11:10 am. DAVIS, Peter B., Williams, Michael L., Bownng, Samuel A. and Ramezani, Jahan Middle Proterozoic Tectonic History of the Central Tusas Mountains, Northern New Mexico, and Comparison with the Baraboo Interval, Southern Lake Superior Region 11:30 a.m. WILLIAMS, M.L., Jercinovic, M.J., and Karlstrom, K.E. Proterozoic Tectonic History of Southwestern North America: Insight from Microprobe Monazite Geochronology 11:50 a.m. HOLM, D., Jercinovic, M.M., and Williams, M. Initial Results of In Situ electron Microprobe (EMP) Age Dating of Monazite from the Southern Lake Superior Region: Confirmation of Widespread Geon 17 Metamorphism xxvi �_____ 12:10 p.m. SCHNEIDER, D.A., Hoim, D.K., and Hamilton, M.A. Directing Timing Constraints of Paleoproterozoic Metamorphism, Southern Lake Superior Region: Results from Shrimp U-Pb Dating of Metamorphic Monazites 12:30 LUNcH BREAK POSTERS removed after Lunch SESSION VII: DEVELOPMENTS IN UNDERSTANDING ARCHEAN GEOLOGYAND ORE DEPOSITS Session Chair: 2:00 p.m. Eric Jerde, Morehead State University HUDAK, George J., Peterson, Dean M., and Morton, Ronald L. New Volume Calculations for the Pyroclastic Eruptions Associated with the Sturgeon Lake Caldera Complex, Northwestern Ontario: Implications for the Scale of Archean Volcanic Processes 2:20 p.m. PETERSON, D.M., Gallup, C., Jirsa, M.A. and Davis. D.W. Correlation of Archean Assemblages Across the U.S.-Canadian Border: Phase I Geochronology 2:40 p.m. JIRSA, Mark A. and Chandler, Val W. Geophysical Answers to Geologic Queries in the Superior Province of Northern Minnesota C., Mason, John K., Schnieders, Bernie R., and Stott, Greg M. A Synopsis of Archean and Proterozoic Platinum Group Element Mineralization in the Thunder Bay District, Ontario 3:00 p.m. SMYK, Mark 3:20 p.m. COFFEEBREAK SESSION VIII: GEOLOGIC SETTINGS OF WEEKEND FIELD TRIPS 3:40 p.m. MUDREY, M.G., Jr., Hunt, T.C., and Czechanski, M.L. Overview of Field Trip 2: 4:00 p.m. Upper Mississippi Valley Zinc-Lead District BROWN, B.A., Luther, F.R., Courter, S.M., Schmitt, J.W., and Lien, 3. Field Trip 3: Economic Geology of the Baraboo and Waterloo Quartzites 5:00 p.m. SINGER, Brad (Department of Geology and Geophysics) Tour of Weeks Hall, University of Wisconsin (Transportation provided) SATURDAY 8:00 a.m. and Weeks End Refreshment Seminar MAY 12 FIELD TRIP 2: Upper Mississippi Valley Zinc-Lead District M.G. Mudrey, Jr. and Thomas C. Hunt Wisconsin Geological and Natural History Survey and University of Wisconsin - Platteville 8:00 a.m. 6:00 p.m. TRIP 3: Industrial Mineral and Aggregate Resources of the Baraboo Interval Quartzites Brown, B.A., Luther, F.R., Courter, S.M., Schmitt, J.W., and Lien, 3. Wisconsin Geological and Natural History Survey, University of Wisconsin - Whitewater, Mathy Construction, Kraemer Company FIELD Return of Field Trips xxvii �POSTER PRESENTATIONS BESKAR, Shawn The Blake Gabbro: A taxitic-tectured gabbro sill south of Thunder Bay, Ontario BIHARI, D.B. and Kissin, S.A. Alteration and Pge-au Mineralization in the North Roby Zone, Lac Des lies Mine, Northwestern Ontario BOERBOOM, Terrence J. Redefined Volcanic and Sedimentary Stratigraphy of the Northern St. Croix Horst in Pine County, Minnesota, and the Application of Arcview to Geologic Mapping BROWN, B.A. and Czechanski, M.L. GIS Applications for Resource Inventory and Land-use Planning in Wisconsin BUCHHOLZ, Thomas W., Faister, Alexander U., and Simmons, Wm. B. Recent Developments in the Mineralogy of the Nine Mile Piuton, Wausau Complex CANNON, W.F. and Woodruff, L.G. Regional Arsenic Anomalies Shown by NURE Stream Sediment and Hydrogeochemical Data in Northern Wisconsin and Michigan CHANDLER, Val W. and Morey, G.B. Paleomagnetic Study of Paleoproterozoic Rocks in the Animikie Group, Northern Minnesota DAHL, D. Structure, Stratigraphy and Punctuated Evolution of Minnesota's Mineral Exploration Archives DANIELS, David L., Nicholson, Suzanne W., Cannon, William F., and Kucks, Robert P. New Aeromagnetic Map of Wisconsin Examined by Regional Context Jerde, Eric A., SAL VATO, DanielJ., Thole, Jeff and Wirth, Karl R. The Early Gabbroic Series of the Midcontinent Rift System: Continued Assessment of Magmatic Origins JOHNSON, Dave Distribution of Arsenic in Wisconsin Groundwater JOHNSON, J.R. and Kissin, S.A. Fluid Inclusion Evidence for a Role for Hydrothermal Activity in the Roby Zone, Lac Des lies Mine, Northwestern Ontario KELLY, Colleen, and Kean, William F. Rock Magnetic Studies of Phyllitic Zones from the Baraboo Syncline, Wisconsin KNOBELOCH, Lynda, Warzecha, Charles. and Nelson, Shelli Health Surveillance in a Community Affected by Arsenic-Contaminated Water LARSON, Phillip C. Potential for Copper Mineralization in the Animikie Group, Minnesota LIVELY, Richard and Morey, G.B. Contributions to the Cultural Geography of the West Mesabi Range, Northern Minnesota xxviii �MILLER, J.D., Jr., Wahi, T.E., Green, J.C.,Chandler, V.W., Severson, M.A., and Peterson, D.E. Digital Geologic Map of Northeastern Minnesota and Associated Databases in GEMS - a Modified Arcview Format MUDREY, M.G., Jr., and Brown, B.A. Structure of the Buried Precambrian Basement in Southwest Wisconsin and Its Influence on Regional Paleozoic Geology and Zinc-Lead Mineralization MUDREY, M.G., Jr., Brown, B.A. and Daniels, Daniels L. Preliminary Analysis of Aeromagnetic Data in Southern Wisconsin: The Role of Precambrian Basement in Paleozoic Evolution NEMITZ, Michael B. and Larson, Phillip C. Mineralogical Variations in Iron-formation in the Thermal Metamorphic Aureole of a Diabase Dike NEWKIRK, Trent T., Hudak, George J., and Hauck, Steven A. Preliminary Lava Flow Morphology Studies at the Five Mile Lake Vms Prospect, Archean Vermilion District, Ne Minnesota: Implications for Volcanic Processes, Volcanic Paleoenvironments, and VMS Exploration NICHOLSON, S.,W., Boerboom, T., Cannon, W.F., Wirth, K., and Isachsen, C.E. A New Look at the 1.1. Ga Chengwatana Volcanics in the St. Croix Horst, Minnesota and Wisconsin ODETTE, Jason D., Hudak, George J., Suszek, Thomas, and Hauck, Steven A. Preliminary Evaluation of Hydrothermal Alteration Mineral Assemblages and Their Relationship to VMS-style Mineralization in the Five Mile Lake Area of the Archean Vermilion Greenstone Belt, Northeastern Minnesota PETERSON, Dean M., Gallup, Christina, Jirsa, Mark A. and Davis. Donald W. Correlation of Archean Assemblages Across the U.S.-Canadian Border: Phase I Geochronology PEYCHAL, C., Kean, W.F., and Schaper, D. Magnetic Survey Near Waterloo Wisconsin PHILLIPS, Erin H., Wirth, Karl R., Veroort, J.D. Gehrels, G.E. Nd and U-Pb Isotope Studies of the Syenitic Aurora Sill, Mesabi Range, Minnesota REID, Daniel D. Freeze/Thaw Testing of Carbonate Aggregate Sources in Wisconsin - a Status Report SANDLAND, Travis 0., Wirth, Karl R., Vervoort, Jeff D., Gehrels, George E., Kennedy, Bryan C. and Harpp, Karen S. Roles of Fractional Crystallization and Assimilation in the Production of Midcontinent Rift Granophyres SMYK, Mark C., Stewart, Jennifer and O'Brien, Mark S. Platinum Group Element Exploration in Northwestern Ontario SNYDER, Stephen L., Ervin, C.Patrick, Geister, Daniel W., and Daniels, David L. A New Gravity Map of Wisconsin SOOFI, M.A. and King, S.D. Post-rift Evolution of the Midcontinent Rift System: Some Numerical Experiments Weissbach, Annette E., HEINEN Elizabeth M., and Lauridsen, Keld B. A Study of Well Construction for Arsenic Contamination in Northeast Wisconsin xxix �Industry and Informational Displays CRONK, William J. Layne Northwest, W229 N5005 DuPlainville Rd, Pewaukee, WI 53072. Phone (262)-246-4646 KIRCHER, Steve Crandon Mine Development, Nicolet Minerals, 7 N. Brown St, 3rd Floor, Rhinelander, WI 54501. Phone (715)365-1450 (Rhinelander office), (715)478-1516 (Crandon office) STEWART, Jennifer Ontario Geological Survey Resident Geologist Program, Northwestern Ontario District, Suite B002, 475 South James Street, Thunder Bay ON, P7E 6E3. Phone (807) 475-1108 SUNDEEN, S. Paul Michigan Department of Environmental Quality, Geological Survey Division, 735 E. Hazel Street, P.O. Box 30256, Lansing, MI 48909. Phone(517) 334-6959. xxx �The Blake Gabbro: A taxitic-textured gabbro sill south of Thunder Bay, Ontario. Shawn Beskar - University of St. Thomas South of Thunder Bay, Ontario, plutonic and hypabyssal rocks associated with the Keweenawan Rift (1109 Ma to 1086 Ma) intrude sedimentary rocks of the Lower Proterozoic (1.9 Ga) Animikie Group. Prior to the discovery of the Blake Gabbro, the igneous terrane south of Thunder Bay was thought to have been comprised of five distinct intrusions (Lightfoot and Lavigne, 1995): (1) Logan Sills; (2) Arrow River Dikes; (3) Pigeon River Dikes; (4) Crystal Lake Gabbro; (5) Pine River - Mount Molly Intrusion. Discovered in 1995, the Blake Gabbro is situated south of Thunder Bay within Blake Township, some 60 km north of the Duluth Complex. The region is characterized by northeast trending diabase-capped ridges and deeply eroded valleys. Positioned beneath a sequence of flat lying Logan Sills, the Blake Gabbro intrudes the argillites of the Rove Formation (Animikie Group). Since 1995, diamond drill cores that intersect the Blake Gabbro have been recovered and logged. From these cores it has been determined that the Blake Gabbro is a northeast trending, sub-horizontal sill of limited plan width but unknown strike length. The maximum thickness intersected by holes drilled thus far is 131 m. The sill thins to less than 20 mat its margins, some 300 m from the center of the body. Samples of the Blake Gabbro have been taken from the diamond drill cores for petrologic study. Initial studies indicate that the Blake Gabbro is a taxitic-textured sulphide-bearing sill. Plagioclase and pyroxene are present in roughly equal quantities. Elongate, cumulus plagioclase grains of variable size are enclosed by optically continuous intercumulus pyroxene. Initial analyses of plagioclase yield compositions ranging from An72 to An83. Minor amounts of olivine and biotite are present. Sulphide minerals consist of pyrrhotite and chalcopyrite. Preliminary whole-rock geochemical data obtained through XRF spectroscopy is presented in Table 1. The significance of the Blake Gabbro is realized upon comparison with the igneous terrane of Noril'sk, Siberia. The region south of Thunder Bay is thought to be equivalent in many respects to the geologic setting of the Noril'sk region and as such, may host large magmatic sulphide deposits (Lightfoot and Lavigne, 1995). The Keweenawan Osler Group Volcanic rocks are similar in composition to the Nadezhdinsky Formation lavas at Noril'sk. Both exhibit large degrees of crustal contamination (as evidenced by their high silica content and LaJSm ratio) and are depleted in nickel and copper. At Noril'sk, chonoliths (subvolcanic tube-like magma channels) containing mineralized picrites and gabbros served as feeders to the overlying sequence of flood basalt. It is thought that the Blake Gabbro may represent such a conduit. Chalcophile elements (nickel, copper and platinum group metals) missing from the associated Osler Group Volcanic sequence may reside within the Blake Gabbro, although the preliminary geochemical data presented seems to suggest otherwise. REFRENCES Lightfoot, P.C. and Lavigne, Jr., M.J. 1995. Nickel, copper, and platinum group element mineralization in Keweenawan intrusive rocks: new targets in the Keweenawan of the Thunder Bay region, northwestern Ontario: Ontario Geological Survey, Open File Report 5928, 32p. 1 �Table 1. Preliminary whole-rock geochemistry Major element data presented as weight per cent Trace element data presented as parts per million BP98.1-1 BP99.1-2 BP99.2-1 BP99.3-2 BP99.3-3 Si02 Ti02 51.42 49.29 48.54 52.00 51.18 3.38 1.43 1.27 2.95 3.48 A1203 13.84 18.52 15.18 13.65 13.97 Fe203 MnO MgO CaO Na20 1(20 P205 17.09 10.91 11.14 16.15 16.54 0.17 0.14 0.14 0.15 0.16 4.44 6.72 10.16 4.62 4.92 6.76 10.85 9.11 6.68 6.99 2.97 2.81 1.58 2.67 2.70 1.35 0.66 0.61 1.38 1.33 0.59 0.25 0.18 0.44 0.48 Total 102.01 101.58 97.91 100.69 101.75 Sc 23.5 25.6 25.2 25.8 26 V 337.5 224 200.9 343.9 396.8 Cr 51.8 157.1 104.8 63.1 48.9 Co 46.1 46.7 58 47.5 45.9 Ni 67 121.1 207 79.2 74.5 Zn Ga Rb Sr 162.5 82.9 90.6 172.9 144.9 25.6 20.9 19.5 23.9 24.1 54.9 23.2 29.3 61.3 60.8 425 306.6 348.8 447.4 501.8 Y 42.4 23.5 20.4 36.2 36 Zr 277.8 101.7 87.3 243.6 233.5 Nb 30.8 9.7 9.5 27.4 28.9 Ba 337.7 167.4 189.3 390.9 442.8 La Ce Pb 32.4 6.6 4.2 32.3 24.8 79.5 26.6 25.6 77.3 68.7 7.5 3.9 3 10.1 6.7 2 �ALTERATION AND PGE-AU MINERALIZATION IN THE NORTH ROBY ZONE, LAC DES ILES MINE, NORTHWESTERN ONTARIO BIHARI, D.B. and KISS1N, S.A., Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E 1, stephen.kissin(Z)lakeheadu.ca The Lac des Ties Complex appears as a linear zone of mafic plutons that trend east to northeast and extends from Lake Nipigon to Atikokan in northwestern Ontario (Sutcliffe, 1986). The complex is situated in Archean granitoids that consist of gneissic tonalites, medium-grained hornblende diorites and quartz diorites. The Lac des lies Complex occurs in a circular outcrop fashion that is approximately 30 km in diameter and is the largest of a series of mafic to ultramafic intrusions (Sutcliffe, 1986). The Roby Zone was the initial site of mining at the Lac des Iles Mine. The North Roby Zone is its narrow northward extension. The North Roby Zone contains a narrow strip (<50 m) of anomalously high PGE and Au values associated with sparse sulfides called "noseeum ore". Five stripped outcrops approximately 50 x 10 m were studied in the North Roby Zone. Alteration of primary pyroxene to talc and pink coloration of recessively weathered plagioclase is strongly suggestive of hydrothermal alteration. The five stripped outcrops reveal a northeasterly striking, steeply dipping sequence of leucogabbro, varitextured gabbro, pyroxenite and east gabbro. A total of 32 hand samples were collected and studied in thin section. From these 21 were selected for whole-rock and trace element analysis in order to compare chemistry of altered and unaltered samples. Hydrothermal alteration appears to have affected the primary ortho- and clinopyroxenes of the host rocks, progressively converting them to talc. Other petrographic indications are obscure, as regional metamorphism has overprinted the Lac des Iles Complex and its mineralized rocks. The grade of metamorphism is the albite-epidote subfacies of greenschist facies as evidence by incipient breakdown of plagioclase to sericite and clinozoisite, chioritization of pyroxenes and formation of tremolite-actinolite, as well as minor metamorphic albite. Chlorite coronas surround mafic minerals, and develops decussate assemblages of chlorite and tremolite-actinolite. Minor penetrative deformation is evident in undulatory extinction in plagioclase and development of weak schistosity. The development chlorite coronas and general overprinting ofmafic minerals by chlorite and sericitization ofplagioclase are significant in distinguishing hydrothermal alteration from subsequent regional metamorphism. Analysis did not generally reveal striking compositional variations in the host rocks; however, more detailed analysis did show chemical effects of alteration. Chondrite-normalized REE plots revealed that all REEs were depleted relative to unaltered rocks; however, in most altered leucogabbro and varitextured gabbro, minor to insignificant depletion of Eu relative to other REEs produced an apparent positive Eu anomaly in the plots. Other chemical changes, because of their subtle expression and obscurity owing the problem of closure, were examined by use of Pearce Element Ratios. Most notable was Na-depletion due to alteration, which can be distinguished from igneous fractionation effects in plagioclase. Sutcliffe, R.H., 1989. Magma Mixing in Late Archean Tonalitic and Mafic Rocks of the Lac des lies Area, Western Superior Province. Precambrian Research, vol. 44, pp.81-101. 3 �________________ REDEFINED VOLCANIC AND SEDIMENTARY STRATIGRAPHY OF THE NORTHERN ST. CROIX HORST IN PINE COUNTY, MINNESOTA, AND THE APPLICATION OF ARC VIEW TO GEOLOGIC MAPPING BOERBOOM, Terrence J. (Minnesota Geological Survey, boerb001@unm.edu) Pine County, Minnesota, located on the northwestern margin of the St. Croix horst (Fig. 1), contains bedrock that ranges from Archean to Pale ozoic in age; however, most of the county is underlain by rocks of the Midcontinent rift (Fig. 2). The southeastern half of the county is underlain by mafic volcanic rocks ofthe St. Croix horst, late rift-filling sedimentary rocks (Hinckley Sandstone and Fond du Lac Formation) underlie the central part of the county, and the far northwestern corner is made up of Archean and Paleoproterozoic rocks (see Boerboom, this volume). This county was recently remapped by the Minnesota Geological Survey (MGS)', but due to generally poor outcrop, the mapping relied heavily on geophysical and drill hole data. ArcView GIS software proved useful in manipulating and integrating these data, particularly the 4000 water wells contained in the MGS County Well Index database. This presentation is intended to demonstrate the usefulness of Arc View software in map construction, and also to show the results of our mapping efforts in T Pine County. Other Arc files to be demonstrated /1 — Pine include maps of depth to bedrock, bedrock topography, and surficial geology. Craddock (1972) provides a review of the history I — St. — -) Croix horst of investigations in Pine County and environs, but some of the notable early accounts of the bedrock geology in Pine County are by Upham (1888) and Hall (1901); Grout (1910) described the occurrence of copper mineralization in Pine County. The most recent published geologic map to cover Pine County is 1:250,000scale(Moreyandothers, 1981). Ourlatest mapping coincides with other mapping projects in — N / Sedimentary rocks Intrusive rocks Volcanic rocks \ I Figure 1. Location of Pine County relative to the St. Croix horst and the Mid-continent rift system. adjacent Wisconsin (Wirth and others, 1998, Cannon and others, 2001; Nicholson and others, 2001). Geologists on both sides of the border benefited greatly from the recent acquisition of high-resolution aeromagnetic data in Wisconsin by the U.S.G.S., as summarized by Cannon and others (2001). The St. Croix horst, part of the Mesoproterozoic Midcontinent rift system, is comprised of subaerial mafic volcanic rocks that have traditionally been lumped together as the Chengwatana volcanic group. Based on our work and that of the U.S.G.S., the term "Chengwatana" is now restricted only to those volcanic rocks that lie between the Douglas and Pine Faults (Fig. 2). The demarcation of Keweenawan sedimentary vs. volcanic rocks (i.e. the Douglas Fault) is welldefined on the basis of water well information. The Pine Fault (Fig. 2) lies inboard of and parallel to the Douglas Fault, and as summarized by Cannon and others (2001), may have been a bounding fault that controlled the distribution of graben-fill volcanic rocks. Hall (1901) first applied the term "Chengwatana Series" to a series of steeply dipping basalt flows and interfiow conglomerates exposed along the Snake River near Pine City, Minnesota. According to Hall: "Thefloodofl898, which tore away the dam at the foot of Cross Lake and poured down the [Snake] river a vast volume of water, cleaned out in an admirable manner for examination the channel of the stream for several miles." In this stretch of outcrop, Hall recognized 65 steeply dipping lava flows and five interfiow conglomerates. Our remapping of this now less wellexposed sequence identified 37 basalt flows that range from 10 to 300 feet thick, and six interfiow conglomerates that range from 10 to 100 feet thick, although a more careful examination might reveal more flows. The interfiow conglomerates contain abundant round boulders of Keweenawan-type granophyric granite and porphyritic basalt from an unknown source. This sequence, the type locality for the Chengwatana volcanic group, dips about 65 degrees east, and starts within a few hundred feet of the Douglas Fault. Other outcrops of the redefined Chengwatana group that are adjacent to the Douglas Fault dip 40 to 70 degrees east, and those near the Pine Fault dip 10 degrees west. This change in dip is consistent with aeromagnetic patterns that indicate the presence of a southward-plunging syncline that merges 4 �into an unnamed fault (Fig. 2). Hall (190 l)recognized this syncline on the basis of the change in dip direction. Sedimentary rocks similar to the Fond du Lac Formation are present on top of the horst at the south edge of the county. The Minong volcanics (approximately 1094 Ma; Nicholson and others, 2001), part of the northeast-plunging Ashland Syncline, lie to the east of the Chengwatana group, and are interpreted to be younger. The Minong volcanics are distinguished from an unnamed central pile of volcanic rocks to the north by divergent linear aeromagnetic patterns. Aeromagnetic lineaments imply that the rocks in this central panel are folded into a doubly-plunging anticline (Fig. 2). Several aeromagnetically-inferred, reverse-polarized diabase dikes cut all three of the volcanic packages in Pine County. The Hinckley fault (Fig. 2) is proposed as a splay from the Douglas Fault that has displaced the contact between the Hinckley Sandstone and Fond du Lac Formation slightly upward, based on outcrop and geophysical data. North of this fault, the Hinckley Sandstone is typical cliff-forming, uniform and fine-grained quartz arenite, whereas south of the fault the sandstone forms subdued outcrops, is slightly more feldspathic, and contains scattered cobbles of quartzite and minor agate. Locally, tributary streams have cut through Sandstone similar to Fond du Lac Formation Figure 2. Simplified geologic map of Pine County, Minnesota. this feldspathic sandstone and exposed conglomeratic sandstone, typified by trough cross-beds with trough bases lined by small basalt cobbles, which is assigned to the underlying Fond du Lac Formation. Although the major distribution of rock types has not changed significantly as a result of this mapping effort, we have been able to refine the volcanic stratigraphy of the St. Croix horst. Pronounced linear trends on aeromagnetic maps, essentially parallel to the strike of bedding in volcanic rocks, outline the different volcanic basins, and the measured orientations of volcanic flows in outcrops match those aeromagnetic trends. 'This work was done as part of the Pine County Geologic Atlas (Minnesota Geological Survey, County Atlas Series, in prep.), which includes data base, bedrock geology, surficial geology, Quaternary stratigraphy, depth to bedrock and bedrock topography, and mineral resource plates. References: Cannon, W.F., Daniels, D.L., Nicholson, S.W., Phillips, J., Woodruff, L.G., Chandler, V.W., Morey, G.B., Boerboom, T.J., Wirth, K., and Mudrey, M.G., Jr., 2001, New map reveals origin and geology of North American Mid-continent rift: Eos, v. 82, no. 8, p. 97. Craddock, C., 1972, Keweenawan geology of east-central and southeastern Minnesota, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota—A centennial volume: Minnesota Geological Survey, p. 4 16-424. Grout, F.F., 1910, Keweenawan copper deposits: Economic Geology, v. 5, p. 471-476. Hall, C.W., 1901, Keweenawan area of eastern Minnesota: Bulletin of the Geological Society ofAmerica, v. 12, p. 312342. Morey, G.B., Olson, B.M., and Southwick, D.L., 1981, East-central Minnesota, bedrock geology: Minnesota Geological Survey, scale 1:250,000. S.W., Boerboom, T.J., Cannon, W.F., and Wirth, K., 2001, Reinterpretation of the Chengwatana volcanics in Nicholson, the St. Croix horst, Minnesota and Wisconsin: Geological Society ofAmerica, North-Central Section Abstracts, 35" Annual Meeting. Upham, W., 1888, The geology of Pine County, in Winchell, N.H., and Upham, W., eds., The geology of Minnesota: Minnesota Geological Survey Final Report 1, v. 2, p. 629-645. Wirth, K., Cordua, W.S., Kean, W.F., Middleton, M., and Naiman, Z.J., 1998, Field guide to the geology of the southeastern portion of the Midcontinent rift system, eastern Minnesota and western Wisconsin: Institute on Lake Superior Geology 44" Annual Meeting, Minneapolis, Minn., Proceedings, v. 44, Pt. 2, Field trip guidebook, P. 33-75. 5 �STRATIGRAPHY OF THE PALEOPROTEROZOIC DENHAM FORMATION-A CONTINENTAL MARGIN ASSEMBLAGE OF BASALT, ARKOSE, AND DOLOMITE BOERBOOM, Terrence J., and JIRSA, Mark A. (Minnesota Geological Survey, boerb001@umn.edu andjirsa001@umn.edu) The Paleoproterozoic Denham Formation, as originally defined, consists of metamorphosed quartz-rich sedimentary rocks, dolomite, and mafic volcanic rocks (Morey, 1978). The type locality of the Denham Formation, in northwestern Pine County, Minnesota (see figure 2 in Boerboom, this volume), consists of a series of outcrops in and near an abandoned glacial outwash channel. The Denham Formation forms a pronounced linear, positive aeromagnetic anomaly that can be traced from the exposures for 40 miles to the west, to Mille Lacs Lake (Boerboom and others, 1999). This anomaly follows the northern margin of the Archean McGrath Gneiss (2550±14 Ma, Van Schmus and others, this volume). The anomaly is produced by scattered chert-magnetite clasts within fragmental volcanic rocks. The Denham area was mapped as part of the Pine County Geologic Atlas (see Boerboom, this volume). In addition to the outcrops, 15 exploratory drill cores and cuttings holes were utilized in the map interpretation, as was information from water wells contained in the Minnesota Geological Survey County Well Index. The results will be published on the forthcoming 1:100,000 scale geologic map of Pine County, which will include a 1:12,000 scale inset map of the type locality. The rocks of the Denham Formation have undergone regional, amphibolite-grade metamorphism and at least two periods of deformation. The mafic volcanic rocks are amphibolitic, but contain well-preserved primary features. The granular sedimentary rocks are recrystallized, but retain much of their primary grain size, shape, and composition. In contrast, layers interpreted as pelitic sedimentary rocks are recrystallized to garnet-staurolite-sericite schist. Dolomite is completely recrystallized to marble, with local relict bedding features. The first of two deformation events was synchronous with metamorphism to the garnet zone of the amphibolite facies (Holm, 1986). It produced Si foliation that typically is parallel to bedding, and a locally strong, shallowly plunging, stretching lineation. The second deformation folded Si and bedding along steeply dipping axes, and was concurrent with or followed by peak metamorphism that produced staurolite. In the Denham valley, the stratigraphic sequence dips variably to the north, having local F2 folds with overturned limbs. North of the valley, bedding and SI in graywacke are nearly horizontal, and are deformed into open F2 folds with local crenulation features. North of these graywacke outcrops, the bedding dips to the south, defining a broad, regional-scale, F2 syncline. Despite deformation and metamorphism, the stratigraphy of the Denham Formation forms a coherent package that is shown schematically on Figures 1 and 2. In this discussion, metamorphic rock names, and the prefix "meta" are omitted for clarity. The base of the Denham consists of interbedded siltstone and cross-stratified pebble conglomerate. This is overlain by coarse-grained and locally conglomeratic arkose that apparently pinches out laterally. The arkose is interbedded 3-10 Graphitic argillite with amygdaloidal basalt flows that grade stratigraphically upward (northward) from Dolomite >500 massive, to pillowed, to fragmental. The volcanic rocks are thickest at the eastern limit of outcrop, where at least four flows of nearly 1000 feet total thickness were c Fragmental mafic ol anic rocks 700 recognized and thin westward to two flows of 300 feet total thickness This distribution implies that the eastern exposures are nearest to the vent, which may lie beneath the DOIOiIC ad arkose Fond du Lac Formation (Fig 2) The overlying arkosic and pelitic strata apparently Shale 350 pinch out to the east where the volcanic package thickens, and are not present in drill holes to the north and east of the Denham valley. The northern-most outcrops in the Pillowed basalt valley consist of very pure dolomite, now marble, having ptygmatically folded and 3001000 strongly lineated quartz veins. Drill cores show that the dolomite is at least 500 feet Dolomiticarkose 200 thick, and is overlain by graywacke that is exposed discontinuously to the north for some distance. The contact between dolomite and overlying graywacke is marked by Siltstone 1100 a thin layer of graphitic argillite. Field and petrographic observations imply that clastic detritus in the Denham Formation was derived in large part from a weathering residuum on the subjacent McGth McGrath Gneiss. Near the contact with the Denham Formation, the McGrath grades Gneiss : abruptly from granite gneiss containing quartz, orthoclase, plagioclase, and biotite; to Figure 1. Stratigraphic column of strongly foliated, quartz- and sericite-rich schist that contains orthoclase, but no Denham Formation; thicknesses plagioclase. The arkosic parts of the Denham Formation similarly lack plagioclase, in feet. Ok 6 �Fraacvolc Dolomitic mble —----- — I r' -— t. t_ L - - Lva_3. Explanation Metagraywactse (Palnoproterozotc) Strike and dip of inclined bedding showing younging dtrection Direction and plunge of lineations inctnding Outcrop — elongate metamohic mineeds, fold axes. on McOuath Ofleiss Strike and dip of Fl cleavage Map urea — Fond On Lac Formation _ Conlacl between basalt flows and elongate pillows -r-10 Strike and dip of inclined bedding, youngiug direclion unknown —r_ Strike and dip of overturned bedding. in this example beds top to northeast but dip southwest Geologic contact Figure 2. Simplified geologic map of the Denham Formation and adjacent McGrath Gneiss. and are composed of quartz and orthoclase grains, together with scattered clasts of granitic gneiss. Studies of saprolite developed beneath Cretaceous sedimentary rocks on Precambrian crystalline rocks in southwestern Minnesota may provide an analog (Setterholm and others, 1989). These studies demonstrate that plagioclase is one of the first minerals to alter to kaolinitic clay during the weathering process, and that orthoclase and quartz are the most resistant to weathering. The basal Cretaceous strata locally consists of reworked saprolite, including beds of cross-stratified sandstone and nearly pure kaolinitic shale, Exposures of basal Cretaceous sedimentary rocks locally contain detrital orthoclase and quartz derived by slight reworking of grus-textured, weathered granite. We infer that the same process occurred in the Paleoproterozoic by erosion and reworking of weathered McGrath Gneiss into beds of arkose and kaolinitic shale. These were subsequently metamorphosed to produce recrystallized arkose and staurolite-garnet- sericite schist. Weathering of orthoclase may have liberated potassium for the inferred conversion of kaolinite to sericite during metamorphism. The Denham Formation is interpreted to represent a rift-margin assemblage deposited during the Paleoproterozoic, genetically similar to, and perhaps temporally equivalent with, the Chocolay Group in Michigan. In this setting, the McGrath Gneiss was part of the continental margin that was weathered and eroded to provide detritus to an evolving rift basin undergoing active, shallow water volcanism. Interbedded arkose and dolomite higher in the stratigraphic section represent foundering of the shelf and deepening water, possibly by subsidence of localized grabens. The lack of arkose in the upper, dolomite-dominated part of the sequence indicates that deposition of coarse detritus was restricted to the shallow, nearshore environment adjacent to the McGrath. The sedimentological gradation of dolomite to graywacke stratigraphically upward indicates further deepening water and associated turbidite deposition. The deformation of the Denham Formation is inferred to be the product of basin closure during the Penokean orogeny. References: Boerboom, T.J., Severson, M.J., and Southwick, DL., 1999, Bedrock geology of the Mule Lacs 30 x 60-minute quadrangle, eastcentral Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-l00, scale 1:100,000. Holm, D.K., 1986, A structural investigation and tectonic interpretation of the Penokean Orogeny: east-central Minnesota: Unpubi. M.S. thesis, University of Minnesota, Duluth, 114 p. Morey, GB., 1978, Lower and Middle Precambrian stratigraphic nomenclature for east-central Minnesota: Minnesota Geological Survey Report of Investigations 21,52 p. Setterhoim, DR., Morey, GB., Boerboom, T.J., and Lamons, R.C, 1989, Minnesota kaolin clay deposits—A subsurface study in selected areas of southwestern and east-central Minnesota: Minnesota Geological Survey Information Circular 27, 99 p. 7 �GIS Applications for Resource Inventory and Land-use Planning in Wisconsin B. A. Brown and M. L. Czechanski Wisconsin Geological and Natural History Survey Madison, Wi Wisconsin has recently enacted comprehensive "smart growth"land-use planning legislation that specifically requires counties and local units of government to consider metallic and nonmetallic mineral resources as they develop and adopt a comprehensive plan by 2010. Wisconsin's nonmetallic mine reclamation rules take effect in 2001. These rules contain provisions to protect undeveloped aggregate deposits from zoning changes, and designate end uses for reclaimed sites, both of which link reclamation into the planning process. Implementation of mandatory reclamation and planning for future supplies both require an inventory of active production sites. In addition, planning requires analysis of geologic data to identify location, extent, and quality of undeveloped resources. Wisconsin Geological and Natural History Survey (WGNHS) is working in cooperation with the U.S. Geological Survey and several state agencies to compile a spatial database of active operations and historic mineral production sites. This database will ultimately link existing state and federal databases containing a variety of information on location, lithology, formation, engineering testing, permit status etc., through a common identification number. WGNIHS is also working with county and local governments to inventory active sites and to assemble and assess the quality of geologic data available for comprehensive planning. Computerized geographic information system (GIS) technology provides powerful new tools for inventory and analysis of mineral resource information. Digital coverages showing the locations of mines, pits, and quarries, and spatial databases documenting the character and extent of deposits can now be easily incorporated with bedrock and surficial geology, soils maps, water resource maps and an ever increasing variety of other environmental, cultural, and political coverages to feed directly into the land-use planning process. We will present an interactive demonstration of statewide and county geologic and mineral resource coverages recently produced by WGNHS and discuss some of the land-use planning methodologies and applications currently under development. 8 �Field Trip 3:Economic Geology of the Baraboo and Waterloo Quartzites Bruce A. Brown (1), Frank R. Luther (2), Susan M. Courter (3), James W. Schmitt (4), and Jennifer Lien (5) Field trip 3 will examine the aggregate and industrial mineral resources of the Proterozoic Baraboo and Waterloo Quartzites of southern Wisconsin. In the Waterloo area twenty miles east of Madison, we will visit a large quarry that produces construction aggregate, breakwater stone, and railroad ballast. We will travel from waterloo to the Baraboo area, where we will visit five operations that produce a variety of aggregate products. The Proterozoic quartzites of southern Wisconsin have long been recognized for their unique hardness and durability, refractory properties, and resistence to weathering. Quarries have operated in both areas for more than a century, producing a variety of industrial mineral products and aggregates. Today the major uses for this hard and durable rock are railroad ballast, riprap and breakwater stone, and crushed stone base material. A variety of specialty aggregates ranging from seal coat chips to leachate collection and filter bed material are produced as well. Stop 1 will be the Michels Materials Waterloo quarry. This operation was opened in 1988 as a source of large stones with high resistance to freeze-thaw loss, for use in constructing breakwaters and erosion control structures on the great lakes. Bedding in the Waterloo Quartzite is up to 2 meters thick and joints are widely spaced, allowing blocks up to 10 tons or larger to be quarried. Crushed material was first produced only as a means of disposing of undersize waste rock. Michels continues to produce breakwater stone that exceeds all Corps of Engineers specifications, but much of the current output is crushed for ballast and construction aggregate. We will not have time to visit the historical quarries located to the south of the Michels Quarry which were operated in the early 1900s for refractory blocks, but the geology of the Michels quarry and the old quarry area is described in previous guidebooks by Luther(1992, 1997). From Stop 1 we will travel northwest across the glaciated landscape of Dane and Colombia counties to the Baraboo Range. This drive will provide a look at a classic drumlin landscape. Stop 2 will be the Williams quarry, operated by the Kraemer Company. This quarry is located on the north limb of the Baraboo syncline and utilizes the Parfreys Glen Formation, a time-transgressive, near-shore deposit that accumulated around the Baraboo bluffs during Cambro-Ordovician submergence. This quarry produces aggregate that is essentially a quartzite gravel. As the quarry goes deeper into the hillside, more solid quartzite is being quarried. The sedimentary structures in the coarse basal conglomerates and overlying sandstones are spectacular. Stop 3 will briefly examine the 1760 Ma. Rhyolite that underlies the quartzite, 9 — �exposed in a road cut on STH 33. Stop 4 will be Milestone Materials Jesse Pit a combination gravel pit/quartzite quarry on top of the south range. Lunch will be in Devils Lake State Park, near the site of a former refractory and abrasive quarry. After lunch we will visit Milestone's Fox Ridge pit and asphalt plant where a variety of quartzite aggregate products are produced, sold, and made into asphalt paving materials. We will next visit the Martin-Marietta Rock Springs Quarry, a historic operation now a major producer of ballast. We will finish at the Kraemer Co, LaRue Quarry on the south range near the site of the historic Sauk and Illinois iron mines. LaRue Quarry contains many examples of sedimentary and tectonic structures as well as examples of quartzite weathering. The trip will return to the Sheraton by 6:00 PM. Luther, F.R., (1992 ), The Waterloo Quartzite at the old Portland Quarry: in The 56th Annual TriState Geology Field conference Guidebook to the Geological setting of whitewater, Wisconsin and surrounding Area, Jack Travis, ed. P51-61. Luther, F.R. (1997), The Precambrian Waterloo Quartzite, Dodge and Jefferson Counties, Wisconsin—Petrology, Structure, and Industrial Use: in Mudrey, M.G., Jr., ed. Guide to Field Trips in Wisconsin and Adjacent areas of Minnesota., 31st Meeting Northcentral Section, Geol. Soc. Am., Madison, WI, p.31-35. 1) Wisconsin Geological Survey, Madison, WI 2 )UW-Whitewater, Whitewater, Wi 3) Michels Materials, Inc., Brownsville, WI 4) D.L.Gasser Construction, Baraboo, WI 5) The Kraemer Company, Plain, WI 10 �RECENT DEVELOPMENTS IN THE MINERALOGY OF THE NINE MILE PLUTON, WAUSAU COMPLEX BUCHHOLZ, Thomas W., 1140 12th Street North, Wisconsin Rapids, Wisconsin 54494; FALSTER, Alexander. U., and SIMMONS, Wm. B., Department of Geology and Geophysics, University of New Orleans, New Orleans, Louisiana 70148 The mid-Proterozoic Wausau Complex is composed of four intrusive centers; from north to south the Stettin, Wausau, Rib Mountain and Nine Mile plutons (Meyers eta!, 1984). The Stettrn intrusion is the oldest and most ailcalic, and the three other plutons are progressively younger and more siicic. The youngest and most silicic intrusion, the Nine Mile Pluton, is an epizonal anorogenic and heterogeneous granitic intrusion, with locally abundant pegmatites, aplites and miarolitic zones. Miarolitic cavities in the pegmatites as well as miaroles within some phases of the granite attest to shallow levels of emplacement of the pluton. At the surface the granite is altered to a friable disaggregated material called "grus" that is extensively quarried for use as road gravel. The Nine Mile pluton's pegmatites and aplites contain a wealth of mineral species (Faister, 1981, 1987, Hanson et a!., 1998. Buchholz eta!., 1999, 2000), which show heterogeneous distribution for many species across the pluton. Titanium oxide species, such as anatase, brookite, and rutile are abundant in pegmatites of the northern part of the pluton and are far less conspicuous in the central and southern portions. Anatase is by far the most abundant polymorph in the Nine Mile pluton. Niobium and tantalum mineralization is sparse in the northern part of the pluton but becomes more abundant in the central and southern parts. As in most other anorogenic pegmatites, Nb> Ta in Nb-Ta oxides such as ferrocolumbite, uranopyrochlore, liandratite/petschekite, and ilmenorutile. However, in some small-scale, restricted environments late-stage Ta-enrichment is manifest as tapiolite, manganotantalite, strueverite and microlite. In Ta-rich areas there is also a dramatic increase in the abundance of fluorite (Buchholz et al., 1999, 2000). In these areas fluorite becomes a significant mineral phase in some of the pegmatites and even in the adjacent granite. Beryllium mineralization is dominated by phenakite and bertrandite. Rare beiyl, bavenite, and euclase tend to be more common in the northern segment of the pluton. Lithium mineralization is absent but elevated contents of Li are found in micas (lithian biotite and zinnwaldite) in the central and northern parts of the pluton, typically associated with more fractionated mineral species of the Nb-Ta oxides. LREE-mineralizalion in the Wausau complex is dominated by phosphates of the monazite group, rhabdophane, and by carbonates of the bastnaesite group. HREE-minerals are essentially restricted to xenotime-group minerals. Both LREE and HREE minerals are found throughout the pluton. Unlike the REEminerals in the South Platte district in Colorado (Simmons Ct a!., 1987), REE-minerals in the Nine Mile pluton occur as small crystals and grains throughout the pegmatites (Hanson et a!., 1998), whereas in the South Platte pegmatites, they form large masses in the core margin and in the replacement units. Zirconium mineralization is restricted to zircon, which occurs as an accessory mineral throughout the complex, but more HI-enriched examples are restricted to the high-F environments. Tin is exceedingly rare but it has been found as cassiterite in the central and western portions of the pluton. Some Nb-Ta-oxide minerals from this area contain elevated Sn-content, as well. Manganese mineralization tends to increase from north to south throughout the pluton. The mineralogy of the Nine Mile pluton exhibits some unusual geochemical trends which are not commonly seen in other anorogenic intrusive systems (which are typical NYF-type environments, i.e. Nb, Y, and F-enriched): The strong Ta-, HI- and minor Li-, and Sn-enrichment, if only in localized environments, are far more characteristic of LCT-type pegmatites (Li, Cs, and Ta-enriched). REFERENCES: Buchholz, T. W., Falster, A. U. & Simmons, Wm. B. 1999. Ta, Nb, U, Y, and REF Minerals of the Koss Quariy, Marathon County, Wisconsin: The 26th Rochester Mineralogical Symposium, Abstracts of Contributed Papers. p.6. Buchholz, T. W., Falster, A. U. & Simmons, Wm. B. 2000. Additional Mineralogy of the Koss Quarry, Miarathon County, Wisconsin: The 27" itochester Mineralogical Symposium, Abstracts of Contributed Papers. P.S. 11 �Falster, A. U. 1981. Minerals of the Wausau Pluton: The Mineralogical Record, 12, P. 93-97. Faister, A. U. 1987. Minerals of the Pegmatitic Bodies in the Wausau Pluton, Marathon Co., Wisconsin: Rocks and Minerals, 62, p. 188-193. Faister, A. U., Simmons, Wm. B., Webber, K. L., & Buchholz, T.W. (in press). Peginatites and Pegmatite Minerals of the Wausau Complex, Marathon C., Wisconsin: Special volume published by the Societa Italiana di Scienze Naturali Hanson, S.L., Faister, A.U., Simmons, W.B., Webber, K.L., Buchholz, T. 1998. Rare-Earth-Element (REE) Mineralization of Pegmatites in the Wausau Complex, Marathon County, Wisconsin: The 25th Rochester Mineralogical Symposium, Abstracts of Contributed Papers. p. 12. Myers, P.E., Sood, M.H., Berlin, L.A. & Faister, A.U. 1984. The Wausau Syenite Complex, Central Wisconsin: Thirtieth Annual Inst. On Lake Superior Geology, Field Trip Guidebook 3. Simmons, W. B., M. T. Lee, and R. H. Brewster 1987. Geochemistry and evolution of the South Platte granitepegmatite system, Jefferson Co., Colorado. Geochimica et Cosmochimica Ada, 51, 455-471. 12 �CAMBRO-ORDOVICIAN STRATIGRAPHY OF SOUTHERN WISCONSIN: SEQUENCE STRATIGRAPHY RULES Byers, C.W., Dept. of Geology and Geophysics, University of Wisconsin, Madison, WI 53706 cwbyers(i).geology.wisc .edu During the past decade, the series of sandstone and carbonate formations that range from Late Cambrian through Late Ordovician have been reinterpreted n terms of the tenets of sequence stratigraphy. Older interpretations relied heavily on the facies concept; while some facies changes are still accepted, others have been superceded by the recognition of subtle unconformities, both at formation contacts and within formations. The new approach breaks the stratigraphic column into numerous unconformity-bounded units, indicating many short-term sea level fluctuations. These cycles are shorter by more than an order of magnitude than the major cratonic sequences originally defined by Sloss. Rarely do they local sequences show the full range of features expected in a complete cycle: subaerial weathering, lowstand sediments, marine transgressive surface, zone of maximum flooding, offlapping deposits. More typically, the sequences are asymmetric and truncated, with their thicknesses dominated by only one phase of the transgressive-regressive cycle. For example, the Cambrian Jordan Sandstone consists of two shaling-upward marine packages (offlaps) separated by a surface of transgression; lowstand and transgressive deposits are lacking. In contrast, the Ordovician St. Peter Sandstone is composed mostly of eolian lowstand and transgressive marine strata, with a thin cap of Glenwood Shale representing the zone of maximum flooding. Offlapping strata are thin or absent. Because southern Wisconsin lies on the flank of a cratonic dome, minor unconformities might be expected to grade into comformable sections downdip into the surrounding basins but reappear on other cratonic highs, if the cycles are eustatic in origin. 13 �A preliminary interpretation of new aeromagnetic and gravity data in Wisconsin W. F. Cannon, David L. Daniels, Stephen L. Snyder, Suzanne W. Nicholson, USGS, Reston, VA This geologic sketch map showing Precambrian basement terranes of Wisconsin is an early interpretation of newly acquired and compiled gravity and aeromagnetic data. Geophysical data are supplemented by bedrock mapping in the north and by limited drill hole information and erosional windows through Paleozoic cover in the south. The map presents a new picture of parts of the Precambrian basement in the southern and western parts of the state where it is largely concealed by a thin cover of Paleozoic strata. The map allows inferences on the mineral resources of shallowly buried basement rocks. NOKEAN FOLD AND THu rfI'?* • Most of the basement of Wisconsin is composed of rocks formed or modified during the Penokean orogeny, roughly 1850 m.y. ago. Isotopic evidence indicates that Penokean crust extends throughout the southern part of the state where younger granite, rhyolite, 14 �and quartzite lie unconformably on it. All were folded and metamorphosed in the foreland of the Mazatzal orogen sometime after 1760 Ma. Archean crust can be confidently traced as far south as the Trempealeau fault, but there is no geologic or isotopic evidence for it south of that structure. The southern Penokean Province appears to be entirely juvenile crust formed during the Penokean orogeny and composes the continental basement for the slightly younger Mazatzal orogeny. Large anorogenic granite plutons were intruded at about 1450 Ma and mafic plutons of unknown age also are widespread. Finally, dikes of diabase, probably related to the Midcontinent rift, cut all other Precambrian units. Diabase dikes Midcontinent Rift- sandstone in flanking basins. I. I Midcontinent Rift- basalt flows and conglomerate in central horst (1100 Ma). Anorogenic granite plutons and related rhyolite. Roughly 1 450 m.y. old. Mafic plutons of unknown age. Identified by circular to ovoid corresponding magnetic and gravity anomalies. Quartzite, lesser argillite and schist, minor iron-formation. Unconformable on 1 760 Ma rhyolite and granite. Strongly folded and variably metamorphosed. Granite plutons (1 760 Ma). Post -orogenic plutons with respect to Penokean orogeny. Rhyolite and epizonal granite (1760 Ma). Contains undifferentiated areas of younger quartzite. Strongly folded in Mazatzal orogeny. ROCKS OF PENOKEAN OROGEN Fold and thrust belt- Early Proterozoic metasedimentary and metavolcanic rocks and Archean basement gneisses. 1' . VV v I V V• Y Pembine-Wausau terrane- Early Proterozoic metavolcanic rocks, syntectonic granite. Archean basement lacking or discontinuous. Marshfield terrane- Archean gneiss and infolded Early Proterozoic metavolcanic and granitic rocks. Mostly granitic gneisses based on low gravity values. Ia—I V V V V V K V V *VXXXX V V V 1.' Marshfield terrane- Archean gneiss and infolded Early Proterozoic metavolcanic and granitic rocks. Mostly mafic gneiss based on high gravity values. Southern Penokean terrane- poorly known unit with high gravity and magnetic anomalies. Probably mostly mafic metavolcanic rocks. Contains undifferentiated areas of quartzite and 1 760 Ma rhyolite and granite. Southern Penokean terrane- poorly known unit with low gravity and magnetic anomalies. Probably mostly felsic rocks. Contains undifferentiated areas of quartzite and 1 760 Ma rhyolite and granite. Northern limit of Paleozoic strata. 15 �REGIONAL ARSENIC ANOMALIES SHOWN BY NURE STREAM SEDIMENT AND HYDROGEOCHEMICAL DATA IN NORTHERN WISCONSIN AND MICHIGAN W. F. Cannon, USGS, Reston, VA L. G. Woodruff, USGS, Mounds View, MN A regional arsenic anomaly in northeastern Wisconsin and the upper peninsula of Michigan is identified in the NURE (National Uranium Resource Evaluation) surveys of stream sediments and ground water. The anomalous region is about 250 miles long in north-south direction and as much as 75 miles wide. Examination of the anomaly with regard to bedrock and glacial geologic features suggests that it is a composite anomaly caused by two different bedrock sources of arsenic and variations in glacial dispersal of arsenic-rich bedrock. The two sources differ in their expression. One, the Michigamme anomaly is expressed mostly in stream sediments and to a lesser degree in well water. The other, the Fox River Valley anomaly is expressed strongly in well water, but has almost no stream sediment signature. Figure 1. Composite arsenic anomaly map of northern Wisconsin and Michigan. Base is map of glacial lobes. The shaded semi-transparent surface shows a combined anomaly from both NURE stream sediment and well water data. The surface shows the more anomalous of the two data sets relative to the regional mean values of 2 ppm As for stream sediments and 0.65 ppb As for well water. Only areas with arsenic above regional mean values are shown in the 3-D surface. The surface is defined by about 3200 stream sediment analyses and 3500 well water analyses. Black unit is arsenic-bearing Michigamme Formation and heavy line is the outcrop trace of arsenic-bearing Ordovician sandstone. 16 �Figure 2. A. The Fox River Valley anomaly shown by well water. Anomaly lies mostly west of arsenic-rich sandstone. B. The Michigamme anomaly shown by stream sediments. Anomaly location is controlled by location of Michigamme Formation and glacial features. Base map as in Figure 1. Arrows show direction of ice movement. The Fox River Valley Anomaly The Fox River Valley arsenic anomaly is best shown by NURE well water data and is only weakly expressed in stream sediment data (see Figure 2A). Arsenic values range up to a maximum of 60 ppb in well water. Interestingly, a great majority of the wells that show high arsenic in the NURE data lie west of the outcrop trace of the gently eastdipping arsenic-bearing sandstone and also west of the area where more recent data has identified an arsenic problem in wells. Bedrock in the western area of the anomaly is mostly Cambrian sandstone and Precambrian crystalline rocks, mostly granite. No arsenic source is known in these rock units. The anomaly is mostly within the area once occupied by the Green Bay glacial lobe and lies in a down-ice direction from the Ordovician sandstone. Glacial transport of arsenic-rich bedrock into the anomalous area appears to be a significant factor, suggesting that the immediate source of arsenic in well water west of the outcrop of the arsenic-rich sandstone is the unconsolidated glacial deposits. The Michigamme Anomaly The Michigamme anomaly is most strongly expressed in stream sediment date, but also occurs in well water data (Figure 2). It is geographically restricted by a combination of bedrock and glacial geology. The northern extent of the anomaly in stream sediments coincides very closely with the northern extent of the outcrop belt of black slate within the Precambrian Michigamme Formation. The eastern extent of the anomaly in northern Michigan is defined by the western margin of the Green Bay glacial lobe, which did not cross arsenic-rich bedrock. To the south, there appears to be glacial dispersal of arsenicrich bedrock in both the Green Bay lobe and Langlade sublobe in northern Wisconsin where high arsenic values in stream sediments and well water extend more than 50 km south of the outcrop belt of the Michigamme black slates. 17 �PALEOMAGNETIC STUDY OF PALEOPROTEROZOIC ROCKS IN THE ANIMIKIE GROUP, NORTHERN MINNESOTA CHANDLER, Va! W. and MOREY, G.B. (Minnesota Geological Survey, chand004@umii.edu and moreyOol @urnn.edu) A pilot study was conducted to investigate paleomagnetism in the Pokegama Quartzite and Biwabik Iron Formation of the Paleoproterozoic Animikie Group. The quartzite was sampled at six sites along the northern margin of the central and western parts of the Mesabi Iron Range, with three to six oriented samples collected per site. The iron-formation was sampled at four sites at a small outlier located to the north of the range near Pike Mountain; one to five oriented samples were collected per site. All samples are fresh and unoxidized. Core and cube specimens were cut from the field samples, and selected specimens were subjected to stepwise alternating-field and thermal demagnetization. Observed directions of magnetization have considerable scatter, and some specimens, especially those of magnetite-rich ironformation, were highly unstable during stepwise demagnetization. Nonetheless, several samples of each unit yielded stable, well-clustered magnetizations which appear to be associated with the early history of the rocks. With regard to thermal demagnetization, high blocking temperatures (greater than 600°C) are consistent with hematite as the primary carrier of stable magnetizations in the iron-formation. Hematite is an original or very early diagenetic phase in the iron-formation, whereas magnetite is a diagenetic phase that may have formed much later in the paragenetic scheme. Unstable or nulled magnetizations above thermal levels of 550°C in Pokegama specimens imply that magnetic minerals other than hematite may be present. Correcting for a bedding dip average of 100 to the south, the stable magnetizations of the Biwabik and Pokegama formations are directed at declinations/inclinations averaging 242°/65° and 289°/85°, respectively. The Pokegama (alpha=13.3°) and Biwabik (alpha=21.9°) directions cannot be discriminated from each other, or from a direction derived previously for the stratigraphically equivalent Gunflint Iron Formation in Canada (Symons, 1966) within 95 percent confidence. Our Animikie directions also overlap with those of a secondary imprint recognized to the north in the pre-Animikie Kenora-Kabetogama dikes by Halls (1986), who attributed it to a regional episode of hydrous alteration that tended to be more pronounced southwards, towards the Animikie basin. A working model that is consistent with present observations proposes that the diagenesis and subsequent magnetization of the Animikie rocks were accompanied by a regional groundwater flow system that may have been concentrated near the base of the Animikie sequence. The results of this study indicate that further paleomagnetic work on the Animikie Group rocks will be valuable, although the selection of sites that will produce useful results may pose some problems. Remaining tasks include determining if the Pokegama and Biwabik directions are truly indiscriminate from each other as well as from other Paleoproterozoic directions reported in the area. Ultimately, tightly constrained paleopole(s) can be combined with high-resolution radiometric dating to significantly improve the Paleoproterozoic apparent polar wander path for North America. References Cited: Halls, H. C., 1986, Paleomagnetism, structure, and longitudinal correlation of middle Precambrian dykes from northwestern Ontario and Minnesota: Canadian Journal of Earth Sciences, v. 23, p. 142-157. Symons, D. T. A., 1966, A paleomagnetic study of the Gunflint, Mesabi, and Cuyuna Iron Ranges in the Lake Superior Region: Economic Geology, v.61, p. 1336-1361. 18 �AN OVERVIEW OF AEROMGANETIC MAPPING IN MINNESOTA CHANDLER, Val W., Minnesota Geological Survey,2642 University Avenue, St. Paul, MN 55114, chand004@tc.umn.edu AN OVERVIEW OF AEROMAGNETIC MAPPING IN WISCONSIN MUDREY, M.G. Jr., Wisconsin Geological and Natural History Survey, 3817 Mineral Point Rd., Madison, WI 53705, mgmudreyfacstaff.wisc.edu MINNESOTA Magnetic methods have long been used by geologists in Minnesota to help investigate poorly exposed Precambrian bedrock. In fact, the Cuyuna iron range, which was discovered in 1904 by dip needle, was the first mining district in the United States that was discovered wholly by a geophysical method. The first large-scale magnetic project in Minnesota occurred after WWII, when George M. Schwartz, then-director of the Minnesota Geological Survey (MGS), made a cooperative arrangement with the U.S. Geological Survey (USGS) for surveying in Minnesota using the newly developed aeromagnetic method. Although the priority of this early work was locating new iron ore resources, the usefulness of the new method in mapping Precambrian geology was quickly realized, and by 1950 over 40,000 square miles of northern Minnesota had been covered by this method. During the 1960s while P.K. Sims was director of the MGS, the USGS aeromagnetic coverage over the entire state was completed, and an integrated program of geologic mapping using aeromagnetic and gravity data began. These efforts culminated in 1970 with the publication of a state geologic map, the first since 1932. By the mid-1970s, the potential of the USGS aeromagnetic data had been realized, and newer, higher resolution data were needed. Through the efforts of Matt Walton, thendirector of the MGS, and Robert Hansen, then-executive director of the Legislative Commission on Minnesota Resources (LCMR), a new state-wide program of high-resolution aeromagnetic surveying began in 1979. Funding came primarily from the LCMR, with additional contributions of data from the USGS, the U.S. Steel Corporation and the Geological Survey of Canada. The data obtained earlier was flown at one mile line spacing with 1000 feet terrain clearance, and much of the new flying was conducted at ¼ mile spacing with 500 feet terrain clearance. In addition, the new data were digital and could be readily subjected to a variety of computer-based processing and enhancement schemes to assist in geologic interpretation and mapping. State-wide coverage was completed in 1991, and the new aeromagnetic data, used in conjunction with an improved gravity database, have dramatically improved Precambrian geologic mapping in Minnesota. Virtually all Precambrian bedrock in the state has been re-mapped at a scale of 1:1,000,000 or larger. The new geologic maps, as well as the geophysical data used to help make them, will be useful to a variety of scientific and economic investigations for many years to come. WISCONSIN T.C. Chamberlin's geologic staff began mapping in northern Wisconsin in 1 870s as a continuation of mapping in southern Wisconsin. It was recognized that conventional geological techniques did not provide sufficient information in the glacially covered, 19 �geologically complex areas of the Gogebic Range. C.E. Wright of the Chamberlin's Wisconsin Geological Survey sent a sketch to instrument maker Gurley in Troy, New York, of a "dipping needle" based on a design he had seen from Sweden. The determination of the inclination and declination of the magnetic field had been well established by R.D. Irving and C.R. Van Hise of the U.S. Geological Survey by the late nineteenth century. Beginning in 1913, W.O. Hotchkiss and colleagues of the Wisconsin Geological and Natural History Survey (WGNHS) mapped large areas of northern Wisconsin by conventional and magnetic methods to assess mineral value for taxation. In 1935, C.K. Leith, R.J. Lund and A. Leith of the U.S. Geological Survey were able to produce a reasonable regional geologic map of the Lake Superior Precambrian that was based on conventional geology, mineral mapping, and extensive magnetic surveys. Using fluxgate magnetometers developed during World War II, G.P. Wollard and his students undertook regional magnetic surveying in Wisconsin in the early 1 960s. Of note is the regional aeromagnetic map of Wisconsin in 1964 by R.W. Patenaude and colleagues, who mapped Wisconsin on a 1 0-km line spacing at 1 000-m elevation. Prior to 1972, more detailed surveys were limited to small areas in support of geologic programs in southwestern and central Wisconsin. In 1972, the WGNHS received a small grant from industry to initiate surveys in central Wisconsin. With this seed and grants from Upper Great Lakes Regional Commission, the WGNHS and University of WisconsinOshkosh professor John Karl conducted a survey of a large area in northern Wisconsin. That survey was completed in 1977 with a "fill in the holes" grant from U.S. Geological Survey. These and subsequent public surveys were flown on lines spaced at 805 m, and oriented in a north-south direction. The altitude was draped to topography at 305 m above ground level. Interest in massive sulfide exploration in the 1 980s resulted in many private surveys, some of which were released to the public through the WGNHS. More recently (1997-1999) the U.S. Geological Survey completed surveying the remaining land areas of Wisconsin. The release of these data on CD-ROM is stimulating a reevaluation of the regional geologic fabric of the upper Midwest. Remaining activities include an adjustment of the surveys to a common base and preliminary analysis of the newly acquired data by W.F. Cannon and others. The statewide coverage, used in conjunction with an improved gravity database, will dramatically improve subsurface geologic mapping. The new geologic maps as well as the geophysical data used to help make them will be useful to a variety of scientific and economic investigations for many years to come. 20 �STRUCTURE, STRATIGRAPHY AND PUNCTUATED EVOLUTION OF MINNESOTA'S MINERAL EXPLORATION ARCHIVES David Dahi, Minnesota Department of Natural Resources, Hibbing, Mn 55746 A project to catalog the content of Minnesota DNR's mineral exploration archives for remote digital access has provided a substantial "opportunity" to better understand the structure and relationships among some 15,000 unpublished mineral exploration documents, and the 100+ exploration programs they came from. Together, the documents provide a mosaic of evolving mineral exploration methods and exploration models, and reflect changing exploration focus over time. In the past, archive users have often had a sense of déjà vu when researching Minnesota's archives. Indeed, during the process of cataloguing the content, some 25% of the archive documents were found to be duplicates of existing information. In some cases, more than a dozen copies of a document existed in the files. The lineage of that duplication, and the reasonable geographic motif that led to that duplication offer several lessons about the geographic nature of mineral exploration programs and storage of mineral exploration data for future geologic research. Within government activities, remote access to the mineral exploration archive provides a needed input for more responsive local and regional planning, and offers a historical background for mineral resource management decisions. In conjunction with online comparison to other data sets such as DOQ's, DRG's, DEM's, Public Land Survey, aeromagnetic data, published geologic maps, land use, landsat, soils and other baseline information, the exploration data archives can now be used to more quickly glean unique insights about previous exploration efforts and to develop insights for new geologic and geophysical exploration and research. 21 �NEW AEROMAGNETIC MAP OF WISCONSIN EXAMINED IN A REGIONAL CONTEXT DANIELS, David L., daveusgs.gov, NICHOLSON, Suzanne W., swnichusgs.gov, CANNON, William F., wcannonusgs.gov, U.S. Geological Survey, MS 954 National Center, Reston, VA 20192, and KUCKS, Robert P., rkucksusgs.gov, U.S. Geological Survey, MS964 DFC Box 25046, Denver, CO 80225 The new aeromagnetic map of Wisconsin portrayed in color at a scale of 1:500,000, is the result of digitally blending grids of 22 surveys flown between 1948 and 1999. The composite grid features 1) a resolution of 250m, and 2) a common elevation; all surveys were either flown at, or digitally continued to, an elevation of 1000 ft (305m) above mean terrain, prior to assembling into a state grid. The U.S. Geological Survey acquired all recent data in the state (1988 to 1999) amounting to about 77,000 line-miles. The digital flightline data for three of these surveys have recently been released on CD-ROMs. These data were added to earlier USGS surveys and 4 surveys acquired by Wisconsin Geological and Natural History Survey. Flight lines are V2-mile apart or less for 95% of the state, giving the aeromagnetic map nearly uniform specifications, and making the map an excellent tool for USGS mineral resource investigations. The Wisconsin grid has also been digitally blended with data from surrounding areas (Chandler, 1991; Hildenbrand and Kucks 1984, 1991) to form a preliminary regional aeromagnetic map of the North-Central US that reflects the structure of Precambrian basement rocks (see gray-scale index map). The regional map will be shown in color at a scale of 1:2,000,000. The aeromagnetic data of the region include some surveys of widely spaced flight lines (3 to 6 miles), particularly in northwestern and central Illinois, Lake Michigan, and the lower-peninsula of Michigan. These are areas where higher resolution data would be helpful to better define basement geology. Aeromagnetic features observed within Wisconsin can be traced into surrounding states. These features include: 1) high-amplitude linear anomalies that record the upturned edges of Keweenawan basaltic lava flows of the Midcontinent Rift System, and the smooth magnetic field of the flanking sedimentary basins; 2) abundant, narrow, linear magnetic anomalies probably generated by diabase dikes show a variety of trends across the region. (These anomalies are prominent only in areas of high-resolution surveys); 3) a strong ENE directed aeromagnetic fabric in areas of exposed Precambrian rocks in northern Wisconsin, Minnesota, Michigan's northern Peninsula, and Canada that records highly-deformed basement rocks, and 4) an aeromagnetic fabric with no preferred trend and abundant circular to arcuate anomalies, that characterizes much of the area to the south. This undirected fabric may reflect large areas of anorogenic igneous rocks, although part of the undirected fabric could also be due to greater depth to basement and lower survey resolutions. In SE Wisconsin a belt of high overall magnetic intensity lies within this area of undirected fabric, and is characterized by a series of high-amplitude, oval to circular anomalies. The belt continues SW through Illinois and Iowa into Missouri and eastward across Lake Michigan (A-A' on figure). The circular to oval anomalies suggest plutonic complexes in the basement. References Chandler, V. W., 1991, Shaded-relief aeromagnetic anomaly maps of Minnesota: Minnesota Geological Survey, one sheet, Scale 1:1,000,000 Hildenbrand, T.G., and Kucks, R.P., 1984, Residual total intensity magnetic map of Ohio: U.S. Geological Survey Geophysical Investigations Map GP-096 1, 1 sheet, scale 1:500,000. Hildenbrand, T.G., and Kucks, R.P., 1991, Total intensity magnetic anomaly map of Missouri: U.S. Geological Survey Open-File Report 9 1-0573, 1 sheet, scale 1:500,000. 22 �Aeromagnetic Anomaly Map of Wisconsin and North-Central US 0 0 _950 9O0 100 0 100 kilometres NAD27/LCC9O 23 200 -85° �MIDDLE PROTEROZOIC TECTONIIC HISTORY OF THE CENTRAL TUSAS MOUNTAINS, NORTHERN NEW MEXICO, AND COMPARISON WiTH THE BARABOO INTERVAL, SOUTHERN LAKE SUPERIOR REGION DAVIS, Peter B 1, WILLIAMS, Michael L. 1, BOWRING, Samuel A 2, and RAMEZANI, Jahan 2. (1) Department of Geosciences, Univ of Massachusetts, Amherst, MA 01003, davis@geo.umass.edu, (2) Earth, Atmospheric & Planetary Sciences, Massachusetts Institute of Technology The similarity between mid-Proterozoic quartzite units of the Baraboo interval, and the Ortega quartzite of northern New Mexico, has been recognized for decades. (Dott 1993) These similarities are based on sedimentary characteristics and the general timing of deposition, deformation and metamorphism of these supermature quartzite units. Plate tectonic reconstruction models also show a trend in the bedrock's mantle separation age extending from southwest to northeast that conned these units (Hoffman 1988)(Figure#1). However, drawing a meaningful correlation between them across a 1000-mile separation is extremely difficult because of the sparse nature of intervening exposure, and the uncertainty in the absolute timing of events. Recent work in the Tusas Mountains of northern New Mexico has further constrained the tectonic history that affected the Ortega quartzite among other units, and therefore might shed light on this tentative correlation. Recent field mapping, geochronology, and petrologic analysis in the Tusas Mountains has focused on a northwest-southeast striking enigmatic discontinuity that is suspected to be a late mesoproterozoic ductile fault. This fault juxtaposes higher-grade complexly deformed supracrustal immature to mature metasediments (which includes the Ortega Quartzite) and felsic metavolcarncs found to the south against lower grade immature metasediments and mafic to felsic metavolcanics and intrusives found to the north (Figure#2). Ductile deformational features present in these rocks can be grouped into three generations. A strong bedding parallel Si foliation is ubiquitous, however there are few Fl folds. D2 features include reclined tight to isoclinal folds with a strong SW dipping S2 foliation, and SW plunging stretching lineations (L2), sub-parallel to fold axes. Kinematic indicators suggest transport on this lineation was to the northeast. These structures are more pronounced south of the discontinuity. D3 produced east-west trending open folds, and either a newer crenulation clevage, or reactivated S2. In some localities D3 has reoriented F2 folds into F3 folds. Metamorphic conditions can also be grouped into three generations. Ml conditions reached lower greenschist facies. M2 reached greenschist facies conditions. M3 is discontinuous across the NW-SE discontinuity from upper greenschist to the north, to sub-amphibolite to the south (550°C -4.5kb). Timing of all three of these deformational events were traditionally correlated with the 1.67-1.65Ga. Mazatzal orogeny. New geochronologic data of 1.67-1.69 Ga. for the wealdy deformed Tusas granite suggests that the strong fabric in the Moppin group host rock was produced before 1.69Ga., possibly during the Yavapai Orogeny. A refined age for the Tres Piedras Granite, and a syn-Di dike, is now 1.67-1.69 Ga. is interpreted to suggest that the first two fabrics in the granite, directly correlative to regional fabrics, are indeed the result of 1.68-1.65Ga. Mazatzal tectonism. These data also further constrain the age of deposition of the Hondo group, which includes the 24 �______ Ortega Quartzite, to older than 1.69 -Ga., but younger than the underlying 1.70-1.7lGa. Burned Mountain rhyolite. D3, which increases in strength from north to south across the Tusas Mountains, was recently constrained in the southern Tusas Mountains to 1.45-1.4OGa. (Bishop et al 1996). Island-Arc Related Mafic to Felsic Volcanic sediments and Intrusives Moppin Group > 1.775 The earliest tectonic event represents an Tusas Granite early phase of thrusting with syn-orogenic - 1.69 Ga plutonism, and is primarly preserved to the SuDracrustal northern portion of the Tusas Mountains. The Sediments Tres second event, thought to be the result of additional and Volcanics thrusting with folding, is preserved throughout the Granite —1.68 Ga Tusas Mountain range. The third event significantly overprinted much of the range both Vadito Group tectonically and thermally. Its effects are thought to -1 .7OGa. be the result of the burial of the range to the midcrust leaving the region to eventually cool isobarically, erasing much of the evidence for earlier events. The effects of this overprint however 5 Miles strong in the southern half of the tusas, diminish at the discontinuity, and are poorly preserved to the Figure#2 - Tectonic Map of north. the Tusas Mountains, NM This refined model for the central Tusas Mountains of northern New Mexico provides a better framework with which to compare the Baraboo Interval in future regional models. Both regions contain a generalized supracrustal sequence of rhyolite followed by laminated to massive supermature quartzites. Constraint on the timing of deposition of both quartzite units is between approximately 1.71 — 1.65 Ga. Deformation is constrained between deposition and respective thermal event around 1.45 Ga. (Medaris 1996). Some have suggested that folding of the Baraboo Interval occured around approximately 1.63 Ga., during which whole-rock Rb-Sr systems were reset over the region. The 1.45 Ga. thermal and tectonic event that affected both the southern Lake Superior Region and the Tusas Mountains is thought to be related to the anorogenic magmatic event (Anderson / 1992). Proterozoic rocks of northern New Mexico and the Baraboo Interval of the southern Lake Superior region are windows into important tectonic processes. These widely separated windows reveal different structural levels and foreland proximities along a general orogenic trend across the Laurentian margin as it grew through mid-Proterozoic time. Hoffman, Paul F. United plates of America, the birth of a craton; early Proterozoic assembly and growth of Laurentia, Annual Review of Earth and Planetary Sciences. 16; P 543-603. 1988. Bishop, Jennifer L. Williams, Michael L. Lanzirotti Antonio. A doubly-looping P-T-t-D history for Proterozoic rocks of northern New Mexico and implications for the tectonothermal behavior of the mid-crust. Abstracts with Programs, Geological Society of America. 28; 7, P 495. 1996. Medaris, Gordon L. The Baraboo Quartzite, Wisconsin; Proterozoic deposition and deformation in the Lake Superior region. Abstracts with Programs, Geological Society of America. 28; 7, P376. 1996. Dott, Robert H. The Proterozoic red quartzite enigma in the north-central United States; resolved by plate collision?. In: Early Proterozoic geology of the Great Lakes region. Memoir Geological Society of America. 160; Pages 129-141. 1983. 25 �SOME OBSERVATIONS FROM THE WILLIAMS QUARRY EXPOSURE: EVIDENCE OF DEBRIS FLOW DEPOSITS IN THE PARFEYS GLEN FORMATION? PHILIP FAUBLE, Wisconsin Department of Natural Resources, P.O. Box 7921, Madison, WI 53707-7921; faub1p@dnr.ta.wi.us JENNIFER LIEN, Kraemer Company, P.O. Box 235, Plain, WI 53577; jen1@mhtc.net A quarry located on the north side of the North Range of the Baraboo Syncline, originally developed to mine sand and gravel from a quartzite conglomerate of the Parfeys Glen Formation, has recently expanded into insitu Baraboo Quartzite. This provides a unique opportunity to observe the Precambrian/Cambrian unconformity and its relationship to the overlying conglomeratic deposits. The undifferentiated quartzite conglomerate and conglomeratic sandstones, found in a broad band on both sides of the North and South Ranges of the Baraboo Hills, were first described in detail by Dalziel and Doll (1970). They inferred that these deposits were the result of wave action eroding and transporting quartzite from the ancestral Baraboo Hills during the transgression of the late Cambrian seas. These deposits were later named the Parfeys Glen Formation by Clayton and Attig (1990), who split the formation into three componants: a talus conglomerate directly adjacent to the quartzite, a conglomeratic sandstone and a sandstone unit that may or may not contain quartzite pebbles. Although the causes of the lithologic differences between individual units within the Parfreys Glen Formation were unclear, they considered it likely that the talus conglomerate was considerably older than the adjacent sandstone. At present, the quarry exposure is approximately 15 meters high, 200 meters long, north facing, and generally oriented east to west, parallel to the axis of the North Range. There are three distinct lithologies exposed along the quarry wall consisting of the following, from the base of the exposure to the top: in-situ quartzite, a very coarse quartzite conglomerate, and a finer-grained conglomeritic sandstone that gradually transitions to a quartz sandstone. In-situ Quartzite: An irregular, rounded outcrop of Precambrian Baraboo Formation quartzite is exposed at the base of the quarry wall. The quartzite is slightly overturned and dips steeply to the north. The main quarry face roughly parallels the strike of the quartzite beds. The largest exposure of quartzite is located just west of the center of the quarry wall and seems to represent a subdued topographic high prior to deposition of the upper units. The most striking feature of the in-situ quartzite is the presence of a smooth, rounded, gently undulating erosional surface along the unconformity between the quartzite and the overlying conglomerate. This smooth surface is present on both vertical and near horizontal exposures. While smooth, this surface is not varnished and does not possess striations, gouges or percussion marks. There are no obvious potholes developed in this surface, but shallow features that resemble small chutes are common. Quartzite Conglomerate: Overlying the smooth erosional unconformity developed on the quartzite is a quartzite conglomerate of varying thickness. This deposit is thinnest on the top of the quartzite knob, thickest within two low areas that flank the western and eastern sides of the knob, and gradually thins to the east. An exposure along the western wall of the quarry indicates that the coarse conglomerate gently dips northward. The conglomerate is composed of large rounded boulders of quartzite suspended in an unsorted, massive matrix of sand, rounded pebbles and cobbles. The conglomerate is poorly cemented and seems to lack any materials smaller than fme sand. All lithic fragments larger than sand are composed of quartzite or quartzite breccia likely derived from the nearby outcrops of Baraboo quartzite. Depending on quarry development, exposed quartzite boulders can range in size from 1 to over 3 meters in diameter along their longest visible axis. The largest boulders appear to be concentrated along the quartzite unconformity and the upper margins of the deposit. As first noted by Dalziel and Dott (1970), boulders larger than 2 meters in diameter tend to be angular or posses smooth rounded features on only one or two faces. Boulders smaller than about 2 meters are almost always completely rounded. There does not appear to be any clear fabric or preferred orientation to the larger clasts within the deposit. Con glomeratic Sandstone: Above and to the north of the quartzite conglomerate is a conglomeratic sandstone that thickens dramatically from less than 3 meters directly above the quartzite knob to over 10 meters at the northern edge of the quarry. It is composed of horizontally bedded strata consisting of alternating layers of sand and pebble conglomerate. This deposit is likely marine in origin, with abundant glauconite, sometimes occurring in thin, discrete beds, and abundant Scolithus burrows in the sand layers. The sandstone is visibly finer grained and much better cemented than the quartzite conglomerate directly below it. In contrast to the large boulders found in the conglomerate, an informal, random sampling of 50 of 26 �the largest pebbles visible in the sandstone exposed along the eastern wall of the quarry indicated a long axis diameter no greater than 20 cm. On the eastern wall of the quarry, the horizontally bedded conglomeratic sandstone can be seen to neatly onlap the north-dipping conglomerate. Above the conglomerate/sandstone unconformity, sandstone drapes the larger boulders projecting above the surface of the conglomerate deposit. Along the southern wall of the quarry, the conglomeratic sandstone extends from above the quartzite conglomerate to within about 3 meters of the top of the exposure where it abruptly transitions into a well cemented quartz aremte. This transition does not appear to be an unconformable surface, but likely reflects the drowning of the pebble source as the transgressing Cambrian seas covered the crest of the North Range. Conclusions: The exposure at the Williams Quarry clearly preserves and records three distinct events that occurred on the North Range of the ancestral Baraboo Hills sometime from the late Precambrian to the late Cambrian. First, topographically prominent exposures of the Baraboo quartzite were eroded into a series of smoothly rounded features. Second, conditions changed and a deposit of unsorted quartzite debris (conglomerate) covered the erosional surface, filling in swales and blanketing hillslopes. Lastly, a marine deposit of sand and locally derived pebbles covered the conglomerate. The exact processes and conditions that produced the smooth weathering/erosional surface on the insitu quartzite are unknown. However, the nature of the unsorted quartzite conglomerate indicates that it is likely the result of subaerial mass movements of material eroded from the crest of the North Range. We suggest that these movements took place in the form of large debris flows that generally followed drainages developed in the ancestral Baraboo Hills. Similar coarse conglomeritic deposits adjacent to Precambrian erosional surfaces have been noted in other regions. The lowest facies of the Upper Cambrian Lamotte Sandstone in southeast Missouri contain locally-derived rhyolite boulder conglomerates that have been interpreted as small alluvial fans formed, in part, by debris flows adjacent to the incised Precambrian bedrock of the ancestral St. Francois Mountains (Houseknecht, 1978). The wide grain size distribution of the conglomerate, ranging from fine sands to boulders several meters in diameter, the larger clasts suspended within in an unsorted mass, and the lack of clear grading or internal structures, closely matches the description of debris flow deposits in Coussot (1996). Debris flow deposits result from the rapid transport and mass emplacement of a highly viscous slurry of water and debris. Evidence that the quartzite debris within the conglomerate was transported and not merely weathered in place can be seen at the unconformable boundary between the in-situ quartzite and the conglomerate, just east of the bedrock knob. The in-situ quartzite at this location contains an extensive quartzite breccia, very similar to the Rock Springs (Abelman's Gorge) breccia (Dalziel and Dott, 1970). Directly above the smooth, truncated surface of the eroded breccia lie large boulders and cobbles that contain no evidence of breccia. Conversely, large grains of breccia-bearing material can be seen in the conglomerate mass above other areas that lack insitu breccia. Many other questions concerning this conglomerate deposit remain unanswered. The source area and the processes that produced the rounded boulders, cobbles and sand that make up the conglomerate remain enigmatic. Thick deposits of Paleozoic and glacial sediments obscure the most likely quartzite source areas near the crest of the North Range, south of the quarry. If the top of the conglomeratic sandstone represents the drowning of the uppermost quartzite outcrop, then the source area for the conglomerate may be no more than a few meters higher in elevation than the top of the quartzite knob exposed in the quarry wall. This doesn't preclude the formation of a debris flow because, once a flow is initiated, it may continue to move over slopes as low as a few degrees. Debris flows are also very poor rounding agents, so it is likely that the quartzite clasts in the deposit were already rounded smooth prior to transport. Features visible in a few of the conglomerate boulders suggest that at least some of the rounding occurred in situ, possibly in response to intense chemical weathering. REFERENCES CITED Clayton, L. and Attig, J., 1990, Geology of Sauk County, Wisconsin, Wisconsin Geological and Natural History Survey Information Circular No. 67, 68p. Coussot, P. and Meunier, M., 1996, Recognition, classification and mechanical description of debris flows, Earth Science Reviews, 40, 209-2TJp. Dalziel, I.W. and Dott, R.H., 1970, Geology of the Baraboo District, Wisconsin, Wisconsin Geological and Natural History Survey Information Circular No. 14, l64p. Houseknecht, D. and Etheridge, F., 1978, Depositional history of the Lamotte Sandstone of southeast Missouri, Journal of Sedimentary Petrology, v.48, 5'75-586p. 27 �Contrasts in the Geologic and Hydrochemical Occurrences of Arsenic Contamination of Groundwater in Eastern Wisconsin Gotkowitz, M.B., Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison, WI 53705 mbgotkow(facstaffvisc .edu Schreiber, M.E., Department of Geological Sciences, Virginia Tech, Derring Hall, Blacksburg, VA 24061 mschreib@vt.edu Simo, J.A., Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 W. Dayton St, Madison, WI 53706 simo@geology.wisc.edu Clusters of arsenic-impacted wells are found in two areas of eastern Wisconsin: the Fox River valley (east-central Wisconsin) and the Lake Geneva (southeastern Wisconsin) areas. In the Fox River valley area, arsenic concentrations up to 12,000 ppb have been measured in groundwater from a CambrianOrdovician sandstone aquifer. An arsenic-rich, sulfide-bearing, secondary cement horizon (SCH) is commonly present at the top of the Ordovician St. Peter Formation. Within the SCH, arsenic occurs in pyrite and marcasite and in iron oxyhydroxides, but not as a separate arsenopyrite phase. Whole rock concentrations of arsenic within the SCH range from about 15 to 675 ppm. Core samples show that arsenic-bearing minerals are also present in the St. Peter below the SCH. Several pieces of evidence support the hypothesis that oxidation of sulfides is the cause of high (>100 ppb) concentrations of arsenic in well water, including 1) the presence of arsenic-bearing sulfides in the aquifer, 2) water-chemistry data that show a positive correlation between arsenic, iron, and sulfate and negative correlation between arsenic and pH; and 3) nearly identical sulfur isotopic signatures in pyrite and dissolved sulfate. There is a strong correlation between high arsenic concentrations and the occurrence of intersecting elevations of the SCH and water levels within wells. This relationship provides the basis for our conclusion that atmospheric oxygen, introduced to the SCH through well boreholes, provides an oxidant to the system. However, the cause of more commonly encountered, moderate (less than 100 ppb) arsenic concentrations found in wells in the Fox River valley is not well understood. The variability of the thickness of the SCH and the associated mass of arsenic within the sulfides, as well as the local availability of an oxidant to fuel the oxidation of the SCH, may contribute to the spatial variability in arsenic concentrations in well water. The available water quality data are not sufficient to determine if other geochemical mechanisms, such as desorption or reductive dissolution of arsenic-bearing iron oxyhydroxides, control the moderate arsenic concentrations measured in well water. Our preliminary work in the Lake Geneva area indicates that geologic and hydrogeologic conditions leading to a cluster of arsenic-impacted wells in this part of the state are not similar to those in the Fox River valley. Arsenic has been detected in wells open to the Quaternary deposits, Silurian dolomite, and the Cambrian-Ordovician sandstone. Arsenic concentrations in rock samples from these aquifers range from 1.4 to 18 ppm; aqueous concentrations in well water range up to 80 ppb. Water chemistry in arsenic-contaminated wells is not consistent with sulfide oxidation, and sulfide mineralization has not been observed in rock samples collected from the area. These results indicate that other geochemical mechanisms of arsenic release, such as the reduction of arsenic-bearing iron-oxides, may also affect Wisconsin groundwater supplies. 28 �THREE NEW ZIRCON DATES FOR THE MIDCONTINENT RIFT, NORTH SHORE, MINNESOTA: MORE DATA, MORE QUESTIONS GREEN,J. C., Geological Sciences, U. of MN Duluth, Duluth, MN 55812; DAVIS, D. W., Royal Ontario Museum, 100 Queen's Park, Toronto, ON M5S 2C6; and SCHMITZ, M. D., Earth and Planetary Sciences, MIT, 77 Mass. Ave., Cambridge, MA 02139 U/Pb zircon dates have been obtained for three significant localities on the North Shore, with some intriguing results. Skeletal zircons from the residual monzodiorite at the top of the Duluth Complex Layered Series in Duluth give an age of 1098.5 +7-1.3 Ma (DWD). This agrees with the 1099.3 +1- 0.3 Ma age determined by Paces and Miller (1993) from a segregation within the upper part of the Layered Series. The skeletal crystal habit assures that the new date represents the magma crystallization age, not inherited crystals. A rhyolite flow exposed below the quarry in the southeast flank of Canton Peak near Tofte gives an age of 1092.6 +7- 2.0 Ma (DWD). This is the youngest date yet found on the North Shore. This local volcanic sequence is apparently isolated by faults from the rest of the North Shore Volcanic Group, including its uppermost sequence, the Schroeder-Lutsen basalts (SLB), and it is clearly younger than the 1096.6 +7- 1.9 Ma Palisade thyolite (Davis and Green, 1997) which unconformably underlies the SLB. Since it has been suested (Miller et al., 1995) that the SLB is an outlier or fringe of the Portage Lake Volcanics (dated at 1096-1094 Ma, Davis and Paces, 1990), this new date implies either that the SLB is actually considerably younger that the PLy, or that the Canton Quarry sequence is an isolated remnant of heretofore unrecognized late volcanic activity on the Minnesota flank of the MRS. Furthermore, the dated quarry sequence is intruded by the anorthosite-xenolith-bearing Carlton Peak diabase, making that still younger and not coeval with dated units of the Beaver Bay Complex (—1096 Ma, Paces and Miller, 1993). Finally, a granite xenolith in the Terrace Point basalt flow southwest of Grand Marais (Green, 2000) is dated at 1096.7 +7- 1.4 Ma (MDS). Since this medium-grained, upper mesozone-looking biotite granite must have crystallized slowly at many kilometers' depth before being broken off and carried to the surface in the basalt magma, this implies that the Terrace Point flow (basal flow of the SLB) is significantly younger than the granite. A coarse-grained segregation vein within the Schroeder basalts was collected and processed, but no zircons or baddelyite were found, leaving the SLB still undated. Davis, D.W. and Green,J.C., 1997, Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution: Can. Jour. Earth Sci., v. 34, p. 476-488 Davis, D.W. and Paces,J.B., 1990, Time resolution of geologic events on the Keweenaw Peninsula and implications for development of the Midcontinent Rift system: Earth and Planet. Sci. Lett., v. 97, p. 54-64 Green, J .C., 2000, Mystery faults of the Cascade River, North Shore, or, What is this granite doing here? (abs.): Proceedings Vol. 46, Part 1: Abstracts and Programs, Forty-Sixth Ann. Institute on Lake Superior Geology, Thunder Bay, ON, p. 14 Miller,J.D.,Jr., Chandler, V.W., Green,J.C. and Witthun, K., 1995, The Finland Tectonomagmatic Discontinuity - a growth fault marking the western margin of the Portage Lake volcanic basin of the Midcontinent Rift System: Basement Tectonics 10, R.W. Ojakangas, A.B. Dickas, andJ.C. Green, Eds, Kiuwer Acad. Publ, p. 35-40 Paces,J.B. and Miller,J.D.,Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochnonological insights to physical, petrogenetic, paleomagnetic, and tectonomagmatic processes associated with the 1.1 Ga Midcontinent Rift System: Jour. Geophys. Res., v. 98, No. B8, p. 13,997-14,013. 29 �INITIAL RESULTS OF IN SITU ELECTRON MICROPROBE (EMP) AGE DATING OF MONAZITE FROM THE SOUTHERN LAKE SUPERIOR REGION: CONFIRMATION OF WIDESPREAD GEON 17 METAMORPHISM IIOLM, D., dhohnkentedu, Dept of Geology, Kent State University, 44242; JERCINOVIC, MJ., and WILLIAMS, M., both at Dept of Geosciences, University of Massachusetts, Amherst MA 01002. Introduction. Like most orogens the pattern and degree of Proterozoic metamorphism preserved across the 1870-1830 Ma Penokean orogemc belt is highly variable. Widespread 1630 Ma metamorphism of much of the Penokean arc terrane in Wisconsin is well documented (see Table 4 of Peterman et aL, 1985, CMP; Romano Ct al., 2000, PreC. Res.) and intrusion of the 1470 Ma Wolf River batholith certainly produced thermal overprinting in central and southern Wisconsin (Hohn et aL, 1998, GSAA; Naymark et al., 2001, GSAA). Until recently, intermediate pressure and higher temperature metamorphism both north and west of the arc terrane (northernmost Wisconsin, northern Michigan and east-central Minnesota) has been attributed to the geon 18 Penokean orogeny (see overview by Geiger and Guidotti, 1989, Geoscience Wise.). Three separate studies published in the late 1990's however (Schneider Ct al., 19%, CJES; Marshak et al., 1997, Geology; Hohu et al, 1998, AJS) proposed the presence of a widespread geon 17 amphibolite facies overprinting metamorphism. In order to better establish the timing of intermediate grade metamorphism in the southern Lake Superior region we have started a microprobe monazite geochronology study of selected metamorphic samples from east-central Minnesota and northern Wisconsin. EMP dating of monazite is currently emerging as a rapid and accurate means of geochronology, and is most easily applied to the Precambrian where the Pb concentration often allows greater analytical precision (Williams et al., 1999, Geology). Results from East-Central Minnesota. We dated monazite from a staurolite-garnet schist outcrop (sample MN-29) of the metamorphosed Little Falls Formation collected on the Mississippi River (Blanchard Dam). Thermobarometric, thermochronologic, and textural analysis of this and other metamorphosed samples led Hoim et al. (1998) to suggest two episodes of aniphibolite facies metamorphism (a geon 18 Penokean Ml and a younger geon 17 M2 associated with emplacement of abundant post-tectonic plutons). Eight EMP spot analyses on three separate monazite grains yielded U-Th-P ages ranging from 1755 Ma down to 1667 Ma (mean age of 1719±21 Ma). The oldest ages are concordant with abundant Ar/Ar mica and hornblende ages obtained throughout the region (Holm and Lux, 19%, Geology; Holin et al., 1998). Results from northern Wisconsin. We have obtained preliminaiy data from two drill core samples of garnet-biotite-sillimanite schist located near Park Falls, Wisconsin. These samples occur within the recently identified fault-bounded, sillimanite-bearing panel just north of the Niagara Fault zone (Park Falls subterrane of Cannon et al., 1998, ILSG). Sample PF-2-311 yielded a mean age of 1710 ± 43 Ma from 3 spots analyzed on 3 separate monazite grains. Sample BL-2-252 (located nearby the first sample) revealed a distinctly chemically zoned grain which produced distinct core and rim ages of 1805 Ma and 1695 Ma, respectively. Finally, we sampled the well known kyanite bearing outcrop located near the town of Powell. This sample is located within the higher pressure fault-bounded panel identified as the Powell subterrane (Cannon et at., 1998). Ten EMP spot analyses on three separate grains yielded U-Th-Pb ages ranging from 1780 Ma to 1747 Ma(meanageofl765±7Ma). Implications and Conclusions. These data, although preliminary, appear to support other independent evidence for a widespread, post-Penokean, geon 17 intermediate grade metamorphic event in the southern Lake Superior region. In Minnesota, geon 17 metamorphism appears concentrated within the internal zone of the orogen where abundant post-tectonic pistons exist To the north in the medial zone earlier geon 18 metamorphic ages are preserved in garnet grade rocks (Schneider et aL, 2001, ILSG). In northern Wisconsin the age of metamorphism in stmctural panels preserving contrasting metamorphic conditions (sillimarnte versus kyanite) appears to be dominantly geon 17 although a hint of earlier Penokean metamorphism is apparently preserved (see also Schneider et at., 2001, for Penokean metamorphic ages preserved north of the Niagara Fault zone in northern Michigan). These metamorphic age constraints suggest that the fault-bounded subterranes may have been juxtaposed after the Penokean orogeny during a period of widespread metamorphism followed by rapid unrooflng (as suggested by geon 17 cooling ages). We conclude that microprobe monazite geochronology holds promise for unraveling the timing of multiple metamorphic and tectonic events in the Lake Superior region as it has for other portions of the southern maigin of Laurentia (Williams et at, 2001, ILSG). 30 �AGE OF THE HUMBOLDT GRANITE, NORThERN MICHIGAN: IMPLICATIONS FOR THE ORIGIN OF THE REPUBLIC METAMORPHIC NODE. HOLM, D., dholni(ã)Jentedu, Dept of Geology, Kent State University, 44242; VAN SCLIMUS, W.R.. and MacNEILL, LC., both at Dept of Geology, Univ. of Kansas, Lawrence, KS, 55045. The alkali-feldspar granite near Humboldt lies on the northeastern edge of a large negative gravity anomaly that is roughly coincident with the Republic metamorphic node. Hornblende, muscovite, and biotite Ar/Ar ages obtained from country rock across the node are 1720-1670 Ma (Schneider et al., 1996). A whole-rock Rb/Sr minimum age of 1733±25 Ma on the granite was reported by Schulz et a!. (1988). The concordance of the whole-rock age with the thermochronologic data led Schneider Ct a!. (1996) to infer that post-tectonic plutonism caused the metamorphic nodal pattern. In order to test this hypothesis we obtained both U-Pb and Ar/Ar mineral age data on the Humboldt granite. U-Pb zircon results. Six single-grain analyses of somewhat tuibid grains separated from a sample of the Humboldt granite (AGR-1, collected by Holin) yielded a linear array on a conventional U-Pb concordia diagram. All six analyses yield an upper intercept age of 1806±21 Ma, although one analysis is clearly off the line. Elimination of this analysis yields an age of 1805 ±7 Ma with a lower intercept of 181 ±64 Ma. One analysis is nearly concordant and has a 201Pb/206Pb age of 1802±6 Ma. Thus it is clear that the ciystallization age is significantly older than the Ar retention age or the Rb/Sr age, which is a common situation in the Lake Superior region. Ar/Ar mica results. We dated three separate mica fractions obtained from a coarse phase of the granite. In order to test for possible intraciystalline age gradients we furnace step-heated the rim and core portion of a coarse muscovite grain. Both analyses yielded essentially concordant plateau dates of 1712±6 Ma (core) and 1703±6 Ma (rim). Step-heating of biotite did not produce a reliable plateau age. Incremental ages increased monotonically with the three highest temperature fractions giving ages between 1670 and 1700 Ma (amounting to 45% of the gas released). These Ar/Ar results are comparable to mica Ar/Ar ages from older bedrock of the Republic region and are concordant within error with the whole-rock Rb/Sr age of the Humboldt granite. The crystallization age of the granite indicates that it could not have provided the heat source needed to form the Republic metamorphic node at ca. 1720 Ma. We cannot rule out the possibility of a younger geon 17 intrusive body existing in the Republic subsurface, although thus far geon 17 plutons have been documented predominantly south of the Niagara fault zone (the sole exception being the 1781 Ma Parlc Falls granite, Van Schmus et a!., 2001). A combination of anomalously high basement heat production rates (Attob, 2000) together with basement remobilization (i.e., Marshak et al., 1997) might better explain geon 17 low P/high T metamorphism in the Republic region of northern Michigan. Attob, K, 2000, Contrasting Metamorphic Record of Heat Production Anomalies in the Penokean Orogen of Northern Michigan: Journal of Geology, v. 108, p. 353-36 1. Marshak, S., Tinkham, D., Allcmim, F., Bruekner, H., and Bornhorst, T., 1997, Dome-and-keel provinces formed during Paleoproterozoic orogenic collapse — core complexes, diapirs, or neither?: Examples from the Quadrilatero Ferrifero and the Penokean orogen: Geology, v. 25, p. 415-418. Schneider, D., Holin, D., and Lux, D., 1996, On the origin of Early Proterozoic gneiss domes and metamorphic nodes, northern Michigan: Canadian Journal of Earth Sciences, v. 33, p. 1053-1063. Schulz, K.J., Sims, P.K., and Peterman, Z.E., 1988, A post-tectonic rare-metal-rich granite in the Southern 34th Ann. Inst. on L. Superior Geology, Marquette, Michigan, Complex, Upper Peninsula, Michigan: p. 95-96. Van Schmus, W.R, MacNeill, L.C., Holm, D.K, and Boerboom, T.J., 2001, New U-Pb ages from Minnesota, Michigan, and Wisconsin: Implications for Late Paleoproterozoic Crustal Stabilization: 47th Ann. Inst. on L. Superior Geology, Madison, Wisconsin, May (this volume). 31 I �206PbI 0.20 0.22 0.24 0.26 0.28 0.30 0.32 0.34 3.0 3.4 3.8 207Pb/235U 4.2 4.6 5.0 5.4 �NEW VOLUME CALCULATIONS FOR THE PYROCLASTIC ERUPTIONS ASSOCIATED WITH THE STURGEON LAKE CALDERA COMPLEX, NORTHWESTERN ONTARIO: IMPLICATIONS FOR THE SCALE OF ARCHEAN VOLCANIC PROCESSES GEORGE J. HUDAK Department of Geology, University of Wisconsin Oshkosh, Oshkosh, WI 54901 DEAN M. PETERSON and RONALD L. MORTON Department of Geology, University of Minnesota — Duluth, Duluth, MN 55812 The Archean Sturgeon Lake Caldera Complex (SLCC) of northwestern Ontario comprises a north-facing, homoclinal, bimodal sequence of caldera-associated, greenschist facies metamorphosed, volcanic, intrusive and sedimentary strata with a composite thickness of nearly 4500 meters and a strike length of at least 25 km (Morton et al., 1991). At least three volcanic-associated massive sulfide (VMS) orebodies were formed as sub-seafloor replacement deposits within quartz-phyric, pumice-rich vitric tuff within the caldera complex (F-Group, Mattabi, and Sturgeon Lake); the other three orebodies in the region (Sub-Creek Zone, Creek Zone, and Lyon Lake) were probably formed from structural deformation of the Sturgeon Lake Mine (Hudak, 1996). Hudak et al. (2000) have divided the caldera complex into three stratigraphic sequences. The Pre-Caldera Sequence (PCS) is composed of a 200-2100 meter thick succession comprising subaerial basalt lavas, scoria-rich volcaniclastic rocks, and very minor rhyolite lavas that formed prior to the development of the caldera complex. The Early Caldera Sequence (ECS) comprises a 650-1300 meter thick succession of subaerial rhyolite ash tuff formed immediately prior to the formation of the caldera edifice, and subaqueous coarse heterolithic breccias, syneruptive aphyric and quartz-phyric pumice-rich vitric tuffs, and andesitic to rhyolitic lava flows formed prior to, or simultaneously with, the Mattabi VMS orebody. The Late Caldera Sequence (LCS) is composed of a complex, 500-1500 meter thick succession of subaqueous quartz- and quartz-plagioclase-phyric vitric tuffs, andesitic to dacitic lava flows, lava domes, and cryptodomes, resedimented syn-eruptive ash-rich mudstones, ash-rich sandstones, and lithic lapillistones, as well as post-eruptive tuffaceous mudstones, tuffaceous sandstones, tuffaceous breccias, and Algoma-type iron formations. Hudak (1996) and Morton et a!. (1998) have shown that the stratigraphic relationships indicate that the SLCC developed in a manner similar to the well-known caldera cycle of Smith and Bailey (1968). The presence of exceptionally well-preserved, often delicate primary lithological textures within the volcaniclastic and volcanic rocks in the south Sturgeon Lake region has allowed individual rock units and their facies equivalent deposits to be distinguished and correlated. These features indicate that only minor amounts of structural deformation, and probably only minor amounts of diagenetic compaction, have occurred within the intracaldera volcaniclastic rocks that are present. This, along with the steeply dipping nature of the strata in the region, allows us to make very accurate areal calculations of the volcaniclastic strata by means of geographic information system (GIS) analysis. Although arguably speculative, volumes of individual pyroclastic eruptions have been calculated using the average thicknesses of each of the major pyrociastic units within the caldera, the strike lengths of the pyroclastic units, and the assumption that these units were deposited within a circular caldera. The results of our eruption volume calculations are contained in Table 1. These calculations indicate that, although Archean in age, the SLCC eruptions were similar in scale to eruptions associated with Cenozoic arc-associated caldera systems. Thus, it appears that not only was the caldera cycle (Smith and Bailey, 1968) established by Late Archean time, but that pyroclastic eruptions occurring in Archean arc-associated caldera complexes were similar in scale to pyroclastic eruptions occurring in more recent arc-associated caldera systems. Although more detailed studies on the evolutionary processes and eruptive volumes need to be completed at other well-preserved Archean caldera complexes, these results may provide us with clues to Late Archean tectonic and petrological processes. 33 �Table 1. Volume Estimates of Sturgeon Lake Caldera Complex Eruptions Eruption Caldera Sequence - Estimated Jackpot Lake High Level Lake Bell River Mattabi Lower "L" Middle "L" Upper "L" Eruption Pre-Caldera Early Caldera Early Caldera Early Caldera Late Caldera Late Caldera Late Caldera VoIu(J 8.8 16.6 1.9 28.7 3.0 6.9 3.5 Table 2. Volume Estimates of Historic Caldera Eruptions Estimated Volume (km3) Source Pinatubo (1991) Krakatau(1883) Santorini (3.6 ka) Tambora (1815) Vandever Mtn Tuff(T-J) Taupo (1.8 Ka) Kuwai (—1450) 4-5 10 25-30 25 13-26 30-35 32-39 Lipman, 2000 Lipman, 2000 Lipman, 2000; Cas and Wright, 1987 Cas and Wright, 1987 Kokelaar and Busby, 1992 Lipman, 2000: Cas and Wright, 1987 Monzier et al., 1994 References Cas, R. A., and Wright, J. V., 1987. Volcanic Succession Modern and Ancient: Allen and Unwin, London, 528 pp. Hudak, G. J., 1996. The physical volcanology and hydrothermal alteration associated with late caldera volcanic and volcaniclastic rocks and volcanogenic massive sulfide deposits in the Sturgeon Lake region of northwestern Ontario: unpublished Ph. D. dissertation, University of Minnesota, Minneapolis, MN, 463 pages. Hudak, G. J., Morton, R. L., Peterson, D.M., and Franklin, J. M., 2000. The Sturgeon Lake Caldera Complex, northwestern Ontario: volcanological evolution of an Archean shallow water VHMS belt: Volcanic Environments and Massive Sulfide Deposits, CODES Special Publication 3, p. 8991. Kokelaar, P., and Busby, C., 1992. Subaqueous explosive eruptions and welding of pyroclastic deposits: Science, v. 257, p. 196-201. Lipman, P. W., 2000. Calderas, in Sigurdsson, H., 2000, Encyclopedia of Volcanoes: Academic Press, San Diego, CA, p. 643-662. Monzier, M., Robin, C., and Eissen, J.-P., 1994. Kuwae (—1425 A. D.): the forgotten caldera: Journal of Volcanology and Geothermal Research, v. 59, p. 207-218. Morton, R. L., Walker, J. S., Hudak, G. J., and Franklin, J. M., 1991. The early development of an Archean submarine caldera complex with emphasis on the Mattabi ash-flow tuff and its relationship to the Mattabi massive sulfide deposit: Economic Geology, v. 86, p. 1002-1011. Morton, R. L., Hudak, G. J., Walker, J. S., Jongewaard, P. K., and Murphy, C. M., 1998. The stratigraphy and physical volcanology of the Archean south Sturgeon Lake Caldera Complex, northwestern Ontario: Geological Association of Canada / Mineralogical Association of Canada Annual Meeting Abstract Volume 23, p. A-128. Smith, R. L., and Bailey, R. A., 1968. Resurgent Cauldrons, in Coats, R. R., Hay, R. L., and Anderson, C. A. (eds), Studies in Volcanology (Howell Williams Volume), Geological Society of America Memoir ll6,p. 153-210. 34 - �A PRACTICAL EXERCISE IN METALLIC MINE RECLAMATION LADYSMITH, WISCONSIN T.C. HUNT, Director of the Reclamation Program, School of Agriculture, University of Wisconsin - Platteville, Platteville, WI 53818 hi November 1968, Great Lakes Exploration, a subsidiary of Kennecott, intersected copper mineralization along the Flambeau River south of Ladysmith, Rusk County, Wisconsin. This discovery and the rising environmental consciousness in Wisconsin lead to nearly 25 years of legislation, engineering evaluation, and environmental regulation. The Flambeau Mine was officially opened on July 31, 1993 and by the time the mine closed in 1997, 1.9 million short tons of ore averaging 8.9 percent copper and 0.10 ounces per ton of gold were produced. This mine was unique in that all of the ore was shipped directly to smelters for metal recovery; there was no beneficiation on site. Reclamation of the property was an important part of the entire mine design. Kennecott Minerals' Flambeau Mine reclaimed their open pit copper mine under the jurisdiction of Wisconsin's environmentally sensitive metallic mining laws; viewed by many as the most strict in the nation. It was alleged that Wisconsin's modem mining laws were prohibitive, but the Flambeau Mine provides a case study demonstrating viable mining and reclamation within the constraints of rigorously protective regulatory requirements can happen. The mine site is adjacent to the Flambeau River, one of Wisconsin's premier whitewater canoeing area, and surface water, groundwater, and mine water were important parts of the mining plan. Reclamation was initiated prior to the completion of mining. Flambeau backfilled the open pit mine by layering mine waste and carbonate rock to control acidity, and reclaimed the surface using state-of-the-science ecological restoration methods. The company brought the surface of the former pit to its approximate original contour, rebuilt pre-existing intermittent stream channels, restored native plant communities, and developed a series of biofilters and wetlands to enhance runoff quality. A trail system opens the site to the public for recreational pursuits such as hiking, cross-country skiing, and bird watching. Bald Eagles and black bears are frequent visitors to the site. Visual inspection and statistical analysis of surface water samples present evidence that the site is stabilizing and that the hydrologic system is functioning as designed. Monitoring continues on the site, and the vegetative sampling results indicate the reclaimed mine site is on the desired trajectory for plant community development, diversity, cover, plant frequency, and productivity. This experience of a well designed and implemented mine plan, in conjunction with a well designed and implemented reclamation plan, illustrates that mining and the environment can be compatible. 35 �THE EARLY GABBROIC SERIES OF THE MIDCONTINENT RIFT SYSTEM: CONTINUED ASSESSMENT OF MAGMATIC ORIGINS JERDE, Eric A. (e.jerde@moreheadst.edu) and SAL VATO, Daniel J. (student), Department of Physical Sciences, Morehead State University, Morehead, KY 40351; THOLE, Jeff and WTRTH, Karl R., Geology Department, Macalester College, St. Paul, Minnesota, 55105 The Early Gabbroic Series of the Midcontinent Rift System (MCR), informally known as Nathan's Layered Series (after Nathan, 1969), is comprised of numerous tabular and sheetlike intrusions just south of the Gunflint Trail in the vicinity of Poplar Lake in extreme northeastern Minnesota. A U-Pb zircon date of 1106.9 ± 0.6 Ma has been obtained for one of the units in this series (Paces and Miller, 1993), making it among the oldest materials associated with the MCR. Nathan (1969) identified 27 separate units, given the letter designations A-Z and AA in their inferred chronological order. The chronological sequence was based entirely on cross-cutting relations and discordances observed. Nathan's own interpretation, plus those of subsequent observers, offers that many of the separate units may be gradational variations, while others may be unique dikes or "sport" varieties (to use the terminology of Nathan (1969). In terms of major magma bodies, the series comprises four principal units: A-B (troctolites to gabbronorites), F-G (oxiderich olivine gabbro), M (gabbronorite), and P-Q (troctolites to gabbros). These intrusions can fairly be considered to be the principal events in the evolution of the Early Gabbroic Series. A key aspect of this series of rocks, and one that led to the initial interest by H.D. Nathan in the I 960s, is that several units contain large amounts of Fe-Ti oxides. In places oxides comprise in excess of 30% of the rock (Nathan, 1969). However, among the major units listed above, only unit F-G is notable for high abundances of oxide, exceeding 60% at some locations. Until now, no chemical analyses have been available for rocks of this series. Initial modeling of magma crystallization (Jerde, 2000) indicated that the principal units of the early Gabbroic Series may be the result of polybaric fractionation. In such a scenario, units A-B, and P-Q formed through crystallization at 6kb, with M being more fractionated. Unit F-G could have formed through extensive crystallization at I kb. Oxide phases are favored by fractionation at lower pressures, resulting in the higher amounts seen in unit F-G. Results of initial chemical analyses of bulk rocks (Table I) show that both high-Al, low-Ti magmas and low-Al, high-Ti magmas are present. This is an interesting result since it has long been assumed that the earliest magmatic products of the MCR were all low-Al and high-Ti. The oxide-rich nature of the Early Gabbroic Series was consistent with this assumption. From this data, it is evident that unit F is significantly different from other units. Unit F may not be associated with unit G at all, and chemically resembles unit A. This is in marked contrast to the interpretation of Nathan (1969). The mineral data for some of the units shows variations consistent with multiple magma compositions, particularly among the pyroxenes. Among pyroxenes, Unit C appears to resemble unit P-Q, which was originally thought to be much younger. The variations in pyroxene may be additional evidence for polybaric fractionation since pressure differences plays a pivotal role in the appearance of pyroxene during crystallization. Most plagioclase is of an intermediate An content, showing very little variation among the units. The latest field efforts have focused on individual units, simply to be systematic. These probably represent individual intrusions, and once each one is examined, they can hopefully be placed into a broader context. The first unit examined was P-Q. This unit is present in a single band extending from Poplar Lake for approximately 15 kilometers to the west. It is interesting to note that Unit P is repeated, appearing twice in the magma stack (Fig. 1). In both instances, unit J is present stratigraphically above unit P. Unit J has a very characteristic appearance, with large ophitic pyroxenes (up to 3 cm) and layers of coarse grained material rich in Fe-Ti oxides. In most instances, when the top of unit P is approached, material very much like unit J appears, suggesting a gradational contact. A definitive contact between P and J was not observed at any location. Chemical and mineral data, along with the completely gradational contact between P (a gabbro) and Q (a troctolite), indicate that these are probably a single intrusion that has undergone continuous fractionation. It is possible that J is simply a late stage fractionate, formed after P. The repetition of unit P may reflect a second pulse of magma, or perhaps faulting (a thrust?) There is some indication that the two layers of unit P may have different compositions (compare sample JO-I 4 with the other unit P compositions in Table 1). One sample does not a difference make, however, and a definitive answer awaits further analysis, which will take place in the next few weeks. Recent studies of anorthosite-bearing intrusions such as the Kiglapait, Laramie, and Nain intrusions, has indicated that anorthosites can be produced through fractionation of a high-Al, high-Fe basaltic magma at pressures of 9-14 kb (Scoates and Lindsley, 2000). Such high-Al, high-Fe melts are consistent with low degrees of melting of an Fe-rich, enriched mantle source. Fractionation of these melts at higher pressures (-.14 kb) produces strong Fe enrichment and oxide phases appear early in the crystal assemblage. In such a scenario, the fractionation produces a ferrodiorite which is in equilibrium with anorthosite of an intermediate (An) composition. Enrichment of Si during fractionation is only possible at low pressures due to a thermal divide between cpx and opx at pressures in excess of 5 kb (Scoates and Lindsley, 2000). If such a magma were parental to the Early Gabbroic Series, it suggests that some of the Early Gabbroic Series units may be siblings to the liquids that formed the anorthositic series further to the south. Additional crystallization modeling is definitely needed to explore this new potential source for rocks of the early stages of the Midcontinent Rift development. 36 �! Table 1. Preliminary whole-rock compositions of major units from the Early Gabbroic Series Sample Unit J9-38 J9-35 J9-32 J9-31 J9-2 J9-46 J9-24 J9-23 J9-22 J9-29 A A B B C F G G G SiO2 TiO2 48.26 0.25 49.47 0.25 52.63 52.66 50.02 51.53 45.65 0.55 0.75 0.50 47.31 4.79 3.05 M M M 47.06 51.48 48.09 48.29 47.03 P 48.31 io-io P P jo-il J0-14 J9-26 J0-4 J0-13 J9-48 J9-41 J9-40 J9-37 J9-27 J9-17 1.12 3.73 20.01 17.93 13.84 16.36 16.23 15.08 15.74 12.98 15.14 49.20 49.30 0.41 1.83 1.80 1.72 22.98 18.93 22.28 17.52 15.56 15.40 14.53 p 4743 2.15 15.81 P 49.63 51.70 1.15 J Q Q Q Y 1.61 1.30 2.64 48.17 49.03 0.93 0.17 0.12 16.14 14.68 18.48 19.92 52.76 1.75 18.81 1.28 1.32 1.54 1.11 1.31 1.26 1.99 1.99 2.19 0.92 1.38 1.26 1.62 1.50 1.45 1.50 1.67 1.30 1.21 1.72 1.48 1.70 !P MiiQ M CaO 8.50 8.80 10.24 7.42 8.72 8.38 13.27 13.26 14.60 6.14 9.20 8.41 10.79 9.98 9.68 9.97 11.14 8.63 8.03 11.46 9.85 11.30 0.12 0.14 0.23 0.16 0.15 0.16 0.18 9.40 10.70 8.03 8.99 8.24 4.73 0.21 5.41 0.20 0.08 0.13 0.09 0.15 0.16 0.16 0.17 0.16 0.15 0.17 0.16 0.14 0.10 4.88 8.75 1.76 7.43 1.76 10.25 7.23 7.15 7.84 6.46 7.25 7.64 8.16 7.12 4.38 8.92 9.05 8.38 11.08 11.68 12.05 9.60 9.39 7.36 9.58 9.74 10.15 7.71 12.81 12.09 12.60 11.80 12.94 12.79 7.12 7.66 3.07 2.94 2.75 2.13 2.65 2.46 2.34 2.87 2.68 3.13 3.80 2.85 3.78 2.63 2.40 2.44 2.32 2.45 2.32 2.58 3.14 3.38 2.92 0.25 0.23 0.22 0.42 0.25 0.33 0.61 0.81 0.93 0.98 0.29 0.48 0.29 0.23 0.37 0.21 0.29 0.19 0.47 0.35 0.35 2.15 !Qs 0.04 0.03 0.02 0.13 0.03 0.10 0.57 0.31 0.76 0.07 0.03 0.11 0.07 0.02 0.13 0.06 0.03 0.02 0.11 0.07 0.06 0.22 99.33 99.39 100.51 100.83 100.37 100.64 98.96 99.14 99.32 99.44 99.40 99.26 98.47 100.07 99.90 100.22 99.39 99.72 100.32 99.00 99.11 99.19 61.8 62.6 62.1 63.0 61.8 60.7 35.9 39.0 34.4 31.0 55.9 24.7 59.9 53.2 53.7 55.3 47.7 56.9 59.9 52.8 53.2 37.8 Analyses performed by XRF in the Department of Geology, Macalester College, St. Paul. Fig. 1. North-South cross section of the Early Gabbroic Series. Note the P-J-Q-P-J sequence in the center of the figure. (from Nathan, 1969). 0 I,,,, References Cited: Jerde, E.A., 2000, Magmatic origins for Nathan's Layered Series: An initial reassessment of the Midcontinent Rift's first major plutonic materials: Institute on Lake Superior Geology Proceedings, 46th Annual Meeting, Thunder Bay, ON, May, v.46, part 1, p. 24-25. Nathan, H.D., 1969, The geology of a portion of the Duluth Complex, Cook County, Minnesota: Ph.D. dissertation, University of Minnesota, Minneapolis, i98p. Paces, J.B. and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic and tectono-magmatic processes associated with the 1.1 Ga Midcontinent Rift System: Journal of Geophysical Research, v.9 8, 13,997-14013. Scoates, J.S. and Lindsley, D.H., 2000, New insights from experiments on the origin of anorthosite: EOS Transactions American Geophysical Union, v.81, no.48, Fall meeting Supplement. 37 �GEOPHYSICAL ANSWERS TO GEOLOGIC QUERIES IN THE SUPERIOR PROVINCE OF NORTHERN MINNESOTA JIRSA, Mark A., and CHANDLER, Va! W. (Minnesota Geological Survey, jirsa001@umn.edu, and chand004@umn.edu) Geophysical investigations play a vital role in geologic mapping in Minnesota. As an example, we present here some highlights from mapping the Archean Superior Province in parts of Minnesota where the bedrock is covered by glacial sediment as thick as one hundred meters. Mapping of the well-exposed Archean rocks, restricted largely to the Vermilion district of northeastern Minnesota (Fig. 1), was completed in the 1960s and 1970s. Although significant, this outcrop mapping suffered from two shortcomings: 1) the area constitutes less than 5 percent of the state's bedrock crust; and 2) the Vermilion district may not be representative of the remainder of the state's Archean terranes that comprise two-thirds of Minnesota's first crystalline bedrock beneath glacial materials. These shortcomings were recognized in the late-1970s by Dr. Matt Walton, then-director of the Minnesota Geological Survey (MGS). He lobbied the State Legislature and received funding (from 1979 to 1991) to collect high-resolution, state-wide, aeromagnetic data. Additional aeromagnetic data were contributed by the U.S. Geological Survey and U.S. Steel Corporation. This was coupled with efforts to map areas having poorly exposed bedrock, acquire gravity and corresponding rock properties data, and drill test-borings in locations determined by the new geophysical data and geologic models. After two decades, the results include aeromagnetic coverage of the entire state, a grid containing more than 56,000 gravity stations (averaging 1 station per 2 square kilometers, state-wide), 2,500 determinations of density and magnetic susceptibility from nearly every major rock type, and drill cores representing many of Minnesota's Archean rocks. Armed with these data, the MGS has produced variably detailed geologic maps, and in the process raised important questions about the nature of the Archean crust. Some of those questions are described below. International VERMILION DISTRICT Falls 9o COOK- ,-' >GRB NIA HORN LIJthb2 SIDE LAKE // Archean rocks Mafic to felsic stocks Granitoid batholiths Schist of sedimentary protolith Gneiss and schist I L_ 0 (JJ Layered mafic complexes 6Omi 0 I I 80km MAP AREA Metavolcanic and metasedimentary rocks Figure 1. Simplified geologic map of the Superior Province in Minnesota showing the location of various map areas discussed in the text (LLSD—Leech Lake structural discontinuity, GRB—Giants Range batholith). 38 �Early thrust nappes Mapping in the Cook-Side Lake area (Fig. 1) was directed toward understanding the geological relationships between well exposed strata of the Vermilion district on the east, and the less well exposed rocks of the Side Lake area on the west. The volcanic sequences in each district are lithologically similar, and form broad anticlines that face stratigraphically toward one another. The intervening area, covering some 600 square kilometers, is marked by scattered exposures composed of turbiditic graywacke and slate of the Lake Vermilion Formation. These strata are complexly deformed, and their interpretation required detailed structural and sedimentological analysis. The model that evolved requires that much of the Lake Vermilion Formation has been exhumed to a level that exposes the bottom of a very large thrust nappe. Geophysical data are consistent with this interpretation, as they show graywacke as a relatively thin, geophysically translucent package overlying a "floor" of denser and variably magnetic volcanic strata. Leech Lake structural discontinuity The broadly folded volcanic and turbiditic strata of the Lake Vermilion Formation are part of what is known as the Soudan belt. The Soudan belt is structurally contrasted with the adjacent Newton belt to the north; the latter forms a series of fault-bounded, mostly north-facing, homoclinal panels. The Newton belt differs further in containing komatiitic flows and abundant mafic to ultramafic sills that are not present in rocks of the Soudan belt. In the well-exposed Vermilion district, the belts are dissected by late faults to such an extent that the relationships between the belts, and even the exact position of the boundary, are unclear. Extending the belts westward away from the zone of coinciding faults using a combination of geophysical, outcrop, and drilling data identified a major state-wide discontinuity between the two contrasting structural panels. Although its origin is not well understood, the informally named Leech Lake structural discontinuity (LLSD on Fig. 1) is considered a significant Archean crustal suture. Mud Lake syncline Because Archean bedrock in the area known as the Virginia horn (Fig. 1) is mantled by magnetic iron-formation of Paleoproterozoic age, recent mapping in the area relied almost exclusively on gravity modeling. The Archean bedrock is marked by a large fold, thought by earlier geologists to be an anticline, having graywacke in the core and volcanic strata on the limbs. The graywacke is complexly folded, and detailed structural analyses of cleavage/bedding relationships produced a contrasting model of an early-formed, west-facing, upright syncline. The model was tested by a series of gravity profiles that outlined a broad synform that is cored by relatively low-density graywacke, and flanked by inwardfacing limbs of dense basaltic crust. Gravity modeling indicates that graywacke along the syncline axis is I to 2 kilometers thick and underlain by dense basaltic rocks. Geophysical boundary at 5 kilometers Constrained by physical property data, gravity and magnetic model studies of Superior Province rocks in Minnesota indicate that density and magnetization contrasts associated with the Archean greenstone-granite belts do not appear to extend to depths much greater than about 5 kilometers. This depth is surprisingly shallow, given the width and near-vertical geometry of most of these belts. The belts could be cut off by low-angle thrusting, which would be consistent with the convergent-margin, accretionary models advanced by the Canadian Lithoprobe project. On the other hand, the limited depth extent of the greenstone-granite belts could represent enhanced assimilation and mixing of magmatic and supracrustal components at depth. Support for the latter model exists along the southern boundary of the Giants Range batholith with adjacent supracrustal sequences that are complexly invaded and locally migmatized by granodiorite. In either interpretation (low-angle thrust or assimilation at depth), gravity and magnetic model studies indicate the observed anomaly signatures associated with Superior Province rocks in Minnesota are largely the result of sources within the uppermost crust, and that relatively little anomaly signature is apparently produced by middle or lower crustal rocks. In summary, aeromagnetic data, supplemented by gravity and rock property data, have had tremendous impact on mapping Superior Province rocks in Minnesota. These geophysical data provide an essential framework and continuity that is not typically available from existing drill holes and outcrops. During the last 20 years, much of the Archean bedrock outside of the Vermilion district has been remapped at scales of 1:1,000,000 or greater. This ongoing mapping provides a valuable base for launching mineral exploration programs, and has revolutionized our understanding of early crustal evolution in Minnesota. 39 �DISTRIBUTION OF ARSENIC IN WISCONSIN GROUNDWATER DAVE JOITThSON, Drinking Water & Groundwater Bureau, Wisconsin Department of Natural Resources, PU Box 7921, Madison, WI 53707-7921, johnsdmmail0.dnr.state.wi.us Iii the past ten years, with the recognition of elevated arsenic is drinking water supplies, the Wisconsin Department of Natural Resources has analyzed over 15,000 individual samples statewide. Ten percent of the statewide samples exceed the 10 ppb proposed Arsenic standard by the US Environmental Protection Agency; in some large areas over 25 percent of the samples exceed this level. The objectives of the sampling program were to (1) identify the distribution of elevated arsenic in water supplies, (2) identify concentration ranges and causes for the elevated arsenic, and (3) determine construction guidelines to minimize the problem. The arsenic appears in many settings, the most common presently recognized being a northeast trending belt in east-central Wisconsin, west of the Fox River. The highest concentration in Wisconsin, 15,000 ppb, occurs in this belt. Numerous studies (Burkel, 1993; Pelczar, 1996; Simo and others, 1996,1997; and Gotkowitz, 2001) have identified a sulfide-rich horizon recognized near the top of the St. Peter sandstone. This appears to be the principal source for As in this area. However, there are some elevated levels west of the western-most subcrop extent of the sandstone unit. Other areas of elevated As have been identified. These include relatively shallow domestic water wells in Silurian dolomite in southeast Wisconsin and in association with the Upper Mississippi Valley Zinc-Lead District in southwestern Wisconsin. Elevated concentrations are also noted in isolated areas in central and northeastern Wisconsin associated with unique Precambrian units. Township-based surveys of private wells in east central Wisconsin are continuing and may permit greater delineation of the As problem and possible solutions. Burkel, R.S., 1993, Arsenic as a Naturally Elevated Parameter in Water Wells in Winnebago and Outagamie Counties, Wisconsin: unpub. MS thesis, University of Wisconsin -Green Bay, 111 p. Pelczar, J.S., 1996, Groundwater Chemistry of Wells Existing Natural Arsenic Contamination in East-central Wisconsin: unpub. MS thesis, University of Wisconsin - Green Bay, 206 p. Simo, J.A., Freiberg, P.G., and Freiberg, KS., 1996, Geologic Constraints on Arsenic in Groundwater with Applications to Groundwater Modeling: Wisconsin Water Resources Center GRR 96-01, 57 p. Simo, J.A., Freiberg, P.G., and Schreiber, M.E., 1997, Stratigraphic and Geochemical Controls on the Mobilization and Transport of Naturally Occurring Arsenic in Groundwater: Implications for Water Supply Protection in Northeastern Wisconsin: Wisconsin Water Resources Center GRR 97-05, 56 p. Gotkowitz, M.B., Scbrieber, M.E., and Simo, J.A., 2001, Contrasts in the geologic and hydrochemical Occurrences of Arsenic contamination of Groundwater in Eastern Wisconsin: thstitute on Lake Superior Geology, this meeting. 40 �FLUID INCLUSION EVIDENCE FOR A ROLE FOR HYDROTHERMAL ACTIVITY IN THE ROBY ZONE, LAC DES ILES MINE, NORTHWESTERN ONTARIO JOHNSON, J.R. and KISSIN, S.A., Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E 1, stephen.kissin@lakeheadu.ca The Roby Zone of the Lac des lies Pd-Pt-Ni-Cu-Au mine, located 90 km north-northwest of Thunder Bay, Ontario, was the initial locus of mining activity in the deposit. A striking aspect of the deposit in the early stages of mining was the coincidence of the ore zone with an envelope of taicose hydrothermal alteration (Michaud, 1998). Based on evidence of hydrothermal activity in off-set and deep, copper-rich deposits at Sudbury (Molnar et al., 1997) and observations of felsic veins and pods in the Roby Zone, an investigation of fluid inclusions was undertaken. From a suite of samples collected during pre- and early-mining stages at the Roby Zone by Michaud (1998), hand specimen were selected. The hand samples used in the study were selected based on the presence and abundance of transparent minerals. From three hand samples (Lac-3 18, Lac-304 and Lac-46) taken from the heterolithic gabbro located in the middle of the Roby Zone, a number of doubly polished sections 100 tim-thick were prepared. Sample Lac-3 18 is composed of pegmatitic gabbro that contain minor veins of plagioclase and epidote. Sample Lac-304 consists of a quartz vein that is emplaced within a medium-grained gabbro. The last sample used, Lac-46, is a mediumgrained gabbro crosscut by a felsic vein network. Four distinct types of fluid inclusions were observed occurring in plagioclase and quartz: Type I - Single-phase liquid inclusions. These were extremely numerous in quartz but less common in plagioclase. Type II- Two-phase inclusions (liquid + vapour). The most numerous type of inclusion that was found in plagioclase. The majority had a vapour content of 10-20% by volume. Type III - Polyphase inclusions (liquid + vapour + one or more solid phases). These were fairly abundant but of a small size, occurring only only in plagioclase. Numerous inclusions contained a single crystal, but only a few were found to contain more than one solid. Type IV - CO2-rich polyphase inclusions. These were found only in plagioclase, containg two solids, two liquids and vapour. The fluid inclusion studies revealed the following results: Type II and Type III inclusions - All but one inclusion homogenized to vapour in the range of 540.1-352.4°C, but the majority were in the range 460-420°C. Eutectic temperatures generally clustered about -52°C or -49.8°C, the eutectics 41 �for NaC1-CaC12-H20 and CaCl2-H20, respectively. Some lower eutectic temperatures observed may be the result of the small size of the inclusions and difficulty in detecting the first melting. Some eutectic temperatures were near -21.2°C, the NaCl-H20 eutectic temperature. These may be attributed to lower concentrations of CaC12, such that its effects on melting behaviour were difficult to observe. Most inclusions in this group had final melting temperature of about -20°C, indicating moderate to high salinity. Type IV inclusion - Only one inclusion containing liquid CO2 and two daughter crystals was observed. The eutectic temperature was -54.2°C with a final melting temperature of -15.5°C, consistent with data from Type II and II inclusions. Liquid CO2 homogenized to vapour at 26.8°C, and the remaining vapour homogenized to liquid at 160.8°C. However, the daughter crystals melted at 230°C and >412°C, the decrepetation temperature. Type I - Liquid only inclusions exhibited eutectic temperatures of--34°C and final melting temperatures of-15°C. These inclusions, although saline, were clearly formed at low temperature. The study has shown that high-temperature saline fluids are intimately associated with the ore zone of the Roby Zone. It is suggested that the present distribution of the ore is the result of hydrothermal activity associated with these fluids, possible of late magmatic origin. Similarity to fluids active in the Sudbury Igneous Complex is striking. Michaud, M.J., 1998. The Geology, Petrology, Geochemistry and Platinum-Group ElementGold-Copper-Nickel Ore Assemblage of the Roby Zone, Lac des Iles Mafic-Ultramafic Comples, Northwestern Ontario. M.Sc. thesis, Lakehead University, Thunder Bay, Ontario. Molnar, F., Watkinson, D.H., Jones, P.C. and Gatter, I., 1997. Fluid inclusion Evidence for Hydrothermal Enrichment of Magmatic Ore at the Contact Zone of the Ni-Cu-PlatinumGroup Element 4b Deposit, Lindsley Mine, Sudbury, Canada. Economic Geology, vol. 92, pp. 674-685. 42 �HYDROGEOCHEMICAL MODELING OF ARSENIC IN MINNESOTA GROUND WAT1R KANTVTSKY, Roman, Minnesota Geological Survey, 2642 University Avenue, St. Paul, MN 55114, kaniv001@umn.edu Concentrations of dissolved arsenic in Minnesota ground water commonly exceed 3pJL (micrograms per liter) and are as much as 157 piL. The highest regional concentration of arsenic in ground water (0.06—91 WL, mean 6 iJL) is documented in the western part of the state where the aquifers are buried bodies of sand, gravel, and silty sand that form discontinuous lenses beneath lake deposits and within till. Because of this, the hydrogeochemical modeling was performed for ground water systems in this part of the state. A surface complexation two-layer sorption model was used to assess the mechanisms controlling arsenic distribution in the Quaternary buried artesian aquifer and Cretaceous aquifer systems. The release of sorbed arsenic from iron hydroxides into ground water was estimated by this model. The test data consisted of concentrations of total arsenic and iron in solution; this was derived from the arsenic and iron concentrations in the geological materials. The result of this model is the distribution of arsenic—either adsorbed to the surface of iron hydroxides, or into the aqueous phase. These results suggest that mobility of arsenic is promoted by the onset of suboxic conditions in aquifer systems where iron hydroxides have sorbed arsenic. The reduction of ferric oxides and hydroxides, together with the reduction of As(V) to the less-strongly adsorbed and more mobile As(III), can release adsorbed arsenic into ground water. This reduction may be driven by microbial degradation of natural organic matter in aquifer systems. The source of the arsenic in the Quaternary buried artesian system is presumed to be iron oxides and iron hydroxides that have sorbed arsenic, and form amorphous coatings on mineral grains within the silty-clayey till. Despite the fact that the arsenic concentrations in the ground water and the geological materials used for the modeling were far apart, the modeling illustrates that the process is probably generic and not limited by geography. Based on the sorption model and the classification of aqueous environments, the working model that explains the mech anisms controlling distribution of arsenic in ground water can be applied to five hydrogeochemical systems in Minnesota: the Quatemary buried artesian aquifer system, the Quaternary water-table aquifer system, the Cretaceous aquifer system, the Paleozoic- Mesoproterozoic artesian basin aquifer system, and the Precambrian crystalline rock aquifer system. The thick, silty-clayey, glacial and lacustrine sediments that cover the aquifers of the Quatemary buried artesian aquifer system in west-central Minnesota serve as a geochemical transition zone associated with the transformation from oxic to suboxic conditions. In areas where the tifi is more sandy, suboxic conditions may not exist. Variability in arsenic concentrations in water of the Quaternary buried artesian aquifer system can thus be explained by changes in geochemical conditions, the variability of the arsenic content in the sediments, or by variability in Waters within other distribution of chemical reductants (e.g. buried organic matter). hydrogeochemical systems typically have low concentrations of arsenic, possibly because they are associated with oxic conditions. High arsenic concentrations associated with sulfide mineralization or iron-formation may be present locally within any system, but are not a regional feature of any of these hydrogeochemical systems. Reference: Kanivetsky, R., 2000, Arsenic in Minnesota ground water: hydrogeochemical modeling of the Quaternary buried artesian aquifer and Cretaceous aquifer systems: Minnesota Geological Survey Report of Investigation 55, 23 p. 43 �ROCK MAGNETIC STUDIES OF PHYLLITIC ZONES FROM THE BARABOO SYNCLINE, WISCONSIN. KELLY*, COLLEEN, AND KEAN, WILLIAM F., Department of Geosciences, University of Wisconsin —Milwaukee, P.O. Box 413, Milwaukee, WI 53201 wkean@uwm.edu. (* student) Preliminary paleomagnetic results based on limited sampling from a variety of rock types, suggested that the magnetism of the metamorphic rocks associated with the Baraboo Syncline was pre-folding (Kean and Mercer,1986). The detailed work by Mercer (1984) based on data from two quartzite sites on the southern limb and one site on the northern limb shows the magnetism is carried by hematite, and is prefolding. The unfolded magnetic direction is Dec=201°, Inc.66.4°, Alpha 95=10.7°, which gives a paleopole at 257°E, 4.67°N. This is consistent with 1.75Ga. paleopole positions for North American. However, the sites with phyllites were notably inconclusive (Mercer, 1984). Possible causes are; incomplete structural information, synfolding magnetization that was not recorded by the quartzite, or anisotropy of magnetization caused during the folding. Nonetheless magnetic results from these strata could provide important clues about the actual age of folding of the syncline. The phyllites result from metamorphism of clay rich layers within the original sandstone sequence, which could also contain hematite, magnetite or maghemite. It is likely that the magnetic mineralogy, and/or the magnetic direction in these layers will change during metamorphism and folding, and therefore provide information on the deformation history. A suite of oriented hand samples, collected from phyllitic zones on both sides of the Baraboo Syncline, were subjected to a variety of magnetic measurements to determine the magnetic mineralogy and remanent directions. The measurements included stepwise alternating field (AF) and thermal demagnetization to either 1 OOmT or 750° C respectively, magnetic susceptibility, saturation isothermal remanent magnetization (SIRM), and hysteresis properties. A.F. demagnetization was mostly ineffective. However, the thermal demagnetization results show that the majority of samples have a single magnetic direction that reduced to zero intensity between 650-750° C. The SIRM plots never reach saturation by 300mT, and the coercivity of remanence (Hcr) values derived from the hysteresis ioops are in the range of 140-180 mT. These results are indicative of hematite with an effective grain size of 15-20 tm, that probably developed during metamorphism. However, additional studies are required to exclude the possibility that the differences in the magnetic directions for these sites are not due to local structural conditions, or anisotropy of remanent magnetization. References: Mercer, D.,1984, Paleomagnetism of the Baraboo Quartzite, Unpublished MS thesis, UWMilwaukee, 294 pp. Kean, W.F. and Mercer, D., 1986, Preliminary Paleomagnetic Study of the Baraboo Quartzite, Wisconsin, Geoscience Wisconsin, Vol. 10, p 46-53. 44 �HEALTH SURVEILLANCE IN A COMMUNITY AFFECTED BY ARSENICCONTAMINATED WATER Lynda Knobeloch and Charles Warzecha, Wisconsin Department of Health and Family Services Shelli Nelson, University of Wisconsin-Madison, Environmental Toxicology Center Many private drinking water wells in Wisconsin's Fox River Valley contain naturally occurring arsenic at levels of health concern (Riewe et al., 2000). In 1993, the Department of Health and Family Services surveyed water use and health status among families living in the affected region. Data from this initiative indicated that residents whose daily arsenic intakes exceeded 50 were approximately three times more likely to report skin cancer than residents with lower arsenic intakes (Haupert et al, 1996). The Department is currently re-evaluating arsenic exposure and health status in this region. Three factors make this re-evaluation timely. In January, the U.S. EPA lowered the safe drinking water standard for arsenic from 50 ug/L to 10 ugIL, increasing the number of wells that will be deemed "unsafe." Since the previous study, the number of families that use private wells in this region has increased dramatically. In addition, repeated sampling of several wells in this region seems to indicate that arsenic concentrations are increasing over time, possibly due to regional drawdown of the aquifer. The current study covers a much broader geographic region than the 1993 study. Each township within Outagamie and Winnebago Counties has been encouraged to participate. To date, nearly 2000 families have submitted well water samples for arsenic analysis and completed a 4-page health questionnaire. This larger study population, combined with a more extensive list of health outcomes, is expected to provide us with a better understanding of arsenic exposure and its impact on the health of families that consume water from private wells in these counties. References: Riewe T, Weissbach A, Heinen L, and R Stoll, 2000. Naturally occurring arsenic in well water in Wisconsin. Water Well J; (June 2000): 24-31. Haupert TA, Wiersma JH, and JM Goldring, 1996. Health effects of ingesting arseniccontaminated groundwater. Wisc Med J; 95(2):100-104. 45 �HYDROGEOLOGIC SETTING OF ELEVATED ARSENIC IN SOUTHEASTERN MICHIGAN KOLKER, Allan, and CANNON, W. F., U.S. Geological Survey, Reston, VA, 20192, HAACK, S. K., and WESTJOHN, D. B., U.S. Geological Survey, Lansing, Ml, 48911, and WOODRUFF, L. G., U.S. Geological Survey, Saint Paul, MN 55112. High levels of arsenic, up to nearly 40 times the EPA standard of 10 tg/L, are present in southeastern Michigan, primarily in private supply wells. To investigate the problem, the USGS, in collaboration with the University of Michigan, sampled more than 100 wells, including public, private and monitoring wells, and examined aquifer materials in the region for possible sources of arsenic. The study area includes nearly all of Genesee, Huron, Lapeer, Livingston, Oakland, Sanilac, Shiawassee, Tuscola, and Washtenaw counties, which have a combined population of more than 2 million. Of the wells sampled by the USGS, more than 50% exceed the current EPA drinking water standard [1,2]. Most of the affected wells are completed in the Marshall Sandstone (Mississippian), the principal bedrock aquifer in the region, but in some cases, water in overlying glacial aquifers, or in the Saginaw (Pennsylvanian) aquifer is also affected. Problem wells are concentrated in the eastern and southeastern parts of the Marshall Sandstone subcrop belt, where Marshall Sandstone is in direct hydrologic contact with permeable Pleistocene glacial deposits [3]. To investigate the possible relation between the composition of aquifer materials and the arsenic content of well water, test wells were drilled by the USGS in Huron County (H-i 5D) and Lapeer County (LP-1), in areas known to contain wells with high arsenic contents. Both of the USGS wells show a gradual decrease in total arsenic of well water with depth, but redox conditions (Eh = 29 to 68 mV; [1]) and the fraction of As III (88 — 100% of total arsenic), measured for H-15D, are essentially uniform with depth (Figure). There is no distinct correlation between arsenic contents of aquifer materials, sampled in 4.5 ft. (1.37 m) intervals (0 to —300 ppm), and waters, sampled largely in 50-ft. (15.7 m) intervals (Figure). Likewise, there is no correlation between As concentration and total Fe or SO4 contents of waters in the region, even for samples having the highest (>50 p.g/L) As contents. These findings suggest that in-situ pyrite oxidation in the Marshall aquifer is very limited. In the Marshall Sandstone and in portions of the overlying Michigan Formation, pyrite is locally present as a cement whose texture indicates growth that has displaced framework sand grains. This pyrite locally contains highly arsenic-enriched domains (up to 8.5 wt. %) occurring as overgrowths on pyrite framboids having much lower arsenic contents, that are in turn incorporated into pyrite having low arsenic contents [4,5]. Well cuttings containing arsenic-rich pyrite have arsenic contents as high as 1020 ppm. Investigation of till samples containing iron oxy-hydroxides, probably derived from weathering of arsenian pyrite, shows arsenic contents up to about 0.7 weight percent. This indicates that a portion of the arsenic is retained during the weathering/oxidation process, or that arsenic is concentrated on oxide surfaces by adsorption of aqueous arsenic. Because there are multiple sources of arsenic in glacial and bedrock aquifers that are in hydrologic continuity, no single process may explain the overall distribution of arsenic in southeastern Michigan wells. Reduction of iron-oxyhydroxides in glacial aquifers by exposure to suboxic ground water, is one possible source. In-situ pyrite oxidation, while limited, may locally be enhanced by high concentrations of bicarbonate [6]. Because of the extreme arsenic contents of some Marshall Sandstone pyrite, small amounts of in-situ oxidation could result in contamination of bedrock aquifers. Likewise, we cannot rule out sporadic paleo-oxidation of pyrite in the bedrock aquifer, resulting from lower water table levels that existed following the last glaciation. All of these processes have likely contributed to the widespread but sporadic nature of arsenic enrichment in southeastern Michigan ground water. 46 �Compilation of analytical results for arsenic in USGS test cores H-15D (A), and LP-1 (C) and waters in corresponding packed intervals (B and D). Data represent bedrock intervals only, with the exception of glacial drift recovered from LP-i. Brackets on water data indicate size of packed intervals represented. Results for arsenic speciation in H-i 5D waters are from M.-J. E a C 0 10 Kim [21. 20 30 Sandstone! 40 Siltstone 60 70 Mudstone 80 90 100 Shale 110 120 As (ppb) As (ppm) Carbonate References and selected geochemical characteristics of [1] Haack, S. K., and Trecanni, S. L., 2000, Arsenic concentration U.S. Geological Survey Water Resources in southeastern Michigan: ground water and aquifer materials Investigations Report 00-4171, 38 p. groundwater of southeast Michigan: Ph.D. dissertation, [2] Kim., M.-J., 1999, Arsenic dissolution and speciation in University of Michigan, Ann Arbor, Ml, 201 p. framework of the Michigan Basin Regional Aquifer [3] Westjohn, 0. B., and Weaver, T. L., 1998, HydrogeologiC Professional Paper 1418, 47 p. System: U.S. Geological Survey pyrite in the Mississippian 0. B., Woodruff, L. G., 1998, Arsenic-rich [4] Kolker, Allan, Cannon, W. F., Westjohn, Michigan ground water: Geological Society anomalous arsenic in southeastern Marshall Sandstone: Source of of America, Abstracts with Programs, v. 30, no. 7, p. A-59. D. B., Haack, S. K., and Kim, M.-J., 1999, Arsenic in [5] Kolker, Allan, Cannon, W. F., Woodruff, L. G., Westjohn, EOS, American Geophysical Union southeastern Michigan ground water: Results of USGS test drilling: S147. Transactions, v.80, no.17, p. S146ions and arsenic dissolution by ground water: [6] Kim, M.-J., Nriagu, Jerome, and Haack, S. K., 2000, Carbonate 34, p. 3094-3100. Environmental Science and Technology, v. 47 �POTENTIAL FOR COPPER MINERALIZATION IN THE ANIMIKIE GROUP, MINNESOTA LARSON, Phillip C., Department of Geological Sciences, University of Minnesota, Duluth, MN 55812, plarson2@d.umn.edu Numerous lines of evidence have recently been identified suggesting the potential occurrence of Keewenawanstyle native copper or White Pine-style sediment-hosted copper mineralization in Animikie Group sediments in northeastern Minnesota. Native copper has been reported in quartz veins in the Wanless Mine near Buhi (Gruner, 1946). Copperstained rocks have been observed in the Butler Taconite mine (D. Ridgeway, pers. comm.). Hydrothermally altered diabase dikes in the National Steel Pellet Company (NPSC) Mine have elevated copper values (up to 950 ppm) associated with a chlorite-quartz alteration assemblage (Larson et a!., 1999). In addition, diagenetic pyrite from the NSPC mine frequently displays a secondary coating of chalcopyrite on bedding and fracture surfaces. Finds of float native copper in glacial drift have occasionally been reported in the Nashwauk-Keewatin area of the Mesabi Range (L. Mattson, pers. comm.). It is improbable that this float copper is derived from the Lake Superior basin, as surficial drift on the Mesabi has either a northwest or northeast provenance; this strongly suggests a local provenance. These occurrences suggest the northern flank of the Animikie Basin has hosted a regional-scale mineralizing event by an oxidized cupriferous fluid. The timing of copper mineralization is constrained by cross-cutting relationships. A series of diabase dikes intrude iron formation in the NSPC mine area; these dikes are tentatively dated at —1 100 Ma. Overprinting of alteration on well-developed cooling structures and post-emplacement shears in the dikes suggests that the copper mineralizing event significantly post-dated dike emplacement — similar to native copper mineralization on the Keewenaw peninsula. In contrast, in the South Stevenson Mine a diabase dike intruded the natural iron-ore body, indicating dike emplacement post-dates natural iron-ore formation. These relationships demonstrate that the natural iron-ore forming hydrothermal event is distinct from, and preceded the copper mineralizing event. Morey (1999) has presented a model for formation of natural iron ore deposits on the Mesabi by alteration of Biwabik iron-formation by a hydrothermal fluid generated deep in the Animikie basin. A topographically-driven regional hydraulic flow system was induced by recharge in a highland area bounding the southern end of the basin. Fluids evolved deep in the basin were driven through relatively permeable strata at the base of the Animikie Group (Pokegama Quartzite). These fluids leaked upward along fractures through the overlying iron-formation toward the margins of the basin. Concentration of iron oxides in natural iron-ore deposits is chiefly a function of silica removal from unaltered iron-formation. Evidence suggests that this process extended down into the underlying Pokegama Quartzite as well. Holway (1956) reported that at the Auburn Mine, quartzite underlying the natural iron-ore was composed of friable sand from which the silica cement had been leached. In most exposures, the Pokegama Quartzite is a well-cemented low permeability rock. However, local desilicification associated with natural iron-ore formation has enhanced permeability along numerous fault- and fracture-hosted fluid pathways. During formation of the Midcontinent Rift System, rift-filling volcanics, intrusives, and sediments truncated the southern margin of the Animikie basin. The base of the Animikie Group is thus presumably in contact with volcanic rocks in the deeper portions of the rift — the likely source of oxidized cupriferous brines associated with Keewenawan native copper mineralization. These same fluids may have been driven from the western flank of the rift into the basal portion of the Animikie Group (Fig. 1). Fluid flow would have been focused through the basal clastic units, and in particular along the preexisting desilicified faults and fractures. Overlying iron-formation and shale units would have served as effective aquitards. Two scenarios for concentration of significant copper mineralization by cupriferous brines discharging along the northern margin of the basin present themselves (Fig. 2). High-porosity, high-permeability desilicified clastic sediments within the Pokegama Quartzite may be the host for native copper mineralization similar to that observed in brecciated flow-tops and conglomerate beds on the Keewenaw Peninsula. Interaction of the brines with the iron-sulfide-rich base of the Virginia Formation may have produced stratiform copper sulfide mineralization similar to that observed at the White Pine and Presque Isle deposits. 48 �Midconfinent Rift Mesabi Range Animikie Basin NW OronfoIBaieId Group Sec mentsJSE Figure 1. Schematic cross-section of post-Midcontinent Rift flow system through the Animikie Basin. k\ \ White Pine-Style Copper Sulfide \Tirginia Formation Figure 2. Schematic cross-section of copper mineralization, showing relationship between faults/fractures, desilicification, and Animikie Group sediments. References Gruner, J.W., 1946, Mineralogy and Geology of the Mesabi Range: Office of the Commissioner of the Iron Range Resources and Rehabilitation, St. Paul, 127 p. Hoiway, W., 1956, Auburn Mine, in Schwartz, G.M., ed., Geol. Soc. America Annual Meeting Field Trip 1 Guidebook, p.160-167. Larson, P. C., Hanttula, J. E. and Price, J. 5., 1999, Proterozoic mafic dikes, western Mesabi Range, Minnesota. GSA Abstracts with Programs, Vol. 31, No. 7: 106. Morey, G. B., 1999, High-grade iron ore deposits of the Mesabi Range, Minnesota — product of a continental-scale Proterozoic ground-water flow system. Economic Geology 94: 133-142. 49 �CONTRIBUTIONS TO THE CULTURAL GEOGRAPHY OF THE WEST MESABI RANGE, NORTHERN MINNESOTA LIVELY, Richard and MOREY, G. B. (Minnesota Geological Survey, lively@umn.edu and morey001@umn.edu) The Mesabi Iron Range in northern Minnesota has produced more than 4 billion tons of iron ore and taconite from more than 500 mines since ore was first shipped from that area in 1892. Mining, confined to an east-northeast trending strip of land some 100 miles long and 4 miles wide, has greatly modified both the original landscapes and associated cultural features. In this discussion, we assess some of those changes utilizing modern data and topographic maps prepared during the summer of 1899 and 1900 by E.C. Bebb and his assistants D.L. Fairchild and Louis B. Weed, all topographers with the U.S. Geological Survey. The original data were compiled at a scale of 1:16,000 with a contour interval of 20 feet, and ultimately were published at a scale of 1:50,000 in 1903 (Leith, 1903). The original field maps were obtained from the Cuyler Adams estate in the early 1 980s and were subsequently digitized by Emily Bauer of the Minnesota Geological Survey to produce a pre-mining topographic rendition of the western half of the Mesabi range. The digitized maps were prepared as part of a larger study of the hydrologic character of the western Mesabi range sponsored by the Legislative Commission on Minnesota Resources (LCMR). We compare here the 1899-1900 data with topographic data obtained in 1999 by the Minnesota Department of Natural Resources as part of the Mesabi Elevation Project (DNR, 1999). As one would expect, open-pit mining has considerably changed the topographic expression along the range, mainly by increasing the relative abundance and depth of topographically low areas and redistributing topographically high areas. In general, the overall spread in elevation has increased by over 100 feet due to excavation, while over the same period the maximum elevations on the range have not changed substantially. When comparing data from the 1999 high-resolution digital surface with data from the 1899-1900 survey in undisturbed areas, the resulting correlations are virtually identical. This implies a high degree of accuracy among the early topographers mapping the Mesabi range. In 1899-1900, the western half of the range contained 11 mines and 17 mine dumps totaling about 0.7 square miles, or about 0.3 percent of the 230 square miles originally mapped. In 1986, mining activity accounted for about 140 square miles of pits and dumps, or about 60 percent of the study area. Mining also significantly affected drainage patterns along the range. In 1899-1900, mapped streams totaled over 400 miles in length, whereas only about 220 miles could be found compiled from topographic maps between 1970 and 1990, a reduction of about 45 percent. Much of that loss involved the removal of headwaters for many southward flowing streams along the south side of the Laurentian drainage divide. The transportation features mapped on the range in 1899-1900 were primarily limited to about 170 miles of roads or trails and about 125 miles of railroad track. Most of these extended along the strike of the range, with relatively little development crossing the range. By 1990, railroad mileage had doubled to about 250 miles and road length had increased to over 1000 miles. In addition to the increased transportation network, mining created and destroyed many towns as it progressed. For example, the entire town of Hibbing was moved several miles after a rich deposit was discovered under its streets. The Leith (1903) report also included a geologic map that was designed to aid exploration by delineating the boundaries of the Biwabik Iron Formation. That map, compiled during the spring of 1900 and the summers of 1900 and 1901, relied heavily on the support and geologic information, especially test pit and drill hole data, provided by J.U. Sebenius of the Lake Superior Consolidated Iron Mines (now owned by the U.S. Steel Corporation). Although the geologic data base was limited to relatively few, sparsely distributed data points, the resulting geologic map read very much as it does today, even with more complete data now available. The only significant difference involves a better understanding of faults and related structures which became apparent as extensive test drilling and mining progressed. References Cited: Leith, C.K., 1903, The Mesabi iron-bearing district of Minnesota: United States Geological Survey Monographs, v. 43, 316 p., 1 plate. Minnesota Department of Natural Resources, 1999, Mesabi Elevation Project: St. Paul, Minnesota. 50 �PRECAMBRIAN GEOLOGY OF S. WISCONSIN: A PANORAMA FROM THE BARABOO RANGE MEDARIS, L.G., Jr., Dept. of Geology & Geophysics, Univ. of Wisconsin-Madison, 1215 W. Dayton Street, Madison, WI, 53706, medarisgeology.wisc.edu Red, supermature quartzites, most notably the Baraboo, Barron, and Sioux Quartzites, have long been recognized as a distinctive Proterbzoic feature in the southern Lake Superior region, signifying depositionon a stable craton under conditions of intense chemical weathering in the presence of significant free atmospheric oxygen (Dalziel & Dott, 1970; Ojakangas & Weber, 1984; Southwick et al., 1986). The quartzites were inferred to be post-1750 Ma in age, to have been folded and metamorphosed at —4630 Ma, and locally intruded by granitic rocks at —1450 Ma. The term, Baraboo interval, was introduced by Dott (1983) for this succession of sedimentation, deformation, metamorphism, and intrusion in the time span of 1750 to 1450 Ma. In the last six years investigators at Wisconsin and Dan Holm at Ball State and coworkers have taken a renewed interest in the Baraboo interval, and their results substantiate the original concept and framework of the Baraboo interval and provide a more detailed understanding of the disparate geological processes that shaped the southern Lake Superior region in mid-Proterozoic time. Igneous Basement of the Baraboo Range The Baraboo Quartzite is underlain by diorite, granite, and rhyolitic lavas and pyroclastic rocks (Dalziel and Doff, 1970). U-Pb zircon ages of the Baxter Hollow granite and rhyolite are indistinguishable, and taken together, yield an age of 1749±12 Ma (Van Wyck, 1995). Chronologically and petrologically, the Baraboo basement is correlative with the subalkalic granite and rhyolite suite of the Fox River Valley (Smith, 1978; Anderson et a!., 1980). Igneous textures are well preserved in all units of the Baraboo basement, although igneous minerals are partly to completely replaced by a variety of greenschist facies minerals. Typically, biotite is replaced by chlorite, plagioclase is transformed to albite and fine-grained epidote (saussurite), and intermediate alkali feldspar is recrystallized to a fine-grained mixture of near end-member microcline and albite. Hornblende in diorite is partly replaced by chlorite and intergrown actinolite and cummingtonite. Paleosols beneath the Quartzites Mature paleosols have been recognized beneath the Sioux, Baraboo, and Barron quartzites (Southwick & Mosler, 1984; Medaris et al., 1997; Medaris, 2000). Among these paleosols, the Barron represents the best standard for comparison, because it has not been affected by later hydrothermal alteration, as have the other two. The Barron paleosol is a saprolite, derived from Penokean metatonalite and consisting of quartz, kaolinite, hematite, traces of sericite, and crandallite-florencite. During weathering of the metatonalite protolith, A12O3, TiO2 and Zr remained immobile, Na20, CaO, MgO, and MnO were effectively removed, Ba, Sr, K2O, and Rb were substantially reduced, and SiO2 decreased by 10%. The extreme chemical maturity of the Barron saprolite is reflected in a high value, 95.7, for its Chemical Index of Alteration. The Baraboo Quartzite is underlain by saprolite, which varies in thickness from 30 to 50 feet and was derived from granite and rhyolite. Chemical changes in the Baraboo saprolite relative to protolith are similar to those in the Barron, except for subsequent hydrothermal addition of K20 and Rb. Common features of the Barron, Baraboo, and Sioux paleosols are the absence of feldspar and a high degree of mineralogical and chemical maturity, similar to present-day weathering profiles in warm, humid climates, where intense chemical leaching is characteristic. Such a climate and a stable tectonic setting were essential for generating the supermature features of the Baraboo interval quartzites. Depositional Age and Composition of Baraboo Interval Sedimentary Rocks A post-Pen okean depositional age for the Baraboo interval quartzites was long recognized, and recent analyses of detrital zircon grains in the Baraboo, Barron, Flambeau, McCaslin, and Sioux quartzites have yielded numerous U-Pb ages from 1782 to 1712 Ma (Van Wyck, 1995; Doff et a!., 1997; Holm Ct al, 1998), demonstrating that the quartzites were deposited no earlier than —1710 Ma. The chemical maturity of the Baraboo 51 �interval sediments was previously inferred from the near absence of detrital feldspar, the abundance of pyrophyllite or kaolinite, and the predominance of zircon, magnetite, rutile, and apatite among heavy minerals. Recent analyses of Barron, Baraboo, and Sioux siltstone, pelite, and their metamorphosed equivalents, yield compositions consisting essentially of Si02, Al203, Fe203, hO2, and H20, with a Chemical Index of Alteration ranging from 96.8 to 98.6. The extreme chemical maturity of the Baraboo interval sediments reflects a source region that experienced extensive chemical leaching and produced detritus consisting largely of quartz, kaolinite, and hematite. 1630 Ma Folding and Metamorphism There was widespread Rb-Sr isotopic resetting at 1635±33 Ma in Fox River Valley granite and rhyolite, Baxter Hollow granite, and Baraboo rhyolite (Dott & Dalziel, 1972; Van Schmus et al., 1975; X = 1.42*1011 yr'), and it was suggested that such isotopic resetting was coincident with folding of the quartzites, which might represent an eastern expression of the Mazatzal deformation in the southwestern U.S. (Dott, 1983; Van Schmus et al., 1993). The extent of foreland deformation is marked by a 1630 Ma tectonic and thermal front in northern Wisconsin, which was located on the basis of 40ArP9Ar cooling ages of mica and hornblende in basement rocks and the distribution of folding in quartzites (Holm et al., 1998; Romano et al., 2000). The Barron Quartzite is unfolded and unmetamorphosed, consisting of quartz, kaolinite, and hematite. In contrast, the Baraboo Quartzite and underlying basement have been folded and recrystallized under low grade conditions (Medaris et al., 1998). The coexistence of quartz and pyrophyllite in the metasedimentary rocks requires a temperature between 285°C and 360°C, at 1 kbar and unit activity of H20. Post-i 630 Ma HydrothermalActivitv Muscovite grains from sub-Baraboo metasaprolite and from muscovite-pyrophyllite-diaspore veins near the base of the Baraboo quartzite yield discordant 39Ar release spectra, with well-defined plateaux ages at 1456±11 and 1467±11 Ma, respectively (Naymark et al., 2001a). An apparent whole-rock Rb-Sr isochron age for saprolite and pedogene is 1336±75 Ma. These data provide the first substantial evidence for a Wolf River-age imprint on the Baraboo Range, due to the effects of hydrothermal fluids that probably were driven along the sub-Baraboo nonconformity by heat from regionally extensive Wolf River magmatism. Muscovite grains from two samples of Sioux pipestone, which contain the assemblage muscovitepyrophyllite-diaspore, also yield discordant 39Ar release spectra, but with significantly younger plateaux ages of 1370±10 and 1268±11 Ma (Naymark et al., 2001b). The geological significance of such ages is unclear at present, and the —100 m.y. difference in ages of the two samples, which occur at the same stratigraphic level, deserves further investigation. Nevertheless, these recent 40ArP9Ar results reveal the existence of important, and possibly widespread, post-1630 Ma hydrothermal activity in the southern Lake Superior region. References Anderson, J.L. et al. (1980) Contrib. Mineral. Petrol., v. 74, 3 11-328; Dalziel I.W.D. & Dott R.H., Jr. (1970) Wis. Geol. Nat. History Survey, Inf. Circ. 14, 164 pp; Dott, RH. Jr. (1983) Geol. Soc. Amer. Memoir 160, 129-141; Dott, R.H., Jr. & Daiziel, I.W.D. (1972) Jour. Geol., v. 80, 552-568; Dott, R.H., Jr. et al. (1997) Geol Soc. Amer. Abstr. with Progr., v. 29, No. 4, 13; HoIm, D. et at. (1998) Geology, v. 26, 907-910; Medaris, L.G. Jr. et al. (1997) 43rd Inst. Lake Superior Geol., 39-40; Medaris, L.G., Jr. et al. (1998) 44th Inst. Lake Superior Geol., 89-90; Medaris, L.G., Jr. (2000) 46th Inst. Lake Superior Geol., 37-38; Naymark, A. et at. (2001a) Geol. Soc. Amer. Abstr. with Progr., v. 33, No. 4, in press; Naymark. A. et al. (2001b) 47th Inst. Lake Superior Geol., in press; Ojakangas, R.W. & Weber, R.W. (1984) Minn. Geol. Surv., Rept. mv. 32, 1-15; Romano, D. et at. (2000) Precambr. Res., v. 104, 25-46; Smith, E.I. (1978) Geol. Soc. Amer. Bull., v. 89, 875-890; Southwick, D.L. et at. (1986) Geol. Soc. Amer. Bull., v.97, 1432-1441; Southwick, D.L. & Mossler,J.H. (1984) Minn. Geol. Surv., Rept. mv. 32, 17-44; Van Schmus, W.R. et at. (1975) Geol. Soc. Amer. Bull., v. 86, 1255-1265; Van Schmus, W.R. et al. (1993) Precambrian: Conterminous U. S., Geol. North America, v. C-2, 270-28 1, Geol. Soc. Amer.; Van Wyck, N. (1995) Ph.D. thesis, Univ. Wis.-Madison, 280 pp. 52 �RECENT ADVANCES IN UNDERSTANDING THE GLACIAL RECORD OF WISCONSIN MICKELSON, D.M., Department of Geology and Geophysics, University of Wisconsin, Madison, WI 53706. Micke1son(dgeology.wiscedu and CLAYTON, LEE, Wisconsin Geological and Natural History Survey, 3817 Mineral Point Rd., Madison, WI 53706. Several glacier advances reached Wisconsin, but the record of early ice advances is sparse, probably because of their age (some >800 ka) and the intense erosion that took place during the subsequent glaciations (Alden, 1918; Bleuer, 1970; Baker, etal., 1983; Miller, 2000). Thin, discontinuous till occurs on the uplands in west central and central Wisconsin and south of the late Wisconsin deposits in southern Wisconsin. Most have been classified into the lithostratigraphic system adopted by the Wisconsin Geological and Natural History Survey, as have younger 1987; Clayton, 1991). More detailed mapping of 1984; Attig, glacial deposits (Mickelson, Quaternary deposits at 1:100,00 scale has been published for about 18 counties, and several more are in preparation. South and west of late Wisconsin and older glacial deposits lies the Dnftless Area. Recognized as driftless since before 1850 because of its higher relief and lack of erratics, the Driftless Area has Paleozoic bedrock close to the surface with only a cover of bess less than lOm thick. The landscape has been produced by fluvial erosion since at least sometime in the Tertiary. Although Black (1960) suggested that it was glaciated, there is no evidence that it was (Mickelson etal., 1982). In all likelihood, the Driftiess Area remained unglaciated because it had to cross the deep, east-west Lake Superior basin. Much ice was diverted into the Green Bay Lobe and into the Superior Lobe, which extended westward into Minnesota. The Driftless Are a is surrounded by glacial deposits of different ages, but was never surrounded by ice as portrayed by early workers and many textbooks. Late Wisconsin ice advanced into the northeast Wisconsin by about 24,000 radiocarbon years ago based on model results and radiocarbon dates in Illinois. There is a relative lack of radiocarbon dates from this time period in Wisconsin, probably because permafrost was thick and tundra vegetation covered the landscape. Also, ice was particularly erosive behind the ice margin compared to further south in Illinois. Although there is no radiocarbon record of the advance, model results indicate that the Lake Michigan Lobe advanced more quickly than lobes that had to traverse the deep, east-west oriented Lake Superior basin (Cutler, j., 2000a) because extensive calving slowed their advance. Ice of the Green Bay, Langlade, Chippewa, and Superior Lobes clearly advanced onto permafrost. This argument is based field observation of ice wedge casts (Clayton, etal., 1997), the lack of buried wood, (Attig, etal., 1989) evidence of tundra vegetation (Maher and Mickelson, 1996), and model results (Cutler, etal., 2000b). Landforms near the maximum ice extent appear to reflect subglacial conditions at the time they were deposited. Relief within end moraines increases from about 10 m in southern Wisconsin to more that 60 m in northern Wisconsin, reflecting the wider, longer lasting frozen-bed zone near the margin. Drumlins are extensive, probably because slow melt out of subglacial permafrost caused inhomogenities in the bed and differential streamlining. Tunnel channels left by large, probably catastrophic, flows of water from beneath the ice sheet are common along the outer margins in press). Tunnel channels and drumlins appear to be absent farther south in (Clayton,., 1999; Cutler, Illinois where buried wood and model results indicate there were warmer temperatures during the advance to the maximum ice advance position. The length of time that ice remained at or near the maximum position in the Green Bay lobe is debated. Maher and Mickelson (1996) have argued that ice remained near its maximum extent until about 15, 000 years ago and that subsquent deglaciation was rapid based on radiocarbon dates from Devils Lake and northeastern Wisconsin. Colgan submitted) argue that deglaciation was slower based on cosmogenic dates and time needed for (1999; Colgan glacial landform development. References Cited Attig, J.W., Clayton, Lee, and Mickelson, D.M., (Eds.), 1988, Pleistocene stratigraphic units of Wisconsin 198487: Wisconsin Geological and Natural History Survey, Information Circular 62, 61 pp. Attig, J.W., Mickelson, D.M., and Clayton, Lee, 1989, Late Wisconsin landform distribution and glacier-bed conditions in Wisconsin, Sedimentary Geology, v. 62, p. 399-405. Baker, R.W., Biehl, J.F., Simpson, T.W., Zelazny, L.W., and Beske-Deihl, S., 1983, Pre-Wisconsinan glacial 53 �stratigraphy, chronology, and paleomagnetics of west-central Wisconsin: Geological Society of America Bulletin, v. 94, p. 1442-1449. Clayton, Lee, Attig, J.W., and Mickelson, D.M., and Johnson, M.D., 1991, Glaciation of Wisconsin: Wisconsin Geological and Natural History Survey, Educational Series 36, 4 p. Clayton, Lee, Attig, J.W., Mickelson, D.M., 1997, Conditions around the margin of the Green Bay lobe during the height of the Wisconsin glaciation: j: Mudrey, M.G., Jr., Guide to Field Trips in Wisconsin and adjacent areas of Minnesota, 31st Annual meeting of the North-central section, Geological Society of America: Wisconsin Geological and Natural History Survey, p. 23-30. Clayton, L, Attig, J.W., and Mickelson, D.M., 1999, Tunnel channels formed in Wisconsin during the last glaciation: In Mickelson, D.M. and Attig, J.A., (Eds.), Glacial Processes Past and Present: Geological Society of America Special Paper 337, p. 69-82. Colgan, P.M. and Mickelson, D.M., 1997, Genesis of streamlined landforms and flow history of the Green Bay lobe, Wisconsin, USA: Sedimentary Geology, v. 111, p. 7-25. Colgan, P.M., 1999, Reconstruction of the Green Bay Lobe, Wisconsin, United States, from 26,000 to 13,000 radiocarbon years B.P. :In Mickelson, D.M. and Attig, J.A., (Eds.), Glacial Processes Past and Present: Geological Society of America Special Paper 337, p. 137-150. Colgan, P.M., Bierman, P.R., Mickelson, and Caffee, Marc, submitted, Variation in glacial erosion near the southern margin of the Laurentide Ice Sheet, south central Wisconsin: implications for cosmogenic dating of glacial terrains: Geological Society of America Bulletin, Cutler, P.M., Colgan, P.M., Mickelson, D.M., and MacAyeal, 2000a, Influence of the Great Lakes on the advance of the southern Laurentide Ice Sheet at the last glacial maximum: Geological Society of America, 2000 Abstracts with Programs, v. 32, no. 7, p. A-330 Cutler, P.M., MacAyeal, D.R., Mickelson. D.M., Parizek, B.R., and Colgan, P.M., 2000b, A numerical investigation of ice-lobe-permafrost interaction around the southern Laurentide Ice Sheet: Journal of Glaciology, v. 46, no. 153, p.311-325. Cutler, P. M. Clayton, Lee, Mickelson, D.M., Colgan, P.M., and Attig, J.W., in press, Tunnel Channels and Associated Fan Deposits in Wisconsin, U.S.A.: Insights into the Plumbing of the Southern Laurentide Ice Sheet: Quaternary International. Maher, L.J. Jr., and Mickelson, D.M., 1996, Palynological and radiocarbon evidence for deglaciation events in the Green Bay lobe, Wisconsin: Quaternary Research, v. 46, p. 251-259. Mickelson, D.M., 1997, Wisconsin's glacial landscapes: In Ostergren, R.C. and Vale, T.R., Wisconsin Land and Life: Madison, University of Wisconsin Press, p. 35-48. Mickelson, D.M., Clayton, Lee, Baker, R.W., Mode, W.N., and Schneider, A.F., 1984, Pleistocene stratigraphic units of Wisconsin: Wisconsin Geological and Natural History Survey, Miscellaneous Paper, 84-1, 199 pp. Miller, J.W., 2000, Glacial stratigraphy and chronology of central southern Wisconsin, west of the Rock River, Wisconsin: Madison, Wisconsin, M.S. Thesis, University of Wisconsin, 147 pp. Mickelson, D.M.,Knox, J.C. and Clayton, Lee, 1982, Glaciation of the Driftless Area: An evaluation of the evidence, In: Quaternary history of the Driftless Area , Knox, J.C., Clayton, Lee, and Mickelson, D.M. (Eds.), Wisconsin Geological and Natural History Survey, Field Trip Guidebook 5, p. 155-169. 54 �THE DULUTH COMPLEX: WHAT IT IS, WHAT IT AIN'T, AND WHAT WE STILL DON'T KNOW p Miller, James D., Jr. (Minnesota Geological Survey, do Natural Resources Research Institute, 5103 Miller Trunk Highway, Duluth, MN 55811 mille066@tc.umn.edu) Since the time of the first Minnesota state geological survey over 100 years ago (1.), several generations of geologists have worked to unravel the mysteries of the igneous rocks of northeastern Minnesota: the aptly named Duluth Complex. Each new level of understanding was brought about by new data or concepts about geological processes. With early survey studies recognizing the general distribution of igneous rock types, Grout's (2-5) work in the Duluth area established many of the broader geologic relationships of the Complex and developed fundamental concepts about how layered mafic intrusions form, many of which are still held today. However, one of his principle ideas, that the Duluth Complex is a large singular lopolithic intrusion (2), was proven to be an oversimplification by a flurry of geologic mapping conducted throughout the complex in the 1950s to 1970s (6-13). These studies were spurred by efforts to establish a geological framework within which to understand the Cu-Ni sulfide deposits first discovered in the late-1940s. Despite the fact that these deposits have yet to prove of economic importance, this intense period of geologic mapping served to formalize the general intrusive stratigraphy of the Duluth Complex and showed it to be a multiply intruded igneous system. In the early-70s, acceptance of the plate tectonic theory and recognition that the Duluth Complex was part of an intracontinental rift system created a new paradigm within which to evaluate the magmatic and tectonic history of the Duluth Complex (14). The current generation of Duluth Complex studies have focused on five major objectives: 1) 2) 3) 4) 5) to interpret the geologic picture of the vast, poorly exposed central part of the Duluth Complex using high-resolution aeromagnetic data acquired in the early-1980s (15, 16); to unravel the intrusive history of the complex with high resolution U-Pb dating of its gabbroic, anorthositic and felsic rocks (17, 18); to delineate the internal igneous stratigraphy of the various layered intrusions of the Duluth Complex with core logging, detailed mapping, and petrologic studies (19-22); to map the intrusive components of the hypabyssal Beaver Bay Complex and distinguish these from intrusions of the deeper Duluth Complex (23); and to evaluate the potential for economic base- and precious-metal deposits in areas of known mineralization and in other unexplored areas of the Duluth Complex (25). This presentation will highlight some of the new ideas that have emerged from this generation of studies and that are currently being summarized in a new 1:200,000-scale digital geologic map of northeastern Minnesota (see Miller and others, this volume) and in a companion report to be published in summer, 2001. It will also attempt to clarify some misconceptions about the Duluth Complex and point out where more study is needed. Keweenawan intrusive igneous rocks compose more than 60 percent of the bedrock geology of northeastern Minnesota, but only about half of them constitute the Duluth Complex. The Duluth Complex refers to those intrusions that were emplaced into the base of the comagmatic volcanic edifice of the North Shore Volcanic Group. Intrusions emplaced higher within the volcanic pile are not considered part of the Duluth Complex, but rather belong to the Beaver Bay Complex or, where isolated, are identified as individual subvolcanic bodies. Other than scattered, isolated masses of strongly hornfelsed mafic volcanic rock, the Duluth Complex is virtually a continuous mass of intrusive igneous rock. Within that mass, four general rock series are distinguished on the basis of age, dominant lithology, internal structure, and structural position within the complex. Each rock series was multiply emplaced and, where possible, individual intrusion names are assigned. 55 �Early Gabbroic Series—layered sequences of dominantly gabbroic cumulates occurring along the northeastern contact of the Duluth Complex. With their reversed magnetic polarity and 1108 Ma U-Pb zircon age (17), these rocks were evidently emplaced during the early magmatic stage of rifting. Two intrusive units are currently recognized: Nathan's Layered Series and the Cucumber Lake gabbro, though the latter has not been mapped in detail (10). Felsic Series—massive granophyric granite with lesser intermediate rocks occur in a semicontinuous string of elliptical bodies along the eastern and central roof zone of the complex. Ongoing age dating and isotopic studies of these various bodies by Vervoort (26, 18) suggest that most were emplaced during early magmatic activity between 1109 to 1102 Ma and came from Paleoproterozoic to Mesoproterozoic crustal sources. The position of major mafic and anorthositic intrusions beneath these granophyre bodies suggests that the felsic rocks acted as density barriers to mafic magmas, thus causing their plutonic emplacement. Anorthositic Series—a structurally complex suite of foliated, but rarely layered plagioclase-rich gabbroic cumulates that evidently formed by multiple emplacement of plagioclase crystal mushes (27). The erratic internal structure of these rocks typically precludes distinguishing individual intrusive bodies. Although commonly intruded by and included in layered series rocks, U-Pb dating indicates comparable ages of 1099 Ma for both rock series (17). Layered Series—previously referred to as the troctolitic series (9-12, 14), this suite is composed of troctolitic to ferrogabbroic cumulates that constitute at least 11 major mafic layered intrusions. These intrusions display a range of internal differentiation from poorly differentiated troctolitic bodies, such as the Partridge River and South Kawishiwi intrusions, to the well-differentiated Duluth Layered Series and Greenwood Lake intrusion (21). Interpretations of geophysical data over the unexposed central and southern parts of the complex (15, 16; Miller and others, this volume) have led to the recognition of several previously unidentified layered intrusions: the Boulder Lake, Western Margin, Greenwood Lake, and Osier Lake intrusions. Aeromagnetic data also imply that emplacement of the thick sheet-like intrusions of the layered series occurred by sequential overplating of previous intrusions beginning in the northwestern part of the complex and progressing southeastward. Although much has been learned about this enormous and complex igneous system in the past 20 years, much more remains to be done before a complete picture of the Duluth Complex is developed. Vast areas of the Duluth Complex and associated subvolcanic intrusions are unmapped in detail (especially in the BWCA). Age dating studies have only begun to unravel the emplacement history of Duluth Complex intrusions and their possible relationship to higher subvolcanic intrusions and volcanic rocks. Determining the igneous stratigraphy of the newly recognized, but poorly exposed layered intrusions (and their potential for stratiform POE deposits) will require systematic drilling and geochemical studies. These and other challenges await the next generation of geologists who dare to tackle the mysteries of the Duluth Complex. References: 1) Winchell, 1899, MGS Final Rpt IV; 2) Grout, 1918a, Am J Sci 46, p.516; 3) Grout, l9l8b, J Geol 26, p.626; 4) Grout, 1918c, J Geol 26, p.481; 5) Grout, 9l8d; J Geol 26, p.439; 6) Grout, Sharp, & Schwartz, 1959, MGS Bull 39; 7) Taylor, 1964, MGS Bull 44; 8) Green, Phinney & Weiblen, 1966, MGS Misc Map M-2; 9) Phinney, 1972, Geol of Mimi: Cent Vol, p.335; 10) Phinney, 1972, Geol of Minn: Cent Vol, p.346; II) Davidson, 1972, Geol of Minn: Cent Vol, p.354; 12) Bonnichsen, 1972, Geol of Minn: Cent Vol, p.361; 13) Green, 1982, MGS Geologic Map of Minn—Two Harbors sheet; 14) Weiblen & Morey, 1980, Am J Sci 280-A, p. 88; 15) Chandler, 1990, Econ Geol 85, p.816; 16) Miller & Chandler, 1999, MGS Misc Map M-l0l; 17) Paces & Miller, 1993, J Geophys Res 98, p.13997; 18) Vervoort & others, this volume; 19) Severson & Hauck, 1990, NRRIJGMIN-TR-89-1l; 3) Severson, 1994, NRRI/TR-93/34; 21) Miller & Ripley, 1996, Layered intrusions, Elsevier, p.257; 22) Severson & Miller, 1999, MGS Misc Map M-91; 23) Miller & Chandler, 1997, GSA Spec Paper 312, p.73; 24) Hauck et a!., 1997, GSA Spec Paper 312, p.137; 25) Miller, 1999, MGS Inf Circ 44; ) Vervoort & Green, 1997, Can J Earth Sci 34, p.521; 27) Miller & Weiblen, 1990, J Pet 31, p.295. 56 �DIGITAL GEOLOGIC MAP OF NORTHEASTERN MINNESOTA AND ASSOCIATED DATABASES IN GeMS—A MODIFIED ARCVIEW FORMAT Miller, J.D., Jr.', WahI, T.E.', Green, J.C.2, Chandler, V.W.', Severson, MA.3, and Peterson, D.E.23 1) Minnesota Geological Survey, 2642 University Ave., St. Paul, MN 55114; 2) Department of Geological Sciences, University of Minnesota—Duluth, Duluth, MN 55812; 3) Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN 55811 The Minnesota Geological Survey (MGS), in collaboration with the Natural Resources Research Institute and the Department of Geological Sciences at the University of Minnesota—Duluth, is currently compiling geologic, structural, drill hole, geophysical and geochemical data from northeastern Minnesota into an ArcViewbased GIS called GeMS (Geologic Mapping System). The main focus of this project, which is being funded by the Minnesota State Legislature through the Minerals Coordinating Committee, is to develop a new 1:200,000-scale geologic map of the Duluth Complex and related Keweenawan igneous rocks. The map will be available in summer, 2001 either as a 1:200,000-scale paper map, as a downloadable image from the MGS website, or as part of a CD-ROM that will also include all related data compiled for the study in an ArcView format. A companion report addressing the geology and mineral potential of the Duluth Complex will also be published in summer, 2001. This presentation will give an overview of the basic components of GeMS and will describe the types and attributes of database themes included in the compilation. GeMS was initially conceived to be a user-friendly interface to the UNIX workstation-based Arclnfo software for the purpose of digitally storing, retrieving and imaging geologic, geophysical and geochemical data (WahI and others, 1995, 1997). GeMS was recently converted to ArcView 3.2 for this and other MGS mapping projects because of ArcView's common usage as GIS software on the PC platform, its expanded data management capabilities, and its more flexible and easy-to-use graphical user interface. ArcView manages geographical data as point, line and polygon themes and links them to attribute tables containing related information. ArcView is particularly well-suited to making geologic maps because of its ability to sort data and interpretive themes by various attributes, to graphically portray the sorted data in a variety of ways, and to accommodate various types of base images (DRGS, geophysical images, orthophotos, etc.). The types of data and information themes that are part of the current version of GeMS are listed in Table 1. Table 1. Data and interpretive themes included in GeMS Data type Theme type Attributes outcrop polygon field station ID, geologist, date visited, data source, observational detail, exposure, outcrop types, # of photos taken, # of samples taken, rock type, field description, map unit sample point field station ID, geologist, date sampled, data source, form of sample, rock type, map unit, field description, # of thin sections, petrographic description, related data available (probe, whole rock, isotope, assay, geochron, rock properties) structure point field station ID, geologist, date measured, data source, structure type, attitude, confidence level, # of averaged measurements, display scale drill hole point drill hole ID, date drilled, logged by, date logged, lessee, elevation, azimuth, plunge, depth, depth to bedrock, first bedrock, last rock type, present core location, core diameter map lines line geologist, date, basis 1, basis 2, line type (e.g., fault, contact, dike, fold axis, etc.) map unit polygon geologist, date, basis I, basis 2, map label, unit description miscellaneous varied gossan zones, test pit locations, etc. 57 �The advantages of moving from the old cartographic methods of making geologic maps to a GIS-based approach are numerous: GIS maps are easily updated as new data become available; several types of geologic data can be viewed at the same time; and different bases can be used. In addition to these advantages common to all GIS map systems, GeMS incorporates some other unique and useful features. These include source identification data (who, when and where). This is especially important for the compilation of the 1:200,000-scale geologic map of northeastern Minnesota, which compiles data and interpretations from various sources. Another important feature is the identification of the basis for geologic interpretations (map lines and units). For example, possible options for the basis for a fault interpretation are: topographic expression, inference from aeromagnetic data, geologic unit offset, air photo lineament, or local observation in outcrop. The database for northeastern Minnesota available on CD-ROM will not be a complete compilation. The amount of detailed outcrop data alone that could potentially be compiled totals more than 100,000 polygons. This project is concentrating on data from areas that are open to mineral exploration. The completeness of the database at present is shown in Table 2. In addition to continuing to fill out this database, some other elements that will be added to GeMS include 1) compiling all geochemical data and linking that database to outcrop and drill hole samples, 2) allowing for drill core logs to be added to the database, and 3) exporting portions of the system to handheld devices to allow direct data capture in the field. As new mapping is conducted and new data are acquired, updated versions of the regional geologic map and its associated database will be published—a task made easier having moved our map-making into the digital age. Table 2. Current Status of Data Compilation for the Northeastern Minnesota Geologic Map Area Data Type Outcrop Areas where compilation nearly complete Areas yet to be compiled —23,000 Duluth area, southern and central Duluth complex, Allen and Babbitt quadrangles, Beaver Bay Complex' Northern Duluth Complex (map areas of Phinney, Davidson, Foose, and Nathan), North Shore (Green) Units compiled Total units to be compiled to date >100,000 Samples2 —5000 —3600 Duluth area, central Duluth Complex, Allen quadrangle, Beaver Bay Complex Samples of Green and misc. uncatalogued samples and thin sections stored at the MGS Structure —8000 >5000 All published geologic maps Unpublished mapping by Phinney and Green —600 All drill hole data are compiled Drill Holes —600 ')only about 30% of outcrop polygons in the Beaver Bay Complex have their attributes compiled, the unattributed polygons thus mark only outcrop location. 2) includes well-located samples for which geochemical, petrographic, or rock property data exist and/or for which a preserved sample or thin section exists. References: 1995 ESRI International User Conference Proceedings, http://www.esri.comllibrary/userconf/proc95/to200/p I 67.html Wahl, T.E., Miller, J.D., Jr., Bauer, E.J., 1995, Bedrock geologic mapping using Arclnfo: Wahi, T.E., Miller, J.D., Jr., Jirsa, M.A., Boerboom, T.J., Chandler, V.W., Runkel, A.C., Dahl, D., Severson, M.J., 1997, Geologic mapping System (GeMS): a digital approach to bedrock geologic mapping: Institute on Lake Superior Geology, Proceedings v. 43, part 1, p. 59-60. 58 �STRUCTURE OF THE BURIED PRECAMBRIAN BASEMENT IN SOUTHWEST WISCONSIN AND ITS INFLUENCE ON REGIONAL PALEOZOIC GEOLOGY AND ZINC-LEAD MINERALIZATION MUDREY, M.G. Jr., Wisconsin Geological and Natural History Survey, 3817 Mineral Point Rd., Madison, WI 53705, mgmudreyfacstaff.wisc.edu BROWN, B.A., Wisconsin Geological and Natural History Survey, 3817 Mineral Point Rd., Madison, WI 53705, babrown1@facstaff.wisc.edu Our recent geologic mapping in southwestern Wisconsin has focused on pre-Sinnipee Group formations in the area north of the historic upper Mississippi Valley Zinc-Lead Mining District and south of the Wisconsin River. The regional stratigraphy can be clearly defined from outcrop exposures and mineral exploration boreholes in the Tunnel City Group, Trempealeau Group, Prairie du Chien Formation, and Ancell Group. The relatively easy recognition of the various units, in some cases to within 3 meters of a member or formation boundary, permits recognition of repeated sections and offset. This and previous mapping permits delineation of regionally significant anticlines, synclines, and faults, some which of have throws of more than 30 meters. Our analysis of recent aeromagnetic data (Bracken and Nicholson, 2000) leads to a better understanding of folds and faults recognized in outcrop and permits extrapolation of some of the faults to the east at least 60 km. The large, broad folds mapped by previous workers were based on detailed examination of mineral exploration boreholes and outcrops. Many of the structures correlate with basement linear features defined from the aeromagnetic data. Some of the folds, such as the Allamakee Anticline, may be related to readjustments along Keweenawan and older structural features (such as the Belle Plaine Fault and equivalent structures). Other east-west features, notably the Meekers Grove Anticline, are coincident with aeromagnetic linears at depth. These linear features are probably faults or faults with coincident dikes and probably define the north tectonic edge of the Illinois Basin. Regional map analysis suggests a steepening of dip of the Paleozoic rock units southward into the Illinois Basin along these regional deep basement trends, from a regional dip of less than 10 feet per mile to over 20 feet per mile. The large, quiet magnetic areas in western Grant County and else where in southern Wisconsin may reflect Wolf River-age plutons within the basement because the signatures are circular and apparently undeformed. Wolf River-aged material has been drilled along the state line between Wisconsin and Illinois (Coates, and others, 1983). Some of the linear anomalies defined from aeromagnetic data are coincident with positive linear gravity anomalies (Geister and Ervin, this meeting) and are probably wide, mafic dikes. The location of the plutons and faults probably influenced sedimentation patterns during the early Paleozoic by localizing the large reef/carbonate bank deposits of Middle Ordovician (Ludvigson and others, 1983). It is commonly thought that deep brines from the Illinois Basin gave rise to the hydrothermal solutions responsible for mineralization (Bethke, 1986). Localization of the 59 �zinc-lead mineralization along small faults and folds in mine workings is well documented from detailed mine mapping; however, this mineralization does not appear to be related to the deeper, more extensive basement structures. Heyl and others (1970) concluded that the broad-scale folds and faults were responsible for the tectonic framework, but the structures controlling ore deposition were developed by solution and slumping during the mineralization stages. We propose that the broader tectonic elements at the periphery of the Illinois Basin controlled ascent of the hydrothermal brines and therefore distribution of regional mineralization. The rapid cooling of those ascending brines along fractures resulted in the mineral concentrations. In this model it is the details of depth of burial of individual geologic units, their uplift and cooling history, and timing of transit of the hydrothermal brines, that are important in mineralization rather than host-rock lithology and local structure. Bethke, C.M., 1986, Hydrologic constraints on the genesis of the Upper Mississippi Valley mineral district from Illinois Basin brines: Economic Geology, v. 81, p. 233-249. Bracken, R.E., and Nicholson, S.W., 2000, Aeromagnetic Surveying in Wisconsn 1998-99: Digital Data Files: U.S. Geological Survey Open-File Report 99-527. Coates, M.S., Haimson, B.C., Hinze, W.J., and Van-Schmus, W.R., 1983, Introduction to the Illinois Deep Hole Project: Journal of Geophysical Research. B, v 88, no. 9, p. 7267-7285. Heyl, A.V., Broughton, W.A., and West, W.S., 1970 (1st edition), Geology of the Upper Mississippi Valley Base-Metal District, Wisconsin Geological and Natural History Survey [3Td Information Circular 16, 45 p. (some sections revised by M.G. Mudrey, Jr. in 1978 edition]). Ludvigson, G.A., Bunker, B.J., Witzke, B.J., and Garvin, P.L, 1983, A burial diagenetic model for the emplacement of zinc-lead sulfide ores in the Upper Mississippi Valley, USA: in Kisvarsanyi, G., Grant, S.K., Pratt, W.P., and Koenig, J.W., eds. International conference on mississippi valley type lead-zinc deposits (proceedings volume): University of Missouri Rolla, Rolla, MO, p. 497-5 15. 60 �___ PRELIMINARY ANALYSIS OF AEROMAGNETIC DATA IN SOUTHERN WISCONSIN: THE ROLE OF PRECA1'LBRIAN BASEMENT IN PALEOZOIC EVOLUTION MUDREY, M.G. Jr., Wisconsin Geological and Natural History Survey, 3817 Mineral Point Rd., Madison, WI 53705, mgmudreyfacstaff.wisc.edu BROWN, B.A., Wisconsin Geological and Natural History Survey, 3817 Mineral Point Rd., Madison, WI 53705, babrown1@facstaff.wisc.edu DANIELS, David L., U.S. Geological Survey, MS954 National Center, Reston, VA 20192, dave@usgs.gov The new aeromagnetic map of Wisconsin was the result of digitally blending the data from 22 surveys flown between 1948 and 1999. The most recent survey (74,000 line-kilometers), acquired by the U.S. Geological Survey, covers much of southern Wisconsin. The flight line data for four surveys acquired by the U.S.Geological Survey during the past 12 years have been released on CD-ROMs. These data were added to earlier U.S.Geological Survey surveys and 4 surveys acquired by Wisconsin Geological and Natural History Survey. Flight lines are 800m apart or less for 95% of the state, giving the aeromagnetic map nearly uniform specifications. All surveys were either flown at or continued to an elevation of 305m above terrain prior to assembling into a state grid. The data interval of the grid is 250 m. In this presentation, we show a preliminary analysis of the aeromagnetic data south of 4350, for which there is little Precambrian information either from outcrop or cuttings from deep boreholes. The resulting aeromagnetic map of southern Wisconsin illustrates the structure of the Precambrian rock underlying Paleozoic and Pleistocene cover. The pattern generally represents a complex Precambrian terrane that may include middle Proterozoic granite-green stone terrace containing small, circular anomalies related to the Wolf River batholith. The dominant Precambrian bedrock in eastern Wisconsin is quartzite of the Baraboo type. This unit is generally magnetically transparent; as a result, the magnetic signature originates from rock of the basement to the quartzite. In places, notably Fond du Lac County, folding within some units in the quartzite sequence is evident and suggests interbedded slate and iron formation. Other basement features include well defined faults in the Precambrian, some of which are clearly of Paleozoic age. In eastern Wisconsin the Waukesha and related faults define a basement terrace boundary, and in southern Wisconsin the Beloit and other faults define the northern edge of the Illinois Basin. The lack of relationship between the Upper Mississippi Valley Zinc-Lead District mineralization and Precambrian basement suggests a minor role for pre-Paleozoic elements during localization of the zinc-lead mineralization. 61 �MISSISSIPPI VALLEY-TYPE MINERALIZATION IN THE FOX RIVER VALLEY, EASTERN WISCONSIN (Modified from 42nd ILSG - Cable, Wisconsin, 1996) MUDREY, M.G. Jr., Wisconsin Geological and Natural History Survey, 3817 Mineral Point Rd., Madison, WI 53705, mgmudrey@facstaff.wisc.edu BROWN, B.A., Wisconsin Geological and Natural History Survey, 3817 Mineral Point Rd., Madison, WI 53705, babrownl@facstaff.wisc.edu FREIBERG, P.G., Department of Geology and Geophysics 1215 W. Dayton St., Madison, WI 53706-1692, SIMO, J.A., Department of Geology and Geophysics 1215 W. Dayton St., Madison, WI 53706-1692, simo@geology.wisc.edu Regional NURE (National Uranium Resource Evaluation Program) geochemical data suggest that anomalous concentrations of arsenic and other mineral exploration path-finder elements are present in the area southwest of Green Bay, Wisconsin where the Sinnipee, Ancell and Prairie du Chien Groups are the uppermost bedrock units. In addition, fluorine levels in groundwater have been known to be high in the Fox River valley between Green Bay and Appleton, where fluorite and other Mississippi Valley-type minerals are reported to be present in well cutting from the Sinnipee Group. Areas of significantly elevated values occupy northwestern Outagamie County and adjacent areas. A clearly defined nickel province that spatially corresponds to the arsenic province suggests that a polymetallic (As, Co, Mo, Ni, Th, V) hydrogeochemical province exists in eastern Wisconsin and may relate to documented faults (Mudrey and Bradbury, 1992). The geology differs from the better documentd five-element (Ni-Co-As-Ag-Bi) veins (Kissin, 1993) by being carbonate hosted rather than shale or volcanic hosted, but are similar in essential mineralogy and elements. Economic concentrations of Mississippi Valley-type mineralization have not been found in Wisconsin outside of Grant, Iowa, and Lafayette Counties, but geologic logs of 16 mineral exploration holes and more than 600 water wells in eastern Wisconsin contain reports of minor mineralization. In addition, more than 100 occurrences of sulfide minerals have been reported from outcrops and quarries throughout southern and eastern Wisconsin (Brown and Maass, 1992). A fairly continuous horizon of sulfide mineralization has been observed in quarries and drill core from Kenosha to Green Bay. Mineralization within this horizon consists of sulfide-cemented sandstone and sulfide infills of vugs and molds of fossil. The principal concentration of mineralization has been observed at or near the top of the St. Peter sandstone, but scattered mineralization is known throughout the Paleozoic section in this region. References: Brown, B.A. and Maass, R.S.. 1992, A reconnaissance survey of wells in eastern Wisconsin for indications of Mississippi Valley type mineralization: Wisconsin Geological and Natural History Survey Open-file Report WOFR 1992-3, 31 p. 62 �Kissin, S.A., 1993, Five-element Ni-Co-As-Ag-Bi) Veins: in P.A. Sheahan and M.E. Cheny, Ore Deposit Models, Volume II, Geoscience Canada Reprint Series,V. 6, p. 87-98. Mudrey, M.G., Jr., and Bradbury, K.R., 1992, Evaluation of NURE hydrogeochemical data for use in Wisconsin groundwater studies: Wisconsin Geological and Natural Histoly Survey Open-file Report WOFR 93-2, 61 p., 1 computer diskette. Mudrey, M.G., Jr., Brown, B.A., Freibeg, P.O., and Simo, J.A, 19%, Mississippi ValleyType Mineralization in the Fox River Valley, Eastern Wisconsin (abs.): Institute on Lake Superior Geology Proceedings, 42nd Aimual Meeting, Cable, WI, 1996, v. 42, part 1, p. 38-39 63 �OVERVIEW OF FIELD TRIP 2: UPPER MISSISSIPPI VALLEY - ZINC-LEAD DISTRICT, WISCONSIN MUDREY, M.G., Jr., Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison WI 535705, mgmudreyfacstaff.wisc.edu HUNT, T.C., Director of the Reclamation Program, School of Agriculture, University of Wisconsin - Platteville, Platteville, WI 53818, huntt@am.uwplatt.edu CZECHANSKI, M.L., Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison WI 535705, mlczecha@facstaff.wisc.edu First recovery of galena in the Driftless Region of Wisconsin, Illinois, Iowa and Minnesota was by native Americans for ornamentation about 1000 C.E. By 1690 Europeans recognized the lead deposits and began mining in what is now known as the Upper Mississippi Valley Zinc-Lead District. In the early 19th century, this was the premier lead mining district in North America. Numerous immigrant groups and developers were attracted to the area and the resulting population in flux played a major role in the eventual formation of the states of Illinois, Iowa, and Wisconsin. Initial mineral development (late 1 8111 and early 1 9t11 centuries) consisted of collecting surface occunences of galena and digging down until the excavations became unstable (badger holes). Deeper mining techniques were initiated, but were hampered by inadequate dewatering techniques below the water table. By 1850 lead production had peaked. Metallurgical developments and techniques to dewater mines led to significant zinc production (zinc ore became abundant with depth), initially from smithsonite (dry bone) and since 1900 from spha!erite. Peak production years were 1917 and 1952. The last mine, Eagle-Picher's Shullsburg Mine, ceased production in 1978. Since 1900, the U.S. Bureau of Mines, the U.S. Geological Survey, and the Wisconsin Geological and Natural History Survey have collected large amounts of information from the area including, detailed mine maps, drillhole locations, assay data, and geologic logs. This information is summarized in detailed geologic maps. Al Hey! and others (1959) summarized information up to 1950s in U.S. Geo!ogica! Survey Professiona! Paper 309. Hey! and others (1970) prepared a shorter summary of Professional Paper 309 for the Geological Society of America Meeting in Milwaukee in 1970. In addition, a large number of detailed geologic maps were prepared for southwestern Wisconsin and adjoining Illinois and Iowa. This area is the type-locality of the Upper Mississippi Valley Zinc-Lead mineralization. Warm (135 to 180 °C) mineral-bearing saline bnnes from the south (Illinois Basin) and southwest (Iowa Basin) are responsible for the mineralization. The bulk of the mineralization occuned in Middle Ordovician strata during the PermianlPennsylvania. Dominant mineralization occurred in the Decorah shaly dolomite, with significant quantities of mineralization in the over- and under-lying dolomite strata. Deeper mineralization has not been significantly tested. Local ore controls include minor folds and faults. Mineralization occurs as replacement and breccia filling in vertical fractures and crevices (gash veins), 64 �dipping fracture planes (pitches) and horizontal bedding planes (flats). Gash veins not uncommonly occur directly over the pitch and flat structures. There is a general vertical ore zonation with lead in the vertical veins and zinc concentrated in pitches and flats. Replacement and solution breccia are common, leading to some bonanza-type mineralization. Wall rock alteration is minimal, suggesting no widespread thermal events, but rather joint and fracture localization. The field trip will examine the Potosi Hill Ordovician geologic exposure, where the entire section from the Ancell Group through the Sinnipee Group is exposed; the Platteville Mining Museum and Bevan Mine which illustrates the regional geology and historical mineral development techniques; Pendarvis State Historical Site which captures the 1 830s spirit of mining;; and modem reclamation at the Shullsburg site, location of the last producing zinc mine in Wisconsin (1978). Heyl, A.V., Agnew, A.F., Lyons, E.J., and Behre, C.H., Jr., 1959, The Geology of the Upper Mississippi Valley Zinc-Lead District: U.S. Geological Survey Professional Paper 309, 310 p. Heyl, A.V., Broughton, W.A., and West, W.S., 1970 (1st edition), Geology of the Upper Mississippi Valley Base-Metal District, Wisconsin Geological and Natural History Survey Circular 16, 45 p. (some sections revised by M.G. Mudrey, Jr. in 1978 (3"' edition)) 65 �RECOGNITION OF POST-1630 MA FLUID-DRIVEN METAMORPHISM IN BARABOO INTERVAL QUARTZITES BY MEANS OF LASER PROBE 40Ar/39Ar GEOCHRONOLOGY NAYMARK, ALISSA, SINGER, BRAD and MEDARIS, L.G., Jr., Deptartment of Geology and Geophysics, Univ. of Wisconsin-Madison, 53706, anaymark@students.wisc.edu, bsinger@geology.wisc.edu, medaris @geology.wisc.edu. The southern Lake Superior region experienced many transformations during Proterozoic time Quartzites of the Baraboo interval, most notably the Baraboo, Barron, and Sioux, are known to have been deposited between —1710 Ma and 1630 Ma on 1750 Ma and older igneous and metamorphic basement. Quartzites south of Hoim et al.'s (1998) tectonic front were folded at —1630 Ma and, presumably, subjected to low-grade metamorphism at the same time. The present investigation, using CO2 laser probe 40Ar/39Ar incremental-heating methods, was undertaken to evaluate the timing and extent of low-grade metamorphism in Baraboo interval sedimentary rocks. Surprisingly, little evidence for 1630 Ma metamorphism was found in the analyzed samples; instead, a strong signature of post-1630 Ma hydrothermal activity was discovered in the Baraboo and Sioux quartzites. The Baraboo Ranke: Age spectra for three muscovite samples are discordant in the low temperature gas steps, but gave similar plateau ages. The initial 30% of gas released typically gave apparent ages beginning at —900 Ma in the vein material, and —1200 Ma in the samples from Baxter Hollow, rising to concordant plateau ages for the last 70% of the gas released. Muscovite from hydrothermal muscovite-pyrophyllite-diaspore veins in the base of the Baraboo Quartzite yielded a plateau age of 1467±10 Ma (±2) [Figure 1A]. Samples of muscovite from metasaprolite at Baxter Hollow yielded plateau ages of 1456± 11 and 1461± 12 Ma [Figure 1B]. These data provide the first evidence that hydrothermal metamorphism coeval with the Wolf River Batholith affected rocks in the Baraboo Range. We propose that this hydrothermal activity was due to large-scale movement of fluids through the crust, driven by heat provided by Wolf River granitic magmatism. The Denzer diorite is a member of the —1750 Ma igneous suite underlying the Baraboo Quartzite. It preserves an igneous texture, but was weakly recrystallized, presumably at 1630 Ma, with plagioclase partly replaced by albite and epidote, biotite altered to chlorite, and hornblende altered to actinolite, cumrningtonite, and chlorite. Biotite from the Denzer diorite yielded a plateau age of 1746±12 Ma [Figure 1D] that may reflect time since crystallization. In contrast, two samples of hornblende with intergrowths of actinolite, cummingtonite, and chlorite, yielded ages of 1596±16 and 1427± 15 Ma, which represent partial and complete Ar resetting of what may have been a 1630 Ma metamorphic assemblage. Sioux Ouartzite: Numerous samples of fine-grained metasedimentary rocks (pipestone) were collected from Pipestone National Monument, SE Minnesota. They were analyzed optically and via electron microprobe, X-ray diffraction, and X-ray fluorescence to determine their mineralogical and chemical compositions. Most samples contain the assemblage: muscovite-pyrophyllite-diaspore, similar to pipestone and hydrothermal veins at Baraboo. Owing to the fine grain size (—20 pm), less than 0.0 1mg whole rock samples were prepared from the SiOux pipestone based on X-ray diffraction patterns indicating that muscovite was the only potassium bearing phase present. The samples with K20 greater than 4.0 wt. % and more than 35% modal muscovite were chosen for the whole rock 40Ar/39Ar experiments. The plateau ages are slightly discordant at low and high temperature. However, 80% of the gas gave plateau ages of 1370±10 and 1268±10 Ma [Figure 1C], suggesting that post-1460 Ma hydrothermal activity affected the Sioux Quartzite. The geological significance of these ages from Sioux pipestone remains unclear, and the 100 million year difference in age between two samples from the same stratigraphic level is problematic. A possible explanation is differential loss of argon via diffusion from multiple small domains within the submicroscopic mica. The Barron Quartzite: The Barron Quartzite, which is located north of Holm et al.'s (1998) inferred tectonic front, was unaffected by 1630 Ma folding and metamorphism, and consists 66 �. predominately of quartz, kaolinite, and hematite. Although muscovite is rare in the Barron sedimentary rocks, t was found immediately beitw the Barron Quartzite in a vein, which cuts metatonalite basement. Both the vein and metatonalite are now saprolites, having been weathered in Proterozoic time, and the muscovite is partly replaced by kaolinite. The muscovite yields a plateau age of 1808±14 Ma, demonstrating that the Barron Quartzite and underlying basement were not affected by 1630 Ma folding and metamorphism, or by 1460 Ma hydrothermal activity. Conclusions: "°Ar/39Ar analyses of Baraboo and Sioux samples that were affected by Kmetasomatism, including Baraboo metasaprolite and hydrothermal veins and Sioux pipestone, revealed post-1630 Ma hydrothermal activity in the southern Lake Superior region. Hydrothermal fluids attending Wolf River magmatism exploited the nonconformity separating quartzites from underlying plutonic and metamorphic rocks. The muscovite 40Ar/39Ar plateau ages most likely record low temperature crystallization (—300CC) assemblages in metasaprolite, hydrothermal veins, and pipestone. Further investigation will be necessary to delineate the scale of this potentially regional fluid flow in the crust. Reference: HoIm, D., D. Schneider, and C. D. Coath. (1998) Age and deformation of Early Proterozoic quartzites in the southern Lake Superior region: Implications for extent of foreland deformation during final assembly of Laurentia. Geology, v. 26, p. 907-9 10. A 2000- B I I I I 2000 OOBOWIa muscovite vein 1467.14 ± 10.55 Ma . 96BHIA Baxter Hollow muscovite 1461.23 ± 11.79 Ma and 1455.93± 11.45 Ma • 1600 : 1600: 4 120O- 1200 , 8O0 800- 400- 400- I I I 20 40 60 C I 80 , 0 iC 0 I 0 uuu. D I OOPNMOI muscovite whole rock 1370.42± 10.30 Ma 20 40 I 60 I 80 10 0 • 1600- 1600- ,1200- OODDOI Denzer Diorite biotite 1742.02±11.91 Ma 1200 I: OOPNMO3 muscovite whole rock 1268.10± 10.63 Ma 800- 400 0 I I I I 20 40 60 80 100 0 20 40 60 80 100 Cumulative 39Ar released % Cumulative 39Ar released % Figure 1: Age spectra discussed in text. Weighted mean plateau ages are reported with 2 errors. 67 �MINERALOGICAL VARIATIONS IN IRON-FORMATION IN THE THERMAL METAMORPHIC AUREOLE OF A DIABASE DIKE NEMITZ, Michael B., and LARSON, Phillip C., Department of Geological Sciences, University of Minnesota, Duluth, MN 55812 The Biwabik Iron-formation in the National Steel Pellet Company Mine near Keewatin, Minnesota is cut by a series of diabase dikes. It has previously been empirically observed that iron-formation adjacent to these dikes is characterized by enhanced magnetite weight recovery, increased silica liberation indices, and increased oxidation reflected as elevated Fe3!Fe2 ratios. Granular cherty iron-formation samples were collected in two transects perpendicular to the contact of a 5-rn thick diabase dike. The transects extended 25-rn along the strike of the ironWhole-rock geochemical analyses, x-ray diffractometry, and reflected- and formation. transmitted-light polarizing microscopy were used to assess the geochemical, mineralogical, and textural variations in iron-formation adjacent to the dikes. Microscopy indicates an increase in the total amount of primary, euhedral magnetite, hematite, and total Fe-oxide proximal to the dike. The total Fe-oxide content appears to increase in an exponential fashion. This trend is reflected in the whole-rock Fe content. Magnetite occurs in all samples as primary euhedral crystals. Hematite occurs in a number of forms: euhedral to subhedral pseudomorphs after magnetite (martite), as inclusions in the lattice of euhedral magnetite, or as fine grained aggregates. Previous to iron-formation oxidation, a hydrothermal alteration event was focused along the dike margins. Alteration leached Mg, Ca, and Na from both diabase and iron-formation. This event is reflected in the iron-formation whole-rock Mg content, which show that Mg has essentially been removed in the 5-m zone proximal to the contact. Carbonate, and to a lesser extent Fe-silicate minerals have essentially been removed in the 10-rn zone proximal to the dike contact. Conversely, void space is most abundant in this same zone. This suggests void space has been created at the expense of carbonate and silicate phases. The increase in void space also correlates with the increase in hematite proximal to the This suggests that the oxidation of Fe-oxides to hematite may be related to increased porosity and permeability of iron-formation due to void space. Increased porosity allowed dike. oxidizing meteoric waters to circulate deeper along the dike margins. This study demonstrates that thermal metamorphism of iron-formation by diabase dikes has caused an increase in Fe-oxide content, both magnetite and hematite, thus resulting in increased magnetite weight recovery. However, the increase in void space due to alteration has allowed increased circulation of meteoric waters, resulting in oxidation of iron-formation and elevated hematite content relative to magnetite. 68 �PRELIMINARY LAVA FLOW MORPHOLOGY STUDIES AT THE FIVE MILE LAKE VMS PROSPECT, ARCHEAN VERMILION DISTRICT, NE MINNESOTA: IMPLICATIONS FOR VOLCANIC PROCESSES, VOLCANIC PALEOENVIRONMENTS, AND VMS EXPLORATION TRENT T. NEWKIRK, GEORGE J. HUDAK Department of Geology, University of Wisconsin Oshkosh, Oshkosh, WI 54901 STEVEN A. HAUCK Natural Resources Research Institute, University of Minnesota-Duluth, Duluth, MN 55811 As part of a two-year grant from the Minerals Coordinating Committee (MCC, State of Minnesota), we have undertaken a field-based, detailed investigation of the pillow lava morphology at the Five Mile Lake volcanic-associated massive sulfide (VMS) prospect, which is located approximately 15 miles southwest of Ely, Minnesota. This prospect is situated within a greenschist facies metamorphosed assemblage of Archean subaqueous, primarily mafic metavolcanic and metasedimentary rocks. These rocks lie within the Lower Member of the Late Archean Ely Greenstone (Peterson and ursa, 1999). Historically, several mineral exploration programs have been conducted in the Lower Member of the Ely Greenstone to evaluate its potential for VMS-style base metal mineralization. One of the most significant exploration programs to date was completed in 1995, when Teck Exploration Ltd. intersected stringer Zn-Cu mineralization in several diamond drill holes at the Five Mile Lake prospect. Cas (1992) and Gibson et al. (1999) and have emphasized the importance to mineral exploration programs of determining the volcanic environments associated with VMS mineralization. Gibson et al. (1999) have noted that mapping the orientation of synvolcanic dikes and sills is perhaps one of the most effective means to identify synvolcanic fault zones in lava flow dominated volcanic settings. This mapping is important because these structures not only dictate the location of volcanic vent sites, but also commonly control the locations of hydrothermal discharge sites responsible for making VMS deposits. Unfortunately, the generally small size of synvolcanic dikes and sills relative to their volcanic products commonly makes them difficult to recognize, especially in ancient volcanic sequences that are often plagued by a general lack of outcrop. Thus, detailed facies mapping in volcanic sequences is also an essential part of determining vent proximal volcanic environments. Studies of modern subaqueous mafic lava flows indicate that flow morphology has a direct relationship to effusion rate, cooling rate, and the slope upon which the lavas are erupted (Kennish and Lutz, 1998). In areas with relative fast effusion rates, sheet flows commonly occur in vent proximal environments, and grade laterally into pillow lavas farther from the volcanic vent. Slow effusion rates favor the immediate development of pillow lavas. It is interesting to note that several studies (Ballard and Moore, 1977; Ballard et al., 1981; Hekininan, 1984; Hekinian et al., 1989) have found that the glassy margins on vent proximal sheet flows tend to be thicker than the glassy margins on more distal, pillow lavas formed from the same eruption. This occurs because pillow lavas generally contain more crystals or crystal nuclei that inhibit the formation of their glassy outer margins (Kennish and Lutz, 1998). Therefore, the thickness of the glassy margins on these lava flows is also a general indicator of proximity to the volcanic vent. We have undertaken detailed mapping (1:120 scale to 1:5000 scale) of the physical characteristics of the extremely well-preserved, relatively undefonned pillow lavas at the Five Mile Lake prospect. We have measured various features of these pillows, including pillow shape, pillow horizontal dimensions, pillow vertical dimensions, pillow vesicularity, pillow selvedge characteristics, and the thicknesses of chlorite-rich, formerly glassy pillow margins. The relatively high vesicularity of these 69 �pillows (commonly between 10% and 15%), as well as the local presence of multiple pillow selvedges suggests that these flows were formed in a relatively shallow (<1 km) subaqueous environment. Our detailed mapping has also allowed us to identify several shallow mafic dikes that can be seen undergoing a vertical, then lateral transformation into pillow lavas. These regions represent the volcanic vent sites from which these pillow lavas issued. In several vent-proximal locations, chert-rich exhalite horizons (up to 1 meter thick) containing traces to several percent pyrite, sphalerite, and chalcopyrite are also present. Thus, our results support the relationships between proximal volcanic environments and mineralization in lava flow dominated sequences indicated by Gibson et al. (1999). Of particular interest are the results of our measurements of the thickness of the formerly glassy margins surrounding the pillows at the prospect. We have found that the formerly glassy pillow margins are clearly thicker (>4.5 cm thick) at these vent proximal locations than they are at locations that appear to be more distal to volcanic vents (where they are generally 2cm or less in thickness). Our results suggest that in pillow-dominated sequences, the thickness of the glassy margin surrounding pillow margins may be an accurate indicator of proximity to volcanic vents. This has significant implications for mineral exploration, as this measurement can be easily and quickly completed during field mapping, and may provide an effective means to identify regions within monotonous pillow sequences that are more likely to contain VMS mineralization. References Ballard, R. D., and Moore, J. G., 1977. Photographic Atlas of the Mid-Atlantic Ridge Rift Valley: Springer-Verlag, Berlin, 114 p. Ballard, R. D., Francheteau, J., Juteau, T., Rangin, C., and Normark, W.,1981. East Pacific Rise at 21°N: the volcanic, tectonic and hydrothermal processes of the central axis: Earth and Planetary Science Letters, v. 55, p. 1-10. Cas, R. A. F., 1992. Submarine volcanism: eruptions styles, products, and relevance to understanding the host rock successions to volcanic-hosted massive sulphide deposits: Economic Geology, v. 87., p. 5 11-541. Gibson, H. L., Morton, R. L., and Hudak, G. J., 1999. Submarine volcanic processes, deposits, and environments favorable for the location of volcanic-associated massive sulfide deposits: Reviews in Economic Geology, v. 8, p. 13-5 1. Hekinian, R., 1984. Undersea Volcanoes: Scientific American, v. 251, p. 46-55. Hekinian, R., Thompson, G., and Bideau, D., 1989. Axial and off-axial heterogeneity of basaltic rocks from the East Pacific Rise at 12°35'N — 12°5 1 'N and 1 1°26'N — I l°30'N: Journal of Geophysical Research, v. 94, p. 17437-17463. Kennish, M. J., and Lutz, R. A., 1998. Morphology and distribution of lava flows on mid-ocean ridges: a review: Earth Science Reviews, v. 43, p. 63-90. Peterson, D. M., and Jirsa, M. A., 1999. Bedrock Geological Map and Mineral Exploration Data, Western Vermilion District, St. Louis and Lake Counties, Northeastern Minnesota. 70 �A NEW LOOK AT THE 1.1 GA CHENGWATANA VOLCANICS IN THE ST. CROIX HORST, MINNESOTA AND WISCONSIN Nicholson, S.W., U.S.Geological Survey, MS 954, Reston, VA 20192; Boerboom, T., Minnesota Geological Survey, St. Paul, MN 55114; Cannon, W.F., U.S.Geological Survey, MS 954, Reston, VA 20192; Wirth, K., Macalester College, St. Paul, MN 55105; and Isachsen, C.E., University of Arizona, Tucson, AZ 85721 The 1.1 Ga Midcontinent rift system (MRS) hosts several classes of hydrothermal and magmatic mineral deposits (Nicholson et al, 1992). Recent speculation about the presence of additional magmatic mineral deposits (e.g., "Voisey's Bay"-type Ni-Cu: Schulz et al., 1998) has focused attention on the chemical compositions of rift-related basalts and the identification of central volcanic complexes as exploration tools. The St. Croix horst contains the most southerly exposure of volcanic and sedimentary rocks related to the MRS, but until now, sufficient chemical, age, and geophysical data have not been available to characterize adequately the nature of the volcanic rocks (Nicholson et al., 1997) and their relationship to the regional chemical stratigraphy established for the MRS. North of the St. Croix horst rift-related rocks are exposed around the margins of Lake Superior and detailed stratigraphic sections are well established. Outcrops within the St. Croix horst are sparse: the volcanic rocks are nearly all subaerial basalt flows and were previously assigned to the Chengwatana Volcanics. Recently Cannon et al. (2001) used new detailed aeromagnetic imaging and age determinations to subdivide the former Chengwatana Volcanics into three units, the Minong Volcanics, the Clam Falls Volcanics, and a newly redefined Chengwatana Volcanics (Fig. 1). More than 200 chemical analyses are now available for volcanic rocks in the St. Croix horst. When compared to the regional chemical stratigraphy recognized previously around western Lake Superior (Nicholson et al., 1997), the three new volcanic units appear to be related as follows. The youngest unit, the Minong Volcanics, is most similar to the Portage Lake Volcanics (1096-1094 Ma). Both are dominated by low- TiO2 (less than about 2.0 wt. % Ti02 ) basalts with low abundances of incompatible trace elements. Like the Portage Lake Volcanics, the Minong Volcanics contain few rhyolites, but a rhyolite flow near the base of the section has been dated at about 1094 Ma (Lake Nelson rhyolite; Zartman et al., 1997), an age comparable to the upper Portage Lake Volcanics. Basalts with depleted compositions similar to N-MORB occur as flows near the top of the Minong Volcanics and as dikes in the upper Portage Lake Volcanics. The Clam Falls Volcanics unconformably underlie the Minong Volcanics. It is also dominantly lowTi02 basalts, but high-Ti02 basalts (more than about 2.5 wt % Ti02; increased abundances of incompatible trace elements) are more common in this unit than in the overlying unit. The high- TiO2 basalts are similar in composition to high- Ti02 basalts in the Portage Lake Volcanics and the underlying upper Kallander Creek Volcanics (dated at 1098 Ma) to the northeast. Although chemically similar, the metamorphic grade of the Clam Falls Volcanics is considerably higher than the Minong, Portage Lake, or Kallander Creek Volcanics, suggesting that this unit represents exhumation of a more deeply buried portion of the rift (Wirth et al., 1997). A rhyolite flow at the base of the Clam Falls Volcanics yields ages between 1100 and 1102 Ma (unpub. data: K.R. Wirth). The oldest unit, the newly redefined Chengwatana Volcanics now confined to the volcanic rocks between the Pine and Douglas faults, is dominated by high- Ti02 basalts with accompanying intermediate and felsic volcanic rocks. This association of high-hO2 basalts with intermediate and felsic volcanic rocks has been postulated elsewhere in the rift to be related to central volcanic complexes, sites of prolonged shallow magma chamber development and accompanying volcanism. The Amnicon gabbroic and granophyric intrusive complex southeast of Duluth cuts the base of the redefined Chengwatana Volcanics and most likely represents the magma chamber for a central volcanic complex. The extensive granophyre in the Amnicon pluton is identical chemically to an overlying rhyolite flow. This rhyolite flow has a preliminary date of about 1106 Ma (unpub. data: C.E. Isachsen). Southwest of the Amnicon pluton the Chengwatana Volcanics include more low- hO2 basalts as the influence of the localized Amnicon magmatic system diminishes. In conclusion, the high- Ti02 basalts and related intermediate and felsic rocks of the redefined Chengwatana Volcanics in the St. Croix horst were probably erupted from localized magmatic sources from about 1107 to about 1102 Ma. This was followed by outpourings of voluminous low-Ti02 flood basalts characteristic of the main stage of nfting of the MRS after about 1102 Ma, now represented in the St. Croix 71 �horst by the Clam Falls and Minong Volcanics. Central volcanic complexes in the St. Croix horst, such as the Amnicon complex, may be potential exploration targets for Cu-Ni sulfide mineralization, if further study can show the availability of a source of sufficient sulfur to produce segregation of Cu-Ni+PGE metals. References: Cannon, W.F., Daniels, D.L., Nicholson, S.W., Phillips, J., Woodruff, L.G., Chandler, Va!, Morey, G.B., Wirth, K.R., and Mudrey, MG., Jr., 2001, New map reveals origin and geology of North America Midcontinent rift: EOS, v. 82, no.8, pp. 97-101. Davis, D.W., and Green, J.C., 1997, Geochronology of the North American Midcontment rift in western Lake Superior and implications for its geodynamic evolution: Canadian Journal of Earth Sciences, v. 34, pp. 476-488. Davis, D.W., and Paces, J.B., 1990, Time resolution of geologic events on the Keweenawan Peninsula and implications for development of the Midcontinent rift system: Earth and Planetary Science Letters, v. 97, pp. 54-64. Nicholson, S.W., Cannon, W.F., and Schulz, K.J., 1992, Metallogeny of the Midcontinent rift system of North America: Precambrian Research, v., 58, pp. 355-386. Nicholson, SW., Shirey, S.B., Schulz, K.J., and Green, J.C., 1997, Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development: Canadian Journal of Earth Sciences, v. 34, pp. 504-520. Schulz, K.J., Cannon, W.F., Nicholson, S.W., and Woodruff, L.G., 1998, Is there a "Voisey's Bay" —type Ni-Cu sulfide deposit in the Midcontinent rift system in the Lake Superior region?: Mining Engineering, v. 50, pp. 57-62. Wirth, K.R., Vervoort, J.D., and Naiman, Z. J., 1997, The Chengwatana Volcanics, Wisconsin and Minnesota: petrogenesis of the southernmost volcanic rocks exposed in the Midcontinent rift: Canadian Journal of Earth Sciences, v. 34, pp. 536-548. Zartman, R.E., Nicholson, SW., Cannon, W.F., and Morey, G.B., 1997, U-Th-Pb zircon ages of some Keweenawan Supergroup rocks from the south shore of Lake Superior: Canadian Journal of Earth Sciences, v. 34, pp. 549-561. Upper Michigan and North-Central Wisconsin Northwest Wisconsin Eastern Minnesota Fig.!: Stratigraphic column for Keweenawan volcanic rocks on the south shore of western Lake Superior. The U-Pb age dates are from the following sources: a, Davis and Green, 1997; b, Zartman et a!., 1977; c, Davis and Paces, 1990; d, unpublished data, K.R. Wirth; e, unpublished data, C.E. Isachsen 72 �OVERVIEW OF ARSENIC OCCURRENCES AND PROCESSES CONTROLLING ARSENIC MOBILITY IN GROUND WATER D. Kirk Nordstrom U.S. Geological Survey Boulder, CO Introduction Arsenic concentrations in ground waters can range from less than a few jtg/L to tens or even hundreds of mg/L in locally contaminated environments. Both anthropogenic and natural sources for arsenic in ground waters occur in many locations world-wide. Natural sources are causing or have caused poisoning of populations in India, Bangladesh, Chile, Argentina, Mexico, Taiwan, Mongolia, Japan, and China. Mining activities are responsible for arsenic poisoning in Thailand. Arsenic mass poisoning in Bangladesh is the largest known, affecting nearly 30 million people. The processes that enrich arsenic in minerals and in ground waters are complex but apprehensible. Sources The geochemical cycle of arsenic from magmatic-hydrothermal processes through weathering, sedimentation, and diagenesis transforms the element in a number of ways that produces an array of present-day natural sources. Probably the single most abundant mineral source of arsenic is arsenian pyrite. Pyrite is ubiquitous in the earth's crust, occurring in sedimentary, metamorphic, and igneous rocks. Arsenic has a strong affinity for the sulfur site in pyrite, substituting up to about 10 wt. % regardless of whether the origin is sedimentary or hydrothermal (Kolkar, Nordstrom, and Goldhaber, 2001). Arsenopyrite contains higher concentrations of arsenic (39-53%) but it is a much rarer mineral. Arsenopyrite and arsenian pyrite are commonly found in association with gold mineralization so that gold mining frequently releases arsenic to the environment. Other arsenic-rich minerals include orpiment, realgar, and enargite. Weathering of these minerals in oxidizing environments solubilizes arsenic as As(III) and ultimately as As(V). Arsenate, or As(V), has a strong adsorption affinity for hydrated iron oxides (Pierce and Moore, 1982) and in oxidized sediments iron oxides can be a source of soluble arsenic if they undergo reductive dissolution during early diagenesis. Geothermal springs are commonly enriched in arsenic, containing 0.1-5 mgIL dissolved arsenic (as both As(llI) and As(V)). Geothermal power plants often have to deal with proper disposal of arsenic-enriched waste waters. Anthropogenic sources of arsenic are numerous. The primary source of industrial and commercial arsenic was arsenic trioxide that was produced as a by-product of metal mining and processing, primarily from copper smelting. More than 300,000 tons of flue dust containing an average of 6.5% arsenic were piled at Anaconda, Montana before removal and disposal. Stockpiles of arsenic trioxide still exist without proper containment and are releasing soluble arsenic to ground waters. Several arsenic insecticides (copper, lead, calcium, magnesium, zinc, and sodium arsenites and arsenates), herbicides (sodium arsenite and methanearsonate, disodium methanearsonate, and cacodylic acid), dessicants (arsenic acid), wood preservatives, animal feed additives, drugs, chemical weapons, and alloys were produced for many years. Roxarsone, an organic arsenical, is still widely used today to 73 �clean parasites out from the stomachs of pigs and poultry (Garbarino et al., 2001). Transformations and Processes Arsenic in surface and ground waters occur dominantly as either arsenite, As(III), or arsenate, As(V). Reduction of arsenic occurs with the possible formation of several methylated species, the most prevalent being monomethly- and dimethylarsenic acid. Several microorganisms including species of fungi, algae, and bacteria catalyze the reduction of arsenic. Methylated arsenic is volatile and is released to the atmosphere in open systems. Oxidation of arsenic is also catalyzed by microbes and it has been demonstrated that soluble As(III) and arsenic sulfide minerals such as arsenopyrite and orpiment can be catalytically oxidized to soluble As(V). More than 25 species of arsenic-oxidizing bacteria have been identified and many more are believed to exist. In geothermal waters, the dominant form of dissolved arsenic is As(ffl) at the source of the discharge but this can be oxidized rapidly to As(V) by microbes that survive at temperatures of 50-95°C. Little is known about the breakdown of feed additives such as roxarsone. Although there has been research on the transformations of arsenic within humans and some other organisms, the pathways are very complicated and much remains to be learned. Water quality environments that encourage solubilization and mobility of arsenic are high pH and oxic conditions, anoxic or moderately reducing conditions with no sulfate reduction, anoxic with strongly reducing conditions with little to no sulfate present, or strongly acidic oxidizing conditions (below the normal solubility of hydrated iron oxides). Environments that encourage low arsenic mobility are moderately acidic to neutral and oxidizing conditions, or organic rich sulfate-reducing environments. References Garbarino, J.R., Rutherford, D.W., and Wershaw, R.L. (2001) Degradation of roxarsone in poultry litter. USGS Workshop on Arsenic in the Environment, Feb. 21-22, 2001, website: wwwbrr.cr.usgs.gov/Arsenic Kolkar, A., Nordstrom, D.K., and Goidhaber, Mi. (2001) Occurrence and micro-distribution of arsenic in pyrite. USGS Workshop on Arsenic in the Environment, Feb. 21-22, 2001, website: wwwbrr.cr.usgs.gov/Arsenic Pierce, M.L. and Moore, C.B. (1982) Adsorption of arsenite and arsenate on amorphous iron hydroxide. Water Res. 16, 1247-1253. 74 �PRELIMINARY EVALUATION OF HYDROTHERMAL ALTERATION MINERAL ASSEMBLAGES AND THEIR RELATIONSHIP TO VMS-STYLE MINERALIZATION IN THE FIVE MILE LAKE AREA OF THE ARCHEAN VERMILION GREENSTONE BELT, NORTHEASTERN MINNESOTA JASON D. ODETTE, GEORGE J. HUDAK, THOMAS SUSZEK Department of Geology, University of Wisconsin Oshkosh, Oshkosh, WI 54901 STEVEN A. HAUCK Natural Resources Research Institute, University of Minnesota Duluth, Duluth, MN 55811 The Minerals Coordinating Committee (MCC, State of Minnesota) recently awarded a two year grant to geologists from the Natural Resources Research Institute (NRRI), University of Minnesota — Duluth (UMD), and the University of Wisconsin Oshkosh to further characterize the geology, volcanology and metamorphosed hydrothermal alteration mineral assemblages associated with several volcanic-associated massive sulfide (VMS) prospects in the Vermilion District of northeastern Minnesota. This investigation includes detailed outcrop mapping, diamond drill core relogging, petrography, lithogeochemistry, and geophysical rock property evaluations. The Five Mile Lake prospect occurs approximately 15 miles southwest of Ely, Minnesota, and is situated within greenschist facies metamorphosed, primarily mafic Archean metavolcanic and metasedimentary rocks within the Lower Member of the Ely Greenstone Sequence of the Vermilion District. In 1994, Teck Exploration Ltd. intersected volcanic-associated massive sulfide (VMS) — style stringer Zn-Cu mineralization in three of four diamond drill holes completed at this prospect. Subsequent studies by the Minnesota Department of Natural Resources (Hudak and Morton, 1999) have suggested that mineralization at Five Mile Lake may be representative of that associated with "Noranda-type" (Morton and Franklin, 1987) Archean VMS We are currently completing a detailed investigation of the mineralogical, chemical, and spatial characteristics of the metamorphosed synvolcanic hydrothermal alteration that occurs at the Five Mile Lake VMS prospect. Our two-month long field program was completed during the summer, 2000, and consisted of two investigative phases. The first phase comprised GPS-assisted geological mapping of the entire prospect at 1:5000 scale, with more detailed mapping of surface-mineralized zones at 1:120 scale. During our mapping, special attention was paid to alteration mineral assemblages present, and their apparent relationships to locally extremely well-preserved volcanic and volcaniclastic textures. Hand samples were collected from each outcrop. When appropriate, a portable, hand-held diamond drill was used to collect samples of adjacent alteration mineral assemblages. During the second phase of our investigation, Teck Exploration Ltd.'s four diamond drill holes (SXL-1, SXL-2, SXL-3 and SXL-4) were relogged, with particular emphasis being paid to the alteration mineral assemblages and textures present. Laboratory investigations are currently being conducted. All outcrop and drill core samples have been slabbed and investigated using a binocular microscope for alteration mineral assemblages, alteration textures, and alteration paragenesis. Prepartion of one hundred ninety-seven thin sections is currently being completed, and petrographic analyses on the available thin sections are being performed. Fiftythree samples have been analyzed for major and trace elements by ALS Chemex Labs, Inc. (Sparks, NV). At the present time, we have concentrated our alteration studies on the mafic pillow lava flows and the diabase dikes that occur at the prospect. Based on our preliminary field and laboratory investigations, all pillow lavas at the prospect have undergone varying degrees of hydrothermal alteration. This is consistent with the findings of Peterson (personal communication, 2001), who indicates that the prospect resides within a semiconformable alteration zone comprising at least 3 0km2 of rocks. Three distinct hydrothermal alteration mineral assemblages occur within the pillow lavas at the prospect. The 75 �least altered assemblage (LA) comprises pillow lavas containing a mineral assemblage composed of albite + epidote + chlorite in proportions which are consistent with greenschist facies metamorphism of an original basalt or basaltic andesite lava composition. A quartz + albite + epidote ± actinolite assemblage (QAE) occurs within pillow cores, whereas a chlorite + actinolite ± epidote assemblage (CA) occurs in pillow selvedges and within the matrix to pillow breccia and pillow hyaloclastite. Locally, CA assemblage filled amygdules occur within the QAE assemblage pillow cores, and these rocks seem to have a close spatial relationship to thin (<Im thick) mineralized exhalite horizons at the prospect. Textural relationships indicate that the paragenesis of these alteration mineral assemblages is early QAE followed by later CA. Mass balance analysis of least altered pillow lavas, QAE assemblage, and CA assemblage rocks have been performed using constant Al203 and best fit (based on Al203, Ti02, Zr, Nb) isocons (Grant, 1986). Relative to least altered rocks, QAE assemblage rocks illustrate gains in Si02 and Na20 and decreases in CaO, Fe203, FeO, MgO, MnO, K20, H20, Cu and Zn. These trends may be indicative of regional silicification and spilitization from rapidly heated, downwelling, silica-saturated hydrothermal fluid. Relative to least altered rocks, CA assemblage pillows are enriched in MgO, Fe203, FeO, Zn and H20 and are depleted in Si02, K20, and CaO. These results may represent an alteration assemblage formed from the mixing of cooler, downwelling Mg-rich seawater and hotter, upwelling Fe- and Zn-rich evolved hydrothermal solutions within permeable pillow selvedges and hyaloclastite. Locally, diabase dikes are altered to epidosite. The epidosite zones comprise rounded to oval, 0.1-1.5 meter diameter patches composed of pale green epidote (locally up to 40%) + quartz ± actinolite. Preliminary x-ray diffraction analyses indicate that the major epidote mineral in these epidosite zones is clinozoisite. Least altered diabase is composed of a mineral assemblage containing quartz-clinochloreferroactinolite and albite with only minor (<5%) epidote and clinozoisite being present. Mass balance analysis of the least altered diabase and the epidosite alteration patches have also been performed using constant A1203 and best fit isocons. These analyses indicate that the epidosite zones are enriched in CaO, and simultaneously depleted in Na20, K20, MnO, Fe203, FeO, 5, Zn, and Cu relative to the least altered diabase. We initially interpret these epidosite patches as areas of locally high water:rock ratio alteration within lower semi-conformable alteration zones associated with high temperature base metal leaching. At the present time we are in the process of further characterizing the alteration mineral assemblages by means of our laboratory studies. Petrographic and x-ray analyses to further characterize the mineralogy and paragenetic sequences of the alteration mineral assemblages is just beginning, and further geochemical classification of the alteration mineral assemblages will be performed. It is believed that in the long term, further characterization of the alteration mineral assemblages at the Five Mile Lake VMS prospect will provide exploration companies with additional data that is needed to conduct efficient and effective mineral exploration programs for VMS deposits in the Vermilion District. References Grant, J. A., 1986. The isocon diagram — a simple solution to Gresen's equation for metasomatic alteration: Economic Geology, v. 81, p. 1976-1982. Hudak, G. J., and Morton, R. L., 1999. Bedrock and Glacial Drift Mapping for VMS and Lode Gold Alteration in the Vermilion — Big Fork Greenstone Belt, Part A, Discussion of Lithology, Alteration, and Geochemistry at the Five Mile Lake, Eagles Nest, and Quartz Hill Prospects: Minnesota Department of Natural Resources Project 326, 136 pages. Morton, R. L., and Franklin, J. M., 1987. Two-fold classification of Archean volcanic-associated massive sulfide deposits: Economic Geology, v. 82, p. 1057-1063. 76 �CORRELATION OF ARCHEAN ASSEMBLAGES ACROSS THE U.S.-CANADIAN BORDER: PHASE I GEOCHRONOLOGY PETERSON, Dean M. (Natural Resources Research Institute, Duluth, MN, dpetersl@nrri.umn.edu); GALLUP, Christina (University of Minnesota-Duluth, cgallupd.umn.edu); uRSA, Mark A. (Minnesota GeologicalSurvey,jirsa OOl@umn.edu); and DAVIS, Donald W. (Royal Ontario Museum, Toronto, ON, dond@rom.on.ca) Past attempts at temporal correlation of Archean stratigraphic assemblages between rocks of the geochronologically well constrained Shebandowan district of Ontario, and the Vermilion district of northeastern Minnesota (Figure 1), have invariably suffered because: I) at some scale, one greenstone belt looks pretty much like another; 2) rocks of both districts are dissected by faults having poorly known displacements; and 3) little geochronologic data exists for the Minnesota rocks. Nevertheless, detailed analyses reveal that there are significant stratigraphic and lithologic relationships in each assemblage that can be compared and contrasted. Recent U-Pb geochronology is beginning to shed light on similarities and differences between assemb1aes in the two areas. Here we report the results of two sets of U-Pb dates on zircons from the Vermilion district. - Locatkrn of dated samples A 2683±1.4 Ma Porphyry B 2722±0.9 Ma Rhyohte - '11'" S ,',S - ''/, .'.''.." '.Northern Light Gneiss Saganaga piuton" ,t,A,,, 'l Western SupedoProvtce Vermilion " wgoo, District A R B j IIIIlIU4Soudari 'Btgfork ' Shebandowan 's.Stratigraphic faong p'Lake \,.... ,E!y',' , ;)" 'SSS, ,-,5555 s'Gmnitnd truss S.ipeno r Twnisiiameg lype conglomeratic sequences - ''" 'S" Wawa Subprovlnce Vermilion Ouetico Subprovlnce Wabigoon Subprovince Graywactra Metasvcanic rocks J1111 Graywacke -KomatutcTholeidic NEWTON BELT Shebandowan GREETCHELL Cak-akaiic/thoIeitic/komatiitic Migmatite. schist. and j granite TIf1 Graywacke SOUDAN BELT SAGANAGONS ASSEMBLAGE LI Cac-aalcflhoeiiiic Figure 1. Preliminary correlation of Late Archean stratigraphic assemblages and belts through the Vermilion district, showing the location of dated samples. Inset shows the map location within the western Superior Province. Rocks of the Vermilion district are subdivided on the basis of stratigraphic and structural contrast into two distinct domains, known as the Soudan and Newton belts (Figure 1). The Soudan belt contains large, broad folds involving caic-alkalic and tholeiitic volcanic strata overlain by, and locally interdigitates with, turbiditic rocks. In contrast, the Newton belt consists of elongate, northeast trending, and mostly northward-younging volcanic and volcaniclastic sequences. Volcanic rocks of the Newton belt differ from those of the Soudan in containing locally abundant komatiitic flows and peridotitic sills (eg. the Newton Lake Formation and Deer Lake sequence). The belts are fault-bounded, and the relationship between stratigraphic units within each belt is largely conformable, though faults obscure contacts locally. In its eastern extension, the Soudan belt is continuous with the Saganagons assemblage that terminates against the Saganaga pluton and Northern Light Gneiss (Figure 1). The Newton belt extends discontinuously eastward into the Shebandowan district, and broadens to form the approximately 2720 Ma-old Greenwater and Burchell assemblages2. The Greenwater assemblage is similar to the Newton Lake Formation in younging predominantly to the north, and in containing komatiitic and tholeiitic flows, mafic and ultramafic sills, and local intermediate to felsic flows and pyroclastic rocks. The Burchell is lithologically similar and temporally identical, but youngs to the south. Intrusions in both districts vary from felsic porphyries demonstrably related to volcanism, to large plutons emplaced posttectonically. Both districts contain unconformable, Timiskaming-type sequences composed of caic-alkalic volcanic rocks, conglomerates, and finer grained sedimentary rocks. These Timiskaming-type rocks include the roughly 2690 Ma-old Shebandowan assemblage2 and some lithologic components of the Knife Lake Group in Minnesota5. Periods of generally N-S-directed compression resulted in three major deformation events that are recognized in rocks of both districts. The earliest, D1, produced broad, locally recumbent folds within the Soudan belt and major fault zones throughout the region. The affect of D1 on rocks of the Newton belt and much of the Greenwater assemblage appears to have been thrust imbrication of large crustal blocks, resulting in mainly northward stratigraphic facing. Field relationships indicate that uplift, faulting, and the deposition of Timiskaming-type sequences in local fault-bounded basins occurred late in D1 deformation. The 77 �second deformation event, D2, produced synchronous regional metamorphism, foliation development, and structures having largely dextral asymmetry. D2 has been constrained in the Vermilion district to the time period 2674 Ma to 2685 Ma', and between about 2680 and 2685 Ma in the Shebandowan2. The abundant NE- and NW-trending faults that dissect the stratigraphic assemblages are assigned to D3. The two samples from the Vermilion district were selected mainly because they had the potential to produce zircons, and their ages would constrain the timing of volcanism and D2 deformation. Zircons were separated and isotopic compositions were measured at the Royal Ontario Museum in Toronto, using methods developed by Krogh4. The samples include: A) Newton belt a weakly deformed, irregularly shaped quartz-feldspar porphyry (QFP) body intruded discordantly along a basalt - iron-formation contact within the Pac Man Pond gold prospect. Field relationships indicate that the QFP was emplaced late in D2, because it (and similar intrusions) cuts D2 shear zones but is itself weakly deformed. B) Soudan belt — quartz-phyric rhyolite lava flow in the Fivemile Lake VMS prospect. Zircons from the Newton belt sample were very light brown, small (—lig after abrasion), subhedral, prismatic, and typically cracked. The zircons from the Soudan belt sample were colorless to light brown or pink, larger (—2-3 jig after abrasion), euhedral to subhedral, prismatic, and except for rare grains having a clear core, lacked observable internal cracks. After abrasion, three zircons from each sample were chosen for dating: the results are shown in Figure 2, plotted as 2 sigma error ellipses relative to concordia. Regression and age calculation follow the method of Davis (1982). Only one zircon without cracks could be found in the Newton belt sample. This produced the datum that is closest to concordia (Figure 2). It defines a line with the other data, giving an upper concordia intercept age of 2681 +1-4 Ma. All data have 207Pb/206Pb ages that are indistinguishable within error. Under the assumption that Pb loss was recent, the average 207Pb/206Pb age of 2683.0 +1- 1.4 Ma gives a more precise value for the age of intrusion. The age of this late-syn D2 porphyry constrains the maximum age of D2 in the Vermilion district, and is similar in age to parts of the Giants Range batholith, which forms the southern margin of the Vermilion greenstone belt. Data from the Fivemile Lake rhyolite zircons plot very near concordia and have indistinguishable 207Pb/206Pb ages that average to 2722.6 ± 0.9 Ma. This is the first age ever reported for the Ely Greenstone, and is similar in age to rhyolites in the Greenwater assemblage in Shebandowan district. .528 .526 D .524 .522 .520 .518 13.4 13.5 13.6 11.6 13.7 12 12.4 12.8 207Pb/235LJ 207Pb/235u Figure 2. U-Pb isotopic compositions for A) Porphyry in the Newton Belt, and B) Rhyolite in the Soudan Belt. 7 The ca 2720 Ma volcanic rocks in the Vermilion and Shebandowan districts, as well the Manitouwadge area6' farther to the east, together with the bracketed ages for D2 deformation that are nearly identical in all three districts, implies that this terrane defines a major orogen extending more than 600 kilometers. The individual belts have been strongly attenuated by deformation and pluton emplacement, but further U-Pb dates will continue to explore the Vermilion district in this broader orogenic context. Boerboom, T.J., and Zartman, R.E., 1993, Geology, geochemistry, and geochronology of the central Giants Range batholith, northeastern Minnesota: Can. J. Earth Sci. 30:25 10-2522. 2Corfu, F., and Stott, G.M., 1998, Shebandowan greenstone belt, western Superior Province: U-Pb ages, tectonic implications, and correlations: GSA Bulletin 110:1467-1484. 3Davis, D.W., 1982. Optimum linear regression and error estimation applied to U-Pb data. Can. J. Earth Sci. 19: 2141-2149. 4Krogh, T.E., 1982, Improved accuracy of U-Pb ages by the creation of more concordant systems using an air abrasion technique. Geoch. et Cosmochim. Acta, 46: 637-649. 5iirsa, M.A., 2000, The Midway sequence: a Timiskaming-type pull-apart basin deposit in the western Wawa subprovince, Minnesota: Can J. Earth Sci., 37: 1-15. 6Zaleski, E., van Breemen, 0., and Peterson, V.L., 1999, Geological evolution of the Manitouwadge greenstone belt and WawaQuetico subprovince boundary, Superior Province, Ontario, constrained by U-Pb zircon dates of supracrustal and plutonic rocks: Can. J. Earth Sci., 36: 945-966. 7Davis, D.W., Schandi, E.S., and Wasteneys, H.A. 1994. U-Pb dating of minerals in alteration halos of Superior Province massive sulfide deposits: syngenesis vs. metamorphism. Contrib. Mineral. Petrol. 115: 427-437. 78 �MAGNETIC SURVEY NEAR WATERLOO WISCONSIN PEYCHALI*, C., KEAN', W. F., and SCHAPER2*, D., Department of Geosciences, University of Wisconsin -Milwaukee'4 Department of Geological Engineering, University of Wisconsin-Madison2. wkean@uwm.edu. (* student) The Waterloo Wisconsin area continues to provide geologic puzzles because of its diverse bedrock. The Waterloo Quartzite is considered by most as cotemporaneous with the Baraboo Quartzite. A mafic dike is known from two quarries about I mile apart along a north-south strike, and a course feldspar rich pegmatite appears within 200 meters north of the quartzite quarry in which the mafic dike is found ( Luther, 1977). The age relationship of these three rock types is still uncertain. Although, the two igneous rock types intrude the quartzite, and both are dated in the range of 1.45 -1.5 Ga.(Van Schmus Ct al. 1978, Brown, 1986), which is not necessarily the primary age. Paleomagnetic studies on the mafic dike provide a well defmed magnetic direction of Dec.=299°, Inc.-43°, N=26, Alpha 95= 13.7°, giving a paleopole at 323° E, 2° S which is interpreted as a 1 .7Ga. pole position ( Schaper and Kean, 1999). The object of this study is to better define the magnetic basement in this region in an attempt to clarify the relationship of these known rocks. A proton precession magnetometer survey covered most of the roads in the vicinity of the old Portland Quarry and the Michels Materials Quarry. A first survey covered the region at 0.1 mile spacing. Selected locations were resurveyed at 50 meter spacing , and also with a continuously reading rubidium vapor magnetometer. Finally one road section was covered at 2 meter spacing. Well construction reports, and the aeromagnetic maps of the region were examined for additional insight into the bedrock. The aeromagnetic maps show a general decreasing trend from west to east, with a 200 nT. anomaly northeast of the Michels Materials quarry, where well construction reports indicate granite. The ground magnetic survey shows a 1000 nT. negative anomaly along Highway 19 that appears to be in line with the known outcrops of the mafic dike. The negative value is consistent with the negative remanent magnetism of the mafic dike. Similar anomalies are found on the south-north section of Hubbleton Rd. which passes in front of the active quarry. We interpret these as the extension of the dike to the north east of its known outcrop. References: Brown, B.A., 1986, The Baraboo Interval in Wisconsin, Geoscience Wisconsin, vol.10, p.1-15. Luther, F. R., 1997, the Precambrian Waterloo Quartzite, Dodge and Jefferson Counties, WisconsinPetrology, structure and Industrial Use. In "Guide to Field Trips in Wisconsin and Adjacent Areas of Minnesota". M.G. Mudrey Jr. Field Trip Coordinator, Wisconsin Geological Survey, 114 pp. Schaper, D., and Kean, W., 1999, Multiple Magnetic Directions in a Proterozoic Dike near Waterloo, Wisconsin. Fall Meeting AGU, abstracts. Van Schmus, W.R., 1978, Geochronology of the Southern Wisconsin Rhyolites and Granites, Geoscience Wisconsin, vol.2, p.1 9-24. 79 �Nd and U-Pb Isotope Studies of the Syenitic Aurora Sill, Mesabi Range, Minnesota Phillips, Erin H., Macalester College, St. Paul, MN 55105; Wirth, Karl R., Geology Department, Macalester College, St. Paul, MN 55105, wirth@macalester.edu; Vervoort, J.D., and Gehrels, G.E., Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721 The Aurora sill is a concordant tabular intrusion approximately 5.6 km long and 6 to 37 meters thick that occurs in the Mesabi Range north of the town of Aurora, Minnesota. The age of the sill is unknown, but it has commonly been assigned a Mesoproterozoic (Keweenawan) age because it intrudes the 1.88 Ga Biwabik Iron-Formation and also because of its resemblance to granitic complexes, or "granophyres", of the Midcontinent Rift (White, 1954). The geochemistry and petrography of the Aurora sill, however, are quite different from the Midcontinent Rift (MCR) granophyres (Phillips et al., 2000). The isotopic data presened ho indicate that the Aurora sill has a Keweenawan age and represents a unique occurrence of this rock type in this portion of the MCR. Mineralogically, the Aurora sill is composed of albite, potassium-feldspar, aegerine, fibrous amphibole, chlorite, Fe-Ti oxides, Fe sulfide, and zircon and is classified as a syenite (< 5% normative quartz or nepheline) based on both modal and normative compositions. The presence of two feldspars indicates subsolvus crystallization and high P1120. Albite is the predominant mineral in the sill and is aligned in fine-grained samples to produce a trachytic texture. Samples from the Aurora sill exhibit limited geochemical variation (Si02=54.6-60.7 wt. percent; Mg#=0.4-0.75; Y=9-19 ppm) suggesting that the intrusion has undergone relatively minor internal fractionation. A large sample from the Aurora sill yielded only a few small (60-80 jim) anhedral zircons. No crystal faces were present on any of the zircon grains but it is unknown whether this is due to fracturing during processing or resorption of the grains. Four U-Pb zircon analyses (each consisting of 3 small zircons) yield a highly discordant regression with an upper intercept of 2970 ± 257 (2; Figure 1). It is improbable that this date represents the age of the sill because it intrudes the Biwabik Iron-Formation that was deposited about 1.88 Ga(Fralick et al., 1998). Our interpretation is that the zircons grew in the Mesoproterozoic with significant inheritance from Archean basement rocks in the region. The lower intercept of 1190 ± 466 Ma indicates either zircon growth or a Pb loss event in the Mesoproterozoic that may have coincided with intrusion of the sill. Nd isotopes provide further evidence of a Keweenawan age for the Aurora sill. A slightly negative epsilon Nd value of -1.4 results when an age of 1,100 Ma is assumed for the Aurora sill (Figure 2). If older ages are used, they result in unrealistically high ENd values (ENd = +9 @ 1,800 Ma; ENd +23 @ 2,900 Ma). The Nd isotopic results corroborate a Keweenawan age (1 .1 Ga) for the sill and are consistent with the lower U-Pb zircon intercept. If the Aurora sill is Keweenawan in age, the sill would represent a relatively rare type of alkaline 0.6 magmatism in this part of the MCR. The sill is distinct from the granophyric complexes of the northwestern limb of the MCR in several respects. The granophyric 0.5 textures and modal quartz that are typical of felsic rocks of the MCR are not present in the Aurora sill. The sill is saturated to weakly undersaturated in silica and 0.4 contrasts sharply with the silica saturated granophyres of the MCR. Furthermore the Aurora sill is characterized by high total alkalis, Nb/Y, and Ce/Zr compared with felsic rocks of the MCR. The Aurora 0.3 sill also lacks the negative Nb anomalies that are 0.2 characteristic of the MCR granophyres. Although there are no known Keweenawan syenites in the vicinity of the Aurora sill, there are several alkaline intrusions in 01 the northern portion of the MCR. The Coldwell Complex and Killala Lake Complex both contain 7Pb* 1235U syenites that have been dated at 1108 ± I Ma, near the Figure 1 80 �beginning of continental rifting (Heaman and Machado, 1992). Preliminary examination of the geochemistry of these syenites displays similarities to the Aurora sill. For example, they exhibit high Nb/Y ratios and follow similar trends on many geochemical plots (e.g., Na20+K20 versus Si02) and have broadly similar ENd values to the early E Nd (T) gabbro phases from the Coldwell complex (Heaman and Machado, 1992). Unlike the Coldwell and Killala Lake complexes, the Aurora sill is not known to be associated with carbonitites. The Nd isotopic and trace-element signatures of the Aurora sill suggest that it was derived from a mantle melt that did not interact with older, evolved crustal materials. However, this Age (Ga) Figure 2 . interpretation is not fully consistent with the U-Pb zircon data which clearly indicate inheritance of older zircons. It is difficult to imagine a process whereby zircons are inherited from the extant Archean crust without that crust contributing a highly unradiogenic Nd isotopic signature (negative ENd values) to the sill. This could occur if the assimilant was of low concentration and/or near-chondritic Sm/Nd ratios, but neither is a common characteristic of zircon bearing lithologies. One possibility is that the zircons in the Aurora sill have been derived from small degrees of assimilation of Biwabik Iron-Formation or related Animikie sediments. These sediments have variable but generally less negative E values in the Mesoproterozoic (Hemming et al., 1995) and also contain Archean detrital zircons. The age of the Aurora sill is important to understanding the timing of ore formation in the Mesabi Range. In 1999, Morey proposed a conceptual model in which the high-grade ores formed in a regional ground-water system during Paleoproterozoic time (1.6-2.5 Ga). Subsequently, Graber and Strandlie (1999) argued that nearby high grade ores must have formed in Mesoproterozoic or later time, because the Aurora sill appears to have controlled ore-forming processes in nearby mines. Since a Keweenawan Aurora sill could not have had an effect on ore-formation in the Paleoproterozoic, it is not possible at this time to confirm the model described by Morey (1999). References Cited Fralick, P.W., S.A. Kissin, and D.W. Davis, 1998, The Age and Provenance of the Gunflint Lapilli Tuff. Institute on Lake Superior Geology, Proceedings and Abstracts, p. 66-67. Graber, Ronald G. and Alan J. Strandlie, 1999, Where are the Metamorphosed Natural Orebodies of the Mesabi Range? Institute on Lake Superior Geology, Proceedings and Abstracts, p. 17-19. Heaman, L.M. and Machado, N., 1992, Timing and origin of the Midcontinent rift alkaline magmatism, North America: Evidence from the CoIdwell complex. Contributions to Mineralogy and Petrology, V 110, p. 289-303. Hemming, S.R., S.M. McLennan, and G.N. Hanson, 1995, Geochemical and Nd/Pb isotopic evidence for the provenance of the Early Proterozoic Virginia Formation, Minnesota. Implications for the tectonic setting of the Animikie basin, Journal of Geology, V 103, p. 147-168. Morey, G.B., 1999, High-Grade Iron Ore Deposits of the Mesabi Range, Minnesota Product of a Continental-Scale Proterozoic Ground-Water System. Economic Geology, V 94, p. 133-142. Phillips, E.H., 2000, Petrogenesis of the Enigmatic Aurora Sill, Mesabi Range, Minnesota, Unpublished honors thesis, Macalester College, St. Paul, MN. Phillips, E.H., K.R. Wirth, and G.B. Morey, 2000, Petrogenesis of the Enigmatic Aurora Sill, Mesabi Range, Minnesota. Institute on Lake Superior Geology, Proceedings and Abstracts, p. 51-52. White, David A., 1954, The Stratigraphy and Structure of the Mesabi Range, Minnesota. Minnesota Geological Survey Bulletin 38. Minneapolis: The University of Minnesota Press, p. 63-66. 81 �Freeze/Thaw Testing of Carbonate Aggregate Sources in Wisconsin — A Status Report Daniel D. Reid, Wisconsin Department of Transportation, 3502 Kinsman BlvcL, Madison, Wisconsin 53704-2507 Delamination and deterioration of exposed pavement aggregates has been a common occurrence on highways in southern and northeastern Wisconsin, and has lead to highway maintenance problems in some areas. The principal cause of these problems is crushed stone aggregate produced from Sinnipee Group (Galena, Decorah and Platteville Formations) rock. Prior to 1999, the Wisconsin Department of Transportation (WisDOT) excluded entire formations and members of the Sinnipee Group as a way of mitigating pavement problems. A study of carbonate aggregate resources in Wisconsin, conducted jointly by WisDOT and the Wisconsin Geologic and Natural History Survey (WGNHS), provided a comprehensive analysis of the stratigraphy and geologic properties of Sinnipee Group aggregate resources. This study identified significant regional variability in Sinnipee Group rock, and concluded that the laboratory freeze/thaw test was the most effective method of identifying problem aggregate sources. Based on this data, WisDOT concluded that specifications excluding certain formations and members were not appropriate for uniform application across the entire state. As a result of the WisDOT/WGNHS carbonate aggregate study, WisDOT developed and implemented specifications for freeze/thaw testing of carbonate aggregate sources used in pavements and bridge decks in October 1999. These specifications mandated freeze/thaw testing in counties where Sinnipee Group rock outcrops, and set the threshold for loss at 18% by weight. Included in the specifications was a clause that allows WisDOT to waive freeze/thaw testing for existing aggregate sources determined to be in the Silurian System or Prairie du Chien Group of carbonate rocks. WisDOT has now completed over 230 independent freeze/thaw tests on carbonate aggregate source material throughout Wisconsin. To date, the results indicate that freeze/thaw testing is performing it's intended function. A decrease in pavement problems has been reported and WisDOT now has an effective method of controlling aggregates that produce excessive delamination. As an added benefit, aggregate producers are now permitted to use aggregate sources that were excluded prior to establishment of the freeze/thaw testing specification, so long as material from these sources tests under the 18% threshold. References: - AASHTO, 1996, Standard Specification for Soundness of Aggregates by Freezing and Thawing, American Association of State Highway and Transportation Officials (AASHTO) Designation T 103-91. Brown, Bruce A., 1999, Aggregate Resources of the Sinnipee Group in Eastern and Southern Wisconsin, Wisconsin Geologic and Natural History Survey Open-File Report 1999-07. Ostrom, M.E., 1967, Paleozoic Stratigraphic Nomenclature for Wisconsin, Wisconsin Geological and Natural History Survey Information Circular No. 8. 82 �___ A Metamorphosed Evaporite Sequence from the Sibley Basin Rogala, B. and Fralick, P.W., Department of Geology, Lakehead University, Thunder Bay, Ontario The Sibley Group sediments were deposited in a subsiding infracratonic basin (Fralick and Kissin, 1995), between 1339 ± 33 Ma (Franklin et aL, 1980) and 1537 +10-2 (Davis and Sutcliffe, 1984). The Group is divided into three main Formations: Pass Lake, Rossport, and Kama Hill, representing deposition in a braided fluvial-mudflatplaya environment (Cheadle, 1986). This study concentrates on a section through a lateral correlative of the cyclic facies contained in the Rossport Formation present in the Noranda drill core NI-92-5. The location of this drill hole is north f iier cored sections and outcrops of the cyclic facies and represents a more basin center environment. The cyclic facies to the south consists of alternating layers of dolomite and red shale with individual layers, in the approximately 40 m thick assemblage, varying from mm- to dm- In drill hole NI-92-5 the layering is at a similar scale, but is composed of alternations between what was first thought to be dolomite-rich and gypsum-rich scale. intervals, reflecting wet and dry seasonality in the central playa environment. SEM and XRD analysis indicate that the sequence was metamorphosed. The progression of metamorphic facies, from lower to higher T towards the diabase sill, is represented by the following reactions: 3CaMg(C03)2 + 4SiO2 +1H20 —' lMg3Si4Oio(OH)2 + 3CaCO3 + 3CO2 and 5Mg3Si4O10(OH)2 + 6CaCO3 + 45i02 —. 3Ca2Mg5Si8O22(OH)2 + 6C02 + 2H20 or 2Mg3Si4Oio(OH)2 +3CaCO3 - lCa2Mg5Si8O22(OH)2+ lCaMg(CO3)2 + 1CO2 +1H20 or 5CaMg(C03)2 + 8SiO2 + 1H2O — lCa2Mg5Si8O22(OH)2 + 3CaCO3 + 7CO2 (Winkler, 1974) Near the diabase sill pargasite, a hornblende with the composition (Na,K)01Ca2Mg4A13Si6O22(OH), is the dominant mineral. Clinochlore, a chlorite mineral with a composition of Mg5Al5Si3Oio(OH), is found throughout the metamorphic series. Both of these minerals are common in magnesian carbonate metamorphism (Pattison and Tracy, 1991). Primary layering has been masked in places by metamorphism. However, S.E.M. analysis clearly shows the mineralogical layering, reflecting variations in primary geochemical constituents between individual laminae. ICP-AES analysis also highlights the varying compositions of the sequence. CafMg ratios indicate the precipitation of gypsum in many layers. This is supported by the observation of gypsum using S.E.M.. The pargasite tends to exist in layers with differing K and Na proportions. The K and Na ions may reflect the incorporation of KCI and NaC1. Both of the minerals are found in S.E.M. sections taken near the middle of the sequence. 83 �Clastic input is variable between individual layers, but shows an increase at the top of the section. This increase is related to the appearance of sand sheets, which typically mark the end of the cyclic facies in the Sibley Group sediments. Clastic material becomes slightly enriched in elements denoting a mafic rock source up-section. The source material is distinctly alkalic. Alkalic material may be associated with plume activity, which would fit the infracratonic theory for the formation of the Sibley Basin. However, suitably alkalic rocks have not been found in the area. References Cheadle, B.A. 1986. Alluvial-playa sedimentation in the lower Keweenawan Sibley Group, Thunder Bay District, Ontario. Canadian Journal of Earth Sciences, 23, 527-542. Davis, D.W. and Sutcliffe, R.H. 1984. U-Pb ages from the Nipigon Plate and Northern Lake Superior. Geological Society of America Bulletin, 96, 1572-1579. Fralick, P. and Kissin, S. 1995. Mesoproterozoic basin development in central North America: implications of Sibley Group volcanism and sedimentation at Redstone Point. in: Petrology and metallogeny if volcanic and intrusive rocks of the midcontinent rift system, Proceedings of the International Geological Correlation Program, Project 336. Franklin, J.M., Mcllwaine, W.H., Poulsen, K.H. and Wanless, R.K. 1980. Stratigraphy and depositional setting of the Sibley Group, Thunder Bay District, Ontario, Canada. Canadian Journal of Earth Sciences, 17, 633-651. Pattison, D.R.M. and Tracy, R.J. 1991. Phase equilibria and thermobarometry of calcareous, ultramafic, and mafic rocks, and iron formations. In: Kerrick, D.M. (ed.), Contact Metamorphism. Reviews in Mineralogy, 26, Mineralogical Society of America. Winkler, H.G.F. 1974. Petrogenesis of Metamorphic Rocks. Springer-Verlag: New York Inc., 320 p. 84 �Roles of Fractional Crystallization and Assimilation in the Production of Midcontinent Rift Granophyres. Sandland, Travis 0., (tsandland@Macalester.edu) & Wirth, Karl R., Geology Department, Macalester College, St. Paul, MN, 55105; Vervoort, Jeff D. & Gehrels, George E., Department of Geosciences, University of Arizona, Tucson, AZ, 85721; Kennedy, Bryan C., Geology Department, Macalester College, St. Paul, MN, 55105; Harpp, Karen S., Department of Geology, Colgate University, Hamilton, N 13346 The granitic complexes of the Midcontinent Rift (MCR), commonly termed granophyres, comprise a significant portion of the Duluth Complex in northern Minnesota. They range in size from 30 to 150 km2 in surface area and are ito 2 km in thickness. They consist of basal diorite and monzodiorite and progress upward to quartz monzodiorite, granodiorite, and granite. This study focuses on the petrogenesis of four of these complexes: the Greenwood Lake, Misquah Hills, Eagle Mountain, and Pine Mountain granophyres. Miller and Vervoort (1996) identified two magmatic stages in this part of the MCR. An "early stage" from 1108-1105 Ma, and a "main stage" from 1100-1094 Ma. The four granophyre complexes addressed in this study have U-Pb zircon ages consonant with this chronology. The older granophyres include the Misquah Hills and Greenwood Lake complexes with ages of 1106±6 Ma and 1106±3 Ma (±2 sigma), respectively, and were emplaced during the early stage of the rift. The Eagle Mountain and Pine Mountain granophyres have ages of 1098±4 and 1095±4 Ma, respectively, and were emplaced during the main magmatic stage. The granophyre complexes vary widely in composition (47-76 wt. % Si02) and plot as linear Figure 1 trends on many Harker variation diagrams. The two granophyre groups (early and main stage) are indistinguishable on such plots and appear to have ci very similar major element chemistry. Incompatible trace elements (including REEs) are enriched in both groups (Fig. 1) although the early stage granophyres generally have higher concentrations than the late stage granophyres. Incompatible trace element ratios (e.g., LaISm, Gd! Yb) are similar for both groups. The early stage granophyres have initial epsilon Nd values between 20 0 and -2. In contrast, the main stage granophyres are isotopically enriched, with initial epsilon Nd Ce values between -3 and -8. The isotopic and incompatible element data suggest both fractional crystallization (FC) and La Pr Tb Ho Tm Lu Nd Sm Gd Dy Er Yb Eu assimilation fractional crystallization (AFC) processes were involved in the evolution of the granophyres. The large negative epsilon Nd values associated with the main stage granophyres indicate AFC processes and suggest contamination by an isotopically enriched source, possibly felsic Archean crustal rocks. This trend can be seen on a graph of epsilon Nd vs. La/Yb (Fig. 2). The main stage granophyres also 85 �have low Nb/Y ratios, a signature of contamination by crustal materials, whereas the Nb/Y ratios of the early stage granophyres are positively correlated with La/Sm (Fig. 3). If AFC processes were active during the formation of the early stage granophyres, the assimilant was likely limited to juvenile mafic rocks with little or no isotopic enrichment. Alternatively, the enrichment of incompatible elements in the early stage granophyres could be the result of fractional crystallization. Figure 2 Figure 3 20 2 FC AFC 10 - Early Stage. Main Stages 0 I -8 -7 I I I -6 -5 -4 I -3 -2 0 I -l La/Sm Epsilon Nd These data support the model of rift evolution as presented by Vervoort and Green (1997). The early stage granophyres were formed either by fractional crystallization, or by AFC processes with contamination from a high Sm/Nd mafic crust. During this stage of rift evolution, the middle to upper crust was probably relatively cold, so assimilation likely occurred in the lower crust due to local heating as a result of plume initiation. The main stage granophyres are isotopically contaminated and suggest AFC processes at work. During this time period, renewed rifling resulted in higher ambient temperatures in the crust, and allowed melting of older, low Sm/Nd, negative epsilon Nd sources, possibly at middle to upper crustal levels. References: Miller, J.D., Vervoort, J.D., 1996, The latent magmatic stage of the Midcontinent rift: a period of magmatic underplating and melting of the lower crust. in 42" Annual Meeting of the Institute on Lake Superior Geology, Cable, Wis., May 1996, Proceedings volume 42, pp 33-35. Vervoort, J.D., and Green, J.C., 1997, Origin of evolved magmas in the Midcontinent rift system, northeast Minnesota: Nd-isotope evidence for melting of Archean crust: Canadian Journal of Earth Sciences, v. 34. p. 52 1-535. 86 �DIRECT TIMING CONSTRAINTS ON PALEOPROTEROZOIC METAMORPHISM, SOUTHERN LAKE SuPERIOR REGION: RESULTS FROM SHRIMP U-PB DATING OF METAMORPHIC MONAZITES Schneider, D.A., Syracuse University, Syracuse, NY, 13244; Hoim, D.K., Kent State University, Kent,OH, 44242; Hamilton, M.A., Geological Survey of Canada, Ottawa, ONT, K1A 0E8 The age of the Penokean Orogeny has been firmly established for decades by U-Pb pluton age data. The timing of post-orogenic cooling and lower temperature overprinting has only been established more recently by thermochronologic investigations. Little information exists however on the timing of the initial higher-grade metamorphism which affected the southern Lake Superior region during the Paleoproterozoic. Such information represents a critical missing link in our understanding of the tectonothermal evolution of the crust during and after Penokean orogenesis. Monazite, a REEphosphate accessory mineral, grows as a common metamorphic phase under amphibolite and higher-grade conditions. Monazite U-Pb ages typically yield reliable estimates for the timing of peak metamorphism, although post-metamorphic deformation and fluid flux can cause lower-temperature monazite dissolution and reprecipitation. We utilized the SHRIMP at the GSC in Ottawa to obtain geochronologic information on distinct mineral domains (e.g., core vs. rim) which may have resulted from particular tectonothermal events. Through BSE imaging prior to analyses, we found rim textures spatially distinct from interior replacement textures. Age data are indistinguishable from each domain and replacement probably resulted from the same thermal event as the rim neogrowth. Age data summarized below include only concordant or near concordant analyses and are reported as weighted average 207Pb/206Pb ages. In this pilot study we separated monazite from an amphibolite grade Paleoproterozoic metasedimentary unit in east-central Minnesota (Kettle River locality) and from an Archean gneiss unit within the Peavy district, northern Michigan (Foster City locality). From Kettle River, a sample of garnet schist, which attained peak metamorphic conditions of 470-520°C at 5-6 kbar, yielded distinct populations of core and rim U-Pb ages from 5 monazite grains. Eight analyses of core domains yielded a U-Pb monazite crystallization age of 1834.0 ± 6.1 Ma, reflecting primary metamorphic growth. A population of monazite rimlreplacement spot analyses, characterized by a higher Th content, yielded an age of 1792.9 ± 4.3 Ma. This latter age is similar to the --1799 Ma U-Pb crystallization age of the deformed Hillman tonalite (Van Schmus et aL, 2000, ILSG and this volume). From Foster city (>500°C and -4 kbar), an Archean muscovitebiotite quartzofeldspathic gneiss yielded a monazite core age of 1834.4 ± 6.0 Ma from 10 spots on 11 grains. Rimlreplacement domain analyses from 9 spots yielded a younger age of 1809.8 ± 6.3 Ma, consistent with high-temperature cooling in the region (1800 and 1785 Ma Ar-Ar hornblende ages; Mancuso et al., ILSG, 1997). 87 �These metamorphic U-Pb ages are the first reliable metamorphic dates from this Paleoproterozoic orogen and indicate a widespread thermal pulse at 1835 Ma in response to accretion-induced crustal thickening. Albeit preliminary, the age of metamorphism is remarkably consistent across the orogen. High temperature Ar-Ar cooling ages on hornblende (—500°C) from north of the Watersmeet dome (—1822 Ma; Schneider et a!., 1996, CJES) and biotite Ar-Ar dates from low-grade structural panels from east-central Minnesota (1840-1830 Ma; Schweitzer et al., 2000, ILSG) are compatible with an —1835 Ma orogenwide metamorphic episode. Further accessory mineral geochronology holds promise for better elucidating the timing of syn-orogenic metamorphism, as well as overprinting post-orogenic events throughout the southern Lake Superior region. Foster City Archean mu-bi quartzofeldspathic schist 0.35 -a a. (p 0 0.33 1: 0.31 0.29 0.27 4.2 4.4 0.33 0.31 0.29 0.27 4.2 4.4 4.6 88 4.8 5.0 5.2 5.4 5.6 �RESULTS OF IGNEOUS THERMOMETRY AND BAROMETRY ON THE EAST-CENTRAL MINNESOTA BATIIOL1TH: EVIDENCE FOR POST-EMPLACEMENT EXHUMATION AND COOLING SCHWEiTZER, D., and HOLM, D., both at Dept of Geology, Kent State University, Kent; OH, 44242; VAN SCHMUS, W.R., Dept. of Geology, Univ. of Kansas, Lawrence, KS, 55045; BOERBOOM, T., Minnesota Geological Survey, 2642 University Avenue, St Paul, MN The internal zone of the Penokean orogen in east-central Minnesota was invaded by abundant plutonism associated with emplacement of the East-Central Minnesota Batholith (ECMB) from 1787 to 1772 Ma (Van Schmus et al., 2000). Previous application of the Aluminum-in-Hornblende (AH) igneous barometer on phases of the ECMB indicate paleodepth estimates that increase from -13 km in the north (Ka-F, K-4; Fig 1) to —18 km in the southeast near St Cloud (SC, Fig 1). HoIm aed others (1998) inferred the depth differences to reflect intrusion of different magma bodies into a rapidly unroofing terrane. This interpretation predicts that progressively younger intrusions would have been emplaced into progressively shallower crustal levels. At the time, their hypothesis was limited by the lack of any precise age data on the intrusions analyzed. The new ECMB U-Pb age data provides the framework to directly test this hypothesis of syn-emplacement exhumation. We selected six new samples for further All barometric work (starred localities, Fig 1). Anderson and Smith (1995) outlined the important role that temperature plays in the Al concentration in homblende rims and proposed a temperature correcting calibration for the application of the All barometer. For this reason, the Hornblende-Plagioclase (HB-PL) thermometer (Blundy and Holland, 1992) was applied to these samples in order to derive a crystallization temperature estimate for each sample. The Schmidt (1992) pressure calibration is presented in order to compare the effects of the temperature correction. The electron microprobe at the University of Maine, Orono was used to obtain compositional data used for both the All barometer and HB-PL thermometer. The results of these analyses are summarized in the following table. Table 1: Summary of igneous thermometry and barnmetry results Schmidt, 1992 A and S, 1995 An Content B and H, 1990 kbar kbar Temperature (°C) (%) Sample ID Rock Unit EC-31 EC-15 DS-99-17 EC-1 EC-35 RFG-7 Granodiorite Mafic Plug Anne Lake Granite Watab Quartz Dionte Granodiorite Reformatory Granodiorite 789 ± 38 49.99 59.35 31.29 40.01 43.77 51.31 884±38 745 ± 38 6% *38 746±38 783 ± 38 9.1 ± 0.6 13.4 ± 0.6 6.0 ± 0.6 5.2 ± 0.6 8.5 ± 0.6 6.4 ± 0.6 <6.5 ± 0.6 <6.2 ± 0.6 4.9 ± 0.6 5.0 ± 0.6 <7.2 ± 0.6 4.4 ± 0.6 Assuming an average overburden density of 2.7 g/cc, the pressures recorded by EC-31, EC-15, and EC-35 correspond to maximum emplacement depths of 23-27km. These appear to be anomalously high when compared to other rocks in the area and probably reflect artificial Al enrichment of hornblende crystal rims due to high An content. Samples RFG-7 (17 lan), EC-1(18 km), and DS-99-17(18 km) when corrected for temperature effects appear to preserve viable estimates since they are consistent with prior barometric results. In the north, samples Ka-F and K-4 (Fig. 1) both preserve emplacement depths of 13 km but their UPb ages are 15m.y. apart. To the south, all samples near St Cloud record —17-18 km depths regardless of age. This implies that very little uplift occurred during emplacement of the ECMB. The bulk of post-tectonic exhumation appears to have occurred after the emplacement of the ECMB. Preserved pressure differences probably reflect deeper emplacement depths (from N to 5) across the batholith. Post-emplacement exhumation is consistent with cooling ages being 10-20 m.y. younger than ciystallization ages from the batholith (Holm et al, 1998; Hohn and Lux, 19%). The ECMB was likely completely exhumed prior to deposition of the Early Proterozoic (1750-1650 Ma) red quartzites. 89 �MSD: \ ,.- H It Is MN-29 :-::•::-: 18±2km I— A l P16 (MSD) ,c—I___ -. ', / -, — _s. \ I_,>. — — -%.— •5_ , / 5—— '11787* ' I<23*2km s. ,,-I, ' 5. Is ; '___-\ I_/'-'/ — I S —' — I -. Is * I <24t2kI — c-i J , — ;, _, __'_l 10 km F/ /' , _s— -, I, / — .,- —. — I / \ 'I'—'' ' — —' - Figure 1: Simplified map of the internal zone of the Pekonean orogen. Localities marked with filled circles represent samples analyzed by Hotm et at (1998). Starred localities denote samples analyzed in this study. MSD = Malmo Structural Discontinuity; SC = St. Cloud; LF = Little Falls. U-Pb ages after Van Schmus et at (2000). References Anderson, J.L, and Smith, D.R, 1 995,The effects of temperature and oxygen fugacity on the Al-in -hornblende barometer: American Mineralogist, v.80, p.549-559. Blundy, J.D., and Holland,TJ.B, 1990, Calcic amphibole equilibria and a new amphibole-plagiodase geothermometer: Contributions to Mineralogy and Petrology, v.104, p.208-224. HoIm, D.K.and Lux, D.R, 1996, Core complex model proposed for gneiss dome development during collapse of the Paleoproterozoic Penokean orogen, MN: Geology, v.24, p.343-346. Holm, D.K., Darrah, KS., and Lux, D.R., 1998, Evidence for widespread —.1760 Ma metamorphism and rapid stabilization of the Early-Proterozoic (1870-1820 Ma) Penokean orogen, MN: Am. J. of Science, v.298, p.60-81. Schmidt, M.W., 1992, Amphibole compoisition in tonalite as a function of pressure: an experimental calibration of the Al-in-hornblende barometer: Contributions to Mineralogy and Petrology, v.1 10, p. 304-310. Van Schmus,W.R., MacNeill, LC., Holm, D.K, Boerboom,TJ., and isa, M.A., 2000,The 1787-1772 Ma east-central Minnesota batholith: Precursor to crustal stabilization in the Lake Superior region: ILSG proceedings, 46th annual meeting, Marquette, Ml, v.46, p.65-66. 90 �A SYNOPSIS OF ARCHEAN AND PROTEROZOIC PLATINUM GROUP ELEMENT MINERALIZATION IN THE THUNDER BAY DISTRICT, ONTARIO SMYK, Mark C., MASON, John K. and SCHNIEDERS, Bernie R., Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7, and STOTT, Greg M. Ontario Geological Survey, Ministry of Northern Development and Mines, Willet Green Miller Centre, 933 Ramsey Lake Road, Sudbury, ON P3E 6B5 Platinum group element (PGE) occurrences and deposils are in both Neoarchean and Mesoproterozoic mafic to ultramafic intrusive rocks in the Thunder Bay District. Recent, dramatic increases in platinum group metal prices have prompted renewed interest in PGE exploration, resulting in the discovery of hitherto unknown hosts and styles of mineralization. Archean There are four broad, temporally diverse settings for Neoarchean PGE mineralization: (1) Pre-tectonic, mafic to ultramafic, subvolcanic(?) intrusions intimately associated with greenstone belts of various ages in the Wawa and Wabigoon subprovinces; (e.g. Haines Gabbro (Shebandowan belt / Wawa; 2722 Ma); Core Zone gabbro (Obonga Lake belt / Wabigoon; 2733 Ma); (2) Post-tectonic, mafic to ultramafic intrusions (Ca. 2692 Ma), related to late plutonism in the Wabigoon Subprovince, hosted by gneissic tonalite-granodiorite (e.g. Lac des Iles and Tib Lake complexes; Buck Lake, Legris Lake, etc.); (3) Syn- to post-tectonic, mafic to ultramafic intrusions (a.k.a. "Quetico-type"; Ca. 2680 to 2688 Ma) hosted by Quetico subprovince metasedimentary rocks (e.g. Samuels Lake, Kawene, Nym Lake, Chief Peter Lake, North Elbow Lake, etc.); and (4) Mafic intrusive rocks occurring within syn- to post-tectonic, diorite-monzodiorite-monzonite suites with sanukitoid affinity (ca. 2680 to 2685 Ma), within the Wabigoon, Quetico and Wawa subprovinces (cf Stern eta!. 1989) (e.g. Roaring River Complex; Entwine Lake). Disseminated to locally net-textured chalcopyrite, Fe-sulphides, pentlandite and magnetite typically characterize PGE-mineralized zones, which are commonly associated with intrusive contacts, polyphase intrusive breccias, as well as sheared and hydrothermally altered zones. Proterozoic Mesoproterozoic intrusive rocks associated with the Midcontinent Rift locally range in age from Ca. 1108 Ma (e.g. reversely polarized Coldwell alkaline complex; Logan diabase) to ages younger than the magnetic polarity reversal that occurred between 1105 and 1102 Ma (Davis and Green 1997). A tabulated synopsis is provided below: 91 �Intrusion I Lithology Mineralization Style Local Examples (Associated PGE-Miizeralized Areas) Disseminated sulphides and native metals in lherzolite, dunite, peridotite,_etc. Disseminated and blebby sulphides in medium- to coarsegrained, varied-textured (a.k.a._'taxitic')_gabbro Disseminated sulphides in Crspinel-bearing cumulate layers Disseminated, intergranular sulphides, fracture fillings Disseminated sulphides in medium- to coarse-grained Leckie Lake (Wolf Mountain); Hele Township; Eva-Kitto townships Normal Magnetic Polarity <(1005-1102 Ma): U Layered(?) picritic ultramafic intrusions Layered gabbro-anorthositic intrusions: Crystal Lake Gabbro Pine Point - Mount Mollie Gabbro Pigeon River and Arrow River diabase dykes Crystal Lake Gabbro (Great Lakes Nickel deposit) (Cr-spinel-bearing, anorthositic gabbro above Cu-Ni deposit) (Mount Mollie; Pine River) (Wallenius; Naomi Island; Jarvis Point) gabbro_and_diabase Reversed Magnetic Polarity (ca. 1108 Ma): Logan diabase sills, cone sheets Tholeiitic to alkaline complexes: CoIdwell alkaline complex Killala Lake alkaline complex I Sparse, disseminated sulphides Disseminated sulphides in medium- to coarse-grained, van-textured (a.k.a. taxitic) gabbro Disseminated sulphides in massive Fe-Ti-oxide cumulate layers (Numerous) Coldwell Two Duck Lake gabbro (Marathon deposit); Geordie Lake gabbro Eastern Border gabbro (Skipper Lake zone) Killala Lake Border gabbro (Sandspit Killala) (Unknown) The development of tectono-magmatic models for these various suites of intrusions is the focus of ongoing research as part of the Ontario Geological Survey's Operation Treasure Hunt. These new data will provide new insights into late Archean subprovince accretion in the Superior Province, as well as the development of the Midcontinent Rift. They will help to elucidate possible links between the age, geochemistry and setting of these intrusive rocks and PGE mineralization processes in order to generate new exploration targets. References Davis, D.W. and Green, J.C. 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution; Canadian Journal of Earth Sciences, V.34, p.476-488. Stern, R.A., Hanson, G.N. and Shirey, S.B. 1989. Petrogenesis of mantle-derived, LILE-enriched Archean monzodiorites and trachyandesites (sanukitoids) in southwestern Superior Province; Canadian Journal of Earth Sciences, v.26, p.1688-1712. 92 �PLATINUM GROUP ELEMENT EXPLORATION IN NORTHWESTERN ONTARIO SMYK, Mark C., S11IWART, Jennifer and O'BRIEN, Mark S. Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ONP7EÔS7 During the past two years there has been a marked increase in exploration for the platinum-group elements (PGE) In iiorthwestern Ontario. This interest has been driven by a dramatic rise in the prices of platinum and palladwm. Renewed PGE exploration in northwestern Ontario has also been significantly influenced by the sticcess of North American Palladium Ltd.'s Lac des Ties Mine which produced 95 116 ounces of -palladium at àcash cost of US$142 per ounce in 2000. Recent exploration efforts at Lac des lies have increased 'the measured and indicated resource to 145 600 000 tonnes at an average grade of 1.57 glt Pd, 0.17 g/t Pt, 0.12 g/t Au, 0.06% Cu and 0.05 % Ni. There were over 100 PGE exploration programs in northwestern Ontario in 2000, accounting for more than half of all mineral exploration in the region. These exploration efforts have focused on both Archean and Proterozoic mafic to ultramafic intrusions, leading to the discovery of many occurrences in areas andlor intrusions not previously known to host PGE mineralization. PGE occur in two main geological settings: (1) In Archean pre- to post-tectonic, mafic to ultramafic intrusions. Mineralization may be associated with: • Disseminated "sparse" sulphides in gabbro and ultramafic rocks (e.g. Lac des Ties complex and satellite intrusive complexes), and localized by: • Zones of hydrothermal alteration • Heterolithic, igneous breccia zones Contact zones and tectonic structures • • (Semi-) massive, sheared, copper-nickel sulphide deposits (e.g. Shebandowan and Thierry mines) in ultramafic rocks • Sheared ultramafic rocks with copper-nickel sulphides intruding banded iron formation (e.g. Trout Bay) • Chromitite within layered ultramafic complexes (e.g. Big Trout Lake, Chrome Lake?) (2) In Mesoproterozoic, Midcontinent Rift-related (ca. 1108-1102 Ma), mafic to ultramafic intrusions near Lake Superior and Lake Nipigon. Mineralization may be associated with: • Disseminated suiphides in coarse-grained to pegmatitic, van-textured ("taxitic") gabbro (e.g. Great Lakes Nickel; Marathon deposits) • Massive oxide (± sulphide) units in layered gabbro (e.g. Coidwell Eastern Gabbro (Ti-Fe-oxides); Great Lakes Nickel (Cr-spinel) • Disseminated sulphide ± native metals in olivine-rich, layered ultramafic rocks (e.g. Wolf Mountain) 93 �A NEW GRAVITY MAP OF WISCONSIN SNYDER, Stephen L., U.S. Geological Survey, MS 954 National Center, Reston, VA 20192, ssnyder@usgs.gov; ERVIN, C. Patrick, Dept. of Geology and Environmental Geosciences, Northern Illinois University, DeKalb, IL 60115, pervin@niu.edu; GEISTER, Daniel W., Dept. of Geology and Environmental Geosciences, Northern Illinois University, DeKalb, IL 60115, z011561@students.niu.edu and DANIELS, David L., U.S. Geological Survey, dave(usgs.gov. A new Bouguer anomaly gravity map of Wisconsin has been created from more than 37,000 gravity measurements collected between 1948 and 2000. North of latitude 44° N., more than 28,000 stations were compiled by C.P. Ervin and M.E. Thompson. These data include stations from the Wisconsin Geological and Natural History Survey (WGNHS), the National Geophysical Data Center (NGDC), the Defense Mapping Agency (DMA), the U.S. Geological Survey (USGS), and Northern Illinois University. An approximate station interval of 1 mile (1.6 km) was established where possible, but limited access in some areas necessitated a greater spacing. Prior to 1999, the data south of latitude 44° N. totaled approximately 3800 stations from NGDC and from G. Randy Keller (University of Texas-El Paso). The current USGS effort, begun in 1999, is directed at upgrading the gravity coverage of the state south of latitude 44° N. with a station density comparable to that north of 44° N. To date we have added measurements at more than 6800 stations to achieve a nominal spacing of one to two miles. These new data replace the older data from the NGDC, which had a nominal station spacing ranging from 3 to 10 miles. Nearly all of the data are tied to the Wisconsin First-order Gravity Base Station Network, which is in turn tied to the International Gravity Standardization Network-1971 (IGSN-71). The 37,000 measurements were then gridded at an interval of 500 m. The map will be presented at a scale of 1:500,000. The rationale for upgrading the gravity coverage has been to provide higher resolution data to assist in the interpretation of the basement geology of southern Wisconsin, much of which is hidden by glacial and Paleozoic cover. The gravity data, along with aeromagnetic data, give clues to the structural evolution of the Precambrian crust, making the map an excellent tool for USGS mineral resource studies. Some notable features shown on this map include 1) the Midcontinent gravity high, which has one of the steepest gravity gradients in the conterminous US, 2) the flanking gravity lows corresponding to Keweenawan sedimentary basins, 3) gravity highs over inferred mafic plutons in southeastern Wisconsin, 4) a low centered on the Precambrian Wolf River batholith, but much larger in area, 5) gravity lows associated with granitic intrusives just south of latitude 46° N., and 6) several northwest trending linear features of unknown origin. The accompanying figure shows a shaded relief image of the gravity map. 94 �92° -89 -91 880 -87° .5 .15 H 21 .— .55 — 27 .— 30 .— 33 — 35— 35 I 42 * 45 — -45 * 51 54 * 50 53 -55 72 -51 500 milhigals 50 50 0 klometres NA027/ LGC-90 95 �POST-RIFT EVOLUTION OF THE MIDCONTINENT RIFT SYSTEM: SOME NUMERICAL EXPERIMENTS Soofi, M. A., and King, S. D., Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907 The Midcontinent Rift system (MCR) is a major geological and geophysical feature of North America. This 1.1 b.y. old feature (Nicholson and Shirey, 1990; Klewin and Shirey, 1992) is believe to have evolved first through the tensile forces of a rift origin and then through the compressive forces from the intraplate collision between North America and Grenville Tectonic Zone (GTZ) (Van Schmus and Hinze, 1985). The support for interaction between the MCR and GTZ come from the geophysical studies which reveal thrust faults and folds along the length of the MCR (e.g., Zhu and Brown, 1986; Chandler et al., 1989; Mariano and Hinze, 1994; Allen et al., 1997). Also, the time of thrusting along the MCR is constrain to be 1060 Ma (Bornhorst et al., 1988; Cannon et al., 1993) which is comparable to 1100-1060 Ma (Easton, 1992) as the duration of collision in Grenville Tectonic Zone. A quantitative study of the late-stage deformation of the MCR under the Grenville tectonism has been performed by Soofi and King (1999). They showed that the distance between the MCR and Grenville Tectonic Zone is appropriate for the two geologic provinces to interact. We expand on their findings and investigate the contribution of various factors involved in the collision between North America and the Grenville terranes, including: the size of colliding terranes of Grenville Tectonic Zone; the location of collision along the eastern and southern boundaries of North America; and the angle of convergence between North America and the Grenville terranes. We use a 2D, viscous, finite element model that treats lithosphere as a thin sheet with stresses averaged over its thickness (for computational method see Houseman and England (1986); for program location see the web site http : / /www.earth.monash.edu.au/Research/Basil). The model boundaries coincide with pre-Grenville collision boundary of North America. In the model, the MCR is considered as a low-strength block. We consider the present shape of the MCR as the shape at the time of collision with the Grenville terranes. The models are run to represent 21 Ma of convergence at the rate of 50 mm/yr. To constrain the model results we use geophysically observed uplifts along the reverse faults of the MCR (e.g., Zhu and Brown, 1986; Mariano and Hinze, 1994; Allen et al., 1997). These uplifts are compared with crustal thickening along the MCR in the model. Based on our results we conclude that the 3 to 8 km variation in uplift along the thrust faults of the western arm of the MCR is the consequence of size of the colliding terranes and location of the collision. The model results also suggest that deformation along the eastern arm of the MCR was comparable to that along the western arm. The reason we do not observe such deformation in geophysical studies (e.g., Zhu and Brown, 1986) is, perhaps, due to later surficial and/or tectonic processes. We also conclude that collision with Crenville Tectonic Zone was active along both the eastern and southern boundaries of North America. This resulted in the non-linear shape of the western arm of the MCR and may also played a role in the formation of the Belle Plaine fault. We do not observe any significant difference in the model results for different angles of convergence between North America and Grenville terranes. Consideration of other constraints in addition to the uplift along the MCR may help to determine the dominant angle of convergence between North America and Grenville Tectonic Zone. 96 �REFERENCES CITED Allen,D.J., Hinze, W.J., Dickas, A.B., and Mudrey, M.G.Jr., 1997, tnterMedgeophysical modeling of the North American midcontineñt rift system: New thterpretations for western Lake Superior, northwestern Wisconsin, and eastern Minnesota: Geological Society of America Special Paper 312, p. 47-71. Bornhorst, T J, Paces, J B, Grant, N K, Obradich D, and Thiber, NkK 1988, Ae f native copper mineralization, Ke-weenaw Peninula, Michigan: conmi (eiogy, v 83, p. 41,19-625 Cannon, W.F., Petermn, Z.E., and Sims, P.K., 193, Crustal- sca1ethrtisting and origin of the Montreal river monocline - A 35 km thick cross-section of the midcontinent rift in northern Michigan and Wisconsin: Tectonics, v. 12, p. 728-744. Chandler, V.W., McSwiggen, P.L., Morey, G.B., Hinze, W.J,, and Anderson, R.R., 1989, Interpretation of seismic reflection, gravity, and magnetic data across middle Proterozoic mid-continent rift system, northwestern Wisconsin, eastern Minnesota, and central Iowa: American Association of Petroleum Geologists Bulletin, v. 73, p. 261-275. Easton, R.M., 1992, The Grenville Province and the Proterozoic history of central and southern Ontario, in Thurston, P.C. Williams, H.R., Sutcliffe, R.H., and Stott, G.M., eds., Geology of Ontario, Ontario Geological Survey, Special volume 4, Part 2, p. 715-904. Houseman, G., and England, P., 1986, Finite strain calculations of continental deformation 1. Method and General results for Convergent zone: Journal of Geophysical Research, v. 91, p. 3651-3663. Klewin, K.W., and Shirey, S.B., 1992, The igneous petrology and magmatic evolution of the midcontinent rift system: Tectonophysics, v. 213, p. 33-40. Mariano, J., and Hinze, W.J., 1994, Structural interpretation of the Midcontinent Rift in eastern Lake Superior from seismic reflection and potential-field studies: Canadian Journal of Earth Sciences, v. 31, p. 619-628. Nicholson, S.W., and Shirey, S.B., 1990, Midcontinent rift volcanism in the Lake Supe- rior region: Sr, Nd, and Pb isotopic evidence for a mantle plume origin: Journal of Geophysical Research, v. 95, p. 10851-10868. Soofi, M.A., and King, S.D., 1999. A modified beam analysis effect of lateral forces on lithospheric flexure and its implication for post-rift evolution of the Midcontinent Rift System, Tectonophysics, v. 306, p. 149-162. Van Schmus, W.R., and Hinze, W.J., 1985, The midcontinent rift system: Annual Reviews of Earth and Planetary Science, v. 13, p. 345-383. Zhu, T., and Brown, L.D., 1986, Consortium for continental reflection profiling Michigan surveys: Reprocessing and results: Journal of Geophysical Research, v. 91, p. 11477-11495. 97 �THE COOL EARLY EARTH: OXYGEN ISOTOPE EVIDENCE FOR CONTINENTAL CRUST AND OCEANS ON EARTH AT 4.4 Ga VALLEY, JW*, PECK, WH, KING, EM, Dept. of Geology + Geophysics, Univ. of Wisconsin-Madison; GRAHAM, CM, Dept. of Geology + Geophysics, Edinburgh Univ., Scotland; and WILDE, SA, School of Applied Geology, Curtin Univ., Bentley, Western Australia, * valley@geology.wisc.edu Zircons preserve the best record of U-Pb crystallization age and oxygen isotope ratios of igneous rocks. The I8O of non-metamict zircon is unaffected even by hydrothermal alteration and high-grade metamorphism. Ion microprobe analysis of detrital zircons from the —3 Ga Jack Hills metaconglomerate (Narryer Gneiss Terrane, Yilgarn Craton, Western Australia) yield UPb ages from 3.1 to 4.4 Ga (Fig. 1, SHRIMP II, ref 1) and I8O from 5 to 8 %c (Cameca 4f, ref 2). The 18O of these zircons averages 6.3, and is 1 per mil higher than that in equilibrium with the mantle and that of normal Archean granitic zircons (Fig. 2; 5.3±0.3 %o, 5.5±0.4 %, respectively; ref 3). The distribution of mantle-like vs. mildly elevated 18O values for magmas is constant from 2.7 to 4.4 Ga, and on 4 continents (Fig. 2). The age of 4.404 ±0.008 Ga from one 200 im zircon is >99% concordant and represents the oldest recognized terrestrial material. This crystal is zoned in ö'80 (5.0±0.7% vs. 7.4±0.7%o) and REEs (La=0.3 to 13.6 ppm), and contains inclusions of Si02. REE patterns are HREE enriched with positive Ce and negative Eu anomalies; calculated melts are LREE enriched. Taken together, these results suggest crystallization from a quartz-saturated granitic magma and thus the existence of continental crust, possibly in a setting like Iceland. The high 6180 portion of the crystal would be in equilibrium with a magma at 6'80(WR)= 8.5-9.5%. There is no known mantle reservoir with such high values. '8O(WR) values above 8.5 are typical of "S-type" granites that have melted or assimilated material that was altered by low temperature interaction with water at the surface of the Earth (i.e., weathering, diagenesis, low T hydrothermal alteration). Thus the high 18O value of the 4.4 Ga zircon suggests that surface temperatures were cool enough for liquid water suggesting that the early steam-rich atmosphere condensed to form oceans at that time. The evidence for liquid water and oceans at 4.4 Ga suggests a Cool Early Earth. This contrasts with the Hot Early Earth and global magma oceans envisioned at 4.5-4.3 Ga based on: an impact origin of the Moon (4.45-4.50 Ga), core formation, higher Hadean radioactive heat production, and intense early meteorite bombardment (4). The surface of the Earth cools quickly to form a crust by radiation, but a magma ocean caused by these processes would persist beneath the initially thin crust for up to 400 m.y. (5) and might erupt as massive flood basalts in response to major meteorite impacts, boiling surface waters. The thermal contrasts presented by these lines of evidence are minimized if the Moon and core formed earlier (-.4.5 Ga), if the Moon formed by a process not involving a Mars-size impactor, or if the early meteorite bombardment was less intense or irregular in timing. It is possible that periods of Cool vs. Hot Early Earth alternated, with 98 �boiling of early oceans after major impact events followed by periods of cooler surface conditions. If life evolved in these seas, multiple extinctions before 3.9 Ga are suggested. 1.20 Detrital Zircon from Jack Hills Metaconglomerate W74 1.10 1 .00 0.90 0.80 206Pb 238 U 0.70 0.60 0.50 0.40 0.30 0.20 0.10. 207 Pb/ 235U -I- 7 A + 6 dD A AAA £ V A 5.3±0.3%c A V A a mantle zircon A A £ V A 0 V A Jack Hills Australia V Barberton + Superior Province 4, Scotland ® Manitouwadge Miscellaneous 2 2600 , 2800 3000 3200 3400 3600 3800 4000 4200 4400 U-Pb Zircon age (Ma) (1) SA Wilde, JW Valley, WFI Peck and CM Graham (2001) Evidence from Detrital Zircons for the Existence of Continental Crust and Oceans on the Earth 4.4 Gyr Ago. Nature. 409: 175-178. (2) WH Peck, JW Valley, SA Wilde, and CM Graham (2001) Oxygen Isotope Ratios and Rare Earth elements in 3.3 to 4.4 Ga zircons: Ion Microprobe Evidence for Early Archean high ö'80 Continental Crust. Geochim Cosmochim Acta, in press (3) WH Peck, EM King, JW Valley (2000) An Oxygen Isotope Perspective on Precambrian Crustal Growth and Maturation. Geology 28: 363-366; EM King, JW Valley, DW Davis, GR Edwards (1998) Oxygen isotope ratios of Archean plutonic zircons from granite-greenstone belts of the Superior Province. Precam, Res 92: 365-387. (4) HN Pollack (1997) Thermal Characteristics of the Archaean, in: de Wit and Ashwal (eds), Greenstone Belts, 223-232 (5) Y Abe (1993) Physical State of the Very Early Earth. Lithos 30:223-235 99 �NW U.PB AGES FROM MINNESOTA, MICHIGAN, AND WISCONSIN: IMPLICATIONS FOR LATE PALEOPROTEROZOIC CRUSTAL STABILIZATION. Van Schmus, W.R. (rvschmus@ku.edu) and MacNeil!, L.C., both at Dept. of Geology, Univ. of Kansas, Lawrence, KS 66045, HoIm, D.K., Dept. of Geology, Kent State University, Kent, OH 44242; and Boerboom, T. J., Minnesota Geological Survey, 2642 University Avenue, St. Paul, MN 55114. As part of a comprehensive study of the East-Central Minnesota Batholith (ECMB: Boerboom and HoIm, 2000; Van Schmus et al., 2000), we have also determined the U-Pb ages of several rock units spatially and (or) temporally related to it. The samples fall into several categories (Table 1): (a) the McGrath Gneiss to the NE of the ECMB in Minnesota; (b) Penokean basement east of the ECMB in Minnesota; (c) post-Penokean, preECMB rocks in Michigan and Minnesota, and (d) units coeval with or slightly younger than the ECMB in Minnesota and Wisconsin. Table 1. New U-Pb Results from Minnesota, Michigan, and Wisconsin. Sample Description MNOO-07 McGrath Gneiss, 2 mi. west of McGrath, MN 2550 ± MNOO-0 1 Bradbury Creek granodiorite, 4 mi. south of Onamia, MN Tonalitic gn., Hillman migmatite?, 3 mi. south of Onamia, MN 1877±15 Ma MNOO-02 MN99-09 MNOO-03 MNOO-04 AGR-1 Late tonalite in Hiliman migmatite, 3 mi. SE of Lastrup, MN Late tonalite in Hilhnan migmatite, 9 mi. west of Onamia, MN Late tonalite in Hilhnan migmatite, 13 mi. west of Onamia, MN Humboldt granite, Humboldt, MI (Hoim et aL, 2001) Multiple East Central Minnesota Batholith Van Schmus et al. (2000); Boerboom and HoIm (2000) Includes EC-2, -4, -5, -15, and -25 of Jima and Chandler (1997) 1787 to 1772 Ma PF-99 VS73-37 Two-mica granite, Park Falls, Wisconsin Radisson Granite, Radisson, Wisconsin 1781 ± 14 Ma 1776 ± 08 Ma V573-08 VS77-251 VS79-85 Amberg granite, 1 mi. north of Amberg, WI Lugerville granite, 1.5 mi. SW Lugerville, WI Lugerville granite, Rock Carry Rapids, 1 mi. E Lugerville, WI 1754±llMa VS74-13 VS73-02 Observatory Hill rhyolite, 6 mi. south of Montello, WI Montello granite, Montello, WI Age 14 Ma 1853 ± 10 Ma 1798 ± 03 Ma (composite of 3 samples) 1805 ± 07 Ma all 1749 ± 04 Ma (pooled; all near) concordia) 1759 ± 02 Ma 1746 ± 03 Ma The oldest sample is from the McGrath Gneiss and confirms the late Archean age of that unit. Two samples yielded Penokean ages. The Bradbury Creek granodiorite was dated by Goldich and Fischer (1986) at 1869 ± 5 Ma, and our date of 1877 ± 15 Ma is fully consistent with this. A tonalitic gneiss (MNOO-02) about one mile to the north-northwest and mapped as Hillman migmatite yields a slightly younger, but statistically indistinct, age of 1853 ± 10 Ma. We are investigating the possibility that the Bradbury Creek unit is more extensive than originally defmed from aeromagnetic data. In any case, normal Penokean ages occur in eastern Minnesota, and Penokean basement forms the host crust for the ECMB. Several samples yielded U-Pb ages close to 1800 Ma. The main units in Minnesota are late, undeformed (or less deformed) tonalite phases within the Hillman migmatite complex. Samples from three separate localities yield a mean age of 1798 ± 3 Ma for the tonalite. For one of these samples (MNOO-03), the tonalite dated intrudes deformed tonalitic gneiss similar to that of MNOO-02. At this time it appears that the protolith for the Hillman migmatite may be Penokean, but that it was extensively injected by younger tonalite about 1800 Ma. 100 �Further detailed sampling and geochronology will need to be done to test this option. In Upper Michigan an alkali-feldspar granite near Humboldt yielded an upper intercept age of 1805 ± 7 Ma (HoIm et al., 2001). These rocks define a distinct post-Penokean, pre-ECMB phase of magmatism in or near the southern part of the Superior Province. As reported previously (Van Schmus et al., 2000), the East-Central Minnesota Batholith was emplaced within a relatively short span of time 1787 to 1772 Ma. We analysed zircons from several drill core samples to the east (ursa and Chandler, 1997), and the results (Table 1: EC-2, EC-4, EC-5, EC-1 5, EC-25) extend the known area of the ECMB. In order to determine whether units of the so-called "1760 Ma" suite of rocks in Wisconsin are coeval with the ECMB, we reanalysed several samples from the senior author's collection, plus one new sample (PF-99) using single-grain analyses and more precise techniques. The westernmost samples (PF-99 and VS7337) yield ages indistinguishable from those of the ECMB, suggesting that magmatism of that episode extended into northwestern Wisconsin. In contrast, samples of the "1760 Ma suite" farther east (Amberg and Lugerville granite) or farther south (Montello granite, Observatory Hill rhyolite) are distinctly younger, with ages of 1746 to 1759 Ma. These ages are similar to Ar-Ar ages reported for the ECMB and indicate that the last major magmatic pulse occurred about 1750 Ma. Our new data now suggest three main post-Penokean pulses of magmatism about 25 m.y. apart in the southern Lake Superior region: ca. 1800, 1775, and 1750 Ma. The origin of the thermal energy in each case is presently unknown. These ages fall within the time span of magmatism in the Yavapai province-northern Central Plains orogen (inner accretionary belt; Van Schmus et al., 1993) and could correlate with pulses of northward directed subduction associated with southward growth of the continent. Interestingly, the two older and deeper-seated igneous bodies currently reside in Paleoproterozoic crust unaffected by younger Mazatzal deformation and reheating (Holm Ct al., 1998). It appears that localization of the earlier pulses in the west (Minnesota) and north (northwesternmost Wisconsin) may have contributed to the overall greater exhumation of these areas (compared to most of Wisconsin) and that this in turn may have dramatically strengthened the crust in those regions. We note that crustal remelting and thinning after orogenesis can both contribute to an overall stronger continental lithosphere. One fmal note: reference to a "1760 Ma" suite in Wisconsin should probably now be discontinued, since those rocks formed in two pulses at about 1775 and 1750 Ma. Boerboom, TJ., and Holm, D.K., 2000, Paleoproterozoic intrusive igneous rocks of southeastern Stearns County, central Minnesota. Minn. Geol. Survey, Rept. mv. 56, 36p + 1 map. Goldich, S.S., and Fischer, L.B., 1986, Air-abrasion experiments in U-Pb dating of zircon. Chemical Geology, v. 58, p. 195-215. Holm, D., Schneider, D., and Coath, C., 1998, Age and deformation of Early Proterozoic quartzites in the southern Lake Superior region: Implications for extent of foreland deformation during final assembly of Laurentia: Geology, v. 26, p. 907-910. Holm, D., Van Schmus, R., Boerboom, T., and Jirsa, M., 1999, Role of post-Penokean granite genesis in crustal stabilization in the Lake Superior region, north-central United States. Geol. Soc. America Abstracts with Programs, v. 31, no. 7, p. A-259. Hoim, D.K., Van Schmus, W.R., and MacNeill, L.C., 2001, Age of the Humboldt granite, northern Michigan: Implications for the origin of the Republic metamorphic node. 47th Ann. Inst. on L. Superior Geology, Madison, Wisconsin, May (this volume). Jirsa, M.A., and Chandler, V.W., 1997, Scientific test drilling and mapping in east-central Minnesota, 19941995: summary of lithologic results. Minn. Geol. Survey, Inf. Circular 42, lO5p. Schweitzer, D.J., Schneider, D.A., Boerboom, T.J., HoIm, D.K., and Van Schmus, W.R., 2000, Assessing the extent of Early Proterozoic Penokean versus 1770-1760 Ma metamorphism in east-central Minnesota. Abstracts, 46th Ann. Inst. on L. Superior Geology, Lakehead University, Thunder Bay, Ontario, May Van Schmus, W.R., Bickford, M.E., and Condie, K.C., 1993, Early Proterozoic crustal evolution, in Reed, J.C., Jr., Bickford, M.E., Houston, R.S., Link, P.K., Rankin, D.W., Sims, P.K., and Van Schmus, W.R., editors, Precambrian: Conterminous U.S. Geological Society of America, The Geology of North America, v. C-2, p.270-281. Van Schmus, W.R., MacNeill, L. C., Holm, D.K., Boerboom, T.J. and Jima, M.A., 2000, The 1787-1772 Eastcentral Minnesota batholith: precursor to crustal stabilization in the L. Superior region. Abstracts, 46th Ann. Inst. on L. Superior Geology, Lakehead University, Thunder Bay, Ontario, May 101 �A STUDY OF WELL CONSTRUCTION FOR ARSENIC CONTAMINATION IN NORTHEAST WISCONSIN ANNETTE E. WEISSBACH, Wisconsin Department of Natural Resources, DNRNortheast Region, Waste Management, Remediation, and Redevelopment, 1125 N Military Aye, P0 Box 10448, Green Bay WI 54307-0448 ELIZABETH M. HEINEN, Wisconsin Department of Natural Resources, DNRManitowoc Field Station, 2220 E CTH V, Mishicot WI 54228, and KELD B. LAURIDSEN, Wisconsin Department of Natural Resources, DNR-Northeast Region, Waste Management, Remediation, and Redevelopment, 1125 N Military Aye, P0 Box 10448, Green Bay WI 54307-0448 Arsenic has been detected in approximately one third of the private drinking water wells in the Fox River valley of Northeast Wisconsin. Concentrations detected are some of the highest found naturally occurring in the world. Research has indicated that presently 3.5% of the wells in Outagamie and Winnebago counties exceed the current drinking water standard of 50 ppb, whereas close to a quarter of the wells may exceed the proposed standard of 10 ppb. Department of Natural Resources study results indicate the geochemical phenomenon causing the elevated levels of arsenic in groundwater of this region is associated with oxidation of a sulfide-mineralized zone located at the top of the deep sandstone aquifer system. A regional decline in water levels may have exposed this sulfide rich zone to oxidation from air within the open boreholes of water wells extending through this zone. This oxidation process can initiate a chemical reaction similar to acid mine drainage. Recommendations have been developed for constructing wells within a delineated advisory area. This guidance recommends constructing wells with well casing pipe to extend through the sulfide rich zone. This study compared arsenic concentrations of wells constructed according to the guidance, with wells constructed to traditional construction standards. Additionally, this study examined data to determine if it was better to replace a contaminated well with a new one, or to reconstruct the existing well with a liner. The results of this study indicate that the guidance gives adequate protection for wells constructed in the arsenic advisory area and that liners are successful at reducing arsenic concentrations, although not as successful eliminating arsenic contamination. 102 �holes from which the samples were collected, and so may be from near the base of the Quaternary section. Low ppm Th I wt.% K ratios, which is the case for most of these high K tills, in general are indicators of potassic alteration. Because there is no apparent process that would create this type of alteration pattern following till deposition, it is proposed that tills with high K contents may have a local bedrock source area characterized by potassic alteration. This alteration may indicate pre-Marathon Foration paleoweathering of bedrock or may indicate local or regional hydrothermal alterat4on. Because potassic alteration can indicate precious metal and sulfide mineralization, high-K tills in the lower part of the Marathon Formation, characterized by some high Zn values, may aid in mineral exploration. Provenance Discrimination Samples of Quatemary deposits collected from closely spaced rotasonic boreholes in the area of the Bend massive sulfide deposit in Taylor County proved to have unique geochemical characteristics that could be xpiained by differences in source areas and sedimentological processes (Woodruff and others, 2000). The classification of till samples in the field was successfully duplicated by discriminate analysis of the geochemical data set using the statistical software package 5+. Based on geochemistry and grain size, samples from the Bend area were statistically grouped into either Copper and a Lake Falls or Marathon Formations, easily distinguishing a carbonate provenance Superior provenance. The same type of discriminate analysis was run on the regional data set using identical element and grain size input. When compared to field classifications, results gave an error of 17% (correctly predicting 38 of 46 samples) for a carbonate provenance and an error of 5% (correctly predicting 51 of 54 samples) for a Lake Superior provenance. However, the utility of this statistical approach for the regional Quaternary data set is questionable. Geochemical fingerprints of correlated Mg Cu and Ti for and Ca values for samples from the Marathon Formation and correlated samples from the Copper Falls Formation identified in the Bend section are less distinct compared to the in the regional data set. Several factors inherent to the regional data, as These Bend area data, may complicate correlations and discrimination of provenance. factors include differences in the method of sample collection (rotasonic core vs. auger glaciofluivial or sampling), which could influence sample identification (e.g., till vs. glaciogenic debris samples), and the existence of geochemical outliers, such as the high larger geographic area K tills, in the regional data set that may reflect sampling from a with many diverse bedrock sources of material. References Wisconsin Attig, J. W., 1993, Pleistocene geology of Taylor County, Wisconsin: Geological and Natural History Survey Bulletin 90, 35 p. Woodruff, L.G., Attig, J.W., and Cannon, W.F., 2000, Geochemical impacts of an undisturbed mineral deposit — results from the Bend deposit, Wisconsin [abs]: Institute on Lake Superior Geology Proceedings, v. 46, p. 72-73. 105 � https://digitalcollections.lakeheadu.ca/files/original/0274839d2a0a8f7d05473aabb3bc01b8.pdf c44648cb3e8f8088276e886956c5658f PDF Text Text ���CONTENTS CONTENTS Proceedings Volume 47 Part 22 -— Field Trips Interval: New TripI: Tripi: Sedimentologic, Tectonic and Metamorphic Metamorphic History of the Baraboo Interval: 11 Evidence from Investigations in the Baraboo Range,Wisconsin Investigations Locality 1: Baxter Hollow 10 10 Locality 2: Hydrothennal Veins, Hwy 12 14 14 Hydrothermal 15 Locality 3: Quartzite and Metapelite, Hwy 12 15 Locality 4: Abelman's Gorge 17 17 Leaders: L. Gordon Medaris, University of Wisconsin -- Madison Madison Robert H. Dott, Jf., Madison Jr., University of Wisconsin -- Madison Trip Trip 2: Geology, Ore Deposits, and Cultural History ofthe of the Upper Mississippi Valley Zinc-Lead District Stop 1: Platteville Mining Museum and Rollo Jamison Museum Stop 2: Potosi Hill - Ordovician Sinnipee Group Stop 3: New Diggings Lead Digs Stop 4: Shullsburg Mine Site - Metallic Mine Reclamation Reclamation Stop 5: Pendarvis State Historical Site (Mineral Point) Historical 23 30 30 36 36 39 Leaders: M.G. M.G. Mudrey, Mudrey, Jf., Jr., Wisconsin Geological and Natural History Survey Thomas C. Hunt, University of Wisconsin - Platteville of the Baraboo and Waterloo Quartzites Trip Trip 3: Economic Geology Geology of Quartzites of Southern Southern Wisconsin Stop Stop 1: Michels Michels Materials Waterloo Quarry Stop 2: The Kraemer Co. Williams Quarry Stop Stop 3: 1,760 1,760 MaRhyolite Ma Rhyolite Stop Stop 4: Milestone Materials Jesse Pit and Quarry Stop Stop 5: Milestone Materials Fox Ridge Asphalt Plant and Sales Yard 6: Martin Marietta Aggregates Aggregates Rock Springs Quarry Stop 6: Stop 7: Kraemer Company LaRue Quarry Leaders: Leaders: Brown, Wisconsin Geological and Natural History HistOlY Survey Bruce A. Brown, Survey University of Wisconsin - Whitewater Frank R. Luther, University Whitewater Susan M. M. Courter, Michels Materials Susan Materials James W. Schmitt, D.L. Gasser Construction Construction Company Jennifer Lien, The Kraemer Company 111 43 46 47 49 50 50 51 52 �This page intentionally left blank �Field Trip 11 Sedimentologic, Tectonic and Metamorphic History of the Baraboo Interval: Interval: New Evidence from Investigations in the Baraboo Range, Range, Wisconsin Wisconsin by H. Dott, Dort, Jr. Ir. L. Gordon Medaris, Ir. Jr. and Robert H. Department of Geology and Geophysics Geophysics of Wisconsin Wisconsin -- Madison Madison University University of Devil's Lake State Park View to the west along the East Bluff, Devil's State Park (foreground), Devil's Lake (background) Baraboo Quartzite (foreground), (background) �This page intentionally left blank �FOREWORD Baraboo Range began began in 1852 and and culminated in in 1970 with with Geological investigations of the Baraboo Dott. publication of the monograph, "Geology of the Baraboo District, Wisconsin", by Dalziel and Dott. Despite its publication 30 years ago, this monograph (with accompanying accompanying detailed geological map) Despite remains today the most comprehensive Baraboo Range. comprehensive treatise on the Baraboo In 1996 a paleosol at the base of the Baraboo Baraboo Quartzite Quartzite was discovered in a drill core taken in in Baxter Hollow (Medaris et al., 1996). Recognition of ofthe paleosol, which was developed at the expense the developed expense of granite underlying the quartzite, resolved any remaining question about the relative ages of the Baraboo Quartzite and Baxter Hollow Granite. This discovery subsequently inspired aa new look at the Baraboo techniques that that were unavailable unavailable 30 years ago, and led led to Baraboo Range, using a variety of analytical techniques depositional age of the quartzite, paleoclimatic environment, elaboration of a number of topics, including depositional data substantiate substantiate the sedimentary geochemistry, and conditions and age of metamorphism. The new data Dott and provide additional insight into into the processes conclusions originally drawn by Dalziel and Don discovery is is responsible for producing the Baraboo Range. An especially exciting and unexpected unexpected new discovery the occurrence of 1,460 Ma hydrothermal activity along the base base of the Baraboo Quartzite, presumably driven by heat from Wolf River-type magmatism. Proterozoic evolution of of the Baraboo This field guide provides an updated overview overview of the Proterozoic and describes several key outcrops, from which Range, based on the results of our recent investigations, investigations, many of the new data were obtained. INTRODUCTION Red, supermature quartzites, most notably the the Baraboo, Barron, and Sioux Quartzites, Quartzites, have long long been recognized as distinctive and important Precambrian Precambrian features in the southern Lake Superior Superior region. region. The physical and chemical characteristics of the quartzites signify deposition in a stable cratonic setting under conditions of intense chemical weathering in the presence of significant free oxygen in the post-1,750 1997; FloIm Holm et a!., al., atmosphere. These quartzites are now known to be post-I, 750 Ma in age (Dott et al., 1997; ~ 1,630 Ma (Romano eta!., et al., 2000), and locally intruded 1998), to have been folded folded and metamorphosed at 1,630 I,450 Ma (Dott and Dalziel, 1972). by granitic rocks at ~—l,450 1972). The term, Baraboo interval, was introduced by Dott DoU (1983) for this distinctive sequence of sedimentation, deformation, and metamorphism in the time span of 1,450 to 1,750 Ma. Our recent investigations not only validate the concept ofthe of the Baraboo interval, but also provide greater insight into the disparate geological events that shaped shaped the southern Lake Superior region in mid-Proterozoic time. REGIONAL SETTING Baraboo interval sedimentary rocks are widely distributed in the southern Lake Superior region metamorphic rocks. The (Fig. 1), where they lie nonconformably on 1,750 Ma and older igneous and metamorphic the general southerly direction paleocurrents, the increase in sediment thickness from north to south. south, direction of paleocurrents, suggest that chemical maturity of the sedimentary rocks, and the occurrence of mature paleosols suggest deposition occurred occurred on a stable, passive continental margin on the southern edge of a Proto-North American craton (Dott, (Dort, 1983). All of the quartzites in Figure 1, 1, except for the Waterloo, have yielded detrital zircon grains with V-Pb ages in the range, 1,712 to 1,778 et aL, 1997; Holm et al., U-Pb 1,778 Ma (Dott (Dort eta!., 1998; Van Wyck, 1995), and these data, combined with the inferred time offolding of folding and deformation at —1,630 Ma, established from 4°ArP9Ar 40Ar/39Ar cooling ages of basement amphibole and mica (Romano ~1,630 (Romano et al., (Dort and Dalziel, Dalziel, 1972; Van Schmus et al., 1975), 2000) and Rb-Sr resetting of granite and rhyolite (Dott eta!., 3 3 �constrain deposition to between between 1,710 and 1,630 Ma. Subsequently, constrain deposition of of the the Baraboo Baraboo interval interval sediments sediments to 1,710 and 1,630 Ma. Subsequently, the easternmost quartzites at McCaslin and Waterloo were intruded by granitic rocks of Wolf Wolf River River age age the easternmost quartzites at McCaslin and Waterloo were intruded by granitic rocks of Dalziel, 1972; 1972; Van Wyck, Wyck, 1995). 1995). (Dott and Daiziel, 200 km NO (, ~-S~D~--~ MN Barron (300 m) • • 1630Ma McCaslin \ A ,. Flambeau ­ ~..t f~-Water - - ­ '~ to ­ B"abOO(~ml ~I ~_MI NE , )~ IA I IN Figure Figure 1. 1. Distribution Distribution of of Baraboo Baraboo and and correlative correlative quartzites quartzites in in the the southern southern Lake Lake Superior Superior region, with meters, and region, with average average paleocurrent paleocurrent directions, directions, thicknesses thicknesses in in meters, and paleosol paleosol localities localities quartzites have (*). Heavy (*). Heavyline lineisisthe theinferred inferred1,630 1,630Ma Matectonic tectonic front, front, south south of of which which quartzites have been folded (Hoim been (Holm et al., aI., 1998) 1998) IGNEOUS IGNEOUS BASEMENT BASEMENT OF OF THE THEBARABOO BARABOO RANGE RANGE The Baraboo Baraboo Quartzite Quartzite is diorite near the town town of of Denzer, Denzer, by The is underlain underlain by by diorite near the by granite granite in in Baxter Baxter both limbs of the syncline in its Hollow, and and by rhyolitic lavas and pyroclastic rocks beneath both limbs of the syncline in its eastern eastern part part Hollow, granite and rhyolite are (Daiziel and Dott, 1970). U-Pb zircon ages of the Baxter Hollow (Dalziel and Dott, 1970). U-Pb zircon ages of the Baxter Hollow granite and rhyolite are indistinguishable, and indistinguishable, and taken taken together, together, yield yield an an age age of of1,749±12 1,749±12 Ma Ma (Van (Van Wyck, Wyck, 1995). 1995). Chronologically Chronologically subalkalic granite and rhyolite rhyolite suite suite of of and petrologically, petrologically, the the subalkalic granite and is correlative correlative with with the and the Baraboo Baraboo basement basement is Fox River River Valley Valley igneous igneous suite suite contains the Fox Fox River River Valley Valley (Smith, (Smith, 1978; Anderson etet al.. a!.. 1980). 1980). The The Fox the 1978; Anderson contains analyses demonstrate that both metaluminous and peraluminous types, and recent chemical both metaluminous and peraluminous types, and recent chemical analyses demonstrate that Denzer Denzer diorite is peraluminous. peraluminous. and and rhvolite rhyolite is is both both peraluminous peraluminous and and diorite is is metaluminous, metaluminous, Baxter Baxter Hollow Hollow granite granite is the Baxter Hollow metaluminous (Fig. (Fig. 2A). 2A). Among Ma silicic silicic lithologies. lithologies. the Baxter Hollow granite granite is is the the metaluminous Amongall allthe the1,750 1,750 Ma Fe-number and least differentiated differentiated in as measured by fe-number in terms terms of of major major elements, elements, as measured by and Ca-number Ca-number (Fig. (fig. 2B). 2B). least of Figure 2. containing 625-700 The Denzer diorite is metaluminous and plots outside the scales The Denzer diorite is metaluminous and plots outside the scales of Figure 2. containing 625-700 ppm ppm Sr. Sr, 0.62-0.68. of 0.70-0.82, Fe-number of 0.46-0.48. and Ca-number of an A1203/(K20+Na20+CaO) AbO)(K20+Na20+CaO) value of 0.70-0.82, fe-number of0.46-0.-l8. and Ca-number of 0.62-0.68. 44 �• 1000 1000 G Baxter Hollow granite rhyolite booBara R granite Hollow Baxter G R Baraboo rhyolite GG C C G cC G G G E P P 100 100 R P 0. 0 .3 R E P R en P C/) R R R R R P granites peraluminous ~~ P P '--- peraluminous granites A M1 MJi'l.MM;P ~ ~ metaluminous granites A graniteS metaluminous PP I"" I"" I"" j_.IPH 1.0 1.1 10 0.9 1.0 I 1.2 I"" 1.3 I",.;"" 10 0.9 1.1 1.2 1.3 1.0 1.0 * + 0.9 + R + C G ~ ".).. R 0 ~~, G a) 0.7 ~...,,~. <$)~ G G a) 0.6 0.61 G 0 E rhyolite boo Bara R granite Ho/low Baxter G B G Baxter Hollow granite B R Baraboo rhyolite t! , o ! , I, ,! I I !! I!!!, I , ! ! ! I, ! I I I CaO/(CaO+Na20) molecular 0.3 0.2 0.1 0.5 0.1 0.2 03 molecular CaO/(CaO+Na20) ! !! !,!! 0.4 ('3 0 " G Li:::J u E G G ~ 07 ~ 0 G ~ G 0+ U- "'' 'oj>. G 0.8 0.8 R 0 ~ * ~ ++ R +,. 0OJ u. R + ++ * + OJ u. metaluminous granites R 6" Cl :2: + 0 granifes meta/uminous J. / + / 0.4 0.5 0 impublished) Medaris, 1980; al., et Anderson 1978; Smith, from (data suite Valley River Fox the of granites (F) peraluminous and (M) metaluminous with (R) rhyolite Baraboo and (G) granite Hollow Baxter of comparison Chemical 2. Figure Figure 2. Chemical comparison of Baxter Hollow granite (G) and Baraboo rhyolite (R) with metaluminous (M) and peraluminous (P) granites of the Fox River Valley suite (data from Smith, 1978; Anderson et aI., 1980; Medaris, unpublished) quartz. q, saussurite; and albite mostly plagioclase, p, hornblende; h, biotite; after chlorite c, biotite; b, feldspar; alkali a, Abbreviations: polarizers). crossed matrix, aphanitic m microphenocrists (plagioclase rhyolite Baraboo and light), polarized plane (micrographic, granite Hollow Baxter light), polarized plane granular, (subhedral diorite Denzer in textures typical of Photomicrographs 3. Figure Figure 3. Photomicrographs of typical textures in Denzer diorite (subhedral granular, plane polarized light), Baxter Hollow granite (micrographic, plane polarized light), and Baraboo rhyolite (plagioclase microphenocrysts in aphanitic matrix, crossed polarizers). Abbreviations: a, alkali feldspar; b, biotite; c, chlorite after biotite; h, hornblende; p, plagioclase, mostly albite and saussurite; q, quartz. 5). (Fig. cummingtonite and actinolite intergrown and chlorite by replaced partly is hornblende diorite. Denzer In 4). (Fig. albite and microcline end-member near of mixture fine-grained extremely an into exsolves feldspar alkali intermediate and (saussurite), epidote fine-grained and albite to transformed is plagioclase chlorite, by replaced is biotite Typically, minerals. facies greenschist of variety a by replaced completely to partly being minerals igneous with extensive, is lization recrystal- although 3), (Fig. basement Baraboo the of units all in preserved well are textures Igneous Igneous textures are well preserved in all units of the Baraboo basement (Fig. 3), although recrystal­ lization is extensive, with igneous minerals being partly to completely replaced by a variety of greenschist facies minerals. Typically, biotite is replaced by chlorite, plagioclase is transformed to albite and fine-grained epidote (saussurite), and intermediate alkali feldspar exsolves into an extremely fine-grained mixture of near end-member microcline and albite (Fig. 4). In Denzer diorite, hornblende is partly replaced by chlorite and intergrown actinolite and cummingtonite (Fig. 5). 5 5 �Figure image, showing Figure 4. 4. Back-scattered electron image, partial of relict igneous alkali feldspar, partial replacement of af (Or 54-70) 54-70) by by K-feldspar, K-feldspar, k (Or 98.5) and albite, af(Or ab p, epidote epidote and and albite albite after plagioclase; ab (Ab (Ab 96.4). 96.4). p, q, q, quartz. of amphibole in Denzer Denzer Figure 5. 5. Ca Ka X-ray map of diorite. Relict igneous hornblende, hb, partly partly chI, and an intergrowth of replaced by chlorite, chl. actinolite, act, and cummingtonite, cum. cum. BARABOO QUARTZITE QUARTZITE BARABOO The 1,500 m thick and is overlain conformably by by the black black Seeley Seeley Slate Slate The Baraboo Quartzite is 1,500 (100 (100 m), m), Freedom Freedom Dolomite Dolomite with with iron formation formation (300 m), Dake Quartzite (65 m), and Rowley Creek Slate The Baraboo Baraboo Quartzite Quartzite consists consists of of 85-90% 85-90% of of quartz quartz arenite and subordinate subordinate quartz wacke, wacke, Slate (45m). (45m). The which are characterized by prominent cross bedding bedding (Fig. (Fig. 6) 6) and and ripple ripple marks, marks, 5-10% 5-10% of ofconglomerate, conglomerate, and Although all of of these rock types have experienced low and 5-10% 5-10% of of siltstone siltstone (Fig. (Fig. 7) 7) and and pelite pelite (Fig. (Fig. 8). 8). Although grade 9) and grade metamorphism, metamorphism, the the original original clastic elastic textures are remarkably well-preserved in quartzite (Fig. 9) Figure 6. 6. Typical Typical quartzite quartzite with with prominent prominent Figure cross bedding. metasiltstone with with refracted refracted Figure 7. Typical metasiltstone Coin isis 2.5 2.5 cm in in cleavage, Field cleavage, Field Locality Locality 2. 2. Coin diameter. 6 6 �fL. Figure 9. Photomicrograph of typical quartzite with relict clastic texture (partly crossed polarizers). polarizers). crossed (partly texture elastic relict with quartzite typical of Photomicrograph 9. Figure __j cleavage. crenulation and folds chevron with Larue, near Metapelite 8. Figure Figure 8. Metapelite near Larue, with chevron folds and crenulation cleavage. polarizers). crossed (partly texture elastic relict with metasiltstone of Photomicrograph 10. Figure Figure 10. Photomicrograph of metasiltstone with relict clastic texture (partly crossed polarizers). intense experienced that region source a implies sediments interval Baraboo the of maturity chemical ages average to compared and shale average to normalized extreme The 11). (Fig. different of shale are compositions rock the when apparent readily (Al:O±K2O+Na2O+CaO) is maturity chemical of degree remarkable The Al203/ 100*molar = CIA radius. ionic decreasing 98.6. to 96.8 from ranging high, exceptionally of order in arranged Elements ages. the over shale average to compared and 1985) McLeiman, and (Taylor is Alteration of Index Chemical The MnO. and MgO, CaO, Na20, K20, of concentrations shale average to normalized rocks, metasedimentary interval Baraboo fine-grained of Compositions 11. Figure low extremely with H20, and Ti02, Fe203, A1203, Si Al Fe Mg Ca Na K Si02, of entirely almost consist chemically 0.001 quartzites Barron and Sioux, Baraboo, the Cenozoic & from metapelite and Metasiltstone minerals. Mesozoic heavy among apatite and rutile, magnetite, 0.01 Paleozoic zircon, of predominance the and pyrophyllite, x of abundance the feldspar, detrital of absence Proterozoic near the from inferred been long has Quartzite x 0.1 Archean Baraboo the of maturity chemical The -9-Sioux metasiltstone. and quartzite in domains interstitial in and 8), (Fig. cleavage crenulation Barron of development by accompanied is it where Baraboo layers, metapelite in only occurred has lization recrystal- Complete 10). (Fig. metasiltstone metasiltstone (Fig. 10). Complete recrystal­ 10 F r - - - - - - - - - - - - - - - - - - _ lization has occurred only in metapelite layers, Baraboo -.­ CIA 50.4 - 65.4 where it is accompanied by development of Barron crenulation cleavage (Fig. 8), and in interstitial .. < SIOUX domains in quartzite and metasiltstone. ".ii x -; The chemical maturity of the Baraboo ~ Archean f 0< Quartzite has long been inferred from the near ~ 0.1f~ ProterozOic absence of detrital feldspar, the abundance of x ii PaleOZOIC pyrophyllite, and the predominance of zircon, E CIA 96.8 - 98.6 magnetite, rutile, and apatite among heavy "' MeSOZOIc & CenozoIc minerals. Metasiltstone and metapelite from the Baraboo, Sioux, and Barron quartzites 0.001 I~-----'---~--~-----'---~--'---' K Na Ca Mg Fe AI Si chemically consist almost entirely of SiOl, Ah03, Fe 203, Ti0 2, and H 20, with extremely low Figure I 1. Compositions of fine-grained Baraboo interval metasedimentary rocks. normalized to average shale concentrations of KlO, Na 20, CaO, MgO, and (Taylor and McLennan, 1985) and compared to average MnO. The Chemical Index of Alteration is shale over the ages. Elements arranged in order of exceptionally high, ranging from 96.8 to 98.6. decreasing ionic radius. CIA = 100*molar AbO/ The remarkable degree of chemical maturity is (AbOJ+K20+N a20+CaO) readily apparent when the rock compositions are normalized to average shale and compared to average shale of different ages (Fig. 11). The extreme chemical maturity of the Baraboo interval sediments implies a source region that experienced intense 10 ~ 1i~ ; ~ : ~I ~ , .. ~ . ' 7 7 �chemical leaching leaching and and produced produced detritus detritus consisting consisting largely largely of of quartz, quartz, kaolinite, kaolinite, and and hematite. hematite. chemical The persistent question about the about the grains of of the Baraboo Quartzite and relative ages of I Individual Individual grains of detrital detrital zircon zircon 0.34 ft from basal Baraboo Quartzite 0.34 from basal Baraboo Quartzite Baxter Hollow granite was was resolved resolved with with the the 1800 Ma UW Radio genic Isotope Lab ! UW Radiogenic Isotope Lab 1800 Ma discovery of the sub-Baraboo paleosol in drill of paleosol in drill Analyst: Analyst: Ron Ron Schott Schott 0.30 0.30 core from Baxter Hollow (Medaris (Medaris et et al., aI., 1996) 1996) 1600 and subsequent recognition of 1600 Ma _ _ 1761 (3) I of saprolite saprolite in in b ,i712(4) •1866 (3) outcrop in Baxter Hollow granite granite and and in in 0.26 1715(12) 6 rhyolite beneath the east end 1400 MaZ °.'222 1691(2) -1866 (3) end of of the syncline syncline 1400 Ma - 1691 (2) al., 1997). (Medaris et aI., 1997). Further Further confirmation confirmation i< •17793 . 0 0.22 -1779 (3) post-1,750 Ma depositional age of of the post-l,750 of the 0- ° CD o quartzite was provided by U-Pb analyses of N M. detrital zircon from near the base of of the the 0.18 1000 Ma quartzite, using the single-grain evaporation quartzite, 1740 (6) -1740 (6) technique. Among Among the the seven seven grains grains analyzed, analyzed, _~I --'---'----'I_ _ J 0.14 0.14 six are slightly discordant and and one one is is more more so, so, 5.0 4.0 3.0 2.0 1.0 1.0 2.0 3.0 4.0 5.0 age of with one onegrain grainyielding yieldinga 207Pb/206Pb a 207Pbpo6Pb age 2O7pb* 207 * / 235 pb U 1,691 1,866 Ma, and the other other six six ranging rangingfrom from 1,691 12. V-Pb U-Pb concordia concordia diagram diagram for Figure 12. Figure for detrital detrital zircon zireon to 1,779 Ma (Fig. (Fig. 12; Dott et al., 1997). Although a1., 1997). Although grains from the Baraboo Quartzite. 207Pb/206Pb ages are grains from the Baraboo Quartzite. 207Pbfo6 Pb ages are of 1,691 1,691 and 1,715 1,715 Ma are the two grains with ages of given for for individual individualgrains; grains;2cr 2 standard given standard deviations deviations are are shown in parentheses. highly radiogenic and may not be reliable, the the in parentheses. shown other grains are "well behaved" and indicate indicate that + hematite deposition may ~ 1,710 Ma. may have have begun begunas aslate lateasas—4,710 qtz :!: hematite diorite, qtz diorite, rutile rutile silt stone granite svanbergite siltstone 5vanbergite granite METAMORPHISM & petite pelite albite & rhyolite & & rhyolite albite r ' I ~ ,I / - ­ 01.1- ;;~; ~i~) "" II 1 chlorite chlorite pale osol pri actinolite paleosol Although the structure of of the Baraboo Range actinolite cummingtonite has been been well well studied, studied, little attention has has been cummingtonite has been mc epidote epidote devoted to metamorphism, other than identifying identifying titanite titanite pyrophyllite in metapelite. ItItisis now (kaolinite, now known known that that all all (kaolinite, retrograde) lithologic units in the Baraboo Baraboo Range Range have have been been retrograde) recrystallized to varying hydrothermal varying degrees degrees by by low-grade low-grade veins metamorphism (Medaris et at., 1998). Because of veins metamorphism (Medaris et aI., 1998). Because Si02 the extreme chemical maturity maturity of ofmany many rock rock types, types, the critical mineral assemblages can be adequately represented in in the the system, system, K20-Al203-Si02K 20-Ab03-SiOz- H20 H20 (KASH) (Fig. 13). Baraboo quartzite. metasiltstone, (KASH) (Fig. 13). Baraboo quartzite. metasiltstone, and metapelite metapelite contain contain quartz and pyrophyllite plus plus and svanbergite (a hematite, rutile, and svanbergite accessory hematite, (a strontian strontian aluminophosphate-sulfate diagenetic diagenetic mineral); mineral); of quartz, quartz, muscovite, muscovite, hematite, hematite, metapaleosol consists of usp and rutile; rutile; hydrothermal hydrothermal veins veins near of the the and near the the base base of A1203 K20 quartzite contain pyrophyllite, muscovite, and Figure compositions and diaspore; and the metaigneous basement is Figure 13. 13. Rock Rock compositions and mineral mineral assemblages assemblages in in the Baraboo Range, projected into the system, the Baraboo Range, projected into the system, KASH. KASH. characterized by quartz, microcline, microcIine, and and albite albite Abbreviations: dsp, diaspore; mc, microcline; ms, mus+/- muscovite, hematite, chlorite, epidote, epidote, titanite, titanite, Abbreviations: dsp, diaspore; me, microeline: ms, mus­ covite; qtz, covite; qtz, quartz: quartz: pri. prl. pyrophyllite. pyrophylJite. actinolite, and cummingtonite, depending depending on on 8 8 ,I �2. and I Localities Field on sections following the in further discussed are results new significant These magmatism. River Wolf from pulse thermal a by driven fluids hydrothermal of activity the to related presumably found, was Ma —1,460 at overprint strong a metamorphism, Ma 1,630 for evidence obtaining than rather However, 2001). eta!., (Naymark Ma 1,630 of age metamorphic a confirm to effort an in taken under- recently was Range Baraboo the in muscovite (1975). and hornblende of investigation 40Ar/39Ar A eta!. Schnius Van and (1972) Dalziel and Dott north. the to ages older and Penokean from south the in of data from recalculated intercept and Age ages post-Penokean separates front tectonic the 2000); Valley. River Fox the and Baraboo from rhyolite and granite for isochron Rb-Sr 15. Figure a!., et Romano 1998; al., et (Holm rocks basement in biotite and hornblende of analyses 40Ar/39Ar 6 S I Ri.r by established was 1) Fig. (see Wisconsin central 40 30 20 10 0 0.6 and northern in deformation foreland of extent 0.70254 the marking front tectonic a of position The 8.1 = MSWD 0.8 time. this at occurred also Range Baraboo the of metamorphism and folding that implication 1.0 5 the with 1993), Schmusetal., Van 1983; (Dott, U.S. southwest the in orogeny Mazatzal 1.2 w Ma -—1,650 the with associated deformation foreland to related metamorphism low-grade 1.4 regional by caused was resetting Rb-Sr granite Valley River Fox V such that suggested was it Subsequently, ). yr rhyohte Valley River Fox = Ma(X 33 ± 1,635 of age isochron BaxterHollowgranite 1.42*10.11 1.6 rhyolite Baraboo A Rb-Sr whole-rock apparent an yielded Valley River Fox the and Baraboo from rocks igneous that and Ma —l,650 at region Superior Lake southern the in systems Rb-Sr of resetting widespread was there that recognized Schmus Van 1975 in However, Range. Baraboo the from available were ages mineral metamorphic meaningful no recently, Until metamorphism. Baraboo of conditions the precisely more establish to effort an in underway are studies inclusion Fluid respectively. C, 335 1. = a(H20) for calculated and 290 and 50°C, 3 and 275 to lowered be would KASH, system, the in reactions Stable 14. Figure limits temperature these example for 0.9 0C T activity, H20 reduced At 345°C. and 305°C 450 400 350 300 250 to further temperature the restrict between 0 Quartzite Baraboo the of base the at veins hydrothermal in diaspore and pyrophyllite of coexistence The kbar. 1 at 60°C 3 and 285°C 2 between to metamorphism of temperature the constrains kyanite) or (andalusite phase 4 aluminosilicate an or kaolinite of absence the in pyrophyllite and quartz of association stable The 14. Figure in shown topology 6 the in resulting 1989), al., et (Brown base data thermodynamic GeoCaic the of means by activity H20 unit at calculated been have 8 KASH, system, the in equilibria Phase product. retrograde a is it that indicate relations textural occurs, kaolinite Where composition. bulk specific specific bulk composition. Where kaolinite occurs, textural relations indicate that it is a retrograde product. Phase equilibria in the system, KASH, 8I I \ , , have been calculated at unit H 20 activity by means of the GeoCalc thermodynamic data base (Brown et al., 1989), resulting in the 6 topology shown in Figure 14. The stable association of quartz and pyrophyllite in the .... (IJ absence of kaolinite or an aluminosilicate ~ 4 phase (andalusite or kyanite) constrains the 0: temperature of metamorphism to between 2 285°C and 360°C at 1 kbar. The coexistence of pyrophyllite and diaspore in hydrothermal veins at the base of the Baraboo Quartzite oI V /1 <1 /' I I I I restrict the temperature further to between 250 300 350 400 450 305°C and 345°C. At reduced H 20 activity, T,oC 0.9 for example, these temperature limits Figure 14. Stable reactions in the system, KASH, would be lowered to 275 and 350°C, and 290 and calculated for a(HzO) = I. 335°C, respectively. Fluid inclusion studies are underway in an effort to establish more precisely the conditions of Baraboo metamorphism. Until recently, no meaningful metamorphic mineral ages were available from the Baraboo Range. However, in 1975 Van Schmus recognized that there was widespread resetting of Rb-Sr systems in the southern Lake Superior region at ~ 1,650 Ma and that igneous rocks from Baraboo and the Fox River Valley yielded an apparent whole-rock Rb-Sr I l. Baraboo rhyolite 1.6 Y Baxter Hollow granite isochron age of 1,635 ± 33 Ma (A = 1.42*10. 11 n Fox River Valley rhyolite yr· l ). Subsequently, it was suggested that such 'V Fox River Valley granite 1.4 Rb-Sr resetting was caused by regional <0 low-grade metamorphism related to foreland ~ (f) 1.2 deformation associated with the ~ 1,650 Ma !'--­ Mazatzal orogeny in the southwest U.S. (Dott, <Xl (jj 1.0 1983; Van Schmus et aL, 1993), with the implication that folding and metamorphism of 0.8 the Baraboo Range also occurred at this time. MSWD = 8.1 The position of a tectonic front marking the 0.6 I I I I I I I I extent of foreland deformation in northern and o 10 20 30 40 central Wisconsin (see Fig. 1) was established by Rrfl7j sr8 6 40 ArP9 Ar analyses of hornblende and biotite in basement rocks (Holm et al., 1998; Romano et aL, Figure 15. Rb-Sr isochron for granite and 2000); the tectonic front separates post-Penokean ages rhyolite from Baraboo and the Fox River Valley. in the south from Penokean and older ages to the north. Age and intercept recalculated from data of A 40ArP9 Ar investigation of hornblende and Dott and Dalziel (1972) and Van Schmus et al. muscovite in the Baraboo Range was recently under(1975). taken in an effort to confirm a metamorphic age of 1,630 Ma (Naymark et al., 2001). However, rather than obtaining evidence for 1,630 Ma metamorphism, a strong overprint at -1,460 Ma was found, presumably related to the activity of hydrothermal fluids driven by a thermal pulse from Wolf River magmatism. These significant new results are discussed further in the following sections on Field Localities 1 and 2. i . . . I — 9 9 ! I �DESCRIPTIONS OF FIELD TRIP LOCALITIES LOCALITIES Figure 16. Figure 16. Perspective Perspective sketch sketch map map of of the the Baraboo Baraboo Range, Range, showthg showing Field Field Trip Trip Localities Localities 1-4, 1-4, of quartzite in in the Baraboo syncline syncline (light stipple), extent extent of of glacial glacial drift drift (parallel (parallel distribution of lines), and and important from map map by by L.J. LJ. Maher). Maher). lines), important geographic geographic features features (modified (modified from Locality 1: Locality 1: Baxter Baxter Hollow Hollow 1 (SW¼, Sec 33, 33, T1IN, TI iN, R6E; (SW i4, Sec R6E; Figure Figure 17) 17) Relation Baxter Hollow Hollow granite granite and and Baraboo Relation between between Baxter Baraboo Quartzite; sub-Baraboo paleosol Quartzite: sub-Baraboo paleosol Note that the are on Note that the outcrops outcrops in in Baxter Baxter Hollow Holloware 0!l private private property, and and pennission permission is is required required for for access. access. property, Although the contact contact between Although the between Baxter Baxter Hollow Hollow granite and Baraboo Quartzite is not exposed, granite and Baraboo Quartzite is not exposed, the the closest outcrops of the two rock types being 20 closest outcrops of the two rock types being 20 feet feet apart, (1970) concluded and Doff Dott (1970) concluded that that quartzite quartzite apart, Daiziel Dalziel and is nonconformable on on granite, granite, because is nonconformable because of of the the absence absence of granitic granitic dikes dikes in quartzite, quartzite of in quartzite, quartzite xenoliths xenoliths in in granite, and contact metamorphic effects in quartzite. granite, and contact metamorphic effects in quartzite. The uppermost uppermost outcrops outcrops of of granite granite in The in Baxter Baxter Hollow Hollow are commonly commonly sheared, sheared, most most likely are likely due due to to concenconcen­ tration along the tration of of deformation deformation along the quartzite-granite quartzite-granite Figure 17. Locality 1, 1, Baxter Baxter Hollow. Hollow. Symbols: Symbols: PEg, PEg, contact by differential slip between two competent contact by differential slip between two competent Baxter Hollow Hollow granite; granite P€b, Quartzite Baxter PEb, Baraboo Quartzite; lithologic units units during during folding. folding. lithologic €, Upper Cambrian quartzite conglomerate and E, and conglomeratic sandstone. This and other locality locality maps from from Dalziel and Dott, 1970. maps 1970. 10 10 �i; I :1 W rrIiiI I . ' c: 11: 18. Composite figure figure of Baraboo Baraboo paleosol paleosol in Drill Drill Core 613. Left: boxed drill core with recovered Figure 18. quartzite. regolith, and saprolite; saprolite; center: enlargements of selected selected intervals; right: photomicrographs of quartzite (note (note Si0 Si022 overgrowths on on detrital detrital quartz quartz grains). grains). regolith (pedogene). and saprolite. quartzite saprolite. 11 11 �The lower lower 100-200 The 100-200 meters meters of of quartzite quartzite exposed exposed uphill uphill to to the the north north are are characterized characterized by by red, red, Pebbles average 30 sand-sized strata strata with with pebble mm in in sand-sized pebble beds beds and and lenses. lenses. Metapelite Metapelite layers layers are are rare. rare. Pebbles average 30 mm formation, milky vein quartz pebbles diameter, but range up to 68 mm near the base. Throughout the diameter, but range up to 68 mm near the base. Throughout the formation, milky vein quartz pebbles The latter probably predominate, predominate, but but red red cherty cherty granules granules and and pebbles pebbles form form aa persistent persistent lesser lesser component. component. The latter probably Stratification here here consists include include clasts clasts of of devitrified devitrified groundmass groundmass from from the the 1,750 1,750 Ma Ma rhyolites. rhyolites. Stratification consists mostly mostly 50 cm thick. Several these of long, low-angle oflong, low-angle cross cross bedding bedding in in both both tabular tabular and and wedge wedge sets sets up up to to 50 cm thick. Several sets sets of ofthese overturning beneath beneath their their upper upper truncation truncation surfaces. surfaces. A cross laminae show syndepositional overturning A sudden sudden and disturbed pore fluid and grain increase of shear velocity scoured grains from the bed increase of shear velocity scoured grains from the bed and disturbed pore fluid and grain packing, packing, causing incipient of the the cross cross bed bed set set deformed deformed as as ififentirely entirely causing incipient liquefaction liquefaction during during which which the the upper upper part part of characteristic of fluvial and liquid. Such Such distorted distorted cross liquid. cross bedding bedding in in low-angle low-angle sets sets is is most most characteristic of fluvial and tidal tidal sands, sands, which experience large fluctuations of current velocity. which experience large fluctuations of current velocity. holes drilled drilled in The quartzite-granite quartzite-granite contact The contact was was penetrated penetrated by by eight eight holes in 1959 1959 by by the the U.S. U.S. Army Army the contact was recovered in only one drill core, this this Corps of Engineers, and although material from the contact was recovered in only one drill core, single recovery was fortuitous, because it demonstrates the the existence existence of of aa paleosol paleosol beneath beneath the the quartzite quartzite pebbly quartzite is separated from underlying (Medaris et et al., al., 1997). 18) overlying overlying pebbly quartzite is separated from underlying (Medaris 1997). In In drill drill core core 613 613 (Fig. (Fig. 18) pedogenic zone, zone, consisting consisting of of fine-grained foot-thick, reddish-purple pedogenic granitic saprolite saprolite by byaa2Y2 2\12 foot-thick, fine-grained hematite, hematite, largely Granitic texture is preserved in saprolite, but biotite quartz, and muscovite (± (± kaolinite). kaolinite). Granitic texture is preserved in saprolite, but biotite is is largely replaced by by muscovite. muscovite. The replaced by hematite, and and feldspar feldspar isis completely completely replaced The first first feldspar feldspar to to appear appear in in of the saprolite drill core is feet below below quartzite. quartzite. Note that is —30 ~30 feet that the the uppermost uppermost part part ofthe saprolite (2" (2" thick) thick) has has aa not in to that pronounced pronounced planar planar fabric fabric (Fig. (Fig. 18), 18), which which is is similar similar in in style, style, if if not in scale, scale, to that in in the the sheared sheared granite granite outcrops. change in oxides, has been calculated by Chemical weathering weathering in as % Chemical in the the paleosol, paleosol, expressed expressed as % change in oxides, has been calculated by unweathered granites from outcrops —30 and 40 feet comparing paleosol paleosol samples comparing samples to to the the average average of of two two unweathered granites from outcrops ~30 and 40 feet (Fig. to be be immobile during weathering below the contact, contact,and andnormalizing normalizingtotoA1203, Al z0 3, which which is is assumed assumed to immobile during weathering (Fig. in K. effective removal of Mg, Ca, and Na, and enrichment 19). 19). The The most most notable notable chemical chemical changes changes are effective removal of Mg, Ca, and Na, and enrichment in K. o ,.-----~-o-r~ 0 o 1 ,--------,---___, 0.1 o ~ .::."....;~. ,.~..... :. " :.......'::..1­ 0 a ~ I s s ss ~ 10 {g :.~ 10 10 ••••••••••••••••••••• •••• ••••••• J••••• GG .. ..... 100 ~--'-'---'--'-~~...L.......l.-o-~ 20 -20 -40 -60 -60 -40 -20 00 20 01 — ~ ..~ s 10 :~ L ..; 100 -100 -100 100 0 0 100 300 200 200 300 % change Fe203 % change Fe203 L . 50 25 0 100-100......--e~-'-'-'-'~~~.L........J -50 -25 -75 -100 -75 -50 -25 MgO 0 25 50 % change Lot % change MgO 0.1 ,------,-~-___, 0 0 0 III ~ 0.1 01 01 b 10 I r i G % % change change S102 5i02 01 ~ 0 0 I peciogenic pedogenic zone z one, 0 0 0 i,: ~(·········t·:~ i·····:::,;::··l···:~·····r·1· 1 I 0) -c 0 1 0 .- s saprO1!t 10 10 is athered ,·················,1; ·········,··;;=:,:;;;;····;1'· ·················'···1·;·'· I_t_II• GIG 100 100 -100 -100 -80 -80 -60 -60 ' -40 -40 -20 -20 % change % change CaO CaO 00 1 0 -60 ' -1000-60 -100 -80 -60 -40 -40 -20 -20 % change Na20 % change Na20 ' 0 0 100-80 -60 -40 -20 0 ' 20 . -80 -60 -40 -20 0 K20 20 % change % change K20 19. Depth Depth variation variation in in % % change change of of selected selected oxides Figure Figure 19. oxides in in the the Baraboo Baraboo paleosol. paleosol. 0, O. pedigene; pedigene; S, S, saprolite; saprolite; G, G, granite. granite. 12 12 40 60 40 60 �___________________________________________ occurred, but that K K was We believe that all feldspar was altered during weathering weathering and that K leaching occurred, reintroduced during later metasomatism 80 " - - - - - - - - - - - - - - - - - - - - - - - , 80 associated with fluid flow along the sub­ subsaprolite comparison comparison 60 Baraboo nonconformity. Note that detrital E 0 ]i feldspar is rare in quartzite, and interlayered, interlayered, 0 ec. 40 0. fine-grained metasedimentary metasedimentary rocks are are 20 .S for impoverished in K, Na, and Ca. A model for ~ \ ,c .... t.:: / .s0 01 Baraboo saprolite, prior to K metasomatism, metasomatism, ~ is is provided by the Barron saprolite, saprolite, which is ~ -20 a) ~ unmetamorphosed and located located north of the the • asa) -40 0) g> inferred 1,630 Ma tectonic front (Fig. 1; 1; 0 ~ -60 -c Medaris, 2000). A comparison comparison of the two ':!< o -80 saprolites (Fig. 20) reveals a close similarity similarity in most elements, except for the enrichment of -100 ! ¥ ! '.~! ! -100 p AI Si Rb Al Rb KK Ba Sr Na Ca Ca Mg Mn Fe Fe Ti K and Rb in Baraboo saprolite. The chemical features of the Baraboo and Barron paleosols Figure Figure 20. Chemical Chemical comparison of are similar to those of modern-day, modem-day, mature weathering weathering Baraboo and Barron saprolites. Baraboo profiles developed in warm, humid climates. profiles - (,l C.) 1 I 1 An apparent whole-rock Rb-Sr isochron age of the paleosol is 1336 ± ± 75 Ma (Fig. 21), and 39 Ar release spectrum, with a well-defined plateau plateau at at metasaprolite yields a discordant 39Ar muscovite from metasaprolite evidence for 1,456 ± 11 (Fig. Naymark et aI., These data provide the substantial evidence a 1,456 11 Ma 22; al., 2001). These data provide first substantial ± Wolf River imprint on the Baraboo Range, due most likely to the the effects of hydrothermal hydrothermal fluids that were channeled along the sub-Baraboo nonconformity and driven by a thermal pulse generated by regionally sub-Baraboo nonconformity extensive magmatism of Wolf River age. 0.85 j ~ 0.80 (jj r-:- co (jj 075 0.75 z 7 I a0 I I 1 I I 1500 ro 1500 1)3 ~ 1250 1250 OJ 'E ~ « 0. c. 0. c. I I I I I 55 I ! L1J~ 1456 t 11 ± Ma (2o) Ma (20") 1000 1000 750 muscovite, metasapralite, metasaprolite, Baxter Hollow muscovite. Hallow 500 500 250 250 o 0 I 6 ThnJI IF=!' C) Cl «0 MSWD = 19.4 194 2 4 2 3 3 4 TI- - - - - - - - - - - - - - - - - - - - , 1750 1750 UW Radiogenic Radio genic Isotope Lab Lab Analyst: Ron Schott — 0.70817 0.70817 ~ 0.70 0.70 \ 2000 I Baraboo paleosol paleosol R regolith (pedogene) (pedogene R S s saprolite saprolite UW Rare Gas Geochronology UWRare Geochronoiogy Lab Lab Analyst: A!,ssa Alissa Naymark Naymark 4naIySf: +-I---~--~---~-------< 40 60 80 20 40 60 80 100 o 0 7 Cumulative 39Ar Released (%) 87; Sr 86 Rb87! Sr86 Rb Figure 21. Rb-Sr whole-rock isochron for the paleoso!. metamorphosed Baraboo paleosol. 39Ar Figure 22. 22. 39 Ar release spectrum for muscovite from the the metamorphosed Baraboo saprolite. 13 13 �Locality 2: 2: Hydrothermal Hydrothermal Veins. Locality Veins, Hwy Hwy 12 12 (SW1/4, NE1/4, Sec Sec 34, 34, TIIN, Ti iN, R6E; (SWl/4, NEl/4, R6E; Figure Figure 23) 23) Diaspore.-muscovite-pyrophyllite veins in in Baraboo Baraboo Diaspore-muscovite-pyrophyllite veins Quartzite, near the base of the section Quartzite, near the base ofthe section Figure 24. 24. Diaspore-muscovite-pyrOPhyllite Figure Diaspore-muscovite-pyrophyllite veins veins in quartzite. in quartzite. Figure 23. Locality 2, 2, hydrothermal hydrothennal vems veins in in quartzite. Symbols as in in Fig. Fig. 17. 17. of Baraboo Quartzite, which contains a A few few meters meters east east of dark red A of Highway Highway 12 12 is is aa dark red outcrop outcrop of Baraboo Quartzite, which contains a consisting of of diaspore, diaspore, muscovite, muscovite, and network ofthin, of thin, white network white hydrothermal hydrothermal veins veins (Fig. (Fig. 24), 24), consisting and pyrophyllite. pyrophyllite. occurring in the center and intergrown muscovite The veins veins are are commonly zoned, with The with diaspore occurring in the center and intergrown muscovite and and pyrophyllite along the margins (Fig. 25). Kaolinite pyrophyllite along the margins (Fig. 25). Kaolinite of pyrophyllite. occurs as a retrograde replacement of pyrophyllite. Locally, fine-grained fine-grained sedimentary Locally, sedimentary rocks, rocks, originally quartz-bearing, near the base of the originally quartz-bearing, near the base of the quartzite have quartzite have been been pervasively pervasively replaced replaced by by diaspore, diaspore, muscovite, pyrophyllite, pyrophyllite, and muscovite, and hematite, hematite, resulting resulting in in aa soft, purple-red stone that was quarried by Native soft, purple-red stone that was quarried by Native Americans for for the the production production of of pipes. pipes. This Americans This mineral mineral assemblage is the same assemblage is the same as as that that in in the the classic classic pipestone (catlinite) (catlinite) from pipestone from the the Sioux Sioux Quartzite Quartzite (Medaris et al., 1999). The formation (Medaris et a!., 1999). The formation of of diaspore diaspore at at 300-350°C in in the the Si0 Si02-rich 300-350°C environment of of the the 2-rich environment Baraboo and and Sioux Baraboo Sioux quartzites quartzites is is surprising, surprising, and and requires the influence of a fluid phase with an requires the influence of a fluid phase with an 0.01. extremely extremely low low activity activity of of Si02, Si0 2, on on the the order order of of 0.0 I. 14 14 05mm Figure 25. Photomicrograph of hydrothermal vein in Figure 25. Photomicrograph ofhydrothennal vein in quartzite (plane (plane polarized polarized light). quartzite light). Abbreviations: Abbreviations: dsp, dsp. diaspore; ms, muscovite: prl, pyrophyllite. diaspore; ms. muscovite: prl. pyrophyllite. �Muscovite in a sample discordant 39Ar 39Ar release release spectrum with with a Locality 2 yields a discordant sample of vein from Locality " --------------------, 2000 plateau age of 1,467 ± 11 Ma, which is within ± 11 error of the 1,456 Ma plateau age for muscovite muscovite 1750 1750 from metasaprolite. Thus, the age of fluid 1500 1500 ..... ro activity responsible for K-metasomatism in the 1250 Baraboo 1,460 Ma. J1250j ~ -1,460 Baraboo Range is well established at ~ 1467 ±± 11 Ma Ma (2o) (20-) 1467 Although migration was C woo 1000 Although large-scale fluid migration !!! probably probably driven by heat from Wolf River fr 750 muscovite, ms-pu-dsp ms-prl-dsp vein, vein, Hwy Hwy 12 12 muscovite, c( magmatism, magmatism, the source and composition of such 500 500 fluid fluid remains remains unknown unknown and deserves UW Rare Gas Gas Geocivoao!ogy GeochronologyLab Lab 250 250 UWRare investigation. Analyst: Alissa Nayma,* Naymarl< AaaIyst I f 11 AhSSa Figure 26. Ar release spectrum for muscovite from ~ 26. 39 Ar hydrothermal vein in quartzite, Highway 12. o 0 I o 0 I 20 20 40 40 60 60 80 80 100 100 CumulatIve 39Ar Released (%) (%) Cumulative Ar Released Locality 3: Quartzite Quartzite and and Metapelite, Metapelite, Hwy 12 12 (NWI/4, IS, Ti TlIN, Figure 27) 27) (NW 1/4, Sec 15, iN, R6E; Figure Sedimentary and metamorphic metamorphic features of Baraboo Baraboo Sedimentwy quartzite and metapelite metapelite "~:Ji£b Qal 27. Locality 3. 3, Baraboo quartzite and meta­ metaFigure 27. Symbols: Qal. Qal, Quaternary alluvium; other pelite. Symbols: as in in Fig. Fig. 17. 17. symbols as outcrop at Locality Locality 3, illustrating illustrating aa Figure 28. Quartzite outcrop cross sets reactivation surface (left arrow) and contorted cross (right arrow). DO NOT HAMMER HAMMER THIS OUTCROP! OUTCROP! The upper stratigraphic stratigraphic portion portion of the Baraboo quartzite differs in several ways ways from the lower part part at at The Baxter Hollow. Pebbly layers are rare, metapelite layers are Baxter are more more common common (the thickest known zone zone isis exposed here), here), and the style style of stratification is different. Cross bedding exposed bedding is higher angle and occurs occurs in in sets sets mostly 10 10 to 20 em cm thick. thick. Master Master bedding surfaces are commonly defined by by thin metapelite. metapelite. Individual mostly are slightly slightly concave upward; sets of cross laminae are mostly tabular, but some cross laminae are some troughtrough­ shaped sets occur in this roadcut. Excellent asymmetric. sinous-crested sinous-crested ripple ripple marks marks visible visible on one of the exposure indicate the dominant dominant south-flowing surface in the middle ofthe south-flowing paleocurrent direction typical typical the entire foonation. formation. for the On the south-facing south-facing cliff, several reactivation surfaces and some some contorted cross cross sets sets are are exposed exposed in cross section section (Fig. 28). Reactivation surfaces are convex-up in convex-up truncations truncations of of cross bed sets. They They spasmodic activation of of dune forms that produce cross bedding, and are most most characteristic characteristic of indicate spasmodic 15 15 �tidal systems. The dominant migrate to to form form cross cross laminations, laminations, then tidal systems. The dominant tidal tidal flow flow activates activates dunes, dunes, which which migrate then the subordinate flow partially erodes the dunes to form the convex-up surface. When the tide the subordinate flow partially erodes the dunes to form the convex-up surface. When the tide turns turns again, again, the dominant of cross cross laminae, which bury the reactivation laminae, which bury the reactivation the dominant flow flow reactivates reactivates the the dunes dunes to to form form aa new new set set of surface. In this cliff, the the dominant dominant currenf current'ss bed bed shear occasionally disturbed disturbed the the grain grain packing packing and and caused deformation of cross laminae as described for Locality 1. Reactivation surfaces suggest that the the caused deformation of cross laminae as described for Locality I. Reactivation surfaces suggest that origin, indicating a gradual northward of the Baraboo Quartzite is of of shallow marine origin, indicating a gradual northward upper half of transgression during formations are are all all considered considered to to be transgression during its its deposition; deposition; the the overlying overlying Precambrian Precambrian formations be normal normal marine. manne. of the A the north end of of the the roadcut roadcut and and at at the the top A prominent prominent metapelite metapelite layer layer is is exposed exposed at at the north end top of the hill, hill, metapelite (Fig. where one one can can see see excellent exposures of and crenulation crenulation cleavage cleavage in in metapelite (Fig. 29). of chevron folds folds and 29). where Red quartzite quartzite boudins are common Red common in in metapelite, metapelite, as as are are folded folded white white quartz quartz veins, veins, which which appear appear to to inferred to to have have originally originally been emanate from from the the quartzite quartzite boudins. boudins. The metapelite is inferred been aa kaolinite-rich kaolinite-rich in metapelite, which shale, based on its bulk chemical composition. composition. The which is is shale, The present present mineral mineral assemblage assemblage in metapelite, with subordinate typical for metapelite metapelite throughout throughout the the Baraboo Baraboo Range, Range, consists consists mostly mostly of ofpyrophyilite pyrophyllite with subordinate small grains grains of of rutile, and minor tablets of of black black hematite, hematite, small rutile, and minor amounts of recrystallized quartz, thin tablets retrograde kaolinite (Fig. 30). Trace of retrograde kaolinite (Fig. 30). Trace amounts amounts of Ce-bearing svanbergite, SrAl3(P04)(S04)(OH)6, Ce-bearing svanbergite, SrAb(P0 4 )(S04)(OH)6, 10 to 20 20 !-lm tm in 10 to in diameter, diameter, are are scattered scattered through through the the metapelite (Medaris and Fournelle, 1998). The metapelite (Medaris and Fournelle, 1998). The occurrence of this this diagenetic diagenetic aluminophosphatealuminophosphate­ occurrence of sulfate mineral is significant because itit bears bears on on sulfate mineral is significant because the phosphorus flux in the oceans. the phosphorus flux in the oceans. Figure 30. Back-scattered Back-scattered electron electron image image of of Figure 30. metapelite. Locality 3. Abbreviations: h, hematite; metapelite, Locality 3. Abbreviations: h, hematite; k, k, kaolinite; kaolinite; p, p, pyrophyllite; pyrophyllite; q, q, quartz. quartz. Figure 29. Chevron folds folds and and crenulation crenulation cleavage cleavage in metapelite, Locality 3. The scale The scale coin, coin, enhanced enhanced by a black circle, is is 2.5 cm em in in diameter. diameter. DO NOT HAMMER THIS THIS OUTCROP! OUTCROP! 16 16 �Gorge Ableman's 4: Locality Locality 4: Ableman's Gorge dunes eolian Cambrian Upper Rock; Hise Van zone; breccia quartzite quartzite; in layers metasiltstone and ripples Quartzite; Baraboo and conglomerate Cambrian Upper between unconformity Angular 31) Figure R5E; T12N, 28/29, Sec (SW1/4, (SW1I4, Sec 28/29, T12N, R5E; Figure 31) Angular unconformity between Upper Cambrian conglomerate and Baraboo Quartzite; ripples and metasiltstone layers in quartzite; quartzite breccia zone; Van Hise Rock; Upper Cambrian eolian dunes 17. Fig. in as symbols other Group; City Tunnel Cambrian Upper €tc, Sandstone; Galesville Cambrian Upper Eg Symbols: dunes. eolian Cambrian and unconformity angular 4D, Rock; Hise Van 4C, breccia; quartzite 4B, Quartzite; Baraboo and conglomerate Cambrian Upper between unconformity angular 4A: Gorge. Ableman's 4, Locality 31. Figure Figure 31. Locality 4, Ableman's Gorge. 4A: angular unconformity between Upper Cambrian conglomerate and Baraboo Quartzite; 4B, quartzite breccia; 4C, Van Hise Rock; 4D, angular unconformity and Cambrian eolian dunes. Symbols: €g Upper Cambrian Galesville Sandstone; Etc, Upper Cambrian Tunnel City Group; other symbols as in Fig. 17. N 8r,,",,- Zon. ­ 1100 1000 .11m:['p'f~~ ~11.EJ51in ",?,.iH I~~~900 D B C 1 ? ? A t 'kh~s;;~.. 5'/0 iOcx ~:;;:;r~I']1'l' ~'fq 'Opo Feel o Feel (1970). Dott and Daiziel from Modified gorge. the of ends both at cliffs quartzite buried against butting strata Cambrian of symmetiy the Note D. - A letters by indicated Localities Field with west), (lookmg Gorge Ableman's through section Cross 32. Figure Figure 32. Cross section through Ableman's Gorge (looking west), with Field Localities indicated by letters A-D. Note the symmetry of Cambrian strata butting against buried quartzite cliffs at both ends of the gorge. Modified from Dalziel and Dott (1970). features guartzite unconformity; angular Precambrian - Cambrian Upper 4A: 4A: Upper Cambrian - Precambrian angular unconformity; quartzite features Baraboo. the of part upper marine, the in be must exposures These waves. by formation indicate which symmetric, and straight relatively are these of crests the 3, Locality at ripples the Unlike bed. overlying the of bottom the on ripples the of casts are see we What board. wash great-grandmother's resembling face marked ripple spectacular a is quarry the of end south The help). (binoculars boulders quartzite coarse with conglomerate Cambrian Upper brown-weathering and quartzite red between unconformity angular the see can one cliff, the of top the At face. quarry old the in visible layers metasiltstone thin and sets bed cross with quartzite vertical exposes 32) (Fig. Gorge Ableman's of side west the on quarry abandoned The The abandoned quarry on the west side of Ableman's Gorge (Fig. 32) exposes vertical quartzite with cross bed sets and thin metasiltstone layers visible in the old quarry face. At the top of the cliff, one can see the angular unconformity between red quartzite and brown-weathering Upper Cambrian conglomerate with coarse quartzite boulders (binoculars help). The south end of the quarry is a spectacular ripple marked face resembling great-grandmother's wash board. What we see are casts of the ripples on the bOllom of the overlying bed. Unlike the ripples at Locality 3, the crests of these are relatively straight and symmetric, which indicate formation by waves. These exposures must be in the marine, upper part of the Baraboo. 17 17 �4B:Qjiartzite 4B: Quartzite Breccia Breccia Zone Exposed on on the the west west wall wall of the gorge Exposed of the gorge is is aa quartzite breccia zone, consisting of angular red quartzite breccia zone, consisting blocks cemented cemented by a stockwork of of white quartz blocks generally massive, veins (Fig. (Fig. 33). 33). The vein quartz is generally massive, veins of which but locally euhedral quartz crystals, some crystals, some of which (a member member of are coated by dickite (a of the the kaolinite kaolinite are The mineral group), group), occur occur in mineral in vugs vugs (Fig. (Fig. 34). 34). The growth euhedral quartz crystals commonly show growth euhedral indicates that zoning, and and preliminary investigation indicates that zoning, isotopes quartz is is zoned zoned with with respect to oxygen isotopes as as quartz 18 from 9.33 to 18.95 %o ranging well, with 18O from 9.33 to 18.95 %0 well, with 8 0 in one one crystal crystal (Fig. (Fig. 35). 35). S. (VSMOW) in (VSMOW) S. W. W. Bailey Bailey for fluid reported a temperature of of 105-107°C 105-1 07°C for fluid reported inclusions in in the the quartz, quartz, which which is inclusions is consistent consistent with with the the (cited in in Daiziel Dalziel and and Doft, Dott, 1970). 1970). presence of dickite (cited In some some places places in in the the breccia breccia zone, zone, the the In slightly separated quartzite fragments fragments appear quartzite appear to to be be slightly separated the impression pieces of a jigsaw jigsaw puzzle, giving the impression that that fragmented by some the quartzite might have been fragmented by the some Perhaps brecciation type of explosive activity. Perhaps brecciation was was type (sub critical caused by passage of a low-temperature caused by passage of a low-temperature (sub critical fluid to to the the vapor vapor point) hydrothermal hydrothermal fluid point) fluid from from the the fluid shallower levels field as it migrated from deeper to shallower levels field Figure 33. Quartzite breccia, Martm-Marietta Quarry, north limb limb of of the the along the the quartzite strata in the north along Figure 33. Quartzite breccia, Martin-Marietta Quarry, along strike and 1 km east of Field Locality 4B. hydrothermal activity activity is syncline. The The age age of such hydrothermal along strike and ­ I km east of Field Locality 4B. is syncline. Precambrian, because unknown, other other than than being being Precambrian, unknown, because Cambrian conglomerate. breccia boulders and cobbles of quartzite occur in in the the overlying overlying Upper Upper Cambrian conglomerate. boulders and cobbles of quartzite breccia occur 1 mm 1 mm UW Stable Isotope Lab Analyst: Mike Spicuzza UW Stable Isotope Lab Analyst Mike Spicu2Za Sample Sample 98BB20 986820 breccia, Figure 34. 34. Polished Polished slab Figure slab of of quartzite quartzite breccia, crystals. showing aa vug vug lined lined by by euhedral euhedral quartz showing quartz crystals. VSMOW) in a zoned Figure 35. Variation of ö'°O 180 (%o, of8 (%0, VSMOW) in a zoned Figure 35. Variation breccia. euhedral quartz crystal from the quartzite euhedral quartz crystal from the quartzite breccia. 18 18 �here. quartzite the in seen be also can cleavage widely-spaced a and bedding Cross idea. better a have not do We surface. unusual this produced probably time Pleistocene during silt and sand by blasting wind that suggested Twenhofel W.H. 1930s, the In polish. exceptional an has quartzite of outcrop an highway, the of west just and north meters 100 32). (Fig. gorge the of end south the at spring artesian the behind seen is as quartzite the in joints and bedding along weathering by produced fissures down filtered that sand Cambrian as it interpret We saw? just we that unconformity the below feet 200 least at here doing this is What quartzite. vertical of outcrop an within quartzite of fragments angular enclosing sandstone quartz Cambrian-type tan-colored, has exposure subtle a zone, breccia the of north meters 100 About alteration. hydrothermal of episodes unrelated and different two by affected was Range Baraboo the whether or 2), and 1 (Localities quartzite the of base the modified that fluids River-age Wolf the of variants shallower and cooler are brecciation with associated fluids thermal hydro- the whether seen be to remains It Range. Baraboo the in feature widespread a thus is breccia quartzite of development The syncline. the throughout scattered are vertical, and east-west oriented also features, breccia incipient with veins quartz thin and kilometers, of-20 distance a over syncline Baraboo the of limb north the along occur breccia quartzite of Outcrops dip. vertical and strike east-west an with layers, quartzite adjoining the to parallel oriented and thick meters -400 is zone breccia The The breccia zone is ~ 100 meters thick and oriented parallel to the adjoining quartzite layers, with an east-west strike and vertical dip. Outcrops of quartzite breccia occur along the north limb of the Baraboo syncline over a distance of ~20 kilometers, and thin quartz veins with incipient breccia features, also oriented east-west and vertical, are scattered throughout the syncline. The development of quartzite breccia is thus a widespread feature in the Baraboo Range. It remains to be seen whether the hydro­ thermal fluids associated with brecciation are cooler and shallower variants of the Wolf River-age fluids that modified the base of the quartzite (Localities 1 and 2), or whether the Baraboo Range was affected by two different and unrelated episodes of hydrothermal alteration. About 100 meters north of the breccia zone, a subtle exposure has tan-colored, Cambrian-type quartz sandstone enclosing angular fragments of quartzite within an outcrop of vertical quartzite. What is this doing here at least 200 feet below the unconformity that we just saw? We interpret it as Cambrian sand that filtered down fissures produced by weathering along bedding and joints in the quartzite as is seen behind the artesian spring at the south end of the gorge (Fig. 32). 100 meters north and just west of the highway, an outcrop of quartzite has an exceptional polish. In the 1930s, W.H. Twenhofel suggested that wind blasting by sand and silt during Pleistocene time probably produced this unusual surface. We do not have a better idea. Cross bedding and a widely-spaced cleavage can also be seen in the quartzite here. Rock Hise Van 4C: 4C: Van Hise Rock wall). washboard ripple-cast the did (as south the to be also must up' 'way the that shows geometry its for this, with consistent is bedding cross truncated The Gorge. Ableman's of south the to axis its with syncline a be must structure that that infer then We fold. large a surface axial the parallels roughly cleavage the that assume we part, a is Rock Hise Van which of structure larger the see not can we Although beds. quartzite two these of each in visible is bedding cross addition, In side. either on quartzite pink coarser the into (flattens) refracts cleavage this that note degrees; 20 about northward dipping cleavage slaty shows it Today Quartzite. Baraboo the of part upper the mudstone silty a as deposited was rock the of center the in band dark vertical The within Landmark. Historical National a declared was Rock the 1999, May, In annually. visit professionals and students of hundreds century; a for trips field for stop essential an this made has Rock Hise Van of clarity and accessibility The 'way-up.' or direction facing structural determining for marks ripple and bedding cross folds, drag cleavage, of use the perfected team Their region. Superior Lake the of geology Precambrian the study to side. either on quartzite cross-bedded by bounded layer central Leith later and Hise Van by headed Madison slaty the with east, the from viewed Rock Hise Van 36. Figure at office district a 1882 in established had Survey Geological U.S. the mining, iron of commencement the by Stimulated country. vegetated in clear not typically are which structures, larger-scale inferring for clue scale outcrop- an as bedding and cleavage slaty between relationship geometric fundamental the demonstrate to laboratory a as this used Leith, K. Charles protégé, his and Hise Van R. Charles sign. historic 1999 a and plaque metal 1923 a by described as geologists, for interest historic special of is 36) (Fig. highway the of side east the on Rock Hise Van Van Hise Rock on the east side of the highway (Fig. 36) is of special historic interest for geologists, as described by a 1923 metal plaque and a 1999 historic sign. Charles R. Van Hise and his protege, Charles K. Leith, used this as a laboratory to demonstrate the fundamental geometric relationship between slaty cleavage and bedding as an outcrop­ scale clue for inferring larger-scale structures, which are typically not clear in vegetated country. Stimulated by the commencement of iron mining, the U.S. Geological Survey had established in 1882 a district office at Figure 36. Van Hise Rock viewed from the east, with the slaty Madison headed by Van Hise and later Leith central layer bounded by cross-bedded quartzite on either side. to study the Precambrian geology of the Lake Superior region. Their team perfected the use of cleavage, drag folds, cross bedding and ripple marks for determining structural facing direction or 'way-up.' The accessibility and clarity of Van Hise Rock has made this an essential stop for field trips for a century; hundreds of students and professionals visit annually. In May, 1999, the Rock was declared a National Historical Landmark. The vertical dark band in the center of the rock was deposited as a silty mudstone within the upper part of the Baraboo Quartzite. Today it shows slaty cleavage dipping northward about 20 degrees; note that this cleavage refracts (flattens) into the coarser pink quartzite on either side. In addition, cross bedding is visible in each of these two quartzite beds. Although we can not see the larger structure of which Van Hise Rock is a part, we assume that the cleavage roughly parallels the axial surface a large fold. We then infer that that structure must be a syncline with its axis to the south of Ableman's Gorge. The truncated cross bedding is consistent with this, for its geometry shows that the 'way up' must also be to the south (as did the ripple-cast washboard wall). 19 19 �and Eolian 4D: Basal Basal Cambrian Cambrian Unconformity and 4D: Eolian Dunes Dunes basal Cambrian unconformity again to the northeast (Fig. From Van Van Hise Hise Rock, From Rock, we we can can see see the the basal Cambrian unconformity again to the northeast (Fig. Cambrian conglomerates. high on on the the wall wall at the top 32, right right end). end). The flat-lying strata high 32, at the top of of the the Gorge Gorge are are Cambrian conglomerates. against buried cliffs of sandstones underlie As shown shown in in Figure 32, Cambrian sandstones As underlie these these and and terminate terminate against buried cliffs of eolian dunes blown up these sandstones quartzite. Large, Large, sweeping cross beds in these quartzite. sandstones were were formed formed within within eolian dunes blown up fell from the cliffs to be angular blocks blocks of of quartzite quartzite occasionally against those those cliffs. cliffs. Rare, Rare, scattered angular against occasionally fell from the cliffs to be Late Cambrian sea is sand. The The arrival the encroaching buried, but but not abraded, abraded, by by the the eolian sand. buried, arrival here here of ofthe encroaching Late Cambrian sea is quartzite conglomerate, which is underlain by a recorded by by aa sharp boundary with overlying rounded recorded rounded quartzite conglomerate, which isparallel underlain by a vertical called Skolithos (closely spaced, straight, sandstone layer with marine worm burrows called sandstone Skolithos (closely spaced, straight, parallel vertical 20 mm from 33 -- 20 tubes about 1 mm in diameter and from mm long). long). pocket sample of most of the important features of the Ableman's Gorge provides a geologic vest of the Ableman's Gorge provides a geologic vest pocket sample of most of the important within afeatures mountain deposited, then then folded folded and Baraboo Hills. Baraboo Quartzite was deposited, and metamorphosed metamorphosed within a mountain Ma ago), an elliptical ring of quartzite hills eroded. In range, which was then eroded. In Late Late Cambrian Cambrian time time (500 (500 Ma ago), an elliptical ring of quartzite hills range, encroached, those hills were converted deposits. Then, began to be buried by wind deposits. began Then, as as the the Cambrian Cambrian sea sea encroached, those hills were converted tropics at about 15 degrees south latitude, so islands. North North America America lay to an an atoll-like atoll-like ring ring of of islands. lay in in the the tropics at about 15 degrees south latitude, so to cliffs and tore away quartzite huge waves, waves, which the sea passing tropical storms generated huge which broke broke upon upon the sea cliffs and tore away quartzite and occasionally swept rounded boulders blocks. Repetition of this scenario rounded boulders up up to to 1.5 1.5 m m in in diameter, diameter, and occasionally swept blocks. the islands became buried by sediment, for half haifaa kilometer some of them them offshore for kilometer or or so. so. Gradually Gradually the islands becameisland burieddisappeared by sediment, some by the clearly revealed than here. The Abieman testimony of which is nowhere more clearly revealed than here. The Ableman island disappeared by the buried until the end of the Ordovician highest islands end of Cambrian time, but the highest islands were were not not finally finally buried until the end ofthe Ordovician end Period (ca 440 Ma). Acknowledgments departmental Weare are indebted indebted to to many of our departmental We many ofour encouragement, and and colleagues for for their interest, encouragement, colleagues John Fournelle technical expertise. These include technical expertise. These include John Fournelle (rare gas (electron microprobe,1, Brad Singer Singer (rare (electron microprobe), Brad gas isotopes), isotopes), John Valley (stable Clark Johnson (radiogenic isotopes), Clark Johnson isotopes), John Valley (stable inclusions), who isotopes), and and Phil Phil Brown Brown ('fluid (fluid inelusiam), who isotopes), made possible possible provided analytical analytical facilities facilities that that made provided Baraboo data. data. Several acquisition ofmany of man of the new ofthe new Baraboo Several acquisition analyses that that are are students and technicians pe/formed performed analyses students Alissa Nayniark, reported here, reported here, including ineluding Robb Robb Bunge, Bunge, Alissa Naymark. Brian Hess Hess prepared prepared Ron Schott, Schott, and and Mike Mike Spicuzza. Spicuzza. Brian Ron thin sections, sections, and and Mary various types of high-quality thin ofhigh-quality Mmy composite diagram of the Diman created created the Diman the marvelous marvelous composite diagram ofthe Baraboo paleosol. Geological and and Natural Natural From the From the Wisconsin Wisconsin Geological provided access to the History Survey. Histor.v Survey. Bruce Bruce Brown Brown provided access to the examination which Baxter Hollow Hollow drill drill core. core, the Baxter the examination of ofwhich take aa new look at at the to take prompted us to new look the Baraboo Baraboo Range, Range, the Barron and Mike Mudrey informed us of paleosol, and Mike Mudrey informed us of the Barron paleosol. unmnodJIedBaraboo Baraboo which serves serves as as aa model for unmodified which model for paleosol. 20 20 Cleopatra's Needle, overlooking Devil's Lake Cleopatra's Needle, overlooking Devil's Lake �REFERENCES Anderson, J.L., Cullers, RL. Anorogenic metaluniinous metaluminous and peraluminous peraluminous granite R.L. and Van Schmus. Sciunus. W.R W.R. (1980) Anorogenic plutonism Wisconsin, USA: Contrib. Mineral. PetroL, 311- 328. plutomsm in the Mid-Proterozoic of Wisconsin, Petrol., v. 74, p. 311PTA-system: a Geo-Calc Geo-Calc software software package package for the Brown, T.H., Berman, RG. and Perkins, E.H. (1989) PTA-system: Mineral., v. 74, 74, p. p. 485-487. 485-487. calculation and display of activity-temperature-pressure phase diagrams: Amer. Mineral., Jr. (1970) Geology of the Baraboo Wisconsin: Wisconsin Geol. GeoL Nat. Dalziel, I.W.D. and Dott, RH. Jr. Baraboo district, Wisconsin: History Survey ml. Inf. Circ. 14. 164 pp. 164 pp. in the the north-central north-central United States: States: Resolved Resolved by by plate plate Dott, RH., R.H., Jr. (1983) The Proterozoic red quartzite enigma in collision?: GeoL p. 129-141. Geol. Soc. Amer. Memoir 160. p. 129-141. Dott, R.H., Jr. and Dalziel, I.W.D. (1972) Age and correlation of of the Precambrian Baraboo Baraboo Quartzite of Wisconsin: 552-568. Jour. Geol., v. 80, p. p. Dott, R.H., Jr., Medaris, L.G., Jr. and Schott. RC. (1997) Doll, Schott, R.C. (1997) Post-1760-Ma deposition of the Baraboo Quartzite: Confinnation Soc. Amer. Abstracts with Programs, Programs. v. Confirmation from detrital zircon ages and new field evidence: Geol. Soc. 29, No.4, No. 4, p.13. p.13. of Early Proterozoic Proterozoic quartzites in in the Holm, Hoim. D.K., Schneider, C.D. (1998) Age and deformation of Schneider, D. and Coath, C.D assembly of southern Lake Superior region: Implications Implications for extent of foreland defonnation deformation during final assembly Laurentia: Geology, V. p. 907-910. v. 26, p. 907-9 10. Dott, R.H.. Jr.. J.H.. Johnson, C.M., Schott, R.C. R.e. and and Baumgartner, L.P. (1996) Age Age Medaris, L.G., Jr., Dolt, Jr., Fournelle, J.H.. CM., Schott, Inst. Lake Superior Geol., Abstracts with and geological significance Superior Geol., significance of the Baraboo Quartzite Quartzite: 42nd 1nst. Programs, v. 42, p. p. 31-32. Dott, R.H.. RH.. Jr. Jr. and McSweeney, K. (1997) (1997) The sub-Baraboo paleosol, Medaris, L.G., Jr., Baumgartner, Bau.mgartner, L.P.. Dolt weathering and metasomatism: metasomatism: 43rd 43rd Inst. Lake Lake Superior Superior Wisconsin: Geochemical Geochemical evidence for Proterozoic weathering 43. p. 3 9-40. p. 39-4U. Geol., Abstracts with Programs, v. 43, R.J. (1998) Post-1.76 Ga low-grade metamorphism of Medaris, L.G., Jr., Brown, Brown. P.B. and Bunge. Rl of the the Baraboo Baraboo v. 44, p.89-90. p.89-90. Quartzite: 44th Inst. Lake Superior Geol.. Abstracts with with Programs, v. Medaris, L.G., Jr., and Foumelk Svanbergite in the Baraboo Baraboo Quartzite: Quartzite: Significance Significance for diagenetic Fournelle. lH J.H. (1998) Svanbergite Geol., Abstracts with processes and phosphorous flux in Precambrian oceans: 44th Inst. Lake Superior GeoL Programs, V. v. 44, p.91-92. p.91-92. Medaris, L.G., Jr., Fournelle, 1.H., Broihan, J.H. (1999) Chemical and mineralogical J.H., Boszhardt, R.F. and Broihan. comparison of Baraboo, Barron. and Sioux argillite. metapelite and pipestone: 45th Inst. Lake Superior Geol., Abstracts with Programs. Programs, v. 45. p.35-36. p.35-36. Medaris, L.G., L.G.. lr. Jr. (2000) The Barron saprolite Confirmation of mature chemical weathering in the the source for Mcdaris, saprolite: Confirmation Paleoproterozoic Paleoproterozoic quartz arenites aremtes in the Lake Superior Superior region: 46th Inst. Lake Superior Geol., Abstracts with Programs, v. 46, p. p. 37-38. Naymark, A., Singer, B.S. and Medaris, L. G.. Jr I) Recognition of Wolf River-age metamorphism in the L.G.. Jr. (200 (2001) Baraboo Ar thermochronology: thermochronology: Geol. Soc. Amer. Abstracts with Programs, v. V. 33, Baraboo Range by means of 4°ArP~ 40Ax/39Ar no. 4, in press. Romano, D., Holm, D.K. and Foland. K.A. (2UOO) (2000) Determining the extent and nature of Mazatzal-related overprinting of the Penokean orogenic belt in the southern Lake Superior region, north-central USA: Res.. v. 104, p. Precambrian Res., p. 25-46 Smith, E.I. El. (1978) Precambrian rhyolites and granites in south-central Wisconsin: Field relations and Smith, 875-890. geochemistry: GeoL Geol. Soc. Amer. Bull Bull.... v 89. p. 875-890. SR. and McLennan, Crust: Its Composition and Evolution: Blackwell, BlackwelL Taylor. S.R McLeiman. S.M. SM. (1985) (1985) The ContlI1ental Continental Crust: Oxford, 312 pp. pp. Van Schmus, W.R., Thurman. E.M. and Peterman. Z.E. ZE (1975) Geology and Rb-Sr chronology chronology of Middle Wisconsin: Geol. Soc. Amer. Bull., v. 86. Precambrian rocks in eastern and central Wisconsin BulL, V. 86, p. p. 1255-1265. K.C. (1993) Early Proterozoic Jr. Proterozoic crustal evolution, in Reed, J.C. J.e. Jr Van Schmus, W.R., Bickford. M.E. and Condie. K.c. U.S.. Geology of North America, America. V. v. C-2, GeoL Geol. Soc. Amer., Boulder, et al., eds., Precambrian: Conterminous US. Boulder, p. 270-281. 21 �This page intentionally left blank �Field Trip 2 Geology, Ore Deposits, and Cultural History of the Upper Mississippi Valley Zinc-Lead District M.G. Mudrey, Jr. Wisconsin Geological and Natural History Survey 3817 Mineral Point Road Madison, Wisconsin 53705-5100 T.C. Hunt University of Wisconsin–Platteville Platteville, Wisconsin 53818 Headframe of a southwestern Wisconsin zinc-lead mine circa 1930 (photograph courtesy of Platteville Mining Museum). �This page intentionally left blank �BACKGROUND The Upper Mississippi Valley Zinc-Lead District of southwestern Wisconsin was one of the oldest continuously producing mining districts in the United States. The largest and most productive parts of the district extended across five Wisconsin counties and into small areas in Illinois and Iowa (fig. 1). Over 1.2 million tons of zinc and nearly 100,000 tons of lead were recovered from the Wisconsin part of the Upper Mississippi Valley District from 1910 to 1974. Heyl and others (1959) suggested that an additional 250,000 tons of zinc and 350,000 tons of lead were produced in the Wisconsin part of the district from 1800 to 1910. This field trip provides a geologic, mineral deposit, and cultural overview of the area. Numerous detailed geologic reports about the area have been prepared; the most significant by Heyl and others (1959) documents ____ R5W. 4 5, 8Z0p8 5 3 4 (lW. 2 ShE. 3 2 N. 5.3* - %J 98 AL CR A N FORD E H 111151(1150 So..0411. , 04 WyLodfl CL 20. Elk.d.,° :: a F/\ PlO1( PlO / 20. Pb ,-51k90R00 'I 90 DE L A W 0th_I:. 08 -: J P1.5*—rn. J3UQU( \ sal 0 U B UO 80* Yb..t a S —- S. RSW '4.zo MADISON — I M(H..b smo LA F A 3 4 1 Y 0AV A S / 084101118(0 — E TT E I N A 4W O \J0 •s I I Pb O L.m (0.o V*-. PSZ.o13 c1( I oa.,...jus. ! CX,I2 SM_S ON 08_i —— b.l W110 P111th do. 5u a—. L F'-—— 94 1*, 0 Ce,18 Mi Pt1 -+44 ,1L58 FL A M A K 97 ° / E Pd - ESS 6 RAE INDEX TO MINES AND DIGGINGS 25 . M0.,.IC,.d01.I08 2 LO&1(jIfld000000 3. Cop,., 0111 ,.io. 4 R1500IO0 00.4 O!W1080 5. 3.01410118011*44401(1 7 24 N 6. AlI leAd Clj1(11884101111t 51(101 C 0100W Il1llI• 9 W800$AORId(OIOOdd00lII*l ID 11111. IIIC5*ff11 0* dIgolliR II W00dlO.4I..d10040 8. f( 12. 0It11Cnl*0p IS. D.195W1111110141 IS. 5101. 811ff SWOOP - IS $o!dgqOfl q00.0, IN 05.100111. 4IWIP II 0(1.10 dIRgIIOP IS St*d(01(&IOSddMO. IS. 1010 C SSZAOP 20. M4*0..A9W14( 2!. *40041.444111(1 22. ML C41011 4089040 250808* 0 EXPLANATION 081.180. .1.5840.4110.110. dSIII.404 e1 — Ml.,, 01 5.1858010 1R Geltilt 0.00,89, and P1110.4111. MIt. II 0410 II• P00.1, dli CA.,, 410115 — 41.0800 Pb.Z0,Co.d 16.4. 8AO0 (005011.01 II Figure 1. Map of the main part of the Upper Mississippi Valley district and mineral deposits in outlying parts of the district (from Heyl and others, 1959). 25 �hundreds of mineral properties in Illinois, Iowa, and Wisconsin. A good summary of the geology and economic geology of the district is given by Heyl and others (1970). In addition, more than 1,000 square miles have been mapped and published on standard 1:24000, 7.5-minute topographic maps. The zinc and lead mines of southwestern Wisconsin are part of the earliest producing zinc-lead mining districts in the United States—the Upper Mississippi Valley District. There was a small amount of production in Minnesota. Production prior to 1800 was very small. Use of galena and mining were reported by explorers Jean Nicolet in 1634 and by Nicolas Perrot in 1692. In 1788 Julien Dubuque obtained permission from the Sauk and Fox to work lead mines in the area.. Dubuque mined, with these Native Americans as his labor force; on the west bank of the Mississippi River in the vicinity of what is now Dubuque, Iowa. This was the principal mining center in the region from 1788 until Dubuque’s death in 1810. Early white miners worked the properties during the summer, returning south in the winter. The arrival of permanent settlers in 1825, the Winnebago Peace Treaty, and “lead rush of 1827” established the Upper Mississippi District as a major mineral producer and resulted in the states of Illinois, Iowa, and Wisconsin. The Mexican War of 1847, the California Gold Rush of 1849, and the cholera epidemic of 1854 brought the district into decline, but by 1859 and the Civil War, production of lead increased, and eventually zinc production began. The District remained in production until 1978 with the close of the Eagle–Picher mine in Shullsburg. Around 1906, systematic mine mapping was begun by students and faculty at the Wisconsin School of Mines, now University of Wisconsin–Platteville. In 1946 systematic geologic mapping was initiated by the U.S. Geological Survey. This mapping program was built on the foundation of mine maps and mineral reserve studies initiated during World War II. Thecooperation of the former U.S. Bureau of Mines, U.S. Geological Survey, University of Wisconsin–Platteville, and the Wisconsin Geological and Natural History Survey led to the development of the Wisconsin Mineral Development Atlas (Heyl and Broughton, 1980). In a series of atlas plates at 1:2400-scale, mine workings, drillhole location and number, and selected surface features (roads, lead digs) are shown. The details of the 30,000 drillholes are kept in approximately two dozen large binders covering Grant, Iowa, and Lafayette Counties. Supplemental data include Green County. From this information Broughton (1991) estimated 12.5 million tons of ore averaging 4.94 percent zinc and 0.47 percent lead remain in place. There are large areas for which there is no modern (post-1900) mineral exploration, and little exploration data below the base of the Sinnipee Group. The field trip begins in Madison, Wisconsin, proceeds to Platteville (Stop 1) for an overview of the district at the Bevan Mine and Rollo Jamison Museum. The Bevan Mine is an 1845 lead mine that produced more than two million pounds of lead ore in one year. The underground workings include two dioramas, one of an 1840 crevice lead mine, and the other a 1935 zinc mine. The Mining Museum includes numerous dioramas about the regional geology and mineral deposits and how miners in the 19th century went about their business. The geologic stop will be west of Dickeyville on Potosi Hill (Stop 2). This road cut contains St. Peter sandstone (Ancell Group) at the base to Galena Dolomite (Sinnipee Group) at the top. Two stops will be made to address mine reclamation: west of New Diggings (Stop 3), where one of the few remaining area of lead digs is preserved (natural reclamation from 1840), and Shullsburg at the former Eagle–Picher mine (Stop 4). The trip ends with a tour of Pendarvis State Historical Site (Stop 5), which recreates the cultural–historical setting of the 1840s mine district. 26 �Geology and Ore Deposits The zinc-lead deposits of southwestern Wisconsin are a classic example of the stratabound Mississippi Valley-type zinc–lead deposits. Although deposits are known to occur in most of the lower Paleozoic dolomite and sandstone units of the district (fig. 2), all important ore bodies are in the Middle Ordovician Sinnipee Group (Platteville, Decorah, and Galena Formations). The most common type of deposits (fig. 3) are (1) those associated with vertical or steeply inclined joints or fractures, called gash-vein or crevice deposits (dominantly galena with gangue); j E ZINC ' z I/ i/z / / 2 A/,/ / I a % k Average Maquoketa shale 200 - Dolomite, buff, cherty; argillaceous near base 110 108—240 // Dolomite, yellowish-buff, thin-bedded, shaly 40 Dolomite, yellowish-buff, thick-bedded;.Receptacuhtes in middle 80 / - ,, 7 /7 1" Dolomite, drab to buff; cherty; Receptac-uhies near base Platteville formation Limestone and dolomite, brown and grayish; green, sandy shale and phosphatic nodules at base St. Peter sandstone Sandstone. quartz, coarse, rounded - 105 35—40 . DISCONFORMITY -DISCONFORMITY— -r-±7 225 - Dolomite, limestone, and shale, green and brown; phosphatic nodules and bentonite near base Decorah formation —— 90 // "/ / 1 0 Dolomite, buff, cherty; Pentamerizs at top, Shale, blue, dolomitic; phosphatic depauperate fauna at base Galena dolomite > — thickn:ss, Description 40+ — — ----- -- Prairie du Chien group (undifferentiated) Trempealeau formation 280- Dolomite, light-buff, cherty; sandy near base and in upper part; shaly in upper part .: - 0— 320 240 .____________ — Sandstone, siltstone, and dolomite 120-150 Franconia sandstone Sandstone and siltstone, glauconitic 110—140 Dresbach sandstone Sandstone •I- -/- - a z a, 0. 0. Eau Claire sandstone 60-- 140 Siltstone and sandstone Mount Simon :.' . -- - -. -: sandstone ---:-:-:•:•:-: Sandstone 700— 1050 440— 780 Figure 2. Simplified stratigraphic section showing relative quantitative stratigarphic distribution of lead and zinc in the Upper Mississippi Valley district (from Heyl and others, 1978). 27 �(2) those associated with inclined and horizontal fractures, called pitch-and-flat deposits (sphalerite with minor galena and gangue); (3) those consisting of fine galena and sphalerite scattered through the country rock, called disseminated deposits; and (4) those consisting of angular breccia or country rock fragments cemented with ore and associated minerals, called breccia deposits. The most abundant gangue minerals with the ore minerals sphalerite and galena are calcite, pyrite, marcasite, barite, and rarely, chalcopyrite. The ore deposits are contained within weak structures produced by gentle folds in the dolomite strata. The ore deposits show vertical and regional zoning. Copper, barium, nickel, and arsenic are abundant in the east-central part of the district. Vertically, lead is greater in the higher part of a mineralized area; zinc, iron sulfides, nickel, and secondary dolomite are more abundant in the deeper deposits, generally conforming to structural control. Details of the district and individual properties are given in Heyl and others (1959). The paragenetic sequence generally involves early deposition of quartz, dolomite, pyrite, marcasite, with several generations of sphalerite and galena. Mineralization of cobaltite, chalcopyrite, chalcocite, millerite, and enargite are reported. Late gangue minerals include most of the calcite. It should be emphasized that the paragenetic sequence is generalized. Deposition Figure 3. Diagrammatic plans and sections illustrating typical patterns of gash-vein lead deposits and underlying pitch-and-flat deposits of the arcuate and linear types and their stratigraphic position to one another (from Heyl and others, 1978). 28 �throughout the general sulfide period took place under conditions of rhythmic oscillations in composition and temperature; such oscillations formed the hundreds of minute color bands characteristic of all the main minerals. McLimans and Barnes (1975) argued that most, if not all, of the distinct color-banding in sphalerite from mine to mine and area to area are coeval. McLimans and Barnes (1975) also argued for warmer temperatures (up to 220°C) for the deposits; most other workers suggest 150°C is a maximum. The high temperature seems to be supported by Rowan and Goldhaber (1996) and Zimmerman (1986), but their interpretations differ. Rowan and Goldhaber argued for a relatively thin Paleozoic cover (less than 3,000 ft) and higher heat flow; Zimmerman argued for normal geothermal gradients, but deeper burial (up to 9,000 ft). Outside of ore deposits, regional maximum temperatures were quite low (50 to 90°C, Blabaum, 1995). I interpret these data to suggest that the region was not deeply buried, but that high temperature mineral solutions were restricted to faults and fracture channels, and hence ore deposits were hotter than country rock. Extensive lead isotope data clearly identify disconformable lead that varies regionally; lead isotope ratios are lowest to the west and south. It is generally thought that mineralization of the district occurred during the Permian–Pennsylvanian by long-distance transport of metalbearing brine from the adjacent Illinois basin. FIELD TRIP The field trip (fig. 4) will visit the Platteville Mining Museum and Bevan Mine (Stop 1), examine Ordovician geologic exposures at Potosi Hill (Stop 2), an historic unreclaimed area of 1830 lead digs near New Diggings (Stop 3), modern metallic mine reclamation at the former Shullsburg zinc–lead mine/mill site (Stop 4), and the Pendarvis State Historical Site (Stop 5). 1 Platteville Museum 2 Potosi Hill 3 New Diggings 4 Shullsburg Reclamation 5 Pendarvis Miles Figure 4. Map showing field trip stops. 29 �Stop 1: Platteville Mining Museum and Rollo Jamison Museum Location: SW¼NE¼ sec. 15, T3N, R1W, Grant County, Wisconsin (Platteville 7.5-minute topographic quadrangle, 1952). Authors: Modified from <http://platteville.wi.us/visitors/mining.html>. Description: The Platteville Mining Museum traces the development of lead and zinc mining in the Upper Mississippi Valley through models, dioramas, artifacts, and photographs. The surface part of the museum includes dioramas of mining and a mosaic of maps showing at 1:24,000scale the geology and structure of the district. The underground part of the Museum, the Bevan Lead Mine, is an 1845 lead mine which produced over two millions pounds of lead ore in one year and is accessed via a walk down decline. The diorama in the mine shows how mining was accomplished in the 1840s (crevice deposits of lead), and in the 1930s (pneumatic jackleg drill and mine carts). Ceiling bolting is extensive in the underground mine; this was a demonstration area in the the past on how to install mine bolts. A small, reconstructed headframe and hoist and a track with a 1931 locomotive and ore cars complement the underground mine. For more information contact: City of Platteville Museum Department, 405 East Main Street, P.O. Box 780, Platteville, Wisconsin 53818; telephone (608) 348-3301. Stop 2: Potosi Hill—Ordovician Sinnipee Group Location: Roadcut at east side of U.S. Highway 61 in the SW¼NW¼ sec. 7, T2N, R2W, Grant County (Potosi 7.5-minute topographic quadrangle, 1972; fig. 5). Author: M.G. Mudrey, Jr. (modified from Ostrom, 1987). Description: This is an excellent and easily accessible exposure of the upper part of the Ancell Group (St. Peter and Glenwood Formations), and the Sinnipee Group (Platteville, Decorah, and Galena Formations). The exposure consists of a lengthy roadcut on the north side of U.S. Highway 61 (Whitlow and West, 1966). At least two east–west faults are recognized in the outcrop, the most significant (about 10 ft of throw) is in the valley between the upper and lower exposures. On the southeast wall of the quarry at the north end of the roadcut can be seen an example of the pitch and flat structure that hosts the zinc–lead mineralization in the district. The Sinnipee Group is the principal host of zinc and lead mineralization in the Upper Mississippi Valley Base-Metal District. This locality has been well studied over the past 50 years. The lithologic description given is that of Agnew (1956) with modifications by Ostrom (1978, 1987), and Mudrey (field reviews from 1976 to 2001). Agnew’s descriptions are the most comprehensive; however, slumping and outcrop deterioration and road construction have changed the ease with which individual parts of the exposure may be examined. The description given here is a composite of Agnew, Ostrom, and Mudrey. Figure 6 shows the geologic section exposed at the Potosi Hill roadcut. The base of the section consists of Ordovician sandstone of the St. Peter Formation, Ancell Group. The Tonti Member is the thickest of the three members in the formation, and consists dominantly of friable fine- to coarse-grained sandstone. Thickness can vary considerably, from absent to over 100 m thick. This variation is attributed to deposition on an erosional surface; deep channels are recognized elsewhere, particularly in La Crosse County. The basal unit of the St. Peter is the clay-rich Readstown Member, which may be a reworked residual regolith. The upper unit of the St. Peter seen in this exposure is a bluish-gray silt to shale, which can be locally absent in the region. Bioturbation in this unit is common. The St. Peter appears to be a near-shore deposit (Dott and others, 1986). In the area of the type locality, St. Paul, Minnesota, it appears to be entirely ma30 �rine; in the Madison and southern Wisconsin area, it is entirely nonmarine (Winfree and Dott, 1983). Locally the St. Peter is well cemented, ranging from silica to hematite. Habermann (1978) interpreted most of the cementation as the result of duricrust development during the Ordovician; however, in places, notably in area of known zinc-lead sulfide mineralization, some of the hematite cementation appears to be the result of weathering of minor sulfide ore bodies. The overlying Ordovician Sinnipee Group consists of a basal Figure 5. Topographic map showing location of field trip stop 2. Platteville Formation of several dolomitic limestone to dolomite members; the Decorah Formation, a shaly dolomite; and the uppermost Galena Formation, a vuggy weathering cherty dolomite. The lowermost member of the Platteville Formation, the Pecatonica Member, was formerly quarried in the district for building stone. The Quimbeys Mill Member, the uppermost member of the Platteville, is a sublithographic dolomite. This less than 1-m thick bed is very distinctive and is used through the district as a marker to the base of the mining horizon (termed glass rock because of the conchoidal fracture). The overlying Decorah Formation has been defined in many ways over the years, but is generally mapped as the shaly, dolomite part of the Sinnipee Group. Members in the Decorah are easily recognized. The basal Spechts Ferry Member is overlain by the Guttenberg (pronounced GUT-ten-berg), which contains brown petroliferous shale partings. This unit recognized in the mining district as “oil rock” and is a source bed for petroleum in Iowa and Michigan, where it is deeply buried. The Ion Member overlies the Guttenberg and consists of a blue unit and a gray unit, both very shaly dolomite with minimal structural strength. Most of the zinc–lead mineralization in the area is hosted by the Ion. Thickly bedded Galena Dolomite overlies the Decorah Formation. In the absence of fossils, field mapping has relied on the abundance of chert to divide the unit into a lower cherty unit and an upper relatively non-cherty unit that is less dolomitized. Weathering of the unit results in a mosaic outcrop pattern (locally termed honey-comb), where the less dolomitic parts of the rock are dissolved in preference to the harder, more indurated well-dolomitzed parts. Receptaculites is abundant in the upper, non-cherty part of the section. 31 �120GALENA FORMATION 110- Potosi Hill Section SE ¼NW ¼ sec, 7, T2N, R2W Grant County, Wisconsin 100OECORAH FORMATION Ion Member 90- 80Guttenberg Member 70 60- 50McGregor Member 40- 30- 20Pecotonice Member 10- 0- Figure 6. Potosi Hill section; explanation on following pages. (Modified from Ostrom, 1987.) 32 �Thickness (ft) Unit Member thickness (ft) Sinnipee Group Galena Dolomite Formation Prosser Member (upper cherty) Dolomite, olive drab to light brown, thick- to thin-bedded; medium to coarse grained, vuggy, abundant white chert; Receptaculties near top 4.0 44+ (lower Receptacultities Zone) Lower Receptaculities Zone; dolomite, olive drab to light brown, thickbedded, medium-to-coarse-grained, abundant chert, abundant Receptaculities 16.0 (lower cherty) Lower Cherty Zone; dolomite, olive drab to light brown, thick- to mediumbedded, medium to coarse grained, bands of chert nodules 14.5 (buff) Lower Buff Zone; dolomite, light brown, slight green mottling; thickbedded 9.5 Decorah Formation Ion Dolomite Member (Gray Unit) Dolomite, olive to gray, medium- to thick-bedded, vuggy, green shale partings throughout, sparry calcite present 13.5 (Blue Unit) Blue unit; dolomite, purplish gray, medium grained, slightly fossiliferous. Green shale present as partings, and as 0.5-ft bed, 0.8 ft below the top of the Interval, calcite present 5.1 Shale, green dolomitic shale in middle of interval 0.9 33 19.5 �Guttenberg Limestone Member (Oil Rock) Limestone, pinkish to purplish brown, fine grained to sublithographic, fossiliferous, upper 1 ft fine- to medium-grained, red-brown shale present as parting, calcite and limonite and iron sulfide present in small amounts 4.6 Shale metabentonite, brownish orange, crumbly, sticky when wet 0.1 Limestone, purplish brown, sublithographic, thin wavy bedding, fossiliferous, brown carbonaceous shale present as thin beds and partings, calcite and limonite present. Thin metabentonite bed at base 9.6 Limestone, brown gray, fine grained, thick-bedded 1.0 15.3 Spechts Ferry Shale Member (Clay Bed) Shale, orange gray, calcareous, and limestone, tan gray, fine grained, limestone 0.4 to 0.7 ft from base of unit 0.8 Limestone, gray, fine-grained, thin bedded 0.6 Shale, gray, green, brown, fissile, some beds fossiliferous, limestone present as thin lenses near middle of the interval 3.2 Limestone, tan with iron oxide mottling, fine grained, thin bedded 0.8 Shale, gray-green-brown. Fissle with thin lenses of gray fine-grained limestone 1.7 Limestone, dark to light gray, thin-bedded fossiliferous 0.7 Shale, brown-green-orange-gray, brown carbonaceous shale parting at top 0.5 Limestone, purplish brown, fine grained, thin-bedded, very fossiliferous, fucoids at base 0.5 Metabentonite, orange, sticky when wet, with brown shale partings 0.2 8.8 Platteville Formation Quimbys Mill Member (Glass Rock) Limestone, dark purplish gray, sublithographic, thick-bedded, conchoidal fracture, irregular upper surface, shale at base 0.8 0.8 14.5 29.5 MacGregor Member (Magnolia) Limestone, light grayish tan, fine grained, dense, partings of yellowish platy shale, very fossiliferous, thin 2 in. to 10 in. beds, upper 5 ft generally thicker bedded 34 �Mifflin Sub-member Limestone, light grayish brown, fine-grained, dense, medium-to thickbedded, discontinuous partings of bluish-green shale, fossiliferous 15.0 Pecatonica Member (Quarry Beds) Dolomite, brownish gray, fine to medium crystalline, sugary texture, thinto medium-bedded, even-bedded, beds 0.1 to 18 in. thick. Weathered surface shows distinct but discontinuous thinner beds 3.0 Dolomite, bluish, medium-grained, granular, sugary textured, argillite 1.0 Dolomite, brownish gray, fine to medium grained, crystalline, sugary texture, thin- to medium-bedded, even-bedded, beds about 1in. thick 8.0 Dolomite, bluish, medium grained, granular, sugary textured, argillite 1.0 Dolomite, brownish gray, fine to medium grained, crystalline, sugary texture, medium-bedded, even-bedded, beds thicker than 1 ft 6.0 Dolomite, brownish gray, fine to medium grained crystalline, sugary texture, medium-bedded, even-bedded, beds about 1 ft thick 4.6 23.6 Ancell Group Glenwood Formation Hennepin Member (Shale) Very silty, sandy dolomite, yellowish brown, abundant phosphatic pellets up to 2 mm in diameter, scattered round medium quartz sand grains, poorly sorted, iron-oxide cemented 0.5 0.5 1.5 1.5 Harmony Hill Member Silty shale, brown and bluish green grading downward to bluish green with some redish brown, little rounded medium grained quartz sand, abundant pale green clay in matrix, reworked/bioturbated texture St. Peter Formation Tonti Member (Sandrock) Sandstone, light yellowish gray, very fine to medium grained, Some light brown stains cross-bedded Base of exposure in drainage ditch 35 �Stop 3: New Diggings Lead Digs Location: Intersection of County Highways J and W, NW¼NW¼SW¼ sec. 22, T1N, R1E, Lafayette County, Wisconsin (New Diggings, 7.5- minute quadrangle, 1952). Author: M.G. Mudrey, Jr. (2001). Description: The area south of the intersection was extensively mined for residual lead in the early part of the nineteenth century. Miners would dig down over surface occurrences of galena, collecting residual galena in the soil. At some depth, depending principally on the competency of the soil profile, the dig was abandoned, and another started adjacent to the first dig. Once the area had been mined, miners would move on to another area. There was no attempt at active reclamation or deeper mining at that time. The area north of the intersection was also heavily mined; however, leveling of the surface in the 1950 for agriculture effectively removed the topographic evidence of mining. The high alkalinity of the soil derived from the Sinnipee Group dolomite neutralizes any acid mine drainage from oxidation of sulfide minerals and allows native plant communities to rapidly recover. Stop 4: Shullsburg Mine Site—Metallic Mine Reclamation Location: East side of Lafayette County O, 2 miles south of Shullsburg, sec. 22, T1N, R1E, Lafayette County (Shullsburg 7.5-minute topographic quadrangle, 1972; fig. 7). Authors: T.C. Hunt (University of Wisconsin–Platteville) and M.G. Mudrey, Jr., 2001. Figure 7. Topographic map showing location of field trip stop 4. 36 Description: The Shullsburg/Blackstone mining unit, known as the Calumet and Hecla Mine, was discovered by Calumet and Hecla Consolidated Copper Company about 1947 in a systematic exploration drilling program. In 1949 a 360-ft shaft was sunk on the property. Typically in the Upper Mississippi Valley Base Metal District, small ore bodies were identified from surface drilling exploration, and cross-cuts and drifts were driven from existing areas of mining to the newly discovered ore. Mining was by room and pillar methods. Within the mine, many of the drifts converged to make this complex one of the largest producers in the district. Galena and sphalerite were the principal ores mined. The most abundant minerals associated with the ore minerals were calcite, pyrite, marcasite, barite, and more rarely, chalcopyrite. Some individual intersections of ore over a 10-ft vertical interval assayed at more than 14 percent zinc. The ore was processed by �an on-site 1,000 ton per day flotation mill. The mill feed ranged between 4 to 6 percent zinc (Heyl and others, 1959; Heyl and others, 1970). The ore was hosted in the Decorah Formation, about 280 ft below the pre-mine groundwater surface. Groundwater pumping to dewater the mine ranged from 4 to 17 million gallons per day, and the cone of depression extended over 12 square miles (Evans and Cieslik, 1985). Eagle Picher Company (EP) acquired the properties in 1954. EP operated this site continuously from 1954 until 1979. The zinc and lead ore body was accessed via a decline that was built to replace the shaft that had succumbed to fire. EP extracted about 1,000 tons per day using a modified room and pillar mining method and processed about 1,500 tons of ore per day in the flotation mill on-site. Ore was also received from the nearby Bear Hole mine (Reinke, 1977). In the 1970s Wisconsin changed its mining laws to require a reclamation plan and financial bonding in conjunction with a mining permit. On April 18, 1978, Eagle-Picher received a permit from the Wisconsin Department of Natural Resources to mine zinc and lead at the site. The permit to mine was secured with only an approved reclamation plan; no bond was required because the operation was permitted as a nonconforming project site (Wisconsin Department of Natural Resources, April 18, 1978). The permitted mining site covered 72 acres at the Shullsburg mine and mill and an additional acre at the Blackstone pump site. In 1981, Inspiration Development Company gained ownership of four nonconforming mining units in southwestern Wisconsin that were under permit with the Wisconsin Department of Natural Resources; the Shullsburg site was among them. Most of these sites had been developed decades earlier, but were not permitted until mid to late 1970s. Only the Bear Hole (a nearby ore deposit) and Shullsburg units produced ore after they were permitted. The mining units in southwestern Wisconsin were shut down permanently during the period of 1978 and 1979. IDC never produced ore after its purchase, but IDC assumed responsibility for the reclamation of the site which is currently in different phases of reclamation. The primary environmental concerns were groundwater and surface water pollution, dusting from waste piles, stockpiles, and roads, aesthetics, and safety concerns. Following the closure of the Shullsburg Mine water quality in some nearby private watersupply wells deteriorated. Affected wells were located within the cone of depression created by pumping to keep the underground mine dewatered (fig. 8). Following mine closure, groundwater from these wells showed increased levels of sulfate, iron, calcium, magnesium, and total dissolved solids. The mechanism of contamination was postulated to be the following sequence: (1) oxidation of sulfide minerals, (2) formation of soluble sulfate mineral phases, (3) breakdown of carbonate host rock by acid produced during sulfide oxidation, and (4) dissolution of soluble materials by groundwater with rock strata that was previously dewatered during active mining (Evans and others, 1983; Evans and Cieslik, 1985). The impacted wells were reconstructed or abandoned and new wells constructed into the underlying sandstone aquifer. Numerous relic mine waste piles exist nearby this site including flotation tailings, jig tailings, waste-rock piles, and junk piles. Waste materials were trucked off-site as merchantable by-product for purposes of construction or agriculture. The decline portal has been backfilled, graded, top dressed, and stabilized with vegetation. Surface drainage has been restored so that runoff water is discharged from the sites without significant erosion. Observable subsidence or caving has not occurred at this site. This site predates topsoil salvage requirements. Generally, no topsoil remained for redistribution during reclamation activities, but where it was available in the form of dikes and berms, it was used to topdress the site. IDC has routinely used cow manure as a substitute for topsoil to 37 �Figure 8. Areal extent of cone of depression developed around the Bear Hole and Shullsburg Mines, Lafayette County, Wisconsin (from Evans and others, 1983). \- 38 �effect the establishment of vegetation. Most of the site is reasonably well stabilized with vegetation. Introduced pasture grasses and legumes were approved by the Wisconsin Department of Natural Resources for the last phase of final reclamation at this site. During the first phase of final revegetation in the mid-1980s, native trees, grasses, and shrubs were installed (Hunt, 1989). The area is not sited in wetland habitat. Proximity to a perennial stream may have impacted the riparian area, but baseline data are scarce. Presently, the impacts appear minimal. The existence of on-site settling ponds have created wetland habitat, albeit small and of marginal value. The designated post-mining land use for the nonconforming Shullsburg mining unit is a designated wildlife area, which is compatible with the adjacent land-use pattern. The current management level of this site is low, but there are plans to conduct a prescribed fire to help revitalize the native vegetation. The topography of this site is modified by waste piles, steepsided settling ponds, and relic mine openings and artifacts. The existing vegetation is a mixture of introduced agronomic pasture grasses and native vegetation. The existing oak groves present on these sites represent pre-settlement vegetation. Stop 5: Pendarvis State Historical Site (Mineral Point) Location: NE¼SE¼ sec. 31, T4N., R3E., Iowa County, Wisconsin (Mineral Point 7.5-minute topographic quadrangle, 1980). Author: Modified from <http://www.shsw.wisc.edu/sites/pend/>. Description: The museum consists of a series of stone buildings salvaged in the 1930s by Robert Neal and Edgar Hellum, who formed a partnership to acquire, restore and rebuild the few remaining cottages built by Cornish miners in the early 19th century. Their venture, called Pendarvis after an estate in Cornwall, preserves the regional cultural legacy of the early history of mining in southwest Wisconsin. Early miners extended their search for lead from northwestern Illinois into Wisconsin around 1820. Much of this early mining was seasonal, with the miners returning south to Illinois in the winter. With the arrival of Cornish miners and their families in the late 1830s, small villages were developed in proximity to the lead mines. In the hill south of Pendaris is the Merry Christmas Mine. Originally lead was mined by recovering residual galena from the soil (the hill side is extensively pockmarked with lead digs). Once the residual lead was recovered, the early miners sunk shallow shafts (above the water table) to recover galena from bedrock. For more information: Pendarvis State Historical Site, 114 Shake Rag Street, Mineral Point, Wisconsin 53565; telephone (608) 987-2122. REFERENCES Agnew, A.F., 1956, in Agnew, A.F. and Sloan, R.E., The Ordovician Rocks of Southwestern Wisconsin and Northeastern Iowa: Second Day, October 29, 1956: in G.M. Schwartz, ed., Guidebook for Field Trips, Minneapolis Meeting, 1956, Geological Society of America, p. 86. Blabaum, J.M., 1995, Origin and maturation of the organic matter in the Middle Ordovician Guttenberg Member of the Decorah Formation of southwestern Wisconsin: Geoscience Wisconsin, v. 15, p. 71–76. 39 �Broughton, W.A., 1991, Zinc and lead reserves of southwest Wisconsin: The undrilled lead digs of southwest Wisconsin: Wisconsin Geological and Natural History Survey Open-File Report 1991-5, 11 p. Dott, R.H., Jr., Byers, C.W., Fielder, G.W., Stenzel, S.R., Winfree, K.E., 1986, Aeolian to marine transition in Cambro-Ordovician cratonic sheet sandstones of the northern Mississippi Valley, USA: Sedimentology, v. 33, no. 3, p. 345–367. Evans, T. J., and Cieslik, M.J., 1985, Impact of groundwater from closure of an underground zinc–lead mine in southwest Wisconsin: Wisconsin Geological and Natural History Survey Miscellaneous Paper 81-01, 16 p. Evans, T.J., Cieslik, M.J., and Hennings, R.G., 1983, Investigation of the effects of recent mine closings on ground-water quality and quantity in the Shullsburg area: Wisconsin Geological and Natural History Survey Open-File Report 1983-1, 36 p. Habermann, G.M., 1978, Mineralogic and textural variations of the duricrust in southwestern Wisconsin: Ph.D. thesis, University of Wisconsin–Madison, 153 p. Heyl, A.V., Jr., Agnew, A.F., Lyons, E.J., Behre, C.H., Jr., and Flint, A.E., 1959, The Geology of the Upper Mississippi Valley Zinc-Lead District: U.S. Geological Survey Professional Paper 309, 310 p. Heyl, A.V. (and Broughton, W.A.), 1980, A very brief history of the Wisconsin mineral development atlas; general information and procedures concerning zinc-lead atlas: Wisconsin Geological and Natural History Survey Open-File Report 1980-4, 6 p. Heyl, A.V., Broughton, W.A., and West, W.S., 1970 (revised 1978), Geology of the Upper Mississippi Valley Base-Metal District: Wisconsin Geological and Natural History Survey Information Circular 16, 45 p. Hunt, T.C. 1989. Mined land reclamation in Wisconsin since 1973: Ph.D. Thesis. University of Wisconsin–Madison. 194 p. McLimans, R.K. and Barnes, H.L., 1975, Sphalerite stratigraphy in the upper Mississippi Valley Pb-Zn deposits: Economic Geology, v. 70, no. 7, p. 1324–1325. Ostrom, M.E., 1978, Potosi Hill Exposure, in Geology of Wisconsin: Outcrop descriptions: Wisconsin Geological and Natural History Survey, Gr-7/2N2W, 4 p. Ostrom, M.E., 1987, Middle Ordovician rocks at Potosi Hill, Wisconsin: in Geological Society of America Centennial Field Guide–North-Central Section, 1987, p. 201–204. Reinke, G. H., December 2, 1977, Eagle-Picher Industries, Shullsburg Mine and Mill Unit Environmental Impact Assessment Worksheet, Bureau of Solid Waste Management: Wisconsin Department of Natural Resources. 40 �Rowan, E.L. and Goldhaber, M.B., 1996, Fluid inclusions and biomarkers in the Upper Mississippi Valley zinc-lead district; implications for the fluid-flow and thermal history of the Illinois: U.S. Geological Survey Bulletin B 2094-F, 34 p. Whitlow, J.W., and West, W.S., 1966, Geology of the Potosi Quadrangle, Grant County Wisconsin, and Dubuque Country, Iowa: U.S. Geological Survey Bulletin 1123-I, p. 533–571. Winfree, Keith, and Dott, R.H., Jr., Progress on the St. Peter Sandstone of the Upper Midwest, in M.G. Mudrey, Jr., Field Trip chairman, Sedimentology of Ordovician Carbonates and Sandstones in Southwestern Wisconsin: Field Trip Guide Book, 17th Annual Meeting, North-Central Section, Geological Society of America, 1983, p. 4–13. Wisconsin Department of Natural Resources. April 18, 1978, Order Number EX-78-32B. Eagle Picher Industries, Shullsburg Mine and Mill Unit Permit to Mine: Wisconsin Department of Natural Resources, 3 p. Zimmerman, R.A., 1986, Fission-track dating of samples of the Illinois drill-hole core: U.S. Geological Survey Bulletin 1622-J, p. 99–108. 41 �This page intentionally left blank �Field Trip 3 Economic Geology of the Baraboo and Waterloo Quartzites of Southern Wisconsin Bruce A. Brown Wisconsin Geological and Natural History Survey 3817 Mineral Point Road Madison, Wisconsin 53705-5100 Frank R. Luther Department of Geology University of Wisconsin–Whitewater Whitewater, Wisconsin 53190 James W. Schmitt D.L. Gasser Construction Box 441 Baraboo, Wisconsin 53913 Susan M. Courter Michels Materials Box 128 Brownsville, Wisconsin 53006 Jennifer Lien The Kraemer Company Box 235 Plain, Wisconsin 53577 Stockpiles of Baraboo quartzite aggregate, LaRue Quarry. �This page intentionally left blank �INTRODUCTION The Proterozoic quartzites of the Baraboo Range and the Waterloo area of south-central Wisconsin have been quarried for a variety of industrial mineral products since the late nineteenth century. The extreme hardness and refractory properties of the quartzite were recognized early, and both areas were important producers of refractory blocks, mill liners, grinding pebbles, and several types of abrasives. Grinding pebbles were used in place of steel balls in mills that ground clay and feldspar for the ceramic industry, talc and other materials for the cosmetic industry, and in grinding some types of metallic ores. The most commonly used pebbles were flint beach pebbles, of 3 to 4 inch size, hand picked from beaches in northern Europe. Wisconsin State Geologist William O. Hotchkiss was instrumental in promoting Wisconsin quartzite for this use when European sources were cut off during World War I. The early operations used pebbles from gravels derived from the Paleozoic conglomerates formed adjacent to the quartzite, such as we will see at Stop 2. Later, grinding balls were manufactured by crushing quartzite, sorting for size and shape, and tumbling to achieve rounding. The last grinding ball producer, the Baraboo Quartzite Co., ceased operations in the 1980s. Today the primary uses for southern Wisconsin quartzite are crushed stone aggregates, riprap, and breakwater stone. For many years, the hard, abrasive nature of the quartzites, which average 98 percent silica, made crushed quartzite aggregates prohibitively expensive due to excessive wear on crushing and screening equipment. Crusher jaws quickly wore out, and steel wire screens commonly had to be replaced daily to maintain tight gradation specifications. Modern alloys, combined with the use of plastic and rubber-faced screens, have increased efficiency and reduced the cost of producing railroad ballast and a variety of quartzite aggregates. This guide is intended to provide the locations of stops and a brief outline of the geology and industrial process and products at each site that we intend to visit. Additional handouts relating to products, testing, specifications, and history will be provided on the day of the trip. We have not provided lengthy detailed descriptions of each stop because the geologic features we will see will likely change with the next round of production blasting. Wherever possible, we have tried to cite appropriate published field guides that explain the geology of nearby or relevant exposures regularly accessible to the public. We will visit six active quarrying operations on Trip 3. Company personnel will be available to describe the geology, mining, and processing methods, and the products made at each site. Because these are active operations, we will need to sign releases and MSHA Part 46 site-specific hazard training forms. Please remember to observe all safety rules and use common sense at all times. Especially stay away from highwalls and unstable slopes. GEOLOGY OF THE QUARTZITES Because the primary focus of this trip is on economic geology, only a brief discussion of the geologic history of the Baraboo Interval rocks of southern Wisconsin is included. The reader is referred to general discussions of tectonic setting (Greenberg and Brown, 1984) and previous guides to the Waterloo area (Luther, 1992, 1997), and the Baraboo area, (Dalziel and Dott, 1970; Malone and others, 1997; Medaris and Dott, 2001). The Baraboo and Waterloo quartzites belong to a group of sedimentary rocks that include the McCaslin, Rib Mountain, Necedah, Hamilton Mound, Flambeau, Barron quartzites of Wisconsin and probably the Sioux Quartzite of Minnesota that were deposited on continental crust formed during the 1850 Ma Penokean Orogeny (Greenberg and Brown, 1984). For many years the exact age of the quartzites and the timing of deformation and metamorphism were hotly debated. Recent work summarized by Medaris and Dott (2001) suggested that the Baraboo quartzite was deposited on a basement of 1,760 Ma. Granite and rhyolite, and that most of the 45 �metamorphism and alteration at Baraboo resulted from hydrothermal activity accompanying emplacement of the Wolf River Batholith at around 1,460 Ma. The Waterloo quartzite is intruded by pegmatite of Wolf river age, suggesting that recrystallization and metamorphism may also be related to this regional thermal event. Both the Baraboo and Waterloo quartzites have been deformed. The Baraboo Range is an isolated doubly plunging syncline, slightly overturned to the south. The Waterloo outcrop area is in the nose of a broad eastward-plunging synclinal structure. Several possible models for deposition and subsequent deformation of the quartzites have been proposed (Greenberg and Brown, 1984), but the timing of deformation is still debated, possibly related to a regional 1,630 Ma thermal event, but certainly after about 1,720 Ma and prior to the 1,460 hydrothermal event. Quartzite, presumably equivalent to the Baraboo and Waterloo quartzites, is the most common basement lithology found in deep wells throughout southeastern Wisconsin. The stratigraphic sequence at Baraboo consists of 1,500 m of red to purple quartzite, overlain by 100 m of Seeley slate, 300 m of Freedom Formation (dolomite and carbonate iron formation), 65 m of pebbly Dake quartzite and 45m of Rowley Creek slate (Dalziel and Dott, 1970). The quartzite is well exposed, but the overlying formations, except for the Dake, are known from drill core. All of the operations that we will visit are located in or adjacent to the lower quartzite sequence. The upper formations are rarely exposed, being covered by Paleozoic sandstone and Quarternary glacial deposits in the interior of the syncline. At Stop 5 we will be near the site of two of the three historic iron mines that, before closing in the early 1920s, briefly mined the iron-rich carbonates of the Freedom Formation. The stratigraphic sequence at Waterloo is poorly known because of very limited exposure. Most of the known outcrop is in the area of the Michels Quarry, and consists of gray pebbly quartzite interbedded with argillite beds that have been metamorphosed to andalusite schist (Luther, 1992, 1997). Outlying exposures that show less evidence of metamorphism tend to have the more typical reddish-purple color and hydrothermal quartz veins and breccias seen in other Baraboo Interval quartzites. Stop 1: Michels Materials Waterloo Quarry Location: NE¼ sec. 33 and NW¼ sec. 34, T 9N, R13E, 1.5 miles east of Portland, Wisconsin, on Highway 19 (Waterloo 7.5-minute topographic quadrangle, 1976; fig. 1). Leaders: Frank Luther, Sue Courter, and Bruce A. Brown. Description: The Michels Waterloo Quarry was opened in 1988 by the Edward E. Gillen Co., a Milwaukee marine contractor involved in harbor and breakwater construction on the Great Lakes. Exploratory core drilling indicated massive bedding up to 2 to 3 m thick that would allow for quarrying of individual blocks up to 20 tons in size. The rock was tested and proved to exceed all Corps of Engineers specifications for breakwater stone, particularly resistant to freezethaw. The quarry site is also conveniently located for transport of blocks to Milwaukee by rail or truck. The Gillen Co. soon began to accumulate a large pile of waste blocks too small for breakwater stone and brought in crushing equipment to convert this material into railroad ballast and other aggregate. A rail loading facility was built 1 mile south of the quarry and ballast is moved to the site by truck. The rock is gray to reddish gray in color and consists of thoroughly recrystallized sandsized quartz grains, with interlayered pebbly beds containing pebbles of quartz, chert, jasper, and iron formation up to 1.5 cm. The massive beds of quartzite are separated by 10 to 20 cm argillite beds that are metamorphosed to andalusite schist. The thick east-dipping beds facilitate quarrying of large blocks, but make maintaining a flat quarry floor difficult. Glacial till overburden thickens to the east, and contains abundant quartzite boulders. Paleozoic sandstone conglomerate similar to the Parfreys Glen Formation of the Baraboo area locally occurs above the quartzite. 46 �- _S - 55 ± -_S— — _ 44' S -. -S — __-44-44.'44— 44—7895 .- - -44- -44-44- z 7 Figure 1. Topographic map showing location of Stop 1. We will examine the quarry geology, mining and crushing methods, and products. From Waterloo we will proceed northwestward across the glaciated terrane of eastern Dane County to Stop 2, located at the east end of the Baraboo Range. On the way we will have the opportunity to see some classic examples of drumlins and other glacial landforms. Stop 2: The Kraemer Co. Williams Quarry Location: SE¼, NE¼ sec. 19, T 12 N, R 8,E, Columbia County, south side of Highway 33, about 3 miles west of I-90-94 (Pine Island 7.5-minute topographic quadrangle, 1975; fig. 2). Leaders: Jennifer Lien, Phil Fauble, and Bruce A. Brown. Description: The Williams Quarry and another operation 0.5 mile to the west were opened in the Parfreys Glen Formation (Clayton and Attig, 1990), a time-transgressive, proximal conglomeratic facies formed by wave and storm action around the Baraboo Range as it was slowly buried by advancing seas during Cambrian and Ordovician time. The face at Williams has now advanced far enough into the hillside to expose three distinct lithologic units. At the base of the highwall, steeply dipping quartzite of the Baraboo north range is exposed. The quartzite is cut by numerous white quartz veins, and is locally brecciated and recemented by white hydrothermal quartz. Vugs and cavities in the breccia zone are filled with white clay and lined with quartz crystals up to 20 cm in length. Overlying the quartzite is a poorly sorted deposit consisting of clasts ranging from sand sized up to rocks 2 to 3 m in diameter (fig. 3). Fauble and Lien (2001) suggested that this unit may have originated as a debris flow because of the lack of sorting and evidence of reworking by wave action. Overlying and lapping onto the debris flow are beds of pebble conglomerate and sandstone typical of the Parfreys Glen Formation. These beds contain sedimentary structures and trace fossils typical of near-shore marine deposits. Abundant glauconite suggests equivalence to 47 �Figure 2. Topographic map showing location of Stop 2. Figure 3. South face at Williams Quarry showing Parfreys Glen conglomerate over coarse debris flow deposit overlying quartzite with white veins and clay pockets. the Tunnel City Group away from the Baraboo Range. The Williams Quarry produces base course and a small amount of riprap. Crushing and screening are done intermittently as needed, using portable equipment, typical of smaller quarry operations throughout rural Wisconsin. 48 �Stop 3: 1,760 Ma Rhyolite Location: SW¼ NE¼ sec. 33, T12N, R7E, south side of Highway 33, 0.3 mile east of the Lower Narrows of the Baraboo River (Lewiston 7.5-minute topographic quadrangle, 1975; fig. 4). Leader: Bruce A. Brown. Description: We will stop at this old road cut to examine the rhyolite that underlies the Baraboo quartzite. This rock is typical of the 1,760 Ma metarhyolites exposed along the northern and southern edges of the Baraboo syncline and to the northeast in the Fox Valley (Smith, 1978). The rock is a dark reddish brown color, and contains quartz and feldspar phenocrysts in a fine-grained matrix. A tuffaceous texture is visible in thin section and hand specimen. Flattened shards and pumice fragments can commonly be seen on weathered outcrop surfaces. The rhyolites have never been commercially quarried in the Baraboo area, but were extensively quarried for paving blocks in the Fox Valley at Berlin and Utley (Buckley, 1898). As you return to the van, notice the shoulder stone along Highway 33, and the aggregate used in the asphalt pavement. The shoulder stone was likely from the Williams Quarry. The asphalt contains a high percentage of quartzite aggregate. Wisconsin Department of Transportation has been slow to embrace quartzite as an aggregate for asphalt mixes because the dense quartzite does not absorb asphalt readily. This pavement seems to be performing well with no sign of deterioration after several years of use. Quartzite has performed well as a concrete aggregate in Wisconsin, outlasting the cement and fine aggregate matrix in many older pavements and sidewalks. In the city of Madison, examples of concrete with quartzite aggregate poured in the 1920s are still in use. We will drive through the Lower Narrows, where the Baraboo River cuts through the North Range on our way to the next stop. The quarry at the southwest end of the narrows was formerly operated by the Baraboo Quartzite Co., a producer of mill linings and grinding balls for many years. This was the last active producer of grinding media in the district, closing in the early 1980s. 15 Figure 4. Topographic map showing location of Stop 3. 49 �Figure 5. Topographic map showing location of Stop 4. Stop 4: Milestone Materials Jesse Pit and Quarry Location: NW¼SW¼ sec. 15, T11N, R7E, 2 miles east of Highway 113 on south side of Tower Road, Sauk County (Baraboo 7.5-minute topographic quadrangle, 1994; fig. 5). Leader: Jim Schmitt. Description: The Jesse Pit began as a sand and gravel operation along the terminal glacial moraine. As the pit was deepened, quartzite was exposed and quarrying began (fig. 6). We are now on the south limb of the Baraboo syncline, and the exposed quartzite dips to the north at a low angle. The dip surface is a bedding plane, and in many areas excellent examples of ripple marks can be seen. The Jesse Pit is an example of a growing number of sand and gravel operations that have encountered bedrock and begun to produce crushed stone as well. Gravel is in demand for concrete aggregate, but many gravel deposits lack sufficient coarse crushing material needed to achieve the percentage of fractured surfaces required to meet modern asphalt mix design standards. Operations such as Jesse Pit have the advantage of being able to furnish concrete and asphalt aggregate plus a variety of other products ranging from sand to landscape boulders. We will examine the quartzite exposures and look at some of the glacial material as well as discuss some of the reclamation activities currently in progress at this site. Stop 5: Milestone Materials Fox Ridge Asphalt Plant and Sales Yard Location: SE¼ sec. 22, T12N, R6E, 2 miles north of Baraboo on Fox Hill Road. Leader: Jim Schmitt. Description: At this stop we will examine some of the 27 products produced at or marketed from the Fox Ridge Pit. The list includes seven quartzite products ranging from base course to washed sand and seal coat chips. The quartzite is hauled in to this site from quarries such as Jesse and Rock Springs. We will discuss the uses of quartzite for asphalt aggregate as well as a variety of construction uses. 50 �Figure 6. View of Jesse Pit looking north. Working face in quartzite in foreground; sand and gravel face behind. Slope in background near tree line is reclaimed pit area ready to be seeded. Stop 6: Martin Marietta Aggregates Rock Springs Quarry Location: SW¼ sec. 28, T11N, R5E, 0.5 miles north of Rock Springs (Rock Springs 7.5-minute topographic quadrangle, 1975; fig. 7). Leader: Bruce A. Brown (Joe Michels). Description: This large quarry was opened in 1958 by the C&NW Railroad and operated exclusively as a source of railroad track ballast for many years. At the time it was opened, the large-scale production of quartzite ballast was not considered cost effective because of the hard abrasive nature of the rock and the resulting equipment maintenance costs. The C&NW ultimately proved that the superior performance of quartzite under heavy, high-speed train traffic was worth the cost in the long run. The angular quartzite interlocked to form a stable, welldrained track base, and did not break down under heavy traffic, as did limestone. Today the quarry is operated by Martin Marietta and it remains one of the largest ballast producers in Wisconsin. We are directly across the Upper Narrows of the Baraboo River from Van Hise rock, and several historic quarries. The old quarries are now part of a Wisconsin Department of Natural Resources Natural Area; originally they were operated for refractory and grinding media, with some early limited production of crushed stone. The bedding in this area is nearly vertical as at Williams Quarry (Stop 2). Hydrothermal breccias cemented with white quartz are also common in this area. The geology of the Narrows area is described in detail by Medaris and Dott (2001) as Locality 4 of Field Trip 1 for this meeting. We will tour the quarry and examine the large modern crushing and screening plant. As we drove up the hill to the quarry office, we passed a large pile of fines created by years of ballast production. This material at one time was eroding and washing into the Baraboo River 51 �below. It has since been stabilized and vegetation is beginning to take hold. Does anyone have any good ideas for marketing this stuff? Stop 7: Kraemer Company LaRue Quarry Location: NW¼ sec. 22, T11N, R5E (Rock Springs 7.5-minute topographic quadrangle, 1975; fig. 8). Leaders: Jennifer Lien and Bruce A. Brown. Description: The LaRue Quarry is a historic operation Figure 7. Topographic map showing location of Stop 6. in the Baraboo region. At one time this quarry produced ballast and was connected to the C&NW at North Freedom by the track now used by the Midcontinent Railway Museum. We may be in time to see the steam train pull into the quarry on one of its runs. Foundations of the old permanent crushing and screening plant and a few ruined buildings remain from the earlier operation. As we drive into LaRue Quarry, the leaders will point out the sites of two historic iron mines also served by this branch line from 1900 to around 1922. The quarry is now operated on an as-needed basis with portable crushing and screening equipment. The principal product is construction aggregate, primarily base course. Rue Figure 8. Topographic map showing location of Stop 7. 52 �Figure 9. East face of LaRue Quarry, showing unconformity of Paleozoic sandstones overlying Baraboo quartzite. LaRue is on the south range and, as at Jesse Pit, beds dip at a low angle to the north. Thin argillite beds are present and cross-bedding can be seen on joint surfaces. Ripple marks are common on bedding surfaces. LaRue provides some excellent views of the unconformity between the quartzite and the onlapping Cambrian-Ordovician sandstones (fig. 9). Alteration along joint surfaces has produced a weathering pattern similar to spheroidal weathering in granite. Rounded boulders can be found in which the interior is fresh purple quartzite surrounded by a rind of bleached sandstone resembling the overlying sediments. REFERENCES Buckley, E.R., 1898, On the Building and Ornamental Stones of Wisconsin: Wisconsin Geological and Natural History Survey Bulletin 4, 544 p. Clayton, L., and Attig, J.W., 1990, Geology of Sauk County, Wisconsin: Wisconsin Geological and Natural History Survey Information Circular 67, 68 p. Dalziel, I.W.D., and Dott, R.H., Jr., 1970, Geology of the Baraboo District, Wisconsin: Wisconsin Geological and Natural History Survey Information Circular 14, 164 p. Fauble, Philip, and Lien, Jennifer, 2001, Some Observations from the Williams Quarry Exposure: Evidence of Debris Flow Deposits in the Parfreys Glen Formation: Abstracts, 47th Annual Institute on Lake Superior Geology, Madison, Wisconsin, p. 26–27. 53 �Greenberg, J.K., and Brown, B.A., 1984, Cratonic sedimentation during the Proterozoic: An anorogenic connection in Wisconsin and the upper Midwest: Journal of Geology, v. 92, p. 159–171. Luther, F., 1992, The Waterloo Quartzite at the old Portland Quarry, in Travis, J., ed., the 56th Annual Tri-State Geology Field Conference, Whitewater, Wisconsin, p. 51–61. Luther, F., 1997, The Precambrian Waterloo Quartzite, Dodge and Jefferson Counties, Wisconsin—Petrology, Structure, and Industrial Use: Field Trip 5, in Guide to Field trips in Wisconsin and Adjacent Areas of Minnesota: Wisconsin Geological and Natural History Survey, Madison, Wisconsin, p. 31–35. Malone, D.H., Van Wyck, N., and Nelson, R., 1997, Field guide for field trip leaders to the Baraboo district, Wisconsin: Field Trip 3 in Guide to Field Trips in Wisconsin and Adjacent Areas of Minnesota: Wisconsin Geological and Natural History Survey, Madison, Wisconsin, p.13–22. Medaris, L.G. Jr., and Dott, R.H., Jr., 2001, Field Trip 1:Sedimentologic, Tectonic, and Metamorphic History of the Baraboo Interval: New Evidence from investigations in the Baraboo range, Wisconsin: 47th Annual Institute on Lake Superior Geology, Madison, Wisconsin, p. 1–21 (this volume). Smith, E.I., 1978, Introduction to Precambrian rocks of south-central Wisconsin: Geoscience Wisconsin, v. 2, p. 1–15. 54 � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2001 Description An account of the resource Institute on Lake Superior Geology. Madison, Wisconsin. May 9-12, 2001. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2001 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/2456791f0692913e884f16bfdc1fc9dd.pdf b199c02ed01d1fc8083e26b36a355f3f PDF Text Text INSTITUTE ON LAKE SUPERIOR GEOLOGY 48TH ANNUAL MEETING PROCEEDINGS VOLUME 48 PART 1 –PROGRAM AND ABSTRACTS Kenora, Ontario – May 12-16, 2002 �INSTITUTE ON LAKE SUPERIOR GEOLOGY 48TH ANNUAL MEETING PROCEEDINGS VOLUME 48 PART 1 –PROGRAM AND ABSTRACTS �CONTENTS Proceedings Volume 48 Part 1—Program and Abstracts Editors: K. O’Flaherty, C. Storey Institutes on Lake Superior Geology, 1955-2002.............................................................. iii Constitution of the Institute on Lake Superior Geology......................................................v By-Laws of the Institute on Lake Superior Geology........................................................ vii Membership Criteria for the Institute on Lake Superior Geology................................... viii Goldich Medal Guidelines ................................................................................................. ix Past Goldich Medalists ...................................................................................................... xi Goldich Medal Committee................................................................................................. xi Citation for 2002 Goldich Medal Recipient ..................................................................... xii Eisenbrey Student Travel Awards .....................................................................................xv Student Travel Award Application Form ........................................................................ xvi Student Paper Awards..................................................................................................... xvii Student Paper Awards Committee .................................................................................. xvii Board of Directors ......................................................................................................... xviii Local Committees .......................................................................................................... xviii Session Chairs.................................................................................................................. xix Banquet Speaker ................................................................................................................xx Report of the Chair of the 47th Annual Institute Meeting.................................................xx Program............................................................................................................................xxv Abstracts ..............................................................................................................................1 Cover Photo: Regina Mine, 1918. The Regina Mine produced intermittently from 1895 to 1943, yielding over 8,000 ounces gold and 1,460 ounces silver. The Regina was the first mine in North America to use cyanide for gold recovery. ii �INSTITUTES ON LAKE SUPERIOR GEOLOGY, 1955-2002 # 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 DATE 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 PLACE Minneapolis, Minnesota Houghton, Michigan East Lansing, Michigan Duluth, Minnesota Minneapolis, Minnesota Madison, Wisconsin Port Arthur, Ontario Houghton, Michigan Duluth, Minnesota Ishpeming, Michigan St. Paul, Minnesota Sault Ste. Marie, Michigan East Lansing, Michigan Superior, Wisconsin Oshkosh, Wisconsin Thunder Bay, Ontario Duluth, Minnesota Houghton, Michigan Madison, Wisconsin Sault Ste. Marie, Ontario Marquette, Michigan St. Paul, Minnesota Thunder Bay, Ontario Milwaukee, Wisconsin Duluth, Minnesota Eau Claire, Wisconsin East Lansing, Michigan International Falls, Minnesota Houghton, Michigan Wausau, Wisconsin Kenora, Ontario Wisconsin Rapids, Wisconsin Wawa, Ontario Marquette, Michigan Duluth, Minnesota Thunder Bay, Ontario Eau Claire, Wisconsin Hurley, Wisconsin Eveleth, Minnesota Houghton, Michigan Marathon, Ontario Cable, Wisconsin CHAIRS C.E. Dutton A.K. Snelgrove B.T. Sandefur R.W. Marsden G.M. Schwartz & C. Craddock E.N. Cameron E.G. Pye A.K. Snelgrove H. Lepp A.T. Broderick P.K. Sims & R.K. Hogberg R.W. White W.J. Hinze A.B. Dickas G.L. LaBerge M.W. Bartley & E. Mercy D.M. Davidson J. Kalliokoski M.E. Ostrom P.E. Giblin J.D. Hughes M. Walton M.M. Kehlenbeck G. Mursky D.M. Davidson P.E. Myers W.C. Cambray D.L. Southwick T.J. Bornhorst G.L. LaBerge C.E. Blackburn J.K. Greenberg E.D. Frey & R.P. Sage J. S. Klasner J.C. Green M.M. Kehlenbeck P.E. Myers A.B. Dickas D.L. Southwick T.J. Bornhorst M.C. Smyk L.G. Woodruff iii �43 44 45 46 47 48 1997 1998 1999 2000 2001 2002 Sudbury, Ontario Minneapolis, Minnesota Marquette, Michigan Thunder Bay, Ontario Madison, Wisconsin Kenora, Ontario R.P. Sage, W. Meyer J.D. Miller, M.A. Jirsa T.J. Bornhorst, R.S. Regis S.A. Kissin, P. Fralick M.G. Mudrey, Jr., B.A. Brown P. Hinz, R.C. Beard iv �CONSTITUTION OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY (Last amended by the Board—May 8, 1997) Article I Name The name of the organization shall be the "Institute on Lake Superior Geology". Article II Objectives The objectives of this organization are: A. To provide a means whereby geologists in the Great Lakes region may exchange ideas and scientific data. B. To promote better understanding of the geology of the Lake Superior region. C. To plan and conduct geological field trips. Article III Status No part of the income of the organization shall insure to the benefit of any member or individual. In the event of dissolution, the assets of the organization shall be distributed to _________ (some tax free organization). (To avoid Federal and State income taxes, the organization should be not only "scientific" or "educational, but also "non-profit") Minn. Stat. Anno. 290.01, subd. 4 Minn. Stat. Anno. 290.05(9) 1954 Internal Revenue Code s.501(c)(3) Article IV Membership The membership of the organization shall consist of persons who have registered for an annual meeting within the past three years, and those who indicate interest in being a member according to guidelines approved by the Board of Directors. Article V Meetings The organization shall meet once a year. The place and exact date of each meeting will be designated by the Board of Directors. Article VI Directors The Board of Directors shall consist of the Chair, Secretary-Treasurer, and the last three past Chairs; but if the board should at any time consist of fewer than five persons, by reason of unwillingness or inability of any of the above persons to serve as directors, the vacancies on the board may be filled by the Chair so as to bring the membership of the board to five members. Article VII Officers v �The officers of this organization shall be a Chair and Secretary-Treasurer. A. The Chair shall be elected each year by the Board of Directors, who shall give due consideration to the wishes of any group that may be promoting the next annual meeting. His/her term of office as Chair will terminate at the close of the annual meeting over which he/she presides, or when his/her successor shall have been appointed. He/she will then serve for a period of three years as a member of the Board of Directors. B. The Secretary-Treasurer shall be elected at the annual meeting. His/her term of office shall be four years, or until his/her successor shall have been appointed. Article VIII Amendments This constitution may be amended by a majority vote (majority of those voting) of the membership of the organization. vi �BY-LAWS OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY I. Duties of the Officers and Directors A. It shall be the duty of the Annual Chairman to: 1. Preside at the annual meeting. 2. Appoint all committees needed for the organization of the annual meeting. 3. Assume complete responsibility for the organization and financing of the annual meeting over which he/she presides. B. It shall be the duty of the Secretary-Treasurer to: 1. Keep accurate attendance records of all annual meetings. 2. Keep accurate records of all meetings of, and correspondence between, the Board of Directors. 3. Hold all funds that may accrue as profits from annual meetings or field trips and to make these funds available for the organization and operation of future meetings as required. C. It shall be the duty of the Board of Directors to plan locations of annual meetings and to advise on the organization and financing of all meetings. II. Duties and Expenses A. Regular membership dues of $5.00 or less on an annual basis shall be assessed each member as determined by the Board of Directors.. B. Registration fees for the annual meetings shall be determined by the Chair in consultation with the Board of Directors. The registration fees can include expenses to cover operations outside of the annual meeting as determined by the Board of Directors. It is strongly recommended that registration fees be kept at a minimum to encourage attendance of students. III. Rules of Order The rules contained in Robert's Rules of Order shall govern this organization in all cases to which they are applicable. IV. Amendments These by-laws may be amended by a majority vote (majority of those voting) of the membership of the organization; provided that such modifications shall not conflict with the constitution as presently adopted or subsequently amended. vii �MEMBERSHIP CRITERIA FOR THE INSTITUTE ON LAKE SUPERIOR GEOLOGY Approved May 8, 1997 A. Membership in the Institute on Lake Superior Geology requires either participation in Institute activities, or an indication on a regular basis of interest in the Institute. Those individuals registering for an annual meeting will remain as members for 4 years unless: 1) they indicate no further interest in the Institute by responding negatively to the statement on meeting circulars "Remove my name from the mailing list"; or 2) two successive mailings in different years are returned by the postal service as address unknown. B. Those individuals who have not registered for an annual meeting in the past 4 years must indicate an interest in the Institute by postal, electronic , or verbal correspondence with the Secretary-Treasurer at least once every two years. Such individuals will be removed from the membership if they indicate no further interest in the Institute or two successive mailing in different years are returned by the postal service as address unknown. C. The Secretary-Treasurer will maintain a list of current members. The list will include the date of the beginning of continuous membership, dates of returned mail, dates of last contact (expression of interest), and the date membership expires, barring a change of status initiated by the member. Those individuals who have become members of ILSG by Section B will have an expiration date listed at 2 years from the upcoming meeting. For example, a member who expresses interest in September of 1997 (the next annual meeting is May, 1998) will have an expiration date of May, 2000, unless the member contacts the Secretary-Treasurer or attends an annual meeting. D. "Member for Life" status is granted to individuals who have been (nearly) continuous participants of the ILSG meetings for 15 years, Goldich Medal recipients, or those who have served as meeting chairs. This status will be further maintained unless the individuals indicate no further interest in the Institute, or 4 mailings in different years are returned by the postal service as address unknown, or they are deceased. E. All members will be mailed the First Circular for the Annual Meeting and the ILSG Newsletter. The Chair of the annual meeting may opt to send the first circular to additional individuals. All returned mail should be reported to the Secretary-Treasurer. F. The Secretary-Treasurer can designate any individual who is on the ILSG membership list (mailing list) as of January 1, 1997 as a member for life based on participation in ILSG activities. G. Members are strongly encourage to send address corrections to the Secretary-Treasurer to avoid unintentional lapse of membership. viii �GOLDICH MEDAL GUIDELINES (Adopted by the Board of Directors, 1981; amended 1999) Preamble The Institute on Lake Superior Geology was born in 1955, as documented by the fact that the 27th annual meeting was held in 1981. The Institute's continuing objectives are to deal with those aspects of geology that are related geographically to Lake Superior; to encourage the discussion of subjects and sponsoring field trips that will bring together geologists from academia, government surveys, and industry; and to maintain an informal but highly effective mode of operation. During the course of its existence, the membership of the Institute (that is, those geologists who indicate an interest in the objectives of the ILSG by attending) has become aware of the fact that certain of their colleagues have made particularly noteworthy and meritorious contributions to the understanding of Lake Superior geology and mineral deposits. The first award was made by ILSG to Sam Goldich in 1979 for his many contributions to the geology of the region extending over about 50 years. Subsequent medalists and this year's recipient are listed in the table below. Award Guidelines 1) The medal shall be awarded annually by the ILSG Board of Directors to a geologist whose name is associated with a substantial interest in, and contribution to, the geology of the Lake Superior region. 2) The Board of Directors shall appoint the Goldich Medal Committee. The initial appointment will be of three members, one to serve for three years, one for two years, and one for one year. The member with the briefest incumbency shall be chair of the Nominating Committee. After the first year, the Board of Directors shall appoint at each spring meeting one new member who will serve for three years. In his/her third year this member shall be the chair. The Committee membership should reflect the main fields of interest and geographic distribution of ILSG membership. The out-going, senior member of the Board of Directors shall act as liaison between the Board and the Committee for a period of one year. 3) By the end of November, the Goldich Medal Committee shall make its recommendation to the Chair of the Board of Directors, who will then inform the Board of the nominee. 4) The Board of Directors normally will accept the nominee of the Committee, inform the medalist, and have one medal engraved appropriately for presentation at the next meeting of the Institute. 5) It is recommended that the Institute set aside annually from whatever sources, such funds as will be required to support the continuing costs of this award. ix �Nominating Procedures 1) The deadline for nominations is November 1. Nominations shall be taken at any time by the Goldich Medal Committee. Committee members may themselves nominate candidates; however, Board members may not solicit for or support individual nominees. 2) Nominations must be in writing and supported by appropriate documentation such as letters of recommendation, lists of publications, curriculum vita's, and evidence of contributions to Lake Superior geology and to the Institute. 3) Nominations are not restricted to Institute attendees, but are open to anyone who has worked on and contributed to the understanding of Lake Superior geology. Selection Guidelines 1) Nominees are to be evaluated on the basis of their contributions to Lake Superior geology (sensu lato) including: a) importance of relevant publications; b) promotion of discovery and utilization of natural resources; c) contributions to understanding of the natural history and environment of the region; d) generation of new ideas and concepts; and e) contributions to the training and education of geoscientists and the public. 2) Nominees are to be evaluated on their contributions to the Institute as demonstrated by attendance at Institute meetings, presentation of talks and posters, and service on Institute boards, committees, and field trips. 3) The relative weights given to each of the foregoing criteria must remain flexible and at the discretion of the Committee members. 4) There are several points to be considered by the Goldich Medal Committee: a) An attempt should be made to maintain a balance of medal recipients from each of the three estates—industry, academia, and government. b) It must be noted that industry geoscientists are at a disadvantage in that much of their work in not published. 5) Lake Superior has two sides, one the U.S., and the other Canada. This is undoubtedly one of the Institute's great strengths and should be nurtured by equitable recognition of excellence in both countries. x �PAST GOLDICH MEDALISTS 1979 Samuel S. Goldich 1991 William Hinze 1980 not awarded 1992 William F. Cannon 1981 Carl E. Dutton, Jr. 1993 Donald W. Davis 1982 Ralph W. Marsden 1994 Cedric Iverson 1983 Burton Boyum 1995 Gene LaBerge 1984 Richard W. Ojakangas 1996 David L. Southwick 1985 Paul K. Sims 1997 Ronald P. Sage 1986 G.B. Morey 1998 Zell Peterman 1987 Henry H. Halls 1999 Tsu-Ming Han 1988 Walter S. White 2000 John C. Green 1989 Jorma Kalliokoski 2001 John S. Klasner 1990 Kenneth C. Card 2002 Ernest K. Lehmann GOLDICH MEDAL COMMITTEE Ron Sage (2002) Ontario Geological Survey, Thunder Bay Rod Johnson (2002) Rod Johnson and Associates, Negaunee, Michigan Frank Luther (2003) University of Wisconsin, Whitewater James D. Miller, Jr., as out-going senior member of Institute Board of Directors, is liaison between Goldich Medal Committee and the Board through 2001 meeting. xi �Citation Ernest K. Lehmann 2002 Goldich Medal Recipient Ladies and Gentlemen: Thank you for being here tonight for this presentation of the 2002 Sam Goldich Medal Award. I don’t know if any of you remember the late Ben Dickerson. He was one of the premier exploration geologists of his time and wrote a column in Skillings Magazine. The column was called “ News and Rumor” from the Bush”. It described current exploration activity particularly in the U.S. during those golden years of the 70’s and 80’s. At that time the exploration community was quite small and Ben seemed to know everyone. His column was cryptic in that he would commonly refer to people in coded names. For example there was the “Great Buawa” and the “Shallow”. Ernie was referred to as the “Bearded Sphinx of the North”. I’m not sure where that came from, but know the “Sphinx” was “good copy” because he was so active in minerals exploration particularly in the Great Lakes region in those days as he certainly is today. I had the good fortune and privilege of working with Ernie for nearly two decades. Over the years, his list of accomplishments in the field of economic geology and mineral exploration are many. It would take hours to go through them all. So I will focus on those related to the Lake Superior area. First some background. Ernie was born in 1929 in Heidelberg, Germany. He was educated in the public schools of New Rochelle, N.Y., attended Williams College, in Williamstown, Massachusetts, where he graduated cum laude in geology in 1951. He attended graduate school in geology at Brown University, Providence, Rhode Island, and completed the Owners and Presidents Management Program of the Harvard Business School in 1984. He is married to Sally Willius Lehmann (his better half) and resides in Minneapolis, Minnesota. Prior to founding Ernest K. Lehmann & Associates, Inc. in 1967, Ernie was an independent consultant and partner in a Minneapolis based geological consulting firm from 1958 to 1967. In 1950, he began his career by working first as a miner and then as geologist for the Signal Mining Company at Bannack, Montana. From 1951 to 1958, Ernie worked for Kennecott Copper Corporation and its exploration arm, Bear Creek Mining Company. Ernie served on active duty as a Terrain Intelligence Analyst in the U.S. Army Corps of Engineers from 1953 to 1955 and was awarded the Commendation Ribbon for his service. As many of you know, Ernie is the founder and CEO of North Central Mineral Ventures Inc. and of Ernest K. Lehmann & Associates, Inc. Under his leadership, the firms have engaged in the planning, management and execution of mineral exploration programs, mineral deposit development, mine appraisal and mineral economic studies particularly in the Great Lakes region. These activities have spanned most important hard mineral commodities including ferrous, non-ferrous, precious and strategic metals, industrial minerals and fertilizer raw materials. In the course of their activities, he and his firms have been active in staffing and managing exploration and mine development projects and acquiring private and public mineral xii �lands in both the U.S. and abroad. The latter activities have included claim staking, mineral leasing of private, state and federal lands, and the creation and management of mineral joint ventures. In addition to extensive work in the US and Canada, he has conducted and managed exploration and consulted on mine development and evaluation Central and South America, Africa and Europe. In the course of his activities, Ernie has specialized in exploration management and in mineral deposit appraisal evaluation. He has appeared extensively as an expert witness on mineral property appraisal and taxation and on mining claim related issues. He has served on advisory committees to the Office of Technology Assessment of the U.S. Congress on strategic and critical minerals and to the state of Minnesota on direct reduction of iron ores. On behalf of the American Institute of Professional Geologists, he has testified before Congress on strategic minerals issues and on the 1872 mining law. He has also testified before Congress on issues related to the Federal Land Management Policy Act. Before state legislative committees, he has testified on mineral property appraisal, mineral taxation, mineral leasing and mine permitting. In 1985, he became president of the American Institute of Professional Geologists, also serving the Institute in various other capacities on a state and national level. In 1987, he was awarded the Ben H. Parker Medal, the Institute’s highest award for service to the geological profession, and has been awarded Honorary Membership in the Institute in 1997. He currently serves as Chairman of the AIPG Foundation and is a member of numerous other technical and professional bodies including the Society of Economic Geologists, The Mining and Metallurgical Society of America, the Society of Mining Engineers, the Society for Geology Applied to Mineral Deposits, and the Northwest Mining Association. He is a registered geologist in Minnesota, California, Georgia, Delaware and Alaska and is accredited by the European Federation of Geologists. He is currently a director and president of the Minnesota Exploration Association and serves on various committees in Minnesota in this capacity, including the Blue Ribbon Committee on Minnesota Minerals, the advisory board to the Natural Resources Research Institute and the State Mapping Advisory Committee. Ernie has also been actively involved in mining related environmental issues again particularly in the Great Lakes area. He was instrumental in initiating and executing the Minnesota Mining Permit Simulation Project, a joint state agency, environmental community and industry effort to examine the mine permitting process and problems for non-ferrous metal mining. He helped organize a workshop on financial assurance in the mining industry in cooperation with the Minnesota DNR, MPCA, and the Audubon Society. Ernie and the firm were consultants to the state of Maine charged with developing metal mining regulations, spanning activities from exploration through operation and closure. The firms were also involved in mine permitting activities in Wisconsin. To this I would like to mention some of the major projects Ernie has initiated in the Great Lakes area. These include: 1) Exploration programs for Bear Creek in 1952, 1956 through 1957 for copper and copper nickel in the Duluth and Mellon Gabbro Complexes and Nonesuch Shale. xiii �2) Copper and copper-nickel exploration projects carried out for Cerro Corp. from 1967-1969 in Wisconsin, Minnesota, Michigan and Ontario. He was one of the first explorationists to recognize the Wisconsin greenstone belts as an important VMS target . 3) VMS exploration joint ventures in Wisconsin that spanned from 1975 through 1993. Such notables as Getty, Chevron, Denison Mines and Asarco were involved with these projects. Under the supervision of the late Ned Eisenbrey, these programs resulted in three discoveries. Of these one, the Bend copper-gold deposit, is and remains potentially economic. The huge amount of geologic and geophysical data generated from these exploration projects also resulted in establishing a more detailed “geologic framework” that was previously lacking for the Wisconsin greenstone belts. This work culminated in several papers published in “Economic Geology” in 1990 and 1994. 4) At the same time Ernie was managing a major VMS program in Archean of northern Minnesota from 1979 through 1985 for Getty and Billiton. 5) From 1980 to 1982, Ernie was one of the first to seriously begin exploring the interior of the Duluth Complex for copper-nickel and PGMs through a joint venture with Billiton. His exploration efforts continue today with a focus on the Birch Lake PGM project that involves Impala. I believe Ernie’s past and present exploration and political efforts in Minnesota today has resulted in the current brisk activity within the Duluth Complex and transformed the state to an attractive part of the world to conduct environmentally responsible mining projects. Finally, with deep gratitude and respect, I would like to present this year’s Sam Goldich medal recipient, my mentor and friend, Ernest K. Lehmann. Theodore A. DeMatties xiv �EISENBREY STUDENT TRAVEL AWARDS The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name "Eisenbrey" was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. "Ned" Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The number of awards and value are determined by the annual Chair in consultation with the Secretary-Treasurer. Recipients will be announced at the annual banquet. The following general criteria will be considered by the annual Chair, who is responsible for the selection: 1) The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. 2) Students who are the senior author on either an oral or poster paper will be given favored consideration. 3) It is desirable for two or more students to jointly request travel assistance. 4) In general, priority will be given to those in the Institute region who are farthest away from the meeting location. 5) Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. The form below is optional. Successful applicants will receive their awards during the meeting. xv �INSTITUTE ON LAKE SUPERIOR GEOLOGY Eisenbrey Student Travel Award Application Student Name: Date: Address: email: Department Head-Typed Department Head-Signature Educational Status: Are you the senior author of an oral or poster paper? YES Will any other students be traveling with you? NO Who? Statement of need (use additional page if necessary) Please return to: xvi �STUDENT PAPER AWARDS Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper— last modified by the Board in 1997—follow: 1) The contribution must be demonstrably the work of the student. 2) The student must present the contribution in-person. 3) The Student Paper Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. 4) In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. 5) The total amount of the awards is left to the discretion of the meeting Chair and SecretaryTreasurer, but typically is in the amount of about $300 US. 6) The Secretary-Treasurer maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. 7) The names of award recipients shall be included as part of the annual Chair's report that appears in the next volume of the Institute. Student papers will be noted on the Program. STUDENT PAPER AWARDS COMMITTEE Peter Hollings Lakehead University, Thunder Bay Neil Pettigrew Avalon Ventures Ltd., Thunder Bay Connie Dickens U.S. Geological Survey, Reston, Virginia xvii �BOARD OF DIRECTORS Mike Mudrey, Chair Wisconsin Geological and National History Survey, Madison, Wisconsin 2000-2003 Steve Kissin, Department of Geology, Lakehead University, Thunder Bay, Ontario 1999-2002 Ted Bornhorst, Department of Geological Engineering and Sciences, Michigan Tech UniversityHoughton, Michigan 1998-2001 Jim Miller, Minnesota Geological Survey, St. Paul, Minnesota Secretary-Treasurer Mark A. Jirsa, Minnesota Geological Survey St. Paul, Minnesota LOCAL COMMITTEE Peter Hinz, Ontario Geological Survey, Resident Geologist Program, Kenora, ON Richard C. Beard, Northwest Mineral Development Services, Kenora, ON Charles E. Blackburn, Blackburn Geological Services, Kenora, ON Christine Blackburn, Blackburn Geological Services, Kenora, ON Kevin O’Flaherty, Kenora, ON Carmen Storey, Ontario Geological Survey, Resident Geologist Program, Red Lake, ON Kathleen McGowan-Hinz, Kenora, ON SESSION CHAIRS xviii �Alasdair Mowat Emerald Fields Resource Corp., Kenora Philip Fralick Lakehead University, Thunder Bay Mark Smyk Ontario Geological Survey, Thunder Bay 2002 BANQUET SPEAKER L. Harvey Thorleifson Geological Survey of Canada Ottawa, Ontario The Search for Diamonds in Canada xix �REPORT OF THE CHAIR OF THE 47TH ANNUAL MEETING Michael G. Mudrey, Jr. Chair ILSG 2001 The 47th Annual Institute on Lake Superior Geology was jointly hosted by the University of Wisconsin-Extension Geological and Natural History Survey and the University of WisconsinMadison, Department of Geology and Geophysics , in Madison, Wisconsin on May 9-12, 2001. Principal local committee members were M.G. Mudrey, Jr . and B.A. Brown, co-chairs, Robert H. Dott, Jr, and L.Gordon Medaris, Jr., Program-cochairs, and Kathleen M. Zwettler, Meeting Coordinator. Other principal individuals are listed in the Proceedings Volume. Attendance at ILSG 2001 A total of 172 professionals and student professionals attended the meeting, 82 of whom preregistered by the April 2, 2001 deadline. A total of 26 students were registered, ten of whom requested and received travel assistance and had registration fees for the meeting waived. Eisenbrey Student Travel Awards 2001 Ten students requested and received travel assistance form the Eisenbrey Student Travel Award Fund established to support student participation at the Annual Institute. Details, including criteria and application forms are available at the Eisenbrey website. Justin Johnson- Lakehead University-Thunder Bay, Ontario - Fluid Inclusion Evidence for a Role for Hydrothermal Activity in the Roby Zone, Lac Des Iles Mine, Northwestern Ontario; Becky RogalaLakehead University-Thunder Bay, Ontario - A Metamorphosed Evaporite Sequence from the Sibley Basin ; Dan BihariLakehead University-Thunder Bay, Ontario - Alteration and Pge-au Mineralization in the North Roby Zone, Lac Des Iles Mine, Northwestern Ontario; Phillip LarsonUniversity of Minnesota-Duluth - Potential for Copper Mineralization in the Animikie Group, Minnesota; Michael NemitzUniversity of Minnesota-Duluth - Mineralogical Variations in Iron-formation in the Thermal Metamorphic Aureole of a Diabase Dike; Lisa LarsonUniversity of Minnesota-Duluth - ; Muhammad Asif Soofi- Purdue University, West Lafayette, Indiana - Post-rift Evolution of the Midcontinent Rift System: Some Numerical Experiments; Jason OdetteUniversity of Wisconsin-Oskosh - Preliminary Evaluation of Hydrothermal Alteration Mineral Assemblages and Their Relationship to VMS-style Mineralization in the Five Mile Lake Area of the Archean Vermilion Greenstone Belt, Northeastern Minnesota ; xx �Trent Newkirk- Dan Schweitzer- University of Wisconsin-Oskosh - Preliminary Lava Flow Morphology Studies at the Five Mile Lake Vms Prospect, Archean Vermilion District, Ne Minnesota: Implications for Volcanic Processes, Volcanic Paleoenvironments, and VMS Exploration ; and Kent State University-Kent Ohio - Results of Igneous Thermometry and Barometry on the East-central Minnesota Batholith: Evidence for Post-emplacement Exhumation and Cooling . Meeting Summary The 47th Annual Institute on Lake Superior Geology Annual Meeting was held at the Sheraton Madison Hotel, the same location as the 1973 meeting. The 2001 meeting focused on hydrogeologic aspects of arsenic in groundwater, completion of acquisition of detailed regional geophysical data; the evolution of the post-Penokean--pre-Keweenawan crystal terrane of North American in 36 poster displays and 32 oral presentations . The two days of technical sessions were preceded by Field Trip 1 - Sedimentologic, Tectonic and Metamorphic History of the Baraboo Interval led by L. Gordon Medaris, Jr. , and Robert H. Dott, Jr. (University of Wisconsin-Madison) , and followed by Field Trip 2 - Upper Mississippi Valley Zinc-Lead District led by M.G. Mudrey, Jr. (Wisconsin Geological and Natural History Survey) and Thomas C. Hunt (University of Wisconsin-Platteville) and Field Trip 3- Industrial Mineral and Aggregate Resources of the Baraboo Interval Quartzites lead by Bruce A. Brown (Wisconsin Geological and Natural History Survey), Frank R. Luther (University of Wisconsin Whitewater), James W. Schmitt (D.L. Gasser Construction), Susan M. Courter (Michels Materials) and Jennifer Lien (The Kraemer Company) The meeting began with regional geologic summaries of the Pleistocene of Southern Wisconsin by D.M. Mickelson (University of Wisconsin-Madison) and Lee Clayton (Wisconsin Geological and Natural History Survey), Sequence Stratigraphic Analysis of the Paleozoic by Charles E. Byers (University of Wisconsin-Madison), and Stratigraphic Metamorphic Analysis of the Baraboo Supracrustal Rocks of the Midcontinent by L. Gordon Medaris, Jr. , (University of Wisconsin-Madison). A series of invited presentations completed the morning of the first technical sessions three presentations on Aeromagnetic Investigations in the Midwest. An invited presentation on an Overview of Arsenic Occurrences and Processes Controlling Arsenic Moblity in Ground Water by D. Kirk Nordstrom (U.S. Geological Survey) kicked off a special session of four papers on the Hydrogeologic Setting of Elevated Arsenic and Heavy Metals in Public and Private Water Supplies. General geologic topics completed the first day. Three papers on the Midcontinent Rift started Friday morning, followed an invited symposium of eight papers on the Thermo-Thermo-Tectonic History of 1800 to 1200 Ma post-Penokean to pre-Keweenawan. The remaining significant presentations dealt with Archean and platinumgroup element geology. The technical program was completed by 3:30 p.m. Proceeding including Pat 1 (Programs and Abstracts) and Part 2 (Field Trip Guidebook) are available from the Institute . xxi �Best Student Paper Awards for 2001 were presented to Alissa Naymark-University of Wisconsin-Madison (oral) Cash award plus chrome-plated drill bit donated by Layne Northwest Travis Sandland-Macalester College (poster) and Erin H. Phillips-Macalester College (poster) Jason D. Odette-University of Wisconsin-Oshkosh (poster) Trent T. Newkirk-University of Wisconsin-Oshkosh (poster) After the break, Mike Mudrey and Bruce Brown regaled the remaining participants with previews of their field trips (Field Trip 3 was eventful in that a flat tire delayed the trip ( How many ILSGers does it take to change a tire? Three! Frank Luther, Mark Jirsa and Bob Reszka), and Brad Singer and John Valley lead the group on an impressive tour of the isotope laboratories in the Department of Geology and Geophysics. The entire group of ILSG participants remaining in Madison joined the Department of Geology and Geophysics Geology Club for the annual Departmental Picnic courtesy of contributions from Steve Kircher, Crandon Mine Development, Nicolet Minerals, and William J. Cronk, Layne Northwest, directly to the Institute. Annual Banquet and Goldich Award At the Annual Banquet Bruce R. Doe prepared, and Michael G. Mudrey, Jr. delivered a biographic history of Samuel S. Goldich who passed away on 20 December 2000 in Applewood, Colorado. After the introduction of 6 of the surviving Goldich recipients, an upbeat Gene LaBerge presented John S. Klasner with the Goldich Medal for 2001 for his contributions to the Institute and Lake Superior Geology. Tom Hunt illustrated the successful reclamation of the Ladysmith Mine of Kennecott Mining for the after dinner address, and Peter Hinz of the Ontario Geological Survey invited participants to the 48 th Annual Meeting in Kenora . Best Student Paper Awards for 2001 Alissa Naymark-University of Wisconsin-Madison (oral) Cash award plus chrome-plated drill bit donated by Layne Northwest Travis Sandland-Macalester College (poster) Erin H. Phillips-Macalester College (poster) Jason D. Odette-University of Wisconsin-Oshkosh (poster) Trent T. Newkirk-University of Wisconsin-Oshkosh (poster) Details of presentations are found in the Proceedings and Abstracts for the 47 th Annual Meeting . After the break, Mike Mudrey and Bruce Brown regaled the remaining participants with previews of their field trips (Field Trip 3 was eventful in that a flat tire delayed the trip ( How many ILSGers does it take to change a tire? Three! Frank Luther, Mark Jirsa and Bob Reszka), and Brad Singer and John Valley lead the group on an impressive tour of the isotope laboratories in the Department of Geology and Geophysics. The entire group of ILSG participants remaining in Madison joined the Department of Geology and Geophysics Geology Club for the annual Departmental Picnic courtesy of contributions from Steve Kircher, Crandon Mine Development, Nicolet Minerals, and William J. Cronk, Layne Northwest, directly to the Institute. xxii �Proceedings including Part 1 (Programs and Abstracts) and Part 2 (Field Trip Guidebook) are available from the Institute . Institute on Lake Superior Geology c/o Mark Jirsa, Executive Secretary 2642 University Avenue St. Paul MN 55114-1057 Phone: (612)-627-4539 Fax: 612-627-4778 e-mail: jirsa001@maroon.tc.umn.edu xxiii �48th ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY Program xxiv �Program of Events Sunday, May 12 06:00 - 18:00 Field Trip 1: Tanco Rare-Element Pegmatite, Southeastern Manitoba Monday, May 13 08:00 - 18:00 Field Trip 2: Quaternary Geology of Southeastern Manitoba 08:00 - 16:00 Field Trip 3: Structure and Sedimentology of the Seine Conglomerate, Mine Centre Area, Ontario 17:00 – 20:00 Registration – Best Western Lakeside Inn, Kenora, Ontario 19:00 – 21:00 Ice-Breaker Social and cash bar in Best Western Lakeside Inn (Authors at poster from 19:30 to 21:00) xxv �TUESDAY MAY 14 8:00 a.m. - 9:00 a.m. REGISTRATION 9:00 a.m. INTRODUCTORY REMARKS P. Hinz Co-Chairman, Ontario Geological Survey, Kenora, ON TECHNICAL SESSION I: Session Chair: Mark Smyk, District Geologist, Ontario Geological Survey, Thunder Bay, ON 9:15 a.m. K. O’Flaherty, (Consulting Geologist, Kenora, ON) A Brief History of Mining in Northwestern Ontario 9:40 a.m. M. Sanborn-Barrie*, T. Skulski, N. Rayner, (Geological Survey of Canada, Continental Geoscience Division, Ottawa, ON) and J.R. Parker (Ontario Geological Survey, Precambrian Section, Sudbury, ON) 300 my evolution of the Red Lake greenstone belt, western Superior Province, Ontario: A synthesis of current constraints on volcanism, sedimentation, deformation, metamorphism and gold mineralization 10:05 a.m. COFFEE BREAK AND POSTER SESSION 10:30 a.m. B. Nitescu*, A. R. Cruden, A. R. Bailey, (University of Toronto, Toronto, ON) Crustal Models of the Western Superior Province from gravity and Lithoprobe seismic data. 10:55 a.m. P. Fralick* and King, (Department of Geology, Lakehead University, Thunder Bay ON) Mesoarchean evolution of Western Superior Province: evidence from metasedimentary sequences near Atikokan 11:20 a.m. M.A. Jirsa, (Secretary-Treasurer, ILSG, St. Paul, MN) A Report on the Secretary-Treasurer's Position LUNCH BREAK ILSG BOARD MEETING (by invitation) POSTERS xxvi �TECHNICAL SESSION II: Session Chair: Alasdair Mowat, Emerald Fields Resource Corp., Kenora, ON 1:30 p.m. D. Peterson, (Natural Resources Research Institute, University of Minnesota-Duluth, Duluth, MN) Cu-Ni-PGE Mineralization in the South Kawishiwi intrusion; North Western Minnesota. Variation due to magmatic processes. 1:55 p.m. B. Rogala*, P.W. Fralick, and G. Borradaile, (Department of Geology, Lakehead University, Thunder Bay, Ontario) New Information from the Sibley Group 2:20 p.m. M.A. Jirsa, (Minnesota Geological Survey, St. Paul, MN) Archean and Paleoproterozoic mafic intrusions in Minnesota 2:45 p.m. COFFEE BREAK AND POSTER SESSION 3:15 p.m. C. Sturm, (Geology Department, Oberlin College, Oberlin, OH) Petrographic study of the Otter Tail Pluton, Superior Province, Northwestern Ontario 3:40 p.m. G. Ferguson* and A. Woodbury, (Department of Civil Engineering, University of Manitoba, Winnipeg, MB) Ground water and heat flow in an interlobate moraine in southwestern Manitoba. 6:00 p.m. SOCIAL - Cash Bar 7:00 p.m. Annual Banquet and Award Presentation • Announcement of 49th Annual Meeting Location • 2002 Goldich Award Presentation to Mr. Ernie Lehmann, Citation by Ted Dematties • Banquet Speaker: Dr. L. Harvey Thorleifson, Geological Survey of Canada The Search for Diamonds in Canada xxvii �WEDNESDAY MAY 15 8:00 a.m. - 9:00 a.m. REGISTRATION 9:00 a.m. INTRODUCTORY REMARKS R.C. Beard Co-Chairman, Northwest Mineral Development Services, Kenora, ON TECHNICAL SESSION III: Session Chair: Philip Fralick, Lakehead University, Thunder Bay, ON 9:15 a.m. H.H. Woodard, (Department of Geology, Beloit College, Beloit, WI) Internal structures within crustal structural slabs, Quetico-Wawa subprovince junction, Quetico Provincial Park, Ontario 9:40 a.m. R. Bernatchez, (Atikokan Resources Ltd., Atikokan, ON) Mesoarchean Base Metal Systems in the Atikokan Area, NW Ontario, Canada 10:05 a.m. COFFEEBREAK AND POSTER SESSION 10:30 a.m. L.G. Medaris*, B.S. Singer, P.E. Brown, B.R. Jiccha, M.E. Smith, (University of Minnesota-Duluth, Duluth, MN) Wolf River age brecciation in the Baraboo quartzite, Wisconsin: implications for Proterozoic tectonics in the Lake Superior Region. 10:55 a.m. D.L. Southwick, (Minnesota Geological Survey Ret’d, Pacitas, NM) Inferences from the Hattemberger deep drill hole, Carlton County, Minnesota, pertinent to regional stratigraphy and mineral potential of the western segment of the Penokean Orogen 11:20 a.m. J.C. Pedersen* and D. Bubar, (Avalon Ventures Ltd., Toronto, ON) Mineralogy and zonation of the Big Whopper Pegmatite, Separation Rapids, Kenora area, Ontario 11:45 a.m. M.A. Jirsa (Secretary-Treasurer, ILSG, St. Paul, MN) Discussion on the Secretary-Treasurer’s Position 12:00 Presentation of Student Paper Awards LUNCH BREAK 3:00 p.m. FIELD TRIP 6: Departs for Red Lake. xxviii �Thursday, May 16 08:00 - 18:00 Field Trip 4: Industrial Minerals and Paleozoic Geology of Southeastern Manitoba 08:00 - 16:00 Field Trip 5: Separation Rapids Rare-Element Pegmatite Field, Ontario 08:00 - 18:00 Field Trip 6: Geology of the Red Lake Camp, Ontario xxix �POSTER PRESENTATIONS Boerboom, T. (Minnesota Geological Survey, St. Paul, MN) The influence of Archean crustal structures on the margin of the Penokean orogen near Pope County, west central Minnesota. Buchholz, T.W. (Wisconsin Rapids, WI); Falster, A.U. and Simmons, Wm. B. (University of New Orleans, New Orleans, LO). Heterogeneity in the nine mile pluton, wausau complex, in terms of high field strength element (hfse) and rare earth element (ree) bearing minerals Cannon, W.F., (USGS, Reston, VA), Laberge, G.L. (Oshkosh, WI), Klasner, J. S., (Macomb, IL), Dicken, C., (USGS, Reston, VA) Geology of the western Gogebic Iron Range, northwest Wisconsin—a record of sedimentation and deformation across the northern margin of the Penokean orogen Fein, E., (Oberlin College, Oberlin, OH) Anisotropy of magnetic susceptibility in the Otter Tail Pluton, Northwestern Ontario Hubin, J. and Cordua, Wm. (University of Wisconsin, River Falls, WI) SEM imaging of fossil nannobacteria from the supergene zone, Flambeau copper mine, Ladysmith Wisconsin. Johnson, J.R., Kissin, S.A. and Hollings, P. (Lakehead University, Thunder Bay, ON) VHMS mineralization and alteration within the Lumby Lake Greenstone Belt, Northwestern Ontario Kelso, I.S. and Kissin, S.A. (Lakehead University, Thunder Bay, ON) Geology and fluid inclusion studies of the Thunder Bay Agate Mine, Northwestern Ontario Miller, J.D. Jr.1, Green2, J.C., Severson3, M.J., Chandler1, V.W., Peterson3, D.M., Hauck3, S.A., and Wahl1 T.E, (1Minnesota Geological Survey, St. Paul, MN, 2University of Minnesota–Duluth, Duluth, MN, 3Natural Resources Research Institute, Duluth, MN) New geologic map and report of the Duluth Complex and related rocks, Northeastern Minnesota. R.L. Patelke, (Natural Resources Research Institute, University of Minnesota-Duluth, Duluth, MN) Digital Drill Logs for the Duluth Complex - Lithology and Assays Peterson, D.M. (Natural Resources Research Institute, University of Minnesota-Duluth, Duluth, MN) xxx �3-Dimensional View Through a Mineralized System: the South Kawishiwi Intrusion, Duluth Complex Severson, M.J. (Natural Resources Research Institute, University of Minnesota-Duluth, Duluth, MN) The Mine Permitting Process in Minnesota - Who, What, Where, and When. Sikkila, K. (Wisconsin Department of Transportation, Superior, WI) Description of a pegmatite occurrence on the eastern margin of the Mellen Granite, State Highway 13, Ashland County, Wisconsin Woodruff, L.G. (U.S. Geological Survey, Mounds View, MN), W. F. Cannon, and C. Dicken, (U.S. Geological Survey, Reston, VA) Impact of fire on the forest floor and mineral soils, Snowbank Lake, Minnesota COMPANY POSTERS Avalon Ventures Ltd., 111 Richmond Street West, Suite 1116 Toronto, ON M5H 2G4 Emerald Fields Resource Corp., 1546 Pine Portage Road Kenora, ON P9N 2K2 Goldcorp Inc., Balmertown, ON P0V 1C0 Nuinsco Resources Ltd., 940 The East Mall, Suite 110 Toronto, ON M9B 6J7 xxxi �48th ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY Abstracts ��Mesoarchean Base Metal Systems in the Atikokan Area, NW Ontario, Canada Bernatchez, R. (Atikokan Resources Ltd., Atikokan, ON) A high grade silver-lead-zinc showing was discovered in the Lumby Lake Metavolcanic Belt in 1995 between Lumby and Herontrack Lake 100 meters north of the creek. Three grab samples taken from this showing returned an impressive 109, 189 and 416 oz Ag/ton with other samples assaying up to 25% lead and 8% zinc. Since 1995, Atikokan Resources has carried out an extensive exploration program consisting of detailed geological mapping, a 2292 sample soil survey, a 150 whole rock geochemical survey, ground geophysical magnetic, electromagnetic and IP surveys ( dipole-dipole and Gradient array) and seven diamond drill holes over a grided area 6.6 km E-W x 2 km N-S from the west end of Lumby Lake to Hutt Lake. All but one diamond drill hole was drilled on the main high grade silver discovery area located between Lumby and Herontrack Lake. This work has been successful in defining a world class size VMS system. This VMS system is located near the base of the southern limb of the Lumby Lake Mesoarchean Syncline. The detailed geological mapping has identified at least three major eruptive cycles of predominantly intermediate to felsic pyroclastic and tuffaceous rocks. Each eruptive cycle has been terminated with the deposition of exhalative chert and sulphides. These three horizons have been identified as the Lumby-Spoon Lake (southern), Delos Lake and Pond Lake (northern) Horizon. Most of the mechanical stripping has been focused on the Lumby-Spoon Lake Horizon. The rocks along the top portion of the Delos and Pond Lake Horizon are poorly exposed. Limited portions of these two horizons were exposed by mechanical stripping near the west end of each lake. The main high grade silver-lead-zinc discovery is located at the top of the Lumby-Spoon Lake Horizon. This horizon has now been traced from outcrop exposure and drilling for a distance of about 3 kilometers. This east-west striking horizon consists of four or more repeated sub-cycles of quartz-phyric rich tuff, lapilli tuffs (coarse and fine) and chert mineralized with sulphides. The mineralization is contained within the chert, tuff, lapilli tuff in the form of dissemination, stringers and semi-massive sulphide beds. The sulphide mineralization consists of sphalerite, galena, native silver, acanthite, pyrite, chalcopyrite and pyrrhotite. Each felsic cycle vary in thickness from a few meters up to 20-25 meters. The upper cherty portion of each cycle contains is usually mineralized with sphalerite, galena and acanthite and/or native silver. The tuff lapilli tuff section generally contains stringer mineralization of chalcopyrite, pyrite with minor sphalerite, galena acanthite and pyrrhotite. The main high-grade silver showing within the Lumby-Spoon Lake has been drill-tested with six diamond drill holes along an east-west strike length of 450 meters and down to a depth of about 400 meters. Mineralization has been intersected in drill holes over core length from 13 metres and up to 82 metres. Native silver was observed in all six holes drilled on the Lumby-Spoon Lake Horizon. The zone is still open at depth and on strike east and west. In total, the four or five sub-cycles forms a mineralized horizon measuring over 200 meters wide in the main showing area. 1 �The geology, mineralogy and alteration of the mapped area have shown strong similarity with other VMS deposits found in the Archean, Properozoic, and Mesozoic Eras. The drilling and litho geochemical rock sampling have shown the classic footwall chloritic and/or talc alteration, sericitic hanging wall alteration, zinc-lead-silver capping and footwall stringer mineralization. The litho geochemical rock sampling has shown areas of sodium depletion and potassium enrichment within the felsic rocks and magnesium enrichment within the footwall. This alteration appears to be parallel to sub-parallel to the stratigraphy. The felsic volcaniclastic rocks appear to have been derived from a highly fractionated sub-volcanic magma and have been classified as F2 and F3a type rhyolites. A new exploration program of detailed geological mapping, litho geochemical sampling and ground geophysical magnetic and electromagnetic surveys carried out in the Richardson Lake area, 12 km east and on strike with the main high grade silver-lead-zinc showing, has identified similar thick felsic bi-modal volcaniclastic rock sequence. It appears to be the easterly extension of the island arc at Lumby Lake. Several new base metal occurrences have been discovered and are contained within similar quartz- spheric felsic volcaniclastic rocks similar to those found in the Lumby Lake felsic rocks. K. Tomlinson of the GSC (2000) has identified island arc volcanism in both the southern and northern limb and at the base of the Lumby Lake Meta-volcanic Belt. The work carried out to date by ARI has shown that at least two island arc systems may exist within the lower portion of the southern limb of the Lumby Lake Meta-volcanic Belt. It is however, conceivable, that more that two island arc systems exist in this part of the belt. Recent work by ARI in the upper portion of the belt, north of the synclinal fold axis in the northern limb, has identified a different style of mineralization contained within sulphide facies IF. It is unknown at this time if thses two styles of mineralization are related to the same volcanic event of island arc building. It is possible that the Lumby Lake Meta-volcanic Belt may contain world class size VMS deposits yet undiscovered. The results obtained to date by Atikokan Resources Inc. shows that potential. ARI continues to explore and find previously identified and unidentified VMS base metal style mineralization in the Lumby and Finlayson Lake Meta-volcanic Belts. 2 �The influence of Archean crustal structures on the margin of the Penokean Orogen near Pope County, West-central Minnesota Boerboom, T. J., (Minnesota Geological Survey, St. Paul, MN (boerb001@umn.edu)) The western margin of the Paleoproterozoic Penokean Orogen in Minnesota is marked by a series of northwest-trending, sawtooth-like fault offsets, or tear faults, that correspond in space but not in orientation to major northeast-oriented block-bounding shear zones within the adjacent Archean Minnesota River Valley subprovince (Figs. 1A and 1B). The origin of these tear faults, whether reactivated Archean structures or structures formed strictly during the Penokean orogenic cycle is not clear, but they may be the result of scissors faulting focused on preexisting weaknesses in the crust during pre-orogenic extension. Pope County was mapped as part of the Minnesota Geological Survey County Atlas geologic mapping program, supported by the Minnesota Department of Natural Resources, Division of Waters. The bedrock there is deeply buried beneath glacial drift, and there are only six drill cores available for the 720 square mile (1900 square kilometer) county. Considering this, it is obvious that any geological interpretation of the bedrock geology is more accurately described as an interpretation of geophysical characteristics; nevertheless, several prominent structural features can be recognized from the geophysical data. The Minnesota River Valley subprovince has been subdivided into four blocks⎯the Benson, Montevideo, Morton, and Jeffers blocks, from north to south (Fig. 1B). Each of these crustal blocks is separated by straight and narrow east-northeast-trending geophysical lineaments that have been shown to be north-dipping shear zones (Southwick and Chandler, 1996; Southwick, 2002). In Minnesota, the western margin of the Paleoproterozoic Penokean Orogen changes from a northeast orientation that is roughly parallel to the Wawa-Minnesota River Valley subprovince boundary, to a north-south orientation that is perpendicular to the regional fabric of the adjacent Minnesota River Valley subprovince (Figs. 1A and 1B). The major sawtooth-notched offsets, or tear faults, are most prominent in the north-south stretch, where they coincide spatially with the major block-bounding faults in the adjacent Minnesota River Valley subprovince. The most prominent of these tear faults, which crosses southern Pope County, separates the Archean Montevideo block from Paleoproterozoic rocks on the east, but to the west is contained wholly within the Archean Benson block (Fig. 1B). The eastern extent of this fault is occupied by a continuous string of late- to post-tectonic, mafic to felsic intrusions (Fig. 1B) that have been verified by drilling to consist of intrusive rocks similar to those exposed in outcrops to the northeast, near St. Cloud. The concentration of these late, ovoid intrusions along the fault plane implies that the fault extends to deep crustal depths. The tear faults may have formed strictly during Paleoproterozoic time, or they may be reactivated Archean structures that were oriented parallel to the direction of maximum extension and shortening of the Penokean deformational belt. In either case, the coincidence of the orogen-bounding tear faults with major northeast-trending shear zones in the adjacent Minnesota River Valley subprovince 3 �implies that preexisting structures in the Archean crust provided zones of weakness which ultimately controlled the location of the tear faults. Boerboom, T.J., in prep., Bedrock geology of Pope County, Minnesota, plate 2 of Harris, K.L., project manager, Geologic atlas of Pope County, Minnesota: Minnesota Geological Survey County Atlas C-15, Part A, scale 1:200,000. Southwick, D.L., 2002, Geologic map of pre-Cretaceous bedrock in southwest Minnesota: Minnesota Geological Survey Miscellaneous Map M-121, scale 1:250,000. Southwick, D.L., and Chandler, V.W., 1996, Block and shear-zone architecture of the Minnesota River Valley subprovinces: Implications for late Archean accretionary tectonics: Canadian Journal of Earth Sciences, v. 33, no. 6, p. 831–847. 4 �Heterogeneity in the Nine Mile Pluton, Wausau Complex, in terms of high field strength element (hfse) and rare earth element (ree) bearing minerals. Buchholz, T.W., (1140 12th Street North, Wisconsin Rapids, Wisconsin 54494); Falster, A. U., and Simmons, Wm. B., (Department of Geology and Geophysics, University of New Orleans, New Orleans, Louisiana 70148). The Nine Mile pluton is the youngest and most silica-rich of the four intrusive centers of the Wausau complex which is exposed in Marathon County, Wisconsin. The complex is approximately 1.5 Ga in age and locally abounds in pegmatitic veins, aplites, and miarolitic granite. Mineralization in these environments is commonly varied and complex, though in many cases restricted to small localized environments. Notable variation exists in minerals of HFSE (high-field strength elements such as Nb, Ta, Zr, Th, etc) and to a lesser extent in REE (rare earth elements). The type and size of these mineral-rich bodies varies throughout the pluton. In the northern portion of the pluton, large and well-defined pegmatitic bodies with sizeable miarolitic cavities containing a wealth of accessory phases are abundant. The size of pegmatitic veins and the size of miarolitic cavities are smaller in the central and southern parts of the pluton. Small miarolitic cavities in dikes and in granite typically host the accessory minerals in the northern area of the pluton. Significant variation also exists with respect to Nb, Ta, Ti and REE+Y mineralization. In the northern part of the pluton, titanium oxides such as anatase, brookite, and rutile are widespread but they are rare in the central and southern portions. Columbite-tantalite group minerals are most abundant in the central part of the pluton, but less abundant in the south and rare in the northern part. Conversely, Nb-dominant euxenite-group minerals which are rare to absent in the central portion of the pluton are more abundant in the southern and western parts. REE+Y mineralization appears to be more evenly distributed but may be slightly more abundant in the central part. Zircons are widespread in small amounts throughout the pluton and show Hf-enrichment, particularly in close proximity with fluorite. This enrichment occurs mainly in the central portion of the pluton, although isolated instances have been noted in the south and west. Cassiterite is exceedingly rare but has been noted in the central, western, and northern parts of the pluton. Fluorite is most abundant in the central portions of the pluton where it is commonly intimately associated with the Nb-Ta-oxide minerals microlite, manganotantalite and tapiolite. The link of high fluorine activity and more highly evolved geochemical fractionation of Nb-Ta oxides is obvious. Such highly fractionated oxides generally do not occur in NYF-type (NYF = Nb-Y-F enriched pegmatites typical of anorogenic origin) pegmatitic environments. The localized niches of high F activity are where these uncommon HFSE and REE phases occur. In all cases of exotic mineralogy, the central portion of the Nine Mile pluton differs from the rest of the pluton; it is also only here that several high-Sc phases have been found. These observations may indicate that the central portion of the Nine Mile pluton may actually be a fifth intrusive center. Buchholz, T. W., Falster, A. U. & Simmons, Wm. B. 1999. Ta, Nb, U, Y, and REE Minerals of the Koss Quarry, Marathon County, Wisconsin: The 26th Rochester Mineralogical Symposium, Abstracts of Contributed Papers. p. 6. 5 �Buchholz, T. W., Falster, A. U. & Simmons, Wm. B. 2000. Additional Mineralogy of the Koss Quarry, Marathon County, Wisconsin: The 27th Rochester Mineralogical Symposium, Abstracts of Contributed Papers. p. 5. Buchholz, T. W., Falster, A. U. & Simmons, Wm. B. 2001. Minerals of the Ladick East Quarry, Marathon County, WI: The 28th Rochester Mineralogical Symposium, Abstracts of Contributed Papers. p. 7. Falster, A. U., Simmons, Wm. B., Webber, K. L., & Buchholz, T.W. 2000. Pegmatites and Pegmatite Minerals of the Wausau Complex, Marathon C., Wisconsin: Memorie della Societa Naturali e del Museo di Storia Naturale di Milano, V. 30, p. 13-28. Hanson, S.L., Falster, A.U., Simmons, W.B., Webber, K.L., Buchholz, T. 1998. Rare-EarthElement (REE) Mineralization of Pegmatites in the Wausau Complex, Marathon County, Wisconsin: The 25th Rochester Mineralogical Symposium, Abstracts of Contributed Papers. p. 12. Keppler, Hans. 1993. Influence of fluorine on the enrichment of high field strength trace elements in granitic rocks: Contributions to Mineralogy and Petrology, V. 114, p. 479-488. Myers, P.E., Sood, M.H., Berlin, L.A. & Falster, A.U. 1984. The Wausau Syenite Complex, Central Wisconsin: Thirtieth Annual Institute On Lake Superior Geology, Field Trip Guidebook 3. 6 �Geology of the western Gogebic Iron Range, northwest Wisconsin—a record of sedimentation and deformation across the northern margin of the Penokean orogen Cannon, W. F., (USGS, Reston, VA), Laberge, G. L. (Oshkosh, WI), Klasner, J. S., (Macomb, IL), Dicken, C., (USGS, Reston, VA) Between 1991 and 1994 we examined most bedrock exposures of the iron-bearing strata and adjacent units of the western Gogebic Iron Range. We also compiled previously mapped features from both published and unpublished sources, including very detailed but unpublished mapping performed by iron mining companies in the 1950’s. The Gogebic Iron Range is a steeply north-dipping monocline of Early Proterozoic strata. The monocline formed at about 1.1 Ga (Cannon and others, 1993) by crustal-scale thrusting on faults such as the Atkins Lake-Marenisco fault that tilted gently south-dipping rocks formed during the Penokean orogenic cycle at approximately 1.9-1.8 Ga. The map pattern therefore is a cross section of the Penokean features that passes from a weakly deformed depositional and deformational foreland in the east to parts of the Penokean orogen toward the west that were more tectonically active both during deposition and deformation. Early Proterozoic strata lie unconformably on Late Archean granitic and metavolcanic rocks. The Bad River Dolomite, the oldest of the Early Proterozoic strata, is thin and discontinuous in the east but forms a continuous unit several hundred meters thick in the Grandview area in the west. Eastward from the Mt. Whittlesey area the basal unit of the Palms Formation commonly is a chert breccia composed of a residuum of chert fragments formed by dissolution of the cherty dolomite of the Bad River. Elsewhere, basal argillite beds of the Palms lie directly on the Bad River or Archean basement. The Palms Formation maintains uniform character and thickness 7 �across the mapped area. It consists of thin-bedded argillite and argillaceous quartzite in the lower part and thick-bedded quartzite in the upper part. The Ironwood Iron-formation lies conformably on the Palms. Near Upson and eastward the Ironwood displays a five-fold internal stratigraphy (Huber, 1959) of wavy-bedded jaspillitic iron-formation interlayered with even-bedded carbonate and silicate iron-formation. This stratigraphy cannot be traced far west from Upson. Jaspillitic and hematitic units are rare from Mt. Whittlesey to the west where the iron-formation has more reduced mineral assemblages and scarcer cherty units. The westernmost exposures in the Grandview area are banded gruneritemagnetite iron-formation, which becomes progressively leaner and more argillaceous to the west. The iron-formation in the Grandview area is roughly 2,000 feet thick as opposed to 5001000 feet farther east. In the westernmost part of the Grandview area drill holes show that the iron-formation becomes interbedded with black shale. The Tyler Formation overlies the Ironwood, probably with low angle unconformity, and is predominantly a thick sequence of graded-bedded turbidites. Most Early Proterozoic rocks were variably metamorphosed at about 1.1 Ga in the broad contact aureole of the Mellen intrusive complex. The Early Proterozoic stratigraphic section records deposition in a relatively shallow stable basin through the time of deposition of the Palms Formation. Beginning with deposition of the Ironwood, there are indications of tectonic instability and variations of depositional setting across the area. In the east iron-formation appears to have accumulated under tectonically quiescent conditions allowing widespread deposition of similar lithologies and thicknesses. To the west, deposition appears to have been in progressively deeper water and to have had a greater input of fine-grained clastic material. Also, in the Grandview area, sills of diabase as much as 1,000 feet thick occur in the iron-formation and enigmatic beds of breccia, possibly of volcanic derivation, suggest a proximity to tectonically and volcanically active parts of the Penokean orogen. The situation may be similar to the eastern end of the Gogebic Range where sills and volcanic rocks interfinger with the Ironwood. Finally, deposition of the turbidites of the Tyler Formation marks the transgression of a deep-water foreland basin in advance of accreting volcanic arcs of the Wisconsin magmatic terranes to the south. Penokean structures are much more intensely developed in the west than the east. Near Upson, structures are limited to rare outcrop-scale folds in the iron-formation and to moderately to weakly developed cleavage in more argillaceous units of the Tyler Formation. From near Mt. Whittlesey to the Mineral Lake area folding and faulting are much more intense. Folds with wavelengths of hundreds of meters are discernable in the iron-formation. Bedding plane faults and other originally low angle faults are also common including a basal decollement that separates the Early Proterozoic strata from Archean basement. Folds appear to have been mostly assymetrical to recumbent and north-verging, but Middle Proterozoic tilting has rotated these so that they are now mostly overturned structures. Likewise, original low angle thrusts are now tilted toward the north and have the appearance of down-to-the-north normal faults. Cannon, W.F., Peterman, Z.E., and Sims. P.K., 1993, Crustal-scale thrusting and origin of the Montreal River monocline—a 35-km-thick cross section of the Midcontinent rift in northern Michigan and Wisconsin: Tectonics, v.12, p. 728-744. Huber, N.K., 1959, Some aspects of the origin of the Ironwood Iron-formation of Michigan and Wisconsin: Econ. Geology, v. 54, p. 82-118. 8 �Anisotropy of magnetic susceptibility in the Ottertail pluton, Northern Ontario. Fein, E. M., Sturm, C. L., and Czeck, D. M., (Department of Geology, Oberlin College, Oberlin OH, 44074, Elizabeth.Fein@oberlin.edu.) INTRODUCTION In this project, we studied the structural and magnetic fabrics of the Ottertail Pluton, a small, granitic body within the Superior Province of Northern Ontario. The Ottertail, part of the Algoman suite of quartz monzonites and granodiorites, intruded surrounding rocks along the Wabigoon- Quetico subprovince boundary around 2.7 Ga (Davis et. al, 1989). From field observation, the emplacement of the Ottertail Pluton has been considered to be postdeformational due to the lack of obvious deformation fabrics evident in other types of rocks at the subprovince boundary (e. g. Davis et. al, 1989; Poulsen, 2000) . This interpretation has been used to constrain the timing of final deformation along the Wabigoon- Quetico boundary associated with microplate collision. PROBLEM On a much smaller scale, within deformed conglomerates, such as the nearby Seine River conglomerates, rigid clasts often display evidence of much less deformation than do more yielding clasts. Could a similar situation have occurred on a much larger scale in the case of the Ottertail Pluton? The relatively rigid granite body could appear to be relatively undeformed in the field, even if the timing of its emplacement was pre- or syn- tectonic. This situation, where a pluton appears undeformed despite being present during deformation, has been shown to occur in some modern tectonic regimes (e. g. Paterson & Tobisch, 1988). A more detailed study of plutonic fabrics and relationships is needed to conclusively determine whether a pluton is truly “post deformational.” METHOD In August of last year, we sampled an E-W (and a shorter N-S) transect across the Ottertail Pluton traveling along Highway 11, west of Mine Centre, Ontario. Thirty-one sites were sampled resulting in a total of 218 oriented cores. The anisotropy of magnetic susceptibility (AMS) of these granitic cores was measured to learn about the history of pluton emplacement. The measurements were conducted at the Institute of Rock Magnetism at the University of Minnesota in Minneapolis in January of this year. In general, the samples have a significant AMS signal contributed by multi-domain magnetite. The AMS data has been complied to describe the attitude of the variations in magnetic fabric within the Ottertail Pluton. To determine the relative timing of pluton emplacement with respect to the deformation, we need to show whether the magnetic fabric is concordant with preserved, regional patterns of strain. The AMS study will be combined with our field observations and a detailed petrographic study. 9 �RESULTS Regional deformation fabrics have already been well documented in the literature (e. g. Poulsen, 1986; Poulsen, 2000; Czeck, 2001). Preliminary results of our study suggest that the magnetic fabrics preserved in the Ottertail are related to the pattern of regional strain fabrics in the surrounding rocks. Further work will be conducted to analyze the magnetic fabrics to determine if, indeed, the Ottertail Pluton is pre- or syn- tectonic. If it can be shown to be pre- or syntectonic, this will have major implications for the relative timing of larger, regional, tectonic events. In particular, if the Ottertail is representative of other Algoman plutons and the deformation is truly regional, it may not be possible to constrain the end of regional deformation using the date of the intrusion of Algoman granites. Czeck, D. M., 2001. Strain analysis, rheological constraints, and tectonic model for an Archean polymictic conglomerate: Superior province, Ontario, Canada. Unpublished Ph. D. Thesis, University of Minnesota, 245 p. Davis, D. W., Poulsen, K. H., Kamo, S. L., 1989. New insights into Archean crustal development from geochronology in the Rainy Lake area, Superior Province, Canada. Journal of Geology 97, 379-398. Paterson, S. R., Tobisch, O. T., 1988. Using pluton ages to date regional deformations: problems with commonly used criteria. Geology 16, 1108-1111. Poulsen, K. H., 1986. Rainy Lake Wrench Zone: An example of an Archean Subprovince boundary in Northwestern Ontario. In: de Wit, M. J., Ashwal, L. D. (Eds.), Tectonic evolution of greenstone belts Technical Report 86-10, pp. 177-179. Poulsen, K. H., 2000. Archean metallogeny of the Mine Centre - Fort Frances area. Ontario Geological Survey Report 266, 121. 10 �Groundwater and Heat Flow in an Interlobate Moraine in Southeastern Manitoba. Ferguson, G. A. G. and Woodbury, A. D., (Department of Civil Engineering, University of Manitoba) Significant deviations from conductive heat flow occur in the Sandilands area of southeastern Manitoba as a result of groundwater flow. The main geological feature of this area is an interlobate moraine that coincides with the subcrop belt of the Ordovician Winnipeg Formation, which is composed primarily of sandstone and shale. The sandstone of the Winnipeg Formation acts as a regional aquifer and regional hydraulic head distribution and hydrogeochemistry suggest that the Sandilands area is an important source of recharge to this aquifer. The sandstone aquifer of the Winnipeg Formation is hydraulically separated from overlying hydrostratigraphic units west of the subcrop belt by a layer of low permeability shale that is thought to be continuous [Betcher, Third Canadian Hydrogeological Conference (1986)]. Overlying the Winnipeg Formation is the Ordovician Red River Formation, consisting of dolomitic limestone in the study area. The Red River Formation contains an aquifer in its upper extent, and previous studies have suggested that the Sandilands is an important recharge area for this aquifer as well. However, Sandilands recharge is likely less important as this carbonate aquifer is also recharged through the tills between the moraine and the Red River [Render, Cdn. Geotech. Journal, v.7, 243-274 (1970)]. Till is also present at the base of the moraine and acts as a semi-confining layer. Deviations from conductive heat flow provide a means of determining the direction and magnitude of groundwater flow, and are an efficient method of investigating the interaction between the groundwater in the surficial glaciofluvial sediments and the underlying bedrock aquifers. The geothermal gradient in the Superior Province of southeastern Manitoba is approximately 0.01 OC/m, which corresponds to a heat flow of approximately 40 w/m2. Temperature profiles through the moraine in the Sandilands area show geothermal gradient as low as 0.006 OC/m below the depth of penetration for seasonal variations. One-dimensional analytical models indicate that downward flow of groundwater must be occurring at a rate of approximately 10-8 m/s to produce the observed curvature seen in the temperature profiles. Analysis of temperature profiles in areas adjacent to the moraine has also aided in interpreting the hydrostratigraphy of the area. Temperature profiles through the upper shale unit of the Winnipeg Formation are linear, indicating that this layer is does act as an effective confining layer for the underlying sandstone aquifer. Discharge areas and areas of predominantly horizontal flow are present in the area surrounding the moraine, suggesting that not all of the water infiltrating into the moraine reaches the underlying bedrock aquifers. 11 �Mesoarchean Evolution of Western Superior Province: Evidence from Metasedimentary Sequences near Atikokan P. Fralick and D. King, (Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1. (pfralick@mercury.lakeheadu.ca)) Paleogeographic reconstructions based on data supplied by metasedimentary sequences provide a basis for understanding plate movements which create sedimentary basins. This combined with geochronology allows sequential developmental stages of a craton to be deciphered. Thus, by interpreting environment of deposition from outcrop data, inferring provenance from composition of the metasedimentary units, and determining the age of detrital zircons and zircons in interstratified or intrusive igneous rocks insight will be gained on the plate movements which caused a basin to subside. The above was applied to a Mesoarchean sequence of metasedimentary and metavolcanic rocks to evaluate the degree basin subsidence could be explained by lateral plate motions similar to those of today. The succession is on the southern margin of Wabigoon Subprovince in the Atikokan area. It consists of, along an eighty kilometre belt from southwest to northeast,: 1) the Steep Rock Group, 2) the Little Falls assemblage, 3) the Finlayson Lake greenstone belt, and 4) the Lumby Lake greenstone belt. The oldest rocks exposed in the sequence are pre 3014 Ma tholeiitic basalts (Figure 1), the dominant rock type in the entire assemblage. The lowermost sedimentary units at Little Falls and Finlayson consist of felsic agglomerates and tuffs laterally transitional into resedimented sandstones and conglomerates. Transport was via high density mass flows, though some beds are possibly traction current deposits. Mafic ash layers are present throughout. Age determinations of the resedimented, pyroclastic zircon population gave 2996.9+-0.8 Ma, almost identical to the compositionally similar (Stone et al, 1992) Marmion Batholith which intrudes the area. Felsic volcanism also occurs in the Lumby Lake belt at this time. Plume derived (Hollings et al, 1999; Hollings and Wyman, 1999; Tomlinson et al, 1999) mafic and ultramafic volcanism continued in these belts until some time after 2828 Ma (age determination of Tomlinson et al, 2001) when eruptions ceased and a coarsening upward sedimentary sequence of iron formation, DE turbidites, classic turbidites, wave reworked coarse sandstones, and conglomerates was deposited at Finlayson The geochemistry and detrital zircon population (2997 to 3002 Ma) of the Finlayson sandstones is similar to Wagita Formation sandstones, a fluvial unit deposited in incised paleovalleys at the base of the Steep Rock Group (Wilks and Nisbet, 1988). Eroded basalt and tonalite detritus moved through Steep Rock paleochannels building out the shoreline in the Finlayson area. The sequence at Lumby consists of classic turbidites interbedded with lenses of mass flow conglomerates near the top, overlain by carbonate and finally iron formation. Although sedimentation was probably sinchronous in the Lumby area, geochemistry indicates komatiites were important source material here. Clastic sedimentation in the entire area was ended by transgression causing backfilling of Wagita fluvial channels followed by rapid drowning of the entire source area. Stromatolitic carbonates were deposited in the shallow ocean, but as subsidence continued the carbonate was replaced by iron formation. At 2780 Ma (Tomlinson et al, in press) the next volcanic cycle began with eruption of the komatiitic Ashrock Formation. 12 �The mafic and ultramafic volcanic units which dominate the sequence have been ascribed to plume generated melts erupting in an ocean plateau environment (Tomlinson et al, 1999; Hollings and Wyman, 1999; Hollings et al, 1999); the presence of the tonalites being ascribed to slab melting in a proximal subduction zone (Hollings et al, 1999). Though this model is elegant the new age determinations render it unworkable. Generation of felsic melts by plume induced heating at the base of the overthickened basaltic plateau is compatible with tonalite geochemistry (Smithies, 2000). The sedimentary sequence is consistent with an ocean plateau environment, but definitely not representative of a continental rift or cover sequence as previously suggested (Thurston and Chivers, 1990). Partial melts generated in the lower basaltic crust formed magma chambers 2 to 4 km below surface. This magma fed felsic eruptions which, in turn, supplied most of the material for clastic layers lower in the volcanic pile. Eruption of the enclosing basalts was probably sporadic, as the area was effected by pulses of plume activity (Tomlinson et al, 1999). These events lasted for over 185 Ma, into the Neoarchean, until finally terminated by a hiatus in volcanic activity. With the end of plume driven crustal heating, thermal decay induced subsidence lead to subareal to shallow water clastic sedimentation and shallow to deep water chemical sedimentation. The existence of an ocean plateau for over 185 Ma, and possibly over 235 Ma, calls into question the operation of plate tectonics in the Mesoarchean, and especially fast spreading, multiple ridge-subduction zone configurations proposed for this time period. If the plateau was undergoing lateral plate motion subduction zones must have been well spaced and movement slow to allow for its activity over such a long time period. This is difficult to reconcile with a hotter earth requiring greater heat dissipation capacity than today. The other alternative: conventional plate tectonics did not operate in the Mesoarchean, but rather heat loss was achieved through vigorous plume activity, possibly similar to the early history of Mars, needs to be explored further. Hollings, P. and Wyman, D., 1999. Lithos, 46, 189-213. Hollings, P., Wyman, D. and Kerrich, R., 1999. Lithos, 46, 137-161. Smithies, R.H., 2000. Earth and Planetary Science Letters, 182, 115-125. Stone, D., Kamineni, D.C. and Jackson, M.C., 1992. Geological Survey of Canada, Bulletin 405. Thurston, P.C. and Chivers, K.M., 1990. Precambrian Research, 46, 21-58. Tomlinson, K., Davis, D.W., Hughes, D.J. and Thurston, P.C., 1998. Lithoprobe Report 65, 35-47. Tomlinson, K., Hughes, D.J., Thurston, P.C. and Hall, R.P., 1999. Lithos, 46, 103-136. Tomlinson, K., Davis, D.W., Stone, D. and Hart, T., 2001. Lithoprobe Report, Western Superior. Tomlinson, K., Davis, D.W., Stone, D. and Hart, T., in press, Precambrian Geology. Wilks, M.E. and Nisbet, E.G., 1988. Canadian Journal of Earth Sciences, 25, 370-391. 13 �EEl Felsic Volcanic Rocks LUMBY SOUTH Mafic Volcanic ______________ 2997 Ages in Million years 2828 WALLACE I AVP Grarntic Rocks —e .— —C U— —U U— FINLAYSON LITTLE PAT T c Fig. 1. Stratigraphic sections of Mesoarchean rocks in the Atikokan area, and Wallace Lake in the Uchi Belt for comparison. Geochronology mainly from Tomlinson et al and Stone et al. 14 �SEM Imaging of fossil nannobacteria from the supergene zone, Flambeau Copper Mine, Ladysmith, Wisconsin Hubin, J. and Cordua, Wm., (Department of Planet and Earth Science, Univeristy of Wisconsin-River Falls, 410 South Third Street, River Falls, WI 54022 william.s.cordua@uwrf.edu) The discovery of nannobacteria fossils in Eocene supergene zones in Chilean copper deposits (Sillitoe et. al. 1996) suggested that similar, but older, nannobacteria may be found in the Cambrian to Precambrian supergene zone of the Flambeau Copper Mine in Rusk County, Wisconsin. A scanning electron microscope study of samples from the Flambeau Mine focused on the interface between pyrite and replacing chalcocite. Images revealed several sites where spherical clusters 0.1 to 0.3 microns in diameter are present (fig. 1). Through visual comparison with existing studies and elimination of other possible explanations, these clusters are interpreted as being nannobacteria. The age of supergene mineralization at the Flambeau Mine predates the 525 million year old Mt. Simons sandstone (May and Dinkowitz, 1996), and may pre-date the deposition of the Barron quartzite (1750 - 1650 m.y.) (Medaris, 2000). If these are fossil nannobacteria rather than recent contaminants, it suggests that the involvement of nannobacteria in the supergene enrichment processes spans a considerable proportion of geologic time. Acknowledgements: We are indebted to Jeff Thole and his assistants for their help in using the SEM facilities at Macalester College for this study. REFERENCES: May, E.R., and Dinkowitz, S., 1996. An overview of the Flambeau supergene enriched massive sulphide deposit: geology and mineralogy, Rusk County, Wisconsin, in LaBerge, G., edited, Volcanic massive sulphide deposits of northern Wisconsin: A commemorative volume: Insitute on Lake Superior Geology Proceedings, 42nd Annual Meeting, Cable, WI, v.42, part 2, p. 67-93 Medaris, G. 2000, The Barron saprolite: confirmation of mature chemical weathering in the source for Paleoproterozoic quartz arenites in the Lake Superior region [abstract] Institute on Lake Superior Geology Proceedings, 46th Annual Meeting. Thunder Bay, ON, 2000, Part 1, p.37-38. Sillitoe, R.H., Folk, R.L. and Saric, N. 1996, Bacteria as mediators of copper sulphide enrichment during weathering, Science, vol. 272, p. 1153-1155. 15 �Figure 1. Suspected nannobacteria (arrows) at pyrite-chalcocite interfaces, Flambeau Mine, Ladysmith, Rusk County, Wisconsin. 20,000x. White bar = 5 microns. 16 �Archean and paleoproterozoic mafic intrusions in Minnesota Jirsa, M. A., (Minnesota Geological Survey, St. Paul, MN (jirsa001@umn.edu)) Exploration for platinum group elements (PGEs) in Minnesota has been restricted largely to mafic intrusions of the Mesoproterozoic Duluth Complex for obvious reasons—the complex contains known PGE occurrences associated with previously outlined coppernickel deposits. A new study by the Minnesota Geological Survey and the Natural Resources Research Institute is looking beyond the Duluth Complex to describe and analyze the varied mafic, ultramafic, and alkalic intrusions in the northern, western, and east-central parts of the state. Although geochronologic data are sparse, field and geophysical evidence implies that most of these intrusions are Archean and Paleoproterozoic in age. Figure 1 shows some of the major mafic intrusions and intrusive complexes for which some “ground truth” (drill core or outcrop) exists. They have been identified by surface and geophysical mapping and drilling; however, few have been thoroughly described and analyzed, and almost none have been analyzed for PGE content. Some of the intrusions are lithologically similar to the so-called “Quetico” and “Atikokan” intrusions that are the targets of exploration in Canada. The geologic settings of significant PGE deposits worldwide include mafic to ultramafic plutons associated with volcanic arc terranes in linear orogenic belts (Ural-type), differentiated sill complexes, ophiolitic complexes, syn- to late-orogenic layered complexes (Bushveld and Stillwater), and alkalic to subalkalic composite plutons. This study is designed to characterize the various types of mafic intrusions in Minnesota and perhaps broaden the exploration horizon to similarly diverse terranes throughout the state. At present, an inventory containing the pertinent facts from more than 150 individual intrusions and thousands of diabasic dikes has been compiled and sampling work has begun. As an initial classification, the known intrusions are grouped below first according to the age of host rocks into which they were emplaced, and secondly on the basis of perceived temporal, mineralogic, and geometric attributes. ARCHEAN HOST ROCKS Layered mafic-ultramafic complexes—Pre-tectonic with respect to the main metamorphic and foliation-forming deformation event. The rocks are variably metamorphosed and have tholeiitic to komatiitic compositions. The presence of layering and cumulate textures indicates differentiation. Rock types include peridotite, pyroxenite, gabbro, anorthositic gabbro, and diorite. Examples: Deer Lake Complex and Newton Lake Formation. Subvolcanic mafic sills—Pre-tectonic, variably metamorphosed, presumably hypabyssal intrusions associated with greenstone sequences. They typically are tholeiitic in composition. Rock types include gabbro, pyroxenite, and diorite. Example: Thistledew Lake sequence. Amphibolitic sills and dikes—Typically schistose, metamorphosed, and folded, narrow sills emplaced into schists and granitic intrusions of the Quetico subprovince. Example: Ash River amphibolite. 17 �Grygla p/Won ( Ash River amphibo/ite N - I Dever P'U(UT' *,'/ N.anLn. Thistledow Lake, Duluth Complex and related rocks sills 1• r, p EXPLANATION anomaly / Aeromagnetic interred lobe diaba&c dike iiiiuuiij,ipuui or containing, matic phases '\\\- &Y Drill hole Figure 1. Generalized map of Minnesota showing the distribution of mafic intrusions and drill holes that intersect mafic intrusive rocks. Many of the drill holes encountered intrusions too small to be depicted. Intrusions discussed in the text are labeled. 18 �Lamprophyric, pyroxenitic, peridotitic, and hornblendic intrusions—Syn- to post-tectonic dikes and small, irregular bodies. They typically are coarse-grained, oxide-rich, and strongly magnetic. Example: Dead River pluton. Sanukitoid composite stocks—Discrete, post-tectonic intrusions of the dioritemonzodiorite-monzonite-syenite suite. They typically are quartz-poor to quartzabsent. Most contain intrusive phases and enclaves of amphibole-pyroxene-micabearing diorite, locally of lamprophyric affinity. Example: Gheen pluton. Tonalite-diorite intrusions—Discrete, probably post-tectonic intrusions that contain magmatic phases varying from tonalite to diorite and gabbro. Example: Grygla pluton and associated intrusions. PALEOPROTEROZOIC AND ARCHEAN HOST ROCKS Gabbroic intrusions—Discrete, typically semicircular intrusions containing gabbro, troctolite, anorthositic gabbro, and pyroxenite. Typically show as magnetic and gravity highs. Example: Lake Washington intrusion. Pyroxenite and peridotite plugs and stocks—Small, circular to irregularly shaped bodies having strong magnetic signatures. Commonly occur along linear geophysical discontinuities. Examples include many of the small intrusions in central Minnesota shown on Figure 1 by drill hole locations. Age is uncertain, but one is known to intrude ~1780 Ma granite. Diabasic to gabbroic dikes—Northwest-trending dikes are part of the Kenora– Kabetogama swarm of Paleoproterozoic age, other dike orientations are both Paleoproterozoic and Mesoproterozoic in age. Outcrop mapping indicates dike widths between 30 and 100 meters. Larger dikes commonly are composite and have central zones composed of granodiorite and leucogabbro. This classification will undoubtedly be modified with the addition of new petrographic and geochemical data. This study is funded by the Minnesota Legislature on recommendation of the Minnesota Minerals Coordinating Committee. 19 �VHMS mineralization and alteration within the Lumby Lake Greenstone Belt, Northwestern Ontario Johnson, J.R., Kissin, S.A. and Hollings, P., (Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1, jrjohnsonca@yahoo.ca) The Lumby Lake greenstone belt, located approximately 40 km northeast of Atikokan, Ontario or 165 km northwest of Thunder Bay, Ontario, has been a focus of exploration activity for the past one hundred years. The belt is located within the Wabigoon Subprovince of the Superior Province. The Wabigoon Subprovince is interpreted as a collage of 3 – 2.7 Ga plutonic and volcanic terranes (Blackburn et al, 1991), where as Tomlinson et al. (1999) have shown that the Lumby Lake belt ranges in age from 28983001 Ma. Mapping done by Jackson (1985a, b) showed the belt to be dominated by mafic metavolcanic units that occur as massive to pillowed flows. Thin felsic volcanics and clastic sedimentary units were found throughout the belt along with less common ultramafic flows, located only towards the top of the stratigraphic sequence. The Marmion Lake Batholith intrudes the southern portion of the belt. In the past the belt has been discounted as a possible base metal camp due to the relative absence of felsic material. Hollings and Wyman (1999) documented that the mafic volcanic rocks of the belt are similar to those of modern ocean plateau tholeiites (associated with plume related volcanism), consistent with the presence of komatiites. These authors invoked a complex model of plume arc interaction to account for the presence of arc related felsic volcanics within the plateau related mafic pile. Hollings and Wyman (1999) reported the presence of two distinct types of felsic volcanic rock, and comparable plutonic phases, within the Lumby Lake belt. One suite characterized by fractionated heavy rare earth elements (HREE) are comparable to the nonprospective F1 of Lesher et al (1996), the second suite with unfractionated HREE are similar to the variably prospective F2 of Lesher. The F2 felsic horizons are found within the Lumby Lake-Spoon Lake area and further to the south within the Marmion Lake Batholith. Tomlinson et al. (1999) found that the felsic volcanics of belt, in general, give the oldest ages, ~3 Ga. Additional exploration, by Atikokan Resources Inc. (ARI) in the last decade, and mapping done, in conjunction with ARI, in the last year has identified the presence of additional extensive felsic flows, tuffs and breccias, predominately in the southern portion of the belt. Areas of base metal mineralization, such as the stratabound pyrite, sphalerite, galena, chalcopyrite, native silver and silver sulphides found in the Lumby Lake-Spoon Lake area (0.2 to 9.2% Zn, #0.25% Cu, 1.0% Zn and 37 – 60 g Ag/t) and the pyrite, sphalerite, galena, pyrrhotite and chalcopyrite located near Richardson Lake, have been revealed. It should be noted that there is a scarcity of volcanic hosted massive sulphide deposits of 3 Ga or older 20 �Examination of the area near Richardson Lake indicates that silicification and chloritization of the rocks is common with sericite alteration less prominent. This alteration is consistent with that found in the footwalls of volcanogenic massive sulphide deposits (Franklin, 1993). Additional mapping and geochemical work, involving the classification of newly located felsic horizons and the relative enrichment and depletion of elements, will lead to further defining these zones of alteration and enable the location of additional base metal targets. In addition geochemical analyses will be utilized to further define the tectonic setting of the Lumby Lake belt. Blackburn, C. E., Johns, G. W., Ayer, J. A., Davis, D. W., 1991, Wabigoon Subprovince, Geology of Ontario. Ontario Geological Survey, p. 303-382. Davis, D. W., and Jackson, M. C., 1988, Geochronology of the Lumby Lake greenstone belt: A 3 Ga complex within the Wabigoon Suprovince northwestern Ontario: Geological Society of America Bulletin, v. 100, p. 818-824. Franklin, J. M., 1993, Volcanic-associated massive sulphide deposits, in Kirkham, R. C., Sinclair, W. D., Thorpe, R. I. And Duke, J. M., eds, Mineral Deposit Modeling: Geological Association of Canada, Special Paper 40, p. 315-334. Hollings, P. and Wyman, D., 1999, Trace element and Sm-Nd systematics of volcanic and intrusive rocks from the 3 Ga Lumby Lake Greenstone belt, Superior Province: evidence for Archean plume-arc interaction. Lithos, v. 46, p. 189-213. Jackson, M. C., 1985a, Geology of the Lumby Lake area, eastern part, districts of Kenora and Rainy River: Ontario Geological Survey Open-File Report 5535, 122 p. Jackson, M. C., 1985b, Geology of the Lumby Lake area, western part, districts of Kenora and Rainy River: Ontario Geological Survey Open-File Report 5534, 151 p. Lesher, C. M., Goodwin, A. M., Campbell, I. H. and Gorton, M. P., 1986, Trace-element geochemistry of ore associated and barren, felsic metavolcanic rocks in the Superior Province, Canada; Canadian Journal of Earth Sciences, v. 23, p. 222-237. Tomlinson, K. Y., Davis, D. W., Thurston, P. C., Hughes, D. J., and Sassevile, C., 1999, Geochemistry, Nd isotopes and geochronology from the Central Wabigoon Subprovince and North Caribou Terrane: regional correlations leading towards a Mesoarchean reconstruction. Lithoprobe Report #70, p. 136-152. 21 �Geology and fluid inclusion studies of the Thunder Bay Agate Mine, Northwestern Ontario Kelso, I.S. and Kissin, S.A., (Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1 stephen.kissin@lakeheadu.ca.) The Thunder Bay Agate deposit, located northeast of Thunder Bay at the junction of Highway 11/17 and Highway 527, was discovered in 1989 and developed into the presently operating Thunder Bay Agate Mine in 1996. The deposit is hosted by carbonate units of the Gunflint Formation and is 50m NE of the contact with olivine diabase of a Logan sill. The deposit has a surface exposure of approximately 160 x 200m. Throughout the area of the deposit, intermittent open spaces within the host-rock are filled by agate. Where the agate-filled spaces are larger than ~50cm high and 1m wide, there is evidence of collapse brecciation of the host-rock. The largest agate-filled space observable is 2m wide and 0.8m high. Thunder Bay agate consists of three mineral phases: chalcedony, quartz, and graphitic carbon. The chalcedony can be subdivided into three varieties: dark amorphous silica (opal-A?), light amorphous silica (opal-CT?), and translucent chalcedony. A consistent sequence of mineralization is observed throughout the deposit. From nucleation against the host-rock, the sequence is rhythmic bands of dark amorphous silica, light amorphous silica, translucent chalcedony, and quartz-encased, carbon-filled vugs. As mineralization radiates from several points of nucleation, interference patterns generate complex, sinuous structures in the sequence. Although graphitic carbon is not always present, where it does occur it is never observed to be in direct contact with the host-rock or chalcedony. Quartz is often the final stage of mineralization and is rarely observed to be in direct contact with the host-rock. The rhythmic bands of each stage vary from <1mm to >1cm in width. Three types of fluid inclusions, according to the terminology of Roeder (1984), were found: Type I: Single-phase inclusions containing liquid or vapour only. A number of Type I inclusions in quartz were examined; however, no change on freezing was observed. Type II: Two-phase liquid + vapour inclusions. These are the most common type of inclusion in quartz. Eutectic, final melting and homogenization temperatures were obtained. Type III: Three-phase liquid + vapour + solid(s) inclusion. Not observed in this study. Type IV: CO2-rich inclusions. These homogenized at <31°C and decrepitated when heated above homogenization. Type II Two-phase inclusions (liquid and vapour) are the most numerous type of primary inclusion within the quartz. These inclusions range in size from 4 to 50µm along their greater axis and have a vapour content of 2% to 7% by volume. All Type II inclusions in the quartz homogenized to liquid in the range of 70 - 190°C with the majority homogenizing within the range of 100-130°C. The eutectic and final melt temperatures of the inclusions clustered around two ranges. Inclusions with eutectic temperatures of 22 �approximately -52°C exhibited final melt temperatures of -34 to -27°C; inclusions with eutectic temperatures of approximately -25°C exhibited final melt temperatures of -7 to 2°C. The higher temperature cluster corresponds to the NaCl-H2O eutectic temperature of -22°C. The lower temperature cluster of inclusions likely contain minor amounts of CaCl in solution with NaCl. The chalcedony is almost exclusively devoid of inclusions; however, one cluster of about 10 Type IV inclusions containing liquid CO2 were observed in translucent chalcedony. In these inclusions the frozen CO2 displays visible melting at -45°C and homogenized at 30.2°C. Fluid inclusion studies indicate the quartz was emplaced in a low-temperature, lowpressure hydrothermal environment. The timing of deposition is delimited between the age of the Gunflint Formation (1878 ma; Fralick et al., in press) and the Logan sills (1109 ma, Davis and Sutcliffe, 1985) which have been seen to contain xenoliths of typical agate (J. Scott, pers. com., 2001). The deposition of agate appears to be related pervasive, early silicification of the Gunflint and other Lake Superior-type iron formations, as summarized by Simonson (1987). He proposed that silicification most likely could be attributed to siliceous thermal springs located nearer the axis of the depositional basin. Compaction could expel the siliceous solutions to the margins of the basin. Rimstidt and Barnes (1980) showed that silica precipitation in response to declining is most effective in a decrease from 300 to 100°C, the approximate mean homogenization temperature observed in quartz-hosted inclusions. Silica solubility also decreases sharply with a drop in pH. Organic matter producing CO2 at the site of agate deposition would both cause silica precipitation and dissolution of carbonate host rocks through lowering of pH. Evidence of such CO2 production is seen in the presence of Type IV CO2-rich inclusions. Organic matter also is believed to provide a favourable nucleation site for silica (Knoll, 1985). Thus, the occurrence of the agate deposit in carbon-rich carbonate rocks can be explained. Fralick, P., Davis, D.W., and Kissin, S.A. in press. The Age of the Gunflint Formation, Ontario, Canada: Single zircon U-Pb age determinations from reworked volcanic ash. Canadian Journal of Earth Science. Davis, D.W., and Sutcliffe, R.H. 1985. U-Pb ages from the Nipigon Plate and northern Lake Superior. Geological Society of America Bulletin 96: 1572-1579. Knoll, A.H. 1985. Exceptional preservation of photosynthetic organisms in silicified carbonates and silicified peats. Philosophical Transactions of the Royal Society of London B311: 111-122. Rimstidt, J.D., and Barnes, H.L. 1980. The kinetics of silica-water reactions. Geochimica of Cosmochimica Acta 44: 1683-1699. Roedder, E. 1984. Fluid Inclusions. Mineralogical Society of America, Reviews in Mineralogy 12. Simonson, B.M. 1987. Early silica cementation and subsequent diagenesis in arenites from four Early Proterozoic iron formations of North America. Journal of Sedimentary Petrology 57: 499-511. 23 �WOLF RIVER-AGE BRECCIATION IN THE BARABOO QUARTZITE, WISCONSIN: IMPLICATIONS FOR PROTEROZOIC TECTONICS IN THE LAKE SUPERIOR REGION Medaris, L.G., Jr., Singer, B.S., Brown, P.E., Jicha, B.R., and Smith, M.E., (Dept. of Geology and Geophysics, Univ. of Wisconsin-Madison, 1215 W. Dayton St., Madison, WI, 53706; medaris@geology.wisc.edu) Breccia zones in Baraboo Quartzite, consisting of angular red quartzite fragments cemented by a stockwork of white quartz veins (Fig. 1), are distributed sporadically throughout much of the Baraboo Range. Larger breccia zones tend to be parallel with associated bedding, although thin quartz veins with incipient breccia features are commonly crosscutting. The most prominent breccia zone, which is well exposed in Ableman's Gorge, is ~100 meters thick and can be traced for ~20 kilometers along the north limb of the Baraboo syncline. Figure 1. Quartzite breccia, Martin-Marietta Quarry, north limb, Baraboo syncline Figure 2. Back-scattered electron image of muscovite (light gray), kaolinite (dark gray), and quartz (medium gray) in breccia cement The breccia cement is composed predominantly of massive vein quartz, which is occasionally accompanied by small amounts of specular hematite. Locally, euhedral quartz crystals occur in vugs, some of which are partly to completely filled by kaolinite. Muscovite is rare in the breccia cement, but muscovite was recently discovered by Phil Fauble in an outcrop of breccia in the bluffs east of Devil's Lake on the south limb of the syncline. At this locality, prominent flakes of muscovite appear to be in textural equilibrium with kaolinite and quartz (Fig. 2). 24 �The stable coexistence of quartz, kaolinite, and muscovite is limited to temperatures below ~300oC by the reaction, kaolinite + quartz = pyrophyllite + H2O (Fig. 3). Quartz in the muscovite bearing sample contains abundant H2O-rich fluid inclusions, which yield homogenization temperatures from 160 to 186oC and freezing point depressions from 1.2 to -6oC, corresponding to maximum salt contents of ~6.8 wt%. The fluid inclusion isochores, combined with phase equilibrium constraints, indicate that brecciation occurred in the range, 200 to 280oC and 500 to 2000 bars (depths of 2 to 8 kms). Such a temperature range is lower than that for the predominant quartz + pyrophyllite assemblage in the Baraboo Quartzite, which is stable between about 300 and 375oC at 2000 bars. Step-heating, using a defocused CO2 laser, of a single muscovite grain (ca. 0.01 mg) from the breccia cement yielded a discordant 40Ar/39Ar age spectrum with a well-defined plateau at 1,459  3 Ma (Fig. 4), representing ~75% of the gas released during the heating experiment. This result is concordant with 40Ar/39Ar plateau ages of 1,456  11 and 1,467  11 Ma for muscovite from metasaprolite and muscovite-pyrophyllite-diaspore hydrothermal veins in the Baraboo Quartzite (Naymark et al., 2001). Results of this investigation provide additional evidence for post-1,630 Ma hydrothermal activity in the Baraboo Range, which was first recognized by Naymark et al. (2001) and attributed to the influence of 1,465 Ma Wolf River magmatism (Medaris, 2001). We suggest that emplacement of the 1.5-1.4 Ga transcontinental belt of A-type granites, of which the Wolf River batholith is part, provided the heat flux necessary for promoting regionally extensive fluid flow and accompanying hydrothermal alteration along permeable channels. It now appears that such hydrothermal activity extended far beyond the Baraboo Range, including the Sioux Quartzite in Minnesota (Naymark, et al., 2001) and the Athabasca Basin in northern Canada (Kotzer, et al., 1992). Further research may reveal that much of the Paleo- and Mesoproterozoic crust of the Lake Superior region was affected by areally extensive, but stratigraphically restricted, hydro- thermal alteration at 1.5-1.4 Ga. REFERENCES Kotzer, T.G., et al. (1992) Can. Jour. Earth Sci., v. 29, p. 1474-1491; Medaris, L.G., Jr. (2001) 47th Inst. Lake Superior Geol., p. 51-52; Naymark, A., et al. (2001) 47th Inst. Lake Superior Geol., p. 66-67. 25 �New geologic map and report of the Duluth Complex and related rocks, Northeastern Minnesota. Miller, J. D. Jr.1, Green2, J. C., Severson3, M. J., Chandler1, V. W., Peterson3, D. M., Hauck3, S. A., and Wahl1 T. E, (1Minnesota Geological Survey, 2642 University Avenue, St. Paul, MN 55114, 2Department of Geological Sciences, University of Minnesota–Duluth, Duluth, MN 55812, 3Natural Resources Research Institute, 5013 Miller Trunk Hwy., Duluth, MN 55811) The Minnesota Geological Survey (MGS), in collaboration with the Natural Resources Research Institute, recently published a 1:200,000-scale map of the Duluth Complex and related rocks of northeastern Minnesota (Miller and others, 2001). A companion report (Miller and others, 2002) describes the geology and mineral potential of the map area; a CD-ROM included in the report contains the digital map and related field data. This ambitious project was funded by a grant from the Minnesota State Legislature on the recommendation of the Minerals Coordinating Committee. The two products constitute the largest and most complete summary of the Duluth Complex ever produced and, undoubtedly, will serve as a benchmark for future exploration and research. Several developments in the last 20 years led to the regional compilation. Gravity and high-resolution aeromagnetic data were used in conjunction with test drilling to map the vast, poorly exposed central part of the Duluth Complex. Detailed mapping in the better exposed part of the complex, as well as core logging along the its base, led to a greater understanding of the petrologic, structural, and stratigraphic relationships in the intrusive sequence. High-resolution radiometric dating aided the unraveling of the intrusive history of the complex, and advances in Geographic Information Systems (GIS) greatly enhanced our ability to produce complicated geologic maps. Finally, and perhaps most importantly, the sharp upturn in PGE exploration in the Duluth Complex created an urgent need for an improved geologic framework. The new geologic map (Miller and others, 2001) has two major components: a 1:200,000-scale geologic map that delineates over 270 rock units, and a more generalized 1:500,000-scale geologic map that classifies the rock units into 72 time–stratigraphic entities. Additional information on the two sheets include (1) a map that shows the relative density of outcrops and drill-out locations; (2) a 1:500,000 gray-scale image of the first vertical derivative of the aeromagnetic data; (3) an index to 64 published and unpublished mapping projects used in the present compilation; (4) a correlation diagram for the 72 generalized time–stratigraphic units; (5) brief descriptions of rock units; and (6) a list of references. Because of the importance of gravity and magnetic data to this compilation, a related map that presents the second vertical derivative of gravity data (color) superimposed on reduced-to-pole, first vertical derivative of magnetic data (black and white) is available at a scale of 1:200,000 (Chandler, 2001). The maps can be purchased from the Minnesota Geological Survey in paper form; they are also available 26 �as PDF files from the Minnesota Geological Survey web site (http://www.geo.umn.edu/mgs). The new 1:200,000-scale geologic map is a greatly improved representation of the Duluth Complex and related rocks; for example, the southern and northwestern parts of the complex are depicted as a composite of several intrusions consisting of troctolitic and gabbroic cumulates. Much of the PGE exploration has focused along the base of the complex in two cumulate bodies, the South Kawishiwi and Partridge River intrusions, but several similar bodies defined by the new map may become exploration targets. The map refines our understanding of the geology of the upper part of the complex, which consists of a partially disrupted cap of anorthositic rocks and several felsic intrusions. Improvements have also been made in mapping the Beaver Bay Complex and other hypabyssal intrusions in the volcanic roof of the complex. One of the bodies, the Sonju Lake intrusion, is being explored for PGE potential, and the new map identifies other mafic cumulate and gabbroic bodies in the roof zone that may be worthy exploration targets. Finally, the North Shore Volcanic Group, which forms the roof (and locally the footwall) of the complex, has for the first time has been subdivided into informal formational entities on a geologic map. The companion report (Miller and others, 2002) provides a general description of the Duluth Complex and related rocks as portrayed on the map and assesses the potential for nonferrous mineral deposits. The following is a summary of each of the 8 chapters and appendix: 1, Formal definition of nomenclature and stratigraphic and rock-type classifications used to describe the Mesoproterozoic (Keweenawan) rocks of northeastern Minnesota; 2, History of geologic mapping and mineral exploration of the complex; 3, Geophysical attributes of the complex and related rocks; discussion of how data aid interpretation of the buried bedrock geology and deeper geologic structure of northeastern Minnesota; 4, Geology and structure of the Archean and Paleoproterozoic rocks that form the footwall of the complex; 5, Volcanic stratigraphy and structure of the comagmatic North Shore Volcanic Group, into which the Duluth Complex and related intrusions were emplaced; 6, Geologic, structural, and stratigraphic relationships of various intrusions of the complex; 7, Geology of the Beaver Bay Complex and related hypabyssal intrusions; 8, Types of mineralization that are known to occur or may occur in the complex and related Keweenawan rocks; identification of new exploration target areas; and Appendix, Description of digital image and data layers included in the CDROM (back pocket) and an explanation of how to access the GIS layers within the ArcView program. The CD-ROM accompanying the report contains ArcView -based digital compilations of the geologic map and related field data, as well as base maps and geophysical images. Attribute tables identify the source and character of data that accompany the geology layers (map units, geologic faults, and contacts) and the related field data layers (outcrops, structure measurements, drill holes, and field samples). An MGS-created 27 �extension, GeMS (Geologic Mapping System) allows the abbreviated attribute data to be decoded. Compilation of all pertinent field data was not possible under the time constraints of the two-year project. Priority was given to data from explorable areas of the Keweenawan in northeastern Minnesota. Updated versions of this digital compilation will be produced as warranted by new mapping and additional archiving of older field data. Chandler, V.W., 2001, Superimposed magnetic on gravity anomaly map of the central Duluth Complex and western part of the Beaver Bay Complex, Lake and St. Louis Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-120, scale 1:200,000. Miller, J.D., Jr., Green J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, 207 p. + compact disc in back pocket. Available by the end of March 2002. Miller, J.D., Jr., Green J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-119, 2 sheets, scales 1:500,000 and 1:200,000. 28 �Crustal models of the Western Superior Province from gravity and Lithoprobe seismic data Nitescu, B., Cruden, A.R., Bailey, R.C., (Department of Geology, University of Toronto, Toronto, ON M5S 3B1, bnitescu@geology.utoronto.ca) The western part of the Superior Province (WS) is characterised by a regional pattern of linear, EW-trending, fault-bounded belts containing distinctive rock types, structures, ages, metamorphic conditions, and geophysical attributes. Detailed geological, geochemical and geochronological studies in the past two decades have led to the formulation of the “accretion hypothesis” in which the WS is interpreted to have assembled progressively from north to south by collisions with various arc, oceanic and continental fragments. To test this hypothesis at depth a Lithoprobe transect was established in the WS, and acquisition of seismic refraction and reflection data, as well as detailed gravity observations were made along several profiles crossing the WS subprovinces. The refraction/wide-angle reflection data indicate a crustal thickness of 4043 km and relatively high average crustal velocities of 6.6-6.7 km/s (White et al. 1997). A high velocity basal crustal layer (7.5-7.7 km/s) was imaged west of Lake Nipigon in the southern WS beneath the Wawa, Quetico and central Wabigoon subprovinces (White et al. 2001). In general, on the seismic reflection sections the lower crust is homogeneously reflective, the middle crust is heterogeneous and strongly reflective, and the upper crust is heterogeneous and often weakly reflective. The Moho is well imaged and shows a flat or slightly dipping geometry below the Berens River Subprovince, a crustal root below the Uchi Subprovince and an upwarp beneath the metasedimentary English River Subprovince (ERS) and its along strike continuation in the Wabigoon indentor. The Bouguer gravity data in the WS show variations between -10 and -90 mgal, gravity highs occurring over the metavolcanic belts, and gravity lows in most of the areas underlain by felsic intrusions. An exception is represented by the metaplutonic Berens River Subprovince, which is characterised by a regional gravity high. The most outstanding gravity feature observed in the WS is a prominent Bouguer gravity high (>20 mgal) coincident with the length of the metasedimentary ERS. The lack of detailed correspondence between this anomaly and the surface distribution of the metasediments suggests a deep linear source (Nitescu and Cruden 2001). A significant positive anomaly also occurs in the Winnipeg River Subprovince east of Kenora, and broadly parallels the surface distribution of the Lount Lake batholith. It is not clear whether this anomaly is related to the ERS gravity anomaly, or some other deep-crustal feature. The aim of the present study is to investigate the crustal structure of the WS based on gravity models constrained by seismic information and to infer the processes responsible for its present-day configuration. We present results obtained from a gravity inversion algorithm (Pilkington and Crossley 1986) employed for the determination of the Moho topography in the WS, and 2.5-D gravity forward models of the WS crust obtained along N-S seismic lines 1 and 2b. The inversion model of the Moho topography reveals the existence of EW-trending crustal roots below the Uchi and Wabigoon subprovinces (Moho depths of 43-44 km) separated by a parallel rise of the Moho to depths of 37-39 km below the ERS. In the 29 �region underlain by the high velocity basal slab the Moho deepens from 41 km below central Wabigoon Subprovince to 47 km below the northern Quetico Subprovince. These results generally agree with the Moho geometry observed along the seismic profiles, although in detail there are differences. For instance, the N-S extent of the Moho upwarp below the ERS is overestimated in the inversion model, which clearly demonstrates that the ERS gravity anomaly cannot be accounted for only by the Moho rise as observed on the reflection sections. The 2.5D gravity forward model along reflection line 2b extends from the Winnipeg River Subprovince to the Berens Subprovince along the Red Lake road. The model indicates that north of the ERS the observed Bouguer gravity field combines the effects of the Moho topography and upper crustal structures (greenstone belts and various intrusive bodies), with no major density variations occurring at the mid- and lower crustal levels in this segment of the crust. Due to poor reflectivity below the ERS, the seismic data does not constrain well the location at depth of the ERS anomaly source, which may correspond to a transparent region observed between 2-4 s TWT, above a zone of weak mid-crustal reflectivity. The lower density of the rocks exposed at surface in the region of the Winnipeg River Subprovince anomaly indicates that its source is also buried. This anomaly could be caused either by a dense body, which may be related to the source of the ERS anomaly, or by uplifted mid-crustal rocks as suggested by the seismic fabric at the southern end of line 2b. In the latter case the modelling results show that the uplifted Winnipeg River Subprovince mid-crustal rocks should extend to the south below the northern Wabigoon Subprovince. The 2.5D forward gravity model of the WS crust along reflection corridor 1 extends for 500 km from the northern Wawa Subprovince to the Sachigo Subprovince. In the model, Moho topography and the interface relief between major crustal layers (as revealed by the refraction data) cause long wavelength gravity anomalies. Short wavelength anomalies are generated by upper crustal metavolcanics and granitoids. The high-density (3.1-3.2 g/cc), high-velocity basal slab imaged in the southern half of the corridor is not reflected in the long wavelength gravity field as a maximum, since the effect of the positive density contrast created in the lower crust is cancelled by the combined effects of the southward deepening of the Moho and the downward depression of the intra-crustal interfaces above the slab. The present study shows that the gravity field in the WS can be successfully modelled in accord with seismic reflection/refraction data by upper crustal density variations and undulations of the Moho and the interfaces between major crustal layers. The structural grain of the subprovincial configuration is not paralleled by density variations in the middle and lower crust, but is reflected in the Moho undulations. The cause of the crustal roots beneath the granite-greenstone subprovinces is not clear. They may be related to greenstone belt loading, but this depends on whether the lower crust was strong enough after the formation of the greenstone belts to prevent the relaxation of the initial Moho depressions through flow. The Moho upwarp below the ERS may be preserved from a ca. 2710-Ma-old extensional episode in the WS which led to the uplift of the Winnipeg River Subprovince and the formation of sedimentary basins on its flanks (Nitescu and Cruden 30 �2001). The deepening trend of the Moho towards the Quetico and Wawa subprovinces is probably related to the load imposed at the lower crustal level by the observed highvelocity/high density basal slab. This slab was previously interpreted as a remnant of a subducted Archean oceanic plate (White et al. 2001), but it could alternatively represent a Proterozoic igneous underplating episode related to Mid-continental rift magmatism. It is likely that the crustal structure of the WS contains not only elements preserved from Kenoran tectonism, but also younger post-orogenic effects. Therefore caution should be used when interpreting the features revealed by the seismic data solely in terms of processes responsible for the accretionary stage of the Superior Province. Nitescu, B., and Cruden, A.R. 2001. Gravity models of the English River Subprovince: implications for its deep structure and tectonic origin. In 2001 Western Superior Transect Seventh Annual Workshop, University of British Columbia (in press). Pilkington, M., and Crossley, D.J. 1986. Determination of crustal interface topography from potential fields. Geophysics, 51, p.1277-1284. White, D., Asudeh, I., Kay, I., Forsyth, D., and Roberts, B. 1997. Preliminary results of the 1996 Lithoprobe Western Superior seismic refraction survey. In 1997 Western Superior Transect Third Annual Workshop, University of British Columbia, Lithoprobe Report #63, p.106. White, D., Musacchio, G., Helmstaedt, H.H., Harrap, R., Thomson, C., Sol, S. 2001. Remnants of Archean subduction in the Western Superior Province: Results from combined Lithoprobe deep seismic studies. In 2001 Western Superior Transect Seventh Annual Workshop, University of British Columbia (in press). 31 �Digital Drill Logs for the Duluth Complex - Lithology and Assays Patelke, R. L., (Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, MN 55811-1442) Since 1988 the Natural Resources Research Institute (NRRI) staff has logged 950 Duluth Complex drill holes of the about 1,500 holes with core available (of the approximately 2,200 holes recorded as penetrating the Complex). This project involves transcribing all of the NRRI logs, plus our interpretation of company logs for drill holes with no core remaining, into a consistent digital format. This downhole “from-to” interval format includes logged rock type, map unit (reconciled from cross-sections and geologic maps where available), and the interval distance up from footwall. This is combined with other data tables, being consolidated from older reporting and projects, which will include: all drill hole specific information (location, company, year drilled, etc.); down hole survey data; all original assays (usually copper-nickel +/sulfur); secondary assays on selected intervals; all available PGE plus gold assays; and eventually all whole rock geochemistry and microprobe data. The format will allow this data to be sorted and filtered geographically, by deposit, by company, by depth or distance from footwall, or other criteria. This data format is intended primarily for use in 3-D mining software and is available on CD-ROM as Excel spreadsheet files, ASCII text files, and as a Gemcom for Windows (Microsoft Access) database. NRRI staff will also format this database to the users needs if practical. The database will also be posted on the NRRI Economic Geology Group website at: http://www.nrri.umn.edu/egg/. The ultimate purpose of this work is to improve the ability to do statistical and spatial analysis of Duluth Complex geology by rapidly combining lithological and assay data for both scientific study and deposit evaluation. This analysis breaks down into two broad categories. Simple 2-D comparisons, such as: comparisons between intrusions or deposit areas; rock type versus grade; ratio of a particular rock type which is mineralized versus part not mineralized; assay grade of a particular unit; or grade as a function of distance from footwall (see Table 1 for examples from Babbitt and Serpentine deposits). More complex 3-D analysis can also be done by using this data to create lithological and grade surfaces, solids, or block models, then intersecting these models; compositing grade by rock type, map unit, level, or distance from a particular point or surface; assessing linear and 3-D variography; contouring by level or distance from a point; and other manipulations of the raw data. Most mining software is very flexible, and the overall data layout will allow NRRI or others to add distance or point data (distance from collar, such as a thin section location) or from-to interval data (such as alteration, grain size, or other assayed elements) as separate tables that can then be compared to the existing data set. The utility in this is the time savings in answering the numerous small questions that arise as well as easing modeling efforts on the Duluth Complex Cu-Ni sulfide and Fe-Ti oxide deposits 32 �Table 1: Consolidated Rock type versus Copper, Nickel, Sulfur, and Copper:Nickel ratio, for Babbit and Serpentine Deposits, St Louis County Minnesota. Surface drill holes only. “Consolidated Rocktypes” are condensed from over 1,000 individual rock types over about 25,000 lithological intervals. “Percent Mineralized” assumes that company sampled all mineralized zones. This is essentially true at Babbitt-Serpentine, but not always true for other deposit areas. Grades are only for the mineralized portions, no unsampled footage (zero value?) was included in the average. "Consolidated Rocktype" Overburden (glacial drift) OUI (Oxide Ultramafic Intrusions) Anorthositic rocks Augite Troctolites Contaminated Rocks Total footage of rock type Percent of rock type in deposit Total mineralized footage of rock type Total unmineralized footage of rock type Percent mineralized of rock type Average copper grade of rock type Average nickel grade Average sulfur grade Average Cu:Ni of rock type of rock type ratio 10,122 2,824 1.8 0.5 447 2,377 0 16 0.31 0.10 1.92 3.29 63,510 63,210 30,792 11.3 11.2 5.5 18,151 28,443 18,248 45,359 34,767 12,544 29 45 59 0.31 0.42 0.44 0.09 0.10 0.11 0.66 1.14 2.25 3.32 4.02 4.06 Gabbroic Rocks Mixed Duluth Complex (not logged) Pegmatitic Rocks Ultramafic Rocks Troctolitic Rocks Dikes, Basaltic Veins, Granitic Massive Sulfides Semi-Massive Sulfides Virginia Formation 3,022 166,344 0.5 29.5 1,348 70,695 1,674 95,649 45 42 0.33 0.39 0.08 0.10 0.85 1.03 4.32 3.74 710 24,473 143,059 698 2,428 415 343 24,760 0.1 4.3 25.4 0.1 0.4 0.1 0.1 4.4 139 7,732 63,929 181 870 413 341 6,279 571 16,741 79,130 517 1,558 2 2 18,481 20 32 45 26 36 100 99 25 0.36 0.36 0.38 0.08 0.29 2.02 1.45 0.38 0.10 0.11 0.10 0.02 0.08 0.57 0.43 0.10 0.71 0.74 0.90 0.35 1.04 12.68 9.69 2.43 3.53 3.17 3.62 2.58 3.51 3.86 4.17 3.04 Bedded Pyrrhotite Unit Virginia Sill MG Sill Sill in Biwabik Iron Formation Biwabik Iron Formation Pokegama Quartzite Giants Range Granites Basalts Massive Chlorite Faults Massive Graphites Hybrids (Hornblendite, etc.) Orthopyroxenites Massive Oxides (not in OIUs) Serpentinites deposit total: deposit averages: 3,834 1,686 9,537 284 6,976 20 420 2,951 12 630 10 505 111 49 76 563,811 0.7 0.3 1.7 0.1 1.2 0.0 0.1 0.5 0.0 0.1 0.0 0.1 0.0 0.0 0.0 100 2,928 311 1,129 4 507 906 1,375 8,408 281 6,469 20 301 2,404 4 443 5 412 20 23 57 330,500 76 18 12 1 7 0 28 19 67 30 60 18 82 55 25 40 0.13 0.09 0.10 0.03 0.11 0 0.14 0.20 0.19 0.36 0.41 0.31 0.44 0.52 0.47 0.04 0.05 0.03 0.01 0.03 0 0.05 0.08 0.06 0.10 0.08 0.12 0.14 0.13 0.11 4.25 0.35 0.49 0.14 0.44 0 0.29 0.85 0.34 1.15 1.31 1.67 2.29 0.56 1.62 2.50 1.58 2.45 2.67 3.05 0 2.34 2.65 2.75 3.66 4.58 2.86 3.79 4.40 4.72 0.39 0.10 1.29 3.66 24.40 34,742 25,445 60,187 5.20 34,314 25,873 60,187 37.10 34,532 25,655 60,187 47.00 34,414 25,773 60,187 119 547 8 187 6 93 91 27 19 223,192 maximums: number of assayed intervals: number of unassayed intervals: total number of intervals: 33 �Mineralogy and Zonation of the Big Whopper Pegmatite, Separation Rapids area, Kenora, Ontario Pedersen, J. C. P.Geo., (Consulting Geologist, Avalon Ventures Ltd.) and Bubar, D. S. (President and CEO, Avalon Ventures Ltd.) The Big Whopper pegmatite is the largest in a field of rare metal pegmatites occurring in the Separation Lake metavolcanic belt, situated approximately 60 km north of Kenora, Ontario. This greenstone belt occurs at the boundary of the English River and Winnipeg River sub-provinces of the Superior structural province. It is interpreted to be the eastern continuation of the Bird River Greenstone belt in Manitoba, which hosts the Cat Lake rare metal pegmatite field and the famous Tanco pegmatite. The Big Whopper pegmatite is 100% owned by Avalon Ventures Ltd. Exploration work to date has defined indicated and inferred resources totalling 11.6 million tonnes grading 1.34% Li2O, 0.30% Rb2O and 0.007% Ta2O5. The deposit is now at the feasibility study stage as a potential producer of lithium-rich feldspars, tantalum and mica. The Big Whopper is a vertically-oriented pegmatite dyke hosted by deformed metavolcanic rocks of lower to middle amphibolite facies metamorphism (amphibolite). A north-directed compressional tectonic event produced flattening and a strong vertically oriented regional schistosity striking WNW at the Big Whopper. This schistosity is folded about a sub-vertical axis. The Big Whopper itself exhibits high-strain features and a tight S-fold geometry, with the thickened central portion of the pegmatite coinciding with the hinge zone, and attenuated limbs extending to the east and west. The pegmatite has been traced over a strikelength of 1.5 km and ranges from 10 to 80 m in width. The dominant economic minerals in the deposit are petalite and columbite-tantalite, the principal ore mineral of tantalum. Petalite is a rare lithium aluminosilicate mineral used in certain specialty glass and ceramics applications, such as thermal shock resistant cookware. Tantalum finds its major use (approximately 60%) in the manufacture of miniature tantalum capacitors, which are currently in high demand for use in popular electronics products such as computers and cell phones. The deposit also contains significant quantities of rubidium potash (Rb-K) feldspar and albite, (both used in the glass and ceramic industries); lepidolite, an important source of rubidium in the chemicals industry, as well as the tin mineral cassiterite. Other potentially valuable industrial minerals include muscovite mica (some with elevated lithium) which may comprise up to 15% of the ore, spodumene averaging 3-5% occurring as SQUI (spodumene-quartz intergrowth) replacement rims on petalite, cassiterite, pale pink spessartine garnet and high-purity quartz. Minor accessory minerals include apatite, zircon, gahnite, monazite, xenotime, rare sulphides, sulphosalts, and thorite. Mineralogical zoning observed in the Big Whopper is characteristic of highly evolved rare metal pegmatites, with well-developed wall zones and internal intermediate zones classified according to their dominant constituent minerals (Figure 1). The mineralogical zones of the Big Whopper identified to date are Wall Zone (predominantly albitite), Megacrystic Feldspar and Quartz-Mica marginal Zones and Petalite (intermediate) Zone. The Petalite Zone is the largest zone defined to date, comprising approximately 80% of the volume of the Big Whopper. Its essential mineralogy consists of petalite, Rb-K-feldspar, albite, quartz and mica. Sub-zones are defined based on subtle textural and mineralogical variations such as the presence of lepidolite mica. 34 �Lepidolite-rich zones tend to occur on the periphery of the deposit and are typically enriched in tantalum. THE BIG W-IOPPER PEGMATITE Detailed Geology lro 700W N I 600W 300W ( EngIMi River .— IiaseIine—* PETALITE ZONE Ato D: A - petalte (white) k-feldspar quartz a lb te B - petalte (pink) k-feldspar albite I C- petalite (blue-grey and pink) albite mica 0 - peta lite lepido lite a lb te (ta ntalum-rich zone >0.01 % Ta 0) I:.: I interdigitated PSLL ZONE (albitite) and FE LDSFR, QUARtZ-MICA ZON ES 100 amphibolite host rocks metre stripped outcrop Fie I: BiWhcwer Geology 35 / �Cu-Ni-PGE Mineralization in the South Kawishiwi Intrusion, Northeastern Minnesota Variation due to Magmatic Processes Peterson, D. M., (Economic Geology Group, Natural Resources Research Institute, University of Minnesota, Duluth email: dpeters1@nrri.umn.edu) INTRODUCTION The Mesoproterozoic South Kawishiwi intrusion (SKI), which is exposed in a 32 x 8-kilometer arcuate band along the northwestern margin of the Duluth Complex, is composed dominantly of troctolitic cumulates. Footwall rocks to the intrusion include, from north to south, the Paleoproterozoic Virginia and Biwabik Iron Formations, and the Late Archean Giants Range batholith. Regional crosscutting features and remnant pillars and xenoliths indicate that the SKI intruded along the boundary between Mesoproterozoic volcanic and Anorthositic Series rocks and the older footwall strata. The SKI abuts the troctolitic Partridge River intrusion on the southwest, is inferred to be semi-conformable to the later Bald Eagle intrusion to the east, and abuts and cuts older Anorthositic Series rocks to the northeast. The basal mineralization within the SKI occurs dominantly in heterogeneous zones of troctolitic, gabbroic, noritic, and ultramafic rocks. This heterogeneity is a function of many factors, including footwall assimilation, dehydration and volatile fluxing from the footwall strata into the overlying magmas, chilling, and repeated magma injection. Miller and Severson (2002) subdivide the intrusion into five major map units from the base upwards: 1) a heterogeneous basal contact zone of sulfide-bearing troctolitic, gabbroic, and noritic rocks; 2) ophitic augite-troctolite; 3) poikilitic leucotroctolite; 4) ophitic troctolite; and 5) homogeneous troctolite. Severson (1994) and Zanko et al. (1994) have further subdivided the marginal zone of the intrusion into 17 different units, with sulfide mineralization dominantly confined to four units: the Basal Augite-Norite (BAN), Basal Heterogeneous (BH), Updip Wedge (UW), and Ultramafic 3 (U3) units. Minor mineralization higher up in the igneous stratigraphy of the intrusion occurs locally in the Ultramafic 1 (U1) and Ultramafic 2 (U2) units. Recent research has identified two dominant styles of mineralization (“Open” and “Confined”) within the intrusion that have distinctive differences in their igneous stratigraphy, metal contents, timing, and mode of origin. Simplified regional geologic and mineralization style maps for the SKI are presented in Figure 1. 36 �Figure 1. Simplified regional geologic (left) and basal mineralization style (right) maps of the South Kawishiwi intrusion. Disseminated basal mineralization within the SKI has traditionally been divided into five distinct deposits: 1) Serpentine, 2) Dunka Pit, 3) Birch Lake, 4) Maturi, and 5) Spruce Road. The disseminated sulfides occur as interstitial grains that average about 1-5% (visual estimation) and range from trace amounts to 10%, with local zones of massive sulfide at the basal contact. Major sulfides are pyrrhotite, chalcopyrite, cubanite, and pentlandite. Pyrrhotite is generally the dominant sulfide, especially closer to the basal contact. Recent work by Wallbridge Mining Company and the author have defined a potential additional deposit area east of the Maturi deposit, informally named the Maturi Extension deposit. Although the style of mineralization in all of the deposits is dominated by disseminated Cu-Ni sulfides, differences occur between the deposits in igneous stratigraphy, Cu-Ni and PGE grade, mineralization thickness, and contained tonnes. Regional analysis of the drill hole assay data for all of the deposits of the South Kawishiwi intrusion has led to the identification of two main styles of mineralization associated with the base of the intrusion (Fig.1). These mineralization types include: 1. “Open” – vertically extensive (> 450 meters) mineralization with low - moderate Cu-Ni grade and low Au+PGE grades. Cu-Ni grades typically increase towards the basal contact although the mineralized zones are typically erratic in their spatial extent and grade and commonly interfinger in a random pattern with zones that are barren of sulfides. Restricted zones of massive sulfide occur locally at the basal contact. This erratic pattern of mineralization, in part, mirrors the lithologic heterogeneity of the basal units. Examples of this open style include the Spruce Road, Serpentine, and Dunka Pit deposits. 2. “Confined” – vertically restricted (< 150 meters) mineralization with moderate - high Cu-Ni grades and moderate to very high (locally) Au+PGE grades. Cu-Ni grades typically are the highest near the top of the mineralized zone (units U3 and BH) and gradually decrease with 37 �depth toward the basal contact, and no zones of massive sulfide at the basal contact have been identified. For example, the upper portion of the mineralized zone within the Maturi deposit consistently exhibits copper values in excess of 1.0% that decrease to ~0.25% at the basal contact. Examples of the confined style include the Maturi, Maturi Extension, and the Birch Lake deposits. The differences between the styles of mineralization reflect the different histories for the rocks hosting the mineralization. It is hypothesized that the early Open-style of mineralization reflects repeated small injections of sulfur-saturated, highly contaminated magma. In contrast, the Confined-style of mineralization reflects much larger batches of more primitive magma that may have incorporated much of its sulfur by assimilation of Open-style mineralized rocks (or remnant magma). Miller, J.D., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geologic Survey, Report of Investigations 58, 200 p. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-119, nominal scale 1:200,000. Peterson, D.M., 2002, Copper-Nickel grade maps for the Spruce Road deposit, South Kawishiwi Intrusion, Duluth Complex: Natural Resources Research Institute, University of Minnesota, Duluth, Report of Investigations, NRRI/RI-2002/03, 99 p. Severson, M.J., 1994, Igneous stratigraphy of the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/TR 93/34, 210 p. (with plates) Zanko, L.M., Severson, M.J., and Ripley, E.M., 1994, Geology and mineralization of the Serpentine copper-nickel deposit, Duluth Complex, Minnesota. Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report, NRRI/GMIN-TR-93-52, 90p. 38 �3-Dimensional View Through a Mineralized System: the South Kawishiwi Intrusion, Duluth Complex Peterson D. M., (Economic Geology Group, Natural Resources Research Institute, University of Minnesota, Duluth email: dpeters1@nrri.umn.edu) A 35x8x2 km three-dimensional AutoCAD™ model of the basal contact surface of the South Kawishiwi intrusion (SKI) has recently been completed. The model is based on the location of ~ 800 drill hole piercing points into footwall strata and geophysical data (Figure 1). Integration of regional geological, geophysical, and geochemical features with the 3-D model has led to new ideas on possible feeder channels for magmas of the northern SKI. The interpreted master magmatic feeder channel of the northern SKI is fed from the central Mid Continent Rift through the Bald Eagle Intrusion gravity high, into a dike-like body of troctolitic rocks (herein termed the BEI Trough) cutting older Anorthositic Series rocks (Fig. 1). The 3-D model has been instrumental in the development of new ideas on the location and origin of the Cu-Ni-PGE mineralization within the SKI. Integration of the magmatic feeder channel concept with lineament analysis and recent PGE assays has led to the development of conceptual models for the formation of the Spruce Road (Peterson, 2002) and Maturi (Peterson, 2001) deposit areas. Early, contaminated magmas that formed the Spruce Road deposit were deflected to the north by a pillar of older Anorthositic Series rocks (Fig. 1) that is located at depth within the northern portion of the SKI (Fig. 2). Moreover, PGE-enriched Cu-Ni mineralization of the Maturi Extension deposit is located beneath the pillar, and a conceptual model for the formation of this deposit area is given in Figure 3. Figure 1. Regional shaded relief 3-D model of the SKI 39 �. Figure 2. Conceptual magma flow model for the Spruce Road area. Figure 5. Conceptual model for the formation of the Cu-Ni deposits of the northern SKI. Sheet-like mineralization of Maturi located to the left of the diagram, and the PGE-enriched Maturi Extension located under the Anorthositic Series block. Peterson, D.M., 2001, Development of a conceptual model of Cu-Ni-PGE mineralization in a portion of the South Kawishiwi Intrusion, Duluth Complex, Minnesota: Laurentian University – Society of Economic Geologists, Second Annual PGE Workshop, Sudbury, Ontario. Peterson, D.M., 2002, Copper-Nickel grade maps for the Spruce Road deposit, South Kawishiwi Intrusion, Duluth Complex: Natural Resources Research Institute, University of Minnesota, Duluth, Report of Investigations, NRRI/RI-2002/03, 99 p. 40 �New Information from the Sibley Group Rogala, B., Fralick, P.W., and Borradaile, G. (Department of Geology, Lakehead University, Thunder Bay, Ontario) The Sibley Group sediments were deposited in a subsiding intracratonic basin (Fralick and Kissin, 1995) between 1537 +10-2 Ma (Davis and Sutcliffe, 1984) and 1339 ± 33 Ma (Franklin et al., 1980). It is presently thought that these sediments were deposited closer to 1537 Ma. The Group was previously divided into three main Formations: Pass Lake, Rossport, and Kama Hill. The Pass Lake Formation consists of the conglomeratic Fork Bay Member and the sheeted sandstones of the Loon Lake Member, representing a braided fluvial environment (Cheadle, 1986). The Rossport Formation is separated into the Channel Island, Middlebrun Bay, and Fire Hill Members. The Channel Island Member is a cyclic dolomite-shale unit interpreted to be playa lake sediments (Cheadle, 1986). The Middlebrun Bay Member is a stromatolitic unit, considered a marker bed for the Sibley Group. It represents a period of relatively stable water and salinity levels. The Fire Hill Member consists of mudcracked red silt with mudchip conglomerates and sand sheet incursions. It signifies a time of tectonic tilting of the basin. The Kama Hill Formation is not subdivided, and is composed of purple shales and siltstones interpreted as mud flat deposits (Cheadle, 1986). The Nipigon Bay Formation is a new addition to the Sibley Group, overlying the Kama Hill Formation. The exposure of this unit is quite limited. It has only been found to crop out on some of the islands in Nipigon Bay, in particular Quarry Island and Vert Island. The Nipigon Bay Formation has also been found in several holes drilled by Falconbridge Limited in 1997. These holes intersected approximately 500 m of well-sorted medium sandstone. The colour of the sandstone can be buff, red, orange, or purple. The sandstone has cross-stratification, which varies from low- to high-angle. The thickness of cross-stratification sets is also variable, ranging from 20 cm to 300 cm, with an average thickness of 100 cm. Figure 1 SiO2 content varies through the Sibley stratigraphy. The upper 500 m is the Nipigon Bay Formation. Si O 2 C o n t e n t 50. 00 55. 00 60. 00 65. 00 70. 00 75. 00 80. 00 85. 00 90. 00 95. 00 1 00. 00 0 1 00 200 300 400 500 600 700 800 900 1 000 Extensive geochemistry was conducted on the sandstones throughout the Sibley stratigraphy, from the Pass Lake Formation to the Nipigon Bay Formation. The geochemistry shows an 41 �interesting series of element peaks within the Pass Lake Formation, indicating sharp, contrasting influxes of source material. However, source-rock modeling has not proven conclusive at this point. The geochemistry indicates an abrupt change at the base of the Nipigon Bay Formation. This Formation is much less variable and shows a different source-rock character than that of the Pass Lake Formation. This is portrayed in Figure 1, using SiO2 content, although the same pattern is repeated in all of the elements that were used for this analysis. The Channel Island Member of the Rossport Formation has also produced some interesting information. Paleomagnetic analysis was conducted on a 50 cm section of core from Noranda’s NI-92-7 drill hole. The inclinations were calculated and graphed to show the variations. The result was the chart in Figure 2 (left), which defines a curve that is believed to represent secular variation. This work will be followed up with isotope geochemistry and scanning electron microscopy for further interpretations. Figure 2 Paleomagnetic inclinations measured at 1 cm intervals along a 50 cm length of core. Inclination (degrees) -50 -40 -30 -20 -10 0 10 20 30 40 0 5 10 15 20 25 30 35 40 45 References Cheadle, B.A. 1986. Alluvial-playa sedimentation in the lower Keweenawan Sibley Group, Thunder Bay District, Ontario. Canadian Journal of Earth Sciences, 23, 527-542. Davis, D.W. and Sutcliffe, R.H. 1984. U-Pb ages from the Nipigon Plate and Northern Lake Superior. Geological Society of America Bulletin, 96, 1572-1579. Fralick, P. and Kissin, S. 1995. Mesoproterozoic basin development in central North America: implications of Sibley Group volcanism and sedimentation at Redstone Point. in: Petrology and metallogeny if volcanic and intrusive rocks of the mid-continent rift system, Proceedings of the International Geological Correlation Program, Project 336. Franklin, J.M., McIlwaine, W.H., Poulsen, K.H. and Wanless, R.K. 1980. Stratigraphy and depositional setting of the Sibley Group, Thunder Bay District, Ontario, Canada. Canadian Journal of Earth Sciences, 17, 633651. 42 �300 my evolution of the Red Lake greenstone belt, western Superior Province, Ontario: A synthesis of current constraints on volcanism, sedimentation, deformation, metamorphism and gold mineralization. Sanborn-Barrie, M.1, Skulski, T.1, Rayner, N.1 and Parker, J.R.2 (1Geological Survey of Canada, Continental Geoscience Division, 601 Booth Street, Ottawa, ON, K1A 0E8; 2Ontario Geological Survey, Willet Green Miller Centre, Sudbury, ON, P3E 6B5) msanborn@NRCan.gc.ca The Red Lake gold camp preserves an extensive record of magmatic and sedimentary activity from 3.0-2.7 Ga, and evidence of multiple episodes of deformation, hydrothermal alteration and metamorphism. The oldest volcanic rocks, which host its major lode gold deposits, are komatiite and tholeiitic basalt of the Balmer assemblage which includes minor 2.99-2.96 Ga intermediate to felsic volcanics and chemical metasedimentary rocks. Some of these basaltic rocks have trace element compositions (Th/Nb) reflecting local interaction with a differentiated crust, however, continental basement has not yet been observed in the belt. A thick sequence of 2.94-2.91 Ga intermediate to felsic calc-alkaline flows and pyroclastic rocks, interbedded with basalt, komatiite and stromatolitic carbonate comprise the overlying Ball assemblage. Ball volcanic rocks show trace element variations (e.g. Th/Nb) in mafic to felsic volcanic rocks that are also consistent with contamination by differentiated crust. Relatively juvenile εNd values of +0.8 to +1 in these felsic rocks suggest that the contaminant was relatively young (~3 Ga) crust. Ball ultramafic rocks have depleted εNd values of +2.2 and may reflect intra-arc extension and upwelling of depleted mantle-derived magma. Quartz-rich fuchsite-bearing clastic rocks of the c. 2.9 Ga Slate Bay assemblage have detrital zircon profiles (GSC SHRIMP, n=140 grains; and Corfu et al., 1998) that indicate derivation from predominantly Ball (with lesser Balmer) sources. This clastic sequence, which extends the length of the belt, overlaps the Ball/Balmer contact, however, north of Red Lake (McInnes Lake), Ball-age volcanic rocks and iron formation are in depositional contact with the Balmer assemblage (Corfu et al. 1998). Ball volcanism was followed by the deposition on Balmer substrate of the 2.894 Ga Bruce Channel assemblage comprising calc-alkaline intermediate pyroclastic rocks overlain by an upward-fining sequence of clastic sediments and chert-magnetite iron formation. Felsic tuff has depleted εNd values of +2 (Henry et al. 1999), reflecting a change between 2.99 and 2.89 Ga toward isotopically depleted magmatism that may reflect crustal growth at a juvenile continental margin (cf. Henry et al. 2000). The overlying Trout Bay assemblage comprises a lower sequence of basalt, 2.85 Ga intermediate pyroclastic and epiclastic rocks, iron formation and an upper sequence of pillowed tholeiitic basalt. The upper basalt sequence has depleted, MORB-like trace element profiles reflected in positive εNd values of +2, similar to model Archean depleted mantle. The Trout Bay and Ball assemblages face toward each other across an interpreted tectonic contact that may reflect telescoping across the continental-oceanic interface. Following a hiatus of some 100 my, the onset of extensive calc-alkaline volcanism is recorded by the Confederation assemblage with c. 2.75-2.74 Ga shallow marine to subaerial calc-alkaline mafic to felsic volcanic rocks of the McNeely sequence, overlain and interstratified by c. 2.74 Ga submarine, mixed calc-alkaline and tholeiitic basalts and FIII-type rhyolite of the Heyson assemblage. McNeely volcanic rocks have an εNd value of +1 to +0.8, whereas Heyson volcanic rocks have depleted εNd values of +2.9. These geochemical and isotopic data are consistent with establishment of a Neoarchean shallow marine arc (McNeely) on the existing Mesoarchean 43 �continental margin sequence, with local intra-arc extension and eruption of high temperature F3 rhyolite and submarine tholeiitic basalt (Heyson). A depositional relationship between the Confederation and Mesoarchean assemblages is supported by a c. 2.75 Ga felsic dyke that cuts the Balmer assemblage, and by Mesoarchean inheritance in c. 2.74 Ga volcanic rocks (Corfu et al., 1998). An angular unconformity is indicated by opposing facing of Balmer and Confederation strata in the Madsen area, consistent with overturning of the Balmer (D0) prior to Confederation volcanism. Local occurrences of coarse clastic rocks, such as ironstone-derived conglomerate on Wolfe Bay, may represent basal conglomerate marking this unconformity. Typically, however, the interface between Neoarchean and Mesoarchean volcanic rocks is covered by a c. 2.735 Ga clastic sequence, the Huston assemblage. Detrital zircon populations from conglomerate samples of the Huston assemblage from the Balmertown and Madsen areas show single, prominent (50-60 grains), c. 2.74 Ga detrital age peaks that indicate derivation from the Confederation assemblage. Undated conglomeratic rocks that underlie the youngest volcanic assemblage in the belt (c. 2.733 Ga Graves) are correlated with the Huston assemblage, as are argillaceous to turbiditic rocks that conformably overlie c. 2.744 Ga McNeely volcanic rocks in central Red Lake. Shallow marine to subaerial, 2.73 Ga calc-alkaline, intermediate to felsic volcanic rocks and synvolcanic plutons of the Graves assemblage reflect Andean-style arc magmatism that culminated in the c. 2.72-2.70 Ga Kenoran orogeny. The youngest supracrustal rock in the belt is a pebble conglomerate, once thought to correlate with the “Austin tuff” ore horizon. This garnetiferous metasedimentary rock yielded a diverse detrital zircon population profile (n=56 grains), with ages that correspond to the Balmer, Ball, Trout Bay and Confederation assemblages, and younger. The maximum depositional age of this sample is 2700 ±6 Ma, the upper intercept of a 5-spot regression for the youngest detrital zircon analysed. This young supracrustal sequence provides a new maximum age for penetrative deformation and amphibolite-facies metamorphism in the Red Lake area. Its relationship to gold mineralization is no longer certain. The Red Lake greenstone belt displays evidence of several episodes of deformation, interpreted to be closely linked with extensive hydrothermal activity and gold mineralization. Nonpenetrative deformation (D0) appears to have involved overturning of the c. 2.99 Ga Balmer assemblage, possibly related to recumbent folding, prior to Neoarchean volcanism. The main stages of penetrative deformation were imposed after ca. 2.74 Ga volcanism. These resulted in northerly trending, south-plunging F1 folds and associated S1/L1 fabrics, superimposed by eastto northeast-trending D2 structures (F2/S2/L2) in western and central Red Lake, and by southeasttrending folds and fabrics (the “mine trend”) in eastern Red Lake. D1 may be bracketed between 2.744 Ga, the age of Confederation volcanic rocks in central Red Lake that contain F1 structures, and c. 2.733 Ga, the age of the Graves volcanic sequence that does not appear to have been affected by D1. It seems probable that deposition of the Huston assemblage took place synchronous with, and as a response to deformation (D1) and that D1 is linked to a change in plate dynamics that took place between Confederation and Graves volcanism. An important constraint on the timing of D2 is provided by the relationship of regionally extensive D2 fabrics to the 2718 ±1 Ma Dome Stock. Supracrustal rocks adjacent to the stock, and occurring as xenoliths within the stock, contain a penetrative S2 fabric. The stock itself contains a weak throughgoing NE-striking foliation coplanar to S2 observed elsewhere. These fabric relationships suggest that the main cleavage-forming stage of D2 predated intrusion of the Dome stock at 2718 Ma, but that shortening was sustained beyond its emplacement. Penetrative strain 44 �appears to have outlasted c. 2700 ±6 Ma, the maximum age of young unconformably overlying conglomeratic rocks that display a penetrative tectonic foliation coplanar to D2 fabrics throughout central Red Lake. Metamorphic mineral assemblages within this sample and surrounding rocks include staurolite-cordierite-garnet-bioite from pelitic compositions and orthoamphibole-garnet from mafic compositions, indicating that amphibolite facies metamorphism also outlasted ca. 2700 ±6 Ma. We interpret tectonometamorphism of the Red Lake belt between c. 2.718 and 2.7 Ga to be the result of orogenic activity related to collision between the c. 3 Ga North Caribou terrane and the c. 3.6-3.0 Ga Winnipeg River terrane, to the north and south of the Red Lake belt, respectively. If the dated “Austin tuff” sample represents an intact relict of the gold ore horizon mined from the Madson area, these new data would suggest gold mineralization of young, unconformably overlying clastic rocks after ca. 2.7 Ga. However, this is in contrast to the timing of gold mineralization in the Balmertown area, which has been shown to predate 2714 ±4 Ma, the age of a post-ore porphyritic dyke from the New Red Lake (formerly A.W. White) mine (Corfu and Andrews, 1987). Corfu, F. and Andrews, A.J., 1987. Geochronological constraints on the timing of magmatism, deformation and gold mineralization in the Red Lake greenstone belt, northwestern Ontario. Can. J. Earth Sci. 24, 13021320. Corfu, F., Davis, D. W., Stone, D. and Moore, M., 1998. Chronostratigraphic constraints on the genesis of Archean greenstone belts, northwestern Superior Province, Ontario, Canada. Precam. Res. 92, 277-295. Henry, P., Stevenson, R.K., Larbi, Y. and Gariepy, C., 2000. Nd isotopic evidence for Early to Late Archean (3.4-2.7 Ga) crustal growth in the western Superior Province (Ontario, Canada). Tectonophysics, v.322, p.135-151. 45 �The Mine Permitting Process in Minnesota - Who, What, Where, and When. Severson, M. J., (Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, MN 55811-1442 e-mail: mseverso@nrri.umn.edu) Interest in the Cu-Ni-PGE deposits of the Duluth Complex has been recently renewed due to advances in hyrometallurgical processes and increased PGE metal prices. Economic evaluations are currently being conducted for several of the deposits, and in some cases, initial steps have also been taken in order to obtain the various permits to mine. In Minnesota, the permitting process for a non-ferrous, metallic mine is complicated; involves dealing with numerous different federal, state, and local agencies; and as yet, has not been carried through to completion for establishment of either an open pit or underground mine. This project outlines each of the necessary permitting steps that are needed to develop a metallic mine, and provides time lines, or length of the various determination processes, for each type of permit that would be required. Also included are lists of agencies, contact persons, phone numbers, and e-mail addresses. All of the data generated in this project will be used to produce a report, and supplemental pamphlet, that fully describes what each permit entails. At the forefront of the mine permitting process in Minnesota is an environmental review process consisting of preparation of an Environmental Impact Statement (EIS), and because an EIS is mandatory, it must be preceded by preparation of a scoping Environmental Assessment Worksheet (EAW). Many of the permits that are mining-related require this process to be completed (Table 1). The scoping EAW is designed to identify potentially significant issues (including possible environmental, sociological, economic, and health risk impacts) that will be associated with a proposed mine and will need further study in the EIS. Thus, the EAW is a “blueprint” for the EIS because it sets limits on what will be discussed further and at what level of detail. It is a standardized six-page questionnaire (31 questions), accompanied by numerous supplemental pages to fully answer the questions, and “typically” takes 90-120 days to complete. The EIS, which typically takes 280 days to complete (legally mandated time), is a thorough study of the issues defined in the EAW and provides information regarding environmental impacts and how they can be avoided/minimized. It considers and sets forth a series of “reasonable” alternatives (including the “no-build” alternative), possible permit conditions, and possible mitigation measures. The EIS does not approve or disapprove of a project; rather, it provides information and alternatives. A determination on the adequacy of the information provided in the EIS must be made before it can be used in determining whether to grant or deny any mine-related permit applications. The Minnesota Department of Natural Resources (DNR) is the Responsible Government Unit (RGU) for both the scoping EAW and EIS. The EAW/EIS process should begin as soon as possible because the entire process could take around 1.5 years before a final adequacy decision can be made. Furthermore, no decisions on most mine-related permits can be made until after the EIS process has been completed (actually 25-90 days after the adequacy decision). It is extremely important to make early contact with all of the agencies that are responsible for granting permits for several reasons that include: 1. to insure that all issues relating to the various permits are included for study in the EIS; 2. to begin preparation on some of the permits so they can be issued as quickly as possible after the EIS is complete (some permits can take up to 1.5 years (or more) before decisions to grant or deny can be made); 3. to avoid duplication of efforts 46 �for the various permits and establish cooperation between the agencies; and 4. to get the public informed and involved to hopefully minimize later delays in the permitting process. Table 1: List of potential permits required to establish a non-ferrous, metallic mine in Minnesota. Permit Agency Approximate Application determination time* Permit to Mine** DNR Minerals 7-8 months WCA-Wetlands Replacement Plan** DNR Minerals 4-6 months Water Appropriation Permit** DNR Waters < 2 months Public Waters Work Permit** DNR Waters < 2 months Dam Safety Permit** DNR Waters < 2 months Part 70 - Air Quality Permit** MPCA 1.5 years NPDES/SDS Stormwater Permits (two - for Construction and Industrial Activity) NPDES/SDS Wastewater Permits** ( two - for industrial process wastewater and sewage) Hazardous Waste Permit** MPCA < 2 months? MPCA 1.5 years MPCA 1-3 years Section 404 Permit (discharges to wetlands)** ACOE Section 10 Permit ? (Affects to “navigable” waters of the US) Section 401 Certification (wetland certification needed before 404 permit can be issued) Water Treatment Plant Permits ACOE 120 days (unless Fed. EIS required?) ? MPCA 60-120 days ? MPCA ? Local Permits - zoning, construction, bonding, etc varies “short” period * = Minimum time to complete (assuming optimal conditions) after all data that are required to be submitted for the permit are complete ** = Permit that by itself may require a mandatory EAW and/or EIS. Abbreviations: DNR = Department of Natural Resources; MPCA = Minnesota Pollution Control Agency; ACOE = Army Corps of Engineers; NPDES = National Pollutant Discharge Elimination System; SDS = State Disposal System; WCA = Wetland Conservation Act. 47 �Description of a pegmatite occurrence on the eastern margin of the Mellen Granite, State Highway 13, Ashland County, Wisconsin. Sikkila, K., (Wisconsin Department of Transportation, District 8, Superior, Wisconsin) A highway construction project supervised by the Wisconsin Department of Transportation, District 8 office took place during the 2001 field season on State Highway 13 immediately north of the City of Mellen in central Ashland County. Construction activity began at the intersection with State Highway 169, extending northward for 3480 meters. The project consisted of a realignment within the existing right-of-way and necessitated the blasting, excavation and removal of approximately 33000 cubic meters of rock. Bedrock geology in the project area consists of intrusive rocks of the Lower Keweenawan (approximately 1.1 Ga) Mellen Complex. The highway right-of-way is positioned over the contact between two discrete rock bodies within the Complex: the gabbroic rocks of the Potato River Intrusion (to the east); and the younger Mellen Granite (to the west). A previously existing roadcut along the right-of-way indicated the presence of an intrusive breccia at this intrusive contact. The primary lithology of the breccia matrix in this previously existing roadcut appears to be granodiorite, with xenoliths of gabbro, lesser amounts of mafic volcanic rocks, and minor amounts of metasediments of probable Lower Proterozoic age. New exposures resulting from construction activity indicate an additional complexity to this brecciated contact margin, culminating in the presence of a small pegmatite body approximately 520 meters to the north of the original intrusive breccia exposure. The petrography of the pegmatite itself appears to consist primarily of coarse-grained potassium feldspar (perthite), quartz and biotite, with accessory epidote, apatite and other currently unidentified minerals. An irregular central “core” contains abundant quantities of myrmekite, various unidentified phosphates(?), and an unidentified pale lilac-colored mineral. Analyses will be performed on these minerals in the near future. The intrusive breccia in the areas proximal to the pegmatite displays moderate to strong potassium metasomatism, with plagioclase altered to potassium feldspar/perthite/myrmekite and the generation of epidote as a residual by-product, the alteration of mafic minerals in gabbroic xenoliths to biotite and/or phlogopite and hornblende, etc. Anastomosing swarms of small irregular felsite (aplite?) dikes also occur in this zone. Evidence for later-stage cataclasis and a retrograde chlorite event is represented by occurrences of chloritized mafic fault gouge and the presence of books of chlorite replacing biotite phenocrysts in areas proximal to these gouge zones. Irregular localized bodies of intrusive rocks of intermediate composition (syenite, monzonite, monzonite porphyry, monzogabbro, quartz diorite, etc.) also occur along the perimeter of this breccia/pegmatite zone. Widely disseminated sulfides (pyrite/chalcopyrite) are also noted. 48 �Inferences from the Hattenberger deep drill hole, Carlton County, Minnesota, pertinent to regional stratigraphy and mineral potential of the western segment of the Penokean Orogen Southwick, D.L., (Minnesota Geological Survey (retired); davidsouthwick@earthlink.net) The Hattenberger core hole, drilled vertically to a depth of 7,440 feet (2,268,3 meters) near Kettle River in Carlton County, Minnesota, penetrates a sequence of metamorphosed and deformed sedimentary and volcanic strata of Paleoproterozoic age within the fold and thrust belt of the Penokean orogen. Continuous core recovered from the depth interval 803 to 7,440 feet (244.8 to 2,268.3 meters) provides a rare depth-dimensional view of the lithostratigraphy and structures in rocks that are not well exposed at the surface. The cored interval below depth 4,780 feet (1,457.3 meters) consists predominantly of quartz-rich mica schist, calcareous mica schist, and dolomitic marble that are interstratified on coarse to fine scales. Relatively minor amounts of mafic volcanic rock (both flows and fragmental types) and recrystallized cherty iron-formation are interstratified with the dominant quartzose, micaceous, and calcareous metasedimentary rocks in the upper part of this lowermost major division of the core. In the upper major division of the core, above depth 4,780 feet, the predominant rock types are (1) dark-colored, thinly bedded graphitic mica schist and phyllite; (2) non-magnetic, graphitic, silicate-carbonate-sulfide iron-formation, parts of which are characterized by prominent porphyroblasts of Fe-rich garnet and amphibole; and (3) an assortment of mafic volcanic rock types that in general form thin stratigraphic units interbedded with the metasedimentary components. Sills of metadiabase chemically identical to the volcanic rocks occur throughout the whole core, in both the lower sequence of quartzose and calcareous schists and the upper sequence of graphitic and iron-rich rocks. Altogether, the sills and allied metavolcanic rocks amount to about half of the total footage drilled, with the greater part being sills. The lithologic and structural attributes of the lower sequence of rocks in the Hattenberger core match well with those of the Denham Formation at its type locality (Morey, 1978; Boerboom and Jirsa, 2001). Likewise, the attributes of the upper sequence match well with those of the Glen Township Formation, the type section for which is contained in several cores drilled about 23 miles (37 kilometers) along regional strike to the west of the Hattenberger locality (Morey, 1978). The assumed Denham–Glen Township contact in the Hattenberger core is occupied by a thick sill of metadiabase, the margins of which are strongly sheared. This leaves the nature of the formational contact open to question. It could be a conformable contact invaded by the sill, with the shearing at sill margins due to mechanical contrasts during complex, multi-stage regional deformation, or it could be a tectonic contact invaded by the sill that was later reactivated. 49 �Highly deformed, variably metamorphosed stratotectonic complexes of calcareous metasedimentary rocks, carbonate-silicate-sulfide iron-formation, and mafic metavolcanic rocks are the regional hosts for economic mineral deposits in several important mining camps, including the Homestake district in South Dakota (Caddey and others, 1991) and the Broken Hill and Mount Isa districts in Australia (Ashley and others, 1998; Painter and others, 1999). The mechanical contrasts among the various rock types in these complexes, together with the geochemical reactivity of the calcareous and iron-rich strata, provide fertile ground for mineral deposition from spatially associated hydrothermal systems. Clearly, the geological attributes revealed in the Hattenberger core are not of themselves meaningful indicators of mineral-deposit probability. However, those attributes taken in conjunction with mineralogical evidence of hydrothermal activity in the Cuyuna mining district (McSwiggen and others, 1995; Melcher and others, 1996) and the presence of barren massive sulfide deposits in the Glen Township area (Han, 1968) suggest that broad areas of east-central Minnesota are intrinsically prospective for a variety of ore-deposit types. Ashley, P.M., Lottermoser, B.G., and Westaway, J.M., 1998, Iron-formations and epigenetic ironstones in the Palaeoproterozoic Willyama Supergroup, Olary domain, South Australia: Mineralogy and Petrology, v. 64, p. 187-218. Boerboom, T.J., and Jirsa, M.A., 2001, Stratigraphy of the Paleoproterozoic Denham Formation—a continental margin assemblage of basalt, arkose, and dolomite [abs.]: Institute on Lake Superior Geology, 47th Annual Meeting, Proceedings, v. 47, Program and Abstracts, pt. 1, p. 6-7. Caddey, S.W., Bachman, R.L., Campbell, T.J., Reid, R.R., and Otto, R.P., 1991, The Homestake gold mine, an Early Proterozoic iron-formation-hosted gold deposit, Lawrence County, South Dakota: U.S. Geological Survey Bulletin 1857-J, p. J1-J67. Han, T.M., 1968, Ore mineral relations in the Cuyuna sulfide deposit, Minnesota: Mineralium Deposita, v. 3, no. 2, p. 109-134. McSwiggen, P.L., Morey, G.B., and Cleland, J.M., 1995, Iron-formation protolith and genesis, Cuyuna range, Minnesota: Minnesota Geological Survey Report of Investigations 45, 54 p. Melcher, F., Morey, G.B., McSwiggen, P.L., Cleland, J.M., and Brink, S.E., 1996, Hydrothermal systems in manganese-rich iron-formation of the Cuyuna North range, Minnesota: Geochemical and mineralogical study of the Gloria drill core: Minnesota Geological Survey Report of Investigations 46, 59 p. Morey, G.B., 1978, Lower and Middle Precambrian stratigraphic nomenclature for east-central Minnesota: Minnesota Geological Survey Report of Investigations 21, 52 p. Painter, M.G.M., Golding, S.D., Hannan, K.W., and Neudert, M.K., 1999, Sedimentologic, petrographic, and sulfur isotope constraints on fine-grained pyrite formation at Mount Isa Mine and environs, northwest Queensland, Australia: Economic Geology, v. 94, no. 6, p. 883-912. 50 �Petrographic Study of the Ottertail Pluton, Superior Province, Northwestern Ontario Sturm, C. L., Czeck, D. M and Fein, E., (Oberlin College Geology Department, Oberlin Ohio 44074. Claire.sturm@oberlin.edu) INTRODUCTION This study concentrates on the Ottertail Pluton at the Wabigoon-Quetico subprovince boundary in the western Superior Province near Mine Centre, Ontario. The Ottertail is one of the Algoman plutons dated at 2686 Ma (Davis et. al, 1989), which have been used to constrain the termination of deformation at the subprovince boundary (e. g. Davis et. al, 1989; Poulsen, 2000). Our goal is to study the mineralogy and microstructures of the pluton in thin section. At the macroscopic level the pluton seems largely undeformed because of the general lack of macroscopic deformation fabrics. This has led many researchers to interpret the Ottertail Pluton to be posttectonic. However, it has been shown that determining the relationship between pluton emplacement and deformation requires more detailed analysis (Paterson and Tobisch 1988). We are using petrographic analysis combined with a magnetic fabric study to better determine the relationship between the pluton emplacement and deformation at the Wabigoon-Quetico boundary. ANALYSIS Using optical petrography and a vibrating sample magnetometer (VSM), we determined the mineralogy and magnetic mineralogy of the pluton. In general, the pluton ranges from quartz monzonite to granite to granodiorite. Mineralogy consists of: quartz, plagioclase, microcline, microcline perthite, hornblende, myrmekite, and some zircon. There are also minor amounts of sericite, clinozoisite, magnetite (and other opaque minerals) in some portions of the pluton. The grain sizes vary. Many quartz crystals have undulose extinction and some have subgrains, which suggest slight deformation by dislocation processes. Minor metamorphism is indicated by sericite in plagioclase crystals, and microcline perthite. At the macroscopic level, the pluton seems largely undeformed, but our study of anisotropy of magnetic susceptibility (AMS) and microstructures show that there is some evidence for deformation. Davis, D. W., Poulsen, K. H., Kamo, S. L., 1989. New insights into Archean crustal development from geochronology in the Rainy Lake area, Superior Province, Canada. Journal of Geology 97, 379-398. Paterson, S. R., Tobisch, O. T., 1988. Using pluton ages to date regional deformations: problems with commonly used criteria. Geology 16, 1108-1111. Poulsen, K. H., 2000. Archean metallogeny of the Mine Centre - Fort Frances area. Ontario Geological Survey Report 266, 121. 51 �Internal structures within crustal structural slabs, Quetico-Wawa subprovince junction, Quetico Provincial Park, Ontario Woodard, H.H. (Department of Geology, Beloit College, Beloit, Wisconsin 53511) The rocks of the Wawa subprovince, adjacent to the Quetico-Wawa subprovince junction, in the region between Agnes Lake and McKenzie Lake in Quetico Provincial Park, Ontario can be grouped into two discrete structural slabs. Detailed mapping of the ductile borders of these slabs indicates that the slabs are approximately 2 km thick and probably form the limbs of major recumbent folds. The base of each slab is defined by a ductile structural discontinuity, possibly representing the lower limb of a nappe or thrust surface. The northeastern McKenzie Lake slab rests upon the underlying Agnes Lake slab. The rocks which make up these structural slabs are amphibolite-grade, migmatitic, mica-quartz-feldspar-garnet schists, amphibolite, and tonalitegranodiorite. All of these rocks contain major deformational structures which were formed before, during and after the emplacement of the enclosing crustal slabs. The earliest recognizable structures are found only in the tonalite-granodiorites. These structures are layers of centimeter-scale thickness, and most are only recognized on outcrop surfaces where the layers are accentuated by superficial weathering. These layers are probably caused by centimeter-scale shearing within the tonalites. Both field measurements and aerial photo interpretation demonstrate that these centermeter-scale layers occur in “structural packages” which often have thicknesses on a scale of hundreds of meters. Hundreds of strike and dip measurements made on these layers, in conjunction with interpretation of large-scale, detailed, aerial photographs, demonstrate that the layers are deformed into a complex series of folds with typical wave lengths ranging from one to two kilometers. Many of the folds appear overturned, but some are upright, and most plunge northeastward, with occasional reversal of plunge. Interlayered and infolded with the tonalites are schists and amphibolites, and primary structures in these meta-sedimentary and metavolcanic rocks sometimes allow determination of stratigraphic top. All of the internal layers and folds are cut and smeared by the last recognizable ductile deformation along the subprovince junction. The internally folded rocks are later broken into individual kilometer-scale structural blocks which are bounded by steeply dipping faults. These faults are rarely exposed in outcrop, but some cut and offset the Quetico-Wawa subprovince ductile junction. Thus, a brittle phase of deformation followed the last ductile deformation at the subprovince junction and the emplacement of the crustal slabs. These faults, which cut the internal folds of the McKenzie Lake crustal slab, appear unrelated to the Burntside Lake brittle fault zone. This fault has been traced in outcrop about 100 km northeastward from its type locality on Burntside Lake, Minnesota and is mapped as terminating against the Quetico-Wawa junction in the vicinity of McKenzie Lake. The internal faults typically lack the intense centimeter-scale brecciation and the intense hydrothermal oxidation so characteristic of the Burntside Lake fault zone. Further, the strike orientation of seventy-five mapped internal faults appear geometrically related to the ductile folding of the Quetico-Wawa junction (and crustal slab development) rather than to the northeast-striking Burntside Lake fault. 52 �Thus, the internal structures within the McKenzie Lake crustal slab initially appear to record intense centimeter-scale shearing within all the rocks. This shearing is followed by ductile folding and complete recrystallization and then by recumbent overfolding of the sheared and folded sequence to produce the crustal slab. Following slab emplacement, the rocks are broken into kilometer-scale fault blocks, the strikes of which suggest a relationship to a later stage of compression across the subprovince junction. The latest ductile deformation along the QueticoWawa subprovince junction cuts the crustal slabs, and all their rocks and internal structures. The Agnes Lake structural slab, which underlies the McKenzie Lake slab, contains a similar suite of rocks and has similar internal structures to the above-described McKenzie Lake slab. However, within the Agnes Lake slab the internal structures are dominated by a series of northeast-striking, gently northwest-dipping, ductile shear zones chiefly at the contacts with tonalite and schistose rock units. These ductile shears are themselves recumbently folded and overturned toward the southeast. Although fieldwork continues on the Agnes Lake slab, the current interpretation of these internal ductile shears is that they represent folded sections of an earlier Quetico-Wawa subprovince junction, which have been sheared and folded into the Agnes Lake slab. 53 �Impact of fire on the forest floor and mineral soils, Snowbank Lake, Minnesota Woodruff, L.G., (U.S. Geological Survey, Mounds View, MN, 55112 (woodruff@usgs.gov)), Cannon, W.F., and Dicken, C., (U.S. Geological Survey, Reston, VA 20192) We are investigating the geochemical effects of fire on the forest floor and mineral soils in an area of the Superior National Forest near Snowbank Lake that was burned in a fuel reduction prescribed fire by the USDA Forest Service on October 11, 2000. The forest around Snowbank Lake, in northern Minnesota, is a mixed stand of older-aged balsam fir, spruce, aspen, red pine and jack pine. Many trees, particularly mature balsam and spruce, were blown down by a severe storm in July 1999. The scheduled Snowbank Lake fire gave us the opportunity to establish 10 study sites in the proposed burn area. At each site we described and collected soils prior to the fire, immediately after the fire, and seven months after the fire to quantify physical and chemical changes in the forest floor as a result of the burning. Initial sampling was in July 2000. At each site, forest floor material (living moss, forest litter, and/or humic layers) and mineral soil horizons (A-, E-, B- and C-horizons as available) were collected for analysis. The thickness of each layer was measured, and representative samples were collected for density calculations. The fire burned much of the fine fuel at the surface and affected 8 of the 10 sample sites. On October 12, prior to any post-fire rainfall, we resampled the sites. Burn severity (a qualitative assessment of the heat pulse directed towards the ground during a fire) was estimated and where possible samples comparable to pre-burn samples were collected. The sites were resampled seven months later in May 2001, following a winter of moderate snow cover. The fuel load at the surface and the moisture content of the forest surface controlled the major impact of the fire on the forest floor. Fire severity was high (100% of organic forest floor material consumed, mineral soil exposed) at 4 sites, moderate at 1 site (some forest floor material burned, mineral soil not exposed), and light at 3 sites (surface material charred with minimal loss at the forest floor). Two sites were untouched by fire. One of the elements of interest in this study was mercury. Gaseous elemental mercury in the atmosphere is transferred into a forest environment by wet and dry deposition onto the forest floor, creating a repository of mercury that is bound to organic carbon compounds. Recent studies show that smoke plumes from forest fires carry substantial mercury, but the origin of that mercury was in question (Friedli et al., 2001). Our preburn sampling shows that much of the mercury at the forest surface is bound to organic material on the forest floor and in organic mineral soils. Severe fire can vaporize much of this mercury as the organic compounds are combusted. Based on pre-burn analyses and measured thickness and density of organic material and assuming that all mercury in the organic layer is emitted during high severity burns when the forest floor is burned down to the mineral soil, average mercury emissions from the Snowbank Lake burn were about 2 mg/m2 of burned surface. This estimate only takes into account mercury bound to organic material on the forest floor and does not include burned foliage or woody fuels. Mercury bound to organic material in A-horizon mineral soils was not lost, even at sites of high burn severity, indicating that the fire’s thermal pulse did not heat soils above the 357oC vaporization temperature of mercury. Because the footprint of an historic fire was apparent in our study of soil geochemistry in Isle Royale National Park (Woodruff and Cannon, 2001) we anticipated that exposure of mineral 54 �soils by fire would lead to loss of both mercury and carbon over time. Loss of organic matter should be accelerated by solar heating of previously shaded soils and by lower input rates of organic matter because of loss of forest canopy. Interestingly, analyses of A-horizon soils collected at Snowbank Lake in May 2001 show no carbon loss and marked increases in mercury rather than decreases (Figure 1A). The reasons for this increase are unknown, although we speculate that it may be the result of leaching of mercury from the overlying ash layer or possibly from enhanced sorption of mercury by exposed organic material in mineral soils. The fact that carbon does not increase concomitant with mercury suggests that physical incorporation of ash and adsorbed mercury is not the cause of higher mercury concentrations. Lead, which much like mercury has an atmospheric source and is enriched in the forest floor and organic mineral soils, is enriched in the uppermost mineral soil samples collected in October after the fire compared to preburn values (Figure 1B), but comparable soil samples collected the following May show a slight decrease in lead, although most values are still higher than July 2000 levels. The increase of lead in soils immediately following the fire may be the result of the concentration of residual lead remaining after the burning of forest vegetation. Lead sorbed onto organic material may not volatilize in the fire, but could collect on surface soils. Additional sampling is planned for May 2002 to monitor the evolving geochemistry of the forest surface. This research shows that there are both immediate and long-term impact from fire, including loss of volatile elements and profound element movement at the forest surface. Figure 1. Mercury and lead versus carbon in A-horizon soil samples from the area of the Snowbank Lake prescribed fire. Friedli, H., L. Radke and J. Lu, 2001, Mercury in smoke from biomass fires: Geophysical Research Letters, v. 28, p. 3223-3226. Woodruff, L.G. and Cannon, W.F., 2001, The effect of fire on mercury and carbon in forest soils: results from northern Michigan and Minnesota: Geological Society of America Abstracts with Programs, v.33, p. A186. 55 � https://digitalcollections.lakeheadu.ca/files/original/9e1527ceb6b24c1bb2230ab8c4120149.pdf 125fa414cc8aa8747725d63da098aa94 PDF Text Text INSTITUTE ON LAKE SUPERIOR GEOLOGY 48th Annual Meeting Proceedings Volume 48 Part 2 - Field Trip Guidebook Kenora, Ontario – May 12-16, 2002 �INSTITUTE ON LAKE SUPERIOR GEOLOGY 48th Annual Meeting May 12-16, 2002 Kenora, Ontario Hosted by: Peter Hinz and Richard C. Beard Co-Chairs Sponsored by the Ontario Geological Survey Proceedings Volume 48 Part 2 - Field Trip Guidebook (Compiled by Blackburn Geological Services) �48th Annual Meeting Institute on Lake Superior Geology Volume 48 contains the following parts: Part 1: Program and Abstracts Part 2: Field Trip Guidebook 1 - Tanco Rare-Element Pegmatite, Southeastern Manitoba 2 - Quaternary Geology of Southeastern Manitoba 3 - Structure and Sedimentology of the Seine Conglomerate, Mine Centre Area, Ontario 4 - Industrial Minerals and Paleozoic Geology of Southeastern Manitoba 5 - Separation Rapids Rare-Element Pegmatite Field, Ontario 6 - Geology of the Red Lake Camp, Ontario Reference to the material in this volume should follow the example below: Lichtblau, A. and Storey, C.C. 2002. Geology of the Red Lake Camp, Ontario: Institute on Lake Superior Geology Proceedings, 48th Annual Meeting, Kenora, Ontario, 2002, v. 48, Part 2, p. 121-138. Volume 48 is published by the Institute on Lake Superior Geology and distributed by the Institute Secretary-Treasurer: Mark Jirsa Minnesota Geological Survey 2642 University Avenue St. Paul, MN USA 55114-1057 (612) 627-4780 email: jirsa001@tc.umn.edu ILSG webstite http://www.ilsgeology.org/ ISSN 1042-9964 Cover Illustration: Geologists examining a dump at the Gold Hill mine, Kirkup Township, 13 km southeast of Kenora, in 1914. Between 1886 and 1893 this mine produced 1090 ounces of gold from 220 tons milled. Four shallow shafts were sunk to a combined depth of 258 feet: the headframe for one of them is seen in the picture. The winning of gold from narrow discontinuous quartz veins was typical of the numerous small-scale mines of the Kenora goldfields in the latter part of the 19th and the early 20th century. ii �CONTENTS Proceedings Volume 48 Part 2 – Field Trips Trip1: The Tanco Rare-Element Pegmatite, Southeastern Manitoba ....................................1 Leaders: Staff, Tantalum Mining Corporation of Canada, Ltd. Trip 2: Quaternary Geology of Southeastern Manitoba..........................................................23 Stop 1: Striated outcrop, West Hawk Lake ................................................................26 Stop 2: West Hawk Lake, Till Section .......................................................................27 Stop 3: West Hawk Lake, Meteorite Impact Structure ..............................................28 Stop 4: Sapping Channels (Upper Cambell Beach) ...................................................30 Stop 5: Upper Campbell Beach of Lake Agassiz .......................................................31 Stop 6: Interglacial Site at Grunthal...........................................................................32 Leaders: E. Nielsen, Manitoba Geological Survey G. Matile, Manitoba Geological Survey Trip 3: Structure and Sedimentology of the Seine Conglomerate, Mine Centre Area, Ontario ........................................................................................................................37 Stop 1: Basal facies, low deformation, Shoal Lake road............................................60 Stop 2: 2D view, sandy lenses, dextral shear, Forest Tour road ................................61 Stop 3: 3D view, moderate deformation, Horsecollar Junction, Hwy 11 ..................61 Stop 4: Ultra deformed conglomerate, Hwy 11 .........................................................61 Stop 5: Small fold, Seine River bridge, Hwy 11 ........................................................62 Leaders: D. Czeck, Oberlin College P. Fralick, Lakehead University Trip 4: Industrial Minerals and Paleozoic Geology of Southeastern Manitoba ....................69 Stop 1: Sungro horticultural shagnum peat bog and plant .........................................90 Stop 2: Cold Spring Granite dimension stone quarry and plant.................................90 Stop 3: Gillis Tyndall Stone quarry and plant............................................................91 Leaders: J. Bamburak, Manitoba Geological Survey R. Bezys, Manitoba Geological Survey iii �Trip 5: Separation Rapids Rare-Element Pegmatite Field, Ontario ......................................95 Stop 1: Big Mack pegmatite.....................................................................................107 Stop 2: Separation Rapids pluton .............................................................................109 Stop 3: Big Whopper pegmatite ...............................................................................110 Stop 4: Marko's pegmatite........................................................................................115 Stop 5: James' pegmatite ..........................................................................................116 Leaders: C. Blackburn, Blackburn Geological Services D. Bubar, C. Pedersen, K. Rees, Avalon Ventures Ltd. C. Galeschuk, Tantalum Mining Corporation of Canada, Ltd. A. Mowatt, Emerald Fields Resource Corp. T. Pryslak, A..P. Pryslak Geological Services Trip 6: Geology of the Red Lake Camp, Ontario ...................................................................121 Stop 1: Meso-neoarchean contact, Woodland Cemetery road .................................130 Stop 2: Calcite carbonatized pillowed flows, Sandy Bay road ................................130 Stop 3: Cofederation/Balmer assemblages contact, Suffel Lake road .....................130 Stop 4: Madsen deposit, power line outcrops ..........................................................131 Stop 5: Buffalo deposit.............................................................................................133 Stop 6: Howey mine .................................................................................................134 Stop 7: Howey Bay-Flat Lake deformation zone.....................................................134 Stop 8: Redcon carbonate zone, Nungesser road .....................................................135 Leaders: A. Lichtblau, Ontario Geological Survey C. Storey, Ontario Geological Survey Staff, Goldcorp Inc. - Red Lake Mine Staff, Placer Dome North America - Campbell Mine iv �Field Trip 1 The Tanco Rare-Element Pegmatite, Southeastern Manitoba Peter Vanstone Chief Geologist Steven Young Mill Superintendent Roland Simard Mine Superintendent Carey Galeschuk Project Geologist Alistair Gibb Chemical Plant Superintendent Tantalum Mining Corporation of Canada Limited P.O. Box 2000 Lac du Bonnet, Manitoba R0E 1A0 "Giraffe" underground at the Tanco mine. �INTRODUCTION Pegmatites throughout the world range in age from late Archean (2,500-2,800 million years) to Miocene (5-23 million years) (Cerny 1989a). Within Canada, there are noticeable concentrations of rare-element pegmatites associated with the following orogenic events: 1. the Kenoran Orogeny (2,750-2,550 million years) in the Archean Superior Province; 2. the Hudsonian Orogeny (1,800 – 1,600 million years) in the Churchill Province; and, 3. the Grenville Orogeny (1,200 – 900 million years) in the Grenville Province. The pegmatite being commercially exploited by Tantalum Mining Corporation of Canada Limited (Tanco) is an example of an extremely fractionated, pollucite-bearing pegmatite which was emplaced during the Kenoran Orogeny. The Tanco pegmatite is located at Bernic Lake in the Canadian Shield of southeastern Manitoba, approximately 180 kilometres by paved and all-weather gravel road northeast of Winnipeg (Figure 1). The nearest communities, Lac du Bonnet and Pinawa, are located approximately 60 kilometres and 75 kilometres, respectively, from the minesite. Tanco has been a significant producer of tantalum concentrates, ceramic-grade spodumene concentrates, pollucite and other materials since the late 1960’s. More recently cesium N (icY ii Winnipeg IdIlUVI du Bois Lac PiIqIM if I Seauj ic o 20 ——— 10 Figure 1. Location of Tanco 2 20 30 40 40 30 50 �chemicals have been produced at the minesite. During this time, the pegmatite has been the subject of numerous studies because of its very limited low temperature alteration, lack of postemplacement, structural deformation and its absence of weathering effects. The geographic location of the pegmatite and the willingness of Tanco to allow access to its extensive diamond drill core library, as well as, the underground workings have also been contributing factors. For more information on the Tanco pegmatite and pegmatites in general, the reader is referred to the Canadian Mineralogist issues by Berry (1972), Cerny (1982), Martin and Cerny (1992), and Anderson et al (1998). Also, Brown and Ewing (1986) edited an issue of the American Mineralogist focused on pegmatites and granitic rocks, and Moller et al (1989) edited the proceedings of a workshop on the lanthanides, tantalum and niobium. Brisbin (1986) discusses pegmatite intrusion mechanisms and Ercit (1986) discusses tantalum mineralogy. An overview of the Tanco pegmatite and the mining/milling operations was published by Crouse, et al (1979) and Thomas (1984) completed a fluid inclusion study of the Tanco pegmatite. Tanco is 100% owned by Cabot Corporation of Boston, Massachusetts and is operated by the Cabot Specialty Fluids division headquartered in Houston, Texas. TANCO HISTORY In 1928, Jack Nutt Mines staked and explored the Bernic Lake area pegmatites for tin. During the following two years, shaft sinking began and a small tin concentrator was established on what is now the Tanco minesite. Feed for this mill came from the nearby exposed pegmatites. At this same time, a four hole drill program was underway to explore the pegmatites at depth. It was during this program that the Tanco pegmatite was intersected in Hole #3. Work on the property continued through 1930, but poor economic conditions forced the company to abandon the property. The claims subsequently reverted back to the Crown. In 1955, Montgary Petroleum Corporation Limited acquired the property and completed an extensive surface drill program. Over the next couple of years, a power transmission line from Pointe du Bois and a mine access road were constructed. Also, the sinking of a threecompartment shaft began and some surface facilities were built. In 1957, American Metals Company, Limited optioned the property from Montgary and completed a surface drill program. It was during this work that the internal zonation of the pegmatite was recognized and documented (Hutchinson 1959), and pollucite was identified. During 1959 and 1960, Chemalloy Minerals Limited (formerly Montgary) completed both surface and underground drill programs, and extracted small quantities of pollucite and quartz. In 1961, the mine was placed on care and maintenance and then allowed to flood in 1962. In 1966, Chemalloy started to evaluate the tantalum potential of the pegmatite. Extensive diamond drilling was carried out from surface and underground over the subsequent three years. The initial result of this activity was the formation in 1967, of Tantalum Mining Corporation of Canada Limited (Tanco), a joint venture between Chemalloy and Northern Goldfield Limited. In 1969, construction of a 500 ton per day tantalum gravity concentrator was completed and Tanco began commercial production. Production of ceramic grade spodumene concentrates began on a pilot scale in 1984, and went commercial in 1986 when the new spodumene concentrator was completed. Although the lithium potential of the pegmatite had been investigated over the years for the production of 3 �ceramic grade spodumene concentrates and lithium carbonate, none of these investigations proceeded beyond the feasibility stage. The Tanco joint venture remained in place until 1993 when Cabot Corporation acquired 100% of the operation. Up to 1993, different companies, in addition to the original two companies, were involved in the joint venture. These include: Manitoba Development Corporation (1972-1993), Kawecki Berylco Industries/Cabot (1974-1993) and Hudson Bay Mining and Smelting Co. Ltd. (1978-1993). In 1996, Cabot formed the Cabot Specialty Fluids division and started construction of the cesium brine plant. In 2001, the plant was expanded to allow for the manufacture of conventional cesium chemicals. RARE-ELEMENT PEGMATITE FORMATION Rare-element pegmatites, like the Tanco pegmatite, occur in synclinoria of metavolcanicmetasedimentary sequences that separate granitoid batholiths from gneissic tonalities. The pegmatites evolve from late orogenic, peraluminous (A/[C+N+K]>1), S-type granites. The resultant pegmatite fields are situated on the lower portions of relatively steep geothermal gradients (±40°-50°C/km.). The complex type pegmatites are commonly emplaced in low pressure/high temperature facies (upper greenschist to lower amphibolite) metamorphic terrains with emplacement generally at a depth of four to six kilometers (Cerny 1989a). The parental, fertile granite is late- to post-tectonic and post-dates the peak of regional metamorphism. These granites are leucocratic and although commonly equigranular, may be porphyritic. At depth, they are biotite bearing and grade upward or laterally into a two-mica granite or a muscovite granite which is capped by a coarse grained to pegmatitic, megacrystic Kfeldspar, graphic leucogranite (Cerny and Meintzer 1988). This pegmatitic granite stage is an integral step in the formation of rare-element pegmatites. The pegmatitic melt forms within the parental, fertile granite through the process of magmatic differentiation. This melt collects in the upper portion of the fertile granite pluton, with the volatiles and other liquidus-depressing constituents such as H2O, F, Li, B and P increasing outward from the parental granite. These constituents, plus even small amounts of exsolved supercritical fluid, reduce the viscosity of the melt. The lower the liquidus temperature and the less viscous the melt, the more mobile the melt becomes and the further out it will migrate. Melt migration occurs when there is sufficient internal pressure and the magma reservoir is tapped by a tectonic disturbance of the outermost solidified shell of the pegmatitic granite. Pegmatite groups form a regional zonation around the parental granite with the complexity of the pegmatites increasing away from the parental granite (Cerny 1991b) (Figure 2). Rare-Element Pegmatite Classification Pegmatites can be divided into two types, the lithium-cesium-tantalum (LCT) type and the niobium-yttrium-fluorine (NYF) type. Over the years, the LCT type pegmatites have been well studied because of their economic significance. These pegmatites have been an economic source of lithium, tantalum, tin, cesium and rubidium minerals with mica, quartz and feldspathic sand by-products. The LCT type of pegmatites can be subdivided into classes based on mineralogy/chemistry and complexity (Table 1). 4 �I I I I / / / / / / / / / 1 / / ' Li,Be,Ta,Sn (Rb,Cs) I \ . Figure 2. Schematic section of a zoned fertile granite-pegmatite system. 1. fertile granite; 2. pegmatitic granite; 3. barren to beryl bearing pegmatites; 4. beryl-type, columbite- to phosphate-bearing pegmatites; 5. complex spodumene (or petalite) bearing pegmatites with Sn, Ta, ±Cs; 6. faults. (modified from Cerny 1989b) LCT type pegmatites have a number of chemical characteristics that distinguish them from the NYF type pegmatites. Some of these features include the following: • the tantalum content exceeds the niobium content; • the tin content can equal tantalum content; • they contain low levels of the light and heavy rare earths; • the pegmatites are enriched in boron and alkali elements; and, • they have low levels of uranium and thorium. Mineralogical characteristics can also be used to distinguish between LCT type and NYF type pegmatites. Some of the characteristics of the LCT type pegmatites include: • a general absence of fluorite (fluorine is tied up in minerals such as topaz, lepidolite, amblygonite); • a common occurrence of lithium and phosphate minerals; • the presence of tourmaline; 5 �• • simpler oxides of tantalum and niobium ±tin with essentially no rare earth element (REE) content; and, beryl can be present. The Tanco pegmatite is a good example of the complex type-petalite subtype of LCT pegmatite and is probably the most studied pegmatite of its type. Pegmatite Types Subtypes Characteristics Examples Beryl (i) (ii) beryl-columbite beryl-columbite-phosphate • relatively simple Greer Lake, MB PEG Group, NWT Complex (iii) spodumene petalite complex internal zonation lithium rich diverse mineralogy primary crystallization is greater than secondary replacement bodies Hugo, SD; Harding, NM (iv) • • • • Complex (v) amblygonite Lepidolite AlbiteSpodumene Albite • high fluorine activity Tanco, MB; Bikita, ZM, Peerless, SD Brown Derby #1, CO Kings Mountain, NC • least common • generally small Hengshan Field, P.R.C. Table 1. Classification of LCT type pegmatites. (modified from Cerny 1991a) GEOLOGIC SETTING The Bernic Lake pegmatite group, of which the Tanco pegmatite is a member, is one of a number of such groups comprising the Winnipeg River Pegmatite Field located in the Archean Bird River Greenstone Belt of the western Superior Province in the Canadian Shield (Figure 3). The Bird River Greenstone Belt is bounded on the north by the English River Subprovince, a belt of highly metamorphosed metasediments and metavolcanics rocks, and mafic to felsic batholiths and plutons (Beakhouse 1991a). To the south, this belt is bounded by the pluton-dominant Winnipeg River Subprovince (Beakhouse 1991b). The Bird River Greenstone Belt is comprised of six formations of the Rice Lake Group. In general terms, the belt consists of mafic to felsic metavolcanic and derived metasedimentary rocks all of which have been intruded by synvolcanic to late tectonic mafic to felsic intrusives. Of these formations, the Eaglenest Formation is the oldest and the Booster Lake Formation is the youngest (Trueman 1980). The six formations are listed below in chronological order. 1) Booster Lake Formation: metapelite and metagreywacke - unconformity 2) Flanders Lake Formation: lithic meta-arenites and metaconglomerate - unconformity 3) Bernic Lake Formation: felsic to mafic metavolcanic and metasedimentary units, felsic and mafic intrusive with porphyry units 6 �- unconformity4) Peterson Creek Formation: metarhyolites and clastic components 5) Lamprey Falls Formation: metabasalts, Bird River Sill, metagabbro 6) Eaglenest Lake Formation: metamorphosed volcanic wacke IVIdflROUd / / / JIILdFIU Sachigo Subprovince Bird River greenstclne belt Berens River SubDrovince --- 250 kflometres _________ Superior Province Southern Province and Nipigon Plate Phanerozoic basin sequence .___— Subprovince boundary Figure 3. Regional geological setting of the Tanco pegmatite (modified from Beakhouse 1991b). 7 �Trueman (1980) defined four major structural events in the area. The first two events were episodes of east-west folding, the second event being associated with the emplacement of large regional batholiths. These events led to associated prograde, low pressure/high temperature metamorphism (Abukuma type) in the Bernic Lake Formation. The third event was major eastwest faulting, with associated retrograde metamorphism. The last major event was a second faulting episode that propagated a series of northwest trending faults. With this event there was localized retrograde metamorphism. Overall the metamorphic grade throughout the Bernic Lake Formation is upper greenschist to lower amphibolite facies (Cerny, et al 1981). Numerous synvolcanic intrusive units intrude the Bird River Greenstone Belt. They range in size from a kilometre up to about 25 kilometres and generally display an elongated east-west shape. The compositions of these intrusives vary from body to body and even within an individual intrusive, and include gabbro, anorthositic gabbro, diorite, quartz monzonite, granodiorite and granite. Quartz and quartz-feldspar porphyries are also found throughout the belt. The Bernic Lake Formation, which is in fault contact with the Booster Lake Formation to the north and the Peterson Creek Formation to the south, consists of a complex array of layers of metamorphosed basalt, andesite, dacite, rhyolite, iron formation, conglomerates, volcanic wackes and sandstone. Lateral continuity is not common among most of the rock types and is only persistent in the mafic to intermediate metavolcanics and, in part, the iron formations. Other volcanic units in the area seem to be composed of flows of limited lateral extent. Syn- to post-tectonic emplacement of granite and pegmatitic granite stocks throughout the Bird River Greenstone Belt provided the source for a number of rare-element pegmatite groups found within the belt. Of all the pegmatites identified in the area, the largest and most economically significant ones occur within the Bernic Lake pegmatite group situated within the Bernic Lake Formation. The major pegmatites of this group include the Tanco, Dibs, Buck, Coe, and Pegli. All of these pegmatites have an east-west elongation, are horizontal to sub-horizontal in orientation and are hosted by either mafic intrusives or associated mafic metavolcanic units. TANCO PEGMATITE GEOLOGY The Tanco pegmatite, situated at the western end of Bernic Lake, is an extremely fractionated, rare-metal, complex type-petalite subtype, LCT pegmatite and is hosted by a late-stage, subvolcanic, metagrabbro (Tanco amphibolite) intrusive. The age of the Tanco pegmatite is 2,650 – 2,550 million years (Cerny 1989a). The pegmatite is completely blind or buried, and only sub-crops in a limited area in the bottom of Bernic Lake. Based on hundreds of diamond drill hole intercepts, the pegmatite has a maximum length of approximately 1,990 metres and a maximum width of 1,060 metres (Figure 4), and is up to 100 metres thick. The total tonnage of the pegmatite has been calculated to be approximately 25 million tonnes (Stilling, 1998). Emplacement is hypothesized to be within the pressure shadow of an easterly trending, dual plunging, anticlinal axis (Cerny, et al 1981). More recent data has given rise to the possibility that dilation may have been aided by a number of pre-existing faults that could have freed up the overlying block of metagabbro, thus allowing the intruding pegmatitic fluid to “hydraulically” lift the overlying host rock. Initial intrusion of this fluid appears to be into a sub-horizontal joint 8 �set that is ubiquitous throughout the Bernic Lake area. The bi-lobate shape of the pegmatite may also be influenced by a possible sinistral offset of the host anticline. OS 1.0 0.6 Tanco Mineral OS 1.0 NI kIIon,41n LI I cd7a-'----T—--—7 I - C' -' 7)-i 0 LI '—-' L7 f Lake Figure 4. Plan view of the Tanco Pegmatite Pegmatite Zonation Internally, the pegmatite is composed of nine discrete mineralogical zones with the different ores of economic interest – those of tantalum, spodumene, cesium and rubidium – each essentially occurring in different zones. Characteristic textures and mineralogical assemblages distinguish each zone. The pegmatite is the host to approximately 100 different minerals (Cerny, et al 1998). A coloured longitudinal section of the Tanco pegmatite is appended and a brief description of each of the internal zones is given below. A more complete description of the pegmatite zonation and mineralogy can be found in Cerny, et al (1998). Of the nine zones comprising the pegmatite, only the Border and Wall Zones occur as concentric shells enveloping the entire pegmatite. When combined, however, the Lower and Upper Intermediate Zones also form a concentric shell within the pegmatite. The Central Intermediate and Lepidolite Zones have both been subjected to late stage micaceous alteration. Unlike the stereotypical zoned pegmatite, the location of the Quartz Zone or core is in the upper portion of the dike for most of the mine and in its more traditional, central location only in the western portion. Border Zone (Zone 10) This is a very thin zone (a few to 30 centimetres) that envelops the pegmatite and is composed of fine-grained, saccharoidal assemblage of albite and quartz with lesser to rare tourmaline, apatite, biotite, beryl and triphylite. In places it may have a layered appearance. 9 �Wall Zone (Zone 20) The Wall Zone consists of very coarse grained, brick-red perthite, quartz, fine to coarse-grained tourmaline, albite, brown to greenish muscovite books and accessory white beryl. In general, the hanging-wall Wall Zone is thinner and coarser grained than the footwall Wall Zone. Increased albite (cleavelandite) content and bands of “footwall” aplite characterize this latter Wall Zone. The tin content of the zone generally exceeds the tantalum content. Aplitic Albite Zone (Zone 30) This zone is one of the main tantalum ore zones and is most prominent in the eastern portion of the pegmatite. The dominant mineral is a pale blue to white, fine-grained, saccharoidal albite. Quartz is a common constituent with subordinate to rare minerals including muscovite, Taoxides (predominantly wodginite), beryl and apatite. Texturally, undulating layers of the saccharoidal albite distinguish the zone. Where the Aplitic Albite Zone is in contact with the Quartz Zone, the contact is generally characterized by increased tantalum along the contact and within the albite in very close proximity to the contact (“nugget effect”). In some places, white beryl crystals that have grown off the albite into the quartz mark this contact. Lower Intermediate Zone (Zone 40) The locally known Mixed Zone is characterized by both its diversity of minerals and grain size. The mineral assemblage that distinguishes this zone is comprised of coarser grained microclineperthite and SQUI pseudomorphs (Spodumene-QUartz Intergrowth after primary petalite) in a finer grained matrix of albite, quartz and micas. Assemblages consisting of quartz pods containing amblygonite and/or spodumene, and radial rims of cleavelandite and lithianmuscovite around feldspar rich assemblages are less common. Common subordinate to rare minerals include lithian-muscovite, lithiophilite, lepidolite, petalite and Ta-oxides. The generally gradational contacts with the tantalum and spodumene zones allow for the selective mining of this zone for both tantalum and spodumene, but only under strict grade control. Upper Intermediate Zone (Zone 50) The Spodumene Zone as it is referred to, has evolved from the Lower Intermediate Zone and is comprised of very coarse-grained microcline-perthite and SQUI with lesser spodumene blades, quartz and amblygonite. Subordinate to rare mineralogy consists of pollucite, lithiophilite, albite, lithian-muscovite, petalite, eucryptite and Ta-oxides (predominantly tantalite). The SQUI, which is an oriented intergrowth of cogenetic spodumene and quartz, has resulted from the isochemical breakdown of the primary petalite. This process occurs under decreasing pressure conditions during the cooling of the intrusion (London (1986). This zone displays the largest crystals in the pegmatite with the petalite pseudomorphs (SQUI) attaining lengths up to approximately seven metres and the microcline-perthite reaching up to ten metres in length. Central Intermediate Zone (Zone 60): The MQM (Muscovite and Quartz after Microcline) Zone is another of the main tantalum ore zones. The zone is comprised of microcline-perthite, quartz, albite and muscovite with 10 �subordinate to rare beryl, Ta-oxides (predominantly wodginite), spodumene, sulphides, and apatite. The minerals are medium to coarse grained. Quartz Zone (Zone 70) This is a massive, monomineralic zone with accessory spodumene (SQUI) and amblygonite. When in contact with the tantalum zones, the contact is commonly characterized by increased tantalum concentration (“nugget effect”) Pollucite Zone (Zone 80) The Pollucite Zone is a sub-zone of the Upper Intermediate Zone with a gradational contact occurring between the two, and consists of a monomineralic core of pollucite enveloped by an assemblage of SQUI, microcline-perthite, amblygonite, petalite and interstitial pollucite. Polygonal fracturing with lithian-muscovite and/or quartz filling is not uncommon within the core. Accessory minerals include apatite and albite. This is the zone from which the cesium ore is mined. Lepidolite Zone (Zone 90) This zone forms two flat lying, east-west elongated sheets within, (at least in part) the Central Intermediate Zone and in contact with the Upper Intermediate Zone. The mineral assemblage consists of fine-grained, purple lithian-muscovite and lepidolite with lesser microcline-perthite, and subordinate to accessory albite, quartz, beryl and Ta-oxides (predominantly microlite). The zone is mined for tantalum and has been mined, on a limited scale, for rubidium. MINING The heart of the Tanco pegmatite is situated some 60 metres (~200 feet) below Bernic Lake, and is accessible from surface either via a shaft or via a 400 metre (~1,300 foot), 20 percent decline. Mining is carried out using the “room and pillar” method. The mine’s shallow depth contributes to lower inherent ground stresses and generally stable ground conditions. After weighing these factors, and considering the diverse mineralogy encountered at Tanco, it was decided that the Room and Pillar mining method would provide the optimum approach for economic extraction at Tanco. The first pillar design saw pillars 16 metres square (50 ft. x 50 ft.), with mining rooms also at 16 metres wide. As mining progressed over the years, ongoing rock mechanics studies showed that the rooms could be increased to 22 metres or 72 feet, without excessively loading the pillars. Pillar reduction has now been done successfully throughout the mine and continues as an integral part of the mining plan. Two-boom hydraulic jumbos perform all drilling for drifts, slashes, benches and arches. During the initial top slice development, the roof is carefully arched, utilizing smooth blasting techniques. The roof arches allow residual ground stresses to be redirected to the post pillars. Ground stress in the Tanco mine is considered low, relative to other hard rock mines, and as such, rock bolting is rarely required. At Tanco, the roof of mature mine workings may often average 20 metres (~65 feet) above the working levels below, and in places, may reach 30 metres (~95 feet). These high backs are 11 �carefully monitored throughout mining operations, utilizing custom designed aerial lift devices (referred to as Giraffes). Where suited, mining is carried out, utilizing a single boom Simba longhole drill. In particular, the longhole method has been the primary approach to pillar reduction. The broken ore is transported utilizing 5 yd3, 6 yd3 and 7 yd3, load-haul-dumps (LHD’s) – mobile, front-end loader units - and a 20 ton truck to various ore-passes, which are located throughout the mine. The ore is broken on grizzlies (metal grates at the top of the ore pass), utilizing either mobile or stationary hydraulic rock breakers. The ore is then passed to an underlying tramming level where it is transported to the shaft by a train of 4 ton, Granby style, side dump ore cars, and hoisted to surface coarse ore bins via 4 ton Kimberly style skips. Tantalum and spodumene ores are stored in one of two loading pockets and skipped on a daily basis up the two-compartment shaft, into dedicated surface coarse ore bins. The mine however, must produce and provide three distinct ores to the mill. To overcome the limitation of the system, one loading pocket and associated coarse ore bin is emptied weekly and an appropriate tonnage of pollucite ore is batched through. Mine ventilation air is downcast from surface through one of two vent raises, one being, in part, the Jack Nutt shaft from 1929/30 and the other, a 1.8-m (6 foot) diameter bore-hole raise. The exhaust mine air up-casts through the access decline. Total fresh air volume exceeds 5300 m3 per minute (190,000 ft3 per minute) and is appropriate for the operation of Tanco’s fleet of diesel mining equipment. A fleet of personnel carriers and service trucks supports mining operations. Tanco maintains all of its mine equipment at its own on-site facilities. MINERAL PROCESSING Due to land constraints, the concentrator is constructed on a peninsula formed by two bays on Bernic Lake. The building is multi-floored, with equipment on a total of six levels. The major items of concentration equipment are on two levels, with feed preparation equipment, filters and driers, on the upper levels, with pumps on the lower levels. The first stage of processing, common to all four mineral products, is crushing, where the coarse ore from underground (-300 mm in size) is broken down to –12 mm. in size. The tantalum, spodumene and pollucite ores are crushed into separate fine-ore, storage bins. The new dry grinding plant supplies ground pollucite for the cesium formate plant. Different processes concentrate each ore. Tantalum is processed by gravity concentration, a process that makes use of the fact that tantalum minerals are much heavier than the waste minerals. Spodumene, on the other hand, is primarily processed by flotation, which makes use of the different physical and chemical characteristics of the surfaces of the various minerals. Pollucite is ground and then subjected to acid leaching and other chemical processing to produce cesium chemicals. Tantalum Processing (Figure 5.) There are three main elements in the gravity concentration of Tanco’s minerals: liberation of the values from the gangue or waste rock; feed preparation of the ground product into different size fractions; and concentration of the different fractions. At Tanco, the plant is split effectively into four fractions – grinding/spiral circuit, coarse sand circuit, fine sand circuit and slime circuit. 12 �Fine ore is first ground to pass 2 mm. The –2 mm. product passes to the spirals, which recover the coarse, free, tantalum minerals, which may otherwise have been ground too fine for effective recovery. The spiral tailing is sized at 0.30 mm. by a Linatex hydrosizer with the underflow recirculating to the main grinding mill. Figure 5. Tanco’s tantalum gravity separation flowsheet. Effective feed preparation is essential for satisfactory separation on shaking tables, and this is carried out with cyclones, followed by Bartles-Stokes hydrosizers. The hydrosizers contain four spigots and an overflow. The spigot products, or sand fractions, are distributed to further banks of spirals. These spirals each produce a low-grade concentrate, a recirculated middling, and a 13 �tailings product. Falcon concentrators scavenge the fine sand tailings products. This centrifugal separator is one of the newest concentration devices, confirming Tanco’s commitment to “leading edge technology” in the pursuit of performance. Rougher concentrates from all sections are collected in a storage tank from which the cleaner section is fed at constant flowrate and density. Classification in cyclones and a hydrosizer sizes feed to four cleaner tables, which produce a fine, 35% Ta2O5 concentrate, a recirculated middling, and a tailing. Overflows from the various cylcones along with the Stokes hydrosizer overflow constitute the feed to the ultrafines circuit. These are thickened in another bank of cyclones and treated on a Mozley MultiGravity Separator (MGS). The MGS produces a rougher concentrate, upgraded on Bartles CrossBelts. Overall recovery of tantalum ranges from 69-72%. During the summer months accumulated tailings can be processed along with the ore; the same flowsheet being used. Recovery from the tailing portion of the feed is of the order of 30%, upgrading the feed from 0.05% to 30% Ta2O5. The specifications of a typical tantalum concentrate produced at Tanco is given in Table 2. Element Typical Concentrate (wt. %) Ta2O5 35% - 38% SnO2 14% - 18% Nb2O5 5% - 8% TiO2 2% - 4% Table 2. Typical tantalum concentrate Tantalum Markets Tanco’s tantalum concentrates are shipped to the Cabot Performance Materials facility in Boyertown, Pennsylvania for conversion to the metal or tantalum compounds. The major uses for tantalum are in the electronics industry and for cutting tools. High quality capacitors are the major single use for tantalum. Europe is the major consumer of tantalum carbide used in production of hardmetal alloys for cutting tools. Other tantalum alloys are important constituents of aero engines, and for acid resistant pipes and tanks used in the chemical industry. One minor but important use of tantalum is in the medical industry, for “spare-part” surgery – tantalum “pins” are used for such areas as hip-joint replacements, as it is the only metal that is not rejected by body fluids. Spodumene Processing (Figure 6) After crushing to –12 mm., the heavy medium Triflo circuit rejects the feldspar from the –12 mm. +0.5 mm. range. Ferrosilicon and magnetite as a 70:30 mixture are used with a feed density of 2.74 kg/l. and effective density of separation of 2.65 kg/l. The –0.5 mm. fraction continues to the grinding circuit. The sink product and the –0.5 mm. fraction are ground in closed circuit with a 2 mm. primary screen and a Linatex hydrosizer with an approximate cut point of 150 micron. Rougher and cleaner spirals recover coarse free tantalum within the grinding circuit. A 5 foot, low intensity drum magnet removes ground steel produced during the grinding process. The grinding circuit product is scavenged for tantalum by two Falcon concentrators. Tantalum from the Falcon concentrators is upgraded on a Double Deck Holman Table with the tailings and middlings returning to the grinding circuit. Coarse tantalum from the cleaner spiral is also 14 �upgraded on a single Holman Table. The tantalum recovered from the spodumene circuit is a valuable by-product. Figure 6. Schematic flowsheet of the spodumene circuit Prior to the amblygonite flotation stage, in order to control phosphate levels, the pulp is deslimed by single stage cycloning. Due to the nugget-like appearance of the amblygonite, close control of this flotation stage must be maintained. Starvation quantities of collector are used based on 15 �feed tonnage and previous tails assays. Starch is used as a depressant for spodumene at pH 9.2. A spodumene-phosphate by-product called Montebrasite is produced to meet market requirements. This concentrate is subjected to wet high intensity magnetic separation to remove weakly magnetic iron materials. This concentrate is pumped to a belt filter and propane fired rotary drier. The dried concentrate goes to a storage bin prior to bagging or bulk shipping to meet the customers’ requirements. Mica is then removed with a single flotation stage. This step assists in the removal of K2O from the final concentrate product. The mica flotation tailings are two stage cycloned to remove starch. Two conditioning stages for automatic pH control and collector addition are carried out prior to rougher flotation. The rougher concentrate goes on to two or three stages of cleaning to The Wet High Intensity Magnetic Separator (WHIMS) non-magnetic fraction is thickened by two stages of cycloning and stored in a holding tank prior to tonnage controlled feeding to the belt filter and propane fired rotary drier. Final product handling is carried out utilizing air slides and dense phase pneumatic pumping to storage bins. The final product can be shipped to the customer in 25 kg, 1,000-kg bags, or bulk, via road or rail. The concentrate is sold to markets worldwide. Water used within the circuits is either fresh (from Bernic Lake) or re-cycled pond overflow depending on the section of the plant. Spodumene Product Specifications Clients can accept different levels of impurities, depending on their specific use of the material. Specifications of Tanco’s 7.25%, -200 Mesh, 6.8%, and Spodulite grade concentrates are shown in Table 3. Spodumene Markets Customers specify tight impurity levels for the use of spodumene concentrates in the glass and ceramics industries, and the process for the production of these concentrates is based on removal of contaminant minerals. Spodumene can be used either as a feedstock for the production of lithium carbonate and metal, or directly, in its mineral form, in the glass and ceramics industries. Since the development of the “salars” in the USA and Chile, most lithium carbonate is recovered from these sources, and little spodumene is now used for chemical production. Lithia is a very powerful flux, especially when used in conjunction with potash and soda feldspars. In ceramics, lithium lowers thermal expansion and decreases the firing temperature. Glasses containing lithia are much more fluid in the molten state than those containing proportionate amounts of sodium or potassium. Lower viscosity and faster melting can be utilized to improve glass quality in terms of fewer defects such an unmelted or partially melted raw material grains, and more rapid removal of small bubbles. Lower viscosity can permit the glassmaker to run a forming machinery at a higher rate, or create more elaborate products such as some perfume bottles. In frits and glazes, lithia is used to reduce the viscosity and thereby increase the fluidity of the coatings. This reduces maturing times and lowers firing temperatures. Small amounts of lithia also increase gloss. 16 �Element/Sizing 7.25% Grade -200 Mesh 6.8% Mesh Spodulite Li2O 7.25 ± -0.1% 7.10% ± -0.2% 6.80% min. 5.00% min. 0.15% max. 0.08% max. 0.10% max. 0.06% ± -0.01% Fe2O3 Na2O 0.35% max. 0.30% max. 0.45% max. 0.75% max. K2O 0.30% max. 0.60% max. 0.40% max. 0.75% max. P2O5 0.27% max. 0.40% 0.27% max. 0.20% max. MnO2 0.04% max. 0.06% max. 0.04% max. 0.05% max. Al2O3 24.0% min. 25.0% min. 23.0% min. 20.0% typ. Tyler 20 Mesh 0.0% max. Tyler 28 Mesh Trace max. Tyler 48 Mesh 1.0% max. Tyler 200 Mesh 50.0% min. 10.0% max. 55.0% min. 80.0% typ. Table 3. Tanco’s spodumene products specifications. CESIUM FORMATE Cesium formate is a clear, water soluble fluid with a specific gravity of 2.3 g/cm3 (i.e. it is two and one third times heavier than water) and a viscosity similar to water. It is used in the oil drilling industry as a drilling fluid, where the properties of low viscosity, high specific gravity and complete solution confer significant benefits over traditional mud (bentonite) based drilling fluids in deep wells greater than 4,575 metres (15,000 ft.). Use of cesium formate eliminates formation damage, particularly skin formation while drilling through the reservoir (oil bearing) rock. This results in improved hydrocarbon flows to the well giving better daily production from the well, in addition to enhanced recoveries from the reservoir in the long term - that is more hydrocarbons may be extracted from the well before well stimulation techniques become necessary. From an occupational health and safety perspective, there are considerable benefits to the use of cesium formate. The pH is between 10 - 11, and skin contact, although undesirable, has no immediate consequences. Low mammalian toxicity is an added benefit. The low environmental toxicity of cesium formate makes it the fluid of choice in areas where environmental sensitivities are particularly acute. Cesium Formate Plant The cesium formate pilot plant was designed, built and commissioned in 1996/97 in response to a potential market for formate brines. The focus of plant production was aimed at the oil and gas industries’ demand for a high-density, solids free drilling fluids. The plant was designed to 17 �readily incorporate process changes and modifications enabling it to produce a wide variety of cesium-based products, thus allowing Tanco and Cabot to rapidly respond to these future markets. The original plant was designed to produce 500 barrels/month of 2.3 g/cm3 specific gravity cesium formate. In 1999, expansion of the plant allowed for the production of 700 bbl/month. In 2001, the plant underwent a further expansion in order to accommodate the manufacturing of conventional cesium chemicals. Since Bernic Lake is a headwater lake and therefore very susceptible to environmental damage, the plant design minimizes environmental impacts on the surrounding area. All areas of the plant are contained to capture any spilled material, and wastes are stored in a lined disposal cell, which eliminates the discharges to the lake. Cesium Formate Manufacture Pollucite ore is mined from the Tanco mine along with the spodumene and tantalum ores. The mine contains approximately 75% of the worlds proven reserves of pollucite. The ore is crushed to –12 mm., and then dry ground in a ball mill to a powder form. Utilizing a series of acid/base reactions, the cesium is extracted from the pollucite ore and converted to a high-density, cesium formate solution. The final product is shipped by container to Aberdeen, Scotland and Bergen, Norway for use by the drilling industry in the North Sea, and to Houston, Texas for use in the Gulf of Mexico. Markets Cesium chemicals are currently used primarily in catalyst and chemical synthesis applications. While current worldwide demand for fine cesium chemicals is approximately 700,000 pounds a year, it is expected that new applications in the oil, gas and chemical industries for these products will increase in demand by more than ten-fold. TANCO UNDERGROUND TOUR STOPS Due to the constant changes underground as a result of on-going mining activities, the stops for the tour will not be determined until closer to the date of the tour. The tour participants will receive a tour stop handout upon arrival at the minesite. 18 �REFERENCES Anderson, A.J., Groat, L.A. and Simmons, W.B.Jr. (eds.) (1998): Granitic Pegmatites: The Cerny – Foord Volume. The Canadian Mineralogist, vol. 36, pt. 2. Beakhouse, G.P. (1991a): Winnipeg River Subprovince, in Thurston, P.C., Williams, H.R., Sutcliff, R.H. and Stott, G.M., (eds.), Geology of Ontario: Ministry of Northern Development and Mines, Special Volume 4, Part 1, pp.239-278. Beakhouse, G.P. (1991b): Winnipeg River Subprovince, in Thurston, P.C., Williams, H.R., Sutcliff, R.H. and Stott, G.M., (eds.), Geology of Ontario: Ministry of Northern Development and Mines, Special Volume 4, Part 1, pp.279-302. Berry, L.G. (ed.) (1972): The Tanco Pegmatite at Bernic Lake, Manitoba. The Canadian Mineralogist, vol. 11, pt. 3. Brisbin, W.C. (1986): Mechanics of pegmatite intrusion. The American Mineralogist, vol. 71. N°s. 3 and 4, pp. 644-651. Brown, G.E., Jr., and Ewing, R.C. (eds.) (1986): R. H. Jahns Memorial Issue: The mineralogy, petrology, and geochemistry of granitic pegmatites and related granitic rocks. The American Mineralogist, vol. 71, N°.’s 3 and 4. Cerny, P. (ed.) (1982): Granitic Pegmatites in Science and Industry. Mineralogical Association of Canada Short Course Handbook 8. Cerny, P. (1989a): Characteristics of pegmatite deposits of tantalum, in Moller, P., Cerny, P. and Saupe, F., (eds.), Lanthanides, Tantalum and Niobium: Society for Geology Applied to Mineral Deposits, Special Publication 7, Springer-Verlag, pp.195-239. Cerny, P. (1989b): Exploration strategy and methods for pegmatite deposits of tantalum, in Moller, P., Cerny, P. and Saupe, F., (eds.), Lanthanides, Tantalum and Niobium: Society for Geology Applied to Mineral Deposits, Special Publication 7, Springer-Verlag, pp.274-302. Cerny, P. (1991a): Rare-element Granitic Pegmatites. Part II: Regional to Global Environments and Petrogenesis. Geoscience Canada, vol. 18, pp. 49-67. Cerny, P. (1991b): Rare-element Granitic Pegmatites. Part 1: Anatomy and Internal Evolution of Pegmatite Deposits. Geoscience Canada, vol. 18, pp.68-81. Cerny, P., Ercit, T.S. and Vanstone, P.J. (1998): Mineralogy and Petrology of the Tanco Rareelement Pegmatite Deposit, Southeastern Manitoba. International Mineralogical Association, Field Trip Guidebook B6, 17th General Meeting, Toronto, Ontario, Canada. Cerny, P. and Meintzer, R.E. (1988): Fertile granites in the Archean and Proterozoic fields of rare-element pegmatites: crustal environment, geochemistry, and petrogenetic relationships, in Taylor, R.P. and Strong, D.F. (eds.), Recent advances in the geology of granite related mineral deposits: Canadian Institute of Mining and Metallurgy, Special Volume 39, pp. 170-207. Cerny, P., Trueman, D.L., Ziehlke, D.V., Goad, B.E., and Paul, B.J. (1981): The Cat LakeWinnipeg River and Wekusko Lake Pegmatite Fields, Manitoba. Manitoba Department of Energy and Mines, Mineral Resources Division, Economic Geology Report ER80-1. Crouse, R.A., Cerny, P., Trueman, D.L. and Burt, R.O. (1979): The Tanco Pegmatite, Southeastern Manitoba. The Canadian Mining and Metallurgy Bulletin, Feb. 1979, pp.142-151. 19 �Ercit, T.S. (1986): The simpsonite paragenesis: the crystal chemistry and geochemistry of extreme Ta fractionation. Ph.D. thesis, University of Manitoba, Winnipeg, Manitoba, Canada. Hutchinson, R.W. (1959): Geology of the Montgary Pegmatite. Economic Geology, vol. 54, pp. 1525-1542.. London, David (1984): Experimental phase equilibria in the system LiAlSiO4-SiO2-H2O: a petrogenetic grid for lithium rich pegmatites. American Mineralogist, vol. 69, pp. 995-1004. Martin, R.F. and Cerny, P. (eds.) (1992): Granitic Pegmatites. The Canadian Mineralogist, vol. 30, part 3. Moller, P., Cerny, P. and Saupe, F. (eds.) (1989): Lanthanides, Tantalum and Niobium. Society for Geology Applied to Mineral Deposits, Special Publication N°. 7, Springer-Verlag, New York, NY. Stilling, A. (1998): Bulk composition of the Tanco Pegmatite at Bernic Lake, Manitoba, Canada. M.Sc. thesis, University of Manitoba, Winnipeg, Manitoba, Canada. Thomas, A.V. (1984): A petrological and fluid inclusion study of the Tanco pegmatite, S.E. Manitoba. M.Sc. thesis, University of Toronto, Toronto, Ontario, Canada. Trueman, D.L. (1980): Stratigraphy, structure, and metamorphic petrology of the Archean greenstone belt at Bird River, Manitoba. Ph.D. thesis, University of Manitoba, Winnipeg, Manitoba, Canada. 20 �21 �22 �Field Trip 2 Quaternary Geology of Southeastern Manitoba Erik Nielsen and Gaywood Matile Quaternary Geologists Manitoba Geological Survey Industry, Trade and Mines 360-1395 Ellice Avenue Winnipeg, Manitoba R3G 3P2 Canada Extensive wetlands that started to form in response to mid-Holocene climate change, are a common feature of the southeastern Manitoba landscape. The photo was taken approximately 25 km east of Sandilands. �INTRODUCTION The climatic, geomorphic and ecological changes that have occurred in northwestern Ontario, southeastern Manitoba and Canada in general, over the last 100 000 years have been nothing short of spectacular. The Sangamonian Interglacial, which was not unlike the present interglacial, lasted from approximately 125 000 to 75 000 years ago, and ended with the advance of the Laurentide Ice Sheet in what was the greatest ecological catastrophe to befall Canada in recent geological time. The Laurentide Ice Sheet flowed southward out of Quebec and Nunavut and covered most of Canada and the northern parts of the United States as far south as New York City and De Moines, Iowa. The southern ice margin fluctuated periodically throughout the Wisconsinan, but northwestern Ontario and southeastern Manitoba and most of the rest of Canada were locked in the icy grip of the continental ice sheet until almost 10 000 years ago. For an estimated 65 000 years, northwestern Ontario and southeastern Manitoba lay devoid of trees, grasses and all living things, under a one kilometre thick ice mass! Ameliorating climate in the late Pleistocene saw the rapid northward and northeastward retreat of the ice margin and the establishment of glacial Lake Agassiz between the retreating ice margin and the high ground to the south, east and west. Lake Agassiz, at times over 200 m deep persisted for about 4 000 years from approximately 11 700 to 7 700 years BP and occupied all of Manitoba below the Cretaceous Escarpment, as well as much of northwestern Ontario. The flat fertile plains of the Red River valley and parts of northwestern Ontario, such as the Fort Francis area, resulted from the deposition of thick deposits of deepwater glaciolacustrine sediments. The numerous beach deposits in northwestern Ontario, southeastern Manitoba and elsewhere, record successive lake levels. Water levels recorded by the beaches relate to differential isostatic rebound and stepwise drainage of Lake Agassiz into the Gulf of Mexico, the Great Lakes and the Arctic Ocean before the lake finally drained into Hudson Bay. POSTGLACIAL VEGETATION AND CLIMATE Little is know of the postglacial vegetation of southeastern Manitoba despite the extensive and detailed work in the region by the Geological Survey of Canada and the Manitoba Geological Survey over the last fifteen years. Information on the postglacial climate and vegetational history of the region is inferred form a single pollen diagram from Hayes Lake near Kenora (McAndrews, 1982). The vegetational history of Hayes Lake suggests that the area was invaded by spruce forest immediately upon deglaciation and regression of Lake Agassiz. The early spruce forest changed to pine, birch, poplar and alder forest after 10 000 years BP. Based on the available data from Hayes Lake, open, mixed woodland existed in the northwestern Ontario and southeastern Manitoba during the early to mid-Holocene. Spruce and fir increased at the expense of alder after about 3 600 years BP, in response to a cooling climate. The present vegetation has remained relatively stable for the past 3 600 years. QUATERNARY GEOLOGY OF SOUTHEASTERN MANITOBA Twelve thousand years ago all of Manitoba, except possibly isolated areas above the Manitoba Escarpment, was completely covered by glacial ice, which at it's maximum extended as far south 24 �as Des Moine, Iowa. Rapid glacial retreat, caused by the rapid amelioration of climate, was enhanced by a proglacial lake environment, which promoted accelerated ice beak-up by means of iceberg calving along the glacier margin. By ten thousand years ago the ice margin was at the south end of Lake Winnipeg, and southeastern Manitoba was ice-free. As ice retreated into southeastern Manitoba it divided into two glacial lobes, the Rainy Lobe which advanced from the northeast and the Red River Lobe which advanced from the northwest. Sediments deposited by the Rainy Lobe typically have a sand-rich matrix and Precambrian-rich clast lithology, whereas sediments carried by the Red River Lobe are typically silt-rich and predominantly Paleozoic carbonate clasts, reflecting the lithologies of the bedrock that the glacier was advancing over. The interlobate position between these two ice-lobes is defined by large, sorted sand deposits in the south, the Sandilands Moraine, and sand and gravel deposits further north (Figure 1). Retreat was rapid, commonly followed by minor glacial readvances that eroded or destroyed previously deposited recessional moraines. Figure 1. Field trip stops plotted on a Digital Elevation Model of a portion of southeastern Manitoba Glaciation in Manitoba blocks the natural northward drainage and consequently a proglacial lake, glacial Lake Agassiz, formed as the ice front retreated north of the continental divide in South Dakota. Lake Agassiz existed for about four thousand years, from about 11 700 years before present (BP) until about 7 700 years BP when it finally drained into Hudson Bay (Thorleifson, 1996). Paleostrandlines and associated radiocarbon dates from southeastern 25 �Manitoba document much of Lake Agassiz history (Figure 1). The initial phase of Lake Agassiz, the Lockhart Phase, during which time the lake drained southward into the Gulf of Mexico, and encompasses the highest levels of the lake lasted until about 11 000 years BP. The Lockhart Phase in southeastern Manitoba is represented by numerous, but poorly defined strandlines along the higher parts of Sandilands Moraine and by most of the clay deposited in the Red River valley to the west. The Lockhart Phase was followed by the Moorhead Phase which ended about 9 900 years BP. The Moorhead Phase is characterized by relatively low water levels, due to glacial retreat in the Lake Nipigon area of northwestern Ontario, which allowed drainage through lower outlets to Lake Superior and the Atlantic Ocean. Several, well-developed beaches and wave-cut escarpments and at least one in-filled abandoned river channel represent the Moorhead Phase north and west of the Sandilands Moraine. The following Emerson Phase, spanned the interval from about 9 900 to 9 300 years BP. A glacial readvance in northern Ontario blocked the eastern outlets. This caused the level of Lake Agassiz to rise to approximately the level it was at the end of the Lockhart Phase. The elevation difference was the result of about 1 000 years of isostatic rebound. Lake Agassiz again drained south into the Gulf of Mexico via the Mississippi River. Four prominent lake levels formed during the Emerson Phase, the Norcross, Tintah, Upper Campbell and the Lower Campbell. The Upper and Lower Campbell levels are the bestdeveloped strandlines in Lake Agassiz and can be traced in this region around the Sandilands Moraine and eastward almost to the Ontario border. A great deal of erosion occurred along the Sandilands Moraine at this time as a result of the prevailing winds coming from the northwest, across the open expanse of the lake. The final phase of Lake Agassiz, the Morris Phase, is represented by a series of regularly spaced, moderately well developed strandlines (Figure 1). The final drainage of the lake, occurred as successively lower eastern outlets opened, first draining through Lake Superior and then through more northerly outlets to the north Atlantic, until the final drainage into Hudson Bay about 7 700 years BP. During the rise and fall of Lake Agassiz water levels, the Sandiland Moraine, a large generally unconfined sand aquifer, rapidly became saturated and de-watered. The rapid de-watering caused the formation of sapping channels throughout the moraine, one of which is truncated by the Upper Campbell, and is therefore clearly related to the final drainage of Lake Agassiz from the area. FIELD TRIP STOPS STOP 1 - STRIATED OUTCROP, WEST HAWK LAKE Northwestern Ontario and the adjacent parts of southeastern Manitoba have been glaciated numerous times throughout the Pleistocene. Each successive glaciation in large part removes the evidence of previous glaciation. Previously deposited sediments are stripped away and bedrock outcrops are molded and striated such that they record only the most recent events. Consequently, the terrestrial glacial record is largely incomplete. The outcrop of pillow basalt at this stop was striated and polished by the last ice flow to affect this area (Figure 2). The striations, orientated towards 230º, are common throughout the area and 26 �record glacier movement out of Hudson Bay towards the southwest. Striations are typically fine scratches on the gentle stoss sides of this outcrop. The plucking and steep sides at the down glacier side of the outcrop, indicates that the ice flow was towards the southwest and not the northeast. Minor variations in striation direction across the outcrop is due to topographic deflection at the glacier sole and is not related to different glacial events. In addition to striations, numerous p-forms, areas that have been eroded by subglacial meltwater under hydraulic head imposed by the glacier, are found on the outcrop. Figure 2. Striated outcrop at West Hawk Lake showing ice flow towards 230º. Striations are best preserved under till or glaciolacustrine sediments. Once glacial polish and striations have been exposed to weathering they don’t usually last very long. STOP 2 – WEST HAWK LAKE, TILL SECTION Till is the material that is directly deposited by the action of glacier ice although there may be considerable influence of subglacial meltwater. It consists of a wide variety of grain sizes from clay to boulders and may be considered as being generally unsorted (Figure 3). Till is generally derived primarily by the comminution of the immediately underlying bedrock with only very small components originating from various up glacial sources. This is the case with the till in the West Hawk Lake area, which is sandy in texture and was derived primarily by the comminution of the underlying volcanic rocks. A small proportion of the till was derived from granitic rocks that outcrop approximately 5 km to the northeast. The till is generally not calcareous, but in places Paleozoic carbonate erratics derived from the Hudson Bay Lowland, 650 kilometres to the 27 �northeast, can be found testifying to long distance glacial transport. Carbonate erratics are however rare, both because of the long glacial transport and the low survival rate of these soft lithologies, but also because of dissolution by near surface weathering in the time since the area was deglaciated. Carbonate erratics, which may be found in the scree were probably ice rafted and subsequently deposited in the glaciolacustrine sediments, which are common in the area. Carbonate erratics are probably not derived from the till at this site. Figure 3. Section at West Hawk Lake exposing sandy till of northeast provenance. STOP 3 - WEST HAWK LAKE, METEORITE IMPACT STRUCTURE West Hawk Lake is almost 4 km wide and nearly circular in shape. It was drilled in the 1960s by the Dominion Observatory and found to be approximately 100 m deep and contain approximately 100 m of sediment overlying the Precambrian basement. The circular shape and great depth of the lake, as well as the presence of shock-metamorphosed quartz and other features indicates the lake was formed by a meteor impact. The oldest sediments in the crater are of Cretaceous age indicating the impact occurred prior to that time, possibly in the Paleozoic. Because the lake is so deep compared to its diameter, it has been believed for many years that sediment deposited in the lake would be protected from erosion during glaciation. Glacier ice moving over the lake would shear over the top of the lake and not penetrate to the bottom. Previously deposited sediment would therefore be unaffected by glaciation. In addition, the great depth and the fact that there are no major rivers draining the lake means that sediments entering the lake would not be eroded or flushed through the lake. Consequently, the possibility exists that a complete Holocene, glacial and pre-glacial record spanning perhaps millions of years 28 �might be preserved in the sediment in-fill at the bottom of the lake. Jim Teller from the Department of Geology, University of Manitoba, has undertaken coring of the upper 15 m of the sediment in-fill and is planning to core the remaining 75-85 m to elucidate the glacial history, the history of glacial Lake Agassiz and climate variability of the mid continent over possibly the last million years or more. En route to Stop 4 we drive west on the Trans Canada Highway. Approximately 12 km west of Falcon Lake we leave the area affected by glaciation from Hudson Bay and northwestern Ontario. The ice flow direction changes to southeasterly (145º) and the associated till becomes fine textured and highly calcareous, having been derived by the comminution of Paleozoic carbonate bedrock in the Manitoba Interlake, northwest of Winnipeg. We will have an opportunity to observe diamicton, similar to this till, in the Grunthal pit at Stop 6. Point of Interest In some sections of the low boreal forest, the construction of the Trans-Canada Highway had a considerable effect on drainage and the height of the water table. Where the Trans-Canada Highway becomes divided, just west of Falcon Lake, a small stand of eastern white cedars (Thuja occidentalis) in the median illustrates the impact of hydrological change on the local vegetation. The growth of these trees began to be affected following highway construction in 1981. Although most of these trees had been growing since the early 1800s, the elevated water table caused their ringwidth and wood density to decline by 50 percent within two to three years. While some cedars were able to survive under the raised water table for several years, the last tree had succumbed to flooding by 1993. Although most are still standing upright, these trees have been dead for 10 to 20 years. Figure 4. Composite tree-ring density curves for eastern white cedar (Thuja occidentalis) from Falcon Lake and East Braintree. 29 �In other areas of southeastern Manitoba, which marks the western limit of cedars, trees growing around undisturbed wetlands can live up to 350 years and possibly longer as is the case around East Braintree. These long-lived cedar trees can potentially provide records of changes in environmental conditions since the mid-17th century and may greatly improve our understanding of the natural variability of climate, forest fire frequency and insect infestations in this region (Figure 4). STOP 4 - SAPPING CHANNELS (UPPER CAMPBELL BEACH) The Upper Campbell beach is Lake Agassiz's most prominent strandline and defines the Sandilands Moraine as an island about 9 500 years BP (Figure 1). This phase of Lake Agassiz relates to the final drainage of the lake as progressively lower eastern outlet were opened by glacial retreat until the lake finally completely drained into Hudson Bay. The lunch spot is located on the back or landward side of the Upper Campbell beach, on the shore of a small lake. This small lake is located at the down slope end of what is believed to be a sapping channel. The head of this channel is 5 km to the southeast (Figure 5). This site clearly indicates that the sapping channel is truncated by the Upper Campbell beach, making it older than the beach. k Peat r%%Upper Cajnpbell WavecuffScarp 'I C SaLing els '1 ciofluvial Norcross Sand . Navecut $carp 5km Figure 5. Surficial geological map draped on a Digital Elevation Model of a portion of southeastern Manitoba. Stop 4 is located at the lower end of the sapping channels. The sapping channels are typically infilled with peat. Conical depressions are found above and below the Norcross wavecut scarp. Sapping channels have only been recognized in the Sandilands area, and are only found above the Upper Campbell beach, in areas where silty sands and fine sands are the predominant 30 �sediments. During times of high lake levels in Lake Agassiz, prior to the formation of the Upper Campbell beach, a high water table would have existed in the Sandilands. With the drop in lake level to the Upper Campbell beach, the water table would have dropped accordingly. This reequilibration of the water table would have taken place rapidly, perhaps in a decade or less, resulting in high gradients in the local hydrogeologic system. These high gradients made it possible to mobilize silt and transport it from the hydrogeologic system. The surficial expressions of this transport are the sapping channels, created where groundwater discharge occurred. Numerous conical depressions occur in the Sandilands that are also likely the result of groundwater movement and are believed to be contemporaneous with the sapping channels. These conical depressions occur almost exclusively in silty sands, above and below the Norcross escarpment, 5 km to the southeast. Although the term piping is commonly used to refer to flushing of sediment from beneath a dam in civil engineering terms, piping can occur in natural settings when there is upward movement of groundwater under high gradients in silts and silty sands (Higgins, 1982). The high gradients enable silt to be removed from the sediment matrix and depressions were formed as the remaining sediment collapses under the influence of gravity. Following a drop in the level of Lake Agassiz, groundwater flowing from higher elevations to lower elevations may have become semi-confined as overlying sediments became finer. The resulting upward gradient combined with presence of overlying silty sands suggests that piping is a viable mechanism for forming these conical depressions. STOP 5 - UPPER CAMPBELL BEACH OF LAKE AGASSIZ During the Emerson phase of Lake Agassiz the Sandilands Moraine was subjected to a tremendous amount of shoreline erosion. The prevailing winds were from the northwest and Sandilands was an island in the southeast part of the Lake Agassiz basin. As a consequence, the Sandilands Moraine was subjected to waves with a fetch in access of 300 km. Evidence of this erosion are two 20 m high wave-cut scarps, the Upper Campbell and Norcross scarps, on the northwest flank of the moraine. Large well-developed spits are situated on both ends of these scarps. The largest of these spits is found south of the lower of the two scarps, the Upper Campbell scarp. Figure 6. Ground penetrating radar profile across the Upper Campbell spit. This west to east profile is 150 metres wide with 30 metre thick foreset beds and several metre thick topsets. There is fine-grained glaciolacustrine sediment at the base of the foresets. 31 �This stop is located on the crest of the southern spit. The spit is approximately 15 k long, 8 k wide and 30 m thick and composed predominantly of sand with minor amounts of gravel. Ground penetrating radar surveys carried out by the Geological Survey of Canada and the Manitoba Geological Survey show the structure of the spit to be large foreset beds which prograde southward in the core and westward on the west flank (Figure 6). Topset beds are several metres thick. STOP 6 - INTERGLACIAL SITE AT GRUNTHAL Although the exposure at the Grunthal pit is poor, it is interesting because musk ox (Ovibos moschatus), extinct bison, (possibly Bison antiquus) and wooly mammoth (Mammuthus primigenius) (Figure 7) bones have been dredged from below the water table. Wood and a variety of organic-rich, fine-textured silt and silty-clay sediments have also been recovered during gravel extraction, but the stratigraphy of the site is speculative. Figure 7. Lower M1 molar of Mammuthus primigenius (V2554) from the Grunthal pit. (A) Lateral aspect. (B) Occlusal aspect. The sediment above the water table and the sand and gravel extracted by the dredge is interpreted to be late Wisconsinan, ice-proximal, glaciofluvial sediment deposited 11-12 000 years ago, when the last glacier ice to affect the area was waning. These sand and gravel deposits are in part capped by diamicton that may have been deposited as debris flows in a proximal glaciofluvial environment. 32 �Little is known about the sediments underlying the late Pleistocene sediments below the water table. The various bones, wood and the organic-rich sediments dredged from the bottom of the pit suggest the underlying deposits are of mid-Wisconsinan interstadial or possibly Sangamonian Interglacial age. Radiocarbon dating of a wood sample gave a finite age of 44 020 ± 1 030 years BP (GX-27643) suggesting an interstadial age, although dates in this range are close to the limit of the radiocarbon technique and must be accepted with some trepidation. The presence of bones of Mammuthus primigenius and Bison antiquus strongly suggest an interglacial age. In addition, a lophar index of 9 (number of lophs per 100 mm of mesiodistal crown length) of an M1 mammoth molar from the deposit (Graham Young and Ed Dobrzanski, per com 2002) is similar to an M1 or M2 molar of Sangamonian age from Bird, Manitoba, (Nielsen et al. 1988) further suggesting an interglacial age for the deposit. Two samples of organic-rich mud, dredged from below the water table, were submitted to Paleotec Services in Ottawa for macrofossil analysis. The plant macrofossil evidence suggests a forested environment dominated by spruce trees and the presence of sedges, buckbean and mosses further indicates the area was poorly drained. The presence of bark beetles (Scolytidae) agrees with the plant fossil evidence of a forested environment. The insect fossils, specifically rove beetles are in agreement with the plant macrofossil data suggesting a stream or slowly moving water in a poorly drained area possibly a pond or wet depression. The absence of aquatic submergent plants and other typical aquatic faunal elements suggest the pond was temporary rather than permanent. The water-worn bones in association with finer textured organic-rich silt and silty clay suggest the deposit may be in part a point bar. Alternatively, the bones became abraded when they were incorporated into the overlying glaciofluvial sediments. Interpretation of the floral and faunal macrofossil assemblage indicates the climate at the time of deposition was probably warm. This conclusion is based on the abundant macrofossil remains of spruce along with bark beetles suggesting the climate was at least warm enough for the growth of boreal forest. However, the absence of fossil evidence of other boreal taxa, specifically pine and deciduous trees such as birches and alders is puzzling. The forest, being composed of only spruce, resembles the boreal forests in northern regions. This, along with the presence of fossil rove beetles (Eucnecosum) which have distributions restricted to northern boreal, arctic, or alpine areas suggests the climate may have been cooler than today. The macrofossil evidence and the presence of M. primigenius, Bison antiquus and Ovibos moschatus at the site are taken to indicate a northern or boreal steppe or steppe-tundra environment. All the taxa from the site except the Pleistocene megafauna can be found living in southern Manitoba today, with the rove beetle being at its southern limit. It is therefore concluded from the macrofossil evidence that the deposit at Grunthal is probably of interglacial age, and it is tentatively assigned to the Sangamonian. Point of interest If time permits a small detour will be made to the Dawson Trail, which was the first ‘road’ linking Fort Garry to eastern Canada. 33 �Simon James Dawson, an engineer and land surveyor, was given the task in 1857 of surveying the country between Lake Superior and the Red River valley in Manitoba. Dawson subsequently proposed a route from Port Arthur’s Landing, which later became Port Arthur and then Thunder Bay, that would use waterways and roads, to prepare the way for the railroad and thereby forestall northward expansion by aggressive American interests. Figure 8. Map showing the Dawson Trail between Port Arthur’s Landing (Thunder Bay) and Fort Garry (Winnipeg). Construction of the Dawson Trail in Manitoba was started in 1868 under the direction of John Allan Snow, as a make-work project after several years of repeated crop failure in the Red River valley, but was then hastened because of potential trouble with the métis. The 1200 man army of Colonel Garnet Joseph Wolseley, which was sent west from Upper Canada in 1870 to quell the métis uprising led by Louis Riel, was in part employed to help finish the construction of the Dawson Trail (Figure 8). The army worked on the road to the point where it was passable and arrived in the Red River settlement in August of 1870. Interestingly the army traveled from Fort Francis to Fort Garry via Lake of the Woods and the Winnipeg River and did not use the Dawson Trail from the Northwest Angle, and across Sandilands. The trip from Port Arthur’s Landing to 34 �Fort Garry lasted approximately one month and was made by approximately 1600 travelers in 1873. With the completion of the Canadian pacific Railroad in 1885, the Dawson Trail was quickly forgotten after having being used for only a few years and never really being finished as Dawson originally envisioned it. However, much of the trail is still in use either as bush roads or snowmobile trails. Parts of the Trans-Canada Highway between Richer and Winnipeg also follow the original road. The road is especially well preserved in sections of Sandilands where in some boggy areas the original corduroy can still be found. ACKNOWLEDGEMENTS We would like to thank Graham Young and Ed Dobrzanski of the Manitoba Museum of Man and Nature for their analysis of the vertebrate bones from the Grunthal pit and their help in making the Sangamonian age assignment of the deposit. We would also like to thank David Riddle of Manitoba Historic Resources Branch for supplying the Dawson Trail map. Grant Ferguson of the Department of Engineering, University of Manitoba wrote the very eloquent description of the formation of the sapping channels for us. REFERENCES Higgins, Charles G. 1982. Piping and sapping: development of landforms by groundwater outflow. pp. 18-59. In: Groundwater as a Geomorphic Agent. R.G. LaFleur (ed.) 1982. Allen and Unwin, Inc. London, U.K. Kerr, D.G.G. 1975. Historical Atlas of Canada. Third revised edition. Thomas Nelson & Sons (Canada) Ltd. McAndrews, J.H. 1982. Holocene environment of a fossil bison from Kenora, Ontario. Ontario Archaeology, vol. 37, pp.41-51. Nielsen E., Churcher, C.S., and Lammers, G.E. 1988. A wolly mammoth (Proboscidea, Mammuthus primigenius) molar from the Hudson Bay Lowland of Manitoba. Canadian Journal of Earth Sciences, vol. 25, pp. 933-938. Thorleifson H. 1996. Review of Lake Agassiz History. In, Teller, J.T., Thorleifson, L.H., Matile, G. and Brisbin, W.C. eds. Sedimentology, geomorphology and history of the central Lake Agassiz basin. Geological Association of Canada Field Trip B2, 101p. 35 �36 �Field Trip 3 Structure and Sedimentology of the Seine Conglomerate, Mine Centre Area, Ontario Dyanna Czeck Department of Geology Oberlin College 52 W. Lorain Street Oberlin, OH 44074 Philip Fralick Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 Moderately deformed Seine conglomerate containing metavolcanic and granitoid clasts, with clast tiling due to dextral deformation. Hwy 11, 1 km east of Horsecollar Junction. �FOREWORD This trip will examine sites related to the development and deformational history of a synorogenic sedimentary unit, the Seine Conglomerate. The unit extends across the Canada/ United States border in the Rainy Lake region, an area that has sparked interest and controversy for American and Canadian geologists for over a century. The Seine is of significant interest because it preserves important structural and sedimentological clues that may lead us to a better understanding of the tectonic history of the Archean Wabigoon-Quetico subprovincial boundary. REGIONAL SETTING Introduction and Tectonic Setting The central portion of the Superior province is characterized by alternating subprovinces of metavolcanic-plutonic and metasedimentary natures (Fig. 1). Figure 1. The Superior Province. From Card and Ciesielski (1986). One popular tectonic interpretation for the central portion of the Superior Province is of repeated island arc, microcontinent collisions. The collisions are evidenced by rocks that can be interpreted as arc sequence subprovinces (metavolcanics) and their corresponding accretionary prism subprovinces (metasediments) (Langford and Morin, 1976; Hoffman, 1989; Percival and 38 �39 Figure 2. Simplified Geologic Map of Mine Centre area showing the extent of the Seine. Geology compiled from Wood, 1980a & b, Stone, 1998a & b, and Czeck, 2001. �Williams, 1989; Card, 1990; Hoffman, 1990). In general, the ages in the greenstone belts are similar along strike, but differ systematically across strike (Hoffman, 1989). This is consistent with the island-arc accretion model. A history of southward accretion has been proposed to explain the juxtaposition of Superior Province terranes (Langford and Morin, 1976; Percival and Williams, 1989; Card, 1990). The Seine Conglomerate, located in the Rainy Lake region of the western Superior Province, was deposited along the boundary between the Wabigoon metavolcanic/plutonic subprovince and the Quetico metasedimentary subprovince (Fig. 2). The structural observations along the Wabigoon–Quetico boundary are consistent with an oblique island-arc microplate collision circa 2.7 Ga. In this part of the Superior Province, it seems likely that first the Quetico acted as a subduction prism during accretion of the Wawa to Wabigoon. Then, it was effectively shifted from the subduction prism setting to a back-arc setting, as subduction shifted (Percival and Williams, 1989). In the Rainy Lake region, a series of lithostratigraphic terranes were assembled together along structurally controlled, stratigraphically discordant boundaries during the collisions. The boundary between the Wabigoon and Quetico Subprovinces in this region is divided into three primary blocks by dextral wrench faults (Poulsen, 1986). Each of the small terranes and subterranes may have undergone a somewhat unique history of formation and deformation. The Quetico Fault forms the northern boundary, separating the granite-greenstone terrain of Wabigoon Subprovince from the Coutchiching Group argillites and the Seine Conglomerate. The Seine River-Rainy Lake Fault forms the southern boundary of these sedimentary sequences with the Quetico turbiditic metasediments to the south. The wedge-shaped area lying between the two major fault systems is itself dissected by splays off the major east-west faults, which isolate the lithic units, destroying stratigraphic integrity. This problem has resulted in historical speculation on the lateral equivalency of the Seine and Coutchiching sediments (Merritt, 1934) and the Coutchiching and Quetico (Lawson, 1913). Geochronology Davis et al (1989) used U-Pb, single zircon geochronology to bracket the ages of the Coutchiching and Seine between 2704+-3 to 2692+-2 and 2696+5-3 to 2686+2-1 respectively. Even though an overlap in age existed, Davis et al (1989) believed that structural considerations indicated that the Coutchiching is slightly older than the Seine. Further detrital zircon geochronology of the Seine was conducted by Davis and reported in Fralick and Davis (1999). Of the two samples analysed one was from the sandstone dominated lithofacies (collected near the Seine River bridge) and the other was from the conglomerate dominated lithofacies (collected near Horsecollar Junction on Highway 17). The detrital zircons in both samples give very consistent ages with the sandstone lithofacies clustering at 2693+-1 (Fig. 3) and the conglomerate lithofacies clustering at 2692+-1 (Fig. 4). These detrital zircon ages are similar to the Bear Pass pluton, a granitic mass which outcrops near the Seine 40 �Conglomerate (Fralick and Davis, 1999). Metamorphic titanite from the pluton gave an age of 2684+-5. Figure 3. Isochron showing U-Pb detrital zircon ages from the upper Seine Group (supplied by D. Davis). Figure 4. Isochron showing U-Pb detrital zircon ages from the lower Seine Group (supplied by D. Davis). Detrital zircons from the nearby Quetico and Coutchiching metasediments have an age distribution that is very different than that from the Seine Conglomerate. Their youngest ages are 2699+-1 for the Quetico and 2704+-3 for the Coutchiching with the population in both rock units extending back past 3000Ma. This is in sharp contrast to the Seine zircons which indicate 41 �dominance of a single source. This source may have evolved slightly through time as the age for the upper sandstone is slightly older than the lower conglomerate, though within error. This inverse age stratigraphy may reflect erosive unroofing of slightly older segments of the source igneous body. In any case, the Seine must be 2692 Ma, or younger, and its detritus was derived from a different source than the Quetico and Coutchiching (Fralick and Davis, 1999). The Bear Pass pluton is a good candidate for the source of the sediment except its zircons have lower Th/U ratios than the Seine. The Seine’s Th/U ratios of 0.74 to 1.07 are more typical of alkaline igneous rocks (Fralick and Davis, 1999). Alkaline igneous rocks 2692 Ma in age are present to the east of the area in the Shebandowan region. Metamorphic titanite in the Rice Bay Dome has given an age of 2693-+3 Ma , coeval with or predating the Seine (Davis et al,1989). The Bear Pass intrusion also predates the Seine and is late-tectonic, probably emplaced into the Rice Bay Dome after the Coutchiching turbidites had been overturned (Fralick and Davis, 1999). Thus, the Seine was deposited after an early period of folding and metamorphism. What can we learn from the Seine? The Rainy Lake region has been metamorphosed, generally to greenschist grade, and deformed in response to the Archean microplate collisions. Most of the preserved deformation is of a ductile nature, and thus occurred at significant depth. The Seine itself is metamorphosed and significantly deformed. From the geochronology and sedimentological evidence (to be described below), we know that the Seine was deposited in a dynamic convergent plate setting. From the significant flattening fabrics and structural evidence, we know that the Seine was subsequently buried and deformed at mid-crustal levels. Therefore, the Seine preserves a record of a conglomerate’s journey through the crust within a dynamic convergent zone. We can hope to interpret from the Seine a relatively late stage record of the microplate collision history at the Wabigoon- Quetico tectonic boundary through a history of syn-deformational deposition, burial, and deformation. Through analysis of the sedimentology, early structures, and late structures, we can hope to interpret various portions of the conglomerate’s path through the crust. SEDIMENTOLOGY OF THE SEINE The Seine Conglomerate was interpreted to have been deposited in a fluvial system by Wood (1980). This system undergoes a gradual transition from conglomerate dominated near its base to sandstone dominated near its top. Channels in the lower portion of the section are Scott type (Miall, 1978), with gravel and cobbles forming both longitudinal bars and interbar channels. Through cross-stratified, medium-grained sandstones, representing chute channels, commonly form small lenses in these sequences. The coarse-grained lithofacies association is transitional upwards into successions which contain thicker layers of trough cross-stratified sandstone. This represents a transition from gravelly main channels to channels dominated by the migration of sand dunes. With further fining upwards, the Formation becomes sandstone dominated. The sandstones are organized into stacked, trough cross-stratified lenses, with rarer large-scale, planar cross-stratified layers and conglomeratic horizons. This reflects development of a South Saskatchewan type braided river (Miall, 1978) with dune migration prevalent in the channels and only minor development of sandy transverse and gravelly longitudinal bars. To further clarify the 42 �relationship between the lithofacies present in the rocks and the environments in which the sediment was deposited, the remainder of this section will discuss the types of sediment deposited in the differing sub-environments of gravelly to sandy braided streams. Braided rivers are multichannel streams with large width to depth ratios that commonly develop in high slope areas, such as alluvial fans and proglacial outwash plains. This type of channel pattern is generally caused by a combination of factors which include: large diurnal and seasonal fluctuations in discharge; high slope, or a rapid increase in slope; high discharge velocities; the dominance of bedload sediment (sand and gravel) in transport; and, meagre vegetation on the floodplain. These factors result in the stream being easily able to erode its banks, spread out laterally and choke its channel with coarse sediment building midchannel islands. The channel morphology of braided rivers is characterised by a series of channels and bars which are occupied at various levels of discharge (Williams and Rust, 1969). During most of the year, with normal to low discharge, one, two or more channels will snake through the assemblage of bars. However, during peak discharge all the previously dry bars and minor channels will be overtopped and the river will develop only one large channel. Flood events, such as this, are the intervals when the coarsest sediment, generally composing the bars, but also flooring the channels in high energy systems, will move. Hammer and Smith (1983) found that bedload sediment transport increases at an exponential rate with river discharge. As the majority of coarse sediments are transported in bedload during high discharge events, with reduction in discharge the ability of the river to continue transporting this material down gradient is also reduced (Burton, 1989). At this point the largest material in transport stops moving. This produces a low velocity shadow downstream from the sedimented material where more detritus accumulates. This process leads to the formation of gravel bars, termed longitudinal bars, within the river channel. If the channel is in a relatively high slope area, the sand and finer material in transport will only be deposited if it is trapped or carried into the pores between the gravel and sedimented as matrix. Here, the main channel is pebble, cobble or boulder dominated with this detritus arranged into a stacked sequence of commonly irregular lenses. These lenses represent areas of scour and deposition on the bottom of a larger channel. Some gravel lenses may be formed by either migration of coarse-grained dunes or lateral fill into scour pits producing cross-stratification. In less energetic systems trough cross-stratified sand lenses will interbed with the gravel. This represents sand in bedload tractive transport as dunes being deposited and not re-eroded. Further reduction in energy levels of the system will eventually cause the main channel deposits to become a series of stacked, trough cross-stratified sandstone lenses. In braided streams carrying gravel, longitudinal bars will develop, splitting the flow at low stage (Figs. 5 & 6). These bars are lozenge-shaped mounds of pebbles, cobbles or boulders in clast support with a coarse-grained matrix. They are usually internally massive and nongraded, though occasionally parallel lamination is present. While the bars are submerged during maximum discharge events they are subjected to extremely turbulent flow, during which time they are modified by both erosion and deposition. Such processes result in the head of the bar being continually reworked producing a well sorted and coarser grained deposit (Burton, 1989). A significant decrease in velocity over the length of the bar results in a corresponding decrease in the size of material being deposited. 43 �Figure 5. Photograph of the North Saskatchewan River in Alberta. The main channel (A) is cutting around a longitudinal bar which grades from a coarse head (B) to a finer pebble tail with a veneer of darker coloured sand (C). The bar tail in the foreground also has darker sandy areas mantling it (C) and is cut by chute channels (D); one of which is building a chute delta (E). If main channel erosion was not occurring at F this would be the site of a bar edge sand wedge supplied by the nonconfined overbar flow. During waning flow, the bar tail will be shielded from the main current in the river as the bar top, in the center of the bar, begins to become emergent. This may cause a thin sand sheet, or patches of sand in lower areas, to be deposited over the bar tail. As the flow stage continues to drop small channels will sporadically develop cutting across the upper surface of the bar, from a main channel upstream to another main channel downstream. These are chute channels which commonly fill with trough cross-stratified sand produced by dunes migrating down the channels. They form sand lenses in the gravelly longitudinal bar. Where chute channels rejoin the main channel sediment may accumulate as a large avalanche face building out into the deeper main channel (chute delta). This produces a large-scale, planar cross-stratified sand or gravel deposit banked up against the side of the longitudinal bar. Similar, but laterally extensive, deposits can also be formed by unconfined sheet flow over the only slightly submerged surface of the bar. Again, where this flow enters the deeper main channel a large-scale, planar cross-stratified wedge of sediment will be banked up against the bar. This is called a bar edge sand wedge (Figs. 5 & 6). All of the above may be present in sand dominated braided systems, but the longitudinal bars will be subordinate and may be absent. The main bar forms in these systems are transverse bars. These are large sand waves, features similar to continuous crested, long wavelength dunes. 44 �Figure 6. Block diagram of a coarse-grained braided stream schematically showing: coarse bar head (A); finer bar trail with thin sand patches which accumulate in areas shielded from the current during waning flow (B); chute channel with dunes (C); chute delta (D); bar edge sand wedge (E); and main channel with dunes (F). Gravelly braided stream deposits similar to these dominate the lower Seine Conglomerate. Figure 7. Block diagram of a sand dominated braided stream schematically showing: the main channel with dune migration producing trough cross-stratification (A); transverse bar migration and stacking producing sandflats which are internally planar cross-stratified (B); minor (to no) development of gravelly longitudinal bars (C). Sandy braided stream deposits, and especially the main channel facies, dominate the upper Seine Conglomerate. Where they are abundant they will pile up next to one another clogging the channel with sand and producing large areas of sand flats (Smith, 1970; Miall, 1985) (Fig. 7). Internally, they are composed of large-scale (commonly>1m thick sets), planar cross-stratification. Transverse bars are quite laterally continuous and are interbedded with other large-scale, planar cross-stratified sand units, in sand flats (downstream accretion macroforms of Miall, 1988), or successions of smaller lenses of trough cross-stratified sand, representing dune migration on the channel floor 45 �(Fig. 7). The transverse bars are mostly active during higher discharge. During low discharge, their tops may become emergent and eroded, and dunes mantle their surface. The next flood event will often result in the bar building in a somewhat different direction. This will result in an apparent change in the angle of the cross-stratification (a reactivation surface). This sloping surface may also show evidence of erosion and contain small lenses of trough crossstratification. This completes the general overview of sediment deposits associated with channel sequences in braided streams. Floodplain lithofacies are usually minor to nonexistent in these successions as the fine grained deposits have little preservation potential. Braided streams commonly avulse, changing the position of their channel and combing the floodplain, eroding the fine-grained deposits and forming stacked channel sequences. The Seine Conglomerate contains lithofacies corresponding to all of the channel subenvironments discussed above. Conglomerate dominated longitudinal bar-channel sequences are the most common in outcrop. However, sandier channel sequences are not rare and gain importance higher in the Formation. STRUCTURAL GEOLOGY Deformed primary fabrics Bedding is often difficult to discern in the Seine. Where it can be identified, it is displayed by variations in grain sizes, often indicated by fine-grained layers interbedded with pebble conglomerates. In some cases, cross bedding can be seen within larger sandy layers. In general, bedding strikes approximately east – west, and is subvertical (Fig. 8). This orientation is similar to bedding attitudes measured throughout much of the Superior Province (Poulsen, 1986; Hudleston et al., 1988; Bauer and Bidwell, 1990; Tabor and Hudleston, 1991; Bauer et al., 1992; Jirsa et al., 1992; Bauer and Hudleston, 1995; Hudleston and Bauer, 1995). Bedding is generally subvertical and subparallel regardless of lithology. However, this fact does not necessarily indicate deformation of a continuous stratigraphic sequence. Based on opposing stratigraphic facing in adjacent rocks, an unconformity between the volcanic units and the base of the Seine conglomerate can be identified (Stop 1) (Lawson, 1913; Poulsen et al., 1980; Poulsen, 2000). There are some exceptions to the general EW, vertical bedding orientation. In the area along Shoal Lake (Stop 1), the bedding strikes more NE/ SW with a shallower (~65°) southeasterly dip. The orientations of the bedding within the Shoal Lake area and adjacent rocks to the north and south combine to create a large, gentle S structure, with shallower dips on the middle section of the S. Several folds on the scale of hundreds of meters have been identified within the Seine metasediments based on stratigraphic facing (scour beds and cross-beds) and lithologic similarity (Hsu, 1971; Wood et al., 1980a; Wood et al., 1980b; Poulsen, 2000). These folds have vertical limbs and are typically upright and isoclinal. The trends of the hinges are roughly parallel to the foliation (EW). The plunges of the hinges are unknown, and cannot be constructed due to the subparallelism of the limbs. In a few locations (see Stop 5), one can see small scale folds that may display horizontal bedding at the hinges. 46 �Figure 8. Equal area stereonets showing structural fabric data near Mine Centre, Ontario. A) Poles to bedding. 46 measurements. B) Poles to foliation. 142 measurements. C) Mineral lineations. 123 measurements. D) Intersections between foliation (cleavage) and bedding. 44 measurements. From Czeck (2001). Ductile deformation Ductile deformation (as evidenced by rocks with pronounced foliations and mineral lineations) is pervasive throughout the entire Wabigoon–Quetico boundary zone. The overall dominance of the foliation over the lineation creates an S-L type fabric. However, strain is also localized into an anastomosing network of more discrete shear zones, including two main zones of localized shear and displacement. These are the Seine River - Rainy Lake Fault and the Quetico Fault, which diverge to the west and merge to the east (Fig. 2). An anastomosing pattern of smaller shear zones links the major shear zones shown in anastomosing, gentle S-like shapes (Fig. 9). The locations of these smaller shear zones have been determined by linear features observed on electromagnetic anomaly maps or are identified as the discordant boundaries of apparently independent lithostratigraphic terranes (Poulsen, 1986; Poulsen, 2000). Direct observation of these smaller shear zones is difficult because they are typically under water or buried beneath recent sedimentary deposits and not exposed, presumably because they are highly erodable. The presence of discrete shear zones implies some strain partitioning between shortening and wrench components of ductile deformation along the Wabigoon–Quetico boundary. It is probable that the discrete shear zones are zones with relatively high wrench influence. Conversely, it is probable that the wide zones of deformation between the shear zones have undergone deformation with a stronger shortening influence. In some instances, small (on the order of a few meters), secondary shear zones may be seen. 47 �Figure 9. Schematic diagram illustrating structural features of Rainy Lake Wrench Zone. Short solid arrows identify downward facing units. From Poulsen (1986). Foliation is moderately to well developed in all rocks of the region, with the exception of some late stage plutons. It is especially well developed along the major shear zones and the Seine River – Rainy Lake and Quetico Faults. The foliations are largely subvertical and at a low angle both to bedding (except in the hinge of folds) and to the subprovince boundary (Fig. 8). An exception to the subvertical foliation is in the area along Shoal Lake (Stop 1). Here, like the bedding, the foliation strikes more NE/ SW with a shallower dip (~65°). Like the bedding, the combined orientations of the foliations within this Shoal Lake area and adjacent rocks to the north and south create a large, gentle S structure, with shallower dips on the middle section of the S. Unlike the bedding, the cleavage is not folded by the major upright folds. It is, however, affected locally by crenulations. In general, there is no consistently oriented intersection lineation between bedding and foliation throughout the Seine. Instead, this lineation defines a great circle corresponding roughly to the planes of foliation and bedding. This is to be expected in the situation in which bedding and foliation are subparallel because slight variations in orientation of the two planar features will have a significant effect on the orientation of the intersection lineation. Typically, chlorite or amphibole forms a mineral lineation, which varies in intensity from weak to strong depending on location. The lineation plunge is highly variable across the Wabigoon– Quetico boundary without any clear systematic change from east to west or from north to south, although there are local domains of similar lineation plunge (Czeck, 2001). The highest concentrations of lineation orientations plunge steeply to the east, and their mean orientation is 66°/076. However, there are also significant numbers of westward and shallowly plunging lineations. The range of lineation orientations is great enough that the “average lineation” may have little geologic meaning. 48 �The lineation referred to thus far is the mineral lineation, a lineation due to the preferred alignment of mineral grains or clusters of grains. This lineation is present in most rock types along the Wabigoon–Quetico boundary. Within the Seine conglomerate, there are, in fact, two distinct linear elements that can be measured independently: the mineral lineation and that defined by the long axes of the conglomerate clasts. Both can be considered penetrative features of the rock fabric. The long axes of clasts within the conglomerate are generally coincident with the mineral lineation as would be expected if the conglomerate clasts were an accurate recorder of strain and the mineral lineation reflects the stretching direction Relatively late-stage sinistral and dextral crenulations locally affect the cleavage. These crenulations are fairly small (usually cm scale). They are most abundant in the most highly sheared rocks. This correlation and the relative timing of the crenulations makes it seem likely that they formed during the latest stage of a continuing saga of transpression. Features of deformed conglomerates The Seine Group provides an excellent opportunity to observe the effects of competency contrasts on deformation. These natural competency contrasts allow us to obtain structural information that is unavailable in more homogeneous rocks, making the conglomerates are excellent tools for structural analysis and tectonic interpretations. Within the conglomerates, asymmetric shear sense indicators are prevalent. In general, these are either in the form of asymmetric pressure shadows at the ends of clasts, wrapped foliation indicating rotation of the most rigid clasts, and clast tiling. All of these shear-sense indicators are most evident on the subhorizontal plane, regardless of lineation orientation. They indicate dextral sense of shear. In general, the conglomerate clasts have been strongly flattened, although the degree of flattening is strongly dependent on lithology. The intensity of flattening strain varies greatly through the field area. We will be viewing several degrees of deformation on the various stops of the fieldtrip. TECTONIC STORY OF THE SEINE METASEDIMENTARY SEQUENCE AND SURROUNDING ROCKS Plate Collisions and Sediment Deposition Comparing the zircon populations of sedimentary units in the Rainy River area with other rock groups in the region generates some interesting trends. The zircon populations of turbidites and conglomerates on the northern margin of Wabigoon Subprovince ( Savant Group and Ament Bay Formation) exhibit similarities with the Coutchiching metasediments (Davis, 1997). The conglomeratic units near the northern margin of Wabigoon Subprovince represent braided fluvial systems (Turner and Walker, 1973; Devaney, 1999). The sediments they were transporting were deposited in the same time bracket (Davis et al, 1988) as some of the sedimentary units near Rainy Lake. If the sedimentary units in the Rainy Lake area represent detritus shed off of upraised blocks during collision-orogeny, as appears to be the case (Davis et al, 1989), 49 �Figure 10. Interpretive sketches showing subduction and plate convergence along the southern margin of Wabigoon Subprovince. The accretionary prism built at 2700 Ma of trench turbidites is represented by the Quetico. Northward subduction in the Schreiber and Shebandowan areas ceased at approximately 2720 Ma, but restarted in the Shebandowan area at 2692 as immanent collision ceased subduction in the Quetico trench. After collision of the Wawa-Abitibi terrain with the Quetico-Wabigoon assemblage, orogenic uplift affected the area. During this interval small basins opened on both sides of the suture zone and accumulated coarse fluvial deposits including the Seine Conglomerate. understanding the sequencing of sedimentary pulses is key to deciphering the tectonic forces which formed the Seine Basin and uplifted its source area. To understand this sequencing, it is necessary to outline the tectonic history of the region, and the sedimentary response to tectonism, from 2720 Ma to 2685 Ma. Sedimentary sequences deposited between 2720 and 2685 Ma in, and adjacent to, Wabigoon Subprovince record the final phases of subduction and collision of this area with landmasses to the north and south. Examined basin fill sequences are divisible into three depositional systems tracts. Sediments in the Beardmore-Geraldton area record progradation of braided streams and fan/braid delta complexes from a volcanically active area to the north (Devaney and Fralick, 1985; Devaney, 1987; Devaney and Williams, 1989). The outbuilding sequence fed detritus to a poorly structured, turbidite ramp/fan assemblage in the forearc basin (Barrett and Fralick, 1989), from which it was rerouted into the Quetico trench, via multiple channel systems (Fralick et al., 1992) (Fig. 10). Sediment geochemistry confirms that the calc-alkaline volcanic rocks present in the Onaman-Tashota area, to the immediate north of the forearc basin, were the source of the sediment (Fralick and Kronberg, 1997). Lower Zr and Y values in sandstones from this area compared to analyses of rocks from the western trench (data from Sawyer, 1986) indicate less involvement of older felsic crust as a sediment source. Zircon geochronology (data from D. Davis) demonstrates that the northern Quetico trench received sediment between approximately 50 �2705-2699 Ma, whereas the forearc basic continued to accumulate sediment until at least 2696 Ma. No zircons older that 2828 were found in these sequences. In contrast the zircon population of samples from the western Quetico (Davis et al., 1990) (Fig. 10) contains both 2900 and 3000 Ma zircons, indicating the erosion of older tonalites in this area. A sedimentary assemblage present to the east of Terrace Bay, in Wawa Subprovince, contains a poorly structured turbidite succession which correlates as the distal equivalent of the Quetico trench deposits (Purdon, 1995) (Fig. 10). The geochemistry of these rocks is very similar to the trench and forearc assemblages to their north, with lower Zr and Y than the trench sandstones to the west. Their geochemistry does not match local sources in the Hemlo and Winston Lake areas. Detrital zircon geochronology matches the Quetico trench sediments to the north, with the exceptions that the main zircon population, which probably reflects age of deposition, is 3 Ma younger, and the sandstones contain a 2900 Ma population. Similar turbidites are present in two other areas of the northern Wawa Subprovince; Shebandowan and Manitouwadge. At the former they are younger than 2700 Ma (F. Corfu, pers. comm.), and at the latter they are younger than 2692 Ma (E. Zaleski, pers. comm.). In the Shebandowan belt, the turbidites are succeeded by a 2692 Ma (Corfu and Stott, 1986), high-Na volcanic assemblage interlayered with near-shore, moderate-to high-slope marine deposits. These are in turn succeeded by <2686 Ma (Corfu and Stott, 1998) braided stream conglomerates eroding crystalline basement. This depositional system tract records 2709-2693 calc-alkaline arc volcanism on the southern margin of Wabigoon Subprovince and its erosion and transport to the Beardmore-Geraldton forearc basin and Quetico trench at approximately 2700 Ma. A trench-full state developed at approximately 2696 Ma and the sediment apron expanded to the south covering tholeiitic basalts and a starved clastic-chemical sequence in the area east of Terrace Bay. Similar sequences in Shebandowan and Manitouwadge probably represent overflow in these areas as well. Diachronous overflow younging to the east suggests oblique closure of the arcs comprising northern Wawa Subprovince with the Quetico trench. Deep-marine sediments and volcanic assemblages in the Shebandowan area were upraised at 2692 Ma to surface levels, while deepmarine sedimentation continued in the Manitouwadge area, further indicating west-side-first scissor closure. The second tract encompasses the English River Subprovince and Warclub Group, on the northern margin of Wabigoon Subprovince. The depositional systems which formed these two units were very similar. They are both primarily composed of unstructured medial to distal turbidite assemblages. The Warclub Group thickens and coarsens upward from a 10m thick basal zone composed of a starved slate-chert assemblage that overlies mafic volcanics. Near its top, minor interbedding with ashes of the Berry River Volcanics occurs. It is sharply overlain by the volcanic unit, which is mostly composed of grainflows of felsic volcanic detritus. The Warclub Group is laterally continuous to the east of Dryden, but does not lithically correlate with sediments in the Minnitaki Lake area. Zircon geochronological patterns (Davis, 1995) for the Warclub Group and English River sediments exhibit a variety of ages. This is in contrast to patterns for sediments on the southern margin of Wabigoon Subprovince, which show a clustering of young ages, and indicates that syndepositional volcanism was not an important sediment source. The youngest detrital zircon age determination for the Warclub Group is 2716 Ma (Davis, 1995), which is in agreement with its stratigraphic position below the 2712 Ma Berry 51 �River Volcanics (Davis, 1995). Volcanics interbedded with the Warclub Group near Vermilion Bay give an age of 2716 Ma (Davis, 1995). The youngest detrital zircon present in the English River assemblage is 2705 Ma (Davis, 1995). Turbidites of the English River Subprovince and the Warclub Group accumulated in a deep water setting; in the case of the latter, accumulation occurred directly on a mafic assemblage. The turbidites fed from the erosion of local rocks which were upraised, probably tectonically. Deposition of the Warclub Group ceased at 2712 Ma when a felsic volcanic episode effected its basin. English River sediments continued to accumulate until at least 2705 Ma (Davis, 1995). The Warclub Group was most likely deposited in a remnant ocean basin between the Wabigoon and Winnipeg River landmasses. The English River sediments may represent either a remnant ocean basin or a classical trench. The third depositional systems tract includes the Abram, Minnitaki, Savant, Sturgeon and Conglomerate Lake Groups, and possibly the Crowduck and White Partridge Bay Groups. These sedimentary sequences represent high-slope, fan delta deposits fed into an east-west linear trough which developed south of the north margin of Wabigoon Subprovince. Basal units are dominated by erosion products from the immediately adjacent, underlying lithologies. There is a rapid upward increase in the amount of felsic volcanic detritus, with some sequences almost entirely composed of this material. Reworked sedimentary clasts, representing all fan delta lithofacies, are important constituents of some sequences. Ages of youngest zircons are variable, ranging from post 2707 Ma (Stott and Davis, 1999) for Conglomerate Lake (probably depositional age) to 2699 Ma for Crowduck Lake (D. Davis, pers. comm.). Zircon populations are varied, indicating erosion of older units rather than penecontemporaneous volcanism. The basin system represents proximal foreland basin deposits which were overridden by thrust sheets. Multiple periods of thrusting are indicated by cannibalism, and variation in youngest zircon ages. The Conglomerate Lake assemblage was deposited between 2703 and 2709 Ma; the basal Savant Group at 2704 Ma (Davis, 1995). These ages are similar to ages for cessation of sedimentation in the English River assemblage, and indicate foreland thrusting may be linked to closure of the English River oceanic system. The three depositional systems tracts are interrelated due to the controlling tectonic processes. Closure of the Warclub remnant ocean initiated development of a foreland thrust belt on the northern margin of Wabigoon Subprovince. During the same period, extensive calc-alkaline volcanism commenced on the southern margin of the subprovince, with commencement of north directed subduction, and fed a systems tract which delivered sediment to two other subprovinces. Probably oblique closure of Wawa arc systems terminated the southern depositional system at 2692 Ma in the west, and resulted in upraising of deep-water environments to shallow depths. By 2686 Ma oceanic deposits, which had formed only 14Ma previously, were being eroded by streams draining the Wawa-Quetico-Wabigoon collision zone. The depositional environment of the Coutchiching turbidites is consistent with a source-distal, off fan or braid delta setting. Its sedimentology is also consistent with a distal Quetico ramp setting but its position on the northern, source-proximal basin margin makes this scenario unlikely. The Coutchiching most likely was deposited as a submarine apron to the south of fandeltas fed by thrust-faulting on the northern margin of the Wabigoon subprovince. The age 52 �distribution of zircons from the Coutchiching Group (Davis et al., 1989) is similar to that of coarse-grained metasedimentary sequences present on the northern boundary of the Wabigoon subprovince, all of which have youngest detrital zircons of 2704 Ma, a few m.y. earlier than the Quetico (Davis 1995, Davis 1990). None of these metasediments show the concentration of ages less than 2710 Ma that is characteristic of the Quetico metasediments, suggesting that they are slightly earlier. The Seine Conglomerate represents the youngest pulse of sedimentation in the evolving collision zone between the Wawa-Abitibi oceanic volcanics, the Quetico trench sediments and the Wabigoon craton. The west side first, scissor closure of the Wawa-Abitibi terrain with the Wabigoon resulted in compression and metamorphism in the Rainy Lake area at 2692 Ma while the Shebandowan area 200 km to the east was just starting to be uplifted and the Manitouwadge area 500km to the east still had an active trench system and subducting ocean floor. Scissor closures such as this denote oblique collision or collision of a promontory. In either circumstance, transpression, or partitioning of oblique deformation into boundary-parallel (wrench) and boundary-perpendicular components (folding or thrust faulting) is likely to result (e. g. Harland, 1971). As blocks slide past one another in wrench settings, strike-slip basins can form as dilation zones open at fault curves, terminations with stepovers, or extensional duplexes. Small rifts can also open due to lateral terrain escape from the compression zone. There are several late-stage conglomerates similar to the Seine in the Superior Province, many bearing a striking resemblance to one another. They are known as Timiskaming type conglomerates (e. g. Pettijohn, 1943). Based on sedimentological evidence, including the large clast size, relative rarity of cross-bedding in quartzites, and the predominance of relatively immature graywackes interbedded with the conglomerates, Timiskaming type conglomerates have long been recognized as forming in a dynamic, tectonic environment (Pettijohn, 1943). Specifically, researchers have concluded that the conglomerates may have formed synkinematically, possibly in wrench related basins (Poulsen, 1986; Poulsen, 2000). This conclusion is based on the fact that, like the Seine Group conglomerates, other Timiskaming type conglomerates are often found along major wrench zones within the Superior Province. Detailed provenance studies of Timiskaming-type conglomerates have supported the wrench basin interpretation, in that source areas have been located both north and south of the deposition area (Legault and Hattori, 1994). Thus, due to the scissor-closure interpreted from the Seine clasts’ provenance and the association of the Seine and other Timiskaming type conglomerates with major wrench zones, it may be appropriate to compare the early structures and stratigraphy in the Seine group to those of more modern strike-slip basins. The aerially limited basin into which the Seine was deposited most likely opened due to some wrench-related process, while the general compression in the area upthrust a possibly yoked source terrain. Plate collisions and Deformation In general, the rocks at this boundary show a history of upper crustal stacking and wrenching followed by ductile transpression. The stacking is evidenced by significant amounts of upright bedding and early folds. As deformation continued and rock units became buried, ductile deformation became dominant. This part of the deformation sequence created the dominant S-L 53 �fabrics and was responsible for most of the strain in the rocks. Continued ductile transpression resulted in late-stage crenulations and kinks in the foliation fabric. There is some evidence for minor, brittle structures that formed during the waning moments of deformation. Along the Wabigoon- Quetico boundary, there are two general deformational phases of collision evidenced by the structural field observations. Note that the distinction of two deformational phases is NOT meant to imply that there were two stages of collision, but only that the structural style evolved during the collisional history. The first phase can be interpreted to have included shortening or stacking of strata, frequently coinciding with boundary-parallel motion, in the upper crust. After the first phase and corresponding crustal thickening, the second phase of deformation that created the dominant structural fabric is interpreted to have involved deeper, ductile transpression. The structures preserved in the Seine and surrounding rocks are a result of both their upper-crustal and deeper level deformation. Structural Evidence of the Tectonic Nature of the Seine Basin The sedimentological evidence supports the interpretation that the Seine Group was deposited in a dynamic, tectonic environment. The structural evidence supports this conclusion as well. The Seine is the latest of supracrustal rocks to have been deposited at the Wabigoon–Quetico boundary (Poulsen et al., 1980; Davis et al., 1989; Fralick and Davis, 1999). Despite not containing as many folds as its neighboring rock units, the Seine has also undergone deformation that caused the bedding to become vertical. Even though both the Seine and its neighboring rock units are vertically oriented, the reversal in stratigraphic facing between the base of the Seine and the directly adjacent volcanic units (Lawson, 1913; Poulsen et al., 1980; Poulsen, 2000) implies that the earlier strata were tilted, at least in part, prior to Seine group deposition. Therefore, it can be interpreted that the early stacking deformation began before and endured during deposition of the Seine. Thus the Seine group was deposited in a dynamic, tectonic environment, most likely in a basin formed through strike-slip faulting processes that would be expected at an obliquely convergent margin. The specific type of strike-slip basin (pull-apart, duplex related, fault splay, …) in which the Seine was deposited remains unclear. However, the geometries of the structures within the Seine Group may provide clues as to the nature of this basin. The dominant vertical nature of the bedding implies that the bedding was probably tilted during the first, upper-crustal stage of deformation by some means (folding, faulting, or both). The few relict folds within the Seine suggest that at least part of the basin was undergoing shortening during basin evolution. This observation leads to the conclusion that the basin was most likely not a simple extensional pullapart basin (Poulsen, 1986), but rather some other type of wrench-related basin that contained significant areas undergoing shortening during its formation. If we assume that shear zones form in relatively weaker zones of rocks, the present–day location and orientations of the bounding ductile shear zones may be indicative of the earlier brittle faults. If we follow this logic, the gentle S-shapes of the bounding faults suggest a restraining bend rather than a releasing bend in a dextral strike-slip regime (Fig. 11). This type of restraining bend would likely be associated with thrusting, overturned folding, and localized areas of subsidence and sediment accumulation (Christie-Blick and Biddle, 1985). This scenario seems 54 �Figure 11. A) Bends in dextral strike-slip fault resulting in either a restraining bend and corresponding thrust faults, overturned folds, and sediment accumulation or a releasing bend and the corresponding normal faulting and pull apart basin. Based on Christie-Blick and Biddle, 1985. B) Possible basin that may have formed if Quetico Fault to the north had relatively more displacement than the Seine River - Rainy Lake Fault to the south. likely for the Seine basin. Alternatively, one could imagine that both the southern Seine RiverRainy Lake Fault and the northern Quetico Fault were active master-faults and the secondary features (now seen as secondary shear zones) were insignificant in the early history (Fig. 11b). In this scenario, one might expect a basin to form at the fault intersection if the displacement on the Quetico Fault was significantly more than the displacement along the Seine River-Rainy Lake Fault. Alternatively, it is possible, given the scarcity of folds within the Seine, that the Seine could have been deposited in a pull-apart basin; however, this would require that the sense of motion along the faults was sinistral rather than dextral during this early history. There is no evidence for such a change in sense of fault motion, but a switch in motion sense would be possible given that such evidence would likely have been later obliterated by the dominant ductile fabrics. A change in fault sense would require a change in plate motion or the geometry of the boundary. Given the above three scenarios, it may not be possible to prove any one of them, but it seems that the first scenario, that of a basin forming at a restraining bend, is most likely. The original nappe-like nature of the folds and the likelihood of thrusting (see below) support this scenario. Evidence of Upper Crustal (Brittle) Deformation: Upright bedding as a result of “Stacking” The major piece of evidence for the first phase of deformation is the ubiquitous steep bedding. One might consider two structural end-members, folding and faulting, that could cause the subvertical tilting of strata. After folding, one would expect to see repetition of stratigraphy and opposing stratigraphic facing directions in adjacent strata. Even in areas with upright, isoclinal folds, one might also expect to see some areas with horizontal bedding corresponding to the fold hinges. After faulting, one would also expect to see repetition of stratigraphy, but not necessarily reversals in stratigraphic facing directions. Within the Seine and surrounding regions, it seems likely that the tilting of bedding was achieved, to some degree, through both faulting and folding, a combination referred to here as “stacking” (Czeck, 2001). 55 �There is significant evidence for folding throughout the Superior Province in general and the Wabigoon–Quetico boundary in particular. Evidence for folding includes visible hinges of some folds and opposing directions of stratigraphic facing (Hooper and Ojakangas, 1971; Bauer, 1985; Poulsen, 1986; Hudleston et al., 1988; Bauer and Bidwell, 1990; Tabor and Hudleston, 1991; Bauer et al., 1992; Jirsa et al., 1992; Bauer and Hudleston, 1995; Hudleston and Bauer, 1995). Several large, upright and isoclinal folds have been identified within the Seine metasediments based on stratigraphic facing (scour beds and cross-beds) and lithologic similarity (Hsu, 1971; Wood et al., 1980a; Wood et al., 1980b; Poulsen, 2000). The present–day upright orientation of folds is consistent with either originally nappe-like or more upright folds. In either case, the second phase of deformation, bulk ductile transpression, would have the effect of rotating the fold limbs from steep to subvertical orientations. However, the combination of several observations including the present-day juxtaposition of adjacent right-way-up and overturned folds (Poulsen et al., 1980; Borradaile, 1982; Poulsen, 2000), suggest that the original orientations of many of the folds were nappe-like. There is less direct evidence for thrusting, due to a general lack of sedimentary markers that would enable one to recognize duplicated sequences of rock. Instead, the most convincing pieces of evidence for the existence of faults are observations that suggest that all of the vertical bedding could not be a consequence of folding alone. First, within the Seine metasedimentary sequence, folds are much less evident than those reported in other areas, but the dominance of vertical bedding is still the rule. Second, unlike in rocks faulted by brittle mechanisms, folded rocks often leave a record of strain. It has been shown in some areas within the Superior Province that after “unstraining” the folded component of the preserved strain, the strata are still dipping vertically (Schultz-Ela, 1988). Thus, folding alone is insufficient to account for the vertical bedding. The lack of evidence for faults may indicate that stacking occurred entirely by folding. However, it is also true that if thrust faults were originally present, one would not expect to see evidence of them, other than tilted bedding, after the major fabric-forming deformation (phase 2) took place. Thus, it may be impossible to conclusively determine to what degree the initial stacking event consisted of folds or thrusts. If modern arcs are a suitable analogy to describe the tectonics of the Wabigoon–Quetico boundary, we may consider that while both folding and faulting play a role, faulting appears to be the dominant process at modern arcs (Karig et al., 1979; Karig et al., 1980). Thus, in all likelihood, both thrusts and folds were operational during the stacking phase of deformation at the Wabigoon–Quetico boundary, and it is possible that faulting, although not directly evident, was the dominant process. The relative contributions of faulting and folding to the overall structure may not be equivalent across granite-greenstone terranes, nor even across different portions of the same boundary, such as the Wabigoon–Quetico subprovince boundary. Lack of key exposures and a detailed stratigraphic framework, allied with obfuscation caused by overprinting fabrics from the second phase of deformation, may make it difficult to ascertain the relative contributions of folding and faulting. The term “transpression” was first introduced by Harland (1971) to describe obliquely convergent motion between two crustal blocks, or motion partitioned into convergent and strikeslip components. Harland, in his original analogue experiments, demonstrated this type of partitioning by generating folds within a deforming medium bounded by two rigid obliquely 56 �convergent plates. In an analogous region of deforming rocks, oblique plate collision may partition motion into strike-slip zones parallel to the boundary and folds and thrusts whose strikes rotate during deformation. The probable contemporaneous history of convergence and strike-slip motions along the Wabigoon–Quetico boundary, as manifested by both faulting/folding (stacking) and wrenching, suggests that Harland-type transpression may have been occurring during the stacking phase of deformation. At least this is likely during the stacking phase that occurred contemporaneously with Seine Group deposition. It should be noted that the “stacking” in the Seine may or may not be contemporaneous with any of the “stacking” record within adjacent lithologic domains, even within the Quetico and Seine River - Rainy Lake fault-bounded wedge. In fact, the opposite facing directions found between the Seine and a directly adjacent volcanic unit within the wedge reinforces the idea that at least some of the stacking in adjacent units occurred prior to Seine deposition. Deeper Ductile deformation: Homogeneous Transpression with Variable Extrusion The second phase of deformation formed the dominant structural fabric in the Seine. It formed the penetrative foliation and lineation fabrics, as well as most of the recorded strain. Figure 12. Idealized transpression model according to Sanderson and Marchini (1984) In general, the foliation is subvertical, and the individual clasts within the Seine conglomerates display a flattening fabric with a subvertical plane of flattening. On subhorizontal planes, the clasts commonly form dextral shear sense indicators, regardless of lineation orientation. Most of these fabric features along the boundary are consistent with a type of ductile transpression first described by Sanderson and Marchini (1984). Sanderson and Marchini (1984) provided a mathematical description of a specialized case of transpression: homogeneous deformation consisting of orthogonal simple shear and pure shear components (Fig. 12). In addition to these two components, their model involves constant volume and confines deformation to a vertically bounded zone. Such an idealized scenario is, perhaps, most likely to correspond to strain in deep, vertical ductile shear zones during oblique convergence. The structural fabrics for this model were predicted by Fossen and Tikoff (1993) and are summarized in Fig. 13. The consistencies of fabrics observed along the Wabigoon–Quetico boundary with the homogeneous transpression model suggest that at least the Wabigoon–Quetico boundary as a whole has undergone quasi homogeneous transpression. 57 �Fig. 13. Generalized transpression. Strain (simple shear and pure shear components) and fabrics (foliation, lineation, conglomerate clast asymmetry) based on Sanderson and Marchini (1984) and Fossen and Tikoff (1993). The front and back sides of the boxes are parallel to the deformation zone boundaries. The mineral fabrics are shown in the general case with fabric oblique to the deformation zone boundaries. As strain accumulates, the foliation becomes progressively closer to subparallel with the deformation zone boundaries. Ellipses (some with “tails”) represent schematic clast traces on each plane. Based on Czeck (2001). Significantly, the mineral lineations along the Wabigoon–Quetico boundary are neither vertical nor horizontal, as predicted by the Sanderson and Marchini (1984) transpression model; they plunge between 0-90° in both east and west directions (Fig. 8). The most likely way to create variable obliquely plunging lineations of this type is through a combination of Sanderson and Marchini style transpression with nonvertical extrusion (Fig. 14) (Czeck, 2001). In the original Fig. 14. Schematic view of strain and deformation fabrics for monoclinic transpression with nonvertical extrusion. Light colored ellipses represent schematic clast traces on each plane. (a) Simple view of transpression with nonvertical extrusion. (b) Schematic view of transpression with overall bulk vertical extrusion and localized zones of nonvertical extrusion. Dark colored ellipses represent schematic "hard" zones that influence local extrusion directions. The relative location of Fig. 8a is indicated. Based on Czeck (2001). 58 �Sanderson and Marchini style transpression and subsequent models (Sanderson and Marchini, 1984; Fossen and Tikoff, 1993; Robin and Cruden, 1994; Dutton, 1997; Jones and Holdsworth, 1998; Lin et al., 1998), extrusion of material was assumed to be vertically upwards. This assumption is logical because, in general, one would expect the direction towards the earth’s surface to provide the least resistance for material movement. However, rocks at depth may have other boundary conditions that cause local pressure gradients to deviate from this first-order assumption. For many reasons, such as the anastomosing of shear zones and influences of large lithologically diverse bodies, the local pressure gradients at depth may cause rocks to extrude in nonvertical directions. Given this model, the large range of lineation orientations in the Seine is not surprising considering the lithologically diverse nature of the subprovince boundary and the intricately anastomosing shear zones. As noted by some authors (e. g. Bauer et al., 1992), there may be a final stage of deformation involving amplification of strike-slip motion along wrench zones. This late stage of deformation has often been described as brittle in nature, and thus represents rock exhumation. The presence of brittle faulting of dextral strike-slip motion without contemporaneous thrusting (Kennedy, 1984), may suggest that the latest stage of deformation was almost entirely strike-slip. However, some late-stage brittle faults associated with N-S shortening have been observed (Tabor and Hudleston, 1991). There may also have been an amplification of shortening between the major strike-slip faults, thus creating a more discretely partitioned deformation toward the end of the tectonic history (Tabor and Hudleston, 1991). Evolution of tectonic styles The stacking and the homogeneous transpression most likely developed as an evolution of structural styles during different stages of the same oblique collisional event. The presently exposed rocks undoubtedly underwent a voyage through different zones in the crust as evidenced by their deposition at the surface and ductile deformation at some depth. This gradual change in position within the crust is probably responsible for the observed evolution in deformation styles. It is important to note that the two phases of deformation described here are similar to the commonly discussed “D1” and “D2” described throughout the Superior Province. While specific structural details are different, the general trend of folds overprinted by intense ductile flattening fabrics is common (Poulsen, 1986; Hudleston et al., 1988; Bauer and Bidwell, 1990; Tabor and Hudleston, 1991; Bauer et al., 1992; Jirsa et al., 1992; Bauer and Hudleston, 1995; Hudleston and Bauer, 1995). Locally, rocks at all the Superior Province microplate boundaries may have undergone the same general story of tectonic stacking and wrenching in the upper crust followed by crustal thickening and ductile transpressive flattening as the rocks were deformed at deeper levels. While this general story may be similar, it is important to consider that the regional or even local correlation of these “D1” and “D2” events is probably inappropriate. While the Seine metasedimentary group was forming in a wrench-related basin and surrounding rocks were being stacked, other rocks- even those located along the same microplate boundary or even within the same fault-bounded wedge- may have already reached a deeper level of the crust and began a more ductile, homogeneous phase of transpression. Thus, it is dangerous to correlate exact styles of deformation (e.g. folds) in the Seine with those in either of the adjacent Wabigoon or Quetico subprovinces. 59 �FIELD TRIP STOPS (Stops are located by UTM co-ordinates based on NAD 83, UTM Zone 15) Drive to outcrops located on the west side of Shoal Lake Road, south of Mine Centre. (0526600E 5394850N) STOP 1 - BASAL FACIES OF THE SEINE CONGLOMERATE, EXAMPLE OF LOW DEFORMATION At this location, we are near the basal contact of the Seine. Based on opposing stratigraphic facing in adjacent rocks, an unconformity between the volcanic units and the base of the Seine conglomerate can be identified (Lawson, 1913; Poulsen et al., 1980; Poulsen, 2000). The clasts within the Seine include more tonalite than is typical further up in the section. A possible saprolite may be observed between the volcanics and the Seine (C. Hemstad, pers. comm.). Strain at this location is unusually low. Foliation and bedding attitudes are atypical, subparallel with a more NE/SW strike and shallower (~65°) southeasterly dip than is typical for the rest of the Seine’s area. In addition, the clasts appear to be much more angular and irregularly shaped than the rest of the Seine. This is a highly unusual outcrop of Seine that poses several interesting questions that are open for discussion. We pose some here with the hope of encouraging some reflection and discussion at the outcrop. The orientations of the bedding and foliation within the Shoal Lake area and adjacent rocks to the north and south combine to create a large, gentle S structure, with shallower dips here on the middle section of the S. There are several different ways to interpret this sigmoidal map pattern. The various interpretations have bearing on the timing of the geometrical arrangement of structures with respect to the phases of deformation, and therefore will also influence our interpretations of the nature of the Seine basin. Five interpretations are suggested here: 1) The variation in orientation of foliation around the Shoal Lake area may be due to a late stage, gentle folding event. This is suggested by the similar changes in strike and dip of both foliation and bedding. The fold axis of this structure is approximately 55°/077. 2) As suggested by Poulsen (2000), the faults and small-scale shear zones could be equivalent to those created in fault models of strike-slip systems such as those of Tchalenko (1970), Lowell (1972), and Wilcox et al. (1973). If this were the case, since these models involve Mohr-Coulomb failure, the orientations of the faults would have been determined relatively early in the deformation history, during Harland-type transpression, when the rock would have behaved in Mohr-Coulomb fashion. In this scenario, the Rainy Lake – Seine River fault was the “master fault,” and the Quetico and smaller shear zones formed later as second order conjugate shears. Since there is significant, deeper level, ductile deformation recorded along these same faults, they must have been reactivated during ductile deformation because they were zones of weakness. 3) Alternatively, the orientation of the faults may have been determined during the later ductile transpressive stage of deformation. In this case, the orientation of the major faults and the minor 60 �shear zones would constitute a mega-scale S-C feature creating the sigmoidal pattern seen on the map (Fig. 9). 4) It is also possible that the largest faults were formed by brittle processes, buried and then reactivated as ductile shear zones, with the smaller shear zones being formed later purely by ductile means. 5) Yet another interpretation is possible. It could be that the sigmoidal shape of the foliation is related to lithological contrasts, with foliation wrapping around more rigid bodies. In such a scenario, the rocks to the east of the Bad Vermilion intrusive complex (the large plutonic unit in the west-central portion of Fig. 2) were caught in a large strain shadow region (Borradaile and Dehls, 1993). This hypothesis is consistent with the lower strain found in this area. The apparent “wrapping” of the fabrics around this intrusion seems analogous to the “wrapping” of foliations and lineations around relatively rigid conglomerate clasts at a smaller scale. Not surprisingly, there exists a similar asymmetry in shape between the rigid conglomerate clasts and large rigid units such as the Bad Vermilion intrusive complex. In addition, the shapes of the clasts here are much more angular than the rest of the Seine clasts. If one were to consider studies that model nonspherical clasts in deformation, we would expect more deformed outcrops of Seine to have barrel or bone shaped deformed clasts (Treagus et. al, 1996; Treagus and Lan, 2000). As we will see at the next outcrops, this is not the case. Why? Drive East along Hwy 11, right onto Forest Tour Road. (0536650E 5398850N) STOP 2 - TWO-DIMENSIONAL HORIZONTAL VIEW OF SEINE CONGLOMERATE, SANDY LENS AND DEXTRAL SHEAR INDICATORS This outcrop allows us to see a large 2D view of the Seine. The deformation here is typical of much of the field area, the granitoid clasts being fairly undeformed while the volcanic clasts reflect significant flattening. Minor amounts of quartzite, BIFs, and other lithologic clast types are also observable. Dextral shear sense indicators are prominent including asymmetric pressure shadows and clast tilings. Many sandy beds and channels, often with graded bedding, can be located within this outcrop. Is it possible to determine stratigraphic facing here? Drive East along Hwy 11 to Horsecollar Junction. (0542150E 5398500N) STOP 3 - THREE-DIMENSIONAL VIEW OF MODERATELY DEFORMED SEINE CONGLOMERATE This outcrop allows us to see a 3D view of the Seine. The deformation here, as at the last stop, is typical of much of the field area with average strain and dominant flattening fabrics. Here we can observe the subparallel, undulatory nature of bedding and foliation. The lineation plunge here is relatively steep (~78°E). The dextral asymmetric indicators are most prominent on the subhorizontal plane. 61 �Drive East along Hwy 11. (0559500E 5398900N) STOP 4 - ULTRA DEFORMED SEINE CONGLOMERATE WITH ALTERATION The conglomerates here are extremely deformed. Both volcanic and plutonic clast types are extremely flattened. Many clasts are flattened beyond recognition, giving the rock a striped appearance. Is it possible to estimate strain in a rock that is this deformed? The lineation plunge here is typical of the field area (~44°E). While this lineation is “average” stretching lineation for the Seine, it is in no ways representative of the typical deformation due to the wide variance in lineation orientation. The lineation orientation does not vary systematically across the field area and is not directly related to the amount of strain (Czeck, 2001). Again, the dextral asymmetric indicators are most prominent on the subhorizontal plane. This is the case throughout the field area, regardless of lineation orientation, a result that one would not expect in either an ideal strike-slip shear zone or homogeneous transpression. The combination of subhorizontal asymmetric shear indicators and variable lineation orientations makes a deformation model of quasi homogeneous transpression with a variable extrusion direction most likely (Czeck, 2001). The extensive carbonate alteration in this rock suggests that fluid flow was an important factor during deformation. This implies the possibility of volume loss in these ultra deformed rocks. There was probably a symbiotic relationship between fluid localization and enhanced deformation. Drive back West along Hwy 11, stop just east of Seine River bridge. (0551850E 5398700N) STOP 5 - SMALL FOLD IN SEINE At this stop, we can observe the sandier facies more typical of the upper part of the Seine sequence. Cross beds and localized deposits of gravel can be observed. The cross beds allow us to recognize stratigraphic facing. The cross beds are deformed, and a good example of a meter-scale fold evident. This fold is asymmetric and upright with its axis oriented 6°/S83W. This subhorizontal fold axis orientation is similar to fold axes orientations described in the Quetico Subprovince and contrasts with the subvertical fold axes described in the Wabigoon subprovince (Borradaile, 1982). Localized prolate strain, atypical of the field area, is evident in the fold hinge. Down the road, just east of this outcrop, another small fold can be observed in the sandy facies. Does this fold have the same subhorizontal fold axis? 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Rainy Lake Wrench Zone: An example of an Archean Subprovince boundary in Northwestern Ontario. In: de Wit, M. J., Ashwal, L. D. (Eds.), Tectonic evolution of greenstone belts Technical Report 86-10, pp. 177-179. Poulsen, K. H., 2000. Archean metallogeny of the Mine Centre - Fort Frances area. Ontario Geological Survey Report 266, 121. Poulsen, K. H., Borradaile, G. J., Kehlenbeck, M. M., 1980. An inverted Archean succession at Rainy Lake, Ontario. Canadian Journal of Earth Sciences 17, 13581369. Purdon, R.H., 1995. Lithostratigraphy and provenance of the Neoarchean McKellar Harbour sequence, Superior Province Ontario Canada. M.Sc. thesis, Lakehead University, 172 p. Robin, P.-Y., Cruden, A. R., 1994. Strain and vorticity patterns in ideally ductile transpression zones. Journal of Structural Geology 16, 447-466. Sanderson, D. J., Marchini, W. R. D., 1984. Transpression. Journal of Structural Geology 6, 449-458. Sawyer, E.W., 1986. 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Ontario Geological Survey Preliminary Map P. 2201, scale 1:15840. 67 �68 �Field Trip 4 Industrial Minerals and Paleozoic Geology of Southeastern Manitoba James D. Bamburak and Ruth K. Bezys Manitoba Geological Survey 360-1395 Ellice Avenue Winnipeg, Manitoba R3G 3P2 Interior of the Manitoba Legislative Building, Winnipeg, built in the early 1900s. Decorative dolomitic limestone, the world famous "tapestry stone" with its unique mottled appearance, is still being quarried from the Selkirk Member, Red River Formation in the Garson area. �FOREWORD Currently, the Province of Manitoba has 18 industrial mineral processing plants and quarries, excluding the production of aggregate (Fig. 1 and Table 1). The 2001 estimated value of industrial mineral production in the province is $78.1 million, including aggregate production that forms slightly less than half of the total. The $78.1 million represents 7% of the province’s mineral production. Fourteen of Manitoba’s industrial mineral processing plants and quarries are situated in southeastern portion of the province. Figure 1. Industrial mineral producing plants and quarries in southern Manitoba Industrial minerals produced in southeastern Manitoba include lithium, cesium, tantalum, sand, aggregate, dimension stone and peat. Peat is considered as a quarry mineral under the Manitoba Mines and Minerals Act. Lithium, cesium, and tantalum are produced for export from the Tanco mine in the Bernic Lake pegmatite (Field Trip No. 1, this volume). Sand and aggregate are quarried for use in local construction. Dimension stone and peat are quarried for local consumption and export. Two companies, Sun Gro Horticulture Canada Ltd., and Premier Horticulture Ltd. extract horticultural quality sphagnum peat. The locations of the peat operations are shown on Figure 1 and listed in Table 1. Five companies, Cold Spring Granite (Canada) Ltd., Gillis Quarries Limited, Carrieres Polycor Inc., Manex Granit Ltd. and Canital Granite Ltd. quarry dimension 70 �Table 1 INDUSTRIAL MINERAL PROCESSING PLANTS AND QUARRIES 2002* No. 1 2 Location Elma (P&Q) Whitemouth (Q) Company Sun Gro Horticulture Canada Ltd. Carrieres Polycor Inc. 3 Bernic Lake (P&Q) Tantalum Mining Corporation of Canada Ltd. 4 Winnipeg (P) Westroc Inc. 5 6 Brandon Winnipeg (P) Nexen Inc. Canital Granite Ltd. 7 Moss Spur (Q) 8 Lac du Bonnet (P&Q) dimension stone 9 Garson (P&Q) 10 Medika (Q) Sun Gro Horticulture Canada Ltd. Cold Spring Granite (Canada) Ltd. Gillis Quarries Limited Manex Granit Ltd. 11 Meditation Lake (Q) Manex Granit Ltd. 12 PR 307&309 (Q) Manex Granit Ltd. 13 14 15 16 17 18 Julius North (P&Q) Caribou Cluster (Q) Giroux (P&Q) Harcus (Q) Faulkner (P&Q) Flin Flon (P) Sun Gro Horticulture Canada Ltd. Premier Horticulture Ltd. PremierHorticulture Ltd. Westroc Inc. Graymont Western Canada Ltd. Hudson Bay Mining and Smelting Co., Limited Product Peat moss Granite, dimension stone Tantalum oxide, spodumene and amblygonite concentrates, cesium and rubidium ores and cesium formate Gypsum wallboard Sodium Chlorate Granite, dimension stone Peat moss Granite, Tyndall stone Granite, dimension stone Granite, dimension stone Granite, dimension stone Peat moss Peat moss Peat moss Gypsum Lime, limestone Sulphur (P) Plant (Q) Quarry * excludes aggregate producers stone. Gillis quarries a dolomitic limestone, the renowned "Tyndall Stone", and the other companies quarry granite. Locations of the stone quarries are shown on Figure 2. INTRODUCTION This one day field trip is designed to visit three industrial mineral sites, including one Lower Paleozoic site. The sites that will be visited include: • the sphagnum peat bog harvesting operation and plant of SunGro Horticulture Canada Ltd. near Elma (Stop 1); 71 �• the granite dimension stone quarry and plant of Cold Spring Granite (Canada) Ltd. near Lac du Bonnet (Stop 2); and, • the Ordovician Tyndall Stone quarry and plant of Gillis Quarries Limited at Garson (Stop 3). Figure 2. Granitic dimension stone quarries in southeastern Manitoba. Sphagnum Peat Canada holds more than a third of the world’s peat resources with 1 223 834 km2 of peatlands, or 12 percent of the total land mass (Tarnocai et al., 1995). Approximately 40% of Manitoba's surface is covered with peat deposits, many of which are inaccessible and/or of uneconomic thickness and quality to be harvested (Dixon and Stewart, 1988). Nevertheless, Manitoba holds vast reserves of peat suitable for horticultural or energy peat production. Two companies, Sun Gro Horticulture Canada Inc. (Stop 1) and Premier Horticulture Inc. harvest 6 bogs in southeastern Manitoba (Fig. 1) for horticultural quality peat. An aerial view of a typical peat extraction area is shown in Figure 3. Peat companies hold almost 4000 hectares of leases in good standing, and 5500 hectares of pending leases in the Interlake and southeastern Manitoba. Peat production in Manitoba 72 �in 2001 was estimated to be worth over $25 million, and this figure is sure to increase as new companies bring their bogs into production. Figure 3. Caribou cluster peat extraction area of Premier Horticulture Inc. Horticultural quality sphagnum is the preserved, but undecomposed, remains of sphagnum moss plants. Sphagnum deposits accumulate in areas of poor drainage where the rate of atmospheric precipitation exceeds the rate of evapotranspiration, i.e. the low boreal forest climatic zone. The accumulation of sphagnum occurs within an acidic, nutrient poor environment, above the level of the local water table. The characteristics of sphagnum that allow it to survive in this environment are the same characteristics that make it valuable to the horticultural industry as a growing medium and soil conditioner (Schmidtke and Bamburak, 1996). These characteristics are: 1. the ability to absorb approximately 20 times its weight in water; 2. a high capacity for cation exchange; 3. a fibrous structure that introduces volume and pore space to a soil mix; 4. compressibility; and, 5. the ability to resume its precompression volume after compression is released. Sphagnum moss is composed largely of rigid walled hyaline cells. The function of these cells is to absorb and hold water. Since the sphagnum must get all its nutrients from the 73 �nutrient poor atmospheric precipitation, it is able to absorb approximately 20 times its weight in water and has a high cation exchange capacity. These properties make it a valuable growing medium in places like Texas where the native soil is fine and does not retain moisture. The hyaline cells are compressible and will resume their shape even after being compressed to a 10:1 ratio. The sphagnum moss can be compressed into bales for efficient transport. These properties are retained after the plant has died and even when it is slightly humified. Since the sphagnum accumulation takes place in an acidic, oxygen poor environment, it is possible for several metres of relatively undecomposed sphagnum to develop. The high quality bogs in southeastern Manitoba accumulated sphagnum to a maximum depth of approximately 2.5 m over a period of 4000 years. A bog must be prepared before peat extraction occurs. First the trees and roots are removed, ditches are dug, and the bog is drained. It may take two years to prepare a bog for harvesting. Sun dries the surface of the bog, which is then raked using a harrower (or cultivator) to loosen the surface peat. This loose, dry sphagnum peat is lifted from the surface with vacuum harvesters (Fig. 4). The harvesters empty the peat into stock piles, or winrows. The stock piled peat is either moved into the plant for processing or is stored in plastic "silage" tubes for future processing when unfavourable weather prohibits harvesting. (Peat can't be extracted if the weather is too wet or too dry, because quarrying equipment can't operate on a bog that has been saturated by rain or melt water and sparks from equipment can ignite peat dust in hot, dry weather). Figure 4. Conga line of peat vacuum harvesters on Elma bog of Sun Gro Horticulture Canada Inc. Once in the plant, the peat is dried (if necessary). Popped perlite is added as a volumizer. The peat is treated with surfactants, which increase absorptive capacity, and fertilizers. 74 �The type and quantity of the chemicals added to the peat are dependent on the intended end use. Specialized mixes are available for several applications, i.e. soil mix for violet plants. The peat is baled, shrink-wrapped on pallets and stored in the warehouse before being loaded onto semi trailers. Some of the peat is sold in local markets, but most goes to nurseries and greenhouses in the southern United States. The peat companies take advantage of backhaul rates by shipping the peat south in trucks that bring produce to Manitoba from the southern United States. Ninety percent of the peat from Sun Gro's Manitoba quarries is exported to Texas. If the Sun Gro operations in Alberta or New Brunswick are unable to produce because of bad weather or labour problems, Manitoba peat is diverted to markets west or east of Texas to cover the shortfall. Both Sun Gro Horticulture and Premier Horticulture operate harvesting operations and plants across Canada (Schmidktke and Bamburak, 1996). Granitic Dimension Stone Four companies quarry Precambrian granitic dimension stone at 5 sites in southeastern Manitoba (Fig. 2). They include: Cold Spring Granite (Canada) Ltd., Carrieres Polycor Inc., Manex Granit Ltd. and Canital Granite Ltd. At all sites, the granitic outcrops possess unique physical features that permit the quarrying of large blocks of stone Schmidtke (1993). They include: 1. widely spaced, preferably orthogonal fractures that will allow removal of blocks with a minimum trimmed size of 2.0 m by 1.25 m by 1.25 m; 2. widely spaced, or preferably the absence of, veining; 3. homogeneous, attractive and fashionable colours and textures; 4. the absence of minerals that pluck when polished, or oxidize and cause unsightly rust spots when exposed to the elements; 5. road access; 6. proximity to transportation routes, finishing facilities and markets; and, 7. acceptable strength values that meet ASTM standards (as per Annual Book of ASTM Standards). Two of the granitic dimension stone cutting and polishing plants are located in southeastern Manitoba (Fig. 1 and Table 1). One plant (Stop 2) is adjacent to the quarry near Lac du Bonnet (Cold Spring Granite), and the other plant is situated in Transcona, on the east side of Winnipeg (Canital Granite). Tyndall Stone 75 �At Garson, Tyndall Stone is quarried by Gillis Quarrries Ltd. (Fig. 1 and Table 1), east of the plant. This famous dimension stone, sometimes called “tapestry stone”, occurs in the lower half of the 43 m thick Selkirk Member of the Ordovician Red River Formation. Its unique appearance comes from a matrix of light-coloured limestone, with mottled areas of dark dolomitic limestone distributed uniformly throughout. The horizontal beds are approximately 60 to 100 cm thick, beneath 1 to 6 m of overburden consisting of soil, lake clays and stony glacial till. The upper four beds in the quarry (total thickness 2 to 4 m) have either a buff and golden-brown buff matrix. The fifth layer down has a matrix, which is transitional into the underlying beds that have a steel-gray matrix (Wilkins, 1986; Coniglio, 1999). After the overburden is stripped, the stone is extracted using two 2.5 m diameter circular saws, drilling and wedging. The 6 to 8 tonne stone blocks are taken into the plant where they are cut and finished using diamond tipped saws or are ground, sheared or split into a variety of products. Finishes available are rubbed (machine smoothed), bushhammered, pointed face, rough cut, sandblasted, split and rustic. Stone is usually marketed as dimension stone, which is cut and shaped to specification or as random ashlar, which are pre-cut into standard or random shapes to be set into mortar (Wilkins, 1986; Coniglio, 1999). 76 �REGIONAL GEOLOGICAL SETTING The Province of Manitoba is completely underlain by 3.0 to 1.7 billion year old Precambrian rock, which is overlain in the southwest and northeast by younger (less than 570 million years old) Phanerozoic sedimentary rocks (Fig. 5). Pleistocene and Holocene deposits, younger than 2 million years in age cover most of these earlier rocks. Industrial minerals in Manitoba range, in age and form, from Precambrian dimension stone to Holocene peat deposits. Figure 5. Principal geological domains of Manitoba Within southeastern Manitoba, the Precambrian surface is exposed in the east half of the area and is known as the Precambrian Shield. The surface dips to the west (Fig. 6), where it is covered by an increasing thickness of Lower Paleozoic strata which also dip gently to the southwest at approximately 2.8 m per km (Fig. 7). The eastern edge of the Paleozoic outcrop belt represents the Manitoba Lowland or First Prairie Level, which is 77 �bounded on the east by the Precambrian Shield, and on the west by the Manitoba Escarpment. Figure 6. Precambrian structure contour map. The Manitoba Escarpment forms the eastern edge of the Second Prairie Level, which is underlain by Cretaceous strata dipping gently to the southwest at 1.5 to 1.9 m per km. The actual Escarpment is composed of soft, easily eroded sands and shales in the lower part of the Cretaceous, underlying a resistant shale cap (Odanah Member of the Pierre Shale). In southeastern Manitoba, industrial minerals are mined from Precambrian, Lower Paleozoic and Holocene localities. Granite quarries (Fig. 2) are situated within late tectonic granitic batholiths and plutons on the western edge of the Archean Superior Province. The Ordovician Tyndall Stone quarry (Gillis Quarry, Fig. 1) occurs within the eastern edge of the Paleozoic outcrop belt. The harvested spahagnum peat bogs (Fig.1) 78 �are located in areas of thick glacio-lacustrine sediments that overlie Archean terrane or Ordovician strata. Figure 7. Structure cross section, southern Manitoba. Precambrian Basement The Precambrian of southeastern Manitoba is comprised of younger Archean granitic batholiths or plutons that have intruded the older greenstone, sedimentary and gneissic formations of the Wabigoon, Winnipeg River, Bird River and English River domains of the Superior Province (Fig. 5). The structure contours on the buried Precambrian surface are shown in Figure 6. All granitic dimension stone production to date has been derived from the Winnipeg River and Bird River domains. The Medika and Betula Lake plutons are situated within the Winnipeg River Domain. The Lac du Bonnet Batholith (LDBB), located to the northwest, lies within the Bird River Domain. For the purpose of the visit to Cold Spring’s Lac du Bonnet Quarry (Stop 1), the following will focus on the latter. The LDBB (outlined on Fig. 2) is the youngest intrusion in the Winnipeg River area (Tammemagi et al., 1980). The batholith is a predominantly pink granite that extends over approximately 1000 km2 from Pointe du Bois southwestward beneath Paleozoic cover. The largest exposures of the batholith occur east of the town of Lac du Bonnet, but isolated outcrops are found as far west as the farmlands directly north of Beausejour. Many of the outcrops of Lac du Bonnet granite have widely spaced fractures, which makes them potential sources of dimension stone (Schmidtke, 1993). 79 �The geology of the Lac du Bonnet Batholith (LDBB) has been the subject of intensive study by Atomic Energy of Canada Ltd. (AECL) as a research site for geological, geotechnical and hydrogeological studies to determine the potential for storing nuclear waste at depth in granite. The regional geology has been described, in detail, by Tammemagi et al. (1980) and McCrank (1985). Lower Paleozoic Stratigraphy The Lower Paleozoic beds (Table. 2) exposed in southeastern Manitoba form part of the Manitoba outcrop belt that is located on the northeastern edge of the Western CanadaSedimentary Basin (WCSB). The WCSB is a composite feature which includes both the Elk Point Basin (Fig. 8), centered in south-central Saskatchewan (which controlled Devonian deposition), and the Williston Basin, centered in northwestern North Dakota (which controlled the depositional patterns throughout the remainder of postCambrian time). Since the Manitoba outcrop belt appears to be situated on the northeastern edge of the sedimentary basin, and roughly parallels the regional structure contours, one might surmise that the strata comprising the outcrop belt would be relatively uniform in lithology. The outcrop would also represent marginal shelf-type deposits relative to the thicker, more basinal sedimentary sequence found to the southwest in the subsurface. However, this is not the case for most Paleozoic formation in southwest Manitoba. The outcrop belts, particularly the Ordovician and Devonian, show marked changes in both thickness and lithology, indicating a complex and varied tectonic and depositional framework (Bezys and McCabe, 1996). The outcrop succession is not marginal to the depositional basin, but rather exposes a series of dip sections of the basin, which show the maximum possible isopach and lithofacies variation. As well, the directions of the dip sections are opposite: basinal Ordovician outcrops occur at the southern end of the outcrop belt, whereas basinal Devonian outcrops occur at the northern (or northwestern) end of the outcrop belt. Regional strike of the Paleozoic strata is approximately north-south, and regional dip increases gradually and uniformly from about 2.6 m/km in the eastern part of southeastern Manitoba to 4.2 m/km in the western part. Despite the regional structural dip to the southwest, isopachs of the Winnipeg and Red River formations all trend eastwest and thicken to the south at up to 0.3 m/km (Fig. 9B and 10B, respectively). This indicates a major change in tectonic framework subsequent to early Paleozoic time, as mentioned previously. The present north-south structural trend probably developed during late Paleozoic to early Mesozoic, due to uplift with associated erosion and eventual exposure of Precambrian bedrock in southeastern Manitoba. A detailed discussion outlining: the evolution of the complex pattern of structural trends in the Lower Paleozoic; a regional tectonic control for apparent anomalies in facies trends; and other related structural and stratigraphic anomalies are described in Bezys and McCabe (1996). 80 �Table 2. Geological formations in Manitoba. 81 �Figure 8. Major structural features of the Elk Point and Williston basins. Ordovician Winnipeg Formation The Winnipeg Formation, a quartzose sandstone interbedded by green, waxy shale with sand and silt interbeds, is exposed in outcrop east at the northeast end of the southeastern Manitoba area, on Black Island and near Seymourville. Structure contours and isopachs 82 �Figure 9A. Winnipeg Formation structure contour map. 83 �Figure 9B. Winnipeg Formation isopach map. 84 �Figure 10A. Red River Formation structure contour map. 85 �I * I •1 Red River Formation IsopachMap - Sm —— Churchill Superior :---- BoundaryZone Outcrop belt suborop I LTt1,1OJW zk Figure 10B. Red River Formation isopach map. 86 I N _______ kilonwtres �for the Winnipeg Formation are shown in Figures 9A and B, respectively. The formation was described, in detail, by McCabe (1978). Red River Formation The Red River Formation consists of two principal subunits, the lower Red River and upper Red River strata (Table 2). In the vicinity of the south basin of Lake Winnipeg, the lower Red River (Baillie, 1952) can be subdivided into three mappable members: a lower Dog Head Member, a medial Cat Head Member, and an upper Selkirk Member (=Tyndall Stone). Lower Red River strata consist of light grey to yellowish- and brownish-buff, prominently mottled, fossiliferous, commonly cherty, dolomitic limestones. The upper Red River strata consist of dolomite and argillaceous cherty dolomite, designated as the Fort Garry Member. A thin, high calcium limestone bed occurs locally at the top of the Fort Garry Member. Stucture contours and isopachs for the Red River Formation are shown in Figures 10A and B, respectively. At Garson, Gillis Quarries Limited (Stop 3) is actively quarrying the Red River Formation (Selkirk Member) for its dimension stone (Tyndall Stone). Stony Mountain Formation The Stony Mountain Formation is subdivided into three members, in ascending order: the Gunn, Penitentiary, and Gunton (Table 2). The Williams Member was once included within the Stony Mountain Formation; however, standardized correlations established for the new Atlas of the Western Canada Sedimentary Basin (Norford et al., 1994) have placed the Williams into the overlying Stonewall Formation (Bezys and McCabe, 1996). The Gunn Member consists of greyish-red to purplish-grey, fossiliferous, calcareous shale with interbeds of relatively clean, fossiliferous limestone. The Penitentiary Member consists of yellowish- to reddish-grey, fossiliferous, argillaceous dolomite. These two members together comprise the lower Stony Mountain. The upper Stony Mountain (Gunton Member) consists of a buff, finely crystalline, sparsely fossiliferous, nodular-bedded dolomite that is relatively uniform in thickness and lithology. All three members are exposed in the Mariash Quarry and abandoned City of Winnipeg quarries at Stony Mountain, an erosional outlier of the Stony Mountain Formation, located 7 km southwest of the Town of Stonewall, near the western edge of the southeastern Manitoba area. The Gunton Member acts as a cap rock for a shallowly buried, east-facing, north-trending escarpment (Gunton Escarpment), 4 km east of the Town of Stonewall. The Gunton Member is extensively used for crushed stone, extracted from quarries in the Stony Mountain and Stonewall areas. The stone has also been used to construct buildings in Winnipeg, Stony Mountain (including the Federal Penitentiary) and in Stonewall. 87 �Ordovician/Silurian Stonewall Formation The Williams Member, is the basal unit of the Stonewall Formation (Bezys and McCabe, 1996). It represents the oldest of a series of so-called “para-time-stratigraphic” markers; thin sandy and/or argillaceous beds that can be traced for many hundreds of kilometres throughout most of the Williston Basin (Table 2, Fig. 8). These marker beds provide the primary means for stratigraphic subdivison of Upper Ordovician and Silurian strata and probably represent deposits related to brief periods of shoaling or even slight uplift and erosion (i.e. non-sequences) (Porter and Fuller, 1959). The Williams Member consists of buff to grey to red, sublithographic dolomudstone. The lower Stonewall beds, above the Williams Member, consist of pale yellowish-grey to yellowish-brown, faintly mottled, medium- to thin-bedded, finely crystalline dolomite with sparse, poorly preserved fossils. A sandy argillaceous marker bed, the “t-marker” or “t-zone”, separates the lower Stonewall Formation from the upper Stonewall Formation. The upper Stonewall Formation consists of light brown to grey, laminated to thin-bedded, sparsely fossiliferous microcrystalline dolomite, which is capped by a grey to buff dolomudstone marker bed, the Upper Stonewall Marker. According to Bezys and McCabe (1996), the t-marker within the upper part of the Stonewall Formation, also marks the position of the Ordovician-Silurian boundary in the Williston Basin. This was confirmed in biostratigraphic studies, based upon outcrop and subsurface investigations. The formation was previously quarried in the Town of Stonewall for lime and aggregate production. Silurian Interlake Group The Interlake Group, consisting of the Fisher Branch, Moose Lake, Atikameg, East Arm and Cedar Lake formations (in ascending stratigraphic sequence), is exposed near the western margin of southeastern Manitoba area. In the subsurface, the group consists of yellow-orange to grey, fossiliferous, oolitic, stromatolitic dolomite, interrupted by sandy, argillaceous marker beds. Jurassic and Cretaceous South of the City of Winnipeg, within the Manitoba Lowland, a large area of Jurassic sediment infills a major pre-Mesozoic channel in the Paleozoic erosion surface. Also, many small Cretaceous outliers have been noted in karst features penetrated by water wells (Bannatyne, 1988). Recent Six peat quarries (Fig.1) are located in areas of thick glacio-lacustrine sediments that overlie Archean terrane or Ordovician strata. Over the past 4000 years, sphagnum plants have contributed organic matter that has accumulated to a maximum depth of approximately 2.5 m. 88 �Near Elma, Sun Gro Horticulture Ltd. (Stop 1) harvests sphagnum peat from spring to fall from a drained bog. Sphagnum peat is also produced on a seasonal basis at 5 other sites, on a seasonal basis, by Sun Gro Horticulture Canada Inc. and Premier Horticulture Inc. 89 �FIELD TRIP STOPS Leave Kenora, on Trans Canada Hwy, travel west for 110 km to Prawda, at junction of Trans Canada Hwy and PR 506. Continue on Trans Canada Hwy (west) to Hwy 11 (11 km), turn right (north) to Sun Gro Horticulture entrance road (12 km), turn left (west), park in visitor’s parking area. STOP 1 – SUNGRO HORTICULTURE SPHAGNUM PEAT BOG AND PLANT The Elma bog (Fig. 1 and Table 1), quarried by Sun Gro Horticulture Canada Ltd., is approximately 3000 acres (12.14 km2) in area and has been in production since 1969. The on site plant was completed in 1972. Peat is quarried using the vacuum harvesting method described above. The bales are loaded at the plant into semitrailers and shipped to the southern United States. A small percentage of the peat is sold for local consumption at retail stores in Manitoba. Reserve estimates have not been published by the company. Leave Sun Gro parking area, travel east on exit road back to Hwy 11, turn left (north) for 50 km to Cold Spring Granite entrance road, turn left (west), park in visitor’s parking area. STOP 2 – COLD SPRING GRANITE DIMENSION STONE QUARRY AND PLANT The quarry and plant of Cold Spring Granite (Canada) Ltd. (Fig. 2) are located in the south central area of the LDBB on a 1220 m long 6 to 8 m high ridge approximately 10 km south of the Town of Lac du Bonnet (Fig. 2). The quarry is accessed via Highway 11. The Precambrian monadnock (Bezys et al., 2001) was first quarried from 1933 to 1949 by a local, Ivor Peterson, for tombstones. An American company, Cold Spring Granite Ltd., reopened the quarry in 1959 and has produced stone from the ridge to the present time. The product is a fine grained pale rose granite sold under a variety of trade names including Lac du Bonnet, Canyon Rose, Colonial Rose and Canadian Mist. Even grained rock is used for monuments and building stone, textured or variegated rock is used for tiles and countertops. The plant at Lac du Bonnet is equipped to make grave markers, countertops, paving and landscaping material and structural panels. Blocks are shipped to the Minnesota plant for finishing into headstones, mausoleums, monuments, columbariums, structural panels, tiles, custom design industrial work, paving and landscaping material. Fine grained, even textured Lac du Bonnet granite is prized for grave markers, monuments and headstones, because sandblasted letters and designs stand out well. It is also a preferred rock for precision industrial applications because it takes a very tight smooth polish, i.e. precision milling surfaces. Rock is sold locally from the Lac du Bonnet quarry, and internationally through Cold Spring Granite's office in Cold Spring, Minnesota. 90 �Figure 11. Granite dimension stone blocks being removed from Cold Spring Quarry in 1982. Prior to 1987, rock was removed from the quarry with wire saws and moved into the plant using hoists (Fig. 11). Since 1987, blocks have been removed by drilling and blasting. Sections of the outcrop are drilled off with portable, track-mounted, hydraulic drills. The drilled sections are then separated from the outcrop by blasting. The blast must move the section of rock without shattering it or inducing microfractures. The sections of rock are then drilled and wedged into smaller blocks that are moved to the plant with a 988 Cat loader. The plant has a 10 wire slab saw, a Salvatore 16 head polishing machine, a JB 110 granite milling machine, a 24" diamond saw and 6', 2', and 1' hydraulic splitters. Blocks are cut into slabs with the wire saw. The slabs are cut and polished with the diamond saw and the polishing machine for use as structural stone. Polished slabs are also manufactured into paving stone and grave markers with the hydraulic splitters and into countertops and furniture with the milling machine. Raw blocks are shipped to plants in Montreal for manufacture into granite tile and to the plant in Cold Spring, Minnesota to be processed for all other applications. Leave Cold Spring parking area, travel east on exit road back to Hwy 11, turn right (south) to PTH 44, turn right (west) to Garson (total 55 km) arrive Gillis Quarries, park in visitor’s parking area. STOP 3 – GILLIS TYNDALL STONE QUARRY AND PLANT Gillis Quarries Limited quarries an 8 m thick section of pale yellowish brown, dolomite mottled, burrowed, fossiliferous micrite of the Selkirk Member of the Ordovician Red River Formation at Garson (Fig. 1 and Table 1). In the quarry, the well known “Tyndall Stone”, is extracted using two 2.5 m diameter tungsten carbide-toothed circular saws, 91 �(Fig. 12), followed by wedging of the blocks along the bedding planes. The stone is finished in the plant, as described earlier. Three colours of stone are produced from various parts of the quarry – buff (a light creamy beige with pastel brown mottles) and golden buff (possibly due to ground water) from the upper beds. And, gray (a pale bluish grey with gray-brown mottles) from the lower beds (Wilkins, 1986; Coniglio, 1999). Figure 12. Carbide toothed circular saw in Gillis Tyndall Stone Quarry. The quarry is noted for its well-preserved fauna of large cephalopods, gastropods, corals, stromatoporoids, bryozoans, crinoids, trilobites, brachiopods, bivalves and calcareous algae, etc. A waste pile is available to hunt for fossils. Gillis Quarries Limited has been a family-owned business since 1915 when August Gillis and his son, Charles, acquired a quarry property in Garson. The stone was finished in Winnipeg, until 1968 when the Garson plant was built. Gillis Quarries has owned all the quarry property in Garson since 1973. The company estimated that based on the 1986 production rate, it had at least 100 to 125 years of stone in reserve (Garson and District History Book Committee, 1990). References: Corehole M-3-69 (internal government core logs) (Garson Quarry, 15-3-136EPM). Go east on Hwy 44, 7 km to Hwy 12, turn right (south) and continue 38 km to the Trans Canada Hwy. Turn left (east) and continue to Kenora (175 km). 92 �REFERENCES Baillie, A. D. 1952. Ordovician geology of Lake Winnipeg and adjacent areas, Manitoba. Manitoba Mines Branch Publication 51-6, 64 p. Bannatyne, B.B., 1980. Sphagnum bogs in southern Manitoba and their identification by remote sensing; Manitoba Energy and Mines, Economic Geology Report ER79-7, 103p. Bannatyne, B.B., 1988. Dolomite resources of southern Manitoba; Manitoba Energy and Mines, Economic Geology Report ER85-1, 4 maps, 39p. Betcher, R. N., McCabe, H. R., and Render, F. W. 1993. The Fort Garry aquifer in Manitoba. Manitoba Energy and Mines, Geological Report GR93-1, 15 p. Bezys, R.K. and McCabe, H.R. 1996. Lower to Middle Paleozoic stratigraphy of southwestern Manitoba – Field Trip Guidebook B4: Geological Association of Canada/Mineralogical Association of Canada Annual Meeting, Winnipeg, Manitoba, May 27-29, 1996. Bezys, R.K., Matile, G.L.D. and Keller, G.R. 2001. Investigation of Precambrian monadnocks (NTS 62I/1 and 62/8); in Report of Activities 2001, Manitoba Industry, Trade and Mines, Manitoba Geological Survey, p. 133-137. Coniglio, M. 1999. Manitoba’s Tyndall Stone; in Wat on Earth; University of Waterloo, Department of Earth Sciences, Spring 1999, pp. 15-18. Dixon R.J. and Stewart, J., 1988. Peatland inventory of Manitoba: III- Interlake region using LANDSAT thematic mapper; Manitoba Department of Mines and Natural Resources, Surveys and Mapping Branch, 21p. Garson and District History Book Committee 1990. Garson, then and now 1890-1990; Derksen Printers Ltd., pp. 14-26. McCabe, H.R., 1978. Reservoir potential of the Deadwood and Winnipeg Formations, southwestern Manitoba, Manitoba Energy and Mines, Geological Paper GP 78-3, 54p. McCrank, G.F.D., 1985. A geological survey of the Lac du Bonnet Batholith, Manitoba; Atomic Energy of Canada Limited, Report AECL-7816, 63p. Norford, B.S., Haidl, F.M., Bezys, R.K., Cecile, M.P., McCabe, H.R., and Paterson, D.F. 1994. Middle Ordovician to Lower Devonian strata of the Western Canada Sedimentary Basin, in Geological Atlas of the Western Canada Sedimentary Basin, G.D. Mossop and I. Shetson (compilers), Calgary, Canadian Society of Petroleum Geologists and Alberta Research Council, p. 109-127. Porter, J.W. and Fuller, J.G.C.M. 1959. Lower Paleozoic rocks of the northern Williston Basin and adjacent areas. American Association of Petroleum Geologists Bulletin, Vol. 43, No. 1, pp. 124-189. Schmidtke, B.E., 1993. Granitic dimension stone potential of southeast Manitoba; Manitoba Energy and Mines Economic Geology Report ER93-1, 52p. 93 �Schmidtke, B.E. and Bamburak, J.D., 1996. Industrial minerals of southeast Manitoba – Field Trip Guidebook B7, Geological Association of Canada/Mineralogical Association of Canada Annual Meeting, Winnipeg, Manitoba, May 27-29. 1996. Tammemagi, H.Y., Kerford, P.S., Requeima, J. and Temple, C.A., 1980. A geological reconnaissance of the Lac du Bonnet Batholith; Atomic Energy of Canada Limited, Report 6439, 68p. Tarnocai, C., Kettles, I.M., Ballard, M., 1995. Peatlands of Canada; Geological Survey of Canada, Open File 3152. 1:6 000 000 map with marginal notes. Wilkins, C., 1986. Manitoba’s magnificent limestone graces Canada’s finest buildings; Canadian Geographic, v. 106, no. 1, pp. 28-37. 94 �Field Trip 5 Separation Rapids Rare-Element Pegmatite Field, Ontario Charles Blackburn Blackburn Geological Services Site 130, Comp. 21 Kenora, Ontario P9N 3W Don Bubar President and CEO Avalon Ventures Ltd. 1116-1111 Richmond Street West Toronto, Ontario M5H 2G4 Carey Galeschuck Project Geologist Tantalum Mining Corporation of Canada Limited Box 2000 Lac du Bonnet, Manitoba R0E 1A0 Alasdair Mowatt President Emerald Fields Resources Corporation 1546 Pine Portage Road Kenora, Ontario P9N 2K2 Chris Pederson Consulting Geologist Karen Rees General Manager Avalon Ventures Ltd. 777 Red River Road Thunder Bay, Ontario P7B 1J9 Tony Pryslak A.P. Pryslak Geological Services 15 Hunterspoint Rd. Winnipeg, Manitoba R3R 3B6 Aerial view of the Big Whopper pegmatite, from the southeast �FOREWORD Separation Lake and surrounding area has in recent years emerged as, if not the most, certainly among the most, important host to rare-element pegmatites in Ontario. The Separation Rapids pegmatite field (Figure 1), located where the English River forest access road crosses the English River near Separation Rapids, was first discovered in the 1993 field season by Fred Breaks of the Ontario Geological Survey (OGS). However, the presence of beryl-bearing pegmatites had been known long before roads were pushed into this region, since at least the 1930s, when a Geological Survey of Canada field crew working its way along the lakes and waterways of the English River noted beryl "in a large pegmatite dyke cutting volcanics on the east shore of English River 2 miles northwest of Separation rapids" (Stockwell 1932). Access in those days was difficult, and so possibilities of exploitation lay dormant for 50 years. Separation Lake and the surrounding area was included in a 37 000 km² reconnaissance survey of the present English River and Winnipeg River subprovinces in the 1970s (Breaks and Bond 1993), and although other previously known rare-metal pegmatites were examined in some detail, little attention was paid to the pegmatites at Separation Lake. Then, in the 1980s, Carmen Storey of the OGS, as part of a broad evaluation of the industrial mineral potential of a large part of northwest Ontario, sampled what was possibly the same dike as Stockwell had examined and noted accessory red garnet and apatite. He also found lithium and berylium assay values in other pegmatites recently exposed along the newly opened right-of-way for the English River road (Storey 1990). It was not until late in the field season of 1993, when Fred Breaks, following a long summer of investigation of the Raleigh lake pegmatites, and taking the opportunity to visit the OGS field camp of Charlie Blackburn at Separation Rapids, knowing of the beryl mineralization and suspecting that the pegmatites there might show some further characteristics of the prized rare-element group, that the real potential began to emerge (Breaks 1993). Blackburn was completing the second year of a broad geological survey of the never-before-mapped Separation Lake greenstone belt. He and Jeff Young (Blackburn and Young 1993) had become involved in the possibilities of base metal potential in the belt, at that time being explored by Champion Bear Resources Ltd., and had not realised that other more exciting and exotic metals lay beneath their feet. Tony Pryslak, working for Champion Bear, had also encountered beryl-bearing pegmatites in their base metal exploration program. A half-day of fieldwork convinced Breaks that he was on to something, and in the next few days he laid the foundation for what was to eventually develop over the next 5 field seasons into the discovery of the Big Whopper and Big Mac pegmatites and numerous other bodies. Breaks (1993) was quick to realise that the Separation Rapids pluton, exposed on islands and along the shoreline of the English River, bears striking similarity in size, constituent granitic units and mineral content to the peraluminous Greer Lake pluton of the Winnipeg River Pegmatite District in Manitoba, the location of the Tanco Mine. He related the pegmatites, that had up to that time only been discovered on the 96 �97 Figure 1. General geology and distribution of rare-element groups in the Bird River-Separation Lake metasedimentary-metavolcanic belt, southeast Manitoba and northwest Ontario. Orthopyroxene-in isograd outlines the Umfreville-Conifer lakes granulite zone. Figure taken from Figure 1 in Breaks and Tindle (2002). �east side of the river, to the pluton, and called the complete package the Separation Rapids Pegmatite Field. In the following field season Breaks began what was to become a detailed investigation of the area around Separation Rapids, in partnership with colleagues Andy Tindle (Breaks and Tindle 1994) and Yuanming Pan (Breaks and Pan 1995). Thanks to painstaking work, discovery of the Big Whopper pegmatite was made by Breaks and Tindle in 1996 on the west side of the English River (Breaks and Tindle 1996, 1997). Following announcement in 1996 of the discovery, the Big Whopper was staked by local prospectors Bob Fairservice and Jim Willis. Further expansion of the Separation Rapids pegmatite field was made to the west. Discovery of the Big Mack was made in 1998 by two other local prospectors, Al Mowatt and Phil Thorgrimson. Meanwhile, Tantalum Mining Corporation of Canada Ltd. (Tanco) geologists Carey Galeschuk and Peter Vanstone were further exploring the numerous pegmatites on ground earlier investigated by Champion Bear to the east of the river. Tanco continues to explore under a joint venture agreement with Gossan Resources Ltd. Other companies that became major stakeholders included Avalon Ventures Ltd. (Big Whopper), Emerald Fields Resources Corp. (Big Mack) and Champion Bear Resources Ltd. (Marko's Pegmatite). Most recently Tony Pryslak and Seymour Sears, working for Champion Bear Resources, enabled further expansion of the field to a minimum 6.5 km strike length, with their discovery of a number of rare metal pegmatites (e.g. the Glitter Zone) further to the west. INTRODUCTION Breaks and Tindle (1997) have pointed out that: "Rare-metal class pegmatites of the complex-type and petalite-subtype represent the most desirable target for tantalum, cesium, rubidium and ceramic quality petalite in Archean terrain settings......Current economic interest is focussed upon the petalite potential.......The widest part of the Big Whopper Pegmatite averages 37% petalite over 60 m which is comparable to the world's premiere petalite deposit at the Bikita Pegmatite of southern Zimbabwe. The tantalum potential is also considerd significant as wodginite, the chief ore mineral for Ta at the Tanco Mine of southeastern Manitoba, is not only widespread in the Separation Lake area, but also exhibits compositional variation unlike any other pegmatite group on a global scale. Cesium.....also has high exploration potential as pollucite, the only ore mineral for the metal, has ...been verified in the area." So-called "fertile" granite/pegmatite systems are typically peraluminous and of an S-type heritage (Breaks and Tindle 1997). The 4 km² Separation Rapids pluton represents a classic example of a fertile granite. It has generated a rare-element pegmatite field with a minimum presently known east-west dimension of 12 km that is 3 km at its widest. The pluton compares in size and constituent granitic units with the Greer Lake pluton (Cerny et al 1981) 55 km to the northwest in the Winnipeg River Pegmatite District of southeast Manitoba (Figure 1). Similarities include presence of cordierite, beryl, cassiterite and the common presence of primary layering between pegmatitic leucogranite, sodic aplite, 98 �99 Figure 2. Distribution of rare-element pegmatite mineralization in the Separation Rapids pegmatite group, and location of field trip stops 1 through 5. Modified from Figure 3 in Breaks and Tindle (2002). �potassic pegmatite and coarse grained granite. Beryl has been found at numerous places that constitute a zone in the southern portion of the pluton, either as a primary phase, or secondary with garnet, muscovite and biotite after cordierite. The pegmatites have recently (Breaks and Tindle 2002) been grouped into an eastern and a western subgroup, based on their position relative to the Separation Rapids pluton. However, both subgroups exhibit a beryl zone and a complex, petalite-bearing zone. The Big Whopper, Big Mack and Glitter Zone pegmatites are located within the petalite subzone in the western subgroup, and the Marko's and James' pegmatites are in the complementary subzone in the eastern subgroup. Audrey's pegmatite lies within the beryl zone, just outside the petalite zone. REGIONAL GEOLOGICAL SETTING The Separation Rapids pegmatite field is set in the heart of the Separation Lake greenstone belt (Figures 1 and 2). Metavolcanic and subordinate metasedimentary rocks occur discontinuously along the English River-Winnipeg River subprovincial boundary from the Ontario-Manitoba border in the west to western Lac Seul in the east, a distance of about 100 km. They represent the eastern extension of the Bird River greenstone belt in Manitoba (Cerny et al. 1981) (Figure 1). The Separation Lake greenstone belt (Figure 1 and 2; Blackburn and Young 2000; Blackburn et al 1994a,b) is the largest segment, extending from the east shore of Umfreville Lake to Helder Lake, a distance of 45 km, and with a maximum width of 5 km. It consists predominantly of a lower sequence of mafic metavolcanic rocks, with intercalated magnetite-bearing iron formations, a single discontinuous clastic metasedimentary unit, and overlying subordinate felsic metavolcanic rocks. Gabbro sills intrude the mafic metavolcanic sequence. A thin unit of polymictic conglomerate and sandstone lies along the northern margin of the belt. Along the length of the belt the volcanosedimentary assemblage faces predominantly to the north. However, the lower sequence is folded about the westerly plunging Separation Narrows anticline, while in the west folding has been about the easterly plunging Paterson Lake antiform. Metamorphic grade is amphibolite throughout the belt. Breaks and Tindle (2002) suggest on the basis of geochronology done in the Bird River portion in Manitoba that the belt has an age range of >2844 Ma to 2740 Ma, while the Separation Rapids pluton has been dated at 2646 +/- 2 Ma (Larbi et al 1999). The English River Subprovince, extending north from the Separation Lake belt to the Uchi Subprovince, is comprised of metasedimentary migmatites (50%), and felsic to intermediate plutonic rocks comprised of a tonalitic suite in the west and a granodiorite to granite suite in the east (Breaks 1991). Metamorphic grade varies from amphibolite to granulite, and has affected all rocks except 100 �those of a peraluminous suite (Breaks 1991). The Winnipeg River Subprovince, south of the Separation Lake belt, is comprised of felsic to intermediate plutonic rocks ascribed to two suites, an early tonalitic suite in the north and a later granitic suite in the south by Beakhouse (1991). Rocks of the tonalitic suite in the subprovince are metamorphosed to medium to high grade, while granitic suite rocks were either synchronous with or postdated regional metamorphism (Beakhouse 1991). GEOLOGY OF THE PEGMATITE FIELD The Separation Rapids pluton and the Separation Rapids rare-element pegmatite field lie entirely within the greenstone belt (Figure 2). There appears to be little direct relationship between the pegmatite field and the stratigraphy of the greenstone belt. However, deformation events could have provided convenient structural traps into which the pluton and the pegmatites were emplaced. A major folding event, represented by folding about the Separation Narrows anticline, preceded emplacement of the cross cutting Separation narrows pluton, dated at 2646 +/- 2 Ma, as noted above. De la Fuente (1998), in a study done for Tanco, interpreted three deformation phases, such that D1 and D2 predated emplacement of the Separation Narrows pluton. He interprets the pluton to therefore be parent to pegmatites that are only weakly deformed, such as the Marko's pegmatite. In his interpretation, the Separation Narrows pluton cannot be parental to the Big Whopper and Big Mack pegmatites, both of which are complexly folded. He suggests that the source of the Separation Narrows pluton may be at depth either within the greenstone belt or "within the mainly undeformed Winnipeg River subprovincelate granites outcropping to the south" (de la Fuente 1998). His analysis that the Treelined Lake granite was involved in D2 and D3 deformation is consistent with this granite being a possible source of the Big Whopper, Big Mack, James and other complexly deformed pegmatites. These structurally based interpretations of relative timing of emplacement of various pegmatites and granitic bodies differ from the interpretation of Fred Breaks and colleagues (eg. Breaks and Pan 1995; Tindle and Breaks 2000: Breaks and Tindle 2002), made on mineralogical and geochemical arguments, that there is a consistent evolutionary trend from the Treelined Lake granite complex through the Separation Narrows pluton, to the various pegmatite groups. Detailed discussion of all other aspects of the Precambrian geology of the Separation Lake area, such as lithology, stratigraphy, metamorphism and mineral deposits, other than those associated with the rare metal pegmatites, has been made by Blackburn and Young (2000). Breaks and Tindle (2002) have recently presented a detailed account of the Separation Narrows pegmatite field, from which much of the rest of this section is paraphrased or quoted in parentheses. 101 �Treelined Lake Granite Complex The Treelined Lake granite complex is a "peraluminous granite mass situated in the adjacent English River subprovince. This granite mass is an irregular-shaped, 3 to 23 by 63 kilometer mass situated mostly in the core of the Umfreville-Conifer lakes granulite centre." (Figure 1). There is an abrupt regional metamorphic discontinuity at the boundary between the English River subprovince and the Separation Lake greenstone belt, jumping from upper amphibolite in the greenstones to granulite in the migmatized clastic metasedimentary rocks to the north. Although not a field trip stop in the present guide, the boundary is described in detail as Stop 1-6 in the Western Superior Province Fieldtrip Guidebook for Precambrian '95 (Beakhouse et al 1995). Rocks of the Treelined Lake granite complex characteristically contain the metamorphic minerals garnet, orthopyroxene, cordierite, while the "southwestern apophysis consists mainly of garnetbiotite and muscovite-biotite granite with local, in situ pegmatite zones that contain rareelement-enriched mineralogy." Breaks and Tindle (2002) discuss such a pegmatite that occurs very close to the boundary adjacent to the Umfreville Road that contains "tourmaline, topaz, cassiterite, gahnite, fluorapatite,....microlite, manganocolumbite and manganotantalite." Separation Rapids Pluton The Separation Rapids pluton (Figures 2) has a "core of coarse-grained, potassiumfeldspar-porphyritic, garnet-biotite-muscovute granite that is enveloped by a larger area composed of various pegmatitic granite units." Variable textural and mineralogical features include: wide range in grain size, from aplite to potassic pegmatite and pegmatitic leucogranite (with potassic megacrysts up to one meter in diameter); graphic, plumose, radial, and unidirectional solidification textures; layering among units; a peraluminous mineralogy of cordierite, primary muscovite, biotite, garnet and schorl tourmaline; and metasomatic rare-element-rich biotite and muscovite along contacts with mafic metavolcanic host-rocks and enclaves. "Rare-element minerals.....are largely confined to...the southeastern part of the pluton. These comprise occurrences of green and white beryl, columbite-tantalite group minerals and cassiterite in potassic pegmatite, various sodium-rare-element-enriched pods and layers (albitite, albite trondhjemite and muscovite-quartz-cleavelandite pods) and more rarely in fine-grained leucogranite." Rocks of the pluton are conveniently exposed on the shore-lines and islands of the English River. Eastern Subgroup Pegmatites The eastern subgroup of pegmatites (Figure 2) covers a 7.5 km² area to the east of the English River. It "comprises a narrow, 0.5 by 5 kilometre, central axis of 11 complex type, petalite subtype pegmatites that is almost completely enveloped by zones of beryltype pegmatites." "The beryl zone contains dikes of pegmatitic leucogranite, potassic pegmatite and minor sodic aplite.....Green and white beryl....is the most widespread rareelement mineral. Cassiterite, ferrocolumbite and gahnite represent widespread accessory 102 �minerals. (In the beryl zone) Wodginite.....has only been documented in Audrey's pegmatite." Topaz has been found in four dikes in the beryl zone. The petalite zone pegmatites characteristically, in addition to petalite, contain a diverse population of oxide minerals, in particular wodginite. The 8 by 130 metre Marko's pegmatite in particular contains a diversity of wodginite species (viz. titanowodginite, ferrowodginite, ferrotitanowodginite, and tungsten-rich wodginite). Marko's pegmatite is notable also for its zonation, containing four primary zones and two replacement units. Features of the primary zones are dominated by megacrysts of petalite and of potassium feldspar. In the replacement zones: spodumene-quartz intergrowths (the SQUI so common at Tanco) occur within petalite megacrysts; lepidolite replaces muscovite; and oxide minerals are especially conspicuous. Southwestern Subgroup Pegmatites The southwestern pegmatite subgroup of pegmatites (Figure 2) occupies a 0.3 to 0.8 by 6.5 kilometre area to the west of the English River. It is divisible into two zones, a beryl type and a petalite subtype zone. The beryl zone contains numerous small and large pegmatites with major mineralogy similar to those of the eastern subgroup. The petalite zone contains nine relatively larger, deformed pegmatite lenses, the largest of which in surface outcrop are the 56 by 650 metre Big Whopper and the 30 by 100 metre Big Mack. "The initial resource estimate of Avalon Ventures Limited....revealed the Big Whopper pegmatite to contain 7.1 million tonnes with an average of 1.285% Li2O, 0.346%Rb2O and 0.007% Ta2O5 over a strike-length of 600 metres and to a depth of 200 metres." At the Big Mack, "a preliminary estimate of 300 000 tonnes averaging 30.5% petalite to a 65 metre depth has been indicated by the initial diamond drilling program." "Petalite content of the core of the Big Whopper and Big Mack pegmatites ranges from 30 - 60%.....Petalite is of optimum quality for use as a direct feedstock in the lithium glass and ceramic industry." Extremely low iron contents, a deleterious metal in the glass-making industry, are indicated in samples taken by Breaks and Tindle that analyzed at <5 to 123 ppm Fe. "Furthermore, Li2O contents (4.6 - 4.7 %) are somewhat higher than the 4.3% average of six Li2O analyses compiled from the literature." "Other phases in the petalite zone pegmatites include garnet...., cordierite, lepidolite, cookeite, spodumene, eucryptite, bikitaite, holmquistite, topaz and chrysoberyl. Oxide phases include cassiterite, gahnite, ferrowadginite, ferrotapiolite, ferrocolumbite, manganocolumbite, ferrotantalite, struverite, and yttro- and yttrian pyrochlore (Tindle and Breaks 2000). Potassium and sodium feldspar minerals, of high purity, represent potential valuable byproducts of the exploitation of petalite pegmatites in the area. Potassium feldspar from the southwestern subgroup also reveals a significant variation in Rb content.....(Those) from the Big Whopper pegmatite indicate a rubidium content mainly in the 1.5 to 2.0 wt. % range with a maximum value of 3.0 wt %." 103 �FRACTIONATION TRENDS IN RARE ELEMENT PEGMATITES Oxide "minerals of the columbite-tantalite group are the most common Nb-Ta species in rare element pegmatites" (Tindle and Breaks 2000). Following Cerny and Ercit (1985), Tindle and Breaks (2000) have used the columbite-tantalite quadrilateral (Fig. 3) to Figure 3. Columbite-tantalite quadrilateral. Vectors describe variation trends in beryltype and complex-type pegmatites. From Fig. 9, Tindle and Breaks (2000). analyse fractionation trends in both beryl-type and complex-type pegmatites of the Separation Narrows rare element pegmatite field. Fractionation trend may be strongly influenced by the activity of fluorine, as indicated in the evolution paths shown in the quadrilateral (Fig. 3). Changes in bulk chemistry, increase in temperature and decrease in pressure result in the formation of petalite, lepidolite and amblygonite subtype pegmatites, all noted for their high fluorine activity. Tindle and Breaks (2000) have subdivided the Separation Rapids pegmatites into Fesuites and Mn-suites on the basis of columbite-tantalite (oxide mineral) compositions. Figures 4, 5,and 6 demonstrate the use of this classification for the four pegmatites to be visited on the field trip. Data for those of the southwest sub-group complex-type pegmatites are shown on Fig. 4, and for the eastern sub-group complex-type pegmatites on Figs. 5 and 6. Data for a number of other pegmatites are also included on the diagrams, taken from Tindle and Breaks (2000). For clarity, envelopes have been added around the data for the Big Mack, Big Whopper and lepidolite unit of the Big Whopper (Fig. 4), Marko's pegmatite petalite core and outer layer (Fig. 5) and James' pegmatite (Fig. 6). 104 �Figure 4. Columbite-tantalite quadrilateral: major SW subgroup petalite pegmatites. Modified from Fig. 13a, Tindle and Breaks (2000). In Fig. 4, the Big Mack pegmatite data clearly fall in the Fe-suite, as do most of the pegmatites of the southwest sub-group. However, data for the Big Whopper pegmatite are spread over a large area of the quadrilateral, while data for the lepidolite unit show extreme fractionation along the high Mn side of the quadrilateral. It is suggested that the apparent randomness of Big Whopper data reflect the complex folding this pegmatite has undergone. The Marko's pegmatite (Fig. 5) is the only Mn-suite petalite-subtype in the eastern subgroup. According to Tindle and Breaks (2000), the fractionation trend is from the outer, earlier crystallizing, layered pegmatite-aplite unit toward the late crystallizing petalite rich core. In Fig. 6, the James' pegmatite data fall in the Fe-suite. The differentiation trend is subvertical in the diagram, indicating fluorine-poor conditions. However, the pegmatite is more evolved than the primitive dikes #9 and #10 (same diagram) and equivalent to approximately 50% of the samples from the Big Whopper (Fig. 4). 105 �Figure 5. Columbite-tantalite quadrilateral: eastern subgroup, manganese suite petalite pegmatite. Modified from Fig. 12b, Tindle and Breaks (2000). Figure 6. Columbite-tantalite quadrilateral: eastern subgroup Fe suite petalite pegmatites. Modified from Fig. 12a, Tindle and Breaks (2000). 106 �THE FIELD TRIP The trip commences in Kenora. Proceed north on Highway 659 to the turn-off on to English River Road, just south of Redditt. Take the English River Road to the turn-off on to the Sand Lake Road, about 7 km south of the Separation Narrows bridge. Proceed along the Sand Lake Road to the Emerald Fields Resources access road, to the Big Mack pegmatite (Fig. 2). STOP 1 - BIG MACK PEGMATITE The Big Mack pegmatite (Fig. 7) is complexly folded and compressed into a 35 m x 100 m lens with several prominent apophyses tapering to the south, southeast and west. These apophyses consist of non-petalite bearing sodic aplites, blocky potassic pegmatite and holmquistite bearing granitic units. Several units will be observed within the Big Mack: A) Wall zone of medium blocky quartz+plagioclase+muscovite+garnet+biotite. Cordierite is common in this unit and in the apohyses. It is generally altered to garnet+mica+holmquistite rich simplectites that give the unit and several metres of the interior petalite zone a spotted appearance. B) Petalite rich, medium to coarse blocky phase of quartz+plagioclase+Kspar+muscovite+petalite. The petalite varies up to 60% in this unit and is identified by light brown weathering. C) Chrysoberyl bearing petalite pegmatite. This unit is restricted to a lens at the south end of the trench. It is grey due to the dominance of biotite over muscovite. Petalite content is generally lower than in unit B (15-20%), and it is generally finer grained but still blocky in nature and contains sporadic lime green 1-15 m chrysoberyl crystals. D) Primary aplite layered with petalite pegmatite. Folds are defined by this unit. E) Replacement albitic unit seen as white weathering layers and pods of muscovite+garnet+albite+quartz+K-spar. This unit is best observed enveloping the mafic metavolcanic screens. F) Post deformation, fracture controlled to vein like features include bikitaite, holmquistite and eucryptite. The eucryptite can be recognized by its grey, recessive weathering, capped by a lacy network of quartz veining. Massive to pillowed mafic flows can be observed on the hill north of the parking area. 107 �108 Figure 7. The Big Mack pegmatite, showing location of units discussed in the text. �Return to the Sand Lake Road, and then to the Separation Narrows bridge. Board boats and travel down stream, passing through Separation Rapids, to Heart Island on the Separation River (Fig. 2). STOP 2 - SEPARATION RAPIDS PLUTON The Separation Rapids Pluton is interpreted by Tanco geologists to be a flat lying, sheetlike, layered, very fractionated pegmatitic granite. The interpretation of the pluton being a sheet rather than a stock-like body is based on an aeromagnetic survey flown for Tanco (Assessment Files, Ministry of Northern Development and Mines, Kenora) that shows a magnetic pattern of similar continuity, amplitude and intensity as in the surrounding volcanic rocks extending beneath the Separation Rapids Pluton. Figure 8. Location of Heart Island in the Separation Rapids pluton. Heart Island (Fig. 8) displays classic pegmatitic granite features such as “bird’s-foot mica”, megacrystic potash feldspars, pegmatitic vugs, and aplite banding. If time permits, Red Handed Island, the larger island to the west of Heart island, will be visited. This island features a flat lying lepidolite-bearing pegmatitic granite that postdates D1 and D2 deformation phases. It is affected by D3 phase open folds with E-W 109 �axes gently plunging to the west. It may represent an external facies of the Separation Rapids pluton. Proceed by boat from Heart Island to Avalon Ventures Ltd.'s boat landing on the west shore of the river. A short walk inland leads to the Big Whopper pegmatite (Fig. 2). STOP 3 - BIG WHOPPER PEGMATITE Introduction Following staking of ground over the Big Whopper by local prospectors Bob Fairservice and James Willis in 1996, the 560 acre property (since expanded to 4480 acres) was optioned to Avalon Ventures Ltd. which has earned a 100% interest, subject to a 2% NSR royalty interest retained by the vendors. Figure 9. Big Whopper pegmatite. Areas 1 and 2 of the field trip stop are indicated. In 1997 and 1998, Avalon Ventures Ltd. began exploring the property by conducting linecutting, ground magnetic, geological and geochemical surveys, overburden stripping, trenching, mineralogical studies and diamond drilling totalling 8,751 metres in 57 holes. This work delineated the Big Whopper (Fig. 9) over a strike length of 1.5 km with widths 110 �ranging from 10 m to 80 m and to a vertical depth of 250 metres where it remains open. Total indicated and inferred petalite resources are estimated at 11.6 million tonnes grading 1.34% Li2O, 0.30% Rb2O and 0.007% Ta2O5. In 1999, a pre-feasibility study was completed on the deposit by Micon International Ltd. which concluded that development of the deposit as a producer of petalite plus feldspars, mica and tantalum, was economically viable and recommended proceeding with a bulk sampling program and full feasibility study. This work has not yet begun. In 2001, in response to higher tantalum prices, Avalon conducted a program of 1,401 metres of diamond drilling in 12 holes, channel sampling, mineralogical and metallurgical studies to better define tantalum distribution within the Big Whopper pegmatite system. The dominant economic minerals in the deposit are petalite and columbite-tantalite, the ore mineral of tantalum. The deposit also contains significant quantities of rubidiumpotash feldspar, albite, lepidolite, and cassiterite. At the Big Whopper, which is hosted entirely within amphibolites, a north-directed compressional tectonic event produced flattening and a strong vertically oriented regional schistosity striking west-northwest. This schistosity is folded about a sub-vertical axis, with minor folds commonly observed both in the pegmatite and the amphibolite host rocks. The Big Whopper itself exhibits tight s-fold geometry, with the thickened central portion of the pegmatite coinciding with the hinge zone, and attenuated limbs extending to the east and west. Fold axes exhibit vertical to sub-vertical orientations. Parallel mineral lineations indicate a strong vertical stretching component, with aspect ratios in the order of 5:1 to 10:1. Small pegmatites flanking the Big Whopper commonly exhibit boudinage structures indicative of a high-strain environment. Mineralogy and Zonation Mineralogical zoning observed in the Big Whopper is characteristic of highly evolved rare metal pegmatites, with well-developed wall zones and internal intermediate zones classified according to their dominant constituent minerals (Figure X). Metallogenic zoning is closely related to mineralogic zoning. The predominant mineralogical zones of the Big Whopper are as follows: 1. Wall Zone (predominantly albitite) 2. Megacrystic Feldspar and Quartz-Mica marginal Zones 3. Petalite (intermediate) Zone 1. The Wall Zone is a narrow 1 to 10 metre wide endocontact zone of albitite consisting essentially of saccharoidal to aplitic albite with accessory K-feldspar, muscovite, quartz, and Mn-rich garnet (spessartine). Proximal dykes and stringers exhibit the same mineralogy as the Wall Zone albitite. Cassiterite and tantalum oxides are commonly associated with the albitite, along with rare gahnite (a zinc spinel). Albitite characteristic of the Wall Zone commonly occurs in intimate association with the Megacrystic Feldspar Zone and these zones together with the Quartz Mica Zone, likely reflect early, more primary phases of pegmatite development. 111 �2. The Megacrystic Feldspar Zone is confined to the eastern and western internal margins of the Big Whopper pegmatite and resembles pegmatitic granite. It consists predominantly of orange-pink to grey-white, coarse to megacrystic K-feldspar and coarse silvery mica aggregates in an albitic matrix. Highly elevated whole rock rubidium values averaging in excess of 0.3% Rb2O are attributed to Rb-K-feldspar as an essential mineral in the pegmatite. The Quartz Mica Zone occurs in intimate association with the Megacrystic Feldspar Zone, and is generally enveloped by or interdigitated with it. It contains 50% quartz, 30% muscovite, and 20% K-feldspar. Elevated whole rock lithium values in this zone (up to 0.5% Li2O) are attributed to the presence of micas of the lithian muscovite to lepidolite series. 3. The Petalite Zone is an intermediate zone of the Big Whopper pegmatite and is the largest defined to date, comprising approximately 80% of its volume. The essential mineralogy consists of petalite, Rb-K-feldspar, albite, quartz and mica. A crude but distinct petalite zoning can be identified within the Big Whopper pegmatite as tightly folded layers with progressive fractionation increasing eastward. Ribbon-like, white petalite displaying schlieren-like habit (Type A) grades to coarse pink and pink-white petalite (Type B), and to blue-grey to pink-grey petalite (Type C). A fourth sub-zone (Type D) is recognized based on the presence of the purple mica lepidolite and occurs peripheral to Types A-C, mainly to the south and east. This zone is enriched in tantalum, typically assaying greater than 0.01% Ta2O5. Type C is a very fine-grained, equivalent of Types B and A, which is very highly foliated, mica-rich unit and commonly occurs interlayered as coarse-grained bands and lenses within Types B and A. Petalite in this unit tends to be partially altered to spodumene-quartz intergrowth (“SQUI”) exhibiting a net texture. Mineralization The main economic minerals of the Big Whopper deposit are petalite, Rb-K-feldspar, albite, lepidolite and columbite-tantalite. The Big Whopper pegmatite system is characterized by unusually high purity end-member compositions of the constituent minerals, a feature reflecting the highly evolved chemistry of the system. Petalite (LiAlSi4O10) is almost stoichiometrically pure, averaging close to the theoretical maximum lithium content of 4.8% Li2O, with only traces of soda, potash, and iron. It averages about 25% of the ore, and varies from white to pale pink in colour. Albite averages 11% soda, 0.10% potash, 0.35% lime, 0.01% iron oxide (Fe2O3). It makes up 40% of the ore, and on average ranges from white to bluish-white in colour. Rb-K-feldspar constitutes 10 to 15% of the ore. Although known in other pegmatites where it generally exhibits perthitic intergrowth, the Big Whopper variety carries only 0.3-0.4% Na2O along with 2.8% Rb2O, 15-16% K2O and. It is generally grey-white in colour and typically occurs as large megacrysts in the petalite ore. Lepidolite (K Rb(Li,Al)2-3(AlSi3O10)(O,OH,F)2) is a distinct purple-coloured mica that occurs in marginal zones of the Big Whopper and in separate flanking dykes. The mineral is an ore of rubidium, containing up to 4% Rb2O, and can comprise up to 15% of the ore. 112 �Columbite-tantalite (Mn,Fe)(Nb,Ta)O6 Manganocolumbite and manganotantalite predominate with rare microlite and ferrocolumbite, all occurring as fine-grained dark brown opaques. Tantalum is well distributed through the deposit but is typically enriched to levels exceeding 0.01% Ta2O5 in marginal lepidolite-rich petalite zones and albitite dykes. Other potentially valuable industrial minerals include muscovite mica (some with elevated lithia) which may comprise up to 15% of the ore, spodumene averaging 3-5% occurring as SQUI replacement rims on petalite, cassiterite, pale pink spessartine garnet and high-purity quartz. Minor accessory minerals include apatite, zircon, gahnite, monazite, xenotime, rare sulphides, sulphosalts, and thorite. Field Stop Descriptions Area #1: Big Whopper Petalite Deposit Main Mass The large stripped exposure of the Big Whopper main mass reveals all of its major mineralogical zones and sub-zones. The Wall Zone and feldspathic zones are exposed on the northwest side of the outcrop. The coarse white and pink petalite zone (Type B) is well exposed in the trench in the central part of the outcrop. It is flanked by Type A and C petalite mineralization with Type D (lepidolite rich) occurring on the south side and in a separate exposure to the east. The surface trench along the top of the Whopper averages 1.58% Li2O, 0.33% Rb2O and 0.007% Ta2O5 across 58.90 metres. Amphibolite screens occur within and at the margins of the Big Whopper. These screens are commonly disjointed with mullioned terminations, but are continuous to depth. Narrow 2 to 5 cm wide albitic haloes characteristically rim most of these screens, and show remarkably little variation in width regardless of the size of the amphibolite screen. These rims are interpreted as reaction fronts or depletion haloes, in which are concentrated lithophile elements (specifically Li, Rb, and Cs) forming mica-rich (glimmerite) selvedges to the amphibolite screens. The white albitic rims are depleted in these elements, but commonly exhibit elevated tantalum values of up to 0.049% Ta2O5. Area #2: Lepidolite Dike Lepidolite-rich petalite pegmatite dikes occur flanking the Main Mass of the Big Whopper pegmatite system mainly to the east. These dikes tend to be enriched in tantalum relative to the Main Mass of the Big Whopper reflecting the greater abundance of more high-evolved tantalum minerals such as microlite and wodginite in the Lepidolite Dyke. Channel sampling of this outcrop has produced an average of 0.023% Ta2O5 over 3.15 metres, with individual assays up to 0.031% Ta2O5. Dark blue fluor-apatite is a common accessory mineral Return to Avalon's landing on the Winnipeg River, and by boat to the Separation Narrows bridge. Proceed north on the English River Road about 3 km to the Umfreville 113 �114 Figure 10. Marko's pegmatite, showing location of zones discussed in the text. �Road turn off. A skidder trail leads from the Umfreville Road to Marko's pegmatite (Fig. 2). STOP 4 - MARKO'S PEGMATITE Marko's pegmatite (Fig. 10) extends along strike for a distance of 190 m in an east-west direction. It has a shallow dip to the south and is discordant to the near vertical dip of the iron formation that is its host rock. The pegmatite has a maximum thickness of 15 m and a maximum down dip extension of 30 m. Drill intersections indicate in cross section that Marko's pegmatite occupies a tension fracture that extends from the gabbro/iron formation contact at surface and progresses south across the iron formation at a relatively shallow angle but near normal to the primary layering in the iron formation. In longitudinal section Marko's pegmatite plunges 5-10º to the west. A second pegmatite, the North Marko's pegmatite, lies along the gabbro/mafic metavolcanic contact at surface, 20 m to the north of marko's pegmatite. Diamond drilling shows that the pegmatite extends to a depth of 40 m as a single, near vertically dipping sheet and then splits into north and south dipping sections. The south limb increases in size and degree of differentiation in the down dip and easterly directions. The North Marko's pegmatite is essentially barren at surface, but drill results show that mineralogy and geochemistry changes abruptly with depth and association with either a roll or flattening of the dike. This is likely due to the entrapment of fluorine rich fluids along these structural features. Both pegmatites are pristine and undeformed by the folding events that affected the Big Mack and Big Whopper pegmatites. Marko's pegmatite is exposed as two lenses at surface (Fig. 10). This is due to a sigmoidal roll in the moderate dip of the intrusion to the south. The internal zones can be readily traced and differentiation trends established with confidence along the surface exposures and in drill core. Zones of Marko's pegmatite to be examined are as follows: A) Wall zone comprised of quartz+albite+muscovite+beryl. Accessory minerals include black oxides (cassiterite, wodginite), green tourmaline and apatite. B) Petalite rich core zone, containing up to 95% petalite. The amount of petalite is locally variable, as seen in the westernmost part of the outcrop which consists essentially of coarse blocks of K-spar up to 2 m across, with interstitial petalite. This represents the crest of the pegmatite. C) Layered pegmatite-aplite. Quartz+garnet+biotite aplite is interlayered with a coarser muscovite+beryl granite. Fine grained black oxide minerals are common in the albite rich unit. A 1.6 m channel sample assayed 0.165% Ta2O5 and 0.10% Sn. D) Grey, fine grained granite withj minor muscovite and fine grained black oxides. 115 �E) Muscovite replacement unit of K-spar. F) Albitization of the petalite core. North Marko's pegmatite will also be examined. It has a simple assemblage at surface of quartz+K-spar+albite+mica with minor bands of aplite. Return to the Umfreville Road/English River Road intersection. Proceed north on the English River Road to a 75 m trail on the left leading to James' pegmatite (Fig. 2). STOP 5 - JAMES' PEGMATITE This highly fractionated pegmatite (Fig. 11), intruded into mafic metavolcanics, is strongly deformed and folded. De la Fuente (1998) has described it as an example of pre D2 deformation phase pegmatite. The Treelined Lake Granite, which shows the same deformation, is suggested (de la Fuente 1998) to be source granite for pre D2 pegmatites and pegmatitic granites Figure 11. Location of James' pegmatite In addition to quartz and feldspars, minerals present include green beryl, lithiophilite, petalite, spodumene blades, abundant black tourmaline, curviplanar mica (“ballpeen 116 �mica”), which is assumed to be lithium rich, and possibly zinnwaldite. Ferrowodginite has been identified in this pegmatite (Fred Breaks, personal communication). This zoned pegmatite is approximately 30 metres by 2 metres. The joint venture of Tanco and Gossan Resources Ltd. drilled the dike in 1996. The best Ta205 grades obtained were 0.035% in drill core and 0.026% at surface. The pegmatite dips to the southwest at about 60° and appears to widen at depth. End of field trip stops. 117 �REFERENCES Beakhouse, G.P. 1991. Winnipeg River Subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p.279-301. Beakhouse, G.P., Blackburn, C.E., Breaks, F.W., Ayer, J., Stone, D. and Stott, G.M. 1995. Western Superior Province Fieldtrip Guidebook, Precambrian '95; Ontario Geological Survey, Open File Report 5924, XXp. Blackburn, C.E. and Young, J.B. 1993. Geology of the Separation Lake greenstone belt; in Summary of Field Work and Other Activities 1993, Ontario Geological Survey, Miscellaneous Paper 162, p.68-73. Blackburn, C.E. and Young, J.B. 2000. Precambrian geology of the Separation Lake area, northwestern Ontario; Ontario Geological Survey, Open File Report 6001, 94 p. Blackburn, C.E., Young, J.B., Searcy, T.O. and Donohue, K. 1994a. Precambrian geology of the Separation Lake greenstone belt, west part; Ontario Geological Survey, Open File Map 241, scale 1:20 000. Blackburn, C.E., Young, J.B., Searcy, T.O. and Donohue, K. 1994b. Precambrian geology of the Separation Lake greenstone belt, east part; Ontario Geological Survey, Open File Map 242, scale 1:20 000. Breaks, F.W. 1991. English River Subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p.239-277. Breaks, F.W. 1993. Granite-related mineralization in northwestern Ontario: I. Raleigh Lake and Separation Rapids (English River) rare-element pegmatite fields; in Summary of Field Work and Other Activities 1993, Ontario Geological Survey, Miscellaneous Paper 162, p.104-110. Breaks, F.W. and Bond, W.D. 1993. The English River Subprovince – an Archean gneiss belt: geology, geochemistry and associated mineralization; Ontario Geological Survey, Open File Report 5846, v. 1 and 2, 884 p. Breaks, F.W. and Pan, Y. 1995. Granite-related mineralization in northwestern Ontario: III. Relationship of granulite metamorphism to rare-element mineralization in the Separation Lake area of the English River Subprovince in Summary of Field Work and Other Activities 1995, Ontario Geological Survey, Miscellaneous Paper 164, p. 79-81. Breaks, F.W. and Tindle, A.G. 1994. Granite-related mineralization in northwestern Ontario: II. Detailed examination of the Separation Narrows (English River) rare-element group in Summary of Field Work and Other 118 �Activities 1994, Ontario Geological Survey, Miscellaneous Paper 163, p. 109-112. Breaks, F.W, and Tindle, A.G. 1996. New discovery of rare-element pegmatite mineralization, Separation Lake area, northwestern Ontario; Ontario Geological Survey, Open File Report 5946, 9p. Breaks, F.W. and Tindle, A.G. 1997. Rare-metal exploration potential of the Separation Lake area: an emerging target for Bikita-type mineralization in the Superior Province of NW Ontario; Ontario Geological Survey, Open File Report 5966, 27p. Breaks, F.W. and Tindle, A.G. 2002. Rare-metal mineralization of the Separation Lake area, northwest Ontario: characteristics of a new discovery of complex-type, petalitesubtype, Li-Rb-Cs-Ta pegmatite in Industrial Minerals in Canada, CIM Special Volume 53, p. 159-178. Cerny, P. and Ercit, T.S. 1985. Some recent advances in the mineralogy and geochemistry of Nb and Ta in rare-element granitic pegmatites; Bulletin Mineralogie, v. 108, p. 499-532. Cerny, P., Trueman, D.L., Ziehlke, D.V., Goad, B.E. and Paul, B.J. 1981. The Cat Lake-Winnipeg River and the Wekusko Lake pegmatite fields, Manitoba; Manitoba Department of Energy and Mines, Economic Geology Report ER80-1, 216p. de la Fuente, F. 1998. Structural analysis of the Tanco's Separation Lake property, western Ontario, Canada; report for Tantalum Mining Corporation of Canada Limited, 29 p. Larbi, Y., Stevenson, R., Breaks, F.W., Machado, N. and Gariepy, C. 1999. Age and isotopic composition of Late Archean leucogranites: implications for continental collision in the western Superior Province; Canadian Journal of earth Sciences, Vol. 36, p. 495-510. Stockwell, C.H. 1932. Beryllium deposits; p.126 in Geology and mineral deposits of a part of southeastern Manitoba, by J.F. Wright; Geological Survey of Canada. Memoir 169, 150p. Storey, C.C. 1990. An evaluation of the industrial mineral potential of parts of the districts of Kenora and Rainy River; Ontario Geological Survey, Open File Report 5718, 259p. Tindle, A.G. and Breaks, F.W. 2000. Tantalum mineralogy of rare-element granitic pegmatites from the Separation Lake area, northwestern Ontario; Ontario Geological Survey, Open File Report 6022, 387p. 119 �120 �Field Trip 6 Geology of the Red Lake Camp, Ontario Andreas Lichtblau and Carmen Storey Ontario Geological Survey Ministry of Northern Development and Mines 227 Howey Street, Box 324 Red Lake, Ontario P0V 2M0 Example of late gold vein stockwork forming part of the High Grade Zone, 32 Level, Red Lake Mine. Estimated contained gold in sample: 298 ounces (at 7,284 ounces gold per ton). Photo courtesy of Goldcorp Inc. �REGIONAL GEOLOGY The Red Lake District (Fig. 1) is underlain by Archean rocks of the Superior Province of the Canadian Shield. Rocks of four subprovinces are found in the Red Lake District: 1. Uchi Subprovince rocks in the Red Lake District comprise the Red Lake and BirchConfederation Lake greenstone belts in which the bulk of exploration and mining activity has taken place. The supracrustal rocks of the Red Lake greenstone belt can be subdivided into several assemblages with ages ranging from ca. 2990 Ma to ca. 2700 Ma (Table 1). Major granitoid intrusions show a range from ca. 2734 Ma to 2699 Ma (Table 2). 2. English River Subprovince rocks, south of the Uchi Subprovince, are predominantly metasedimentary and host minor intrusive rocks similar to those in the Quetico Subprovince. 3. To the north, the Berens River Subprovince formed the core of a microcontinent. This area is underlain by ca. 2750-2690 Ma felsic plutonic rocks interpreted as a magmatic arc formed at an Andean-style margin that culminated in the Kenoran orogeny. These plutonic rocks intruded an older subtratum (North Caribou terrane) on which Mesoarchean volcanic rocks of the Red Lake belt are also interpreted to have formed. 4. The Sachigo Subprovince comprises crustal blocks ranging from Paleoarchean (>3.4 Ga) to Neoarchean (ca. 2.7 Ga) in age. Figure 1. Western Uchi Subprovince (modified from Percival et al. 2000) 122 �123 Figure 2. General geology of the Red Lake greenstone belt (after Parker 2000). Tour stops as indicated. �GEOLOGY OF THE RED LAKE BELT (adapted from Sanborn-Barrie et al. 2001) The Red Lake greenstone belt (Fig. 2) is dominated by the (ca. 2990 Ma) maficultramafic Balmer assemblage, an oceanic plain sequence; minor calc-alkalic volcanic rocks of arc-like affinity terminate the assemblage. The majority of lode gold deposits in the camp are hosted by the basal mafic-ultramafic sequence. A later diverse lithologic association, the Ball assemblage, appears to represent a shallow marine, volcanic edifice built upon the Balmer substrate. Table 1. Summary of supracrustal lithologies and radiometric ages in the Red Lake greenstone belt (modified from Parker 2000; with new ages from Sanborn-Barrie et al. 2001 and Skulski et al. 2001; final error estimates are not cited for the new unpublished ages of T. Skulski). Supracrustal assemblage English River ? U-Pb Age (Ma) <2700±6 Rock types and descriptions References Polymictic pebble conglomerate. Thought to correlate with the Austin tuff, host to the Madsen gold deposit. Strongly calc-akaline rocks. Andesitic to dacitic pyroclastic rocks SanbornBarrie et al. 2001 Corfu and Andrews 1987 ConfederationGraves (north Red Lake) 2733±1.5 Huston (Cemetery) ConfederationHeyson (southeast Red Lake) ≤2743 Well-bedded argillite and turbiditic wacke; polymictic conglomerate. Basal sequence is commonly tholeiitic to calcalkaline with lobe-hyaloclastite rhyolite flows; intermediate pyroclastic rocks; basalt; and feldspar-phyric andesite. Calc-alkaline rocks are more abundant at higher stratigraphic levels. Skulski et al. 2001 Corfu and Wallace 1986 ConfederationMcNeely (central and SE Red Lake) Trout Bay 2742; 2748+10/-5 Dominated by calc-alkaline, intermediate lapilli-tuff breccia and lapilli tuff SanbornBarrie et al. 2001 2853 Lower tholeiitic basalt sequence with associated gabbroic rocks overlain by finegrained clastic metasedimentary rocks (wacke, argillite) interlayered with subordinate intermediate pyroclastic rocks and chertmagnetite iron formation. Overlain by tholeiitic, pillowed basalts. Strongly calc-alkaline intermediate pyroclastic rocks overlain by pebble conglomerate, thinly bedded wacke and capped by chert-magnetite iron formation Interlayered, feldspathic wacke, lithic wacke and argillite; lenses of pebble and cobble conglomerates and quartz-rich pebble conglomerate and quartz arenite. Typically calc-alkaline intermediate pyroclastic rocks and rhyolite flows; komatiitic to tholeiitic SanbornBarrie et al. 2001 2748+10/-5 to 2739±3 Bruce Channel 2894±1.5; 2894±2 Slate Bay ≤2916 Ball 2940±2; 2925±3 124 Corfu and Wallace 1986; Corfu and Andrews 1987 Corfu et al. 1998 Corfu and Wallace 1986 �Balmer 2992+20/-9; 2989±3; 2964+5/-1 basalt; overlain by chert-magnetite iron formation and dolomitic marble which contains stromatolites. Tholeiitic basalt, basaltic komatiite and komatiite interlayered with subordinate chertmagnetite iron formation; minor clastic metasedimentary rocks; minor intermediate to felsic pyroclastic rocks; and rhyolite. Corfu and Andrews 1987 Table 2. Summary of lithologies and radiometric ages for major granitoid intrusions in the Red Lake greenstone belt (modified from Parker 2000; new ages cited in Sanborn-Barrie et al. 2001 and elsewhere do not have final error estimates assigned, as this U-Pb data is not yet published). Granitoid intrusion Cat Island pluton U-Pb Age (Ma) 2699 Rock types and descriptions References Potassium feldspar granodiorite SanbornBarrie et al. 2001 Noble 1989 Walsh Lake pluton 2699 Killala-Baird batholith 2704±1.5 Hammel Lake batholith Dome stock 2717±2 2718±1 McKenzie stock 2720±2 Red Crest stock 2729±1.5 Potassium feldspar- and quartz-phyric monzogranite; xenolith-rich, diorite or granodiorite; possible oxidized phase at Ranger Lake with broad magnetic anomaly Potassium feldspar- and quartz-phyric monzogranite; xenolith-rich, diorite or granodiorite, diorite or granodiorite; oxidized, magnetite-bearing marginal phase. Potassium feldspar and quartz porphyritic monzogranite; associated anorthositic intrusion. Granodiorite and augite porphyritic diorite/gabbro. Augite porphyritic diorite-gabbro; some ultramafic rocks; granodiorite Augite porphyritic diorite-gabbro Little Vermilion batholith Douglas Lake pluton 2731±3 Hornblende tonalite-granodiorite 2734±2 Biotite tonalite Corfu and Andrews 1987 McMaster 1987 Corfu and Andrews 1987 Corfu and Andrews 1987 Corfu and Andrews 1987 Corfu and Andrews 1987 Corfu and Stone 1998 Widespread ca. 2894 Ma calc-alkaline volcanism is represented in Red Lake by the Bruce Channel assemblage. Overlying this is the ca. 2850 Ma Trout Bay assemblage which includes substantial basaltic and gabbroic rocks in western Red Lake which are prospective for PGE mineralization, and which includes minor intermediate pyroclastic rocks throughout central Red Lake. The Trout Bay assemblage may correlate with Woman assemblage rocks of the Confederation Lake belt. A regional angular unconformity is interpreted to separate the Mesoarchean assemblages from the Neoarchean Confederation assemblages. Volcanogenic massive sulphide mineralization is associated with the younger sequence. A significant number of felsic 125 �units are classed as FII and FIII type rhyolites, considered highly prospective for large (Kidd Creek/Noranda type) massive sulphide deposits (Parker 1999). A newly recognized component of the Neoarchean supracrustal package is the Huston sedimentary assemblage that includes polymictic cobble- to pebble-conglomerate and argillite; clasts include jasperoidal chert iron formation, massive sulfide pebbles, and mafic flow (?) rocks, as well as well-bedded, graded turbiditic wacke and argillite. The U-Pb age of detrital zircons give single age peaks of 2743 and 2746 Ma at the cemetery and Madsen sites respectively (Skulski et al. 2001), indicating erosion of pre-existing Confederation age rocks, and deposition after ca. 2743 Ma. Recent age dating (Skulski et al. 2001) has also yielded multiple ages of detrital zircons from a fragmental unit thought to correlate with the Austin "tuff" ore zone at the former Madsen mine. Most of the Meso- and Neoarchean assemblages exposed in Red Lake are represented in this unit. Maximum age of deposition is consequently ≤2700±6 Ma. DEFORMATION (adapted from Sanborn-Barrie et al. 2001) The Red Lake greenstone belt has undergone at least three phases of deformation: 1) D0, a non-penetrative, early (pre-2748 Ma) event involving overturning of the Balmer assemblage; 2) D1, (bracketed between 2733-2742 Ma) resulted in a north trending foliation that is axial planar to F1 folds and involved east-west shortening; and 3) D2, (ca. 2720-2700 Ma) resulted in a dominantly east- to northeast-striking foliation that refolds F1 folds. A local 'deflection' of S2 around the McKenzie stock created an east-southeast striking corridor of heterogenous strain forming the "Mine Trend", from Cochenour through the Balmertown area, hosting the major gold deposits of the camp. HYDROTHERMAL ALTERATION (adapted from Parker 2000) The Red Lake greenstone belt has been affected by a large-scale (10's of kilometres) hydrothermal alteration system, resulting in approximately contemporaneous a) strong to intense, distal calcite carbonatization that affects rocks of all ages; and b) less extensive (kilometres), proximal, strong to intense ferroan-dolomite and potassic alteration, found in almost all areas hosting gold mineralization. Carbonate alteration affects both the Dome (2718±1 Ma) and McKenzie (2720±2 Ma) stocks and is overprinted by calcsilicate, skarn-like alteration formed during the intrusion of the Killala–Baird batholith (2704±1.5 Ma) and the Walsh Lake pluton (2699 Ma). The significant carbonate alteration event is therefore bracketed between 2718 and 2704 Ma, during D2. The main macroscopic features of carbonate alteration are pervasive replacement of rock matrix, open-space filling/replacement of primary porosity (vesicles, pillow selvages, hyaloclastite matrix), filling of extension veins with massive, colloform, crustiform and cockade breccia textures, networks of variably oriented veins and "jigsaw puzzle" breccia veins. 126 �Multiple stages of carbonate alteration and veining have been recognized, indicating continuous carbonatization during D2 deformation. Potassic metasomatism takes the form of sericite/muscovite alteration in greenschistfacies rocks; in ferroan-dolomite altered ultramafic rocks fuchsite occurs instead of sericite. Potassic alteration in amphibolite-facies mafic and ultramafic rocks takes the form of pervasive biotite ± muscovite. Centimetre- to metre-wide, strong to intense, biotite ± calcite ± ferroan-dolomite ± disseminated pyrite alteration halos often enclose ferroan-dolomite veins in amphibolite-facies mafic rocks. Biotite altered zones in amphibolite-facies rocks are characterized by a diverse assemblage of aluminosilicate minerals such as andalusite, staurolite and cordierite, with garnet, chloritoid, cummingtonite and anthophylite. Barren, pervasive silicification within proximal alteration zones may be due to release and remobilization of silica during periods of pervasive carbonatization. The majority of gold deposits in the Red Lake belt are quartz and arsenopyrite rich selective replacement zones of colloform-crustiform ferroan-dolomite veins and breccia. GEOLOGY OF THE CAMPBELL-RED LAKE GOLD DEPOSIT (adapted from Dubé et al. 2002) Gold has been continuously produced from the Campbell-Red Lake (formerly known as the Campbell-Dickenson) deposit since 1948: current production levels and reserves are given in Table 3. Historical production figures for the Red Lake greenstone belt are shown in Table 4. Table 3. Current gold production and reserves, Red Lake greenstone belt Mine Production to end of 2000 Production in 2001 Tonnage Grade Tonnage @ Grade @ Total Commodity Reserves at end of 2001 Total Commodity Tonnage Grade Goldcorp Inc. Red Lake Mine 74 148 tons @ 1.57 ounces per ton 85 115 ounces Au 246 618 tons @ 2.26 opt Au (223 728 tonnes @77.50 g/t) 503 385 ounces Au 3 208 000 tons (2 910 000 tonnes) (1) 1.34 opt Au (46.04 g/t ) Placer Dome (CLA) Ltd. Campbell Mine 473 000 tonnes @ 15.7 g/t Au 229 408 ounces Au 438 000 tonnes @13.3 g/t 178 139 ounces Au 1 941 000 tonnes (2 139 600 tons) (2) 16.7 g/t Au (482 800 tons @ 0.388 opt Au) (1) News release, Goldcorp Inc. February 7, 2002 (2) News release, Placer Dome (CLA) Ltd. February 14, 2002 127 (0.487 opt Au) �Table 4. Historical gold production, Red Lake greenstone belt GOLD PRODUCTION IN THE RED LAKE GREENSTONE BELT to December 31, 2001 MINE YEARS PRODUCTION OF CAMPBELL RED LAKE GOLDCORP (DICKENSON) MADSEN COCHENOUR-WILLANS MCKENZIE RED LAKE HOWEY HASAGA STARRATT OLSEN H.G. YOUNG MCMARMAC GOLD EAGLE RED LAKE GOLD SHORE BUFFALO ABINO LAKE ROWAN RED SUMMIT MOUNT JAMIE 1949 - PRESENT(1) 1948 - PRESENT(1,2) 1938 - 1976, 1997(4) - 1999 1939 - 1971 1935 - 1966 1930 - 1941, 1957(7) 1938 - 1952 1948 - 1956 1960 - 1963 1940 - 1948 1937 - 1941 1936 - 1938 1981 - 1982 1985 - 1986 1986 - 1988 1935 - 1936 1976 TOTAL ORE MILLED (SHORT TONS) GOLD PRODUCED TROY OUNCES OUNCES PER TON 17,979,851 8,619,008 8,678,143 2,311,165 2,353,833 4,630,779 1,515,282 907,813 288,179 152,978 180,095 86,333 31,986 2,733 13,023 591 552 10,335,248 3,736,704 2,452,388 1,244,279 651,156 421,592 218,213 163,990 55,244 45,246 40,204 21,100 1,656 1,397 1,298 277 265 0.575 0.434(3) 0.283(5) 0.538(6) 0.277 0.091(8) 0.144 0.181 0.192 0.296 0.223 0.244 0.052 0.511 0.100 0.469 0.480 47,752,344 19,390,257 0.406 NOTES: (1) Includes final production figures for 2001. (2) For 1997, 1998 and 1999 no production due to strike by unionised employees. (3) From 1970, includes production from Robin Red Lake. (4) Includes clean up ore and materials from the mine site. (5) Historic grade, actual grade for 1999 was 0.14 ounce per ton gold. (6) Includes production from Annco and Wilmar properties. (7) Continuous production 1930 to 1941; includes 268 ounces recovered from clean up in 1957. (8) The ore mined at Howey, before sorting totalled 5,158,376 tons. The average production from run-of-mine ore was therefore 0.0817 ounce per ton gold. Alteration facies in the High Grade Zone at Goldcorp Inc.'s Red Lake Mine have been described by Dubé et al. 2002: 1. an outer, metre-wide, garnet-chlorite-magnetite alteration with chlorite-amphibole-andalusite and locally associated centimetre- to metre-wide 'bleached zone' containing andalusitemuscovite-quartz-ilmenite ...; 2. a proximal, centimetre- to metre-wide, massive to laminated, reddish-brown, biotite-carbonate alteration with disseminated pyrite (3-5%) and carbonate veinlets in well foliated basalt; and 3. a gold-rich, strongly foliated, silicified zone with abundant fine-grained arsenopyrite, sericite, and rutile, and lesser amounts of pyrite, pyrrhotite, magnetite, and stibnite (≤15%). This third alteration facies is adjacent to the silicified auriferous carbonate veins and replaces the biotite-carbonate-rich alteration. 128 �The chronology of gold-rich replacement textures suggests a syn-D2 mineralizing event, dominated by silicification of carbonate veins, contemporaneous with boudinage of the veins. The silicified carbonate veins are hosted mainly by basalt; areas of high-grade gold mineralization are controlled by F2 fold hinges deforming the basalt-ultramafic contact. Multiple periods of silicification and gold deposition overprint and replace the carbonatization in these lower pressure hinge zones. The extremely high grade ore (>2.0 oz/t Au) currently mined at Goldcorp Inc.'s Red Lake Mine, is possibly due to a combination of factors, including the presence of a lowpermeability ultramafic cap, allowing the build-up of very high fluid pressure in the footwall basalt; the high iron content of the tholeiitic basalt, creating a chemical, as well as structural, trap for the auriferous fluids; multiple D2 strain events; repeated episodes of gold deposition and remobilization into a low pressure F2 fold hinge hosting the High Grade Zone. SUMMARY OF STOPS, SURFACE FIELD TRIP, RED LAKE BELT The first outcrops after the underground tours will traverse both the proximal-distal alteration facies and the Neo–Mesoarchean boundary. ¾ The tour then continues to the south-central portion of the Red Lake greenstone belt, within the metamorphic aureole of the Killala-Baird batholith. ¾ Outcrops at the interface of Meso- and Neoarchean assemblages expose rock units similar to those mined at the past producing Madsen mine, a high temperature, disseminated, stratabound gold deposit, quite dissimilar to deposits in the 'Mine Trend'; ¾ this is followed by a visit to the Dome stock, a mineralized granodiorite intruded into the volcanics in the central portion of the belt; ¾ followed by stops in the town of Red Lake to view the site of the Howey mine, and to examine intensely deformed rocks within the purported Howey Bay – Flat Lake deformation zone; ¾ the last stops will be within strongly altered and veined Balmer rocks in the northeast portion of the belt. 129 �STOP 1 - MESO-NEOARCHEAN CONTACT Woodland Cemetery Road and Hwy. 125 (Fig. 2) These outcrops show altered relatively low-strain pillowed basaltic komatiite flows of the Balmer Assemblage unconformably overlain by polymictic conglomerate of the Huston assemblage. The exposures are in the transition from calcite carbonatization (distal alteration) to ferroan-dolomite (proximal) alteration. The pillowed and minor massive flows show extensive iron carbonate alteration as well as iron carbonate and quartz veins. Fuchsite is present in the central part of the outcrops on the west side of the highway (cemetery side). The Campbell Mine is approximately 1.5 km to the north While the mafic flows have not been directly dated at this locality, they are typically variolitic, and show a geochemical similarity with known Balmer age rocks elsewhere; the massive and pillowed flows here can be traced to Balmertown, where an intercalated rhyolite at the Campbell mine was dated at 2989 ± 3 (Corfu and Andrews 1987). Variolitic flows occur in the northern part of the outcrops on the east side of the highway. However, they are unconformably overlain by Huston polymictic conglomerate further south along the outcrop. The conglomerate contains a large proportion of rounded cherty, jasperoidal and pyritic fragments. It represents an apron of Confederation assemblage (McNeely age-2743 Ma) detritus deposited at the break in the paleoslope between the Confederation volcanic centre and its Balmer age substrate. STOP 2 - CALCITE CARBONATIZED PILLOWED FLOWS: DISTAL CARBONATE ALTERATION FACIES Outcrops on west side of Hwy. 125 and Sandy Bay Road (Fig. 2) Slightly deformed pillows of the Balmer assemblage show pervasive calcite carbonatization, calcite veins and pods. Amygdules are also filled (replaced?) with calcite. Jig-saw puzzle breccias (created by fluid overpressure at depth) are cemented by calcite. This represents the distal, outer halo of carbonate alteration. STOP 3 - CONTACT BETWEEN CONFEDERATION AND BALMER ASSEMBLAGES Suffel Lake Road and Hwy. 618 (Figs. 2 and 3) Exposures on the south side of the highway are part of the lowermost units of the Neoarchean Confederation assemblage. The outcrops here are amphibolite-facies 130 �tholeiitic, quartz-feldspar-porphyritic lapilli-crystal tuff, with thin, dark grey, collapsed pumice fragments; occasional lapilli sized lithic clasts are also observed. Strike of the rocks is generally northeast, facing and dipping steeply southeast. A sample from this unit, 800 m northeast of the intersection, gave an age of 2744 ± 1 Ma (Corfu and Andrews 1987). The north side of the road exposes highly altered tholeiitic, mafic volcaniclastic rocks of the Balmer assemblage. Abundant garnet and biotite rims clasts; minor andalusite is present. This outcrop, barren at this locality, forms part of the Austin "tuff" ore zone, described further below. STOP 4 - MADSEN DEPOSIT, POWER LINE OUTCROPS (Figs. 2 and 3) Time limitations of the tour do not permit a complete visit of the Madsen deposit; a brief description of the deposit follows: Geology of the Madsen Deposit (adapted from Dubé et al. 2000) Madsen is a stratabound, replacement-style, disseminated gold deposit, exhibiting two alteration facies, the mineralogy of which is now represented by two amphibolite-facies zones: 1) a pervasive aluminous, metre- to tens-of-metres-wide, low-strain, outer zone, containing andalusite-garnet-biotite-staurolite-amphibole; metre-wide stockwork amphibole veins and veinlets alternate with the pervasive alteration. Timing of this alteration is pre- to syn-D1, but its relationship to gold mineralization is not yet known; indeed, it could be classified as the amphibolite-facies equivalent of volcanogenic massive sulfide (VMS) type alteration, related to a Confederation age syn-volcanic hydrothermal alteration system; 2) an inner zone comprising a banded-laminated texture, characterized by bands of actinolite-hornblende-microcline-calcite-tourmaline, alternating with biotiterich bands. The amphibole is commonly randomly oriented. Diopside locally forms disseminated crystals up to 7-8 cm long, or veinlets. Ore zones occur within the inner alteration zone, and comprise finely layered, sulfide-rich lenses up to a few metres wide. Sulfides (8-10%) comprise pyrrhotite, pyrite and/or arsenopyrite with trace chalcopyrite, and are found as disseminations or veinlets parallel to lamination/foliation. Gold occurs in the native state as inclusions in silicate minerals and locally as coatings on sulfide minerals. Highest grade is found in areas of most intense alteration, represented by quartz-biotite-muscovite-microcline assemblage in mm-cm bands or layers. Crenulation of alteration bands, sulfides and calcite veinlets by S2 as well as the large-scale deformation and folding of Austin ore lenses by F2 folds are consistent with pre- to early D2 timing of gold mineralization. A minimum age on the deposit is 2699 ± 4 Ma (Corfu and Andrews 1987), the age of a cross-cutting post-ore granodiorite dyke. 131 �1/ I 400m / // J // //// d//// oO// / Figure 3. Geology of the Madsen mine area (modified from Dubé et al. 2000) 132 �Proximal alteration and style of mineralization may indicate the Madsen deposit to be related to higher temperature (400º-600ºC) gold deposits and gold-skarn deposits hosted by mafic volcanics (Parker 2000). South Austin Zone – Powerline Section The base of this series of poorly exposed outcrops is a well banded/layered example of Austin "tuff", from which the bulk of the 2.5 million ounces gold of the Madsen deposit were mined between 1938-1976 (Table 4). At this locality the Austin is a strongly altered (biotite, amphibole, garnet) mafic volcaniclastic/epiclastic rock, with wacke and conglomerate clasts, occupying the position of the unconformity between Balmer and Confederation assemblages. Further up the hill the Confederation age quartz-feldspar porphyritic lapilli tuff unit from Stop 1 forms the structural and stratigraphic hangingwall of the deposit and marks the beginning of Confederation time. Overlying this unit is an altered (biotite, garnet) polymictic conglomerate outcrop, part of the Huston assemblage, that yielded a single peak in detrital U-Pb zircon ages of ≤ 2746 Ma (Sanborn et al. 2001a). At the top of the hill, feldspar phyric tuff of the Confederation assemblage is exposed. STOP 5 - BUFFALO DEPOSIT - DOME STOCK MINERALIZATION (Fig. 2) The approximately 7 km diameter hornblende-biotite granodiorite stock (Table 2) has been dated at 2718 ± 1 Ma (Corfu and Andrews 1987) and is interpreted to have been emplaced during D2 (Sanborn-Barrie et al. 2001). The stock is variably iron-carbonate, sericite, and chlorite altered and deformed. Exposures to be visited (Figure 4) are at its southern contact; here it intrudes, and contains xenoliths of, foliated Balmer assemblage mafic volcanic rocks (Figure 4). The stock hosts several gold occurrences and two past-producing mines: the Red Lake Gold Shore produced 21,100 ounces gold, and the Buffalo Mine produced 1656 ounces gold. The Buffalo prospect was discovered in 1925 and explored several times since then. Note the adit reopened by Claude Resources Ltd. in October 1998 to further explore the Buffalo deposit. Gold is hosted within two sets of quartz-tourmaline-pyrite-calcite veins in conjugate orientation (centimetre-wide NE veins: 239°/73° N, and decimetre-wide NW veins: 119°/76° S; Pettigrew 1999). Their orientation may be as a result of the intersection of two previously interpreted (Durocher and Hugon 1983) deformation zones (St. Paul BayMartin Bay and Flat Lake-Howey Bay Deformation Zones). The dominant vein set strikes NW; primary quartz vein fill was replaced by tourmaline, concomitant with bleached pink metasomatic halos developing around tourmaline-rich portions of the veins. Gold is concentrated in the calcite-albite-sulfide halos, in particular at its outer fringe, where chalcopyrite and tellurides were deposited. A second stage of gold 133 �mineralization is associated with Bi-tellurides in fractures and cavity fillings in quartz and late fracture-filling pyrite, hosted within the qtz-tourmaline-pyrite-calcite veins. mafic volcanics granodiorite '---1 vein / shear Figure 4. Detailed geology of south side of Buffalo Pit (from Lavigne et al. 1986) STOP 6 - HOWEY MINE (Fenced in pit - drive by: Fig. 2) On the north side of Hammell Road a cement foundation marks the site the former Howey mine. Behind the fenced off area is the site of the crown pillar mined out in the final stages of the mine. The Howey Mine was the first producer (1930-1941) in the Red Lake camp and remains the lowest grade profitable gold mine in Canadian mining history (final average grade 0.08 opt Au, having produced 422 000 ounces gold). The Howey (and adjacent Hasaga) ore bodies occur in a boudinaged, variably sericitized and silicified quartz-feldspar porphyry dyke trending approximately 065°/80°S. Centimetre-wide, auriferous quartz veinlets trend 080°, making an angle of 15° with the contacts of the dyke and dip 80°S. Gold-bearing quartz veinlets formed as the last of three episodes of quartz veining. Gold is associated with pyrite-sphalerite-galena-tourmaline ± tellurides. Small flat outcrops between the highway and the fence are highly deformed intermediate rocks of the Howey Mine hanging wall. This site lies within the northeast trending Howey Bay – Flat Lake deformation zone and comprises Confederation age rocks. STOP 7 - HOWEY BAY-FLAT LAKE DEFORMATION ZONE (Fig. 2) The Howey Bay – Flat Lake deformation zone was defined by Durocher and Hugon (1983), and was interpreted to be part of a belt-wide system of transcurrent shear zones hosting most of the major gold deposits. Recent detailed work has led to a reevaluation of this concept (Sanborn-Barrie et al. 2000). 134 �This stop is approximately 750 m southwest of the Howey mine. Intense deformation at this stop has destroyed most primary textures that might be used to identify the rocks. The dominant rock type is mylonitized intermediate tuff. Pink felsic dikes that cut the intermediate rock are also mylonitized. Iron-carbonate veins are boudinaged and transposed into the shear direction. The far western extremity of the outcrops exposes deformed quartz-feldspar dyke (similar in appearance to the Dome stock) containing mafic xenoliths and quartz-tourmaline veinlets. STOP 8 - REDCON CARBONATE ZONE: Proximal ferroan-carbonate alteration; carbonate veining West and east sides of Nungessor Road (Fig. 2) This area is approximately 4 km north of the Campbell–Red Lake deposit, still within the proximal, ferroan-carbonate alteration facies. The outcrops are weakly foliated (145°), dominantly massive to pillowed Balmer assemblage basalts, occurring within the amphibolite-facies metamorphic aureole of the Walsh Lake pluton. The stripped area on the east side of the Nungesser road was mapped in detail (Figure 5) by Redcon Gold Mines in 1981 (assessment files) and now forms part of Goldcorp Inc.'s holdings. Here, a 1-2 m wide carbonate vein is exposed near its southeastern termination. The vein can be traced in outcrop and drilling for approximately 750 m to the west-northwest and will be seen at the next stop on the west side of the road. Gold occurs in north-northwest trending, irregular, centimetre-thick quartz-actinolite stringers within the carbonate vein. After an initial, pervasive biotite alteration event, cross-cutting relationships suggest the following sequence of formation (from Lavigne et al. 1986): 1. amphibole-quartz-calcite cross-fractures 2. quartz-calcite veins 3. ferroan-dolomite veins 4. mafic dyke 5. auriferous quartz veins Silicification evident in the pillowed flow on the northern half of the outcrop is barren and apparently not related to the gold-rich silicification event, rather, it may be due to local silica dumping following pervasive carbonate metasomatism. A "black line" fault occurs in the northern wall rocks of the main carbonate vein. A mafic (or lamprophyre) dyke (unit 4, above) cuts the vein, but is itself cut by late quartzactinolite-gold stringers. 135 �TYPE A VEINS TYPE C VEINS I \\\\\\\ss\\1 hb+q+pI+blo 1i2:J Pillow basalt Type A veins re aorom,e (type ci veine EI Matic dike (D) II Quartz ±Au (tvoe E) veina Fault Prominent Joints Figure 5. Detailed geology of the Redcon prospect (modified from Lavigne et al. 1986) The western outcrops are approximately 300 m west-northwest of the previous exposures. Things to note on the western series of outcrops: • differing colours of cross-cutting carbonate veins • colloform/crustiform textures in carbonate veins • andalusite-garnet-biotite alteration of pillows cut by calc-silicate veins (diopside ± calcite, quartz, tourmaline; retrograding to epidote, tremolite, actinolite/hornblende, magnetite) • calc-silicate veins cross-cut by carbonate veins • folding of carbonate veins by the "Mine trend" S2 136 �REFERENCES AND BIBLIOGRAPHY OF RECENT RESEARCH Chi, G., Dubé, B. and Williamson, K. 2002. Preliminary fluid-inclusion microthermometry study of fluid evolution and temperature-pressure conditions in the Goldcorp High-Grade zone, Red Lake mine, Ontario, in Current Research 2002C27, geological Survey of Canada, 14p. Dubé, B., Balmer, W., Sanborn-Barrie, M., Skulski, T. and Parker, J. 2000. A preliminary report on amphibolite-facies, disseminated-replacement-style mineralization at the Madsen gold mine, Red Lake, Ontario; in Current Research 2000-C17, Geological Survey of Canada, 12p. Dubé, B., Williamson, K., and Malo, M. 2001. Preliminary Report on the Geology and Controlling Parameters of the Goldcorp Inc. High Grade Zone, Red Lake Mine, Ontario; Geological Survey of Canada, Current Research 2001-C18, 13 p. Dubé, B., Williamson, K., and Malo, M. 2002. Geology of the Goldcorp Inc. High Grade zone, Red Lake mine, Ontario: an update, in Current Research 2002-C26, Geological Survey of Canada, 15p. Durocher, M.E. and Hugon, H., 1983. Structural geology and hydrothermal alteration in the Flat Lake-Howey Bay deformation zone, Red Lake area, in Summary of Field Work, 1983, Ontario Geological Survey, Miscellaneous Paper 116, p. 216 to p. 219. Gulson, B.L., Mizon, K.J. and Atkinson, B.T. 1993. Source and timing of gold and other mineralization in the Red Lake area, northwestern Ontario, based on lead-isotope investigations, Canadian Journal of Earth Science v. 30, pp. 2366-2379. Horwood, H.C., 1940. Geology and mineral deposits of the Red Lake area, in Fortyninth Annual Report of the Ontario Dept. of Mines, vol. XLIX, Pt. II, 231p. Lavigne Jr., M.J., Hugon, H., Andrews, A.J. and Durocher, M.E. 1986. Gold deposits of the Red Lake District, Relationships of gold mineralization to regional deformation and alteration in the Red Lake greenstone belt, Ontario, in Gold '86, Excursion Guidebook, ed. Pirie, J. and Downes, M.J., p.167 to p.211. Parker, J.R. 1999. Exploration potential for volcanogenic massive sulphide (VMS) mineralization in the Red Lake greenstone belt; in Summary of Field Work and Other Activities 1999, Ontario Geological Survey, Open File Report 6000, p.19-1 to 22-26. Parker, J.R. 2000. Gold mineralization and wall rock alteration in the Red Lake greenstone belt: a regional perspective; in Summary of Field Work and Other Activities 2000, Ontario Geological Survey, Open File Report 6032, p.22-1 to 22-27. 137 �Parker, J.R. 2001. Intermediate to Felsic Plutons in the Red Lake Greenstone Belt: Relationship to Deformation and Gold Mineralization; in Summary of Field Work and Other Activities 2001, Ontario Geological Survey, Open File Report 6070, p. 191 to 19-10. Penczak, R.S., and Mason, R. 1999. Characteristics and origin of Archean premetamorphic hydrothermal alteration at the Campbell Gold Mine, Northwestern Ontario, Canada, Economic Geology, v. 94. pp. 507-528. Penczak, R.S., and Mason, R. 1997. Metamorphosed Archean epithermal Au-As-Sb-Zn(Hg) vein mineralization at the Campbell Mine, Northwestern Ontario, Economic Geology, v.92, pp 696-719. Percival, J.A., Bailes, A.H., Corkery, M.T., Dubé, B., Harris, J.R., McNicoll, V., Panagapko, D., Parker, J.R., Rogers, N., Sanborn-Barrie, M., Skulski, T., Stone, D., Stott, G.M., Thurston, P.C., Tomlinson, K.Y., Whalen, J.B., and Young, M.D. 2000. An integrated view of Western Superior crustal evolution: highlights of 2000 NATMAP studies, in Summary of Field Work and Other Activities 2000, Ontario Geological Survey, Open File Report 6032, p.13-1 to p.13-17. Pettigrew, N., 1999. Structural and alteration history of the Buffalo Gold Deposit, Red Lake, Ontario; B.Sc. Thesis, University of New Brunswick, 154p. Pirie, J. and Downes, M.J., eds., 1986. Gold '86 Excursion Guidebook. Sanborn-Barrie, M., Skulski, T., and Parker, J. 2001. Three hundred million years of tectonic history recorded by the Red Lake greenstone belt, Ontario, in Current Research 2001-C19, Geological Survey of Canada, 32p. Sanborn-Barrie, M., Skulski, T., Parker, J. and Dubé, B., 2000. Integrated regional analysis of the Red Lake greenstone belt and its mineral deposits, western Superior Province, Ontario, in Current Research 2000-C18, Geological Survey of Canada, 16p. Skulski, T., Sanborn-Barrie, M. and Sanborn, N., 2001. New U-Pb geochronology in the Red Lake greenstone belt, Western Superior NATMAP, unpublished poster. Stone, D. and Hallé, J. 2000. Geology of the Blackbear, Yelling and Stull Lake areas, Northern Superior Province, Ontario, in Summary of Field Work and Other Activities 2000, Ontario Geological Survey, Open File Report 6032, p. 15-1 to 15-9. 138 � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2002 Description An account of the resource Institute on Lake Superior Geology. Kenora, Ontario. May 12-16, 2002. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2002 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/144f6c8afe6c22b14b0587341cb54d7e.pdf c67d0082182c4da907410c829b7469e8 PDF Text Text 49TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY IRON MOUNTAIN, MICHIGAN MAY 7 — 11, 2003 Proceedings 49- a- Volume - ------PART -PROGRAMS PROGRAMS AND ABSTRACTS ABSTRACTS PART 11— AND �INSTITUTE INSTITUTE ON ON LAKE LAKESUPERIOR SUPERIOR GEOLOGY GEOLOGY 49TH ANNUAL MEETING MAY 7-11, 2003 IRON MOUNTAIN, MICHIGAN HOSTED HOSTEDBY: BY: LAUREL G. LAUREL G.WOODRUFF WOODRUFFAND AND WILLIAM WILLIAMF. F.CANNON CANNON Co-Chairs Co-Chairs U.S.GEOLOGICAL GEOLOGICAL SURVEY U.S. SURVEY With With assistance assistance from from Michigan MichiganTechnological TechnologicalUniversity University and and John JohnGartner, Gartner,Coleman ColemanEngineering EngineeringCompany Company Volume Volume 49 49 Part Proceedings and Abstracts Abstracts Part 11 — - Proceedings Compiled Compiledand and edited edited by byLaurel LaurelWoodruff, Woodruff,U.S. U.S. Geological GeologicalSurvey Surveyand and Theodore Theodore Bornhorst, Bornhorst, Michigan Michigan Technological University University Cover CoverPhoto: Photo:Berkshire BerkshireShaft, Shaft,Menominee MenomineeRange, Range,Michigan. Michigan.Photo Photofrom fromthe theMichigan Michigan Technological University Mining Engineering Department Collection. Technological University Mining Engineering Department Collection. �49TH 4gTHINSTITUTE INSTITUTE ON LAKE LAKE SUPERIOR SUPERIOR GEOLOGY GEOLOGY VOLUME VOLUME 49 49CONSISTS CONSISTSOF: OF: PART PART1: 1:PROGRAM PROGRAMAND ANDABSTRACTS ABSTRACTS PART2: 2: FIELD FIELDTRIP TRIPGUIDEBOOK GUIDEBOOK PART OVERVIEW: OVERVIEW: PALEOZOIC PALEOZOICSTRATIGRAPHY STRATIGRAPHYAND ANDTECTONICS TECTONICSALONG ALONG THE THE NIAGRA NIAGRA SUTURE SUTURE ZONE, MICHIGAN MICHIGANAND AND WISCONSIN WISCONSIN TRIP MAGMATIC 1:PEMBINE-WAUSAU PEMBINE-WAUSAU MAGMATICTERRANE TERRANE TRIP 1: TRIP 2: MENOMINEE MENOMINEEIRON IRONDISTRICT DISTRICT TRIP 2: TRIP 3:STRATRIGRAPHY STRATRIGRAPHYAND ANDSTRUCTURE STRUCTUREOF OFTHE THEIRON IRONRIVER RIVER—TRIP3: CRYSTAL CRYSTALFALLS FALLSBASIN BASIN TRIP - THE THE REPUBLIC REPUBLICMINE MINE TRIP4: 4: LIFE LIFECYCLE CYCLEOF OFAN ANIRON IRONDEPOST DEPORT— FROM FROM ORE ORE GENESIS GENESIS TO TO MINE MINERESTORATION RESTORATION Reference to to material in Part 1 1 should should follow the t h e example below: Rogala, Rogala, B., B., Fralick, Fralick,P., P., and and Borradaile, Borradaile,G., G., 2003, 2003, AA magnetostratigraphic magnetostratigraphicand andsecular secularvariation vari$ion 4gth study study of of the the Sibley Sibley Group Group [abstract]; [abstract];Institute Instituteon onLake LakeSuperior SuperiorGeology GeologyProceedings, Proceedings, 49 Annual Ml, v. v.49, Annual Meeting, Meeting, Iron Iron Mountain, MI, 49, part part 1, 1,p.65-66. p. 65-66. and distributed by Publishedby bythe the 49th 4gth Institute Instituteon on Lake Lake Superior Geology Geology a n d distributed bythe the Published ILSG Secretary-Treasurer: ILSG MarkJirsa Jirsa(through (through2003) 2003) Mark Minnesota Minnesota Geological GeologicalSurvey Survey 2642 University UniversityAvenue Avenue 2642 Paul, MN MN55114-1 55114-1057 St. Paul, 057 In In 2004 2004 contact: contact: USA USA JirsaOOl @tc.urnn.edu @tc.umn.edu JirsaOOl Peter Peter Hollings Hollings Lakehead Lakehead University University Department of of Geology Geology Department Thunder Thunder Bay, Bay, ON ON P7B P7B5E1 5E1 CANADA CANADA peter.hollinas@lakeheadu.ca Deter.hollinps@lakeheadu.ca ILSG website: http://www.ilscieolopy.orp htt~://www.ilsaeoloav.org ILSG ISSN 1042-9964 �CONTENTS CONTENTS PROCEEDINGS PROCEEDINGS VOLUME VOLUME 49 49 PART PART1—PROGRAM 1-PROGRAM AND AND ABSTRACTS ABSTRACTS Institutes Instituteson on Lake LakeSuperior SuperiorGeology, Geology,1955-2003 1955-2003............................................................ iviv Constitution Constitutionof of the the Institute Instituteon onLake LakeSuperior Superior Geology Geology ................................................... vivi vii By-Laws of of the the Institute Instituteon onLake LakeSuperior Superior Geology Geology ....................................................... vii By-Laws ... MembershipCriteria Criteria ...................................................................................................... Membership viH VIII GoldichMedal MedalGuidelines Guidelines............................................................................................... Goldich ix ix Goldich GoldichMedal MedalCommittee Committee ............................................................................................... xx Past Goldich GoldichMedallists Medallists ..................................................................................................xi Past xi Citation Citation for for 2003 2003Goldich GoldichMedal MedalRecipient Recipient..................................................................... xD xii Eisenbrey EisenbreyStudent StudentTravel TravelAwards Awards................................................................................. xiv xiv xiiv Student Travel Travel Award Award Application ApplicationForm Form ....................................................................... xHv Student Student Student Paper PaperAwards Awards................................................................................................... xv xv Student Paper Paper Awards Awards Committee Committee ................................................................................ xv xv Student SessionChairs Chairs .............................................................................................................. Session xv xv Board of of Directors Directors ........................................................................................................ Board xvi xvi Local Committees Committees......................................................................................................... Local xvi xvi BanquetSpeaker Speaker.......................................................................................................... Banquet xvi xvi xvii Report of the the Chair Chair of of the the 48th 48thAnnual AnnualMeeting Meeting............................................................ xvii Report Program ....................................................................................................................... Program )O(i List of of Contributors Contributors ....................................................................................................... List X)(iI xxii Abstracts ................................................................................................................... Abstracts iii III xxi ... )O(VItI xxvm �INSTITUTES LAKE SUPERIOR INSTITUTESON LAKE SUPERIOR GEOLOGY GEOLOGY # YEAR YEAR CHAIRS CHAIRS PLACE PLACE 1 1955 Minneapolis, Minneapolis,Minnesota Minnesota C.E. Dutton C.E. Dufton 2 1956 Houghton, Houghton,Michigan Michigan A.K. A.K. Snelgrove Snelgrove 3 1957 East EastLansing, Lansing,Michigan Michigan B.T. B.T. Sandefur Sandefur 4 1958 Duluth, Duluth,Minnesota Minnesota R.W. Marsden R.W.Marsden 5 1959 Minneapolis, Minneapolis,Minnesota Minnesota G.M. G.M. Schwartz Schwartz && C. C. Craddock Craddock 6 1960 Madison, Madison,Wisconsin Wisconsin Eli. E.N.Cameron Cameron 7 1961 Port Port Arthur, Arthur, Ontario Ontario E.G. E.G. Pye Pye 8 1962 Houghton, Houghton,Michigan Michigan A.K. A.K. Snelgrove Snelgrove 9 1963 Duluth, Duluth, Minnesota Minnesota H. H.Lepp Lepp 10 1964 lshpeming, Ishpeming,Michigan Michigan A.T. A.T. Broderick Broderick 11 1965 St. St. Paul, Paul,Minnesota Minnesota P.K. P.K. Sims Sims && R.K. R.K. Hogberg Hogberg 12 1966 Sault SaultSte. Ste.Marie, Marie,Michigan Michigan R.W. R.W. White White 13 1967 East EastLansing, Lansing,Michigan Michigan W.J. W.J.Hinze Hinze 14 1968 Superior, Superior,Wisconsin Wisconsin A.B. A.B. Dickas Dickas 15 1969 Oshkosh, Oshkosh,Wisconsin Wisconsin G.L. G.L. LaBerge LaBerge 16 1970 Thunder ThunderBay, Bay,Ontario Ontario M.W. E.Mercy Mercy M.W. Bartley& Bartley &E. 17 1971 Duluth, Duluth,Minnesota Minnesota D.M. D.M. Davidson Davidson 18 1972 Houghton, Houghton,Michigan Michigan J. J. Kalliokoski Kalliokoski 19 1973 Madison, Madison,Wisconsin Wisconsin M.E. M.E. Ostrom Ostrom 20 1974 Sault SaultSte. Ste.Marie, Marie,Ontario Ontario P.E. P.E.Giblin Giblin 21 1975 Marquette, Marquette,Michigan Michigan J.D. J.D. Hughes Hughes 22 1976 St. St.Paul, Paul,Minnesota Minnesota M. M.Walton Walton 23 1977 Thunder ThunderBay, Bay,Ontario Ontario M.M. M.M.Kehlenbeck Kehlenbeck 24 1978 Milwaukee, Milwaukee,Wisconsin Wisconsin G. G.Mursky Mursky 25 1979 Duluth, Duluth,Minnesota Minnesota D.M. D.M.Davidson Davidson 26 1980 Eau EauClaire, Claire,Wisconsin Wisconsin P.E. P.E.Myers Myers 27 1981 East EastLansing, Lansing,Michigan Michigan W.C. W.C.Cambray Cambray 28 1982 International InternationalFalls, Falls,Minnesota Minnesota D.L. D.L.Southwick Southwick 29 1983 Houghton, Houghton,Michigan Michigan T.J. T.J.Bornhorst Bornhorst iv �30 1984 Wausau, Wisconsin Wisconsin G.L. LaBerge G.L. LaBerge 31 1 985 Kenora, Ontario Ontario C.E. C.E. Blackburn Blackburn 32 1986 Wisconsin Rapids, Wisconsin Rapids, Wisconsin Wisconsin J.K. Greenberg Greenberg 33 1987 Wawa, Wawa,Ontario Ontario 34 1988 Marquette, Michigan Michigan J. S. Klasner Klasner 35 1989 Duluth, Minnesota Minnesota J.C. J.C. Green Green 36 1990 Thunder Bay, Bay, Ontario Ontario M.M. Kehlenbeck M.M. 37 1991 Eau Claire, Wisconsin P.E. Myers Myers 38 1992 Wisconsin Hurley, Wisconsin A.B. A.B. Dickas Dickas 39 1993 Eveleth, Minnesota Minnesota D.L. Southwick 40 1994 Houghton, Michigan Michigan T.J. Bornhorst Bornhorst 41 1995 Marathon, Ontario Ontario M.C. Smyk M.C. Smyk 42 1996 Cable, Wisconsin Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario Ontario Meyer R.P. Sage & W. Meyer 44 1998 Minneapolis, Minnesota Minnesota J.D. Miller & M.A. J.D. M.A. Jirsa Jirsa 45 1999 Marquette, Michigan Michigan Regis T.J. Bornhorst Bornhorst & R.S. Regis 46 2000 Thunder Bay, Ontario Ontario S.A. Kissin & P. Fralick S.A. Fralick 47 2001 Madison, Wisconsin Wisconsin M.G. Mudrey, Jr. & & B.A. B.A. Brown Brown 48 2002 Kenora, Ontario Ontario P. Hinz Beard Hinz & R.C. Beard 49 2003 Iron Mountain, Michigan Michigan L.G. L.G. Woodruff & W.F. Cannon Cannon E.D. Frey & R.P. Sage Sage E.D. V �__________(some CONSTITUTION OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY (Lastamended amendedby bythe theBoard—May Board-May 8,8,1997) (Last 1997) ArticleII Article Name The The name nameof of the the organization organizationshall shall be bethe the "Institute "Instituteon onLake Lake SuperiorGeology". Geology". Superior ArticleIII1 Article Obiectives Obiectives Theobjectives objectivesof of this thisorganization organizationare: are: The A. A. To Toprovide provideaameans meanswhereby wherebygeologists geologistsininthe theGreat GreatLakes Lakesregion regionmay may exchangeideas ideasand andscientific scientificdata. data. exchange B. B. To Topromote promotebetter betterunderstanding understandingofofthe thegeology geologyofofthe theLake LakeSuperior Superiorregion. region. C. field C. To Toplan planand andconduct conductgeological geological fieldtrips. trips. . Status No No part part of of the the income income of of the the organization organizationshall shallinsure insureto to the the benefit benefitof ofany any member memberor or individual. individual.In Inthe the event eventof of dissolution, dissolution,the theassets assetsof ofthe theorganization organization (sometax tax free freeorganization). organization). shall shallbe bedistributed distributedto to Article IllIll Article a (To (To avoid avoid Federal Federal and and State State income incometaxes, taxes, the the organization organizationshould shouldbe benot notonly only "scientific" "scientific"or or "educational, "educational, but but also also "non-profit") "non-profit") ArticleIV IV Article Article VV Article ArticleVI Vl Article Article VII Vll Article ArticleVIII Vlll Article Minn. Minn. Stat. Stat. Anno. Anno. 290.01, 290.01, subd. subd.44 Minn. Minn. Stat. Stat. Anno. Anno. 290.05(9) 290.05(9) 1954 1954 Internal InternalRevenue RevenueCode Codes.501 s.501(c)(3) (c)(3) Membership Membership The The membership membership of of the the organization organizationshall shall consist consist of of persons persons who who have have registered registeredfor for an an annual annualmeeting meetingwithin withinthe thepast pastthree threeyears, years, and andthose thosewho who indicate a member according to guidelines approved by the indicate interest interestin in being being a member according to guidelines approved by the Board Board of of Directors. Directors. Meetinas Meetings The The organization organization shall meet once a year. The Theplace placeand and exact exact date date of of each each meeting meeting will will be be designated designated by by the the Board Boardof of Directors. Directors. Directors Directors The The Board Boardof of Directors Directorsshall shall consist consist of of the the Chair, Chair, Secretary-Treasurer, Secretary-Treasurer,and andthe the last last three three past past Chairs; Chairs; but if the the board board should should at at any any time time consist consist of of fewer fewer than than five five persons, persons, by by reason reasonof of unwillingness unwillingnessor or inability inabilityof of any any of of the the above above persons persons to to serve serve as as directors, directors, the the vacancies vacancieson on the the board boardmay may be be filled filled by by the the Chair Chairso so as as to members hi^ of of the the board boardto to five five members. members. to bring brina the the membership 0fficeA Officers The officers officers of this organization organization shall shall be be aa Chair Chair and and Secretary-Treasurer. Secretary-Treasurer. A. The TheChair Chairshall shallbe beelected electedeach each year year by by the the Board Board of of Directors, Directors, who who shall shall give give due consideration consideration to the wishes of any group group that may may be be promoting promoting the the next next annual meeting. His/her Hislherterm term of of office officeas asChair Chairwill will terminate terminate at at the the close close of of the the annual annual meeting meeting over which which he/she helshe presides, presides, or when his/her hislher successor successor shall shall have been appointed. He/she Helshe will will then serve serve for aa period period of three years as a member member of the Board Board of Directors. Directors. B. The meeting. His/her TheSecretary-Treasurer Secretary-Treasurershall be be elected at the annual meeting. Hislher term of office shall be four years, or until his/her successor shall have term of office shall be four years, or until hislher successor shall havebeen been appointed. appointed. ~mendments Amendments This This constitution constitution may may be be amended amended by by aa majority majorityvote vote (majority (majorityof of those thosevoting) voting) of of the the membership membership of of the the organization. organization. vi �BY-LAWS OF BY-LAWS OFTHE THE INSTITUTE INSTITUTE ON LAKE LAKE SUPERIOR SUPERIOR GEOLOGY GEOLOGY I. Duties of the Officers and Directors I. Directors A. It shall be the duty of the Annual Annual Chairman Chairman to: to: I1.. Preside Presideat at the the annual annualmeeting. meeting. 2. Appoint Appointall allcommittees committeesneeded neededfor for the theorganization organizationof of the theannual annualmeeting. meeting. 3. Assume Assume complete completeresponsibility responsibility for for the the organization organization and and financing financing of of the the meeting over which annual meeting which he/she hershe presides. presides. B. It shall be the duty of the Secretary-Treasurer B. Secretary-Treasurer to: 1. Keep I. Keepaccurate accurateattendance attendance records recordsof of all all annual annual meetings. meetings. 2. Keep Keepaccurate accurate records recordsof of all all meetings meetings of, of, and and correspondence correspondence between, the Board of Directors. Directors. 3. Hold all funds Hold all funds that that may may accrue accrue as as profits profits from from annual annual meetings meetings or field trips for the organization and operation of of and to make these funds available for future future meetings meetings as as required. required. of the the Board of Directors to plan locations of annual C. It shall be the duty of C. organization and meetings and to advise on the organization and financing financing of of all all meetings. meetings. II. Duties 11. Dutiesand and Expenses Expenses $5.00 or or less less on on an an annual basis shall be A. Regular Regularmembership membershipdues dues of of $5.00 assessed each member as determined by the Board of Directors.. B. Registration Registrationfees fees for for the the annual annual meetings meetings shall shall be be determined determined by by the Chair in the Board Board of of Directors. Directors. The consultation with the The registration registration fees fees can can include include expenses to cover operations outside of the annual meeting as determined determined by recommended that registration the Board Board of Directors. Directors. It is strongly recommended registration fees be be attendance of students. kept at a minimum to encourage attendance Ill. Rules Ill. Rules of of Order Order The rules contained in Robert's Rules of Order shall govern this organization organization in in all cases cases to which which they they are are applicable. applicable. IV. IV. Amendments Amendments These by-laws by-laws may be amended by a majority majority vote (majority (majority of those those voting) voting) of of the the membership of the organization; provided that such modifications modifications shall shall not conflict conflict with the constitution constitution as presently presently adopted adopted or subsequently subsequently amended. amended. - Last 996 May, 11996 Last Amended Amended— May, vii vii �MEMBERSHIP MEMBERSHIPCRITERIA CRITERIA FOR FOR THE THE INSTITUTE ON LAKE LAKE SUPERIOR INSTITUTE SUPERIOR GEOLOGY GEOLOGY Approved May 8, 1997 8,1997 A. Membership Membershipin inthe theInstitute lnstituteon onLake LakeSuperior Superior Geology Geology requires requires either either participation participationin in Institute activities, activities, or an indication on aa regular basis of of interest interest in in the the lnstitute, Institute. Those Those individuals registering for an annual annual meeting meeting will remain remain as members members for for 44 years years unless: unless: 1) they indicate no further interest interest in in the Institute Institute by by responding respondingnegatively negatively to to the the statement on meeting meeting circulars circulars "Remove "Remove my my name name from from the the mailing mailinglist"; list";or or 2) 2)two two different years are successive mailings in different are returned returned by by the the postal postal service service as as address address unknown. unknown. B. Those individuals individuals who have have not not registered registered for an an annual annual meeting meeting in in the thepast past44years years must indicate indicate an interest interest in the Institute by postal, electronic, or verbal correspondence lnstitute by or verbal correspondence with the Secretary-Treasurer Secretary-Treasurer at least once every two two years. years. Such Suchindividuals individualswill will be be removed from the membership membership if they indicate indicate no no further further interest interestin in the the Institute lnstituteor ortwo two different years are successive mailing in different are returned returned by by the postal postal service service as as address address unknown, unknown. C. The The Secretary-Treasurer Secretary-Treasurer will maintain a list of current members. The The list listwill will include include the date of the beginning beginning of continuous continuous membership, membership, dates of returned returned mail, dates dates of last contact (expression of interest), and the date membership expires, barring a change of status initiated by by the the member. member. Those Those individuals individuals who have have become become members membersof of ILSG ILSG by by Section B will will have have an an expiration expiration date date listed listed at at 22 years years from from the the upcoming upcoming meeting. For For example, a member who expresses interest interest in in September of 1997 1997 (the (the next nextannual annual 2000, unless meeting is May, 1998) will have an expiration date of May, 2000, unless the the member member contacts contacts the Secretary-Treasurer Secretary-Treasureror or attends attends an an annual annual meeting. meeting. 0. "Member D. "Memberfor forLife" Life"status statusisisgranted grantedto to individuals individualswho who have have been been (nearly) (nearly) continuous participants of the ILSG meetings meetings for 15 years, Goldich Medal recipients, or those who have sewed served as as meeting meeting chairs. chairs. This maintained unless the This status will be further maintained individuals indicate no further further interest in the the lnstitute, Institute, or 4 mailings in different years are returned by the postal service as address unknown, or they are are deceased. deceased. E. All All members memberswill will be be mailed mailedthe the First FirstCircular Circularfor for the the Annual Annual Meeting Meetingand andthe theILSO ILSG Newsletter. The Chair of the annual meeting may opt to to send the the first first circular to Newsletter. additional individuals. All All returned returnedmail mail should shouldbe be reported reported to to the the Secretary-Treasurer. Secretary-Treasurer. F. The TheSecretary-Treasurer Secretary-Treasurercan candesignate designateany any individual individualwho who is is on on the the ILSG ILSGmembership membership list (mailing list) as of January 1, 1997 as a member for life based on participation participation in in ILSG ILSG activities. activities. corrections to the SecretaryG. Members Membersare are strongly strongly encouraged encouraged to send address corrections Secretay Treasurer to avoid unintentional unintentional lapse lapse of membership. membership. viii viii �GOLDICH GOLDICHMEDAL MEDALGUIDELINES GUIDELINES (Adopted by the the Board Board of Directors, (Adopted Directors, 1981; 1981; amended amended1999) 1999) Preamble Preamble documented by the fact that the 27th The Institute lnstitute on Lake Lake Superior Superior Geology Geology was born in 1955, as documented meeting was held in 1981. The annual meetino TheInstitute's Institute'scontinuing continuina objectives obiectivesare are to to deal dealwith with those those aspects of geiogy geology that are related geographically to to Lake Lake .&p&ior; Superior; to to encourage encourage the the discussion discussion of of ispects subjects and geologists from and sponsoring sponsoring field trips that will bring bring together geologists from academia, academia, government government surveys, and industry; maintain an informal industty; and to maintain informal but highly effective mode of operation. the course of its existence, the membership membership of the Institute During the lnstitute (that (that is, those geologists who indicate an interest in the the objectives of the ILSG ILSO by attending) attending) has has become aware of the fact fact that certain of their colleagues meritorious contributions to the colleagues have have made made particularly parlicularly noteworthy and meritorious understanding of Lake understanding Lake Superior Superior geology geology and and mineral mineraldeposits. deposits. The first award award was made made by by ILSO ILSG to Sam Goldich Goldich in in 1979 1979 for his his many many contributions contributions to to the the geology of the region region extending over about 50 years. Subsequent Subsequentmedallists medallistsand andthis thisyear's year'srecipient recipientare are table below. listed in the table below. Award Guidelines 1) The Themedal medalshall shallbe beawarded awardedannually annuallyby bythe the ILSG ILSGBoard Boardof of Directors Directorsto toaageologist geologistwhose whosename name is associated with a substantial interest in, and contribution contribution to, to, the the geology geologyof of the the Lake LakeSuperior Superior region. region. 2) The TheBoard Boardof of Directors Directorsshall shall appoint appoint the the Goldich Goldich Medal Medal Committee. The Theinitial initialappointment appointmentwill will of three three members, one one to to serve serve for for three three years, years, one onefor fortwo twoyears, years,and andone onefor forone oneyear. year. The The be of member with the the briefest incumbency incumbency shall shall be be chair chair of ofthe the Nominating Nominating Committee. Committee. After the first the Board of of Directors shall appoint at each spring meeting meeting one new member year, the member who will serve for three years. In the chair. chair. The Committee In his/her hisher third year this member shall be the Committee membership the main main fields fields of of interest interest and and geographic geographic distribution distributionof of ILSG ILSG membership. membership. The outshould reflect the going, senior senior member of the Board of Directors going, Directors shall act as liaison liaison between between the the Board Board and and the Committee for aa period Committee period of of one one year. year. 3) By Bythe the end end of of November, November, the Goldich Goldich Medal Medal Committee Committee shall make make its its recommendation recommendation to the of the Board of of Directors, who will then inform the Board of the nominee. Chair of 4) The The Board Boardof of Directors Directors normally normallywill will accept the nominee nominee of the Committee, Committee, inform inform the medallist, and have one medal engraved appropriately appropriately for presentation medal engraved presentation at the next next meeting meetingof of the the Institute. lnstitute. 5) recommended that the Institute lnstitute set set aside aside annually annually from from whatever whatever sources, sources, such such funds funds as as will will 5) It is recommended support the continuing continuing costs be required to supporl costs of of this this award. award. Nominating Procedures Nominatina Procedures shall take 1) The The deadline deadline for nominations is November 1. The The Goldich Goldich Medal Medal Committee shail at any anytime. nominations at time. Committee Committeemembers membersmay maythemselves themselves nominate nominate candidates; however, for or supporl support individual Board members may not solicit for individual nominees. nominees. 2) Nominations Nominationsmust mustbe bein inwriting writingand andsupported supportedby by appropriate appropriatedocumentation documentationsuch such as as letters letters of recommendation, publications, curriculum curriculum vita's, and recommendation. lists of ~ublications. and evidence evidenceof of contributions contributionsto to Lake Lake Superior geologiand geology and to the Institute. superior lnstitute. 3) Nominations Nominationsare are not not restricted restricted to Institute lnstitute attendees, but but are are open open to anyone who has worked on contributed to to the the understanding of Lake Superior geology. and contributed ix �Selection Guidelines Guidelines Selection 1) Nominees Nomineesare aretotobe beevaluated evaluatedon onthe thebasis basisofoftheir theircontributions contributionsto toLake LakeSuperior Superiorgeology geology 1) (sensulato) lato)including: including: (sensu a) importance importanceofofrelevant relevantpublications; publications; a) b)promotion promotionofofdiscovery discoveryand andutilization utilizationofofnatural naturalresources; resources; b) c) c) contributions contributionstotounderstanding understandingofofthe thenatural naturalhistory historyand andenvironment environmentof ofthe theregion; region; d') generation generat'onofofnew newideas ideasand andconcepts; concepts;and and d) e) and e) contributions contributionstotothe thetraining tra~ning andeducation educationof ofgeoscientists geoscientistsand andthe thepublic. public, 2) 2) Nominees Nomineesare aretotobe beevaluated evaluatedon ontheir theircontributions contributionsto tothe theInstitute lnstituteas asdemonstrated demonstratedby by attendance attendanceat at Institute lnstitutemeetings, meetings,presentation presentationof oftalks talksand andposters, posters,and andservice sewiceon onInstitute lnstituteboards, boards, committees, committees,and andfield fieldtrips. trips. 3) 3) The Therelative relativeweights weightsgiven giventotoeach eachofofthe theforegoing foregoingcriteria criteriamust mustremain remainflexible flexibleand andatatthe the discretionof of the the Committee Committeemembers. members. discretion 4) There Thereare areseveral severalpoints pointstotobe beconsidered consideredby bythe theGoldich GoldichMedal MedalCommittee: Committee: 4) a) a)' An Anattempt attemptshould shouldbe bemade madetotomaintain maintainaabalance balanceofofmedal medalrecipients recipientsfrom fromeach eachofofthe the three threeestates—industry, eslales-inoustry, academia, academia,and andgovernment. government. b) ItIt must mustbe benoted notedthat that industry industrygeoscientists geoscientislsare are at ataadisadvantage disadvantageininthat thatmuch muchofoftheir the.r b) work work ininnot notpublished. published. 5) 5) Lake LakeSuperior Superiorhas hastwo twosides, sides,one onethe the U.S., U.S.,and and the the other other Canada. Canada. This Thisisisundoubtedly undoubtedlyone oneof of the the Institute's Institute'sgreat greatstrengths strengthsand andshould shouldbe benurtured nurturedby byequitable equitablerecognition recognitionofofexcellence excellenceininboth both countries. countries. GOLDICH GOLDICH MEDAL MEDALCOMMITTEE COMMITTEE Serving Sewing through through the the meeting meeting year year shown shown in inparentheses parentheses Frank Frank Luther Luther(2003) (2003) University University of Wisconsin, Whitewater Ron Sage Sage(2004) (2004) Ron Ontario Ontario Geological Geological Survey Survey (retired) (retired) David DavidMeineke Meineke(2005) (2005) Meriden Meriden Engineering, Engineering, Hibbing, Hibbing, Minnesota Minnesota Steve Kissin,as asout-going out-goingsenior seniormember memberof ofInstitute lnstituteBoard Boardof ofDirectors, Directors,isisliaison liaison Steve Kissin, between Goldich Medal Committee and the Board through the 2004 meeting between meeting x �2003 2003GOLDICH GOLDICHMEDAL MEDALRECIPIENT RECIPIENT Klaus Klaus J. Schulz Schuiz U.S. Geological Geological Survey Survey Reston, Virginia Reston, Virginia GOLDICH GOLDICHMEDALISTS MEDALISTS 1979 1979 Samuel SamuelS. S. Goldich Goldich 1991 William WilliamHinze Hinze 1991 1980 1980 not notawarded awarded 1992 William WilliamF.F.Cannon Cannon 1981 1981 Carl Carl E. E. Dutton, Dutton, Jr. Jr. 1993 Donald DonaldW. W. Davis Davis 1982 1982 Ralph RalphW. W.Marsden Marsden 1994 Cedric CedricIverson Iverson 1983 I983 Burton BurtonBoyum Boyurn 1995 Gene GeneLaBerge LaBerge 1984 1984 Richard RichardW. W.Ojakangas Ojakangas 1996 I996 David DavidL.L.Southwick Southwick 1985 1985 Paul PaulK. K.Sims Sims 1997 1997 Ronald RonaldP. P.Sage Sage 1986 1986 G.B. G.B.Morey Morey 1998 Zell ZellPeterman Peterman 1998 1987 I987 Henry HenryH. H.HaIls Halls 1999 1999 Tsu-Ming Tsu-MingHan Han 1988 1988 Walter WalterS. S.White White 2000 2000 John JohnC. C.Green Green 1989 1989 Jorma JormaKalliokoski Kalliokoski 2001 2001 John John S. S. Klasner Klasner 1990 1990 Kenneth KennethC. C.Card Card 2002 2002 Ernest Ernest K. K. Lehmann Lehmann xi �CITATION KlausJ. J. Schulz Schulz Klaus 2003 Goldich Goldich Medal Medal Recipient Recipient 2003 Klaus Klaus Schulz Schulz has has had had aa long long and and productive productivecareer career spanning spanning more morethan than30 30years years as asaa geologist geologistininthe theLake LakeSuperior Superiorregion. region.He Hewas wasintroduced introducedto tothe thegeology geologythrough throughhis his educationin inthe thearea, area,he hecompleted completedgraduate graduatestudies studiesininthe theregion, region,performed performedseveral several education summers summers of of field field work work for for mining miningcompanies companies in in aa number number of different different areas, areas, and and has has conducted conductedextensive extensiveresearch researchas as aa scientist scientistwith withthe theU.S. US. Geological GeologicalSurvey. Survey.This This extensive extensiveand anddiverse diverse experience experiencehas hasmade madehim himaa real realauthority authorityon onthe thegeology geologyof ofthe the Lake Superior Superiorregion. region. Lake Klaus Klaus received receivedhis his B.S. B.S. degree degree in in geology geology from from the the University Universityof of WisconsinWisconsinOshkosh Oshkoshinin1971. 1971.He Hecompleted completedhis hisMasters Mastersdegree degreeatatthe theUniversity UniversityofofMinnesotaMinnesotaDuluth Duluthin in 1974, 1974,with with aa thesis thesis project project in in the the Vermilion Vermilion district districtof of northern northernMinnesota. Minnesota.He He received receivedhis his Ph.D. Ph.D. from from the the University Universityof of Minnesota Minnesotain in1977 1977with withaadissertation dissertationon onthe the petrology petrology of volcanic rocks rocks in the Vermilion district. Klaus spent the next two years as a National NationalResearch Research Council Council Research ResearchAssociate Associate with with NASA NASA at at the the Johnson JohnsonSpace SpaceCenter Center in in Houston, Houston, where he he studied Archean basaltic and ultramafic magma types as analogs analogs of early early planetary planetary crust. In In 1982, 1982, after three years as a faculty member member at Washington Washington University University in in St. St. Louis, Louis, Klaus Klaus resigned resigned his his teaching teachingposition positionand andjoined joinedthe theU.S. U.S.Geological Geological Survey Survey in in Reston, Reston, VA, fulfilling a long-standing long-standing dream dream of his. his. During During the next next twenty twenty years with with the the USGS USGSKlaus Klaus was was aa research research scientist scientist and and administrator administrator with with a strong strong interest interestin in the the geology geologyof of the theLake LakeSuperior Superiorregion. region. The The traits traits that that have have made made Klaus Klaus a success success were evident early early in his his career. In In his his undergraduate undergraduate days days at Oshkosh, Oshkosh, Klaus Klaus distinguished distinguished himself as as an an avid avid reader reader of the the geological geological literature. literature. As a junior in 1970, 1970, he wrote an outstanding outstanding research research paper paper discussing greenstone belts and modern modern island island arcs. He He discussing the similarities similarities between Archean greenstone worked worked several summers doing fieldwork for Bear Creek Mining Mining Company Company in in central central Wisconsin Wisconsin and and northern northern Michigan, Michigan, and for U.S. Steel Steel Corp. Corp. in in the the Vermilion Vermilion district district of of northern northern Minnesota. Minnesota. This combination of field work and and a thorough thorough knowledge knowledge of of the the literature literature has has continued continued to be a hallmark hallmark of his his professional professional career, career, and and has has led led to to aa number number of of significant significant contributions contributions to the geology geology of of the the Lake LakeSuperior Superior region. region. In William Spence In the summer of 1971, Klaus and William Spence discovered discovered the Lake Lake Ellen Ellen kimberlite working as exploration geologists in the kimberlite near Crystal Falls, Michigan, while working area. area. Klaus Klaus was very much involved involved in the recognition recognition of the rock rock as as aa kimberlite. kimberlite. This This the first first kimberlite kimberlitediscovered discovered in in the the Lake LakeSuperior Superiorregion. region. was the xii xv �His Masters thesis involved considerable mapping in the Ely greenstone greenstonebelt belt in in Minnesota, and and geochemical geochemical studies studies for his his Ph.D. Ph.D. dissertation dissertationshowed showedthat thatthe theNewton Newton Lake to komatiites. komatfltes. This was the first Lake Formation Formation was a high-magnesium high-magnesium basalt, similar to documented occurrence rocks in in the the Lake Superior region. documented occurrence of komatHtic komatiitic rocks 1980's, his field mapping and and geochemistry geochemistry of of rocks in the Pembine Pembine In the early 1980's, area of the Wisconsin Wisconsin magmatic magmatic terranes demonstrated demonstrated the presence presence of ophiolitic ophiolitic rocks. rocks. the Lake Superior region, Again, this was the first documented ophiolite in the region, and and showed that the Wisconsin magmatic magmatic terranes were, at least in part, an oceanic oceanic island island arc. arc. His His subsequent model model for the evolution of the Marquette Range Range Supergoup Supergoup on on the the continental continental margin during familiarity with the rocks during the Penokean Penokean orogeny is an extension of his familiarity rocks in in the region of the the geologic literature region combined combined with his encyclopedic knowledge of literature on the evolution of continental continental margins. margins. the GLIMPCE program, program, which which ultimately provided provided Klaus also contributed to the significant insight insight into the structure and origin of the Mid-continent Mid-continent rift, rift, and and into into its its magmatic origin origin and metallogeny. metallogeny. He has authored and co-authored more than than 120 publications, maps, maps, abstracts abstracts and field guides, including 1992, including field guides for the 1984, 1984, -1 992, and and 2003 2003 Institute Institutemeetings. meetings. Klaus' contributions have provided aa better better understanding understanding of the Archean, the the the Middle Proterozoic, and and the the Phanerozoic history of the Lake Early Proterozoic, the Lake Superior region. And he continues to be an active contributor on a global global stage, stage, taking takingthe the knowledge knowledge and experience experience that he has gained in the Lake Lake Superior region region and and applying applying itit to international international projects. projects. Therefore, itit is Schulzas as is my my distinct pleasure pleasure and honor honor to present present Klaus KlausJuergen JuergenSchulz the 2003 recipient recipient of the Goldich Medal Medal "For Outstanding Outstanding Contributions ContributionsTo To The The Lake Lake Superior Superior Region". Region". Submitted by Submitted by Gene Gene L. L. LaBerge LaBerge xui xiii �__________________________________________ __________________________ EISENBREY STUDENT TRAVEL AWARDS The The 1986 1986Board BoardofofDirectors Directorsestablished establishedthe theILSO ILSGStudent StudentTravel TravelAwards Awardstotosupport supportstudent student participation of the the Institute. Institute. The participation at the annual meeting of The name name Eisenbrey" "Eisenbrey"was was added added to to the the awardin in1998 1998totohonor honorEdward EdwardH.H.Eisenbrey award Eisenbrey(1(1926-1985) 926-1 985) and and utilize utilize substantial substantialcontributions contributions made madeto to the the 1996 1996Institute Institutemeeting meetingin in his his name. name. "Ned" "Ned"Eisenbrey Eisenbreyisiscredited creditedwith withdiscovery discoveryofof significant massive sulfide sulfide deposits deposits in in Wisconsin, significant volcanogenic volcanogenic massive Wisconsin, but his his scope scope was was much much broader—he broader-he has has been been described described as as having having unique unique talents talents as as an anore orefinder, finder,geologist, geologist,and and These awards are intended help defray some of the direct travel costs ofof teacher. These awards intended to help defray some of the direct travel costs teacher. attending Institute meetings, and include a waiver of registration fees, but exclude expenses attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, meals, lodging, lodging, and andfield fieldtrip tripregistration. registration. The The annual annual Chair Chair ininconsultation consultation with with the the for Recipients Secretary-Treasurer determinesthe the number number of of awards be Secretary-Treasurer determines awards and andvalue. value. Recipients will be announcedat at the the annual annualbanquet. banquet. announced The The annual annual Chair, Chair, who who isisresponsible responsible for for the theselection, selection, will will consider consider the thefollowing followinggeneral general criteria: criteria: 1) 1) The Theapplicants applicantsmust musthave haveactive activeresident resident(undergraduate (undergraduateor or graduate) graduate)student studentstatus statusat at the the time time of of the the annual annual meeting meeting of of the the Institute, Institute, certified certified by by the department departmenthead. head. 2) 2) Students Studentswho whoare arethe thesenior seniorauthor authoron oneither eitheran anoral oralor orposter posterpaper paperwill willbe begiven givenfavored favored consideration. consideration. 3) ItIt is is desirable desirablefor for two two or or more morestudents studentsto to jointly jointly request requesttravel travelassistance. assistance. 3) 4) 4) InIngeneral, general,priority prioritywill willbe begiven giventotothose thoseininthe theInstitute Instituteregion regionwho whoare arefarthest farthestaway awayfrom from the the meeting meetinglocation. location. 5) 5) Each Eachtravel travelaward awardrequest requestshall shallbe bemade madeininwriting writingto to the the annual annualChair, Chair,and andshould shouldexplain explain need, need, student and author status, and other significant details. The Theform formbelow belowisisoptional. optional. Successful Successful applicants applicants will will receive receive their their awards awards during during the the meeting. meeting. - NSTITUTE ONLAKUPERIORGEOLOGY ONLAKESUPERIORGEOLOGY INSTITUTE EisenbreyStudent Student Travel Travel Award Application Application Date: Date: StudentName: Name: student Address: Address: email: Department Head-Typed Department DepartmentHead-Signature Head-Signature Educationalstatus: Status: Educational Are you author of anoforal poster paper? paper? YESNO-NO_ Are youthe thesenior senior author anororal or poster YES_ who? Who? Willany anyother other students studentsbe betraveling travelingwith withyou? you? Will statement Statementofofneed need(use (useadditional additionalpage pageififnecessary) necessary) Please Pleasereturn returnto: to: xiv �STUDENTPAPER PAPERAWARDS AWARDS STUDENT Each year, year, the the Institute Instituteselects selects the the best best of of the the student student presentations presentations and and honors honors Each presenterswith with aa monetary monetary award. Funding Fundingfor for the theaward awardisis generated generated from from registrations registrations presenters of the theannual annualmeeting. meeting.The TheStudent StudentPaper PaperCommittee Committeeisisappointed appointedby bythe theannual annualmeeting meeting of Chairin insuch suchaamanner manneras asto to represent representaabroad broadrange rangeof of professional professionaland andgeologic geologic Chair expertise. Criteria Criteriafor forbest beststudent studentpaper—last paper-last modified modifiedby bythe theBoard Boardinin2001—follow; 2001-follow: expertise. 1) The Thecontribution contributionmust mustbe bedemonstrably demonstrablythe thework workofofthe thestudent. student. 1) 2) 2) The Thestudent studentmust mustpresent presentthe thecontribution contributionin-person. in-person. 3) The TheStudent StudentPaper PaperCommittee Committeeshall shalldecide decidehow howmany many awards awards to to grant, grant, and and whether whether or or 3) not to to give give separate separate awards awards for for poster postervs. vs.oral oralpresentations. presentations. not 4) 4) InIncases casesofofmultiple multiplestudent studentauthors, authors,the theaward awardwill willbe bemade made to to the the senior senior author, author, or or the award awardwill willbe beshared sharedequally equallyby byall allauthors authorsof ofthe thecontribution. contribution. the 5) The Thetotal totalamount amountofofthe theawards awardsisisleft leftto tothe thediscretion discretionof of the themeeting meetingChair Chairand and 5) Secretary-Treasurer, but typically is in the amount of about $500 US (increase approved Secretary-Treasurer, but typically is in the amount of about $500 US (increase approved by Board, Board,10/01). 10101). by 6) 6 )The TheSecretary-Treasurer Secretary-Treasurermaintains, maintains,and andwill willsupply supplyto to the the Committee, Committee, aa form form for for the the numerical numerical ranking ranking of presentations. This Thisform form was was created createdand and modified modifiedby by Student Student Paper Paper Committees Committees over over several several years years in in an an effort to to reduce reduce the the difficulties difficultiesthat that may mayarise arise from from selection selectionby by raters raters of diverse background. The Theuse useof of the theform formisisnot notrequired, required,but butisis left to to the the discretion discretionof of the the Committee. Committee. left 7) The Thenames namesofofaward awardrecipients recipientsshall shallbe beincluded includedas as part partof of the the annual annualChair's Chair'sreport report 7) that that appears appearsininthe thenext nextvolume volumeof ofthe theInstitute. Institute. Student Student papers papers will will be be noted noted on on the the Program. Program. 2003STUDENT STUDENTPAPER PAPERAWARDS AWARDSCOMMITTEE COMMITTEE 2003 Theodore - Michigan TheodoreBornhorst Bornhorst - MichiganTechnological TechnologicalUniversity, University,Houghton, Houghton, Ml MI --- Chair Chair Kevin KevinSikkila Sikkila—-Wisconsin Wisconsin Department Department of of Transportation, Transportation,Superior, Superior,WI Wl Anne Purdue University University Fort Fort Wayne, Wayne, Fort Wayne, IN AnneArgast Argast—-Indiana IndianaUniversity University — - Purdue IN Tim St. Norbert TimFlood Flood— -St. Norbert College, College, De De Pere, Pere,WI Wl 2003 2003SESSION SESSIONCHAIRS CHAIRS Peter Geological Survey, Kenora, PeterHinz Hinz— - Ontario Geological Kenora,ON ON Eric Jerde Morehead State University, Morehead, Eric Jerde - Morehead State University, Morehead, KY KY James Miller Miller -- Minnesota Minnesota Geological GeologicalSurvey, Survey, Duluth, Duluth, MN MN James Mike MikeMudrey, Mudrey,Jr. Jr.— - Wisconsin Geological Geological and and Natural Natural History History Survey, Survey, Madison, Madison,WI Wl xv �2003BOARD BOARDOF OFDIRECTORS DIRECTORS 2003 Board Boardappointment appointmentcontinues continuesthrough throughthe theclose closeofofthe themeeting meetingyear yearshown shownininparentheses, parentheses,ororuntil untilaa successor successorisisselected selected Laurel Laurel Woodruff Woodruff Co-Chair Co-Chair2003 2003meeting meeting(2006) (2006) U.S. U.S. Geological GeologicalSurvey, Survey,St. St.Paul, Paul,MN MN Peter Hinz Hinz(2005) (2005) Peter Ontario OntarioGeological GeologicalSurvey, Survey,Kenora, Kenora,ON ON Jr. (2004) (2004) Michael C. G. Mudrey, Mudrey,Jr. Michael Wisconsin WisconsinGeological Geologicaland andNatural NaturalHistory HistorySurvey, Survey,Madison, Madison,WI Wl Stephen A. Kissin Kissin(2003) (2003) Stephen A. Lakehead LakeheadUniversity, University,Thunder ThunderBay, Bay,ON ON Hollings-Secretary-Treasurer(2006) (2006) Peter Hollings-Secretary-Treasurer Peter Lakehead LakeheadUniversity, University,Thunder ThunderBay, Bay,ON ON Mark A. A. Jirsa-Secretary-Treasurer-"emeritus" Mark Jirsa-Secretary-Treasurer-"emeritus" (in transition) transition) Minnesota MinnesotaGeological GeologicalSurvey, Survey,St. St.Paul, Paul,MN MN 2003LOCAL LOCALCOMMITTEES COMMITTEES 2003 General General Co-Chairs Co-Chairs Laurel LaurelC. G. Woodruff Woodruff—-U.S. U.S.Geological GeologicalSurvey, Survey,St. St. Paul, Paul, MN MN WilliamF.F.Cannon Cannon—-U.S. U.S. Geological GeologicalSurvey, Survey,Reston, Reston,VA VA William Program Program and Abstracts Abstracts Editors Editors Laurel U.S. LaurelG. G. Woodruff Woodruff-- -US.Geological GeologicalSurvey, Survey,St. St.Paul, Paul,MN MN Theodore J. Bornhorst — Michigan Technological University, Houghton,MI MI Theodore J. Bornhorst - Michigan Technological University,Houghton, Field Field Trip Trip Guidebook Guidebook Editor Editor WilliamF.F.Cannon Cannon— U.S. U.S.Geological GeologicalSurvey, Survey,Reston, Reston,VA VA William - Acting Acting Local LocalCommittee, Committee, Iron IronMountain Mountain John Coleman Engineering, Engineering, Iron IronMountain, Mountain, Ml MI JohnGartner Gartner—- Coleman Connie ConnieDicken Dicken— - U.S. U.S. Geological GeologicalSurvey, Survey, Reston, Reston, VA VA Sally - Oshkosh, WI Wl SallyLaBerge LaBerge— 2003 BANQUET SPEAKER Susan Susan Martin Martin Department Department of of Social Social Sciences Sciences Michigan Michigan Technological Technological University University Houghton, Houghton, Michigan Michigan The indigenous indigenous people of the Lake Superior Superior Basin: Basin: Understanding Understandingthe the links links among among environment, geology and religious belief belief xvi xvi �48TH Report Reportof of the theChair Chair of of the the 48'" Annual Annual Meeting Meeting 2002 Peter Hinz, Hinz, Co-Chair Co-ChairILSG ILSG2002 Peter The 48thAnnual AnnualInstitute Instituteon onLake LakeSuperior SuperiorGeology Geologywas was hosted hostedby by the the Ontario Ontario Geological Geological The Survey 2001. Principal Surveyon on May May9-12, 9-12,2001. Principallocal localcommittee committeemembers memberswere werePeter PeterHinz Hinzand and Richard C. C. Beard, Beard, co-chairs, co-chairs,Carmen CarmenC. C.Storey, Storey, and andKevin KevinO'Flaherty O'FlahertyProgram Programco-chairs, co-chairs, Richard CharlesE. E. Blackburn, Blackburn,Field FieldTrip TripCo-ordinator, Co-ordinator,M. M.Kathleen KathleenMcGowan-Hinz, McGowan-Hinz,Treasurer, Treasurer, Charles and Christine ChristineC. C. Blackburn, Blackburn,Secretary. Secretary.Other Other principal principalindividuals individuals are are listed listed in in the the and Proceedings ProceedingsVolume. Volume. 2001 AttendanceatatILSG ILSG2001 Attendance A A total total of of 97 97 professionals professionalsand andstudent student professionals professionalsattended attendedthe the meeting, meeting, 39 39 of of whom whom pre-registered pre-registeredby by the the April April 2, 2, 2001 2001 deadline. deadline. A A total total of of 88 students studentswere were registered, registered, 77of of whom whom requested requestedand andreceived receivedtravel travelassistance. assistance. Eisenbrey Student StudentTravel TravelAwards Awards2001 2001 Eisenbrey Seven Seven students students requested requested and and received received travel assistance assistance from the Eisenbrey Eisenbrey Student Student Travel Travel Award Award Fund Fundestablished establishedto to support support student student participation participationat at the the Annual Annual Institute. Institute. Details, website. Details, including includingcriteria criteria and andapplication applicationforms, forms, are areavailable availableat atthe theILSG ILSGwebsite. BogdanNitescu Nitescu Bogdan Claire Sturm Sturm Claire ElizabethFein Fein Elizabeth Justin Johnson Johnson Justin Becky Rogala Rogala Becky WilliamJahn Jahn William DanielaVallini Vallini Daniela University of Toronto, Toronto, Toronto, Toronto, ON ON University Oberlin OberlinCollege, College, Oberlin, Oberlin,OH OH Oberlin Oberlin College, College, Oberlin, Oberlin,OH OH Lakehead University, University, Thunder Thunder Bay, Bay, ON ON Lakehead Lakehead Lakehead University, University, Thunder Thunder Bay, Bay, ON ON University University of Minnesota Minnesota-- Duluth, Duluth, Duluth, Duluth, MN MN University University of Western Western Australia, Nedlands, Nedlands, WA WA MeetingSummary Summary Meetin The 48 Annual AnnualInstitute Instituteon onLake LakeSuperior SuperiorGeology GeologyAnnual AnnualMeeting Meetingwas washeld heldat atthe theBest Best Western Western Lakeside Lakeside Inn Inn and and Convention Convention Centre, Centre, the the same same location locationas as the the 1985 1985meeting. meeting. The The one-and-a-half one-and-a-halfdays days of of technical technicalsessions sessionswere werepreceded precededby: by:Field FieldTrip Trip11— - Tanco Rare-Element Rare-Element Pegmatite, Peamatite. Southeastern Southeastern Manitoba Manitoba led led by bv staff of the the Tantalum Tantalum Mining Mining Corporation by Quaternary Geology corporation of of Canada ~ a n a dLtd.; a~ t d .followed followed ; byField FieldTrip Trip22—-Quaternary Geologyofof Southeastern Southeastern Manitoba Manitoba led ledby by E. E. Nielsen Nielsenand and Gaywood GaywoodMatile Matile(Manitoba (ManitobaGeological Geological Survey); Survey); and and Field Field Trip 33- Structure structure and and Sedimentology Sedimentologyof the the Seine Seine Conglomerate, Conglomerate, Mine Mine Centre Centre Area, Ontario Ontario lead lead by by Dyanna Dyanna Czeck (Department (Department of Geology, Geology, Oberlin Oberlin College) College) and Philip Fralick (Department (Department of Geology, Geology, Lakehead Lakehead University) University) Due Due to the small number number of talks talks submitted, submitted, the the Technical Technical Session Session Chairs Chairs were were unable unableto to group group talks into into session session themes. The The meeting meeting began began with with an an anecdotal anecdotal history historyof of mining mininginin northwestern northwestern Ontario Ontario presented presented by by Kevin Kevin O'Flaherty, followed followed by by regional regional scale scale talks talks on on the Western Superior Province. The ~uperior~rovince. Theremainder remainderof of the the technical technicalsessions sessionsincluded includedaa broad broad range range of talks talks focusing focusing on on ground ground water, water, petrography, petrography, sedimentology, sedimentology, mineralogy mineralogyand and structural structural topics. The The final final session session ended ended at noon, noon, allowing allowing for for an an early early departure departureof of Field Field Trip 66 to Red Red Lake. Lake. Post Post meeting meeting trips trips included: included: Field Field Trip Trip 4— 4 - Industrial Industrial Minerals and and Paleozoic Separation Rapids Paleozoic Geology Geology of of Southeastern SoutheasternManitoba; Manitoba;Field FieldTrip Trip55— -Separation RapidsRareRareElement Pegmatite Geology of of the the Red Lake Camp. All Pegmatite Field, Ontario; and Field Field Trip 6— 6 - Geology field field trips ran ran smoothly smoothly considering considering the the frigid frigid conditions conditions of of early early May May in in northwestern northwestern Ontario. ILSG Secretary -Treasurer, Mark Jirsa was the lone participant Ontario. 1LSG Secretary -Treasurer, Mark Jirsa was the lone participantof of Field FieldTrip Trip66 successful in obtaining samples from Goldcorp's Red Lake Mine in Red Lake. He successful in obtaining samples from Goldcorp's Red Lake Mine in Red Lake. Hewas was able to do this by cunningly cunningly embedding embedding the samples samples in in the the back back of of his his neck. neck. Upon Upon returning to Kenora the samples were proudly displayed in a baggy kindly supplied supplied by by the staff of Red Red Lake's Lake's Margaret MargaretCochenour Cochenour Memorial MemorialHospital Hospitalemergency emergencyroom. room. - - - ~ xvii �Annual Banquet and Goldich Goldich Award At the Annual Annual Banquet Banquet Ted Ted DeMatties DeMatties presented presented the the citation citation for Ernest Ernest K. K. Lehmann, Lehmann, recipient recipient of the the Goldich Goldich Medal Medal for 2002 2002 for for his his contributions contributionsto to the the Institute Instituteand andLake Lake Superior Superior Geology. Geology. L. L. Harvey Harvey Thorliefson, Thorliefson, Geological GeologicalSurvey Survey of Canada, Canada, provided provided aa scintillating scintillating discussion discussion on The Search Search for Diamonds Diamondsin in Canada Canada for the the after after dinner dinner address. Laurel Laurel Woodruff Woodruff and and Bill Bill Cannon Cannon of of the the U.S. U.S. Geological GeologicalSurvey Survey invited invited 49th Annual in participants participants to the the 49Ih~ n n u aMeeting Meeting l in Iron Iron Mountain, Mountain, Michigan. Michigan. 2002 Best Student Paper Awards 1) Becky BeckyRogala Rogala--Lakehead LakeheadUniversity,Thunder University,ThunderBay, Bay, Ontario Ontario ($400, ($400, oral oral presentation) presentation) New in formation from from the Sibley New information Sibley Group Group OberlinCollege, College, Oberlin, Oberlin, Ohio Ohio ($50, ($50, poster; poster; Co-authors Co-authorsC.L. C.L. Sturm Sturm 2) Elizabeth ElizabethFein Fein--Oberlin Anisotropy of magnetic magnetic susceptibility susceptibility in in the the Ottertail Ottertailpiuton, and D.M. Czeck) Anisotropy pluton, Northern NorthernOntario Ontario 3) Claire ClaireSturm Sturm--Oberlin OberlinCollege, College,Ohio Ohio($50, ($50, oral; oral; Co-authors Co-authors D.M. D.M. Czeck Czeck and and E. E. Fein) Fein) Petro graphicstudy studyof of the the Offertail Ottertailpluton, pluton, Superior Superior Province, Province, Northwestern Northwestern Ontario Petrographic 2002 Eisenbrey Student Student Travel Awards Awards University of of Toronto, Toronto, Toronto, Toronto, ON ON ($250) ($250) 1) Bogdan BogdanNitescu Nitescu-- University 2) Claire ClaireSturm Sturm--Oberlin Oberlin College, College, Oberlin, Oberlin, Ohio Ohio ($200) 3) Elizabeth ElizabethFein Fein-- Oberlin OberlinCollege, College,Oberlin, Oberlin, Ohio Ohio($200) ($200) 4) Justin JustinJohnson Johnson--Lakehead LakeheadUniversity, University,Thunder Thunder Bay, Bay, ON ON($150) ($150) LakeheadUniversity, University,Thunder Thunder Bay, Bay, ON ON ($150) ($150) 5) Becky BeckyRogala Rogala--Lakehead 6) William WilliamJahn Jahn--University Universityof of Minnesota, Minnesota, Duluth, Duluth,MN MN($150) ($150) 7) Daniela DanielaVallini Vallini -- University University of Western Western Australia, Nedlands, WA ($400) 2002 Goldich Goldich Medal Recipient 2002 Recipient Lehmann Ernest K. Lehmann MTII MTU Archives Donation Donation A check for $100 Technological University Archives, as required $100 was sent to Michigan Technological required by Board agreement ($1 per participant per meeting), for maintenance of ILSG proceedings proceedings archives. Proceedings including Part 1 (Programs and Abstracts) Abstracts) and Part 2 (Field Trip Guidebook) Guidebook) are available from the Institute: Institute: Institute on Lake Superior Geology do c/oMark MarkJirsa, Jirsa, Secretary Secretary -- Treasurer Treasurer Minnesota Geological Survey Survey 2642 University 2642 University Avenue St. Paul MN 55114-1057 551 14-1057 Phone: 612.627.4539 612.627.4539 Fax: Fax: 612.627.4778 612.627.4778 jirsaool @tc.umn.edu e-mail: jirsa001 @tc.umn.edu xvhi xviii �4&h1 ANNUAL INSTITUTE LAKE SUPERIOR GEOLOGY BOARD 4dhANNUAL INSTITUTE ON LAKE BOARD OF DIRECTOR'S DIRECTOR'S MEETING MEETING Board of Directors Directors Peter Hinz (2002 (2002General GeneralChair) Chair) Michael Mudrey Mudrey (2001 (2001 Co-chair) Co-chair) Steve Kissin Kissin (2000 (2000 Co-chair) Co-chair) Laurel Woodruff: Proxy Proxy for Ted Ted Bornhorst Bornhorst (1999 (1999 Co-chair Co-chair and and liaison liaison with Goldich Goldich committee) committee) Mark Jirsa (Institute (InstituteSecretary-Treasurer) Secretary-Treasurer) Guests Guests Phil Fralick Fralick (2000 (2000 Co-chair) Co-chair) Carmen Storey storey (2003 Program program Chair) Kevin O'Flaherty (2003 (2003 Program Program Chair) Chair) Bill Cannon Cannon (proposed (proposed 2003 2003 Co-chairs) Co-chairs) Rod Johnson (Goldich Rod Johnson (Goldich Committee) Committee) Frank Luther Luther (Goldich (Goldich Committee) Committee) based on the secretaries' notes and recollection; recollection; any omissions or The following is based misstatements unintentionaL Motions by the the Board Boardof of Directors Directors are generally generally misstatements are unintentional. Motions by paraphrased—"approved" paraphrasedÑ'approve or or "accepted" "accepted implying implyingthat that aa motion motionwas was made, made, seconded, and passed unanimously. The and passed The expression expression "generally "generally agreed" carries carries less formality, be pursued. pursued. Some formality, but but indicates indicates a directive that will be Some issues issues that that were were resolved resolved after after the Board Boardmeeting, but but during during the conference conference are are included includedhere here for for closure. closure. MINUTES MINUTES 1. Accepted Acceptedreport reportof of the the Chairs Chairs for for the the 47th 47th ILSG, ILSG, Madison, Madison, Wisconsin; Wisconsin; as as printed printed in in the the Proceeding Proceeding Volume (Mudrey), and minutes of last Board meeting, meeting. May 10, 2001 (Jirsa) (Jirsa) 2. 2, Received, Received,discussed, discussed, and andaccepted accepted2001-2002 2001-2002 ILSG ILSG Financial FinancialSummary Summary (Jirsa). (Jirsa). (4gth 3. Discussed location—Iron Mountain, Discussedand andapproved approved2003 2003 ( 4 9 annual) meeting location-Iron Mountain, Michigan, and tentative co-chairs Laurel Woodruff and Bill Cannon, Cannon, USGS. As currently currently envisioned, envisioned, Ted Bornhorst Bornhorst will handle handle logistics logistics of field trips. 4. Approved ApprovedPeter PeterHinz Hinzas as on-going on-goingILSG ILSG Board Boardmember. member. 5. Discussed Discussedreplacing replacingRod RodJohnson Johnson as as the the "member "member from from industry" industry" on on Goldich Goldich Committee Committee (end (end of term term 2002) 2002) with several several candidates candidates including including Dave Dave Meineke Meineke of of Meriden Meriden Engineering, Hibbing, Minnesota. Dave later accepted the position and was welcomed, and Rod was thanked for for his service to to the the Institute, during during the the annual annual banquet. banquet. Dave's Dave's term will end after Goldich Goldich selection for the the meeting meeting of of 2005. 6. Discussed Discussedreplacement replacementof of Mark MarkJirsa Jirsa as as ILSG ILSGSecretary-Treasurer Secretary-Treasurer(end (endof of 4-year 4-year term term 2002). A new member to the Institute, Peter Hoflings, Lakehead University in Thunder Institute, Peter Hollings, Lakehead University in Thunder Bay, was installed installed as "Secretary-Treasurer in-training," pending pending a vote vote by by the the general general membership membership (as required required in in By-Laws). By-Laws). Because Because of his his newness newness to the the Institute, Institute,the the board board generally agreed that Peter Peter would serve serve 2 years of the the 4-year 4-year term term concurrently concurrentlywith with Mark Mark in a period of transition. At the end end of the the 22 years years (following (following the the 2004 2004 meeting), meeting),the the finances and records finances records of the institute, institute, and and responsibilities responsibilities of the the position positionwould would fall fall to to Peter. Peter. This was presented to to the membership after after the the Board meeting, and and was was generally accepted. accepted. 7. Other Otherbusiness: business: a) Discussed Discussed the offer by by Mike Mike Mudrey Mudrey to take take over over as as ILSG ILSG webmaster—It webmaster-lt was was generally agreed that Mike Mike could could do that, assuming assuming Ted Ted was was busy busy with with other other obligations and probably probably would not mind the relief. relief. Subsequent Subsequent discussions discussionsindicate indicate xix �that Ted Ted would like like to to continue continue in in this this endeavor, endeavor, and and has has already already paid paid in in advance advance for for 5 years of web service service to to continue. continue. ItIt remains remains in in Ted's Ted's hands. hands. b) Discussed efforts by Graham Wilson to list ILSG publications as part of his —Steve Kissin MINLIB project and website (www.turnstone.ca) (www.turnstone.ca)ÑStev Kissin volunteered volunteered to contact Graham and see if there is anything that the ILSG can and should do to assist, assist. c) Discussed Discussed the prospect prospect of extending extending aa "free "free ride" ride" to to annual annualGoldich GoldichMedal Medal recipients. It was generally generally agreed agreed that registration registration costs costs should should be be paid paid by by the the annual meeting annual meeting committee, and and that that lodging, lodging, meals, meals, and and travel travel costs costscould couldbe bepaid, paid, at the discretion discretion of the the annual annual meeting meeting chairs. chairs. d) Discussed Discussed the the ILSG ILSG Newsletter—Peter NewsletterÑPete Hinz Hinz has has offered offered to to write write itit beginning beginningin in that transition. The topic of of 2004 or so. He can coordinate with Ted Bornhorst about that whether the Newsletter Newsletter should should remain remain paper, or be be changed changedto to aa wholly wholly electronic electronic format was discussed discussed and and tabled. tabled. Most Most seemed seemed to to think think we we should shouldeventually eventuallyswitch switch with email notification. This This raised a further further to a web-based newsletter, perhaps with issue issue that members must be encouraged to notify the secretary-treasurer of changes in email email address address or or other other status. status. sampling—An issue was raised that that at at least one group of regular e) Questionable sampling-An meeting meeting participants participantshas has a tradition tradition of using using guidebooks guidebooksto to locate locateplaces placesfor formassive massive sampling programs. In this one case, samples are sold to Wards or other rock rock and mineral mineral specimen specimen dealers. The The problems problems are are 1) 1) some some of of the the localities localitiesdiscussed discussedinin guidebooks guidebooks are on private land (and therefore trespassing is likely), and 2) taking large amounts of sample from some localities limits the use of these sites sites to future generations. It was generally agreed that ILSG would print in their guidebooks a Policy Statement that warns of this "questionable "questionable sampling sampling practice." practice." Mark Mark Jirsa Jirsa will will create such language create language for inclusion inclusion in in future future guidebooks. guidebooks. abstracts—Peter f) Discussed Discussed digital submission of a b s t r a c t e e t e r Hinz warns from experience that for preparers, particularly this practice can easily turn into a nightmare for particularly if the submitters don't follow (or the host organization doesn't specify) rigid rigid guidelines guidelines for submission formats. This includes both text and illustration formats. Adjournment submission formats. both text and illustration formats. Adjournment Respectfully Respectfully submitted on January 27, 2003 to Peter Peter Hinz, Hinz, Chair Chair of of the the 48th 48thannual annual meeting, for incorporation into the Report of the Chair to appear in Proceedings Proceedings Volume 49. Mark Jirsa, Secretary-Treasurer, Institute on Lake Superior Geology xx �PROGRAM xxi �49th The following companies made made generous generous contributions contributions to to the the 49"' Annual Annual Meeting. Meeting. We thank them and and John John Gartner Gartner of the the Local Local Committee Committee for their commitment to the the Institute Institute on on Lake Lake Superior Superior Geology. Geology. For For almost almost 50 years this this organization organization has has thrived thrived through through the the sustained sustained interests interests of of individuals, and government agencies in the individuals, corporations, corporations, universities, universities, and government agencies the international geologic community. community.This Thisdedication dedicationtoto an an exchange of international geologic exchange of scientific ideas and aa passion passion for for field field trips trips (even (even in in driving driving rain rain or or snow) snow) has enabled enabled the ILSG ILSG to to fulfill fulfill one one of of its its primary primary objectives: objectives: to to promote promote better better understanding understandingof of the thegeology geology in in the the Lake Lake Superior Superiorregion. region. Kleiman Well Drilling, Kleiman Pump Pump &&Well Drilling,Inc. Inc. P.O. 704 P.O. Box 704 Iron Mountain, Michigan 49801-0704 49801-0704 Prime Prime Meridian Meridian Resources ResourcesLtd. Ltd. N7478 N7478 Niagara Niagara Lane Lane Lac, WI Wl 54935 54935 Fond du Lac, Coleman Coleman Engineering Engineering Company Company Circle Drive Drive 635 Circle Iron Mountain, MI Ml 49801 49801 xxii �WEDNESDAY MAY 7,2003 WEDNESDAY 7, 2003 8:00 TRIP 1: 1: WISCONSIN MAGMATIC TERRANE 8:00a.m. a.m.FIELD FIELD TRIP WISCONSIN MAGMATIC TERRANE(#1 (#1ININGUIDEBOOK) GUIDEBOOK) Klaus Klaus Schulz, Schulz, U.S. U.S. Geological GeologicalSurvey Survey Gene Oshkosh,emeritus emeritus GeneLaBerge, LaBerge,University UniversityofofWisconsin Wisconsin—- Oshkosh, - OF FIELD TRIP TRIP 2: 2:THE THEREPUBLIC REPUBLICMINE MINE— LIFE LIFECYCLE CYCLE OFAN ANIRON IRONORE ORE 8:00 a.m. a.m. FIELD 8:00 DEPOSIT (#4 IN INGUIDEBOOK) GUIDEBOOK) DEPOSIT FROM FROMGENESIS GENESIS TO RECLAMATION RECLAMATION(#4 William WilliamCannon, Cannon,U.S. U.S.Geological GeologicalSurvey Survey John John Meler, Meier,Cleveland ClevelandCliffs Cliffs Iron Iron Company Company 6:00 Return of 6:00 p.m. Return of Trips Trips11 and and22 4:00 p.m. p.m. - 8:00 Registration 4:00 8:00 p.m. p.m. Registration 7:00 p.m. p.m. -- 9:00 and Poster Poster Setup Setup 7:00 9:00 p.m. p.m. Ice Breaker Social and THURSDAYMAY 8,2003 THURSDAY 8, 2003 - 8:00 8:00 a.m. a.m.- 9:00 9:00a.m. a.m.REGISTRATION REGISTRATION Note: Note: Technical TechnicalSessions Sessionsare arein inWhite White spruce, Spruce,Pine PineMountain MountainResort Resort 4• *: Denotes Denotes Student Student Presentation Presentation 8:15 a.m. a.m.INTRODUCTORY INTRODUCTORYREMARKS REMARKS 8:15 Laurel G. G. Woodruff and and William WilliamF. F. Cannon, Cannon,Co-Chairs Co-Chairs TECHNICAL TECHNICALSESSION SESSIONII Session Session Chair: Chair: Jim Jim Miller, Miller, Minnesota Minnesota Geological Geological Survey, Survey,Duluth, Duluth,MN MN - 8:30 - Menominee Range 8:30 a.m. a.m. Harold HaroldBernhardt Bernhardt Menominee RangeHistorical HistoricalFoundation FoundationMuseum Museum A A brief brief history historyof of iron ironmining mining on on the theUpper UpperPeninsula's Peninsula'sMenominee MenomineeIron IronRange Range 9:00 9:00 a.m. Cannon, W.F., W.F., LaBerge, G.L. G.L. and Klasner, Klasner, J.S. J.S. Niagara Niagara suture suture zone, northern Michigan Michigan and and Wisconsin—tectonics Wisconsin-tectonics in in the the 1.85 1.85 Ma Ma arc-continent arc-continent collisional collisional boundary boundary K. 9:30 a.m. Schulz, K. A A Paleoproterozoic Paleoproterozoic suprasubduction suprasubductionzone ophiolite-island ophiolite-island arc complex complex in northeastern northeasternWisconsin Wisconsin 10:00 10:OO a.m. a.m.COFFEE COFFEEBREAK BREAKAND ANDPOSTER POSTER SESSION SESSION 10:40 a.m. am. Schneider, D.A., HoIm, D.K., D.K., O'Boyle, O'Boyle, C. C., Hamilton, M. and Jercinovic, Jercinovic, M. D.A., Holm, , Hamilton, M. Paleoproterozoic development of a gneiss dome corridor in the southern Lake Paleoproterozoic development gneiss dome corridor in the southern Lake Superior Superior region, region,USA USA 11:00 a.m. Holm, HoIm, D.K., D.K., Van Van Schmus, Schmus, W.R., W.R., MacNeill, MacNeill, L.C., L.C., Boerboom, Boerboom, T.J., 11:OO a.m. T.J., Schweitzer, D. and Schneider, D.A. Schweitzer, D. and Schneider, D.A. Late Paleoproterozoic Paleoproterozoic (1900-1600 (1900-1600 Ma) Ma) tectonic tectonic history history of of the the northern northernmidmidcontinent, continent, U.S.A.: Implications Implicationsfor for crustal crustalstabilization stabilization 11:20a.m. 11:20 a.m. Medaris, L.G., L.G., Jr. and and Dofl, Dott,R.H., R.H., Jr. Jr. The sedimentology, metamorphism, The Sioux Sioux Quartzite Quartzite revisited: sedimentology, metamorphism,geochemistry geochemistryand and the the origin origin of of pipestone pipestone 11:40p.m. 11:40 p.m. Smyk, Smyk, M.C. M.C. The planned activities activities and objectives objectives The Lake Lake Nipigon Nipigon Geoscience Geoscience Initiative Initiative— - planned xxiii �12:00 Meeting (by invitation) 12:OO p.m. p.m. Lunch LunchBreak— Break -Poster PosterSession Sessionand andILSG ILSGBoard Board Meeting (by invitation) TECHNICAL TECHNICALSESSION SESSIONIIII Session SessionChair: Chair:Mike MikeMudrey, Mudrey,Jr., Jr.,Wisconsin WisconsinGeological GeologicalSurvey, Survey,Madison, Madison,WI Wl 1:30 1:30 p.m.4 p.m. +Heggie, Heggie,C. G. and and Hollings, Hollings,P. P. Geochemistry Geochemistry.and and mineralization mineralizationof of the the Seagull Seagull Intrusion, Intrusion, Northern Northern Ontario Ontario +:. 1:50 Johnson, J.R., 1:50 p.m. p.m. + Johnson, J.R., Hollings, Hollings,P. P.and andKissin, Kissin,S.A. S.A. Mineralization Mineralizationof of the theNorton NortonLake LakeCu-Ni-POE Cu-Ni-PGEdeposit deposit 2:10 2 3 0 p.m. p.m. Miller, Miller,J.J.D., D., Jr. Jr. Petrology Petrologyand and POE PGE potential potential of of the theGreenwood GreenwoodLake Lake Intrusion, Intrusion,central centralDuluth Duluth Complex, Complex,Lake LakeCounty, County,Minnesota Minnesota 2:30 2:30 p.m. p.m. + + Joslin, Joslin,G.D., G.D., Miller, Miller,J.D., J.D., Jr. Jr. and andRowell, Rowell,W.F. W.F. Stratiform StratiformPd-Pt-Au Pd-Pt-Aumineralization mineralizationininthe theSonju SonjuLake LakeIntrusion, Intrusion,Lake LakeCounty, County, Minnesota Minnesota 2:50 2:50 p.m. p.m. + + Marma, Marma, J., Brown, Brown,P. P.and andHauch, Hauch,S. S. Magmatic Magmaticand and hydrothermal hydrothermalPOE PGE mineralization mineralizationof of the the Birch Birch Lake LakeCu-Ni-POE Cu-Ni-PGE Deposit Depositin in the theSouth South Kawishiwi, Kawishiwi,Duluth Duluth Complex, Complex, northeast northeast Minnesota Minnesota 3:10 3:10p.m. p.m.COFFEE COFFEEBREAK BREAKAND ANDPOSTER POSTERSESSION SESSION 3:30 3:30 p.m. p.m. Waggoner, Waggoner,1. T. AAhydrothermal hydrothermalcomponent componentofofIron IronFormations Formations—A -A Marquette Marquette Range Rangeperspective perspective 3:50 3:50 p.m. p.m. Tsu-Ming Tsu-MingHan Han Mode of occurrence Mode of occurrence of of trona trona and and thermonatrite thermonatriteand andtheir theirpossible possibleorigin originininthe the Negaunee Iron-Formation of the Marquette Range, Lake Superior District, Negaunee Iron-Formation of the Marquette Range, Lake Superior District,USA USA 4:10 4:10 p.m. p.m. Blaske, Blaske,A.R. A.R. Geology of Valley type mineralization mineralization at Bellevue, Geology of the the MississippiMississippi-Valley Bellevue, Michigan Michigan 4:20 4:20 p.m. p.m. +ç Larson, Larson,P. P. Mean Mean transport transport length length in in tills tills of of the the southern southernportion portion of of the theLaurentide Laurentideice icesheet: sheet: implications implications for for drift drift exploration explorationin in the the Lake LakeSuperior Superiorregion region .:. 4:50 4:50 p.m. p.m. + Marlow, Marlow, L., L., Mooers, Mooers,H. H. and andLarson, Larson,P. P. Glacial Glacial Lakes Aitkin and Upham: Upham: their their origin origin and and environmental environmentalhistory history 5:10 530 p.m. p.m. Trow, Trow,J. J. Five wan copper copper sulfides sulfides in in Ontario and Five gold gold possibilities in some Keweena Keweenawan Michigan Michigan xxiv �- - ICEBREAKER BREAKER— MIXER MIXER— CASH BAR CASH BAR 6:00 p.m. p.m. ICE 6:00 BANQUET 7:00 p.m. p.m. ANNUAL ANNUAL BANQUETAND ANDAWARD AWARDPRESENTATION PRESENTATION 7:00 Announcement of of 50th 50"'Annual Annual Meeting Meeting Location Location • Announcement Presentationto Klaus Klaus Schulz Schulz 2003 Goldich Award Presentation • Banquet Address Address • 2003 Banquet Dr. Technological University Dr. Susan Martin, Michigan Technological University The indigenous indigenous people people of of the the Lake Lake Superior Superior Basin: Understanding links among geology Understanding the links among environment, geology and and religious religious belief belief . Ing participants Meeting participantswho who are arenot notregistered registeredfor forthe thebanquet banquetare arewelcome welcometo tothe thebanquet banquetaddress address Meet FRIDAY 9, 2003 FRIDAY MAY 9,2003 TECHNICAL TECHNICAL SESSION SESSION Ill Ill Session Chair: Chair: Eric Jerde, Jerde, Morehead State State University, University,Morehead, Morehead, Kentucky Kentucky 8:20 8:20 a.m. a.m. INTRODUCTORY INTRODUCTORYREMARKS REMARKS Laurel 0. G.Woodruff Woodruffand andWilliam WilliamF. F.Cannon, Cannon,Co-chairs Co-chairs 8:30 a.m. a.m. Hollings, Hollings, P., P., Fralick, Fralick, P. P. and and Kissin, Kissin, S. S. Geochemista'y andgeodynamic geodynamic implications implications of of the the 1537 1537 Ma Redstone Redstone Point Geochemistry and Point anorogenic anorogenic granite, granite, Ontario, Ontario, Canada Canada 8:50 a.m. a.m. 8:50 Buttram, R.M. P.M. and Bjornerud, Bjornerud, M. M. Textural constraints on holith Textural constraints on the the origin of rapakivi rapakivi textures textures in in the theWolf WolfRiver RiverBat Batholith 930 a.m. 9:10 am. -:Sequence .Sandin, NA. N.A. and and Bornhorst, Bornhorst, T.J. T.J. Marquette County, Sequence of Precambrian Precambrian mafic dikes in Marquelte County, Michigan, with +: - emphasis emphasis on on the the Sugarloaf Sugarloaf Mountain Mountain and and Republic Republic areas areas 9:30 a.m. a.m. Jerde, Jerde, E.A. E.A. Gabbro/granophyre relations relations of of the the Crocodile CrocodileLake Lake Intrusion: Intrusion: a possible vent for the Hovland Hovland Lavas? Lavas? 9:50 a.m. < + Vislova, T. T. Evaluation of initial magma compositions for the Bald Eagle Intrusion Intrusion and associated rocks associated rocks 10:10 COFFEEBREAK BREAKAND AND POSTER POSTER SESSION SESSION 10:lOa.m. a.m.COFFEE 10:30a.m..:• Charkoudian, K., 10:30 a.m. +: Charkoudian, K., Tikoff, B. 6. and and Bauer, Bauer, R. R. Stike -slipseparation separation of of the the Burntside Burntside trondhjemite trondhjemite and and the Wakemup Stike-slip WakemupBay Bay tonatlite, tonatlite, Northern Northern Minnesota Minnesota -:. 10:50 a.m. a.m. + Garbowicz, Garbowicz, A. A. and Bjornerud, Bjornerud,M. M. Paleostress eastern part of the Paleostress inferences from slip vectors in the eastern the Wisconsin Wisconsin segment of the the Midcontinent rift rift 11:10 am.. :+e Potter, E.G. and Mitchell, Ri-I. 1 1 :10 a.m. E.G. and R.H. The rare and exotic mineralogy mineralogy of of the Western Subcomplex of the Deadhorse Deadhorse Creek Diatreme, Northwestern Northwestern Ontario Ontario xxv �11:30 a.m. a.m. Brown, Brown, B.A., Jr., Czechanski, M.L., 11:30 B.A., Mudrey, M.G., M.G., Jr., M.L., Reid, Reid, D.D. D.D. and and Hunt, Hunt, T.C. T.C. Highway construction, construction, mine reclamation, and land-use planning challenges in the the historic historic Upper Upper Mississippi Mississippi Valley Valley lead-zinc lead-zinc district district of of southwest southwestWisconsin Wisconsin 11:50a.m. 1 1:50 a.m. Wattrus, N. N. High-resolution High-resolution multibeam bathymetry in Lake Superior 12:10 p.m. p.m. LUNCH LUNCHBREAK— BREAK -POSTERS POSTERSREMOVED REMOVED AFTER LUNCH LUNCH TECHNICAL TECHNICAL SESSION SESSIONIV Chair: Peter Session Chair: Peter Hinz, Hinz,Ontario OntarioGeologicai GeologicalSurvey, Survey,Kenora, Kenora,ON ON 1:40 a.m. a.m. 1:40 Vatlini, D.A., N.J., Rasmussen, Rasmussen, B., B., Fletcher, I. + Vallini, D.A., McNaughton, N.J., I. and Griffin, B.J. B.J. xenotime U-Pb geochronology geochronology to to unravel unravel the the history history of of Proterozoic Using xenotime sedimentary basins: aa study study in in Western WesternAustralia Australia and and the the Lake Lake Superior Superior region 2:00 p.m. p.m. Kissin, Kissin, S.A., S.A., Vallini, Vallini, D.A., Addison, W.D. W.D. and and Brumpton, Brumpton, G.R. 2:00 D.A., Addison, G.R. zircon ages from the flint and and Rove Rove Formations, Formations, northwestern northwestern Ontario New zircon the Gun Gunflint Ontario 2:20 p.m. p.m. +Richardson, • Richardson, A., 2:20 A., Fralick, P. and Hollings, P. Sibley zircon and whole rock geochemical Sibley Basin sediment provenance using zircon methods: Possible Possiblesource source areas areas of of the the Pass Pass Lake Lake Formation Formation + 2:40 p.m. p.m. + Rogala, Rogala, B., B., Fralick, Fralick, P. P. and and Borradaile, G. 2:40 magnetostratigraphic and secular variation study of the A magnetostratigraphic the Sibley Sibley Group Group 3:00 p.m. COFFEE COFFEE BREAK BREAK p.m. Argast, Argast, A. 3:20 p.m. What does sediment chemistry tell us about rocks like those those from the the Fern Fern Creek Formation? 3:40 p.m. p.m. Bartnik, Bartnik, P. J. and 3:40 and Evans, B. W. Geology and hydro geology in in the the Kingsford, Michigan Geology hydrogeology Michigan area area 4:00 p.m. p.m. Presentation Presentation of Student Student Paper Paper Awards Ted Bornhorst, Michigan Technological University: University: Student Student Paper Paper Committee Committee SATURDAYMAY MAY10,2003 10, 2003 SATURDAY (#2 IN 8:00 a.m. a.m.FIELD FIELDTRIP TRIP 3: 3: MENOMINEE MENOMINEE IRON IRON RANGE (#2 IN GUIDEBOOK) Gene Oshkosh, emeritus Gene LaBerge, LaBerge, University University of of Wisconsin Wisconsin— - Oshkosh, John Kiasner, Klasner, University University of Western Western Illinois, Illinois, emeritus emeritus William Cannon, William Cannon, U.S. US. Geological GeologicalSurvey Survey 6:00 p.m. p.m. Return Return of Trip Trip 3 6:00 SUNDAYMAY MAY11, 2003 SUNDAY 11,2003 8:00 a.m. FIELD TRIP4:4:Iron Iron River River —Crystal Crystal Falls Falls Iron Iron District 8:00 FIELD TRIP District(#3 (#3ININGUIDEBOOK) GUIDEBOOK) LaBerge, University Oshkosh, emeritus emeritus Gene LaBerge, University of Wisconsin Wisconsin— - Oshkosh, John Klasner, Klasner, University of Western Western llllnois, Illinois, emeritus emeritus William Cannon, William Cannon, U.S. U.S. Geological Geological Survey Survey 6:00 p.m. p.m. Return Return of Trip Trip 4 6:00 EM �POSTER PRESENTATIONS PRESENTATIONS Brown, B.A., Brown, B.A., Czechanski, Czechanski, M.L., M.L., Mudrey, Mudrey,M.G., M.G., Jr. Jr. and and Reid, Reid,D.D. D.D. Wisconsin Wisconsin mineral resource resource GIS GIs and and related related digital digital map map and anddatabase databaseproducts products— -a progress report Boerboom, T. Boerboom, Bedrock geologic maps of Keweenawan Bedrock geologic Keweenawan volcanic volcanic and intrusive intrusive rocks rocks in in the the Lake wood, French French River, River, and and Knife Knife River 7.5' quadrangles, North Lakewood, 7.5'quadrangles, North Shore Shore of of Lake Lake Superior, Superior, Minnesota Minnesota Easton, P.M. Easton, P.M. Geology and mineral potential of Proterozoic intrusions in the Geology Proterozoic mafic intrusions the northern northern Grenville Province of Ontario Grenville Ontario Hart, T.R. Hart, rocks of of the Lake Nipigon and Crystal Keweena wan mafic and ultramafic intrusive rocks Keweenawan and . northwestern Ontario Lake areas, northwestern Ontario S.A., Oreskovich, Oreskovich, J.A. and Hauch, S.A., and Severson, Severson, M.J. M.J. Geology, drill holes, mineral leases, and geophysics in the andgeophysics the Duluth Duluth and and Beaver Beaver Bay Bay Compexes, northeastern Minnesota: Integration Integration of of various various GIS G I sdatabases databasesto totell tellaa story of the history of past and current current Cu-Ni-PGE Cu-Ni-PGE mineral mineral exploration exploration • Heiling, 4Heiling,C.D. C.D. Oaf vertLake Lake Volcanic VolcanicComplex, Complex,St. St Louis Louis County, Minnesota Peperites of the Gafvert •4. Hooker, Hocker,S.M., S.M., Hudak, Hudak, G.J., G.J., Odette, Odette,J.D. J.D. and andNewkirk, Newkirk,T.T. T.T. Chemistry of alteration mineral phases phases at at the Five Five Mile Mile Lake Lake volcanic-hosted prospect, NE Minnesota massive sulfide prospect, Minnesota Jirsa, M. .+:.Keatts, Keatts, M.J., Jirsa, M. and and Hoim, Holm,D. D. 40ArPArsingle-grain single-grainanalyses analyses of of Precambrian Precambrian mafic intrusions intrusions in Results of 4oArf'Ar in northern northern north-central Minnesota and north-central Minnesota • McKenzie, Â¥: McKenzie, M.A., M.A., Hoim, D.K., D.A. and and Jercinovic, Jercinovic, M. Holm, D.K., Schneider, D.A. I . of EMP monazite monazite geochronology geochronology in in E-C E-C Minnesota: Minnesota: Evidence Results of Evidence for for largelargescale geon 17 post-tectonic plutonism 17 metamorphism associated with post-tectonic •:-Metsaranta, Metsaranta,R., R.,Fralich, Fralich, P. P.and and Hollings, Hollings, P. P. investigation of Mesoarchean volcanic and and rnetasedimentary metasedimentary A geochemical investigation Mesoarchean meta metavolcanic Uchigreenstone greenstone belt belt rocks from the the Birch Birch — - Uchi . :+ Nicholson, and Cannon, Nicholson,S. S. W. W. and Cannon, W.F. W.F. and structure structure of Keweenawan Keweena wan rocks rocks of of the the St. St. Croix horst, northwestern northwestern Stratigraphy and Wisconsin Wisconsin W.,Parker, Parker,J.R., JR.,Straub, Straub, K.J. K.J. and and Tomllnson, Y. Stott, G.M., G.M., Davis, 13. D. W., Tomlinson, K. K.Y. goon Subprovince, Archean tectonostratigraphic tectonostratigraphicassemblages assemblages of of eastern eastern Waba Wabagoon Subprovince, northwestern north western Ontario Ontario xxvii �ABSTRACTS Xxviii �WHAT DOES SEDIMENT SEDIMENT CHEMISTRY TELL US ABOUT ROCKS FORMATION? LIKE THOSE FROM THE FERN CREEK FORMATION? Argast, Anne, Department of Geosciences, Geosciences, Indiana Indiana University Purdue Purdue University University Fort Wayne, Fort Wayne, IN 46805-1499, 46805-1499, Argast@ipfw.edu Argast@ipfw.edu Bulk chemical analyses are accepted and powerful tools for the the study of of metamorphic and igneous rocks. Bulk chemical chemical analyses analyses are are less accepted accepted and and less less widely widely used used for for the the study studyof of sedimentary rocks. This is at least partly the result of a widely-held view that chemical data result a widely-held view that chemical dataare are unreliable unreliable indicators indicators of sedimentary sedimentary events events due due to to the the post-burial post-burial diagenetic diageneticalteration alterationof of the the sediment. sediment. In recent years, this view has been reinforced reinforced with with studies studies indicating indicatingthe thepotential potential for for extreme diagenetic diagenetic alteration alteration of sediments, sediments, with with km-scale km-scale transport transportproposed proposedin insome somesystems systems (e.g., Wintsch and and Kvale, Kvale, 1994). 1994). Potassium is often singled-out as an especially mobile component in diagenetic systems. For example, Awwiller (1993), working in the Gulf Coast Tertiary, postulates an increase from 2.0 to 3.8 wt. percent K20 in mudrocks, due significantly to to the the transport transport of of K K as aspart part of of 1UJ0 pore volumes of fluid passing through the system in the depth range from 1500 to 4000 m below the surface. surface. Donnelly, 1987) maintains maintains KzO K20 is a generally conservative An alternate view (Argast and Donnelly, that observed variations in KzO K20 content preserve element in diagenetic settings, and that preserve compositional characteristics present at and before accumulation. Depending on your point-ofdiagenetically and metamorphically metamorphicaily altered view, the chemistry of diagenetically altered sedimentary sedimentaryrocks rocks may may (or (or not) provide provide useful information information about about provenance, provenance, weathering and other qualities of the may not) sedimentary system. system. Unconsolidated Unconsolidated sediments, sediments, delivered as turbiditic pulses of siliciclastic siliciclastic debris debris eroded eroded from from the the Himalaya Mountains, accumulated on the Bengal Pan Fan (DSDP 218) and now at subbottom subbottom depths from 12 12 to 729 m, produce chemical trends very similar to those previously noted in lithified lithified sedimentary rocks. rocks. The The similarity similarity in in chemical chemical trends trends across this broad (and metamorphosed) sedimentary range of conditions and environments suggests sedimentary chemical trends arise from fundamental conditions imposed upon the system before burial, and are are not necessarily necessarily the the result of extensive extensive diagenetic alteration alteration at depth. depth. Proterozoic, Lower Chocolay Group) is well exposed along the The Fern Creek Formation (Early Proterozoic, Sturgeon River near the dam northeast of Loretto, Michigan. These rocks have been interpreted interpreted as glaciogenic in origin, and the diamictites they contain used as evidence for glacially-derived dropstone units. Others have interpreted the Fern Creek Formation as nonglaciogenic nonglaciogenic in in origin origin lagoonal or or with the sediments accumulated in fluviatile environments grading upward into into lagoonal estuarine environments. Field and textural qualities (to be discussed discussed as as part part of of aa post-meeting post-meeting hon Range) fieldtrip in the Menominee Iron Range) support support aa glaciogenic glaciogenicorigin. origin. 1 �Bulk rock chemistry chemistry also also supports supports aa glaciogenic origin (Argast, (Argast, 2002) . The The chemical chemical data, data, including sediments from including the the absence absenceof of aacorrelation correlationbetween between K20 K2Oand andA1203, A1203show sediments from the Fern Creek Formation Formation were were deposited deposited without without extensive extensivesorting sorting or or demixing demixing of of hydraulically hydraulically coarsercoarserNa20/K20 and atomic + K)/Al K)/A1 atomic and (2Ca (2Ca ++ Na + and finer-grained fractions. Other data, including the NazO/K^O ratio ratio suggest suggest sediment sediment accumulated accumulated with with abundant abundant original original feldspar. feldspar. The The chemical chemical index index of of alteration (CIA) Gowganda diamictite diamictite (CIA) ranges ranges from from 50 50 to 61, 61, similar similar to the average average CIA of 57 in Gowganda matrices. The accessory accessory suite suite is complex complex and includes includes poorly rounded zircons. zircons. These These attributes attributes are consistent consistent with an an origin origin as as aa glacial glacial till. till. . and/or thorium were identified identified in the Several minerals enriched in rare earth elements (REE) andlor Fern Creek Creek Formation. Formation.These Theseinclude includemonazite, monazite,huttonite huttonite(monoclinic (monoclinicThS1O4) ThSi04) and a fluorfluorhydroxy-REE mineral. Th concentrations concentrations as high as 53 53 ppm were noted in one one bulk bulk analysis. analysis. Efforts to obtain obtain aa chemical chemical date date on these minerals minerals have so so far far been unsuccessful. unsuccessful. The Camey Carney Lake Lake Gneiss Gneiss is is aa chemically-compatible chemically-compatiblepossible possible source source for for the the Fern Fern Creek Creek Formation. Formation. REFERENCES REFERENCES Argast, A., A,, 2002, 2002, The The lower lower Proterozoic Proterozoic Fern Creek Creek Formation, Formation,northern northern Michigan: Michigan:mineral mineral and and J. Earth Earth Sci., Sci., v. v. 39, 39,p. p. 481481bulk geochemical geochemical evidence evidence for for its glaciogenic glaciogenic origin: origin: Can. Can. J. 492. 492. Argast, S. and Donnelly, Donnelly, T.W., T.W., 1987, 1987, The chemical chemical discrimination discrimination of of clastic clastic sedimentary sedimentary components: components: J. J. Sed. Sed. Pet., Pet., v. v. 57, 57, p. p. 813-823. 813-823. Awwiller, D.N., 1993, 1993, Illite/smectite Illitelsmectite formation formation and and potassium potassium mass mass transfer transfer during duringburial burial diagenesis of mudrocks: a study from the Texas Gulf Coast Paleocene-Eocene: Sed, diagenesis mudrocks: study Texas Coast Paleocene-Eocene:J.J. Sed. 501-512. Pet., v. 63, 63, p. 501-512. of Wintsch, R. P. and Kvale, C. M., 1994, 1994, Differential mobility of elements in burial diagenesis of siliciclastic J. Sed. Sed. Res., Res., v. v. 64A, 64A, p. p. 349-361. 349-361. siliciclasticrocks: rocks: 3. 2 �GEOLOGY AND HYDROGEOLOGY IN THE KINGSFORD, MICHIGAN MICHIGAN AREA BARTNTK, PATRICK PATRICK J., J., pbartnik0arcadis-us.com, pbartnik@)arcadis-us.com, ARCADIS ARCADIS G&M, (I&M, Inc., BARTNIK, Inc., Kingsford, Kingsford, Michigan, 49802; and bevans@arcadis-us.com, ARCADIS EVANS, BRUCE W., bevans@arcadis-u.s.com, ARCADIS G&M, G&M, Inc., Inc., Milwaukee, Milwaukee, Wisconsin, 53202 Wisconsin, 53202 Investigations have been undertaken in a portion of the City Investigations City of Kingsford, Kingsford, Michigan Michiganand and Michigan (the Breitung Township, Michigan (the study study area) area) to determine determine the the geologic geologic and and hydrogeologic of glacial sediments and and bedrock. bedrock. The hydroeeologic characteristics of The study study area area is is located located Dickinson in ~ i c k i n s oCounty County i in the south-central south-central Upper Upper Peninsula Peninsula of of Michigan. Michigan. 1EPA)t ffpT I 7 .' '. I . . / IUNGSEOkD .. ,*-. 4 C&y md L v". ] """ l—% Cit;ofKingsford 1. f.I if, P'1 \ — — Study Area * ty 1ownship Kinysford City Supply Well The ARCADIS investigations were largely completed between April 1997 1997 and January January 2001, but are continuing. During During the the investigations, investigations, over 300 300 soil borings were were completed, along with with 47 47 test test pits pits and and 9.5 9.5 miles miles of of geophysical geophysical study. study. The topography is by three three distinct distinct landform terraces (Upper, Lower, and Riverside), which characterized by range in elevation from approximately 1,120 feet above mean sea level (ft msl) to 1,045ftftmsl. msl. The Upper Terrace contains several isolated glacial kettles. approximately 1,045 elevation of of the the Menominee Menominee River River is is approximately approximately 1,037 1,037 ftft msl. msl. The The geology geology is The elevation 3 �comprised of unconsolidated glaciofluvial and glaciolacustrine deposits of clay, silt, sand, that exhibit complex horizontal horizontal and and vertical vertical spatial spatialvariability. variability. These and gravel that Michigamme Slate and the Lower sediments overlie the Middle Precambrian Michigarnme Formation. Depth to groundwater in the Precambrian metavolcanic Quinnesec Formation. unconsolidated deposits ranges from about 10 feet below land surface (bis) near the to more more than than 50 feet feet bls bis in in the theUpper UpperTerrace. Terrace. Groundwater flow Menominee River to follows irregular pathways toward the Menominee River, but generally flows from northeast to southwest. Vertical Vertical hydraulic hydraulic gradients range from +0.863 ft/ft in upland —0.012ft/ft ftlftnear nearthe theMenominee MenomineeRiver. River. Hydraulic Hydraulic conductivities conductivities range from iO3 areas to -0.012 10'~ per second (cdsec) (cm/see) to to 10" 10' cm/sec material to to 1.03 xx 1i03 centimeters per c d s e c for coarser-grain material 0'~ i05 0 "cm/sec c d s e c for for the the very very fine-grain sand and sandy silt. The Thebedrock bedrockisis ccm/sec d s e c to 3.94 x 1 considered impermeable. impermeable. Groundwater generally considered Groundwater flow velocities range from approximately 3 ft/day to 280 ft/day in coarser-grain units, units, and and from from approximately approximately0.1 0.1 fine-grain sand and sandy silt. ft/day to 3 ft/day in the very fine-grain To aid in the understanding of the complex complex geology within the study study area, area, threethreetopographic surface, dimentional modeling of the geology was undertaken using the topographic surface, and glacial glacial sediments. sediments. Thirteen geologic units identified from the bedrock surface, and borehole data were categorized in to 3 units, based on permeability and anticipated anticipated effects on groundwater flow. flow. The modeling and visualization of the geology were The Geostatistical Software Library (GSUB), (GSLIB), developed at Stanford completed using a Geostatistical FORTRAN programs, and Environmental Visualization System University, FORTRAN System (EVS) Development Corporation. Corporation. software developed by the C Tech Development 4 �GEOLOGY GEOLOGY OF OF THE THE MISSISSIPPI-VALLEY MISSISSIPPI-VALLEYTYPE TYPE MINERALIZATION MINERALIZATIONAT MICHIGAN BELLEVUE, MICHIGAN BLASKE, Allan R., Blaske Geoscience, BLASKE, Allan R., Blaske Geoscience, 8313 8313 Hartel, Hartel, Grand Grand Ledge, Ledge, Ml MI48837 48837 The Bayport Bayport Limestone is exposed exposed in in quarrying quarrying operations operations at at Bellevue, Bellevue, in in southwestern southwestern Eaton Baton County, Michigan. Mining has active around around Bellevue County, Michigan. Mining has been been active Bellevue since since the mid-1800's. Approximately 25 25 feet of the Bayport is exposed in the quarrying Approximately quarrying operations, and consists of a buff colored thin-bedded limestone. gray to buff limestone. The Bayport Bayport limestone limestone is is late Mississippian in age, age, and comprises the upper portions of the The Mississippian in the Grand Rapids Group. Grand Group. It is underlain underlain by the the Michigan Michigan Formation, Formation, also of of the the Grand Grand Rapids Rapids below the Grand Group. The The early earlyMississippian Mississippian Marshall Marshall Sandstone and Coldwater Shale lie below Rapids Group. The TheBayport Bayportisisoverlain overlainby bythe theearly earlyPennsylvanian PennsylvanianSaginaw SaginawFormation Formation(within (within the Michigan Basin), but covered only by glacial sand and gravel at the quarry site. gravel at the quarry site. Mineralogy of the deposit is simple, Mineralogy of simple, consisting consisting predominantly of pyrite, marcasite, and calcite. Pyrite is most commonly found as encrustations of cubic crystals, formed directly on limestone. limestone. Mamasite Marcasite is generally generally lighter in color than the pyrite, and often in iridescent, iridescent, platy crystal groups. Marcasite is by by far the dominant iron sulfide. sulfide. Two observed. Early Marcasite is dominant iron Two generations generations of calcite are observed. Early calcite is found as calcite as small small crystals crystals lining lining cavities cavities as as drusy drusycoatings. coatings. The second generation of calcite is found in large, large, euhedral euhedral crystals and cleavable masses. Trace Traceamounts amountsof ofsphalerite, sphalerite, barite, and fluorite are present. Fluorite Fluorite was was the the earliest earliest to to form, form, as as small small brown brown crystals crystals directly directly on the Pyrite was was formed on the limestone. limestone. Pyrite formed in association association with the the early early calcite. calcite. Marcasite and and sphalerite pyrite. Second sphalerite are later than the early calcite and pyrite. Second generation generationcalcite calcitebegan began slightly slightlyafter after the marcasite. Tiny Tiny crystals crystals of of marcasite marcasite can also also be be found found on on the the large large calcite, calcite, indicating indicating that that formation of of marcasite continued to to the end of formation marcasite continued of mineralization. mineralization. Barite Barite appears appears later than the sulfides, but before the end of the calcite formation. sulfides, calcite formation. Mineralizationisis present present predominantly predominantlyinin brecciated brecciated zones zones and and vein structures Mineralization structures within within the the The most Bayport Limestone. Limestone. The most common common type type of breccia breccia consists consists of of small, small, angular angular clasts clasts surrounded by by open-space open-spacefilling fillingofofsulfides sulfidesand andcalcite. calcite. A second type of breccia consists consists of of surrounded type of larger, rounded rounded clasts, clasts, with with the the interstitial larger, interstitial spaces spaces filled filled with aa muddy muddy limestone limestone and and pyrite. pyrite. Orientation and and size size of of the mineralized mineralized zones zones within within the the limestone limestone is is not not known, due to lack of Orientation within the the quarry quarry and and insufficient insufficienthistorical historicalmapping. mapping. Fine-grained iron sulfide is also exposure within observed as replacement structures, along apparent solution fronts within the massive massive limestone. limestone. The geochemistry of the the sulfides indicates indicates the the simplicity simplicity of of the the mineralization. mineralization. 36-element ICP geochemistry of analysis of of pyrite pyrite and and marcasite marcasite separates, separates, as as well well as a composite breccia sample, indicate very analysis concentrations of of trace trace elements. elements. Copper, lead and low concentrations and zinc zinc are are found found at at concentrations concentrations of of less less than 60 ppm; ppm; nickel nickel is is less less than than 30 30 ppm; ppm; and and cadmium cadmium and and cobalt cobalt less lessthan than 55ppm. ppm. Barium is generally less less than than 20 20 ppm. ppm. Manganese is high in the also low, generally the breccia breccia (385 (385 ppm), ppm), and and lower lower in the sulfide separates (64 to 109 ppm), while chromium is high in the sulfides (150 ppm) and low 5 �ppm). Arsenic in the breccia (31 ppm). Arsenic isis present present in in the the breccia breccia (7 (7 ppm), ppm), but but low low in in the the iron iron sulfides sulfides at at less than 5 ppm. ppm. Sulfur isotopic were analyzed analyzed on on separated samples of Sulfur isotopic compositions compositions were of pyrite, pyrite, marcasite, marcasite, and and sphalerite. Sulfur (6S) ofofthe Sulfur isotopic compositions (S^S) thesulfide sulfidephases phasesfrom fromthe theBayport BayportLimestone Limestone are for These are 14.5°/ 14S0Ioofor forthe the pyrite, pyrite, 12.8°/ 12.8°/o forthe themarcasite, marcasite, and and 19.8°/ 19.8'loofor forthe thesphalerite. sphalerite. These compositionsindicate indicatethat that the the mineralizing mineralizingfluids fluids were were basinal basinal brines brines from from within compositions within the the Unpublished data obtained from the surrounding Mississippian and Pennsylvanian Pennsylvanian formations. formations. Unpublished USGS (Westjohn, I). B., pets. pers. comm.) comm.) as part part of the the RASA RASA program program indicate indicate aa large large range range of of (Westjohn, D. 34 . 8SSSininsamples samplescollected collectedfrom fromthe theunderlying underlyingMarshal! MarshallSandstone Sandstone and and Michigan Michigan Formation, Formation, as well as the overlying Saginaw Formation Formationand andthe theJurassic Jurassic Red Red Beds. Beds. Pore well overlying Saginaw Pore water, water, whole-rock, whole-rock, sulfide, and sulfate sulfur isotope isotope compositions compositions for the the underlying underlying formations formations exhibit exhibitaverage average o/ 20°/c,, while the average 634S withinthe the overlying overlying formations formations is near 17 S^S within 17 'loo. S^S near 20°/oo Temperature Temperature of the mineralization has been determined using fluid inclusions in calcite (Panter, (Panter, K. 5., S., 2001). 2001). Calcite afforded afforded the only mineral phase with with inclusions inclusions for formicrothermometric microthermometric study. Temperatures Temperaturesof of homogenization homogenization indicate a bimodal distribution, with aa low low temperature temperature mean of approximately 5g°C 58°C, and and a high high temperature mean mean of of approximately 138OC. 138°C. The The mean mean temperature of all inclusions These temperatures are similar to those temperature of inclusions analyzed analyzed was was 107°C. 107OC. These temperatures are those compositions in authigenic minerals the Mississippian observed observed using using isotopic isotopic compositions authigenic minerals in Mississippian and and Pennsylvanian sandstones (Westjohn, (Westjohn, D. B., 1994). 1994). Fluid salinities salinities based based on on freezing freezing point point Pennsylvanian sandstones equivalent weight percent depression range from 2.6 to 9.5 equivalent percent NaCI. NaCl. The quarries quarries at at Bellevue Bellevue are are located located within within55 to to 66 miles miles to the north The north and and west west of of the the known known northwest end of the northwest the Albion-Scipio Albion-Scipio Oil Oil Field Field Trend. Trend. This oil oil field field (dolomitized (dolomitized fracture fracture and and solution cavities) structure is located within the Trenton-Black River (Middle (Middle Ordovician) Ordovician)rocks, rocks, 4,000 feet feet deeper deeper than than the the Bayport Bayport Limestone. Limestone. The structure structure is related related to to faulting faulting within within some 4,000 basement rocks. rocks. Evidence the basement Evidence of of the the structure, structure, however, however, is is present present in in the the lower lowerMississippian Mississippian Sunbury Shale Shale Formation, Formation, (approximately (approximately 3,000 3,000 feet feet higher higher than than the Middle Ordovician rocks), the Coldwater the Sunbury), and the the Coldwater Shale Shale (overlying (overlying the Sunbury), and the Marshall Marshall Sandstone Sandstone (overlying (overlying the If movements associated with this are evident Coidwater). Coldwater). If movements associated with this structure structure are evident in in the theformations formations immediately below the Bayport Limestone, it seems likely that the Bayport Bayport Formation Formation would would also also be affected by faulting associated with with the structure. Faulting Faulting associated associatedwith with the theTrend Trendisislikely likely responsible for small in the Bayport, allowing allowing for for brecciation, brecciation, subsequent subsequent fluid fluid responsible for small structures structures in the Bayport, precipitation of the mineralization. migration, and precipitation mineralization. REFERENCES REFERENCES Panter, K. 5., S., 2001. 2001. A Preliminary Preliminary Microtermometric Study of Fluid Inclusions in Calcite Calcite from data, Bowling Bowling Green Green State University, OH Bellevue, Michigan, unpublished data, of Westjohn, D. B., 1994, Michigan Basin RASA Solid-Phase Investigation, in Geohydogeology of Carboniferous Aquifers of the Michigan Basin, Great Lakes Section-SEPM, 1994 Fall Carboniferous Aquifers of the Michigan Section-SEPM, 1994 Fall Field Conference, September 23-24, 1994, 1994, Lansing, MI 6 �GEOLOGIC MAPS MAPS OF OFKEWEENAWAN KEWEENAWAN VOLCANIC AND INTRUSIVE ROCKS BEDROCK GEOLOGIC IN THE LAKEWOOD, FRENCH FRENCH RIVER, RIVER, AND KNIFE RIVER 7.5' QUADRANGLES, NORTH SHORE OF OF LAKE LAKE SUPERIOR, SUPERIOR,MINNESOTA MINNESOTA f., Minnesota Geological Survey, St. Paul, Paul, MN, MN, boerbOOI@umn.edu boerb001@umn.edu BOERBOOM, Terrence J., Geological Survey, Survey, with with partial partial funding funding by by the U.S. The Minnesota Minnesota Geological U.S. Geological Geological Survey Survey STATEMAP STATEMAP geologic mapping program, has recently published detailed bedrock geologic geologic maps maps of of three threequadrangles quadrangles located along the North North Shore of Lake Superior northeast of Duluth, Minnesota Minnesota (Fig. 1; located 1; Boerboom and others, 2002a, b). Field Field mapping mapping was was completed completedatat aa scale of 1:12,000, others, 1:12,000, and compiled compiled at a scale scale of of has shown that some some flow flow sequences sequences can can he be traced traced inland inland as as far as 10 1:24,000. This mapping 10 to 12 12 1:24,000. mapping has shown that of individual individual flows flows within within the thelarger larger flow flow units. units. Several kilometers, and has identified hundreds of Several mafic to felsic, subcordant to discordant discordant sills sills and and intrusions intrusions have have also also been been mapped. mapped. Prior to this mapping, bedrock geologic geologic maps maps for for this this area were at aa scale mapping, the only only published published bedrock scale of of (for example example Miller Miller and others, 2001). 2001), and other work was concentrated concentrated along the shoreline 1:200,000 (for shoreline of Lake Superior. Superior. Brannon Brannon(1984) (1984)sampled sampled160 160successive successivevolcanic volcanic flows, flows, starting startingabove abovethe theLester LesterRiver River geochemical study. study. Green sill and ending in Two Harbors, as part of an exhaustive geochemical Green and and others others (1977) (1977) included this this area area as part of aa more included more broad broad coastal coastal zone zone management management study. study. Schwartz and Sandberg Sandberg published a paper on the diabase sills near Duluth that included some of the (1940) published the sills sills mapped mapped during during this study. Sandberg Sandberg(1938) (1938) mapped mapped the the stratigraphy stratigraphy of the flows exposed exposed at the the shoreline shorelinefrom from Duluth Duluth to to Two Harbors, identifying some 180 180 lava lava flows. flows. Although all of these these studies studies made made some someincursions incursions inland from the shore, none of them provided systematic mapping away from the shoreline shoreline proper. Bedrock exposure in the map area varies greatly, from nearly continuous outcrop along the shoreline shoreline and many of the short short streams streams along along the the slope slope into into Lake Lake Superior, Superior, to to variably variablyabundant abundant outcrop outcropin in the the hills inland from the lakeshore. Throughout Throughout the the map map area, area, there there are are many many closely closelyspaced spacedstreams streams that that bedrock perpendicular perpendicular to to the the strike strike of of the the volcanic volcanic stratigraphy. stratigraphy. Hence, have eroded into the bedrock Hence, many of the individual flows could could be be traced traced for for a great individual flows great distance distance along along strike strike by tying tying them them together together from from one one streamcut to the next, in combination combination with with the the shoreline outcrops. outcrops. In In contrast, contrast,the the more more resistant resistantintrusive intrusive typically exposed exposedon onthe thetops topsand andslopes slopesofofhigh highhills. hills. The northeast northeast part part of of the map area is rocks are typically aeromagnetic poorly exposed and thus much of the bedrock geology in that area is constrained constrainedlargely largelyby by aeromagnetic data. data. Green (2002) has proposed a subdivision of the North Shore Volcanic Group into a series series of informal informal sequences and formations formations that that are are separated by major lithological breaks or by sequences and separated by lithological and and geochemical geochemical breaks by Within the the area of the maps shown here, these include the Larsmont basalts, Sucker River intrusions. Within intrusions. Lakewood lavas, lavas, and and the the Lakeside Lakeside lavas lavas (Fig. (Fig. 2). 2). The detailed detailed bedrock geologic geologic maps maps shown basalts, the Lakewood here subdivide these informal informal formations formationsinto into multiple multiple layers layers comprised comprisedof of lava lava flows of here subdivide these of similar similar documented. composition and texture in which multiple flow contacts have been documented. REFERENCES Boerboom, T.J., T.J., Green, J.C., J.C., and Jirsa, Jirsa, M.A., M.A., 2002a, 2002a, Bedrock Bedrock geology geology of of the French French River and Lakewood quadrangles, St. Louis County, Minnesota: Minnesota Geological Survey quadrangles, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Miscellaneous Map M128, scale 1:24,000. 1:24,000. Bedrock geology geology of of the Knife River quadrangle, St. Louis and Lake Counties, -2002b, 2002b, Bedrock Counties, Minnesota: Survey Miscellaneous Miscellaneous Map Map M-129, scale 1:24,000. Minnesota Geological Survey Brannon, J.C. J.C. 1984, Geochemistry Geochemistry of of successive successive lava lava flows flows of of the Keweenawan North Shore Volcanic Group: St. dissertation,312 312p. p. Group: St. Louis, Louis, Washington Washington University, University, Ph.D. dissertation, 7 �Green, Green, J.C., J.C., 2002, 2002,Volcanic Volcanicand andsedimentary sedimentary rocks rocksofofthe theKeweenawan KeweenawanSupergroup Supergroupininnortheastern northeastern Minnesota, Minnesota,Chapter Chapter55of of Miller, Miller,J.D., J.D., Jr., Jr., Green, Green,J.C., J.C.,Severson, Severson,M.J., M.J., Chandler, Chandler,V.W., V.W.,Hauck, Hauck,S.A., S.A., Peterson, T.E.,Geology Geologyand and mineral mineralpotential potentialof ofthe theDuluth DuluthComplex Complexand andrelated related Peterson,D.M., D.M., and andWahl, Wahl,T.E., rocks rocksof of northeastern northeasternMinnesota: Minnesota: Minnesota MinnesotaGeological GeologicalSurvey Survey Report Report of of Investigations Investigations 58, 58, p. p. 9494105. 105. Green, Green, J.C., J.C., Jirsa, Jirsa, MA., M.A.,and andMoss, Moss,C.M., C.M., 1977, 1977,Environmental Environmental geology of the North North Shore Shore of of Lake Lake Superior:Minnesota MinnesotaGeological GeologicalSurvey, Survey,99 99p.p. Superior: Miller, J.C.,Severson, Severson,M.J., M.J., Chandler, Chandler,V.W., V.W., and andPeterson, Peterson,D.M., D.M., 2001, 2001,Geologic Geologicmap map Miller, J.D., J.D., Jr., Jr.,Green, Green,J.C., of of the the Duluth Duluth Complex Complex and and related related rocks, northeastern Minnesota: Minnesota Minnesota Geological Geological Survey Survey MiscellaneousMap MapM-119, M-119,scale scale1:200,000. 1:200,000. Miscellaneous Sandberg, Sandberg, A.E., 1938, 1938, Section Section across across Keweenawan Keweenawan lava lava flows flowsatatDuluth, Duluth,Minnesota: Minnesota: Geological Geological Society of of America America Bulletin, Bulletin,v. v. 49, 49, p. p. 795-830. 795-830. Society Schwartz, Schwartz, G.M., G.M., and and Sandberg, Sandberg, A.E., A.E., 1940, 1940, Rock series series in in diabase diabase sills sillsatatDuluth, Duluth,Minnesota: Minnesota: Geological Society of America Bulletin, v.51, v. 51,p. p. 1135-1172. 1135-1172. Geological AmericaBulletin, 92' 47 47' map showing showing location location of of Figure 1.1. Index map Figure mapped quadrangles. quadrangles. Work Work is is currently currently in in progress on the the Two Two Harbors Harbors and and Castle Castle progress Dangerquadrangles. quadrangles. Danger Index map map showing showing the the Figure 2.2. Index relative positions positions of the the informal informal volcanic units of Green (2002). volcanic Green (2002). Intrusive rocks Volcanic rocks Boundary of informal volcanic units units volcanic 88 �MINERAL RESOURCE WISCONSIN MINERAL RESOURCEGIS CISAND AND RELATED RELATED DIGITAL DIGITALMAP MAP AND AND DATABASE PRODUCTS -A PROGRESS REPORT REPORT B:A.1,CZECHANSKI, CZECHANSKI,M.L.', Mi.', MUDREY, BROWN, B.A.', MUDREY,M.G., M.G., Jr.', ~ r . and ' ,andREID, REID,Daniel DanielD.2 D.' (1) (1) Wisconsin Geological and Natural History Survey, Univ. of Wisconsin-Extension, 3817 Survey, Wisconsin-Extension, 3817Mineral Mineral Point Road, Madison, Madison, WI 53705, 53705, babrownl@facstaff.wisc.edu, babrownl @facstaff.wisc.edu, (2) (2) Wisconsin Dept of Transportation, 3502 Transportation, 3502 Kinsman Blvd, Madison, Madison, WI WI 53704-2507 53704-2507 information on 1,302 A new Mines, Pits and Quarries (MPQ) database containing information throughout Wisconsin Wisconsin has has been been completed by the Wisconsin significant nonmetallic mining sites throughout Geological and Natural History Survey (WGNHS) in cooperation cooperation with the the U.S U.S Geological Geological Survey (USGS). Locations were digitized from county-based digital orthophotography orthophotographywherever wherever (MASIMIIILS available and by site site visits. visits. Data Datatables tableswere werelinked linkedtotoexisting existingUSGS USGSdatabases databases (MAS/MILS and MRDS) and to Wisconsin Department of of Transportation Transportation (WDOT) (WDOT) aggregate test data; this linkage of all previous digital and analog analog databases is the first updated updated inventory inventory since since1980. 1980. Future versions will be augmented with current site information, collected under the nonmetallic nonmetallic reclamation program of Wisconsin Department of Natural Natural Resources, and additional historic sets such as the Road Material Survey WGNHSIWDOT. Survey sites sites of of the the WGNHSIWDOT. orthophotography, soil, Georeferenced maps layered with digital geology, topography, orthophotography, soil, and so forth provide a valuable land-use planning resource. Concern Concernfor forsafety safetyand andconstruction construction reconstruction of U.S. Highway 151 through the historic Upper Mississippi problems in the reconstmction District, southwest Wisconsin, Wisconsin, made possible the scanning and Valley Base-Metal Mining District, georeferencihg of the the Wisconsin Wisconsin Mineral MineralDevelopment DevelopmentAtlas. Atlas. The Mineral Development Atlas georeferencing of is a detailed set of of 1,450 section-scale maps (1 (I inch equal inch equal 200 200 feet) feet) of of mine mine workings, workings, drill-hole drill-hole dating from 1900 until until mining ceased ceased in 1979. These maps were location and ancillary data dating maintained by the WONTIS and the the University University of Wisconsin-Platteville Wisconsin-Platteville and WGNHS and and were were scanned scannedby by the the WDOT. WDOT. All Wisconsin water well construction reports for 1936-1988 are now available available on CDRom. They provide an extensive data set for geologic mapping as well as environmental environmental and and water resource analysis. New WGNHS map products are being produced in digital digital form and a variety of analog maps including the 1:24,000 1:24,000 USGS geologic quadrangle maps of the lead-zinc lead-zinc district are being converted to digital as resources allow. digital as resources allow. This presentation will provide an interactive demonstration demonstration of these these data data sets sets and and GIS GIs layers, a review of available map data such as regional geophysics, and an update on the status of geologic geologic mapping mapping at at the the WGNHS. WGNHS. 9 �HIGHWAY CONSTRUCTION, CONSTRUCTION,MINE MINERECLAMATION, RECLAMATION,AND AND LAND-USE LAND-USEPLANNING PLANNING CHALLENGES CHALLENGES IN THE THE HISTORIC HISTORICUPPER UPPER MISSISSIPPI MISSISSIPPI VALLEY VALLEY LEAD-ZINC LEAD-ZINC DISTRICT OF OF SOUTHWEST SOUTHWEST WISCONSIN BROWN, B.A.1, M.L.', REID, B.A.', MUDREY, M.G., Jr.1, ~ r . ' CZECHANSKI, , M.L.', REID, Daniel Daniel D.2, D.', and HUNT, T.C.3, Wisconsin Geological Geological and T.c.~,(1) Wisconsin and Natural History Survey, Univ. of Wisconsin-Extension, Wisconsin-Extension, babrown1@facstaff.wisc.edu, 3817 Mineral Point Road, Madison, WI 53705, babrownl@facstaff.wisc.edu, mgmudrey@wisc.edu, (2) Wisconsin Dept. of of Transportation, 3502 Kinsman Blvd, Madison, WI mgmudrey@wisc.edu, 53704-2507, (3) Reclamation Program, Univ. of Wisconsin-Platteville, 712 Pioneer Tower, Platteville, Platteville, WI WI 53818 53818 The Upper Mississippi Valley Valley Lead-Zinc District of Wisconsin, Illinois, Iowa, and Minnesota produced nearly 10 10 million tons of lead-zinc ore from the 1820s 1820s until the last last mine mine closed closed in 1978. The district will probably never be mined again, but but problems problems related to mineralization and past mining activity pose significant problems for highway construction and post-mining land use. Specific hazards and engineering problems problems include include (1) Highly altered and unstable rock and construction, (2) leachate from shallow abandoned mine workings encountered during highway construction, roaster-pile waste and (3) locally degraded degraded groundwater groundwaterfrom fromlead-zinc lead-zincsulfide sulfidemines. mines. As rural residential development increases, increases, the abandoned workings, particularly poorly sealed sealed shafts, can be a hazard. Most low-sulfide low-sulfide waste rock has been recycled as aggregate, and carbonate-rich tailings overgrown with vegetation vegetation make it difficult to find any surface evidence of small, small, older mine sites sites that may cause cause problems. problems. High sulfate in groundwater samples was noted in 1978 following closure 1978 following closure and and flooding flooding in an an area where large mines had operated for more than 50 years and a drawdown cone had bad developed developed over a 20-square mile area. A well-replacement program near Shullsburg Shullsburg restored potable water supplies. Onsite reclamation consisted consisted of establishing establishing vegetation vegetation on the tailings tailings and and crushing crushing the the coarse waste rock for aggregate. Leachate from zinc roaster waste piles produced over 100 100 years Mineral Point. The roaster piles were resulted in highly acidic and metal-rich surface water near Mineral successfully reclaimed by surface grading and contouring contouring along along with neutralization and fertilization to allow vegetation to establish. This was accomplished accomplished at aa fraction fraction of of the the cost cost of of removal of the roaster waste waste piles. piles. Previously undiscovered sulfide sulfide mineralization mineralization and and associated associated rock rock alteration alterationexposed exposedduring during construction along U.S. Highway 151 near Mineral Mineral Point Point resulted in the unanticipated highway construction unanticipated structures. The need to identify identify areas areas of need for engineering redesign of major roadcuts and structures. potentially unstable slopes led to scanning of the Wisconsin Mineral Development Development Atlas, Atlas, which which has proven to be invaluable in identifying areas of mineralization, alteration, alteration, and and abandoned abandoned workings in the path of construction. These detailed maps (1 inch to 200 200 feet) feet) of of mine mine workings workings and exploration drillhole locations are now being used by county and regional planners planners and and zoning authorities to identify and incorporate potential mining related hazards hazards into into land-use land-use planning. planning. 10 �TEXTURAL CONSTRAINTS TEXTURES IN IN THE THE WOLF WOLF RIVER TEXTURAL CONSTRAINTSON ON THE THE ORIGIN ORIGIN OF RAPAKIVI TEXTURES RIVER BATHOLITH BATHOLITH R. Michele Buttram Buttrarn and and Marcia Marcia Bjornerud Bjomemd Geology Department, Department, Lawrence University, Appleton, WI 54912 The Wolf River Batholith of north-central Wisconsin, a 1.47 Ga composite anorogenic pluton, includes some of the world's finest examples of 'rapakivi' granite, granite,in in which which large large plagioclase. Although potassium feldspar crystals are mantled by plagioclase. Although rapakivi rapakivi granites graniteshave have origin of of this this distinctive distinctive texture, texture, both both in the been described for more than a century, the origin elsewhere, remains remains controversial. controversial. Some workers argue that Wolf River complex and elsewhere, point under equilibrium rapakivi mantles are coronae formed at a peritectic or eutectic point crystallization conditions. Others Others maintain that rapakivi rapaldvi textures record disequilibrium associated with magma mixing mixing and and or or sudden sudden changes changes in in pressure. pressure. While most previous investigations investigations have focused on the chemistry of rapakivi granites, this study study examined examinedthe thephysical physicalcharacter characterofofthe theWolf WolfRiver Riverrocks rocks—- specifically, specifically, the size, shape, orientation and distribution rapakivi-type feldspar crystals. Among distribution of the rapahvi-type Amongthe the most striking characteristics of these rocks is the large size of the feldspars (up to 7 cm in length). Statistical Statisticalanalyses analyses show show that there is no significant significant difference in size or aspect with and and without without the therapakivi rapakivimantle. mantle. However, the K-feldspar ratio between crystals with tend to be be rounder rounder (less euhedral) euhedral) than than non-rapakivi non-rapakivi cores of the rapakivi-type crystals tend grains, suggesting prior to to the growth of the suggesting that they experienced significant resorption prior plagioclase mantle. A Aweak weak grain grain shape shape fabric fabric and and random random juxtaposition of rapakivi rapaldvi and non-rapakivi grains must also be explained by any viable model for the origin of the grains also explained texture. Our Our data data appear appearto to be be most most consistent consistent with the magma mixing model, which is compatible with earlier geochemical studies of the Wolf River complex. compatible complex. 11 �Niagara suture suture zone, zone, northern northernMichigan Michiganand andWisconsin—tectonics Wisconsin-tectonics in Ma arc-continent arc-continent collisional boundary the 1.85 Ma W.F. G.L. LaBerge, W.F. Cannon, Cannon,(U.S. (U.S.Geological GeologicalSurvey, Survey,Reston, Reston,VA VA20192, 20192, wcannon@usgs.Rov) wcannon(Susss.sov) G.L. LaBerge, (University of of Wisconsin-Oshkosh Wisconsin- Oshkosh(retired) (retired)and andU.S. U.S.Geological GeologicalSurvey), Survey),John JohnS.S.Klasner Kiasner (Western Illinois University (retired) (retired)and and U.S. U.S.Geological Geological Survey) Survey) The Niagara suture suture zone, zone, as as used used here, here, is is aa belt belt varying varying in in width width from fromabout about66 km krnto to40 40km kmlying lying north of the Niagara fault. It separates the accreted accreted volcanic volcanic arcs arcs of the the Wisconsin Wisconsinmagmatic magmatic terranes (WMT) on the south from the autocthonous and parautochtonous continental margin metavolcanic rocks rocks of of rocks on the north. ItIt consists consists of of Paleoproterozoic Paleoproterozoic metasedimentary and metavolcanic upon which they they were the epicratonic Marquette Range Supergroup Supergroup and Archean basement rocks upon deposited. The TheArchean Archean rocks rocks constitiute constitiute the the southern southern margin of the Superior craton, which was rifted and eventually separated during extensional eventually separated during extensional phases of the Penokean orogenic orogenic cycle, and then thrust northward northward during Penokean convergence. The suture zone is marked by very high strain and widespread multiple steeply to vertically plunging folds. folds. The suture zone is one of whose hierarchy hierarchy of of component component parts parts is is shown numerous subdivisions of the Penokean orogen whose below. Michigamme subterrane subterrane Michigamme Foreland fold Foreland fold and thrust "Niagara \ Niagarasuture suture zone Park Falls panel /, I Watersmeet Watersmeet panel Beechwood Beechwood panel Iron River River panel Menominee panel cianel /1 north north PENOKEAN OROGEN\ — - Niagara Niagara fault --PENOKEAN OROGEN --ioihh — — \\ south\ Pembine-Wausau (northern ~ e m b i n e - ~ a u s terrane aterrane u (northernpart part of ofWMT) WMT) - --- Mars hfleld terrane (southern (southern part Marshfield part of ofWMT) WMT) The The map pattern shown here was derived from published detailed maps in the east east (Bayley and and others, 1966; Dutton, 1971; others, 1961) others. 1971: James and others 1968; and James and others. 1961) and from our recent work in the west, where outcrops are scarce but access to recent exploration recentwork exploration drill electromagnetic data have aided in information as well as proprietary detailed aeromagnetic and electromametic clarifying the geologic>el~tionships geologic relationships (Cannon clarifying (Cannon and and others, others,1998). 1998). Each of the five fault panels of the Niagara suture suture zone has a unique unique set set of characteristics. characteristics. Watersmeet panel- Paleoproterozoic Paleoproterozoic strata are mostly pelitic schists schists and and gneisses gneisses containing containing ferruginous near the the base. base. They ferruginous strata and locally dolomite near They were were deposited deposited on on aa basement basement of Archean gneiss. gneiss. Both basement and cover were deformed deformed into into gneiss gneiss domes. domes. High-pressure metamorphism metamorphism produced kyanite-bearing assemblages. assemblages. Park Falls panel- Generally similar to Watersmeet panel except that metamorphism metamorphismwas lower lower pressure and sillimanite-bearing sillimanite-bearing assemblages assemblagesare arepredominant. predominant. Beechwood panel- Consists of Paleoproterozoic Paleoproterozoic graywacke graywacke and shale and mafic volcanic rocks rocks in in equal parts. parts. Archean basement is not exposed. exposed. Folds are roughly equal are ENE-trending and have subhorizontal axes. Rocks are are in greenschist greenschistfacies. facies. hon Iron River River panel- Rocks are the Paint River Group, including the Badwater Greenstone. Archean basement is not exposed. Strata are multiply folded creating a complex complex fold fold interference interference map map pattern. Most Most fold fold plunge steeply. Metamorphosed Metamorphosedto to greenschist greenschist or or sub-greenschist sub-greenschist facies. 12 �_____________________________ 89' 90' 88' t 4, '4— 46'- 3i •"'$ — ., S, 1' nt titer-i' V '7 A•S 50 0 I I I 7 AL-) L nt nit n-i' rlLp,1reflorniflee A 7 7 ,a 7 50 L'/AtvAkva 7 AS 7 A•3 7 , CL, L LVA tY' tvA tvA C. 7 7 L KM - 7,,. 7 7 89' 88' Maoshowing showingthe thefive fivestructural structuralpanels panels(Park (ParkFalls, Falls.Watersmeet, Watersmeet, Beechwood. IronRiver, River. Map Beech wood, Iron and that the Niagara andMenominee) ~enominee) thatconstitute constitutethe ~ i a ~ asuture suture r a zone. zone.Faults Faultsthat thatbound boundthe thepanels panelsare are Flambeau FlambeauFlowage Flowagefault fault(FFF), (FFF),Powell Powellfault fault(PF), (PF),Elmwood Elmwoodfault fault(EF), (EF),Paint PaintRiver Riverfault faull(PRF), (PRF), Badwater Badwaterfault fault(SF), (BF),North NorthRange Rangefault fault(NRF), (NRF),and andSouth SouthRange Rangefault fault(SRF). (SRF). 9'l' Menomineejanelpanel-Rocks Rocksare areentirely entirelyof ofPaleoproterozoic Paleoproterozoic age. age.No NoArchean Archeanbasement basementisisexposed. exposed. Menominee Strain Strainwas wasextreme. extreme. Commonly Commonlyallallstructural structuralelements, elements,including includingfold foldaxes, axes,are aresubvertical. subvertical. Metamorphism Metamorphismisislower lowertotoupper uppergreenschist greenschistfacies faciesand andlargely largelypost-tectonic. post-tectonic. These Thesepanels, panels,and andthe theNiagara Niagarasuture suturezone zone that that they they constitute, constitute,differ differfrom fromthe theMichigamme Michigamme subterrane subterraneto tothe thenorth. north.There Therewas waslittle littlepenetrative penetrativedeformation deformationofofArchean Archeanbasement basementisisininthe the Michigamme Michigamme subterrane. subterrane. Paleoproterozoic Paleoproterozoicstrata stratawere weremoderately moderately to toweakly weakly deformed. deformed. Folds, Folds, for postforthe themost mostpart, part,are aresimple simpleand andgently gentlyplunging. plunging.Metamorphic Metamorphicgrade gradeisisvariable variableand andmostly mostlyposttectonic. Thus, the Niagara suture zone documents a range of tectonic styles unique to the very tectonic. Thus, the Niagara suture zone documents a range of tectonic styles unique to the very high strains strainsin inaabelt beltno nomore morethan than aa few few tens tens of of kilometers kilometerswide, wide, along alongwhich which differential differential high movement craton margin margin on on the the north north was was movement between between the the accreting accreting arcs arcs on on the the south south and and the the craton concentrated. concentrated. References References Bayley, Bayley,R.W., R.W.,Dutton, Dutton,CE., C.E.,and andLamey, Lamey,C.A., C.A.,1966, 1966,Geology Geologyofofthe theMenominee Menomineeiron-bearing iron-bearingdistrict, district, Dickinson DickinsonCounty, County,Michigan Michiganand and florence Florenceand andMarinette MarinetteCounty, County,Wisconsin: Wisconsin:U.S. U.S.Geological GeologicalSurvey Survey Professional 96p. ProfessionalPaper Paper513, 513.96~. Cannon, Cannon,WE., W.F.,LaBerge, LaBerge,G.L. G.L.Kiasner, Klasner,J.S., J.S.,and andSchulz, Schulz,K.J., K.J.,1998, 1998,Reinterpretation Reinterpretationof of the the Penokean Penokean 44th continental part of ofnorthern northern Wisconsin Wisconsin and Michigan Michigan (abs.): (abs.):Proceedings Proceedingsof of 44' Annual AnnualInstitute Institute continentalmargin margin in in part on onLake LakeSuperior SuperiorGeology, Geology,v.v.44, 44,p.p.52-53. 52-53. Dutton. Dutton,CE., C.E.,1971, 1971,Geology Geologyofofthe theFlorence Florencearea, area,Wisconsin Wisconsinand andMichigan: Michigan:U.S. US. Geological GeologicalSurvey Survey Professional 54 p.p. ProfessionalPaper Paper633, 633,54 James, James,H.L., H.L.,Clark, Clark,L.D., L.D.,Lamey. Lamey,C.A., C.A.,and andPettijohn, Pettijohn,EU., F.J.,1961, 1961,Geology GeologyofofCentral CentralDickinson DickinsonCounty, County, Michigan: Michigan:U.S. US. geological geological Survey Survey Professional ProfessionalPaper Paper 310, 310,176 176p.p. James, James, H.L., H.L., Dutton, Dutton, CE., C.E.,Pettijohn, Pettijohn,F.J., F.J., and and Weir, Weir,K.L., K.L.,1968, 1968,Geology Geologyand andore oredeposits depositsof ofthe theIron IronRiver River US. Geological Crystal Falls Falls district, district,Iron Iron County, County, Michigan: US. GeologicalSurvey SurveyProfessional Professional Paper Paper 570, 570,134 134p. p. -- Crystal 13 �Strike-slip separation Strike-slip separation of the Burntside Burntside trondhjemite and and the the Wakemup Wakemup Bay Bay tonalite, Northern Minnesota Karoun Charkoudian, Basil Tikoff Department of Geology and Geophysics, Wisconsin. Madison WI, WI, 53706 53706 Geophysics, University of Wisconsin, Robert Bauer Department of Geological GeologicalSciences, Sciences,University University of of Missouri, Missouri,Columbia, Columbia,MO, MO, 65211 65211 INTRODUCTION The INTRODUCTION TheVermilion Vermilionfault faultisisaalocal localtectonic tectonicboundary boundary in in the the southern southern Canadian Canadian Shield juxtaposing the Quetico subprovince (granites and schists) with the Wawa greenstones. Quetico subprovince greenstones. Burntside trondhjeniite The Bumtside trondhjemite and the Wakemup Bay tonalite are small, elliptical, Archean granites granites separated by 35 k km of right right lateral offset offset on on the the Vermilion Vermilionfault faultin innorthern northernMinnesota. Minnesota. The m of Vermilion fault is interpreted interpreted as initially initially active as a normal fault, juxtaposing the shallow shallow Wawa the north (figure 1, greenstone to the south with the deeper granites and migmatized schists to the 1, reactivated as a strike-slip trondhjemite from stage stage 1). 1). It was later reactivated strike-slip fault, separating the Burntside trondhjemite from the Wakemun Wakemup Bay tonalite (figure 1, stage 2). Although the Vermilion fault is the regional Although the Vermilion fault is the regional 1. . Bav, . boundary between between the the Quetico Quetico and Wawa Wawa subprovinces, the Haley fault lies to the south south of of the the Vermilion fault and contains contains Quetico schists schists that belong- [ Stage N s t a x II Queticp gnttanaschsts on the north side side of of the Vermilion vermilion fault fault (figure 1) 1). The purpose of this study is to compare pluton setting fabrics, fabrics composition, of emplacement setting, composition, and shape of the two plutons to to determine determine if they constitute constitute a piercing point on the Vermilion fault. In In addition, addition, we have 20km determined din on on the the Vermilion Vermilion fault, fault. constrained constrained ..---~ the dip I emplacement history of the Wakemup tonalite. tonalite, and the emplacement the determined determined a potential potential cause cause for the the isolated isolated fault fault block block Queticp Sge 2 granites and schists that now contains contains the the Wakemup WakemupBay Bay pluton pluton (figure (figure11, 44 Bumtslde stage 2). stage Wakenup 110 The Burntside Bumtside trondhjeimte trondhjemite is is aa small small lenticular lenticular s pluton that intruded intruded the schist schist that lies lies to to the the north north of of the the ShaawaLake Bumtside continuation of the Vermilion Burntside Lake fault, a continuation fault at its eastern end. The TheWakemup Wakemup Bay Bay pluton pluton is is aa biotite-bearing tonalite that intruded biotite-bearing intruded the schist schist that lies lies Figure Figure 11 just to to the the north north of of the the Haley Haley fault. fault. - - -çffr ~ ~ tr ~ n'" The Anisotropy of Magnetic Susceptibility (AMS) is a rapid, nonAMS ANALYSIS ANALYSIS used in in granitic granitic studies studies to to obtain obtain magnetic magnetic fabrics. fabrics. Principle destructive technique, commonly used Principle magneticfoliation foliationisisdefined definedasas k-k1 kmx-k,,,, AMS ellipsoid axes are defined as km>kht>k,nin. The magnetic thethe is defined defined as as the the orientation orientation of of km,. plane, and the magnetic lineation is Wakemup Bay Bay tonalite tonalite (500-8500pSI) (500-8500pSI) The bulk susceptibility varies widely in both the Wakemup Burntside trondhjemite (SOO-3500pSI). This range range of of susceptibility susceptibility is attributed to the and the Bumtside (500-3500pSI). This content throughout throughout these these bodies. bodies. The large variation in magnetite content The AMS foliations foliations parallel parallel the the in both both plutons. plutons. Lineations measured field foliation in Lineations in in the the Wakemup Wakemup Bay tonalite tonalite dip dip shallowly shallowly to the E and W, and lineations lineations in the Burntside Bumtside trondhjemite trondhjemite dip dip shallowly shallowlyto to the the ENE ENEand andWSW. WSW. consistently parallel Magnetic lineations consistently parallel the long long axis axis of of the the plutons. plutons. The Bumtside and Wakemup plutons were selected selected for a gravity gravity study study GRAVITY STUDY lithology (biotite (biotite schist) with with a significant and because they both contain aa single surrounding lithology gicc). In consistent density contrast (Adensity (Adensity == -0.08 -0.08 to -0.1 glcc). In addition, addition, the gravity gravity data allows allows us to model the dip of the Vermilion Vermilion fault. fault. 14 �Lacoste and Romberg Romberg gravity gravity meter meter model modelGGwas wasused usedfor forboth bothareas. areas. After A Lacoste Burntside pluton and corrections, a forward model approach was used to interpret the depth of the Bumtside Vermilion fault geometry using WinGLink, WinGLink, a geophysical geophysical interpretation interpretation software software program. program. The Burntside pluton pluton is a thick body body between between 2-3 2-3 km km in inthickness. thickness. The Vermilion fault is a steeplyBumtside dipping to to vertically vertically oriented orientedfeature. feature. Using a gravimetric gravimetric three-dimensional thee-dimensional iterative north dipping technique on on the the Wakemup Wakemup Bay Bay pluton pluton resulted resulted in in aa good goodfirst-order first-orderpicture pictureof ofthe thepluton. pluton. Most technique thin, less less than than 0.5km 0.5km thick. thick. There are two root zones of up to 4 km krn depth, of the pluton is very thin, both of which lie on the southern portion of the Wakemup pluton, furthest away away from from the the Vermilion fault. fault. INTERPRETATION We interpret the Burntside Bumtside and Wakemup plutons as as part part of of the the same same granitic complex complex prior prior to to strike-slip strike-slip faulting faulting on on the theVermilion Vermilionfault. fault. These igneous bodies are granitic composition and both both have undergone solid-state deformation. deformation. The The plutons have similar similar in composition settings. The structural settings. The Wakemup Wakemup Bay pluton intrudes an F3 fold hinge and the Burntside pluton has refolded F2 folds at its southern end. Given Given the the separation, separation, the the folding folding episodes episodes may may or or may may not correlate correlate exactly. exactly. The gravity inversion and AMS study on the Wakemup pluton provide constraints constraints on on pluton emplacement. The The pluton pluton has has an an average average thickness thickness of of 0.5 km. Because the pluton reflects the true thickness thickness of the pluton. pluton. The TheAMS AMS contains a roof of wallrock, this estimate reflects F3fold. fold. foliation and lineation parallel the the fold fold limbs limbs and and fold fold hinge, hinge, respectively, respectively, of of aa km-scale km-scale F3 Therefore we interpret the Wakemup F3 fold fold hinge. hinge. Wakemup as syntectonically intruding intruding an an F3 We use a forward gravity model to estimate the dip on the Vermilion fault, which dips 70° N N and and vertical. vertical. This interpretation requires that the section of the Vermilion fault between 70' Bumtside pluton was not active as a south-side down normal fault. south of the Burntside We propose the following tectonic tectonic model model (figure (figure 1). The Burntside Bumtside pluton and the Bay pluton pluton were were initially initially part part of of the the same same granitic granitic complex. complex. The Wakemup Bay The Vermilion Vermilion fault fault was was 1, stage 1), l), which which juxtaposed the the aznphibolite amphibolite facies facies Quetico Quetico initiated as a normal fault (figure 1, sub-province with with the the greenschist greenschist facies facies Wawa Wawa belt. belt. The The Wakemup Wakemup tonalite, tonalite, with with aa thick thick root root on on promontory in the the fault system. The its south side, acted as a promontory The Vermilion Vermilion fault fault was was then then reactivated as a strike-slip fault (figure 1, stage 2), cutting through the thinnest (NW) section of the Wakemup Bay pluton. This This created created the the fault-bounded fault-bounded block block that that contains containsthe theWakemup WakemupBay Bay pluton. Therefore, Therefore, itit isis evident evident that that the the pluton pluton shape shape has played a crucial role in controlling Vermilion fault orientation, both for the early normal faulting and later strike-slip faulting. REFERENCES REFERENCES Bauer, R.L., 1985. 1985, Norwegian Bay Quadrangle, St. Louis County, Minnesota. Minnesota MinnesotaGeological Geological Survey, Miscellaneous Map series. 1:24,000. Survey, Miscellaneous series, Map Map M-59, M-59, 1:24,000. Bauer, R.L., 1986, 1986, Multiple folding and pluton emplacement in Archean migmatites of the southern Vermilion granitic complex, complex,northeastern northeasternMinnesota. Minnesota.Can. Can.1. J.Earth EarthSci., Sci.,v.v.23, 23, p. 1753-1764. 1753-1764. R.L., and Bidwell, Bidwell, M.E., ME., 1990, Bauer, R.L., 1990,Contrasts Contrasts in the response to dextral transpression across the Queticoboundary in innortheastern northeasternMinnesota. Minnesota. Can. J. Earth Sci., v. v.27, Wawa subprovince boundary 27, p. p. 1521-1535. 1521-1535. Sims, MG., 1972, Sims, P.K., and Mudrey, M.G., 1972,Burntside Burntsidegranite granitegneiss, gneiss,Vermilion Vermiliondistrict, district,ininSims, Sims,P.K., P.K.,etetal., al.,eds., eds., Geology of Minnesota: A Centennial Centennial Volume: Volume: St. Paul, Minnesota Minnesota Geological Survey, p. 98-101. Vigneresse, emplacement by regional deformation: Tectonophyiscs, Tectonophyiscs, v. 249, p. 1995, Control of granite emplacement Vigneresse, J.L., 1995, 17 3-186. 173-186. 15 �MINERAL POTENTIAL POTENTIAL OF PROTEROZOIC PROTEROZOIC MAFIC GEOLOGY AND MINERAL MAFICINTRUSIONS INTRUSIONS IN THE THE NORTHERN PROVINCE OF ONTARIO NORTHERN GRENVILLE GRENVILLE PROVINCE ONTARIO R.M. EASTON, EASTON, Ontario OntarioGeological GeologicalSurvey, Survey,933 933 Ramsey Ramsey Lake Lake Road, Road, Sudbury, Sudbury,Ontario OntarioP3E P3E6B5, 6B5, mjke.easton@ndm.gov.on.ca imke.easton@ndm.eov.on.ca Since 1998, 1998, mafic intrusions near the Grenville Front in Ontario have been prime exploration targets targets for for Cu-Ni-PGE Cu-Ni-PGE mineralization. mineralization. To To assist assist in this this effort, effort, the the Ontario Ontario Geological Geological has conducted detailed mapping in in high high potential areas of of the Grenville Province between between Survey has and 2002. This poster summarizes 1999 and summarizes the results results of these these mapping mapping efforts. efforts. East Bull Lake East Lake intrusive intrusivesuite, suite,including includingthe theRiver RiverValley Valleyintrusion: intrusion:Country Countryrocks rocksto to East Bull Lake intrusive intrusive (EBU) (EBU)suite suiterocks rocks in in the thearea area are areinferred inferredto to be be mainly mainlyArchean Archean in in age, age, and and are grouped into 4 gneiss associations. Metamorphic grade is upper amphibolite associations. Metamorphic amphibolite facies; facies; country country rocks to the mafic mafic intrusions intrusions are are commonly commonlymigmatitic. migmatitic. The Paleoproterozoic Paleoproterozoic EBLI intrusions emplaced EBU suite consists of several mafic layered intrusions emplaced between 2490 and kin, roughly and 2468 2468 Ma Ma (James (Jameset et al. al. 2002) 2002)that that occur occurover overaadistance distanceofof—250 -250 km, centered on the present site site of Sudbury. Sudbury. The largest of these bodies in the Grenville Grenville is the River Valley intrusion, intrusion, which underlies underlies roughly roughly 100 100kin2 km2 of Dana and Crerar Crerar townships. townships. Previous Previousmaps maps correlated correlated mafic rocks west of Crerar Township with the River Valley intrusion. intrusion. This This study indicates that at least 3 separate separate intrusions are present, each emplaced emplaced into into different different country country rocks, and with different different stratigraphy stratigraphyand and mineral mineral potential. potential. EBU EBU suite suiterock rock types types range range in composition composition from anorthosite anorthosite to melanorite, troctolite and rarely peridotite; leucogabbronorite leucogabbronorite and gabbronorite dominante. The crystallization crystallization order order of primocryst phases is most commonly plagioclase (An80.62), olivine (Fo7659), orthopyroxene primocryst phases is most commonly plagioclase (An80.62), olivine (Fo76-59), orthopyroxene (En7558), titanomagnetite,and and clinopyroxene. clinopyroxene. In In Dana Dana Township, Township, the (En75.58),titanomagnetite, the River River Valley Valley intrusion intrusion locally exhibits primary mineralogy and well preserved igneous textures. textures. Phase Phase layering varies from cm- to m-scale, which is discernable discemable in outcrop, outcrop, and dm or or larger, larger, which which is is identified identified by by detailed mapping. Isomodal layering is most common; mineral and size graded graded layers are less common. Cryptic layering is well documented for the River Valley intrusion. Pearce-element Pearce-element chondrite-normalized REE diagrams illustrate illustrate that each ratio and chondrite-normalized each body formed formed from from one one or or more more cogenetic magmas (James et al. 2002). A high-Al, low-Ti tholeiite tholeiite composition composition can can explain explain the the dominant leucocratic rock compositions in in the the EBU EBU suite suite (James (James et et al. al. 2002). 2002). EBLI suite, contact-type Cu-Pd-Pt mineralization (1 Within the EBU (1 to 10 10g/t g/t Pd+Pt+Au) Pd+Pt+Au) occurs in the matrix of an inclusion and/or fragment-bearing gabbronorite to leucogabbronorite andlor fragment-bearing gabbronorite to leucogabbronoriteat at the base or side of the intrusions where the primary igneous contact contact is preserved. preserved. A A second, second, similar, zone of mineralization may occur 100-200 100-200 m above above the contact. contact. Examples Examples occur occur throughout the EBLI EBU suite, suite, however, the most consistent grades grades have been reported from the River Valley intrusion in Dana Township. Chalcopyrite and lesser lesser pyrrhotite pyrrhotite form form 1-3 1-3 % % sulfide, sulfide, either finely disseminated or as local cm-sized patches. PGE mineralization mineralization is is commonly commonly associated with suiphide. sulphide. Study of the East Bull Lake intrusion indicates indicates that that mineralization mineralization originates from the intrusion and subsequent subsequent dynamic dynamic mixing of S-saturated, S-saturated, inclusion-bearing, inclusion-bearing, second-stage second-stage (PGE enriched, i.e. 20-100 ppb PGE) magmas magmas that that entered enteredthe the magma magma chamber chamber carrying liquid sulfide droplets (James et al. 2002). Reef-style mineralization mineralization has has yet yet to to be be documented within the EBLI suite. suite. 16 �GeologicaJ history history between Sudbury Geological Sudburyand andRiver RiverValley: Valley:Archean Archeanrocks rocksininthis thisarea arearecord record aa sequence of events similar to that observed observed in the Levack Gneiss complex complex and and high-grade high-grade portions of the Quetico subprovince, subprovince, but unlike the Pontiac subprovince. The following geological history is inferred. After deposition of greywackes south of the Temagami Temagami greenstone greenstone belt, invasion by tonalitic to granodioritic plutons, probably accompanied accompanied by burial, burial, formed formed the the migmatitic migmatitic gneisses gneisses now represented represented by the Pardo Pardo and Red Cedar Cedar Lake Lake gneiss gneiss associations, associations,likely likely between 2685 and 2675 2675 Ma. This This was was followed followed by by a second second period period of of tonalitic tonaliticto togranodioritic granodioritic magmatism, deformation deformation and metamorphism metamorphism at at mid-crustal mid-crustal levels levels between between 2670 2670and and2660 2660Ma. Ma. products of of this this latter activity. activity. Subsequent felsic The Crerar gneiss association represents the products magmatism at roughly 2640 Ma was accompanied accompanied by emplacement emplacement of pegmatite pegmatiteveins. veins. A logical extension of this work is to interpret the gneiss associations as a southwarddeepening deepening section of the crust. As interpreted, interpreted, Archean metawackes metawackes exposed exposed immediately immediatelynorth north of the Grenville Front Front represent high-levels high-levels of of the the crust. crust. The The Pardo Pardo gneiss, gneiss, immediately immediately south of of the Grenville represents the middle part of aa 10-15 Grenville Front, represents 10-15 km km thick thick upper upper crustal crustallayer layerdominated dominated by supracrustal and intrusive rocks. The Red Cedar Cedar Lake gneiss and the the Street Street gneiss gneissassociation association represent the basal portion of this upper crustal layer, with with the the former former derived derived from from a sequence and the latter metasedimentary rock sequence latter from a greenstone greenstone sequence. sequence.Intrusive Intrusiverocks rocks of of the the Crerar gneiss association are part of aa 10-15 km thick thick middle middle crustal crustallayer. layer.This Thiscrustal crustalsection sectionisis 10-15 km roughly equivalent to that observed across the Wawa gneiss gneiss domain. domain. Emplacement Emplacementof ofEBLI EBLIsuite suite bodies occurs at several levels within this this crustal crustal section. section. Flett Township Townshipmafie maficIntrusions: Intrusions:Evidence Evidencefor foraamafic maficand andA-type A-typegranite granite magmatic magmatic province in the northern Grenville Province was discovered while examining mafic intrusions near Temagami that that occur in in Tomiko Tomiko domain, near near its contact with with the Grenville Front tectonic zone. rocks consist consist of of gneissic gneissic granite, granite, with with minor minor mafic mafic and quarztose gneiss and Proterozoic country rocks The Fall Fall Lake Lake intrusion intrusion consists consists of of little little metamorphosed metamorphosed gabbro and metaconglomerate. The Fanny Lake Lake intrusion intrusion consists consists of of olivinite olivinite and and troctolite. troctolite. Igneous texture is leucotroctolite. The Fanny well preserved, but metamorphic coronas occur occur around around primary olivine olivine and and clinopyroxene. clinopyroxene. indicates that that both both bodies bodies are are slightly slightly alkalic, alkalic, compositionally compositionally similar to the Geochemistry indicates diabase dike dike swarm swarm dated dated at at 1238  ± 44 Ma, Ma, and and have have affinities affinities to to within-plate within-plate basalts. Sudbury diabase intrusion yielded pristine baddeleyite, with with 3 concordant or just just slightly The Fall Lake intrusion slightly discordant 1235±22Ma. Ma.The TheFanny Fanny Lake Lake sample discordant grains grains giving givingan an average average207Pb/206Pb 2 0 7 ~ b / 2 0age 6age ~ bofof1235 yielded baddeleyite, baddeleyite, with with some some grains grains having thin thin zircon zircon overgrowths, consistent with the presence of corona textures in the body. Two concordant grains without overgrowths overgrowths gave an 1238  ± 2 Ma. Ma. average average 2207Pb/2°6Pb 0 7 ~ b / 2 fage fage i ~ of bof1238 Both intrusions are spatially associated with the A-type Mulock granite, granite, dated previously at 12444L3 Ma. Intrusions Intrusions of of similar similar age include include the the Sudbury dike dike swarm, Mercer anorthosite, 1244+4/.3Ma. the West West Bay Bay and and Powassan Powassan granitoid granitoid plutons. plutons. The The new new age age data data provides provides further evidence and the of aa bimodal bimodal magmatic province active from 1270-1235 for the presence of 1270-1235 Ma in the Laurentian margin of the Grenville Province. Province. The The tectonic setting is interpreted as an extensional extensional rift that of a continental continental arc active on on the southern margin of North America formed inboard of America between 1450-1300 Ma. Ma. This This setting setting resembles resembles that that of of the the Cenozoic Cenozoic Columbia River Basalt Group. Group. R.S., Easton, Easton, R.M., R.M., Peck, Peck. D.C. D.C. and Hrominchuk, Hrominchuk, J.L. J.L. 2002. 2002. The East Bull Lake intrusive James, R.S., intrusive suite: suite: remnants of a —2.48Ga Galarge largeigneous igneousand andmetallogenic metallogenicprovince provinceininthe theSudbury Sudburyarea areaof of the the Canadian Canadian Shield; Economic -2.48 v.97, p.1577-1606. Geology, v.97, 17 �PAIEOSmES5 INFERENCES FROM FAULT PALEOSTRESS INFERENCES FROM FAULTSLIP SLIPVECTORS VECTORSIN INTHE THEEASTERN EASTERNPART PARTOF OFTHE THE WIscoNSIN SEGMENT MIDCONTINENT Rwr WISCONSIN SEGMENTOF OF THE MIDCONTINENT Amy Garbowicz, Marcia Marcia Bjomerud, Bjomerud, 54912 Geology Department, Lawrence Lawrence University, Appleton, WI 54912 Building accurate models models for for both Building accurate both the opening opening and closing of of the the Midcontinent Midcontinent Rift Rift requires an understanding of the the evolution of regional stresses over over time. This study requires an understanding of evolution of regional stresses study paleostress indicators indicators in in the the portion portion of of the Rift exposed near focused on slickenfibers as paleostress northeasternmost Wisconsin. Wisconsin. The the southern shore of Lake Superior in northeastemmost The orientations orientations of slickenfibers were were used used to determine slickenfibers determine slip vectors vectors on outcrop-scale outcrop-scale faults faults within within riftriftrelated igneous and sedimentary rocks. Rocks sampled span the entire range of related and sedimentary rocks. Rocks sampled span the range the the Keweenawan Supergroup, from the Tyler Formation to the Freda Sandstone, with most of the sampling in the Porcupine Volcanics, the Kallander Creek Volcanics, and the Mellen Gabbro. for their their age, age, Gabbro. . The mineral composition of the slickenfibers was used as a proxy for based on the based the known known regional regional sequence sequence of of secondary secondary mineralization mineralization within within the the Rfit. Rfit. Chlorite and epidote slickenfibers slickenfibers were grouped together and considered considered older older since since these these were among were among the first first minerals minerals precipitated precipitated by by hydrothermal hydrothermal fluids following following the the main main of calcite and zeolite were considered to be magmatic interval. interval. Slickenfibers of be younger. younger. Some individual faults were observed to have multiple generations of slickenfibers slickenfibers with reactivation or or continuous continuous slip over aa protracted protracted different compositions, indicating either reactivation Data from the period of time. period time. Data the field field were were analyzed analyzed using using Fault Fault Kinematics Kinematics (by (by R. R. Allmendinger, Cornell Unviersity), a program that calculates best-fit paleostress tensors Allmendinger, Cornell Unviersity), a program that from fault slip slip information. information. The calculated tensors all indicate indicate normal normal stress stress regimes regimes (maximum principal stress subvertical), even even for the latest (maximum principal latest generations generations of of slickenfibers. slickenfibers. This contrasts with the the results This contrasts with results of studies studies on on the the Keweenaw Keweenaw Peninsula, Peninsula, which which have have documented two distinct stress documented two stress regimes. regimes. There, There, early early normal normal faulting faulting gives way to to far-field stresses associated with the Grenville reverse faulting, possibly as a response to far-field Orogeny. Orogeny. The The absence absence of of reverse-slip reverse-slip vectors vectors in in the the northeastern northeastern Wisconsin Wisconsin segment segment of of the Midcontinent Rift may reflect the misorientation of this part of of the the rift rift with withrespect respectto to those far-field stresses. stresses. . 18 �Mode of Occurrence their Possible Possible Origin in in the the Negaunee Occurrence of Trona and Thermonatrite and their Iron-Formation of the Marquette Range, Range, Lake Superior District, USA Tsu-Ming Han (Retired) (Retired) Research Research Laboratory, Cleveland-Cliffs Cleveland-CliffsInc. Inc. A white colored substance Negaunee Ironsubstance is often seen on the surface of the silicate-bearing Negaunee Formation of low metamorphic grade on the Marquette Range, Michigan. This substance is mostly of a mixture containing hydrous hydrous sodium sodium carbonates carbonates(trona (tronaand andthermonatrite) thermonatrite) It occurs as thin coatings coatings coatings along bedding (Figure 1-A), and in fractures cutting across the bedding; as coatings and colloform colloform clusters on bedding planes (Figure 1-B); and as contour patterns distributed between the fractures fractures of bedding surfaces. Furthermore, nearly pure trona was developed quickly as as dendrites dendrites and minute minute dots dots on on the the cut cut surfaces surfaces of of some somehand hand specimens specimensin instorage storage(Figure (FigureC). C). To To the writer's writer's knowledge, knowledge, these these minerals minerals have have not not been been previously previously reported reportedfrom from Precambrian Precambrian BIF BIF of of the the equivalent equivalentmetamorphic metamorphicgrade gradein in other otherdistricts. districts. 4' 4fr.! Figures Mode of of occurrence occurrence of of trona trona and and thermonatrite. thermonatrite. Figures1 1—- Mode A-As white B-As colloform bedding plane. plane A—As whitecoatings coatings along along bedding. bedding. B—As colloform clusters clusters on a bedding C- As specimenin in storage. storage. C— Asdendrites dendriteson onthe the cut cut surface surface of a hand specimen The iron-formation is composed of magnetite, siderite, siderite, ankerite, ankerite, and stilpnomelane.. stilpnomelane.. The Minnesotaite more than Na in in these these minerals minerals Minnesotaite is also locally present in noticeable quantities. quantities. K is more as is the case in nearly all of the As aa as the Precambrian iron-formations iron-formations of of low low metamorphic metamorphic grade. grade. As 19 �_____________________ general general rule, rule, stilpnomelane stilpnomelanecontains containsmore moreKK and and Na Na than than the theminnesotaite. minnesotaite. However, However, the the K20:Na20 ratio these minerals and K20:Na20 ratioinin these minerals andin..the in theiron ironformation& formationsmay may vary vary substantially.. substantially. . Based Basedon onthe theresults resultsfrom fromthe thehighly highlypurified purifiedwater waterleaching leachingtests testson onmore morethan thantwenty twentydifferent different samples, practicallyinsoluble insoluble samples, the theNa Nain inthe theiron-formation iron-formationisiswater-soluble water-solublewhereas whereasthe theKKisispractically (Figures 2A and and B). B).The TheXRD XRDand and analytical analyticaldata datashow showaagood goodcorrelation correlationbetween between the theamount amount (Figures2A of Na20, K20 and the the amounts amounts of ofNa20, K 2 0and andA1203 A1203(Figures (Figures3A 3Ato to C). C). of stilpnomelane stilpnornelaneand 1L40 0.35 0.30 0.25 0.20 0.15 0.34 p.06 0.04 0.30 y25.66x+2.1167 = 0.7495 25, 0.9 20. 15 0.7 ar. I In .BTst':T 5 0 0 0.05 0 0.10 0.15 0.1 0.2 0.3 20 0.4 0.5 0.6 0.7 0.8 0.20 - Figure Figure 22-AAand andBB Solubility Solubilityof ofK20 K 2 0and and Na20 Na20 in the the silicate-bearing silicate-bearingiron-formation iron-formationwith with high high in and low K20:Na20 K20:Na20ratios. ratios. and - 0.9 Figure33- A A to to CC Relationship Relationshipofofstilpnomelane stilpnomelane Figure to K20, K20,Na20 Na20and andA1203. A1203. to 5 . 5 0. 0. c/ 00 % #4203 Y m A1203 0.5 0.5 1.0 1.0 1.5 1.5 2.0 2.0 2.5 2.5 It It may be logically concluded concluded that most of the sodium was was derived derived from from the stilpnomelane, stilpnomelane, which which was leached leached out by meteoric water. The mixture mixture of of the the hydrous sodium sodium carbonates carbonates was then developed through evaporation under the atmospheric conditions. developed through evaporation under the atmospheric conditions. 20 I1 3.0 3.0 �and Ultramafic Ultrainafic Intrusive Intrusive Rocks of the Lake Nipigon and Keweenawan Mafic and Ontario Crystal Lake areas, northwestern Ontario 933Ramsey RamseyLake LakeRoad, Road, Sudbury, Sudbury,Ontario Ontario P3E R.,Ontario OntarioGeological GeologicalSurvey, Survey,933 Hart,Thomas ThomasR., Hart, 6B5; tom.hart@ndm.gov.on.ca tom.hart@ndm.gov.on.ca 6B5; The Keweenawan diabase sills in the the Lake Lake Nipigon and and Crystal Crystal Lake Lake areas, areas, northwest of Lake Lake Superior, Superior, consist of two two distinct distinct geochemical geochemical and and geographical geographicalgroups groups with each area also hosting a number of unique intrusions intrusions that suggest suggest different different tectonic tectonic processes. Mapping by Smith and Sutcliffe (1987) in in the the Crystal Crystal Lake Lake area area identified identified a processes. series of 6 Logan diabase sills >5 m thick that gently dip dip to to the the southwest, and intrude into the early Proterozoic Rove Formation. Formation. Northeast Northeast trending dykes of of the the Pigeon Pigeon River swarm range from olivine to quartz diabase in composition, and include dykes that crosscut the Logan sills and dykes that appear to merge with the sills. crosscut sills. The The layered layered gabbro gabbro — anorthosite anorthosite -- troctolite Crystal Lake Gabbro crosscuts and contains inclusions of Pigeon of the Logan sills can can be be subdivided into into a low Ti02 Ti02 -Th/Yb - ZrN Zr/Y River dykes. Samples of group and group (OGS (OGS2002). 2002). The high Ti02 group group and aa high high Ti02 Ti02-ThJYb -Th/Yb - ZrIY group The high Ti02 group waswas asbeing beingquartz quartz normative, comparable theLogan type sills Logan by Sutcliffe identified as normative, andand comparable to the to type by sills Sutcliffe (1991). Samples identified as Pigeon River dykes exhibit high degree of variability (1991). Samples identified as Pigeon River dykes exhibit a highadegree of variability suggestingthat thatthey they represent at least unrelated intrusions. Oneofsubset suggesting represent at least three three unrelated intrusions. One subset the of the Pigeon TiOz group of Logan sills, and and another another Pigeon River River dykes dykes is is comparable comparable to the the low low TiO2 subset is comparable to the Crystal Lake Lake Gabbro. Gabbro. Most Most of of aa third third subset subset of of dyke dykesamples samples are located along Highway 61 61 close close to to the the Pigeon Pigeon River, River, and and contain contain lower lower trace element than the the other other intrusions intrusions in the area. Gabbro samples from the abundances and ratios than Crystal Lake Gabbro intrusion display some overlap with the low Ti02 hO2 group groupof of Logan Logan Zr/Y, Th/Yb, and Th/Ta ThITa ratios. ratios. The Logan sills but also includes samples with higher ZrIY, diabase diabase sills sills are confined to the area to the south of Thunder Bay, with the Nipigon diabase diabase sills sills located locatedto to the the north. north. intrusive event in the the Lake Nipigon Nipigon area is probably The initial Keweenawan intrusive flat lying lying to to shallowly shallowly dipping dipping peridotites peridotites located located in the represented by the relatively flat Disraeli, Leckie Seagull -- Fox Leckie— - Seagull Fox Mountain, Mountain, Hele, and Kitto areas that form intrusions a Disraeli, few kilometres in diameter. The peridotites are composed of orthocumulate orthocumulate to mesocumulate mesocumulate textured wehrlite to lherzolite, containing containing 11 to 2% reddish brown mica mica and and commonly a discontinuous olivine gabbro border phase (e.g. Sutcliffe 1987; 1987; Hart et al. 2002). The Disraeli, Seagull and Hele Hele peridotites are characterized characterized by higher MgO and Zr/Y ThiTa ratios than the Nipigon diabase series of of ZrIY and Th/Yb values but lower Th/Ta diabase sills. sills. A series stratigraphically below the Nipigon sills, as exposed 0.5 to 3.0 3.0 m thick sills are located stratigraphically exposed along Highway 17 at Kama Kama Hill. Hill. These These sills have MgO, Th/Yb ThIYb and Th/Ta Itt/Ta values values intermediate intermediate between the peridotites and Nipigon sills, and subdivided subdivided into into higher higher Th/Ta Th/Ta and lower ThITa may be be possible possible with with additional sampling. These sills have Th/Ta subgroups may ThITa and LdYb La/Yb ratios comparable to the high TiO2 Th/Ta Ti02 group of Logan sills, but generally generally have lower trace element abundances. The The Kitto peridotite also has Th/Yb, Th/Ta and and Zr/Y ratios that overlap with these sills rather than the other peridotites. The olivine ZrIY these peridotites. The olivine tholelite m thick, thick, and are chilled against the peridotite peridotite tholeiite Nipigon diabase sills are up to 200 200 in work indicates indicatesthat that some some sills sills were were formed formed by bymultiple multiple pulses pulses of of intrusions. Previous work Hart et et al. al. 2002), but the chemistry of the sills over the magma (e.g. Sutcliffe, 1987; Hart 21 �entire Lake Nipigon area area displays displays little little variation. Geochemical Geochemical differences differencesbetween between the the peridotites and Nipigon diabase sills the variations observed in the sills are comparable to the Osler Ti02 values Osier Group volcanic rocks (e.g. Sutcliffe, Sutcliffe, 1991). The Nipigon sills have Ti02 values comparable but higher ThTa ThiTa and lower LaNb La/Yb Ti02 group of Logan sills but comparable to the low Ti02 ratios. The differences differences in the the geochemistry geochemistry of the diabase diabase sills sills between between the the Lake LakeNipigon Nipigon and Crystal Lake areas is similar to the differences observed observed in in the the volcanic volcanic rocks of of a number of flood basalt provinces (e.g., Mantovani et al. 1985). The regional extent of the geochemical groups within the Keweenawan Keweenawan intrusions intrusionsisisnot notknown. known. An initial examination Complex (Ripley examination of troctolites troctolites from from the Babbit deposit of the Duluth Complex (Ripley et al. ThITa, ThlYb, ThIYb, and ZrN Zr/Y ratios comparable comparable to to the Nipigon peridotites 1999) indicates Th/Ta, peridotites rather than the intrusions intrusions of the the Crystal Crystal Lake area. area. References Hart, T.R., terMeer, M., and Jolette, C. C. 2002. 2002. Precambrian Precambrian geology of Kitto, Eva, Summers, Summers, Dorothea Dorothea and andSandra Sandra Townships, 206 Townships, Beardmore Beardmore area, area, northwestern northwesternOntario; Ontario; Ontario Ontario Geological Geological Survey, Survey, Open OpenFile FileReport Report6095, 6095,206 p. P. L.S., de Sousa, M.A., MA., Civetta, Innocenti, F., 1985. Mantovani, M.S.M., Marques, L.S., Civetta, L., L.,Atalla, L., and Innocenti, 1985.Trace Trace element and strontium isotopic constraints constraints on the origin and evolution evolution of of Parana Paranacontinental continental flood flood basalts basalts of of Santa Santa strontium Catarina State (southern Brazil); Journal of of Petrology. Petrology, v.v.26, 26, p. 187-209. 187-209. Ontario Geological Survey. Survey, 2002 2002. Proterozoic Volcanic volcanic and Intrusive Data associated associated wilh with Oniano fntrusive Whole Rock Geochemical Daia the Keweenawan Kewcenawan Midcontinent Midconunent Rift, Rift.Lake Lake Superior Suoenor Area, Area. Ontario; Onlano. Ontario Onlano Geological Geological SurveyMiscellaneous Miscellaneous . Survey Release—Data 114. Release-Data 114. Ripley, EM., Sm-Nd, and Pb isotonic isotopic consl~aints constraints on mantle Rinlev. Larnbert. D.D., D.D.. and and Frick, Frick. L.R., L.R.. 1999. 1999. Re-Os, Re-0s. Sm-Nd. mantle and crustal ..r.., , E.M.. - ,Lambert, contributions to magmatic contributions magmatic sulfide sulfide mineralization in the Duluth Complex; complex; Geochimica Geochimica et et Cosmochimica Cosmochimica Acta, Acta, v. 62, p.3349-3365. p.3349-3365. v. Smith, AR. and 7987. andSutcliffe, Sutcliffe.RH. R.H. 1987.lCeweenawan Keweenawanintrusive intrusive rocks of the Thunder TTiunder Bay area; in Summary Summary of Field Smith. A.R. Work and Other 137, p. Other Activities, Activities. Ontario Ontano Geological Geological Survey Miscellaneous Paper paper-137. p.248-255. 248-255. Sutcliffe, LakeNipigon, Nioigon. Canada;Contributions Conmbuiions Sutclifie. R.H., R.H.. 1987. 1987.Petrology Pctrolow Middle Proterozoic Proterozoic diabase diabase and and picrites niintes from from Lake -.ofof Middle . - Canada; to Mineralogy p.201-211. Mineralogy and Petrology, v.96, p. 201-211. Sutcliffe, R.H., RH., 1991. 1991.Proterozoic Proterozoicgeology geologyof of the theLake Lake Superior Superior area; area; in Geology of Ontario, Ontario, Ontario Ontario Geological Geological Survey Special Volume Volume4, 4,Part Part!, 1,p.627-658. p. 627-658. 22 �DRILL HOLES, HOLES, MINERAL MJNERALLEASES, LEASES,AND ANDGEOPHYSICS GEOPHYSICSIN GEOLOGY, DRILL NTHE DULUTH AND BEAVER BAY COMPLEXES, NORTHEASTERN NORTHEASTERN MINNESOTA: MINNESOTA: INTEGRATION INTEGRATION OF GIS DATABASES DATABASES TO TELL A STORY OF THE HISTORY OF PAST AND VARIOUS GIs CURRENT CU-NT-PGE MINERAL EXPLORATION CURRENT CU-NI-PGE MINERAL EXPLORATION Steven Hauck, Julie Julie A. A. Oreskovich, Oreskovich, and and Mark J. J. Severson, Severson,Economic EconomicGeology GeologyGroup, Group, Steven A. Hauck, Natural Resources Resources Research Research Institute Institute (NRRI), (NRRI), University University of Minnesota, Minnesota, Duluth, Duluth, 5013 5013 shauck@nni.umn.edu Miller Trunk Highway, Duluth, MN 55811-1442, shauck@nrri.umn.edu Mineral exploration in the Duluth Complex began in 1948 on Spruce Road when two prospectors found sulfide mineralization. mineralization. Subsequent Subsequentcore coredrilling, drilling,geological geologicalmapping, mapping,and and airborne airborneand and ground geophysics by more than 28 exploration companies (including (including the NRRI, MGS MGS Dept. of Natural Resources, Minnesota Geological Survey, Minnesota Survey, and the DNR - Dept. Resources, Division Division of Lands Lands and Minerals), over the next 52 copper-nickeltplatinum-groupelement element 52 years years led led to to the the discovery discovery of copper-nickel±platinum-group mineralization along the basal contact of of the the Duluth Duluth Complex Complex (Fig. 1). Ten (PGE) mineralization Ten Cu-Ni-PGE Cu-Ni-PGE orPGE-Cu-Ni years. Over or PGE-Cu-Ni deposits depositswere were defined defined by by drilling drilling during these years. Over 2,142 2,142 drill drill holes holes have have with 1,666 of of these holes being drilled been drilled into the Duluth and Beaver Bay complexes with the basal basal contact. Over along the Over 954,000 954,000 ft. ft. of of drill drill core from the the basal basal contact contact has has been relogged by NRRI, and their results are discussed in many publications. publications. Geophysical Geophysical exploration exploration began began as as early as 1956, by Bear Bear Creek Creek Mining Mining Company, and and continues continues today. today. The State of Minnesota State Minnesota (MGS), with funding from the Legislative Commission on Minnesota Minnesota Resources, Resources, flew flew high high this area area as as well well as asthe therest restof ofthe thestate. state. The MGS has also resolution aeromagnetics over this collected and produced a gravity map covering both both complexes. complexes. Peak Peak exploration exploration(1966-1978) (1966-1978) began with the the leasing leasing of of State State of of Minnesota Minnesotamineral mineralrights rightsinin 1966 1966(Fig. (Fig. 1). 1). Exploration and began resource calculations) continued through development (drilling, bulk sampling, shaft sinking, resource In 1998, 1998, State State and and Federal Federal mineral mineral leasing and exploration exploration drilling drilling began to to increase increase with with 1978. In rise in price price of PGEs; PGEs; 2) possible use of new new hydrometallurgical hydrometallurgical techniques to more the: 1) rise recover copper copper and nickel; nickel; and and 3) 3) introduction introduction of of new new PGE PGE exploration exploration models efficiently recover (sulfide saturation; Miller et al., 2002) for intrusions in in the Beaver Bay Complex, i.e., at Sonju and the the Duluth Duluth Complex, i.e., Greenwood Lake Lake and and Layered Layered Series Series at at Duluth). Duluth). The Lake, and The maps maps poster illustrate the relationship between geology, geophysics, geophysics, drilling, drilling, and and mineral in this poster were produced produced in ArcView (GIs). (GIS). The The maps were were also also compiled compiled by by using using leasing and were the DNR's online information from: 1) the online (minarchive.dnr.state.mn.us) (minarchive.dnr.state.rnn.us)attributeattribute- and and GIS-based GIs-based database of non-ferrous minerals' information information and and State State mineral mineral rights rights holdings; holdings; 2) 2)U.S. U.S.Forest Forest leases, permits, permits, and applications database; database; and and 3) 3) NRRI NRRI in-house GIS data on the history Service leases, of Cu-Ni-PGE Cu-Ni-PGE mineralization. mineralization. Using the resulting GIs GIS database, the spatial relationships in the of in drilling, leasing, leasing, etc. with with time and place place were were then then combined combined with geological changes in information from Miller et al. (2002) to better understand the past and present exploration exploration areas areas to assist in in defining new new areas in in which to to explore explore for for non-ferrous non-ferrous minerals. and to References References Miller, J. D., Jr., Green, J.C., Severson, Severson, M.J., Chandler, V.W., V.W., Hauck, Hauck, S.A., S.A., Peterson, Peterson, D.M., D.M., and and Wahl, T.E., T.E., 2002, Geology and mineral potential of of the Duluth Complex and related rocks rocks of of 207 p. northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, 58,207 23 �s-ale so Deposit/Area as as as as as as N1i •ChiIL,LSSdJOeSideLHGLOi •1u0b14/HGs SPRUCE RD INCO & t '0 0 0 CU 0 as as t as 50 as as !essk;L Ij_ Rod & Mstsse-i R Spiace - Ieoo MATURI CU as Leek - Spease Rood —1— —f-—I— Il—eorCoeekHII BABBITT (M in no ma x) o(E9e! III AMAX (Mesabo) USS DUNKA ROAD I oeeoo (NorthMel) PolyNeti I I -— Ceeeo WYMAN CREEK USSIITa .1.1 11111 Ii: DUNKA PIT ; Begs yi,—os—foeesotios dells og 11 ErIch WETLEGS <ii AMAX * Duval Duval — fl"II WATER HEN(OUI) • Bear B e a rCreek Creek i_- - SOFILSONCRECK WESTERN MARGIN II It • : Exxon Exxon heIst Dodge! I I I • t/Des o,-e/Aoeecoe Seed • Other O t h e r Companies Companies I —— — LOCCOOS BIRCH LAKE 11 II hee,icoo She d/N(COR LONCNOSE (Out) -ii INCO•. SCATTERED Reosceek.. BeoCeeek••I OR I - • Neeost RECONNAISSANCE Ducol DRILLING SCATTERED DRILLING I USS toXXp • •Ul — / Si * so OS 0 5 Beg, Cooed o sO Si so I o ! '0 so C - Stereos S Ctsfffft ! GUNFLINT TRAIL = CONCENTRATED DRILLING St SI-il 0 320koJg-WhItesIde JJjOekJoyfl as 0 fl I-os--s-so Si LEGEND LEGEND • INCO INCO uss uss 0 0 Si 0 0 '0 0 0 0 0 0' 0 Ot 5 as 2 Figure 1. History Historyof of Cu-Ni-PGE Cu-Ni-PGEexploration explorationin in the the Duluth DuluthComplex Complex (after (afterMiller Milleret etal., al., 2002). 2002). as 0 - as Si �Geochemistry and Mineralization of the Seagull Intrusion, Northern Ontario Heggie, G., P., P., (Depanment of Geology, Lakehead Heggie, G.,and andHollings, Hollings, (Department of Geology, LakeheadUniversity, University,955 955Oliver Oliver Road, Thunder Bay, On, P7B 5E1, gheggie@mail.lakeheadu.ca) gheggie@mail.lakeheadu.ca) Platinum Group Group Elements Elements (POE-Platinum, (PGE-Platinum, Palladium, Osmium Osmium and and Iridium) Indium) have have seen seen substantial increases in demand over the last 30 years, as industrial and and commercial commercial users users have increased their consumption. consumption. Canadian Canadian production of these metals has until until recently recently been limited to by-products from from nickel Sudbury). Opening Opening of of the the copper mines (e.g., Sudbury). lies mine in Ontario Ontario demonstrated demonstrated the the Lac des fles economic PGE deposits potential for economic deposits in in Canada. Further work on deposit deposit and and exploration models is essential essential to to identifying identifying new targets and prospective prospective host rocks. rocks. 4 Lake N(pfgono 1'> •z SeaguU I Quetico Subprovrnce I Lake Superhr • • bIeyGro The Seagull Lake intrusion is found found within the n's Osler Grp Nipigon Embayment, approximately 70km 1). north east east of Thunder ThunderBay, Bay,Ontario Ontario(Fig. (Fig.1). Relative age dating places the age of the intrusion to be younger Seagull Lake intrusion younger than than Figure 1. Map Map showing location of Seagull Intrusion Intrusion Figure the Sibley Group sedimentary sedimentary sequence, sequence, (1339±33 (1339±3Ma) (Franklin et et al., al., 1980), 1980), and regional regionalgeology. geology. as part of the intrusion has been seen to cross cut Sibley Sibley stratigraphy. stratigraphy.A A chilled chilledmargin margin has has been been Intrusion and and the the younger younger Nipigon Nipigon Sills defining defining an upper age of of observed between the Seagull Intrusion approximately 1.1 Ga (Davis (Davis and and Sutcliffe, Sutcliffe,1985). 1985).This This falls falls within within the the time time of of mid-continental mid-continental region. Volcanic Volcanic activity activity was was responsible responsible for for production production of of thick thick rifting in the Lake Superior region. al., 1989) beneath beneath and around the shores of Lake Superior and the basaltic sequences (Cannon et al., mafic to ultramafic complexes complexes (e.g., Duluth complex). emplacement of numerous mafic C The Seagull Intrusion Intrusion is is currently currently under under exploration exploration by by East East West West Resource Resource Corporation, It is a ultramafic intrusion intrusion consisting consisting of of cumulate cumulate olivine, olivine, and oxide minerals with pyroxene layered ultramafic oikocrysts and interstitial feldspar. feldspar. Lithological Lithological phases phases include include dunites, lherzolites, olivine gabbronorites, gabbros, and and pryoxenites. pryoxenites. A A distinctive distinctive olivine gabbronorite is found within the but this exhibits chilled margins and is thought to post date the formation of the rest of intrusion but intrusion. the intrusion. 25 �Ni (ppm) Interval (m) Cu (ppm) 1160 269 4.0 375.0 WMOO-01 1413 501 408.0 4.5 1565 12.0 779 572.0 987 6.0 1180 546.0 WM98-02 1647 112 379.0 8.0 WM98-05 1841 1843 569.0 6.0 1455 1220 6.0 579.0 Figure2.2.Table Tableofofmetal metalcontents contentsfrom fromassay assay(Caven, (Caven,It,R.,2000) 2000) Figure DDH Depth (m) Pt (ppb) 307 336 363 535 336 693 458 Pd (ppb) 383 418 438 566 393 847 537 Mineralization Mineralizationoccurs occursininthe theform formofofPGE PGEminerals mineralsassociated associatedwith withdisseminated disseminatedFeNi FeNisulfides sulfides (pentlandite).Pentlandite Pentlanditeisisfound foundininhigher higherabundances abundancesatatdiscrete discreteintervals intervalsthroughout throughoutthe the (pentlandite). intrusion,with witha ageneral generalincrease increasetowards towardsthe thebase baseofofthe theintrusion. intrusion. intrusion, Work Workisiscurrently currentlybeing beingundertaken undertakentotounderstand understandthe thestratigraphy stratigraphyofofthe theintrusion, intrusion,the thenature natureofof the theplatinum platinumgroup groupmineralization, mineralization,and andformational formationalcontrols controlsononthe themineralized mineralizedzones, zones,which whichare are present presentininthe theintrusion intrusionininorder ordertotoaid aidininthe thedevelopment developmentand andrefinement refinementofofexploration exploration techniques,and anddeposit depositmodels. models. techniques, Cannon, J.C., Cannon,W.F., W.F.,Green, Green,A.G., A.G.,Hutchinson, Hutchinson,D.R., D.R.,Lee, Lee,M., M..Milkereit, Milkereit,B., B.,Behrendt, Behrendt,J.C., J.C.,Halls, Halls,H.C., H.C.,Green, Green, J.C., Dickas, Dickas,A.B., A.B.,Morey, Morey,GB., G.B.,Sutcliffe, Sutcliffe,R., R.,and andSpencer, Spencer,C., C.,1989, 1989,The TheNorth NorthAmerican AmericanMidcontinent Midcontinentrift rift beneath beneathLake LakeSuperior Superiorfrom fromGLIMPCE GLIMPCESeismic SeismicReflection Reflectionprofiling. profiling.Tectonics, Tectonics,v.v.8,8,p.p.305-332. 305-332. Caven, R.J., R.J., 2000, 2000,Progress ProgressReport Reporton onthe theWolf WolfMountain Mountainand andDisraeli DisraeliProperties Propertiesfor forEast EastWest WestResource Resource Caven, Corporation, Ltd. Corporation,Canadian CanadianGolden GoldenDragon DragonResources ResourcesLtd. Ltd.and andAvalon AvalonVentures VenturesLtd. Davis, Davis, D.W., D.W., and and Sutcliffe, Sutcliffe, R.H., R.H., 1985, 1985, U-Pb U-Pb ages agesfrom fromthe theNipigon Nipigonplate plateand andNorthern NorthernLake LakeSuperior. Superior. GeologicalSociety SocietyofofAmerican AmericanBulletin, Bulletin,v.96, v.96,p.p.1572-1579. 1572-1579. Geological Franklin, Franklin,J.M, J.M.McJlwaine, Mcllwaine,WIT., W.H., Poulsen, Poulsen,K.H., K.H., and and Wanless, Wanless,R.K., R.K., 1980, 1980,Stratigraphy Stratigraphyand anddepositional depositionalsetting settingofof the theSibley SibleyGroup, Group,Thunder ThunderBay Baydistrict, district,Ontario, Ontario,Canada. Canada.Canadian CanadianJournal JournalofofEarth Earth Sciences, Sciences,v.17, v.17,p.p. 633-65 1. 633-651. 26 �PEPERITES OF PEPERITES OF THE THE GAFVERT GAFVERTLAKE LAKE VOLCANIC VOLCANIC COMPLEX, COMPLEX, ST. LOUIS MINNESOTA COUNTY, MINNESOTA Heiling, Carrie Came D., Department Geological Sciences, Sciences, University of Minnesota Minnesota Duluth, Duluth, Department of Geological 1114 Kirby Drive, Drive, Duluth, Duluth, MN, 55812; cheiling@d.umn.edu cheiling@d.umn.edu The Gafvert Lake area, located within the Upper Ely member of the Ely Greenstone Greenstone of forms part of of aa the Vermilion District in Northeastern Minnesota (Figure 1) (Card, 1990), forms Morton (personal (personal communication) communication)has has large, Archean, felsic volcanic complex. Morton late stage caldera interpreted the complex to be a composite volcano that underwent late collapse. This Thisstudy studyhas has focused focused on on aa two two square square mile area in the central part of the complex. Here Herethe the complex, complex,from from the the oldest oldest to to the the youngest rocks, is composed of a) coarse, heterolithic breccias (interpreted (interpreted to represent meso-and mega breccias) breccias) (Morton, (Morton, personal communication), b) more than 3000 feet of massive to bedded pumice-rich communication), and beds beds of of chert chert and and massive to lapilli tuff, c) dacitic lavas and domes, and d) lenses and pyrite (Figure 2). The The breccias breccias and and lapilli lapilli tuffs tuffs have have been been intruded intruded by by aa semi-massive pynte swarm of feldspar porphyry dacite dikes that represent feeders to the domes domes and/or andlor flows. flows. Peperites are rocks formed by the in situ disintegration of magma intruding and mixing unconsolidated sediment sediment or or ash ash (Skilling (Skillingetetal., al., 2002). 2002). At Gafvert Lake the with wet unconsolidated dikes intruded and peperites formed near the top of the complex where dacite porphyry dikes mixed with wet, unconsolidated pumice-rich pumice-rich lapilli lapilli tuff. tuff. This mixing led to quenching quenching of the the dacitic magma magma and disruption disruption and and vesiculation vesiculation of the lapilli and fragmentation of tuffs. The Thepeperites peperites occur occur within within 100 100 feet feet of dike contacts though they form much more extensive areas where where several several dikes dikes occur occur close close together. together. Angular and finger-like blocks of dike material occur within the peperite, peperite, locally locally these these are are connected connectedto toaanearby nearbydike. dike. Macrotextures in outcrop and microtextures in thin section helped identify and classify classify the following fragment types and internal structures within the peperites: a) blocky juvenile juvenile fragments fragments with chilled chilled rims rims and and occasional occasional jig-saw fit fit textures, textures, b) b) platy platyto to ragged ragged juvenile juvenile fragments fragments with curviplanar curviplanar surfaces surfaces and and broken gas gas bubbles, bubbles, c) c) ameboid ameboidto to juvenile fragments, fragments, d) abundant pumice which exhibits variable vesicularity. vesicularity. globular juvenile Most of this pumice is juvenile to the lapilli lapilli tuffs tuffs but a small small percentage percentage contains contains feldspar feldspar to those those found found in in the the dikes possibly indicating local, rapid vesiculation crystals identical to of dike material. Close Close to to dike dike margins margins feldspar feldspar crystals are broken and internally with fractures fracturesfilled filledby bylapilli lapillituff. tuft Pumice, fractured with Pumice, close close to to dike dike contacts, may be broken or disaggregated into into several several small small jigsaw-fit jigsaw-fitpieces. pieces. Locally the ash matrix to radiating away from from dike margins. the lapilli tuffs is amygdaloidal with amygdules radiating References References K.D., 1990, A A review review of of the the Superior Superior Province Province of of the the Canadian Canadian Shield, a product of Card, K.D., Archean accretion: Precambrian Precambrian Research, Research,v.v. 48, 48,pp. pp. 99-156. 99-156. Ely, Minnesota, 2003, Mapquest, www.mapquest.com. Mapquest, www.mapquest.com. Morton, R.L., communication, University of Minnesota-Duluth. R.L.,2003, personal communication, White, J., J., McPhie, McPhie, J., J., 2002, 2002, Peperite: Peperite: a review of magma-sediment mingling, Journal Skilling, I., White, of Volcanology Volcanology and Geothermal GeothermalResearch, Research,xc v. 114, 114,pp pp 1-17. 1-17. 27 �___ \ 01 crane Lake Agnes Lake St. Lóujc Buyck--. 0% - !L h__c1tQ/iF 6urrit'i'1 : - For. . it Trout Lake I RObIF1SOr'' Forestcenter0 lear island st,L120K -—22 .••' -- ,Babbitt — — ;Emlrarrass Erit :-°_V S Virgirii&.J_c.Auroja 11001 9 1 L4SAbCIIA — . r Murphy Cit HoYt Lakes IToirñi . .0Yihyte Fifllaç_fl — (Mapqucst, 2003). Location of Gafvert Lake complex 1: Location Figure Figure 1: complex (Mapquest, 2003). Exp I a n at io fl Explanation - -? ' Tuft II - • Peperite samples samples Fault Zone '" ''Contacts Contacts / \" /\/ Railroad ' R a i l r o a dgrade grade Mud Road 4, "Mud Creek Road / -Â¥ 7 ) - p '*__ -ç. \ .'f -,Qg __DE" I 'S-- ,' , "- " N UI Tuff Qac / /A/ . Diab " Zrtc c'_ Bx N ,. - - - -. I Mbas -- Metabasalt Metabasalt Q Qfp-QtzfeldPorphyry - Qtzfeld Porphyry Cht - Black Chert Cht Diab -- Diabase Diabase Dikes Dikes Dior -- Diorite Dior Diorite Dac -- Dacite Dac Dacite Dikes Dikes luff Tuft Tuff - Lapilli Lapilli Tuff Bx -- Breccia Breccia Bx -- Siltstone &&Iron IronFm Fm 81st SIst - - I 01p , Sisi It METERS Wa to O 0 of Gafvert Lake Lake volcanic complex. Figure 2: Generalized map of a portion of 28 28 �CHEMISTRY CHEMISTRY OF OF ALTERATION ALTERATION MINERAL MINERAL PHASES PHASES AT THE FIVE MILE MILE LAKE LAKE VOLCANIC-HOSTED MASSIVE SULFIDE PROSPECT, NE MINNESOTA VOLCANIC-HOSTED MINNESOTA Hocker, J., Odette, Odette, J.J. D., D.,and andNewkirk, Newkirk, T. T.T., T.,Department DepartmentofofGeology, Geology, Hocker, S. M., Hudak, Hudak, G. J., University of Wisconsin Blvd., Oshkosh, WI 54901, hudak@uwosh.edu Wisconsin Oshkosh, Oshkosh, 800 800 Algoma Blvd., Alteration mineral assemblage assemblage mapping mapping at at the Five Mile Lake Alteration mineral Lake Prospect Prospect in in the the Vermilion Vermilion District of northeastern Minnesota has identified two distinct types of alteration alteration zones zones within within 2.7 2.7 billion year-old billion year-old volcanic volcanic and and volcaniclastic volcaniclastic rocks rocks associated associated with withvolcanic-hosted volcanic-hosted massive massive sulfide (VHMS) (VHMS) mineralization mineralization (Hudak (Hudak etet al., al., in press; al., 2001a, 2001a, 2001b; 2001b; Peterson, Peterson, press; Odette et al., alterationzones zones are are composed composed of of various 2001). 2001). Regional Regional semi-conformable semi-conformable alteration various proportions of ± amphibole ± chlorite + epidote quartz epidote 5 amphibole 5 chlorite ± quartz + i plagioclase plagioclase feldspar. feldspar. These regional, semiconformable alteration alteration zones zones are locally cross-cut by several relatively narrow, northeastsemiconformable northeasttrending disconformable alteration alteration zones composed composed of fine-grained fine-grained chlorite and/or sericite that are closely associated with synvolcanic synvolcanic fault fault zones. zones. Electron microprobe microprobe analyses analyses of of the various Electron various alteration alteration mineral mineral phases phases (epidote (epidote group group minerals, chlorite, chlorite, amphibole, amphibole, white white mica, mica, and and feldspar) feldspar) have have been been conducted conducted in an effort minerals, effort to to associated with the better understand hydrothermal processes processes associated the development development of the the semiconformableand and disconformable disconformablealteration alterationzones zonesatat the the Five Five Mile Lake prospect. semiconformable prospect. These These that: a) epidote group minerals range in composition from zoisite/clinozoisite zoisite/clinozoisite to analyses indicate that: pistacite; pistacite; b) chlorite chlorite is dominantly dominantly ripidolite; ripidolite; c) amphibole amphibole is is primarily primarily actinolite actinolite and and ferroferroactinolite, with magnesio-homblende and ferro-hornblende ferro-hornblendealso alsopresent; present;d)d) sericite sericite is fineactinolite, with magnesia-hornblende and finegrained muscovite; and e) e) plagioclase plagioclase feldspar feldsparisisdominantly dominantlyalbite. albite. Alteration mineral chemistry chemistry at at the the Five Mile Lake Alteration mineral Lake Prospect Prospect is remarkably remarkably similar similar to to that that from the the Noranda Noranda VHMS VHMS mining mining camp camp of of Canada, Canada, as as well well as as other other VHMS VHMS mining mining camps around the world. world. This alteration mineral chemistry chemistry suggests the presence of of aa complex, complex,long-lived long-lived hydrothermal system that that evolved (hundreds of of meters) hydrothermal system evolved from seafloor-proximal seafloor-proximal (hundreds meters) to to deeper deeper subseafloor environments (-1-3 (—4-3kilometers) kilometers)asasthe the volcanic volcanic rocks rocks were buried subseafloor environments buried by by apparently apparently mafic to intermediate volcanism and associated sedimentation. This rapid, dominantly effusive mafic suggests that in addition to the Five Mile Mile Lake Lake Prospect, Prospect, the the uppermost uppermost several several hundred hundred meters meters of the Lower Member of the Ely Greenstone also has excellent exploration potential for VHMS mineral deposits. deposits. References References Galley, A,, A., Bailes, A., Hannington, M., Hollc, G., Katsube, 1, Paquette, S., Bailes, A., Hannington, M., Holk, G., Katsube, J., Paquette, F., F., Paradis, Paradis, S., Galley, Santaguida, F., Database for Santaguida, F., and Taylor, Taylor, B., 2002, Database for CAMIRO CAMIRO Project Project 94E07: 94E07: Interrelationships between subvolcanic subvolcanic intrusions, intrusions, large-scale large-scale alteration alteration zones, zones, and VMS Interrelationships between VMS of Canada Open File Report 4431 (CD-ROM). deposits: Geological Survey of Hudak, G. J., Heine, J., J., Newkirk, Newkirk, T., T., Odette, Odette, J., J., and Hauck, S., S., in press. press. Comparative Comparativegeology, geology, stratigraphy, and lithogeochemistry lithogeochemistryof of the the Five Five Mile Mile Lake, Lake, Quartz Hill, and Skeleton stratigraphy, and Skeleton Lake Lake Vermilion District, District,NE NE Minnesota: Minnesota: A report to the Minerals Coordinating VMS occurrences, Vermilion Coordinating Committee, DNR Minerals Division, State State of Minnesota. 29 �Kranidiotis, Kranidiotis,P. P.and andMacLean, MacLean,W. W.H., H.,1987, 1987,Systematics Systematicsofofchlorite chloritealteration alterationatatthe thePhelps PhelpsDodge Dodge Massive MassiveSulfide SulfideDeposit, Deposit,Matagami, Matagami,Quebec: Quebec:Economic EconomicGeology, Geology,v.v.82, 82,p.p.1898-1911. 1898-1911. Odette, Odette, J. D., D., Hudak, Hudak, G. G. J., J., Suszek, Suszek, T., T.,and andHauck, Hauck, S. S.A., A,,2001a, 2001a,Preliminary Preliminary evaluation evaluation of hydrothermal alteration alteration mineral mineral assemblages assemblages and and their their relationship relationship to to VMS-style VMS-style hydrothermal mineralization mineralization in the Five Mile Mile Lake Lake area areaof ofthe theArchean ArcheanVermilion VermilionGreenstone GreenstoneBelt, Belt, NE NE 47th Minnesota: Institute Institute on on Lake Lake Superior SuperiorGeology, Geology, 47thAnnual Annual Meeting, Meeting, Proceedings ProceedingsVolume Volume Minnesota: 47, Part Part 1-Program 1-Programand and Abstracts, Abstracts,p. p. 75-76. 75-76. 47, Odette, Odette, J. D., D., Hudak, Hudak, G. G. J., J., Suszek, Suszek,T., T.,and andHauck, Hauck,S.S.A., A.,2001b, 2001b,Preliminary Preliminaryevaluation evaluation of of hydrothermal alteration alteration mineral mineral assemblages assemblages and and their their relationship relationship to to VMS-style VMS-style hydrothermal Archean Vennilion Vermilion Greenstone Greenstone Belt, Belt, NE NE mineralization in the Five Mile Lake Lake area area of of the the Archean mineralization Minnesota: Minnesota: Geological Society Society of of America America Abstracts Abstracts and and Programs Programs Volume Volume 33, 33, No. No. 6, 6,p. p. AA420. 420. Peterson, Peterson, D. M., M., 2001, 2001, Development Development of of Archean Archean lode-gold lode-gold and and massive massive sulfide sulfidedeposit deposit exploration exploration models using geographic geographic information information system system applications: applications: targeting targeting mineral mineral exploration exploration in northeastern northeastern Minnesota from analysis analysis of of analog analogCanadian Canadianmining miningcamps: camps: unpublished unpublished Ph. Ph. D. D. dissertation, dissertation,University Universityof of Minnesota, Minnesota,Duluth, Duluth, Minnesota, Minnesota,503 503p.p. 18-cm 16 14 H —I P 1111111 0.0 O4'.B 0.2 0.8 0.0 PSEUOOThUNPNGOT CQRVNOOflILIT! -J "ffiff. 12 10 — C— •FM.CnICaC!*I AU. o'A•*TPff C ow—5—o'.w TALC .C1tGTh 00 \ TOk..S. 6- 'RI 10 20 0.D 30 1.0 tM. — 3.0 2.0 4.0 5.0 6.0 A WTIG ON EPIDOTE NA C0RUIUM 1.0 0.9 0,8 0.7 0.6 ft. :? 0.5 0.4 0.3 0.2 0.1 ThCCLASE 0.0 K&A 6.0 analyses for for epidote-group epidote-group minerals minerals (A), (A),chlorites chlorites Figure 1.1. Summary of electron microprobe analyses Figure (B), amphiboles (C), and white white micas (D) from the Five Mile Mile Lake Lake Prospect Prospect and andselected selectedVHMS VHMS mines. Compositional fields for Noranda minerals determined from Galley et al. (2002). (2002). 30 �GEOCHEMISTRYAND AND GEODYNAMIC GEODYNAMICIMPLICATIONS IMPLICATIONS THE 1537 1537 MA GEOCHEMISTRY OFOFTHE MA POINT ANOROGENIC REDSTONE POINT ANOROGENICGRANITE, GRANITE,ONTARIO, ONTARIO,CANADA CANADA Hollings, Fralick, P.P.and andKissin, Kissin,S.S.(Department (DepartmentofofGeology, Geology,Lakehead LakeheadUniversity, University, 955 955 Hollings, P., P., Fralick, Rd., Thunder Thunder Bay, Bay, Ontario, P7B 5E1, 5E1, Canada; Canada; Peter.HollinRs@lakeheadu.ca) Oliver Rd., Peter.Hollings@lakeheadu.ca) The The Redstone Redstone Point granite granite is is a Mesoproterozoic felsic igneous Mesoproterozoic felsic igneous complex (1537+101-2 Ma; Davis Davis and complex (1537+10/-2 Ma; Sutcliffe, 1984) located in the the northern northern portion of the Sibley Sibley Basin on the west shore of Lake 1). It It is is shore Lake Nipigon Nipigon (Fig. (Fig. 1). unconformably overlain overlain by by arenites of unconfonnably the Pass Lake Sibley Lake Formation Formation of of the the Sibley Group. These sediments are in turn Group. These sediments are turn intruded and overlain overlain by by an an extensively extensively developed sills developed Sequence sequence of diabase diabase sills related to an early related to early stage stage of of the the MidMidevent. The Continent Rifting event. The entire entire sequence has been gently folded into aa shallowly, easterly plunging plunging succession succession of open synclines and anticlines, open synclines and anticlines, with with Figure 1. Map Map showing Figure showing the the location location of the the Redstone Redstone Point Point dips not usually usually exceeding exceeding 15°. 15". Outcrop Outcrop granite in relation relation to Proterozoic granite Proterozoic anorogenic anorogenic granite granite complexes of ofNorth North America. America. Modified after after Anderson complexes Anderson (1983) (1983) density density of of igneous igneous units units is very very good good along the shoreline of Lake Nipigon, in contrast contrast to to sedimentary sedimentary sequences, sequences,which which only only provide provide small, scattered scattered outcrops. outcrops. The igneous rocks of Redstone Point have been briefly described by Davis and and Sutcliffe Sutcliffe (1985), (1985), wherein they emphasised that the rocks are wherein they emphasised that are anorogenic anorogenic granites granites gradational gradational to rhyolites rhyolites and and fragmental rhyolites and and dacites. In fact, fragmental rhyolites fact, presently presently accessible accessible outcrop outcrop indicates indicates that that extrusive extrusive members dominate the the magmatic magmatic rocks rocks of of the area. Porphyritic texture with members dominate with volcanic volcanic features features including vesicles, flow flow structures, agglomeratic units, units, rubbly rubbly flow flow tops including vesicles, structures, agglomeratic tops and and segregation segregation cylinders differentiate extrusive extrusiverocks rocks from from more more limited cylinders differentiate limited exposures exposures of of uniformly uniformly textured textured intrusive rocks. As As contacts between between units units are are generally unexposed unexposed and the base base of of the section is nowhere exposed, thicknesses thicknesses of of units units and and of the nowhere exposed, the entire entire succession succession are areunknown; unknown;however, however, continuous outcrop outcrop in in cliff-forming cliff-formingunits unitsindicates indicatesthat that aa minimum of lOOm continuous minimum thickness thickness of 100m of volcanic rock is present in in the the area. area. The igneous rocks are distinctively brick red, red, suggesting suggesting the the dominace dominace of of ferric ferric iron iron in the The igneous rocks distinctively brick the various mineral hosts but especially in trace amounts in feldspars. feldspars. The The intrusive intrusivemember memberdisplays displays equigranular phaneritictexture texturewith withmost mostmineral mineralgrains grains11 to to 5 mm in diameter. equigranular phaneritic diameter. The volcanic rocks are true porphyries with phaneritic phenocrysts of alkali feldspar, quartz and hornblende hornblende in an aphanitic matrix of of the the same minerals. minerals. Quartz Quartz phenocrysts phenocrystsare areeuhedral euhedraland and 11 to to 33 mm in aphanitic matrix diameter associated with alkali feldspar phenocrysts occasionally exhibiting exhibiting synneusis synneusis twinning twinning of microcline. Homblende and magnetite are less as well as as albite-pericline albite-pencline twins indicative indicative of microcline. Hornblende less 31 �_________________________________________________ abundant and finer grained than in the intrusive rocks. Near flow tops the porphyries grade into textured aphanitic uniformly textured aphanitic rhyolites with sparse phenocrysts. The samples samples from the the Redstone Redstone Point Point intrusive intrusive complex complex are all all characterised characterised by high high Si02 SiO, contents (73-83 (73-83 wt%) wt%) and and elevated elevatedK,0 1(20and andNN;0 contents q 0 abundances abundances (2-7 (2-7 wt% wt% and and 0.2-3.5 0.2-3.5 wt% wt% respectively). respectively). They They are are typically typically LREE relatively LREE enriched enriched with with relatively unfractionated HREE HREE (La/Sm, (La/Smn = = unfractionated 2.8-5.1; Gd/Yb,, GdIYb == 1.1-1.6; 1.1-1.6; Fig. 2) characterised by elevated elevated and are characterised and Nb Nb contents. contents. Samples Samples Zr, Y and from the Redstone Redstone Point Point igneous igneous 02.13 complex fulfil fulfil the detailed complex detailed trace trace 02-H element criteria of of Whelan element criteria Whelan et al. al. y (1987) (1987) for anorogenic anorogenic granites. granites. o . I 1 . . . . Rb Tli Rb Ra BaTh Pr S SrNd U HISmEu, fl GdThOy Er YbLu U NbLa Nb La Cc Cc Fr t hid Zr Hf Sin Eu ll Gd 7% D! Tile Y ifc Er Yb Lu Al A1 U V V Sc SP Figure Representative primitive mantle normalised diagram diagram for Figure 2. Representative for samples from from the the Redstone RedstonePoint Pointigneous igneous complex Similarities Similarities between between Proterozoic Proterozoic basin sequences basin sequences (e.g., (e.g., Athabaska, Athabaska, Thelon, Hornby Bay and Sibley basin fill sequences) imply that basin genesis and developmental developmental controls were similar. The setting, setting, architecture, depositional systems and deformational deforrnational histories of all four basins strongly infer that they are intracratonic, forming as a result of heating cratonic lithosphere. The heating event is represented represented in in northern northern Canada by by numerous 1790 to 1730 Ma anorogenic, syenogranite batholiths and and comagmatic ash-flow tuffs occurring syenogranite batholiths occurring west of of Hudson Hudson Bay. In the western Great Lakes region a heating event produced the 1537 Ma Redstone western Great the 1537 Ma Redstone Point Point assemblage and and other 1500 records a assemblage 1500 Ma Ma anorogenic anorogenic batholiths. The southern southern mid-continent mid-continent records lithospheric heating event with anorogenic granite production production from from approximately approximately 1480 1480to to 1320 1320 Ma (Fig. 1). 1). These These events events outline outline aa progressive progressive southward southward displacement displacement of lithospheric heating from a maximum age of approximately 1750 ma in northern Canada to aa minimum minimum age age of of 1310 1310 Ma in the As heat transfer Ma the southwestern southwestern United United States. States. As transfer from from the the asthenosphere asthenosphere is the the only only heating, drift of North America over hotter than mechanism for producing extensive lithospheric heating, regional ages of of heating, drift rates of approximately 1.1 average asthenosphere is implied. Using regional 1.1 cm/year are necessary, and agree in magnitude with present to 1.4 cmlyear present rates. rates. REFERENCES Anderson, J., J., 1983. Proterozoic anorogenic anorogenic granite granite plutonism plutonism of of North North America. In: Medaris et Anderson, al., (Eds), Proterozoic Proterozoic geology. Geological Society Society of America America Memoir Memoir 161, 161,133-154. 133-154. Davis, D., D., and Davis, and Sutcliffe, Sutcliffe, R., R., 1985. 1985. U-Pb ages ages from from the the Nipigon Nipigon Plate Plate and and Northern Northern Lake Lake Superior. Bulletin, 96, Superior. Geological Geological Society of America Bulletin, 96, 1572-1579. 1572-1579. Whelan, J., Currie, Currie, K., K., and andChappeli, Chappell, B., B.,1987. 1987.A-type A-typegranites: granites:geochemical geochemicalcharacteristics, characteristics, discrimination Contributions to 407-419. discrimination and petrogenesis. Contributions to Mineralogy Mineralogy and and Petrology, Petrology, 95, 95,407-419. 32 �PALEOPROTEROZOIC (1900-1600 Ma) TECTONIC HISTORY OF LATE PALEOPROTEROZOIC OF THE THE NORTHERN NORTHERN FOR CRUSTAL STABILIZATION STABIUZATION MID-CONTINENT, U.S.A: IMPLICATIONS FOR HOLM, D.K., Dept. of Geology, Geology, Kent Kent State StateUniversity, University, Kent, Kent, OH OH44242; 44242;VAN VANSCHMIUS, SCHMUS, MacNEILL, L.C., Dept. of Geology, University of of Kansas, Lawrence, KS 66045; W.R., and MacNELL, Minnesota Geological BOERBOOM, T.J., Minnesota Geological Survey, 2642 University Avenue, St. St. Paul, Paul, MN MN SCHWEiTZER, D., Dept. of Geology, 55114; SCHWEITZER, Geology, Kent State State University, University, Kent, OH OH 44242; 44242; SCHNEIDER, D.A., Dept. of Geological Geological Sciences, University, Athens, SCHNEJDER, Sciences, Ohio University, Athens, OH OH 45701 45701 We propose that the the late late Paleoproterozoic Paleoproterozoic igneous and and deformational deformational history history preserved preserved in in the the southern Lake Lake Superior region is the southern Superior region the result resultofofnorthwest-directed northwest-directed convergence convergence during during and and New U-Pb zircon ages indicate indicate that that late to post-Penokean following geon 18 Penokean accretion. New magmatism began ca. 1800 1800 Ma and and generally generally migrated migrated southeastward southeastward across across the thenewly newlyaccreted accreted terrane. Magmatic Magmaticpulses pulses atatca. ca.1800, 1800,1775, 1775,and and1750 1750Ma Mamay maycorrelate correlatewith withnorthwest-directed northwest-directed growth of of the North North American mid-continent. We suggest subduction associated with southward growth suggest that geon 17 Yavapai-age slab rollback caused continental arc magmatism to step step southeastward southeastward between 1800 and 1750 Ma (Fig. (Fig. 1A). As As the the slab slab steepened, steepened, the reduced compressional compressional stresses thermal input allowed for collapse of the overthickened and increased thermal overthickened portions of of the thePenokean Penokean In northern collapse involved involved the the formation formation of of gneiss crust. cmst. In northern Wisconsin, Wisconsin, collapse gneiss domes domes and and their their exhumation panels brought brought up up from depth via exhumation within discrete fault-bounded panels via tectonic tectonic extrusion extmsion (Schneider (Schneider et al., al., ILSG, ILSG, 2003). 2003). Collapse of the thePenokean Penokeanorogen orogen—- and possibly temporary temporary cessation of crustal stabilization and deposition of Baraboo Interval cessation of slab slabsubduction subduction—- resulted in cmstal quartzites between between 1750 and 1650 in aa long-lived quartzites 1650 Ma. Ma. However, However, in long-lived orogen orogen model, model,renewed renewed tectonism to the the south south resulted resulted in in the the eventual accretionofof aa Mazatzal Mazatzal arc arc (Fig. tectonism to eventual accretion (Fig. IB) 1B) with with widespread deformation deformationand andmild mild reheating reheatingof of Penokean Penokean crust crust to to the north. The age of of this this widespread Ar/Ar step-heating studies on basement rock deformation is inferred from conventional ArIAr rock beneath beneath undeformed Baraboo BarabooInterval Intervalquartzites. quartzites. The 1900 to 1600 deformed and undeformed 1600 Ma tectonic history of the north-central United States, States, not not surprisingly, records the the southward growth and the north-central United surprisingly, records and tectonic tectonic development of the southern Laurentian margin. New and published 40Arf39Ar mineral agesdelineate delineatethe thenorthern northern and and western western extent of geon New 4 0 ~ r / 3 9mineral ~r ages geon crustal deformation. deformation. Interestingly, 16 crustal Interestingly, only lower-grade crust intruded by the the shallower-level shallower-level ca. 1750 Ma Ma plutons (and associated associated rhyolites) rhyolites) were weredeformed deformedsignificantly significantlyduring duringgeon geon 16. 16. Deeper level collapsed crust and crust pervasively invaded by the older magmatic pulses are level collapsed cmst and cmst pervasively invaded by magmatic pulses arelargely largely unaffected by Mazatzal deformation deformation and and reheating. reheating. We suggest suggest that post-orogenic post-orogenic intrusions intrusions and and crustal thinning was an important step in strengthening strengthening and stabilizing the the crust crust in inthe thesouthern southern Lake Superior Superior region. region. O'Boyle, Hamilton, and Jercinovic, 2003, 2003, Paleoproterozoic Paleoproterozoic development of of a Schneider, Holm, O'Boyle, gneiss dome corridor corridor in in the the southern southern Lake LakeSuperior Superiorregion, region,USA: USA: Institute on Lake Superior Geology Abstracts (this (this volume). volume). 33 �avapai subduction (ca. 1750 Ma) w -southward propagation of magmatism from 1775 Ma ECMB to 1750 Ma granites and rhyolites in Wl Mazatzal orogeny (1650-1630 Ma) deformation of during ost-Penokean" quartzites 8azatzal accretion " age Figure 11:: A) A) Subduction Subduction rollback model model proposed to explain magmatic magmatic age progression the Penokean convergence, preprogressionacross across the Penokean orogen, orogen, ca.Yavapai convergence, accretion. Mazatzal ECMB East-centralMinnesota Minnesota batholith. batholith. B) Mazatzal ECMB ==East-central B) Mazatzal Mazatzal accretion model (modified aL,2000) to to explain explain deformation deformation accretion model (modifiedafter after Romano Romano et al.,2000) of Baraboo Interval Interval quartzites quartzites and and southward southward growth growth of of Laurentia. Laurentia. 34 �GABBRO/GRANOPHYRE OF THE THE CROCODILE CROCODILELAKE LAKEINTRUSION: INTRUSION: A GABBROIGRANOPHYRERELATIONS OF POSSIBLE VENT FOR THE HOVLAND LAVAS? (e.jerde@morehead-st.edu), Department of Physical Sciences, Morehead State JERDE, Eric A. (e.jerde@morehead-st.edu). State University, Morehead, KY 40351 40351 One of the notable notable characteristics characteristics of the Midcontinent Midcontinent Rift Rift is the presence of large large amounts amounts of of felsic felsicmaterial. material. Indeed, the nature and origins of this abundant abundant silicious silicious material material has been the source source of numerous numerous studies studies(e.g., (e.g., Nelson, 1991; Green and and Fitz, Fitz, 1993; 1993; Vervoort Vervoort and andGreen, Green, 1997; 1997;Kennedy Kennedyeteta!., 2000;Sandland Sandlandetetal., al.,2001). 2001). To the Nelson. al., 2000, Series is a pronounced ridge south of the Early Gabbro Series ridge composed composed of of this this felsic felsic material, material, properly properlytermed termedaa Series layers are granophyre. The Early Gabbro Series are inc!ined inclined to the south, south, thus are below the granophyre granophyre stratigraphically. This This felsic felsic rock rock was was noted and and described described by Nathan (1969) (1969) as a very late-stage material, and has presumed to to have have formed formed significantly significantlyafter afterthe the gabbros gabbrosin inthe theregion. region. However, several severa! generally been presumed observations !ater than than the the granophyres. granophyres. These observationsindicate indicate that the gabbro was emplaced later These include include gradational gradational contacts, contacts, with some chilling of the gabbro. Another described as Another observation observation is is the the abundance abundance of material described as "intermediate "intermediate rock" by various investigators aL, 1959). This This materia! material is always found between the investigators in the past (e.g., Grout et al., gabbro gabbro and granophyre, granophyre,and is is presumably presumably the the result of of assimilation assimilationof of granophyre granophyreby by an an intruding, intruding,hot hotgabbro. gabbro. Investigations Investigations into into possible origins of the the granophyres granophyres(Sandland (Sandlandet et al., al., 2001; 2001; Karl Karl Wirth, Wirth,pers. pers.comm.) comm.) included included radiometric radiometric age age determinations, determinations, and and revealed revealed that that the the granophyres granophyresadjacent adjacentto to the theEarly EarlyCiabbro Gabbro Series Seriesare, are, like the gabbros, Ga),and andessentially essentiallycontemporaneous. contemporaneous. Because gabbros, among among the earliest earliest rocks of the the rift rift (—1107 (-1 107 Ga), Because silicious silicious materia! material generally is a late-stage product of magma evolution, evolution, the surprising antiquity of the the granophyres granophyres adds to questions thheir origin. questionssurrounding their The early age for the grar granophyres however, suggest suggest an an origin origin fifor the layered layered nature nature of of the the Early Early Gabbro Gabbro 3r the 1013hyres does, however, Series stratigraphically. Senes located below them stri :ti;graphically. Due granophyric material would have created created aa Due to to their their low low density, density, the granophyric . that .. . reiaraea . ~ ~ > - > . . . - -. ~ ~.~material . . coming ~~. up "from below. barrier retarded the buoyant. gamroic gabbroic up rrom below. These harrier mat me rise of buuyani These rising rising !iquids liquids would would have have beeni forced to spread !aterally, resulting in the the ;aP apparent layering that isis obserl observed, andproviding providing aa cap, cap, blocking blocking any any laterally, resultingin bee] red, and parent layeringthat further ler rise of gabbroic gabbroic material. material. furti east of of the the layered layered Earl: EarlyY (Gabbro Series of of Nathan Nathan (19 (1969) is another another occurrence occurrence of Immediately to the east 69) is Jabbro Series ~ . ~ ~ > & - L . c. . . , (i.e., . —1107 < granophyre, also determined Ga;Karl Kar!Wirth, Wirth,pers. pers. comm.). comm.). This 01 an early early origin origin n.e., -1 I U I Ga; This granopnyre, aeiermmea to be ne among among those mose of termed the the Crocodile Crocodile Lake Lake Intrusion by by Miller Miller et et al. al. (2001). (2001), and and the rocks are interpreted to be rock group has been termed geophysica! evidence, evidence, and and aa few few sample sample examinations examinations(Babcock, (Babcock, 1959). 1959). Work gabbroic based on geophysical Work done done between between 1913 edges of of this this intrusion, intrusion, and and indicates indicates that that they they are are basalt basalt lavas lavas and gabbroic 1913 and 1948 1948 included the very edges intrusions, along with "red "red rock" (Grout et al., 1959) 1959) that is now known to refer to granophyre. (1969), there is a body of gabbroic material stratigraphically stratigraphica!ly below below the Like the series mapped by Nathan (1969). granophyre. granophyre. During the past year, a reconnaissance reconnaissance was was made made into into the the Crocodile Crocodile Lake Lake Intrusion Intrusion to to examine examine some of the rock rock re!ations (Fig. 1). Traverses by forest forest blowdown, blowdown,but butthe theoutcrops outcropsare arenumerous. numerous. Several relations Traverses were greatly hampered by gabbro units are present, as well we!l as as aa band band of of "intermediate "intermediate material" materiaP' at at the the very very top top of of the the gabbro, below the granophyre. Within Withinthe thegranophyre granophyreitself, itself, several several bodies bodies of of gabbro gabbrowere were found found to to have have actually actuallyintruded intrudedthe the granophyre. In Inthe thecoarse-grained coarse-grainedinteriors interiors of of these these bodies, bodies,the thegabbro gabbroisisindistinguishable indistinguishablefrom fromthe thegabbros gabbros (i.e., below the granophyre granophyre stratigraphically). stratigraphically). observed further further north (i.e., south of of the the granophyre granophyre are are prominent prominent knobs knobs and and ridges ridges that that are are composed composed of basalt. These Immediately to the south These part of the Hovland Lavas, which are are reversely reversely polarized, polarized, and were extruded extruded during the earliest earliest are mapped as pan discontinuous bodies (and other stringers stringers and local dikes) period of the the rifting. rifting. It is perhaps possible that the discontinuous dikes) within within feeder conduits for the eruptive eruptive basalts basa!ts immediately immediately to to the the south south (shown (shown schematically schematically the granophyre represent the feeder are basaltic basaltic stringers stringers and and small small dikes. dikes. Surrounding in Fig. 2). In In several several other other places within the granophyre, there are Surrounding !arger gabbroic bodies bodiesare areobvious obviousreaction reactionzones zoneswhere wheregranophyre granophyrehas hasbeen beenassimilated assimilatedinto intothe thegabbro. gabbro. In In the larger to the the south, south,numerous numerous inclusions inclusions are arepresent present that that are are pinkish pinkish in in color, color. along with with one of the flows immediately to felsic stringers stringers and irregular masses of felsic felsic material. to assess assess the the relation relation between between the the gabbros gabbros within within the the granophyre granophyre and the lavas to the Further work is planned to south. IfIf this thisisisindeed indeed aa feeder feeder system, system, itit might provide provide insight insight into into the the mechanism mechanism of magma magma emplacement emplacement and and the the eventual "breakthrough" "breakthrough" to the surface, during during the the onset onset of of rifling. rifting. h he earl^ ~ .~~~ . ~ . ~ ~ ~ . ~ . ~. ~~~ ~ ~ ~~ ~~ . . ~ .~ ~ ~ . 35 P-. �ReferencesCited: Cited: References (1959)MS. M.S.thesis, thesis.University UniversityofofWisconsin, Wisconsin.Madison, Madison,4747p.p. Babcock,R.C., R.C..Jr. Jr.(1959) Babcock, Geothermal Research, Journal of Volcanological and Green. J.C. and Fitz. T.J., 1993. Journal of Volcanological and Geothermal Research,54, 54,177-196. 177.196. Green, J.C. and Fitz, T.J., 1993, Bulletin 39, 163p. G.M. 1959 Minnesota Geologica Survey Grout, F.F., Sharp, R.P., and Schwm, G.M. 1959 Minnesota Geologica Survey Bulletin 39, 163p. Grout, F.F., Sharp, R.P., and Schwartz, Union Jerde,BA. E.A.and andKennedy, Kennedy.B.C., B.C.,2000, 2000.American AmericanGeophysical Geophysical Union2000 2000Fall FallMeeting, Meeting,San SanFrancisco. Francisco. Jerde, and Wirth, KR. 2001, ILSG 47, 36-37. Jerde, E.A., Salvato. D.J. Thole. J., and Wirth, K.R. 2001. ILSG 47.36-37. Jerde, BA., Salvato, D.J, Thole, S., 29-30. Kennedy,B.C., B.C.,Wirth, Wirth,K.R., K.R.,and andVervoort, Vervoort,J.D., J.D.,2000, 2000,ILSG ILSG46, 46.29-30. Kennedy, M.J., Chandler, V.W., and Miller,J.D., J.D..Jr., Jr.,Green, Green,J.C., J.C.,Severson, Severson, M.J., Chandler, V.W., andPeterson, Peterson,D.M., D.M.,2001, 2001,Minnesota MinnesotaGeological Geological Miller, M119. SurveyMiscellaneous MiscellaneousMap MapSeries Series M-119. Survey 198p. University Nathan.HI)., H.D..1969, 1969. Ph.D.dissertation, dissertation. UniversityofofMinnesota, Minnesota.Minneapolis, Minneapolis, 198p. Nathan, Ph.D. University of Minnesota. Duluth. Nelson. N. 1991. M S . Thesis. University of Minnesota. Duluth. Nelson, N. 1991, MS. Thesis, K.S. 2001, LSG 47, 85-86. Gehrels, G.E., Kennedy, B.C., Sandland,TO., T.O.,Wirth, Wirth,KR., K.R.,Vervoort, Vervoort,J.D., J.D., Gehiels, G.E., Kennedy, B.C.,and andHarpp, H q p , K.S. 2001, ILSG 47,8546. Sandland, Vervoort,3D. J.D.and andGreen, Green.J.C., J.C.,1997, 1997.Canadian CanadianJournal JournalofofEarth EarthSciences, Sciences,34,521-535. 34.521-535. Vervoort, Lake, showing location of Fig.i.1. Reconnaissancegeologic geologicmap mapofofthe theregion regionjust justsouth southofofCrocodile Crocodile Lake, showing location of Fig. Reconnaissance Lake Intrusion gabbros sabhrobodies bodiesininthe thegranophyre granophyre thatforms formsthe thecap capabove abovethe theCrocodile Crocodile Lake Intrusion gabbros gabbro that andintermediate intermediaterocks. rocks. and '4 possible feeder for the Hoviand Lavas. Schematic N-Scross crosssection sectionofofFig. Fig.1 1showing showingthe the possible feeder for the Hovland Lavas. Fig.2.2.Schematic Fig. N-S 36 �East West Resource ResourceCorporation Corporation(EWR) (EWR) has has undertaken undertakenaa detailed detailedexploration explorationprogram program in the vicinity of Norton Lake, including an extensive drilling program. Detailed Detailed examination examination of of being paid paid to to the the mineralized mineralized 'main' zone drill core has been undertaken with special attention being zone to to determine the exact nature of the mineralization. Preliminary Preliminaryresults results indicate indicate the the deposit denosit consists consists of massive pyrrhotite with pentlandite, magnetite, chalcopyrite and pyrite. pyrite. The The platinum group element's element's (PGE's) (PGE's)are arefound foundforming formingdiscrete discreteplatinum platinumgroup groupminerals mineralsand andare arealso alsobelieved believedto to form a solid solution with the sulphides. sulphides. Results ~ e s u i tshow sshowthat that in inaddition additionto toprimary primarymineralization mineralization a secondary, hydrothermal, enrichment enrichment of PGE's PGE's has has taken takenplace. place. Mineral Mineral Pyrrhotite Pyrrhotite Pentlandite Pentlandite Pyrite Chalcopyrite Chalcopyrite Manganoan fllmenite Illmenite 1 Magnetite Michenerite Hessite Hessite Formula Fe,.$ Fei.S (Fe,Ni)9S8 (Fe,NihSs FeS2 CuFeS2 CuFeSi (Fe,Mn)Ti03 (Fe,Mn)Ti03 Fe304 1 Fe304 1 PdBiTe Ag2Te 1 Ag2Te Elements Minor Elements Ni Co Co Co Co Ni Notes Notes Main mineral mineral Secondary Secondary Trace Trace, also also veins More common common than magnetite, magnetite, easily mistaken for magnetite in polished section section 1 Sb, Pt Table 1: Table 1:Summary Summaryof of the mineralogy mineralogy of Norton Lake Lake deposit. deposit. Corfu F. and Stott G.M. 1996. 1996. Hf isotopic composition and age constraints constraints on on the evolution evolution of of the Archean Central Uchi Subprovince, Ontario, Canada. Precambrian Research, v. 78, p 53-63 East West Resources Resources Corporation, Corporation, 2001. 2001. Annual Annual Report. Report. PGM Ventures, Ventures, 2003. 2003. www.pgm-ventures.com Stott 0. G.M. M.and andCorfu Corfu F. F. 1991, 1991,Uchi Uchi Subprovince, Subprovince, in Geology of Ontario, Ontario, Ontario Geological Survey Survey Special Special Volume Volume 4, 4, Part Part1.1. 38 �STRATIFORM STRATIFORM Pd-Pt-Au Pd-Pt-AUMINERALIZATION MINERALIZATION IN IN THE THE SONJU SONJU LAKE LAKE INTRUSION, INTRUSION, LAKE COUNTY, MINNESOTA JOSLIN, 1114 Kirby JOSLIN,Gregory GregoryD. D.Department Departmentof ofGeological Geological Sciences, Sciences, University of Minnesota-Duluth, 11 14 Kirby Drive, MILLER, James D., Jr., Drive, Duluth, Duluth, MN MN 55812, 55812,email: email:ioslOOl3@d.umn.edu; josl0013@d.urnn.edu;MILLER, Jr., Minnesota Minnesota do NRRI, 5013 Miller Trunk TrunkHwy, Hwy,Duluth, Duluth,MN MN 5581 55811; and ROWELL, ROWELL, Geological Survey, c/o 1; and 6th William, St., Minneapolis, MN 55402. 55402. William, F., Franconia Minerals MineralsCorp., Corp.,12 12S. S. 6 St., The Sonju Lake intrusion intrusion (SLI) is a 1200 1200 m thick, closed-system, well-differentiated, tholeiitic, layered intrusion located within the Mesoproterozoic Midcontinent Rift-related Beaver Bay Complex of northeastern Minnesota (Miller and and Chandler, Chandler, 1997). 1997). In the late 1990's, 1990's. outcrop outcrop sampling by Miller (1999) indicated indicated the presence of meter-scale stratiform Pd-Pt-Au mineralized POE reef) within the oxide gabbro interval (or PGE gabbro unit of the SLI, located about 2/3 of the way up intrusion. In from the basal contact of the intrusion. In June of 2002 Franconia Minerals Corp. conducted exploratory drilling exploratory drilling through through the the Pd-Pt-Au Pd-Pt-An enriched enriched zone. zone. In hand sample, the mineralized interval appears as a homogeneous oxide gabbro, with no enrichment. However, geochemically the location of the visible indication of precious metals enrichment. mineralization is distinct. Three Three drill drill cores, cores, spanning spanning aa strike strike length of approximately 800 m, define define and are are correlated correlated on the the basis basis of of aa distinctive distinctive Cu-Au Cu-Au break datum (Fig. 1). 1). With the exception exception of localized Pt enrichment enrichment associated with an interval interval enriched in olivine olivine about about 110 110m m below the Cu-Au horizon, all Pd-Pt-Au enrichment occurs over an interval of 0 to 90 m below the defined datum (Fig. 2). In In general general precious precious metals metals peaks are stratigraphically stratigraphically offset from one another, progressing upward upward in in the the succession successionPd->Pt->Au. Pd-*Pt-)Au. Maximum Maximumgrades gradesin in 0.3m 0.3m long long core core samples are 410 ppb Pd, 275 ppb Pt, and 1080 ppb ppb Au. Au. Above the Cu-Au break, all precious metals are very strongly depleted. Strong Strong correlation correlation between Fe. Fe, Al A1 and modal olivine with precious metals peaks indicates indicates a possible possible connection connection between subtle subtle modal layering layering of of plagioclase, plagioclase, oxide, oxide, and and olivine olivine with withmineralization. mineralization. The oxide oxide gabbro-hosted POE PGE reef in the Sonju Sonju Lake intrusion intrusion shows shows marked similarities, similarities, with some differences, Skaergaard intrusion intrusion of of East East differences, to stratiform stratiform POE PGE mineralization in the Skaergaard Greenland (Andersen et al., 1998), 1998). the Rincon del Tigre Complex of Bolivia (Prendergast, (Prendergast, 2000), 2000). intrusions throughout throughoutthe the world. world. Whole rock and many other tholeiitic mafic layered intrusions geochemistry, clinopyroxene clinopyroxene and olivine compositions, compositions, and petrographic data are are consistent consistent with with an orthomagmatic origin for the mineralization related to the fractional fractional segregation segregation of of sulfide sulfide magma. The melt from silicate magma. The homogeneity homogeneity of the host rock, the thickness of the mineralized interval, and the offset of metal concentrations concentrations imply that sulfide sulfide saturation saturation was was passively passively thggered by fractional crystallization of the Sonju magma. Mungall (2002) recently triggered Mungall(2002) recently argued argued that that stratigraphic POE reefs can be satisfactorily stratigraphic offsets of Pd, Pt, Au and Cu peaks common to many PGE satisfactorily model of of sulfide liquation liquation and and settling. settling. The model shows explained by a kinetic model shows that that the the degree degree will be be controlled controlled by by kinetic kinetic factors, factors, such such as as the the diffusivity diffusivity of of of offset and metal enrichment will of sulfide supersaturation, sulfide sulfide droplet droplet size, and its its settling chalcophile elements, the degree of result in in variability of of the apparent apparent silicate/sulfide silicate/sulfide melt melt ratio ratio (R (R factor). factor). The velocity, which result The correlation correlation of multiple multiple peaks peaks of POE PGE with with subtle, subtle, broad broad modal modal variations variations may may be be related related to to repeated convective overturn caused by the crystallization of of magnetite magnetite in in an environment environment of of sulfide over-saturation, over-saturation, as suggested by Prendergast (2000) (2000) to explain a similar similar correlation correlation in in the the Rincon del Tigre Complex. Some Some evidence evidence of late-stage sulfide dissolution and remobilization remobilization exists, but it appears to have little to no effect effect upon upon the the distribution distribution of of precious precious metals. metals. 39 �SLO2-3 SLO2-2 5L02-1 mno,3 ibo,e Co'Aob,,ak rlTrm,T +70.0 — +70.0 +80.0 — +600 —+000 4105 — +40.0 +300 — +35.0 +10.0 — +20.0 .300,0 +30,0 — lddk!4- Fig. Fig.1:1:Correlation Correlationofofdrill drill cores 1, 5L02-2, coresSLO2SL02-1, SL02-2,and and SLO2-3 SL02-3showing showingdistinctive distinctive 00 1=11— .10.0 — ''0.0 -10.0 — '20.0 — 30,0 WWIII— — .40.3 -40.0 Cu-Au Cu-Aubreak. break.The TheCu-Au Cu-Au break breakisisused usedtotoprovide provideaa datum datumtotowhich whichall all stratigraphic stratigraphicplots plotsare are correlated, correlated,and andposition positioninin stratigraphy stratigraphyisismeasured measuredasas meters metersabove aboveororbelow belowCu-Au Cu-Au break. break. '50.0 -10,0 — '00.0 11214- — '70.0 -700 — '00+ 11414- Will- — '90,0 ._,I000 —-lion -100.0 112111— -110.0 —'120.0 -l 00.0 — -000,0 [It'i—' Cu'Au break 8 • Au(ppb) Cu ppm) mete's above Cu-Au break 5L02-3 SLO2-2 SL02-1 — +700 — +700 — +700 t00.0 — +600 — -+600 +50,0 —*500 —+50,0 +400 — 5400 — +400 300 — +30.0 — +30.0 200 — +20.0 — 420.0 — +100 — +10.0 +10.0 — -10.0 — -100 — -20,0 — - 00i 20,0 — -500 — -30,0 — 420 — '400 — .000 — -50.0 '600 — -00.0 -700 — -700 — .70.2 000 — .800 — -00.0 — 900 — -00.0 — -00.0 1000 __..,1000 —100.0 7100 —110.0 —-110.0 —-1200 —010.0 —'120.0 300 — '132.5 — -130.0 —00 -400 888 Fig. Fig.2: 2:Correlation Correlationof of Pd Pd and Pt in drill holes SLO2and Pt in drill holes SL021,1,SLO2-2, SL02-2,and andSLO2-3. SL02-3. Notice Noticemultiplicity multiplicityof of spikes spikesand andoffset offsetbetween between Pt Ptand andPd Pdpeaks. peaks. I 2 § § U Pd)ppb) W Po(ppb) References: References: Andersen, Rasmussen, Andersen. J.J. C. C. 0., O., Rasmussen,H., H.,Nielsen, Nielsen,T. T. F. F.D., D., Ronsbo, Ronsbo,J.J. G., G., 1998, 1998,The The Triple TripleGroup Groupand andthe the Platinova PlatinovaGold Goldand andPalladium PalladiumReefs Reefsininthe theSkaergaard SkaergaardIntrusion: Intrusion:Stratigraphic Stratigraphicand andPetrographic Petrographic Relations. 488-509. Relations.Economic EconomicGeology. Geology.Vol. Vol.93, 93,pp. pp.488-509. Miller, Miller,J.J. D. D. Jr., Jr.,1999, 1999,Geochemical GeochemicalEvaluation Evaluationof ofPlatinum PlatinumGroup GroupElement Element(PGE) (PGE)Mineralization Mineralizationininthe the Sonju 44,31 3 1p. p. SonjuLake Lake Intrusion, Intrusion, Finland, Finland,Minnesota: Minnesota: Minnesota Minnesota Geological Geological Surv. SUN.Information InformationCircular Circular44, of the Beaver Miller, J. D., Jr., and Chandler, V. W., 1997, Geology, petrology, and tectonic significance Miller, J. D.. Jr., and Chandler, V. W.. 1997, Geology, petrology. and tectonic significance of the Beaver Bay Bay Complex, Complex,northeastern northeasternMinnesota, Minnesota,inin Ojakangas, Ojakangas,R. R.W., W., Dickas, Dickas,A. A.B., B., Green, Green,J.J.C., C.,eds., eds.,Middle Middle Proterozoic Proterozoicto toCambrian CambrianRifting, Rifting, Central CentralNorth North America: America:Geological GeologicalSociety Societyof of America AmericaSpecial SpecialPaper Paper 312, p. 73-96. 312, p.73-96. Mungall, E.,2002, 2002,Kinetic KineticControls Controlson onthe thePartitioning Partitioningof ofTrace TraceElements ElementsBetween BetweenSilicate Silicateand andSulfide Sulfide Mungall,J.J. E.. Liquids. Liquids. Journal Journal of of Petrology. Petrology. Vol.43, Vol. 43, pp.749-768 pp. 749-768 Prendergast, Tire Complex, D., 2000,Layering 2000,Layering and Precious Metals Mineralization in the Rincon del Tigre Complex, Prendergast, M. M. D.. Eastern Bolivia. Economic Geology. Vol. 95, pp. 113-130. Eastern Bolivia. Economic Geology. Vol. 95, pp. 113-130. 40 �RESULTS OF SINGLE-GRAINANALYSES ANALYSESOF OFPRECAMBRIAN PRECAMBRIAN MAFIC MAFIC RESULTS OF40Ar/39Ar '")~r/^'>~r SINGLE-GRAIN INTRUSIONS IN IN NORTHERN NORTHERN ANI) AND EAST-CENTRAL EAST-CENTRALMINNESOTA MINNESOTA INTRUSIONS KEA'ITS, M.J., Kent, OH OH44242; 44242;JIRSA, uRSA, M., KEATTS, MJ., Dept. of Geology, Kent State University, Kent, St. Paul, MN 551 14-1057; Minnesota Geological Geological Survey, Survey, 2642 University University Avenue West, St. Minnesota 55114-1057; HOLM, HOLM, D., Dept. Dept. of of Geology, Geology, Kent Kent State StateUniversity, University, Kent, Kent,OH OH44242 44242 Age information from mafic mafic intrusive intrusive suites suites is critical Age information from critical for for proper proper interpretation interpretation of the the geologic history and for mineral deposit models in the Lake Superior region. region. As As part of an effort to evaluate evaluate PGE potential potential in mafic mafic intrusions intrusions in Minnesota, Minnesota, several several plutons plutons have been dated dated using the CO2 Ar/Ar incremental heating technique at the University of of Wisconsin-Madison Wisconsin-Madison COi laser ArIAr Rare Rare Gas Gas Geochronology Geochronology Laboratory. Laboratory. For late-stage late-stage shallow shallow plutons plutons containing containing primary primary magmatic homblende, Ar/Ar ArIAr mineral mineral ages are likely likely to to closely closely approximate approximate the crystallization crystallization magmatic hornblende, age. In regions regions with with aamore moreprotracted protractedthermal thermalhistory history(i.e., (i.e.,low-grade low-grademetamorphism, metamorphism, slowslowcooling, etc.), the Ar/Ar intrusions from ArIAr data provide minimum ages for the mafic plutons. Mafic intmsions Minnesota selected for for this study study represent aa broad range range of of geologic geologic settings, settings, including including1) 1)small small mafic supracrustal and and intrusive intrusive rocks rocks within mafic plutons plutons emplaced emplaced into Paleoproterozoic Paleoproterozoic supracrustal within the Penokean orogen (samples 264, R17); and 2) varied, varied, primarily latelate- to post-tectonic post-tectonic intrusions intrusions in in supracrustal rocks of the Archean Archean Wabigoon Wabigoon (samples Al, Al, B21, UBD) and Wawa (samples K15, LP, ANA) subprovinces of Superior Province. We We report report here here the initial results from LP, subprovinces of Superior Province. from eight eight separate intrusions (Fig. 1). separate intrusions (Pig. 1). A homblende East-central Minnesota. (R17) from East-central Minnesota. A hornblende grain grain (R17) from aa sample sampleofofmedium-grained medium-grained hornblendite from the Tibbett's Brook intrusion cutting the homblendite the East-central East-central Minnesota Minnesota batholith batholith in in Morrison Co. yields yields aa plateau date of 0.006 Ga from contiguous increments from 44 contiguous increments Momson Co. plateau date of 1.770 1.770 ±Â 0.006 constituting 74% of of the gas released. constituting 74% released. AAbiotite biotitegrain grain(264) (264)from fromaasample sampleofofcoarse-grained coarse-grained biotitic olivine gabbronorite gabbronorite cutting the Little Falls Formation in Morrison Momson Co. yields yields aa plateau plateau constituting 68% of the gas released. date of 1.791 ± Â0.008 0.008 Ga from 5 contiguous increments constituting Wawa Subprovince. A hornblende grain (ANA) from a sample sample of of prismatic prismatic homblende hornblendediorite diorite collected near Red Lake in Beltrami Co. yields a near-plateau date of 2.587 ±0.012 a . 0 1 2 Ga Ga in in 55 nonnoncontiguous incrementsconstituting constituting50% 50%ofofthe thegas. gas. A A biotite biotite grain grain (K15) (K15) from from a sample contiguous increments sample of biotite granodiorite porphyry collected in Norman Co. yields aa plateau date date of 2.639 2.639±Â 0.007 Ga 0.007 Ga from 6 contiguous increments constituting 79% 79% of of the gas released. A biotite grain (LP) from a sample of porphyritic syenite collected at the the Wawa-Quetico Wawa-Quetico subprovince subprovinceboundary boundary in in St. St.Louis Louis Co., 0.006 Ga from from 77contiguous contiguous Co., in the the Linden Linden Pluton, Pluton, yields yields aa plateau plateau date date of 2.666 2.666 ±Â 0.006 increments constituting constituting 88% of the gas released. grain (£321 (B21) from the Oaks intrusion leucodiorite sampled Wabigoon Subprovince. A hornblende grain near Fault in in Roseau 1 ±Â 0.008 0.008 Ga from 88 near the Vermilion Vermilion Fault Roseau Co. yields yields aa plateau plateau date date of of 2.67 2.671 contiguous increments increments constituting constituting75% 75%ofofthe thetotal totalgas gasreleased. released. A A hornblende homblende grain grain (Al) (Al) from the Black River gabbro, collected in Roseau Co., yields a plateau date of 2.685 ± 0.011 Ga from from 2.685  0.011 11 contiguous increments increments constituting constituting90% 90%of of the the total total gas gas released. released. A hornblende hornblende (UBD) (UBD) from a sample collected in Koochiching Co. north of the sample of homblende-biotite hornblende-biotite gabbro collected the Rainy Rainy LakeLakeSeine River Fault Fault yields yields aa plateau plateau date date of of 2.695 Seine River 2.695 ±  0.007 0.007 Ga from 66 contiguous contiguous increments increments constituting 49% of the gas released. released. 41 �_____________ __________________________ ___ The mineral age data data from mafic mafic plutons plutons from from the the Wabigoon Wabigoon subprovince subprovince are are synchronous synchronous Mafic plutons plutons from from the the with the last deformation event (D2) dated in the range 2.685-2.674 Ga. Ga. Mafic Wawa subprovince give an 80 m.y. Interestingly, the Linden m.y. age age range range from from 2.58 2.58 to to 2.66 2.66 Ga. Ga. Interestingly, Linden Pluton gives a biotite Pluton biotite date date concordant concordant (within (within error) with the youngest youngest mafic pluton from the Wabigoon subprovince. subprovince.The Theyounger youngerspread spreadofof ages ages from from the Wawa Wabigoon Wawa are are consistent consistent with with southward growth of the Superior Province Province during the the latest latest Archean. Archean. Mafic plugs plugs evident evident from aeromagnetic maps in in east-central Minnesota are comagmatic with the circa 1.775 aeromagnetic maps east-central Minnesota comagmatic with 1.775 Ga Ga EastEastcentral Minnesota batholith. batholith. Further constraining the temporal framework of mafic mafic intrusions intrusions may contribute to mineral deposit models models for for PGE may contribute to mineral deposit PGE in these these intrusions intrusions and and their their analogs analogs in in adjacent states states and and provinces. provinces. Fig.1 °Ar/"Ar age Fig.l "ArPAr age spectra: spectra: t, = plateau age, age, çt, == total total gas gas age. age. 2.0 2.0 2.0 R17 I 264 1.5 o t_.._1 :. e 1.8 1.0 0.5 3.0 Btite MSWD Biotite MSWD2.57 2.57 Amphibole MSWD 1.78 1.770±0.006 t, = 1.770  0.006 Ga Ga t9 = 1.653 1.653  ± 0.005 0.005 Ga p Ga [.1 t==1.791 t, 1.791±0.008 t 0.008 Ga Ga 1.782  ± 0.007 Ga t,t9 == 1.782 1.5 3° ANA .. o H- — 2 25 Amphibole MSWD MSWD 0.30 = 2.671 ± 0.008 Ga t9 = 2.705 ± 0.013 Ga Amphibole MSWD Amphibole MSWD 0.71 0.71 = 2.587 ± 0.012 Ga = 2.550 ± 0.009 Ga 2.0 3.0 2.0 3.0 K15 — t: 2.5 TTL..c.H 2.5 . 2 Amphibole Amphibole MSWD MSWD1.81 1.El Biotite MSWD MSWD 2.48 2.46 t, = 2.639 ±Â 0.007 Ga Ga 2.685 ± t, = 2.685 Â0.011 0.011 Ga Ga = 2.640 ± 0.007 Ga t = 2.700 ± 0.010 Ga 2.0 — LP % Ir11 — m —LI 2.5 2.5 2 Amphibole MSWD MSWD 1.03 1.03 ± 0.007 Ga t, = 2.695  = 2.730 ± 0.006 Ga Biotite MSWD 1.11 = 2.666 ± 0.006 Ga = 2.657 ± 0.006 2.0 - 0 0 2.0 100 0 $00 0 —. . 100 $ 20 20 30 30 40 50 SO 60 60 700 7 80 Cumulative "Ar "Ar released (%) 90 SO 42 10 10 20 20 30 40 S O 50 60 60 7 700 80 "Ar released Cumulative "Ar released (%) 90 90 300 100 �Ontario New zircon ages from from the the Gunflint and Rove Formations, Formations, northwestern northwestern Ontario Kissin, Kissin, S.A., Department Departmentof of Geology, Geology,Lakehead Lakehead University, University,Thunder ThunderBay, Bay,ON, ON,P7B P7B5E1 5E1Canada, Canada, stephen.kissin@lakeheadu.ca stephen.kissin@lakeheadu.ca ;;Vallini, Vallini, D.A., D.A., University University of Western Australia, Australia, 35 35 Stirling Stirling Hwy, Crawley, 6009, W.A., iWO, Canada; W.A., Australia; Australia; Addison, Addison, W.D., RR 2, Kakabeka Kakabeka Falls, Falls, ON, ON, POT 1W0, Canada; Brumpton, Brumpton, G.R.., 211 21 1 Henry Henry St, St, Thunder Thunder Bay, ON, P7E 4Y7, Canada. Canada. Previous work based on U-Pb U-Pb geochronology geochronology from presumed volcanogenic volcanogenic zircons zircons obtained obtained from a tuff layer at the lower/upper ± 2Ma, lowerlupper Gunflint Formation boundary yielded an age of 1878 1878  2Ma, believed to approximate the age of deposition of of the the unit unit (Fralick et et al., al., 2002). This This age age corresponds closely with the age of the correlative Hemlock Formation of Michigan (1874 ±  corresponds closely Schneider et al., 9Ma; Schneider a]., 2002). 2002). 9Ma; We report here preliminary age determinations determinations based on SHRIMP SHRIMP analyses of zircons extracted from three volcanic ash layers; one lying in the Gunflint Gunflint Formation, Formation, and and two within the overlying Rove Formation. The The Gunflint-Rove Gunflint-Rovecontact contact is an an important important reference reference point. Pufahl and Fralick (2000) placed it at the top of a sequence sequence of chert-carbonate chert-carbonate grainstones grainstones which which is overlain by carbonaceous shales of of the the Rove Rove Formation. Formation. The Gunflint Gunflint exposure exposure outcrops at Little Falls, on lOm (topographically) (topographically) below the -10m Little on the the south southside sideof of the theKakabeka KakabekaFalls FallsGorge, Gorge,— Gunflint lapilli tuff dated by Fralick et al. (2002). A Rove volcanic ash ash exposure exposure at at Oliver Oliver Creek Creek is estimated (stratigraphically)above abovethe theGunflint-Rove Gunflint-Rove contact. contact. Zircons were also estimated to be —70m -70m (stratigraphically) extracted from an ash layer within Falconbridge Pine River (PR98-1) (PR98-1) drilicore drillcore (688.24m (688.24m down down contact. hole), located -Am -4m above the Rove-Gunflint contact. The Oliver a mean 207Pb/206Pb age of of 1821 1821 ± ""~b/^Pbage  16 single The OliverCreek Creekzircons zirconsrecorded recorded a mean 16 Ma while a single age of 1840Ma 1840Ma was obtained from the drillcore PR98-1 sample. sample. The The errors errors cited cited are are at at the the one one standard deviation (10) ( l a ) and and 95% 95% confidence confidence level level and and the the analyses analysesare are less lessthan than5% 5%discordant. discordant. There recordedfrom from each each locality locality which are assumed to There are are also also two two younger younger ages ages of of —1786Ma -1786Ma recorded outliers. be outliers. The Little Falls zircons, zircons, which are are somewhat somewhat rounded rounded and and fractured, fractured,yielded yielded various various than 2000Ma. 2000Ma. Most Most of of the the ages ages are more more than 10% 10% discordant, discordant,and and these these samples samples ages, all older than may have suffered lead loss. As As well, well, there there are are some some indications indicationsof of admixture admixtureof of shalely shalelymaterial material in the ash layer at this locality. Older Older zircons zircons from from the the lapilli lapilli tuff tuff layer layer at at the the lower/upper lowerlupper Gunflint Gunflint contact (Fralick (Fralick et al., a]., 2002) 2002) were also found found to be admixed admixedwith with Paleoproterozoic Paleoproterozoic zircons. zircons. Using the stratigraphic column of of Pufahl and Fralick (2000), we estimate that the drillhole (PR9S-i) Gunflint lapilli (PR98-1)samples samplesare—i are -1 lOm 10m above the Gunflint lapilli tuff tuff layer layer containing containingthe the zircons zircons dated dated by Fralick et al. (2002), (2002),while while we we estimate estimatethe theOliver OliverCreek Creeksamples samplestotobebe—iSOm -150m above this same layer. The ages reported here indicate that a slow sedimentation sedimentationrate ratemust must have havebeen been required in order to account for the age difference difference between between the the lapilli lapilli tuff tuff of Pufahl and Fralick Fralick and the two sets of Rove dates reported here. here. This This slow Rove Rove sedimentation sedimentation rate is comparable that reported in banded iron formations formations of the early early Proterozoic Proterozoic Campbell CampbellGroup, Group,Griqualand, Griqualand, West Sequence, Sequence, South Africa (Barton et al., 1994). 1994). 43 �Oliver Creek reasonable agreement The Oliver Creek ages ages reported here are are in reasonable agreement with the the 1833 1833±Â 6Ma age reported by Schneider Schneider et a!. al. (2002) for the Tobin Lake Pluton, which is undeforined undeformed by Penokean deformation and intrudes presumed Hemlock Volcanic equivalents. However, However,the the with the the Penokean Penokean zircons from the Rove Formation suggest that volcanic activity associated with continued for for at at least least 40 40 m.y. m.y. Further Orogeny continued Furtherstudies studiesare are underway underway to clarify some of the questions raised by our results. results. Barton, E.S., Altermann, Smith, C.B. 1994. U-Pb Altermann, W., William, 1.5., I.S., amd Smith, U-Pb zircon zirconage agefor foraatuff tuff in in the Campbell Group, Griqualand West Sequence, South Africa: Implications for Early Proterozoic Proterozoic rock rock accumulation accumulation rates. rates. Geology Geology 22: 22:343-346. 343-346. Fralick, P., Davis, D.W., and Kissin, S.A. 2002. 2002. The Ontario, The age age of the Gunflint Formation, Ontario, reworked volcanic ash. Canadian Canada: single zircon U-Pb age determinations from reworked Journal of Earth Earth Science Science39: 39:1089-1091. 1089-1091. Pufahl, P. and Fralick, P. 2000. Fieldtrip 4: Depositional environments environments of the Paleoproterozoic Paleoproterozoic Gunflint Formation. Proceedings of the Institute Institute on on Lake Lake Superior SuperiorGeology, Geology, 46, 46, pt.2. D.A., Bickford, Bickford, M.E., M.E., Cannon, Cannon, W.F., W.F.,Schulz, Schulz,K.J., K.J.,and andHamilton, Hamilton,M.A. M.A.2002. 2002. Age of of Schneider, D.A., formations, Marquette volcanic rocks and syndepositional iron formations, Marquette Range Range Supergroup: Supergroup: implications Paleoproterozoic iron formations implications for for the tectonic tectonic setting setting of Paleoproterozoic formations of the Lake Lake Superior Superior Region. Canadian Canadian Journal Journal of of Earth Science Science 39: 39:999-1012. 999-1012. 44 �OF THE THE SOUTHERN SOUTHERN PORTION OF O FTHE THELAURENTIDE LAURENTIDE ICE ICE MEAN TRANSPORT TRANSPORT LENGTH LENGTH IN TILLS OF SHEET: IMPLICATIONS FOR DRIFT EXPLORATION EXPLORATION IN THE THE LAKE LAKE SUPERIOR SUPERIOR REGION REGION LARSON, Phillip Phillip C., C., Department of Minnesota, MN 55812, 55812, LARSON, Department of Geological Geological Sciences, University University of Minnesota, Duluth, Duluth, MN plarson2@'d.umn.edu plarson2@d.umn.edu Introduction Bedrock - till, till, Bedrock in the the Lake is typically typically covered covered by by aamantle mantleofofglacigenic glacigenicsediments sediments— Lake Superior region is outwash, that presents significant challenge challenge to successful successful application application of of surficial surficial outwash, and lacustrine lacustrinesediments sediments—- that presents a significant geochemical techniques techniques widely widely used used to help targets. The glacial glacial environment environment is is very very complex, complex, geochemical help generate generate drilling drillingtargets. with sediments produced produced by by a range of processes. with sediments processes. Till represents the ideal sampling sampling media media in in these theseenvironments, environments, direction) is attached to since a vector (ice flow direction) to the the composition composition at any location location indicating indicatingthe thedirection directiontotothe thesource source of any defined anomaly. anomaly. However, recent work has led to recognition that both the the magnitude magnitude of of aa till till geochemical geochemical anomaly andthe the potential potential transport transportdistance distancetotoits itssource sourcemay mayhave haveaawide widerange rangeofofvalues. values. This is a reflection of anomaly and transport distance of till-forming material, material, and is related the mean transport related to the the fundamental fundamental sediment sediment transport transport process process responsible for forming forming the till. responsible till. Theory The concentration concentration of an indicator (a distinct distinct lithologic or geochemical component derived from a discreet in till till is the direct product of the physical processes processes of of glacial glacial erosion, erosion, transport, transport, and and deposition. Indicator source) in Indicator concentrationisis controlled controlledby by aa number number of of variables, including substrate substrate hardness hardness and and the the efficacy of the glacial concentration variables, including erosional regime. ice flow indicator mass regime. Under Under steady steady state state ice flow conditions conditions and and uniform uniform bed bed erosion erosion rates, rates, indicator mass concentration c in (1) down-ice down-ice of of an an indicator indicator source sourceof of finite finiteflow-line flow-linelength lengthLL(1) (1) is: is: concentration ci intill till atatany anytransport transportlength lengthTT(1) — L (1) where X X isis the the erosion erosion length length scale scale (I). (1).For Fortills tills down-ice down-ice of of the the indicator indicator source, under steady state state conditions, conditions, the decrease &/ST assumes assumes aa quasi-exponential quasi-exponential form. form. decrease in indicator indicator concentration concentration with with increasing increasing transport transport length length bc/8T Erosion length length scale, scale, \X,isisrelated relatedto to the the spatial spatial bed bed erosion erosion rate rate E E ((mL3) and the thickness of the debris layer in Erosion m r ) and (m-1.): transport md 1d (mV2): (2) increases, so so does the mean transport XXis is closely related to the mean transport distance distance of till-forming material; as X X increases, distance. distance. vs. Long-Distance Long-Distance Transport: Transport: Examples Short- vs. Tills in the Lake Superior Superior region can be broadly broadly grouped grouped into two two categories categories based based on onthe themean meantransport transport length of the till forming material. mean transport length Tills characterized characterized by short-distance short-distance mean length are commonly commonly composed composed of of coarse-grained coarse-grained material containing abundant abundant angular angular clasts, clasts, and and display rapid rapid decrease in indicator material containing indicator concentration concentration with with transport transport length. This This isis exemplified exemplified by by tills tills overlying overlying the the Vermilion Vermilion greenstone greenstone belt of of northern northern Minnesota, Minnesota, which which displays displays rapid decrease decrease in in concentration concentrationofofnumerous numerousindicator indicatorlithologies; lithologies; X for range from 2.Oi0m. m. A ?for . thisthis tilltill range from 1.01.0 to to 2.0-103 rapid dispersal train composed composedofof clasts clasts of of Nipigon Nipigon diahase diabase in in till east dispersal train east of of Lake Lake Nipigon. Nipigon, Ontario Ontario displays displays similar similar characteristics. Dispersal in indicator indicatorconcentration; concentration;calculated calculated?\for forthis this Dispersal isis characterized characterizedby by aa similar similar rapid rapid decrease decrease in till range 102 m over relatively relatively soft greenstone greenstone to to 2.4-lo4 2.4 i04 m m over over hard hard diabase. diabase. Both range from from5.5 5.5-10' Both tills tills are are interpreted interpreted to to have formed by by erosion of of hard hard bed bed by quarrying and abrasion, and englacial transport. transport. Tills characterized by long-distance Tills characterized by long-distance mean mean transport transport length are are commonly commonly composed composed ofoffine-grained fine-grained material with aa relatively low abundance of rounded material with relatively low abundance of rounded clasts, and display display little little apparent apparent decrease decrease in inindicator indicator with transport transport length. length. Cretaceous shale grains in Des Moines Lobe tills of the Minnesota concentration with Minnesota River valley show relatively relatively little little decrease decrease in in concentration concentration along along the the flow flow axis axis extending extending down down the the valley valley (Matsch, 1972); the calculated X Xfor for this this till till is is 5.0-lo5 5.01 5 m. m. Carbonate-bearing Carbonate-bearing tills overlying the Canadian Shield north of Lake Lake Superior Superior (Thorleifson and and Kristjansson, Kristjansson, 1993) show similar long-distance transport of Paleozoic Paleozoic carbonate carbonateand andProterozoic Proterozoic greywacke clastswith withlittle little decrease decrease in in concentration; concentration;the thecalculated calculated1.Xfor forthis thistill till is is o.oio5 6.0-10m. m. Both tills are are greywacke clasts have deposited deposited from deforming subglacial sediment layers (deformation (deformation tills). interpreted to have 45 �Discussion Discussion tills are are characterized characterizedby by1).that that are 10 to to 100 times The data indicate deformation tills times higher than those of of thin, thin, coarse-grained tills. Tills characterized by intermediate intermediate values of the erosion erosion length length scale scale X 1. have not as as yet yet been been recognized in the Lake Superior region, and perhaps perhaps do do not not exist. This Thisgap gapininrecognized recognized values values suggest suggest there there are are two main processes by which which bed bed material material is is eroded eroded and and entrained, entrained,transported, transported, and anddeposited deposited— - entrainment entrainment and and transport by a deforming subglacial subglacial layer, and erosion by quarrying and abrasion and transport transport as as an an englacial englacial debris debris load with deposition deposition by lodgement or meltout. Deformation tills form with with little little accompanying accompanying erosion of underlying underlying hard hard bedrock. bedrock. Their formation is is consistent with with redistribution of unconsolidated regolithoror sediment derived from consistent redistribution of unconsolidated regolith sediment derived from aa preglacial preglacial reservoir. reservoir. Consequently,till till composition composition reflects reflectsthat that of of distant distant (>100 Consequently, (>lo0 km) km)bedrock. bedrock. Tills characterized characterized by short short mean mean restricted erosion erosion and and entrainment and and transport of hard bedrock. transport length indicate spatially and temporally restricted Their formation is consistent consistent with erosion erosion by quarrying quarrying and abrasion abrasion of of hard hard bedrock bedrockwith withsubsequent subsequentextensive extensive textural Till composition km) bedrock. bedrock. textural modification during during transport. transport. Till composition closely reflects reflects that thatofofnearby nearby(—10 (-10 1cm) Consequently, these tills have enormous potential value as geochemical sampling sampling media. Recognition of the process process responsible responsible for till till formation formation in in aa given given area areaisiscritical criticalfor forsuccessful successfulapplication application of surficial geochemical and boulder tracing exploration techniques techniques in the Lake Lake Superior Superior region. region. Limited scope scope orientation surveys surveys aimed aimed at at characterizing characterizingthe theerosion erosionlength lengthscale scale 1?. provide provide a means orientation means of of quickly quickly assessing assessing the potential for successful application of drift exploration techniques on both both regional and property scales. 46 �Glacial Glacial Lakes Lakes Aitkin and Upham: their origin origin and and environmental environmental history history PhillipLarson Larson LisaMarlow, Marlow,Howard HowardMooers, Mooers,&&Phillip Lisa Department Departmentof ofGeological GeologicalSciences, Sciences,University Universityof of Minnesota, Minnesota, Duluth, Duluth, Minnesota Minnesota55812 55812 Glacial Minnesota bounded Glacial Lakes Lakes Aitkin Aitkin and and Upham Upham occupied a basin in north-central Minnesota bounded on on the the north northby by the the Giants GiantsRange Range and and to to the the east, east, south, south,and and west west by by hummocky hummocky moraines moraines of of the the Rainy Rainy lobe, lobe, Superior Superior lobe, lobe, and St. Louis sublobe. The The lakes lakescaine came into into existence existence with the the retreat retreat of of the the Rainy Rainy lobe lobefrom fromthe theSt. St.Croix Croix phase phase sometime sometimeafter after15,000 15,000yr yr BP BP (Clayton (Clayton and and Moran, Moran,1982; 1982; Mooers Mooersand andLehr, Lehr, 1997). 1997).The Thebasin basinwas waslater lateroveridden overiddenby by the the St. St. Louis Louis sublobe sublobe from from the the northwest. northwest.With Withthe thewastage wastageofofthe theice iceof ofthe theSt. St.Louis Louissublobe, sublobe,the the basin basin was was again againoccupied occupiedby by lakes; as Lake Lake AitkinAJpham I and the later phase as lakes; the the earlier earlierphase phase is is referred referred to as as Lake AitkinlUpham Aitkin/Upham II.Sediment Sedimentof ofthe theearly earlylake lakephase phaseisispreserved preserved at at aa few localities. localities. One Onesuch such AitkinAJphamII. locality locality preserves preserves aa sequence sequence that helps redefine the glacial glacial chronology. AAsedimentary sedimentarysequence sequence located locatedin in the thenortheast northeastcorner comerof of the the Upham Upham basin basin reveals reveals sub-aqueously sub-aqueously deposited depositedRainy RainyLobe Lobe outwash outwashbeneath beneathglaciotectonically glaciotectonically deformed deformedfine-grained fine-grained lake lake sediments sediments deposited depositedby by the theSt. St. Louis Louis sublobe. sublobe. This, This,along alongwith withother othergeomorphic geomorphicrelationships relationships(P.C. (P.C.Larson, Larson,unpublished unpublisheddata) data) indicates indicatesthat thatthe theRainy RainyLobe Lobe ice ice margin margin was was coincident coincidentwith with the the Giants Giants Range Range when when the theSt. St. Louis Louissublobe sublobeadvanced advancedacross acrossthe thelake lakebasin. basin. Using @EM) the the elevation elevationof of lake lakebasin basin was was adjusted adjustedfor forisostatic isostatic Using aa Digital DigitalElevation Elevation Model Model (DEM) rebound based on the highest lake level, then tilted incrementally through several stages rebound based on the highest lake level, then tilted incrementally through several stagesto toassess assess beaches, beaches, inlets, inlets, and and outlets outlets over time. AAseries seriesof ofsuccessively successivelylower lower outlets outletsdraining drainingto tothe theSt. St. Louis Louis River Riverserved servedas as outlets outlets for for Glacial Glacial Lakes Lakes Aitkin Aitkin and and Upham Upham (Hobbs, (Hobbs,1983; 1983;Farnum, Farnum,1964; 1964; Wright, Norwood through through the the Embarrass Embarrass Wright, 1972). 1972).Meltwater Meltwaterentered enteredthe thelakes lakesfrom fromGlacial GlacialLake LakeNorwood gap, gap,and and later laterfrom fromGlacial GlacialLake LakeKoochiching Koochichingalong alongthe thePrairie PrairieRiver. River. During During this this time time Aitkin Aitkin and and Upham Upham were were confluent, confluent,and and the the outlet outlet was was established establisheddown down the the modern modem St. St.Louis LouisRiver. River. The The lakes lakes were were separated separated by by aa sill sill ca ca 11,500-10,100 11,500-10,100yr yr BP, BP, after after inflow inflow from from Koochiching Koochichingwas was divertedto to Glacial GlacialLake LakeAgassiz. Agassiz. diverted Extensive Extensive dune dune fields fields formed following initiation initiation of drainage drainage of the lakes. Granulometry Granulometry indicates throughout the thebasin. basin. Maximum indicates a 4p 4(pgrain grain size size signature characterizes dunes throughout Maximum dune dune amplitude metersand anddune dunemorphologies morphologiesrecord recordprominent prominentnorthwesterly northwesterlywinds. winds. Dunes amplitude is is —3 -3 meters overly overly source source areas, areas, which include include an underfiow underflow fan fan deposited deposited by by the the Prairie Prairie River Riverinlet inletand andthe the western margin marginof of Lake Lake Upham. Upham. western A sediment sediment core core collected collected from Hay Lake (93°W, (93'W, 52°N), 52ON), located located within aa dunefield dunefieldat atthe theedge edge of Glacial Glacial Lake Lake Upham, records three prominent peaks in whole-core whole-core magnetic magnetic susceptibility susceptibility between No clastic clasticinput inputisisevident evidentafter after6,600 6,600yr yrB.P., B.P.,suggesting suggestingdune dune between 10,100 10,100and and 6,600 6,600 yr BP. No stability. The timing of dunes within the basin has important implications for other dunes throughout Minnesota. Eolian Eolianevents eventsrecorded recordedin in the the core coreare areinterpreted interpreted as as the the result resultof of lake lake drainage drainage and exposure exposure of abundant abundant source material material during during Late Late Glacial Glacial and and Early Early Holocene Holocene rather rather than than landscape landscape destabilization destabilization because of mid-Holocene aridity (Keen et al., 1990; 1990; Grigal Grigal et et al., al., 1976; 1976;Dean Dean et et al., 1996; 1996; Dean, 1997). 1997). Additionally, Additionally,this thissediment sedimentcore core places places aa minimum minimum age on the drainage of of Glacial Lakes Lakes Aitkin Aitkin and and Upham Upham II. II. Lake Upham must have drained drained after 47 �age on the drainage of Glacial Lakes Aitkin and Upham IiT. I. Lake Upham must have drained drained after 11,500 B.P. and Lake Aitkin may have persisted until ca. 7,000 10,100 yr B.P. 7,000 yr 11,500 yr BP and before 10,100 BR BP. Clayton, L. and Moran, S.R. 1982, 1982, Chronology Chronology of late Wisconsinan glaciation glaciation in in middle middle North North America. Quaternary Quaternary Science ScienceReviews Reviews11(1), (I),55-82. 55-82. Ahlbrandt, T.S., Dean, W.E., Ahlbrandt, T.S., Anderson, Anderson, R.Y., R.Y., Bradbury, Bradbury, J.P., 1996. 1996.Regional Regional aridity aridityin in North North America during the middle Holocene. Holocene. The The Holocene Holocene 6 (2), 145-155. 145-155. Dean, W.E., 1997. 1997. Rates, Rates, timing, timing, and and cyclity cyclity of Holocene eolian activity activity in in north-central United United States: 1-334. States: Evidence Evidence from from varved varvedlake lakesediments. sediments.Geology Geology25 25(4), (4),33 331-334. H.E., Jr. Jr. 1964. A Late-Wisconsin Late-Wisconsin buried soil near Farnham, R.S., McAndrews, J.H., and Wright, H.E., Minnesota, and its paleobotanical setting. 393-412. Aitkin, Minnesota, setting. American American Journal Journal of of Science Science262, 262,393-412. Drexier, 1985. 1985. Late-Wisconsinan and Holocene Holocene History History of of the the Lake Lake Superior Superior Farrand, W.R. & Drexler, In Karrow, Karrow,Quaternary Quaternary evolution evolution of of the the Great Great Lakes, Lakes, Geological Geological Association Association of of Canada Canada Basin. In Special Paper 30, 30, 17-32. 17-32. D.F., Severson, R.C., R.C., Golz, G.E., 1976. Evidence Evidence of of eolian eolian activity activity in north-central Grigal, D.F., Geological Society Society of America America Bulletin Bulletin 87, 87, 1251-1254. 1251-1254. Minnesota 8,000 to 5,000 yr. ago. Geological Aitkin, Upham, and early Lake Hobbs, H.C. 1982, 1982, Drainage relationships of Glacial Lakes Aitkin, Agassiz in northeastern Minnesota. In In Teller, Teller,J.T., J.T., and and Clayton, Clayton,Lee, Lee,eds., eds.,Glacial GlacialLake LakeAgassiz: Agassiz: Geological Association of Canada Special Paper 26, 245-259. Canada Special Paper 26,245-259. K.L., Shane, L.C.K, L.C.K, 1990. A continuous record record of of Holocene Holocene eolian eolian activity and Keen, K.L., Lake Ann, east-central east-central Minnesota. Minnesota. Geological vegetation change at Lake Geological Society Societyof of America America Bulletin Bulletin 102, 1646-1657. 102,1646-1657. Mooers, Mooers, H.D., H.D., Lehr, J.D., 1997. 1997. Terrestrial record of Laurentide ice sheet reorganization reorganization during Heinrich events. events. Geology Geology 25 25 (11), (ll), 987-990. 987-990. H.E. 1972, Quaternary Quaternary history historyof ofMinnesota. Minnesota In In Sims, P.K., and Morey, G.B., GB., eds., Wright, H.E. eds., Geology of of Minnesota: A 15-578. A Centennial Centennial Volume: Volume: Minnesota MinnesotaGeological GeologicalSurvey, Survey,5515-578. 48 �and Hydrothermal HydrothermalPGE PGEMineralization Mineralizationof ofthe theBirch BirchLake LakeCu-Ni-PGE Cu-Ni-PGEDeposit Deposit Magmatic and in the South northeast Minnesota South Kawishiwi Intrusion, Duluth Complex, northeast Minnesota John Marma, Mama, Phil Brown and Steve Steve Hauck* Hauck* University of of Wisconsin, Wisconsin, Madison, Madison, Wisconsin Wisconsin 53706, USA Department of Geology Geology and Geophysics, University *Natat Resources Research *Natural Resources ResearchInstitute, Institute,University Universityof ofMinnesota, Minnesota, Duluth, Duluth, Minnesota 55811, 55811, USA USA The The Birch Birch Lake Lake Cu-Ni-POE Cu-Ni-PGE Deposit is located 12 12 miles south south of Ely, MN in the the South South Kawishiwi Intrusion (SM) (SKI) of of the the Duluth Duluth Complex (DC). The TheSKI SKIisisone oneof of two twolayered layered mafic mafic intrusions intrusions along along the the basal contact contact of the the DC to host sub-economic Cu-Ni-POE Cu-Ni-PGE deposits. deposits. Mineralization is is dominantly dominantly hosted by the U3 layer, the lower-most of three ultramaficultramafictroctolite packages characterized characterized as a zone of alternating ultramafic (picrite-peridotite) (picrite-peridotite) and troctolite troctolite horizons horizons with with lenses lenses and and pods of oxide-bearing oxide-bearing (>5%) (>5%) ultramafic ultramafic and/or and/or massive massive oxide. oxide. The purpose of this study study was to locate, describe, describe, and characterize the textural relationships relationships among platinum group group minerals minerals (POM), (PGM), sulfides, sulfides, and silicate silicate phases to help help delineate delineate the the relative significance significanceof primary and remobilized platinum group group element element (POE) (PGE) concentrations. concentrations. Samples Samples from from 4 drill drill holes transecting the Birch Lake Deposit were obtained obtained from from the the (NRRI) located located in in Duluth, Duluth, MN. MN. EMPA and Natural Resource Research Institute (NRRI) and detailed detailed c15.tm in petrography were used to locate POE PGE bearing minerals, averaging <l5yan in diameter, diameter,and andto to geochemistry. Identifying POM textural relationships with characterize the host mineral geochemistry. Identifying the PGM other phases is critical to understanding the mechanism mechanism by by which which PGMs POMs were were deposited. deposited. Data from this study study will aid exploration exploration in locating other other deposits deposits and and guide guide metallurgists metallurgists in in improving recovery recovery techniques. techniques. POEs occur most often as various Pd minerals with associated PGEs associated Pt, Pt, Os, Os, Ir, Ir, Ru, Ru, Au, Au, Ag, Ag, Te, Te, Bi minerals categories of silicate-sulfide-PGM silicate-sulfide-PGMtextural textural minerals and and were were grouped grouped into into the following following 4 categories relations: PGMs that occur in "halos" "halos" residing most commonly commonly in anorthite-enriched anorthite-enriched zones zones in• in relations: 1) PGMs primary plagioclase around either interstitial sulfide sulfide (dominantly (dominantly chalcopyrite), chalcopyrite), interstitial interstitial sulfide sulfide and silicate silicate (dominantly (dominantly chalcopyrite, chalcopyrite, clinopyroxene, clinopyroxene, and hydrous silicates silicates (amphibole (amphibole and and biotite)), or silicate or hydrous hydrous silicate) silicate) (Figure (Figure 1). This This style style of silicate (dominantly clinopyroxene or POMs identified. 2) 2) Remobilized RemobilizedPOMs PGMsthat thatoccur occurin in mineralization hosted 58% of the total PGMs chlorite, serpentine, or secondary magnetite. magnetite. This chlorite, This style style of of mineralization mineralization hosted hosted 21% 21% of of the the total total PGMs identified. 3) 3)Random RandomPOMs PGMsthat that occur occur in in poikilitic poikilitic anorthite-rich anorthite-richplagioclase plagioclase(An (An75-An 75-An POEs sometimes residing in disseminated 95) and clinopyroxene (Wo 30-Wo 50) with PGEs chalccipyriteor orhydrous hydroussilicate silicate pockets, pockets,but butno no association association with with "halos". "halos". This style of chalcopyrite POMs identified. 4) 4)In In interstitial interstitialsulfides sulfidesor orsilicates silicatesthat that mineralization hosted 11% 11% of the total PGMs include (?) textures, or include hydrous silicates, silicates, chalcopyrite, chalcopyrite, clinopyroxene, clinopyroxene, sulfides sulfides with symplectite symplectite ('1) calcite. This style calcite. style of mineralization mineralization hosted 10% 10% of the total total POMs PGMs identified. identified. POM concentrations in The following following is a summarized summarized model for the formation of high PGM the Birch Lake deposit. The SKI begins as a magma body that is replenished The SKI begins as a magma that is replenished with with multiple multiple saturated. The injections of magma, which becomes sulfur saturated. The magma magma body body is is relatively relatively POE PGE poor, poor, the conduits with a PGE POE enriched due to partial loss of sulfides during emplacement leaving the segregation of the total sulfide. The The sulfides sulfides in in the the magma magma body body scavenge scavenge available availablePOEs PGEs and and grains. Primary crystallize as disseminated, interstitial sulfide grains. Primary hydrous hydrous phases phases form form at at this this time time 49 �from a fluorine-rich, deuteric fluid. fluid. A Cu-, PGE-rich, sulfide-poor fluid A Cl-, Cu-, fluid enters enters the the magma magma chamber at its base via the original magma conduit(s) and/or faults. faults. The The fluid fluidmigrates migrates along along grain boundaries, and interacts with the larger interstitial sulfides. A A dynamic dynamicenvironment environmentisis created created in which the fluid, fluid, containing containing aa significant significantconcentration concentration of of dissolved dissolvedmetals, metals,begins beginsto to consume and use sulfur from the larger grains to produce more more sulfides. At At the the same sametime, time, the the fluid is reacting with neighboring neighboring grains, grains, specifically specifically plagioclase, plagioclase, and and through through aa cation cation exchange exchange reaction, alters the plagioclase rims, enriching them in calcium. This This reaction reaction causes causesaa volume volume loss that is filled with precipitated (±chlorite) producing the disseminated precipitated sulfides and POMs PGMs (±chlorite "halos" Finally, another anotherfluid fluid migrated migrated through through "halos" around the larger interstitial grains (Figure 1). Finally, the intrusion that remobilized POMs on a small scale. intrusion remobilized PGMs on a small scale. Based on evidence evidence solely from the Birch Lake deposit, deposit, POM PGM mineralization mineralizationappears appears "compartmentalized". This concentrated or "compartmentalized. This could could be the result of two possible mechanisms: 1) Areas of high PGM POM concentrations are dependent on on their proximal proximal distance to "feeder" "feeder" zones (i.e. conduits or faults) where fluids can be introduced; introduced; and/or 2) High POM PGM concentrations concentrationsare are that localize fluid movement. due to structural controls within these heterogeneous rocks that of primary primary vs. vs. remobilized remobilized This study contributes to the current debate on the roles of (deuteric?) PGE PGE mineralization mineralization in in layered mafic mafic intrusions. intrusions. For the Birch fdeuteric?) Birch Lake Lake deposit, deposit, the the data data suggest both mechanisms played important roles in the origin of the ore minerals. Figure Figure 11 a.) Photomicrograph Photomicrograph in in plane-polarized plane-polarizedlight lightof ofthin thinsection sectionBL BL89-2 89-22516.4 2516.4—- locations locations A-L. chalcopyrite and and pyroxene pyroxene cross-cut cross-cut by by vertical verticalchlorite chlorite veins, veins, all all of of which which are Large interstitial chalcopyrite by a disseminated, dominantly dominantly chalcopyrite chalcopyrite halo halo that that is is in in An-enriched An-enriched plagioclase surrounded by rims. Notice Notice all all PGMs, PGMs, except except one, either occur in the halo; in chlorite veins; in interstitial biotite; or in clinopyroxene. The The altered altered plagioclase plagioclase and altered pyroxene on the left side of the occurrences -this —thisincludes includesareas areaswithin withinthe theoriginal originalhalo. halo. This image are devoid of any PGM occurrences fluid event that that removed PGMs and altered the minerals, which it suggests a second alteration fluid passed through. Dashed line represents the extent of halo and An-enrichment in the adjacent plagioclase grains. grains. White stars represent PGM POM occurrences. occurrences. Cpx=Clinopyroxene, plagioclase Opx=Orthopyroxene, Bi=Biotite, Bi=Biotite, Chl=Chlorite, Chl=Chlorite, Cpy=Chalcopyrite, Plag=Plagioclase Plag=Plagioclase 50 �EMP MONAZITE RESULTS OF EMP MONAZITEGEOCHRONOLOGY GEOCHRONOLOGYIN IN B-C E-C MINNESOTA: MINNESOTA: EVIDENCE EVIDENCE FOR LARGE-SCALE GEON 17 17 METAMORPHISM ASSOCIATED WITH POSTPLUTONISM TECTONIC PLUTOMSM MCKENZIE, M.A., and HOLM, D.K., both at Dept. of Geology, Kent State University, Kent, OH; SCHNEIDER, D.A., Dept. of Geological Sciences, Ohio University, Athens, OH; OH, SCHNEIDER, JERCII4OVIC, M., M., Dept. of Geosciences, Geosciences, University of Massachusetts, Massachusetts, Amherst, JERCINOVIC, Amherst, MA MA Determination of the Determination the timing timing and and extent extent of of poly-phase metamorphism metamorphism is is essential essential in in unraveling the the tectonic tectonic history history of of a region. region. The The pattern and degree of metamorphism preserved across the Penokean orogenic belt in the southern Lake Superior region is highly variable. dates fromeast-central east-centralMinnesota Minnesotaindicate indicatewidespread widespread cooling cooling at -1760 0~r/3Q dates ~ r from Abundant 440Ar/39Ar 1760 Ma shortly after the emplacement emplacementof ofthe theeast-central east-centralMinnesota Minnesotabatholith batholith(ECMIB) (ECMB)atat—1775 -1775 Ma Ma (Hoim (Holm et al., 1998; in review). Yet YetU-Pb U-PbSHRIIvIP SHRIMP monazite ages from three localities localities across the northern MI, region (e-c MN, northern MI,and andnorthern northernWI) WI)record recordonly onlyaauniform uniform—1830 -1830 Ma metamorphic metamorphic episode and episode and aa secondary secondaryyounger younger—1800 -1800 Ma thermal pulse linked linkedto toaarecently recentlyidentified identified—1800 -1800 Ma magmatic event event (Schneider et al., in in review). review). This study utilizes the total Pb electron (EMP) monazite age dating technique to better constrain the extent of thermal thermal microprobe (EMP) overprinting surrounding the batholith. overprinting surrounding batholith. in situ situ metamorphic metamorphic monazite monazite ages ages from from three three For this study we obtained obtained in Paleoproterozoic metasedimentary garnet-staurolite schist samples and one garnet-cordierite garnet-cordierite from the the plutonic plutonic zone of of east-central east-centralMinnesota Minnesota (Figure (Figure 1). 1). Schist gneiss sample from Schist sample sample AMpredominantly elongate monazite grains displaying a mottled chemical variation in 016 contains predominantly Th content. content. This ± 33 Ma from 79 79 spots spots on on seven seven Y and Th This sample sample yielded yielded aa mean age of 1746 1746  Maand and 1760 1760Ma. Ma. A third grains. Two Two age age domains domainswere were recognized recognizedatat—1738 -1738 Ma third less less prominent —1780Ma Maage agedomain domainwas wasobtained obtainedon onsome somehigh highYYregions. regions. Schist Schist sample -1780 sample MN-29 MN-29 contains contains sub-euhedral monazite monazite displaying prominent prominent regions regions of of high highTh Th content. content. This This sample sample yielded a 10 Ma from 92 spots 1764±Â 10 spots on five grains. A A single singleprominent prominent age agedomain domain was was mean age of 1764 recognized atat— 1772 1772 Ma. Schist Schistsample sampleP-16 P-16contains containsmonazite monazitewith with very very irregular irregular grain grain recognized numerous inclusions, inclusions, and variable Th Th content. content. This sample sample yielded yielded aa mean age age of boundaries, numerous grains. Two 1772 1772±Â 33 Ma from 92 spots on seven grains. Two age domains are identified: aaprominent prominent age age domain at —1770 Maand andaasmaller smaller population population age domain at —1800 Ma. Lastly, the Sartell -1770 Ma -1800 Ma. Sartell euhedral monazite gneiss, sample S-2, contains euhedral monazite grains displaying displaying distinctive distinctive core/rim corelrim textures. textures.  3 Ma from 102 spots on seven grains. Three Three age age This sample yielded a mean age of 1756 1756 ± domains are identified: two 1750 Ma and 1770 1770Ma Ma and and aa third third less less twoprominent prominentdomains domainsatat— 1750 Ma on high U cores. cores. prominent domain domain at at —1800 -1800 Ma Mathermal thermalimprint imprint associated associated with intrusion reveal aa profound profound —1770 -1770 Ma Our EMP results reveal of the 1775 Ma Ma ECMB. ECMB. The The considerable considerable distance distance of of some some of these these samples samplesfrom from the the western western edge of of the exposed exposed batholith (30-40 km) and the absence of Penokean metamorphic ages absence metamorphic ages suggests that that the the thermal thermal pulse pulse must have been been dramatic. dramatic. However, KHowever, the the garnet-schist garnet-schist sample sample KR (east of Mille Lacs) that records only geon 18 18 SHRIMP ages lies north of the the region of of thermal thermal of the the batholith. batholith. We influence of We note note that that sample sample K-R K-R is is located located just north north of of the the Malmo MaimoStructural Structural Discontinuity (MSD) (MSD) and and sample sample AM-016 is is located located south south of of it. it. Our Discontinuity Our data data reveal reveal that that the the MSD MSD juxtaposes rocks of different metamorphic metamorphic age (geon 18 18 metamorphism to the north from from geon - - 51 �ba' C) CD . '° <o ,__._ toaaa C') Epp CD .-,. 0 •L rQ5o o ECDCD -t r> to u — — " Un J•fl1 tr4 0 t nfl ni C)g <'o— rM -t. flH hU >E;•<'z g :h UH 17 metamorphism to the south). We propose, therefore, that the MSD is a geon 17 structure that exhumed the plutonic terrane of east-central Minnesota. West of Mille Lacs, a significant portion of the MSD juxtaposes post-Penokean plutons to the south against older metamorphic rocks to the north. This clearly supports our interpretation that this structure was active well after Penokean orogenesis. z __c_ C C S S a a I Frequncy P-16 Composite - AllSamples All Ages Histogram F) 01 52 CD 0 0, 'C to 0 I Figure 1: Histograms of BMP Th-U-total Pb in situ monazite spot ages. . ¶. a 0 ) .:po S. H 0 a Frequency 5 5 Frequency a —— I, •1 0 C 0, a C, 0 3 <'c C, .. C) MN-29 Conposte AM-016 Composite t o C,, pp —— Holm, D.K., Dan-ah, K., andLux, D., 1998, American Journal of Science, 298,60-81. Holm. D.K., Van Schmus, W.R., MacNeill, L., Boerboom, T., Schweitzer, D., and Schneider, D.A., in review, Geological Society of America Bulletin. Schneider, D.A., Holm, D.K., O'Boyle, C.,Hamilton, M., Jercinovic, M., in review, Geological Society of America Special Volume "Gneiss Domes and Orogeny." �THE SIOUX QUARTZITE REVISITED: SEDIMENTOLOGY, METAMORPHISM, GEOCHEMISTRY AND THE ORIGIN OF PIPESTONE GEOCHEMISTRY MEDARIS, L.G., L.G., Jr., Jr., and DOTT, R.H., Jr., Dept. of Geology & Geophysics, University University of Wisconsin-Madison, Madison, medaris@geology.wisc.edu; Wisconsin-Madison, Madison,WI, WI,53706; 53706; medarisgeology.wisc.edu;rdott@geology.wisc.edu rdottgeoIogy.wisc.edu Red, supermature supermature quartzites of the Baraboo Interval were deposited after 1.75 1.75 Ga on a stable craton craton in in the presence of free atmospheric oxygen under under conditions conditions of of intense intense chemical chemical weathering. weathering. Some atmospheric oxygen Some quartzites quartzites (Baraboo and and Flambeau) Flambeau) were were folded folded and and recrystallized recrystallizedatat 1.63 1.63 Ga Ga (HoIm ci aT, al., 1998), 1998), and and many many (Baraboo (Holm et quartzites were hydrothemally hydrothermallyaltered alteredatat1.46 1.46 Ga Ga (Medaris (Medaris etci al., 2002, quartzites were 2002, in in press), press), presumably presumably in in response promoted by by continental continental scale scale A-type A-type magmatism. magmatism. These discoveries response to brine migration migration promoted discoveries have prompted us to reevaluate the sedimentology, metamorphism, and geochemistry of the Sioux Quartzite. Sedimentolozy The Sedimentolo.eg TheSioux SiouxQuartzite, Quartzite, which which isisseveral several hundred hundred meters meters thick, thick, is is composed composed mostly mostly of quartz sandstone with interstratified ci al., 1986). 1986). Heterogeneous Heterogeneous interstratified lenses of red mudstone (Southwick et cobble conglomerate occurs at the base and finer pebbly layers are scattered throughout the lower half half or so, whereas mudstones Sedimentary structures in the sandstones mudstones occur chiefly chiefly within the upper upper half. half. Sedimentary sandstones include predominant - 15 15 cm in in thickness, thickness, rare rare predominant festoon-style, festoon-style, nested trough cross cross beds beds averaging averaging 10 10— zones sets, a few examples zones of planar-tabular planar-tabular sets, examples of herring herring bone bone cross cross bedding, bedding, and and both both asymmetric asymmetric and and symmetric ripple marks. The mudstones are mostly massive, massive, but but parallelparallel- laminated laminated and andripple-laminated ripple-laminated varieties are also also present. present. In In most most cases, cases, quartz quartz silt silt and and fine fine sand in a finer varieties are sand grains grains are disseminated disseminated in rare graded graded laminations laminations are arealso alsopresent. present. Some mudstones mudstones show polygonal polygonal 'mud' 'mud' cracks, matrix, but rare cracks, and and the overlying sandstones commonly contain intraclasts ripped up from such cracked beds Interpretations of the Sioux depositional environment include shallow marine and braided fluvial Interpretations (Doff, 1983; 1986). In In the the latter scenario, the cross bedded et a!., a!., 1986). bedded sandstones sandstones represent represent river river (Dott, 1983; Southwick Southwick ci' channel deposits, and the mudstones, channel deposits, mudstones, slack water deposits in in ponds ponds between between active active channels. channels. However, However, this interpretation is inconsistent with the the rarity of scoured channel bases bases and and tabular sets of planar cross inconsistent with laminations, which would would have have formed formed by by laterally laterally migrating migrating bars, bars, and and the existence of wave laminations, which wave ripples, ripples, which expected in in the the sands sands of of aa braid braid plain. plain. Ojakangas (1984) suggested which are not expected Ojakangas and Weber (1984) suggested that that the the one-third of the Sioux formation formation was was deposited deposited in a shoreline marine setting with tidal influences, upper one-third accounting for the polygonal desiccation desiccation cracks, cracks, and the herringbone herringbone cross bedding, wave ripples, polygonal and thickness and extent of certain certain mudstone mudstone layers layers (now (now pipestone). pipestone). Interpretation Interpretation of of the the Sioux Sioux as asaa fluvial-to-marine fluvial-to-marine transgressive succession successionwould wouldconform conformtoto the present transgressive present interpretation interpretation of of the thecorrelative correlativeBaraboo Baraboo Quartzite, which has wave ripples ripples and and reactivation reactivation surfaces surfaces in in its upper upper half half (Medaris (Medaris et et al., al., in press). Metamorphism Metamorvhism Mineral Mineralassemblages assemblages in in finefine5 grained grained Sioux sedimentary rocks can be 5 expressed in the expressed in the system, system, KASH, KASH, as as portrayed portrayed in in a:4 0:4 Figure 1, where rock compositions are projected compositions projected onto the anhydrous onto anhydrous plane, plane, K-Al-Si, K-Al-Si, and and two two 3 critical dehydration reactions reactions are plotted. Additional phases include abundant hematite and Additional 22 a Ti02 TiO; phase, phase, either either anatase anatase in in the theCottonwood Cottonwood 1989), or rutile in Basin (CB) Basin (CB) (Stelz, (Stelz, 1989), in the Pipestone Basin (PB). The stable of 11 Pipestone (PB). The stable existence existence of the CB kaolinite in the CB (Stelz, (Stelz, 1989) 1989) requires temperatures below -300°C -100°C, whereas temoeratures whereas pyronvro250 350 T, 300 phyllite in in the the PB PB is is stable stable above above-300°C -100°C. The T, 0C OC phyilite 250 300 53 �quartz-pyrophyllite assemblageininthe the PB PB (0, (0, Fig. quartz-pyrophyllite assemblage Fig. 1), I), ininwhich which vermicular vermicular kaolinite kaolinite has has been been replaced replacedby bypyrophyllite pyrophyllite(Fig. (Fig.2A), 2A), most likely likely represents represents higher highertemperature, temperature, largely largelyisochemical isochemical most recrystallization of ofaaquartz-kaolinite quartz-kaolinite protolith, protolith, like likethat thatininthe theCB CB recrystallization The occurrence occurrence of muscovite muscovite in both both basins basins isis (0, Fig. Fig. 1). 1). The (C>, attributed to to K-metasomatism K-metasomatism related related to to 1.46 1.46Ga Gahydrothermal hydrothermal attributed (+,Fig. Fig.1)1)isisaametasomatic metasomaticrock rock composed composed of of activity. Pipestone Pipestone(+, activity. pyrophyllite, muscovite, diaspore, diaspore, hematite, hematite, and and rutile, rutile, ininwhich which pyrophyllite, former quartz quartz grains grains have have been been completely completely replaced replaced by bydiaspore, diaspore, former Because the Sioux 2B). Because the Sioux pyrophyllite, and muscovite (Fig. pyrophyllite, muscovite (Fig. 2B). Quartzite Quartzite is is largely largely undeformed undeformed and and lies lies north north of of the the extrapolated extrapolated trend et al., al.,1998), 1998), trend of of the the 1.63 1.63 Ga GaMazatzal Mazatzal tectonic tectonic front front (HoIm (Holm et we we suggest suggest that that all all metamorphic metamorphic features features of of the theSioux SiouxQuartzite Quartzite are are due due to to 1.46 1.46Ga Gahydrothermal hydrothermal activity, activity, rather rather than than aaMazatzal Mazatzal event. event. Geochemistry Geochemistq Where Whereunmodified unmodifiedby byK-metasomatism, K-metasomatism, finefinegrained grained sedimentary sedimentary rocks rocks of of the theBaraboo BarabooInterval Intewalare areremarkably remarkably mature, mature, being being practically devoid devoid of of K, K, Na, Na, Ca, Ca, Mg, Mg, and and Mn Mn (Fig. (Fig. 3), values of 97 to 99. 31, and and having Critical Index of Alteration Alteration values 99.InIn such 3) and and such rocks rocks the the wide wide range range ininproportion proportion of of Si Si to to Al A1(Fig. (Fig. 3) of quartz quartz to to kaolinite kaolinite in inthe the I), reflects the original original proportion proportion of quartz to to kaolinite, kaolinite, or orpyrophyllite pyrophyllite(Fig. (Fig. 1), quartz protolith sediments. protolith sediments. has stabilized stabilized K-metasomatism has muscovite muscovite in both the CB and and PB, PB, but but the muscovite-bearing muscovite-bearing rocks in the the CB CB 0 150 2 record aa lower lower temperature temperature and and higher higher record C ratio ratio of of Si/Al SiIAl compared compared to to pipestone pipestone in in 1uJ (Fig. 1). 1). The Theclassic classicpipestone, pipestone, the PB (Fig. o in addition addition to to substantial substantial KK contents, contents, in contains contains lower lower Si and and higher higher Al A1than than pyrophyllite in associated associated quartz quartz ++pyrophyllite that in samples (Figs. (Figs. 1 & 3). Assuming Assuming Zr Zr to to samples be an an immobile immobile element, element, isocon isocon be calculations indicate indicate that that the the mean mean calculations -50 pipestone pipestone composition composition was produced by by 65% Si02, S O 2 ,45 45 to to 55% 55% removal of 20 to 65% -100 Al Si Mn Fe 11 K Na Ca Mg Ti02, Ti02,35 35 to to 65% 65% Fe203, Fe203, and addition of 15 to 45% 45% Al203 A1203and and—800% -800% K20, K20, 15 pyrophyllite samples. samples. compared to the average average compositions compositions of of the two Si-rich Si-rich and and two two Al-rich Al-rich quartz quartz ++pyrophyllite compared The composition of of one one pipestone pipestone sample sample(*, (*, Fig. Fig. 1) 1) requires requires removal removal of of 68% 68% Si02 Si02 and and The reconstructed reconstructed composition addition addition of of50% 50%A1203 A1203during during metasomatism. metasomatism. Further investigation investigation is underway to provide a more more detailed detailed evaluation evaluation of of brine brinecompositions compositions and and metasomatic processes involved in this important, regional scale, 1.46 Ga hydrothermal event. metasomatic 1.46 Ga hydrothemal event. References Geol. Amer. References Dott, Dott,RH. R.H.Jr.Jr.(1983) (1983) Geol.Soc. SOC. h e r .Memoir Memoir160, 160,129-141; 129-141;HoIm, Holm,U. D. etetatal.(1998) (1998)Geology, Geology,v.v.26, 26, 907-910; Medaris, L.G., Jr. et at (2002) 48th Inst. Lake Superior Geol., 24-25; Medaris. L.G., Jr. eta!. L.G., Jr. et 01.(in (inpress) press) 907-910; Medaris, L.G., Jr. et aL (2002) 48th Inst. Lake Superior Geol., Jour. Ojakangas, R.W. Geol. Jour. Geol.; Ojakangas, RW. & & Weber, Weber,R.W. R.W.(1984) (1984)Minn. Mi. Geol. Sun'., S w . ,Rept. Rept.mv. Inv.32, 32,1-15; 1-15;Soutkwick, Southwick,D.L. D.L. etet at Amer. al.(1986) (1986)Geol. Geol,Soc. SOC. Amer.Bull., Bull.,v.v.97, 97,1432-1441; 1432-1441;Stelz, Stelz,D.E. D.E.(1989) (1989)M.S. MS.Thesis, Thesis,Wichita WichitaState StateUniv., Univ.,140 I40pp. pp. 850 54 �_____________ A geochemical investigation investigation of of Mesoarchean Mesoarchean metavoIcanic metavolcanic and and metasedirnentary metasedimentary rocks from from the the Rirch-Uchi Birch-Uchi greenstone greenstone belt Metsaranta, R., Hollings, P.P(Department of of Geology, Lakehead Metsaranta, R.,Fralick, Fralick,P.P.and and Hollings, . (Department Geology, LakeheadUniversity, Universio,Thunder ThunderBay Bay ON CAN, CAN, P7B 5 SE)) EI) Most Mesoarchean greenstonebelts belts in in the Western Most Mesoarchean grtGlfibL"fifie Western Superior Province are comprised comprised primarily of komatiite-tholeiite komatiite-tholeiite sequences and associated associated sedimentary sedimentary rocks (Thurston (Thurston and Chivers 1990). 1990). These These—2.9-3.0 -2.9-3.0 Ga Ga assemblages assemblages have have been been interpreted interpretedto to represent represent plume generated volcanism in oceanic plateau settings (for example, Hollings et al. al. 1999, 1999, Tomlinson et aL1999). a1.1999). This study is aa preliminary preliminary investigation investigation of metavolcanic metavolcanic and metasedimentary strata strata from from the Mesoarchean metasedimentary Mesoarchean Balmer assemblage assemblage of the the Birch-Uchi Birch-Uchi Rogers et et al. greenstone belt. belt. Rogers al. (2000) (2000) have have suggested suggested that, that, given given their theirgeochemical geochemical affinities and Nd isotopic evidence for contamination by older crust, volcanic rocks of the may represent represent aa continental continentalarc arcsetting. setting. This implies that the Balmer Balmer assemblage may Assemblage may represent a distinct tectonic setting from those proposed Assemblage may represent a distinct tectonic setting from proposed for other other B Mesoarchean rocks in the the Superior Superior Province. F'rovince. Sediment geochemistry geochemistry and and depositional depositional environment studies studies along along ¶C — / :t ...,/ with igneous igneous geochemistry will will be ..-:?;_ applied to provide further further constraint constraint on on applied / -, - — the possible tectonic setting of these the possible tectonic setting these — rocks. rocks. — r* '— •M ç' The The Birch-Uchi Birch-Uchi greenstone greenstone belt is located in the central located in central portion portion of of the the W It is Uchi (Fig.1). is Uchi Subprovince Subprovince (Fig.!). comprised comprised of three three volcanic units termed termed the Balmer, Balmer, Nanow Narrow Lake Lake and and spanning Woman assemblages, Woman assemblages, spanning approximately approximately 250 Ma. Ma. The Balmer Balmer assemblage is the assemblage the oldest oldest of these these volcanic units and volcanic units and has has U-Pb U-Pb zircon zircon ages from felsic volcanic horizons that suggest an an age of ca. suggest ca. 2975-2989 2975-2989 Ma Ma (Rogers et al., 2000). The stratigraphy (Rogers et al., The stratigraphy of the the Balmer Balmer assemblage assemblage is is divided divided into four four suites: suites: aalower lowersedimentary sedimentary sequence, aa mafic sequence, mafic volcanic volcanic suite and and petrographically distinct distinct felsic two petrographically felsic volcanic suites suites (Rogers volcanic (Rogers et et a1. al. 2000). 2000). Samples collected for for this study Samples collected study are are located in the the southern southern portion portion of of the the located 1- Location Location and generalized generalized geology geology of of study study Figure 1area and Birch-Uchi Greenstone belt (modified from from assemblage in the Woman Woman Balmer assemblage Stott and Corfu stott Cofi 1991) 1991) River/Bear RiverBear Lake area. These These comprise comprise 16 samples from the lower sedimentary sequence and 34 samples of the mafic mafic volcanic volcanic suite (2000). suite of Rogers Rogersetetal. a1.(2000). , J: - - —— L 144)t,Iun, Lake 55 �Field observations suggest that the the Balmer Balmer assemblage assemblage sedimentary rocks are turbiditic. turbiditic. Sediment geochemistry geochemistrywill will be be applied to constrain the source rocks compositions for Sediment constrain the these sediments. As As no no contact contact with with underlying underlying older rocks has been identified this might provide valuable about the preexisting provide valuable information information about preexisting older older crust. crust. Alternatively, the the may be be derived derived from the Balmer assemblage volcanics volcanics and and this this could support a sediments may hypothesis that that the the Balmer assemblage represents aa continental continental arc arc setting with hypothesis assemblage represents with the the sediments deposited in a fore-arc trench. trench. Volcanic rock samples samples appear The first first is aa Volcanic rock appear to fall fall into intotwo twocompositional compositional trends. trends. The tholeiitic trend trend comprised comprised of of primarily primarily tholeiitic tholeiiticbasalts basaltsand andandesites. andesites. The second tholeiitic second is is aa calc-alkaline trend trend of of andesitic to rhyodacitic The geochemistry of these calc-alkaline rhyodacitic compostion. compostion. The geochemistry of samples will be be applied setting for for these rocks samples will applied to suggest suggest a possible possible tectonic tectonic setting rocks and and implications of this in relation to to other other Mesoarchean Mesoarchean terranes. terranes. 400 C = C C * 300 Rhyolite Tr ac 0.1 N C Rhyodacite/ a >. 200 rachyA Andesite 0.01 N • I SIIJNP1 AndeslQ asa ..I 100 AIk.Bas SubAlkaline Basa t .001 .01 0.1 1 10 0 200 100 300 Zr ZT Nb/Y N bN Lithology Figure 3- Lithology Discrimination diagram for Assemblage Balmer Assemblage volcanics. Circles volcanics. Circles are q w e s are Tholeiitic trend ssquares caic-alkaline trend. calc-alkaline Figure 3-A plot of V vs Zr showing compostional compostional groups groups in Balmer Assemblage Assemblage volcanics. volca~cs. Circles are Tholeiitic trend squares are calc-alkaline calc-alkaline trend. trend. References: References: Hollings, P., Wyman, Wyman, D. D. and and Kerrich, Kerrich, R. R. 1999. Komathte-basalt-rhyolite Hollings. P,, Komatiite-basalt-rhyolite associations northern Superior Province greenstone belts: belts: significance of plume-arc volcanic associations interaction in the the generation generationof ofthe theproto protocontinental continentalSuperior SuperiorProvince. Province.Lithos Lithos 46: 137-162. interaction in 137-162. Rogers, N., McNicoll, V., V., van van Stall, C.R., C.R., and and Todinson, Tomlinson, K.Y. K.Y.2000. 2000. Lithogeochemical Lithogeochemical in the the Uchi-Confederation lJchi-Confederation greenstone greenstone belt, northwestern O Ontario: implications for studies in n h o : implications for Archean Archean Tectonics. Geological 16: 1lip. lp. GeologicalSurvey Surveyof of Canada, Canada,Current Current Research 2000-C 2000-C16: G.M., and andCorfu, Corfu,F. F. 1991. 1991. Uchi Subprovince. Tn: of Ontario, Ontario, special Stott, G.M., In: Geology of volume 4, 4, part part 1. Ontario OntarioGeological GeologicalSurvey, Sumey,pp pp 145-238. 145-238. P.C. and and Chivers, Chivers,K.M. K.M. 1990. 1990. Secular Thurston, P.C. Secular variations in greenstone sequence development emphasizing Superior Superior Province, Province,Canada. Canada. Precambrian Research. 46: 21-58 emphasizing Tomlinson, K.Y., K.Y.,Hughes, Hughes,D.J., D.J., Thurston, Thurston,PP.C., andHall, Hall,R.P. R.P. 1999. 1999. Plume Todinson, C , and magmatism and and cmstal crustal growth at 2.9 2.9 to 3.0 Gain Ga in the Steeprock and Lumby Lake Lake area, area, Western Western Superior Province. Province. Lithos 46: Superior 46: 103-136. 103-136, 56 �PETROLOGYAN1I ANJl PGE POTENTIAL GREENWOODLAKE LA= INTRUSION, INTRUSION, PETROLOGY POTENTIAL OF THE THE GREENWOOD CENTRAL CENTFULDULUTH DULUTH COMPLEX, COMPLEX,LAKE LAKECOUNTY, COUNTY,MINNESOTA MINNFSOTA MILLER, MILLER,James, James,D., D., Jr., Jr., Minnesota MinnesotaGeological GeologicalSurvey, Survey,mille066@tc.umn.edu milleO66@tc.umn.edu This This report report summarizes summarizes the results results of of aa petrologic petrologic and and metallogenic metallogenic study study of of drill drill core coreand and outcrop outcrop samples samples that that profile profile the the Greenwood Greenwood Lake Lake intrusion intrusion (GLI) (GLI) of of the the central centralDuluth Duluth Complex Complex (Fig. 1). The Thelittle littlethat thatwas wasknown knownabout aboutthis thisvery very poorly poorly exposed exposedlayered layeredmafic maficintrusion intrusionprior priorto to (Fig. 1). this this study study came came from from interpretation interpretation of of its itsaeromagnetic aeromagneticsignature, signature,seven sevendrill drillcores, cores,and andsparse, sparse, localized localized outcrop. outcrop. The Thepurpose purposeofofthis thisstudy studywas wastotoestablish establishthe theigneous igneousstratigraphy stratigraphyofofthe theGLI GLI and and to to evaluate evaluate its itspotential potentialfor forPGE PGEreef reefmineralization. mineralization. The The GLI GLI isis an anapproximately approximately two two kilometer-thick, kilometer-thick, sheet-like sheet-like intrusion intrusion that that dips dips gently (approximately (approximately10°) 10")to to the east east and covers an area of about in length) about 300 square square kilometers. For For this this study, study, 19 19bedrock bedrock drill drill cores cores (20 (20 to to 80' 8O'in length) were were acquired acquired in inearly early2002 2002along alongthe thewest—northwest-trending west-northwest-trending Erie/LTV ErieLTV railroad railroad and andpowerline powerline west west of of Lake Lake County County Highway Highway 22(Fig. (Fig. 1). I). Samples from these these cores cores and andfrom fromintermittent intermittent outcrops outcrops along the eastern eastern extent extent of of the the railroad railroad grade grade were weresubjected subjectedtotopetrographic petrographicstudy studyinin transmitted transmitted and reflected light, light, microprobe microprobe analyses of olivine olivine and and pyroxene pyroxene composition, composition, and and whole whole rock rock analyses analyses of of their theirlithogeochemistry, lithogeochemistry, including including platinum, platinum, palladium, palladium, and and gold gold concentrations. concentrations. The The results results of of the the drilling drilling and and petrographic petrographic study study show show that that the the igneous igneousstratigraphy stratigraphy of of the the GLI GLI can can be be grossly grossly subdivided subdivided into into aa lower lower troctolitic troctolitic zone zone(GLtr, (GLtr,0-650 0-650meters), meters),composed composed mostly mostly of of leucotroctolitic leucotroctoliticcumulates, cumulates,aa medial medial gabbroic gabbroiczone zone(GLog, (GLog,650-1800 650-1800meters), meters),composed composed of (GLfg,1800-2130 1800-2130meters), meters), of olivine olivine oxide oxide gabbro gabbro cumulates, cumulates,and and an an upper upper ferrogabbroic ferrogabbroiczone zone(GLfg, composed largely of magnetite magnetite gabbro gabbro (Fig. (Fig. 2). 2). The troctolitic troctolitic zone zone contains contains abundant, abundant, large large anorthositic and oxide gabbroic gabbroic inclusions, inclusions, presumably derived derived from from anorthositic anorthositic series seriescountry country rock. Although Althoughthe theGLI GLIisisaawell-differentiated well-differentiated intrusion intmsion that that formed formedas asan anopen openmagma magma system, system, microprobe data show that cryptic layering trends (such as Fo in in olivine, olivine,Fig. Fig. 2) 2) are are inconsistent inconsistent with formation by in situ crystallization crystallization differentiation. differentiation. This This and and other other evidence evidence (such (such as abrupt abmpt changes in lithology, lithology, leucocratic leucocratic compositions of troctolitic troctolitic rocks, rocks, and and suspect suspectcumulus cumulustextures textures of troctolitic troctolitic rocks) suggest suggest that that the the differentiated differentiated character character of of the theGLI GLIwas wasprobably probablyinherited inherited from from aa deeper deeper crustal crustal magma chamber, chamber, which which was itself itself undergoing undergoingopen opensystem systemdifferentiation. differentiation. The chemostratigraphy chemostratigraphy of chalcophile chalcopbile elements elements through through the the GLI GLI are are difficult difficultto tointerpret interpretinin such such a complex open magma system, but suggest that some potential for PGE reef mineralization may occur in the lower part of the gabbroic zone (Fig. 2). Below Belowthis thislevel, level,recharging rechargingmagmas magmas appear to have been undersaturated in sulfide, and copper copper and and sulfur sulfurconcentrations concentrationshigher higherin inthe the pabbroic zone (above 800 meters) indicate An unexpected unexpected result of this gabbroic indicate intermittent intermittent saturation. saturation. An study was the discovery of aa large, large, sulfide-bearing sulfide-bearing oxide oxide gabbro gabbroinclusion inclusionwithin withinthe thetroctolitic troctolitic zone. Aeromagnetic Aeromagnetic data data suggest suggest that this inclusion is aa conformable conformable tabular mass with a strike length of about 8 kilometers. The magnetic data further suggest The magnetic further suggest that that similar similar rock rock types types form form part of the footwall to the GLI, GLI. The The possibility possibility of of sulfur sulfur contamination contamination in in the the contact contact aureole aureole around this inclusion and along the base of the intrusion intmsion warrants further exploration of these areas for contact-type contact-typeCu-Ni-PGE Cu-Ni-PGE sulfide sulfidemineralization. mineralization. ., Funding for this project was provided to the Minnesota Geological Geological Survey Survey by aa grant grant from from the Minnesota State Legislature Legislature on the recommendation of the Minerals Minerals Coordinating CoordinatingCommittee. Committee. 57 �Generalized Generalized geology geology of of the the Greenwood GreenwoodLake Lake intrusion intrusionand and the the central ceneal Duluth Duluth Complex. Small Small dots denote denote drill d i l l hole hole and and Complex. diamonds denote denote outcrop outcrop locations locations along along diamonds ErieLTV railroad railroad tracks. tracks. Long Long dashed dashed lines lines Erie/LTV denote denote faults. faults.Intrusive Intrusiveunits unitsare: are: Figure 1. 1. Figure GLtr—-GLI GLtr-GLl troctolitic troctoliticzone zone GIog-GLIgabbroic gabbroiczone zone GIog—GLI GLtg—GLI GLfpGLlferrogabbroie ferrogabbroiczone zone MW—Mt. MW-Mt. Weber Weber granophyre granophyre CLLS—Cloquet CLLS-Cloquet Lake Lakelayered layeredseries series BEI-Bald Eagle Eagle intrusion intrusion BEI—Bald SKI—South SKI-South Kawishiwi Kawishiwi intrusion intrusion PAl—Partridge PRI-Partridge River Riverintrusion intrusion WMI—Western WMI-Western Margin Margin intrusion intrusion sew Layered Series Ferrogabbroic Femgatbr& Gabbroic Gabbmic S T-IIrc L____J Trociolluc Fetsc Series ri Morihositic l___J Series Norih Shore a Group Virginia Forniston Biwabik ranFormation Giants Range I,-: Granite o0 10 10 20 20 Kilometers Kilometers Sample locationo I I Au concentrations of Fo Foin in olivine olivine and and of of Cu, Cu, Pt + Figure 2. 2. Chemostratigraphy Chemostratigraphy of + Pd, Pd, and and Au wncentrations through thmugh the the Figure locations of of drill drill core Lake intrusion. intmsion. Stratigraphic Stratigraphic locations core (boxes) (boxes) and and outcrop outcrop (diamonds) (diamonds) Greenwood Lake samples and general are shown shown in in the left columns. g e n e d lithostratigraphy lithostratigraphy are columns. Large Largeinclusions inclusionsofofanorthositic anorthositic (ox gb) gb) are denoted. Abrupt series rocks (AS) and oxide gabbro (ox Abmpt increases increasesin inCu/Pd CuPd(arrows) (mows)may maymark mark sulfide saturation events. The The zone found most favorable favorable to to host host PGE PGE reef reef mineralization mineralizationis is identified. identified. 58 �Stratigraphy and the St. St. Croix Croix horst, horst, northwestern northwestern Stratigraphy andstructure structureof ofKeweenawan Keweenawan rocks of the Wisconsin U.S.Geological Survey, Sun'ey, Reston, VA S.W. Nicholson, and W.F.Cannon, U.S.Geologica1 The St. St. Croix Croix horst horst is is the the partially inverted inverted central graben graben of the the Midcontinent Rift System System (MIRS)that thatextends extendssouthwestward southwestwardfrom from western westernLake Lake Superior. Superior. It It is is bounded by the (MRS) Douglas fault on the the northwest northwest and and the Atkins Lake fault on the the southeast. southeast. Both Both are arenow now reverse faults, but may have been graben-bounding normal faults during rifting and and northern limit of of the the horst horst is is White's White's Ridge, a subsurface basement high volcanism. The northern evident in both seismic seismic and and gravity gravity data, data, which did not subside subside substantially substantiallyduring duringrifting rifting and against which rift volcanic and sedimentary rocks pinch out or become much much thinner. thinner. Lake Superior from the St. Croix White's Ridge Ridge effectively effectively separates separates the MRS in western Lake horst and the volcanic, sedimentary, sedimentary, and structural history of the two rift segments segments differ differ in several aspects. High-resolution aeromagnetic, gravity, and seismic data permit the the tracing of flow sequences for long distances and to great depth. This geometry geometry combined combined with with the the chemistry of the volcanic rocks allows us us to to decipher decipher aa volcanic volcanic stratigraphy stratigraphy in in spite spite of of widespread cover by glacial deposits and Paleozoic sedimentary rocks (Cannon (Cannon et et al., al., 2001). interpretations (Chandler Our interpretation, aided by previous gravity and seismic interpretations (Chandleret et al., al., is that that the original structure of of the St. Croix horst was an asymmetric graben, or 1989), is possibly a half graben, like those of the Lake Superior portion of the MRS. MRS. The The Lake Lake Owen Owen fault was a major growth fault fault on the southeast southeast side side of the the graben graben and and the the volcanic volcanicfill fill fault, The Douglas fault on the thickened toward and terminated against the fault. the northwest northwest side side of of the horst is not clearly a growth feature and may be simply a thrust formed formed during during rift rift inversion. Thrust displacement on the Douglas Douglas fault fault must must be be 20 km or more because it juxtaposes of the base of a thick volcanic sequence sequence over the younger younger Bayfield Bayfield Group. Group. Cannon et al. (2001) (2001) and Nicholson et al. (2001) used chemical chemical and and aeromagnetic aeromagneticdata datato to define the Minong Volcanics, the underlying Clam Clam Falls Falls Volcanics, and the Chengwatana Volcanics as three formations making up the graben-filling graben-filling volcanic volcanic sequence. sequence.The Thethree three have similar chemistry, chemistry, but were defined defined by structure structure and and geochronology. geochronology.The Thethree-part three-part division no longer seems justified justified and the Chengwatana and Clam Falls Volcanics are Volcanics, as earlier earlier defined, were restricted restricted combined into a single unit. The Chengwatana Volcanics, to a fault-bounded belt belt between between the theDouglas Douglasand andPine Pinefaults faultsand andtheir theirstratigra'phic stratigrahic not known known directly. directly. We now believe, based on seismic relationships to other volcanics were not data, that the Pine fault does not extend into the the northern northern part of of the horst, where the previously defined Chengwatana and Clam Falls units units appear to be a continuous depositional sequence of compositionally indistinguishable flows that we propose be called entirely Chengwatana. basalts about Chengwatana. The Minong Volcanics, a sequence sequence of low-Ti02 low-TiOibasalts about 33km kmthick, thick, overlie the Chengwatana, Chengwatana, along an apparent low angle angle disconformity disconforrnitybased based on onaeromagnetic aeromagnetic form lines. These form lines also show a disconfonnity disconformity within the Minong volcanics on the of the the Ashland Ashland syncline. syncline. A A lower lower unit, unit, not not present present on the northwest northwest limb, is southeast limb of mostly high-Ti02 high-Ti02 basalt. basalt. Based Based on on the the presence presence of of abundant abundanthigh-TiO2 high-Ti02 basalts basaltsand andmore more magmatic center center was was active in this area sometime evolved rocks, we infer that a localized magmatic flow in in the the upper upper part of of this sequence. A second, but before 1095 1095 Ma, the age of a rhyolite flow may have have existed existed on on the western margin of the graben near apparently older, volcanic center may 59 �_____ the Amnicon Amnicon Complex Complex where the Chengwatana Volcanics are mostly high-Ti02 high-Ti02basalts, basalts, andesites and rhyolites. rhyolites. Clastic sedimentary sedimentary rocks of the Oronto Group overlie the volcanic rocks. Only the basal unit, the Copper Copper Harbor Conglomerate, Conglomerate, is preserved in most of the St. St. Croix horst where as as much as 2 km of sandstone and conglomerate lie along the axis of the Ashland syncline. The krn sandstone conglomerate Ashland syncline. Copper Harbor thins to only a few tens of meters toward the northern end of the horst in the same areas where the volcanic section also shows substantial thinning. Apparently the area now comprising comprising the northern part of the St. Croix horst did not subside nearly as deeply deeply as parts farther to the southwest. southwest. This relatively positive relief persisted throughout throughout volcanic volcanic activity and deposition deposition of the Copper Harbor Conglomerate. Conglomerate. The overlying overlying Nonesuch Shale Shale maintains a relatively relatively uniform thickness around the northern part of the Ashland syncline, syncline, suggesting suggesting that that the the topographic topographic high high was was buried buried by by that that time. time. 91'00' EXPLANATION EXPLANATION 4V00' Bayfield and BayfieldGroup Group and equivalent sandstones Freda Sandstone Nonesuch Shale copper Harbor Conglomerate Gabbro and granophyre Minong Volcanicslow-Ti basalts Minong volcanicshigh-Ti basalts Chengwatana volcanics Kallander Creek Kallander CreekVolcanics Volcanicl Siemens Creek Volcanics volcanics Siemens Archean and Paleoproterozoic Paleopmterozoic 46'OO,i__ 46'O 92'OO' 92W 0 rocks 91°00 90 30 KM WY., Daniels, Cannon, Cannon, W.F., Daniels, D.L., D.L., Nicholson, Nicholson, S.W., Phillips, J., Woodruff, Woodruff, L.G., Chandler, Chandler, V.W., V.W., Morey, Morey, G.B., G.B., Boerboom, T., Wirth, K.R., and Mudrey, M.G., MG., Jr., Boerboom. Jr., 2001, 2001,New New map mapreveals revealsorigin originand andgeology geologyof ofNorth North American v. 82. 82, no. no. 8, 8, pp. pp. 97-101 97-101 American Midcontinent Midcontinent rift: rift: EOS, EOS, v. V.W., McSwiggen, P.L., P.L., Morey, Morey, G.B., GB., Hinze, W.J., W.J., and Anderson, R.R., R.R., 1989, Interpretation of Chandler, V.W., seismic seismic reflection, gravity, and magnetic data across Middle Proterozoic Mid-continent Rift system, northwestern Wisconsin, eastern Minnesota, and central Iowa: American Association of Petroleum Petroleum Geologists Geologists v. 73, 73, p. p. 261-275. 261-275. Bulletin, v. SW., Boerboom, W.F., Wirth, K. and Isachsen, C.E., C.E., 2001, A new look at the the 1.1 Ga Ga Nicholson, S.W., Boerboom, T., Cannon, W.F., Chengwatana horst, Minnesota Minnesota and Wisconsin, Institute Institute on Lake Superior Superior Geology, Chengwatana Volcanics in the St. Croix horst, 1,p. 71-72. 71-72. v. 47, part 1, 60 �TheRare Rareand andExotic ExoticMineralogy Mineralogyof of the theWestern WesternSubcomplex Subcomplexof of the the Deadhorse DeadhorseCreek Creek The Diatreme,Northwestern NorthwesternOntario. Ontario. Diatreme, G. Potter Potterand andRoger RogerH. H. Mitchell Mitchell EricG. Eric egpotter@mail.lakeheadu.ca egpotter@mail.lakeheadu.ca Dept. Dept.of of Geology, Geology,Lakehead LakeheadUniversity, University,955 955Oliver OliverRoad, Road,Thunder ThunderBay, Bay,ON. ON.P7B P7B5E1 5El The Themain main mineralized mineralizedzone zoneof ofthe thewestern westernsubcomplex subcomplexof ofthe theDeadhorse DeadhorseCreek Creekdiatreme diatreme exhibits complex complexmineralization mineralizationinvolving: involving: first first and and second second order ordertransition transitionmetals metals exhibits (specifically PEE; Be; Th; and U. The (specifically Sc, Ti, V, Cr, Mn, Fe, Zr and Nb); REE; Themineralization mineralization is Nbis manifested manifested by by the the presence presence of of the the following followingminerals: minerals: thortveitiite, thortveitiite,Sc-V-aegirine, Sc-V-aegirine, NbV-rutile, V-mtile, V-crichtonite, V-crichtonite, Ba-Mn-hollandite, Ba-Mn-hollandite, zircon, zircon, monazite-Ce, monazite-Ce, xenotime-Y, xenotime-Y, uraninite, uraninite, thorite, thorogummite, thorogurnmite, barite, barite, barylite, barylite, tyuyamunite, tyuyamunite, phenacite, phenacite, pyrite, pyrite, hematite, hematite, thorite, magnetite magnetite and and several several as as of of yet yet unnamed unnamed mineral mineral species species (Platt and Mitchell, Mitchell, 1996; 1996;Smyk Smyk et al., 1993; this study). Of interest in this presentation are: Nb-V-rutile, crichtonite et al., 1993; this study). Of interest in this presentation are: Nb-V-mtile, crichtoniteand and Sc-V-aegirine. Sc-V-aegirine. The The Nb-V-rutile Nb-V-mtile isis enriched enriched in in Cr2O3, with concentrations concentrations reaching Cr203, with reaching 30 6.49 6.49 wt.%. wt.%. The enrichment enrichment of of - Cr2O3 and Nb205 is similar to that Cr203 and Nb2O5 similar to that of of rutile mtile reported reported in in alkaline alkalineigneous igneous :1 20 rocks, as illustrated in an atomic rocks, as illustrated in an atomic - percent plotofof~Cr percent plot + r 3 4++' Nb5 Nb5+++ Ta5 ~ a ^ 1991). vs. Ti4 (Haggerty, vs. ~ i ^ (Haggerty, 1991). z 10 + However, Nb205 contents contents are are However, the the Nb2O5 - unusually unusually high high compared comparedto toalkaline alkaline M ~ ÑT ~ ~ ~igneous & ~ rocks igneous rocks in in general, general, with with " 00 29.32 concentrations reaching 95 100 80 85 90 50 55 55 60 65 70 75 75 80 85 90 95 100 concentrations reaching 60 65 70 50 Ti4 (Atomic%) Ti4* ( A ~ O ~%)C wt.%. Such such Nb2O5 Nb205 contents contents&re re wt.%. I similar similar to those those reported in in ilmenorutile, ilmenorutile, whiph which is historically found in pegmatites. pegrnatites. Also Also unique to the rutile is the distinct the Deadhorse Deadhorse Creek: Creek futile distinct enrichment enrichment of V2O3 V203 (up to 10.52 10.52 wt.%) and and the the lack lack of of tantalum. tantalum....' wt.%) : The Sc-V-aegirines Sc-V-aegirines present present at at Deadhorse Deadhorse Creek Creek contain contain the the highest highest reported reported The concentrations of Sc2O3 and and V203 V2O3(16.46 (16.46and and11.99 11.99wt.%, wt.%,respectively). respectively). The only only other other have been occurrences of of VV- and andSc-enriched Sc-enriched aegirine aegirine have been reported reported from from alkaline alkaline occurrences metasomatites metasomatites associated with iron-ore deposits in Ukraine (Valter et al., 1994; 1994;Pavlishin Pavlishin et al., presence thortveitiite and al., 2000). 2000). OfOfnote noteisisthethe presenceofofboth both thortveitiite(Sc2Si2Oi) (Sc2Si2O7) andSc-enriched Sc-enriched aegirine within the main mineralized zone. zone. Although Although the the source source of the the Sc Scin inthe theaegirine aegirine remains conjectural,itit appears appears that that the the Sc, V and Na remains somewhat somewhat conjectural, Na was was scavenged scavengedfrom from alteration of the main mineralized mineralized zone zone by Fe-rich fluids. fluids. The V-rich V-rich crichtonites crichtonites are best termed termed vanadium-rich vanadium-rich analogues of crichtonite-(Sr) crichtonite-(Sr) and and senaite-(Pb). The in the the crichtonites crichtonites isis peculiar, peculiar, as as the the presence of of Nb205in The enrichment enrichment in Nb2O5 Nb has has been been aa distinguishing distinguishing feature feature of of the the mantle-derived mantle-derivedend end members memberslindsleyite-(Ba) lindsleyite-(Ba) 61 �the crichtonites plot ininthe Interestingly, the theupper-mantle upper-mantle and mathiasite-(K) mathiasite-(K) (LIMA). (LIMA). Interestingly, and LIMA quadrant of FeO + Fe203 + MgO vs. Ti02 (Haggerty, 1991), near + F e B 3 + MgO vs. Ti02 (Haggerty, 1991), near the the LIMA LIMA LIMA quadrant of compositions compositionsdue dueto tothe thereplacement replacementof of iron ironby by vanadium. vanadium. The mtiles and and The Nb-enriched Nb-enriched rutiles have believed to crichtonite are crichtonite are believed to have formed fonned relatively relatively early early in in aa 30 30 crichtrmite (s$ Crichtnnite(Sr) multistage-alteration Armalcolite Ooadrant • (?EE) multistage-alteration sequence sequence of of the the diatreme — Deadhorse Creek &n,irc am,(Pb) w). Deadhorse Creek diatreme by by O 25 reaction reaction of of stoichiometric stoichiometricrutile mtilewith with Lovamgitc (Ca) + hydrous hydrous alkaline alkaline solutions solutions ennched enriched 20 in Nb Nb and and V. V. These These hydrous hydrous alkaline alkaline solutions solutions likely likely also alsoaltered altered Armalcolite Ouandrant DHC Crichinnite 15 zircon zircon to to an an unnamed unnamed hydrated hydrated awhich is zirconosilicate, calcium zirconosilicate, which is — calcium LIMACrptonites— with the found found in in association association with the 10 72 64 68 56 60 52 52 56 60 64 68 crichtonite and rutile. crichtonite and mtile. Textural Textural and and TtO2çVt. /o) TiO, (Wt.%) suggest that compositional data compositional data suggest that subsequent alteration alteration formed fonned the the Sc-V-aegirines Sc-V-aegirines and and imparted imparted the the pervasive pervasive subsequent hematitization hematitization to to the the main main mineralized mineralizedzone. zone. I I I I I I n - ~ i ~ b Crichtonite ~Crichtooite ~ ~ i t i ~ Non-Kimberlitic - - - p • - — I I I I I ' References References of the upper Oxide Minerals: Minerals: mineralogy of upper mantle. mantle. In: In: Oxide Haggerty, S.E. S.E. (1991): (1991): Oxide mineralogy petrologic and and magnetic magnetic significance. significance. Reviews Reviews in in Mineralogy, Mineralogy, 25, 25,Mineral. Mineral. Soc. Soc. Amer., 335-416. Amer., 335-416. Platt, Platt, R.G. R.G. and and Mitchell, Mitchell, R.H. R.H. (1996): (1996): Transition Transition metal metal rutiles mtiles and andtitanates titanatesfrom fromthe the Miner. Miner. Deadhorse Creek Diatreme complex, northwestern Ontario, Canada. Deadhorse Creek Diatreme complex, northwestern Ontario, Canada. Mag., 403-413. 60,403-413. Mag., 60, Pavlishin, Baldan, F.G., F.G., Bugaenko, V.M., Voznyak, Voznyak, D.K., D.K., Galaburda, Pavlishin, V.1., V.I., Baklan, Bugaenko, V.M., Galaburda, Yu, A., A., G.O., Mel'nikov, Kulchic'ka, Dekhtulins'ky, E.S., Donskey, O.M., Krivdik, S.G., Krivdik, G.O., Mel'nikov, Donskey, V.S., Radzivill, A, Ya. Ya. And And Zimbal, Zimbal, S.M. S.M. (2000): (2000): Science-based Science-based perspectives perspectives of of improvement of mineral mineral resources resourcesor orrare raremetals metalsinin Ukraine. Ukraine. Mineral., Journal, improvement of 22, no.1, no.l,5-20. (in Russian) Russian) 22, 5-20. (in Smyk, M.C., Taylor, R.P., Jones, Smyk, M.C., Taylor, R.P., Jones, P.C. P C . and and Kingston, Kingston, D.M. D.M. (1993): (1993): Geology Geology and and geochemistry geochemistry of the West Dead Horse Horse Creek Creek rare-metal rare-metal occurrence, occurrence,northwestern northwestern Ontario. Explor. Mining. Geol., 2, no. 3, 245-25 1. Ontario. Explor. Mining. Geol., 2, no. 3,245-251. Valter, V.M., Sharkin, O.P. O.P. and Yakolev, Valter, A.A., Khomenko, V.M., Yakolev, V.M. V.M. (1994): (1994): AA vanadian vanadian aegirine in alkaline metasomatites from Zheltye Vody. Vody. Doklady Doklady Akademii Akademii Nauk Nauk Ukrainy, Ukrainy, No. 3, 3, 110-116. 110-116.(in (in Russian) Russian) 62 �Sibley Sibley Basin Basin sediment sediment provenance provenanceusing using zircon zircon and andwhole whole rock rockgeochemical geochemical methods: methods: Possible Possible source sourceareas areasof of the thePass PassLake LakeFormation Formation Richardson, Hollings, P.P. (Department Geology, A.,Fralick, Fralick,P.,P.,and and Hollings, (Departmentofof Geology,Lakehead LakeheadUniversity, University,955 955 Richardson,A., Oliver OliverRd., Rd., Thunder ThunderBay, Bay,Ontario, Ontario,P7B P7B5E1, 5E1,Canada; Canada,ajrichar@mail.lakeheadu.ca) airichar@mail.lakeheadu.ca) The TheSibley SibleyGroup Groupconsists consistsof of Proterozoic Proterozoicsediments sedimentsthat that outcrop outcropdiscontinuously discontinuouslyover overaa 15000 15000sq. sq. km km region region in in the the area area surrounding surrounding central central and and southern southern Lake Lake Nipigon. Nipigon. Its Its age age is is bracketed Redstone Point Point Complex Complex(1537 (1537 bracketedby by the theunderlying underlyingRedstone ++lo/-2 101-2 Ma; +I-33 33Ma Ma Ma;Davis Davisand andSutcliffe, Sutcliffe.1985) 1985)and anda a1339 1339+1Rb-Sr Rb-Srage ageon ondiagenetically diageneticallyaltered alteredSibley Sibleysediments sediments(Franklin, (Franklin, 1978). SibleyGroup Groupwas was divided divided into into three three formations: formations:the the 1978).The The Sibley Kama Kama Hill Hill Formation Formation(top), (top),the theRossport RossportFormation, Formation,and and the the Pass PassLake LakeFormation Formation(bottom), (bottom),by by Franklin, Franklin, et et al. al. (1980). (1980).The The Kama Kama Hill Hill Formation Formationconsists consistsof of aalaminated laminatedshale shalefacies, facies,the the Rossport Rossportof of mudstone mudstoneand and stromatolitic stromatoliticfacies, facies,and and the the Pass Pass Lake Lakeof of aaconglomeratic conglomeraticfacies faciesand andaaplane-bedded plane-beddedor orcrosscrossbedded bedded sandstone sandstonefacies facies(Cheadle, (Cheadle,1986). 1986).This Thisstudy study investigates investigatesthe thesources sourcesthat thatfed fedsediment sedimentto tothe thePass PassLake Lake Formation Formationin in the thesouthern southernportion portion of of the the basin. basin. 30km LEGEND Proterozoic 1097 Ma rn Li_J Oslar Group 1110 Ma DI. base :I. ,1339Ma Regional Regional granitic granitic sources sourcesmay may include: include: the the Mesoproterozoic Mesoproterozoic Redstone Redstone Point Point anorogenic anorogenic intrusion, intrusion, Sample Locations Neoarchean Neoarchean peraluminous peraluminous Quetico Queticogranites, granites, and and Granite •,, Redstone Point Point Granite McKenzie McKenzie granites. granites. Of Ofthese, these,the the Redstone Redstone Point Point isis more more Artisan highly evolved than the others and contains abundant highly evolved than the others and contains abundant • Regional Granites ~ ~ it^^~ l ~ ~ ~ l Granitic Rocks Metasedimentaty zircon zircon and and aa distinct distinctgeochemical geochemical signature signaturewith withvery very elevated PassLakeFm. elevated values values for forthe thehigh highfield field strength strengthelements elements Rocks (HFSE). (HFSE). Samples Sampleswere were collected collectedfrom fromsurface surfaceexposures exposuresatatseveral several Figure 1. 1. Regional Regional geologic geologic map map with with sample sample Figure locations (Fig. 1). Representative samples of the Pass Lake locations (Fig. 1). Representative samples of the Pass Lake locations. locations. Formation Formation of of the the Sibley SiblevGroup Grouowere were taken taken from from aa cliff cliff section section directly directlyacross acrossfrom fromPass PassLake Lakeon on Hwy. Hwy. 587. 587. Individual Individualbeds beds were were grouped grouped into into assemblages assemblages consisting of up to 16 16 beds. Bed Bedthickness thicknessbecame became finer finer and and thinner up section. AAtotal totalof of 26 grained sandstone. sandstone. Two 26hand hand samples sampleswere were obtained from the Pass Lake cliff and consisted of fine to medium grained Two additional Pass Lake Formation samples were obtained from road cuts cuts further furtherup-section up-section that that consisted consistedof of medium medium grained grained sandstone. sandstone. Additional granitic samples were were obtained from road cuts along Hwy 11/17 11/17 and each location. Samples and Hwy. 527 (Fig. 1). One One sample sample was was taken taken from fromeach Samples of of Redstone Point sandstones, and and granite granite samples samples were were previously obtained by by P. P. Fralick from from the the English English Bay Bay region region of of Lake Lake Nipigon Nipigon (Fig. (Fig. 1). 1). S Sibley Graup 1537 Ma Granite and Rhyolite 1900 Ma AnImikle Group • 1 I - Figure x-Ray Figure 2. 2. Backscatter X-Ray SEM-EDS SEM-EDSimage image of of aa zircon zircon AR-01. fromsample sampleAR-Ol. from ICP-AES Plasma -- Atomic Emission ICP-AES (Inductively Coupled Plasma Emission Spectroscopy) Spectroscopy) Samples were cut into approximately 4 x 3 x 0.5 cm sections and crushed to included to aa fine fine powder nowder of <30 microns. Chemical Chemicalpreparation preparation included - hydrofluoric hydrofluoric acid acid digestion digestion to to remove remove all all silica silica and and allow allow complete complete solution solutionof of samples. Prepared samples were analysed at the Lakehead University Prepared samples were analysed at the Instrument Laboratory. Laboratory. 63 �A SEM-EDS SEM-EDS (Scanning (Scanning Electron ElectronMicroscope Microscope- - Energy EnergyDispersive Dispersive X-Ray X-Ray Microanalysis) Microanalysis) Samples Samples were were ground ground to to 30 30micron micron thin thinsections sectionsand andcut cutinto into discs discs suitable suitable for for the the SEM SEM stage. stage. Before Beforeanalysis, analysis,samples samples were carbon coated to prevent charge build- up while being analysed. Samples analysed for with were analysed. carbon Samples coatedwere were to prevent analysed charge for50 build50seconds seconds up while withan being an accelerating voltage of 20 KeV, and a beam current 0.475 accelerating voltage of 20 KeV, and a beam current ofof0.475 t, ..'$?Â¥: . .:. pA using using aa JEOL JEOL 5900 5900 SEM SEMwith withaasystem systemresolution resolutionof of139 139 pA . - .. eV, at at Lakehead Lakehead University Instrument Instrument Laboratory. Laboratory. Images Images eV, . .A* .:.. . . were taken using a backscatter-electron detector. Zircons were were taken using a detector. Zircons were .. analysed Zr,Y, Y,Th, Th,U, U,and andHf. Hf. analysed for for five five elements: elements: Zr, The The use use of of zircons zirconsin insediment sedimentprovenance provenancestudies studieshas hasbeen been 10Y+TKU IO-Y+T~+U WIO Hrlo limited to to work work done done by by Owen Owen(1987) (1987)which whichinvolved involved limited employing employing hafnium hafnium content of of detrital detritalzircons zirconsin indetermining determining zirconsfrom from Figure Figure3.3. SEM-EDS analyses of zircons the upper Jackfork the source source of of the hupper JackforkSandstone Sandstoneand andthe theParkwood Parkwood Pass PassLake LakeFm Fm sandstones sandstones(points), (points)'Redstone Formation. He Hecame cameto tothe theconclusion conclusionthat thathafnium hafniumcontent content Formation. Point Pointsandstones sandstones(squares), (squares),Redstone RedstonePoint Point of of these these zircons zircons agreed agreed with with optical opticaland andcathodoluminescence cathodoluminescence (+),and andregional regionalArchean Archeanand and Granites(+), Granites modal modal analyses, analyses, and and is is aa viable viable method methodfor forprovenance provenance Neoarchean Neoarcheangranites granites(triangles), (triangles). determination. determination. This This study study is is the the first first to touse useSEM-EDS SEM-EDSmethods methodsasaswell wellas as analyses for Y, Th, and U. Fig. 3 shows zircon analysis results. The majority of zircons plot at Zr/Hf ratio analyses for Fig. 3 shows zircon analysis The majority of zircons plot at Zr/Hf ratio of of approximately Y, Th, Th,and and U, U, but but aa significant significantpopulation populationshow showaaY, Y,Th, Th,UU approximately 40 40 with with relatively relatively low low amounts amounts of of Y, enrichment trend. The Thegeochemical geochemical signature signature of zircons from from both sandstones sandstones and and granites granites show show similarity, similarity, and and indicates indicates local local sourcing sourcingof of sediment sediment • £ with Archeanand and with aa possible possible influence influence of of regional regionalArchean Proterozoic Proterozoic felsic felsic igneous igneous intrusives. intmsives. 2000 Whole Whole rock rock interpretation interpretation of ofICP-AES ICP-AES ppmZr/%T02 ppm Zr/%T102 geochemistry geochemistry (Fig. (Fig. 4) 4) trends trends agree agree with with SEM SEM 1000 loGo distribution within samples. Immobile elemental distribution within samples. Immobile elemental element element ratios ratios of of the the Pass Pass Lake Lake sandstones sandstonestend tendto to - A fall on on aa mixing trend between between enriched enrichedand and nonnonfall I 125 75100 25 50 enriched 0 25 wm 75 Nb/%Ti02 ' 150 enriched sources. This Thisstudy studyhighlights highlights the the possible possible usefulness of using SEM-EDS generated data usefulness of using SEM-EDS generated datain in concert concert with with more more traditional traditionalchemical chemicalanalyses analysesin in Figure Figure4. 4. Immobile Immobileelement elementplot plotofofICP-AES ICP-AESanalyses. analyses. provenance provenancestudies. studies, Pass PassLake Lakesandstones sandstones(points) (points)plot plotininsimilar similarfield fieldtoto sandstones derived from, and overlying Redstone Point sandstones derived from, and overlying RedstonePoint granite granite (squares). (squares). Redstone (+), and and other other Redstonepoint point granite granite (+), granites granites (triangles) (triangles)are arealso alsoshown. shown. . I I I , 1 . A I I I 1 References References Cheadle, Alluvial-playasedimentation sedimentationininthe thelower lowerKeweenawan KeweenawanSibley SibleyGroup, Group,Thunder ThunderBay BayDistrict, District, Cheadle, BA. B.A.(1986) (1986)Alluvial-playa Ontario; Journal of of Earth EarthSciences, Sciences,v.v.23, p.527-542. Ontario; Canadian Journal 23, p. 527-542. Davis, Davis, D., and Sutcliffe, Sutcliffe,R., (1985) (1985) U-Pb U-Pb ages ages from from the Nipigon Plate Plate and Northern Lake Lake Superior. Superior.Geological GeologicalSociety Society of of America 1572-1579. America Bulletin, Bulletin, 96, 96, 1572-1579. Franklin, in Rubidium-strontium Rubidium-strontiumisochron isochron age age studies, studies. Report Report2,2, geological geological Franklin, J.M., (1978) (1978)The The Sibley Sibley Group, Group, Ontario; Ontario; in Survey of Canada, Canada, Paper Paper77-14, 77-14,p.p. 31-34. 31-34. Survey Franklin, McIlwaine, W.H., W.H., Poulsen, Poulsen, K.H. K.H. and and Wanless, Wanless. R.K. R.K. (1980) (1980)Stratigraphy Stratigraphyand anddepositional depositionalsetting settingofofthe the Franklin, J.M., Mcllwaine, Sibley 17,p.633-651. p.633-651. Sihley Group, Group, Thunder Thunder bay District District Ontario, Ontario, Canada; Canada; Canadian Canadian Journal Journal of of Earth Earth Sciences, Sciences,v.v. 17, Owen, Sedimentary Petrology, Vol 57, No.5., NO.^., 1987., 1987..p.p.831-838. 831-838. Owen, M. (1987) (1987) Hafnium in Detrital Zircons: Journal of Sedimentary Vol57, 64 �Magnetostratigraphic and Group A Magnetostratigraphic and Secular SecularVariation Variation Study Study of the the Sibley Group Rogala, P. and Borradaile, G. (Department of Geology. Lakehead University, Thunder Rogala,B., B.,Fralick, Fralick, P. and Borradaile, G. (Department of Geology, Lakehead University, ThunderBay, Bay, Ontario, P711 SEI, brogala@lakeheadu.ca) brogala@lakeheadu.ca) P7B 5E1, The Sibley Sibley Group Group is is aa red red bed bed sequence sequence that that was was deposited deposited in in aasubsiding subsidingintracratonic intracratonic basin (Fralick and Kissin, 1995) 1995) overlying, in part, a 1537+10-2 1537+10-2 Ma (Davis (Davis and and Sutcliffe, Sutcliffe, 1984) 1984) anorogenic granite-rhyolite complex. The Group was previously divided into three main anorogenic granite-rhyolite complex. The Group previously divided three main Rossport, and and Kama Kama Hill. Hill. An unnamed Formations: Pass Lake, Rossport, unnamed Formation and the Nipigon Bay Formation have have recently recently been been added. added. The Formation The Pass Pass Lake Lake Formation Formation consists consists of of the the conglomeratic conglomeratic Loon Lake Lake Member of the Loon Member and the the sheet-like sheet-like sandstones sandstones of the Fork Fork Bay Bay Member, Member, representing representing aa braided fluvial environment environment (Cheadle, (Cheadle, 1986). 1986). The Rossport Formation Formation is separated separated into the the braided Channel Island, Island, Middlebrun MiddlebrunBay, Bay,and andFire FireHill HillMembers. Members. The Channel Member is is a Channel Channel Jsland Island Member cyclic dolomite-shale dolomite-shale unit unit interpreted interpretedtoto be be playa cyclic playa lake lake sediments sediments (Cheadle, (Cheadle, 1986). 1986). The Middlebrun Bay Bay Member, considered a marker marker bed bed for for the Sibley Group, is aa stromatolitic Middlebrun stromatolitic unit unit that represents a migrating strandline. The The Fire Fire Hill Hill Member Member consists consistsof of mudcracked mudcrackedred red silt siltwith with mudchip conglomerates and sand sand sheet sheet incursions. incursions. It signifies a time of of tectonic tectonic tilting tilting of the the basin. The Kama basin. Kama Hill Hill Formation Formation is not not subdivided, subdivided, and is is composed composed of of purple purple shales shales and and siltstones interpreted as mud flat flat deposits deposits (Cheadle, (Cheadle, 1986). 1986). The The unnamed Formation is divided divided two unnamed unnamed Members. Members. These into two These represent represent a deltaic deltaic and fluvial fluvial environment. environment. The The Nipigon Nipigon Bay Formation Formation consists of of cross-stratified sandstonebeds, beds, and and is is thought to denote Bay cross-stratified sandstone denote an an acolian aeolian environment. environment. Samples were were taken taken from Samples from the Pass Pass Lake, Lake, Rossport, Rossport, Kama Kama Hill, Hill, and and Nipigon Nipigon Bay Bay Formations for a paleomagnetic study. study. The unnamed Formation was not sampled due to the lack of exposure. of exposure. The ThePass PassLake, Lake,Kama Kama Hill, Hill, and and Nipigon Nipigon Bay Bay Formation Formation were were used to to conduct conduct a preliminary of the the Sibley Sibley Group. Group. The TheRossport RossportFormation Formation was was preliminary study of the magnetostratigraphy of sampled from unoriented drill core, core, thus could only be used to study study secular secular variation. variation. The paleopoles calculated from the Pass Lake, Lake, Kama Kama Hill, Hill, and andNipigon NipigonBay BayFormations Formations have been been plotted along an apparent apparent polar wander path (APWP) defined defined by Elston Elston et et at. al.(2002) (2002) (Figure 1). 1). The samples samples from from the Pass Pass Lake Lake Formation Formation have been divided into into sample sample groups groups corresponding to Quarry Island, Transitional to to the Rossport Formation, and and an an outcrop outcrop atat Pass Pass The paleopole of the with a diagenetic Lake. Lake. The paleopole of the Quarry Quarry Island Island Group Group corresponds corresponds with diagenetic event at at 1978), and and the approximately approximately 1339±33 1339±3 Ma Ma (Franklin, (Franklin, 1978), the latter latter two two groups groups have havepaleopoles paleopoles associated with an The Kama has an older associated with an early earlyKeweenawan Keweenawan overprint. overprint. The Kama Hill Formation Formation has older discordant paleopole and and a younger younger paleopole paleopole that that isis located located within within the 1500 Ma section of the discordant paleopole that the the Sibley Sibley Basin Basin formed formed prior prior to to this, as is apparent polar polar wander path. is apparent path. This This suggests suggests that supported by by the recent discovery of sedimentary xenoliths within the 1537 supported 1537 Ma Ma Redstone Redstone Point Point granite. that lie on the APWP granite. The The Nipigon Nipigon Bay Bay Formation Formation has paleopoles paleopoles that APWP near near 1400 1400 Ma Ma and and 1100 Ma. Ma. The The first first paleopole paleopole may may be primary or related to the diagenetic diagenetic event that affected the Pass Lake samples at 1339 1339 Ma. The Thelatter latter paleopole paleopole correlates correlateswith with the the Osler OsierVolcanics. Volcanics. Formation revealed a The paleomagnetic study on a 90 cm core section from the Rossport Formation When this this curve was compared secular variation curve. curve. When compared to typical typical secular secular variation variation curves curves (Butler, 1998; Tauxe, Tauxe, 1998), the the time-span time-spanfor for Sibley Sibleydeposition depositioncan canbe beestimated. estimated. The 90 cm section was estimated to to represent represent 2500 2500 to to 3000 3000 years. years. This can can be be extrapolated extrapolated to to estimate estimate that that the Rossport Formation could potentially potentially represent represent 75 75 000 000 years years of of deposition. deposition. 65 �210'S — 240'S Figure 4.12 Paths (APWP) is plotted Figure 4.12 AAwell-defined well-defined Proterozoic Proterozoic Apparent Apparent Polar Polar Wander Paths al., 2002). The components are ThePass PassLake LakePCA PCAcomponents are designate designatewith withQI, QI,T, T, (after Elston et at., and 0 0 to the Quany to indicate indicatethe Quarry Island, Island, Transitiona', Transitional, and and Outcrop Groups. The The Kama Kama Hill Hill KH and and the the Nipigon Nipigon Bay Bay Formation Formation isisNB. NB. The PCA, PCB, and Formation is designate KH PCC components are denoted respectively by A, B or C after the Formation short form. form. Note that NB-C is a reversed reversed pole pole on on the the back back side of of the the globe. globe. Elston Blston et al. al. (2002) has provided a lower (Sl) (Si) and provided and upper upper (S2) (S2) Sibley Sibley Group pole based on data from Robertson well as as aa pole pole for for the the Keweenawan Keweenawan Osier Osler Group Group (Kl) (Ki) and lower Powder Mill (1973), as well Volcanics (K2). (K2). 1998. Paleomagnetism: magneticdomains domains to to geological Butler, R.F. R.F. 1998. Paleornagnetism: magnetic geological terranes, terranes, Department Department of of Geosciences Geosciences University of Arizona, (originallypublished published by by Blackwell University Arizona, http://www.geo.arizona.edu/Paleomag/bookl http://www.eeo.arizona.edu/Paleomag/book/ (originally Blackwell Scientific Publications Publications in 1992) 1992) Cheadle, B.A. BA. 1986. Alluvial-playasedimentation sedimentationininthe thelower lowerKeweenawan KeweenawanSibley SibleyGroup, Group,Thunder ThunderBay Bay District, District, 1986.Alluvial-playa 527-542. CanadianJournal Journalof ofEarth EarthSciences, Sciences, 23, 23,527-542. Ontario. Canadian Davis, D.W. and Sutcliffe, Suteliffe,R.H. RH. 1984. 1984.U-Pb U-Phages agesfrom fromthe theNipigon NipigonPlate Plateand and Northern Northern Lake Superior. Geological Geological Society of of America America Bulletin, Bulletin, 96, 1572-1579. 1572.1579. D.P., Enldn, Enkin, R.J., R.J., Baker, Baker, J. J. and Kisilevsky, Kisilevsky, D.K. D.K. (2002). Elston, D.P., (2002). Tightening Tighteningthe theBelt: Belt:paleomagnetic-stratigraphic paleomagnetic-stratigraphic constraints on deposition, correlation, and and deformation of of the Middle Proterozoic (ca. 1.4 1.4 Ga) Ga) Belt-Purcell Belt-Purcell United States States and andCanada. Canada. Geological Society ofAmerica Bulletin, 114, Supergroup, United 114, 619-638. 619-638. basin development development in in central central North North America: of America: implications of Fralick, P. and and Kissin, Kissin, S. S. 1995. 1995. Mesoproterozoic basin Sibley Group volcanism volcanism and sedimentation at Redstone Point. In: Petrology and metallogeny metallogeny of volcanic volcanic and intrusive system, Proceedings Proceedings of of the International International Geological Geological and intrusive rocks rocks of of the themid-continent mid-continent rift system. Correlation Program, Project 336. 336. in: Wanless, Wanless, R.K. R.K. and andLoveridge, Loveridge, W.D., W.D., Rubidium-strontium Rubidium-strontium 1978. The Sibley Group, Group, Ontario, Ontario, in: Franklin, J.M. J.M. 1978. report 2. 2. Geological 3 1-34. isotopic age studies, report Geological Survey Survey of Canada Canada Paper Paper77-14, 77-14.31-34. Robertson, W.A. WA. (1973a). Robertson, (1973a). Pole position position from thermally thermally cleaned cleaned Sibley Group sediments sediments and its its relevance relevance to to Proterozoic magnetic magneticstratigraphy. stratigraphy. Canadian Canadian Journal Journal of Earth Eon/i Sciences, Sciences 10, 10, 180-193. 180-193. Tauxe, L. 1998. 1998.Paleomagnetic Paleomagneticprinciples principlesand andpractice, practice,Kiuwer KluwerAcademic Academic Publishers, Publishers, Netherlands, Netherlands,299 299p.p. 66 �Mafic Dikes in Marquette County, County, Michigan with with Sequence of Precambrian Precambrian Mafic emphasis on the Sugar/oaf Sugarloaf Mountain and and Republic Republic Areas N.A. Sandin and T.J. Bornhorst Engineering and Bomhorst (Department (Department of Geological and Mining Engineering and Sciences, Michigan Technological University, Sciences, University, Houghton, Houghton, MI 49931) 4993 1) Precambrian mafic dikes are very common throughout Marquette County, Michigan. Michigan. These dikes Ga). Past Past studies studies by Kantor dikes have have ages agesfrom fromArchean Archean(—2.7 (-2.7 Ga) to middle Proterozoic Proterozoic (—1.1 (-1.1 Ga). Kantor (1968), Gair (1969), Cannon (1974), and Baxter and Bomhorst Bornhorst (1988) have suggested up to six suggested different mafic dike events in Marquette County. These events were interpreted to consist of (from old to young): 1) Archean mafic dikes post-Archean post-Archean volcanism volcanism and before Archean granitoid intrusions intrusions which cut the Archean volcanic rocks; 2) Archean mafic dikes that cut Archean granitoid intrusions, but are subjected to Archean deformation; 3) Archean mafic dikes sedimentary rocks of the that cut Archean basement rocks, but do not cut Early Proterozoic sedimentary dikes that cut Marquette Range Marquette Range Supergroup; 4) Early Proterozoic mafic dikes Supergroup sedimentary rocks rocks prior to Penokean metamorphism and deformation; 5-6) Supergroup 5-6) N-S and E-W Keweenawan mafic dikes. This study has confirmed much of Baxter and and Bornhorst Bomhorst (1988), (1988), however, new data indicate significant significant modifications. modifications. This study focused on the Sugarloaf Sugarloaf Mountain area near Marquette, Ml MI because because of of the the excellent exposures on shore and adjacent to Lake Superior, and previous work by Kantor exposures adjacent previous work by Kantor (1968), who identified mafic dikes of multiple ages. In the Sugarloaf Mountain area over over 300 mafic dikes intruding Archean tonalitic basement were identified and mapped using using aa GPS GPS and pace pace method. method. Dikes receiver and the compass and Dikes identified identified as as critically critically important important to to understanding the sequence of events were sampled for microscopic and chemical chemical study. study. Bornhorst (1988) interpreted interpreted thin, thin, discontinuous, discontinuous, tabular tabular mafic mafic bodies bodies at Baxter and Bomhorst in the the Sugarloaf Sugarloaf Mt. Mt. area as as being being Archean Archeanpost-plutoniclpre-deformation post-plutonic/pre-defonnation Wetmore Landing in above). While While this this interpretation interpretation is is still still possible, possible, the the favored favored mafic dikes (number 22 above). interpretation here is that these mafic bodies are xenoliths that were deformed deformed during during the the Archean along with the host plutonic rocks. plutonic rocks. In Marquette Marquette County, Baxter and Bornhorst Bomhorst (1988) (1988) as well as as previous previous workers workersrecognized recognized the numerous mafic intrusives intrusives that cut Marquette Marquette Range Supergroup Supergroupsedimentary sedimentaryrocks rocksprior prior to to Penokean metamorphism and deformation. deformation. These These were were presumed presumed to to be be of of generally generallythe the same same that in the Sugarloaf Mt. area, three age separate mafic intrusive age. This study indicates that intrusive events events of this age are present. Based on cross-cutting cross-cutting relationships, the sequence sequenceconsists consistsof of diabase diabase dikes trending N20¡E N20°E, diabase diabase dikes dikes trending trending N60¡E N60°E, and anddiabase diabase dikes dikes trending trending east-west. east-west. In In discriminated from addition to cross-cutting relationships, these groups can be discriminated from each each other otherby by trace trace elements. elements. N20°E diabase diabasedikes dikesare arethe theoldest oldest of ofthe theEarly EarlyProterozoic Proterozoicdikes. dikes. In the Sugarloaf Mt. The N2093 area, these dikes N20°E and range in width from NO593 to N20% from one one to to 25 dikes vary in trend from from N05°E 25 feet. feet. Mafic dikes of this age are are the the most most common common of of the Early Early Proterozoic Proterozoic dikes dikes in the Sugarloaf Mountain area. These These dikes dikes exhibit exhibit aa varying varying texture texture from from porphyritic porphyritic to to phaneritie phaneritic from fromthe the dike interiors interiors to the margins. They consist of hornblende, pyroxenes, pyroxenes, chlorite, chlorite, plagioclase, plagioclase, epidote, and sericite. The The REE REE patterns patterns are are enriched enriched in in light-REE light-REEwith with aa moderate moderateslope. slope. Compared to the REE patterns of the sills sills from from the the Marquette Marquette Range Range Supergroup, Supergroup,the theN20°E N2093 concentration of of light-REE, light-REE, is is less less depleted depleted in in heavy-REE, heavy-REE, and has a series has a higher concentration shallower slope. Thus, Thus, our our initial initial interpretation interpretationis is that that these these dikes dikes are are not not related relatedto tothe thesills. sills. 67 �The TheN60°E N60"Emafic maficdikes dikesare areintermediate intermediateEarly Early Proterozoic Proterozoic age. age. They Theyvary varyinintrend trendfrom from N45°E and N45% to N60t N60"E andrange rangeininwidth widthfrom fromone onetoto6060feet. feet.These Theseare arethe theleast leastcommon commonof ofthe theEarly Early Proterozoic Proterozoicdikes dikesininthe theSugarloaf SugarloafMountain Mountainarea. area.They Theygenerally generallyhave havethinly thinlyfoliated foliatedmargins margins with with aa massive, massive,fine-grained fine-grainedinteriors. interiors. These Thesedikes dikeshave haveaaphaneritic phaneritic texture textureand and consist consist of of hornblende, hornblende,pyroxenes, pyroxenes,chlorite, chlorite,plagioclase, plagioclase,epidote, epidote,sericite, sericite,and andminor minoramounts amountsofofcarbonate. carbonate. These Thesedikes dikescross-cut cross-cutthe theN20°E N20TE diabase diabase dikes. REE REEpatterns patternsare areenriched enrichedininlight-REE light-REEand andhave have aasteep steepslope. slope.Compared Comparedtotothe theN20°E N20%series seriesand andthe thesills sillsof ofthe theMarquette MarquetteRange RangeSupergroup, Supergroup, the theN60°E N60"E dikes dikes are are more more enriched enriched in light-REE light-REE with a steeper slope. slope. The TheN60°E N60"Edikes dikes are are more more depleted depletedin inheavy-REE heavy-REEthan thanthe theN20°E N20"Eseries. series.Our Our initial initialinterpretation interpretationisisthat thatthese thesedikes dikesare areaa distinct distinctmagmatic magmaticevent eventwith withrespect respecttotothe theearlier earlierN20°E N20%dikes dikesand andthe themafic maficsills. sills. The Theeast-east east-eastdiabase diabasedikes dikesare arethe theyoungest youngest series seriesof of the the Early Early Proterozoic Proterozoicdikes. dikes. They Theyvary vary in inwidth widthfrom fromfive fivetoto75 75feet feetwide. wide.These Thesedikes dikesgenerally generallyhave have thinly thinlyfoliated foliatedmargins margins with with aa massive, massive,fine-grained fine-grainedinterior, interior,although althoughtwo two dikes dikeshad had porphyritic porphyritic interiors. interiors. They They have have aa phaneritic phaneritictexture textureand andconsist consistof of homblende, hornblende,pyroxenes, pyroxenes,chlorite, chlorite,plagioclase, plagioclase,epidote, epidote,and and sericite. sericite.These Thesedikes dikescross-cut cross-cutthe theN20°E N20"Ediabase diabasedikes dikesand andthe theN60°E N60"Ediabase diabasedikes. dikes. Compared Compared to tothe theREE REEpatterns patternsof ofthe thesills sillsfrom fromthe theMarquette MarquetteRange RangeSupergroup, Supergroup,the theeast-west east-west dikes dikeshave haveaa higher concentration of light-REE and a steeper slope. The east-west dikes are depleted in the higher concentration of light-REE and a steeper slope. The east-west dikes are depleted in the heavy-REE heavy-REEcompared comparedtotothe theN20°E N20%dikes. dikes. They They are are lower lowerin in light-REE light-REEand andhave haveaashallower shallower slope than the N60°E series. Our initial interpretation is that these dikes are a distinct slope than the N60% series. Our initial interpretation is that these dikes are a distinctmagmatic magmatic event eventfrom fromthe theearlier earlierdikes dikesand andthe themafic maficsills. sills. There Thereare arethree threedistinct distinctmafic maficdike dikeevents eventsin in the theSugarloaf SugarloafMt. Mt.Area. Area.We Wepropose proposethat thatthese these dikes dikesare are not not related related to tothe themafic maficsills sillsthat that cut cut the theMarquette MarquetteRange RangeSupergroup. Supergroup.IfIf true, true, then then there theremust must be be atatleast least4, 4,and andlikely likelymore, more, Early Early Proterozoic Proterozoicmafic mafic magmatic magmaticpulses pulses in in the the Marquettearea. area. Marquette Two Twogroups groupsof of unmetamorphosed unmetamorphoseddiabase diabasedikes dikeswere were identified identifiedin inthe theSugarloaf SugarloafMt. Mt.area, area, consistent consistent with with Baxter Baxter and and Bornhorst Bomhorst (1988). (1988). These Thesedikes dikesare areKeweenawan Keweenawanin inage ageand and consist consist of of north-southtrending trendingseries seriesand andan aneast-west east-westtrending trendingseries. series. Both Both dikes dikeshave haveaadiabasic diabasictexture texture aanorth-south andvary vary from from10 10toto75 75 feet feet wide. wide. and In In the the Republic Republicarea, area,Baxter Baxterand and Bornhorst Bornhorst (1988) (1988) suggested suggestedthat thatsome somemetamorphosed metamorphosed mafic maficdikes dikes with with distinct distinctplagioclase plagioclase phenocrysts phenocrysts are are older older than than the themetamorphosed metamorphosedProterozoic Proterozoic dikes dikesin in the the Sugarloaf Sugarloaf area. area.They They proposed proposed that that these these dikes dikes might might correlate correlatewith withthe theMatechewan Matechewan dike dikeswarm swarmnorth north of of Lake Lake Superior Superiorin in Canada. Canada. We We tested tested this this hypothesis hypothesisby bydoing doingchemical chemical analysis analysis of these these dikes. dikes. The The REE REE data data for for these dikes dikes are are similar similar to to Matechewan Matechewandikes dikesfrom from elsewhere elsewhereand and support supportthe the hypothesis hypothesisproposed proposed by by Baxter Baxter and and Bornhorst Bornhorst(1988). (1988). References References Baxter, Baxter,D.A. D.A. and and Bornhorst, Bornhorst,TI., T.J.,1988, 1988,Multiple MultipleDiscrete DiscreteMafic MaficIntrusions IntrusionsofofArchean ArcheantotoKeweenawan KeweenawanAge, Age, western 2 pp. western Upper Upper Peninsula, Peninsula,Michigan: Michigan:Institute Institute on Lake Lake Superior Superior Geology Geology Proceedings Proceedingsand andAbstracts, Abstracts,v. v. 34, 34.2 pp. Cannon, Cannon, W.F., W.F.,1975, 1975,Bedrock Bedrock Geological Geological Map Mapof of the the Republic Republic Quadrangle, Quadrangle,Marquette MarquetteCounty, County,MI: MI:U.S. U S .Geological Geological Survey, Survey, Miscellaneous MiscellaneousInvestigations InvestigationsSeries SeriesMap, Map,1-862. 1-862. Gair, MI: U.S. US. Gair, J.E. J.E. and and Thaden, Thaden, RE., R.E.,1968, 1968,Geology Geologyof ofthe the Marquette Marquetteand andSands SandsQuadrangles, Quadrangles,Marquette MarquetteCounty, County, MI: Geological 77 pp. Geological Survey SurveyProfessional ProfessionalPaper Paper397, 397,77 pp. Halls, Halls, ll.C. H.C.and andPhinney, Phinney,W.C., W.C.,2001, 2001,Petrogenesis Petrogenesisof ofthe theEarly EarlyProterozoic ProterozoicMatachewan MatachewanDyke DykeSwarm, Swarm,Canada, Canada,and and Implications 22 Implications for for Magma Magma Emplacement Emplacementand Subsequent Subsequent Deformation: Deformation: Canadian CanadianJournal Journal of of Earth Earth Sciences Sciences 38, 38,22 pp. PP. Kantor, Swarms in in the the Sugarloaf Sugarloaf Mountain Mountain Area, Area, Marquette Marquette County, County,MI: MI:M.S MS. Kantor, J.A., J.A., 1969, 1969, Assimilation Assimilation and Dike Swarms Thesis, Thesis, Michigan Michigan Technological Technological University, University, Houghton, Houghton, MI, MI, 83 83 pp. pp. 68 �PALEOPROTEROZOIC DOME CORRIDOR CORRIDOR IN THE THE PALEOPROTEROZOIC DEVELOPMENT OF A GNEISS DOME SOUTHERN SOUTHERN LAKE LAKE SUPERIOR SUPERIOR REGION, REGION, USA USA SCHNEIDER, SCHNEIDER, D.A., Dept. of Geological Geological Sciences, Sciences,Ohio Ohio University, University, Athens, Athens, OH OH 45701; 45701; HOLM, D.K., and O'BOYLE, C., C., Dept. Dept. of of Geology, Geology, Kent State State University, Kent, OH 44242; HAMILTON, HAMILTON, M., M., Continental Continental Geoscience Geoscience Division, Division, Geological Geological Survey Survey of of Canada, Canada, Ottawa, Ottawa,ON ON Canada; and JERCINOVIC, MA 01003 01003 JERCINOVIC, M., Dept. of Geosciences, Geosciences, U-Mass, Amherst, Arnherst, MA Paleo-reconstruction of the Penokean orogen at Ca. Ma reveals reveals the presence of of a Paleo-reconstruction ca. 1750-1700 Ma narrow corridor corridor of Archean cored Paleoproterozoic gneiss domes just north of and parallel Paleoproterozoic gneiss domes of and parallel to to the the main suture Wisconsin, and and northern northern Michigan. Michigan. Penokean suture zone zone in Jvlinnesota, Minnesota, Wisconsin, Penokean (ca. (ca. 1850 1850Ma) Ma) metasedimentary rocks infolded within the domes metasedimentary domes give give predominantly 1750-1700 1750-1700Ma Ma cooling cooling ages and are overlain depositionally depositionally by ca. 1700 1700 Ma Baraboo Baraboo interval interval quartzites. quartzites. We We conducted conducted SHRIMP and total-Pb EMP geochronometry to obtain metamorphic timing constraints on U-Pb SHRIMP distinct monazite monazite mineral domains domains from from amphibolite amphibolite grade grade rocks rocks sampled sampledacross acrossthe the entire entirelength length of the gneiss dome corridor. Based Based on on metamorphic metamorphicmonazite monazite crystallization crystallization ages, ages, midcrustal midcrustal amphibolite facies metamorphism (Ml) (Ml) peaked peaked around around 1830 1830Ma Ma and and was was concurrent concurrent with with late late reliably recorded recorded at at ca. ca. 1800 Ma Ma (M2) and and Penokean plutonism; subsequent thermal pulses are reliably again at ca. 1765 1765 Ma (M3), (M3), both also also coeval coeval with magmatic magmatic activity. activity. The youngest monazite monazite ages overlap overlap with abundant abundant Ar-Ar mineral mineral age age data, data, which which indicate indicate widespread cooling of of the gneiss dome comdor corridor immediately immediately following M3. M3. We We propose propose that that the the lime during structural modification of of the the tectonically tectonically buried buried gneiss domes formed at this time continental margin rocks. In In our ourconceptual conceptual model model (Fig. (Fig. 1), l), northward northward vertical vertical extrusion extrusion of of aa decoupled midcrustal block containing the gneiss dome corridor accommodated gravitational collapse of overthickened crust. Elevated Elevatedcountry countryrock rock temperatures temperaturesaccompanied accompaniedwith with profuse profuse melting (i.e., intrusion of the East-central Minnesota batholith) promoted doming of the lower density Archean basement into the more dense overlying Paleoproterozoic metasedimentary rocks, ultimately enabling its complete decoupling from the remaining lower crust. This Thisprocess, process, primarily driven by buoyancy forces, allows for the redistribution of crustal mass from thick to horizontal crustal crustal extension. extension. Tectonic extrusion and crustal thin regions without significant horizontal thinning at this stage may have been facilitated by a decrease in horizontal compressive compressive stresses acting on the region from the south (i.e., Yavapai slab rollback as proposed by Holm HoIm et al., ILSG, 2003). 2003). In Inour ourmodel model(Fig. (Fig.1), l),the thefaults faultsbounding boundingthe thegneiss gneissdome domecorridor comdorare areca. ca.1765 1765 structures, although some, like the Niagara Fault zone, are am reactivated Penokean structures. Ma structures, We note that in east-central Minnesota, a significant portion of the Malmo Structural Structural discontinuity juxtaposes post-Penokean plutons to the south south against older older metamorphic metamorphic rocks to the north (west of Mille Lacs). This This clearly clearly supports supports our our interpretation that this structure structure (and the Flambeau Flowage fault equivalent in northern Wisconsin) Wisconsin) was active active well after Penokean orogenesis. orogenesis. D.K., Van Schmus, W.R., W.R., MacNeill, L.C., Boerboom, Boerboom, T.J., Schweitzer, Holm, D.K., Schweitzer, D., and and Schneider, D.A., 2003, Late Paleoproterozoic (1900-1600 Ma) tectonic history of the northern U.S.A.: Implications for for crustal crustal stabilization: stabilization: Institute mid-continent, U.S.A.: Institute on Lake Superior Superior Geology abstracts abstracts (this (this volume). volume). 69 �Penokean Penokean orogen, orogen, Ml: MI: 1830 2830 Ma Ma to to M2: M2:1800 1800Ma Ma $S NN warm, warm, Penokean Penokean orogen, orogen, M3: M3: 1765 1765 Ma Ma N N SS GNEISS DOME DOMECORRIDOR COftRlDOR GNEISS ARCHEAN GRANITE-GREENSTONE V.1SONSIN MAGMA11C TERRANE Quvenile island alt) ARCHEAN GNEISS PALEOPROTEROZOC ROCKS (supracwstal} F ~ g ~1. 1. x eSchematic SchemaucN-S N-Scross-sections cross-secttonsat at1830-1800 1830-1800Ma Ma(A) (A)and and Figure 11765 765 Ma Ma (6) (B) dep~cling depicting the [he proposed proposed evolution evoluttonot of the thegneiss gnelssdome 6ome corridor corndor in In northern northern Wisconsin. Wtsconsin. Note Noterelative relativelocations locattons ol of gray gray circles clrcies that represent depth of crustal blocks. that represent depth of crustal blocks. 70 �A Paleoproterozoic suprasubduction suprasubduction zone arc complex zone ophiolite-island ophiolite-island arc in northeastern northeastern Wisconsin Wisconsin Schulz, Klaus Schulz, Klaus J., J., (U.S. (U.S.Geological GeologicalSurvey, Survey,Reston, Reston,VA VA 20192, 20192, kschulz@usgs.gov) kschulz@us~s.~ov) The Paleoproterozoic Paleoproterozoic volcanic volcanic and and associated associated intrusive intmsive rocks rocks exposed exposed in in northeastern northeastern Wisconsin Wisconsin are are the the easternmost easternmost exposures exposures of of the the Pembine-Wausau Pembine-Wausau terrane, terrane, the the northernmost northernmost of of the the two two Wisconsin Wisconsin magmatic magmatic terranes terranes that that were were accreated accreated to to the the southern southern margin of the Archean Atchean Superior SuperiorCraton during during the the Penokean Orogeny Orogeny (Sims (Sims and and others, others, 1989). The The rocks rocks of of the the Pembine-Wausau Pembine-Wausau terrane terrane are are separated separated from the epicratonic sedimentary rocks of the Marquette Range Supergroup to the north in Michigan by the Niagara fault fault zone. zone. The volcanic rocks of the the Pembine-Wausau terrane terrane exposed exposed northeastern northeastern Wisconsin, Wisconsin, formed at about 1,870 1,870Ma Ma and and are are cut cut by a variety variety of intrusive intrusive rocks ranging ranging from from synsynvolcanic gabbros, diorites, and tonalities to syn-and post-tectonic granitoids (i.e., Dunbar Dunbar Oneiss Gneiss and related rocks). The Thevolcanic volcanicrocks rocks are are divided divided into into four four fault-bounded fault-bounded units, units, McAllister, Beecher, and Pemene formations. These the Quinnesec, McAllister, Theseunits units are are interpreted interpreted to record the evolution evolution of of aa Paleoproterozoic Paleoproterozoic suprasubduction suprasubductionzone zoneophiolite-island ophiolite-islandarc arc complex, the Pembine Pembine ophiolite-arc ophiolite-arc complex. complex. of The Quinnesec Formation is the oldest volcanic unit and consists predominantly of lava flows pillowed basalt flows and massive diabase, but includes andesite and rhyolite lava and fragmental rocks locally. Several Several large large gabbro gabbro sills sills are are present, present, particularly particularly near the Niagara fault zone, some with peridotite and pyroxenite layers. In In addition, addition, aa large large serpentinized peridotite-gabbro pendotite-gabbro body that produces a large large positive positive magnetic magnetic anomaly anomaly is is exposed exposed south south of Timms Timms Lake Lake (Morgan (Morgan County County Park) Park) east east of of Pembine, Pembine,Wisconsin. Wisconsin. is locally Serpentinized peridotite is dominant in the western part of this body where it is cut by coarse-grained (1-5 cm) dikes of of pyroxenite. Layered Layered and and massive massive gabbro gabbro and and masses of strongly foliated-lineated gabbro are dominant in the eastern part of the body where they are cut by numerous mafic dikes with cliabasic microdioritic textures; some diabasic to microdioritic some of the dikes dikes appear appear to to be be sheeted. sheeted. The rocks of the Quinnesec Formation appear to record the birth and youth stages of a suprasubduction zone ophiolite (Shervais, 2001). 2001). Rocks Rocks formed formed during during the initial phase of ophiolite evolution evolution typically include include layered layered and and isotropic isotropic plutonic plutonic gabbros, gabbros,sheeted sheeted dikes, and a "lower" "lowe?' volcanic volcanic section consisting consisting of low-K tholeiitic basalt and basaltic andesite with MORB MORE and primitive arc tholeiite tholeiite affinities. Gabbros Gabbros formed formed during during this stage are often ductilely deformed deformed (foliated (foliated or or boudinaged) boudinaged) in in response response to to syn-magmatic syn-magmatic extension. Rocks Rocks formed formed during during the the second second or or youth stage of ophiolite formation include intrusive mafic-ultramafic sills and diabase dikes, and an "upper" volcanic unit unit characterized by basalt and andesite with with highly depleted incompatible trace element compositions (i.e., low-Ti basalt, high-Mg andesite and boninite) (Shervais, (shemais, 2001). basalts and gabbros are tholeiitic, with generally low Compositionally, the Quinnesec basalts Ti02 and other high field strength element abundances, Ti02 other abundances, and and flat to extremely extremely light REE REE 71 �some of of the the basalts, basalts, gabbros, and and depleted patterns (Sims and others, 1989). Tn In addition, some Ti02 and REE REF abundances, abundances, but relatively high Cr and Ni andesites have very low Ti02 contents. The Thetrace traceelement element characteristics characteristicsof of the the mafic mafic rocks rocks overlap overlap those those of of mid-ocean mid-ocean ridge basalts and primitive island-arc tholeiite suites whereas the andesites show with fore-arc-related fore-arc-relatedboninites. boninites. The presence in in the upper upper part part of of compositional affinities with the Quinnesec Quinnesec Formation Formation of mafic mafic rocks rocks derived derivedfrom from highly highlyrefractory refractorymantle mantle isis particularly diagnostic of a relationship to the early stages of of intraoceanic subduction and formation (Shervais, 2001). 2001). This fomation in a forearc setting (Shemais, This also implies that the Quinnesec Formation and associated rocks did not form in a back-arc basin near or on the margin of the Superior Johnson, 1997), Superior Craton, as has recently been proposed (Van Wyck and Johnson, 1997), but rather formed as an intraoceanic ophiolite-arc system above a southward dipping (in present coordinates) coordinates) subduction subduction zone. zone. The McAllister, Beecher and Pemene formations consist of volcanic and volcaniclastic calc-alkaline rocks ranging from andesite (McAllister) to rhyolite (Pemene), all with calc-alkaline compositions characteristic of of mature mature oceanic oceanic arcs, arcs. These volcanic rocks and associated intrusives, such as the Newingham Tonalite and Twelve Foot Falls Quartz Diorite, appear appear compatible with the third or maturity stage of suprasubduction ophiolite evolution compatible suprasubduction zone ophiolite evolution (Shervais, 2001). Characteristic (Shemais, Characteristic of this stage are intrusive rocks, such as hornblende diorite, quartz diorite, and tonalite, as well as volcanic rocks ranging from basalt to diorite, to calc-alkaline caic-alkaline compositions. compositions. Volcanism typically becomes rhyolite, all with transitional to in these sequences. sequences. In many cases, rocks of this stage have not more silicic with time in of the subjacent ophiolite, ophiolite, but rather have been attributed attributed to postbeen considered part of ophiolite arc volcanism (Shervais, ophiolite (Shervais, 2001). 2001). It appears appears likely that growth of the Pembine ophiolite-arc complex was terminated terminated by its its collision with and obduction onto the passive passive southern southern margin margin of the the Superior SuperiorCraton. Craton. Because subduction appears to be largely driven by slab pull, the southward southward subduction subduction of oceanic lithosphere lithosphere attached to the Superior Superior continental continental margin would would have have pulled pulled the the continental lithosphere lithosphere along with it as it descended descended into into the the subduction subduction zone zone below below the the ophiolite-arc system. With With detachment detachment of of the the subducting subducting oceanic oceanic lithosphere, lithosphere, the the buoyancy of the continental lithosphere lithosphere would have led to its rapid uplift along along with with the the leading edge of the the ophiolite-arc ophiolite-arc complex complex ((Shervais, 2001). This S h e ~ a i s2001). , This stage stage is is recorded recorded by by the deformation sequence and by the intrusion of the synSF- to post-tectonic post-tectonic deformation of the ophiolite-arc sequence units of the the Dunbar Dunbar dome. dome. Shervais, J.W., 2001, Birth, death, and resurrection: the life life cycle cycle of suprasubduction suprasubduction zone zone ophiolites: ophiolites: Geochemistry Geophysics Geosystems, Geosystems, v01.2, vol.2, Paper Paper number 2000GC0~080. 2000GC000080. On-line publication publication at at htto://g-cubed.org. hm:I/g-cubed.org. Sims, W.R., Schulz, Schulz,K.J., K.J., and and Petennan, Peterman, Z.E., Z.E., 1989, 1989, Tectono-stratigaphic Tectono-stratigraphic evolution evolution of of Sims, P.K., Van Schmus, W.R., Proterozoic Wisconsin magmatic terranes of the Penokean Orogen: Canadian Canadian Journal the Early F'roterozoic Journal of of Earth Earth Sciences, v. v. 26, 26,p.p.2145-2'58. 2145-2158. Van Wyck, N., N., and Johnson, C.M., C.M., 1997, Common lead, Sm-Nd, and U-Pb constraints constraints on on petrogenesis, petrogenesis, crustal architecture, and tectonic setting of the Penokean orogeny (Paleoproterozoic) cmstal (Paleoproterozoic)in in Wisconsin: Wisconsin: Geological Society of America Bulletin, Bulletin, v, v. 109, p, p. 799-808. 72 �- PLANNW PLANNED ACTIVITIES ACTIVITIES AND THE LAKE LAKB NIPIGON GEOSCIENCE INITIATIVE OBJECTIVES SMYK, Mark C., Ontario Geological Survey, Ministry SMYK, Ministry of Northern Development Development and Mines, Suite 8002, 435 James Suite B002,43S JamesSt. St.South, South,Thunder ThunderBay, Bay,ON ONP7E P7E6S7, 6S7,and andmembers membersof ofthe theScientific Scientificand and Implementation Committees. Committees, Lake Lake Nipigon NipigonGeoscience GeoscienceInitiative, Initiative,c/o do Ontario Prospectors Association, 1000 1000 Alloy Drive, Drive, Thunder Bay, ON P7B 6A5 The Lake Nipigon Geoscience Initiative Initiative (LNGI) (LNGI) was created in 2002 as a $7.0 M Cdn. Cdn. project projectaimed aimed at at attracting mineral investment to the area around Lake Nipigon. The O Ontario Prospectors Association's attracting n t ~ Prospectors o Association's (OPA) portion of the project is funded funded through through an agreement agreement with the Northern Ontario Onmio Heritage Heritage Fund. The partnering with the Ontario Geological Survey (OGS), the Ministry of Northern Development and OPA is partnering Canadian Mining Industry Research Organization (CAMRO), (CAMIRO), Lakehead University, Mines (MNDM), the Canadian partners and communities in the L&e Lake Nipigon area. It will focus focus on four key as well as with private sector pmners objectives: Lake Nipigon Nipigon region region through collection collection of high Maintain and then increase mineral investment in the Lake 1. 1. quality geological data and provision of interpretations that meet the needs and priorities priorities of of the the mineral mineral industry and that maintain maintain or attract attractmineral mineral investment investmentto to Ontario; Ontario; exploration discovery discovery rate by addressing "masking and deep search search challenges challenges 2. Increase the mineral exploration and skill gap" in the area; area; recently recognized 3. Respond to, and evaluate, new and exciting mineral deposit models recently 3. recognized for for nickelnickelcopper, palladium-platinum, and gold-copper mineralization in the region; 4. Reinforce and demonstrate an innovative economic development model based on local local community, community$ partnerships in geoscience that result in mineral resource economic industry, and government partnerships economic development in the local communities, communities, the the region, region, and and Ontario. Ontario. The LNGI predominantly of Emhayment, which consists predominantly LNGI is focused on the Nipigon Basin Basin I1 Embayment, Mesoproterozoic, Midcontinent Midcontinent Rift-related, RiR-related, ultramafic ultramaficto mafic mafic intrusions intrusions that that have have intruded intruded Mesoproterozoic Sibley Sibley Group sedimentary sedimentary rocks and Archean Archean basement basement rocks rocks of of the the Quetico Queticoand and Wabigoon Wabigmn subprovinces. snbprovinces. comprehensive geoscience geoscience database database that that will will assist assist in in mineral mineral exploration. exploration. The The project will develop a comprehensive consultations that helped LNGI evolved through a series of community and industry consultations helped define define the the project project parameters. AAthorough thorough compilation compilationof of previous previous exploration exploratiou and and geological data data provided provided aa baseline baseline for for the project and identify potential potential gaps in the the geoscience database. database. The The main main components componentsof of the the initiative initiative include: • • • • • • • • Precambrian Section, Detailed geological mapping, undertaken by Precambrian Section, OGS OGS Airborne Airborne magnetic survey survey Gravity Gravity survey survey Quaternary (surficial) (surticial) case studies, studies, undertaken undertaken by Sedimentary SedimentaryGeoscience GeoscienceSection, Section,OC)S OGS Geochronology Geochronology Physical property studies studies Geographic Information Systems (GIs) (GIS) compilation Complementary Complementary research at Lakehead University • Sibley Fralick) Sibley Group studies (P. Fmlick) intrusion studies (P. Hollings; G. Borradaile) • Nipigon mafic intmsion Bomadaile) • Sulphide mineralization mineralization studies studies(S. (S. Kissin) Kissin) . 73 �The TheOntario OntarioGeological GeologicalSurvey Surveywill willhelp helpacquire acquireand and publish publish the the results results of of the the geoscience geosciencestudies studies as as maps, maps, reports,and and digital digitaldata datasets. sets.The Theinformation informationwill willthen then be be available available over over the the Internet Internet through though the tbe MNDM's MiWM's reports, ERMES ERMESand andCLAIMap CLAIMapsystems. systems.This Thisvaluable valuableinformation informationwill be be used to to globally globally market market the tbe resource resoutce potentialand andinvestment investmentappeal appealofofthe theLake LakeNipigon Nipigonregion. region. potential 74 �TECTONOSTRATIGRAPHIC ASSEMBLAGES OF EASTERN ARCHEAN TECTONOSTRATIGRAPHIC WABIGOON SUBPROVINCE, NORTHWESTERN ONTARIO ONTARIO STOTT Greg STOTT GregM., M., Ontario OntarioGeological GeologicalSurvey, Survey,Sudbury, Sudbury, ON, ON, P3E P3E 6B5 6B5 (greg.stott@ndm.gov.on.ca), DAVIS, Don. W., Department of Geology, University of of (greg.stott@ndm.gov.on.ca), DAVIS, of ON, PARKER, PARKER, Jack Jack R., R., Ontario Ontario Geological Geological Survey, Survey, Sudbury, ON, Toronto, Toronto, ON, STRAUB, Kristan J., Laurentian University, University,Sudbury, Sudbury,ON ONand andTOMLINSON, TOMLINSON, Kirsty STRAUB, Knstan J., Y., Geological Survey of Canada, Y., Canada, Ottawa, Ottawa, ON ON The Archean Wabigoon Subprovince Subprovince is a complex of volcanic and sedimentary sedimentary of Mesoarchean to Neoarchean age. The supracrustal assemblages and granitoid suites of of this subprovince, which includes the Onaman-Tashota greenstone easternmost part of greenstone belt east of Lake Nipigon, preserves a history of over 250 million years of volcanism. This area has recently recently been been treated treated to to aa regional regional mapping, mapping, geochemical geochemical and and geochronological part of of the the Western Western Superior NATMAP project. A 1:250 000 compilation synthesis as part map (Stott et al. 2002) arising from this project illustrates the subdivision subdivision of of the the OnamanOnamanTashota (0-T) (O-T) greenstone belt belt into into tectonostratigraphic tectonostratigraphic assemblages assemblages (Figure (Figure 1), I), based on stratigraphic geochronological and geochemical similarities similarities and stratigraphic correlations, geochronological and contact contact component of of this this map, map, summarized summarized in in Figure Figure 2, 2, is is the the relationships. A more interpretive component delineation delineation of the assemblages in terms of the environment environment of crystallization of of volcanic volcanic and plutonic rocks and deposition of sedimentary sedimentary rocks. This is based on on lithologic lithologic and and geophysical characteristics, whole-rock geochemical classification, and where available, available, Nd isotopic isotopic signatures. signatures. Onaman-Tashota greenstone belt straddles the width of the eastern The Onaman-Tashota eastern Wabigoon Wabigoon Subprovince between the English River and Quetico metasedimentary subprovinces. Subprovince metasedimentary subprovinces. ItIt isis mainly composed composed of Neoarchean Neoarchean(dominantly (dominantly2.74 2.74— - 2.72 2.72 Ga) basaltic and dacitic dacitic flows, flows, volcanic rocks occur autobreccia and pyroclastic rocks. Mesoarchean (3.05 (3.05 —2.92 - 2.92 Ga) volcanic occur in in the northwest and along the western margin of the belt. Widespread Nd isotopic isotopic evidence evidence in the northern part of the Onaman-Tashota belt suggests that Neoarchean volcanism volcanism erupted thmugh half of the belt through Mesoarchean basement. Basement in the northern half contains an older component component than than that that south of of the Humboldt Humboldt Bay High High Strain Zone. The 2.74 Ga Willet assemblage tholeiitic basalts of ocean floor affinity dominate 2.74 dominate the the northern northern half of the 0-T O-T belt. belt. This This assemblage is flanked to the the north north and and south by calc-alkalic of continental continental margin arc affinity that border metasedimentary subprovinces assemblages of subprovinces composed of of flysch-like wacke wacke derived derived from from the the erosion erosion of of the the 0O-T belt and plutons plutons - T belt during orogenesis at at circa circa 2.7 2.7 Ga. Ga. Most Most sedimentary units units within within the the 0-T O-T belt form the supracrustal assemblages, assemblages, reflecting reflecting erosion of the underlying volcanic and youngest supracrustal and plutonic rocks towards the English English River River and Quetico basins basins to the north and south. Reference Reference Stott, G.M., Davis, D.W., D.W., Parker, Parker, J.R., J.R., Straub, Straub, K.J. ILI. and Tomlinson, K.Y. K.Y. 2002. Geology and Assemblages, eastern eastern Wabigoon Wabigoon Subprovince, Subprovince, Ontario; Ontario; Ontario Ontario Geological Geological Survey, Tectonostratigraphic Assemblages, Preliminary Map P.3449, scale 1:250 1:250 000. KY., Stott, Tomlinson, G.M.and and Davis, D.W. 2000. Nd isotopes in the eastern Wabigoon subprovince: Tomlinson, K.Y., Stott, G.M. for crustal crustalrecycling recyclingand andcorrelations correlationswith withthe thecentral centralWabigoon; Wabigoon;in inHarrap, Harrap, R.M. R.M. and and implications for I-Ielmstaedt,H.H. H.H.(eds.), (eds.),2000, 2000,Western WesternSuperior SuperiorTransect Transect Sixth Sixth Annual Annual Workshop, Workshop, Lithoprobe Report Report #77, #77, Helmstaedt, Lithoprobe Lithoprobe Secretariat, Secretariat, University University of British British Columbia, Columbia, p.119-126. p.119-126. 75 �__________ Figure1.1. Figure Tectonostratigraphic Tectonostratigaphic assemblagesof of the the assemblages Onaman-Tashota Onaman-Tashota greenstone greenstonebelt beltand and Proterozoic Proterozoicdiabase diabasedike dike swarms, swarms,Eastern Eastern Wabigoon WabigoonSubprovince. Subprovince. i o__o 1e o Kilameires Presreetlo = 0' Moos Is mlanreediala nwIarasw ass.eblagos jmeppsdi&, Is aMOOsodeaso asseaamagas Niri5000labauseilIl Ares.., Uas1jsdtstad plusast — CSaO,c rn,100asbnaSaW assamb)aga, C— m 1T!2J EnlolsIlar — Heml MalmUp-Veaus unsabdisidad Nipiger iKasreeasaasl slabs. didps (0100 Ma) and Marathon diahaaa dId. errors 12101 and 2120 Ma) *4,-p .__+ + '-•l- + -_. to. Is-a as eJ it) Wtle thdahth) Oreanar leelsilda) Zi' lhrsaloaandlh.bee Isslaidle) canasru East )thseasr Marshall Shear elena Albad-OlSdhll 4 Z222 ElethasSRlcMly (saJbaOnaas) fl staladnewas dials,, sib, salem )244E hI 247.1 Ma) arId kasadasi tIlt ftaashlisn Uscsrredlled dbMs so andatasal age. possibly ralal,dIo Biac0005Il,a 2170 Ma) UI Malaffoeer (2101 and 2121 Ma) dika swaass Figure2. 2. Tectonic Tectonic Figure affinities affinitiesassigned assignedtoto volcanicand and volcanic sedimentary sedimentary assemblages assemblagesand and plutonic plutonicsuites. suites. + + 1- +.+,+.+ 0Nakina 4 )a rss--'a------C-a't + -a-/ p Lake Nopigon1., Is°—.1 + •4 + + + + + + + + -p 4Th—---rI-n- — + + + + + + + + + + + + + -p 4 •-0- Proterozoic i continental plume related a'nyMyM Ocean floor Oceanic unsubdivided Archean 4 Orogenic plutons I } continental arc conbnental margin arc + 4.4—cc + ::-:;1 Orogenic sediments 1 Unknown tectonic affinity 20 0 Kilomalyas Meaoarcliean continental unsubdivided 76 �FIVE GOLD GOLD POSSIBILITIES SOME KEWEENAWi4N KEWEENAWAN COPPER SULFIDES FIVE POSSIBILITIES IN IN SOME SULFIDES IN IN ONTARIO AND AND MICHIGAN MICHIG?d Prow, Trow, Jim, Jim, Geological Geological Sciences, Sciences, Michigan Michigan State State University, University, emeritus, 540 Lake Avenue *2, HanCock, emeritus, 540 #2, HancoCk, Michigan 49930 49930 Most fire—assayed fire-assayed "invisible" "invisible"gold, gold, from from .12 -12to to 2.50 2.50 oz oz Au/st, Au/st, occurs in "blue "blue chalcocite" chalcocite" (with (with minor covellite) covellite) but not in black black chalcocite chalcocite (with (with no covellite) covellite) on on the the adit, adit, 1st, lst, 2nd, 2nd, and and 3rd levels Mamainse Point, Point, Ontario. Ontario. levels of of the the Coppercorp Coppercorp mine mine at at Mamainse Both Both occur occur with with specular specular hematite. hematite. Copper Copper mineral mineral zoning zoning exextending from from carbonates and oxides through through native native copper, copper, black chaldocite and specularite, chalcocite specularite, "blue "blue chalcocite" chalcocite" and specularite, specularite, to to bornite and chalcopyrite is related to to nearness nearness to the the Keweenaw Keweenaw and related faults faults apparently down which circulated oxidizing solutions The solutions during during an an upward-migrating upward-migrating hydrothermal hydrothermal episode. episode. The former faults electrical anomalies, whereas former faults display positive SP electrical anomalies, whereas nearly perpendicular perpendicular cross faults faults with commercial commercial ores display display negative SP SP anomalies anomalies of this convective convective hydrothermal hydrothermal cell cell (Trow). (Trow). Such progressive hydrothermal fluids Such progressive oxidation of hydrothermal fluids is is suggested suggested for for the Keweenawan of Michigan by the USGS1s USGS's Woodruff, Woodruff, Cannon, Cannon, and and Back. Back. Ontario, Trow For Ontario, Trow deduces thermochemical thermochemical calculations calculations with with standard standard free free energies energies and typical activities activities for for constituents constituents (except (except for for oxygen, oxygen, whose whose activities activities are are the the unknowns). unknowns). These are arrayed on on a logarithmic logarithmic scale scale which mimics the the observed observed copper copper mineral mineral zones, zones, and in that sequence sequence AAuS U S -first ~firstoxidized oxidized to to deposit deposit gold gold at at the the which chalcopyrite chalcopyrite first same oxygen activity at which first oxidized to to covellite covellite and and specularite. specularite. At the the present present it it is is uncertain uncertain if if the the "blue chalcocite" chalcocite" exsolved exsolved ivto into chalcocite chalcodite and covellite from from llblue at low low temperatures, of original covellite digenite at temperatures, or if most of original covellite was was replaced replaced by by late late chalcocite chalcocite at at roughly roughly 2,500 2,500 times times the the oxygen oxygen activity activity at at which which covellite covellite originally originally formed. formed. Essentials Essentials for for gold at Coppercorp include include 1) 1) Keweenawan Keweenawan permeable permeable basaltic vesicular beds and conglomerates, conglomerates, 2) 2) felsite felsite intrusives intrusives with with permeable permeable border border breccias as as conduits conduits for for rising rising hydrohydrothermal solutions, solutions, 3) 3) nearness to the the Keweenaw Keweenaw and and related related faults faults with with positive positive SP SP anomalies, anomalies, 4) 4) mineralized mineralized cross cross faults faults with with ores ores yielding negative negative SP anomalies, anomalies, and 5) 5) "blue ''blue chalcocite". chalcocite". In Michigan, Michigan, field examination examination of ore ore deposits deposits and and structures structures mapped by the the USGS USGS shows shows that that the the major major lodes lodes (Baltic, (Baltic, Ashbed, Ashbed, Isle Royale, Pewabic, Osceola, Calumet Isle Royale, Pewabic, Osceola, Calumet conglomerate, conglomerate, and and Kearsarge) Kearsarge) and the Cliff, Cliff, Central, Central, and and Delaware Delaware fissure fissure deposits deposits all all displayS display negative SP anomalies. anomalies. The Keweenaw, Keweenaw, Hancock, Hancock, Mayflower, Mayflower, and and negative SF Gratiot—Suffolk 5? anomalies, anomalies, approapproGratiot-Suffolk faults faults all display positive SP priate for downward oxidative oxidative contamination contamination of of rising rising hypogene hypogene (not supergene) supergene) mineralization. (not mineralization. the best best matches to Canadian Canadian gold gold in From southwest to northeast the Michigan Michigan , so so far examined, examined, occur 1) 1) from from Mass Mass City City to to the the Indiana Indiana mine adjacent intrusives and the Keweenaw fault fault in adjacent to felsite intrusives in Houghton and and Keweenaw Counties the Allouez Ontonagon County, 2) In Houghton Gap Gap fault fault between Copper City and New Allouez is is near the Copper , 77 �2 City felsite felsite and and the the Keweenaw Keweenaw fault. fault. According to to Bornhorst, Bornhorst, page 132, 132, Randy Weege of C & H H thought thought that that this this fault fault perhaps perhaps was was aa fluid fluid pathway pathway for for 60% 60% of of the the district's district's copper copper production. production. Further, Further, it it replicates replicates and and improves improves upon upon the the best best geophysical geophysical signature signature at Coppercorp, Coppercorp, the the persistent persistent SB SB zone, zone, with with flanking flanking negative SP anomalies negative anomalies in in the the midst midst of of which which is is aa positive positive SP SP anomaly. anomaly. In Michigan, the positive "core" "core" anomaly splinters splinters the northern northern end end of of the the negative negative anomalies anomalies in the westward of offf the vicinity of vicinity of Abmeek. Ahmeek. This This part of the the district district contains contains arsenic, arsenic, which accompanies accompanies gold in in many western western mining mining camps. camps. 3) 3 ) In In 1999 1999 million tonnes tonnes of chalcocite Maki and and Bornhorst Bornhorst reported reported on on the the 4½ 4% million chalcocite amygduloids of the in drilled amygduloids the Gratiot Gratiot deposit deposit in in Keweenaw Keweenaw County, County, This lode where these these beds beds are are intruded intruded by by dacite dacite (felsite). (felsite). This lode appears appears at at the the intersection intersection with with the the southward southward extension extension of of Trow's Trow's negative negative SF SP anomaly anomaly as as observed observed at at the the Central Central mine mine and and 2¼ 2% miles miles 4) In Keweenaw County, County, the the USGS's USGS's Hank Hank Cornwall Cornwall on on the SSE. SSE. 4) to the pages 166—167 166-167 describes describes minor minor traces traces of of gold gold with with mainly mainly chalco— chalcocite and specularite specularite and some some covellite covellite and and chalcopyrite chalcopyrite in in an an amygduloid near near the the top top of of the the Greenstone Greenstone flow. flow. This This is is not not near near the Keweenaw fault, N.4°E. vertical fault fault, but it is is cut by a N.4OE. fault with a negative SF SP anomaly, anomaly, which must be intersected at depth by a a N.4°E., SF anomaly, anomaly, where it N.40~., 35°—45°NW. 350-45oNW. fault fault with a positive SP it is is exposed exposed to to the the east east of of the the vertical vertical fault. fault. There exists exists aa possipossibility for for aa horizontal horizontal ore ore shoot shoot at at these these faults' faults' intersection. intersection. These geologic map These four four possibilities possibilities are plotted on the latest geologic of the the Keweenaw Keweenaw peninsula, Peninsula, by by Cannon Cannon and and Nicholson. Nicholson. Not yet yet reconnoitered reconnoitered possililities possililities may may occur occur to to the the northeast northeast of of these. these. Remember, Remember, from from 1849 1849 to 1961 1961 the the old timers all missed the the Carlin Carlin "invisible" gold. ttinvisible" gold. Nevada is is now now the the biggest gold producing producing state state because of the observations, observations, thinking, thinking, and and Perseverance perseverance of of the the USGS's USGSss Ralph Ralph Roberts Roberts and and Newmont's NewmontlsJohn John Livermore. Livermore. REFERENCES REFERENCES CITED Bornhorst, T. P. J., 3., 1997, Tectonic context of native copper Bornhorst, 1997, Tectonic copper deposits deposits Midcontinent Rift System, System, in Geological of the North American Midcontinent in Geological 127-136. Society of America Special Special Paper Paper 312, 312, p. p. 127—136. Cannon, Nicholson, S. W., 2001, Geologic map of Cannon, W. F. and Nicholson, 2001, Geologic of the the Michigan, USGS USGS Geological Keweenaw Peninsula and adjacent area, area, Michigan, Geological Investigations Investigations Series Series Map Map 1—2696. 1-2696. Cornwall, Differentiation in lavas of the Cornwall, H. R., 1951, 1951, Differentiation the Keweenawan Keweenawan series Michigan, series and the origin of the copper deposits of Michigan, Geological Society no.2, p. p. 159—201. 159-201. v. 62, 62, no.2, Geological Society of of America America Bull. Bull. v. Maki, 3. Gratiot chalcocite Maki, J. C., 1999, 1999, The Gratiot chalcocite deposit, deposit, Keweenaw Keweenaw Peninsula, Michigan, Michigan Technological University, M.S. Peninsula, Technological University, M.S. Thesis, 71 Thesis, 71 p. p. Trow, Trow, 3., J., 1992, 1992, Inductive Inductive electrostatic electrostakic gradiometry gradiometry (IESG) (IESG) deciphers deciphers Keweenawan copper plumbing system, system, Soc. SOC. Mining, Mining, Metall. and and Expl. Phoenix Phoenix Meeting, Meeting, Preprint Preprint 92—32, 92-32, 22 22 p. p. Woodruff, L. G., Cannon, Cannon, W. W. F., and Back, 3. M., M., 1994, Woodruff, Back, J. 1994, Chalcocite Chalcocite Portage Lake volcanics, volcanics, Keweenaw mineralization in the Portage Keweenaw Peninsula, Peninsula, Michigan, Michigan, 40th 40th Ann. Inst. Inst. on on Lake Lake Superior Superior Geology, Geology, Houghton, Houghton, Abstracts, p - 77—78. 77-70. Abstracts, p. 78 �Using xenotime U-Pb geochronology tto unravel the the history of Proterozoic U-Pb geochronology o unravel Proterozoic sedimentary sedimentary basins: a study study in inWestern Western Australia Australia and and the the Lake Superior Region N.J., Rasmussen, B., Fletcher, dvallini@aeol.uwa.edu.au, McNaughton, N.J., Fletcher,I., I., Griffin, Griffin, B.J., B.J., Vallini, D.A., D.A., dvallini@cieol.uwa.edu.au, University of Western Western Australia, 35 University 35 Stirling StirlingHwy, Hwy,Crawley, Crawley, 6009, Australia Diagenetic xenotime ('(P04) (YPOJ is is aa trace trace constituent constituent in in aa wide wide variety variety of of siliciclastic siliciclasticsedimentary sedimentaryrocks. rocks, It typically forms pyramidal crystals of only a few microns in size, rarely exceeding exceeding 10 10 pm, pm, growing on (isostructural] detrital zircons. A recent study by Vallini et al. (2002) [isostructural] detrital (2002) showed showed convincing convincingpetrographic petrographic and age relationships that demonstrate this U-bearing phosphate phosphate could could begin begin forming forming in sediments at or just below the sediment-water sediment-waterinterface, interface, shortly shortly after burial. burial. A few years years ago ago itit was was discovered discoveredthat that itit is possible 0 pm possible to date xenotime crystals 1210 pmininsize, size,using usingthe theSHRIMP SHRIMP (Sensitive (SensitiveHigh HighResolution ResolutionIon Ion Microprobe), providing a robust Microprobe), providing robust isotopic isotopic age age for its its formation, formation, hence an age for for early early diagenesis diagenesisand andaa close proxy for sediment deposition. Xenotime is especially useful in that it has has very high high U U contents contents and remains remains closed to to radiogenic radiogenic parent-daughter parent-daughter mobility, unlike most most other other dateable dateablediagenetic diagenetic mineral. Diagenetic Diageneticxenotime xenotimeU-Pb U-Pbgeochronology geochronology has has the the potential potential to unravel unravel the the chrono-stratigraphy chrono-stratigraphy of unfossiliferous unfossiliferous sedimentary sedimentary basins, especially especially those sequences sequences devoid devoid of of dateable dateableinterlayered interlayered volcanic rocks. basins where where aa lack of aareliable main application application is in Precambrian Precambrian basins reliabletemporal temporal volcanic rocks. Its main framework hinders evolution and framework hinders an an understanding understanding of basin basin evolution and maturation, maturation, tectonic tectonic affiliations, metallogeny metallogeny and and value value as as exploration explorationtargets. targets. Xenotime also forms during post-diagenetic fluid flow events, such as alteration, mineralisation mineralisation and and metamorphism, as well as being metamorphism, as being aa magmatic magmatic mineral mineral and and aadetrital detritalheavy heavymineral. mineral.The Theexceptional exceptional its excellent range excellent properties in situ range of of formation formation conditions conditions of xenotime, xenotime, coupled coupled with with its properties for in situ geochronology, provide many many new new opportunities opportunities in in establishing establishing the the timeframe timeframe of events geochronology, provide events in inmany many hitherto hitherto poorly poorly understood understoodsedimentary sedimentarybasins. basins. Unusually coarse (up to 200 Unusually coarse 200 microns) microns) and and abundant abundant diagenetic diagenetic xenotime xenotime crystals c!ystals in inthe themetametasandstones of the greenschist greenschist facies Mount Mount Barren Barren Group, Group, southwestern southwestern Australia, Australia, allow allowthe thedetailed detailed study of xenotime within a phosphatic phosphatic sandstone interval and and is study xenotime and its host rock. rock. Xenotime occurs within present in multiple different styles styles - as cement present multiple morphologically morphologically different cement overgrowths overgrowths on zircons, zircons, pyramidal pyramidal overgrowths on on zircons, zircons, cement (no zircon) in shale laminations, replacement of shale (?) (7) intraclasts intraclasts overgrowths and as fluid events events from from early diagenetic to low and as xenotime xenotime crystals crystals within within intraclasts. intraclasts, Multiple Multiple fluid diagenetic to low temperature/early temperaturelearly hydrothermal, hydrothermal, prior prior to metamorphism, metamorphism, were recorded recorded within within single singlexenotime xenotimecrystals. crystals. U/Pb geochronology, accompanied by Ob~eNati0nS observations of of petrographic relationships relationships between SHRIMP UlPb of xenotime and other diagenetic the various various generations generations of xenotime and between between xenotime xenotime and diagenetic minerals minerafs and pyrobitumen,allowed allowedfor for the the construction construction of of a temporal for the diagenetic pyrobitumen, temporal framework framework for diagenetic and and early early hydrothermal events that that occurred within these rocks; (1) ca 1700 1700 Ma: deposition deposition of of partly partlyre-worked re-worked phosphatic siliciclasticsediments sedimentson on the the seafloor seafloor was was followed followed by by in-situ of the in-situ phosphatisation phosphatisation of the phosphatic siliciclastic sediments formation (mean (mean age age of of 1697i 1697± 77 Ma), (2) With burial, sediments and an initial period of xenotime formation burial, an an early pore-filling pore-filling carbonate cement was introduced into parts of the interval, i n t e ~ a las , well well as as early early diagenetic diagenetic cuboid pyrite growth, (3) ca 1650 1650Ma: Ma: during during burial burial diageriesis, diagenesis, aa fluidfluid- movement movementevent eventcaused causedthe the dissolution of of primary pore pore space space and formation of of xenotime xenotime (mean age of 1646 Ma),with with partial dissolution 1646 ±t 88 Ma), accompanying phosphate remobilisation, (4) Oil migration event, (5) Several accompanying Several silica silica cement cementgenerations generations introduced around this time, (6) ca ca 1560 1560Ma: Ma: minor minor addition addition of of xenotime xenotime rims rimsto toexisting existingovergrowths, overgrowths, (7) ca ca 1480 1480 Ma: Ma: addition addition of of xenotime xenotime cement cement (no (no zircon) zircon) in in shale shale interlaminations, interlaminations, (8) (8) ca ca1200 1200Ma: Ma: peak peak of metamorphism. metamorphism. Wavelength Dispersive Spectrometer (WDS) microprobe analysis of each type type of of xenotime xenotime showed showed a gradual from LREE enrichment enrichment to to MREE MREE enrichment, enrichment,with withtime. time. Due Due to to this this radational gradual change change from gradational nature, discrete boundaries between generations, based based on on chemistry, chemistry, could could not not be established, established. This study of diagenetic to hydrothermal hydrothermal xenotime dramatically dramatically improved improved the the estimated estimatedage agerange rangeof of the Mount Mount Barren Group, which was was previously previously constrained constrained to 1200 1200 Ma Ma(peak (peakmetamorphism) metamorphism)and and 1790 Ma Ma (youngest (youngest detrital detrital zircon zircon population), population), and and discounted discounted some some previous previous tectonic tectonic models models concerning the the timing of collision between between major major cratons cratons within western Australia Australia and andthese these cratons cratons with East East Antarctica. Antarctica. Using the information gleaned gleaned from the study of xenotime in the Mount Mount Barren Barren Group, Group, aa similar similar study study basin in in the the Lake is currently underway on another Proterozoic sediment-dominated basin Lake Superior Superior Region Region containing the the Marquette Range Supergroup Supergroup and and its its equivalents, equivalents, the the North Range, Mille Marquetle Range Mille Lacs Lacs and and containing The early Proterozoic strata consist of of three unconformity-bounded unconformity-bounded lithostratigraphic Animikie Groups. The lithostratigraphic groups consisting of glaciogenics, quartzites, dolomite, iron iron formation, greywacke greywacke and andshale shaleand andminor minor intercalated volcanics. Sedimentation Sedimentation is is thought thought to to have begun Ma (correlation of Chocolay intercalated volcanics. begun —2240 -2240 Ma Chocolay 79 �Group with with Gowganda Gowganda Fm, Fm, upper upper Huronian Huronian Supergroup, Ontario) (Fairbain (Fairbain et al., 1969) 1969) and and ceased ceased by by Group —1850Ma Ma(coinciding (coincidingwith with orogen-normal orogen-normalarc arc collision collision along along the the Niagara Fault zone and the Malmo -1850 Malmo Discontinuity, during during the the Penoken Penoken Orogeny) Orogeny)(Sims (Simsetetal., al., 1993). 1993).Part Partof of the the study study is is to to determine Discontinuity, determine if xenotime-rich horizons, such such as as that in the Mount xenotime-rich horizons, Mount Barren Barren Group, can be located located in in this this stratigraphy stratigraphy and to to document document the thesedimentological, sedimentological,structural structuralor orstratigraphical stratigraphicalfeatures featuresthat thatthey theyhave haveinincommon. common. Ceflain rock units units from from the the different different sequences sequences over over the whole whole region region were were targeted targeted for for xenotime xenotime Certain analysis using proposed proposed sedimentological sedimentoiogical controls for xenotime formation that were determined from the Mount Mount Barren BarrenGroup Groupstudy. study. One sedimentary feature favourable to xenotime formation may be the presence of large quantities of sedimentary sedimentary apatite apatite within within the the host host rock rock or or adjoining adjoining rocks. rocks. A A field fieid sample sample of of low low greenschist greenschist facies facies phosphatic chert-conglomerate,atat the the base base of the Baraga phosphatic chert-conglomerate, Baraga Group, Group, from from aa documented documented phosphorite phosphorite locality locality in in the Dead Dead River River Basin, Basin, northern norlhern Michigan, contains large quantities of xenotime ranging from <30 <30 pm pm pitted pittedovergrowths overgrowthson on detrital detritalzircons, zircons,to to>100 >I00micron micronxenotime xenotimecements. cements. Other rock units units that that contain contain xenotime xenotime overgrowths overgrowths and cements of appreciable appreciable size and quantity, were; were; (i) (i) quartzite quartzite beds beds ininseveral several drillholes drillholesthrough through the the Mahnomen Mahnomen Formation, Formation, Mille Mille Laos Lacs Group, Group, Cuyuna Range, Range, contain up up to to 50 50 xenotime xenotime crystals crystals per per thin thin section, section, some someofofthese theseup uptoto—60 -60 pm pmin in size, (ii) (ii) aa sandstone sandstone bed bed within drillcore from the base of the Baraga Group in Dead River Basin- its largest was 60 o b s e ~ e dwas 60 pm pm(Di) (iii) aagrit-pebble grit-pebbleconglomerate conglomerate and and very very coarse-grained coarse-grained largest xenotime xenotime observed sandstone sandstone outcrop outcrop at Slate Slate River River Kill Hill locality, locality, Baraga Baraga Basin, Basin, which is is assumed assumed to to lie lie at at the the base base of of the the xenotime grains grains per per thin thin section which are pm in in size, and (iv) Baraga Group, averaged averaged —15 -15 xenotime are up up to to —60 -60 prn conglomerate at Big Eric's Crossing locality, Baraga Basin, contains the basal basal Baraga Baraga Group hematitic conglomerate up to 55 xenotime xenotime grains grains per per thin thin section, section, some some of of these these are are up uptoto—100 -100 pm pm in insize. size. Pokegema Pokegema Quartzite Quartzite samples, samples, West West Mesabi Mesabi Range, Range, showed showed minor <30 pm prn xenotime xenotime overgrowths overgrowths on on zircons. zircons. All of of the therock rocksamples samplesdescribed describedabove aboveare arevery verycoarse-grained coarse-grainedsandstone/conglomerate sandstone/conglomerate beds beds which are either which either located located near near aastratigraphic stratigraphic boundary boundary and/or andlor are are interbedded interbedded with with shale shalebeds. beds. Xenotime Xenotirne from these localities were analysed on the SHRIMP SHRIMP and and revealed revealed several several age age groups; groups; (i) (i) xenotime xenotime in in the the Mahnomen MahnomenFormation Formationdrillcore drillcorerevealed revealedages agesofof—1870 -1870 Ma Maand and—1770 -1770 Ma (1760-1790 (1760-1790 Ma), (U) One large xenotime overgrowth from the Dead drillcore,gave gavean anage ageofof—2600 -2600 (ii) One Dead River RiverBasin Basindtilfcore, Ma, (Di) xenotime contained contained within within the the Slate River Hill outcrop (iii) xenotime outcrop yielded yielded ages ages of of —2500 -2500 Ma, Ma, (iv) the samples from Ma. The The samples from Big Big Eric's Eric's Crossing Crossingcontained containedxenotime xenotimeshowing showingages agesofof—2550 -2550 Ma Ma and and —1750 -1750 Ma. Ouartzite in in the the West West Mesabi Range, Range, contained contained xenotime xenotime with with an an age age of of Pokegem Quartzite sample from the Pokegema -2300 -2300 Ma Maand and—1770 -1770 Ma. Ma. Xenotime yielding yielding ages ages of of ca 2500 Ma Xenotime Ma or or older older may may be befrom fromrecycled recycleddetrital detrital(magmatic) (magmatic) grains. grains. The younger Ma (1760-1790 (1760-1790 Ma), Ma), occurs occurs in in xenotime from widespread localities younger age age of of —1770 -1770 Ma localities across across the Lake Lake Superior Superior Region Region and may may reflect reflect an an epigenetic epigenetic thermal thermal event event across across the the region. region. The The age age Maof of anorogenic anorogenic magmatism, magmatism, pluton pluton emplacement emplacement and and appears to correlate correlate with an an episode episode at at —1760 -1760 Ma gneissic doming gneissic doming recorded recorded throughout throughout Wisconsin, Wisconsin, northern northern Michigan Michigan and and central centralMinnesota. Minnesota. ItIt partial melting of crustal rocks postdates the Penoken Orogeny and involved paflial rocks as as aa result result of of continentcontinentcontinent or continent-arc collision to the south south of the the region region (Sims, (Sims, 1996). 1996). This This event event is is approximately approximately coeval with the development of the Central Plains Plains Orogen (1800-1630 Ma) to the south and may be a consequence consequence of the the accretion accretion of this this terrane terrane to to the the North North American American continent continent(Sims, (Sims, 1996). 1996). This study highlights highlights the sensitivity of in-situ xenotime geochronology to identifying identifying cryptic fluid flow events within within basins. basins. This This study study will be be ongoing ongoing in in 2003-2004. 2003-2004. Fairbafrn H.W., Hurley, P.M., P.M., Card, K.D. K.D. and and Knight, C.J., C.J., 1969, Correlation and radiometric ages of Fairbaim H.W., Nipissing metasediments with Proterozoic Canadian Journal Journal of Nipissing Diabase Diabase and Huronion Huronlon metasedlments Proterozoic events in Ontario: Canadian Earth Sciences, v. 6, p. D. 489-497. Sims, P.K., Proterozoic Penokean eds., Archean and P.K., 1996, 1996,Early Ear1y'~roterozoic PenokeanOrogeny, Orogeny,ininSims SlmsP.K. P.K.and andCarter, Cafier, L.M.I-1., L.M.H., eds., and Late Survey Late Proterozoic Proterozoic Geology Geoloav of the Lake Lake Superior Suoerior Region, Realon. U.S.A.. 1993: 1993:U.S. US.Geological Geoloaical S u ~ eProfessional vProtessional -, of - . U.S.A., u Paper 1556, p.28-60. 1556, p. 28-60. Sims, al., 1993, Region and Trans-Hudson Trans-Hudson Orogen, in J.C., Jr., Jr., and Slms, P.K., P.K., et al., 1993, The Lake Superior Region in Reed, Reed, J.C., and others, eds., eds., Precambrian:Conterminous Society of of America. America, the the Precambrian:Conterminous U.S.: US.: Boulder, Colorado, Geological - Souetv Geology of ~ North v. C-2, p. o r l America, hAmerica, v. p. 11-120. 11-120. Vallini, B.,. Kranez. Krapez, B.,. Fletcher. Fletcher, l.R., Vallini.. D., D...Rasmussen, Rasmussen. 5.. 1.R.. and and McNaughton, McNauahton. N.J.. 2002, 2002.. Obtaining Obtainina diagenetic diaoenetlc , . B.. - . N.J., ages from metamorphosed sedimentary rocks: U-Pb of unusually metamorphosed sedimentary U - ~ dating dating b unusually coarse coarse xenotime xenotirne cement cement in In phosphatic v.30, phosphatic sandstone: Geology, v. 30,p. p.1083-1086. 1083-1086. ~ ~~ 80 . ~-~ ~ �EVALUATION OF INITIAL MAGMA COMPOSITIONS COMPOSITIONS FOR FOR THE THE BALD BALD EAGLE EAGLE INTRUSION INTRUSION AND ASSOCIATED ROCKS VISLOVA, Tatiana, Tatiana, Department Department of of Geology Geology and and Geophysics, Geophysics, University University of of Minnesota Minnesota VISLOVA, funnel-shaped concentrically-zoned concentrically-zoned Bald the Duluth Duluth The funnel-shaped BaldEagle Eagle Intrusion Intrusion in in the Complex is characterized characterized by very very restricted mineral compositions, and consists consists of of only only two units: units: ananolivine—plagioclase olivine-plagioclase cumulate cumulateand andananolivine—plagioclase-clinopyroxene olivine-plagioclase-clinopyroxene cumulate (Weiblen, 1980). In terms terms of of differentiated differentiated units units (Weiblen, 1965; Weiblen Weiblen and Morey, 1980). expected in the Bald to be expected in a typical typical layered layered intrusion, intrusion, the Bald Eagle Eagle Intrusion Intrusion appears appears to be petrologically petrologically incomplete. incomplete. This This has has raised raised the question question whether the four-phase (olivineplagioclase-clinopyroxene-oxide)cumulates, cumulates, assigned assignedto to the the Greenwood Lake Intrusion plagioclase-clinopyroxene-oxide) found to to the south of the Bald Eagle (Miller et al., 2002), and granophyre granophyre found Eagle Intrusion Intrusion are genetically related to the Bald Eagle Intrusion Intrusion (Weiblen (Weiblen and and Morey, Morey, 1980). 1980). New petrographic studies and and microprobe analyses (Vislova, (Vislova, 2003) 2003) make make it New petrographic studies microprobe analyses possible to evaluate for the Bald possible evaluate parent parent magma magma compositions compositions for Bald Eagle Eagle Intrusion, Intrusion, and and quantitatively assess possible petrogenetic relationships between the Bald Bald Eagle Eagle Intrusion Intrusion and spatially associated rocks. Computer programs programs (MELTS, Ghiorso and Sack, Sack, 1995; 1995; Ariskin et et al., al., 1993) were used to investigate and COMAGMAT, COMAGMAT, Ariskin investigate these questions. questions. A A primitive North North Shore Volcanic Group olivine tholeiite tholeiite (P-melt) (P-melt) was used as as an an initial initial composition (Miller magma composition (Miller and and Ripley, Ripley, 1996). 1996). Equilibrium crystallizationofof P-melt, P-melt, calculated calculated by by MELTS Equilibrium crystallization MELTS at at I1 atm total total pressure and oxygen quartz-fayalite-magnetite pressure oxygen fugacity fugacity near near ororbelow belowthethe quartz-fayalite-magnetite buffer, buffer, reproduces the crystallization order and mineral assemblages observed in the Bald Eagle Intrusion. The The calculated calculated composition compositionof ofthe thefirst firstclinopyroxene clinopyroxene(mg (mg— 81) 81) equals the one of the first plagioclase and olivine observed, however calculated compositions of olivine are are much much higher than those This could be ascribed higher those observed. observed. This ascribed to to the the dynamics dynamics of ofcrystal-melt crvstal-melt - than segregation in in a flowing segregation flowing magma system. Until the crystals crystals suspended suspended in magma magma grow grow large enough they might be carried away, erupted, and found as phenocrysts in lavas. lavas. At -—7 At 7 % melt melt remaining remaining MELTS MELTS reproduces reproduces the most most evolved evolved mineral mineral compositions in in the Bald Eagle This suggests that the Bald compositions Eagle Intrusion Intrusion (Fig. (Fig. 1). This suggests that Bald Eagle Eagle Intrusion might might be be a complete sequence with with a few Intrusion complete crystallization crystallization sequence few percent percent remaining remaining melt. the question melt. It leaves leaves unanswered unanswered the question of of the the origin origin of of four-phase four-phase cumulate cumulate and and granophyre. Modelingshows showsthat that aa more more evolved evolved high high Ti Ti and and high high Fe melt Modeling melt (D-melt) (D-melt) is required for crystallization of the evolved evolved units units in in the the Greenwood GreenwoodLake Lake Intrusion Intrusion (Fig. (Fig.1). 1). This melt of P-melt in an melt can can be be produced produced by by fractional fractional crystallization crystallization of an intermediate intermediate magma chamber at 2-3 kbar total total pressure. Equilibrium crystallization of of D-melt D-melt at 1 atm Equilibrium crystallization atm reproduces reproduces the the crystallization crystallization order, the the appearance of Fe-Ti Fe-Ti oxides, oxides, and and the the compositions most of order, appearance of compositions ofof most of the the units units associated with the Bald Eagle Intrusion (Fig. I). 1). However, However, the the most most evolved evolvedrocks rocks in in the the Greenwood LakeIntrusion Intrusion(ferrogabbro (ferrogabbrowith withFoFo<c 50) 50) and Greenwood Lake and granophyre granophyre were were not not reproduced crystallization. These units might require fractional reproduced by equilibrium equilibrium crystallization. require fractional crystallization or assimilation. assimilation. - 81 �. 65 c. C x 0 I- 6•, >1 . ,. I • 4: A -. 265 65 A£As Zig, A • 1 4 .f s0 E , 6060 A 5 A AA - A . 55 55 1 <>Bald EagleIntrusion Intrusion oBald Eagle tGreenwood AGreenwoodLake LakeIntrusion Intrusion +Calculated from P-melt P-melt • Calculated from sCalculated A Calculatedfrom from D-melt D-melt I 30 30 40 40 50 60 50 60 Mg/ (Mg+Fe) (Mg+Fe)in in olivine olivine Atom % % Mgf 70 70 60 80 MgI(Mg+Fe) variations variations in in coexisting olivine and Fig. 1. Mg/(Mg+Fe) and clinopyroxene. clinopyroxene. References: References: Ariskin, A.A., Frenkel, M.Y., MY., Barmina, Barmina,G. G. S., S., and and Nielsen, Nielsen,R. R. L., L.,1993, 1993,COMAGMAT; COMAGMAT;aa differentiation processes. processes, Computers & Geosciences, 19 FORTRAN program to model magma differentiation (8), p. 1155-1170. 1155-1170. Ghiorso, mass transfer transfer in magmatic processes; IV, A Ghiorso, M. S., and Sack, Sack, R.O., 1995, 1995, Chemical mass revised and internally consistent thermodynamic model for the interpolation interpolation and internally consistent and extrapolation extrapolation elevated temperatures and pressures, of liquid-solid equilibria in magmatic systems at elevated Contributions to Contributions to Mineralogy Mineralogvand and Petrology, Petrolopy, 119 119(2-3), (2-3),p. p. 197-212. 197-212. Hauck, S.A., Peterson, Peterson, D.M., D.M., and Miller, J.D., Jr., Green J.C., Severson, Severson, M.J., Chandler, V.W., Hauck, Wahl, T.E., 2001, Geology and mineral potential of the Duluth Complex Complex and related related rocks rocks of Geological Survey SurveyReport Report of of Investigations Investigations 58,207 58, 207pp. pp. + + northeastern Minnesota: Minnesota Geological disc in back pocket, 2002. compact compact pocket, 2002. Miller J.D., Jr. and and EM. E.M.Ripley, Ripley,1996, 1996,Layered Layeredintrusions intrusionsofofthe theDuluth DuluthComplex, Complex,Minnesota, Minnesota, USA. In: In: Cawthorn Cawthom R.G. (ed.) (ed.)Layered Lavered Intrusions, Intrusions, 531 pp. Vislova, T., 2003, Petrology of the Bald Eagle Intrusion and associated rocks rocks and its its relevance relevance to to crystallization in dynamic magma chambers in in the the Midcontinent Midcontinent Rift, Ph.D. Dissertation, crystallization Dissertation, University of Minnesota, Minnesota, 226 226 pp. pp. Weiblen, P.W. and Morey, Ci. B., 1980,AAsummary summaryof ofthe thestratigraphy, stratigraphy, petrology, petrology, and and structure structure G. B., 1980, of the Duluth Complex. In: In: Irving, Irving, A. J., and Dungan, M. A. A. (ed.), (ed.),1980, 1980,The TheJackson Jacksonvolume, volume, p. 88-133. 88-133. American Journal of Science, Science,Vol. Vol. 280-A, 280-A, Part Part 1,1,p. funnel-shaped, gabbro-troctolite gabbro-troctolite intrusion in the Weiblen, P.W., 1965, 1965, A funnel-shaped, the Duluth Duluth Complex, Complex, Lake Lake Ph.D. Dissertation, Dissertation, University County, Minnesota, Ph.D. University of Minnesota, 161 161pp. pp. 82 �A Hydrothermal Component A Component of Iron Formations-A Marquette Range Perspective T.D., 141 141 Chippewa, Chippewa, Negaunee, MI MI 49866 49866 Waggoner, T.D., The origin of of Lake Lake Superior Superior banded banded iron iron formations formations (BIF) (BIF) has has been been aa contentious contentious issue issue for at least aa century century and and aa half. half. Concepts of origin include include weathering, weathering, volcanic and organic activity activity whereby ions are carried Clear organic whereby ions carried in in and andprecipitated precipitated from from solution. solution. Clear definition of of the definition the source, source, mode mode of of transport transport orordepositional depositionalmechanisms mechanisms isis generally generally lacking. This Thispaper paperwill willaddress addressthe the strong strongevidence evidence for for aa hydrothermal hydrothermal source for "hard ores" found in the upper parts of ores" of the the Negaunee Negaunee Iron Formation (NIP) (NIP)and by extension a possible source BIP portion. The source for for the precursor hematite in the BIF TheRange Range was was formed formedin in aa tectonically active area believed to be an extensional rifting environment not unlike those active an extensional rifting environment unlike those found in Fe-Oxide (Cu, U, U,Au, REE) and some VIIMS VHMS deposits deposits The Marquette Range portion of the the Lake Lake Superior SuperiorIron Iron District District displays displays many many features features similar to other large Lake Superior BIFs found around the world and, thus, making it an excellent study subject subject for for the source excellent study source and and role role played played by by igneous igneous and andsedimentary sedimentary processes. The TheNegaunee Negauneeand andbasal basalGoodrich Goodrichunits unitsexhibit exhibitBlIP, BIF, soft soft supergene supergeneenriched enriched concentrationsand and "hard "hard ores" ores" as massive concentrations massive bodies, bodies, banded banded jaspilites jaspilites and and detrital detrital "Hard ores" conglomerates. "Hard conglomerates. ores" are generally generally dense silver gray to black black massive massive metallic metallic magnetite or schistose metallic hematite associated with jaspilite jaspilite and contain in excess of of of the the origin of the 60% iron. iron. Discussion Discussion of the "hard "hard Ores" Ores" on on the theMarquette Marquette Range Range has has metamorphism or revolved revolved around around supergene supergene enrichment enrichment prior to metamorphism or hydrothermal hydrothermal enrichment associated associated with the Penokian Orogeny. Orogeny. of the field field geology geology do donot not support support with with either eitherof ofthese thesepositions. positions. First, the Many features of cobble and pebbles of jasper hematite in the the basal basal Goodrich Goodrich conglomerate conglomerate show show random random orientation of of the 'schistose' orientation 'schistose' hematite hematite indicating indicating the the schistose schistose nature nature of of the the hematite hematite not a result of metamorphism. In existed prior to emplacement and not In addition addition many many of of the the with the "hard rocks associated associated with "hard ores" ores" exhibit exhibit hydrothermal hydrothermal minerals minerals including including sericite, sericite, tourmaline, chlorite, chloritoid, high aluminous silicates, garnet, hematite, magnetite and tourmaline. The lower and Menominee below the the NIP lower Proterozoic Proterozoic Chocolay Chocolay and Menominee sediments sediments below NIF exhibit exhibit Specular, multiple examples examples of of high-grade hematite that that can be interpreted multiple high-grade hematite interpreted as as vents. vents. Specular, microplaty and and bytroidal bytroidalhematite hematiteare arefairly fairlycommon commonininmany manyoutcrop outcropareas. areas. Some Some of of microplaty All the sites these have been described previously in literature while others have not. previously others have not. All sites were subject to exploration for iron ore during century and most exhibit exhibit were exploration for during the the late late 119th9 century shallow shafts. The The major major components components are are chert, chert, jasper and and vein quartz along along with coarse specular, microplaty microplaty and and bytrioidal bytrioidal hematite hematite contained contained in in breccia breccia zones zones that exhibit specular, exhibit episodic reworking. reworking. There episodic There are are alterations alterations to the host host rock rock as as some some occurrences occurrences exhibit exhibit chlorite, silica, silica, k-spar and aluminous alurninous silicates. silicates. A large area in sections sections 21, 22, and and 23, 23, 47-26 47-26 contain contain multiple multiple enriched enriched hematite hematite sites breccia zones adjacent to northwesterly trending faults. that form two northwest trending breccia In addition there is aa In addition to the the silica silica flooding, flooding, brecciation brecciation and and hematite hematite concentration concentration there significant area area of of andalusite cordierite and and chloritoid chloritoid adjacent adjacent to to the eastern significant andalusite cordierite eastern linear linear 83 �breccia zone in section 23. These section 23. Theseminerals minerals are are present present in aa much much broader lower regional chlorite zone of metamorphism metamorphism and most likely are a result of the hydrothermal event that impacted the three square square miles miles referenced referenced above. above. A conglomerate in Sec. 22 and 23, 47-26 has been previously described as "unusual" and 23,47-26 is sandwiched sandwiched between between lower Chocolay Chocolay argillite argillite units. units. Clasts causing dimpling in the underlying argillite were described as rafted elasts clasts from a glacial interlude. interlude. It is unlikely reef growth growth during during the the same period of of that a glacial event coincided with significant algal reef time. The "unusual" is extremely coarse, tightly tightly packed packed and and shows no time. "unusual" conglomerate conglomerate is extremely coarse, sedimentary features, sedimentary features. In addition significant rinds of k-spar have formed on the the granite granite gneiss cobbles. The The cobbles cobbles and and matrix matrix contain contain euhedral euhedral magnetite, martite and specular hematite suggesting suggesting this this area was tectonically active and may well have been been an an active active vent area over a period vent period starting starting at the the earliest earliest extensional extensional period period and continued continued to be active beyond resembles some some of of the breccias beyond the Ajibik Ajibik time. time. The conglomerate conglomerate resembles breccias at Olympic Dam and could well be a hydrothermal breccia. REE chondrite normalized analysis of of the hematite vents match quite closely closely with with both both the hematite, mametite magnetite and combination the hematite. combinationhematite/magnetite hematitelmasnetite "hard ores" found found throughout the the Range. Range. Recent hematite confirms throughout Recent work work on on the the Brockman Brockman microplaty microplaty hematite confirms a hydrothermal hydrothennal origin due to the recognition of surrounding alteration to the host. - - Initially the the vent vent areas areas were were studied studied in in relation relation to to the "hard "hard Ores" Ores" but the fact fact that that the the vents are all hematite suggests they could be the source for the precursor hematite seed hematite the source for the precursor hematite seed cores that Han Han identified identified in low low metamorphic metamorphic grade grade BIF BIF units. units. These seed cores cores have have been been identified identified in in the theNegaunee, Negaunee, Biwabik, Biwabik, Brockman, Brockman, Solcoman, Sokoman, Temiscamie Temiscamie and and Kuruman Iron hon Formations. Formations. that the extensional phase of rifting ceased ceased near near the the end of NIP time It is quite plausible that ofNIF and and aa reverse reverse compressional compressional event event started started causing causing faulting faulting that that produced producederosional erosional material for the basal Goodrich conglomerate. conglomerate. Reference: Reference: Origin of of Magnetite hon Formations Han, T.H., 1988, 1988, Origin Magnetite in Precambrian Precambrian Iron Formations of Low Low Han, T.H., Metamorphic Grade, Grade, Proceedings Proceedings of of the Seventh Metamorphic Seventh Quadrennial Quadrennial IAGOD Symposium, Symposium, p. 641-656. 641-656. 84 �nigh-Resolution High-ResolutionMultibeam Multibeam Bathymetry Bathymetry in in Lake Lake Superior. Superior. N. J. N. J. Wattrus Large Large Lakes Lakes Obsewatory, Obseruotory,University Universityof of Minnesota, Minnesota, Duluth, Duluth,MN MN 55812 55812 Like all large large lakes, lakes, the the composition composition and and shape shapeof ofthe thelake lakefloor floorof of Lake Lake Superior Superior reflects the processes that well as as in in the past. that shape shape its its formation formation today as well Maps of of the the lake floor floor made made with withtraditional traditional echosounders echosounders lack the resolution resolution to these processes preserved preserved in in permit scientists to read the subtle subtle "fingerprint' "fingerprint" of of these the lake-floor. advent of of modem, modem, high-resolution high-resolution multibeam multibeam sonar has has lake-floor. The advent revolutionized the the mapping mappingof of the the sea-floor. sea-floor. In a traditional echosounder, echosounder, the depth depth to to the thelake-floor lake-floor below the ship ship is measured measured by by timing timing how how long long It it takes for an an acoustic acousticping ping to to travel travel to to and back to the ship. the lake-floor and ship. the delay, the deeper The longer the deeper the the lake This type of surveying lake floor floor is. This surveying provides high-resolution bathymetric information along the trackline trackline followed by the the survey survey boat. is followed by boat. Nothing Nothing is about the known about the lake lake floor floor either either side. side. High-resolution multibeams use a fan of acoustic acoustic beams beams (over (over100) 100)to measure the shape of the measure the shape of the lake lake floor floor along a "swath'. "swath".By sailing a series of overlapping swaths, It it is possible to achieve complete complete coverage coverage of the lake lake floor at high resolution. resolution. Backscatter Backscatter information informationcollected collected with with the bathymetric data can can be used to to create psuedo-sidescan images of the create psuedo-sidescan images These can can be used to map lakefloor. These map spatial variations variations in the spatial the composition composition of the the lake lakefloor. floor. from the the catalog catalog of ofmultibeam multibeam surveys surveys conducted conducted by the Examples, drawn from Large Lakes LakesObservatory, Observatory,are arepresented. presented.These Theseillustrate ifiustrate the the potential potential of of this this Large formapping mapping the the subtle subtle signal processes technology for signal of past geologic processes superimposed on superimposed on the the lake lakefloor. floor. 85 � https://digitalcollections.lakeheadu.ca/files/original/8525569c75ccfe6981beac1369d7b7e6.pdf 8131941cb506976ea49cb08392f013c4 PDF Text Text INSTITUTE ON ON LAKE LAKE SUPERIOR SUPERIOR GEOLOGY GEOLOGY INSTITUTE 49t 4 9 'Annual ~ n n u aMeeting Meeting l Proceedings Volume 49 Part 2- Field Trip Guidebook QuinnesecMine, Mine.Menominee MenomineeIron IronDistrict District Quinnesec Wausecapyritic pyriticslate slate Wauseca A' Refoldedfold foldstyle styleof ofIron IronRiver-Crystal River-CrystalFalls Fallsallochthon allochthon Refolded Iron Mountain, Mountain, Michigan Michigan Iron May 7-11, 7- 11,2003 May 2003 �INSTITUTE ON LAKE SUPERIOR GEOLOGY 49TH ANNUAL MEETING MAY 7-12, 2003 IRON MOUNTAIN, MICHIGAN HOSTED BY: HOSTED Laurel Laurel G. Woodruff and William William F. F. Cannon Co-chairs Co-chairs U.S. Geological Geological Survey Survey With assistance from Michigan Technological University University and and John Gartner, John Gartner, Coleman Engineering Engineering Company Company Local committee committee representative representative Proceedings Proceedings Volume Volume 49 49 Part 2 2— Field Trip Trip Guidebook Guidebook - Field Compiled Compiled and and edited editedby byWilliam WilliamF. F.Cannon, Cannon,USGS USGS Illustrations prepared Dickenand andStacy StacySaari, Saari,USGS USGS Illustrations preparedby byConnie ConnieL.L.Dicken �CONTENTS Proceedings Volume Volume 49 49 Proceedings Part 2— 2 - Field Trips Part Trips Overview—Paleoproterozoic stratigraphy and Overview-Paleoproterozoic stratigraphy and tectonics tectonics along along 1 the the Niagara Niagara suture suture zone, Michigan Michigan and Wisconsin Wisconsin. ....... .1 Trip Trip 1. 1. Pembine-Wausau Pembine-Wausau magmatic magmatic terrane Trip Trip 2. Menominee Menominee Iron Iron District District. ................. . 3333 .......................... . 47 47 Trip Trip 3. Stratigraphy Stratigraphyand and structure structureof of the the Iron Iron RiverRiver644 Crystal Crystal Falls Falls basin basin. ................................ . 6 Trip Trip 4. Life Lifecycle cycleof ofan aniron irondeposit—the deposit-the Republic RepublicMine Mine 877 from from ore ore genesis genesis to mine mine restoration restoration ................ . 8 Cover illustrations: illustrations: Wauseca Wauseca Pyritic Pyritic Member Member of Dunn Dunn Creek Creek Slate. AAsulfide sulfidefacies faciesiron-formation iron-formationfrom fromthe the Iron Iron River-Crystal River-Crystal Falls district, Michigan. Original Original photograph photograph appears appears in USGS in USGS Professional ProfessionalPaper Paper 570. 570. Schematic illustration illustration of cross-folding cross-folding in in the the Iron Iron River-Crystal River-CrystalFalls Falls district district as as depicted depicted USGS Professional ProfessionalPaper Paper 570. 570. in USGS Quinessec Mine in the Menominee Iron District at at Quinnesec, Quinnesec, Michigan. Michigan. Photograph by Photograph by Elizabeth Heinen. Heinen. Elizabeth �PALEOPROTEROZOIC STRATIGRAPHY AND TECTONICS ALONG THE PALEOPROTEROZOIC STRATIGRAPHY NIAGARA NIAGARA SUTURE ZONE, MICHIGAN AND WISCONSIN G.L. LaBerge1, ~ a ~ e r g eW.F. ' , cannon2, J.S.Klasne,3, ~lasne?, W,Ojakangas4 0jakangas4 Cannon2, K.J. K.J. Schulz2, Schuli, J.S. R.R.W. U.S. University Wisconsin-Oshkosh(retired) (retired)and and U.S. U.S. Geological GeologicalSurvey, Survey,2* U.S. University of Wisconsin-Oshkosh Geological Survey, Geological Survey, Western Illinois University (retired) and U.S. U.S. Geological GeologicalSurvey, Survey, University (retired) 4 University of Minnesota Minnesota Duluth Duluth (retired) (retired) INTRODUCTION INTRODUCTION The Niagara Niagara suture zone formed during the Penokean Penokean orogeny at about 1875 1875 Ma by collision of island island arcs of the Wisconsin magmatic magmatic terranes with the southern margin of the Superior craton and its epicratonic cover of the Marquette Marquette Range Supergroup. The Niagara Niagara fault, fault, generally generally considered considered the the principal principalsuture suture Supergroup. boundary, has an arcuate trace across northern Wisconsin defined defined primarily from geophysical data, sparse sparse outcrops, outcrops, and and widely widely spaced spaced drill drill core core data data(fig. (fig.1). 1).In In geophysical more detail, the Niagara fault is but one of a family of subparallel anastamosing anastamosing faults that bound bound structural panels of Paleoproterozoic Paleoproterozoic rocks, which together comprise the Niagara suture zone (fig. 2). The rocks within these panels have a distinctive structural style marked marked by by tight tight folds folds with with widely widely varying varying but but commonly commonly steeply plunging axes. These probably formed by refolding of simpler, gently plunging folds, which are widely recorded recorded in in correlative correlative strata strata north north of of the the suture suture zone. A blanket of glacial material material over most of the region, the on-lap of Paleozoic thick cover cover of of Keweenawan Keweenawanrocks rocksof of Paleozoic rocks rocks in northern northern Michigan Michigan and and aa thick the Midcontinent rift system in northwestern Wisconsin and eastern Minnesota Minnesota have obscured the suture zone over most of its length in the southern Lake Superior region. region. The area area described described in in this this guide guide is is the the only only area areawhere where extensive exposures allow direct observations observations of stratigraphic stratigraphic and and structural structural features along features along the collision collision zone. The purpose of these field trips is to examine exposures along and on both sides of the Niagara suture zone. Stops were chosen to illustrate: 1) the Paleoproterozoic Paleoproterozoic sedimentary sedimentary sequence sequence developed developed on on the the continent continent margin; margin;2) 2) magmatic terranes; and 3) igneous rocks that constitute parts of the Wisconsin magmatic some of the structural collision of the island island structural features produced as a result of the collision arcs with the continent margin, and their influence influence on interpretations interpretationsof the stratigraphy. stratigraphy. 1 �92' 84' 88' EXPLANATION 46' Pajeozoic strata- limestone, dolomite, sandstone shale Grenville Province- Middle Proterozoic gneiss and Ed'dáiplutonic rocks fiIi.± Midcontinent Rjft- Middle Proterozoic flood basalt, rhyolite, sandstone, and gabbroic intrusive rocks Anorogenic pluton- Middle Proterozoic granite, " anorthosite Marquette Range Supergroup andAnimikie GroupEarly Proterozoic metasedimentary and meta vol conic rocks. Iron ranges in black Wisconsin Maginatic Terranes- Early Proterozoic metavolcanic, metasedimentary rocks, andplutons Huronian Supergroup- Early Prolerozoic sedimentary rocks, gabbroic intrusions Superior Province- Archean granilic and - ' metavolcanic rocks — — — Gravity gradient, southern edge ofArchean craton MAJOR JR ON RANGES A Gogebic B i'il'arquette ( I \• C Menominee 42' D Iron River Crystal Falls E Cuyuna F Mesabi 2 TGGUnfltit 92' Figure Figure1. 1.Generalized Generalizedgeologic geologicmap mapof ofthe theLake LakeSuperior Superiorregion regionshowing showingthe themajor majoriron iron ranges. ranges. The Thesouthern southernpart partof ofthe theupper upperpeninsula peninsulaofofMichigan Michiganand andadjacent adjacentparts partsofofnortheastern northeastern Wisconsin Wisconsincontain containexposures exposuresofofArchean Archeanrocks rockswith withoverlying overlyingPaleoproterozoic Paleoproterozoic metasedimentary metasedimentaryand and metavolcanic metavolcanicrocks rocksof of the theMarquette MarquetteRange RangeSupergroup, Supergroup,as aswell well as as aa wide wide variety varietyof of volcanic, volcanic, sedimentary, sedimentary,and andintrusive intrusiverocks rocksof ofthe theWisconsin Wisconsin magmatic magmaticterranes. terranes. The The modern moderngeologic geologicframework frameworkofofthe thearea areahas hasbeen beenestablished established largely 1940'sthrough through largely by by mapping mappingby by the the U.S. U.S. Geological GeologicalSurvey Surveyconducted conductedfrom fromthe the1940's the Jamesand andothers, others,1968; 1968;Sims, Sims, the1980's 1980's(Bayley, (Bayley,and andothers, others,1966; 1966;Dutton, Dutton,1971; 1971;James 1990; 1990; Sims Sims and and Schulz, Schulz, 1993). 1993). Beginning Beginningininthe thelate late1970's, 1970's,the theimportance importanceof ofthe the Niagara Niagarafault fault zone zone as as aa suture suturebetween betweenoceanic oceanicterranes terraneson onthe thesouth southand andaacontinental continental assemblage assemblageon onthe thenorth northwas wasdocumented documented(Cambray, (Cambray,1978; 1978;Greenberg Greenbergand andBrown, Brown, 1983; Larue and Sloss, 1980; Larue, 1983). This led to a period of study of the 1983; Larue and Sloss, 1980; Larue, 1983). This led to a period of study of thestructural structural geology geology of of the the region regionin inorder order to todecipher decipherthe thestructural structuralevents eventsconsequent consequentto tosuturing suturing (Larue (Larue and and Ueng, Ueng, 1985; 1985;Sedlock Sedlockand andLarue, Larue,1985; 1985;Ueng Uengand andLarue, Larue,1987). 1987).This This guidebook guidebook draws draws heavily heavily on on these these previous previousstudies, studies,supplemented supplementedin inplaces placesby byour our own own observations. observations. 2 �8900 8800 - 8800' - Diabasedike dike Diabase (Middle (MiddleProterozoic) Proterozoic) of the the Niagara of Niagara fault /\/ Faults set set /Av A/Faults Faults Cambrian Cambrian El Munising MunisingSandstone Sandstone Paleoproterozoic 46 00, 0° Metagabbro Metagabbro Paint Grroup, Paint River Grroup, iron-formation iron-formationin inblack black Baraga Group Group Menominee MenomineeGroup, iron-formation iron-formation in in black black Chocolay Chocolay Group Group Wisconsin MagmaticTerranes MagmaticTerranes Archea n Archean Granite Granite and and gneiss gneiss BF BLF CT FT NF NRF -4530 45¡30PRF PRF SRF 30 0 30 60 Badwater fault fault Badwator Bush Lake Lake fault fault Bush Calumet trough Calumet trough Felch Felchtrough trough Niagara Niagara fault fault North North Range Range fault fault Paint Paint River River fault fault South South Range Range fault fault 90 Kilometers Kilometers Figure 2. Geologic Geologic map map of the Niagara Niagara suture suture zone and surrounding surrounding terranes terranes showing the location of named named features referred to in the text. Figure �CONTINENT CONTINENT MARGIN MARGIN SEQUENCE SEQUENCE A thick succession of dominantly dominantly sedimentary rocks that were deposited deposited on Archean basement is widely distributed in northern Michigan, Wisconsin and Minnesota, and, to a lesser extent, in northern northern Ontario. The succession records a wide variety of geologic iron-formations in environments and includes the extensive and economically important iron-formations the Lake Superior Superior region. region. STRATIGRAPHY Archean Archean Carney Lake Gneiss: The Carnev Theonly onlyextensively extensivelyexposed exposedArchean Archean unit unit in in the the field field trip trip area areais is the Carney 2). The The Carney Carney Lake, Lake, like like other other Archean Archean rocks rocks farther farther Carney Lake Lake Gneiss Gneiss (fig. (fig. 2). north, is exposed in the core of a Penokean bounds the Menominee Penokean structural uplift. It bounds Menominee Range on the north north and and is is the the basement basement on on which which the the Paleoproterozoic Paleoproterozoiciron-bearing iron-bearing sequence sequence of the Menominee Menominee Range Range was deposited. The Carney Lake Lake Gneiss is seen at stops 2-2 and 2-3, where it is exposed beneath the unconformity unconformity at the base of the Paleoproterozoic Fern Creek Formation. According to Treves (1 (1966) 966) granitic gneiss forms forms about 85 85 percent percent of the the unit. unit. Inclusions Inclusionsof of amphibolite, amphibolite, biotite biotite schist schist and and some some quartzite quartzite constitute constitute about about 10 10 percent, the the remainder remainder being being granodiorite granodiorite and and syenite syenite dikes. AmphiboUte inclusions are are more abundant in the northern Amphibolite inclusions northern part part of the the complex, complex, whereas biotite biotite schist is is more more common common in in the southern part. part. According to Davis Davis and and others others (1960) (1960) the Carney Carney Lake Lake Gneiss Gneiss is is about 2,700 million million years old. Foliation Foliation and tabular inclusions inclusions define a complex internal internal folding pattern pattern in the gneiss, which is mostly mostly a result of Archean Archean deformation. deformation. Paleoproterozoic Paleoproterozoic The Paleoproterozoic continent continent margin margin sequence is is comprised comprised of sedimentary and volcanic rocks rocks at at least least several severalkilometers kilometersthick thickininthe theMenominee Menomineeand andIron IronRiver River--Crystal Crystal Falls districts and probably much thicker in much of the area. The stratigraphic relationships are shown in figure 3, which compares the stratigraphy in the field trip area relationships with that in other well-studied well-studied areas to the north north and east. Originally Originally referred referred to as "Huronian" by (1911), and andlater lateras as"Animikie "AnimikieSeries" Series"by byJames James by Van Hise Hise and and Leith Leith (1911), (1958), (1 958), the sequence was renamed renamed the Marquette Range Supergroup, comprised of the Chocolay, Menominee, Menominee, Baraga Baragaand and Paint PaintRiver RiverGroups, Groups,by byCannon Cannonand andGair Gair(1970). (1970). The stratigraphic stratigraphic succession succession of the the first three three groups groups is is well well established. established. However, However, the stratigraphic stratigraphic position position of the Paint Paint River River Group, Group, along along with the Badwater Badwater Greenstone, is is of four less certain. The Paint River Group was originally considered the youngest of stratigraphically stratigraphically superposed superposed groups groups by by James James (1958), (1958), but but more more recent recent interpretations interpretations generally emplaced over the Baraga Group. generally consider consider it to be be an allochthon allochthon structurally structurally emplaced The Paint Paint River Group Group may may be be a distal, deeper water equivalent of the Menominee Menominee and Baraga Groups, which has been thrust onto the continental margin during Penokean compressive compressive deformation. deformation. 4 �Areas north and east of Areas north Menominee MenornineeRange Range Iron River-Crystal River-CrystalFalls Falls Iron field trip trip NortheasternWisconsin Wisconsin Northeastern 1 , allochthon allochthon Amberg Granite I 1833 Ma Diabase and gabbro ] 11 1 Goodrich Quartzite Ne 0.1 unconformity unconfmitY Amasa AmasaIron Iron - formation 1- — Negaunee ic lrontormation5 aj -- unconformity — I Piblk AjibikQuartzite Quartzite - ' -I860 Ma —1860 ...Ma 1 Volcanic Volcanic and and mafic mafic intrusive intrusiverocks rocks —1870 -1870 Ma Ma Ophiolite Dunn Creek Slate I Feich FelchFormation Formation Badwater Greenstone uhconfomiity unconformity Kona KonaDolomite Dolomite Randviile Dolomite Dolomite Randville Mesnard Quartzite Sturgeon SturgeonQuartzite Quartzite Enchantment Lake Formation Fern Fern Creek Creek Formation Formation giaciogenic gladogenicsediments sediments unconformity -- unconformity Granite Graniteand and gnelss gneiss n I unconformity 0n 1 Qieciogenlc sediments Archean Archean 1 Granite Graniteand and gneiss gneiss . Riverton Iron-Formation Riverton Iron-Formation 1874 Ma ISiamo Siamo Slate Slate 1 21 2 Hiawatha Graywcke CD z . Vulcan Vulcan Iron-formation iron-formation Volcanics FortuneLake LakeSlate Slate Fortune I 0. I Stambaugh Stambauah Formation Formation Michigamme Michigamme Formation Formation (0 1835 Ma u Diabase and gabbro 2 Michigamme Formation 0 Spikehom Creek Granite I 'Tobin Lake Granite Diabase and gabbro 1752 Ma 1 detachment detachment surface surface Camey Lake Gneiss —2700 Ma Camey!-$;p I Metasedimentary ro Metasedimentaryrocks Note: The Thestratigraphic stratigraphicrevisions revisions shown shown in in Note: diagram diagram are are not not official official USGS USGS revisions revisions to to stratigraphic stratigraphic usage usageas as of of2003. 2003. Figure 3. Correlation Correlation chart chart of Paleoproterozoic Paleoproterozoic strata strata in in Menominee Menominee and and Iron IronRiverRiverFigure Crystal Crystal Falls Falls and and surrounding surrounding terranes. terranes. Includes Includeschanges changesto to previous previoususage usagenot notyet yet officially officiallyadopted adoptedby bythe the USGS. USGS. Chocolay Chocolay Group Group Fern Creek Creek Formation: Formation: Francis FrancisJ.J.Pettijohn Pettijohn(1943) (1943) described describedand andnamed namedthe the Fern Paleoproterozoic Fern Fern Creek Creek Formation, Formation,verified verified itit as as aa basal basal sedimentary sedimentaryunit unit resting resting Paleoproterozoic unconformablyupon upon Archean Archean granitic granitic basement basement of of the the Carney Carney Lake LakeGneiss, Gneiss,and and unconformably suggested aa glacial glacial origin. origin. Additional Additional descriptions descriptionswere were made made by by Trow Trow (1948), (1948), by by suggested Freedman and and others others (1961) (1961) and and by by Bayley Bayley and and others others (1966). (1966). Prior Prior to to Pettijohn's Pettijohn'swork, work, Freedman the relationships relationships of these these sedimentary sedimentary rocks rocks to the granitic rocks of the region region were the debated debated by by Bayley Bayley (1904), (1904), Lamey Lamey (1937), (1937), and and Dickey Dickey (1936). (1936). Two other other basal basal Paleoproterozoic Paleoproterozoicformations formationsin in the the region, region,the theEnchantment EnchantmentLake Lake Two Formation Formation and and the the Reany Reany Creek Creek Formation, Formation, are are present present in in the the Marquette MarquetteTrough, Trough,50-55 50-55 miles miles north north of the the Fern Fern Creek Creek exposures. exposures. ItIt has has been beensuggested suggestedby byseveral severalworkers workersthat that these rock rock units unitsare arealso alsoglaciogenic glaciogenicand andcorrelative correlativewith with the the Fern FernCreek CreekFormation Formation these (e.g., Gair and and Thaden, 1968; 1968; Puffett, Puffett, 1969; 1969; Gair, 1981; 1981; Ojakangas, 1984). The two exposures of the Fern Creek Formation seen on these the Fern Creek Formation seen on these field field trips trips (stops (stops2-2 2-2and and2-3) 2-3)are are of more morethan than local localsignificance. significance.Correlation Correlationwith with the the Gowganda GowgandaFormation Formationininthe the of Huronian Huronian Supergroup Supergroup ca ca 120 120 miles miles to to the the east east in in Ontario Ontario and and with with the the Snowy Snowy Pass Pass Supergroup 900 900 miles miles to the the WSW in in Wyoming Wyoming has has been been proposed proposed by by various various workers Supergroup (e.g., (e.g., Puffett, Puffett, 1969; 1969; Young, Young, 1970, 1970, 1973, 1973, 1983; 1983; Ojakangas, Ojakangas, 1984, 1984, 1985, 1985, 1988; 1988; Roscoe Roscoe and Card, Card, 1993). 1993). Young Young (1970) (1970) suggested suggestedthat that all allof of these theseunits, units,and andothers othersininQuebec Quebec and 5 �and the NW Territories, Territories, are are remnants remnants of of aa continental-scale continental-scaleglaciation. glaciation. Furthermore, Furthermore, correlation correlation with glaciogenic glaciogenic units units on the Fennoscandian Fennoscandian Shield in Finland Finland and adjacent Karelia, Russia, has also been proposed (Marmo and Ojakangas, 1984; Ojakangas, Ojakangas and others, 2001). 1985, 1988; Ojakangas and others, 1991; 1991; Ojakangas strengthened by the presence of It must be emphasized that the correlations are greatly strengthened similar stratigraphic stratigraphic sequences sequences in in all of the above-named above-named areas and regions. This is is as follows, moving stratigraphically stratigraphically upward: glaciogenic rocks, paleosols paleosols (or remnants thereof), orthoquartzites, orthoquartzites, and and carbonates carbonates (Ojakangas, (Ojakangas,1997; 1997; Ojakangas Ojakangas and and others, others, 2001). Mafic 2.15 Ga cut cut all all the the aforementioned aforementioned sedimentary - 2.1 5 Ga Mafic dike dike swarms swarms dated dated at at 2.2 2.2 — sequences. It is possible possible that the glaciogenic glaciogenic units units of North North America and and the Baltic Baltic region (Marmo and Ojakangas, 1984) were formed on a single supercontinent, Kenorland, at about 2,300 Ma Ma (Ojakangas, (Ojakangas, 1988). 1988). The breakup breakup of Kenorland Kenorland occurred occurred at 2.2 — 2.1Ga Gawith withthe theemplacement emplacementof of the the Nipissing Nipissing mafic mafic sills sills and and dike dike swarms of of the - 2.1 Canadian Canadian Shield and the Jatulian mafic rocks rocks of the Fennoscandian Fennoscandian Shield. The Fern Fern Creek Formation Formation is exposed in only a few small areas adjacent to the Archean Carney Lake Gneiss Gneiss (figs. 2, 4). These These exposures exposures likely likely represent represent erosional erosional remnants remnants of more more widespread widespread glaciogenic glaciogenic deposits depositspreserved preservedin intopographic topographic lows lowson onthe theArchean Archean bedrock bedrock surface. Post-glacial Post-glacial weathering, erosion, and sorting sorting by wind and water resulted stratigraphic sequence of glaciogenic deposits, paleosol resulted in the stratigraphic paleosol (sericitic (sericitic schist), and quartz sand (now the Sturgeon Sturgeon Quartzite) Quartzite) that will be seen at stop 2-2 (Fern Creek locality) Black Creek Creek (sec. (sec. locality) and and stop stop 2-3 2-3 (Sturgeon (SturgeonRiver Riverlocality). locality).Two Twoother otherlocalities localities—- Black 6, T. 39 N., R. 28 W.), and Pine Creek (sec. 32, T. 41 N., N., R. 29 W.) W.) (Freedman (Freedman and and others 1961) are relatively relatively inaccessible inaccessible and and not as well exposed. These others 1961) — - are These four four small small areas of exposure are located along a 17-mile portion of the northwest-trending exposure located 17-mile portion northwest-trendingcontact between the Carney Lake Gneiss and these basal Paleoproterozoic units (fig. 4). The two field stops are within 5 miles of each other. Sturcieon Quartzite: At thick, light light colored, colored, Sturaeon Quartzite: At most most places places in in the the Menominee Menominee district district aa thick, vitreous quartzite forms the basal basal Paleoproterozoic unit on the Carney Lake Gneiss (Bayley and others, 1966). 1966). The type exposures exposures of the Sturgeon Sturgeon Quartzite Quartzite are along the Sturgeon Sturgeon River in the Felch Felch district, about six mites miles north of the Menominee district (see fig 2). In In the Menominee Menominee district the quartzite quartzite forms a continuous continuous belt belt along along the southwest margin of the Carney Lake Gneiss. ItIt is well exposed at stops 2-2 and 2-3. The Sturgeon Sturgeon Quartzite Quartzite ranges ranges from 1,000 1,000 to 2,000 feet thick, thick, is is composed composed primarily primarily of white, gray, green or pink vitreous vitreous quartzite, quartzite, commonly showing ripple marks and crossbedding. It is composed composed almost entirely of quartz. quartz. Trow (1948) (1948) showed that the crossbedding bedding data suggest that the quartz quartz sand sand of the Sturgeon Sturgeon was derived derived from the northwest, and Pettijohn Pettijohn (1957) (1957) showed that many of the Paleoproterozoic quartzites of the Lake Superior Superior region region had had a source source area area to the west or northwest. The Sturgeon Sturgeon Quartzite is considered considered to have formed during a marine marine transgression transgression onto the Archean craton, possibly to the northwest. Bayley and others (1966) interpreted the quartzite to (1966) interpreted be conformable conformable and gradational gradational with the Fern Fern Creek Formation, and to be conformable conformable with the overlying overlying Randville Randville Dolomite. Dolomite. Randville Dolomite: The TheRandville RandvilleDolomite Dolomiteoverlies overliesthe the Sturgeon SturgeonQuartzite Quartzitein in the the Menominee and Felch districts. It takes its name from exposures near Randville, north of of southeast-trending belts belts in the Menominee Menominee Iron Mountain. The dolomite occurs in three southeast-trending 6 �district because because of repetition repetition by by faulting faulting (Bayley (Bayley and others, 1966). 1966). ItIt is estimated to be be thick. Classic Classic exposures exposures of deformed deformed stromatolitic dolomite dolomite are are present present about 2,000 feet thick. at stop stop 2-6. The Randville Randville is composed mainly of massive massive clastic dolomite with thick- and thinbedded sandy dolomite, dolomitic and quartzose slate, and pebbly dolomite 3-12 conglomerate (Bayley (Bayley and others, 1966). 1966). Domal stromatolites 1-3 1-3 inches high and 3-1 2 common and widespread in the formation. They form reefs reefs as inches in diameter are common much much as 50 feet thick and and are are of great great aerial aerial extent. extent. The The stromatolites stromatolites are usually in thinbedded sandy dolomite and conglomeratic dolomite of shallow water origin. The clastic dolomite consists mainly of rounded rounded carbonate fragments that range range in size from sand to cobbles. Internally the fragments consist of fine-grained fine-grained dolomite. dolomite. Bayley and others (1966) (1966) stated stated that the the presence presence of of stromatolites, stromatolites, oscillation oscillationripple ripplemarks, marks,mud mudcracks cracksand and (1981) clastic beds indicates deposition in very shallow water. Larue (1 981) reported that remnants of anhydrite and gypsum gypsum crystals, along with the bedding features, suggest deposition of the Randville Randville Dolomite in a paleo-sabkha environment. Thus, the Chocolay Group may record continuous deposition from a glacial environment (the Fern Creek Formation) Formation) to an an arid arid sub-tropical sub-tropical environment environment(the (theRandville RandvilleDolomite). Dolomite). Menominee Group Menominee Feich Felch Formation: The TheFelch FelchFormation Formationisisaasericitic sericiticslate slateand andquartzite quartzitethat thatoverlies overliesthe the Randville Randville Dolomite. Dolomite. ItIt consists consists of thin-bedded thin-beddedsericitic sericitic slate slateand andphyllite phylliteand andintercalated intercalated thin-bedded thin-bedded quartzite, with the quartzite quartzite layers layers more more prevalent prevalent near near the top of the (Bayley and and others, others, 1966). 1966). ItIt is is about about 100 100 feet feet thick thick in in the the Menominee Menominee district, district, formation (Bayley formation but thickens to about 500 feet in in the Feich Felch district district to the north. Bayley and others (1966) considered the Felch Formation Formation to be correlative with the Ajibik Quartzite and Siamo Slate of the Marquette district and the Palms Palms Formation Formation of the Gogebic district. They state that although the Felch Felch Formation Formation is structurally concordant on the Randville Dolomite, both local and regional relationships suggest that the Felch Formation is unconformable on the Randville Dolomite. However, the Felch Formation is conformable and gradational gradational with the overlying overlying Vulcan Vulcan Iron-formation. Iron-formation. Iron-formation: The Vulcan Iron-formation: The Vulcan Vulcan Iron-formation Iron-formationis is the the major major iron-bearing iron-bearing unit of the iron-formation is divided divided into four units, two composed mainly of of Menominee district. The iron-formation granular iron-formation and two composed of slate and slaty iron-formation. In iron-formation composed In succeeding order the units are the Traders Iron-bearing Iron-bearingMember, the Brier Slate, the Iron-bearing Member, and the Loretto Curry Iron-bearing Loretto Slate. The Traders and Curry Members layers of granular granular jasper alternating alternating with layers layers of magnetite magnetite and and hematite. hematite. The The contain layers Brier and Loretto Members are mainly laminated siliceous iron-rich slate, which locally Loretto Members contains laminae of detrital quartz, feldspar, micas, micas, zircon and tourmaline. According to (1958), 958), the iron-formation iron-formation is about 1,000 feet thick, of which about 730 feet is is Dutton (1 ferruginous 330 feet, Loretto LorettoSlate Slate == 400 400 feet) feet) and and 270 270 feet feet is is ferruginous slate slate (Brier (Brier Slate Slate == 330 iron-formation (Traders granular iron-formation (Traders = 100 100 feet, Curry Curry = = 170 170 feet). The The Vulcan is is seen seen at 2-4 and stops 2-4 and 2-5. 7 �Group Baraga Group In the area of the field trips, rocks rocks of the Baraga Baraga Group Group are are mostly mostly in in the the Menominee Menominee Range, where they consist of a variety of rock types generally combined combined into into the Michigamme Formation. The belts underlain by the Michigamme Michigamme Formation are very poorly exposed, exposed, which accounts, at least in part, for for the lack lack of of detailed detailed mapping mapping of of what what may well be otherwise discernible map units. According to Bayley and others (1966) (1966) the Michigamme Formation consists chiefly of slate, especially quartzose, micaceous, and Michigamme graphitic varieties, subgraywacke, subgraywacke, quartzite, quartzite, conglomerate, conglomerate, dolomite, dolomite, dolomitic dolomitic quartzite, quartzite, iron-formation.More More recent recent exploration exploration drilling drilling also has has identified identified units units of and some iron-formation. rocks. An unconformity unconformity between between the Michigamme Michigamme and and underlying underlying Vulcan mafic volcanic rocks. Iron-formation is indicated indicated by the presence of widespread basal conglomerate, Iron-formation clasts of iron-formation iron-formationand and other other Menominee Menominee and and Chocolay Chocolay Group Group containing clasts lithologies, and by regional truncation truncation of pre-Baraga pre-BaragaGroup Group units units beneath beneath the the basal basal lithologies, Michigamme units. The Michigamme Michigamme Formation Formation is is the youngest youngest Paleoproterozoic Paleoproterozoic unit Michigamme preserved preserved in in the the Menominee MenomineeRange. Range. The Michigamme Michigamme Formation Formation extends extends westward westward and is present widely, although poorly exposed, in fault panels panels lying lying between between the Wisconsin magmatic magmatic terranes terranes and and the the Iron Iron River-Crystal Falls allochthon. has provided providedaa detailed detaileddescription description of of these these River-Crystal allochthon. Dutton Dutton (1971) (1971) has lithologically as their equivalents in the Menominee rocks, which are equally as varied lithologically Range. Farther north, the Michigamme Michigamme Formation is structurally beneath the Iron Iron RiverCrystal Falls allochthon and is widely exposed north and east of the allochthon. This area also has has considerable lithologic diversity, but graded-bedded graded-bedded graywackes, graywackes, pelitic politic schist and slate, and and impure impure quartzite become become more more dominant dominant toward toward the north north and and interlayered interlayered mafic volcanic rocks rocks and iron-formation iron-formationbecome become volumetrically volumetrically minor. minor. Total thickness of the Michigamme Michigamme Formation Formation is is not not known, known, but but itit is is probably probably several several thousand feet or more. Dutton Dutton (1971) (1971) stated stated that that the the Michigamme Michigamme might might be be as as much muchas as 20,000 feet thick in in the Florence, Florence, Wisconsin Wisconsin area. area. Barovich Barovich and and others others (1989) (1989) used used Nd Nd isotope data to show that the Michigamme Michigamme Formation Formation in in the field trip area area was derived derived from a Paleoproterozoic Paleoproterozoic source. Sims and others (1993) (1993) suggest that the source was the Wisconsin Wisconsin magmatic magmatic terranes terranes to to the the south, south, with with deposition deposition in in aa foredeep foredeep environment environment during docking docking of the the Wisconsin Wisconsinmagmatic magmatic terranes terranes with with the the continent continent margin. margin. Paint River Group Group Rocks assigned to the Paint River Group consist of about 6,500 feet of sedimentary sedimentary strata, which overlie as much much as 15,000 volcanic rocks. rocks. They form the of 15,000 feet of volcanic They form the bedrock bedrock of the Iron Iron River-Crystal River-Crystal Falls Falls district district (James (James and and others, 1968). 1968). Five Five formations formations were were assigned assigned originally to the Paint Paint River River Group. Here, Here, we add add a sixth sixth formation, the the Badwater Greenstone, as discussed in more detail below. The stratigraphic position position of the Paint Paint River River Group Group has has been been problematical problematical for more more than than half half aa century. century. The The group group was interpreted (e.g. Leith Leith and and interpreted to be be part part of the the Michigamme MichigammeFormation Formation in in older older reports reports (e.g. others, 1935), 1935), but but was interpreted interpreted to to be be aa separate separate group group (younger (younger than than the the Michigamme) by James and others (1968) Michigamme) (1968) and Cannon and Gair (1970). (1970). However, as discussed discussed below, below, more recent recent studies studies have have suggested suggested that the the Paint Paint River River Group Group may may be a fault-repeated fault-repeated sequence sequence correlative correlative with the Baraga Baraga and/or and/or Menominee Menominee Groups Groups (cf. (cf. Sims Sims and 1993). Below, Below, we provide provide evidence evidence that the Paint Paint River Group Group is an and others, others, 1993). is an 8 �structurally emplaced emplaced over the Michigamme Michigamme Formation Formation and consists of deepallochthon structurally water distal distal equivalents equivalents of the Menominee Menominee and Baraga Baraga Groups. Groups. The 6,500 feet of Paint Paint River River Group Group sedimentary sedimentary strata strata have have some some special special Larue and and Sloss Sloss (1980) (1980) point point out that presence presence of turbidites indicates indicates characterisitics. Larue that they were deposited in a subsiding basin. Likewise, 986) noted Likewise, Cannon Cannon (1 (1986) noted "the group consists of a very unusual unusual sequence sequence of extremely extremely ferruginous ferruginous slate, greywacke, and carbonate carbonate iron-formation. iron-formation.The The abundance abundance of pyritic pyritic and and graphitic graphitic slate, slate, the absence absence of more more oxidized oxidized facies facies of iron-formation, iron-formation, and and the the drastic drastic lateral lateral facies facies changes changes of of some some units suggest that the group was deposited in deep, anaerobic water in a tectonically environment." unstable environment." Badwater Greenstone: The The Badwater Badwater Greenstone Greenstone is is aa thick sequence of massive massive and units (agglomerates) (agglomerates) along with minor pillowed basalt, with the pillows and fragmental units interbedded slate and iron-formation iron-formation(James (James and and others, 1968). 1968). ItIt is is seen seen at stop stop 3-7. 3-7. interbedded The Badwater Badwater is estimated estimated to be be 3,000 3,000 to 8,000 feet feet thick, but but may may be be up up to to 15,000 15,000 feet thick in the Iron relative age of the Badwater Iron River area (James and others, 1968). The The relative Greenstone Greenstone has has been been uncertain uncertain for many many years, but but most most recent recent interpretations interpretations(e.g., 1993) consider consider it to be be correlative correlative with the lithologically and chemically Sims and others, 1993) Hemlock Formation. Formation. As such, itit would be be equivalent to part of the Menominee similar Hemlock Group Group deposited deposited about 1875 1875 Ma. We here here suggest suggest that that the the Badwater Badwater be be placed placed within the Paint Paint River River Group in contrast contrast to its its previous previous assignment assignment to the Menominee Menominee or Baraga Baraga Groups Groups for reasons reasons discussed discussed more more fully fully below. Dunn Creek Slate: The TheDunn DunnCreek Creek Slate Slate is is composed composed of 400 400 to 1,500 1,500 feet of strata strata between the Badwater Badwater Greenstone and the Riverton Riverton Iron-formation Iron-formation(James (James and others, 1968). It is seen at stop stop 3-5. 3-5. The The Dunn Dunn Creek Creek is is aa lithologically lithologically varied varied unit unit comprised comprised of of aa sequence sequence of well-bedded well-bedded to laminated laminated argillite argillite and cherty argillite, argillite, with units units of somewhat coarser impure impure quartzite, and and thin cherty iron-formations. iron-formations. The term "slate" is a misnomer misnomer in that most most of the rock rock has, has, at at best, best, aa moderately moderately developed developed cleavage, cleavage, and true slates are rare. rare. The upper upper part part of the the formation formation is is aa distinctive distinctive highly highly graphitic graphitic argillite and argillite breccia unit, the the Wauseca Wauseca Pyritic Member (stop (stop 3-3), 3-3), which is present throughout the Iron Iron River-Crystal River-Crystal Falls Falls district district and and forms the footwall of the Iron-formation (James and others, 1968). The Wauseca is an example of Riverton Iron-formation sulfide facies iron-formation sulfide iron-formationas as defined defined by by James James (1954). (1954). ItIt contains contains from from 15 15 to to 25% 25% Fe Fe from 10 and from 10 to 30% 30% S. In In the the Iron Iron River River area area parts parts of the the Wauseca Wauseca Member Member contain contain carbon that has has not been been altered altered to graphite, graphite, suggesting suggesting that that the the rocks rocks have have undergone undergone little, if any metamorphism. metamorphism. Tyler and and others others (1957) (1957) showed that some of the carbonaceous carbonaceous material material in the Iron Iron River River area area is, in in fact, coal, coal, and and as such, this is is one one of the oldest known occurrences of coal in the world. The lower part of the Dunn Creek known occurrences coal in The part Dunn Slate Slate is is poorly poorly exposed, but but evidently evidently varies varies considerably considerablyin incharacter characterwithin withinthe thedistrict district (James Badwater (James and others, 1968). 1968). The basal basal contact contact of the the Dunn Dunn Creek Creek with the Badwater Greenstone Greenstone is poorly poorly known, but probably probably conformable. conformable. The great lateral lateral changes changes in thickness of the Dunn thickness Dunn Creek suggest suggest that that itit was was deposited deposited over aa surface surface with considerable relief and that the relief was buried buried before before the later later phases phases of deposition deposition so that thin units units of the Wauseca Wauseca Member Member are are continuous continuous over over the the entire entire district. The The Dunn Dunn Creek is conformable conformable with and and locally locally gradational with the basal basal units of the Riverton Riverton Iron-formation. Iron-formation.This contact contact is is seen seen at at stop stop 3-3. 3-3. The The Riverton Riverton is is also also seen seen at stops stops 3-3 3-3 and 3-4. 3-4. 9 �The 10 10to to 800-foot-thick 800-foot-thickRiverton Riverton Iron-formation Iron-formationis is conformable Riverton Iron-formation: The and gradational with the underlying Dunn Creek Slate, and is the main iron-bearing unit in the Iron River-Crystal Falls district. It is described in detail by James and others (1 968). Where it is is oxidized oxidized it consists consists primarily of thin-bedded chert and iron-rich iron-rich (1968). carbonate, layers of stilpnomelane and disseminated graphite. However, in most areas iron-formationis is altered, altered, with with the the iron iron oxidized oxidized to limonite limonite and and goethite, goethite, and and the the chert chert the iron-formation being variably leached (James and others, 1968). The best natural natural exposure of the unoxidized phase of the Riverton is on the apron of the dam on the Paint River in Crystal Falls, Michigan Michigan (stop (stop 3-6). 3-6). The Hiawatha Hiawatha Graywacke Graywacke ranges in thickness from 0 -- 500 feet Hiawatha Grawacke: Graywacke: The and disconformably overlies the Riverton Iron-formation. Iron-formation.The basal part of the formation formation breccia of chert fragments in is a breccia in a graywacke graywacke matrix, matrix, which is well exposed along the Paint River in Crystal Falls (James and others, 1968). The chert fragments are commonly commonly an inch inch or more more in in size, but but locally locally range range up up to as much much as 2 feet in in length. length. Above the basal basal breccia breccia unit, unit, most most of the the Hiawatha Hiawatha Graywacke Graywacke is is a dark gray gray massive massive medium grained greywacke in which clastic grains are readily visible. In the western part Iron River River area area the graywacke graywacke is is particularly particularlycoarse coarse and and contains contains aa large large amount amount of the Iron of clastic feldspar (James (James and and others, others, 1968). 1968). They They suggested suggested that that the the change change from from chemical ironchemical to clastic deposition deposition was the result result of structural structural disturbance disturbance that halted halted ironformation formation deposition. deposition. Stambaugh Formation: The Stambauoh The Stambaugh Stambaugh Formation Formation is is about 100 100 feet thick and is composed composed of a lower lower laminated laminated cherty unit overlain by massive massive chloritic slate and some graywacke (James and others, 1968). 1968). Much of the unit is moderately moderately to strongly magnetic, a feature that was very helpful in resolving the structure in the district. helpful Fortune Lakes Slate: The The Fortune Fortune Lakes Lakes Slate Slate is is the uppermost uppermost formation of the Paint River Group and the youngest Precambrian Precambrian unit in the district (James and others, 1968). 1968). It also underlies underlies the largest area and, because of poor exposure, is the least known known formation. ItIt is is at at least least 4,000 feet feet thick thick and and is is composed composed dominantly dominantly of of slates slates with with interbedded graywacke and minor iron-formation. Graded-bedded graywacke composes Graded-bedded about 25 percent of the formation. Graded Graded units units range range in in thickness from 1-30 1-30feet (James and others, 1968). 1968). Stratigraphic Synthesis Stratigraphic Synthesisof of the the Marquette MarquetteRange Range Supergroup Supergroup The foundation foundation for modern modern stratigraphic stratigraphic terminology terminology of of the the Paleoproterozoic Paleoproterozoicstrata strata of of the the southern southern Lake Lake Superior Superior region region was established established by James James (1958), (1958), who defined the fourfourfold group group designation designation still still in in use use and and introduced introduced the term term Animikie Animikie Series Series for the the stratigraphically superposed sequence of Chocolay, Menominee, Baraga, presumably stratigraphically and and Paint Paint River River Groups. Groups. The term term "Marquette "Marquette Range Range Supergroup" Supergroup" was introduced introduced by Cannon and Gair (1970) (1970) to to replace replace "Animikie "Animikie Series" Series"in in compliance compliancewith with the the North North American Stratigraphic Stratigraphic Code; an an assemblage assemblage of groups groups is is aa supergroup, supergroup, not not aa series, series, and the name name "Animikie" "Animikie" was already already aa well established established group group name namein inMinnesota Minnesotaand and Ontario. In more recent years, two principal principal changes have been proposed. First is the recognition that the Menominee Group contains thick volcanic formations that are temporal equivalents of the major major iron-formations. iron-formations.This This relationship relationship is is best best documented documented in the eastern Gogebic range where the Emperor Volcanic Complex (Trent, 1976) Emperor 1976) interfingers with the Ironwood interfingers Ironwood Iron-formation Iron-formation and is unequivocally unequivocally part of the Menominee 10 �(Klasner and and others, 1998). 1998). Likewise, Likewise, the Hemlock Hemlock Formation, Formation, aa thick thick Group succession (Klasner rocks long long considered considered aa part part of the the Baraga Baraga Group, Group, was was succession of volcanic rocks reassigned reassigned to the Menominee Menominee Group Group (Cannon, 1986). 1986). Although Although relationships relationships are not not as definitive as in the Gogebic definitive Gogebic Range Range because because of structural structural complications complications and and poor poor exposures, the correlation correlation is supported supported by interbedded interbedded iron-formations iron-formationswithin the Hemlock and an extensive iron-formation (AmasaIFence River) River) overlying the Hemlock, Hemlock, iron-formation (Amasa/Fence which are believed to be the westward continuation continuation of the Negaunee Negaunee Iron-formation Iron-formationfrom the Marquette Marquette Range. Also, the Amasa Formation Formation is is unconformably unconformably overlain overlain by by a conglomerate lithologically lithologically indistinguishable from basal basal conglomerate conglomerate of the Goodrich Goodrich Quartzite of the Marquette Marquette Range. Range. Taken Taken together, together, the the weight weight of of evidence evidence suggests suggests that that Emperor and Hemlock Hemlock volcanic rocks rocks were erupted erupted simultaneously simultaneously with ironboth the Emperor formation deposition and interfinger interfinger with the Ironwood Ironwood and Negaunee Negaunee Iron-formations. Iron-formations. EmperorIIronwood assemblage assemblage and and the Hemlock/Amasa Hemlock/Amasa assemblage assemblage are are Both the Emperor/Ironwood unconformably overlain by a conglomerate marking marking the base base of the Baraga Baraga Group Group and and unconformably thus thus seem seem properly properly assigned assigned to to the the Menominee Menominee Group. Group. second principal principal change change to to stratigraphic stratigraphiccorrelation correlation is is the the recognition recognition that that the the Paint Paint A second River Group is likely an allochthon allochthon structurally emplaced over the Baraga Group and is therefore not necessarily therefore necessarily a younger sequence. sequence. As we propose propose here, certain lithologic similarities between Paint River units and Menominee and Baraga Group units suggest similarities that the Paint Paint River River is equivalent to both both the Menominee Menominee and Baraga Baraga Groups. The five formations originally assigned formations assigned to the Paint Paint River River Group Group lie in their entirety within the Iron Iron River-Crystal River-Crystal Falls basin (referred (referred to as the Iron Iron River-Crystal River-Crystal Falls district by James and others, 1968). 1968). Over the years there have have been been differences differences in in opinion opinion concerning concerning the relative ages of the strata strata within the Iron Iron River-Crystal River-Crystal Falls Falls basin, the underlying underlying Badwater Greenstone, and nearby nearby Baraga Baraga and Menominee Group strata. Leith Leith and others (1935) (1935) interpreted interpreted the sedimentary sedimentary rocks rocks of the district to to be be roughly roughly equivalent equivalent to the Michigamme Michigamme Formation, Formation, but but they they were were uncertain uncertain about about the the stratigraphic stratigraphic position position of of what they called Pentoga belts belts of greenstone greenstone (Badwater (Badwater Greenstone Greenstone called the Paint Paint River River and and Pentoga of current usage). James James (1958), (1958), however, however, proposed strata of the Iron Rivercurrent usage). proposed that the the strata of the Iron RiverCrystal Falls district comprised the uppermost the Paint River Group of four Paint River Group Crystal uppermost stratigraphic groups that make stratigraphic make up up the the Marquette Marquette Range Range Supergroup. Supergroup. The The Badwater Badwater Greenstone Greenstone was considered to be part of the Baraga Baraga Group. Larue Larue and and Sloss Sloss (1980) (1980) discussed sedimentation of the Marquette Marquette Range Range Supergroup Supergroup and and accepted accepted James' James' interpretation, interpretation, whereas others questioned questioned the stratigraphic stratigraphic position position of the Paint Paint River River Group. For example, Cambray Cambray (1978) (1978) suggested suggested that the Paint Paint River River Group Group is is stratigraphically equivalent to the Menominee 1992), stratigraphically Menominee Group. More More recently, Sims (1990, (1990, 1992), and Sims and Schulz (1993) (1993) proposed proposed that the Paint Paint River Group Group is the stratigraphic equivalent of the Baraga Baraga Group Group and and they correlated correlated the Badwater Badwater Greenstone Greenstone with the Hemlock Hemlock Formation Formation in in the the Menominee MenomineeGroup. Group. is some some lithologic lithologic similarity between between units units of the Paint Paint River River As shown in figure 3, there is Group and other nearby nearby successions, successions, the most most obvious obvious being being a thick and and extensive extensive banded iron-formation iron-formation overlain disconformably disconformably by clastic rocks rocks including including ferruginous conglomerate. In In this this regard, regard, the the Riverton Riverton Iron-formation/Hiawatha Iron-formation1HiawathaGraywacke Graywacke VulcanlMichigamme succession succession of succession of the Paint Paint River River Group Group is is similar to the Vulcan/Michigamme the Menominee Menominee Range Range and and the the Negaunee/Goodrich NegauneelGoodrichsuccession successionof of the theMarquette MarquetteRange. Range. We suggest suggest that that the the Riverton/Hiawatha RivertonIHiawathadisconformity disconformity is is equivalent equivalent to to the the Menominee/Baraga formations MenomineelBaraga unconformity unconformity elsewhere elsewhere and that the Riverton Riverton and older formations of the Paint Paint River Group Group are are equivalent equivalent to Menominee Menominee Group Group units, whereas the Hiawatha and younger parts parts are equivalent to the Baraga Baraga Group. Group. Thus, the thick clastic, 11 �units of the upper upper part part of the the Paint Paint River River Group Group are are correlatives correlativesof rocks rocks in part turbiditic, units of similar depositional setting setting in the Baraga Baraga Group. The Badwater Badwater Greenstone Greenstone may may be be the equivalent of the Hemlock Hemlock Formation Formation in the Menominee Menominee Group. We here here suggest suggest Badwater Greenstone should be placed placed in the Paint River Group rather than the that the Badwater Structural evidence presented Menominee Group. Structural presented below indicates that the Badwater is sequence, as indicated indicated by the widespread occurrence of part of the allochthonous sequence, steeply plunging integral part part of the Paint Paint plunging folds characteristic characteristic of the allochthon, and is an integral being the the volcanic volcanic base base on on which which younger younger units unitswere were deposited depositedwith with River succession, succession, being is more more aptly included included with the Paint Paint River Group rather than apparent conformity. ItIt thus is separated and thus has unknown unknown in other groups from which it is structurally separated stratigraphic relations. stratigraphic relations. Structure Structure The area of the field trips spans the Paleoproterozoic Paleoproterozoic suture separating the Archean Superior craton and Paleoproterozoic Paleoproterozoic epicratonic rocks on the north from Paleoproterozoic Paleoproterozoic oceanic and island arc rocks of the Wisconsin magmatic magmatic terranes on the south. The most commonly commonly accepted model model for accretion is is northward northward emplacement emplacement south-directed subducting slab, eventually leading to the of volcanic arcs over a south-directed of Penokean culmination of Penokeandeformation deformation as as the the arcs arcs collided collided with the southern edge of the Superior Superior craton. craton. The Niagara fault zone, a zone of intense intense shearing as much much as several kilometers kilometers wide in some areas, is the feature which most completely separates volcanic rocks on the south from epicratonic sedimentary rocks on the north north and is generally considered to be the surface surface trace trace of the the suture. suture. However, However, itit is is but but one one of of aa series series of of anastamosing anastamosingfaults faults structural panels in which volcanic and sedimentary rocks are intermixed. that bound structural Niagara These panels compose a belt as much much as 25 km wide, which we refer to as the Niagara northward suture zone. Available structural data indicates that these panels were thrust northward during collision and are allochthonous with regard regard to Archean basement rocks and, in some cases with regard regard to lower parts parts of the sedimentary sedimentary succession. succession. The allochthons are characterized by a complex structural structural history in which refolding refolding of early folds has resulted resulted in diversely diversely and and commonly commonly steeply steeply plunging plunging folds. folds. Terminology Terminology Because Because various names have been applied over the years to structural elements composing what we now refer to as the Niagara Niagara suture zone, the following discussion (1966) correlates the terms used used here with previous terminology. Both Bayley and others (1 966) and Dutton Dutton (1971) (1971) defined defined the structural structural panels panels of the region region and and the map map geometry geometry has has remained remained essentially unchanged unchanged since their work. They, however, did not use the term "Niagara fault", although they did recognize and map the fault as a major boundary boundary between between the dominantly volcanic rocks to the south and dominantly sedimentary rocks to the north. They did name name other faults and and also also applied applied names names to various structural structural blocks. We have have retained retained their names names throughout throughout most most of this this report. report. With With recognition recognitionof of and Ueng Ueng (1985) (1985) introduced introducedthe the significance of the Niagara Niagara fault as aa suture, suture, Larue Larue and the fault as term "Florence-Niagara "Florence-Niagara terrane" to encompass encompass the eight structural structural panels panels defined defined by Bayley and others (1966) roughly ten-kilometer-wide zone of (1966) and Dutton (1971) (1971) as a roughly rocks exhibiting internal deformation unique suture exhibiting very intense internal unique to the tectonics of the suture zone. An additional terrane, the Crystal Falls terrane, was proposed by Ueng Ueng and Larue Larue 12 �(1987) (1 987) and considered to be part of the terrane-boundary tectonic assemblage because its complex, although although less less intense intense deformational deformational history. history. In In this report report we have have used used of its the term "Niagara terrane and "Niagara suture zone" to encompass encompass both both the Florence-Niagara Florence-Niagara terrane Crystal Falls Falls terrane. The suture suture zone is is bounded bounded on the south by the Niagara Niagara fault zone. The rocks rocks of the suture suture zone zone all all show show a complexity complexity and intensity of folding much much exhibited by correlative strata strata farther north. north. The northern northern boundary boundary of the greater than exhibited suture zone is the Badwater Badwater fault and and Paint Paint River River fault, both both probably probably originally thrusts, which transported transported Paleoproterozoic RiverPaleoproterozoic strata strata northward. northward. We use use the term term "Iron "Iron RiverFalls allochthon" allochthon" as as an an equivalent equivalent to to the the "Crystal "CrystalFalls Fallsterrane" terrane"of of Ueng Uengand and Crystal Falls Larue Larue (1987) (1987) and and "Iron "Iron River-Crystal River-CrystalFalls Fallsbasin" basin"of of many manyolder older reports. reports. Niagara Niagara Fault Fault Zone Zone The Niagara Niagara fault zone is is a zone of highly highly strained strained rock most most commonly commonly up to a few hundred hundred meters meters wide wide although although there there are arefew few places placeswhere where ititisisexposed exposedand andits itswidth width can be estimated. Its Its location location (see (see fig. 1) 1) is is relatively relatively well defined in the regions regions covered by this field guide, but lack of outcrops outcrops to the west make make its its location location somewhat problematic. In In those regions regions its its location location is is based based largely on interpretation of aeromagnetic maps maps on which the fault is is expressed expressed as a discontinuity of structural structural patterns. On geologic maps maps itit is is generally generally portrayed portrayed as a single line line separating mostly mostly metasedimentary metasedimentary rocks rocks on the north north from metavolcanic metavolcanic rocks rocks on the south, but but itit is is likely likely to have numerous numerous splays extending southward into the volcanic terranes, only some of which have have been been seen seen in in outcrop. outcrop. On the field trips we will examine two of the best-exposed best-exposed areas of the fault zone. Highly sheared rocks of the fault zone are are well exposed exposed in in Piers Piers Gorge Gorge (stop (stop 2-1). Although this locality locality is about a kilometer south of the mapped mapped fault, there is is little little doubt that shearing shearing seen here is a splay of the Niagara Niagara fault. Rocks Rocks within the fault zone are severely flattened and stretched stretched and and foliation foliation strikes strikes generally generally WNW and and dips dips steeply steeply south. High High strain has has resulted resulted in in rotation rotation of fold axes axes to parallelism parallelism with the direction of maximum maximum Larue, 1985). 1985).Based Basedon onstretch stretchlineations, lineations,which whichplunge plunge600 60' elongation (Sedlock (Sedlock and and Larue, SW, Sims Sims and and Schulz (1993) (1993) suggest suggest that tectonic tectonic transport transport was northeastward, northeastward, perpendicular to the trace of the fault, and and onto onto the exposed exposed part part of the continental continental margin margin to the north. north. They They infer inferthat that the thefault faultzone zonedips dipsabout about700 70' to to the the south. south. The The fault fault zone is also seen at stop 3-1 3-1 at the Pine Pine River dam. There, very highly strained rocks rocks of the Michigamme Michigamme Formation Formationare are well well exposed. exposed. Although the Niagara Niagara fault has has been been widely accepted as a paleosuture, gravity studies by Attoh and and Klasner Klasner (1989) (1989) suggest suggest that that the the Archean Archean cratonic cratonicrocks rockscontinue continuesouthward southward at depth to a steep gravity gradient that trends northeastward northeastward across north-central north-central Wisconsin (fig. Wisconsin (fig. 1). 1). By By that that interpretation interpretationthe the Niagara Niagara fault fault is is aa major major thrust, thrust, which which has has transported arc rocks transported rocks for aa least least aa few few tens tens of of kilometers kilometersnorthward northward over over the the southern southern edge of the Superior Superior craton. Based Based on on gravity gravity model model studies, studies, Klasner Klasner and Osterfeld Osterfeld (1984) suggested that magmatic (1984) had had previously suggested magmatic domes, such such as the Dunbar Dunbar Dome, Dome, are allochthonous, detached at depth and thrust northward northward in the hanging hanging wall of the Niagara Niagara fault, which flattens flattens at depth depth toward toward the south. 13 �Niagara Niagara suture suturezone zone Both Both the the Menominee Menominee and and Iron Iron River-Crystal River-Crystal Falls Falls districts districts lie lie near near the the southern southern edge edge of of the exposed continental margin of the Penokean orogen and both lie within the Niagara the exposed continental margin of the Penokean orogen and both lie within the Niagara suture suture zone zone as aswe we define defineitithere. here.The Thecontinental continentalrocks rocksnorth northof ofthe theNiagara Niagarafault faultare are highly highly faulted faulted and and divided divided into into aa series series of of structural structuralblocks. blocks. Sedlock Sedlock and and Larue Larue(1985) (1985) refer refer to to these these blocks blocksas as "fault "faultbounded boundedtectonostratigraphic tectonostratigraphicterranes", terranes", but butwe we prefer preferto to use use the the term term "structural "structuralpanels" panels" because becausethey they do do not not fit fit the the strict strictdefinition definitionof of aa tectonostratigraphic tectonostratigraphic unit, unit,which whichisisdefined definedin inthe the Glossary Glossaryof of Geology Geologyas asaa"mixture "mixtureof of rock faultrock stratigraphic stratigraphicunits unitsresulting resultingfrom from tectonic tectonic deformation." deformation."Rather Ratherthese theseare arefaultbounded boundedslices slices of of rocks rocksor orrock-stratigraphic rock-stratigraphicsequences sequencesthat thathave haveaaunique uniquestructural structural signature. signature. Although Althoughboth bothdistricts districtsare arepart partof ofthe theNiagara Niagarasuture suturezone, zone,they theyare aredistinctly distinctly different differentfrom fromeach eachother otherstructurally. structurally.Thus Thusthey theywill willbe bediscussed discussedindividually individuallybelow. below. Menominee Menominee District District The The Menominee Menomineedistrict districtlies lieswithin within the the Niagara Niagarasuture suture zone zone at at the the southern southern edge edge of of aa series series of of uplifted upliftedblocks blocksof of Archean Archeanbasement basement(figs. (figs.1,1,2). 2).Those Thoserocks rocksnorth northofofthe the Menominee Menomineerange rangecontain contain fault-bounded fault-boundedtroughs troughsof of tightly tightlyappressed appressedPaleoproterozoic Paleoproterozoic strata—the Felch and and Calumet Calumet troughs troughs (fig. 2)-2 ) - down-faulted down-faulted between between blocks blocks of of Archean Archean strata-the Felch rocks. rocks.Studies Studiesby byKlasner Klasnerand andothers others(1989) (1989)and andKlasner Klasnerand andSims Sims(1993) (1993) suggest suggestthat that these Archean blocks were uplifted and thrust northward along Bush Lake fault, a these Archean blocks were uplifted and thrust northward along Bush Lake fault, a master masterfault fault that, that, they theysuggest, suggest,carried carriednow noweroded erodedPaleoroterozoic Paleoroterozoicand andArchean Archean rocks rocks onto onto the the continental continentalhinterland hinterlandto tothe thenorth. north.AAseries seriesof of south-verging south-vergingthrusts thrustsoccurs occursinin the the Felch Felchand andCalumet Calumettrough troughareas. areas. These Thesemay mayhave havedeveloped developedas as back-thrusts back-thrustsduring during the thenorthward northwardthrusting thrustingevent, event,or orthey theymay mayhave havedeveloped developedlater laterduring duringthe theMazatzal Mazatzal orogeny orogeny (HoIm (Holm and and others, others, 1999; 1999;Romano Romanoand and others, others, 2000). 2000). The The general generalstructure structureof of the theMenominee Menomineeiron irondistrict district(fig. (fig.4) 4)isisaasouth-facing south-facinghomocline homocline of of Paleoproterozoic stratain which stratigraphic arecreated createdby Paleoproterozoicstrata inwhich stratigraphicrepetitions repetitionsare bythree three major major faults (Bayleyand andothers, others,1966). 1966).The Thefaults faultscut cutthe the faultsand andby byfolding foldinginternal internalto tofault faultslices slices(Bayley folds foldslongitudinally, longitudinally,approximately approximatelyalong alongthe thefold foldaxes, axes,repeating repeatingthe thePaleoproterozoic Paleoproterozoic sequence sequencethree three times timesand andforming formingthree three"ranges". "ranges".Farthest Farthestnorth, north,the theCarney CarneyLake Lake Gneiss forms the core of a broad anticlinal structure. The Paleoproterozoic Gneiss forms the core of a broad anticlinal structure. The Paleoproterozoicstrata stratalie lie unconformably unconformablyon on the the gneiss gneissand anddip dipsteeply steeplyto to the the south southor orare areoverturned overturned(as (asat atstop stop 2-3) 2-3)and anddip dipsteeply steeplynorth northand andface facesouth. south.Farther Farthersouth, south,the thePaleoproterozoic Paleoproterozoicstrata strataare are repeated repeatedtwice twice by bymajor majorfaults faultsto toform formthe thetwo tworanges rangesof ofthe thedistrict. district.These Thesefaults faultswere were named 4). namedthe theNorth NorthRange Rangefault faultand andSouth SouthRange Rangefault faultby byBayley Bayleyand andothers others(1966) (1966)(fig. (fig.4). The Thefaults faultshave havesteep steepdips dipsatatthe thepresent presentlevel levelofofexposure exposureand andconsistently consistentlyshow showsouthsouthside-up side-updisplacement. displacement.Most Mostrecent recentinterpretations interpretations(e.g., (e.g.,Sims Simsand andSchulz, Schulz,1993) 1993) consider them to have been thrust faults, which were steepened by continued consider them to have been thrust faults, which were steepened by continuedshortening shortening of of the thethrust thrustpanels. panels.The Therocks rocksininthe thehanging hangingwall wall(south (southside) side)ofofthese thesefaults faultshave haveno no indications indicationsof of Archean Archeanbasement basementrocks, rocks,inincontrast contrastto tothe thearea areaimmediately immediatelyto tothe thenorth north where wherethe theCarney CarneyLake LakeGneiss Gneissisisan anintegral integralpart partofofthe thestructure. structure.The Thenorth northrange rangeand and south southrange rangepanels panelsmay maybe beallochthons allochthonsdetached detachedfrom frombasement basementand andthrust thrustnorthward northward over overthe themore moreautochthonous autochthonoussequence sequenceofofthe thenorthern northernpart partofofthe thedistrict. district.The The Menominee range is bounded on the south by the Niagara fault (stop 2-1), along Menominee range is bounded on the south by the Niagara fault (stop 2-I), alongwhich which itit isisinincontact contactwith withvolcanic volcanicrocks rocksof ofthe theWisconsin Wisconsinmagmatic magmaticterranes. terranes. 14 �87 4630' •r •( •( •( • . . a , 4552'3O — I • • '2 —Ferr Cre,!Ic Fm Fern Cr,ck rm 88 730" 87 5700 2 I 90 55 2 4 87 4630 I 00 5 10 ip 8 6 I I 1'O 10 12 12 I 15 15 Miles Miles Kilometers Kilometers EXPLANATION EXPLANATION Cambrian Cambrian MunisingSandstone Sandstone Munising North NorthofofNiagara Niagarafault fault Paleoproterozoic Paleoproterozoic SouthofofNiagara Niagarafault fault South Paleoproterozoic Paleoproterozoic Metadiabase Metadiabase E Michigamme Formation Michigamme Formation -graywacke graywacke LI. Badwater Greenstone Badwater Greenstone Vulcan VulcanIron-formation Iron-formation L HoskinsLake LakeGranite Granite Hoskins MarinetteQuartz QuartzDiorite Diorite ........ Marinette Metagabbro Metagabbro QuinnesecFormation Formation Quinnesec Randville RandvilleDolomite Dolomite Sturgeon SturgeonQuartzite Quartzite Fern FernCreek CreekFormation Formation - — fault fault Archean Archean Granitic Graniticrocks rocksand andgneiss gneiss 0 L:.' Carney Lake Gneiss Carney Lake Gneiss Figure Figure4. 4. Geologic Geologicmap mapof ofpart partofofthe theMenominee MenomineeIron IronRange Rangeshowing showingthe thelocation locationofof field trip stops. Geology is from Bayley and others (1966). field trip stops. Geology is from Bayley and others (1966). Thesefaults faultscontinue continuetotothe thenorthwest northwestinto intothe theFlorence, Florence,Wisconsin Wisconsinarea areaand andalong alongthe the These southside sideof ofthe theIron IronRiver-Crystal River-CrystalFalls Fallsdistrict, district,where whereDutton Dutton(1971) (1971)mapped mappedfour fourfaultfaultsouth boundedstructural structural"blocks". "blocks".He Henamed namedthe theblocks, fromnorth northtotosouth, south,the BruleRiver blocks,from theBrule River bounded block,the theKeyes KeyesLake Lakeblock, block,the thePine PineRiver Riverblock, block,and andthe thePopple PoppleRiver Riverblock. block.The The block, 15 �Menominee Menominee Range Range and contains, in in its its western portions, portions, an an extensive extensive dolomite dolomite (Saunders Formation) Formation) generally generally accepted accepted as the equivalent equivalent of the the Randville Randville Dolomite Dolomite of (Saunders Menominee Range. Range. The The occurrence occurrence of of these these shelf-deposited shelf-depositedrocks rocksin inthe the the Menominee southernmost and and structurally structurally highest highest thrust thrust panel panelof of the the continental continentalforeland forelandindicates indicates southernmost the continental continental shelf shelf originally originally extended extended substantially substantiallysouth south of of the the present presentNiagara Niagara that the fault. fault. - Iron District Iron River - Crystal Falls District 88 1800" EXPLANATION EXPLANATION North North of o f Niagara Niagara fault Tobin TobinLake LakeGranite Granite Metagabbro Metagabbro - Paint Group - undivided Paint River River Group undivided Fortune FortuneLake LakeSlate Slate Riverton Riverton Iron-formation Iron-formation Dunn DunnCreek CreekSlate Slate 463'00" Badwater BadwaterGreenstone Greenstone a Michigamme MichigammeFormation, Formation, graywacke graywackeand andvolcanic volcanicrocks rocks Michigamme MichigammeFormation Formation --quartzite quartzite Michigamme MichigammeFormation Formation --graywacke graywacke Amasa Iron-formation Hemlock Volcanics undivided undivided HemlockVolcanics Randville Dolomite Randville Dolomite :k.j_ Saunders SaundersFormation Formation Dickinson DickinsonGroup Groupundivided undivided ,n South South of of Niagara Niagara fault 4552'30" ,-,!;..:+ Bush Lake Lake granite Bush granite Granite and tonalite Graniteand tonalite Quinnesec Formation Formation Quinnesec EI] Metasedimentary Metasedimentaryrocks rocks - — faults faults • field fieldtrip tripstops stops 88 1800" 22 I I 55 22 00 I I I I 00 I 44 66 I I I F 55 Miles Miles 88 F I I I 10 10 Kilometers Kilometers NF-Niagara fault NF-Niagarafault SRF- South South Range Range fault fault NRF-North Range fault NRF-North Range fault BF-Badwater fault BF-Badwaterfault PRF-Paint River PRF-Palnt River fault fault CS-Commonwealthsyncline svncline CS-Commonwealth MA-Mastodon MA-Mastodonanticline anticline TBS-Tim TBS-TimBowers Bowerssyncline syncline Figure 5. 5. Geologic Geologicmap mapof of the theFlorence Florencearea areaand andeastern easternpart partof ofthe theIron IronRiver-Crystal River-Crystal Figure (1971)and and Falls district district showing showing the the location locationof of field fieldtrip trip stops. stops. Geology Geologyisisfrom fromDutton Dutton(1971) Falls James James and and others others (1968). (1968). 16 �The Iron Iron River-Crystal River-Crystal Falls District (fig. 1, 2, 5) is a triangular-shaped basin of tightly tightly triangular-shaped basin folded strata of the Paint Paint River Group. Structural studies by James and others (1968) (1968) showed that sedimentary strata of the Iron Iron River-Crystal River-Crystal Falls basin are tightly and multiply folded. Dips of less than 60 degrees are rare. For the most part the trends of the parallel or sub-parallel sub-parallel to the principal principal axis of the triangular triangular district. fold axes are parallel Stratigraphically overturned beds are common. There are a dozen or more faults having displacements measured in thousands of feet. In In addition, James and others (1968, (1968, page 87) noted "It is the opinion of one of the authors (FJP) that the folds in in the Michigamme Michigamme Slate are unrelated to those in the Badwater Greenstone and Dunn Creek suggestingthat thatthat thatthe theMichigamme Michigammehas hasaa different different structural structuralsignature signature than than slate...", suggesting Francis J. Peftijohn Pettijohn (FJP) the Badwater Greenstone and overlying Dunn Dunn Creek Creek Slate. Slate. Francis suggested that aa north-trending north-trendingfault fault separates separates the the Michigamme Michigamme from from the the futher suggested Badwater and Paint River Group strata along the eastern edge of the district. More recent structural studies (Ueng and Larue, 1987) identified identified four phases of deformation, including two major folding events that resulted in the extremely complex fold interference pattern characteristic of the district. This refolded interference refolded fold geometry geometry is exceptionally well displayed displayed at stop stop 3-6. 3-6. exceptionally Uncertainty concerning the stratigraphic position of the Paint River Group and Badwater (1990) (1993) to Greenstone, as discussed above, led Sims (1 990) and Sims and Schulz (1 993) to propose that a thrust (detachment) fault (the Badwater thrust fault) lies at the base of the Badwater Greenstone Greenstone and that the Badwater Badwater and and overlying overlying Paint Paint River River Group Groupstrata strataare are allochthonous, allochthonous, tectonically emplaced above Baraga Group strata of the continental margin. Although the Badwater thrust fault was not directly observed, they based their interpretation on unpublished geochemical data by K. J. Schulz, and and the the presence of of interpretation "extensive thrust faulting" faulting" of continental 'extensive continental margin margin strata. strata. To test this this interpretation, interpretation, Klasner Klasner and and others (1999) (1999) compared compared the structural structural signature signature (primarily orientation of fold axes) in the Paint River Group strata and Badwater Greenstone with the structural signature in underlying Baraga Group strata. Based on measurement of 123 measurement 123 fold axes throughout the Iron Iron River-Crystal River-Crystal Falls basin, they found that most most fold fold axes axes plunge plunge steeply steeply (approximately (approximately 85 degrees) north (fig. 6A). There There are are at least least three other trends in in the plot plot of the the fold fold axes, indicating indicating that the rocks rocks in in the the Paint River Group are multiply deformed. In contrast, fold axes in the area north and east of the Iron Iron River-Crystal River-Crystal FaIls Falls basin plunge at low angles to the west-northwest or east-southeast east-southeast (see (see fig. 6B). 6B). These These measurements measurementswere were made made mainly mainly in in Michigamme Michigamme Formation strata of the Baraga Baraga Group. The striking striking difference in in structural structural history history of these two regions separates regions supports the proposal proposal of Sims Sims (1990) (1990) that aa thrust fault separates these two regions. regions. Thus, the Paint Paint River including the Badwater Greenstone, Greenstone, River Group, including appears appears to be allochthonous, allochthonous, having having been thrust onto the autochthonous autochthonous Baraga Baraga Group Group strata. strata. The metamorphic metamorphic grade grade of rocks rocks surrounding surrounding the the Paint Paint River River Group Group ranges ranges from from greenschistto greenschist toamphibolite amphibolite(James (Jamesand andothers, others,1968; 1968;Bayley Bayleyand andothers, others, 1966; 1966;Dutton, Dutton, 1971), but Paint River Group rocks of the Iron Iron River-Crystal Falls district reach only greenschist or lower metamorphic metamorphic grade. This feature, too, suggests suggests that that there there is is aa fault fault with considerable displacement separating the Paint River Group from surrounding considerable displacement Paint surrounding rocks. rocks. 17 �A 0 0 •0 0 0 4 123 fold axes 123 fold axes in in rocks rocks of of the theIron IronRiver-Crystal River-CrystalFalls Falls 0 allochthon allochthon 0 .00 .0 0 f 36 Michigamme 36 fold fold axes axes in Michigamme Formation in footwall of allochthon. allochthon.Black Blackdot dotisis beddingbeddingcleavage cleavage intersection intersection at at stop stop 3-8. 3-8. Figure Figure 6. Stereoplots Stereoplots (lower (lower hemisphere hemisphere equal equal area area projections) projections) showing showing the the orientation orientation of fold fold axes axes within within the the Iron IronRiver-Crystal River-CrystalFalls Falls allochthon allochthon(A) (A)and and in in the the Michigamme Michigamme Formation Formation north north and and east of the the allochthon allochthon (B). (B). north-northeasterly-orientedcross cross section sectionconstructed constructedfrom fromunderground undergroundmapping mappinginin A north-northeasterly-oriented mine mine workings workings (James (James and and others, others, 1968) 1968)in inthe the Iron IronRiver Riverarea areaprovides providesan anexample exampleof of the complexity complexity of folding folding typical typical of of this this district. district. The The structure structureseen seenhere heresuggests suggeststhat thatthe the initial folding folding in in the the Iron Iron River-Crystal River-CrystalFalls Fallsbasin basin may may have have been beenrecumbent recumbent (fig. (fig. 7A). 7A). initial synclinein in Hiawatha Hiawatha Graywacke Graywackeand and overlying overlying The cross cross section section shows shows aa recumbent recumbent syncline The Riverton Iron-formation. Iron-formation.In Inthe the third thirddimension dimensionthis this fold foldplunges plungessteeply steeplyinto intothe thesection section Riverton NW). ItItisisstrongly stronglyoverprinted overprintedby byother otherfolds foldsof of diverse diverseorientations. orientations.The Thesyncline syncline (to the the NW). (to closes toward the southwest and the overturned upper limb in which the stratigraphically closes toward the southwest and the overturned upper limb in which the stratigraphically older Riverton Riverton Iron-formation Iron-formationlies liesabove above Hiawatha HiawathaGraywacke Graywacke suggests suggestsaa northward northward structural structural vergence. vergence. The The orientation orientationof of fold foldaxes axesmeasured measuredin inoutcrops outcropsnear nearthe themine mineare are shown shown in in figures figures 7B 7B and and 7C. 7C. 18 �___ ____ ___ NE A 1500" 1500' 1000' 1000' 1000 1 EXPLANATION deposits 500 500' 500 Metodiabose dike HioessthssGoywoeke Riverton Iron-Formation Dunn Creek Slate Mined ore bodies LEVE SEA LEVEL LEVEL 200 0 200 400 600feet Figure Figure7. 7.AA-Geologic Geologiccross crosssection sectionthrough throughthe theBuck BuckMine MineininIron IronRiver, River,Michigan, Michigan,about about 12 12miles mileswest westof ofCrystal CrystalFalls. Falls.Cross Crosssection sectionisisininsec. sec.1,1,T.T.42 42N., N.,R.R.35 35W., W.,and andsec. sec.6,6, T. T.42 42N., N.,R.R.34 34W. W.Reproduced Reproducedfrom fromJames Jamesand andothers others(1968) (1968)totoshow showlarge largerefolded refolded recumbent recumbentsyncline, syncline,the thevery verytight tightfolding, folding,and andwide widediversity diversityofofaxial axialsurfaces surfacesofoffolds. folds. Lower Lowerhemisphere hemisphereequal equalarea areastereograms stereogramsofoffold foldaxes axesininoutcrops outcropsofofRiverton RivertonIronIronformation formation(B) (B)and andHiawatha HiawathaGraywacke Graywacke(C) (C)near nearthe themine. mine. 19 �History of Iron History Iron Mining Mining the Menominee district district in 1848 by by two two explorers, explorers, J.W. J.W. Foster Iron ore was discovered in the and S.W. Hill, Hill, according according to Winchell Winchell (1895). (1895). However, However, iron iron mining mining did did not not begin begin until until 1870, when when N.P. N.P. Saxton started digging pits and trenches on the site of of the Breene the first first ore being shipped in 1873 (Bayley and and others, others, 1966). All All the the major Mine, with the mines had been located located by 1878. 1878. Production Productioncontinued continued until until 1958, 1958, with with aa total total production production from the district of approximately 85,000,000 tons (Bayley and others, 1966). 1966). Seven mines produced produced nearly 77,000,000 tons of ore, with a majority of the production production from the district coming coming from three three mines, mines, the the Chapin Chapin (27,500,000 tons), the the Penn Penn (21,700,000 tons) and the Aragon (11,200,000 (11,200,000 tons) (Dutton, 1958). 1958). Production Production from the Chapin Mine ended in 1934 1934 with a major collapse of the workings. The subsidence from Chapin this collapse collapse formed the lake lake on the north north side of Iron Iron Mountain. Mountain. A causeway across the district was Highways U.S.-2 and U.S.-141. Ore from the district lake now carries the traffic on Highways hauled hauled by rail to Escanaba, Ml, MI, from where it was carried carried by boat to steel mills mills on the lower Great Lakes. All of the ore shipped shipped from the district was high-grade high-grade natural natural iron iron ore. Although the iron-formation iron-formation in the Menominee Menominee district was studied as a possible source of beneficiating beneficiating ore (a "taconite "taconite ore"), ore"), no no commercial commercial operation operation has has been been undertaken. undertaken. Harvey Mellen, a United States land surveyor, first discovered discovered iron iron ore ore in in the the Iron IronRiverRiverCrystal Falls district in 1851 commenced in 1881 1851 (James (James and others, 1968). Mining commenced I881 and the first ore ore was shipped shipped in in 1882. 1882. Except Except for for the the early early years years of of mining, mining, when when many manymines mines recovered by underground underground were operated as small open pits, nearly all of the ore was recovered mining mining (James (James and others, 1968). 1968). Mine exposures and maps of the numerous mines greatly helped helped resolve the complex structure of the district. A major major hazard hazard to mining mining was caused by the pyritic slate of the Wauseca Pyritic Member of the Dunn Creek Slate, to air in the the mines. During the the 92-year which would burn spontaneously when exposed to period 974, approximately approximately 205,000,000 205,000,000 tons tons of iron ore were shipped from period from 1882-1 1882-1974, the district. district. Although there there were were 121 121 mines mines in in the the district, district, aa majority majorityof of the the ore ore came came from from about a dozen mines. about a dozen mines. In the Florence district, iron iron ore ore was recovered recoveredfrom from six six relatively relativelysmall small mines minesbetween between 1980. Only about about 8,000,000 8,000,000 tons tons of of predominantly soft soft hematite and limonite 1880 and 1960. ore with a high phosphorous produced (Dutton, 1971). The Florence phosphorous content was produced Florence mine was the most productive productive mine in the district with 3,680,000 tons shipped between 1880 and 1931. Nearly Nearly all of the production production from the district district was from the Riverton Riverton IronIronformation (Dutton, formation (Dutton, 1971). 1971). PEMBINE-WAUSAU MAGMATIC PEMBINE-WAUSAU MAGMATIC TERRANE Introduction Introduction northeastern Wisconsin The volcanic and plutonic plutonic rocks rocks that are widely distributed in northeastern south of the Menominee Menominee and Iron Iron River-Crystal River-Crystal Falls iron-bearing iron-bearing districts in Michigan are the easternmost easternmost exposures of a major major east-trending east-trending belt belt of volcanic and plutonic plutonic 20 �Pembine-Wausau terrane, the northernmost of the two Wisconsin Wisconsin rocks known as the Pembine-Wausau magmatic terranes (Sims and others, 1989). 1989). The occurrences in Marinette and Florence represent the best-exposed best-exposed portion of this suite of rocks in the Lake Counties, Wisconsin represent Superior region region and include a dismembered dismembered ophiolite sequence (Schulz, 1987). The magmatic terranes represent represent complex magmatic magmatic arcs accreted accreted to to the Wisconsin magmatic southern margin Paleoproterozoic Penokean Penokean margin of the Archean Superior Craton during the Paleoproterozoic orogeny orogeny (Sims (Sims and and others, others, 1989). 1989). The Pembine-Wausau Pembine-Wausau terrane is is mainly composed composed of tholeiitic and calc-alkaline volcanic and others, others, 1989). A A more rocks that formed between 1860 and 1889 Ma (Sims and restricted calc-alkaline volcanic succession restricted succession was deposited deposited between between 1835 1835 and and 1845 1845Ma Ma on the older rocks rocks along along the southern margin margin of the terrane terrane in in Marathon Marathon County. Granitoid rocks constitute nearly half of the exposed rocks in the terrane and range in age from about 1870 1870 to 1760 1760 Ma. These intrusive rocks are mainly mainly granodiorite and tonalite but include gabbro, diorite and granite (Sims and others, 1993). 1993). An older suite of granitoids ranging ranging in age from about 1870 1870 to 1840 1840 Ma is broadly syn-orogenic whereas post-collsional alkali-feldspar younger post-colisional alkali-feldspar granite suites were emplaced at about 1835 Ma and 1760 1760 Ma. The magmatic magmatic rocks rocks of the Pembine-Wausau Pembine-Wausau terrane terrane are are separated separated from from epicratonic rocks of the Marquette Marquette Range Range Supergroup Supergroup to the north north by the Niagara Niagara fault zone (see Bayley and others, 1966, and Dutton, 1971) and from the Marshfield magmatic terrane to the south along the Eau Eau Plaine Plaine shear zone (Sims (Sims and and others, others, 1989). 1989). General Geology Geology Volcanic rocks are relatively well exposed in the the northern and eastern part of Marinette County and eastern Florence Florence County in northeastern Wisconsin, where they form an arcuate belt around the large Dunbar gneiss-granitoid dome (fig. 8) (Sims (Sims and Schulz the Quinnesec Formation by James 1993). The supracrustal rocks, formally named the (1958), (1958), consist of metamorphosed metamorphosed basalt, andesite, dacite, and rhyolite lava flows and volcaniclastic rocks, and locally, sedimentary rocks including greywacke, graphitic slates, and iron-formation. iron-formation.Pyritic to pyrrhotitic pyrrhotitic massive sulfide bodies are also present Cummings, 1982; 1983). Gabbro Gabbro sills sills are are common, common, locally (Hollister and Cummings, 1982; LaBerge, 1983). particularly in the northern part of the sequence (Bayley and others, 1966). Serpentinite bodies, bodies, commonly with some some associated associated gabbros, gabbros, are are also also present presentlocally. locally. Jenkins (1973) (1973) noted noted that at least four lithologically distinct volcanic units could be defined in northeastern northeastern Wisconsin with three of the units units sufficiently sufficiently different different from the the lithologies (Prinz,1958; 1958;Bayley Bayleyand andothers, others, lithologies of the the type type area area of of the the Quinnesec Quinnesec Formation Formation(Prinz, 1966) to warrant their their separate designation. designation. He proposed proposed the the informal names McAllister formation, Beecher formation, and Pemene Pemene formation, listed listed in the order of progressively progressively (1973) that this this also represents the order of more silicic units, and Jenkins (1 973) suggested that decreasing age. More recently, DePangher (1982) (1982) proposed that the Quinnesec the Quinnesec Group consisting consisting of of five five lithostratigraphic lithostratigraphic units Formation be designated the having formational status. For the purposes of this field guide, the informal nomenclature proposed by by Jenkins Jenkins (1973) (1973) will will be be used used for for the the volcanic volcanic rocks rocksin inthe the area. area. proposed 21 �88" 0845" 87" 5830" 87° 4815" 45" 4100"- 45° 4100" I 88" 0845" 22 I I 87° 4815" 87° 58' 30" 00 ' I ' 22 I 88 , I 00 55 66 44 ' 10 10 55 10 10 'I I I I 15 15 12Miles 12 Miles I Kilometers Kilometers EXPLANATION EXPLANATION Munising MunisingSandstone Sandstone(Cambrian) (Cambrian) Paleoproterozoic rocks rocks north northof of Niagara Niagara fault fault Paleoproterozoic Michigamme MichigammeFormation Formation -graywacke graywacke Vulcan VulcanIron-formation Iron-formation Randville RandvilleDolomite Dolomite Paleoproterozoicrocks rocksof ofWisconsin Wisconsin Magmatic MagmaticTerranes Paleoproterozoic Terranes Intrusive trusiverocks rocks Volcanicrocks rocks Volcanic . Spikehorn SpikehornCreek CreekGranite Granite formation' . . . Rhyolite Rhyoliteand and dacite dacite"Pemene "Pernene formation" Bush BushLake LakeGranite Granite Basaltic andesitic breccia "McAllister "McAllister formation" formation" Basaltic and andesitic r"J Hoskins HoskinsLake LakeGranite Granite , ,,=.*: Rhyolite Beecher formation Rhyolite,felsic felsictuff tuff,graywacke graywackenBeecher formation" TwelveFoot FootFalls FallsQuartz QuartzDiorite Diorite Twelve Marinette MarinetteQuartz QuartzDiorite Diorite Serpentiniteand andgabbro, gabbro, base base of of ophiolite ophiolitecomplex complex Serpentinite NewinghamTonalite NewinghamTonalite QuinnesecFormation Formation Quinnesec ' NewinghamTonalite, NewinghamTonalite, . , Athelstane AthelstaneQuartz QuartzMonzonite Monzonite megacrystic megacrysticfacies facies .. . DunbarGneiss Dunbar Gneiss Metagabbro Metagabbro Volcanic Volcanicand andgranitic graniticrocks rocksundivided undivided - — faults faults •0 field fieldtrip tripstops stops Figure 8. 8. Generalized Generalizedgeologic geologic map map of of part partof of northeastern northeasternWisconsin Wisconsinshowing showingthe the Figure location locationof fieldtrip stops. of field tripstops. The units unitsof of the theDunbar Dunbargneiss-granitoid gneiss-granitoiddome domeintrude intrudethe thevolcanic volcanicrocks rocksininnorthern northern The MarinetteCounty, County, and andthe the Athelstane AthelstaneQuartz QuartzMonzonite Monzoniteintrudes intrudesthem themininthe thesouth. south. Marinette 22 �Small Small intrusive intrusive bodies bodies ranging ranging from hornblendite hornblendite and gabbro to quartz quartz diorite, diorite, granite granite and and calc-alkaline calc-alkalinelamprophyre lamprophyre are are widespread, widespread, particularly particularly in in the southeastern southeastern parts parts of of the the exposed exposedvolcanic volcanic sequence. sequence. To To the the north north and and northeast, northeast, the the volcanic volcanic sequence sequenceis is truncated truncated by by the the Niagara Niagarafault fault (Bayley (Bayley and and others, others, 1966; 1966; Dutton, Dutton, 1971), 1971),which which marks marksaa major major discontinuity discontinuity in in the rocks rocks of the the region. region. North North of this this fault, fault, rocks rocksof of the the Michigamme Michigamme Formation Formation and other units units of the Marquette Marquette Range Range Supergroup Supergroup occur, occur, along Archeangneissic gneissicrocks. rocks. alongwith with basement basementuplifts upliftsof of Archean The The supracrustal supracrustalrocks rocks and and associated associatedsubvolcanic subvolcanic intrusives intrusives are are variably variably altered altered to to greenschist facies mineral assemblages throughout the eastern outcrop area but contain greenschist facies mineral assemblages throughout the eastern outcrop area but contain assemblages assemblages as as high high as as amphibolite amphibolitefacies facies adjacent adjacent to to the the Dunbar Dunbar dome dome and andfurther further to to the the west. west. The The rocks rocksare areregionally regionallyfolded foldedon onnorthwest-trending northwest-trendingaxes, axes,but butthey theycommonly commonly lack lack aa strong strongpenetrative penetrativecleavage cleavage in in the the east. east. As As aa result, result, primary primarytextures textures and and structures structuresare are generally generally well well preserved preservedin in the the eastern eastern outcrop outcrop area. area. Units Units generally generally face face outward Dunbardome dome and and Athelstane Athelstane intrusion. intrusion. outwardfrom from the the margins marginsof of the the Dunbar Contacts Contactsbetween between the the various various volcanic units units are not not exposed, but but are are interpreted interpreted to to be be high-angle high-anglefaults faults (Jenkins, (Jenkins, 1973; 1973; Sims Sims and and Schulz, Schulz, 1993). 1993). Because Because of of uncertainties uncertaintiesin in the the amount amount of of displacement displacement on on these these faults faults and and the the complexity complexity of of folding, folding, detailed detailed correlations correlationsbetween between units units have have not not been been possible. possible. Present Present geologic geologic data data support support the the interpretationof of the the Quinnesec Quinnesec Formation Formation(as (as used used by by Jenkins) Jenkins) as as the the oldest oldest volcanic volcanic interpretation unit. unit. The The relative relativeages agesof ofthe theother otherunits, units,however, however,remain remainuncertain. uncertain.The Theunits unitsmay mayhave have been beenstructurally structurallyemplaced emplaced and and may may not not be be in in their their original original stratigraphic stratigraphic position. position. The The regional McAllisterformation formation may may be be younger younger than than the the regionalstructure structuresuggests suggeststhat that the the McAllister Beecher Beecher formation formation but but older older than than the the Pemene Pemeneformation. formation. Further Furtherwork work isisrequired requiredto to resolve resolvethe the age ageand andstratigraphic stratigraphicrelationships relationshipsof ofthese theseunits. units. Until Until relatively relativelyrecently recently the age age of the the volcanic volcanic rocks rocks in in northeastern northeasternWisconsin Wisconsin was was aa point point of of controversy. controversy.Van VanHise Hiseand andBayley Bayley(1900) (1900)and andBayley Bayley(1904) (1904)originally originally Archeanbecause becauseof ofthe thestriking strikingsimilarity similarityofofthese these interpretedthe the"Quinnesec "Quinnesecschists" schists"as asArchean interpreted rocks rocksto to Archean Archean rocks rockselsewhere elsewhere in in the the Lake Lake Superior Superior region. region. Van Van Hise Hiseand and Leith Leith (1911) subsequentlyassigned assignedthe theQuinnesec Quinnesecto toaapost-Michigamme post-Michigamme(i.e. (i.e. (1911)subsequently Paleoproterozoic) Paleoproterozoic) age age on on the the basis basis of of the the interpretation interpretationof of Hotchkiss Hotchkissand and others others (1915) (1915) that the Michigamme Formation graded upward into volcanic rocks in Florence County, that the Michigamme Formation graded upward into volcanic rocks in Florence County, Wisconsin.Dutton Dutton(1971) (1971)later laterreinterpreted reinterpretedthe therelationship relationshipininthis thisarea areaand andplaced placedaa Wisconsin. faultbetween betweenthe the volcanic volcanic units unitsto to the the south south and andthe the Michigamme MichigammeFormation Formationto to the the fault Bayleyand andothers others(1966) (1966) and andDutton Dutton(1971), (1971), favored favoredan anArchean Archean age, age,although although north.Bayley north. acknowledgingthat thatdefinitive definitivefield fieldevidence evidencewas was lacking lackingto to establish establishthe the age ageof of the the acknowledging Quinnesec QuinnesecFormation. Formation. Banks Rebello(1969) (1969) reported reportedaa U-Pb U-Pbzircon zircon age age of of 1,866±39 1,866±3Ma Mafor for aa felsic felsic Banksand andRebello volcanic sample sample from from south south of of the the Dunbar Dunbardome. dome. This Thisage, age, which which isisnot notresolvable resolvablefrom from volcanic the theages agesofofthe therocks rocksofofthe theDunbar Dunbardome dome(Sims (Simsand andothers, others,1984), 1984),isisnow nowgenerally generally taken takenas asthat thatofofthe thevolcanic volcanicsequence sequenceininnortheastern northeasternWisconsin, Wisconsin,although althoughthis thislocality locality isisisolated 1,870±5Ma Mawas was isolatedfrom fromthe themain mainareas areasof of outcrop. outcrop. More Morerecently, recently,an anage ageof of 1,870±56 obtained for the basalts of the Quinnesec Formation by whole-rock Sm-Nd isotopic obtained for the basalts of the Quinnesec Formation by whole-rock Sm-Nd isotopic systematics(Beck (Beckand andMurthy, Murthy,1991). 1991).Thus, Thus,the theage ageof ofthe thevolcanic volcanicrocks rocksinin systematics Archean as as northeasternWisconsin Wisconsinisisnow nowestablished establishedas as Paleoproterozoic Paleoproterozoicand andnot notArchean northeastern once oncethought. thought.Their Theirage ageisisgenerally generallysimilar similarto tothat thatobtained obtainedfor forthe themassive massivesulfide sulfide depositsnear nearCrandon, Crandon,Monico Monicoand andLadysmith Ladysmithto tothe thewest west (Sims, (Sims,1976) 1976)and andto toages agesof of deposits 23 �magmatic terrane terrane (Sims and other volcanic and and plutonic plutonic rocks rocks of the the Pembine—Wausau Pembine-Wausau magmatic others, 1989). 1989). Volcanic Units Units The four volcanic units that comprise the supracrustal sequence in Marinette and Florence Counties, as well as some of the granitoid bodies that intrude them, are briefly (1989), described below. Further information can be found in Sims and others (1 989), Sims and (1993). 993). Although the rocks are mostly others (1992), (1992), and Sims and Schulz (1 metamorphosed fades, the metamorphosed at greenschist facies, the prefix prefix "meta" "meta"is is generally generally omitted omitted below belowfor for simplicity. simplicity. Quinnesec Formation: The Quinnesec Formation is the dominant volcanic unit exposed in northeastern Wisconsin. Its Its stratigraphic thickness is not known known because because of uncertainties in the degree of folding and faulting but is probably on the order of several thousand meters. The Quinnesec Formation consists predominantly of basalt lava flows fragmental andesite and diabase in the north, but includes includes pillowed pillowed and fragmental andesite in in the the south. south. The The basalt is commonly pillowed, with the pillows locally variolitic. The pillowed flows are in commonly pillowed, pillows locally The pillowed flows are in some areas overlain by thick (tens of meters) sections of pillow breccia and hyaloclastite breccia (N. V2, ½, sec. 11, T. 37 N., A. R. 21 E., E., Faithorn Faithorn 7.5 minute minute quadrangle). quadrangle). Andesite Andesite breccia increases increases in abundance abundance southward southward in in the formation, formation, and and is is generally generally plagioclase plagioclaseand and clinopyroxene-phyric clinopyroxene-phyric and amygdaloidal. amygdaloidal. Fresh, Fresh, glacially glacially polished polished outcrops outcrops around aroundthe the new location location of the Kremlin mine pit east of Pembine (S. V2, ½, sec. 26, T. 37 N., R. 21 E., Faithorn Faithorn 7.5 minute minute quadrangle) quadrangle) provide provide excellent excellent exposures exposures of of andesite andesitebreccia. breccia.Felsic Felsic tuff and breccia breccia are also present locally in the Quinnesec, particularly in the northern Menominee River. Felsic Felsic fragmental fragmental units unitsare are also also present presentin inthe the portion near the Menominee LaSalle LaSalle Falls Falls area along along the Pine Pine River River in in Florence Florence County County (Bayley (Bayley and and others, others, 1966; 1966; Dutton, Dutton, 1971). 1971). tholeiitic, with with generally generally low low Ti02 Ti02 and and other other Compositionally, the Quinnesec basalts are tholeiitic, high-field-strength element abundances, and flat to extremely light rare-earth element high-field-strength rare-earth (REE) depleted patterns (Sims and others, 1989). In addition, some of the basalts basalts and andesites have have very very low low Ti02 Ti02and and REE REEabundances, abundances, but butrelatively relativelyhigh highCr Crand andNi Ni the andesites contents. The trace element characteristics of the mafic volcanic rocks overlap those of mid-ocean ridge basalt mid-ocean basalt (MORB) (MORB) and and primitive primitive island-arc island-arc tholelite tholeiite suites suites whereas whereasthe the with boninites (fig. 9), although none of the andesites show compositional affinities with andesites are as high in MgO as true boninites. The felsic volcanic rocks have low potassium compositions with relatively low REE flEE abundances abundances and flat REE REE patterns. patterns. They are are similar in in composition composition to tholeiitic plagiogranite/rhyolite plagiogranitelrhyolite (Schulz, 1987; Sims and others, 1989). 1989). The compositional data suggest that the original basaltic magmas that gave rise incompatiblerise to the Quinnesec rocks rocks were derived from a variably incompatiblevalue element-depleted mantle source. This is supported by a large positive epsilon Nd value 1991), indicative indicative of of derivation derivation from from aa mantle mantlewith with of 4.2 for the basalts basalts (Beck (Beck and and Murthy, Murthy, 1991), long-term depletion in light rare-earth rare-earthelements. elements. 24 �2.0 ' I -L 1.6 1.2 - Beecher Beecher andesites andesites A A Phanerozoic Phanerozoic boninites 0.8 Pemene Pemene - rhyol ites 0.4 0.0 40 50 60 70 80 S102 (wt. %) 2.0 I I I a I I I I 1.6 0 1.2 Beecher andesites 0.8 Pemene rhyolites Phanerozoic boninites 0.4 .1 L 0.0 0 50 100 150 200 Zr (ppm) Figure 9.Plots Plotsof of Ti02 TiOaversus versusSi02 SiOg(upper) (upper)and andZr Zr (lower) (lower)for forvolcanic volcanicrocks rocks(triangles) (triangles) Figure9. andrelated relatedgabbros gabbrosand anddiabase diabase(circles) (circles)from fromthe theQuinnesec QuinnesecFormation, Formation,northeastern northeastern and Wisconsin.Note Notethat thatseveral severalofofthe thesamples samplesplot plotininthe thefield fieldofofPhanerozoic Phanerozoicboninites. boninites. Wisconsin. Fields Fieldsalso also are areshown shownfor for Beecher Beecherandesites andesitesand andPemene Pemenerhyolites. rhyolites. Sedimentaryrocks rocksappear appeartotobe berare rarewithin withinthe theQuinnesec QuinnesecFormation. Formation.Where Wherepresent, present, Sedimentary they theyconsist consistmostly mostlyofofchert, chert,graywacke, graywacke,slate slateand andiron-formation. iron-formation.Iron-formation Iron-formationoccurs occurs asthin thinunits unitsinterlayered interlayeredwith withvolcaniclastic volcaniclasticsedimentary sedimentaryrocks rocksand andtuffs, tuffs,and andconsists consistsofof as 25 �interlayered interlayered chert and siderite (Cummings, 1978). 1978). A small massive massive sulfide deposit, containing containing pyrrhotite pyrrhotiteand and chalcopyrite, chalcopyrite, in in a felsic tuff, and and in in aa fine-grained fine-grainedblack black slate slate that that occurs occurs between between the the felsic felsic tuff tuff and and aa mafic mafic unit unit is is exposed exposed at at Pine Pine Rapids Rapids (locally (locally known "LaSalle Falls") Falls") on on the the Pine Pine River River (LaBerge, (LaBerge,1983). 1983). known as "LaSalle In In addition to volcanic rocks, the Quinnesec Formation Formation also contains a number of gabbroic gabbroic and and ultramafic ultramafic rocks rocks (Sims (Sims and and Schulz, Schulz, 1993). 1993). Numerous Numerous large large gabbro gabbro bodies bodies are are present, present, particularly particularly near near the Niagara Niagara fault zone. These These bodies bodies are are more more or or less less conformable conformable with with the the basaltic basalticflows flows and andprobably probablyrepresent representsynvolcanic synvolcanicsills, sills,although although Bayley Bayley and and others others (1966) (1966) considered considered the gabbroic gabbroic sills to be be "post-Animikie" "post-Animikie"in in age. age. Gabbro Gabbro and and anorthositic anorthositic gabbro gabbro comprise comprise the the bulk bulk of the the sills. sills. Some Some sills sills also also contain contain peridotite peridotite(serpentinite) (serpentinite) and and pyroxenite pyroxenite layers layers and magnetite-rich magnetite-richgabbro. gabbro. Trace Trace element element and and isotopic isotopic data data for for the the gabbros gabbros show show they are are comagmatic comagmatic with with the the basalts basalts of of the the Quinnesec Quinnesec Formation Formation(Schulz, (Schulz, unpublished unpublisheddata). data). Several Several serpentinized serpentinized peridotite peridotite bodies bodiesof of varying varyingsizes sizesoccur occurwithin withinthe thevolcanic volcanic succession, succession, but but the the largest largest and and best-exposed best-exposed occurs occurs south south of Timms Timms Lake Lake (Morgan (Morgan County County Park) Park) east east of Pembine, Pembine, Wisconsin. Wisconsin. The The body body trends trends east east from from outcrops outcropsin in the the NE. sec.19, 19,T. T. 37 37 N., N., R. R. 21 21 E., for for a distance of about %, sec. about 4.5 km kmto to the theNorth NorthBranch Branch NE. 1/4, Pemebonwon %, sec. 22, T. 37 37 N., N., R. R. 21 21 E. E. The The body body produces produces aa large large Pemebonwon River River in in the NE. ¼, magnetic magnetic anomaly. Serpentinized peridotite is dominant in the western part body anomaly. Serpentinized peridotite is dominant in part of the the body where (1-5 cm) dikes of pyroxenite pyroxenite now now replaced replaced by by where itit is is locally locally cut by by coarse coarse grained grained (1-5 amphibole. amphibole. The textures, although The serpentinite serpentinite generally generally shows few primary primary textures, although primary primary compositional compositional layering layering is is suggested suggested locally locally by differential differential weathering of bands bands in in some some outcrops. outcrops. Veins Veins of of carbonate carbonate and and cross-fiber cross-fiber asbestos asbestos are are common. common. Layered Layered and and massive massive gabbro, gabbro, along along with local local masses masses of strongly strongly foliated-lineated foliated-heated gabbro, gabbro, are are dominant dominant in in the the eastern eastern part part of of the the body. body. The The gabbroic gabbroic rocks rocks are are cut cut by by numerous numerousmafic mafic dikes, some of which appear to be sheeted, with diabasic to microdioritic textures. The dikes, some of which appear to be sheeted, diabasic microdioritic The strongly strongly foliated-lineated foliated-lineatedgabbro gabbro masses masses appear appear as as screens screens between between the the dikes. dikes. The The foliation in in the the gabbro gabbro screens screens is is at at aa high high angle angle to to the the contacts contacts of of the the body body with with the the foliation Quinnesec Quinnesec basalts. basalts. The The strong strong deformational deformationalfabric fabric of of the the gabbro gabbro screens screens is is not not present present in in most most of of the the associated associated rocks rocks of of the the body, body, including including the the dikes. dikes. This This suggests suggeststhat that the the deformation deformation of of the the gabbro gabbro in in the the screens screens occurred occurred prior prior to emplacement emplacement of the dikes dikes and and to to the the regional regionaldeformation. deformation.The Thetrend trendof of the the mafic maficdikes dikesisisabout aboutparallel parallelto tothe thetrend trendof of the serpentinite-gabbro serpentinite-gabbrobody body(—E-W) (-E-W) and generally at a high high angle angle to to the the foliation foliation of of the the the gabbro gabbro screens. screens. The The dikes dikes do do not not appear appear to to extend extend into into the the surrounding surrounding pillow pillow basalts. basalts. These These features features suggest suggest that that the the serpentinite-gabbro serpentinite-gabbro body body may may be be fault-bounded fault-bounded and and tectonically emplacedwithin within the the Quinnesec Quinnesec Formation. Formation. The The compositions compositions of of the the gabbros gabbros tectonically emplaced and and diabasic diabasic dikes dikes within within this this body bodyrange rangefrom fromMORB MORBto todepleted depletedisland-arc island-arctholeiite tholeiiteand and high-magnesiumandesite andesite with with boninitic boninitic affinities; affinities; these these mafic mafic rocks rocks are are similar similar in in high-magnesium composition compositionto to the the basalts basaltsin inthe the Quinnesec QuinnesecFormation Formation(fig. (fig.9). 9). The The lithologies lithologiesand andtheir their arrangement arrangementin inthis this ultramafic-gabbroic ultramafic-gabbroicbody, body, along alongwith with those those of of the the Quinnesec QuinnesecFormation Formationgenerally, generally,are are similar similar to to those those that that characterize characterize Phanerozoic Phanerozoic ophiolite sequences sequences(fig. (fig. 10). 10).This This includes includes(from (frombottom bottomto to top): top): mafic-ultramafic mafic-ultramafic ophiolite plutonic plutonic rocks, rocks, aa dike dike(sheeted?) (sheeted?)complex, complex,extrusive extrusivepillowed pillowedand andmassive massivebasalt basaltlava lava flows, flows, and and overlying overlying arc-related arc-relatedvolcaniclastic volcaniclastic rocks. rocks. Tectonized Tectonized ultramafic ultramafic rocks rocks representing suboceanic suboceanic mantle mantle have have not not been been recognized. recognized. Also, based based on on the present present representing exposure,the the ultramafic-mafic ultramafic-maficcumulates cumulatesand anddike dikecomplex, complex,although although present, present,are aremuch much exposure, less less extensive extensive than than in in the ideal ideal ophiolite sequence sequence (fig. (fig. 10). 10). This This may may be be aa function function of 26 �limited limited exposures exposures in in the region region and/or and/or lack lack of of preservation preservationof of the the lower lowerpart partof of the the ophiolite ophiolite sequence sequenceduring during later later magmatic magmaticand andtectonic tectonicevents, events.Incomplete Incompletesequences sequences are are common commonin in ophiolites ophiolitesfound found ininmany manyorogenic orogenic belts belts(Moores, (Moores,2002). 2002).The Thelithologic lithologic association associationobserved observedin in the the Quinnesec Quinnesec and andthe the compositions compositionsof of rocks, rocks,ranging rangingfrom from MORB MORE3 to depleted depleted island-arc island-arc tholeiite tholeiite and and high-magnesium high-magnesiumandesite andesiteof of boninitic boniniticaffinity, affinity, and plagio-rhyolite, are similar to Cenozoic suprasubduction zone ophiolites like those and plagio-rhyolite, are similar to Cenozoic suprasubduction zone ophiolites like those of of the the Coast Coast Ranges Rangesin inCalifornia California(Shervais (Shervaisand andKimbrough, Kimbrough,1985; 1985;Shervais, Shervais,2001). 2001). J 1 1 , ^ , I, Shallow-water Shallow-water or terrestrial terrestrial sedimentary sedimentaryrocks rocks Unconformity Unconformity Pelagic sediments Pelagic sediments sediments or or abyssal abyssal deep-sea fan sediments or or volcanic volcanicarc arc deposits deposits Mafic pillow breccia, Mafic pillow pillowlava, lava, pillow breccia, and and massive massive flows Mafic Mafic sheeted sheeted dike dike complex complex \( 1/ Massive plagiogranite Massive gabbro, diorite, and plagiogranite Cumulate Cumulatesection: section: ultramafic-mafic ultramafic-maficcumulates cumulatesatatbase, base, more toward top. more felsic toward top. Commonly Commonlycyclic, cyclic,common common contorted contortedlayering layeringand and other other evidence evidence for deformation deformation Petrologic Moho Ultramafic peridotite with Ultramafic tectonite; peridotite with discontinuous discontinuous layers of of dunite dunite (D) and and concentrations concentrations of chromite (Cr) chromite(Cr) Figure 10. 10. Schematic Schematic cross-section cross-section of of aa complete completeophiolite ophiolite(after (afterMoores, Moores,2002). 2002). Figure The Quinnesec Quinnesec Formation Formationis is intruded intruded by by units units of of the the Dunbar Dunbar dome dome including includingthe the Marinette Marinette Quartz Quartz Diorite Diorite and and the the Spikehorn Spikehorn Creek Creek Granite, Granite, as as well well as as by by the the Newingham Newingham Tonalite Tonalite and and Twelve Foot Foot Falls Falls Quartz Quartz Diorite Diorite (Sims (Sims and and Schulz, Schulz, 1993) 1993) and and numerous numerous 27 �To the the southeast the the Quinnesec is in small lamprophyre dikes and plugs locally. To apparent fault contact with the Pemene formation (Sims and Schulz, 1993). McAllister formation: The The McAllister McAllisterformation formationappears appearsto to be be in in fault fault contact contactwith with the the the area (Jenkins, 1973). It occurs in a roughly east-west belt adjoining volcanic units in the between the Pemene formation to the north and the Beecher formation formation to the south (Sims and Schulz, 1993). It ranges in thickness from about 300 meters in the west to W and ~.70-80'W 3,000 meters in the east with units facing north and generally striking N.70-80° consists of of calc-alkaline caic-alkalirie basaltic to to andesitic andesitic dipping near vertical. The McAllister consists volcanic breccia breccia with aa lithic lithic tuff matrix, matrix, and and locally locally pillowed pillowedand and massive massivelavas. lavas. Fragments in the breccia breccia are distinctive distinctive in in that they they generally generally contain contain large largepyroxene pyroxene crystals crystals that are now replaced replaced by hornblende. Amygdules are also common in many gradation in fragment fragments. Vertically, the McAllister formation shows no consistent gradation size. However, laterally, fragment fragment size size increases from from west west to to east east (Jenkins, 1973). Near the Menominee suggesting the Menominee River, blocks more than 15 cm in diameter are common, suggesting source area for this dominantly fragmental fragmental unit may be located located to the east in Michigan. source formation: The Beecher formation: The Beecher Beecher formation formation extends extends in a north-facing, east to southeasttrending belt belt south of the McAllister formation and north north of the Athelstane Quartz Quartz and Schulz, Schulz, 1993). The The formation formation is at at least 3,000 3,000 meters thick thick with aa Monzonite (Sims and thicker (2,000 - 3,000 m) lower unit consists dominantly of of lower and upper unit. The thicker caic-alkaline flows and and lesser calc-alkaline plagioclaseplagioclase- and pyroxene-phyric andesite and dacite lava flows interbedded felsic volcaniclastic rocks. The thinner (up to 300 m) upper unit consists of interbedded rounded ash, crystal tuff, lapilli tuff, and coarser fragmental rocks, some with distinctive rounded pink to white felsite fragments. Some units show grading whereas others are unsorted. Black slates are also present present locally in the upper upper part part of the the formation. formation. The lower part of the Beecher Beecher formation, where intruded intruded by the Athelstane Quartz Monzonite, faces away from the intrusion intrusion and and has has aa well well developed developed foliation foliation and and steeply steeply plunging lineation. Dikes plunging Dikes of Athelstane Quartz Quartz Monzonite Monzonite extend extend only only aa short short distance distance formation. into the Beecher formation. Pemene formation: The The Pemene Pemeneformation formationisisinterpreted interpretedto to be bethe the youngest youngest volcanic volcanic unit in eastern Marinette County. It occupies a broad broad oval oval area area whose whose outline outline is is well well in the the northern part part of of the Miscauno Island 7.5 7.5 minute expressed in the local topography in 2,000 meters thick and consists predominantly predominantly of of quadrangle. The Pemene is at least 2,000 thick (45 300 m) calc-alkaline calc-alkaline rhyolite lava flows typically composed of a flow-top (45 -- 300 hyaloclastite microspheruliticcentral central core. core. hyaloclastite breccia, an underlying flow-banded flow-banded unit, and a microspherulitic The microspherulitic microspherulitic core constitutes the bulk of each flow and contains plagioclase plagioclase phenocrysts, commonly in glomeroporphyritic clusters, clusters, and and locally locally phenocrysts phenocrystsof of blue blue and lower contacts contacts quartz. The flow-banded unit, where present, has gradational upper and and laminar to highly-contorted highly-contorted banding. The The top top of each each flow flow has has aa layer layer of of hyaloclastite hyaloclastite breccia 3 to 75 m thick that probably formed by quench fragmentation as the hot rhyolite flows sec. 13, 13, flows came came in in contact contactwith with cold coldexternal externalwater. water.Exposures Exposuresininthe theNE NE1/4, 14, NW ¼, 14, sec. T. 36 N., R. 21 E., E., (Miscuano (Miscuano Island Island 7.5 minute quadrangle) provide an excellent cross sectional view of one of these rhyolite flows. Here, large open outcrops show a transition from massive massive microspherulitic microspheruliticrhyolite rhyoliteupward upward into into flow-banded flow-banded rhyolite rhyolitebreccia brecciawith with clasts to about 30 cm (a "crackle "crackle breccia"?). breccia"?). This in turn grades grades upward upward into a spectacular fine hyaloclastite hyaloclastite consisting of black rhyolite fragments, some with fine internal banding and white rinds rinds (figs. 11 11 and and 12). 12). Thin Thin sedimentary sedimentary units unitswith with graded graded 28 �bedding are present between some rhyolite flows. The general characteristics characteristics of the Pemene rhyolites is similar to those of other submarine rhyolite lava flow and dome complexes such that on the Island Island of Ponza, Italy (Scutter and others, 1998). Autobrecciated Autobrecciated and and hyaloclastite hyaloclastitecarapace carapace banded and Flow banded and microspherulitic interior Figure 11. Schematic cross-section of of the the upper part of of a subaqueous rhyolite lava flow in the Pemene Pemene formation. formation. Photograph of the rhyolite hyaloclastite in the upper carapace of a Figure 12. Photograph subaqueous Pemene subaqueous Pemene rhyolite rhyolite lava lava flow. 29 �The Pemene Pemene formation shows little evidence of a penetrative structural fabric. The flows show a southward dip in the north and dip nearly vertically in the south. Jenkins (1 (1973) 973) interpreted the structure of the Pemene as an east-trending, asymmetric, doubly interpreted of plunging plunging syncline. syncline. Major Intrusive Intrusive Rocks Rocks A variety of intrusive rocks are present within the supracrustal sequence in northeastern Wisconsin. They range tonalite to range from synvolcanic synvolcanic gabbro, diabase, diorite, and tonalite syntectonic intermediate to felsic granitoids (including those related related to the Dunbar dome), lamprophyric lamprophyric dikes and plugs, and post-tectonic granites and diabase dikes. Twelve Foot Falls Quartz Diorite: The Twelve Foot Falls Quartz Diorite comprises a some 20 20 km km by by 55 km km in size size to to the the west west of of Beecher, Wisconsin Wisconsin large "sill-like" intrusion some of gray, gray, generally medium to to coarse-grained coarse-grained (Sims and Schulz, 1993). It is composed of quartz quartz diorite diorite containing containing subhedral subhedral crystals crystals of of sodic sodic andesine, andesine,subhedral subhedralhornblende, hornblende, and anhedral bluish quartz. In composition it is similar to calc-alkaline calc-alkaline andesites in the McAllister and Beecher formations (Sims (Sims and others, others, 1992). 1992). Atheistane Atheistane Quartz Monzonite intrudes the Beecher Athelstane Quartz Monzonite: The Athelstane formation and extends for an unknown formation and extends for an unknown distance distance to to the the south southand andwest west(Sims (Simsand andSchulz, Schulz, 1993). It consists dominantly of of medium- to to coarse-grained quartz monzonite and locally contains contains numerous numerous metavolcanic metavolcanicinclusions. inclusions. The The Athelstane Athelstane Quartz QuartzMonzonite Monzoniteisisdated dated at 1,836±15 1,836±1Ma Ma (Banks (Banks and and Cain, 1969). 1969). The The Amberg Amberg Granite, Granite,which which intrudes intrudesthe the Athelstane Quartz Monzonite west and north of of Amberg (Sims and Schulz, 1993), 1993), is dated 1,756±1Ma Ma (Van (Van Schmus, Schmus, 1980). 1980). dated at 1,756±19 Dunbar Dunbar Dome Dome Only a brief summary of of the aeoloav geology of of the Dunbar dome is presented presented here for the Note. Onlv purposes of the present trips. More More comprehensive comprehensive accounts accounts are are available available in in aa guidebook auidebook by bv Sims Sims and and others others (1984), (19841, and and in in U.S. U.S. Geological Geoloaical Survey Survey Professional ProfessionalPaper Paper 1517 by Sims and and others others (1992). (1992). 1517 bv The Dunbar dome is one of several granitoid domes in northern Wisconsin that have have cores of gneiss, migmatite migmatite and granitoid rocks rocks and are mantled mantled by metavolcanic metavolcanic and Pembine-Wausau magmatic terrane (Sims and others, metasedimentary rocks in the Pembine-Wausau 1985). Where ages have been determined, both the core of the domes and mantle or cover rocks are of Paleoproterozoic age. The Dunbar dome is a complex antiformal structure consisting of a central core of Dunbar Gneiss, Marinette Quartz Diorite, and from the core composed of Hoskin Lake Granite, and three lateral protuberances (lobes) from the west, west, Spikehorn Creek Granite on on the the east, and Newingham Bush Lake Granite on the Tonalite on the south (Sims and others, 1992). structural evolution of 1992). The intrusive and structural the dome spanned the relatively short time of about 30 Ma, from syn-tectonic syn-tectonic events at about 1865 1865 Ma to post-tectonic post-tectonic at about about 1835 1835 Ma. Ma. Conclusions Conclusions The volcanic and associated intrusive rocks in northeastern Wisconsin south of the fault, the Pembine ophiolite, are are interpreted to to record the the evolution of of a Niagara fault, 30 �Paleoproterozoicsuprasubduction suprasubductionzone zone ophiolite-island ophiolite-islandarc arc sequence. sequence. Shervais Shervais(2001) (2001) Paleoproterozoic has has shown shown that suprasubduction suprasubductionzone zone ophiolites ophiolites tend tend to to display displayaa consistent consistentsequence sequence of events tectonic processes. events during their formation formation and evolution in response response to similar tectonic processes. This This sequence sequence includes includesthe the following following(after (afterShervais, Shervais,2001): 2001): (1) Birth, Birth, which which entails entails the the initiation initiationof ophiolite ophiolite formation formation during during extension extension above above aa (1) reconfigured intraoceanic subduction zone. Rocks formed newly forming or reconfigured during during the the initial initial phase phase of ophiolite ophiolite formation include include layered layered and isotropic isotropic plutonic gabbros, sheeted dikes, and a "lower" volcanic section consisting lowplutonic gabbros, sheeted dikes, and a "lower" volcanic section consistingof oflowK K tholeiitic tholeiitic basalt basalt and and basaltic basaltic andesite andesite with with MORB MORB and and primitive primitive arc arc tholeiite tholeiite affinities. Gabbros Gabbros formed formed during during this this stage stage are are often often ductilely ductilely deformed deformed (foliated (foliated affinities. or or boudinaged) boudinaged)in in response responseto to syn-magmatic syn-magmaticextension. extension. (2) (2) Youth, Youth, during duringwhich which continued continuedmelting meltingof of previously previouslydepleted depletedasthenospheric asthenospheric mantle mantle occurs occurs in in response response to to increased increased fluid flux from from the the subducting subducting slab. Rocks Rocks formed during during the the second second phase phase of ophiolite ophiolite formation formation include include intrusive intrusive maficmaficformed ultramafic ultramafic sills sills and and additional additional dikes, dikes, and andan an"upper" "upper"volcanic volcanicunit unitcharacterized characterizedby by basalt and andesite with highly depleted incompatible trace element basalt and andesite with highly depleted incompatible trace element compositions compositions (i.e., (i.e., low-Ti low-Tibasalt, basalt, high-Mg high-Mgandesite andesiteand andboninite). boninite). (3) Maturity, Maturity,during duringwhich which the the subduction subductionzone zone stabilizes stabilizes and and the the rate rate of of crustal crustal (3) spreading slows. slows. Rocks Rocks formed formed during during this this phase phase include include hornblende hornblende diorite, spreading quartz quartz diorite, diorite, tonalite, and and volcanic volcanic rocks rocks ranging ranging from from basalt basalt to to rhyolite, rhyolite, all all with with transitional transitional to tocalc-alkaline calc-alkalinecompositions. compositions.Volcanism Volcanismtypically typicallybecomes becomesmore more silicic silicic with time. In In many many cases, these rocks rocks have have not been been considered considered part part of the subjacent subjacent ophiolite, ophiolite, but but rather rather have have been been attributed attributed to post-ophiolite post-ophiolitearc volcanism. volcanism. (4) Death, Death,which which results resultsfrom from the the demise demiseof of active activespreading spreadingand andsubduction. subduction.In Inthe the (4) case case where where death death results results from from collision collision with with an an active activeocean oceanspreading spreadingcenter, center, dikes dikes and and lavas lavas with with oceanic oceanic basalt basalt compositions compositionsmay maycrosscut crosscutand andoverlie overliethe the older older ophiolite-arc ophiolite-arcsection. section. (5) Resurrection, Resurrection,which which accompanies accompaniesemplacement emplacementby by obduction obduction onto onto aa passive passive (5) continental margin marginor or accretionary accretionary uplift uplift with with renewed renewedsubduction. subduction. In In the the case case continental where where death death is is the the result result of of collision collision with with aa passive passive margin, margin, death death and and resurrection of of the the ophiolite ophiolite sequence sequencemay mayoccur occuressentially essentiallysimultaneously. simultaneously. resurrection The rocks rocks of of the the Quinnesec Quinnesec Formation Formationappear appear to to record record the the first first two two stages stagesof of The suprasubduction zone zone ophiolite evolution. The presence presence in the upper upper part of the suprasubduction Quinnesec Formation Formation of basalt basalt and and andesite andesite lavas lavas and dikes derived from highly highly Quinnesec refractory mantle mantle is is particularly particularly diagnostic diagnostic of aa relationship relationship to to the the early early stages stages of refractory intraoceanic forearcsetting setting(Shervais (Shervaisand andKimbrough, Kimbrough, intraoceanicsubduction subductionand and formation formationin inaaforearc 1985; Beccaluva Beccaluvaand and Serri, Serri, 1988). 1988).This This further furtherimplies impliesthat that the the Quinnesec QuinnesecFormation Formationand and 1985; associated rocks rocks did did not not form form in in aa back-arc back-arcbasin basin near near the the margin margin of of the the Superior Superior associated Craton, but but probably probablyformed formedas asan anintraoceanic intraoceanicophiolite-arc ophiolite-arcsystem systemabove aboveaasouthward southward Craton, dipping dipping(in (inpresent presentcoordinates) coordinates)subduction subductionzone. zone. McAllister, Beecher Beecherand and Pemene Pemene formations formationsand and The calc-alkaline calc-alkalinevolcanic volcanic rocks rocks of of the the McAllister, The associated associatedintrusives intrusivessuch such as as the the Newingham Newingham Tonalite Tonalite and and Twelve Twelve Foot Foot Falls Falls Quartz Quartz Diorite appear appear compatible compatiblewith with the the third thirdstage stage(maturity) (maturity)of ofsuprasubduction suprasubductionzone zone Diorite ophiolite evolution. evolution. Shervais Shervais (2001) (2001) notes notes that that for for aa suprasubduction suprasubduction zone zone ophiolite ophiolite to to ophiolite reach maturity maturity requires requires that that the the ocean ocean basin basin being being subducted subducted be be large large enough enough to to reach complete completethe the first first two two stages stageswithout without disappearing. disappearing.This This suggests suggests that that the the 31 �Paleoproterozoic significant in size (at Paleoproterozoic ocean basin that was subducted was probably significant least many many hundreds hundreds of kilometers). kilometers). It appears likely that growth of the Pembine Pembine ophiolite-arc system system was terminated terminated by by its its collision obduction onto onto the passive passive margin margin of the the Superior Superior craton. craton. Since Since collision with with and and obduction subduction subduction appears appears to to be be largely largely driven driven by by slab slab pull, pull, the the southward southward subduction subduction of of oceanic oceanic lithosphere lithosphere attached attached to to the the Superior Superior continental continental margin marginwould would have have pulled pulled the the continental lithosphere along with it as it descended into the subduction zone below continental lithosphere along with it as it descended into the subduction zone below the the ophiolite-arc ophiolite-arcsystem. system. With With detachment detachment of of the the subducting subductingoceanic oceanic lithosphere, lithosphere,the the buoyancy buoyancy of the the continental continental lithosphere lithosphere would have have led led to its its rapid rapid uplift uplift along along with with the the leading leading edge edge of of the the ophiolite-arc ophiolite-arc system system (Shervais, (Shervais, 2001). 2001). This This interpretation interpretationsuggests suggests that the the volcanic volcanic and and associated associated rocks rocks of of northeastern northeastern Wisconsin Wisconsinare are allochthonous, allochthonous,as as is is also also suggested suggested by by gravity gravity and and magnetic magnetic data data for for the the region region (Klasner (Klasner and and others, others, 1985; 1985; Attoh Attoh and and Klasner, Klasner, 1989). 1989). This This stage stage is is recorded recorded by by the the deformation deformationof of the the ophiolite-arc ophiolite-arc sequence Dunbardome. dome. ItItisis sequence and and by by the the intrusion intrusionof the the syn-tectonic syn-tectonic units units of of the the Dunbar possible possible that the shallow-water shallow-water sedimentary sedimentary rocks rocks (carbonates) (carbonates) along along the the west west margin margin of of the Dunbar Dunbar dome dome (Sims (Sims and and Schulz, Schulz, 1993) 1993) represent represent Chocolay Chocolay Group Group rocks rocks of of the the Marquette Marquette Range Range Supergroup Supergroup that that were were uplifted upliftedfrom from the the continental continentalmargin marginbasement basement below below during during formation formation of of the the Dunbar Dunbardome. dome. 32 �FIELD FIELDTRIP TRIP1I PEMBINE-WAUSAU PEMBINE-WAUSAU MAGMATIC MAGMATICTERRANE TERRANE Klaus Klaus J. J. Schulz, USGS, Reston, VA VA and and Gene Gene L. LaBerge, University University of of Wisconsin-Oshkosh Wisconsin-Oshkosh (retired), (retired), Oshkosh, WI Wl and andUSGS USGS Pillowed flows of high-Mg high-Mg andesite of the Quinnesec Quinnesec Formation, Formation, part part of the Pillowed Pembine Pembine ophiolite ophiolite complex, complex, Quiver Quiver Falls, Falls, Wisconsin Wisconsin �FIELD FIELDTRIP TRIP1I PEMBINE-WAUSAU MAGMATIC MAGMATIC TERRANE TERRANE PEMBINE-WAUSAU Klaus J. Schulz, Schulz, USGS, Reston, VA and Gene L. LaBerge, University University of WisconsinWisconsinOshkosh Oshkosh (retired), (retired),Oshkosh, Oshkosh,WI Wland andUSGS USGS The Paleoproterozoic Paleoproterozoicvolcanic volcanic and andassociated associatedintrusive intrusiverocks rocksexposed exposedininthe theeastern eastern The part of of Marinette MarinetteCounty County in in northeastern northeastern Wisconsin Wisconsin are are the the easternmost easternmost exposures exposures of of part the Pembine-Wausau Pembine-Wausauterrane, terrane, the northernmost northernmost of the two Wisconsin magmatic magmatic terranes the (Sims (Sims and others 1989). 1989). The volcanic rocks are composed composed of tholeiitic tholeiitic and calc-alkaline volcanic and and volcaniclastic volcaniclastic rocks rocks that formed at about 1870 1870 Ma Ma (Sims (Sims and and others, others, 1989). 1989). volcanic They They are are cut cut by by aa variety variety of intrusive intrusive rocks rocks ranging ranging from from syn-volcanic syn-volcanic gabbros, gabbros, diorites, diorites, and tonalities tonalities to to syn-and syn-andpost-tectonic post-tectonicgranitoids granitoids(i.e., (i.e., Dunbar DunbarGneiss Gneissand andrelated relatedrocks). rocks). and The The magmatic magmatic rocks rocks of of the the Pembine-Wausau Pembine-Wausauterrane terrane are are separated separated from from the the epicratonic epicratonic sedimentary sedimentary rocks rocks of of the the Marquette Marquette Range Range Supergroup Supergroup in in Michigan Michigan by by the Niagara Niagara fault fault zone. The The lithologic lithologic units units present present in in eastern eastern Marinette Marinette County County and and their chemistry chemistry zone. strongly strongly suggest suggest that that these these rocks rocks represent represent aa Paleoproterozoic Paleoproterozoicsuprasubduction suprasubduction zone zone ophiolite, the the Pembine Pembine ophiolite ophiolite (Schulz, 1987; 1987; Sims Sims and others, 1989). 1989). The ophiolite ophiolite, and and associated associated island-arc island-arcrocks rocks were accreted accreted to the southern southern margin margin of the the Archean Archean Superior Superior Craton Craton during during the the Penokean PenokeanOrogeny. Orogeny. On On this this field field trip trip we will examine examine the major major lithologies lithologies that comprise comprise the Pembine Pembine ophiolite. This includes examples of ultramafic rocks (serpentinite), layered ophiolite. This includes ultramafic rocks (serpentinite), layered and and massive massive gabbros cut cut by by mafic mafic dikes, dikes, pillow pillow basalts basalts and and andesites, andesites, and and several overlying calcgabbros alkaline alkaline arc-related arc-relatedvolcanic volcanic and andvolcaniclastic volcaniclasticrocks rocks(fig. (fig.1-1). 1-1). 34 �88 0845' 8T 5830" 88°08'45" 8758'30" 87 4815" 45 41 00 22 t I 0 0 ' 5 ' 2 I 0 0 55 6 6 4 'II 5 5 ' 8748'15" 88 , II 10 10 10 10 •II 15 15 12 Miles Miles 12 II Kilometers Kilometers EXPLANATION EXPLANATION Munising Sandstone Munising Sandstone(Cambrian) (Cambrian) Paleoproterozoic north of Paleoproterozoic rocks north o f Niagara fault Formation-graywacke Michigamme Formation graywacke Iron-formation Vulcan lron-formation Dolomite Randville Dolomite Paleoproterozoic rocks of Terranes Paleoproterozoic of Wisconsin Magmatic MagmaticTerranes Intrusive trusive rocks rocks Volcanic rocks . E Spikehorn Creek Spikehorn CreekGranite Granite and dacite dacite "Pemene formation" formation" . . ...., Rhyolite Rhyoliteand Granite Bush Lake Granite Basaltic and andandesitic andesitic breccia breccia"McAllister "McAllisterformation" formation Hoskuns Lake Lake Granite Granite Hoskins Rhyolute tuff graywacke Beecher formation" formation Rhyolite, felsuc felsic tuff,graywackel'Beecher Athelstane Athelstane Quartz Monzonite Monzonite Twelve Foot FallsQuartz Quartz Diorite Diorite Twelve Marinette Quartz Diorite Diorite Marinette Serpentinite of ophiolite ophiolite complex complex Serpentinite and and gabbro, base of NewinghamTonalite jJ NewinghamTonalite Quinnesec Formation Quinnesec Formation NewinghamTonalite, NewinghamTonalite, megacrystic facies megacrystic .. . Metagabbro Metagabbro Volcanic and rocks undivided undivided and granituc granitic rocks — faults - faults • field field trip trip stops stops Figure 1-1. Generalized geologic map of part of northeastern northeastern Wisconsin Wisconsin showing showing the the location of field trip stops stops 1-1 1-1 through through 11-8. See figure figure 3-2 3-2(p. (p. 67) 67) for for location locationof of stops stops1-9 1-9 location -8. See and 1-10. 1-10. 35 �____ _______ __ ____________ ____ _______ ° '6 / 1' + ET. -r1I k N 32 ' - ' aveH Pt -f •. - --- I && • ./ — c( - - . ./ ;. L 9Xt I - — - N ' . . . \ . / - — * . .. ) - • 2Y -- I . ( T. \ - — Approximatel 10 miles to Dunbar - 26O . . k. . ••• -' - n — . '- iaN . - . .— . - l . IZ I K" w 3 I) 5000 0 — ,_ — 0 10000 METERS I I 10000 20000 30000 — -= 400CR FEET Figure Figure1-2. 1-2.Part Partofofthe theEscanaba Escanaba1:100,000-scale 1:I 00,000-scaletopographic topographicmap mapshowing showingthe thelocation location of of field fieldtrip tripstops stops1-1 1-1through through1-7 1-7and andsupplemental supplementalstops stops1-8 1-8and and1-11. 1-11. Stop side U.S. 141, SW NE Stop1-1. 1-1.Spikehorn SpikehornCreek CreekGranite Granite(west (west side U.S.Highway Highway 141, SW¼,?h, NE1/4 '%sec. Isec. , 36, 36,T. T.38 38N., N.,R. R.20 20E.). E.). The Theoutcrop outcropon onthe thewest westside sideofofU.S. U.S.Highway Highway141 141(fig. (fig.1-2) 1-2)isisrepresentative representativeofofthe the Spikehorn SpikehornCreek CreekGranite, Granite,aagray graytotopinkish pinkishgray, gray,finefine-totomedium-grained medium-grainedmassive massive granite granitewith withscattered scatteredpotassium potassiumfeldspar feldspargrains grainsas asmuch muchas as22cm cminindiameter. diameter.The The granite graniteisiscomposed composedofofplagioclase plagioclase(sodic (sodicoligoclase) oligoclase)with withweak weaknormal normalzoning, zoning, 36 �microclinemicroperthite, microperthite,quartz, quartz,biotite, biotite,sphene, sphene,and andopaque opaqueoxides. oxides.Accessory Accessoryminerals minerals microcline include zircon, allanite, apatite (rare), and fluorite (rare). The granite has sharp intrusive include zircon, allanite, apatite (rare), and fluorite (rare). The granite has sharp intrusive contacts contactswith with the the Quinnesec Quinnesecvolcanic volcanicrocks rocksand andthe theMarinette MarinetteQuartz QuartzDiorite. Diorite. The The Spikehorn SpikehornCreek Creek Granite Granite isis aa post-tectonic post-tectonicdiapiric diapiric intrusion intrusion on on the the northeast northeast side side of of the the Dunbar Dunbar dome dome with with an an age age of of 1,835±6 1,835*6 Ma Ma (Sims (Sims and and others, others, 1992). 1992). ItIt is is similar similar in in composition LittleTobin Lake Lake Granite, Granite, which which intrudes intrudes the the Badwater Badwater composition and and age age to to the the LittleTobin Greenstone Greenstonenorth north of of the the Niagara NiagaraFault Faultzone zone (Schneider (Schneiderand and others, others, 2002; 2002; see see also also field field trip trip 3, 3, stop stop 3-9). 3-9). Both Both the the Spikehorn Spikehornand and Little LittleTobin Tobin Lake Lakegranites granites represent represent"stitching" "stitching" plutons, plutons, emplaced emplaced after after collision collision of of the the Pembine-Wausau Pembine-Wausaumagmatic magmaticterrane terranewith with the the passive passive margin margin of of the the Superior Superior Craton. Craton. As As such, such, the the age age of of these these granites granites provides providesaa minimum minimumage agefor for the thePenokean PenokeanOrogeny. Orogeny. Stop Stop 1-2. 1-2. Exposures Exposuresof of Serpentinite, Serpentinite,Gabbro. Gabbro,and and Mafic Mafic Dikes Dikes(i.e., 0.e.. Ophiolite) Ophiolite)East East of (NW1/4, %, NW ¼, sec. sec. 22, 22ÂT. T. 37 37N., N.?R. R. 21 21E.) Em) ofPembine Pembine(NW % Follow Follow the the red red flags flags and and trail trail north north to to outcrops outcrops of of serpentinite serpentinite on on the the south south side side of of the the North NorthBranch BranchPemebonwon PemebonwonRiver. River.The Theserpentinite serpentiniteininthe thesurrounding surroundingoutcrops outcropsshows shows variable variable features features including includinglayering layering(fig. (fig. 1-3), 1-3),brecciation, brecciation,and andcarbonate carbonateand andserpentine serpentine veining. veining. Chromite Chromiteisisclearly clearly evident evident in insome some samples. samples. Some Some serpentinite serpentinite is is highly highly magnetic magnetic whereas whereas other other samples samples are are not; not; this this may may reflect reflectoriginal original variations variations in in the the proportion proportionof of olivine olivineand andpyroxene pyroxenein in the the ultramafic ultramafic rocks. rocks.Locally, Locally,dikes dikes of of pyroxenite pyroxenite (now (now altered alteredto to talc-serpentine) talc-serpentine)have havebeen beenobserved observedcutting cuttingthe theserpentinite. serpentinite. Figure Figure1-3. 1-3.Photograph Photographof ofnearly nearlyvertical verticallayering layeringininultramafic ultramafic(serpentinite) (serpentinite)rocks rocksat at stop 1-2. stop1-2. 37 �As we walk south we will first first see further exposures of serpentinite followed after a few hundred hundred meters meters by by a series series of of outcrops outcrops with with variable variable proportions proportionsof of layered layeredgabbro, gabbro, foliated-lineated gabbro, massive diabase and quartz diabase (dikes?), all cut by foliated-lineated reddish-brown-weathering reddish-brown-weathering mafic dikes (figs. 1-4 1-4 and 1-5). The foliation in the foliated gabbro is variable in strike between N.15 W. to N.15 E. and dips steeply either E or W. The mafic dikes generally strike about E-W parallel parallel to the overall trend of the serpentinite-gabbro body and dip steeply. The dikes do not appear to extend outside the body into the surrounding pillow basalts. The serpentinite-gabbro body appears to be fault-bounded and and tectonically tectonically emplaced emplaced within within the the Quinnesec Quinnesec Formation. Formation. Figure Figure 1-4. Photograph Photograph of rusty weathering mafic dikes with gabbro screens at stop 1-2 1-2 (Note, dike dike margins margins are are highlighted). highlighted). The lithologies lithologies and their arrangement in this ultramafic-gabbroic ultramafic-gabbroic body along with those of the Quinnesec Quinnesec Formation Formation generally are similar to those that characterize recent ophiolite sequences (Moores, 2002). This includes (from bottom to top): mafic-ultramafic mafic-ultramafic plutonic rocks, rocks, aa dike dike (sheeted?) (sheeted?) complex, complex, extrusive extrusivepillowed pillowedand andmassive massivebasalt basaltlava lava flows, and overlying volcaniclastic sedimentary rocks. The compositions of the gabbros and mafic island-arc tholeiite and MORB to depleted island-arc mafic dikes within this body range from MORB high-magnesium high-magnesium andesite with boninitic affinities; the mafic rocks rocks are similar in composition to the gabbros and basalts basalts in the Quinnesec Formation Formation (Sims and others, 1989; Schulz, 1987 1987 and unpublished unpublished data). The The lithologic lithologic association association and and chemistry chemistry are are similar to recent suprasubduction zone ophiolites like those of the Coast Ranges in California California (Shervais and Kimbrough, 1985; Shervais, 2001). These data, along with the presence of overlying overlying calc-alkaline caic-alkaline island-arc island-arc volcanic and volcaniclastic rocks, suggest formation of the Quinnesec Quinnesec as as a suprasubduction suprasubduction zone ophiolite associated with forearc extension during the early stages of subduction subduction and island arc formation. 38 �Figure Figure 1-5. 1-5. Close-up Close-up photo photo of of rusty rusty weathering weathering mafic mafic dike dike cutting cuttingfoliated foliated gabbro gabbro at at stop stop 1-2. 1-2. Note Note cleavage cleavage development development along along the the upper upper margin margin of of dike dike (dike (dike contacts contactsare are hightlighted). hightlighted). Stopl-3. Stop1-3.Pillow Pillowbasalt basaltofofthe theQuinnesec QuinnesecFormation. Formation,Quiver QuiverFalls Fallson onthe theMenominee Menominee River sec. 24, T. 37 River(NE (NE1/4, %, SW SW 1/4, %, sec. 37 N., N., R. R.21 21E.) E.) - Basalt Basaltand andandesite andesitepillow pillow lavas lavasand and pillow pillow breccias brecciasare are common common in in the the Quinnesec Quinnesec Formation. three-dimensionalview viewofofpillows pillowscan canbe beseen seenininthis thisexposure exposurealong alongthe the Formation.AA three-dimensional banks banksof of the theMenominee MenomineeRiver Riverat at Quiver QuiverFalls Falls(fig.1-6). (fig.l-6). Here, Here,south-facing south-facingelongate elongate closely closely packed packedbasalt basaltpillows pillowscan can be be seen seen with with excellent excellent preservation preservation of of features features due due to to the thelow lowdegree degreeof ofdeformation. deformation.Locally, Locally,the thebasalt basalthere hereisisalso alsohighly highlyvariolitic variolitic(fig. (fig.1-7) 1-7) with with large large(1-2 (1-2cm) cm)round roundpinkish pinkishsiliceous-appearing siliceous-appearingvarioles. varioles. Chemically Chemicallythe the basalt basalt isis characterized (9.8 wt.%), wt.%), and andvery very low low Ti02 Ti02(0.35 (0.35 wt.%), wt.%), Zr Zr (35 (35 characterizedby byrelatively relativelyhigh highMgO MgO(9.8 ppm), ppm),and andREE REE(flat (flatpattern patternatat—6 -6 xx chondrites) chondrites) contents. contents. ItIt is is similar similar in in composition composition to to some someof of the themassive massivediabasic diabasicgabbro gabbroseen seenat atstop stop1-2. 1-2. 39 �Fig1-ire 1-6. Figure 1-6. Photograph Photograph of of pillow pillow basalt basalt in in the the Quinnesec Quinnesec Formation Formation at at Quiver Quiver Falls Falls on oi the the Menominee MenomineeRiver River(stop (stop1-3). 1-3). FigiJre 1-7. 1-7.Photograph Photographof of variolitic variolitic basalt basalt at at Quiver Quiver Falls Falls on on the the Menominee Menominee River River (stop (s1 Figure 1-3). 1-3)I. 40 �Stop 1-4. 1-4. Andesite Andesite Breccia Brecciaat atthe theNew NewKremlin KremlinMine MinePit Pit(5(S½, Vs,sec. sec.26, 26,T.T.37 37N., N.,R. R. Stop 21 21 E.) E.) The fresh fresh glacially glacially polished polished outcrops outcrops around around the the new new Kremlin Kremlin mine mine pit pit at at this this stop stop The provide excellent exposures of andesite breccia breccia in the upper upper part part of the Quinnesec Quinnesec provide Formation. The Kremlin Kremlin mine mine processes processes the Quinnesec rocks rocks for roofing roofing granules. granules. The The Formation. andesite breccia breccia consists consists of of angular angular to to sub-rounded sub-roundedvolcanic volcanic fragments fragments ranging rangingfrom from andesite about 55 cm cm to to at at least least 40 40 cm cm across across in in aa matrix matrixof of 0.5 0.5 to to 2.0 2.0 cm cmhyaloclastite hyaloclastitefragments. fragments. about The sub-rounded sub-roundedfragments fragments and and hyaloclastite hyaloclastitematrix matrix suggest suggest that that the the unit unit may mayrepresent represent The andlor subaqueous subaqueous debris debris flows. flows. Note Note the the black black pyroxene pyroxene phenocrysts phenocrysts in in pillow breccias brecciasand/or pillow some of of the the andesites andesites clasts. clasts. Locally Locallyto to the the west, west, similar similar rocks rocksare are interlayered interlayeredwith with some rhyolite rhyolite flows flows and and tuffs. tuffs. Stop1-5. 1-5.McAllister McAllister Formation Marek Road %,sec. sec.22, 22,T.T.36 36N., N.,R.R.21 21 Stop Formation onon Marek Road (NE(NE 1/4,%, NENE 1/4, E.) The McAllister McAllister formation formation consists consists of of caic-alkaline calc-alkaline basaltic basalticto to andesitic andesitic volcanic volcanic breccia breccia The with aa crystal crystal lithic lithic tuff tuff matrix, matrix, and and locally locally pillowed pillowedand and massive massive lavas. lavas. This outcrop outcrop is is with typical of of the the McAllister McAllister volcanic volcanic breccia. breccia. Fragments Fragments in in the the breccia brecciaare are distinctive distinctive typical because they they generally generally contain contain large large pyroxene pyroxene crystals crystals that that are are now now replaced replaced by by because hornblende. Amygdules Amygdules are are also also common common in in many many fragments. fragments. The The size size of of fragments fragments in in hornblende. the McAllister McAllister formation formation increases increases from from west west to to east. east. Near Near the the Menominee Menominee River, River, blocks blocks the over 15 15 cm cm in in diameter diameter are are common common suggesting suggestingthe the source source area areafor for this this dominantly dominantly over fragmental fragmental unit unit may may be be located locatedto to the the east east in in Michigan. Michigan. Stop onon Marek Road (NW 1/4,%,NW Stop1-6. 1-6.Beecher BeecherFormation Formation Marek Road (NW NW1/4, %, sec. sec. 26, 26,T. T. 36 36N., N.,R. R.21 21 E.) E.1 The Beecher Beecherformation formationconsists consistsof oftwo twounits: units:aathick thick(—2,000-3,000 (-2,000-3,000 m) m) lower lower unit unit The consisting consisting dominantly dominantly of of calc-alkaline calc-alkaline plagioclaseplagioclase- and and pyroxene-phyric pyroxene-phyric andesite andesite and and dacite lava lavaflows flows and andlesser lesserpyroclastic pyroclasticrocks, rocks,and andaathinner thinner(—300 (-300 m) upper upper unit, which which dacite consists consists of of interbedded interbedded felsic felsic ash, ash, tuff tuff and andcoarser coarser fragmental fragmental rocks. rocks.The Theexposures exposuresat at this lapillituff, tuff, and and this stop stop are are in in the the upper upper unit unit and and show show bedded bedded crystal crystal tuff tuff (fig. (fig. 1-8), 1-8),lapilli coarser coarser units, units, some some with with distinctive distinctive rounded rounded pink pink to to white white felsite felsite fragments fragments (fig. (fig. 1-9). 1-9). Some Some units units are are graded graded whereas whereas others others are are unsorted. unsorted. Graded Graded bedding bedding in in some some layers layers indicates tops tops to to the the northeast. northeast. Jenkins Jenkins(1973) (1973) suggested suggestedthat that at at least leastsome someof of the the rocks rocks indicates of of the the Beecher Beecherformation formationwere weredeposited depositedsubaerially. subaerially. 41 �4L - -. — —. - 4 —? S — -;.. —-— :;-— :-— :t_ —. ;:;b -- - ,- —- ,- -. __4 --- . __( . : Figure 1-8. 1-8. Photograph Photograph of bedded bedded crystal crystal tuff in in the the upper upper unit unit of of the the Beecher Beecherformation, formation, Figure stop stop1-6. 1-6. Figure 1-9. 1-9.Photograph Photographof of aa distinctive distinctive fragmental fragmental unit unitwith with rounded roundedpink pinkto to white white felsite felsite Figure 1-6. fragmentsininthe theupper upperpart partof ofthe theBeecher Beecherformation, formation,stop stop1-6. fragments 42 42 �Stop 1-8. 1-8. Pemene Pemene Formation Formationat at Pemene Pemene Falls Fallson onthe theMenominee MenomineeRiver River(NW (NW corner, corner, Stop Sec. sec. 23, T. 37 37 N., N., R. R. 25 25 W.) W.) Exposed along along the bank bank of the the Menominee Menominee River River at Pemene Pemene Falls Falls is is rhyolite rhyolite of the Exposed Pemene Formation. Formation. The The rocks rocks are are dark dark gray gray to to reddish reddish gray, gray, contain contain few few feldspar feldspar Pemene phenocrysts, and are generally microspherulitic. Phenocrysts, many of which are phenocrysts, and are generally microspherulitic. Phenocrysts, many glomeroporphyritic, consist consist mainly mainly of euhedral euhedral to subhedral subhedral albite; however, phenocrysts phenocrysts glomeroporphyritic, blue quartz quartz are are present present locally. locally. The The microspherules microspherules consist consist of radial radial intergrowths intergrowths of of blue quartz quartz and and albite. This phase phase of the the Pemene Pemene probably probably represents represents the devitrified devitrified interior interior portion portion of of aa rhyolite rhyoliteflow. flow. In In areas areas to to the the west, Pemene Pemene rhyolite rhyolite lava lava flows show internal internal gradations gradations from massive massive microspherultic rhyolite rhyolite at their their centers centers to flow-banded flow-banded rhyolite rhyolite and and finally autobreccia autobreccia microspherultic and hyaloclastite hyaloclastite carapaces. carapaces. This suggests suggests the Pemene Pemene rhyolite rhyolite flows were deposited deposited and subaqueously. Locally, Locally, felsic dikes dikes are found found cutting cutting the rhyolite. rhyolite. subaqueously. SupplementalStop Stop 1-8. 1-8. Dunbar Gneiss Dunbar, Wisconsin Supplemental Gneiss West and North of Dunbar, Wisconsin Stop 1-8a. 1-8a. Dunbar Gneiss on the West Side of of County CountyRoad Road U U Stop of the the Intersection Intersection of Y4, SW SW ¼, Y4, sec. sec.26, 26,T. T.37 37N., N.,R. R.18 18E.). E.). withU.S. U.S.Highway Highway8 8(SW (SW1/4, with The low low outcrops outcrops here here are are composed composedmainly mainly of of megacrystic megacrysticgranite granite gneiss gneiss that that contains contains The rafts of layered layered amphibolite amphibolite (fig. (fig. 1-10). 1-10). This This lithology lithology is is similar similar to to that that dated dated from from an an rafts outcrop to to the the north northwith with aa U-Pb U-Pbzircon zircon concordia concordia upper upper intercept intercept age age of of 1,862±5 1,862~5 Ma outcrop Ma N. (Sims and and others, others, 1992). 1992).Lineation Lineationin inthe theamphibolite amphiboliteplunges plungesgenerally generallyabout about20-25° 20-25' N. (Sims 85-90' E. E.Locally, Locally, the the amphibolite amphiboliteisisrefolded refoldedby byfolds foldshaving havingN. N.50° 50' W. W.steep steepaxial axial 85-90° N. 70° 70' W. foliation. foliation. The The granite granitegneiss gneiss surfaces. The The granite granite gneiss gneiss has pervasiveN. hasaa pervasive surfaces. composition, and is interpreted plutonic protolith protolith (Dunbar Gneiss) tonalitic in Gneiss) is is tonalitic in composition, and is interpreted as a plutonic (Dunbar (Sims (Sims and and others, others, 1992). 1992). Stop1-8b. 1-8b.Migmatitic MigmatiticDunbar DunbarGneiss Gneiss (Center sec.15, 15,T.T.3737N., N.,R.R.18 18E.). E.). Stop (Center sec. The exposures exposures on on the the east east side side of of the the road roadare are of of migmatitic migmatiticDunbar DunbarGneiss. Gneiss.The Thegneiss gneiss The here here consists consistsmainly mainly of of compositionally compositionally layered layered rocks, rocks, biotite biotite gneiss, gneiss, and and lesser lesser amphibolite, intruded intrudedby by megacrystic megacrystic biotite biotitegneiss, gneiss, granite granitepegmatite, pegmatite, and and aplite. aplite. All All amphibolite, N. 50-55° 50-55' W. at at 90°. 90'. The The foliation foliation is is defined defined by by rocks are are deformed deformed with with foliation foliation striking striking N. rocks biotiteand andhornblende hornblendealignment alignmentand andisisgenerally generallyparallel paralleltotocompositional compositionallayering. layering. biotite 43 �_ — ..— a.— * it,r * -- qr * - ..._ - ..- ' -. ,—. - •* - - I . -. — .4 T Figure 1-10. 1-10. Photograph Photograph of an an outcrop outcrop of of megacrystic megacrystic Dunbar Dunbar Gneiss Gneiss with with folded folded Figure amphibolite amphiboliterafts raftsat atstop stop1-8a. 1-8a. SupplementalStop Stop1-9. 1-9. Sulfide Sulfide deposit depositat at"LaSalle "LaSalleFalls" Falls"on onthe thePine PineRiver. River.(NW (NW Supplemental %, SE 1/4, SE%, ¼,sec. sec.30, 30,T.39 T.39N., N.,R.18 R.18E.E.See Seefigure figure3-1 3-1and and3-2 3-2for for location.) location.) The Pembine-Wausau Pembine-Wausauterrane terraneisishost hostto to aanumber numberof of volcanogenic volcanogenicmassive massivesulfide sulfide The deposits, deposits, two two of of which whichare areknown knownin innortheastern northeasternWisconsin Wisconsin(Cummings, (Cummings,1978; 1978;LaBerge, LaBerge, 1983). 1983). The The deposit deposit on on the the Pine PineRiver Riverisisthe theonly onlyknown knownnaturally naturallyexposed exposedmassive massive sulfide sulfidedeposit depositin inWisconsin. Wisconsin. LaSalle Falls) Falls) on on the the Pine Pine River River occurs occurs in in The deposit deposit at at Pine Pine Rapids Rapids(locally (locally known known as as LaSalle The 3-2). the Quinnesec Quinnesec Formation Formationabout about one one mile mile south south of of the the Niagara NiagaraFault Faultzone zone (fig. (fig. 3-2). the LaSalleFalls Fallsis is formed formed where where the the Pine Pine River River flows flows over over aa resistant resistant unit unit of of rhyolite rhyolite LaSalle breccia brecciaonto onto an an easily easily eroded eroded unit unit of of sulfide-bearing sulfide-bearing schist schist that that occurs occurs between betweenthe the rhyolite rhyolite and and aa unit unit of of mafic maficvolcanic volcanic rocks. rocks.Removal Removalof of the the easily easily eroded eroded schist schist has has formedaa narrow narrowgorge gorge on on the the Pine PineRiver Riverfor for several severalhundred hundredfeet feet below belowthe the falls. falls. The The formed withaaprominent prominentlineation lineationthat thatplunges plunges rocksstrike strikenearly nearlyeast-west east-westand anddip dip60-70° 60-70' SSwith rocks 50-60°, 50-60, SS40° 40' W. W. The deposit deposit was was discovered discovered during during an an airborne airborne geophysical geophysical survey survey in in the the 1970's, 1970's, and and The has has been beendrilled. drilled. The Themain mainpart partof ofthe thegeophysical geophysicalanomaly anomalyextends extendsdownstream downstreamwithin within theriver riverchannel channelbetween betweenexposures exposuresof of rhyolite rhyoliteon onthe the north northand andbasaltic basalticrocks rockson onthe the the south. Drill Drillcores cores show show that that the the rhyolite rhyolite consists consists of of coarse coarse fragments fragmentsin inaa relatively relatively south. sulfide-richmatrix, matrix, aa typical typical "stringer "stringer ore". ore". Rhyolite Rhyoliteexposed exposed at at the the falls falls is is very very pitted, pitted, sulfide-rich due due to to breakdown breakdownof of the the sulfide-rich sulfide-rich(pyrrhotite-chalcopyrite) (pyrrhotite-chalcopyrite)matrix matrix material. material. Exposures Exposures 44 �immediately below below the falls, at the stratigraphic stratigraphic level level of the the main main EM EM anomaly, consist consist of immediately approximately approximately18 18m m of of sulfide-bearing sulfide-bearingschist schist and and cherty cherty units. units. The Thesedimentary sedimentaryunit unit isis mainly laminated laminated chloritic chloritic and and sericitic sericitic schist with pyritic lenses lenses up to 3 mm mm thick. mainly Garnets Garnets are are common commonin insome some layers. layers. Sprays Sprays of of black black tourmaline tourmaline are are present present in in the the mafic mafic volcanic volcanic rocks rocks on on the the south south side side of of the the river. river. The The structure structure in inthe the area areaisissomewhat somewhat puzzling. puzzling.Based Basedon on regional regional geology, geology, Dutton Dutton (1971) Bayley and and others others (1966) (1966) concluded concluded that that the the Quinnesec Quinnesec Formation Formationfaces faces (1971) and and Bayley north northnear near the the Niagara NiagaraFault. Fault. Sims Simsand andothers others(1984) (1984) also also reported reportedthat that the the Quinnesec Quinnesec faces DunbarDome, Dome, which which lies lies south south of of "LaSalle "LaSalleFalls". Falls". In In faces northward northwardaway away from from the the Dunbar addition, although although deformation deformationhas hasobscured obscuredfacing facingdirection directionindicators, indicators,such suchas as pillows, pillows, addition, %, NE NE ¼, %, sec. sec. 26, 26, T. T. 34 34N., N., A. R. in most mostareas, areas,north-facing north-facingpillows pillowsare areexposed exposedininthe theSW SW1%, in 17 E., about abouttwo two miles mileswest westof of"LaSalle "LaSalleFalls". Falls".However, However,the thelithologic lithologicsequence sequenceand and 17E., pattern patternof of mineralization mineralizationat at "LaSalle "LaSalleFalls" Falls"suggests suggests that that the the rocks rockshosting hostingthe the mineralization mineralization face face southward. southward. The The zone zone of of "stringer "stringerore" ore" isisnorth northof of the the main mainsulfide sulfide zone, zone, and and the the sequence sequence is is "overlain"(?) "overlain1'(?)by by mafic mafic volcanic volcanic rocks rocksto to the the south. south. SupplementalStop Stop1-10. 1-10.Pine PineRiver RiverPegmatite Peqmatite bodies. (NW NE V*.sec. sec.22, 22, TT .39 .39 Supplemental bodies. (NW ¼,%,NE 1/4, N., figures 3-1 and 3-2 for for location.) location.) N., R.17 R.17 E.) See figures - (WARNING: (WARNING: - Because Because this thisexposure exposureisisnear nearthe thePine PineRiver RiverWILD WILDRIVERS RIVERSAREA. AREA, no no collecting collectinuis ispermitted permittedat at this this locality.) locality.) Dutton Dutton (1971) (1971) reported reportedthe the occurrence occurrenceof of pink pinktourmaline tourmalinein inaa pegmatite pegmatitedike dikefrom fromthis this area. area. The Thepegmatites pegmatitesare arelocated locatedapproximately approximately150 150feet feetwest westof ofthe thePine PineRiver Riverand and approximately approximately300 300feet feetsouth southof of Highway Highway101 101in inFlorence FlorenceCounty County(fig. (fig.3-1). 3-1).The Thelocation location is is about about aa mile milesouth southof of the theNiagara NiagaraFault. Fault. The Thepegmatite pegmatitebodies bodiesare areup upto toaafew fewmeters meterswide wideand andcut cutfelsic felsicvolcanic volcanicrocks rocksof ofthe the Quinnesec QuinnesecFormation. Formation.The Thepegmatites pegmatitesare aresub-parallel sub-parallelto tothe thefoliation foliationininthe thevolcanic volcanic rocks, rocks,strike strikenearly nearlynorth-south, north-south,and anddip dipabout about50 50degrees degreeswest. west. AA number numberof of small small lithium-rich lithium-richpegmatite pegmatitebodies bodiesininthe thearea areacontain containspodumene, spodumene,lepidolite, lepidolite,and andelbaite elbaite tourmaline tourmalineas aswell wellas asquartz, quartz,albite albiteand andmicrocline. microcline.Some Somepegmatites pegmatitescontain containabundant abundant pink tourmaline crystals 0.5 1.0 cm wide and 2.5 5.0 cm long, commonly oriented pink tourmaline crystals 0.5 - 1.0 cm wide and 2.5 - 5.0 cm long, commonly oriented roughly roughlyperpendicular perpendicularto to the theupper uppercontact contactof of the thepegmatite. pegmatite.Some Sometourmalines tourmalinesare arecolor color zoned, zoned, with withaapink pinkcore coreand andblue-green blue-greenrind. rind.The Thepegmatites pegmatitesare arecomposed composeddominantly dominantly of of aplitic apliticquartz-feldspar quartz-feldsparwith withsome somelepidolite. lepidolite.The Thepegmatites pegmatitesrepresent representhighly highlyevolved evolved granitic graniticmelts meltsrelated relatedto tothe thenearby nearbyBush BushLake LakeGranite Graniteassociated associatedwith withthe theDunbar Dunbar gneiss-granitoid gneiss-granitoiddome dome(Sims (Simsand andothers, others,1992). 1992). Supplemental SupplementalStop Stop1-11. 1-11. Metasedimentary MetasedimentarvRocks Rockson on the the Northwest NorthwestSide Sideof ofthe the Dunbar DunbarDome Dome(SW (SW1/4 %, SE SE¼, VA, sec. sec.7,7,T.T.37 37N., N.,R.R.18 18E.) E.) Metasedimentary Metasedimentaryrocks rocksare areexposed exposedintermittently intermittentlyon onthe thewest westside sideof of the theDunbar Dunbardome dome and andadjacent adjacentto toand andnorthwest northwestof ofthe theBush BushLake Lakelobe lobe(Sims (Simsand andSchulz, Schulz,1993). 1993).Although Although included includedby byDutton Dutton(1971) (1971)ininthe theQuinnesec QuinnesecFormation, Formation,subsequent subsequentmapping, mapping, geophysical geophysicaldata, data, and andcore coredrilling drillingby byaaprivate privatecompany companyshow showthat that these these strata strataunderlie underlie the theQuinnesec. Quinnesec.The Theexposed exposedmetasedimentary metasedimentaryrocks rocksare aremetamorphosed metamorphosedatatamphibolite amphibolite grade gradeand andare aremainly mainlyquartz-rich quartz-richschist, schist,impure impuremarble, marble,calc-silicate calc-silicaterocks, rocks,and andbiotite biotite schist. schist.Drilling Drillingand andelectromagnetic electromagneticdata dataindicate indicatethat thataagraphitic graphiticschist schistlies liesalong alongthe the 45 �west west side side of of the the Bush Bush Lake Lake lobe lobe stratigraphically stratigraphically below below the the exposed exposed metasedimentary metasedimentary rocks. rocks. The The large large exposure exposure at at this this stop stop on on the the south south side side of of Macintire MacintireCreek Creek consists consistsof of interbedded interbeddedcaic-silicate calc-silicaterocks rocks and and biotite biotite schist schist that that are are cut cut by by granite granite pegmatite pegmatite and and apilite apilite dikes, dikes, identical identical to to those those exposed exposed within within the the western western part part of the the Dunbar Dunbar dome. dome. At At another SW %, A, sec. VA, SW sec. 11, 11, T. T. 38 38 N., N., R. R.17 17E.), E.), aa another outcrop outcrop area areato tothe thenorth north(NW (NW1/4, succession 100m mthick thick of of marble, marble,calc-silicate calc-silicaterocks, rocks,and andthin thininterbeds interbedsofofbiotite biotite succession at at least least 100 schist schist and and ferruginous ferruginous quartzite quartzite is is exposed. exposed. The The marble marble at at this this location locationhas has structures structures suggestive suggestive of of stromatolites. stromatolites. In In many many respects respects the the metasedimentary metasedimentaryrocks rocks exposed exposed on on the the west west side side of of the the Dunbar Dunbar dome dome resemble resemble those those of of the the Chocalay ChocalayGroup Group of of the the Marquette MarquetteRange Range Supergroup, Supergroup,as as exposed exposed in in the the Menominee Menominee iron iron range range to to the the north north (Bayley (Bayley and and others, others, 1966). 1966). These These metasedimentary metasedimentaryrocks rocks may may compose compose aa tectonic tectonic slice slice of of continental-margin continental-margin rocks rocks that that is is interleaved Dunbar dome dome and and volcanic volcanic rocks rocks of the Quinnesec interleaved with with granitoid granitoid rocks rocks of of the the Dunbar Formation. Formation. Alternatively, Alternatively, they they may may have have been been uplifted uplifted from from beneath beneath the the over-thrust over-thrust Quinnesec Formation during the formation of the Dunbar dome. Quinnesec Formation during the formation of the Dunbar dome. 46 �FIELD FIELD TRIP TRIP 22 MENOMINEE MENOMINEE IRON IRON DISTRICT DISTRICT Gene Gene L. L. LaBerge, LaBerge, University Universityof of Wisconsin-Oshkosh Wisconsin-Oshkosh(retired) (retired)and andUSGS; USGS;John JohnS. S. Kiasner, Klasner, Western WesternIllinois IllinoisUniversity University(retired) (retired)and andUSGS; USGS;William WilliamF. F.Cannon, Cannon, USGS; gas, University USGS; Richard Richard W. W.Ojakan Ojakangas, University of Minnesota Minnesota Duluth Duluth (retired) (retired) The Quinnesec Quinnesec Mine Mine near near Quinnesec, Quinnesec, Michigan Michigan produced produced about 500,000 tons of siliceous siliceous iron iron ore between between 1887 1887 and 1935 1935 from open pits pits and stopes in the Vulcan IronIronformation. The Theworkings workingsseen seenhere hereare areon onthe the overturned overturnednorthern northern limb limb of a syncline. The The locality locality is is also also noted notedfor for the the excellent excellentexposures exposuresof of the thebasal basalCambrian Cambrian unconformity. The The roof roof of of the the working working in in the the upper upper right right is is the base base of the Munising Sandstone, Sandstone, which which lies lies with with an an angular angular unconformity unconformityon onthe theoverturned overturnedVulcan VulcanIronIronPhotographby byElizabeth ElizabethHeinen. Heinen. formation. Photograph �FIELD TRIP TRIP 22 FIELD MENOMINEE MENOMINEE IRON IRONDISTRICT DISTRICT Gene L. LaBerge, University University of Wisconsin-Oshkosh Wisconsin-Oshkosh(retired) (retired)and and USGS; USGS;John John S. S. Gene F. Cannon, Cannon,USGS; USGS; Klasner, Western WesternIllinois IllinoisUniversity University(retired) (retired)and and USGS; USGS;William WilliamF. Kiasner, Richard W. Ojakangas, Ojakangas, University University of of Minnesota MinnesotaDuluth Duluth(retired) (retired) Richard W. 88" 730 87 5700 87 46 30" 11 Fern Creek Fe, 'reek Fm 88 730 22 0 I 55 22 44 I 00 66 88 12 12 10 10 I I 55 I 10 10 15 15 Miles Miles Kilometers Kilometers EXPLANATION EXPLANATION Cambrian Cambrian MunisingSandstone Sandstone Munising North Northof ofNiagara Niagarafault fault Paleoproterozoic Paleoproterozoic . South South of of Niagara Niagara fault fault Paleoproterozoic Paleoproterozoic Metadiabase Metadiabase Hoskins HoskinsLake LakeGranite Granite MichigammeFormation Formation.graywacke Michigamme graywacke Marinette MarinetteQuartz QuartzDionte Diorite Badwater Badwater Greenstone Greenstone Metagabbro Metagabbro Vulcan Vulcan Iron-formation Iron-formation Quinnesec QuinnesecFormation Formation Randville Randville Dolomite Dolomite llh1llJ Sturgeon Sturgeon Quartzite Quartzite Fern Fern Creek Formation Formation - — fault fault Archean Archean Granitic Graniticrocks rocksand andgneiss gneiss Carney CarneyLake LakeGneiss Gneiss Figure 2-1. Geologic Geologic map map of of part part of of the the Menominee MenomineeIron-district Iron-districtshowing showingthe thelocation locationof of Figure field trip trip stops. stops. Geology Geologysimplified simplifiedfrom fromBayley Bayleyand andothers others(1966) (1966)and andSims Simsand andSchulz Schulz field (1993). (1993). 48 �0 5000 I I 1 217 10000 t IUt Kmbeyç 5000 I 1 / 20000 — - M ion 2-4rway I I 30000 I / I 10000 T I 0 I 40000 ciP etQp 'L 15 000 I Stfl —=-- NNii 5J L- r I n ci' ( I I I ,— I 70000 FEET 20 000 METERS 288k _— _- _—''2 g3j I ' c—_? — Figure 2-2. Part of the Escanaba 1:100,000-scale topographic map showing the location of field trip stops 2-1 through 2-5. See figure 3-2 for the location of stop 2-6. 0 —— 1000 700 57 - J" �This This trip trip examines examines rocks rocks of of the the Menominee Menomineeiron-bearing iron-bearingdistrict, district, with with emphasis emphasis on on units units of the the Paleoproterozoic PaleoproterozoicMarquette Marquette Range Range Supergroup. Supergroup.Archean Archean rocks rocks of of the the Carney Carney of Lake Lake Gneiss Gneiss are are seen seen at at two two stops stops and and volcanic volcanic rocks rocks of of the the Wisconsin Wisconsinmagmatic magmatic terranes, intensely intensely deformed deformed in in the the Niagara Niagara fault fault zone, zone, are are also also seen. seen. The The geologic geologic map map terranes, of the the field field trip trip area area isis shown shown in in figure figure 2-1 2-1and and aa road roadmap mapof of the the field fieldtrip tripstops stopsisisinin of figure figure2-2. 2-2. Stop River. (SE 1/4, NE 1/4, Stop2-1. 2-1. Piers PiersGorge Gorgeon onthe theMenominee Menominee River. (SE 114, NE %, sec. sec.24, 24,1. T.39 39N., N.,R. R. 30 30W.) W.) Rocks Rocksexposed exposedalong alongthe the Menominee MenomineeRiver Riverat atPiers PiersGorge Gorgeare arealmost almostcertainly certainlyaabranch branch of of the theNiagara Niagarafault faultzone zoneand andrepresent representone oneof of the thefew fewexposures exposuresof ofthe thefault faultzone. zone.This This location location is is about about one one kilometer kilometer south south of of the the mapped mappedtrace trace of of the the Niagara Niagarafault. fault. The Thehill hill lying lying north north of of the the gorge, gorge, but but still still south south of of the the mapped mappedfault, fault, isisunderlain underlainby bymetagabbro metagabbro that that isismuch muchless lessdeformed deformedthan thanthe therocks rocksininthe thegorge. gorge.These Theserelationships relationshipsindicate indicatethat that strain strain along alongthe the fault fault was was distributed distributedvery very heterogeneously heterogeneouslyand andconcentrated concentratedin indiscrete discrete zones zones of of very very high highstrain strainsurrounding surroundingislands islandsof of weakly weakly deformed deformedrocks. rocks.The Therocks rocksininthe the gorge gorgeare arehighly highlyfoliated foliatedand andlineated lineatedquartz-sericite quartz-sericiteschists schistsand andchloritic chloriticschists, schists, probably probablydeveloped developedfrom from felsic felsic and andmafic maficvolcanic volcanicrocks. rocks.Felsic Felsicand andmafic maficvolcanic volcanicrocks rocks with with only only weak weak foliation, foliation, along along with with mafic maficsills sills with with little little internal internaldeformation, deformation, are are exposed exposed on onboth bothsides sides of of this this strongly stronglyfoliated foliatedzone. zone. Metagraywacke Metagraywackeof of the the Marquette MarquetteRange Range Supergroup Supergroupisisexposed exposedin inNorway, Norway,about about 22miles milesnorth northofofthis thislocality, locality,and andvolcanic volcanicand and plutonic plutonicrocks rocksof of the the Wisconsin Wisconsinmagmatic magmaticterranes terranesare areexposed exposedalong alongthe theMenominee Menominee River Riverininthis thisarea. area. The 80'-85' W W and anddips dips 800850 80'-85' N. N. and and has has aa stretch stretch lineation lineation Thefoliation foliationhere herestrikes strikesNN800850 that plunges 600650, N 85° W. that plunges 60'-65', N 85' W. As As the the recognized recognizedboundary boundarybetween between the the dominantly dominantly sedimentary sedimentary rocks rocks of the the Marquette Marquette Range RangeSupergroup Supergroupto tothe thenorth northand andthe theWisconsin Wisconsinmagmatic magmaticterranes terranesto tothe thesouth, south,the the Niagara Niagarafault faultzone zoneisiscommonly commonlyreferred referredto toas asaasuture. suture.However, However,ititlacks lackssome somefeatures features (such (suchas asaamélange) melange)that thatare aretypical typicalof of suture suturezones. zones.Geophysical Geophysicalevidence evidence(Attoh (Attohand and Klasner, LaBergeand andKlasner, Klasner,2001) 2001) suggests suggeststhat thatthinned thinnedcontinental continentalcrust crust Klasner,1989; 1989;and andLaBerge of of the theSuperior Superiorcraton cratonhas hasbeen beenoverridden overriddenby bythe theWisconsin Wisconsinmagmatic magmaticterranes, terranes,and and extends extendsin inthe the subsurface subsurfacefor for 10-50 10-50miles milessouth southof of the theNiagara Niagarafault faultzone. zone.IfIfthis thisisisthe the case, case, the the Niagara Niagarafault faultzone zonemay maybe bethe the frontal frontalthrust thruston onwhich whichoceanic oceanicrocks rocksof of the the Wisconsin Wisconsinmagmatic magmaticterranes terranesoverrode overrodethe thecontinent continentmargin marginassemblage assemblageof ofthe the Marquette MarquetteRange RangeSupergroup. Supergroup.Continued Continuedcompression compressionof of the thesuture suturezone zoneresulted resultedin inthe the steepening of the thrust surfaces into their present, nearly vertical orientation. steepening of the thrust surfaces into their present, nearly vertical orientation. Stop Stop2: 2: Fern FernCreek Creeklocality locality—-Archean Archean basement. basement.Fern FernCreek Creek Formation, Formation,and and Sturgeon SturqeonQuartzite. Quartzite.(N(N½, Vi,sec. sec.34, 34,T.T.40 40N., N.,R. R.29 29W.). W.). Take TakeCounty CountyRoad Road573 573to tothe thenortheast northeastoff offofofU.S. U.S.2,2,about about11mile milenorthwest northwestofofthe thetown town of ofNorway. Proceedabout about22miles milesand andturn justbeyond beyondthe thebridge overPine Pine Norway.Proceed turnnorth north(left) (left)just bridgeover Creek, milesto toFern FernCreek, Creek,which whichcrosses crosses Creek, onto ontoaasecondary secondaryroad. road.Proceed Proceedabout about1½ 1Vz miles the about 1% thesecondary secondaryroad roadatataasharp sharpbend bendininthe theroad. road.Proceed Proceed about 34 mile milefarther farthertotoaasmall small intermittent intermittentcreek creekthat thatalso alsocrosses crossesthe theroad. road.Park Parknear nearhere. here.See Seefigure figure2-3 2-3for foraa detailed 2-4for foraageneralized generalizedrock rockcolumn. column. detailedmap, map,and andFigure Figure2-4 50 �r OuTCffOP M A P Figure2-3. Location Location map map of of stop stop 2-2. 2-2. (from (fromPettijohn, Pettijohn,1943.) 1943.) Figure2-3. FERN FERN CREEK FM FM M (FERN LOCALITY) (FERN CREEK LOCALITY) 150 100 50 -°.o: • DIAMICTITE DIAMICTITE ARGILLITE ARGILLITE DROPSTONES DROPSTONES 0 CONGLOMERATE CONGLOMERATE Figure2-4. 2-4. Generalized Generalizedstratigraphic stratigraphiccolumn columnat at Fern FernCreek Creeklocality. locality.SQ SQat atthe thetop topof ofthe the Figure column column designates designates the the Sturgeon SturgeonQuartzite. Quartzite. 51 �situated in a NW-trending NW-trendingvalley about 175 175 m m wide, between between two Note that the road is situated prominent topographic highs. The Carney Lake Lake Gneiss Gneiss forms the prominent prominent bluff bluff and and prominent Sturgeon Quartzite Quartzite forms aa prominent prominent upland on the northeast side of the road, and the Sturgeon Fern Creek ridge on the southwest side of the road. The valley is situated on the Fern Formation, Formation, of which the the lowest lowest 60 60 to to 70 70 m mare are somewhat somewhatexposed. exposed. Archean-Paleoproterozoic contact, a major unconformity unconformity representing representing a few The Archean-Paleoproterozoic hundred million years of erosion, is now subvertical, as are the Fern Creek Formation Formation substop is is at this this unconformity, unconformity, and and then we will and the Sturgeon Quartzite. Our first substop move up-section up-section through the subunits of the Fern Fern Creek Formation Formation and end in the Sturgeon Sturgeon Quartzite. Quartzite. Substop subs to^ 1. From the road, move about 100 100 m up the small creek bed bed to the subvertical unconformity, unconformity, where we can can observe observe both both the the Carney CarneyLake LakeGneiss Gneissand andthe theoverlying overlying basal gneiss-fragment (i.e., arkosic) conglomerate conglomerate of the Fern Fern Creek Creek Formation. Formation. Note Note basal the angular to subangular nature nature of the clasts, obviously locally derived. Minor beds of red arkosic sandstone are also present. The entire entire sequence has stratigraphic tops toward the south. Interpretation: High-velocity High-velocityfluvial fluvialand andin-situ in-siturubble/debris rubbleldebrisflow flow deposits. deposits. Substop 2. Reverse Reverse direction and head back toward the road, moving up the stratigraphic section. This poorly exposed subunit, 60-75 feet thick, is a laminated laminated finescattered larger stones. Many of these stones show clear evidence grained argillite with scattered fine-grained sediment from above, causing a bowing of having been "dropped" into the fine-grained penetration of the underlying laminae. Others do not show these downward and/or a penetration relationships relationships and are therefore therefore called called "lonestones" "lonestones"rather rather than than "dropstones" "dropstones"(fig. (fig.2-6). 2-6). Note the E-W E-W slatey slatey cleavage cleavage that that crosses crossesthe thebedding, bedding,which whichstrikes strikesabout aboutNN500 50' W. W. S that the stone has Figure 2-5. Thin-bedded argillite and siltstone with dropstone. Note that both both pierced pierced and and bowed bowed down down the the underlying underlying laminae. laminae. 52 �Figure Figure 2-6. 2-6. Large Large (35 (35 cm) cm) lonestone lonestone (beneath (beneath hammer) hammer) in in vertical vertical laminated laminated siltstone/sandstone siltstone/sandstone beds. beds. Some Some smaller smaller lonestones lonestones are are also also present. present. Interpretation: Deposition Depositionof of fine-grained fine-grainedsediment sedimentin in aa body body of of water water (marine?) (marine?) near near a melting melting glacier, glacier, with with the the larger larger clasts clasts dropped dropped in in from from icebergs icebergs and/or and/or aa floating floating ice ice shelf. shelf. Substop Substop 3. The next next subunit subunit is is a diamictite, diamictite, aa matrix-supported matrix-supported conglomerate conglomerate with with rather rather sparse clasts clasts set in in a massive massive graywacke graywacke matrix matrix (fig. (fig. 2-7). 2-7). Note Note the the total total lack lack of of bedding bedding sparse and and the presence presence of aa crude crude schistosity schistosity that causes causes the the rock rock to to break break into into thick thick slabs. slabs. Interpretation: (i.e.tillite) tillite)or orby bythe the"raining "rainingout" out" Interpretation: Deposition Depositiondirectly directlyby byglacial glacialice ice(i.e. of detritus detritus from from icebergs icebergsor or aa floating floating glacier glacier onto onto aa basin basin floor floor lacking lackingcurrents currentsto to generate (i.e., "rainout "rainouttill"). till"). generate lamination lamination (i.e., Substop Cross the road, road, following following the marked marked trail trail about about 100 100m m across across aa low-lying low-lying Substop 4. Cross area area without outcrops, base of the the ridge ridge that that lies southwest of the road. The first outcrops, to the the base lies southwest of the road. The first low-lying low-lying exposure at the base base of the ridge ridge is is poorly poorly exposed exposed and and is is composed composed of argillite/sericite m thick. thick. The The low-lying low-lyingarea areamay maybe be argillite/sericiteschist schist and and sericitic sericiticquartzite, quartzite, about about 55 m totally totally or in part underlain by by this rock, which is softer softer than than the rocksof substops or in part underlain this sericitic sericitic rock, which is the rocks of substops 1-3 1-3 and and the overlying overlying vitreous quartzite. The The prominent prominent ridge ridge is is composed composed of the the Sturgeon Sturgeon Quartzite Quartzite (fig. (fig. 2-8). 2-8). ItIt is is well well cemented cemented with with silica silica and and is is totally totally recrystallized, recrystallized, making making itit aa very resistant resistant rock rock unit. unit. ItIt is is composed composed almost almost totally totally of of well-sorted, well-sorted, finefine- to to 53 �medium-grained quartz sand; few grains are coarser than 1 mm. Original Original grain boundaries are easily seen boundaries seen only only in in the the sericitic sericitic quartzite. quartzite. Diamictite with scattered scattered stones stones and and crude crude schistosity. schistosity. Figure 2-7. Diamictite Interpretation: The The sericite sericite schist schist is is a paleosol and the sericitic quartzite is aa reworked paleosol that that was developed upon the the Fern Creek Creek Formation during during aa long period of extensive weathering in a subtropical or tropical climate. The overlying period Sturgeon Quartzite is is the product product of the reworking reworking and and sorting sorting of the the weathered weathered detritus detritus Sturgeon by wind and water. The quartzite here is about 325 m thick, and elsewhere in the region region it has a maximum thickness of 600 m (Freedman and others, 1961). It obviously represents the transported resistant quartz sand fraction of a very broad, deeply chemically weathered surface that was largely developed upon granitic rock over a long period period of time. There is is no no diagnostic diagnostic evidence evidence within the quartzite itself of a terrestrial terrestrial (fluvial) versus a marine environment environment of deposition. deposition. However, However, an an apparently apparently conformable conformable relationship with the overlying thick Randville Randville Dolomite Dolomite (300-430 (300-430m), m), which which is is stromatolitic and contains detrital detrital quartz grains and thin interbeds interbeds of quartzite, is strongly marine environment environment for the Sturgeon Sturgeon Quartzite. In In addition, suggestive of a shallow marine high in the formation suggests suggests a gradational gradational contact contact with the diopside-rich quartzite high and others, others, 1961). Detrital quartz grains in the the sericitic Randville Dolomite (Freedman and schist show a marked 1966), indicative of marked elongation and alignment (Bayley and others, 1966), shearing shearing along along this this softer softer zone zone beneath beneaththe the quartzite. quartzite. 54 �Figure Figure 2-8. 2-8. Ripples Ripplesin inSturgeon SturgeonQuartzite Quartzite Stop Stop2-3: 2-3: Sturgeon SturqeonRiver RiverDam Damlocality. locality.Archean Archeanbasement, basement,Fern FernCreek CreekFormation, Formation, and andSturgeon SturgeonQuartzite. Quartzite.(E(E½,95sec. ,sec.8,8,T.T.3939N., N.,R.R.29 29W.). W.). From Stop Stop 2, 2, backtrack backtrack about about 1½ 1V2 miles milesto to County CountyRoad Road573. 573. Turn Turn left left (east) (east) and and From proceed proceedabout about3½ 3%miles. miles.Turn Turnleft left(east) (east)on onSwede SwedeSettlement SettlementRoad, Road,which whichleads leadstoward toward the mile.Turn Turnleft left(north) (north)on onaasmaller smallerroad road thepower powerstation/dam stationldamand andproceed proceedfor for about about1½ 1V2 mile. that V2 mile mileto tolocked lockedgate. gate.Park Parkhere. here.IfIf thatleads leadsto to the thepower powerstation/dam. stationldam. Proceed Proceedabout about½ approaching approachingthis stopfrom fromthe thevillage villageof Loretto,take takeCounty CountyRoad Road573 573north outof thisstop of Loretto, northout of Loretto V2 mile. mile.Then Thenturn turnright right(east) (east)on onSwede SwedeSettlement SettlementRoad, Road,which whichleads leads Lorettofor forabout about½ miles.Turn Turnleft left(north) (north)on onaa towardthe the power powerstation/dam, stationldam,and andproceed proceedfor for about about1½ 1V2 miles. toward smaller smallerroad roadthat that leads leadsto to the the power powerstation/dam. stationldam. Proceed Proceedabout about½ V2 mile mileto to locked lockedgate. gate. Park Parkhere. here.As As of of this thiswriting writinginin2003, 2003, the thedam damisisslated slatedfor forremoval removalininthe therelatively relativelynear near future. future. Here Herethe theSturgeon SturgeonRiver Riverhas hascut cutaadeep deepgorge gorgethrough throughthe theSturgeon SturgeonQuartzite; Quartzite;the the formation formationwas wasnamed namedfor forthis thislocality. locality.This Thissmall smallarea areahas hasbeen beenwell wellstudied, studied,especially especially because Archean-Paleoproterozoiccontact contactat at the thedam. dam.The Thearea area becauseof of the thepresence presenceof of the theArchean-Paleoproterozoic has hasbeen beendescribed describedby byCredner Credner(1869), (1869),Brooks Brooks(1873), (1873),Rominger Rominger(1881), (188l), Irving Irving(1890), (1890), Bayley Bayley(1904), (1904),Lamey Lamey(1937), (1937),Pettijohn Pettijohn(1943), (1943),and andTrow Trow(1948). (1948). SubstoD subs to^ 1. 1. Walk Walk past past the the gate gate to to the the end end of of the the road roadat at the the powerhouse powerhouseand and dam. dam. We We will willtraverse traverseback theroad roadto vehicles,thus thusobserving observingthe therock unitsinin backup upthe tothe thevehicles, rockunits stratigraphic stratigraphicsequence. sequence.The Thedam damwas wasconstructed constructedon onSturgeon SturgeonRiver RiverFalls, Falls,which whichwas was held heldup upby byaa thick thick mafic maficdike dikethat thatcan canbe beseen seenininthe thewoods woodsoff offthe theeast eastend endof ofthe thedam. dam. The ArcheanCarney CarneyLake LakeGneiss Gneissand andthe thePaleoproterozoic Paleoproterozoic Theunconformity unconformitybetween betweenthe theArchean Fern FernCreek CreekFormation Formationcan canbe beseen seenininaasmall smallground-level ground-levelexposure exposureadjacent adjacenttotothe thedam dam 55 �(fig. 2-9). The lowest lowest bed bed in in the the Fern Fern Creek Creek is is aa diamictite diamictite at at this this spot, spot, whereas whereas aa short short distance to the west on the river bottom bottom by by the power power station, station, the the lowest lowest unit unit is is arkosic arkosic sandstone with rare oversized oversized stones. Figure Figure 2-9. Unconformity Unconformity at Sturgeon Sturgeon River River dam. dam. Hammer Hammer head head rests rests on on Archean Archean Carney Lake Lake Gneiss Gneiss and and hammer hammer handle handle is is on on basal basal diamictite diamictite of the the Fern Fern Creek Creek Formation. Formation. Nearby Nearby in river bottom, bottom, the the basal basal unit unit is is arkosic arkosic sandstone sandstone with with rare rare dropstones, illustrated 1. dropstones, illustratedininfigure figure2-1 2-11. FERN FERN CREEK FM FM (STURGEON LOCALITY) (STURGEONRIVER RIVER LOCALITY) M - 75 SANDSTONE ARGILLITE 50 ARKOSE • 25 / 25 M 20 CONGLOMERA GcK' SANDSTONE SANDSTONE SANDSTONE SANDSTONE DROPSTONES L 2Fm5nw DIAMITITE DIAMICTITE DIAMICTITE DIAMICTITE DIAMICTITE DIAMICTITE SANDSTONE SANDSTONE ARGILLITE ARGILLITE 10 CONGLOMERATE GRAYWACKE ARGILLITE DIAMICTITE DIAMICTITE SANDSTONE SANDSTONE DROPSTONES DROPSTONES ONFSTONFS DIAMICTITE DIAMICTITE SANDSTONE SANDSTONE DROPSTONES DROPSTONES 2-10. Figure 2-1 0. Stratigraphic column at Sturgeon Sturgeon River River locality. SQ at the the top of the the column column designates the Sturgeon designates Sturgeon Quartzite. Quartzite. 56 �2-10 Figure 2-1 0 is a measured measured column column of of the the Fern FernCreek Creek Formation. Formation.The Thelower lower25 25m mare arewell well exposed when there is no water in the channel. Note that this portion of the formation consists of five beds of diamictite (matrix-supported (matrix-supportedconglomerate) as thick as 2.5 m, arkosic sandstone sandstone beds beds as as thick as as 2.6 m m with rare rare oversized oversized stones, stones, stacked stacked three arkosic sandstone beds beds with minor intercalated intercalated siltstone and argillite argillite laminae, laminae, an argillite argillite arkosic sandstone 15 cmconglomerate. conglomerate.Because Becausethe thewater waterlevel levelisiscommonly commonlyhigh, high, bed 4.5 cm thick, and a 15cm figures 2-11 2-11 and and 2-12 2-12 are are included included here here to illustrate illustrate some important important features. figures 2-11. stratigraphic column. Note Figure 2-1 1. Granitic dropstone in lowest sandstone of the stratigraphic that the stone has pierced and bowed down the underlying laminations. stone has pierced and bowed down the underlying laminations. the river bottom is not found found on the west Interestingly, the well-exposed section seen in the bank mof of conglomeratic conglomeratic rock is present present there. 1%m there. Apparently Apparently the the more more bank of the the river; river;only only11/2 complete section is preserved preserved in a topographic low on the Archean surface. However, faulting may may be a factor factor as as well, for weathered weathered pyrite pyrite is present along a fault between the Archean basement basement and the Fern Fern Creek west of the powerhouse. The middle middle 25 m m of the Fern Fern Creek Creek Formation Formation is is relatively relatively poorly poorly exposed; figure 2-10 2-10 shows this portion portion consisting of conglomerate, conglomerate, graywacke graywacke sandstone sandstone with oversized stones, stones, and and arkosic arkosic sandstone sandstonewith with oversized oversizedstones. stones. 57 �beds of stratigraphic stratigraphic column column of Figure Figure 2-10. View Figure 2-12. Second, third, and fourth beds is to west. Beds 2 and 4 are diamictites below and above arkosic sandstone with rare dropstones. dropstones. Interpretation: This This isisaa glaciogenic glaciogenic sequence. sequence. The The diamictites may be thin tills suggested by deposited beneath glacial ice, but more likely are debris flow deposits as suggested one diamictite bed that grades upward upward into sandstone. Some of the conglomeratic beds beds are difficult to clearly classify classify as either matrix-supported matrix-supported or clast-supported. One 20 cm bed at the 15 15 m level level in the section is graded from medium medium sand to clay, suggestive of a turbidity current mechanism. Several of the oversized stones in the sandstone and greywacke beds show either a bowing down of the underlying laminae or an actual penetration, indicating indicating that the stones were dropped into the basin from above and are indeed dropstones. Other lonestones lonestones may be dropstones, too, but clear evidence is lacking. The likely likely mechanism mechanism for deposition deposition of dropstones dropstones is release from melting melting icebergs icebergs or from from aa floating floatingglacier. glacier. Substop 2: The 50 and and 75 75 m m on Figure Figure 2-9 is intermittently intermittently The25 25m msection section between between 50 exposed on the west bank of the river, but this area is usually inaccessible because of high water. It includes beds of sericitic sericitic quartzite interbedded interbedded with sericite schist. The sericitic nature of this interval is illustrated by sericitic nature of this interval is illustrated by aa small small road-level road-leveloutcrop outcropbetween betweenthe the the river just just north of the quartzite ridge. This is a sericitic quartz pebble road and the conglomerate with sericite clay chips, some reddish rather than yellow-green in color. Interpretation: This Thissericitic sericitic portion portion of of the the column column is is interpreted interpreted as a reworked reworked paleosol that formed on the Fern Fern Creek Formation Formation during a warm climatic period period that followed glaciation. Trow (1948) first suggested suggested that this might might be be a paleosol. paleosol. Substop 3: Sturgeon SturgeonQuartzite Quartzite ridge. ridge. Note Note that that the the bedding bedding is is slightly overturned towards the south, and that cross-bedding cross-bedding indicates that stratigraphic tops are to the 58 �Cross-bedding is is of both both trough trough and and planar planar types. types. According According to to Trow Trow (1948), (1948), the the south. Cross-bedding general cross-bedding cross-bedding indicates indicates aa paleocurrent paleocurrent trend trend from from the the northwest northwest toward toward the the general southeast. southeast. Interpretation: Abundant Abundantasymmetrical asymmetricalripple ripplemarks markshave havelow lowripple rippleindices indices Interpretation: lengthlripple height) height) indicative indicative of deposition deposition by water rather rather than by wind. The (wave length/ripple beds are generally generally of even thickness, indicative indicative of aa shallow shallow marine marine rather rather than than aa fluvial fluvial beds environment of of deposition. deposition. See See text text for for substop substop 44 of of stop stop 2-2 2-2for for additional additionalinterpretation interpretation environment of the the genesis genesis of of the the Sturgeon SturgeonQuartzite. Quartzite.The TheSturgeon Sturgeonhas haslong longbeen beencorrelated correlatedwith with of the the Mesnard Mesnard Quartzite Quartzite of of the the Marquette MarquetteTrough. Trough. Stop 2-4. Brier Brier Slate Member of the Vulcan Iron-formation inNorway, Norway,MI. MI. (Modified (Modified Stop Iron-formation in fromDutton, Dutton,1958). 1958).(E(E1/2, Vz, SE SE ¼, Vn, sec. sec. 5, 5, T. T. 39 39N., N., R. R. 18 18W.) W.) from Three iron iron mines, mines, the the Aragon, Aragon, Cyclops, Cyclops, and and Norway Norway mines mines were were developed developed in in the the Three northern part part of of the the city city of of Norway, Norway, MI, MI, and and subsidence subsidence of of these these abandoned abandonedmines mines has has northern limited development development in in this this part part of of the the city. city. The The Aragon Aragon mine mine was was the the third third largest largest limited producer of of iron iron ore ore in in the the Menominee Menominee district district (Dutton, (Dutton, 1958), 1958), and and the the head headframe frame of of the the producer mine still still stands stands about about 55 blocks blockseast east of of this this stop. stop. Figure Figure2-13 2-13 shows shows the the location locationof of mine several interesting interestinggeological geological features, features, but but the the trip trip will will visit visit only only the the Brier Brier Slate Slate Member Member several type type locality. locality. According to to Dutton Dutton (1958), (1958), outcrops outcrops along along the the ridge ridgenorth northof of Norway Norwayare are Randville Randville According Dolomite with with aa rather rather extensive, extensive, but but incomplete, incomplete, cover cover of breccia breccia composed composed of of dolomite dolomite Dolomite fragments that that are are slightly slightly to to thoroughly thoroughly silicified. silicified. (This (This breccia breccia is is similar to that at the top fragments of the the correlative correlative Bad Bad River River Dolomite Dolomite in in the the Gogebic Gogebic district, district, some some 120 120miles miles westwestof northwest of here.) here.) The Thesilicification silicificationisisbelieved believedto to be bethe the result result of of surficial surficial weathering weathering in in northwest pre-iron-formationtime. time. The The dolomite dolomite dips dips about about 60 60 degrees degrees southward, southward, post-Randville -- pre-iron-formation post-Randville and the the convexity convexity of stromatolites stromatolites in the dolomite face in the dolomite indicate indicate that that the the beds beds also also face and southward. Small Small outliers outliers of of Cambrian Cambrian sandstone sandstone overlying overlying the the Randville Randville Dolomite Dolomite are are southward. also (e.g., at at Rochon RochonSt. St. and and Curry Curry Lane Lane along along the the ridge). ridge). also present present (e.g., The TheRandville Randvilleisisoverlain overlain by by the the Felch FelchFormation, Formation,the the basal basalunit unit of of the the Menominee Menominee Group. Group. One One of of the the few few exposures exposures of of the the Felch FelchFormation Formationis is at at this this locality locality in in Norway Norway (Dutton, 1958). 1958). The The Felch FelchFormation Formationconsists consists of of sericitic sericitic slate slate and and thin thin layers layers of of (Dutton, quartzite. quartzite. The The uppermost uppermostpart part of of the the formation formation isisaa ferruginous ferruginous quartzite quartzitethat that isisaamarker marker bed bed throughout throughout the the district. district. The The quartzite quartziteis is aa transitional transitional unit unit between betweenthe the dominantly dominantly clastic, clastic, non-ferruginous non-ferruginousstrata strataof of the theFelch FelchFormation Formationand andthe theiron-rich iron-richchemical chemical sediments of the Vulcan Iron-formation (Dutton, 1958). sediments of the Vulcan Iron-formation (Dutton, 1958). Three Threemembers membersof of the theVulcan VulcanIron-formation Iron-formationwere wereexposed exposedininNorway Norwayin in1958, 1958, accordingto to Dutton Dutton(1958). (1958).Exposures Exposuresof of the thebasal, basal,thin-bedded thin-beddedoxide oxidefacies faciesTraders Traders according Member are are present present at at the the northeastern northeastern end end of of aa southwesterly southwesterly trending trending area area of of Member outcrops.The TheTraders Tradersisispresently presentlyexposed exposednorth northof of Sixteenth SixteenthAve. Ave. and andMain MainSt., St., in inthe the outcrops. oldCyclops Cyclopsmine mineworkings, workings, which which isisbeing beingused usedas as aa landfill landfillby by the the city city of of Norway. Norway. The The old Brier Brier Slate Slate Member Member (this (this stop) stop) is is exposed exposed on on an an outcrop outcrop knob knob at at Eleventh Eleventh Ave. and and the the Aragonlocation, location,southwest southwestof of the theTraders TradersMember Memberlocation. location.In Infact, fact, the theBrier BrierSlate Slatewas was Aragon named namedfor for exposures exposureshere. here. Several Several small-scale small-scale folds folds that that plunge plunge about about 45 45 degrees degrees easterlyare areexposed exposedalong alongthe the old oldrailroad railroadcut cut on onthe the north northside sideof of the theoutcrop. outcrop. easterly According theoxide oxidefacies, facies,oolitic ooliticand andgranular granulariron-formation iron-formationof ofthe the Accordingto to Dutton Dutton(1958), (1958),the 59 �Curry member member of of the theVulcan Vulcan Iron-formation Iron-formationwas wasexposed exposedat atthe thesouthwest southwestend endof ofthe the Curry outcroparea. area. outcrop The Michigamme MichigammeFormation Formationunconformably unconformablyoverlies overlies the the iron-formation iron-formationand andisisexposed exposedin in The several low low roadcuts roadcutsalong alongU.S. U.S. Hwy-8 Hwy-8on on the the southern southernoutskirts outskirtsof of Norway. Norway. several 00 I 1I2 112 I I 1 1 1 i Miles Miles Figure 2-13. 2-13. Part Partofofthe theNorway Norway7½' 7 %quadrangle quadrangleshowing showingthe thelocation locationof of stop stop2-4 2-4and and Figure some other features of geologic interest. some other features of geologic interest. Stop 2-5. 2-5. Quinnesec Quinnesec mine, mine, just just northwest northwestof of Quinnesec, Quinnesec,Ml, MI, (Modified (Modifiedfrom fromDutton, Dutton, Stop T. 39 39 N., N., R. R. 30 30W.) W.) 1958)(SW (SW1/4 Y4, SE SE 1/4 34, sec. 34, T. 1958) sec. 34, (NOTE: For this property. property. For For (NOTE; For safety safety reasons reasons aa security security fence fence surrounds surrounds this MI, Ph (906-774-4471). permission to enter contact Joe Massie, Quinnesec, permission to enter contact Joe Massie, Quinnesec, MI, Ph (906-774-4471). The abandoned abandonedworkings workingsof of the theQuinnesec Quinnesecmine mineare aremainly mainlyininthe theTraders Tradersiron-bearing iron-bearing The member of the Vulcan Iron-formation. The mine lies on the overturned north, limbofof aa member of the Vulcan Iron-formation. The mine lies on the overturned north, limb 60 second order syncline (fig. 2-1 4). The Precambrian strata at the mine dip about second order syncline (fig. 2-14). The Precambrian strata at the mine dip about 60 degreeshorth, north,but butface facesouthward, southward,inasmuch inasmuchas asthe theBrier Brierslate slatemember memberofofthe theVulcan Vulcanisis degrees 60 �along the south side side of the the excavated excavated approach approach to to the the mine, mine, and and the the Felch FelchFormation Formationisis along the north north wall of the the workings. Note the Cambrian sandstone north side side of the the sandstone overlying the mine workings along the north hill, providing an unusual view of an unconformity unconformity (fig. 2-15). 2-15). The basal basal portion portion of the the sandstone contains contains numerous numerous angular angular slabs slabs of of oxidized oxidized iron-formation, iron-formation,iron iron ore, ore, and and sandstone slate in a sandy matrix. matrix. Clearly, this area area was a small island island as the Cambrian Cambrian sea sea advanced over the area. The clasts clasts of iron iron ore ore in in the the basal basal conglomerate conalomeratealso also indicate indicate the area. that the ore here here was formed formed before before the the Cambrian Cambrian sea sea covered coveredthe area. R. 30W. R. 30 W. I I 1000 feet 0 <") 1 1 I xmj Xm Michigamme MichigammeFormation Formation Vulcan Iron-formation Iron-formation [&c Curry Curry Member Member Iron-bearing Iron-bearing Member Member Xvb Brier Slate Member Xvt Traders Iron-bearing Iron-bearing Member :1;X 1 1 xfXf 1 1IXrI Xr 1 Open pit ..>.Exposed Exposedbedrock bedrock J<^_) Small Small area of exposed exposed bedrock bedrock $j Strikeanddipof Strike and dip ofbeds beds X 50 Strike Strikeand and dip dip of overturned overturned beds beds Beds dip 70°-80° 70'-80' except Felch Formation Formation --*- Randville Dolomite Randville Dolomite as noted 2-14. Figure 2-1 4. Map Map of of the the Quinnesec Quinnesec mine mine and and vicinity vicinity at stop stop 2-5 showing that the workings were developed developed in the overturned overturned northern northern limb limb of aa small small syncline. syncline. Map Map from from Bayley Bayley (1957). (1957). 61 �5. Abandoned workings at Quinnesec mine. Ore bodies were mined from the Figure 2-1 2-15. Iron-bearingmember member of Vulcan Iron-formation. Iron-formation.View View looking looking west west shows shows beds beds Traders Iron-bearing but facing facing south. Roof Roof of workings workings is is base base of of the the Munising Munising Sandstone Sandstone of of dipping north, but of the the basal Cambrian unconformity. Cambrian age and provides an excellent view of Photo Photo by Elizabeth Elizabeth Heinen. Heinen. Stop 2-6. Marqaret St. on Lake Antoine in Iron Mountain. Mountain, 2-6. Randville Randville Dolomite Dolomite alonq along Margaret in Iron (Modified from 1958) (SE 1/4, MI. (Modified fromDutton, Dutton, 1958) (SE %,NE NE1/4, %, sec. 29, 29, T. T. 39 39 N., N., RR30 30 W.) W.) - This exposure exposure of the Randville Randville Dolomite Dolomite on the south shore of Lake Lake Antoine Antoine was formerly the site of a small small quarry quarry for the the production production of of road road material. material. Operation Operation of of the the quarry was halted during the late 1930's through the influence of geologists and other interested people who wanted the the site preserved for for future geologic examination interested (Dutton, 1958). 1958). The glacially scoured scoured outcrop shows a variety of sedimentary sedimentary and structural structural features. The rocks rocks are are dipping dipping nearly nearly vertically, and face southward. southward. Sedimentary Sedimentary features features preserved here include abundant stromatolites (fig. 2-15), typically 3-4 inches high and preserved 2-15), 4-8 inches inches in diameter. The old old quarry quarry face provided provided a vertical vertical view of the stromatolites now largely obscured by graffiti. Thin layers of quartz sand are interbedded interbedded with the dolomite, and and ripple ripple marks marks and and mud mud cracks cracks are are present present in in places. places. These These features features suggest a very shallow water environment of deposition, and several authors (e.g. Larue, 1981) have suggested that the Randville Dolomite may have been deposited in a paleo paleo sabkha sabkha environment. environment. - 62 �Structural Structural features features exposed exposed here here include include deformed deformed stromatolites. stromatolites. Several Several layers layers of stromatolites that are are strongly strongly skewed skewed in in a right-lateral right-lateraldirection direction are are present. present. The The dolomite dolomite evidently evidently recrystallized recrystallized readily, readily, allowing allowing ductile ductile deformation deformationof of the the stromatolites. stromatolites. Possible fracture is nearly nearly "axial planar" to the deformed deformed Possible fracture cleavage is oriented so that it is stromatolites, and and is is roughly roughly parallel parallel with the regional regional structure. Figure interbedded Randville Dolomite at stop 2-6. Laminated Laminated dolomitic layers layers are interbedded Figure 2-16. 2-16. Randville with with stromatolitic stromatolitic dolomite. dolomite. Stromatolite Stromatolite mounds mounds are are deformed deformed and and show show an an asymmetry asymmetry indicating right lateral lateral sense of shear. View is looking looking down at a horizontal surface and indicating a right beds bedsdip dip vertically. vertically. 63 �FIELD TRIP 3 STRATIGRAPHY AND STRUCTURE OF THE IRON RIVERCRYSTAL FALLS BASIN BASIN William F. Cannon, USGS, USGS, Reston, VA; VA; John John S. S, Kiasner, Klasner, Western WesternIllinois Illinois (retired) and USGS; University (retired) USGS; Gene Gene L. LaBerge, LaBerge, University Universityof of WisconsinWisconsinOshkosh Oshkosh (retired) (retired) and and USGS USGS Tightly folded Riverton Iron-formation Michigan. Tight Iron-formation near Stager Lake, Michigan. complex folds such as these are typical of the structural style of the Iron RiverCrystal Falls Crystal Falls allochthon. allochthon. �FIELD TRIP 3 FIELD BASIN STRATIGRAPHY AND STRUCTURE OF THE IRON RIVER-CRYSTAL FALLS FALLS BASIN William F. Cannon, USGS, Reston, VA; John John S. Klasner, Kiasner, Western Illinois University (retired) and USGS; Gene L. LaBerge, University University of Wisconsin-Oshkosh Wisconsin-Oshkosh (retired) (retired) and (retired) USGS USGS The Iron Iron River-Crystal River-Crystal Falls iron district was mined extensively for high-grade high-grade "soft" iron 1882 until the early 1970's 1970's producing producing more more than than 200 200 million million tons of ore. ores from 1882 Because of of the high economic interest in the region, the availability of many underground mine workings, workings, and extensive diamond drilling, drilling, a detailed detailed stratigraphy stratigraphy and and deciphered in in what what otherwise otherwise would would have havebeen beenaa largely largelyunknown unknownterrane, terrane, structure was deciphered mostly concealed by the nearly continuous cover of glacial deposits. The most detailed region was conducted by a large group of USGS USGS and affiliated affiliated geologists geologists study of the region 1943 and continuing 10 years. Surface exposures exposures were mapped mapped beginning in 1943 continuing for about 10 in detail, as were most underground underground mine workings. Ground magnetic surveys helped delineate the surface surface trace trace of of certain certain units unitsand and the the extensive extensivecollection collectionof of exploration explorationdrill drill core was examined. One of the earliest aeromagnetic surveys was conducted here II.The The work of the the USGS USGS when the technique was still classified shortly after World War II. was aided immeasurably immeasurably by the cooperation of the numerous mining companies active The results results of that that painstaking painstakingwork work were weresummarized summarizedininUSGS USGSProfessional Professional in the area. The Paper 570 (James and others, 1968), which remains the the only comprehensive comprehensive account account of of the geology geology of the region. Much Much of the the descriptive descriptive material material in in this this guide guide is is taken taken from from that that work. Additional detailed studies of the Florence area, Wisconsin were done by Dutton 1971) and also were instrumental (Dutton, 1971) instrumental in determining the geological relationships Iron River-Crystal River-Crystal Falls basin, including areas visited in along the southern extent of the Iron the first three stops of this this trip (figs. (figs. 3-1 3-1 and and 3-2). 3-2). The Iron Iron River-Crystal River-Crystal Falls basin is a triangular structure, with an area of about 300 square miles, underlain underlain by strata of the Paint Paint River Group. It is surrounded, except on part of the eastern side, by volcanic rocks of the Badwater Greenstone. Greenstone. Our current current interpretation of the area is that the Paint Paint River Group, as originally defined, and the Badwater northward during the Badwater Greenstone, are an allochthon, and were thrust northward driven by arc arc collision collision south south of of the the Niagara fault. fault. This trip trip traverses traverses Penokean orogeny, driven from the Niagara fault, as seen at Pine River Flowage in northern Wisconsin, northward Niagara Pine River Flowage northward across the complexly deformed deformed fault panels of the Niagara Niagara suture zone, and onto the structurally simpler rocks rocks north of the Iron Iron River-Crystal River-Crystal Falls Falls allochthon. Most stops examine the lithology and structure of rocks of the Paint Paint River River Group. Principal Principal observations are the preponderance preponderance of steeply plunging folds in all panels of the suture particularly in the Iron Riverzone rocks, the extraordinarily complex fold patterns, particularly Crystal Crystal Falls Falls allochthon, and the contrast with the simpler, gently plunging folds north of the suture suture zone. zone. 65 �EXPLANATION EXPLANATION North North of o f Niagara Niagara fault Tobin Tobin Lake Lake Granite Granite Metagabbro Metagabbro El Paint undivided Paint River Group - undivided Fortune FortuneLake LakeSlate Slate Riverton Riverton Iron-formation Iron-formation Dunn DunnCreek CreekSlate Slate X . " El El Badwater Badwater Greenstone Greenstone Michigamme MichigammeFormation. Formation, graywacke and graywacke and volcanic volcanicrocks rocks Michigamme MichigammeFormation Formation --quartzite quartzite Michigamme MichigammeFormation Formation -- graywacke graywacke Iron-formation Amasa Iron-formation Hemlock Volcanics undivided undivided HemlockVolcanics Randville Dolomite Randville Dolomite Saunders Formation Formation Saunders Dickinson undivided Dickinson Group undivided South o off Niagara fault fault Bush Lakegranite Granite and tonalite Quinnesec Formation Formation Qulnnesec El Metasedimentary rocks Metasedimentary rocks — faults - faults 2 I field field trip tripstops stops NE-Niagara fault fault NF-Niagara SRF-South South Range Range fault fault SRF- 0 I 5 5 • I I 2 4 I I 8 6 I I I I 0 0 Miles 5 5 10 10 Kilometers NRF-North Range fault NRF-North Range fault BF-Badwater fault fault BF-Badwater Riverfault fault PRF-Paint River CS-Commonwealth syncline MA-Mastodon anticline anticline MA-Mastodon Bowers syncline syncline TBS-Tim Bowers Figure 3-1. Geologic map of the eastern eastern part part of of the the Iron Iron River-Crystal River-CrystalFalls Falls basin basin showing the location location of stops stops for for field field trip trip 3. 3. 66 �Figure Figure3-2. 3-2. Part Partof of the theIron IronMountain Mountain1:100,000-scale 1:100,000-scaletopographic topographicmap mapshowing showingthe the location locationof of field fieldtrip tripstops stops3-1 3-1through through3-4 3-4and and1-9, 1-9,1-10. 1-10. Stop3-1. 3-1. Michiqamme MichiciammeFormation FormationatatPine PineRiver RiverFlowage Flowaue(NE (NE1/4, 114,SW SW1/4, 114,sec. sec.28, 28,T. T. Stop 39 39N. N.R. R.18 18E.) E.) This Thisoutcrop outcroplies lieswithin withinthe thePine PineRiver Riverstructural structuralblock blockas asdefined definedby byDutton Dutton(1971). (1971). Large Largeexposures exposuresare arealong alongthe thegorge gorgeof ofthe thePine PineRiver Riverjust justdownstream downstreamfrom fromthe thePine Pine River River dam. dam. The The Pine PineRiver Riverblock blockisiscomposed composedalmost almostentirely entirelyof ofthe theMichigamme Michigamme Formation Formationconsisting consistingmostly mostlyof ofgraywacke graywackeand andlesser lesserquartzite quartziteand andconglomerate conglomerate(stop (stop 3-2). 3-2).Although Althoughpenetrative penetrativedeformation deformationisisintense intenseand andthere thereare aremany manysmall-scale small-scalefolds folds well wellexposed exposedat atthis thisstop, stop,the theoverall overallstructure structureofofthe theblock blockseems seemstotobe beaauniformly uniformly south-facing south-facingsuccession successionas asindicated indicatedby bycross crossbeds bedsand andgraded gradedbeds. beds. The Theoutcrops outcropsseen seenhere hereare arevery veryclose closetotothe theNiagara Niagarafault faultwhose whoselocation locationhere hereisiswell well constrained constrainedby byrather ratherabundant abundantoutcrops outcrops(see (seefig. fig.3-3). 3-3).The Therocks rocksexposed exposedbelow belowthe the dam damare areno nomore morethan than500 500feet feetnortheast northeastof of the thevolcanic volcanic rocks rocksof of the theQuinnesec Quinnesec Formation, Formation,part partof of the theWisconsin WisconsinMagmatic MagmaticTerranes. Terranes.The TheMichigamme MichigammeFormation Formationininthis this vicinity vicinitywas waswell welldescribed describedby byDutton Dutton(1971) (1971)and andthe thefollowing followingisisextracted extractedfrom fromhis his report. report."The "Therock rockisisgray grayand andwell-bedded well-beddedininlayers layersone-fourth one-fourthtotoone-half one-halfinch inchthick. thick.The The percentage percentageofofminerals mineralsininthe therocks rocksisisapproximated approximatedas asquartz quartzfrom from20 20toto50; 50;sericite sericiteand and muscovite to 40. 40. Dark Dark muscovitefrom from20 20to to70; 70;biotite biotitefrom from10 10to to25; 25; and andchlorite, chlorite,ifif present, present,from from55 to red redgarnets garnetsare areabundant abundantininthe theschist schistnear nearthe thequartzite; quartzite;they theylocally locallyare areconcentrated concentratedinin layers layersand andlenses, lenses,but butthey theymay maybe beminor minorororabsent. absent.Some Somegarnets garnetshave havebeen beenrotated rotated 67 �formation as as much much as as 25 25 degrees, degrees,as as shown shownby bythe the angular angulardiscordance discordancebetween between after formation general foliation foliation and and the the layers layers of of very very fine fine opaque opaque grains grainswithin within the the garnets. garnets. Deflected Deflected general foliation at at the the boundary boundarysurfaces surfacesof of the thegarnets garnetsalso alsoindicates indicatesdirection directionbut butnot notamount amount foliation of of rotation." rotation." 'so O EXPLANATION 1T S . n Michigamnie Slate qu.rte slate. Locally includes,nleor conglomerate (cQi) 055. agglomerate and tremoiite orhist nsl qC QUuttottlc cungluma,ate. Locally In. g:un,ritic (at) or magnetuLic cludea (m..t Iron-formation a. oes.tnblage of this units. Locally includes graphitic slat. (gal), guotte. mice slate (Cl). quarts grsywaclte (gw), grunerittc achiot (Qru). and amphi bout. (am) If 1000 FEET A L 0 Outcrop Outcrop or orgroup group of of small smalloutcrops outcrops Metaf,lsic cod q5arcz. Uetafelalczocks.toetazhyoiite? rocks-metarhyolll~? andquartzmuscovite m u s c o v i f achier; schist: locally locsltyincludes tncludos in. intertoiiated fsch) t e r t o f l à § tactual ~schlat(schi Strikeand and dip dipof ofbode beds Strike Direction uf of top tv@ detennined detçwaineby bygraded çrçd Direction bedding bedding z Contact L.ong daubed wfl eta .pprox4mately 1*. hott dashed where Inferred; fl rated: dotted where concealed; queried Øzare Strike and dip of beds beddlu Dlrectinh of top detenulned by cross doubtful 0 Iii UJ 0 Probable Probablefault fault aDotted i?otted wh,re whewcOncealed conceiled II.. U. upthcown tiptftrown U. Q. slde;D. d d à §0 downthrawnalde;querlatd downtbtowniildfquer~~d wftwe where doubtful do~bttui d5 Quinnesec Portnation ...... T s - Vertical Inclined Inclined Vertical Strikeand anddip dipof ofbeds beds Strike Strikeand anddip dipof of foliation foliation Strike *t_ Strike and anddip di ofofbed bedand andplunge plunge Strike o&eatiou of lineation SB Abandonedshaft shaft Abandoned x Test Testpit pit Figure3-3. 3-3. Geologic Geologicmap mapof of part partof of the theFlorence Florencearea, area,Wisconsin Wisconsinshowing showingthe thelocation locationof of Figure 3-2.Map Maporiginally originallypublished publishedby byDutton Dutton(1971, (1971,figure figure3). 3). fieldtrip tripstops stops3-1 3-1and and3-2. field 68 �A STOP 3-1 flU dots- stretched concretions diamonds- mineral mi squares- fold axes Figure 3-4. 3-4. Lower Lower hemisphere hemisphereequal equalarea areastereoplots stereoplotsshowing showingorientation orientationof of fold foldaxes axes Figure and andrelated relatedstructures structuresat atvarious variousstops stopsfor forfield fieldtrip trip3.3. 69 �The rocks rocks near near the dam dam are are aa strongly strongly foliated foliated and and lineated lineated chlorite-garnet chlorite-garnetschist. Primary Primary layering is well preserved preserved but but has has been been transposed transposed parallel parallelto to S1 8, foliation. Dextral drag folds in the transposed layering have steeply plunging axes (see fig. fig. 3-4,A) that are parallel parallel to mineral mineral lineations lineations on foliation foliation surfaces. Axes of maximum maximum elongation in deformed clasts and andconcretions concretionsplunge plunge60 60 degrees degrees toward toward the south, parallel parallel to the in mineral mineral lineation. lineation. These These highly highly strained strainedrocks rockshave havean anS1 S1foliation foliation that that dips dips steeply steeply south south as do the elongation elongation (stretching) (stretching) axes. These These steep, steep, south south plunging plunging structures structures seem seem to be be characteristic characteristic of the the Niagara Niagara fault fault zone. Similarly Similarly oriented, steeply south plunging plunging structures structures occur south south of the the Niagara Niagara Fault Fault zone, probably probably in in splays of the Niagara Niagara fault in in volcanic rocks rocks of the Wisconsin Wisconsin magmatic magmatic terrane terrane that that crop crop out out aa few few miles miles south south of of Pine River River Dam (Sims (Sims and others, 1985). 1985). The structural structural fabric in in this region region reflects reflects the Pine overthrusting--with overthrusting-with a right right -lateral -lateral component--of component--of the Wisconsin Wisconsin magmatic magmatic terrane terrane from from the south onto the continental margin margin and and later steepening steepening of the thrusts thrusts to their their present present orientation. orientation. Stop3-2. Conglomerate of the Michigamme Stops-2. Quartzite and Conqlomerate Michiqamme Formation Formationnear near Pine Pine River sec. 28, T. 39 (NE1/4, %, NW 1/4 %, sec. 39 N., N., R. I?.18 18E.) E.) RiverDam. Dam.(NE The quartzite conglomerate conglomerate exposed exposed here (fig. (fig. 3-3) 3-3) is is the most most prominent prominent and and bestbestexposed andothers (1935)considered considered exposed unit in the Pine River Block (Dutton, 1971).Leith Leithand others(1935) (Dutton,1971). it to be be a separate separate formation (the (the "Breakwater "Breakwater Quartzite"). Quartzite1').However, However, according to Dutton Dutton (1971), (1971), it appears appears to be be a lens lens within the Michigamme Formation Formation and he did not give it a separate separate stratigraphic stratigraphic name. name. Figure Figure 3-5. 3-5. Conglomerate ConglomerateininMichigamme MichigammeFormation Formationatatstop stop3-2 3-2showing showingstrongly strongly stretched figure 5. 5. stretched and and aligned aligned pebbles. pebbles. Photograph Photograph from from Dutton Dutton (1971), (1971), figure 70 70 �The quartzite conglomerate conglomerate is about 700 feet thick and extends northwestward northwestward about 3 near the center of sec. 28, T. 39 39 N., N., R. R. 18 18 E., E., to to the the NE NE corner, corner, sec. sec. 24, 24, T. T. 39 39 miles from near R. 17 17 E., E., (Dutton, (Dutton, 1971). 1971). Cross-bedding Cross-bedding indicates indicates that statigraphic statigraphic tops tops are are toward toward N., A. the southwest. Layers and lenses of quartzite with flat (or flattened and stretched) pebbles and cobbles (fig. 3-5) of recrystallized recrystallized chert and iron-formation iron-formation are the dominant lithology. (Note: This locality is is only only about about one one half half mile mile north northof ofthe theNiagara Niagarafault.) fault.) The matrix within the pebbly units is composed of quartz, fine-grained fine-grained hematite or magnetite, 965), who showed that or both. The unit was studied studied in detail by Nilsen (1 (1965), that it consists of of two conglomeratic subunits separated by a quartzite and pebbly quartzite subunit. His paleocurrent analysis indicates a predominant current flow toward the southeast in a shallow, near-shore near-shore basin. basin. Structurally, there appears appears to to be be aa cleavage-parting cleavage-partingparallel parallelwith with bedding. bedding.Bedding Bedding 65' SW. Stretched Stretched N55OW, parallel parallel to the strike of the Niagara Niagara fault, and and dips 65° strikes N55°W, pebbles pebbles plunge plunge steeply steeply southwest. southwest. Iron-formation and Stop 3-3. Riverton Iron-formation and Wauseca Wauseca Pyritic Pvritic Member Member of Dunn Dunn Creek Slate (SW %, SW SW¼, %, sec. sec.34, 34,T. T. 40 40N., N.,R. R. 18 18 E.) E.) Slate (SW¼, - named by Dutton (1971) and This stop is a'ong along the axis of the Commonwealth Commonwealth syncline named is within Dutton's Dutton's Brule Brule River River block, block, now now included includedas as the the southeastern southeasternextension extensionof of the the River-Crystal Falls allochthon. Roadcuts Iron River-Crystal Roadcuts on Highway Highway N N about about 2 miles miles southeast southeast of of Florence, Wisconsin were made made after Dutton's Dutton's mapping mapping of the area, but but reveal reveal geology geology very much as inferred inferred on his maps. The roadcuts consist of alternating units of black, pyritic slate of the Wauseca Pyritic Member, the uppermost uppermost member of the Dunn Creek iron-formation of the overlying Riverton IronSlate, and cherty carbonate and silicate iron-formation formation. Individual Individual lithologic units of slate and iron-formation iron-formation are generally a few tens of feet thick and and the contact between between them is well exposed in many places along the roadcut. roadcut. The interleaving interleaving of the the two two units units is is probably probably aa result result of of repetition repetitionby by tight tight folding, folding, contacts cannot be traced around fold hinges within the limits of the roadcut. although contacts The lithologies lithologies could could be be stratigraphically stratigraphically interlayered, interlayered, but but such such broad-scale broad-scaleinter-bedding inter-bedding is not known elsewhere in the district where a few feet, feet, at most, of transitional beds occur between workings. We think it more between the two units where exposed in many mine workings. likely that folds with amplitudes greater than the height of the roadcut roadcut cause the repetitions with a geometry like those displayed in parts of figure 7 (p. 19) at the Buck mine in which fold amplitudes are many times greater then fold wave-lengths. Smallscale tight folds showing this type of geometry are common within individual lithologic units in this roadcut. The Wauseca consists consists of multiply multiply deformed, black, ferriginous, and and highly pyritic slate with thin - up to a few inches thick - beds of chert. The Wauseca Wauseca was classified as a sulfide fades iron-formation classified sulfide facies iron-formationby (1954) and and was his principal byJames James (1954) was his principal example for defining defining this facies. Foliation is generally parallel to the bedding and axial Foliation planar folds in N55OW,85°NE, 85ONE, and and is is parallel parallelto to the the trend trend in bedding. bedding. ItIt is is roughly roughly oriented oriented N55°W, of the Brule Brule River River block. block. The The rock rock is is folded foldedisodlinally isoclinally with with axes axes that that plunge plungevariably variablybut but most plunge steeply northwest (see stereoplot on fig. 3-4,B). These small folds are most likely likely parasitic to the Commonwealth Commonwealthsyncline. syncline. The Wauseca Wauseca Pyritic Pyritic Member Member has has some anomalous anomalous chemical characteristics. Ongoing studies by the USGS of the geochemistry geochemistry of black slates in the region found that a roadcut contained contained 1230 1230 parts parts per million composite of 30 feet of black black slate in in this roadcut arsenic and 14 14 parts per million selenium, both values being the highest that we have 71 �detected in northern northern Wisconsin Wisconsin and the upper peninsula detected peninsula of Michigan. Michigan. Studies are continuing Wauseca might might contribute contribute to aa regional regional continuing to determine to what degree the Wauseca arsenic anomaly icial materials. surficial materials. arsenic anomaly in insurf 3-4. Riverton Riverton Iron-formation Iron-formation along Stop 3-4. alonq eastern limb limb of of Iron Iron River-Crystal River-CrystalFalls Falls basin (NW basin (NW¼, ?A,sec. sec.31, 31,T.T.42 42N., N., R. R. 32 32 W.). W.). Typical Riverton Riverton Iron-formation Iron-formationis is exposed exposed in in cuts cuts along along two sub-parallel sub-parallel abandoned abandoned railroad single-track road. The exposures railroad spurs, the northern of which is drivable as a single-track are approximately in the middle of the Riverton, which in this area strikes NNE and dips of the area are steeply to the west into the Iron Iron River-Crystal Falls basin. Detailed maps of Professional Paper 570 by James and others (1 (1968) 968) and detailed included in USGS Professional lithologic descriptions of the Riverton are in the same publication. In general, the Riverton, where unaffected unaffectedby by secondary secondary oxidation, oxidation, which which is is widespread widespreadin inthe the district, district,isis thin-bedded and consists mostly of interbedded chert and siderite. Iron silicate minerals, thin-bedded and consists mostly interbedded and Iron silicate minerals, mostly stilpnomelane, are only locally important. Thin partings of argillaceous and carbonaceous carbonaceous material material are common common and and some pyritic pyritic layers layers are also widespread. Iron-formation at stop 3-4. Figure 3-6. Tight folds in the Riverton Iron-formation The rock seen here here is is generally only weakly oxidized so preserves many of the original sedimentary minerals and structures. Like Like all of the Iron Iron River-Crystal River-Crystal Falls allochthon, 72 I..,— �___ metamorphic metamorphicgrade gradeisisextremely extremelylow lowand andno nometamorphic metamorphiceffects effectsare aredetectable detectableininhand hand specimens. specimens.Small-scale Small-scalefolds foldsare arevery verywell welldeveloped. developed.Most Mostplunge plungegently gentlyto tomoderately moderately toward towardthe thenorth northor orsouth south(fig. (fig.3-4,C). 3-4,C). -N ) ' V — 7 Xr (J'-',p / - - - () v4 / - - La M — u3': 1UO1S - - 1650 iom 5000 reek ., ' I \ 4 -— - - 632W 4Q 1DJO METERS 5000 0 I 0 10000 4O FEET 20000 Figure Figure3-7. 3-7.Part Partofofthe theIron IronRiver River1:100,000-scale 1:100,000-scaletopographic topographicmap mapshowing showingthe thelocation location ofoffield trip stops 3-5 through 3-9. field trip stops 3-5 through 3-9. 73 �Stop 3-5. 3-5. Dunn DunnCreek CreekSlate Slatenear nearAlpha, Alpha,MI MI(NW (NW SE1/4, %, sec. sec. 7, 7,T. T. 42 42 N., N., R. R. 32 32 W). Stop ¼,%,SE The Dunn Creek Slate is the lowermost lowermost unit of the Paint River Group as defined by James (1958). (1958). It lies, probably conformably, on the Badwater Greenstone Greenstone and has a gradational upper contact with the Riverton Iron-formation. Iron-formation. It is a unit of greatly varied Iron River-Crystal River-Crystal Falls basin, and is lithology and thickness considering the entire Iron defined more as a stratigraphic interval interval than by a distinctive lithology. James and others (1968) described the variations in lithology and thickness. The area near the village of (1968) Alpha contains the best exposures and probably the greatest stratigraphic thickness of Dunn Creek. The The detailed detailed mapping mapping of the the area area by by the the USGS USGS as as part part of the the Iron Iron the Dunn 968) also produced a series River-Crystal Falls study presented presented in James and others (1 (1968) of more detailed reports published by the Geological Survey of Michigan. The report on area (Pettijohn (Pettijohn and and others, 1969) 1969) subdivided subdivided the Dunn Dunn Creek into into three the Alpha area mappable units based on a unit of distinctive laminated slate that forms the middle part of the formation and separates upper and lower units of gray to black, cherty, in part part sideritic, slate. The mapping of these units units was very useful in tracing the northward northward extension of the Mastodon Mastodon anticline, but for reasons not known to us these internal internal units on maps in in Professional Professional Paper Paper 570. 570. According to Pettijohn and others were not shown on (1969) (1969) the exposures seen at this stop are in the lower unit of the Dunn Creek Slate and lie about 1500 feet west of the trace of the axial plane of the Mastodon Mastodon anticline. This MI (fig. outcrop is located on the north side of Highway N about one mile east of Alpha, Ml 3-7). The outcrop consists of black ferruginous slate with more massive, massive, openly folded, cherty layers. There is a slatey foliation parallel to bedding in places. Elsewhere foliation N40°W is axial planar to the open open folds folds in in the the massive massivelayers layers and and is is generally generallyoriented orientedN40°W, 75ONE. 75°NE. A stereoplot of of fold fold axes from the broader region in this area (fig.3-4,D) shows that the folds plunge plunge steeply steeply to gently gently north-northwest. north-northwest. Fold Fold axes axes reported reported by Pettijohn Pettijohn plunged from 40-80 degrees northwest. and others (1969) (1969) also plunged northwest. It is clear from the relationships mapped in this area that even regional folds like the Mastodon anticline have steep plunges plunges within the allochthon. allochthon. Iron-formation at Stop 3-6. Riverton Iron-formation at the the Paint Paint River River Dam Dam in in Crystal Crystal Falls. Falls. Mt. MI. (Center, sec. (Center, sec. 20, 20, T. T. 43 43 N., N., R. R. 32 W.) 500-800 feet thick in the Crystal Falls area and is mostly The Riverton Iron-formation Iron-formation is 500-800 inter-laminated siderite Exposures inter-laminatedchert and siderite; Exposuresjust just below belowPaint PaintRiver RiverDam Damprovide providethe the of the the Riverton and of of the best example in the district both of the primary Iithology lithology of characteristic of the Iron extraordinary structural complexity of the deformation characteristic Iron RiverCrystal Falls allochthon. A sketch map of the outcrop, published by James and others (1968), (1968), is reproduced reproduced here to provide a view of the entire outcrop, some of which is flooded periodically, depending on the rate of flow of the Paint River. The exposure chert-siderite lithology, which comprises the bulk of of the Riverton IronIronshows the typical chert-siderite formation throughout the district. district. There are are also good good examples examples of a silicate silicate ironironformation, a lithologic type unique to the Crystal Falls area. These were described by "......at at the the apron apron of of the the Paint PaintRiver River dam dam in in Crystal Crystal James and others (1968) (1968) as follows: ".. Falls the chert-siderite chert-siderite iron-formation iron-formation contains layers that consist dominantly of stilpnomelane. These layers, which are from a fraction of an inch inch to several inches thick, distinguished in the outcrop from the chert and siderite by their inferior are readily distinguished inferior hardness hardness and by by their their peculiar peculiar closely closely spaced spaced cross cross fracture fracture that that resembles resemblesthat that of ofsome some coal." Some Somebeds bedsare arealso alsorelatively relativelyrich richin inpyrite. pyrite. 74 �Inclined Vertical Overturned Strike and dip of bedding . ......... ... .......... .... . Bedding plane Trace of selected bedding plane Traced o r r o ~ nm Daubed w h e w u.vproxiwzlely locnted -.... . t k oulcw~ Fault,showing relative movement Dotted where concealed 1010 I L 00 , , I l 1010 I I 20 20 I I 30 30FOCI FEET I SurveyedbybyH.HL. L. Jamesand and Srveyod James W W 5, S Why,, White 948 1948 I Figure3-8. 3-8.Sketch Sketchmap mapofofRiverton RivertonIron-formation Iron-formationatatPaint PaintRiver RiverDam, Dam,Crystal CrystalFalls, Falls, Figure Michigan. Michigan.(James (Jamesand andothers, others,1968, 1968,fig fig21) 21) showsthat thatmost mostfolds foldsplunge plungesteeply steeplyto tothe thewest west(note (notethat thatnorth northisistoward toward Figure3-4, 3-4,EEshows Figure 55degrees degreestotovertical. vertical.But, But, theupper upperright rightininfigure figure3-8). 3-8).Plunges Plungesare aresteep, steep,varying varyingfrom from55 the onthe themost mostnortheasterly northeasterlypart partofofthe theoutcrop outcropfolds foldsplunge plungetoward towardthe thenorth northatatabout aboutright right on angles anglesto tothe theother otherfolds. folds.These Theseabrupt abruptvariations variationsininfold foldgeometry geometryare arecharacteristic characteristicofof the thedistrict districtas asrevealed revealedininthe themany manyunderground undergroundmines minesthat thatwere weremapped mappedduring duringthe the periodof ofmining miningininthe theearly earlytotomid-i mid-1900's. additiontotothe thefold foldaxes axesthat thatwe wehave have period 900's. InInaddition Larue(1987), (1987),who who measured,this thisexposure exposurewas wasstudied studiedinindetail detailby byUeng Uengand andLarue measured, recognized44 phases phasesof of deformation deformationand andprovide provideaadetailed detaileddiscussion discussionof ofthe thestructural structural recognized history.To Togeneralize generalizetheir theirconclusions, conclusions,the thefolds foldsthat thatplunge plungesteeply steeplytotothe thewest westformed formed history. ininan second aninitial initialmajor majorfolding foldingphase phaseand andthose thosethat thatplunge plungesteeply steeplytotothe thenorth northare areaasecond phase. phase. 75 �Stop 3-7. Badwater Greenstone north northof ofCrystal CrystalFalls Falls(sec. (sec.18, 18,T.T.43 43N., N.,RR32 32 W.) W.) Badwater Greenstone Alloutcrops outcropsininthis thisarea areaare areon onprivate privateland landand andshould shouldnot notbe bevisited visitedwithout without NOTE: All prior permission permission of property property owners. owners. The Badwater Greenstone is a thick succession of submarine mafic volcanic rocks that underlies the Dunn Creek Slate, probably conformably, although evidence is scant. As the Paint River Group because discussed above, we propose to include the Badwater in the structural evidence indicates that it is part of the Iron Thus structural Iron River-Crystal Falls allochthon. Thus it has definable stratigraphic stratigraphic relationships relationships to to the the overlying overlying Paint Paint River River strata stratabut but isisin in fault fault contact with all other surrounding units. James and others (1968) (1968) described rocks of the Badwater as .somewhat varied in detail, but in in general general they they are are massive massivefine-grained fine-grained Badwater as ". ...somewhat dark-greenish-gray rocks that consist chiefly of chlorite, actinolite, hornblende, dark-greenish-gray rocks consist chiefly of chlorite, actinolite, hornblende, albite, albite, clinozoisite-epidote, and carbonate. carbonate. Ellipsoidal Ellipsoidal and and agglomeratic agglomeratic structures structuresare are common. common. fragmental volcanics . The rocks rocksalmost almostcertainly certainly originated originatedas as sub-marine sub-marine flows and fragmental entirely, of of primary primary basaltic basaltic composition." composition." The formation and are dominantly, ifif not entirely, carbonate-rich slate and thin ironcontains minor sedimentary rocks consisting of carbonate-rich formation. James James estimated estimated that the the Badwater Badwater is is as as much much as as 15,000 15,000 feet thick thick in in places, places, but its thickness varies varies greatly greatly within within the the district. district. I'. . . At this stop the Badwater Greenstone is a strongly deformed pillow basalt and pillow stretched volcanic clasts and breccia. The basalt is weakly to strongly foliated, and has stretched N80°W85°NE. 85ONE. Stretched Stretched clasts clasts and and pillows pillows pillows. Foliation is generally oriented oriented N80°W, plunge steeply northwest as shown in figure 3-4, F. These steeply plunging structures, similar to those inherently part of the Niagara suture zone rocks from here southward to the Niagara Niagara fault, are are the the principal principal evidence evidence that the Badwater Greenstone Greenstone is is part part of the Iron Iron River-Crystal River-Crystal Falls Falls allochthon allochthon and and is is structurally structurally detached detachedfrom from more moresimply simply deformed rocks to the north that will be seen at stop 3-8. Stop 3-8. 3-8. Michiciamme Michiqamme Formation Formation north northof of Crystal CrystalFalls, Falls,MI MI(SW (SW¼, %, NE NE ¼, %, sec. 12, T.43N., T. 43 N., R.33W.) R. 33 W.) The Michigamme Formation, a thick sequence of clastic rocks, dominantly graded and bedded graywackes and thinner-bedded siltstones, underlies a very large area north and east of the Iron River-Crystal Falls allochthon. Broad, open folds with steeply dipping, Iron River-Crystal Falls more or less less east-west east-west trending, trending, axial axial planes planes and and shallowly-plunging shallowly-plungingfold foldaxes axes scattered outcrops of varicolored slate are characterize deformation. At this locality scattered located on the north side of the Paint River, about six-tenths six-tenths of a mile west from the Highway-141 this characteristic fold fold geometry. Prominent slatey Highway-141 bridge and display well this cleavage cleavage is oriented oriented N75°W, N75¡W80°NE, 80°NEand and intersections intersections of of bedding beddingand andcleavage cleavageindicate indicate that fold axes plunge 15 15 degrees west. Outcrop-scale Outcrop-scale folds with this orientation are also present here. This nearly horizontal, westerly plunge of the fold axes is characteristic characteristic of fold axes found in the Michigamme Formation Formation along the north and east sides of the Iron Iron River-Crystal Falls district, and contrasts contrasts sharply with the steeply plunging orientation of Greenstone and overlying Paint fold axes and stretch stretch lineations lineations found in the Badwater Greenstone the rocks here and those at River Group strata. This contrast in structural style between the all other stops seen to this point indicate that there is a fundamental structural structural boundary boundary between the Badwater Greenstone and other rocks of the allochthon and the Michigamme Michigamme Formation. The outcrops visible south of the River are Badwater Greenstone, Greenstone, indicating indicating that the boundary boundary lies immediately immediately south of these Michigamme Michigamme 76 �probably beneath beneath the river. We have have proposed proposed the name name Paint Paint River River fault for outcrops, probably structure and interpret interpret itit to be be the basal basal detachment of the allochthon. this structure Stop 3-9. Little Tobin Lake Lake Granite Granite (NE (NE 1/4, %, see. Stop Little Tobin sec.21, 21,T. T. 42 42 N., R. 32 32 W.) A dike-like dike-like body of granite, called called the little Tobin Lake Lake dike dike by James and others others (1968), intrudes the Badwater intrudes Badwater Greenstone Greenstone and forms a body about 2 miles long long and a quarter quarter of a mile wide (fig. 3-1). The The rocks rocksare are gray gray to to reddish reddish gray gray and and finefine- to to medium-grained medium-grained and are composed dominantly of microcline microcline microperthite, microperthite, albite, and mica. In In this area the Badwater Badwater is intensely intensely folded on the Mastodon anticline and the adjacent Tim Bowers syncline, syncline, both both of which which are are traversed traversed by by the the dike, dike, which which maintains maintainsaa nearly nearlystraight straighttrace. trace. emplaced after the major major folding of the Paint Paint River River Group. Group. The dike clearly seems to be emplaced It yielded a U-Pb zircon date of 1833+/-6 Ma (Schneider and others, 2002), which places emplacement of the Iron Iron River-Crystal River-CrystalFalls Falls allochthon. a younger limit on the age of emplacement bodies of granite granite lie between here and Crystal Falls, about 5 miles Several other smaller bodies to the north. north. These, too, are apparently apparently post-tectonic post-tectonic and and occur both both within the allochthon and in the structurally lower Michigamme Formation. James and others (1968) (1 968) states "The granitic granitic bodies bodies all all have have been been sheared sheared to to some some extent, extent, and andall allhave have been metamorphosed". Perhaps these granites record a younger, circa 1650 Ma been record circa 1650 deformation as has been been proposed proposed for this area by Romano Romano and others (2000). (2000). deformation 77 �REFERENCES REFERENCES CITED CITED K., and and Klasner, Klasner, J.S., J.S., 1989, 1989,Tectonic Tectonicimplications implicationsof of metamorphism metamorphismand andgravity gravity Attoh, K., field in in the Penokean Penokean orogen of northern northern Michigan: Tectonics, Tectonics,v. v. 8, 8, p. p. 911-933. 91 1-933. 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Shervais, J.W., 2001, Birth, Birth, death, death, and and resurrection: resurrection:the the life life cycle cycle of of suprasubduction suprasubduction zone ophiolites: Geochemistry Geophysics Geosystems, vol.2, Paper ophiolites: Geochemistry Geophysics Geosystems, Paper number number 2000GC000080 http://g2000GC000080 [20,925 [20,925 words, 88 figures, figures, 33 tables]. tables]. On-line On-linepublication publicationat at http:llgcu bed .o rg cubed.org. Shervais, J.W. and and Kimbrough, Kimbrough, D.L., D.L., 1985, 1985, Geochemical Geochemicalevidence evidence for for the the tectonic tectonic setting of the Coast Range Range ophiolite: ophiolite: aa composite composite island island arc-oceanic arc-oceanic crust crust terrane terrane in in western v. 13, 13, p. p. 35-38. 35-38. western California: California: Geology, Geology, v. 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Sims, P.K., P.K., Schulz, K.J., DeWitt, DeWitt, E., E., and and Brasaemle, Brasaemle,B., B., 1993, 1993, Petrography Petrographyand and geochemistry geochemistry of Early Early Proterozoic Proterozoicgranitoid granitoid rocks rocks in in Wisconsin Wisconsin magmatic magmaticterranes terranesof of the the Penokean orogen, northern Penokean northern Wisconsin—a Wisconsin-a reconnaissance reconnaissance study: study: U.S. U.S. Geological Geological Survey Bulletin Bulletin 1904-J, lgO4-J, p. p. J1-J31. J1-J31. Trent, V.A., 1976, 1976, The The Emperor Emperor Volcanic Volcanic Complex Complex of of the the eastern easternGogebic Gogebic Range, Range, G.V., and Wright, Wright, W.B.,eds., Changes Changes in in stratigraphic stratigraphic nomenclature nomenclature Michigan, in Cohee, G.V., by the U.S. Geological Geological Survey, 1975: U.S. Geological Survey Bulletin 1422-A, p. 69-74. 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Great 85 �FIELD FIELD TRIP TRIP 44 LIFE LIFE CYCLE CYCLE OF OF AN AN IRON IRON DEPOSIT—THE DEPOSIT-THE REPUBLIC REPUBLIC MINE MINE FROM ORE GENESIS GENESIS TO MINE MINE RESTORATION RESTORATION William F. Cannon, USGS, Reston, VA; VA; John G. G. Meier, Cleveland Cleveland Cliffs Cliffs Iron Iron Co., Ishpeming, Ishpeming, MI; Thomas Thomas Waggoner, Waggoner,Geologic Geologic Consultant, Consultant, Negaunee, Negaunee, MI MI Republic pit pit in in 1973, 1973, approximately approximatelyat at mid-point mid-pointof of production. production. The Republic The The Republic RepublicWetlands Wetlands Preserve Preserveinin2002, 2002, constructed constructedon onthe theformer formertailings tailings basin basin of of the the Republic Republicmine. mine. �FIELD FIELDTRIP TRIP 44 LIFE LIFE CYCLE CYCLE OF OF AN AN IRON IRONDEPOSIT—THE DEPOSIT-THE REPUBLIC REPUBLICMINE MINEFROM FROMORE OREGENESIS GENESIS TO TO MINE MINERESTORATION RESTORATION William William F. Cannon, Cannon, USGS, USGS, Reston, Reston, VA; VA;John John G. G.Meier, Meier, Cleveland ClevelandCliffs CliffsIron IronCo., Co., lshpeming, Ishpeming, MI; MI; Thomas ThomasWaggoner, Waggoner,Geologic GeologicConsultant, Consultant,Negaunee, Negaunee, MI MI Brief Brief history historyof of mining miningand andgeologic geologicstudies studies Mmmci: M i n i : The The Republic RepublicMine Minein inwestern westernMarquette MarquetteCounty, County,Michigan Michiganproduced produced more more than than 75 75million milliontons tonsof ofiron ironore oreduring duringtwo twogenerations generationsofofmining. mining.The TheRepublic RepublicIron Iron Company was was organized organized in in Marquette, Marquette, Michigan Michigan in in October October 1870 1870 and and mining mining at at what is is Company now now the the site site of of the theclosed closedRepublic RepublicMine Minebegan beganinin1871. 1871.Operated Operatedby bythe theRepublic RepublicIron Iron Company, Company, the the mine mine was was producing producing 235,000 235,000 tons tons of of hard, hard, specular specular hematite hematite by by 1880. 1880. Mining Mining operations operations were were conducted conducted in in open open pits pits and and 14 14 shafts. shafts. The The greatest greatest shaft shaft depth depth was was 2,910 2,910 feet. feet. The TheCleveland-Cliffs Cleveland-CliffsIron IronCompany Companyacquired acquiredthe the mine minein in1914 1914and and operated operateditituntil until1926 1926when whenititclosed. closed.Shipments Shipmentsfrom from the the stockpile stockpilecontinued continueduntil until1937. 1937. The 937 was Thetotal totalore oreshipped shippedfrom from1872-1 1872-1937 was more morethan than 8.5 8.5 million milliontons. tons.After Afterthat, that,the the mine 950s. minesat satidle idleuntil untilthe theearly early11950s. In In 1952, 1952, construction construction of of the the modern-day modern-dayfacilities facilities began began and and the the open open pit pit mine mine was was in in production productionby by1956, 1956,mining mininglow-grade low-gradeiron ironore ore that that was was concentrated concentratedat at the the mine's mine'splant plant and andpelletized pelletizedat at the the former formerEagle EagleMills Millspellet pelletplant plantin inNegaunee NegauneeTownship. Township. In Inthe the early early 1960's 1960'san anexpansion expansionof of the themine, mine,including includingpelletizing pelletizingand andadditional additionalconcentrating concentrating equipment, equipment, took tookplace. place.This Thisexpansion expansionwas was completed completedin in1964, 1964,at at which which time time Republic Republic was was producing producingmore morethan than22 million milliontons tons of of iron ironore orepellets pelletsannually. annually. AA unique uniquepart partof of the the mining miningoperation operationat at Republic Republicwas was the the in-pit in-pitcrushing crushing system, system, which which performed performedcoarse coarse crushing crushingnear nearthe the floor floor of of the thepit pitand andtransported transportedthe thecrushed crushedore orefrom fromthe thepit pitto tothe the plant, plant,aavertical verticallift liftof of 647 647feet, feet,via viaaa2,814-foot 2,814-footconveyor conveyorininan aninclined inclinedtunnel tunnelin inthe the footwall footwallof of the theore orebody. body. In In1981, 1981,with with the the iron ironore oreand andsteel steelindustry industryin inaa deep deep recession, recession,operations operationsat at Republic Republic were were suspended. suspended. When Whenhope hopefaded fadedthat thatmarket marketconditions conditionswould wouldimprove improveto to allow allowpellet pellet production productionto to resume, resume,Republic Republicwas was permanently permanentlyclosed closed in in early early 1996. 1996. More More than than 45 45 million milliontons tonsof of pellets pelletswere wereproduced producedatatthe theRepublic Republicplant plantbetween between1964 1964and and1981. 1981. Including Includingpellets pelletsproduced producedat at the theEagle EagleMills Millsplant plantand andsome someproduced producedat at the the Humboldt Humboldt plant plantwith withRepublic Republicore, ore,Republic Republicaccounted accountedfor formore morethan than63 63million milliontons tonsofofpellets pelletssince since 1956. 1956.When Whenthe themine mineclosed closedinin1981, 1981,Republic Republicemployed employedmore morethan than700 700people. people. In Inaddition additionto to being beingaamajor majoriron-producing iron-producingarea areafor for more morethan thanaacentury, century,the thegeologic geologic relationships relationshipsshown shownininthe themine minearea areaand andother othernearby nearbyexposures exposureshave haveplayed playedan an important importantrole rolein indeveloping developingconcepts conceptsof of the theorigin originof of iron-formations iron-formationsof ofvarious various sedimentary sedimentaryfacies, facies, their theiralteration alterationthrough throughregional regionalmetamorphism, metamorphism,and andaarather rather complex complex history historyof of iron ironconcentration concentrationthat that led ledto tothe theformation formationof of the thehigh-grade high-gradeores oreson on which whichthe theinitial initialmining miningwas wasbased. based.This Thistrip tripvisits visitssome someof of these theseexposures exposuresand andprovides provides aareview reviewof of the the rich richhistory historyof of studies studiesof of the thegeology geologyof of iron irondeposits depositsininthis thispart partof of the the Lake LakeSuperior Superiorregion. region. 87 �Geologic Geoloqic studies: The Thefirst firstrecorded recordedgeologic geologicobservations observationsat at Republic Republic were made by Government Government Land Office surveyors in 1846 1846 while surveying the township and range grid of the area. They clearly recognized recognized the importance importance of the ore that was readily visible on what was then a prominent prominent hill hill along along the the Michigamme MichigammeRiver. River. Five years later the area was visited by J.W. Foster Foster and and J.D. Whitney, Whitney, U.S. U.S. Government Government Geologists, "The 1851): "The Geologists, who provided provided the the following description description (Foster (Foster and and Whitney, Whitney, 1851): largest mass mass observed by us in this region occurs on the left bank of the Machi-gamig, Machi-gamig, in section section 7, of township township 46, range range 29, 29, and and traces traces of of itit are are to to be be observed observedin in several several of of the the adjoining adjoining sections. sections. ItIt here here rises rises in in aa nearly nearly vertical vertical cliff to to the the height height of of one one hundred hundredand and thirteen thirteen feet, and and is is somewhat somewhat variable variable in in purity. purity. For For the the most most part part itit has has aa slaty slaty cleavage, cleavage, and, on on close close inspection, inspection, is is observed observed to be be composed composed of alternating alternatingbands bands of of micaceous micaceous specular iron and quartz, tinged red by the peroxide of iron: but there are occasional occasional belts belts which display display a granular texture, and and apparently apparently possess possess a greater greater degree degree of purity. purity. These These laminae laminae are are nearly nearly vertical, vertical, exhibiting exhibiting few contortions, contortions, and and range range with so much much uniformity, uniformity, that the observer observer would would be be inclined inclined to refer refer both both the the slates slates and and the iron iron to aa common common origin. origin. Interlaminated Interlaminatedwith with itit is is aa band band of of rock rock composed composedmainly mainly of of white, granular granular quartz, quartz, with with traces traces of feldspar, feldspar, through through which which are are disseminated disseminated particles, particles, as as well as rounded rounded masses, of specular specular iron. iron. ItIt is is difficult difficult to to pronounce pronounce whether whether this this is is aa conglomerate conglomerate or or breccia." breccia." During the ensuing 20 years, the wave of exploration and development along the Marquette Marquette Range reached reached Republic. In 1871, shortly before before mining was initiated, the ore was was described described by by Swineford Swineford (1871). (1871). His His description description of Smith Smith Mountain, Mountain, as as itit was then then called, called, was "The "The mountain mountain rises rises to aa height height of nearly, nearly, ifif not not quite, quite, 1,000 1,000 feet feet above above the the waters waters of the the Michigammi Michigammi River, River, which which runs runs near near its its base, base, and and the explorations explorations made made last summer summer reveal reveal the the existence existence of an an immense immense body body of ore, which can be traced over a last mile mile by by outcrops outcrops alone. The The writer writer visited visited this this mountain mountain last last summer summer and and has has no no hesitation hesitation in in saying saying the he he believes believes itit to be be by by far the the most most valuable valuable property property yet discovered. discovered. The The ore ore is is aa very pure pure magnetic, magnetic, similar similar to to that that of of the the Washington Washington and and Champion. Champion. The The elevation elevation is is such such that the ore ore can can be be mined mined at aa comparatively comparatively trifling trifling cost, cost, and and itit would would be be an an easy easy matter matter to to mine mine and and ship ship aa hundred hundred thousand thousand tons tons in in the the first first year after after commencing commencing operations." He Hefurther furtherstates states"It "It was was originally originally discovered discovered by by S.C. Smith, Smith, Esq., Esq., of of Marquette, Marquette, from from whom whom itit takes takes its its name, name, and and who who threw threw away away an an immense immense fortune fortune in its sale at a nominal price." Apparently Apparently Mr. Mr. Swineford Swineford had had a rather optimistic optimistic eye, both both in in estimating estimating topographic topographic relief relief and and ore ore reserves. reserves. In 1873, 1873, T.M. Brooks Brooks published published aa comprehensive comprehensive survey survey of the iron-bearing iron-bearingrocks rocks of of the the In Marquette range range including including both both descriptions descriptionsand and an an atlas. atlas. At Republic Republic he he determined determined the the Marquette detailed detailed internal internal stratigraphy stratigraphy of of the the iron-formation iron-formationand and produced produced the the first first detailed detailedmap map of of the deposit. deposit. He He recognized recognized the synclinal synclinal character character of the host host rocks rocks and and described described the the several several lithologic lithologic types of iron-formation iron-formation and and ores ores exposed exposed in in the area. Like Like Swineford, Swineford, Brooks was was very very impressed impressed by by the the large large ore ore exposure exposure and and wrote wrote "The "The immense immense mass mass of of Brooks pure Va of of the the pure specular specular ore, which which was was naturally naturally exposed exposed near near the center center of the the north north ½ southeast Sec.7, 7,T. 1. 46, 46, R. R. 29, 29, could could leave leave no no reasonable doubt in the mind mind of the southeast 1/4 % ofofSec. experienced experienced observer, observer, that this deposit of ore ore was one of the the largest, this deposit was one largest, ifif not not the the largest, largest, in in the the Marquette Marquette region. region. This This outcrop, outcrop, the the extent extent of of which which is is shown shown on on the the map map of of the the Republic Mountain, Mountain, being being there marked marked "pure "pure specular specular ore", is, is, so so far far as as II know, know, the the Republic largest outcrop outcrop of of any any equally equally rich rich ore, ore, ever ever found found in in the the United United States." States." largest 88 �more detailed detailed and accurate accurate description of the geology geology was published published in 1897 1897 by Henry A more of the classic USGS Monograph 28 by Van Hise and Bayley Lloyd Smyth as Chapter VI of on the geology geology of the "Marquette "Marquette Iron-bearing Iron-bearingDistrict" District" (Van (Van Hise Hise and Bayley,1897). Bayley,1897). By original mines mines were well established and an understanding understanding of the that time, the original processes of ore-formation ore-formation was being being developed developed by Van Hise and Bayley. Smyth's Republic were an important part of that research research and are discussed in observations at Republic more detail below. Among Smyth's contributions were recognition of the detailed sequence of the Early Early Proterozoic Proterozoic strata, which has has not been been significantly stratigraphic sequence modified since, with with the exception exception of some some changes changes in in nomenclature, nomenclature,and anddelineating delineatingthe the modified With regard regard to to the the ore ore deposits, deposits, Smyth's Smyth's maps maps plunging synclinal geometry of the strata. With were the first (and (and only) ones that showed showed a distinct separation between between dominantly hard specularite ores and dominantly magnetite ores. He correctly surmised that the hard ores formed formed by secondary secondary concentrations concentrations within the iron-formation, iron-formation,in part part through leaching of original original siliceous beds, and also recognized recognized the detrital detrital character of iron iron enrichments Goodrich Quartzite. Quartzite. enrichments in the basal basal conglomerates conglomerates of the Goodrich Much later, in the early 1950's, 1950's, Harold Harold James of the USGS USGS published published two fundamental papers papers in which Republic Republic played a major role. His work on zones of regional metamorphism metamorphism (James, 1955) 1955) was strongly influenced influenced by the Republic Republic area, which was at the center of a zone of sillimanite grade grade metamorphism metamorphism of his his Republic Republic metamorphic metamorphic node. node. The The Republic Republic area area was was the the sole sole locality locality where where several severallithologic lithologictypes typesof of ironironformation were within within this this high-grade high-gradezone zone and and was was the the principal principalexample exampleupon uponwhich whichhe he defined the very high-grade high-grade metamorphic metamorphic effects effects on iron-formation. iron-formation. A A fundamental fundamental outcome outcome of that study was that even very intense metamorphism metamorphism does not change change the majority of the iron, iron, an an idea idea contrary contrary to widely held held concepts original oxidation state of a majority at that time. The relationships relationships upon upon which he based based that conclusion are clearly displayed in a glacially polished outcrop at the Kloman Kloman Mine (fig. 4-1, stop 4-1). James's studies of the importance importance of sedimentary sedimentary conditions in determining the mineralogic mineralogic character of iron-formations, published published as his his famous "Sedimentary "Sedimentary facies of iron-formation" iron-formation" (James, 1954), also drew heavily heavily on relationships relationships seen seen in in the the Republic Republicarea, area, and and again again the the exposures exposures at Kloman Kloman mine mine were were instrumental instrumentalin inshowing showingthe therelationship relationshipbetween between bedding bedding characteristics characteristicsand and mineralogy mineralogyof of iron-bearing iron-bearingphases. phases. The most most recent recent comprehensive comprehensive geologic geologic study of the Republic Republic area area was conducted by Cannon (1975) who mapped mapped the geology of the iron-bearing iron-bearingsequence and and surrounding surrounding Cannon (1975) the rocks, including detailed mapping in the open-pit. Because the pit was developed in the same stratigraphic unit unit that contained contained the high-grade high-gradeores ores mined mined previously previously underground, exposures of those high grade ores and their relationships to host rocks rocks were abundant in the pit and were a fundamental component of Cannon's study of the component Cannon's origin of hard hard iron iron ores of the Marquette Marquette Range Range (Cannon, (Cannon, 1976). 1976). Unfortunately Unfortunately those those exposures exposures are are now now flooded flooded in in the the abandoned abandonedpit. pit. Geology of the Republic Republic area The Republic Republic area area has has many many similarities similarities to the Marquette Marquette Iron Iron Range Range to the north north and and east and and is is generally generally considered considered to be be an extension of it. Paleoproterozoic Paleoproterozoic strata strata of the Marquette Range Supergroup are preserved in a syncline, the Republic trough, between uplifted blocks of Archean gneisses. The stratigraphic units defined in the Marquette Iron Range, can be applied in the Republic trough as well, and to some extent can be traced directly into into the Marquette Marquette Range. Range. The The Paleoproterozoic Paleoproterozoic units units in the Republic Republic area area consist of a basal sequence of the Ajibik Ajibik Quartzite Quartzite and and Siamo Siamo Slate Slate which were basal clastic sequence 89 �mapped mapped as as aa single single unit unit by by Cannon Cannon (1975) (1975) because because very limited limited exposure exposure does does not not allow allow accurate accurate mapping mappingof of aa contact contactbetween betweenthem. them.These Theseare areoverlain overlainby bythe theNegaunee Negaunee Iron-formation, Iron-formation,the the principal principaliron-bearing iron-bearingunit. unit.The TheGoodrich GoodrichQuartzite Quartzitelies liesunconformably unconformably on on the the Negaunee. Negaunee. The The unconformity unconformity was was well well exposed exposed in in the Republic Republic pit pit before before flooding. flooding. The Thebasal basalpart partof of the theGoodrich Goodrichisisaaferruginous ferruginousconglomerate, conglomerate,which whichgrades grades upward into quartzite. The youngest strata are biotite-garnet schist with beds of upward into quartzite. The youngest strata are biotite-garnet schist with beds of impure impure quartzite, quartzite, which which make makeup upthe the Michigamme MichigammeFormation. Formation.The The Michigamme Michigammealso also contains contains aa basal unit unit of silicate-magnetite silicate-magnetiteiron-formation. iron-formation. Several Several sills sills of of diabase diabase were were intruded intrudedinto into basal the the strata, strata, mostly mostlyin inthe the iron-formation, iron-formation,and andhave havebeen beenfolded foldedwith with it. it. The synclineof of The fundamental fundamentalstructure structure of of the thearea areaisisaanarrow, narrow,deep, deep,northwest-plunging northwest-plungingsyncline Paleoproterozoic Paleoproterozoicstrata strata of of the the Marquette MarquetteRange Range Supergroup Supergroupflanked flanked by by Archean Archean gneisses. gneisses.Gravity Gravitystudies studiesindicate indicatethat that along along Highway Highway95, 95, where where the the trough trough is is about about 3,000 3,000 feet feet wide, wide, ititisisabout about5,000 5,000 feet feet deep deep(Klasner (Klasnerand andCannon, Cannon,1974). 1974).Axes Axes of of minor minor folds foldsmeasured measuredin inthe the Republic Republicpit, pit, which which follows follows the the iron-bearing iron-bearingbeds beds around around the the keel keel of to the 45'to the northwest. northwest.The The tight tightcompression compressionwithin within the the of the the trough, trough, plunge plungeabout about45° syncline syncline has hasproduced producedintense intensesmall small scale scale folding folding in in some some units, units, such such as as the the schists schists of of the 4-2(fig. (fig.4-1). 4-1).This Thisdeformation deformationapparently apparentlyalso also the Michigamme MichigammeFormation Formationseen seenat at stop stop4-2 has hascaused causedsevere severeattenuation attenuationof of the the iron-formation iron-formationalong alongthe thelimbs limbsof of the thesyncline synclineas as shown shown by by the the drastic drasticthinning thinningseen seen in in traversing traversingfrom from the the Republic Republicpit pitat at the the keel keelof of the the structure structure along alongstrike strikeonto ontothe thenearly nearlyvertical verticallimbs. limbs. The Thecontact contactbetween betweenthe the Paleoproterozoic Archean basement basement gneisses gneisses is is interpreted interpretedto to be be aa fault fault Paleoproterozoicstrata strata and and Archean along along both bothlimbs limbsof of the the syncline. syncline.The Thefault fault on onthe thesouthwestern southwesternlimb limbwas was well well exposed exposedin in the thepit pitand andcompletely completelytruncates truncatesthe the Negaunee Negauneeand andAjibik/Siamo AjibikISiamostrata strataproducing producingthe the fish-hook fish-hook shape shapeof of the thetaconite taconiteorebody. orebody. Bedding Beddingininthe theiron-formation iron-formationnear nearthe thefault, fault,as as well Archeanrocks rocksalong alongthe thefault, fault,generally generallydips dipsfrom from75°-85° 75'-85' totothe the well as asshear shearfoliation foliationininArchean southwest indicating that the fault is a high-angle reverse fault at the present exposure southwest indicating that the fault is a high-angle reverse fault at the present exposure level. level.The The fault fault along alongthe the northeast northeastlimb limbisisnot notas aswell well documented documentedby byexposures, exposures,but but beds bedson on the the northeast northeastlimb limbare arecommonly commonlyoverturned overturnedand anddip dip about about 85° 85' to to the the northeast northeast suggesting suggestingthat that the thenortheast northeastlimb limbfault faultisisalso alsoaahigh highangle anglereverse reversefault. fault.Thus Thusthe the Republic Republictrough troughhas hasan anunusual unusualgeometry geometryininbeing beingaasyncline synclinethat thatwidens widenssomewhat somewhat below belowthe thepresent presentsurface. surface. The Theintense intensePenokean Penokeandeformation deformationrecorded recordedininthe thePaleoproterozoic Paleoproterozoicstrata strataof of the the Republic Archean gneisses gneisseswhere where intense intense Republictrough troughisisnot notpresent presentwithin withinthe the surrounding surroundingArchean multiple multiplefolding foldingevents eventsdocumented documentedby byTaylor Taylor (1967), (1967),Cannon Cannon(1975), (1975),and andHoffman Hoffman (1987) (1987)appear appearto tobe beentirely entirelyaaresult resultof of late lateArchean Archeantectonism. tectonism.Cannon Cannon(1973) (1973)used used metadiabase metadiabasedikes dikesas asstructural structuralmarkers markersto toindicate indicatethe thegeneral generalabsence absenceof of penetrative penetrative Penokean Archeanrocks. rocks.These Thesedikes, dikes,which whichoccur occurinin Penokeandeformation deformationininthe theareas areasof of Archean northeastnortheast-and andnorthwest-trending northwest-trendingswarms, swarms,are aremetamorphosed metamorphosedby byPenokean Penokean metamorphism metamorphismof of the theRepublic Republicnode nodeand, and,ininplaces, places,have havesheared shearedmargins marginscaused causedby by Penokean Penokeandeformation, deformation,but buteverywhere everywheremaintain maintaintheir their vertical, vertical, planar planardike dikegeometry geometryand and have haveessentially essentiallyno nointernal internaltectonic tectonicfabric. fabric.Relict Relictdiabasic diabasictextures texturesare arecommonly commonlywell well preserved preservedeven evenininthe themost mosthighly highlymetamorphosed metamorphoseddikes. dikes.Thus Thusititappears appearsthat thatPenokean Penokean deformation Archeanbasement basementrocks rockswas waslargely largelyconfined confinedto tozones zonesof of deformationwithin withinareas areasof of Archean shearing, shearing,which whichseparate separatelarger largerblocks blocksof of rock rockthat thatremained remainedrigid. rigid.The TheRepublic Republictrough, trough, therefore, therefore,isisaagraben grabenwith withrespect respecttotoArchean Archeanrocks. rocks.The Thesynclinal synclinalform formofofthe thetrough troughofof Paleoproterozoic strata is a result of those strata being molded around the structural Paleoproterozoic strata is a result of those strata being molded around the structural form formcreated createdby bydifferential differentialmovement movementofofindividual individualdiscrete discreteblocks blocksofofArchean Archeanbasement basement rocks. rocks. 90 �____ EXPLANATION EXPLANATION Mesoproterozoic Mesoproterozoic Diabase Diabase Paleoproterozoic Paleoproterozoic Metadia Metadiabase base E Michigamme Fm. Fm. --Michigamme lower lower slate slate member member MichigammeFm. Fm. Michigamme -- banded banded silicate-magnetite silicate-magnetite iron-formation iron-formation Goodrich GoodrichQuartzite Quartzite ::-: Goodrich Qua rtzite Goodrich Quartzite -- basal conglomerate basal conglomerate Negaunee NegauneeIron-Fm Iron-Fm hematite-richoxide oxidefades facies -- hematite-rich Negaunee NegauneeIron-Fm Iron-Fm -- magnetite-rich magnetite-richoxide oxidefacies facies Negaunee NegauneeIron-Fm Iron-Fm -- iron ironsilicate silicatefacies facies Siamo Slate and Ajibik Siamo Ajibik Quartzite, undifferentiated undifferentiated Quartzite, Archean Archean Granitic Graniticgneiss gneiss Mafic gneiss Lii. Mafic gneiss open open pit pit minedumps dumps EJ mine - — faults faults 8800 1/2 0 ,I 1 I miles miles Figure 4-1. Geologic Geologic map map of of the the Republic Republicarea, area, Michigan Michiganshowing showingthe the location locationof of field field Figure trip trip stops. stops. Geology Geologyfrom from Cannon Cannon(1975). (1975). t 91 �Economic geology geology The iron iron ores produced produced at Republic Republic were of two different different types: early production production was of high-grade high-gradeores ores that occur within the upper upper part part of the the Negaunee Negaunee Iron-formation Iron-formationand and lower lower part part of the overlying overlying Goodrich Goodrich Quartzite. Quartzite. These These are are locally locally known known as "hard ores" because because of their their compact, compact, coarse coarse crystalline crystalline nature. nature. The The more more recent recent production productionof iron iron concentrate concentrate and and pellets pellets was was from from lower-grade lower-gradematerial materialtypical typical of of upper upperstratigraphic stratigraphicunits units of the Negaunee Iron-formation. Negaunee Iron-formation. Concentrating-grade Concentratinci-aradeore (taconite): (taconite): The The modern modern open open pit pit of the the Republic Republic mine mine was developed developed to mine mine the uppermost uppermost parts parts of the Negaunee Negaunee Iron-formation Iron-formationconsisting consisting mostly mostly of hematitic hematitic jasper (fig. (fig. 4-2). 4-2). The The ore ore horizon horizon varied varied from from 400-600 400-600 feet feet thick in in the pit. The ore horizons horizons dip approximately approximately vertically along in the northern part of the pit, which which lies lies along along the the northeast northeast limb limb of the the Republic Republic syncline, syncline, but but dips dips flatten flatten to to about about 45 45 degrees degrees in in the the southern southern part part of of the the pit pit near near the the synclinal synclinal axis. axis. Coarse-grained Coarse-grainedspecular specular hematite hematite was the most important important ore mineral mineral but but some some magnetite magnetite was also produced produced from from aa thin thin unit unit of of banded bandedchert-magnetite chert-magnetiteiron-formation iron-formationthat that formed formedaacontinuous continuousunit unit no no more more than a few tens of feet feet thick stratigraphically stratigraphically below below the hematitic hematitic jasper. Some Some iron Goodrich iron production production also came from highly highly ferruginous conglomerate conglomerate of the Goodrich Quartzite, mostly present as a lens lens as much much as several hundred hundred feet thick in in the southwest part part of the pit. Essentially Essentially all of the iron iron was deposited as primary sedimentary sedimentary accumulations. accumulations. The iron iron content content of the ore ore was no no greater than than typical of the Negaunee Negaunee elsewhere in the region. region. The economics economics of the deposit deposit were controlled controlled principally by the structural structural geometry geometry of the ore ore beds beds and and by by the the oxide oxide mineralogy mineralogy and and grain grain size. Lower Lower units units of the Neguanee, grunerite)- magnetite magnetite iron-formation, iron-formation, Neguanee, never mined, consist of silicate silicate (mostly (mostly grunerite)which which was was not not amenable amenable to to concentration concentration by by the the techniques techniques used used at at Republic. Republic. These These rocks, rocks, along along with three diabase diabase sills, underlie underlie the prominent prominent ridge ridge along the northeast northeast and and southeast flanks flanks of the pit pit where they dip dip nearly nearly vertically. The uppermost uppermost diabase diabase sill the orebody orebody (figs. (figs. 4-3, 4-3, 4-5). 4-5). The The unusual unusual strength strength and and stability stability of sill forms forms the the footwall of the this this metadiabase metadiabaseallowed allowed the the development development of of aa high high footwall footwall of of the the pit, pit, which which stood stood several several hundred hundred feet high high as a vertical vertical to slightly overhanging overhanging rock face. This This situation situation was was an an important important economic economic factor factor in in that that itit eliminated eliminated the the need need for for the the large large amount amount of of waste footwall waste rock rock removal removal that that would would have have been been necessary necessary to to maintain maintain aa more more typical typical footwall pit pit slope. slope. In In addition to the occurrence occurrence of the hematite hematite and and magnetite magnetite iron-formation, iron-formation, other other geologic geologic factors factors were critical critical in in enhancing enhancing the economics economics of the Republic Republic mine. The The geometry geometry of the syncline syncline created created the moderately-dipping moderately-dipping and thick sequence sequence of ironironformation formation along along the the keel, keel, which which was was required required to to extract extract ore ore with with an an economically economically permissible permissible amount amount of of removal removalof of underlying underlyingiron-silicate iron-silicateand anddiabase, diabase,and andoverlying overlying Goodrich Goodrich Quartzite Quartzite waste rock. rock. Also, the the high high degree degree of metamorphic metamorphicrecrystallization recrystallization was was vital vital in in enhancing enhancing the the liberation liberationand and concentration concentration of of iron iron minerals, minerals, in in that that grinding grinding to to very-fine very-finegrain grain size size was was not not required required to to allow allow separation separation of of iron-minerals iron-mineralsfrom from chert chert by by the the flotation flotationprocess processused usedat atRepublic. Republic. 92 �:f • • 4- • - • • • .1: • • • •;r. • :-•'.. :•' - • • -- - •.•• - - • . I— - • • -• • • • - :. Ygure 4-2. Wavy-bedded jaspilite typical of the ore zone for the Republic open pit. .ighter layers are specular hematite and darker layers are lenticular beds of ietamorphosed jasper. �Figure Figure 4-3. 4-3. The The Republic Republicopen open pit pit in in 1973. 1973. View View looking lookingSE SE along along the the NE NE limb limb of of the the Republic Republicsyncline. The high wall on the left side of the pit the upper upper contact contact of syncline. The high wall on the left side of the pit is is the of aa metadiabase metadiabasesill, sill,now nowdipping dippingvertically. vertically. Hard Hardores: ores: The Thefirst firstphase phaseofofmining miningatatRepublic Republicwas wasbased basedon onhigh-grade high-grade(60-65% (60-65%Fe) Fe) concentrations concentrationsof of specular specularhematite hematiteand andmagnetite, magnetite,which whichwere were referred referredto toas ashard hardore ore (fig. (fig.4-4), 4-4),inincontrast contrastto tothe theearthy earthymasses massesof ofiron ironoxides oxidesand andhydroxides, hydroxides,the thesoft softores, ores, widely widely mined minedin inthe the eastern easternparts partsof of the theMarquette Marquettedistrict. district. The Theorigin originof ofhard hardores oreshas has been investigated since the geologic studies of Van Hise and Bayley (1897), who were been investigated since the geologic studies of Van Hise and Bayley (1897), who were the thefirst firstto torecognize recognizeaaconnection connectionbetween betweenthe theNegaunee-Goodrich Negaunee-Goodrichunconformity unconformityand and occurrence occurrenceof of hard hardore. ore.They Theyproposed proposedthat thatoxidation oxidationof of siderite sideriteand andleaching leachingof ofsilica silica from fromthe theiron-formation iron-formationby bygroundwater groundwaterformed formedhematite hematiteconcentrations. concentrations.They They envisioned envisionedtectonism tectonism as as important importantin inproducing producingaapermeable permeablecrushed crushedzone zoneat at the the unconformity unconformityto toaccentuate accentuategroundwater groundwaterflow. flow. Later, Later,Van VanHise Hiseand andLeith Leith(1911) (1911) proposed surficialweathering weatheringand and proposedthe theclassic classictheory theorythat thatthe thehard hardores oresformed formedby bysurticial leaching leachingof of the theNegaunee NegauneeIron-formation, Iron-formation,during duringthe theuplift upliftpreceding precedingdepositon depositonofofthe the Goodrich Quartzite, and are paleosupergene concentrations; this weathered and Goodrich Quartzite, and are paleosupergene concentrations; this weathered and leached leachedmaterial materialwas was later laterdeformed deformedand andmetamorphosed metamorphosedto toproduce producethe thepresent presenthard hard ore. ore. This Thistheory theorywas was widely widely accepted acceptedand andmost mostrecently recentlysupported supportedby byGair Gair(1975) (1975) based based largely largelyon onobservations observationsin inthe the Cliffs CliffsShaft Shaftmine mineininlshpeming. Ishpeming.Others Othershave haveproposed proposedthat that the thehard hardores oresare areat atleast leastpartly partlyhydrothermal hydrothermal(Roberts (Robertsand andBartley, Bartley,1943; 1943;Crump, Crump,1948; 1948; Anderson, Anderson,1968; 1968;Marsden, Marsden,1968). 1968). AAcomprehensive comprehensivereexamination reexaminationof of the the hard hardores oresof of the theMarquette MarquetteRange Rangeby byCannon Cannon (1976), (1976),including includingcritical criticalrelationships relationshipsthen thenexposed exposedin inthe the Republic Republicpit, pit, suggested suggestedthat that there thereare aretwo twotypes typesofofhard hardore, ore,each eachformed formedby byaasubstantially substantiallydifferent differentset setofof processes. processes.He Herecognized recognizedaadistinction distinctionbetween betweendominantly dominantlyspecular specularhematite hematiteores, ores, 94 �possessed Penokean Penokean tectonic fabrics, and dominantly magnetite magnetite ores, which commonly possessed massive and appear to post-date which generally are massive post-date tectonism. The hematite ores show geologic relationships relationships that are fully consistent with the original original paleosupergene paleosupergene origin proposed by Van Hise and Leith and are still believed to record an interval interval of oxidative proposed hiatus between between the Negaunee Negaunee IronIronweathering and leaching during the stratigraphic hiatus Goodrich Quartzite. The geologic geologic map map of the Republic Republic area area published published by by formation and Goodrich (in Van Hise and Bayley, 1897) clearly distinguished distinguished hematite hematite and and magnetite Smyth (in separate ore lenses. lenses. Cannon Cannon types of ores, which occurr in distinct and to some extent separate documented that a similar duality of ore types is widespread in the western Marquette documented range, most particularly at the Greenwood and Champion mines. The magnetite ore is invariably post-tectonic, commonly commonly contain contain euhedral euhedral quartz quartz crystals crystals in vugs, vugs, and and occurs occurs Cannon proposed proposed that the magnetite magnetite ores formed formed from from aa within hematitic host rocks. Cannon hydrothermal fluid that was reducing with respect to the hematitic host rock. He suggested that fluids fluids released released by metamorphic metamorphic devolatilization of the iron-formation, iron-formation, suggested which was rich in both hydrous and carbonate minerals in its primary state, was the origin of the hydrothermal hydrothermal fluids. Metamorphism Metamorphism of the Republic Republic node node is is known known to have origin begun begun during the closing phases phases of Penokean Penokean deformation deformation but but to have have outlasted outlasted and reached its peak after deformation. Thus, the magnetite magnetite hard hard ores were interpreted interpreted to have been deposited shortly after formation of the Republic syncline and to be precipitates from reduced metamorphic fluids carrying ferrous iron from stratigraphically precipitates lower parts of the iron-formation iron-formation to the upper parts where reaction with hematite hematitc beds caused caused precipitation precipitationof magnetite. magnetite. Figure 4-4. Former in Republic pit showing onleft leftand Former exposure exposure in Republic pit showing wavy-bedded wavy-beddedjaspilite jaspilite on and magnetite hard ore on right. The hard truncates jaspilite bedding hard ore truncates bedding at a nearly nearly right angle. Within a few centimeters of the contact, much much of the jasper is is converted converted to to milky milky crystals have quartz and partly removed removed from the rock, creating vugs into into which quartz crystals grown. The loss of volume resulted in brecciation brecciation of the iron-formation, which was later healed and largely obliterated by magnetite deposition. Magnet is about 12 cm long. 95 �pit and mill The Republic pit mill metallurgical In 1947 Cleveland Cliffs Iron Co. began research on devising a viable metallurgical concentrating scheme scheme to upgrade upgrade the banded banded specular specular chert iron-formation iron-formationat at Republic Republic to a suitable grade of concentrate. concentrate. The crude crude ore composition composition was 38% 38% Fe, Fe, 42.5% Si02, SiOa, 0.033% P, P, 0.90% 0.90% MgO, MgO, 0.53% 0.53% CaO, CaO, 0.72% 0.72%A1203, A1203,0.03% Na20, NaaO,and and 0.04% 0.04% K20. K20.AA concentration process process was needed needed to produce a product with roughly 65% Fe and 5% Si02. The Company settled on a hot anionic flotation of the specularite after the crude Si02. was ground to 90% passing passing 325 mesh mesh size. After the board board of directors directors approved approved a new new mining operation at Republic, Republic, the company company teamed teamed with with three three partners partnerswho who would would consume & LL Steel Steel Corp., Corp., and and consume the pellet pellet product (i.e. Wheeling Pittsburgh, Pittsburgh, JJ & International Harvester Harvester Co.). Operations International Operations started started with with site site clearing clearingin in1952 1952foUowed followed by construction construction of a concentrator concentrator capable capable of producing producing an initial initial 600,000 long long tons tons (It) of concentrate. concentrate. Pelletization Pelletization (agglomeration) (agglomeration)was was initially initiallyconducted conductedat at the the Eagle EagleMills Millspellet pellet plant located located southwest of the City of Marquette Marquette starting starting in in 1956. 1956. In In 1962, 1962, a pelletizing facility located at the mine was brought on line with production increasing to 2.6 million It of pellets pellets per per year. year. The open pit was developed by a conventional bench-berm system from surface to an of +940 feet. A unique feature of the Republic ultimate depth of Republic Mine was the development of the vertical footwall on the northeast side of the pit that was vertical to slightly overturned overturned footwall referred required steel mesh screen referred to as the highwall (fig. 4-5). This distinctive design required spalled rock from falling into work areas. held in place by rock bolts to prevent spatted the pit crusher was relocated from from the pit crest to an During the middle 1970s the aditkhamber located located in the high high wall on the +1130 +I130 bench (lower left of fig. 4-5). A 28002800adit/chamber grade had had been been driven from surface surface to the chamber chamber to to house house foot-long gallery at 11% 11% grade foot-long the conveyor used to move the ore from the crusher to the plant plant stockpile (upper (upper right right of fig. 4-5). 4-5). Mining was conducted using conventional shovel-truck shovel-truck equipment equipment to to get the the ore to to the crusher. Initial Initial haulage haulage units units were 34 tons Euclid Euclid trucks. However, by the time the mine was idled idled in in 1981, 1981, 80-ton 80-ton units units were were standard. standard. Republic crude was extremely hard hard and and initial initial production production drilling drilling used used the the Linde LindeJet Jet Piercing machine machine in which fuel oil and spa11the the rock rock and oxygen oxygen were combusted combusted to to heat heat and and spall for drilling. During 970s the the mining equipment industry developed During the the 11970s developed rotary rotary machines machines capable of delivering 90,000 lbs. Ibs. of down pressure to the bit, which allowed the mine to change change over to conventional blast hole drilling. Blasting Blasting required required the use of ammonium nitrate and fuel oil (ANFO) at a high usage exceeding one pound per ton whereas other required results for half the powder factor. mines could get the required dewatering wells wells on on the the pit pit perimeter perimeter that that Rock stability at the mine was enhanced by dewatering drew down water keeping keeping it from the pit faces and and further stabilizing stabilizing the pit pit walls. This was especially important on the northwest side of the pit where the Michigamme Michigamme River had been partly diverted to allow pit development. 96 �Figure 4-5. The highwall at Republic. View is looking southeast. The steep rock face is the upper upper contact of a metamorphosed metamorphosed diabase diabase sill, which was emplaced emplaced at the base base of the oxide-facies ore unit. Other rock faces in upper upper left are underlying beds of magnetitegrunerite grunerite iron-formation, iron-formation,which were removed removed as waste. The units units now now dip vertically or are slightly overturned so that, in in places, places, the highwall highwall projects projects slightly outward over the pit. The two portals portals in in lower center are the access access to the primary primary crusher. Crushed Crushed ore ore was moved through an inclined moved to stockpile (on horizon horizon in upper right) by conveyor conveyor through inclined tunnel. The highwall highwall was about 300 feet high high at the time time of this this photograph photograph near near the the end end of pit pit operation. operation. Crude Crude ore underwent underwent three crushing crushing stages stages to provide provide a product product less less than than ½-inch %-inch size, which was fed to the grinding grinding section. Grinding Grinding consisted consisted of a conventional conventional rod rod mill mill with a steel rod rod charge charge to effect effect size size reduction. reduction. The The crude crude was then then sent sent to to aa ball ball mill mill containing containing small steel steel balls balls to complete complete the grinding grinding to aa size size suitable to liberate liberate ore ore minerals from gangue. The crude crude was discharged at 90% passing passing 325 mesh. Flotation Flotation using a fatty acid reagent separated the iron iron from the gangue gangue by by floating floating the iron iron minerals (anionic). The coarse nature of the product required required further grinding grinding to make make itit suitable for pelletizing. Concentrate Concentrate was reground reground in in ball ball mills, heated, heated, and and the pulp pulp was sent hot hot to roughers, roughers, cleaners cleaners and and scavengers. scavengers. The The final final product product was was dewatered, dewatered, balled, balled, and finally sent to the Allis Chalmers Grate Kiln system to be heated to 2440° F to Chalmers Grate Kiln system be heated 2440' F produce a tough pellet capable of withstanding handling and transport to the furnace. The product product was carried carried to Marquette Marquette by by rail rail and and then shipped shipped via lake lake cargo vessels vessels to steel furnaces in in the lower lower Great Great Lakes. Lakes. Scrubber Scrubber and and electrostatic electrostatic precipitators precipitators 97 �removed removed 99% 99% of the the particulate particulatematter matter from from the the discharge discharge air. air. Tailings Tailings from from the the process process settled in in vast settling settling ponds ponds located located southeast southeast of the the plant plant that that have have subsequently subsequently were settled been been converted converted to to viable viable wetlands. wetlands. From 1956 1956 to 1981 1981 the mine mine produced produced 62 62 million million tons tons of pellets pellets recovered recovered from from 145 145 From million million tons tons of of crude crude ore. ore. Considerable resources resources remain remain in in the ground ground at currently currently subeconomic status. The The Considerable resources, as as estimated estimatedby byCleveland ClevelandCliffs CliffsIron IronCo., Co., include includeboth bothspecular specularhematite hematiteore, ore, resources, the traditional traditionalore ore mined minedat at Republic, Republic,and and magnetite-silicate magnetite-silicateiron-formation, iron-formation,which which was was the not amenable amenable to concentration concentration by by the the process process used used at at Republic. Republic. The The estimated estimated not resources in million of long tons (MLT) of ore are: resources in million long tons (MLT) ore are: Ore Resource Grade SiOsininconc. cone. Oretype type Grade Fe Fe Recovery Recovery Fe Feininconcentrate concentrate Si02 Spec. hematite 63 MLT ML T 38% Fe 44% Spec. hematite 38% Fe 44%Wt. Wt.Rec. Rec. 65.0% 65.0%Fe Fe 5.00% 5.00% Mag. 60 MLT 27% Wt. 27%Fe Fe 33% 33% Wt,Rec. Rec. 67.0% 67.0% Fe Fe 5.80% Mag.-Silicate -Silicate 60 MLT 5.80% Talc is is common common in in footwall footwall ore ore in in the the area areaof of the the axial axial keel keelof of the the syncline. syncline. MgO MgO Talc analyses analyses range range from from 1% 1% to 4.7%. Schistose Schistoseconglomerates conglomeratescontain containsericite, sericite, chlorite, chlorite, epidote and and tourmaline. tourmaline. Most Most ore ore in in the the pit pit contained contained 0.20% 0.20% Ti02 TiOpbut but the the southwest southwestPark Park epidote City area area had had elevated elevated values values in in the the .50 .50 to to 1.65% 1.65%Ti02 Ti02as as rutile rutile associated associated with with hematite hematite City Mine Mine closure closureand and restoration restoration From tailings tailinas basin basin to to the the Republic RepublicWetlands Wetlands Preserve: Preserve:The The Republic RepublicMine Mine in in Marquette Marquette From County, Michigan Michiganceased ceasedoperations operationsinin1981, 1981,but butwas wasnot notofficially officiallyclosed closeduntil until1996. 1996. County, Planswere were made madeto to reclaim reclaimaasubstantial substantialportion portionofofthe theRepublic RepublicMine Mineas asaawetland wetland Plans mitigationproject projectto to serve serve the the needs needsof of the the nearby nearbyEmpire Empireand and Tilden Tilden mines minesand and other other mitigation Cliffs-managedproperties. properties. The The Cleveland-Cliffs Cleveland-CliffsIron IronCompany Companyand and its its partners partnersin in the the Cliffs-managed two active active mines mines agreed agreed to to proceed proceedwith with what what is is called called the the Republic Republic Wetlands Wetlands Preserve Preserve two (RWP). The The Michigan MichiganDepartment Departmentof of Environmental EnvironmentalQuality Quality and and the the U.S. U.S. Environmental Environmental (RWP). Protection Agency were involved from the inception of the project. Protection Agency were involved from the inception of the project. NorthernEcological EcologicalServices, Services,Inc. Inc.(NES) (NES)and andCliffs CliffsMining MiningServices ServicesCompany Company(CMSC) (CMSC) Northern formed the the project project team team involved involvedin in the the planning, planning, design, design, construction, construction, and and monitoring monitoring of of formed createdlrestoredon on iron iron the RWP RWPproject. project. Approximately Approximately650 650 acres acres of of wetlands wetlands were were created/restored the tailingsand andreuse reusewater water basins basinsat atthe theRepublic RepublicMine Mine(fig. (fig.4-6). 4-6). Another Another 1,650 1,650acres acresof of tailings wetlands and and uplands uplands were were included included in in the the RWP, RWP, bringing bringingthe the total total acreage acreage'to 2,300 wetlands to 2,300 acres. acres. Water Water levels levelsin in the the tailings tailings basins basins were were managed managedto to create create optimal optimal conditions conditions for for wetlandvegetation, vegetation, with with dormant dormantseeding seedingand andfertilizing fertilizingbeing beingdone, done, as aswell well as as aerial aerial wetland application application of of seed seed and and fertilizer fertilizer on on less less accessible accessible areas. areas. Over Over 250,000 250,000 trees trees were were planted plantedin inthe the wetlands. wetlands. 98 �Figure 4-6. Wildlife trail in a forested and emergent wetland developed on the former tailings basin tailings basin at the Republic Republic Mine, Mine, now now the the Republic RepublicWetlands WetlandsPreserve. Preserve. RWP completed its fifth, and final, year of monitoring monitoring and and the In 2002, phase I of the RWP wetland plant growth and wildlife use is nothing short of spectacular. Peregrine Peregrine falcons, a federal endangered endangered species, have have been been documented documented on a number of occasions using pair of bald bald eagles eagles nests nests there, as as well as as osprey osprey and and common common loons, loons, all the RWP. A pair state-threatened species. There is also a great blue blue heron heron nesting nesting colony, with approximately 60 nests active each year. Over 100 100 species of birds have been documented documented using the site. The land has been transformed from former mined lands to a diverse component component of the landscape landscape in about five years' time. The wetland credits credits that have not been been used to compensate compensate for unavoidable unavoidable wetland impacts at the Empire Empire and Tilden Tilden mines mines are in the process process of being being placed placed in in a wetland mitigation bank to be used for future mine-related mitigation mine-related projects projects in Marquette Marquette County. In summary, the RWP RWP is is a classic classic example example of how how the mining mining industry industry and and government government a valuable natural regulators can work together to reclaim former mine land and create 9 asset that benefits requirements. benefits area area wildlife and satisfies satisfies the mitigation mitigation requirements. FIELD FIELD TRIP TRIP STOPS STOPS Several Several previous previous ILSG ILSG field field trips trips have have visited visitedthe the Republic Republicarea. area. The Thefollowing following descriptions are taken largely from guidebooks guidebooks prepared prepared by by Cannon Cannon and Klasner Klasner (1972) and Cannon Cannon and and others others (1975). (1975). Stop Iron-formation at Stop 4-1. Negaunee Iron-formation at Kloman Klomanmine. mine. Kloman (also (also known known as Columbia) Columbia) Mine was a small, early mine that produced The Kloman about 95,000 tons of hard hard ore between between 1873 1873 and 1883. 1883. The ore body lay along the 99 �Negaunee Iron-formation Iron-formationand and Goodrich Goodrich Quartzite. Quartzite. The The pit pit is is visible visible contact with the Negaunee inside of the fenced area but is not available for examination because of safety concerns. The principal principal interest interest at this this stop stop is is the the 'arge, large, glacially glacially polished polishedoutcrop outcrop of of the the concerns. Negaunee Iron-formation Iron-formation lying just northeast northeast of the fenced fenced area. Here the Negaunee exhibits a small-scale interbedding of several different lithologic types of iron-formation. iron-formation.A A detailed detailed section section is is shown shown here here as as originally originally presented presented by by Cannon and Klasner Klasner (1972) and much much of the description description is is likewise likewise taken taken from that that Cannon earlier field guide. The outcrop outcrop is is on the northeast northeast limb limb of the the Republic Republic syncline. syncline. Beds Beds earlier N 45°W 45OW and anddip dip vertically. vertically. As As aa point pointof of reference referenceto tothe thedetailed detailedsection sectionthe the strike about N 42 feet feet on on the the section. section. The The large eye-bolt set into into the the outcrop outcrop corresponds corresponds to to the the bed bed at at 42 'arge Negaunee as seen here shows a marked lateral facies change from the Republic Republic open Whereas the the ore ore horizon horizon in in the the pit pit is is entirely entirely pit only about 3,000 feet along strike. Whereas with aa few tens tens of of feet feet of of chert-magnetite chert-magnetiteiron-formation iron-formationat at the the base, base,the the same same jaspilite, with ironchert-magnetite and and chert-magnetite-silicate chert-magnetite-silicate ironhorizon here contains many units of chert-magnetite formation interbedded interbedded with with the the jaspilite. jaspilite. correspondence between characteristics and mineralogy A close correspondence between bedding characteristics mineralogy suggests that the present mineralogy, although a product of sillimanite-grade mineralogy, product sillimanite-grade metamorphism, metamorphism, still still differences in the sediments, particularly reflects original differences particularly in oxidation state of the iron. Hematite-bearing units units are are typically typically wavy-bedded wavy-beddedand and contain contain red redor ormaroon maroonjasper, jasper, Hematite-bearing much much of which has abundant bright red red granules, possibly possibly originally oolites. This is in contrast to the magnetite-rich magnetite-richand and silicate-bearing silicate-bearingunits, units, which are are typically evenbedded. Presumably Presumably the wavy-bedded wavy-bedded hematitic rocks were deposited in relatively shallow water, above wave base, where currents were sufficiently intense to produce disturbed bedding and oolites, and the water was sufficiently sufficiently oxygenated to result in a disturbed even-bedded magnetite and silicate rocks, on the thoroughly oxidized sediment. The even-bedded other hand, are presumed presumed to have have formed formed in in deeper deeper water, below wave base, where a combination of quiet bottom bottom conditions and relatively unoxygenated unoxygenated water allowed the accumulation of ferrous ferrous iron this is is so, so, the rocks rocks here here iron minerals minerals in in uniform uniform layers. layers. IfIf this indicate indicate deposition deposition in progressively shallower shallowerwater severalfluctuations eitherinin in progressively water with with several fluctuationseither depth of water or depth of wave action action to produce produce the interlayering interlayering of lithologic types in in the transition zone between the dominantly ferrous sediments at the base of the section transition between sediments base and the ferric sediments sediments at the top of the section. section. The correspondence correspondence between between bedding bedding characteristics characteristics and mineralogy mineralogy is convincing evidence that these rocks represent represent a primary primary oxide facies facies of iron-formation iron-formationand and that the the specularite and magnetite magnetite are metamorphic metamorphic derivatives derivatives of primary primary hematite hematite and magnetite or some precursor minerals with similar oxidation states. Some minerals that are considered formed by diagenetic or early metamorphic considered primary may have formed metamorphic changes; changes; however, the the oxidation oxidation state state of of each bed must reflect the composition of bottom or interstitial interstitial waters, for adjacent beds beds are are commonly commonly of markedly markedly different different oxidation oxidation states. The relationships relationships shown in this outcrop were instrumental instrumental in the recognition recognition that some iron-formation is a primary facies of iron-formation hematitic iron-formation iron-formation (James, 1954). 1954). 100 �____ _______ feet GoodrichQuartzite Quartzite Goodrich meters . 0 unconformity EXPLANATION EXPLANATION massivespecularite specularite with with variable Hard ore: massive magnetite and amounts of magnetite and little little or or no no chert. chert. 5 :-:: yq ,--- Wavy-bedded discontinuous beds Wavy-bedded jaspillite: discontinuous beds jasper or or jasper-mantled jasper-mantled chert, much much with with of jasper interbedded with with specularite granular texture, interbedded layers with minor to moderate amounts of layers with moderate amounts magnetite. to22 inches inchesthick. thick. magnetite.Beds Beds typically typically11to 10 c 50 --H E Even-bedded chert-magnetite chert-magnetite iron-formation: iron-formation: typically in 1/2 to to 11 inch inch thick. thick. Chert Chert is is typically in beds beds 1/2 white or gray and interbedded magnetite is white interbedded magnetite is fine-grained and fine-grained andmassive. massive. 20 — - —1—— -- Even-bedded chert-magnetite-silicate chert-magnetite-silicate iron-formation: similar similar to to chert-magnetite chert-magnetite iron-formation: iron-formation iron-formationbut butwith withaaselvage selvage of of grunerite grunerite at chert-magnetite chert-magnetite contacts. contacts. 25 1±TTIT chert-silicate iron-formation: iron-formation: Even-bedded chert-silicate chert and and grunerite grunerite with with thin thin interlayered chert of magnetite magnetite within within grunerite grunerite laminae of layers layers / —, , iron-formation: grunerite Silicate-magnetite iron-formation: grunerite with magnetite; with laminae laminae and and disseminations of magnetite; non-cherty. non-cherty. 14, b - - 4 -"i.kLi><-!" covered covered Figure 4-7. Detailed stratigraphic section section across across the outcrop outcrop of Negaunee NegauneeIron-formation Iron-formation at the Kloman Kloman mine mine (from (from Cannon Cannon and and Klasner, Klasner, 1972). 1972). The rocks rocks here here were metamorphosed metamorphosedto to sillimanite sillimanite grade grade during during the the Penokean Penokeanorogeny orogeny (James, 1955), as indicated by the coarse grain size of chert and by mineral (James, 1955), as indicated by the coarse grain size of chert and by mineral assemblages assemblages in in nearby nearby pelitic pelitic and and mafic mafic rocks. rocks. Of particular particular interest interest at at this this outcrop outcrop is is the the 101 �lack of equilibration of oxidation states between between adjacent units units in spite of the intense metamorphism. metamorphism. Dominantly Dominantly hematitic units are in sharp contact with dominantly magnetitic units, and although hematitic beds are nowhere in direct contact with silicatebearing beds, they are separated by only thin beds in many places. These relationships bearing attest to the inability of metamorphic metamorphic fluids to induce induce widespread oxidation or reduction of solid phases and are a classic illustration illustration of rocks in which the chemical potential potential of oxygen during metamorphism metamorphism was buffered buffered by the solid phases. 4-2. Michigamme Michigamme Formation Formation garnet-amphibole garnet-amphibole schist. schist. Stop 4-2. roadcut on the east east side side of Highway Highway 95 95 shows shows tightly tightly folded folded schist of the Michigamme Michigamme A roadcut Formation, which here is mostly an iron-rich meta-argillite, meta-argillite, now consisting of biotitegarnet-amphibole (grunerite) (grunerite) schist containing a few inch-thick inch-thick layers layers of impure quartzite. Although the rock is in the sillimanite zone of metamorphism, metamorphism, sillimanite is not present here because of the lack of appropriately aluminous compositions. Blades and rosettes of light-colored rosettes light-colored iron-amphibole iron-amphibole are common and have been mistaken for sillimanite in field examination of this outcrop by many geologists. It is interesting interesting to note Harold James James who defined defined the Republic Republic metamorphic metamorphic node, and the sillimanite zone that Harold its core, in in his his classic classic paper paper on zones of regional regional metamorphism metamorphism (James, (James, 1955) 1955) never never at its observed sillimanite in outcrops. The only occurrence was in a glacial erratic boulder of of Michigamme-type schist, which he presumed was transported only a short distance. The Michigamme-type zone was defined more on the basis of assemblages in mafic rocks than on the distribution of sillimanite. Later, Later, detailed detailed mapping mapping of the region region (Cannon (Cannon and and Klasner, Klasner, distribution (fibrolite) in in one one outcrop outcrop of of the the Michigamme MichigammeFormation Formationseveral several 1976) did find sillimanite (fibrolite) miles northeast of this locality. An additional sillimanite occurrence was reported in drill core about three miles miles northwest northwest of of here here by by Haase Haase(1979). (1979). This outcrop outcrop is is approximately approximately on the axis of the Republic Republic syncline. Minor folds with attenuated limbs are common amplitudes much greater than wavelengths and greatly attenuated and and reflect reflect the gross gross geometry geometry of of the the Republic Republictrough, trough, which which along along the the highway highway is is estimated estimated to be be about about 5,000 feet feet deep deep by by gravity gravity models models (Klasner (Klasner and and Cannon, 1974) 1974) but is only about 3,000 feet wide. The folding is markedly non-cylindrical at outcrop scale, and domains of homogeneous homogeneous strain are very small, being measurable measurable in a few tens tens of square square feet. feet. Although Although most most minor minor folds folds plunge plunge northwest northwest in in accord accord with with the the plunge of the Republic Republic syncline, the plunge of minor folds vary from about 15 15 to 65 degrees. degrees. The The non-cylindrical non-cylindrical nature nature of the folding folding may may be be aa reflection reflectionof an an earlier earlier folding, which has has been been strongly overprinted overprinted by by the development development of the Republic Republic syncline. The style of folding roadcut folding is is especially especially well shown near the north north end of the roadcut (fig. 4-8) where a 13-inch-thick quartzite bed bed is is repeated repeated many many times by folds. Axes 1- to 3-inch-thick of adjacent folds, only a few inches apart, have plunges that diverge by as much as 60 degrees. degrees. At the next next stop stop (stop (stop 4-3) 4-3) we will see a major major contrast contrast between between the high high degree degree of structures seen here and their near absence in nearby Archean penetrative Penokean Penokean structures basement rocks. basement rocks. 102 �Figure 4-8. Vertical face about 1 1 meter high high showing tight folds in Michigamme Michigamme biotitie-garnet Formation at stop 4-2. Folds are shown by a thin quartzite layer (light) in biotitie-garnet schist. Note Note thickened thickened hinge hinge regions regions and and attenuated attenuated limbs. limbs. Fold Fold at right right is is completely completely dismembered, a geometry geometry reflecting reflecting the the Republic Republic syncline syncline itself. itself. 4-3. Archean Archean gneiss gneiss and younger younger dikes. Stop 4-3. This roadcut roadcut on the west side of Highway Highway 95 near near the intersection intersection with Old Old Highway Highway 95 shows a widespread variety of granitic gneiss, which makes makes up part of the southern southern complex, complex, the basement basement on on which Paleoproterozoic Paleoproterozoic strata strata were deposited. The most most abundant abundant rock type in in the southern southern complex complex is is aa coarsely coarsely megacrystic megacrystic granite, granite, named named the Bell Bell Creek Creek Gneiss Gneiss by Cannon Cannon and and Simmons Simmons (1973). (1973). Rock Rock typical typical of this unit unit is is exposed roadcut. The The rocks rocks near near the the center center exposed in the northern northern and and southern southern parts parts of this this roadcut. are finer-grained finer-grained and and somewhat sheared sheared granite. granite. The The relationships relationships in in the southern southern part part of the outcrop outcrop are are sketched sketched in in figure figure 4-9 4-9 and and illustrate illustratetwo two major major periods periods of of orogeny orogeny that that affected Archean rocks; rocks; the the older older being beingan an intense intense Late Late Archean Archean folding folding and and the the affected the Archean younger being being non-penetrative non-penetrative deformation deformation during during the Penokean Penokean orogeny. orogeny. The The oldest oldest rock is the granitic gneiss, which was deformed and metamorphosed metamorphosed in the Archean event. Northwest-trending Northwest-trendingplanar planar structures structures are are shown shown mostly mostly by by preferred preferred orientation orientation of large microcline megacrysts and, in places, by a subtle compositional layering expressed expressed as variations in in abundance abundance of the megacrysts. megacrysts. A northeast-trending northeast-trending truncates the northwest-trending porphyritic dike of metadiabase truncates northwest-trending structures. The dike trends roadcut exaggerating exaggerating its its true true thickness. thickness. Typically, Typically, such such dikes dikes trends subparallel to the roadcut are no no more more than a few tens tens of feet feet thick. thick. This This dike dike truncates truncatesthe the granite granite foliation, foliation, but butisis metamorphosed to a grade typical of this part of the Republic massive, although metamorphosed Republic 103 �metamorphic metamorphic node. Thus, it must must have have been been emplaced emplaced after after the Archean deformation deformation but before the Penokean Penokean metamorphism. metamorphism. Its Its massive massive nature, nature, including including preserved preserved diabasic texture in thin section, and its straight trace indicates indicates that itit was not not deformed deformed during the Penokean Penokean orogeny, even though its its emplacement emplacement must must predate predate the orogeny. orogeny. Such dikes are very common throughout the southern southern complex complex and consistently show these same relationships of retaining retaining planar planar dike dike geometry geometry and relict diabasic textures. Deformation, Deformation, presumably presumably of Penokean Penokean age, is is seen seen in in some dikes as sheared sheared margins, margins, but even in those cases, the planar dike geometry geometry is not not altered. Good Good examples examples of this can be seen in other roadcuts roadcuts north north of here here heading heading toward Humboldt. Humboldt. The occurrence occurrence of such planar metadiabse metadiabse dikes at literally literally hundreds hundreds of localities throughout the southern complex is prime evidence that the complex was not penetratively deformed basement during the Penokean Penokean orogeny. Rather, Penokean Penokean deformation deformation of the Archean basement appears to have been been accomplished by relative movement between discrete, rigid, faultbounded blocks. This indicates indicates that the intense intense deformation deformation of Paleoproterozic Paleoproterozic strata, does not not extend extend into into the basement. The The fault blocks blocks of such as just seen at stop 4-2, does Archean rocks rocks appear to have have provided a rigid rigid form around around which the Paleoproterozoic Paleoproterozoic strata were molded. In In the case of the Republic Republic trough, the structure with respect respect to Archean rocks is a deep, narrow narrow graben graben between between two high-angle high-angle reverse faults. The syncline developed in the Paleoproterozoic rocks as a result of their compression between the two bounding fault uplifts. Estimates Estimates of metamorphic metamorphic pressures pressures by Hasse Hasse (1979) (1979) and Attoh and Klasner Klasner (1989) (1989) indicate indicate that the Republic Republic area was buried buried beneath kilometers of strata during Penokean roughly eight kilometers Penokean deformation and was heated to 550' to 600° metamorphic conditions, conditions, which slightly postdated postdated temperatures temperatures of 5500 600°C at peak metamorphic the major deformation. Under Under those conditions the Archean granitic gneisses gneisses apparently apparently did not deform plastically, but rather retained retained considerable strength strength so that the shape shape of fault-bounded basement the individual fault-bounded basement blocks blockscontrolled controlledthe the geometry geometryof of structures structuresin inthe the overlying overlying strata. strata. roadcut is is a Mesoproterozoic Mesoproterozoic diabase dike related related to the A final feature in this roadcut Midcontinent rift and part of the Baraga dike swarm of reversed magnetic polarity dikes intruded has a chilled chilled contact against both the granitic intruded at roughly roughly 1.1 Ma. The dike has gneiss and and metadiabase metadiabase dike virtually unmetamorphosed. unmetamorphosed. dike and and is is virtually 104 �or 0 0 0 '0 0 ip 10 feet feet 9O M Figure Figure 4-9. Sketch Sketch map map of southern southern part part of of roadcut roadcut showing showing relationships relationshipsbetween between Archean granitic gneiss and diabase dikes of two different ages. Archean granitic gneiss and diabase dikes of two different ages. Ref erences References Anderson, JJ G, G, 1968, 1968, The The Marquette Marquettedistrict, district, in inRidge, Ridge,J. J. D. D.ed., ed., Ore Oredeposits depositsof of the the United United States States 1933-1967 1933-1967 (Graton-Sales (Graton-SalesVol.): Vol.): New New York, York, American American Institute Instituteof of Mining, Mining, Metallurgical, Metallurgical, and Petroleum Petroleum Engineers, Engineers, v. 1, p.508-517. p. 508-517. K, and and Klasner, Klasner, J.S., J.S., 1989, 1989,Tectonic Tectonicimplications implicationsof of metamorphism metamorphismand andgravity gravity Attoh, K, field in in the the Penokean Penokean orogen orogen of of northern northernMichigan: Michigan:Tectonics, Tectonics,v. v. 8, 8, p. p. 911-933. 91 1-933. field Brooks, T.B., 1873, 1873, Iron-bearing Iron-bearingrocks rocks(economic): (economic):Michigan MichiganGeological GeologicalSurvey, Survey,Upper Upper Brooks, Peninsula, Peninsula, v. v. 1, 1,pt. pt. 1, 1,319 319p. p. Cannon, inYoung, Young, G.M., G.M., ed., ed., Cannon, 1973, 1973, The The Penokean Penokean orogeny orogenyin in northern northernMichigan, Michigan, in Huronian stratigraphy stratigraphy and and sedimentation: sedimentation:Geological GeologicalAssociation Associationof of Canada Canada Special Special Huronian Paper Paper 12, 12, p.251-271. p. 251-271. Cannon, Cannon, W.F., 1975, 1975,Bedrock Bedrockgeologic geologic map mapof ofthe theRepublic Republicquadrangle, quadrangle,Marquette Marquette County, Michigan: Michigan: U.S. U.S. Geological Geological Survey Survey Miscellaneous Miscellaneous Investigations InvestigationsSeries Series map map I-I862, 862, scale scale 1:24,000. 1:24,000. 105 �1976, Hard Hard iron iron ore ore of of the the Marquette MarquetteRange, Range, Michigan: Michigan: Economic Economic Cannon, W.F., 1976, Geology, v. 71, P. p. 1012-1028. 1012-1028. and Klasner, Klasner, J.S., 1972, 1972, Guide Guide to to Penokean Penokean deformational deformationalstyle style and and Cannon, W.F., and regional metamorphism metamorphismof the the western western Marquette MarquetteRange, Range,Michigan: Michigan:Proceedings Proceedingsofof18th 18th Institute on Lake Lake Superior Superior Geology, v. 18, 18, p. B1-B38. B1-B38. Annual Institute and Kiasner, Klasner, J.S., 1976, 1976, Geologic Geologic map map and and geophysical geophysical interpretation interpretationof of Cannon, W.F., and the Witch Witch Lake Lake quadrangle. quadrangle. Marquette, Marquette, Iron, Iron, and and Baraga BaragaCounties, Counties, Michigan: Michigan: U.S. U.S. Geological Survey Miscellaneous 1:62,500. Geological Miscellaneous Investigation InvestigationSeries Series Map Map 1-987, 1-987, Scale 1 :62,500. Cannon, W.F., and and Simmons, Simmons, G. G. C., C., 1973, 1973, Geology Geology of of part part of of the the southern southerncomplex, complex, Marquette district, Michigan: Journal of Research of the U.S. Geological Geological Survey, v. 1, p. 165-172. 165-1 72. Gair, J.E., Kiasner, Klasner, J.S., J.S., and and Boyum, Boyum, B.H., B.H., 1975, 1975, Marquette MarquetteIron IronRange: Range: Cannon, W.F., Gair, Proceedings of 21St Proceedings 21'' Annual Institute on Lake Superior Geology, v. 21,p. p.125-1 125-174. v.21, 74. Crump, R.M., 1948, Ph.D. 1948, Origin Origin of hard hard iron iron ores ores of the the Marquette Marquette district: district: unpublished unpublishedPh.D. dissertation, University of Wisconsin-Madison, Madison, Wisconsin, 87 p. dissertation, University Madison, 87 J.W., and Whitney, J.D., 1851, Foster, J.W., 1851, Report Report on on the the geology geology of of the the Lake Lake Superior Superior land land district, part 2, the iron region, together with the general geology: geology: U.S. U.S. 32nd 32'' Congress, Congress, Special Session, Session, Senate Senate Executive Executive Document, Document,v. v. 3, 3, no. no.4, 4, 406 406p. p. Gair, J.E., J.E., 1975, 1975, Bedrock geology and ore deposits deposits of the Palmer quadrangle, Marquette County, Michigan: U.S. Geological Geological Survey Survey Professional ProfessionalPaper Paper769, 769, 159 159p. p. C.S., 1979, Metamorphic petrology of the Negaunee Iron-formation, Haase, C.S., Iron-formation, Marquette Marquette district, northern northern Michigan: Michigan: Ph. Ph. D. D. Dissertation, Dissertation, Indiana Indiana University, University, 246 p. Hoffman, M.A., M.A., 1987, The southern complex: geology, geochemistry, mineralogy mineralogy and mineral chemistry of selected uranium- and thorium-rich granites: unpub. Ph. D. dissertation, dissertation, Michigan Michigan Technological Technological University, Houghton, Houghton, Michigan, 382 p. James, H.L., 1954, 1954, Sedimentry Sedimentry facies facies of iron-formation: iron-formation:Economic Economic Geology, Geology, v. v. 49, 49, p. p. 235-293. 235-293. James, H.L, H.L., 1955, 1955,Zones Zonesof of regional regionalmetamorphism metamorphismin in the the Precambrian Precambrianof of northern northern Michigan: Geological Geological Society Society of America America Bulletin, Bulletin, v. v. 66, 66, p.1455-1488. p.1455-1488. Kiasner, J.S. and Cannon, W.F., W.F., 1974, Klasner, J.S. 1974, Geologic interpretation interpretation of gravity gravity profiles profiles in in the western Marquette Marquette district, district, northern northern Michigan: Michigan: Geological Geological Society Society of America Bulletin, v. 8. 85, p. p. 213-21 213-218. Marsden, R.W., 1968, 1968, Geology Geology of the iron iron ores ores of the the Lake Lake Superior Superior region region in in the the united united in Ridge, J.D., ed., ed., Ore Ore deposits depositsof of the theUnited UnitedStates States1933-1 1933-1967 967 (Graton-Sales (Graton-Sales States, in Vol.): New York, American Institute Vol.): Institute of Mining, Metallurgical Metallurgical and Petroleum Petroleum Engineers, v. 1, p 489-507. 489-507. 106 �M.W., Roberts, H.M., and Bartley, M.W ., 1943, 1943, Hydrothermal replacement in deep seated iron ore deposits deposits of the Lake Lake Superior region: region: Economic Economic Geology, v. 38, p.1-24. p.1-24. Swineford, A.P., 1871, 1871, Swineford's Swineford's history historyof of the the Lake LakeSuperior Superioriron irondistrict-its district-itsmines minesand and furnaces: Marquette, Michigan, Marquette Marquette Mining Mining Journal, 98 p. Taylor, W.E.G., 1967, 1967, The The geology geology of of the the lower lower Precambrian Precambrianrocks rocks of of the the ChampionChampionRepublic area of upper upper Michigan: Michigan: Northwestern NorthwesternUniversity University Report Report 13, 13, 33 33 p. p. Van Hise, Hise, C.R., and and Bayley, Bayley, W.S., W.S., 1897, 1897,The TheMarquette Marquetteiron-bearing iron-bearingdistrict districtof ofMichigan: Michigan: Geological Survey Monograph U.S. Geological Monograph 28, 608 608 p. p. Van Hise, C.R., and and Leith Leith C.K., 1911, 1911, The geology of the Lake Lake Superior Superior region: region: U.S. U.S. Geological Survey Monograph Geological Monograph 52, 641 p. p. 107 ������Proceedings Volume 49 PARTI - PROGRAMS AND ABSTRACTS �INSTITUTE ON LAKE LAKE SUPERIOR GEOLOGY GEOLOGY 49TH ANNUAL MEETING MAY 7-11, 2003 IRON MOUNTAIN, MICHIGAN HOSTED BY: BY: LAUREL G. G. WOODRUFF AND WILLIAM WILLIAM F. LAUREL WOODRUFF F. CANNON CANNON Co-Chairs Co-Chairs U.S. GEOLOGICAL SURVEY U.S. GEOLOGICAL SURVEY With assistance Technological University assistance from Michigan Michigan Technological University and John John Gartner, Gartner, Coleman Coleman Engineering Engineering Company Company Volume Volume 49 49 —Proceedings Proceedingsand and Abstracts Abstracts Part 1 edited by Compiled and edited by Laurel Laurel Woodruff, Woodruff, U.S. U.S. Geological Geological Survey Survey and and Theodore Bornhorst, Bornhorst, Michigan Michigan Technological Technological University University Cover Photo: Cover Photo:Berkshire BerkshireShaft, Shaft,Menominee Menominee Range, Range, Michigan. Michigan. Photo from the Michigan Technological Technological University University Mining Mining Engineering EngineeringDepartment DepartmentCollection. Collection. �49TH INSTITUTE ON LAKE SUPERIOR GEOLOGY 4gTH GEOLOGY VOLUME VOLUME49 49CONSISTS CONSISTS OF: OF: PART 1 1::PROGRAM AND PART PROGRAM AND ABSTRACTS ABSTRACTS PART 2: 2: FIELD TRIP PART FIELD TRIPGUIDEBOOK GUIDEBOOK OVERVIEW: PALEOZOIC PALEOZOICSTRATIGRAPHY STRATIGRAPHYAND AND TECTONICS TECTONICS ALONG THE NIAGRA SUTURE ZONE, ZONE, MICHIGAN AND WISCONSIN 1: PEMBINE-WAUSAU PEMBINE-WAUSAUMAGMATIC MAGMATICTERRANE TERRANE TRIP 1: MENOMINEEIRON IRONDISTRICT DISTRICT TRIP 2: MENOMINEE TRIP TRIP 3: 3:STRATRIGRAPHY STRATRIGRAPHYAND ANDSTRUCTURE STRUCTUREOF OFTHE THEIRON IRONRIVER RIVER—CRYSTAL CRYSTAL FALLS FALLS BASIN BASIN TRIP TRIP 4: 4: LIFE LIFECYCLE CYCLEOF OFAN ANIRON IRONDEPOST DEPOST— - THE THE REPUBLIC REPUBLIC MINE MINE RESTORATION FROM ORE GENESIS TO MINE RESTORATION to material in Part 1 should should follow follow the Reference to the example example below: below: Rogala, B., Fralick, Fralick, P., and Borradaile, G., 2003, A magnetostratigraphic magnetostratigraphicand and secular secular variation varittion 49th study of the Sibley Sibley Group Group [abstract]; [abstract]; Institute Institute on Lake Lake Superior Geology Geology Proceedings, Proceedings, 49 Annual Annual Meeting, Meeting, Iron Iron Mountain, Mountain, Ml, MI, v. v. 49, 49, part part 1, 1, p. p. 65-66. 65-66. 49th Published by the 4gth Institute on Lake Superior Geology and distributed distributed by Published Institute by the Secretary-Treasurer: ILSG Secretary-Treasurer: Mark Jirsa (through (through 2003) 2003) Minnesota Minnesota Geological Geological Survey 2642 University University Avenue Avenue St. Paul, MN 551 55114-1057 14-1057 USA USA @tc.umn.edu JirsaOOl @tc.umn.edu In 2004 contact: contact: Peter Hollings Hollings Lakehead University University Department Department of Geology Geology Thunder Bay, ON P7B P7B5E1 5E1 CANADA peter.hollinas@ lakeheadu.ca Deter.hollincl@lakeheadu.ca I LSGwebsite: website: httr://www.ilscjeolociy.orci htt~://www.ilsaeoloav.org ILSG ISSN 1042-9964 1042-9964 �CONTENTS CONTENTS PROCEEDINGS VOLUME VOLUME 49 PART 1—PROGRAM I-PROGRAM AND AND ABSTRACTS ABSTRACTS Institutes Institutes on Lake Lake Superior Superior Geology, 1955-2003 1955-2003............................................................ iv Constitution of the Institute Constitution Institute on on Lake Lake Superior Superior Geology Geology ...................................................vi vi By-Laws vii By-Laws of the Institute Institute on Lake Lake Superior Geology Geology ....................................................... ... Membership Criteria Criteria ...................................................................................................... VIII viii ix Goldich Medal Medal Guidelines Guidelines............................................................................................... ix Goldich Medal Committee ............................................................................................... xx Past Goldich Medallists Medallists .................................................................................................. xi Citation xii Citation for 2003 2003 Goldich Goldich Medal Medal Recipient Recipient ..................................................................... xii Eisenbrey Student Eisenbrey Student Travel Travel Awards Awards ................................................................................. xiv Student Student Travel Award Application Application Form Form ....................................................................... xiiv Student Student Paper Paper Awards Awards ................................................................................................... xv Student xv Student Paper Paper Awards Committee Committee ................................................................................ Session Session Chairs Chairs .............................................................................................................. xv Board xvi Board of Directors Directors ........................................................................................................ xvi Local xvi xvi Local Committees Committees......................................................................................................... Banquet xvi Banquet Speaker Speaker.......................................................................................................... xvi Report Report of the Chair of the the 48th 48th Annual Annual Meeting Meeting............................................................ xvii Program xxi xxi Program ....................................................................................................................... List of Contributors xxii Contributors ....................................................................................................... ... Abstracts ................................................................................................................... xxviii Abstracts XXVIII III iii �INSTITUTES LAKE SUPERIOR GEOLOGY INSTITUTES ON LAKE SUPERIOR GEOLOGY # YEAR PLACE PLACE CHAIRS CHAIRS 1 1955 Minneapolis, Minnesota Minnesota C.E. Dutton Dutton 2 1956 Houghton, Michigan Houghton, Michigan A.K. Snelgrove Snelgrove 3 1957 East Lansing, Michigan Michigan B.T. Sandefur 4 1958 Duluth, Minnesota Minnesota R.W. Marsden Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz & C. Craddock Craddock 6 1960 Wisconsin Madison, Wisconsin Cameron E.N. Cameron 7 1961 Port Arthur, Ontario Ontario E.G. Pye Pye 8 1962 Houghton, Michigan Michigan A.K. Snelgrove A.K. Snelgrove 9 1963 Duluth, Minnesota Minnesota H. Lepp Lepp 10 1964 lshpeming, Michigan Ishpeming, Michigan A.T. Broderick Broderick 11 1965 St. Paul, Paul, Minnesota Minnesota P.K. Sims & R.K. Hogberg Hogberg 12 1966 Sault Ste. Marie, Marie, Michigan Michigan R.W. White 13 1967 Lansing, Michigan East Lansing, Michigan W.J. Hinze Hinze 14 1968 Superior, Superior, Wisconsin Wisconsin A.B. Dickas A.B. Dickas 15 1969 Oshkosh, Wisconsin Wisconsin LaBerge G.L. LaBerge 16 1970 Thunder Bay, Ontario Ontario M.W. Bartley E. Mercy Mercy Bartley & E. 17 1971 Duluth, Minnesota Minnesota D.M. Davidson Davidson 18 1972 Houghton, Houghton, Michigan Michigan J. Kalliokoski Kalliokoski 19 1973 Madison, Wisconsin Wisconsin M.E. Ostrom Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin Giblin 21 1975 Marquette, Marquette, Michigan Michigan J.D. Hughes Hughes 22 1976 St. Paul, Minnesota Minnesota M. Walton 23 1977 Thunder Bay, Ontario Ontario M.M. Kehlenbeck Kehlenbeck 24 1978 Milwaukee, Wisconsin Wisconsin G. Mursky Mursky 25 1979 Duluth, Duluth, Minnesota Minnesota D.M. Davidson Davidson 26 1980 Eau Claire, Wisconsin Wisconsin P.E. Myers Myers 27 1981 East Lansing, Michigan Michigan W.C. Cambray Cambray 28 1982 International International Falls, Falls, Minnesota Minnesota Southwick D.L. Southwick 29 1983 Houghton, Houghton, Michigan Michigan T.J. Bornhorst Bornhorst iv �30 1984 Wausau, Wausau, Wisconsin Wisconsin G.L. LaBerge LaBerge 31 1985 Kenora, Ontario Ontario G.E. C.E. Blackburn Blackburn 32 1986 Wisconsin Wisconsin Rapids, Rapids, Wisconsin Wisconsin J.K. Greenberg Greenberg 33 1987 Wawa, Wawa,Ontario Ontario 34 1988 Marquette, Michigan Marquette, Michigan J. S. Klasner Klasner 35 1989 Duluth, Minnesota Minnesota J.C. Green Green 36 1990 Thunder Bay, Bay, Ontario Ontario M.M. Kehlenbeck Kehlenbeck 37 1991 Wisconsin Eau Claire, Wisconsin P.E. Myers Myers 38 1992 Hurley, Wisconsin A.B. Dickas A.B. Dickas 39 1993 Eveleth, Minnesota Minnesota D.L. Southwick Southwick 40 1994 Houghton, Michigan Michigan T.J. Bornhorst Bornhorst 41 1995 Marathon, Ontario Ontario M.C. Smyk 42 1996 Cable, Wisconsin Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario Ontario R.P. Sage & W. Meyer Meyer 44 1998 Minneapolis, Minneapolis, Minnesota Minnesota J.D. Miller Miller & M.A. M.A. Jirsa Jirsa 45 1999 Marquette, Marquette, Michigan Michigan T.J. Bornhorst R.S. Regis Regis Bornhorst & R.S. 46 2000 Thunder Bay, Ontario Ontario S.A. Kissin & P. Fralick Fralick 47 2001 Madison, Wisconsin Wisconsin B.A. Brown Brown M.G. Mudrey, Jr. & B.A. 48 2002 Kenora, Ontario Ontario P. Hinz & R.C. Beard Beard 49 2003 Iron Iron Mountain, Mountain, Michigan Michigan L.G. Woodruff & W.F. Cannon Cannon E.D. Frey & R.P. Sage E.D. V �__________(some CONSTITUTION SUPERIOR GEOLOGY CONSTITUTION OF THE INSTITUTE ON ON LAKE LAKE SUPERIOR GEOLOGY (Last 1997) (Last amended amended by by the the Board—May Board-May 8, 8,1997) Article Article II Article Article IIII Article Ill Article III Name Name The name name of the the organization organization shall shall be be the the "Institute "Instituteon on Lake Lake Superior Geology". Geology". Objectives Objectives The objectives objectives of this this organization organization are: are: A. To Toprovide provideaameans meanswhereby wherebygeologists geologistsin inthe the Great GreatLakes Lakesregion regionmay may exchange exchange ideas ideas and and scientific scientificdata. data. B. To Topromote promotebetter betterunderstanding understandingof of the the geology geology of of the the Lake LakeSuperior Superior region. region. C. To plan and conduct geological field trips. To plan and conduct geological field trips. Status Status No part of the income income of the organization organization shall insure insure to the the benefit benefit of of any any member or individual. individual. In the event of dissolution, dissolution, the assets assets of the the organization organization distributed to shall be distributed (some tax free free organization). organization). (To avoid avoid Federal Federal and State State income income taxes, the organization organization should should be be not not only only "scientific" 'scientific" or "educational, "educational, but but also also "non-profit") "non-profit") Article Article IV IV Article V Article Article VI Article VI Article VII Article VII Article Article VIII VIII Minn. Stat. Anno. 290.01, subd. 44 Minn. Stat. Anno. 290.05(9) 1954 Internal 1954 Internal Revenue RevenueCode Codes.501 s.501(c)(3) (c)(3) Membership Membership The membership membership of the the organization organization shall shall consist consist of of persons personswho who have have registered for an annual meeting meeting within the past three years, and and those those who who indicate interest in being a member according to guidelines guidelines approved approved by by the Board of Directors. Directors. Meetings Meetings The organization organization shall meet once a year. The The place placeand andexact exact date dateof of each each meeting meeting will be designated designated by the Board Board of Directors. Directors. Directors Directors The Board Board of Directors Directors shall shall consist consist of the the Chair, Chair, Secretary-Treasurer, Secretary-Treasurer,and andthe the last three past past Chairs; Chairs; but but if the the board board should should at at any any time time consist consist of of fewer fewer than than five persons, persons, by reason reason of unwillingness unwillingness or inability inability of any any of of the the above above persons persons to serve as directors, directors, the vacancies on the board board may may be be filled filled by by the the Chair Chair so so as as to bring bring the membership membership of the the board board to to five five members. members. Officers Officers The officers of this organization organization shall shall be be aa Chair Chair and and Secretary-Treasurer. Secretary-Treasurer. A. The TheChair Chairshall shallbe beelected electedeach each year year by by the the Board Board of of Directors, Directors, who who shall shall give due consideration consideration to the wishes of any group group that may may be be promoting promoting the the next annual meeting. His/her Hislher term term of of office officeas as Chair Chair will will terminate terminate at at the the close close of of helshe presides, presides, or when his/her hislher successor successor shall shall the annual meeting meeting over which he/she have been appointed. He/she Helshe will will then then serve serve for aa period period of of three three years years as as aa member of the Board Board of Directors. Directors. B. The TheSecretary-Treasurer Secretary-Treasurershall shall be elected at the annual meeting. His/her Hislher term of office shall shall be be four years, or until until his/her hislher successor successor shall shall have have been been appointed. appointed. Amendments Amendments This constitution may be amended amended by a majority majority vote (majority (majority of of those those voting) voting) of of the membership membershipof of the the organization. organization. vi �BY-LAWS OFTHE THE INSTITUTE INSTITUTE ON LAKE LAKESUPERIOR SUPERIOR GEOLOGY BY-LAWS OF GEOLOGY I. Duties I. Duties of the Officers Officers and Directors Directors A. A. It shall be the duty of the Annual Chairman to: 1. Preside Presideat at the theannual annualmeeting. meeting. Appointall allcommittees committeesneeded neededfor for the theorganization organizationof of the theannual annualmeeting. meeting. 2. Appoint 3. Assume Assumecomplete completeresponsibility responsibilityfor for the theorganization organizationand andfinancing financing of of the the annual annual meeting meeting over over which which he/she helshepresides. presides. B. It shall be the duty of the Secretary-Treasurer B. Secretary-Treasurer to: 1. Keep Keepaccurate accurateattendance attendancerecords recordsof of all all annual annual meetings. meetings. 2. Keep accurate records of all meetings of, and correspondence Keep accurate records of all meetings of, and correspondencebetween, between,the the Board of Directors. Directors. 3. Hold Holdall allfunds fundsthat thatmay mayaccrue accrueas asprofits profitsfrom fromannual annualmeetings meetingsor or field fieldtrips trips and to make make these these funds funds available available for the the organization organization and and operation operationof of future meetings meetings as as required. required. C. C. It shall be the duty of the Board Board of Directors Directors to plan plan locations locations of of annual annual meetings meetings and and to advise on the organization organization and and financing financing of of all all meetings. meetings. II. Duties Duties and and Exrenses Expenses A. Regular Regularmembership membershipdues duesof of $5.00 $5.00 or or less lesson on an an annual annual basis basis shall shall be be assessed each member member as determined determined by by the Board Board of Directors.. Directors.. B. Registration be determined Registrationfees feesfor forthe theannual annualmeetings meetingsshall shallbe determinedby by the the Chair Chair in in of Directors. The consultation with the Board of The registration registrationfees fees can can include include expenses to cover operations operations outside of the annual annual meeting meeting as as determined determinedby by the Board Board of Directors. Directors. It is strongly recommended recommended that registration registration fees fees be be kept at a minimum minimum to encourage encourage attendance attendance of students. students. III. Ill. Rules Rules of Order Order The rules contained contained in Robert's Robert's Rules Rules of Order Order shall shall govern govern this this organization organizationin in all all cases to which they they are are applicable. applicable. IV. Amendments Amendments amended by a majority These by-laws by-laws may be amended majority vote (majority (majority of those those voting) voting) of of the the membership of the organization; membership organization; provided provided that such modifications modifications shall shall not not conflict conflict with the constitution constitution as as presently presently adopted adopted or or subsequently subsequentlyamended. amended. Last Last Amended Amended— - May, 1996 1996 vii vii �MEMBERSHIP CRITERIA FOR MEMBERSHIP CRITERIA FOR THE THE INSTITUTE LAKE SUPERIOR GEOLOGY INSTITUTE ON LAKE SUPERIOR GEOLOGY Approved May May 8, 8, 1997 1997 A. Membership Membershipininthe theInstitute Instituteon onLake LakeSuperior SuperiorGeology Geologyrequires requireseither either participation participationin in on aa regular regular basis basis of of interest interest in in the the Institute. Institute. Those Institute activities, or an indication on Those individuals years unless: unless: individuals registering registering for an an annual annual meeting meeting will remain remain as as members membersfor for 44 years 1) they indicate no further interest in the Institute Institute by by responding responding negatively negatively to to the the 2) two two statement on meeting meeting circulars circulars "Remove "Removemy my name name from from the the mailing mailinglist"; list";or or 2) statement successive mailings mailings in different different years are are returned returned by by the the postal postal service service as as address address successive unknown. unknown. Those individuals individuals who have have not not registered registered for for an an annual annual meeting meetingin in the the past past44 years years B. Those must indicate indicate an interest interest in in the the Institute Institute by by postal, postal, electronic, electronic, or or verbal verbal correspondence correspondence Secretary-Treasurer at least once every two two years. years. Such with the Secretary-Treasurer Such individuals individuals will be removed from the membership membership ifif they they indicate indicate no no further further interest interest in in the the Institute Instituteor or two two successive mailing mailing in different different years are are returned returned by by the the postal postal service service as as address address successive unknown. unknown. C. The TheSecretary-Treasurer Secretary-Treasurerwill will maintain maintain a list of current members. The The list list will will include include of returned mail, dates of of last the date of the beginning of continuous membership, dates of contact (expression (expression of interest), interest), and and the the date date membership membership expires, expires, barring barringaa change changeof of status initiated by the member. Those individuals who have become members of ILSO Those individuals who have become members ILSG by by Section B will have an expiration date date listed listed at at 22 years years from from the the upcoming upcoming meeting. meeting. For For example, a member who expresses interest interest in September of 1997 1997 (the (the next next annual annual meeting is May, 1998) will have an expiration date of May, 2000, unless unless the member member contacts the Secretary-Treasurer Secretary-Treasurer or attends attends an an annual annual meeting. meeting. D. "Member "Memberfor for Life" Life"status statusisisgranted grantedto to individuals individualswho who have have been been (nearly) (nearly) continuous continuous 15 years, years, Goldich Goldich Medal Medal recipients, recipients, or or those those who who participants of the ILSG ILSG meetings meetings for 15 have served as meeting chairs. This This status status will will be be further further maintained maintained unless unless the the mailings in in different different years years are are individuals indicate no further interest interest in the Institute, Institute, or 4 mailings returned by the postal service as address unknown, unknown, or they are deceased. deceased. E. All All members memberswill will be be mailed mailedthe the First First Circular Circular for for the the Annual Meeting Meeting and and the ILSG ILSG Newsletter. The The Chair Chair of of the the annual annualmeeting meetingmay may opt opt to to send send the the first first circular circular to to additional individuals. All All returned returned mail mail should should be be reported reported to the Secretary-Treasurer. Secretary-Treasurer. F. The TheSecretary-Treasurer Secretary-Treasurercan candesignate designateany any individual individual who is is on the ILSG ILSG membership list (mailing list) as of January January 1, 1, 1997 1997 as a member for life life based based on on participation participationin in ILSG ILSG activities. activities. G. Members Membersare are strongly stronglyencouraged encouragedto to send send address address corrections corrections to to the the SecretarySecretaryTreasurer Treasurer to avoid avoid unintentional unintentional lapse lapse of of membership. membership. VIII viii �GOLDICH MEDAL GUIDELINES (Adopted by the 1981; amended 1999) 1999) the Board of of Directors, 1981; Preamble Preamble Institute on on Lake Lake Superior Superior Geology Geology was was born born in in 1955, 1955, as as documented documentedby bythe the fact fact that thatthe the27th 27th The Institute annual meeting was held in 1981. The TheInstitute's Institute'scontinuing continuingobjectives objectives are are to to deal deal with with those those aspects of geology geology that are are related related geographically geographicallyto Lake Lake Superior; Superior; to to encourage encourage the the discussion discussionof of aspects subjects and sponsoring field trips that will bring together together geologists geologists from from academia, academia, government and industry; industry; and and to maintain maintain an informal informal but but highly highly effective effective mode modeof of operation. operation. surveys, and During the course of its existence, existence, the membership membership of the the Institute Institute (that (that is, is, those thosegeologists geologistswho who indicate an interest interest in in the objectives objectives of the the ILSG ILSG by by attending) has has become become aware awareof of the the fact fact that that indicate colleagues have have made made particularly particularly noteworthy noteworthyand and meritorious meritorious contributions contributionsto to the the certain of their colleagues understanding understanding of Lake Lake Superior Superior geology geology and and mineral mineral deposits. deposits. The first award award was made made by by ILSG ILSG to to Sam Sam Goldich Goldich in in 1979 1979for for his his many manycontributions contributionsto tothe thegeology geology of the region extending over about 50 years. years. Subsequent Subsequent medallists medallists and and this year's recipient recipient are listed listed in the table table below. below. Guidelines Award Guidelines 1) The The medal medalshall shall be beawarded awardedannually annually by by the ILSG Board of Directors to a geologist whose name is associated with a substantial interest in, and contribution contribution to, the geology of the Lake Lake Superior region. region. 2) The TheBoard Boardof of Directors Directorsshall shall appoint appoint the Goldich Medal Committee. The The initial initial appointment appointment will for two two years, years, and and one one for for one one year. year. The be of three members, one to serve for three years, one for The briefest incumbency shall be chair chair of of the the Nominating Committee. Committee. After member with the briefest After the the first first year, the Board Board of Directors Directors shall shall appoint at each each spring spring meeting meeting one one new new member memberwho who will will serve serve years. In the chair. chair. The Committee for three years. In his/her hislher third third year this member shall be the Committee membership membership geographic distribution distribution of of ILSG membership. membership. The should reflect the main fields of interest and geographic The outoutgoing, senior member member of the the Board Board of of Directors Directorsshall shall act act as as liaison liaisonbetween betweenthe the Board Boardand andthe the Committee for a period Committee period of one one year. 3) By Bythe theend endof of November, November,the the Goldich GoldichMedal Medal Committee Committee shall shall make make its its recommendation recommendationto to the the Chair of the Board Board of Directors, Directors, who will then inform inform the the Board Board of the the nominee. nominee. 4) The TheBoard Boardof of Directors Directorsnormally normallywill will accept accept the the nominee nominee of of the the Committee, Committee, inform informthe the medallist, medallist, medal engraved and have one medal engraved appropriately appropriately for presentation presentation at the next next meeting meetingof of the the Institute. Institute. recommended that the Institute 5) It is recommended Institute set aside aside annually from whatever sources, sources, such such funds funds as as will will required to support the continuing be required continuing costs of this award. award. Nominating Procedures Nominatina Procedures 1) The The deadline deadline for nominations nominations is November 1. The The Goldich Goldich Medal Medal Committee Committee shall shall take time. Committee nominations at any time. Committeemembers membersmay maythemselves themselves nominate nominatecandidates; candidates; however, however, Board members may may not not solicit for or or support support individual individual nominees. nominees. 2) Nominations Nominationsmust mustbe beininwriting writingand andsupported supportedby byappropriate appropriatedocumentation documentationsuch such as as letters letters of of recommendation, lists recommendation, lists of publications, publications, curriculum curriculum vita's, vita's, and and evidence evidenceof of contributions contributionsto to Lake Lake Superior geology and and to the the Institute. Institute. 3) Nominations Nominationsare arenot notrestricted restrictedto to Institute Instituteattendees, attendees, but but are are open open to anyone anyone who has worked on and and contributed contributedto the the understanding understandingof of Lake LakeSuperior Superiorgeology. geology. ix �Selection Guidelines Nomineesare areto tobe beevaluated evaluatedon on the the basis basis of of their their contributions contributionsto to Lake Lake Superior Superior geology geology 1) Nominees (sensu (sensu lato) lato) including: including: a) importance importanceof of relevant relevant publications; publications; b) promotion promotionof of discovery discoveryand andutilization utilization of of natural natural resources; resources; c) contributions contributionsto tounderstanding understandingof of the the natural naturalhistory history and and environment environment of the the region; region; d) generation generationof of new newideas ideasand and concepts; concepts; and and e) contributions contributionsto to the thetraining trainingand and education education of of geoscientists geoscientists and and the the public. public. 2) Nominees Nomineesare areto tobe beevaluated evaluatedon ontheir their contributions contributions to to the the Institute Instituteas as demonstrated demonstratedby by attendance attendance at at Institute Institutemeetings, meetings, presentation presentationof of talks talks and andposters, posters,and andservice serviceon onInstitute Instituteboards, boards, committees, committees, and and field field trips. trips. 3) The Therelative relativeweights weightsgiven givento toeach eachof of the theforegoing foregoing criteria criteria must must remain remainflexible flexible and andat at the the discretion discretion of the the Committee Committeemembers. members. 4) There Thereare areseveral severalpoints pointsto tobe beconsidered consideredby bythe the Goldich Goldich Medal Medal Committee: Committee: Anattempt attemptshould shouldbe bemade madeto tomaintain maintainaabalance balanceof of medal medalrecipients recipientsfrom from each eachof of the the a) An three estates—industry, estates-industry, academia, academia, and and government. government. b) It must be noted noted that industry industry geoscientists are are at aa disadvantage disadvantage in in that that much muchof of their their work in in not not published. published. 5) Lake of LakeSuperior Superiorhas hastwo two sides, sides, one one the U.S., and the other Canada. This This is is undoubtedly undoubtedly one of the Institute's Institute's great great strengths strengths and and should should be be nurtured nurturedby by equitable equitable recognition recognition of of excellence excellenceininboth both countries. countries. GOLDICH GOLDICH MEDAL MEDAL COMMITTEE COMMITTEE Serving through the meeting year shown in parentheses Frank Frank Luther Luther(2003) (2003) University of Wisconsin, Whitewater Ron R o n Sage Sage (2004) (2004) Ontario Geological Geological Survey Survey (retired) (retired) David David Meineke Meineke (2005) (2005) Meriden Engineering, Engineering, Hibbing, Hibbing, Minnesota Minnesota Steve Kissin, Kissin,as asout-going out-goingsenior seniormember member of of Institute Institute Board Board of Directors, Directors, is is liaison between Goldich Medal Committee and the Board through the 2004 meeting meeting x �2003 2003GOLDICH GOLDICHMEDAL MEDALRECIPIENT RECIPIENT Klaus Klaus J. Schulz Schulz U.S. Geological Geological Survey U.S. Reston, Virginia Virginia GOLDICH MEDALISTS MEDALISTS 1979 Samuel SamuelS. S.Goldich Goldich 1991 Hinze 1991 William Hinze 1980 not notawarded a warded 1992 William WilliamF. F.Cannon Cannon 1981 1993 1 993 Donald DonaldW. W. Davis Davis Carl E. Dutton, Dutton, Jr. 1982 Ralph RalphW. W. Marsden Marsden 1994 Cedric CedricIverson Iverson 1983 Burton BurtonBoyum Boyum 1995 Gene GeneLaBerge LaBerge 1984 Richard RichardW. W. Ojakangas Ojakangas 1996 David DavidL. L.Southwick Southwick 1985 1997 Ronald RonaldP. P. Sage Sage Paul K. Sims Sims 1986 G.B. G.B. Morey Morey ZelI Peterman 1998 Peterman 1998 Zell 1987 Henry HenryH. H. Halls Halls 1999 Tsu-Ming Tsu-MingHan Han 1988 Walter Walter S. S. White White 2000 John JohnC. C.Green Green 1989 Jorma JormaKalliokoski Kalliokoski 2001 2001 John S. Klasner Klasner 1990 Kenneth KennethC. C. Card Card 2002 Ernest ErnestK. K.Lehmann Lehmann xi �CITATION Klaus J. Schulz 2003 Goldich Medal Recipient Recipient ning more Klaus Schulz has had had a long long and and productive productivecareer careerspan spanning more than than 30 30 years years as a geologist geologist in in the the Lake Lake Superior Superior region. region. He He was was introduced introducedto to the the geology geologythrough throughhis his education education in the area, he he completed completed graduate graduate studies studies in in the the region, region,performed performedseveral several summers of field work for mining mining companies in a number of different areas, and has conducted extensive extensive research research as aa scientist scientist with the U.S. U.S. Geological Geological Survey. Survey. This This conducted extensive extensive and diverse diverse experience experience has has made made him him a real real authority authority on on the the geology geology of of the the Lake Superior region. Klaus Klaus received received his B.S. degree degree in in geology geology from the the University Universityof of WisconsinWisconsinOshkosh Oshkosh in 1971. 1971. He He completed completed his his Masters Masters degree degree at at the the University Universityof of MinnesotaMinnesotaDuluth Duluth in 1974, with a thesis project project in in the the Vermilion district district of northern northern Minnesota. Minnesota. He He received received his Ph.D. from the University University of Minnesota Minnesota in in 1977 1977 with aa dissertation dissertation on on the the petrology petrology of volcanic rocks rocks in in the Vermilion district. district. Klaus Klaus spent spent the the next next two two years years as as aa National National Research Research Council Council Research Research Associate Associate with NASA at the Johnson Johnson Space Space Center Center in Houston, where he studied basaltic and ultramafic ultramafic magma magma types types as as analogs analogs of of studied Archean basaltic early planetary crust. In 1982, after three years as a faculty faculty member at at Washington Washington University University in St. Louis, Louis, Klaus Klaus resigned resigned his his teaching teaching position position and and joined the the U.S. U.S. Geological Geological Survey in Reston, VA, fulfilling a long-standing long-standing dream dream of of his. his. During During the the next nexttwenty twenty years years with the USGS Klaus Klaus was a research research scientist scientist and administrator administrator with a strong strong interest interestin in the geology of the Lake Lake Superior Superior region. region. The traits that have have made made Klaus Klaus a success success were evident early early in in his his career. career. In In his his undergraduate days at Oshkosh, undergraduate Oshkosh, Klaus Klaus distinguished distinguished himself himself as as an an avid avid reader reader of of the the geological literature. literature. As a junior in in 1970, 1970, he he wrote wrote an an outstanding outstanding research research paper paper discussing the similarities similarities between between Archean greenstone greenstone belts belts and and modern modern island island arcs. arcs. He He worked several summers doing fieldwork fieldwork for Bear Bear Creek Creek Mining Mining Company Company in in central central Wisconsin and northern Michigan, and for U.S. U.S. Steel Corp. Corp. in the the Vermilion Vermilion district district of of northern Minnesota. This combination combination of field work and and a thorough thorough knowledge knowledgeof of the the literature has continued continued to be be a hallmark hallmark of his his professional professional career, career, and and has has led led to to aa number of significant significant contributions contributions to the the geology geology of the the Lake Lake Superior Superiorregion. region. In the summer of 1971, 1971, Klaus Klaus and and William William Spence Spence discovered discovered the the Lake LakeEllen Ellen kimberlite near Crystal Falls, Falls, Michigan, while working as exploration exploration geologists geologists in in the the area. Klaus was very much involved involved in the recognition recognition of the rock rock as as aa kimberlite. kimberlite. This This was the first kimberlite kimberlite discovered discovered in in the the Lake Lake Superior Superior region. region. xl' �Masters thesis involved involved considerable mapping in the Ely Ely greenstone greenstone belt belt in in His Masters considerable mapping studies for his his Ph.D. Ph.D. dissertation dissertation showed showed that that the the Newton Newton Minnesota, and geochemical geochemical studies Lake Formation Formation was a high-magnesium high-magnesium basalt, basalt, similar to komatiites. komatiites. This This was was the the first first documented occurrence occurrence of of komatiitic komatiiticrocks rocksin in the the Lake LakeSuperior Superiorregion. region. documented In the early 1980's, his field mapping mapping and geochemistry geochemistry of rocks rocks in in the the Pembine Pembine area area of the the Wisconsin Wisconsin magmatic magmaticterranes terranesdemonstrated demonstratedthe thepresence presenceof ofophiolitic ophioliticrocks. rocks. Again, this was the the first first documented documentedophiolite ophiolite in in the the Lake Lake Superior Superiorregion, region,and andshowed showed magmatic terranes were, at least in part, an oceanic island arc. His that the Wisconsin magmatic model for the the evolution evolution of of the the Marquette MarquetteRange RangeSupergoup Supergoupon onthe thecontinental continental subsequent model margin during during the the Penokean Penokean orogeny orogeny is is an an extension extension of of his his familiarity familiarity with with the the rocks rocksininthe the margin region combined combined with his his encyclopedic knowledge knowledge of the geologic geologic literature literature on on the the region evolution of continental continental margins. margins. Klaus also contributed contributed to the GLIMPCE GLIMPCE program, program, which ultimately ultimately provided provided significant insight into the structure and origin of the Mid-continent rift, and into its magmatic magmatic origin origin and and metallogeny. metallogeny. He has authored authored and and co-authored co-authored more more than 120 120 publications, publications, maps, maps, abstracts abstracts and field guides, including including field field guides guides for the the 1984, 1984, 1992, 1992, and and 2003 2003 Institute Institutemeetings. meetings. Klaus' contributions contributions have provided provided a better understanding understanding of the the Archean, Archean, the the Early Proterozoic, Proterozoic, the Middle Proterozoic, and the Phanerozoic history history of the the Lake Lake Superior region. And he continues continues to be be an active active contributor contributor on on aa global global stage, stage, taking taking the the knowledge experience that he knowledge and experience he has gained gained in the Lake Lake Superior Superior region region and and applying applying itit to international international projects. projects. Therefore, it is my my distinct distinct pleasure pleasure and and honor honor to present present Klaus Klaus Juergen Juergen Schulz Schulz as as the 2003 recipient recipient of the Goldich Medal "For Outstanding Contributions To The Lake Superior Region". Region". Submitted by Gene Submitted Gene L. L. LaBerge LaBerge xiii �____________________________________________ __________________________ EISENBREY STUDENT TRAVEL AWARDS The 1986 1986 Board Board of of Directors Directors established established the the ILSG ILSG Student Student Travel Travel Awards Awards to to support support student student participation at at the annual meeting of of the the lnstitute. Institute. The The name name "Eisenbrey" "Eisenbrey" was added added to the award in 1998 to honor honor Edward Edward H. Eisenbrey Eisenbrey (1926-1985) (1926-1985) and utilize substantial contributions made to the 1996 Eisenbrey 1996 Institute lnstitute meeting meeting in his name. "Ned" "Nedt1 Eisenbreyisiscredited creditedwith withdiscovery discoveryof of significant scope was was much much significant volcanogenic volcanogenic massive massive sulfide sulfide deposits deposits in in Wisconsin, Wisconsin, but but his scope broader—he broader-he has been described as having having unique unique talents as an an ore ore finder, finder,geologist, geologist,and and teacher. These are intended to help defray teacher. These awards awards are intended to defray some some of the the direct direct travel travel costs costs of of attending Institute lnstitute meetings, and include a waiver waiver of registration fees, but but exclude exclude expenses expenses The annual Chair in for meals, meals, lodging, lodging, and and field field trip trip registration. registration. The in consultation consultation with the the Secretary-Treasurerdetermines determinesthe the number number of of awards will be Secretary-Treasurer awards and and value. value. Recipients Recipients will be announced at the annual banquet. banquet. The annual Chair, who is is responsible responsible for the the selection, selection, will will consider consider the the following following general general criteria: criteria: 1) The Theapplicants applicantsmust musthave haveactive activeresident resident (undergraduate (undergraduateor or graduate) graduate) student student status status at at the time of the the annual annual meeting meetingof of the the Institute, lnstitute,certified certifiedby bythe thedepartment departmenthead. head. 2) Students Studentswho who are arethe the senior senior author author on on either either an an oral oral or or poster poster paper paper will will be be given given favored consideration. consideration. 3) It is desirable for two or more more students students to jointly request request travel travel assistance. assistance. 4) InIngeneral, general,priority prioritywill willbe begiven givento tothose thoseininthe the Institute lnstituteregion regionwho who are are farthest farthest away away from from the meeting meeting location. location. 5) Each Eachtravel travel award award request requestshall shall be be made made in in writing writing to to the the annual annual Chair, and and should should explain other significant details. details. The need, student and author status, and other The form form below below is is optional. optional. Successful applicants will receive receive their awards during the meeting. meeting. n INSTITUTE ONLAKUPERIOAGEOLOGY Application Eisenbrey Student Travel Award Application Student Student Name: Name: Date: Date: Address: email: Department Department Head-Typed Head-Typed Department Educational Status: Educational Status: Department Head-Signature Head-S~gnature Areyou youthe the senior author anororal or paper? poster paper? YES_ Are senior author of anof oral poster YESNO- NO_ Will any other other students students be be traveling travelingwith with you? you? Who? Who? Statement Statement of need need (use (use additional additional page page ifif necessary) necessary) Please Please return return to: to: xiv �STUDENT PAPER AWARDS Each year, the lnstitute student presentations presentations and and honors honors Institute selects the best of the student Funding for for the the award award is is generated generated from from registrations registrations presenters with a monetary award. Funding of the annual meeting. The TheStudent StudentPaper PaperCommittee Committeeisis appointed appointedby by the the annual annual meeting meeting Chair in such a manner as to represent represent aa broad broad range range of of professional professionaland andgeologic geologic expertise. Criteria Criteriafor for best beststudent student paper—last paper-last modified modifiedby by the the Board Board in in 2001—follow: 2001-follow: Thecontribution contributionmust mustbe bedemonstrably demonstrably the the work work of of the the student. student. 1) The 2) The student must present the contribution in-person. The student must present the contribution in-person. TheStudent StudentPaper PaperCommittee Committeeshall shalldecide decidehow how many manyawards awards to to grant, grant, and and whether whether or or 3) The not to give separate awards for poster poster vs. oral presentations. presentations. casesofofmultiple multiplestudent studentauthors, authors,the theaward awardwill will be bemade madeto to the the senior senior author, author,or or 4) InIncases award will be be shared shared equally by by all all authors authors of the the contribution. contribution. the award Thetotal totalamount amountof ofthe theawards awardsisisleft leftto tothe thediscretion discretionof of the themeeting meetingChair Chairand and 5) The Secretary-Treasurer,but but typically typically is is in in the the amount amount of of about about $500 $500 US US (increase (increaseapproved approved Secretary-Treasurer, by Board, Board, 10/01). 10101). TheSecretary-Treasurer Secretary-Treasurermaintains, maintains,and andwill will supply supply to to the the Committee, Committee,aa form form for for the the 6) The numerical ranking of presentations. presentations. This Thisform formwas wascreated createdand andmodified modifiedby byStudent Student numerical Paper Committees over several years in an effort to reduce reduce the difficulties difficultiesthat that may may arise arise from selection by raters of of diverse diverse background. background. The use of the form is not required, but is left to the discretion discretion of the the Committee. Committee. Thenames namesof ofaward awardrecipients recipientsshall shall be beincluded includedas as part part of of the the annual annual Chair's Chair's report report 7) The that appears appears in the next next volume of the the Institute. lnstitute. Student papers papers will be noted noted on the Program. Program. 2003 STUDENT PAPER AWARDS COMMITTEE Theodore TheodoreBornhorst Bornhorst- -Michigan MichiganTechnological TechnologicalUniversity, University, Houghton, Houghton, MI MI -- Chair Kevin WI KevinSikkila Sikkila— - Wisconsin Department of Transportation, Superior, Wl Anne Argast PurdueUniversity University Fort FortWayne, Wayne, Fort Wayne, Wayne, IN Anne Argast— - Indiana Indiana University University — - Purdue Tim Tim Flood Flood— - St. Norbert Norbert College, De De Pere, Pere, WI Wl 2003 SESSION CHAIRS Peter Ontario Geological Survey, Survey, Kenora, Peter Hinz Hinz— - Ontario Kenora, ON ON Eric Jerde Eric Jerde--Morehead MoreheadState StateUniversity, University, Morehead, Morehead, KY KY James James Miller Miller- -Minnesota MinnesotaGeological GeologicalSurvey, Survey,Duluth, Duluth, MN MN Mike Mudrey, Wisconsin Geological Mike Mudrey,Jr. Jr.— -Wisconsin Geological and and Natural Natural History History Survey, Survey, Madison, Madison, WI Wl xv �2003 BOARD OF 2003 BOARD OFDIRECTORS DIRECTORS appointment continues continues through through the the close close of of the the meeting meeting year year shown shown in in parentheses, parentheses, or oruntil untilaa Board appointment successor is selected Laurel Woodruff Woodruff Co-Chair Co-Chair2003 2003meeting meeting(2006) (2006) U.S. Geological Geological Survey, Survey, St. St. Paul, Paul, MN MN Peter Hinz Hinz (2005) (2005) Geological Survey, Kenora, ON Ontario Geological Michael G. Mudrey, Jr. (2004) Michael (2004) Wisconsin Wisconsin Geological Geological and and Natural NaturalHistory HistorySurvey, Survey, Madison, Madison, WI Wl Stephen A. A. Kissin Kissin(2003) (2003) Lakehead University, Lakehead University, Thunder Thunder Bay, Bay, ON ON Hollings-Secretary-Treasurer (2006) Peter Hollings-Secretary-Treasurer (2006) Lakehead University, Thunder Lakehead Thunder Bay, Bay, ON ON A. Jirsa-Secretary-Treasurer-"erneritu~~~ Jirsa-Secretary-Treasu rer-"emeritus" (in (in transition) transition) Mark A. Minnesota Geological Geological Survey, Minnesota Survey, St. Paul, Paul, MN MN 2003 COMMITTEES 2003LOCAL LOCAL COMMITTEES General General Co-Chairs Co-Chairs Laurel Laurel G. G. Woodruff Woodruff—-U.S. U.S.Geological GeologicalSurvey, Survey, St. St. Paul, Paul, MN MN William F. Cannon — U.S. Geological Survey, Reston, VA William F. Cannon - U.S. Geological Survey, Reston, VA and Abstracts Abstracts Editors Program and Laurel Laurel G. G. Woodruff Woodruff----U.S. U.S.Geological GeologicalSurvey, Survey,St. St. Paul, Paul, MN MN Theodore J. Theodore J. Bornhorst Bornhorst—-Michigan MichiganTechnological TechnologicalUniversity, University,Houghton, Houghton,MI MI Field Trip Guidebook Editor Editor William F. William F.Cannon Cannon—- U.S. U.S. Geological GeologicalSurvey, Survey, Reston, Reston, VA VA Acting Local Local Committee, Iron lron Mountain John Gartner John Gartner— - Coleman Coleman Engineering, Engineering, Iron lron Mountain, Mountain, MI MI Connie Dicken Connie Dicken— - U.S. Geological Geological Survey, Reston, Reston, VA Sally LaBerge Sally LaBerge— - Oshkosh, Oshkosh, WI WI 2003 BANQUET SPEAKER Susan Martin Martin Department of Social Department Social Sciences Sciences Michigan Technological University University Houghton, Houghton,Michigan Michigan The indigenous indigenous people of the Lake Superior Superior Basin: Basin: Understanding Understanding the links links among environment, geology and religious religious belief xvi xvi �48TH Report of the Chair of the 4aTHAnnual Meeting Meeting Hinz, Co-Chair Co-Chair ILSG ILSG 2002 2002 Peter Hinz, 48th The 4athAnnual Institute lnstitute on Lake Lake Superior Superior Geology Geology was hosted hosted by by the the Ontario Ontario Geological Geological 9-12, Survey on May 9-1 2, 2001. Principal Principal local committee members members were Peter Peter Hinz Hinz and and Richard C. Beard, co-chairs, co-chairs, Carmen C. Storey, and and Kevin Kevin O'Flaherty OrFIahertyProgram Programco-chairs, co-chairs, Richard McGowan-Hinz,Treasurer, Charles E. Blackburn, Blackburn, Field Field Trip Co-ordinator, Co-ordinator, M. Kathleen Kathleen McGowan-Hinz, and Christine C. Blackburn, Blackburn, Secretary. Secretary. Other principal principal individuals individuals are listed listed in in the the Proceedings ProceedingsVolume. Volume. Attendanceat atILSG ILSG2001 2001 Attendance A total of 97 97 professionals professionalsand and student student professionals professionalsattended attendedthe the meeting, meeting,39 39of of whom whom pre-registered by the April 2, 2001 deadline. A total of 8 students pre-registered students were were registered, registered,77 of of whom requested requested and and received received travel travel assistance. Eisenbrey Student Eisenbrey Student Travel Travel Awards Awards2001 2001 Seven students students requested requested and and received received travel travel assistance assistance from from the the Eisenbrey EisenbreyStudent Student Travel Award Fund Fund established established to support student student participation participation at at the the Annual Annual Institute. lnstitute. Details, including criteria and application application forms, are available available at the ILSG ILSG website. Bogdan Nitescu Bogdan Nitescu Claire Claire Sturm Sturm Elizabeth Fein Elizabeth Fein Justin Johnson Johnson Becky Rogala Rogala William Jahn Jahn Daniela Daniela Vallini Vallini University of Toronto, Toronto, Toronto, ON University ON Oberlin Oberlin College, College, Oberlin, Oberlin, OH OH Oberlin Oberlin College, College, Oberlin, Oberlin, OH OH Lakehead University, Thunder Bay, Lakehead Bay, ON ON Lakehead University, Thunder Bay, Lakehead Bay, ON ON University of Minnesota - Duluth, Duluth, Duluth, Duluth, MN MN of Western Australia, Nedlands, University of Nedlands, WA Meetin Meetin Summary Summary tf The 48 Annual AnnualInstitute lnstituteon onLake LakeSuperior SuperiorGeology GeologyAnnual Annual Meeting Meeting was held held at the Best Western Lakeside Lakeside Inn Inn and and Convention Convention Centre, Centre, the the same same location locationas as the the 1985 1985meeting. meeting. The one-and-a-half days of technical sessions were preceded by: Field Trip 1 — Tanco one-and-a-half days of technical sessions were preceded by: Field Trip 1 - Tanco Rare-Element Rare-Element Pegmatite, Pegmatite, Southeastern Southeastern Manitoba Manitoba led led by by staff of of the the Tantalum TantalumMining Mining Corporation of Canada Quaternary Geology of - Quaternary Corporation Canada Ltd.; Ltd.; followed followedby byField FieldTrip Trip22— Southeastern Manitoba Nielsen and Gaywood Matile Matile (Manitoba (Manitoba Geological Geological Southeastern Manitoba led led by E. Nielsen Survey); and Field Field Trip 3- Structure Structure and and Sedimentology Sedimentology of the the Seine Seine Conglomerate, Conglomerate,Mine Mine Centre Area, Ontario Ontario lead lead by by Dyanna Dyanna Czeck (Department (Department of Geology, Geology, Oberlin Oberlin College) College) and Philip Fralick Fralick (Department (Department of Geology, Geology, Lakehead Lakehead University) University) Due to the small number number of talks talks submitted, submitted, the the Technical Technical Session Session Chairs Chairs were were unable unable to to group talks into into session session themes. The The meeting meeting began began with with an an anecdotal anecdotal history historyof of mining miningin in northwestern Ontario presented O7FIaherty,followed followed by by regional regional scale scale talks talks on on northwestern presented by by Kevin Kevin O'Flaherty, the Western Western Superior Superior Province. Province. The Theremainder remainderof ofthe thetechnical technicalsessions sessionsincluded includedaabroad broad range of talks focusing focusing on on ground ground water, water, petrography, petrography,sedimentology, sedimentology,mineralogy mineralogyand and structural topics. The final final session session ended ended at noon, noon, allowing allowing for an an early early departure departure of of Field Field Trip Trip 66 to to Red RedLake. Lake. Post Postmeeting meetingtrips tripsincluded: included:Field FieldTrip Trip44— - Industrial IndustrialMinerals Mineralsand and Paleozoic Geology 5 - Separation SeparationRapids RapidsRareRareGeology of Southeastern SoutheasternManitoba; Manitoba;Field FieldTrip Trip5— Element Pegmatite Geology of of the Red Lake - Geology Lake Camp. Camp. All All Pegmatite Field, Field, Ontario; Ontario;and andField FieldTrip Trip66— field field trips trips ran ran smoothly smoothly considering considering the the frigid frigid conditions conditionsof of early earlyMay Mayin innorthwestern northwestern Ontario. ILSG Secretary Secretary -Treasurer, -Treasurer, Mark Mark Jirsa Jirsa was the the lone lone participant participant of of Field FieldTrip Trip 66 successful in obtaining samples samples from from Goldcorp's Goldcorp's Red Red Lake Lake Mine Mine in in Red Red Lake. Lake.He Hewas was able to do this this by by cunningly cunningly embedding embedding the the samples samples in in the the back back of of his hisneck. neck.Upon Upon returning to Kenora Kenora the samples were proudly proudly displayed displayed in a baggy baggy kindly kindly supplied supplied by by the the staff of Red Red Lake's Lake's Margaret MargaretCochenour CochenourMemorial MemorialHospital Hospitalemergency emergencyroom. room. xvii xvii �Goldich Award Annual Banquet Banquet and Goldich the Annual Annual Banquet Banquet Ted Ted DeMatties DeMatties presented presentedthe the citation citation for for Ernest ErnestK. K.Lehmann, Lehmann, At the Goldich Medal Medal for 2002 for his his contributions contributions to to the the Institute lnstituteand and Lake Lake recipient of the Goldich Superior Geology. L. Harvey Harvey Thorliefson, Geological Survey of Canada, Canada, provided provided aa Thorliefson, Geological scintillating discussion on The Search Search for Diamonds Diamonds in in Canada Canada for the the after after dinner dinner address. Laurel Woodruff and and Bill Bill Cannon Cannon of the the U.S. U.S. Geological Geological Survey Survey invited invited 49th participants the 4gth Meeting in in Iron Iron Mountain, Mountain, Michigan. Michigan. participants to the Annual Meeting 2002 Best Student Student Paper Awards Awards 1) Becky Becky Rogala Rogala-- Lakehead LakeheadUniversity,Thunder University,Thunder Bay, Bay, Ontario Ontario ($400, oral presentation) presentation) formation from the Sibley New in information Sibley Group Group 2) Elizabeth ElizabethFein Fein-- Oberlin OberlinCollege, College, Oberlin, Oberlin, Ohio Ohio ($50, ($50, poster; poster; Co-authors Co-authors C.L. C.L. Sturm Sturm and D.M. Czeck) Anisotropy of magnetic susceptibility in the Ottertail Ottertailpluton, pluton, Northern Ontario Ontario 3) Claire ClaireSturm Sturm-- Oberlin OberlinCollege, College, Ohio Ohio($50, ($50, oral; oral; Co-authors Co-authorsD.M. D.M. Czeck Czeck and and E. E. Fein) Fein) Petrographic study of the Petrographic the Ottertall Ottertail pluton, Superior Superior Province, Province, Northwestern NorthwesternOntario Ontario Student Travel Awards 2002 Eisenbrey Student Awards 1) Bogdan BogdanNitescu Nitescu-- University Universityof of Toronto, Toronto, Toronto, Toronto, ON ON ($250) ($250) 2) Claire OberlinCollege, College,Oberlin, Oberlin,Ohio Ohio($200) ($200) ClaireSturm Sturm--Oberlin 3) Elizabeth ElizabethFein Fein-- Oberlin OberlinCollege, College,Oberlin, Oberlin, Ohio Ohio ($200) ($200) 4) Justin LakeheadUniversity, University,Thunder Thunder Bay, Bay, ON ON ($150) ($150) JustinJohnson Johnson--Lakehead 5) Becky Rogala Lakehead University, Thunder Bay, ON ($150) 5 ) Becky Rogala - Lakehead University, Thunder Bay, ON ($150) 6) William WilliamJahn Jahn--University Universityof of Minnesota, Minnesota, Duluth, Duluth, MN MN ($150) ($150) 7) Daniela Universityof of Western WesternAustralia, Australia,Nedlands, Nedlands, WA WA ($400) ($400) DanielaVallini Vallini--University 2002 Goldich Goldich Medal Recipient 2002 Ernest K. Lehmann Lehmann Archives Donation MTU Archives Donation A check check for for $100 $100 was was sent sent to to Michigan Michigan Technological TechnologicalUniversity UniversityArchives, Archives, as as required requiredby by Board agreement ($1 per per participant participant per per meeting), meeting), for for maintenance maintenanceof of ILSG ILSGproceedings proceedings archives. archives. Proceedings Proceedings including Part Part 11 (Programs (Programs and and Abstracts) and and Part Part 22 (Field (Field Trip Trip Guidebook) Guidebook) are available available from from the the Institute: lnstitute: Institute on Lake Superior Geology lnstitute do c/o Mark Mark Jirsa, Jirsa, Secretary Secretary -- Treasurer Treasurer Minnesota Minnesota Geological Geological Survey 2642 University University Avenue St. Paul Paul MN MN 55114-1 55114-1057 057 612.627.4539 Phone: 61 2.627.4539 Fax: Fax: 612.627.4778 612.627.4778 e-mail: jirsaOO1 jirsaool @tc.umn.edu @tc.umn.edu xviiiii xvi �48th ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY BOARD OF DIRECTOR'S MEETING Board of Directors Directors Peter Hinz (2002 (2002General GeneralChair) Chair) Michael Mudrey Mudrey (2001 (2001 Co-chair) Co-chair) Steve Kissin Kissin (2000 (2000 Co-chair) Co-chair) Laurel Woodruff: Proxy Proxy for Ted Ted Bornhorst Bornhorst (1999 (1999 Co-chair Co-chairand and liaison liaisonwith with Goldich Goldich committee) committee) Mark Jirsa (Institute (Institute Secretary-Treasurer) Secretary-Treasurer) Guests Guests Phil Fralick Fralick (2000 (2000 Co-chair) Co-chair) Carmen Carmen Storey Storey (2003 (2003 Program ProgramChair) Chair) O'Flaherty (2003 Kevin O'Flaherty (2003 Program ProgramChair) Chair) Bill Cannon (proposed 2003 Co-chairs) (proposed 2003 Co-chairs) Rod Johnson (Goldich (Goldich Committee) Committee) Frank Luther (Goldich (Goldich Committee) The The following following is based on the secretaries' notes notes and and recollection; recollection; any any omissions omissions or or unintentionaL Motions misstatements are unintentional. Motions by by the the Board Board of Directors Directors are are generally paraphrased—"approved" paraphrased-"approved" or or "accepted" "accepted"implying implying that that aa motion motionwas wasmade, made, seconded, and passed unanimously. The Theexpression expression"generally "generallyagreed" agreed" carries carriesless less be pursued. pursued. Some formality, but indicates a directive that will be Some issues issues that that were were conference are are included included here here for for resolved after the Board meeting, but during the conference closure. closure. MINUTES MINUTES 1. Accepted Acceptedreport reportof of the the Chairs Chairsfor for the the 47th 47th ILSG, ILSG, Madison, Madison, Wisconsin; Wisconsin; as as printed printed in in the the Proceeding Volume (Mudrey), and minutes of last Board Board meeting, meeting, May May 10, 10, 2001 (Jirsa) (Jirsa) 2. Received, Received,discussed, discussed, and andaccepted accepted2001-2002 2001-2002 ILSG ILSG Financial FinancialSummary Summary (Jirsa). (Jirsa). (4gth 3. Discussed annual) meeting Discussedand and approved approved 2003 2003 (4gth meeting location—Iron location-Iron Mountain, Mountain, Michigan, Michigan, and tentative co-chairs co-chairs Laurel Laurel Woodruff Woodruff and and Bill Bill Cannon, Cannon,USGS. USGS.As As currently currently envisioned, Ted Bornhorst Bornhorst will handle handle logistics logistics of field field trips. 4. Approved ApprovedPeter PeterHinz Hinzas ason-going on-goingILSG ILSGBoard Boardmember. member. 5. Discussed Discussedreplacing replacingRod RodJohnson Johnsonas as the the "member "member from from industry" industry" on on Goldich Goldich Committee Committee (end (end of term term 2002) 2002) with with several several candidates candidates including including Dave Dave Meineke Meinekeof of Meriden Meriden Engineering, Hibbing, Minnesota. Dave later accepted the position and was welcomed, Engineering, Minnesota. Dave later accepted the position and was welcomed, and Rod was thanked for his service to the Institute, during the the annual banquet. Dave's Dave's term will end after Goldich Goldich selection selection for the the meeting meeting of of 2005. 2005. 6. Discussed Discussedreplacement replacementof of Mark MarkJirsa Jirsaas as ILSG ILSGSecretary-Treasurer Secretary-Treasurer(end (endof of 4-year 4-year term term 2002). A new member member to the the Institute, Institute, Peter Peter Hollings, Hollings,Lakehead LakeheadUniversity Universityin in Thunder Thunder Bay, was installed installed as "Secretary-Treasurer "Secretary-Treasurer in-training," pending pending a vote vote by by the the general general membership membership (as (as required required in in By-Laws). By-Laws). Because Because of his his newness newness to to the the Institute, Institute,the the board board generally agreed that Peter Peter would serve 2 years of the the 4-year term term concurrently concurrentlywith with Mark Mark in a period period of transition. At the the end end of the the 22 years years (following (followingthe the 2004 2004 meeting), meeting),the the finances and records records of the the institute, institute, and and responsibilities responsibilitiesof the the position positionwould would fall fall to to Peter. Peter. was generally generally This was presented to the membership after the Board meeting, and was accepted. accepted. 7. Other Other business: business: a) Discussed Discussed the offer by by Mike Mike Mudrey Mudrey to take take over as as ILSG ILSG webmaster—It webmaster-It was was generally agreed that Mike Mike could could do that, assuming assuming Ted Ted was was busy busy with with other other obligations and probably probably would not mind mind the relief. relief. Subsequent Subsequent discussions discussions indicate indicate xix xix �that Ted Ted would would like like to to continue continuein in this this endeavor, endeavor, and and has has already alreadypaid paidin inadvance advancefor for that 55 years years of of web web service service to to continue. continue. ItIt remains remains in in Ted's Ted's hands. hands. b) Discussed Discussedefforts effortsby by Graham GrahamWilson Wilsonto to list listILSG ILSGpublications publicationsas aspart partofofhis his b) MINLIB project and S t e v e Kissin Kissin volunteered volunteered to and website (www.turnstone.ca) -—Steve contact Graham Graham and and see see ifif there there isis anything anythingthat that the the ILSG ILSGcan canand andshould shoulddo doto to contact assist. assist. Discussedthe the prospect prospect of of extending extending aa "free "free ride" ride" to to annual annual Goldich GoldichMedal Medal c) Discussed recipients. ItIt was was generally generally agreed agreed that that registration registrationcosts costs should shouldbe bepaid paidby bythe the recipients. annual meeting meeting committee, committee, and and that that lodging, lodging, meals, meals, and and travel travel costs costscould couldbe bepaid, paid, annual at the the discretion discretion of of the the annual annual meeting meetingchairs. chairs. Discussed the the ILSG ILSG Newsletter—Peter NewsletterÑPete Hinz Hinz has has offered offeredto to write write ititbeginning beginninginin d) Discussed 2004 or or so. so. He Hecan cancoordinate coordinatewith with Ted TedBornhorst Bornhorstabout aboutthat thattransition. transition.The Thetopic topicofof 2004 whether the the Newsletter Newslettershould shouldremain remainpaper, paper, or or be bechanged changedto toaawholly whollyelectronic electronic whether format was was discussed discussedand andtabled. tabled. Most Mostseemed seemedto to think think we weshould shouldeventually eventuallyswitch switch format to to aa web-based web-basednewsletter, newsletter,perhaps perhapswith with email emailnotification. notification.This Thisraised raisedaafurther further issue that members members must must be be encouraged encouraged to to notify notify the the secretary-treasurer secretary-treasurerof of issue changes in email address or other status. changes in email address or other status. e) Questionable Questionablesampling—An sampling~An issuewas was raised raisedthat that at at least leastone onegroup groupof ofregular regular e) issue meeting meeting participants participants has has aa tradition tradition of of using using guidebooks guidebooksto to locate locateplaces placesfor formassive massive sampling programs. programs. In In this this one one case, case, samples samples are sold sold to Wards Wards or other other rock rock and and sampling 1) some some of of the the localities localitiesdiscussed discussedin in mineral specimen specimen dealers. dealers. The The problems problems are are 1) mineral guidebooksare are on on private privateland land(and (andtherefore thereforetrespassing trespassingisislikely), likely),and and2) 2)taking taking guidebooks large amounts amountsof of sample samplefrom fromsome somelocalities localitieslimits limitsthe theuse useof ofthese thesesites sitestotofuture future large generations.ItItwas was generally generallyagreed agreedthat that ILSG ILSGwould would print printinintheir theirguidebooks guidebooksaa generations. Policy Statement Statement that that warns warns of this this "questionable "questionablesampling sampling practice." practice." Mark Mark Jirsa Jirsa will will Policy create create such such language languagefor for inclusion inclusionin infuture futureguidebooks. guidebooks. f)f) Discussed Hinz abstracts~peter Hinzwarns warns from fromexperience experiencethat that Discusseddigital digitalsubmission submission of of abstracts—Peter this practice practice can can easily easilyturn turn into intoaa nightmare nightmarefor for preparers, preparers,particularly particularlyififthe the this submitters don't don't follow follow (or (or the the host host organization organization doesn't specify) specify) rigid rigid guidelines guidelines for submitters submission formats. Adjournment Adjournment submission formats. This This includes includes both both text and illustration formats. Respectfully Respectfullysubmitted submittedon on January January 27, 27, 2003 2003to to Peter PeterHinz, Hinz,Chair Chair of of the the48th 48thannual annual meeting, meeting, for for incorporation incorporationinto intothe theReport Reportof of the theChair Chairto toappear appearininProceedings ProceedingsVolume Volume 49. 49. Mark Mark Jirsa, Jirsa, Secretary-Treasurer, Secretary-Treasurer,Institute Instituteon onLake LakeSuperior SuperiorGeology Geology xx �0 0 -o PROGRAM >< x xxi �49th The following companies made generous contributions to the 4gth Annual Meeting. Meeting. We thank them and and John John Gartner Gartner of of the the Local Local Committee Committee for for their commitment to the Institute on Lake Superior Geology. For almost commitment the Institute on Lake Superior Geology. For almost 50 years this sustained interests interests of this organization organization has thrived through through the sustained individuals, corporations, universities, and government agencies individuals, corporations, universities, and government agencies in the the international geologic community. community. This This dedication dedicationtoto an an exchange of international geologic exchange of scientific ideas and aa passion for for field field trips trips (even (even in in driving driving rain rain or or snow) snow) has enabled the ILSG to to fulfill fulfill one one of of its its primary primary objectives: objectives: to promote better understanding of the geology in the Lake Superior region. Kleiman Pump & Well Drilling, &Well Drilling, Inc. Inc. P.O. Box Box 704 704 Iron Mountain, Michigan 49801-0704 49801-0704 Prime Meridian Meridian Resources Resources Ltd. Ltd. N7478 Niagara Niagara Lane Lane Fond du Lac, Lac, WI Wl 54935 54935 Coleman Engineering Engineering Company Company 635 Circle Drive Drive Iron Mountain, MI 49801 49801 xxii �+ WEDNESDAY MAY 7, 2003 8:00 a.m. TRIP 8:00 a.m.FIELD FIELD TRIP1:1WISCONSIN : WISCONSINMAGMATIC MAGMATICTERRANE TERRANE (#1 (#I IN IN GUIDEBOOK) GUIDEBOOK) Klaus Schulz, U.S. Geological Geological Survey Oshkosh, emeritus Gene LaBerge, LaBerge, University University of of Wisconsin Wisconsin— - Oshkosh, 2: THE ORE 8:00 a.m. FIELD TRIP 2: THEREPUBLIC REPUBLICMINE MINE— - LIFE LIFE CYCLE CYCLEOF OF AN IRON ORE FROM GENESIS GENESIS TO TO RECLAMATION RECLAMATION(#4 (#4 IN GUIDEBOOK) GUIDEBOOK) DEPOSIT FROM William William Cannon, U.S. US. Geological Geological Survey John Meier, Meier, Cleveland Cleveland Cliffs Cliffs Iron Iron Company Company 6:00 p.m. p.m. Return Return of Trips 11 and 6:00 and 2 4:00 p.m. p.m. -- 8:00 8:00p.m. p.m. Registration Registration 7:00 p.m. p.m. -- 9:00 9:00p.m. p.m. Ice Breaker Social and Poster Setup THURSDAY MAY 8, 2003 8:00 a.m. -- 9:00 9:00a.m. a.m.REGISTRATION REGISTRATION Note: Technical Technical Sessions Sessionsare are in inWhite White Spruce, Spruce, Pine Pine Mountain Mountain Resort Resort + Denotes Denotes Student Presentation Presentation 8:15 a.m. 8:15 a.m. INTRODUCTORY INTRODUCTORYREMARKS REMARKS Laurel Laurel G. Woodruff and and William William F. F. Cannon, Cannon, Co-Chairs Co-Chairs TECHNICAL TECHNICAL SESSION SESSION II Session Chair: Jim Miller, Minnesota Minnesota Geological Survey, Duluth, MN MN - 8:30 a.m. Harold HaroldBernhardt Bernhardt- Menominee MenomineeRange RangeHistorical HistoricalFoundation FoundationMuseum Museum A brief history history of iron iron mining mining on on the the Upper Upper Peninsula's Peninsula's Menominee MenomineeIron Iron Range Range 9:00 a.m. a.m. Cannon, Cannon, W.F., W.F., LaBerge, LaBerge, G.L. GL. and Klasner, Klasner, J.S. J.S. Niagara suture suture zone, northern northern Michigan Michigan and and Wisconsin—tectonics Wisconsin-tectonics in in the the 1.85 1.85 Ma arc-continent arc-continent collisional collisional boundary boundary 9:30 a.m. a.m. Schulz, Schulz, K. 9:30 A Paleoproterozoic Paleoproterozoic suprasubduction suprasubduction zone ophiolite-island ophiolite-island arc arc complex complex in northeastern northeastern Wisconsin Wisconsin 10:00 10:OO a.m. a.m. COFFEE COFFEEBREAK BREAKAND ANDPOSTER POSTER SESSION SESSION 10:40 a.m. a.m.Schneider, Schneider, D.A., D.A., Holm, HoIm, D.K., D.K., O'Boyle, O'Boyle, C., C., Hamilton, Hamilton, M. and Jercinovic, Jercinovic, M. 10:40 M. Paleoproterozoic development of a gneiss dome corridor in the the southern southern Lake Lake Superior region, USA USA 11:00 a.m. Holm, Hoim, D.K., D.K, Van 11:OO a.m. Van Schmus, Schmus, W.R., W.R., MacNeill, MacNeill, L.C., L.C., Boerboom, Boerboom, T.J., T.J., Schweitzer, Schweitzer, D. D. and and Schneider, Schneider,D.A. D.A. Late Paleoproterozoic (1900-1600 (1900- 1600 Ma) tectonic history of the the northern northern midmidU.S.A.: Implicationsfor for crustal crustalstabilization stabilization continent, U. S.A. :Implications 11:20 a.m. Medaris, Medaris, L.G., L.G., Jr. Jr. and Dott, 11 :20 a.m. Dott, R.H., R.H., Jr. The Sioux Quartzite metamorphism, geochemistry Quartzite revisited: sedimentology, metamorphism, geochemistry and and the origin of pipestone 11:40 p.m. Smyk, M.C. 1 1:40 p.m. The planned activities activities and The Lake Lake Nipigon Nipigon Geoscience GeoscienceInitiative Initiative— - planned and objectives objectives xxiii xxiii �12:00 p.m. Lunch Poster Session Session and 12:OO p.m. LunchBreak Break— - Poster and ILSG ILSG Board BoardMeeting Meeting(by (byinvitation) invitation) TECHNICAL SESSION SESSION IIII TECHNICAL Session Chair: Mike MikeMudrey, Mudrey, Jr., Jr., Wisconsin Wisconsin Geological Geological Survey, Survey, Madison, Madison, WI Wl 1:30 p.m. •: Heggie, Heggie, G. G. and and Hollings, Hollings, P. 1 :30 p.m. P. Geochemistry and mineralization of the Seagull Intrusion, Northern Ontario Geochemistry Ontario +:+ p.m.++:•:•Johnson, Johnson,J.R., J.R.,Hollings, Hollings, P. P. and and Kissin, Kissin, S.A. 1:50 p.m. Mineralization Mineralization of the Norton Lake Cu-Ni-PGE Cu-Ni-PGE deposit 2:10 p.m. Miller, J. J. D., Jr. 2:lO Petrology and PGE potential of the Greenwood Lake Intrusion, central central Duluth Complex, Lake County, County, Minnesota Minnesota p.m. •: Joslin, Joslin, G.D., Miller, J.D., Jr. and 2:30 p.m. G.D., Miller, and Rowell, Rowell,W.F. W.F. Stratiform Pd-Pt-Au mineralization in the Sonju Lake Intrusion, Lake County, Minnesota Minnesota +:+ 2:50 p.m. p.m. + + Marma, J., J., Brown, Brown, P. 2:50 P. and and Hauck, Hauck, S. S. Magmatic Magmatic and hydrothermal hydrothermal PGE mineralization mineralization of the Birch Birch Lake Lake Cu-Ni-PGE Cu-Ni-PGE Deposit Deposit in the the South South Kawishiwi, Duluth Complex, northeast northeast Minnesota Minnesota 3:10 p.m. 3:lO p.m.COFFEE COFFEEBREAK BREAKAND ANDPOSTER POSTER SESSION SESSION Waggoner, T. 3:30 p.m. Waggoner, T. Marquette Range perspective A hydrothermal hydrothermal component component of of Iron IronFormations Formations—A -A Marquette p.m. Tsu-Ming Tsu-Ming Han 3:50 p.m. Han Mode of occurrence occurrence of trona trona and and thermonatrite thermonatrite and and their their possible possible origin origin in in the the Iron-Formation of the Marquette Negaunee Iron-Formation Marquette Range, Lake Superior District, District, USA USA 4:10 p.m. Blaske, Blaske, A.R. 4:lO A.R. Mississippi-Valley at Bellevue, Michigan Geology of the MississippiValley type mineralization at 4:20 p.m. p.m. + Larson, Larson, P. 4:20 P. Mean transport length in tills of the southern portion of the Laurentide Laurentide ice sheet: implications for drift exploration exploration in the Lake Superior region +:+ 4:50 p.m. p.m. 63+ Marlow, Marlow, L., Mooers, H. and Larson, Larson, P. 4:50 P. environmental history Glacial Lakes Aitkin and Upham: their origin and environmental 5:10p.m. TrowJ. 5:10 p.m. Trow, J. Five gold possibilities in some wan copper copper sulfides sulfides in Ontario some Keweena Keweenawan Ontario and Michigan Michigan xxiv �6:00 p.m. CASH BAR 6:00 p.m. ICE BREAKER BREAKER— - MIXER — - CASH 7:00 p.m. p.m. ANNUAL ANNUALBANQUET BANQUETAND ANDAWARD AWARDPRESENTATION PRESENTATION 50th • Location Announcement of 5oth Annual Meeting Location • 2003 Klaus Schulz Schulz 2003 Goldich Goldich Award Presentation Presentation to Klaus • 2003 Banquet Banquet Address 2003 Dr. Susan Martin, Martin, Michigan Michigan Technological Technological University University The indigenous indigenous people of the Lake Superior Basin: Understanding Understanding the links links among environment, environment, geology geology and religious religious belief Meeti ng participants Meeting participantswho whoare arenot notregistered registeredfor forthe thebanquet banquetare arewelcome welcometo tothe thebanquet banquetaddress address FRIDAY MAY 9, 2003 TECHNICAL SESSION Ill Ill Session Session Chair: Eric Jerde, Morehead Morehead State State University, Morehead, Morehead, Kentucky Kentucky 8:20 8:20 a.m. a.m. INTRODUCTORY INTRODUCTORYREMARKS REMARKS Laurel G. Woodruff Woodruff and and William William F. F. Cannon, Cannon, Co-chairs Co-chairs Laurel 8:30 a.m. a.m. Hollings, Hollings, P., P., Fralick, Fralick, P. P. and and Kissin, Kissin,S. S. Geochemistiy and of the the1537 1537Ma Ma Redstone Redstone Point Geochemistry and geodynamic geodynamicimplications implications of Point anorogenic anorogenic granite, Ontario, Canada Canada 8:50 a.m. a.m. +*.:. Buttram, P.M. R.M. and and Bjornerud, M. : Buttram, M. Textural constraints in in thethe Wolf River BatBatholith ho 11th Textural constraintson onthe theorigin originofofrapakivi rapakivitextures textures Wolf River 9:10 a.m. a.m. + .:.Sandin, Sandin, N.A. N.A. and and Bornhorst, Bornhorst, T.J. Sequence of Precambrian Precambrian mafic dikes in Marquette County, County, Michigan, Michigan, with with emphasis on the Sugar/oaf Sugarloaf Mountain and Republic areas 9:30 a.m. a.m. Jerde, Jerde, E.A. E.A. Gabbro/granophyre theCrocodile CrocodileLake Lake Intrusion: Intrusion: aa possible possible vent vent Gabbro/granophyre relations relations ofofthe for the Hovland Lavas? for the Hovland Lavas? 9:50 Vislova, 9:50 a.m.. :.:• 0 Vislova, T. T. Evaluation of initial initial magma compositions compositions for the Bald Bald Eagle Eagle Intrusion and Evaluation of magma for the Intrusion and associated rocks rocks associated 10:10 a.m. COFFEE 10:10 COFFEEBREAK BREAKAND AND POSTER POSTER SESSION SESSION 10:30 a.m. a.m. + •:•Charkoudian, Charkoudian, K., K., Tikoff, Tikoff, B. and :+ and Bauer, Bauer, R. R. Stike -slipseparation separationofof Burntside trondhjemite Wakemup Bay Bay Stike-slip thethe Burntside trondhjemite andand thethe Wakemup tonatilte,Northern NorthernMinnesota Minnesota tonatlite, 10:50 1O:5O a.m. <+ .:.Garbowicz, Garbowicz, A. A. and Bjornerud, Bjornerud, M. M. Wisconsin Paleostress inferences from slip vectors the eastern eastern part part of the Paleostress inferences from slip vectors in in the the Wisconsin segment rift segmentofofthe theMidcontinent Midcontinent rift 11:10 Potter, E.G. E.G.and and Mitchell, Mitchell, R.H. 11:10 a.m. +*•:• : Potter, R.H. The and exotic exoticmineralogy mineralogyofofthe the WesternSubcomplex Subcomplex of of the theDeadhorse Deadhorse The rare rare and Western Creek Diatreme, Northwestern Ontario Creek Diatreme, Northwestern Ontario xxv XXV �B.A., Mudrey, Mudrey, M.G., M.G., Jr., Jr., Czechanski, Czechanski, M.L., M.L., Reid, Reid,D.D. D.D. and andHunt, Hunt,T.C. T.C. 11:30 a.m. a.m. Brown, Brown, B.A., Highway construction, construction, mine mine reclamation, reclamation, and and land-use land-use planning planning challenges challenges in in the historic historic Upper Upper Mississippi Mississippi Valley Valley lead-zinc district of southwest southwest Wisconsin Wisconsin 11:50a.m. 11:50 a.m. Wattrus, Wattrus, N. N. High-resolution multibeam bathymetry in Lake Lake Superior Superior High-resolution 12:10 p.m. p.m.LUNCH LUNCHBREAK BREAK—-POSTERS POSTERSREMOVED REMOVEDAFTER AFTER LUNCH LUNCH 12:10 \ TECHNICAL SESSION IV IV TECHNICAL Session Chair: Peter PeterHinz, Hinz,Ontario OntarioGeological GeologicalSurvey, Survey, Kenora, Kenora, ON ON 1:40 a.m. a.m. +*:•:•Vallini, Vallini,D.A., D.A.,McNaughton, McNaughton,N.J., N.J.,Rasmussen, Rasmussen,B., B.,Fletcher, Fletcher, I. I. and and Griffin, Griffin, B.J. B.J. U-Pb geochronology to unravel the history history of Proterozoic Proterozoic Using xenotime U-Pb sedimentary basins: basins: aastudy studyininWestern WesternAustralia Australia and andthe theLake LakeSuperior Superiorregion region sedimentary 2:00 p.m. p.m. Kissin, S.A., S.A., Vallini, D.A., D.A., Addison, W.D. W.D. and Brumpton, Brumpton, G.R. G.R. New zircon ages from from the the Gun Gunflint Ontario flint and and Rove Formations, northwestern northwestern Ontario 2:20 p.m. p.m. +%+ Richardson, Richardson, A., A., Fralick, P. and Hollings, Hollings, P. P. Sibley Basin sediment provenance using zircon and whole rock geochemical methods: Possible Possiblesource sourceareas areasof of the thePass Pass Lake Lake Formation Formation G. 2:40 p.m. p.m. ++:•:•Rogala, Rogala,B., B.,Fralick, Fralick, P. P. and and Borradaile, Borradaile, G. A magnetostratigraphic magnetostratigraphic and secular variation study study of of the Sibley Group 3:00 3:00 p.m. p.m. COFFEE COFFEE BREAK BREAK 3:20 p.m. Argast, A. sediment chemistry tell us about rocks like those from the What does sediment the Fern Fern Creek Formation? 3:40 p.m. p.m. Bartnik, Bartnik, P. J. and and Evans, B. B. W. ford, Michigan area Geology and hydrogeology hydrogeology in in the the Kings Kingsford, 4:00 p.m. p.m. Presentation Presentation of Student 4:00 Student Paper Paper Awards Awards Bornhorst, Michigan Michigan Technological University: University: Student Paper Committee Ted Bornhorst, Committee SATURDAYMAY MAY10,2003 10, 2003 SATURDAY 8:00 a.m. a.m. FIELD FIELDTRIP TRIP3: 3:MENOMINEE MENOMINEEIRON IRONRANGE RANGE (#2 (#2 IN IN GUIDEBOOK) GUIDEBOOK) Gene emeritus Gene LaBerge, LaBerge, University Universityof of Wisconsin Wisconsin— - Oshkosh, emeritus John Klasner, University University of of Western WesternIllinois, Illinois,emeritus emeritus William Cannon, U.S. William U.S. Geological Geological Survey 6:00 p.m. p.m. Return Return of Trip 3 6:00 3 SUNDAYMAY MAY11, 2003 SUNDAY 11,2003 TRIP4:4:Iron Iron River River — Crystal Falls Falls Iron ININGUIDEBOOK) 8:00 a.m. FIELD TRIP - Crystal IronDistrict District(#3 (#3 GUIDEBOOK) Gene Oshkosh, emeritus Gene LaBerge, LaBerge, University Universityof of Wisconsin Wisconsin— - Oshkosh, John Klasner, Klasner, University University of of Western WesternIllinois, Illinois, emeritus emeritus William Cannon, Cannon, U.S. Geological Survey 6:00 p.m. p.m. Return Return of of Trip 4 6:00 xxvi xxvi �POSTER PRESENTATIONS Brown, Czechanski, M.L., Mudrey, MG., M.G., Jr. Jr. and andReid, Reid, D.D. D.D. Brown, B.A., Czechanski, Wisconsin mineral resource GIS GIs and related digital map and database database products Wisconsin —a a progress report Boerboom, Boerboom,T. T. Bedrock geologic geologic maps maps of of Keweena Keweenawan intrusive rocks rocks in in the the Bedrock wan volcanic and intrusive North Shore Shoreof of Lake Lake Lakewood, Lake wood, French French River, River, and and Knife River 7.5' quadrangles, quadrangles, North Superior, Minnesota Easton, Easton, R.M. P.M. Geology and mineral potential of Proterozoic Proterozoic mafic intrusions in the the northern northern Grenville Province of Ontario Grenville Ontario Hart, Hart, T.R. Keweena wan mafic ma f/cand andultramafic ultramaficintrusive intrusive rocks rocks of of the Lake Nipigon Nipigon and Keweenawan and Crystal Crystal Lake areas, areas, northwestern Ontario Ontario S.A., Oreskovich, Oreskovich, J.A. and Hauck, S.A., and Severson, Severson, M.J. M.J. and Beaver Bay Geology, drill holes, mineral leases, and geophysics in the Duluth and Integration of of various various GIS GIs databases databases to to tell tell a Compexes, northeastern Minnesota: Integration story story of the the history history of of past past and and current currentCu-Ni-PGE Cu-Ni-PGEmineral mineral exploration exploration •+ Heiling, C.D. Heiling, :+ C.D. Peperites of the Gafvert Gafvert Lake Volcanic Volcanic Complex, Complex, St. Louis County, County, Minnesota Minnesota Hocker, S.M., S.M., Hudak, Hudak, G.J., G.J., Odette, Odette, J.D. J.D. and Newkirk, ++•:. : Hocker, Newkirk,T.T. T.T. alteration mineral phases at the Five Mile Lake volcanic-hosted Chemistry of alteration volcanic-hosted massive massive sulfide sulfide prospect, NE NE Minnesota Minnesota ++•: Keatts, Keatts,M.J., M.J., Jirsa, Jirsa,M. M. and and Hoim, Holm, D. D. single-grainanalyses analyses of of Precambrian Precambrian mafic intrusions intrusions in Results of 40ArI9Ar ^Ar^Ar single-grain in northern northern and north-central north-central Minnesota Minnesota Mckenzie, M.A., and Jercinovic, Jercinovic, M. ++ McKenzie, M.A., Hoim, Holm, O.K., D.K., Schneider, D.A. and M. Evidence for largeResults of EMP monazite geochronology in in E-C E-C Minnesota: Minnesota: Evidence scale geon 17 17 metamorphism metamorphism associated associated with with post-tectonic post-tectonic plutonism plutonism Metsaranta,R., R.,Fralich, Fralich, P. P. and and Hollings, Hollings, P. ++:•:•Metsaranta, geochemical investigation volcanic and metasedimentary A geochemical investigation of of Mesoarchean Mesoarcheanmeta metavolcanic metasedimentary rocks from Uchi greenstone greenstone belt from the theBirch Birch— - Uchi Nicholson, S. Nicholson, S.W. W. and Cannon, Cannon, W.F. W.F. Stratigraphy wan rocks of the St. Stratigraphy and and structure structure of of Keweena Keweenawan St. Croix Croix horst, horst, northwestern northwestern Wisconsin Wisconsin Stott, G.M., G.M., Davis, D. W., K.J. and Tomlinson, Tomlinson,K. K.Y. Y. Stott, W.,Parker, Parker,J.R., JR., Straub, 1(4. Archean tectonostratigraphic assemblages of eastern Waba goon Subprovince, Archean tectonostratigraphicassemblages of eastern Wabagoon Subprovince, northwestern north western Ontario Ontario xxvii ��ABSTRACTS xxvffl ��WHAT DOES SEDIMENT CHEMISTRY TELL US ABOUT ROCKS LIKE THOSE FROM THE FERN CREEK FORMATION? Department of Geosciences, Indiana University Argast, Anne, Department University Purdue Purdue University University Fort Fort Wayne, Fort Wayne, IN 46805-1499, 46805-1499, Argast@ipfw.edu Argast@ipfw.edu Bulk chemical chemical analyses analyses are are accepted accepted and and powerful powerful tools tools for for the the study study of of metamorphic metamorphicand and igneous rocks. rocks. Bulk Bulk chemical chemical analyses analyses are are less less accepted accepted and and less less widely widely used used for for the the study studyof of sedimentary rocks. This is at least partly the result of a widely-held view that chemical chemical data data are are unreliable indicators indicators of of sedimentary sedimentary events events due due to to the the post-burial post-burial diagenetic diageneticalteration alterationof of the the sediment. In recent years, this view has been reinforced with studies sediment. studies indicating indicating the the potential potential for for extreme extreme diagenetic diagenetic alteration alteration of of sediments, sediments,with with km-scale km-scale transport transport proposed proposedin insome somesystems systems (e.g., Wintsch and Kvale, 1994). 1994). Potassium Potassium is often singled-out singled-out as an especially especially mobile component in diagenetic systems. For example, example, Awwiller (1993), (1993), working working in in the the Gulf Gulf Coast Coast Tertiary, Tertiary,postulates postulatesan an increase increasefrom from2.0 2.0to to 3.8 wt. percent K20 1 0 pore K20in mudrocks, mudrocks, due due significantly to the transport of K as part of i03 volumes of fluid fluid passing through through the the system system in in the the depth depth range range from from 1500 1500to to 4000 4000m mbelow below the the surface. surface. An alternate alternate view (Argast (Argast and and Donnelly, Donnelly, 1987) 1987)maintains maintains K20 K20isis aa generally generallyconservative conservative element in diagenetic settings, and that observed variations in K K20 2 0 content preserve compositional compositional characteristics characteristicspresent present at at and and before before accumulation. accumulation.Depending Dependingon on your your point-ofpoint-ofview, the chemistry chemistry of diagenetically diageneticallyand metamorphically metamorphically altered altered sedimentary sedimentaryrocks rocks may may (or (or may not) provide useful information information about about provenance, provenance, weathering weathering and and other other qualities qualitiesof of the the sedimentary sedimentary system. system. Unconsolidated sediments, Unconsolidated sediments, delivered delivered as turbiditic pulses of siliciclastic debris eroded from the Mountains, accumulated Himalaya Mountains, accumulated on the the Bengal Fan (DSDP (DSDP 218) 218) and and now now at at subbottom subbottomdepths depths from 12 noted in lithified 12 to 729 m, produce chemical trends very similar to those previously noted (and metamorphosed) sedimentary sedimentary rocks. The similarity in chemical trends across across this this broad broad range of conditions conditions and and environments environmentssuggests suggests sedimentary sedimentary chemical chemical trends trends arise arisefrom from fundamental conditions conditions imposed imposed upon upon the the system system before before burial, burial, and and are are not not necessarily necessarily the the result result of extensive extensive diagenetic diagenetic alteration alteration at at depth. depth. The Fern Creek Formation Formation (Early (Early Proterozoic, Proterozoic, Lower Lower Chocolay Chocolay Group) Group) is is well well exposed exposedalong along the the Sturgeon Sturgeon River near the dam dam northeast of of Loretto, Loretto, Michigan. Michigan. These These rocks rocks have have been been interpreted interpreted as glaciogenic in origin, origin, and and the diamictites diamictites they they contain contain used as as evidence evidence for for glacially-derived glacially-derived dropstone dropstone units. units. Others Others have have interpreted interpretedthe the Fern Fern Creek Creek Formation Formation as as nonglaciogenic nonglaciogenicin in origin origin with the sediments accumulated environments grading upward into lagoonal or accumulated in fluviatile environments estuarine environments. environments. Field and textural qualities (to be discussed as part of a post-meeting fieldtrip in the Menominee Menominee Iron Range) Range) support support aa glaciogenic glaciogenic origin. origin. 1 �chemistry also also supports supports aa glaciogenic glaciogenic origin origin (Argast, (Argast, 2002) 2002) . The chemical data, Bulk rock chemistry chemical data, including the absence absenceof of aa correlation correlationbetween betweenK20 K 2 0and andA1203, A1203,show sediments sedimentsfrom from the the Fern Fern including deposited without without extensive extensivesorting sortingor or demixing demixingof of hydraulically hydraulicallycoarsercoarserCreek Formation Formation were deposited + Na ++ K)/A1 and finer-grained finer-grained fractions. Other data, including the Na20/K20 Na20/K20 and (2Ca + K)/Al atomic atomic ratio suggest sediment accumulated with abundant original feldspar. The The chemical index of of 61, similar similar to to the average average CIA CIA of 57 57 in in Gowganda Gowganda diamictite diamictite alteration (CIA) ranges from 50 to 61, matrices. The accessory suite suite is complex and and includes includes poorly poorly rounded rounded zircons. zircons. These These attributes attributes are consistent with an origin as as aa glacial glacial till. till. . Several minerals enriched enriched in rare earth elements elements (REE) (REE) and/or andlor thorium thorium were were identified identifiedin in the the Fern Creek Formation. These include include monazite, huttonite huttonite (monoclinic (monoclinic ThSiO4) ThSi04) and a fluorfluorhydroxy-REE mineral. Th concentrations concentrations as high as 53 53 ppm were noted in one bulk analysis. Efforts Efforts to obtain obtain aa chemical chemical date date on on these these minerals minerals have have so sofar far been been unsuccessful. unsuccessful. The Carney Lake Gneiss Gneiss is aa chemically-compatible chemically-compatible possible possible source source for for the the Fern Fern Creek Creek Formation. Formation. REFERENCES Argast, A., 2002, 2002, The The lower lower Proterozoic Proterozoic Fern Fern Creek Creek Formation, Formation,northern northern Michigan: Michigan:mineral mineraland and bulk geochemical evidence J. Earth Earth Sci., Sci.,v. v. 39, 39,p. p.481481evidence for for its its glaciogenic glaciogenic origin: origin: Can. Can. J. 492. 492. Argast, S. and Donnelly, T.W., 1987, of clastic sedimentary 1987, The chemical discrimination of components: J. Sed. Pet., v. 57, p. 813-823. 813-823. components: Awwiller, D.N., 1993, Illite/smectite formation and potassium mass transfer during burial 1993, Illite/smectite diagenesis Sed. diagenesis of mudrocks: aa study study from from the Texas Texas Gulf Coast Coast Paleocene-Eocene: Paleocene-Eocene:J.J. Sed. Pet., v. 63, p. 501-512. 501-512. of 1994, Differential mobility of elements in burial diagenesis of Wintsch, R. P. and Kvale, C. M., 1994, siliciclastic siliciclastic rocks: J. Sed. Sed. Res., Res., v. v. 64A, 64A, p. p. 349-361. 349-361. 2 �GEOLOGY AND HYDROGEOLOGY IN THE KINGSFORD, MICHIGAN MICHIGAN AREA GEOLOGY BARTNIK, PATRICK J., pbartnik@arcadis-us.com, pbartnik@arcadis-us.com,ARCADIS ARCADIS G&M, G&M, Inc., Inc., Kingsford, Kingsford, Michigan, 49802; and EVANS, ARCADIS G&M, G&M,Inc., Inc.,Milwaukee, Milwaukee, EVANS, BRUCE BRUCE W., W., bevans@arcadis-us.com, bevans @ arcadis-us.com,ARCADIS Wisconsin, 53202 Wisconsin, 53202 Investigations have been undertaken in a portion of the City of Kingsford, Investigations Kingsford, Michigan Michigan and and Breitung Township, Michigan (the study study area) to determine determine the geologic geologic and and of glacial sediments sediments and and bedrock. bedrock. The hydrogeologic characteristics of The study study area is located located south-central Upper Peninsula Peninsula of Michigan. Michigan. in Dickinson County in the south-central The ARCADIS investigations investigations were largely largely completed between April April 1997 1997and and January January 2001, but are continuing. During Duringthe theinvestigations, investigations,over over300 300soil soil borings borings were were completed, along with 47 test pits and 9.5 miles of geophysical study. The The topography topography is is characterized by three distinct characterized distinct landform terraces (Upper, (Upper, Lower, and and Riverside), Riverside), which which range in elevation elevation from from approximately approximately 1,120 1,120feet feet above above mean sea sea level level (ft (ft msl) msl) to to approximately approximately 1,045 1,045 ft msl. The The.Upper Upper Terrace Terrace contains contains several several isolated isolated glacial glacial kettles. kettles. The elevation of the Menominee River is approximately 1,037 ft msl. The Thegeology geology is is 3 �comprised of unconsolidated unconsolidated glaciofluvial and and glaciolacustrine glaciolacustrinedeposits deposits of of clay, clay, silt, silt,sand, sand, and gravel that exhibit complex horizontal and vertical spatial variability. These These the Lower sediments overlie the Middle Precambrian Michigamme Slate and the Precambrian metavolcanic Quinnesec Formation. Formation. Depth to groundwater in the unconsolidated unconsolidated deposits deposits ranges from about 10 10 feet below land surface (bls) near the Menominee bis in the Upper Terrace. Groundwater Menominee River to more than 50 feet bls Groundwaterflow flow follows irregular pathways toward the Menominee River, but generally flows from northeast to southwest. +0.863ft/ft ft/ftin in upland upland southwest. Vertical Verticalhydraulic hydraulicgradients gradientsrange range from from +0.863 ft/ftnear nearthe theMenominee Menominee River. River. Hydraulic areas to —0.012 -0.012 ft/ft Hydraulicconductivities conductivitiesrange rangefrom from1 O3 centimeters per second (cdsec) (cm/sec) to to 10.' 101ccm/sec forcoarser-grain coarser-grainmaterial materialtoto1.03 1.03x x1 i0 d s e c for 0'~ cm/sec " c~d s e cfor forthe thevery veryfine-grain fine-grainsand sand and and sandy sandy silt. The Thebedrock bedrock is c d s e c to 3.94 xx 1i00cm/sec generally considered impermeable. Groundwater Groundwaterflow flow velocities velocities range range from from approximately 3 ft/day to 280 ft/day in coarser-grain units, and from approximately approximately0.1 0.1 approximately ft/day ft/day to 33 ft/day ftlday in in the the very very fine-grain fine-grain sand sandand andsandy sandysilt. silt. To aid in the understanding of the complex geology within the study area, threedimentional dimentional modeling of the geology was undertaken using the topographic surface, bedrock surface, and glacial sediments. sediments. Thirteen geologic units identified from the borehole data were categorized in to 3 units, based on permeability and anticipated effects on groundwater flow. The The modeling modeling and visualization of the geology were completed completed using aa Geostatistical GeostatisticalSoftware SoftwareLibrary Library (GSLIB), (GSLEB), developed developed at at Stanford Stanford University, FORTRAN programs, and and Environmental Environmental Visualization VisualizationSystem System(EVS) (EVS) software developed by the C Tech Development Corporation. Corporation. 4 �GEOLOGY GEOLOGY OF OF THE THE MISSISSIPPI-VALLEY MISSISSIPPI-VALLEYTYPE TYPE MINERALIZATION MINERALIZATIONAT BELLE VUE, MICHIGAN MICHIGAN BELLEVUE, BLASKE, Allan Allan R., R., Blaske BlaskeGeoscience, Geoscience,8313 8313Hartel, Hartel,Grand GrandLedge, Ledge,ME MI 48837 48837 BLASKE, The Bayport Limestone is exposed in quarrying quarrying operations at Bellevue, in southwestern southwestern Eaton Eaton County, Mining has County, Michigan. Michigan. Mining has been been active active around around Bellevue Bellevue since since the mid-1800's. Approximately 25 feet of the Bayport is exposed in the the quarrying quarrying operations, and consists of a gray to buff colored thin-bedded thin-bedded limestone. limestone. Bayport limestone Mississippian in comprises the upper portions portions of The Bayport limestone is is late Mississippian in age, age, and comprises the upper of the Grand Rapids Group. Group. It is underlain underlain by by the the Michigan Michigan Formation, Formation, also of the the Grand Grand Rapids Rapids Group. The Theearly earlyMississippian MississippianMarshall Marshall Sandstone Sandstone and and Coldwater Coldwater Shale Shale lie lie below below the the Grand Grand Rapids Group. The TheBayport Bayportisisoverlain overlainby bythe theearly earlyPennsylvanian PennsylvanianSaginaw SaginawFormation Formation(within (within the Michigan Basin), Basin), but but covered covered only only by by glacial glacial sand sand and and gravel gravel at at the the quarry quarry site. site. Mineralogy of the deposit deposit isis simple, simple,consisting consistingpredominantly predominantly of of pyrite, pyrite, marcasite, marcasite, and and calcite. calcite. Pyrite is most commonly commonly found as encrustations encrustations of cubic cubic crystals, crystals, formed directly directly on limestone. limestone. Marcasite is generally generally lighter lighter in in color color than than the the pyrite, pyrite, and and often often in in iridescent, iridescent,platy platycrystal crystalgroups. groups. Marcasite is by by far the dominant iron sulfide. sulfide. Two of calcite calcite are are observed. observed. Early Marcasite is dominant iron Two generations generations of calcite is found as small coatings. The small crystals crystals lining cavities as drusy drusy coatings. The second second generation generation of calcite is found in large, masses. Trace large, euhedral euhedral crystals and cleavable masses. Trace amounts amountsof of sphalerite, sphalerite, barite, and fluorite are present. Fluorite was the earliest to form, as small brown crystals Fluorite was the earliest to form, as small brown crystalsdirectly directly on the Marcasite and Pyrite was was formed formed in association association with with the the early early calcite. calcite. Marcasite and the limestone. limestone. Pyrite sphalerite sphalerite are later than the early calcite and pyrite. Second Second generation generation calcite calcite began slightly slightly after the marcasite. Tiny Tinycrystals crystalsof of marcasite marcasite can can also alsobe befound foundon onthe thelarge largecalcite, calcite,indicating indicatingthat that Barite appears later than formation of marcasite continued to the end of mineralization. formation the end of mineralization. appears than the the sulfides, sulfides, but before the end end of of the the calcite calcite formation. formation. Mineralization present predominantly predominantlyinin brecciated brecciated zones zones and and vein Mineralization isis present vein structures structures within within the Bayport Limestone. Limestone. The The most most common common type type of of breccia breccia consists consists of of small, small,angular angular clasts clasts surrounded by open-space open-space filling filling of of sulfides sulfides and and calcite. calcite. A second surrounded by second type type of of breccia breccia consists consists of larger, rounded clasts, clasts, with with the interstitial larger, rounded interstitial spaces spaces filled with aa muddy muddy limestone limestone and and pyrite. pyrite. Orientation Orientation and size of the mineralized zones within the limestone is not known, known, due due to to lack lack of of mapping. Fine-grained exposure within the quarry and insufficient historical mapping. Fine-grained iron iron sulfide sulfide is is also also observed as replacement structures, structures, along along apparent apparent solution solution fronts fronts within within the the massive massivelimestone. limestone. 36-element ICP of the sulfides indicates the the simplicity simplicity of of the the mineralization. mineralization. 36-element The geochemistry geochemistry of analysis of pyrite and marcasite separates, as well as as aa composite composite breccia breccia sample, sample, indicate indicate very very low concentrations of of trace elements. Copper, Copper, lead lead and andzinc zinc are arefound foundatatconcentrations concentrationsof of less less than 60 ppm; nickel is less than 30 is than 30 ppm; ppm; and and cadmium cadmium and cobalt less than 5 ppm. Barium is also low, generally less than 20 ppm. Manganese is high in the breccia (385 ppm), and lower in Manganese is high in the breccia (385 pprn), and lower in the sulfide separates (64 to 109 109 ppm), pprn), while chromium is high in the sulfides sulfides (150 (150 ppm) and and low 5 �in the breccia (31 ppm). Arsenic Arsenic isis present present in in the the breccia breccia (7 (7 ppm), ppm), but low in the the iron iron sulfides sulfides at less than 5 ppm. Sulfur isotopic were analyzed on separated Sulfur isotopic compositions compositions were analyzed on separated samples samples of pyrite, pyrite, marcasite, marcasite, and and sphalerite. Sulfur Sulfur isotopic isotopic compositions compositions (34S) (S^S) of of the the sulfide sulfide phases phases from from the Bayport Limestone are 14.5°/ for for for are 14.5°/o forthe the pyrite, pyrite, 12.8°/ 12.8°/o forthe themarcasite, marcasite, and and 19.8°/ 19.8°/o forthe thesphalerite. sphalerite. These compositionsindicate indicatethat that the the mineralizing fluids were were basinal basinal brines compositions mineralizing fluids brines from within within the the surrounding Mississippian Mississippian and and Pennsylvanian Pennsylvanian formations. formations. Unpublished surrounding Unpublished data data obtained obtained from from the the USGS (Westjohn, D. D. B., pers. comm.) as part of the RASA program indicate a large range range of of program indicate 6S ininsamples S^S samplescollected collectedfrom fromthe theunderlying underlyingMarshall MarshallSandstone Sandstoneand andMichigan Michigan Formation, Formation, as well as the overlying Formation and and the the Jurassic Jurassic Red Red Beds. Beds. Pore well overlying Saginaw Formation Porewater, water,whole-rock, whole-rock, sulfide, and sulfate sulfur sulfur isotope isotope compositions compositions for the the underlying underlying formations formations exhibit exhibit average average 345 20°/, while S^S near 20°/oo whilethe the average average S^S within the overlying overlying formations formationsisisnear near170/00. 17 Oleo. Temperature of the mineralization has been determined determined using fluid inclusions in in calcite calcite (Panter, (Panter, K. 5., S., 2001). 2001). Calcite afforded the only only mineral mineral phase phase with with inclusions inclusionsfor formicrothermometric microthermometric study. Temperatures Temperaturesof of homogenization homogenization indicate indicate aa bimodal bimodal distribution, distribution, with with aa low low temperature temperature mean of approximately approximately 58°C, 5g°Cand a high temperature mean of approximately approximately 138°C. 138OC. The Themean mean These temperatures are similar to those temperature temperature of all all inclusions inclusions analyzed analyzed was was 107°C. 107OC. These temperatures are those the Mississippian observed using isotopic compositions in authigenic minerals in observed using isotopic compositions authigenic minerals Mississippian and and Pennsylvanian sandstones (Westjohn, (Westjohn, D. B., Pennsylvanian sandstones B., 1994). 1994). Fluid salinities salinities based based on on freezing freezing point point depression range from 2.6 to 9.5 9.5 equivalent equivalentweight weight percent percent NaCI. NaCl. The quarries at Bellevue are located located within within55 to to 66 miles miles to the north and west of the quarries at Bellevue are the known known northwest end of the northwest the Albion-Scipio Albion-Scipio Oil Oil Field Field Trend. Trend. This oil field field (dolomitized (dolomitized fracture fracture and and solution cavities) structure is located within the Trenton-Black River (Middle Ordovician) rocks, cavities) structure is located within the Trenton-Black River (Middle Ordovician) rocks, some 4,000 feet deeper than the Bayport Limestone. The The structure structureisis related relatedto tofaulting faultingwithin within the basement basement rocks. Evidence Evidenceof ofthe thestructure, structure,however, however, isispresent present ininthe thelower lowerMississippian Mississippian Sunbury Shale Formation, (approximately 3,000 feet higher than the Middle Middle Ordovician Ordovician rocks), rocks), the Coldwater the Coldwater Shale (overlying (overlying the Sunbury), Sunbury), and and the the Marshall Marshall Sandstone Sandstone (overlying (overlying the the are evident Coldwater). Coldwater). If movements movements associated associated with this this structure structure are evident in in the theformations formations immediately below the Bayport Limestone, Limestone, itit seems seems likely likely that that the the Bayport Bayport Formation Formationwould would also also with the the structure. structure. Faulting associated with the Trend is likely be affected by faulting associated with responsible for for small structures responsible structures in the the Bayport, Bayport, allowing allowing for for brecciation, brecciation, subsequent subsequent fluid fluid migration, and precipitation of the the mineralization. mineralization. REFERENCES REFERENCES of Fluid Inclusions in Calcite from 2001. A A Preliminary Preliminary Microtermometric Study of Panter, K. S., 2001. Bellevue, Michigan, unpublished data, Bowling Green State University, OH Westjohn, D. B., 1994, of 1994, Michigan Basin RASA Solid-Phase Solid-Phase Investigation, in Geohydogeology of Carboniferous Aquifers Aquifers of of the Michigan Basin, Great Lakes Carboniferous Lakes Section-SEPM, Section-SEPM, 1994 1994 Fall Fall Field Field Conference, Conference, September September23-24, 23-24, 1994, 1994,Lansing, Lansing, MI MI 6 �WAN VOLCANIC AND INTRUSIVE ROCKS BEDROCK GEOLOGIC GEOLOGICMAPS MAPSOF OFKEWEENA KEWEENAWAN IN THE THE LAKE WOOD, FRENCH RIVER, AND KNIFE RIVER 7.5' QUADRANGLES, QUADRANGLES, NORTH LAKEWOOD, FRENCH KNIFE SHORE OF LAKE LAKE SUPERIOR, SUPERIOR, MINNESOTA MINNESOTA BOERBOOM, Terrence Terrence J., Minnesota Minnesota Geological GeologicalSurvey, Survey, St. St. Paul, Paul, MN, MN, boerb001@umn.edu boerb001 @umn.edu The Minnesota Minnesota Geological Geological Survey, with with partial partial funding funding by by the theU.S. U.S.Geological GeologicalSurvey SurveySTATEMAP STATEMAP geologic mapping program, has recently recently published published detailed detailed bedrock bedrock geologic geologic maps maps of of three three quadrangles quadrangles located along the North Shore of Lake Lake Superior Superior northeast northeast of of Duluth, Duluth, Minnesota Minnesota (Fig. (Fig. 1;1;Boerboom Boerboomand and others, 2002a, was completed completed at at a scale of 1:12,000, at a scale 2002a, b). b). Field mapping mapping was 1:12,000, and compiled compiled at scale of 1:24,000. This mapping has shown that some flow sequences can can be traced inland as far as 1:24,000. as 10 10to to 12 12 kilometers, and has identified hundreds of of individual flows flows within within the the larger larger flow flow units. units. Several mafic to felsic, subcordant subcordant to discordant discordant sills sills and and intrusions intrusionshave have also also been been mapped. mapped. Prior to this mapping, bedrock geologic geologic maps maps for for this this area area were were at at a scale of mapping, the only only published published bedrock 1:200,000 (for example Miller and others, 2001), and other work was concentrated along the shoreline of Lake Superior. Brannon Brannon(1984) (1984)sampled sampled160 160successive successivevolcanic volcanic flows, flows, starting starting above above the the Lester Lester River River sill and ending in Two Two Harbors, Harbors, as part part of of an an exhaustive exhaustive geochemical geochemical study. Green Greenand andothers others(1977) (1977) included this area as part included this part of of aamore morebroad broadcoastal coastalzone zonemanagement managementstudy. study. Schwartz and Sandberg Sandberg (1940) published a paper on the diabase sills near Duluth that included some of the the sills sills mapped mapped during during this study. Sandberg Sandberg(1938) (1938)mapped mapped the the stratigraphy stratigraphyof of the the flows flows exposed exposed at the shoreline shoreline from Duluth to Two Harbors, identifying some 180 180 lava lava flows. flows. Although all of these these studies studies made made some some incursions incursions inland from the shore, shore, none none of them them provided systematic systematicmapping mapping away away from fromthe the shoreline shorelineproper. proper. Bedrock exposure in the map area area varies varies greatly, greatly, from from nearly nearly continuous continuous outcrop outcrop along alongthe the shoreline shoreline and many of the short short streams streams along along the the slope slope into into Lake Lake Superior, Superior, to variably variably abundant abundant outcrop outcrop in the hills inland hills inland from the lakeshore. Throughout Throughout the the map map area, area, there there are are many many closely closely spaced spaced streams that have eroded into the bedrock perpendicular to the strike of the volcanic stratigraphy. Hence, Hence,many many of of the the individual flows flows could could be be traced traced for for a great individual great distance distance along along strike by tying tying them them together together from from one one the shoreline shoreline outcrops. outcrops. In contrast, the more resistant intrusive streamcut to the next, in combination with the rocks are typically typically exposed exposed on on the the tops tops and and slopes slopes of of high high hills. hills. The rocks The northeast northeast part of the the map map area area is is poorly exposed and thus much of the bedrock geology in that area is constrained largely by aeromagnetic aeromagnetic data. data. Green (2002) has proposed a subdivision subdivision of of the the North North Shore Shore Volcanic Volcanic Group Group into intoaa series seriesof of informal informal sequences and formations that are separated sequences and formations that separated by major major lithological lithological and and geochemical geochemical breaks breaks or or by by intrusions. Within intrusions. Within the the area area of of the themaps mapsshown shown here, here, these these include include the the Larsmont Larsmont basalts, Sucker River River basalts, the Lakewood lavas, and and the Lakeside Lakeside lavas lavas (Fig. (Fig. 2). The Thedetailed detailedbedrock bedrockgeologic geologicmaps mapsshown shown here subdivide these informal into multiple layers comprised comprised of of lava flows here subdivide these informal formations formations into multiple layers flows of of similar similar composition and texture in which which multiple multiple flow flow contacts contacts have have been been documented. documented. REFERENCES REFERENCES Boerboom, T.J., Green, J.C., and Jirsa, M.A., 2002a, 2002a, Bedrock Bedrock geology geology of the French French River River and and Lakewood quadrangles, St. Louis County, Minnesota: Minnesota quadrangles, MinnesotaGeological Geological Survey Survey Miscellaneous Miscellaneous Map Map MM128, scale 1:24,000. 1:24,000. 2002b, Bedrock geology of the Knife River River quadrangle, quadrangle, St. St. Louis Louis and and Lake Lake Counties, Counties, Minnesota: Minnesota: Minnesota Minnesota Geological Geological Survey Survey Miscellaneous MiscellaneousMap Map M-129, M- 129,scale scale1:24,000. 1:24,OOO. Brannon, J.C. 1984, 1984, Geochemistry Geochemistry of successive successive lava flows of the the Keweenawan Keweenawan North North Shore Shore Volcanic Volcanic Group: St. St.Louis, Louis,Washington WashingtonUniversity, University,Ph.D. Ph.D. dissertation, dissertation,312 312p. p. 7 �Green, J.C., J.C., 2002, 2002, Volcanic Volcanic and and sedimentary sedimentary rocks rocks of of the theKeweenawan Keweenawan Supergroup Supergroup in in northeastern northeastern Green, Minnesota, Minnesota, Chapter Chapter 55 of of Miller, Miller, J.D., J.D.,Jr., Jr.,Green, Green,J.C., J.C.,Severson, Severson,M.J., M.J.,Chandler, Chandler,V.W., V.W.,Hauck, Hauck,S.A., S.A., and Wahi, Wahl, T.E., T.E., Geology Geology and and mineral mineral potential of of the the Duluth Duluth Complex Complex and and related related Peterson, D.M., and rocks of of northeastern northeastern Minnesota: Minnesota MinnesotaGeological GeologicalSurvey SurveyReport Report of of Investigations Investigations 58, 58, p. p. 9494105. 105. Green, Green, J.C., J.C., Jirsa, Jirsa, M.A., M.A., and and Moss, Moss, C.M., C.M., 1977, 1977,Environmental Environmental geology geology of of the the North North Shore Shoreof ofLake Lake Superior: Superior: Minnesota MinnesotaGeological GeologicalSurvey, Survey,99 99 p.p. Miller, J.D., J.D., Jr., Jr., Green, Green, J.C., J.C., Severson, Severson,M.J., M.J.,Chandler, Chandler,V.W., V.W.,and andPeterson, Peterson,D.M., D.M.,2001, 2001,Geologic Geologicmap map Miller, of of the the Duluth Duluth Complex Complex and and related related rocks, rocks, northeastern northeastern Minnesota: Minnesota: Minnesota MinnesotaGeological GeologicalSurvey Survey Miscellaneous MiscellaneousMap MapM-1 M- 119, 19,scale scale1:200,000. 1:200,000. Sandberg, Sandberg, A.E., 1938, 1938, Section Section across across Keweenawan Keweenawan lava flows flows atatDuluth, Duluth,Minnesota: Minnesota: Geological Geological Society Societyof of America AmericaBulletin, Bulletin,v.v.49, 49, p. p. 795-830. 795-830. Schwartz, G.M., G.M., and Sandberg, Sandberg, A.E., 1940, 1940, Rock Rock series series in in diabase diabase sills sills atatDuluth, Duluth,Minnesota: Minnesota: Schwartz, Geological Geological Society Society of of America America Bulletin, Bulletin, v.51, v. 51, p. p. 1135-1172. 1135-1172. Figure Figure 1.1. Index Index map map showing showing location location of of mapped Work isis currently currently inin mapped quadrangles. quadrangles. Work progress progress on on the the Two Two Harbors Harbors and andCastle Castle Danger Dangerquadrangles. quadrangles. diabase Figure Figure 2.2. Index Index map map showing showing the the relative positions positions of of the the informal informal relative volcanicunits unitsof ofGreen Green(2002). (2002). volcanic - LIII Intrusiverocks rocks Intrusive Volcanicrocks rocks Volcanic Boundary of informal volcanic units 8 �WISCONSIN MINERAL RESOURCE RESOURCE GIS GIs AND RELATED DIGITAL DIGITAL MAP MAP AND DATABASE PRODUCTS RELATED PRODUCTS -A REPORT A PROGRESS REPORT BROWN, BA.1, B:A.', CZECHANSKI, CZECHANSKI,M.L.', M.L.',MUDREY, MUDREY,M.G., M.G.,Jr.1, ~ r . 'and ,andREID, REID,Daniel DanielD.2 D .(1) (1) ~ BROWN, Geological and and Natural Natural History History Survey, Survey, Univ. Univ. of of Wisconsin-Extension, Wisconsin-Extension,3817 3817Mineral Mineral Wisconsin Geological Point Road, Madison, Madison, WI 53705, 53705, babrowni babrownl @facstaff.wisc.edu, @facstaff.wisc.edu,(2) Wisconsin Dept of Transportation, Transportation, 3502 3502 Kinsman Kinsman Blvd, Blvd, Madison, Madison,WI WI 53704-2507 53704-2507 A new Mines, Mines, Pits Pits and and Quarries Quarries (MPQ) (MPQ) database database containing containing information information on on 1,302 1,302 significant nonmetallic mining sites sites throughout Wisconsin has has been been completed completedby by the the Wisconsin Wisconsin Geological and Natural History History Survey Survey (WGNHS) (WGNHS) in cooperation cooperation with with the the U.S U.S Geological Geological Survey (USGS). Locations Locations were digitized digitized from from county-based county-based digital digital orthophotography orthophotographywherever wherever available (MASIMILS available and by site visits. Data Datatables tableswere were linked linked to to existing existingUSGS USGS databases databases(MAS/MILS and MRDS) and to Wisconsin Wisconsin Department Department of of Transportation Transportation(WDOT) (WDOT)aggregate aggregatetest testdata; data;this this linkage linkage of all previous digital digital and and analog analog databases databases is is the the first first updated updated inventory inventorysince since1980. 1980. Future versions will be augmented with current site information, collected under the nonmetallic reclamation program of Wisconsin Department of Natural Resources, and additional historic sets such as the Road Material WGNHSJWDOT. Material Survey Survey sites sites of of the the WGNHS/WDOT. Georeferenced maps layered with digital geology, topography, orthophotography, Georeferenced orthophotography,soil, soil, and so forth provide a valuable land-use planning resource. Concern Concern for for safety safety and and construction construction the historic historic Upper Upper Mississippi problems in the reconstruction of U.S. Highway 151 through the possible the scanning and Valley Base-Metal Mining District, southwest Wisconsin, made possible georeferencing of the Wisconsin Mineral Development Atlas. The The Mineral Mineral Development DevelopmentAtlas Atlas of mine workings, drill-hole is a detailed set of 1,450 1,450 section-scale section-scale maps (1 inch equal 200 feet) of location and ancillary ancillary data dating from from 1900 1900 until mining mining ceased ceased in in 1979. 1979.These These maps maps were were maintained by the WGNHS and and the University University of Wisconsin-Platteville Wisconsin-Plattevilleand and were were scanned scannedby by the the WDOT. WDOT. All Wisconsin water well construction reports for 1936-1988 1936-1988 are now available on CDmapping as well well as environmental environmental and Rom. They provide an extensive data set for geologic mapping water resource analysis. analysis. New WGNHS map products are are being produced produced in in digital digital form form and andaa variety of analog maps including including the 1:24,000 1:24,000USGS geologic geologic quadrangle quadrangle maps maps of of the the lead-zinc lead-zinc district are being converted converted to digital as as resources allow. allow. This presentation will will provide provide an an interactive interactivedemonstration demonstrationof of these thesedata datasets setsand andGIS GIs layers, a review of available available map data such as regional geophysics, and and an update update on on the the status status of of geologic mapping at the WGNHS. WGNHS. 9 �HIGHWAY CONSTRUCTION, CONSTRUCTION,MINE MINERECLAMATION, RECLAMATION,AND ANDLAND-USE LAND-USEPLANNING PLANNING HIGHWAY HISTORIC UPPER UPPER MISSISSIPPI MISSISSIPPIVALLEY VALLEY LEAD-ZINC LEAD-ZINC CHALLENGES IN THE HISTORIC DISTRICT DISTRICT OF OF SOUTHWEST SOUTHWEST WISCONSIN BROWN, B.A.1, B.A.~,MUDREY, MUDREY,M.G., M.G.,Jr.1, ~ r . 'CZECHANSKII, ,CZECHANSKI, M.L.1, M.L.', REID, REID, Daniel DanielD.2, D . ~and and , HUNT, HUNT, BROWN, T.C.3, T.c/, (1) Wisconsin Geological and and Natural History Survey, Survey, Univ. Univ. of Wisconsin-Extension, Wisconsin-Extension, Wisconsin Geological 3817 38 17 Mineral Point Road, Madison, Madison, WI 53705, 53705, babrown1@facstaff.wisc.edu, babrown 1@ facstaff.wisc.edu, mgmudrey@wisc.edu,(2) Wisconsin Dept. of Transportation, 3502 Kinsman Blvd, Madison, WI mgmudrey@wisc.edu, 53704-2507, Wisconsin-Platteville,712 712 Pioneer PioneerTower, Tower, 53704-2507, (3) Reclamation Program, Univ. of Wisconsin-Platteville, Platteville, Platteville, WI 53818 538 18 of Wisconsin, Illinois, Illinois, Iowa, Iowa, and and Minnesota Minnesota The Upper Mississippi Valley Lead-Zinc District of produced nearly 10 million tons of lead-zinc ore from the 1820s until the last mine closed in 1978. The district will probably never be mined again, but problems related related to to mineralization mineralization and and past mining activity pose significant significant problems problems for for highway highway construction construction and and post-mining post-miningland land use. Specific hazards and engineering problems include (1) (1) Highly Highly altered altered and and unstable unstable rock rock and and shallow abandoned leachate from from abandoned mine workings workings encountered encountered during during highway highway construction, construction,(2) (2) leachate from lead-zinc lead-zinc sulfide sulfide mines. mines. As roaster-pile waste and (3) locally degraded groundwater from As rural rural residential development increases, increases, the the abandoned abandoned workings, workings, particularly particularlypoorly poorly sealed sealedshafts, shafts,can can be a hazard. Most low-sulfide low-sulfide waste waste rock rock has has been recycled recycled as as aggregate, aggregate, and and carbonate-rich carbonate-rich tailings overgrown with vegetation make make it difficult difficult to to find find any any surface surface evidence evidenceof of small, small,older older mine sites that may cause problems. problems. in an an High sulfate in groundwater samples was noted in 1978 following closure and flooding in area where large mines had operated for more than 50 drawdown cone cone had had developed developed 50 years and a drawdown over a 20-square mile area. area. A well-replacement well-replacement program program near Shullsburg Shullsburg restored restored potable potable water water supplies. supplies. Onsite Onsite reclamation consisted consisted of of establishing establishingvegetation vegetation on on the the tailings tailingsand andcrushing crushingthe the coarse waste rock for aggregate. Leachate from zinc roaster waste piles produced over 100 years over 100 years resulted in highly acidic and metal-rich surface surface water near Mineral Point. Point. The The roaster roaster piles piles were were successfully reclaimed by surface contouring along with neutralization surface grading and contouring neutralization and and of the cost of of fertilization to allow vegetation to establish. This was accomplished at a fraction of removal of the roaster waste piles. piles. rock alteration exposed exposed during Previously undiscovered sulfide mineralization and associated rock highway construction along U.S. Highway 151 near Mineral Point resulted in the 151 resulted in the unanticipated unanticipated engineering redesign of major roadcuts roadcuts and structures. structures. The The need need to to identify identify areas areas of of need for engineering the Wisconsin Wisconsin Mineral Mineral Development Development Atlas, which potentially unstable slopes slopes led to scanning scanning Of of the of mineralization, mineralization, alteration, alteration, and and abandoned has proven to be invaluable in identifying areas of workings in the path of construction. construction. These These detailed detailed maps maps (1 (1 inch inch to to 200 200 feet) feet) of of mine mineworkings workings exploration drillhole and exploration drillhole locations locations are are now now being being used used by by county county and and regional regional planners planners and and potential mining mining related related hazards hazards into into land-use zoning authorities to identify and incorporate potential planning. 10 �TEXTURAL CONSTRAINTSON ONTHE THEORIGIN ORIGINOF OF RAPAKIVI TETURE IN IN THE THEWOLF WOLF RIVER RiVER TEXTURAL CONSTRAINTS RAPAKIVI TEXTURES BATHOLITH BATHOLITH R. Michele Buttram Buttram and and Marcia Marcia Bjornerud Bjornerud Geology Department, Lawrence University, Appleton, WI 54912 549 12 The Wolf River Batholith of north-central Wisconsin, a 1.47 Ga composite anorogenic pluton, includes some of the world's finest examples of 'rapakivi' granite, granite, in in which which large large potassium feldspar crystals are mantled by plagioclase. Although Although rapakivi rapakivi granites granites have have been described for more than a century, the origin of of this distinctive texture, both in the Wolf River complex and elsewhere, remains remains controversial. controversial. Some workers argue that rapakivi mantles are coronae formed at a peritectic or eutectic point under equilibrium crystallization conditions. Others Others maintain maintain that rapakivi textures record disequilibrium associated with magma mixing and or sudden sudden changes in pressure. While most previous previous investigations investigations have have focused focused on on the the chemistry chemistry of of rapakivi rapakivi granites, granites, specifically, this study study examined examinedthe thephysical physicalcharacter characterofofthe theWolf WolfRiver Riverrocks rocks—- specifically, the feldspar crystals. crystals. Among size, shape, orientation and distribution of the rapakivi-type feldspar Among the most striking characteristics of these rocks is the large size of the feldspars (up size feldspars (up to to 77 cm cm in in length). Statistical Statistical analyses analysesshow show that that there there is is no no significant significant difference difference in in size size or or aspect aspect ratio between crystals with and without the rapakivi mantle. However, However, the the K-feldspar K-feldspar cores of the rapakivi-type rapakivi-type crystals crystals tend to be rounder (less (less euhedral) euhedral) than than non-rapakivi non-rapakivi grains, suggesting that they experienced significant resorption prior to the growth growth of of the the plagioclase mantle. AAweak weakgrain grainshape shapefabric fabric and and random random juxtaposition of of rapakivi rapakiviand and non-rapakivi grains must must also also be be explained explained by by any any viable viable model model for for the the origin origin of of the the texture. Our Ourdata data appear appearto to be be most most consistent consistent with with the the magma magma mixing mixing model, model, which which isis compatible with earlier geochemical studies of the Wolf River River complex. complex. 11 �Niagara suture suture zone, zone, northern northernMichigan Michiganand andWisconsin—tectonics Wisconsin-tectonics in Ma arc-continent arc-continent collisional boundary boundary the 1.85 Ma W.F. Cannon, Cannon,(U.S. (U.S. Geological GeologicalSurvey, Survey, Reston, Reston, VA VA 201 92, wcannon @ usgs.gov) G.L. 20192, wcannon@usgs.gov) G.L. LaBerge, LaBerge, (University of of Wisconsin-Oshkosh Wisconsin-Oshkosh(retired) (retired)and andU.S. US.Geological GeologicalSurvey), Survey),John JohnS. S.Klasner Kiasner illinois University (retired) (retired) and (Western Illinois and U.S. U S . Geological Geological Survey) Survey) The Niagara suture suture zone, as used here, is a belt varying varying in width from from about about 6 km to 40 km lying north of the Niagara Niagara fault. fault. It separates the accreted volcanic arcs of the Wisconsin magmatic terranes (WMT) on the south south from from the autocthonous autocthonous and parautochtonous parautochtonous continental continentalmargin margin rocks on the north. ItItconsists consistsof of Paleoproterozoic Paleoproterozoic metasedimentary metasedimentary and and metavolcanic metavolcanic rocks of epicratonic Marquette Marquette Range Range Supergroup Supergroup and and Archean Archean basement basement rocks rocks upon upon which which they they were were the epicratonic deposited. The TheArchean Archeanrocks rocksconstitiute constitiutethe thesouthern southernmargin margin of of the the Superior Superiorcraton, craton, which which was was deposited. rifted and eventually eventually separated separated during during extensional extensional phases phases of of the the Penokean Penokean orogenic orogeniccycle, cycle,and and during Penokean Penokean convergence. convergence. The The suture suture zone zone is is marked marked by by very very high high then thrust northward during strain and widespread multiple steeply to vertically plunging folds. The suture suture zone zone is is one one of of subdivisions of the Penokean Penokean orogen whose hierarchy of component component parts parts is is shown shown numerous subdivisions below. below. Michigamme subterrane Foreland fold fold and thrust i Foreland Niagara suture \~ i a ~ a suture ra zone /, Park Falls panel Watersmeet Watersmeet panel Beechwood Beechwood panel Iron Iron River River panel Menominee panel Menominee oanel j J / I I/ PENOKEAN OROGEN PENOKEANOROGEN,, \\ \ north north - - -south south- - Niagara fault - - - - Pembine-Wausau (northern part ~ e m b i n e - ~ a u s aterrane u (northern part of of WMT) WMT) Mars hfield terrane (southern (southern part Marshfield part of ofWMT) WMT) -- The map pattern shown here was derived from published detailed detailed maps in the east east (Bayley (Bayley and and Dutton, 1971; others, 1966; 1966; Button, 1971; James and others others 1968; 1968; and James and others, others, 1961) 1961)and and from from our our but access access to to recent recent exploration exploration drill drill recent work in the west, where outcrops are scarce but information as well as proprietary detailed aeromagnetic aeromagnetic and electromagnetic electromagneticdata data have have aided aided in in relationships (Cannon clarifying the geologic relationships (Cannon and others, others, 1998). 1998). Each of the the five five fault fault panels panels of of the the Niagara Niagara suture suture zone zone has has aa unique unique set set of of characteristics. characteristics. Watersmeet panel- Paleoproterozoic Paleoproterozoic strata strata are are mostly mostly pelitic pelitic schists schists and and gneisses gneissescontaining containing near the the base. base. They ferruginous strata and locally dolomite near They were were deposited deposited on on a basement of deformed into into gneiss gneissdomes. domes. High-pressure Archean gneiss. Both Both basement and cover were deformed metamorphism produced kyanite-bearing kyanite-bearing assemblages. assemblages. Park Falls panelpanel- Generally Generally similar similar to to Watersmeet Watersmeet panel panel except except that that metamorphism metamorphismwas was lower lower pressure and sillimanite-bearing sillirnanite-bearing assemblages assemblages are are predominant. predominant. Beechwood Beechwood panelpanel- Consists Consists of of Paleoproterozoic Paleoproterozoic graywacke graywacke and shale shale and and mafic mafic volcanic volcanic rocks rocks in in roughly equal parts. Archean Archean basement basement is not exposed. Folds Folds are are ENE-trending ENE-trending and have subhorizontal axes. Rocks are in greenschist facies. facies. Iron River panelvanel- Rocks are the Paint River Group, Group, including including the the Badwater Badwater Greenstone. Greenstone.Archean Archean basement is not exposed. exposed. Strata Strata are are multiply multiply folded folded creating creating aa complex complex fold fold interference interferencemap map pattern. Most Mostfold foldplunge plunge steeply. steeply. Metamorphosed Metamorphosedto togreenschist greenschist or or sub-greenschist sub-greenschist facies. facies. 12 �88' 9•1 46' c-is tic r-,c V A a V - A r-ic a r-tc A ?AA .A vA I I I a LV" I_VA A 0 A a '- A 7 a Lv A A a Fembine— "ausau terrane ,%/AA — -l LV i_L V IL Vi L) 50 A— cv' 1.-V1 c-is) .1 I_V1 v a LVA cv" 1 50 1KM ai ,a , 1a i 91' 89' 88' 0• Map showing showing the the five five structural structuralpanels panels (Park (ParkFalls, Falls,Watersmeet, Watersmeet,Beechwood, Beechwood,Iron IronRiver, River, Map and Menominee) that that constitute constitute the Niagara Niagara suture zone. Faults Faultsthat thatbound boundthe thepanels panelsare are Flambeau Flambeau Flowage Flowage fault fault (FFF), (FFF), Powell Powell fault (PF), Elmwood fault (EF), Paint River fault (PRF), (PRF), Badwater Badwater fault fault (BF), (BF),North NorthRange Rangefault fault(NRF), (NRF),and andSouth SouthRange Rangefault fault(SRF). (SRF). Menominee Menominee panel- Rocks are entirely of Paleoproterozoic age. No No Archean Archean basement basement is is exposed. exposed. Strain Strain was extreme. Commonly Commonlyall allstructural structuralelements, elements,including including fold fold axes, axes, are are subvertical. subvertical. Metamorphism Metamorphism is is lower lower to to upper upper greenschist greenschist facies faciesand and largely largely post-tectonic. post-tectonic. These These panels, panels, and and the the Niagara Niagara suture suturezone zone that that they they constitute, constitute,differ differfrom fromthe theMichigamme Michigamme subterrane Archeanbasement basementisisin in the the subterrane to the north. There Therewas waslittle littlepenetrative penetrativedeformation deformationof of Archean Michigamme subterrane. Paleoproterozoic deformed. Folds, Paleoproterozoic strata strata were moderately to weakly deformed. for for the the most part, are are simple simpleand and gently gently plunging. plunging. Metamorphic Metamorphic grade grade is is variable variableand and mostly mostly postposttectonic. Thus, Thus,the theNiagara Niagarasuture suturezone zonedocuments documentsaarange range of of tectonic tectonic styles styles unique unique to to the the very very high strains strains in in aa belt belt no no more more than than aa few few tens tens of of kilometers kilometers wide, wide, along along which which differential differential movement between the accreting arcs on the south and the craton margin on the accreting arcs on the south and the craton margin on the north north was was concentrated. concentrated. References References Bayley, Bayley, R.W., R.W., Dutton, Dutton, C.E., C.E.,and and Lamey, Lamey, C.A., C.A., 1966, 1966,Geology Geologyof of the the Menominee Menomineeiron-bearing iron-bearingdistrict, district, Dickinson Dickinson County, County, Michigan Michigan and and Florence Florence and and Marinette MarinetteCounty, County,Wisconsin: Wisconsin:U.S. U.S. Geological GeologicalSurvey Survey Professional 96p. Professional Paper 513, 5 13,96p. Cannon, Cannon, W.F., LaBerge, G.L. Klasner, J.S., and Schulz, K.J., 1998, 1998, Reinterpretation of the Penokean 44th continental margin in part of northern Wisconsin and Michigan (abs.): Proceedings Proceedings of 44thAnnual Annual Institute Institute continental on on Lake Lake Superior Superior Geology, Geology,v. v. 44, 44, p. p. 52-53. 52-53. Dutton, Dutton, C.E., 1971, 1971, Geology Geology of the Florence area, Wisconsin and Michigan: U.S. Geological Survey Professional Paper 633, 633,54 p. Professional 54 p. James, James, H.L., H.L., Clark, Clark, L.D., L.D., Lamey, Lamey, C.A., C.A., and and Pettijohn, Pettijohn,F.J., F.J., 1961, 1961,Geology Geologyof ofCentral CentralDickinson DickinsonCounty, County, Michigan: Michigan: U.S. U.S. geological geological Survey SurveyProfessional ProfessionalPaper Paper 310, 3 10,176 176p.p. James, H.L., Dutton, C.E., C.E., Pettijohn, F.J., F.J., and Weir, K.L., 1968, 1968, Geology and ore deposits of the Iron River Professional Paper Paper 570, 134 p. -- Crystal Crystal Falls district, Iron County, Michigan: U.S. Geological Survey Professional 13 �Strike-slip separation separation of the Burntside trondhjemite and and the Wakemup Wakemup Bay tonalite, Northern Minnesota Northern Minnesota Karoun Charkoudian, Basil Tikoff Department of Geology and Geophysics, Geophysics, University of Wisconsin, Wisconsin, Madison Madison WI, WI, 53706 53706 Robert Bauer Department of Geological Geological Sciences, Sciences, University of Missouri, Missouri, Columbia, Columbia, MO, MO, 65211 6521 1 INTRODUCTION The INTRODUCTION TheVermilion Vermilionfault faultisisaalocal localtectonic tectonicboundary boundary in the southern Canadian Shield juxtaposing the Quetico Quetico subprovince subprovince (granites (granites and and schists) schists) with with the the Wawa Wawa greenstones. greenstones. Burntside trondhjemite The Bumtside trondhjemite and and the Wakemup Bay tonalite tonalite are are small, small, elliptical, elliptical, Archean Archeangranites granites separated km of right lateral offset on the Vermilion Vermilion fault fault in in northern northernMinnesota. Minnesota. The separated by 35 35 krn normal fault, fault, juxtaposing juxtaposing the the shallow Wawa Wawa Vermilion fault is interpreted as initially active as a normal greenstone to the south with the deeper deeper granites granites and and migmatized schists schists to to the the north north (figure (figure1,1, stage 1). fault, separating the the Burntside Burntside trondhjemite trondhjemite from from 1). It was later reactivated as a strike-slip fault, stage Bay tonalite (figure 1, stage 2). 2). Although the Vermilion fault is the regional the Wakemup Bay boundary between between the the Quetico Quetico and and Wawa Wawa subprovinces, subprovinces,the th Haley fault lies to the south of the Vermilion fault and contains Quetico Quetico schists schists that belong belong on the north side of the Vermilion Vermilion fault fault (figure (figure 1). 1). The purpose purpose of this this study study is is to to compare compare emplacement setting, fabrics, composition, composition, and shape shape of the two plutons to determine if they constitute constitute a piercing piercing point on the Vermilion fault. In In addition, addition, we have determined the dip on the Vermilion Vermilion fault, constrained constrained the emplacement emplacement history of the the Wakemup Wakemup tonalite, tonalite, and and determined a potential cause for the isolated fault block that now contains contains the the Wakemup Wakemup Bay pluton (figure (figure 1, 1, stage 2). The Burntside trondhjemite trondhjemite is a small lenticular pluton that intruded the schist that lies to the north of the Burntside Bumtside Lake fault, fault, a continuation continuation of of the the Vermilion Vermilion fault at its eastern end. The The Wakemup Wakemup Bay pluton is a biotite-bearing biotite-bearing tonalite tonalite that that intruded intruded the the schist schistthat that lies lies Figure 11 just to to the the north north of of the the Haley Haley fault. fault. Figure g'one The Anisotropy AMS ANALYSIS ANALYSIS Anisotropy of Magnetic Magnetic Susceptibility Susceptibility (AMS) (AMS) is is aa rapid, rapid, nonnondestructive technique, commonly used in granitic studies to obtain obtain magnetic magnetic fabrics. fabrics. Principle Principle The magnetic foliation is defined as the kmx-kint AMS ellipsoid axes axes are are defined defined as as knBx>kint>kmin. plane, and the magnetic lineation is defined as the orientation of kmx. The bulk susceptibility j.tSI) susceptibilityvaries varies widely widelyin inboth boththe theWakemup WakemupBay Baytonalite tonalite(500-8500 (500-8500pS1) and the Burntside trondhjemite (5OO-350OSI). (500-3500pSI). This This range of susceptibility susceptibility is attributed attributed to the large variation in magnetite content throughout these bodies. The The AMS AMS foliations foliations parallel parallel the the measured field foliation in both plutons. Lineations Lineationsin inthe the Wakemup Wakemup Bay Bay tonalite tonalite dip dip shallowly shallowly to the E and W, and lineations in the Burntside trondhjemite dip shallowly to the ENE and WSW. WSW. Magnetic lineations lineations consistently consistently parallel parallel the the long long axis axis of of the the plutons. plutons. The Burntside GRAVITY GRAVITY STUDY STUDY Burntside and Wakemup plutons were selected selected for for aa gravity gravity study study because they both contain a single surrounding lithology (biotite schist) with a significant and g/cc). In consistent density density contrast contrast (Adensity (Mensity = -0.08 to -0.1 glcc). = -0.08 In addition, addition,the the gravity gravitydata data allows allowsus us to model the dip of the Vermilion Vermilion fault. fault. 14 �meter model model G G was was used usedfor forboth both areas. areas. After A Lacoste and Romberg gravity meter corrections, a forward forward model approach approach was used to interpret interpret the depth of the Burntside Burntside pluton pluton and and Vermilion fault geometry using WinGLink, WinGLink, aa geophysical geophysical interpretation interpretation software software program. program. The km in in thickness. thickness. The Vermilion Vermilion fault is a steeplysteeplyBurntside pluton is a thick body between 2-3 km north dipping to vertically oriented feature. Using Using aa gravimetric gravimetric three-dimensional three-dimensional iterative iterative technique on the Wakemup Bay pluton pluton resulted resulted in in aa good good first-order first-order picture picture of of the the pluton. pluton. Most of the pluton is very thin, less than 0.5km thick. There are are two root zones zones of up to to 4 km depth, thick. There both of which lie on the southern southern portion portion of the the Wakemup pluton, pluton, furthest furthest away away from from the the Vermilion fault. INTERPRETATION We interpret interpret the the Burntside Burntside and and Wakemup Wakemup plutons plutons as as part part of of the the same same on the the Vermilion Vermilion fault. fault. These granitic complex prior to strike-slip faulting on These igneous igneous bodies bodies are are similar in composition and both have undergone solid-state deformation. The plutons have similar structural settings. The TheWakemup WakemupBay Bay pluton pluton intrudes intrudes an an F3 F3 fold hinge hinge and the Burntside Burntside pluton Given the the separation, separation, the the folding folding episodes episodes may may or may has refolded F2 folds at its southern end. Given not correlate correlate exactly. exactly. The gravity gravity inversion inversion and and AMS study study on the Wakemup Wakemup pluton provide provide constraints constraints on on pluton emplacement. The km. Because The pluton pluton has has an average thickness of 0.5 km. Because the pluton contains a roof of wallrock, this estimate reflects the true thickness of the pluton. The TheAMS AMS foliation foliation and and lineation lineation parallel parallel the the fold fold limbs limbs and and fold fold hinge, hinge, respectively, respectively,of of aakm-scale km-scale F3 F3fold. fold. Therefore Therefore we interpret interpret the the Wakemup Wakemup as as syntectonically syntectonicallyintruding intruding an an F3 F3 fold fold hinge. hinge. We use a forward forward gravity model to estimate estimate the dip dip on on the the Vermilion Vermilion fault, fault, which whichdips dips between 70° 70' N and vertical. This Thisinterpretation interpretationrequires requiresthat thatthe the section sectionof of the theVermilion Vermilionfault fault south of the Burntside pluton was not active active as a south-side south-side down normal normal fault. fault. We propose the following tectonic model (figure 1). The The Burntside Burntside pluton pluton and and the the granitic complex. complex. The Wakemup Bay pluton were initially part of the same granitic The Vermilion Vermilion fault fault was was initiated as a normal fault (figure 1, juxtaposed the amphibolite facies Quetico 1, stage 1), I), which juxtaposed with the the greenschist greenschist facies facies Wawa Wawa belt. belt. The Wakemup tonalite, with a thick root on sub-province with its south side, acted as a promontory in the fault system. The The Vermilion Vermilion fault fault was was then then reactivated as a strike-slip fault (figure 1, stage 2), cutting through the the thinnest thinnest (NW) section of of the Wakemup Bay pluton. This This created createdthe the fault-bounded fault-bounded block block that that contains contains the the Wakemup Wakemup Bay pluton. Therefore, Therefore,ititisisevident evidentthat thatthe thepluton plutonshape shapehas has played played aa crucial crucialrole role in in controlling controlling Vermilion Vermilion fault orientation, orientation, both both for for the the early early normal normal faulting faulting and and later later strike-slip strike-slipfaulting. faulting. REFERENCES REFERENCES Minnesota. Minnesota Bauer, R.L., 1985, 1985, Norwegian Bay Quadrangle, St. Louis County, Minnesota. Minnesota Geological Geological Survey, Miscellaneous Miscellaneous Map series, series, Map Map M-59, M-59, 1:24,000. 1:24,000. Bauer, R.L., 1986, 1986, Multiple Multiple folding folding and and pluton pluton emplacement emplacementin in Archean Archeanmigmatites migmatitesof of the the southern southern Vermilion granitic 1753-1764. granitic complex, complex, northeastern northeastern Minnesota. Minnesota. Can. Can. J. J. Earth Earth Sci., Sci., v. v. 23, 23, p. p. 1753-1764. Bauer, R.L., and Bidwell, M.E., 1990, 1990, Contrasts in the response to dextral transpression across the QueticoWawa subprovince boundary in northeastern Minnesota. Can. Can. J. J. Earth Earth Sci., Sci., v. v. 27, 27, p. p. 1521-1535. 1521-1535. Sims, P.K., and Mudrey, M.G., 1972, district, in in Sims, P.K., P.K., et et al., eds., eds., 1972, Burntside granite gneiss, Vermilion district, Geology of Minnesota: Centennial Volume: St. St. Paul, Minnesota Minnesota Geological Geological Survey, Survey, p. p. 98-101. 98-101. Minnesota: A Centennial Vigneresse, Vigneresse, J.L., J.L., 1995, 1995,Control Control of of granite graniteemplacement emplacementby by regional regional deformation: deformation:Tectonophyiscs, Tectonophyiscs,v.v.249, 249,p.p. 173-186. 173-186. 15 �GEOLOGY AND MINERAL MINERAL POTENTIAL POTENTIAL OF PROTEROZOIC PROTEROZOIC MAFIC MAFIC INTRUSIONS IN THE NORTHERN GRENVILLE GRENVILLE PROVINCE PROVINCE OF ONTARIO ONTARIO R.M. EASTON, Ontario Geological Survey, Survey, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5, mike.easton@ndm.gov.on.ca rnike.easton@ndm.gov.on.ca Since 1998, 1998, mafic intrusions intrusions near the Grenville Grenville Front in Ontario Ontario have have been been prime prime exploration targets for Cu-Ni-PGE Cu-Ni-PGE mineralization. mineralization.To To assist assist in in this this effort, effort, the the Ontario OntarioGeological Geological Survey has conducted detailed mapping in high potential areas of the Grenville Grenville Province Province between between 1999 and 2002. This poster summarizes the results of these mapping efforts. efforts. East Bull Bull Lake Lakeintrusive intrusivesuite, suite,including includingthe theRiver RiverValley Valleyintrusion: intrusion:Country Countryrocks rockstotoEast East East Bull Lake intrusive intrusive (EBLI) (EBLI) suite suite rocks in the area area are inferred to be mainly Archean Archean in in age, age, and and are grouped into 4 gneiss associations. associations. Metamorphic grade is upper amphibolite amphibolite facies; fades; country country rocks to the mafic intrusions intrusions are are commonly commonly migmatitic. migmatitic. The Paleoproterozoic PaleoproterozoicEBLI EBLI suite suite consists consists of several several mafic mafic layered layered intrusions intrusions emplaced emplaced between 2490 krn, roughly roughly 2490 and and 2468 2468 Ma Ma (James (Jamesetetal. al.2002) 2002)that thatoccur occurover overaadistance distanceofof—250 -250 km, site of Sudbury. Sudbury.The The largest largest of these bodies in in the the Grenville Grenvilleisis the the River River centered on the present site Valley intrusion, intrusion, which underlies roughly 100 of Dana Dana and Crerar townships. Previous maps 100 km2 krn2 of correlated correlated mafic rocks west of Crerar Crerar Township Township with with the the River River Valley Valley intrusion. intrusion.This This study study indicates indicates that at least least 3 separate separate intrusions intrusions are are present, present, each each emplaced emplacedinto into different differentcountry country rocks, and with different different stratigraphy stratigraphy and mineral potential. EBLI suite rock types range in composition from anorthosite anorthosite to melanorite, melanorite, troctolite troctolite and and rarely peridotite; leucogabbronorite leucogabbronorite and gabbronorite dominante. dominante. The The crystallization crystallizationorder order of of olivine orthopyroxene (An80-62), olivine (Fo7659), orthopyroxene primocryst phases phases is is most mostcommonly commonlyplagioclase plagioclase(An8062), (En7558), titanomagnetite,and andclinopyroxene. clinopyroxene. In In Dana Dana Township, Township, the the River River Valley intrusion (En75-58),titanomagnetite, locally exhibits primary mineralogy and well preserved igneous textures. textures. Phase Phase layering layering varies from cm- to m-scale, m-scale, which is discernable discernable in outcrop, outcrop, and and dm dm or or larger, larger, which which is is identified identifiedby by detailed mapping. Isomodal layering is most common; common; mineral and and size size graded graded layers layers are are less less common. Cryptic layering is well documented for the River Valley intrusion. intrusion. Pearce-element Pearce-element ratio and chondrite-normalized chondrite-normalizedREE REE diagrams diagrams illustrate illustrate that that each each body formed formed from from one one or or more more tholeiite composition composition can can explain explain the the cogenetic magmas (James et al. 2002). A high-Al, low-Ti tholeiite al. 2002). 2002). dominant leucocratic rock compositions compositions in the the EBLI EBLI suite suite (James (James et al. Within g/t Pd+Pt+Au) Pd+Pt+Au) Within the the EBLI EBLI suite, suite, contact-type contact-type Cu-Pd-Pt Cu-Pd-Pt mineralization mineralization (1 (1 to to 10 10g/t occurs in the matrix of an inclusion and/or fragment-bearing gabbronorite to leucogabbronorite inclusion fragment-bearing gabbronorite to leucogabbronoriteat at the base or side of the intrusions intrusions where the primary igneous contact is preserved. preserved. A A second, second, m above the the contact. contact. Examples Examples occur occur similar, zone of mineralization may occur 100-200 m throughout the EBLI suite, however, the most consistent consistent grades have been reported reported from from the the % sulfide, sulfide, River Valley intrusion in Dana Township. Township. Chalcopyrite and lesser lesser pyrrhotite pyrrhotite form form 1-3 1-3% disseminated or as local cm-sized either finely disseminated cm-sized patches. PGE mineralization mineralizationis is commonly commonly associated with sulphide. sulphide. Study Study of the East Bull Lake intrusion indicates indicates that that mineralization mineralization originates from the intrusion and subsequent subsequent dynamic dynamic mixing of S-saturated, S-saturated,inclusion-bearing, inclusion-bearing, second-stage (PGE enriched, i.e. 20-100 20-100 ppb PGE) magmas that entered entered the the magma magma chamber chamber second-stage carrying liquid sulfide sulfide droplets droplets (James (James et et al. al. 2002). 2002). Reef-style Reef-style mineralization mineralizationhas has yet yet to to be be documented within the the EBLI EBLI suite. suite. 16 �history between Geological history between Sudbury Sudburyand andRiver RiverValley: Valley:Archean Archeanrocks rocksin in this this area area record a sequence of events similar to that observed in the Levack Gneiss Gneiss complex complex and and high-grade high-grade portions of the Quetico subprovince, subprovince, but unlike the Pontiac subprovince. The following geological history is inferred. inferred. After deposition of greywackes greywackes south south of the the Temagami Temagarni greenstone greenstone belt, invasion by tonalitic tonalitic to to granodioritic granodioritic plutons, plutons, probably probably accompanied accompaniedby by burial, burial, formed formed the the migmatitic gneisses gneisses now represented represented by the Pardo Pardo and and Red Red Cedar Cedar Lake Lake gneiss gneiss associations, associations,likely likely between 2685 and 2675 2675 Ma. Ma. This This was was followed followedby by aa second second period period of of tonalitic tonalitic to to granodioritic granodioritic magmatism, deformation deformation and and metamorphism metamorphism at at mid-crustal mid-crustal levels levels between between 2670 2670and and2660 2660Ma. Ma. The Crerar gneiss association association represents represents the the products products of of this this latter latter activity. activity.Subsequent Subsequentfelsic felsic magmatism at roughly 2640 Ma was accompanied accompanied by emplacement of pegmatite pegmatite veins. veins. A logical extension of this work is to interpret the gneiss associations as a southwarddeepening section of the crust. crust. As interpreted, interpreted, Archean Archean metawackes metawackes exposed exposed immediately immediatelynorth north of the Grenville Grenville Front represent high-levels of the crust. crust. The The Pardo Pardo gneiss, gneiss, immediately immediatelysouth south of of the Grenville Grenville Front, represents represents the middle middle part of aa 10-15 10-15 km km thick thick upper upper crustal crustallayer layer dominated dominated supracrustal and intrusive by supracrustal intrusive rocks. rocks. The Red Cedar Cedar Lake gneiss and and the Street Street gneiss gneiss association association represent the basal portion of this upper crustal crustal layer, layer, with the former former derived derived from from aa metasedimentary rock sequence and the latter latter from a greenstone sequence. sequence. Intrusive Intrusive rocks rocks of of the the km thick middle middle crustal crustal layer. layer. This This crustal crustal section section is is Crerar gneiss association are part of a 10-15 10-15 km roughly equivalent to that observed observed across the Wawa gneiss domain. domain. Emplacement Emplacementof of EBLI EBLI suite suite bodies occurs at several levels levels within this this crustal crustal section. section. Flett Evidence forfora amafic Flett Township Townshipmafic maficIntrusions: Intrusions: Evidence maficand andA-type A-typegranite granitemagmatic magmaticprovince province in the northern Grenville Grenville Province Province was discovered while examining mafic intrusions near Temagami that occur occur in Tomiko Tomiko domain, domain,near near its its contact contact with with the the Grenville GrenvilleFront Fronttectonic tectoniczone. zone. Proterozoic Proterozoic country country rocks consist consist of of gneissic gneissic granite, granite, with with minor minor mafic mafic and andquarztose quarztosegneiss gneissand and metaconglomerate. metaconglomerate. The Fall Lake intrusion consists of little metamorphosed gabbro and leucotroctolite. The Fanny Lake intrusion consists of olivinite leucotroctolite. olivinite and and troctolite. troctolite. Igneous Igneous texture texture is is well preserved, but metamorphic coronas coronas occur occur around around primary olivine olivine and and clinopyroxene. clinopyroxene. Geochemistry indicates that both bodies are slightly alkalic, alkalic, compositionally compositionally similar similar to to the the ±44 Ma, Ma, and and have have affinities affinities to to within-plate within-plate basalts. basalts. Sudbury diabase dike swarm dated at 1238  The Fall Lake intrusion intrusion yielded yielded pristine pristine baddeleyite, baddeleyite, with 33 concordant concordantor or just slightly slightly discordant grains 1235±22Ma. Ma. The The Fanny Fanny Lake sample 2 0 7 ~ b / 2 0age 6age ~ bofof1235 sample discordant grainsgiving givingan anaverage average207Pb/206Pb yielded baddeleyite, with some grains having thin zircon overgrowths, overgrowths, consistent consistent with with the the presence of corona corona textures textures in in the the body. body. Two Two concordant concordant grains grains without without overgrowths overgrowthsgave gavean an average 207Pb/206Pb 1238  ± 2 Ma. 2 0 7 ~ b / 2 0age 6age ~ bofof 1238 average Both intrusions intrusions are are spatially spatially associated associatedwith with the the A-type A-type Mulock Mulock granite, granite,dated dated previously previously at 12444L3 1 2 4 4 1 - Ma. ~ Intrusions of similar age include the Sudbury dike swarm, swarm, Mercer Mercer anorthosite, anorthosite, granitoid plutons. The new age data data provides provides further further evidence evidence and the West Bay and Powassan granitoid for the presence of a bimodal magmatic province active from 1270-1235 1270-1235Ma Ma in in the the Laurentian Laurentian margin of the Grenville Province. The tectonic setting is interpreted as an extensional rift nft that formed formed inboard inboard of aa continental continental arc arc active active on on the the southern southern margin margin of of North North America Americabetween between 1450-1300 1450-1300 Ma. This setting setting resembles that of the Cenozoic Cenozoic Columbia ColumbiaRiver River Basalt BasaltGroup. Group. James, R.S., Easton, Easton, R.M., R.M., Peck, Peck, D.C. D.C. and and Hrominchuk, Hrorninchuk, J.L. J.L. 2002. 2002.The TheEast EastBull BullLake Lakeintrusive intrusivesuite: suite:remnants remnantsof ofaa —2.48Ga Galarge largeigneous igneousand andmetallogenic metallogenicprovince province in in the the Sudbury Sudbury area of the Canadian Canadian Shield; -2.48 Shield; Economic Economic p.1577-1606. Geology, v.97, p. 1577-1606. 17 �PALEOSTRESS INFERENCES PALEOSTRESS INFERENCES FROM FROMFAULT FAULT SLIP SLIP VECTORS VECTORS IN IN THE THE EASTERN EASTERNPART PARTOF OFTHE THE WISCONSIN SEGMENT SEGMENTOF OFTHE i'H MIDCONTINENT Rwr WISCONSIN MIDCONTINENT RIFT Amy Garbowicz, Garbowicz, Marcia Marcia Bjornerud, Bjornemd, Geology Department, Department, Lawrence University, Appleton, WI 54912 549 12 Building accurate accurate models models for for both both the opening Building opening and closing closing of the the Midcontinent Midcontinent Rift Rift requires an an understanding of the evolution requires understanding of evolution of regional regional stresses stresses over over time. time. This This study study paleostress indicators indicators in in the the portion portion of of the Rift exposed near focused on slickenfibers as paleostress of Lake Superior in in northeasternmost northeasternmost Wisconsin. Wisconsin. The orientations of the southern shore of slickenfibers were were used used to determine slickenfibers determine slip vectors vectors on on outcrop-scale outcrop-scale faults within riftspan the the entire range of the related igneous and related and sedimentary sedimentary rocks. rocks. Rocks Rocks sampled sampled span the Keweenawan Supergroup, from the Tyler Formation to the Freda Sandstone, with most of the sampling in the Porcupine Porcupine Volcanics, Volcanics, the Kallander Kallander Creek Creek Volcanics, Volcanics, and and the the Mellen Mellen Gabbro. . The The mineral composition of the slickenfibers was used as a proxy for their age, Gabbro. based on the based the known known regional regional sequence sequence of of secondary secondary mineralization mineralization within within the Rfit. Rfit. Chlorite and epidote slickenfibers were grouped together and considered older since these slickenfibers were among the first were first minerals minerals precipitated precipitated by hydrothermal hydrothermal fluids following the main main of calcite and zeolite were magmatic interval. interval. Slickenfibers of were considered considered to to be be younger. younger. Some individual faults were observed to have have multiple multiple generations generations of of slickenfibers slickenfiberswith with different compositions, indicating indicating either either reactivation reactivation or or continuous continuous slip slip over a protracted protracted different period Fault Kinematics Kinematics (by R. period of time. time. Data Data from the the field field were were analyzed analyzed using using Fault Ailmendinger, Cornell Unviersity), a program that calculates Allmendinger, calculates best-fit best-fit paleostress paleostress tensors tensors from fault slip tensors all indicate normal stress slip information. information. The calculated tensors stress regimes regimes (maximum principal principal stress subvertical), even for the (maximum the latest latest generations generationsof ofslickenfibers. slickenfibers. This This contrasts contrasts with with the results results of of studies studies on on the theKeweenaw Keweenaw Peninsula, Peninsula, which which have have documented two two distinct stress documented stress regimes. regimes. There, There, early early normal normal faulting faulting gives gives way way to reverse faulting, possibly as a response to far-field far-field stresses stresses associated associated with the the Grenville Grenville Orogeny. The Orogeny. The absence absenceof of reverse-slip reverse-slipvectors vectors in in the the northeastern northeastern Wisconsin Wisconsin segment segment of of the Midcontinent Midcontinent Rift may reflect the misorientation of this part of the rift with respect to those far-field far-field stresses. stresses. . 18 �Possible Origin in in the the Negaunee Negaunee Mode of Occurrence of Trona and Thermonatrite and their Possible Iron-Formation of the Marquette Range, Lake Superior District, USA Tsu-Ming Han (Retired) (Retired) Research Laboratory, Cleveland-Cliffs Cleveland-Cliffs Inc. the silicate-bearing Negaunee Negaunee IronIronA white colored substance substance is often seen on the surface of the Formation of low metamorphic Range, Michigan. This substance is metamorphic grade grade on the Marquette Marquette Range% thermonatrite) ItIt occurs mostly of a mixture containing hydrous sodium carbonates (trona and thermonatrite) occurs as as thin coatings along bedding (Figure (Figure 1-A), 1-A), and in fractures cutting cutting across the bedding; as coatings patterns distributed distributed and colloform clusters on bedding planes (Figure 1-B); and as contour patterns Furthermore, nearly pure trona trona was developed quickly quickly between the fractures of bedding surfaces. Furthermore> dendrites and minute dots as dendrites dots on the cut cut surfaces surfaces of of some some hand specimens specimens in in storage storage(Figure (FigureC). C). To the writer's knowledge, knowledge, these these minerals minerals have not been previously reported reported from Precambrian Precambrian BIF B E of of the the equivalent equivalent metamorphic metamorphicgrade grade in in other other districts. districts. ILl Figures - Mode Mode of occurrence occurrence of trona and thermonatrite. thermonatrite. Figures 11— A—As whitecoatings coatingsalong along bedding. bedding. B-As B—Ascolloform colloformclusters clusters on on aa bedding bedding plane. A-As white C— Asdendrites dendriteson onthe thecut cut surface surface of of aa hand hand specimen in storage. C- As storage. The iron-formation is composed composed of of magnetite, magnetite, siderite, siderite, ankerite, ankerite, and and stilpnomelane.. stilpnomelane.. Minnesotaite Minnesotaite is also locally present in noticeable quantities. K is more than Na in these minerals as is the case in nearly nearly all all of of the the Precambrian Precambrian iron-formations iron-formationsof oflow lowmetamorphic metamorphicgrade. grade. As As aa 19 . �general generalrule, rule,stilpnomelane stilpnomelanecontains containsmore moreKKand andNa Nathan thanthe theminnesotaite. rninnesotaite.However, However,the the K20:Na.20 K20:NaZOratio ratioininthese theseminerals mineralsand andinthe in theiron ironformationa formationsmay mayvary varysubstantially.. substantially. . Based Basedon onthe theresults resultsfrom fromthe thehighly highlypurified purifiedwater waterleaching leachingtests testson onmore morethan thantwenty twentydifferent different samples, samples,the theNa Naininthe theiron-formation iron-formationisiswater-soluble water-solublewhereas whereasthe theKKisispractically practicallyinsoluble insoluble (Figures 2Aand andB). B).The TheXRD XRDand andanalytical analyticaldata datashow showaagood goodcorrelation correlationbetween betweenthe theamount amount (Figures2A ofofstilpnomelane Na20, K20 ofNa.20, K20and andA1203 A1203(Figures (Figures3A 3AtotoC). C). stdpnomelaneand andthe theamounts amountsof y = 9.9759x- 0645 0.05 0 0.10 0.15 020 - Figure22 AAand andBB Solubility Solubilityof ofK20 K 2 0and andNa20 Na20 Figure in the silicate-bearing iron-formation with high in the silicate-bearing iron-formation with high andlow low K20:Na20 K20:Na20ratios. ratios. and - 25: R 2O 15 15 - Figure33- AAto to C C Relationship Relationship of of stilpnomelane stilpnomelane Figure toK20, K20,Na20 Na20and andA1203. Al203, to 00 0.5 0.5 1.0 1.O 1.5 1.5 2.0 2.0 2.5 2.5 ItItmay may be be logically logicallyconcluded concludedthat thatmost mostof ofthe thesodium sodiumwas wasderived derivedfrom fromthe the stilpnomelane, stilpnomelane, which ofthe thehydrous hydroussodium sodiumcarbonates carbonateswas was whichwas was leached leachedout outby bymeteoric meteoricwater. water.The Themixture mixtureof then thendeveloped developedthrough throughevaporation evaporationunder underthe theatmospheric atmosphericconditions. conditions. 20 3.0 3.0 �Keweenawan Mafic and and Ultramafic Intrusive Intrusive Rocks of the Lake Nipigon and Ontario Crystal Lake areas, northwestern Ontario Hart, Hart, Thomas ThomasR., R.,Ontario OntarioGeological GeologicalSurvey, Survey,933 933Ramsey RamseyLake LakeRoad, Road, Sudbury, Sudbury, Ontario Ontario P3E 6B5; tom.hart@ndm.gov.on.ca tom.hart @ndm.gov.on.ca The Keweenawan diabase diabase sills sills in the Lake Lake Nipigon and and Crystal Crystal Lake Lake areas, areas, northwest of Lake Superior, Superior, consist consist of of two distinct distinct geochemical geochemical and and geographical geographicalgroups groups with each area also hosting hosting aa number number of unique unique intrusions intrusions that that suggest suggest different differenttectonic tectonic processes. Mapping Mapping by Smith Smith and Sutcliffe Sutcliffe (1987) (1987) in the Crystal Crystal Lake Lake area area identified identified aa series of 6 Logan diabase diabase sills sills >5 >5 m thick that gently gently dip dip to the southwest, southwest,and and intrude intrude into the early Proterozoic Proterozoic Rove Rove Formation. Formation. Northeast Northeast trending trending dykes dykes of of the the Pigeon Pigeon River River swarm range from olivine olivine to quartz diabase diabase in composition, composition,and and include includedykes dykes that that crosscut the Logan sills and dykes that appear to merge with the sills. sills. The The layered layered gabbro gabbro — anorthosite anorthosite -- troctolite troctolite Crystal Lake Gabbro crosscuts and contains inclusions of Pigeon -Tb/Yb - ZrIY Zr/Y River dykes. Samples of the Logan sills sills can be subdivided subdivided into into aa low low TiO2 Ti02 -TWYb group and -Tb/Yb - Zr/Y (OGS2002). 2002).The The high Ti02 group ZrIY group group (OGS high Ti02 group waswas and aa high high TiO2 Ti02 -Th/Yb identified as asbeing beingquartz quartz normative, comparable theLogan type Logan by Sutcliffe normative, andand comparable to thetotype sills bysills Sutcliffe (1991). Samplesidentified identified Pigeon River dykes exhibit high degree of variability (1991). Samples as as Pigeon River dykes exhibit a higha degree of variability suggesting that they represent represent at least three three unrelated unrelated intrusions. intrusions. One One subset subset of of the the comparable to the low TiO2 of Logan Logan sills, and another Ti02 group of Pigeon River dykes is comparable Gabbro. Most subset is comparable to the Crystal Lake Gabbro. Most of of aa third third subset subset of of dyke dyke samples samples contain lower lower trace trace element element are located along Highway 61 close to the Pigeon River, and contain abundances and ratios than the other other intrusions intrusions in the area. area. Gabbro Gabbro samples samplesfrom from the the Crystal Lake Gabbro intrusion display some overlap with the low TiO2 group of Logan Ti02 of Tb/Ta ratios. The Th/Yb, and TWTa sills but also includes samples samples with higher Zr/Y, ZrIY, ThIYb, The Logan diabase sills are confined to the area to the south of Thunder Thunder Bay, with the Nipigon Nipigon diabase sills located to the north. north. The initial initial Keweenawan Keweenawan intrusive intrusive event event in in the the Lake Lake Nipigon Nipigon area area isis probably probably represented by the relatively flat lying to shallowly dipping dipping peridotites peridotites located located in in the the Disraeli, Seagull - Fox Mountain, Hele, and Kitto areas that form intrusions a - Seagull Leckie— Disraeli,Leckie few kilometres in diameter. The peridotites are composed of orthocumulate to mesocumulate textured wehrlite to lherzolite, containing 1 to 2% reddish brown mica and commonly a discontinuous (e.g. Sutcliffe Sutcliffe1987; 1987;Hart Hart et et al. al. discontinuous olivine olivine gabbro gabbro border border phase phase (e.g. 2002). The Disraeli, Seagull Seagull and Hele peridotites are characterized characterized by higher higher MgO and and Zr/Y Tb/Yb values but lower ThITa Th/Ta ratios than the Nipigon diabase sills. A series of of ZrIY and TWYb 0.5 to 3.0 m thick sills are located stratigraphically below below the Nipigon sills, as exposed Tb/Yb and Tb/Ta along Highway 17 at Kama Hill. These sills have MgO, TWYb TWTa values values intermediate Tb/Ta intermediate between the peridotites and Nipigon sills, and subdivided into higher Th/Ta and lower Tb/Ta subgroups may be possible with additional sampling. These sills have TWTa subgroups with additional sampling. These sills have Tb/Ta La/Yb ratios comparable to the high Ti02 group of Logan sills, but generally ThITa and LalYb have lower trace element abundances. The Kitto peridotite also has ThIYb, Th/Yb, Th/Ta Tb/Ta and Zr/Y ratios that overlap with these sills rather than the other peridotites. The olivine ZrIY tholeiite Nipigon diabase sills are up to 200 m thick, and are chilled against the peridotite pendotite intrusions. Previous work indicates that some sills were formed by multiple pulses of of magma (e.g. Sutcliffe, Sutcliffe, 1987; 1987; Hart et al. al. 2002), 2002), but the the chemistry chemistry of of the the sills sills over overthe the 21 �entire Lake Nipigon area area displays displays little little variation. variation. Geochemical Geochemical differences differencesbetween between the the entire peridotites and Nipigon diabase diabase sills are comparable comparable to the variations observed in the peridotites Osier values Osler Group volcanic rocks (e.g. Sutcliffe, 1991). The The Nipigon Nipigon sills sills have have Ti02 Ti02 values Ti02group group of of Logan sills sills but higher higher ThITa Th/Ta and and lower lower La/Yb LdYb comparable to the the low low Ti02 comparable differences in the the geochemistry geochemistry of the the diabase diabase sills sills between between the the Lake Lake Nipigon Nipigon ratios. The differences and Crystal Lake areas is similar to the differences observed in the volcanic volcanic rocks of aa flood basalt provinces provinces (e.g., (e.g., Mantovani Mantovani et et al. al. 1985). 1985).The The regional regionalextent extentof of the the number of flood An initial initial geochemical groups within the Keweenawan intrusions is not known. An examination of troctolites troctolites from from the the Babbit Babbit deposit deposit of of the the Duluth Duluth Complex Complex(Ripley (Ripleyetetal. al. examination 1999) Th/Yb, and and Zr/Y Zr/Y ratios ratios comparable to the Nipigon peridotites peridotites ThITa, Th/Yb, 1999) indicates ThTFa, rather than the intrusions intrusions of the the Crystal Crystal Lake area. area. References References Hart, T.R., terMeer, M., and Jolette, C. 2002. Precambrian geology of Kitto, Eva, Summers, Dorothea and Sandra Townships, Beardmore area, northwestern Ontario; Ontario Geological Survey, Open File Report 6095, 206 206 p. PL.S., de de Sousa, Sousa, M.A., MA., Civetta, L., Atalla, L., and Innocenti, Innocenti, F., F., 1985. Trace Trace element element and and Mantovani, M.S.M., Marques, L.S., strontium isotopic constraints on the origin and evolution of Parana continental flood basalts of of Santa Santa Catarina State (southern Brazil); Journal of Petrology, v. 26, p. 187-209. Ontario Geological Survey, 2002. Proterozoic Volcanic and Intrusive Whole Rock Geochemical Data associated with the Keweenawan Midcontinent Rift, Lake Superior Area, Ontario; Ontario Geological Survey Miscellaneous Release—Data 114. Release-Data 114. EM., Lambert, Pb isotopic isotopic constraints constraints on on mantle mantle and and crustal crustal Ripley, E.M., Lambert, D.D., and Frick, L.R., 1999. 1999. Re-Os, Sm-Nd, and Pb to magmatic magmatic sulfide mineralization in the Duluth Complex; Geochimica et Cosmochimica Acta, contributions to v. V. 62, p.3349-3365. Smith, A.R. and Sutcliffe, R.H. 1987. Keweenawan intrusive rocks rocks of of the the Thunder Bay area; in Summary Sunnnary of Field Work and Other Activities, Ontario Geological Survey Miscellaneous Paper 137, 137, p. 248-255. 1987.Petrology Petrology of of Middle Middle Proterozoic Proterozoic diabase diabase and picrites from Lake Nipigon, Sutcliffe, R.H., RH., 1987. Nipigon, Canada; Canada; Contributions Contributions to Mineralogy and Petrology, v.96, p. 201-211. 201-21 1. Sutcliffe, R.H., RH., 1991. 1991.Proterozoic Proterozoic geology geologyof of the the Lake Lake Superior Superior area; area; in in Geology Geology of of Ontario, Ontario, Ontario Ontario Geological Geological Survey Survey Part Special Volume 4, P art 1, 1, p. 627-658. 22 �GEOLOGY, DRILL HOLES, MINERAL MINERAL LEASES, AND GEOPHYSICS GEOPHYSICS IN THE THE DULUTH DULUTH GEOLOGY, INTEGRATIONOF OF AND BEAVER BAY COMPLEXES, NORTHEASTERN MINNESOTA: INTEGRATION VARIOUS GIS GIs DATABASES DATABASES TO TO TELL TELL A A STORY STORYOF OFTHE THE HISTORY HISTORYOF OFPAST PASTAND AND VARIOUS CURRENTCU-NI-PGE CU-NI-PGEMINERAL MINERALEXPLORATION EXPLORATION CURRENT Steven A. Hauck, Hauck, Julie Julie A. A. Oreskovich, Oreskovich,and and Mark Mark J.J. Severson, Severson,Economic EconomicGeology GeologyGroup, Group, Steven Natural Natural Resources ResourcesResearch Research Institute Institute(NRRI), (NRRI),University University of of Minnesota, Minnesota,Duluth, Duluth,5013 5013 MN 55811-1442, 55811-1442,shauck@nrri.umn.edu shauck@nrri.umn.edu Miller Trunk Trunk Highway, Highway, Duluth, Duluth,MN Miller Mineral exploration exploration in the Duluth Duluth Complex Complex began in 1948 1948 on Spruce Spruce Road when two prospectors found found sulfide sulfide mineralization. mineralization. Subsequent Subsequentcore coredrilling, drilling,geological geologicalmapping, mapping,and andairborne airborneand and ground ground geophysics geophysics by more more than than 28 28 exploration explorationcompanies companies (including (includingthe the NRRI, NRRI,MGS MGS-Minnesota Minnesota Geological Survey, Survey, and and the DNR -- Dept. Dept. of Natural Resources, Resources,Division Divisionof of Lands Lands and and Minerals), over the next 52 years led to the discovery of copper-nickel±platinum-group element Minerals), over the next 52 years led to the discovery of copper-nickelkplatinum-group element (PGE) mineralization mineralization along the basal contact of the Duluth Complex (Fig. 1). Ten Ten Cu-Ni-PGE Cu-Ni-PGE or or PGE-Cu-Ni deposits deposits were were defined by drilling during these years. Over Over2,142 2,142drill drill holes holes have have been drilled drilled into into the Duluth Duluth and and Beaver Bay complexes with 1,666 1,666 of these holes being drilled along along the basal contact. contact. Over Over954,000 954,000ft. ft.of ofdrill drillcore corefrom fromthe thebasal basalcontact contacthas hasbeen beenrelogged relogged by NRRJ, publications. Geophysical NRRI, and their results are discussed in many publications. Geophysical exploration exploration began began as as early as 1956, 1956, by Bear Creek Mining Company, and continues today. The The State State of of Minnesota Minnesota (MGS), with funding funding from from the Legislative Legislative Commission Commission on on Minnesota MinnesotaResources, Resources,flew flewhigh high resolution aeromagnetics aeromagnetics over over this area as well as the rest of the state. The TheMGS MGShas hasalso also collected collected and produced a gravity map covering both complexes. Peak Peakexploration exploration(1966-1978) (1966-1978) began with the leasing leasing of State State of Minnesota Minnesota mineral rights in 1966 1966 (Fig. 1). Exploration Explorationand and development (drilling, bulk sampling, shaft sinking, resource calculations) continued through development (drilling, bulk sampling, shaft sinking, resource calculations) continued through 1978. 1978. In In1998, 1998,State Stateand andFederal Federal mineral mineral leasing leasing and and exploration explorationdrilling drilling began began to to increase increase with with the: 1) 1) rise in in price price of of PGEs; PGEs; 2) 2) possible possible use use of of new new hydrometallurgical hydrometallurgicaltechniques techniquesto tomore more efficiently efficiently recover recover copper copper and and nickel; nickel; and and 3) 3) introduction introductionof of new new PGE PGE exploration explorationmodels models (sulfide i.e.,atatSonju Sonju (sulfide saturation; saturation;Miller Miller et et al., al., 2002) 2002) for for intrusions intrusionsin in the the Beaver BeaverBay BayComplex, Complex,i.e., Lake, and the Duluth Complex, Duluth). The Complex, i.e., Greenwood Lake and Layered Series at Duluth). The maps maps in this this poster poster illustrate illustrate the the relationship relationshipbetween between geology, geology,geophysics, geophysics, drilling, drilling,and andmineral mineral leasing leasing and were produced in ArcView (GIS). (GIs). The Themaps maps were were also alsocompiled compiledby by using using information from: 1) (minarchive.dnr.state.mn.us)attributeattribute-and and GIS-based GIs-based DNR's online online(minarchive.dnr.state.mn.us) 1) the DNR's database database of non-ferrous non-ferrous minerals' minerals' information information and and State State mineral mineral rights rights holdings; holdings; 2) 2) U.S. U.S. Forest Forest Service GIs data dataon onthe thehistory history Service leases, leases, permits, permits, and and applications applicationsdatabase; database; and and3) 3) NRRI NRRI in-house in-houseGIS of Cu-Ni-PGE Cu-Ni-PGE mineralization. mineralization. Using Usingthe theresulting resultingGIS GIsdatabase, database,the thespatial spatialrelationships relationshipsin inthe the changes in drilling, drilling, leasing, leasing, etc. etc. with with time time and and place place were were then then combined combinedwith withgeological geological information information from from Miller Miller et et al. al. (2002) (2002)to to better better understand understandthe the past past and andpresent presentexploration explorationareas areas and and to assist assist in in defining definingnew new areas areas in in which which to to explore explorefor fornon-ferrous non-ferrousminerals. minerals. References References Miller, Jr., Green, Green, J.C., J.C., Severson, Severson, M.J., M.J., Chandler, Chandler, V.W., V.W., Hauck, Hauck, S.A., S.A., Peterson, Peterson, D.M., and Miller, J. D., Jr., Wahl, T.E., 2002, 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: 207 p. Minnesota: Minnesota MinnesotaGeological Geological Survey SurveyReport Report of of Investigations Investigations58, 58,207 p. 23 �- CU S CU SC INCO 0 CD Creeku Beer Creek I = U (Ii U Ui S Ut S S 0 0 S 0 CU USS S N 1 t S 0 S 0 Dovl Coo Neweori 0 S 10 INCO Beer Creek' I S N 10 S OJ N S 0 N I I I - CU = (S Theel U XX CU 0 CU CU CU CU I CU CU CU a 0' INc0 0' CU 0 S Di 10 I S 0' CU S = 0 - - I PotyMeti 10 0 0 0' CU 0' Xl 0' - CU 0' - - 0 = = CONCENTRATED DRILLING - II IU \felkrdgeU• S 0 Di = SCATTERED DRILL NG Arerieer S/ic d/NICDR CK Lehcerrr I - 0 CU SSS CU 10 N 0 N CU CU S 5555555555555 N !hosorr 1 oxocL1 Cleveerd CliP Fe I • N CD 11111 0 S 55 N Blerkerkerg—Wkiteside U I orrotioc dril eg) I r4 I AMAX - S CU uss •I Devot helps D ooge CD I (costly iroc Beer Creek IIHIIIIHHIII BI I I k/S Moore/A ercerr ShijId Creek I S - sPLie Rood Mokori S ---Beor Ceek •L,L Lrek1 Beer Creeki Bee B Sproce Rood cUr Creek INCO •1US'BM1MGS S CU - Metri = S IIIjIflIII S UCkidere eed Cdhiteeide S CU Figure 1. History of Cu-Ni-PGE exploration in the Duluth Complex (after Miller et at., 2002). GUNFLINT TRAIL DRILLING RECONNAISSANCE OR SCATTERED LONGNOSE (Oul) BIRCH LAKE WATER HEN (OUI) WESTERN MARGIN SO. FILSON CREEK WETLEGS DUNKA PIT WYMAN CREEK (NorthMef) DUNKA ROAD (Minnamax) (Mesaba) BABBITT MATURI SPRUCE RD & Deposit/Area kin.iesolo sow C—Ni nvwol INCO • Duval • Exxon • Other Companies AMAX • Bear Creek !I USS • LEGEND �Geochemistry and Mineralization of the Seagull Seagull Intrusion, Northern Ontario Heggie, P., P., (Department of Geology, Lakehead Heggie,G., G.,and andHollings, Hollings, (Department of Geology, LakeheadUniversity, University,955 955 Oliver Oliver Road, Thunder Bay, On, P7B 5E1, gheggie gheggie@mail.lakeheadu.ca) @ mail.1akeheadu.ca) Platinum Group Elements Elements (PGE-Platinum, (PGE-Platinum, Palladium, Osmium Osmium and Iridium) Indium) have seen seen substantial increases in demand over the last substantial 30 years, as industrial and commercial users have increased their consumption. consumption. Canadian Canadian production of these metals has until recently been limited to by-products from nickel Sudbury). Opening of the copper mines (e.g., Sudbury). Lac des mine in in Ontario demonstrated demonstrated the des Ties Iles mine the potential for economic economic PGE PGE deposits deposits in in Canada. Further work on deposit and exploration exploration models is is essential essential to to identifying identifying new targets and prospective prospective host rocks. rocks. The Seagull Lake intrusion intrusion is found found within within the the Nipigon Embayment, approximately 70km approximately 70km north east of Thunder Thunder Bay, Ontario Ontario (Fig.1). (Fig.1). Relative Relative age dating dating places the age age of the Seagull Lake intrusion to be younger than Map showing location location of Seagull Intrusion Intrusion Figure 1. Map the Sibley Sibley Group sedimentary sedimentary sequence, sequence, and (1339±33 Ma) (Franklin et al., 1980), and regional regionalgeology. geology. (1339k33 1980), as part of the intrusion has been seen to cross cut Sibley Sibley stratigraphy. A chilled chilled margin has been observed between the Seagull Seagull Intrusion and the younger Nipigon Sills defining defining an upper age of approximately 1.1 Ga (Davis (Davis and and Sutcliffe, Sutcliffe,1985). 1985).This This falls falls within within the the time time of of mid-continental mid-continental rifting in the Lake Superior region. Volcanic activity was responsible for production Superior region. Volcanic activity responsible for productionof of thick thick basaltic sequences sequences (Cannon (Cannon et et al., al., 1989) 1989)beneath beneath and and around around the the shores shoresof of Lake LakeSuperior Superiorand andthe the emplacement of numerous mafic to ultramafic complexes (e.g., Duluth complex). complexes (e.g., Duluth complex). The Seagull Seagull Intrusion is currently currently under exploration exploration by East East West West Resource Resource Corporation, Corporation,ItIt isis aa consisting of cumulate layered ultramafic intrusion consisting cumulate olivine, olivine, and oxide oxide minerals minerals with with pyroxene pyroxene oikocrysts and interstitial feldspar. feldspar. Lithological phases include include dunites, dunites, iherzolites, lherzolites, olivine olivine gabbronorites, gabbros, and pryoxenites. A distinctive distinctive olivine olivine gabbronorite gabbronorite is is found found within within the the gabbronorites, intrusion but this exhibits chilled chilled margins and is thought to post date the formation of the rest of the intrusion. intrusion. 25 �DDH WMOO-01 WMOO-01 Depth (m) (m) Depth 375.0 375.0 408.0 572.0 546.0 379.0 Interval Interval (m) (m) 4.0 4.0 4.5 12.0 6.0 8.0 6.0 6.0 6.0 Cu Cu (ppm) (ppm) 269 269 Ni (ppm) Ni ( P P ~ 1160 1160 Pt pt (ppb) (PP~) 1413 1565 779 WM98-02 1180 987 WM98-05 112 1647 569.0 1843 1841 579.0 1220 1455 579.0 1220 1455 Figure Figure 2. Table Table of metal metal contents contents from from assay assay (Caven, (Caven, R., R., 2000) 501 307 307 336 363 535 336 693 458 458 Pd (ppb) pd (PP~) 383 383 418 438 566 393 847 537 Mineralization occurs in the form of PGE minerals associated with disseminated FeNi sulfides (pentlandite). Pentlandite is found in higher abundances at discrete intervals intervals throughout throughout the the intrusion, with a general increase increase towards the base base of of the the intrusion. intrusion. Work is currently being undertaken to understand the the stratigraphy stratigraphy of the the intrusion, intrusion, the the nature nature of of the platinum group mineralization, mineralization, and and formational formational controls controls on on the the mineralized mineralized zones, zones,which whichare are present in the intrusion in order to aid in the development and refinement of exploration techniques, and deposit models. Cannon, W.F., Green, A.G., Hutchinson, D.R., Lee, M., Milkereit, B., B., Behrendt, Behrendt, J.C., J.C., Halls, Halls, H.C., H.C., Green, J.C., Dickas, A.B., Morey, G.B., Sutcliffe, Dickas, Sutcliffe, R., and and Spencer, Spencer, C., C., 1989, 1989,The TheNorth NorthAmerican AmericanMidcontinent Midcontinent rift rift Reflection profiling. profiling. Tectonics, Tectonics, v. 8, p. 305-332. beneath Lake Superior from GLIMPCE Seismic Reflection Caven, R.J., 2000, Progress Caven, R.J., Progress Report Report on on the the Wolf Wolf Mountain Mountain and and Disraeli Disraeli Properties Properties for for East EastWest WestResource Resource Corporation, Corporation, Canadian Canadian Golden Golden Dragon Dragon Resources Resources Ltd. Ltd. and and Avalon Avalon Ventures VenturesLtd. Ltd. Davis, D.W., D.W., and R.H., 1985, Davis, and Sutcliffe, Sutcliffe, R.H., 1985, U-Pb U-Pb ages ages from from the the Nipigon Nipigon plate plate and andNorthern Northern Lake LakeSuperior. Superior. Geological Geological Society Society of American American Bulletin, Bulletin, v.96, v.96, p. 1572-1579. 1572-1579. Franklin, J.M, Mcllwaine, McIlwaine, W.H., Poulsen, K.H., and and Wanless, Wanless, R.K., R.K., 1980, 1980,Stratigraphy Stratigraphy and and depositional depositional setting setting of of the Sibley Group, Thunder Thunder Bay Bay district, district, Ontario, Ontario, Canada. Canada. Canadian Canadian Journal of of Earth Earth Sciences, v.17, p. p. 633-65 633-651. 1. 26 �PEPERITES PEPERITESOF OF THE THEGAFVERT GAFVERTLAKE LAKE VOLCANIC VOLCANIC COMPLEX, COMPLEX,ST. ST. LOUIS COUNTY, MINNESOTA MINNESOTA Heiling, Carrie D., Department of Geological Geological Sciences, Sciences, University University of of Minnesota Minnesota Duluth, Duluth, 1114 1114 Kirby Drive, Duluth, MN, 55812; cheiling@d.umn.edu cheiling@d.umn.edu The Gafvert Lake area, located within the Upper Ely member of the Ely Greenstone Greenstone of the Vermilion District in Northeastern Minnesota (Figure 1) (Card, 1990), forms part part of of a large, Archean, felsic volcanic complex. Morton Morton (personal (personal communication) communication) has has interpreted the complex to be a composite volcano that underwent late stage caldera collapse. This Thisstudy studyhas has focused focused on on aa two two square square mile mile area area in in the the central central part part of of the the complex. Here Herethe thecomplex, complex,from fromthe theoldest oldestto to the the youngest youngest rocks, rocks, is is composed composed of of a) a) coarse, heterolithic breccias (interpreted (interpreted to represent meso-and mega breccias) breccias) (Morton, (Morton, personal communication), b) more than 3000 3000 feet feet of massive massive to to bedded bedded pumice-rich pumice-rich lapilli tuff, c) dacitic lavas and domes, and and d) d) lenses lenses and and beds of of chert chert and and massive massive to to semi-massive pyrite (Figure 2). The The breccias breccias and and lapilli lapilli tuffs tuffs have have been intruded by a swarm of feldspar porphyry dacite dikes that represent feeders to the domes and/or and/or flows. flows. Peperites are rocks formed by the in situ disintegration of magma intruding and and mixing mixing with wet unconsolidated sediment et al., al., 2002). At At Gafvert Gafvert Lake the sediment or or ash (Skilling (Skilling et peperites formed near the top of the complex where dacite dacite porphyry dikes intruded intruded and and unconsolidated pumice-rich pumice-rich lapilli lapillituff. tuff. This mixing led to quenching mixed with wet, unconsolidated and fragmentation of the dacitic magma and disruption and vesiculation vesiculation of of the the lapilli lapilli tuffs. The peperites occur within 100 feet of dike contacts though they form much The peperites occur within 100 feet of dike contacts though they form much more together. Angular and extensive areas where several dikes occur close together. and finger-like blocks of dike material occur within the peperite, locally these are connected to aa nearby nearby dike. dike. Macrotextures in outcrop and and microtextures microtextures in thin thin section section helped helped identify identify and and classify classify the following fragment types and internal structures within the peperites: a) a) blocky blocky jig-saw fit textures, b) platy to ragged juvenile fragments with chilled rims and occasional jig-saw juvenile fragments with curviplanar surfaces and broken gas bubbles, c) ameboid to globular juvenile fragments, d) abundant pumice which exhibits variable variable vesicularity. vesicularity. Most of this pumice is juvenile to the lapilli tuffs but a small percentage contains feldspar crystals identical to those found in the dikes possibly indicating local, local, rapid rapid vesiculation vesiculation of dike material. Close Closeto todike dikemargins marginsfeldspar feldspar crystals crystals are are broken broken and and internally internally fractured with fractures filled by lapilli tuff. Pumice, Pumice, close close to to dike dike contacts, contacts, may may be jigsaw-fit pieces. pieces. Locally the ash matrix to broken or disaggregated into several small jigsaw-fit the lapilli tuffs is amygdaloidal amygdaloidal with amygdules amygdules radiating away from dike margins. margins. References References Card, K.D., 1990, Province of of the the Canadian Canadian Shield, Shield, aa product product of of 1990, A review of the Superior Province Archean accretion: Precambrian Research, v. 48, pp. 99-156. Precambrian Research, v. 48, pp. 99-156. Minnesota, 2003, Mapquest, Ely, Minnesota, Mapquest, www.mapquest.com. www.rnapquest.com. communication, University University of Minnesota-Duluth. Minnesota-Duluth. Morton, R.L., 2003, personal communication, Skilling, Skilling, I., White, J., McPhie, McPhie, J., J., 2002, 2002, Peperite: Peperite: aa review of magma-sediment magma-sediment mingling, mingling, Journal Journal of Volcanology Volcanology and Geothermal Geothermal Research, Research, v. 114, 114, pp 1-17. 1-17. 27 �Figure 1: 1: Location Location of of Gafvert Gafvert Lake Lake complex complex (Mapquest, (Mapquest, 2003). 2003). Explanation Explanation • Peperite samples Fault Zone Contacts / '" ' Railroad grade grade , " Mud Creek Road / Mud Road f' * A/ - Mbas - Metabasalt Metabasalt SIst METERS 200 00 J0 Â¥s 410 400 600 oo 800 Figure Figure 2: Generalized Generalized map of a portion of Gafvert Gafvert Lake volcanic complex. complex. 28 Q Qfo- Qtzfeld - QtzfeldPorphyry Porphyry Black Chert Cht -- Black Cht Diab -- Diabase Diabase Dikes Dikes Dior Dior -- Diorite Diorite Dac -- Dacite Dikes Dikes Dac Tuff -- Lapilli Lapilli Tuff luff Bx Bx -- Breccia Breccia SIst SIst - Siltstone &&Iron Iron Fm Fm �CHEMISTRY OF OF ALTERATION ALTERATION MINERAL MINERAL PHASES PHASES AT THE FIVE MILE MILE LAKE LAKE CHEMISTRY VOLCANIC-HOSTED MASSIVE MASSIVE SULFIDE SULFIDE PROSPECT, PROSPECT,NE NE MINNESOTA MINNESOTA VOLCANIC-HOSTED locker, Hocker, S. S.M., M.,Hudak, Hudak, G. G.J.,J.,Odette, Odette,J.J.D., D.,and andNewkirk, Newkirk,T.T.T., T.,Department Department of ofGeology, Geology, University of Wisconsin Wisconsin Oshkosh, Oshkosh, 800 800 Algoma Algoma Blvd., Oshkosh, WI 54901, hudak@uwosh.edu hudak@uwosh.edu Alteration mineral Lake Prospect Prospect in in the the Vermilion Vermilion Alteration mineral assemblage assemblage mapping mapping at at the the Five Mile Lake identified two distinct types of alteration zones within 2.7 District of northeastern Minnesota has identified year-old volcanic volcanic and and volcaniclastic volcaniclastic rocks rocks associated associated with withvolcanic-hosted volcanic-hosted massive massive billion year-old 2001a,2001b; 2001b;Peterson, Peterson, mineralization (Hudak et a!., al., in in press; press; Odette Odette et et al., al.,2001a, sulfide (VHMS) mineralization 2001). 2001). Regional Regional semi-conformable semi-conformable alteration alterationzones zones are are composed composed of of various various proportions proportions of of quartz quartz ++ epidote epidote ±Â amphibole amphibole ± chlorite chlorite ±Â plagioclase plagioclase feldspar. feldspar. These regional, regional, These are locally locally cross-cut cross-cut by by several severalrelatively relativelynarrow, narrow,northeastnortheastsemiconformable alteration zones are composed of fine-grained fine-grained chlorite chlorite and/or andlor sericite sericite that that trending disconformable alteration zones composed are closely closely associated associated with with synvolcanic synvolcanicfault faultzones. zones. Electron microprobe microprobe analyses the various various alteration alteration mineral mineral phases phases (epidote (epidote group group Electron analyses of of the minerals, chlorite, feldspar) have conducted in an effort effort to to minerals, chlorite, amphibole, amphibole, white white mica, mica, and and feldspar) have been been conducted better understand understand hydrothermal hydrothermal processes processes associated associated with with the the development development of the the semiconformable and Mile Lake Lake prospect. prospect. These These semiconformable and disconformable disconformable alteration alteration zones zones at at the the Five Mile zoisite/clinozoisite to analyses indicate that: a) epidote epidote group group minerals minerals range in composition composition from zoisite/clinozoisite analyses pistacite; b) chlorite chlorite is is dominantly dominantly ripidolite; ripidolite; c) c) amphibole amphibole is is primarily primarily actinolite actinolite and and ferroferropistacite; actinolite, with magnesio-hornblende magnesio-hornblende and is fineheactinolite, and ferro-hornblende ferro-hornblendealso also present; present; d) d) sericite is grained grained muscovite; muscovite; and and e) e) plagioclase plagioclasefeldspar feldsparisisdominantly dominantlyalbite. albite. chemistry at the Five Five Mile Mile Lake Lake Prospect Prospect is is remarkably remarkably similar similar to that that Alteration mineral mineral chemistry VHMS mining mining camp camp of of Canada, Canada,as as well well as as other other VHMS VHMSmining miningcamps campsaround around from the Noranda VHMS the world. world. This alteration alteration mineral mineral chemistry chemistry suggests the presence of aa complex, complex, long-lived long-lived hydrothermal system evolved from from seafloor-proximal seafloor-proximal (hundreds meters) to deeper deeper hydrothermal system that that evolved (hundreds of of meters) subseafloor environments kilometers) as as the the volcanic volcanic rocks rocks were wereburied buriedby byapparently apparently subseafloor environments (-1-3 (1 -3 kilometers) dominantly effusive effusive mafic mafic to to intermediate intermediate volcanism volcanism and and associated associated sedimentation. sedimentation. This This rapid, dominantly addition to the the Five Five Mile Mile Lake Lake Prospect, Prospect, the the uppermost uppermost several several hundred hundred meters meters suggests that in addition Lower Member Member of of the the Ely Ely Greenstone Greenstonealso also has has excellent excellentexploration explorationpotential potential for for VHMS VHMS of the Lower mineral deposits. deposits. References References F., Paradis, Paradis, S., S., Hannington, M., Holk, G., G., Katsube, Katsube, J., J., Paquette, Paquette, F., Galley, A., Bailes, A., Hannington, Galley, A., Bailes, M., Holk, Santaguida, Santaguida, F., F., and and Taylor, Taylor, B., B., 2002, 2002, Database Database for CAMIRO CAMIRO Project Project 94E07: 94E07: Interrelationships between subvolcanic intrusions, intrusions, large-scale large-scale alteration zones, and and VMS VMS Interrelationships between subvolcanic deposits: deposits: Geological Geological Survey Surveyof ofCanada CanadaOpen OpenFile FileReport Report4431 443 1(CD-ROM). (CD-ROM). and Hauck, Hauck, S., S., in in press. press. Comparative Comparativegeology, geology, Newkirk, T., T., Odette, Odette, J., and Heine, J., J., Newkirk, Hudak, G. J., Heine, Mile Lake, Lake, Quartz Quartz Hill, Hill, and and Skeleton SkeletonLake Lake stratigraphy, and lithogeochemistry of the Five Mile VMS VMS occurrences, occurrences, Vermilion Vermilion District, District, NE Minnesota: AAreport reportto tothe theMinerals MineralsCoordinating Coordinating Committee, Minerals Division, State of Minnesota. DNR Minerals Minnesota. Committee, DNIR 29 �Kranidiotis, Kranidiotis, P. P. and and MacLean, MacLean, W. W. H., H., 1987, 1987,Systematics Systematicsof of chlorite chlorite alteration alteration at at the the Phelps Phelps Dodge Dodge Massive Sulfide Sulfide Deposit, Deposit,Matagami, Matagami,Quebec: Quebec:Economic EconomicGeology, Geology,v.v.82, 82, p. p. 1898-1911. 1898-1911. Massive Odette, Odette, J. D., Hudak, Hudak, G. G. J., J., Suszek, Suszek, T., T., and and Hauck, Hauck, S. S.A., A.,2001a, 2001a. Preliminary Preliminary evaluation evaluation of hydrothermal hydrothermal alteration alteration mineral mineral assemblages assemblages and and their their relationship relationship to to VMS-style VMS-style Archean Vermilion Vermilion Greenstone Greenstone Belt, Belt, NE NE mineralization in the Five Mile Mile Lake Lake area area of of the theArchean mineralization 470h Minnesota: Institute Institute on on Lake LakeSuperior SuperiorGeology, Geology, 47thAnnual Meeting, Meeting, Proceedings Proceedings Volume Volume 47, 47, Part Part 1-Program 1-Programand and Abstracts, Abstracts, p. p. 75-76. 75-76. Odette, D., Hudak, Hudak, G. G. J., J., Suszek, Suszek, T., T.,and andHauck, Hauck, S.S.A., A.,2001b, 2001b,Preliminary Preliminary evaluation evaluation of Odette, J. D., hydrothermal hydrothermal alteration alteration mineral mineral assemblages assemblages and and their their relationship relationship to to VMS-style VMS-style mineralization in the Five Five Mile Mile Lake Lake area area of of the theArchean Archean Vermilion Vermilion Greenstone Greenstone Belt, Belt, NE NE mineralization Minnesota: Geological Geological Society Society of of America America Abstracts Abstracts and and Programs Programs Volume Volume 33, 33,No. No. 6,6, p. p. AAMinnesota: 420. 420. Peterson, Peterson, D. M., M., 2001, 2001, Development Development of of Archean Archean lode-gold lode-gold and and massive massive sulfide sulfidedeposit deposit exploration models models using using geographic geographic information information system system applications: applications: targeting targeting mineral mineral exploration exploration in northeastern northeastern Minnesota Minnesota from analysis analysis of of analog analog Canadian Canadian mining mining camps: camps: exploration Duluth, Minnesota, Minnesota, 503 503 p. p. unpublished Ph. Ph. D. D. dissertation, dissertation, University University of of Minnesota, Minnesota, Duluth, unpublished Fe (total) 18 0.2 0.4 0.6 16 14 12 10 0 AlzQ, CORUNDUM CORUNDUM 0.9 0.8 0.7 0.6 La :' 0.5 0.4 0.3 0.2 0.1 I 6.0 6.5  Flm Mite Lake Amphlboto I 7.0 7.0 7.5 8.0 ORTHOCLASE ORTHOCLtSE KAISiiO. KOJSieOe Figure Figure 1.1. Summary Summary of of electron electronmicroprobe microprobeanalyses analysesfor forepidote-group epidote-groupminerals minerals(A), (A),chiorites chlorites (B), (B), amphiboles amphiboles (C), (C), and andwhite whitemicas micas(D) (D)from fromthe theFive FiveMile MileLake LakeProspect Prospectand andselected selectedVHMS VHMS mines. mines.Compositional Compositionalfields fieldsfor forNoranda Norandaminerals mineralsdetermined determinedfrom from Galley Galleyetetal. al.(2002). (2002). 30 30 �GEOCHEMISTRY AND GEODYNAMIC GEODYNAMICIMPLICATIONS IMPLICATIONSOFOFTHE THE 1537 GEOCHEMISTRY AND 1537 MA MA REDSTONE REDSTONE POINT POINT ANOROGENIC ANOROGENICGRANITE, GRANITE,ONTARIO, ONTARIO,CANADA CANADA Hollings, (DepartmentofofGeology, Geology,Lakehead Lakehead University, University, 955 955 Hollings, P., Fralick, Fralick, P.P.and andKissin, Kissin,S.S.(Department Oliver Rd., Rd., Thunder Bay, Bay, Ontario, Ontario, P7B P7B 5E1, 5E1, Canada; Canada; Peter. Peter.Hollines@lakeheadu.ca) Hollinss @ lakeheadu. ca) The Redstone Point Point granite is is aa felsic igneous Mesoproterozoic Mesoproterozoic felsic igneous complex Ma; Davis complex (1537+10/-2 (1537+10/-2 Ma; Davis and and Sutcliffe, 1984) located in the northern northern portion of the Sibley Sibley Basin on the the west west shore of Lake Nipigon (Fig. 1). shore of Lake Nipigon (Fig. 1). It is is unconformably overlain by by arenites unconformably overlain arenites of of the Pass Lake Lake Formation Formation of of the the Sibley Sibley are in turn Group. Group. These These sediments sediments are turn intruded and overlain by an an extensively extensively sills developed developed sequence sequence of diabase diabase sills related to an early stage of the Midrelated to early stage of the Midevent. The entire Continent Continent Rifting Rifting event. entire sequence sequence has been gently folded into aa shallowly, easterly plunging plunging succession succession of open synclines and anticlines, with open synclines and anticlines, with of the the Redstone Point Point Figure 1. Map showing the location of dips usually exceeding 150. Outcrop not usually exceeding 15O. Outcrop granite in relation relation to Proterozoic Proterozoic anorogenic anorogenic granite granite density of of igneous igneous units units is very very good good of North North America. America. Modified after after Anderson (1983) density complexes of Anderson(1983) sedimentary sequences, sequences, which which only only provide provide along the shoreline of Lake Nipigon, in contrast contrast to to sedimentary small, scattered scattered outcrops. outcrops. The igneous rocks of Redstone Point Point have have been been briefly briefly described describedby by Davis Davisand andSutcliffe Sutcliffe(1985), (1985), of Redstone wherein they they emphasised emphasised that that the the rocks are to rhyolites and wherein are anorogenic anorogenic granites gradational gradational to fragmental rhyolites rhyolites and and dacites. dacites. In fact, presently fragmental presently accessible accessible outcrop indicates that extrusive extrusive members dominate dominate the the magmatic magmatic rocks rocks of the area. members area. Porphyritic Porphyritic texture texture with with volcanic volcanic features features including vesicles, vesicles, flow flow structures, units, rubbly rubbly flow tops including structures, agglomeratic agglomeratic units, tops and andsegregation segregation cylinders differentiate differentiate extrusive extrusive rocks rocks from from more limited cylinders limited exposures exposures of of uniformly uniformly textured textured intrusive rocks. As contacts between units are generally unexposed and the base of the section is nowhere exposed, thicknesses thicknesses of of units units and of nowhere exposed, of the the entire entire succession succession are are unknown; unknown; however, however, continuous outcrop in in cliff-forming units indicates indicates that that a minimum continuous outcrop cliff-forming units minimum thickness thickness of of lOOm 100m of of volcanic rock is present in the area. area. The igneous rocks are the dominace of ferric are distinctively distinctively brick brick red, suggesting suggesting the dominace of ferric iron iron in in the the various mineral hosts but especially in trace amounts in feldspars. The intrusive member displays equigranular phaneritic phaneritic texture texture with with most most mineral mineral grains grains 11 to to 5 mm in diameter. equigranular diameter. The The volcanic volcanic rocks are true porphyries porphyries with with phaneritic phaneritic phenocrysts phenocrysts of of alkali alkali feldspar, feldspar,quartz quartzand andhornblende hornblendein in an aphanitic matrix of of the same minerals. Quartz phenocrysts are are euhedral euhedraland and 11 to to 3 mm in an aphanitic matrix Quartz phenocrysts diameter associated with alkali feldspar phenocrysts occasionally exhibiting synneusis twinning as well as as albite-pencline albite-pericline twins twins indicative indicative of of microcline. microcline. Hornblende Hornblende and and magnetite magnetite are are less less 31 �abundant and finer grained than in the the intrusive intrusive rocks. rocks. Near Near flow flow tops tops the the porphyries porphyriesgrade grade into into uniformly textured aphanitic rhyolites with sparse phenocrysts. textured rhyolites with sparse phenocrysts. The samples from the The the Redstone Redstone Point Point intrusive intrusive complex complex are all all characterised characterised by by high high Si02 Si02 contents (73-83 (73-83 wt%) wt%) and elevated contents elevated K20 &O and and Na20 Na,0 abundances abundances (2-7 wt% wt% and and 0.2-3.5 0.2-3.5 wt% wt% respectively). They They are respectively). are typically typically LREE enriched LREE enriched with relatively unfractionated IIREE (La/Sm,, unfractionated HREE (La/Smn = = 2.8-5.1; Gd/Yb,, = = 1.1-1.6; 1.1-1.6; Fig. 2) 2.8-5.1; Gd/Ybn 2) and are are characterised characterised by by elevated elevated Zr, Y Y and and Nb Nbcontents. contents. Samples Samples from the Redstone Redstone Point Point igneous igneous complex fulfil fulfil the complex the detailed detailed trace trace criteria of of Whelan element criteria Whelan et al. al. (1987) for anorogenic anorogenic granites. granites. . f _J Rb BaTii Tb UU Nb La Ce C <b Ba Nb La Pr Pr Sr Nd Sr Nd Zr Ba T La M Zr Hf llf Sn, Sin Eli Ti Gd (a Tb Tb fly W) 7Y Ho 110 P Fr Yb Yb LII 41 V Sc Sc mantle normalised Figure 2. Representative primitive mantle normalised diagram diagram for for samples from the the Redstone Redstone Point Pointigneous igneouscomplex complex Similarities Similarities between between Proterozoic Proterozoic basin sequences sequences (e.g., (e.g., Athabaska, Athabaska, Thelon, Hornby Bay and and Sibley Sibley basin fill fill sequences) sequences) imply that basin genesis genesis and and developmental developmental controls were similar. The setting, architecture, depositional systems and deformational histories of all four basins strongly infer that they are intracratonic, forming as a result of heating cratonic The heating heating event is represented represented in in northern northern Canada by by numerous 1790 to 1730 Ma lithosphere. The anorogenic, syenogranite syenogranite batholiths batholiths and comagmatic ash-flow tuffs occurring west of anorogenic, comagmatic ash-flow of Hudson Hudson Bay. In the the western western Great Great Lakes Lakes region region aa heating heating event event produced produced the the 1537 1537Ma Ma Redstone Redstone Point Point assemblage and and other 1500 records a assemblage 1500 Ma anorogenic anorogenic batholiths. The southern southern mid-continent mid-continent records heating event with anorogenic granite granite production production from from approximately 1480 to to 1320 lithospheric heating Ma (Fig. 1). These events outline a progressive southward displacement of lithospheric heating 1). events outline a progressive southward from a maximum age of approximately 1750 1750 ma in northern Canada to aa minimum minimum age age of of 1310 1310 Ma in the Ma the southwestern southwestern United States. States. As heat heat transfer transfer from from the the asthenosphere asthenosphere is the the only only mechanism for producing extensive lithospheric heating, drift of North America over hotter than average asthenosphere is implied. Using regional ages of heating, drift rates of approximately 1.1 1.1 cm/year are necessary, and agree in magnitude magnitude with present rates. rates. to 1.4 1.4 cmlyear REFERENCES REFERENCES Anderson, J., 1983. Proterozoic anorogenic granite granite plutonism plutonism of of North North America. America. In: In: Medaris Medaris et Anderson, al., (Eds), Proterozoic Proterozoic geology. geology. Geological Society of America Memoir Memoir 161, 161,133-154. 133-154. Davis, and Sutcliffe, Sutcliffe, R., 1985. 1985. U-Pb U-Pb ages ages from from the the Nipigon Nipigon Plate Plate and and Northern Northern Lake Lake Davis, D., and Superior. Superior. Geological Geological Society Society of America Bulletin, Bulletin, 96, 96, 1572-1579. 1572-1579. Whelan, J., Currie, Currie, K., K., and andChappell, Chappell,B., B.,1987. 1987.A-type A-typegranites: granites:geochemical geochemicalcharacteristics, characteristics, Whelan, discrimination and petrogenesis. Contributions to Mineralogy and Petrology, 95, 407-419. discrimination and petrogenesis. Contributions to Mineralogy and Petrology, 95,407-419. 32 �PALEOPROTEROZOIC (1900-1600 (1900-1600 Ma) Ma) TECTONIC TECTONIC HISTORY HISTORY OF OF THE THE NORTHERN NORTHERN LATE PALEOPROTEROZOIC MID-CONTINENT, MID-CONTINENT, U.S.A: IMPLICATIONS IMPLICATIONS FOR CRUSTAL CRUSTAL STABILIZATION STABILIZATION HOLM, D.K., Dept. of Geology, Geology, Kent State State University, Kent, OH OH 44242; 44242; VAN VAN SCHMUS, SCHMUS, W.R., and MacNEILL, L.C., Dept. of Geology, Geology, University of Kansas, Lawrence, Lawrence, KS 66045; 66045; BOERBOOM, T.J., Minnesota Geological Survey, 2642 University University Avenue, Avenue, St. Paul, Paul, MN MN 55114; SCHWEITZER, D., Dept. of Geology, Kent State University, Kent, OH 44242; 551 14; SCHWEITZER, D., Dept. of Geology, State Kent, OH 44242; SCHNEIDER, SCHNEIDER, D.A., Dept. of Geological Geological Sciences, Sciences, Ohio Ohio University, University, Athens, Athens, OH OH 45701 45701 We propose that the late late Paleoproterozoic Paleoproterozoicigneous igneous and and deformational deformationalhistory history preserved preserved in in the the southern Lake Superior southern Lake Superior region region is the the result result ofofnorthwest-directed northwest-directed convergence convergence during during and following geon 18 18 Penokean accretion. accretion. New U-Pb zircon ages indicate that late to post-Penokean magmatism began ca. 1800 1800 Ma Ma and and generally generally migrated migrated southeastward southeastwardacross across the the newly newlyaccreted accreted terrane. Magmatic Magmatic pulses pulses atatca. ca.1800, 1800,1775, 1775,and and1750 1750Ma Mamay maycorrelate correlatewith withnorthwest-directed northwest-directed subduction associated with southward growth of the North American mid-continent. mid-continent. We suggest suggest that geon 17 Yavapai-age slab rollback caused continental arc magmatism to step southeastward 17 continental to step southeastward As the the slab slab steepened, steepened, the reduced compressional compressional stresses between 1800 and 1750 Ma (Fig. 1A). As and increased thermal input allowed allowed for collapse collapse of of the the overthickened overthickenedportions portions of of the the Penokean Penokean In northern collapse involved involved the the formation of gneiss crust. In crust. northern Wisconsin, Wisconsin, collapse formation of gneiss domes domes and and their their within discrete fault-bounded panels panels brought brought up up from depth via tectonic exhumation within tectonic extrusion extrusion (Schneider - and possibly temporary temporary (Schneider et al., al., ILSG, ILSG, 2003). 2003). Collapse of the the Penokean Penokeanorogen orogen— resulted in crustal stabilization stabilization and deposition deposition of of Baraboo Baraboo Interval Interval cessation of of slab slabsubduction subduction—- resulted However, in a long-lived orogen model, renewed quartzites between 1750 and 1650 Ma. quartzites between 1650 Ma. However, in long-lived orogen model, renewed tectonism to to the the south of a Mazatzal tectonism south resulted resulted in the the eventual eventual accretion accretion of Mazatzal arc (Fig. (Fig. 1B) 1B) with with widespread deformation deformation and and mild mild reheating reheating of of Penokean widespread Penokean crust to the the north. north. The age of this this deformation is inferred from conventional Ar/Ar ArIAr step-heating studies on basement rock beneath deformed and undeformed undeformed Baraboo Baraboo Interval Intervalquartzites. quartzites. The 1900 to 1600 Ma tectonic history of of United States, not surprisingly, records the southward the north-central north-central United surprisingly, records southward growth growth and tectonic tectonic development of the southern southern Laurentian margin. New and published mineral agesdelineate delineatethe the northern northern and and western extent of geon ^ ~ r / ^ ~mineral r ages published 40Ar/39Ar 16 crustal deformation. Interestingly, Interestingly, only only lower-grade lower-grade crust intruded by the shallower-level ca. 1750 Ma plutons (and associated rhyolites) were were deformed significantly during during geon geon 16. Deeper Deeper level by the older level collapsed crust and crust crust pervasively pervasively invaded by older magmatic magmatic pulses pulses are are largely largely by Mazatzal deformation and reheating. reheating. We suggest that post-orogenic intrusions and unaffected by crustal thinning was an important step in strengthening strengthening and stabilizing the crust in the the southern southern Lake Superior Superior region. region. Schneider, Holm, O'Boyle, Hamilton, Hamilton, and and Jercinovic, Jercinovic, 2003, 2003, Paleoproterozoic Paleoproterozoic development of a gneiss dome corridor in the southern Lake Superior region, region, USA: USA: Institute Institute on on Lake Lake Superior Superior Geology Abstracts (this (this volume). volume). 33 �subduction (ca' vapai subduction (ca. 1750 1750 Ma) Ma) w —southward pro propagation "Â¥southwar agation of magmatism magmattsm I from to 1750 Ma granites granites and and rhyolites from 1775 1775 Ma Ma ECMB ECMB to 17 0 Ma rhyolites in WI Wl S N NFZ NFZ 2J750-1630 7 5 0 4 6 3 0Ma Maquartzites quartzites /_— ,. ', ,f — — 1...AVRENtIA -I . '- ' '—-— C -' Mazatzal orogen orogeny Mazatzal y (1650-1630 (1650-1630 Ma) Ma) 2 deformation deformation of "post-Penokean ost-Penokean"quartzites quartettes during Mazatzal azatzai accretion accretion A) Subduction Subduction roHback model proposed proposed to to explain magmatic magmatic age Figure 1: 1: A) rollback model progression the Penokean progressionacross across the Penokean orogen, orogen,ca.Yavapai ca.Yavapai convergence, preaccretion. ECMB = East-central East-central Minnesota Minnesota batholith. batholith. B) Mazatzal accretion. ECMB = 6)Mazatzal Mazatzal accretion model (modified after Romano et al , 2000) to explain deformation to explain deformation accretion model (modified after Romano et a]., quartzites and and southward southward growth growth of of Baraboo Interval quartzites of Laurentia, Laurentia, 34 �GABBRO/GRANOPHYRE LAKE INTRUSION: INTRUSION: A GABBROIGRANOPHYRE RELATIONS OF THE CROCODILE LAKE POSSIBLE VENT FOR THE HOVLAND LAVAS? JERDE, Eric A. (e.jerde@morehead-st.edu), (e.jerde@morehead-st.edu),Department Department of Physical Physical Sciences, Sciences,Morehead Morehead State State University, Morehead, KY 40351 4035 1 Morehead, KY One of the notable characteristics characteristics of of the the Midcoritinent Midcontinent Rift is the presence of large large amounts amounts of felsic felsic material. material. Indeed, the nature and origins origins of this this abundant abundant silicious silicious material material has has been been the the source sourceof of numerous numerousstudies studies(e.g., (e.g., Nelson, 1991; a!., 2000; 2000; Sandland et al., 2001). To 1991; Green and Fitz, 1993; 1993; Vervoort and Green, 1997; Kennedy et al., Tothe the south of the Early Gabbro Series is a pronounced ridge composed of of this this felsic felsic material, material, properly properly termed termed aa granophyre. granophyre. The Early Gabbro Gabbro Series Series layers layers are are inclined inclined to to the the south, south,thus thus are are below below the the granophyre granophyre stratigraphically. This This felsic felsic rock rock was was noted noted and and described described by by Nathan Nathan (1969) (1969) as as aa very very late-stage late-stage material, material, and has generally been presumed to have formed significantly after after the the gabbros gabbros in inthe theregion. region. However, several observations indicate that the gabbro was emplaced later than than the the granophyres. granophyres. These These include include gradational gradational contacts, contacts, with some chilling of of the the gabbro. Another Another observation observation is is the the abundance abundance of material described as "intermediate "intermediate rock" by various investigators in the past (e.g., Grout et al., 1959). This This material material isis always always found found between between the gabbro and granophyre, granophyre, and is is presumably presumably the the result result of assimilation assimilationof granophyre granophyre by by an an intruding, intruding,hot hot gabbro. gabbro. Investigations into possible origins origins of the granophyres granophyres (Sandland (Sandland et et al., al., 2001; 2001; Karl Karl Wirth, Wirth,pers. pers.comm.) comm.) included radiometric age determinations, determinations, and revealed revealed that that the the granophyres granophyresadjacent adjacent to to the the Early EarlyGabbro GabbroSeries Seriesare, are, of the rift rift (-1 (-.1107 Ga), and and essentially essentially contemporaneous. contemporaneous. Because like the gabbros, among the earliest rocks of 107 Ga), silicious material generally is a late-stage late-stage product of magma evolution, evolution, the surprising surprising antiquity antiquity of the the granophyres granophyres adds to questions questions surrounding surrounding their their origin. origin. The early age for the granophyres granophyres does, does, however, suggest suggest an origin origin for the the layered layered nature nature of of the the Early Early Gabbro Gabbro Series located below them stratigraphically. Due Due to to their their low low density, density, the granophyric material would have created a barrier that retarded the rise of buoyant gabbroic material coming up up from from below. below. These rising liquids liquids would have layering that that is is observed, observed, and and providing providing aa cap, cap, blocking blocking any any been forced to spread laterally, resulting in the apparent layering further rise of gabbroic gabbroic material. material. of the layered Early Early Gabbro Gabbro Series Series of of Nathan Nathan (1969) (1969) is is another another occurrence occurrence of of Immediately to the east of Ga;Karl KarlWirth, Wirth,pers. pers. comm.). comm.). This granophyre, also determined to be among those those of an early early origin origin (i.e., (i.e., —1107 -1 107 Ga; by Miller Miller et et al. al. (2001), (2001), and and the the rocks rocks are are interpreted interpreted to to be be rock group has been termed the Crocodile Lake Intrusion by sample examinations examinations (Babcock, (Babcock, 1959). Work gabbroic based on geophysical evidence, and a few sample Work done done between between 1913 and 1948 1948 included included the the very very edges edges of of this this intrusion, intrusion, and and indicates indicatesthat that they they are are basalt basalt lavas lavasand andgabbroic gabbroic intrusions, along with "red rock" (Grout (Grout et et al., al., 1959) 1959)that that isis now now known known to to refer refer to to granophyre. granophyre. Like the series mapped by Nathan (1969), there is a body of gabbroic material stratigraphically below the granophyre. granophyre. was made made into into the the Crocodile Crocodile Lake Lake Intrusion Intrusionto to examine examine some someof ofthe therock rock. During the past year, a reconnaissance was relations (Fig. 1). Traverses Traverseswere weregreatly greatly hampered hampered by by forest forest blowdown, blowdown, but the outcrops are are numerous. numerous. Several Several gabbro units are present, as well as a band of "intermediate "intermediate material" material" at the the very very top top of of the the gabbro, below below the the granophyre. Within Within the the granophyre granophyreitself, itself, several several bodies bodies of gabbro gabbro were were found to have actually actually intruded the granophyre. In In the the coarse-grained coarse-grainedinteriors interiorsof of these these bodies, bodies, the the gabbro gabbro is is indistinguishable indistinguishable from from the gabbros gabbros below the granophyre granophyre stratigraphically). stratigraphically). observed further north (i.e., below of the the granophyre granophyre are are prominent prominent knobs knobs and andridges ridgesthat thatare arecomposed composed of ofbasalt. basalt. These Immediately to the south of are mapped as part of the Hovland Hovland Lavas, Lavas, which which are are reversely reversely polarized, polarized, and and were were extruded extrudedduring duringthe theearliest earliest period of the rifting. ItIt isisperhaps perhapspossible possiblethat that the the discontinuous discontinuousbodies bodies (and (and other other stringers stringers and and local local dikes) dikes) within within the granophyre represent the feeder feeder conduits conduits for for the the eruptive eruptive basalts basalts immediately immediatelyto to the the south south (shown (shownschematically schematically in Fig. 2). In In several severalother otherplaces places within within the the granophyre, granophyre,there there are are basaltic basaltic stringers stringers and small dikes. Surrounding Surrounding has been been assimilated assimilated into into the the gabbro. gabbro. In the larger gabbroic bodies are obvious reaction zones where granophyre has immediately to the south, numerous inclusions are present that are pinkish one of the flows immediately pinkish in in color, color, along along with with felsic stringers and irregular masses masses of felsic material. material. Further work is planned to assess assess the relation relation between the gabbros gabbros within within the granophyre granophyre and and the the lavas lavas to to the the south. IfIf this and the the this isis indeed indeedaa feeder feeder system, system, itit might might provide provide insight insight into the mechanism of magma emplacement and "breakthrough" to the eventual "breakthrough" the surface, surface, during during the the onset onset of of rifting. rifting. 35 �References Cited: References Cited: Babcock, R.C., Jr. (1959) M MS. S .thesis, thesis,University Universityof of Wisconsin, Wisconsin,Madison, Madison, 47 47 p. p. Research, 54, 177-196. 177-196. Green, J.C. and Fitz, T.J., 1993, 1993, Journal of Volcanological and Geothermal Research, Grout, SurveyBulletin Bulletin39, 39, 163p. l63p. Grout, F.F., F.F., Sharp, Sharp, R.P., and Schwartz, Schwartz, G.M. 1959 Minnesota Geologica Survey Jerde, E.A. and Kennedy, Kennedy, B.C., B.C., 2000, 2000, American American Geophysical Geophysical Union 2000 2000 Fall Meeting, Meeting, San San Francisco. Francisco. Jerde, Jerde, Salvato, D.J, D.J, Thole, Thole, J., J., and and Wirth, Wirth, K.R. K.R. 2001, 2001, ILSG ILSG 47, 47, 36-37. 36-37. Jerde, E.A., Salvato, Kennedy, B.C., B.C., Wirth, Wirth, K.R., K.R., and and Vervoort, Vervoort,J.D., J.D.,2000, 2000,ILSG ILSG46, 46,29-30. 29-30. Jr., Green, J.C., Severson, Severson, M.J., Chandler, Chandler, V.W., and Peterson, D.M., 2001, Minnesota Minnesota Geological Geological Miller, J.D., Jr., Survey 19. Survey Miscellaneous MiscellaneousMap MapSeries SeriesM-1 M-119. Nathan, H.D., 1969, of Minnesota, Minnesota, Minneapolis, Minneapolis, 198p. l98p. 1969, Ph.D. dissertation, University of Nelson, N. 1991, 1991,M.S. M.S. Thesis, Thesis, University Universityof of Minnesota, Minnesota,Duluth. Duluth. Sandland, TO., Wirth, 47, 85-86. Wirth,K.R., K.R.,Vervoort, Vervoort,J.D., J.D.,Gehrels, Gehrels,G.E., G.E.,Kennedy, Kennedy, B.C., and Harpp, K.S. 2001, ILSG 47,8586. Sandland, T.O., Vervoort, J.D. and Green, J.C., 1997, 34, 521-535. 1997, Canadian Canadian Journal of Earth Sciences, 34,521-535. Fig. 1. Reconnaissance Reconnaissancegeologic geologicmap map of of the the region region just south south of Crocodile Crocodile Lake, Lake, showing showing location location of gabbro bodies in the the granophyre granophyre that forms forms the the cap cap above above the the Crocodile Crocodile Lake LakeIntrusion Intrusiongabbros gabbros and intermediate intermediate rocks. rocks. .4 SchematicN-S N-S cross crosssection sectionof of Fig. Fig. 11showing showing the the possible possible feeder feeder for for the the Hovland Hovland Lavas. Lavas. Fig. 2. Schematic 36 �MINERALIZATION MINERALIZATION OF OF THE THE NORTON NORTON LAKE LAKE Cu-Ni-PGE Cu-Ni-PGEDEPOSIT DEPOSIT JOHNSON, JOHNSON, J.R., J.R.,HOLLINGS, HOLLINGS,P.P.and andKISSIN, KISSIN,S.A. S.A.Department DepartmentofofGeology, Geology,Lakehead Lakehead University, Thunder Bay, ON, P7B 5E1, jrjohnsonca@yahoo.ca 5E1, jrjohnsonca0 yahoo.ca The Norton Lake Cu-Ni-PGE lun northeast northeast of of Fort Fort Cu-Ni-PGE deposit deposit is located located approximately approximately50 50 km Hope, within the Miminiska-Fort Hope Greenstone belt of the Uchi Subprovince, northwest Hope, Miminiska-Fort Hope Greenstone belt of the Uchi Subprovince, northwest Ontario (Figure 1). Only Onlylimited limitedgeological geological investigations investigationshave have been been undertaken undertaken within within the the belt due to both its remote location and sparse outcrop. As As aa result result of of this this the the belt belt has has been been subdivided based on limited structural and regional stratigraphic considerations considerations (Stott (Stott and and Corfu, Corfu, 1991). The The Norton Norton Lake Lake deposit depositis is located located within within an an unnamed assemblage assemblage comprising comprising basaltic flows with magnetite iron formations. formations. ItItisisthought thought that that this this assemblage assemblagecan can be be correlated, correlated, through similar rock types and aeromagnetic trends, with with the the -2900 Ma Northern Pickle Pickle terrane of the Pickle Lake greenstone greenstone belt (Corfu (Corfu and and Stott, Stott, 1996). 1996). Figure Figure1:1:A-Map A-Mapof of Superior SuperiorProvince Provinceshowing showinglocation location of of Uchi Uchi Subprovince. Subprovince. B Simplified Simplified geology geology map of the Miminiska-Fort Miminiska-Fort Hope Hope Greenstone Greenstonebelt belt (after Stott and Corfu, Corfu, 1991). 1991). The Norton Lake area consists consists of massive massive to pillowed pillowed basalts basalts with with rare rare ultramafic ultramaficflows. flows. The deposit itself is hosted within a sheared amphibilite amphibolite with minor gabbroic units. Previous Previous work determined 944 500 500 tonne tonne nickel-copper nickel-copper deposit depositcontaining containing0.72% 0.72%Ni Ni determined the deposit deposit to to be be aa 944 West Resource Resource Corporation, Corporation, 2001). 2001). The and 0.56% Cu with an undefined PGE potential (East West The geological setting, host rock and mineralization of the Norton Lake deposit deposit are are comparable comparableto to that of the Thierry Deposit, Pickle Lake, Ontario. The The Thierry Thierry Mine Mine is is currently currently undergoing renewed exploration exploration to to determine determineits its viability viability as as aa PGE PGE deposit deposit (PGM (PGM Ventures). Ventures). 37 �East West Resource Resource Corporation Corporation (EWR) has undertaken a detailed exploration program of Norton Lake, including an an extensive extensive drilling drilling program. program. Detailed in the vicinity of Detailed examination examination of of being paid paid to to the the mineralized mineralized 'main' zone drill core has been undertaken with special attention being zone to to determine the exact nature of the mineralization. Preliminary Preliminary results results indicate indicate the the deposit deposit consists consists chalcopyrite and and pyrite. pyrite. The platinum group of massive pyrrhotite with pentlandite, magnetite, chalcopyrite element's s) are element's (PGE' (PGE's) are found found forming discrete discrete platinum group minerals and are also believed to form a solid solution solution with the suiphides. sulphides. Results Resultsshow showthat thatin inaddition additionto toprimary primarymineralization mineralization a secondary, hydrothermal, enrichment of PGE's has has taken taken place. place. Mineral Mineral Pyrrhotite Pyrrhotite Pentlandite Pentlandite Pyrite Pyrite Chalcopyrite Chalcopyrite Manganoan Manganoan Illmenite Illmenite Formula Fei-xS FeiS (Fe,Ni)9S8 (Fe,Ni)9S8 FeS2 FeS2 CuFeS2 CuFeS2 (Fe,Mn)Ti03 (Fe,Mn)Ti03 Magnetite Michenerite Michenerite Hessite Hessite Fe304 PdBiTe PdBiTe Ag2Te Ag2Te Minor Elements Elements Ni Co Co Co Ni Notes Main mineral Secondary Secondary Trace Trace, also veins More common common than magnetite, magnetite, easily mistaken for magnetite magnetite in polished section section Sb, Sb, Pt Table Table 1: 1: Summary Summary of the mineralogy mineralogy of Norton Norton Lake Lake deposit. deposit. Corfu F. and Stott G.M. 1996. 1996. Hf isotopic composition and age constraints on the evolution of the Archean Central 78,pp53-63 53-63 Central Uchi Subprovince, Subprovince,Ontario, Ontario,Canada. Canada.Precambrian Precambrian Research, Research,v. v. 78, East West Resources Resources Corporation, Corporation,2001. 2001. Annual Annual Report. Report. PGM Ventures, 2003. 2003. www.pgm-ventures.com www.pgm-ventures.com Stott G. M. and and Corfu Corfu F. F. 1991, 1991,Uchi Uchi Subprovince, Subprovince,in in Geology Geology of of Ontario, Ontario,Ontario OntarioGeological Geological Survey Special Special Volume 4, 4, Part 1. 1. 38 �Pd-Pt-Au MINERALIZATION STRATIFORM Pd-Pt-AU MINERALIZATION IN THE SONJU LAKE INTRUSION, LAKE COUNTY, MINNESOTA JOSLIN, GregoryD. D.Department Departmentof of Geological GeologicalSciences, Sciences,University University of of Minnesota-Duluth, Minnesota-Duluth, 1114 1114 Kirby Kirby JOSLIN, Gregory joslOOl3@d.umn.edu;MILLER, MILLER, James D., Jr., Minnesota Drive, Duluth, MN 55812, email: josl0013@d.umn.edu; Geological Survey, Survey, c/o do NRRI, 5013 Miller Miller Trunk TrunkHwy, Hwy,Duluth, Duluth,MN MN558 55811; andROWELL, ROWELL, 11; and William, F., Franconia Minerals MineralsCorp., Corp.,12 125. S.6th 6"' St., Minneapolis, MN 55402. The Sonju Sonju Lake intrusion intrusion (SLI) (SLI) is a 1200 1200m m thick, closed-system, closed-system, well-differentiated, well-differentiated,tholeiitic, tholeiitic, layered intrusion located within the the Mesoproterozoic Mesoproterozoic Midcontinent Midcontinent Rift-related Rift-relatedBeaver Beaver Bay Bay Complex of of northeastern northeastern Minnesota Minnesota (Miller (Millerand andChandler, Chandler,1997). 1997). In In the the late late 1990's, outcrop sampling by Miller (1999) (1999) indicated the presence of meter-scale meter-scale stratiform stratiformPd-Pt-Au Pd-Pt-Au mineralized mineralized interval (or PGE reef) within the oxide oxide gabbro gabbro unit of the the SLI, SLI, located about about 2/3 213 of of the the way way up up from the basal contact of the intrusion. In In June June of of 2002 2002 Franconia Franconia Minerals Minerals Corp. Corp. conducted conducted exploratory drilling drilling through through the the Pd-Pt-Au Pd-Pt-Au enriched enriched zone. zone. exploratory In hand sample, the mineralized interval appears as a homogeneous oxide gabbro, gabbro, with no visible indication of precious metals enrichment. However, However, geochemically the location location of the mineralization is distinct. Three Threedrill drillcores, cores,spanning spanningaastrike strikelength length of of approximately approximately 800 800m, m, define define and are are correlated correlatedon on the the basis basis of of aa distinctive distinctiveCu-Au Cu-Au break break datum datum(Fig. (Fig.1). 1). With the exception of localized Pt enrichment associated with an interval enriched in olivine about 110 m m below the Cu-Au horizon, horizon, all Pd-Pt-Au enrichment occurs over an interval interval of 0 to to 90 90 m m below the the In general general precious precious metals metals peaks peaks are are stratigraphically stratigraphicallyoffset offset from from one one defined datum (Fig. 2). In Maximumgrades gradesin in 0.3m 0.3m long core another, progressing progressingupward upwardininthe thesuccession successionPd+Pt+Au. Pd-)Pt-Au. Maximum samples are 410 ppb Pd, 275 ppb Pt, and 1080 ppb ppb Au. Au. Above Above the Cu-Au break, all precious metals are very strongly depleted. Strong A1 and and modal modal olivine olivine with Strongcorrelation correlationbetween between Fe, Fe, Al precious metals peaks indicates a possible connection between subtle modal modal layering layering of of plagioclase, plagioclase, oxide, oxide, and olivine olivine with with mineralization. mineralization. The oxide gabbro-hosted gabbro-hosted PGE reef in the Sonju Sonju Lake Lake intrusion intrusion shows shows marked marked similarities, similarities, with some differences, differences, to stratiform stratiformPGE PGE mineralization mineralization in in the the Skaergaard Skaergaard intrusion intrusionof of East East Greenland (Andersen et al., 1998),the the Rincon Rincon del del Tigre Tigre Complex Complex of of Bolivia Bolivia (Prendergast, (Prendergast,2000), 2000), al., 1998), and many other tholeiitic mafic layered intrusions throughout throughout the the world. world. Whole Whole rock rock geochemistry, clinopyroxene clinopyroxene and olivine compositions, and petrographic data data are are consistent consistentwith with an orthomagmatic origin for the mineralization related to the fractional fractional segregation segregation of of sulfide sulfide melt from silicate magma. The The homogeneity homogeneity of the host rock, the thickness of the mineralized interval, interval, and the offset offset of metal metal concentrations concentrationsimply imply that that sulfide sulfide saturation saturationwas was passively passively triggered by fractional crystallization crystallization of the Sonju magma. magma. Mungall (2002) recently argued Mungall(2002) argued that stratigraphic offsets offsets of Pd, Pd, Pt, Pt, Au Au and and Cu Cu peaks common to many PGE reefs can can be satisfactorily satisfactorily explained by a kinetic model of sulfide liquation and settling. The The model model shows shows that that the the degree degree of offset and metal enrichment will be controlled by by kinetic kinetic factors, factors, such as as the the diffusivity diffusivity of of chalcophile elements, the degree of sulfide supersaturation, supersaturation, sulfide droplet size, and its settling chalcophile velocity, which result in variability of the apparent silicatelsulfide silicate/sulfide melt melt ratio ratio (R factor). The The correlation of multiple peaks of PGE with subtle, correlation subtle, broad broad modal variations variations may may be be related related to to by the crystallization crystallization of of magnetite magnetite in in an an environment environment of of repeated convective overturn caused by sulfide over-saturation, as suggested by Prendergast (2000) to explain a similar correlation in sulfide over-saturation, suggested (2000) explain a similar correlation in the the Rincon del Tigre Complex. Some Someevidence evidence of of late-stage late-stage sulfide sulfide dissolution dissolution and remobilization exists, but it appears appears to have little little to to no no effect effect upon upon the the distribution distributionof of precious precious metals. metals. 39 �nrelorl ,bovo *60.0 - - -' Ft!lI1lI—._+50.0 ' +40,0 " *30,0 -. .. +70.0 *70.0 *60.0 *60.0 *50.0 *50.0 *40.0 *40.0 +30.0 *20.0 - -'-' ' ' ';'*. - ,,::-- +10_A 410.0 0.0 -10.0 -10.0 -10.0 .20.0 -20.0 -20.0 -30.0 -30.0 -40.0 .40.0 -500 -50.0 -5 0.0 1AJJ114i1_ -40,0 -70.0 '60,0 -60.0 -70.0 -70.0 -80.0 -60.0 -90.0 -90.0 I—.— " -30.0 1111111— 85000001 5 -40.0 11111111— . , -ao,o 00.0 1+00 -110.0 4 -110.0 '110.0 -120.0 , -120.0 -120.0 -130.0 'break +200 0.0 0.0 Cu-Au *30.0 920.0 *100 5L02-3 SLO2-2 SLO2—1 Cu-Au break +70,0 r -130.0 Fig. 1: Correlation Correlation of of drill cores SLO2-1, SL02-1, SLO2-2, SL02-2, and SLO2-3 showing showing distinctive SL02-3 break. The Cu-Au break. The Cu-Au Cu-Au break is used to provide a datum to which all stratigraphic plots are correlated, and position in stratigraphy is measured as meters above above or or below below Cu-Au Cu-Au break. break. -130.0 0 • Au ppb) Cu (ppm) 5L02-3 SLO2-2 SLO2-1 '*70.0 '+70.0 +60.0 .660.0 —.50.0 *50.0 *40.0 -+40.0 *30.0 +30.0 *20.0 -620.0 *10.0 *10.0 -10.0 -10.0 -20.0 -20.0 -30.0 -30.0 .40.0 '' -40.0 - -50.0 -50.0 -60.0 '60.0 .70.0 -70.0 .80.0 -80.0 -90.0 -90.0 l00.0 -100.0 -510.0 -110.0 '-520.0 -120.0 Fig. 2: Correlation Correlation of Pd Pd and Pt Pt in in drill drillholes holesSLO2SL021, I, SLO2-2, SL02-2, and and SLO2-3. SL02-3. Notice multiplicity of spikes spikes and offset offset between Pt and Pd peaks. -130.0 -130.0 S • Pd)ppb) S Pt(ppb) References: References: 0., Rasmussen, Andersen, J. C. O., Rasmussen, H., H., Nielsen, Nielsen, T. T. F. F. D., Ronsbo, Ronsbo, J. U., G., 1998, 1998, The The Triple Triple Group Group and and the the Platinova Gold and Palladium Reefs in the Skaergaard Intrusion: Stratigraphic and Petrographic Petrographic Relations. Relations. Economic Economic Geology. Geology. Vol. Vol. 93, 93, pp. pp. 488-509. 488-509. Miller, J. D. Jr., 1999, 1999, Geochemical Geochemical Evaluation Evaluation of Platinum Group Element (PGE) Mineralization in the Sonju Lake Intrusion, Finland, Minnesota: Minnesota: Minnesota Minnesota Geological Geological Surv. Surv. Information Information Circular Circular 44, 44, 31 3 1p. p. Miller, J. D., Jr., Jr., and and Chandler, Chandler, V. W., 1997, 1997, Geology, Geology, petrology, and tectonic significance of the Beaver Bay Complex, northeastern Minnesota, in Ojakangas, R. R. W., W., Dickas, A. A. B., B., Green, J. C., eds., Middle Proterozoic Proterozoic to to Cambrian Cambrian Rifling, Rifting, Central CentralNorth North America: America: Geological GeologicalSociety Societyof of America AmericaSpecial SpecialPaper Paper 312, p. 73-96. 73-96. Mungall, Mungall, J. E., 2002, 2002, Kinetic KineticControls Controlson on the thePartitioning Partitioningof of Trace TraceElements ElementsBetween BetweenSilicate Silicateand andSulfide Sulfide Liquids. Journal Journalof of Petrology. Petrology. Vol. Vol. 43, 43, pp. pp. 749-768 749-768 Prendergast, M. D., 2000,Layering 2000,Layering and Precious Metals Mineralization in the Rincon del Tigre Complex, Eastern Bolivia. Bolivia. Economic Economic Geology. Geology.Vol. Vol. 95, 95, pp. pp. 113-130. 113-130. 40 �RESULTS OF OF 40Ar/39Ar 4 0 ~ r / 3 9SINGLE-GRAIN ~r ANALYSES MAFIC RESULTS SINGLE-GRAIN ANALYSESOF OF PRECAMBRIAN PRECAMBRIAN MAFIC INTRUSIONS IN NORTHERN NORTHERN AND EAST-CENTRAL EAST-CENTRAL MINNESOTA MINNESOTA KEAYFS, M.J., Dept. of Geology, Kent State University, Kent, OH 44242; JIRSA, KEATTS, JIRSA, M., Minnesota Geological Geological Survey, 2642 University Avenue West, St. St. Paul, Paul, MN MN 55114-1057; 55114-1057; HOLM, D., Dept. of Geology, Kent State University, Kent, OH 44242 Age information from mafic mafic intrusive intrusive suites suites is is critical of the Age information from critical for proper proper interpretation interpretation of geologic history and for mineral deposit models in the Lake Superior region. As part of an effort to evaluate evaluate PGE potential potential in in mafic mafic intrusions intrusions in in Minnesota, Minnesota, several several plutons plutons have have been been dated dated CO2 laser ArIAr Ar/Ar incremental heating technique at the University University of Wisconsin-Madison using the C 0 2 laser Rare Rare Gas Gas Geochronology Geochronology Laboratory. Laboratory. For late-stage late-stage shallow shallow plutons plutons containing containingprimary primary magmatic hornblende, ArIAr Ar/Ar mineral ages are are likely likely to to closely closely approximate approximate the the crystallization crystallization age. In regions age. regions with with aamore moreprotracted protractedthermal thermalhistory history(i.e., (i.e.,low-grade low-grademetamorphism, metamorphism, slowslowcooling, etc.), the Ar/Ar data provide minimum ages for the mafic plutons. Mafic intrusions ArIAr data minimum ages plutons. intrusions from from Minnesota selected for this study represent a broad range of geologic settings, including 1) small mafic and intrusive mafic plutons plutons emplaced emplaced into into Paleoproterozoic Paleoproterozoic supracrustal supracrustal and intrusive rocks rocks within within the the Penokean orogen (samples 264, R17); and 2) varied, primarily late- to post-tectonic intrusions in rocks of of the Archean Wabigoon Wabigoon (samples (samples Al, Al, B21, UBD) and Wawa (samples K15, supracrustal rocks of Superior Province. Province. We We report report here the initial LP, ANA) subprovinces subprovinces of initial results results from from eight eight separate intrusions (Fig. (Fig. 1). 1). East-central ~Minnesota. A hornblende ~ast-central innesota.A hornblende grain grain (R17) (R17) from from aa sample sampleofofmedium-grained medium-grained homblendite batholith in hornblendite from the Tibbett's Brook intrusion cutting the East-central Minnesota batholith Morrison Co. yields yields aa plateau date of from 44contiguous contiguousincrements increments Morrison Co. plateau date of 1.770 1.770 ±Â 0.006 Ga from constituting 74% of of the the gas released. constituting 74% released. AAbiotite biotitegrain grain(264) (264)from fromaasample sampleofofcoarse-grained coarse-grained biotitic olivine gabbronorite cutting the Little Falls Formation in Morrison Co. Co. yields yields aa plateau plateau date of 1.791 1.791 ± Â0.008 0.008 Ga from 5 contiguous increments constituting constituting 68% of the gas gas released. released. twa Subprovince. A Ahornblende Wawa Subprovince. hornblendegrain grain(ANA) (ANA)from fromaasample sampleofofprismatic prismatichornblende hornblende diorite diorite collected near Red Lake in Beltrami Co. yields a near-plateau date of 2.587 ±0.012 ~ 0 . 0 1Ga 2Ga in in 55 nonnoncontiguous incrementsconstituting constituting50% 50%ofofthe the gas. gas. A A biotite biotite grain grain (K15) (K15) from from a sample contiguous increments sample of collected in in Norman Norman Co. Co. yields yields aa plateau plateau date date of of 2.639  ± 0.007 biotite granodiorite porphyry collected 0.007 Ga from 6 contiguous increments constituting 79% 79% of of the gas released. A biotite grain (LP) from a sample of porphyritic syenite collected at the Wawa-Quetico subprovince boundary in St. Louis Co., from 77contiguous contiguous Co., in the the Linden Linden Pluton, Pluton, yields yields a plateau plateau date date of 2.666 2.666 ±Â 0.006 Ga from increments increments constituting constituting 88% 88% of of the the gas gas released. released. Wabigoon Subprovince. A hornblende grain (B21) from the Oaks intrusion leucodiorite leucodionte sampled 0.008 Ga from 88 near Fault in Roseau 1 ±Â 0.008 near the Vermilion Vermilion Fault Roseau Co. Co. yields yields aa plateau plateau date date of of 2.67 2.671 contiguous increments constituting 75% 75% of of the the total total gas gas released. released. A hornblende grain (Al) (Al)from from 0.0111 Ga from the Black River gabbro, collected in Roseau Co., yields a plateau date from date of of 2.685 2.685 ±Â 0.01 11 contiguous increments increments constituting constituting 90% 90% of of the the total total gas gas released. released. A A hornblende hornblende (UBD) (UBD) from a sample Co. north north of the sample of of hornblende-biotite hornblende-biotite gabbro collected collected in Koochiching Koochiching Co. the Rainy Rainy LakeLakeSeine River Fault Fault yields yields a plateau date of Seine River plateau date of 2.695 2.695 ±  0.007 Ga from from 66 contiguous contiguous increments increments constituting constituting 49% of the gas released. released. 41 �The mineral age data from from mafic mafic plutons plutons from from the the Wabigoon Wabigoon subprovince subprovince are are synchronous synchronous Mafic plutons plutons from from the the with the last deformation event (D2) dated in the range 2.685-2.674 Ga. Mafic Wawa subprovince give give an an 80 m.y. age range from 2.58 to 2.66 Ga. Interestingly, Interestingly,the theLinden Linden Pluton gives a biotite biotite date date concordant concordant (within error) with the youngest youngest mafic pluton from the Pluton Wabigoon subprovince. Wawa are are consistent consistent with with Wabigoon subprovince.The Theyounger youngerspread spreadofof ages ages from from the Wawa southward growth of of the Superior Province Province during during the the latest latest Archean. Archean. Mafic plugs evident from aeromagnetic maps 1.775 Ga Ga EastEastaeromagnetic maps in in east-central Minnesota Minnesota are are comagmatic comagmatic with with the circa 1.775 central Minnesota batholith. Further constraining the mafic intrusions intrusions the temporal framework of mafic may contribute contribute to mineral deposit deposit models these intrusions intrusions and and their their analogs analogs in in may to mineral models for for PGE PGE in these adjacent states states and and provinces. provinces. Fig.i agespectra: spectra:t t, ==plateau plateau age, age, t, tq == total total gas age. Fig.l 40Ar/39Ar "Ar/^Ar age age. 2.0 0 2.0 Ri 7 264 1.5 2:: fl.... 1 1.8 J0.5 3.0 Biotite MSWD MSWD 2.57 2.57 Amphibole MSWD 1.78 Amphibole MSWD 1.78 t, tp== 1.770 1.770±Â0.006 0.006Ga Ga t9= t = 1.653±0.005 1.653 0.005Ga Ga I I 6t ==1.791 1.791±Â0.008 0.008Ga Ga t, = 1.782 1.782±  0.007 0.007Ga Ga 1.5 3.0 ANA B21 —--————ur Ct 0 2.5 1 Amphibole 0.71 Amphibole MSWD MSWD0.71 2.587  ± 0.012 0.012 Ga tp= 2.587 tg 2.550  ± 0.009 0.009 Ga Ga t, == 2.550 2.0 2.0 2.0 3.0 " 3.0 Ki 5 .__.__.__ Ct 0 . Al ti. ...:::.::::::: 2.5 Biotite MSWD Biotite MSWD 2.48 2.48 2.639 ± t, = 2.639  0.007 0.007Ga Ga = 2.640 2.640±Â 0.007 0.007Ga Ga 3.0 3.0 LP t:,Vt:::::::ZV:. : Amphibole MSWD Amphibole MSWD 1.81 1.81 t, = 2.685 2.685 ± tp  0.011 0.011 Ga Ga t9== 2.700 2.700  ± 0.01 0.010 tg 0 Ga Ga 2.0 -. 2.0 UBD —iF :1:2.:, 0 2.5 - Amphibole Amphibole MSWD MSWD 0.30 0.30 t ==2.671 2.671 ±Â0.008 0.008Ga Ga tto==2.705 2.705±Â0.013 0.013Ga Ga 2.5 Biotite MSWD Biotite MSWD1.11 1.I1 t,, = 2.666 2.666±Â 0.006 0.006Ga Ga t9 2.657 ± 6 == 2.657  0.006 0.006 2.0 0 0 10 10 20 20 30 30 40 40 50 50 60 60 700 7 60 80 Cumulative released (%) Cumulative39Ar "Ar released (%) 2.0 900 1100 0 9 00 0 42 , 10 10 Amphibole MSWD MSWD 1.03 1.03 2.695 ± $, = 2.695  0.007 0.007Ga Ga 2.730 ± 6 .= 2.730  0.006 0.006Ga , --"-" *- - , 20 30 30 40 40 50 60 60 700 7 80 80 Cumulative Cumulative 9Ar "Ar released released (%) (%) r-- 90 90 100 100 �New zircon New zircon ages agesfrom fromthe the Gunflint Gunflint and and Rove RoveFormations, Formations,northwestern northwestern Ontario Ontario Kissin, S.A., Department Department of Geology, Geology, Lakehead University, Thunder Thunder Bay, ON, P7B 5E1 5E1 Canada, Canada, stephen.kissin@lakeheadu.ca; stephen.kissin@lakeheadu.ca ;Vallini, Vallini, D.A., D.A.,University University of of Western Western Australia, Australia,35 35 Stirling Hwy, Stirling Hwy, Australia; Addison, W.D., W.D., RR 2, 2, Kakabeka Kakabeka Falls, Falls, ON, ON, POT POT iWO, 1W0,Canada; Canada; Crawley, 6009, W.A., Australia; Brumpton, G.R.., 211 Bmmpton, 21 1 Henry St, Thunder Thunder Bay, ON, P7E 4Y7, Canada. Canada. Previous work based on U-Pb geochronology geochronology from presumed volcanogenic zircons zircons obtained obtained 1878 ±Â 2Ma, from a tuff layer at the lower/upper lowerlupper Gunflint Formation boundary yielded an age of 1878 believed to approximate the age of of deposition of of the unit unit (Fralick et et al., 2002). 2002). This This age age corresponds closely with the age of the correlative correlative Hemlock Formation Formation of Michigan Michigan (1874 (1874 ±: 9Ma; Schneider 9Ma; Schneider et a!., al., 2002). We report report here preliminary preliminary age age determinations determinations based on on SHRIMP SHRIMPanalyses analyses of of zircons zircons Formation, and and two two within within the the extracted from three volcanic ash layers; one lying in the Gunflint Formation, Rove Formation. Formation. The overlying Rove The Gunflint-Rove Gunflint-Rovecontact contact is is an an important important reference reference point. point. Pufahi Pufahl and Fralick (2000) placed it at the top of a sequence sequence of chert-carbonate chert-carbonategrainstones grainstones which which is is of the the Rove Rove Formation. Formation. The Gunflint exposure exposure outcrops outcrops at at overlain by carbonaceous shales of The Gunflint Little Little Falls, Falls, on on the the south southside sideof of the theKakabeka KakabekaFalls FallsGorge, Gorge,--1Om -10m (topographically) (topographically) below below the the Gunflint lapilli tuff dated by Fralick et a!. al. (2002). A Rove volcanic ash exposure exposure at Oliver Oliver Creek Creek is estimated to be -70m (stratigraphically) (stratigraphically) above above the Gunflint-Rove Gunflint-Rove contact. contact. Zircons were also extracted extracted from an ash ash layer layer within within Falconbridge Falconbridge Pine Pine River River (PR98-1) (PR98-1)drillcore drillcore(688.24m (688.24mdown down hole), above the Rove-Gunflint Rove-Gunflint contact. hole), located located —4m -4m above contact. The a mean 207Pb/206Pb of~1821 16 Ma while aa single single ^ ~ b l age ~age~of 1821 ~ b±Â 16 The Oliver OliverCreek Creekzircons zirconsrecorded recorded a mean of 184OMa wasobtained obtained from from the the dnllcore drilicore PR98-1 PR98-1 sample. sample. The The errors errors cited cited are at the one age of 1840Ma was discordant. deviation ((la) standard deviation l o ) and and 95% 95% confidence confidence level and the analyses are less than 5% discordant. There are recorded from from each each locality locality which which are assumed to are also also two two younger younger ages ages of of —1786Ma -1786Ma recorded be outliers. outliers. The Little Falls zircons, zircons, which are are somewhat somewhat rounded rounded and and fractured, fractured, yielded yielded various various ages, all older than 2000Ma. Most Most of of the the ages ages are are more more than 10% 10% discordant, discordant, and and these these samples samples may have suffered lead loss. As As well, well, there thereare are some some indications indicationsof of admixture admixtureof of shalely shalelymaterial material in the ash layer at this locality. Older Olderzircons zirconsfrom from the the lapilli lapilli tuff tuff layer layer at at the the lower/upper lowerlupper Gunflint Gunflint contact contact (Fralick (Fralick et al., al., 2002) 2002) were were also also found found to to be be admixed admixedwith with Paleoproterozoic Paleoproterozoic zircons. zircons. Using the stratigraphic stratigraphic column of Pufahl and Fralick (2000), we estimate that the drillhole lOmabove abovethe theGunflint Gunflint lapilli lapilli tuff layer containing containing the drillhole (PR98-1) (PR98-1)samples samplesare are—1 -1 10m the zircons zircons dated by Fralick Fralick et a!. (2002), while we estimate the Oliver Creek samples to be —150m above al. (2002), while we estimate the Oliver Creek samples to be -150m this same layer. The ages reported reported here indicate indicate that aa slow slow sedimentation sedimentationrate rate must must have have been been required in order to account account for for the age age difference difference between between the lapilli lapilli tuff tuff of of Pufahl Pufahl and andFralick Fralick and the two sets of Rove dates reported here. This slow Rove sedimentation rate is comparable that reported in banded iron iron formations formations of of the the early early Proterozoic Proterozoic Campbell CampbellGroup, Group,Griqualand, Griqualand, West Sequence, Sequence, South South Africa Africa (Barton (Barton et et a!., al., 1994). 1994). 43 �The Oliver Creek ages reported here are in reasonable agreement with the 1833 1833±Â 6Ma undeformed by by age reported by Schneider et al. (2002) for the Tobin Lake Pluton, which is undeformed Penokean deformation and intrudes presumed Hemlock Volcanic However, the the Volcanic equivalents. equivalents. However, zircons from the Rove Formation Formation suggest suggest that volcanic activity activity associated associated with with the the Penokean Penokean continued for for at at least least 40 40 m.y. m.y. Further Orogeny continued Furtherstudies studies are are underway to clarify some of the questions raised questions raised by our our results. results. Barton, E.S., Altermann, W., William, I.S., amd Smith, C.B. 1994. U-Pb U-Pb zircon zircon age age for for aa tuff tuff in in the Campbell Campbell Group, Group, Griqualand GriqualandWest West Sequence, Sequence,South South Africa: Africa: Implications Implicationsfor forEarly Early Proterozoic 22: 343-346. 343-346. Proterozoic rock accumulation accumulation rates. rates. Geology Geology 22: Fralick, P., Davis, D.W., and Kissin, S.A. S.A. 2002. The Theage ageof of the the Gunflint GunflintFormation, Formation,Ontario, Ontario, Canada: single zircon U-Pb age determinations determinations from reworked volcanic ash. ash. Canadian Canadian Journal 1089-1091. Journal of Earth Earth Science Science 39: 39: 1089-1091. Pufahi, Pufahl, P. and and Fralick, Fralick, P. P. 2000. 2000. Fieldtrip Fieldtrip 4: 4: Depositional Depositionalenvironments environmentsof of the thePaleoproterozoic Paleoproterozoic Gunflint 46, pt.2. pt.2. Gunflint Formation. Formation. Proceedings Proceedings of of the the Institute Institute on on Lake Lake Superior Superior Geology, Geology, 46, W.F., Schulz, K.J., K.J., and and Hamilton, Hamilton, M.A. M.A. 2002. Age Schneider, D.A., Bickford, M.E., Cannon, W.F., Age of of volcanic rocks and and syndepositional syndepositional iron iron formations, formations, Marquette Marquette Range Range Supergroup: Supergroup: implications implications for for the the tectonic tectonic setting setting of of Paleoproterozoic Paleoproterozoiciron iron formations formationsof of the theLake Lake Superior Region. Canadian Journal of Earth Science 39: 999-1012. Canadian Journal of Earth Science 39: 999-1012. 44 �THE SOUTHERN PORTION OF OF THE THE LAURENTIDE LAURENTIDE ICE ICE MEAN TRANSPORT LENGTH LENGTH IN TILLS OF THE SHEET: IMPLICATIONS IMPLICATIONS FOR FOR DRIFT DRIFT EXPLORATION EXPLORATION IN THE THE LAKE SUPERIOR SUPERIOR REGION REGION LARSON, Department of Geological Sciences, Sciences, University Minnesota, Duluth, 55812, LARSON, Phillip Phillip C., Department of Geological University of of Minnesota, Duluth, MN MN 55812, plarson2 @d.umn.edu plarson2@d.umn.edu Introduction Introduction Bedrock the Lake Lake Superior Superior region region isistypically typically covered covered by byaamantle mantleofofglacigenic glacigenicsediments sediments—- till, Bedrock in the outwash,and andlacustrine lacustrinesediments sediments — presents presents a significant - that significant challenge to successful successful application application of of surficial surficial outwash, geochemical techniques techniques widely widely used used to help geochemical help generate generate drilling drilling targets. targets. The glacial glacial environment environment is is very very complex, complex, with sediments produced produced by by a range of processes. with processes. Till represents represents the the ideal ideal sampling samplingmedia media ininthese theseenvironments, environments, since a vector (ice flow direction) is attached to the composition composition at any location indicating the direction to the source of any defined anomaly. anomaly. However, recent work work has led led to to recognition recognition that that both both the themagnitude magnitudeof ofaatill tillgeochemical geochemical anomaly and and the the potential potential transport transport distance distance to to its its source source may may have have aa wide wide range range of of values. values. This anomaly This isis aa reflection reflection of of material, and and is related to the the mean mean transport distance of till-forming till-forming material, the fundamental fundamental sediment sediment transport process responsible for forming forming the the till. till. Theory The concentration of an an indicator indicator (a (a distinct distinct lithologic lithologic or or geochemical geochemical component component derived derived from from aa discreet discreet in till till is the direct product of the physical processes processes of of glacial glacial erosion, erosion,transport, transport,and anddeposition. deposition. Indicator Indicator source) in concentration concentration is controlled by a number number of of variables, variables, including including substrate substrate hardness and the the efficacy efficacy of of the theglacial glacial erosional regime. regime. Under Under steady steady state state ice ice flow flow conditions conditions and and uniform uniform bed bed erosion erosion rates, rates, indicator indicator mass mass concentration ci c in till at any transport transport length length TT (1) (1) down-ice of an indicator source source of finite finite flow-line flow-linelength length LL (1) (I) is: cTO — L (1) where For tills down-ice of of the indicator source, under steady state conditions, the where X. is the erosion length length scale scale (1). (1). For decrease decrease in indicator indicator concentration concentration with with increasing increasingtransport transportlength lengthcäc/5T 8c/8T assumes assumes aaquasi-exponential quasi-exponential form. form. Erosion length scale, scale, X, X,isis related related to to the the spatial spatial bed bed erosion erosion rate rate EE (m-1'") (ml3) and and the the thickness thickness of the the debris debris layer layer in in Erosion length transport transportmd md (m12): (m-l'2): (2) E X closely related related to to the the mean transport distance of till-forming material; as ?X increases, X isis closely increases, so so does does the the mean mean transport transport distance. distance. ShortTransport: Examples Short- vs. Long-Distance Transport: Examples Tills in the Lake Superior Superior region region can can be be broadly broadly grouped grouped into into two two categories categories based based on onthe themean meantransport transport length of the till forming forming material. material. Tills characterized by short-distance short-distance mean transport transport length length are are commonly commonly composed composed of of coarse-grained coarse-grained material containing abundant abundant angular angular clasts, and display material containing display rapid decrease decrease in in indicator indicator concentration concentration with with transport transport length. This Thisisisexemplified exemplifiedby bytills tillsoverlying overlyingthe theVermilion Vermiliongreenstone greenstonebelt beltof ofnorthern northernMinnesota, Minnesota,which whichdisplays displays rapid in concentration concentrationof ofnumerous numerousindicator indicatorlithologies; lithologies; range from 2.0 l0m. m. A XX forfor thisthis tilltill range from 1.01.0 to to 2.0-lo3 rapid decrease decrease in dispersal train composed composed of of clasts clasts of of Nipigon dispersal train Nipigon diabase diabase in till till east east of of Lake LakeNipigon, Nipigon, Ontario Ontario displays displays similar similar characteristics. Dispersal for this this Dispersalisischaracterized characterizedby byaasimilar similarrapid rapiddecrease decreaseininindicator indicatorconcentration; concentration;calculated calculatedXX for till range from 5.5-10' 5.5.102 m m over over relatively relatively soft soft greenstone greenstone to to 2.4-lo4 2.4 i04 m m over over hard hard diabase. diabase. Both Both tills tills are areinterpreted interpretedto to have formed by erosion of hard bed by quarrying quarrying and abrasion, abrasion, and and englacial englacial transport. transport. Tills characterized Tills characterized by long-distance long-distance mean transport transport length length are are commonly commonly composed composed of offine-grained fine-grained material with with a relatively material relatively low low abundance abundance of rounded rounded clasts, clasts, and and display display little little apparent apparent decrease decrease in in indicator indicator with transport length. Cretaceous concentration with Cretaceous shale shale grains grains in in Des Moines Lobe tills of the Minnesota Minnesota River valley show relatively little decrease in concentration along the flow axis extending down down the valley valley (Matsch, 1972); the XI.for forthis thistill tillisis5.0-10 5.0 l0 m. m.Carbonate-bearing Carbonate-bearingtills tillsoverlying overlyingthe the Canadian Canadian Shield Shield north of Lake Superior calculated ? (Thorleifson and Kristjansson, 1993) 1993) show show similar similar long-distance long-distance transport transport of of Paleozoic Paleozoic carbonate carbonateand andProterozoic Proterozoic m. Both tills greywacke clasts concentration; the 6.0-10' tills are are greywacke clasts with with little little decrease decrease in in concentration; the calculated calculated XXfor forthis this till till is 6.0 I m. interpreted interpreted to have deposited deposited from from deforming deforming subglacial subglacialsediment sediment layers layers (deformation (deformationtills). tills). 45 �Discussion by A,? that are 10 to 100 The data indicate deformation tills are characterized by 100 times higher than those of thin, by intermediate intermediate values values of of the the erosion erosion length length scale scale 'k? have have not as yet been coarse-grained tills. Tills characterized characterized by recognized in in the Lake Lake Superior Superior region, region, and and perhaps perhaps do do not not exist. exist. This recognized This gap gap in in recognized recognized values values suggest suggest there are entrainment and two main main processes by which bed material two material is is eroded eroded and and entrained, entrained, transported, transported, and and deposited deposited— - entrainment transport by a deforming subglacial layer, and erosion by quarrying and abrasion and transport as an englacial debris load with deposition by lodgement lodgement or meltout. Deformation tills form with little little accompanying accompanying erosion of underlying underlying hard bedrock. bedrock. Their formation formation is is consistent with redistribution of unconsolidated regolith or or sediment derived from consistent with redistribution of unconsolidated regolith sediment derived from aa preglacial preglacial reservoir. reservoir. Consequently, till composition reflects that of distant Consequently, till composition reflects distant (>100 (>I00 km) krn) bedrock. bedrock. Tills characterized characterized by short short mean mean transport length indicate spatially and temporally restricted erosion and entrainment and transport of hard bedrock. Their formation is consistent consistent with with erosion erosion by by quarrying quarrying and and abrasion abrasion of of hard hard bedrock bedrockwith withsubsequent subsequentextensive extensive Till composition closely reflects reflects that textural modification modification during during transport. transport. Till composition closely that of of nearby nearby (—10 (-10 km) krn) bedrock. bedrock. Consequently, Consequently, these tills have enormous enormous potential potential value as geochemical geochemical sampling sampling media. media. Recognition of the process responsible responsible for till till formation formation in aa given given area area is is critical critical for forsuccessful successfulapplication application of surficial surficial geochemical geochemical and boulder tracing tracing exploration exploration techniques in the Lake Lake Superior Superior region. region. Limited scope scope orientation surveys aimed aimed at characterizing the erosion length provide a means orientation surveys characterizing the length scale X A, provide means of of quickly quickly assessing assessing the the potential for successful successful application application of of drift drift exploration explorationtechniques techniqueson on both both regional regionaland andproperty propertyscales. scales. 46 �Glacial Lakes Aitkin Aitkin and and Upham: Upham: their their origin and environmental history Lisa Marlow, Howard Mooers, & Phillip Phillip Larson Geological Sciences, Department of Geological Sciences, University of Minnesota, Minnesota, Duluth, Duluth, Minnesota Minnesota 55812 55812 Aitkin and Upham occupied a basin in north-central Minnesota Glacial Lakes Aitkm Minnesota bounded bounded on on the the north by the Giants Range and to the east, south, south, and west by hummocky moraines moraines of of the the Rainy Rainy The lakes lakes came came into into existence existence with with the the retreat retreat of of the the lobe, Superior lobe, and St. Louis sublobe. The Rainy lobe from the St. St. Croix Croix phase phase sometime sometime after after 15,000 15,000yr yr BP BP (Clayton (Clayton and and Moran, Moran,1982; 1982; Mooers and Lehr, 1997). 1997). The Thebasin basinwas waslater lateroveridden overidden by by the the St. St. Louis Louis sublobe sublobefrom from the the northwest. With sublobe,the the basin basin was was again again occupied occupied by by Withthe thewastage wastageof of the theice iceof of the theSt. St.Louis Louis sublobe, Aitkin/Upham I and the later phase as as Lake Lake lakes; the earlier phase is referred to as Lake AitkinJUpham AitkinflJpham at a few few localities. localities. One Sediment of of the the early early lake phase is preserved at One such AitkinAJpham II. Sediment locality preserves a sequence that helps redefine the glacial chronology. chronology. A sedimentary sedimentary sequence located in the northeast Upharn basin basin reveals reveals sub-aqueously sub-aqueouslydeposited depositedRainy Rainy Lobe Lobe northeast corner comer of the the Upham outwash beneath glaciotectonically glaciotectonicallydeformed deformed fine-grained fine-grained lake lake sediments sediments deposited depositedby by the the St. St. Louis sublobe. This, This,along alongwith with other othergeomorphic geomorphicrelationships relationships(P.C. (P.C. Larson, Larson, unpublished unpublisheddata) data) indicates indicates that the Rainy Lobe ice margin was coincident coincident with the the Giants Giants Range Range when when the the St. St. sublobe advanced across the lake lake basin. Louis sublobe Using a Digital Elevation Elevation Model Model (DEM) (DEM) the elevation elevation of of lake lake basin basin was was adjusted adjustedfor forisostatic isostatic rebound based on the highest lake lake level, level, then then tilted tilted incrementally incrementally through through several severalstages stagesto to assess assess beaches, inlets, and outlets over time. AAseries seriesof of successively successively lower lower outlets outlets draining draining to to the the St. St. and Upham Upham (Hobbs, (Hobbs, 1983; 1983;Farnum, Farnum,1964; 1964; Louis River served as outlets outlets for for Glacial Glacial Lakes Aitkin and Wright, 1972). Meltwater Meltwaterentered enteredthe the lakes lakes from from Glacial Glacial Lake Lake Norwood Norwood through through the the Embarrass Embarrass gap, and later from Glacial Lake Koochiching along the the Prairie Prairie River. River. During During this this time time Aitkin Aitkin and Upham were confluent, confluent, and and the outlet outlet was was established established down down the the modern modem St. St.Louis Louis River. River. The lakes were separated separated by a sill ca 11,500-10,100 11,500-10,100yr BP, after after inflow inflow from from Koochiching Koochiching was was diverted to Glacial Glacial Lake Lake Agassiz. Agassiz. of the lakes. Granulometry Extensive dune fields formed following initiation of drainage of Granulometry 4ip grain grain size size signature signaturecharacterizes characterizesdunes dunesthroughout throughoutthe thebasin. basin. Maximum dune indicates a 4(p amplitude metersand anddune dunemorphologies morphologiesrecord recordprominent prominentnorthwesterly northwesterlywinds. winds. Dunes amplitude is —3 -3 meters overly source areas, which include include an underfiow underflow fan deposited deposited by the Prairie Prairie River River inlet inlet and and the the western margin of Lake Upham. Lake Upham. A sediment core collected collected from from Hay Lake (93°W, (93OW, 52°N), 52ON), located within aa dunefield dunefieldat at the the edge edge of Glacial Lake Upham, records records three prominent peaks in whole-core whole-core magnetic magnetic susceptibility susceptibility between 10,100 10,100 and 6,600 yr BP. No No clastic clasticinput input isis evident evident after after 6,600 6,600 yr yr B.P., B.P., suggesting suggestingdune dune stability. The timing of dunes within the basin has important implications for other dunes throughout Minnesota. Eolian Eolianevents eventsrecorded recorded in in the the core core are are interpreted interpretedas as the the result result of of lake lake drainage and exposure of abundant source material during Late Glacial and Early Holocene exposure source material during Glacial and Early Holocene rather than landscape landscape destabilization destabilization because because of mid-Holocene mid-Holocene aridity aridity (Keen (Keen et et al., al., 1990; 1990;Grigal Grigal et al., 1976; Dean et a!., 1996; Dean, 1997). Additionally, this sediment core places a minimum 1976; al., Additionally, this sediment core places a minimum age on the drainage of of Glacial Lakes Aitkin Aitkin and Upham Upham II. II. Lake Upham must have drained after 47 �Lakes Aitkin Aitkin and and Upham Upham II. II. Lake Upham must have drained after age on the drainage of Glacial Lakes 11,500 yr BP and before 10,100 10,100 yr B.P. and Lake Aitkin may have persisted until ca. 7,000 7,000 yr 11,500 BP. BP. Clayton, L. and Moran, S.R. S.R. 1982, 1982, Chronology Chronology of late late Wisconsinan Wisconsinan glaciation glaciation in in middle middleNorth North America. Quaternary Quaternary Science Science Reviews Reviews 11 (1), (I), 55-82. 55-82. Dean, W.E., Ahlbrandt, Ahlbrandt, T.S., T.S., Anderson, Anderson, R.Y., R.Y., Bradbury, Bradbury, J.P., J.P., 1996. 1996.Regional Regional aridity aridityin in North North America during the middle (2), 145-155. 145-155. middle Holocene. Holocene. The The Holocene Holocene 66 (2), Dean, W.E., 1997. 1997. Rates, Rates, timing, timing, and and cyclity cyclity of Holocene Holocene eolian eolian activity activity in in north-central north-central United United States: States: Evidence from varved lake lake sediments. sediments. Geology Geology 25 25 (4), (4), 331-334. 331-334. Farnham, R.S., McAndrews, J.H., and Wright, H.E., Jr. 1964. 1964. A Late-Wisconsin buried soil near Famham, Aitkin, Minnesota, and 393-4 12. and its its paleobotanical paleobotanical setting. setting. American American Journal Journal of of Science Science262, 262,393-412. History of of the the Lake Lake Superior Superior Farrand, W.R. & Drexler, 1985. 1985. Late-Wisconsinan and Holocene History Basin. In In Karrow, Karrow,Quaternary Quaternaryevolution evolution of of the the Great Great Lakes, Lakes, Geological Geological Association Association of of Canada Canada Special Paper 30, 30, 17-32. 17-32. Grigal, D.F., Severson, Severson, R.C., Golz, Golz, G.E., G.E., 1976. 1976.Evidence Evidence of of eolian eolian activity activity in in north-central north-central 1-1254. Minnesota 8,000 8,000 to 5,000 5,000 yr. ago. ago. Geological GeologicalSociety Societyof ofAmerica AmericaBulletin Bulletin87, 87,125 1251-1254. Upham, and and early early Lake Lake Hobbs, H.C. 1982, 1982, Drainage Drainage relationships relationships of of Glacial Glacial Lakes Lakes Aitkin, Aitlun, Upham, Agassiz in northeastern Minnesota. In Teller, J.T., and Clayton, Lee, eds., Glacial Lake In Teller, J.T., and Clayton, Lee, eds., Glacial Lake Agassiz: Agassiz: 245-259. Geological Association of Canada Canada Special Special Paper Paper 26, 26,245-259. Keen, K.L., Shane, Shane, L.C.K, 1990. 1990. A continuous continuous record of Holocene Holocene eolian eolian activity activity and and vegetation change at Lake Ann, east-central Minnesota. Minnesota. Geological Geological Society Society of America America Bulletin Bulletin 102, 102, 1646-1657. 1646-1657. Mooers, H.D., Lehr, J.D., 1997. 1997.Terrestrial Terrestrial record of Laurentide Laurentide ice ice sheet sheet reorganization reorganization during during Heinrich events. Geology Geology 25 (11), (1 I), 987-990. 987-990. Sims, P.K., P.K., and and Morey, Morey, G.B., G.B.,eds., eds., Wright, H.E. 1972, 1972, Quaternary Quaternary history of Minnesota. Minnesota. In Sims, Geology of of Minnesota: A 15-578. A Centennial CentennialVolume: Volume:Minnesota MinnesotaGeological GeologicalSurvey, Survey,5515-578. 48 �Magmatic and Hydrothermal HydrothermalPGE PGEMineralization Mineralizationof of the the Birch Birch Lake LakeCu-Ni-PGE Cu-Ni-PGEDeposit Intrusion, Duluth Complex, northeast northeast Minnesota in the South Kawishiwi Intrusion, John Marma, Marma, Phil Phil Brown Brown and and Steve Steve Hauck* Hauck* Department Department of Geology Geology and Geophysics, Geophysics, University of Wisconsin, Wisconsin, Madison, Madison, Wisconsin Wisconsin53706, 53706,USA USA *Natural *Natural Resources Research Research Institute, Institute, University of Minnesota, Minnesota, Duluth, Duluth, Minnesota Minnesota 55811, 5581 1,USA USA The Birch Lake Cu-Ni-PGE Cu-Ni-PGE Deposit Deposit is located located 12 12 miles miles south south of of Ely, Ely, MN MN in in the the South South Kawishiwi Intrusion (SKI) of the Duluth Complex (DC). The The SKI SKIis is one one of of two two layered layered mafic mafic intrusions along the basal contact of the DC to host sub-economic Cu-Ni-PGE deposits. intrusions contact of the DC to host sub-economic Cu-Ni-PGE deposits. Mineralization Mineralization is dominantly dominantly hosted hosted by by the the U3 U3 layer, layer, the lower-most lower-most of of three three ultramaficultramafictroctolite troctolite packages characterized characterized as as aa zone zone of alternating alternatingultramafic ultramafic (picrite-peridotite) (picrite-peridotite)and and troctolite (>5%) ultramafic ultramaficand/or andlormassive massiveoxide. oxide. troctolite horizons horizons with lenses lenses and and pods pods of of oxide-bearing oxide-bearing(>5%) The purpose of this study study was to locate, locate, describe, describe, and and characterize characterizethe the textural textural relationships relationships among platinum group group minerals minerals (PUM), (PGM), sulfides, sulfides,and and silicate silicatephases phases to to help help delineate delineatethe the relative significance significance of primary and remobilized remobilized platinum platinum group group element element (PGE) (PGE)concentrations. concentrations. Samples drill holes transecting transecting the the Birch Birch Lake Lake Deposit Deposit were were obtained obtainedfrom fromthe the Samples from 4 drill Natural Resource Research Institute (NRRI) located in in Duluth, Duluth, MN. MN. EMPA EMPA and and detailed detailed petrography were used to locate PGE bearing minerals, minerals, averaging averaging e15pm <15tm in in diameter, diameter,and and to to characterize characterize the host mineral geochemistry. Identifying Identifying the PGM PGM textural textural relationships relationships with with other phases is critical to understanding the mechanism by by which which PGMs PGMs were deposited. Data Data from this study will aid exploration in locating other deposits and guide metallurgists in improving improving recovery techniques. techniques. Ir,Ru, Ru, Au, Au,Ag, Ag, Te, Te, PGEs occur most often often as various various Pd minerals minerals with with associated associatedPt, Pt, Os, Os,Ir, categories of silicate-sulfide-PGM silicate-sulfide-PGMtextural textural Bi minerals and were grouped into into the following following 44 categories relations: 1) PGMs PGMs that occur in "halos" residing most commonly in anorthite-enriched relations: anorthite-enrichedzones zones in in primary plagioclase around either interstitial interstitial sulfide sulfide (dominantly (dominantly chalcopyrite), chalcopyrite),interstitial interstitial sulfide sulfide and silicate silicate (dominantly chalcopyrite, chalcopyrite, clinopyroxene, and hydrous silicates silicates (amphibole (amphiboleand and silicate) (Figure (Figure 1). This This style style of biotite)), or silicate (dominantly clinopyroxene or hydrous silicate) of the the total total PGMs PGMs identified. identified. 2) Remobilized PGMs that occur in mineralization hosted 58% of chlorite, serpentine, or secondary magnetite. This This style style of of mineralization mineralization hosted hosted 21% 21% of of the the total total PGMs identified. 3) 3)Random RandomPGMs PGMsthat thatoccur occurin inpoikilitic poiluliticanorthite-rich anorthite-richplagioclase plagioclase(An (An 75-An 75-An 95) and clinopyroxene clinopyroxene (Wo 30-Wo 50) with PGEs sometimes residing in disseminated but no no association with with "halos". "halos". This style of chalcopyrite or hydrous silicate pockets, but mineralization hosted 11% 4)In In interstitial interstitialsulfides sulfidesor orsilicates silicatesthat that mineralization 11% of the total PGMs identified. 4) include hydrous silicates, (?) textures, textures,or or silicates, chalcopyrite, chalcopyrite, clinopyroxene, sulfides sulfides with symplectite symplectite(?) calcite. This style of mineralization mineralization hosted hosted 10% 10% of the the total total PGMs PGMs identified. identified. of high PGM concentrations in The following is a summarized summarized model for the formation of the Birch Lake deposit. The TheSKI SKIbegins begins as as aa magma magma body body that is replenished replenished with multiple multiple injections of magma, which becomes sulfur saturated. The The magma magma body body is is relatively relatively PGE PGE poor, due to partial loss of sulfides during emplacement leaving the conduits with a PGE enriched segregation of the total sulfide. The Thesulfides sulfidesin in the the magma magma body body scavenge scavengeavailable available PGEs PGEs and and crystallize as disseminated, interstitial sulfide grains. Primary Primary hydrous hydrous phases phases form form at at this this time time 49 �fro:maafluorine-rich, fluorine-rich,deuteric deutenc fluid. fluid. A Cl-, Cu-, PGE-rich, sulfide-poor from sulfide-poor fluid enters the magma faults. The chamber at its base via the original magma conduit(s) and/or faults. The fluid fluid migrates migrates along along grain boundaries, and interacts with the larger interstitial interstitial sulfides. A A dynamic dynamic environment environment is is created in which the fluid, containing a significant concentration of dissolved metals, begins containing significant concentration of dissolved metals, begins to to to produce produce more more sulfides. sulfides. At consume and use sulfur from the larger grains to At the the same same time, time, the the fluid is reacting with neighboring grains, specifically plagioclase, and through aa cation cation exchange exchange them in in calcium. calcium. This reaction, alters the plagioclase rims, enriching them This reaction reaction causes causes aa volume volume (±chlorite) producing producing the the disseminated disseminated loss that is filled with precipitated sulfides and PGMs (±chlorite "halos" around the larger interstitial grains (Figure 1). 1). Finally, Finally, another another fluid fluid migrated migrated through through "halos" the intrusion that remobilized remobilized PGMs on on aa small small scale. scale. Based on evidence evidence solely solely from from the Birch Lake deposit, deposit, PGM PGM mineralization mineralizationappears appears concentrated or "compartmentalized". This Thiscould couldbe be the the result result of of two two possible possiblemechanisms: mechanisms: 1) 1) of high PGM concentrations are dependent on on their their proximal proximal distance distance to to "feeder" "feeder" zones Areas of 2) High PGM PGM concentrations concentrationsare are (i.e. conduits or faults) where fluids can be introduced; introduced; and/or and/or 2) due to structural controls within these these heterogeneous heterogeneous rocks that localize localize fluid fluid movement. movement. This study contributes to the current debate on the roles of of primary vs. remobilized mafic intrusions. intrusions. For (deuteric?) PGE mineralization in layered mafic For the the Birch Lake Lake deposit, deposit, the data data suggest both mechanisms played im of the ore minerals. )ortant roles in the sussest nlaved imiortant orisin ore minerals. Figure Figure 11 locations A-L. A-L. a.) Photomicrograph in a.)~hotomicro~ra~h inplane-polarized plane-polarizedlight lightofofthin thinsection sectionBL BL89-2 89-22516.4 25 16.4—- locations Large interstitial chalcopyrite chalcopynte and pyroxene cross-cut by vertical chlorite veins, all of which are surrounded by a disseminated, disseminated, dominantly chalcopyrite halo that is in An-enriched plagioclase rims. Notice Notice all allPGMs, PGMs,except except one, one, either either occur occur in the halo; in chlorite veins; in interstitial biotite; or in clinopyroxene. clinopyroxene. The Thealtered alteredplagioclase plagioclase and and altered alteredpyroxene pyroxene on on the the left left side side of of the the image are devoid includesareas areaswithin withinthe theoriginal originalhalo. halo. This devoid of any PGM occurrences occurrences —this -this includes suggests a second alteration fluid event that removed PGMs and altered altered the the minerals, minerals,which which itit passed through. Dashed Dashedline linerepresents representsthe theextent extentof of halo halo and andAn-enrichment An-enrichment in in the the adjacent adjacent plagioclase grains. White Cpx=Clinopyroxene, White stars stars represent represent PGM occurrences. occurrences. Cpx=Clinopyroxene, Opx=Orthopyroxene, Bi=Biotite, Bi=Biotite, Chl=Chlorite, Opx=Orthopyroxene, Chl=Chlorite, Cpy=Chalcopyrite, Cpy=Chalcopyrite, Plag=Plagioclase Plag=Plagioclase 50 �RESULTS OF OF EMP EMP MONAZITE MONAZITE GEOCHRONOLOGY GEOCHRONOLOGYIN E-C MINNESOTA: MINNESOTA: EVIDENCE EVIDENCE FOR LARGE-SCALE GEON 17 METAMORPHISM ASSOCIATED WITH POSTLARGE-SCALE GEON 17 METAMORPHISM ASSOCIATED WITH POSTTECTONIC PLUTONISM MCKENZIE, M.A., and HOLM, D.K., D.K., both at at Dept. Dept. of Geology, Geology, Kent State State University, University, Kent, Kent, OH; SCHNEIDER, SCHNEIDER, D.A., D.A., Dept. Dept. of of Geological Geological Sciences, Sciences, Ohio Ohio University, University,Athens, Athens,OH; OH; JERCINOVIC, M., Dept. of Geosciences, Geosciences, University of Massachusetts, Massachusetts,Amherst, Amherst, MA MA Determination of the timing and extent of poly-phase metamorphism is essential in Determination unraveling the tectonic history of a region. The The pattern pattern and and degree degree of of metamorphism metamorphism preserved preserved across the Penokean orogenic belt in the southern Lake Superior Superior region is highly variable. Abundant 40Ar/39Ar dates from east-centralMinnesota Minnesotaindicate indicatewidespread widespread cooling cooling at at -1760 - 1760 Ma ~ r l dates ~ from r east-central shortly after the emplacement emplacementof of the the east-central east-centralMinnesota Minnesotabatholith batholith(ECMB) (ECMB)atat—1775 -1775 Ma (Hoim (Holm et al., 1998; 1998; in review). Yet Yet U-Pb U-PbSHRIMP SHRIMPmonazite monaziteages ages from from three three localities localitiesacross across the the Ma metamorphic MN, northern northern MI, MI, and and northern northern WI) WI)record recordonly onlyaauniform uniform—1830 -1 830 Ma region (e-c MN, episode 1800 Ma thermal thermal pulse pulse linked linkedtotoaarecently recentlyidentified identified—1800 -1800 episode and and aa secondary secondaryyounger younger—1800 Ma magmatic event (Schneider et al., a!., in in review). review). This This study study utilizes utilizes the the total total Pb Pb electron electron microprobe (EMP) monazite age dating technique to better constrain the the extent of of thermal overprinting overprinting surrounding surrounding the batholith. batholith. metamorphic monazite monazite ages ages from from three three For this study study we obtained obtained in situ metamorphic Paleoproterozoic gamet-staurolite schist samples and one garnet-cordierite Paleoproterozoic metasedimentary garnet-staurolite gneiss sample from the plutonic zone of east-central Minnesota (Figure 1). Schist Schistsample sampleAMAM016 contains contains predominantly elongate elongate monazite monazite grains grains displaying displaying aa mottled mottled chemical chemical variation variation in in Y and Th content. This Thissample sampleyielded yieldedaa mean mean age age of of 1746 1746±Â 3 Ma from 79 spots spots on seven grains. Two Maand and1760 1760Ma. Ma. A third less prominent Twoage agedomains domainswere were recognized recognized at at —1738 -1738 Ma —1780 Maage agedomain domainwas wasobtained obtainedon onsome somehigh highYYregions. regions. Schist sample MN-29 contains -1'780 Ma sub-euhedral monazite displaying prominent regions of of high high Th Th content. content. This This sample sampleyielded yielded aa 10 Ma from 92 spots on five grains. A mean age of 1764  10 A single singleprominent prominentage age domain domain was was 1764 ± recognized 1772 Ma. Schist recognizedatat— 1772 Schistsample sampleP-16 P-16contains containsmonazite monazitewith withvery veryirregular irregulargrain grain boundaries, numerous inclusions, and variable Th content. This This sample sampleyielded yielded aa mean mean age age of of 1772 Two age age domains domains are identified: aaprominent prominent age age 1772 ±Â 33 Ma from 92 spots on seven grains. Two domain Maand and aa smaller smaller population population age domain Ma. Lastly, the Sartell domain at at —1770 -1770 Ma domain at at —1800 -1800 Ma. Sartell gneiss, corelrimtextures. textures. gneiss, sample sample S-2, S-2, contains contains euhedral euhedral monazite monazite grains grains displaying displaying distinctive distinctivecore/rim Ma from 102 spots on seven grains. Three This sample yielded a mean age of 1756 ± 3 Three age age sample 1756  3 domains 1750 Ma and 1770 1770 Ma and and aa third third less less domains are are identified: identified:two twoprominent prominentdomains domainsatat— 1750 prominent 1800 Ma on high U U cores. cores. prominent domain domainatat—1800 Our EMP Ma thermal thermal imprint imprint associated with intrusion EMP results results reveal reveal aa profound profound—1770 -1770 Ma intrusion of the 1775 1775 Ma ECMB. The Theconsiderable considerabledistance distanceof of some someof of these thesesamples samplesfrom fromthe the western western edge of the exposed batholith (30-40 km) and the absence of Penokean metamorphic ages exposed batholith (30-40 krn) and the absence of Penokean metamorphic ages suggests that the thermal pulse must have been dramatic. However, However, the the garnet-schist garnet-schistsample sample KKR (east of Mille Mule Lacs) that records only geon 18 SHRIMP SHRiMP ages lies north of the region of thermal influence of the batholith. We Wenote notethat thatsample sampleK-R K-R isislocated locatedjust just north north of of the theMalmo MalmoStructural Structural of it. Our Discontinuity (MSD) and sample AM-016 is located south of Our data datareveal reveal that that the the MSD MSD juxtaposes rocks of different different metamorphic age (geon 18 18 metamorphism to the north from geon - - - - 51 �17 metamorphism to the south). We We propose, propose, therefore, therefore, that the the MSD MSD is is aa geon geon 17 17 structure structure that exhumed the plutonic terrane of east-central Minnesota. West West of of Mille Mille Lacs, Lacs, aa significant significant juxtaposes post-Penokean plutons to the south against older metamorphic portion of the MSD juxtaposes rocks to the north. This Thisclearly clearlysupports supportsour our interpretation interpretationthat that this this structure structurewas was active activewell well after Penokean orogenesis. orogenesis. Hoim, D.K., Darrah, K., and Lux, D., 1998, 60-8 1. 1998,American American Journal Journal of of Science, Science,298, 298,60-8 Holm, T., Schweitzer, D., and Schneider, Holm, D.K., Van Schmus, W.R., MacNeill, L., Boerboom, T., D.A., in review, review, Geological GeologicalSociety Society of of America America Bulletin. Bulletin. Schneider, D.A., Holm, Hoim, D.K., O'Boyle, C., C., Hamilton, Hamilton, M., Jercinovic, Jercinovic, M., in review, review, Geological Society of America Special Volume "Gneiss Domes and Orogeny." Special volume Domes and Orogeny." Composite MN-29 Composite AM-Ol 6 Composite 0 7 0 1700 04 7720 2 1740 7 4 1760 1760 1780 1780 1800 1800 Mo8 More 1700 1700 1720 1720 1740 1740 Age (Ma) AW (Ma) 1760 1760 1780 1780 !do0 1000 More Mo. Age ~ g (Ma) (Ma) e Composite S-2 Composite P-16 Composite 20 - 1520 10- IS U. 5- 1 0 1700 71720 7 4 1740 1760 1760 1780 1780 1800 1000 More Mo,. 1 1 0-' 1700 1700 1720 1720 1740 1740 1760 1760 1780 1780 1800 WOO Mor. More Age (Ma) Ag. (Ma) - Histogram AllSamples Samples - All Ages Histogram All 700 1720 1740 1760 780 1800 1820 Age (Ma) - Figure 1: Histograms Histogramsof of EMP EMP Th-U-total Th-U-total Pb in situ s i t ~monazite spot ages. 52 �THE SIOUX QUARTZITE REVISITED: SEDIMENTOLOGY, SEDIMENTOLOGY, METAMORPHISM, METAMORPHISM, GEOCHEMISTRY GEOCHEMISTRY AND THE THE ORIGIN ORIGIN OF PIPESTONE PIPESTONE Geology & & Geophysics, Geophysics, University University of of MEDARIS, L.G., Jr., and DOTT, R.H., Jr., Dept. of Geology Wisconsin-Madison, Madison,WI, WI,53706; 53706;medaris@geology.wisc.edu; medarisgeology.wisc.edu;rdott@geology.wisc.edu rdottgeoIogy.wisc.edu Wisconsin-Madison, Madison, Red, supermature quartzites of the Baraboo Baraboo Interval were deposited after after 1.75 1.75 Ga Ga on on aa stable stablecraton craton ininthe the presence of free atmospheric oxygen oxygen under under conditions of intense intense chemical chemical weathering. weathering. Some presence Some quartzites quartzites (Baraboo and and Flambeau) Flambeau) were were folded folded and and recrystallized recrystallizedatat 1.63 1.63 Ga Ga (Holm (HoIm et et al., 1998), 1998), and and many many (Baraboo quartzites were were hydrothermally hydrothermallyaltered alteredatat 1.46 1.46 Ga Ga (Medaris (Medaris et et a!., quartzites al., 2002, 2002, in in press), press), presumably presumably in in response to brine migration promoted by by continental continental scale scale A-type A-type magmatism. magmatism. These response migration promoted These discoveries discoveries have have prompted us to reevaluate the sedimentology, metamorphism, and geochemistry of the Sioux Sioux Quartzite. Quartzite. Sedimentolo'y SedimentologvThe TheSioux SiouxQuartzite, Quartzite,which whichisisseveral severalhundred hundredmeters metersthick, thick,isiscomposed composed mostly mostly of quartz sandstone with interstratified interstratified lenses of red red mudstone mudstone (Southwick (Southwick et et al., al., 1986). 1986).Heterogeneous Heterogeneous cobble conglomerate occurs at the base and finer finer pebbly layers layers are are scattered scattered throughout throughout the the lower lower half half or or Sedimentary structures structures in the sandstones so, whereas mudstones mudstones occur chiefly chiefly within within the the upper upper half. half. Sedimentary sandstones include 15 cm in in thickness, thickness, rare rare include predominant predominant festoon-style, festoon-style, nested trough trough cross cross beds bedsaveraging averaging1010—- 15 zones of planar-tabular sets, a few zones planar-tabular sets, few examples examples of of herring herring bone bone cross cross bedding, bedding, and and both bothasymmetric asymmetric and and symmetric ripple marks. The mudstones are mostly massive, but parallel- laminated and ripple-laminated varieties are are also also present. present. In In most cases, quartz quartz silt silt and and fine sand grains are disseminated in a finer varieties most cases, disseminated in finer rare graded graded laminations laminations are are also also present. present. Some mudstones show polygonal polygonal 'mud' cracks, matrix, but rare cracks, and and the overlying sandstones commonly contain intraclasts ripped up from such such cracked cracked beds beds Interpretations of the Sioux depositional environment include shallow marine and and braided fluvial fluvial (Doll, 1983; Southwick et al., a!., 1986). 1986). In the latter latter scenario, scenario, the cross cross bedded bedded sandstones sandstones represent represent river river (Dott, channel deposits, and the mudstones, mudstones, slack slack water water deposits deposits in in ponds ponds between between active activechannels. channels.However, However, this interpretation interpretation is inconsistent with the rarity of scoured channel bases and tabular sets sets of of planar planar cross cross laminations, which would would have have formed formed by laterally bars, and the existence laminations, which laterally migrating migrating bars, existence of of wave wave ripples, ripples, Ojakangas and and Weber Weber (1984) (1984) suggested suggested that that the the which are not expected in the the sands of a braid plain. Ojakangas expected in braid plain. one-third of the Sioux formation was deposited in a shoreline upper one-third shoreline marine marine setting setting with with tidal tidal influences, influences, polygonal desiccation cracks, and accounting for the the herringbone herringbone cross cross bedding, bedding, wave ripples, polygonal and thickness thickness and extent of certain certainmudstone mudstonelayers layers(now (nowpipestone). pipestone). Interpretation Interpretation of of the the Sioux Siouxas asaafluvial-to-marine fluvial-to-marine transgressive succession would conform to the present interpretation of the correlative Baraboo transgressive succession would conform to the present interpretation of the correlative Baraboo Quartzite, which has wave ripples and reactivation surfaces surfaces in in its its upper upper half half (Medaris (Medariseteta!., al.,ininpress). press). Metamorphism Metamorphism Mineral Mineralassemblages assemblages in in finefine55 grained Sioux sedimentary rocks rocks can be 9 expressedininthe the system, system,KASH, KASH,asasportrayed portrayedin inac- 4 _ expressed Figure 1, 1, where rock rock compositions compositions are are projected projected onto the anhydrous onto anhydrous plane, plane, K-Al-Si, K-Al-Si, and and two two 3critical dehydration reactions are plotted. are plotted. Additional phases phases include abundant hematite Additional hematite and 2a Ti02 TiOz phase, phase, either either anatase anatase in in the the Cottonwood Cottonwood (Stelz, 1989), 1989), or rutile in the Basin (CB) Basin (CB) (Stelz, 11 The stable existence Pipestone Basin (PB). Pipestone (PB). The existence of - co~onwood the CB kaolinite in the CB (Stelz, (Stelz, 1989) 1989) requires temperatures —300°C, whereas pyrotemperatures below -300°C pyroI /I I I phyllite ininthe thePB PB isis stable stableabove above-300°C 300°C. The phyllite The 250 300 350 T, OC 53 �quartz-pyrophyllite assemblageininthe thePB PB(0, (0, Fig. 1), quartz-pyrophyllite assemblage I), in in which which vermicular kaolinite has been replaced by by pyrophyllite pyrophyllite (Fig. (Fig. 2A), 2A), most likely most likely represents represents higher higher temperature, temperature, largely largely isochemical isochemical recrystallization of a quartz-kaolinite protolith, like that in the CB CB The occurrence of muscovite in both (0, Fig. 1). 1). The occurrence of muscovite in both basins basins is is attributed to K-metasomatism related to 1.46 attributed K-metasomatism related 1.46 Ga Ga hydrothermal hydrothermal ("I",Fig. Fig. 1)1)isisaametasomatic metasomatic rock composed of activity. Pipestone Pipestone (+, pyrophyllite, muscovite, diaspore, hematite, and and rutile, pyrophyllite, muscovite, diaspore, hematite, rutile, in in which which former quartz grains have been completely former completely replaced by diaspore, diaspore, pyrophyllite, and and muscovite pyrophyllite, muscovite (Fig. (Fig. 2B). 2B). Because Because the Sioux Sioux undeformed and lies north of the extrapolated Quartzite is largely undeformed trend of the 1.63 al., 1998), 1998), 1.63 Ga Ga Mazatzal Mazatzal tectonic tectonic front front (Hoim (Holm et al., we suggest suggest that all all metamorphic metamorphic features features of of the the Sioux Sioux Quartzite Quartzite 1.46Ga Ga hydrothermal hydrothermal activity, activity, rather rather than than aa Mazatzal Mazatzal are due to 1.46 event. event. Geochemistry Where Where unmodified unmodified by by K-metasomatism, K-metasomatism, finegrained grained sedimentary rocks of the Baraboo Interval are remarkably mature, being practically Mg, and and Mn Mn (Fig. (Fig. mature, practically devoid devoid of K, Na, Ca, Mg, 3), and having Critical Critical Index Index of of Alteration Alteration values values of of 97 97 to to 99. 99. In In such rocks rocks the the wide wide range range in in proportion proportion of of Si Si to to A1 Al (Fig. 3) and such quartz to kaolinite, 1), reflects reflects the the original original proportion proportion of quartz to kaolinite in the kaolinite, or or pyrophyllite pyrophyllite (Fig. (Fig. I), protolith sediments. sediments. K-metasomatism has stabilized 200 muscovite in both both the the CB and PB, but but muscovite in .4i the muscovite-bearing rocks in the CB muscovite-bearing rocks CB 150 150 record record a lower lower temperature temperature and and higher higher ratio of Si/Al SiIAl compared to pipestone in the PB (Fig. 1). The Theclassic classicpipestone, pipestone, 5 100 loo .s 8 in addition addition to to substantial substantial KK contents, contents, contains lower Si Si and higher contains lower higher Al A1 than than 8oa.to 50 that in associated associated quartz + + pyrophyllite pyrophyllite fca"!& samples (Figs. 1 & & 3). 3). Assuming samples (Figs. Assuming Zr to ~ g ? be an immobile element, isocon ID calculations indicate the mean indicate that the mean E -50 pipestone composition was produced by 0 removal of 20 to 65% Si02, removal of Si02, 45 to 55% 55% -100 Ti02, Si Fe TI Ti02, 35 to 65% 65% Fe203, Fe203,and addition of K Na Mn Fe Ti Al Si K Na Ca Mg Mg Mn 15 to to 45% 45%A1203 A1203and and—800% -800% 1(20, K20, compared to the average compositions of the two Si-rich Si-rich and two Al-rich Al-rich quartz quartz ++ pyrophyllite samples. samples. The reconstructed composition of of one one pipestone pipestone sample sample (*, (*, Fig. Fig. 1) 1) requires requires removal removal of of 68% 68% Si02 Si02and and reconstructed composition addition during metasomatism. addition of of 50% 50%A1203 A1203during metasomatism. Further Further investigation investigation is underway underway to provide a more more detailed detailed evaluation evaluation of of brine brine compositions compositions and and metasomatic processes involved in this important, important, regional scale, scale, 1.46 1.46 Ga Ga hydrothermal hydrothermal event. event. References Dott, Dott,R.H. R.H.Jr. Jr.(1983) (1983)Geol. Geol.Soc. Soc.Amer. Amer.Memoir Memoir 160, 160,129-141; 129-141;HoIm, Holm, D. eta!. et al.(1998) (1998)Geology, Geology, v. 26, 907-9 10; Medaris, L.G., Jr. eta!. (2002) 48th Inst. Lake Superior Geol., 24-25; Medaris. L.G., Jr. Geol., 24-25; Medaris. L.G., Jr. eta!. et al.(in (in press) press) 907-910; Medaris, L.G., Jr. et al. (2002) 48th Inst. Lake Jour. Geol.; Ojakangas, Ojakangas, R.W. R.W. & & Weber, Weber,R.W. R.W.(1984) (1984)Mimi. Minn. Geol. Geol. Surv., Surv., Rept. Inv. 32, 1-15; 1-15; Southwick, D.L. et a!. al. (1986) (1986) Geol. Soc. Soc. Amer. Bull., v. 97, 1432-1441; 1432-1441;Stelz, Stelz, D.E. D.E. (1989) (1989)M.S. M.S. Thesis, Thesis,Wichita Wichita State StateUniv., Univ.,140 140pp. pp. (, Cd, C.) d . = - r 5 54 �A geochemical investigation investigation of of Mesoarchean Mesoarchean metavolcanic and metasedimentary metasedimentary rocks from the Birch-Uchi Birch-Uchi greenstone belt Metsaranta, Hollings, P.P. (Department of of Geology, Metsaranta,R., R.,Fralick, Fralick,P.P.and and Hollings, (Department Geology,Lakehead LakeheadUniversity, University,Thunder ThunderBay Bay ON CAN, P7B P7B 5E1) 5E1) Most Mesoarchean belts in the Mesoarchean greenstone greenstone belts the Western Western Superior Superior Province Province are are comprised comprised primarily of komatiite-tholeiite komatiite-tholeiite sequences sequences and and associated associated sedimentary sedimentary rocks rocks (Thurston (Thurston and Chivers Chivers 1990). 1990). These These—2.9-3.0 -2.9-3.0 Ga Ga assemblages assemblages have been been interpreted interpreted to to represent represent plume generated generated volcanism volcanism in in oceanic oceanic plateau plateau settings settings(for (forexample, example,Hollings Hollingsetetal. al.1999, 1999, Tomlinson et et al.1999). al.1999). This study study isis aapreliminary preliminary investigation investigation of of metavolcanic metavolcanic and and metasedimentary strata from the metasedimentary strata the Mesoarchean Mesoarchean Balmer Balmer assemblage assemblage of of the theBirch-Uchi Birch-Uchi greenstone belt. belt. Rogers Rogers et al. al.(2000) (2000)have havesuggested suggested that, that,given giventheir theirgeochemical geochemical affinities affinities and and Nd isotopic isotopic evidence evidence for for contamination contamination by by older older crust, crust, volcanic volcanic rocks rocks of of the the Balmer assemblage may represent a continental arc arc setting. This This implies implies that that the the Balmer Balmer Assemblage may represent those proposed proposed for for other other sent aa distinct tectonic tectonic setting from those Mesoarchean Mesoarchean rocks rocks in in the the Superior Superior P \ B,U' Province. Province. Sediment geochemistry geochemistry and and depositional depositional environment environment studies studies along along with igneous igneous geochemistry geochemistry will will be M8 applied to provide provide further further constraint constraint on on the possible tectonic setting of these possible tectonic setting of these — Ss.thviti LSO rocks. rocks. A i:3 I N N The Birch-Uchi Birch-Uchi greenstone greenstone belt belt is is located in the located in the central central portion portion of of the the MNI NM It Uchi (Fig.1). It is is Uchi Subprovince Subprovince (Fig.1). volcanic units three comprised comprised of three volcanic units termed the Balmer, Balmer, Narrow Narrow Lake Lake and and spanning assemblages, Woman Woman assemblages, spanning approximately 250 Ma. Ma. The Balmer Balmer assemblage assemblage is is the the oldest oldest of these these volcanic units and has volcanic units has U-Pb U-Pb zircon zircon ages from felsic felsic volcanic volcanic horizons horizons that that suggest suggest an age age of of ca. ca. 2975-2989 2975-2989 Ma Ma (Rogers et al., 2000). The Thestratigraphy stratigraphy of the the Balmer Balmer assemblage assemblage is is divided divided into four four suites: suites: aalower lowersedimentary sedimentary sequence, sequence, a mafic mafic volcanic volcanic suite suite and and two two petrographically petrographically distinct distinct felsic felsic volcanic volcanic suites suites (Rogers (Rogers et et al. al. 2000). 2000). Samples for this study Samples collected collected for study are are located in the the southern southern portion portion of of the the Figure Figure 11-Location Locationand andgeneralized generalizedgeology geologyof ofstudy study area area and and Birch-Uchi Birch-UchiGreenstone Greenstonebelt belt(modified (modifiedfrom from Balmer assemblage assemblage in in the Woman Woman Stott Stott and and Corfu Corfu 1991) 1991) River/Bear RiverIBear Lake area. These Thesecomprise comprise 16 samples from the 34 samples samplesof of the themafic maficvolcanic volcanic the lower lower sedimentary sedimentary sequence sequence and and 34 suite suite of of Rogers Rogers et et al.(2000). al.(2000). 55 �Field observations suggest that the the Balmer Balmer assemblage assemblage sedimentary sedimentary rocks are are turbiditic. turbiditic. Sediment geochemistry geochemistry will will be be applied applied to constrain the source rocks compositions Sediment compositions for these sediments. As Asno nocontact contactwith with underlying underlying older older rocks rocks has been identified identified this might provide provide valuable valuable information information about about the the preexisting preexisting older older crust. crust. Alternatively, the the sediments may be derived from the Balmer assemblage volcanics and this could support a hypothesis that the Balmer hypothesis that Balmer assemblage assemblage represents represents aa continental continental arc setting setting with with the the sediments deposited in a fore-arc fore-arc trench. Volcanic rock samples appear Volcanic rock appear to fall fall into intotwo twocompositional compositionaltrends. trends. The The first first is aa tholeiitic trend trend comprised comprised of of primarily primarily tholeiitic tholeiitic basalts basalts and and andesites. andesites. The The second second is is aa caic-alkaline trend of andesitic to rhyodacitic compostion. The geochemistry of these calc-alkaline trend of andesitic rhyodacitic geochemistry of samples will will be be applied samples applied to suggest suggest aa possible possible tectonic tectonic setting setting for these these rocks rocks and and implications ten-anes. implications of this this in relation relation to to other other Mesoarchean Mesoarchean terranes. 400 0 300 * 200 0 U II I I 0.01 100 .001 .01 I U 0 0.1 1 0 10 200 100 300 Zr Nb/Y NbN Figure 33- Lithology Lithology Discrimination diagram for Balmer Assemblage volcanics. Circles Circles are Tholeiitic trend squares are calc-alkaline trend. calc-alkaline trend. Figure 3-A plot of V vs Zr showing compostional groups in Balmer Assemblage Assemblage volcanics. volcanics. Circles are Tholeiitic trend squares are calc-alkaline calc-alkaline trend. trend. References: References: Hollings, P., Wyman, D. and Kerrich, R. 1999. Komatiite-basalt-rhyolite Komatiite-basalt-rhyolite volcanic associations associations northern northern Superior Superior Province Province greenstone greenstonebelts: belts: significance significanceof of plume-arc plume-arc interaction in the generation of of the the proto proto continental continentalSuperior SuperiorProvince. Province. Lithos Lithos 46: 137-162. 137-162. Lithogeochernical Rogers, N., McNicoll, V., van Stall, C.R., and Tomlinson, K.Y. 2000. Lithogeochemical studies in the Uchi-Confederation Uchi-Confederation greenstone greenstone belt, belt, northwestern northwestern Ontario: Ontario: implications implicationsfor for Archean Archean Tectonics. Geological 2000-C 16:1lip. GeologicalSurvey Survey of of Canada, Canada, Current Research 2000-C16: lp. Stott, G.M., and Corfu, F. F. 1991. Uchi Uchi Subprovince. Subprovince. In: In:Geology Geology of of Ontario, Ontario, special special volume 4, part 1. 8. 1. Ontario OntarioGeological GeologicalSurvey, Survey,pp pp145-23 145-238. Thurston, P.C. and Chivers, K.M. 1990. 1990. Secular Secularvariations variationsin ingreenstone greenstonesequence sequence development development emphasizing Superior Province, Canada. Precambrian PrecambrianResearch. Research. 46: 46: 21-58 21-58 D.J., Thurston, Thurston,P.C., P.C., and andHall, Hall,R.P. R.P. 1999. 1999. Plume Tomlinson, K.Y., Hughes, D.J., magmatism and crustal growth at 2.9 to 3.0 Ga in the Steeprock Steeprock and and Lumby Lumby Lake Lake area, area, Western Western Superior Province. Province. Lithos 46: 46:103-136. 103-136. 56 �PETROLOGY PETROLOGYAND AND PGE POTENTIAL POTENTIAL OF OF THE THE GREENWOOD GREENWOOD LAKE LAKE INTRUSION, INTRUSION, COMPLEX, LAKE CENTRAL DULUTH COMPLEX, LAKE COUNTY, COUNTY, MINNESOTA MILLER, James, D., Jr., Minnesota Minnesota Geological Geological Survey, Survey, mille066@tc.umn.edu mille066@tc.umn.edu MILLER, This report summarizes the results of a petrologic and metallogenic of drill core and metallogenic study of outcrop samples samples that profile profile the the Greenwood Greenwood Lake Lake intrusion intrusion (GLI) (GLI) of of the the central centralDuluth DuluthComplex Complex (Fig. 1). 1). The Thelittle littlethat thatwas wasknown knownabout aboutthis thisvery verypoorly poorlyexposed exposedlayered layeredmafic maficintrusion intrusionprior priorto to this study came from interpretation interpretation of its its aeromagnetic aeromagnetic signature, seven drill cores, and sparse, sparse, localized outcrop. The Thepurpose purposeof of this thisstudy study was was to to establish establish the the igneous igneous stratigraphy stratigraphy of of the GLI and to evaluate evaluate its its potential potential for for PGE PGEreef reef mineralization. mineralization. The GLI is an an approximately approximately two two kilometer-thick, sheet-like 10') to to the the east east and and covers covers an kilometer-thick, sheet-like intrusion intrusion that that dips dips gently gently (approximately (approximately10°) area of about 300 square kilometers. For For this this study, study,19 19bedrock bedrock drill drill cores cores (20 (20 to to 80' 80'in in length) length) Erie/LTV railroad railroad and powerline were acquired acquired in in early early 2002 2002along alongthe thewest—northwest-trending west-northwest-trending ErieILTV west of Lake 1). Samples Samples from from these these cores and west Lake County County Highway Highway 22 (Fig. (Fig. 1). and from from intermittent intermittent outcrops along the eastern extent of the railroad grade were subjected subjected to petrographic study in transmitted and reflected light, microprobe microprobe analyses analyses of olivine olivine and and pyroxene pyroxene composition, composition, and and whole rock whole rock analyses analyses of of their theirlithogeochemistry, lithogeochemistry, including including platinum, platinum, palladium, palladium, and gold gold concentrations. concentrations. The results of the drilling drilling and and petrographic petrographic study study show show that the the igneous igneous stratigraphy stratigraphy of of the the GLI can be grossly (GLtr, 0-650 0-650meters), meters), composed composed GLI grossly subdivided subdivided into into aa lower lower troctolitic troctolitic zone zone (GLtr, mostly of leucotroctolitic leucotroctolitic cumulates, cumulates, aa medial gabbroic gabbroic zone (GLog, 650-1800 meters), composed of olivine oxide gabbro (GLfg, 1800-2130 1800-2130 meters), gabbro cumulates, cumulates, and and an an upper upper ferrogabbroic ferrogabbroic zone (GLfg, composed largely of magnetite magnetite gabbro gabbro (Fig. (Fig. 2). 2). The troctolitic zone contains contains abundant, abundant, large anorthositic and oxide oxide gabbroic gabbroic inclusions, inclusions, presumably presumably derived derived from from anorthositic anorthositicseries seriescountry country rock. Although Althoughthe theGLI GLIisisaawell-differentiated well-differentiatedintrusion intrusionthat that formed formedas asan anopen openmagma magma system, system, microprobe data show that cryptic cryptic layering layering trends trends (such (such as as Fo Fo in in olivine, olivine, Fig. Fig. 2) 2) are areinconsistent inconsistent with formation by in situ crystallization differentiation. This This and and other other evidence evidence (such as abrupt changes in lithology, lithology, leucocratic leucocratic compositions compositions of of troctolitic troctolitic rocks, rocks, and and suspect suspect cumulus cumulus textures textures of troctolitic rocks) suggest of the GLI was suggest that the the differentiated differentiated character of was probably inherited from a deeper crustal crustal magma magma chamber, chamber, which which was was itself itself undergoing undergoing open opensystem systemdifferentiation. differentiation. The chemostratigraphy of chalcophile chalcophile elements elements through through the GLI GLI are are difficult difficult to to interpret interpretin in such a complex open magma system, system, but suggest that some potential for PGE reef mineralization may occur in the lower part of the gabbroic gabbroic zone (Fig. 2). Below Belowthis thislevel, level,recharging rechargingmagmas magmas appear to have been undersaturated undersaturated in in sulfide, sulfide, and and copper copper and and sulfur sulfur concentrations concentrationshigher higherin inthe the gabbroic zone (above 800 meters) indicate intermittent intermittent saturation. saturation. An unexpected result of this study was the discovery discovery of of aa large, large,sulfide-bearing sulfide-bearingoxide oxide gabbro gabbro inclusion inclusion within within the thetroctolitic troctolitic zone. Aeromagnetic Aeromagneticdata datasuggest suggestthat thatthis thisinclusion inclusionisisaaconformable conformabletabular tabularmass masswith withaastrike strike length of about 8 kilometers. The The magnetic magnetic data data further further suggest suggest that similar similar rock types form part of the footwall to the GLI. The Thepossibility possibilityof of sulfur sulfurcontamination contamination in the contact aureole around this inclusion and along the base of the intrusion of these areas for intrusion warrants further exploration of contact-type contact-type Cu-Ni-PGE Cu-Ni-PGE sulfide sulfide mineralization. mineralization. Funding for this project was provided to the Minnesota Geological Survey by a grant from recommendation of the Minerals Coordinating the Minnesota State Legislature on the recommendation Coordinating Committee. Committee. 57 �Figure 1. 1. Generalized geology of the Greenwood Lake intrusion and the central Duluth Small dots dots denote Complex. Small denote drill drill hole hole and and diamonds diamonds denote outcrop locations locations along ErieILTV ErieILTV railroad tracks. tracks. Long Long dashed dashed lines lines denote denote faults. faults. Intrusive Intrusiveunits units are: are: GLtr—GLI GLtr-GLI troctolitic zone Glog—GLI Glog-GLI gabbroic gabbroic zone zone GLfg—GLI GLfg-GLI ferrogabbroic ferrogabbroic zone MW—Mt. Weber granophyre MW-Mt. granophyre CLLS—Cloquet CLLS-Cloquet Lake Lake layered layered series series BEI—BaId Eagle intrusion BEI-Bald intrusion SKI—South Kawishiwi intrusion SKI-South PRI—Partridge River PRI-Partridge River intrusion intrusion WMI—Western WMI-Western Margin Margin intrusion intrusion Layered Layered Series Series Ferrogabbroic Ferrogabbroic Gabbroic Gabbroic Troclolitlc Troclolillc 3 L: ] Felsic Series k Anorthositic Series North Shore Volcanic Group Group Virginia Formation Biwabik IronFormation Giants Range Granite • • meters 20 Kilometers Kilometers 10 10 0 0 Who rock Whole rockgeochemistry geochemistry OHvine 2000 / 1600 / ,Pt+Pd * re .1 1200 * P. 0 800 400 C -,* I. 0 r— = = • 807060504030 Fo : 0 400 800 1200 Cu (ppm) 0 = •. —— 204060 Pti-Pd PttPd && Au Au (ppb) (ppb) Cu/Pd (xlO Figure Figure 2. Chemostratigraphy Chemostratigraphyof of Fo Fo in in olivine olivine and and of Cu, Pt ++ Pd, and Au concentrations concentrations through through the the Greenwood Lake Stratigraphic locations locationsofof drill drill core (boxes) Lake intrusion. intrusion. Stratigraphic (boxes) and and outcrop outcrop (diamonds) (diamonds) samples and general lithostratigraphy are are shown shown in in the the left columns. columns. Large Large inclusions inclusions of of anorthositic anorthositic series rocks (AS) and oxide gabbro (ox gb) are denoted. Abrupt (arrows) may mark Abrupt increases in Cu/Pd Cu/Pd (arrows) mark increases in sulfide saturation events. The The zone zone found foundmost most favorable favorable to host PGE reef mineralization mineralization is identified. identified. 58 �Stratigraphy and northwestern Stratigraphy andstructure structureof ofKeweenawan Keweenawan rocks rocks of the St. Croix horst, northwestern Wisconsin Wisconsin S.W. Nicholson, and W.F.Cannon, U.S.Geological U.S.Geologica1Survey, Survey, Reston, Reston, VA VA graben of of the Midcontinent Rift System The St. Croix horst is the partially inverted central graben (MRS) that extends southwestward southwestward from western Lake Lake Superior. Superior. ItIt is is bounded bounded by by the the Douglas fault on the northwest and the Atkins Lake fault on the southeast. Both are now reverse faults, but may have been graben-bounding normal faults during rifting and volcanism. The northern northern limit of the horst is White's Ridge, a subsurface basement high volcanism. evident in both seismic and gravity data, which did not subside substantially during rifting and against which rift volcanic and sedimentary rocks pinch out out or or become become much much thinner. thinner. White's Ridge Ridge effectively effectively separates separates the the MRS MRS in in western western Lake Lake Superior Superior from from the the St. St. Croix Croix horst and the volcanic, volcanic, sedimentary, sedimentary, and and structural structural history history of of the the two two rift rift segments segmentsdiffer differin in High-resolution aeromagnetic, gravity, gravity, and and seismic seismic data data permit permit the the tracing tracing of of several aspects. High-resolution flow sequences for long distances and to great depth. This geometry combined with the chemistry of the volcanic rocks allows us to decipher a volcanic volcanic stratigraphy in spite of widespread cover by glacial deposits deposits and and Paleozoic Paleozoic sedimentary sedimentary rocks rocks (Cannon (Cannon etetal., al.,2001). 2001). Our interpretation, interpretation, aided aided by previous gravity gravity and and seismic seismic interpretations interpretations (Chandler (Chandleretetal., al., 1989), is that the original structure of the St. Croix horst was an an asymmetric asymmetric graben, graben, or or possibly a half graben, like like those those of of the the Lake Lake Superior Superior portion portion of of the the MRS. MRS. The TheLake LakeOwen Owen fault was a major growth fault on on the the southeast southeast side side of of the the graben graben and and the the volcanic volcanic fill fill thickened toward and terminated against the fault. The Douglas fault on the northwest side of of the horst is not clearly a growth feature and may be simply a thrust formed during rift rift km or more because it inversion. Thrust displacement on the Douglas fault must be 20 krn juxtaposes of the base of a thick volcanic sequence over the younger Bayfield Group. Cannon et al. (2001) and Nicholson et al. (2001) used chemical and aeromagnetic data to underlying Clam Falls Volcanics, and the Chengwatana define the Minong Volcanics, the underlying Volcanics as three formations making up the graben-filling graben-filling volcanic volcanic sequence. sequence. The The three three three-part have similar chemistry, but were defined by structure and geochronology. The three-part division no longer seems seems justified and and the the Chengwatana Chengwatana and and Clam Clam Falls Falls Volcanics Volcanicsare are combined into a single single unit. The The Chengwatana Chengwatana Volcanics, Volcanics, as as earlier earlier defined, defined, were were restricted restricted to a fault-bounded belt between the Douglas and and Pine Pine faults faults and and their their stratigraphic stratigraphic relationships to other other volcanics were were not known known directly. directly. We We now now believe, believe, based based on on seismic seismic data, that the Pine fault does not extend into the northern northern part of the horst, where the depositional previously defined Chengwatana and Clam Falls units appear to be a continuous depositional sequence of compositionally indistinguishable indistinguishable flows flows that that we we propose propose be be called called entirely entirely Chengwatana. The Minong Volcanics, aa sequence krn thick, thick, low-Ti02basalts basalts about about 33 km sequence of of low-Ti02 overlie the Chengwatana, along along an an apparent apparent low low angle angle disconformity disconfomity based based on on aeromagnetic aeromagnetic disconformity within within the Minong volcanics on the form lines. These form lines also show a disconformity southeast limb of the Ashland syncline. syncline. A A lower lower unit, unit, not not present present on on the the northwest northwestlimb, limb,isis mostly high-Ti02 high-Ti02 basalt. Based on the presence of abundant high-Ti02 basalts and more evolved rocks, we infer infer that aa localized magmatic center center was was active active in in this this area area sometime sometime upper part part of of this sequence. A second, but but before 1095 Ma, the age of a rhyolite flow in the upper apparently older, volcanic center may have existed existed on the the western western margin margin of of the the graben graben near near 59 �______ Volcanics are mostly mostly high-TiOz high-Ti02 basalts, the Amnicon Complex where the Chengwatana Volcanics andesites and rhyolites. rhyolites. Clastic sedimentary rocks of the Oronto Oronto Group overlie overlie the volcanic volcanic rocks. rocks. Only Only the the basal basal unit, the Copper Harbor Conglomerate, Conglomerate, is is preserved in most of of the the St. St. Croix Croix horst horst where where as as krn of sandstone and conglomerate conglomerate lie along along the the axis axis of of the the Ashland Ashland syncline. syncline. The The much as 2 km Copper Harbor thins to only a few tens of meters toward the northern northern end end of of the the horst horst in in the the same areas where the volcanic section also shows substantial substantial thinning. thinning. Apparently Apparently the the area area now comprising the northern part of the St. St. Croix Croix horst did did not not subside subside nearly nearly as as deeply deeply as as parts farther to the southwest. This relatively positive relief persisted throughout throughout volcanic volcanic activity and deposition of the Copper Harbor Conglomerate. Conglomerate. The The overlying overlying Nonesuch Nonesuch Shale Shale Ashland syncline, syncline, maintains a relatively uniform thickness around the northern part of of the the Ashland suggesting that the topographic high was buried by that time. time. 9 I OOO' 92W 47OO 47-00, EXPLANATION EXPLANATION Bayfield Bayfield Group Group and and equivalent sandstones i:: Freda Sandstone Nonesuch Shale Copper Copper Harbor Harbor Conglomerate Conglomerate Gabbro granophyre Gabbro and and granophyre \\\ Minong Volcanicslow-Ti basalts Minong Minong VolcanicsVolcanics- Y11 high.Ti basalts Chengwatana Volcanics Chengwatana Volcanics Kallander Creek Kallander Creek Volcanics Volcanics 1 Siemens Creek Siemens Creek Volcanics Volcanics Archean and Paleoproterozoic Paleoproterozoic rocks 46OO 46'00' 92°OO 0 30 60 90 KM Cannon, W.F., Daniels, D.L., Nicholson, S.W., SW., Phillips, Phillips, J., J., Woodruff, Woodruff, L.G., L.G., Chandler, Chandler, V.W., V.W., Morey, Morey, G.B., G.B., Boerboom, T., Wirth, KR., K.R.,and andMudrey, Mudrey,M.G., M.G., Jr., Jr., 2001, 2001, New New map map reveals reveals origin origin and and geology geology of North American American Midcontinent Midcontinent rift: EOS, EOS, v. v. 82, 82, no. no. 8, 8, pp. pp. 97-101 97-101 Chandler, Chandler, V.W., V.W., McSwiggen, McSwiggen, P.L., P.L., Morey, Morey, G.B., G.B., Hinze, Hinze, W.J., W.J., and and Anderson, Anderson,R.R., R.R.,1989, 1989,Interpretation Interpretationof of seismic seismic reflection, reflection, gravity, gravity, and and magnetic magnetic data data across across Middle Middle Proterozoic Proterozoic Mid-continent Mid-continentRift Rift system, system, northwestern northwestern Wisconsin, Wisconsin, eastern eastern Minnesota, Minnesota, and and central central Iowa: Iowa: American American Association Associationof of Petroleum PetroleumGeologists Geologists Bulletin, Bulletin, v. 73, 73, p. p. 261-275. 261-275. Nicholson, S.W., Boerboom, T., Cannon, Cannon, W.F., W.F., Wirth, Wirth, K. K. and and Isachsen, Isachsen, C.E., C.E., 2001, 2001, A A new new look look at at the the 1.1 1.1Ga Ga Boerboom, T., Chengwatana Chengwatana Volcanics Volcanics in the St. St. Croix Croix horst, horst, Minnesota Minnesota and Wisconsin, Wisconsin, Institute Institute on on Lake Lake Superior Superior Geology, Geology, v. 47, part 1, 1, p. 71-72. 71-72. 60 �of the Western Subcomplex of of the the Deadhorse Deadhorse Creek The Rare and Exotic Mineralogy of Diatreme, Northwestern Ontario. Ontario. Eric G. Potter and Roger Roger H. Mitchell egpotter@mail.lakeheadu.ca egpotter@mail.lakeheadu.ca Dept. of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON. P7B 5E1 5E1 The main mineralized zone of the western western subcomplex of the Deadhorse Creek diatreme exhibits complex involving: first first and second exhibits complex mineralization mineralization involving: second order order transition transition metals metals (specifically Sc, Ti, Ti, V, V, Cr, Mn, Mn, Fe, Fe, Zr Zr and and Nb); Nb); REE; REE; Be; Be; Th; Th; and and U. U. The The mineralization mineralization (specifically Sc, is manifested by the presence of of the the following following minerals: minerals: thortveitiite, thortveitiite, Sc-V-aegirine, Sc-V-aegirine,NbNbV-rutile, V-crichtonite, Ba-Mn-hollandite, zircon, monazite-Ce, monazite-Ce, xenotime-Y, uraninite, tyuyamunite, phenacite, thorite, thorogummite, thorite, thorogummite, barite, barite, barylite, tyuyamunite, phenacite, pyrite, pyrite, hematite, magnetite and several as of yet unnamed mineral species (Platt (Platt and and Mitchell, Mitchell, 1996; 1996;Smyk Smyk et a!., al., 1993; 1993; this study). Of Ofinterest interestininthis thispresentation presentation are: are:Nb-V-rutile, Nb-V-rutile, cnchtonite crichtoniteand and Sc-V-aegirine. Sc-V-aegirine. I I The Nb-V-rutile The Nb-V-rutile is enriched in Cr203, Cr203,with concentrations concentrations reaching Alkaline Igneous Suites 30 30 ,--. The 6.49 wt.%. wt.%. The enrichment enrichment of of &: Cr203 and Cr2O3 and Nb2O5 Nb2O5isis similar similar to that 80 A Xenoliths in Kimberlite of rutile reported in alkaline igneous reported alkaline igneous 20 2o rocks, as illustrated in an atomic rocks, illustrated an atomic t-1+ - percent percentplot plotofofcr3+ Cr3 ++ Nb5* Nb5 + ~Ta5a ~ ^ "0 1991). (Haggerty, vs. Ti4i ~ (Haggerty, vs. ~ * 1991). z+ lo However, the Nb205 However, the Nb205 contents contents are are 3 unusually high compared to alkaline compared Lunar Meteorites igneous igneous rocks in general, general, with 0 0 29.32 90 95 50 55 60 65 75 80 85 50 55 60 65 70 75 80 85 90 95 100 100 concentrations concentrations reaching 29.32 , <.a+ Ti 11 (Atomic (Atomic °') o,/o) wt.%. Such such Nb2O5 m205contents contents are are wt.%. which is is historically historicallyfound foundin in pegmatites. pegmatites. Also similar to those reported in ilmenorutile, which unique to the Deadhorse Creek Creek rutile rutile is the distinct of V203 (up to 10.52 unique distinct enrichment enrichment of V203 (up 10.52 wt.%) and the lack of tantalum. tantalum. I I I I I I I !! Deadhorse Creek contain the The Sc-V-aegirines present at Deadhorse The Sc-V-aegirines the highest reported of Sc203 Sc203 and V203 V203 (16.46 (16.46 and and 11.99 wt.%, wt.%, respectively). respectively). The concentrations of The only only other other have been occurrences of occurrences of VV- and andSc-enriched Sc-enriched aegirine aegirine have been reported reported from from alkaline alkaline a!., 1994; metasomatites associated with iron-ore deposits in Ukraine (Valter et al., 1994; Pavlishin presence thortveitiite (Sc2Si2O7) et al., al., 2000). 2000). OfOfnote noteis isthethe presenceofofboth both thortveitiite (Sc2Si207)and andSc-enriched Sc-enriched within the main main mineralized mineralized zone. zone. Although aegirine within Although the the source source of of the the Sc Sc in in the the aegirine aegirine remains somewhat somewhat conjectural, conjectural,itit appears appearsthat thatthe the Sc, Sc, V V and Na was remains was scavenged scavenged from alteration of the main mineralized mineralized zone by Fe-rich fluids. fluids. analogues of of crichtonite-(Sr) and The V-rich crichtonites are best termed vanadium-rich analogues senaite-(Pb). The Nb2O5in the crichtonites is peculiar, as the presence presence of Theenrichment enrichmentin inNb2O5 Nb has been a distinguishing distinguishing feature feature of of the the mantle-derived mantle-derived end end members members lindsleyite-(Ba) lindsleyite-(Ba) 61 �_____________________ ___________________ __________________ and mathiasite-(K) mathiasite-(K) (LIMA). (LIMA). Interestingly, Interestingly, the the crichtonites crichtonites plot plot in the theupper-mantle upper-mantle LIMA quadrant quadrant of of FeO ++ Fe203 LIMA Fe203++ MgO MgO vs. vs. TiO2 Ti02(Haggerty, (Haggerty, 1991), 1991), near the the LIMA LIMA compositions due to the the replacement replacement of of iron iron by vanadium. vanadium. The Nb-enriched rutiles mtiles and crichtonite crichtonite are believed to to have have Non-Kimberlitic Crichtpnite in formed aa formed relatively early in 30 C,,chtornte (Sr). Davthtc (UREB) Armalcolite Ouadrant multistage-alteration sequence of the the Deadhorse Creek diatreme by Senaite (Pb), 25 of stoichiometric rutile with reaction mtile Lovetingite (Ca). hydrous alkaline solutions hydrous solutions enriched enriched 20 1 These hydrous Nb and V. in Nb and V. hydrous Upper-Mantle LIMA Crichtonite alkaline solutions likely also also altered altered Armalcolite Armalcolite Ouand.rant Ouandrant 15 unnamed hydrated zircon to to an unnamed hydrated -DFIC Crichtomtc calcium zirconosilicate, zirconosilicate, which is is JIMA cric(ttonites__Pptle 10 found in association with the 56 52 56 60 64 68 72 60 64 68 crichtonite and rutile. rutile. Textural and and /o) TiO, (Wt.%) 1102 (Wt. compositional data suggest suggest that imparted the pervasive subsequent alteration alteration formed the Sc-V-aegirines Sc-V-aegirines and imparted pervasive hematitization hematitization to the the main main mineralized mineralized zone. zone. 35 I I I — I I References References Haggerty, S.E. (1991): of the upper (1991): Oxide Oxide mineralogy mineralogy of upper mantle. mantle. In: In: Oxide Oxide Minerals: Minerals: petrologic and 25, Mineral. Mineral.Soc. Soc. and magnetic magnetic significance. significance. Reviews Reviewsin inMineralogy, Mineralogy, 25, Amer., 335-416. 335-416. Platt, R.G. and and Mitchell, Mitchell, R.H. R.H. (1996): (1996): Transition Transition metal metal rutiles mtiles and titanates titanates from from the the Miner. Miner. Deadhorse Creek Diatreme complex, northwestern Ontario, Canada. Deadhorse Diatreme complex, northwestern Ontario, Canada. 403-413. Mag., 60, 60,403-413. Pavlishin, V.I., V.1., Baklan, Baklan, F.G., F.G., Bugaenko, V.M., Voznyak, D.K., Galaburda, Pavlishin, Bugaenko, V.M., Voznyak, D.K., Galaburda, Yu, A., A., Dekhtulins'ky, E.S., Donskey, O.M., Krivdik, S.G., Kulchic'ka, G.O., Mel'nikov, V.S., Radzivill, A, Ya. Ya. And And Zimbal, Zimbal, S.M. S.M. (2000): (2000): Science-based Science-basedperspectives perspectives of of improvement of of mineral mineral resources resourcesor orrare raremetals metalsinin Ukraine. Ukraine. Mineral., Journal, improvement 22, no.1, 1, 5-20. 5-20. (in Russian) Russian) 22, no. Smyk, M.C., Taylor, R.P., Jones, Smyk, M.C., Taylor, R.P., Jones, P.C. and and Kingston, Kingston, D.M. D.M. (1993): (1993): Geology Geology and and geochemistry of the West Dead Dead Horse Horse Creek Creek rare-metal rare-metal occurrence, occurrence, northwestern northwestern Ontario. Explor. Mining. Geol., Ontario. Geol., 2, no. 3, 245-251. 245-251. Sharkin, O.P. and and Yakolev, Yakolev, V.M. V.M. (1994): (1994): AAvanadian vanadian Valter, A.A., Khomenko, V.M., Sharkin, aegirine in alkaline alkaline metasomatites from Zheltye Vody. Dokiady DokladyAkademii Akademii Nauk Ukrainy, No. 3, 110-116. 110-116. (in Russian) 62 �methods: Possible Sibley Basin sediment provenance using zircon and and whole whole rock rock geochemical methods: source areas of the Pass Lake Formation Richardson, A., A., Fralick, Fralick,P., P.,and andHollings, Hollings,P.P.(Department (DepartmentofofGeology, Geology,Lakehead LakeheadUniversity, University, 955 Richardson, SE!, Canada; Oliver Rd., Thunder Bay, Ontario, P7B 5E1, Canada; ajrichar@mail.lakeheadu.ca) airichar@mail.lakeheadu.ca) [CANADA A. +4+ - + + 48 $upfls. + +4+1 + + + +1 USA L. Huron 4__u_,r ' —+ 4 4 +++ +4+ 4 30km 4+ ++4 ÷4+ LSGEND II. Proterozoic 4+4. 1097 Ma V Odor Group 1110 Ma *4 1.. 1 / 1537 EJ GranfteMa and Rhyolit. l800Ma Arilmikie Group AnImikle Group Archean Granific anmitic Rocks R W ~ ^] Metasedimentary Metasedlrnentary a MÇ.VOIC;~, .4* 4+ ++ + epa.s ÷+ >1339Ma Sibley Group 9 + Rocks Rocks 1 .1 Lake Superior The Sibley Group consists of Proterozoic Proterozoic sediments sediments that outcrop discontinuously over a 15000 15000 sq. km region in the area surrounding central and southern Lake Nipigon. Its age is bracketed bracketed by the underlying underlying Redstone Redstone Point Point Complex Complex (1537 (1537 +101-2 Ma; Davis and Sutcliffe, +/- 33 33 Ma Ma +lo/-2 Ma; Sutcliffe, 1985) 1985)and andaa1339 1339+1Rb-Sr age on diagenetically diagenetically altered altered Sibley Sibley sediments sediments(Franklin, (Franklin, 1978). The Sibley Group was divided into three formations: the Kama Hill Formation Formation (top), (top), the the Rossport Rossport Formation, Formation, and and the the Pass Lake Formation (bottom), by Franklin, et al. (1980). The The Kama Hill Formation Formation consists consists of aa laminated laminatedshale shale facies, facies,the the Rossport of mudstone mudstone and stromatolitic stromatolitic facies, facies, and and the the Pass Lake of a conglomeratic conglomeratic facies facies and and aa plane-bedded plane-beddedor or crosscrossbedded sandstone sandstone facies (Cheadle, (Cheadle, 1986). 1986).This This study study investigates investigates the sources sources that fed sediment sediment to to the the Pass Pass Lake Lake Formation in the southern portion of the basin. Formation southern portion basin. Regional granitic granitic sources sources may include: include: the the Mesoproterozoic Redstone Point anorogenic intrusion, Redstone Point anorogenic intrusion, mpjjQfions Neoarchean peraluminous Quetico Quetico granites, granites, and and Redstone Point Granite McKenzie granites. Of Of these, these,the theRedstone RedstonePoint Point isis more more highly evolved than the others others and contains contains abundant abundant Regional Granltes Granites R Regional zircon and a distinct distinct geochemical geochemical signature signaturewith with very very • Pass PassLake LakeFm, elevated values for the high field Fm. field strength strength elements elements (HFSE). (HFSE). Samples Samples were collected collected from from surface surface exposures exposuresat at several several locations l o c a t i o n(Fig. s ( ~ i1). ~ . Representative Representativesamples samplesof of the the Pass Pass Lake Lake Formation of the Sibley Group Group were were taken taken from from aa cliff cliff section section directly across from Pass Lake on Hwy. 587. Individual Individual beds beds were grouped into assemblages consisting of of up up to to 16 beds. beds. Bed thickness total of of thickness became finer and thinner up section. AA total 26 hand samples were obtained from the Pass Lake cliff and consisted of of fine fine to to medium medium grained grained sandstone. sandstone. Two additional additional Pass Lake Formation Formation samples samples were were obtained obtained from from road road cuts cuts further further up-section up-sectionthat that consisted consistedof of medium medium grained grained sandstone. sandstone. Figure Fiigure 1. 1. Regional Regionalgeologic geologicmap mapwith withsample sample locations. locations. Additional granitic granitic samples were obtained from from road cuts cuts along along Hwy Hwy 11/17 11/17 Hwy. 527 527 (Fig. (Fig. 1). One sample was was taken taken from from each each location. location. Samples and Hwy. of Redstone Point sandstones, and granite samples were previously obtained obtained by P. Fralick Fralick from the English Bay region of Lake Nipigon (Fig. (Fig. 1). 1). Figure Fiigure 2. 2. Backscatter Backscatter X-Ray X-Ray SEM-EDS image of a zircon from sample AR-Ol. AR-01. ICP-AES (inductively Coupled Plasma -- Atomic Emission Spectroscopy) ICP-AES (Inductively Spectroscopy) Samples were cut into approximately 4 x 3 x 0.5 cm sections sections and crushed to a fine powder of <30 microns. Chemical Chemical preparation preparationincluded included hydrofluoric acid digestion to remove all silica and allow complete complete solution solution of samples. Prepared Prepared samples samples were were analysed analysed at at the the Lakehead University Instrument Laboratory. 63 �SEM-EDS (Scanning (Scanning Electron Microscope -- Energy Dispersive X-Ray Microanalysis) SEM-EDS X-Ray Microanalysis) Samples were ground to 30 micron thin sections and cut into discs suitable for the SEM stage. Before Before analysis, analysis, samples samples were carbon coated to prevent charge build- up while being analysed. Samples analysed. Samples were were analysed analysed for for 50 50 seconds seconds with with an an accelerating voltage of 20 KeV, and a beam current of 0.475 pA using a JEOL 5900 5900 SEM SEM with a system system resolution resolution of of 139 139 University Instrument Instrument Laboratory. Laboratory. Images eV, at Lakehead University taken using using aa backscatter-electron backscatter-electron detector. detector. Zircons were taken Zircons were analysed for five elements: Zr, Y, Th, U, and Hf. The use of zircons in sediment provenance studies has been 1o*Y+Th+U limited to work done by Owen (1987) (1987) which involved employing hafnium content of detrital zircons in determining Figure 3. SEM-EDS SEM-EDS analyses analyses of zircons zircons from from the source of the upper Jackfork Sandstone and the Parkwood Fm sandstones (points), (points) Redstone Pass Lake Fm Formation. He Hecame came to to the the conclusion conclusion that that hafnium hafnium content content Point sandstones (squares), Redstone Point these zircons agreed with optical and cathodoluminescence of optical and cathodoluminescence (ranites (+), Granites (+),and andregional regional Archean Archean and modal analyses, and is a viable method for provenance Neoarchean granites (triangles), (triangles). determination. determination. This study is the first to use SEM-EDS SEM-EDS methods methods as as well well as as analyses for Y, Th, and U. U. Fig. results. The Fig. 33 shows shows zircon analysis results. The majority majority of of zircons zircons plot at Zr/Hf ZdHf ratio of significant population population show show aa Y, Y, Th, Th, U approximately 40 with relatively low amounts of Y, Th, and U, but a significant enrichment trend. The enrichment trend. The geochemical geochemical signature signature of zircons from both sandstones sandstones and granites show show 3000 of sediment similarity, and indicates local sourcing sourcing sediment A with a possible influence influence of of regional regional Archean Archean and and . Proterozoic felsic igneous intrusives. 2000 2000 0 Whole rock interpretation interpretation of of ICP-AES ICP-AES ppm Zr/%Ti02 geochemistry (Fig. 4) trends agree with SEM 11000 ow elemental distribution within samples. Immobile Immobile element ratios of the the Pass Pass Lake Lake sandstones sandstones tend tend to to enriched and nonfall on a mixing trend between J 0 0 25 50 125 150 175 sources. This O 25 50 ppm 75Nb/%Ti02 loo Iz5 150 175 enriched sourc&. This study study highlights highlights the possible ppm Nb/%T02 usefulness of using SEM-EDS generated data in concert with more traditional chemical chemical analyses analyses in Figure 4. Immobile Immobileelement elementplot plot of of ICP-AES ICP-AESanalyses. analyses, provenance studies. Pass Lake Lake sandstones sandstones (points) (points)plot plot in in similar similarfield field to to sandstones derived from, and overlying overlying Redstone Redstone Point granite (squares). Redstone Redstone point granite (+), and other (triangles) are also shown. granites (triangles) shown. . ' I A I References References Cheadle, Cheadle, B.A. (1986) (1986)Alluvial-playa Alluvial-playa sedimentation sedimentationin in the the lower lower Keweenawan Keweenawan Sibley Sibley Group, Group,Thunder ThunderBay BayDistrict, District, Ontario; Ontario; Canadian Canadian Journal Journal of of Earth Earth Sciences, Sciences,v. v. 23, 23, p. p. 527-542. 527-542. Davis, D., and Sutcliffe, of Sutcliffe, R., (1985) (1985) U-Pb ages from the Nipigon Plate and Northern Lake Superior. Geological Society of America Bulletin, Bulletin, 96, 96, 1572-1579. 1572-1579. Franklin, age studies, Report Report 2, geological Franklin, J.M., (1978) (1978) The Sibley Sibley Group, Group, Ontario; Ontario; in Rubidium-strontium isochron age Survey Survey of Canada, Canada,Paper Paper 77-14, 77-14,p.p.331-34. 1-34. Franklin, J.M., J.M., McIlwaine, Mcllwaine, W.H., Franklin, W.H., Poulsen, Poulsen, K.H. and Wanless, R.K. (1980) (1980) Stratigraphy and depositional setting of the Sibley Sibley Group, Thunder bay District Ontario, Ontario, Canada; Canadian Journal of Earth Sciences, v. 17, p.633-651. Owen, M. (1987) 57, No.5., 1-838. (1987)Hafnium Hafnium in in Detrital Detrital Zircons: Zircons: Journal Journal of of Sedimentary SedimentaryPetrology, Petrology,Vol Vol57, NOS.,1987., 1987.,p.p.83 831-838. 64 �A Magnetostratigraphic Magnetostratigraphic and andSecular SecularVariation VariationStudy Studyof of the theSibley Sibley Group Group Rogala, P. and Borradaile, G. (Department of Geology, Lakehead University, Thunder Rogala,B., B.,Fralick, Fralick, P. and Borradaile, G. (Department of Geology, Lakehead University, ThunderBay, Bay, brogala@lakeheadu.ca) Ontario, P7B 5E1, brogala@lakeheadu.ca) The Sibley Group is aa red red bed bed sequence sequence that that was was deposited deposited in in aasubsiding subsidingintracratonic intracratonic basin (Fralick and Kissin, 1995) 1995)overlying, overlying, in part, part, aa 1537+10-2 1537+10-2Ma Ma (Davis (Davisand andSutcliffe, Sutcliffe,1984) 1984) The Group anorogenic anorogenic granite-rhyolite granite-rhyolite complex. complex. The Group was was previously previously divided divided into into three three main main Formations: Pass Lake, Rossport, and Kama Kama Hill. An An unnamed unnamed Formation Formation and and the the Nipigon Nipigon Bay Bay Formation have recently recently been been added. added. The The Pass PassLake LakeFormation Formation consists consists of of the theconglomeratic conglomeratic Formation have Loon of the Fork Loon Lake Member Member and the the sheet-like sheet-like sandstones sandstones of Fork Bay Bay Member, Member, representing representing a The Rossport braided environment (Cheadle, (Cheadle, 1986). 1986). The Rossport Formation Formation is separated separated into the the braided fluvial environment Channel Island, Middlebrun Bay, and Fire Hill Members. The Channel Island Member is Channel Island, Middlebrun Bay, and Fire Hill Members. The Channel Island Member aa The cyclic unit interpreted interpreted to to be playa cyclic dolomite-shale dolomite-shale unit playa lake lakesediments sediments(Cheadle, (Cheadle, 1986). 1986). The Middlebrun Bay Member, considered a marker bed for the Sibley Sibley Group, Group, isis aastromatolitic stromatoliticunit unit that represents a migrating strandline. The The Fire Fire Hill Hill Member Member consists consists of mudcracked mudcracked red silt with mudchip conglomerates and sand of tectonic tectonic tilting tilting of of the the sand sheet sheet incursions. incursions. It signifies a time of The Kama basin. basin. The Kama Hill Formation Formation is not not subdivided, subdivided, and is is composed composed of of purple purple shales shales and and siltstones interpreted as mud flat deposits (Cheadle, (Cheadle, 1986). The Theunnamed unnamed Formation Formation isis divided divided These represent represent aa deltaic deltaic and and fluvial fluvial environment. environment. The TheNipigon Nipigon into two unnamed Members. Members. These Bay Formation consists of cross-stratified sandstone beds, beds, and and is thought to denote denote an an aeolian aeolian environment. environment. Samples were taken taken from Samples were from the the Pass Pass Lake, Lake, Rossport, Rossport, Kama Kama Hill, Hill, and and Nipigon Nipigon Bay Bay Formations for a paleomagnetic paleomagnetic study. study. The unnamed Formation was not sampled due to the lack of exposure. The ThePass PassLake, Lake,Kama KamaHill, Hill,and andNipigon Nipigon Bay Bay Formation Formation were were used used to to conduct conduct aa preliminary study of of the magnetostratigraphy of the the Sibley Group. The TheRossport RossportFormation Formation was was magnetostratigraphy of sampled from unoriented drill drill core, core, thus thus could could only only be be used used to to study study secular secularvariation. variation. The paleopoles calculated from the Pass Lake, Lake, Kama Kama Hill, Hill, and and Nipigon Nipigon Bay Bay Formations Formations have been plotted along an apparent polar wander path (APWP) defined by Elston et al. (2002) (2002) Elston et al. an apparent polar wander (APWP) defined (Figure 1). 1). The samples samples from from the the Pass Pass Lake Lake Formation Formation have have been been divided dividedinto intosample samplegroups groups corresponding to Quarry Island, Transitional to the Rossport Formation, and and an an outcrop outcrop at at Pass Pass The paleopole of the with a diagenetic Lake. Lake. The paleopole of the Quarry Quarry Island Island Group Group corresponds corresponds with diagenetic event event at 1978), and and the latter 1339±3 Ma (Franklin, (Franklin, 1978), latter two two groups groups have havepaleopoles paleopoles approximately 1339±33 approximately The Kama has an older associated with with an an early earlyKeweenawan Keweenawan overprint. overprint. The Kama Hill Formation Formation has older associated discordant paleopole paleopole and a younger paleopole paleopole that that is is located within within the 1500 Ma section of the that the the Sibley Basin formed formed prior prior to to this, this, as is apparent apparent polar wander wander path. path. This This suggests suggests that Sibley Basin supported by the recent discovery discovery of sedimentary sedimentary xenoliths within the 1537 1537 Ma Ma Redstone Redstone Point Point Formation has has paleopoles paleopoles that that lie lie on on the APWP near 1400 granite. granite. The Nipigon Bay Formation 1400 Ma and and 1100 Ma. The Thefirst firstpaleopole paleopolemay maybe beprimary primary or orrelated related to to the thediagenetic diageneticevent eventthat thataffected affectedthe the Pass Lake samples at 1339 OsierVolcanics. Volcanics. 1339 Ma. The Thelatter latterpaleopole paleopole correlates correlateswith with the the Osler The paleomagnetic study on a 90 90 cm cm core core section section from from the the Rossport Rossport Formation Formationrevealed revealed aa When this curve variation curve. curve. When curve was was compared compared to to typical typical secular secular variation variation curves curves secular variation (Butler, 1998; Tauxe, Tauxe, 1998), the the time-span time-span for for Sibley Sibley deposition depositioncan canbe be estimated. estimated. The 90 (Butler, 90 cm cm section was estimated to represent represent 2500 2500 to to 3000 years. years. This This can can be be extrapolated extrapolated to estimate estimate that the Rossport Formation could potentially represent 75 000 years of deposition. Rossport Formation could potentially represent 75 000 years of deposition. 65 �Figure Figure4.12 4.12 AAwell-defined well-definedProterozoic ProterozoicApparent ApparentPolar PolarWander WanderPaths Paths(APWP) (APWP)isisplotted plotted (after Elston et al., al., 2002). 2002). The ThePass PassLake LakePCA PCAcomponents componentsare aredesignate designatewith with QI, QI,T, T, and indicatethe theQuarry QuarryIsland, Island,Transitional, Transitional, and and Outcrop Groups. The TheKama Kama Hill Hill and 00totoindicate Formation is designate KH and the Nipigon Bay Bay Formation Formation is is NB. NB. The The PCA, PCB, and PCC PCC components components are are denoted denoted respectively respectively by A, B or C after the Formation short short form. Note that NB-C is a reversed pole on the back side of of the the globe. globe. Elston Elston et et al. al. (2002) (2002) has has provided a lower (Si) (Sl)and andupper upper (S2) (S2) Sibley Sibley Group Group pole pole based on data from Robertson (1973), as well as a pole for the Keweenawan Keweenawan Osler Osier Group (Ki) (Kl)and and lower lower Powder Powder Mill Mill Volcanics (K2). (K2). magnetic domains domains to to geological Butler, R.F. R.F. 1998. 1998. Paleomagnetism: Butler, Paleomagnetism: magnetic geological terranes, terranes, Department Department of of Geosciences Geosciences University (originally published published by http://www.~eo.arizona.edu/Paleomae/book/ (originally by Blackwell Blackwell University of of Arizona, Arizona,http://www.geo.arizona.edu/Paleomag/book/ Scientific Publications Publications in in 1992) 1992) Scientific Cheadle, B.A. B.A. 1986. Alluvial-playa Cheadle, Sibley Group, Group, Thunder Thunder Bay Bay District, District, Alluvial-playasedimentation sedimentationin inthe thelower lowerKeweenawan Keweenawan Sibley CanadianJournal JournalofofEarth EarthSciences, Sciences,23, 23,527-542. Ontario. Canadian 527-542. Davis, D.W. and Sutcliffe, R.H. R.H. 1984. Geological 1984.U-Pb U-Pbages agesfrom fromthe theNipigon NipigonPlate Plate and and Northern Northern Lake Lake Superior. Superior. Geological 96, 1572-1579. 1572-1579. Society of America America Bulletin, 96, Elston, D.P., Enkin, R.J., Baker, Baker, J.J. and and Kisilevsky, Kisilevsky, D.K. D.K. (2002). (2002).Tightening Tighteningthe theBelt: Belt:paleomagnetic-stratigraphic paleomagnetic-stratigraphic constraints constraints on deposition, deposition, correlation, correlation, and and deformation deformation of of the the Middle Middle Proterozoic Proterozoic (ca. (ca.1.4 1.4Ga) Ga)Belt-Purcell Belt-Purcell GeologicalSociety Society of of America Bulletin, 114, 114,619-638. Supergroup, United States and Canada. Geological 619-638. basin development in central North America: and Kissin, Kissin, S. S. 1995. Fralick, P. and 1995. Mesoproterozoic Mesoproterozoic basin development in America: implications implications of Sibley Group volcanism volcanism and and sedimentation sedimentation at at Redstone Redstone Point. Point. In: Petrology Petrology and and metallogeny metallogeny of of volcanic volcanic Proceedings of of the the International International Geological Geological and intrusive intrusive rocks rocks of of the themid-continent mid-continent rift rift system, system, Proceedings and Correlation Program, 336. Program, Project Project 336. Franklin, J.M. J.M. 1978. Franklin, in: Wanless, Wanless,R.K. R.K.and andLoveridge, Loveridge,W.D., W.D.,Rubidium-strontium Rubidium-strontium 1978. The Sibley Sibley Group, Group, Ontario, Ontario, in: isotopic age studies, report report 2. Geological Geological Survey Survey of Canada Paper Paper 77-14, 77-14, 31-34. 3 1-34. W.A. (1973a). (1973a). Pole position position from from thermally thermally cleaned Sibley Sibley Group Group sediments sediments and and its its relevance relevance to to Robertson, W.A. Proterozoic magnetic stratigraphy. Canadian Canadian Journal Journal of of Earth Earth Sciences Sciences,10, 10,180-193. 180-193. Tauxe, L. 1998. KluwerAcademic Academic Publishers, Publishers, Netherlands, Netherlands, 299 p. 1998.Paleomagnetic Paleomagneticprinciples principlesand andpractice, practice,Kluwer 66 �Mafic Dikes Dikes in in Marquette Marquette County, County,Michigan Michigan with with Sequence of Precambrian Precambrian Mafic Sugarloaf Mountain Mountain and and Republic Republic Areas emphasis on the Sugarloaf Sandin and and T.J. T.J. Bornhorst Bornhorst (Department of Geological and Mining Engineering and N.A. Sandin Sciences, Sciences,Michigan Michigan Technological TechnologicalUniversity, University,Floughton, Houghton,MI MI49931) 49931) Precambrian Precambrian mafic mafic dikes dikes are arevery very common common throughout throughout Marquette MarquetteCounty, County,Michigan. Michigan.These These 1.1 Ga). dikes Archean(—2.7 (-2.7 Ga) Ga)to tomiddle middleProterozoic Proterozoic(— (-1.1 Ga). Past Past studies studiesby by Kantor Kantor dikes have haveages agesfrom fromArchean (1968), (1968), Gair (1969), (1969), Cannon Cannon (1974), (1974),and and Baxter Baxter and and Bornhorst Bornhorst (1988) (1988)have havesuggested suggestedup upto tosix six different of different mafic dike events in Marquette County. These events were interpreted to consist of (from Archean mafic mafic dikes dikes post-Archean post-Archean volcanism volcanism and and before before Archean Archean (from old to young): young): 1) 1) Archean granitoid intrusions intrusions which cut the Archean volcanic rocks; 2) Archean mafic dikes that cut Archean granitoid intrusions, intrusions, but but are are subjected subjectedto to Archean Archean deformation; deformation;3) 3)Archean Archeanmafic maficdikes dikes that cut Archean basement rocks, but do not cut Early Proterozoic sedimentary rocks of the that cut Archean basement rocks, but do not cut Early Proterozoic sedimentary rocks of the Marquette 4) Early Early Proterozoic Proterozoic mafic mafic dikes dikesthat thatcut cutMarquette MarquetteRange Range Marquette Range Range Supergroup; Supergroup;4) Supergroup Supergroup sedimentary sedimentaryrocks prior prior to to Penokean Penokean metamorphism metamorphism and and deformation; deformation;5-6) 5-6)N-S N-Sand and E-W E-W Keweenawan Keweenawan mafic mafic dikes. dikes. This This study study has confirmed confirmed much of Baxter and Bomhorst Bornhorst (1988), however, however, new new data data indicate indicatesignificant significantmodifications. modifications. This This study study focused focusedon on the the Sugarloaf SugarloafMountain Mountain area areanear near Marquette, Marquette,MI MIbecause becauseof ofthe the excellent excellent exposures exposures on on shore shore and and adjacent adjacent to to Lake Superior, Superior, and previous work by Kantor (1968), (1968), who identified identified mafic mafic dikes dikes of of multiple multiple ages. ages. In In the the Sugarloaf Sugarloaf Mountain Mountainarea areaover over300 300 mafic dikes Archean tonalitic tonalitic basement basement were were identified identifiedand andmapped mappedusing usingaaGPS GPS dikes intruding intruding Archean receiver receiver and and the the compass compass and and pace method. Dikes Dikesidentified identifiedas ascritically criticallyimportant importanttoto understanding the the sequence sequence of of events events were were sampled sampled for for microscopic microscopic and andchemical chemicalstudy. study. Baxter Baxter and and Bornhorst Bornhorst (1988) (1988) interpreted interpreted thin, thin, discontinuous, discontinuous, tabular tabular mafic mafic bodies at Wetmore post-plutoniclpre-deformation Wetmore Landing Landingin in the the Sugarloaf SugarloafMt. Mt.area areaas asbeing beingArchean Archeanpost-plutonic/pre-deformation mafic dikes (number (number 2 above). While this interpretation interpretation is still possible, the favored interpretation interpretation here is that these mafic bodies are xenoliths that were deformed during the Archean along with the the host host plutonic plutonic rocks. rocks. In Marquette Marquette County, County, Baxter Baxter and and Bornhorst Bornhorst (1988) (1988)as as well well as asprevious previousworkers workersrecognized recognized the numerous mafic intrusives intrusives that that cut cut Marquette Marquette Range Range Supergroup Supergroupsedimentary sedimentaryrocks rocks prior prior to to Penokean metamorphism metamorphism and and deformation. deformation.These These were were presumed presumed to to be be of of generally generallythe the same same age. This study indicates indicates that in the the Sugarloaf Sugarloaf Mt. Mt. area, area, three three age age separate separatemafic mafic intrusive intrusiveevents events of this age are present. present. Based Based on on cross-cutting cross-cuttingrelationships, relationships,the the sequence sequenceconsists consistsof of diabase diabase dikes trending N20°E, N60°E, and and diabase diabase dikes dikes trending trending east-west. In N20°Ediabase dikes trending N60°E In addition to cross-cutting cross-cuttingrelationships, relationships,these these groups groups can can be be discriminated discriminatedfrom fromeach eachother otherby by trace elements. elements. trace The of the Early Proterozoic Proterozoic dikes. dikes. In The N20°E N20° diabase dikes are the oldest of In the the Sugarloaf SugarloafMt. Mt. area, these these dikes dikes vary vary in in trend trend from fromN05°E N05OE to to N20°E N 2 0 2 and range in width width from from one one to to25 25feet. feet. Mafic dikes of this age are the most common of the Early Proterozoic dikes in the Sugarloaf Mountain area. These Thesedikes dikesexhibit exhibitaavarying varyingtexture texturefrom fromporphyritic porphyritic to to phaneritic phaneritic from fromthe the dike dike interiors interiors to to the the margins. margins. They They consist consist of of hornblende, hornblende, pyroxenes, pyroxenes, chlorite, chlorite,plagioclase, plagioclase, epidote, epidote, and sericite. sericite. The TheREE REEpatterns patternsare areenriched enrichedin inlight-REE light-REEwith withaamoderate moderateslope. slope. Compared to the REE patterns of the sills from the Marquette Range Supergroup, the Compared REE patterns of the sills from the Marquette Range Supergroup, theN20°E N20° series has a higher concentration concentrationof of light-REE, light-REE,is is less less depleted depletedin in heavy-REE, heavy-REE, and andhas hasaa shallower shallower slope. Thus, Thus,our ourinitial initialinterpretation interpretationisisthat thatthese thesedikes dikesare arenot notrelated relatedto tothe thesills. sills. 67 �The N60°E mafic dikes dikes are intermediate intermediate Early Early Proterozoic Proterozoic age. age. They vary in trend from N60° mafic N45°E N60°E and and range range in in width width from from one one to to 60 60 feet. feet. These These are are the the least least common of the Early N45OE to N60° Proterozoic dikes in the Sugarloaf Mountain area. They generally have They generally have thinly thinly foliated foliated margins margins fine-grained interiors. interiors. These dikes have a phaneritic phaneritic texture texture and and consist consist of of with a massive, fine-grained hornblende, pyroxenes, chlorite, chlorite, plagioclase, plagioclase, epidote, epidote, sericite, sericite,and and minor minoramounts amountsof of carbonate. carbonate. diabase dikes. dikes. REE N20°diabase REE patterns are enriched enriched in light-REE light-REE and have These dikes cross-cut the N20°E N20% series and the sills of the Marquette Range Supergroup, Supergroup, a steep slope. Compared Comparedto to the the N20°E the N60°E dikes are more enriched in light-REE with a steeper N60°dikes steeper slope. The TheN60°E N60°dikes dikesare aremore more depleted in heavy-REE interpretation is that these heavy-REE than the N20°E N20° series. Our initial interpretation these dikes dikes are are aa distinct distinct magmatic event event with with respect respect to to the the earlier earlier N20°E N20° dikes and the mafic sills. sills. of the the Early Early Proterozoic Proterozoic dikes. dikes. They The east-east diabase dikes are the youngest series of They vary in width from from five five to to 75 75 feet feet wide. These dikes generally have thinly foliated foliated margins with aa massive, fine-grained fine-grained interior, interior, although although two two dikes dikes had had porphyritic porphyritic interiors. interiors.They They have have aa phaneritic texture and consist of hornblende, pyroxenes, chlorite, plagioclase, epidote, and sericite. sericite. These Thesedikes dikescross-cut cross-cutthe theN20°E N20Z diabase diabasedikes dikesand andthe theN60°E N60Z diabase diabasedikes. dikes.Compared Compared to the REE patterns of the sills from the Marquette Range Supergroup, the east-west dikes have a higher concentration of light-REE and a steeper slope. The The east-west east-westdikes dikes are are depleted depletedin in the the compared to the N20°E N20% dikes. They are lower in light-REE and have a shallower shallower heavy-REE compared slope than the N60°E interpretation is that these dikes are N60° series. Our initial interpretation are a distinct distinct magmatic magmatic event from the earlier earlier dikes dikes and and the mafic mafic sills. sills. Mt. Area. We propose that these There are three distinct mafic dike events in the Sugarloaf Mt. dikes are not related to the mafic mafic sills sills that that cut cut the the Marquette MarquetteRange Range Supergroup. Supergroup.IfIf true, true,then then and likely likely more, more, Early Proterozoic mafic magmatic magmatic pulses pulses in in the the there must be at least 4, and Marquette area. Marquette area. Two groups groups of unmetamorphosed diabase dikes were identified in the Sugarloaf Mt. area, consistent with Baxter and Bornhorst (1988). These Thesedikes dikesare are Keweenawan Keweenawan in in age age and and consist consist of of diabasictexture texture a north-south trending series and an east-west trending series. Both Both dikes dikes have have aa diabasic and vary from 10 10 to 75 75 feet wide. that some some metamorphosed metamorphosed In the Republic area, Baxter and Bornhorst (1988) suggested that mafic dikes with distinct distinct plagioclase plagioclase phenocrysts phenocrysts are are older older than the the metamorphosed metamorphosedProterozoic Proterozoic dikes in the Sugarloaf area. area. They proposed that these dikes dikes might might correlate correlate with with the the Matechewan Matechewan dike swarm north of Lake Superior Superior in Canada. Canada. We We tested tested this this hypothesis hypothesis by by doing doing chemical chemical analysis of these dikes. The REE data data for for these dikes dikes are are similar similar to to Matechewan Matechewan dikes dikesfrom from elsewhere and support support the hypothesis proposed proposed by Baxter Baxter and and Bornhorst Bornhorst (1988). (1988). References References Baxter, D.A. and Bornhorst, T.J., 1988, 1988, Multiple Discrete Mafic Intrusions of Archean to Keweenawan Age, 2 pp. western Upper Peninsula, Michigan: Michigan: Institute Institute on on Lake Lake Superior Superior Geology Geology Proceedings Proceedings and and Abstracts, Abstracts,v.v.34, 34,2 pp. U.S. Cannon, W.F., 1975, 1975, Bedrock Geological Map of the Republic Quadrangle, Marquette County, MI: U S . Geological Miscellaneous Investigations Survey, Miscellaneous InvestigationsSeries SeriesMap, Map,1-862. 1-862. Gair, J.E. and Thaden, R.E., 1968, 1968, Geology Geology of the Marquette and Sands Quadrangles, Marquette County, MI: U.S. 77 pp. Geological Survey Survey Professional Professional Paper Paper 397, 397,77 pp. Hails, H.C. and Phinney, Halls, Phinney, W.C., 2001, 2001, Petrogenesis Petrogenesisof of the the Early Early Proterozoic ProterozoicMatachewan MatachewanDyke DykeSwarm, Swarm,Canada, Canada,and and Implications for Magma Emplacement 22 38,22 Implications Emplacement and Subsequent Subsequent Deformation: Deformation: Canadian Canadian Journal Journal of of Earth Earth Sciences Sciences38, pp. PP. Kantor, J.A., 1969, 1969, Assimilation Assimilation and and Dike Dike Swarms Swarms in in the the Sugarloaf Sugarloaf Mountain Mountain Area, Area, Marquette MarquetteCounty, County,MI: MI:M.S. M.S. Thesis, Michigan Technological Technological University, University, Houghton, Houghton, MI, MI, 83 83 pp. pp. 68 �PALEOPROTEROZOIC DEVELOPMENT OF A GNEISS DOME CORRIDOR IN THE SOUTHERN LAKE SUPERIOR SUPERIOR REGION, REGION, USA SCHNEIDER, D.A., Dept. of Geological Sciences, Sciences, Ohio Ohio University, University, Athens, Athens, OH OH 45701; 45701; HOLM, D.K., and O'BOYLE, O'BOYLE, C., C., Dept. Dept. of of Geology, Geology, Kent Kent State State University, University, Kent, Kent, OH OH 44242; 44242; HAMILTON, M., Continental Continental Geoscience Division, Geological Survey of Canada, Ottawa, ON Canada; M., Dept. Dept. of of Geosciences, Geosciences, U-Mass, U-Mass, Amherst, Amherst,MA MA 01003 01003 Canada; and JERCINOVIC, JERCINOVIC, M., Paleo-reconstruction of a Paleo-reconstruction of the Penokean orogen at ca. 1750-1700 1750-1700 Ma reveals the presence of narrow corridor corridor of Archean Archean cored cored Paleoproterozoic Paleoproterozoic gneiss gneiss domes domes just north north of of and andparallel parallel to tothe the main suture zone in Minnesota, Wisconsin, and northern Michigan. Penokean Penokean (ca. (ca.1850 1850Ma) Ma) metasedimentary rocks infolded within the domes give predominantly 1750-1700 1750-1700 Ma cooling ages and are overlain depositionally depositionally by ca. ca. 1700 1700Ma Ma Baraboo Baraboo interval interval quartzites. quartzites.We Weconducted conducted U-Pb SHRIMP and total-Pb EMP geochronometry to obtain metamorphic timing constraints on distinct distinct monazite mineral domains domains from from amphibolite amphibolitegrade grade rocks rocks sampled sampledacross acrossthe theentire entirelength length of the gneiss dome corridor. Based Basedon onmetamorphic metamorphicmonazite monazite crystallization crystallizationages, ages,midcrustal midcrustal amphibolite amphibolite facies metamorphism (Ml) (Ml) peaked peaked around around 1830 1830Ma Ma and and was was concurrent concurrentwith with late late Penokean plutonism; subsequent thermal pulses are reliably recorded at ca. 1800 Ma (M2) and again at ca. 1765 1765 Ma (M3), (M3), both also also coeval coeval with with magmatic magmatic activity. activity. Ar-Ar mineral mineral age data, data, which which indicate indicate The youngest monazite ages overlap with abundant Ar-Ar widespread cooling cooling of the gneiss gneiss dome dome corridor corridor immediately immediately following followingM3. M3. We We propose propose that that the the of the tectonically buried gneiss domes formed at this time during structural modification of continental margin rocks. In Inour ourconceptual conceptual model model (Fig. (Fig.1), I), northward northward vertical vertical extrusion extrusionof of aa midcrustal block containing decoupled midcrustal containing the the gneiss gneiss dome dome corridor corridor accommodated accommodatedgravitational gravitational collapse of overthickened crust. Elevated Elevatedcountry country rock rock temperatures temperatures accompanied accompaniedwith with profuse profuse promoted doming of the lower melting (i.e., intrusion of the East-central Minnesota batholith) promoted density Archean basement into the more more dense dense overlying overlying Paleoproterozoic Paleoproterozoicmetasedimentary metasedimentary rocks, ultimately enabling its complete decoupling from the remaining lower crust. This This process, process, redistribution of of crustal crustal mass from thick to primarily driven by buoyancy forces, allows for the redistribution thin regions without significant horizontal crustal extension. Tectonic Tectonicextrusion extrusionand andcrustal crustal thinning at this stage stage may have have been facilitated facilitated by aa decrease decrease in in horizontal horizontal compressive compressivestresses stresses acting on the region from from the south south (i.e., (i.e., Yavapai Yavapai slab slab rollback rollback as as proposed proposed by by Hoim Holm et et a!., al., ILSG, 2003). In In our ourmodel model(Fig. (Fig.1), I),the thefaults faultsbounding bounding the the gneiss gneissdome domecorridor corridorare areca. ca. 1765 1765 Ma structures, structures, although some, some, like like the the Niagara Niagara Fault Fault zone, zone, are are reactivated reactivatedPenokean Penokean structures. structures. of the Malmo Structural We note that in east-central Minnesota, a significant portion of discontinuity juxtaposes post-Penokean plutons to the south against older metamorphic rocks to discontinuity of Mille Mule Lacs). This the north (west of This clearly clearly supports supports our interpretation that this structure (and the Flambeau Flowage fault equivalent in northern Wisconsin) was active well after Penokean orogenesis. orogenesis. Holm, D.K., Van Schmus, W.R., MacNeill, L.C., Boerboom, T.J., Schweitzer, D., and Schneider, Schneider, D.A., 2003, 2003, Late Paleoproterozoic Paleoproterozoic(1900-1600 (1900-1600 Ma) Ma) tectonic tectonic history history of of the the northern northern mid-continent, U.S.A.: Implications for crustal stabilization: Institute on Lake Superior mid-continent, Institute on Lake SuperiorGeology Geology abstracts (this abstracts (this volume). volume). 69 �Penokeanorugm, orogen,Ml: Ml: 1830 Ma to M2: Penohan 1830 Ma M2: 1800 1800 Ma Ma N N s$ warm, Penokean orogen, M3: 1768 Ma N ONEISS GNEISS DOME DOME CORRtDOR CORRIDOR WISCONSIN WISCONSIN MAGMATIC MAGMATIC TERRANE TERRAME (uveniIe Quvimiie island iaiad an;) arc) ARCHEAN GRANITE-GREENSTONE GRAWE-GREENSTONE ARCHEAN GNEISS E PALEOPROTEROZOIC ROCKS (supracrustal) Figure Figure 1. 1. Schematic SchematicN-S N-Scross-sections cross-sectionsatat1830-1800 1830-1800Ma Ma (A) and (B) depicting depictingthe the proposed proposed evolution evolution of ofthe the gneiss gneiss dome 1765 1765 Ma Ma (6) northern Wisconsin. Note corridor in northern Note relative relative locations locationsof of gray gray circles circles that represent represent depth depth of of crustal cruslaf blocks. blocks. 70 s$ �suprasubduction zone A Paleoproterozoic suprasubduction zone ophiolite-island ophiolite-island arc arc complex complex northeastern Wisconsin in northeastern Wisconsin Schulz, Klaus Schulz, Klaus J., J., (U.S. (U.S.Geological GeologicalSurvey, Survey,Reston, Reston,VA VA20192, 20192,kschuiz@usRs.Rov) kschulz@usgs.gov) The Paleoproterozoic Paleoproterozoic volcanic and associated associated intrusive rocks exposed in northeastern Wisconsin are the easternmost easternmost exposures exposures of the Pembine-Wausau terrane, the northernmost of the two Wisconsin magmatic terranes that were accreated to the southern margin of the Archean Archean Superior SuperiorCraton Craton during during the the Penokean Penokean Orogeny Orogeny (Sims (Simsand andothers, others, 1989). The The rocks rocks of of the the Pembine-Wausau Pembine-Wausau terrane are separated separated from the epicratonic epicratonic sedimentary rocks of the Marquette Marquette Range Supergroup sedimentary Supergroup to the north in Michigan Michigan by by the the Niagara fault fault zone. zone. The volcanic rocks of the Pembine-Wausau terrane exposed northeastern Wisconsin, formed synformed at about about 1,870 1,870Ma Ma and and are are cut cut by by aa variety variety of intrusive intrusive rocks rocks ranging ranging from from synvolcanic gabbros, diorites, and tonalities to syn-and post-tectonic granitoids (i.e., Dunbar Gneiss and related rocks). The Thevolcanic volcanicrocks rocksare aredivided divided into into four fourfault-bounded fault-boundedunits, units, the Quinnesec, Quinnesec, McAllister, Beecher, and Pemene formations. These Theseunits units are areinterpreted interpreted to record the evolution evolution of of aa Paleoproterozoic Paleoproterozoicsuprasubduction suprasubductionzone zone ophiolite-island ophiolite-islandarc arc complex, complex, the Pembine Pembine ophiolite-arc ophiolite-arccomplex. complex. The Quinnesec Formation is the oldest volcanic unit and consists predominantly of pillowed basalt flows and massive diabase, but includes andesite and rhyolite lava flows and fragmental rocks locally. Several Severallarge largegabbro gabbro sills sills are are present, present, particularly particularly near near the Niagara fault zone, some with peridotite and pyroxenite layers. In In addition, addition,aa large large serpentinized serpentinized peridotite-gabbro peridotite-gabbrobody that produces produces aa large large positive positive magnetic magneticanomaly anomalyisis exposed Timrns Lake Lake (Morgan (Morgan County County Park) Park) east east of of Pembine, Pembine,Wisconsin. Wisconsin. exposed south south of Timms Serpentinized Serpentinized pendotite peridotite is is dominant dominant in in the the western western part of of this this body where it is locally locally cut by coarse-grained coarse-grained (1-5 cm) dikes of pyroxenite. Layered Layered and massive massive gabbro gabbro and and masses of strongly strongly foliated-lineated foliated-lineatedgabbro gabbro are are dominant dominant in in the the eastern eastern part part of of the the body body where they are cut by numerous mafic dikes dikes with diabasic diabasic to to microdioritic microdioritictextures; textures;some some of the dikes dikes appear appear to be sheeted. sheeted. The rocks of the Quinnesec Quinnesec Formation Formation appear appear to to record record the the birth birth and and youth youth stages stagesof of aa zone ophiolite ophiolite (Shervais, (Shervais, 2001). 2001). Rocks formed fonned during the initial phase suprasubduction zone of ophiolite ophiolite evolution evolution typically typically include include layered layered and and isotropic isotropic plutonic plutonic gabbros, gabbros,sheeted sheeted dikes, and a "lower" volcanic section consisting of low-K tholeiitic basalt and basaltic consisting MORB and primitive arc tholeiite affinities. Gabbros Gabbros formed formed during during this this andesite with MORE stage are often ductilely deformed (foliated or boudinaged) in response response to syn-magmatic syn-magmatic extension. extension. Rocks Rocksformed formedduring duringthe thesecond secondor oryouth youth stage stageof of ophiolite ophioliteformation formationinclude include intrusive intrusive mafic-ultramafic mafic-ultramafic sills and diabase dikes, and an "upper" volcanic volcanic unit characterized characterized by basalt and and andesite andesite with highly highly depleted depleted incompatible incompatibletrace trace element element compositions compositions (i.e., low-Ti basalt, high-Mg andesite and boninite) (Shervais, 2001). Compositionally, Compositionally,the Quinnesec Quinnesec basalts basalts and and gabbros gabbros are are tholeiitic, tholeiitic,with with generally generallylow low Ti02 and TiOz and other other high field field strength strength element abundances, and flat to extremely light REE 71 �depleted patterns (Sims and others, 1989). 1989). In In addition, addition,some some of of the the basalts, basalts,gabbros, gabbros, and and andesites REE abundances, abundances, but but relatively relativelyhigh high Cr Cr and andNi Ni andesites have very low Ti02 TiOzand and REE contents. The Thetrace traceelement elementcharacteristics characteristicsof of the the mafic mafic rocks rocks overlap overlapthose those of of mid-ocean mid-ocean ridge basalts and primitive island-arc tholeiite suites whereas the andesites show fore-arc-relatedboninites. boninites. The presence in the upper part part of of compositional affinities with fore-arc-related Quinnesec Formation Formation of mafic rocks derived the Quinnesec derived from from highly highly refractory mantle mantle is is diagnostic of a relationship particularly diagnostic relationship to the early stages of intraoceanic intraoceanic subduction subductionand and This also also implies implies that that the the Quinnesec Quinnesec formation in a forearc setting (Shervais, 2001). This Formation Formation and associated associated rocks did not form in a back-arc basin near or on the margin of the Superior 1997),but but Superior Craton, Craton, as has recently been proposed (Van Wyck and and Johnson, Johnson, 1997), rather formed as an intraoceanic intraoceanic ophiolite-arc ophiolite-arc system system above a southward southward dipping dipping (in (in coordinates) subduction zone. present coordinates) The McAllister, McAllister, Beecher and Pemene formations consist of volcanic and volcaniclastic rocks ranging from from andesite andesite (McAllister) (McAllister)to to rhyolite rhyolite (Pemene), (Pemene), all all with with caic-alkaline calc-alkaline compositions compositions characteristic characteristic of mature oceanic arcs. These Thesevolcanic volcanic rocks rocksand andassociated associated Twelve Foot Foot Falls Falls Quartz Diorite, Diorite, appear intrusives, such as the Newingham Tonalite and Twelve compatible compatible with the third or maturity stage of suprasubduction zone ophiolite evolution (Shervais, 2001). Characteristic Characteristicof of this this stage stageare are intrusive intrusive rocks, rocks, such such as as hornblende hornblende rocks ranging ranging from from basalt basalt to to diorite, quartz diorite, and tonalite, as well as volcanic rocks rhyolite, all with transitional transitional to calc-alkaline calc-alkaline compositions. Volcanism Volcanismtypically typicallybecomes becomes more silicic with time in these sequences. In In many many cases, cases, rocks of of this this stage stage have have not been considered considered part of the subjacent subjacent ophiolite, but rather have been attributed to postophiolite ophiolite arc volcanism volcanism (Shervais, (Shervais,2001). 2001). It appears likely that growth of the Pembine Pembine ophiolite-arc ophiolite-arccomplex complex was was terminated terminatedby by its its collision obduction onto the passive southern margin of the Superior Craton. collision with and obduction Because subduction appears appears to to be be largely largely driven driven by slab slab pull, pull, the the southward southwardsubduction subduction of oceanic lithosphere lithosphere attached attached to the Superior continental margin would have pulled the continental continental lithosphere lithosphere along with it as it descended into the subduction zone below the ophiolite-arc system. With With detachment detachment of of the the subducting subducting oceanic oceanic lithosphere, lithosphere,the the buoyancy of the continental continental lithosphere lithosphere would have have led led to to its its rapid rapid uplift uplift along alongwith withthe the leading edge of the ophiolite-arc ophiolite-arc complex (Shervais, 2001). This Thisstage stageisisrecorded recordedby by the the deformation of the syn-to to post-tectonic post-tectonic the ophiolite-arc ophiolite-arcsequence sequence and and by by the the intrusion intrusion of of the the synunits of the Dunbar Dunbar dome. dome. Shervais, J.W., 2001, Birth, death, and resurrection: resurrection: the life cycle of suprasubduction zone ophiolites: Geochemistry Paper number number 2000GC000080. 2000GC000080. On-line publication publication at Geochemistry Geophysics Geosystems, vol.2, Paper http://g-cubed.org. http://e-cubed.org. P.K., Van Schmus, W.R., W.R., Schulz, K.J., K.J., and and Peterman, Peterman, Z.E., Z.E., 1989, Tectono-stratigraphic Tectono-stratigraphic evolution evolution of of Sims, P.K., the Early Proterozoic Proterozoic Wisconsin Wisconsinmagmatic magmatic terranes terranesof of the the Penokean PenokeanOrogen: Orogen:Canadian CanadianJournal Journalof ofEarth Earth Sciences, v. 26, 158. Sciences, 26, p. p. 2145-2 2145-2158. Van Wyck, N., and and Johnson, Johnson, C.M., C.M., 1997, 1997,Common Common lead, lead,Sm-Nd, Sm-Nd,and and U-Pb U-Pbconstraints constraintson onpetrogenesis, petrogenesis, crustal architecture, and tectonic setting of the Penokean orogeny (Paleoproterozoic) in Wisconsin: crustal architecture, and tectonic setting of the Penokean orogeny (Paleoproterozoic) in Wisconsin: Geological Geological Society Society of America America Bulletin, Bulletin, v. v. 109, 109,p. p. 799-808. 799-808. 72 �THE THE LAKE NIPIGON GEOSCIENCE GEOSCIENCE INITIATIVE INITIATIVE -- PLANNED ACTIVITIES ACTIVITIES AND AND OBJECT! VES OBJECTIVES SMYK, Mark C., C., Ontario Ontario Geological Geological Survey, Survey, Ministry Ministryof ofNorthern NorthernDevelopment Developmentand andMines, Mines, SMYK, Mark Suite B002,435 B002, 435 James James St. St. South, South, Thunder Thunder Bay, Bay, ON ON P7E P7E 6S7, 6S7,and andmembers membersof ofthe theScientific Scientificand and Implementation Committees, Conmiittees, Lake Lake Nipigon Nipigon Geoscience Geoscience Initiative, Initiative,c/o do Ontario Prospectors Association, 1000 Association, 1000 Alloy Drive, Drive, Thunder Thunder Bay, Bay, ON ON P7B P7B 6A5 6A5 was created created in in 2002 2002 as as aa $7.0 $7.0 M M Cdn. Cdn. project project aimed at The Lake Nipigon Geoscience Initiative (LNGI) was the area area around Lake Lake Nipigon. Nipigon. The The Ontario OntarioProspectors Prospectors Association's Association's attracting mineral investment to the (OPA) portion of the project project is is funded funded through through an an agreement agreementwith with the the Northern Northern Ontario OntarioHeritage HeritageFund. Fund.The The OPA is partnering with the Ontario Ontario Geological Geological Survey Survey (OGS), (OGS), the the Ministry Ministry of Northern Northern Development Developmentand and Mines (MNDM), the Canadian Canadian Mining Mining Industry Research Research Organization Organization (CAMIRO), (CAMIRO),Lakehead Lakehead University, University, as well as with private sector partners and communities communities in the Lake Nipigon area. area. It will will focus focus on on four four key key objectives: objectives: 1. Maintain 1. Maintain and then increase increase mineral mineral investment investmentin in the the Lake Lake Nipigon region region through through collection collectionof of high high quality geological data and provision provision of interpretations interpretationsthat meet meet the the needs needs and and priorities priorities of of the the mineral mineral industry and that maintain or attract mineral investment investment to Ontario; 2. 2. Increase the mineral exploration discovery discovery rate by addressing addressing "masking and deep deep search search challenges challenges and skill gap" in the area; area; 3. Respond nickel3. Respond to, and and evaluate, evaluate, new new and and exciting exciting mineral mineral deposit deposit models models recently recently recognized recognizedfor fornickelcopper, copper, palladium-platinum, palladium-platinum,and and gold-copper gold-coppermineralization mineralizationin in the the region; region; 4. 4. Reinforce Reinforce and and demonstrate demonstrate an an innovative innovativeeconomic economic development developmentmodel model based based on on local localcommunity, community, industry, industry, and government government partnerships partnerships in in geoscience geoscience that result result in in mineral mineral resource resource economic economic development development in the the local local communities, communities, the the region, region, and and Ontario. Ontario. The LNGI is Embayment, which which consists consists predominantly predominantly of of is focused focused on on the the Nipigon Nipigon Basin Basin // Embayment, Mesoproterozoic, Mesoproterozoic, Midcontinent MidcontinentRift-related, Rift-related,ultramafic ultramaficto to mafic mafic intrusions intrusionsthat that have haveintruded intruded Mesoproterozoic Mesoproterozoic Sibley Sibley Group Group sedimentary sedimentaryrocks rocks and and Archean Archean basement basementrocks rocks of of the theQuetico Queticoand and Wabigoon Wabigoon subprovinces. subprovinces. The project will develop a comprehensive will assist in in mineral mineral exploration. exploration. The comprehensive geoscience database that will The LNGI evolved evolved through aa series series of of community community and industry industry consultations consultations that helped define define the the project project parameters. AAthorough thoroughcompilation compilationof of previous previousexploration explorationand and geological geological data data provided provided aa baseline baseline for for the project and identify the geoscience database. The identify potential potential gaps gaps .in in the Themain main components componentsof ofthe theinitiative initiative include: include: • • • • • • • • Detailed geological geological mapping, mapping, undertaken undertaken by by Precambrian Precambrian Section, Section, OGS OGS Airborne Airborne magnetic magnetic survey survey Gravity Gravity survey survey Quaternary Quaternary (surficial) (surficial) case case studies, studies, undertaken undertaken by Sedimentary SedimentaryGeoscience Geoscience Section, Section,OGS OGS Geochronology Geochronology Physical property studies studies Geographic (GIs) compilation compilation Geographic Information Information Systems Systems (GIS) Complementary Complementary research at Lakehead Lakehead University • Sibley Group studies studies (P. (P. Fralick) Fralick) • Nipigon mafic intrusion intrusion studies studies (P. (P. Hollings; Hollings; G. G. Borradaile) Borradaile) • Sulphide Sulphide mineralization mineralization studies studies (S. (S. Kissin) 73 �The Ontario Ontario Geological Geological Survey Survey will will help acquire acquire and and publish publish the the results results of of the the geoscience geosciencestudies studiesas asmaps, maps, reports, and digital data sets. The Theinformation informationwill will then then be be available available over over the the Internet Internet through through the the MNDM's MNDM's ERMES and CLAIMap CLAIMap systems. systems. This valuable valuable information information will be used to globally globally market market the the resource resource potential and investment investment appeal appeal of the the Lake Lake Nipigon region. region. 74 �TECTONOSTRATIGRAPHIC ARCHEAN T E C T O N O S T U T I G M P H I C ASSEMBLAGES ASSEMBLAGES OF O F EASTERN EASTERN WABIGOON SUBPROVINCE, NORTHWESTERN ONTARIO STOTT Greg ONyP3E P3E 6B5 6B5 STOTT GregM., M.,Ontario OntarioGeological Geological Survey, Survey, Sudbury, Sudbury, ON, (greg.stott@ndm.gov.on.ca), DAVIS, Don. W., Department of Geology, University of of (greg.stott@ndm.gov.on.ca)y Toronto, Toronto, Toronto, ON, ONyPARKER, PARKER,Jack JackR., R.,Ontario OntarioGeological GeologicalSurvey, Survey,Sudbury, Sudbury,ON, ON, Toronto, STRAUB, Kristan J., Laurentian University, University, Sudbury, Sudbury, ON ON and andTOMLINSONy TOMLINSON, Kirsty Y., Geological Survey of Canada, Ottawa, ON The Archean Wabigoon Subprovince Subprovince is a complex of volcanic and sedimentary supracrustal assemblages supracrustal assemblages and granitoid suites suites of Mesoarchean Mesoarchean to Neoarchean Neoarchean age. age. The The easternmost Onaman-Tashota greenstone belt easternmost part of this subprovince, subprovince, which includes the Onaman-Tashota east of Lake Nipigon, preserves aa history history of over over 250 250 million million years years of of volcanism. volcanism.This This area has recently been treated to a regional mapping, geochemical and geochronological 11250000 000 compilation compilation synthesis as part of the Western Superior Superior NATMAP project. A 1:250 map (Stott et al. 2002) arising arising from this project illustrates the subdivision of the OnamanTashota (O-T) (Figure 1), based (0-T) greenstone greenstone belt into tectonostratigraphic assemblages (Figure based on on stratigraphic correlationsy correlations, geochronological and geochemical similarities stratigraphic similarities and and contact contact relationships. Figure 2, 2,isisthe the relationships. A more interpretive interpretive component of this map, summarized in Figure delineation of the assemblages assemblages in in terms terms of of the the environment environmentof of crystallization crystallizationof of volcanic volcanic and plutonic rocks and deposition deposition of sedimentary rocks. This is based on lithologic and geophysical characteristics, characteristics, whole-rock whole-rock geochemical geochemical classification, classification,and and where where available, available, Nd isotopic isotopic signatures. signatures. The Onaman-Tashota Onaman-Tashotagreenstone greenstone belt straddles straddles the the width width of of the the eastern easternWabigoon Wabigoon Subprovince Subprovince between the English English River River and and Quetico Queticometasedimentary metasedimentarysubprovinces. subprovinces.ItIt isis mainly dacitic flows, flows, mainly composed composedof ofNeoarchean Neoarchean(dominantly (dominantly2.74 2.74—- 2.72 Ga) basaltic and dacitic autobreccia and autobreccia and pyroclastic pyroclasticrocks. rocks.Mesoarchean Mesoarchean(3.05 (3.05—- 2.92 2.92 Ga) volcanic rocks occur occur in the northwest northwest and and along along the the western western margin margin of of the the belt. belt. Widespread Widespread Nd Nd isotopic isotopicevidence evidence in the northern part of of the the Onaman-Tashota Onaman-Tashota belt belt suggests suggests that that Neoarchean Neoarchean volcanism volcanism erupted erupted through Mesoarchean Mesoarchean basement. basement. Basement Basement in in the the northern northern half half of of the the belt belt contains contains an older older component component than than that that south south of of the the Humboldt Humboldt Bay Bay High High Strain StrainZone. Zone.The The 2.74 Ga Willet Willet assemblage assemblagetholeiitic tholeiitic basalts basalts of of ocean ocean floor floor affinity affinity dominate dominatethe thenorthern northern half of the O-T calc-alkalic 0 - T belt. belt. This This assemblage assemblage is is flanked flanked to to the the north north and and south southby by calc-alkalic assemblages assemblages of continental continental margin margin arc arc affinity affinity that border border metasedimentary metasedimentarysubprovinces subprovinces composed of flysch-like wacke derived from the erosion of the O-T belt and composed flysch-like derived from the erosion of the 0 - T belt and plutons plutons during orogenesis orogenesis at circa circa 2.7 2.7 Ga. Ga. Most Most sedimentary sedimentary units units within within the the O-T 0 - T belt belt form formthe the youngest supracrustal supracrustal assemblages, assemblages, reflecting erosion of the the underlying underlying volcanic volcanic and and plutonic rocks towards towards the the English English River River and and Quetico Queticobasins basins to to the the north northand andsouth. south. Reference Reference Stott, G.M., Davis, D.W., Parker, J.R., J.R., Straub, K.J. and Tomlinson, K.Y. 2002. Geology and Tectonostratigraphic Tectonostratigraphic Assemblages, Assemblages,eastern eastern Wabigoon Wabigoon Subprovince, Subprovince,Ontario; Ontario;Ontario OntarioGeological GeologicalSurvey, Survey, 000. Preliminary Map P.3449, scale 1:250 1~250 Tomlinson, K.Y., Stott, G.M. and Davis, D.W. 2000. Nd isotopes in the eastern Wabigoon subprovince: implications for crustal recycling and correlations with with the the central central Wabigoon; Wabigoon; in in Harrap, Harrap, R.M. R.M. and Helmstaedt, H.H. (eds.), 2000, Western Superior Transect Sixth Annual Workshop, Lithoprobe Report #77, Lithoprobe Secretariat, Secretariat, University University of of British British Columbia, Columbia,p.119-126. p. 119-126. 75 �Figure Figure1. 1. Tectonostratigraphic Tectonostratigraphic assemblages assemblages of of the the Onaman-Tashota Onaman-Tashota greenstone greenstone belt and and Proterozoic diabase dike swarms, Eastern Wabigoon Subprovince. Subprovince. , +•t!4+ •-4..:L--•, .....a.4 ++ ++ + +++ •++ +÷++÷ + + + + .4 P + + + + + .4 + 4 + + ++ 4 + + + + .4 + + + + + + + + + 4 + + + .4 + .4 + + 4. + .4 + + + + 4 +ZP+ .4 + F ( + + + .._+ + ++ +4 + + .4 4+ + + + 4 + + + + + .4 + .4 .4 + + .4 + + + .4 + + + + + + + + + .4 + + + 4* + .4 + + + + Figure 2. Tectonic Figure Tectonic affinities assigned to volcanic and sedimentary sedimentary assemblages and assemblages plutonic suites. suites. + + .4 + +4+ + + + .4 + + 4 .4 + + + .4 ++t#4J + 4+4+ +±+ + + + + + •# 4 .4 + + + r++.— + '4.4 + + + + —-,"+ + +_ + + + 4.' + + + + 4 .t(__... (4. + + + +,I. + +1) + / / Lake Nipigon + + + + + + +c--.4+ + + + Proterozoic Proterozoic [ I- ' I I plume Continental p i m e related related m 4-L 11 - Orogenic plutons plutons orogenic orogenic sediments sedimen& Orogenic Continental arc Contmental Continental rnargm margin arc Cont~nsntal unsubdivided Continental unsubdivided 3 Ocean Ocwan floor floor Oceanic Oceanic unsubdivided unsubdivided Archean Archean Unknown Unknown tectonic tecton~caffinity afflnlty I0 10 0 0 7 Mesoarchean .Mesoarchean ..- - - -.-..- - Kilometres Kllometres ?//////A 76 20 20 ÷ �FIVE GOLD POSSIBILITIES IN SOME KEWEENAWAN COPPER SULFIDES SULFIDES IN IN ONTARIO AND MICHIGAN Trow, Jim, Trow! Jiml Geological Sciences, Sciences! Michigan State State University, University! emeritus, emeritusl 540 540 Lake Lake Avenue *2, #Z1 Hancoek, HancoC!kl Michigan Michigan 49930 49930 gold, Most fire—assayed fire-assayed "invisible" ltinvisiblell goldl from from .12 .12 to to 2.50 2.50 oz oz Au/st, Au/str occurs in minor covellite) in "blue I1bluechalcocite" c h a l ~ o c i t e(with (with ~~ covellite) but but not not in in black chalcocite (with (with no no covellite) covellite) on on the the adit, adit! 1st, lstl 2nd, 2nd1 and and 3rd levels of the Pointl Ontario. Ontario. the Coppercorp Coppercorp mine mine at at Mamainse Mamainse Point, Both occur occur with with specular specular hematite. hematite. Copper mineral zoning zoning exextending from carbonates and oxides through native copper, copper! black chaleocite and specularite, chalcocite specularitel "blue Itbluechalcocite" c h a l ~ o c i t eand ~ ~ specularite, specularitel to to bornite and chalcopyrite is related to nearness to the Keweenaw and related faults faults apparently down which circulated oxidizing solutions The solutions during an an upward-migrating upward-migrating hydrothermal hydrothermal episode. episode. The anomalies! whereas whereas former faults display positive SP electrical anomalies, nearly perpendicular cross faults faults with commercial ores ores display negative SP anomalies of this convective hydrothermal cell cell (Trow). (Trow). Such progressive oxidation of hydrothermal fluids fluids is is suggested suggested for for the Keweenawan of Michigan by the USGS's USGSts Woodruff, Woodrufff Cannon, Cannon! and and Back. Ontario, Trow deduces thermochemical calculations For Ontario! calculations with standard standard free energies and typical activities for constituents constituents (except (except for for oxygen, whose activities oxygenl activities are are the the unknowns). unknowns). These are arrayed arrayed on on a logarithmic scale which mimics the the observed copper copper mineral zones, zonesl AuS- first and in that sequence AUS-I first oxidized oxidized to to deposit deposit gold gold at at the the same oxygen activity at which chalcopyrite first first oxidized oxidized to to covellite and and specularite. specularite. At the present it it is is uncertain if if the the "blue chalcocite" iito chalcoCite and covellite from I1blue c h a l ~ o c i t eexsolved ~~ ivto chalcocite digenite at low temperatures, temperatures, or if if most of original covellite covellite was replaced by 2!500 times times the the oxygen oxygen by late late chalcocite chalcocite at at roughly roughly 2,500 activity at which which covellite covellite originally originally formed. formed. Essentials for gold at Coppercorp Coppercorp include include 1) 1) Keweenawan permeable permeable vesicularbeds conglomeratesl 2) 2) felsite felsite intrusives intrusives basaltic vesicular beds and and conglomerates, with permeable permeable border border breccias breccias as as conduits conduits for for rising rising hydrohydrothermal solutions, solutionsl 3) 3) nearness to to the the Keweenaw Keweenaw and and related related faults faults with positive SP anomalies, anomaliesl 4) 4) mineralized cross cross faults faults with with ores ores yielding negative SP anomalies, anomaliesr and 5) 5) "blue 'Iblue chalcocite". chalc~cite~~. In Michigan! Michigan, field examination of ore ore deposits deposits and and structures structures (Balticl Ashbed, Ashbed! mapped by the USGS shows shows that the the major lodes lodes (Baltic, Isle Royale, Royalel Pewabic, Pewabicl Osceola, Osceola, Calumet conglomerate, conglomeratet and and Kearsarge) Kearsarge) and the Cliff, Cliff! Central, Central! and Delaware Delaware fissure fissure deposits deposits all all display display Keweenaw, Hancock, Mayflowerl Mayflower, and negative SP SP anomalies. anomalies. The Keweenaw! Gratiot—Suffolk faults Gratiot-Suffolk faults all all display positive positive SF SP anomalies, anomaliesl approappropriate for downward oxidative contamination hypogene contamination of of rising rising hypogene (not supergene) supergene) mineralization. mineralization. From southwest southwest to northeast the the best matches to to Canadian Canadian gold gold in in Michigan examined, occur 1) from Mass City to the Indiana so far examinedl Michigan so Indiana mine adjacent to felsite felsite intrusives intrusives and and the the Keweenaw Keweenaw fault fault in in Ontonagon County, Countyl 2) 2) In In Houghton Houghton and and Keweenaw Keweenaw Counties Counties the the Allouez Allouez Gap fault between Copper City and and New Allouez Allouez is is near near the the Copper Copper , 77 �2 City felsite felsite and the the Keweenaw Keweenaw fault. fault. According to to Bornhorst, Bornhorstl page 132, 132! Randy Weege of C & H thought thought that this this fault fault perhaps perhaps was a fluid pathway for 60% of the district's districtls copper copper production. production. Further, it replicates replicates and improves Further! improves upon the the best geophysical geophysical signature Coppercorp, the persistent SB signature at Coppercorpl SB zone, zone! with with flanking flanking negative negative SP anomalies anomalies in in the midst of which which is is aa positive positive SP SP anomaly. llcorell splinters anomaly. In Michigan! Michigan, the positive "core" anomaly splinters westward off the northern northern end of the the negative negative anomalies anomalies in in the the vicinity vicinity of of Ahmeek. Ahmeek. This part of the the district district contains contains arsenic, arsenicl which accompanies accompanies gold in in many western western mining camps. camps. 3) In In 1999 1999 Bornhorst reported on the Maki and Bornhorst the 4½ 4% million million tonnes of chalcocite chalcocite in drilled amygduloids of the Gratiot deposit in in Keweenaw Keweenaw County, Countyl where these these beds are intruded intruded by dacite dacite (felsite). (felsite). This lode This lode appears appears at the intersection intersection with the the southward southward extension extension of of Trow's Trowls negative SP anomaly as as observed at at the the Central Central mine mine and and 2¼ 2% miles miles negative to to the the SSE. SSE. 4) 4) In Keweenaw Keweenaw County, Countyt the the USGS's USGS1s Hank Hank Cornwall Cornwall on on 166-167 describes describes minor traces traces of gold with with mainly mainly chalco— chalcopages 166-167 specularite and some some covellite covellite and and chalcopyrite chalcopyrite in in an an cite and specularite amygduloid amygduloid near near the the top top of of the the Greenstone Greenstone flow. flow. This is is not not near near faultl but it it is is cut cut by aa N.4°E. N.dOE. vertical vertical fault fault with with the Keweenaw fault, negative SP anomaly, anomalyl which which must be intersected intersected at at depth depth by by aa a negative N.4°E., 35°-45°NW. N.4OE.! 35O-45O~W.fault fault with a positive positive SP SP anomaly, anomalyt where where it it is is exposed to to the the east east of of the the vertical vertical fault. fault. There exists exists a possibility for a horizontal horizontal ore ore shoot shoot at at these 5 h b e faults' faults4 intersection. intersection. possibilities are plotted on on the the latest latest geologic geologic map map These four possibilities of the the Keweenaw Keweenaw peninsula, peninsulal by by Cannon Cannon and and Nicholson. Nicholson. Not yet yet reconnoitered possililities possililities may occur occur to to the the northeast northeast of of these. these. Remember, Remember! from from 1849 1849 to to 1961 1961 the the old timers timers all all missed the the Carlin Carlin "invisible" gold. llinvisiblell gold. Nevada is is now now the the biggest biggest gold gold producing producing state state because of the observationst observations, thinkingl thinking, and and Perseverance perseverance of of the the USGS's Ralph Roberts USGS1s Roberts and Mewmont's Newmontls John John Livermore. Livermore. REFERENCES CITED Bornhorst, T. J., Bornhorstl J e t1997, 1997! Tectonic Tectonic context context of of native native copper copper deposits deposits of the North A~erican American Midcontinent Midcontinent Rift Rift System, Systeml in in Geological Geological Society of America Special Special Paper Paper 312, 3121 p. 127—136. 127-136. Cannon, Cannon! W. F. and Nicholson, Nicholsonl S. S. W., W e 12001, 2O0lt Geologic Geologic map map of of the the Keweenaw Peninsula Peninsula and adjacent adjacent area, areal Michigan, Michigan! USGS USGS Geological Geological Investigations Investigations Series Series Map Map 1—2696. 1-2696. Cornwall, Cornwalll H. R., R e 1 1951, 19511 Differentiation Differentiation in in lavas lavas of of the the Keweenawan Keweenawan series and the origin Michigan! origin of the the copper copper deposits deposits of of Michigan, Geological Geological Society Society of of America America Bull. Bull. v. v. 62, 62# no.2, no.2# p. p. 159-201. 159-201. Maki, J. C., Makit C.! 1999, 199g1 The The Gratiot Gratiot chalcocite chalcocite deposit, deposit! Keweenaw Keweenaw Peninsulal Technological University, Universityl M.S. M.S. Peninsula, Michigan! Michigan, Michigan Technological Thesisl 71 p. p. Thesis, 71 Trow, J., Trow, J o t 1992, 1992! Inductive Inductive electrostatic electroskakic gradiometry gradiometry (IESG) (IESG) deciphers Keweenawan Keweenawan copper plumbing system, systeml Soc. SOC. Mining, Miningl Metall. and and Expl. Expl. Phoenix Phoenix Meeting, Meetingl Preprint Preprint 92—32, 92-32! 22 22 p. p. Woodruff, Woodrufft L. L. G., G e lCannon, Cannon! W. W. F., F.! and and Back, Back! J. J. M., M.! 1994, 1994# Chalcocite Chalcocite mineralization mineralization in in the the Portage Portage Lake Lake volcanics, volcanicst Keweenaw Keweenaw Peninsula, Peninsulal Michigan, 40th Michiganl 40th Ann. Inst. Inst. on on Lake Lake Superior Superior Geology, Geologyl Houghton, Houghtonl Abstracts, p . 77—78. 77-78. Abstracts! p. 78 �Using xenotime U-Pb geochronology to to unravel the history U-Pb geochronology history of ofProterozoic Proterozoicsedimentary sedimentary basins: a study basins: study in in Western Western Australia Australia and and the the Lake Lake Superior Superior Region Region McNaughton, N.J., N.J., Rasmussen, Rasmussen, B., B., Fletcher, Fletcher, I., I., Griffin, B.J., Vallini, D.A., dvallini@peol.uwa.edu.au, dvallini@aeol.uwa.edu.au, McNaughton, B.J., University of Western Australia, 35 Stirling Hwy, Crawley, 6009, Australia Diagenetic xenotime (YPO4) is aa trace trace constituent in a wide variety of siliciclastic (YPOJ is siliciclastic sedimentary sedimentary rocks. rocks. It typically forms pyramidal crystals of only aa few few microns microns in in size, size, rarely rarelyexceeding exceeding10 10pm, vm,growing growingon on [isostructural] detrital detrital zircons. zircons. A A recent study by Vallini et al. (2002) [isostructural] (2002) showed showed convincing convincing petrographic petrographic and age relationships that demonstrate demonstrate this U-bearing U-bearing phosphate phosphate could begin begin forming forming in in sediments sediments at at or just below below the sediment-water sediment-water interface, interface, shortly shortly after burial. burial. A few years ago itit was discovered discovered that itit 10 pm is possible to date xenotime crystals 210 vmininsize, size,using usingthe theSHRIMP SHRIMP(Sensitive (SensitiveHigh HighResolution ResolutionIon Ion for its formation, hence an age for early diagenesis and a Microprobe), providing aa robust isotopic age for close proxy for sediment deposition. Xenotime especially useful useful in in that that it has very high U contents Xenotime is especially and remains remains closed closed to toradiogenic radiogenicparent-daughter parent-daughter mobility, mobility, unlike unlike most mostother otherdateable dateablediagenetic diagenetic mineral. Diagenetic Diageneticxenotime xenotimeU-Pb U-Pbgeochronology geochronologyhas hasthe thepotential potentialto tounravel unravelthe thechrono-stratigraphy chrono-stratigraphy of unfossiliferous unfossiliferous sedimentary sedimentary basins, especially especially those sequences sequences devoid devoid of of dateable dateable interlayered interlayered volcanic basins where where aa lack lack of aa reliable volcanic rocks. Its Its main main application application isis in Precambrian Precambrian basins reliable temporal temporal affiliations, framework basin evolution evolution and framework hinders hinders an an understanding understanding of basin and maturation, maturation, tectonic affiliations, metallogeny and value as exploration metallogeny exploration targets. targets. Xenotime also forms during post-diagenetic Xenotime post-diagenetic fluid flow events, events, such such as as alteration, alteration,mineralisation mineralisationand and metamorphism, as as well well as being a magmatic metamorphism, magmatic mineral mineral and aa detrital detrital heavy heavy mineral. mineral. The The exceptional exceptional in situ its excellent excellent properties range of of xenotime, range of formation formation conditions conditions of xenotime, coupled coupled with with its properties for in situ geochronology, provide many many new new opportunities opportunities in in establishing establishing the the timeframe timeframe of of events geochronology, provide events in many many hitherto poorly understood understood sedimentary sedimentary basins. basins. Unusually coarse coarse (up (up to 200 crystals in the metametaUnusually 200 microns) microns) and and abundant abundant diagenetic diagenetic xenotime xenotime crystals sandstones of the greenschist greenschist facies facies Mount Mount Barren Barren Group, Group, southwestern Australia, allow the detailed detailed study of xenotime xenotime and its its host host rock. rock. Xenotime Xenotime occurs occurs within within aa phosphatic phosphatic sandstone sandstone interval interval and is is present in multiple different styles styles - as as cement cement overgrowths overgrowths on on zircons, zircons, pyramidal pyramidal present multiple morphologically morphologically different overgrowths on zircons, cement (no (no zircon) zircon) in in shale shale laminations, laminations, replacement replacement of of shale shale (?) (7)intraclasts intraclasts fluid events and as intraclasts. Multiple Multiple fluid events from from early early diagenetic diagenetic to low low as xenotime xenotime crystals crystals within within intraclasts. temperature/early temperaturelearly hydrothermal, hydrothermal,prior prior to to metamorphism, metamorphism,were were recorded recordedwithin withinsingle singlexenotime xenotimecrystals. crystals. U/Pb geochronology, accompanied by observations of petrographic relationships between SHRIMP UlPb between the various various generations generations of xenotime xenotime and and between between xenotime xenotime and other other diagenetic diagenetic minerals minerals and and pyrobitumen,allowed allowedfor for the the construction of a temporal pyrobitumen, construction of temporal framework framework for the the diagenetic diagenetic and and early early events that that occurred within these rocks; (1) ca 1700 hydrothermal events 1700 Ma: Ma: deposition deposition of of partly partly re-worked re-worked phosphatic siliciclastic siliciclastic sediments sedimentson on the the seafloor seafloor was was followed followed by in-situ of the phosphatic in-situ phosphatisation phosphatisation of sediments and an initial period 1697k 7 Ma), Ma), (2) (2) With With burial, burial, an an period of xenotime xenotime formation (mean age of 1697± early pore-filling pore-fillingcarbonate carbonate cement cement was introduced introduced into into parts parts of of the the interval, interval, as as well well as as early earlydiagenetic diagenetic cuboid pyrite ca 1650 1650Ma: Ma:during duringburial burialdiagenesis, diagenesis, aafluidfluid-movement movementevent eventcaused causedthe the pyrite growth, growth, (3) (3) ca Ma), with with partial dissolution of primary pore space and formation of xenotime (mean age of 1646 1646 ± 88 Ma), accompanying phosphate remobilisation, remobilisation, (4) Oil migration migration event, (5) Several Several silica silica cement cement generations generations ca 1560 1560Ma: Ma: minor minor addition addition of of xenotime xenotimerims rimsto toexisting existingovergrowths, overgrowths, introduced around this time, (6) (6) ca (7) ca 1480 1480 Ma: Ma: addition addition of of xenotime xenotime cement cement (no (no zircon) zircon) in in shale shale interlaminations, interlaminations, (8) (8) ca ca 1200 1200Ma: Ma: peak of metamorphism. metamorphism. Wavelength Wavelength Dispersive Dispersive Spectrometer Spectrometer (WDS) (WDS) microprobe microprobe analysis of each each type type of of xenotime xenotimeshowed showed with time, time. Due to this a gradual gradual change from LREE LREE enrichment enrichment to MREE MREE enrichment, enrichment, with this gradational gradational could not be established. nature, discrete boundaries between generations, based on chemistry, could This study study of of diagenetic diagenetic to to hydrothermal hydrothermalxenotime xenotimedramatically dramaticallyimproved improvedthe the estimated estimatedage agerange rangeof of the Mount Mount Barren Barren Group, Group, which which was was previously previously constrained constrained to to 1200 1200Ma Ma(peak (peakmetamorphism) metamorphism)and and 1790 1790 Ma Ma (youngest (youngest detrital detrital zircon zircon population), population), and and discounted discounted some some previous previous tectonic tectonic models models concerning the timing of of collision collision between between major major cratons cratons within within western western Australia Australia and andthese thesecratons cratons with East East Antarctica. Antarctica. Using the information gleaned from the study of xenotime xenotime in the Mount Mount Barren Barren Group, a similar study basin in in the Lake Superior Region is currently underway on another Proterozoic Proterozoic sediment-dominated basin containing and its equivalents, the North Range, containing the Marquette Marquette Range Supergroup Supergroup and equivalents, the Range, MilIe Mille Lacs Lacs and and Animikie Groups. Groups. The early early Proterôzoic Proterozoic strata strata consist consist of of three three unconformity-bounded unconformity-bounded lithostratigraphic groups consisting consisting of glaciogenics, glaciogenics, quartzites, quartzites, dolomite, dolomite, iron iron formation, formation, greywacke greywacke and and shale shale and and minor minor volcanics. Sedimentation is thought intercalated volcanics. thought to to have havebegun begun—2240 -2240 Ma Ma (correlation (correlationof of Chocolay Chocolay * 79 �Group with Gowganda Gowganda Fm, upper Huronian Huronian Supergroup, Ontario) (Fairbain et al., 1969) 1969) and ceased ceased by —1850Ma Ma(coinciding (coincidingwith with orogen-normal orogen-normalarc arccollision collision along along the the Niagara Fault zone and -1850 and the the Malmo Malmo Discontinuity, during during the the Penoken Penoken Orogeny) Orogeny)(Sims (Simsetetal., al., 1993). 1993). Part Partof of the the study study is to determine if Discontinuity, xenotime-rich horizons, such such as as that in the Mount xenotime-rich horizons, Mount Barren Barren Group, can be be located located in in this this stratigraphy stratigraphy and to document the sedimentological, sedimentological, structural structural or or stratigraphical stratigraphical features features that thatthey theyhave haveinincommon. common. Certain rock rock units from the different Certain different sequences sequences over over the whole region region were targeted targeted for xenotime xenotime analysis using proposed proposed sedimentological controls for xenotime formation that were determined determined from the Mount Mount Barren Barren Group Group study. study. feature favourable favourable to to xenotime xenotime formation formation may may be be the the presence of of large quantities One sedimentary feature of sedimentary sedimentary apatite apatite within the host host rock rock or or adjoining adjoining rocks. rocks. A field field sample sample of of low lowgreenschist greenschist facies facies phosphatic chert-conglomerate, chert-conglomerate,atatthe the base base of of the Baraga Group, phosphatic Group, from a documented documented phosphorite phosphorite locality in the Dead Dead River River Basin, Basin, northern northern Michigan, Michigan, contains contains large large quantities quantities of of xenotime xenotimeranging rangingfrom from <30 <30 pm pm pitted pitted overgrowths overgrowthson on detrital detrital zircons, zircons,to to>100 >I00micron micronxenotime xenotimecements. cements. Other rock units units that that contain contain xenotime xenotime overgrowths overgrowths and cements of appreciable size and quantity, were; were; (i) (i) quartzite quartzite beds beds in inseveral several drillholes drillholes through through the the Mahnomen Mahnomen Formation, Formation, Mille Mille Lacs Lacs Group, Group, Cuyuna Range, contain up to 50 50 xenotime xenotime crystals crystals per per thin thin section, section, some some of ofthese theseup uptoto—60 -60 pm pm in in size, (ii) a sandstone sandstone bed bed within within drillcore drillcore from from the the base baseof of the theBaraga BaragaGroup GroupininDead DeadRiver RiverBasinBasin-its its largest was 60 pm (iii) (iii)a agrit-pebble grit-pebbleconglomerate conglomerateand andvery verycoarse-grained coarse-grained largest xenotime xenotime observed observed was 60 pm sandstone outcrop at Slate River Hill Hill locality, Baraga Basin, which is assumed to lie at the base base of the Baraga xenotime grains grains per thin section which are pm in size, and (iv) Baraga Group, averaged averaged —15 -15 xenotime are up up to to —60 -60 pm the basal basal Baraga Baraga Group Group hematitic hematitic conglomerate conglomerate at at Big Big Eric's Eric's Crossing Crossing locality, locality, Baraga BaragaBasin, Basin,contains contains up to 55 xenotime xenotime grains grains per per thin thin section, section, some some of of these these are are up uptoto—100 -100 pm pm in in size. size. Pokegema Pokegema Quartzite samples, West Mesabi Mesabi Range, Range, showed showed minor <30 pm xenotime overgrowths on zircons. All of of the therock rocksamples samplesdescribed describedabove aboveare arevery verycoarse-grained coarse-grainedsandstone/conglomerate sandstone/conglomerate beds beds which are either which either located located near near aastratigraphic stratigraphic boundary boundary and/or and/or are are interbedded interbedded with with shale shale beds. beds. Xenotime from from these localities were were analysed on the SHRIMP Xenotime SHRIMP and revealed revealed several age groups; groups; (i) xenotime in xenotime in the the Mahnomen MahnomenFormation Formationdrillcore drillcorerevealed revealedages agesofof—1870 -1 870Ma Maand and—1770 -1 770Ma Ma(1760-1 (1760-1790 790 Ma), (ii) One One large large xenotime xenotime overgrowth overgrowth from the Dead Dead River River Basin Basin drillcore, drillcore, gave gave an anage ageofof—2600 -2600 Ma, (iii) xenotime xenotime contained contained within the the Slate Slate River River Hill Hilloutcrop outcropyielded yielded ages agesofof—2500 -2500 Ma, Ma, (iv) (iv) the the samples from from Big BigEric's Eric'sCrossing Crossingcontained containedxenotime xenotimeshowing showingages agesofof—2550 -2550Ma Maand and—1750 -1 750 Ma. Ma.The The sample from the Pokegema Quartzite in the West Mesabi Mesabi Range, Range, contained contained xenotime with an an age age of of -2300 -2300 Ma Maand and—1770 -1 770 Ma. Ma. Xenotime yielding yielding ages ages of of ca 2500 Ma Xenotime Ma or or older older may may be be from from recycled recycleddetrital detrital (magmatic) (magmatic) grains. grains. The younger age age of of —1770 -1770 Ma (1760-1790 Ma), occurs in xenotime from from widespread widespread localities localitiesacross across thermal event event across across the region. The age the Lake Superior Superior Region and may reflect reflect an epigenetic epigenetic thermal anorogenic magmatism, appears to correlate correlate with with an anepisode episodeat at—1760 -1760 Ma of anorogenic magmatism, pluton pluton emplacement emplacementand and It gneissic doming gneissic doming recorded recorded throughout throughout Wisconsin, Wisconsin, northern northern Michigan Michigan and and central central Minnesota. Minnesota. It postdates the Penoken Orogeny and involved partial melting of of crustal rocks as a result result of of continentcontinentcontinent-arc collision to the south of the region (Sims, 1996). This event is approximately continent or continent-arc coeval with the development development of the the Central Central Plains Plains Orogen Orogen (1800-1630 (1800-1630 Ma) Ma) to to the the south south and and may may be be aa consequence of the accretion of this terrane to the North American continent (Sims, 1996). 1996). This study highlights highlights the sensitivity sensitivity of of in-situ in-situxenotime xenotime geochronology geochronology to to identifying identifyingcryptic cryptic fluid fluid flow flow events within basins. This study will be be ongoing in 2003-2004. Fairbairn Fairbairn H.W., Hurley, Hurley,P.M., P.M., Card, Card, K.D. K.D. and and Knight, Knight, C.J., C.J., 1969, 1969,Correlation Correlationand and radiometric radiometricages agesof of Nipissing Nipissing Diabase Diabase and and Huronion Huronion metasediments metasedimentswith with Proterozoic Proterozoicevents events in in Ontario: Ontario: Canadian CanadianJournal Journalof of Earth Earth Sciences, Sciences, v. v. 6, 6, P. p. 489-497. 489-497. Sims, P.K., 1996, in Sims Sims P.K. P.K. and and Carter, Carter, L.M.H., L.M.H., eds., eds., Archean Archean and and 1996, Early Early Proterozoic Proterozoic Penokean Penokean Orogeny, Orogeny, in Late Proterozoic Geology Geology of the Lake Lake Superior Superior Region, Region, U.S.A., 1993: U.S. Geological Geological Survey Professional Professional Paper Paper 1556, 1556, p. p. 28-60. 28-60. Sims, P.K., et al., 1993, in Reed, Reed,J.C., J.C., Jr., Jr., and and 1993, The Lake Lake Superior Superior Region Regionand and Trans-Hudson Trans-HudsonOrogen, Orogen, in others, eds., Precambrian:Conterminous Precambrian:ConterminousU.S.: U.S.: Boulder, Boulder, Colorado, Colorado, Geological GeologicalSociety Society of America, the Geology of North America, v. C-2, p. p. 11-120. 11-120. Vallini, D., Rasmussen, Rasmussen, B., B., Krapez, Krapez, B., B., Fletcher, Fletcher,l.R., I.R., and and McNaughton, McNaughton, N.J., N.J., 2002, 2002, Obtaining Obtainingdiagenetic diagenetic ages from metamorphosed metamorphosedsedimentary sedimentary rocks: rocks: U-Pb U-Pbdating dating of of unusually unusuallycoarse coarse xenotime xenotimecement cementinin phosphatic phosphatic sandstone: sandstone: Geology, Geology, v. 30, 30, p. p. 1083-1086. 1083-1086. 80 �EVALUATION OF INITIAL MAGMA COMPOSITIONS FOR THE BALD EAGLE INTRUSION AND ASSOCIATED ROCKS VISLOVA, Tatiana, Department of Geology and Geophysics, University of Minnesota The funnel-shaped concentrically-zoned Bald Eagle Intrusion in the Duluth Complex is characterized by very restricted mineral compositions, and consists of only two units: an olivine—plagioclase cumulate and an olivine—plagioclase-clinopyroxene cumulate (Weiblen, 1965; Weiblen and Morey, 1980). In terms of differentiated units expected in a typical layered intrusion, the Bald Eagle Intrusion appears to be petrologically incomplete. This has raised the question whether the four-phase (olivineplagioclase-clinopyroxene-oxide) cumulates, assigned to the Greenwood Lake Intrusion (Miller et al., 2002), and granophyre found to the south of the Bald Eagle Intrusion are genetically related to the Bald Eagle Intrusion (Weiblen and Morey, 1980). New petrographic studies and microprobe analyses (Vislova, 2003) make it possible to evaluate parent magma compositions for the Bald Eagle Intrusion, and quantitatively assess possible petrogenetic relationships between the Bald Eagle Intrusion and spatially associated rocks. Computer programs (MELTS, Ghiorso and Sack, 1995; and COMAGMAT, Ariskin et al., 1993) were used to investigate these questions. A primitive North Shore Volcanic Group olivine tholeiite (P-melt) was used as an initial magma composition (Miller and Ripley, 1996). Equilibrium crystallization of P-melt, calculated by MELTS at 1 atm total pressure and oxygen fugacity near or below the quartz-fayalite-magnetite buffer, reproduces the crystallization order and mineral assemblages observed in the Bald Eagle Intrusion. The calculated composition of the first clinopyroxene (mg 81) equals the one observed, however calculated compositions of the first plagioclase and olivine are much higher than those observed. This could be ascribed to the dynamics of crystal-melt segregation in a flowing magma system. Until the crystals suspended in magma grow large enough they might be carried away, erupted, and found as phenocrysts in lavas. At -7 % melt remaining MELTS reproduces the most evolved mineral compositions in the Bald Eagle Intrusion (Fig. 1). This suggests that the Bald Eagle Intrusion might be a complete crystallization sequence with a few percent remaining melt. It leaves unanswered the question of the origin of four-phase cumulate and granophyre. Modeling shows that a more evolved high Ti and high Fe melt (D-melt) is required for crystallization of the evolved units in the Greenwood Lake Intrusion (Fig. 1). This melt can be produced by fractional crystallization of P-melt in an intermediate magma chamber at 2-3 kbar total pressure. Equilibrium crystallization of D-melt at 1 atm reproduces the crystallization order, the appearance of Fe-Ti oxides, and the compositions of most of the units associated with the Bald Eagle Intrusion (Fig. 1). However, the most evolved rocks in the Greenwood Lake Intrusion (ferrogabbro with Fo < 50) and granophyre were not reproduced by equilibrium crystallization. These units might require fractional crystallization or assimilation. 81 �85 . ,. *A G. 0 0 x I- 0. 0 .E 75 0 U+ A tL±&& A , --. 0) 6565 1 # E E 0 o <60 60 BaId Eagle Intrusion AGreenwood LakeIntrusion Intrusion Greenwood Lake A • Calculated 4 Calculated from P-melt P-melt Calculated from D-melt Lsf4. 55 55 30 30 40 50 50 60 Atom % % Mg/ (Mg+Fe) in olivine (Mg+Fe) olivine 70 70 80 80 Mg/(Mg+Fe) variations variations in in coexisting olivine and clinopyroxene. Fig. Fig. 1. Mg/(Mg+Fe) References: References: Ariskin, A.A., Frenkel, M.Y., Barmina, Barmina, G. S., and Nielsen, R. L., 1993, 1993, COMAGMAT; COMAGMAT; aa FORTRAN program to model magma differentiation processes, Computers Computers & Geosciences, Geosciences, 19 19 (8), p. 1155-1170. 1155-1170. IV, A Ghiorso, M. S., and Sack, R.O., 1995, 1995, Chemical Chemical mass transfer in magmatic processes; processes; IV, revised and internally internally consistent consistent thermodynamic model for the interpolation interpolation and and extrapolation extrapolation of liquid-solid equilibria equilibria in magmatic systems at elevated elevated temperatures temperatures and and pressures, pressures, Contributions Contributions to Mineralogy Mineralogy and and Petrology, Petrology, 119 119 (2-3), (2-3), p. 197-212. 197-212. Miller, J.D., Jr., Green Green J.C., J.C., Severson, Severson, M.J., M.J., Chandler, Chandler,V.W., V.W., Hauck, Hauck, S.A., S.A., Peterson, Peterson, D.M., D.M., and and mineral potential potential of of the the Duluth Duluth Complex Complex and and related related rocks rocks of of Wahl, T.E., 2001, Geology and mineral Report of of Investigations Investigations 58,207 58. 207 pp. pp. ++ northeastern Minnesota: Minnesota Geological Survey Report compact disc in back pocket, 2002. 2002. Miller J.D., Jr. and andE.M. E.M. Ripley, Ripley,1996, 1996,Layered Layeredintrusions intrusionsof of the the Duluth Duluth Complex, Complex, Minnesota, In: Cawthorn Cawthorn R.G. R.G. (ed.) (ed.)Layered LayeredIntrusions, Intrusions,531 53 1 pp. USA. In: Vislova, T., 2003, Petrology Petrology of the Bald Eagle Intrusion Intrusion and associated associated rocks rocks and and its relevanceto to its relevance crystallization in dynamic Midcontinent Rift, Ph.D. Dissertation, Dissertation, crystallization dynamic magma chambers in the Midcontinent University of Minnesota, 226 pp. Weiblen, Weiblen, P.W. and and Morey, Morey, G. G. B., B., 1980, 1980,A A summary summary of of the the stratigraphy, stratigraphy,petrology, petrology,and and structure structure of the Duluth Complex. In: In: frying, Irving, A. A. J., J., and and Dungan, Dungan, M. A. A.(ed.), (ed.),1980, 1980,The TheJackson Jacksonvolume, volume, American Journal of Science, Science, Vol. 280-A, Part 1, 1, p. 88-133. 88-133. Weiblen, P.W., 1965, 1965, A funnel-shaped, funnel-shaped, gabbro-troctolite gabbro-troctolite intrusion intrusion in in the the Duluth Duluth Complex, Complex,Lake Lake County, Minnesota, Ph.D. Dissertation, University of Minnesota, 161 pp. University of Minnesota, 161 82 �A Hydrothermal Hydrothermal Component Component of Iron Iron Formations-A Formations-A Marquette Marquette Range Range Perspective Perspective Waggoner, T.D., 141 141 Chippewa, Chippewa,Negaunee, Negaunee, MI MI49866 49866 Waggoner, T.D., The origin of Lake Superior Superior banded iron formations (BIF) has been a contentious contentious issue for at least a century century and aa half. half. Concepts of origin origin include include weathering, weathering, volcanic volcanic and and organic activity activity whereby whereby ions ions are are carried in and organic and precipitated precipitated from from solution. solution. Clear definition of of the source, definition source, mode mode of of transport transport or ordepositional depositional mechanisms mechanisms is is generally generally lacking. This Thispaper paperwill willaddress addressthe thestrong strongevidence evidencefor for aa hydrothermal hydrothermal source source for for "hard "hard ores" found in the upper upper parts parts of of the the Negaunee Negaunee Iron Iron Formation Formation (NIF) (NIF) and and by extension extension aa possible source for the precursor hematite in the BIF portion. Range was formed formed in a portion. The Range tectonically active area believed to be an extensional extensional rifting rifting environment environment not not unlike unlike those those REE) and and some some VHMS deposits deposits found in Fe-Oxide (Cu, U, Au, REE) The Marquette Range portion of of the the Lake Lake Superior SuperiorIron Iron District Districtdisplays displaysmany many features features BJFs found around the world and, thus, making it an similar to other large Lake Superior BIFs excellent study subject subject for for the source excellent study source and and role role played played by by igneous igneous and andsedimentary sedimentary processes. The Goodrich units units exhibit exhibit BIF, BIF, soft soft supergene supergene enriched TheNegaunee Negaunee and and basal basal Goodrich concentrations and "hard concentrations and "hard ores" ores" as as massive massive bodies, bodies, banded banded jaspilites jaspilites and and detrital detrital conglomerates. conglomerates. "Hard "Hard ores" are are generally generally dense silver gray gray to to black black massive massive metallic metallic magnetite or schistose metallic hematite associated with jaspilite jaspilite and contain in excess of 60% iron. of the the origin iron. Discussion Discussion of origin of of the the "hard "hard Ores" Ores" on on the the Marquette Marquette Range Range has has revolved revolved around around supergene supergene enrichment enrichment prior prior to metamorphism metamorphism or or hydrothermal hydrothermal enrichment enrichment associated associated with the the Penokian Penoluan Orogeny. Orogeny. of the field geology geology do do not not support support with witheither eitherof ofthese thesepositions. positions. First, the Many features of Goodrich conglomerate conglomerate show show random random cobble and pebbles of jasper hematite hematite in in the the basal basal Goodrich orientation of the 'schistose' orientation of 'schistose' hematite hematite indicating indicating the schistose schistose nature nature of of the the hematite hematite existed prior to emplacement and not a result of metamorphism. In In addition addition many many of of the the rocks associated with the "hard "hard ores" ores" exhibit exhibithydrothermal hydrothermal minerals minerals including including sericite, sericite, chlorite, chloritoid, chloritoid, high aluminous aluminous silicates, silicates,garnet, garnet, hematite, hematite, magnetite magnetiteand and tourmaline. tourmaline. The lower NIF exhibit exhibit lower Proterozoic Proterozoic Chocolay Chocolay and Menominee Menominee sediments sediments below below the NW multiple examples of of high-grade hematite that that can be interpreted multiple examples high-grade hematite interpreted as vents. vents. Specular, Specular, microplaty and bytroidal bytroidal hematite hematite are are fairly fairly common commoninin many many outcrop outcropareas. areas. Some microplaty and Some of of these have been described described previously in literature while others have not. not. All the sites sites were lgth century and most exhibit exhibit were subject to exploration exploration for iron ore during during the late late 19th shallow shafts. The Themajor majorcomponents componentsare arechert, chert,jasper jasper and andvein veinquartz quartzalong alongwith withcoarse coarse specular, microplaty and and bytrioidal hematite contained contained in in breccia zones specula, microplaty bytrioidal hematite zones that that exhibit exhibit episodic reworking. reworking. There There are are alterations alterations to to the the host hostrock rockas assome someoccurrences occurrencesexhibit exhibit chlorite, silica, silica, k-spar k-spar and aluminous aluminous silicates. silicates. A large area in sections sections 21, 21, 22, 22, and 23, 47-26 contain contain multiple multiple enriched enriched hematite hematite sites sites that form two northwest trending trending breccia zones zones adjacent adjacent to to northwesterly northwesterlytrending trending faults. faults. In addition there isis a In addition to the the silica silicaflooding, flooding, brecciation brecciation and hematite hematite concentration concentration there significant area of of andalusite and chloritoid adjacent to the significant area andalusite cordierite cordierite and chloritoid adjacent the eastern eastern linear linear 83 �breccia zone in section 23. These Theseminerals minerals are are present present in in aa much much broader broader lower lower regional chlorite zone of metamorphism and most likely are a result of the hydrothermal event that impacted impacted the three square square miles referenced above. above. A conglomerate in Sec. 22 and 23, 47-26 has been previously described as "unusual" and 23,47-26 is sandwiched sandwiched between lower lower Chocolay Chocolay argillite argillite units. units. Clasts causing dimpling in the the underlying argillite were described as rafted clasts from a glacial glacial interlude. interlude. It is unlikely unlikely reef growth during the the same period period of of that a glacial event coincided with significant algal reef coarse, tightly tightly packed packed and shows no time. The "unusual" "unusual" conglomerate conglomerate is extremely extremely coarse, sedimentary sedimentary features. features. In addition significant rinds of k-spar have formed formed on on the the granite granite gneiss cobbles. The Thecobbles cobblesand andmatrix matrix contain contain euhedral euhedral magnetite, magnetite, martite and specular specula hematite suggesting this area was tectonically tectonically active and may well have been an an active active vent area over a period vent period starting starting at at the the earliest earliest extensional extensional period period and continued to be resembles some some of of the breccias active beyond beyond the Ajibik Ajibik time. time. The conglomerate conglomerate resembles breccias at at Olympic Dam and could well be a hydrothermal breccia. analysis of of the hematite match quite closely with both REE chondrite normalized analysis hematite vents match "hard ores" the hematite, hematite, magnetite magnetite and and combination combination hematite/magnetite hematitelmagnetite "hard ores" found throughout the the Range. Range. Recent throughout Recent work work on onthe theBrockman Brockrnan microplaty microplaty hematite hematite confirms confirms aa hydrothermal origin due to the recognition of surrounding surrounding alteration alteration to the the host. host. Initially the the vent vent areas areas were were studied studied in in relation relation to to the the "hard "hard Ores" Ores" but but the the fact that that the Initially vents are all hematite suggests they could be the source source for the the precursor precursor hematite hematite seed cores have have cores that Han identified in low low metamorphic metamorphic grade BIF units. These seed cores been been identified identified in in the theNegaunee, Negaunee,Biwabik, Biwabik, Brockman, Brockrnan, Sokoman, Sokoman, Temiscamie Temiscamie and and Kuruman Iron Formations. Formations. It is quite NIF time time quite plausible plausible that that the the extensional extensional phase phase of of rifting rifting ceased ceased near the end end of of NIP and aa reverse and reverse compressional compressional event event started started causing causing faulting faulting that that produced produced erosional erosional material for the basal Goodrich conglomerate. Reference: Reference: Han, T.H., T.H., 1988, 1988, Origin Origin of Magnetite Magnetite in in Precambrian Precambrian Iron Iron Formations Formations of of Low Low Metamorphic Grade, Grade, Proceedings Proceedings of of the Seventh Metamorphic Seventh Quadrennial Quadrennial IAGOD IAGOD Symposium, Symposium, p. 641-656. 641-656. 84 �High-Resolution Multibeam Bathymetry Bathymetry in Lake Superior. Superior. N. N. JJ.. Wattrus Large Lakes 558122 Lakes Observatory, Observatory,University Universityof of Minnesota, Minnesota,Duluth, Duluth,MN MN 5581 Like all large large lakes, lakes, the composition and and shape shapeof of the the lake lakefloor floor of of Lake Superior reflects the processes that that shape shape its itsformation formation today as a s well as in the past. Maps of of the the lake floor floor made made with with traditional traditional echosounders echosounders lack the the resolution resolution to to permit scientists scientists to to read read the the subtle subtle"fingerprint" "fingerprint" of these processes preserved in the lake-floor. The The advent advent of of modem, modem, high-resolution high-resolution multibeam sonar sonar has has revolutionized the the mapping mapping of of the the sea-floor. sea-floor. In a traditional echosounder, the the depth depth to to the the lake-floor lake-floor below the ship ship is measured measured by by timing timing how how long long it takes takes for for an a n acoustic acoustic ping ping to to travel travel to to the lake-floor and back to the ship. lake-floor and ship. longer the the delay, the the deeper the The longer lake floor is. This type of of surveying provides high-resolution bathymetric information along the trackline followed by the the survey boat. Nothing is followed by known about the the lake lake floor floor either side. side. High-resolution multibeams use a fan High-resolution multibeams of acoustic beams 100)to beams (over (over 100) measure the the shape shapeof of the the lake lake floor floor along a "swath". By sailing sailing aa series of "swath". By overlapping swaths, swaths, it is is possible possible to to achieve complete coverage coverage of the lake lake floor aatt high resolution. Backscatter information Backscatter information collected collected with the bathymetric data can be used to create psuedo-sidescan psuedo-sidescan images images of of the These can can be be used used to map lakefloor. These spatial variations in in the the composition composition of of the lake floor. lake floor. Examples, drawn from the the catalog of of multibeam multibeam surveys surveys conducted conducted by the Large Lakes Observatory, are presented. These illustrate the potential of Large Lakes Observatory, are presented. These illustrate of this this for mapping mapping the the subtle subtle signal processes technology for signal of past geologic processes superimposed superimposed on on the the lake lakefloor. floor. 85 � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2003 Description An account of the resource Institute on Lake Superior Geology. Iron Mountain, Michigan. May 7-11, 2003. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2003 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/fbb8f910e05cdd14c47cd729c9a998c9.pdf 7d8d30bac0d37353bfa2cdd70c285dea PDF Text Text 50TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY DULUTH, MINNESOTA, MAY 4-9, 2004 �INSTITUTE ON LAKE SUPERIOR GEOLOGY 50TH ANNUAL MEETING MAY 4-9, 2004 DULUTH, MINNESOTA HOSTED BY: STEVEN A. HAUCK AND MARK J. SEVERSON Co-Chairs NATURAL RESOURCES RESEARCH INSTITUTE, UNIVERSITY OF MINNESOTA DULUTH WITH ASSISTANCE FROM THE NRRI ECONOMIC GEOLOGY GROUP, MINNESOTA GEOLOGICAL SURVEY, AND THE DEPARTMENT OF GEOLOGICAL SCIENCES, UMD Volume 50 Part 1 – Proceedings and Abstracts Compiled and edited by Steven A. Hauck, Dean Peterson, and Julie Oreskovich Cover Photos: Upper Left – Soudan Iron Formation, 25th Level West, Soudan Underground Mine State Park, Soudan, MN; Center – ILSG 2003 Quinnesec Mine, WI, Menominee Iron District Field Trip; Lower Left – Natural Ore Auburn Mine, Eveleth MN, Looking Northwest Along the Auburn Fault. 2 �50TH INSTITUTE ON LAKE SUPERIOR GEOLOGY VOLUME 50 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: Volcanic Stratigraphy, Hydrothermal Alteration, and VMS Potential of the Lower Ely Greenstone, Fivemile Lake to Sixmile Lake area. TRIP 2: Geologic Highlights of New Mapping in the Southwestern Sequence of the North Shore Volcanic Group and Beaver Bay Complex. TRIP 3: Late Wisconsinan Superior-lobe Deposits in the Lake Superior Basin Northeast of Duluth. TRIP 4: Geology of the Eastern Mesabi Iron Range, Northeastern Minnesota. TRIP 5: Classic Outcrops of Northeastern Minnesota. TRIP 6: Glacial and Postglacial Landscape Evolution in the Glacial Lake Aitkin and Upham Basin, Northern Minnesota. TRIP 7: Economic Geology of Archean Gold Occurrences in the Vermilion District, Northeast of Soudan, Minnesota. TRIP 8: Geology of the Western Contact of the Duluth Complex, Partridge River and South Kawishiwi Intrusions, Northeastern Minnesota. Reference to material in Part 1 should follow the example below: Holm, D.K., Van Schmus, R.W., and Schneider, D.A., 2004, The Influence of Radiometric Dating for Unraveling the Precambrian Geologic History of the Lake Superior Region [abstract]; Institute on Lake Superior Geology Proceedings, 50th Annual Meeting, Duluth, MN, v. 50, part 1, p. 80-84. Published by the 50th Institute on Lake Superior Geology and distributed by the ILSG Secretary-Treasurer: Peter Hollings Lakehead University Department of Geology Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca ILSG website: http://www.lakesuperiorgeology.org ISSN 1042-9964 3 �CONTENTS PROCEEDINGS VOLUME 50 PART 1—PROGRAM AND ABSTRACTS Institutes on Lake Superior Geology, 1955-2004 5 Constitution of the Institute on Lake Superior Geology 7 By-Laws of the Institute on Lake Superior Geology 8 Membership Criteria 9 Goldich Medal Guidelines 10 Goldich Medal Committee 11 Past Goldich Medalists 12 Citation for 2004 Goldich Medal Recipient 13 Eisenbrey Student Travel Awards 14 Student Travel Award Application Form 14 Student Paper Awards 16 Student Paper Awards Committee 16 Session Chairs 16 Board of Directors 17 Local Committees 17 Banquet Speaker 18 Report of the Chair of the 49th Annual Meeting 19 Program 25 List of Contributors 26 Abstracts 37 4 �INSTITUTES ON LAKE SUPERIOR GEOLOGY # YEAR PLACE CHAIRS 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz & C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims & R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley & E. Mercy 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray 28 1982 International Falls, Minnesota D.L. Southwick 5 �29 1983 Houghton, Michigan T.J. Bornhorst 30 1984 Wausau, Wisconsin G.L. LaBerge 31 1985 Kenora, Ontario C.E. Blackburn 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey & R.P. Sage 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage & W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller & M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst & R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin & P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey, Jr. & B.A. Brown 48 2002 Kenora, Ontario P. Hinz & R.C. Beard 49 2003 Iron Mountain, Michigan L.G. Woodruff & W.F. Cannon 50 2004 Duluth, Minnesota S.A. Hauck & M.J. Severson 6 �CONSTITUTION OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY (Last amended by the Board—May 8, 1997) Article I Article II Article III Name The name of the organization shall be the "Institute on Lake Superior Geology". Objectives The objectives of this organization are: A. To provide a means whereby geologists in the Great Lakes region may exchange ideas and scientific data. B. To promote better understanding of the geology of the Lake Superior region. C. To plan and conduct geological field trips. Status No part of the income of the organization shall insure to the benefit of any member or individual. In the event of dissolution, the assets of the organization shall be distributed to _________ (some tax free organization). (To avoid Federal and State income taxes, the organization should be not only "scientific" or "educational, but also "non-profit") Article IV Article V Article VI Article VII Article VIII Minn. Stat. Anno. 290.01, subd. 4 Minn. Stat. Anno. 290.05(9) 1954 Internal Revenue Code s.501(c)(3) Membership The membership of the organization shall consist of persons who have registered for an annual meeting within the past three years, and those who indicate interest in being a member according to guidelines approved by the Board of Directors. Meetings The organization shall meet once a year. The place and exact date of each meeting will be designated by the Board of Directors. Directors The Board of Directors shall consist of the Chair, Secretary-Treasurer, and the last three past Chairs; but if the board should at any time consist of fewer than five persons, by reason of unwillingness or inability of any of the above persons to serve as directors, the vacancies on the board may be filled by the Chair so as to bring the membership of the board to five members. Officers The officers of this organization shall be a Chair and Secretary-Treasurer. A. The Chair shall be elected each year by the Board of Directors, who shall give due consideration to the wishes of any group that may be promoting the next annual meeting. His/her term of office as Chair will terminate at the close of the annual meeting over which he/she presides, or when his/her successor shall have been appointed. He/she will then serve for a period of three years as a member of the Board of Directors. B. The Secretary-Treasurer shall be elected at the annual meeting. His/her term of office shall be four years, or until his/her successor shall have been appointed. Amendments This constitution may be amended by a majority vote (majority of those voting) of the membership of the organization. 7 �BY-LAWS OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY I. Duties of the Officers and Directors A. It shall be the duty of the Annual Chairman to: 1. Preside at the annual meeting. 2. Appoint all committees needed for the organization of the annual meeting. 3. Assume complete responsibility for the organization and financing of the annual meeting over which he/she presides. B. It shall be the duty of the Secretary-Treasurer to: 1. Keep accurate attendance records of all annual meetings. 2. Keep accurate records of all meetings of, and correspondence between, the Board of Directors. 3. Hold all funds that may accrue as profits from annual meetings or field trips and to make these funds available for the organization and operation of future meetings as required. C. It shall be the duty of the Board of Directors to plan locations of annual meetings and to advise on the organization and financing of all meetings. II. Duties and Expenses A. Regular membership dues of $5.00 or less on an annual basis shall be assessed each member as determined by the Board of Directors.. B. Registration fees for the annual meetings shall be determined by the Chair in consultation with the Board of Directors. The registration fees can include expenses to cover operations outside of the annual meeting as determined by the Board of Directors. It is strongly recommended that registration fees be kept at a minimum to encourage attendance of students. III. Rules of Order The rules contained in Robert's Rules of Order shall govern this organization in all cases to which they are applicable. IV. Amendments These by-laws may be amended by a majority vote (majority of those voting) of the membership of the organization; provided that such modifications shall not conflict with the constitution as presently adopted or subsequently amended. Last Amended – May, 1996 8 �MEMBERSHIP CRITERIA FOR THE INSTITUTE ON LAKE SUPERIOR GEOLOGY Approved May 8, 1997 A. Membership in the Institute on Lake Superior Geology requires either participation in Institute activities, or an indication on a regular basis of interest in the Institute. Those individuals registering for an annual meeting will remain as members for 4 years unless: 1) they indicate no further interest in the Institute by responding negatively to the statement on meeting circulars "Remove my name from the mailing list"; or 2) two successive mailings in different years are returned by the postal service as address unknown. B. Those individuals who have not registered for an annual meeting in the past 4 years must indicate an interest in the Institute by postal, electronic, or verbal correspondence with the Secretary-Treasurer at least once every two years. Such individuals will be removed from the membership if they indicate no further interest in the Institute or two successive mailing in different years are returned by the postal service as address unknown. C. The Secretary-Treasurer will maintain a list of current members. The list will include the date of the beginning of continuous membership, dates of returned mail, dates of last contact (expression of interest), and the date membership expires, barring a change of status initiated by the member. Those individuals who have become members of ILSG by Section B will have an expiration date listed at 2 years from the upcoming meeting. For example, a member who expresses interest in September of 1997 (the next annual meeting is May, 1998) will have an expiration date of May, 2000, unless the member contacts the Secretary-Treasurer or attends an annual meeting. D. "Member for Life" status is granted to individuals who have been (nearly) continuous participants of the ILSG meetings for 15 years, Goldich Medal recipients, or those who have served as meeting chairs. This status will be further maintained unless the individuals indicate no further interest in the Institute, or 4 mailings in different years are returned by the postal service as address unknown, or they are deceased. E. All members will be mailed the First Circular for the Annual Meeting and the ILSG Newsletter. The Chair of the annual meeting may opt to send the first circular to additional individuals. All returned mail should be reported to the Secretary-Treasurer. F. The Secretary-Treasurer can designate any individual who is on the ILSG membership list (mailing list) as of January 1, 1997 as a member for life based on participation in ILSG activities. G. Members are strongly encouraged to send address corrections to the Secretary-Treasurer to avoid unintentional lapse of membership. 9 �GOLDICH MEDAL GUIDELINES (Adopted by the Board of Directors, 1981; amended 1999) Preamble The Institute on Lake Superior Geology was born in 1955, as documented by the fact that the 27th annual meeting was held in 1981. The Institute's continuing objectives are to deal with those aspects of geology that are related geographically to Lake Superior; to encourage the discussion of subjects and sponsoring field trips that will bring together geologists from academia, government surveys, and industry; and to maintain an informal but highly effective mode of operation. During the course of its existence, the membership of the Institute (that is, those geologists who indicate an interest in the objectives of the ILSG by attending) has become aware of the fact that certain of their colleagues have made particularly noteworthy and meritorious contributions to the understanding of Lake Superior geology and mineral deposits. The first award was made by ILSG to Sam Goldich in 1979 for his many contributions to the geology of the region extending over about 50 years. Subsequent medallists and this year's recipient are listed in the table below. Award Guidelines 1) The medal shall be awarded annually by the ILSG Board of Directors to a geologist whose name is associated with a substantial interest in, and contribution to, the geology of the Lake Superior region. 2) The Board of Directors shall appoint the Goldich Medal Committee. The initial appointment will be of three members, one to serve for three years, one for two years, and one for one year. The member with the briefest incumbency shall be chair of the Nominating Committee. After the first year, the Board of Directors shall appoint at each spring meeting one new member who will serve for three years. In his/her third year this member shall be the chair. The Committee membership should reflect the main fields of interest and geographic distribution of ILSG membership. The out-going, senior member of the Board of Directors shall act as liaison between the Board and the Committee for a period of one year. 3) By the end of November, the Goldich Medal Committee shall make its recommendation to the Chair of the Board of Directors, who will then inform the Board of the nominee. 4) The Board of Directors normally will accept the nominee of the Committee, inform the medallist, and have one medal engraved appropriately for presentation at the next meeting of the Institute. 5) It is recommended that the Institute set aside annually from whatever sources, such funds as will be required to support the continuing costs of this award. Nominating Procedures 1) The deadline for nominations is November 1. The Goldich Medal Committee shall take nominations at any time. Committee members may themselves nominate candidates; however, Board members may not solicit for or support individual nominees. 2) Nominations must be in writing and supported by appropriate documentation such as letters of recommendation, lists of publications, curriculum vita's, and evidence of contributions to Lake Superior geology and to the Institute. 3) Nominations are not restricted to Institute attendees, but are open to anyone who has worked on and contributed to the understanding of Lake Superior geology. Selection Guidelines 1) Nominees are to be evaluated on the basis of their contributions to Lake Superior geology (sensu lato) including: 10 �a) b) c) d) e) importance of relevant publications; promotion of discovery and utilization of natural resources; contributions to understanding of the natural history and environment of the region; generation of new ideas and concepts; and contributions to the training and education of geoscientists and the public. 2) Nominees are to be evaluated on their contributions to the Institute as demonstrated by attendance at Institute meetings, presentation of talks and posters, and service on Institute boards, committees, and field trips. 3) The relative weights given to each of the foregoing criteria must remain flexible and at the discretion of the Committee members. 4) There are several points to be considered by the Goldich Medal Committee: a) An attempt should be made to maintain a balance of medal recipients from each of the three estates— industry, academia, and government. b) It must be noted that industry geoscientists are at a disadvantage in that much of their work in not published. 5) Lake Superior has two sides, one the U.S., and the other Canada. This is undoubtedly one of the Institute's great strengths and should be nurtured by equitable recognition of excellence in both countries. GOLDICH MEDAL COMMITTEE Serving through the meeting year shown in parentheses George Hudak (2006) University of Wisconsin, Oshkosh Ron Sage (2004) Ontario Geological Survey (retired) David Meineke (2005) Meriden Engineering, Hibbing, Minnesota Steve Kissin, as out-going senior member of Institute Board of Directors, is liaison between Goldich Medal Committee and the Board through the 2004 meeting 11 �2004 GOLDICH MEDAL RECIPIENT Paul W. Weiblen Department of Geology & Geophysics University of Minnesota, Minneapolis GOLDICH MEDALISTS 1979 Samuel S. Goldich 1992 William F. Cannon 1980 not awarded 1993 Donald W. Davis 1981 Carl E. Dutton, Jr. 1994 Cedric Iverson 1982 Ralph W. Marsden 1995 Gene LaBerge 1983 Burton Boyum 1996 David L. Southwick 1984 Richard W. Ojakangas 1997 Ronald P. Sage 1985 Paul K. Sims 1998 Zell Peterman 1986 G.B. Morey 1999 Tsu-Ming Han 1987 Henry H. Halls 2000 John C. Green 1988 Walter S. White 2001 John S. Klasner 1989 Jorma Kalliokoski 2002 Ernest K. Lehmann 1990 Kenneth C. Card 2003 Klaus J. Schultz 1991 William Hinze 2004 Paul W. Weiblen 12 �Citation Paul W. Weiblen 2004 Goldich Medal Recipient Paul W. Weiblen, or P.W., has been a friend and professional colleague for more than 40 years. We have worked together on more projects than I can remember since P.K. Sims selected us to help implement his programs at the Minnesota Geological Survey in the early 1960s. Therefore, it is my distinct honor and privilege to serve as P.W.’s citationist for the 2004 Goldich Medal. P.W. was born and raised in Miller, South Dakota, and after graduation from high school in 1945, he entered the U.S. Army. In the summer of 1945, World War II had ended in Europe, but we were still in combat with Japan. Luckily, before P.W. finished training, the war ended, and he was sent to Germany. After military service P.W. returned to college and earned a B.A. degree at Wartburg College in Waverly, Iowa (1950), and an M.A. in History at the University of Minnesota (1952). P.W. came into geology in sort of a roundabout way. Apparently, he was in Istanbul working as an agent for American Express when he met a geologist who exposed him to the wonders of the profession. Consequently, he returned to the University of Minnesota in 1959, received an M.S. degree (1962) and Ph.D. degree (1965). He stayed at the University in the Geology and Geophysics Department as an Assistant Professor (1965), Associate Professor (1969), Professor (1980), and Professor Emeritus (1997). He was hired specifically to organize and supervise the Department’s Electron Microprobe Laboratory (1965-1980). In the 1960s, electron microprobes were at the cutting edge of modern research, and this facility was one of the first in the country. Along the way he served as Curator of the petrology collection (1970-1997) and supervisor of the scanning electron microscope facility (1970-1997). He took a year off from the University to work at NASA Headquarters in Washington, D.C., and served as Director of the University’s Space Science Center (1985-1990). As an academic, P.W. served on an array of academic, professional, and service committees. That service included the Board of Directors of the Campus Club (1994-1996) and its President (1996-1997). He regularly taught courses in physical geology, igneous and metamorphic petrology, optical mineralogy/electron microprobe techniques, and numerous seminars covering all kinds of geologic topics. He served as mentor for 10 Ph.D. theses—by students including M.G. Mudrey, Jr., K.J. Schulz, R.W. Copper, R. Bauer, W. Day, J.D. Miller, Jr., S.W. Nicholson, and B. Saini-Eidukat—and 13 master’s theses. P.W. was first and foremost an igneous petrologist with eclectic interests, and he could generate ideas faster than anyone I know, which he was always willing to share. Much of the research that I have received credit over the years started out in P.W.’s brain as a throwaway. Unlike many research geologists today who focus on one narrow topic, P.W. concentrated on five separate research topics over much of his career. One subject included activities that focused primarily on the petrogenesis of the Midcontinent Rift System, especially the Duluth Complex. A second subject focused on the origin of copper- and nickel-sulfide mineralization in the Duluth Complex, especially as it relates to metal 13 �recovery from these possible ores. A third topic revolved around the origin of Archean greenstones in northern Minnesota and the high-grade gneisses in the Minnesota River Valley. A fourth subject, which he researched with Ed Roedder of the U.S. Geological Survey, focused on petrologic and geochemical attributes of melt inclusions in lunar samples obtained on various Apollo missions. Lastly, P.W. has been active in research to improve quantitative chemical analyses using electron beam techniques. Lately P.W. has delved into high-voltage electrical pulse methods for disaggregating rocks to produce clean mineral separates. All of these activities produced well over 100 publications including many presented here at the Institute. All are marked by the careful use of data, acquired both in the field and in the laboratory, and a strong intellectual component. Some of his contributions have been controversial, but they have always made us think. That thinking has led us to a better understanding of geologic processes, and for us in the Institute, a better understanding of early earth history in the Lake Superior region. Before I close, I would like to say a few words about P.W. the man. You never really know a person until you have to live with them in a tent camp after five days of rain. P.W. was always an easy-going, personable, considerate guy who was a pleasure to be around. I would go into the bush with him any day. It is my distinct honor to present to the Institute, Paul W. Weiblen as its 2004 recipient of the Goldich Medal for “Outstanding contributions to the geology of the Lake Superior region”. Submitted by G. B. Morey April 2004 14 �EISENBREY STUDENT TRAVEL AWARDS The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name "Eisenbrey" was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. "Ned" Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The annual Chair in consultation with the Secretary-Treasurer determines the number of awards and value. Recipients will be announced at the annual banquet. The annual Chair, who is responsible for the selection, will consider the following general criteria: 1) The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. 2) Students who are the senior author on either an oral or poster paper will be given favored consideration. 3) It is desirable for two or more students to jointly request travel assistance. 4) In general, priority will be given to those in the Institute region who are farthest away from the meeting location. 5) Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. The form below is optional. Successful applicants will receive their awards during the meeting. I NSTITUTE ONLAKE SUPERIOR GEOLOGY Eisenbrey Student Travel Award Application Student Name: Date: Address: email: Department Head-Typed Department Head-Signature Educational Status: Are you the senior author of an oral or poster paper? YES Will any other students be traveling with you? NO Who? Statement of need (use additional page if necessary) Please return to: 15 �STUDENT PAPER AWARDS Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper—last modified by the Board in 2001—follow: 1) The contribution must be demonstrably the work of the student. 2) The student must present the contribution in-person. 3) The Student Paper Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. 4) In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. 5) The total amount of the awards is left to the discretion of the meeting Chair and SecretaryTreasurer, but typically is in the amount of about $500 US (increase approved by Board, 10/01). 6) The Secretary-Treasurer maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. 7) The names of award recipients shall be included as part of the annual Chair's report that appears in the next volume of the Institute. Student papers will be noted on the Program. 2004 Student Paper Awards Committee Glenn Adams (Chair) – Doe Run Company, Viburnum, MO Angelique Magee – Ontario geological Survey, Thunder Bay, ONT Jim Small – Edward Kraemer and Sons, Burnsville, MN 2004 Session Chairs David Dahl – MN Dept. of Natural Res., Lands & Minerals Div., Hibbing, MN Jayne Englebert – MSA Professional Services, Baraboo, WI Sidney Hemming – Lamont-Doherty Earth Observatory, Palisade, NY Douglas Hunter – Wallbridge Mining Company, Lively, Ont. Jill Peterman – Wisconsin Department of Transportation, Superior, WI Mark Smyk – Ontario Geological Survey, Thunder Bay, Ont. Wanda Taylor – University of Nevada at Las Vegas, Las Vegas, NV Scott Wolter – American Petrographics Service Company, St. Paul, MN 16 �2003 BOARD OF DIRECTORS Board appointment continues through the close of the meeting year shown in parentheses, or until a successor is selected Steven A. Hauck Co-Chair 2004 Meeting (2007) Natural Resources Research Institute, University of Minnesota Duluth, Duluth, MN Laurel Woodruff (2006) U.S. Geological Survey, St. Paul, MN Peter Hinz (2005) Ontario Geological Survey, Kenora, ONT Michael G. Mudrey, Jr. (2004) Wisconsin Geological and Natural History Survey, Madison, WI Peter Hollings-Secretary-Treasurer (2006) Lakehead University, Thunder Bay, ONT 2004 LOCAL COMMITTEES General Co-Chairs Steven A. Hauck – Natural Resources Research Institute, Univ. Minn. Duluth Mark J. Severson – Natural Resources Research Institute, Univ. Minn. Duluth Program and Abstracts Editors Steven A. Hauck -- Natural Resources Research Institute, Univ. Minn. Duluth Dean Peterson -- Natural Resources Research Institute, Univ. Minn. Duluth Julie Oreskovich -- Natural Resources Research Institute, Univ. Minn. Duluth Field Trip Guidebook Editor Mark J. Severson – Natural Resources Research Institute, Univ. Minn. Duluth . Acting Local Committee, Duluth, MN Barbara Hauck – Duluth, MN John Heine – Natural Resources Research Institute, University of Minnesota Duluth Julie Heinz – Natural Resources Research Institute, University of Minnesota Duluth Charles Matsch - Department of Geological Sciences, University of Minnesota Duluth James D. Miller, Jr. – Minnesota Geological Survey, Duluth, MN Penny Morton – Department of Geological Sciences, University of Minnesota Duluth Julie Oreskovich - Natural Resources Research Institute, University of Minnesota Duluth Richard Patelke - Natural Resources Research Institute, University of Minnesota Duluth Dean M. Peterson - Natural Resources Research Institute, University of Minnesota Duluth Lawrence M. Zanko - Natural Resources Research Institute, University of Minnesota Duluth 17 �2004 BANQUET SPEAKER R. H. Dott, Jr. Department of Geology & Geophysics University of Wisconsin, Madison, WI 53706 THE VAN HISE ARMY AND OTHER PIONEERS OF LAKE SUPERIOR GEOLOGY 18 �Report of the Chair of the 49th Annual Meeting REPORT OF THE 49TH ANNUAL MEETING OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY Iron Mountain, Michigan The U.S. Geological Survey, with assistance from Michigan Technological University, hosted the 49th Annual Institute on Lake Superior Geology on May 7 – 11, 2003 at the Pine Mountain Resort in Iron Mountain, Michigan. The meeting consisted of two days of technical sessions with two pre- and two post-technical session field trips. John Gartner and Ted Bornhorst provided pre-meeting assistance. Sally LaBerge, Gretchen Klasner, and Suzanne Nicholson provided valuable logistical assistance on-site at Pine Mountain. Connie Dicken was media czar for the technical sessions, keeping all presentations on track while ignoring all needless advice. Pre-meeting registration was 104 students and professionals with an additional 61 on-site registrations, for a total of 165 registrants. Proceedings Volume 49 was published in two parts. Part I – Program and Abstracts, edited by Laurel Woodruff and Ted Bornhorst – the volume contains 45 published abstracts, for 33 oral and 12 poster presentations; and Part 2 – Field Trip Guidebook, edited by William Cannon, with assistance from Connie Dicken and Stacy Saari. The 49th meeting marked the first time in its history that an ILSG meeting was held in this part of Michigan. Field trips visited areas new to the ILSG, which may have resulted in the excellent subscription for all the trips. On Wednesday, May 7, Bill Cannon and staff from Cleveland Cliffs Mining lead a field trip to the Republic Mine, where the life cycle of the deposit, from ore genesis to mining and restoration, was covered. Three other field trips were developed to examine exposures along and on both sides of the Niagara suture zone, a major structural and geologic feature that marks the boundary between the Superior craton and the Wisconsin magmatic terranes. Klaus Schulz and Gene LaBerge lead the Wednesday field trip to the Wisconsin magmatic terrane on the southern side of the suture zone. On Saturday and Sunday, May 10 and 11, Bill Cannon, Gene LaBerge, John Klasner, and Dick Ojakangas were co-leaders for successive field trips through the Menominee Iron District (Saturday) and the Iron River – Crystal Falls area (Sunday). The Field Trip Guidebook for these three trips drew on previous studies in the area and the more than 100 cumulative years of Lake Superior geology expertise of the field trip leaders to provide a comprehensive and definitive compilation of the Paleoproterozoic stratigraphy and structure of the Niagara suture zone in this part of Michigan and Wisconsin. One hundred and twenty participants attended the banquet on Thursday night. This year’s banquet speaker was Susan Martin of Michigan Technological University. Dr. Martin is a professor of industrial archeology at Michigan Technological University and the author of the book Wonderful Power: The Story of Ancient Copper Working in the Lake Superior Basin. The title of Dr. Martin’s post-banquet talk was: The indigenous people of the Lake Superior Basin: understanding the links among environment, geology, and religious belief. As always, a highlight of the banquet was the presentation of the 2003 Goldich medal to Klaus Schulz of the U.S. Geological Survey, recognizing his long and productive career as a geologist in the Lake Superior region. 19 �The technical session began with three invited presentations. The first was by Harold Bernhardt of the Menominee Range Historical Foundation Museum on the mining history of the Menominee Iron Range. The following two talks, by Bill Cannon and Klaus Schulz, were on the Paleoproterozic rocks of the Niagara suture zone, established the context for the three field trips on that topic. The student paper committee had a difficult job this year. Twenty of the presentations in the technical sessions were from students – 15 oral and 5 posters. In the end, three awards were given: Best Student Paper ($300) went to Karoun Charkoudian (University of Wisconsin-Madison) for her talk titled: Strike-slip separation of the Burntside trondhjemite and the Wakemup Bay tonalite, Northern Minnesota. In recognition of the large number of excellent student presentations, two additional students were chosen for Honorable Mention ($100 each) - Amy Garbowicz (Lawrence University) and Stephanie Hocker (University of Wisconsin – Oshkosh). The Student Paper Award fund was supplemented by silent auction of an original volume of the classic Butler and Burbank USGS Professional Paper 144 on Copper Deposits of Michigan. Eisenbrey Student Travel Grants were given to 15 students: Greg Joslin and Phillip Larson – University of Minnesota, Duluth; Merida Keatts and Mary McKenzie – Kent State; John Marma and Karoun Charkoudian, – University of Wisconsin, Madison; Amy Garbowicz – Lawrence University; Daniela Vallini – University of Western Australia; Stephanie Hocker – University of Wisconsin-Oshkosh; and Becky Rogola, Geoff Heggie, Justin Johnson, Riku Metsaranta, Eric Potter and Adam Richardson, all from Lakehead University. All awards were presented at the conclusion of the technical sessions. The Institute’s Board of Directors met on May 8, 2003 and a brief overview of the meeting is provided below: 1. Accepted the Report of the Chair for the 48th ILSG from Peter Hinz and minutes of last Board meeting, May 14, 2002 from ILSG secretary-treasurer, Mark Jirsa. 2. Accepted the 2002-2003 ILSG Financial Summary from Mark Jirsa. 3. Approved one co-chair from the 49th meeting, Laurel Woodruff, as on-going board member. 4. Nominated George Hudak of the University of Wisconsin, Oshkosh to replace Frank Luther on the Goldich Committee, a position that George later graciously accepted. 5. Approved Duluth, Minnesota as the location for the 2004 (50th annual) ILSG and cochairs Steve Hauck and Mark Severson. 6. Discussed the transition of the Secretary Treasurer position from Mark Jirsa to Peter Hollings and accepted a proposal that both be appointed co-treasurers during the transition period. 7. Discussed the transition and evolution of the ILSG webpage and procedures required to move the ILSG into the electronic era. th 8. Discussed possible activities related to 50 meeting to commemorate the longevity and impact of the ILSG. The 49th ILSG meeting was a great success and we wish to thank all the people who contributed to that success. The staff of Pine Mountain was professional and responsive to the needs of a large group. Kleiman Pump and Well Drilling, Iron Mountain, MI, Prime Meridian Resources, Ltd., Fond du Lac, WI, and Coleman Engineering Co., Iron Mountain, MI provided generous monetary contributions. The field trips this year had a large number of participants, and thanks are due to field trip leaders, van drivers, and everyone else who stepped up when needed to drive, hand out lunches, unlock gates, or keep the crowds 20 �moving. As always, everyone who attended the 49th ILSG was willing to help as necessary or adapt to any situation that developed. The meeting this year was well attended and we are heartened by the excellent student participation and attendance, a trend we hope continues. Because of the outstanding response to the meeting and field trips, the 49th ILSG generated several thousand dollars for the ILSG general fund. We both are very happy with the outcome of the 49th meeting and hope that others think it was a success. An ILSG meeting requires a lot of work and time for all involved, but the assistance of the larger ILSG community makes the job of the co-chairs almost bearable, and we encourage others to take on the task. Laurel Woodruff and Bill Cannon Co-Chairs, 49 21 �INSTITUTE ON LAKE SUPERIOR GEOLOGY BOARD OF DIRECTORS MEETING 49th Annual Institute Meeting Thursday, May 8, 2003 Iron Mountain, Michigan Board of Directors Laurel Woodruff (2003 General Chair) William Cannon (2003 Co-chair) Peter Hinz (2002 Co-chair) Michael Mudrey (2001 Co-chair) Steve Kissin (2000 Co-chair) Peter Hollings (Institute Secretary-Treasurer) Guests Mark Jirsa (Emeritus Institute Secretary-Treasurer) Steve Hauck (proposed 2004 Co-chair) Mark Severson (proposed 2004 Co-chair) Ted Bornhorst (Communications Coordinator) Ron Sage (Goldich Committee) Frank Luther (Goldich Committee-outgoing member) The following is based on the secretaries' notes and recollection; any omissions or misstatements are unintentional. Motions by the Board of Directors are generally paraphrased—"approved" or "accepted" implying that a motion was made, seconded, and passed unanimously. The expression "generally agreed" carries less formality, but indicates a directive that will be pursued. Some issues that were resolved after the Board meeting, but during the conference are included here for closure. MINUTES 1. Peter Hinz presented his report on the 48th meeting and stressed the importance of ensuring that the audiovisual component of the sessions runs smoothly. Hinz remarked that the folder of advice to meeting chairs has been replaced by an electronic version, which Woodruff agreed to forward to the next Chairs. Jirsa noted that the full minutes of the Board of Directors Meeting is not normally published in the Proceeding Volume. Accepted report of the Chairs for the 48th ILSG, Kenora, Ontario; as printed in the Proceeding Volume (Hinz), and minutes of last Board meeting, May 14, 2002 (Jirsa). 9. Received, discussed, and accepted 2002-2003 ILSG Financial Summary (Jirsa). Bornhorst agreed to close the ILSG Michigan accounts this year. 10. Approved Laurel Woodruff as on-going board member 11. Discussed replacing Frank Luther as “academic member” on Goldich Committee (end of term 2003). George Hudak of the University of Wisconsin, Oshkosh was proposed and accepted by the Board. George later accepted the position and was welcomed. 12. Discussed and approved 2004 (50th annual) meeting location—Duluth, Minnesota, and co-chairs Steve Hauck and Mark Severson. Hauck and Severson presented a list of seven potential field trips proposed for the meeting. Bornhorst advised the co-chairs to keep field trips close to home in order to avoid impacting upon future meetings. 13. Discussed the transition of the Secretary Treasurer position from Jirsa to Hollings. Distribution of publications to be transferred to Hollings this year with transfer of the Archives and funds at the 50th Annual Meeting. Hollings to open Canadian bank accounts and arrange non-profit status for ILSG in Canada. Accepted a proposal that Jirsa and Hollings be appointed co-treasurers during the transition period. 22 �14. Discussed the transition of Webmaster from Bornhorst to Mudrey. It was agreed that the website would be moved from Michigan Tech to a commercial ISP with Mudrey and Bornhorst to coordinate the transition 15. Discussed and accepted transition from a paper to an electronic newsletter. Mudrey suggested establishing an Email list and agreed to moderate this. Woodruff, Jirsa, Mudrey and Hollings to form a committee to investigate the possibility of making the proceedings volumes available online. Committee to report to the board at the 50th annual meeting. 16. Discussed and agreed that long service pins are created for members who have been involved with the Institute for more than 15 years. Woodruff has the attendance lists and will pursue for presentation at the 50th annual meeting. 17. Other business a. Bill Cannon proposed a compilation of 50 years of ILSG photos. This would be prepared as a CD to coincide with the 50th annual meeting. Cannon will be assisted by Gene LaBarge. A request for photographs was presented to the membership at the banquet. b. It was requested by Luther that GPS locations of outcrops be included in future field guides c. Mark Smyk and Pete Hollings offered to organize 51st Annual meeting in Nipigon, Ontario in 2005, to coincide with the culmination of new geoscience initiatives in the region. Adjournment Respectfully submitted on May 13, 2003 to Laurel Woodruff, Co-chair of the 49th annual meeting, for incorporation into the Report of the Chair to appear in Proceedings Volume 50. Pete Hollings and Mark Jirsa Secretary-Treasurer, Institute on Lake Superior Geology 2003 Best Student Paper Awards Best Student Paper ($300): Karoun Charkoudian, University of Wisconsin, Madison; for her presentation co-authored with Basil Tikoff; Strike-slip separation of the Burntside trondhjemite and the Wakemup Bay tonalite, northern Minnesota. Honorable Mentions ($150 each): Amy Garbowicz, Lawrence University, Appleton, Wisconsin; for her presentation co-authored with Marcia Bjornerud; Paleostress inferences from fault slip vectors in the eastern part of the Wisconsin segment of the Midcontinent Rift. Stephanie Hocker, University of Wisconsin, Oshkosh; for her poster co-authored with G. Hudak, J. Odette, and T. Newkirk; Chemistry of alteration mineral phases at the Fivemile Lake volcanic-hosted massive sulfide prospect, northeastern Minnesota. 2003 Eisenbrey Student Travel Awards 1) Geoff Heggie, Lakehead University ($100) 2) Adam Richardson, Lakehead University ($100) 3) Justin Johnson, Lakehead University ($100) 4) Becky Rogala, Lakehead University ($100) 5) Eric Potter, Lakehead University ($100) 6) Riku Metsaranta, Lakehead University ($100) 7) Greg Joslin, University of Minnesota, Duluth ($100) 8) Phillip Larson, University of Minnesota, Duluth ($100) 9) Merida Keatts, Kent State, Ohio ($100) 10) Mary McKenzie, Kent State, Ohio ($100) 11) John Marma, University of Wisconsin, Madison ($100) 12) Karoun Charkoudian, University of Wisconsin, Madison ($100) 23 �13) Amy Garbowicz, Lawrence University, Appleton, Wisconsin ($100) 14) Stephanie Hocker, University of Wisconsin, Oshkosh ($100) 15) Daniela Vallini, University of Western Australia ($200) 2004 Goldich Medal Recipient Paul W. Weiblen, Department of Geology & Geophysics, University of Minnesota MTU Archives Donation Proceedings including Part 1 (Programs and Abstracts) and Part 2 (Field Trip Guidebook) are available from the Institute: Institute on Lake Superior Geology c/o Mark Jirsa, Secretary - Treasurer Minnesota Geological Survey 2642 University Avenue St. Paul MN 55114-1057 Phone: 612.627.4539 Fax: 612.627.4778 e-mail: jirsa001@tc.umn.edu 24 �PROGRAM 25 �The following companies made generous contributions to the 50th Annual Meeting. We thank them for their commitment to the Institute on Lake Superior Geology. For 50 years this organization has thrived through the sustained interests of individuals, corporations, universities, and government agencies in the international geologic community. This dedication to an exchange of scientific ideas and a passion for field trips (even in driving rain or snow) has enabled the ILSG to fulfill one of its primary objectives: to promote better understanding of the geology in the Lake Superior region. Franconia Minerals Corporation, Spokane, WA Idea Drilling Incorporated, Virginia, MN Iron Mining Association, Duluth, MN Lehmann Exploration Management, Minneapolis, MN Meriden Engineering, LLC, Hibbing, MN Minerals Processing Corporation, Duluth, MN Minnesota Exploration Association (MExA), Minneapolis, MN Minnesota Minerals Coordinating Committee, St. Paul, MN Teck Cominco American Incorporated, Spokane, WA Wallbridge Mining Company, Lively, ONT 26 �Tuesday May 4, 2004 7:30 a.m. Field Trip 1: Volcanic stratigraphy, hydrothermal alteration, and VMS potential of the lower Ely Greenstone, Fivemile Lake to Sixmile Lake area. Field Trip Leaders: George Hudak (UWO), John Heine (NRRI), Mark Jirsa (MGS), Dean Peterson (NRRI). 6:00 p.m. Overnight in Tower, MN at Fortune Bay Casino. 8:00 a.m. Field Trip 2: Geologic Highlights of New Mapping in the Southwestern Sequence of the North Shore Volcanic Group and Beaver Bay Complex. Field Trip Leaders: Terry Boerboom (MGS), Jim Miller (MGS), and John Green (UMD – Dept. of Geol. Sci.). 6:00 p.m. Overnight in Duluth on your own. Wednesday May 5, 2004 8:00 a.m. Field Trip 2: Geologic Highlights of New Mapping in the Southwestern Sequence of the North Shore Volcanic Group and Beaver Bay Complex. Field Trip Leaders: Terry Boerboom (MGS), Jim Miller (MGS), and John Green (UMD – Dept. of Geol. Sci.). 8:00 a.m. Field Trip 3: Late Wisconsinan Superior-lobe Deposits in the Lake Superior Basin Northeast of Duluth. Field Trip Leader: H. Hobbs (MGS). 8:00 a.m. Field Trip 4: Geology of the Eastern Mesabi Iron Range, Northeastern Minnesota. Field Trip Leaders: R. Ojakangas (UMD – Dept. of Geol. Sci.), M. Severson (NRRI), P. Jongewaard (United Taconite), D. Halverson (Northshore mining), J. Arola (Inland), J. Evers (Cliffs-Services). 6:00 p.m. - Return of Trips 1, 2, 3, and 4 4:00 p.m. - 8:00 p.m. Registration 7:00 p.m. - 9:00 p.m. Ice Breaker Social and Poster Setup 27 �Thursday May 6, 2004 7:00 a.m. – 4:00 p.m. REGISTRATION 8:00 a.m. – 8:05 a.m. INTRODUCTORY REMARKS Steven A. Hauck and Mark J. Severson, Co-Chairs SPECIAL TECHNICAL SESSION I The History of Geologic Investigations in the Lake Superior Region Session Chairs: Mark Smyk, Ontario Geological Survey, Thunder Bay, ONT Sidney Hemming, Lamont-Doherty Lab., Columbia Univ., NY 8:05 a.m. – Old Prospector “Gold is Where You Find It! So Is Ag and Cu and Fe!” The OLD PROSPECTOR: Gold Rushes and Mineral Prospecting, 1848 to 1900 in Western North America and The Lake Superior Region 8:35 a.m. – Johnson, A.M. Douglass Houghton’s 1840 Field Excursion to Lake Superior 8:55 a.m. – Miller, J.D., Jr. N.H. Winchell's Study of the Keweenawan Supergroup Rocks of Northeastern Minnesota, 1872-1900 9:15 a.m. – Smyk, M.C., and Magee, A. Silver Threads and Golden Needles: Geological Milestones in Northwestern Ontario 9:35 a.m. – Holm, D.K., Van Schmus, R.W., and Schneider, D.A. The Influence of Radiometric Dating for Unraveling the Precambrian Geologic History of the Lake Superior Region 9:55 a.m. - 10:20 a.m. COFFEE BREAK AND POSTER SESSION 10:20 a.m. – Chandler, V.W., Boerboom, T.J., and Jirsa, M.A. Promontory Tectonics of the Penokean Orogen in Minnesota: A Gravity and Magnetic Perspective 10:40 a.m. – Medaris, G., Jr., and Singer, B. Geochronology of Precambrian Rocks in Central Wisconsin: A Review and New 40 Ar/ 39Ar Analyses 11:00 a.m. – Southwick, D.L. Late Paleoproterozoic Rhyolite-Quartzite Sequences in the Southwestern U.S.: Speculative Relationship to Rocks of the Baraboo Interval 11:20 a.m. – Ormand, C.J., and Czeck, D.M. Three-Dimensional Geometry and Strain of the Baraboo Syncline: Kinematic Implications 28 �11:40 a.m. – Fralick, P., and Pufahl, P.K. Oxygenation of the Archean Hydrosphere: Evidence from the Eagle Island Deltaic Complex 12:00 p.m. – 1:10 p.m.–LUNCH BREAK – POSTER SESSION and ILSG BOARD MEETING (by invitation) TECHNICAL SESSION II Session Chairs: Jill Peterman, Wisconsin Department of Transportation, Superior, WI Dave Dahl, MN Dept. Natural Resources, Lands & Minerals Div., Hibbing, MN 1:10 p.m. – Metsaranta, R.T.*, and Fralick, P.W. Geochemistry and Petrography of Altered Basement Rocks Underlying the Middle Proterozoic Sibley Group 1:30 p.m. – Heggie, G.*, and Hollings, P. Multiple Intrusive Stages Associated with Keweenawan Rifting: The Leckie Stock, Seagull Intrusion, and Nipigon Sill. 1:50 p.m. – Richardson, A.*, and Hollings, P. A Geochemical Study of the Sills of the Nipigon Basin, Ontario 2:10 p.m. – Boerboom, T.J. Newly Recognized Diatreme Breccia Dikes on Lake Superior Near Two Harbors, Minnesota 2:30 p.m. – 2:55 p.m. COFFEE BREAK AND POSTER SESSION 2:55 p.m. – Hollings, P. Trace Element Geochemistry of the Osler Group Volcanics – Implications for Mid-Continent Rifting 3:15 p.m. – Hart, T.R. Geochemistry of the Proterozoic Intrusive Rocks of the Nipigon Embayment 3:35 p.m. – MacDonald, C.A. and Tremblay, E. Lake Nipigon Region Geoscience Initiative: Results of Bedrock Mapping in the Northern Part of the Western Nipigon Embayment, Northwestern Ontario, Canada 3:55 p.m. – Schneider, R.V. Depth Migration of Seismic Reflection Data: An Example for Lake Superior Studies Ballroom Must Be Empty by 4:30 p.m. for Banquet Setup. 6:00 p.m. ICE BREAKER – MIXER – CASH BAR 7:00 p.m. ANNUAL BANQUET AND AWARD PRESENTATION • • • Announcement of 51st Annual Meeting Location 2004 Goldich Award Presentation to Paul Weiblen 2004 Banquet Address Meeting participants who are not registered for the banquet are welcome to attend the banquet address. 29 �Friday May 7, 2004 8:00 a.m. – 8:05 a.m. INTRODUCTORY REMARKS Steven A. Hauck and Mark J. Severson, Co-Chairs TECHNICAL SESSION III Session Chairs: Jayne Englebert, MSA Professional Services, Baraboo, WI Douglas Hunter, Wallbridge Mining Company, Lively, Ontario, Canada 8:05 a.m. – Johnson, J.R.,* Hollings, P., and Kissin, S. Regional Geochemistry Surrounding the Norton Lake Cu-Ni-PGE Deposit, Uchi Subprovince, Ontario 8:25 a.m. – Rossell, D.M., and Coombes, S. The Geology of the Eagle Nickel-Copper Deposit: Marquette County, Michigan 8:45 a.m. – Mahin, R.A., Quigley, T.O., and Lynott, J.S. The Discovery and Geology of the L-K Massive Sulfide Deposit, Menominee County, MI 9:05 a.m. – Bornhorst, T.J., and Robinson, G.W. Precambrian Aged Supergene Alteration of Native Copper Deposits in the Keweenaw Peninsula, Michigan 9:25 a.m. – Severson, M.J., and Hauck, S.A. Whatever Happened to Those Cu-Ni Deposits? 9:45 a.m. – Shafer, P.L.,* and Ripley, E.M. Hydrogen Stable Isotopic Evidence for Hydrothermal Alteration and PGE Concentration Involving Meteoric Water in the Birch Lake Area, Duluth Complex, MN 10:05 a.m. - 10:30 a.m. COFFEE BREAK AND POSTER SESSION 10:30 a.m. – Hudak, G.J., Newkirk, T.T., Drexler, H., Odette, J.D., and Hocker, S.M. Neoarchean Peperites in the Vicinity of Fivemile Lake, Vermilion District, NE Minnesota 10:50 a.m. – Han, T.-M. Effect of Mineralogy on Processing of Low Grade Iron Ores From the Negaunee IronFormation on Marquette Range of the Lake Superior District 11:10 a.m. – Fosnacht, D., Iwasaki, I., and Bleifuss, R. Iron Nodule Research at the Natural Resources Research Institute, UMD 11:30 a.m. – Zanko, L.M., Oreskovich, J.A., and Niles, H.B. Taconite Aggregate Potential of Coarse Tailings from the Biwabik Iron Formation, With an Emphasis on Geology, Mineralogy, and Microscopy 11:50 a.m. – Larson, P.C., Regional Till Sampling in the Vermilion Greenstone Belt, Minnesota: Preliminary Results and Interpretations 30 �12:10 p.m. - 1:00 p.m. LUNCH BREAK – POSTERS REMOVED AFTER LUNCH SPECIAL TECHNICAL SESSION IV Department of Geological Sciences, University of Minnesota Duluth Fifty Years of Geological Contributions to Lake Superior Geology and Other Geological Areas 1:00 p.m. – Introductory Remarks on 50 years of Geology at University of Minnesota, Duluth: Penny Morton Session Chairs: Scott Wolter, American Petrographics Service Company, St. Paul, MN Wanda Taylor, University of Nevada at Las Vegas, Las Vegas, NV 1:05 p.m. – Ojakangas, R.W., and Ojakangas, G.W. Deposition of Paleoproterozoic Siliciclastics and Iron-Formation in a Tidally Influenced Shelf Environment, Animikie Basin, Lake Superior Region 1:25 p.m. – Breckenridge, A.* The Lake Superior Varve Stratigraphy and Implications for Eastern Lake Agassiz 14 Outflow From 10,700 to 8,900 YBP (9.5-8.0 C KA) 1:45 p.m. – Syverson, K.M. Origin of Pre-Wisconsinan Glacial Units in Northern Wisconsin Based on Lithologic Characteristics 2:05 p.m. – Jirsa, M.A. Mapping by the Minnesota Geological Survey in Support of Land-use and Water Planning on the Mesabi Iron Range 2:25 p.m. – Brown, T.R. Recent Geophysical and Geochemical Applications to Exploration Activities in Cripple Creek Mining District, Colorado 2:45 p.m. – 3:10 p.m. 3:10 p.m. the COFFEE BREAK AND POSTER SESSION Student Paper and Travel Awards 3:20 p.m. – Vervoort, J.D., and Wirth, K.R. Origin of the Rhyolites and Granophyres of the Midcontinent Rift, Northeast Minnesota 3:40 p.m. – Schmidt, S.Th., and Seifert, K. Ocean-Floor-Type-Alteration of Drilled MRS Volcanic Rocks in Iowa 4:00 p.m. – Davidson, D.M., Jr. Heller, Sims and Marsden: Mentors Extraordinaire 4:20 p.m. – Grant, J.A. Isocon Analysis: How to Make It Work for You 4:40 p.m. – Morton, R. Twenty-One Years in a Caldera: UMD Geology Students and Sturgeon Lake, Ontario 31 �5:30-7:30 p.m. Buffet at the Depot Sponsored by the UMD Dept. of Geological Sciences (All ILSG Registrants are invited) Saturday May 8, 2004 8:00 a.m. Field Trip 5: Classic Outcrops of Northeastern Minnesota. Field Trip Leaders: M. Jirsa (MGS), T. Boerboom (MGS), R. Ojakangas (UMD-Dept. Geol. Sci.), J. Miller (MGS), J. Green (UMD-Dept. Geol. Sci.), G.B. Morey (MGS), D. Peterson (NRRI), M. Severson (NRRI), R. Patelke (NRRI). 6:00 p.m. Overnight in Tower, MN at Fortune Bay Casino. 8:00 a.m. Field Trip 6: Glacial and Postglacial Landscape Evolution in the Glacial Lake Aitkin and Upham Basin, Northern Minnesota. Field Trip Leaders: L. Marlow, P. Larson, H. Mooers (UMD-Dept. Geol. Sci.). 6:00 p.m. Return of Trip to Radisson Hotel, Duluth Minnesota. 7:00 a.m. Field Trip 7: Economic Geology of Archean Gold Occurrences in the Vermilion District, Northeast of Soudan, Minnesota. Field Trip Leaders: D. Peterson (NRRI), R. Patelke (NRRI). 6:00 p.m. Return of Trip to Radisson Hotel, Duluth Minnesota. 8:00 a.m. Field Trip 8: Geology of the Western Contact of the Duluth Complex, Partridge River and South Kawishiwi Intrusions, Northeastern Minnesota. Field Trip Leaders: M. Severson (NRRI), J. Miller, Jr. (MGS). 6:00 p.m. Return of Trip to Radisson Hotel, Duluth Minnesota. Sunday May 9, 2004 8:00 a.m. Field Trip 5: Classic Outcrops of Northeastern Minnesota. Field Trip Leaders: M. Jirsa (MGS), T. Boerboom (MGS), R. Ojakangas (UMD-Dept. Geol. Sci.), J. Miller (MGS), J. Green (UMD-Dept. Geol. Sci.), G.B. Morey (MGS), D. Peterson (NRRI), M. Severson (NRRI), R. Patelke (NRRI). 6:00 p.m. Return of Trip to Radisson Hotel, Duluth Minnesota. 32 �POSTER PRESENTATIONS Boerboom, T.J. Bedrock Geologic Maps of the Two Harbors and Castle Danger 7.5-Minute Quadrangles, North Shore of Lake Superior, Minnesota Buchholz, T.W., Falster, A.U., and Simmons, Wm.B. A Greisen-like Mineral Assemblage from the Nine Mile Pluton, Marathon County, Wisconsin Cordua, W.S. Enigmatic 1300 – 1400 Ma Mafic Pluton from the Koss Pit, Marathon County, WI Drexler, H.L.*, Hudak, G.J., and Peterson, D.M. A Field and Laboratory Study to Evaluate the Genetic Relationships Between the Purvis Pluton and Volcanic Rocks and Volcanic-Associated Mineralization in the Vermilion District of NE Minnesota Erickson, M.L.*, and Barnes, R.J. Late Wisconsin Till and Arsenic Contamination in Upper Midwest Groundwater Fitzpatrick, F.A. Influence of Geologic Setting on Hydrogeomorphic Characteristics of Southern Lake Superior Tributaries Green, J.C., and Miller, J.D., Jr. The Geology of the Duluth Complex and the North Shore Volcanic Group Portrayed in New 7.5' Quadrangle Maps of the Duluth Metropolitan Area Hart, T.R., and Magyarosi, Z. Precambrian Geology and Mineralization of the Northern Black Sturgeon River area, Nipigon Embayment Hemming, S.R., and Roy, M. 40 Ar/39Ar Hornblende Evidence for Provenance of Ice Rafted Detritus in the North Atlantic: Implications for Tracking Past Changes in the Extent and Dynamics of Northern Hemisphere Ice Sheets Hoffman, A.T.*, Peterson, D.M., Patelke, R.L., and Hudak, G.J. Preliminary Petrography and Hydrothermal Alteration of the Soudan Mine Area, Vermilion District, Northeastern Minnesota Jirsa, M.A. Regional Compilations of Bedrock Geology in Northern Minnesota: The Vermilion, Ely, and Basswood Lake Quadrangles Kaukonen, R.J., and Alapieti, T.T. Platinum Mineralization at Drill Hole A4-11 of the Wetlegs Area of the Partridge River Intrusion, Duluth Complex, Northeast Minnesota 33 �Kean, Wm.F. Magnetic Susceptibility Anisotropy and Remanent Magnetism of Quartzite and Phyllite from Baraboo, Wisconsin Klawiter, B. Lithic Materials and Archaeology in the Western Lake Superior Region Larson, P.C., Mooers, H.D., and Marlow, L.M. Early Advance of the St. Louis Sublobe: A Revised Chronology of the Deglaciation of Northeastern Minnesota MacDonald, C.A., and Tremblay, E. Precambrian Geology of the South Armstrong–Gull Bay Area, Nipigon Embayment, Northwestern Ontario, Canada Maes, S., Tikoff, B., Brown, P., and Ferré, E. Magnetic Fabric Constraints on Magmatic Flow: Insizwa Sill, South Africa and the Sonju Lake Intrusion, Minnesota. Magee, A. Mining and Exploration Activities in Northwestern Ontario Mahin, R.A., Quigley, T.O., and Lynott, J.S. The Geology of the L-K Massive Sulfide Deposit, Menominee County, MI McSwiggen, P.L., and Morey, G.B. Mineral Chemistry and Stratigraphy of the Biwabik Iron Formation, Near the Virginia Horn, Mesabi Iron Range, Minnesota Mudrey, M.G., Jr., and Cannon, W.F. Status of Publicly Available Mid-Continent Reflection Seismic Data Patelke, R., and Severson, M. Duluth Complex Bulk Samples Patelke, R., Severson, M., and Peterson, D. Untested Targets in the Duluth Complex Peterson, D.M., and Patelke, R.L. The Proposed National Underground Science and Engineering Laboratory at the Soudan Mine, Northeastern Minnesota: A Geological Site Investigation Piercey, P., Schneider, D.A., and Holm, D.H. Petrotectonic Evolution of Paleoproterozoic Granitic Rocks Across the Central Penokean Orogen, Northern MI and WI Planavsky, N.*, and Bjornerud, M. Blowing in the Wind: The Copper Harbor Stromatolites Revisited Ruhanen, R.W. Geologic Reconnaissance of the Spaulding Mine Area, Cook County, Minnesota 34 �Schneider, R.V. Depth Migration of Seismic Reflection Data: An Example for Lake Superior Studies Stott, G.M. Close Proximity of Kimberlite Pipes to Diabase Dykes: Structural Controls and Predictiveness in the James Bay Lowlands, Ontario Trow, J. Dowsing Employs Classical Mechanics and Static Electricity to Locate SelfPotential Anomalies Inductively and Rapidly NOTE: Asterisk * denotes a student eligible for a Best Student Paper Award. 35 �ABSTRACTS 36 �BEDROCK GEOLOGIC MAPS OF THE TWO HARBORS AND CASTLE DANGER 7.5MINUTE QUADRANGLES, NORTH SHORE OF LAKE SUPERIOR, MINNESOTA BOERBOOM, Terrence J., Minnesota Geological Survey, boerb001@umn.edu Two recent quadrangle-scale geologic maps of an area adjacent to Lake Superior have been published by the Minnesota Geological Survey as part of the U.S. Geological Survey STATEMAP program. This ongoing mapping effort has resulted in five published quadrangle geologic maps in an area stretching from Duluth to Castle Danger (Boerboom and others, 2002a, b, 2003a, b; Fig. 1a). Work is currently in progress on a sixth geologic map (Split Rock Point); and another (Two Harbors NE) will be mapped in the coming year. Field mapping for all these maps was or will be conducted at a scale of 1:12,000, and map compilations are at 1:24,000. The North Shore is experiencing ever-increasing development of both commercial property and private residences, creating a concomitant demand on ground-water resources, which near Lake Superior come mainly from bedrock reservoirs, particularly layered volcanic flows and interflow sedimentary rocks. Development also brings an increased need for quality aggregate materials for both construction and shoreline preservation projects. Although some volcanic flows may be suitable for crushed-rock aggregate, the intrusive rocks are likely to provide the best source of this material. Thus, the goal of this mapping is to refine the stratigraphy of volcanic rocks in the southwest limb of the North Shore Volcanic Group, and to identify the extent, mineralogy, and relationships of intrusive rocks emplaced into the volcanic pile. This mapping will not only lay the groundwork for present and future resource demands, but also further our understanding of the geologic aspects of these rocks. 37 �Green (2002) subdivided volcanic rocks of the Keweenawan North Shore Volcanic Group into a series of informal lithostratigraphic units based on the relative thickness and composition of the volcanic flows or by areas separated by intervening intrusions. Several of these units were extended into the Two Harbors and Castle Danger quadrangles from the southwest (Fig. 1b). Two new lithostratigraphic units were added to those of Green (2002) based on this mapping⎯the Stewart River basalts and the Crow Creek lavas (Fig. 1b). The Stewart River basalts include several flows of diabasic-textured basaltic lavas and an ambiguous unit that has attributes of both a lava flow and an intrusion, interpreted as a shallow-level subvolcanic intrusion that breached the surface to form flows along its upper margin. The Crow Creek lavas are named for olivine tholeiitic and andesitic flows that outcrop mainly along Crow Creek. These lavas contrast with flows east of the Lafayette Bluff diabase and west of the Silver Creek diabase, implying that the intrusions have created a major disruption of the volcanic stratigraphy. Mapping has refined the shape and intrusive relationships of known intrusions, and has identified several new intrusions. Most noteworthy among these is the previously known but poorly understood Two Harbors intrusion, which has been shown to form a small, zoned, east-plunging, synform-shaped body. This intrusion grades from troctolitic diabase through poikilitic olivine gabbro to well-foliated intergranular gabbro, from the lowest exposed to the highest exposed portions of the body. Another smaller body of more massive olivine-rich ophitic olivine diabase may be related to the Two Harbors intrusion, but does not exhibit any consistent modal or textural layering. Of particular interest is the recognition of a diatreme-like breccia that cuts volcanic rocks near the mouth of Crow Creek (Boerboom, 2004). This diatreme contains clasts from less than 1 millimeter to 5 meters in size, in a matrix of zeolite- and chlorite-cemented rock flour. The clasts include a heterogeneous mixture of fine-grained porphyritic basalt, amygdaloidal to ophitic to intergranular basalt, and interflow sedimentary rocks. Fine-grained felty-textured prismatic ferromonzodiorite also occurs as intrusions into basalt adjacent to the diatreme, and as clasts in the diatreme, indicating a possible cogenetic relationship between the two. References Boerboom, T.J., 2004, Newly recognized diatreme breccia dikes on Lake Superior near Two Harbors, Minnesota [abs.]: Institute on Lake Superior Geology (this volume). Boerboom, T.J., Green, J.C., and Jirsa, M.A., 2002a, Bedrock geology of the French River and Lakewood quadrangles, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-128, scale 1:24,000. ———2002b, Bedrock geology of the Knife River quadrangle, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map M-129, scale 1:24,000. Boerboom T.J., Green, J.C., and Miller, J.D., Jr., 2003a, Bedrock geologic map of the Castle Danger quadrangle, Lake County Minnesota: Minnesota Geological Survey Miscellaneous Map M-140, scale 1:24,000. ———2003b, Bedrock geologic map of the Two Harbors quadrangle, Lake County Minnesota: Minnesota Geological Survey Miscellaneous Map M-139, scale 1:24,000. Green, J.C., 2002, Volcanic and sedimentary rocks of the Keweenawan Supergroup in northeastern Minnesota, Chapter 5 of Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, p. 94-105. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2002, Geologic map of the Duluth Complex and related rocks, Northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map M-119, scale 1:200,000. Sandberg, A.E., 1938, Section across Keweenawan lava flows at Duluth, Minnesota: Geological Society of America Bulletin, v. 49, p. 795-830. Schwartz, G.M., and Sandberg, A.E., 1940, Rock series in diabase sills at Duluth, Minnesota: Geological Society of America Bulletin, v. 51, p. 1135-1172. 38 �NEWLY RECOGNIZED DIATREME BRECCIA DIKES ON LAKE SUPERIOR NEAR TWO HARBORS, MINNESOTA BOERBOOM, Terrence J., Minnesota Geological Survey, boerb001@umn.edu Recent mapping by the Minnesota Geological Survey during the 2002 and 2003 field seasons (see Boerboom, 2004) has identified two nearly identical diatreme-like breccias located about 15 kilometers apart, both of which intrude volcanic rocks of the North Shore Volcanic Group. One is located near the mouth of Crow Creek, 3 kilometers southwest of Castle Danger, and the other is located near the mouth of the Split Rock River. The diatremes contain clasts that range up to four meters in diameter. The clasts are composed of typical North Shore volcanic group rocks, including intergranular to ophitic basalt, porphyritic basalt, and minor interflow sedimentary rocks, as well as felty-textured prismatic monzodiorite that also occurs as intrusions into basalts adjacent to the diatremes. At both places the clasts are heavily replaced by zeolites (mainly laumontite) and chlorite, and are surrounded by a matrix of zeolites and chlorite mixed with rock flour. Irregularly distributed brown gossans that probably represent zones of pyritic matrix have also been identified at Crow Creek. The Crow Creek diatreme is the best exposed of the two diatremes, but outcrops are mainly accessed by water. Exposures on a shore-parallel high cliff wall and in cliffs of basalt that are cut by the breccia and protrude into the lake show the dike to be approximately 10 meters wide and vertical in orientation, with clasts up to 4 meters in size. At the shore the dike is oriented approximately east–west, but other scattered, low outcrops inland imply that the dike may have an overall 0.25 mile diameter ring-like structure or a very irregular strike direction. Monzodiorite associated with this diatreme is poorly exposed, and where visible contains abundant angular basalt clasts and small amygdules of chlorite and calcite. The same monzodiorite also occurs as clasts in the diatreme, thus it must have preceded the diatreme in timing but may have been closely associated with it. The Split Rock River diatreme is exposed just southeast of the river mouth as a 6-meter wide, northstriking, subvertical dike that truncates amygdule-layered tholeiitic basalts. This dike contains clasts of both intergranular and ophitic basalt up to 20 centimeters in diameter, and small clasts of interflow sedimentary rocks and ferromonzodiorite petrographically identical to that at Crow Creek. The latter also occurs alone in an outcrop near the diatreme. Narrow, dike-parallel, anastamosing zones of brecciation cut both basalt and ferromonzodiorite in the vicinity of the diatreme dike. In both cases, the brecciated texture, multiple clast varieties of North Shore volcanic group-related rocks and sharp cross-cutting contacts with adjacent volcanic rocks imply that the diatremes were emplaced as an explosive, gascharged intrusion that cut across the volcanic strata. The amygdaloidal and deuterically-altered Lafayette Bluff diabase occurs in close proximity to the Crow Creek diatreme (Boerboom and others, 2003), which may have provided a source of volatiles as it cooled. These diatremes are soft and easily eroded and thus unlikely to form significant outcrops inland, and they could easily be misidentified as a volcanic flow breccia. Careful examination of isolated or seemingly out of sequence flow breccias for disparate clast lithologies may reveal more of these diatreme dikes. REFERENCES Boerboom, T.J., 2004, Bedrock geologic maps of the Two Harbors and Castle Danger 7.5-minute quadrangles, North Shore of Lake Superior, Minnesota [abs.]: Institute on Lake Superior Geology (this volume). Boerboom, T.J., Green, J.C., and Miller, J.D., Jr., 2003, Bedrock geology of the Castle Danger quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-140, scale 1:24,000. 39 �Precambrian Aged Supergene Alteration of Native Copper Deposits in the Keweenaw Peninsula, Michigan BORNHORST, Theodore J., and ROBINSON, George W., A.E. Seaman Mineral Museum, Michigan Technological University, Houghton, MI 49931 The Keweenaw Peninsula within the Midcontinent Rift System is host to a world-class native copper mining district from which about 6 billion kg of refined copper was mined from 1845 to 1968. By far the principal economic minerals mined were native copper and native silver. Supergene alteration minerals occur irregularly throughout the district. The most abundant of these are cuprite, tenorite, malachite, and chrysocolla in close association with native copper. Less common are azurite and paramelaconite, though about fifty additional species are known (Table 1). These minerals typically occur as thin coatings on or replacing native copper, irregular thin coatings along fractures, and euhedral crystals in small pockets filling remaining pore space. For the most part supergene minerals are found near the surface (< 300 m) and are well below the current water table. As might be expected, sulfates are conspicuously uncommon, owing to the general absence of sulfide ores in the district. Likewise, the arsenates (e.g., annabergite, tyrolite, erythrite, etc.) are associated with the copper arsenide minerals domeykite and algodonite. One group, however, the chlorides (e.g., paratacamite, calumetite, etc.), may have formed by reaction with fossil brines, rather than strictly supergene means, as these minerals have been found in isolated areas 1375 m below surface with no visible means of connection to downward-moving meteoric waters. The rocks of the Midcontinent Rift System were deposited between roughly 1.1 and 1.0 Ga, and late in the evolution of the rift (about 1.05 Ga) the rift was subjected to regional compression resulting in reverse faulting, uplift, and sediment deposition. During this compression event, native copper was deposited from a large-scale hydrothermal system along with numerous other hydrothermal minerals. Uplift related to reverse faulting likely continued for some time after the formation of the native copper deposits, likely ending about 1.0 Ga. The rift rocks of the Keweenaw Peninsula were subjected to erosion. If enough cover was removed, the native copper could have been sufficiently near the surface to be exposed to oxygenated groundwater, which would form supergene minerals. Erosion ended by the end of Precambrian time, when the rift rocks were unconformably overlain by Paleozoic sediments, which still cover major portions of the rift. These sediments sheltered the rift rocks of the Keweenaw Peninsula from weathering and erosion. The Paleozoic sediments were removed by Pleistocene glaciers, again making supergene alteration possible. Since the last glacial retreat it is probable that the water table has not been significantly lower than it is today. In turn, Pleistocene supergene alteration should be more or less restricted to above the current water table. Because most observed supergene alteration is below the water table, it is unlikely that it formed since the glaciers removed the Paleozoic cover. Thus, supergene alteration must have occurred during/near the end of the protracted period of erosion in late Precambrian time. The copper deposits had to be reasonably near the surface at that time for supergene alteration to occur. While the Pleistocene glaciers removed significant thickness of Paleozoic cover they could not have removed any significant thickness of rift rocks, else the supergene altered zones would have been removed too. Therefore, the current surface of the Keweenaw Peninsula is likely near the same stratigraphic/erosional level that was present when the Keweenawan rocks were buried by Paleozoic sediments. 40 �acanthite annabergite anthonyite ardennite ? atacamite azurite brochantite buttgenbachite calcite calumetite carbonate-cyanotrichite chalconatronite chrysocolla connellite covellite crednerite cuprite dioptase erythrite gerhardtite goethite guerinite gypsum halite humboldtine ? hydromagnesite kaolinite langite lavendulan likasite malachite manganite mirabilite montmorillonite moolooite ? nantokite olivenite paramelaconite paratacamite pharmacolite picropharmacolite plancheite posnjakite pseudomalachite rauenthalite spertiniite tenorite tyrolite vaterite veszelyite vladimirite whewellite ? unknown Ca-Cl mineral unknown Cu-Cl minerals unknown Cu-Ca-Cl mineral Table 1. Supergene minerals found or reported from the Keweenaw native copper deposits. 41 �THE LAKE SUPERIOR VARVE STRATIGRAPHY AND IMPLICATIONS FOR EASTERN LAKE AGASSIZ OUTFLOW FROM 10,700 TO 8,900 CAL YBP (9.5 - 8.0 14C KA) BRECKENRIDGE, Andy, Large Lakes Observatory, University of Minnesota Duluth, 10 University Drive, 109 RLB, Duluth, MN 55812-2496, brec0027@d.umn.edu Glaciolacustrine rhythmites from Lake Superior record the regional recession of the Laurentide Ice Sheet (LIS) from 10,700 to 8,900 cal ybp (ca. 9.5-8.0 14C ka). LIS retreat from Superior opened eastern Lake Agassiz outlets so that the rhythmites reflect the combined impacts of sediment-laden meltwater and Lake Agassiz discharge. Using sediment cores retrieved from Lake Superior, I present rhythmite stratigraphies, a time series analysis of the thickness measurements, and high-resolution inorganic carbonate data to demonstrate that this is an annual record (varved). The varve thickness records primarily document regional ice margin dynamics: thick varve sequences at 9,100 cal ybp (~8.1 14C ka) and 10,400-10,200 cal ybp (~9.29.0 14C ka) record two periods of moraine formation (the Nakina and Nipigon). General varve cessation is associated with the circumvention of Lake Agassiz and glacial meltwater into Lake Ojibway at 9,040 cal ybp (~8.1 14C ka), although adjacent to meltwater inlets, rhythmic sedimentation persisted for another 200 years. Positively identifying Lake Agassiz catastrophic discharge events remains speculative but seems feasible. The initial influx of Lake Agassiz water is expected at around 10,600 cal ybp (~9.4 14C ka), but at this time, most of eastern and northern Lake Superior was covered by ice. Three sets of thick-thin varves in western Lake Superior perhaps record influxes of Lake Agassiz at around 10,630, 10,600, and 10,570 cal ybp (~9.4 14C ka). Varve formation in Superior coincides with high lake levels in Lake Huron, suggesting that high lake levels in Huron correspond to periods of high Agassiz and/or meltwater flow into Lake Superior. 42 �Recent Geophysical and Geochemical Applications to Exploration Activities in the Cripple Creek Mining District, Colorado BROWN, Timothy R., Cripple Creek and Victor Gold Mining Company, P.O. Box 191, Victor, CO 80863, tbrown@anglogoldna.com Approximately 23 million ounces of gold have been produced from underground and surface operations since gold was discovered in the Cripple Creek Mining District in 1891. This total includes nearly 2 million ounces produced by Cripple Creek and Victor Gold Mining Company (CC&V) over the past 10 years. Gold has been produced from numerous veins in a 30 MA alkaline diatreme complex and is strongly associated with intense potassic alteration. Current production is mining low-grade disseminated haloes around the major historic producers from two open pits and the gold is recovered from a heap leach facility. This rate of production, and future production, in a mature mining district could not take place without an aggressive, ongoing exploration program that utilizes every available tool. Airborne geophysical surveys conducted in 1999 included magnetics, resistivity, and radiometrics. The magnetic survey clearly outlines the diatreme and shows numerous geological, structural, and cultural features inside the diatreme while contributing to the understanding of the alteration. Resistivity has given a better understanding of the zones of clay, potassic alteration, and possibly water. All of these suggest the location of structural features. The radiometric survey, and especially K, outlines large zones of alteration, however this is strongly influenced by historical and recent disturbances as well as the leach pad. IP and CSAMT surveys were initiated in 2001 and the results were encouraging enough to launch a much larger survey in 2002 that covered 37 line miles. The density of data allowed us to model and display the information in several different ways that include plans, sections, and 3D shells. CC&V utilizes a survey tool that captures oriented joint and fracture information with a down-hole camera. This tool captures important geotechnical data as well as structural orientations that can be cross-referenced to assay data. The geochemical relationships between Au and Te and the potassic alteration have long been known in the district. Recent studies have shown a consistent relationship between gold and other elements (As, Hg, Sb, and V) and have suggested high-grade gold always occurs with strong potassic alteration although strong potassic alteration does not always indicate zones of high-grade gold. Application of the knowledge gained from a better understanding of the district’s geology, alteration, and structure has led to focused, efficient drilling programs. The results of the exploration programs have continued to add to the operations as mid-year 2003 reserves stand at 4.2 million ounces and production is scheduled to continue through 2013. 43 �A GREISEN-LIKE MINERAL ASSEMBLAGE FROM THE NINE MILE PLUTON, MARATHON COUNTY, WISCONSIN BUCHHOLZ, Thomas W., 1140 12th Street North, Wisconsin Rapids, Wisconsin 54494, FALSTER, Alexander U., and SIMMONS, Wm.B., Department of Geology and Geophysics, University of New Orleans, New Orleans, Louisiana 70148. The Nine Mile pluton is the youngest and most silicic of the four known intrusive centers of the approximately 1.5 Ga Wausau complex, exposed in Marathon County, Wisconsin. Mineralization in this epizonal pluton is often varied and complex, though frequently restricted to small localized environments in pegmatitic veins, aplites, and miarolitic granite. Recently a greisen-like mineral assemblage was identified in the Maguire weathered granite quarry, located approximately ¼ mile west of the Ladick East (or Charneski) quarry in the south central portion of the pluton. Quarrying here has intersected several small aplite-pegmatite dikes ranging in thickness from approximately 2” (5 cm) to about 15” (40 cm). These consist of layered banded aplite, commonly with thin 1-3” thick pegmatitic cores, and locally exhibit a highly peraluminous composition that is unusual in the predominantly alkalic Nine Mile granite. Due to the rapid progress of pit operations and ongoing reclamation work, most dikes could not be studied in their original position, but one briefly exposed gently dipping subhorizontal dike was observed in situ. Piles of discarded aplite-pegmatite boulders indicate the probable locations of additional dikes, possibly as many as three based upon varying characteristics. Most dikes were probably roughly vertically oriented, as adjacent pit walls show no signs of the dikes. Much material was recovered from piles of mingled boulders; sample characteristics generally allow attribution to one of the surmised four dikes. Exotic mineralization is often present in both the aplite and pegmatitic portions, but is concentrated along and adjacent to aplite-pegmatite contacts and along thin discordant mica-rich veinlets cutting across visible banding or layering. Late-stage mineralization in these dikes, generally confined to the vicinity of the pegmatitic cores, shows mineralization indicative of a greisen-like environment. Minerals identified to date include topaz (19-20% F content) as masses intergrown with albite and other minerals and as tiny clear crystals to 0.4 mm. Ferberite/hübnerite is found as well-formed dark red-brown to black crystals to 1.5 mm on and in albite and quartz. Most crystals have a high Mn content, but some portions of crystals are Mn dominant and are therefore hübnerite. Cassiterite, though less common than ferberite/hübnerite, has been found as dark red-brown crystals and masses to 3 mm. It appears that these represent the first reports of topaz, ferberite and hübnerite from in-situ occurrences in the Wausau complex, and indeed from the state of Wisconsin. Cassiterite has been observed in trace amounts from several other sites within the Nine Mile Pluton, but its relative abundance at this site is highly unusual. Monazite- (Ce) is common as clear to translucent yellow-orange crystals up to 4.5 mm in size. Columbite-group minerals are common as well-formed crystals to approximately 4 mm, and are ferrocolumbite with significant Mn and Ta. Some crystals have W-contents of over 12%, making them tungstenian columbite-tantalite. Zircon is locally very abundant, and is Hfrich. Xenotime is sparse, and appears to show significant enrichment in HREE. Other minerals 44 �noted include pyrite, chalcopyrite, barite (excellent clear platelets), an unidentified bright blue Cu sulfide, ilmenite, quartz, microcline, albite, siderophyllite, muscovite and possibly zinnwaldite. Rare phases include tiny inclusions of stolzite/raspite in ferberite, microlite, a Bi mineral forming minute tabular crystals, a beam-sensitive W mineral, an ilmenorutile like mineral, a grayite-like Th phosphate and silvery black crystals of a Nb-dominant ixiolite-like mineral with octahedral morphology. Interestingly, considering the abundance of fluorite at other Nine Mile Pluton sites, fluorite is much less common here. Nonetheless, the high F content of topaz and associated micas indicates that these were very F-rich systems. The association of topaz, cassiterite and ferberite, and the highly peraluminous nature of the mineralization indicate that a greisen-like environment prevailed in the latter stages of crystallization of these dikes. To the knowledge of the authors this is the first observation of such an environment in the Nine Mile pluton, in the Wausau Complex, and probably in the state of Wisconsin. 45 �Promontory Tectonics of the Penokean Orogen in Minnesota: A Gravity and Magnetic Perspective CHANDLER, V.W., BOERBOOM, Terrence J., and JIRSA, Mark A., Minnesota Geological Survey, chand004@umn.edu The sharp southward bend of the Penokean orogen in central Minnesota and possible continuation southwest into Iowa along the Spirit Lake trend (Anderson and Black, 1983; Van Schmus and others, 1993) have been cited as evidence for a promontory in the pre-collisional margin of the Superior Province craton (Schulz and Sims, 1993). Recent geologic investigations in this area have clarified the structure of both the orogen and the proposed promontory. Owing to a lack of bedrock exposure and drill holes, these investigations have relied to varying degrees on geophysical data. Of particular importance have been the high quality gravity and aeromagnetic databases in Minnesota (Chandler, 1991; Chandler and Schaap, 1991) and Wisconsin (Daniels and Snyder, 2002). Gravity and magnetic data have guided recent re-interpretations of the high-grade gneissic rocks in the Minnesota River Valley subprovince, which forms the core of the proposed promontory. On the basis of distinctive anomaly signatures, Southwick and Chandler (1996) divided the subprovince into 4 blocks bounded by three major east- to northeast-striking shear zones, which most likely formed during late Archean accretion of the Minnesota River Valley rocks onto the Superior craton. These shear zones, which are inferred from geophysical models and seismic data to have a moderately steep north dip (Southwick and Chandler, 1996), were likely reactivated during the roughly north–south convergence of the Penokean orogen. On a broad scale, the gravity and magnetic signatures of the Minnesota River Valley subprovince differ significantly from those associated with the Archean gneissic rocks of the Marshfield terrane in Wisconsin. This supports the view that the two Archean gneiss terranes are unrelated, and were most likely brought together during events related to the Penokean orogeny (Schulz and others, 1993). Geologic mapping supported by geophysical data reveals that significant changes in the structure of the Penokean orogen occur near the proposed promontory. Derivative enhanced gravity and magnetic data indicate that the structural grain of the orogen changes from northeast–southwest to nearly north–south (Boerboom and others, 1995; Jirsa and Chandler, 1997), and neither external zone fold-and-thrust belts nor foreland basin deposits appear to extend appreciably south of lat 45°15’N. Part of the internal zone of the orogen includes syntectonic granitic and metavolcanic rocks that are similar to those of the Penokean magmatic terranes of northwestern Wisconsin, but a large proportion of the internal zone is comprised of post-orogenic granitic rocks (Jirsa and Chandler, 1997). In fact, these post-orogenic granites, which are collectively referred to as the east-central Minnesota batholith (Holm and others, in press) are interpreted to constitute the dominant part of the orogen south of lat 45°15’N. In that area, the orogen is characterized by a broad magnetic low, the source of which is tentatively interpreted by geophysical modeling to represent non-magnetic, metasedimentary rocks that occur at a depth of 5 to 15 kilometers beneath Paleoproterozoic granitic rocks. Gravity and magnetic signatures indicate a somewhat complicated structure for the orogen in southern Minnesota. The orogen is truncated near lat 44°45’N. along a fault bounded block of Minnesota River Valley subprovince rocks. Both the orogen and Minnesota River Valley rocks are covered to the east by rocks of the Mesoproterozoic Midcontinent Rift System, but south of lat 44°15’N., rocks of the orogen are interpreted to re-emerge from the Midcontinent Rift System and extend southwest along the Spirit Lake trend. A broad magnetic low, combined with the drilling data from Minnesota (Southwick, 1994) and Iowa (Van Schmus and others, 1993), indicate that the geology along this part of the Spirit Lake trend may be similar to the internal zone of the orogen in east-central Minnesota. In conclusion, recent geologic interpretations in central and south-central Minnesota have helped refine our understanding of the effects of the large continental promontory with the western margin of the 46 �Penokean orogen. In fact, some of the changes in orogen structure may be explainable by the promontory. For example, pre-existing crustal weaknesses within the promontory may have enhanced thrusting, sedimentation, igneous activity, and crustal thickening during Penokean convergence. Increased crustal thickening near the promontory, in the area now occupied by the east-central Minnesota batholith, could have led to the removal of foreland fold-and-thrust belts and foreland basin deposits, and to increased melt generation of the lower crust to produce the post-orogenic granites. REFERENCES Anderson, R.R., and Black, R.A., 1983, Early Proterozoic development of the southern Archean boundary of the Superior Province in the Lake Superior region [abs.]: Geological Society of America Abstracts with Programs, v. 15, no. 6, p. 515. Boerboom, T.J., Setterholm, D.R., and Chandler, V.W., 1995, Bedrock geology, pl. 2 of Meyer, G.N., project manager, Geologic atlas of Stearns County Minnesota: Minnesota Geological Survey County Atlas C-10, scales 1:100,000 and 1:200,000. Chandler, V.W., 1991, Aeromagnetic anomaly map of Minnesota: Minnesota Geological Survey State Map S-17, scale 1:500,000. Chandler, V.W., and Schaap, B.D., 1991, Bouguer gravity anomaly map of Minnesota: Minnesota Geological Survey State Map S-16, scale 1:500,000. Daniels, D.L., and Snyder, S.L., 2002, Wisconsin gravity and aeromagnetic maps and data: A web site for distribution of data: U.S. Geological Survey Open File Report 02-493 <http://pubs.usgs.gov/of/2002/of02-493/index.htm>. Holm D.K., Van Schmus, W.R., MacNeil, L.C., Boerboom, T.J., Schweitzer, D., and Schneider, D., in press, U-Pb zircon geochronology of Paleoproterozoic plutons from the northern mid-continent, U.S.A.: Evidence for subduction flip and continued convergence after geon 18 Penokean orogenesis: Geological Society of America Bulletin. Jirsa, M.A., and Chandler, V.W., 1997, Scientific test drilling and mapping in east-central Minnesota, 1994-1995: Summary of lithologic results: Minnesota Geological Survey Information Circular 42, 105 p. Schulz, K.J., Sims, P.K., and Morey, G.B., 1993, Tectonic synthesis, the Lake Superior region and TransHudson orogen, in Reed, J.C., Jr., Bickford, M.E., Houston, R.S., Link, P.K., Rankin, D.W., Sims, P.K., and Van Schmus, W.R., eds., Geology of North America: Precambrian: Conterminous U.S.: Boulder, Colo., Geological Society of America, v. C-2, p. 60-64. Southwick, D.L., 1994, Assorted geochronologic studies of Precambrian terranes in Minnesota: A potpourri of timely information, in Southwick, D.L., ed., Short contributions to the geology of Minnesota: Minnesota Geological Survey Report of Investigations 43, p. 1-19. Southwick, D.L., and Chandler, V.W., 1996, Block and shear zone architecture of the Minnesota River Valley Subprovince: Implications for late Archean accretionary tectonics: Canadian Journal of Earth Sciences, v. 33, p. 831-847. Van Schmus, W.R., Bickford, M., and Condie K., 1993, Early Proterozoic crustal evolution, in Reed, J.C., Jr., Bickford, M.E., Houston, R.S., Link, P.K., Rankin, D.W., Sims, P.K., and Van Schmus, W.R., eds., Geology of North America: Precambrian: Conterminous U.S.: Boulder, Colo., Geological Society of America, v. C-2, p. 279-281. 47 �Enigmatic 1300 – 1400 Ma Mafic Pluton from the Koss Pit, Marathon County, WI CORDUA, William S., Dept. Plant and Earth Science, University of Wisconsin - River Falls, 10 South Third Street, River Falls, WI 54022, william.s.cordua@uwrf.edu In late Fall, 2002, an unusual body of mela-diorite cutting granite was found by mineral collector Tom Buchholz in a newly excavated area of the Red Rock quarry #510 (A.K.A. Koss Pit), on the east side of County O, SW 1/4 sec 2 T27N R6E, SW of Wausau, Marathon County, WI. The quarry is dominantly in the 1520 –1480 Ma Nine Mile granite (Buchholz, et. al., 2000). The unusual rock is exposed as rubble in a teardrop shaped area approximately 2 meters wide by 15 meters long, trending N55oE. This rock contains spectacular euhedral twinned crystals of plagioclase up to 20 cm. long. It has chilled margins against the granite and contains granite xenoliths. It is cross-cut by thin pinkish diorite dikes less than a centimeter wide. Samples were collected for thin section work, major and minor trace element analysis and radiometric dating. Study of 8 thin sections revealed the pluton is a mela-diorite consisting of 7 – 20 % plagioclase phenocrysts in a matrix of medium to fine -grained hornblende (20- 24%), biotite (27 - 37%), titanite (3-4%) and felsic material (22 -28%). The felsic material is an equigranular mosaic consisting mostly of oligoclase (about An 14). The hornblende and biotite occasionally form clots of coarser crystals. No feldspathoids, olivine or pyroxenes were found. The plagioclase phenocrysts were rounded and embayed, suggestions partial resorption. Their compositions were andesine (about An 43) but showed zoning on the rims and along cleavages to oligoclase. The phenocrysts had numerous inclusions of biotite and hornblende identical to that found in the groundmass. This suggests that the plagioclase crystals are not xenocrysts. Chemical analyses were done of 2 samples: WSC-03-01 was from near the contact with the granite. WCS-O3-02 is from the center of the body. Both samples are nepheline normative (WSC-03-01 ne = 0.63; WSC-03-02 ne = 3.41) and olivine normative (WSC-03-01 ol = 9.16 WSc-03-02 ol = 10.46). The less undersaturated nature of WSC-03-01 may be due to contamination by the granite country rock. These rocks are similar chemically to lamprophyres. Some of their major element trends fall within the shoshonite fields (Joplin, 1968). Mineralogically, however, the rocks are distinct from typical lamprophyres in the conspicuous plagioclase phenocrysts and lack of modal pyroxene or olivine (Rock, 1991). Trace element chemistry of these two samples is consistent with trends from shonshonitic and alkalic rocks formed in mid-plate regions (Pierce, 1982). Spider gram plots show the Koss Pit mela-diorite is enriched in both compatible and incompatible lithophile elements relative to MORB (figure 1). Its pattern resembles that of lamprophyres, such as the calc-alkaline lamprophyre series (Pierce, 1982, Rock, 1991). REE distribution shows a negative slope and enrichment in all REEs relative to Chengwatana volcanic rocks (Wirth, et al., 1997) and local Penokean granites. They are inconsistent with a Keweenawan body contaminated with granitic basement. They are consistent with the partial melting of a metasomatically enriched lithospheric mantle. A possible source of enrichment could be volatile- rich material subducted during the Penokean orogeny. K-Ar ages were determined by Activation Labs Ltd. Argon was determined by isotope dilution procedure on noble gas mass spectrometry. K concentrations were determined by ICP. The whole age measured was 1307.2 +/- 41 Ma. A biotite separate was determined to have an age of 1410.8 +/- 47 Ma. These ages are unique for central Wisconsin, which had shown a gap in igneous activity between the Nine-Mile granite at 1520 - 1480 Ma, and Keweenawan events at 1100 Ma. The ages are consistent with the field data, in that the mela-diorite clearly cross-cuts the Nine-Mile 48 �Granite but is chemically and mineralogically dissimilar to younger Keweenawan bodies. Unless its radiometric age has been reset due to uplift, the Koss Pit mela-diorite pluton represents a newly discovered igneous event in central Wisconsin. Fieldwork by the author has found other thin, highly altered dikes of mafic to lamprophyric character cutting older granites elsewhere in central Wisconsin. Their altered character make petrographic and radiometric work difficult. One may speculate that these are related in age to the Koss Pit mela-diorite. The small size, easily eroded character, and extensive glacial cover make the extent and relationships of such mafic igneous rocks difficult to determine. Trace element Spidergram 1000 100 WSC-03-01 10 WSC-03-02 Av. Calc-Alk Lamp 1 Sr K2O Rb Ba Th Ta Nb Ce P2O5 Zr Hf Sm TiO2 Y Yb Sc Cr 0.1 Trace element Buchholz, T.W., A.U. Falster and W.B. Simmons, 2000, “Ta, Nb, U, Y and REE Minerals of the Koss Quarry, Marathon County, Wisconsin” [abstract], 26th Rochester Mineralogical Symposium, Rocks and Minerals, vol. 75, p. 170-171. Joplin, J.A., 1968, “The Shoshonite Association: a review”, Geological Society of Australia, vol. 15 #2, p. 275-294. Pierce, J.A., 1982, “Trace element characteristics of lavas from destructive plate boundaries” in Andesites: Orogenic andesites and related rocks edited by R.S. Thorpe, John Wiley and Sons Pub., p. 525-548. Rock, N.M.S., 1991, Lamprophyres, New York, Van Nostrand Reinhold, 285 p. Wirth, K; J.D. Vervoort and Z.J. Naiman, 1997, “The Chengwatana Volcanics, Wisconsin and Minnesota: petrogenesis of the southernmost volcanic rocks exposed in the Midcontinent Rift”, Canadian Journal of earth Sciences, vol. 34, p. 536-548. 49 �Heller, Sims and Marsden: Mentors Extraordinaire DAVIDSON, Donald M., Jr., P.O. Box 2571, Tubac, AZ 85646 In considering the measure of my “contributions to the science”, particularly as related to UMD, it became apparent that three men played essential roles in this pilgrim’s progress. Robert L. Heller was instrumental in building an outstanding geoscience department at UMD. Basically he hired people, including myself, who were committed to effective teaching as well as research. But above all, he set the pattern for hiring people largely based on “how well we got along” and then “what they did”. This premise served me well in building departments at both UTEP and NIU. Bob was also highly committed to teaching himself and thus was not afraid to ask considerable of his department. I distinctly remember a 27 contact hour quarter my first year in harness! However, he was equally quick to encourage educational innovation and that led to what I consider an outstanding series of team taught courses and course sequences: Earth Materials (integrated mineralogy-petrology); Earth Structure (integrated sedimentationtectonics); Geology of North America; and Precambrian Geology to name a few. Although more difficult to set in place under the semester system, I tended to encourage such thinking at other schools and the team-teaching process served me well at Exxon Research. Finally, through his activities at AGI Bob helped me truly realize that “the whole was indeed bigger than the sum of the parts” and thus I worked diligently at getting AGU back under the AGI umbrella during my tenure at GSA, I believe for the betterment of the science. Paul K Sims is well known to this Institute. His professional work represents to me the essence of doing “good science”. He was a patient reviewer, particularly for neophytes, and thus influenced my approach to graduate student thesis supervision as well as “in-house” publications at Exxon. It was a real pleasure to work for him summers under the sponsorship of the Minnesota Geological Survey mapping in the Boundary Waters with outstanding assistants and colleagues such as Paul Weiblen. While he demonstrated considerable administrative skill as Survey Head, I was fortunate to see such talent in action again during his tenure as SEG President and President of the Economic Geology Publishing Company. This outstanding role model of organization stood me in good stead both serving as director of graduate studies at UMD, but in preparing for and carrying out the first graduate program review. Ralph W. Marsden, as well as helping found the ILSG, was the consummate Department Chair. He was infinitely patient and delegated frequently and well. More importantly, once he gave you a responsibility, he left you alone to work on it, although always available for advice. He also gave young faculty license to develop and grow - in my case organizing informal departmental sessions in 1969 on a new paradigm for how the earth works: Plate Tectonics. Yet I believe Ralph will best be remembered for his service to the science through society work. He was extremely active both in SEG and AIME and encouraged young faculty to join and participate in societies of their interest. Under his tutelage I became the Business Manager for Economic Geology, and later Treasurer of SEG. I am truly pleased SEG named its distinguished service medal after him. None could be more deserving. Happy Fiftieth Birthday UMD Geology and ILSG! 50 �THE VAN HISE ARMY AND OTHER PIONEERS OF LAKE SUPERIOR GEOLOGY DOTT, R.H., Jr., Department of Geology & Geophysics, University of Wisconsin, Madison, WI 53706 Sir William Logan, first Director of the Canadian Geological Survey (1842-1869), initiated investigations of Precambrian rocks. He coined Laurentian for the complex granitic and gneissic rocks of southern Ontario and adjacent Quebec and he assumed that these represented the original continental crust. The Algoman granite and overlying Huronian sedimentary series were soon named north of Lake Huron. These three names were thought to represent universal Precambrian subdivisions for many years. The first comprehensive surveys south of Lake Superior were initiated by Congress to investigate mineral deposits. The first two were led by David Dale Owen in 1839-40 to the lead mining district of the upper Mississippi Valley and in 1848-49 farther north to Lake Superior. In 1850-51 J.W. Foster and J.D. Whitney surveyed northern Michigan and adjacent Wisconsin, where copper and iron deposits were known. Minnesota, Wisconsin, and Michigan funded state surveys during the 1860s and 1870s, but these were parochial and uneven in quality. Most important were surveys under N.H. Winchell in Minnesota, T.C. Chamberlin in Wisconsin, and T.B. Brooks and R.J. Pumpelly in northern Michigan. For the Tenth National Census of the United States, Congress mandated that the U.S. Geological Survey catalogue the nation’s mineral resources. In 1880 several university geologists, including T.C. Chamberlin and Roland D. Irving of Wisconsin, were recruited to help accomplish this formidable task. At the conclusion of the census, Wisconsin’s Irving suggested that an integrated geological investigation of the Precambrian of the Lake Superior region was needed to facilitate the understanding and exploitation of the iron ranges. U.S. Geological Survey Director J.W. Powell agreed, and in 1882 created a Lake Superior Division to be located at Madison with Irving in charge. The first of nine large USGS Monographs to be published by the Division was Irving’s Copper-Bearing Rocks of Lake Superior (1883). In this he recognized the Lake Superior syncline and presented petrographic analyses of varied Keweenawan rocks, an early application of that important, new technique. In 1888 Irving died suddenly, so his young assistant, Charles R. Van Hise, immediately became both Director of the Division and Professor of Geology. The program went forward at a fast pace with a small army of young geologists fanning out across the different iron ranges. Four more Monographs and two Bulletins appeared during the 1890s. In 1903, Van Hise was chosen President of the University of Wisconsin, so his protégée, Charles K. Leith, took over both of his mentor’s former offices. Leith’s own Mesabi Range Monograph (1903) and four others plus one Bulletin were published under his direction. Besides an overarching synthesis of all of the work of the army, which was published in 1911 (Monograph 52), Van Hise and Leith devoted much attention to the development of universal principles for deciphering complex structures and metamorphism in terms of fundamental mechanics and chemistry. To cope with the scattered nature of rock exposures in a recently glaciated region of complex geology, the USGS group honed techniques for determining the relations between visible outcrop-scale (mesoscopic) and very obscure larger-scale (macroscopic) structures. Slaty cleavage and drag folds were employed early as valuable tools for such analysis, and in 1910 William O. Hotchkiss, a student of the Van Hise-Leith school (and about-to-be Wisconsin State Geologist), recognized the value of cross bedding and graded bedding for determining ‘way up.’ These fundamental insights gained by the Van Hise army and ground-breaking textbooks by Leith of structural geology (1913), metamorphic geology (1915; 51 �with his protégée Warren J. Mead), and economic geology (1921) thrust the University of Wisconsin’s Department of Geology into international prominence. Soon many students came from Canada, China, Japan, and Britain for postgraduate work in the ‘Wisconsin School of Precambrian Geology.’ The largest contingent was from Canada after 1910 when Director R.W. Brock of the Geological Survey of Canada adopted the policy that survey geologists henceforth must have the Ph.D. degree. The flow from the north to Wisconsin and other U.S. universities continued until the 1960s when more Canadian institutions began granting the Ph.D. By 1900 there was a need to reconcile some Canadian and U.S. interpretations of the major divisions of the Precambrian rocks of the Lake Superior region. Of particular note was a discrepancy in the Rainey Lake area along the Minnesota-Ontario border, where Andrew C. Lawson had inferred in 1887 that the oldest rocks were sediments of the Coutchiching formation, but U.S. geologists believed that the Keewatin volcanic complex was older and that the Coutchiching was equivalent to the younger Knife Lake series in Minnesota. In 1905 an international committee of survey geologists from both countries reviewed the evidence and favored the U.S. interpretation. In response, Lawson restudied the area in 1911, but stubbornly reaffirmed his original belief in spite of the fact that his field assistant, J.D. Trueman, a PhD student of Leith’s, showed him the value of cross bedding for determining ‘way up,’ which should have led recalcitrant Lawson to see his error. It was not until 1925 that Frank F. Grout of Minnesota disproved Lawson definitively using both graded bedding and cross bedding. The Minnesota-Ontario border region also became the burial ground of the long-standing dogma that Laurentian granites and gneisses represented the original crust of North America, for here the Keewatin volcanic complex now claimed that title. By this time the commonly accepted divisions of the Lake Superior Precambrian record had become Archean (including Keewatin, Algoman and Huronian) and Proterozoic (including Animikie and Keweenawan). By the mid-twentieth Century, the major stratigraphic divisions of the region were established and isotopic dating was beginning at last to provide a sound basis for long-range correlations. A.O. Nier and S.S. Goldich at the University of Minnesota were especially important in applying the new techniques to the Precambrian of this region, which allowed the recognition and dating of several additional tectonic and metamorphic events. Meanwhile, Francis J. Pettijohn had pioneered the study of Precambrian sedimentary rocks with his classic Archean Sedimentation (1943) and many subsequent studies with his University of Chicago graduate students. Stanley A. Tyler’s discovery in 1953 of the Gunflint fossils on the north shore of Lake Superior and Preston Cloud investigations while at the University of Minnesota in the 1960s revolutionized thinking about Precambrian life. Meanwhile the study of the region was greatly enhanced by the widespread application of geophysical techniques such as aeromagnetic surveys and geochemical studies. Finally, since 1970 plate tectonics has revolutionized our understanding of the evolution of the Lake Superior region. 52 �A Field and Laboratory Study to Evaluate the Genetic Relationships Between the Purvis Pluton and Volcanic Rocks and Volcanic-Associated Mineralization in the Vermilion District of NE Minnesota DREXLER, H.L.*, HUDAK, G.J., Geology Department, University of Wisconsin Oshkosh, 800 Algoma Blvd., Oshkosh, WI 54901; drexlh92@uwosh.edu PETERSON, D.M., Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN 55811 The Purvis Pluton is an east-west trending, moderate- sized (~3km3), sill-like multiphase dioritic to tonalitic intrusion with a strike length of 5.7 km and a thickness, which ranges from 100-1200 meters (Peterson, 2001). This intrusion occurs near the base of the eastern part of the Ely Greenstone – Lower Member (Peterson and Jirsa, 1999; Jirsa et al., 2001). Peterson (2001) has suggested that the Purvis Pluton represents a felsic, synvolcanic sill-like subvolcanic intrusion that may have been the heat engine, which drove subseafloor hydrothermal activity, which produced VMS-like Cu-Zn mineralization at the Eagles Nest and Purvis Road prospects. The evidence for this interpretation is based on the local presence of an east-west oriented D2 foliation within the Purvis Pluton, the lack of a significant contact aureole adjacent to the intrusion, and the intimate relationship between the uppermost margins of the pluton and intense, semi-conformable quartz + epidote alteration zones. Volcanic rocks in the Ely Greenstone – Lower Member consistently contain the D2 foliation, which is constrained by age dates to have occurred during regional deformation between 2674 and 2683 Ma (Boerboom and Zartman, 1993; Peterson et al., 2001). We have conducted detailed field mapping, petrographic and lithogeochemical studies to further evaluate the spatial, mineralogical, and chemical characteristics of the Purvis Pluton. Field mapping, supported by subsequent petrographic studies, indicates that the Purvis Pluton contains several distinct phases. These include: 1) xenolithic hornblende diorite; 2) xenolithic hornblende tonalite; 3) xenolithic leucotonalite; 4) leucotonalite and trondhjemite; and 5) leucotonalite dikes. Paragenetic relationships between these phases have been determined in the field based and by petrography based on cross-cutting relationships and the xenolith contents of the various phases. Angular, coarse-grained gabbro/diorite lapilli, which have rare earth element characteristics similar to the other phases of the pluton (Figure 1a), are common in the xenolithic hornblende tonalite and xenolithic leucotonalite phases, and appear to represent an early product of Purvis Pluton crystallization. The xenolithic hornblende diorite, xenolithic hornblende tonalite, and xenolithic leucotonalite commonly contain basalt-andesite lapilli, epidote + quartz-altered basalt-andesite lapilli, and lapilli and blocks of oxide facies iron formation. These three xenolithic phases are intruded by the main phase of the intrusion, leucotonalite/trondhjemite. Leucotonalite dikes are a minor phase of the intrusion, and when present, consistently cut through the other plutonic phases. A sample of the hornblende tonalite has been submitted for geochronological analysis, and we are anxiously awaiting the results. Preliminary lithogeochemical evaluations indicate that the Purvis Pluton is calc-alkalic (Figure 1b). The various phases of the intrusion are classified as tonalite and trondhjemite using O’Connors normativebased granitic rock classification scheme (Figure 1c). Based on trace element characteristics, all phases of the Purvis Pluton were formed in a volcanic arc setting (Figure 1d), which is consistent with the geochemical affinity of the volcanic strata in the Ely Greenstone – Lower Member (Hudak et al., 2002). Recent studies by Galley (2003) have evaluated the physical and chemical characteristics of synvolcanic intrusions spatially and temporally associated with VMS deposits in Canada and Scandinavia. We are currently completing our evaluation of the lithogeochemical features of the Purvis Pluton, and our comparison of these features to VMS-associated synvolcanic intrusions associated with VMS deposits. 53 �Figure 1. a) REE diagram illustrating trends associated with Purvis Pluton phases, gabbro/diorite xenoliths, and amphibolite xenoliths; b) normative alkali - total iron – magnesium tertiary plot (after Irvine and Baragar, 1971); c) normative feldspar tertiary plot for the Purvis Pluton (after O’Connor, 1965); d) Rb – (Y + Nb) discriminant diagram for the Purvis Pluton (after Pearce et al., 1984). References Boerboom, T. J., and Zartman, R. E., 1993, Geology, geochemistry, and geochronology of the central Giants Range batholith, northeastern Minnesota: Canadian Journal of Earth Sciences, v. 30, p. 2510-2522.Boerboom and Zartman, 1993 Galley, A., 2003, Composite synvolcanic intrusions associated with Precambrian VMS-related hydrothermal systems: Mineralium Deposita, v. 38, p. 443-473. Hudak, G. J., Heine, J., Newkirk, T., Odette, J., and Hauck, S., 2002. Comparative geology, stratigraphy, and lithogeochemistry of the Five Mile Lake, Quartz Hill, and Skeleton Lake VMS occurrences, Vermilion District, NE Minnesota: NRRI Technical Report NRRI/TR-2002/03, 390 pages. Jirsa, M. A., Boerboom, T. J., and Peterson, D. M., 2001, Bedrock geological map of the Eagles Nest Quadrangle, St. Louis County, Minnesota: Minnesota Geological Survey Misc. Map Series M-114. O’Connor, J. T., 1965, A classification for quartz-rich igneous rocks based on feldspar ratios: USGS Professional Paper 1965:B79-B84. Pearce, J. A., Harris, N. B. W., and Tindle, A. G., 1984, Trace element discrimination diagrams for the tectonic interpretation of granitic rocks: Journal of Petrology, v. 25, p. 956-983. Peterson, D. M., 2001, Development of Archean lode-gold and massive sulfide deposit exploration models using geographic information system applications: targeting mineral exploration in northeastern Minnesota from analysis of analog Canadian mining camps: unpublished Ph. D. dissertation, University of Minnesota, Duluth, Minnesota, 503 p. Peterson, D. M., Gallup, C., Jirsa, M. A., and Davis, D. W., 2001, Correlation of Archean assemblages across the U.S.- Canadian border: Phase I geochronology: 47th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 47, Part 1 – Programs and Abstracts, p. 77-78. Peterson, D. M., and Jirsa, M. A., 1999, Bedrock Geological Map and Mineral Exploration Data, Western Vermilion District, St. Louis and Lake Counties, Northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-98, scale 1:48,000. 54 �Late Wisconsin Till and Arsenic Contamination in Upper Midwest Groundwater ERICKSON, Melinda L.*(Water Resources Sciences), and BARNES, Randal J., (Civil Engineering), University of Minnesota, 500 Pillsbury Dr. SE, Minneapolis, MN 55455, eric0984@umn.edu Exposure to arsenic, a recognized human carcinogen, is a widespread public health problem, with more than 150 million people worldwide estimated to be exposed to unsafe levels of arsenic from their drinking water. To reduce arsenic exposure from drinking water, the US Environmental Protection Agency recently adopted a new, more protective arsenic drinking water standard, 10 µg/l. All public water systems must comply with the new arsenic standard by January 2006. In the upper Midwest, arsenic in ground water is a widespread, naturally occurring contamination problem regionally impacting both public and private drinking water wells. Hundreds of upper Midwest public water systems serving over a million people are affected by the change in the arsenic standard. Additionally, in Minnesota alone, 150,000 – 250,000 people are estimated to obtain drinking water from private wells with arsenic concentrations exceeding 10 µg/l. Private well owners are not forced to comply with federal drinking water standards. Figure 1 illustrates that groundwater arsenic concentrations in excess of 10 µg/l are associated with the lateral extent of northwest-sourced late Wisconsin sediment. Statistical analysis supports the visual observation: 10.7% of public water systems located within the footprint of the Late Wisconsin till exceed 10 µg/l, and only 2% of public water systems outside the footprint exceed 10 µg/l. Our research results indicate that the elevated arsenic concentrations in upper Midwest groundwater are not primarily due to high arsenic concentrations in Late Wisconsin till. Sediment analyses indicate that Late Wisconsin sediment in western Minnesota does not have particularly high arsenic concentrations, and water arsenic and sediment arsenic concentrations measured in our research are not correlated (Figure 2). We hypothesize that the specific physical characteristics of the Late Wisconsin till, such as its fine-grained matrix, entrained organic carbon, and active anaerobic biological activity, create the geochemical environment favorable to a regional-scale mobilization of arsenic via desorption. In western Minnesota aquifer sediments, we measured that 0.5 – 0.7 mg/Kg of the total arsenic is adsorbed arsenic. Adsorbed arsenic is labile and can be readily desorbed, especially in suboxic and reduced aquifers. 55 �# S S # S # # S S # S # S # S # S # S # S # S # # S S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # # S S # S # S # S # S # S # S # S # S # S # S # S # S # S #S S # S# # S # S # S # S # # # S S S # S # S # S # S # S # S # North Dakota S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S# # S S # S # S # # S S # S # S # S # S # # S # S S # # S S # S # S # S # S # S # S # S # S # S # S # S # S # # S S # S # S # # S S # S # S # # S S # # S S # S # S # S # S # S # S # S # S # S # S # S # S # # S S # # S S # S # # S S # S # S # S # S # S # S # S # S # S # SS# # S S # S# # S# # S S# # S S # S# # S # # S S S S# # S S # S # # S S# S# S # S # S# # S# # S# ## S # S# S# S # S S S# S S# # S S# S# # S S # # S S# # S# S# # SS # S S # S# # S # S S S # # S # S # S # S # S # S # S # S # S # S # # S S # S # S # S# # S S # S # S # S # S # S # S # S # S # S # S # S # S # ## S S S # S # S # S # # S S # S # S # S # S # S # S # S # S # 75 0 75 S # S # # S S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # # S S # # S S # S # S # S # # S # S S # S # S # S # # S S # S # S # S # # S # S S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # # S S # S # S # # S S # # S S # S # # S S # S # S # S # S # S # # S S # # S S # S # S # S # S # S # # S S # S # ##S S # S S # S # # S # S # S # S # S S # # S S # S # S # S # S # # S S # S # S # S # S # S # S # S # S # # S S # S # S # S # S # S # S # S # S # S # S # S # S # # S S # # S # S S # S # S # S # S # S # # S # S S # S # S # S # # S # S S # S # S # S# # S S ## S S # S # S # S # S # ##S # S S # S ##S#S S # S S # S # # S S # S # S # S # # S S # S # S # S # S # S # S # S # # S # S # S S # S # S # S # S # S # S # S # # S S # # S S # S # S # S # S # S # S # S # S # S # S # S # # S S # # S # S S # S # S # S # S # # S S # # S # S S # S # S # S # S # # S # S # S # S S # S # S # S # S # S # S # S # S # S # S # S # S # # S # S S # S # # S # S # S S # S # S # S # S # # S S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # # S S # # S S # S # S # S # S # # S S # # S # S S # S # S # S # S # S # S # S # S # S # # S S # S # # S S # S # S # S # S # S # # S # S S # # S S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # # S S # # S S # S # # S S # S # # S ## S S # S # S # S Iowa S # S # S # S # S # S # S # # S S # S # S # S # S # S # S # S # S # S # S # S # S # # S S # S # S # S # # S S # S # S # S # S # S # S # # S S # S # S # S # # S S # S # S # S # S # # S S # S # S # S # S # S # S # # # #S S S # S # #S S S # S # S # S # # S S # S # S # S # S # # S # S S # S # S # # S S # # S # S S # S # S # S # # S S # S # S # # S S # S # S # # S S # # S ## S S S # S # S # # S S # S # S # S # S # S # # S S # S # S # S # S # S # S # S # S # S # S # S # # S S # S # S # S # S ## S S # # S S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # # S S # S # S # S # S # S # # S S # S # S # S # S # S # S # S # S # # S # S # S # #S S #S S # S # # S S # S # S # S # S # S # S # S # S # S # # S S # S ## S S # S # # S S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # # S S #S # S # S # S # S # S # ## S S S # S # S # S# # S S # S # S # S # S # S # S # S # S # S # S # S # # S S # S # S S # # S # S # S # S # S # S # S # S # S # # S S # S # S # S # S # S # S S # # S # S # Arsenic Concentration 0 - 10 ug/l 10 - 20 ug/l # 20 - 50 ug/l S Late Wisconsin Till S # # S S # S # S # S # S # # S S # S# # S# ## S S S# S S # S # S # S # S # # S S # S # S # S# # S # S S # S # # S S# # S S# # S S # S # S # # S S # S # S # S # S # S # # S S # S # S # S # S # S # S # # S S # S # # S # S# S S # S # S # # S S # S # S # S # S # S # S # # S S # # S S # S # # S S # # S # S S # S # S # S # S # S # 150 Miles S # S # S # S # S # # S S # S # S # S # S # # S # S S # S # S # S # S # S # S # S # S # S # S # # S S # S # S ## S S # S # S # S # S# # S# S# S # # S S S # S # S # S # S # S # # S S # S # S # S # S # # S # S # S S # S # S # S # S # S # S # S ## S S # S # S # S # S# # S# S # S S# S# # S# # S# # S S S S# S# # S S # S # S# S# # S# S S# # S S# S S# S # S# S# # S # S # S # S # S # S # S # S # S # S # S # S # # S S # S # # S S # S # S # S # S # S # S # S # S # S # # S S # # S # S S # # S S # S # S # S# # S S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # ## S S S # S # S # # S S # S # S # S # # S S # S # S # S # S # S# # S S # S # S # S # S # S # S # S # S # S # S # S # S # S # ## S S # S# S S # S # S # S # S # # S S # S # S # S # S S# # S # S # SS # S# ## S S# S # ## S S S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # # S S # S # S # S # S # S # S # S # S # # S S # S # S # S # S # ## S S S # S# # S # S # S S # S# S # # S S # S# # S S ## S S# # S# S S # S# # S# S S # S # S # S # S # S # S # S # S # S # # S S # S # # S S # S # S # S # S # # S S # S # S # # S S # S # S # S # S # S # # S S # S # S # S S# # S# S # S # S # S # S# # S # S S S S# # S# # S # S # S # # S# S# S# # S # S S # S# S S# S S S S # S# # S# S S# # S # S# S# # S# # S # S # S # # S# # S# S S # S# # S# # S S# # S S S# S SS # S # S S# # S S S # S# # S# S# # S # S# S# S# S # # S# S # S# S S# S S# # S S# # S# S# S# # S# S # S SS # S# # S# # S# S S S S # S# S # S # # S S# S# # S # # S# S# S# S# # S# S# S# SS # S# # S S# S # S# S# # S S # S # # S S# # S # S# S # S S# S# S# S# # S SS # S# # S S# # S # S # # S S# # S# S# S S # S # S# # S# S S# # S S# # S# S # # S# S # # S # S S # # S S S# # S# # S# S # S S # S# S # S# S S# # S # # S S# # S S# S# S# S# S # S# S # S# # S# # S# S S# S# S# S # S # S S # S# # S# S# S# # S ## S # # S# # S# # S S# S# S S # S# # S S S S # S# S# S# S# S # S# # S # S # # S# S S # S S # S # S S # S S S # S # S# # S# # S S # S # # S# S# S# S S # # S S S S# # S S# # S# # S # S # S# # S S# S# S # S # # S S# S S # S S# # S # S# # SS# S# S S # S # S# # S# S # # S# # S# S# S# # S # S# S # S S S# S S# # S # S# S S# # S# S# S # S S # S # S # S # S# # S S# S# S # S# # S S # S # S# # S S # # S # S # S S# # S S # # S S# # S S # S # S# # S # S # S S # S # S # S # S # S # S S # # S # S # S # #S S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # # S S # S # #S S # S # S # # S S # S # S # S S # # S # S # S # S # S# # S S # S # S S S # # ## S S# # S# S# S# # S S S S ## S # S # S # S# # S# # S S S # S # S S # # S # S # S # S # S # S # S # S # S# ## S S S # S# # S S # S # S # S # S # S # S # S # # S S # S # S # S # S # S # S# # S S # S # ##S S S # # S# S # S S # S # S# # S S# # S# S# S S# # S S ## S S ## S ## S S # S S# # S S # S # S # S # S # # S S # S # S # S # N S # S S# # # S S # # S S # S # S # S# # S S # # S S # S # S # S # S # S # S # S # S # S # S # S S # S# # S# # S S # S S # # S # # S S # S # S # S # S # S # S # S # S # S # S # # S S # S # S# # S S # S # S # S # S# # S# S S # # S # S S S # # S # S # S # S # S # S # S # ## S SS # S # S # S # # S S # S # S # S# # S # S S # # S S # ## S S S # S # S # S # S # S # S # S # # S S # # S S # S # S # # S S # S # S # S # S # S S # # S # S S # # S# S # S # S # S # S # S #S#S #S#S # S # S # S # S # S # S # # S S # S # S # S # S# # S S # S # S # S # S # S # S # S S# # S # S # S # S # S # S # S # S # S # S # S # S # # S S # S # S# # S S # # S S # S # S # S ## S S # S # S # S # S # S # S # # S S # S # S # S # S # S # South Dakota S # S # S # S # ## S S S # S # S # S S # # S # S # S # S # S # S # S # S # S # S # S # S # S # S # # S # S S # S # S # S # # S ## S S S ## S S # S # S # # S S # S # S # S # S # S # S # # S S # S # S # S # S # S# # S S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S# # S S # S # S # S # S # S # S # S# # S# S S # S # S# # S S # S # S # S# # # S S S # S # Minnesota S # S # S # # S S # S # S # S # S # S # S # S # S # # S S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S# S S# # # S S # S # S # S # ## S S S # S # S # S # S ## S S # S # S# # S # S # S S # S # S # # S # S S # S # S # S # S # S # S # S # # S S # S # S # S # S ## S # S ## S S S # ## S S S # S# # S S # S # S ## S # S S # S # S # S # S # # S S # S # S # S # S # S # S # S # # S S # S # S # S # S # S # S# S # # SS S # # S # # S S # S # S # S # S # S # S # S # S # S # S # ## S S S # S # S # S # S # # S S # S # S # S # S # S # # S S # S # S # S # S # # S ##S S # S S # S # S # # S # S S # S # S # S # S # S # S # S # # S S # S # S # S # S # S # S # # S S # S # S # S # S # # S S # S # S# # S S # S# # # S S S # # S S # S # S # S # S # S # S # # S S # S # S # S # S S # # S # S # S # # S S # S # S # S # S # S # S # S # # S S # S # S # S# # S S # S # S # S # # S S # S# # S S # S # # S # S S # S # S # S # S # S # S # S # S # S # S # S # # S S # S # S # S # S # S # S # S # S # S # S # ## S SS # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # S # # S S # S # S # S # # S # S S# # S S S # S# # S# # S # S S # # S S # # S S # S # S # S # S # S # S # S S # ## S S # S # S # S # S # S # S# # S S # S# # S S S ## S S# # S# S# # S S # S # S # S # S # # S S # S # S # S # # S S # S # S # S # S # # S S # S # S # S # S # S # S # S # S # S # S # # S S # S # S # # S S # S # # S S # Figure 1 – Arsenic concentrations in upper Midwest public water systems Figure 2 – Measured arsenic concentrations in water and sediment collected from western Minnesota. 56 �INFLUENCE OF GEOLOGIC SETTING ON HYDROGEOMORPHIC CHARACTERISTICS OF SOUTHERN LAKE SUPERIOR TRIBUTARIES FITZPATRICK, Faith A., U.S. Geological Survey, 8505 Research Way, Middleton, WI 53562, fafitzpa@usgs.gov The U.S. Geological Survey has conducted assessments of historical and current geomorphic, sediment, and flooding characteristics for several Lake Superior tributaries in the Duluth, Minnesota and Bayfield County, Wisconsin areas. These assessments were done to determine historical natural and human-caused alterations in aquatic habitat and to provide base line data for stream restoration activities. For streams with little or no bedrock control, such as those in the Bayfield County area, historical geomorphic responses to clear-cut logging and burning in the late 1800s and subsequent agriculture in the early 1900s included channel incision and lateral migration in upstream reaches and aggradation, widening, and lateral migration in downstream reaches (Fitzpatrick et al., 1999). However, these geomorphic responses to changes in land cover were dependent on composition of glacial deposits, type of glacial or glaciolacustrine landforms, spatial position within the watershed, and relative timing of large floods. Geomorphic responses to increased runoff and sediment inputs were manifested during extreme floods that occurred one to two decades after maximum agricultural activity (Fitzpatrick and Knox, 2000). For streams with bedrock control, such as those in the Duluth area, the main geomorphic response to land clearing, such as urban sprawl, is channel widening. The major source of sediment in most of the southern Lake Superior tributaries has been and continues to be from channels down-cutting through relict glacial lake shorelines. Longitudinal profiles of the tributaries are a reflection of a combination of glacial and glacio-lacustrine landforms and bedrock topography (fig. 1). Slopes are steep where streams intersect glacial lake shorelines and (or) bedrock near the surface. Longitudinal profiles are a useful reconnaissance tool for identifying stream reaches prone to erosion or sedimentation. Inflection points on the longitudinal profiles represent reaches with slope transitions. These transitional reaches tend to be the most sensitive to changes in water and sediment inputs. 1,500 Miller Creek, Duluth, MN 1,400 ALTITUDE, IN FEET 1,300 1,200 Cranberry River, Herbster, WI 1,100 1,000 900 800 700 North Fish Creek, Ashland, WI 600 25 20 15 10 5 0 RIVER MILE FROM MOUTH Figure 1. Longitudinal profiles for three Lake Superior tributaries. REFERENCES Fitzpatrick, F.A., and Knox, J.C., 2000, Spatial and Temporal Sensitivity of Hydrogeomorphic Response and Recovery to Deforestation, Agriculture, and Floods: Physical Geography 21(2): 89-108. Fitzpatrick, F.A., Knox, J.C., and Whitman, H.E., 1999, Effects of Historical Land-Cover Changes on Flooding and Sedimentation, North Fish Creek, Wisconsin: U.S. Geological Survey Water-Resources Investigations Report 99-4083, 12 p. 57 �IRON NODULE RESEARCH AT THE NATURAL RESOURCES RESEARCH INSTITUTE, UMD FOSNACHT, Donald R., Center for Applied Research and Technology Development, Natural Resources Research Institute, University of Minnesota Duluth, Duluth, MN IWASAKI, Iwao, and BLEIFUSS, Rodney, Coleraine Minerals Research Laboratory, Natural Resources Research Institute, University of Minnesota Duluth, Coleraine, MN Synopsis A program jointly funded by the Economic Development Administration and the University of Minnesota Permanent University Trust Fund has been underway at the Coleraine Minerals Research Laboratory over the last two years. During this period, significant research and development has taken place using a variety of new furnace capabilities to test the responsiveness of iron ore taconite concentrates to reduction and smelting under a variety of test conditions. During the coarse of the investigation, over 1150 tests were undertaken using a laboratory tube furnace, these were supplemented by over 200 tests from a 2-stage laboratory box furnace, and finally by dozens of test from a 3-stage pilot-scale linear hearth furnace. During the program, the conditions for producing satisfactory iron nodules with low sulfur, gangue, and tramp impurity levels were elucidated. A variety of reductants, slag fluidizers, and iron ore mixtures were employed during the test program. Laboratory Tube Furnace Tests The test program was initiated using a tube furnace (see Figure 1) with a 2” dia. x 48” long mullite tube, which takes 1” wide x 4” long and 1” high graphite boat, to screen the test conditions for use in laboratory box and pilot plant linear hearth furnaces. Major parameters investigated included such raw materials as: (1) taconite concentrates with different levels of silica content as well as pellet plant wastes and screened pellet fines, (2) different carbonaceous reductants including Eastern anthracite, low-, medium- and high-volatile bituminous and Western sub-bituminous coals as well as their carbonized char and coke, and (3) different types of additives, such as balling binders and some specific additives for slag fusion temperature reduction and iron nugget sulfur control. Furnace operating conditions included temperature and time at temperature, furnace atmosphere, hearth layer materials, iron nugget and slag chemistries as well as iron nugget size. Environmental issues of concern are slag disposal and/or utilization and effluent emission of mercury, NOX, SOX and particulate matter. Figure 1: Laboratory Tube Furnace In the tube furnace, over 1150 different conditions have been tested. Test results demonstrated that larger-sized iron nuggets can be routinely produced by feeding dry raw material mixtures without prior agglomeration. Taconite concentrates with different levels of silica indicated that magnetic concentrates with 6% SiO2 produced metallic iron nuggets more readily than a more expensively produced super-concentrate of 2% SiO2. Pellet plant wastes and screened pellet fines produced satisfactory iron nuggets, but consisting mainly of hematite, these raw materials appeared to require somewhat different conditions. Laboratory Box Furnace Tests A laboratory, electrically-heated box furnace (see Figure 2), having two 12”x12”x12” heating chambers with the two chambers capable of controlling temperatures up to 1450°C (2642°F) independently, and which accepts a 5” wide x 6” long x 1-1/2” high graphite tray was designed and constructed during the project. Over 220 different conditions have been tested in the box furnace, confirming the results obtained in the tube furnace for both dry balled feed and a feed without prior agglomeration. In the box furnace, a major emphasis was placed in developing 58 �methods to produce larger-sized iron nuggets by feeding dry raw material mixtures in an attempt to circumvent costly balling and drying steps. A series of different size iron nuggets were produced, ranging from 5/15” to 2-1/2” (7 to 65 mm) in size. Figure 2: Laboratory Box Furnace Pilot-plant Linear Hearth Furnace Tests (Rotary Hearth Simulator) The natural gas-fired pilot-scale linear hearth furnace simulator has been installed and commissioned. The furnace is a forty-foot long iron reduction furnace (see Figure 3), consisting of three individual heating zones and a final cooling section. Sample trays are conveyed through the furnace by a hydraulically driven walking beam system. Zones are controlled individually according to temperature, pressure and feed rate, making this furnace capable of simulating several reduced iron processes and operating conditions. An Allen Bradley PLC micro logic controller coupled to an Automation-Direct PLC for the walking beam mechanism controls the furnace through a user-friendly PC interface. The PLC control system regulates individual zone burners to manage zone temperatures. A pair of 450,000 BTU/hr natural gas fired burners heats zones one and two. Zone one is rated for a continuous operating temperature of 2000 oF, while zone two can be continuously operated up to 2400 oF. Zone three is fired by a pair of 1 Million BTU/hr burners that were required to achieve the operating temperatures of 2600 oF in reasonable time to complete testing. Each zone has an individual exhaust duct and control damper to regulate pressure in that zone. A manually controlled exhaust fan damper is also installed to reduce the capacity of the exhaust fan, and allow the individual duct control dampers to manipulate pressures to desired set-points. Reducing the burner air, to operate the burners sub-stoichiometric, and operating zone pressures positive is required to reduce oxygen levels to 0.0% and provide acceptable furnace atmospheres for iron reduction. Figure 3: Pilot Scale Linear Hearth Furnace (Rotary Hearth Simulator) Major differences in the test conditions from laboratory electric furnaces were the low CO/CO2 ratio and high turbulence of the furnace gas and the sample pallets acting as an unexpectedly large heat sink. Research work focused on careful adjustment in the amount of coal addition, and on the type and amount of additives in order to minimize the generation of micro nuggets. The Linear Hearth Furnace has routinely been used to test a variety of the test variables shown to be important from the box furnace and tube furnace tests. It is possible to make various size iron nuggets at this scale that have low amounts of micro-nuggets and which have low levels of undesirable tramp elements. The furnace is extremely useful for testing a multiplicity of test parameters in a very short period of time. 59 �OXYGENATION OF THE ARCHEAN HYDROSPHERE: EVIDENCE FROM THE EAGLE ISLAND DELTAIC COMPLEX FRALICK, Philip, Department of Geology, Lakehead University, Thunder Bay, ON, Canada, P7B 5E1, philip.fralick@lakeheadu.ca PUFAHL, Peir K., Department of Geological Sciences and Geological Engineering, Queens University, Kingston, ON, Canada, K7L 3N6, pufahl@geol.queensu.ca The Eagle Island Group is located in southern Uchi Subprovince, Canadian Shield, at the west end of Lake St. Joseph. It overlies 2713 Ma volcanic rocks and is deformed by an event, which probably occurred prior to 2702 Ma (Stott and Corfu 1991). Outcrop along the shores of Eagle Island is excellent and provides a rare opportunity to document an Early Precambrian depositional system including abundant iron formation. The lowermost 35 m of section contains three coarse-grained parasequences separated by assemblages of graded sandstone beds and iron formation (IF) (Fig. 1). The IF consists of three types: (a) dominantly magnetite-rich sediment with mm- to cm-scale, graded or ungraded siltstone interbeds: (b) cm-scale graded to sharply bounded siltstone layers either contiguous or separated by mm-thick laminae of magnetite-rich sediment: (c) sandstone beds in places separated by magnetiterich intervals. The lowermost coarse-grained parasequence consists of graded sandstones and conglomerates separated by a-type IF units. The second parasequence contains: graded sandstones and conglomerates separated by magnetite; graded sandstone lenses surrounded by magnetite; crossstratified sandstones with magnetite drapes on reactivation surfaces; low angle, laterally accreting sandstone and conglomerate packages with internal magnetite laminae; multistory conglomerates with internal magnetite drapes; and ripple laminated sandstones in a-type IF with mm-thick magnetite laminae draping avalanche surfaces in ripple trains. The upper parasequence is composed of cross-stratified, coarse-grained sandstones interlayered with conglomeratic lenses. This is sharply overlain by 73 m of b-type, gradational to a-type, IF. A 182 m thick succession of graded, medium-grained sandstone beds overly the IF and these are succeeded upwards by 67 m of trough cross-stratified, coarse-grained sandstones. Next is a 60 m thick sandstone-conglomerate assemblage consisting of sharp-sided, nongraded laterally extensive, thin conglomerate lenses interlayered with well-sorted medium-grained sandstones with internal, shallowly inclined pebble stringers. Approximately 50% of the clasts are IF. This assemblage is overlain by a thin package of turbidites capped by a-type IF. 60 �The Eagle Island Group was previously considered a deep-water submarine fan deposit (Meyn and Palonen 1980, Berger 1981). The lithofacies associations present and their architectural organization make this unlikely. The thick succession of graded beds is interpreted as shallow-water storm deposits and the trough cross-stratified sandstones represent either nearshore sands, a distributary mouth bar complex or braided fluvial channels. The overlying conglomerate-sandstone unit consists of layers characteristic of foreshore beach deposits interlayered with fluvial mouth gravel bars. The 400 m thick, coarsening upwards sequence represents a wave modified delta. Thirty-five meters of strata underlying the coarsening-upwards delta is also progradational with three parasequences building from subaqueous (lower) to strandline (middle) to braided fluvial (upper). The parasequences are separated by IF developed on flooding surfaces. The most interesting features exist in the middle (strandline) parasequence. Here, IF was deposited at periods of low stream discharge in the very proximal distributary mouth environment. Magnetite laminae drape all scales of reactivation surfaces developed in this proximal nearshore setting. In contrast the 73 m thick IF was deposited on the delta top during a major flooding event and represents a portion of the transgressive and highstand systems tract (Fig. 2) where chemical sedimentation kept pace with relative sealevel rise. The accumulation of IF only in the nearshore of this depositional system limits possible precipitation mechanisms. IF accumulation models relying on relatively constant Fe precipitation in the world ocean combined with siliciclastic starvation are not compatible with data presented here. Freshwater influx into the nearshore, and resultant increase in pH, may have been responsible for iron precipitation but accompanying fresh water dilution of marine waters makes this unlikely. Photo-synthetically induced oxygenation of the shallow nearshore is the probable cause of iron precipitation. Berger, B.R. 1981. Stratigraphy of the west Lake St. Joseph greenstone terrain, northwestern Ontario. Unpub. M.Sc. Thesis, Lakehead University, 117p. Meyn, H.D. and Palonen, P.A. 1980. Stratigraphy of an Archean submarine fan. Precambrian Research, Vol. 12, 257-285. Stott, G.M. and Corfu, F. 1991. Uchi Subprovince. In: Geology of Ontario. Ont. Geol. Sur. Spec. Vol. 4, Pt. 1, 145-238. 61 �Isocon Analysis: How To Make It Work For You. GRANT, James A., Department of Geological Sciences, University of Minnesota Duluth, MN 55812 Isocon analysis (Grant, 1986) is a simple and effective means of quantitatively estimating changes in mass or volume or concentrations in mass transfer. The method has been applied to such diverse phenomena as hydrothermal alteration, replacement, migmatites, shear zones, paleosols, silcretes, sedimentary exhalative deposits and fumarolic deposits. It may be accomplished graphically by plotting an altered composition (CiA) against an original composition (CiO) with no significant manipulation of the data. Species that have remained immobile in the process define the isocon, which is a straight line through the origin. Data points falling above the isocon represent gain, and those below represent loss, of the corresponding chemical species, and the slope of the isocon gives the mass change in the process. It is critical to obtain as close an approximation to the original rock composition as possible, since that rock no longer exists. Sampling the altered rock is generally less problematic, even if the alteration is zoned. Given zonation or general heterogeneity of the rocks, judicious sampling and averaging of samples is necessary. Fortunately it is painless to try different combinations and arrive at a reasonable compromise. Scaling should be important only in producing a satisfactory isocon diagram to portray the model in question. If the slope of the isocon is based on CiA/CiO values, scaling cannot affect the results because the scale factor cancels out. Scaling can however affect the perception of the results especially if points are crowded close to the origin. The choice of immobile species can be determined by inspection of a well-constructed isocon diagram, by inspection of CiA/CiO values, by statistical methods like that of Baumgartner and Olsen (1995), or by plotting pairs of species like Cail and Cline (2001). In any case the geochemical characteristics of the species and of the process involved need to be considered thoughtfully. Characterization of an isocon based on immobile species (as opposed to constant mass, volume or predetermined species) devolves to defining a slope for the isocon. This may be done graphically, by a least squares method of linear regression, or by averaging the slopes for the immobile species. Commonly there is some range within which reasonable isocons could be chosen. Data points that lie close to a chosen isocon within such ranges would correspond to small gains and losses, and not much significance should be placed on them. Data points far from the range of possible isocons will not be affected significantly by the minutiae of the choice of isocon. Log-log plots do not add anything to the analysis, except the possibility of confusion, and should be avoided. 62 �RFERENCES Baumgartner, L. P. and Olsen, S. N., 1995. A least-squares approach to mass transport calculations using the isocon method. Economic Geology, 90, 1261-1270. Cail, T. L., and Cline, J. S., 2001. Alteration associated with gold deposition at the Getchell Carlin-type gold deposit, North-central Nevada. Economic Geology, 96, 1343-1359. Grant, J. A., 1986. The isocon diagram - a simple solution to Gresens' equation for metasomatic alteration. Economic Geology, 81, 1976-1982. 63 �THE GEOLOGY OF THE DULUTH COMPLEX AND THE NORTH SHORE VOLCANIC GROUP PORTRAYED IN NEW 7.5' QUADRANGLE MAPS OF THE DULUTH METROPOLITAN AREA GREEN, J.C., Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 (jgreen@d.umn.edu) MILLER, J.D., Jr., Minnesota Geological Survey, c/o Dept. of Geological Sciences, University of Minnesota, Duluth, MN 55812 The most current bedrock geologic map of the Duluth area is the 40 year old map by R.B. Taylor (1964), which was the first detailed-scale (1:24,000), full color map of Precambrian bedrock geology published by the Minnesota Geological Survey. This map, which built on the earlier field studies of Grout (1918) and Schwartz (1949), served as a companion to MGS Bulletin 44 and focussed mainly on the geology and petrology of the Duluth Complex. Taylor's map divided the complex into two major series, the layered series and the anorthositic series, and several subordinate units. The volcanic rocks of the North Shore Volcanic Group (NSVG), which compose the hanging wall and part of the footwall of the Duluth Complex, were not subdivided, but the major mafic sills that intrude the hanging wall volcanic rocks were delineated. The first author (Green) has conducted reconnaissance mapping in the Duluth area since 1960, focussing mostly on the volcanic rocks of the Duluth Quadrangle. In 1992, the Minnesota Geological Survey received funding from the Minnesota Minerals Coordinating Committee to conduct detailed mapping in nine quadrangles encompassing the Duluth metropolitan area. Approximately five months of intense field mapping resulted in the production of a 1:48,000-scale open-file map (Miller, Green and Chandler, 1993). We have conducted intermittent mapping over the past 11 years, which has continued to improve our understanding of the Duluth area. These field data are currently being digitally compiled into new 1:24,000-scale maps for the Duluth, Duluth Heights, and West Duluth quadrangles and parts of the Esko and Adolph quadrangles. Preliminary versions of these maps will be displayed as a poster presentation. One of the most important contributions of these new maps is their detailed delineation of the igneous stratigraphy of the layered series. As Taylor (1964) recognized, the layered series at Duluth (DLS) is a well-differentiated, 3- to 4.5-km-thick, moderately east-dipping, sheet-like mafic layered intrusion. Our mapping has subdivided the DLS into six major zones and various subzones on the basis of dominant cumulate rock types. A 150-300m-thick basal contact zone is composed of coarse-grained, taxitic olivine gabbro and augite troctolite. This is overlain by a 300-600m thick zone of complexly layered cumulate rocks including feldspathic dunite, oxide peridotite, melatroctolite, augite troctolite and olivine gabbro that comprise the melanocratic zone. These rock types commonly define several macrocyclic subzones grading from dunite/melatroctolite upward to augite troctolite/olivine gabbro. However, in many areas, melatroctolite transgresses augite troctolite. The structural complexities of this zone are interpreted to represent the effects of multiple intrusions during the early inflation stage of the DLS magma chamber. The next zone up is the troctolite zone. It is 700-1200m thick, consists mostly of homogeneous foliated troctolitic cumulates. The cyclic zone forms the medial section of the DLS and is characterized by cyclical variations in cumulus mineralogy between troctolitic and gabbroic cumulates. At least four major troctolite-gabbro macrocycle subzones are delineated within the well-exposed southern extent of the cyclic zone. Miller and Ripley (1996) suggested that the cyclicity formed by pressure fluctuations attending magma venting episodes. The persistent occurrence of gabbroic cumulates defines the next unit, the 600m- to 1700m-thick gabbro zone. Gabbroic cumulates, in turn, grade upward into nonfoliated (noncumulate) apatitic quartz ferromonzodiorite, which composes most of the 50m- to 200m-thick upper contact zone. This quartz ferromonzodiorite complexly mixes with a fine-grained biotitic ilmenite ferrodiorite, which forms the "chilled" DLS contact with anorthositic series rocks. A couple of small bodies of melanogranophyre, which irregularly cut through the anorthositic series, probably represent the uppermost differentiate of the DLS. This igneous stratigraphy, which is complimented by cryptic 64 �layering of cumulus mineral compositions, implies that the DLS formed by bottom-up, open-system fractional crystallization of a moderately evolved, olivine tholeiitic magma. The structurally and lithologically complex anorthositic series (AS), which caps the layered series, is subdivided into four units: 1) a troctolitic anorthosite unit that fringes the lower part of the series; 2) an ophitic olivine leucogabbro unit that occurs mainly as inclusions in the upper part of the layered series; 3) a plagioclase-phyric gabbro unit that occurs along the upper contact; and 4) a main unit of undifferentiated gabbroic anorthosite that comprises 90% of the AS. Although their physical relations clearly indicate that the DLS intruded the AS, precise U/Pb zircon dating (Paces and Miller, 1993) shows essentially identical ages of 1099 Ma for both series. A more complete picture of geologic structure in the Duluth area is also portrayed by these new maps. Much of the area is cut by ENE to ESE-trending faults. With the exception of the fault zone exposed in Stewart Creek, these faults are speculative and inferred from topography, aeromagnetic data, and geologic offset. One of the more enigmatic structural features of the area is an antiform-synform duplex defined by foliation and layering in the Spirit Mountain area (West Duluth quadrangle). The limb between these fold structures is near vertical and their N-S fold axes are doubly plunging. By their orientation, they do not appear to be related to faulting. Perhaps the folds developed by collapse of the cumulate pile overlying a feeder zone. Another major improvement on the geologic picture of the Duluth area is in the subdivision of the North Shore Volcanic Group and related hypabyssal intrusions, which form the Duluth Complex hanging wall. The AS intruded normal-polarity lavas of the NSVG, of which approximately 2445 m form the section within the Duluth quadrangle. These volcanic rocks are intruded by the 600m thick Endion sill near the middle, the lensing Northland sheet in the upper part, and the Lester River sill at the top, plus two minor diabasic intrusions along with several late basaltic dikes. Several distinctive and mappable volcanic and sedimentary units within this section include five large felsic flows that constitute 37% of the sequence (Green and Fitz, 1993). From lowest to highest, these are the City Hall icelandite, the Congdon Park rhyolite, the 40th Ave. East icelandite (dated at 1098.4+/-1.9Ma, Davis and Green, 1997), the 42nd Ave. East rhyolite, and the Lester Park icelandite. The lowermost lavas are contact-metamorphosed by the Duluth Complex to pyroxene-hornfels to hornblende-hornfels facies; the remainder show greenschist and laumontite-prehnite-pumpellyite assemblages due to burial metamorphism (Schmidt, 1993). References Davis, D.W, and Green, J.C., 1997, Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution. Canadian Journal of Earth Science 34, p. 476-488. Green, J.C., and Fitz, T.J.III, 1993, Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent Rift plateau volcanics, Minnesota: petrographic and field recognition. Journal of Volcanology and Geothermal Research 54, p. 177196. Grout, F.F., 1918, Internal structures of igneous rocks; their significance and origin with special reference to the Duluth Gabbro. Journal of Geology 26, 439-458 Miller, J.D., Jr., Green, J.C., & Chandler, V.W., 1993, Preliminary geologic map of the Duluth area, St. Louis County, Minnesota. Minnesota Geological Survey Open-file Report 93-2, scale 1:48,000 Miller, J.D., Jr., and Ripley, E.M., 1996, Layered intrusions of the Duluth Complex, Minnesota, USA. In Cawthorne, R.G., ed., Layered Intrusions: Amsterdam, Elsevier Science, p. 257-301. Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic, and tectonomagmatic processes associated with the 1.1 Ga Midcontinent Rift System. Journal of Geophysical Research 98, No B8, p. 13,997-14,013. Schmidt, S.Th., 1993, Regional and local patterns of low-grade metamorphism in the North Shore Volcanic Group, Minnesota, USA. Journal of Metamorphic Geology 11, p. 401-414. Schwartz, G.M., 1949, The geology of the Duluth metropolitan area. Minnesota Geological Survey Bulletin 33, 136p. Taylor, R. B., 1964, Geology of the Duluth Gabbro Complex near Duluth, Minnesota. Minnesota Geological Survey Bulletin 44, 63 pp. 65 �EFFECT OF MINERALOGY ON PROCESSING OF LOW GRADE IRON ORES FROM THE NEGAUNEE IRON-FORMATION ON MARQUETTE RANGE OF THE LAKE SUPERIOR DISTRICT HAN, Tsu-Ming, Senior Research Scientist (Retired), Cleveland-Cliffs Inc., Ishpeming, Michigan, USA During the past half-century, Cleveland-Cliffs has investigated and processed three types of lowgrade iron ores from the Negaunee Iron-Formation on the Marquette Range of the Lake Superior District. The three ore types are: (1) Specular hematite ore of medium metamorphic grade with the principal gangue of chert and locally some sericite, garnet, epidote and grunerite. (2) Magnetite ore of low metamorphic grade with the principal gangue of chert, siderite, ankerite, stilpnomelane, minnesotaite and some clastics. (3) Oxidized magnetite ore in which martite is the principal ore mineral. Ultrafine-grained hematite, microplaty hematite, goethite and earthy hematite are locally present in substantial quantities. Chert and clastic quartz are the principal gangue with local distribution of some gypsum and clay minerals (kaolin, dickite, and montmorillonite). Some minor minerals in the above ores have caused problems in the concentration process and concentrate grade control either in plant practice or from the laboratory testing. Others have caused the physical and chemical quality of the final pellet product. This paper reports the mode of occurrence of these minerals and their negative effects affecting the plant operation and the quality of the pellet product. I – Mineralogy affecting the process of concentration A – Laboratory data showed that gypsum causes total flocculation during desliming due to the release of calcium ions into solution. It prevents the rejection of slimes from flotation feed. This minimizes the selectivity of amine flotation resulting in lower the weight and iron unit recoveries as a result. B– Montmorillonite absorbs amine killing froth leading the catastrophic failure in separating ore minerals from gangue. It can easily be detected by a simple procedure referred to as “Shake Test”, or by analyzing MgO content of the crude ore before processing, i.e., in most cases, the concentrate grade is determined by the MgO content of the crude ore. II – Ore mineralogy that can affect concentrate grade A – Ultrafine-grained magnetite is typically present in the iron-formation adjacent to chloritized basic dikes and sills. The magnetite may represent as much as 40% of the ironformation. However, its grain size is generally finer than 5 microns and it is practically impossible to produce a magnetic concentrate with an acceptable grade by any feasible mechanical means from this type of material. B – Ultrafine-grained hematite occurs as irregular grains and microplates of a few microns or less finely disseminated in chert. Such a hematite and chert relationship has been referred to as 66 �“Hematitic Chert”. It is the host of martite and is considered as waste, which cannot be rejected by the magnetic oxide conversion process (MOC) but can be partially removed by the amine flotation. III – Mineralogy that can affect pellet physical quality Graphite can be rejected during desliming and flotation. However, some of it is entrapped in the magnetic concentrate for palletizing. It causes internal fusion of pellets due to the differential rate of heat transfer and oxygen diffusion toward center of the pellets. This leads to the development of concentric cracking in the pellets and consequently lowers the physical quality of the pellets during transfer. The resulting structural weakness in the pellets can be minimized by the addition of hematite to the balling feed to enhance oxygen availability IV – Mineralogy that can affect pellet chemical quality A – Phosphorus in the oxidized ore occurs as apatite, which has been designated as P1, and as an impurity in goethite and dove-tailed hematite, as P2. Most of the P1 can be rejected during the desliming and flotation stages, whereas the P2 increases with the increase of iron in the concentrate. In order to produce an acceptable final product containing less than 0.03% P from some of the highly oxidized ore, a substantial amount of these ore minerals has to be rejected by high intensity magnetic separation. B – Titanium is present as rutile in some specular hematite. It is practically impossible to separate from its specular hematite host by any known mechanical means. Consequently, it reports with the iron in the concentrate from the specular hematite. C – Stilpnomelane and minnesotaite contain the potassium and sodium. These constituents are a detriment to blast furnace refractory. Most of these minerals are rejected during magnetic separation and flotation. Some of them are entrapped in the magnetite fines as a result of magnetic flocculation. Generally, the alkaline content in the pellets has not been a major problem. Specular hematite concentrate is no longer produced. Oxide ore containing montmorillonite or gypsum remain to be successfully processed. Magnetite iron-formation adjacent to the intrusives has been considered as “waste” and removed by selective mining. 67 �Geochemistry of the Proterozoic Intrusive Rocks of the Nipigon Embayment HART, T.R., Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5 (tom.hart@ndm.gov.on.ca) The Nipigon Embayment is an approximately 19 000 km2 area of Proterozoic igneous and sedimentary rocks centred on Lake Nipigon, north of Lake Superior and approximately 110 km northeast of Thunder Bay. There are three geochemically distinct Proterozoic intrusive rock types in the Nipigon Embayment, the southern ultramafic intrusions, the Kitto-Jackfish intrusions and the Nipigon Sill Complex (Hart, 2003). The southern ultramafic intrusions consist of the Disraeli, Seagull-Leckie, and Hele intrusions located in the area between Lake Nipigon and Lake Superior. These intrusions are composed of a pyroxene peridotite core with irregular olivine gabbro zones along their margins, and intrude the rocks of the Archean Quetico Subprovince and the Proterozoic Sibley Group. Their present geometry is sill-like, but all three intrusions are composed predominantly of cumulate wehrlite and lherzolite and may represent the remnants of large bodies. The intrusions have calc-alkaline affinities with [La/Yb]mn (mantle normalized) ratios of 5.3 to 18.53 and have weakly depleted Th and little to no Nb anomalies ([Th/Yb]mn ratios of 0.78 to 1.1). The broad range in composition is probably a result of a combination of igneous processes within the magma chambers and assimilation of country rock. Evidence of assimilation is most evident in small, irregular monzogabbro pods that are geochemically similar to the surrounding olivine gabbro but have higher K2O contents. All three intrusions have REE and HFSE ratios that are comparable to ocean island basalts as has been noted for the Seagull Intrusion by Heggie and Hollings (2004). The Kitto-Jackfish intrusions range from an approximately 750 m thick sill-like body at Kitto to an approximately 50 m thick sill at Jackfish Island, English Bay area (MacDonald, 2004), and sills up to a few metres thick at Kama Hill, Nipigon Bay of Lake Superior. The Kitto Intrusion is composed of a pyroxene peridotite core with an irregular olivine gabbro border zone comparable to the southern peridotites (Hart et al., 2002). These sills intrude the rocks of the Quetico and Wabigoon subprovinces and the Sibley Group. The Kitto-Jackfish sills have calc-alkaline affinities with [La/Yb]mn ratios of 6.3 to 10.7, weak Th depletion and weak negative Nb anomalies ([Th/Nb] mn ratios of 1.1 to 1.8) and rare earth element (REE) and high field strength element (HFSE) contents similar to the southern peridotites. The 68 �trend of these intrusions on a La/Sm - Gd/Yb diagram could be a result of either a higher degree of assimilation than the southern peridotites or an indication of a different magma source. The presence of platinum group element (PGE) mineralization in the Seagull and Kitto intrusions suggests that differences in geochemistry are not a useful tool in area selection during exploration. A series of generally flat-lying to shallow-dipping diabase sills of the Nipigon Sill Complex ranging from a few metres to greater than 100 m in thickness intrude both the southern ultramafic intrusions and the Kitto intrusion. The 1109 Ma sills (Davis and Sutcliffe, 1985) have tholeiitic affinities with [La/Yb]mn ratios of 1.61 to 3.29 and moderate negative Nb anomalies ([Th/Nb]mn ratios of 1.8-3.1). Although the sills probably formed as a result of multiple injections of magma, geochemistry suggests that many of the sills are single cooling units. There is geochemical evidence of assimilation of the country rock, which is supported by field relationships. The effects of assimilation are most evident in the chilled margins, although sills of similar thickness appear to display variability in composition suggesting that the degree of assimilation depends on the composition of the country rock. The Nipigon diabase sills have lower TiO2 (0.87 to 2.0 wt.%), Zr/Y (4.2 to 2.7) and [La/Yb]mn ratios than the Logan sills (TiO2: 3.45 to 3.79 wt.%; Zr/Y: 5.5 to 7.7; [La/Yb]mn: 6.5 to 7.8) located south of Thunder Bay. The [La/Yb]mn ratios of the Logan sills are comparable to the ultramafic intrusions of the Nipigon Embayment, but their REE and HFSE contents are about 3 times higher than the intrusions. Similar geographic related geochemical variations have been observed in some flood basalt provinces (e.g., Mantovani et al. 1985). References Davis, D.W. and Sutcliffe, R.H. 1985. U-Pb ages from the Nipigon plate and northern Lake Superior; Geological Society of America Bulletin, v.96, p.1572-1579. Hart, T.R. 2003. Keweenawan Mafic and Ultramafic Intrusive Rocks of the Lake Nipigon and Crystal Lake areas, northwestern Ontario; in Part 1: Programs and Abstracts, Institute on Lake Superior Geology, Proceedings Volume 49; Iron Mountain, Michigan, May 7-11, 2003. Hart, T.R., terMeer, M. and Jolette, C. 2002. Precambrian Geology of Kitto, Eva, Summers, Dorothea and Sandra Townships, Beardmore Area, Northwestern Ontario. Ontario Geological Survey, Open File Report 6095, 206p. MacDonald, C.A. 2004. Precambrian geology of the south Armstrong-Gull Bay area, Nipigon Embayment, northwestern Ontario; Ontario Geological Survey, Open File Report 6136, 42p Mantovani, M.S.M., Marques, L.S., de Sousa, M.A., Civetta, L., Atalla, L. and Innocenti, F. 1985. Trace element and strontium isotopic constraints on the origin and evolution of Parana continental flood basalts of Santa Catarina State (southern Brazil); Journal of Petrology, v.26, p.187-209. 69 �Precambrian Geology and Mineralization of the Northern Black Sturgeon River area, Nipigon Embayment HART, T.R., and MAGYAROSI, Z., Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5 (tom.hart@ndm.gov.on.ca) A 1:50 000 scale bedrock mapping project in the northern Black Sturgeon River area was conducted to investigate the regional geological setting of the platinum group element (PGE) bearing mafic to ultramafic intrusion in the Seagull Lake area, in the southern portion of the Nipigon Embayment (Hart and Magyarosi, 2004). The Nipigon Embayment is an approximately 19 000 km2 area of Proterozoic igneous and sedimentary rocks centred on Lake Nipigon, north of Lake Superior. This mapping was completed by the Ontario Geological Survey as part of its commitment of in-kind support to the Lake Nipigon Region Geoscience Initiative (LNRGI). The LNRGI is a geoscience-based geological data acquisition and compilation program operated by the Ontario Prospectors Association (OPA) and funded through an agreement with the Northern Ontario Heritage Fund Corporation (NOHFC). The LNRGI also includes partnerships with the private sector, Lakehead University and communities in the Lake Nipigon area. The initiative also includes airborne magnetic and radiometric surveys, ground gravity surveys, and targeted surficial geochemical and geochronological studies. The north Black Sturgeon 280000 River map area is located approximately 110 km northeast of Thunder Bay and south of Lake Nipigon, and is underlain by 5600000 metamorphosed and deformed Archean volcanic and sedimentary rocks of the southern Wabigoon Subprovince and metamorphosed feldspathic wackes and siltstones of the Quetico Subprovince. Sedimentary rocks of the relatively flat-lying 1340 Ma Sibley Group (Franklin, 1978), consisting of conglomerates, sandstones, LLaakkee mudstones, siltstones, limestone, N Niippiiggoonn unconformably overlie the Archean rocks. Proterozoic mafic to ultramafic intrusions in the Disraeli Lake and the Seagull–Leckie lakes areas intrude the Quetico Subprovince and Sibley Group. Both intrusions are composed of a pyroxene peridotite core with an irregular olivine gabbro zones along the margin. A series of undulating, generally flat-lying to shallow-dipping 1109 Ma diabase sills of the Nipigon Sill Complex 5400000 (Davis and Sutcliffe, 1985) intrude 430000 all other rock units in the map area. The Black Sturgeon fault zone consists of a series of north and northwest-trending faults which form an asymmetric basin or half-graben. Less prominent northeasttrending faults probably represent reactivation of structures in the Archean basement rocks. 70 �The location of alteration and the ultramafic intrusions appear to be controlled by the distribution of faults within the Black Sturgeon fault zone. The Disraeli and Seagull ultramafic intrusions occur along the same series of north trending faults that appear to correlate with north-trending structures in the northern Gull Bay area about 80 km to the north. A parallel series of north-trending faults about 25 km to the east hosts the Hele ultramafic intrusion. The most significant PGE mineralization in the Seagull Intrusion was intersected at or near the basal contact of the peridotite with metasedimentary rocks of the Quetico Subprovince. The mineralization is interpreted by Heggie and Hollings (2004) to be magmatic, and formed as a result of sulphur saturation of the magma during initial stages of emplacement. Biotite is ubiquitous in the ultramafic intrusions, but textural evidence is unclear as to whether the biotite is a primary igneous mineral. Preliminary microprobe analyses from the Disraeli Intrusion indicates that biotite may contain up to 5 wt.% Cl, which is similar to the high Cl and F contents reported in biotite associated with PGE minerals in the Coldwell Complex (Watkinson and Ohnenstetter, 1992). The ultramafic intrusions are also cut by the faults suggesting a later reactivation. Metre to tens of metre wide zones of intense hematization occur in along these faults in the Seagull intrusion, and a 40 m thick interval in one drill hole is reported to contain a sylvite rich brine. One sample from the brine rich interval has 3.4 ppm Pt and 1.2 ppm Pd along with elevated Cu and K2O contents suggesting that late fluids could remobilize, and possibly concentrate, the PGEs. Further work is required to understand the timing of this late alteration and the metallogenic significance. References Davis, D.W. and Sutcliffe, R.H. 1985. U-Pb ages from the Nipigon plate and northern Lake Superior; Geological Society of America Bulletin, v.96, p.1572-1579. Franklin, J.M. 1978. The Sibley Group, Ontario; in Rubidium-strontium isochron age studies, report 2; ed. R.K. Wanless and W.D. Loveridge; Geological Survey of Canada, Paper 77-14, p.31-34. Hart, T.R. and Magyarosi, Z. 2004. Precambrian Geology of the northern Black Sturgeon River and Disraeli Lake Area, Nipigon Embayment, northwest Ontario; Ontario Geological Survey Open File Report 6138, 56 p. Heggie, G.J., and Hollings, P., 2004. Controls on PGE Mineralization in the Seagull Intrusion, Northwestern Ontario; Geological Association of Canada-Mineralogical Association of Canada, Joint Annual Meeting, St. Catharines 2004, Program with Abstracts. Watkinson, D.H. and Ohnenstetter, D. 1992. Hydrothermal origin of platinum-group mineralization in the Two Duck Lake Intrusion, Coldwell Complex, northwestern Ontario; Canadian Mineralogist, v.30, p.121-136. 71 �Multiple Intrusive Stages Associated with Keweenawan Rifting: The Leckie Stock, Seagull Intrusion, and Nipigon Sill HEGGIE*, G., and HOLLINGS, P., Department of Geology, Lakehead University, 955 Oliver Rd., Thunder Bay, Ontario, P7B 5E1, Canada; gheggie@lakeheadu.ca Igneous activity related to Keweenawan Rifting (ca. 1108 Ma, Davis and Green, 1997) has produced a wide spectrum of lithologies and numerous igneous suites (Osler Volcanics, North Shore Volcanics, Mamainse Point Volcanics, Duluth Complex, Logan Sills, Nipigon Sills, Coldwell Complex, English Bay Complex, Eva Kitto Intrusion, Seagull Intrusion, Leckie Stock). Found on the periphery of Lake Superior and Lake Nipigon these igneous bodies have been studied for 95 years (first mapped in Canada by Wilson 1910), as work further expands our understanding of the magmatic history, the more complex igneous relationships become. Rift related intrusive rocks outcrop extensively around Lake Nipigon (Figure 1). The most abundant intrusions in this region are the Nipigon Sills (Sutcliffe 1986, Hart and McDonald 2003). Other igneous bodies have been identified in the process of PGE, Cu and Ni exploration. These intrusive bodies, including the Seagull Intrusion and Leckie Stock, although not as expansive as the Nipigon Sills still play an integral part in the development of a unified petrogenetic model. Figure 1. Location of intrusive, extrusive, and sedimentary rocks associated with the Keweenawan Rifting event. Modified after Sutcliffe, 1991. Contact relationships between the three intrusive suites (Leckie Stock, Seagull Intrusion, and Nipigon Sill) are not yet fully understood. Chill margins on a Nipigon Sill crosscutting the Leckie Stock have been identified in drill core, establishing that the Nipigon Sill post dates the 72 �Leckie Stock. Age relationships between the Seagull Intrusion and Nipigon Sill are still unknown, as is the relationship between Seagull Intrusion and Leckie Stock. Lithologically, geochemically, and mineralogically it is possible to distinguish between the three bodies. The Leckie Stock is characterized by ultramafic lithologies, with rare earth element plots similar to ocean island basalt with (La/Sm)n values of 1.01-5.21 and (Gd/Yb)n of 1.82-3.73. Mineralogically the Leckie Stock is dominated by olivine with an average composition of Fo82. This compares to the mafic lithologies (olivine gabbro - gabbro norites) of the Nipigon Sills with (La/Sm)n of 1.51-1.61 and (Gd/Yb)n values of 1.46-1.49. Olivine mineralogy in the Nipigon Sill varies more than in the Leckie Stock, and has an average olivine composition of Fo50. Lithologically the Seagull Intrusion, displays the greatest variation, varying from olivine gabbros, - gabbros to granophyres. Olivine analysed from this body has an average composition of Fo67. Mineralogical and geochemical data suggest that the Leckie Stock is the most primitive magma of the three intrusive suites. The Seagull Intrusion and Nipigon Sills are similar in terms of mineralogy and petrology in that they display overlapping olivine compositions, but REE data (Hart 2002) suggests that the Seagull Intrusion is more closely related to the ultramafic Leckie Stock, if a cross cutting relationship exists between the two (Seagull Intrusion crossing Leckie Stock). The Leckie Stock could not be a feeder zone for either the Seagull Intrusion or the Nipigon Sill, but a feeder for some yet unidentified igneous body higher up in stratigraphy. Davis, D.W., and Green, J.C., 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution. Canadian Journal of Earth Sciences. V.34, p. 476-488. Hart, T., 2002, Keweenawan Mafic and Ultramafic Intrusive Rocks on the Lake Nipigon and Crystal Lake areas, northwestern Ontario, in Institute on Lake Superior Geology, Proceedings Volume 49, Part 1 –Programs and Abstracts, p. 21-22. Hart, T., and MacDonald C.A., 2003. Lake Nipigon Region Geoscience Initiative. Proterozoic and Archean Geology of the South-Central and North areas of the Western Nipigon Embayment. Ontario Geological Survey, Summary of Field work and other Activities. Open File Report 6120. Sutcliffe, R.H., 1991. Proterozoic Geology of the Lake Superior area, in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p. 627-658. Sutcliffe, R.H., 1986. The Petrology, Mineral Chemistry and Tectonics of Proterozoic rift-related igneous rocks at Lake Nipigon, Ontario. Unpublished PhD. Thesis, The University of Western Ontario, London, Ontario. Wilson, A.W., 1910, Geology of the Nipigon Basin, Ontario. Memoir No.1 Canada Department of Mines, 152 p. 73 �40 Ar/39Ar Hornblende Evidence for Provenance of Ice Rafted Detritus in the North Atlantic: Implications for Tracking Past Changes in the Extent and Dynamics of Northern Hemisphere Ice Sheets HEMMING, Sidney R., and ROY, Martin, Lamont-Doherty Earth Observatory of Columbia University, Rt. 9W, Palisades, NY 10964 The concentration and provenance of terrigenous sand grains from North Atlantic sediment cores from pelagic environments provide important constrains on the dynamics of former Northern Hemisphere ice sheets. The association of this ice rafted detritus (IRD) signal with proxy records for climate conditions in the North Atlantic, brings additional constraints on ice sheet dynamics in a paleoclimate context. Studies at proximal locations identify configuration changes in specific ice sheets, while studies of cores in the open ocean can yield a history of variation where relative timing of iceberg discharges from different source regions as constrained by simple stratigraphic principles. An example is the case of the Laurentide ice sheet (LIS) where the distinctive variations in age provinces underlying the LIS from south to north can be used to constrain its extent and ice stream activity during the last glacial cycle. 40Ar/39Ar hornblende data from IRD indicate the ice reached Grenville and Appalachian provinces along the margin after about 30 kyr (e.g., and the Last Glacial Maximum, LGM). The increase in flux of IRD at the LGM is coincident with the increase in southeastern LIS provenance components. The last glacial cycle was punctuated by Heinrich events, massive discharges of icebergs from the Hudson Strait ice stream. The general trend in LIS development and identification of major ice stream events that are captured by the marine sediment record show the potential power of this approach for understanding the evolution of ice sheets in general. This approach can be applied throughout the Pleistocene to understand the temporal and spatial development of large volume ice sheets and the transition to the 100-ky cycle that dominated the past 700 kyrs. We seek to expand the studies both geographically and through the Pleistocene. 74 �PRELIMINARY PETROGRAPHY AND HYDROTHERMAL ALTERATION OF THE SOUDAN MINE AREA, VERMILION DISTRICT, NORTHESTERN MINNESOTA HOFFMAN, A.T.*, Dept. of Geosciences, University of Minnesota, Duluth, hoff0578@d.umn.edu PETERSON, D.M., PATELKE, R.L., Economic Geology Group, Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth MN 55811 HUDAK, G.J., Geology Department, University of Wisconsin Oshkosh, Oshkosh, WI The Ely Greenstone of the western Vermilion District is a lithologic and structurally diverse setting located in the southern extension of the Wawa Subprovince of the Superior Province of the Canadian Shield. The Wawa Subprovince is the host for a significant number of mineral deposits and showings, most notably lode gold and volcanogenic massive sulfides (VMS) (Williams et al., 1991). In the last decade, there has been a significant effort put forth to document areas in the Vermilion district that seem geologically and mineralogically compatible for hosting these deposits. This work includes reconnaissance mapping in the eastern Murray shear zone for mesothermal gold mineralization (Peterson, 2001; Peterson and Patelke, in prep.) and the re-evaluation of the Fivemile Lake, Purvis Lake, Eagles Nest, Skeleton Lake, and Needleboy Lake VMS prospects (Peterson, 2001; Hudak et al., 2002a; Hudak et al., 2002b). In the summer of 2003 mapping for a National Underground Science and Engineering Laboratory (NUSEL) was completed near the Soudan Mine in northeastern Minnesota (Peterson and Patelke, 2003). This work revealed localized lode gold and volcanogenic massive sulfide type alteration. This alteration may correlate with alteration located in the adjacent Fivemile Lake Prospect (Hudak et al., 2002b) and Murray Gold Prospect (Peterson, 2001). The goal of this project is to better understand the volcanic stratigraphy, and syn- and post hydrothermal alteration near the Soudan Mine. Eighty thin sections were analyzed to defined primary syn-volcanic alteration assemblages in the Soudan Mine area. Classification of mineral assemblages followed Hudak et al. (2002b) in effort to establish consistent hydrothermal mineral alteration nomenclature across the Vermilion District. Four mineral assemblages dominate the area: 1) Epidote + Quartz + Actinolite ± Chlorite; 2) Epidote + Quartz ± Chlorite; 3) Mottle Epidote + Quartz ± Actinolite ± Chlorite ± Albite (Epidosites); and 4) localized and lesser amounts of Garnet + Magnetite. Alteration assemblages 1 (Epidote + Quartz + Actinolite ± Chlorite) and 2 (Epidote + Quartz ± Chlorite) likely represent early-formed, semi-conformable zones associated with down welling seawater. Alteration assemblages 3 (Mottle Epidote + Quartz ± Actinolite ± Chlorite ± Albite (Epidosites)) and 4 (localized Garnet + Magnetite) appear to represent hydrothermal fluid upflow zones that may be proximal to syn-volcanic structures (Harper, 1999; Gibson et al., 1999). These results suggest that volcanogenic massive sulfide targets may be present near the Soudan Mine. Petrographic analysis is ongoing and a detailed field analysis during summer 2004 will better constrain these relationships. 75 �References Harper, G.D., 1999, Structural Styles of Hydrothermal Discharge in Ophiolite/Sea-Floor Systems: Reviews in Economic Geology v. 8, p. 53-73 Hovis, S. T., 2001, Physical Volcanology and Hydrothermal Alteration of the Archean Volcanic Rocks at the Eagles Nest Volcanogenic Massive Sulphide Prospect, Northern Minnesota. Unpublished M.S. Thesis, University of Minnesota Duluth, Duluth, Minnesota, 102 p. Hudak, G.J., Heine J., Hocker, S.M., Hauck, S., 2002a, Geologic Mapping of the Needleboy Lake-Six Mile Lake Area, Northeastern Minnesota: A Summary of Volcanogenic Massive Sulfide Potential. Report of Investigations NRRI/RI-2002/14. Hudak, G.J., Heine, J., Newkirk, T., Odette, J., and Hauck, S., 2002b. Comparative Geology, Stratigraphy, and Litho-Geochemistry of the Five Mile Lake, Quartz Hill, and Skeleton Lake VMS Occurrences, Vermilion District, NE Minnesota. NRRI Technical Report NRRI/PR-2002/03, 390 p. Peterson, D.M., Patelke, R. L., 2003 National Underground Science and Engineering Laboratory (NUSEL): Geologic Site Investigation for the Soudan Mine, Northeastern Minnesota. NRRI Technical Report NRRI/TR-2003/29, 87 p. Peterson, D.M., 2001 Development of Archean lode gold and massive sulfide exploration models using Geographic Information System applications: Targeting mineral exploration in northeastern Minnesota from analysis of analog Canadian mining camps: Unpublished Ph.D. dissertation, University of Minnesota Minneapolis, 502 p. Peterson, D.M., and Patelke, R.L., in Prep, Neo-Archean gold mineralization in the Mud Creek area, Northern St. Louis County, Minnesota: Natural Resources Research Institute, University of Minnesota Duluth. Williams, H.R., Stott, G.M., Heather, K.B., Muir, T.L., and Sage, R.P., 1991, Wawa Subprovince, in Thurston, P.C., Williams, H.R., Sutcliffe, R.H., and Stott, G.M., eds., Geology of Ontario: Ontario Geological Survey Special Volume 4, Part 1, p. 485-539. 76 �Trace Element Geochemistry of the Osler Group Volcanics – Implications for Mid-Continent Rifting HOLLINGS, Pete, Department of Geology, Lakehead University, 955 Oliver Rd., Thunder Bay, Ontario, P7B 5E1, Canada; peter.hollings@lakeheadu.ca To date three major Proterozoic events have been recognized on the northern margins of Lake Superior (Sutcliffe, 1991); 1) the Paleoproterozoic Animikie Group sediments (~1.86 Ga), 2) ~1.54 Ga Mesoproterozoic anorogenic granites and Sibley Group sediments, 3) Mesoproterozoic rifting at ~1.11 to 1.09 Ga during which the Keweenawan Supergroup was deposited. The Osler Group volcanics are found towards the base of the Keweenawan Supergroup. The Osler Group volcanics comprise a bimodal sequence of basalts and less abundant rhyolites that are well exposed on a series of islands off Rosport on the northern shore of Lake Superior and also as sparse outcrop on the Slate Islands off Terrace Bay (Fig. 1). The Osler Group has been described as a thick sequence of tholeiitic flood basalts with up to 2800m exposed on the Black Bay Peninsula (Sutcliffe, 1991). The majority of the sequence comprises magnetically reversed Lower Keweenawan flows with a small section of magnetically normal flows at the top of the sequence (Halls, 1974). Davis and Sutcliffe (1985) have reported U-Pb ages of 1107.5 +4/-2 Ma for a rhyolite from the base of the sequence and 1097.6 ±3.7 Ma for a rhyolite towards the top. Figure 1. Location of the intrusive, extrusive and sedimentary rocks associated with Keweenawan Rifting. Modified after Sutcliffe (1991). 77 �Detailed sampling traverses were undertaken on Wilson and Vein islands in the summer of 2002, in order to investigate geochemical variations within the Osler Group. Exposure on the two islands is excellent, beginning at the northern end of the islands with a basal conglomerate that includes abundant clasts of Sibley sediments. The conglomerate is strongly heterolithic and indicates an approximately northward flow direction. The volcanic units comprise numerous flows ranging from a few centimeters to a few metres in thickness. Individual flows are frequently marked by rubbly flow tops or ropey, pahoehoe textures, while thicker flows may show well developed columnar jointing (Fig. 2). Figure 2. A) Columnar basalts within the Osler Group on Simpson Island. B) Pahoehoe texture in thin basalt flow on Wilson Island. SiO2 and MgO contents of the Osler Group volcanics on Wilson and Vein Islands range from 4754 wt. % and 5-15% respectively, consistent with data from earlier studies. Basalts are characterized by weak to moderate LREE enrichment (La/Smn = 1-5), weakly fractionated HREE (Gd/Ybn = 2-4) and display primitive mantle normalised patterns comparable to modern Ocean Island Basalts. Preliminary examination of REE data for the Group suggests that the La/Smn ratio, a good indicator of crustal contamination, increases towards the top of the sequence (ranging from ~1.5 near the base to ~5 near the top). References Davis, D. and Sutcliffe, R., 1985. U-Pb ages from the Nipigon plate and northern Lake Superior. Geological Society of America Bulletin, 96, 1572-1579. Halls, H. C., 1974. A paleomagnetic reversal in the Osler volcanic group, northern Lake Superior. Canadian Journal of Earth Sciences, 11, 1200-1207. Sutcliffe, R., 1991. Proterozoic geology of the Lake Superior area. In. Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1. 627-660. 78 �The Influence of Radiometric Dating for Unraveling the Precambrian Geologic History of the Lake Superior Region HOLM, Daniel K., Dept. of Geology, Kent State University, Kent, OH 44242, dholm@kent.edu VAN SCHMUS, R.W., Dept. of Geology, Univ. of Kansas, Lawrence, KS 66045 SCHNEIDER, D.A., Dept. of Geological Sciences, Ohio University, Athens, OH 44701 The paucity of biostratigraphic controls in Precambrian rocks has long made radiometric dating of primary importance for unraveling the complex geologic history in the Lake Superior region (Goldich, 1968). The first ILSG meeting in 1955 occurred very close to the time of initiation of a radiometric dating program by S.S. Goldich and A.O Nier in 1956. Subsequent geochronologic investigations laid the theoretical and practical foundation upon which current radiometric studies are expanding. Thus the tremendous influence of geochronologic studies in this region, only briefly summarized here, has been well chronicled at ILSG meetings over the past 50 years. Pioneering studies in K-Ar, Rb-Sr, and U-Pb dating. Early studies by Goldich et al. (1961, 1970), Aldrich et al. (1965), and Peterman (1966) were pioneering applications of radiometric dating which provided a broad-brush means of correlation (based on age rather than on degree of metamorphism and deformation) and contributed to the formation of a world-wide time scale for the Precambrian (Goldich, 1968), flatteringly referred to by some as “Goldich’s time scale”. The early results represented a significant breakthrough in Precambrian geology, yet proved difficult to interpret as co-existing minerals gave different ages for the same decay scheme. Comparative mineral studies by Aldrich et al. (1965) and Hanson and Gast (1967) led to the recognition of metamorphic effects on various isotopic systems (especially K-Ar and Rb-Sr). For instance, Van Schmus et al. (1975a) used Rb/Sr age data to document the existence of a widespread but poorly understood low-grade 1650 Ma metamorphic event in Wisconsin. The 2700 Ma age of the greenstone-granite belts in Minnesota and southern Ontario was documented early on by whole-rock Rb-Sr ages (Jahn and Murthy, 1975), K-Ar, and U-Pb ages (Peterman et al., 1972). The large analytical uncertainty in these data, however, precluded their use for resolving the time of important, short-lived, geological events (plutonism, deformations) that occurred during their formation and accretion. Likewise, the errors on initial U-Pb zircon ages on Mid-continent Rift rocks would not allow for differentiation of magmatic pulses, but simply suggested “a sharp pulse of igneous activity” at 1115+15 Ma (Silver and Green, 1963). Early U-Pb zircon work in the Minnesota River Valley temporarily produced some of the world’s oldest dated rocks (Catanzaro, 1963; Goldich et al., 1970; Goldich and Hedge, 1974), and clearly demonstrated that K-Ar and Rb-Sr mineral ages reflect younger metamorphic events. For instance, using K-Ar and Rb-Sr ages, Goldich et al. (1961) first bracketed the Penokean orogeny as having occurred between 1800 and 1600 Ma. Similarly young K-Ar biotite ages (compared to U-Pb zircon ages from the same rock) from Precambrian shields and orogenic belts worldwide led to the concept of a ‘metamorphic veil’ (Armstrong, 1966) which reflects younger overprinting events (common in Precambrian terranes) and obscures the age of older rock forming events. Modern U-Pb dating (bulk aliquot to single crystal to spot dating). In the 1970’s and 1980’s, advances in conventional U-Pb zircon analyses (Krogh, 1973, 1982) allowed geochronologists to see through the ‘metamorphic veil’ and firmly established the age spectrum of igneous activity in the region (Silver and Green, 1972; Van Schmus et al., 1975b; Van Schmus, 1976; 1980). For instance, the age of the Penokean orogeny was definitively bracketed between 1870-1830 Ma by Van Schmus (1976, 1980), who first invoked the plate tectonic concept of a southern magmatic arc colliding with a northern passive margin. Archean age gneisses (Marshfield terrane) were dated south of the Penokean magmatic arc rocks (Van Schmus and Anderson, 1977) and Early Archean ages were obtained from gneiss dome rocks north of the Niagara fault zone (Peterman et al., 1980). The greatly improved accuracy in U-Pb zircon dating has allowed researchers (Boerboom and Zartman, 1993; Corfu and Stott, 1986, 1998; Davis et al., 1989; Zaleski et al., 1999; Ayers et al., 2002) to document the time of short-lived magmatic and deformational events commonly involved in the 79 �formation of late Archean granite–greenstone belts. Enough high precision data has been obtained (i.e., Shebandowan, Vermillion, and Manitouwadge) to suggest correlation of volcanic assemblages and their subsequent deformation along 600 km of strike (Peterson et al., 2001). New U-Pb single-crystal zircon studies continue to refine the earlier plutonic ages, which were obtained on mg-size fractions. Post-Penokean “1760 Ma” plutonism (Sims et al., 1989) is now known to have actually occurred in southeast younging pulses at ca. 1800, 1775, and 1750 Ma, possibly reflecting flip in subduction polarity and slab-rollback of Yavapai-age oceanic lithosphere (Holm et al., 2004). The Penokean orogenic belt twice formed the source region for generation of crustal-melt batholiths; first at ca. 1775 Ma with intrusion of the East-central Minnesota batholith (Holm et al., 2004) and again at 1470 Ma in central Wisconsin (Van Schmus et al., 1975). The new single crystal results show that the Eastcentral Minnesota batholith was emplaced over a ca. 20 m.y. duration (Holm et al., 2004; Keatts et al., 2004), whereas Wolf River magmatism was relatively short-lived (with nine different phases intruded over a <5 m.y. interval; Dewane and Van Schmus, 2003). New important U-Pb zircon results have also been obtained from volcanic rocks over the last decade or so (see Van Schmus and Hinze, 1985 for a summary of earlier age data). High precision U-Pb geochronology on the volcanics of the Midcontinent Rift system indicate rapid eruption over a relatively short duration (1108-1094 Ma; Davis and Paces, 1990; Paces and Miller, 1993; Davis and Green, 1997; Zartman et al., 1997). The great volume of magma generated over this short time span and the limited degree of extension strongly favor a mantle plume origin. New geochronology on interlayered volcanics within the Marquette Range Supergroup yield essentially identical ages of 1875 Ma for the Hemlock (Schneider et al., 2002) and Gunflint (Fralick et al., 2002) Formations. The revised age constraining the timing of deposition of the Supergroup across the Animikie basin suggests these sediments were laid down coeval with arc formation to the south and supports the hypothesis of Hoffman (1987), who first proposed that the iron formations are syn-Penokean foredeep deposits. Conventional single crystal and SHRIMP U-Pb dating of detrital zircons (Van Wyck, 1995; Holm et al., 1998; Medaris et al., 2003) finally resolved the controversy over the maximum age of the Baraboo Quartzite and similar red quartzites in northern Wisconsin and Minnesota (<1750 Ma). A completely different geochronologic database described next provided firm constraints on the minimum age of quartzite deposition (>1650 Ma). Ar-Ar thermochronology. Thermochronology (the study of the time-temperature evolution of rocks) was born from the realization than many mineral-isotopic systems could be used to determine the time of cooling of a terrain through a series of predictable temperatures. Because an intimate relationship exists between the thermal and tectonic processes operating during orogenesis and subsequent events, reconstructing the thermal history of a region ultimately aids in constraining its tectonic evolution. Thermochronologic data from the region was initially dominated by the Rb/Sr method on biotite (Peterman and Sims, 1988). In the 1990’s, application of the Ar-Ar incremental heating technique on hornblende, muscovite, and biotite yielded considerable information bearing on the Proterozoic thermal history of the southern Lake Superior region (Holm and Lux, 1996, 1998; Schneider et al., 1996; Holm et al., 1998a; Romano et al., 2000). Especially important was the proposal that a separate, perhaps widespread, geon 17 amphibolite facies metamorphic event occurred after the Penokean orogeny, followed by an episode of rapid crustal exhumation (Holm et al., 1998a). Additionally, the widespread geographic distribution of basement Ar/Ar biotite ages serendipitously provided a key minimum age constraint (ca. 1630 Ma) on the overlying Proterozoic red quartzites and led to a greater appreciation of the role of younger tectonism (i.e. Mazatzal deformation) in the region (Holm et al., 1998b). New minerals, new techniques, and new directions. Rapid advances in technology (allowing higher precision, increased mass and spatial resolution, and in situ capabilities) and the application of new chronometers (monazite and xenotime) are now verifying earlier hypotheses for widespread tectonothermal events following Penokean orogenesis. Recent results of U-Pb monazite dating using both ion and electron microprobe techniques now recognize distinct metamorphic pulses at 1835, 1800, and 1770 Ma – thermal pulses which can be directly tied to known magmatic events (Schneider et al., 2004). New U-Pb xenotime ages (Vanelli et al., 2003, and in progress) from coarse sandstone/conglomerate beds 80 �of the Marquette Range Supergroup identify a widespread 1790-1760 Ma fluid flow event – an event that may have been ultimately responsible for forming the high-grade iron ore deposits in this region (Morey, 1999). Recent single crystal Ar-Ar dating of fine-grained, low-temperature minerals within the Baraboo Interval Quartzites (Medaris et al., 2003) has documented a much younger, but apparently even more vast hydrothermal fluid system generated by Mesoproterozoic plutonism. Finally, Ar-ion laser dating results reveal significant age gradients (~200 to 500 m.y.) in coarse muscovite from basement samples interpreted to signify partial resetting by fluids during both Mazatzal orogenesis and Mesoproterozoic magmatism (McKenzie, 2004; Rose, 2004). In addition to direct dating of metamorphism and fluid flow, geochronology is now being used to put absolute time (not simply time constraints) on important deformation features in the Lake Superior region. The age of the Mazatzal tectonic front in northwest Wisconsin is dated by the age of reset biotite in basement beneath the deformed quartzites. The Malmo Structural discontinuity in Minnesota and the Flambeau Flow fault in Wisconsin are now thought to be geon 17 exhumation structures (formed during orogenic collapse) on the basis of differing metamorphic ages across these faults (McKenzie et al., 2003; Schneider et al., 2004). Current monazite work on tectonite samples of the Niagara fault zone (Rose, 2004) and the Eau Pleine shear zone (Loofboro, 2005) should provide direct time constraints on their formation (Williams and Jercinovic, 2002). Beginning this month we anticipate the publication of a large number of new ages in just the next few years – in terms of raw numbers, perhaps more than has been published in the Lake Superior region in the entire past 50 years! For instance, Van Wyck and Martin (2004) exploit the relatively fast LA-ICP-MS technique to report over 200 U-Pb detrital zircon ages from the Baraboo and Hamilton Mounds quartzites. Geochronologic work accepted and in progress (Schneider et al., 2004, 2005; Holm et al., 2004, 2005; Vanelli et al., in progress; Bickford et al., in progress; McKenzie, 2004; Rose, 2004; Keatts, 2004, Loofboro, 2005; Schmitz and Bowring, in progress; Medaris et al., in progress; et al. that we may be unaware of) will certainly continue to refine our knowledge of the Precambrian geologic history of this region and very likely yield surprises which can not be anticipated. Aldrich, L.T., Davis, G.L., and James, H.L., 1965, Ages of some minerals from metamorphic and igneous rocks near Iron Mountain, Michigan: Journal of Petrology, 6, 445-472. Armstrong, R.L., 1966, K-Ar dating of plutonic and volcanic rocks in orogenic belts. In O.A. Schaeffer and J. Zahringer, eds., Potassium argon dating, 117-131. Springer-Verlag, New York, 234 p. Ayer, J., Amelin, Y., Corfu, F., Kamo, S., Ketchum, J., Kwok, K., and Marquis, R., 2002: Evolution of the southern Abitibi greenstone belt based on U-Pb geochronology; autochthonous volcanic construction followed by plutonism, regional deformation and sedimentation: Precambrian Research, 115, 63-95. Boerboom, T.J., and Zartman, R.E., 1993, Geology, geochemistry, and geochronology of the central Giants Range batholith, northeastern Minnesota: Canadian Journal of Earth Sciences, v. 30, p. 2510-2522. Catanzaro, E.J., 1963, Zircon ages in southwestern Minnesota: Journal of Geophysical Research, v. 68, p. 20452048. Corfu, F., and Stott, G.M., 1986, U-Pb age for late magmatism and regional deformation in the Shebandowan Belt, Superior Province, Canada: Canadian Journal of Earth Sciences, 23, 1075-1082. Corfu, F., and Stott, G.M., 1998, Shebandowan greenstone belt, western Superior Province: U-Pb ages, tectonic implications, and correlations: Geological Society of America Bulletin, 110, 1467-1484. Davis, D.W., and Green, J.C. 1997, Geochronology of the North American Midcontinent Rift in western Lake Superior and implications for its geodynamic evolution: Canadian Journal of Earth Sciences, 34, 476-488. Davis, D.W., and Paces, 1990, Time resolution of geologic events on the Keweenaw Peninsula and implications for the development of the Midcontinent rift system: Earth and Planetary Science Letters, 97, 54-64. Davis, D.W., Poulsen, K.H., and Kamo, S.L., 1989, New insights into Archean crustal development from geochronology in the Rainy Lake area, Superior Province, Canada: Journal of Geology, 97, 379-398. Dewane, T.J., and Van Schmus, W.R., 2003, Detailed U-Pb geochronology of the Wolf River batholith, northcentral Wisconsin: Evidence for a short-lived magmatic event ca. 1470 Ma: Geological Society of America Abstracts, 37, 92. Fralick, P., Davis, D., and Kissin, S., 2002, The age of the Gunflint Formation, Ontario, Canada: single zircon U-Pb age determinations from reworked volcanic ash: Canadian Journal of Earth Sciences, 39, 1085-1091. Goldich, S.S., Nier, A.O., Baadsgaard, H., Hoffman, J.H., Krueger, H.W., 1961, The Precambrian Geology and geochronology of Minnesota, Minnesota Geology Survey Bulletin 41, 193 p. 81 �Goldich, S.S., Hedge, C.E., and Stern, T.W., 1970, Age of the Morton and Montevideo gneisses and related rocks, southwestern Minnesota: Geological Society of America Bulletin, 81, 3671-3696. Goldich, S.S., 1968, Geochronology in the Lake Superior region: Canadian Journal of Earth Sciences, 5, 715-724. Goldich, S.S., and Hedge, C.E., 1974, 3800-Myr granitic gneiss in south-western Minnesota: Nature. 252, 467-468. Hanson, G.N., and Gast, P.W., 1967, Kinetic studies in contact metamorphic zones: Geochim. Cosmochim. Acta, 31, 1119-1153. Hoffman, P., 1987, Early Proterozoic foredeeps, foredeep magmatism and Superior-type iron formations of the Canadian shield. In Proterozoic lithospheric evolution. Edited by A. Kroner, American Geophysical Union Geodynamics Series, 17, 85-98. Holm, D.K., and Lux, D., 1996, Core complex model proposed for gneiss dome development during collapse of the Paleoproterozoic Penokean orogen, Minnesota: Geology, 24, 343-346. Holm, D.K, and Lux, D., 1998, Depth of emplacement and tilting of the Middle Proterozoic (1470 Ma) Wolf River batholith, Wisconsin. Ar-Ar thermochronologic constraints: Canadian Journal of Earth Science, 35, 1143-1151. Holm, D., Darrah, K., and Lux, D., 1998a, Evidence for widespread ~1760 Ma metamorphism and rapid crustal stabilization of the Early Proterozoic Penokean orogen, Minnesota: American Journal of Science, 298, 60-81. Holm, D., Schneider, D., and Coath, C., 1998b, Age and deformation of Early Proterozoic quartzites in the southern Lake Superior region: Implications for extent of foreland deformation during final assembly of Laurentia: Geology, 26, 907-910. Holm, D.K., Van Schmus, W.R., MacNeill, L., Boerboom, T., Schweitzer, D., and Schneider, D., 2004, U-Pb zircon geochronology of Paleoproterozoic plutons from the northern mid-continent, U.S.A.: Evidence for subduction flip and continued convergence after geon 18 Penokean orogenesis: Geological Society of America Bulletin, in press. Jahn, B.M., and Murthy, V.R., 1975, Rb-Sr ages of the Archean rocks from the Vermilion district, northeastern Minnesota: Geochimica et Cosmochimica Acta, 39, 1679-1689. Keatts, M., Holm, D., Jirsa, M., and Boerboom, T., 2004, Generation of a 1790-1770 Ma continental arc batholith in east-central Minnesota: Compass, v. 82 (in press). Krogh, T. E., 1973, A low contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determinations: Geochimica et Cosmochimica Acta, 37, 485-494. Krogh, T.E., 1982, Improved accuracy of U-Pb zircon ages by the creation of more concordant systems using an air abrasion technique: Geochim. Cosmochim. Acta, 46, 637-649. Loofboro, J., 2005, Timing and nature of Proterozoic poly-metamorphism in central Wisconsin: M.S. thesis, Kent State University, Kent, OH (in progress). McKenzie, M.M., 2004, Age pattern and nature of Paleoproterozoic metamorphism of the internal zone of the Penokean orogen, east-central Minnesota: M.S. thesis, Kent State University, Kent, OH, 105 p. McKenzie, M.M., Holm, D.K., Schneider, D.A., and Jercinovic, M.J., 2003, Results and implications of monazite geochronology from the western Penokean orogen, Minnesota: Geological Society of America Abstracts with Programs, 35, 272. Medaris, G., Singer, B.S., Dott, R.H., Naymark, A., Johnson, C.M., and Schott, R.C., 2002, Late Paleoproterozoic climate, tectonics and metamorphism in the southern Lake Superior region and Proto-North America: Evidence from Baraboo interval quartzite: Journal of Geology, 111, 243-257. Morey, G.B., 1999, High-grade iron ore deposits of the Mesabi Range, Minnesota – Product of a continental scale Proterozoic ground-water flow system: Economic Geology, 94, 133-142. Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic and tectonomagmatic processes associated with the 1.1. Ga Midcontinent Rift system: Journal of Geophysical Research, 98, 13997-14013. Peterman, Z.E., 1966, Rb-Sr dating of Middle Precambrian metasedimentary rocks, Minnesota: Bulletin of the Geological Society of America, 77, 1031-1044. Peterman, Z.E., and Goldich, S.S., Hedge, C.E., and Yardley, D.H., 1972, Geochronology of the Rainy Lake region, Minnesota-Ontario, in Doe, B.R., and Smith, D.K., eds., Studies in Mineralogy and Precambrian Geology (John W. Gruner Volume): Geological Society of America Memoir 135, 193-215. Peterman, Z.E., and Sims, P.K., 1988, The Goodman Swell: A lithospheric flexure caused by crustal loading along the Midcontinent Rift System: Tectonics, 7, 1077-1090. Peterman, Z., Zartman, R., and Sims, P., 1980, Tonalitic gneiss of early Archean age from northern Michigan: Geological Society of America Special Paper 182, 125-138. Peterson, D.M., Gallup, C., Jirsa, M.A., and Davis, D., 2001, Correlation of Archean assemblages across the U.S.Canadian border: Phase I Geochronology: ILSG Abstracts, 47, 77-78. 82 �Romano, D., Holm, D., and Foland, K., 2000, Determining the extent and nature of Mazatzal-related overprinting of the Penokean orogenic belt in the southern Lake Superior region, north-central USA: Precambrian Research, 104, 25-46. Rose, S., 2004, The age and extent of metamorphism within the Paleoproterozoic Penokean orogen, northern Wisconsin and Michigan: M.S. thesis, Ohio University, Athens, OH, 105 p. Schneider, D., Holm, D., and Lux, D., 1996, On the origin of Early Proterozoic gneiss domes and metamorphic nodes, northern Michigan: Canadian Journal of Earth Sciences, 33, 1053-1063. Schneider, D., Bickford, M., Cannon, W., Shulz, K., and Hamilton, M., 2002, Age of volcanic rocks and syndepositional iron formations, Marquette Range Supergroup: implications for the tectonic setting of Paleoproterozoic iron formations of the Lake Superior region: Canadian Journal of Earth Sciences, 39, 999-1012. Schneider, D., Holm, D.K., O’Boyle, C., Hamilton, M., and Jercinovic, M., 2004, Paleoproterozoic development of a gneiss dome corridor in the southern Lake Superior region, U.S.A.: Geological Society of America Special Paper (in press). Silver, L.T., and Green, J.C., 1963, Zircon ages for Middle Precambrian rocks of the Lake Superior region: Trans. Am. Geophysical Union, 44, 107. Sims, P.K., Van Schmus, W.R., Schulz, K.J., and Peterman, Z.E., 1989, Tectonostratigraphic evolution of the Early Proterozoic Wisconsin Magmatic Terranes of the Penokean orogen: Canadian Journal of Earth Sciences, 26, 2145-2158. Vallini, D.A., et al., 2003, Using xenotime U-Pb geochronology to unravel the history of Proterozoic sedimentary basins: a study in Western Australia and the Lake Superior Region: ILSG abstr, 49, 79-80. Van Schmus, W.R., 1976, Early and Middle Proterozoic history of the Great Lakes area, North America. Royal Society of London Philosophical Transactions, 280, 605-628. Van Schmus, W.R., 1980, Chronology of igneous rocks associated with the Penokean orogeny in WI. In Morey, G., and Hanson, G., (eds.) Selected studies of Archean gneisses and lower Proterozoic rocks, southern Canadian Shield. Geological Society of America Special Paper, 182, 159-168. Van Schmus, W.R., Thurman, E.M., and Peterman, Z.E., 1975a, Geology and Rb-Sr chronology of middle Precambrian rocks in eastern and central Wisconsin: Geol. Society of America Bulletin, 86, 1255-1265. Van Schmus, W.R., Medaris, L.G., and Banks, P.O., 1975b, Geology and age of the Wolf River batholith, Wisconsin: Geological Society of America Bulletin, 86, 907-914. Van Schmus, W.R., and Anderson, J.L., 1977, Gneiss and migmatite of Archean age in the Precambrian basement of central Wisconsin: Geology, 5, 45-48. Van Schmus, W. R., and Hinze, W. J., 1985, The Midcontinent Rift System. Annual Reviews Earth Planetary Sciences, 13, 345-383. Van Wyck, N., 1995, Major and trace element, common Pb, Sm-Nd, and zircon geochronology constraints on petrogenesis and tectonic setting of pre- and Early Proterozoic rocks in Wisconsin: Ph.D. thesis, University of Wisconsin-Madison, 280 p. Van Wyck, N., and Martin, N., 2004, Detrital zircon ages from Paleoproterozoic Quartzites: Measured by laser – ablation ICP-MS: Journal of Geology (in press). Williams, M.L. and Jercinovic, M.J., 2002, Microprobe monazite geochronology: putting absolute time into microstructural analysis. Journal of Structural Geology, 24, 1013-1028. Zaleski, E., van Breemen, O., and Peterson, V.L., 1999, Geological evolution of the Manitouwadge greenstone belt and Wawa-Quetico subprovince boundary, Superior Province, Ontario, constrained by U-Pb zircon dates of supracrustal and plutonic rocks: Canadian Journal of Earth Sciences, 36, 945-966. Zartman, R.E., Nicholoson, S.W., Cannon, W.F., and Morey, G.B., 1997, U-Th-Pb zircon ages of some Keweenawan Supergroup rocks from the southern shore of Lake Superior: Canadian Journal of Earth Sciences, 34, 549-561. 83 �NEOARCHEAN PEPERITES IN THE VICINITY OF FIVEMILE LAKE, VERMILION DISTRICT, NE MINNESOTA HUDAK, G. J., NEWKIRK, T. T., DREXLER, H., ODETTE, J. D., and HOCKER, S. M., Department of Geology, University of Wisconsin Oshkosh, 800 Algoma Blvd., Oshkosh, WI 54901, hudak@uwosh.edu The Ely Greenstone – Lower Member in the vicinity of Fivemile Lake contains a well-studied sequence of ~2722 Ma, more or less east-west striking, steeply north-dipping and north-facing, arc-associated submarine basalt-andesite pillow lavas, tuffs and lapilli tuffs, rhyodacite to rhyolite tuffs, lapilli tuffs, and lavas, and synvolcanic diabase and diorite sills and dikes (Peterson et al., 2001; Hudak et al., 2002). Volcanic facies mapping at 1:5000 scale, supported by grants from the Natural Resources Research Institute (University of Minnesota Duluth), the University of Wisconsin Oshkosh, and the Minerals Diversification Plan of the Minnesota State Legislature, identified a northeast- trending zone of basalt - andesite tuffs, lapilli tuffs, and tuff-breccias which contain zones of northeast-trending coherent facies andesite–dacite that display pillowed, amoeboid, and commonly, jigsaw-puzzle fit morphologies. The southern extent of this zone occurs along a peninsula on the southwestern shoreline of Fivemile Lake. The northern extent of this zone extends approximately 150 meters north of the north-central shoreline of Fivemile Lake. Over the past year, extremely detailed volcanic facies mapping (1:50-1:100 scale), as well as petrographic and lithogeochemical studies, have been performed in an attempt to determine the genesis of these apparently genetically related coherentand volcaniclastic facies-bearing, northeast-trending zones. On the south shoreline of Fivemile Lake, the volcaniclastic facies is typically matrix-supported. The matrix consists of non-bedded basalt-andesite tuff, and locally contains amoeboid-shaped zones that contain up to 60% <1 mm quartz amygdules. Localized zones adjacent to northeast-striking coherent facies andesite–dacite comprise lapilli tuffs and tuff-breccias. Clasts in these deposits are commonly curviplanar to angular and jigsaw puzzle-fit in shape (Figure 1a), are moderately to highly vesicular (10-50%), have extremely finegrained margins, and locally contain <1-2 cm wide, apparently contact metamorphosed zones adjacent to their margins. These blocks and lapilli are compositionally similar to, more common adjacent to, and have their long axes aligned with, northeast-trending, sheet-like to pillow-shaped coherent facies andesite-dacite. The deposits on the north-central shoreline of Fivemile Lake have slightly different morphologies. A sharp, northsouth contact occurs between east-west striking, steeply north-dipping, bun- to mattress-shaped pillowed basalt-andesite and basalt-andesite tuffs and lapilli tuffs that comprise the base of a 700 meter wide, 200 meter thick, shallow submarine tuff cone (Hudak et al., in prep.). The tuffs and lapilli tuffs at this location are intruded by northeast-trending, synvolcanic, amoeboid (Figure 1b), pillow-like, and lobe-like zones of coherent-facies andesite-dacite which appear to have locally undergone in-situ fragmentation to produce localized lapilli tuffs and tuff-breccias that contain moderately to highly vesicular curviplanar, ameoboid, and blocky jigsaw puzzle-fit lapilli and blocks. These fragments have very fine-grained margins that are also rimmed by fine-grained, apparently contact metamorphosed zones. Locally, numerous thin (up to 1cm wide), parallel, highly vesicular zones occur within the volcaniclastic deposits that more or less mimic the orientations the margins of amoeboid coherent facies andesite–dacite or the margins of andesite-dacite blocks. The northeast-trending zones containing coherent and volcaniclastic facies are interpreted to comprise peperites with associated synvolcanic dikes. Peperites are fragmental rocks that form from the intimate mixing of hot magma with unconsolidated, typically wet sediments (Batiza and White, 2000; Schmidt and Schminke, 2000). They are common in submarine arc-related volcano-sedimentary sequences (Skilling et al., 2002), and commonly occur in proximity to synvolcanic fault zones. The interpretation that the volcaniclastic deposits represent peperites is based on the morphology of the lapilli and blocks which comprise the deposits, the consistent grain size variations from the margins to the centers of the lapilli and blocks, their spatial relationships adjacent to, and orientations parallel to, synvolcanic sheet-like to pillowed coherent facies andesite-dacite, and the presence of amygdules within the fine-grained tuffaceous matrix. The interpretation that the sheet-like to pillowed, coherent facies andesite-dacite represents dikes rather than lava flows is based on the northeast strike of the pillowed dikes (which is parallel to synvolcanic structures mapped in the Fivemile Lake region (Hudak et al., 2002)), the lack of consistent facing directions in the pillowed coherent 84 �facies, and the similarity in the chemical compositions of the coherent domains and the lapilli and blocks within the lapilli tuffs and tuff-breccias. The differences in the morphologies of the peperites on the southwestern and north-central shorelines of Fivemile Lake may represent their different levels of formation in the shallow sub-seafloor. Blocky peperites south of Fivemile Lake appear to have formed in lava flowconfined submarine aquifers located several hundred meters below the paleoseafloor, whereas more amoeboid peperites found north of Fivemile Lake appear to have formed near the seafloor within water-saturated submarine vent-fill deposits associated with a shallow submarine tuff cone volcano. Discordant zones of peperites can be used to identify synvolcanic fault zones that may be important in the exploration for volcanichosted massive sulfide deposits. Figure 1. Photographs of peperites deposits south (A) and north (B) of Fivemile Lake. References Batiza, R., and White, J. D. L., 2000, Submarine Lavas and Hyaloclastite, in Sigurdsson, H., 2000, Encyclopedia of Volcanoes: Academic Press, San Diego, California, p. 361-381. Hudak, G. J., Heine, J., Hocker, S., and Hauck, S. A., 2002, Geological mapping of the Needleboy Lake – Six Mile Lake area, northeastern Minnesota: a summary of volcanogenic massive sulfide potential: Natural Resources Research Institute Report of Investigation NRRI/RI – 2002/14, 16 p. Hudak, G. J., Heine, J., Newkirk, T. T., and Hocker, S. M., in prep. Comparative geology, stratigraphy, and lithogeochemistry of the Needleboy Lake to Sixmile Lake area, Vermilion District, NE Minnesota: University of Minnesota Permanent University Trust Fund Research Report. Peterson, D. M., 2001, Development of Archean lode-gold and massive sulfide deposit exploration models using geographic information system applications: targeting mineral exploration in northeastern Minnesota from analysis of analog Canadian Mining camps: unpublished Ph. D. dissertation, University of Minnesota, Duluth, Minnesota, 503 p. Peterson, D. M., Gallup, C., Jirsa, M. A., and Davis, D. W., Correlation of Archean assemblages across the U.S. – Canadian Border: phase I geochronology: ILSG Proceedings Volume 47, p. 77-78. Schmidt, R., and Schminke, H.-U., 2000, Seamounts and Island Building, in Sigurdsson, H., 2000, Encyclopedia of Volcanoes: Academic Press, San Diego, California, p. 383-402. Skilling, I. P., White, J. D. L., and McPhie, J., 2002, Peperite: a review of magma-sediment mingling: Journal of Volcanology and Geothermal Research, v. 114, p. 1-17. 85 �MAPPING BY THE MINNESOTA GEOLOGICAL SURVEY IN SUPPORT OF LANDUSE AND WATER PLANNING ON THE MESABI IRON RANGE JIRSA, Mark A., Minnesota Geological Survey, jirsa001@umn.edu This presentation describes two mapping projects on the Mesabi Iron Range underway by the Minnesota Geological Survey: 1) Hydrogeologic base maps of the Mesabi Iron Range—Funded by the Legislative Commission on Minnesota Resources; and 2) Bedrock and Quaternary geologic maps of the Mesabi Iron Range—Funded by the Minnesota Minerals Coordinating Committee. In recent years, a number of mining companies have been working with the University of Minnesota’s Department of Landscape Architecture on development concepts for the Mesabi Iron Range. Although not directly a part of those efforts, the Minnesota Geological Survey’s mapping projects are designed to provide technical data to address issues—primarily related to water—that require consideration prior to development. When mining and associated pumping ceases in any particular locality, water enters the mine from a variety of sources: rain and runoff, percolation through porous unconsolidated sediment layers, and flow through fractures and weathered zones in bedrock. The movement of ground water through these diverse materials is complex and poorly understood; however, it is clear that the boundary between the bedrock and overlying unconsolidated sediments is the single most important interface in the movement of ground water. Project 1, the hydrogeologic maps, addresses the location and shape of this hydrologically important interface. Some of this information for the western half of the Mesabi Range has already been published (Fig. 1; Jirsa and others, 2002; Lively and others, 2002). Project 2 will provide details about the lithologic content, structure, and water-bearing characteristics of the materials above and below that interface. Among other objectives, the bedrock geologic mapping is designed to identify other structures that may be important hydrologic interfaces, such as fractures, faults, and bedding surfaces. The bedrock mapping will modify and digitize the most recent existing map of the Mesabi Range compiled by a large group of scientists (Meineke and others, 1999). This will be compiled with other archived maps and new fieldwork. These efforts are reconnaissance in scope, and considerably more detail could be extracted from mine exposures, mining company records, and surface geologic mapping. Nevertheless, these projects will produce framework geologic maps and site-specific data in digital format that will be useful in resolving issues related to ground and surface waters, aggregate resource management, and land-use planning. They are intended to lay the groundwork for more detailed investigations into specific areas of future concern. 86 �Figure 1. Generalized geologic map of the Mesabi Iron Range showing the subcrop location of Paleoproterozoic Biwabik Iron Formation (gray) and Mesoproterozoic Duluth Complex, and various taconite mining operations (black; asterisk indicates inactive mine). Dashed polygons outline western and eastern map areas. None of these projects would be possible without the contributions of drill hole and geologic data and insight from members of the Minnesota Department of Natural Resources, the Natural Resources Research Institute, and most particularly, the mining companies. REFERENCES Jirsa, M.A., Setterholm, D.R., Bloomgren, B.A., and Lively, R.S., 2002, Bedrock topographic and depth to bedrock maps of the western half of the Mesabi Iron Range, northern Minnesota: Minnesota Geological Survey Miscellaneous Map M-126, scale 1:100,000. Lively, R.S., Morey, G.B., and Bauer, E.J., 2002, One hundred years of mining: Alterations to the physical and cultural geography of the western half of the Mesabi Iron Range, northern Minnesota: Minnesota Geological Survey Miscellaneous Map M-118; 4 pls., scale 1:100,000. Meineke, D.G., Buchheit, R.L., Dahlberg, E.H., Morey, G.B., and Warren, L.E., comps., 1999 Geologic map of the Mesabi Iron Range, Minnesota (2nd ed.): Hibbing, Minn., Mesabi Range Geological Society, scale 1:62,500. 87 �REGIONAL COMPILATIONS OF BEDROCK GEOLOGY IN NORTHEASTERN MINNESOTA: THE VERMILION, ELY, AND BASSWOOD LAKE QUADRANGLES JIRSA, Mark A., Minnesota Geological Survey, jirsa001@umn.edu Compilations of geologic mapping, although not typically portraying new field information in great detail, serve the important functions of providing regional mapping consistency, framework for various lithologic and geochemical surveys, and regional guidance for exploration and environmental issues. With this in mind, several regional bedrock geologic compilations at scale 1:100,000 have recently been completed and are underway for areas in northeastern Minnesota (Fig. 1), funded in large part by the STATEMAP program of the U.S. Geological Survey. The Vermilion Lake 30' x 60' quadrangle was completed in 2003 (Jirsa and Boerboom, 2003), and is displayed and described here. The adjacent Ely and U.S. portion of Basswood Lake 30' x 60' quadrangles are slated for completion in 2004. Perhaps as important as the functions listed above, the new compilations integrate disparate analog mapping, including some from the late 1800s and early 1900s, into a lithostratigraphically consistent, digital (GIS) format. Figure 1. Generalized geologic map of northeastern Minnesota showing the location of 30' x 60' quadrangles. The Vermilion Lake quadrangle covers most of the exposed area of the Vermilion district of northeastern Minnesota that lies outside of the Boundary Waters Canoe Area Wilderness (BWCAW), and is thus open in the broad sense to mineral exploration and mining. The Vermilion district is a local designation applied to the portion of the Neoarchean Vermilion greenstone belt in which there has been mining activity in the past, primarily of highgrade iron ore. The quadrangle also covers the east end of the Mesabi Iron Range, a world-class mining district in Paleoproterozoic strata that for decades has been the principal economic force in this part of Minnesota. Nevertheless, there is interest in finding alternative industries for cities along the iron range and alternative commodities for a diversified mining industry. At the present time there are encouraging indications that one or more viable polymetallic mines may develop in the basal contact zone of the Mesoproterozoic Duluth Complex. The zone of prime exploration and development interest in this regard crosses the southeastern corner of the Vermilion Lake map sheet. For the first time, the Vermilion Lake compilation merges well-exposed Neoarchean rocks of the Tower–Soudan area with more scattered exposures to the west into a single structural and lithologic interpretation. The map highlights a large, regional anticlinal structure that is cored by the steeply dipping Tower–Soudan anticline (Fig. 2). Tracing this fold structure westward into sedimentary rocks of the Lake Vermilion Formation, it becomes a broad and complex nappe structure that extends at least 30 kilometers. The Vermilion greenstone belt is comparable in its geologic attributes to other Neoarchean belts in Canada that contains significant deposits of gold, copper, zinc, and other metals. This general analogy has been recognized for decades and has stimulated several waves of mineral 88 �exploration in the area. Although past exploration efforts have been substantial, they have not been exhaustive. A major handicap was the lack of detailed geologic mapping, a situation that has been largely corrected over the past 25 years through the efforts of the Minnesota Geological Survey and the exploration industry itself. Figure 2. Schematic geologic map of the Vermilion Lake 30' x 60' quadrangle (Jirsa and Boerboom, 2003) showing folds, faults, and stratigraphic facing in Archean strata. Archean intrusions are gray. Three generations of folds are shown: F1—solid; F2—long dash; F3—short dash. Open arrows indicate horizontal facing directions in steeply dipping supracrustal sequences and upright facing directions where bedding is shallower. Solid arrows indicate the direction of downward stratigraphic facing. The bold fault line is the boundary between the Quetico subprovince to the north, and the Wawa subprovince to the south. The regional compilations of mapping data at scale 1:100,000 may prove to be valuable tools for the next cycle of exploration, which inevitably will come when local and global economic conditions appropriately conjoin. The compilation effort underway for the Ely and U.S. portions of the Basswood Lake 30' x 60' quadrangles involves many of the same geologic entities as depicted on the Vermilion Lake sheet. Because it includes areas within the BWCAW, which are generally protected from exploration and mining, this map will serve a slightly different purpose from that of the Vermilion Lake map, by providing regional geologic context. This effort will pave the way for such products as a “geologic user-guide” to the BWCAW, which has been initiated by the Minnesota Geological Survey. REFERENCE Jirsa, M.A., and Boerboom, T.J., 2003, Bedrock geology of the Vermilion Lake 30’ x 60’ quadrangle, northeast Minnesota: Minnesota Geological Survey Miscellaneous Map M-141, scale 1:100,000. 89 �Douglass Houghton’s 1840 Field Excursion to Lake Superior JOHNSON, Allan M., Professor Emeritus, Department of Geological & Mining Engineering & Sciences, Michigan Technological University, Houghton, Michigan 49931 In 1840, as the first State Geologist of Michigan, Douglass Houghton was near the end of the first geological survey of the state. In 1837, when Michigan entered statehood, and 1838 he was involved with and oversaw the reconnaissance surveys of the southern peninsula. In 1839 the survey work progressed to the northern peninsula, along the north shores of Lakes Huron and Michigan. For 1840, the program was much more ambitious, for he and his assistants (Bela Hubbard and C.C. Douglass) would undertake a survey along the south shore of Lake Superior. Detroit businessman, Charles Penny, also accompanied the group. The party left Detroit near the end of May 1840 and through the spring and early summer made their way from Sault Ste. Marie westward along the south shore of Lake Superior in their flotilla consisting of a larger Mackinaw boat and several smaller craft. This survey resulted in the mapping of rocks exposed adjacent to the shore, with more detailed mapping along major streams and rivers flowing into the big lake. Special attention was given to mapping volcanic and sedimentary rocks of the Keweenaw Peninsula and taking samples of previously known locations of copper bearing rocks (copper oxide at Copper Harbor and native metal from the copper boulder on the Ontonagon River). At the end of July, Houghton’s assistants returned to southern Michigan to continue with ongoing geologic investigations including boring a salt well at Grand Rapids. Houghton remained on Lake Superior with his voyageurs through mid-September. When his assistants left from LaPointe on Madeline Island, Houghton continued reconnaissance mapping at Isle Royale, the Porcupine Mountains and Portage Lake, and more detailed work on the Black and Eagle Rivers. It was on the Eagle River where Houghton discovered numerous occurrences of native copper and the presence of native silver which gave him the confidence to express cautious optimism in the potential for copper mining on the Keweenaw Peninsula in his early 1841 report to the Michigan Legislature. Word of this report soon led to the US War Department establishing a land office at Copper Harbor to issue mining permits and ultimately to the first great mining rush in the United States. For 150 years the native copper district of the Keweenaw was active in supplying a growing nation and the world with high purity lake copper with over 12 billion pounds of production. It is of interest to note that the 1840 journals of Hubbard and Penny have been published, while Houghton’s 1840 geologic notes became ‘lost’. It was not until 1978 that the notes appeared at public auction in New York City advertised as “the notes of an early Michigan surveyor”, where they became the property of Central Michigan University. 90 �References Carter, James L., and Rankin, Ernest H., eds., 1970, North to Lake Superior, The Journal of Charles W. Penny, 1840, The John M. Longyear Research Library, Marquette, Michigan, 84 p. Cummings, John, September 14, 2000, personal communication in Mount Pleasant, Michigan regarding purchase of Douglass Houghton’s 1840 field notes in New York City in 1978. Fuller, George N., 1928, Geological Reports of Douglass Houghton, Michigan Historical Commission, Lansing, 700 p. Houghton, Douglass, 1840, Original Field Notes of 1840 Expedition to Lake Superior, Clarke Library, Central Michigan Univ., Mount Pleasant, MI 325 p. Merrill, George P, 1906, Contributions to the History of American Geology, Smithsonian Institution, Washington Government Printing Office, 733 p. Mason, Philip, P., ed., 1958, Schoolcraft’s 1832 Expedition to Lake Itasca, Michigan State University Press, East Lansing, 390 p. Peters, Bernard C., ed., 1983, Lake Superior Journal, Bela Hubbard’s Account of the 1840 Houghton Expedition, Northern Michigan University Press, Marquette, 113 p. Rintila, Edsel K., 1954, Douglass Houghton, Michigan’s Pioneer Geologist, Wayne University Press, Detroit, 119 p. Williams, Mentor L., ed., Schoolcraft’s 1821 Narrative Journal of Travels through the Northwestern Regions of the United States, Michigan State University Press, East Lansing, 1992, 520 p. 91 �REGIONAL GEOCHEMISTRY SURROUNDING THE NORTON LAKE Cu-Ni-PGE DEPOSIT, UCHI SUBPROVINCE, ONTARIO JOHNSON, J.R.*, HOLLINGS, P., and KISSIN, S., Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1 The Norton Lake Cu-Ni-PGE deposit (delineated in 1981 as 944,000 tonnes at 0.72% Ni and 0.56% Cu) is located within the northern most unnamed assemblage of the Miminiska-Fort Hope Greenstone Belt approximately 50 km northeast of Fort Hope, northwest Ontario (Fig. 1). The deposit is hosted within a sheared amphibolite unit that is typically found at, but not limited to, the contact between an upper basalt and a lower sedimentary unit. Due to both its remote location and sparse outcrop only limited geological investigations have been undertaken within the belt to date. Consequently, the belt has been subdivided based on available regional stratigraphic and structural interpretations. Previous work by the Ontario Geological Survey has tentatively correlated the unnamed assemblage with the McGruer assemblage of the North Caribou, located to the west, based on a similarity of rock types and a pervasive aeromagnetic anomaly that extends between the two (Stott and Corfu, 1991). Figure 1: Uchi Subprovince, northwestern Ontario, study area outlined. A 12 km east-west by 6 km north-south block surrounding the Norton Lake Deposit has been mapped and sampled to allow characterization of the volcanic assemblages. In addition to field mapping, drill core left in the area by previous exploration programs was examined (covering an area of 20 km by 10 km). Samples selected to provide good areal coverage and minimal alteration were analysed by XRF and ICP-MS. Three suites can be recognised within the assemblage; Suite I basalts are characterized by weakly depleted to enriched LREE [(La/Sm)n=0.9-1.2] and weakly fractionated HREE [(Gb/Yb)n=1.1-1.2] in conjunction with flat to positive Nb anomalies. Suite II is characterized by LREE depleted basalts [(La/Sm)n=0.4-0.7] and unfractionated HREE [(Gb/Yb)n=0.9-1.1], while Suite III consists of basalts to dacites that are LREE enriched with weakly fractionated HREE [(La/Sm)n=2.6-4.7, (Gd/Yb)n=0.8-2.4] and pronounced negative Nb anomalies. 92 �Sm-Nd isotope work on select samples is currently underway with initial epsilon-Nd values indicating little to no crustal contamination of the samples. Preliminary interpretations suggest the Norton Lake area formed in a tectonic setting analogous to modern island arcs and is best compared with the Northern Pickle Assemblage of the Pickle Lake belt rather than the McGruer assemblage of the North Caribou terrane. Hollings P., 1998, Geochemistry of the Uchi Subprovince, northern Superior Province: and evaluation of the geodynamic evolution of the northern margin of the Superior Province ocean basin; Ph. D. thesis, University of Saskatchewan, Saskatoon, Saskatchewan, 229 p. Stott G. M. and Corfu F., 1991, Uchi Subprovince, in Geology of Ontario, Ontario Geological Survey Special Volume 4, Part 1. Young, M. D., 2003. New structural, geochronological, and geochemical constraints on the tectonic assembly of the Archean Pickle Lake greenstone belt, Uchi subprovince, western Superior Province; M. Sc. thesis, Queen’s University, Kingston, Ontario, 182 p. 93 �Platinum Mineralization at Drill Hole A4-11 of the Wetlegs Area of the Partridge River Intrusion, Duluth Complex, Northeast Minnesota KAUKONEN, R.J., and ALAPIETI, T.T., University of Oulu, Oulu, Finland Apparently either the more peculiar PGE mineralization at DU-15 of the South Kawishiwi Intrusion or the vast but low grade deposits of disseminated base-metal sulfides may have been the reason why, as it seems, the platinum potential of the Wetlegs area of the Partridge River Intrusion has almost been forgotten. The whole rock analyses from drill core A4-11 (Gladen, 1990) show significant PGE values (Fig.1), which undoubtedly warrant an investigation. Unfortunately the best parts of the drill core A4-11 have been previously used for analysis and hence material for thin sections was not available throughout the core. The cryptic variation paths of the main rock-forming silicates are presented in Fig. 1. Compared to data from drill core A4-14 (Kaukonen, 1994), the crystallization paths of olivine and pyroxenes show distinctive similarities. The general upward increasing trend of primitiveness is particularly evident in olivine. Plagioclase compositions on the other hand don’t really present any clear trends aside from perhaps a slight decrease down the drill hole in the lower half of the core. Dpeth in Drill Hole (m) 0 0 100 100 200 200 300 300 400 400 500 500 600 600 700 40 50 60 70 80 30 40 50 60 70 40 50 60 70 80 30 Plag An% Oliv Fo% Opx En% 40 Cpx En% 50 0 700 4000 8000 Pt+Pd+Au ppb Fig.1. Cryptic variation paths of the main rock-forming silicates and the determined concentrations of noble metals in drill hole A4-11. The best PGE values are found between 70-80 meters down hole. The highest Pt+Pd+Au value is about 7 ppm, of which Au accounts for just less than 1 ppm (Fig.1). Petrologically the mineralization seems to be a somewhat ’classic’ base-metal sulfide type PGE mineralization where the platinum-group minerals are mainly associated with disseminated base-metal sulfides, mainly chalcopyrite and pyrrhotite. The most significant difference to the classic or often referred to as orthomagmatic PGE mineralizations is probably related to the silicates rather than sulfides, as the host rock is essentially troctolite rather than orthopyroxenite. This feature, however, is well in accordance with the crystal mush theory of Miller and Weiblen (1990). 94 �As each mineralization has some unique characteristics, the A4-11 PGE mineralization also has its own signature. One of the most striking features is the notable occurrence of native copper, which is widely present. The grain size of copper is usually very small, yet it is readily distinguishable with an ore microscope using a relatively small magnification. The presence of native copper is possibly an indication of a reducing environment, and here it is mainly an alteration product of chalcopyrite. The platinum-group mineralogy of the A4-11 mineralization is also rather special. The most common PGMs are stannides and plumbates (Komppa, 1998). The minerals identified and analyzed for this investigation included atokite (Pd2Sn) and zvyagintsevite (Pd3Pb). However, as pointed out by Cawthorn et al. (2002), the platinum-group mineralogy of any given deposit is not necessarily a reflection of the primary processes involved in the enrichment of the PGE in a stratiform deposit, but rather a product of secondary processes that shape the mineralization further. References: Cawthorn, R.G., Lee, C.A., Schouwstra, R.P., and Mellowship, P., 2002, Relationship between PGE and PGM in the Bushveld Complex: Can. Min., Vol. 40, pp. 311-328. Gladen, L., 1990, Duluth Project: Report on 1989 Exploration in the Duluth Complex, Minnesota Department of Natural Resources, Internal Report, 11 p. Kaukonen, R.J., 1994, Cryptic Variation in the Partridge River Intrusion of the Duluth Complex, unpublished M.Sc thesis, University of Oulu, Finland, 66 p., (in Finnish with English Abstract). Komppa, U.E., 1998, Oxide, Sulphide and Platinum Mineralogy of the South Kawishiwi and Partridge River Intrusions of the Duluth Layered Intrusion Complex, Minnesota, U.S.A. M.Sc. thesis, University of Oulu, Finland, 104 p., (in Finnish), English version published as Natural Resources Research Institute, Report of Investigation in 2002, NRRI-RI2002/02 Miller, J.D., Jr., and Weiblen, P.W., 1990, Anorthositic Rocks of the Duluth Complex: Examples of Rocks Formed from Plagioclase Crystal Mush: Jour. Petrol., Vol. 31, Part 2, pp. 295339. 95 �MAGNETIC SUSCEPTIBILITY ANISOTROPY AND REMANENT MAGNETISM OF QUARTZITE AND PHYLLITE FROM BARABOO WISCONSIN KEAN, William F., Department of Geosciences, UW-Milwaukee, P.O. Box 413, Milwaukee WI 53201, wkean@uwm.edu The Baraboo quartzite and the associated phyllite are paleomagnetically well behaved rocks, with single magnetic directions carried by hematite, but with different directions for the rock types. The magnetism in the quartzite is associated with single domain hematite grains, is pre-folding, and is consistent with an ~1760 Ma age of formation (Kean and Mercer, 1986). The phyllite is dominated by multidomain hematite, (Kelly and Kean 2001), and although the magnetic directions are internally consistent at any one site, they are scattered between sites both before and after a fold test. The presence of multidomain hematite grains (up to 100 µ) suggests the remanence was developed during metamorphism which could have occurred during the folding in the region, or from fluids introduced at a later time when the radiometric ages were reset (Medaris et al., 2003). In an attempt to better understand these differences in magnetic directions, a large block of rock that contained quartzite and phyllite was collected from the south limb of the Baraboo Syncline at an outcrop on Highway 12. A series of 2.5-cm. diameter cores were drilled from each of the layers for thermal demagnetization and anisotropy of magnetic susceptibility (AMS) measurements. The results presented in Figure 1 show that the phyllite has higher values of AMS (P`) than the quartzite, and the magnetic grains are more oblate in shape (T) compared to the quartzite. 1 0.8 Quartzite 0.6 SHAPE T Phyllite 0.4 0.2 0 1 1.1 1.2 1.3 1.4 1.5 -0.2 -0.4 Anisotropy P' Figure 1. Anisotropy of Magnetic Susceptibility (AMS) P’ versus shape factor T for samples from the Baraboo region. T values below zero are prolate in shape, values near zero are neutral, and values above zero are oblate in shape. The phyllite shows large values of anisotropy and is strongly oblate. The quartzite is close to neutral in shape and of lower anisotropy. 96 �The comparison between remanent magnetic directions and AMS ellipsoid axes for the two rock types is presented in Figure 2. The magnetic direction for the phyllite is perpendicular to the minimum anisotropy axis and is rotated toward the bedding plane. The magnetic direction for the quartzite appears to be unrelated to the susceptibility anisotropy. Figure 2: Comparison of paleomagnetic directions and AMS for quartzite and phyllite samples at Highway 12 outcrop. It appears that the disparity in the magnetic directions in the phyllite zone is related to the amount of local strain that occurred during the metamorphism and formation of the Baraboo syncline, and not related to later fluid injection associated with the Wolf River batholith intrusive event. Limited AMS results from other phyllite sites in the region provide similar results. References: Kean, W.F. and Mercer, D., 1986, Preliminary Paleomagnetic Study of the Baraboo Quartzite, Wisconsin, Geoscience Wisconsin, Vol. 10, p 46-53. Kean, W.F. and Kelly, C., 2001, Rock Magnetic Studies of Phyllite Layers from Baraboo Interval Rocks in Wisconsin, Abstracts with Programs, GSA Annual Meeting, 33, p. A143. Medaris, L.G., Singer, B.S., Dott, Jr., R.H., Naymark, A., Johnson, C.M., and Schott, R.C., 2003, Late Paleoproterozoic Climate and Tectonics in Southern Lake Superior Region and Proto-North America: Evidence from Baraboo Interval Quartzites. Journal of Geology, Vol. 111, p 243-257. 97 �Lithic Materials and Archaeology in the Western Lake Superior Region KLAWITER, Brian, Superior National Forest, Duluth, Minnesota 55808, USA The most prevalent and enduring artifacts left behind by prehistoric cultures are those made of stone. The study of these cultural material remains shows that prehistoric people were very perceptive and discerning “geologists” when it came to choosing the stone materials that they would use. The available lithic materials had a broad range of useful properties that were to be carefully considered when choosing the raw material for a particular tool or purpose. Prehistoric people developed a keen eye for the desired physical properties of stone, and they knew where to get it. One enduring legacy from these prehistoric and early historic people is in the form of place-names such as Gunflint Lake and Knife Lake; both translated from the earlier Ojibwe names (Upham, 1920), and both of which have subsequently passed their names along to major geologic features: the Gunflint Iron Formation and the Knife Lake Group metavolcanics and metasediments. This presentation will display and describe many of the geologic materials and their prehistoric uses as discovered by archaeologists working in the western Lake Superior region. In addition to studying prehistoric cultures, it is often equally interesting to study the modern archaeologists who study the ancient cultures. Most archaeologists are students of the Social Sciences whose occupation requires them to acquire a basic working knowledge of geology. This geologic education is often acquired in the field or in the lab by peer interaction, rather than in the classroom by professional geologists. As a result, the field of archaeology has in many ways developed its own sub-culture of geology, which is best displayed in the naming systems applied to the geologic materials recovered from archaeological excavations. This ad hoc (and locally territorial) nomenclature is often confusing to archaeologists and bewildering to geologists. For those interested in the overlap of geology and archaeology, this presentation outlines the archaeological nomenclature applied to geologic formations and materials in the western Lake Superior region, and highlights the efforts of this author and others to “reform” the archaeological nomenclature to more generally conform to geologic standards. Keywords: archaeology, lithics, Gunflint, Knife Lake, nomenclature, Superior National Forest Upham, W., 1920, Minnesota Geographic Names: Their Origin and Historic Significance: Minnesota Historical Society, Saint Paul, Minnesota. 98 �REGIONAL TILL SAMPLING IN THE VERMILION GREENSTONE BELT, MINNESOTA: PRELIMINARY RESULTS AND INTERPRETATIONS LARSON, P.C., Department of Geological Sciences, University of Minnesota, Duluth, MN 55812, plarson2@d.umn.edu A project of regional till sampling was undertaken over the Vermilion greenstone belt during the 2001 and 2003 field seasons. Samples were collected to provide coverage of ~1 sample per 3 km2 in the area between Tower and Ely, MN. The silt+clay fraction (<63µm) of B- and Chorizon till samples was analyzed for gold, platinum, and palladium by fire assay and for 47 major- and trace-elements by ICP-MS and ICP-AES. The regional sampling program provides information on the background concentrations of various elements in till, as well as the presence and location of anomalous gold and base metal indicator concentrations. In conjunction with the sampling, observations on the character and distribution of glacial sediments overlying the greenstone belt were made to provide a framework for interpretation of the till geochemical data set. The study area records a simple record of Quaternary events. Till cover is generally thin and discontinuous over the study area, with the exception of the Vermilion and Wahlsten moraines. Essentially all glacial sediments were deposited during the final retreat of ice from the study area (the Vermilion Phase). Moraine orientations indicate ice was flowing at 180° to the Wahlsten moraine, and 195° to the slightly younger Vermilion moraine. Elements in till associated with the granite-gneiss Vermilion granitic complex to the north of the greenstone belt (Be, K, Rb, Ba) show a systematic decrease in concentration along glacial flowlines south of the Vermilion fault (the contact between the granitic complex and greenstone belt). This demonstrates that the composition of till overlying the greenstone is overwhelmingly a function of physical transport by glacial action. Till overlying the greenstone is therefore a mixture of material transported from the Vermilion granitic complex and material eroded from the greenstone belt; quantification of this mixing trend indicates a mean transport length of ~8 km for till-forming material. The till sampling program has clearly identified a number of areas with anomalous precious and base metal concentrations. Highlights include a number of samples with >50 ppb gold, including one sample with 940 ppb gold. Copper values up to 314 ppm, zinc values up to 368 ppm, and platinum values of up to 8 ppb were also reported. Higher density follow-up sampling is recommended to more clearly define and determine the significance of anomalies, as well as determine the location of potential source rocks. The results of this survey clearly demonstrate the utility of till compositional surveys for exploration in the Vermilion greenstone belt, as well as nearby areas with a similar glacial geological setting. 99 �EARLY ADVANCE OF THE ST. LOUIS SUBLOBE: A REVISED CHRONOLOGY OF THE DEGLACIATION OF NORTHEASTERN MINNESOTA LARSON, P.C., MOOERS, H.D., and MARLOW, L.M., Department of Geological Sciences, University of Minnesota, Duluth, MN 55812, plarson2@d.umn.edu The general retreat of the Laurentide Ice Sheet (LIS) from Minnesota during the Late Wisconsin was characterized by a fluctuating boundary between ice originating in the Keewatin and Labradoran accumulation centers. Recent investigations into the glacial geology of central and northern Minnesota have newly recognized stratigraphic relationships prompting revision of the deglacial chronology. A prominent event during deglaciation was the advance of the Keewatin-provenance St. Louis Sublobe (Alborn phase) from the Red River Valley across northern Minnesota into the area vacated by the retreating Labradoran-provenance Rainy Lobe. This advance has previously assigned a relatively late age (~12 ka) and correlated with post-Vermilion phase margins of the Rainy Lobe, implying that ice-free conditions existed between the Rainy Lobe and the Giant’s Range at the time of the advance. However, the absence of Keewatin till north of the Giant's Range despite the presence of such till at the crest of the Giant's Range, combined with evidence for abundant stagnant Rainy Lobe ice south of the Giant’s Range, suggests the St. Louis Sublobe advance occurred while active Rainy Lobe ice was present immediately north of the Giant's Range. A newly identified Rainy Lobe ice margin confirms this relationship; this margin is probably correlative with the previously described Allen moraine. We refer to the phase of the Rainy Lobe responsible for its formation as the Northofnashwauk phase. The relationships above indicate that the Alborn phase occurred significantly earlier than previously believed. Retreat of the St. Louis Sublobe from its Alborn phase maximum occurred by stagnation and wastage of a large mass of ice. The next prominent recessional moraine formed at the suture between active and stagnant ice in southern Beltrami County (Rabideau moraine). Here, ice-contact meltwater channels emanated outward from the ice margin, terminating as outwash fans at the edge of the stagnant ice. The Rabideau moraine is correlative with the post-Northofnashwauk Big Rice phase and moraine of the Rainy Lobe. Much of the meltwater drained into Glacial Lake Sucre, a western extension of Glacial Lakes Aitkin and Upham II. Sucre drained after abandonment of the Hellwig Creek outlet of Upham II. The Rainy and St. Louis Sublobes retreated from the Big Rice and Rabideau moraines to the Vermilion and Big Stone(?) moraines, respectively. These newly recognized stratigraphic relationships indicate that the St. Louis Sublobe advanced immediately after recession of the Rainy Lobe. Consequently, Glacial Lakes Aitkin-Upham I were much shorter lived, and Aitkin and Upham II much longer lived, than previously believed. 100 �Figure 1. Ice margin positions during Northofnashwauk and Alborn Phases, ca. 13.5 kyr BP. Figure 2. Ice margin positions during Big Rice Phase, ca. 13 kyr BP. Figure 3. Ice margin positions during Vermilion Phase, ca. 12.5 kyr BP. 101 �Lake Nipigon Region Geoscience Initiative: Results of Bedrock Mapping in the Northern Part of the Western Nipigon Embayment, Northwestern Ontario, Canada MacDONALD, C.A. and TREMLAY, E. Precambrian Geoscience Section, Ontario Geological Survey, Ministry of Northern Development and Mines, Sudbury, Ontario, Canada, P3E 6B5 As part of the Lake Nipigon Regional Geoscience Initiative (LNRGI), a 2-year, 1:50 000 scale mapping program was begun in 2003 to better understand the geology of the western Nipigon Embayment. This talk presents results based on mapping of an area of roughly 1200 km2 during the 2003 field season, and focuses on Proterozoic rocks in the map area (Fig. 1) (MacDonald 2004; MacDonald et al. 2004a, 2004b). This project is funded by the Northern Ontario Heritage Fund Corporation (NOHFC) through the Ontario Prospectors Association. The 2003 map area is located approximately 190 km north of Thunder Bay and is partially bordered by the shore of Lake Nipigon on its east margin (Fig. 1, 2). Rocks of both the Superior and Southern provinces occur in the map area and include Archean rocks of the volcanic-plutonic central Wabigoon Subprovince that are disconformably overlain by Mesoproterozoic sedimentary rocks of the Sibley Group, all of which are intruded by Mesoproterozoic mafic igneous rocks. Archean rocks consist dominantly of variably foliated to gneissic, felsic to intermediate, granitoid rocks and mafic to felsic metavolcanic rocks. A 30 metre thick flatlying succession of mafic volcanic pillowed flows occurs in the northern part of the map area. The undeformed nature of these flows, combined with the well-preserved Figure 1. Key map showing the location of the nature of relatively delicate features such as hyaloclastite 2003 map area. suggests that these rocks may be Proterozoic rather than Archean in age. If so, they would be the first Proterozoic mafic volcanic rocks reported this far north of the Midcontinent Rift. Alternatively, they could represent a previously unrecognized greenstone belt within the Superior Province. Pre-Keweenawan rocks include anorogenic igneous and associated metavolcanic rocks of the English Bay complex and sedimentary rocks of the Sibley Group. The English Bay complex (~1540 Ma, Davis and Sutcliffe 1985) consists mainly of massive quartz and feldspar crystal tuffs with a variety of fragment types suggesting an extrusive, volcanic origin for most of the complex. Sibley Group clastic and chemical sedimentary rocks are few, and a combination of paleotopography on the Archean surface as well as the erosional level of the overlying diabase sills probably controls their distribution. At least one olivine gabbro sill, referred to as the Jackfish Island sill, intrudes the English Bay complex. The Jackfish Island sill has geochemical affinities (Fig. 3) with the Kitto peridotite intrusion located on the east side of the Nipigon Embayment (Hart 2003). Both the Jackfish Island and Kitto peridotite are roughly coeval with the ~1110 Ma Nipigon diabase sills (Davis and Sutcliffe, 1985). Nipigon diabase sills intrude and overlie all previously noted rock types and are part of the 1.11 to 1.09 billionyear-old Midcontinent Rift. The Nipigon sills are generally massive, medium-grained, intergranular-textured and gabbroic in composition with local variations including ophitic and poikilitic textured diabase, oikocrystic diabase and magnetite-rich to locally glomeroporphyritic magnetite phases. Coarse-grained pods, veins and/or monzogabbroic phases also occur within the diabase and could represent a late magmatic phases of the sills or assimilation of Sibley Group sedimentary rocks, or both (Hart and Magyarosi, 2004). The diabase sills intrude and typically overlie all other rock units. The diabase sills are tentatively subdivided into 2 types: 1) The Inspiration sills, located in the northern part of the map area, which have higher trace element ratios (Fig. 3) and normal 102 �remnant magnetization than typical Nipigon sills, and 2) Sills located in the southern part of the map area that are reversely polarized and which have lower trace element ratios. This southern group of sills are grouped with the typical Nipigon sills (Fig. 3). A series of subparallel north- and northwest-trending faults that occur within the Black Sturgeon Fault corridor may be related to the formation of the Midcontinent Rift (Fig. 2). These faults may also in part control the location of Archean inliers. Vertical displacement on both fault sets range from 200 to 400 metres. Figure 2. General geology of south Armstrong–Gull Bay area Figure 3. La/Sm versus Gd/Yb for samples from the English Bay complex, Jackfish Island Sill, Nipigon sills and Inspiration sills. References Davis, D.W. and Sutcliffe, R.H. 1985. U-Pb ages from the Nipigon plate and northern Lake Superior; Geological Society of America Bulletin, v.96, p.1572-1579. Hart, T.R. 2003. Keweenawan mafic and ultramafic intrusive rocks of the Lake Nipigon and Crystal Lake areas, northwestern Ontario; in Programs and Abstracts, Proceedings of the Institute on Lake Superior Geology, v.49, Pt. 1, p.21-22. Hart, T.R. and Magyarosi, Z. 2004. Precambrian geology of the northern Black Sturgeon River and Disraeli Lake area, Nipigon Embayment, northwestern Ontario; Ontario Geological Survey, Open File Report 6138, 56p. MacDonald, C.A. 2004. Precambrian Geology of the South Armstrong-Gull Bay area, Nipigon Embayment, Northwestern Ontario; Ontario Geological Survey Open File Report 6136, 42p. MacDonald, C.A., terMeer, M., Lepage, L., Prefontaine, S. and Tremblay, E. 2004a. Precambrian Geology of the Waweig-Wabinosh Lakes area, western Nipigon Embayment, Northwestern Ontario; Ontario Geological Survey, Preliminary Map P.3536, scale 1:50,000. MacDonald, C.A., terMeer, Lepage, L., Prefontaine, S. and Tremblay, E. 2004b. Precambrian Geology of the English Bay-Havoc Lake area, western Nipigon Embayment, Northwestern Ontario; Ontario Geological Survey, Preliminary Map P.3537, scale 1:50,000. 103 �Precambrian Geology of the South Armstrong–Gull Bay Area, Nipigon Embayment, Northwestern Ontario, Canada MacDONALD, C.A., and TREMBLAY, E. Precambrian Geoscience Section, Ontario Geological Survey Ministry of Northern Development and Mines, Sudbury, Ontario, Canada, P3E 6B5 As part of the Lake Nipigon Regional Geoscience Initiative (LNRGI), a 2-year, 1:50 000 scale mapping program was begun in 2003 to better understand the geology of the western Nipigon Embayment. As part of LNRGI, this study mapped an area of roughly 1200 km2 during the 2003 field season (MacDonald 2004; MacDonald et al. 2004a, 2004b). The project was funded by the Northern Ontario Heritage Fund Corporation (NOHFC) through the Ontario Prospectors Association. This poster presentation highlights some of the preliminary results of this mapping program. The south Armstrong–Gull Bay area is located approximately 190 km north of Thunder Bay and is partially bordered by the shore of Lake Nipigon on its east margin. Rocks of both the Superior and Southern provinces occur in the map area and include Archean rocks of the volcanic-plutonic central Wabigoon Subprovince that are disconformably overlain by Mesoproterozoic sedimentary rocks of the Sibley Group, all of which are intruded by Mesoproterozoic mafic igneous rocks. Archean rocks consist dominantly of variably foliated to gneissic, felsic to intermediate, granitoid rocks and mafic to felsic metavolcanic rocks. A 30 metre thick flat-lying succession of mafic volcanic pillowed flows occurs in the northern part of the map area. The undeformed nature of these flows, combined with the well-preserved nature of relatively delicate features such as hyaloclastite suggests that these rocks may be Proterozoic rather than Archean in age. If so, they would be the first Proterozoic mafic volcanic rocks reported this far north of the Midcontinent Rift. Alternatively, they could represent a previously unrecognized greenstone belt within the Superior Province. Pre-Keweenawan rocks include anorogenic igneous and associated metavolcanic rocks of the English Bay complex and sedimentary rocks of the Sibley Group. The English Bay complex (~1540 Ma, Davis and Sutcliffe 1985) consists mainly of massive quartz and feldspar crystal tuffs with a variety of fragment types suggesting an extrusive, volcanic origin for most of the complex. Sibley Group clastic and chemical sedimentary rocks are few, and their distribution is controlled by a combination of original paleotopography on the Archean basement as well as the erosional level of the overlying diabase sills. At least one olivine gabbro sill, referred as the Jackfish Island sill, intrudes the English Bay complex. The Jackfish Island sill has geochemical affinities with the Kitto peridotite intrusion located on the east side of the Nipigon Embayment (Hart 2003). Both the Jackfish Island and Kitto peridotite are roughly coeval with the ~1110 Ma Nipigon diabase sills. Nipigon diabase sills intrude and overlie all previously noted rock types and are part of the 1.11 to 1.09 billion-year-old Midcontinent Rift. The Nipigon sills are generally massive, medium-grained, intergranular-textured and gabbroic in composition with local variations including ophitic and poikilitic textured diabase, oikocrystic diabase and magnetite-rich to locally glomeroporphyritic magnetite phases. Coarse-grained pods, veins and/or monzogabbroic phases also occur within the diabase and could represent a late magmatic phases of the sills or assimilation of Sibley Group sedimentary rocks, or both (Hart and Magyarosi, 2004). The diabase sills intrude and typically overlie all other rock units. The diabase sills are tentatively subdivided into 2 types: 1) The Inspiration sills, located in the northern part of the map area, which have higher trace element ratios and normal remnant magnetization than typical Nipigon sills, and 2) Sills located in the southern part of the map area that are reversely polarized and which have lower trace element ratios. This southern group of sills are grouped with the typical Nipigon sills. 104 �Several northwest- and north-trending faults are prominent in the map area. Northwest-trending faults appear to correlate with structures in the central Wabigoon Subprovince and may control the distribution of Archean inliers. A series of subparallel north-trending faults, which include the Black Sturgeon fault, may represent faults related to the formation of the Midcontinent Rift. The vertical displacement on both fault sets range from 200 to 400 metres. The mineral potential within the map area consists of previously undocumented areas of Archean mafic and felsic metavolcanic rocks that may hold potential for Pb-Cu-Zn volcanogenic massive sulphide (VMS) deposits. Two areas of brittle-ductile deformation within the metavolcanic rocks may also hold potential for shear zone-hosted gold deposits. The mafic to ultramafic portions of an Archean sanukitoid multiphase intrusion should be investigated for platinum group element potential, since gabbroic to pyroxenitic rocks with disseminated chalcopyrite, pyrrhotite and pyrite from the Roaring River Complex have returned assays up to 2.1 g/t Pt+Pd+Au (Schnieders et al. 2002). Cr-Ni-Cu-platinum group element potential exists in both Proterozoic mafic to ultramafic bodies within the map area. Several areas within the Nipigon Embayment may have potential to host iron oxide-copper-gold (i.e., Olympic Dam type) deposits, particularly the English Bay complex. References Davis, D.W. and Sutcliffe, R.H. 1985. U-Pb ages from the Nipigon plate and northern Lake Superior; Geological Society of America Bulletin, v.96, p.1572-1579. Hart, T.R. 2003. Keweenawan mafic and ultramafic intrusive rocks of the Lake Nipigon and Crystal Lake areas, northwestern Ontario; in Programs and Abstracts, Proceedings of the Institute on Lake Superior Geology, v.49, Pt. 1, p.21-22. Hart, T.R. and Magyarosi, Z. 2004. Precambrian geology of the northern Black Sturgeon River and Disraeli Lake area, Nipigon Embayment, northwestern Ontario; Ontario Geological Survey, Open File Report 6138, 56p. MacDonald, C.A. 2004. Precambrian Geology of the South Armstrong-Gull Bay area, Nipigon Embayment, Northwestern Ontario; Ontario Geological Survey Open File Report 6136, 42p. MacDonald, C.A., terMeer, M., Lepage, L., Prefontaine, S. and Tremblay, E. 2004a. Precambrian Geology of the Waweig-Wabinosh Lakes area, western Nipigon Embayment, Northwestern Ontario; Ontario Geological Survey, Preliminary Map P.3536, scale 1:50,000. MacDonald, C.A., terMeer, Lepage, L., Prefontaine, S. and Tremblay, E. 2004b. Precambrian Geology of the English Bay-Havoc Lake area, western Nipigon Embayment, Northwestern Ontario; Ontario Geological Survey, Preliminary Map P.3537, scale 1:50,000. Schnieders, B.R., Scott, J.F., Smyk, M.C., Parker, D.P. and O’Brien, M.S. 2002. Report of Activities 2001, Resident Geologist Program, Thunder Bay South Regional Resident Geologist Report: Thunder Bay South District; Ontario Geological Survey, Open File Report 6081, 45p. 105 �Magnetic Fabric Constraints on Magmatic Flow: Insizwa Sill, South Africa and the Sonju Lake Intrusion, Minnesota. MAES, Stephanie, TIKOFF, Basil, BROWN, Phil, Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 FERRÉ, Eric, Department of Geology, Southern Illinois University, Carbondale, Illinois 62901 Layered mafic intrusions are an important aspect of plate tectonics, commonly related to large igneous provinces and potentially acting as conduits for continental flood basalts. Although they are the source for most of the world’s PGE (platinum group element) and Cr deposits, as well as important Cu and Ni reserves, a large degree of uncertainty exists as to how these systems evolve. Magnetic fabrics, which often closely relate to the mineral fabric, allow us to document magmatic structures and map out the direction(s) of flow within an intrusion. Magnetic fabrics are determined using anisotropy of magnetic susceptibility (AMS) analysis. Knowledge of flow direction may permit us to place constraints on the location of ore deposits as well as to test models of magmatic sulfide deposition. In addition, these techniques allow us to evaluate the importance of vertical cumulus processes versus horizontal particle flow in the formation of layering. When applied to the study of mafic intrusions, standard magnetic techniques such as low field AMS do not always provide a fabric that reflects the flow field. This is due to the complex magnetic behavior of mafic rocks. Mafic rocks often contain multiple magnetic carriers and magnetite domain sizes, which can negatively affect the AMS fabric. A new magnetic approach, high field AMS (HFAMS), has been used to separate these complex anisotropies. By applying HFAMS techniques the ferromagnetic (magnetite) and paramagnetic (mafic silicate) components of the anisotropy can be separated. Hysteresis properties are then used to identify the domain size of magnetite. Our approach is to compare magmatic fabrics in two intrusions with contrasting genetic histories. In a closed system, such as the Sonju Lake intrusion, petrologic models predict a gradual upward increase in magnetite. In contrast, open systems, where episodes of recharge, eruption and/or assimilation can occur, result in a multimodal distribution of magnetite. The Insizwa sill, South Africa represents a system open to magmatic recharge. Bulk magnetic susceptibility measurements of closely spaced samples from vertical borehole cores within the Insizwa sill have documented significant variation in magnetic properties with depth (see attached figure). Variation in the magnetic signal results from changes in concentration and/or mineralogy of the magnetic material. In addition, a strong correlation exists between petrology of particular layers and bulk susceptibility. Preliminary high field results show consistent high field slopes, indicating a constant paramagnetic contribution to the susceptibility. Hysteresis ratios indicate magnetite is predominantly pseudosingle- and multi-domain, with minor single domain in the lower portions of the sill. In the Sonju Lake intrusion, a more straightforward interpretation of the magnetic properties is expected due to the nearly closed system nature. The same magnetic techniques will be applied to Sonju Lake to fully describe the intrusion and provide insight to the various fabric forming processes occurring in open and closed systems. 106 �107 �MINING AND EXPLORATION ACTIVITY IN NORTHWESTERN ONTARIO MAGEE, Angelique, Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 CANADA Northwestern Ontario experienced a significant upswing in mining and mineral exploration in 2003. Six mines produced a total of 1.8 million ounces of gold in 2003, approximately 70% of Ontario’s total. Gold producers included: Campbell Mine (Placer Dome (CLA) Ltd.); David Bell Mine (Teck Cominco Limited and Barrick Gold Corporation); Golden Giant Mine (Newmont Canada Limited); Musselwhite Mine (Placer Dome (CLA) Limited / Kinross Gold Corporation); Red Lake Mine (Goldcorp Inc.); and Williams Mine (Teck Cominco Limited and Barrick Gold Corporation). North American Palladium Ltd. produced 288 000 ounces of palladium and 24 000 ounces of platinum at its Lac des Iles Mine and recently announced it would develop an underground operation below its open pit mine. There are approximately 300 active exploration projects in the northwest, the vast majority of which are focused on gold. Areas receiving the most interest from exploration companies were, the Red Lake greenstone belt, Shoal Lake area, Dogpaw Lake area, Shebandowan greenstone belt, Fort Hope greenstone belt, Onaman-Tashota belt and the Pickle Lake greenstone belt. If metal prices continue their upward trend, northwestern Ontario may well experience levels of exploration activity not seen since the mid-1980s. 108 �THE DISCOVERY AND GEOLOGY OF THE L-K MASSIVE SULFIDE DEPOSIT, MENOMINEE COUNTY, MI MAHIN, Robert A., Aquila Resources Corp., Duluth, MN QUIGLEY, Thomas O., Minerals Processing Corp, Duluth, MN LYNOTT, Jeffrey S., Environmental Compliance Consultants, Inc, Rhinelander, WI. The L-K zinc-gold deposit is a recently discovered major volcanogenic massive sulfide (VMS) of the early Proterozoic Penokean volcanic belt. Several events led to the eventual drilling of discovery hole LK-2, (37 meters @ 9.15 % Zn and 5.85 grams/tonne Au) none of which were part of earlier Wisconsin VMS exploration efforts. Prior AEM surveys to the north, west, and south failed to include the immediate deposit area, an area that despite proximal outcrops of intensely sericite-pyrite-altered rhyolite tuffs, was mapped as granitic intrusives and Paleozoic cover. The discovery of L-K can be attributed to Harry Kleiman, a water-well driller who deepened a local camp owner’s water well, his partner Rich Lassen, who recognized the sphalerite-rich drill cuttings as a potential VMS and subsequently located a nearby outcropping gold-rich gossan, and Tom Quigley, who later joined the partnership and pinpointed a significant gravity/conductive anomaly that turned out to be the L-K deposit. A joint venture with American Copper and Nickel Company, a subsidiary of Inco, resulted in a drilling program of 71 holes for over 20,000 meters of core. The drilling outlined a significant zinc-gold rich VMS within what appears to be a major felsic center. Significant portions of the deposit contain consistent grades of zinc in excess of 10 percent. Gold grades with in massive sulfides range from 1 to 6 grams/tonne. Significant gold mineralization also occurs in proximal sulfide stringers (e.g., 4.2 meters @ 25.3 grams/tonne Au), and in peripheral silicified zones in rhyolite tuffs and QFP dikes (e.g., 12.5 meters @ 7.4 grams/tonne Au). The near-surface gossan averages approximately 16 grams/tonne Au. The stratigraphy of the deposit consists of a stacked series of quartz-feldspar rhyolite crystal tuffs, locally fragmental, with intermittent fine-grained ash-tuff horizons, overlain by fine-grained, laminated tuffaceous sediments. Mineralization occurs as massive (>80%) pyrite+sphalerite+chalopyrite+galena localized at two stratigraphic contacts. Host rocks are intensely sericitized and silicified quartz-feldspar rhyolite tuffs and tuffaceous sediments. Structurally, the deposit and surrounding host rocks have been folded into a west/southwest-plunging, south verging antiform. The deposit is located both in the core of the antiform, where it appears to be tectonically thickened, and in the limbs of the fold. Numerous syntectonic, west-southwest-striking, steeply dipping, quartz-feldspar dikes intrude proximal to the deposit. Drilling has traced the massive sulfide along strike for 700 meters with vertical projections of the deposit ranging from 50 to 270 meters wide. Down-plunge, massive mineralization has been traced to a depth of approximately 300 meters. The deposit remains open at depth and along strike. 109 �THE GEOLOGY OF THE L-K MASSIVE SULFIDE DEPOSIT, MENOMINEE COUNTY, MI MAHIN Robert A., Aquila Resources Corp., Duluth, MN QUIGLEY, Thomas O., Minerals Processing Corp, Duluth, MN LYNOTT, Jeffrey S., Environmental Compliance Consultants, Inc., Rhinelander, WI The L-K deposit is a newly discovered volcanogenic massive sulfide deposit in the Beecher Formation of the early Proterozoic Penokean volcanic belt in western Menominee County, Michigan. The major economic metals at L-K are zinc and gold, with locally high concentrations of silver and copper. Mineralization occurs as massive (greater than 80%) pyrite+sphalerite+chalcopyrite+galena localized at two stratigraphic contacts within a large felsic volcanic center dominated by intensely sericitized and silicified quartz-feldspar, rhyolite tuffs. The stratigraphy consists of a stacked series of quartz-feldspar rhyolite crystal tuffs, locally fragmental, with intermittent fine-grained ash-tuff horizons, overlain by fine-grained, laminated tuffaceous sediments. The main-zone massive sulfide is hosted by altered hanging wall and footwall rhyolite crystal tuffs that are visually identical, but readily distinguishable by wholerock geochemistry, e.g., ratios of Al and Ti (Cattalani 2003, Shriver 2003). Tuffaceous zone mineralization, while not as consistently thick as the main zone, contains impressive grades and is focused along the contact between hanging wall rhyolites and overlying tuffaceous sediments. The massive sulfides are enveloped by gold (and to a lesser extent copper) bearing stockwork stringer sulfides (10 to 80% sulfide) that grade into a disseminated pyritic halo. Structurally, the deposit and surrounding host rocks have been folded into a west-southwest-striking, southwest plunging, south verging asymmetric antiform. Tectonically thickened portions of the deposit occupy the core of the fold and large sections of mineralization are also found in the limbs. Numerous syntectonic, west-southwest striking, steeply dipping quartz-feldspar dikes intrude the deposit. Faulting has resulted in minor offsets. Drilling has traced the massive sulfide along strike for 700 meters, with vertical projections of the deposit ranging from 50 to 270 meters wide. Portions of the deposit sub-crop at the east-northeast up-plunge extension where an ironoxide-rich, precious metal-enriched gossan has developed. Down-plunge, massive mineralization has been traced to a vertical depth of approximately 300 meters and remains open. Gradewise, drill intercepts of greater than 10 meters in excess of 10% zinc are common. Gold grades with in massive sulfides range from 1 to 6 grams. Significant gold mineralization also occurs in proximal sulfide stringers (e.g., 4.2 meters @ 25.3 grams/tonne Au), and in peripheral silicified zones in rhyolite tuffs and QFP dikes (e.g., 12.5 meters @ 7.4 grams/tonne Au). Intercepts of the near-surface gossan average 3 meters thick and approximately 16 grams/tonne Au. References Cattalani, S., 2003, Lithochemistry and chemostratigraphy, Back 40 project, Michigan: INCO private report, 7p. Shriver, N.A., 2003, Michigan: Back 40 chemostratigraphy and deformation of the Back 40 massive sulfide based upon lithogeochemical interpretation: INCO private report, 6p. 110 �MINERAL CHEMISTRY AND STRATIGRAPHY OF THE BIWABIK IRON FORMATION, NEAR THE VIRGINIA HORN, MESABI IRON RANGE, MINNESOTA McSWIGGEN, Peter L., and MOREY, G.B., McSwiggen & Associates, P.A., 2855 Anthony Lane South, Suite B1, St. Anthony, MN (PMcS@McSwiggenAssoc.com; morey001@umn.edu) The mineralogy of the Biwabik Iron Formation changes dramatically from west to east as the formation nears the basal contact of the Duluth Complex. This reflects a contact metamorphism that took place with the emplacement of the igneous Duluth Complex. However, the mineralogy of the Biwabik Iron Formation also varies vertically through the stratigraphy of the unit. A number of detailed studies have been done on the mineralogy of the contact zone (Bonnichsen, 1975). This investigation focused on the western Mesabi Range through the characterization of the E.J. Longyear Drill Hole #1 (S1/2, SW1/4, SE1/4, sec 23, T57N, R18W), which penetrated the entire section of the Biwabik Iron Formation from the overlying Virginia Formation through to the underlying Pokegama Quartzite (Fig.1). The iron-formation has been subdivided into four broad stratigraphic units (Fig. 2), lower cherty, lower slaty, upper cherty, and upper slaty, and into four lateral mineralogical zones (1-4) that reflect the zonation resulting from the contact metamorphism. Zone 1, the westernmost zone and the one in which the Longyear drill hole is located, is characterized by quartz, magnetite, hematite, carbonates, talc, chamosite, greenalite, minnesotaite and stilpnomelane. Talc and minnesotaite are the Mg- and Fe-end members, respectively, of the talc group [Mg6Si8O20(OH)4 – Fe6Si8O20(OH)4]. Minnesotaite is the dominant end member in the Biwabik Iron Formation, and typically occurs as a fibrous mineral, though it is also found in a tabular form. Chamosite is a Fe-chlorite [(Fe, Al)6 (Si, Al)4O10(OH)8], and generally occurs as a platy mineral in granules within the iron-formation. These granules are sand size gains that are the result of the reworking of previously deposited materials, which reflects a higher energy depositional environment. Greenalite is the iron serpentine [Fe6Si4O10(OH)8]. It is reported generally as having a platy, lizardite-like structure, and in the iron-formation occurs in granules. Stilpnomelane [(K, Na, Ca)0.6(Fe, Mg)6Si8Al (O,OH)27.2-4H2O] is a sheet silicate, but in the iron-formation it can occur in either a platy form or in a fibrous form. The above-described silicates do not occur uniformly throughout the stratigraphic section. In the Longyear core, chamosite + stilpnomelane is the dominant silicate assemblage in the upper three units (upper slaty, upper cherty and lower slaty). The assemblages minnesotaite + stilpnomelane, minnesotaite + talc, and greenalite + minnesotaite are the main silicate assemblages in the lower cherty. The carbonates have a wide range of compositions and include calcite, dolomite-ankerite, siderite, and kutnahorite, the manganese equivalent of ankerite. Carbonates occur throughout the Biwabik Iron Formation, but become more dominant in the lower cherty interval. The manganese component of the carbonates varies greatly, ranging from less than 1 mole% MnCO3 to as much as 25 mole percent. Bonnichsen, B., 1975, Geology of the Biwabik Iron Formation, Dunka River area, Minnesota: Economic Geology, v. 70, no 2, p319-340. Jirsa, M.A., Miller, J.D., Jr., and Morey, G.B., 2004, Geology of the Biwabik Iron Formation and Duluth Complex, (in press). 111 �Figure 1. Simplified geologic map of the Mesabi Iron Range. The main geologic unit is the Biwabik Iron Formation (shown in gray) (after Jirsa and others (in press)). Figure 2. Stratigraphic section and mineralogy of the E.J Longyear Drill Hole #1. It was drilled as a long stratigraphic test hole and was finished in 1910. 112 �GEOCHRONOLOGY OF PRECAMBRIAN ROCKS IN CENTRAL WISCONSIN: A REVIEW AND NEW 40Ar/39Ar ANALYSES MEDARIS, Jr., Gordon and SINGER, Brad, Dept. of Geology & Geophysics, Univ. Wisconsin Madison, Madison, WI, 53706; medaris@geology.wisc.edu; bsinger@geology.wisc.edu Most of the Precambrian evolution of Wisconsin is recorded by basement rocks in the central part of the state. Reported here are new 40Ar/39Ar analyses of hornblende, muscovite, and microcline, which bear on the thermal history of this important area. Because this is the 50th meeting of ILSG, it seems appropriate to provide a selective review of geochronological investigations over the past half century and to place our new results in a historical context. Review The framework for a modern classification of Precambrian rocks in the Great Lakes region was provided in 1961 by the seminal work of Goldich et al., who analyzed a large number of igneous and metamorphic rocks by the K/Ar and Rb/Sr methods. A three-fold Precambrian division was proposed, with boundaries at 2.5 and 1.7 Ga, corresponding to the Algoman and Penokean orogenies, although it was recognized that the apparent K/Ar and Rb/Sr ages might reflect subsequent metamorphism. In 1975 Van Schmus et al. established an apparent Rb/Sr age of ~1.65 Ga for a wide variety of igneous and metamorphic rocks in central and eastern Wisconsin. The few available U/Pb zircon data for such rocks yielded protolith ages of 1.8-1.9 Ga, and it was suggested that a widespread, low-grade metamorphic event, whose origin was poorly understood, affected the western Great Lakes region at 1.65 Ga. Also in 1975, Van Schmus et al. recognized the Wolf River batholith as a major igneous component in Wisconsin and determined equivalent Rb/Sr whole rock and U/Pb zircon ages of 1468±34 and 1485±15 Ma, respectively. With the continued acquisition of U/Pb analyses of zircon in the 1970s and 1980s, the Marshfield terrane in central Wisconsin was shown to consist of Archean gneiss (2.87-2.52 Ga) intruded and overlain by a wide variety of granitic rocks and associated felsic to intermediate volcanic rocks, ranging in age from 1.89 to 1.82 Ga (Sims et al., 1989). In 1983 Dott suggested that folding of Baraboo Interval quartzites was the result of plate collision to the south at ~1.65 Ga, and this concept was expanded by Van Schmus et al. (1993), who related 1.65 Ga deformation and low-grade metamorphism in the Great Lakes region to emplacement of the Outer Tectonic Belt onto the southern margin of Laurentia during the Mazatzal Orogeny. In 1998 Holm et al. located the tectonic and thermal front of Mazatzal deformation in northern Wisconsin, based on the distribution of folded and flat-lying quartzites and cooling ages (Rb/Sr, K/Ar, 40Ar/39Ar) of mica in basement rocks, e.g. 1.75-1.70 Ga vs. <1.63 Ga. The position and nature of the Mazatzal front in Wisconsin was confirmed by Romano et al. (2000), who showed that micas from basement rocks outside the front yield 40Ar/39Ar plateau ages of 1.76-1.75 Ga, and those inside, 1.61-1.58 Ga. Five samples of hornblende from Archean and Paleoproterozoic rocks within the front yield 40Ar/39Ar plateau ages of 1830, 1796, 1782, 1733, and 1638 Ma, which are thought to represent partial to complete Mazatzal resetting. Subsequently, 40Ar/39Ar ages of 1.45-1.47 Ga were obtained for muscovite in Baraboo Interval quartzites, reflecting widespread, but stratigraphically localized, hydrothermal activity related to Wolf River magmatism (Medaris et al., 2003). Central Wisconsin Precambrian Rocks The basement in NE Wood and NW Portage counties consists predominantly of Archean gneiss and a variety of Paleoproterozoic igneous rocks, including tonalite, granodiorite, granite, and associated felsic to intermediate volcanic rocks. U/Pb zircon ages are ~2780 Ma for migmatitic gneiss at Linwood Quarry, and 1892, 1851, 1841, and 1824 Ma for different varieties of tonalite along the Wisconsin River (Sims et al., 1989; Van Wyck, 1995). Many of the igneous rocks have been deformed and recrystallized, exhibiting a range of foliated and lineated fabrics. Amphibolite layers in Archean gneiss at Conants Rapids yield a temperature of 665 ºC and amphibolite (metadiabase) dikes cutting foliated tonalite at Biron Dam give 700 ºC (calculated by the hornblendeplagioclase geothermometer at P = 4 kbar; Holland & Blundy, 1994). Laser step-heating yields plateau ages of 1672 Ma for hornblende in metadiabase at Biron Dam (Fig. 1), 1516 and 1533 Ma for hornblende from two samples of amphibolite at Conants Rapids (Fig. 2), 113 �1530 Ma for muscovite in low-grade schist from the Eau Pleine shear zone (Fig. 3), and 981 Ma for microcline in Wolf River granite and 897 Ma for microcline in Baxter Hollow granite (located in the Baraboo Range) (Fig. 4). We interpret the ages of hornblende at Conants Rapids, 3.9 miles from the Wolf River batholith, and muscovite in the EPSZ, 6.7 miles from the batholith, to represent partial resetting by the Wolf River thermal pulse. The age of hornblende at Biron Dam, 13.3 miles from the batholith, may also reflect partial resetting by Wolf River heating, although this age lies within the range for hornblende reported by Romano et al. (2000) and ascribed by them to Mazatzal disturbance. The closure temperature of microcline can be as low as 150 oC, thereby allowing for the possibility that it may record cooling of the craton in central Wisconsin, following 1.1-1.0 Ga Keweenawan rifting and magmatism. The Ar ages reported here seem to result from degassing in response to regional thermal events, rather than pervasive internal deformation and recrystallization (also observed by Romano et al., 2000). Thus, although there has been widespread disturbance of Rb/Sr and Ar isotopic systems in Wisconsin Precambrian basement, many rocks in central Wisconsin have preserved their Penokean structures, textures, and mineralogical compositions. References Dott (1983) GSA Mem. 160 129-141; Goldich et al. (1961) Minn. Geol. Sur. Bull 41; Holland & Blundy (1994) Contrib. Mineral. Petrol. 116 433-447; Holm, D. et al. (1998) Geology 26 907-910; Medaris et al. (2003) J. Geol. 111 243-257; Romano et al. (2000) Precam. Res. 104 25-46; Sims et al. (1989) Can. J. Earth Sci. 26 2145-2158; Van Schmus et al. (1975a) GSA Bull. 86 1255-1265; Van Schmus et al. (1975b) GSA Bull. 86 907-914; Van Schmus et al. (1993) GSA Geology of North America C-2 270-281; Van Wyck (1995) Ph.D. thesis, Univ. Wis.-Madison, 280 pp. 114 �Geochemistry and Petrography of Altered Basement Rocks Underlying the Middle Proterozoic Sibley Group METSARANTA, R.T.*, and FRALICK, P.W., Department of Geology, Lakehead University, 955 Oliver Rd., Thunder Bay, Ontario, P7B 5E1, Canada. rtmetsar@lakeheadu.ca The Sibley Group is a relatively thin Mesoproterozoic mixed carbonate-clastic succession deposited, at least in part, under restricted shallow marine or lacustrine conditions (Franklin et al., 1980; Cheadle, 1986; Rogala 2003). A diverse mixture of genetically distinct chemical sediments is preserved within various depositional sub-environments of the Sibley Basin. These include: dolomitic mudstones and sandstones, stromatolitic carbonates, calcretes and nodular gypsum/anhydrite. In addition to these chemical sediments, possible weathering profiles are developed in various underlying basement lithologies and at higher stratigraphic positions. Given the diversity of chemical sediment types and possible weathering profiles within the basin the potential exists to develop a detailed model for the hydrology and paleoenvironmental evolution of the Sibley Group. Weathering is an important control on the composition of clastic sediments (e.g. Nesbitt et al., 1996) and also restricted basin brines (Rosen, 1994). As a first step towards an overall model of the hydrology of the Sibley Basin, the purpose of this paper is to investigate the petrography and geochemistry of altered rocks at the unconformity between the Sibley Group and underlying Archean and Paleoproterozoic lithologies. To be of value in an analysis of overall basin hydrology and paleoenvironmental conditions, the relative importance of primary pedogenically induced geochemical changes vs. later diagenetic or hydrothermal effects must be addressed. Suites of samples were collected from 4 areas spanning a variety of basement lithological types. Altered Quetico sandstones and Neoarchean granite were sampled from drill core at variable depths below the Sibley Group contact. Altered and unaltered Paleoproterozoic black shales of the Rove Formation were sampled from outcrop as were Mesoproterozoic anorogenic granites and overlying pebbly sandstones. Polished thin sections were cut from each sample and analysed petrographically using both optical and scanning electron microscopy. Whole rock powders were obtained for each sample and were analysed at Lakehead University via ICP-AES for a selection of major and trace elements. Petrographic evidence such as the progressive destruction of plagioclase upwards in the profiles coupled with extensive development of Fe-Ti oxides along grain boundaries and cleavage planes in biotite and chlorite is consistent with a weathering profile. Pebbles in sandstones associated with the anorogenic granite profile are also strongly iron enriched containing up to 30% total iron suggesting a highly oxidizing atmosphere during pre-Sibley Group weathering conditions. The presence of authigenic, euhedral potassium feldspars and the potassium-rich nature of clay minerals in the altered horizons suggest secondary potassium enrichment. The presence of barite, flourite and sulphide mineralization near the Sibley Group contact with Neoarchean granites also reveals that hydrothermal alteration may have had an influence on primary pedogenic geochemical signatures. Preliminary assessment of the major element geochemistry using a simple alteration index (CIA) and A-CN-K diagrams (e.g. Nesbitt and Young, 1982; Nesbitt et al., 1996) also suggests that primary pedogenic alteration may have been effected by later potassium enrichment. 115 �Al2O3 0 10 100 90 20 80 30 70 40 60 50 50 60 40 70 30 Granite Quetico Rove 80 90 20 10 100 CaO+Na2O 0 0 10 20 30 40 50 60 70 80 90 100 K2O Figure 1. Altered samples from three profiles plotted as molar proportions Al, Ca+Na, and K (as oxides), showing divergence of observed alteration trends towards more K-rich compositions from those expected due to weathering (arrow) Figure 2. Left: backscatter SEM image of K-rich clay mineral aggregates associated with altered primary K-feldspar (altered Neoarchean granite) and right: altered biotite grains with extensive development of Fe and Ti oxides (altered Quetico metasediment). References Cheadle, B.A., 1986. Alluvial and playa sedimentation in the lower Keweenawan Sibley Group, Thunder Bay District, Ontario. Canadian Journal of Earth Sciences. 23:527-542 Franklin, J.M., McIlwaine, W.H., Poulsen, K.H., and Wanless, R.K., 1980. Stratigraphy and depositional setting of the Sibley Group, Thunder Bay District, Ontario, Canada. Canadian Journal of Earth Sciences. 17:633-651. Nesbitt, H.W., Young, G.M., 1982. Early Proterozoic climates and plate motions inferred from major element chemistry of lutuites. Nature. 299:715-717. Nesbitt, H.W., Young, G.M., McLennan, S.M. and Keays, R.R. 1996. Effects of chemical weathering and sorting on the petrogenesis of siliciclastic sediments, with implications for provenance studies. Journal of Geology. 104:525-542. Rogala, B., 2003. The Sibley Group: A lithostratigraphic, geochemical and Paleomagnetic study. Unpublished M.Sc. thesis, Lakehead University, Thunder Bay, Ontario, Canada. 254p. Rosen, M.R. 1994. The importance of groundwater in playas: a review of playa classification and the sedimentology of playas. In Rosen, M.R., ed., Paleoclimate and Basin Evolution of playa systems. Geological Society of America Special Paper 289. Boulder Co. pp. 1-18. 116 �N.H. Winchell's Study of the Keweenawan Supergroup Rocks of Northeastern Minnesota, 1872-1900 MILLER, James D., Jr., Minnesota Geological Survey, mille066@umn.edu In 1872, fourteen years after its admission to the Union, the state legislature of Minnesota granted an initial appropriation of one thousand dollars per year to the University of Minnesota to create a geological and natural history survey of the state. In July of that year, Newton Horace Winchell (1839-1914) was hired to lead this survey, a task to which he committed 28 years of his life. There is little debate as to the contribution Winchell made to our understanding of diverse aspects of Minnesota's geology, especially in light of the paucity of existing research. Much of the survey's observations and interpretations of Paleozoic and Quaternary geology still stand up today. However, Winchell's greatest difficulties came in his attempts to make sense of the "crystalline rocks" of northeast Minnesota. As he stated in the preface to the fourth volume of the Final Report: "Here [among the crystalline rocks] the geologist is deprived of his usual guides and guys, and finds himself floundering in a muddy sea of innumerable conflicting currents" (Winchell, 1899, p. xiv). This talk focuses on Winchell's struggles with deciphering the complexities of what is now known as the Keweenawan Supergroup in northeastern Minnesota. My interest in this topic began while researching the history of geologic mapping in the Duluth Complex (Miller and others, 2002). My characterizations and interpretations of Winchell's ideas in this presentation come from selected readings of the 24 annual reports of the Geological and Natural History Survey of Minnesota, the final two volumes of the Final Report (Winchell, 1899, 1900), and Geological and Natural History Survey of Minnesota Bulletins 1, 2, 6, and 8. From this still incomplete sampling of Winchell's prolific writing on the Keweenawan rocks, I have come to conclude that most of his ideas, which seem unconventional by today's standards, were based on paradigms that were accepted by many, if not a majority, of geologists in his day. In the late nineteenth century the principles of stratigraphy and sedimentology were already well established, but the science of igneous and metamorphic petrology was in its infancy. At that time, numerous different ideas about the origin of magmas, volcanic processes, and progressive metamorphism existed. There was no accepted conventional wisdom. Winchell took advantage of the new field of petrographic petrology, which saw widespread use and growth in the last half of the century. He did not, however, have the benefit of the knowledge gained by experimental petrology, which started to emerge just as the survey was being completed. At the beginning of the Survey, Winchell's views of Lake Superior geology were strongly influenced by the early federal survey of the region (Owen, 1852), as well as by reports of Canadian geologists and the preeminent geologists from his native New York and New England. Over the course of the survey, Winchell regularly compared his observations to those of his U.S. and Canadian colleagues, but more often than not, he seemed to go his own way with many of his interpretations of the crystalline rocks. In fact, he often disagreed with the interpretations of his fellow survey colleagues, many of whom were his relatives—Alexander Winchell (his brother), Horace V. Winchell (his son), and U.S. Grant (his brother-in-law). One of his more curious relationships was with R.D. Irving of the University of Wisconsin, and later of the U.S. Geological Survey. Following up on his work on the Keweenawan rocks of northwestern Wisconsin for the third Wisconsin Geological and Natural History Survey report in 1880, Irving published the first thorough summary of Keweenawan geology in the Lake Superior region in a U.S. Geological Survey monograph (Irving, 1883). In this report, Irving recognized the volcanic character of the mafic and felsic rocks of the shore, and the intrusive nature of the gabbros. It included a remarkably accurate map of the geology of Minnesota's North Shore, in which Irving estimated that the volcanic pile from Duluth to the Temperance River exceeds 18,000 feet, relatively close to current estimates of 28,000 feet (Miller and others, 2002, Chapter 5). Irving relied exclusively on his own fieldwork and gave only passing acknowledgement to the work of the Minnesota survey, then in its tenth year. Winchell, for his part, rarely cited Irving's work, and then commonly only to point out some inconsistency or discrepancy with his observations or interpretations. Winchell's displeasure with the growing influence and overreaching of the U.S. Geological Survey, perhaps triggered by Irving's work, prompted him to found and edit the American Geologist journal in 1888. The journal was meant to highlight North American geology, but regularly featured articles critical of the U.S. Geological Survey (Bain, 1916). Irving died the year of its initial publication. Some of the major aspects of Keweenawan geology that Winchell wrestled with over the course of the state survey were: Potsdam Sandstone—With a strong belief in uniformitarianism, Winchell consistently held to the notion that all red quartzose sandstone lying unconformably on crystalline rocks was equivalent to the Lower Cambrian Potsdam Sandstone of upstate New York and New England, which has a similar lithology and unconformable geologic setting. Despite their lack of fossils, Winchell opined that the Puckwunge, Nopeming, Fond du Lac, and Hinckley sandstones, and even the Sioux Quartzite, were all Potsdam equivalents. He suggested that the lava 117 �flows overlying the Puckwunge and Nopeming Sandstones and interlayered with stratigraphically higher sandstone units (such as the sandstone at Cutface Creek near Grand Marais) represented localized volcanism during what was generally a time of sandstone deposition, a period he termed the Manitou epoch. Red rock—Winchell also consistently believed that all granophyre and rhyolite were metamorphosed and fused sedimentary rocks—a commonly held view at the time. He considered the "granophyre range," which arcs through the central part of the Duluth Complex, as a raised ridge of Animikie sediments that were fused by the eruption of the gabbro (see below). Rhyolites were thought to be strongly metamorphosed sandstone interleaved with and metamorphosed by basalt flows. In Winchell's view, quartz and feldspar-phyric flows represented less severe metamorphism because phenocrysts of quartz and feldspar were considered to be preserved detrital grains. Basaltic lava flows—Early on, Winchell adopted J.G. Norwood's interpretation (Owen, 1952) that the basaltic lava flows of the North Shore were intrusive sills (flow interiors) into volcanic sediments (amygdaloidal tops). Midway through the survey he recognized the physical stratigraphy of basalt flows. Duluth Complex—Winchell variably characterized the Duluth Complex as "the great gabbro flood," "the crowning overflow," "the great gabbro outflow," "a basic eruptive," and "the gabbro eruption." From these and other descriptors, it is clear that he saw the gabbro as a great volcanic outpouring of basic magma that spilled out over (and metamorphosed) its Animikie rampart and flowed downslope toward the Lake Superior basin. The Beaver Bay Complex and the gabbros at Duluth represent the distal part of this great gabbro flood. Anorthosite inclusions—Winchell interpreted the anorthosite inclusions of the Beaver Bay Complex as earlierformed feldspathic gabbros of the Duluth Complex (what is now termed the anorthositic series) that were picked up by later surges of the great gabbro eruptive. This idea was commonly accepted until recently (see Stop 24 in Field Trip 5). Age of the gabbro—Winchell changed his interpretation of the age of the gabbro many times over the course of the survey. Based on interlayering of the gabbro with rocks he thought to be upper Keewatin (the Pewabik Quartzite, which is actually metamorphosed Pokegama Quartzite), he saw the gabbro as syn-Keewatin to preAnimikie. Until Lawson (1893) showed that the Logan sills are intrusive into the Animikie Group rocks, rather than interlayered as lava flows, Winchell saw them and the gabbro as syn-Animikie. In the end, Winchell believed the gabbro to be younger than the Animikie Group and older than the Puckwunge Sandstone and overlying lavas. Winchell called the epoch during which the gabbros were emplaced the Norian and later the Cabotian. Origin of the gabbro magma—Winchell does not speculate on the origin of the basic magma that formed the gabbro and related volcanics until the Final Report. Following the idea that all granites formed by fusion of siliceous sediments, he concluded that the gabbro formed by the melting of greenstone-derived sediments. He considered hornfels basalt inclusions, which he called muscavodyte, an intermediate stage of metamorphism of these basic sediments. In the end, Winchell was certain that the facts of his numerous and well recorded observations would hold up to future scrutiny, and unquestionably most have. He was admittedly less confident in his interpretations, however and speculated that new facts, "…not included in our field of observation, will in the future place different interpretations on those which we have attempted..." (Winchell, 1899, p. xiv). This has happened to be sure, but more significantly, the changes in our geologic paradigms and our greater understanding of igneous petrology have allowed those of us who follow in his footsteps to reinterpret his many detailed observations. REFERENCES Bain, H.F., 1916, N.H. Winchell and the American Geologist: Economic Geology, p. 51-62. Irving, R.D., 1883, The copper-bearing rocks of Lake Superior: U.S. Geological Survey Monograph 5, 464 p. Lawson, A.C., 1893, The laccolithic sills of the north-west coast of Lake Superior: Geological and Natural History Survey of Minnesota Bulletin 8, p. 24-48. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, 207 p. Owen, D.D., 1852, Report on a geological survey of Wisconsin, Iowa, and Minnesota: Philadelphia, U.S. Department of the Treasury, 638 p. Winchell, N.H., 1899, The geology of Minnesota: Geological and Natural History Survey of Minnesota, Final Report, v. 4, 629 p., 100 pls. ———1900, The geology of Minnesota: Geological and Natural History Survey of Minnesota, Final Report, v. 5, 1025 p., 6 pls. 118 �Twenty-One Years in a Caldera: UMD Geology Students and Sturgeon Lake, Ontario MORTON, Ron, Department of Geological Sciences, University of Minnesota Duluth Over the past 20 years students from the University of Minnesota-Duluth have been involved in studying the Sturgeon Lake Caldera Complex located in northwestern Ontario. Their work, based on outcrop mapping, logging of more than 200,000 meters of diamond drill core, thin section, microprobe, and chemical studies has shown that the complex comprises a wellpreserved, north facing, moderately to steeply north-dipping, homoclinal sequence that is up to 30 km in strike length and contains more than 3000 m of subaerially and subaqueously deposited volcanic and sedimentary strata. The Pre-Caldera Sequence, worked on by Groves (1984)*and Heine (1985) contains subaerial to shallow subaqueous basalt to andesite lava flows, tuffs and lapilli-tuffs with subordinate rhyolite lava flows. The Early-caldera Sequence, worked on by Walker (1993), Hudak (1989), Beszenek (1992) and Drews (1990) along with Rog (1991) and Murphy (1994) comprises subaerially to subaqueously deposited polymict breccias and aphyric or quartz-phyric, rhyodacite-rhyolite tuffs with reworked tuffs and minor andesite to rhyolite lava flows. The Latecaldera Sequence, worked on by Jongewaard (1989) and Hudak (1996) contains syn-eruptive and reworked quartz- and plagioclase-phyric rhyodacite to rhyolite tuffs and lapilli-tuffs, and subaqueous basalt-andesite to rhyolite lava flows, domes, and cryptodomes. Volcanic-hosted massive sulfide deposits occur in the Early and Late Caldera Sequences. Synthesizing these studies, with additional contributions by Peterson (2001), provides a detailed picture of caldera development and associated massive sulfide formation and hydrothermal alteration. The work of the above geologist’s shows this piecemeal caldera complex formed upon a subaerial to shallow subaqueous basalt -andesite shield volcano with associated local fields of scoria and tuff cones. Caldera formation was characterized by more than a kilometer of collapse accompanied by voluminous pyroclastic eruptions, deposition of laterally extensive meso-and mega-breccias, minor effusive volcanism, and massive sulfide ore deposition with widespread hydrothermal alteration. Later stages of caldera development were dominated by submarine effusive volcanism and intracaldera sedimentation, with subordinate pyroclastic volcanism and ore deposition. The lithostratigraphic sequence, caldera diameter, and estimated eruption volumes, determined from geographic information system (GIS) analysis, are consistent with those that characterize modern ash flow caldera complexes. The Sturgeon Lake Caldera Complex illustrates the following: 1) The necessity of long term, detailed field and laboratory studies to understand complex volcanic systems. 2) That evolutionary and mineralizing processes associated with "ash flow" calderas have been remarkably similar since at least Neoarchean time. 3) That good students do very good work and make professors look great! * Year of degree or year started work at Sturgeon Lake 119 �Status of Publicly Available Mid-Continent Reflection Seismic Data MUDREY, M.G., Jr. Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison, WI 53705 mgmudrey@wisc.edu CANNON, W.F., U.S. Geological Survey, National Center MS 954, Reston, VA 22092 wcannon@usgs.gov The Midcontinent Rift was identified by potential field and geologic studies in the 1950s. However, much of the detail of this largely buried structure was obscure except in a few well exposed areas around Lake Superior. Interest in frontier petroleum rose in the 1970s and the recognition of documented petroleum seeps from the Nonesuch Formation led to the acquisition and evaluation of industrial reflection seismic data. From 1978 to 1986 a series of seismic reflection profiles was obtained by a combination of academic (COCORP), petroleum company, and government (GLIMPCE) research programs. The U.S. Geological Survey and the Geological Survey of Canada acquired 536 line-km in Lake Superior, L.D. McGinnis and Argonne National Laboratory purchased 1,816 line-km in Lake Superior from Grant-Norpac, and A.B. Dickas University of Wisconsin-Superior negotiated the release of 1,336 line-km from Lake Superior south to Iowa from Petty-Ray/Geosource. The profiles provided access to a wealth of new details of the lateral and vertical extent of the rift and its internal structure and stratigraphy from Lake Superior to Kansas. Some of the major findings include: • Particularly significant was the discovery that 20 to 30 km of basalt flows and secondary syn-rift volcaniclastic and post-basalt sedimentary rock produced exceptionally strong and coherent reflections that enabled accurate estimates of the volumes of basalt. • Moho reflections recorded in Lake Superior over the rift range from 46 to 58 km in contrast to 36 to 42 km beneath the surrounding Great Lakes. This provides evidence of magmatic underplating and intrusions within the lower crust and upper mantle contemporaneous with crustal extension...in effect, the mantle was changed into crust by a decrease in seismic velocity. ! Individual basins within the Midcontinent Rift have been delineated by long reflection lines and cross lines -- clearly developed unconformities remove ambiguities of correlation among various volcanic sections within and between basins. Although individual seismic lines have been interpreted, the entire collection of lines, along with newly acquired gravity and magnetic data, have yet to be collectively used to refine our understanding of this 2,000 km rift. GLIMPCE data are available from Robin R. Warnken, National Geophysical Data Center, NOAA/NESDIS/NGDC/ Mail Code E/GC3, 325 Broadway, Boulder, CO USA 80305-3328. phone: (303) 4976338. Email: Robin.R.Warnken@noaa.gov Argonne National Laboratory/Grant-Norpac data are available from 120 �McGinnis, L.D., and Mudrey, M.G., Jr., 2003, Seismic reflection profiling and tectonic evolution of the Midcontinent Rift in Lake Superior: Wisconsin Geological and Natural History Survey Miscellaneous Report MP 91-2, 15 pl. 1 CD-ROM. Files on CD are in PDF format. Petty-Ray/Geosource data are available from Dickas, A.B. and Mudrey, M.G., Jr., 2002, Regional-Scale Geologic Interpretation of Seismic Reflection, Gravity, and Magnetic Profiles Collected along the Western Arm of the Midcontinent Rift System, Upper Peninsula of Michigan, Wisconsin, Minnesota and Iowa: Wisconsin Geological and Natural History Survey Open-file Report 2002-01, 1 CD-ROM. Files on CD are in HTML format. 1. Location of reflection seismic lines: GLIMPCE, heavy dashed; Grant/Norpac, light solid; Petty-Ray/Geosource, medium solid. 121 �“GOLD IS WHERE YOU FIND IT! SO IS Ag AND Cu AND Fe!” (THE OLD PROSPECTOR: GOLD RUSHES AND MINERAL PROSPECTING, 1848 TO 1900 IN WESTERN NORTH AMERICA AND THE LAKE SUPERIOR REGION) OJAKANGAS, Richard W. (a.k.a., The Old Prospector), University of Minnesota Duluth, Duluth, MN 55812, rojakang@d.umn.edu Gold has long provided a chance for the “little guy” to make a fortune. The “Old Prospector” relates factual and illustrative information about several gold rushes, beginning with the 1849 gold rush in California. (“Go West, young man, go West!”) Where did the 200,000 “Forty-Niners” come from? From everywhere! Because it took 11 months for the news to reach the east coast, people living on the Pacific Rim got there first. How many (how few?) hit it rich? Even Captain Sutter and the actual discoverer, Jim Marshall, died poor. The Comstock Lode in Nevada, “discovered” by Henry Comstock, led to the gold and silver rush of 1860, with many of the fortune-seekers crossing back over the Sierra Nevada with their donkeys. A group on their way to California in 1850 found gold in Colorado at the present site of Denver, but the rich California gold was their goal. There were several discoveries in Colorado over the next decades. It was the gold at Cripple Creek, discovered by cowboy Bob Womack in 1890 that really brought the prospectors in. Bob sold his claim for $300 -- $ 7.5 million came out of that gulch! Looking for gold, silver and copper in Montana attracted many prospectors in the ’60s and ‘70s. The copper ore was “so pure that it could be shipped to hell and back for smelting and still make a profit.” (Swansea, Wales, was the location of early smelters.) In 1876, the rush was to the Black hills for gold. General Custer had gone in to look for gold in 1874, and his favorable report brought in a horde of prospectors. All of this was in violation of the Fort Laramie Treaty of 1868. The Indians were embittered, and Custer was sent in to force the Indians back onto their reservations. In the Battle of the Little Big Horn on June 25, 1876, all 265 soldiers in 5 companies of the U.S. 7th Cavalry died. Meanwhile, what was going on in the Lake Superior region? Native copper had been discovered on the Keweenaw Peninsula by Douglas Houghton in 1840, and mining began soon after. Actually, the copper was “rediscovered” by Houghton, as “Old Copper Complex Indians” had been mining it from numerous pits on the peninsula and on Isle Royale for the previous 6000 years. Iron ore was discovered in the UP near Negaunee in 1844 by William Burt and his party, who were running a line using a sun compass and noticed a large deviation in the magnetic azimuth of a regular compass. Chief Manjekijik of the Chippewas led explorers to another outcrop of iron-formation the next year. Major ore shipments began in 1855. Ore in the Menominee district was discovered in 1845 and began production in 1877. Iron ore on the Gogebic was first noted in 1848 by A. Randall and Charles Whittlesey, but production didn’t begin until 1884. 122 �Gold was mined from the Ropes Mine in the Marquette area in the early 1880s, and sporadically thereafter. In 1868, silver was discovered at Silver Islet near Thunder Bay. In 1865, there was a gold rush to Lake Vermilion. Had it been based on a valid discovery of gold, or was the rush a ruse to generate business for suppliers? (There are no authentic reports of gold having been found there.) The gold activity in the Lake Vermilion area led to the 1865 discovery of iron ore by George Stuntz, a surveyor who had a trading post on Minnesota Point in Duluth... The first ore was shipped in 1882 from Soudan, and in a few years 5 mines were producing at Ely. Many men had crossed the Giants Range (the “Mis-sa-be” hills of the Indians) on their trek to Lake Vermilion (Ona-ma-sa-ga-i-gan, or the “lake of the beautiful sunset”) along the 84mileVermilion Trail surveyed and built by Stuntz Some, including Lewis Merritt of Duluth, noted the presence of magnetic iron-formation in the eastern Mesabi. However, it was not until 1890 when the Merritt brothers (sons of Lewis Merritt), with their concept of ore “basins” within the iron-formation, discovered the first high-grade soft ore at Mountain Iron and a year later at Biwabik. (These discoveries ended the fledging developments on the Gunflint Range.) Within a few years, mines reached 30 miles westward, discovered by Frank Hibbing, Archibald Chisholm, Erwin Eveleth, and others. Within 20 years, most of the Mesabi ore had been located. In 1904, 58 Mesabi mines produced more ore than the 78 mines of Michigan and the Vermilion district. Meanwhile, the search for gold continued wherever bedrock was exposed. In 1894, it was discovered in a quartz vein on Little American Island on Rainy Lake, and a mine was developed to a depth of 212 ft. This was Minnesota’s only legitimate gold mine, having produced $5600 worth of gold. Rainy Lake City grew quickly, and died almost as quickly. There were not many steam gravels on bedrock to be panned in this glaciated country, although to the north in Ontario, prospecting continued unabated. The news of the discovery of the Klondike gold in the Yukon Territory by George Carmack in 1896 reached the West Coast, and even Minnesota, in 1897. Several Minnesota prospectors joined 100,000 others who were heading north to Dawson. Perhaps 40,000 made it, maybe 20,000 prospected, and about 4,000 shared in the total gold find of $10,000,000 (an average of $2,500 each). The good stream beds were rapidly staked. In 1898 near Nome, Alaska, on the Bering Sea, 1,000 miles west of the Klondike, gold was discovered by the “Three Lucky Swedes” (Jafet Lindeberg, Eric Lindblom, and John Brynteson, all greenhorns) who became millionaires. Then gold was found in the beach sands of Nome in 1900, and the beaches became “the poor man’s paradise”. More than 23,000 people sailed there from Seattle, Portland, and San Francisco to get rich in one way or another. At the height of beach mining, 2,000 men, women and children were at work, and produced about $2,000,000 worth of gold, an average of $1,000 each. This was the last great placer gold stampede in North America, and really lasted for only the 3 months of the summer of 1900. 123 �Selected References Boyum, Burton H., 1977, The Saga of Iron Mining in Michigan’s Upper Peninsula: John M. Longyear Research Library, Marquette, Michigan, 48 p. Campbell, L.T., 1992, Skagway—a Legacy of Gold: Alaska Geographic, v. 19, # 1, 96 p. Clark, Henry W., 1930, History of Alaska: the Macmillan Company, N.Y., 208 p. Cole, Terrance, 1984, Nome: “City of the Golden Beaches”: Alaska Geographic, v. 11, #1, 183. Davis, E.W., 1964, Pioneering With Taconite: Minnesota Historical Society, 246 p. DeKruif, Paul, 1929, Seven Iron Men: Harcourt, Brace and Company, N.Y., 241 p. Emanuel, Richard P., 1997, The Golden Gamble: Alaska Geographic, v. 24, #2, 96 p. Green, William, 1963, The Bonanza West: University of Oklahoma Press, 430 p. McCourt, Edward, 1969, The Yukon and Northwest Territories: St. Martin Press, N.Y., 236 p. Morgan, Murray and Hegg, E.A., 1967, one Man’s Gold Rush—A Klondike Album: University of Washington Press, 213 p. Oliver Iron mining Company, 1912, Iron Industry of Minnesota: 48 p. Van Barnevald, Charles E., 1912, Iron mining in Minnesota: University of Minnesota, School of Mines Experiment Station, Bulletin No. 1, 214 p. Walker, David A., 1974, Lake Vermilion Gold Rush: Minnesota History, Minnesota Historical Society, Summer 1974, p. 43-54. Walker, David A., 1979, Iron Frontier—The Discovery and Early Development of Minnesota’s Three Ranges: Minnesota Historical Society Press, 315 p. Welbanks, Wallace P. and Woodbridge, Dwight E., 1905, Minnesota Iron Mines: Welbanks, Crandall and Co., Duluth, Minnesota, 46 p. 124 �DEPOSITION OF PALEOPROTEROZOIC SILICICLASTICS AND IRONFORMATION IN A TIDALLY INFLUENCED SHELF ENVIRONMENT, ANIMIKIE BASIN, LAKE SUPERIOR REGION OJAKANGAS, Richard W., University of Minnesota Duluth, Duluth, MN 55812, rojakang@d.umn.edu; OJAKANGAS, Gregory W., Drury University, Springfield, MO 65802; gojakang@drury.edu The Paleoproterozoic Animikie Basin is interpreted as a northward-migrating foreland basin situated north of the Penokean orogen. Basal siliciclastic units are the Pokegama Formation on the Mesabi Range in Minnesota, the Palms Formation on the Gogebic Range in Michigan-Wisconsin, and the Kakabeka Quartzite on the Gunflint Range in Ontario and adjacent Minnesota. The overlying iron-formations are the Biwabik, Ironwood, and Gunflint, respectively. The iron-formations of these three ranges are in turn overlain by the Virginia Formation, the Tyler and Copps Formations, and the Rove Formation, respectively. We interpret the siliciclastics and the iron-formations to have been deposited on the northern edge (i.e., the peripheral bulge and foreland) of the basin about 1900 Ma. Those in Michigan-Wisconsin were likely continuous with those farther north in Minnesota and Ontario prior to their separation by the development of the Mesoproterozoic Midcontinent Rift System at 1100 Ma. However, the units are likely diachronous, with those in Michigan and Wisconsin somewhat older than those in Minnesota and Ontario. They are interpreted to have been deposited on a shelf near a peneplaned surface on Archean rocks. The siliciclastics were deposited near shore and the iron-formations were deposited farther seaward. As the sea transgressed northward, the iron-formations were deposited upon the siliciclastics. Walther’s Law applies, with the vertical facies indicating the lateral facies. The siliciclastic formations consist of lower argillaceous members, middle members of argillite, siltstone, and sandstone, and upper members of mature sandstone (all gradational), interpreted to have been deposited, respectively, in upper tidal, middle tidal, and lower tidal (subtidal?) environments in a transgressing sea. The well-exposed Palms Formation exhibits abundant tidal evidence including bimodal-bipolar paleocurrent plots (N=250) for the formation as a whole and also for specific localities, tidal bedding (lenticular, wavy, and flaser), and minor flat-topped ripple marks and mudcracks. Also present in the middle member are thin sandtextured beds composed of iron silicates that were apparently transported shoreward into the siliciclastic zone. The Pokegama Formation is poorly exposed, but tidal evidence can be interpreted from limited exposures and a few drill cores. Sequences of thicker and thinner laminae in siltstone beds of the lower member are interpreted as evidence of the diurnal inequality that is an alternation in the heights of successive high tides in a twice-daily tidal environment. The diurnal inequality occurs when the moon is above or below Earth’s equatorial plane, because under these conditions any non-equatorial location will pass through different parts of the tidal deformation ellipsoid during each successive high tide. Ideally the diurnal equality disappears at the equator, and therefore our data are suggestive of a non-equatorial depositional location. Poorly exposed packets of progressively thinner and progressively thicker laminae may indicate neap and spring tidal cycles. These investigations are continuing, with a search for longer sequences of laminae in drill cores. 125 �The iron-formations have thick-bedded and granular (‘sandy”) members and thin-bedded and fine-grained (“muddy”) members. The former make up the lower and upper “cherty” members and the latter comprise the lower and upper “slaty” members. The granules are composed of iron oxides, iron silicates, iron carbonates, and chert, whereas the fine-grained units are made up largely of iron silicates and iron carbonates. It has commonly been thought that fine-grained precipitates of silica, iron carbonates and/or iron silicates formed on the shelf edge where upwelling waters supplied the iron and silica to a location below wave-base. Reworking of these fine-grained precipitates by tidal and/or storm currrents resulted in the formation of the granules. The granules were then transported into higher energy locations shoreward of the deeper shelf. The Ironwood Iron Formation is poorly exposed, whereas the Biwabik and Gunflint are well exposed. The Biwabik is exceptionally well exposed in taconite pits. In the Minorca Mine just northeast of Virginia, a paleocurrent plot of 102 cross-beds, including rare herringbone cross-beds, is strongly unimodal to the NNE, perpendicular to the paleogeographically determined shoreline, but with 10 % of the readings in the opposite sense. Therefore, flood tides were dominant. In some pits, such as at Minntac, channels of granular iron-formation are cut into the fine-grained and thinly bedded iron-formation. These are as wide as 1 km and 25 m deep, are oriented perpendicular to the paleogeographically determined shoreline, and are interpreted to be tidal channels in which granular sediment was transported seaward into the realm of fine-grained precipitates. A shallow water environment for the deposition of the granular members is supported by the rounded nature of the grains, the cross-bedding, and two major stromatolite horizons. Stromatolite columns in a bed that has since been mined away were all inclined at 30 degrees to the vertical, suggestive of an environment of deposition at about 30 degrees latitude. The vertical sequence of members—lower cherty, lower slaty, upper cherty, and upper slaty—is due to transgression, regression, and transgression. Selected References Morey, G.B., 2003, Paleoproterozoic Animikie Group, related rocks and associated iron-ore deposits in the Virginia Horn: in Jirsa, M.A. and Morey, G.B., eds., Contributions to the geology of the Virginia Horn Area, St. Louis County, Minnesota, Minnesota Geological Survey, Report of Investigations 53, p. 74-102. Ojakangas, R.W., 1983, Tidal deposits in the early Proterozoic basin of the Lake Superior region—The Palms and the Pokegama Formations: Evidence for subtidal-shelf deposition of Superior-type banded iron-formation: in Medaris, L.G., ed. Early Proterozoic geology of the Great Lakes region: Geological Society of America Memoir 160, p. 49-66. 126 �THREE-DIMENSIONAL GEOMETRY AND STRAIN OF THE BARABOO SYNCLINE: KINEMATIC IMPLICATIONS ORMAND, Carol J., Department of Geology, Wittenberg University, Springfield OH 45501, cormand@wittenberg.edu CZECK, Dyanna M., Department of Geosciences, University of Wisconsin - Milwaukee, P.O. Box 413, Milwaukee, WI 53201 INTRODUCTION Paleoproterozoic sedimentary rocks of the “Baraboo interval” were deposited on a stable cratonic margin with subdued topography, in a warm, humid climate, as evidenced by their physical and chemical maturity (e.g. Dott, 1983; Medaris et al., 2003). The southern deposits, including the Baraboo Quartzite and associated rocks, subsequently underwent both low-grade thermal metamorphism and simultaneous deformation during the Mazatzal orogeny, ~1650-1630 Ma (Holm et al., 1998; Romano et al., 2000). During this collisional event, the flat-lying marginal sediments were crumpled into tight, asymmetric, southward-verging folds. The foreland fold-and-thrust belt of this collision is preserved in isolated outcrops in southern Wisconsin including the Baraboo hills (LaBerge and Klasner, 1986). Approximately 40 kilometers long by 15 kilometers wide, the geometry of the Baraboo Syncline is strikingly three-dimensional. The fold axis trends approximately N80E. The northern limb of the fold is subvertical to slightly overturned; the southern limb dips around 35 degrees northward; the eastern termination plunges approximately 35 degrees westward; and the western termination plunges approximately 25 degrees eastward. DEFORMATION FEATURES Strain on the limbs of the Baraboo Syncline is as three-dimensional as the fold itself. Both limbs of the fold have axial planar phyllitic cleavage, refracted into quartzitic strata. Within the southern limb, however, where quartzite beds are sandwiched within phyllitic layers, threedimensional pinch-and-swell structures (“chocolate tablet boudinage”) show extension parallel to layering, both along strike and down dip. Strain data from quartz grain shapes also indicate three-dimensional strain, with extension either layer-parallel or layer-normal (McKiernan, 2002; Craddock, pers. comm.). In addition, slickensides on the southern limb of the fold indicate one paleostress direction, while slickensides on the northern limb show multiple paleostress solutions (Kirschner et al., 1989). KINEMATIC MODEL Both the fold geometry and the extension within the gently dipping limb of the syncline are consistent with formation in a top-to-the-south simple shear environment (Cambray, 1987). In such an environment, non-cylindrical fold trains would verge southward. In this model, the longer, north-dipping fold limbs are favorably oriented for localized layer-parallel extension, while the shorter, steeply dipping limbs rotate and shorten during deformation. The location of boudinage exclusively on the south limb is consistent with this model. The multiple paleostress directions on the north limb, inferred from slickensides (Kirschner et al., 1989), are also consistent with the rotation of the north limb explicit in the model. Therefore, this simple shear 127 �model is consistent with the majority of the field data. However, it is a two-dimensional model that does not account for the strongly three-dimensional fold geometry. To account for the threedimensional shape of the syncline, we invoke a component of non-plane strain. We envision a variation in degree of shearing along strike, resulting in extension along the fold axis. This model therefore could explain both the strong change in plunge along trend and the threedimensional boudinage within the southern limb of the Baraboo Syncline. We are analyzing microstructural data on both limbs of the fold to further evaluate this kinematic model. REFERENCES Cambray, F. W., 1987. The Baraboo syncline; the shape and refolding explained as a result of superposition of simple shear on a pre-existing fold. GSA Abstracts with Programs, 192. Dott, R. H., Jr., 1983. The Proterozoic red quartzite enigma in the north-central United States: resolved by plate collision? GSA Memoir 160, 129-141. Holm, D., Schneider, D., Coath, C. D., 1998. Age and deformation of Early Proterozoic quartzites in the southern Lake Superior region: implications for extent of foreland deformation during final assembly of Laurentia. Geology 26, 907–910. Kirschner, D., Pershing, J., Teyssier, C., 1989. Nature and kinematics of fault surfaces in the Baraboo Syncline (WI). GSA Abstracts with Programs, 17. LaBerge, G. L., Klasner, J. S.., 1986, Evidence for a major south-directed early Proterozoic thrust sheet in south central Wisconsin. GSA Abstracts with Programs, 664. McKiernan, A., 2002. Stress-strain analysis in Precambrian quartzites from Wisconsin: evidence for eastward continuation of the ca. 1650 Ma Mazatzal and Central Plains orogenies. Honors Paper, Macalester College, MN. Medaris, L. G. , Jr., Singer, B. S. , Dott, R. H. , Jr., Naymark, A., Johnson, C. M., Schott, R. C. , 2003. Late Paleoproterozoic Climate, Tectonics, and Metamorphism in the Southern Lake Superior Region and Proto–North America: Evidence from Baraboo Interval Quartzites. Journal of Geology 111, 243–257. Romano, D., Holm, D. K., Foland, K. A., 2000. Determining the extent and nature of Mazatzalrelated overprinting of the Penokean orogenic belt in the southern Lake Superior region, north-central USA. Precambrian Research 104, 25–46. 128 �DULUTH COMPLEX BULK SAMPLES PATELKE, Richard, and SEVERSON, Mark, Natural Resource Research Institute, University of Minnesota Duluth, Duluth Minnesota 55811, rpatelke@nrri.umn.edu, mseverso@nrri.umn.edu Since copper-nickel exploration began in the Duluth Complex with the first drill hole in 1951, there have been numerous bulk samples taken for metallurgical testing. Our current project is an attempt to develop a narrative history of this work. The project began for two reasons: 1) there is a perception that these bulk samples have usually returned metal grades lower than was expected by the pre-excavation testing; and 2) there was no consolidated listing for where data about these projects might be found. The issue of sample head grade in bulk samples being lower than expected is real, but is not documented well enough to discern an overall cause or to propose a solution beyond more rigorous outcrop stripping, mapping, and drilling before choosing a test site. Most sites have been located based on a single drill hole and as far as we can tell no sites have been rejected after a location has been chosen or once excavation began. There is probably a distinction between a geologist’s definition of a successful test and a metallurgist’s definition. A geologist’s definition of a successful bulk sample is one where the overall grade and mineralogy is what was predicted from assaying, drilling, or outcrop study (a “scientific success”). A metallurgist might define a successful bulk sample as one that represents, or is typical of, the bulk composition and the mineralogical ratios of the deposit, and therefore allows reliable conclusions to be drawn from testing certain steps in the beneficiation process (a “practical success”). The primary source of information for this study is company publications and records in files at MDNR and NRRI, little was found at other locations. Samples in the South Kawishiwi intrusion include: a small outcrop (and drill core?) sample by the USBM; two surface bulk samples from the INCO Spruce Road deposit; samples from the shaft and drift of the INCO Maturi deposit; large excavation and incidental exposure at the Dunka Pit iron mine (over 14 million tons of Duluth Complex material on the surface); and a small incidental exposure of Duluth Complex rock and massive-sulfide related to iron-formation stripping in the Peter Mitchell Taconite mine. Bulk samples at the Babbitt (Mesaba) deposit in the Partridge River intrusion include: work on one surface sample pit and multiple samples from the shaft and drift by AMAX; numerous drill core composites by AMAX; two test pits by Arimetco in the 1990s; one test pit by Teck Cominco in 2001; and a 50,000 ton pit planned by Teck Cominco in the near future. The Dunka Road (NorthMet) deposit had three samples at two locations by USS; a 1991 drill core composite from new large diameter holes by Nerco and Fleck; and a large pilot plant sample by PolyMet from reverse-circulation cuttings. The Longnose Oxide-bearing Ultramafic intrusion (OUI) had two samples taken from pits for beneficiation and process testing of the Fe-Ti oxides. Two complete drill holes in the Water Hen OUI were taken for process testing by the USBM in the 1980s. The major bulk samples are listed in Table 1, the references for the table are in the document below. Reference: Patelke R., and Severson M.J., in prep., 2004, A history of copper-nickel and titanium-oxide test pits, bulk samples, and related metallurgical testing in the Keweenawan Duluth Complex, northeastern Minnesota, Natural Resources Research Institute, Technical Report NRRI/TR-XX, ~100 pages. 129 �SOUTH KAWISHIWI INTRUSION Project Responsible party Year(s) Tonnage Comment Spruce Road USBM Source of sample uncertain, test work done uncertain No data found. INCO 3 holes drilled in 1953, report issued in 1955 19661,150 tons 1967 1974 10,000 tons INCO 1968 INCO 1968? Serpentine Reserve Mining Dunka Pit Erie / LTV Before 1989 1975 to 1998? Comment Grades Reference 0.43% Cu, Ni est. at 0.12% Various files in AMAX archive at MDNR INCO Maturi Grades Lab / bench tests on composite from 3 drill cores and / or outcrop samples? Reported head grade of 0.38% Cu, 0.14% Ni, 0.88% S Reference Grosh et al., 1955, USBM Report 5177 1974 INCO project description on file at MDNR in AMAX archive Pit along south side of Spruce Road, processed by INCO at Sudbury (?) Reported head grade of 0.47% Cu, 0.15% Ni, 1.08% S 1974 INCO project description on file at MDNR in AMAX archive 700 tons (?) Shaft at Maturi, sample sent to INCO lab at Sudbury (?) No data found. 1974 INCO project description on file at MDNR in AMAX archive Drift at Maturi, some drilling done from drift, but little information in NRRI or MDNR files. Assume some material must have been sent for Misc. files at MDNR and NRRI metallurgical tests. Uncertain tonnage An exposure of massive sulfide assumed to be similar or related to the Serpentine deposit is seen in the Peter Mitchell Mine. Exposed during Ruhanen, 2001 for MDNR, Severson South iron-formation stripping. Assayed by MDNR in 2001. Kawishiwi report 14-20 million tons in Stockpiles at Dunka Pit represent Duluth Complex material removed for 0.23% Cu, 0.09% Ni, 2.20% S are the approximate values from Files at MDNR, and Ron Graber at CCI stockpiles? iron ore mine development. exploration drilling. MDNR reports values of 0.29% CuO, 0.10% NiO (uncertain about whether these are oxide or sulfide assays) PARTRIDGE RIVER INTRUSION Project Responsible party Babbitt (Mesaba) AMAX At B1-341 deposit AMAX Dunka Road (NorthMet) deposit Year(s) Tonnage 1978 1976 AMAX 1976 Arimetco at B1-374 1994 Arimetco at B1-411 19951996 Teck Cominco at B1-321 2001 150 tons sent to CMRL, January 1996 5,000 Teck Cominco at B1-321 USS Bulk No 1 Future 50,000 1971(?) unknown tonnage, but small USS Bulk No 2 1971 USS Bulk No 3 1971 Fleck / Nerco 1990 PolyMet Composite 19982000 Longnose Fe-Ti oxide (OUI) 1,150 ton Surface pit in NE corner of deposit. Sample may actually have been taken excavation, 560 tons in South Kawishiwi intrusion, not Partridge River intrusion sent as sample Shaft samples listed as "disseminated." Some of this material used by MDNR for various leaching and ARD tests Drift samples listed as "massive" or "semi-massive." Some of this material used by MDNR for various leaching and ARD tests 200 tons excavated, Surface pit. Sample probably in weakly mineralized pegmatitic zone of sample split to 85 Unit 3 and 115 ton portions American Shield 1984 American Shield 1999 Surface pit. Sample in western part of deposit, location in Unit 1. Reasonably typical material. Various disseminated ore samples. Est. at 0.43% Cu, 0.13% Ni, Various files in AMAX archive at MDNR S unknown Various massive and semi-massive sulfide samples. Various files in AMAX archive at MDNR 0.22% Cu, 0.06% Ni, 0.52% S from blast holes; sorted sample had head grade of 0.30% Cu, 0.08% Ni, 0.63% S 0.61% Cu, 0.12% Ni, 1.02% S from blast holes; CMRL reports MDNR and NRRI files head grade at 0.36% Cu, 0.08% Ni, 0.76% S Surface pit at location drilled by Severson for Arimetco, in Unit 1, near Severson estimate 460 tons at 0.62% Cu center of north edge of deposit. Very typical material. Final sample much larger than 460 tons outlined by Severson. Planned surface pit, at same location as Teck Cominco B1-321. EAW approved by State in 2003 Surface pit near drill hole (26058) with mineralization only in top few feet. Drill hole 26058 grade from 8 to 20 ft. was 0.82% Cu, 0.20% Small pit, found by Zanko and Severson in 1995, no definitive records Ni, 1.21% S; below that hole is not mineralized for hundreds of available. feet; head grade of bulk sample was 0.39% Cu, 0.14% Ni, 0.50% S 300 tons Surface pit near drill hole 26105. Intended to intercept mineralization seen Expected grade based on ddh 26105 was 0.77% Cu, 0.28% in that hole. Material contaminated with hornfels, poor grade. Ni(?), 1.23% S; head grade of sample was 0.40% Cu, 0.13% Ni, 0.97% S 20 tons Surface pit near drill hole 26105. Re-entry of Bulk No 2 site to get material Expected grade based on ddh 26105 was 0.77% Cu, 0.28% not contaminated with hornfels Ni(?), 1.23% S; head grade of sample was 0.58% Cu, 0.22% Ni, 0.98% S 2 large diameter core PolyMet report states they have no records for this work, other than that it No data holes was done in 1991. Two large diameter holes and two smaller twins for submission to state. Nerco holes twin two existing USS drill holes. At least 37 tons Reverse circulation drilling composite from about 55(?) reverse circulation Head grade in 1999 SME/AIME presentation is 0.43% Cu, shipped to testing holes. Data not available on how many or which holes constituted the bulk 0.12% Ni, PolyMet has not published sulfur numbers. laboratory sample. Surface pit, sample sent to CMRL for process testing Surface pit, sample sent to CMRL for process testing Drilled in about 400 ft. of drill USBM samples to test reduction processes on Fe-Ti ore; with goal of Water Hen Fe-Ti Water Hen drill holes SL-27 and SL- 1975 core producing saleable or processable titanium slag product. Work done in Oxide (OUI) 28 1985 Table 1. Major bulk samples of the Duluth Complex. 130 MDNR and NRRI files Study concluded that a high TiO2 product could be made, but that concentration that removes iron is important. High MgO content is not mentioned as a processing issue Communication w/Teck Cominco and NRRI files EAW approved June, 2003 NRRI files NRRI files NRRI files PolyMet January 2000 Prospectus PolyMet press releases and 2001 pre-feasibility study CMRL reports in 1990s on projects, but ore sources for individual projects uncertain CMRL reports in 1990s on projects, but ore sources for individual projects uncertain Nafziger and Elger, 1987, USBM report; drill hole desc. in Ross, 1985 �UNTESTED TARGETS IN THE DULUTH COMPLEX PATELKE, Richard, SEVERSON, Mark, and PETERSON, Dean, Natural Resource Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth Minnesota 55811, rpatelke@nrri.umn.edu, mseverso@nrri.umn.edu, and dpeters1@nrri.umn.edu Introduction There are at least four, presently sub-economic, large, copper-nickel +/-PGE deposits defined along the western and northwestern margins of the Duluth Complex, as well as many smaller prospects (more than ten?). Development has been slowed by metallurgical problems and the perception of permitting difficulty. Exploration for more of the large low-grade deposits would seem unneeded until some of the known ones are developed. We present a few exploration ideas for less studied areas of the Duluth and Beaver Bay Complexes (with profuse apologies to all who may also have mentioned these in the past). We hope to encourage a discussion about smaller, higher-grade, targets. All of the targets listed here point to the need for detailed field mapping and integration of geophysical data. Remember that five hundred million tons of rock forms a cube about 1,750 feet on a side, and that a viable deposit can be relatively small. The Schroeder-Forest Center crustal ridge target Regional aeromagnetics and gravity data indicate that the Duluth Complex occupies two deep “bowls”. The separation between these two deep zones is the Schroeder-Forest Center crustal ridge. In general, mapped surface geology does not carry across this basement ridge zone. The presence of inclusions of Archean supracrustal rocks could also indicate that the base of the Complex is closer to the surface in this area (Boerboom, 1994). The Wilder Lake intrusion strikes parallel to the ridge and dips northeastward, also indicating that there is a division in the Complex. So, if one applies a model based on what we see for the copper-nickel-PGE deposits, i.e., most economic mineralization is close to the basal contact, then determining the depth to the basal contact along the ridge could open up areas for future exploration if the footwall is found to be reasonably shallow. Also, this area would probably be a much different structural regime than we see at the western and northwestern margins of the Duluth Complex. Cloquet Lake layered series This area is grossly similar to the situation along the Schroeder-Forest Center crustal ridge or the western margin. A group of three holes, drilled near the edge of this funnel-shaped intrusion in 1982, went to the base of the intrusion and hit massive sulfide with magnetite. PGE values were low and no further drilling was done, but scattered thin intercepts (3 ft.?) of about 0.5% copper were seen within 250 ft. of the surface. In this case, the footwall was older Keweenawan intrusive rocks. PGE in oxide-rich, pegmatitic, and other rocks The highest average value in the Complex for combined Pt + Pd + Au are in oxide-rich rocks, particularly those at Birch Lake. Pegmatitic zones, massive sulfides, anorthositic rocks, and massive chlorite also return higher than average values. However, these five rock types represent a small percentage of the over 52,000 feet of sample in Severson and Hauck (2003) that record assays for all three metals (troctolitic rocks make up about three fourths of the Pt + Pd + Au assay footage). These five minor rock types are not often mineralized, but when they are, the mineralization is copper-rich. Could there be larger, near-surface zones of these anomalous rock types? So far, localized copper-rich, semi-massive sulfides (some with high PGE values) have been documented at Dunka Road, Dunka Pit, on trend with the Siphon Fault, and at Skibo. Oxide-bearing Ultramafic Intrusions (OUIs) While the Fe-Ti rich OUIs are not particularly rich in sulfides or PGE, and have not yet proved economic as sources of titanium, they are still viable targets for chromium and vanadium oxides. There is less than 1,000 feet of assaying for these oxides available for the Complex (Severson and Hauck, 2003; Patelke 2003). The 131 �OUIs are enigmatic: the alignments of large OUIs in the Western Margin and Partridge River intrusions would indicate a relation to large and as yet undefined faults. Similar fault-related OUI are present south of the Babbitt deposit. However, information from drill core gives no hint to the geometric nature of the root zones of the OUIs. As the OUI formed along fault zones, often late in the crystallization history of the various intrusions, they may mark zones with potentially vigorous hydrothermal activity that could have concentrated PGE. For example, drill hole SL-19A at the Water Hen OUI has a thin platinum and chrome-bearing horizon that has had no known follow up work for PGE, or chromium and vanadium. INCO records show extremely high copper and nickel values, with widely varying ratios, near the Skibo OUI; again with little follow up. Overall, the OUIs may be worthy of attention beyond being a source of titanium. Structural targets Two major scissor-like faults that lie perpendicular to the line of the basal contact near the Babbitt and Dunka Road deposits are relatively unexplored. The Siphon Fault begins in the former LTV pit and extends some distance into the Complex. The Grano Fault starts in the Peter Mitchell taconite pit, passes through the Serpentine deposit, and lies along the border between the Partridge River intrusion and the South Kawishiwi intrusion. The Grano Fault is inferred to be a vent that formed high-grade copper-PGE enriched massive sulfide at the Local Boy ore zone of the Babbitt deposit. Both of these large faults cross mapped W-NW to E-SE trending faults passing though the Dunka Road deposit and to the south of the Babbitt deposit. The intersection of these systems has not been well examined either by drilling or field mapping. These fault intersections could present two types of targets: 1) massive sulfide in locations where space opened during faulting, especially in the footwall (a Sudbury or Local Boy model); and 2) pathways for altering and /or mineralizing fluids to intersect particular geologic horizons (such as in the Birch Lake model or postulated for the top of Unit 1 in the Partridge River intrusion). Voisey’s Bay targets Peterson’s discussion of “feeder zone sulfide mineralization” in Miller et al., (2002), points to the possibility of such a target south of the Spruce Road area (a.k.a. the Highway One Corridor area). There, the potential conduit between the Bald Eagle intrusion and the South Kawishiwi intrusion is confined below a large raft or pillar of earlier formed anorthosite. Mapping of assay copper-nickel grade and ratios indicates that the highest grades in the area may be in a channel or vent area below this raft. References Boerboom, T.J., 1994, Archean crustal xenoliths in a Keweenawan hypabyssal sill, northeastern Minnesota. White was right!: Institute on lake Superior Geology, 40th Annual Meeting, Houghton Mich., Proceedings, v. 40, Program and Abstracts, pt. 1, p. 5-6. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Peterson, D.M., and Wahl, T.E., 2002, Geology and Mineral Potential of the Duluth Complex and related rocks of northeastern Minnesota, Minnesota Geological Survey Report of Investigations 58, 207 p., one CD-ROM. Patelke, R.L., 2003, Exploration drill hole lithology, geologic unit, copper-nickel assay, and location database for the Keweenawan Duluth Complex, northeastern Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Tech. Report., NRRI/TR-2003/21, 97 pages, 1 CD-ROM. Severson, M.J., and Hauck, S.A., 2003, Platinum-group elements (PGEs) and platinum-group minerals (PGMs) in the Duluth Complex: Natural Resources Research Institute, University of Minnesota Duluth, Technical Report, NRRI/TR-2003/37, 296 pages, 1 CD-ROM. 132 �THE PROPOSED NATIONAL UNDERGROUND SCIENCE AND ENGINEERINGLABORATORY AT THE SOUDAN MINE, NORTHEASTERN MINNESOTA: A GEOLOGICAL SITE INVESTIGATION PETERSON, Dean M., Natural Resources Research Institute, Duluth, MN, dpeters1@nrri.umn.edu PATELKE, Richard L., Natural Resources Research Institute, Duluth, MN, patelke@nrri.umn.edu In 2000, the National Science Foundation (NSF) convened a committee chaired by Dr. John Bahcall, of Princeton University, to evaluate the scientific justification for a national facility for deep underground science. The committee's charge was to evaluate the potential physics research that could b completed by the next generation of solar neutrino, double beta decay, proton decay, dark matter, and related background-sensitive experiments. In addition, the committee considered the possible relevance of such a facility to other disciplines, including geoscience, microbiology, materials development and technology, and monitoring nuclear tests. NSF received the recommendations of the Bahcall Report titled “Underground Science” in 2001 (available online at http://www.sns.ias.edu/~jnb/Laboratory/science.pdf). One of the results of the NSF-sponsored workshop on Neutrinos and Subterranean Science in 2002 was the June 2003 publication “EarthLab, A Subterranean Laboratory and Observatory to Study Microbial Life, Fluid Flow, and Rock Deformation”. This publication sets a framework for geological research that could be undertaken in a deep underground setting, and is available online at http://www.earthlab.org. Three unsolicited proposals to fund (each ~ $275 million) the development of a National Underground Science and Engineering Laboratory (NUSEL) were submitted to NSF, and include the University of Washington's Homestake Mine site at Lead, South Dakota (http://mocha.phys.washington. edu/nusel/proposal.html), the University of Minnesota's Soudan Mine site in northeastern Minnesota (http://www.sudan.umn.edu/NUSEL/), and the University of California Irvine's Mt. San Jacinto site near Palm Springs, California (http://www.ps.uci.edu/~SJNUSL/). On May 28, 2003, a NSF site panel report on developing a NUSEL concluded that the Homestake Mine in South Dakota was the most favorable. In addition, the panel considered the Soudan Mine a possible back up site for NUSEL, and that the San Jacinto site is not a viable NUSEL candidate. The evaluation criteria of each of the sites were partitioned into two broad categories: (1) geological suitability; and (2) relative costs. Geological suitability issues for the Soudan Mine included the uncertainty of the geology and rock mass conditions at depth. On June 2, 2003, Barrick Mining Company, the owner of the Homestake Mine, turned off the pumps and began flooding the deep portions of the Homestake mine, which consequently jeopardizes most of the earth science initiatives outlined in EarthLab. On February 6, 2004, the NSF returned without prejudice all of the unsolicited NUSEL proposals, and will soon publish a three-stage request for new NUSEL proposals, which will include: Stage 1 - develop a preliminary plan of research activities requiring deep underground access, to aggregate the plans into science modules, and to define the physical requirements needed for each module; Stage 2 - fund grants for conceptual planning of infrastructure as related to each site; and Stage 3 - fund grants for technical designs for the underground infrastructure, detailed geological characterization and environmental permitting, and development of management plans. Between mid April and mid June of 2003, geologists from the Economic Geology Group of the Natural Resources Research Institute, University of Minnesota Duluth completed detailed geologic mapping around the proposed NUSEL site at the Soudan Mine, and wrote a detailed report describing the results of the study (Peterson and Patelke, 2003). The geological report, geological maps, and GIS data files generated as a result of this work are available online at http://www.nrri.umn.edu/egg/. This poster presents the three plates that accompany the geological site investigation report of the suitability of the Soudan Mine area for hosting a NUSEL. Based on this recent detailed field mapping and interpretation, the geological and structural setting of the Soudan Mine is perfectly suited for hosting a NUSEL. In addition, a Soudan Mine NUSEL contains all the requirements outlined in the EarthLab document. The report by 133 �Peterson and Patelke (2003) outlines the geological suitability of the area for a NUSEL and an integrated EarthLab. The five major themes addressed in this report include: Bedrock Geology - The Neo-Archean bedrock geology of the Soudan Mine area is divided into five major lithostratigraphic units. These units include the: (1) Fivemile Lake sequence, moderate to shallowwater bimodal volcanic rocks; (2) Central Basalt sequence, deep-water tholeiitic basalts; (3) Upper Sequence, Algoma-type iron formation, tuff, and epiclastic rocks; (4) intrusive rocks, felsic porphyries, granodiorite, diorite, gabbro, and lamprophyre; and (5) sheared rocks, distinct curvilinear zones of chloritesericite-ankerite-pyrite schists. Structural Geology - The field area is divided into four main structural domains that include: (1) the Murray shear zone; (2) the Mine Trend shear zone; (3) the Linking Zone; and (4) the Collapsed Hinge Zone. These domains appear to be internally structurally coherent, and are separated from each other either by areas of relatively undeformed rocks or discrete sheared boundaries. NUSEL Site Selection - The geological criteria deemed most important for NUSEL construction include: (1) definition of a competent rock mass for excavation of large caverns at depths of 1,450 m and 2,500 m; (2) minimizing the occurrence of major lithologic contacts that would be encountered during construction of shafts, drifts, and the helical decline; and (3) minimizing the occurrence of major structural features in the area proposed for construction of the helical decline. A large area in the competent pillowed basalts of the Central Basalt sequence appears to meet all of the criteria for construction of the helical decline, and large laboratories could probably be excavated out of a large, highly indurated dioritic sill. Compatibility with EarthLab - One of the main requirements for EarthLab is a very large, instrumented rock volume and access to great depths. At Soudan, the volume of rock that can be reasonably be accessed for EarthLab research in a Soudan Mine NUSEL is approximately 30 km3. The scientific themes of study proposed for EarthLab are: (1) microbial life at depth; (2) the hydrologic cycle; (3) rock fracture and fluid flow; (4) rock-water chemistry; (5) deep seismic studies; and (6) geophysical imaging. The compatibility of a Soudan Mine NUSEL with each of these themes is favorable. Although the conceptual design plans for the Soudan Mine NUSEL requires relatively high-cost new construction at depth, the pristine nature of this geological environment is highly desirable for EarthLab research. In addition, the close proximity of the Soudan Mine NUSEL to the four structural domains minimizes the cost of drilling and drifting into these structural settings for EarthLab research. Outstanding Geological Research Opportunities - The geological and structural setting of the rocks within and adjacent to the proposed Soudan Mine NUSEL provides a unique opportunity for advances in several areas of earth science. Ideas on outstanding research opportunities include: (1) the structural control of lode-gold mineralization; (2) the hydrothermal alteration of subaqueous volcanic rocks and associated massive sulfide copper-zinc mineralization; (3) the origin of massive hematite ore bodies within Algomatype iron-formation; (4) the genetic evolution and temporal development of a Neo-Archean volcanic arc; (5) the Neo-Archean tectonic architecture of the southern Laurentian margin; (6) the Pleistocene hydrogeology of the Superior Craton; and (7) the permeability of crystalline bedrock. References Peterson, D.M and Patelke, R.L., 2003, National Underground Science and Engineering Laboratory (NUSEL); Geological site investigation for the Soudan Mine, northeastern Minnesota: Natural Resources Research Institute, Technical Report NRRI/TR-2003/29, 97 p., 3 plates, 1 cd-rom. 134 �PETROTECTONIC EVOLUTION OF PALEOPROTEROZOIC GRANITIC ROCKS ACROSS THE CENTRAL PENOKEAN OROGEN, NORTHERN MI & WI PIERCEY, P., SCHNEIDER, D.A., Department of Geological Sciences, Ohio University, Athens, OH 45701 USA HOLM, D.H., Department of Geology, Kent State University, Kent, OH 44242 USA Recent U-Pb single-crystal zircon geochronology of Paleoproterozoic post-Penokean granitic rocks of northern Michigan and Wisconsin, historically interpreted as an "anorogenic suite," has revealed a distinct age trend: magmatic pulses apparently migrated southward from ca. 1800 to 1750 Ma, after cessation of Penokean orogenesis (Holm et al., 2004; figure1). Yavapai-aged subduction slab rollback has recently been hypothesized to explain this magmatic pattern. In this model, the subducting slab steepens, and while depth of melting remains static, the locus of melting migrates trenchward (figure 2). Granitoid bodies intrude the Archean gneissic basement, Proterozoic metasedimentary marginal sequences, and an accreted juvenile arc (the Wisconsin Magmatic Terrane) across the breadth of the orogen. Nine samples from eight localities (the Radisson granite was sampled twice due to significant differences in mineralogy) were analyzed petrologically and geochemically, using major-, trace-, and rare-earth element analysis, to discriminate the tectonic setting into which emplacement occurred. Petrologic analysis shows a grain size decrease to the east. Major-element classification (K2O vs. SiO2) indicates a calcalkaline to shoshonite trend for all localities sampled, indicating subduction-related genesis. Nevertheless, trace element tectonic discrimination diagrams after Pearce et al. (1984) indicate a correlation to the local lithology rather than tectonic setting. That is, Humboldt and Montello granites, which intrude Archean gneissic basement, are classified as within-plate granite (WPG); Park Falls syenite, intruding Proterozoic metasedimentary sequences, is categorized as a collisional granite (COLG); and the remaining granites (Radisson, Lugerville, Jennings, Amberg, and Chequamegon) that intrude the accreted arc of the Wisconsin Magmatic Terrane are classed as volcanic arc granites (VAG). For this reason, caution must be exercised in using this technique, especially on single localities. A rare-earth element spidergram, normalized to chondritic values, shows the COLG as the most evolved and WPG as the least, highlighting relative continental crust evolution/component. In this study, the geochemical analyses did not illustrate an age or geographical trend as expected, but rather a correlation of source area and/or relative crustal contribution. Therefore, the suite is interpreted as products of subduction-induced melting across a variable source terrane. References: Holm, D.K., Van Schmus, W.R., MacNeill, L.C., Boerboom, T.J., Schweitzer, D. and Schneider, D.A., 2004, U-Pb zircon geochronology of Paleoproterozoic plutons from the northern midcontinent, U.S.A.: evidence for subduction flip and continued convergence after Geon 18 Penokean orogenesis: Geological Society of America Bulletin, in press. Pearce, J.A., Harris, N.B.W., Tindle, A.G., 1984, Trace Element Discrimination Diagrams for the Tectonic Interpretation of Granitic Rocks: Journal of Petrology, 25, 956983. 135 �136 �BLOWING IN THE WIND: THE COPPER HARBOR STROMATOLITES REVISITED PLANAVSKY*, Noah, and BJORNERUD, Marcia, Geology Department, Lawrence University, Appleton WI 54912 The Copper Harbor Conglomerate is the basal unit of the sediment-dominated upper part of the Keweenawan series. This sedimentary sequence was deposited within the central basin of the Mid-Continent Rift System beginning at approximately 1087 Ma. The coarse grain size, rounded clast shapes, low clay content, large trough cross bed sets, channel structures and ripple marks observed in the conglomerate are best explained by a prograding alluvial fan complex in an arid environment with rugged topography (Daniels, 1982). Locally, the conglomerate is matrixsupported, indicating that debris flows also contributed to its formation. The clasts within the conglomerate appear to have been derived exclusively from the underlying volcanic flows of the rift, suggesting that the basin was only a relative low within a topographically elevated region – probably a thermally supported high similar to the modern East African rift. All sedimentary features of the Copper Harbor Conglomerate point to a nonmarine depositional setting. In the upper part of the Copper Harbor Conglomerate, thin, discontinuous clayey layers with dessication cracks are interbedded with the coarse and thick conglomerate strata, suggesting that ephemeral playa lakes existed within the basin. Some of the lacustrine deposits contain finely laminated, hemispherical, calcareous stromatolitic structures. In many cases these structures drape around the upper surfaces of large boulders that are partly enclosed by clayey sediment. The external shape and fine internal layering of the hemispheroids as well as the presence of detrital sand grains on their inclined sides (indicating organic stabilization) all suggest that they are true biogenic stromatolites (Elmore, 1983). That is, the Copper Harbor stromatolites appear to record a terrestrial microbial community that established itself far above sea level in middle Proterozoic time. Most modern microbial lithifying mats are complex symbiotic communities of more than one genus (Stal, 2000), although there are modern freshwater stromatolites formed exclusively by cyanobacterial activity (Eggleston and Dean, 1976). The current paradigm is that the mucilaginous sheath excreted by cyanobacteria is essential for formation and lithification of the stromatolite structure. Whether the Copper Harbor stromatolites represent multi-genus communities or simple cyanobacterial mats, it is interesting to consider how the microbes might have found their way into a high, dry basin that had previously been the filled with ponded basaltic lavas. Most modern and fossil stromatolites occur in near-shore marine settings, and migrate over time via aqueous fragmentation or microbial mat bifurcation. The Copper Harbor Basin, however, was clearly high and isolated. The nearest continental margin is thought to have lain some 800 km away, beyond the Grenville front (Ojakangas et al., 2001). Paleocurrent directions and the absence of extrabasinal clasts in the conglomerate, furthermore, exclude the possibility of fluvial transport from another terrestrial community. We suggest, therefore, that the basin was colonized through akinete anemochory, or wind transport of dormant reproductive structures. Colonization by this means is consistent with the inferred wind patterns of the time. Localized aeolian dune deposits within the Copper Harbor 137 �conglomerate indicate paleo-wind directions that were orthogonal to fluvial paleocurrent directions, blowing from the southeast (in paleogeographic coordinates), the azimuth of the Grenville coast. This is consistent with the inferred paleolatitude of 20° N, which would have placed the Copper Harbor basin in a paleo-trade wind region (Taylor and Middleton, 1990), with prevailing winds blowing from the Grenville coast toward the Lake Superior region. Long distance aeolian transport of akinete structures is physically plausible. Cyanobacterial akinetes are on the order of 10µm in diameter (Lang and Whitton, 1973), and several recent studies have documented regional and even transoceanic transport of fine sediment (Prospero, 1999), spores (Josefsson, 2002) and pollen grains (Rogers and Levetin, 1998) of similar size. The stromatolites of the Copper Harbor Conglomerate are among the few known records of Proterozoic terrestrial biological communities, and they provide a glimpse of the early stages of colonization of a barren landscape. Aeolian dissemination of microbes and nutrients may have been an important mechanism of dispersal in the Precambrian biosphere. References: Daniels, P. A., Jr., 1982. Proterozoic sedimentary rocks: Oronto Group, Michigan- Wisconsin, in Wold, R.J., and Hinze W.J., eds., Geology and Tectonics of the Lake Superior Basin, GSA Memoir 156, 107-133. Elmore, R.D., 1983. Precambrian non-marine stromatolites in alluvial fan deposits, the Copper Harbor Conglomerate, Upper Michigan. Sedimentology, 30, 829-842. Eggleston, J.R. and Dean, W.E., 1976. Freshwater stromatolitic bioherms in Green Lake, New York. In Walter, M.R., ed., Stromatolites. Developments in Sedimentology, 20, Elsevier, Amsterdam, 479-488. Josefsson, H., 2002. Long Distance Dispersal in Wood Decaying Basidiomycetes (MS Thesis}: University of Umeå University, Umeå, Sweden. Lang, N.J. and Whitton B.A. 1973. Arrangement and structure of thylakoids. In Carr, N.G. and Whitton, B.A., eds., The Biology of Blue Green Algae. University of California Press: Berkeley, 66-79. Ojakangas, R., Morey, G.B., and Green, J.C., 2001. The Mesoproterozoic Mid-continent Rift System, Lake Superior region, USA. Sedimentary Geology, 141-142, 421-442. Prospero, J. 1999. Long range transport of mineral dust in the global atmosphere: Impacts of African dust on the environment of the Southeastern United States. Proceedings of the. National Academy of Sciences, 96, 3396-3403. Rogers, C.A. and Levetin, E.1998. Evidence of long distance transport of mountain cedar pollen into Tulsa, Oklahoma. International Journal of Biometeorology, 42, 65-72. Stal, L.J., 2000 Cyanobacterial mats and stromatolites, in Whitton, B.A., ed., Ecology of Cyanobacteria. Kluwer Academic, Dordrecht, 61-120. Taylor, I. and Middleton, G., 1990. Aeolian sandstones in the Copper Harbor Fm., late Proterozoic, Lake Superior basin, Canadian Journal of Earth Sciences, 27, 1339-47. 138 �A Geochemical Study of the Sills of the Nipigon Basin, Ontario RICHARDSON*, A., and HOLLINGS, P., Department of Geology, Lakehead University, 955 Oliver Rd., Thunder Bay, Ontario, P7B 5E1, Canada; ajrichar@lakeheadu.ca Introduction and Background The Nipigon Sills are a relatively flat-lying Proterozoic tholeiitic diabase sequence associated with the Keweenawan midcontinent rift (MCR) event centered on the Lake Superior region. The MCR was active 5540000N approximately 1100 My ago with published U-Pb zircon ages of 1108.8 +4/-2 Ma, and 1097.6 ±3.7 Ma from the Nipigon Sills and Osler volcanics respectively (Davis and Lake Sutcliffe, 1985). These sills are up to 200 5510000N Nipigon metres in thickness and dominate the geology of the Lake Nipigon basin. The sills currently cover an area of approximately 11 000 km2 representing a minimum volume of 10 000 km3 (Fig. 1; Sutcliffe, 1986). 5480000N Although related to the Logan sills found further south in the Thunder Bay area, they differ in rare earth geochemistry, mineralogy, and paleomagnetic character. First noted by Sir William Logan in a report to the crown in 5450000N 1863, the region was not mapped in detail until the first decade of the twentieth century Legend by Andrew Wilson, whose report in 1910 Nipigon Sill Metavolcanic Drill Hole Collar Scale N included an accurate representation of the Sibley Sediments Granitic Sample 0 20 Km English Bay sills, in addition to Archean geology and Metasediment 1537 +10/-2 Ma (Davis & Sutcliffe, 1985) limited geology of the Sibley Group Figure 1. Regional Geology with sample sediments. Wilson’s report brought to light locations the question of whether the sills represent an extrusive flood basalt sequence or a hypabyssal intrusive unit. The lack of extrusive characteristics (pillows, ropy flow tops, vesicles, etc.) as well as a billion years of erosion makes interpretation based on direct observation difficult. 6 7 10 11 4 15 3 18 1 19 22 24 25 167 149 62 160 119 120 42 124 63 70 89 35 134 90 92 85 31 77 97 69 10 Project This research is funded as part of the Lake Nipigon Regional Geoscience Initiative (LNRGI) in order to further the understanding of the Lake Nipigon region and promote mineral exploration and development through a public/private sector partnership. The aim of this study is to develop a model of sill emplacement using detailed whole rock and isotope geochemical data, as well as petrographic work and mineral chemistry, to develop a formational model for the sills. 139 �Preliminary Results Of the 170 outcrop samples and 530 diamond drill hole (DDH) samples collected in 2003 (Fig. 1), 80 have been analyzed using XRF, ICP-AES, and ICP-MS methods. Preliminary results point to the sills being a remarkably uniform sequence of olivine basalt with a pronounced negative Nb anomaly, 100 LREE enrichment and slightly fractionated HREE (Fig. 2, grey) with La/Smcn and Gd/Ybcn ratios for normal sill averaging 1.74 and 1.40 respectively. A geochemically distinct sill with Gd/Ybcn and La/Smcn 10 ratios of 3.64 and 13.3 respectively was found in the top of drill holes DDH3 and DDH5. This distinct signature was also observed in the basal unit of drill hole SW08-5 1 Th Nb La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Sc of the ultramafic portion of the Seagull intrusion. A Figure 2. REE profiles of three sill types. Type 1 (black), Type 2 third distinct sill, identified in (grey), Type 3 (dashed) two outcrop samples along the southern shore of Lake Nipigon was found to have La/Smcn, and Gd/Ybcn ratios of 6.46 and 1.89 respectively. Though petrographically indistinguishable, this geochemistry indicates that there were at least three magma sources for the sill complex, of which all possess a significant Nb anomaly indicative of crustal contamination References Davis, D.W., Sutcliffe, R.H. (1985) U-Pub ages from the Nipigon plate and northern Lake Superior. Geological Society of America Bulletin, v. 96, p.1572-1579. Heggie, G., Hollings, P. (2003) Lake Nipigon Region Geoscience Initiative. Petrology and Mineral Chemistry of the Seagull Intrusion. In: Summary of Fieldwork and other Activities 2003, Ontario Geological Survey, Open File Report 6120, p. 48-1 to 48-5. Sutcliffe, R.H (1986) The Petrology, Mineral Chemistry and Tectonics of Proterozoic Rift-Related Igneous Rocks at Lake Nipigon, Ontario. Unpublished PhD thesis, University of Western Ontario, Ontario. 253p. Wilson, A (1910) Geology of the Nipigon Basin, Ontario. Canada Department of Mines Geological Survey Branch Memoir No.1. 152p. 140 �THE GEOLOGY OF THE EAGLE NICKEL-COPPER DEPOSIT: MARQUETTE COUNTY, MICHIGAN ROSSELL, D.M., dean.rossell@kennecott.com, Kennecott Exploration Company, 10861 N. Mavinee Dr. #141, Oro Valley, AZ. 85737 COOMBES, S., Kennecott Canada Exploration Inc., #354-200 Granville Street, Vancouver, B.C. V6C 1S4 Kennecott’s discovery of the Eagle nickel-copper deposit in 2002 marked the culmination of more than a decade of exploration work by Kennecott in the Baraga Paleoproterozoic sedimentary basin. The discovery hole, YD02-02 completed in July 2002, intersected 84.2m of massive sulfide mineralization averaging 6.3% Ni and 4.0% Cu. The resource estimate for the Eagle deposit at the end of 2003 was 5 million tonnes at 3.68% Ni, 3.06% Cu and 0.1% Co. The Eagle deposit is hosted in the westernmost of two small peridotite bodies historically referred to as the Yellow Dog Peridotite. The Yellow Dog intrusions, which lack penetrative foliations and truncate Penokean tectonic fabrics in the surrounding meta-sediments, are believed to be Keweenawan in age (Klasner, et. al., 1979). The intrusions are mainly comprised of coarsegrained, variably serpentinized peridotite and feldspathic peridotite. A fine-grained, olivine poor phase is found along the margins of the intrusions and as xenoliths within the peridotite. Possible amygdules in the olivine poor phase(s) suggest a shallow level of intrusion. Three principal types of sulfide mineralization are recognized in the Eagle deposit: disseminated (blebby), semi-massive (matrix) and massive. Although the nickel contents of massive sulfides are relatively uniform throughout the deposit, copper contents vary significantly. Platinum group metals (PGM) and gold values are significantly higher in the copper rich massive sulfides. Copper rich veins and disseminations, with significant PGM and gold, in the surrounding meta-sediments may constitute a fourth type of ore. Massive and semi-massive sulfide ore types in the Eagle deposit are irregularly distributed. The contacts between different ore types are sharp and show little evidence of the gradation that might be expected if gravity driven accumulation of sulfides from an overlying, sulfide saturated, silicate magma was the principle mechanism of ore formation. Sequential emplacement of various mixtures of silicate and sulfide magma and cumulus minerals, derived from a lower stratified magma chamber, may provide a better model. Klasner, J.S., Snider, D.W., Cannon, W.F., and Slack, J.F., 1979. The Yellow Dog Peridotite and a possible buried igneous complex of lower Keweenawan age in the northern peninsula of Michigan. Geologic Survey of Michigan DNR report of investigation 24, 31 pp. 141 �Geologic Reconnaissance of the Spaulding Mine Area, Cook County, Minnesota RUHANEN, Richard W., 1746 Janet Park Dr. Hibbing, MN 55746 The Spaulding “mine” consists of a series of exploration pits, shafts and trenches constructed on a fissure vein at the east end of Spaulding Lake in northeastern Cook County, MN. William P. Spaulding prospected this area during the late 19th century, from about 1875 until the summer of 1897, with the target being silver mineralization. Silver was being mined in Ontario during this time at the Rabbit Mountain and Silver Mountain areas just 20 miles to the northeast in veins of a similar nature, as well as at Silver Islet near the Sibley Peninsula on the shore of Lake Superior. The Spaulding Lake area is remote, accessible only by canoe, and consists of east-striking hills and ridges with predominantly north-facing cliffs. Porphyritic diabase sills cap the hills and ridges while the low lands are occupied by lakes and swamps underlain by Rove formation sediments. The rocks dip 12 – 16 degrees to the south. Spaulding’s work exposed an east-striking vein on the south side of Spaulding Lake at the base of a north-facing cliff of Rove formation capped by a sill. The vein consists of a breccia of angular, porphyritic sill and Rove formation fragments cemented by quartz, calcite and perhaps barite. Vugs lined with drusy quartz crystals are common. Sparse pyrite is the only mineralization seen, no silver or other sulfide minerals have been observed. Dump piles near the pits and shafts consist almost entirely of this breccia vein material. During October 2003, two traverses were made by the author on the north shore of Crystal Lake 1 and 2 miles, respectively, to the west of the exposed vein. Rocks noted are porphyritic sill(s), granophyre, small basaltic dike-like bodies, and hornfelsed to partially melted blocks of Rove sediments. At each traverse location, a north-south fracture forms a 1 to 2 foot scarp facing west. Gabbroic rocks of the Duluth Complex crop out ¼ mile to the south, forming a low ridge on the southwest shore of Crystal Lake. Inclusions of Rove formation are common in the sills and in the Duluth Complex rocks. At the Spaulding “mine” location, the sill – Rove formation contact rises in elevation to the east while curving towards the south. The exploration pits were sunk along depressions thought at the time to be ancient mining features created by copper culture peoples. At the shaft, the last excavation on the vein to the east, the breccia consists entirely of Rove formation. On one dump pile near the eastern extent of the working, granophyre with quartz-filled fractures was found, indicating that granophyre occurs at some depth beneath the sill. The vein disappears under glacial drift to the east of the main shaft. At the southwest end of Spaulding Lake, an east-west striking zone of fault gouge cuts a sill of porphyritic diabase, indicating that the vein structure may continue further west of the Spaulding workings. 142 �OCEAN-FLOOR-TYPE-ALTERATION OF DRILLED MRS VOLCANIC ROCKS IN IOWA SCHMIDT, Susanne Th., Département de Minéralogie, Rue des Maraîchers 13, CH1205 Genève, Switzerland, susanne.schmidt@terre.unige.ch SEIFERT, Karl, Department of Geological & Atmospheric Sciences, Iowa State University, Ames, IA 50011, kseifert@iastate.edu In the Thor Group of the buried Midcontinent Rift System of Iowa well cuttings and cores of basalts and diabases were studied from five sites to determine the alteration pattern within a subsurface zone of 250 x 50 km (Fig. 1). The metamorphic assemblage, metamorphic grade, bulk rock composition and the chemical composition of the metamorphic minerals such as actinolite, pumpellyite, epidote, and chlorite, were determined. Equilibrium phase diagrams were calculated using the DOMINO-Theriak Software. At sites 3 and 5 the metamorphic assemblage is similar and contains the minerals pumpellyiteactinolite-chlorite (Mg-rich)-albite. However, at site 3 this assemblages is restricted to a thin alteration band along a vein and the rock is unaltered away from the vein, whereas at site 5 the rock is almost totally altered and relicts of the primary magmatic minerals, such as clinopyroxene or Ca-rich feldspar, are rarely observed in thin section. At site 4 the assemblage pumpellyitealbite-chlorite is present in a diabase. For site 2 chlorite and epidote were determined. Chlorites are present at all sites showing a wide range of composition (Fig. 2). Chlorite is Mg-rich in less altered units and Fe-rich in strongly altered units. Cross cutting relationships between these minerals imply the later formation of Fe-chlorites. The observed pattern points to greenschistfacies conditions in site 3 and 5 and probably higher temperatures at sites 2 and 1 (epidote-chlorite assemblage). The fact that alteration is focused along veins such as observed at site 3 indicates that infiltration of a fluid at elevated temperatures occurred at some stage of the alteration. For the assemblage amphibole-chlorite-albite-epidote in the pervasively altered diabase of site 5, an equilibrium diagram was calculated using the DOMINO-THERIAK Software (de Capitani. 1994). It restricts the assemblage to a temperature interval between 210 to 260 °C and a pressure of up to 3 kbar. The presence of pumpellyite in another sample of the same site and its upper temperature stability limit of ca. 250 °C at 3 kbar (Potel et al., 2002) are in agreement with the calculated P-T field. References Seifert, K.E. & Anderson, R.E. (1996) Geochemistry of buried Midcontinent Rift Volcanic rocks in Iowa, Data from well samples. Jour. Iowa Acad. Sci. 103, 63-73. De Capitani, C. (1994) Gleichgewichtsphasendiagramme: Theorie und Software: Beihefte zum European Journal of Mineralogy, v. 72. Jahestagung der Deutschen Mineralogischen Gesellschaft, p. 48. 143 �Potel, S., Schmidt, S.Th. & de Capitani, C. (2002) Composition of pumpellyite, epidote and chlorite from New Caledonia – How important are metamorphic grade and whole rock composition? Schweiz. Miner. Petrograph. Mitt. 82, 229-252 Fig. 1 Location of drill sites in the buried MCR in Iowa (after Seifert & Anderson, 1996) Fig. 2 Composition of chlorite in the Thor group 144 �Depth Migration of Seismic Reflection Data: An Example for Lake Superior Studies SCHNEIDER, Robert V., Energy Institute, University of Louisiana at Lafayette, P.O. Box 43612, Lafayette, LA 70504-3612, USA 2-D reflection seismic profiling is a useful geophysical tool for understanding geologic structures in the Earth’s subsurface. For example, it is the primary tool of choice in hydrocarbon exploration on a worldwide basis. A seismic section, under the right circumstances, can provide an accurate picture of geologic formations. The main differences between seismic and geologic sections are: 1) Seismic data are recorded and displayed in time, not depth; 2) Seismic reflections are caused primarily by velocity changes, which may (or may not) be coincident with geologic boundaries. A key process in preparing a seismic data set for interpretation is application of a concept called migration. This step moves seismic amplitudes from where they are recorded (i.e. at the receiver) to where the reflection actually occurs in the geologic section (Gray et al., 2001). Because seismic data are recorded in time, this requires an accurate understanding of the velocities in the subsurface, which carries the seismic wave field. Seismic time processing, however, depends on several assumptions to simplify complexities in seismic wave theory. Of fundamental importance is the presence of a horizontally smoothly varying velocity field. Where this assumption is violated, the resulting seismic profile is degraded below the velocity change. In other words, structures displayed in such sections are misplaced both horizontally and vertically. In extreme cases, even the edges of bodies with significantly different velocities than the surrounding country rock are difficult to image (Larner et al., 1989). Near-surface velocity changes, especially in marine acquisition, are relatively rare examples of this problem. Where they occur, the subsurface image may be degraded even after the application of time migration. An example is found at the Florida Escarpment in the eastern Gulf of Mexico (Figure 1a). Here the steep slope of the ocean bottom, which approaches 45◦ over a 2000m interval, creates a lateral step change in velocity from water to a rock column. To minimize the effects of this problem, careful velocity analysis must be performed in depth (Schneider et al., 2000). The resulting velocity model was used to migrate the seismic data, which were recorded in time, and to output an image in depth (Figure 1b). The depth of the water bottom of Lake Superior varies over short horizontal distances (Figure 2). This similarity to the Florida Escarpment indicates that a reflection profile acquired over portions of the lake may suffer similar effects. Resulting interpretations may therefore be error-prone. Experience suggests that careful velocity modeling in depth followed by depth migration will be required to maximize our understanding of crustal structure in this region. 145 �a b Figure 1. (a) Example of seismic mis-imaging due to time processing across the abrupt Florida Escarpment velocity change. (b) Depth migrated data showing improvement in the image of the water bottom and the subsurface image (data courtesy of TGS-NOPEC). Figure 2. Bathymetric profile showing rapid variation in depth in Lake Superior (Natural Resources Research Institute, 1998). Gray, S. H., Etgen, J., Dellinger, J, and Whitmore, D., 2001, Seismic migration problems and solutions: Geophysics, 66, 1622-1640. Larner, K., Beasley, C. J., and Lynn, W., 1989, In quest of the flank: Geophysics, 54, 701-717. Natural Resources Research Institute, 1998, Lake Superior Bathymetry Map, http://oden.nrri.umn.edu/lsgis/bathy.htm, accessed April 9, 2004. Schneider, R. V., Gordon, M. K., Sempere, J., Willacy, C., Hightower, S., and Scholz, S.F., 2000, Prestack depth imaging in the eastern Gulf of Mexico: The Leading Edge, 19, 1340-1343. 146 �WHATEVER HAPPENED TO THOSE Cu-Ni DEPOSITS? SEVERSON, Mark J., and HAUCK, Steven A., Natural Resources Research Institute, University of Minnesota Duluth Large resources of low-grade copper-nickel sulfide ore that locally contain anomalous Platinum Group Element (PGE) concentrations are well documented by drilling in the basal zones of the Partridge River and South Kawishiwi intrusions. At least nine subeconomic deposits have been delineated in the basal 100 to 300 meters of both intrusions. The mineralization consists predominantly of disseminated sulfides that collectively constitute over 4.4 billion tons of material averaging 0.66% Cu and 0.20% Ni (Listerud and Meineke, 1977). Serious exploration for Cu-Ni deposits at the base of the Duluth Complex (Complex) began about 13 km (8 miles) to the southeast of Ely, MN, in 1948, when strongly mineralized rocks were uncovered in an excavation used to build a forest service road (Spruce Road). Local prospector Fred S. Childers of Ely noted copper stains in the material and he, along with Roger V. Whiteside of Duluth, began searching along the basal contact in the vicinity of the Kawishiwi River. In 1951, they diamond drilled a 57 meter (188 feet) deep hole and intersected mineralized gabbro that averaged 0.36% Cu and 0.13% Ni. In 1952, both Bear Creek Mining Company (BMC) and the International Nickel Company (INCO) began intensive exploration efforts along a 61 km-long zone (38 miles) that coincided with the basal contact. INCO eventually picked up the Childers-Whiteside properties (Spruce Road and Maturi deposits); whereas, BMC concentrated most of their effort near the town of Babbitt (Babbitt and Serpentine deposits). By 1960, these exploration efforts indicated that large tonnages of disseminated Cu-Ni deposits were present along the basal contact. However, the low-grade nature of the deposits and the unavailability of state-owned mineral lands led to suspension of activities. In 1966, state mineral leases were offered by the Minnesota Department of Natural Resources (DNR) and were awarded to successful bidders. Since 1966, over 20 companies have been actively involved in exploration for Cu-Ni and Fe-Ti-V deposits along the basal contact of the Complex and over 1,700 holes totaling over 1.5 million feet of core have been drilled. During the early 1970s, the Spruce Road and Babbitt deposits came the closest to development. Mining plans were submitted, test shafts were sunk (one each at the Maturi and Babbitt deposits), surface bulk samples were collected (3 deposits), and various landuse and water-use permits were requested from State and Federal agencies. Many of these activities drew strong opposition from environmental groups and some state legislators. In 1974, the Environmental Quality Board required that a regional Environmental Impact Statement (EIS) be conducted prior to acceptance of any site-specific EIS mining-related proposals. The DNR discontinued lease sales of State lands (1974-1982) until completion of the regional EIS. However, by the time the regional EIS was submitted in 1979, development of the Cu-Ni deposits was put on hold by the mining industry due to weakened copper and nickel markets, smelter-related problems with cubanite in the copper concentrate, and other financial reasons. Enter the “PGE era.” During the early period of drilling (prior to 1980), all of the exploration companies recognized that the Cu-Ni deposits had some potential for hosting PGEs. Based on very limited sampling, the companies assumed that the typical Cu-Ni ore contained no more that a few hundred parts per billion (ppb) combined platinum and palladium. In 1985, the DNR and Minerals Resource Research Center (MRRC of the U of M) conducted a geochemical evaluation of portions of drill hole Du-15, from the Birch Lake area, and found significant values of 9 parts per million (ppm) combined Pt and Pd (Sabelin and Iwasaki, 1986). A short time later, Morton and Hauck (1987) compiled all of the known PGE data for the Complex and reported the presence of anomalous PGE values, often associated with high Cu values, at several other Cu-Ni deposits. These discoveries sparked renewed interest in the Cu-Ni deposits as potential polymetallic deposits (Miller et al., 2002; and references therein). Additional drill holes were sampled and 147 �analyzed for PGEs throughout the Duluth Complex, and as a result, significant PGEs were found at several more deposits. Some of the PGE-enriched zones were found to be “stratabound” in that they are correlative with certain units of the igneous stratigraphy as determined by Severson and Hauck (1990), for the Partridge River intrusion, and Severson (1994), for the South Kawishiwi intrusion. Still other PGE-enriched zones were found to be related to either localized structural conditions (Local Boy massive sulfide zone of the Babbitt deposit; Severson and Barnes, 1991) and/or combinations of stratigraphy and structure (Birch Lake). For example, four stratabound horizons, each containing generally 1.0 ppm Pd, have been documented at the Dunka Road deposit (Geerts, 1991) and appear to be related to magma mixing. A single stratabound PGE-enriched horizon is present at the Birch Lake PGE prospect and also appears to be related to magma mixing (albeit, the PGE-mineralization is also related to variably digested iron-formation inclusions). However, the PGE-horizon at Birch Lake is quite variable (thickness and PGE contents) and cases can also be made that favor a late hydrothermal origin and redistribution of PGE along a fault zone or an early magmatic origin based on proximity to a feeder zone along the same fault. At present, close proximity to a vent, along with local magma mixing, appears to have been the major factor in controlling the PGE tenor in the above cases (Hauck et al., in prep). Localized modification of the PGE content by a later hydrothermal event, while not ruled out, appears to have been of lesser importance. Enter the “hydromet era.” In the mid to late 1990s, the potential of developing the Cu-Ni deposits using hydrometallurgical techniques has once again sparked renewed interest in the Duluth Complex. PolyMet Mining Corporation has acquired the Dunka Road deposit (NorthMet deposit) and plans to use its patented PlatSol technique to recover Cu, Ni, Co, and PGE. Teck Cominco has leased the Babbitt deposit (Mesaba deposit) and plans to use its patented CESL (Cominco Engineering Services Laboratory) process to recover the same metals. If it can be proven that these processes are feasible and economical, the next phase (the “permitting era”) in developing the low-grade Cu-Ni deposits of Minnesota could begin in the near future. The “permitting era” is anticipated to span at least a 2.5-3.0-year interval wherein an Environmental Assessment Worksheet (EAW), EIS, and applications for eight mining-related permits would be submitted. References: Geerts, S.G., 1991, Geology, stratigraphy, and mineralization of the Dunka Road Cu-Ni prospect, northeastern Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Technical Report NRRI/TR-91-14, 63 p. Listerud, W.H., and Meineke, D.G., 1977, Mineral resources of a portion of the Duluth Complex and adjacent rocks in St. Louis and Lake Counties, northeastern Minnesota: Minnesota Department of Natural Resources, Hibbing, MN, Division of Minerals Report 93, 74 p. Morton, P., and Hauck, S.A., 1987, PGE, Au, and Ag contents of Cu-Ni sulfides found at the base of the Duluth Complex, northeastern Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Technical Report NRRI/GMIN-TR-87-04, 81 p. Miller, J.M., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., and Peterson, D.M., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, 207 p. Sabelin, T., and Iwasaki, I., 1986, Evaluation of platinum group metal occurrence in Duval 15 drill core from the Duluth Complex: Internal report, Minerals Resource Research Center, University of Minnesota, Minneapolis, MN, 23 p. 148 �Hydrogen Stable Isotopic Evidence for Hydrothermal Alteration and PGE Concentration Involving Meteoric Water in the Birch Lake Area, Duluth Complex, MN SHAFER, Paula L., and RIPLEY, Edward M., Department of Geological Sciences, Indiana University, Bloomington, IN 47405 The Birch Lake prospect, located along the western margin of the 1.1 Ga Duluth Complex, contains local concentrations of platinum group elements (PGEs) of up to 8 ppm. Footwall rocks in the area are the Early Proterozoic Biwabik Iron Formation and the Archean Giant's Range Batholith. Petrographic analyses indicate minor to extensive late stage alteration of the troctolitic sequence of the Duluth Complex in the Birch Lake area. Olivine has been converted to serpentine, plagioclase has been locally replaced by chlorite, albite and sericite, and pyroxene has been partially converted to a mixture of chlorite and amphibole. Previous studies, both mineralogic and isotopic, (Sabelin and Iwasaki, 1986, Sabelin, 1987, Marma, et al., 2002, Shafer and Ripley, 2002, and Shafer, et al., 2003) have noted that: 1) some, but not all, PGE mineralization is associated with Cr enrichment, 2) oxygen isotopic studies are not supportive of Biwabik Iron Formation assimilation as a major control on either Cr or PGE enrichment, and 3) Re-Os isotopic studies clearly indicate extensive involvement of crustally derived Os in the ore forming process. Due to the intensity of alteration in the Birch Lake area, hydrothermal fluid transport or concentration of PGEs has been proposed as a key factor in the enrichment process. Hydrogen isotopic studies (primarily involving serpentine and sericite) have been undertaken to accompany our previous isotopic measurements, and to aid in the assessment of the alteration process. δD values of serpentine (-87‰ to -96‰) and sericite (-79‰ to -84‰) are very similar to whole rock values found previously (-78‰ to -98‰). Assuming a temperature between ~200º and 350ºC for serpentinization (Allen and Seyfried, 2003), computed δD values for the water in equilibrium with serpentine and sericite range from -75‰ to -84‰. When compared to the average δD of fluid liberated from the Virginia Formation (-53‰), an origin from a metamorphic fluid is unlikely. Considering the suspected low temperature nature of the extensive alteration and low δ18O values of serpentinized oxide melatroctolites (2.35‰ to 3.39‰), a magmatic fluid is also considered unlikely. The δ18O value of water in equilibrium with serpentine at temperatures between 200º and 300ºC is in the range of -3‰ to 3‰. Although mixing between a magmatic fluid and a variously evolved meteoric water can not be ruled out (Fig. 1), we suggest that the geologic and isotopic data are more consistent with hydrothermal alteration involving a fluid of primarily meteoric origin. The water isotopic compositions could be attained via exchange with either igneous rocks of the Complex or with country rocks. The lack of evidence for widespread 18 O exchange in the country rocks suggests that fluid flow was dominantly via fractures, and that isotopic exchange occurred over long path lengths at low time-integrated water/rock ratios. This fluid has locally concentrated PGEs, possibly as a result of differential solubility and removal of previously present sulfide minerals. 149 �References 10 0 -10 -20 -30 -40 -50 -60 -70 -80 -90 W at e rL in e SMOW M et eo ric δ D(o/oo VSM OW ) Allen, Douglas E., and Seyfried Jr., W. E., 2003, Compositional controls on vent fluids from ultramafic hosted hydrothermal systems at mid-ocean ridges: An experimental study at 400ºC, 500 bars, Geochimica et Cosmochimica Acta, vol. 67, no. 8, p. 1531-1542. Marma, John C., Brown, Phil E., Hauck, Steve A., 2002, Magmatic and hydrothermal PGE mineralization of the Birch Lake Cu-Ni-PGE deposit of the South Kawishiwi Intrusion, Duluth Complex, Northeast MN, In: Geological Society of America, 2002 Annual Meeting: Abstracts with Programs, vol. 34, no. 6, p. 112. Sabelin, T., and Iwasaki, I., 1986, Evaluation of platinum group metal occurrence in Duval 15 drill core from the Duluth Complex: Minneapolis, Minerals Resources Research Center, University of Minnesota, Internal Report. Sabelin, T., 1987, Association of platinum deposits with chromium occurrences: An overview with Implications for the Duluth Complex, Skillings Mining Review, p. 4-7. Shafer, Paula L., and Ripley, Edward M., 2002, Stable isotopic studies of PGE mineralization in the Birch Lake area, South Kawishiwi Intrusion, Duluth Complex, MN, In: Geological Society of America, 2002 Annual Meeting: Abstracts with Programs, vol. 34, no. 6, p. 112. Shafer, Paula L., Ripley, Edward M., Li, Chusi, and Hauck, Steve A., 2003, Re-Os isotope Characteristics of PGE mineralization in the Birch Lake Area, South Kawishiwi Intrusion, Duluth Complex, MN, In: Geological Society of America, 2003 Annual Meeting: Abstracts with Programs, vol. 35, no. 6, p. 230. Magmatic Water Mafic igneous rocks H2O in equilibrium with serpentine mixing exchange -15 -10 -5 0 5 18 δ Ο (o/oo VSMOW) Figure 1. Isotopic values of water in equilibrium with serpentine in the Birch lake area, and potential mixing/exchange paths. 150 10 �SILVER THREADS AND GOLDEN NEEDLES: GEOLOGICAL MILESTONES IN NORTHWESTERN ONTARIO SMYK, Mark C., and MAGEE, Angelique, Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 CANADA Northwestern Ontario has been the focus of much geological inquiry and noteworthy discoveries over the past 140 years, many of which put the region in a worldwide limelight and continue to attract research interest. Some of these geological milestones form the basis of this retrospective paper. Reconnaissance mapping and seminal works by pioneers such as Logan, Macfarlane, Bell, McKellar, Lawson and Wilson helped establish the fledgling Geological Survey of Canada in the mid- to late-19th and early 20th centuries and developed the basic framework of local Precambrian geology. The area was part of the first geologic map in Canada, Geology of Canada, in 1869. Earlier mineral discoveries in the lake Superior area, followed by the discovery of silver in the Thunder Bay District in 1868 at Silver Islet prompted not only detailed surveys of mineral deposits, but also the development of rudimentary mineral policy in Canada. Despite its short-lived (1870-1884) history, Silver Islet was host to a number of “firsts”, including the introduction of the Burleigh drill, the Frue Vanner and the first use in Canada of the diamond drill. The Thunder Bay silver district also attracted E.D. Ingall and Ontario Bureau of Mines geologists, including a young N.L. Bowen, who later attained international fame as an igneous petrologist and geochemist. Evidence of the oldest life in North America is found in the ca. 3.0 Ga stromatolites at Steep Rock Lake. The discovery by Lawson in 1911 of the pseudofossil Atikokania there, named and described by Walcott (1912), led to its hailing as the world’s oldest fossil. Despite the ensuing debate and the widespread ascription of Atikokania to inorganic processes, the region once again gained notoriety when Tyler and Barghoorn (1954) discovered the best-preserved and most diverse microfossil assemblage in North America in the Paleoproterozoic Gunflint Formation. At the time of their discovery, they were deemed the world’s oldest fossils, "the oldest structurally preserved organisms that clearly exhibit cellular differentiation and original carbon complexes which have yet been discovered in pre-Cambrian sediments". It was a benchmark, a monumental "first" in global paleontology. Several minerals’ type localities are found in northwestern Ontario. The discovery of the Hemlo gold deposit in the 1980’s near Marathon yielded criddleite (TlAg2Au3Sb10S10), vaughnite (TlHgSb4S7) and hemloite ((As,Sb)2(Ti,V,Fe,Al)12O23OH). Nisbite (NiSb2); and paracostibite (CoSbS) were discovered in drill core at Trout Bay, Mulcahy Township, in the Red Lake area. The secondary minerals romarchite (SnO) and hydroromarchite (Sn2+3O2(OH)2) were discovered on tin pannikins lost from a voyageur’s overturned canoe between 1801 and 1821, and found 4.5 m below the surface of the Winnipeg River at Boundary Falls, near Kenora. The discovery of these tin-bearing minerals is fittingly related to northwestern Ontario’s colourful history. REFERENCES Tyler, S.A. and Barghoorn, E.S. 1954. Occurrence of structurally preserved plants in Precambrian rocks of the Canadian Shield; Science, v.119, no.3096, p.606-608. Walcott, C.D. 1912. Notes on fossils from limestone of Steeprock series, Ontario, Canada; Geological Survey of Canada, Memoir 28, p.16-23. 151 �Late Paleoproterozoic Rhyolite-Quartzite Sequences in the Southwestern U.S.: Speculative Relationship to Rocks of the Baraboo Interval SOUTHWICK, D.L. (Minnesota Geological Survey (retired); davidsouthwick@earthlink.net) At least 14 mappable units of supermature quartz arenite (quartzite) occur within Proterozoic terranes in the southwestern U.S. These typically are hundreds of meters in thickness, contain relatively minor interbeds of conglomerate and aluminous shale, and quasi-conformably overlie thick sections of high-silica rhyolite and/or rhyolitic pyroclastic rocks (Williams, 2003). Sedimentological features suggest that the quartzite units were deposited in fluvial to shallow marine environments, the latter having been influenced locally by tidal currents (Soegaard and Eriksson, 1985). The rhyolitic rocks beneath the quartzites range in age between 1665 and 1710 Ma. Extrusion apparently peaked in two pulses, the earlier one about 1700 Ma and the later one about 1670 Ma. Various geochronological methods indicate that several of the quartzite units are no more than a few million years younger than the subjacent rhyolite. Thus, as a first approximation, the quartzite formations were deposited in the interval 1700-1660 Ma with apparent depositional maxima around 1700-1695 Ma and 1665-1660 Ma. The rhyolite-quartzite sequences rest unconformably on previously deformed and metamorphosed Proterozoic rocks, and have themselves been involved in one to three fabric-forming tectonothermal events (Williams, 1991; Williams and others, 1999). Although the deformational history of the rhyolite-quartzite sequences is complex and is the topic of continuing research and debate, there is general agreement that the older fabrics were imposed during one or more pulses of the ~1650 Ma Mazatzal orogeny. Some of the younger fabrics and preserved metamorphic assemblages developed during a 1400 Ma tectonothermal event that probably was related to "anorogenic" plutonism in the southern Rocky Mountains and the southern Midcontinent. There are striking temporal, sedimentological, geochemical, and petrographic similarities between the 1700-1660 Ma quartzites in the Southwest and the 1712-1630 Ma quartzites of the Baraboo interval in the Upper Midwest (Medaris and others, 2003). However, the nearly ubiquitous stratigraphic association of quartzite with slightly older rhyolite observed in the Southwest has not been documented in the Upper Midwest. Essentially undeformed rhyolite occurs locally below the basal unconformity of the Baraboo Quartzite in Wisconsin. Clasts of undeformed porphyritic rhyolite and devitrified rhyolite tuff occur very locally in the Sioux Quartzite of extreme southwestern Minnesota (Southwick and others, 1986; Southwick, 1994), and 19th-century drilling in northwestern Iowa reportedly intersected rhyolite units (poorly described) interbedded with and beneath basal strata of the Sioux (Beyer, 1893; 1897). These rhyolites are texturally pristine and unmetamorphosed, although the Sioux occurrences are metasomatically altered. They have been interpreted as extrusive equivalents of late Penokean plutonism (ca. 1770 Ma) (Southwick, 1994), but as yet there is no solid geochronological evidence of their actual crystallization age. If future geochronological investigations should demonstrate a significantly post-Penokean age for the rhyolites beneath quartzites of the Baraboo interval (say 1715-1700 Ma), the possibility of a tectonic connection between the Baraboo rocks and the rhyolite-quartzite sequences of the southwest would become much more tenable. Specifically, the documentation of pre-quartzite rhyolitic volcanism would strengthen the speculative hypothesis that the Sioux Quartzite and related units were deposited in fault-bounded depressions (Southwick and others, 1986) that 152 �originally were volcanically active graben-like basins. Such basins may have been a far-field response to crustal stretching associated with Yavapai and transitional Yavapai-Mazatzal tectonism. References cited: Beyer, S.W., 1893, Ancient lava flows in northwestern Iowa: Iowa Geological Survey Annual Report, v. 1, p. 163-169. Beyer, S.W., 1897, The Sioux Quartzite and certain associated rocks: Iowa Geological Survey Reports and Papers, v. 6, p. 69-112. Medaris, L.G., Singer, B.S., Dott, R.H., Jr., Naymark, A., Johnson, C.M., and Schott, R.C., 2003, Late Paleoproterozoic climate, tectonics, and metamorphism in the southern Lake Superior region and proto-North America: Evidence from Baraboo interval quartzites: Journal of Geology, v. 111, p. 243-257. Soegaard, K., and Eriksson, K.A., 1985, Evidence of tide, storm, and wave interaction on a Precambrian siliciclastic shelf: The 1,700 M.Y. Ortega Group, New Mexico: Journal of Sedimentary Petrology, v. 55, p. 672-684. Southwick, D.L., Morey, G.B., and Mossler, J.H., 1986, Fluvial origin of the Lower Proterozoic Sioux Quartzite, southwestern Minnesota: Geological Society of America Bulletin, v. 97, p. 1432-1441. Southwick, D.L., 1994, Assorted geochronologic studies of Precambrian terranes in Minnesota: A potpourri of timely information [with data contributed by Z.E. Peterman, L.W. Snee, and W.R. an Schmus], in Southwick, D.L. ed., Short contributions to the geology of Minnesota, 1994: Minnesota Geological Survey Report of Investigations 43, p. 1-19. Williams, M.L., 1991, Heterogeneous deformation in a ductile fold-thrust belt: The Proterozoic structural history of the Tusas Mountains, New Mexico: Geological Society of America Bulletin, v. 103, p. 171-188. Williams, M.L., 2003 [abs.], Proterozoic rhyolite-quartzite sequences of the Southwest: Syntectonic "cover" and stratigraphic breaks (~1695 and ~1660 Ma) between orogenic pulses: Geological Society of America Abstracts with Programs, v. 35, no.5, p. 42. Williams, M.L., Karlstrom, K.E., Lanzirotti, A., Read, A.S., Bishop, J.L., Lombardi, C.E., Pedrick, J.N., and Wingsted, M.B., 1999, New Mexico middle-crustal cross sections: 1.65-Ga macroscopic geometry, 1.4-Ga thermal structure, and continued problems in understanding crustal evolution: Rocky Mountain Geology, v. 34, p. 53-66. 153 �Close Proximity of Kimberlite Pipes to Diabase Dykes: Structural Controls and Predictiveness in the James Bay Lowlands, Ontario STOTT, G.M., Ontario Geological Survey, Sudbury, ON P3E 6B5, greg.stott@ndm.gov.on.ca In the northernmost region of Ontario, Paleozoic sedimentary rocks (Figure 1a) cover the Archean Superior Province to form the Hudson Bay (HBL) and James Bay (JBL) lowlands. Diamondiferous kimberlite pipes of Early Jurassic (circa 190 Ma) and Mesoproterozoic (circa 1100 Ma) age occur in two separate clusters in the James Bay Lowlands (Figure 1b). Early Jurassic kimberlite pipes, including the Victor diamond deposit of De Beers Canada Exploration Inc., are close to the Winisk Fault, a major Archean dextral transpressive fault. However, the presence of this fault by itself does not adequately account for the linear chain of these kimberlite pipes near the Victor deposit nor the more scattered distribution, farther west, of Mesoproterozoic “Kyle” kimberlite pipes. Not all of the Kyle intrusions lie close to the Winisk Fault. More significantly, there is a spatial association between Proterozoic diabase dykes and these two clusters of kimberlite pipes. A geological interpretation of regional aeromagnetic maps is being completed of the Precambrian basement underlying the Phanerozoic cover in the JBL and HBL. This analysis includes identification of the various Proterozoic diabase dyke swarms in that region. From this there is reason to suspect a correspondence between both of these kimberlite pipe clusters and two diabase dyke swarms. The Early Jurassic pipes mainly lie in a linear northwestward trend close to a Matachewan (ca. 2446 Ma) diabase dyke. This dyke is part of a parallel bundle of northwest-striking dykes across an approximately 20-kilometre width near the Winisk Fault. It is suggested here that deep crustal fractures associated with these dykes, arising from the giant Matachewan magmatic event, were reopened during subsequent episodes of displacement along the Winisk Fault. Over 90 km farther west, the 2121 Ma Marathon swarm forms an approx. 20 kilometre wide bundle of dykes trending northwards in the vicinity of the Kyle kimberlite pipes. Individual pipes lie close to aeromagnetic traces of the dykes. The occurrence of both sets of kimberlite pipes, close to but generally not on the Winisk Fault, implies the possibility that dyke-associated fracture swarms served as secondorder, extensional “splays” near this major fault and provided preferred emplacement pathways for pipe intrusions at least in the middle to upper crust. Similar observations have been made in the Lac de Gras area in the Slave Province where Paleoproterozoic dykes show a moderate to strong spatial association with kimberlite pipes (Wilkinson et al., 2001). In the context of this model, other areas of potential exploration interest include: 1) an area approximately 60-80 km farther east of the Victor deposit, where there is an overlap of another set of Matachewan and Marathon dyke bundles (see Figure 2) transected by the east-trending Winisk Fault, and where no kimberlite pipes have as yet been discovered; 2) 2) a set of aeromagnetic anomalies near a Marathon dyke 120 km east of Fort Hope; and 3) 3) an area that straddles the Manitoba – Ontario border near the Hudson Bay Lowlands where a set of reversely magnetised, north-striking dykes (and fractures?) occurs between the North Kenyon fault and the Winisk fault. This dyke swarm lies “up-ice” from an area of glacially deposited kimberlite indicator minerals found to the southwest (Stone 2001). These three areas might serve as exploration tests of this empirically apparent correlation between pipe intrusions and dyke and fracture swarms, especially in proximity to the Winisk Fault. It is to be expected that the aeromagnetic expression of the pipes might be masked by the presence of these dykes. This is a testable hypothesis and further research requires dating fracture materials in these diabase/fracture swarms and episodic movement along the Winisk Fault. The apparent correspondence between kimberlite pipe emplacements and geophysically traceable bundles of diabase dykes and accompanying fractures provides a potentially important structural control, especially where subjected to reactivated tensile stress near major transcurrent faults. 154 �References: Stone, D. 2001. A study of indicator minerals for kimberlite, base metals and gold: northern Superior Province of Ontario; Ontario Geological Survey, Open File Report 6066, 140p. Wilkinson, L., Kjarsgaard, B.A., LeCheminant, A.N. and Harris, J. 2001. Diabase dyke swarms in the Lac de Gras area, Northwest Territories, and their significance to kimberlite exploration: initial results; Geological Survey of Canada, Current Research 2001-C8, 17p. Figure 1a. Map of Ontario showing the location of the James Bay and Hudson Bay lowlands. The Winisk fault underlies the Phanerozoic cover rocks of the lowlands. Location of Figure 1b is outlined. ! 1a 1b Figure 1b. A map highlighting the distribution of diabase dyke swarms under the Phanerozoic cover rocks of the James Bay Lowlands and the spatial correlation with kimberlite pipes. Early Jurassic kimberlite pipes, the Attawapiskat kimberlites, including the Victor diamond deposit, concentrate in a train parallel to one dyke in a group of northwest-striking Matachewan (2446 Ma) diabase dykes and inferred associated fractures. Mesoproterozoic (1100 Ma) kimberlite pipes, the Kyle kimberlites, are spatially concentrated near individual diabase dykes in a group of north-striking Marathon (2110-2121 Ma) dykes. 155 �ORIGIN OF PRE-WISCONSINAN GLACIAL UNITS IN NORTHERN WISCONSIN BASED ON LITHOLOGIC CHARACTERISTICS SYVERSON, Kent M., syverskm@uwec.edu, Dept. of Geology, University of Wisconsin, Eau Claire, WI 54702 Till and outwash units deposited before the Wisconsinan Glaciation are found in weathered, isolated erosional remnants that are difficult to interpret. The goal of this paper is to obtain input from other geologists with regards to lithologic trends observed in pre-Wisconsinan till and outwash units in northern Wisconsin (Syverson and Johnson, 2001; Syverson, 2004). Pierce Fm. till in western Wisconsin and till of the Medford Mbr. (Marathon Fm.) in central Wisconsin are very dark gray to yellowish brown, silty, and calcareous (Table 1, Fig. 1). Potential carbonate sources include Hudson Bay and the Winnipeg Lowland. Baker and others (1987) proposed that a pre-Illinoian Des Moines Lobe from the Winnipeg Lowland deposited the Pierce and Medford tills during the same glacial event based on lithologic similarities, reversed paleomagnetic signatures, and boulder trains and till fabrics suggesting ice flow from the NW/NNW. Thornburg and others (2000) noted much higher kaolinite concentrations in Pierce till than in Marathon Fm. till (Table 1). Syverson and Johnson (2001) proposed four possible origins for the Pierce and Medford tills and suggested that the Pierce and Medford tills may have been deposited synchronously by different lobes flowing from the northwest. Question: Do different source areas exist along reasonable flow lines that would cause higher kaolinite values in western Wisconsin than in central Wisconsin? Table 1. Data for calcareous, pre-Wisconsinan till units (from Thornburg and others, 2000; Syverson, 2004). Mean values are reported (the carbonate ratio is for weight % coarse silt fraction; K=kaolinite; V=vermiculite; number of samples in parentheses). Till Unit Pierce Fm. Snd:Slt:Cl % 39:37:24 (41) Calc:Dolo 4.2:1 (5) %K 23.3 (13) V/K 0.5 (13) Edgar Mbr., Mar. Fm. 40:40:20 (283) 0.7:1 (105) 6.8 (3) 2.4 (3) Medford Mbr., Mar. Fm. 35:46:19 (26) 0.3:1 (8) 5.7 (2) 3.2 (2) The easterly extent of the Superior Lobe before the Wisconsinan Glaciation is also uncertain. Reddishbrown, sandy till of the River Falls Fm. unconformably overlies the Pierce till in western Wisconsin (Fig. 1). Syverson (2004) has mapped River Falls Fm. outwash in western Chippewa County that is up to 35 m thick, extremely eroded, and enriched in pedogenic clay to depths of 5 m below the land surface. This outwash commonly contains ice-proximal cobbles, boulders, and large Lake Superior agates. Late Wisconsinan outwash of the Chippewa Lobe in adjacent areas is cobble poor, is relatively unweathered, and rarely contains Lake Superior agates. It is proposed that the Superior Lobe flowed into western Chippewa County (farther east than previously thought) before the Wisconsinan Glaciation. Agate-rich outwash of the River Falls Fm. in Chippewa County might have been deposited in the interlobate junction between the Superior and Chippewa Lobes as the ice wasted from its maximum extent. Baker, R.W., Attig, J.W., Mode, W.N., Johnson, M.D., and Clayton, L., 1987, A major advance of the preIllinoian Des Moines Lobe: Geological Society of America Abstracts with Programs, v. 19, no. 4, p. 187. Clayton, L., Attig, J.W., Mickelson, D.M., and Johnson, M.D., 1992 (revised), Glaciation of Wisconsin: Wisconsin Geological and Natural History Survey Educational Series 36, 4 p. Syverson, K.M., and Johnson, M.D., 2001, Origin of the calcareous, pre-Wisconsinan Pierce and Marathon Formations, Wisconsin: Geological Society of America Abstracts with Programs, v. 33, no. 4, p. A18. 156 �Syverson, K.M., and Colgan, P.M., 2004, The Quaternary of Wisconsin: a review of stratigraphy and glaciation history, in Ehlers, J. and Gibbard, P.L., eds., Quaternary Glaciations -- Extent and Chronology, Part II: North America: Amsterdam, Elsevier Publishing, in press. Syverson, K.M., 2004, Pleistocene geology of Chippewa County, Wisconsin: Wisconsin Geological and Natural History Survey Bulletin, accepted pending revisions. Thornburg, K.L., Syverson, K.M., and Hooper, R.L., 2000, Clay mineralogy of till units in western Wisconsin: Geological Society of America Abstracts with Programs, v. 32, no. 7, p. A270. Figure 1. Pleistocene lithostratigraphic units of Wisconsin (from Syverson and Colgan, 2004; modified from Clayton and others, 1992). The Medford Mbr. of the Marathon Fm. (mentioned in the text) is found in the subsurface below the Edgar Mbr. of the Marathon Fm. 157 �DOWSING EMPLOYS CLASSICAL MECHANICS AND STATIC ELECTRICITY TO LOCATE SELF-POTENTIAL ANOMALIES INDUCTIVELY AND RAPIDLY TROW, Jim, Geological Sciences, Michigan State University, emeritus, 540 Lake Avenue #2, Hancock, Michigan 49930 At last year’s 49th ILSG I showed where dowsing in the Michigan Copper Country could identify SP anomalies associated with four new (-) ore targets, seven historic (-) lodes, three historic (-) fissure veins, and four major (+) faults. A few of these examples are here illustrated as geophysical profiles. This year I should like to show you how this procedures works. A French physicist and a Czech physicist independently ascribed the dowsing phenomenon to magnetics, but this cannot be because famous Russian dowsers could not detect the enormous Kursk magnetic anomaly. The two physicists elicited human dowsing response in magnetic fields caused by direct electric currents. The Czech thought that a head shield of low-reluctance metal blocked the magnetic stimulus (but of course the metal was also a low-resistance Faraday cage which blocked electrical fields.) My experiments with permanent magnets have drawn a complete blank; experiments with static electric charge producers have been a big success. Try them! Over the past 31 years of examining ore deposits in six states and one province, one learns that the linear forces of static electric attraction and repulsion interact with mechanical lever arm, torque, moment of inertia, kinetic energy, and power factors to enable many humans to detect SP anomalies with 3/16” – diameter bare low-fuming bronze welding rods of two configurations: for me, 1) the more sensitive two-pronged 6” x 15” U-rods identify gentle SP gradients (dv/dx), and repeated turnings indicate the electrical magnitude of the SP anomaly by iteration of gradients, whereas 2) the less sensitive single-pronged 6”x15” L-rods identify steep or vertical SP gradients over target boundaries. One needs both kinds. At waist level, with elbows against your sides hold the short rod segments vertically in your hands with the long segments horizontal at essentially the same altitude, free to rotate in horizontal planes, immune from gravity. Extend your hands in front of you at body width with your lower arms horizontal, and aim the horizontal long rod segments away from you, parallel to each other. This is the standard state. Point the horizontal segments in the direction of your traverse, which should be at more than 45 degrees mapwise from the strike of an anticipated anomalous mass. As you walk, keep your eyes straight ahead on the traverse. Head orientation is critical, as the twin sensors are in the brain, as demonstrated by Faraday cage masking of the head and by family members’ CT Scans after strokes which obliterated their dowsing skills. For most people, as a negative SP anomaly is approached each rod swings inwards 90 degrees. At each such turning record location, reset the rods to the standard state, and walk on, recording every 90 degree turning and its location. If you do not reset the rods to the standard state the rods cannot indicate additional small dv/dx increments, and you will forfeit valuable information concerning the magnitude of the anomaly. Eventually, you will encounter a point where each rod swings outwardly 90 degrees for a positive dv/dx, as you finally climb out of the SP electrical “valley”. These readings may spread over hundreds of feet with these rods, which through the sum of incremental gradients, give you an idea of total SP voltage (v) magnitude. 158 �To determine the precise location and width of the shallow part of the anomalous mass, rerun the traverse with the 6” x 15” L-shaped rods. The 90 degrees inward turning of each rod indicates the steep to vertical negative dv/dx of the negative SP anomaly. Record data, reset the rods to the standard state and traverse a short distance more until the 90 degree outward turning of each rod at the positive steep to vertical dv/dx, as you start to climb out of the negative SP “valley”. Trench or drill between these – and + turnings. While one’s head contains the twin “sensors”, one’s rods, arms, body, and legs constitute the “meter”. The static charge to attract or repel the positively charged rods’ ends emanates from one’s body and legs, when activated by an involuntary signal from one’s brain. These conclusions follow from Faraday cage masking of one’s waist-to-knee area (which kills the “meter”), or by one’s shuffling side-ways along a traverse while one’s “eyes-right” head aims along the traverse as usual. Mechanics calculations show that the two sensitive 6” x 15” U-rods require a power of .0064 Watts for both to turn 90 degrees, whereas the less sensitive 6” x 15” L-rods require a power of only .0008 Watts for the same 90 degrees rotation. However, these sensitive U-rods each rotate 90 degrees with half of the torque-causing linear electric force per prong that it takes to rotate each less sensitive L-rod 90 degrees. For simplest interpretation start every traverse on electrically neutral ground. It would take a week to illustrate all the caveats and intricacies of the dowsing art, and then no two of us would react in the same way unless we individually standardized the lengths of our rods over known SP anomalies. To avoid physiological differences among humans, eliminate the human components of the electrical circuits: Connect a 13- or 20-cm.-diameter hollow spun aluminum sphere (the sensor) to an insulated rod handle, and connect the sphere by a wire to a rugged battery-powered center-zero electrometer (the meter) which records millivolts, and observe induced total SP millivoltage, v (not dv/dx). However, use station spacing of five feet or less if you want to make a second derivative (curvature, d2v/dx2) analysis of the data. Thusly, an ancient art transfigures into science. Trow, J., 1992, Inductive electrostatic gradiometry (IESG) deciphers Keweenawan copper plumbing system, Soc. Mining, Metall. and Expl. Phoenix Meeting, Preprint 92-32, 22 p. 159 �ORIGIN OF THE RHYOLITES AND GRANOPHYRES OF THE MIDCONTINENT RIFT, NORTHEAST MINNESOTA VERVOORT, J.D., Department of Geology, Washington State University, Pullman, WA, 99164, vervoort@wsu.edu WIRTH, K.R., Geology Department, Macalester College, St. Paul, MN, 55105. Throughout Earth history continental rift volcanism has been characterized by voluminous outpourings of dominantly mafic magma. Rhyolites and other evolved compositions are occasionally erupted in these rifts—sometimes in significant proportions (e.g., Paraná)—but in most cases they are subordinate or absent. This is also generally true in the Midcontinent Rift (MCR) where magmatism is predominantly basaltic with subordinate rhyolite; intermediate compositions are rare. An exception to this is in northeastern Minnesota where the rhyolite and granitic intrusive complexes comprise a significant percentage (10-25%; Green and Fitz, 1993) of the magmatic volume. Here we present U-Pb zircon and Nd isotopic data from the rhyolites and granitic complexes in order to constrain the timing and origin of these evolved magmas in the context of the dynamic evolution of the rift. Most of the rhyolitic volcanism is contained within a few exceptionally large eruptive units, the largest of which may have exceeded 600 km3 in volume (Green and Fitz, 1993). The rhyolites are characterized by having large areal extent, large aspect ratios, and high-T mineral assemblages all indicative of high-temperature, superliquidus eruptions (Green and Fitz, 1993). The granitic complexes are of comparable size to the rhyolites and occur as tabular concordant intrusions (up to 35 km by 1 km) that consist dominantly of granitic compositions and exhibit granophyric textures. Although dominantly silicic, the granophyres have a wide range of compositions (e.g., 47-76 wt. % SiO2) and plot along linear trends on many variation diagrams. The major and trace element compositions of the rhyolites and the silicic portions of the granophyre complexes are broadly similar and it is tempting to think of the granophyres as rhyolite magmas that never reached the surface. The magmatism of the MCR in the northeast limb of the rift is divided into two main magmatic stages (Miller and Vervoort, 1996): an “early stage” (1108-1105 Ma) of reversed magnetic polarity and a “main stage” (1100-1094 Ma) of normal magnetic polarity (Figure 1). Previous U-Pb zircon geochronology on the rhyolites (Davis and Green, 1997) indicate that whereas a few rhyolitic units were erupted during the early magmatic stage, the majority of these units, including the most voluminous rhyolites, were erupted at 11001097 Ma during the later main stage Figure 1. Plot of U-Pb zircon ages of granophyres compared with stages of rift evolution proposed by Miller and of magmatism. The granophyre Vervoort (1996). complexes we examined have U-Pb 160 �zircon ages consistent with this chronology. The stratigraphically lowest complexes (Misquah Hills, Greenwood Lake) have reversed magnetic polarity and ages of 1106±6 Ma and 1106±3 Ma (2 σ SE), respectively. The southern complexes (Eagle Mountain, Pine Mountain) are higher in the stratigraphy, have normal magnetic polarity, and have ages of 1098±4 to 1095±4 Ma. The rhyolites and granophyres have Nd isotopic compositions that are related to magmatic stage (Figure 2). Compared with the mafic volcanic rocks of the rift (epsilon Nd 0 ± 2), the early-stage rhyolites have slightly negative initial epsilon Nd values (~-4) and the younger, more voluminous, main-stage rhyolites have highly negative initial epsilon Nd values (-10 to -15). A similar correlation exists in the granophyres. The early stage granophyres are isotopically homogenous with epsilon Nd values of 0 to –2. In contrast, the main stage granophyres have more highly negative initial epsilon Nd values (-3 to -8). Thus for both the rhyolites and the granophyres, the main stage magmas appear to have been more highly contaminated by older evolved crust than those of the early stage. Based on these isotopic and age constraints we propose the following scenario for the magmatic evolution of the MCR. During the early stage, rapid extension allowed magmas to migrate through the crust with minimal interaction. Melting higher Sm/Nd (mafic) material in the lower crust at this time may have generated small volumes of silicic melts with slightly negative epsilon Nd values. Reduced extension during the latent stage (1105-1100 Ma) led to ponding of magma near the base of the crust and widespread crustal heating. Renewed extension during the main magmatic stage led to increased magma migration through the crust; prolonged crustal heating and magma flux resulted in increased crustal melting. The main stage rhyolites and granophyres represent melts of older, more evolved (lower Sm/Nd) compositions (Animikie sediments, Archean TTGs) perhaps at Figure 2. Plot of epsilon Nd versus SiO2 values for North mid- to upper-crustal levels. Shore Volcanic Group (NSVG) rhyolites, and granophyres. tholeiites, References Cited Davis, D.W., and Green, J.C., 1997, Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution: Can. J. Earth Sci., v. 34, p. 476-488. Green, J.C., and Fitz, T.J., 1993, Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent Rift plateau volcanics, Minnesota: petrographic and field recognition: Journal of Volcanology and Geothermal Res., v. 54, p. 177-196. Miller, J.D., Jr., and Vervoort, J.D., 1996, The latent magmatic stage of the Midcontinent Rift: A period of magmatic underplating and melting of the lower crust: 42nd Annual Meeting of the Institute of Lake Superior Geology, Cable, WI, Proceedings v. 42, p. 33-35. Vervoort, J. D., and Green, J.C., 1997, Origin of evolved magmas in the Midcontinent Rift system, northeast Minnesota: Nd-isotope evidence for melting of Archean crust: Can. J. Earth Sciences, v. 34, p. 521-535. 161 �Taconite Aggregate Potential of Coarse Tailings from the Biwabik Iron Formation, With an Emphasis on Geology, Mineralogy, and Microscopy ZANKO, Lawrence M., ORESKOVICH, Julie A., Economic Geology Group, Center for Applied Research and Technology Development, Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN 55811 NILES, Harlan B.(retired), Coleraine Minerals Research Laboratory, Natural Resources Research Institute, One Gayley Avenue, Box 188,Coleraine MN 55722 A study by Zanko et al. (2003) was undertaken to assemble a body of technical data that could be used to better assess the potential of using a crushed taconite mining byproduct (coarse tailings) for more widespread construction aggregate purposes, primarily in roads and highways. Fundamental to the project was the collection and generation of geological and mineralogical data. The major Biwabik Iron Formation stratigraphic units from which the samples were derived were identified, and their relative contribution to each sample was quantified. An important part of the mineralogical assessment included X-ray diffraction (XRD) analyses and microscopic evaluation of the size and shape (morphological) characteristics of potentially respirable microscopic mineral particles and fragments. Quantitative mineralogy, based on XRD analyses, showed that the dominant mineral in all samples was quartz (55 to 60 percent), followed by much smaller amounts of iron oxides, carbonates, and silicates. Specialized microscopic analyses and testing performed by the RJ Lee Group, Monroeville, PA, on both pulverized (-200 mesh, or 0.075 mm) and as-is sample composites showed that no regulated asbestos minerals or amphibole minerals were present. A very small number of mineral cleavage fragments/mineral fibers were detected, but these were mostly minnesotaite, a silicate mineral common to the Biwabik Iron Formation. Amphibole minerals, absent in coarse tailings samples from the five western Mesabi Range taconite operations, were present in a single eastern Biwabik Iron Formation sample collected in 2003 for Lake County from the Cliffs Northshore operation in Silver Bay, MN. Importantly, the Superfund Method for the Determination of Releasable Asbestos in Soils and Bulk Materials, EPA 540-R-97-028 (1997), as modified by Berman and Kolk (2000) failed to generate any protocol fibers, i.e., fibers longer than 5mm and thinner than 0.5mm, from either the western coarse tailings samples or the single eastern Biwabik Iron Formation sample. The combined findings suggest coarse tailings and other taconite mining byproducts should be treated with the same common sense safety and industrial hygiene approach practiced for all mineralbased materials that have the potential to generate respirable dust. Overall, the project showed that: 1) a significant source of fine aggregate (coarse taconite tailings) is available at each of the taconite operations studied; 2) coarse tailings are suitable for use in road construction applications; 3) no amphibole or asbestos minerals were present in any of the tailings samples evaluated; and 4) low-cost transportation, workable distribution logistics, and market acceptance will be key for expanded use of taconite mining byproducts like coarse tailings in markets beyond northeastern Minnesota. Furthermore, making greater use of materials that have long been considered “waste” byproducts makes environmental sense, because doing so maximizes the utilization of resources that have already been mined and crushed, and it could reduce pressures to expand existing, or develop new, “natural” aggregate sources. Permitting of pit and quarry expansions and new aggregate mines has become more complex and difficult, both 162 �environmentally and socially, given the growing “not in my back yard” (NIMBY) reaction to such projects. Future research initiatives intend to compile and generate baseline technical information on the quality of potential higher-value aggregate products (e.g., Class A-type aggregate, concrete aggregate, railroad ballast) derived from the major stratigraphic units within the Biwabik Iron Formation. Because the Biwabik Iron Formation is not monolithic, the goal will be to map and identify units that are the best potential sources for various construction aggregate applications. Its major cherty and slaty members, while laterally persistent, have variable geological, mineralogical, physical, and chemical characteristics across the Mesabi Range. Reference: Zanko, L.M., Niles, H.B., and Oreskovich, J.A., 2004, Properties and Aggregate Potential of Coarse Taconite Tailings from Five Minnesota Taconite Operations: Minnesota Local Road Research Board, Final Report 2004-06, 294 p., and Natural Resources Research Institute, University of Minnesota Duluth, Technical Report, TR-20003/44/ 163 � https://digitalcollections.lakeheadu.ca/files/original/bf5768fd04d62dab530753479e5e73ac.pdf b74b74f1c621be41df0769e12085bd07 PDF Text Text 50TH ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY 50TH ANNUAL MEETING MAY 4-9, 2004 DULUTH, MINNESOTA HOSTED BY: STEVEN A. HAUCK AND MARK J. SEVERSON Co-Chairs NATURAL RESOURCES RESEARCH INSTITUTE Proceedings Volume 50 Part 2 – Field Trip Guidebook Compiled and edited by Mark J. Severson and Julie Heinz, NRRI Cover Photos: Upper left – Dick Ojakangas, Peter Jongewaard and John Arola at the United Taconite mine, MN (2004); Upper right – Hal James and trip participants near Champion, MI (1964); Bottom – Carl Dutton, Bill Cannon and Matt Walton, Northern MI (1975). �CONTENTS PROCEEDINGS VOLUME 50 PART 2—FIELD TRIPS Trip 1. Volcanic Stratigraphy, Hydrothermal Alteration, and VMS Potential of the Lower Ely Greenstone, Fivemile Lake to Sixmile Lake Area................................................ 1 Trip 2. Geologic Highlights of New Mapping in the Southwestern Sequence of the North Shore Volcanic Group and in the Beaver Bay Complex .......................................... 46 Trip 3. Late Wisconsinan Superior-Lobe Deposits in the Lake Superior Basin northeast of Duluth ...................................................................................................................... 86 Trip 4. Geology of the Eastern Mesabi Iron Range, Northeastern Minnesota ............... 99 Trip 5. Classic Outcrops of Northeastern Minnesota ................................................... 129 Trip 6. Glacial and Postglacial Landscape Evolution in the Glacial Lake Aitkin and Upham Basin, Northern Minnesota .................................................................................... 170 Trip 7. Economic Geology of Archean Gold Occurrences in the Vermilion District, northeast of Soudan, Minnesota............................................................................ 200 Trip 8. Geology and Mineralization of the Western Contact of the Duluth Complex, Partridge River and South Kawishiwi Intrusions, Northeastern Minnesota............ 227 �FIELD TRIP 1 VOLCANIC STRATIGRAPHY, HYDROTHERMAL ALTERATION, AND VMS POTENTIAL OF THE LOWER ELY GREENSTONE, FIVEMILE LAKE TO SIXMILE LAKE AREA George J. Hudak1, John Heine2, Mark Jirsa3 and Dean Peterson2 1 2 University of Wisconsin Oshkosh Natural Resources Research Institute, University of Minnesota – Duluth 3 Minnesota Geological Survey INTRODUCTION The Vermilion district of northeastern Minnesota contains one of the classic granite-greenstone terranes in the United States. This district comprises the south-central part of the Wawa subprovince of the Superior Province of the Canadian Shield, and has been broadly correlated with the Saganagons Assemblage of the Wawa subprovince in northwestern Ontario (Peterson et al., 2001; Peterson and Patelke, 2003). In Canada, the Wawa subprovince hosts numerous lode gold (e.g. the Hemlo and Renabie districts) and volcanic-hosted massive sulfide (VMS) orebodies (e.g. the Winston Lake, Willroy, Big Nama Creek, Willecho, and Geco deposits; Fyon et al., 1992). The Vermilion district is well known for its numerous, previously mined, massive hematitic iron ore deposits. These iron deposits were discovered in the early 1880s, and virtually all subsequent exploration efforts in the region were targeted on similar iron-formation hosted hematite deposits. However, the discovery of world-class ore deposits in Ontario (the Kidd Creek VMS deposit in 1964 and the Hemlo gold deposit in 1980) led to short periods of both base-metal and gold mineral exploration in the Vermilion district. To date, no lode gold and/or VMS orebodies have been discovered in the Vermilion district to date. This field trip will explore recent research efforts to better understand the potential for VMS mineralization within the Lower member of the Ely Greenstone (Lower Ely). Peterson and Patelke (2004, this volume) explore new research regarding the potential for lode gold mineralization within the Newton Belt of the Vermilion district. Since the mid-1990’s, the Minnesota Department of Natural Resources, the Minerals Diversification Plan of the Minnesota Legislature through the Minerals Coordinating Committee, the Permanent University Trust Fund of the University of Minnesota, the Natural Resources Research Institute (University of Minnesota Duluth), and the University of Wisconsin Oshkosh have provided funding to support a series of systematic studies to better understand the VMS potential of the Lower Ely. Early studies (Lawler and Riihilouma (1997); Hudak and Morton, 1999; Peterson and Jirsa, 1999a; Peterson, 2001) showed that numerous base metal anomalies suggestive of localized VMS mineralization exist in the soils, rocks, and lake sediments within this region. Beginning in 2000, research efforts regarding the VMS potential of the Lower Ely have been focused in an approximately two mile wide corridor extending from Skeleton Lake northward to Sixmile Lake (Newkirk et al., 2001a, 2001b; Odette et al., 2001a, 2001b; Hudak et al., 2002a, 2002b; Hocker et al., 2003; Hudak et al., in prep. a). This area was chosen for several reasons: 1) there are historical base metal prospects in the area; 2) lake sediment samples in the area contain the highest copper (Sixmile Lake) and zinc (Fivemile Lake) anomalies within the state of Minnesota (Peterson, 2001); 3) the area contains several untested geophysical anomalies that have orientations roughly parallel to the strike of the strata (Peterson, 2001); 4) the area contains regionally extensive quartz-epidote alteration zones (Peterson and Jirsa, 1999a, 1999b; Peterson, 2001) that may be related to ancient, potentially VMS depositproducing hydrothermal systems; and 5) the bulk of the area comprises state land, and is easily accessible. These research efforts have been multidisciplinary, and have included several months of detailed field Page 1 1 �mapping (1:50 to 1:5000 scale) for stratigraphy and metamorphosed hydrothermal alteration mineral assemblages, petrographic studies (600+ thin sections), whole rock lithogeochemical analyses (~200), xray diffraction studies and electron microprobe investigations. REGIONAL GEOLOGICAL SETTING Supracrustal rocks in the Vermilion district consist of volcanic-dominated stratigraphic sequences of the Wawa subprovince of the Superior Province of the Canadian Shield. Rocks of the Wawa subprovince in northern Minnesota are divided on the basis of stratigraphic and structural setting into: (1) the Soudan belt, to the south, and (2) the Newton belt, to the north (Jirsa et al., 1992; Southwick et al., 1998). The boundary between these contrasting structural panels can be traced geophysically across the width of Minnesota, and was designated informally as the Leech Lake structural discontinuity (Jirsa et al., 1992). In the region west and north of the Soudan Mine, the Leech Lake structural discontinuity occurs along the Mud Creek shear zone (Hudleston et al., 1988), small segments of the Vermilion and Wolf Lake faults (Sims and Southwick, 1985), and the Bear River fault (Jirsa et al., 1992). The Soudan belt contains large, broad folds involving calc-alkalic and tholeiitic volcanic strata overlain by, and locally interdigitated with, turbiditic rocks. In contrast, the Newton belt consists of elongate, northeast-trending, and mostly northward-younging volcanic and volcaniclastic sequences. Volcanic rocks of the Newton belt differ from those of the Soudan belt in containing locally abundant komatiitic flows and peridotitic sills. The two belts are fault-bounded, and the relationship between stratigraphic units within each belt is largely conformable, although faults obscure contacts locally. In its eastern extension, the Soudan belt is continuous with the Saganagons assemblage in Ontario and terminates against the Saganaga pluton and Northern Light Gneiss. The Newton belt extends discontinuously eastward into the Shebandowan District of Ontario to form the Greenwater and Burchell assemblages. Intrusive rocks in both belts vary from gabbroic and felsic porphyries demonstrably related to volcanism, to large plutons emplaced posttectonically. Both districts contain unconformable, Timiskaming-type sequences composed of calc-alkalic volcanic rocks, conglomerates, and finer grained sedimentary rocks. A simplified regional geological map of the Neo-Archean terranes of northeastern Minnesota and adjacent Ontario is presented in Figure 1-1. Figure 1-1. Simplified correlation map of Neoarchean assemblages across the U.S. - Canada border (modified from Peterson et al., 2001). Inset illustrates major subprovinces of the southwestern Superior Province. Page 2 2 �Lithostratigraphic units in the western Vermilion district include: (1) the Lower member, Soudan Iron Formation member, and Upper member (Upper Ely) of the Ely Greenstone, the Lake Vermilion Formation (including the informally named Britt and Gafvert Lake sequences), and the Knife Lake Group of the Soudan belt; (2) the Bass Lake sequence (Peterson and Jirsa, 1999a) and the Newton Lake Formation of the Newton belt; and, (3) syn- to post-tectonic granitoid intrusions of the Giants Range batholith, and a suite of post-tectonic alkalic stocks and plutons (Fig. 1-2). Contacts between the different units are typically conformable, although considerable overlap in time and space is documented between volcanic and sedimentary sequences (Southwick, 1993). Rock types associated with the lithostratigraphic units in the area are presented in Table 1-1. 560001 + + 4+4 —t++'.'+— 4 • • 4 + 4-- 4 + + + RI .4• - + + - 4 •t 4 + + + I 4: -Fl. -±---± "-4-+4, +)+ + * * + + + + '+ + +,--) * + + /+ + + * I 4 * + * + + + + -F' + +2+ *+ — + +-r. +j* + * + + -F' 4 1 * 4 -J-/ * + * -t>. 4 -•+ + ' + ++ -+ + .4 Late Intrusive Rocks * VMS Prospect in Lower Ely) t' + '' Newton Lake Formation + +y+ + t(+ + + 4., + + + -# + + + + + + • * 590000 -P 4 + SBG000 , KnifeLakeGroup LakeVermilion Formation :: Britt Sequence Gafvert Lake Sequence :' Upper Member Ely Greenstone , Vermilion Granitic Complex + Giants Range Batholith 550000 + 570000 yt Duluth complex (1.1 Ga) + 4 + +,I + + + , • + + +. 4 4 ÷ +-_ ,- + +/ + + +* +*++ + 4- +\. 7+ -r *t_[+- *4 * I- +•+ + + IA Soudan Iron Formation A Lower Member Ely Greenstone -- -- C Bass Lake Sequence Figure 1-2. Simplified lithostratigraphic map of the western Vermilion district with the location of VMS prospects in the Lower Member of the Ely Greenstone (from Peterson and Jirsa, 1998a). UTM Nad 83 coordinates in meters. Page 3 3 �Table 1-1. Lithostratigraphic units within the western Vermilion district (from Peterson and Jirsa, 1998a). Intrusive Rocks Late Intrusions Vermilion Granitic Complex Giants Range Batholith Supracrustal Rocks Newton Belt Newton Lake Formation Bass Lake Sequence Soudan Belt Knife Lake Group Lake Vermilion Formation Gafvert Lake Sequence Britt Sequence Upper Ely Greenstone Soudan Iron Formation Lower Ely Greenstone Plutons and stocks of syenite, monzonite, diorite, and lamprophyre Granite, schist, amphibolite, and schist-rich migmatite Granite, granodiorite, monzodiorite, and schist-rich migmatite Tholeiitic and komatiitic basalt flows, intrusions, and clastic strata Tholeiitic basalt lava flows, iron-formation, and felsic porphyries Graywacke, slate, conglomerate, & sheared equivalents Graywacke, slate, dacitic tuff, and minor conglomerate Dacitic to trachyandesitic lava flows, tuffs, and intrusions Tholeiitic basalt lava flows Tholeiitic basalt lava flows and iron-formation Layered cherty iron-formation, epiclastic rocks, and tuff Calc-alkalic & tholeiitic basalt-rhyolite lava flows, tuffs, epiclastic rocks and minor iron-formations The Lower Ely and Soudan Iron Formation (Morey et al., 1970) comprise the strata we will be investigating during this field trip. The Lower Ely consists dominantly of volcanic arc-associated, pillowed and massive basalt and andesite flows of calc-alkalic and tholeiitic composition (Fig. 1-3). Hypabyssal diabase, gabbro, and diorite sills, isolated dacite, rhyodacite, and rhyolite lava flows, domes, tuffs, lapilli tuffs, and tuff-breccias (Fig. 1-4), and local chemical, volcaniclastic and epiclastic sedimentary strata occur throughout the sequence. Pillows in the lowermost sections of the Lower Ely (named the Fivemile Lake sequence by Peterson and Patelke, 2003) typically are irregularly shaped and well-vesiculated, indicating moderate to shallow water depths of formation (Schulz, 1982; Peterson, 2001; Hudak et al., 2002a). Rare massive to bedded scoria deposits interstratified with the pillowed flows also indicate shallow subaqueous, and perhaps locally, subaerial volcanism (Peterson, 2001; Hudak et al., 2002a). Pillowed basaltic rocks in the uppermost parts of the Lower Ely (named the Central Basalt Sequence by Peterson and Patelke, 2003) typically are sparsely-vesiculated and ovoid, and scoria-rich primary and reworked volcaniclastic rocks are not prevalent (Hudak et al., 2002b; Hudak et al., in prep. a; Peterson and Patelke, 2003). These features suggest a general subsidence of the volcanic pile to deepwater conditions. Stratigraphically overlying the volcanic rocks of the Lower Ely is the Soudan Iron Formation, which consists dominantly of laminated Algoma-type iron-formation, with interstratified basalt lava flows and volcaniclastic and epiclastic strata derived from basaltic to dacitic volcanic strata. A gradational contact over several tens to hundreds of meters occurs between the underlying Lower Ely and the overlying Soudan Iron Formation member of the Ely Greenstone (Hudak et al., 2002b; Peterson and Patelke, 2003). In general, the exhalative nature of many of the rocks within the Soudan Iron Formation member represent deep-water chemical deposition throughout a period of quiescence, which began during the latest stages of volcanism associated with the Lower Ely. The stratigraphic thickness of the Soudan Iron Formation member varies from 50 to 3,000 meters, and averages approximately 700 meters. The thickest sections occur in the nose of the Tower-Soudan anticline (Fig. 1-2), and probably represent stratigraphy that is repeated by shearing and thickened by folding. The time period over which the Soudan Iron Formation was deposited is poorly constrained, as geochronological data is very limited within the Ely Greenstone. Nevertheless, the size (thickness and strike length) of the Soudan Iron Formation is much larger than typical Algoma-type iron-formations, and Page 4 4 �its formation probably occurred during a profound break in effusive and pyroclastic volcanism that was accompanied by a long-lived period of hydrothermal activity. The upper contact of the Soudan Iron Formation is more diverse; overlying stratigraphic units along strike include the Upper Ely, the Gafvert Lake Sequence, and the Lake Vermilion Formation. Figure 1-3. Lithogeochemical characteristics of mafic and intermediate volcanic and volcaniclastic rocks of the Lower member of the Ely Greenstone in the vicinities of Fivemile, Needleboy, and Sixmile Lakes (after Hudak et al., 2002a; Hudak et al., in prep. a). Figures 1-4a and 1-4b modified from Winchester and Floyd (1977), Figure 1-4c modified from Wood (1980), Figure 1-4d modified from Winchester and Floyd (1976), Figures 1-4e and 1-4f modified from Barrett and MacLean (1999). Page 5 5 �Figure 1-4. Lithogeochemical characteristics of felsic volcanic and volcaniclastic rocks of the Lower member of the Ely Greenstone in the vicinities of Fivemile, Needleboy, and Sixmile Lakes (after Hudak et al., 2002a; Hudak et al., in prep. a). Figures 1-5a and 1-5b modified from Winchester and Floyd (1977), Figure 1-5c modified from Pearce et al. (1984), Figure 1-5d modified from Harris et al. (1986), Figures 1-5e and 1-5f modified from Piercey et al. (2001) and Barrie et al. (1993). STRUCTURAL GEOLOGY Periods of generally N-S directed compression resulted in three major regional deformation events in the Neoarchean terranes of northern Minnesota. The earliest deformation event (D1) produced broad, locally recumbent folds within the Soudan belt and major fault zones throughout the region. In the Page 6 6 �Newton belt, D1 was accommodated by thrust imbrication of large crustal blocks, resulting in mainly northward stratigraphic facing. Field relationships indicate that uplift, faulting, and the deposition of Timiskaming-type clastic sedimentary sequences in local fault-bounded basins occurred late in D1 deformation (Jirsa, 2000). A large, map-scale structure related to D1 deformation in the western Vermilion district is the Tower-Soudan Anticline, which is a west-plunging anticline within which the axis and plunge changes orientation along strike from nearly vertical in basalts to shallow NE plunging in the western sedimentary rocks. Axial-planar cleavage associated with this early fold typically is lacking, although Bauer (1985), Hooper and Ojakangas (1971), Hudleston (1976), and Jirsa et al. (1992) have described early cleavage (S1) locally. A second deformation event (D2) associated with synchronous regional metamorphism resulted in foliation development and structures having largely dextral asymmetry. D2 is constrained in the Vermilion district to the time period 2674 to 2685 Ma (Boerboom and Zartman, 1993), and between about 2680 and 2685 Ma in the Shebandowan (Corfu and Stott, 1998). Because D2 deformation affected all of the supracrustal rocks in the area and is reasonably constrained by geochronology, the regional foliation (S2) can be used in the field to temporally relate other structural, intrusive, and deformation events. The relationship between S2 fabric and shear structures indicates that most shearing occurred relatively late in the D2 event. Major shearing that produced the Mud Creek and related shear zones (e.g., the Murray shear zone, Fig. 1-1) is attributed to the late stages of D2 dextral transpression. Structures related to the third deformation event (D3) include abundant NE- and NW-trending faults that dissect the stratigraphic assemblages. Named structures related to D3 include the NE-trending Waasa and Camp Rivard faults east of the Soudan Mine area, and the WNW-trending, crustal-scale Vermilion and related faults that form the Wawa-Quetico Subprovince boundary north of the Soudan Mine (Fig. 1-1). VMS PROSPECTS AND EXPLORATION HISTORY Since the mid-1860’s, numerous mineral exploration programs have been conducted in the Vermilion district. Most of these exploration programs focused on identifying minable deposits of massive hematitic iron ores, such as those mined between 1883 and 1962 in the Soudan Iron Formation member at the Soudan Mine. During the 1980s and early 1990s, subeconomic lode gold mineralization was discovered in close proximity to the east-west-trending Murray shear zone, which dissects the Lower Ely, and in close proximity to the Mud Creek shear zone, which separates the Soudan belt from the Newton belt to the north (Fig. 1-1). Four VMS prospects (Fig. 1-2) occur within the Lower Ely, and occur in close proximity upsection from a semiconformable quartz-epidote alteration zone that extends for at least 19km along strike in the north limb of the Tower-Soudan Anticline (Peterson, 2001). These include the Skeleton Lake prospect (drilled by Exxon, 1972), the Eagles Nest prospect (drilled by Newmont, 1988), the Fivemile Lake prospect (drilled by Teck, 1994), and the Purvis Road prospect (drilled by Rendrag, 1999). The stratigraphy, hydrothermal alteration, and mineralization associated with the Skeleton Lake, Eagles Nest, and Fivemile Lake prospects have been recently re-evaluated (Hudak and Morton, 1999; Hudak et al., 2002a; Hudak et al., 2002b). Detailed studies of the Fivemile Lake, Eagles Nest, and Skeleton Lake prospects indicate that copper-zinc sulfide mineralization occurs proximal to relatively narrow (10s of meters wide) disconformable zones of chlorite and/or sericite alteration within felsic volcanic and volcaniclastic rocks (Hudak and Morton, 1999; Hudak et al., 2003). Hudak et al. (2003) and Hudak et al., (in prep. a) have noted that these disconformable alteration zones and associated mineralization commonly occur near high concentrations of synvolcanic mafic dikes, suggesting that these disconformable hydrothermal alteration zones occurred within synvolcanic fault zones (c.f. Gibson et al., 1999). Page 7 7 �VMS-ASSOCIATED VOLCANIC AND HYDROTHERMAL ALTERATION PROCESSES The composition and distribution of hydrothermal alteration mineral assemblages in the Lower Ely (Table 1-2) is similar to that described in major lava-flow dominated VMS mining districts worldwide (e.g. the Noranda Camp, Quebec; Morton and Franklin, 1987; Franklin, 1996; Gibson et al., 1999; Hudak and Morton, 1999; Peterson, 2001; Hudak et al., 2002; Hudak et al., in prep. a). Results of recent studies (Peterson, 2001; Odette et al., 2001a, 2001b; Hudak et al., 2002; Hudak et al., in prep. a) indicate that not only are the compositions and geometries of the regional alteration mineral assemblages identical to those present in many lava flow dominated massive sulfide mining districts, but that detailed alteration mineral chemistries (Hocker et al., 2003) are also consistent with those associated with the VMS ore deposits in these mining camps. These two observations suggest that the processes that formed the alteration mineral assemblages in the Lower Ely were similar to those that formed equivalent alteration zones in well-established VMS mining camps. Thus, one of the most perplexing questions plaguing economic geologists is why economically significant VMS deposits have not yet been discovered in the Vermilion district. A general genetic model for the formation of VMS deposits and associated hydrothermal alteration zone, as recently presented by Franklin et al. (1998), requires convective metalliferous hydrothermal fluid generation in the sub-seafloor environment via heating of down-welling seawater and leaching of metals from the enclosing volcanic and sedimentary strata (Fig. 1-5). The size of a convective hydrothermal system is a function of the abundance of heat in the upper two kilometers of the sub-seafloor crust (Franklin, 1996; Franklin et al., 1998). The intrusion of hypabyssal synvolcanic dikes and/or sills into the shallow sub-seafloor may vigorously enhance the dynamics of convective hydrothermal cells (Campbell et al., 1981). On reaching a critical reaction temperature of ~ 350°C, sustained acid pH in the hydrothermal fluid (evolved fluid) is achieved, and metals are leached from the rocks into the evolved fluid via primary mineral breakdown by calcium metasomatism, silicification, and hydrolysis reactions (Seyfried et al., 1999). In basalt-dominated systems (such as that in the Lower Ely), leaching-related alteration of mafic "source" zones (lower semi-conformable alteration) forms a mineral assemblage composed of albite-epidote-zoisite/clinozoisite-actinolite-quartz. These zones are variably metal-depleted, and are characterized by patchy silicification and epidotization associated with areas metasomatically enriched in silica and calcium. In lava flow-dominated stratigraphic sequences, regionally confined discordant “pipe-like”, and more regionally extensive “semiconformable” alteration zones are present (Morton and Franklin, 1987). The “pipe-like” semi-conformable alteration zones are closely associated with zones of cross-stratal permeability (e.g. synvolcanic fault zones), and are characterized by well-defined vertically extensive alteration zones containing anomalous abundances of sericite, chlorite (both Fe- and Mg-rich varieties), actinolite/ferroactinolite, quartz, pyrite, and locally, chalcopyrite and/or pyrrhotite. Semiconformable alteration zones extend for several kilometers to tens of kilometers in the rocks stratigraphically beneath and adjacent to VMS mineralized horizons (Santaguida et al., 2002a; Santaguida et al., 2002b). In maficdominated volcanic environments, such alteration typically is associated with regional zones of spilitization (an alteration assemblage composed of albite + quartz + Mg-rich chlorite ± sericite), silicification (quartz ± albite), and epidote-quartz alteration (epidote + quartz ± actinolite ± carbonate) (Morton and Franklin, 1987; Gibson et al., 1999; Santaguida et al.. 2002a, Santaguida et al., 2002b). Regional semiconformable alteration zones in felsic rocks in VMS producing camps such as Noranda (Quebec) or Sturgeon Lake (Ontario), typically comprise extensive zones of spilitization, silicification, and sericitization (sericite + quartz ± Mg-rich chlorite) (Morton and Franklin, 1987; Gibson et al., 1999). Page 8 8 �Table 1-2. Greenschist facies metamorphosed hydrothermal alteration mineral assemblages recognized in the Lower Ely (Hudak et al., 2002a; Hudak et al., in prep. a). Alteration Mineral Assemblage No apparent alteration (unaltered) Least altered Silicified Quartz + Chlorite (chlorite > 15%) Epidote + Quartz ± Chlorite ± Albite (Epidote >10%, Actinolite <10%, Fe-chlorite <20%) Epidote + Quartz ± Chlorite ± Albite (Epidote >10%, Actinolite <10%, Fe-chlorite >20%) Albite + Quartz ± Epidote ± Actinolite ± Chlorite (Epidote <10%, Actionlite <10%) Secondary Plagioclase Feldspar ± Chlorite ± Sericite Biotite ± Fe-/Mg-chlorite Iron Oxide / Hematite Geometry Not applicable Semiconformable Semiconformable Semiconformable Semiconformable Semiconformable Semiconformable Semiconformable Semiconformable Epidote + Quartz + Actinolite ± Chlorite ± Albite (Epidote>10%, Actinolite > 10%) Mottled Epidote +Quartz ± Actinolite ± Chlorite ± Albite (“Patchy Epidosite”) Semi/Disconformable Semi/Disconformable Actinolite ± Fe-chlorite ± Epidote ± Albite (Actinolite >10%, Epidote <10%) Fe-chlorite + Sericite (Fe-chlorite >10%, Sericite >5%) Fe-chlorite + Sericite (Fe-chlorite >10%, Sericite<10%, Fe-chlorite + Sericite >25%) Fe-chlorite + Fe-carbonate ± Sericite ± Actinolite ± Epidote (Fe-chlorite >25%, Fecarbonate > 5%, Actinolite, Sericite, Epidote < 5%) Sericite ± Pyrophyllite ± Fe-chlorite (Sericite/Pyrophyllite >5%, Fe-chlorite <10%) Sericite ± Carbonate (Sericite > 5%, calcite/dolomite/ankerite present, chlorite absent) Mg-chlorite ± sericite Disconformable Disconformable Disconformable Disconformable Disconformable Disconformable Disconformable Chlorite + Fe-carbonate ± Sericite Sericite + Green Mica ± Carbonate Shear Zones Shear Zones Tourmaline + quartz Veins Semiconformable Both discordant and semiconformable alteration zones have been discovered in the Vermilion district (Table 1-2), and have been described by Hudak and Morton (1999), Peterson (2001), Odette et al. (2001), and Hudak et al. (2002a). Semiconformable alteration zones in the Lower Ely are dominated by mineral assemblages containing various proportions of quartz, epidote, zoisite/clinozoisite, Fe-chlorite, Mg-chlorite, actinolite, ferroactinolite, sericite/pyrophyllite, and albite. Odette et al. (2001a, 2001b) and Hudak et al. (2002a) have shown via mass balance analysis that semiconformable quartz + epidote ± actinolite ± albite ± chlorite alteration mineral assemblages in the Fivemile Lake area are metasomatically enriched in calcium and silica, and depleted in base metals (copper and zinc) by 50-90%. Pipe-like, northeast-trending disconformable alteration zones in the Lower Ely are largely composed of Fe-rich chlorite, sericite/pyrophyllite, actinolite and/or ferroactinolite. Pipe-like alteration zones that have been mapped up-section have, to date, not led to the discovery of economically significant VMS deposits, but have been instrumental in locating potential base metal sulfide-bearing stratigraphic horizons and localized chemical exhalites. The mineralogy (Hudak et al., 2002a) and chemical compositions of the alteration minerals (Hocker et al., 2003) in these disconformable zones are consistent with those associated with economic VMS deposits in both Canada and Scandinavia (c.f. Hannington et al., 2002). Page 9 9 �VMS Deposit Impermeable Silicified Zone Zoisite/Ciinozoisite 7/ \ ha/fr - Magmatic Water ' ÷ Si - Cu +ca -Zn +Na - Fe Reservoir Zone Epk Quartz —400t-— \ / / .\ / / — \Subvolcanic Intrusion — — - Figure 1-5. Simplified schematic model of a convective hydrothermal system associated with the formation of Noranda–type (Morton and Franklin, 1987) or lava-flow dominated-type (Gibson et al., 1999) VMS deposits (modified from Franklin , 1996). Water depth controls aspects of associated VMS deposit composition (metal ratios) and alteration assemblages (Franklin, 1986; Franklin, 1993). Field evidence from the recent studies in the Lower Ely (Peterson, 2001; Hudak et al., in press) indicates that both shallow and deep subaqueous environments were present in the Vermilion district, as represented by the Fivemile Lake and Central Basalt Sequences. Such environments are not only associated with base-metal VMS deposits, but may also contain precious metal-rich (e.g. gold, silver) VMS deposits (Sillitoe et al., 1996; Hannington et al., 1999). Thus, the Vermilion district, and in particular, the rocks within a few kilometers of the area we are investigating on this field trip, may contain VMS deposits with a variety of base- and precious metal compositions. Page 10 10 �FIELD TRIP STOPS – DAY 1 (MAY 4) Figure 1-6. Field trip stop locations for Day 1 of the Vermilion district field trip. STOP 1-1: ARCHEAN SOUDAN IRON FORMATION No hammering please! Location: (NE, NE, S. 27, T.62N., R.15W., NAD83 UTM 557120E, 5296660N) Description: This classic exposure of the Soudan Iron Formation (Fig. 1-6) lies on the north limb of the Tower-Soudan anticline, and at the stratigraphic top of the volcanic sequences known collectively as the Lower member of the Ely Greenstone—the focus of this field trip. In a general way, the upper kilometer or so of strata assigned to the Lower Ely displays an upward-stratigraphic progression marked by increasing numbers and thicknesses of iron-formation-rich units, and corresponding decreasing thicknesses of the interleaved basaltic flows. This represents a general cessation of volcanism over some time. It is interesting to observe the rhythmic microlaminations (1 mm or so thick) in various cherty beds exposed here and speculate about the paleoenvironment—that is, whether these represent daily heating/cooling cycles, tidal, climatic, annual, or some other repetitive influence in the depositional environment. What is known about iron-formation units in the Ely Greenstone is that their deposition occurred during periods of relative volcanic and tectonic quiescence by the slow, subaqueous “rain” of chemical precipitates. The Soudan marks an abrupt transition from a period of waning basaltic volcanism in comparatively deep water, to one of mixed, locally emergent volcanic and associated sedimentary deposition represented by exposures to the north and west. The latter include dacitic rocks of what is Page 11 11 �known as the Gafvert Lake sequence, and tuffaceous graywacke of the Lake Vermilion Formation. Whether the transition at Soudan represents a significant unconformity remains enigmatic. This exposure also represents a microcosm of deformation in the western Vermilion district. It displays two generations of close folding in delicate laminations of chert (creamy white), chert-hematite jasper (red), and magnetite-chert (black to silver-colored). The second generation of folds (F2) is tectonic in origin, having subvertical axial surfaces that trend eastward, and steeply plunging axes. Most display Z-asymmetry. The earlier folds (F0-1) appear to have been sharply refolded to produce complex interference patterns. Lundy (1985) studied folding at this locality and concluded that some of the apparent interference structures are the product of early-formed sheath folds that did not involve refolding by D2. The F1 structures are predominantly intrafolial, and exhibit a great variety of style and orientation; implying they formed by layer-parallel, soft-sediment slumping. Work by Hudleston (1976) and Jirsa et al. (1992) extends these types of observations to regional tectonic interpretations of an early (D1) folding that included the development of the Tower-Soudan anticline in rigid volcanic complexes, and associated large nappe structures in the more ductile marginal sedimentary strata, followed by D2 metamorphism and transpressive deformation. STOP 1-2: XENOLITHIC HORNBLENDE DIORITE, PURVIS PLUTON Location: (NE, SW, S. 30, T.62N., R.13W., NAD 83 UTM 571755E, 5296815N) Description: The Purvis pluton is an east-west trending, moderate-sized (~3km3), sill-like multiphase dioritic to tonalitic intrusion with a strike length of 5.7 km and a thickness that ranges from 100-1200 meters (Peterson, 2001). This intrusion occurs in the lower stratigraphic section of the north limb of the Tower-Soudan anticline (Peterson and Jirsa, 1999a; Jirsa et al., 2001). Recent work by Drexler and Hudak (in press) indicate that the intrusion has several phases, including 1) xenolithic hornblende diorite; 2) xenolithic hornblende tonalite; 3) xenolithic leucotonalite; 4) leucotonalite and trondhjemite; and 5) leucotonalite dikes (Fig. 1-7). Detailed field mapping by Peterson (2001), Hovis (2001) and Drexler and Hudak (in press) suggest that the Purvis pluton is a synvolcanic intrusion based on the following characteristics: 1) it lacks a contact metamorphic aureole; 2) its uppermost contact is proximally associated with intense, semiconformable quartz + epidote alteration zones; 3) D2 deformation fabrics occur in both the intrusion and the surrounding country rocks; and 4) early xenolithic diorite phases are cross-cut by thin, commonly D2-deformed dikes of younger tonalite and trondhjemite phases. Galley (2002) and Galley (2003) have indicated that these characteristics are key features of synvolcanic intrusions temporally associated with the genesis of many Precambrian VMS deposits. Peterson (2001) has suggested that the Purvis pluton may have been the heat source that drove hydrothermal systems that produced the Eagles Nest and Purvis Road VMS prospects. A sample of the leucotonalite phase has been submitted to Dr. Daniel Holm (Kent State University) for geochronological analysis, but this age is not yet available. Based on the presence of the D2 fabric in the pluton, U/Pb zircon dates of a D2-deformed felsic lava dome in the Lower Ely in the Fivemile Lake area, and a post-D2 quartz feldspar porphyry in the Newton Belt (Peterson et al., 2001), we anticipate an age between 2683 Ma and 2722 Ma for the Purvis pluton. This locale offers an opportunity to investigate the xenolithic hornblende diorite phase of the Purvis pluton. The outcrop adjacent to the road predominately contains four types of xenoliths: 1) dark green xenoliths of amygdaloidal (5-8%) basalt-andesite pillow lavas which locally have preserved selvedges and interpillow hyaloclastite, and are locally contact metamorphosed along their margins; 2) pale green epidote + quartz-altered basalt-andesite lava xenoliths that are up to 15 cm in diameter; 3) rare coarse-grained gabbro/diorite xenoliths up to 3 cm in diameter; and 4) rare <1-2 cm diameter subangular Page 12 12 �chert xenoliths. Large iron-formation xenoliths up to several meters in diameter, and amphibolite xenoliths up to several centimeters in diameter, may be observed in xenolithic hornblende tonalite outcrops located east of this location (NAD 83 UTM coordinates 0574920E, 5297600N). * - Hr ft / Figure 1-7. Xenolithic hornblende tonalite (A) and xenolithic hornblende diorite (B) phases of the Purvis pluton.. Iron-formation xenoliths present in outcrops east of here were likely derived from iron-formation horizons that occur immediately southwest of Purvis Lake. Basalt and altered basalt fragments also were derived from the surrounding Lower Ely. The presence of epidote-quartz altered mafic xenoliths suggests that this phase of the Purvis pluton stoped its way upward into an earlier-formed proximal zone of quartzepidote alteration formed from high temperature seawater-rock interaction (e.g. Galley, 2003). Amphibolite xenoliths are believed to represent contact metamorphosed basalt fragments based on petrographic similarities (Drexler and Hudak, in press). Drexler and Hudak (in press) have shown that coarse-grained gabbro/diorite fragments likely represent xenoliths of the earliest phases of the pluton. A short hike up to the top of the ridge allows investigation of a newly exposed (and still lichenfree) outcrop displaying the complicated textures between medium-grained diorite and xenoliths in this phase of the Purvis pluton. Collectively, six types of xenoliths comprise 55-60% of the intrusion at this location. These xenolith types include: 1) pale to dark green, subangular to subround basalt-andesite clasts which range from 1-30cm in diameter and make up 15-20% of the rock; 2) subround to subangular, coarse-grained gabbro/diorite xenoliths that are up to 20cm in diameter; 3) subround epidote + quartzaltered basalt-andesite xenoliths that are up to 20cm in diameter; 4) subangular, dark grayish-green finegrained diorite xenoliths which are up to 10cm in diameter; 5) dark green, coarse-grained, actinolite-rich amphibolite fragments up to 12 cm in diameter; and 6) rare pillow selvedge / interpillow hyaloclastite xenoliths which locally make up 1% of the unit. Moving east along the ridge, one can observe additional outcrops of xenolithic diorite. Locally (UTM NAD 83 coordinate 571798E, 5296828N) ENE-trending 10-20cm wide leucotonalite dikes cut through the xenolithic diorite phase, indicating the age relationships between these two units. Farther east along the ridge, the complicated contact between the Purvis pluton and the overlying basalt-andesite pillow lavas can be observed. These lavas contain patchy epidote + quartz alteration, and have locally been metamorphosed to hornfels immediately adjacent to the intrusion. Studies of ancient VMS deposits has documented that the deposits commonly occur in depressions on the paleo-seafloor (3rd-order basins) while modern deposits on the seafloor are found on high-standing structures, such as ridges. These differences are probably more apparent than real, in that Page 13 13 �the modern deposits are generally confined to the axial graben, or depression, of what are otherwise are high-standing structures. In addition, both ancient and modern deposits occur in areas of anomalously high heat flow, generally linked to synvolcanic intrusions beneath the hydrothermal systems. The recent mapping in the Purvis Road area has shown the presence of all of the attributes of typical VMS-forming hydrothermal systems. These attributes include a synvolcanic intrusive heat source (the Purvis pluton), a paleotopographic high-standing structure, VMS-style alteration mineral assemblages, and the presence of Cu and Zn-rich massive sulfide (recent logging in this area has exposed numerous angular boulders of massive sulfide in the basal till). The VMS attributes of the Purvis road area are presented in Figure 1-8. Figure 1-8. Simplified regional lithostratigraphic sequences draped on a 3D view (looking due west) of a residual field aeromagnetic anomaly map of the western Vermilion district (same map area as Fig. 1-2). The lower two photographs show massive sulfide boulders exposed north of the Purvis pluton. Page 14 14 �STOP 1-3. SILICIFIED FIVEMILE LAKE SEQUENCE PILLOW LAVAS / REGIONAL SEMICONFORMABLE ALTERATION Location: (SE, SE, S. 22, T.62N., R.14W., NAD83 UTM 567818E, 5297818N) Description: Geological mapping by Peterson (2001) has indicated the presence of a regional semiconformable quartz-epidote alteration zone extending for at least 19 km along strike within the Lower Ely along the north limb of the Tower-Soudan anticline. This type of alteration is a common feature in many Archean VMS camps (e.g. Noranda (Gibson, 1989) and Snow Lake (Skirrow and Franklin, 1994)), and is attributed to silica- and calcium-dumping that occurs in the deep, sub-seafloor as downwelling hydrothermal fluids are heated to temperatures in excess of 350°C (Franklin, 1986; Franklin, 1993). Semiconformable alteration zones associated with VMS systems are generally much larger in area than their associated mineralization, and therefore provide exploration geologists regional areas in which to concentrate more detailed, follow-up field mapping, geochemical studies, and geophysical surveys for identifying VMS targets. Semiconformable quartz-epidote alteration zones are commonly metal-depleted. Peterson (2001), Odette et al. (2001a, 2001b), and Hudak et al. (2002a) have shown that that quartz-epidote altered basalts within this regional semiconformable alteration zone are commonly depleted in base metals by as much as 50-90%. Peterson (2001) completed simplified mass balance calculations of Cu and Zn depletion associated with the regionally extensive quartz-epidote alteration zone within the Lower Ely, and this work reveals the importance of this alteration to the VMS mineral potential of this area. The surface expression of the quartz-epidote alteration is approximately 19 kilometers long, and averages approximately 1.5 kilometers in width. The area of this zone is on the order of 30.5 km2. Using Southwick et al. (1998) values for Cu and Zn of 59 and 115 ppm, and a mean density of the rocks equal to 2.8g/cm3, the total contained copper and zinc for various volumes of rock from the Lower Ely are calculated, and presented in the top of Table 1-3. These volumes include a 1.0 kilometer thick slab (volume = 30.5 km3), a 2.0 kilometer thick slab (volume = 61 km3), a 3.0 kilometer thick slab (volume = 91.5 km3), and a circular disk (volume = 591 km3). Percent leaching calculations for copper and zinc are presented in the lower portions of Table 1-3, with values of metal leached (in pounds) from the four calculated volumes of rock. The leaching models include 5%, 10%, 25%, and 50% metal leached from the rock. Given the patchy nature of this alteration, an estimate of somewhere between 10 and 25% metal leaching seems reasonable, as does the 91.5 km3 volume. Therefore, one can estimate that many billions of pounds of copper and zinc have been leached out of the volcanic rocks in the north limb of the Lower Ely Greenstone. The concentration of these base metals into specific sites by a VMS style hydrothermal system would have created significant tonnages of massive sulfide. This data clearly reflects the potential for VMS mineralization up-section from this zone. At this location we can observe part of the regionally extensive quartz-epidote alteration zone. The outcrop contains relatively undeformed bun- and mattress-shaped pillows. Interpillow hyaloclastite zones are generally pale to dark green in color, and are chlorite and/or actinolite-rich. Minor red-brown staining locally occurs in these zones, and is indicative of the presence of trace amounts of pyrite and/or chalcopyrite. Pillow selvedges commonly contain up to 10% round to oval, pipe-like quartz-epidote and/or actinolite chlorite amygdules. The cores of the pillows are typically pale green gray in color due to nearly wholesale replacement of the original igneous minerals by quartz and epidote. Page 15 15 �Table 1-3. Copper and zinc leaching calculations for the regional semiconformable quartz-epidote alteration zone present in the Lower Ely (Peterson, 2001). Volume (Km3) 30.5 11,084,920,000 21,606,200,000 61 22,169,840,000 43,212,400,000 91.5 33,254,760,000 64,818,600,000 591 214,793,040,000 418,664,400,000 5% Cu leached (lbs) 10% Cu leached (lbs) 25% Cu leached (lbs) 50% Cu leached (lbs) 554,246,000 1,108,492,000 2,771,230,000 5,542,460,000 1,108,492,000 2,216,984,000 5,542,460,000 11,084,920,000 1,662,738,000 3,325,476,000 8,313,690,000 16,627,380,000 10,739,652,000 21,479,304,000 53,698,260,000 107,396,520,000 5% Zn leached (lbs) 10% Zn leached (lbs) 25% Zn leached (lbs) 50% Zn leached (lbs) 1,080,310,000 2,160,620,000 5,401,550,000 10,803,100,000 2,160,620,000 4,321,240,000 10,803,100,000 21,606,200,000 3,240,930,000 6,481,860,000 16,204,650,000 32,409,300,000 20,933,220,000 41,866,440,000 104,666,100,000 209,322,200,000 Total Contained Cu (lbs) Total Contained Zn (lbs) STOP 1-4: WELL-PRESERVED PILLOW LAVAS, SOUTH OF FIVEMILE LAKE Location: (NE, SW, S. 29, T.62N., R.14W., NAD83 UTM 563195E, 5296660N) Description: The Fivemile Lake Prospect comprises an extremely well preserved, and locally intensely hydrothermally altered, bimodal package of basalt-andesite pillows lavas and coherant and volcaniclastic facies felsic metavolcanic strata which have locally been intruded by diabase and/or diorite synvolcanic sills and dikes. Recent geological studies at the Fivemile Lake Prospect have included detailed field mapping, diamond drill core relogging, petrographic, lithogeochemical, x-ray diffraction, and electron microprobe investigations (Hudak and Morton, 1999; Peterson, 2001; Newkirk et al., 200a, 2001b; Odette et al., 2001a, 2001b; Hudak et al., 2002a; Hocker et al. 2003). These studies were undertaken to a) better understand the geology (in particular, the physical volcanology) of these regions; b) identify and evaluate the composition and distribution of the metamorphosed hydrothermal alteration mineral assemblages that occur at each of these prospects; c) evaluate and compare the chemical compositions of alteration mineral phases to those found at VMS deposits of various ages in various parts of the Canadian Shield, the western United States, and Tasmania; and d) evaluate the potential for VMS and/or lode gold mineralization in these areas. Field mapping and diamond drill core relogging indicate that the Fivemile Lake Prospect comprises thirteen distinct types of strata that can be subdivided into six major lithological classes (Hudak et al., 2002a). These include: a) calc-alkaline and tholeiitic, mafic to intermediate volcanic and volcaniclastic rocks (basalt-andesite pillow lavas, pillow breccias, hyaloclastite deposits, tuff, lapilli tuffs and monogenic volcanic breccias); b) chemical sedimentary rocks (semi-massive sulfide exhalite); c) calc-alkaline to tholeiitic, intermediate to felsic volcanic and volcaniclastic rocks (massive quartz-phyric rhyodacite lava flows, and aphyric to quartz-phyric rhyodacite to rhyolite tuff, lapilli tuff, and tuff breccia deposits); d) synvolcanic intrusive rocks (diorite sill-dike complexes, diabase dikes and sills, and massive gabbro); e) post-volcanic intrusive rocks (feldspar porphyry dikes and sills and quartz-feldspar porphyry dikes); and f) post-volcanic schists (quartz-sericite-ankerite schists, chlorite schists, and quartz-sericite- Page 16 16 �Figure 1.9. Generalized geological map of the Fivemile Lake – Needleboy Lake – Sixmile Lake area (modified after Peterson and Jirsa (1999a), Jirsa et al. (2001), and Hudak et al. (2002a). Page 17 17 �green mica schists). Supracrustal strata at the prospect vary from east-west to southeast-northwest striking, dip steeply to the north, and are north-facing. Because of this geometry, the geological map of the area (Figure 1-9) essentially illustrates a cross-section through the Lower Ely crust. Volcanic and volcaniclastic rocks have chemical characteristics indicative of formation with an ancient volcanic arc setting (Hudak and Morton, 1999; Hudak et al., 2002a). At this stop, we will investigate two outcrops that display exceptionally well-preserved pillow lavas and associated interpillow hyaloclastite. Detailed analysis (Newkirk et al., 2001a, 2001b; Hudak et al., 2002a) indicates that the pillow lavas at Fivemile Lake are morphologically similarly to undeformed Cenozoic pillow lavas from New Zealand and Tasmania. The nomenclature used for describing pillow lavas is illustrated in Figure 1-10). Intiapiliow Cavity r— Coiicentdc Cooling Joints Piliow Bud (non sight to loft) Blocky HyalocJastite — Radial Cooling Joint Neck and Knob Pipe .anygdule — Seivedge Formerly Glassy L Amygdulssririow — inteipiliow Pillow Margin -lyaloclastito Figure 1-10. Nomenclature used for describing pillow lavas and associated hyaloclastite (after Cas and Wright (1987) and Gibson (1989). At the first outcrop (NAD 83 UTM coordinates 563195E, 5296660N) we can observe bun- to lens-shaped, quartz-epidote-actinolite-chlorite-altered basalt-andesite pillow lavas. The general strike of the pillow lavas at this location is approximately 085°. The pillows vary from 0.4-1.0 meters high and 0.5-2.0 meters long, and contain zones of chlorite-rich interpillow hyaloclastite that ranges from 0.5-3.0 cm wide. Local brownish-red sulfide staining may be observed in the interpillow hyaloclastite zones. The pillow selvedges vary from 1-6 cm wide, are commonly silicified and/or quartz-epidote altered, and contain approximately 15% 1-5mm oval to round, quartz ± carbonate filled amygdules. Multiple pillow rinds, separated by 4-8 cm, may be locally observed. Reentrant pillow crusts and apparent pillow buds at this location suggest flow from west to east. The cores of the pillows at this location contain up to 5% 25 cm rounded quartz amygdules, and are faintly plagioclase feldspar-phyric (3-5% <1mm tabular phenocrysts). Faint radial cooling joints are also locally preserved in several of the pillows on this outcrop. Page 18 18 �A second spectacular outcrop of well-preserved, east-west striking pillow lavas occurs approximately 30 meters to the east (NAD 83 UTM coordinates 563225E, 5296660N). Pillow morphology at this outcrop is quite variable, ranging from bun- and mattress-shaped to amoeboid. Topping directions based on pillow morphology are consistently north. Pillows here are typically 50-80 cm in height and 1-1.5 meters long, although pillows as small as 30 cm in diameter and up to 2 meters long may be locally observed. Pillows selvedges vary from 1-7 cm wide (average 2-3 cm) and contain 15-20%, 3-7 mm diameter, round quartz amygdules. Pillow cores generally contain 5-10% 3-10mm round to oval quartz amygdules. Several pillows on the western and north-central part of the outcrop contain multiple pillow rinds. A pillow displaying exceptionally well-preserved “neck and knob” structure may be observed on the east central part of the outcrop. Vesicle size and abundance in pillow lavas (McPhie et al., 1993 and references therein) may be used with other textural and facies characteristics to interpret pillow lavas erupted in “relatively shallow” (<1-2 km water depth) versus “relatively deep” (>2 km water depth) submarine environments. Multiple pillow rind structures are common features in pillow lavas erupted in “relatively shallow” water environments, and are formed by implosion and rupture of the pillow skin resulting from condensation of exsolved gases concentrated in the upper part of the pillow (Kawachi and Pringle, 1988). The relatively high vesicularity of the pillows, coupled with the presence of multiple rind structures, indicate that the pillow lavas in this part of the Lower Ely were formed in a relatively shallow submarine environment. STOP 1-5: PILLOW DIKES, PEPERITES, AND SYNVOLCANIC FAULTS, SW SHORE, FIVEMILE LAKE No hammering please! Location: (SW, NW, S29, T. 62N, R. 14W; NAD 83 UTM Coordinates 0563305E, 5296955N) Description: Peperite is defined as a breccia-like volcaniclastic deposit formed from the in-situ disintegration and mixing of magma or lava with wet, poorly consolidated sediment (Batiza and White, 2000; Schmidt and Schminke, 2000). Peperite is a common rock type in submarine arc-related volcano sedimentary sequences, and is associated with synvolcanic intrusions. It often occurs in phreatomagmatic vent filling deposits and along contacts between synvolcanic intrusions and poorly consolidated, wet, submarine sediments. Key components that aid in the identification of peperites include evidence of partial fluidization of the sediment, local contact metamorphism, and in some rare instances, vesiculation within the sediment (Skilling et al., 2002). In general, the end-member compositions in a peperite are the igneous component (a synvolcanic dike or sill, or a lava flow) and the sedimentary component. Igneous component domains in peperites vary from tabular to lobe-like to pillow-like, and can commonly have irregular, interconnected, fold like, and pod-like shapes. Mixing that results from quenching, hydromagmatic explosions, magma-sediment density contrasts, and/or mechanical stresses resulting from inflation or movement of magma, causes the igneous component of the peperite to be broken-up and dispersed within the wet sediment to form peperite (McPhie and Orth, 1999). Factors such as magma and/or sediment composition, magma injection velocities, volatile content of the magma, relative volumes of sediment and magma involved, volume of pore water heated, presence or absence of shock waves, and the confining pressure all influence the morphology of peperites. Juvenile clast morphologies in peperite are extremely variable, and may be irregular, blocky (commonly jigsaw-fit), pumiceous or scoriaceous, irregular (amoeboid), elongate, wispy, platy or ragged. It is not uncommon for mixed juvenile clast populations to be present within a single peperite deposit (Skilling et al., 2002). This stop encompasses a series of outcrops that occur on a small peninsula along the southwest shoreline of Fivemile Lake (informally and affectionately known as “Pike Point” based on the exceptionally large pike that can be caught from this shoreline). A generalized geological map of this Page 19 19 �area, which points out key features to observe at this stop, is illustrated in Figure 1-11. The area was originally mapped as a series of pillow lavas and felsic debris flow deposits (Peterson and Jirsa, 1999a; Peterson, 2001); however, if the pillow lavas represent a stratigraphic unit, they are oriented approximately 40°-45° to the strike of the supracrustal strata in this area, suggesting that either a) deposition occurred upon a steep paleotopographic surface (which is not supported by the pillow morphology); or b) a fold may be locally present (evidence for this is not currently supported by regional mapping). (V >1. Fivemile — 5296925 Coherant Pillows Fades Volcanidas& Fades LapllI T'fl / TuIT-Br.a X H1cooct 1ñofSonoatI Difiusa planar stralitpcaton a! deals SVl Ice and topping direct on of pillow SyTwolcanlc Fault 0 I cL) I Oulciop Locaa' 7] Approximate Trail 5 10 20 Lobon Figure 1-11. Geological map of the southwestern shoreline (“Pike Point”) area of Fivemile Lake (after Hudak et al., in prep. b). Detailed facies mapping at scales ranging from 1:50 to 1:100 by Hudak et al. (2002a) and Hudak et al. (in press) now suggests that “Pike Point” comprises a series of northeast-trending andesite-dacite pillowed dikes that intruded a deposit of wet, unconsolidated basalt-andesite tuff to form blocky to amoeboid peperites. Pillows may be formed within water-saturated hyaloclastite or sediments within the sub-seafloor provided that the adjacent rocks are poorly consolidated and saturated with water (McPhie et al., 1993 and references therein). The interpretation that the pillowed igneous domains represent dikes rather than pillowed lava flows is based on several characteristics of the dikes: 1) the strike of the pillows is northeast, whereas the strike of the volcanic strata in this region is general more or less east-west; 2) topping directions measured in the pillows are variable and inconsistent; 3) on the most northwestern of the outcrops, apophyses of the dikes can clearly be seen propagating to the north-northwest; 4) the Page 20 20 �chemical compositions of the pillow domains and the blocky to amoeboid fragments in the volcaniclastic deposits is identical; and 5) the trend of the pillows, when followed to the north shore of Fivemile Lake, leads to a series of chemically identical, northeast-trending dikes and associated peperite deposits that occur within the vent facies of a 700 meter (strike length) by 200 meter (height) tuff cone volcano mapped by Hudak et al. (in prep. a, in prep. b; we will see these deposits tomorrow at field trip Stop 1-9). The interpretation that the volcaniclastic deposits represent peperites is based on the following characteristics of the deposits: 1) at several locations along the point, blocky jigsaw puzzle-fit fragments indicate in-situ fragmentation of northeast-trending pillow dikes; 2) fragment shapes in the volcaniclastic deposits vary from amoeboid to blocky and jigsaw puzzle-fit, to curviplanar, all indicative of fragmentation by external water; 3) vesiculation is common within the matrix of the volcaniclastic deposits; and 4) the general trend of the long directions of the fragments is consistent with the strikes of the pillow dikes. Several textures that may be observed at this location are illustrated in Figure 1-12. Figure 1-12. Textures within peperites that occur along the southwestern shoreline of Fivemile Lake: 1-12a) northeast-trending pillow dikes on the northwest side of the point; 1-12b) fractured synvolcanic dike, west-central part of point; 1-12c) synvolcanic dike and blocky peperite with vesiculated sediments, south-central part of point; 112d) northeast-trending pillowed dikes, east-central part of point. Page 21 21 �STOP 1-6: EPIDOTE – QUARTZ ALTERED DIABASE SILLS AND DIKES, SOUTH OF FIVEMILE LAKE Location: (NE, SW, S.29, T.62N., R.14W., NAD 83 UTM 563515E, 5296610N) Description: This stop affords us the opportunity to investigate both synvolcanic and post-volcanic hydrothermal alteration in the Fivemile Lake area. Field, petrographic, x-ray diffraction, electron microprobe, and lithogeochemical studies have identified twelve distinctive metamorphosed hydrothermal alteration mineral assemblages at the Fivemile Lake prospect (Odette et al., 2001a, 2001b; Hudak et al., 2002a; Hocker et al., 2003). Synvolcanic, semi-conformable alteration assemblages have been identified on the basis of the modal percentages of the minerals epidote, actinolite, and chlorite, and include: a) epidote + quartz ± chlorite ± albite; b) epidote + quartz + chlorite ± albite; c) epidote + quartz + actinolite ± chlorite ± albite; and d) albite + quartz ± epidote ± actinolite. These quartz + epidote-rich alteration assemblages are interpreted to represent “seafloor metasomatism” associated with hydrothermal fluids that moved through the subaqueous rocks. Detailed mapping and lithogeochemical trends suggest that early, relatively low temperature hydrothermal alteration mineral assemblages (e.g., albite + quartz ± epidote ± actinolite) were later overprinted by higher temperature alteration mineral assemblages (e.g., epidote + quartz ± chlorite ± albite, epidote + quartz + chlorite ± albite, epidote + quartz + actinolite ± chlorite ± albite) as the volcanic pile was progressively buried by volcanism deeper below the seafloor. It is believed that the early, near seafloor environment, which was locally base metal enriched by VMSstyle mineralization, eventually became depleted in base metals from later, deep-seated hydrothermal circulation. Synvolcanic, disconformable alteration mineral assemblages are commonly proximal to synvolcanic fault zones and VMS-style mineralization at the prospect, and include: a) actinolite + quartz ± iron-rich chlorite ± epidote ± albite; b) iron-rich chlorite + sericite; c) iron-rich chlorite ± sericite ± ironcarbonate ± actinolite ± epidote; d) sericite ± iron-rich chlorite; and e) mottled epidote + quartz ± actinolite. These alteration zones likely represent areas of high temperature metasomatism associated with VMS mineralization at, or up-section from, the Fivemile Lake prospect. Post-volcanic hydrothermal alteration mineral assemblages at the prospect include: a) carbonate (ankerite, dolomite, or calcite); b) sericite + carbonate (ankerite, dolomite, or calcite); and c) sericite + green mica ± carbonate (ankerite, dolomite or calcite). The close association of structurally deformed rocks and these alteration zones, and the orientations of these alteration zones, suggest that these alteration mineral assemblages resulted from syntectonic hydrothermal processes associated with the D2 Murray Shear Zone deformational event. A hydrothermal alteration mineral assemblage map for the Fivemile Lake prospect is illustrated in Figure 113. The mottled epidote + quartz + actinolite alteration zone present in this outcrop is confined to a diabase dike-sill complex located immediately south of Fivemile Lake. This 400 meter by 500 meter disconformable to semiconformable alteration zone locally cross-cuts semiconformable alteration zones comprising the epidote + quartz ± chlorite ± secondary feldspar assemblage, the epidote + quartz + ironchlorite ± secondary feldspar assemblage, and the secondary feldspar + quartz ± epidote ± actinolite alteration assemblage. The mottled epidote + quartz + actinolite alteration zone is characterized by a dark green, actinolite and epidote-rich groundmass that contains discrete 0.1-2.0 meter diameter, round to lens-shaped masses containing anhedral granular epidote and zoisite intergrown with polygonal quartz. These rounded epidote-quartz-rich masses can easily be mistaken for individual pillow lavas; however, they can be distinguished from pillows because they lack abundant amygdules, and have no discrete pillow selvedges or interpillow hyaloclastite zones. Petrographic, x-ray, and electron microprobe analyses indicate the presence of iron-rich chlorite, iron-rich actinolite, pistacite and zoisite/clinozoisite within this assemblage (Odette et al., 2001a, 2001b; Hocker et al., 2003). Page 22 22 �Figure 1-13. Hydrothermal alteration map of the Fivemile Lake prospect (after Hudak et al., 2002a). The mineralogy, mineral compositions, and pronounced calcium enrichment and copper depletion that characterize the mottled epidote + quartz + actinolite alteration assemblage (Odette et al, 2001a, 2001b) are consistent with formation within a synvolcanic, semiconformable to disconformable alteration zone formed by high temperature, high fluid to rock ratio interactions deep within a sub-seafloor hydrothermal cell (c. f. Franklin, 1996; Seyfried et al., 1999). Similar clinozoisite/zoisite bearing alteration mineral assemblages described by Hannington et al. (2002) that occur down-section from the world-class VMS deposits in the Noranda mining camp in Quebec are believed to have formed from anomalous fluid flow at high water to rock ratios and high temperatures. Large zones of base metaldepleted epidosite in the Troodos ophiolite complex have been interpreted by Richardson et al. (1987) to represent deep-seated fossil reaction zones where metaliferous “black smoker” fluids are produced. The confinement of this alteration assemblage to the base of the northeast-trending diabase dike / sill complex, indicates that this alteration assemblage occurs in the deep subsurface of a northeast-trending synvolcanic structure that focused magma, as well as high temperature, perhaps base metal-rich hydrothermal fluids, up-section. Base metal exhalite mineralization occurs approximately 1 km northeast of this location (Stops 1-7 and 1-8, Fig. 1-13). An east-southeast – west-northwest-trending post-volcanic shear zone is present in the northcentral part of the outcrop. The orientation of the schistosity adjacent to the shear zone indicates a dextral sense of shear. The shear zone is one of several east-southeast – west-northwest trending shears that occur at the Fivemile Lake Prospect. Similarly oriented dextral shear zones mapped by Peterson and Patelke (2003) between the north edge of the Murray Shear Zone and the Mine Shear zone in Sections 25 and 26, T. 62N, R. 15 W (approximately 2-3 km west of this location) have been interpreted as Reidel R structures. Page 23 23 �STOP 1-7: ALTERED PILLOW LAVAS, BASE METAL EXHALITE, SCORIA LAPILLI TUFF, NE OF FIVEMILE LAKE Location: (SE, NE, S.29, T.62N., R.14W., NAD83 UTM 564260E, 5297175N) Description: A 40-meter long, east-northeast striking, <1-2 meter thick chert-pyrite-chlorite ± chalcopyrite exhalite horizon crops out at this location approximately 50 meters northeast of Fivemile Lake (Fig. 1-13). Although this unit is volumetrically insignificant within the Fivemile Lake area, its presence is extremely important in terms of understanding potentially ore forming processes in this part of the Lower Ely. A lithogeochemical analysis of the exhalite (Hudak et al., 2002a) indicates slightly elevated Cu (470-509 ppm) and Zn (66-135 ppm). D. M. Peterson (personal communication, 1996) first recognized this unit as being similar in morphology and mineralogy to chemical exhalites associated with, and locally hosting, volcanic-associated massive sulfide mineralization in the Archean basalts and andesites within the Noranda mining camp of Quebec, Canada. This exhalite horizon appears to be on strike with the northernmost of four untested EM conductors that have been identified within Fivemile Lake (Peterson, 2001; Peterson and Jirsa, 1999a). Extremely detailed mapping (1:120 scale) has been completed (Hudak et al., 2002a) to further evaluate the stratigraphic and structural controls on this exhalite deposit (Fig. 1-14). The exhalite is relatively easy to distinguish from its surrounding rocks based on its deep red-brown, gossan-like, sulfidestained weathered surface. When broken, the exhalite is composed of vaguely banded chert and chlorite that contains 5-10% disseminated to banded pyrite. Locally, anhedral disseminated chalcopyrite and locally, extremely fine-grained, finely-disseminated, honey- to dark brown sphalerite grains are present in trace amounts. Detailed mapping indicates that the exhalite unit separates epidote + quartz + actinolite ± chlorite-altered basalt-andesite pillow lavas from overlying iron-chlorite- and sericite-altered basaltandesite tuffs and lapilli tuffs. The exhalite unit stratigraphically and conformably overlies the pillow lavas, and its sharp basal contact illustrates how the chemical sediment draped over the pillow-dominated paleotopography of the seafloor at the time of deposition. Locally, exhalite deposits cut through the pillow lavas, replacing the formerly glassy interpillow hyaloclastite. This relationship suggests that the interpillow hyaloclastite zones represented areas of high permeability, and these regions were the locus of hydrothermal fluid migration upward to, and possibly within the immediate subsurface, of the paleoseafloor. These features are consistent with the interpretation that this represents a region of ventproximal hydrothermal activity. The contact between the exhalite and the overlying lapilli tuff and tuff deposits is also sharp. It is important to note that fragments of the exhalite have not been observed in the basal parts of these overlying volcaniclastic deposits. In addition, no soft sediment deformation has been observed that could be attributed to syndepositional tractional deformation associated with the sedimentation of the overlying volcaniclastic rocks. This suggests that the exhalite deposit was relatively competent at the time the overlying tuffs were deposited. Such field evidence, combined with the lack of intense quartz-epidoteactinolite ± chlorite alteration (which is so prevalent in the footwall pillow basalts) in the overlying tuffs, also suggests that the hydrothermal activity associated with the genesis of the exhalite deposit was completed by the time that the overlying volcaniclastic rocks were deposited. Three other features suggest that exhalite deposition was closely associated with a volcanic ventproximal setting. First, property-scale mapping at Fivemile Lake (Hudak et al., 2002a) clearly illustrates that a synvolcanic diabase intrusion (observed in the previous stop) cross-cuts the stratigraphy in the immediate vicinity of the exhalite deposit (Fig. 1-9). Second, detailed mapping clearly indicates that the exhalite unit is abruptly terminated at its western margin by north to northwest striking pillow dikes that are similar in morphology to the pillow dikes that occur in close association with the peperite deposits Page 24 24 �Figure 1-14. Detailed geological map of exhalite mineralization northeast of Fivemile Lake (from Hudak et al., 2002a). Note the stratigraphic positioning of the chemical exhalite horizon between basalt-andesite pillow lava and basalt-andesite lapilli tuff, suggesting a period of volcanic quiescence which was accompanied by hydrothermal activity prior to the deposition of the lapilli tuff unit. Also note that the western part of the exhalite is cut by pillowed basalt-andesite dikes, once again suggesting the presence of a synvolcanic structure. Note that all UTM coordinates on the diagram are in NAD 27 coordinates. Page 25 25 �along the southwestern shoreline of Fivemile Lake. The location of both the diabase dikes and the pillow dikes clearly indicates the local presence of a major synvolcanic structure. Thick interpillow hyaloclastite deposits in the vicinity of the exhalite also suggest a vent-proximal environment (Newkirk et al., 2001a, 2001b; Hudak et al., 2002a). In summary, semi-massive sulfide exhalite associated mineralization clearly has formed within a volcanic (and associated hydrothermal) vent-proximal environment at Fivemile Lake. Such environments are commonly associated with economic massive sulfide mineralization in other lava-flow dominated Archean volcanic settings, such as at Noranda, Quebec (Gibson, 1989; Gibson et al., 1999; Santaguida, 1999). STOP 1-8: RHYOLITE LAVA DOME, NE OF FIVEMILE LAKE Location: (SE, NE, S.29, T.62N., R.14W., NAD 83 UTM 564440E, 5297160N) Description: The first day of our field trip concludes with an investigation of a 200-meter long, 15-45 meter thick unit of quartz-phyric, rhyodacitic to rhyolite lava that occurs approximately 200 meters northeast of Fivemile Lake (Figs. 1-9 and 1-13). The U/Pb zircon age for this unit is 2722.6 ± 0.9 Ma (Peterson et al., 2001). This biscuit shaped, relatively thick mass of rhyodacitic to rhyolitic is morphologically similar to felsic lava domes and cryptodomes (Cas and Wright, 1987). The presence of quartz-phyric lava fragments in epiclastic deposits approximately 400 meters southeast of this location, as well as the presence of the overlying subaqueously deposited volcaniclastic sediments and intermediate to mafic lava flows, suggests that this unit was at least partially emergent as a subaqueous lava dome. The uppermost contact of this unit is not exposed in outcrop. Where exposed in drill core (diamond drill hole SXL-2), the massive quartz-phyric lava is interstratified with, and overlain by, 1-2 meters of felsic tuff, which in turn, is overlain by pillowed basalt-andesite lava flows. The basal contact of the unit is exposed in diamond drill holes SXL-2 and SXL-3. In SXL-2, the basal contact is represented by a 3-meter thick shear zone composed of quartz, carbonate, sericite and green mica (fuchsite or mariposite?). In SXL-3, the base of the unit is immediately underlain by a 2-meter thick quartz-feldspar porphyry dike that, in turn, is underlain by a shear zone composed of quartz-carbonate-sericite-chlorite schist. This shear zone locally contains 1-3%, 1-8mm wide veins and bands composed of red to redbrown sphalerite. The presence of this shear zone conservatively precludes using the pillowed basaltandesite lava flows as reliable bounding facies; however, the lack of evidence for extensive structural transposition of the felsic lavas, the extremely thick sequence of pillow lavas stratigraphically below the felsic lavas, and the presence to the southeast of submarine epiclastic strata with felsic lava fragments identical in composition to the dome, suggest that the felsic lavas were formed in a subaqueous environment. Drill core intersections of these lava flows are characterized by a massive to faintly amygdaloidal (up to 1% 5mm rounded gray quartz amygdules), pale gray (least altered) to pale yellow green (sericitized) aphanitic groundmass which contains 1-4% grayish blue to colorless euhedral square to subhedral, locally broken, <1-2.5mm quartz phenocrysts (Fig. 1-15a). Pale brown to honey-colored sphalerite veins are locally present in the drill core and range from <1-8cm in width (Figure 1-15b). Two types of veins are present: a) sphalerite veins which are associated with quartz and/or chlorite, which display no indication of structural deformation (in-situ sphalerite veins); and b) sphalerite veins which are associated with local domains of quartz-sericite-ankerite schist, and appear to have been formed by local remobilization of sphalerite along post-volcanic shear zones (structurally remobilized sphalerite veins). The presence of in-situ sphalerite veins, and the abundance of structurally remobilized sphalerite veins in the quartz-phyric lava flows relative to other units in the Fivemile Lake area, suggests that the original Page 26 26 �sphalerite mineralization in the Fivemile Lake area was synvolcanic and associated with the massive quartz-phyric lava flows. As seen in this exposure, the quartz-phyric lava flows contain an aphanitic, pale-gray to gray groundmass. These lavas contain 5-8% 1-3mm diameter subhedral to square euhedral quartz phenocrysts that are locally broken. Rare, 2-4mm subhedral to euhedral quartz glomerocrysts are locally present. Faint, pale gray <1-2mm tabular feldspar phenocrysts are also present, and comprise up to 5% of the rock. Petrographic analysis indicates that lavas are characterized by a very fine-grained quartzo-feldspathic groundmass and subhedral to euhedral quartz phenocrysts that are locally resorbed. Alteration minerals that occur in the groundmass include sericite, epidote, clinozoisite/zoisite, iron-carbonate and iron-rich chlorite. Also present on the southwest side of this outcrop is a northeast-trending, post-volcanic quartzfeldspar porphyry dike. Figure 1-15. Drill core appearance of unmineralized, sericite-altered rhyolite dome (A, SXL-2-173’) and silicified rhyolite dome cut by red-brown “honey-colored” sphalerite veins (B, SXL-2-280’). Modified from Hudak et al. (2002a). Scale bar units are centimeters. FIELD TRIP STOPS – DAY 2 (MAY 5) Figure 1-16. Locations of field trip stops 1-9, 1-10, and 1-11. Page 27 27 �STOP 1-9: FLUIDAL PEPERITES AND VOLCANIC VENT FACIES, NORTH SHORELINE OF FIVEMILE LAKE No hammering please! Location: (SE, NW, S.29, T.62N., R.14W., NAD 83 UTM 563590E, 5297215N). Note: We will be accessing the north side of Fivemile Lake via a private road. Obtain permission to use this road from people in the first house on the right as you turn onto this private drive. Description: Detailed (1:100 to 1:2000 scale) mapping by Hudak et al. (in prep. a) and Hudak (in press) has identified the paleo-vent of a relative small basalt-andesite tuff cone volcano immediately north of this location (Figs. 1-9 and 1-16). The vent facies here is well exposed in a series of outcrops along the north shoreline of Fivemile Lake, and is extremely complicated (Fig. 1-17a). At this location, we can observe steeply north dipping, east-west striking basalt-andesite pillow lavas that occur stratigraphically beneath the tuff cone, and the contact between the pillowed flows and the vent fill deposits, which are composed of exceptionally well preserved blocky to amoeboid peperites (Figs. 1-17 b and 1-17c). The contact between the pillowed flows and the vent fill deposits is interpreted to be a synvolcanic fault zone. Several jigsaw puzzle-fit, more or less in-situ pillow lava blocks can be observed within the vent fill deposits near this contact. The basalt-andesite tuff comprising the vent fill deposits have been intruded by a series of andesite-dacite dikes that are compositionally identical to the pillowed dikes observed southwest of this location at “Pike Point”. Interactions between the wet, unconsolidated vent fill tuffs and the dikes have produced an absolutely spectacular deposit of amoeboid peperite. The peperite displays several exceptional textures, including: 1) jigsaw puzzle-fit igneous component lapilli and blocks indicative of in-situ fragmentation; 2) local contact metamorphosed zones ranging from a few millimeters to a few centimeters in width immediately adjacent to the lapilli and blocks within the peperite; 3) local chilled margins on the peperite fragments indicating rapid quenching and fragmentation of the igneous component from interactions with relatively cool, wet, unconsolidated vent fill tuffs; 4) complicated amoeboid peperite, especially on the northeastern part of the outcrop; and 5) the presence of 1-2mm diameter, commonly quartz-filled vesicles within the vent fill tuffs. Also note the presence of 1-2 cm wide, several centimeter-long parallel bands containing high abundances (20-50%) of 1-2mm vesicles. These bands are generally more or less parallel with the margins of adjacent igneous component peperite fragments, but locally, these bands show no preferred directional relationship with the margins of nearby clasts. Petrographic observations (Hudak et al., in press; Hudak et al., in prep. b) indicate that these zones are composed of parallel bands of highly vesicular, locally plagioclase-phyric fragments that are compositionally identical to the coherent igneous component clasts in the peperite. The origin of the bands may be related to collapse of thin vapor films along the margin of the hot igneous component as occurs in laboratory simulations of peperite formation (Zimanowski and Buttner, 2002). The parallel bands in this outcrop may have formed when the margins of the igneous component shattered due to explosive interaction with external water as the vapor film collapsed. The parallel bands observed on the outcrop indicate that this process occurred periodically and continually. Such a process is consistent with the pulsating phreatic and phreatomagmatic eruptions that occur in shallow submarine environments. Page 28 28 �DETAILED GEOLOGICAL MAP FIVE MILE LAKE PEPERITE Pillow Basalt - Andesite Pepe.te - Igneous Componenl [j] Pepenle - Sedimentary Component [] Miyg.iuie-ric, banding Figure 1-17. Detailed geological map (A) of the vent facies of a basalt-andesite tuff cone volcano located north of Fivemile Lake. Photographs A and B illustrate amoeboid peperite and parallel, vesicle-rich zones present in the outcrop. Page 29 29 �STOP 1-10: CENTRAL BASALT SEQUENCE SHEET FLOWS, PILLOW LAVAS, AND PERLITIC HYALOCLASTITE, HILLTOP S OF HIGHWAY 169 Location: (SE, SW, S.19, T.62N., R.14W., NAD 83 UTM 56200E, 5297800N) Description: The Central Basalt sequence (Peterson and Patelke, 2003) comprises a steeply northdipping (75°- vertical), north-facing sequence of sparsely amygdaloidal pillowed and massive lava flows of basalt composition that are believed to be correlative with the tholeiitic Armstrong Lake volcanic sequence mapped by Jirsa et al. (2001) in the Eagles Nest quadrangle. Relative to massive and pillowed basalt and andesite flows in the Fivemile Lake sequence, Central Basalt sequence lavas flows are notably less amygdaloidal, and lack multiple pillow rind structures. In addition, the Central Basalt sequence lacks the thick sequences of scoriaceous basalt-andesite lapilli tuffs that are commonly interstratified with lava flows in the Fivemile Lake sequence. These characteristics of the Central Basalt sequence indicate eruption and deposition in a deeper submarine environment than the stratigraphically older Fivemile Lake sequence, and suggest overall increasing water depth during the temporal development of the Lower Ely. This field trip stop (Fig. 1-16) displays exceptional preservation of fine-scale volcanic textures that are characteristic of the Central Basalt sequence. The outcrop comprises two east-southeast striking massive basalt flows, ranging from at least five to nine meters in thickness, that are separated by a ten meter thick flow unit comprising pillows and pillow lobes (Fig. 1-18). Flow 1, at the southern part of the outcrop, is composed of a pale- to dark green, faintly feldspar-phyric (~10% 0.5-1 mm laths), sparsely amygdaloidal, basalt sheet flow that locally exhibits tortoise-shell jointing formed in response to contraction during cooling. The uppermost 10-40 cm of the coherent part of Flow 1 is generally silicified and epidotized. Petrographic observations (Hudak et al., in prep. b) indicate that this section of the flow also contains up to 70% <0.1 cm round spherulites. An irregular contact occurs between the coherent basalt flow and an overlying one- to two meter thick unit of dark green, exceptionally well-preserved perlitic in-situ hyaloclastite and associate self-peperite (c.f. Batiza and White, 2000). The hyaloclastite formed from non-explosive fracturing of the basalt glass developed on the flow top due to quenching by water, whereas the perlite formed following deposition by hydration of volcanic glass. Irregular, amoeboid apophyses of coherent basalt, ranging from 10-30 cm thick and up to 1.5 meters long, locally intrude into the hyaloclastite. These apophyses have pale green cores and dark green, very fine-grained margins which grade outward into in-situ, jigsaw-fit perlitic hyaloclastite. An irregular contact occurs between the hyaloclastite and Flow 2, which is composed of north-facing mattress- to bun-shaped pillow lavas and pillow lobes with numerous “neck and knob” structures. Individual pillows have well developed perlitic hyaloclastite margins that range from 1-4 cm in width. Pillow buds indicate propagation from east to west, suggesting the volcanic vent was located east of this location. A NNE-trending D3 normal fault with a 10 cm displacement occurs in this unit on the southeastern part of the outcrop. The coherent pillows and lobes are overlain by up to 2.5 meters of hyaloclastite breccia that contains 20-40% subround to subangular pale gray green basalt lapilli in a jigsaw puzzle-fit dark green perlitic hyaloclastite matrix. The upper contact of Flow 2 and the overlying basalt sheet flow (Flow 3) is irregular, and is marked by thin (1-8 cm thick), sheet-like basalt fragments that are up to 1.6 meters in length. These fragments locally appear to be isoclinally folded about an east-west-trending fold hinge. Although the genesis of this structure is currently not well understood, it may be due to syneruptive deformation of either thin slabs of hot, basal flow margin crust from the overlying flow, or thin injections of basalt magma into the hyaloclastite from either the underlying pillows or the overlying sheet flow. Flow 3 comprises an at least ten-meter thick pale green-gray, slightly feldspar-phyric, sparsely amygdaloidal sheet flow. Steep, NNE-trending west dipping D3 joints are well developed in this unit, as are lensshaped psuedo-pillows that are up to 50 cm in diameter. Page 30 30 �Figure 1-18. Detailed geological map of sheet flows, pillow lavas, hyaloclastite, and associated “self-peperite” at field trip stop 1-10. Page 31 31 �STOP 1-11: Location: RHYODACITE BLOCK AND ASH FLOW DEPOSITS, NORTH SHORELINE OF NEEDLEBOY LAKE (Optional Stop, NW, SW, S.21, T.62N., R.14W., NAD 83 UTM 564855E, 5298360N) This is an optional stop, as it requires a moderately strenuous 20-30 minute hike along a poorly maintained, easy to trip upon trail. Be extremely careful and watch for protruding boulders and cobbles on the trail. Description: A 50 meter-thick unit of quartz-phyric rhyolite tuff and lapilli tuff crops out in a series of several closely-spaced outcrops along the northwestern shoreline of Needleboy Lake (Fig. 1-16). The pale gray to green-gray matrix contains 1-3% subhedral to angular quartz phenocrysts and crystal shards, 10-15% 1-5 cm subangular to subround pumice lapilli, and locally, up to 15% 2-11 cm subround to subangular quartz-phyric rhyolite lava lapilli and bombs which locally display a crude jigsaw puzzle-fit. Both the pumice and the felsic lava clasts are locally crudely stratified. Pumice lapilli altered by epidote commonly occur as pale green fragments within the deposits. The central part of the outcrop contains a well-developed, east-west trending, steeply north-dipping shear zone. Boudinage locally occur in the rhyolite deposits within this shear zone. A 30-35 meter thick gabbro sill occurs immediately north of this shear zone. This unit is characterized by an equigranular medium gray-green to brownish-green color, with pale green epidote altered feldspar and deep green actinolite present. Petrographic observations (Hudak et al., in prep. a) indicate that the gabbro has a sub-ophitic texture. Locally, steeply-dipping northsouth-trending D3 joints and east-west-trending quartz epidote veins may be observed on the northeastern side of this series of outcrops. A large east-west-trending outcrop occurs approximately 40 meters northwest of the northern-most outcrop near the shoreline. The southernmost 5 meters of the outcrop comprises a mafic lapilli tuff that contains up to 5% subangular silicified mafic lapilli. This tuff is progressively deformed into dark green chlorite schist with a well-developed, east-west-trending, steeply south-dipping S2 foliation. The chlorite schist is in sharp contact with a massive, 3-12 meter-thick quartzand feldspar-phyric rhyodacite sill. The sill is overlain by a mafic lapilli tuff containing up to 25% subangular to subround, locally jigsaw puzzle-fit amygdaloidal basalt and scoria clasts. Felsic volcaniclastic deposits at the base of the outcrop are interpreted to represent a sequence of variably-altered rhyolite block and ash flow deposits. Block and ash flow deposits are unsorted deposits with an ash-matrix containing both monolithologic, low-vesicularity lava lapilli and blocks and pumice lapilli and blocks that may display normal or reverse coarse-tail grading (Cas and Wright, 1987; Freundt et al., 2000). The deposits are formed during the explosive collapse of lava domes, and can occur in either the subaerial or submarine environment (Hudak, 1996; Gibson et al., 1999). These deposits are believed to have been initially overlain by a series of basalt-andesite tuffs and lapilli tuffs; however, a synvolcanic gabbro sill intruded the contact between and subsequently separated these two units. Deformation that is concentrated along the contacts between this sill and the adjacent supracrustal strata is attributed to strain partitioning due to different rheological properties during the D2 deformation event. STOP 1-12: SYNVOLCANIC FAULT ZONE WITH “EPIDOSITES”, SE OF SIXMILE LAKE Location: (NE, SE, S.21, T.62N., R.14W., NAD 83 UTM 565860E, 5298535N) During the hike through the swampy area, be extremely careful to walk on areas with vegetation and tree roots. Description: The next several stops (1-12 through 1-17) represent a north-south section through the uppermost parts of the Lower Ely (Fig. 1-19). Basalt-andesite lava flows in this region vary from massive to pillowed, and are generally less amygdaloidal than the basalt-andesite lava flows in the southern part of the section near Fivemile Lake. The purpose of this stop is to investigate a northeast-trending, Page 32 32 �disconformable zone of intensely quartz + epidote ± actinolite altered pillow lavas (epidosites), and to observe one of the post-volcanic quartz-feldspar porphyry dikes that commonly occur within the volcanic sequence. Figure 1-19. Detailed geological map of the area southeast of Sixmile Lake. Field trip stop locations 1-12 through 1-17 are also illustrated on the map. The first outcrop we will investigate is composed of intensely epidote + quartz-altered basaltandesite pillows and lobes. The pillows may be difficult to see due to the intense synvolcanic hydrothermal alteration, but may be located by identifying actinolite ± chlorite-altered zones of interpillow hyaloclastite, as well as concentrated zones of 0.1-1.0 cm round to oval actinolite ± chloritefilled amygdules which occur within the pillow selvedges. Dark gray veins of magnetite with minor amounts of pyrite, chalcopyrite, and malachite up to 1 cm in width occur primarily within interpillow hyaloclastite zones, but also locally cross-cut the interiors of the pillows and lobes. The patchy pistachiogreen and pink coloration on the outcrop is due to the presence of pistacite epidote (green) and zoisiteclinozoisite (pale pink). Santaguida et al. (2002a, 2002b) indicate that epidote compositions within epidote-quartz altered rocks in the Noranda Volcanic Complex of Quebec are sensitive to small-scale variations in fluid:rock ratios within individual alteration zones due to primary permeability differences in the strata being altered. Hannington et al. (2002) note that within the Proterozoic Kristineberg VMS district of Sweden, zoisite-clinozoisite formation occurs in regions altered at relatively high fluid:rock ratios and high temperatures by acidic hydrothermal fluids. The second outcrop to observe occurs approximately 20 meters to the southwest of the first outcrop, and is located near the top of a prominent east-northeast trending ridge. Compositionally, the rock is identical to the first outcrop. Texturally, the rock is intensely brecciated, containing lapilli- to block-sized subangular fragments of intensely epidote + quartz-altered amygdaloidal basalt. Disconformable zones containing jigsaw-fit epidosite occur proximal to synvolcanic fault zones in the Josephine Ophiolite (Harper, 1999). Based on detailed mapping, the northeast trending, disconformable Page 33 33 �zone of epidosite comprising these outcrops has been interpreted by Hudak et al. (2002b) as being proximal to a synvolcanic fault zone. The third outcrop occurs approximately due north of the second outcrop visited. Exposed at this location is a relatively fresh quartz-feldspar porphyry dike. Similar quartz-feldspar porphyry dikes commonly occur throughout this region of the Vermilion district. Figure 1-20. Photographs of intensely epidote-altered basalt-andesite lava flows from the area southeast of Sixmile Lake. Photo A shows brecciated, intensely epidote–quartz altered pillow lavas. Photo B shows typical relationships between intense epidote alteration, stockwork quartz veins, magnetite stringers, and sulfide staining. STOP 1-13: SYNVOLCANIC FAULT ZONE WITH EPIDOSITES, SE OF SIXMILE LAKE Location: (SE, NE, S.21, T.62N., R.14W., NAD 83 UTM 565850E, 5298560N) Description: Several relatively small outcrops of intensely epidote + quartz ± actinolite ± chloritealtered basalt-andesite pillow lavas occur along the southwestern part of an east-west trending ridge at this location. These rocks are mineralogically similar to those investigated at the previous outcrop; however, despite their intense alteration, they display extremely well preserved primary volcanic textures. The pillows at this location are primarily bun-shaped, and range from approximately 1-1.5 meters in diameter. Interpillow hyaloclastite zones range from <1-2 cm in width, and are pale yellow green in color due to the presence of epidote, actinolite, and chlorite. Pillow selvedges and cores once again vary in color from pale green to pinkish green due to variable percentages of pistacite and clinozoisite/zoisite. The pillow selvedges and cores contain up to 5% 0.1-1.0 cm diameter round to oval quartz ± actinolite ± chlorite-filled amygdules. STOP 1-14: STRINGER COPPER MINERALIZATION IN BASALT–ANDESITE, AND SERICITE-ALTERED RHYODACITE TUFFS, SE OF SIXMILE LAKE Location: (SE, NE, S.21, T.62N., R.14W., NAD 83 UTM 565910E, 5298655N) Description: Another small group of outcrops occurs approximately 50 meters northeast of the previous outcrops investigated (Fig. 1-19). Stratigraphically, we are now positioned within the uppermost 100 to 200 meters of the Lower Ely. At this location, we can observe locally brecciated, strongly epidote + quartz-altered, and locally actinolite- and chlorite-altered basalt-andesite pillow lavas. Pillows at this outcrop are significantly smaller (generally approximately 0.5 meters in diameter) than those observed in the previous outcrop, suggesting that we are positioned near the top of a pillowed flow unit (Dimroth et Page 34 34 �al. (1978) and Staudigal and Schminke (1984) have shown that pillows generally decrease in size near the top of a pillowed unit, and attribute this to change in pillow size due to decreasing effusion rates as an individual eruption comes to an end). The outcrop also locally contains up to 5% dark gray stringer-like veins of magnetite with associated pyrite, chalcopyrite, malachite, and associated deep-red sulfide burn (Fig. 1-21a). These veinlets are commonly parallel to a series of quartz stockwork veins that trend approximately 080° and dip steeply to the north. Both the magnetite-sulfide veins and quartz stockwork veins are consistent with cooling of a high temperature, iron-, base metal-, and silica-rich hydrothermal fluid in the shallow seafloor, and may represent stringer mineralization that occurs immediately downsection from massive sulfide mineralization. Figure 1-21. Mineralization and alteration present at field trip stop 1-14. Photograph A illustrates malachite mineralization associated with magnetite-chalcopyrite veins within altered basalt pillow lavas. B shows sericitealtered felsic tuff deposits located immediately up-section from the altered mafic lavas in photograph A. The pillow lavas are immediately overlain by a 10-20 meter thick sequence of sparsely quartz and plagioclase-phyric rhyodacite tuffs (Fig. 21b). These volcaniclastic deposits typically are brownish-gray in color due to the presence of abundant sericite. Close-up observation of the unit reveals the presence of approximately 1% sub-1mm anhedral to subhedral quartz crystal chips, and 2-3% 1 mm diameter subhedral to euhedral tabular plagioclase feldspar phenocrysts. Rare pumice lapilli can be observed with careful observation; they range from 1-2 cm in diameter, are generally rounded to oval in shape, and contain up to 50% sub-1mm round vesicles (originally carbonate vesicles which have now been weathered-out based on petrographic observations by Hudak et al. (in prep. a)). The pumice lapilli in this outcrop increase in abundance from the base toward the top of the unit. Such inverse grading of pumice may be the result of subaerially erupted air fall deposited into the ocean, as large, cold pumice generally take longer to saturate and sink than small, cold pumice (Whitham and Sparks, 1986). This tuff horizon may represent the distal equivalent to fragmental felsic rocks mapped by Peterson and Patelke (2003) that occur stratigraphically below the Ely Greenstone – Soudan Member and the Soudan Mine. Sericite alteration is common in submarine felsic volcaniclastic rocks (Gibson et al., 1989), and may be associated with either vertically extensive discordant pipe-like alteration zones associated with hydrothermal upflow zones, or laterally extensive semiconformable alteration zones formed by extensive shallow seafloor seawater metasomatism of permeable submarine strata. More detailed mapping will be necessary to determine with certainty the geometry of the sericite alteration zone observed in this outcrop. The exposures of felsic tuff dive abruptly to the north into a swamp. Hand auger boring in this swamp (NAD 83 UTM location 0565938E, 5298671N) yielded dark black, organic-rich soil containing abundant, angular, variably weathered clasts of massive pyrite. Stringer copper mineralization, sericitealtered felsic tuffs, the presence of massive sulfide in the subsurface beneath the swamp, and the presence Page 35 35 �of rare sulfide lapilli in volcaniclastic deposits north of the swamp (seen in the next outcrop) suggests that the swamp may be underlain by a massive sulfide horizon (Hudak et al., 2002b). STOP 1-15: CHERT, BANDED IRON-FORMATION AND SEDIMENTARY STRATA, SE OF SIXMILE LAKE ASSOCIATED CLASTIC Location: (SE, NE, S.21, T.62 N., R.14W., NAD 83 UTM 565840E, 5298760N) Be extremely careful when walking on these outcrop surfaces and descending into the small valley immediately south of these outcrops, as they are commonly very slippery. Description: North of the swamp, a southeast-trending outcrop ridge comprises a sequence of volcaniclastic and chemical sedimentary strata (Fig. 1-19). The purpose of this stop is to observe the variety of interflow sedimentary rocks that occur in this upper part of the Lower Ely, to observe wellpreserved sedimentary textures, and to see further evidence for the presence of ancient hydrothermal activity southeast of Sixmile Lake. A series of small outcrops occurring along the northwestern part of the outcrop ridge comprises west-northwest striking, steeply north dipping 10-30 cm thick chemical exhalites (chert horizons and banded magnetite + chert ± jasper banded iron-formation horizons) are that are interbedded with 10-60 cm thick, well-stratified horizons of mudstones, wackes, and matrix-supported polymict diamictites. The base of the stratigraphic sequence occurs at the southern end of the outcrop, where a 20 cm thick magnetite-rich horizon of banded iron-formation occurs (Fig. 1-22a). This chemical sedimentary horizon is overlain by a several meter-thick sequence of interbedded, laminated to bedded, commonly normally graded, mudstones and polymict diamictites interpreted to have formed from submarine debris flows and turbidity currents. The polymict diamictites range from 10-60 cm thick, and contain up to 5% 1-2.5 cm lens-shaped chert clasts, 10-40% silicified or chlorite-altered mafic volcanic lapilli, and rare (<1%) <12cm sulfide-rich fragments. Locally, clasts within the diamictites are vaguely imbricated. Interbedded mudstone horizons are up to 40 cm thick, and locally display cross beds that indicate stratigraphic topping direction is north. Iron-formation horizons at this location indicate that low temperature exhalative hydrothermal activity was occurring in this part of the Lower Ely. It is interesting to note that these iron-formation horizons occur in the area that would be immediately up-section from the possible sulfide horizon within the swamp to the south. Several VMS horizons in similar volcanic sequences within Canada are capped by iron-formations (Franklin et al., 1981). Debris flows and/or turbidity currents that deposited the polymict diamictites and associated mudstones periodically interrupted periods of relatively lowtemperature hydrothermal activity that formed the iron-formation horizons. Clasts within the diamictites are similar to the volcanic and chemical sedimentary strata that occur in the immediate vicinity of the deposits based on detailed mapping (Hudak et al., in prep. a). Chert clasts within the diamictites were likely derived from the iron-formation horizons. Rare sulfide-rich fragments locally contained within the diamictites also suggest that VMS mineralization occurred on or near the paleoseafloor in the vicinity of the area that is now Sixmile Lake, supporting the interpretation that a massive sulfide horizon may be present beneath the swamp which separates the outcrops at this stop from those at the previous stop. Page 36 36 �_J\ \.a' Figure 1-22. Sedimentary strata in the uppermost parts of the Lower Ely. Figure 1-22a shows oxide-facies banded iron-formation and chert exhalites located up-section from altered tuffs seen at field trip stop 1-15. Figure 1-22b shows diamictites that contain abundant chert clasts and rare sulfide clasts. These deposits are also located stratigraphically up-section from the felsic tuffs seen at the previous stop. STOP 1-16: EPIDOTE-QUARTZ ALTERATION ON MARGIN OF DIABASE/GABBRO SILL, SE OF SIXMILE LAKE Location: (SE, NE, S.21, T.62N., R.14., NAD 83 UTM 565960E, 5298790N) The outcrop at this stop occurs on the Sixmile Lake Road. Be attentive for automobiles, trucks, and four-wheelers when observing this outcrop. Description: This relatively small (3 meters by 5 meters) outcrop exposes the northern margin of the east-west-trending gabbro-diorite sill that extends from at least southwest of Sixmile Lake to at least 500 meters east of this location (Fig. 1-19; Hudak et al., in prep.). The southern two-thirds of the outcrop comprises pale green to pinkish green, epidote- and clinozoisite/zoisite-altered subophitic gabbro which contains up to 1% 1-5mm quartz- and/or actinolite-filled amygdules. Numerous north-northeast, steeply west-dipping joints were developed during the regional D3 deformation. The uppermost one-third of the outcrop contains an in-situ hyaloclastite breccia comprising pale-to dark green angular, locally jigsaw puzzle-fit, iron chlorite + actinolite + zoisite/clinozoisite-altered gabbro lapilli in a pinkish-tan to tan matrix of sericite, epidote, and zoisite/clinozoisite (Fig. 1-23a). This outcrop illustrates, in part, the difficulty in determining hydrothermally altered flow top breccias from hyaloclastite breccias and/or peperites that form from intrusion of sills into wet, relatively unconsolidated sediments that occur within the shallow seafloor. The intense hydrothermal alteration that occurs at the northern margin of the outcrop indicates that high water:rock ratio, high temperature (>300°C) hydrothermal alteration was focused in the permeable, sill-marginal, hyaloclastite or peperite (?) breccia. Synvolcanic sills intruded into the shallow seafloor are important heat sources that drive hydrothermal circulation cells that form VMS deposits (Campbell et al., 1981). Sills such as the one exposed at this outcrop may have contributed significantly to the formation of Algoma-type banded ironformation within the overlying Soudan Iron Formation, and perhaps, VMS mineralization within the uppermost parts of the Lower Ely or the Soudan Iron Formation. Page 37 37 �Figure 1-23. Synvolcanic sills and banded iron-formations in the uppermost parts of the Lower Ely. Figure 1-23a shows the uppermost margin of a gabbro/diorite sill that has apparently intruded wet, unconsolidated sediments to form an extremely altered hyaloclastite or sill-marginal peperite. Figure 1-23b shows folded banded iron-formation horizons that occur approximately 10 meters up-section from the sill. STOP 1-17: BANDED IRON-FORMATION, NORTH OF SIXMILE LAKE ROAD Location: (SE, NE, S.21, T.62N., R.14W., NAD 83 UTM 565953E, 5298823N) Description: Two outcrops located within 30 meters north of the Sixmile Lake Road contain relatively thin (<1-2 meters) magnetite-chert horizons interbedded with mafic to intermediate composition volcaniclastic sedimentary strata, and illustrate features common to the uppermost parts of the Lower Ely. Based on detailed mapping by Peterson and Patelke (2003) and Hudak et al. (in prep. a), the uppermost 50-200 meters of the Lower Ely is transitional into the base of the Soudan Iron Formation. Basalt – andesite lava flows and volcaniclastic sedimentary strata decrease in abundance, whereas Algoma-type banded iron-formations become more common as one progresses up-section through this transition zone. This sequence records the change from a period of dominantly effusive submarine volcanism to a period marked by chemical sedimentation. The outcrop closest to the road contains strongly deformed magnetite-chert iron-formation interbedded with iron-chlorite-rich metasediments. Microfossils are locally present in the Vermilion district iron-formations, and hint at a biogenic contribution to their formation (LaBerge, 1973). Deformation has produced more or less north-northwest striking, steeply dipping beds. The deformation that occurs in these strata is not regionally observed, and is believed to be a local phenomenon resulting from syndepositional intrusion of the mafic sill observed in the previous stop. A later quartz-feldspar porphyry dike occurs at the northeastern end of this small outcrop. Note the localized sulfide staining along the contact between the intrusion and adjacent banded iron-formation. The second outcrop, located approximately 30 meters north of the road, contains poorly exposed mafic – intermediate volcaniclastic strata and associated banded iron-formation horizons. The best exposures of the banded iron-formations occur at the southeastern edge of the outcrop. At this location, the magnetite-chert iron-formations locally contain veins and stringers of malachite, along with up to 20% pale brown garnet (x-ray analysis performed by Hudak et al. (in prep. a) indicate that the garnet is andradite). The presence of malachite is indicative of the local high concentrations of copper within the uppermost parts of the Lower Ely. Similar copper mineralization within the Soudan Iron Formation has been identified by Peterson and Patelke (2003) in the 12th level of the Soudan Mine. Page 38 38 �REFERENCES Barrett, T. J. and MacLean, W. H., 1994, Chemostratigraphy and hydrothermal alteration in exploration for VHMS deposits in greenstones and younger volcanic rocks: in Lentz, D. R. (ed.), Alteration and alteration processes associated with ore-forming systems: Geological Association of Canada Short Course Notes, v. 11, p. 433-467. Barrett, T. J., and MacLean, W. H., 1999, Volcanic sequences, lithogeochemisty, and hydrothermal alteraton in some bimodal volcanic-associated massive sulfide systems: Reviews in Economic Geology, v. 8, p. 101-131. Barrie, C. T., Ludden, J. N., and Green, T. H., 1993, Geochemistry of volcanic rocks associated with CuZn and Ni-Cu deposits in the Abitibi subprovince: Economic Geology, v. 88, p. 1341-1358. Batiza, R., and White, J. D. L., 2000, Submarine lavas and hyaloclastite: in Sigurdsson, H., Encyclopedia of Volcanoes: Academic Press, San Diego, CA, p. 361-381. Bauer, R. L., 1985, Correlation of early recumbent and youger upright folding across the boundary between an Archean gneiss belt and greenstone terrane, northeastern Minnesota: Geology, v. 13, p. 657-660. Boerboom, T. J., and Zartman, R. E., 1993, Geology, geochemistry, and geochronology of the central Giants Range batholith, northeastern Minnesota: Canadian Journal of Earth Science, v. 30, p. 2510-2522. Campbell, I. H., Franklin, J. M., Gorton, M. P., Hart, T. R., and Scott, S. D., 1981, The role of synvolcanic sills in the generation of massive sulfide deposits: Economic Geology, v. 87, p. 511541. Cas, R. A. F., and Wright, J. V., 1987, Volcanic Successions: London, Allen and Unwin, 528 p. Corfu, F. and Stott, G. M., 1998, Shebandowan greenstone belt, western Superior Province: U-Pb ages, tectonic implications, and correlations: Geological Society of America Bulletin, v. 110, p. 14671484. Dimroth, E., Cousineau, P., Leduc, M., and Sanschagrin, Y., 1978, Structure and organization of Archean subaqueous basalt flows, Rouyn-Noranda area, Quebec, Canada: Canadian Journal of Earth Science, v. 15, p. 902-918. Drexler, H., and Hudak, G. J., and Peterson, in press, A field and laboratory study to evaluate the genetic relationships between the Purvis pluton and volcanic rocks and volcanic-associated mineralization in the Vermilion district of NE Minnesota: 50th Annual Meeting, Institute on Lake Superior Geology, Proceedings and Abstract Volume 50. Franklin, J. M., 1986, Volcanic associated massive sulfide deposits – an update: in Andrew, C. J., Crowe, R. W. A., Finlay, S., Pennell, W. M., and Pyne, J. F. (eds.), Geology and Genesis of Mineral Deposits in Ireland, Irish Association for Economic Geology, p. 49-69. Franklin, J. M., 1993, Volcanic-associated massive sulfide deposits: in Kirkham, R. V., Sinclair, W. D., and Thorpe, R. I. (eds.), Geology of Canadian Mineral Deposit Types: Geological Survey of Canada, no. 8., p. 158-183. Franklin, J. M., 1996, Volcanic-associated massive sulfide base metals; in Eckstrand, O. R., Sinclair, W. D., and Thorpe, R. I. (eds.), Geology of Canadian Mineral Deposit Types; Geological Survey of Canada, no. 8, p. 158-183. Page 39 39 �Franklin, J. M., Hannington, M. D., Jonasson, I. R., and Barrie, C. T., 1998, Arc-related volcanogenic massive sulfide deposits, in Metllogeny of Volcanic Arcs: British Columbia Geological Survey, Short Course Notes, Open File 1998-8, Section N. Franklin, J. M., Sangster, D. M., and Lydon, J. W., 1981, Volcanic associated massive sulfide deposits: Economic Geology 75th Anniversary Volume, p. 485-627. Freundt, A., Wilson, C. J. N., and Carey, S. N., 2000, Ignimbrites and block and ash flow deposits: in Sigurdsson, H., Encyclopedia of Volcanoes: Academic Press, San Diego, CA, p. 581-599. Fyon, J. A., Breaks, F. W., Heather, K. B., Jackson, S. L., Muir, T. L., Stott, G. M., and Thurston, P. W., 1992, Metallogeny of metallic mineral deposits in the Superior Province of Ontario: in Thurston, P. C., Williams, H. R., Sutcliffe, R. H., and Stott, G. M. (eds.), Geology of Ontario, Special Volume 4, Part 2, p. 1091-1176. Galley, A., 2002, Characteristics of composite subvolcanic intrusive complexes associated with Precambrian VMS districts: in Galley, A., Bailes, A., Hannington, M., Holk, G., Katsube, J., Parquette, F., Paradis, S., Santaguida, F., and Taylor, B., 2002, CAMIRO Project 94E07: Interrelationships between subvolcanic intrusions, large-scale alteration zones, and VMS deposits: Geologic Survey of Canada Open-File Report 94E07, p. 1-40. Galley, A., 2003, Composite synvolcanic intrusions associated with Precambrian VMS-related hydrothermal systems: Mineralium Deposita, v. 38, p. 443-473. Gibson, H. L., 1989, The Mine Sequence of the Central Noranda Volcanic Complex: Geology, Alteration, Massive Sulfide Deposits, and Volcanological Reconstruction: unpublished Ph. D. dissertation, Carleton University, Ottawa, Ontario, Canada, 800 p. Gibson, H. L., Morton, R. L., Hudak, G. J., 1999. Submarine volcanic processes, deposits and environments favorable for the location of volcanic-associated massive sulphide deposits: Reviews in Economic Geology, v 8, p. 13-51. Hannington, M. D., Kjarsgaard, I. M., Santaguida, F., and Cathles, L. M., 2002, Mineral-chemical studies of regional-scale hydrothermal alteration in the Central Blake River Group, Western Abitibi Subprovince: Part 1. Summary of results from the Noranda Volcanic Complex: Geologic Survey of Canada Open-File Report 94E07, p. 47-76. Hannington, M. D., Poulsen, K. H., Thompson, J. F. H., and Sillitoe, R. H., 1999, Volcnaogenic gold in the massive sulfide environment: Reviews in Economic Geology, v. 8, p. 325-356. Harper, G. D., 1999, Structural styles of hydrothermal discharge in ophiolite / sea-floor systems: Reviews in Economic Geology, v. 8, p. 53-73. Harris, N. B. W., Pearce, J. A., and Tindle, A. G., 1986, Geochemical characteristics of collision-zone magmatism: in Coward, M. P., and Reis, A. C. (eds.), Collision Tectonics, Special Publication of the Geological Society 19, p. 67-81. Hocker, S. M., Hudak, G. J., and Heine, J., 2003, Electron microprobe analysis of alteration mineralogy at the Archean Fivemile Lake volcanic-associated massive sulfide mineral prospect in the Vermilion district of northeastern Minnesota: Natural Resources Research Institute Report of Investigations NRRI/RI-2003/17, 49 p. Hooper, P., and Ojakangas, R., 1971, Multiple deformation in the Vermilion district, Minnesota: Canadian Journal of Earth Sciences, v. 8, p. 423-434. Page 40 40 �Hovis, S. T., 2001, Physical volcanology and hydrothermal alteration of the Archean volcanic rocks at the Eagles Nest volcanogenic massive sulfide prospect, northern Minnesota: unpublished M. S. thesis, University of Minnesota – Duluth, Duluth, Minnesota, 137 p. Hudak, G. J., 1996, The physical volcanology and hydrothermal alteration associated with late caldera volcanic and volcaniclastic rocks and volcanogenic massive sulfide deposits in the Sturgeon Lake region of northwestern Ontario, Canada: unpublished Ph. D. dissertation, University of Minnesota, Minneapolis, Minnesota, 463 p. Hudak, G. J., Heine, J., Newkirk, T., Odette, J., and Hauck, S., 2002a, Comparative geology, stratigraphy, and lithogeochemistry of the Fivemile Lake, Quartz Hill, and Skeleton Lake VMS occurrences, Vermilion district, NE Minnesota: A report to the Minerals Coordinating Committee, DNR, Minerals Division, State of Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2002/03, 390 pages. Hudak, G. J., Heine, J., Hocker, S. M., and Hauck, 2002b, Geological mapping of the Needleboy Lake – Sixmile Lake area, northeastern Minnesota: a summary of volcanogenic massive sulfide potential: Natural Resources Research Institute Report of Investigation NRRI/RI-2002/14, 15 p. Hudak, G. J., Heine, J., Newkirk, T. T., and Hocker, S., in prep. a, Comparative geology, stratigraphy, and lithogeochemistry of the Needleboy Lake to Sixmile Lake area, Vermilion district, NE Minnesota: Natural Resources Research Institute Technical Report. Hudak, G. J., and Morton, R. L., 1999, Mineral Potential Study, Minnesota Department of Natural Resources Project 326, Bedrock and Glacial Drift Mapping for VMS and Lode Gold Alteration in the Vermilion – Big Fork Greenstone Belt, Part A: Discussion of Lithology, Alteration, and Geochemistry at the Fivemile Lake, Eagles Nest, and Quartz Hill Prospects: Minnesota Department of Natural Resources Division of Minerals Project 326 Report, 136 p. Hudak, G. J., Morton, R. L., Franklin, J. M., and Peterson, D. M., 2003, Morphology, distribution, and estimated eruption volumes for intracaldera tuffs associated with volcanic-hosted massive sulfide deposits in the Archean Sturgeon Lake Caldera Complex, NW Ontario: American Geophysical Union Geophysical Monograph 40, Explosive Subaqueous Volcanism, p. 345-360. Hudak, G. J., Newkirk, T. T., Drexler, H., Odette, J., and Hocker, S. M., in press, Neoarchean peperites in the vicinity of Fivemile Lake, Vermilion district, NE Minnesota: 50th Annual Meeting, Institute on Lake Superior Geology, Proceedings and Abstract Volume 50. Hudak, G. J., Newkirk, T. T., Drexler, H., Hocker, S. M., Peterson, D. M., and Heine, J., in prep. b, Neoarchean peperites in the Fivemile Lake area of the Vermilion district, NE Minnesota. Hudleston, P.J., 1976, Early deformational history of Archean rocks in the Vermilion district, northeastern Minnesota: Canadian Journal of Earth Sciences, v. 13, p. 579-592. Hudleston, P. J., Schultz-Ela, D., and Southwick, D. L., 1988, Transpression in an Archean greenstone belt, northern Minnesota: Canadian Journal of Earth Sciences, v. 25, p. 10601068. Jirsa, M. A., 2000. The Midway sequence: a Timiskaming-type pull-apart basin deposit in the western Wawa subprovince, Minnesota: Canadian Journal of Earth Sciences, v. 37, p. 1-15. Jirsa, M. A., Boerboom, T. J., and Peterson, D. M., 2001, Bedrock geological map of the Eagles Nest Quadrangle, St. Louis County, Minnesota: Minnesota Geological Survey, Miscellaneous Map M-114, scale 1:24,000. Page 41 41 �Jirsa, M.A., Southwick, D.L., and Boerboom, T.J., 1992, Structural evolution of Archean rocks in the western Wawa subprovince, Minnesota: refolding of precleavage nappes during D2 transpression: Canadian Journal of Earth Sciences, v. 29, p. 2146-2155. Kawachi, Y., and Pringle, I. J., 1988, Multiple-rind structure in pillow lava as an indicator of shallow water: Bulletin of Volcanology, v. 50, p. 161-168. LaBerge, G. L., 1973, Possible biological origin of Precambrian iron-formations: Economic Geology, v. 68, p. 1098-1109. Lawler, T., and Riihiluoma, D., 1997, Minralized clast study, greenstone belt boulder tracing, Ely-Bigfork area, northern Minnesota, Township 60-65 north, Range 11-17 west: Minnesota Department of Natural Resources Division of Minerals Report 318, 41 p. Lundy, J.R., 1985, Clues to structural history in the minor folds of the Soudan Iron Formation, northeastern Minnesota: Unpublished M.S. thesis, University of Minnesota, Minneapolis, 144p. McPhie, J., Doyle, M., and Allen, R., 1993, Volcanic Textures – a guide to the interpretation of textures in volcanic rocks: Centre for Ore Deposit and Exploration Studies, University of Tasmania, 198 p. McPhie, J., and Orth, K., 1999, Peperite, pumice, and perlite in submarine volcanic successions: implications for VHMS minralisation: Proceedings of PACRIM’99, Bali, Indonesia, p. 643-648. Morey, G. B., Green, J. C., Ojankangas, R. W., and Sims, P. K., 1970, Stratigraphy of the lower Precambrian rocks in the Vermilion district, northeast Minnesota: Minnesota Geological Survey, Report of Investigations 14, 33 p. Morton, R. L., and Franklin, J. M., 1987, Two-fold classification of Archean volcanic-associated massive sulphide deposits: Economic Geology, v.82, p. 1057-1063. Newkirk, T., Hudak, G. J., and Hauck, S. A., 2001a, Preliminary lava flow morphology studies at the Fivemile Lake VMS prospect, Vermilion district, NE Minnesota: Implications for volcanic processes, volcanic paleoenvironments, and VMS exploration: Institute on Lake Superior Geology, 47th Annual Meeting, Proceedings Volume 47, Part 1- Program and Abstracts, p. 69-70. Newkirk, T., Hudak, G. J., and Hauck, S. A., 2001b, Preliminary lava flow morphology studies at the Fivemile Lake VMS prospect, Vermilion district, NE Minnesota: Implications for volcanic processes, volcanic paleoenvironments, and VMS exploration: Geological Society of America Abstracts and Programs Volume 33, No. 6, p. A-398. Odette, J. D., Hudak, G. J., Suszek, T., and Hauck, S. A., 2001a, Preliminary evaluation of hydrothermal alteration mineral assemblages and their relationship to VMS-style mineralization in the Fivemile Lake area of the Archean Vermilion Greenstone Belt, NE Minnesota: Institute on Lake Superior Geology, 47th Annual Meeting, Proceedings Volume 47, Part 1-Program and Abstracts, p. 75-76. Odette, J. D., Hudak, G. J., Suszek, T., and Hauck, S. A., 2001b, Preliminary evaluation of hydrothermal alteration mineral assemblages and their relationship to VMS-style mineralization in the Fivemile Lake area of the Archean Vermilion Greenstone Belt, NE Minnesota: Geological Society of America Abstracts and Programs Volume 33, No. 6, p. A-420. Pearce, J. A., Harris, N. B. W., and Tindle, A. G., 1984, Trace element discrimination diagrams for the tectonic interpretation of granitic rocks: Journal of Petrology, v. 25, p. 956-983. Page 42 42 �Peterson, D. M., 2001, Development of Archean lode-gold and massive sulfide deposit exploration models using geographic information system applications: targeting mineral exploration in northeastern Minnesota from analysis of analog Canadian mining camps: unpublished Ph. D. dissertation, University of Minnesota, Duluth, Minnesota, 503 p. Peterson, D. M., Gallup, C., Jirsa, M. A., and Davis, D. W., 2001, Correlation of Archean assemblages across the U.S.- Canadian border: Phase I geochronology: 47th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 47, Part 1 – Programs and Abstracts, p. 77-78. Peterson, D. M., and Jirsa, M. A., 1999a, Bedrock Geological Map and Mineral Exploration Data, Western Vermilion district, St. Louis and Lake Counties, Northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-98, scale 1:48,000. Peterson, D. M., and Jirsa, M. A., 1999b, Lode gold and massive sulfide prospects in the Archean western Vermilion district: Minnesota Exploration Association, Minnesota Exploration Conference, Field Trip Guidebook, 10 maps, 30 p. Peterson, D. M., and Patelke, R. L., 2003, National Underground Science and Engineering Laboratory (NUSEL): Geological site investigation for the Soudan Mine, Northeastern Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2003/29, 88 p. Peterson, D. M., and Patelke, R. 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L., (compiler), 1993, Bedrock geologic map of the Soudan-Bigfork area, northern Minnesota: Minnesota Geological Survey, Miscellaneous Map M-79, scale 1:100,000. Southwick, D. L., Boerboom, T. J., and Jirsa, M. A., 1998, Geologic setting and descriptive geochemistry of Archean supracrustal and hypabyssal rocks, Soudan-Bigfork area, northern Minnesota: implications for metallic mineral exploration: Minnesota Geological Survey, Report of Investigations 51, 69 p. Staudigal, H., and Schminke, H.U.-, 1984, The Pliocene seamount series of La Palma, Canary Islands: Journal of Geophysical Research, v. 89, p. 11195-11215. Whitham, A. G., and Sparks, R. S. J., 1986, Pumice: Bulletin of Volcanology, v.48 , p. 209-223. Williams, H. R., Stott, G. M., Heather, K. B., Muir, T. L., and Sage, R. P., 1991, Wawa Subprovince: in Thurston, P. C., Williams, H. R., Sutcliffe, R. H., and Stott, G. M. (eds.), Geology of Ontario, Special Volume 4, Part 1, p. 485-542. Winchester, J. A., and Floyd, P. A., 1976, Geochemical magma type discrimination; application to altered and metamorphosed basic igneous rocks: Earth and Planetary Science Letters, v. 28, p. 459-469. Winchester, J. A., and Floyd, P. A., 1977, Geochemical discrimination of different magma series and the differentiation products using immobile elements: Chemical Geology, v. 20, p. 325-343. Wood, D. A., 1980, The application of a Th-Hf-Ta diagram to problems of tectonomagmatic classification and to establishing the nature of crustal contamination of basaltic lavas of the British Tertiary Volcanic Province: Earth and Planetary Science Letters, v. 50, p. 11-30. Zimanowski, B., and Buttner, R., 2002, Dynamic mingling of magma and liquefied sediments: Journal of Volcanology and Geothermal Research, v. 114, p. 37-44. Page 44 44 �FIELD TRIP 2 Geologic Highlights of New Mapping in the Southwestern Sequence of the North Shore Volcanic Group and in the Beaver Bay Complex Leaders: Terry Boerboom, Jim Miller Minnesota Geological Survey, University of Minnesota and John Green Department of Geological Sciences, University of Minnesota Duluth Split Rock Lighthouse Point (Stop 2-8) Page 45 �FIELD TRIP 2 Geologic Highlights of New Mapping in the Southwestern Sequence of the North Shore Volcanic Group and in the Beaver Bay Complex by Terry Boerboom, Jim Miller Minnesota Geological Survey, University of Minnesota and John Green Department of Geological Sciences, University of Minnesota Duluth INTRODUCTION For the past 20 years, the Minnesota Geological Survey has been actively conducting detailed (1:24,000-scale) bedrock geologic mapping in northeastern Minnesota. This effort began in 1985 with a mapping project focussed on delineating the geology of the intrusive Beaver Bay Complex in the central part of the region. This eight-year project was partially supported by USGS's COGEOMAP program and resulted in publication of five geologic maps (Miller, 1988; Miller and others, 1989, 1993, 1994; Boerboom and Miller, 1994) covering ten 7.5' quadrangles (Fig. 1). More recently, geologic mapping in northeastern Minnesota has focused on the shoreline quadrangles between Duluth and Split Rock Point, where it joins up with the Beaver Bay mapping. This ongoing mapping, which began in 2001, has produced four geologic maps (Boerboom and others, 2002a, 2002b, 2003a, 2003b) with another currently in production (Fig. 2-1). Mapping in the Two Harbors NE quadrangle, scheduled for this coming field season, and publication of quadrangles of the Duluth metropolitan area later this year, will result in 1:24,000-scale coverage of mappable bedrock along the North Shore from Duluth to Tofte. Long-range plans are to continue to systematically map the shore to the Canadian border. Figure 2-1. North Shore 7.5' quadrangles mapped by the MGS through the USGS-sponsored COGEOMAP and STATEMAP programs. Page 46 �This field trip is intended to highlight some of the characteristic and more intriguing aspects of North Shore geology that the recent detailed mapping has revealed. Of course, there are many more interesting geologic features than we can show in two days. Hopefully, you will take advantage of the geologic maps that have been produced of this area to explore more of its fascinating geology. VOLCANIC AND SEDIMENTARY ROCKS OF THE SOUTHWESTERN SEQUENCE OF THE NORTH SHORE VOLCANIC GROUP by John Green This overview is a slightly modified expert from the MGS Report of Investigations 58, Chapter 5: "Volcanic and sedimentary rocks of the Keweenawan Supergroup in Northeastern Minnesota" by John Green (Miller and others, 2002). Magmatic activity related to the 1.1 Ga Midcontinent Rift produced a more than 10 kilometer thick edifice of lava flows and subvolcanic intrusions that are exposed along Minnesota's north shore of Lake Superior. The lava flows and minor sedimentary rocks are referred to as the North Shore Volcanic Group (Goldich and others, 1961) and the intrusive rocks are variably assigned to the Duluth Complex, the Beaver Bay Complex, and miscellaneous intrusions of the Midcontinent Rift Intrusive Supersuite (Fig. 2-3). Although previous publications have subdivided the North Shore Volcanic Group into informal volcanic suites and distinctive flows (Green, 1972, 1982), the 1:200,000-scale map (M-119, Miller and others, 2001) accompanying the report of Miller and others (2002) represents the first time that the North Shore Volcanic Group has been subdivided into formational entities on a geologic map. A brief description of the North Shore Volcanic Group and associated sedimentary formations of the Keweenawan Supergroup is given here to supplement the information provided on the geologic map about these formational units. Rock classification, recognition, and textures As a coherent tholeiitic compositional suite, the volcanic rocks of the North Shore Volcanic Group can be described using only a few rock names (Fig. 2-2). The most primitive rocks are olivine tholeiites, which form an iron-enrichment trend with further evolution. They display ophitic textures and pahoehoe surfaces nearly everywhere. Most olivine tholeiites are aphyric, but those that are porphyritic contain dominantly plagioclase phenocrysts, less commonly olivine. Transitional basalts contain somewhat higher alkalies and other incompatible elements than the olivine tholeiites, but generally not enough to classify them as alkalic. Their texture is typically intergranular and fine- to medium-grained. Porphyritic varieties generally contain small phenocrysts of plagioclase, olivine, clinopyroxene, and magnetite. The reversed-polarity Hovland lavas, however, are characterized by transitional basalts (grading to basaltic andesites) that contain abundant, large, tabular, plagioclase phenocrysts. Transitional basalt flow surfaces are generally smooth (pahoehoe), although a few show breccia tops. The basaltic andesites and andesites (greater than 52 percent SiO2) are tholeiitic, rather than calcalkaline; they show iron enrichment and contain only anhydrous ferromagnesian minerals. These rocks are typically fine-grained and intergranular to felty or pilotaxitic, and many contain small phenocrysts of plagioclase, olivine, clinopyroxene, and magnetite. The andesites generally weather to a red-brown color, and have flow-brecciated (aa) tops but not bases. Many flows that contain 50 to 55 percent silica show millimeter-scale oxidation lamination (Green, 1989) parallel to the base. A few highly iron-enriched flows, separable only by chemical analysis, can be called ferroandesites. Page 47 �Figure 2-2. AFM compositional diagram for the lavas of the North Shore Volcanic Group (modified from Green, 1982). The boundary between tholeiitic and calc-alkalic rocks is modified from Irvine and Barager (1971). FeO* = FeO + 0.9Fe2O3. Carmichael (1964) first used the name "icelandite" for rocks intermediate in character between andesites and rhyolites in the Tertiary lavas of eastern Iceland. They might be considered the tholeiitic equivalent of calc-alkaline dacite in orogenic suites. Other examples of these rocks have been described from the Galapagos (McBirney and Williams, 1969) and the Miocene of Nevada–Oregon (Wallace and others, 1980). Very large flows of similar composition in the Etendeka volcanics of Namibia have been referred to as quartz latites by Milner and others (1992). Icelandites in the North Shore Volcanic Group (Green and Fitz, 1993) are characterized chemically by SiO2 contents ranging from 60 to 68 percent, high FeO* (averaging 7 percent), K2O + Na2O values between 6.5 and 9 percent, a potassium/sodium atomic ratio of about 0.9, and an Mg number [Mg/(Mg + Fe) atomic] averaging 0.14. Petrographically, North Shore Volcanic Group icelandites grade continuously from the andesites to somewhat paler colors (brown or tan), but have a similar phenocryst assemblage. Quartz and alkali feldspar are common in the groundmass but never occur as phenocrysts. Flowtop features (crusty to coarsely brecciated) indicate that the icelandites were erupted as lavas. The rhyolites have higher silica and total alkali contents, and lower FeO than the icelandites. They are generally light gray, pink, or red. Several are very thick, extensive, and voluminous (up to several hundred cubic kilometers; Green and Fitz, 1993). Although most rhyolites are porphyritic (phenocrysts of quartz, alkali feldspar ± plagioclase, and altered fayalite ± ferroaugite), some lack quartz and alkali feldspar phenocrysts, and rare flows are aphyric. Groundmass textures are fine-grained holocrystalline, typically with a meshwork of platy quartz paramorphs after primary tridymite, which may show a flow structure. A "snowflake" texture is common, in which poikilitic quartz patches (coalesced ex-tridymite grains) enclose small, dusty alkali-feldspar grains. In quartz-phyric flows, these poikilitic quartz patches are in optical continuity with adjacent quartz phenocrysts (Green, 1990). Outcrop-scale flow structure, including folding, is common near flow tops and bases. In some flows, distinct fiamme, deformed to varying degrees, are recognizable. These imply an explosive eruption that produced a pyroclastic flow, which welded and remobilized to produce a rheoignimbrite. Page 48 �Structure and Lithostratigraphy As stated above, the Midcontinent Rift volcanic rocks and interbedded redbeds in northeastern Minnesota comprise the North Shore Volcanic Group (Goldich and others, 1961; Green, 1972, 1977, 1982; Basaltic Volcanism Study Project, 1981). In general, these rocks form an arcuate stack that is slightly tilted toward the southeast and forms the roof rocks into and under which the Duluth Complex and associated hypabyssal intrusions were emplaced. At the southwest end of the North Shore Volcanic Group near Duluth, the volcanic rocks strike north with a 10º to 20º easterly dip; at the northeast end at Grand Portage, the flows strike east–west with a 10º southerly dip (Fig. 2-3). Thus, traveling northeast along the Lake Superior shore northeast from Duluth, and southwest from Grand Portage, one encounters successively higher flows in the volcanic stratigraphy until the Tofte–Lutsen area in southern Cook County, where the highest exposed flows crop out. Exposure is generally excellent along the eroding lakeshore, and along the lower, high-gradient stretches of tributary streams, providing good control on the stratigraphy. A stack of volcanic rocks approximately 9.7-kilometers-thick has been measured in the “southwest limb” (Table 2.1), and another stack of volcanic rocks about 7.2-kilometers-thick has been measured in the “northeast limb” (See Table 5.2, Miller and others, 2002). This implies nearly continuous subsidence during the rifting process. The difference in stratigraphic thickness between the two limbs reflects major complications in the central area, which appears not to have subsided at the same rate as in the limbs, and into which many of the subvolcanic intrusions were emplaced. Except for the capping Schroeder–Lutsen basalt sequence, no stratigraphic unit can be traced from one limb to the other; each limb has its own stratigraphic column. To aid in the correlation of intrusive and extrusive rock units throughout the Midcontinent Rift system, their paleomagnetic polarity has been used. Nearly all of the igneous and sedimentary rocks associated with the Midcontinent Rift were formed either during an earlier, reversed-polarity interval or a succeeding normal-polarity interval. Thus, in each limb of the North Shore Volcanic Group, the lower stratigraphic units show reversed polarity, and the upper sequences show normal polarity. This polarity reversal forms the basis for distinguishing upper and lower sequences in the northeast and southwest limbs. U-Pb zircon dates demonstrate that the reversed-polarity magmatism occurred mainly in the time interval from 1108 to 1107 Ma; whereas, around the Lake Superior basin, normal-polarity magmatism occurred mainly in the interval from 1099 to 1094 Ma (for example Davis and Paces, 1990; Paces and Miller, 1993; Davis and Green, 1997). These two pulses were separated by a magmatically inactive time (at least in the upper crust), which appears to be expressed as a slight unconformity in the volcanic sequence on the north shore. However, because intrusions subsequently penetrated along this horizon in the North Shore Volcanic Group, this unconformity has not been recognized in outcrop. The Duluth Complex separates the upper and lower sequences of the southeastern limb (Fig. 2-3, Table 2.1). One other significant gap in the stratigraphic continuity of the North Shore Volcanic Group occurs near the stratigraphic top, where the Schroeder–Lutsen sequence overlies the upper units of the northeast and southwest sequences (Fig. 2-3, Table 2.1). In the northeast limb, southwest of Grand Marais (Fig. 2-3), the basal flow of the Schroeder–Lutsen sequence (Terrace Point basalt member) overlies a thick sandstone and siltstone unit (Cut Face Creek sandstone), which in turn conformably overlies the Good Harbor Bay andesites of the upper northeast sequence (See Table 5.2, Miller and others, 2002 for details). However, in the southwest limb, the basal Schroeder–Lutsen sequence flow overlies a thinner sandstone and conglomerate unit (the Little Marais conglomerate) that in turn rests in sharp angular unconformity atop structurally disturbed flows of the Belmore Bay lavas near Little Marais (Table 2.1). Furthermore, the gently dipping Schroeder basalts have not been penetrated by the abundant hypabyssal intrusions of the Beaver Bay Complex that complicate the underlying volcanic sequence in this mid-shore area (Green, 1992; Miller and others, 1993). Attempts to date the Schroeder–Lutsen sequence have been unsuccessful to this point. Page 49 �______ ______ ' MESOPROTEROZOIC (KEWEENAWAN) INTRUSIVE ROCKS Beaver Bay Complex and Felsic rocks Misc. Intrusions 1 Mafic rocks Mafic rocks Duluth Felsic rocks Complex NoRTH SHORE VOLCANIC GROUP PALEOPROTERZOIC RoveNirgin ia/Thomson Fm ' General attitude of volcanic rocks Gunflint/Bwabik Fe-Fm ARCH EAN I Southwest Lower 0 10 20 30 40 50 Kilometers Granitoid Intrusions Supracrustal Rocks Sequence Figure 2-3. Simplified geology of northeastern Minnesota showing the major sequences of the North Shore Volcanic Group (taken from Fig. 5.2, Miller and others, 2002). The five lithostratigraphic sequences comprising the two limbs of the North Shore Volcanic Group are further subdivided into informal formational units (Table 2-1). Some formational units are individual flows of distinctive lithology and/or substantial thickness and lateral extent. Most are suites of lava flows that have distinct lithologic characteristics or that are separated by intrusions. Some lava formations contain distinct flows or sedimentary rock units within an otherwise homogeneous package of lavas. Such units are given informal member rank (such as the Silver Beaver rhyolite member of the Baptism River lavas, Manitou transitional basalt member of the Schroeder basalts, and Indian Camp sandstone member of the Lutsen basalts). See Chapter 1 of RI58 (Miller and others, 2002) for more details on stratigraphic nomenclature of Keweenwan rock in northeastern Minnesota. Page 50 �Table 2-1. Generalized stratigraphy of the southwest limb of the North Shore Volcanic Group showing U/Pb ages (Davis and Green, 1997; Green and others, 2001). Positions of intrusions denote approximate stratigraphic level affected and not age of emplacement. Thickness(m) Lithostratigraphic units Lithologic character U/Pb ages 9735 945 Total section 150 Carlton Quarry lavas (fault bounded) Schroeder–Lutsen sequence (normal polarity) 900 Schroeder basalts <45 Little Marais conglomerate basalt, andesite, and rhyolite flows 1094.3±2.0 ophitic olivine tholeiite basalt flows; includes Manitou transitional basalt and Pork Bay breccia polymict volcanic conglomerate and sandstone angular unconformity 8275 Upper southwest sequence (normal polarity) 565 Bell Harbor lavas 100 Palisade Head rhyolite mostly quartz tholeiite basalt and basaltic andesite flows gray-pink, porphyritic rhyolite flow ~1096 Beaver Bay Complex 700 1096.6±1.7 mixed lavas, mostly basalt; includes Baptism River lavas 165-meter-thick Silver Beaver rhyolite 20 Silver Bay porphyritic basalt ophitic basalt flow with abundant large plagioclase phenocrysts 730 Gooseberry River basalts mixed basalt flows, mostly ophitic Lafayette Bluff, Silver Creek diabase intrusions 315 Two Harbors basalts 550 Larsmont basalts mixed aphyric basalt flows; quartz tholeiite flows at base ophitic olivine tholeiite flows Stony Point–Knife Island diabase sheet 1500 Sucker River basalts mixed basalt flows, mostly ophitic 1350 Lakewood lavas mostly basalt flows; rhyolite, icelandite, and ferroandesite at base Lester River diabase sill 1285 mixed basalt, andesite, icelandite, and rhyolite flows Lakeside lavas 1098.4±1.9 Endion diabase sill 1160 mixed basalts, andesites Leif Erickson Park lavas ~1099 Duluth Complex 370 Lower southwest sequence (reversed polarity) 370 Ely's Peak basalts porphyritic, diabasic, and ophitic basalts; pillowed and pyx-phyric basal flow >8 Nopeming Sandstone white to tan quartzite and conglomerate angular unconformity Thomson Formation (Paleoproterozoic) Page 51 �Physical volcanology The volcanic rocks of the Midcontinent Rift, including the North Shore Volcanic Group, represent one of the world’s oldest and best-preserved examples of plateau lavas. However, they are not typical in that they contain a greater thickness of flows and, in the North Shore Volcanic Group, a higher proportion of evolved compositions. They are similar physically and chemically to the Tertiary lavas that make up eastern Iceland (Sigvaldason, 1974; Green, 1977; Wood, 1978), and they formed similarly over a plume at another major rift. The rocks also resemble the late Tertiary and Quaternary volcanic rocks of the southern Snake River Plain, Idaho and southeastern Oregon, because of their interbedded basalts and large rhyolites (e.g., Bonnichsen and Kauffman, 1987; Manley, 1996). The basalts range in character from typical flood flows as voluminous as tens of cubic kilometers to more modest, “plains-type” flows (Greeley, 1982) and thin flow units less than a meter thick. At Duluth in the southwest limb, and Grand Portage in the northeast limb, the lowest flows in the volcanic sequences are pillowed, and thus inferred to have erupted subaqueously; however, nearly all of the other flows were erupted subaerially. The flows show different physical characteristics, closely tied to their chemical compositions and viscosities (Green, 1989; Green and Fitz, 1993). Olivine tholeiites, which dominate the North Shore Volcanic Group, all have pahoehoe surfaces, with or without ropy structures. Other physical characteristics of the various rock types were previously discussed in the “Rock classification, recognition, and textures” section of this guide. All of the flows ranging in composition from basalts to icelandites were erupted as lavas. The rhyolites are notable in their abundance relative to other plateau-lava sequences, their size (up to several hundred cubic kilometers), and extent (Green and Fitz, 1993). Several rhyolites show textural evidence of rheomorphic flow after eruption as ash-flow tuffs, though some were lavas. One of the largest, the Devil Track rhyolite in Cook County, which is as thick as 250 meters and can be traced for 40 kilometers along strike, has ambiguous features that make its mode of eruption difficult to discern; it may be a lava flow. Nearly all the icelandites and rhyolites show evidence of an unusually high temperature of eruption, such as magmatically crystallized groundmass tridymite. The evidently low viscosity of these large rhyolites is attributed to their high temperature, high iron and fluorine contents, and low oxidation state (Green and Fitz, 1993). Geochemistry and chemostratigraphy The North Shore Volcanic Group constitutes a subalkalic, tholeiitic suite that ranges continuously from rather primitive olivine tholeiite to rhyolite, and shows a strong iron-enrichment trend (Fig. 2-2; also Basaltic Volcanism Study Project, 1981; Brannon, 1984). However, relative abundances are strongly bimodal; basalts are greatly predominant, but rhyolites make up 10 to 25 percent of the section. The basalts show trace element and isotopic evidence of derivation mostly from a mantle plume (Nicholson and others, 1997), whereas most of the rhyolites include major contributions from partial melting of the Archean basement (Vervoort and Green, 1997). The most common basalt type, ophitic olivine tholeiite, is generally aluminum-rich (16 to 18 percent Al2O3); the most primitive flows have Mg numbers of about 0.65 to 0.68. The basal few flows in both limbs of the North Shore Volcanic Group have a unique geochemical and petrographic character. Typically they contain augite phenocrysts, are aluminum-poor, and are rich in both compatible (chromium and nickel) and incompatible elements (titanium, phosphorus, and lanthanum) with steep chondrite-normalized lanthanum/ytterbium ratios. This suggests derivation by relatively small-fraction melting of the initial plume head (Nicholson and others, 1997; Green, 1995). In general, there is little stratigraphic regularity of compositional change within the North Shore Volcanic Group, with the following exceptions. In the middle of the upper southwest sequence, there is a marked upward progression toward more primitive compositions through a 3.4-kilometer section from rhyolite east of the Lester River into a thick group of primitive olivine tholeiites in the Knife River–Two Page 52 �Harbors area (Brannon, 1984). This includes the Lakewood lavas, the Sucker River basalts, and the Larsmont basalts. In contrast, in the lower northeast sequence, the approximately 1-kilometer-thick basal Grand Portage lavas progress upsection from basalt to increasingly evolved compositions, ending with Red Rock rhyolite (Green, 1995). As mentioned above, the Schroeder–Lutsen sequence, the youngest in the North Shore Volcanic Group, is composed almost entirely of olivine tholeiites. All of the North Shore Volcanic Group has been affected to some degree by hydrothermal/burial metamorphism. The more permeable (fractured, vesicular) tops and bases of the flows have undergone considerable mineralogical change (deposition of amygdule minerals, alteration of primary minerals), but in many cases the massive flow interiors are remarkably little-altered. Where alteration has approached equilibrium, mineral assemblages range from lower greenschist facies at the base of the North Shore Volcanic Group to zeolite facies at the top (Schmidt, 1993; Schmidt and Robinson, 1997). Interflow Sedimentary Rocks Clastic redbed strata occur at many horizons within the North Shore Volcanic Group (Jirsa, 1984). They are lenticular and range in thickness from a few centimeters to about 100 meters. As these rocks are relatively soft and erodable compared to the adjacent volcanic flows, they are mostly covered and are exposed only along actively eroding sites such as streambeds and the lakeshore. They are predominantly red to brown, well sorted sandstone, with minor conglomerate, siltstone, and shale. Conglomerate beds are most abundant in the midshore area from Little Marais to Lutsen. Compositionally, these redbeds are mainly immature lithic arkose and feldspathic lithic arenite (see Fig. 5.3, Miller and others, 2002). The angular to subrounded clasts are mainly plagioclase, mafic to felsic volcanic rock fragments, clinopyroxene, and Fe-Ti oxides; devitrified or replaced volcanic ash particles and shards are present in a few beds. Quartz is uncommon to absent. The framework grains have been variably cemented with hematite, calcite, prehnite, and a variety of zeolites, depending on the local hydrothermal/burial/contact metamorphic conditions. In some places hydrothermal minerals have replaced many or most of the clasts. A few of these redbed units have thicknesses in excess of 25 meters. These include a crossbedded sandstone in Leif Erickson Park in Duluth (35 meters), which disconformably overlies an eroded basalt flow; the Little Marais conglomerate (and sandstone) exposed in the Manitou River area near Little Marais (as thick as 45 meters); the Indian Camp sandstone (68 meters) northeast of Lutsen; and the Cut Face Creek sandstone southwest of Grand Marais (100 meters), which can be traced for at least 4 kilometers along strike. Of these, the Little Marais and Cut Face Creek units occur at the base of the Schroeder–Lutsen basalt sequence. The sandstone in these units is typically planar- or cross-bedded, and some beds are ripple-marked or mud-cracked. The rocks are inferred to be dominantly fluvial, deposited by moderate-gradient, east- to southwest-flowing streams from sources nearly entirely within the subsiding Midcontinent rift basin (Jirsa, 1984). Many flow-top breccias of andesite and basalt with aa structure contain laminated red sandstone as a matrix because sand filtered down from the flow surface. Similarly, red, laminated sandstone and siltstone form clastic dikes or crevice-fillings a few centimeters wide in the upper parts of some lava flows. Page 53 �INTRUSIONS WITHIN THE SOUTHWESTERN SEQUENCE OF THE NSVG by Terry Boerboom Several hypabyssal sills and sheet-like intrusions occur within the 7-kilometer-thick section of volcanic rocks exposed near Lake Superior from Duluth to Split Rock Point. From lowest to highest in the section, these consist of the Endion Sill, the Northland sill, the Lester River Sill, the Stony Point diabase, the Silver Creek diabase, the Lafayette Bluff diabase, and the Split Rock intrusive felsite. These sheets are variably concordant to the volcanic strata, range from tens of meters to over 500 meters thick, and generally dip gently eastward. They commonly form prominent topographic highlands. This discussion will be limited to those sills visited by this field trip. For a more complete description, see Miller and others, 2002. Lester River diabase The Lester River diabase is named for the outcrops adjacent to the mouth of the Lester River (Schwartz and Sandberg, 1940). The diabase forms a prominent topographic high (Moose Mountain) that extends about 6.5 miles inland from Lake Superior. The diabase forms a sill some 400-m (1,300-feet) thick that dips an average of 19 degrees southeast, conformable with the dip of the surrounding volcanic rocks. The sill generally overlies basalt and underlies rhyolite, the latter of which probably formed a density barrier that trapped the more dense mafic magma as it was emplaced. The upper margin of the sill contains abundant intermediate to felsic rocks that form a continuous, mappable unit along the entire mapped length of the sill, in contrast to the lower margin which contains only thin and discontinuous pods of the hybrid felsic rocks. The hybrid cap rocks likely formed by partial melting of the overlying rhyolite by heat generated from the underplating sill. Jerde (1991) provides considerable geochemical, petrographic, and petrologic data on the Lester River Sill. Most of the Lester River diabase is a dark grayish-black, medium- to coarse-grained, massive to locally weakly porphyritic diabase. The diabase contains approximately 60% plagioclase, 13% intergranular to subophitic augite, 12% variably altered olivine, 0-4% orthopyroxene, 1-4% Fe-Ti oxides, 0-10% quartz and granophyre, 1-3% diktytaxitic chlorite, and less than one percent apatite, biotite, and hornblende. Quartz commonly contains wispy rutile needles. Silver Creek diabase The Silver Creek diabase is named for proximity to Silver Creek, near Silver Cliff, located about 4 miles northeast of Two Harbors. This diabase is composed almost entirely of medium-grained ophitic olivine diabase, with augite oikocrysts from 1 to 5 centimeters in diameter. It forms a subcordant to discordant subhorizontal sill-like intrusion, at least 60-m (200-feet thick), that forms a prominent highland projecting inland about 7 kilometers north-northeast of Lake Superior, and is exposed intermittently for several kilometers beyond. In the area of the Encampment River, the diabase is in subvertical contact with surrounding volcanic and earlier intrusive rocks; this may have been the feeder to the sill. The diabase is free of anorthosite inclusions and contains only rare xenoliths of volcanic rocks. Pope (1976) studied the Silver Creek diabase as part of Master’s thesis under the guidance of Dr. John Green. The Silver Cliff tunnel on Highway 61 cuts through this diabase. Lafayette Bluff diabase The Lafayette Bluff diabase is named after prominent exposures at Lafayette Bluff, forms prominent knobs near Lake Superior but is typically weathered to a brown crumbly grus. It is an irregular, discordant, sheet- to dike-like body composed of soft, dark greenish-black, amygdaloidal and porphyritic Page 54 �diabase, with local differentiated masses of coarse-grained to pegmatitic ferromonzodiorite inland from Lake Superior. The diabase contains locally abundant 1-3 meter diameter inclusions of anorthosite and scattered xenoliths of metamorphosed basalt. The Lafayette Tunnel on Highway 61 has been excavated through the Lafayette Bluff diabase. Split Rock intrusive felsite The Split Rock intrusive felsite (rhyolite) is a pink, fine-grained, weakly porphyritic, flow-banded rock that has a prominent shingle parting to it (hence: Split Rock River). At a given location, the rhyolite has all the physical attributes of a rhyolite flow, but detailed mapping shows that it forms a north-striking, 3.4 kilometer wide body that cuts north from Lake Superior across the stratigraphy of the surrounding volcanic rocks. The felsite forms cliffs on Lake Superior south of the Split Rock River, and in the Split Rock River and nearby streams forms small, sharp canyons. Field measurements show that the flow banding dips gently towards the central axis of the body, implying that the intrusion is keel-shaped. The lower margins of the rhyolite are intruded by, or commingled with, a dark gray, fine-grained ferrodiabase or ferrodiorite. This felsite is exposed in the Split Rock Point 7.5’ quadrangle, which is currently a work in progress as one of the USGS STATEMAP series of geologic maps to be published by the Minnesota Geological Survey. Due to time constraints, this field trip will not visit this unit. GEOLOGY OF THE BEAVER BAY COMPLEX by Jim Miller The Beaver Bay Complex (BBC) is a hypabyssal, multiple-intrusive igneous complex that was emplaced into the upper part of the NSVG over a 600-km2 area in northeastern Minnesota (Fig. 2-4). Much of this area was the focus of detailed bedrock mapping by the Minnesota Geological Survey between 1985 and 1992 (Miller, 1988; Miller and others, 1989, 1993a, 1994; Boerboom and Miller, 1994). Three general areas of the BBC, southern, northern, and eastern, are distinguished on the basis of distinctive rock types and intrusion form (Miller and Chandler, 1997). This field trip will investigate some of the units composing the southern and northern BBC. The relationship of BBC intrusions to other subvolcanic intrusions within the NSVG (Fig. 2-4) is unclear, because of poor exposure to the southwest and insufficient mapping to the northeast. Within the mapped area of the BBC, thirteen intrusive units have been identified that represent at least six major intrusive events (Miller and Chandler, 1997). Most intrusive activity forming the BBC occurred around 1096 Ma based on U-Pb dates of 1095.8±1.2 Ma for a Silver Bay intrusion, the youngest unit of the BBC, and 1096.1±0.8 Ma for the Sonju Lake intrusion (Paces and Miller, 1993). Whether activity overlapped the main stage of Duluth Complex magmatism at 1099 Ma is unknown, because attempts to date the oldest component of the BBC were not successful (Paces and Miller, 1993). The boundary between the BBC and Duluth Complex is generally marked by a northeast-trending keel-shaped intrusion in the northern BBC (Houghtaling Creek troctolite) that separates largely dike and sill intrusions of the BBC to the southeast from massive granophyric granite and extensive areas of structurally complex gabbroic anorthosite to the northwest that are typical of the roof zone of the Duluth Complex. Page 55 �Figure 2-4: Geology of the southern and northern Beaver Bay Complex showing the locations of Stops 8-16. After Miller and others (2001). Units labels are: nsb - NSVG basaltic volcanics; nss - NSVG Schroeder basalts; nsf - NSVG felsic volcanics; asa anorthositic series of the Duluth Complex; fs - felsic series of the Duluth Complex ; slid - Shoepack Lake inclusion-rich diorite; ccpd - Cabin Creek porphyritic diorite; hct Houghtaling Creek trocolite; blg - Blesner Lake diorite; llg - Lax Lake gabbro; frg Finland granophyre; frqm - Finland qtz ferromonzonite; slt - Sonju Lake troctolitic zone; slg - Sonju Lake gabbroic zone; brd Beaver River diabase; sbi - Silver Bay intrusions. See map M-119 for more details. Page 56 �The range of BBC parent magma compositions is similar to the olivine tholeiite and transitional basalt compositions that dominate the NSVG (Fig. 2-5A). Moreover, like the NSVG, the sequence of intrusion of BBC magmas generally involved progressively more primitive compositions. Compositional variations within the various intrusive units developed as a result of in situ magmatic differentiation (Fig. 2-5B), assimilation of footwall rocks, and/or composite intrusions of evolved magma from deeper staging chambers (Fig. 2-5C). The tightly clustered trend of BBC parent magma compositions evident on an AFM diagram (Fig. 2-5A) and the systematic variation of other elemental abundances suggest that all mafic BBC magmas evolved from a common olivine tholeiitic primary magma type. Such a primary composition, which is approximated by the most primitive, high-Al olivine tholeiites of the NSVG, is thought to have given rise to most Midcontinent Rift magmas, especially in later stages of magmatism (Green, 1982; Miller and Weiblen, 1990; Klewin and Shirey, 1992). That even the most primitive of the BBC intrusions, the Beaver River diabase, is significantly evolved from a primitive olivine tholeiite composition (Fig. 2-5A) indicates that all BBC parent magmas were generated in turn by magmatic differentiation of such a primary composition in deeper staging chambers. Petrologic modeling of some BBC intrusions and other hypabyssal bodies that intruded the NSVG (Jerde, 1991) suggests that most magmas experienced multistage, polybaric fractionation between their extraction from the mantle and their subvolcanic emplacement. Although the available radiometric ages do not indicate an overlap of magmatic activity between the BBC and the Duluth Complex (Paces and Miller, 1993), additional dates and more detailed petrologic studies may show that some Duluth Complex intrusions acted as the final levels of staging and differentiation of some BBC-bound magmas. The focus of emplacement of BBC intrusions appears to have migrated toward the rift axis and toward higher stratigraphic levels with time, perhaps reflecting plate drift and thickening of the volcanic pile. Over the exposed extent of the BBC, intrusion shapes appear to have been controlled by a shallow crustal ridge (Schroeder Forest Center Crustal Ridge) which trends northwest across the BBC. The presence of this buried crustal ridge is indicated by a pronounced saddle in the gravity high over northeastern Minnesota (Chandler, 1990) and the presence of Archean-like inclusions of granitic gneiss, biotite schist, metagraywacke and granodiorite in early intrusions over the gravity low (Boerboom, 1994). The broad network of dikes and sheets, characterizing the southern BBC, becomes tightly focused into a narrow zone of subparallel dikes in the northern portion of the BBC that is situated over the gravity minimum. The eastern BBC opens up again into thick sheet intrusions. Based on geologic, geochronologic, geophysical, and geochemical evidence, Miller and Chandler (1997) suggested that the BBC, particularly the youngest Beaver River diabase dike and sheet network, acted as a magma conduit and structural boundary to the formation and infilling of the western end of the Portage Lake Volcanic basin during the main to late stages of rift volcanism and graben formation. On the North Shore, these volcanic rocks are represented by the Schroeder-Lutsen basalts (Fig. 2-2). The second half of this trip will investigate seven intrusive units of the BBC and rocks comprising the roof zone of the Duluth Complex (Fig. 2-4). Stops 2-8 through 2-11 will investigate the composite intrusions of the Beaver River diabase and Silver Bay intrusions, the youngest intrusive components of the BBC. Stop 2-12 will visit typical exposures of the Finland granophyre. Stop 2-13 will traverse the middle cumulate units of the Sonju Lake Intrusion - the most completely differentiated intrusion exposed in the Midcontinent Rift system. Stop 2-14 will examine the units defining the northeastern limit of the Beaver Bay Complex - the Houghtaling Creek troctolite, Dam Five gabbronorite, and the Shoepack Lake inclusion-rich diorite. Finally, Stops 2-15 and 2-16 will view anorthositic and granophyric rocks that comprise the roof zone of the Duluth Complex. For more detailed information on these and other units of the Beaver Bay Complex see reports by Miller and Chandler (1997) and Miller and others (2002, Chapter 7) and the 1:24,000 bedrock geologic maps of the area (Miller, 1988; Miller and others, 1989, 1993, 1994; Boerboom and Miller, 1994). Page 57 �Figure 2-5: AFM diagrams of Beaver Bay Complex (BBC) intrusions. A) Plot of estimated parental magma compositions to various BBC intrusions (see Miller and Chandler, 1997 for unit descriptions and details) compared to major NSVG lava compositions: OT - olivine tholeiite, TB - transitional basalt, A - andesite, FA - ferroandesite, I icelandite, R - rhyolite (after Green, 1983). NSVG-pot is a primitive NSVG olivine tholeiite composition. B) Calculated liquid line of descent of the Sonju Lake intrusion through troctolitic (slmt-sld-slt), gabbroic (slg-slfgslad) and monzodiorite (slmd) intervals of the layered sequence. Also plotted are whole rock compositions of Finland granite (frg) and quartz ferromonzodiorite (frpm). C) Whole rock composition plots of Beaver River diabase (brd; ophitic margins and coarse subophitic interiors distinguished), Silver Bay intrusions (sbi; coarse marginal facies, layered ferrogabbroic cumulate interiors, and granophyric compositions distinguished), and composite intrusions from the northern BBC (nbbc). Sonju Lake intrusion differentiation trend (dashed line) is also shown. Page 58 �FIELD TRIP STOPS The general locations of all stops are shown on Figure 2-6. A more detailed location map is included with each stop description, with a base made at an appropriate scale from a portion of a 7.5’ quadrangle map. This should allow the return visitor to accurately locate the features for each stop. The UTM coordinates listed are all given in NAD83, with easting listed first and northing listed second. The regional setting of the various rock units are given in the introduction, and only facts pertinent to each specific stop are given below. Figure 2-6. Location map for stops for Field Trip 2. Page 59 �DAY 1 GEOLOGY OF THE SOUTHWESTERN SEQUENCE OF THE NORTH SHORE VOLCANIC GROUP FROM DULUTH TO SPLIT ROCK POINT Drive Highway 61 from Duluth towards Two Harbors. At the outskirts of Duluth, just after the Lester River, veer right onto Scenic Highway 61 for approximately one-half mile, then turn right on the road that leads to Kitchee Gammee park. Park near the northeast end of this road and walk down to lakeshore. STOP 2-1: Lester River Diabase in contact with rhyolite T.50N., R.13W., Sec. 4 Lakewood 7.5’ quadrangle. UTM: Start at SW 576929E, 5188417N; End at NE 577390E, 5188981N Highlights: Lester River diabase (lrd), hybrid granophyric rocks (lrf, lrg), rhyolite (nr); late diabase dikes (bdb), glacial striations. The Lester River Diabase is named for the outcrops adjacent to the mouth of the Lester River (Schwartz and Sandberg, 1940). The diabase sill dips southwest about 19 degrees, and forms a prominent topographic high (Moose Mountain) that extends about 6.5 miles inland from Lake Superior. This diabase is mostly intergranular in texture, but does locally exhibit ophitic textures, as seen at this stop. The dominantly intergranular to relatively fine ophitic (less than 1.5 centimeter) texture of the Lester River diabase contrasts with most other Keweenawan diabases such as the Silver Cliff and Beaver River diabases, which typically exhibit a coarse ophitic texture. This stop starts in the massive interior of the sill, and traverses northeast through increasingly felsic rocks that range from ferrodiorite to granite, and ends at a cliff of fine-grained chilled diabase that cuts the preceding rocks. Felsic rocks first show up as irregular felsic dikes that cut the diabase, but moving northeast towards the rhyolite not only do the felsic dikes increase in abundance, but the diabase itself becomes increasingly granophyric until it grades into dioritic rocks. The granophyric felsic hybrids blend into pink, fine-grained, porphyritic rhyolite, informally termed the Lakewood rhyolite (Green and Fitz, 1993). The exact contact between rhyolite and granophyric hybrid rocks is hard to pinpoint due to the similarity in textures between recrystallized rhyolite and the hybrid melts. Note the thin, shorelineparallel, fine-grained diabase dikes that cut the Lester River diabase. On a map scale, the granophyric hybrid phases of the diabase are thick and continuous along the upper sill margin, in contrast with the sill base where hybrid rocks are thin and discontinuous. The diabase sill is sandwiched between rhyolite in the hanging wall, and basalt in the footwall, indicating that the rhyolite acted as a density barrier that trapped and ponded the mafic magma beneath it. The irregularly mixed and distributed hybrid rocks that cap the Lester River diabase are likely due to mixing of remobilized (melted) rhyolite with the underlying mafic magma. Page 60 �--- Climb up bank , walk NE approximately 0.1 mile to small clearing on east side of highway, climb down to lakeshore, stop 2.2. Alternatively walk to next wayside, climb down, and walk back southwest along lakeshore to basalt/rhyolite contact.--- STOP 2-2: Flow contact between rhyolite and basalt of olivine tholeiite composition T.51N., R.13W., Sec. 34 Lakewood 7.51 quadrangle UTM: 577648E, 5189145N Highlights: Contorted flow-banded rhyolite (nr), contact of basalt (nbd) with rhyolite, ropy flow tops in olivine tholeiite (‘diabasic’-textured) basalt flows. Walk southeast to the pink-tinted rhyolite flow that exhibits highly contorted and folded flow banding. Rhyolite contains amygdules of calcite and minor fluorite (please don’t beat up the outcrop with hammers), and is capped by a thin, discontinuous layer of fine-grained sandstone. Move northeast over four basalt flows and observe flow features such as (bottom to top of flow) pipe vesicles, vesicle cylinders, amygdaloidal zones in upper portions of flows, and ropy pahoehoe upper surfaces. The pahoehoe crusts indicate a general southeast flow direction. These basalt flows are of weakly alkaline, transitional intermediate olivine tholeiite composition, with typical olivine tholeiite flow structures. On recently published 1:24,000 scale geologic maps (e.g. Boerboom and others, 2002a), flows such as this are classified as “diabasic-textured’ flows. Our field mapping has shown that the textures of the basalt flows does not always correspond to a unique chemical composition, but rather may have more to do with the amount of volatiles in the lava. Some of the most coarse-grained diabasic-textured basalt flows were mapped in the Sucker River upstream from Old North Shore Road. Despite being relatively thin flows (1-3 meters thick), they exhibit a medium- to coarsegrained felty diabasic texture. Some have a coarse pegmatitic zone located about one-third of the way down from the top of the flow, attributed to the concentration of volatiles in this part of the flow as cooling proceeded from the top and bottom. These coarse-grained flows also have unusually prominent pipe amygdules at the base overlain by a zone of bead-like pipe amygdules/vesicle cylinders up to one meter in length that end below the most coarse-grained part of the flow. The flows observed in stops 2-1 through 2-3 are all part of the Lakewood lavas (see table in introduction), a lithostratigraphic unit characterized by multiple thin ‘packages’ that range in composition from rhyolite to olivine tholeiite. Each flow package consists of multiple flows of like composition. Brannon (1984) conducted a detailed sampling regime of virtually every flow exposed along the shoreline from Duluth to Two Harbors, starting with the rhyolite just above the Lester River diabase. Her geochemistry verifies the mapped stratigraphy and shows that the most silicic rocks (rhyolite, icelandite, and andesite) occur at the base of the Lakewood lava sequence, whereas primitive tholeiitic basalts dominate the top of the sequence. Vervoort and Green (1997) have shown, through Nd isotopic studies, that the NSVG rhyolites are the product of melting of Archean crustal rocks by underplated primitive Page 61 �mafic magma, whereas icelandite and basalt are most likely derived directly from a mantle plume source. In their model, early volcanism (lower reversed sequence) tapped a mantle source with little crustal contamination. This was followed by a period of volcanic quiescence (the 1107 -1100 Ma ‘volcanic hiatus’) during which a mantle plume underplated and melted the crust, generating superheated felsic melts that were later erupted as rhyolite flows and rheoignimbrites, of unusually wide areal extent given their thickness (Green and Fitz, 1993) --- Climb up bank of small stream to wayside rest (this is the third wayside rest on Scenic Highway 61 past Brighton Beach Road), drive or walk approximately 0.3 miles northeast to wayside rest directly across from Old North Shore Cottages Resort, stop 2-3.--- STOP 2-3: Interflow sandstone between icelandite and basaltic andesite flows T.51N., R.13W., Sec. 34 Lakewood 7.51 quadrangle UTM: 578287E; 5189652N Highlights: Interflow sandstone above icelandite (ni), below basaltic andesite (nba) Park at wayside across from North Shore Cottages Area A: Slide down the embankment below wayside (over red clays of Glacial Lake Duluth) to outcrop of sandstone on shoreline. Sandstone is approximately 12-15 feet thick, planar and trough crossbedded. Jirsa (1984) reports the sand grains are predominantly plagioclase (44-53%) and mafic volcanic rock fragments (18-25%), with lesser quartz (1-4), opaques (2-8%), K-spar, felsic volcanic rock fragments, and mudstone fragments (less than 2% each). The sandstone is cemented by a matrix of calcite, zeolite, K-feldspar, and quartz. Crossbedding indicates an easterly paleocurrent direction. The sandstone overlies an icelandite flow which has a rubbly flow top (and unfortunately is quite inaccessible due to cliffs), and is overlain by a flow of basaltic andesite composition (will examine these at site B). The upper surface of this interflow sandstone contains curly molds and bits of baked-on chilled basalt from the overlying flow, which is now eroded away. This sandstone is typical of the thin interflow sedimentary rocks, which are immature, fine- to coarsegrained sediments that were deposited in fluvial and lacustrine environments, although some may interpret some of the bedding in this outcrop to be aeolian in nature. Overall, paleocurrent indicators for the interflow sandstones indicate a transport direction generally toward Lake Superior from a highland of both Keweenawan and pre-Keweenawan sources (Jirsa, 1984). ---Climb back up bank and move about 100 feet northeast and return to lakeshore outcrops.--- Page 62 �Area B: Basaltic andesite. The base of this flow sequence was observed at stop 2-3a. These flows are dark purplish-gray, fine-grained basaltic andesite, which locally have pipe vesicles at the base, and amygdule-rich upper parts that exhibit billowy lobate forms defined by concentrated amygdules. Flow tops are commonly brecciated. Some flows contain sparse plagioclase phenocrysts. Due to time constraints we will not progress up the shore, but the return visitor is encouraged to walk up the shore over at least eight more flows of this composition that are in turn overlain by ophitic basalt flows. Brannon (1984) classifies these flows as ferroandesite, but they are morphologically more similar to flows of basaltic composition. ---- Climb back up back to highway, back to cars. Continue northeast on the Scenic Highway approximately 8 miles to Alseth Road. Turn right on Alseth Road and follow around to a point where shoreline is close to road, park and walk to shore (Stony Point).--- STOP 2-4: Stony Point diabase T.51N., R.12W., Sec. 1 Knife River 7.5’ quadrangle UTM: 590326E; 5197791N Highlights: Stony Point diabase (spd) The diabase forms a sheet-like intrusion that strikes parallel to the shoreline and is discordant to the surrounding volcanic rocks. It is exposed in numerous outcrops between here and Knife River, a distance of nearly 3 miles. It is composed of coarsely ophitic olivine diabase with scattered thin pegmatitic segregations. The island offshore from Knife River forms the northeast-most outcrop of this diabase. The exposed thickness is estimated to be about 100 meters, but a positive aeromagnetic anomaly that corresponds to this diabase, and continues offshore beneath Lake Superior, implies that this diabase is part of a larger intrusive mass whose thickness is unknown. The diabase is chilled against adjacent volcanic rocks, and contains rare small anorthosite xenoliths (neither visible at this stop). ---Continue northeast on the Scenic Highway, to Highway 61 into Two Harbors. Continue through Two Harbors to the northern outskirts of town, turn right on 1st Street and continue south to the crest of the hill adjacent to the picnic grounds. Walk east to lakeshore.--- Page 63 �STOP 2-5: Two Harbors Town Park T.52N., R.10W., Sec. 6 Two Harbors 7.5’ quadrangle UTM: 601851E, 5208673N to 601847E, 5208132N Highlights: Basaltic flows of quartz tholeiite composition (ntb), rubbly flow tops, diabase dike (db) This sequence of quartz tholeiite basalt flows is part of the Two Harbors lithostratigraphic unit (see table in introduction). These flows are comprised of brownish-gray to gray, fine-grained, fresh intergranular basalt that typically exhibits a pronounced oxidation-lamination type of flow banding. The upper parts of these flows typically contain a thick and irregular section of rubbly aa rich in laumontite, and red siltstone is common as thin lenses and crack fillings in the flow tops. Walk south along the shoreline and observe the transition from massive, oxidation-laminated flow interiors to amygdaloidal, rubbly flow tops, and contacts with overlying flows. Each rocky point along this traverse corresponds to the massive, more resistant base of a flow that is underlain by a rubbly, easily eroded, flow top. A N28W striking diabase dike that cuts the basalt can be seen along the south side of the north-most point, but due to time constraints this field trip will not go there. This dike follows a set of closely spaced brittle fractures in the basalt. :---Continue northeast of Two Harbors on Highway 61 approximately 4.5 miles to the Silver Cliff Tunnel. Just past tunnel turn right into an informal parking area adjacent to highway.--*** TRAFFIC HAZARD ****** USE EXTREME CAUTION *** Page 64 �STOP 2-6: Silver Cliff Tunnel T.53N., R.10W., Sec. 15 and 22 Castle Danger 7.5’ quadrangle UTM: 606993E, 5213952N (park area) to 606755E, 5213486N Highlights: Silver Creek diabase (scd), contact with subjacent andesitic flows nca), fault The Silver Creek diabase forms an irregular subcordant, subhorizontal intrusion that is at least 60-m (200ft) thick. The diabase forms a prominent highland that projects inland several miles from Silver Cliff at Lake Superior. The Highway 61 tunnel has created excellent exposures of the contact between both the top and bottom of the diabase with adjacent volcanic rocks, and has exposed a north striking, 55-degree east dipping brittle fault that cuts the base of the diabase. The andesitic flows beneath the sill show a very irregular, rather chaotic flow contact, a rubbly flow top, vuggy quartz-lined stretched vesicles and amygdules of gray agates that have been recrystallized by contact metamorphism from the adjacent diabase. The margins of the diabase are marked by a mix of commingled fine-grained, strongly magnetic, dark gray diabasic rock and pink granophyre. The lower diabase contact strike is approximately N-S, 70 degrees west, and the upper contact (exposed at the south end of the tunnel) strikes N40E, 70 degrees northwest. Out on the old roadbed the diabase displays prominent columnar joints that plunge approximately 60-65 degrees east. From the north edge of the old roadbed, one can look back across the highway down the length of the brittle fault that cuts the base of the diabase. The fault is about 3 meters thick and filled with a mixture of pink zeolites and calcite-filled voids around altered diabase gouge. The old roadbed affords an excellent view of the Lake Superior coast. ----Continue northeast up Highway 61. The next tunnel is cut through the Lafayette Bluff diabase, a dark green, porphyritic, amygdaloidal diabase which bears distinct textural differences from the Silver Creek diabase, and that locally contains anorthosite inclusions up to 2 meters in diameter. As we drive past, note the thick cap of reddish-brown grus developed on top of the Lafayette Bluff diabase, which is typical of this unit. Just inland, the grus was formerly excavated for use as road surfacing material. We will see blocks of the Lafayette Bluff diabase at the parking area for the next stop.-----Continue past Lafayette tunnel, pass Gooseberry State Park. From Gooseberry River, go 3.8 miles to a pulloff on lakeward side of road approximately 300 feet before crossing the Split Rock River. Park at wayside and walk northeast to small point to gain access to shore.--- Page 65 �STOP 2-7: Split Rock River diatreme T.54N., R.8W., Sec. 7 Split Rock Point 7.5’ quadrangle UTM: 620570E, 5226484N to 620497 E, 5226229N Highlights: Split Rock Creek diatreme breccia (dt), brecciated basalt, basalt/andesite flow contact Outcrops to north along shore are of weakly to moderately brecciated ophitic basalt that has apparently been slightly disrupted by the adjacent diatreme dike. A small “island” just offshore of the biggest point here consists of dark gray, medium-grained, ferromonzodiorite that petrographically consists of felty plagioclase, prismatic augite, skeletal to blocky oxides, and red-dusted anhedralinterstitial K-feldspar. The edge of this outcrop nearest the shore shows a dramatic brecciated texture due to diatreme emplacement, and the cliff adjacent to that exposes the diatreme breccia proper. The best diatreme breccia is exposed further south along the beach. Here, the breccia contains visible clasts of massive ophitic basalt, fine-grained amygdaloidal basalt, and intergranular basalt that vary from sub-millimeter to 25 cm in diameter and are all strongly magnetic. Small fragments of interflow sedimentary rocks in the breccia have been noted in thin section. The zeolitic matrix varies from pink to pale green in color. At the south end of this breccia, one can observe the sharp, vertical contact between the diatreme and layered amygdaloidal basalt. The contact here strikes approximately N20W, but overall it is more or less north-south. A short distance south of the diatreme contact are vertical cliffs formed by the blocky jointing pattern of a dark gray, very hard and fresh basaltic andesite flow which overlies amygdule-layered flows that exhibit minor low-amplitude billowy surfaces. The flow contact can be observed southward down the beach to the next point out into the lake. The top of this andesitic flow may be visible across the bay to the north, where it is overlain by massive ophitic basalt. The diatreme exposed here is identical to another to the south, near Crow Creek, that is much more well exposed but unfortunately only accessible by water. There, the diatreme forms a vertical, 10 meter thick, cliff-forming dike that cuts ophitic basalt flows along the shoreline, and in some small outcrops inland. Although poorly constrained, the Crow Creek diatreme may be in the form of a ¼ mile diameter ring-like circular dike, or it may simply have an irregular, ‘wandering’ strike direction that varies from north-south to east-west. The Crow Creek diatreme is a heterolithic breccia that contains clasts of basalt up to 3 meters in length, and has irregular pyrite-rich pods within it. Like the diatreme here, the Crow Creek diatreme has prismatic ferromonzodiorite associated with it, which is itself brecciated, and contains varied types of basalt clasts as well as clasts of interflow sedimentary rocks. Page 66 �The varied types of basalts coupled with interflow sedimentary rocks that make up the clasts in both places demonstrate that the diatreme has cut through a substantial thickness of volcanic strata. Although prismatic ferromonzodiorite is apparently associated with the diatreme in some fashion, the abundance of zeolitic matrix mixed with rock flour implies that the diatremes were formed by a gaseous, volatile-rich explosive mechanism. The volatile source is not known. However, the Lafayette Bluff diabase, which occurs in close proximity to the Crow Creek diatreme, contains abundant zeolitic (mainly laumontite) amygdules, and overall exhibits moderately strong deuteric alteration, which implies that the magma may have been gas-charged. Thus in a highly speculative scenario, degassing during cooling of the Lafayette Bluff diabase may have led to a buildup of volatiles that subsequently exploded through the overlying basalts. Both the Crow Creek and Split Rock river diatreme form vertical dike-like bodies which brecciated clasts of ophitic and intergranular basalt, interflow sandstone and siltstone, fine-medium-grained ferrodioritic rocks, isolated unit quartz grains, and isolated blocky plagioclase grains. At this locality the clasts are generally less than 0.5 meter in diameter, but the diatreme near Crow Creek contains clasts of basalt as large as 4 meters in length. Medium-grained prismatic ferrodioritic intrusive rocks are exposed next to the diatreme dikes at both localities, and at both places the ferrodiorites are cut by and hence predate the diatreme dikes. ---Continue up Highway 61 to Split Rock Lighthouse State Park and History Center. Pull into park lot for the History center and proceed toward the lighthouse. To visit the lighthouse area (Area A) during summer tourist season, the history center charges a fee or requires Historical Society membership. A fee is not required to access the shoreline exposures at Area B. STOP 2-8: Split Rock Lighthouse Anorthosite T.55N., R.8W., Secs. 32 and 33 Split Rock Point NE 7.5' quadrangle UTM: Area A- 623690E, 5228678N Area B - 623625E, 5228665N Highlights: large anorthosite inclusions (anor), ophitic diabase (brd), lower chill, basaltic lava flow with silt-filled aa flow top (nsb) The Beaver River diabase is the most areally extensive intrusive phase of the entire BBC and is found in contact with most other BBC units. In the southern BBC (Fig. 2-4), it occurs as a series of dikes, sills, and sheets of ophitic olivine gabbro that grades into coarser and more subophitic to intergranular gabbro in the medial portions of thicker sheets. One of the most distinctive characteristics of the diabase is that it commonly hosts large (as much as several hundred meters in diameter), rounded to angular inclusions of nearly pure anorthosite. These inclusions, which locally are brecciated and recrystallized (Morrison and others, 1983), are particularly common in the upper and lower margins of the larger diabase sheets. As reported for this stop in the Field Trip 7 guidebook (stop 24), these inclusions have been the focus of geologic debate for over 150 years. Page 67 �Area A: Outcrops of fine-grained, ophitic olivine diabase with centimeter-wide augite oikocrysts, typical of the margins of Beaver River diabase, are exposed just northeast of the lighthouse atop a sheer 30-mhigh sea cliff. The diabase here forms a sill that dips gently (<15°) into the lake and whose basal contact with a basalt flow top is exposed around the base of the point. The diabase is a moderately evolved (mg#=57), high-Al, olivine tholeiite (Fig. 2-5, BRD margin). The prominent point just to the northeast (Rusty Point) is held up by a very large (~200 m) inclusion of medium-grained leucogranite lying at the base of the sill. The diabase is slightly chilled around this granite inclusion, which closely resembles the Finland granophyre (see Stop 2-12). Most of Split Rock Lighthouse Point is held up by a single, large (>35m), layered anorthosite inclusion. Around the base of the lighthouse, the inclusion displays meter-scale modal layering of coarsegrained granular (cataclastic?) anorthosite (>99% Pl) and noritic anorthosite (20% Opx [En70Fs28Wo2], 80% Pl [An60-80]; Morrison and others, 1983). The steeply dipping layers are cut by thin dikes of mediumgrained, granular augite leuconorite (An56, En73Fs24Wo3, En46Fs12Wo41, Morrison and others, 1983). .....Head down the tramway path to the lakeshore and carefully make way along base of slope toward lighthouse. PROCEED CAREFULLY OVER LARGE TALUS BOULDERS. Area B: To the southwest of the lighthouse along the shore, the base of the sill can be observed to conformably overlie an amygdaloidal flow top breccia with a matrix of horizontally bedded siltstone. The diabase here contains a variety of types and sizes of anorthosite inclusions, which stand out as unjointed masses within the highly jointed fine-grained diabase. The diabase is strongly chilled at the basal contact against the flow top, but shows no sign of chilling against the anorthosite inclusions. The variety of anorthosite types can be viewed in the talus blocks at the base of the slope. Looking to the northeast, one can see that the near-vertically, layered inclusion beneath the lighthouse extends to lake-level. This and other anorthosite inclusions hosted in the Beaver River diabase are different from most anorthositic series rocks of the Duluth Complex; the latter are rarely layered, more compositionally evolved, and rarely contain cumulus hypersthene. These inclusions are similar, however, to some anorthosite inclusions within the anorthositic series. The highly disordered structural state of plagioclase (Miller, unpublished data) and the absence of any discernable chill of the diabase against the anorthosite indicate that these inclusions were derived from a middle to lower crustal source. Isotopic and traceelement compositions of these crustal xenoliths suggest that they may be pre-Keweenawan in age (Morrison and others, 1983), but the data are overall ambiguous. Alternatively, if a plagioclase crystal mush origin for Duluth Complex anorthositic rocks is correct (Miller and Weiblen, 1990), a corollary of such a model is that significant amounts of Keweenawan anorthosite, generated by plagioclase flotation under high pressure, should have formed in the deep crust prior to BBC magmatism at 1096 Ma. Under deep crustal conditions, such plagioclase cumulates would probably be distinctive in texture and composition from their shallow crustal counterparts. Moreover, the ambiguous isotopic compositions of the inclusions may indicate that anorthosite-forming Keweenawan magmas were contaminated by older crust, rather than older anorthosite being contaminated by interaction with Keweewawan magmas, as concluded by Morrison and others (1983). Return to Duluth for the evening. DAY 2 Page 68 �GEOLOGY OF THE BEAVER BAY COMPLEX Drive Highway 61 from Duluth past Two Harbors to Lake Co Rd 3. Take this road approximately 25 miles. About 2.3 miles past the Silver Bay municipal airport, look for a prominent roadcut on the adjacent railroad grade on the north side of the road. Pull into gravel pullout. STOP 2-9: Fanning columnar joints in Beaver River Diabase dike/sill T.55N., R.8W., Sec. 9 Silver Bay 7.5' quadrangle UTM: 624590E, 5235865N Highlights: columnar jointed ophitic diabase (brd), anorthosite inclusion (dark outcrops), contaminated and inclusion-rich diabase (brid) The western and northern extent of the Beaver River diabase is marked by a 110-km-long dike (or dike set) across which vertical downward displacement of 1.5 to as much as 6 km on the southeastern (rift axis) side is indicated by offset of older geologic units (Miller and Chandler, 1997). The feature, which has a very pronounced aeromagnetic expression (Fig. 2-7), is termed the Finland tectono-magmatic discontinuity (FTMD). Within the concavity of the FTMD are several other large, often bifurcating dikes and large sheets dipping gently southeast. The complex of sheets holding up tabletop highlands in the southern BBC (Figs. 2-4 and 2-7) may be part of an originally continuous, nearly horizontal sheet that has locally been eroded through to expose volcanic rocks forming the footwall. The fanning pattern of columnar jointing exposed in this railroad cut through Bear Lake Ridge reflects the transition from the FTMD dike into a thick sub-horizontal sheet developed to the east of the ridge. The rock type here is a fine-grained ophitic olivine diabase. The larger railroad cut 150 m to the east reveals the irregular, well-jointed base of the diabase sheet overlying the amygdaloidal flow-top breccia of a deeply weathered ophitic basalt (CAUTION, OUTCROP FACE IS UNSTABLE). Abundant weathered anorthosite inclusions and some quartz- and feldspar-phyric rhyolite inclusions (similar to the Palisade rhyolite) are evident in the diabase. This diabase sheet holds up the extensive 120-m cliff visible across (south of) the Beaver River. Page 69 �Figure 2-7. Shaded-relief image of total magnetic field data over the southern Beaver Bay Complex and generalized geologic map (from M-119) at the same scale. FTMD - Finland Tectono-magmatic Discontinuity. Labelled intrusions of the BBC include the Cloquet Lake layered series (CLLS), the Houghtaling Creek troctolite (HCT), the Sonju Lake intrusion (SLI), the Finland granophyre (FG), the Lax Lake gabbro (LLG), the Blesner Lake diorite (BLD), the Beaver River diabase (BRD), and the Silver Bay intrusions (SBI). Duluth complex units include the anorthositic series (AS), the felsic series (FS), the Osier Lake intrusion (OLI), and the Greenwood Lake intrusion (GLI). North Shore Volcanic Group rocks are denoted as V. Solid lines denote inter-intrusion contacts; dashed lines denote intra-intrusion contacts. See M-119 for subunits of intrusions. Number labels are NAD 83-based UTM grid coordinates. Modified from Figure 7-2 (Miller and others, 2002). Two types of diabase are exposed in the upper ledge of the railroad cut, which is accessible from the east. The more common type is the same fine-grained ophitic olivine diabase observed in the dike, but here it contains scattered anorthosite inclusions and locally is plagioclase porphyritic. Exposed across the eastern half of the ledge, the ophitic diabase is found in sharp contact with a dense, black, aphanitic, intersertal diabase rich in quartz and feldspar xenocrysts, numerous small, blebby inclusions of felsite, and larger inclusions of anorthosite. The presence of anorthosite inclusions argues against it being a volcanic xenolith. Near the western end of the ledge, the ophitic diabase is noticeably chilled and columnar jointed adjacent to a sharp steep contact with the inclusion-rich diabase. In the center of the ledge, the inclusion-rich diabase is itself pseudo-columnar jointed near its sharp contact with the ophitic diabase, but the joints are not orthogonal to the contact. This inclusion-rich diabase is locally observed near the margins of Beaver River diabase intrusions throughout the BBC. It probably represents strong contamination of early anorthosite-bearing Beaver River magmas as they were emplaced into brittle conduits in the volcanic edifice. .... Continue east on Lake Co Rd 3 to junction with Co Rd 4. Turn right and proceed to junction with Highway 61 in Beaver Bay. Turn left, after crossing the Beaver River pull on to shoulder. Walk paths down to beach. Page 70 �STOP 2-10: Inclusion-rich Contact Zone of the Beaver River Diabase at Beaver Bay T.55N., R.8W., Sec. 12 Silver Bay 7.5' quadrangle UTM: 629115E, 5235420N to 629240E, 5235770N Stop 10 Highlights: porphyritic rhyolite inclusion (nspr), hornfels basalt inclusions (nsb) with granophyre ring dikes, anorthosite inclusions (black masses), inclusion-rich Beaver River diabase (brid) Exposed along shoreline outcrops on the north side of the Beaver River is a complex mix of rock types that is commonly characteristic of the basal? contact of the Beaver River diabase. Exposed in the first outcrop north of the Beaver River sandbar is hybrid mix of fine-grained diabase and irregular masses of felsic rock in various stages of mixing and assimilation. While some of the felsic material looks to be rounded irregular melted? inclusions in the diabase, some felsic material clearly occurs as dikes in the diabase. At the steep north end of this exposure is a quartz-feldspar porphyritic rhyolite inclusion that is very similar to the Palisade rhyolite, which holds up Palisade Head and Shovel Point several miles to the north of here. At the base of the steep north side of the outcrop, the rhyolite is in sharp contact with a fine-grained diabase. The diabase is riddled with irregular blebs of felsic material that likely represent melted fragments of the rhyolite inclusion. Wieland Island in the northern part of Beaver Bay is composed of a similar porphyritic rhyolite. The cliffs across the mouth of the river to the southeast are held up by large inclusions of aphyric flow-banded rhyolite and medium-grained massive granophyre. Progressing to the northeast, we cross a small outcrop of an epidotized fragmental mafic rock. This rock may be contaminated diabase or, as implied by similar appearing rock we shall see to the north, it is more likely a metamorphosed flow top breccia of a basalt inclusion. The next low outcrop is of deeply altered (green-tinted) diabase charged with similarly altered anorthosite inclusions. Coming on to a large expanse of semi-continuous outcrop along the rest of the shore, several rock types are observed. Most of the exposure is altered fine-grained diabase. Another prominent rock type is granophyre, which occurs both as irregular masses of varied size and as a curving network of granophyre dikes ranging from several centimeters to over a meter wide. In some areas, granophyre composes almost half of the exposure. In several locations, granophyre dikes are found encircling blocks of dense basaltic hornfels. The basaltic hornfels are sometimes only distinguishable from the fine-grained diabase by a very prominent orthogonal joint pattern and the ring of granophyre. In one block occupying a low spot in the outcrop, a flow contact between massive and flow breccia basalt is recognizable. These blocks of basalt flows were likely thermally metamorphosed by their incorporation into the diabase. The origin of the granophyre is less clear, however. Melting of rhyolite inclusions has evidently given rise to at least some of the granophyre, especially that occurring as irregular masses. However, given the mantling relationship of some granophyre ring dikes to the basaltic hornfels inclusions, is seems possible that some of the granophyre dikes were "sweated out" from the originally hydrated basalt inclusion. Alternatively, the Page 71 �ring dikes may represent rhyolite-derived felsic melts that simply intruded along contacts between the diabase and the basalt inclusions. Following the exposure further to the north, this mix of diabase, granophyre, and basalt abruptly gives way to diabase heavily charged with large anorthosite inclusions. At this point, we will head upslope to the highway and back toward the river. .... Continue along Highway 61 to the northeast past the North Shore taconite plant at Silver Bay and Tettegouche State Park. Proceed 3.0 miles past the junction with MN Highway 1 to a road cut across from (NW of) a Jehovah's Witness Hall. STOP 2-11: Silver Bay intrusions, Beaver River diabase and overturned lava flows T.56N., R.7W., Sec. 1 Illgen City 7.5' quadrangle UTM: A: 637890E, 5247155N B: 638150E, 5247660N C: 638421E, 5247980N Highlights: ferrogabbro layered intrusion (sblg), overturned lava flows (nsb), monzodioritic composite intrusions (sbg) into diabase (brd) containing anorthosite and basalt inclusions The Illgen City quadrangle contains some of the most complex geology exposed in the Keweenawan system (Miller and others, 1988). This complexity resulted from block faulting, displacement, and rotation of volcanic rocks caused by the dike and sill emplacement of the Beaver River diabase and later composite intrusions. The present level of exposure along the shoreline has cut into the footwall of a major sheet-like intrusion of diabase that holds up the highlands about 2 km in from the shore. This is perhaps the same sheet that the Beaver River cuts through near Stop 2-9. The footwall is highly dissected with a complex network of large (50-800m wide) diabase dikes and the intervening blocks of volcanic rocks have been significantly displaced and rotated into a variety of orientations (Fig. 2-8). Multiple composite intrusions of ferrogabbroic to intermediate magmas have produced a variety of intrusions hosted by the Beaver River diabase, which are collectively termed the Silver Bay intrusions. Three roadcuts along a one-and-a-half kilometer stretch of the northwest side of Highway 61 display some of the best examples of the magmatic and structural complexities attending the emplacement of the Beaver Bay Complex. Roadcut A: This roadcut exposes a nearly complete cross section through a small, zoned Silver Bay intrusion. This intrusion, termed the Jehovah body, has the form of a broad, shallow asymmetric synform that plunges about 15° to the east and measures about 450 m north to south. It is intrusive into the axial portion of a Beaver River diabase dike about 500-600 m wide, which dips steeply to the north. Page 72 �A B C Figure 2-8. Geologic cross-section of the shoreline in the vicinity of STOP 2-11 (From Miller and others, 1989). Unit abbreviations are: nsb- basalt; nsob-ophitic basalt; nsr2-rhyolite; nss-interflow sandstone; brd-Beaver River diabase; sbg-Silver Bay gabbro; sblg-Silver Bay laminated gabbro;, gr-granite inclusion. Approximate locations of roadcuts A, B, and C projected to the shore are shown. No vertical exaggeration. On either end of the roadcut west of Highway 61 (Fig. 2-9A) are exposures of a deeply weathered, varitextured but generally coarse-grained, nonfoliated apatitic ferromonzodiorite forming the margins of the intrusion (unit sbg). Toward the center of the roadcut, the ferromonzodiorite abruptly grades into a medium-grained, well-foliated, subprismatic ferrodiorite with poikilitic olivine (unit sbpg). The poikilitic olivine ferrodiorite is composed of about 50-55% prismatic to lath-shaped plagioclase (An45-15); 15-20% subprismatic and partially uralitized augite (En'62-42) and minor intergrowths of pigeonite (En'42-36); 5-10% subequant to bladed iron-titanium oxide; 5-15% subpoikilitic to poikilitic, altered olivine (<Fo48); and 515% granophryic mesostasis and trace apatite. Olivine is almost completely altered to a glassy black sheet silicate mineral, called hisingerite by Gehman (1957). In thin section, the hisingerite appears to be mostly dark-greenish-brown biotite, partially altered to chlorite, with some serpentine and iron oxide. The oxidation of iron oxide, which is especially common around the outer margin of the olivine oikocrysts, produces purplish coronas around the black clots. Variations in the abundance and size of olivine oikocrysts (1.5 to 5 cm across) impart a layering to the gabbro, which is parallel to plagioclase foliation. This internal structure defines an asymmetric synform whose axial plane projects through the northern half of the intrusion. Cryptic variations in the Mg/Mg+Fe contents of mafic minerals are noted upward through this sequence (Fig. 2-9A) and are suggestive of magmatic differentiation. We interpret the coarse-grained, decussate rocks to represent equilibrium crystallization of a stagnant margin around a small convecting core of fractionally crystallizing magma, which formed the foliated gabbro. The dikes of coarse-grained nonfoliated ferrodiorite internal to the zoned intrusion may represent late intrusions of parental magma or remobilization of interstitial magma from within the intrusion. Roadcut B: The next major roadcut to the northeast displays a series of basalt lava flow that have been overturned 125° from horizontal. These flows are part of a large (~1 x 2 km) block of basalts that is completely enclosed by intrusive rocks of the Beaver River diabase and Silver Bay intrusions (Fig. 2-8 and location map). The basalts range in composition from olivine tholeiites, with smooth upper flow surfaces and pipe amygdules at their base, to quartz tholeiites with brecciated (aa) flow tops (Fig. 2-9B). They range in thickness from 3 to 20 meters. Two interflow sandstone units occur beneath the thickest lava flows. These flows are part of the Bell Harbor lavas, the uppermost unit of the upper southwestern sequence of the NSVG (Table 2.1). Page 73 �Figure 2-9. Schematic diagrams of geologic relationships exposed along three roadcuts at STOP 2-11. Foliation development portrayed in Roadcut A by alignment of tick marks. Units: sbg- nonfoliated ferromonzodiorite; sbpg - foliated poikilitic olivine ferrodiorite; sblg - foliated intergranular (non-poikilitic) olivine ferrodiorite. Compositions of augite shown by En content (= MgO/(MgO+FeO), mole %). Roadcut C: A complex sequence of intrusive and inclusive relationships are exposed in the 70' high roadcut to the northeast (Fig. 2-9C). This is one of the tallest roadcuts on the North Shore. Starting at the southernmost end of the roadcut (0 pace) is a medium-grained ophitic olivine diabase with 2cm augite oikocrysts. The diabase gradually coarsens to where by 80 paces the oikocrysts are about 5 cm across. Beginning at about 40 paces, the diabase displays an intense oxidation/alteration that persists through about 200 paces, where it is in contact with hornfels basalt. Between 90 and 130 paces is a coarse grained anorthosite inclusion, which has near vertical contacts with the enclosing diabase and displays the same alteration as the diabase. This anorthosite holds up the crest of the hill that the roadcut traverses. On the north side of the inclusion, the diabase is more subophitic, with coarse (~2 cm) augite clots, and it contains granophyric patches. Progressing north, augite gradually becomes more prismatic and granophyre content increases to where by 160 paces the rock is a ferromonzodiorite typical of the marginal phases of Silver Bay intrusions. Between 170 and 175 paces, two steeply inclined granophyre dikes cut the ferromonzodiorite. At about 180 paces, a subophitic texture again is evident. This rock gradually becomes more ophitic and more medium-grained as a contact with hornfels basalt is approached at 200 paces. The northern contact of this basalt inclusion occurs at a low spot in the roadcut and is obscured by two shallow dipping granophyre dikes. The larger dike (~1.5m wide) cuts medium finegrained ophitic diabase, which contains another hornfels basalt inclusion near the base of the exposure at about 240 paces. The exposure from 240 to 295 paces is relatively unaltered, medium fine to mediumgrained diabase. Between 270 to 280 paces, near the base of the roadcut, is an odd inclusion of coarse- Page 74 �grained, subophitic leucogabbro with apophyses? of very fine, amygdaloidal mafic rock. The inclusion is mantled on its upper right margin by a thin rind of granophyre. The northernmost 10 paces of the roadcut leading to a gravel side road is hornfels basalt. The sequence of events interpreted from the rocks displayed in Roadcut C are: 1) intrusion of diabase dike carrying anorthosite inclusions from depth and incorporating locally derived basaltic wall rock. 2) composite intrusion of ferromonzodioritic magma into the semi-molten core of the diabase dike; 3) injection of granophyre dikes (possibly auto-intrusions of late-stage felsic melt segregated from the ferromonzodiorite); and 4) fluxing of oxidizing fluids through the central zone of the dike. --- Continue on Hwy. 61 to Lake Co. Hwy 6. Take this to MN Hwy 1 in Finland. Turn right on Hwy 1 about 1/4 mile and turn right on to Lake Co Hwy 7. Just past the Finland Community Center and just before the pavement ends, turn left on to the road to the site of a former USAF Radar Station. Take the paved road about 1.4 miles to gravel road crossing and pull out to the left.--- STOP 2-12: Finland granophyre and hybrid dikes T.57N., R.7W., Sec. 4 Finland 7.5' quadrangle UTM: 633180E, 5256520N Highlights: granophyric leucogranite (frg), locally with blebby mafic enclaves (dgh) Exposed over a 40-km2 area in the northwestern part of the southern BBC (Fig. 2-4) is a distinctive, oval-shaped mass of leucogranite and quartz ferromonzodiorite, termed the Finland Radar Station granophyre by Miller and others (1993), or simply the Finland granophyre. The main phase of the granophyre is a homogeneous, salmon-colored, micrographic leucogranite with abundant miarolitic cavities (unit frg, Fig. 2-4). To the north, this granitic phase, which contains less than 5% Fe-silicates and oxides, abruptly grades to a quartz ferromonzodiorite characterized by 5-20% prismatic, iron-rich pyroxene, amphibole, and locally olivine and 20-40% micrographic felsic matrix (unit frqm, Fig. 2-10). At this stop, the typical character of the granitic phase of the Finland granophyre is displayed in the roadcuts on either side of the road to the former USAF radar station (decommissioned in 1974). In the rubble at the north end of this roadcut, the medium-grained granophyre contains small rounded enclaves of fine-grained mafic rock. Some of the rubble blocks are composed completely of this finegrained rock. This occurrence is in line with what to the north is clearly a mafic to intermediate dike cutting the granophyre. This dike is part of an orthogonal network of dikes composed of heterogenous mixtures of mafic and felsic rock types that cut the Finland granite and the underlying Sonju Lake Page 75 �intrusion and that parallel the adjacent Beaver River diabase (BRD-SLI hybrid dikes in Fig. 2-10A). Miller and Chandler (1997) have interpreted these dikes to represent hybrid mixtures of Beaver River diabase magma and residual felsic melts in the upper Sonju Lake intrusion and overlying Finland granophyre. The textures displayed here are consistent with those resulting from two magma mixing, which would imply that the Finland granite was significantly molten when the mafic dikes were emplaced. The intrusive relationships of the Finland granophyre, the older Lax Lake gabbro along its southern margin (llg, Fig. 2-4), and the younger Beaver River diabase at its eastern extent (unit brd, Figs. 2-4 & 210A) are well-established by several exposures of sharp contacts. However the genetic relationship of the granophyre to the underlying Sonju Lake layered intrusion (Stop 13) along its gradational northern boundary (Fig. 2-10A) is more problematic. Stevenson (1974), noting aplitic granophyric dikes cutting the mafic cumulates of the Sonju Lake intrusion, considered the Finland granophyre to be intrusive into the Sonju Lake intrusion. However, mapping by Miller and others (1993) showed these aplite dikes to be phases of the late hybrid dikes (Fig. 2-10A) that cut the Sonju Lake intrusion and both phases of the Finland granophyre. Again, the termination of such a dike is thought to give rise to the mafic enclaves at this stop. Given the parallel zonation of the two phases of the granophyre with the strike of Sonju Lake cumulate units (Figs. 2-4 & 2-10; also see Miller and others, 1993a) and the generally smooth compositional variations across these units (Fig. 2-10B), a comagmatic relationship resulting from crystallization differentiation could be envisioned. Such an interpretation is inconsistent, however, with the large amount of granophyre relative to the layered mafic rocks apparent from the geologic map (Figs. 2-4 & 2-10). Modeling of gravity and aeromagnetic data across these units (Miller and others, 1990) confirms that the volume of granophyre is at least as great as the volume of underlying mafic rocks and thus is too great to be a differentiate of the layered intrusion. Also, preliminary Sm-Nd and Rb-Sr data indicate that the granophyre has an isotopic composition distinct from the mafic rocks and therefore could not have evolved from them (J. Vervoort, unpublished data). A more plausible interpretation is that the mafic magma that produced the Sonju Lake intrusion was trapped by the less dense Finland granophyre. This underplating resulted in partial melting of the granophyre and the development of a diffuse contact. Petrologic and isotopic studies are needed to determine the extent of assimilation across the mafic-felsic contact. --- Return downhill to Lake Co Rd 7. Turn left and proceed 5.6 miles to Sonju Lake Forest Rd. Take the Sonju Rd 3.1 miles to an area of low outcrops--- Page 76 �STOP 2-13: Sonju Lake Intrusion T.58N., R.7W., Sec. 27 Finland 7.5' quadrangle UTM: 633180E, 5256520N Highlights: cumulate stratigraphy of Pl+Ol (slt) -> Pl+Cpx+Ol (slg) -> Pl+Cpx+Ox+/-Ol (slfg) The Sonju Lake intrusion (SLI) is the most completely differentiated mafic layered intrusion recognized in the Keweenawan magmatic system (Stevenson, 1974; Weiblen, 1982). The intrusion occurs as a shallow (15-30°) south- to southeast-dipping, 1200-m-thick sheet of mafic cumulates underlying a granophyre body of similar thickness. The SLI is exposed in outcrop over an area extending about 3 km west from its abrupt truncation by the Beaver River diabase dike of the FTMD structure (Fig. 2-10A). However, it can be traced beneath glacial cover by its aeromagnetic signature for a strike length of about 15 km (Fig. 2-4). Above a basal contact zone of fine- to medium-grained melatroctolite (unit slmt) intrusive into older gabbroic rock, five map units within the SLI are distinguished by their cumulus mineralogy: Ol (unit sld)→ Pl+Ol (unit slt)→Pl+Cpx+Ol (unit slg)→Pl+Cpx+Ox±Ol (unit slfg)→Pl+Cpx+Ox+Ol+Ap (unit slad). At the upper contact with the overlying Finland granophyre is a complex unit (slmd) characterized by a cyclical loss of igneous lamination and enrichment in granophyre at the expense of mafic phases. This igneous stratigraphy is complimented by a smooth, logarithmic cryptic layering exemplified by decreasing mg# values of pyroxene (Fig. 2-10B). Taken together, these characteristics are consistent with formation of the SLI by closed-system fractional crystallization of a moderately evolved tholeiitic basaltic magma (Miller and Ripley, 1996). At this stop, we will traverse the middle cumulate units of the SLI. Exposed in low outcrops on either side of the Sonju Lake Forest Road is coarse-grained, moderately foliated, ophitic olivine gabbro typical of the upper part of the slt unit. It is a Pl+Ol cumulate with abundant (15-20%) augite oikocrysts up to 8 cm in diameter. Downsection from here, the slt unit is more typically an ophitic augite troctolite with olivine about twice as abundant as augite and the latter as 1- to 3-cm oikocrysts (Fig. 2-11). The increased mode and size of augite oikocrysts here presages the approaching cumulus arrival of augite. The arrival of cumulus augite is indicated by its abrupt change from ophitic to granular habit (Fig. 211), which can be observed in intermittent outcrop about 70 paces (~100 m) south of the road. This singular cumulus phase transition from Pl+Ol to Pl+Cpx+Ol is traceable across the entire exposure area of the SLI and serves as a useful datum horizon (Fig. 2-10). The slg unit is about 70 m thick and is typically composed of 60-65% plagioclase, 28-32% subprismatic augite, 5-10% olivine and 1% interstitial Fe-Ti oxide. The rock is consistently coarse-grained and well foliated. Continuing south across a low swamp, the outcrop exposed on the north face and crest of the next ridgeline is medium-grained, well foliated oxide gabbro of the slfg unit. The cumulus arrival of Fe-Ti oxide is defined by its granular habit and its increase in modal abundance to 7-10% (Fig. 2-11). Olivine is commonly in low abundance or absent. This unit is over 200m thick, and in its medial section hosts an 80m-thick interval enriched in PGE (Miller, 1999; Joslin, 2004). Page 77 �91 12 10" — • sidp SNA-li Slit //> Map Units 91 10 4730 sig —. frqm ? :-: e:itei//ii e METER frg . 800 slmd r l1d 600 . 400 n 200 -3- c: x+ slig fl Beaver River diahase -3- /23 BRD-Sl I hybrid dikes Slid • Augile Arrival Soaju Lake Intrusion 7i0hç sirwt olivine lerromonzodiorite si3d apatite ol dionte (PAP05543 slig oxide gtihhio (PAS) sig gahhro sit nociohte P0) slit dumte (0) stmt line inelutioctoliie (OP) Os < -200 (PA 0) —47 27' 30' sld t micrc,graphic granite quaii lenoinoniodiotile -400 8CpxQpx Finland Oranophyrc slmt S + . Western Profiles (1 &2) Eastern Profiles (3-5) harahboiclc 501) oaks SLI-1 Drill Core -600 Volcanic rocks 100 80 60 40 20 MgO/(MgO+FeO) Figure 2-10. A) Geology of the Sonju Lake intrusion and overlying Finland granophyre showing the locations of stops 12 and 13 (modified from Miller and others, 1993). B) Cryptic variation of MgO/(MgO+FeO) (mole %) in augite and orthopyroxene through the Sonju Lake intrusion and Finland granophyre (from Miller and Ripley, 1997). Figure 2-11. Textures of cumulate rocks from the middle units of the Sonju Lake intrusion. Light phase is plagioclase, outlined light gray phase is olivine, dark gray phase is augite, and black phase is Fe-Ti oxide. Page 78 0 �--- Return to Co Rd 7. Proceed north (to left) about 10 miles to Hoist Lake Rd (just past the Trestle Inn on Crooked Lake). Turn left and go 4.5 miles to junction with snowmobile trail heading to right-- STOP 2-14: Northwestern margin of the Beaver Bay Complex T.59N., R.7W., Sec. 12 Cabin Lake 7.5' quadrangle UTM: A - 638780E, 5274975N B - 638460E, 5274980N C - 638125E, 5274755N D - 638565E, 5275005N Highlights: Houghtaling Creek trocolite (hct); Dam Five gabbronorite (dfgn) with inclusion of gabbroic anorthosite (klga); Shoepack Lake inclusion-rich diorite (slid) This sequence of four exposure areas shows the major units comprising the northwestern margin of the Beaver Bay Complex (Miller and others, 1994). The first exposure (Area A) is of a medium coarsegrained, moderately foliated, layered troctolite cumulate with minor (<5%) ophitic augite forming the northwestern margin of the Houghtaling Creek troctolite (Figs. 2-4 and 2-12). The layering and foliation dip about 30º SE. This intrusion occurs as a 2-5 km wide, northeast-trending macrodike, which can be traced along strike for over 50 kilometer in outcrop and by its distinctive aeromagnetic signature (Fig. 27). Intermittent olivine layering and foliation define an asymmetric trough structure with its shorter limb on the southeast side (Fig. 2-12B). ---Following the snowmobile trail north of the road for about 100 m to a junction of trails. Take the trail to the left (west)--Pavement outcrops on the snowmobile trail and in a larger outcrop on the north side of the trail (Area B) are medium-grained, well-foliated gabbronorite cumulates of the Dam Five gabbronorite (dfgn, Fig. 2-12). The rock is composed of 50% plagioclase, 20% subprismatic augite, 27% subprismatic inverted pigeonite, and 3% subpoikilitic Fe-Ti oxide and would classify as a Pl+Cpx+IPig cumulate. Well-developed foliation dips about 27º SSE. This unit occurs exclusively along the northwestern margin of the Houghtaling Creek troctolite. Although a contact between the troctolite and gabbronorite is not observed and their internal structures are nearly conformable, the troctolite does show some fining toward the inferred contact and is considered to be intrusive into the gabbronorite. The very different cumulus mineralogies of the two units, which cannot be related by any conventional differentiation mechanism, is further evidence that these are distinct intrusive units (Fig. 2-12B). --- Continue along snowmobile trail to junction with main road. After crossing Hoist Creek bridge and meeting another road junction, take the trail to the south and then follow flags to Area C outcrops --- Page 79 �The exposures at the crest of this hill (Area C) are of coarse-grained gabbroic anorthosite, which occurs as a large inclusion in the Dam Five gabbronorite. The inclusion is composed of 90-95% moderately foliated plagioclase, 3-7% ophitic augite and 2-3% poikilitic oxide. It is mapped as an inclusion of the Katydid Lake gabbroic anorthosite (klga; see location map), but its ophitic texture is more like lithologies found in the deeper anorthositic series of the Duluth Complex (as, Fig. 2-4; olga, Fig. 212A). --- Return to road junction and continue west and north to clear cut area. Follow flags over crest of hill to outcrop ledge--NW SE A. B. Figure 2-12. Interpretive cross-sections through the northern Beaver Bay Complex from geologic maps by A) Boerboom and Miller (1994) and B) Miller and others (1994). Map unit abbreviations: nsf- felsic volcanics, slid - Shoepack Lake inclusion-rich diorite; slAi - Archean inclusions in slid; whgg - Whitefish Lake granophyre, felsic series of the Duluth Complex; olga, sclg, klga - phases of the anorthositic series of the Duluth Complex; dfgn - Dam Five gabbronorite; hct - Houghtaling Creek troctolite. Exposed in a 5' ledge on the western crest of the clear cut hill (Area D) is a typical exposure of the earliest intrusive unit of the Beaver Bay Complex - the Shoepack Lake inclusion-rich diorite (slid). This unit is composed of a heterogenous mix of aphanitic to very fine-grained diorite and inclusions of predominantly felsite (icelandite and K-feldspar-phyric rhyolite) and lesser amounts of basalt, gabbroic anorthosite, gabbro and locally Archean? rock types (granite, gneiss, amphibolite, schist). Inclusions range in size from single quartz xenocryts to blocks 50m across. Most inclusions exposed here are of felsic volcanic rocks irregularly mixed and partially assimilated into the diorite. However, at the northern end of the exposure, is an inclusion of a cross-bedded interflow sediment hornfels. The rock is composed of a granular mix of plagioclase, oxide and minor pyroxene, with oxide and plagioclase defining graded bedding. The pyroxene (~5-10%) is probably metamorphic. To the north, Archean xenoliths are common. These occurrence reinforce the interpretation that the pronounced gravity low in this area reflects a prominent southeast-trending basement ridge that divides the Keweenawan igneous rocks into two basins of intrusion and eruption (Boerboom, 1994; Miller and Chandler, 1997). The Shoepack Lake diorite is interpreted to be an intrusive breccia that was the first northeast-trending intrusion to breach this crustal ridge and intrude into a rhyolite-dominated part of the NSVG overlying the ridge (Fig. 2-12). This breach of the crustal ridge paved the way for subsequent intrusions, including the Dam Five gabbronorite and the Houghtaling Creek troctolite, to intrude along this same crustal weakness and thus create the composite grouping of northeast-trending intrusions that form the northern BBC (Figs. 2-4, 2-12). The presence of anorthositic series inclusions in this rock suggests that it may be one of the older intrusions in the greater Duluth Complex, perhaps correlative with early stage magmatism. Page 80 �--- Return east to Co Rd 7. Turn left and proceed to Forest Rd 170. Go right about 2.5 miles and then turn left on the secondary forest road to Whitefish Lake. Proceed about 4.0 miles to a gravel pit road on left. Park and walk about 150m south on the forest road to low outcrops on the west side of the road. --- STOP 2-15: Contact between the felsic and anorthositic series of the Duluth Complex T.60N., R.6W., Sec. 2 Wilson Lake 7.5' quadrangle UTM: 646050E, 5286185N Highlights: Contact between Whitefish Lake granophyre (whgg) and flow foliated gabbbroic anorthosite of the Outlaw Lake gabbroic anorthosite (olga). In the low pavement exposure, a sharp contact can be observed between medium-grained pink granophyre and medium-grained, swirly foliated, ophitic oxide gabbroic anorthosite. A bimodal grain size of well-foliated plagioclase in the gabbroic anorthosite imparts a trachytic texture to the rock. Several lines of evidence indicate that the gabbroic anorthosite is intrusive into the granophyre: 1) the gabbroic anorthosite shows a subtle decrease in grain size toward the contact whereas the granophyre show no change in texture as the contact is approached. Gabbroic anorthosite in gravel pit outcrops, about 50 m to the north, is considerably coarser grained than observed here. Two miles to the northeast, a more obvious chill of the gabbroic anorthosite against granophyre is noted. 2) well-developed, but irregular-oriented foliation in the gabbroic anorthosite appears to be concordant within a meter of the contact; and 3) felsic apophyses emanate from the granophyre and into the gabbroic anorthosite, probably caused by back veining of partially melted granophyre at the intrusive contact. This exposure provides rare field evidence for the older age of the felsic series relative to the anorthositic series of the Duluth Complex. This inferred age relationship was reaffirmed by U-Pb zircon dating, which yielded an 1109.6±4 Ma age for the Whitefish granophyre (Sandland and others, 2001) compared to 1099.1 and 1099.6 Ma ages for two anorthositic series samples dated by Paces and Miller (1993). The early stage emplacement of felsic magmas, which gave rise to the granophyre bodies that now dot the upper contact of the Duluth Complex (Fig. 2-3), helps explain the subsequent emplacement of the multiple, massive intrusions of the Duluth Complex. These felsic bodies probably acted as density traps that triggered underplating of first, ferrogabbroic magmas, creating the early gabbro series, then, plagioclase-phyric magmas (Fig.2-12A), creating the anorthositic series, and finally, aphyric mafic magmas, creating the many discreet intrusions of the layered series (Miller and Ripley, 1996; Miller and others, 2002). In the gravel pit are exposures of several varieties of anorthositic rocks that show complex lithologic and structural relationships typical of the anorthositic series (Miller and Weiblen, 1990). Page 81 �--- Return south to Forest Rd 170, then west to Co Rd 7. Proceed north (right) to Forest Rd 172 (Wanless Rd). Go left on 172 for about 2.9 miles to quarry entrance on right side of road (Permission required for entry) --- STOP 2-16: "Lake Superior Green"quarry T.60N., R.7W., Sec. 35 Silver Island Lake 7.5' quadrangle UTM: 636532E, 5278342N Highlights: Dimension stone quarry; porphyritic gabbroic anorthosite (klga); diabase dikes. Cold Spring Granite Company has been operating a dimension stone quarry at this site for the past 10 years. The rock is marketed as Lake Superior Green, but its geologic name is the Katydid Lake gabbroic anorthosite (Boerboom and Miller, 1994; Miller and others, 1994). The rock is a coarse-grained plagioclase-porphyritic gabbroic anorthosite and is composed of 60-70% large (2-3 cm), blocky plagioclase phenocrysts in a medium-grained matrix composed 65% plagioclase, 20% prismatic augite, 510% granophyric mesostasis, and 5-10% accessory phases of oxide, biotite, amphibole, and apatite. The subequant habit of the plagioclase precludes the development of a foliation, though locally some hint of alignment is evident. Locally, small (10-30 cm) rounded inclusions of coarse-grained anorthosite or granulated (tectonized) anorthosite are found. Along the southern wall of the quarry, the gabbroic anorthosite is cut by 10- to 50-m-wide dikes of fine-grained diabase. This rock is thought to be an offshoot from the main mass of the anorthositic series, which occurs about 2 kilometers to the north and is locally called the Outlaw Lake gabbroic anorthosite (Boerboom and Miller, 1994). END OF TRIP ---Continue west along Forest Road 172 10 miles to MN Hwy. 1. Turn left and proceed southeast to Hwy 61 at the shore. Turn right and proceed to Duluth--- Page 82 �References Basaltic Volcanism Study Project (BVSP), 1981, Pre-Tertiary continental flood basalts: in Basaltic Volcanism on the Terrestrial Planets, NY, Pergamon Press, p. 30-77 Boerboom, T.B., 1994, Archean crustal xenoliths in a Keweenawan hypabyssal sill, northeastern Minnesota. White was right!: Proceedings of the 40th Annual Institute on Lake Superior Geology, Program and Abstracts, p. 5-6. Boerboom, T.J., Green, J.C., and Jirsa, M.A., 2002a, Geologic map of the French River and Lakewood quadrangles, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-128, scale 1:24,000 Boerboom, T.J., Green, J.C., and Jirsa, M.A., 2002b, Geologic map of the Knife River quadrangle, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map M-129, scale 1:24,000 Boerboom, T.J., Green, J.C., and Miller, J.D., Jr., 2003a, Geologic map of the Two Harbors quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-139, scale 1:24,000 Boerboom, T.J., Green, J.C., and Miller, J.D., Jr., 2003b, Geologic map of the Castle Danger quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-140, scale 1:24,000 Boerboom, T. J., and Miller, J. D., Jr., 1994, Geologic map of the Silver Island Lake, Wilson lake, and western Toohey Lake quadrangles, Cook and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, Map M-81, scale 1:24,000 Bonnichsen, B. and Kauffman, 1987, Physical features of rhyolite lava flows in the Snake River Plain volcanic province, southwestern Idaho: in The Emplacement of Silicic Domes and Lava Flows, J. H. Fink, ed., p. 119145 Brannon, J.C., 1984. Geochemistry of successive lava flows of Keweenawan North Shore Volcanic Group: unpublished Ph.D. Thesis, Washington University, St. Louis, 212pp. Carmichael, I.S.E., 1964, The petrology of Thingmuli, a Tertiary volcano in eastern Iceland: Jounral of Petrology, v. 5, p. 435-460 Chandler, V.W., 1990, Geologic interpretation of gravity and magnetic data over the central part of the Duluth Complex, northeastern Minnesota: Economic Geology, v. 85, p. 816-829 Davis, D.W., and Green, J.C., 1997, Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution: Canadian Journal of Earth Science, v. 34, p. 476-488 Davis, D. W. and Paces, J. B., 1990, Time resolution of geologic events on the Keweenaw Peninsula and implications for development of the Midcontinent Rift system: Earth and Planetary Science Letters, v. 97, p. 54-64. Gehman, H.M., Jr., 1957, The petrology of the Beaver Bay Complex, Lake County, Minnesota: unpublished Ph.D. dissertation, University of Minnesota, Minneapolis, 300p. Goldich, S.S., Nier, A.O., Baadsgaard, H., Hoffman, J.H., and Krueger, H.W., 1961, The Precambrian geology and geochronology of Minnesota: Minnesota Geological Survey Bulletin 41, 193 p. Greeley, R., 1982, The Snake River Plain, Idaho: Representative of a new category of volcanism: Journal of Geophysical Research, v. 87, N. B4, p. 2705-2712. Green, J.C., 1972, North Shore Volcanic Group, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota—A centennial volume: Minnesota Geological Survey, p. 294-332. Green, J.C., 1977, Keweenawan plateau volcanism in the Lake Superior region, in Baragar, W. R. A., Coleman, L. C., and Hall, J. M., eds., Geological Association of Canada Special Paper 16, p. 407-422. Green, J.C., 1982, Geologic Map Atlas of Minnesota, Two Harbors Sheet: Minnesota Geological Survey, scale 1:250,000. Green, J.C., 1989, Physical volcanology of mid-Proterozoic plateau lavas: The Keweenawan North Shore Volcanic Group, Minnesota: Geological Society of America Bulletin, v. 101, p. 486-500. Green, J.C., 1990, Primary tridymite crystallization and inferences from tridymite and quartz textures in high-T Keweenawan rhyolites and granophyres, Minnesota (abs.): Geological Soc. of America, Abstracts with Programs, v. 22, p. A289-290. Green, J.C., 1992, Geologic map of the north shore of Lake Superior, Lake and Cook Counties, Minnesota: Part 1. Little Marias to Tofte: Minnesota Geological Survey Miscellaneous Map Series, M-71, scale 1:24,000 Page 83 �Green, J.C., 1995, Chemostratigraphy at the fringe of the Midcontinent Rift System: The northeast limb of the North Shore Volcanic Group, Minnesota (ext. abs.): in Proceedings, Petrology and Metallogeny of Volcanic and Intrusive Rocks of the Midcontinent Rift System: 1995 IGCP Project 336 Field Conference and Symposium, p. 55-56 Green, J. C. and Fitz, T. J. III, 1993, Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent Rift plateau volcanics, Minnesota: petrographic and field recognition: Journal of Volcanology and Geothermal Research, v. 54, p. 177-196 Green, J.C., Davis, D.W., and Schmitz, M.D., 2001, Three new zircon dates for the Midcontinent Rift, North Shore, Minnesota: more data, more questions: 47th Institute on Lake Superior Geology, Proceedings and Abstracts, p. 29. Irvine, T.N., and Baragar, W.R.A., 1971, A guide to the chemical classification of the common volcanic rocks: Canadian Journal of Earth Sciences, v. 8, no. 5, p. 523-548. Jerde, E.A., 1991, Geochemistry and petrology of hypabyssal rocks associated with the Midcontinent rift, northeastern Minnesota: unpublished Ph.D. Thesis, University of California, Los Angeles, 305 pp. Jirsa, M. A., 1984, Interflow sedimentary rocks in the Keweenawan North Shore Volcanic Group, northeastern Minnesota: Minnesota Geological Survey Report of Investigations 30, 20 pp. Joslin, G.A., 2004, Stratiform Pd-Pt-Au mineralization in the Sonju Lake intrusion near Finland, Minnesota: unpublished M.S. Thesis, University of Minnesota Duluth. Klewin, K.W., and Shirey, S.B., 1992, The igneous petrology and magmatic evolution of the Midcontinent Rift System: Tectonophysics, v. 213, p. 33-40. Manley, C. R., 1996, Physical volcanology of a voluminous rhyolite lava flow: The Badlands lava, Owyhee Plateau, southwestern Idaho: Journal of Volcanology and Geothermal Research, v. 71, p. 129-153 McBirney, A.R., and Williams, H., 1969, Geology and petrology of the Galapagos Islands: Geological Soc. of America Memoir 118, 197p. Miller, J.D., Jr., 1988, Geologic map of the Split Rock Point NE and Silver Bay quadrangles, Lake County, Minnesota: Minnesota Geological Survey, Miscellaneous Map Series, M-68, scale 1:24,000 Miller, J.D., Jr., 1999, Geochemical evaluation of platinum group element (PGE) mineralization in the Sonju Lake intrusion, Finland, Minnesota: Minnesota Geological Survey Information Circular 44, 32 p. Miller, J.D., Jr., and Chandler, V.W., 1997, Geology, petrology, and tectonic significance of the Beaver Bay Complex, northeastern Minnesota: in Ojakangas, R.J., Dickas, A.B., Green, J.C., (eds.) Middle Proterozoic to Cambrian Rifting, Central North America: Geological Society of America Special Paper 312, p. 73-96. Miller, J.D., Jr., and Ripley, E.M., 1996, Layered intrusions of the Duluth Complex, Minnesota, USA: in Cawthorne, R.G., ed., Layered Intrusions: Amsterdam, Elsevier Science, p. 257-301 Miller, J.D., Jr., and Vervoort, J.D., 1996, The latent magmatic stage of the Midcontinent Rift: a period of magmatic underplating and melting of the lower crust: 42nd Annual Institute on Lake Superior Geology, Proceedings Volume, Part I – Program and Abstracts, p. 33-35 Miller, J.D., Jr. and Weiblen, P.W., 1990, Anorthositic rocks of the Duluth Complex: Examples of rocks formed from plagioclase crystal mush: Journal of Petrology 31, p. 295-339 Miller, J.D., Jr., Boerboom, T.B., and Jerde, E.A., 1994, Bedrock geologic map of the Cabin Lake and Cramer 7.5minute quadrangles, Lake and Cook Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-82, scale 1:24,000 Miller, J.D., Jr., Green, J.C., Boerboom, T.B., & Chandler, V.W., 1993, Geology of the Doyle Lake and Finland quadrangles, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-72, scale 1:24,000 Miller, J.D., Jr., Green, J.C., and Boerboom, T.B., 1989, Geology of the Illgen City quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-69, scale 1:24,000 Miller, J.D, Jr., Schaap, B.D., and Chandler, V.W., 1990, The Sonju Lake intrusion and associated Keweenawan rocks: geochemical and geophysical evidence of petrogenetic relationships: Proceedings of the 36th Annual Institute on Lake Superior Geology, Part 1. Abstracts, p. 66-68 Page 84 �Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-119, scale 1:200,000 Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, 207 p. Milner, S.C., Duncan, A.R., and Ewart, A. , 1992, Quartz latite rheoignibrite flows of the Etendeka Formation, northwestern Namibia: Bulletin of Volcanology, v. 54, p. 200-219 Morrison, D.A., Ashwal, L.D., Phinney, W.C., Shih, C., and Wooden, J.L., 1983, Pre-Keweenawan anorthosite inclusions in the Keweenawan Beaver Bay and Duluth Complexes, northeastern Minnesota: Geological Society of America Bulletin, v. 94, p. 206-221. Nicholson, S. W., Shirey, S. B., Schulz, K. J., and Green, J. C., 1997, Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development: Canadian Journal of Earth Science, v. 34, p. 504-520. Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: geochonological insights to physical, petrogenetic, paleomagnetic and tectonomagmatic processes associated with the 1.1 Ga Midcontinent rift system: Journal of Geophysical Research, v. 98, no.B8, p. 13,997-14,013. Pope, N.M., 1976, Petrology and structure of the Late Precambrian mafic sills east of Silver Creek, Lake County, Minnesota: Unpublished M.S. thesis, University of Minnesota Duluth, 157 p. Sandland, T.O., Wirth, K.R., Vervoort, J.D., Gehrels, G.E., Kennedy, B.C., and Harpp, K.S., 2001, Roles of fractional crystallization and assimilation in the production of Midcontinent rift granophyres: 47th Institute on Lake Superior Geology, Abstracts and Proceedings, p. 85-86. Schmidt, S. Th., 1993, Regional and local patterns of low-grade metamorphism in the North Shore Volcanic Group, Minnesota, USA: Journal of Metamorphic Geology, v. 11, p. 401-414. Schmidt, S. Th. And Robinson, D., 1997, Metamorphic grade and porosity and permeability controls on mafic phyllosilicate distributions in a regional zeolite to greenschist facies transition on the North Shore Volcanic Group, Minnesota: Geological Society of America Bulletin, v. 109, No. 6, p. 683-697. Schwartz, G.M., and Sandberg, A.E., 1940, Rock series in diabase sills at Duluth, Minnesota: Geological Society of America Bulletin, v. 51, p. 1135-1172. Sigvaldason, G. E., 1974, Basalts from the centre of the assumed Icelandic mantle plume: Journal of Petrology, v. 15, p. 497-524. Stevenson, R.J., 1974, A mafic layered intrusion of Keweenawan age near Finland, Minnesota: unpublished M.S. thesis, University of Minnesota Duluth, 160 pp. Vervoort, J. D. and Green, J. C., 1997, Origin of evolved magmas in the Midcontinent Rift System, northeast Minnesota: Nd-isotope evidence for melting of Archean crust: Canadian Journal of Earth Sciences, v. 34, p. 521-535. Wallace, A.B., Drexter, J.W., Grant, N.K., and Noble, D.C., 1980, Icelandite and aenigmatite-bearing pantellerite from the McDermitt caldera complex, Nevada-Oregon: Geology, v. 8, p. 380-384. Weiblen, P.W., 1982. Keweenawan intrusive igneous rocks. In: Wold, R.J. & Hinze, W.J. (eds.) Geology and tectonics of the Lake Superior Basin: Geological Society of America Memoir 156, 57-82. Wood, D. A., 1978, Major and trace element variations in the Tertiary lavas of eastern Iceland and their significance with respect to the Iceland geochemical anomaly: Journal of Petrology, v. 19, p. 393-436 Page 85 �FIELD TRIP 3 LATE WISCONSINAN SUPERIOR-LOBE DEPOSITS IN THE SUPERIOR BASIN NORTHEAST OF DULUTH Howard Hobbs Minnesota Geological Survey INTRODUCTION The recession of the Superior lobe was marked by many readvances. We will examine glacial striations and tills of the Superior lobe along the scenic North Shore of Lake Superior from Duluth to Two Harbors. The stratigraphy in this area will be tied to ice margins and events elsewhere, leading to a consistent glacial history. Each time the ice lobe retreated into the Superior basin, it created a glacial lake, and each subsequent readvance incorporated reddish-brown lake sediment. Each of these lakes is here considered to be a phase of glacial Lake Superior in a broad sense. Each readvance covered a smaller area and reached a lower elevation than the last, and deposited a finer-grained till than the previous one. STRATIGRAPHY Cromwell Formation The oldest glacial sediment exposed in these stops is a rocky, gravelly, reddish-brown (5YR 4/4) till. Its matrix texture is typically a loam, but some samples are extremely low in clay (Fig. 3-1). It is non-to-slightly calcareous, and is deeply leached where exposed at the surface, even where calcareous at depth. This till and associated meltwater sediment has been named the Cromwell Formation (Wright and others, 1970). The characteristic reddish color and abundant red and black clasts, which define the Superior provenance, are derived from the passage of the ice through the Superior basin, and are presumably absent from deposits up-ice from the basin. The till matrix derived its reddish color and some of its red clasts from pulverized red sedimentary rock from the floor of the basin: sandstone, siltstone, and shale. The other red and black rock fragments are primarily extrusive and intrusive igneous rocks from the northwestern lake shore: basalt, rhyolite, diabase, and gabbro. Black slate fragments from the Proterozoic Rove Formation are locally present. In most of its extent (down-ice from the basin), the Cromwell Formation contains many clasts other than those from the Superior basin: gray to black Proterozoic metamorphic clasts, Archean granite and greenstone, and Paleozoic carbonate. These clasts are mainly derived from older Quaternary sediment and local bedrock. Within the Superior basin, the clast assemblage in the Cromwell Formation in many places is dominated by Superior basin clasts. I call this assemblage a “hyper-Superior” assemblage, as opposed to the mixed Superior assemblage down-ice. Inasmuch as the ice itself did not originate inside the Superior basin, the scarcity of clasts from up-ice of the basin implies that the rate of bedrock erosion is much higher within the basin than outside, which explains why the basin formed in the first place. The standard grain size for clast counting and determining provenance is 1-2 millimeters (very coarse-grained sand), and this size is used unless otherwise specified. Barnum Formation The remainder of the glacial and glaciolacustrine sediment in the area is referred to as the Barnum Formation (Fig. 3-2). The Barnum Formation was originally defined to include clayey Page 86 �till overlying the Cromwell Formation (Wright and others, 1970). Hobbs (2002, 2003a, b) recognized three texturally distinct tills above the Cromwell Formation. The tills are overlain and separated in places by reddish lake and deltaic sediment, which ranges in texture from silt to clay (Fig. 3-1), and contains very little sand and almost no coarse-grained fragments. This sediment ranges from massive to laminated, and from grayish-brown to reddish-brown. More detail is provided in the stop descriptions. The individual tills contain inclusions of similar lake sediment in places. These tills and lacustrine sediments are to be included in a modified Barnum Formation (Johnson and others, unpub. data). Until this work is published, the member names will be used informally. Lakewood member The lowest till in the Barnum Formation is the Lakewood member. It is typically reddishbrown (5YR 4/3 to 3/4) noncalcareous silt loam (Fig. 3-1). Its average clay content is only a little more than that of the Cromwell Formation. Its proportion of coarse-grained fragments is typically less than that of the Cromwell till, but more than that of the other tills of the Barnum Formation. In contrast to the Cromwell Formation, large pebbles and boulders are not common. Most samples have a hyper-Superior clast assemblage in the 1-2 millimeter size fraction. A few samples have a small number of Paleozoic carbonate clasts. The till is relatively thin and patchy in the field trip area; it probably averages less than 2 meters in thickness. Moose Lake member The middle till in the Barnum Formation is the Moose Lake member. It is typically reddishbrown (5 to 2.5YR 5/3 to 4/4) calcareous clay loam to silty clay loam (Fig. 3-1). Coarse-grained fragments are sparser than in the brown silty till of the Lakewood member, and large rocks are rare. The typical clast assemblage is Superior, but most are not hyper-Superior. The Moose Lake Page 87 �member contains more clasts from outside the Superior basin (mostly granitic) than does the Lakewood member. Many samples contain secondary carbonate precipitated by soil processes, but fewer than half of the samples have primary carbonate clasts. The number of primary carbonate clasts is small, even in samples that appear to be quite calcareous, which suggests that the bulk of carbonate is in the finer grain sizes. The average thickness of the till in the field trip area is about 3 meters. Page 88 �Wrenshall member Laminated red and gray lacustrine silt and clay in the Nemadji lake plain was named the Wrenshall Formation by Wright and others (1970). It is stratigraphically above the Moose Lake member, and behind the Nickerson moraine at the margin of the Moose Lake member. I am redefining the Wrenshall Formation as a member of the Barnum Formation, because it is a part of the same stratigraphic package of glacial lake sediment and lake-influenced tills. This member has not been observed in the field trip area. Knife River member The upper till of the Barnum Formation is informally named the Knife River member. It is typically reddish-brown (2.5 YR 5/3 to 4/4) calcareous clay (Fig. 3-1), containing few coarsegrained fragments or large rocks. A rough field criterion for differentiating the Moose Lake and Knife River tills is the sound they give off to a shovel. The Moose Lake member goes “clink,” but the Knife River goes “clunk.” That is, almost every shovel thrust into the Moose Lake till encounters a pebble, but most shovel thrusts into the Knife River do not. Where the Knife River till overlies the till of the Moose Lake member, its color is commonly on the same color chip, but a little lighter and redder. The clast assemblage is commonly Superior, rarely hyper-Superior, and occasionally a mixed Superior-Winnipeg assemblage (the Winnipeg assemblage is found in tills deposited by ice that advanced through the Winnipeg lowland. It is characterized by common to abundant Paleozoic carbonate, granitic grains that are more common than dark grains, few red grains, and little to no Cretaceous shale). More than half of the samples contain primary carbonate grains, and many samples contain common to abundant secondary carbonate. Its average thickness is 3 to 4 meters, but appears to be absent in many places close to Lake Superior. Its clay texture is atypical for glacial till, which led Moss (1977) and Gross (1982) to interpret this till as massive glacial lake clay. GLACIAL HISTORY During the most recent glaciation, the field trip area was dominated by the activities of the Superior lobe, which advanced through the axis of the Superior basin from northeast to southwest as far south as the St. Croix moraine (Wright, 1972). The advance to the maximum, and the earlier retreatal phases, deposited till and associated sediments of the Cromwell Formation. Many retreatal or readvance ice margins have been recognized between the St. Croix moraine and the Superior basin (Mooers, 1988; Patterson, 2001), but only the most recent ones are discussed here. As long as the ice did not front a substantial glacial lake between advances, the texture of the till remained more or less the same: rocky loam to sandy loam. The first retreat into the Superior basin did not create a large continuous lake, but rather a set of small ice-marginal lakes trapped between the ice and the sides of the basin. Meltwater escaped by a series of ice-marginal channels, building ice-contact deltas of sand and gravel into ponds on its way out of the basin to the southwest (Stop 3-8). The next advance of the Superior lobe overrode these deltas and deposited a loamy Cromwell till that ends at the Mille Lacs–Highland moraine. The texture of this till does not reflect any large amount of overridden lacustrine sediment. An outwash plain graded to the Highland moraine buries the adjacent part of the Toimi drumlin field. It is graded to a set of channels that seems to end at the St. Louis River. The present day St. Louis River flows east and south, into the Superior basin, so meltwater flow from the Highland moraine maximum must have flowed west (reversed) into glacial Lake Upham I. The flowpath out of Lake Upham I is unknown; deposits of the St. Louis sublobe later covered this whole area (Fig. 3-2). As the ice receded from the Highland moraine, a set of meltwater channels was formed between the receding ice in the basin, and the higher ground to the northwest. These channels are discontinuous, and must have partly formed on the ice itself. Their general flow direction is to Page 89 �the southwest, and they probably exited the Superior basin at its southwestern corner, where bedrock elevations are the lowest. Deposits of subsequent readvances have since covered this area, but the bottom of the most recently used lake spillway is about 1,050 feet in elevation. Gravel in these channels is somewhat younger than the gravel in the ice-contact deltas, but is also included in the Cromwell Formation. Further retreat opened up a lake in the southwestern corner of the Superior basin that extended about as far northeast as Two Harbors. The advance that followed was named the Split Rock phase by Wright (1972). It spilled out of the basin and deposited a thin, reddish-brown (5YR) till, which I have correlated to the Lakewood member of the Barnum Formation. This advance extended well out of the Superior basin deep into Pine County, where it apparently formed the Split Rock, Askov-Lookout Tower, and Kerrick ice margins. This correlation is uncertain, and the till texture is not exactly the same in Pine County as in the field trip area, but till behind this set of margins is finer-grained than that of the earlier phases (Patterson, 2001). The Lakewood member does not exhibit a morphologically distinct ice margin in the field trip area. Its margin is recognized only by the change in texture of the surface till, from that typical of the Cromwell Formation to that typical of the Barnum Formation. This change occurs fairly consistently between 1,150 and 1,200 feet in elevation. All three till members of the Barnum Formation have fairly consistent margin elevations in the field trip area. The ice was confined in a steep-sided basin, so their margins are similar to bathtub rings. Theoretically, one would expect the margins to rise to the northeast, both to account for the down-ice slope of the glacier, and differential postglacial rebound. There is little evidence of this rise in the margin of the Lakewood member. This is partly because the data points on which it is based are widely separated, and partly because the mapped margin of the member is not necessarily the actual ice margin, but the margin of preserved deposits. A thin patchy till deposited on a sloping surface is subject to extensive erosion, both by waves and by sheet runoff. In addition, the area mapped so far is rather small, and the expected rise in ice margin is not great. Another retreat opened up a larger portion of glacial Lake Superior; it is unclear how much of the basin was ice-free at this time. Apparently, this lake accumulated a considerable amount of clay, because the advance that followed deposited the clayey Moose Lake member and built the Nickerson moraine at the southwestern margin of the basin. In the field trip area, the Moose Lake member was deposited up to about 1,100 to 1,150 feet in elevation. If the glacier were merely recycling its own clay, it is puzzling why the Moose Lake member is so much more clayey than the Lakewood member. Another puzzle is why the Moose Lake member is so calcareous compared to the Lakewood member. One possibility is a change in the nature of debris carried into the Superior basin from the central Laurentide Ice Sheet. For example, Mooers and Lehr (1997) proposed that the change from calcareous to noncalcareous debris carried by the Rainy lobe (in a broad sense) was correlated to the migration of the ice divide south of the Hudson Bay lowland, so that the Paleozoic carbonates in the lowland could no longer be transported south. The eastern boundary of these lowlands is up-ice from the Superior basin, and could have supplied some carbonate. In fact, a small number of Paleozoic dolomite grains are sporadically present in the otherwise noncalcareous Cromwell Formation and Lakewood member tills, so a Hudson Bay component is quite possible. However, the mechanics of how a reverse transition from noncalcareous to calcareous would work in this setting is difficult to envision. Another possibility is that glacial Lake Upham I drained into Lake Superior between the Split Rock and Nickerson advances. This lake may have been fed by meltwater from the calcareous Itasca or Koochiching lobes to the north, or even the incipient St. Louis sublobe. If so, some of the meltwater probably funneled down the St. Louis River to glacial Lake Superior. Fine-grained calcareous lake sediment was thus potentially available for entrainment into the Moose Lake till. Although the sediment carried down the St. Louis River should have been mostly brown and Page 90 �gray, there was abundant red clay deposited in the lake from the melting Superior lobe, and the red color tends to dominate in mixtures (the gray color is not the result of a pigment, but the lack of pigment). The later advance of the St. Louis sublobe filled the Aitkin-Upham basin and buried the deposits of Lake Upham I. This advance coincided more or less with the Nickerson moraine, as shown by diversion channels around the moraine that can be traced upstream to the St. Louis River (Wright and others, 1970). There is no way to be sure which of these channels were fed by outwash from the St. Louis sublobe, and which were fed by overflow from glacial Lake AitkinUpham. The general pattern is that the earlier, higher diversion channels are narrower, and presumably carried less water than the later, lower ones. One might speculate that the earlier diversion channels carried meltwater from the ice margin, and that the later ones carried water from the glacial lake. At times, Lake Aitkin-Upham was fed by overflow from Lake Koochiching (the eastern arm of Lake Agassiz) at its highest levels (Hobbs, 1983). Thus, a much larger flow of water was available than could have been generated by local melting in the St. Louis River watershed. The succeeding retreat first opened up a small ice-marginal lake in the southwestern corner of the basin, called glacial Lake Nemadji by Winchell (1899). This lake appears to have been small and fronted by ice at first—the rapid decline of its depositional surface from the proximal edge of the Nickerson moraine to Duluth can be most easily explained if the Superior lobe still occupied most of the basin. The outlet of Lake Nemadji was the Portage River channel, which cuts through the Nickerson moraine and joins a diversion channel of the St. Louis River at the town of Moose Lake. The bottom of the Portage River outlet is about 1,050 feet in elevation, but lakesmoothed topography and minor beach ridges can be recognized up to 1,080 feet. This does not mean that the water in the outlet channel was necessarily 30 feet deep; the higher wave-washed topography could have formed while the channel was being deepened. A great thickness of laminated glacial clay and silt (Wrenshall member) accumulated at this time, between the proximal side of the Nickerson moraine and the ice remaining in the basin. Further retreat of the ice uncovered the lower Brule outlet in Wisconsin, which lowered the level of the lake to about 1,020 feet and dried up the Portage outlet (Leverett, 1932). This lower level of the lake has been called glacial Lake Duluth, but I prefer to use Duluth and Nemadji as levels or phases of glacial Lake Superior, rather than giving every phase a separate lake name. A large outflow from glacial Lake Aitkin-Upham II came down the St. Louis River after the Superior lobe melted back from the Nickerson moraine. It washed a large area (about 10 square miles) down to bedrock, upstream from the Nemadji lake plain. Although this scouring event was extremely widespread, it did not expose any substantial area of bedrock below 1,050 feet in elevation, suggesting that the local base level was no lower than 1,050 at the time of the event. Subsequent stream erosion has incised a narrow valley into the bedrock below 950 feet in elevation, but this cutting seems to have been accomplished by long erosion of a much smaller stream graded to a much lower base level. Thus, the last major meltwater flood down the St. Louis River seems to have come during the short time that glacial Lake Superior stood at the Nemadji level. As the Superior lobe retreated, the lake declined in stages as lower outlets were uncovered, until the entire lake basin was ice-free. The water level was much lower than it is today (at least in the field trip area). As the ice retreated north of Lake Superior, Lake Agassiz spilled east into Lake Nipigon, into Lake Superior, and then out into Lake Huron (Clayton, 1983). A considerable amount of suspended sediment from glacial Lake Agassiz was deposited in the basin. Combined with the earlier sediment from Lake Aitkin-Upham, the Superior basin now had a considerable amount of clay-rich calcareous sediment to be entrained by the next glacial advance. Lake Superior remained ice-free for about 1,000 years, from 1100 to 1000 B.P. (Drexler and others, 1983). Page 91 �The last, or Marquette advance, filled the basin with ice again, depositing a stone-poor, very clayey 2.5YR till, which I have included in the Knife River member of the Barnum Formation. This till is much more clayey on average than the clayey Moose Lake member (Fig. 3-1); its matrix is more calcareous, and it contains more actual carbonate clasts. These differences are attributed mainly to incorporation of sediment from Lakes Agassiz and Aitkin-Upham. In the field trip area, the Knife River member extends up to a little less than 1,050 feet in elevation; it extends laterally over the Wrenshall member as far as the proximal side of the Nickerson moraine (Howard Mooers, unpub. data). The field trip takes place in the area mapped in the French River, Lakewood, Knife River, Two Harbors, and Castle Danger quadrangles (Fig. 3-3). I have recently mapped the surficial deposits of the first 4 quadrangles under the USGS STATEMAP program (Hobbs, 2002, 2003a, b). A surficial map of the Castle Danger quadrangle is in progress. I combined the knowledge gained from this mapping with earlier mapping and interpretations in the region to create this new interpretation of Superior lobe recessional history. Page 92 �FIELD TRIP STOPS 8:00 a.m. Leave Radisson Hotel, Duluth; get onto I-35/ Highway 61, heading northeast through Duluth. Just beyond the where US Highway 61 becomes a 4-lane expressway, turn right onto the Scenic North Shore Drive (County Road 61). Continue about 6 miles to the village of French River. Stop 3-1. Superior bluff cut at French River. French River Quad., T.51N., R. 12 W., Sec 19 NE corner; 583815E, 5194075N NAD83 UTMs This wave-cut bluff exposes about 25 feet of glacial sediment above basalt bedrock. The top of the bedrock is 3 to 4 feet above lake level. It has been broken up by weathering and wave action in most places, but there are some surfaces that have been smoothed by glacial abrasion. Striations strike northeast, almost parallel to the shore but a little more westerly. These striations were carved by the main flow of the Superior lobe through the Superior basin. Wherever striations are exposed by wave action along the shoreline, they have a similar orientation. The lower third of the cut is the Lakewood member of the Barnum Formation [(5) 28 63 9] over till of the Cromwell Formation [(18) 52 42 6]. Numbers in brackets represent individual texture samples: (gravel) sand, silt, and clay. Sand, silt, and clay are considered matrix, and their numbers add to 100 percent, rounded to the nearest 1 percent. The number in parentheses is the total of all fragments larger than 2 millimeters, and is expressed as a percent of the original sample: matrix plus coarse-grained fragments. Large rocks were avoided for the texture samples. Both tills are firm but not hard, with a platy structure parallel to the bluff face. The platy structure is probably a weathering phenomenon. These tills are a little less red than they usually appear—7.5 YR rather than 5YR. The Cromwell sample has a hyper-Superior grain assemblage; the Lakewood sample has a Superior assemblage with a trace of carbonate. The middle third of the bluff cut is red clayey till of the Moose Lake member: [(1) 13 29 58 and [(2) 19 30 51]. Its lower contact is flat and sharp. Both the middle and upper tills are fairly firm, with a strong angular blocky structure. The structure and consistency are the best way to distinguish in-place till from the soft structureless mudflow sediment that coats the bluff. Two samples of the Moose Lake member have Superior grain assemblage with a trace of carbonate. The upper third of the bluff is red clay till of the Knife River member: [(0.1) 4 22 74] and [(0.2) 2 26 72]. Both tills are reddish-brown (2.5 YR 5/4); the contact between them was not observed, but inferred from the texture of the samples. This till contains irregular, subhorizontal clay inclusions, which look grayish by contrast to the red till. They range from pinkish gray (7.5YR 6/2) to light brown (7.5YR 6/3). These inclusions are more calcareous than the red till; they are interpreted as incompletely digested clumps of Lake Agassiz-derived clay. The texture samples from this interval contain some of these clay inclusions, which explains why they are so poor in gravel and sand, and so rich in clay, even for till of the Knife River member. Each sample contains only a tiny number of 1-2 millimeter grains—one is a Winnipeg assemblage, one is a Superior-Winnipeg mixture. NEXT: Continue along the scenic drive about 4.5 miles. Turn right on a road leading to a scenic overlook. Park and scramble down the bluff. Notice how the stone wall has broken and dropped down in places where the red clay till it was built on has slumped. This bluff is actively retreating by wave action and slope failure. The stop is the bluff just across a small gully west of the scenic overlook. Page 93 �Stop 3-2. Superior bluff by overlook. Knife River Quad., T.51N., R.12W., Sec 2, SE of SW of NW; 5888830E, 5197725N NAD 83 UTMs The bluff is about 28 feet high, but only the upper half is exposed (unless a storm has washed the lower part since I saw it). The top of the exposed bluff is not at its original elevation. There are a few slump scarps hidden in the trees above the top, and their combined offset makes the top of the bluff 10 to 12 feet lower than the original surface. No bedrock is exposed at this site. The lowest part of the exposure is 4 feet of till of the Cromwell Formation [(5) 36 59 5] and [(14) 43 48 9]. This till is unusually low in clay, even for Cromwell till. It is calcareous, apparently protected from leaching by the calcareous tills above it. Both samples of Cromwell till contain hyper-Superior grain assemblages. Above the Cromwell till is a layer of brown silty till [(11) 17 59 24] only 2 feet thick, overlain by a foot of massive silt [(1) 7 73 20], which is similar in color and texture to the silty till, but lacks stones. The brown silty till is included in the Lakewood member of the Barnum Formation; the brown silt could also be included in the Lakewood member. It could either be massive lacustrine, or actual till that happens to contain few coarse-grained fragments. More digging may help us figure this out. Both are calcareous. Both contain hyper-Superior grain assemblages with no carbonate grains, but the 1-2 millimeter sample from the massive silt is very small. Above the massive silt is a layer of interbedded red (2.5YR 4/4) and gray (10YR 4/1) calcareous lacustrine clay just over a foot thick. A sample of the red clay is slightly more clayey [0.2) 3 15 82] than the gray [(0.7) 3 18 79], but the gray clay feels more slippery, possibly because of a higher content of expandable clays. Both have very small Superior grain assemblages. The main red and gray beds are about an inch thick, but there are red and gray micro-laminations within each bed. There are no silt beds, making it unlikely that these beds represent varves. The bedding is more or less horizontal, but not very regular. Beds pinch and swell; is this original, or were the beds deformed by overriding of the next glacier? There are numerous slickensided joints running through both the brown silt and the red and gray clay. Were these caused by overriding ice, or by modern slumping? The uppermost layer exposed in this bluff is about 2 feet of calcareous red clayey till [(5) 31 25 44] of the Moose Lake member. It is weak red (2.5 YR 4/2) in the lower part, and reddish brown (2.5YR 4/4) in the upper part. A sample has a Superior grain assemblage. Red clay till of the Knife River member is not exposed here, but it may be present at the site. The top of the exposure is at least 3 feet below the bluff top, and the bluff top itself is probably not the original surface. A soil boring taken along the Scenic Highway from a bluff about 20 feet higher than this one penetrated about 16 feet of red clay till over lacustrine sediment, with an intercalated contact. NEXT: Leave the parking lot, drive back north to the Scenic Highway, turn left and go west about 0.5 mile to the intersection of Homestead Road (County Road 42). Turn right and go north across the railroad tracks and the Highway 61 expressway, and turn left on Old North Shore Road (County Road 290). Proceed about 0.5 mile west into the Big Sucker Creek valley, and park by the bridge. Walk upstream about 0.3 mile, following the trail on the northeast side of the stream. The trail traverses a slumped stream cut. Stop 3-3. Big Sucker Creek cut. Knife River Quad., T.51N., R. 12W., Sec 4, SW of NE of SE of NE; 586740E, 5198325N NAD83 UTMs The creek has cut down to and into bedrock in this whole area. At this site the basal 2 feet of the stream cut is basalt. The base of the drift section is rocky till of the Cromwell Formation, about 4 feet thick. Color is dark reddish brown (5YR 3/2). The lower sample is gravelly and very low in clay [(31) 48 46 6]. The upper sample is more clayey but even more gravelly than the Page 94 �lower one [(37) 34 47 19]. The till is slightly calcareous, but the grain assemblage is hyperSuperior with no carbonate grains. The Cromwell till is overlain by about 9 feet of the Lakewood member; this is one of the thickest Lakewood layers exposed in the field trip area. The trail crosses the cut at the level of the Lakewood member. This till is dark reddish brown (5YR 3/2), slightly to moderately calcareous, less rocky than the Cromwell till, but much more so than the overlying till. The results of three texture samples were: [(12) 24 54 22], [(6) 16 50 34], and [(11) 17 51 32]. All samples have a hyper-Superior grain assemblage, with only one carbonate grain among the three. The Moose Lake member is not exposed here. The contact between the Lakewood member and the overlying Knife River member is above the trail, and is not exposed. What appears to be the contact is actually a slump plane. Till of the Knife River member is not well exposed, and only one sample was obtained [(1) 7 22 71]. It is reddish brown (2.5YR 5/4), slightly to moderately calcareous, and has a Superior grain assemblage, with a few carbonate grains and one grain of spherical frosted quartz derived from Paleozoic sandstone. NEXT: Return to the road, drive back to Homestead Road, and turn right (south). Turn left (east) on the Highway 61 expressway, and drive about 3.5 miles, cross the Knife River, and turn right at the first street leading to the town of Knife River. Turn right at the first intersection, by the school, and head back south and west to a rest area overlooking the river. Stop 3-4. Lunch at Knife River rest area. Knife River Quad, T.52N., R.11W., Sec 31, NE of SW of NW; 591965E, 5200050N NAD83 UTMs While eating lunch, we can observe the erosion along the Knife River, and attempts to control it. This stream, like most streams emptying into Lake Superior, is fairly short, with a steep gradient and a greatly fluctuating discharge. Most of the time the level is low, but snowmelt and big rainfalls can turn it into a roaring torrent. NEXT: Return to Highway 61. Drive about 10 miles northeast, through Two Harbors. About 2.5 miles northeast of Two Harbors turn left (northwest) onto Lake County Road 3. Drive up the hill, and down into the Silver Creek valley. Park where the road crosses Silver Creek, about 2 miles from the turnoff. Walk 200 feet west (upstream) along the creek to a cut on the outside of a meander bend. Stop 3-5. Silver Creek cut. Castle Danger Quad., T.53N., R. 10W., Sec 16, SE of NW of SW of SW; 604470E, 5213910N NAD83 UTMs This is a small cut, but it illustrates some things rarely seen in this area: a good exposure of postglacial stream channel and overbank sediment, and a layer of artificial fill. The total height of the cut is about 7 feet. It exposes a foot of red clay fill over alluvium (overbank and channel sediment) over red clay till of the Knife River member, over brown sandy till of the Cromwell Formation. The Cromwell till is reddish-brown (5YR 4/3), moderately calcareous, fairly hard and very pebbly [(10) 30 44 26]. Its grain assemblage is Superior, with some calcite rhombs. They all appear to be amygdule fragments. Even though the stream has only cut a few feet into the till, it has developed a continuous pavement of rocks on its bottom. The Cromwell till is overlain by clay till [(2) 10 27 63] of the Knife River member, with a fairly sharp, wavy contact. This till is reddish-brown, on the same color chip but slightly lighter and redder than the color of the Cromwell Formation. It is calcareous, with a medium to coarsegrained blocky structure. Its grain assemblage is hyper-Superior, with no carbonate grains. Page 95 �The overlying alluvium was not sampled. It consists of 1.5 to 2 feet of dirty gravel channel sediment, overlain by 1 to 1.5 feet of silt and fine-grained sand overbank sediment. There is a thin soil (possibly truncated) in the top of the overbank sediment. The gravel contains clasts up to boulder size, but large clasts are much less common in this layer than in the currently active channel sediment. The difference is that at the time the gravel was being deposited, the stream was still eroding the clay till; the stream is now eroding till of the Cromwell Formation, which has more and larger coarse-grained clasts. The uppermost layer here is a layer of red clay that I interpret as recent fill dirt. It is texturally indistinguishable from the red clay till of the Knife River member, and was probably excavated from that material. But it is out of order stratigraphically, being underlain by a soil developed in recent alluvium, and not itself having a soil cover. NEXT: Return to vehicle. Drive 0.75 mile uphill (northeast) and down into another small valley, a tributary of Silver Creek. Park near the double culvert. Walk downstream (south) 200 feet to a slumped cut on the outside of a meander. Stop 3-6. Tributary to Silver Creek. Castle Danger Quad., T.53N., R.10W., Sec 16, SW of SW of NE; 605195E, 5214645N NAD83 UTMs This cut is a series of slumps, so there is a mixture of good exposure and covered areas. The description is sort of a composite section. The base of the exposure is loamy till of the Cromwell Formation [(16) 39 34 27]. This texture is somewhat clayey for Cromwell till, but not unusual for inside the Superior basin. The till is slightly calcareous, unoxidized brown (7.5YR 4/2); it is not especially pebbly for Cromwell till. The grain assemblage is hyper-Superior; a large 1-2 millimeter sample contains one grain of dolomite. The Cromwell till is overlain by red clay till of the Knife River member. There is an inclusion of Cromwell till near the base of the clay till, and another several feet higher. It is not clear whether the inclusions were created by glacial erosion, or if they are related to slumping on this slope. Elsewhere on the slope is a contact between red clayey till of the Moose Lake member [(13) 31 30 39] and red clay till of the Knife River member, at a higher elevation than the contact between the Cromwell Formation and the Knife Lake member. This suggests that some or all of the contacts may have shifted downslope. Near the top of the cut is a contact between red clay till of the Knife River member and red lacustrine clay. Here the texture of the till is [(3) 4 24 72] and the lacustrine texture is [(0) 3 18 79]. There is scarcely any difference other than the number of coarse-grained fragments. The till has a small hyper-Superior grain assemblage with no carbonate rock fragments, though there is a grain of secondary carbonate. The lacustrine sample has a very small 1-2 millimeter fraction, half of which is secondary carbonate. NEXT: Return to vehicle and continue northeast and north about 2 miles to the intersection of Gun Club Road. Turn left (west) and follow the road 1 mile west, 0.5 mile south, and 2 more miles west. Turn right on Lake County 2; go 0.5 mile north and turn left (west) on Reeves Road, an unpaved township road. Go up the hill, under a power line, and stop in a clearing about 0.5 mile west of County Road 2. Stop 3-7. Striated bedrock. Two Harbors Quad., T.53N., R.11W., Sec 1, SE of SE of NW; 600155E, 5217670N NAD83 UTMs There are several small hornfels outcrops here, flush with the ground. They are striated northwest–southeast, about 90 degrees from the striations we saw earlier today. These striations reflect ice moving up and out of the Superior basin toward the Highland moraine. This is the Page 96 �general orientation of striations in the highlands. The landforms in this area tend to be streamlined northwest–southeast parallel to the striations. These landforms, mainly bedrock, but partly mantled by drift, are named the Highland Flutes (Wright, 1970). This hill is one of the southwesternmost of the flutes. They extend northeast inside the Highland moraine as far as Lutsen. NEXT: Return to Lake County Road 2; turn right and go south 2.5 miles; turn right on Lake County 12, and go 3 miles west and 2.5 miles south where it dead-ends on Lake County 11 (Valley Road). Turn right and travel 7 miles south and west to Homestead Road. Lake County Road 12 becomes St. Louis County Road 41 at the county line. County Road 41 ends at County Road 42, which turns and becomes Homestead Road. Turn left on Homestead Road, travel 1.3 miles south to the entrance of a gravel pit. Stop 3-8. Peterson Gravel Pit. Knife River Quad., T. 52N., R.12W., Sec 15, NE of SE of NW; 587620E, 5204895N NAD83 UTMs This pit is developed in one of the many ice-contact deltas that developed between the Superior lobe and the North Shore highlands. They occur in the elevation range of 1,150 to 1,210 feet, and have been mapped in the French River, Knife River, and Two Harbors quadrangles. They have not been observed east of the Two Harbors quadrangle. The general stratigraphy here is sand and gravel of the Cromwell Formation overlain by till of the Cromwell Formation overlain by till of the Lakewood member of the Barnum Formation. The section currently exposed may be different from the one described, due to continued mining. The stratified unit ranges from fine-grained sand to medium-grained gravel, moderately well sorted. The color of the finer beds is dark reddish-brown (5YR 3/4); the color of the coarsegrained beds is dominated by the individual rock particles, which are mostly red, black, and dark gray. The unit is non-to-slightly calcareous; two samples show hyper-Superior grain assemblages with no carbonate grains. The gravel is subangular to subrounded; it is more rounded than the gravel in the till, but not much. It does not appear to have been transported far by water. Depending on the orientation of the cut, the sand and gravel appears to have either trough crossbedding, or foreset bedding. On the south side of the pit, the foreset beds dip west. The sand and gravel delta is overlain by a noncalcareous loamy till [(4) 39 52 9], ranging from 3 to 8 feet thick. The contact is sharp and wavy. Some of the underlying beds are deformed and truncated, and there is a considerable amount of sand and gravel incorporated into the till in places. The till is hard and dense, with a coarse-grained platy structure; this suggests that it was deposited under the weight of a glacier, rather than as a mudflow deposit. The grain assemblage is hyper-Superior, with no carbonate grains. On the east side, there is a discontinuous layer of silty till [(3) 20 71 9] that overlies the Cromwell till and the delta sand and gravel. Where sampled, it overlies the delta with no intervening till. This is interpreted as till of the Lakewood member. It is noncalcareous, reddishbrown (5YR 4/4), and contains only a few stones. Its grain assemblage is hyper-Superior, with no carbonate grains. The uppermost layer at this site is a sandy, silty, unbedded deposit, 1 to 2 feet thick, interpreted as colluvium. It does not bury a soil, but the surface soil is developed in it. It must therefore not be much younger than the glacial sediments. NEXT: Return to Homestead Road, turn right, and travel south to the Highway 61 expressway; turn right to return south to Duluth. Page 97 �REFERENCES Clayton, L., 1983, Chronology of overflow to Lake Superior, in Teller, J.T., and Clayton, L., eds., Glacial Lake Agassiz: Geological Association of Canada, Special Paper 26, p. 291-307. Drexler, C.W., Farrand, W.R., and Hughes, J.D., 1983, Correlation of glacial lakes in the Superior basin with eastward discharge events from Lake Agassiz, in Teller, J.T., and Clayton, L., eds., Glacial Lake Agassiz: Geological Association of Canada, Special Paper 26, p. 309-329. Gross, L.B., 1982, The stratigraphy and lithology of the glaciogenic sediments of the Two Harbors area, northeastern Minnesota: Minneapolis, Minn., University of Minnesota, M.S. thesis, 151 p. Hobbs, H.C., 1983, Drainage relationships of glacial Lakes Aitkin and Upham and early Lake Agassiz in northeastern Minnesota, in Teller, J.T., and Clayton, L., eds., Glacial Lake Agassiz: Geological Association of Canada, Special Paper 26, p. 245-259. ———2002, Surficial geology of the French River and Lakewood quadrangles, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-127, scale 1:24,000. ———2003a, Surficial geology of the Knife River quadrangle, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map M-137, scale 1:24,000. ———2003b, Surficial geology of the Two Harbors quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-138, scale 1:24,000. Leverett, F., 1932, Quaternary geology of Minnesota and parts of adjacent states: U.S. Geological Survey Professional Paper 161, 149 p., 5 pls. Mooers, H.D., 1988, Quaternary history and ice dynamics of the late Wisconsin Rainy and Superior lobes, central Minnesota: Minneapolis, Minn., University of Minnesota, Ph.D. dissertation, 200 p. Mooers, H.D., and Lehr, J.D., 1997, Terrestrial record of Laurentide Ice Sheet reorganization during Heinrich events: Geology, v. 25, no. 11, p. 987-990. Moss, C.M., 1977, The surficial and environmental geology of the French River quadrangle, St. Louis County: University of Minnesota Duluth, M.S. thesis, 69 p. Patterson, C.J., 2001, Surficial geology, pl. 4 of Boerboom, T.J., project manager, Geologic atlas of Pine County, Minnesota: Minnesota Geological Survey County Atlas C-13, 7 pls., scale 1:100,000. Winchell, N.H., 1899, The geology of Carlton County, chapter 1 of Final report of the Minnesota Geological and Natural History Survey: Minnesota Geological and Natural History Survey, v. 4, 528 p. Wright, H.E., 1972, Quaternary history of Minnesota, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 515-547. Wright, H.E., Mattson, L.A., and Thomas, J.A., 1970, Geology of the Cloquet quadrangle; Minnesota Geological Survey Geologic Map GM-3, 30 p., 1 pl. Page 98 �FIELD TRIP 4 Geology of the Eastern Mesabi Iron Range, Northeastern Minnesota by Richard W. Ojakangas Emeritus, Department of Geological Sciences, University of Minnesota Duluth Mark J. Severson Natural Resources Research Institute, University of Minnesota Duluth Peter K. Jongewaard United Taconite Mining Company John L. Arola Ispat Inland Mining Company Joel Thomas Evers Retired – LTV Mining Company and Douglas G. Halverson Northshore Mining SCALE N 0 5 0 10 10 20 15 20 MILES 30 40 KILOMETERS Babbitt Mesaba Location Map Virginia Mt. Iron ITASCA CO. Buhl Biwabik Virginia Horn Coleraine Grand Rapids ST. LOUIS CO. LAKE CO. Hibbing Keewatin Nashwauk Duluth C omplex Eveleth CASS CO. AITKIN CO. DULUTH CARLTON CO. ke ior La per Su Generalized map of the Mesabi Range (cross-hatched). Note Duluth Complex at east end. Page 99 �INTRODUCTION Iron-formation was described as early as 1866 by Henry Eames for rocks on what was to become the Mesabi Range. Several attempts were made by individuals on their way to the iron mines of the Vermilion Range to find ore on the Mesabi. However, it was not until November 16, 1890, that the first rich iron ore on the Mesabi Range was discovered by the Merritt brothers near what is now Mountain Iron, MN. In 1892, the first shipment from this mine was 4245 tons (White, 1954). Exploration for iron ore ensued and within the next few years, most of the productive parts of the Mesabi Range were discovered. The Mesabi Range is the largest iron range in the United States and is one of the largest in the world. It is 0.25 - 3.0 miles wide and 120 miles long (cover page). The Biwabik Iron Formation, as thick as 750 ft, in general dips gently to the southeast at an angle of about 7-15 degrees. The iron-formation, called taconite, typically contains 30-40% iron and 40-50% SiO2, plus other components (Morey, 1992). In numerous places along the length of the range, silica was leached out, thereby enriching the iron content to 55%+. These pockets became the highgrade natural ore mines; there were more than 500 individual mines prior to merging into larger mines as the ore between adjacent properties was removed. These were very important in making the United States an industrial giant, and were instrumental in providing raw material for WWI and WWII. As the high-grade ore was becoming depleted, the taconite process was developed. In 1967, taconite production exceeded natural ore production. Currently, six taconite plants are in production (Fig. 4-1). Babbitt Chisholm N Ispat-Inland Hibbing Taconite Minorca Pit Buhl 5 4 Keewatin Nashwauk MDDP-7 Butler Taconite (closed) 3 2 Calumet Cliffs-Erie Site (LTV closed) 6 Keewatin Tac Dunka Pit Iron Mine (closed) 12 Minntac 8 Ispat-Inland Virginia MDDP-5 Laurentian Pit United Taconite Evel 7 Hibbing 11 Biwabik eth Aurora MDDP-2 Coleraine Northshore 10 9 Hoyt Lakes 1 MDDP-8 Grand Rapids 0 187000E 192000E 600 197000E 6 202000E 12 18 207000E 24 Miles 217000E 212000E 222000E 227000E 232000E 600 Looking North 8 500 500 U.S. Mined Taconite Intervals 5 4 300 2 1 U.S. U.S. 200 100 10 6 400 Upper Slaty U.S. 3 Slaty Upper U.S. 400 U.S. U.S. U.C. U.S. U.C. Upper Slaty U.C. U.C. L.S. Lower Slaty U.C. L.S. 100 L.S. Lower Slaty L.S. L.S. 0 L.C. L.C. -100 Lower Cherty Lower Cherty L.C. L.C. L.C. Lower Cherty L.C. 300 U.S. 200 U.C. L.S. 12 U.S. U.C. Cherty Upper U.C. 11 U.C. Upper Cherty U.S. U.C. 0 9 7 L.C. L.C. L.C. -100 L.C. -200 -200 -300 Virginia Horn Area -300 Generalized Stratigraphic Sections -400 187000E 192000E 197000E 202000E 207000E ILSG - May, 1993 212000E 217000E 222000E 227000E -400 232000E Figure 4-1: Generalized map of the Mesabi Range showing taconite pits (black) and cities, and in the lower half, a longitudinal section of the Biwabik Iron Formation (compiled by Henry Djerlev, 1993). Mined taconite intervals are shown as black columns adjacent to sections. Modified from Morey, 1993. Page 100 �The name of Biwabik Iron Formation was chosen by Van Hise and Leith (1901, p. 356), “because the word Biwabik is the Chippewa word for a piece or fragment of iron.” The word taconite is also used in discussions pertaining to hard, unoxidized portions of the iron-formation. H.V. Winchell (1882, p. 135) originally called portions of the BIF “taconyte” because he thought the rocks correlated with lower Cambrian rocks in the Taconic Mountains in northern New England. Since that time, many geologists have used taconite in their descriptions of the ironformation and it has thus become firmly established. Perhaps a more proper definition for “taconite” is an economic term for iron-formation from which iron can be profitably extracted after fine-grinding, followed by magnetic separation and pelletizing (Morey, 1993). REGIONAL GEOLOGY The peneplaned Archean craton in the Lake Superior region formed a platform upon which a Paleoproterozoic continental margin assemblage was deposited in Minnesota, Michigan, and Wisconsin. Extension resulted in localized rifts that received thicker accumulations of sediments and volcanic rocks than did adjacent parts of the platform. Seas transgressed onto the continent one or more times and an ocean basin opened south of present-day Lake Superior. Island arcs that formed during southward subduction collided with the craton margin to the north as the ocean basin closed. A remnant of this oceanic crust is poorly preserved as a dismembered ophiolite sequence in Wisconsin (Schulz, 1987, 2003). The arc volcanics are preserved as the Wisconsin magmatic terranes. The collision resulted in a fold-and-thrust belt known as the Penokean orogen. To the north of the fold-and-thrust belt, a northward-migrating foreland basin—the Animikie basin—developed as the stacked thrusts weighed down the crust (Fig. 4-2). Thick turbidite successions were deposited along the basin axis, and terrigenous clastics and Lake Superior-type iron-formation were deposited on the shelf along the northern margin (i.e., the foreland or peripheral bulge) of the basin. See Ojakangas et al. (2001) and Severson et al. (2003) for more detailed summaries on Paleoproterozoic basin development in the Lake Superior region. The development of the Midcontinent Rift System (MRS) at 1.1 Ga severed the basin into northwestern and southeastern segments (Fig. 4-2). If the MRS rocks are removed from the geologic map, the different portions of the Animikie basin become contiguous and the fold-andthrust belt rocks of Minnesota and Wisconsin-Michigan become continuous (Fig. 4-3). Figure 4-4 is an interpretive cross-section of the Animikie basin during its formative stages, with sediments derived from the Archean basement to the north and from the fold-and-thrust belt to the south. The Paleoproterozoic supracrustal rocks in the northwestern segment, including east-central and northeastern Minnesota and the adjoining part of Ontario, are for the most part poorly exposed. However, mining of iron ore on the Mesabi and Cuyuna Ranges and continued mining of taconite on the Mesabi Range have resulted in excellent artificial exposures and an abundance of drill hole information. Geophysical surveys and stratigraphic test drilling by the Minnesota Geological Survey also have been major sources of information (e.g., Southwick et al., 1988). Animikie Group The Animikie Group unconformably overlies the Mille Lacs and North Range Groups to the south and the Archean basement to the north (Southwick and Morey, 1991). Magnetic data show North Range structures are present beneath Animikie strata to the east of the exposed North Range Group (Chandler, 1993). Page 101 �Figure 4-2: Generalized geologic map showing the distribution of Precambrian rocks and structural elements of the Lake Superior region, modified from Ojakangas 1994 and references therein (from Ojakangas, et al., 2001). The group consists of three conformable major formations on both the Mesabi and Gunflint ranges. The respective units on the two ranges are the Pokegama Formation and the Kakabeka Quartzite (the lowest units), the Biwabik and Gunflint Iron Formations (the middle units) and the Virginia and Rove Formations (the upper units, composed of graywacke and shale). The Thomson Formation in the northern part of east-central Minnesota is correlative with the Virginia and Rove Formations. The Biwabik and Gunflint are on strike with each other and were probably continuous prior to the intrusion of the Duluth Complex at about 1100 Ma. The Animikie Group in Minnesota-Ontario on the Mesabi and Gunflint ranges and the Baraga Group of Michigan-Wisconsin on the Gogebic range were both deposited in the Animikie foreland basin. The basal units comprised of siliciclastic sediment derived from the Archean basement, and the overlying iron-formation, were deposited in a shallow sea on the northern edge (i.e., the peripheral bulge or foreland) of the northward-migrating Animikie basin (e.g., Ojakangas, 1994). Additional details are provided below in the section titled, Environments of Deposition. The siliciclastic and iron-formation units are exposed on the Gogebic Range of northern Michigan and Wisconsin (Palms Quartzite and Ironwood Iron Formation), on the Mesabi Range of northern Minnesota (the Pokegama Formation and the Biwabik Iron Formation), and on the Gunflint range of northeasternmost Minnesota and Ontario (the Kakabeka Quartzite and the Page 102 �Figure 4-3: Schematic hypothesized paleogeography at the time of sedimentation of the Animikie Group turbidites that overlie shelf deposits in the Animikie basin. The rocks of the 1.1 Ga Midcontinent Rift System have been removed from the map, and Michigan and Wisconsin are thus positioned 60 miles closer to Minnesota-Ontario than they were after the formation of the Midcontinent Rift System. Arrows denote generalized transportation directions of sediment from major source areas. Compare with Fig. 4-2. Modified from Ojakangas (1994) and references included therein (from Ojakangas, et al., 2001). Gunflint Iron Formation), and are lithostratigraphic equivalents. They probably were continuous from south to north prior to development of the Midcontinent Rift System in Mesoproterozoic time. A consequence of this model is that they are diachronous, with the units in MichiganWisconsin (located about 60 miles to the south of the Mesabi range during deposition) thus somewhat older than those in Minnesota-Ontario. The thickest and uppermost units in the basin, essentially lithostratigraphic correlatives but probably differing somewhat in age, are the Michigamme, Tyler, and Copps Formations of the southeastern segment and the Thomson, Virginia and Rove Formations of the northwestern segment. These are typical turbidite-shale (flysch) sequences, with graded beds and intercalated muddy “rain-out” sediment. Figure 4-5 is a regional correlation chart of the aforementioned units, as well as others that cannot be discussed herein. Page 103 �Figure 4-4: Schematic cross-section depicting deposition of the Animikie Group turbidites that overlie shelf deposits in the Animikie basin, with sediment derived from both the north south. The southern area, the fold-and-thrust belt, comprises a complex assemblage including: 1. accreted Paleoproterozoic volcanic and plutonic rocks and volcanic rocks of the Wisconsin magmatic terranes; 2. accreted Archean miniplate terranes; 3. older Paleoproterozoic passive-margin sedimentary rocks and volcanic rocks produced during initial rifting of the continental margin, both scraped off the southward-subducting Archean Superior craton; and 4. recycled initial foredeep deposits, possibly including basal shallow water sandstones deposited in the transgressing sea of the northward-migrating foreland basin. The peripheral bulge comprises a source-rock assemblage of Archan granitic rocks and Archean volcanic-sedimentary (greenstone) belts. Scale is approximate. Compare with Fig. 4-3. Modified from Ojakangas (1994) and references included therein (from Ojakangas, et al., 2001). Ages Along the Mesabi Range, the Pokegama Formation rests unconformably on diabase dikes of the Kenora-Kabetogama dike swarm that gives a Rb-Sr isochron age of 2125 +/- 45 Ma (Southwick and Day, 1983; Beck, 1988) and this provides a maximum age for deposition of the Pokegama. A minimum age of 1930 +/- 25 Ma (Pb/Pb) for the Pokegama was obtained by Hemming et al. (1990) from quartz veins that cut the Pokegama. A U/Pb age on euhedral zircons from an ash layer in the lower Gunflint Iron Formation of Ontario is 1878 +/- 2 Ma (Fralich and Kissin, 1998; Fralick et al., 2002). A similar age of 1874 +/- 9 Ma was obtained on zircon from rhyolite in the Hemlock Formation that is adjacent to (and is interlayered with?) the Negaunee Iron Formation in the Marquette Range Supergroup of Michigan (Schneider et al., 2002). A zircon age from an ash layer near the base of the Virginia Formation is 1850 Ma (Hemming et al., 1996), and an age of 1821 +/-16 Ma has been obtained from an ash layer in the Rove Formation about 70 m above the Gunflint Iron Formation (Kissen et al., 2003). Several of these ages are shown on Figure 4-5. Page 104 �Gunflint Range Mesabi Range Cuyuna Range Emily District Gogebic Range Gogebic Range Menominee Range Baraga Basin Pokegama Fm Michigamme Fm Ironwood IF unconformity Palms Fm North Range Group Rabbit L. Fm 2125-1930 Ma deposition occurred sometime during this range Bijiki IF unconformity Goodrich Qtzite unconformity unconformity Negaunee IF Fence River IF Vulcan IF Siamo Slate Hemlock Volc. Hemlock Volc. unconformity Trout Lake Dolo Bad River Dolo Mille Lacs Group Sunday Qtzite Michigamme Fm Marquette District Lower Member Emperor Volcs Trommald IF Iron River/Crystal Falls Michigami Fm Mahnomen Fm 1870 Ma unconformity Mille Lacs Group Greenwood Iron-fm Thomson Fm unconformity Ajibik Qtzite Wewe Slate Kona Dolomite unconformity unconformity 1874 Ma Randville Dolo. Mesnard Qtzite Sturgeon Qtzite (Glen Township) (Denham Fm) Fern Creek Fm. Baraga Group Menominee Group Menominee Range Iron River/Crystal Falls Fortune Lake Slate Michigami Fm Stambaugh Fm Michigami Fm Goodrich Quartzite unconformity Negaunee Iron-fm Amasa Iron-fm Hemlock Volcs 1874 Ma unconformity Vulcan Iron-fm Siamo Slate Felch Fm Ajibik Quartzite Chocolay Group Marquette Range Supergroup Marquette Range unconformity Paint River Group Iron-formation Copps Fm Menominee Group Biwabik Fm Pokegama Qtzite Upper Member Tyler Fm Chocolay Group Gunflint IF 1878 Ma Kekabeka Qtzite 1850 Ma Virginia Fm Animikie Group 1821 Ma Virginia Fm Animikie Group Rove Fm Animikie Group Animikie Group Michigami Fm Hiawatha Graywacke unconformity Riverton Iron-fm Dunn Creek Slate Badwater Grnstn unconformity Randville Dolomite Kona Dolomite Sturgeon Quartzite Mesnard Quartzite Enchantment L. Fm Fern Creek Fm. detachment surface Figure 4-5: Generalized correlation chart of Paleoproterozoic strata in the Lake Superior region (after Morey and Southwick, 1995). Note that recently obtained age dates are shown for the Gunflint Formation, Mahnomen Formation, Hemlock Volcanics, and the Rove and Virginia Formations (see text for references). Also included is a correlation chart (lower right corner) of strata in Menominee, Iron RiverCrystal Falls and surrounding terranes (LaBerge et al., 2003 – includes changes to previous usage not yet officially adopted by the USGS). Iron-formations are shaded. Modified from Severson et al., 2003. Pokegama Formation The formation has long been called the Pokegama Quartzite, but because it contains appreciable argillite and siltstone, the name Pokegama Formation is more appropriate. It has been studied by several workers since it was named by Winchell (1893) for exposures at the western end of the Mesabi Range. Much of the previous work has been summarized by Morey (1972, 1973, 2003). Few natural exposures exist, as thick glacial drift generally covers the formation. Outcrops, roadcuts, and mine cuts occur at a few places along the length of the range, but most exposures are in the central portion of the range. A few drill holes have penetrated the entire formation. One is located just south of Eveleth (NE 1/4, NE1/4, Section 5, T. 57 N., R. 17 W.) and another is southwest of Mountain Iron (SE 1/4, SE 1/4, Section 8, T. 58 N., R. 18 W.); the thicknesses are 167 ft and 85 ft, respectively (Fig. 4-6). Three other drill cores, recently rediscovered, have not yet been studied. Numerous drill holes have penetrated only the upper few feet of the formation, as the drilling was generally undertaken in relation to iron ore exploration and development. The Pokegama is thin at the eastern end of the range and thickens to the western end where it may be more than 300 ft thick. The formation is composed of three main rock types—argillite, siltstone, and quartzite. The latter is generally a silica-cemented quartz sandstone, and is therefore an orthoquartzite rather than a metaquartzite. (Morey, 2003, has determined that mineralogical changes in the Pokegama and the Biwabik are the result of diagenesis rather than metamorphism, except at the eastern end of the range adjacent to the Duluth Complex.) These three rock types make up three gradational Page 105 �members-- lower, middle, and upper-- respectively, as shown in Figure 4-6. Minor thin conglomerates occur at the base of the formation, and seem to represent a weathered residuum on the surface of Archean rocks, perhaps reworked by fluvial processes. The Pokegama unconformably overlies Archean metavolcanic, metasedimentary, and plutonic rocks. There may be as much as 100 ft of relief on the Archean surface (Grout and Broderick, 1919), but the surface was, nevertheless, essentially a peneplain. Some Archean “knobs” were islands when the Pokegama was being deposited, and are present in the wooded areas between Eveleth and Virginia. The Pokegama-Biwabik contact is gradational, with some cherty horizons in the upper Pokegama. Various geologists have placed the contact at different stratigraphic levels. EVELETH M BIF 50 SANDSTONE, FINE-MEDIUM, SHALE MINOR CHERT SANDSTONE, COARSE 40 UPPER MEMBER SANDSTONE, FINE, SHALE MINOR MOUNTAIN IRON SANDSTONE, FINE M 30 SILTSTONE, SHALE 30 BIF SANDSTONES, THIN SANDSTONE, COARSE SANDSTONE, FINE MIDDLE MEMBER 20 20 SILTSTONE, SILTSTONE, SHALE SHALE SANDSTONES, THIN 10 10 SHALE, SILTSTONE, MINOR SHALE, SILTSTONE CONGLOMERATE 0 Archean 0 LOWER MEMBER Archean Figure 4-6: Measured sections from two drill holes that penetrate the entire Pokegama Formation. Dark shading represents shale, thin blank units represent siltstone, and dotted pattern represents sandstone. BIF is Biwabik Iron Formation. After Ojakangas (1983). Biwabik Iron Formation This is one of the world’s major iron-formations, and the largest in the United States. The formation is 200 to 750 ft thick and consists of four divisions as defined by Wolff (1917). These lithostratigraphic units, now informal members, are from the bottom up, the Lower Cherty, the Lower Slaty, the Upper Cherty, and the Upper Slaty. (These are miners’ terms, and do not indicate metamorphism.) The cherty members are dominantly granular (i.e., sand-textured), thick-bedded (several inches to several feet), and are largely composed of chert and iron oxides. The slaty members are dominantly fine-grained (i.e., mud-textured), thin-bedded (<1 inch) and Page 106 �composed mostly of iron silicate and iron carbonate with local chert beds. However, these two rock types are interbedded on all scales and are generally gradational. They contain about the same high quantities of silica, 42-47% (Morey, 1992). The Lower Slaty is not present at the far western end of the range. There are some diagnostic marker units within the formation. Two stromatolite-bearing intervals several feet thick are present, one at the base of the Lower Cherty and the other in the middle of the Upper Cherty. The black “intermediate slate” at the base of the Lower Slaty is reportedly an ash-fall tuff containing about 4 to 5.5 % aluminum oxide (Morey, 1992). At the top of the Upper Slaty are several feet of limestone and dolomite. Most of these marker units pinch out to zero in the vicinity of Nashwauk, about 40 mi from the west end of the range (Morey, 1992). Virginia Formation There are rare exposures of the Virginia in mines at the east end of the Mesabi Range where it has been metamorphosed by the mafic intrusions of the Mesoproterozoic Duluth Complex. Several holes drilled south of the range to study the underlying iron-formation have been drilled through the Pleistocene cover and have intersected as much as 1443-ft of the preserved lower part of the formation (Lucente and Morey, 1983). The lower portion of the formation in the drill holes is dominantly black shale. The upper portion of the drill core, while still dominantly shale, contains beds of siltstone and finegrained feldspathic graywacke comprising thickening- and coarsening-upward turbidite sequences. Ash-fall tuffs, cherty sideritic iron-formation, chert, and limestone are minor rock types low in the formation. The contact with the underlying Biwabik Iron Formation is gradational. The clastic rocks were largely derived from the Archean rocks to the north, with some contributions from lower Proterozoic rocks to the south (Lucente and Morey, 1983). The Virginia Formation is correlated with the Thomson Formation (Morey and Ojakangas, 1970) that is exposed 60 miles to the south in the vicinity of Carlton and Cloquet, and also with the Rove Formation in northeasternmost Minnesota and adjacent Ontario (Morey, 1967). Environments of Deposition, Animikie Group The Pokegama is interpreted to have been deposited in a tidally influenced shallow marine setting near the shoreline, having received clastics from the Archean basement to the north (Ojakangas, 1983). In this model of a transgressing sea, the lower (argillaceous) member was deposited at the shoreline in the upper tidal flat, the middle member of intercalated argillaceous and silty sediment was deposited seaward in the middle tidal flat, and the upper member of quartz sand was deposited still further seaward in a lower tidal flat/subtidal environment. This is illustrated in Figure 4-7. Walther’s Law is applicable here, with the vertical facies showing the relationships of the lateral facies. The lowermost Pokegama contains siltstone beds that contain alternating thicker and thinner laminae that have been interpreted as evidence of the diurnal inequality, and are being investigated further for possible clues to the Paleoproterozoic lunar orbit (G. Ojakangas, 1996). Page 107 �la nep e P in High Tide Low Tide NT ME DI SE Mud, Mud Silt, Sand Sand Sand Upper FA ES CI Middle Lower Shoal or Barriers TIDAL FLAT SUBTIDAL "Cherty" ironformation "Slaty" ironformation Shallower Pelagic mud, Turbidites Deeper SHELF SLOPE Figure 4-7: Sedimentation model showing lateral relationships of the siliciclastic tidal facies of the Pokegama Formation, the two main facies of the Biwabik Iron Formation, and the Virginia Formation (on the slope?). Thicknesses and geography not to scale. From Ojakangas (1983). The Biwabik is interpreted to have been deposited seaward of the Pokegama on a shallow marine, tidally dominated shelf (Figure 4-7). Precipitation of iron minerals including iron carbonate, iron silicate, silica, and perhaps some hematite, occurred on the outer shelf in waters below wave base, giving rise to the mud-textured (slaty) iron-formation. These minerals were likely related to upwelling waters from the deeper part of the basin. The two sand-textured members (Lower Cherty and Upper Cherty) formed in a shallowwater, high-energy environment, as indicated by stromatolites, cross-bedding, and rounded (locally oolitic) grains of iron minerals and chert. Shoreward-moving tidal currents (i.e., flood tides) and/or storms may have disrupted the mud-textured sediment (i.e., precipitates) and transported sand-sized aggregates into shallower water where they were altered by seafloor processes and early diagenetic processes. Thus these granules are “intraclasts” derived from within the basin. Shallow channels up to a mile wide and tens of feet deep were cut into the Lower Slaty member and filled with sand-textured grains of iron minerals and chert at Minntac where this is called the IBC unit (interbedded chert unit). These grains apparently were derived from shallow water and carried seaward into the deeper water environment in which the iron minerals were precipitating. Ebb-flow tidal currents are interpreted as the erosion and transportation agent. A plot of 102 cross-bed measurements in the Minorca Mine on the northeast edge of the Virginia Horn (Fig. 4-2) shows 90 % of the readings making a very prominent mode to the northnortheast and a minor broader mode to the south (Fig. 4-8). This distribution is interpreted as the product of a strong flood tide toward the paleogeographically determined northern shoreline and a much weaker ebb tide. Page 108 �N Figure 4-8: Paleocurrent rose diagram of 102 cross-bedded measurements from the Lower Cherty in the Minorca Mine. 102 A study of the orientations of stromatolite mounds in the stromatolite horizon within the Upper cherty member was conducted by Kevin Boerst (1999) as a UROP (Undergraduate Research Opportunity Project) at the University of Minnesota Duluth. His map is presented here as Fig. 4-9). A paleocurrent plot of mound elongation (Fig. 4- 9) is interpreted as the result of shore-normal tidal currents and shore-parallel longshore currents in shallow water. The repetition of the cherty and slaty members has long been interpreted as the result of transgression and regression (White, 1954). The Virginia Formation was deposited seaward of the iron-formation, probably in a slope-type environment (Fig. 4-7) where episodic turbidity currents deposited graded beds. Some volcanic ash falls evidently settled into the basin forming graded beds with a totally volcanic composition. The dominance of black, fissile shale suggests the “raining out” of clay (i.e., settling through the water column) and deposition in deep, anoxic water below wave base. Minor thin sandstone lenses were deposited by bottom currents (Lucente and Morey, 1983). N 12 N 10 50 70 70 Feet 50 12 90 90 110 110 130 130 10 10 12 10 12 K. Boerst, 1999 (Unpub.) K. Boerst, 1999 (Unpub.) 150 150 20 0 20 40 80 60 100 120 140 160 180 Feet Figure 4-9: Mapped stromatolite mounds in the Algal submember (I submember) in the Upper Cherty of the LTV 2E pit. The rose diagram represents the elongation of the mounds. From unpublished work by Kevin Boerst (1999). Page 109 �Mineralogy The detailed origins of the iron minerals are exceedingly complex and are beyond the scope of this introduction. It has to suffice here to say that Eh and pH are major controls on the stability of the various iron minerals in both the depositional and diagenetic environments, and in the easternmost Mesabi, in the metamorphic environment as well. Recrystallization and replacement of the granules during diagenesis has been extensive, and probably consisted of a number of discrete events. Earlier work on the oxidized taconites of the western Mesabi was accomplished by Bleifuss (1964). He showed that late hematite was developed by the oxidation and pseudomorphic replacement of magnetite octahedra, that layers of goethite were precipitated from solutions likely derived from the oxidation of siderite, and that some goethite formed by the oxidation of acicular iron silicate minerals. Some of the hematite inclusions and crystals in magnetite are similar to those illustrated by Han (1982). He proposed that much of the magnetite formed by the replacement of, and overgrowth on, pre-existing hematite that served as nuclei. Han further suggested that ionic diffusion of ferrous iron was a key process in the formation of the magnetite. Organic carbon may have acted as a reductant in this process. The nature of the major hydrologic events that removed the 40-60% of the silica and oxidized the iron minerals, thus forming the high-grade (natural) ore bodies, has long been debated. Were they descending cool meteoric waters or ascending hydrothermal waters related to igneous activity? Did this occur during the Cretaceous (the age of conglomerates composed of clasts of high-grade hematite), or prior to that time? Morey (1999) provided an excellent review of the arguments. He then proposed that a large-scale topography-driven hydrothermal groundwater system moved waters northward through the sands of the underlying Pokegama Formation, from the vicinity of the regional Penokean orogenic uplift in northern Wisconsin and east-central Minnesota, 40 to 80 miles to the south. Production Figures – Iron Ore and Taconite The annual amount of direct shipped and taconite produced are shown in Figure 4-10. Production and shipping of direct ore started in 1892 and rose steadily until 1953 when a maximum 76 million tons were produced in one year (note the precipitous drop in direct ore production corresponding to the Depression). At around 1955, there was a dramatic decrease in the amount of direct ore as the various mines became depleted. This also corresponds to the initial start-up of taconite mining, using a concentrating and pelletizing method developed by E.W. Davis of the University of Minnesota. Reserve Mining opened the first taconite operations in 1955 (Peter Mitchell Mine) and was shortly followed by Erie Mining in 1957 (old LTV site). Six more taconite operations were added in the 1960s, and by 1967, annual taconite production exceeded direct ore production. The mid-1980s marked a serious depression in the iron ore and steel industry that resulted in the closure of one operation (Butler Taconite) and the bankruptcies of two other taconite producers. More recently, LTV Steel and Eveleth Taconite have closed; Evtac has since reopened as United Taconite. Page 110 �80,000,000 TACONITE DIRECT ORE 70,000,000 TONS PRODUCED 60,000,000 50,000,000 40,000,000 30,000,000 20,000,000 10,000,000 18 92 18 97 19 02 19 07 19 12 19 17 19 22 19 27 19 32 19 37 19 42 19 47 19 52 19 57 19 62 19 67 19 72 19 77 19 82 19 87 19 92 19 97 20 02 0 YEAR Figure 4-10: Annual production figures for direct ore (includes all forms of direct ore) and taconite for the period 1892-2003. Data and graph from James Sellner, Minnesota Department of Natural Resources, Lands and Minerals Division, Hibbing, MN. What’s in a Name? (Those confusing iron-formation submembers!) As early as 1917, the Biwabik Iron Formation (BIF) has been informally broken down into four major lithostratigraphic members, or subdivisions, known as (from bottom to top): Lower Cherty, Lower Slaty, Upper Cherty, and Upper Slaty (Wolff, 1917). The cherty iron-formation members are generally thick-bedded and contain round grains (0.5-2.0 mm) of chert that are referred to as granules. These “cherty” members typically contain higher percentages of iron oxides (magnetite, hematite and/or goethite). In contrast, the “slaty” members are thin-bedded (0.5-3.0 mm thick beds) and very fine-grained. They are composed mostly of Fe-silicates and Fecarbonates. Both cherty and slaty iron-formation types are interlayered at all scales. However, one rock type often predominates in each of the four lithostratigraphic members, and are sonamed due to this dominance, i.e., thick-bedded cherty iron-formation is dominant in the cherty members, whereas thin-bedded iron-formation is dominant in the slaty members. The 1917 four-fold stratigraphy of a Lower and Upper Cherty and a Lower and Upper Slaty members is still used at each of currently operating (and inactive) taconite mines on the Mesabi Range. However, each of the mining companies further subdivides the BIF into several Page 111 �submembers based on bedding types (Fig. 4-11) and mineral assemblages. It is at this point that the BIF stratigraphy becomes very complicated and at times confusing. This is mainly due to the following reasons: • There are localized lateral facies changes between mines (and even within a single mine). Some mines reconcile these differences by splitting out numerous submembers (each with a distinct bedding type, texture, ore grade, and/or mineral assemblage), whereas, other mines lump many of these same differences within a single submember. • There are significant lateral facies changes over several miles between mines. For example a particular horizon may be massive-bedded at one location but is regularbedded a few miles away. This is particularly troublesome within the Upper Cherty member in the western 2/3rds of the Mesabi Range. • Not all mines use the same numbering system – some use abbreviations (LC for Lower Cherty, etc.) followed by a number (as in LC-5 at the top of the Lower Cherty). However, other mines use an alphabet system, devised by Gundersen and Schwartz (1962), starting with the A submember at the top of the Upper Slaty (in this system the top of the Lower Cherty corresponds to the R submember). And further still, another mine refers to the Lower Cherty as the number 1 unit and subdivides it into eight submembers, with 1-8 at the top of the Lower Cherty. • Some mines label downward in their numbering system, whereas other mines label upward in their numbering system. Textures associated with granular rocks Disseminated Diffuse Granules Mottled Patches Textures associated with laminated rocks Regular, Sharp Wavy/Irregular, Sharp Regular, Diffuse Wavy/Irregular, Diffuse Shaly Thin-bedded Figure 4-11: Textural characteristics of the Biwabik Iron Formation (modified from Pfleider et al., 1968, from a classification scheme developed by geologists of the Hanna Mining Company). Page 112 �The submember nomenclature that is used at each of the mines is summarized in Figure 412. It can readily be seen on this summary that any particular submember name changes nomenclature from one mine to the next. This is because there are few good marker horizons within the BIF, and even these can exhibit gradual lateral facies changes or pinch-and-swell relationships to each other. A few of the potential marker horizons within the BIF are presented below. • Top contact of the BIF with the Virginia Formation – In the eastern half of the Mesabi Range a carbonate horizon is present at the very top of the Upper Slaty and the contact between the BIF and Virginia Formation is easily recognized (Gruner, 1924). However to the west of Hibbing, the carbonate layer is absent and lenses of thin-bedded Fe-carbonate iron-formation are present in the Virginia Formation, and the top of the BIF is not easily discerned. • Submember I/Algal unit – a thin unit containing algal stromatolites and jasper-bearing intraformational conglomerate is present near the top of the Upper Cherty. This submember is easily recognized but is not present to the west of Hibbing. • Lower Slaty member – The Lower Slaty has a very-well defined “Intermediate Slate” (also referred to as the Q submember or Paint Rock in Fig. 4-12) at its base which is characterized by a black, carbon-rich, thin-bedded, slaty unit that commonly contains pyrite. This unit is readily evident at all of mines on the range. However, the upper contact of the Lower Slaty is “indefinite and a gradual change to other slaty phases takes place” … [which makes] … the dividing line between the two [Upper Cherty and Lower Slaty] somewhat arbitrary” (Gruner, 1924, p.20). The upper contact of the Lower Slaty is particularly troublesome in the Virginia Horn area. Gruner (1946, p. 45) included lenses of cherty and wavy-bedded taconite (referred to as the Interbedded Chert -- IBC unit at Minntac; Fig. 4-12) in the Lower Slaty, whereas White (1954) included these same units in the overlying Upper Cherty member. • Base of the BIF - The base of the Lower Cherty is generally characterized by thin-bedded iron-formation (also called the “red basal unit”) with localized algal stromatolite and basal conglomerate horizons. However, at many localities the base of the BIF exhibits a gradational contact with the underlying Pokegama Formation. In the Virginia Horn area the base of the BIF contains an iron-bearing sandstone (White, 1954) that some mines include with the iron-formation, whereas others lump this type of material with the Pokegama Formation. From the above description it is readily evident that there are few good marker horizons within the iron-formation; even the upper and lower contacts of the iron-formation are gradational and subject to various interpretations. The “Intermediate Slate” and the algal horizon in the Upper Cherty are the only easily recognizable marker units. However, even using these horizons as markers, one can see from Fig.4-12 that there are problems. Clearly, much additional work needs to be done in understanding how submembers at one mine correlate with submembers at an adjacent mine. These types of studies could inevitably be important in determining why ore grades, and waste rock characteristics, change between mines and even within a single mine. For example, some of the best ore-grade taconite corresponds to the wavy-bedded or irregular-bedded taconite present in both the Lower or Upper Cherty members. The sedimentary environment that produced this type of taconite ore has not been fully documented nor have any recent detailed sedimentological studies been attempted. A better understanding of the various sedimentological textures in the BIF could ultimately lead to an increased ability to better predict changes in ore grades as they relate to facies changes. Page 113 �"Eastern" Mesabi Range LTV (Cliffs-Erie) EVTAC (T-Bird So.) Upper Cherty United (T-Bird No.) LC-5 mass-bdd? LC-4 wavy-bdd LC-3 straight-bdd LC-2 thin-bdd LC-1 ss & congl Lower Cherty Ispat Inland (Minorca pit) upper Upper Cherty Algal MUC waste - mixed-bdd MUC - thick/mass-bdd LUC waste - thin-bdd LUC waste - irreg/mass-bdd LUC - reg-bdd LUC - wavy/thin-bdd LUC - slaty LS - thin-bdd er Intermediate Slate Low TLC - thick/thin-bdd TLC - mass congl TLC - irreg/mass-bdd BLC - wavy-bdd Footwall Lean - thin-bdd Footwall Ore - reg-bdd Footwall Taconite Slaty y Slat US-2 slaty pper US-1 thick & thin-bdd U UC-7 thick-bdd UC-6 thick-bdd Ispat Inland UC-5 algal/congl (Laurentian pit) UC-4 thick & thin-bdd US UC-3 mass-bdd UC-3 thick-bdd UC-2 thick & thin-bdd UC-2 reg/thin-bdd UC-1 thin-bdd UC-1 reg-bdd LS-3 slaty IF LS LS-2 thin-bdd Intermediate Slate Intermediate Slate LC-5B transition zone LC-6 thick-bdd LC-5A mass-bdd LC-5 irreg-bdd LC-4 wavy-bdd LC-4 wavy-bdd LC-3 reg-bdd LC-3 reg-bdd LC-2 thin-bdd LC-2 thin-bdd LC-1 sandstone & congl LC-1 ferrug seds & congl A - limestone B - vague-bdd C - thin-bdd D - thin & wavy-bdd E - mass-bdd F - thin-bdd G - wavy/reg-bdd H - thin/wavy-bdd I - algal/congl J - wavy-bdd K - wide-spaced wavy-bdd L - wavy-bdd M - reg-bdd O - wide-spaced wavy-bdd P - thin-bdd Q - carbonaceous IF R - thick-bdd S - vauge wavy-bdd T - wavy-bdd J - thin/reg-bdd V - thin-bdd W - algal/congl Northshore A - chert & marble B - chert & diopside C - thin-bdd Upper Slaty D - thin & wavy-bdd E - mass-bdd F - thin & wavy-bdd G - mass-bdd H- wavy-bdd I - algal/congl J - thick-bdd K- wavy-bdd L - wavy-bdd Upper Cherty M - thin/reg-bdd N - ?-bdd O - ?-bdd P - thick-bdd Lower Slaty Q - graph. arg. IF R - thick-bdd S - ?-bdd T - ?-bdd Lower Cherty U - reg-bdd? V - w/congl MUC = middle Upper Cherty LUC = lower Upper Cherty TLC = top of Lower Cherty BLC = bottom of Lower Cherty Virginia Horn Upper Cherty Upper Slaty Keewatin (National) & Butler US UC LS paint rock LC-1 thick-bdd LC-2 irreg-bdd LC-3 thick-bdd LC-4A Upper irreg-bdd LC-4A Middle thick-bdd LC-4A Lower wavy/thick/thin-bdd LC-4B irreg-bdd LC-4C Upper irreg-bdd LC-4C Middle laminated LC-4C Lower reg/thin-bdd LC-5A reg/thin-bdd LC-5B thin-bdd LC-6 ?-bdd jasp & thin-bdd thin-bdd congl UC Slaty Lower2-1 thin-bdd 1-8 reg-bdd 1-7 wavy-bdd 1-6 wavy-bdd 1-5 wavy-bdd 1-4 thin/wavy-bdd 1-3 even-bdd 8-3 slaty 1-2 mass-bdd 1-0 red basal slaty algal/congl Hibbtac (Buhl-Kinney) 2-1 thn-bdd 1-8 mass-bdd 1-7 wavy-bdd 1-6 wavy-bdd 1-5 mass-bdd 1-4 slaty 1-3 even-bdd 1-2 slaty 1-1 thin-bdd 1-0 congl Red Basal Lower Cherty Lower Slaty US UC-3 thick-bdd UC-2 thick-bdd UC-1 reg-bdd LS paint rock LC-5 thick/reg-bdd LC-4 wavy-bdd LC-4 wavy-bdd LC-2 reg-bdd LC-1B thin-bdd jasper thin-bdd algal/congl LC-1A Upper Slaty Coleraine area (USS) Dolomite Upper Slaty Upper Slaty - thin-bdd UC-16 - irreg/thin-bdd UC-15 algal/congl y er t UC-14 slaty Ch UC-13 thin-bdd r e p UC-12 congl w/algal frags Up UC-11 slaty LS-10 even-bdd LS-9 even/thick/mass-bdd LS-8 slaty LS-7 (IBC) wavy-bdd Ispat Inland LS-6 thin-bdd (Minorca pit) carbonaceous IF LC-5B thick-bdd LC-5 mass-bdd? LC-5A mass-bdd LC-4 wavy-bdd LC-4 wavy-bdd L LC-3 straight-bdd ow LC-3 wavy-bdd LC-2 thin-bdd er LC-2 even-bdd Ch LC-1 ss & congl LC-2 thick/mass-bdd ert LC-1 arg IF y LC-1 thin-bdd LC-1 algal/congl we rS lat y Hibbtac (Hibbing) Minntac Lo "Western" Mesabi Range Figure 4-12: Correlation chart of submembers, at each of the mines/areas within the Biwabik Iron Formation, as deciphered from published descriptions and mine handouts (modified from Zanko et al., 2001). All columns are hung on the base of the Lower Slaty (“Intermediate Slate”). It is important to note that this summary is preliminary as it has not been field-checked. No scale is implied and the true thickness of each submember is not portrayed. Bars to the left of the columns indicate mined taconite ore zones. Note that there are several consistent submembers within the Lower Cherty as opposed to very few laterally persistent submembers in the Upper Cherty. Page 114 �STOP DESCRIPTIONS Babbitt Hywy 21 Minn 135 Minn 169 4-10 4-11 Du nka Ro ad Embarrass Hywy 21 U.S. 53 nn Mi Minn 135 169 Du n oad ka R 4-9 T59N lut u R13W D Biwabik 4-6 Minn 169 T58N T57N Virginia 4-5 Gilbert 4-4A 4-4C 4-1 4-4B 4-2 Eveleth 4-3 Aurora 4-7 nn Mi lex p om C h R12W Hywy 110 135 Hoyt Lakes Minn 37 U.S. 53 R18W R17W R16W R15W R14W Figure 4-13: Locations of stops that will be visited on the field trip. Generalized contacts of the Biwabik Iron Formation, for the eastern half of the Mesabi Range, and Duluth Complex are shown. Note that in order to observe Stops 4-1, 4-2, and 4-3 in stratigraphic order, we will be back-tracking for short distances. STOP 4-1A: Pokegama/Archean Unconformity Location: On U.S. 53 in Eveleth, there is a stoplight at Grant/Industrial Avenue. Proceed northward past the stoplight for about 0.4 mi north to a gas station. Turn right and immediately turn left on the frontage road (Midway Drive). Drive past a church and past the first street on the right (Mesabi Drive). Watch for a low, small outcrop in the trees on the right, just a few feet off the road. Eveleth 7.5’ quadrangle, T.58N., R.17W, Sec 20, NW of SW of SE; 535445E, 5259520N (NAD83 UTMs) Description: This is the only easily accessible exposure of the unconformity. A close examination will reveal the presence of a thin smear of Pokegama conglomerate composed of schist and vein quartz clasts upon Archean schist with a near-vertical foliation. Note that the foliated schist clasts are flat and have a subhorizontal orientation. NO HAMMERING, PLEASE! STOP 4-1B: Argillaceous lower member of the Pokegama Formation Location: From stop 4-1A, CAREFULLY walk across both lanes of U.S. 53 to the long, roadcut on west side of highway. Eveleth 7.5’ quadrangle, T.58N., R.17W, Sec 20, NW of SW of SE; 535380E, 5259555N (NAD83 UTMs) Page 115 �Description: This cut is about 500 feet long and 5-10 ft high, and is the only exposure of this member. It consists largely of shale and siltstone with minor fine-grained sandstone. It has been interpreted as having been deposited in a low-energy upper tidal flat environment in a sea that transgressed onto the peneplaned surface of Archean rocks (Ojakangas, 1983). Minor channeling is common at the bases of the thicker sandstone beds, and at one spot, 0.5 m of section has been eroded. Small-scale cross-bedding is present in some siltstone beds, and elongated sole marks are visible on the bottoms of some sandstone and siltstone beds. A total of 57 of these paleocurrent indicators show that the currents were generally oriented in a north-south direction. A few concretions as large as 6 inches in diameter are present, as is soft-sediment deformation. Hemming et al. (1991) illustrated the soft-sediment folding and interpreted it as evidence that the Animikie basin was tectonically active during deposition of the Pokegama and the Biwabik. Alternatively, it is interpreted herein as soft-sediment slumping into tidal channels. Fine laminations and sequences of laminations in the sandstone beds have been interpreted as tidal rhythmites (G. Ojakangas, 1996). STOP 4-1C: Jaspillite on Archean metaconglomerate Location: From stop 4-1A, drive a long block northward to Merritt Drive. Turn right. About 100 ft farther, turn right on Mesabi Avenue. About another 100 ft farther, there is a fork in the street. Park on Mesabi Avenue and walk uphill on the street on the left to a broad, flat rock exposure in a driveway. Eveleth 7.5’ quadrangle, T.58N., R.17W., Sec 20, SW of SE (No. 7 Mesabi Lane); 535610E, 5259460N (NAD83 UTMs) Description: This is an excellent exposure of jaspillite resting unconformably on subvertical volcanogenic metaconglomerate. The outcrop is an estimated 30 to 40 ft topographically higher than stop 4-1A, and was also that much higher when the jaspillite was deposited. Is it an erosional remnant of basal Biwabik Iron Formation or is it a local chemical precipitate at the base of the Pokegama Formation? NO HAMMERING, PLEASE! NOTE: The middle member of the Pokegama Formation (a unit of intercalated beds of sandstone, siltstone, and argillite) was once poorly exposed in the flat area across the highway from the U.S. Hockey Hall of Fame. However, the best part of the poor exposure has since been covered by a frontage road. STOP 4-2: Upper member of the Pokegama Formation Location: Continue driving down Mesabi Drive to the frontage road (Midway Drive). Turn right and drive past stop 4-1A to a stop sign. Turn left to intersection with U.S. 53. Cross the northbound lane and turn left on highway. Drive south past stop 4-1B, past the Grant/Industrial Avenue stoplight, past the U.S. Hockey Hall of Fame on the right, and stop at the Rustic Rock Inn (you are now opposite a large roadcut on the east side of the highway). Walk across the highway, CAREFULLY! Eveleth 7.5’ quadrangle, T.58N., R.17W, Sec 32, E ½ of SE; 536055E, 5256775N (NAD83 UTMs) Description: This is the upper member of the Pokegama Formation, composed of silicacemented quartz sandstone. It is the rock type found immediately beneath the Biwabik Iron Formation; it was penetrated by countless drill holes during mining and exploration, and resulted in the formation being originally named the Pokegama Quartzite. Note the massive beds Page 116 �separated by thin beds of shale or siltstone. Silica cementation likely obscured some original cross-bedding. This member was interpreted by Ojakangas (1983) as having been deposited in a high-energy, lower tidal or subtidal environment. STOP 4-3: Lower Cherty member, Biwabik Iron Formation Location: Proceed south on U.S. 53 for 0.6 miles (passing over an overpass and past a long roadcut of iron-formation on the right ), to a left-turn lane. Make a U-turn and proceed north on the highway for about 0.2 miles to roadcut on the off-ramp for Highway 37 (right side of highway). Eveleth 7.5’ quadrangle, T.57N., R.17W, Sec 5, E ½ of NE of NE; 536230E, 5256285N (NAD83 UTMs) Description: Note that this member of the Biwabik Iron Formation overlies the sandy member of the Pokegama Formation of the last stop, and that both units dip gently to the southeast. Observe the thick wavy bedding, the trough cross-bedding, and the sandy texture of the iron minerals and chert grains. The cross-beds are best observed in this eastern roadcut, but are also present at both ends of the longer cut on the other side of the highway. Cross-bedding measurements (Fig. 4-16), although not definitive, are suggestive of a tidally-influenced marine environment. This, coupled with Walther’s law of succession of sedimentary facies (i.e., the facies observed vertically are also similarly related laterally), places the deposition of the ironformation seaward of the Pokegama Formation. N Figure 4-14: Rose diagram of 28 crossbedded measurements, mostly from the roadcut of Stop 4-3. 28 Stops 4-4A through 4-4C are at the Thunderbird North Mine of United Taconite (formerly Eveleth Taconite, or Evtac). Simplified stratigraphic submembers of the iron-formation, according to terminology used by Evtac, are portrayed in Figure 4-15. From stop 4-3, proceed north along U.S. 53 past stop 4-2 to the stoplight at Grant/Industrial Avenue. Turn left on Grant Avenue and follow the road past a cemetery to the entrance road to United Taconite (U-Tac). Page 117 �EVTAC Thunderbird South Virgina Fm 50' Upper Slaty US-2 Slaty silicates +/- irreg chert North 20' US-1 thick + thin BDD thick chert + thin BDD, cherty zones 20' Upper Upper Cherty UC-7 Thick BDD +/- slaty UC-6 170' UC-3 thick-BDD salt & pepper textu re 45' slaty wavy BDD salt & pepper 12' Footwall Lea n thin-B DD +/- graphite Footwall Ore Lower Cherty BLC 45' Bottom Lower Cherty LS-2 Thin BDD sil IF Vertical scale (in feet) K Lower Slaty LS Keweenawan Sill wide sp aced thin wavy beds L Wavy BDD w/ intraform rip ups M Reg BDD w/ wavy-straight BDD Thin BDD Silicate IF Algal 20-35' wide spaced wavy beds 85-130 ' 60' LC-4 wavy BDD cherty IF 55' LC-3 Reg BDD = thin beds o f chert & sil P LC-5A massive BDD w/ mottles +/- th in MGT bnds 45' LC-1 Ferrug con gl Graywacke ss = qtz grns in chl matrix Keweenawan Sill 10-14' carb clasts 65' Thin BDD Non-mag D Wavy BDD 2-6' E Sep t. cracks 13-20' F Thin & wavy-BDD Sep t. crack s 25-34' Mass BDD, MGT G granules, +/- thin BDD zones 10-20' Q carbonaceous 10-20' salt & pepper BDD w/ R Thick poor BDD/mott 45-95' LC-4 wavy BDD 4 5' LC-3 straight BDD w/ 4-5" chert bnds S Vague, wavy BDD +/- mott T Wavy BDD +/- mott Wavy BDD +/- congl K 30 -60' Thin-reg (Hem) LC-2 Thin BDD Silicate IF W LC-1 10' SS +/-congl+/-CH Upper Cherty Wav y BDD Silicate IF (minor congl) 9-2 0' Abundant MGT M (thin + reg BDD) 2-5' N Fayalite-q tz 6-17' O Fayalite-q tz Low MGT content MGT g ran ules 60' Thick BDD, green, MGT poor Lower Slaty P The most metamorph & reconstituted U Thin-reg BDD Thick + thin BDD 25-45' Thin BDD V + Arg + jasp 30' Algal / co ngl Upper Slaty 8-11' H Wavy BDD 2-5 ' I Algal/congl 15-24' Granule w/ pebb les J Thick BDD 30-47' L 50' LC-2 thin BDD = jasp, hem slaty, green silicates 26-43' C Thin BDD 40-80' 60' some salt & p epper not drilled or mined 130' 3-5 ' Int slate 15' LC-5B Transition Zone 50 Keweenawan? Sill 3-6 ' A Chert/marble 13-20' B Chert + Diop <5' Algal congl I Virginia Formation Virgina Fm 15-40' LC-5 irreg BDD cherty silicate IF Footwall Taconite Northshore H Thin wavy BDD O 145' thin-BDD rose-colored with depth Mass w/ septarian cracks, High Pho s (congl @ base) 30' Reg-BDD salt & pepper thin-BDD gray-colored Thin&wavy BDD +/- chert pebbles E J Wavy BDD, mott UC-1 Mott, Reg-BDD LS-3 Dark, slaty silicate IF int slate - graphite 25' LC-6 Thick BDD w/ irreg green sil layers 65' TACONITE ORE ir reg- to mass-BDD +/- mott 30' Lower Cherty Lower Slaty Mass congl pinkish TACONITE ORE Lower Cherty Thick- to thin- BDD 15' 0 UC-2 Reg/thin-BDD 30' 45' TLC Top Lower Cherty D 40-75' Green, wavy reg G BDD UC-2 Thick + th in BDD (ch ert/silicate) 40' Lower Slaty 0 50' 35' UC-1 Thin BDD Thin chert layers w/ MGT beds + wavy bnds LS thin-BDD black to dark green Thin BDD Thin BDD F +/- gran jasp beds TAC ORE wavy- to thin-BDD 90' C 25 35-50' TAC ORE Reg-BDD w/congl Vagu e BDD, Chert + Fe Sil 25-55' TAC ORE 40' Lo wer Upper Cherty UC-3 mass BDD, mott w/ irreg beds TACONITE ORE irreg- to massBDD & mott Upper Cherty Lower Upper Cherty Waste 50' ) Algal unit present but not described B 50 TACONITE ORE Thick + thin BDD cherty silicate thick- to mass-BDD congl w/dissem magnetite ( TAC ORE UC-4 35-55' A Limestone TACONITE ORE 50' Middle Upper Cherty thin-B DD slaty TACONITE Upper Cherty TACONITE ORE 65' 45' Virgina Fm UC-5 jasp/ algal/ congl w/ Mn! Mixed : thin-BDD, congl, mass BDD Vertical Scale (in feet) Middle Upper Cherty Waste Thick BDD pink carb motts Cliffs-Erie Site (old LTV) US Green to Grey Thin BDD, Non-magnetic SIPHON FAULT Algal <20' Upper Cherty 15' 25 Upper Slaty 50' TACONITE ORE TACONITE ORE >50' Ispat Inland Laurentian Pit TACONITE ORE EVTAC Thunderbird 26' GraphiticArgillaceous IF Q 11' R Thick BDD 8' S ? BDD/MGT rich 5' T Granules 10' U Reg BDD? 3' V Granule/qtzose Congl Lower Cherty Figure 4-15: Submember nomenclature for the Biwabik Iron Formation as used at the various taconite mines and idled mine sites that will be visited on this field trip. Modified from Plate II in Zanko et al., 2001. Note that the nomenclature portrayed here is preliminary in nature and future correlations of the various submembers used at the different mines still needs field checking and modification. The stratigraphic nomenclature is currently being revised for the Thunderbird North Mine by United Taconite, but was unavailable at press time and is not included in this figure. Page 118 �STOP 4-4A: Submember LC-3 near the base of the Lower Cherty Location: Northeast end of the Thunderbird North Mine. Eveleth 7.5’ quadrangle T.58N., R.17W., Sec 20 , 535290E, 5260255N (NAD 83) Description: The LC-3 submember (new U-Tac terminology or “footwall ore” in the old Evtac terminology) is characterized by regular-bedded and wavy-bedded cherty taconite wherein most of the magnetite is contained within dark-colored wavy (irregular) beds that locally show good pinch-and-swell relationships. This is the lowest unit mined at the Thunderbird North Mine and contains approximately 20-23% mag-Fe and 2% Davis Tube Si. The Auburn Fault can also be viewed at this locality. The fault zone consists of a 20-footwide zone of broken-up and highly oxidized vuggy rock. To the northwest is the Auburn Mine that was originally developed as an underground mine by the Minnesota Iron Company in 18941902. It was reopened as an open pit by the Oliver Iron Mining Company (a subsidiary of United States Steel Corp.) in 1951 and was essentially closed in the 1960s; scram operations continued intermittently until 1999. The mined ore was located along a main ore trough that coincides with the northwest-trending Auburn Fault. Mined material was obtained from the Upper Cherty and Lower Slaty members, and locally from the Lower Cherty member. Jim Small (personal communication to Peter Jongewaard) reported that he was unable to find any evidence of the Auburn Fault anywhere on the newly-scraped floor of the mine during the final scramming period prior to allowing the pit to fill with water. United Taconite is currently filling the mine with waste rock. STOP 4-4B: Submember LC-4 near the base of the Lower Cherty Location: Thunderbird North Mine. Eveleth 7.5’ quadrangle T.58N., R.17W., Sec 31, 533930E, 5257810N (NAD 83) Description: The LC-4 submember (BLC submember – old Evtac terminology) exemplifies typical wavy-bedded taconite (see Fig. 4-16 at end of this report) that is characteristic of much of the taconite ore of the Mesabi Range. This unit varies from the previous stop in that wavy-beds are dominant and locally exhibit cross-bedded features. At this stop the magnetite is in: 1. the wavy beds; 2. disseminated throughout the cherty bands; and 3. within mottles that are generally less than 1 cm in diameter and cored by Fe-carbonate (siderite). The LC-4 is 40-45-ft-thick and contains 20-27% mag-Fe and 1-2% Davis Tube Si. The LC-4 submember is easily recognized in drill core due to the wavy beds and a salt-and-pepper texture that is defined by disseminated magnetite. STOP 4-4C: Submembers LC-5 through LC-8 near the top of the Lower Cherty Location: Thunderbird North Mine. Eveleth 7.5’ quadrangle T.58N., R.17W., Sec 19; 533915E, 5259540N (NAD 83) Description: At the base of this pit exposure is the LC-5 submember (according to the new UTac terminology, or TLC in the old Evtac terminology) characterized by thick- and irregularbedded, mottled chert with 23-28% mag-Fe. The mottles are pink Fe-carbonate; they are not readily evident on fresh break but appear after a short weathering period. Overlying the LC-5 is a 1-1.5-ft-thick intraformational conglomerate (LC-6 submember – U-Tac terminology) that Page 119 �contains abundant red jasper fragments. The next overlying unit is the LC-7 submember (U-Tac terminology) characterized by thin magnetite-rich beds interlayered with green, Fe-silicatebearing, chert beds (1-6 inches thick). The LC-7 is ore with 20-25% mag-Fe, whereas, the overlying LC-8 submember, which physically resembles the LC-7, is waste as the thin beds no longer contain magnetite – rather Fe-silicates are dominant in the thin beds. This is the last stop at the Thunderbird North Mine. From here we will return to U.S. 53 and proceed about 1.5 miles north to a gated entrance on the right to the “Mineview in the Sky” – follow the signs to the overlook on top of a waste rock dump. Stop 4-5: Mineview in the Sky Overlook Location: Eveleth 7.5’ quadrangle, T.58N., R.17W., Sec 17, Ne of NW of SE; 535710E, 5261650N (NAD83 UTMs). Description: The open pit immediately below the overlook is the Rouchleau Mine that connects northward with additional mines. The Rouchleau Mine produced 52,000,000 tons of high-grade “natural ore” containing 50-55% iron from 1920 to 1976. This area is located on the “Virginia Horn” which is a large Z-shaped bend in the otherwise straight ENE trend of the Mesabi Range. Six miles to the northwest on the horizon is United States Steel’s Minntac taconite operation. Steam visible to the northeast of Virginia is from the Ispat Inland taconite plant. From the Mine View we will proceed to the Minorca Mine of Ispat Inland by driving north on U.S. 53 approximately 4 miles around the town of Virginia. About 1 mile north of the third stop light, make a right turn onto an unnamed gravel road (this road is 0.4 miles north of the road to the golf course). Follow this gravel road across 9th Ave. (paved) and continue to the Minorca mine gate. Stop 4-6 (optional): Cross-bedded LC-3 submember, Minorca Mine Location: Near the northwest of the Minorca Mine (UTM coordinates will be acquired at the site during the field trip). Description: Cross-bedding is exceptionally well exposed at one spot in this mine. Most crossbeds here indicate paleocurrents to the north, but one excellent herringbone cross-bedded set shows paleocurrent trends toward azimuth 0º (lower cross-bed) and azimuth 220º (upper crossbed). See Figure 4-8. If stop 4-6 is visited, we will cross the Laurentian Divide on private company mining roads to gain access to the Laurentian Mine of Ispat Inland. If stop 4-6 is not visited we will proceed southward on U.S. 53 from the Mineview overlook and head west on Highway 135 (about 3 miles) to the town of Gilbert. At the junction of highways135and 37, turn left and proceed north to the gated entrance to the Laurentian Mine of Ispat Inland. The stratigraphic terminology of submembers in the Laurentian Mine is portrayed on Figure 4-15. Page 120 �STOP 4-7A: Submember LC-5a and LC-5b at the top of the Lower Cherty Location: Laurentian Mine. Gilbert 7.5’ quadrangle T.58N., R.17W., Sec 24, NW of SE of NE; 542240E, 5260650N (NAD 83) Description: This stop picks up where we left off at the last stop at the Thunderbird North Mine. At this locality the LC-5a and LC-5b submembers are exposed. The LC-5a is a wavy-bedded taconite ore unit with pink to brown Fe-carbonate patches, lenses, and mottles. The overlying LC-5b submember consists of green Fe-silicate-rich granular cherty material. STOP 4-7B: Lower Slaty member and the “Intermediate Slate” at the base of the Lower Slaty Location: Laurentian Mine. Gilbert 7.5’ quadrangle T.58N., R.17W., Sec 24, NW of SE of NE; traverse from 542315E, 5260700N to 542485E, 5260840N (NAD 83) Description: The entire stratigraphic section of the Lower Slaty (130-ft-thick) can be viewed at this impressive face within the Laurentian Mine. The “Intermediate Slate,” at the base of the Lower Slaty, is characterized by thin-bedded, black, organic-rich slate that locally exhibits bright shiny graphitic surfaces with bedding-parallel slickensides. Pyrite is common to this submember and is present as both disseminated fine- to medium-grained cubes and as thin disks (marcasite?) along bedding planes. All of the Lower Slaty constitutes waste rock. Note that the name “slate” has been applied to all thin-bedded rocks in the BIF, but nearly every writer has pointed out that this term is a misnomer, because these rocks do not have the cleavage of a true slate but merely a parting parallel to bedding (White, 1954). Morey (1993) reported that the “Intermediate Slate” is an ash-fall tuff; however the senior author has yet to see any evidence of volcanic shards in thin-sections collected from this unit. STOP 4-7C: UC-1 submember at the base of the Upper Cherty Location: Laurentian Mine. Gilbert 7.5’ quadrangle T.58N., R.17W., Sec 24, SE of NE of NE; 542545E, 5260790N (NAD 83) Description: The UC-1 submember is a regular-bedded (1-2 inches straight/even beds) granular cherty unit that contains magnetite-rich bands, pink Fe-carbonate mottles, and local crossbedding. This material constitutes ore in that it contains 16-40% mag-Fe (the mag-Fe is extremely variable in this unit) and 2-4% Davis Tube Si. Localized pods of very Fe-rich taconite (up to 40% mag-Fe!) were present but have been recently mined out. STOP 4-7D: UC-2 submember at the middle of the Upper Cherty Location: Laurentian Mine. Gilbert 7.5’ quadrangle T.58N., R.17W., Sec 24, SE of NE of NE; 542590E, 5260740N (NAD 83) Description: This submember exhibits both regular-bedded and thin-bedded features and consists of both taconite ore zones (18-20% mag-Fe) and waste zones. At this particular site are numerous bedding-parallel quartz-carbonate veins with chlorite slickensides. The chlorite is Page 121 �particularly troublesome as it cannot be selectively removed from the blasted ore and ultimately “gums up” in the concentrator. Interbedded with the magnetite-rich beds are red-colored hematite-rich beds that appear to be primary – they are pervasive throughout the area and do not appear to be related to a later oxidation event that produced the direct shipping ores of the Mesabi Range. Also at this locality are localized red jasper beds and intraformational conglomerate lenses with red jasper fragments. STOP 4-7E: UC-3 submember at the top of the Upper Cherty Location: Laurentian Mine. Gilbert 7.5’ quadrangle T.58N., R.17W., Sec 24, SE of NE of NE; 542635E, 5260785N (NAD 83) Description: At this locality is a mixed thick-bedded, regular-bedded, and diffuse-bedded granular cherty unit with brown Fe-carbonate-rich beds (irregular to wispy appearance) as well as Fe-carbonate mottles and patches. The basal contact with the UC-2 submember is present at this locality and is marked by a seven-foot-thick thin-bedded zone at the base of the UC-3. A quartzcarbonate vein marks the contact. Within the middle of this unit is an algal stromatolite-bearing unit which will be viewed at the next stop in LTV pit 2E. This is the last stop at the Laurentian Mine. Return to the mine entrance (the one that is north of Gilbert), turn left on State Highway 135 and proceed west to Biwabik and Aurora. Follow Co. Road 110 through Aurora to Hoyt Lakes. At the stop sign in Hoyt Lakes take a left and proceed down Co. Road 666 (approximately 4-5 miles) to the guard shack near the office buildings at the Cliffs-Erie site (old LTV offices). After getting permission to enter the property, take an immediate right and follow this road about 3.5 miles to a T-intersection with the Dunka Road (private company road). Follow this road about 2 miles through a sharp right turn and a sharp left turn. At the last turn is a series of closed mining roads angling off to the south – walk down the road that points in the more southerly direction to the brow of the hill (Stop 4-8). The stratigraphic nomenclature for the Cliffs-Erie site is presented in Figure 4-15. STOP 4-8: Algal submember (I submember) near the top of the Upper Cherty Location: Cliffs-Erie site (old LTV Pit 2E). Allen 7.5’ quadrangle T.59N., R.14W., Sec 23, N1/2 of NW; 568202E, 5270625N (NAD 83). Access to this site is via the Dunka Road which is a private mining company road. Description: Algal structures were first described by Leith (1903) as “contorted bedding.” Grout and Broderick (1919) are the first who assigned an organic origin to them. The algal submember within the Upper Cherty consists of mounds of fossilized algal colonies that are separated by jasper-bearing intraformational conglomerate; both the algal and conglomerate units exhibit a combined thickness of 2-20 feet. This horizon occurs only in the eastern half of the range (not present west of Hibbing) and is only sporadically present between Hibbing and Chisholm. This locality is an excellent place to view a nearly horizontal portion of the iron-formation that contains abundant individual mounds of algal stromatolite. Stripping of glacial overburden in this area has revealed a dip slope the size of a football field that contains the stromatolite mounds Page 122 �(Graber, 1993). See Figure 4-9 in this field guide. Internally, the mounds are characterized by many individual columnar finger-like structures that are convex upward. The mounds protrude up through a thin veneer of the overlying thin- to wavy-bedded H submember. Studies of a nearby zone in this horizon showed that all the columnar stromatolites were inclined at 30 degrees to the vertical. Unfortunately, this particular site has been removed by mining activities. Stromatolite samples will be able to be collected at the extreme eastern edge of this exposure. Also at this locality the J and H submembers locally contain anthraxolite, which is an organic bitumen containing 95% or more carbon that is black with a vitreous luster and conchoidal fracture and resembles obsidian (Morey, 1994). Morey (1994) reported that anthraxolite is present throughout the iron-formation but it is most common beneath the carbon-rich “Intermediate Slate.” Furthermore, he suggested it formed via a mechanism of concentrating carbon from a mass-kill phenomenon, followed by later migration of a carbon-rich liquid to form the anthraxolite. After this site we will proceed eastward about 14 miles down the Dunka Road (through a locked gate) to the Peter Mitchell mine operated by Northshore Mining. The stratigraphic nomenclature of submembers in the Peter Mitchell Mine is portrayed on Figure 4-15. STOP 4-9: Submembers C, D (Upper Slaty), F, G, H, and I (Upper Cherty) Location: Peter Mitchell Mine. Babbitt 7.5’ quadrangle T.60N., R.13W., Sec 26, S ½ of NE. Several subunits of the Upper Cherty can be viewed within short walking distances at this particular stop (see descriptions and locations listed below). Description: Submembers G (wavy-bedded taconite ore), H (wavy-bedded taconite ore) and I (stromatolitebearing unit) at 578533E, 5278414N (NAD83 UTMs). The I submember is present at the base of the exposure and is overlain by the wavy-bedded H submember. At the top of H is a 0.5-1.0-ft-thick intraformational conglomerate that separates the H and G subunits (the G submember is actually present within the Lower Slaty according to Gundersen and Schwartz, 1962). Note that both H and G subunits constitute taconite ore and both are characterized by wavy-bedding. This same bedding characteristic was also evident in the taconite ore zones within the Lower Cherty that were seen earlier on this field trip. Note also that the bedding characteristics of the Upper Cherty at this locality are wavy rather than massive as at the Laurentian pit. Submember F with small septaria-like structures at 578857E, 5278440N (NAD83 UTMs) The F submember is thin- to wavy-bedded and locally contains small septaria-like structures that consist of whitish quartz-filled subvertical fractures in the granular cherty layers. Even though the F submember contains appreciable magnetite, it is classed as waste material because the magnetite is too fine to be economically concentrated. Submember F with small septaria-like structures and submember G with minor garnets (optional) at 578841E, 5278503N (NAD83 UTMs) Submember C (with Keweenawan sill) and submember D (optional) at 578730E, 5278206N (NAD83 UTMs) Both the C and D submembers are thin-bedded units of the Upper Slaty, however, the D submember is different in that it contains slightly thicker beds and lenses of chert. The “contact” Page 123 �between the C and D is exposed on this bench. A 20-ft-thick Keweenawan sill is also present at this stop (see below for geologic description) and is positioned about in the middle of the C submember. STOP 4-10: Submembers A, B, and C within the Upper Slaty, partially-melted Virginia Formation, two Keweenawan sills, and base of the Duluth Complex Location: Peter Mitchell Mine. Babbitt 7.5’ quadrangle T.60N., R.12W., Sec 16, W ½ of SW. Several subunits of the Upper Slaty, the Virginia Formation, and three Keweenawan intrusive rocks can be viewed within short walking distances at this particular stop (see descriptions and locations listed below). Description: Keweenawan Sill (“BIFSill”) within the C submember at 584051, 5280958N (NAD83 UTMs) A 2-18-ft-thick sill is present in the middle of the C submember at the Peter Mitchell and Dunka Pit areas, and within the J submember in the LTV 2E pit. The sill is generally fine- to mediumgrained with locally very coarse-grained plagioclase phenocrysts and polygonal jointing. A granoblastic texture is evident in thin-section indicating that the sill was emplaced in the early Keweenawan and was later metamorphosed by intrusion of the Duluth Complex. Hauck et al. (1997) noted that this sill is chemically similar to the Logan sills, to the northeast in the Rove Formation, and have informally called this sill a “Logan-type” sill. C submember at 584129E, 5281033N (NAD83 UTMs) The C submember is dominated by well-laminated, thin-bedded, slaty iron-formation containing magnetite, fayalite, ferrohypersthene, and chert. Submembers A, B, and C at 584193E, 5281142N (NAD83 UTMs) At the very top of the BIF is a 2-6-ft-thick chert and marble unit (A submember) that corresponds to the carbonate horizon that is present in only the eastern half of the Mesabi Range. This unit is locally absent in some areas (non-depositional unconformity) and extremely thick in other areas. The B submember is characterized by alternating chert and diopside bands up to one foot thick; marble layers are locally present. In some areas at this stop, pink granophyric veins locally cut the B submember. These veins exhibit pinch-and-swell relationships in that the veins thicken within the diopside bands and pinch in the chert bands. Keweenawan Sill (“VIRGSill”) at base of the Virginia Formation at 584226E, 5281159N (NAD83 UTMs) At the very base of the Virginia Formation is a 2-100-ft-thick sill that consists of a fine-grained, granoblastic, rock with varying amounts of plagioclase, clinopyroxene, orthopyroxene, hornblende, olivine, and biotite. The informal term of “Cr-bearing sill” was first used by Hauck et al. (1997) to highlight the relatively high chromium contents (600-1,200 ppm) that are characteristic of this sill. This sill exhibits two varieties: 1. a fine-grained, massive, gray-colored unit (this exposure) that is extremely difficult to distinguish from the hornfelsed Virginia Formation; and 2. a medium- to coarse-grained unit that is olivine- and/or hornblende-rich and is easily recognized. Page 124 �Partially-melted Virginia Formation in close proximity to the Duluth Complex at 584261E, 5280908N (NAD83 UTMs) In close proximity to the Duluth Complex, the well-bedded sediments of the Virginia Formation are typically transformed into a rock that at first appearance looks like an intrusive rock due to the presence of randomly oriented biotite. This rock is informally referred to as the “recrystallized unit” but is more properly classed as a diatexite (Sawyer, 1999). During emplacement of the Duluth Complex, the sediments of the Virginia Formation were heated, generating 20-40% pervasive partial melts, that literally enabled these rocks to flow in response to stresses that were applied during emplacement. All bedding planes are obliterated and what remains is a mediumgrained recrystallized rock that contains plagioclase, cordierite, orthopyroxene, and decussate biotite. Within this recrystallized matrix are blocks/boudins of more structurally competent siltstone and calc-silicate hornfels (originally limey layers). Basal contact of the Duluth Complex at 584266E, 5280874N (NAD83 UTMs) At this locality the basal contact of the South Kawishiwi intrusion (SKI) is irregular with localized “fingers” of the footwall Virginia Formation protruding upward into the intrusive rocks. Rocks of the SKI consist of weakly to moderately mineralized, fine- to medium-grained, ophitic augite troctolite to olivine gabbro. Cu-Ni values are unknown for this exposure. Return to Duluth via highways 21, 169, and U.S. 53. References Beck, J.W., 1988, Implications for Early Proterozoic tectonics and the origin of continental flood basalts, based on combined trace element and neodymium/strontium isotopic studies of mafic igneous rocks of the Penokean Lake Superior belt, Minnesota, Wisconsin, and Michigan: Unpublished Ph.D. Dissertation, University of Minnesota, 262 p. Boerst, K., 1999, Stromatolites in the LTV 2E pit, Mesabi Range, northeastern Minnesota: unpublished Undergraduate Research Opportunity Project (UROP), University of Minnesota Duluth. Chandler, V.W., 1993, Geophysical characteristics: in Sims, P.K., ed. (chap. 2) The Lake Superior region and TransHudson Orogen, Precambrian: Conterminous US: Geologic Society of America: The Geology of North America, v. C-2, p. 81-89. Fralick, P.W., and Kissin, S.A., 1998, The age and provenance of the Gunflint lapilli tuff [abs.]: 44th Annual Institute on Lake Superior Geology, Minneapolis, MN, 1998, v. 44, part 1, p. 66-67. Fralick, P.W., Davis, D.W., and Kissin, S.A., 2002, The age of the Gunflint Formation, Ontario, Canada: Single zircon U-Pb age determinations from reworked volcanic ash: Canadian Journal of Earth Science, v. 39, p. 1089-1091. Graber, R.G., 1993, Field trip guidebook (Trip 1) – LTV Steel Mining Company: 39th Annual Institute on Lake Superior Geology, Eveleth, MN, 1993, v. 39, part 2, p. 39-42 and 52-59. Grout, F.F., and Broderick, T.M., 1919, The magnetite deposits of the eastern Mesabi Range, Minnesota: Minnesota Geological Survey Bulletin, v. 44, 58 p. Gruner, J.W., 1924, Contributions to the Geology of the Mesabi Range: With special reference to the magnetites of the iron-bearing formation west of Mesaba: Minnesota Geological Survey Bulletin 19, 71 p. Page 125 �Gruner, J.W., 1946, The mineralogy and geology of the taconites and iron ores of the Mesabi Range, Minnesota: Office of the Commissioner Iron Range Resources and Rehabilitation, St. Paul, in cooperation with the Minnesota Geological Survey, 127 p. Gundersen, J.N., and Schwartz, G.M., 1962, The geology of the metamorphosed Biwabik Iron Formation, eastern Mesabi district, Minnesota: Minnesota Geological Survey Bulletin 43, 139 p. Han, T.M., 1982, Iron formations of Precambrian age: Hematite-magnetite relationships in some Proterozoic iron deposits – microscopic observations, in G.C. Amstutz and others, eds., Ore Genesis: The State of the Art: New York, Springer-Verlag, p. 451-459. Hauck, S.A., Severson, M.J., Zanko, L.M., Barnes, S.-J., Morton, P., Alminas, H., Foord, E.E., and Dahlberg, E.H., 1997, An overview of the geology and oxide, sulfide, and platinum-group element mineralization along the western and northern contacts of the Duluth Complex; in Ojakangas, R.W., Dickas, A.B., and Green, J.C., eds., Middle Proterozoic to Cambrian rifting, central North America: Boulder, Geological Society of America Special Paper 312, p. 137-185. Hemming S., McLennan, S.M., Hanson, G.N., and Krogstad, K.M., 1990, Pb isotope systematics in quartz [abs]: Eos (American Geophysical Union Transactions), v. 71, no. 17, p. 654-655. Hemming, S., Hanson, G.N., McLennan, S.M., and Sharp, W.D., 1991, Isoclinal slump-folds in the lower Pokegama Quartzite: Evidence for seismicity and slope instability during deposition of the Animikie Group [abs]: Abstracts and Proceedings, 37th Annual Institute on Lake Superior Geology, Eau Claire, WI, 1991, v. 37, part 1, p. 56-58. Hemming, S.R., McLennan, S.M., and Hanson, G.N., 1996, Geochemical source characteristics and diagenetic trends of the Virginia Formation, Mesabi Iron Range, Minnesota [abs]: 42nd Annual Institute of Lake Superior Geology, Cable, WI, 1996, v. 42, part 1, p. 13. Kissin, S.A., Vallini, D.A., Addison, W.D., and Brumpton, G.R., 2003, New zircon ages from the Gunflint and Rove Formations, northwestern Ontario [abs.]: 39th Annual Institute on Lake Superior Geology, Iron Mountain, MI, 2003, v. 49, part 1, p. 43-44. LaBerge, G.L., Cannon, W.F., Schulz, K.J., Klasner, J.S., and Ojakangas, R.W., 2003, Paleoproterozoic stratigraphy and tectonics along the Niagara suture zone, Michigan and Wisconsin: Field Trip Guidebook, 49th Annual Institute on Lake Superior Geology, Iron Mountain, MI, 2003, v. 49, part 2, p. 1-32. Leith, C.K., 1903, The Mesabi iron-bearing district of Minnesota: United States Geological Survey Monograph 43, 3163 p. Lucente, M.E., and Morey, G.B., 1983, Stratigraphy and sedimentology of the Lower Proterozoic Virginia Formation, northern Minnesota: Minnesota Geological Survey Report of Investigations 28, 28 p. Morey, G.B., 1967, Stratigraphy and sedimentology of the Middle Precambrian Rove formation in northwestern Minnesota: Journal of Sedimentary Petrology, v. 37, no. 4, p. 1154-1162. Morey, G.B., 1972, Mesabi Range: in Sims, P.K. and Morey, G.B., eds., Geology of Minnesota: A Centennial Volume, Minnesota Geological Survey, p. 204-217. Morey, G.B., 1973, Stratigraphic framework of Middle Precambrian rocks in Minnesota: in Young, G.M., ed., Huronian stratigraphy and sedimentation, Geological Association of Canada, Special Paper 12, p. 211-249. Morey, G.B., 1992, Chemical composition of the eastern Biwabik Iron Formation (Early Proterozoic), Mesabi Range, Minnesota: Economic Geology, v. 87, p. 1649-1658. Morey, G.B., 1993, Geology of the Mesabi Range: Field trip guidebook (Trip 1): 39th Annual Institute on Lake Superior Geology, Eveleth, MN, 1993, v. 39, part 2, p. 1-18. Morey, G.B., 1994, Anthraxolite in the Early Proterozoic Biwabik Iron Formation, Mesabi Range, northern Minnesota: in Southwick, D.L., ed.: Short Contributions to the Geology of Minnesota, 1994: Minnesota Geological Survey Report of Investigations 43, p. 39-47. Page 126 �Morey, G.B., 1999, High-grade iron ore deposits of the Mesabi range, Minnesota – Product of a continental-scale Proterozoic ground-water flow system: Economic Geology, v. 94, 133-142. Morey, G.B., 2003, Paleoproterozoic Animikie Group, related rocks, and associated iron-ore deposits in the Virginia horn: in Jirsa, M.A. and Morey, G.B., eds., Contributions to the geology of the Virginia horn area, St. Louis county, Minnesota: Minnesota Geological Survey Report of Investigations 53, p. 74-102. Morey, G.B., and Ojakangas, R.W., 1970, Sedimentology of the Middle Precambrian Thomson Formation, east-central Minnesota: Minnesota Geological Survey Report of Investigations 13, 32 p. Ojakangas, G.W., 1996, Cyclic tidal laminations in the Early Proterozoic Pokegama Formation: digital image analysis and computer modeling [abs.]: 42nd Annual Institute on Lake Superior Geology, 1996, v. 42, part 1, p. 44-45. Ojakangas, R.W., 1983, Tidal deposits in the early Proterozoic basin of the Lake Superior region – The Palms and the Pokegama Formations: Evidence for subtidal-shelf deposition of Superior-type banded iron-formation; in Medaris, L.D., Jr., ed., Early Proterozoic geology of the Great Lakes region: Geological Society of America, Memoir 160, p. 49-66. Ojakangas, R.W., 1994, Sedimentology and provenance of the Early Proterozoic Michigamme Formation and the Goodrich Quartzite, northern Michigan: Regional stratigraphic implications and suggest correlations: United States Geological Survey Bulletin 1904, 31 p. Ojakangas, R.W., Morey, G.B., and Southwick, D.L., 2001, Paleoproterozoic basin development and sedimentation in the Lake Superior region, North America: Sedimentary Geology 141-142, Elsevier Science B.V., p. 319-341. Pfleider, E.C., Morey, G.B., and Bleifuss, R.L., 1968, Mesabi deep drilling project: Progress report no. 1: in Mining Symposium, 29th Annual, and American Institute of Mining and Metallurgical Engineers, Minnesota Section, 41st Annual Meeting, Duluth, 1968 [Proceedings]: University of Minnesota, p. 52-92. Sawyer, E.W., 1997, Criteria for the recognition of partial melting: Physics and Chemistry of the Earth, v. 24, p. 269279. Schneider, D.A., Bickford, M.E., Cannon, W.F., Schulz, K.J., and Hamilton, M.A., 2002, Age of volcanic rocks and syndepositional iron formations, Marquette Range Supergroup: Implications for tectonic setting of Paleoproterozoic iron formations of the Lake Superior Region: Canadian Journal of Earth Sciences, v. 39, p. 999-1012. Schulz, K.J., 1987, An Early Proterozoic ophiolite in the Penokean orogen [abs]: Geologic Association of Canada Program Abstracts 12, p. 87. Schulz, K.J., 2003, A Paleoproterozoic suprasubduction zone ophiolite-island arc complex in northeastern Wisconsin [abs]: 49th Annual Institute on Lake Superior Geology, Iron Mountain, MI, 2003, v. 49, part 1, p. 71-72. Severson, M.J., Zanko, L.M., Hauck, S.A., and Oreskovich, J.A., 2003, Geology and SEDEX potential of Early Proterozoic rocks, east-central Minnesota; Natural Resources Research Institute, University of Minnesota Duluth, Technical Report NRRI/TR-2003/35, 160 p. Southwick, D.L., Morey, G.B., and McSwiggen, P.L., 1988, Geologic map (scale 1:250,000) of the Penokean orogen, central and eastern Minnesota, and accompanying text: Minnesota Geological Survey Report of Investigations 37, 25 p. Van Hise, C.R., and Leith, C.K., 1901, The Mesabi district: United States Geological Survey Annual Report, v. 21, part 3, p. 351-370. White, D.A., 1954, The stratigraphy and structure of the Mesabi Range, Minnesota: Minnesota Geological Survey Bulletin 38, 92 p. Winchell, N.H., 1882, The Potsdam sandstone: Minnesota Geological Survey Annual Report, v. 10, p. 123-136. Winchell, N.H., 1893, Twentieth annual report for the year 1891: Minnesota Geological Natural History Survey, 344 p. Page 127 �Wolff, J.F., 1917, Recent geologic developments on the Mesabi Iron Range, Minnesota: American Institute of Mining and Metallurgical Engineers, Transactions, v. 56, p. 229-257. Zanko, L.M., Severson, M.J., Oreskovich, J.A., Heine, J.H., Hauck, S.A., and Ojakangas, R.W., 2003, Oxidized taconite geological resources for a portion of the western Mesabi Range (west half of the Arcturus Mine to the east half of the Canisteo Mine), Itasca County, Minnesota – A GIS-based resource analysis for land-use planning: Natural Resources Research Institute, University of Minnesota Duluth, Technical Report NRRI/TR-2001/40, 85 p. Figure 4-16: Typical example of wavy-bedded taconite (also referred to as irregular-bedded taconite) that is mined from both the Upper Cherty and Lower Cherty members of the Biwabik Iron Formation on the Mesabi Range of northeastern Minnesota. Photo of the LC-4 submember at the Thunderbird North Mine of United Taconite. Page 128 �FIELD TRIP 5--CLASSIC OUTCROPS OF NORTHEASTERN MINNESOTA Co-leaders and contributors: Mark A. Jirsa1, Terrence J. Boerboom1, John C. Green2, James D. Miller, Jr. 1, G.B. Morey1, Richard W. Ojakangas2, and Dean M. Peterson3 1 Minnesota Geological Survey (staff and emeritus) University of Minnesota-Duluth (emeritus) 3 Natural Resources Research Institute 2 This manuscript has not been reviewed to conform to editorial standards of the Minnesota Geological Survey. INTRODUCTION We propose on this field trip to present the great diversity of Precambrian rock types in northeastern Minnesota using some of the most illustrative and accessible outcrops. We also hope to portray our present understanding of that geologic framework in the context of history— highlighting contrasts between “what we thought we knew” in the earlier years of geologic study, with “what we think we know now.” The trip will revisit many of the outcrops on which so many historical discussions, many of them heated, occurred. Geologic controversies were begun, and in some cases resolved, through discussions of the geologic relationships exhibited at some of these localities. This approach is not intended to criticize earlier works or workers; as each has contributed incrementally to the overall geologic picture. Geologic knowledge, like that in most sciences, is “vertical”— that is, each new iteration stands on the shoulders of previous efforts. Obviously, the trip visits only those places that are reasonably accessible from Duluth and from larger roads (Fig. 5.1). Figure 5.1 Generalized geologic map of northeastern Minnesota showing route of Field Trip #5. Page 129 �Although we consider many of these to be classic outcrops, it should also be noted that— because of access and location—these are only some of the classics; some of the most instructive outcrops are comparatively less accessible. It is likely that we have omitted some favorites, for which we apologize; however, many other fine outcrops can be found in field trip guide books in the accompanying references, including guides of the Institute on Lake Superior Geology, the Minnesota Geological Survey Guidebook Series, and the Geological Society of America (e.g., Biggs, 1987). HISTORICAL CONTEXT The Institute on Lake Superior Geology (ILSG) originated in Minnesota—as the “Lake Superior Institute”—with its first meeting held in Minneapolis in 1955, and has met the past 50 years in a variety of localities around the Great Lakes. Including this year’s meeting, the Institute has met in Minnesota a total of 13 times; five meetings were headquartered in the Twin Cities, six were held in Duluth, and two in more remote localities such as International Falls and Eveleth. Many, though apparently not all of those meetings, involved field trips demonstrating the diverse geology and mineral resources of the state. Unfortunately, the records of field trips during the earliest meetings are scant. Nevertheless, archives show that field trip topics fall into the following general geologic categories, listed geochronologically (with Minnesota meeting years in parentheses) below: Quaternary <1.75 Ma Twin Cities area (1976, 1998) Northern Minnesota (1979, 2004) Paleozoic <545 Ma Southeastern Minnesota (1976, 1979, 1998) Mesoproterozoic—1600-900 Ma North Shore Volcanic Group/Midcontinent rift (1971, 1979, 1989, 1998, 2004) Duluth Complex (1965, 1971, 1979, 1993, 2004) Paleoproterozoic—2500-1600 Ma St. Cloud “granite” district/Penokean orogen (1965, 1979, 1989, 1998) Virginia/Rove Formations (1963, 1971, 2004) Mesabi Iron Range/Biwabik Iron Formation (1963, 1971, 1979, 1993, 2004) Gunflint Iron Formation (1971) Archean—Neoarchean 2900-2500 Ma and Mesoarchean 3400-2900 Ma Vermilion and other greenstone belts (1971, 1979, 1989, 1993, 2004) Vermilion Granitic Complex/Quetico subprovince (1982, 2004) Minnesota River Valley gneiss terrane (1976, 1998) Although there apparently was no companion field trip for the first ILSG meeting in 1955; the “hot topics,” as judged from printed abstracts of that meeting, included the following: • Age, stratigraphic setting, origin, and lithologic subdivision of Lake Superior ironformation (presentations by Harold James, Robert Schmidt, and J.F. Wolff) • Composition and origin of iron ores; addressing competing theories on the creation of “natural ores” by rising hydrothermal vs. descending meteoric waters (Burton Boyum, Stanley Tyler, David White, N.K. Huber, Alan Broderick, Henry Lepp, Tsu Ming Han) • Geophysical characteristics of the “Lake Superior Syncline” (Edward Thiel, Harold Mooney, George Wollard, Lloyal Bacon) • Copper-nickel potential in Duluth gabbro, discovered in 1948 (Gerald Anderson and Donald Yardley) Not surprisingly, most of these same geological topics are important today, and are featured in the various presentations and field trips of this 50th anniversary meeting. Some of the more Page 130 �significant are represented by this field trip to the “classic” outcrops of northeastern Minnesota geology. Many of the controversies during early geologic study of the Lake Superior region involve attempts to correlate geologic units, and the consequent application of stratigraphic nomenclature. In Minnesota and adjacent Ontario, this type of controversy is typified by the debate surrounding the “Couchiching series” in the Rainy Lake area, and the stratigraphic and temporal relationships of Precambrian units in the region. Based on field work, largely in Canada, Lawson (1887) proposed that there was a sequence of sedimentary rocks older than Keewatin greenstone, and named these rocks the Couchiching series. Other workers, notably those with the U.S. Geological Survey, strongly disagreed. The question was so fundamental to understanding the regional geology that it was addressed by a special committee of geologists from the U.S. Geological Survey, Geological Survey of Canada, Michigan Geological Survey, and the Ontario Department of Mines, who visited field localities on both sides of the border. The result was a new model which was applied, rightly or wrongly, to the geology of other greenstone sequences. In retrospect, the controversies arose from the fact that geologists working in different areas observe or infer field relationships and attempt (as a natural consequence of human endeavor) to apply those relationships everywhere, including far afield from their immediate areas of knowledge— the “blind man and the elephant” syndrome. Furthermore, lacking geochronologic markers and dating methods, early workers applied temporal constraints based on generalized rock types; for example, implying that all volcanic rocks are “Keewatin,” all sedimentary rocks are either Seine (younger than Keewatin greenstone) or Couchiching (older than Keewatin greenstone), and recognizing two distinct periods of granitic rock emplacement as either Laurentian (pre-Seine) or Algoman (post-Seine). We now know from additional field work, advances in the understanding of depositional environments, and geochronologic studies that many periods of volcanism, sedimentation, and pluton emplacement have occurred. Much of what was known about the Precambrian geology in Minnesota by the 1950s—when the Institute on Lake Superior Geology (ILSG) was in its infancy—was the result of efforts by F.F. Grout, J.W. Gruner, G.M. Schwartz, and G.A. Thiel, all professors of geology at the University of Minnesota, and their graduate students working for the Minnesota Geological Survey. They summarized their views in a long paper entitled “Precambrian stratigraphy of Minnesota” (Grout and others, 1951). This publication served as backbone for a Geological Society of America field trip in 1956 (Schwartz and others, 1956). This was pioneering work that described major rock units and formalized their classification. It divided Precambrian time into a general three-fold classification scheme of Earlier, Medial, and Later—each Era separated by “great unconformities” in the rocks. The efforts established a geologic framework that was tested and refined by many workers, commonly reporting their results during the early years of the ILSG. For example, geochronologic and geologic studies in the 1960s established the Laurentian as an orogenic event, and much of the evidence for that was presented at ILSG meetings and field trips. FIELD TRIP FACTS The field stops include rocks ranging in age from Archean to Mesoproterozoic (Fig. 5.1 and Table 5-1). For the most part, the stops are presented in geographic rather than geochronologic order. Temporal settings can be deduced from the various figures, descriptions, and Table 5-1. Five topical areas are represented: 1) Virginia Horn—Archean and Paleoproterozoic (Stops 5-1 to 5-7) 2) Tower-Soudan—Archean (Stops 5-8 to 5-14) 3) Ely area—Archean (Stops 5-15 to 5-18) 4) North Shore and Duluth—Mesoproterozoic (Stops 5-19 to 5-25) 5) West of Duluth—Paleoproterozoic and Mesoproterozoic (5-26 to 5-28) Page 131 �TABLE 5-1—Stratigraphic and temporal classification of Precambrian rocks in northern Minnesota (after Sims, 1972). Ages are referenced in the text. Circles enclose the approximate chronostratigraphic position of field trip #5 stops. Considerably more stops appear in this guide than can reasonably be visited during a two-day trip. Other stops are included to provide a more thorough view of the regional geology and to facilitate self-led excursions. Some of these stops, as noted below, are described in detail in other field trip texts in this guide book, and not repeated here. Furthermore, much of the regional geologic framework is covered in the introductory material of those field trips, as noted below. The descriptions of many of the stops contain brief discussions of the “Historical Perspective,” providing the contrast between early and modern understanding of the geology. All UTM locations are given in NAD 83, Zone 15 coordinates. Section subdivisions read from smallest to largest quarter; e.g., “NW, SE, SW” should be read “NW quarter of the SE quarter of the SW quarter.” The small map insets showing stop locations are taken from USGS 7.5-minute topographic quadrangles listed with each stop. Page 132 �FIELD TRIP STOPS Virginia Horn area: Stops 5-1 to 5-7 (refer to Figures 5.2 and 5.3) Regional geologic context of the Archean and Paleoproterozoic rocks described below is reviewed in the introductory material to Field Trips#1, #4, and #7. Figure 5.2 Simplified geologic map of Vermilion Lake 30X60’ quadrangle (modified from Jirsa and Boerboom, 2003) showing location of field stops 5-1 to 5-13. Insets show location of additional figures. Page 133 �Figure 5.3 Generalized geologic map of the Virginia horn area (modified from Jirsa and others, 1998) showing details of field trip stops 5-1 to 5-7. Page 134 �STOP 5-1. Archean pillowed and massive greenstone Location: T.58N., R.17W., sec.23 NW, SE, SW; north edge of athletic fields, Gilbert Junior High School. Gilbert 7.5-minute quadrangle UTM: 539,820E/5,259,750N DESCRIPTION: Outcrop of pillowed and massive basalt is part of the Archean Mud Lake sequence, metamorphosed to low greenschist-grade. Pillow shapes indicate stratigraphic facing is to the northwest, which places this outcrop on the south side of a major D1 structure known as the Mud Lake syncline. Note also the presence locally of fractures filled with reddish jasper, presumably deposited in depressions on the rock surface by overstepping of Paleoproterozoic seas during deposition of the Biwabik Iron Formation. HISTORICAL PERSPECTIVE STOP 5-1: Early work by Gruner (1941) and later by Sutton (1963) interpreted the Archean volcanic and graywacke succession now known as the Mud Lake sequence as an anticline, based on few stratigraphic top indicators, and perhaps influenced by early discussions of the “Couchiching series” (Lawson, 1887) that placed metasedimentary rocks of the Rainy Lake area stratigraphically beneath “Keewatin” volcanic rocks. Detailed structural study by Jirsa and others, (1998) and Jirsa and Boerboom (2003) demonstrate that tholeiitic (Stop 5-1) and calcalkalic volcanic rocks and tholeiitic intrusions are conformably overlain by graywacke and slate (Stop 5-2). In detail, the succession forms a broad, twice-deformed syncline that has been segmented by faults of several generations. NEXT: Return to Highway 37, travel northwest to Hwy 135, thence west on 135 approximately 2.5 miles to Bourgin Road. Turn left (south) on Bourgin Road and continue about 0.4 mile to large cut on left (east) side of road (Stop 5-2). Page 135 �STOP 5-2. Archean graywacke and slate, intruded by quartzofeldspathic porphyry. Location: T.58N., R.17W., sec.21 SW, SW; road cuts on east side of the Bourgin Road. Eveleth 7.5-minute quadrangle. UTM: 536,311E/5,260,659N DESCRIPTION: Outcrops along this side of the road expose quartzofeldspathic porphyry (QFP) intruded into variably deformed graywacke, siltstone, and slate of the Mud Lake sequence. The sedimentary rocks here are moderately deformed, but much of that deformation is inferred to predate the main cleavage-forming event D2, and some may be soft-sediment in origin. The QFP is large and continuous to the east, but at this locality it appears to be segmented into a zone of multiple dikes. Both graywacke and QFP are intensely altered to some combination of iron-carbonate minerals (ankerite, ferroan dolomite) and sericite. Regionally, this style of alteration is commonly, though not always associated with QFP intrusions—presumably because the QFP remained more structurally rigid than the enclosing sedimentary rocks during the shear-related deformation event that accompanied alteration late in D2. Most gold mineralization in the area is closely allied to this alteration, yet this outcrop is surprisingly barren. HISTORICAL PERSPECTIVE STOP 5.2: One of the earliest gold discoveries in Minnesota was made by J.W. Gruner (in Grout, 1937) in a railroad cut not far from stop 5-2. The cut exposes graywacke intruded by quartzofeldspathic porphyry, having visible gold associated with small quartz veins. Despite several episodes of prospecting and systematic study of the region, no economic gold deposits have been discovered. NEXT: Follow Bourgin road to the south and west to a frontage road on the east side of Hwy 53. Turn north (right) on the frontage road and travel about 0.2 miles to first road to right, turn up-hill and continue to #7 Mesabi Lane (Stop 5-3). Page 136 �STOP 5-3. Archean conglomerate Private driveway! Location: T.58N., R.17W., sec.20 SW, SE, No. 7 Mesabi Lane; village of Midway. Eveleth 7.5-minute quadrangle. UTM: 535,713E/5,259,459N DESCRIPTION: Archean conglomerate and lithic sandstone that form the driveway here are part of the northeast-trending Midway sequence, containing these strata types locally interbedded with subaerially deposited, calc-alkalic (trachyandesitic) volcanic rocks. The sequence is inferred to have formed after earliest deformation (D1) of the enclosing graywacke and basaltic rocks of the Mud Lake sequence, but before the cleavage-forming D2 deformation that affected both sequences. The conglomerate contains clasts of basalt, graywacke, porphyritic trachyandesite, and quartzofeldspathic porphyry (QFP). This provenance indicates that the older Archean rocks of the Mud Lake sequence were intruded by QFP, deformed, and uplifted, to provide detritus to what was probably a successor or “pull-apart” basin developed along a major structure now occupied by the Pike River fault zone. Note also the presence of remnant skins of red jasper as at stop 5-1. HISTORICAL PERSPECTIVE: Midway sequence conglomerate has previously been interpreted as a basal sediment (Sutton, 1963), and as a proximal turbidite fan deposit (Levy, 1991), depositionally transitional with graywacke and slate of the Mud Lake sequence. Subsequent work (Jirsa, 2000) indicates that the conglomerate is part of a Timiskaming-type clastic and volcanic sequence that unconformably overlies the older volcanic strata. Deposition of the Midway sequence required uplift, subaerial erosion, continental volcanism, and deposition in isolated basins along a major structural break. Interestingly, Gruner, 1941, inferred that these rocks may be equivalent to similar units of conglomerate in the Knife Lake area, which are now also considered to represent successor-basin deposits that post-date deposition of older greenstone by as much as 30 million years (Corfu and Stott, 1998). The rocks share many attributes with other Timiskaming-type sequences, including the namesake Timiskaming Group near Timmins and the Shebandowan Group near Thunder Bay. The use of the term Timiskaming in this context is quite different from that of Goldich and others, (1961), who applied it as a system term to include the Seine, Knife Lake, and other presumably correlative metasedimentary units. NEXT: Return to frontage road paralleling Hwy 53; turn north and travel about 500 feet to low outcrop (Stop 5-4) on east side of road. Page 137 �STOP 5-4. Archean/Paleoproterozoic unconformity Location: T.58N., R.17W., sec.20 SW, SE; hamlet of Midway. Eveleth 7.5-minute quadrangle. UTM: 535,441E/5,259,520N DESCRIPTION: In the brush on the east side of the frontage road is a small exposure of Archean conglomerate, having nearly vertical foliation and lithologic content much like that at stop 5-3, capped by a thin and discontinuous “skin” of subhorizontally foliated conglomerate. The latter represents basal deposition of the Paleoproterozoic Pokegama Quartzite, the lowest of a tripartite sequence of formations that constitute the Animikie Group. Age dates are somewhat speculative; however, the Archean rocks of the various sequences in the horn probably are about 2.7 Ga (e.g., Peterson and others, 2001). Based on a date from tuffaceous rocks in the Gunflint Iron Formation (Fralick and others, 2002) that is inferred to be equivalent to the Biwabik Iron Formation, the Animikie Group is about 1.8 Ga. Thus, this unconformity represents a geological hiatus of approximately 900 million-years— almost twice that of all Phanerozoic time. The large road cut visible on the west side of Highway 53 is the argillaceous lower member of the Pokegama Quartzite. The road cut exposes shale, siltstone, and minor sandstone, which has been interpreted by Ojakangas (1993) as having been deposited in a low-energy upper tidal flat environment in a sea that peneplaned the Archean surface. Minor channeling is common at the base of thicker sandstone beds, and small-scale cross-bedding occurs in some siltstone beds. Local soft-sediment deformation may be the product of syn-depositional tectonism, or alternatively may represent localized collapse near tidal channels. HISTORICAL PERSPECTIVE STOPS 5-4 and 5-5: The Paleoproterozoic rocks exposed at Stops 5-4 and 5-5, and discussed at stop 5-6 were previously considered to represent a “geosynclinal sequence,” the precise tectonic mechanism of which was poorly understood. Work by a number of authors (notably Southwick and others, 1988) demonstrates that the tripartite formations exposed along the Mesabi Iron Range were deposited along the leading edge of a foredeep known as the Animikie basin that transgressed north over the Archean craton during the Penokean orogeny. In detail, deposition of basal quartzite (Pokegama), medial chemical (Biwabik) and upper turbiditic (Virginia) sediments represents a transgression of near-shore, shelf, and slope environments, respectively (Ojakangas, 1993). NEXT: Travel south on Highway 53 about 1.5 miles; stop on Highway and cross (carefully) to large outcrop on east side (Stop 5-5A) Page 138 �STOP 5-5. Paleoproterozoic Pokegama Quartzite (A) and Biwabik Iron Formation (B) Location: T.58N., R.17W., sec.32 SE, SE, and adjacent, junction of Highways 37 and 53. Eveleth 7.5-minute quadrangle. UTM: scattered outcrops extend from 535,956E/5,256,913N on the north (stop 5-5A), to 536,263E/5,256,200N on the south (stop 5-5B). DESCRIPTION 5-5A: This is the sandy, upper member of the Pokegama Quartzite. It is characterized by coarse grain size and massive beds as thick as 1.5 m. Massive beds are separated by thin beds of shale and siltstone. Ojakangas (1993) interpreted the deposition of this facies as within high-energy, lower tidal or subtidal environment. NEXT: Return to vehicle and continue south on Highway 53 past Highway 37. Turn around and drive north to large outcrop near the SE corner of the junction of Highways 53 and 37 (stop 5-5B) DESCRIPTION 5-5B: This exposure of gently southeast-dipping strata is part of the Lower Cherty member of the Biwabik Iron Formation. It overlies and is generally in transition with the Pokegama Quartzite at stop 5-5A. Notice that both formations have sandy textures and cross-bedding, implying a moderately high-energy depositional environment. The most significant difference between these two units is the abrupt change in sediment source from the extrabasinal quartz grains in the Pokegama, to recycled, chemically precipitated chert in the Biwabik. Measurements of crossbedding in the iron-formation are bimodal, implying deposition in a tidally influenced marine environment (Ojakangas, 1993). NEXT: Drive north on Highway 53 approximately 4 miles to entrance to Mineview overlook (Stop 56), just northwest of the junction of Highways 53 and 135. Follow driveway to overlook. Page 139 �STOP 5-6. Mineview in the Sky Overlook Location: T.58N., R.17W., sec.17 NW SE. North of the junction of hwys 53 and 135. Virginia 7.5-minute quadrangle. UTM: Top of overlook approximately 535,710E/5,261,650N DESCRIPTION: North from this overlook is a 3-mile-long complex of abandoned mining properties, known collectively as the Rouchleau mine; all developed within the Paleoproterozoic Biwabik Iron Formation. In detail, there were some 15 separately named mines within view that shipped ore during the period 1893-1986. All of them, and nearly 500 more along the 150-mile long Mesabi Iron Range, extracted oxidized (hematite- or goethite-rich) and leached iron-formation generally referred to as “natural ore.” The ore deposits here are localized along a set of fault zones (Fig. 5.3) that presumably provided the plumbing system for fluids that first oxidized the formation and produced permeability; and secondly, leached silica from the porous zones. Natural ores contained as much as 50 percent iron and less than 10 percent silica. By contrast, the mammoth open-pit mine in the distance to the northwest, US Steel’s Minntac mine, is developed in unoxidized, magnetite-rich ore containing about 30 percent iron, and 50 percent silica. This type of ore at Minntac, and five other open-pit mines currently in operation along the range, is the source of the iron concentrate known as taconite. The name taconite has also been applied generally to unoxidized iron-formation containing sufficient iron to be mined for a profit using today’s technology. HISTORICAL PERSPECTIVE STOP 5-6: Nearly 70 percent of the 3.6 billion metric tons of iron ore produced on the Mesabi range was extracted as natural ores. Although it has been generally accepted that these ores formed along fracture, bedding, and fault planes by processes of oxidation and leaching, the source of altering solutions has been the subject of considerable debate among economic geologists for nearly 70 years. Much of the literature and geologic observations on the issue are reviewed in Morey (2003). Many writers support the concept of descending meteoric waters to account for the dissolution of silica and oxidation of iron minerals. Others, including Gruner (1930) believed the geologic features were better explained by ascending hydrothermal solutions. Gruner’s theory failed to gain common acceptance, in part because no driving mechanism for such a hydrothermal system could be envisioned. The integration of Animikie Group strata into the tectonic context of the Penokean orogen in east-central Minnesota revived the theory of hydrothermal fluid flow within the Pokegama Quartzite and ultimately the iron-formation, as part of a continent-scale, gravity-driven ground-water system (Morey, 1999). The debate continues, fueled in part by the observation that no oxidized (and subsequently metamorphosed) iron-formation exists on the eastern-most Mesabi range that was metamorphosed by the Mesoproterozoic Duluth Complex. The precise origin of the structural bend in iron-formation known as the Virginia horn has long been questioned. It has been explained as a paired anticline-syncline vs. a modified warping Page 140 �around a fault-bounded horst (Morey, 2003). A topic for future research involves the extent to which vertical tectonism may have been operating during deposition of the iron-formation. NEXT: Return to Highway 53 and travel north through the city of Virginia to wayside rest approximately 2 miles north of town (Stop 5-7). STOP 5-7. Archean Giants Range batholith at “Confusion Hill,” Laurentian Divide Location: T.59N., R.17W., sec.19 SE, SE; wayside off Highway 53. Virginia 7.5-minute quadrangle. UTM: 534,337E/5,269,458N DESCRIPTION: Exposed near this wayside and in road cuts on both sides of the highway is an array of variably layered intrusions having both tonalitic (white) and dioritic (black) compositions. A cursory look shows intrusive relationships that conclusively demonstrate that diorite was emplaced into tonalite at one locality, and at another, tonalite was emplaced into diorite. In detail, all compositions intermediate between the two end members are also present locally. Although the dioritic component is abundant here, the bulk of the mapped unit is tonalitic. Emplacement of this unit, now known as the Lookout Mountain tonalite, probably involved some degree of magma mingling. Dikes of tonalite that cut the adjacent high-grade supracrustal rocks of the Minntac sequence contain metamorphic fabrics, yet little evidence of metamorphic origin can be seen in the interior of the body, implying it is syntectonic with respect to D2 deformation. U-Pb zircon dates (Boerboom and Zartman, 1993) of two components of the batholith exposed to the north bracket the age of D2 deformation between about 2674 and 2682 Ma. Exposures at Confusion Hill are a small part of the Giants Range batholith, which forms the core bedrock of the Laurentian (drainage) divide. The batholith is a 40-mile wide belt of intrusions that can be traced on geophysical maps and outcrop east to the Mesoproterozoic Duluth Complex, and west beyond the western border of Minnesota. It separates Archean supracrustal sequences in the Virginia horn from those of the Tower-Soudan area—making stratigraphic correlation between the two districts speculative. HISTORICAL PERSPECTIVE STOP 5-7: It was once generally thought that the mafic component of this mixed rock represented xenolithic rafts of subjacent amphibolite grade metavolcanic strata. In the process of regional mapping, Boerboom (in Jirsa and Boerboom, 2003) recognized that the intimately co-mingled tonalitic and mafic phases are the product of magma mixing. NEXT: Travel north on Highway 53 to the junction with Highway 169; follow the latter to the northeast approximately 28 miles to County Road 77. Turn left (northwest) on 77 and proceed about 0.5 mi. to Pike River (Stop 5.8). Page 141 �Tower-Soudan Area (refer to Figures 5.2 and 5.4) Much of the framework geology for the Vermilion district, including the Tower-Soudan area, is covered in the prelude to other trips in this guide, notably field trips #1 and #7. HISTORICAL PERSPECTIVE STOPS 5-8 to 5-18; Vermilion District The application of formal stratigraphic nomenclature to these rocks that are poorly constrained by incomplete exposure and scant geochronologic control produced considerable confusion over the years. For example, early workers considered all metabasaltic sequences equivalent, and applied the name “Ely Greenstone” to similar rocks exposed in a broad area of northern Minnesota. It is clear from subsequent mapping and geochronologic work that “greenstones” in the various belts are not all correlative. Use of the term continues today; but is restricted to specific sequences in the Ely and Tower-Soudan areas. In his publication of 1887, Lawson interpreted rocks of the International Falls area as depicting “Couchiching” sedimentary rocks to lie beneath “Keewatin” volcanic strata, and these interpretations were extended to explain stratigraphic relationships throughout much of Minnesota including the Vermilion district. Lawson’s criteria were based in part on stratigraphic younging in metasedimentary sequences. More recent interpretations recognize that these strata have experienced at least two deformations, the earliest of which (D1) involved inversion of large packages of strata into great thrust nappes prior to the major cleavage-forming event (D2). Thus, the boundaries between sedimentary and volcanic rocks in many areas are more structural than stratigraphic, and the use of younging directions in these strata requires detailed structural mapping to fully evaluate. In addition to structural mapping (e.g., Hudleston, 1976; Hudleston and others, 1988; Schultz-Ela and Hudleston, 1991; Jirsa and others, 1992), the study of geochemistry (e.g., Arth and Hanson, 1975; Schulz, 1980; Southwick and others, 1998) and U-Pb zircon geochronology (e.g., Boerboom and Zartman, 1993; Peterson and others, 2001) in Minnesota and adjacent Ontario are beginning to resolve some of the temporal issues. Much work remains in this endeavor. STOP 5-8. Archean graywacke at Pike River Dam Location: T.61N., R.16W., sec. 3, NW, SW; west side of County Road 77, on N side of river. [Note that Fortune Bay Casino—the overnight hotel—lies to the north off of CR 77]. Tower 7.5-minute quadrangle. UTM: 547,300E/5,293,340N HISTORICAL PERSPECTIVE STOP 5-8 Prior to about the 1950s, no depositional mechanism could satisfactorily explain the coincidence in graywacke of 1) coarse sand derived from a source many kilometers distant and having an altered clayey matrix; 2) interbedded black slate; and 3) the lack of evidence for reworking in shallow water (indicative of deposition below wave base). This was changed when the concept of turbidity currents was introduced to the geological profession by Kuenen and Migliorini (1950). Despite widespread publication on turbidites in more modern geologic Page 142 �Figure 5.4. Geologic map of the western Vermilion district (modified from Peterson and Jirsa, 1999), including the Tower-Soudan and Ely areas, and showing details of field trip stops 58 to 5-18. Page 143 �settings through the 1950s and 1960s, the facies model was not refined and applied to Archean and Proterozoic strata in the Lake Superior region until somewhat later (Morey, 1965; Ojakangas, 1966). NEXT: Return to Highway 169 and turn east. Continue approximately 1.7 miles to just east of the junction of 169 and County Road 526. STOP 5-9. Multiply folded Archean graywacke Location: T.61N., R.16W., sec. 2, NE, NE; south side of Highway 169 just east of CR 526. Tower 7.5-minute quadrangle. UTM: 550,050E/5,294,000N DESCRIPTION: This outcrop at the road and several smaller ones in the bush nearby show the superposition of two generations of folds in thin-bedded, well-graded graywacke of the Lake Vermilion Formation. The second-generation folds (F2) are associated with a regional axial plane cleavage in which sedimentary clasts are flattened. The earlier F1 folds have no associated cleavage and tend to be erratic in form, trend, and distribution. Folds display “eye” and “mushroom” shapes that locally are interpreted to be sheath folds (Hudleston and others, 1987). These characteristics are consistent with deformation of poorly lithified sediment. The superposition of deformation events is manifest in the transection of F1 folds by cleavage related to D2. In this area and to the west, one can find anticlinal synclines and synclinal anticlines, indicating stratigraphic inversion prior to D2 folding. HISTORICAL PERSPECTIVE STOP 5-9: The complex structure of the Vermilion district was poorly understood in the 1950s and 1960s. Subsequent structural study (Hooper and Ojakangas, 1971; Hudleston, 1976; Ojakangas and others, 1978; Jirsa and others, 1993) demonstrated two distinct periods of deformation: D1 that includes largely soft-sediment deformation represented at Stop 5-9, and a D2 transpressive deformation and metamorphic cleavage-forming event. Much of the early D1 deformation produced broad areas of down-facing strata formed by large thrust nappes (Jirsa and others, 1992). Prior to this recognition, there was much confusion about temporal relationships of individual volcanic and clastic sequences. NEXT: Continue east on Highway 169 approximately 0.7 mile to Stop5-10. Page 144 �STOP 5-10. [Optional] Archean dacitic tuff/ Paleoproterozoic or Mesoproterozoic diabase dike Location: T.61N., R.16W., sec. 1, SW, NE; Highway 169 road cut. Tower 7.5-minute quadrangle. UTM: 551,160E/5,293,960N DESCRIPTION: These road cuts expose outcrops of white, dacitic tuffaceous sedimentary rock, a component of the Lake Vermilion Formation. Regionally, the formation consists of all compositional gradations between what appears to be first-cycle tuff, tuffaceous graywacke, and mixed-source graywacke, interbedded on all scales. In a general way, the tuffaceous component increases in proportion to the east toward the Tower-Soudan anticline, presumably the source region of volcanic detritus. Rocks of the anticline are interpreted to represent the core of what was probably a large, composite volcanic shield complex, bordered by irregular basins composed of detritus shed from the shield, now represented by the Lake Vermilion Formation. The northeast-trending, steeply dipping, seven-meter wide diabase dike that cuts tuffaceous rocks has been the source of considerable debate. It’s petrographic (olivine-bearing) and geochemical (silica undersaturated) composition is similar to Mesoproterozoic dikes (Schmitz, 1994); yet it lies nearly along strike with, though east of, dikes of the Paleoproterozoic Kenora-Kabetogama dike swarm. NEXT: Continue east on Highway 169 approximately 1.5 miles to road cut. STOP 5-11 [Optional] Archean fragmental volcanic rocks Location: T.62N., R.15W., sec. 32, SW, SW; Highway 169 road cut, west edge village of Tower. Tower 7.5-minute quadrangle. UTM: 553,380E/5,294,430N DESCRIPTION: This outcrop consists of fragmental, variably reworked volcanic conglomerate and tuffaceous rocks of the Lake Vermilion Formation. The presence of this rock type demonstrates the Page 145 �eastward-increasing proximity to volcanic source rocks from stop 5-9 to here. The rock is composed of about 85-95 percent dacitic detritus, 3-5 percent gray clasts of graywacke, slate, and basaltic andesite, and a small percentage of magnetic and sulfidic fragments. Fragments range in size from a few millimeters to 20 cm. The generally poorly developed sorting and bedding, together with varied clast composition, implies a debris-flow origin. NEXT: Travel east on Highway 169, through the city of Tower, and continue east to the village of Soudan. Follow signs to left toward Soudan State Park for approximately 0.3 mi.—at this point, State Park entrance is on your left. Do not follow Park signs, but rather, continue on roadway turning toward the right and follow the road to junction about 0.4 mi. to east. Turn left and follow road for about 0.5 mi., more or less straight past the Soudan Fire Station and up the hill toward the back side of the Park and Stuntz Bay. Disembark at mine buildings and walk about 150 feet north and up-hill to outcrop on the right. STOP 5-12. Archean Soudan iron-formation member of Ely Greenstone No hammering please! Location: T.62N., R.15W., sec. 27, NE, NE; Soudan Mine State Park. Soudan 7.5-minute quadrangle. UTM: 557,120E/5,296,660N DESCRIPTION: This classic exposure of the Soudan iron-formation member of the Ely Greenstone lies on the north limb of the Tower-Soudan anticline, and at the stratigraphic top of the volcanic sequences known collectively as the Lower member of the Ely Greenstone. The outcrop displays two generations of tight folding in delicate laminae of chert (creamy white), chert-hematite jasper (red), and magnetite-chert (black to silver-colored). The second generation of folds (F2) is tectonic in origin, having subvertical axial surfaces that trend east, and steeply plunging axes. Most display Z-asymmetry. The earlier folds (F0-1) appear to have been sharply refolded to produce complex interference patterns. Lundy (1985) studied folding at this locality and concluded that some of the apparent interference structures are the product of early-formed sheath folds that did not involve refolding by D2. The F1 structures are predominantly intrafolial, and exhibit a great variety of style and orientation; implying they formed by layer-parallel, softsediment slumping. It is interesting to observe the rhythmic microlaminae (1 mm or so thick) in various cherty beds exposed here and speculate about the paleoenvironment—that is, whether these represent daily heating/cooling, tidal, climatic, annual, or some other repetitive influence in the depositional environment. What is known about units of iron-formation in the Ely Greenstone, of which there are many, is that deposition occurred during periods of relative volcanic and tectonic quiescence by the slow subaqueous “rain” of chemical precipitates. The deep excavations in this area are the early workings of the Soudan iron mine, the first in Minnesota. The mine produced about 16 mt of high-grade hematite ore (60-63 percent iron Page 146 �content) from 1884 until 1962, when the land was deeded to the State of Minnesota and converted to a park. Most of the production came from underground workings that began here in 1900, and which now can be visited on guided tours. The mine also houses an underground physics research facility at 2340 feet below the surface. A massive expansion of that facility is under consideration to create a national underground laboratory at considerably greater depths (Peterson and Patelke, 2003). NEXT: Return to vehicle and drive to east-bound Highway 169. Travel east approximately 7.5 miles to Mud Creek Road. The Highway follows in a general way, the strike of the nearly vertically dipping Soudan iron-formation, bounded by volcanic rocks of the upper member of the Ely Greenstone on the north and the lower member on the south. Turn north (left) on the Mud Creek Road and follow it for approximately 3.8 miles to Mud Creek. STOP 5-13. Mud Creek shear zone Location: T.62N., R.14W., sec. 5, SE, SE; Outcrop just northwest of Mud Creek near road. Chad Lake 7.5-minute quadrangle. UTM: 564,230E/5,302,800N DESCRIPTION: This outcrop shows highly strained rocks in the Mud Creek shear zone. The rock type is a quartz-iron carbonate-sericite schist, having quartz and tourmaline knots, abundant pyrite, and trace amounts of gold. Its protolith is unknown, because of the intense deformation, but could be any of several rock types in the region, including quartzofeldspathic porphyry, metavolcanic rock, or graywacke. The shear fabric trends east-northeast, and lineations plunge at shallow angles to the east. Development of this shear zone, which occupies most of the valley of Mud Creek, is a product of largely dextral transpressive deformation that has been partitioned into discrete zones, presumably late in D2 deformation. It is generally believed that gold-bearing mineralization was introduced during these later deformation events, and the Mud Creek shear zone and environs continue to attract considerable attention as a gold target (Field Trip #7). The Mud Creek shear is a broad, anastomosing zone that forms the boundary between rocks of the Ely Greenstone and Lake Vermilion Formation on the south, and volcanic and ironformation-bearing rocks known informally as the Bass Lake sequence on the north. The Bass Lake rocks may be equivalent to parts of the Newton Lake Formation exposed north of Ely, but a complex series of faults in the intervening area makes this correlation speculative. NEXT: Drive south from Mud Creek to first gravel trail to the west; turn west and proceed about 0.2 to gravel pit. Page 147 �STOP 5-14 [Optional] Archean graywacke and slate of Lake Vermilion Formation Location: T.62N., R.14W., sec. 8, NW, NE. Soudan 7.5-minute quadrangle. UTM: 564,100E/5,302,400N DESCRIPTION: This recently exposed outcrop of the Lake Vermilion Formation consists of siliceous slate having a 1-2 meter thick unit of thinly bedded tuffaceous graywacke. The slate carries a strong S2 cleavage and is cut by shears; both trending to the east-northeast. Bedding in the graywacke trends north-northeast and youngs to the southeast—this contrasts with the regional setting which strikes to the east-northeast and youngs to the north. The reason for this anomalous trend is unclear, but it may be the result of fault drag along the Mud Creek shear zone. NEXT: Return to Mud Creek Road, travel south to Highway 169, then turn east on 169 and continue approximately 12 miles to Ely. [Enroute, one might visit a spectacular outcrop of pillowed flows described in Field Trip #1, Stop 1-13; Hilltop S of Highway 169 at SE, SW, S19, T.62N, R14W, UTM:056200E, 5297800N]. Pass through Ely and continue approximately 1 mile east of town to junction of 169 and County Road 88; then proceed 0.3 mi farther east to road cut. Ely and vicinity (refer to Figures 5.1 and 5.4) Geologic framework and outcrop locations are shown on Green and Schulz, 1982. STOP 5-15 [Optional] Archean Ely Greenstone Location: T.63N., R.12W., sec. 25, NW, SW; Road cut on Highway 169 approximately 0.3 mi. east of junction with County Road 88 (Spaulding Bay Road). Ely 7.5-minute quadrangle. UTM: 588,350E/5,307,075N Page 148 �DESCRIPTION Pillowed basalt of the Ely Greenstone. Flows and complexly interlayered iron-formation in the area are strongly folded into anticlines and synclines striking predominantly to the northeast. Stratigraphic younging to the north can be established from pillow structures in this exposure and others nearby. High grade iron ore was mined from iron-formations in the noses or axes of many of the folds. The productive Zenith, Chandler, and Pioneer mines, for example, lie within one such fold, the Ely trough. NEXT: Return to junction of Highway 169 and County Road 88, turn north on 88 and travel approximately 2 miles to Echo Trail. Turn right (north) on Echo trail and proceed 0.3 miles north to road cut on west side of the road. STOP 5-16. Archean Newton Lake Formation— variolitic basalt Location: T.63N., R.12W., sec. 22, NE, NW; road cut on Echo Trail. Ely 7.5-minute quadrangle. UTM: 585,750E/5,309,360N DESCRIPTION This road cut exposes pillowed and variolitic flows and hyaloclastite breccia of the Newton Lake Formation. Pillow sizes vary from “normal” to “mattress”, and their shapes indicate younging to the south. The Newton Lake is separated from the Ely Greenstone to the south by a complex zone of faulting (Shagawa Lake and Sibley faults) developed within sedimentary rocks of the Knife Lake Group. Although relatively undeformed conglomerate and sedimentary rocks of the Knife Lake Group are exposed just a few miles to the east, they are typically so sheared and altered in this area as to obscure lithologic and sedimentary interpretations, and will not be visited. The Newton Lake Formation differs from the Ely in that the former contains abundant diabasic sills and rare iron-formation. In addition, lava flows of the Newton Lake typically have larger MgO and incompatible element contents than those of the Ely, and some are classed as komatiites (Schulz, 1980). The Newton Lake Formation (and possibly equivalent Bass Lake sequence) is considered to be the youngest Archean supracrustal sequence in the Vermilion district. Rocks having nearly identical composition and stratigraphic/structural setting occur in Itasca County some 80 kilometers to the west (Jirsa, 1990). NEXT: Continue north on Echo Trail for 0.8 mi. to outcrop 5-17A on left (west) side of road. Page 149 �STOP 5-17. Archean Newton Lake Formation— felsic tuff and mafic intrusions Location: T.63N., R.12W., sec. 15 SE, NW; road cuts on Echo Trail. Ely 7.5-minute quadrangle. UTM: 5-17A=585,775E/5,310,660N 5-17B=585,810E/5,310,700N DESCRIPTION: 5-17A Road cut on west side of Echo Trail is aphanitic to fine-grained, laminated, light-colored felsic tuff and probable reworked tuff, cut by diabase. Minor graded bedding indicates younging to the north; however, it is likely at this exposure that the tuff represents a large block rafted in diabasic intrusions. NEXT: Cross to east side of Echo Trail and walk north to first outcrop. 5-17B This road cut exposes layered metadiabase intruded by hornblende-pyroxene-biotitebearing lamprophyre; both presumed to be intrusions of the Newton Lake Formation. Lamprophyre is coarse- to very coarse-grained and contains inclusions of the diabase. Overall, the Newton Lake Formation contains abundant and locally differentiated sills that vary in composition from diabase and gabbro to pyroxenite and peridotite. NEXT: Return to vehicle and proceed north on Echo Trail, which winds around the east shore of Burntside Lake, for approximately 3.5 miles to junction with County road 803 (Passi Road). Turn left (west) on Passi Road and stop. STOP 5-18. Archean schist (metagraywacke), paragneiss, and trondhjemite of Quetico subprovince Location: T.63N., R.12W., sec. 8, SW, NW; outcrop in SW corner of junction Echo Trail and Passi road. Shagawa Lake 7.5-minute quadrangle. UTM: 581,660E/5,312,050N Page 150 �DESCRIPTION: About one mile south en route to this stop, we passed a sharp, narrow valley that marks the trace of the Burntside Lake Fault. The fault forms a major boundary between comparatively low (greenschist) grade rocks on the south that typify the Wawa subprovince, with high-grade (amphibolite) rocks of the Quetico subprovince on the north. Rocks here include biotite schist, migmatite, and trondhjemite, a part of what’s known as the Vermilion Granitic Complex. Despite the amphibolite grade of metamorphism, the schist retains some evidence of graded bedding locally that indicates a graywacke protolith. NEXT: Backtrack to Highway 169, turn west and continue to Highway 1 at the east side of Ely. Follow Highway 1 south toward the north shore of Lake Superior, a distance of approximately 50 miles. Though we will not stop, this route takes us out of the Archean Wawa subprovince, crossing parts of the Mesoproterozoic Keweenawan Supergroup, including the Duluth Complex and North Shore Volcanic Group. North shore and Duluth areas (refer to Figures 5.1 and 5.5) For regional framework geology, refer to introductory material to Field Trip #2. Figure 5.5 Geologic map of northeastern Minnesota (modified from Miller and others, 2002) showing field trip stops on the north shore and in the Duluth area Page 151 �STOP 5-19. Top of Mesoproterozoic Palisade rhyolite flow (North Shore Volcanic Group) Location: T.56N., R.7W., SEC. 11 NE, SW; 0.2 mi. N of Highway 61 on Highway 1. Illgen City 7.5-minute quadrangle. UTM: 636,600E/5,245,850N DESCRIPTION: Road cuts on the west side of Highway 1 expose the upper zone of the Palisade rhyolite, a very large and extensive porphyritic flow that is part of the North Shore Volcanic Group. Much of the flow, seen for example on Palisade Head and Shovel Point and the Hwy 61 cut just SW of the junction with Hwy 1, has the appearance of a normal lava flow, but features here at the top, and at the inaccessible base of the cliff at Palisade head, show clearly that it was erupted explosively. The hot ash settled and welded, and the resulting hot, devolatilized mass flowed as “reconstituted lava” or ignimbrite. This road cut in the rapidly chilled top part of the flow is made of flow-brecciated, flow-folded welded tuff; microscopic study reveals deformed shards and fiamme in some samples. (More obvious shards are visible in the chilled welded tuff at the flow’s base). Pneumatolytic or hydrothermal alteration has kaolinitized much of the feldspar at this locality. In exposures of the massive flow interior, flow-aligned platy quartz after tridymite is abundant in the groundmass, implying a high temperature of eruption (greater than 870 degrees C). HISTORICAL PERSPECTIVE: Little attention was paid to the rhyolites in the North Shore Volcanic Group (NSVG) in earlier accounts. In some of the earliest work by N.H. Winchell (1889), he attributed all north shore rhyolites to the process of melting of sedimentary rocks. More recently, the importance of these rocks has been increasingly recognized because of their abundance compared to intermediate compositions (bimodal tholeiitic suite), and the opportunity they present for geochronologic study (Goldich and others, 1961). With the further development of the U/Pb zircon dating method, many rhyolites have provided age calibration for magmatism in the Midcontinent Rift. The Palisade rhyolite, for instance, has been dated at 1098.6 ±1.7 my (Davis and Green, 1997). Whereas Bowen-inspired models of magma evolution assumed rhyolites such as this to be the products of crystal fractionation of the dominant basaltic parent, their high abundance in the NSVG, along with recent Sm/Nd analyses (Vervoort and Green, 1997) show this and many others to have been produced by partial melting of Archean basement. Finally, detailed field studies and petrography of this and other large felsic flows has indicated that they were erupted at unusually high temperatures for rhyolite, were highly mobile, and that many are rheoignimbrites (Green and Fitz, 1993). NEXT: Continue south on Highway 1 for 0.3 miles to Highway 61. Turn northeast (left) on 61 and drive approximately 14 miles to turn-in for Sugarloaf Cove Scientific and Natural Area. Walk to Sugarloaf Point. Page 152 �STOP 5-20. Mesoproterozoic basalt at Sugarloaf Cove Location: T.58N., R.5W., Sec. 29 NE, NE. Little Marais 7.5-minute quadrangle. UTM: outcrops begin about 652,020E/5,261,120N Note: This stop is on property of the Sugarloaf interpretive Center Association (a non-profit) and the State of Minnesota. The beach and point are part of a Scientific and Natural Area (MN DNR) established to preserve these features. Please do not use hammers, or remove samples of any kind. Thanks. DESCRIPTION: Many of the attributes of olivine tholeiite lava flows of the NSVG are well exposed on the beach, mainland shore, and point. Flows range from several tens of meters thick to thin flow units of a meter or less. Massive interiors are ophitic: each “ophite” is made of an augite oikocryst enclosing many small plagioclase tablets. Abundant tiny olivine grains occur with plagioclase between the ophites. Pipe amygdules are common in the flow bases, and small vesicle cylinders occur in the massive interior of the thick flow that caps the sequence (The Sugarloaf) on the point. These are all pahoehoe flows, and ropy surfaces are well displayed, as are columnar joints and red sandstone-filled fractures (“clastic dikes”) in the interiors. HISTORICAL PERSPECTIVE: Although considerable petrographic study and some geochemistry had been done on the basalts of the Cu-bearing lavas of the Keweenaw Peninsula of Michigan (e.g., Butler and Burbank, 1929; Broderick, 1935), the NSVG only began to receive such attention in the mid- to late 1960’s, as the world geochemistry community began to focus on basalts and their parent ultramafic rocks of the mantle. The evolution of the plate-tectonics paradigm soon provided a model for the raison d’etre for these basalts, and the concept of the Midcontinent Rift System was born (White, 1972; Green, 1977). Further attention to the physical and geochemical aspects of basalts has generated more data on the NSVG (e.g., Nicholson and others, 1997, and references therein); olivine tholeiites such as these are interpreted as being mainly mantle plume melts, with minor lithospheric contribution. NEXT: Return southwest on Highway 61 approximately 28 miles to entrance to Split Rock Lighthouse State Park. Turn into the park and walk to base of lighthouse. Note that traveling southwest through this western limb of the NSVG, one traverses down-stratigraphic section. Page 153 �STOP 5-21. Mesoproterozoic (Keweenawan) Beaver Bay Complex at Split Rock Lighthouse Location: T.55N., R.8W., Sec. 33 SW, SW; Split Rock Point Lighthouse History Center and State Park. Split Rock Point NE 7.5-minute quadrangle. UTM: base of lighthouse at 623,640E/5,228,680N DESCRIPTION 5-21A: Outcrops of fine-grained, ophitic olivine diabase of the Beaver River diabase, which hosts the majority of anorthosite inclusions from Split Rock to Grand Marais, are exposed just northeast of the lighthouse atop a sheer 30-m-high sea cliff. The centimeter-wide augite oikocrysts obvious here are typical of the lower portion of this diabase sill, which dips gently (<15°) into the lake. The prominent point just to the northeast (Rusty Point) is held up by a very large (>200 m) inclusion of medium-grained granophyric granite lying at the base of the sill. Making our way to the base of the lighthouse, we come upon a good exposure of the large anorthosite inclusion that holds up the entire sea cliff. The coarse-grained (± 1 cm) anorthosite here displays meter-scale modal layering of noritic anorthosite (20% En72 hypersthene; 80% An6080 plagioclase) and anorthosite (>99% Pl). This inclusion is altered and displays a moderate to severe cataclastic and granulated texture indicative of having been recrystallized and/or tectonized (Morrison and others, 1983). The steeply dipping layers are cut by thin dikes of medium-grained, igneous-textured augite leuconorite (Pl-An56; Opx-En75; Cpx-En66). DESCRIPTION 5-21B. Follow the footpath south to the pump house on the lakeshore. Here, black, massive basalt is exposed in wave-washed outcrops. The brecciated, amygdaloidal AA flow top of this basalt flow is exposed in the rubbly bluff to the northeast in the direction of the lighthouse. Laminated siltstone in the matrix of the flowtop dips gently toward the lake. Carefully scrambling over some large boulders of ophitic diabase to the northeast, we can view the inclusion-rich base of the diabase sill conformably overlying the basalt flow top. Note that the diabase is chilled against the basalt, but not against the anorthosite inclusions. The abundance of inclusions disrupts the development of regular columnar jointing in the diabase. Numerous boulders along the base of the cliff display the textural and mineralogical varieties of anorthosite inclusions present. Looking to the northeast, one can see that the vertically layered inclusion beneath the lighthouse extends to lake level. HISTORICAL PERSPECTIVE: Occurrences of nearly pure plagioclase rock holding up various points of high ground between Split Rock Point and Grand Marias have held a special fascination for Lake Superior geologists for over 150 years. These enormous (up to 500m across), light-colored, coarse-grained masses embedded in dark, fine-grained mafic rock were first recognized by J.G. Norwood in 1849 (Fig. 5.6) as part of the federal Owen Survey (Owen, 1852). During the initial state geological survey (1872-1900), N.H. Winchell noted the resemblance of the feldspar rock, as he called it, to the Rice Point Gabbro at Duluth (anorthositic series of the Duluth Complex). He Page 154 �suggested that the anorthosites were blocks plucked from older feldspathic phases of the Duluth gabbro by later outpourings of the great gabbro flood, which he fancied to be an enormous extrusive (Winchell, 1900). He noted that the inclusions became somewhat smaller and more dispersed toward the lake and interpreted this to be due to disaggregation as they traveled downslope (to the southeast) from their source. Midway through the survey, Winchell commissioned A.C. Lawson to study these masses in greater detail. Lawson (1893) concluded that the anorthosite represented the peaks of Archean mountains that where inundated and largely buried by outpouring of the great gabbro flood. The inclusion character evident in most anorthosite occurrences was attributed to spalling of large blocks from the mountain peaks. In the early 1900's the search for corundum in northern Minnesota followed well publicized discoveries of the mineral in Canada. In 1902, several property owners mistook the light green anorthosite on the north shore for corundum. They formed a company, attracted investors, built a plant complete with processing, storage, and transport facilities---all without ever analyzing the rock! The first sale of mined product was also the last when it was discovered that the rock contained no corundum, but instead is composed of the much softer feldspar mineral, anorthite. Despite the setback, the investors regrouped and eventually formed the Minnesota Mining and Manufacturing company (now 3M); demonstrating an important principal: the lessons learned from small failures sometimes lead to great successes. One of the most complete inventories of the anorthosites of the North Shore was reported in Minnesota Geological Survey Bulletin 28 "The Geology of the Anorthosites of the Minnesota Coast of Lake Superior" by Grout and Schwartz (1939). In this report, they documented most known anorthosite occurrences in six 1:31,500 scale township maps. In considering the origin of the anorthosite, Grout and Schwartz clearly recognized them as inclusions of pre-existing rock that were derived from a deeper crustal source and were transported in diabasic magma to their present site by virtue of their low density. They agreed with Winchell that the source was likely the anorthositic rocks of the Duluth Complex. Figure 5.6—Woodcut print from J.G. Norwood’s report on the geology of the north shore (in Owen 1852) showing massive anorthosite inclusions in well-jointed basalt. More detailed considerations of the petrographic and geochemical attributes of the anorthosite inclusions led Phinney (1968) to conclude that Duluth Complex gabbroic anorthosites were an unlikely source for the inclusions. The inclusions, which commonly show signs of severe recrystallization and cataclasis, were more plagioclase-rich (>95%), coarser-grained, and more anorthitic (An70-75) than most anorthositic rocks of the complex. Also, Duluth Complex anorthositic rocks rarely contain orthopyroxene, a common mafic phase in the inclusions. Instead, he speculated that the inclusions were derived from deeper anorthosite bodies in the mid- Page 155 �to lower crust. Proterozoic anorthosite massifs seemed an attractive source by their depth of occurrence and plagioclase-rich composition; however, massif anorthosites are less anorthitic (An45-55) than even Duluth Complex anorthositic rocks (An55-65) and tend to be strongly recrystallized. In a follow-up petrographic and geochemical (Rb-Sr, Sm-Nd, and REE) study (Morrison and others, 1983), Phinney and his coworkers uncovered evidence of crustal contamination in the anorthositic inclusions from a source older than 1.9 Ma. They concluded from this that the inclusions were derived from early Proterozoic anorthosite bodies formed in the lower crust. As a corollary to a model for the origin of plagioclase crystal mush parent magmas to the anorthositic series of the Duluth Complex, Miller and Weiblen (1990) speculated that these anorthositic inclusions may be Keweenawan in age. In their model, plagioclase-rich magmas were generated by ponding of mantle-derived melts in the lower crust. Under the high pressures of the lower crust, plagioclase will be significantly buoyant in mafic magma and would likely give rise to anorthositic cupolas in these deep magma chambers. Early lower crustal intrusions may have been contaminated by interaction with the lower crust and thus imparted crustal isotopic signatures into these early anorthosite cumulates. As rifting progressed and the lower crust was displaced, later magmas may have picked up these early Keweenawan anorthosites and transported them tens of kilometers into subvolcanic dikes and sills of the Beaver River diabase. NEXT: Return to Highway 61, turn southwest (left), and continue approximately 15 miles to Silver Cliff Tunnel. STOP 5-22 [Optional] Mesoproterozoic (Keweenawan) diabase and andesitic flows at Silver Cliff Tunnel Location: T.53N., R.10W., Sec. 15 and 22. Castle Danger 7.5-minute quadrangle. UTM: 606,993E/5,213,952N to 606,755E/5,213,486N; DESCRIPTION: The Silver Creek diabase is an irregular, subcordant, subhorizontal intrusion at least 200 feet thick. The diabase forms a prominent highland that projects inland several miles from Silver Cliff at Lake Superior. Excavation of the Highway 61 tunnel has created excellent exposures of the contact between volcanic rocks and both the top and bottom of the diabase, and has exposed a north striking, 55-degree east-dipping fault that cuts the base of the diabase. Refer to Field Trip 2, stop 2-6 for details. NEXT: Continue southwest on Highway 61 for approximately 30 miles. Near the City of Duluth, 61 becomes London Road; continue southwest on London Road. Do NOT turn onto Interstate Highway 35, but clock mileage and continue approximately 1.25 miles to parking area for Leif Ericson City Park near 12th Avenue East. Walk into the park and to the shoreline. Page 156 �STOP 5-23. Mesoproterozoic (Keweenawan) interflow sedimentary rocks at Leif Ericson Park Location: T.50N., R.14W., Sec. 23 SW. Duluth 7.5-Minute quadrangle. UTM: 570,050E/5,182,950N DESCRIPTION: This classic location exposes an estimated 110’ of interflow sandstone, lying on slightly eroded amygdaloidal flows of the gently southeast-dipping Leif Ericson Park Lavas. The sandstone is fine- to medium-grained, moderately well sorted, and derived almost totally from underlying and adjacent lava flows—no extrabasinal detritus has been detected (Jirsa, 1984). The presence of planar-tabular and trough cross-bedding, together with the lenticular distribution of interflow units, implies the strata represent occasional stream flow deposits in depressions on the evolving lava surfaces. This is one of many interflow sedimentary units, most of which are “red bed-like” in appearance (considered in the early works of N.H. Winchell as Potsdam equivalents). By contrast, this interflow lies very near the base of the North Shore Volcanic Group, and thus has experienced burial metamorphism to nearly greenschist facies—though it is possible that some of the elevated grade above the zeolite facies typical of rocks just to the northeast, is an aureole effect of the adjacent Endion and other diabasic sills. NEXT: Follow signs to Interstate Highway 35, travel southwest on 35 to Highway 53. Turn north on 53 (Piedmont Avenue) and continue about 1 mile to intersection with Skyline Parkway. Turn left (southwest) on Skyline and travel 0.4 mi. to crossing with 28th Ave. W; continue 0.2 mi. farther southwest on Skyline to outcrop on the right (Stop 5-24). STOP 5-24 [Optional] Mesoproterozoic Duluth Complex; Layered Series “Chill”, Granophyre, and Anorthositic Series Location : T50N, R14W, Sec 32, SW, SW; Skyline Parkway about 1/5 mi. SW of 28th Ave. Duluth Heights 7.5-minute quadrangle. UTM: 564,880E/5,179,640N Page 157 �DESCRIPTION: Exposed at the northeast end of this roadcut is a fine-grained mafic rock with intermingled granophyre that together cut coarse-grained olivine gabbroic anorthosite of the anorthositic series. Since early mapping by Grout in the Duluth area in 1911, the relationships displayed by this rather innocuous exposure has been critical in interpreting the intrusive history of the Duluth Complex. The distinct, but complexly intermingled domains of mafic and felsic rock displayed here and commonly observed in the upper parts of the Duluth Complex led Grout (1918c) to the interpretation that these rocks formed by liquid immiscibility. This idea was strongly at odds with the newly introduced notion of granite formation by differentiation of mafic magma touted by Bowen (1928) and led to years of debate between these two famous petrologists. Another significant feature displayed by this exposure of more parochial interest is the relationship between the fine-grained gabbro and the coarse-grained olivine gabbroic anorthosite. The fine-grained mafic rock exposed here can be traced up over the ridge to the west where it merges into the upper contact zone of Layered Series (see Green and Miller, this abstract volume for general summary of the geology of the Duluth Complex at Duluth). Grout and later Taylor (1964) saw this exposure as evidence that the anorthositic series had cooled considerably when the layered series was intruded and thus was considerably older. This paradigm was accepted by all subsequent workers on the Duluth Complex up through the 1990’s. Consequently, it came as something of a shock when high resolution U-Pb ages (Paces and Miller, 1993) revealed that the anorthositic series and the layered series were virtually identical in age (within 0.5 Ma relative to the 22 Ma span of Midcontinent Rift magmatic activity). This precipitated a major paradigm shift in the perception of the intrusive relationships between these two series here and throughout the Duluth Complex (Miller, 1992). A closer look at this layered series “chill” reveals that it is not a thermal quench of the Layered Series at Duluth (DLS) at all. This rock type is found at the contact with the anorthositic series throughout most of this area and has a remarkably homogeneous, evolved composition (MgO/(MgO+FeO) = ~ 37; see Table 2 in Miller and Ripley, 1996). In thin section, it is a subprismatic biotitic oxide ferrodiorite. Phase equilibrium modeling of its composition indicates that it should be in equilibrium with evolved compositions of augite, ilmenite and plagioclase phases that comprise gabbroic cumulates found in the cyclic zone and gabbro zones of the DLS. In sum, this rock is much too evolved to have produced the entire cumulate pile of the layered series. Rather than this being a thermal quench, Miller and Ripley (1996) have interpreted this rock to represent decompression quenching of an evolved, water-saturated layered series magma during venting at a time when the cyclic zone was crystallizing. Whereas decompression of less than water saturated magma will result in superheating and a suspension of crystallization (or at least a significant change in phase equilibrium), decompression under water saturated conditions will cause supercooling and quenching. This model fits nicely with the explanation for the cyclic zone with which this composition is apparently comagmatic (see Stop 25). How does this scenario of venting of a hydrous magma relate to the mafic-felsic duplex which so intrigued Grout? The lobate contacts between the irregular masses of medium-grained granophyre and the fine-grained ferrodiorite host clearly gives the appearance of two-magma mixing. Grout's (1918a) idea that these two magmas formed by silicate liquid immiscibility of hydrous mafic magmas in the roof zone of the Duluth Complex seems more plausible today than when he first proposed it. The concept of silicate liquid immiscibility, which had come to be totally disparaged during Grout's day, has regained some respectability as an plausible, albeit, uncommon petrologic process (Roedder, 1979). However, alternative explanations for this compositional dichotomy can be suggested. One is that these felsic magmas were derived from anatectic melting of various inclusions carried into the layered series chamber. Because of their high silica content and low density, these felsic melts did not readily mix or assimilate with mafic melt, but rather rose to the roof zone where they ponded beneath the anorthositic series cupola. Page 158 �During magma venting from the chamber, the felsic melts became entrained and irregularly mixed with the mafic magmas. While decompression under water saturated conditions caused rapid crystallization of the mafic magma, the felsic melt became irregularly entrapped in the quenched mafic host and cooled more slowly to a medium-grained texture. NEXT: Return to Interstate 35 and follow it southwest to exit for Thomson Hill Rest Area near the crest of the hill west of Duluth. Follow signs to rest area (Stop 5-25). STOP 5-25. Mesoproterozoic (Keweenawan) Duluth Complex at Duluth Location: T.49N., R.15W., Sec. 14 SE, NW; Thomson Hill Rest Area on Skyline Parkway. West Duluth 7.5-minute quadrangle. UTM: 560,700E/5,175,380N HISTORICAL PERSPECTIVE: The well-exposed gabbroic rocks forming the escarpment above the city of Duluth have long been recognized as the type section of the Duluth Complex. While early surveys recognized the presence of two distinct rock types in the Duluth area - normal gabbros and feldspathic gabbros (Winchell, 1900), Grout was the first to interpret the layered gabbros as a product of convection and magma differentiation (Grout, 1918a-c). Taylor (1964) produced the first detailed-scale (1:24,000) geologic map of the complex in the Duluth area wherein he distinguished the stratiform gabbro cumulates of the layered series from the older, structurally complex anorthositic series. He interpreted the structural complexity of the anorthositic series as having formed from multiple injections of a plagioclase crystal mush. He also recognized the differentiated character of the layered series and pointed out that it was basically similar to the Skaergaard Intrusion - the classic example of a well-differentiated intrusion formed by fractional crystallization of a tholeiitic magma (Wager and Deer, 1939). More recently, detailed mapping and petrologic studies in the Duluth area have added substantially to our understanding of emplacement and crystallization history of these two series here at the type locality of the Duluth Complex (Miller and Ripley, 1996, Miller and others, 2002). See Green and Miller, this abstract volume for more details about the geology of the Duluth Complex at Duluth. In two outcrops at this stop, we can view exposures that demonstrate the inclusion/intrusion relationship between of anorthositic and layered series rocks and that illustrate the macrocyclic nature of the cumulate rocks which characterizes the medial part of the Layered Series at Duluth, an interval called the cyclic zone. DESCRIPTION 5-25A: In the road cut north of the rest area parking lot is an exposure of intermittently layered, wellfoliated olivine oxide gabbro hosting several meter-sized, flattened inclusions of olivine anorthosite. The gabbro is a four-phase cumulate of plagioclase, augite, Fe-Ti oxide and olivine. This cumulate typifies the upper gabbroic parts of troctolite→gabbro macrocycles that Page 159 �characterize the cyclic zone. In this three-dimensional view, the anorthositic inclusions clearly have a pancake shape that is conformable to layering and foliation in the host gabbro. This may represent the original shape of the inclusions or it may indicate compaction of a partially molten blocks. In either case, the occurrence of anorthositic inclusions in layered series rocks is a ubiquitous feature throughout the Duluth Complex. That the opposite relationship is rarely if ever observed has reinforced the longstanding interpretation that the anorthositic series is older than layered series. However, high precision U-Pb dating (Paces and Miller, 1993) has shown that this age difference is less than 1 million years relative to the 20-million year magmatic history of the Midcontinent Rift. DESCRIPTION 5-25B: In the layered sequence of gabbroic rocks exposed along the 200-m-long roadcut below the observation deck (Fig. 5.7), changes in cumulus mineral assemblages are displayed that characterize the cyclic zone of the Duluth Layered Series. The approximately 1-km thick cyclic zone is composed of five to six major macrocycles, ranging in thickness between 50 and 200 m, within which troctolitic cumulates (Pl+Ol) grade upward into gabbroic cumulates (Pl+Cpx+FeOx±Ol). Boundaries between the macrocycles are defined by abrupt cumulate reversals back to troctolitic cumulates and are also commonly marked by the occurrence of anorthosite inclusions and microgabbro cumulates at the top of the lower cycle. This exposure traverses the boundary between the third and fourth macrocycles. Figure 5.7. Geology and cryptic variation of mineral compositions along Skyline Parkway road cut near Thompson Hill rest area, Stop 5-24B. View is to the north. Dip of lamination and layering is exaggerated; it averages about 20° to the east. Contacts between units are gradational over thicknesses of 10 cm to 1 m. Mineral composition parameters are An in plagioclase = CaO/(CaO+Na2O+K2O), En' in augite and Fo in olivine = MgO/(MgO+FeO); all in mole%. Cumulus rock codes (in parentheses) list abbreviations of minerals in decreasing order of abundance (P/p plagioclase, C/c-clinopyroxene (augite); F/f -Fe-Ti oxide; O/o-olivine; upper case = cumulus mineral, lower case = intercumulus minerals). Samples A-G are noted by dark circles. The west end of the road cut begins with a coarse-grained, moderately laminated, subophitic olivine gabbro, which locally is intergranular and elsewhere is leucocratic (sample A, Fig. 5.7). Augite in this rock is marginally cumulus but becomes definitely so quickly up-section where it consistently has an anhedral granular to subprismatic habit (sample B). Beyond a poorly exposed interval, this gabbro is interlayered with a 2-m-thick interval in which minor olivine becomes Page 160 �subpoikilitic and concentrated in layers (sample C). Another 3 m above this, the coarse gabbro passes into a medium- to medium fine-grained, well-laminated, subpoikilitic olivine-bearing oxide gabbro (samples D and D') which displays layering of olivine oikocryst concentration and elsewhere isomodal layers rich in Fe-Ti oxide and pyroxene. The very strong foliation and subhedral to euhedral habit of cumulus phases (plagioclase, pyroxene, and ilmenite) impart an adcumulate texture to this rock. Over a poorly exposed interval about 15 m long is an altered, coarse-grained, ophitic gabbroic anorthosite (sample E) that is texturally and mineralogically identical to rocks in the anorthositic series. At the beginning of the next well-exposed section of roadcut, several similar gabbroic anorthosite inclusions are found in a coarse-grained, subophitic to intergranular olivine gabbro (sample F), which gradually grades upward into a more consistently subophitic texture over the remainder of the roadcut (sample G). This rock type closely resembles that at the west end of the roadcut and indicates a downgrading in the cumulus status of pyroxene (and oxide?) and a reemergence of cumulus olivine. At first glance, a sensible interpretation of this cumulus regression is that it represents a magma recharge event. However, the cryptic variation of En and Fo across the cumulus regression exposed in this section (Fig. 5.7) is the reverse of what would be expected from magma recharge. An alternative explanation to magma recharge is that the textural and compositional variations across this interval reflect decompression of the chamber due to eruption to the surface. Decompression of a volatile-enriched magma would cause supercooling and multiple saturation of the magma and thereby explain the abrupt decreases in grain size and cumulus phase changes without much compositional variation. Magma expulsion through the roof of the layered series would also explain the occurrence of a gabbroic anorthosite inclusion at the cumulate regression. The hydrothermally altered nature of the gabbroic anorthosite is consistent with a volatile-rich environment in the anorthositic series cupola of the Duluth Layered Series magma chamber. The cumulus reversal to ophitic olivine gabbro without an increase in mg# could be explained by repressurization of a devolatilized magma. NEXT: Return to Interstate Highway 35 and travel southwest on it approximately 3 miles to Highway 13, Midway Road. Turn north on Midway road and continue 0.8 mi. to road to east (right). Follow road east approximately 0.2 mi., disembark and walk east to base of bluff. West of Duluth (refer to Figures 5.1, 5.5, and 5.8) STOP 5-26. Mesoproterozoic /Paleoproterozoic Unconformity Location: T.49N., R.15W., Sec. 17 SE, SW. Esko 7.5-minute quadrangle. UTM: 555,580E/5,174,380N Page 161 �DESCRIPTION: This area contains exposures showing the unconformable relationship between the Paleoproterozoic Thomson Formation (will see at Stop 5-28), and the Mesoproterozoic (Keweenawan) North Shore Volcanic Group. Scattered outcrops in the brushy area just west of the bluff are folded and metamorphosed feldspathic graywacke, siltstone, and mudstone or slate of the Thomson Formation. By contrast the Keweenawan strata exposed in the bluff have been little affected by tectonism since deposition, as shown by their gentle dip to the east. The unconformity is not exposed; but, inferring from the two sets of exposures, it appears to be an angular one. It represents a nearly 800 million-year hiatus, based on an age of roughly 1100 Ma for the Lakeside lavas northeast of here (Davis and Green, 1997) versus a date of about 1880 Ma for tuff interbedded with iron-formation in the Animikie Group, of which the Thomson Formation is a component (Fralick and others, 2002). The Keweenawan stratigraphic section in this area consists of a basal sedimentary unit, the Nopeming formation, which is conformably overlain by the Ely’s Peak basalts. The latter comprise the oldest volcanic strata of the North Shore Volcanic Group. The Nopeming consists of approximately 30 feet (10 m) of interbedded conglomerate and quartz arenite, with minor siltstone beds in the uppermost part of the unit. Much of the sandstone is medium- to coarsegrained, well-sorted and rounded, quartz arenite. The uppermost, silty parts of the Nopeming contain load casts and other structures indicating soft-sediment deformation. The overlying Ely’s Peak basalts locally contain well-developed pillow structures, indicating subaqueous deposition. Interestingly, a nearly identical stratigraphic sequence—the Puckwunge Formation—is observed 150 miles to the northeast near Grand Portage (Mattis, 1972). Together, these sequences are representative of a broad depositional and tectonic setting at the onset of Keweenawan deposition in this part of the Lake Superior region. HISTORICAL PERSPECTIVE: The stratigraphic position of the Nopeming at the base of the Keweenawan and its correlation with the Puckwunge Formation in the Grand Portage area and the Bessemer Quartzite in the Ironwood, Michigan area has long been accepted. However, a curious complication to this simple interpretation is presented by the 120 year old record of a 500' deep well drilled about 3 miles due south of this site. The log of the Short Line Park well, as reported by Winchell (1889), encountered sandstone and conglomerate, very similar in appearance to the Nopeming that was interlayered with basaltic lavas over a 140-foot interval. Surprisingly, the lower 84 feet of the hole is reported to be exclusively in basalt. This has led some to question whether the Nopeming is the basal unit or just an interflow sedimentary unit (Mattis, 1972). If the latter is correct, it implies a north-south fault along the base of the slope at this site. NEXT: Return to Interstate 35, cross under it, and continue south to join with Becks Road. Follow Becks Road southeast to Gary-New Duluth and Highway 23. Turn south (right) on 23 and follow it through town. At the south edge of town, Highway 23 turns west (right); follow it into the city of Fond du Lac. At the west edge of Fond du Lac, turn northwest (right) on Highway 210 and turn very shortly afterward to left into city park. Walk to northwest to edge of bluff (Stop 5-27). Page 162 �Figure 5.8. Geology and location of stops in the Fond du Lac, Jay Cooke State Park, and Thomson Dam areas (modified from Jirsa and Morey, 1987). STOP 5-27. Mesoproterozoic Fond du Lac Formation Location: T.48N., R.15W., sec. 6 SE, SE along west shore of St. Louis River near city park. Esko 7.5-minute quadrangle (SEE Fig. 5.8 for location). UTM: 554,670E/5,168,140N DESCRIPTION: The Fond du Lac Formation consists of nearly 800 feet of red sandstone, shale, and minor basal conglomerate, of which only a few hundred feet are exposed here along the lower St. Louis River. It is part of an accumulation of as much as 20,000 feet of clastic sediments deposited in the central Lake Superior region following Keweenawan volcanism. The Fond du Lac is considered to be equivalent to the Orienta Sandstone of the Bayfield Group in Wisconsin, and the Jacobsville Sandstone of Michigan. In this area (though not visible at this locality), the Fond du Lac unconformably overlies rocks of the Paleoproterozoic Thomson Formation, Ely’s Peak basalts, and the Duluth Complex (Morey, 1967). The sandstone consists of poorly sorted arkose to subarkose, containing clasts of quartz, chert, microcline, micas, and minor volcanic rocks; implying a source from both outside and inside of the Midcontinent rift. Depositional structures, primarily trough cross-bedding, indicate transport to the east. These sedimentary structures, and the presence of mud cracks and rain imprints, indicate fluvial deposition by streams that meandered across a broad alluvial plane (Morey and Ojakangas, 1982; see also papers in Ojakangas and others, 1997). NEXT: Return to northwest-bound Highway 210 and follow it for about 5 miles to entrance to Jay Cooke State Park. Turn in at main park building area on south side of 210. Page 163 �STOP 5-28, Location 5-28A Paleoproterozoic Thomson Formation Location 5-28A: T.48N., R.16W., Sec. 9 NW, SE; Jay Cooke State Park. Esko 7.5-minute quadrangle. UTM: 548,120/5,166,870N DESCRIPTION 5-28A: The walking bridge over the river south of the park buildings provides a vantage (depending on river level) to steeply dipping beds of graywacke, siltstone, and slate of the Thomson Formation. The Thomson is the upper stratigraphic unit of the Paleoproterozoic Animikie Group, presumably equivalent with the Virginia Formation discussed in stops 5-4, 5, and 6. Geochronologic data indicate that the Thomson Formation was deposited approximately 18701880 Ma (broadly inferred from Fralick and others, 2002), and was deformed during the Penokean orogeny (ca 1850 Ma). As shown in Figure 5.8, the Thomson is unconformably overlain by redbeds of the Mesoproterozoic Fond du Lac Formation, exposed discontinuously just to the east. The unconformity, marked by a basal quartz-pebble conglomerate, can be seen with some difficulty in the river bed at low-water levels near Oldenburg Point, and up Little Creek to the northeast. HISTORICAL PERSPECTIVE In the 1950’s, the Thomson Formation was considered by a number of authors (e.g., Grout and others, 1951) to be equivalent with the Knife Lake Group in northeastern Minnesota. The two rocks share many of the same structural and lithologic attributes. Most significantly, they both unconformably overlie Archean granitic rocks locally. For clarity, the Thomson is part of a sequence of strata that was known to unconformably overlie “Laurentian” granite. As compelling as this argument is, the Thomson is broadly equivalent to the Virginia Formation on the Mesabi range, with which continuity can now be established from geophysical and drill hole data, and the Rove Formation in the Gunflint district. Thus, the Thomson is now assigned as the uppermost unit of the Paleoproterozoic Animikie Group; and the Knife Lake is interpreted to have formed in part within a successor basin during latest Archean. NEXT: Return to Highway 210 and continue northwest approximately 1.6 miles to Thomson Dam. Page 164 �STOP 5-28, Location 5-28B Paleoproterozoic Thomson Formation T.48N., R.16W., sec. 5 SW SW; Thomson Dam. Cloquet 7.5-minute quadrangle. UTM: 545,610E/5,168,100N DESCRIPTION 5-28B: This is the type locality of the Thomson Formation, represented by approximately 650 feet of strata exposed between the dam north of Highway 210 and the RR bridge south of it. These exposures contain about equal proportions of graywacke, siltstone, and slate; metamorphosed to the greenschist facies. The formation contains abundant carbonate-rich concretions that locally are useful for delineating bedding in otherwise massive graywacke beds. Graywacke units range in thickness from 1 inch (2 cm) to 14 feet (4 m), and commonly display sedimentary structures indicative of turbidite deposition, included graded bedding, cross-bedding, sole marks, flute casts, and flame and ball structures (Morey and Ojakangas, 1970). Cross-bedding indicates flow to the south; though the trends of other structures imply more diverse and localized paleoslope directions. Structures in the Thomson Formation include gentle to open folds on varied scales, presumably related to deformation during the Penokean orogeny (Holst, 1984). The fold axes trend east, have vertical to steep south dips, and plunge gently east and west. A well developed axial-planar cleavage is present in the slaty beds, and concretions and mud chips are flattened in the plane of cleavage. The cleavage is deformed locally by kink bands. Quartz veins ranging in width from several cm to 3 meters are common in the formation. One of the largest, just north of the Highway 210 bridge, occupies an extensional fracture near the crest of a large anticline. Smaller veins are more contorted, and presumably were folded along with the adjacent rock. Several dikes of ophitic microgabbro ranging in width from a few inches to 200 feet (65 m) form a dike swarm that occupies northeast-trending joints (Fig. 5.8). The dikes are generally fine- to medium-grained and have chilled, fine-grained margins. Their precise age is unknown; however, their northeast trend and composition implies that they are related to the Mesoproterozoic Midcontinent rift. END OF TRIP REFERENCES Arth, J.G., and Hanson, G.N., 1975, Geochemistry and origin of the early Precambrian crust of northeastern Minnesota: Geochimica et Cosmochimica Acta 39:325-362. Biggs, D.L., ed., 1987, Centennial Field Guide Volume 3, Geological Society of America, North-Central Section; pages 47-73 describe various localities in Minnesota. Boerboom, T.J., and Zartman, R.E., 1993, Geology, geochemistry, and geochronology of the central Giants Range batholith, northeastern Minnesota: Canadian Journal of Earth Sciences 30:2510-2522. Page 165 �Borradaile, G.J., 1982, Tectonically deformed pillow lava as an indicator of bedding and way-up: Journal of Structural Geology, 4:469-479. Bowen, N.L., 1928, The evolution of igneous rocks: Princeton University Press, Princeton, 334 p. Broderick, T.M., 1935, Differentiation in lavas of the Michigan Keweenawan: Geological Society of America Bulletin 46:503-558. Butler, B.S., and Burbank, W.S., 1929, The copper deposits of Michigan: U.S. Geological Survey Professional Paper 144, 238 p. Cannon, W.F., 1992, The Midcontinent rift in the Lake Superior region with emphasis on its geodynamic evolution: Tectonophysics 213:41-48. Corfu, F., and Stott, G.M., 1998, Shebandowan greenstone belt, western Superior Province: U-Pb ages, tectonic implications, and correlations: Geological Society of America Bulletin 110:1467-1484. Davis, D.W., and Green, J.C., 1997, Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution: Canadian Journal of Earth Sciences 34:476-488. Fralick, P., Davis, D.W., and Kissin, S.A., 2002, The age of the Gunflint Formation, Ontario, Canada: single zircon U-Pb age determinations from reworked volcanic ash: Canadian Journal of Earth Sciences 39:1085-1091. Goldich, S.S., Nier, A.O., Baadsgaard, H., Hoffman, J.H., and Krueger, H.W., 1961, The Precambrian geology and geochronology of Minnesota: Minnesota Geological Survey Bulletin 41, 193p. Green, J.C., 1977, Keweenawan plateau volcanism in the Lake Superior region, in Baragar, W.R.A.,ed., Volcanic regimes in Canada: Geological Association of Canada Special Paper 16, p. 407-422. Green, J.C., 1982, Geology of Keweenawan extrusive rocks, in Wold, R.J. & Hinze, W.J. (eds.) Geology and tectonics of the Lake Superior Basin. Geological Society of America Memoir 156, p. 47-56. Green, J.C., and Fitz, T.J.III., 1993, Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent Rift plateau volcanics, Minnesota: petrographic and field recognition: Journal of Volcanology and Geothermal Research 54:177-196. Green, J.C., and Miller, J.D., Jr., 2004, Geology of the Duluth Complex at Duluth as portrayed in new 7.5' geological maps. Abstracts and Proceedings of the 50th Institute on Lake Superior Geology. Green, J.C., and Schulz, K.J., 1982, Geologic map of the Ely quadrangle, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map M-50, scale 1:24,000. Grout, F.F., 1918a, Internal structures of igneous rocks; their significance and origin with special reference to the Duluth Gabbro: Journal of Geology 26:439-458. Grout, F.F., 1918b, Two-phase convection in igneous magmas: Journal of Geology 26:481-499 Grout, F.F., 1918c, A type of igneous differentiation: Journal of Geology 26:626-58. Grout, F.F., 1937, Petrographic study of gold prospects of Minnesota: Economic Geology 37:56-68. Grout, F.F., Gruner, J.W., Schwartz, G.M., and Thiel, G.A., 1951, Precambrian stratigraphy of Minnesota: Geological Society of America Bulletin 62:1017-1078. Grout, F.F., and Schwartz, G.M, 1939, The geology of anorthosites of the Minnesota coast of Lake Superior: Minnesota Geological Survey Bulletin 28, 119 p. Gruner, J.W., 1930, Hydrothermal oxidation and leaching experiments; their bearing on the origin of Lake Superior hematite-limonite ores: Economic Geology 21:697-719, 837-867. Gruner, J.W., 1941, Structural geology of the Knife Lake area of northeastern Minnesota: Geological Society of America Bulletin 52:1577-1642. 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Page 166 �Hudleston, P.J., Schultz-Ela, D.D., and Southwick, D.L., 1988, Transpression in an Archean greenstone belt, northern Minnesota: Canadian Journal of Earth Sciences 25:1060-1068. Jirsa, M.A., 1984, Interflow sedimentary rocks in the Keweenawan North Shore Volcanic Group, northeastern Minnesota: MGS Report of Investigations 30, 20 p. Jirsa, M.A., 1990, Bedrock geologic map of northeastern Itasca County, Minnesota: Minnesota Geological Survey, Miscellaneous Map M-68, scale 1:48 000. Jirsa, M.A., 2000, The Midway sequence: A Timiskaming-type, pull-apart basin deposit in the western Wawa subprovince, Minnesota: Canadian Journal of Earth Sciences 37:1-15. Jirsa, M.A., and Boerboom, T.J., 2003, Bedrock geology of the Vermilion Lake 30’X60’ quadrangle, northeastern Minnesota: Minnesota Geological Survey, Miscellaneous Map M-141, scale 1:100,000. Jirsa, M.A., and Boerboom, T.J., 2003, Geology and mineralization of Archean bedrock in the Virginia horn: in Jirsa, M.A., and Morey, G.B., eds., Contributions to the geology of the Virginia horn area, St. Louis County, Minnesota: Minnesota Geological Survey Report of Investigations 53, p. 74-102. Jirsa, M.A., Boerboom, T.J., and McSwiggen, P.L., 1993, Geology of Archean greenstone-granite terrane in the Cook to Side Lake area: Field Trip 3 in Institute on Lake Superior Geology Proceedings, 39th Annual Meeting, Eveleth Minnesota, v. 39, Part 2, p. 97-128. Jirsa, M.A., Boerboom, T.J., and Morey, G.B., 1998, Bedrock geologic map of the Virginia Horn, Mesabi Iron Range, St. Louis County, Minnesota: Minnesota Geological Survey, Miscellaneous Map Series M-85, scale 1:48 000. Jirsa, M.A., and Boerboom, T.J., and Peterson, D.M., 2001, Bedrock Geologic Map of the Eagles Nest Quadrangle, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-114, scale 1:24,000. Jirsa, M.A., and Morey, G.B., 1987, Jay Cooke State Park and Grandview areas: Evidence for a major Early Proterozoic-Middle Proterozoic unconformity in Minnesota: in Biggs, D.L., ed., Centennial Field Guide Volume 3, Geological Society of America, North-central Section; p.6772. Jirsa, M.A., Southwick, D.L., and Boerboom, T.J., 1992, Structural evolution of Archean rocks in the western Wawa subprovince, Minnesota: refolding of precleavage nappes during D2 transpression: Canadian Journal of Earth Sciences 29:2146-2155. Kuenen, P.H., and Migliorini, C., 1950, Turbidity currents as a cause of graded bedding: Journal of Geology, 58:91-127. Lawson, A.C., 1887, Geology of the Rainy Lake region: American Journal of Science 33:473-480. Lawson, A.C., 1893, The anorthosytes of the Minnesota coast of Lake Superior: Geological and Natural History Survey of Minnesota Bulletin 8, p. 1-23. 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Morey, G.B., and Ojakangas, R.W., 1970, Sedimentology of the Middle Precambrian Thomson Formation, east-central Minnesota: Minnesota Geological Survey Report of Investigations 13, 32 p. Morey, G.B., and Ojakangas, R.W., 1982, Keweenawan sedimentary rocks of eastern Minnesota and northwestern Wisconsin, in Wold, R.J., and Hinze, W.J., eds., Geology and tectonics of the Lake Superior basin: Geological Society of America Memoir 156, p. 135-146. Nicholson, S.W., Shirey, S.B., Schulz, K.J., and Green, J.C., 1997, Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development; in Bornhorst, T.J., ed., Petrology and metallogeny of intraplate mafic and ultramafic magmatism: Canadian Journal of Earth Sciences 34:504-520. Ojakangas, R.W., 1966, Precambrian stratigraphy and structure of the Tower, Minnesota quadrangle (abs): Institute on Lake Superior Geology Proceedings, 12th Annual Meeting, Sault Ste. Marie, Michigan, Proceedings p. 17. Ojakangas, R.W., 1993, Pokegama Quartzite: in Institute on Lake Superior Geology Proceedings, 39th Annual Meeting, Eveleth Minnesota, v. 39, Part 2, p.19-21 and 46-48. Ojakangas, R.W., Dickas, A.B., and Green, J.C., 1997, Middle Proterozoic to Cambrian rifting, central north America: Geological Society of America Special Paper 312, 322 p. Ojakangas, R.W., Sims, P.K., and Hooper, P.R., 1978, Geologic map of the Tower quadrangle, St. Louis County, Minnesota: U.S. Geological Survey Geologic Quadrangle Map CQ-1457, scale 1:24,000. Owen, D.D., 1852, Report on a geological survey of Wisconsin, Iowa, and Minnesota: U.S. Department of the Treasury, Philadelphia, 638 p. Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochonological insights to physical, petrogenetic, paleomagnetic and tectono-magmatic processes associated with the 1.1 Ga Midcontinent rift system: Journal of Geophysical Research 98:13,997-14,013. Peterson, D.M., Gallup, C., Jirsa, M.A., and Davis, D.W., 2001, Correlation of Archean assemblages across the U.S.-Canadian border: Phase I geochronology (abs): Institute on Lake Superior Geology, 47th Annual Meeting, Madison, Wisconsin, Proceedings v. 47, Part 1, p. 77-78. Peterson, D.M., and Jirsa, M.A., compilers, 1999, Bedrock geologic map and mineral exploration data, western Vermilion district, St. Louis and Lake Counties, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map M-98, scale 1:48,000. Peterson, D.M., and Patelke, R.L., 2003, National underground science and engineering laboratory (NUSEL): geological site investigation for the Soudan Mine, northeastern Minnesota: University of Minnesota Natural Resources Research Institute, Technical Report NRRI/TR-2003/29, 88p. Phinney, W.C., 1968, Anorthosite occurrences in Keweenawan rocks of northeastern Minnesota: in Isachsen, Y.W., ed., Origin of anorthosite and related rocks: Albany, New York. New York State Museum and Science Service, Memoir 18, p. 135-147. Roedder, E., 1979, Silicate liquid immiscibility in magmas. In Yoder, H.S., Jr., ed., The evolution of the igneous rocks. Princeton, New Jersey. Princeton University Press, p. 15-57. Schmitz, M.D., 1994, Origin and petrogenesis of the Early Proterozoic Kenora-Kabetogama mafic dike swarm: unpublished seniors thesis, Macalester College, 110 p. Schultz-Ela, D.D., and Hudleston, P.J., 1991, Strain in an Archean greenstone belt of Minnesota: Tectonophysics 190:233-268. Page 168 �Schulz, K.J., 1980, The magmatic evolution of the Vermilion greenstone belt, NE Minnesota: Precambrian Research 11:215-245. Schwartz, G.M., Goldich, S.S., Marsden, R.W., eds., 1956, Field Trip 1: Precambrian of northeastern Minnesota: GSA Guidebook Series, 235 p. Sims, P.K., 1972, History of geologic investigations: in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 21-26. Southwick, D.L., Boerboom, T.J., and Jirsa, M.A., 1998, Geologic setting and descriptive geochemistry of Archean supracrustal and hypabyssal rocks, Soudan-Bigfork area, northern Minnesota: Minnesota Geological Survey Report of Investigations 51, 69p. Southwick, D.L., Morey, G.B., and McSwiggen, P.L., 1988, Geologic map of the Penokean orogen, central and eastern Minnesota, and accompanying text: Minnesota Geological Survey Report of Investigations 37, 25p. Sutton, T.C., 1963, Geology of the Virginia horn area: Minneapolis, University of Minnesota, M.S. thesis, 97 p. Taylor, R. B., 1964, Geology of the Duluth Gabbro Complex near Duluth, Minnesota. Minnesota Geological Survey Bulletin 44, 63 p. Vervoort, J.D., and Green, J.C., 1997, Origin of evolved magmas in the Midcontinent rift system, northeast Minnesota: Nd-isotope evidence for melting of Archean crust: Canadian Journal of Earth Sciences 34:521-535. Wager, L.R., and Deer, W.A., 1939, Geological investigations in East Greenland, Part III. The petrology of the Skaergaard intrusion, Kangerdlugssuaq, East Greenland. Meddr. Gronland. 105, 352 p. Winchell, N.H., 1889, 18th Annual Report of the Minnesota Geological and Natural History Survey. Winchell, N.H., 1899, The Geology of Minnesota. Geological and Natural History Survey of Minnesota, Final Report v. 4, 629 p. Winchell, N.H., 1900, The Geology of Minnesota, Geological and Natural History Survey of Minnesota, Final Report v. 5, 1025 p. White, W.S., 1972, The base of the Upper Keweenawan, Michigan and Wisconsin: U.S. Geological Survey Bulletin 1354-F, 23 p. Page 169 �FIELD TRIP 6 GLACIAL AND POSTGLACIAL LANDSCAPE EVOLUTION IN THE GLACIAL LAKE AITKIN AND UPHAM BASIN, NORTHERN MINNESOTA Lisa Marlow; Phil Larson; Howard Mooers, Department of Geological Sciences, University of Minnesota - Duluth Glacial Lake Aitkin and Upham Road Log with numbered field trip stops, geographic markers and discussion sites: F: Farnum site, B: Baker Spider Creek site, and Hay Lake. Image from 30 meter Digital Elevation Model of Minnesota Page 170 �GLACIAL AND POSTGLACIAL LANDSCAPE EVOLUTION IN THE GLACIAL LAKE AITKIN AND UPHAM BASIN, NORTHERN MINNESOTA Lisa Marlow; Phil Larson; Howard Mooers, Department of Geological Sciences, University of Minnesota – Duluth INTRODUCTION Recent investigations into the glacial geology of northeastern Minnesota have considerably revised the glacial chronology. This field trip examines features (landforms and stratigraphy) of the Glacial Lakes Aitkin and Upham basins that record a complex sequence of events related to advance and retreat of the Laurentide Ice Sheet. Deglaciation and formation of glacial Lakes Aitkin and Upham were followed by drainage of the lakes and eolian activity on exposed lacustrine sediments. Glacial Lakes Aitkin and Upham occupied a basin bounded by the Giants Range to the north and moraines of the Superior and Rainy Lobes and St. Louis Sublobe to the south, east, and west; respectively. As the Rainy Lobe retreated during the Late Wisconsin, meltwater ponded in front of the ice forming proglacial Lakes Aitkin and Upham I. As the Rainy Lobe continued to retreat northeastward across the basin to a margin roughly coincident with the Giants Range, the St. Louis sublobe advanced into the basin (Fig. 6-1). Stagnation and melting of the St. Louis sublobe resulted in formation of Glacial Lakes Aitkin and Upham II (Hobbs 1983). Continued retreat of the Rainy Lobe northward from the Giants Range resulted in formation of Glacial Lakes Norwood and Koochiching, both of which drained southward into Aitkin and Upham II. The evolution of the lakes by a combination of downcutting of the outlets and isostatic rebound is recorded by a series of beaches, wave-cut scarps, and multiple outlets. Dune formation in the Glacial Lakes Aitkin and Upham II basin was strongly controlled by sediment availability. Dune clusters show a strong correlation with the presence of underlying fine-grained nearshore lacustrine sand. Dune formation was likely episodic, coinciding with periods of rapid lake level lowering and exposure of nearshore sands. Cessation of eolian activity resulted from more gradual stabilization of dunefields. Peaks in eolian activity are indicated at 9.8, 9.3 and 7.4 kyr BP (all dates Page 171 �given in C14 years BP) by the magnetic susceptibility record of Hay Lake, a small lake near the Glacial Lake Upham II shoreline (Figs. 6-2 and 6-3). These peaks in eolian activity may coincide with episodes of drainage of Upham II and Aitkin II. Figure 6-1. The Glacial Lake Aitkin and Upham basin. Beaches are noted by shaded polygons; dunes appear as black dots in the basin; boundaries and channels noted as black lines. Field trip stops will illustrate examples of the following: beach, dune, underflow fan, rhythmites, alluvial fan, Lake Upham sediments in Rainy lobe outwash and a wave-washed esker. Map is modified from Hobbs (1983). Page 172 �Figure 6-2. Location of Hay Lake. The lined region indicates the Lake Upham upland area; white polygons are dunes. (Vanduse 7.5’ USGS quadrangle) (gyttja) (marl) C14 Figure 6-3. Hay Lake sediment core (8.57m): 1. Lithology 2. Age-depth model 3. Magnetic susceptibility 4. Composition 5. Mass accumulation rate. Page 173 �The dunefields in the Glacial Lakes Aitkin and Upham II basin display similarity to other Quaternary eolian deposits throughout Minnesota and the Midwest (Figs. 6-4 and 6-5). Despite well over a century of research on Minnesota's glacial landscape, these paleodunefields have received relatively little attention. Although mentioned by Upham (1896), Winchell (1896), Hall and Sardeson (1898), Elftman (1898), Leverett and Sardeson (1919) and Leverett (1932), the first comprehensive work on Minnesota's eolian landscape was that of Cooper (1935, 1938). Figure 6-4. North American dune and loess distribution. (Brady and Weil, 2000) 96° 94° 92° 90° Conifer/ Hardwood 48° 4 Lake Aitkin and Upham basin 3 1 46° 2 Prairie Deciduous 44° 0 100km Figure 6-5. Location of sites in Minnesota associated with eolian sedimentation referenced in the text. 1) Hay Lake 2) Lake Ann 3) Elk Lake 4) Lake Winnibigoshish. Filled areas are prominent dune colonies. (Modified from Keen and Shane 1990) Page 174 �More recently, Grigal and others (1976) examined a dunefield exposed along the southeast shore of Lake Winnibigoshish (Fig. 6-5). They described several buried soil horizons, which yielded radiocarbon dates in the range of 7910 to at least 5040 yr BP suggesting episodic dune formation during the mid-Holocene The formation of these dunes was related to lake-level instability and episodic exposure of littoral areas to wind erosion during the mid-Holocene (Larson and Mooers 2003). The mid-Holocene climate in the upper Midwest was considerably warmer and drier than the modern climate (Webb et al. 1983; Bartlein et al. 1984; Dean et al. 1984; COHMAP 1988; Dean et al. 1996). The warmer, drier conditions resulted in an eastward shift of prairie ecotone (Fig. 6-5). Lower groundwater levels were accompanied by lake level lowering, particularly in groundwater-dominant closed-basin lakes and ponds, and increased fire frequency and vegetational stress locally resulted in landscape destabilization. Grigal and others (1976) speculated that the dunes of the Anoka Sand Plain may be midHolocene in age, an idea Keen and Shane (1990) explored in detail in their paleoenvironmental investigation of Ann Lake (Fig. 6-5). They concluded that there was a relatively continuous record of eolian activity throughout the mid-Holocene, a reversal of the ideas of earlier workers (Hall and Sardeson 1898; Cooper 1935). Keen and Shane’s (1990) whole-core magnetic susceptibility record from Ann Lake records increased silt and fine sand input to the lake from about 8.0 until 4.5 kyr BP with discrete peaks in clastic input at ca. 7.4, 5.8, and 4.9 kyr BP. However, it is difficult to distinguish whether this sediment was exported to the lake directly from active dunes on the shore, eroded from the shoreline due to lake level instability, or deposited suspended airborne sediment. Despite these ambiguities, they concluded that the increased clastic input to Ann Lake was due to landscape destabilization and widespread dune activity on the Anoka Sand Plain. The distinction between lake level lowering and landscape destabilization as triggers of eolian activity is subtle but important. Although the prairie ecotone shifted eastward during the mid-Holocene, this in itself did not create an environment conducive to widespread landscape destabilization. At present in the central Dakotas there are numerous locations with ample sediment availability, but no widespread dune formation because of the stabilizing vegetation of the prairie environment. Landscape destabilization and dune formation in the existing prairie Page 175 �environment occurs by excessive drying such as in the Nebraska Sand Hills (Winspear and Pye 1996). Viewed in this context, while Grigal and others (1976) obtained dates from paleosols buried by eolian events, the Keen and Shane (1990) and Dean (1997) magnetic susceptibility records may merely be a record of dryer, dustier conditions during the mid-Holocene rather than pervasive local eolian activity. Today the Glacial Lakes Aitkin and Upham basin is patchwork of small ‘islands’ occupied by upland vegetation formed on stabilized dunes. The ‘islands’ are interrupted by vast areas of peatland developed on the poorly drained, low-relief lakebed composed of fine-grained lacustrine sediment. Locally, scattered topographic highs are underlain by older glacial sediments predating formation of the lakes. STOP 6 – 1. Glacial Lake Duluth underflow fan. (NW¼ SW¼ Sec. 30, T48N, R15W, Carlton County, Frogner 7.5’ USGS quadrangle; 553728E, 5161950N, NAD83) This location preserves the record of several glacial events that affected the western Lake Superior region and provides insight into their relative chronologies. Here, red Glacial Lake Duluth clays are overlain by a subaqueous fan deposited by meltwater from Glacial Lake Aitkin and Upham II (Fig. 6-6). These in turn are overlain by Superior Lobe till deposited by of the Marquette advance. These relationships indicate meltwater drained from Glacial Lake Upham into Glacial Lake Duluth (Table 1) (Mooers and Lehr 1997; Clayton and Moran 1982; Wright 1972; Wright et al. 1973). During the last phase of Glacial Lakes Aitkin and Upham II, the two lakes were thought to have separated and drained through different outlets; Upham through the St. Louis River and Aitkin through the Mississippi River (Hobbs 1983). A digital elevation model (DEM) reconstruction correcting the basin for isostatic rebound confirms the separation of the two lakes (Marlow 2004). The Mississippi River outlet to Aitkin is a relatively narrow passage through collapsed ice-cored terrain. It is unlikely this underdeveloped outlet carried significant meltwater. Final drainage of Aitkin therefore occurred after significant meltwater contributions to the lake basins had ceased. Page 176 �Figure 6-6. Stop 6-1. Underflow fan at the mouth of the St. Louis River drainage in Glacial Lake Duluth basin. The sediments were deposited by meltwater from Glacial Lake Upham II. Page 177 �Table 6-1. The glacial chronology of northeastern Minnesota outlined resulted from a compilation of several resources (Mooers & Lehr, 1997; Larson (unpublished); Marlow, 2004; Bjork, 1990; Wright, 1972; Clayton & Moran, 1982; Hobbs, 1983; Fenton, 1983; Clayton, 1983; Lehr & Hobbs, 1992) Phase Moraine Lobe Glacial Lakes Other events C14 Date (kyr BP) drainage of Aitkin II Marquette Superior Vermilion Vermilion Big Rice Big Rice/ Wampus 9.9 Rainy Rainy Aitkin II Upham II Allen Rainy Alborn Culver St. Louis Split Rock Cloquet Superior ~11.7 12.0 St. Louis River outlet Inflow from Embarrass gap Us-Kab-WanKa/Chicken/ Hellwig/Birch/Spider outlets formation of Goodland esker Nickerson Nickerson Superior North of Nashwau k Duluth drainage of Upham II cessation of meltwater inflow to Aitkin and Upham II L. Duluth underflow fan Inflow from Prairie River ~13.0 Aitkin Upham I I Automba Sandy Lake Outing St. Croix St. Croix Rainy Rainy ~15.516 Rainy Page 178 �STOP 6 – 2. Hellwig Creek outlet and Glacial Lake Upham II basin overlook. (SE ¼ SE ¼ Sec. 34, T53N, R17W, St. Louis County, Canyon 7.5’ USGS quadrangle; 540174E, 5207618N, NAD83) This stop is near beaches and outlets that existed during the earliest phases of Glacial Lake Upham II, and provides a good overlook from the Upham shore into the basin (Fig. 6-7). Figure 6-7. Stop 6-2. Location of early outlets to Lake Upham; Triangle marks location of core taken by Baker (1965) containing marl. Circle indicates stop 2 site. Page 179 �Following stagnation of St. Louis Sublobe ice meltwater flowed through a combination of surface and englacial channels toward the ice margin then along the ice margin through a chain of small proglacial lakes. Among the earliest and highest outlets in the Upham basin from which these small lakes drained were the Us-Kab-Wan-Ka River (427 m; 1400’) and Chicken Creek (434 m; 1425’) channels. These channels drained to the southeast along the margin of the Superior Lobe eventually reaching the St. Croix River (Hobbs, 1983). Proglacial lakes and subglacial meltwater in the Aitkin basin drained southward through the Snake channel (381 m; 1250’) to the St. Croix River and perhaps southwest to the Mississippi River (378 m; 1240’) (Hobbs 1983). All of these outlets lie above the highest main beaches of Glacial Lakes Aitkin and Upham II, and the DEM correction for isostatic rebound indicates ice must have been present in the basin for them to function (Marlow 2004). Therefore they were likely associated with small proglacial lakes rather than the main stages of the lakes. After the St. Louis Sublobe advance, the margin of the Rainy Lobe retreated north of the Giants Range resulting in the ponding of water between the retreating ice and the Giants Range, forming Glacial Lake Norwood (Fig. 6-8) (Winchell 1901; Hobbs 1983). Glacial Lake Norwood drained south through the Embarrass Gap entering Glacial Lake Upham II at an elevation of 436 m (1450’) (Hobbs 1983). This was the first major meltwater inlet to Glacial Lakes Aitkin and Upham II. During this time a significant amount of stagnant ice remained in the basin, and it is possible that Aitkin and Upham were separated by stagnant ice along the Swan River Sill (Fig. 68). As the Rainy Lobe continued to retreat northward, Glacial Lake Norwood increased in extent. Downcutting in the Embarrass Gap lowered the level first to 436 m (1430’) and eventually to 427 m (1400’). This lower stage is known as Glacial Lake Koochiching (Nikiforoff 1947; Hobbs 1983; Lehr and Hobbs 1992; Leverett 1932). While Glacial Lake Norwood was in existence, the upper strandlines (411 to 421 m; 1350-1380’) in the extreme northeastern part of Glacial Lake Upham II formed. These levels of Upham likely correspond to outlets at Hellwig Creek (409 m; 1330’), Birch (406 m; 1320’), and Spider Creek (400 m; 1300’). A combination of isostatic rebound and downcutting eventually led Page 180 �to successive abandonment of the Hellwig Creek, Birch, and Spider Creek outlets and lake level drop. Baker (1965) reported a bulk radiocarbon date of 13,000±400 (W-1234) yr B.P from a sequence of lacustrine marl within the Spider Creek outlet (Fig. 6-7; and “s” in Fig. 6-8). The marl must post-date the cessation of drainage through the channel since marl formation requires shallow still water. Baker (1965) expressed concern that this date was too old due to possible contamination by lignite. However, this date is consistent with the other evidence presented in this guide. The Spider Creek date places the minimum age of Glacial Lakes Aitkin and Upham II, and therefore the maximum limit of the St. Louis Sublobe, prior to 13.0 kyr B.P. Figure 6-8. Significant features associated with the recession of the Rainy Lobe after the St. Louis sublobe advance. Glacial Lake Norwood (LN) extent is indicated by a dashed line. Page 181 �STOP 6 – 3. Wave-washed esker island (NE¼ NE¼ Sec. 1, T54N, R20W, St. Louis County, Toivola 7.5’ USGS quadrangle, 514367E, 5226982N, NAD83). There are many landforms within the Glacial Lakes Aitkin and Upham II basins that predate development of the lakes. This exposure is an example of an esker deposited during retreat of the Rainy Lobe that was later modified by waves. (Fig. 6-9). This esker and others like it became wave-washed “islands” once Glacial Lakes Aitkin and Upham I and II formed. Exposed at the base of the sequence are coarse gravels. The gravels contain abundant northeast-provenance material and locally derived mudstone and shale from the Paleoproterozoic Virginia Formation and perhaps younger Cretaceous strata. They were deposited by a beaded esker system during retreat of the Rainy Lobe. Overlying the gravels is a fine-grained till deposited by the St. Louis Sublobe. On top of the till is a sequence of nearshore sands and gravels. These presumably eroded from that portion of the esker rising above the level of Glacial Lake Upham II; the strandline formed at about 397 m (1200’) elevation. The uppermost portion of the sequence is a blanket of eolian sediment exported from the surrounding lake plain to the upland after final drainage of Upham. Figure 6-9. Stop 6-3. Wave-washed esker island in Glacial Lake Upham II. Page 182 �STOP 6 – 4. Rainy Lobe collapsed outwash, Glacial Lake Upham I sediments, and St. Louis Sublobe till (NE¼ SE¼ Sec. 15, T57N, R19W, St. Louis County, Kirk 7.5’ USGS quadrangle; 520373E, 5251907N, NAD83) The Glacial Lakes Aitkin and Upham basins were occupied by glacial lakes on two separate occasions during the Late Wisconsin glaciation. The retreat of the Rainy and Superior Lobes from their maximum positions formed a series of ice-cored recessional moraines bordering the lake basins to the south and west leading to ponding of water and formation of Glacial Lakes Aitkin and Upham I (Wright 1972; Hobbs 1983; Lehr and Hobbs 1992). The initial retreat of the Rainy Lobe from the Mille Lacs and Outing moraines (Mooers 1988) led to the formation of Aitkin I (Fig. 6-10). Continued retreat of the Rainy Lobe from the Sandy Lake moraine led to the formation of Upham I. Although the exact time of formation of Aitkin and Upham I is not known, it occurred after the Rainy Lobe retreated from the St. Croix moraine about 15.5 kyr B.P. (Clayton and Moran 1982; Mooers and Lehr 1997), and before the St. Louis Sublobe advanced into the Aitkin and Upham basins from the northwest. Exposed at the base of the sequence are steeply south-dipping foreset beds of a subaqueously deposited fan. These sediments are Rainy Lobe provenance, deposited along the southern margin of stagnant ice lying on the southern slope of the Giant’s Range, an area now characterized by collapsed ice-cored topography. The upper portion of the sequence is a St. Louis Sublobe till. Between the fan sediments and till are a number of elongate slabs of fine-grained lacustrine sediment derived from Glacial Lake Upham I. This lacustrine sediment was eroded from deeper water and thrust onto the fan during the advance of the St. Louis Sublobe (Figs. 6-11 and 6-12). The relationships visible in this exposure indicate that the St. Louis Sublobe advanced while a substantial amount of Rainy Lobe ice was still present south of the Giant’s Range. Page 183 �Figure 6-10. Prominent moraines in the Glacial Lakes Aitkin and Upham basin with some modern features included. Modified from Mooers (1988) and Lehr and Hobbs (1992). Figure 6-11. Subaqueously deposited Rainy Lobe outwash exposed in a gravel pit near Cherry, MN. Page 184 �Figure 6-12. St. Louis sublobe till containing Glacial Lake Upham I sediment. STOP 6 – 5. Eolian dunes. (SW¼ SW ¼ Sec. 31, T56N, R20W, St. Louis County, Riley 7.5’ USGS quadrangle, 504626E, 5237098N, NAD83) Exposed in the sand pit is a small (<2 m) dune composed of characteristic 4-φ sand (62.5 microns or 230 mesh). Most of the sand exposed in the pit is massive and structureless. However, relict cross bedding can be observed in small patches near the base of the sequence. Primary sedimentary structures in the upper portion of the exposure have been obliterated by bio and crioturbation. Clusters of eolian dunes are widely distributed throughout the Glacial Lakes Aitkin and Upham basins. They formed as Glacial Lakes Aitkin and Upham II incrementally drained, exposing areas of littoral sediment to wind erosion. Dune heights range from 1 to 5 meters and are composed of a characteristic fine to very fine grained sand (Marlow 2004). They occur as parabolic or longitudinal dunes, but are commonly distorted in morphology as a result of forming in a variable hydrologic environment (Fig. 6-13) (Bagnold 1941) There are a large number of longitudinal dunes and elongate clusters of dunes oriented in a NW-SE direction, suggesting they formed under prevailing NW winds (Fig. 14). Bagnold’s (1941) description of dunes indicates that they “tend to occur in belts or chains, whose direction coincides with that of the resultant long-period Page 185 �sand vector Q”, with Q being sand flow because of the sum of the strong and gentle wind directions, and the width of the belt at right angles to Q.” Figure 6-13. Distorted dunes in the northern end of Lake Aitkin basin showing a combination of crescentic and longitudinal dunes (Jacobson and Split Hand 7.5 minute USGS quadrangles). Page 186 �Figure 6-14. Trend in dune clusters indicating a NW-SE wind direction. Elongate polygons are beaches. Circles indicate stops 8, 9, and 10 sites. STOP 6 – 6. Glacial Lake Upham II Shoreline. (SE¼ SE¼ Sec. 33, T56N, R21W, St. Louis County, Silica 7.5’ USGS quadrangle, 499595E, 5236726N, NAD83) After the time of upper beach formation, Lake Upham experienced a drop in water level documented at Townline beach (411 m; 1350’) through the use of Ground Penetrating Radar (Fig. 6-15). The GPR results are interpreted as a down stepping of shoreline deposits (Fig. 6-16). The progradation of the bedform resulted in a constructional shoreline and indicates regression of the lake. The regression is an example of Forced Regression as discussed by Posamentier and Allen (1999). Forced regression takes place when there is a relative sea-level fall that progressively exposes the sea (or lake) floor, thereby causing the shoreline to migrate seaward (Posamentier and Allen 1999). Page 187 �Figure 6-15. Shoreline of Glacial Lake Upham II along Townline Road south of Hibbing. Shoreline indicated by black dotted line. Heavy solid line indicates where GPR data was collected (Silica & Riley 7.5 ‘ USGS quadrangle). Figure 6-16. Ground Penetrating Radar profile of Glacial Lake Upham II shoreline. Vertical data represents ~9 meter; horizontal data ~121 meters. (Data processing courtesy of Nigel Wattrus at Large Lakes Observatory, University of Minnesota-Duluth) Page 188 �STOP 6 – 7. Goodland Esker. (SE¼ NE¼ Sec. 9, T55N, R23W, Itasca County, Calumet 7.5’ USGS quadrangle, 479736E, 5234496N, NAD83) This stop is a gravel pit exposure in the Goodland esker, one of the most prominent glacial landforms in Itasca County. The location of the esker and details of its formation place important constraints on the timing of the St. Louis Sublobe advance and deposition of the Alborn drift (Fig. 6-17). The morphology of the esker system is best described by the model of Shreve (1985). The proximal (with respect to the Giants Range) segment is an ~600 m wide, 10 km long channel incised into bedrock and older drift (tunnel valley), occupied by a multicrested esker. The medial segment consists of a single broad-crested esker segment deposited in a 10 km long channel incised into the overlying ice. The distal component of the esker system is a supraglacial fan complex. The base of the subglacial channel drops from 428 m at the crest of the Giant’s Range to <400 m at a point 6 km downstream, then rising to 409 m over the next 4 km. Downstream of this point, the elevation of the top of the broad-crested esker rises 19 m over 10 km, from 434 to 455 m at the apex of the distal fan. The transition from a multi-crested to broad-crested esker is apparently triggered by an increase in the adverse slope up which the subglacial channel flowed. Multi-crested eskers form in areas of moderate upgradient flow as the channel tends to migrate laterally, rather than upward, in the ice, while broad-crested eskers form in areas of steeper upgradient flow due to freezing of the tunnel walls, a process favoring low, wide tunnel geometries (Shreve 1985). Lithologies in the multi-crested segment esker are dominantly Archean granitics and greenstones transported from north of the Giant’s Range. The broad-crested segment has a markedly different lithologic assemblage than the multi-crested segment. In addition to Archean lithologies, an abundance of Paleoproterozoic Animikie Basin lithologies are present including quartzite, sulfidic mudstones, iron formation, and pisolitic lateritic iron formation. The abundance of locally-derived Animikean lithologies indicates material in the broad-crested segment was deposited as incision of the Nchannel was taking place upstream. The broad-crested segment is therefore older than the multi-crested segment. The wide range of mean sizes of the various Animikean Page 189 �lithologies attests to the strong control of primary rock characteristics on particle size. Note in particular the abundance of Pokegama quartzite as large (~1 m) boulders and its paucity in smaller size fractions. Deposition of the distal fan occurred throughout the period the esker system was active. The initial phase was characterized by deposition of a supraglacial fan on stagnant ice. Fan deposits are identifiable up to 10 km from the fan apex, and cover in excess of 100 km2. The maximum elevation on the fan complex is about 477 m, indicating at least 20 m of ice was present above the outlet of the englacial channel at the fan apex. During later stages of the esker system, underlying stagnant ice melted collapsing the earliest deposited fan sediment. The final phase of fan formation was characterized by incision of the collapsed fan head down to about 450 m. The subglacial drainage system that deposited the esker was probably short-lived. It could not have formed before the St. Louis Sublobe occupied the area around Grand Rapids, blocking the natural southwesterly flow of meltwater from the watershed of the present-day Prairie River. Similarly, it could not have persisted after St. Louis Sublobe ice wasted away and the Prairie River began flowing into Glacial Lake Aitkin II. Consequently, it may have been active a few hundred years at most. Despite its short duration, the large size of the esker system – it is one of the largest in Minnesota – attests to an enormous discharge of meltwater responsible for its formation. Meltwater was gathered from an interlobate zone that existed to the north of the Giant’s Range between the St. Louis Sublobe and the Rainy Lobe. This interlobate area may have drained well in excess of 1000 km2 of the ice sheet. The elevation difference between the fan apex (477 m) and the surrounding (subglacial) landscape (400 m) indicate that a continuous cover of Rainy Lobe ice at least 75 m thick was present south of the Giant’s Range at the time of esker formation and advance of the St. Louis Sublobe. This argues strongly against the postulated ice-free zone between the northern margin of the St. Louis Sublobe and the southern margin of the Rainy Lobe. Previous workers have postulated a relatively late advance for the St. Louis Sublobe (ca. 11.7 kyr BP), correlating it with the Vermilion Phase (ca. 12.0 kyr BP), or an even later phase, of the Rainy Lobe (Wright 1972; Hobbs 1983). However, the presence of active Rainy Lobe ice just north of the Giant’s Range at the time of the Page 190 �advance of the St. Louis Sublobe and esker formation provides further evidence that the St. Louis Sublobe advanced at a significantly earlier date. Figure 6-17. Main features of the Goodland esker system. (Image from 30 m DEM of MN) Page 191 �STOP 6 – 8. Prairie River underflow fan in Glacial Lake Aitkin II. (NE¼ NE¼ Sec. 35, T53N, R24W, Itasca County, Jacobson 7.5’ USGS quadrangle, 473831E, 5209702N, NAD83) Glacial Lakes Aitkin and Upham II had two successive major meltwater inlets, the Embarrass Gap and the Prairie River. Inflow of the Prairie River into Aitkin II resulted in deposition of a large underflow fan extending 50 km from its apex (Hobbs 1983). The fan was later incised by the Mississippi River. The cutbank at this stop exposes ~9 m of sandy underflow fan sediments overlying a lacustrine clay (Fig. 6-18). Visible within the underflow sediments are several Bouma sequences. Outflow from Glacial Lake Koochiching into Glacial Lake Upham II through the Embarrass Gap ceased when a lower outlet opened north of Grand Rapids because of collapse of stagnant ice-cored terrain. Meltwater then flowed down the Prairie River into Glacial Lake Aitkin II, lowering the level of Koochiching from 427 to 411 m (1400’ to 1350’) (Hobbs 1983) The Prairie River entered Glacial Lake Aitkin II at an elevation of 396 m (1300’) (Fig. 6-1). Although it is not known exactly when the Embarrass Gap was abandoned, it must have occurred prior to a 10.2 kyr B.P. transition from predominantly clastic to organic lake sedimentation in Sabin Lake located in the Embarrass Gap (Björk 1988; Lehr and Hobbs 1992). Fenton (1983) suggests that the Prairie River outlet was initiated between 12.3 and 10.8 kyr B.P. based on the chronology of the Lake Agassiz basin to the west. Clayton (1983) suggests 11.5 kyr B.P. for the inception of the Prairie River outlet. The Prairie River inlet was abandoned once Glacial Lake Koochiching began to flow into Glacial Lake Climax. However, Glacial Lakes Aitkin and Upham II persisted after abandonment on the last meltwater inlet. Upham finally drained as the St. Louis River outlet was incised to its modern level. Hobbs (1983) indicates that water could have still been flowing into Aitkin after drainage of Upham by way of the Mississippi River, which has a prominent terrace at 389 m (1275’). Farnham and others (1964) obtained a date of 11,635 yr BP on a paleosol in the southwestern part of the Glacial Lake Aitkin II basin. This paleosol is overlain by a thin Page 192 �marl and 3 feet of clay, indicating Aitkin was in existence well after this time. A radiocarbon date of 10,000 yr BP was obtained from a marl from the Ball Bluff quadrangle in the northeastern part of the Aitkin basin (Hobbs 1983) suggests the lake persisted at least until this time. There are two alternative explanations explaining how the southwestern portion of the Glacial Lake Aitkin II basin was successively drained and re-inundated. The close similarity between the presumed initiation of the Prairie River inlet at 11.5 kyr BP (Clayton 1983) and the re-inundation of the southwestern part of the Glacial Lake Aitkin II basin at 11,635 yr BP suggests a causal relationship. Glacial Lake Aitkin II may have assumed a lower, stable level by ca. 11.6 kyr BP. The relatively rapid lowering of Glacial Lake Koochiching would have sent a surge of meltwater into the lake, resulting in a rise in lake level even if only temporarily. The lacustrine sediment overlying the Farnham (1964) paleosol was thus derived from the Prairie River inlet. The alternate explanation is that much of the Aitkin basin was drained through Glacial Lake Upham II by downcutting of the St. Louis River outlet. The Swan River Sill controlled the water level in the remaining lake. As isostatic rebound of the basin progressed, the sill on the northeastern side of the basin rose at a higher rate than the southwestern portion of the basin. Lake level transgression resulted in re-inundation of lake bottom exposed by drainage of Upham. Page 193 �Figure 6-18. Stop 6-8. Exposure of the Prairie River underflow fan at a Mississippi River cutbank. STOP 6 – 9. Rhythmite in Glacial Lake Aitkin II. (SW¼ NW¼ Sec. 4, T52N, R23W, Aitkin County, Jacobson 7.5’ USGS quadrangle, 479288E, 5207689N, NAD83) A rhythmite is overlain by cross-bedded sands, which is then overlain by underflow sediments. This is an interesting site that may document a dramatic inception of the Prairie River inlet into a basin that was previously depositing clays. Page 194 �STOP 6 – 10. Eolian dunes in Glacial Lake Upham II basin. (NW¼ NE¼ Sec. 18, T 52 N R 22 W, Aitkin County, Vanduse Lake 7.5’ USGS quadrangle, 486857E, 5204509N, NAD83) Exposed at this site are eolian sand dunes overlying lacustrine clay of Glacial Lake Upham II (Fig 6-19). Dunes in the Glacial Lakes Aitkin and Upham basins are most densely concentrated near the inlet of the Prairie River to the Aitkin basin and east of the Swan River Sill in the Upham basin (Fig. 6-14). The 4-φ mean grain size of the dunes sampled in these areas in the basin match that of sediments from the underflow fan sediments seen at Stop 8. These sediments have been mapped in the Aitkin and Itasca Soil Surveys (Nyberg 1987, 1999) as the Zimmerman, Cowhorn, and Wawina soil series, all composed of fine to very fine grained sands of lacustrine origin. From the inception of Glacial Lakes Aitkin and Upham II net flow of water was from Aitkin to Upham. The initiation of the Prairie River inlet greatly increased this discharge. The flow of water from Aitkin to Upham resulted in formation the Ball Bluff Spit (Marlow, 2004) extending eastward from the Swan River Sill (Fig. 6-19). The dense dune clusters east of the Swan River Sill in the Upham basin formed from sediment transported from the underflow fan. Page 195 �Figure 6-19. Location of the Ball Bluff spit located within the dashed line. Circle at stop 10 indicates where lacustrine clay is overlain by 1 m. of sand derived from the underflow fan. Triangle indicates site where lake clay is exposed at the surface. REFERENCES Arbogast, A.F., Wintle, A.G., Packman, S.C. 2001. Widespread middle Holocene dune formation in the eastern Upper Peninsula of Michigan and the relationship to climate and outlet-controlled lake level. Geology 30, 55-58. Bagnold, R.A. 1941. The Physics of Blown Sand and Desert Dunes. William Morrow & Company, 265 p. Baker, R.G. 1965. Late-glacial pollen and plant macrofossils from Spider Creek, So. St. Louis County, MN. Geological Society of America Bulletin 45, 645-665. Ballantine, J.W. 1991. Late-Wisconsin Stratigraphy and Glacial History of Southwestern St. Louis County, Minnesota. Unpublished Masters Thesis, University of Minnesota Duluth. 154 p. Bartlein, P.J., Webb, T., and Fleri, E. 1984. Holocene climatic change in the northern Midwest: Pollen derived estimates. Quaternary Research 22, 361-374 Björk, S. 1990. Late-Wisconsinan history North of Giants Range, northern Minnesota, inferred from complex stratigraphy. Quaternary Research 33, 18-36. Brady, N.C. and Weil, R.R., 2000, Elements of the Nature and Properties of Soils. 12th Edition, Prentice Hall, New Jersey. P.558. Page 196 �Clayton, L. 1983. Chronology of Lake Agassiz drainage to Lake Superior, in Teller, J.T., and Clayton, Lee, eds., Glacial Lake Agassiz: Geological Association of Canada Special Paper 26, 291-307. Clayton, L. and Moran, S.R. 1982. Chronology of late Wisconsinan glaciation in middle North America: Quaternary Science Reviews 1, 55-82. COHMAP Members. 1988. Climatic changes of the last 18,000 years: Observations and model simulations. Science 241,1043-1052. Cooper, W.S. 1935. The history of the Upper Mississippi River in late-Wisconsinan and post-glacial time. Minnesota Geological Survey Bulletin 26. Cooper, W.S., 1938, Ancient dunes of the upper Mississippi Valley as possible climatic indicators. American Meteorological Society Bulletin, v. 19, p. 193-204. Dean, W.E. 1997. Rates, timing, and cyclity of Holocene eolian activity in north-central United States: Evidence from varved lake sediments. Geology 25, 331-334. Dean, W.E. 1999. The carbon cycle and biogeochemical dynamics in lake sediments. Journal of Paleolimnology 21, 375-393. Dean, W.E., Ahlbrandt, T.S., Anderson, R.Y., Bradbury, J.P., 1996, Regional aridity in North America during the middle Holocene. The Holocene 6, 145-155. Dean, W. E., Bradbury, J.P., Anderson, R.Y., Barnosky, C.W. 1984. The variability of Holocene climate change: Evidence from varved lake sediments. Science 226, 1191-1194. Elftman,, A.H. 1898. The Geology of the Keweenawan area in northeastern Minnesota. The American Geologist 18, 225-226. Farnham, R.S., McAndrews, J.H., and Wright, H.E. 1964. A Late-Wisconsin buried soil near Aitkin, Minnesota, and its paleobotanical setting. American Journal of Science 262, 393-412. Farrand, W.R. and Drexler. 1985. Late-Wisconsinan and Holocene History of the Lake Superior Basin. in Karrow, Quaternary evolution of the Great Lakes. Geological Association of Canada Special Paper 30, 17-32. Fenton, M.M.(Moran, S.R.; Teller, J.T.; Clayton, Lee). 1983. Quaternary stratigraphy and history in the southern part of the Lake Agassiz basin, in Teller, J.T., and Clayton, Lee, eds., Glacial Lake Agassiz. Geological Association of Canada Special Paper 26, 49-74. Grigal, D.F., Severson, R.C., Golz, G.E. 1976. Evidence of eolian activity in north-central Minnesota 8,000 to 5,000 yr. ago. Geological Society of America Bulletin 87, 1251-1254. Hall and Sardeson, 1898, Eolian deposits of Eastern Minnesota. Bulletin of the Geological Society of America, v. 10, p. 349-360. Hobbs, H.C. 1983. Drainage relationships of Glacial Lakes Aitkin and Upham and early Lake Agassiz in northeastern Minnesota. In Teller, J.T. and Clayton, L., eds., Glacial Lake Agassiz. Geological Association of Canada Special Paper 26, 245-259. Keen, K.L., Shane, L.C.K, 1990, A continuous record of Holocene eolian activity and vegetation change at Lake Ann, east-central Minnesota. Geological Society of America Bulletin 102, 1646-1657. Larson, P.C., and Mooers, H.D. 2003. Holocene drainage evolution of the Mississippi Headwaters, Minnesota: implications for mid-Holocene eolian activity in the North American midcontinent. GSA Abstracts with Programs 35 (7). Lehr, J.D. and Hobbs, H. 1992. Field Trip Guidebook for the Glacial Geology of the Laurentian Divide Area, St. Louis and Lake Counties, Minnesota. Minnesota Geological Survey Guidebook Series 18. Leverett, F., with contributions by Sardeson, F.W. 1932. Quaternary Geology of Minnesota and Parts of Adjacent States. United States Geological Survey Professional Paper 161. Leverett, F. and Sardeson, F.W. 1919. Surface formations and agricultural conditions of north-eastern Minnesota. Minnesota Geological Survey Bulletin 13. Marlow, L. 2004. Late Glacial and Early Holocene history of the Glacial Lakes Aitkin and Upham basin, NorthCentral Minnesota: Implications for the timing of post-glacial eolian activity. Unpublished Masters Thesis, University of Minnesota Duluth, 82 p. Mooers, H.D. and Lehr, J.D. 1997. Terrestrial record of Laurentide ice sheet reorganization during Heinrich events. Geology 25, 987-990. Mooers, H.D. 1988. Quaternary history and ice dynamics of the late Wisconsin Rainy and Superior lobes, central Minnesota. Unpublished Doctoral Thesis, University of Minnesota, 200 p. Nikiforoff, C. 1947. The life history of Lake Agassiz: alternative interpretation. American Journal of Science 245, 205-239. Page 197 �Nyberg, P.R. 1985. Soil Survey of Itasca County, Minnesota. United States Department of Agriculture Soil Conservation Service, 197 p. Nyberg, P.R. 1999. Soil Survey of Aitkin County, Minnesota. United States Department of Agriculture Natural Resource Conservation Service. 163 p. Ojakangas, R.W. and Matsch, C.L., 1982, Minnesota’s Geology. University of Minnesota Press, 255 p. Posamentier, H.W. and Allen, G.P. 1999. Siliciclastic Sequence Stratigraphy-Concepts and Applications. SEPM Concepts in Sedimentology and Paleontology 7. Seppala, M. 1993. Climbing and falling sand dunes in Finnish Lapland. in Pye, K., ed., The Dynamics and Environmental Context of Aeolian Sedimentary Systems. Geological Society London. Shreve, R.L. 1985. Esker characteristics in terms of glacier physics, Katahdin esker system, Maine. Geological Society of America Bulletin 96, 639-646. Upham, W. 1896-1899. The geology of Aitkin County. in Minnesota Geological and Natural History Survey Final Report: The Geology of Minnesota 4, 25-54. Webb, T. III, Cushing, E.J., and Wright Jr., H.E., 1983, Holocene changes in the vegetation of the Midwest. In Wright Jr., H.E., ed., Late-Quaternary Environments of the United States: the Holocene, 109-127. Winchell, N.W. 1901. Glacial Lakes of Minnesota. Geological Society of America Bulletin 12, 109-128. Winchell, N.W. 1896-1899. The geology of the northern part of St. Louis County. in Minnesota Geological and Natural History Survey Final Report: The Geology of Minnesota 4, 222-265. Winspear, N.R. and Pye, K. 1996. Textural, geochemical and mineralogical evidence for the sources of Aeolian sand in central and southwestern Nebraska, U.S.A. Sedimentary Geology 101, 85-98. Wright, H.E. 1972. Quaternary history of Minnesota. in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A Centennial Volume. Minnesota Geological Survey. Wright, H.E., Matsch, C.L. and Cushing, E.J. 1973. Superior and Des Moines Lobes. in Black, R.F., Goldthwait, R,P., and Willman, H.B., eds., The Wisconsinan Stage. Geological Society of America Memoir 136, 153-185. Road Log MILES Begin in Duluth, MN. 0-3 Interstate 35 south to Grand Avenue/MN 23 exit. 3-22 14 miles to County 4. East (left) 2 miles to railroad crossing. Stop 1 Glacial Lake Duluth underflow fan. 22-39 County 4 west ~12 miles to Interstate 35. 39-49 North on Interstate 35 ~12miles to MN 33. 49-69 North on MN 33 through Cloquet for ~18 miles to US 53. 69-73 North on US 53 for ~4 miles. Cloquet River Hellwig Creek Stop 2 Hellwig Creek outlet and Glacial Lake Upham II basin overlook. 73-84 North on US 53 for ~12 miles to County 52 (Arkola Road) at Cotton, MN. 84-99 West on County 52 for ~16 miles to County 5. 99-101 North on County 5 for ~2 miles to gravel pit on left. Stop 3 Wave-washed esker island (Toivola, MN). 101-115 North on County 5 from previous stop for ~13 miles to MN 37. 115-119 East on MN 37 for ~3.5 miles to County 25. 119-120 North on County 25 for ~1.5 miles to the gravel pit on the left. Stop 4 Rainy Lobe collapsed outwash, Glacial Lake Upham I sediments, and St. Louis Sublobe till (Cherry, MN). 120-121 South on County 25 to MN 37. 121-135 West on MN 37 for ~9.5 miles to County 57 (1st Avenue). Page 198 �135-140 South on County 57 for ~8 miles. Stop just before County 442 (Townline Road South). Stop 5 Eolian dunes. 140-140 South on County 57 to County 442 (Townline Road South). 140-144 West on County 442 for ~3 miles. Stop at gravel pit access road on north side. Stop 6 Glacial Lake Upham II Shoreline. 144-145 West on County 442 (Townline Road South) to MN 73. 145-145 North on MN 73 for ~0.5 miles to County 16. 145-156 West on County 16 for ~11 miles to County 20. 156-164 West on County 20 for 0.25 miles to MN 65. 164-168 North on MN 65 for ~8.5 miles to County 12. 168-169 West on County 12 for ~3.5 miles to County 70. 169-170 South on County 70 for ~1 mile to County 434. 170-170 South on County 434 for ~1 mile to gravel pit on right Stop 7 Goodland Esker. 170-175 Continue south on County 434 for ~5 miles to County 10. 175-176 West on County 10 for ~1.5 miles to County 71. 176-181 South on County 71 for ~4 miles to US 2. 181-181 Cross US 2 and continue for ~ 0.25 miles to County 441 (Bluebird Lane) 181-184 West on County 441 for ~2.5 miles to County 3 (River Road). 184-197 South on County 3 for ~2.5 miles to cutbank along Mississippi River on left. Stop 8 Prairie River Underflow fan in Glacial Lake Aitkin II. 197-200 Continue southbound on County 3 (River Road) for ~2 miles to Mississippi River Boat Landing Road on the left Stop 9 Rhythmite in Glacial Lake Aitkin II. 200-201 Return to County 3. 201-202 South on County 3 to MN 200. 202-203 East on MN 200 for 1.5 miles to MN 65. 203-203 North on MN 65 for 0.5 miles to MN 200. 203-208 East on MN 200 for ~4 miles to 154th Avenue. 208-208 North on 154th Avenue. Stop 10 Eolian dunes in Glacial Lake Upham II basin. To return to Duluth: 208-208 Return to MN 200. 208-213 East on MN 200 to US 2. 213-262 East on US 2 to Interstate 35. North on Interstate 35 to start point. Page 199 �FIELD TRIP 7 ECONOMIC GEOLOGY OF ARCHEAN GOLD OCCURRENCES IN THE VERMILION DISTRICT, NORTHEAST OF SOUDAN, MINNESOTA By Dean M. Peterson and Richard L. Patelke Natural Resources Research Institute, University of Minnesota Duluth INTRODUCTION A widespread area of gold mineralization occurs in numerous prospects east of Lake Vermilion, within the Vermilion greenstone belt of northeastern Minnesota. The mineralization occurs in rocks of the Neoarchean (~2.7 Ga) Bass Lake sequence (Peterson and Jirsa, 1999a) of the Wawa subprovince of the Canadian Shield. This zone of abundant gold mineralization is bounded to the south by the Mud Creek shear zone and to the north by the Vermilion fault (Fig. 7-1). The main access to these prospects is along the Mud Creek road (St. Louis County 38). A brief period of mineral exploration for lode-gold deposits in this immediate area of the Vermilion district occurred in the mid 1980s to early 1990s. These programs typically consisted of grid-based geologic mapping, bedrock sampling, ground geophysics, and the completion of soil geochemical surveys. Conversations with many of the people involved in gold exploration programs in the immediate field trip area (centered on Section 6, Township 62 North, Range 14 West), and compilation of all exploration data from the district as a whole (data from the terminated lease files of the Minnesota DNR), has led to the conclusion that interpretation of linear structural elements exposed in outcrops were not used in designing exploratory drilling plans in the field trip area. Therefore, many of the prospects discovered as a result of these exploration programs remain untested by drilling. The authors recently completed a project (Peterson and Patelke, 2004) consisting of detailed geologic and structural mapping within many of the known gold prospects in the field trip area, and will highlight some of the insights generated as a result of this work during the course of this one day field trip. This recent work consisted of detailed outcrop mapping (at scales ranging from 1:1,000 to 1:3,000) that was focused on structural (i.e. shear zones, lineations, intersecting foliations and small-scale folds), geological (i.e. contact relationships, competence contrasts), geochemical (i.e. gold assays, trace element characteristics), and alteration (Fe-bearing carbonate, sericite, pyrite, silicificaton) features within and around areas of gold mineralized exposures. Based on detailed mapping of features described above, the project's goal was to try to determine the location and orientation at depth of gold-rich mineralized zones exposed in outcrops. The mapped geological features were incorporated into the data compiled and described by Peterson (2001), and are available online as a 1:12,000 scale geological map, as well as ArcView GIS data files at http://www.nrri.umn.edu/egg/. Page 200 �Figure 7-1. Location, simplified geology, lode gold prospects, and field trip #7 stops. LODE GOLD ORE DEPOSIT MODEL The brief description of Archean lode-gold deposits that follows is presented as both a basic reference and also to highlight the important features of the model that will be seen during the field trip. Archean lode-gold deposits are one category of ore deposit classified as mesothermal lode-gold deposits (Hodgson, 1993). This deposit type has also been called orogenic gold (Groves et al., 2000), greenstone gold (Robert et al., 1991), Archean lode-gold, mesothermal gold-quartz veins, shear-hosted gold, low-sulfide gold-quartz veins (Berger, 1986b), lode-gold, Mother Lode veins (Bohlke and Kistler, 1986), and iron formation-hosted gold deposits (Berger, 1986a; McMillan, 1996; Rye and Rye, 1974; Fripp, 1976; Kerswill, 1993; Thorpe and Franklin, 1984; and Vielreicher et al., 1994). Whatever the name, they are a widespread group of epigenetic ore deposits that have formed in similar settings throughout geologic time. In general, the deposits form during Page 201 �compressional or transpressional deformation at convergent plate margins in accretionary or collisional orogens (Fig.7-2). They form over a large crustal-depth range (2 to 20 km) from deep-seated, low-salinity H2O-CO2 ± CH4 ± N2 ore fluids, with Au transported as reduced sulfur complexes. The ore fluids are believed to be generated during lower crustal metamorphism from dehydration reactions. Regional structures provide the main control on distribution of lode gold deposits and mining camps. In many terranes, first-order faults or shear zones appear to have controlled regional fluid flow, with greatest ore-fluid fluxes in, and adjacent to, subsidiary faults, shear zones and/or large folds. Highly competent and/or chemically reactive rocks are the most common hosts to the larger deposits. Gold deposition occurs late during the evolutionary history of the host terranes, normally within D3 or D4 in a D1-D4 deformation sequence. Absolute ages of mineralization support their late-kinematic timing, and, in general, suggest that deposits formed diachronously towards the end of the evolutionary history of hosting orogens. Figure 7-2. Generalized tectonic model for the formation of mesothermal gold deposits, after Groves et al., 2000. The late timing of lode-gold deposits is critical to geology-based exploration methods, and hence mineral potential evaluations for these deposits. The late timing is critical because of the present structural geometry of: (1) the deposits, (2) the mining camps, and (3) the enclosing geologic terranes are essentially all similar to the structural geometry during gold mineralization. Therefore the interpretation of bedrock geological maps and cross-sections can be used to discern the physical conditions that existed at the time of ore deposition. Exploration for mesothermal lode-gold deposits should incorporate various aspects of the ore deposit model into criteria that can vector into the most favorable areas for hosting such mineralization. The most fundamental characteristic of this class of deposit is the spatial association of the deposits to regional structures in metamorphic terranes. Zones of widespread carbonate alteration (adjacent to regional structures) should be identified and used to focus subsequent exploration. Within carbonate alteration zones, gold is typically only in areas containing quartz veins, silicification, and/or sericite alteration (with or without sulfides). Two general structural controls on the orientation of lode gold ore shoots include deflections and curvatures of shear zones, and where high strain zones intersect favorable geological elements (Poulsen and Robert, 1989). A generalized sequence of events associated with the formation of typical Archean mesothermal gold deposits is outlined below. The possible observation of many of these important features during the course of the field trip is highlighted by the bracketed, bold text. Page 202 �I. Deposition of Archean Supracrustal Strata (Volcanic and Sedimentary Rocks) II. Compression and Early (D1) Folding [stop 7-1] III. Renewed Compression and Transpression (D2) 1) Development of Major Crustal Shear Zones [stop 7-1] 2) Strain Partitioning and Development of Subsidiary Shear Zones [stop 7-3, 7-4] 3) Intrusion of Porphyritic Intrusions, Continued D2 Deformation [stop 7-2, 7-3 and 7-5] -Local Development of Competency Contrasts [stop 7-3, 7-5] 4) Lower Crustal Dehydration and Fluxing of CO2-rich Fluids up Structures - Fe-Carbonate Alteration Zones [stop 7-3, 7-4, 7-5] 5) Fluxing of Gold-Bearing Hydrothermal Fluids up Subsidiary Shear Zones, Latest D2 - Gold Mineralization Associated with: A) Quartz-Pyrite Veins [stop 7-2, 7-3, 7-4, and 7-5] B) Silicification with Pyrite [stop 7-4] C) Pyritization of Fe-Rich Host Rocks [stop 7-5] D) Quartz-Sericite-Pyrite Schists/Phyllites [stop 7-4] E) Contact Zones Between Rock Units [stop 7-5] IV. Erosion REGIONAL GEOLOGIC SETTING OF THE VERMILION DISTRICT The field trip area is located in the Neo-Archean (~2.7 Ga) Vermilion Greenstone Belt of the classic Vermilion district of northeastern Minnesota. Supracrustal rocks in the Vermilion district consist of volcanic-dominated stratigraphic sequences of the Wawa subprovince of the Superior Province of the Canadian Shield. Rocks of the Wawa subprovince in northern Minnesota are divided on the basis of stratigraphic and structural setting into: (1) the southern Soudan belt and (2) the northern Newton belt (Jirsa et al., 1992; Southwick et al., 1998). The boundary between these contrasting structural panels can be traced geophysically across the width of Minnesota, and was designated informally as the Leech Lake structural discontinuity (Jirsa et al., 1992). In the Vermilion district, the Leech Lake structural discontinuity occurs along the Mud Creek shear zone (Hudleston et al., 1988), small segments of the Vermilion and Wolf Lake faults (Sims and Southwick, 1985), and the Bear River fault (Jirsa et al., 1992). The Soudan belt contains large, broad folds involving calc-alkalic and tholeiitic volcanic strata overlain by, and locally interdigitated with, turbiditic rocks. In contrast, the Newton belt consists of elongate, east- to northeast-trending, and mostly northward-younging volcanic and volcaniclastic sequences. Volcanic rocks of the Newton belt differ from those of the Soudan belt in containing locally abundant komatiitic flows and peridotitic sills. The two belts are faultbounded, and the relationship between stratigraphic units within each belt is largely conformable, although faults obscure contacts locally. In its eastern extension, the Soudan belt is continuous with the Saganagons assemblage in Ontario and terminates against the Saganaga pluton and Northern Light Gneiss. The Newton belt extends discontinuously eastward into the Shebandowan District of Ontario to form the Greenwater and Burchell assemblages. Intrusive rocks in both belts vary from gabbroic and felsic porphyries, demonstrably related to volcanism, to large plutons emplaced post-tectonically. Both districts contain unconformable, Timiskaming- Page 203 �type sequences composed of calc-alkalic volcanic rocks, conglomerates, and finer grained sedimentary rocks. A simplified regional geological map of the Neo-Archean terranes of northeastern Minnesota and adjacent Ontario is presented in Figure 7-3. Figure 7-3. Simplified correlation map of Neo-Archean assemblages across the U.S. - Canada border, modified from Peterson et al. (2001). Inset shows major subprovinces of the southwestern Superior Province. Periods of generally N-S directed compression resulted in three major regional deformation events in the Neoarchean terranes of northern Minnesota. The earliest deformation event (D1) produced broad, locally recumbent folds within the Soudan belt and major fault zones throughout the region. In the Newton belt, D1 was accommodated by thrust imbrication of large crustal blocks, resulting in mainly northward stratigraphic facing. Field relationships indicate that uplift, faulting, and the deposition of Timiskaming-type clastic sequences in local faultbounded basins occurred late in D1 deformation (Jirsa, 2000). A large, map-scale structure related to D1 deformation in the Soudan Mine area is the Tower-Soudan Anticline, which is a west-plunging anticline within which the axis and plunge changes orientation along strike from nearly vertical in basalts to shallow NE plunging in the western sedimentary rocks. Axial-planar cleavage associated with this early fold typically is lacking, although Bauer (1985), Hooper and Ojakangas (1971), Hudleston (1976), and Jirsa et al. (1992) have described early cleavage (S1) locally. A second deformation event (D2) associated with synchronous regional metamorphism resulted in foliation development and structures having largely dextral asymmetry. D2 is constrained in the Vermilion district to the time period 2674 to 2685 Ma (Boerboom and Zartman, 1993), and between about 2680 and 2685 Ma in the Shebandowan (Corfu and Stott, 1998). Because D2 deformation affected all of the supracrustal rocks in the area and is reasonably constrained by geochronology, the regional foliation (S2) can be used in the field to Page 204 �relate other structural, intrusive, and deformation events. The relationship between S2 fabric and shear structures indicates that most shearing occurred relatively late in the D2 event. Major shearing that produced the Mud Creek and related shear zones is attributed to the late stages of D2 dextral transpression. Regional D2 strain patterns in the Vermilion district vary from north to south in the belt (Hudleston et al., 1988; Schultz-Ela and Hudleston, 1991). These patterns include flattening strains that occur to the north, near the present Vermilion Fault and constrictional strains to the south. The field trip area lies within the zone of flattening strain of Hudleston et al. (1988) and Schultz-Ela and Hudleston (1991). Schultz-Ela and Hudleston (1991) mathematically modeled the observed strain patterns as deformation paths that produced flattening strains (west plunging ë 1 axes) by dextral shear of the pre-existing constrictional strains (east plunging ë 1 axes). The linear E-W strain patterns and minor rotations about horizontal axes during the deformation, as described by Hudleston et al. (1988) and Schultz-Ela and Hudleston (1991), preclude early theories (Hooper and Ojakangas, 1971; and Hudleston, 1976) on the structural setting of the greenstone belt by infolding and shear off the flanks of rising granitic diapirs. An estimate of 50% north-south shortening across the belt is proposed by Schultz-Ela and Hudleston (1991), and their model favors an origin of the Vermilion district rocks at a convergent margin, most likely as a N-dipping subduction complex with shallow slab dip. The origin of the southern constrictional strains remains enigmatic. Structures related to the third deformation event (D3) include abundant NE- and NWtrending faults that dissect the stratigraphic assemblages. Named structures related to D3 include the NE-trending Waasa and Camp Rivard faults SSE of Ely, and the WNW-trending, crustalscale Vermilion and related faults that form the Wawa-Quetico Subprovince boundary immediately north of the field trip area. LOCAL GEOLOGIC SETTING; THE BASS LAKE SEQUENCE The informally named Bass Lake sequence (Peterson and Jirsa, 1999a) of the Newton Belt occurs in an east-west trending, fault-bounded panel that widens to the west. The sequence is bounded by segments of the Mud Creek and Shagawa shear zones, and the Bear River, Haley, Vermilion, Burntside Lake, and Wolf Lake faults (Fig. 7-4). The continuation of stratigraphic and geophysical trends from areas of excellent exposure (east of Lake Vermilion) into poorly exposed areas (to the west) have led to the inclusion of rocks of the informally named Cook sequence (Southwick, 1993; Southwick et al., 1998) in the Bass Lake sequence. The western portion of the sequence consists of linear belts of graywacke and basalt cut by late felsic to intermediate composition intrusions. East of Lake Vermilion, the geology of the Bass Lake sequence is dominated by six basic rock types, which include: (1) Tholeiitic pillowed basalt flows (see Fig. 7-4) interpreted to have formed in a deep-water setting based on volcanic textures; (2) Gabbro sills interpreted as synvolcanic in age due to their stratigraphic continuity and similar deformation as the enclosing pillowed basalts; (3) Felsic porphyries (feldspar porphyry and quartz-feldspar porphyry) interpreted to have intruded during late stages of D2 deformation based on field relationships and geochronology (quartz-feldspar porphyry from the Pac Man Pond prospect returned a 207Pb/206Pb age of 2683.0 +/- 1.4 Ma (Peterson et al., 2001)); (4) Algoma-type iron-formation; (5) Thinly- Page 205 �bedded argillite and siltstone; and (6) Sheared rocks, which are dominated by chlorite-rich schist, phyllite, and phyllonite. In addition, localized areas of fragmental felsic volcanic rocks occur stratigraphically below distinct iron-formation horizons. Regional and detailed geologic maps of the sequence are presented in Fig. 7-5. In the last twenty years, numerous gold prospects have been discovered in the eastern portion of the sequence (Fig. 7-1). These prospects generally fall into one of three categories; (1) auriferous quartz-carbonate-pyrite veins and sulfidized zones in iron-formation; (2) auriferous quartz-sericite-ankerite-pyrite schists; and (3) felsic intrusive-hosted auriferous quartz veins and stockworks. All of the prospects are found within areas of moderate to strong ironcarbonate alteration, with the best mineralization commonly found within sericitic alteration zones. Numerous equigranular and porphyritic felsic intrusions occur within the areas of alteration and gold mineralization, and are a good guide for locating mineralized structures. The gold mineralization is generally related to deformation in subsidiary structures associated with movement along the D2 Mud Creek shear zone. Geological and geochemical descriptions of many of the gold prospects have previously been given by Peterson and Jirsa (1999b). Figure 7-4. Jensen cation plot (Jensen, 1976) for volcanic and hypabyssal rocks of the Bass Lake sequence. Undoubtedly the most striking structural feature in the immediate study area is the juxtaposition of the Soudan and Newton belts along the Mud Creek shear zone. In addition, numerous east-northeast trending highly strained zones occur to the north of the Mud Creek shear zone and these subsidiary sheared zones host a majority of the gold prospects in this area of the Bass Lake sequence. A stereonet projection of planar and linear structural features within the field trip #7 area of the Bass Lake sequence is shown in Fig. 7-6. Page 206 �Figure 7-5. Regional to detailed geological maps of the Bass Lake sequence. Geology simplified from Peterson (2001) and Peterson and Patelke (2004). Page 207 �Figure 7-6. Stereonet projections of planar and linear structural features from the field trip area. GLACIAL HISTORY Late Pleistocene glacial deposits (Late-Wisconsin glaciation) in the Vermilion district are associated with the stepwise retreat of the Rainy Lobe of the Laurentide Ice Sheet, approximately 14,000 to 12,000 years ago. The repeated glaciations of the Pleistocene epoch modified the preexisting topography of northeastern Minnesota, i.e. the surface was scoured by glacial ice, exposing fresh bedrock, and new surficial materials were deposited following the retreat of the glaciers. During the retreat of the glacier, the margin of the ice-sheet blocked the natural drainage to the north, and pro-glacial lakes formed in front of this barrier (glacial lakes Norwood, Koochiching, and Agassiz). Subglacial streams left sinuous ridges of sorted sand and gravel (eskers), and delta/fan complexes formed where these streams exited the ice margin and entered the pro-glacial lakes. The field trip area lies within the scoured bedrock terrain of northeastern Minnesota, immediately north of the Vermilion Moraine. Regional and detailed digital elevation maps of the area are presented in Figure 7-7, and display the pronounced recent bedrock scouring of the area, especially along the Vermilion fault and Mud Creek shear zone. Late Pleistocene surficial deposits and landforms that unconformably overlie the Neoarchean bedrock in the field trip area include a thin veneer of basal till, local outwash deposits, and sinuous eskers. Page 208 �Figure 7.7. Regional (top) and local (bottom) digital elevation maps depicting superimposed glacial landforms. Grid values in the detailed map are UTM coordinates, in meters. The locations of field trip stops are depicted in the lower image. FIELD TRIP STOPS Stops for this field trip will include short traverses that visit a number of outcrops for each locality. The traverses are designed to highlight specific features of the lode-gold ore deposit model for each area, though time constraints preclude detailed analysis of each area. Page 209 �STOP 7-1: MUD CREEK SHEAR ZONE Location: (SE, SE, Sec. 5, T.62N., R.14W., NAD83 UTM 564200E, 5302800N) Description: The regional scale Mud Creek shear zone occupies the east-northeast trending valley of Mud Creek, which is clearly visible at this location. This shear zone separates rocks of the Newton Belt (here the Bass Lake sequence) to the north and rocks of the Soudan Belt (Gafvert Lake sequence and the Upper Ely Greenstone Formation) to the south. The Mud Creek shear zone is analogous with major faults (Destor-Porcupine fault) and “breaks” (Cadillac-Larder Lake break) of major lode-gold mining districts in Canada. Historic gold assays taken from rocks of the shear zone itself are essentially devoid of gold, as is the case for most major structures within Archean lode-gold mining camps. The short field trip traverse is located within the northern margin of the internal highly strained zone of the shear, and will visit outcrops of: (1) ankerite-sericite-quartz-green mica-pyrite schist with quartz and tourmaline knots, and (2) highly folded and compositionally banded phyllites with quartz veins (Fig. 7-8). A simplified geology and field trip traverse map of stop 7-1 is presented in Figure 7-9. Figure 7-8. Outcrop photographs of ankerite-sericite-quartz-green mica-pyrite schist (A and B), and highly folded and compositionally banded phyllites (C and D). Page 210 �Figure 7-9. Simplified geology and field trip traverse map of stop 7-1, geology from Peterson and Patelke, 2004. STOP 7-2: SECTION 6 GOLD PROSPECT Location: (SE, SE, Sec. 6, T.62N., R.14W., NAD83 UTM 561895E, 5303410N) Description: Gold mineralization within the Section 6 prospect is dominantly associated with quartz-pyrite veins and stockworks in feldspar ± quartz porphyry. The stockwork-style mineralization is generally not apparent until one takes a detailed look at the outcrops, then the thin quartz veining is seen seemingly everywhere. The highest-grade gold assays taken to date occur in areas of strong sulfide oxidation (sulfide burn) in the porphyry. Mineralization also occurs in sulfidized and epidote-altered basaltic rocks adjacent to the felsic porphyries, and may Page 211 �locally contain significant quantities of chalcopyrite. In addition to porphyry and basaltic rocks, thin dikes and sills of diorite, peridotite, and inclusion-rich lamprophyre will be observed during the field trip stop. Outcrop photographs and a geologic map of the Section 6 prospect area are presented in Figures 7-10 and 7-11, and a table of anomalous gold assays is given in Table 7-1. Figure 7-10. Outcrop photographs from the Section 6 prospect. (A) 1 Ft.-wide dioritic dike near a basalt-porphyry (with BIF inclusion) contact; (B) strong sulfide burn within feldspar porphyry; (C) stockwork quartzveins (black lines) and local sulfide burn within feldspar porphyry; and (D) sulfide burn in quartzveined feldspar porphyry. Table 7-1. Anomalous gold assays from the Section 6 prospect. Description Au (ppb) Siliceous, sericitic and strongly pyritic basalt Carbonatized quartz-feldspar porphyry with pyrite Carbonatized feldspar porphyry with pyrite and quartz veins Pyritic selvage about a 3' quartz vein in basalt Pyritic selvage about a 3' quartz vein in basalt Epidote-silica-hematite altered feldspar porphyry with pyrite-rich veins Page 212 1,460 1,360 1,135 940 890 860 �Description Au (ppb) Pyritic quartz-feldspar porphyry Chloritic feldspar porphyry with ankerite, 1-3% pyrite, and quartz-ankerite-pyrite veins Chlorite-epidote-silica altered basalt with pyrite and chalcopyrite Carbonatized quartz-feldspar porphyry with pyrite Chlorite-epidote-silica altered basalt with pyrite and chalcopyrite 855 840 790 780 445 Figure 7-11. Simplified geology and field trip traverse map of stop 7-2, geology from Peterson and Patelke, 2004. Page 213 �STOP 7-3: SECTION 6 EAST GOLD PROSPECT Location: (SE, SE, Sec. 6, T.62N., R.14W., NAD83 UTM 562840E, 5302905N) Description: Three features of the generalized Archean lode-gold ore deposit model are beautifully exposed along the traverse of this stop. These features include: (1) intense ironcarbonate alteration, (2) competency contrast and associated auriferous quartz veining, and (3) shear deformation. Anomalous gold was first discovered by soil sampling in this locality in 1988 by Chevron Resources. Gold mineralization is generally confined to quartz-pyrite veins in ironcarbonate altered feldspar-porphyry intrusive rocks. The felsic porphyries are generally located adjacent to linear zones of intense strain, and are believed to have intruded along these structural breaks during D2 deformation. Outcrop photographs and a geologic map of the Section 6 East prospect area are presented in Figures 7-12 and 7-13, and anomalous gold assays in Table 7-2. Figure 7-12. Outcrop photographs from the Section 6 East prospect. (A) iron-carbonate altered feldspar porphyry with quartz veins; (B) close-up of quartz-pyrite veins in carbonatized feldspar porphyry; (C) large quartz ± pyrite ± galena veins in massive iron carbonate; and (D) intense iron carbonate alteration Page 214 �Table 7-2. Anomalous gold assays from the Section 6 East prospect. Description 6" quartz vein in sheared feldspar porphyry Feldspar porphyry with ankerite, pyrite, and quartz veins Feldspar porphyry with ankerite, pyrite, and quartz veins Feldspar porphyry with local fracturing and shearing Feldspar porphyry with a rusty quartz-pyrite vein Ankerite-sericite altered basalt with pyrite Au (ppb) Description Au (ppb) 6,210 2,400 1,760 1,380 1,130 1,030 Soil Sample Soil Sample Soil Sample Soil Sample Soil Sample Soil Sample 1,520 622 615 294 170 101 Figure 7-13. Simplified geology and field trip traverse map of stop 7-3, geology from Peterson and Patelke, 2004. Page 215 �STOP 7-4: KERR MCGEE GOLD PROSPECT Location: (SE, SE, Sec. 31, T.63N., R.14W., NAD83 UTM 562480E, 5304310N) Description: The Kerr McGee gold prospect is hosted within an extensive zone of highly strained rocks, interpreted to be a subsidiary structure associated with the Mud Creek shear zone. Moderate to high-grade gold mineralization at the Kerr McGee prospect occurs within multiple thin (0.2 – 2.0 meter) zones of quartz-sericite-ankerite-pyrite ± green mica ± tourmaline schist hosted by an extensive zone of essentially gold-barren chlorite-rich schist. Thin and probably boudined iron-formation horizons occur locally in the chlorite-rich schist, and locally are strongly mineralized in this area. Mineralized zones locally contain extensive foliation and shear parallel quartz, ankerite, and/or quartz-ankerite veins, and may widen in zones of silicification. The style of gold mineralization exposed in the Kerr McGee prospect is similar to both the Clear Cut (~½ mile west) and Railroad Zone (1½ miles east) prospects. In fact, the sericitic zone that hosts the mineralization may have continuity to both of these other prospects. The field trip traverse will take us to numerous outcrops in the prospect area, and discussions will highlight the style of mineralization, possible continuity to other prospects, and shear deformation. Anomalous gold assays from the Kerr McGee prospect are presented in Table 7-3. Outcrop photographs and a geologic map of the prospect area are presented in Figures 7-14 and 7-15 respectively. Table 7-3. Anomalous gold assays from the Kerr McGee prospect. Description 1' wide zone of quartz-sericite-ankerite-pyrite tourmaline schist Quartz-pyrite-sericite knot in ankeritic sericite schist 1' wide zone of quartz-sericite-ankerite-pyrite tourmaline schist 1' wide zone of sericite-quartz-pyrite-ankerite phyllite 1' wide zone of sericite-quartz-pyrite-ankerite phyllite 1' wide zone of sericite-quartz-pyrite-ankerite phyllite 0.5' wide zone of sericite-quartz-pyrite-ankerite phyllite 1.1' wide zone of sericite-quartz-pyrite-ankerite phyllite Sericite-chlorite schist 0.33' wide zone of sericite-quartz-ankerite-pyrite phyllite 0.5' wide zone of sericite-quartz-ankerite-pyrite phyllite Road cut of siliceous quartz-sericite-pyrite schist with quartz veins 1.2' wide zone of sericite-quartz-pyrite-ankerite phyllite 1.1' wide zone of sericite-quartz-pyrite-ankerite phyllite Chlorite-sericite-ankerite schist with 10-20% pyrite Chlorite-quartz-ankerite schist with pyritic (5-10%) siliceous zones Chlorite-quartz-ankerite schist with pyritic (5-10%) siliceous zones Chlorite-quartz-ankerite schist with 1-5% pyrite Quartz-ankerite-pyrite (2-3%) vein Chlorite-sericite-ankerite schist with 10-20% pyrite Brecciated ankeritic-pyritic (5-10%) chert Sericite-chlorite-quartz-ankerite-pyrite (5%) schist Page 216 Au (ppb) DDH 8,010 5,030 4,991 4,660 4,410 3,430 2,770 2,740 2,440 1,675 1,140 980 695 685 1,980 RC-3 1,470 RC-3 1,040 RC-3 620 RC-3 613 RC-3 448 RC-3 369 RC-3 334 RC-3 Interval (ft) 1.0 3.0 4.0 7.0 1.0 1.0 2.0 2.0 �Figure 7-14. Outcrop photographs from the Kerr McGee prospect. (A) Road side outcrop of quartz-sericite-pyriteankerite-green mica-tourmaline schist; (B) minor northwest-trending fault offsetting highly foliated chloritic and sericitic schists; (C) close-up view of complex deformation fabric associated with quartz veins and knots; (D) channel samples taken across thin mineralized zones; (E) more channel samples taken across thin mineralized zones in sericite-rich schist, barren chlorite-rich schist at the bottom of the photograph; and (F) dextral shear fabrics in a mineralized zone at the east end of the outcrop area. Page 217 �Figure 7-15. Simplified geology and field trip traverse map of stop 7-4, geology from Peterson and Patelke, 2004. Three-dimensional visualization (Fig. 7-16) of the detailed lithological and structural mapping by Peterson and Patelke, (2004) within the Kerr McGee prospect area reveals important information that can be used to design drilling plans that significantly increase the chance of intersecting gold mineralization exposed in outcrop at the surface. For example, drill hole RC-3, which is located 100 meters east of the map presented in Figure 7-15, was drilled due north (at a dip of 45º) and targeted to intersect the mineralization exposed in outcrop at the Kerr McGee Page 218 �showing. Chevron Resources drilled this hole in 1987, at the western boundary of their lease property (the Kerr McGee prospect was then held by Kerr McGee). Detailed structural mapping in these outcrops reveals that the rocks within the mineralized zone have moderate to strong elongation and intersection (foliation and shear planes) lineations trending 60º and dipping northeast at 72º. The best interpretation of the down-dip orientation of the mineralized zone is this lineation trend and plunge, and drill hole RC-3 never intersected the mineralized zone. A three-dimensional view of these relationships is given in Figure 7-16. Figure 7-16. Three-dimensional view of the relationship between structural boundaries, the mineralized zone exposed on the surface at the Kerr McGee prospect, and drill hole RC-3. Upward extension to the surface of the two anomalous zones (> 1,000 ppb gold) intersected in hole RC-3 would place these zones in the black spruce and cedar swamp located south-southeast of the prospect. As briefly described in the preceding paragraphs, the sericitic-schist hosted style of mineralization at the Kerr McGee prospect is similar to the mineralization at both the Clear Cut and Railroad Zone prospects, and may form a continuous zone of anomalous to ore-grade gold mineralization over a strike length of > 2.5 miles. By analogy to many Canadian and Australian Archean lode gold deposits, the odds of discovering high-grade zones within this trend would be significantly increased if possible future exploration ventures utilized structural elements exposed in outcrop. A simplified geologic map encompassing the highly strained zone that hosts the Clear Cut, Kerr McGee, and Railroad Zone gold prospects is presented in Figure 7-17. In addition, the trend and plunge of measured lineations are given in both the plan map of Figure 717 and the underlying east-west cross-section. Although the field trip will not visit the Clear Cut Page 219 �and Railroad Zone prospects, anomalous gold assays from these associated prospects are presented in Tables 7-4 and 7-5 respectively. Figure 7-17. Highly simplified geologic and lineation map of the highly strained shear zone that hosts the Clear Cut, Kerr McGee, and Railroad Zone gold prospects. The total lengths of the lineation arrows depicted in the east-west cross-section (lower portion of the figure) are all equal, their apparent difference in length is related to their individual trend and plunge in relation to the view angle of the section. Table 7-4. Anomalous gold assays from the Clear Cut prospect. Description Au (ppb) DDH Interval (ft) 2' wide zone of quartz-sericite-pyrite schist Quartz-sericite-pyrite schist Quartz-sericite-pyrite schist Quartz-sericite-pyrite schist Quartz-ankerite vein with a sericite-pyrite selvage in ankerite-chlorite schist 1/2" wide quartz-pyrite-arsenopyrite vein Oxide-facies iron-formation with 10% pyrite and 5% arsenopyrite Quartz-sericite-pyrite schist Oxide-facies iron-formation with 1-2% cross-cutting pyrite Oxide-facies iron-formation with 10% pyrite and 5% arsenopyrite Chlorite-pyrite phyllite with quartz veining and 5% pyrite Chlorite phyllite with trace to 5% pyrite Chlorite-pyrite phyllite with quartz veining and 1% pyrite Chlorite-pyrite phyllite with quartz veining and 5% pyrite Chlorite-pyrite phyllite with quartz veining and 5% pyrite Page 220 22,000 7,100 2,940 1,970 1,970 598 590 568 440 402 1,510 715 550 528 471 RC-5 RC-5 RC-5 RC-5 RC-5 2.0 2.0 5.0 2.0 2.0 �Table 7-5. Anomalous gold assays from the Railroad Zone prospect. Description Grab sample from large boulder encased in the roots of an overturned tree Sericite-chlorite-ankerite-pyrite phyllite with quartz-ankerite-pyrite veins 4' Sericite-chlorite-ankerite-pyrite phyllite with quartz-ankerite-pyrite veins Chlorite-ankerite-pyrite schist Siliceous sericite schist with ankerite and pyrite Chlorites schist with pyrite and sericite Chlorite schist with pyrite and sericite Chlorite schist with pyrite and sericite Siliceous chlorite schist with pyrite Massive pyrite lens in sericite-ankerite schist Chlorite-ankerite-pyrite phyllite with quartz-ankerite veins Chlorite schist with pyrite and sericite 5' zone of very rusty and weathered, sericite-ankerite-quartz-pyrite phyllite Chlorite-ankerite phyllite with numerous quartz-ankerite-pyrite veins Sericite-quartz-ankerite-pyrite phyllite with 5% pyrite Quartz-sericite-pyrite phyllite with 5% pyrite Au (ppb) DDH Interval (ft) 37,324 17,100 13,623 3,110 2,670 2,410 970 960 960 776 705 620 605 777 V-2 1.5 713 V-4 1.8 591 V-1 6.0 STOP 7-5: PACMAN POND GOLD PROSPECT Location: (SE, SE, Sec. 6, T.62N., R.14W., NAD83 UTM 562225E, 5303780N) Description: The Pac Man Pond gold prospect was explored by Noranda in the late 1980s and early 1990s. They completed a quite extensive program that consisted of geological mapping, bedrock sampling, till prospecting, trenching, and drilling. Three rock types, that include pillowed basalt, iron-formation, and intrusions of feldspar ± quartz porphyry, dominate the prospect geology, with rare occurrences of argillaceous and graphitic sediments as well as carbonate facies iron-formation intersected in drill holes. Gold mineralization is hosted dominantly by brecciated, quartz-pyrite veined, and sulfidized iron-formation, with locally anomalous gold in sulfidized basalt and porphyry. The outcrops observed on the field trip will be concentrated on one of the areas that Noranda completed extensive trenching and gold assaying. Outcrop photographs from the Pac Man Pond gold prospect are given in Figure 7-18, and a geologic map of the Pac Man Pond gold prospect area is presented in Figure 7-19. Anomalous gold assays from bedrock exposures and Noranda drill hole intersections are given in Table 7-6. Page 221 �Figure 7-18. Outcrop photographs from the Pac Man Pond prospect. (A) Strongly iron-carbonate altered quartzfeldspar porphyry; (B) sheared and laminated pyrite and chlorite at the contact between foliated to sheared basalt and sulfidized iron-formation; (C) irregular contact between carbonatized quartzfeldspar porphyry and sulfidized iron-formation; and (D) strong sulfide burn on highly sulfidized iron-formation. Sawed samples of gold-bearing, pyritic iron-formation reveal one of the classic chemical models for the precipitation of gold out of hydrothermal solutions. In this model, when silica-saturated auriferous hydrothermal fluids (with gold carried by HS complexes) encounter a rock rich in FeO (such as Fe-tholeiitic basalt or iron-formation), quartz, pyrite and gold are precipitated out of solution. The classic chemical equation is presented below. 2HAu(HS)2 aq + ½O2 aq + 2FeO s ↔ Page 222 2FeS2 s + 2Au s + 3H2O aq �Figure 7-19. Simplified geology and field trip traverse map of stop 7-5, geology from Peterson and Patelke, 2004. Page 223 �Table 7-6. Anomalous gold assays from the Pac Man Pond prospect. Description Large angular boulder of pyrite-rich (5-50%) iron-formation Sulfidized iron-formation Siliceous, pyritic iron-formation with limonite Siliceous, pyritic iron-formation Sulfidized iron-formation Sulfidized iron-formation with quartz-pyrite veins Limonite stained iron-formation with 5-15% pyrite Sulfidized (10-70% pyrite) basalt Siliceous, pyritic iron-formation Iron-formation with trace pyrite Sulfidized iron-formation Laminated iron-formation with trace pyrite Pyrite-rich iron-formation near contact with quartz-feldspar porphyry Sulfidized iron-formation with 5-20% pyrite Quartz-feldspar porphyry with a rusty quartz vein Laminated iron-formation with trace pyrite 6' wide zone of sulfidized iron-formation with 3-10% pyrite Mineralized iron-formation with 5-15% pyrite Brecciated iron-formation with 5-20% pyrite in matrix Brecciated iron-formation with 5-20% pyrite in matrix Iron-formation with 1-2% pyrite in veinlets Brecciated iron-formation with quartz-pyrite (10%) veins Thin-bedded and folded iron-formation with 3% pyrite Brecciated iron-formation with chlorite-quartz-pyrite (5-10%) matrix Brecciated iron-formation with 5-10% pyrite Weakly brecciated iron-formation with 3-10% pyrite in matrix Fine-grained mafic dike Quartz-feldspar porphyry with trace pyrite Oxidized iron-formation with ankerite and 8% pyrite Au (ppb) 2,701 2,190 1,940 1,810 1,750 1,635 1,020 985 890 875 845 845 815 811 785 760 758 705 28,460 5,400 4,212 2,255 1,747 1,364 1,026 960 856 856 782 DDH Interval (ft) V89-1 V89-1 V-90-2 V-90-4 V-90-7 V-90-2 V-90-3 V89-1 V-90-4 V-90-4 V-90-3 2.0 1.7 3.5 2.0 1.8 1.8 3.2 2.3 1.0 1.0 2.0 REFERENCES Bauer, R.L., 1985, Correlation of early recumbent and younger upright folding across the boundary between an Archean gneiss belt and greenstone terrane, northeastern Minnesota: Geology, v. 13, p. 657-660. Berger, B. R., 1986a, Descriptive model of Homestake Au: in Mineral Deposit Models, Cox, D.P. and Singer, D.A., eds., U.S. Geological Survey, Bulletin 1693, p. 245- 247. Berger, B. R., 1986b, Descriptive model of low-sulphide Au-Quartz veins: in Mineral Deposit Models, Cox, D.P. and Singer, D.A., eds., U.S. Geological Survey, Bulletin 1693, p. 239-243. Page 224 �Boerboom, T.J., and Zartman, R.E., 1993, Geology, geochemistry, and geochronology of the central Giants Range batholith, northeastern Minnesota: Canadian Journal of Earth Sciences v. 30, p. 2510-2522. Bohlke, J.K. and Kistler, R.W., 1986, Rb-Sr, K-Ar and stable isotope evidence for the ages and sources of fluid components of gold-bearing quartz veins in the Northern Sierra Nevada Foothills Metamorphic Belt, Economic Geology, v. 81, p. 296- 422. Corfu, F., and Stott, G.M., 1998, Shebandowan greenstone belt, western Superior Province: U-Pb ages, tectonic implications, and correlations: Geological Society of America Bulletin, v. 110, p. 1467-1484. Fripp, R.E.P., 1976, Stratabound gold deposits in Archean banded iron-formation, Rhodesia; Economic Geology, v. 71, p. 58-75. Groves, D.I., Goldfarb, R.J., Knox-Robinson, C.M., Ojala, J., Gardoll, S., Yun, G.Y., and Holyland, P., 2000, Late-kinematic timing of lode-gold deposits and significance for computer-based exploration techniques with emphasis on the Yilgarn Block, Western Australia; Ore Geology Reviews, v. 17, Issues 1-2, Pages 1-38. Hodgson, C.J., 1993, Mesothermal lode-gold deposits: in Mineral Deposit Modeling. Kirkham, R.V., Sinclair, W.D., Thorpe, R.I., Duke, J.M. eds., Geological Survey of Canada, Special Paper 40, p. 635–678. Hooper, P., and Ojakangas, R., 1971, Multiple deformation in the Vermilion district, Minnesota: Canadian Journal of Earth Sciences, v. 8, p. 423-434. Hudleston, P.J. 1976, Early deformational history of Archean rocks in the Vermilion district, northeastern Minnesota: Canadian Journal of Earth Sciences, v. 13, p. 579-592. Hudleston, P.J., Schultz-Ela, D., and Southwick, D.L., 1988, Transpression in an Archean greenstone belt, northern Minnesota: Canadian Journal of Earth Sciences, v. 25, p. 1060-1068. Jensen, L.S., 1976, A new cation plot for classifying subalkalic volcanic rocks: Ontario Department of Mines, Miscellaneous Paper 66. Jirsa, M.A., 2000, The Midway sequence: a Timiskaming-type pull-apart basin deposit in the western Wawa subprovince, Minnesota: Canadian Journal of Earth Science, v. 37. p. 1-15. Jirsa, M.A., Southwick, D.L., and Boerboom, T.J., 1992, Structural evolution of Archean rocks in the western Wawa subprovince, Minnesota: Refolding of pre-cleavage nappes during D2 transpression: Canadian Journal of earth Sciences, v. 29, p. 2146-2155. Kerswill, J.A., 1993, Models for Iron-formation-hosted Gold Deposits: in Mineral Deposit Modeling, Kirkham, R.V., Sinclair, W.D., Thorpe, R.I. and Duke, J.M., eds., Geological Association of Canada, Special Paper 40, p. 171-200. McMillan, R.H., 1996, Iron formation-hosted Au, in Lefebure, D.V. and Hoy, T, eds., Selected British Columbia Mineral Deposit Profiles, Volume 2 - Metallic Deposits, British Columbia Ministry of Employment and Investment, Open File 1996-13, p. 63-66. Page 225 �Peterson, D.M., 2001, Development of Archean lode-gold and massive sulfide deposit exploration models using geographic information system applications: Targeting mineral exploration in northeastern Minnesota from analysis of analog Canadian mining camps: Unpublished Ph.D. thesis, University of Minnesota, 503 p. Peterson, D. M., and Jirsa, M.A., 1999a, Bedrock geologic map and mineral exploration data, western Vermilion district, St. Louis and Lake Counties, northeastern Minnesota: MGS Miscellaneous Map M-98, scale 1:48,000. Peterson, D. M., and Jirsa, M.A., 1999b, Lode gold and massive sulfide prospects in the Archean western Vermilion district; Minnesota Exploration Conference 1999, Field trip guidebook, 10 maps, 30 p. Peterson, D.M and Patelke, R.L., 2004, Bedrock Geology and Lode Gold Prospect Data Map of the Mud Creek Road Area, Northern St. Louis County, Minnesota: Natural Resources Research Institute, Map Series NRRI/MAP-2004-01. Peterson, D.M., Gallup, C., Jirsa, M.A., and Davis, D.W., 2001, Correlation of Archean assemblages across the U.S. - Canadian border; Phase I geochronology, abstract and oral presentation, Institute on Lake Superior Geology, 47th Annual Meeting, Thunder Bay, Ontario, v. 47. Poulsen, K.H., and Robert, F., 1989, Shear zones and gold: Practical examples from the southern Canadian Shield: in Bursnall, J.T. ed., Mineralization and Shear Zones, Geological Association of Canada, Short Course Notes Volume 6, p. 239-266. Robert, F., Sheahan, P.A., and Green, S.B., eds., 1991, Greenstone gold and crustal evolution: NUNA Conference Volume, Geological Association of Canada, Mineral Deposits Division. Rye, D. M. and Rye, R. O., 1974, Homestake Gold Mine, South Dakota: I. Stable Isotope Studies; Economic Geology, v. 69, p. 293-317. S Schulz-Ela, D.D., and Hudleston, P.J., 1991, Strain in an Archean greenstone belt of Minnesota: Tectonophysics, v. 190, p. 233-268. Sims, P.K., and Southwick, D.L., 1985, Geologic map of Archean rocks, western Vermilion district, northern Minnesota: U.S. Geological Survey, Miscellaneous Investigations Map I-1527, scale 1:48,000. Southwick, D.L., Boerboom, T.J., and Jirsa, M.A., 1998, Geologic setting and descriptive geochemistry of Archean supracrustal and hypabyssal rocks, Soudan-Bigfork area, northern Minnesota: Implications for metallic mineral exploration: Minnesota Geological Survey Report of Investigations 51, 69 p. Southwick, D.L., compiler, 1993, Bedrock geologic map of the Soudan-Bigfork area, northern Minnesota: Minnesota Geological Survey Miscellaneous Map M-79, scale 1:100,000. Thorpe, R.I and Franklin, J.M., 1984, Chemical-sediment-hosted Gold: in Canadian Mineral Deposit Types: A Geological Synopsis, Eckstrand, O.R., Editor, Economic Geology Report 36, Geological Survey of Canada, p. 29. Vielreicher, R.M., Groves, D.I., Ridley, J.R. and McNaughton, N.J., 1994, A Replacement Origin for the BIF-hosted Gold Deposit at Mt. Morgans, Yilgarn Block, W.A; Ore Geology Reviews, v. 9, p. 325-347. Page 226 �FIELD TRIP 8 Geology and Mineralization of the Western Contact of the Duluth Complex, Partridge River and South Kawishiwi Intrusions, Northeastern Minnesota Leaders: Mark Severson Natural Resources Research Institute, University of Minnesota Duluth and Jim Miller Minnesota Geological Survey, University of Minnesota Railroad-cut exposures in the Wetlegs area of the Partridge River Intrusion (Stop 8-3) Page 227 �FIELD TRIP 8 Geology and mineralization of the western contact of the Duluth Complex, Partridge River and South Kawishiwi intrusions, northeastern Minnesota by Mark Severson Natural Resources Research Institute, University of Minnesota Duluth and Jim Miller Minnesota Geological Survey, University of Minnesota INTRODUCTION The Duluth Complex and associated Keweenawan intrusions in northeastern Minnesota constitute one of the largest mafic intrusive complexes in the world, second only to the Bushveld 2 Complex of South Africa. These rocks cover an arcuate area of over 5,000 km (Fig. 8-1) and give rise to two strong gravity anomalies (+50 & +70 mgal) that imply intrusive roots to more than 13 kilometers depth (Allen and others, 1997). The intrusive rocks of northeastern Minnesota were emplaced into a comagmatic volcanic edifice during formation of the Midcontinent Rift between 1108 and 1086 Ma. The Keweenawan geology of northeastern Minnesota has been recently reinterpreted in a 1:200,000 scale geologic map (MGS Miscellaneous Map, M-119; Miller and others, 2001) and in a comprehensive report that focused on the geology and mineral potential of the Duluth Complex (MGS Report of Investigation, RI-58; Miller and others, 2002). The general description of the Duluth Complex given below is largely excerpted from that report. The Duluth Complex is physically defined as a more or less continuous mass of mafic to felsic plutonic rocks that extends in an arcuate fashion from Duluth to nearly Grand Portage (Fig. 8-1). It is bounded by a footwall of predominantly Paleoproterozoic and Archean rocks, a hanging wall of largely mafic volcanic rocks and hypabyssal intrusions, and internally, it contains scattered bodies of strongly recrystallized mafic volcanic and sedimentary hornfels. Defining the Duluth Complex more genetically, it is composed of multiple discrete intrusions of mafic to felsic tholeiitic magmas that were episodically emplaced into the base of a comagmatic volcanic edifice in two general stages - an early stage at about 1108 Ma and an main stage at 1099 Ma. Within the Duluth Complex, four general rock series are distinguished on the basis of age, dominant lithology, internal structure, and structural position. These are: Felsic Series – massive granophyric granite and smaller amounts of intermediate rock that occurs as a semicontinous mass of intrusions strung along the eastern and central roof zone of the complex and was emplaced during early stage magmatism (~1108 Ma). Early Gabbro Series – layered sequences of dominantly gabbroic cumulates that occur in two major intrusions along the northeastern contact of the Duluth Complex and were also emplaced during early stage magmatism (~1108 Ma) Anorthositic Series – a structurally complex suite of foliated, but rarely layered, plagioclase-rich gabbroic cumulates that was emplaced throughout the complex during main stage magmatism (~ 1099 Ma). Layered Series – a suite of stratiform troctolitic to ferrogabboic cumulates that comprise at least 11 variably-differentiated mafic layered intrusions and occurs mostly along the base of the Page 228 �Duluth Complex. These intrusions were emplaced during main stage magmatism, but generally after the anorthositic series. The Partridge River (PRI) and South Kawishiwi (SKI) intrusions, the focus of this field trip, occur along the northwestern margin of the Duluth Complex (Fig. 8-1) and are two of the earliestformed intrusions of the Layered Series. These two intrusions are most renown for hosting the largest tonnage of Cu-Ni sulfide ore in the world (Naldrett, 1997). The bedrock geology of the northwestern margin of the Duluth Complex, as portrayed on the regional geologic map of northeastern Minnesota (M-119; Miller and others, 2001), is shown in Figure 8-2. Detailed mapping in the Babbitt NE, Babbitt SE, and Babbitt SW quadrangles, which was conducted since the publication of M-119 (Miller, Severson, and Foose, 2002; and unpublished field mapping, Miller, 2002-03), has revealed a somewhat different picture of the geology that has yet to be incorporated into the regional map. Mapping in this area will continue this summer with the objective of producing 1:24,000-scale maps of all four of the Babbitt 7.5' quadrangles in the summer of 2005. Figure 8-1. Generalized geology of northeastern Minnesota showing the field trip area. Partridge River intrusion (PRI) and South Kawishiwi intrusion (SKI) are labeled. Page 229 �Figure 8-2. Geology of the northwestern margin of the Duluth Complex showing field stops for Field Trip 8 (see Miller and others, 2001 for more details). Geologic units: Agr - Giants Range granite, PbifBiwabik Iron Fm, Pvf - Virginia Fm, pri - Partridge R. intr., ski-South Kawishiwi intr., bei-Bald Eagle intr.; gltr-Greenwood Lake intr-troctolitic zone; glgb-Greenwood Lake intr, gabbroic zone; glfgGreenwood Lake intr, ferrogabbroic zone; mwgp-Mount Weber granophyre, asau-anorthositic series undifferentiated; asa-anorthositic series, anorthositic rocks; asg-anorthositic series, gabbroic rocks; nsv-NSVG undifferentiated; nsbh-NSVG basaltic hornfels; nssh-NSVG sedimentary hornfels. Page 230 �Figure 8-3. Location of copper-nickel sulfide deposits, Fe-Ti±V deposits, and other exploration areas along the western base of the Duluth Complex (from Miller and others, 2002; Fig. 2.3) Page 231 �Partridge River Intrusion The Partridge River intrusion (PRI, Bonnichsen, 1974) is perhaps one of the most well studied intrusions of the Duluth Complex because it hosts at least four sub-economic Cu-Ni deposits and at least seven potential Fe-Ti deposits. The PRI consists mainly of troctolitic cumulates that are exposed in an arc-shaped area that extends from the Waterhen Fe-Ti deposit area (T.57N., R.14W., Sec. 27) to the Babbitt Cu-Ni deposit (T.60N., R.12W., Secs 31-33; Fig. 83). Miller and Ripley (1996) estimate that the PRI is 2.5 km thick. The footwall of the PRI includes the Paleoproterozoic Virginia Formation (slate and graywacke), and to a lesser extent, the Biwabik Iron Formation. The top of the PRI is in complex contact with anorthositic rocks (unit asa, Fig. 8-2), gabbroic rocks (unit asg), mafic volcanic hornfels (unit nshb), and in one location, an unusual fine-grained and cross-bedded sedimentary hornfels (Colvin Creek hornfels of Bonnichsen, 1972; Patelke, 1996; unit nssh). This assemblage of anorthositic, gabbroic, and hornfelsic rocks are also present as large inclusions within the interior of the PRI (Severson and Miller, 1999) and are thought to represent earlier roof zone septa that were overplated and isolated by later injections of PRI magma. The lower 900 meters of the PRI is known in great detail from the abundance of exploration drill core. This marginal zone, consisting of varied troctolitic and gabbroic rock types, is subdivided into seven stratigraphic units (Severson, 1991; 1994; Severson and Hauck, 1990; 1997; Geerts, 1991) that can be correlated over a strike length of 24 km (Fig. 8-4). All of these igneous units generally exhibit shallow dips (10-20°) to the southeast. The stratigraphy shown in Figure 8-4 is based on the relogging of almost 700 drill holes, or 672,000 feet of drill core. The units of the Partridge River marginal zone are briefly described below starting at the base of the PRI. Units I and III will be viewed at Stop 8-3. For locations of ore deposit areas mentioned below, see Figure 8-3. Unit I - This lower unit consists of a heterogeneous mixture of ophitic troctolitic to gabbroic rocks that contain abundant inclusions of hornfelsic sedimentary footwall rocks and minor thin, discontinuous layers of melatroctolite and peridotite. It is the dominant sulfide-bearing member of the PRI. Noritic rocks are common at the basal contact and peripheral to sedimentary hornfels inclusions, probably due to contamination of the magma. An ultramafic interval, consisting of an oxide-bearing peridotite overlying an oxide-bearing pyroxenite, is present at the base of Unit I in areas where the PRI is in direct contact with the Biwabik Iron Formation (extreme southeastern portion of the Babbitt deposit). Unit II - This unit exhibits considerable variation from one Cu-Ni deposit to the next. At the Dunka Road and Babbitt deposits, Unit II consists of homogenous troctolitic rocks, with minor sulfide mineralization, and a fairly persistent basal ultramafic layer that separates Unit II from Unit I. The upper contact with Unit III is gradational at the Dunka Road deposit. At the Wetlegs deposit, Unit II is either 1) a single ultramafic layer immediately beneath Unit III, and/or 2) the Wetlegs Layered Interval (Fig. 8-3) which consists of repeated, thin cyclic units that internally grade upward from ultramafic rock to troctolite. Still farther to the west at the Wyman Creek deposit, Unit II consists of a single ultramafic horizon that separates sulfide-bearing and heterogeneous troctolitic rocks of Unit I from homogenous troctolitic rocks of Unit IV. Unit III – This unit consists of poikilitic leucotroctolite that commonly grades into poikilitic/ophitic augite troctolite. Because of its fine-grained texture and distinctive olivine oikocrysts that impart a mottled appearance, Unit III is a useful marker horizon, although it is absent from the Wyman Creek deposit and portions of the Babbitt deposit. Hornfelsed basalt inclusions, or roof rocks, are commonly associated with Unit III at the Dunka Road deposit. This relationship, and the highly gradational contact of Unit III Page 232 �with Unit II, suggest that Unit III may have formed as a roof cumulate during crystallization of Unit II. Unit IV - Homogenous ophitic augite troctolite that contains a local basal unit of ultramafic rock. Unit IV typically exhibits a highly gradational upper contact with Unit V. A cyclic sequence of alternating troctolitic and ultramafic layers, termed the Bathtub Layered Interval, is present in the Bathtub ore zone of the Babbitt deposit. Unit V – Unit V consists of homogenous, coarse-grained leucotroctolite. At the Wyman Creek deposit, drill hole relationships suggest that Unit V cuts downward into the lower units (Fig. 8-3). Units VI and VII - Each of these units consist of homogenous leucotroctolite that locally grades into ophitic augite troctolite; both also contain a fairly persistent ultramafic base. Several other similar units (Unit VIII and up) are present above these two units but they are not defined by drilling. OUI - Several late-stage pegmatitic plugs and vertical lenses of oxide-bearing ultramafic intrusions (OUI) intrude the troctolitic rocks of the PRI. The acronym OUI was first used by Severson and Hauck (1990) to designate cross-cutting pegmatitic bodies of peridotite, melatroctolite, melagabbro, and clinopyroxenite that contain a high percentage of coarsegrained oxides (15-100%). Known OUI bodies in the PRI, with Fe-Ti potential, occur at Section 17, Longnose, Longear, Wyman Creek, Section 22, Skibo, Skibo-South, and Waterhen. In all instances, the OUI are spatially arranged along linear trends suggesting that structural control was important to their genesis. In addition, the Longnose-LongearSection 17 group of OUIs are positioned over a window in the basal contact where the Biwabik Iron-Formation is the footwall rock. This relationship suggests a genetic link between assimilation of iron-formation at the basal contact and formation of OUI along a coincident fault zone. Figure 8-4. Generalized stratigraphy of the marginal zone of the Partridge River intrusion (modified from Severson, 1994). Stratigraphic relationships for the area between the Wyman Creek deposit and the Water Hen deposit are poorly understood and are not portrayed (taken from RI-58 figure 6.10). Page 233 �Attributes of the more heterogeneous units (I and II) near the base of the PRI are interpreted to indicate rapid magma replenishment in a progressively developing magma chamber, and magmatic contamination from assimilated footwall rocks. The more homogeneous upper units (IV-VII), each floored by a persistent ultramafic layer, were probably emplaced later in a welldeveloped magma chamber. The ultramafic layers in the upper units, and abundant ultramafic layers of the Wetlegs and Bathtub layered intervals, probably represent the inception of episodic magma injection that crystallized more primitive ultramafic layers before mixing with the resident magma. The contact zone between the PRI and South Kawishiwi Intrusion (SKI) is poorly exposed and poorly drilled (Fig. 8-2). All of the units of the PRI marginal zone, including even the upper units, become unrecognizable in the contact zone with the SKI and correlation of igneous units in one drill hole to a nearby drill hole is tenuous at best. Severson (1994) reasoned that the heterogeneous contact zone of the PRI originally formed at the contact with pre-Keweenawan footwall rocks, and that later emplacement of the SKI effectively removed the footwall portion and positioned SKI intrusive rocks up against PRI intrusive rocks. Complicating the picture of the contact zone is the coincident Grano fault (Fig. 8-16), which appears to have been repeatedly reactivated during, and after, emplacement of the PRI and SKI. The Grano fault may have also served as a feeder channel to the massive sulfides of the Local Boy ore zone at the Babbitt deposit (Fig. 8-15). More petrologic studies have been conducted on the PRI than any other intrusion in the Duluth Complex. These studies pertain to drill holes in the Babbitt deposit (see Miller and others, 2002 for references). Most of the studies pertain to a stratigraphic section intersected in one or a few drill holes that represent only a small fraction of the PRI magmatic system. Divergent progenetic interpretations have resulted from these studies and have been summarized by Miller and Ripley (1996). South Kawishiwi Intrusion The South Kawishiwi intrusion (SKI) of Green and others (1966) hosts at least five subeconomic Cu-Ni deposits and a potential PGE-Cu-Ni deposit (Fig. 8-2). The SKI is dominantly composed of troctolitic cumulates that are exposed in an 8- x 32-kilometer arcuate band. Footwall rocks include the Virginia Formation, in the Serpentine and Dunka Pit deposits, the Biwabik Iron Formation in the Dunka Pit and Birch Lake deposits, and the Archean Giants Range Batholith in the Dunka Pit deposit north to the Spruce Road deposit. The presence of Biwabik Iron Formation as inclusions as far north as the Spruce Road deposit indicates that the majority of Paleoproterozoic units were assimilated and removed from the footwall during emplacement of the SKI leaving the Giants Range Batholith as the dominant footwall rock type. Also present as inclusions in the Dunka Pit and Serpentine deposits, are mafic volcanic hornfels (probable North Shore Volcanic Group) and quartz sandstone hornfels (probably either the Puckwunge or Nopeming sandstones. Anorthositic rocks (unit asau Fig. 8-2) abut the SKI on the northeast and enclose a possible SKI feeder dike that extends farther northeast. To the east, the SKI is inferred to be in semi-conformable contact with the Bald Eagle Intrusion (unit bei; Fig. 8-2). However, based on their relative cross cutting relationships to the Greenwood Lake intrusion, it is clear that the Bald Eagle is younger than the SKI (Chandler, 1990; Miller and others, 2002). On the regional Duluth Complex map (M-119, Miller and others, 2001), the SKI is subdivided into five major map units. These are, from the base upward, 1. a basal contact zone that is a heterogeneous mix of sulfide-bearing troctolitic, gabbroic, and noritic rocks with abundant hornfels inclusions (unit skcz, M-119); 2. a thick unit of subophitic to ophitic augite troctolite (unit skat, M-119) that contains an internal ophitic olivine gabbro unit (unit skog, M-119) ; Page 234 �3. discontinuous and localized layers of poikilitic leucotroctolite (unit skpt, M-119); 4. a thick homogeneous sequence of ophitic troctolite (unit sktr, M-119) ; and 5. an uppermost thick sequence of homogeneous troctolite (unit skta, M-119) that contains numerous anorthositic layers (designated as unit skan where anorthositic layers are mapped in detail). Detailed mapping of the Babbitt NE 7.5' quadrangle in 2001 and 2002 and published as an MGS open-file map (Miller, Severson and Foose, 2002), has subdivided the southern part of the SKI into eight stratiform map units. Most of these units show considerable variation in thickness and some pinch out altogether. Moreover, whereas Foose and Cooper (1978) have interpreted most of the anorthositic rock occurrences in the upper part of the SKI to be conformable plagioclase cumulate layers interleaved within the largely troctolitic cumulates, we have interpreted most anorthositic rock occurrences to be inclusions. Severson (1994) and Zanko and others (1994) further subdivide the marginal zone of the SKI (including units skcz, skat, skog, and sktr, Fig. 8-2) into 17 different lithostratigraphic units (Fig. 8-5) that are present in over 180 drill holes over a strike length of 31 kilometers. Sulfide mineralization is confined to the BH, BAN, UW, and U3 units, and to a lesser extent the U1 and U2 units. Major marker horizons that are correlated in drill hole include three horizons with abundant cyclic ultramafic layers (U1, U2, and U3 units) and a pegmatite-bearing unit (PEG Unit) that was initially recognized by Foose (1984). A large anorthositic inclusion (> 1 km thick) is intersected in six deep drill holes in the Highway One corridor area (AN-G Unit in Fig. 8-5). Figure 8-5. Generalized stratigraphy of the marginal zone of the South Kawishiwi intrusion (modified from Severson, 1994; taken from RI-58 figure 6.12). The lowest units of the marginal zone (Fig. 8-5) are the most varied with respect to textures, rock types, and sulfide content. They are very unevenly distributed along the strike length of the SKI in a “compartmentalized” fashion, suggesting a complicated intrusive history. The lowest units were emplaced early into several restricted magma chambers via repeated and close-spaced magmatic pulses. The U1, U2, and U3 units (Fig. 8-5) represent periods of rapid and continuous magma injection that crystallized more primitive ultramafic layers before mixing with the resident magma. The U3 Unit is unique among the lower units in that it contains several massive Page 235 �oxide pods (titanomagnetite-rich) along its entire length. A spatial correspondence between the U3 Unit and footwall iron-formation suggests that most of the massive oxide pods are iron-rich “restite” produced by the magmatic digestion of iron-formation. The U3 Unit also contains the majority of the high PGE values that have been sampled to date within the SKI. The upper units of the SKI are more laterally continuous throughout the intrusion, though recent mapping in the Babbitt NE quadrangle shows significant variation in thickness and, in some cases, pinching out of these upper troctolitic units. Still, the generally greater lateral continuity and monotonous troctolitic composition of these upper units suggest that they crystallized in a more quiescent and open magmatic system characterized by widely-spaced, large volume magmatic pulses. The occurrence of abundant anorthositic inclusions, especially in the uppermost unit (Fig 8-2), implies that much of the growth of the SKI chamber was accomplished by emplacement of new magma between previous troctolitic injections and an anorthositic series hanging wall. Disseminated Cu-Ni Sulfide Mineralization in the PRI and SKI Large resources of low-grade copper-nickel sulfide ore that locally contain anomalous PGE concentrations are well documented by drilling in the basal zones of the Partridge River and South Kawishiwi intrusions. At least nine subeconomic deposits (Fig. 8-3) have been delineated in the basal 100 to 300 meters of both intrusions. The mineralization consists predominantly of disseminated sulfides that collectively constitute over 4.4 billion tons of material averaging 0.66% Cu and 0.20% Ni (Listerud and Meineke, 1977). Overall, the copper to nickel ratio averages 3.3:1; however, there are wide variations in the Cu:Ni ratio from one Cu-Ni deposit to another deposit and also internally within each of the deposits. PGE concentrations average about 10 ppm Pt+Pd (recalculated to 100% sulfide), but may range as high as 50 ppm Pt+Pd (recalculated to 100% sulfide) in associated stratabound zones, such as at the Dunka Road and Birch Lake deposits. The disseminated sulfide deposits are hosted by taxitic troctolitic to gabbroic rocks that contain abundant inclusions of footwall rock types. Within the Partridge River intrusion, the basal unit - Unit I of Severson and Hauck (1990; Fig. 8-4) hosts the vast majority of the disseminated sulfides. Similarly, mineralization within the South Kawishiwi intrusion is confined to the bottom-most units (Fig. 8-5) that include the following units: BH (basal heterogeneous); BAN (basal augite troctolite and norite); three ultramafic units (U1, U2, and U3); and UW (updip wedge). The disseminated sulfide minerals (dominantly pyrrhotite, chalcopyrite, cubanite, and pentlandite) occur as interstitial grains that make up between trace amounts and 10% of the rock by volume (visual estimation). Pyrrhotite is generally the dominant sulfide, especially closer to the basal contact. Although this mineralization type is categorized as being present within the basal portions of the intrusions, it is important to stress that the rock units do not always contain sulfides throughout their entire vertical section. Mineralized zones are independent of rock type and are typically extremely erratic in their spatial extent and ore grades. Zones that are barren of sulfides commonly “interfinger” with mineralized zones in a random pattern. This erratic pattern of mineralization, in part, mirrors the lithologic heterogeneity of the basal units. The only exception to this random mineralization pattern is the Maturi deposit, and its downdip extension (Maturi Extension; Peterson, 2001), where the uppermost portions of the BH Unit generally exhibit copper values in excess of 1.0% that gradually decrease with depth toward the basal contact. Contrasting with this heterogeneity of rock types and mineralization, some internal PGEbearing sulfide zones within the lower units of both intrusions exhibit a stratabound relationship to the igneous stratigraphic section. Examples include Dunka Road and Birch Lake (see discussion below). Page 236 �Basal Cu-Ni Massive Sulfide Mineralization In a few localized areas along the basal zones of the SKI and PRI, semi-massive to massive sulfide mineralization is present at the basal contact. In most cases, the massive to semi-massive sulfide is proximal to either sulfide-rich footwall rocks or structures such as faults and preComplex folds. Massive sulfide zones that are spatially related to sulfide-rich footwall rocks are intersected in scattered drill holes in the Babbitt, Serpentine, and Dunka Pit deposits (Severson and others (1994), Zanko and others (1994), and Severson (1994), respectively). All of these massive sulfides are pyrrhotite-rich (with generally <2% Cu) and are present at, or slightly above, the basal contact. In all cases, a pyrrhotite-rich member of the footwall Virginia Formation (BDD PO unit) is located at the basal contact and is situated up-dip of the massive sulfide occurrences. This relationship suggests that the BDD PO unit acted as a local sulfur source, which generated a Cu-poor, sulfide-rich melt that was gravitationally concentrated downdip, along the basal contact. The massive sulfide occurrence in the Local Boy ore zone of the Babbitt deposit (Fig. 8-3) is clearly structurally controlled. At this locality, the massive sulfide zones are Cu-rich (generally 5-25% Cu) and are situated along the axis of an anticline defined by the footwall rock units. The highest PGE values (11 ppm Pd and 8 ppm Pt) yet found within the Duluth Complex are associated with these structurally-controlled Cu-rich massive sulfides. The massive sulfides are almost exclusively hosted by the Virginia Formation, present as both inclusions above the basal contact and in the footwall rocks below the basal contact. These relationships, plus sulfide textures that are indicative of structural preparation, suggest that the massive sulfides were "injected" into the footwall rocks. Ripley (1986) and Severson and Barnes (1991) propose that an immiscible sulfide melt, formed in an auxiliary magma chamber at depth, was injected into structurally prepared zones in the footwall rocks along the anticline to form the Local Boy ores. Late movement of Cl-rich fluids, along the axis of the anticline, further redistributed and concentrated the PGEs (Severson and Barnes, 1991). Recent studies (Severson and Zanko, in prep.) indicate that there is an overall increase in the Cu-PGE content of the massive sulfide in an east-to-west direction (Fig. 8-6); this is perhaps the result of fractional crystallization of immiscible sulfide melt as it migrated into the footwall rocks. In this scenario, the north-south trending Grano fault is inferred to be a potential feeder zone. Structurally-controlled veins and irregular pods of massive sulfide are locally present within granitic footwall rocks immediately beneath the SKI. These occurrences are intersected in scattered holes that outline two northeast-trending belts (Fig. 8-7). The linearity of the belts suggests they are fault controlled. One of these belts crudely aligns with the Birch Lake fault zone which trends through the Birch Lake PGE prospect. The veins are moderately Cu-enriched due to fractional crystallization of the sulfide melt as it moved down through the footwall rocks (Bonnichsen and others, 1980; Severson, 1994). The occurrence of local massive sulfide veins near and below the basal contact of the Duluth Complex is an indication that larger, potentially economic footwall massive sulfide deposits may yet be found. In the Sudbury Complex, pooling of a monosulfide solid solution (mss) melt at the basal contact appears to be an important prerequisite to the injection of fractionated sulfide melts (Naldrett, 1997). Stratabound PGE Mineralization Most of the PGE-enriched zones in both the Partridge River and South Kawishiwi intrusions are stratabound in nature. These stratabound PGE horizons are intimately associated with the CuNi sulfide mineralization and with one or more ultramafic layers that indicate magma recharge events. Thus, their PGE-enrichment appears to be related to magma mixing. However, there are also indications that the PGE content in some of these horizons was locally modified by later Clrich hydrothermal solutions. Page 237 �231 2400E 3600E 2800E 3200E 4400E 4000E 211 3600S 3600S Axis of Bathtub Syncline 189 149 253 ? 197 4000S 144 217 4000S 230 ? ? ? A-4 228 ? 234 156 A-3 4400S A-5 121 4400S 154 A-6 ? A-2 158 ? U D ? SHAFT A-1 4800S 162 146 138 142 4800S 152 ? ? Axis of Local Boy Anticline ? ? ? 159 143 ? ? 124 B-3 5200S 133 U D B-1 161 132 B-1A B-2 B-2A 136 5200S B-5 B-4 B-3A 150 C-0 Kulas Fault ? ? ? C-1 119 C-1A ? ? D-5 D-4 D-3 D-2 130 160 ? C-2A 116 5600S ? C-2 D-1 ? ? 148 ? 5600S C-3 139 105 127 135 C-3A U C-4 D C-4A 2400E C-5 C-5A C-6 153 6000S 141 134 129 131 137 ? ? Grano Fault 163 140 120 N MASSIVE SULFIDE TYPES: 1. SEMI-CONTINUOUS MASSIVE SULFIDE HORIZONS ASSOCIATED WITH FOOTWALL ROCKS OR HORNFELS INCLUSIONS ABOVE THE BASAL CONTACT Pyrrhotite Dominant i.e. >85% of sulf. is PO Pyrrhotite Rich i.e. 50-85% of sulf. is PO Copper Rich (i.e. PO is 50%, Cu minerals 50%) SCALE 0 100 FT. 2. WIDELY SCATTERED PODS OF SEMIMASSIVE TO MASSIVE SULFIDE ASSOCIATED WITH EITHER HORNFELS INCLUSIONS OR TROCTOLITIC ROCKS (Pyrrhotite Dominant, >85% PO) UNDERGROUND DRIFT B-5 UNDERGROUND DRILL FAN 153 SURFACE DRILL HOLE Figure 8-6. Potential distribution of semi-massive to massive sulfide types, relative to the Grano fault and Local Boy anticlinal axis, at the Local Boy ore zone of the Babbitt deposit (from Severson and Zanko, in prep.; taken from RI-58, Figure 8.4). Page 238 �Figure 8-7. Linear distribution of massive sulfide, disseminated sulfide, and copper-rich veins in the Giants Range granitic footwall rocks beneath the South Kawishiwi intrusion (after Severson, 1994; taken from RI58, Figure 8.5). Stratabound PGE-enriched horizons, with low to moderate sulfide concentrations (0.05-1.0 wt.% S), are commonly associated with ultramafic layers in the Dunka Road, Babbitt, Wetlegs, and Birch Lake deposits (Fig. 8-3). Elevated Cu and PGE concentrations at the Dunka Road deposit occur at the extreme top of Unit I immediately beneath a laterally persistent ultramafic layer. This stratabound horizon (red horizon of Geerts, 1991, 1994) averages about 10 meters thick and contains an average of 1.0 ppm Pd+Pt. Recent work by Theriault and others (1997, 2000) suggests that the sulfur was largely derived from the mafic magma and that this stratabound horizon was formed as a result of magma mixing. Two similar stratabound PGEenriched horizons, related to laterally discontinuous ultramafic layers, occur toward the middle of Unit I at Dunka Road (orange and yellow horizons of Geerts, 1991; 1994). To the west of Dunka Road, the stratabound PGE horizon at the top of Unit I is also present at the Wetlegs and Wyman Creek deposits. There however, the overall Pd content in this horizon exhibits a definite decrease in an east-to-west direction suggesting that as the magma was intruded it became progressively impoverished with respect to PGE (Severson and Hauck, 1997; Theriault and others, 1997). Additional PGE-enriched stratabound horizons are situated much further above the basal contact in the Partridge River intrusion. These horizons are also often associated with ultramafic layers and are present at Dunka Road, Wetlegs and the Fish Lake area (Sassani, 1992; Severson, 1995). Another example of a PGE stratabound horizon is at the Birch Lake PGE prospect within the SKI. There, PGE contents as high as 9 ppm Pd+Pt, and Cr2O3 contents locally as high as 10 wt.%, are associated with a wide variety of rock types within the U3 Unit. The U3 Unit consists of alternating troctolitic and ultramafic layers in which variably sulfide-mineralized zones and discontinuous pods of Cr-bearing massive oxide both occur (Severson, 1994). The massive oxide pods are interpreted, based on empirical relationships, to have been produced by assimilation and partial melting of the Biwabik Iron Formation. This oxide-rich partial melt may have initially acted as a trap that concentrated Cr and Ti, and through further assimilation and contamination of Page 239 �the magma, may have led to precipitation of PGE. However, because the ultramafic layers of the U3 Unit are interpreted to record new influxes of more primitive magma (Severson, 1994), magma mixing may have played a more significant role in PGE mineralization. Furthermore, the presence of Cl-rich drops on the surface of drill core from the Birch Lake area suggests that a hydrothermal model of concentrating the PGE could also be invoked. A model involving ascending Cl-rich hydrothermal fluids, depicted in Figure 8-8 and similar to a model proposed by Boudreau and McCallum (1992), may have remobilized and further concentrated the PGE along the northeast-trending Birch Lake fault (Severson, 1994). A possible explanation, depicted in Figure 8-8, is that the Birch Lake area represents an area where there was a local increase in the amount of upward-moving Cl-rich solutions that were concentrated, or funneled, along the Birch Lake fault. Another explanation is that the Birch Lake Fault may have initially served as a subsidiary feeder zone to the South Kawishiwi intrusion (Hauck and others, in prep.). According to their model, uncontaminated PGE-enriched magmas were vented in the Birch Lake area. Upon mixing with the resident magma, which was contaminated due to interaction with the Biwabik Iron Formation, PGEs were deposited in zones of turbulent mixing. As the magma migrated away from the vent area (up-dip?) it became progressively impoverished with respect to PGEs. Figure 8-8. Schematic diagram showing the possible role that the Birch Lake fault may have played in funneling upward-moving Cl-rich solutions and the resultant reconcentration of significant magmatic PGE within the U3 Unit at the Birch Lake PGE area (modified from Severson, 1994; taken from RI-58, Figure 8.8). Page 240 �Oxide Ultramafic Intrusions (OUI) Exploratory drilling for Cu-Ni mineralization encountered several OUIs, that later were evaluated for their Fe-Ti±V potential. Many of the OUIs are expressed as aeromagnetic highs, commonly with an associated electromagnetic conductor, and thus they were initially drilled in search of conductive sulfide mineralization. At least thirteen OUIs have been intersected in drill holes along the basal contact of the Duluth Complex (Fig. 8-3). The OUIs are plugs or pipe-like bodies that commonly have irregular apophyses. They intrude troctolitic rocks of the Partridge River, Western Margin, and Boulder Lake intrusions. The Waterhen OUI appears to be rootless as defined by detailed drilling; the three-dimensional configurations of the other OUIs are unknown due to insufficient drilling. In general, the OUIs are spatially arranged along linear trends suggesting that structural control was important to their genesis. Almost all of the OUIs are cross-cutting. Rock types include coarse-grained to pegmatitic clinopyroxenite, dunite, peridotite, melatroctolite, and minor melagabbro; all rock types are oxide-bearing. Some OUIs exhibit a crude zonation from an olivine-rich core (dunite, peridotite, melatroctolite) to an outer clinopyroxenite margin, whereas others consist of only one dominant rock type. Oxide content is variable in the OUIs and ranges from 15% in disseminated zones to 100% in localized massive oxide zones. OUIs that contain thick intervals of massive oxide include Longnose (up to 30 meters thick; Linscheid, 1991) and Section 34 (up to 40 m thick; Severson, 1995). Titanomagnetite is dominant in some of the OUIs whereas ilmenite is dominant in other OUIs. Sulfide minerals (predominantly pyrrhotite) are ubiquitous in all the OUIs and range from trace amounts to 5% in disseminated zones to >70% in localized net-textured and massive sulfide zones, as at Waterhen, Boulder Lake South, and Fish Lake (Fig. 8-3). Page 241 �STOP DESCRIPTIONS STOP 8-1: Hornfels Virginia Formation, Duluth Complex Footwall at Linwood Lake Location: Approximately 300 feet southwest of the intersection of Linwood Lake Road and Reagan Road. Harris Lake 7.5’ quadrangle, T.56N., R.14W., Sec 22, NE of SE of SW; 567865E, 52407780N (NAD83) Description: Outcrops of the Virginia Formation at Linwood Lake show a progressive increase in the amount of deformation, metamorphism, and degree of partial melting towards the basal contact of the Duluth Complex (which based on limited drilling in this area exhibits a nearvertical cliff-like geometry). In this area, outcrops furthest from the contact consist of interbedded argillite and fine-grained distal graywacke (Bouma sequence B with minor C) that exhibit shallow dips of 15° to the southeast. However, slightly closer to the Complex, bedding dips increase to 40-60° to the northeast and small wisps (<5 mm) of partial melt are present along bedding planes. Still closer to the Complex (Stop 8-1) the grade of metamorphism and associated deformation progressively increase and at least two metamorphic varieties are superimposed on the original sedimentary package. Both of these metamorphic varieties are visible at this stop and consist of the following: Disrupted unit (Fig. 8-9)– In close proximity to the Complex the well-bedded sediments of the Virginia Formation are typically transformed into a highly deformed rock or metatexite (Sawyer, 1999). Textures that characterize this rock are bedding planes that are extremely chaotic and random in orientation due to small-scale folding, faulting, and brecciation. Superimposed on this chaotic pattern are abundant zones of partial melt that are also chaotic and folded. Albeit differences in grain size, the partial melts are composed of the same mineralogy as the surrounding sedimentary rocks (feldspar, quartz, orthopyroxene, cordierite, and biotite). Figure 8-9. Photograph of disrupted unit of metamorphosed and folded Virginia Formation with abundant partial melt lenses and wisps in close proximity to the Duluth Complex at Linwood Lake, MN. Page 242 �Recrystallized unit (Fig. 8-10)– This unit is a higher-grade metamorphic equivalent of the disrupted unit, and is properly classed as a diatexite (Sawyer, 1999). By contrast, rocks of the recrystallized unit were heated, generating 20-40% pervasive partial melts, which literally enabled the rocks to flow in response to stresses that were applied during emplacement of the Duluth Complex. All bedding planes are obliterated and what remains is a medium-grained recrystallized rock that contains plagioclase, cordierite, orthopyroxene, and decussate biotite. Within this recrystallized matrix are blocks/boudins of more structurally competent siltstone and calc-silicate hornfels (originally limey layers). Figure 8-10: Photograph of recrystallized unit of metamorphosed Virginia Formation in close proximity to the Duluth Complex at Linwood Lake, MN. STOP 8-2: Anorthositic Series, Duluth Complex, Skibo Vista Location: Roadcut on St. Louis Co. Highway 110, approx. 3.7 mi. north of Co. Hwy 16. Bird Lake 7.5' quadrangle; T.57N., R.13W., Sec 17, SE of NW; 574140E 5252806N (NAD83) Description: Exposed in roadcuts on either side of Co. 110 is an exposure of poikilitic troctolitic anorthosite that is a common lithology of the anorthositic series of the Duluth Complex. The rock varies in texture and modal mineralogy on a meter scale over the outcrops ranging from medium- to medium coarse in grain size, from subpoikilitic to poikilitic in olivine habit, and from 80 to 90% in plagioclase mode. Plagioclase foliation is locally well developed and varies in orientation throughout the exposures. Olivine oikocrysts range from 1 to 5 cm in diameter (larger ones on north side of exposures) and are locally concentrated in planes that parallel foliation and thereby impart a subtle layering. This exposure lies at the western extent of a large expanse of plagioclase-rich gabbroic rocks that comprise the Anorthositic Series of the Duluth Complex (Fig. 8-1). West of here, troctolitic cumulates of the Western Margin intrusion are inferred to occur (Figs. 8-1, 8-2). The lithologic characteristics and structural complexities of the Anorthositic Series, which are hinted at here, Page 243 �have lead many workers (Grout, 1918; Taylor, 1964; Miller and Weiblen, 1990) to conclude that these rocks formed from multiple injections of plagioclase-enriched mafic magmas (plagioclase crystal mushes) early in the main stage of Duluth Complex magmatism. The effectively identical ages of Anorthositic and Layered Series rocks (~1099Ma, Paces and Miller, 1993) implies that intrusion of normal (crystal-free) magmas, forming layered series intrusions like the Western Margin Intrusion, followed soon after the emplacement and crystallization of the Anorthositic Series. STOP 8-3a: Erie hornfels – mafic volcanic unit overlying the Virginia Formation, Erie/LTV RR Grade Location: Access via Dunka Road (private mining company road) approximately 5 miles west the LTV guard shack. Allen 7.5’ quadrangle, T.59N., R.13W., Sec 18, NW of SE of SW; 571815E 5271060N (NAD83) Description: Inclusions of hornfelsed basalt bodies are documented in many places in the Duluth Complex – the earliest detailed studies were by Kilburg (1972), for the Ely’s Peak basalts beneath the Complex near Duluth, MN, and Bonnichsen (1972), for several inclusions located within the Complex. The Erie hornfels was originally discussed by Bonnichsen (1972), and was more recently described in detail by Tyson (1976). Detailed geologic mapping of the area (Severson and Miller, 1999) indicates that the Erie hornfels directly overlies the Virginia Formation (based on the geology in surrounding drill holes) and is most likely situated near the base of the North Shore Volcanic Group in this area. The Erie hornfels is exposed in a 215-meter long railroad cut. Tyson (1976) described it as a series of basalt flows, dipping 55° to the southeast, that are characterized by fine-grained granoblastic rocks containing variable amounts of plagioclase, olivine, and augite with lesser amounts of hornblende and biotite. Relict amygdules (plagioclase- or augite-filled ovoids) and relict plagioclase phenocrysts are present in many places in the railroad cut but the amygdules appear to be more concentrated in the upper portions of the flows. Tyson (1976) also reports that biotite-rich layers are locally present in the exposures and that these layers were probably formed from either interflow sedimentary rocks or altered flow tops. STOP 8-3b: Marginal Units of the Partridge River Intrusion, Erie/LTV RR Grade Location: Erie/LTV RR Grade approximately 1/3 mile east of previous stop. Allen 7.5’ quadrangle, T.59N., R.13W., Sec 18, SE of SE, and Sec 19, SW of SW; traverse from 572430E, 5271135N to 572960E, 5271205N (NAD83) Description: The Wetlegs Cu-Ni deposit was initially drilled by Bear Creek Mining in 19581960 (referred to as the A4 grid) on the basis of electromagnetic conductors which were found to be related to graphitic portions of the footwall Virginia Formation. Some indications of Cu-Ni mineralization were found in the initial drilling and after acquiring state mineral leases in 1966 Bear Creek conducted some follow-up drilling. However, the Cu-Ni grades were low overall, and Bear Creek abandoned further mineral exploration endeavors in this area of the Complex. Exxon Minerals Corp. leased the same lands and conducted drilling programs in 1975 and 1979. They estimated that the Wetlegs deposit contained 38 million tons of material grading 0.29% Cu and 0.10% Ni using a cutoff grade of 0.15% Cu. Recent sampling for platinum group elements (PGE) indicates that there are at least three potential stratabound zones at Wetlegs (Severson and Hauck, 2003). Page 244 �A west to east traverse along the LTV railroad tracks will be conducted at this field stop. The traverse, approximately 0.5 miles long, will start at the basal contact and progress upwards through the igneous stratigraphy of the Partridge River intrusion. Outcrops of Units I and III will be viewed. A generalized geologic map of the Wetlegs area is shown in Figure 8-11. Detailed descriptions of the railroad cuts are listed below. Oxide Ultramafic Intrusion Troctolite (V) Augite Troctolite(IV) PoikiliticTroctolite (III) Melatroctolite (II) Contact Zone (I) EEl MaficVolanic H or n fe Is Virginia Formation Figure 8-11: Geology of the Partridge River Intrusion in the vicinity of the Wetlegs Cu-Ni deposit showing stops 8-3a (Erie hornfels) and 8-3b (Wetlegs deposit). Roman numerals correspond to stratigraphic units of Severson and Hauck (1990). Modified from Severson and Miller (1999). Feet (to the East) Description 0 ft. 100-110 ft. (left side of tracks) Railroad culvert – Longnose Creek Sulfide/gossan-cemented till consisting mostly of large boulders of a pyrrhotite- and graphite-rich member of the Virginia Formation (the BDD PO unit). Note the rounded granite cobbles beneath the BDD PO boulders. Drilling indicates that the BDD PO subcrops very close to this location. 337-355 ft. Very fine-grained (chilled) gabbronorite at basal contact with 60% (left side of tracks) plagioclase, 30% orthopyroxene, 5 % clinopyroxene, trace to 2 % biotite, and 3 % oxides. The outcrop contains well-assimilated “streaks” of Virginia Formation inclusions (very hard to see). 400-1025 ft. Unit I – taxitic medium- to coarse-grained ophitic augite troctolite (POcf) to (both sides of olivine gabbro (PcOf) with patches and lenses of augite-rich pegmatite. tracks) The outcrops are sulfide bearing with the sulfides unevenly distributed along patches, spots, lenses, and joint faces (at numerous orientations!). Trace amounts to 5% uralite due to pervasive deuteric alteration is present. 1245-1355 ft. Unit I – taxitic, pervasively uralitized, medium- to coarse-grained ophitic (two outcrops on olivine gabbro (PcOf) to augite troctolite (POcf) with trace to 1% sulfides. left side of tracks) Also present within these outcrops are very coarse-grained anorthosite inclusions that vary from 10 cm across to 3x5 meter blocks. The edges of the inclusions are sharp, and straight to highly lobate. Within the inclusions, the plagioclase foliation is subvertical and highly variable. Unit I – taxitic, medium- to coarse-grained ophitic augitic troctolite (POcf) 1385-1685 ft. (many outcrops on with abundant irregular pegmatitic patches and lenses. The pegmatites Page 245 �both sides tracks) of contain variable amounts of saussurized plagioclase, clinopyroxene, oxides, biotite, and uralite, with minor quartz, k-feldspar, graphic granite, and sulfides. Most of the sulfides are either within or adjacent to the pegmatites. Uralite is common throughout the outcrops as pervasive replacement products in irregular patches and along joints. 2355-2480 ft. Unit III – mottled, poikilitic/ophitic troctolite (Po(cf)) to augite troctolite (four outcrops on (Pocf). This is a major marker bed due to the presence of medium- to highboth sides of the density olivine oikocrysts up to 10 cm across. This unit is easily recognized tracks) in drill core due to the mottled texture, and the relatively finer-grained plagioclase (1-5 mm). Dunka Road and Babbitt Deposits As the bus proceeds to the next field stop, we will traverse across the southernmost limits of the Dunka Road deposit (originally drilled by United States Steel, now held by PolyMet Mining and called the NorthMet deposit) and the Babbitt deposit (originally drilled by Bear Creek and Amax, now held by Teck Cominco and called the Mesaba deposit). There is very little to see in the way of mineralized exposures for either of these deposits. However, one point to keep in mind is the immense size of these low-grade deposits. The Dunka Road/NorthMet deposit extends for about 3 miles along the road, and the Babbitt/Mesaba deposit extends for another 2.5 miles along the road (the deposits are separated by undrilled and untested area that is about 1 mile wide). STOP 8-4: Hornfels mafic volcanic inclusion and oxide ultramafic intrusions near the margin of the Partridge River Intrusion Location: Erie/LTV RR Grade approximately 2 miles west of the locked gate on the Dunka Road. Babbitt NE 7.5’ quadrangle T. 60N., R.13W., Sec 33.; 584510E, 5275715N (NAD83) Description: The Dunka Railroad hornfels is a large mafic volcanic inclusion located near the eastern margin of the Partridge River intrusion and just south of the Babbitt/Mesaba deposit. Outcrops and drill hole information indicates that the inclusion is about 900 x 1,500 meters across. The hornfels is a fine-grained, granoblastic to poikiloblastic basaltic hornfels that contains variable amounts of plagioclase, augite, hypersthene, inverted pigeonite, and olivine. Both massive and meta-amydaloidal varieties are present. In addition to the basaltic hornfels, several small bodies of late intrusive OUIs are also present in the railroad cuts. The abundance of OUIs at this locality is related to proximity to the north-trending Grano Fault, which may have served as a feeder vent for the massive sulfides at the Local Boy area of the Babbitt/Mesaba deposit. These OUIs, and a wide variety of granitic rocks, occur as lenses and bodies that cut the troctolitic rocks of the Partridge River intrusion. They are extremely common in drill holes, within a 400-580 meter wide zone, on the west side of the Grano Fault. The OUIs at this locality are characterized by medium- to coarse-grained clinopyroxenite with 75-80% augite, 0-15% olivine, <5% interstitial plagioclase, and 5-15% oxides (ilmenite is dominant). Contact relationships with the basaltic hornfels are sharp but highly irregular and lobate. Granitic dikes and veins, present within a NNE-trending zone, are also evident in the exposures. They are rarely observed in close proximity to the OUI, but where they are, the dikes crosscut the OUI. Page 246 �STOP 8-5: Cu-Ni sulfide mineralization, Spruce Road deposit, South Kawishiwi Intrusion Location: Barrow pit 200 feet west of Spruce Road (Forest Rd 181) approximately 3.5 miles north of Highway One. Bogberry Lake 7.5’ quadrangle, T.62N., R.11W., Sec 24, NE of SE of SW; 599295E, 5299010N (NAD83) Description: It all started here! Serious exploration for Cu-Ni deposits at the base of the Duluth Complex began somewhere near this site in 1948 when strongly mineralized and gossanous rocks were uncovered in an excavation into weathered gabbro rubble used to build the Spruce Road. Local prospector Fred S. Childers of Ely, MN, noted copper stains in the material and began searching the outcrops along the basal contact in the vicinity of the Kawishiwi River. Roger V. Whiteside of Duluth, MN, later joined him in further exploration. In 1951, they diamond drilled a 57 meter (188 feet) deep hole located several miles to the southwest of this site in what is now referred to as the Maturi deposit (Fig. 8-3). This hole intersected disseminated sulfides in gabbroic rock that averaged 0.36% Cu and 0.13% Ni (Watowich and others, 1981). In 1952, both Bear Creek Mining Company and the International Nickel Company (INCO) began intensive exploration efforts along a 61 km-long zone (38 miles) that coincided with the basal contact (an area stretching from south of the town of Hoyt Lakes northeastward to this site at Spruce Road). INCO eventually picked up the Childers-Whiteside properties and began drilling activities in 1954; whereas, Bear Creek concentrated most of their effort near the town of Babbitt and drilled several properties during 1957-1960. During this same period, the Minnesota Geological Survey, in cooperation with the U.S. Bureau of Mines, began an examination of the Maturi-Spruce Road area that culminated in the drilling of three holes in 1953. All of these exploration efforts indicated that large tonnage, but low grade, disseminated Cu-Ni deposits were present along the basal contact. In 1966, the Minnesota Department of Conservation adopted rules for state mineral leases. The leases were offered through the Department of Natural Resources (DNR) and were awarded to successful bidders. Since 1966, over 20 companies have been actively involved in exploration for Cu-Ni and Fe-Ti-V deposits along the basal contact of the Complex and over 1.700 holes totaling over 1.5 million feet of core have been drilled. In regards to the Spruce Road deposit, INCO drilled over 150 holes on the property at approximately 200-foot spacings. They calculated that the deposit contains 248 million tons of low-grade material averaging 0.46% Cu and 0.17% Ni. INCO collected at least two bulk samples from Spruce Road: 1) a 1,150 ton sample from several unknown small pits in 1966-67 (Watowich and others, 1981) and; 2) a 10,000 ton sample in 1974. It is uncertain if this barrow pit is one of the 1966-67 INCO bulk sample sites or if this pit was only used for road material. Rock types at this pit consist of sulfide-bearing, taxitic augite troctolite and olivine gabbro of the BH Unit (Basal Heterogeneous Unit) in the Marginal Zone of the South Kawishiwi intrusion. The BH Unit is approximately 360 meters thick (1,200 feet) at Spruce Road. Footwall rocks at Spruce Road consist dominantly of granitic rocks of the Archean Giants Range batholith; but in local areas the Biwabik Iron Formation is still preserved beneath the basal contact. At this locale, the pit is situated about 150 meters (480 feet) above the basal contact. Cu-Ni grades in the top of a nearby drill hole (34801) are 0.51-0.63% Cu and 0.15-0.17% Ni. Page 247 �Figure 8-12: Location map of Stop 8-5 at the Spruce Road Cu-Ni deposit. Two possible INCO bulk sample sites are shown (this stop included), as well as, the reclaimed location of a 10,000 ton bulk sample collected by INCO in 1974. STOP 8-6: Quarry exposure of homogeneous troctolite, South Kawishiwi Intrusion Location: Approx. 3.0 miles south of Spruce Road on Highway 1; follow old access road to abandoned quarry about 800m S of Highway 1. Bogberry Lake 7.5' quad; T.61N., R.11W., Sec 11, SE of NW; 597940E 5293550W (NAD83) Description: This abandoned quarry provides a 3-dimensional look at the remarkable homogeneity of troctolitic cumulates, which compose most the South Kawishiwi (and Partridge River) intrusion. The average rock type is a medium-grained, moderately foliated, augite-poor troctolite. Moderately aligned, cumulus plagioclase makes up 70-75% of the rock, though locally up to 80% plagioclase occurs and the rock would be classified as a leucotroctolite. Subhedral granular olivine composes 20-25% of the average rock, though here too, local olivine enrichment Page 248 �of up to 30% occurs in thin layers parallel to foliation. Interstitial augite and Fe-Ti oxide compose 3-7% of the rock. The textural and mineralogic homogeneity of the troctolitic cumulates composing the middle to upper sections of the SKI (and PRI), and displayed here, are matched by limited variation in mineral compositions of plagioclase, olivine and pyroxene. Phinney (1969) reported a range of plagioclase and olivine compositions in troctolitic units of SKI (excluding the marginal rocks) of An57-72 and Fo50-63, respectively. Similar results are displayed by unpublished microprobe data from troctolitic rocks from the Babbitt NE quadrangle, which indicate olivine compositions of Fo50-65 and augite compositions of En68-75. Many see this lithologic homogeneity and lack of significant cryptic variation as indicating that the SKI and PRI magma systems were open to frequent recharge of relatively primitive magma (Severson, 1994; Lee and Ripley, 1996; Miller and Ripley, 1996). New recharge events are commonly marked by the intermittent occurrences of olivine-rich (melatroctolite) intervals separating otherwise subtle differences in troctolite texture and mode (Severson, 1994). These melatroctolite intervals show a significant increase in An, Fo, and En compositions consistent with the recharge of a more primitive magma. An alternative explanation has been put forth by Chalokwu (Chalokwu and Grant, 1990; Chalokwu and others, 1993). He has suggested that this homogeneity is indicative of single stage emplacement of a viscous semi-crystallized olivine-plagioclase mush that experienced little in situ differentiation. However, his sampling routinely overlooked the melatroctolite intervals. STOP 8-7: Anorthositic inclusion? in troctolite, South Kawishiwi Intrusion Location: Road cut on Tomahawk Rd. about 7.5 miles west of Highway 1; Babbitt NE 7.5' quad; T.60N., R.11W., Sec 18, center of NE; 592355E 5282430W (NAD83) Description: Scattered throughout the entire sequence of monotonous troctolitic cumulates are masses of coarse- to medium-grained troctolitic anorthosite to olivine gabbroic anorthosite. In recent mapping of the Babbitt NE quadrangle (Miller, Severson and Foose, 2002), we (Miller and Severson) have consistently interpreted these anorthositic masses as inclusions. We believe that they represent pieces of the anorthositic series roof pendant that was upwardly displaced and dissaggregated with successive recharge events into the SKI magma chamber. In an earlier detailed field mapping of this mixed anorthosite-troctolite terrane, mainly to the northeast of here, Foose and Cooper (1978, 1981) interpreted many of these anorthositic bodies as comagmatic layers within the troctolite. They saw their elongate and conformable relationship with the internal structure of the enclosing troctolite as evidence that most of these bodies are lensoidal plagioclase cumulate intervals intermittently layered within the troctolite. They used the apparent offset of these layers as evidence for a complex fault pattern (Fig. 8-13). This stop along the Tomahawk road will examine one such example of a troctolitic anorthosite body within leucotroctolitic cumulates. This interval of the SKI is particularly rich in anorthosite bodies. The relationships displayed here are difficult to reconcile with either the Miller and others (2002) or the Foose and Cooper (1978,1981) interpretations. An irregularly shaped anorthosite block projects into the leucotroctolite and appears to cut across the foliation in the enclosing leucotroctolite (Fig. 8-13). Page 249 �Figure 8-13. Geologic map of the Harris Lake area, which is northeast of Stop 8-7 from by Foose and Cooper (1981). Map units are: 1 - augite troctolite, 2 - troctolite, 2a - ophitic augite troctolite layer, 2b - traceable anorthosite layer, 3 - anorthosite haAn.csgaLx ,!aa 44/ •i Page 250 Figure 8-14. Photo of road cut at Stop 8-7 showing an irregularly shaped troctolitic anorthosite mass in leucotroctolite. Foliation in the leucotroctolite is approximately parallel to the hammer. �STOP 8-8: Grano fault zone, Partridge River Intrusion (optional) Location: Small exposure north of Forest Rd 113, 100m east ; Babbitt SE 7.5' quad; T.59N., R.12W., Sec 16, NW of NW; 584486E 5272519W (NAD83) Description: The Grano fault is a major N-S trending structure that was first recognized in the subsurface by compiling exploration drill hole data related to the Babbitt and Serpentine deposit areas (Severson, 1994). As shown in Fig. 8-15, the fault creates a significant (~60m) displacement (down to the east) of the Duluth Complex footwall and affects both the Partridge River and the South Kawishiwi Intrusions. Drill holes into the fault zone in the southeastern portion of the Babbitt deposit shows that to the immediate west of the fault is a 400-580 meterwide (up to 2,000 feet) zone that contains abundant subvertical lenses of granophyre and OUI. To the north along the fault trace the zone with late subvertical lenses pinches down appreciably (not present at Serpentine) and the amount of motion along the fault diminishes as well, indicating that the fault is a “scissors-type” fault. The massive sulfides of the Local Boy ore zone of the Babbitt deposit have been inferred to have been “vented” from the Grano fault. The surface expression of the fault is a prominent linear valley that can be traced over 10 kilometers south from the basal contact (Fig. 8-16). This stop is in the axis of that valley. The rock types exposed on the margins of this valley are a complex mixture of augite leucotroctolite with poikilitic olivine, ophitic augite troctolite to olivine gabbro, poikilitic olivine gabbroic anorthosite, and locally granophyric gabbroic pegmatite. This assemblage represents a complex mix of anorthositic series inclusions and unmineralized marginal zone rocks of the Partridge River Intrusion. The granophyric gabbroic pegmatite is evidently related to a late, volatile-rich mafic magma intruded into Grano fault zone. In the small pavement exposure near the road, a medium-grained poikilitic augite leucotroctolite is cut by gabbroic pegmatite. Figure 8-15. Grayscale image of the footwall topography of the Partridge River and South Kawishiwi intrusions near their contact showing the subsurface expression of the Grano Fault (image generated by Dean Peterson) Page 251 �I•i-_• -- '-.• 2000 m Figure 8-16. Grayscale image of surface topography of the Partridge River and South Kawishiwi intrusions near their contact showing the surface expression of the Grano Fault. Lines show geologic contacts and faults taken from MGS Misc. Map M-119. SKI - South Kawishiwi intrusion; PRI - Partridge River intrusion; AS - anorthositic series; PG - "Powerline Gabbro" of Bonnichsen (1974); HB hornfels basalt; BIF - Biwabik Iron-formation; VF - Virginia Formation. STOP 8-9: Volcanic hornfels and plagioclase-phyric leucogabbro, Partridge River Intrusion Location: Roadcuts along Erie/LTV RR grade approx 1km west of crossing by Forest Rd 113 Babbitt SW 7.5' quad; T.59N., R.12W., Sec 18, SW of SW; 581500E 5271020W (NAD83) Description: Scattered throughout the homogeneous troctolites of the Partridge River intrusion are kilometer-scale inclusions that are composed of various mixtures of anorthositic rocks, volcanic hornfels and oxide gabbro (Fig. 8-2). This assemblage is thought to represent initial intrusions of anorthositic series magmas into the base of the volcanic pile during the main stage of Duluth Complex magmatism (~1099 Ma). The anorthositic rock lithologies, which dominate the Anorthositic Series throughout the Duluth Complex, are interpreted to have crystallized from plagioclase crystal mushes (evolved basaltic melts laden with suspended plagioclase crystals). However, in this area, these anorthositic rocks are locally intruded by oxide gabbro. Bonnichsen (1974b) speculated that the oxide gabbro, which he termed the Powerline Gabbro, may be a differentiate of the Partridge River intrusion. However, detailed mapping in the Allen quadrangle to the west (Severson and Miller, 1999) demonstrated that this gabbro is intruded by troctolitic rocks of the PRI. We consider the gabbro to be a plagioclase-poor component of the Anorthositic Series. In a roadside exposure on the forest road, we can see a typical example of the "Powerline Gabbro". It is a coarse-grained, non-foliated, subophitic olivine oxide gabbro composed of 55% plagioclase, 20% granular olivine, 15% subophitic augite, 10%anhedral Fe-Ti oxide, and trace amounts of hornblende and biotite. The mafic phases locally show incipient granoblastic Page 252 �recrystallization textures and plagioclase and augite display moderate clouding by the exsolution of oxide needles. Both features are indicative of thermal metamorphism. About 200 m through the woods to the north of the forest road, we come upon the Erie/LTV railroad grade. Exposed along a 500m-long road cut on both sides of the grade is a complex mixture of two general rock types. Most of the exposure is a very fine-grained, dense mafic volcanic hornfels that is sparsely plagioclase porphyritic. In thin section, the hornfels displays a perfect granoblastic texture (Fig. 8-17). In sharp to abrupt contact with this volcanic hornfels is a medium grained plagioclase porphyritic leucogabbro. In thin section (Fig. 8-17), the leucogabbro is composed of strongly zoned plagioclase phenocrysts, which show deep clouding by oxide needles, in a matrix of granoblastic-textured oxide melagabbro (40%Pl, 30% Aug, 30% Fe-Ti Ox). At the contacts between the hornfels and the leucogabbro (best seen on the upper surface of the northern roadcut), the plagioclase phenocrysts in the leucogabbro show a strong alignment parallel to a steep contact with the hornfels. One possible interpretation of this leucogabbro is that it represents a chilled intrusion of plagioclase crystal mush that was emplaced into the volcanic pile early in the emplacement of the anorthositic series. Subsequent intrusions of more voluminous anorthositic series magma and later layered series magma forming the Partridge River intrusion produced the strong thermal metamorphism now evident in both the mafic volcanic and the porphyritic leucogabbro. Figure 8-17. Photomicrographs of mafic volcanic hornfels and plagioclase-phyric leucogabbro observed at Stop 8-9. Page 253 �STOP 8-10: Cross-bedded Colvin Creek hornfels, Partridge River Intrusion Location: Approximately 0.5 miles south of where Forest Rd 113 crosses the south branch of the Partridge River and 520 meters (1700 feet) east along a flagged trail. Babbitt SW 7.5' quadrangle; T.59N., R.13W., Sec 25, NE of SW of SE; 580255E 5267943N (NAD83) Description: Magnetic basalt hornfels inclusions (most notably the Colvin Creek hornfels) within the Duluth Complex were first observed by Bonnichsen (1972) and first described in some detail by Tyson (1976). According to Tyson, the magnetic basalt flows of the Colvin Creek hornfels are characterized by fine-grained granoblastic rocks composed of plagioclase, augite, and magnetite with relict plagioclase- and augite-filled amygdules. Tyson (1976) noted a major difference between the magnetic basalts and the more common non-magnetic basalt inclusions (as in stops 8-3a and 8-4) and theorized that the magnetic basalts were derived from weathered and oxidized basalt flows that were subsequently metamorphosed by the Complex. Tyson’s work was of a reconnaissance nature and much more detailed work has since been completed on the Northern Colvin Creek Body by Patelke (1996). The Northern Colvin Creek Body is a large inclusion (2,500 X 800 meters), associated with a magnetic high, that has been rotated to near vertical and exhibits excellent stratigraphic tops to the northwest. Patelke (1996) subdivided the northern Colvin Creek Body into five mappeable units including two metavolcanic units, two intrusive gabbroic sill units, and a cross-bedded sedimentary unit. Within the metavolcanic units, each consisting of multiple basalt flows, Patelke (1996) identified several volcanic features that include: pipe amygdules, sheeted amygdules, and local convoluted flow bases. Figure 8-18. Photograph of cross-bedded “microgabbroic” sediment (X-BDD Unit) of the Northern Colvin Creek Body. Enigmatic cross-bedded sedimentary rocks, with near-vertical dips, were first discovered in the Northern Colvin Creek hornfels and reported by Severson and Hauck (1990). They mapped a X-BDD Unit that exhibited a strike length of over 1,600 meters and an exposed thickness of over 240 meters. Overall the cross-bedded rocks are gabbroic in composition and consist of very finegrained granoblastic concentrations of plagioclase, diopsidic augite, and magnetite with lessor amounts of ilmenite, orthopyroxene, and poikiloblastic pyroxene – the rocks do NOT contain Page 254 �quartz or biotite, nor a basal conglomeratic unit where it overlies magnetic basalt (Patelke, 1996). Because sedimentary-like features were found in the cross-bedded rocks, and in the overlying gabbroic rocks (both with near-vertical dips), Severson and Hauck (1990) postulated that the cross-bedded rocks were deposited via magmatic density currents (a concept they no longer believe in). More recent studies by Patelke (1996) suggests that these cross-bedded rocks were sedimentary and were most likely deposited in a restricted basin as an eolian sediment that was derived from a strictly basaltic terrain (thus no quartz). Whatever their origin, these rocks exhibit beautiful sedimentary structures that include: bedding, cross-bedding, density-graded modal layering, and scour and fill structures (Patelke, 1996). Patelke (1996) suggests that the crossbedded rocks of the Colvin Creek hornfels are similar to a thin sandstone unit near Phantom Lake, north of Two Harbors, MN. However, he also cautions that neither of these sedimentary units are analogous to any of the typical interflow sandstones of the North Shore Volcanic Group as described by Jirsa (1984). Cross-bedded sediments with a gabbroic composition have been found at six locations within the Duluth Complex (most recently shown on a map in Severson, 1995; and in Severson and Miller, 1999). The last stop of this field trip is at one of the more “easily” accessible crossbedded localities. At this locale, the cross-bedded sediments exhibit shallow dips to the northeast and only about 10 feet of stratigraphic section are exposed. The best exposures are present in the northern Colvin Creek Body which is located about two miles to the west of this stop in sections 27, 33, and 34, T.59N., R.13W. A more complete description of the units exposed at the northern body, and a thorough examination of their origin, can be found in Patelke (1996). Figure 8-19: Scanned polished thin section image and photomicrographs of X-BDD Unit of the Northern Colvin Creek Body. Page 255 �References Allen, D.J., Hinze, W.J., Dickas, A.B., and Mudrey, M.G., Jr., 1997, Integrated geophysical modeling of the North American Midcontinent Rift System: New interpretations for western Lake Superior, northwestern Wisconsin, and eastern Minnesota. In: Ojakangas, R.J., Dickas, A.B., Green, J.C., (eds.) Middle Proterozoic to Cambrian Rifting, Central North America: Geological Society of America Special Paper 312, p.47-72. Bonnichsen, B., 1972, Southern part of the Duluth Complex. In: Sims, P.K. & Morey, G.B. (eds.) Geology of Minnesota - A centennial volume. Minnesota Geological Survey, p. 361-388 Bonnichsen, B., 1974a. Copper and nickel resources in the Duluth Complex, northeastern Minnesota. Minnesota Geological Survey Information Circular 10, 24p. Bonnichsen, B., 1974b. Geology of the Ely-Hoyt Lakes district, northeastern Minnesota. Unpublished report by the Minnesota Geological Survey for the Minnesota Department of Natural Resources, 30 p. Bonnichsen, B., Fukui, L.M., and Chang, L.L.Y., 1980, Geologic setting, mineralogy, and geochemistry of magmatic sulfides, South Kawishiwi Intrusion, Duluth Complex, Minnesota: Proceedings, IAGOD Symposium, 5th Stuttgart, Germany, p. 545-565. Boudreau, A.E., and McCallum, I.S., 1992, Concentration of platinum group-elements by magmatic fluids in layered intrusions: Economic Geology, v. 87, p.1830-1848 Chalokwu, C.I. & Grant, N.K., 1990. Petrology of the Partridge River intrusion, Duluth Complex, Minnesota: I. Relationships between mineral compositions, density, and trapped liquid abundance. J. Petrology 31, 265-93. Chalokwu, C.I. & Grant, N.K., Ariskin, A.A., & Barmina, G.S., 1993., Simulation of primary phase relations and mineral compositions in the Partridge River intrusion, Duluth Complex, Minnesota: implications for the parent magma composition. Contrib. Mineral. Petrol. 114, 539-49. Chandler, V.W., 1990, Geologic interpretation of gravity and magnetic data over the central part of the Duluth Complex, northeastern Minnesota. Economic Geology, v. 85, p. 816-829 Foose, M.P., 1984, Drill logs and correlations within part of the South Kawishiwi intrusion, Duluth, Minnesota. U.S. Geological Survey Open-file Report 84-14, 230p. Foose, M.P., and Cooper, R.W., 1978, Preliminary geologic report on the Harris Lake area, northeastern Minnesota. U.S. Geological Survey Open-file Report 78-385, 24pp. with map, scale 1:12,000 Foose, M.P., and Cooper, R.W., 1981, Faulting and fracturing in part of the Duluth Complex, northeastern Minnesota. Canadian Journal of Earth Science, v. 18, p. 810-814. Geerts, S.D., 1991. Geology, stratigraphy, and mineralization of the Dunka Road Cu-Ni prospect, northeastern Minnesota. Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report, NRRI/TR-91-14, 63p. Geerts, S.D., 1994, Petrography and geochemistry of a platinum group element-bearing horizon in the Dunka Road prospect, (Keweenawan) Duluth Complex, northeastern Minnesota: Unpubl. M.S. thesis, Univ. Minn., Duluth, 155 p. Green, J.C., Phinney, W.C., & Weiblen, P.W., 1966. Gabbro Lake quadrangle, Lake County, Minnesota. Minnesota Geological Survey Miscellaneous Map M-2, scale 1:24,000 Grout, F.F., 1918, Internal structures of igneous rocks; their significance and origin with special reference to the Duluth Gabbro. Journal of Geology 26, 439-458 Hauck, S.A., Miller, J.D., Jr., Severson, M.J., in prep., Petrographic, geochemical, and oxide, sulfide, and platinum-group mineral relationships between the PEG and U3 Units, Birch Lake area, South Kawishiwi intrusion, Duluth Complex, Minnesota: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report. Jirsa, M. A., 1984, Interflow sedimentary rocks in the Keweenawan North Shore Volcanic Group, northeastern Minnesota. Minnesota Geological Survey Report of Investigations 30, 20 pp. Kilburg, J.A., 1972, Petrology, structure, and correlation of the Upper Precambrian Ely's Peak basalt. Unpublished M.S. thesis, University of Minnesota, Duluth, 97 p. Lee, I., and Ripley, E.M., 1996, Mineralogic and stable isotopic studies of the South Kawishiwi intrusion, Spruce Road Area, Duluth Complex, Minnesota. Journal of Petrology, v. 37, p. 1437-1461. Page 256 �Linscheid, E.K., 1991, The petrology of the Longnose peridotite deposit and its relationship to the Duluth Complex: Unpubl. M.S. thesis, Univ. Minn., Duluth, 121 p. Listerud, W.H., and Meineke, D.G.., 1977, Mineral resources of a portion of the Duluth Complex and adjacent rocks in St. Louis and Lake Counties, northeastern Minnesota: Minnesota Department of Natural Resources, Division of Minerals, Report 93, 74p. Miller, J.D., Jr., and Ripley, E.M., 1996, Layered intrusions of the Duluth Complex, Minnesota, USA. In Cawthorne, R.G., ed., Layered Intrusions: Amsterdam, Elsevier Science, p. 257-301. Miller, J.D., Jr. and Weiblen, P.W., 1990, Anorthositic rocks of the Duluth Complex: Examples of rocks formed from plagioclase crystal mush. Journal of Petrology 31, p. 295-339. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Minnesota Geological Survey Miscellaneous Map Series, M-119, scale 1:200,000 Miller, J.D., Jr., Severson, M.J., and Foose, M.P., 2002, Bedrock geologic map of the Babbitt NE 7.5' quadrangle, St. Louis and Lake Counties, Minnesota. Minnesota Geological Survey Open-file Map, scale 1:24,000 Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, 207 p. Naldrett, A.J., 1997, Key factors in the genesis of Noril’sk, Sudbury, Jinchuan, Voisey’s Bay, and other world class Ni-Cu-PGE deposits: implications for exploration: Australian Journal of Earth Sciences, v. 44, p. 283-315. Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: geochonological insights to physical, petrogenetic, paleomagnetic and tectono-magmatic processes associated with the 1.1 Ga Midcontinent rift system: Journal of Geophysical Research, v. 98, no.B8, p. 13,997-14,013. Patelke, R.L., 1996, The Colvin Creek body, a metavolcanic and metasedimentary mafic inclusion in the Keweenawan Duluth Complex, northeastern Minnesota: Unpublished M.S. thesis, University of Minnesota, Duluth, 232 p. Peterson, D.M., 2001, Development of a conceptual model of Cu-Ni-PGE mineralization in a portion of the South Kawishiwi Intrusion, Duluth Complex, Minnesota: Laurentian University – Society of Economic Geologists, Second Annual PGE Workshop, Sudbury, Ontario. Phinney, W.C., 1969, The Duluth Complex in the Gabbro Lake quadrangle, Minnesota. Minnesota Geological Survey Report of Investigation 9, 20 p. Ripley, E.M., 1986, Origin and concentration mechanisms of copper and nickel in Duluth Complex sulfide zones – a dilemma: Economic Geology, v. 81, p. 974-978. Sassani, D.C., 1992, Petrologic and thermodynamic investigation of the aqueous transport of platinumgroup elements during alteration of mafic intrusive rocks: Unpubl. Ph.D. thesis, Washington Univ., St. Louis, MO, 2 vols., 952 p. Sawyer, E.W., 1996, Melt segregation and magma flow in migmatite: Implications for the generation of granite magmas: Earth Sciences, v.87, p. 85-94. Severson, M.J., 1991, Geology, mineralization, and geostatistics of the Minnamax/Babbitt Cu-Ni deposit (Local Boy area), Minnesota, Part I: Geology: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/TR-91/13a, 96 p. (with plates) Severson, M.J., 1994, Igneous stratigraphy of the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/TR 93/34, 210 p. (with plates) Severson, M.J., 1995, Geology of the southern portion of the Duluth Complex: Natural Resources Research Institute, University of Minnesota-Duluth, Technical Report NRRI/TR 95/26, 185p. (with plates) Severson, M.J., and Barnes, R.J., 1991, Geology, mineralization, and geostatistics of the Minnamax/Babbitt Cu-Ni deposit (Local Boy area), Minnesota, Part II: Mineralization and geostatistics: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/TR-91/13b, 221 p. Page 257 �Severson, M.J., and Hauck, S.A., 1990, Geology, geochemistry, and stratigraphy of a portion of the Partridge River intrusion: Natural Resources Research Institute, University of Minnesota-Duluth, Technical Report, NRRI/GMIN-TR-89-11, 236p. (with plates). Severson, M.J., and Hauck, S.A., 1997, Igneous stratigraphy and mineralization in the basal portion of the Partridge River intrusion, Duluth Complex, Allen Quadrangle, Minnesota: Natural Resources Research Institute, Univ. Minn., Duluth, Tech. Rept. NRRI/TR-97/19, 102 p. Severson, M.J., and Hauck, S.A., 2003, Platinum group elements (PGEs) and platinum group minerals (PGMs) in the Duluth Complex: Natural Resources Research Institute, Univ. Minn., Duluth, Tech. Rept. NRRI/TR-2003/37, 296p. Severson, M.J., and Miller, J.D., Jr., 1999, Bedrock geologic map of Allen quadrangle, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-91, scale 1:24,000 Severson, M.J., and Zanko, L.M., in prep., The Babbitt Cu-Ni deposit, Part D: Footwall hosted massive sulfide at the Local Boy ore zone (revisited): Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report Severson, M.J., Patelke, R.L., Hauck, S.A., and Zanko, L.M., 1994, The Babbit copper-nickel deposit, Part B: structural datums: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report, NRRI/TR-94/21b, 48p. (with plates) Taylor, R. B., 1964. Geology of the Duluth Gabbro Complex near Duluth, Minnesota. Minnesota Geological Survey Bulletin 44, 63 pp. Theriault, R.D., Barnes, S.-J., and Severson, M.J., 1997, The influence of country-rock assimilation and silicate to sulfide ratios (R factor) on the genesis of the Dunka Road Cu-Ni-platinum-group element deposit, Duluth Complex, Minnesota: Canadian Journal of Earth Science, v. 34, p. 375-389. Theriault, R.D., Barnes, S.-J., and Severson, M.J., 2000, Origin of Cu-Ni-PGE sulfide mineralization in the Partridge River intrusion, Duluth Complex, Minnesota: Economic Geology, v. 95, p. 929-943. Tyson, R.M., 1976, The mineralogy and petrology of the Partridge River troctolite in the Babbitt-Hoyt Lakes region of the Duluth Complex, northeastern Minnesota. Unpublished Ph.D. dissertation, Cornell University, 179p. Watowich, S.N., Malcolm, J.B., and Parker, P.D., 1981, A review of the Duluth Gabbro Complex as a domestic source of critical and strategic metals. Society of Mining Engineers of AIME, preprint 81351, 9 p. Zanko, L.M., Severson, M.J., and Ripley, E.M., 1994, Geology and mineralization of the Serpentine copper-nickel deposit, Duluth Complex, Minnesota. Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report, NRRI/GMIN-TR-93-52, 90p. Page 258 � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2004 Description An account of the resource Institute on Lake Superior Geology. Duluth, Minnesota. May 4-9, 2004. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2004 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/22858aa3bc9f4b3560ee5a63f7e169ab.pdf ad8628deb5689276c27baa82945c876c PDF Text Text 51st ANNUAL MEETING Nipigon, Ontario - May 24-28, 2005 INSTITUTE ON LAKE SUPERIOR GEOLOGY e- <(j. . I Part 1 – Proceedings and Abstracts 51st ILSG Nipigon 2005 wwwIakesuperiorgeology.org �51st ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY May 24-28, 2005 Nipigon, Ontario HOSTED BY: Mark Smyk and Pete Hollings Co-Chairs Ontario Geological Survey and Lakehead University Proceedings - Volume 51 Part 1 – Proceedings and Abstracts Edited by Mike Easton (Ontario Geological Survey) & Pete Hollings (Lakehead University) Cover Photos: Left - pahoehoe texture in basalts of the Osler volcanic group, Wilson Island, Middle - diabase sills on the shores of Lake Nipigon, Right - No. 1 Shaft headframe, MacLeod-Cockshutt Mine, Geraldton. �Proceedings of the 51st ILSG Annual Meeting - Part 1 51ST INSTITUTE ON LAKE SUPERIOR GEOLOGY VOLUME 51 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: GEOLOGY AND FOLD MINERALISATION OF THE BEARDMORE-GERALDTON GREENSTONE BELT TRIP 2: QUATERNARY GEOLOGY OF THE BEARDMORE – NIPIGON AREA TRIPS 3 & 6: A STRATIGRAPHIC TRANSECT ACROSS THE NORTHERN FLANK OF THE MIDCONTINENT RIFT NEAR ROSSPORT TRIP 4: GEOLOGY AND RARE ELEMENT PEGMATITES OF THE QUETICO SUBPROVINCE NEAR NIPIGON TRIP 5: GEOLOGY OF THE BLACK STURGEON AREA Reference to material in Part 1 should follow the example below: Albers, P.B., and Miller, J.D.., 2005. The Geology and Petrology of the Leveaux Porphyritic Diorite, Cook County, MN: Investigating Possible Magmatic Relationships to the Anorthositic Series of the Duluth Complex. In; Easton, M. and Hollings, P. (Eds.), Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Part 1 - Proceedings and Abstracts, v.51, part 1, 3-4. Published by the 51st Institute on Lake Superior Geology and distributed by the ILSG Secretary: Pete Hollings - ILSG Secretary Department of Geology Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Email: peter.hollings@lakeheadu.ca ILSG website: www.lakesuperiorgeology.org ISSN 1042-9964 -i- �Proceedings of the 51st ILSG Annual Meeting - Part 1 Table of Contents Institutes on Lake Superior Geology, 1955-2005 ............................................................. iii Constitution of the Institute on Lake Superior Geology .....................................................v By-Laws of the Institute on Lake Superior Geology ....................................................... vii Goldich Medal Guidelines .............................................................................................. viii Goldich Medallists ............................................................................................................ ix Goldich Medal Committee .................................................................................................x Citation for Goldich Medal Recipient.................................................................................x Eisenbrey Student Travel Awards ..................................................................................... xi Eisenbrey Student Travel Award Application .................................................................. xii Student Paper Awards ..................................................................................................... xiii Student Paper Awards Committee................................................................................... xiii Session Chairs ................................................................................................................. xiii Membership Criteria for the Institute on Lake Superior Geology .................................. xiv Board of Directors.............................................................................................................xv Local Committee...............................................................................................................xv Banquet Speaker ...............................................................................................................xv Report of the Chair of the 50th Annual Meeting ........................................................... xvi Acknowledgements ....................................................................................................... xviii Program ........................................................................................................................... xix Abstracts .............................................................................................................................1 Author Index .....................................................................................................................70 - ii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Institutes on Lake Superior Geology, 1955-2005 # Date Place Chairs 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz & C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims & R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley & E. Mercy 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray 28 1982 International Falls, Minnesota D.L. Southwick 29 1983 Houghton, Michigan T.J. Bornhorst 30 1984 Wausau, Wisconsin G.L. LaBerge 31 1985 Kenora, Ontario C.E. Blackburn 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey & R.P. Sage - iii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 # Date Place Chairs 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage & W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller & M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst & R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin & P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey & Jr., B.A. Brown 48 2002 Kenora, Ontario P. Hinz & R.C. Beard 49 2003 Iron Mountain, Michigan L. Woodruff & W.F. Cannon 50 2004 Duluth, Minnesota S. Hauck & M. Severson 51 2005 Nipigon, Ontario M. Smyk & P. Hollings - iv - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Constitution of the Institute on Lake Superior Geology (Last amended by the Board—May 6, 2004) Article I - Name The name of the organization shall be the “Institute on Lake Superior Geology”. Article II - Objectives The objectives of this organization are: A. To provide a means whereby geologists in the Great Lakes region may exchange ideas and scientific data. B. To promote better understanding of the geology of the Lake Superior region. C. To plan and conduct geological field trips. Article III - Status No part of the income of the organization shall insure to the benefit of any member or individual. In the event of dissolution, the assets of the organization shall be distributed to _________ (some tax free organization). (To avoid Federal and State income taxes, the organization should be not only “scientific” or “educational”, but also “non-profit”) Minn. Stat. Anno. 290.01, subd. 4 Minn. Stat. Anno. 290.05(9) 1954 Internal Revenue Code s.501(c)(3) Article IV - Membership The membership of the organization shall consist of persons who have registered for an annual meeting within the past three years, and those who indicate interest in being a member according to guidelines approved by the Board of Directors. Article V - Meetings The organization shall meet once a year. The place and exact date of each meeting will be designated by the Board of Directors. Article VI - Directors The Board of Directors shall consist of the Chair, Secretary, Treasurer, and the last three past Chairs; but if the board should at any time consist of fewer than six persons, by reason of unwillingness or inability of any of the above persons to serve as directors, the vacancies on the board may be filled by the Chair so as to bring the membership of the board to six members. Article VII - Officers The officers of this organization shall be a Chair, a Secretary and a Treasurer. A. The Chair shall be elected each year by the Board of Directors, who shall give due consideration to the wishes of any group that may be promoting the next annual meeting. His/her term of office as Chair will terminate at the close of the annual meeting over which he/she presides, or when his/her successor shall have been appointed. He/she will then serve for a period of three years as a member of the Board of Directors. -v- �Proceedings of the 51st ILSG Annual Meeting - Part 1 B. The Secretary shall be elected at the annual meeting. His/her term of office shall be four years, or until his/her successor shall have been appointed. C. The Treasurer shall be elected at the annual meeting. His/her term of office shall be four years, or until his/her successor shall have been appointed. The terms of the Secretary and Treasurer shall be staggered so that there will always be a two year overlap between the two. Article VIII - Amendments This constitution may be amended by a majority vote (majority of those voting) of the membership of the organization. - vi - �Proceedings of the 51st ILSG Annual Meeting - Part 1 By-Laws of the Institute on Lake Superior Geology (Last amended by the Board—May 6, 2004) I. Duties of the Officers and Directors A. It shall be the duty of the Annual Chairman to: 1. Preside at the annual meeting. 2. Appoint all committees needed for the organization of the annual meeting. 3. Assume complete responsibility for the organization and financing of the annual meeting over which he/she presides. B. It shall be the duty of the Secretary to: 1. Keep accurate attendance records of all annual meetings. 2. Keep accurate records of all meetings of, and correspondence between, the Board of Directors. 3. Maintain an up-to-date mailing list 4. Act as the first point of contact for all enquires about the Institute 5. Respond to requests for back issues of the Proceedings of the Institute C. It shall be the duty of the Treasurer to: 1. Hold all funds that may accrue as profits from annual meetings or field trips and to make these funds available for the organization and operation of future meetings as required. 2. Store the Goldich medals and each year ensure one is engraved with the name of that years winner D. It shall be the duty of the Board of Directors to plan locations of annual meetings and to advise on the organization and financing of all meetings. II. Duties and Expenses A. Regular membership dues of $5.00 or less on an annual basis shall be assessed each member as determined by the Board of Directors.. B. Registration fees for the annual meetings shall be determined by the Chair in consultation with the Board of Directors. The registration fees can include expenses to cover operations outside of the annual meeting as determined by the Board of Directors. It is strongly recommended that registration fees be kept at a minimum to encourage attendance of students. III. Rules of Order The rules contained in Robert’s Rules of Order shall govern this organization in all cases to which they are applicable. IV. Amendments These by-laws may be amended by a majority vote (majority of those voting) of the membership of the organization; provided that such modifications shall not conflict with the constitution as presently adopted or subsequently amended. - vii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Goldich Medal Guidelines (Adopted by the Board of Directors, 1981; amended 1999) Preamble The Institute on Lake Superior Geology was born in 1955, as documented by the fact that the 27th annual meeting was held in 1981. The Institute’s continuing objectives are to deal with those aspects of geology that are related geographically to Lake Superior; to encourage the discussion of subjects and sponsoring field trips that will bring together geologists from academia, government surveys, and industry; and to maintain an informal but highly effective mode of operation. During the course of its existence, the membership of the Institute (that is, those geologists who indicate an interest in the objectives of the ILSG by attending) has become aware of the fact that certain of their colleagues have made particularly noteworthy and meritorious contributions to the understanding of Lake Superior geology and mineral deposits. The first award was made by ILSG to Sam Goldich in 1979 for his many contributions to the geology of the region extending over about 50 years. Subsequent medallists and this year’s recipient are listed in the table below. Award Guidelines 1) The medal shall be awarded annually by the ILSG Board of Directors to a geologist whose name is associated with a substantial interest in, and contribution to, the geology of the Lake Superior region. 2) The Board of Directors shall appoint the Goldich Medal Committee. The initial appointment will be of three members, one to serve for three years, one for two years, and one for one year. The member with the briefest incumbency shall be chair of the Nominating Committee. After the first year, the Board of Directors shall appoint at each spring meeting one new member who will serve for three years. In his/her third year this member shall be the chair. The Committee membership should reflect the main fields of interest and geographic distribution of ILSG membership. The out-going, senior member of the Board of Directors shall act as liaison between the Board and the Committee for a period of one year. 3) By the end of November, the Goldich Medal Committee shall make its recommendation to the Chair of the Board of Directors, who will then inform the Board of the nominee. 4) The Board of Directors normally will accept the nominee of the Committee, inform the medallist, and have one medal engraved appropriately for presentation at the next meeting of the Institute. 5) It is recommended that the Institute set aside annually from whatever sources, such funds as will be required to support the continuing costs of this award. Nominating Procedures 1) The deadline for nominations is November 1. Nominations shall be taken at any time by the Goldich Medal Committee. Committee members may themselves nominate candidates; however, Board members may not solicit for or support individual nominees. 2) Nominations must be in writing and supported by appropriate documentation such as letters of recommendation, lists of publications, curriculum vita’s, and evidence of contributions to Lake Superior geology and to the Institute. 3) Nominations are not restricted to Institute attendees, but are open to anyone who has worked on and contributed to the understanding of Lake Superior geology. - viii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Selection Guidelines 1) Nominees are to be evaluated on the basis of their contributions to Lake Superior geology (sensu lato) including: a) importance of relevant publications; b) promotion of discovery and utilization of natural resources; c) contributions to understanding of the natural history and environment of the region; d) generation of new ideas and concepts; and e) contributions to the training and education of geoscientists and the public. 2) Nominees are to be evaluated on their contributions to the Institute as demonstrated by attendance at Institute meetings, presentation of talks and posters, and service on Institute boards, committees, and field trips. 3) The relative weights given to each of the foregoing criteria must remain flexible and at the discretion of the Committee members. 4) There are several points to be considered by the Goldich Medal Committee: a) An attempt should be made to maintain a balance of medal recipients from each of the three estates— industry, academia, and government. b) It must be noted that industry geoscientists are at a disadvantage in that much of their work in not published. 5) Lake Superior has two sides, one the U.S., and the other Canada. This is undoubtedly one of the Institute’s great strengths and should be nurtured by equitable recognition of excellence in both countries. Goldich Medalists 1979 Samuel S. Goldich 1992 William F. Cannon 1980 not awarded 1993 Donald W. Davis 1981 Carl E. Dutton, Jr. 1994 Cedric Iverson 1982 Ralph W. Marsden 1995 Gene LaBerge 1983 Burton Boyum 1996 David L. Southwick 1984 Richard W. Ojakangas 1997 Ronald P. Sage 1985 Paul K. Sims 1998 Zell Peterman 1986 G.B. Morey 1999 Tsu-Ming Han 1987 Henry H. Halls 2000 John C. Green 1988 Walter S. White 2001 John S. Klasner 1989 Jorma Kalliokoski 2002 Ernest K. Lehmann 1990 Kenneth C. Card 2003 Klaus J. Schulz 1991 William Hinze 2004 Paul Wieblen 2005 Goldich Medal Recipient Mark Smyk Ontario Geological Survey, Thunder Bay, Ontario - ix - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Goldich Medal Committee Serving through the meeting year shown in parentheses David Meineke (2005) Meriden Engineering, Hibbing, Minnesota George Hudak (2006) University of Wisconsin, Oshkosh Tom Hart (2007) Ontario Geological Survey David Meineke, as out-going senior member of Institute Board of Directors, is liaison between Goldich Medal Committee and the Board through the 2005 meeting. Citation for Goldich Medal Recipient Mark Smyk It is a pleasure to acknowledge the many contributions of Mark C. Smyk to the understanding of the geology of the Lake Superior region at the 51st annual meeting of the Lake superior Institute on Lake Superior Geology. This is the 26th Goldich medal awarded by the Institute to individuals that have made significant contributions to Lake Superior geology. Mark was born in Dryden, Ontario, in 1961 and received his Honors Bachelor of Science degree in Geology from Lakehead University in 1984 upon completion of a thesis titled “A comparative study of silver occurrences, Island Belt Silver Region, Thunder Bay District, Ontario”. From Lakehead University Mark entered Carleton University where he received his Masters of Science degree in geology upon completion of a thesis on silver veins in the Cobalt silver area in 1987. The thesis was titled “Geology of Archean Interflow Sedimentary Rocks and their relationship to Ag-Bi-Co-Ni-As Veins, Cobalt Silver Area, Ontario”. Mark completed contract geological mapping in the Schreiber-Hemlo and Swayze greenstone belts in 1984. He has also completed contracts with David Bell Geological Services Inc. and Saarberg-Interplan Ltd. From 1998 to the present Mark has served as guest lecturer at Lakehead University, Thunder Bay. He has also lectured and prepared course curriculum in geology for the Qikiqtaaluk Corporation, Iqalut, Northern Territory. Mark has long been active in the Institute on Lake Superior Geology and co-hosted the 41st and 51st annual meetings. He is a Registered Professional Geologist in the Province of Ontario, fellow in the Geological Association of Canada, member of the Northwest Prospectors Association and guest editor for “Exploration and Mining Geology”, Canadian Institute of Mining and Metallurgy. Since 1987 Mark has worked with the Ontario Geological Survey. During this period he has authored or co-authored 7 reports, 14 abstracts and 7 guidebooks. The Ontario Geological Survey library lists 62 items as authored or co-authored by Mark. Since working with the Ontario Geological Survey Mark has advanced to become Regional Resident Geologist, Thunder Bay North. His work entails geology related publications, land use planning and interaction with the public, prospectors and industry. Submitted by Ron Sage -x- �Proceedings of the 51st ILSG Annual Meeting - Part 1 Eisenbrey Student Travel Awards The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name “Eisenbrey” was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. “Ned” Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The number of awards and value are determined by the annual Chair in consultation with the Secretary and Treasurer. Recipients will be announced at the annual banquet. The following general criteria will be considered by the annual Chair, who is responsible for the selection: 1) The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. 2) Students who are the senior author on either an oral or poster paper will be given favored consideration. 3) It is desirable for two or more students to jointly request travel assistance. 4) In general, priority will be given to those in the Institute region who are farthest away from the meeting location. 5) Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. The form below is optional. Successful applicants will receive their awards during the meeting. - xi - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Eisenbrey Student Travel Award Application Student Name : __________________________________ Address: Date: ____________ __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ email: __________________________________________________________ Educational status: _____________________________________________________ Are you the senior author of an oral presentation or poster? Yes ____ No _____ Will any other students be traveling with you? Yes ____ No _____ If yes, then who? ___________________________________________________ ___________________________________________________ Statement of need (use additional page if necessary): __________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ Signature: ____________________________________________________ Department Head: ____________________________________________________ - xii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Student Paper Awards Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper—last modified by the Board in 2001—follow: 1) The contribution must be demonstrably the work of the student. 2) The student must present the contribution in-person. 3) The Student Paper Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. 4) In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. 5) The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). 6) The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. 7) The names of award recipients shall be included as part of the annual Chair’s report that appears in the next volume of the Institute. Student papers will be noted on the Program. Student Paper Awards Committee Penelope Morton, University of Minnesota Duluth Greg Stott - Ontario Geological Survey, Sudbury, Ontario Wally Rayner - Toronto, Ontario Session Chairs Charlie Blackburn, Blackburn Geological Services Terry Boerboom, Minnesota Geological Survey William Cannon, United States Geological Survey Mike Easton, Ontario Geological Survey Mary Louise Hill, Lakehead University Peter Hinz, Ontario Geological Survey Tom Lane, CAMIRO Mark Severson, Natural Resources Research Institute - xiii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Membership Criteria for the Institute on Lake Superior Geology Approved May 8, 1997. Amended by the Board—May 6, 2004 A. Membership in the Institute on Lake Superior Geology requires either participation in Institute activities, or an indication on a regular basis of interest in the Institute. Those individuals registering for an annual meeting will remain as members for 4 years unless: 1) they indicate no further interest in the Institute by responding negatively to the statement on meeting circulars “Remove my name from the mailing list”; or 2) two successive mailings in different years are returned by the postal service as address unknown. B. Those individuals who have not registered for an annual meeting in the past 4 years must indicate an interest in the Institute by postal, electronic, or verbal correspondence with the Secretary at least once every two years. Such individuals will be removed from the membership if they indicate no further interest in the Institute or two successive mailing in different years are returned by the postal service as address unknown. C. The Secretary will maintain a list of current members. The list will include the date of the beginning of continuous membership, dates of returned mail, dates of last contact (expression of interest), and the date membership expires, barring a change of status initiated by the member. Those individuals who have become members of ILSG by Section B will have an expiration date listed at 2 years from the upcoming meeting. For example, a member who expresses interest in September of 1997 (the next annual meeting is May, 1998) will have an expiration date of May, 2000, unless the member contacts the Secretary or attends an annual meeting. D. “Member for Life” status is granted to individuals who have been (nearly) continuous participants of the ILSG meetings for 15 years, Goldich Medal recipients, or those who have served as meeting chairs. This status will be further maintained unless the individuals indicate no further interest in the Institute, or 4 mailings in different years are returned by the postal service as address unknown, or they are deceased. E. All members will be mailed the First Circular for the Annual Meeting and the ILSG Newsletter. The Chair of the annual meeting may opt to send the first circular to additional individuals. All returned mail should be reported to the Secretary. F. The Secretary can designate any individual who is on the ILSG membership list (mailing list) as of January 1, 1997 as a member for life based on participation in ILSG activities. G. Members are strongly encouraged to send address corrections to the Secretary to avoid unintentional lapse of membership. - xiv - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Board of Directors Board appointment continues through the close of the meeting year shown in parentheses, or until a successor is selected Mark Smyk/Pete Hollings - General Chair 2005 meeting (2008) - Ontario Geological Survey/Lakehead University Steve Hauck (2007) - University of Minnesota, Duluth Laurel Woodruff (2006) - U.S. Geological Survey Peter Hinz (2005) - Ontario Geological Survey Peter Hollings - Secretary (2006) - Lakehead University, Thunder Bay, Ontario Mark A. Jirsa - Treasurer (2007) - Minnesota Geological Survey Local Committee Co-Chairs Mark Smyk - Ontario Geological Survey, Thunder Bay, Ontario Pete Hollings - Lakehead University, Thunder Bay, Ontario Program and Abstracts Editors Mike Easton - Ontario Geological Survey, Sudbury, Ontario Pete Hollings - Lakehead University, Thunder Bay, Ontario Field Trip Guidebook Editor Pete Hollings - Lakehead University, Thunder Bay, Ontario Organising Committee Mary Louise Hill - Lakehead University, Thunder Bay, Ontario Phil Fralick - Lakehead University, Thunder Bay, Ontario Bill Addison - Thunder Bay, Ontario Ryan Tuomi - Ontario Geological Survey, Thunder Bay, Ontario Levina Collins - Township of Nipigon Banquet Speaker Jim Franklin, Franklin Geosciences Ltd. Mineral Resources for the Future: The Resource Potential of Northern Lake Superior (See abstract on page 19) - xv - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Report of the Chair of the 50th Annual Meeting Duluth, Minnesota The Natural Resources Research Institute (NRRI) of the University of Minnesota Duluth hosted the 50th Annual Institute on Lake Superior Geology on May 4 – 9, 2004 at the Radisson Hotel Duluth-Harborview in Duluth, Minnesota. The meeting consisted of two days of technical sessions with four pre- and four posttechnical session field trips. Julie Ann Heinz, Barbara Hauck, and Jeanne Lukkarila provided excellent on-site assistance. John Heine was responsible for: the 50th ILSG website and online registration; AV expertise that kept the sessions flowing on time; and worked with Mark Severson on logistics for the field trips. Julie Oreskovich was the poster session czar. Richard Patelke was invaluable as the data base manager and the designer of the 50th ILSG T-shirt. Bill Cannon and Gene LaBerge compiled a CD of photographs of past ILSG members and field trips. Other members of the local committee included: Charlie Matsch, Jim Miller, Penny Morton, Dean Peterson, and Larry Zanko – all of whom provided much need expertise. Pre-meeting registration was 275 students and professionals, with an additional 24 on-site registrations, for a total of 299 registrants. This was the first time that on-line registration was offered, and 222 individuals registered on-line. Proceedings Volume 50 was published in two parts: Part 1 – Program and Abstracts, edited by Steven Hauck, with 69 published abstracts, for 39 oral and 30 poster presentations; and Part 2 – Field Trip Guidebook, edited by Mark Severson. The 50th meeting marked the sixth time (1958, 1963, 1971, 1979, 1989, 2004) the ILSG annual meeting was held in Duluth, Minnesota. The nine field trips (one trip with two sections) were well attended with 315 total participants attending one or two field trips. On Tuesday, May 4th, marked the beginning of two two-day field trips: George Hudak and co-leaders lead a field trip to examine the volcanic stratigraphy, hydrothermal alteration, and VMS potential of the Lower Ely Greenstone; and Terry Boerboom and co-leaders lead a trip to visit newly mapped outcrops in the Southwestern Sequence of the North Shore Volcanic Group and in the Beaver Bay Complex. On Wednesday, May 5th, there were two one-day field trips: Howard Hobbs led a trip to look at the Late Wisconsinan Superior-lobe deposits northeast of Duluth; and Richard Ojakangas, Mark Severson, and co-leaders examined the geology of the eastern Mesabi Range. On Saturday, May 10th, there were four field trips: Mark Jirsa, Terry Boerboom, and co-leaders led two sections on a two-day investigation of the classic outcrops of northeastern Minnesota; Lisa Marlow and co-leaders examined the glacial and post-glacial landscape evolution in the Glacial Lake Aitkin and Upham basin; Dean Peterson and Richard Patelke led a field trip to the Vermilion District, northeast of Soudan, MN, to look at the economic geology of Archean gold occurrences; and Mark Severson and Jim Miller visited outcrops along the western contact of the Duluth Complex to illustrate the geology and Cu-Ni mineralization. One hundred and sixty-three participants attended the annual banquet on Thursday night. This year’s banquet speaker was Bob Dott of the University of Wisconsin, Madison. Dr. Dott is professor emeritus from the Department of Geology and Geophysics. Dr. Dott’s post-banquet presentation was: The Van Hise army and other pioneers of Lake Superior geology. The other highlight of the evening was the presentation of the 2004 Goldich medal to Paul Weiblen of the Department of Geology and Geophysics at the University of Minnesota, Minneapolis, which recognized his efforts in many areas in the Lake Superior region, especially in the Duluth Complex. The technical sessions began with a special session on “The History of Geologic Investigations in the Lake Superior Region”. The “Old Prospector” (Richard Ojakangas) gave a brief historical overview of geology between 1848 and 1900 that was followed by four other presentations on the geology, mineralization, and geochronology of the Lake Superior region. As always, the student paper committee had a difficult time of picking a winner. There were six oral presentations and four poster presentations. The winners were: 1) Andy Breckenridge – Large Lakes Observatory ($250, Winner, best oral presentation) - xvi - �Proceedings of the 51st ILSG Annual Meeting - Part 1 2) Heidi Drexler – University of Wisconsin Oshkosh ($125, Winner, best poster presentation) 3) Justin Johnson, Lakehead University ($125, Honourable mention) 4) Adam Hoffman - University of Minnesota ($125, Honourable mention, best poster presentation) In addition five Eisenbrey Student Travel awards were presented in 2004, they winners were: Paula Shafer, Indiana University ($250); Justin Johnson - Lakehead, Thunder Bay ($125); Geoff Heggie - Lakehead, Thunder Bay ($125); Adam Richardson - Lakehead, Thunder Bay ($125); and Riku Metsaranta - - Lakehead, Thunder Bay ($125) The Institute’s Board of Directors met on May 6, 2004, and brief summary of the meeting follows: 1. Accepted the Report of the Chair for the 49th ILSG from Laurel Woodruff, and the minutes of the last Board meeting, May 8, 2003 from Pete Hollings. 2. Accepted the 2003-2004 ILSG Financial Summary from Mark Jirsa. 3. Approved Steven Hauck to continue on as a Board member. 4. Approved Nipigon, Ontario as the 51st annual meeting location with Mark Smyk and Pete Hollings as co-chairs. 5. Replaced Ron Sage, who was on the Goldich Committee with Tom Hart, who had agreed to serve. 6. Discussed splitting the Secretary-Treasurer position. Pete Hollings was nominated and elected secretary by the ILSG members, and Mark Jirsa was nominated and elected treasurer. The 50th ILSG meeting was a great success, and we wish to thank all of the individuals who contributed to this success. The staffs of the Radisson Hotel Duluth-Harborview, UMD Food Service, and the Fortune Bay Casino (field trips) were always professional and responsive to our evolving needs to handle a very large group. The following organizations provided generous monetary contributions: Department of Geosciences, University of Minnesota Duluth, MN; Franconia Minerals Corporation, Spokane, WA; Idea Drilling Incorporated, Virginia, MN; Iron Mining Association, Duluth, MN; Lehmann Exploration Management, Minneapolis, MN; Meriden Engineering, LLC, Hibbing, MN; Minerals Processing Corporation, Duluth, MN; Minnesota Exploration Association (MExA), Minneapolis, MN; Minnesota Minerals Coordinating Committee, St. Paul, MN; Teck Cominco American Incorporated, Spokane, WA; and Wallbridge Mining Company, Lively, ONT. The field trips were very well attended, and our appreciation is due to the field trip leaders, van drivers, and everyone else who contributed to the success of the 50th ILSG meeting. Because of the large attendance for the meeting and field trips, the 50th ILSG meeting generated several thousand dollars to the ILSG general fund. The 50th ILSG meeting also coincided with the 50th anniversary of the Department of Geosciences at the University of Minnesota Duluth (UMD). A special session on Friday afternoon featured papers by UMD faculty, students, and alumni. A banquet celebrating this occasion was held at the Duluth Depot on Friday evening after the ILSG meeting, which was attended by many UMD alumni, faculty, and students, and ILSG members. Both of us are very happy with the results of 50th meeting, and we hope that future ILSG meetings can be as well attended as this meeting. We also hope that the attendees were as happy with the meeting as the local committee was. This meeting, like every ILSG meeting requires a lot of work and time on behalf of the cochairs and the local committee. The additional assistance of ILSG members at-large helped make the meeting a success. Respectively Submitted, Steven Hauck and Mark Severson Co-Chairs, 50th ILSG meeting - xvii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Acknowledgements The following organizations made generous contributions to the 51st Annual Meeting. We thank them for their commitment to the Institute on Lake Superior Geology. For the past 50 years this organization has thrived as a result of the interest of individuals, corporations, universities and government agencies. The dedication to an exchange of scientific ideas and a passion for field trips has enabled the institute to provide one of it’s primary objectives – to promote better understanding of the geology of the Lake Superior Region. North Western Ontario Prospectors Association Lake Nipigon Region Geoscience Initiative Ontario Prospectors Association Ministry of Northern Development and Mines – Ontario Geological Survey Township of Nipigon Municipality of Greenstone Canadian Institute of Mining and Metallurgy, Thunder Bay Branch Department of Geology, Lakehead University David Malouf, Roxmark Mines Limited Chaltrek Geological Supplies Inc. - xviii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Program Tuesday May 24 8:00 a.m. Field Trip 1: Geology and gold mineralization of the Beardmore-Geraldton greenstone belt Leaders: Mark Smyk & Tom Hart (OGS) and Phil Fralick (Lakehead University) 6:00 p.m. Overnight in Beardmore Wednesday May 25 8:00 a.m. Field Trip 1: Geology and gold mineralization of the Beardmore-Geraldton greenstone belt Leaders: Mark Smyk & Tom Hart (OGS) and Phil Fralick (Lakehead University) 8:00 a.m. Field Trip 2: Quaternary Geology of the Beardmore – Nipigon area Leader: Peter Barnett (OGS) 8:00 a.m. Field Trip 3: A Stratigraphic transect across the Northern flank of the Midcontinent Rift near Rossport (I) Leaders: Peter Hollings and Phil Fralick (Lakehead University) 6:00 p.m. Return of Trips 1,2 and 3 4:00 p.m. - 8.00 p.m. Registration (Nipigon Community Centre) 7:00 p.m. - 9.00 p.m. Ice Breaker Social (Nipigon Curl-a-Drome) and Poster Setup (Nipigon Community Centre) Thursday May 26 8:00 a.m. - 4:00 p.m. Registration (Nipigon Community Centre) 9:00a.m. - 9:05 a.m. Introductory Remarks - Mark Smyk and Peter Hollings, Co-Chairs Technical Session I Session Chairs: T. Boerboom (Minnesota Geological Survey), W. Cannon (US Geological Survey) 9:05 a.m. Addison, W., Brumpton, G., Vallini, D., McNaughton, N., Davis, D., Kissin, S., Fralick, P. and Hammond, A. Discovery of distal ejecta from the 1850 Ma Sudbury impact 9:25 a.m. Schnieders, B. and Scott, J. Mining and exploration activity in the Thunder Bay South District 9:45 a.m. Hill, M.L. and Smyk, M. Penokean fold-and-thrust deformation of the Paleoproterozoic Gunflint Formation near Thunder Bay, Ontario 10:05 a.m. Middleton, R. and Heggie, G. Seagull Intrusion; A unique PGE-Ni-Cu Setting. 10:25 a.m. - 11:00 a.m. Coffee Break and Poster Session 11:00 a.m. Maric, M.*, and Fralick, P. Sedimentology of the Rove and Virginia Formations and their tectonic significance - xix - �Proceedings of the 51st ILSG Annual Meeting - Part 1 11:20 a.m. Easton, R.M. The Grenvillian Tomiko quartzites of Ontario: correlatives of the Baraboo quartzites of Wisconsin, the Mazatzal orogen of New Mexico, or unique? Implications for the tectonic architecture of Laurentia in the Great Lakes region. 11:40 a.m. Medaris, G. and Dott, B. Riddle of the sands (Proterozoic) solved by quartzites at Hamilton Mounds Wisconsin 12:00 p.m. - 1:30 p.m. Lunch Break and Poster Session (ILSG Board Meeting by invitation) Technical Session II Special Session - Lake Nipigon Regional Geoscience Initiative Session Chairs: Tom Lane (CAMIRO), Mark Severson (Natural Resources Research Institute) 1:30 p.m. Heaman, L. and Easton, R.M. Proterozoic history of the Lake Nipigon area, Ontario: Constraints from U-Pb zircon and baddeleyite dating 1:50 p.m. Smyk, M. Mineral deposits and metallogeny of the Midcontinent Rift in Ontario 2:10 p.m. Fralick, P., Metsaranta, R., and Rogala, B. Stratigraphy of the Mesoproterozoic Sibley Group and Nipigon Sills 2:30 p.m Hart, T. and MacDonald, C. Mesoproterozoic diabase sills of the Nipigon Embayment, northwest Ontario 2:30 p.m. - 3:00 p.m. Coffee Break and Poster Session 3:00 p.m. Richardson, A.* and Hollings, P. Geochemical variation within the Mesoproterozoic Nipigon diabase sills 3:20 p.m. Metsaranta, R.* and Fralick, P. Depositional setting of the Pass Lake and Rossport Formations (Sibley Group) inferred from a combined sedimentologic/geochemical approach 3:40 p.m. Laarman, J.* and Hollings, P. Petrogenesis and PGE mineralization of the Eva Kitto Intrusion, Northern Ontario 4:00 p.m. Magee, A.*, Hollings, P. and Fralick, P. Preliminary stratigraphy and geochemistry of the Mesoproterozoic Pillar Lake volcanics, Wabigoon subprovince, Superior Province, Armstrong, Ontario, Canada 6:00 p.m Annual Banquet and Award Presentation (Royal Canadian Legion) Announcement of 52nd Annual Meeting Location 2005 Goldich Award Presentation to Mark Smyk 2005 Banquet Address - Dr. J. Franklin Meeting participants not registered for the banquet are welcome to attend the address - xx - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Friday May 27 9:00 a.m. - 12:00 p.m. Registration Technical Session III Session Chairs: Mary Louise Hill (Lakehead University), Mike Easton (Ontario Geological Survey) 9:00 a.m. Holm, D., Cannon, W., Chandler, V., Schneider, D., Schulz, K., & Van Schmus, W. The incredible shrinking Penokean orogen: a new look at the accretionary history of the southern Lake Superior region. 9:20 a.m. Vallini, D.*, Cannon, W. and Schulz, K. New age data for the Chocolay Group, Marquette Range Supergroup: implications for the Paleoproterozoic evolution of the Lake Superior and Lake Huron regions 9:40 a.m. Knudson, D.*, Saini-Eidukat, B., Miller, J., and Daniels, P. Structural state of plagioclase phenocrysts in porphyritic rocks of the Midcontinent rift, northeastern Minnesota 10:00 a.m. Albers, P.* and Miller, J. The geology and petrology of the Leveaux porphyritic diorite, Cook County, MN: Investigating possible magmatic relationships to the Anorthositic Series of the Duluth Complex 10:20 a.m. - 11:00 a.m. Coffee Break and Poster Session 11:00 a.m. Conly, A. and MacDonald, J. Origin of high-sulphate waters of the Hogarth Pit Lake, Steep Rock iron mine, Atikokan, Ontario. 11:20 a.m. Breckenridge, A. and Johnson, T. Lake Superior’s oxygen isotope record suggests overflow to Lake Ojibway between 10,000 and 9,400 CAL BP (~8.9-8.4 14C KA) 11:40 a.m. Blackburn, C. and Kor, P. Control of Quaternary erosional and depositional landforms at the Eastern outlet of glacial Lake Agassiz by Precambrian bedrock and structure, Ottertooth-Pantagreul Lakes area, northwestern Ontario 12:00 p.m. - 1:30 p.m. Lunch Break Technical Session IV Session Chairs: Peter Hinz (Ontario Geological Survey), Charlie Blackburn (Blackburn Geological Services) 1:30 p.m. Weiblen, P., Peterson, D., Vislova, T. Implications of Midcontinent Rift and oceanic ridges analogies and 3D interpretations of the subsurface structure of the Bald Eagle Intrusion in the Duluth Complex and the East Pacific Rise 1:50 p.m. Halls, H. Dyke swarms around the Lake Superior Region define Proterozoic (~2 Ga) deformation of the Archean Superior Province associated with evolution of the Kapuskasing Zone 2:10 p.m. Dahl, D. Gold grains, pathfinder elements, and till clast composition in a portion of the Vermilion Greenstone Belt, Northeastern Minnesota 2:30 p.m Miller, J. and Jirsa, M. - xxi - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Geology, geochemistry and PGE potential of mafic and ultramafic intrusions in Minnesota, excluding the Duluth Complex 2:50 p.m Presentation of Best Student Paper Award and Eisenbrey Awards 3:00 p.m. - 3:30 p.m. Coffee Break and Poster Session (Posters removed after the break) 7:00 p.m Mixer - Cash Bar (Nipigon Curl-a-Drome) Poster Presentations Bucholz, T., Falster, A., and Simmons, W. Mineralogy of pegmatites and spatially associated metasomatized zones, Michels Materials Quarry, Waterloo, WI Cannon, W., Schulz, K., Daniels, D., Anderson, R., Chandler, V., Holm, D., Schneider, D., Van Schmus, R. Geology of Precambrian basement rocks in Iowa and the southern parts of Wisconsin and Minnesota Jirsa, M. and Miller, J. Geologic implications of bedrock mapping in the Ely and Basswood Lake quadrangles, Northeast Minnesota Lane, C.* and Hollings P. Geochemistry and petrography of the Rabbit Islands Breccia, North Central Lake Nipigon MacTavish, A. MetalCORP Ltd. Big Lake Ni-Cu-PGE, Cu-Zn-Ag, and Mo Property Magee, A. Mining and exploration activity in northwestern Ontario Planavsky, N.* and Murphy, J.* New thoughts on old circles: A reexamination of spheroidal Gunflint taxa Rossell, D. and Coombes, S. The geology of the Eagle Lake Nickel-Copper deposit: Marquette County, Michigan Severson, M. PGE and gold potential of the Archean Deer Lake Complex, Minnesota, USA Shareef, S. and Craven, J. A view into the Nipigon Embayment: preliminary results of the largest magnetotelluric study ever in Ontario. Stott, G. and Rayner, N. Discrimination of Archean terranes in the Sachigo subprovince and relevance to volcanogenic masive sulphide exploration Trow, J. and Young, C. Correlation between self-potential and dowsing (IESG) at the Quincy Mine and at the Calumet and Hecla Mine, Michigan - xxii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Weiblen, P., Peterson, D., and Vislova, T. Implications of Midcontinent Rift and oceanic ridges analogies and 3D interpretations of the subsurface structure of the Bald Eagle Intrusion in the Duluth Complex and the East Pacific Rise Zieg, M., Forsha, C., and Habarka, J. Textural examination of Kama Point diabase sill, Nipigon, Ontario NOTE: Asterisk * denotes a student eligible for a Best Student Paper Award Saturday May 28 8:00 a.m. Field Trip 4: Geology and Rare Element-bearing pegmatites of the Quetico Subprovince Leaders: Mark Smyk (OGS) and Steve Kissin (Lakehead University) 8:00 a.m. Field Trip 5: Geology of the Black Sturgeon Area Leader: Tom Hart (OGS) 8:00 a.m. Field Trip 6: A Stratigraphic Transect Across the Northern Flank of the Midcontinent Rift near Rossport (II) Leaders: Peter Hollings and Phil Fralick (Lakehead University) 6.00 p.m. Return of Trips 4, 5 and 6 - xxiii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Discovery of Distal Ejecta from the 1850 Ma Sudbury Impact ADDISON, W.D., R.R. 2, Kakabeka Falls, Ontario, P0T 1W0, Canada; BRUMPTON, Gregory, R., 211 Henry Street, Thunder Bay, Ontario, P7E 4Y7, Canada; VALLINI, Daniela A, McNAUGHTON, Neal, J., Centre for Global Metallogeny, School of Earth and Geographic Sciences, University of Western Australia, Nedlands, Western Australia, 6009, Australia; DAVIS, Don W., Department of Geology, Earth Sciences Centre, University of Toronto, 22 Russell Street, Toronto, Ontario, M5S 3B1, Canada; KISSIN, Stephen A., FRALICK, Philip W., HAMMOND, Anne L., Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada A 25–70-cm-thick, laterally correlative layer near the contact between the Paleoproterozoic sedimentary Gunflint Iron Formation and overlying Rove Formation and between the Biwabik Iron Formation and overlying Virginia Formation, western Lake Superior region, contains shocked quartz and feldspar grains found within accretionary lapilli, accreted-grain clusters, and spherule masses, demonstrating that the layer contains hypervelocity impact ejecta. Smectite replaced microtektites, spherules and crushed spherules, and sphere-insphere features make up the bulk of the ejecta. Accretionary lapilli appear near the middle of the ejecta column in the Ontario Gunflint Formation cores but they are not present in the Minnesota Virginia Formation cores. Zircon geochronologic data from tuffaceous horizons bracketing the layer reveal that it formed between circa 1878 Ma (Fralick et al., 2002) and 1836 Ma. The Sudbury impact event, which occurred 650–875 km to the east at 1850±1 Ma (Krogh et al., 1984), is therefore the likely ejecta source, making these the oldest ejecta linked to a specific impact. Shock features, particularly planar deformation features (French, 1998) are remarkably well preserved in localized zones within the ejecta, whereas in other zones, mineral replacement, primarily carbonate, has significantly altered or destroyed ejecta features. LEGEND racks younger than Paleoproterozoic Paleoproterozoic rocks, BIF in black Archean greenstone granite terrane Figure 1. Location of drill holes in relation to the Sudbury structure. The Sudbury impact is the only known impact location close enough to have produced a craton-sourced (quartz and feldspar grains), westward-thinning ejecta layer (70 cm thinning to 25 cm) this thick, in these locations, given the time constraints established by zircon dating. References Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N J., Davis, D.W., Kissin, S.A., Fralick, P.W. and Hammond, A.L. 2005. Discovery of distal ejecta from the 1850 Ma Sudbury impact: Geology, 33, 193-196. Fralick, P., Davis, D.W. and Kissin, S.A. 2002. The age of the Gunflint Formation, Ontario, Canada: Single zircon U-Pb age determinations from reworked volcanic ash: Canadian Journal of Earth Sciences, 39, 1085–1091. French, B.M. 1998. Traces of catastrophe: Lunar and Planetary Institute Contribution 954, 120p. Krogh, T.E., Davis, D.W. and Corfu, F. 1984. Precise U-Pb zircon and baddeleyite ages for the Sudbury area, in Pye, E.G., Naldrett, A.J., and Giblin, P.E., eds., The geology and ore deposits of the Sudbury structure: Ontario Geological Survey Special Volume 1, 431–446. -1- �Proceedings of the 51st ILSG Annual Meeting - Part 1 Figure 2. Overview of the Gunflint and Rove Formations based on drill core 89MC-1s. The 1878 Ma tuff age is from Fralick et al (2002). Detail from column 1 showing the Gunflint-Rove boundary zone and the relationship of the ejecta to the dated Rove tuff. The Minnesota cores and other Ontario cores show similar features in this zone. Detail of the ejecta layer in drill core BP99-2, the most complete and least altered ejecta section in all the cores. The possible carbonate-replaced fireball layer fades into the overlying carbonate. Minnesota ejecta layers are thinner and lack accretionary lapilli. LEGEND C Possible fireball rayer. carbonate replaced Acoretionary lapilli Ejecta P Possible ejecla. carbonate replaced S [] Inaclastjo Grainstone — Ei Fine - grained chemical sedimentwy rooks Siliclaslic Sandstone [II — Fine-grained siliastics Recrystalljzed and silioffied carbonates -2- �Proceedings of the 51st ILSG Annual Meeting - Part 1 The Geology and Petrology of the Leveaux Porphyritic Diorite, Cook County, MN: Investigating Possible Magmatic Relationships to the Anorthositic Series of the Duluth Complex ALBERS, Paul B., Department of Geological Sciences, University of Minnesota-Duluth, Duluth, MN 55812, albe0167@d.umn.edu and MILLER, James D., Jr., Minnesota Geological Survey, c/o NRRI, 5013 Miller Trunk Hwy, Duluth, MN 55811, mille066@umn.edu The Leveaux porphyritic diorite (LPD) is a 72-m thick, discontinuous, hypabyssal, sheet-like intrusion located within the Mesoproterozoic (1.1 Ga) Midcontinent Rift-related Beaver Bay Complex of northeastern Minnesota (Miller and Chandler, 1997). The LPD makes up five prominent, cuesta-like ridges (Figure 1) that trend parallel to Lake Superior over a 20 km distance. In the Caribou Lake region, the LPD makes up smaller knob-like outcrops that dip in multiple directions and are segmented by the younger Beaver River diabase. The LPD is composed of an upper porphyritic zone, which contains 40-50% of 1-4 cm labradorite megacrysts in a fine-grained ferrodiorite matrix (Figure 2), and a lower aphyric zone composed of fine-grained ferrodiorite with rare megacrysts. The aphyric and porphyritic zones are separated by a 20-30 cm thick gradational transition. The orientation of this planar transition and sheet joints from the aphyric section indicate a gentle (10-20 degrees) southeast dip. The upper contact of the LPD is fault bounded. In two localities, the lower contact is observed to be chilled against a thin arkosic sandstone unit, which in turn is underlain by an ophitic basalt. Few outcrops of a sparsely porphyritic (5-20% plagioclase megacrysts) ferrodiorite off the trend of the main sheet are found and may represent feeder zones to the LPD. Figure 1. Cuesta-like ridge of LPD at Moose Mountain dipping gently toward Lake Superior (looking southeast). The ridge to the right is the Beaver River diabase. Figure 2. Nonfoliated, lath-shaped labradorite megacrysts (1-4 cm) in a dark, fine-grained ferrodiorite matrix at Oberg Mountain. Geochemical and petrographic characteristics of the aphyric zone and the matrix material of the porphyritic zone are nearly identical vertically and laterally throughout the sheet. Mineralogically, the ferrodiorite is composed of plagioclase, augite, K-feldspar, Fe-Ti oxides, and minor accessory minerals of quartz, inverted pigeonite, calcite, and apatite, which collectively display an intergranular, nonfoliated texture. Modally, the ferrodiorite varies between diorite and quartz monzonite. XRF analyses indicate a normative An (An/An+Ab+Or) matrix composition of 39, while microprobe analyses range from 19-64. Matrix augite contains a constant composition (mg # 55-61) throughout the intrusion. Microprobe traverses and Nomarski DIC microscopy of plagioclase megacrysts indicate subtle zoned cores (~An 65) and strongly zoned rims with An compositions comparable to matrix plagioclase. XRD analyses indicate that the plagioclase megacrysts have an intermediate ordered structural state (see Knudsen and others, this volume). Based on these observations, we conclude that the LPD formed by the shallow emplacement of a single plagioclase-phyric ferrodiorite magma. Density calculations indicate that the upper porphyritic section may have -3- �Proceedings of the 51st ILSG Annual Meeting - Part 1 formed by plagioclase megacryst flotation in the host ferrodiorite magma. The average composition, zonation patterns, and structural state of LPD plagioclase megacrysts are similar to cumulus plagioclase of the anorthositic series of the Duluth Complex. Also, the evolved compositions of the LPD ferrodiorite are similar to the estimated anorthositic series trapped liquid compositions (Miller and Weiblen, 1990). With these petrogenetic links, we conclude the LPD is a hypabyssal equivalent to the anorthositic series, which is thought to have formed from plagioclase crystal mushes derived from deep crustal magma chambers. References Miller, J. D., Jr., and Chandler, V. W. 1997. Geology, petrology, and tectonic significance of the Beaver Bay Complex, northeastern Minnesota, in Ojakangas, R.W., Dickas, A.B., Green, J.C., eds., Middle Proterozoic to Cambrian Rifting, Central North America: Geological Society of America Special Paper 312, 73-96. Miller, J. D., Jr., and Wieblen, P. W. 1990. Anorthositic rocks of the Duluth Complex, Minnesota: Examples of rocks formed from plagioclase crystal mush: Journal of Petrology, 31, 295-339. -4- �Proceedings of the 51st ILSG Annual Meeting - Part 1 Control of Quaternary Erosional and Depositional Landforms at the eastern outlet of Glacial Lake Agassiz by Precambrian Bedrock and Structure, Ottertooth-Pantafreul Lakes area, Northwestern Ontario BLACKBURN, C.E., Blackburn Geological Services, Victoria, B.C. and KOR, P.S.G., Ontario Parks, Peterborough, Ontario During late Wisconsinan/early Holocene time, glacial Lake Agassiz debouched into the Nipigon basin in catastrophic bursts at the continental divide through a number of spillway channels. Two protected areas managed by MNR Ontario Parks, Ottertooth Conservation Reserve and Pantagreul Creek Provincial Nature Reserve, encompass the eastern portion of the 15-km-wide Kaiashk Spillway corridor, the southern-most of five major channel complexes (Figure 1). The power of the bursts can be appreciated by the fact that water flowing through the outlet would have fallen from the 457 m elevation of the divide to about the 320 m level of Lake Nipigon - a drop of 137 m over a distance of 20 km. 44 1# C, C, t I. to Erosional features resulting from this outflow include plunge basins that formed at the break in slope from the Archean gneissic basement into the Nipigon lowlands. Two plunge basins are present. The more spectacular of the two, Devils Crater, has walls on the order of 100 m high down which today two water falls cascade. The second un-named plunge basin lies some 1.5 km to the southwest. Streams drain to the southeast from both plunge basins, the one from Devilʼs Canyon being through a deeply incised, narrow gorge. Similar though less prominent features suggesting plunge of water over a pre-existing bedrock “lip” can be seen: a) at the north end of Rabelais Lake; b) about 3 km to the northeast of Devils Crater, and c) near the west shore of Pantagruel Creek. Notably, all of these features lie along a NE-trending line over a distance of ~20 km. Depositional features resulting from the outflow include an alluvial fan at the mouth of the more southerly of the channel-ways exiting from the two plunge basins, which unites with the main spillway channel through which Pantagruel Creek now flows. An “island” of braided eskers forms an erosional remnant in the centre of the channel. Figure 1. Location of the Kaiashk Spillway system, and its relation to four other systems that linked Lake Agassiz with Lake Nipigon during the Nipigon Phase (from Teller and Thorleifson, GAC Special Paper 26, 1983, Fig. 7). Underlying Archean and Proterozoic bedrock geology had a profound influence on development of the above Quaternary features. To the northwest of the northeasterly-trending line joining points of plunge of Agassiz lake waters, Archean gneissic bedrock was peneplained prior to deposition of Proterozoic Sibley Group sediments, and exhumed during the time of outlet from Lake Agassiz. The result is a broad, level plain. Flat-lying Sibley sedimentary rocks were intruded by Nipigon diabase sills, the erosional remnants of which form numerous scarps and cuestas. Evidence of regional scale faulting post dating intrusion of the sills is seen at Devilʼs Crater, where Archean gneisses exposed on its northwest face are uplifted relative to Proterozoic diabase that is exposed throughout the rest of Devilʼs Crater and its gorge. Faulting is probably present at the smaller plunge basin, and at the north end of Rabelais Lake where there is also transition from peneplained Archean gneiss to the incised channel-way of Rabelais Creek. However this latter channel has eroded completely through the diabase “cap” into the gneisses below. Interpretation of -5- �Proceedings of the 51st ILSG Annual Meeting - Part 1 such a regional scale fault is supported by presence of similar NE to NNE-trending regional-scale faults that have been mapped throughout the region. Projection of the regional-scale NE-trending fault is coincident with the five identified points of plunge of Agassiz lake waters during late Wisconsinan/early Holocene time. Other such fault structures, as yet undetected, may have had considerable influence on development of landform features at the eastern exit from glacial lake Agassiz, possibly along the entire length of the spillway channels northwest of present Lake Nipigon (Figure 1). References Teller, J.T. and Thorleifson, L.H. 1983. The Lake Agassiz–Superior connection; in Glacial Lake Agassiz, Geological Association of Canada, Special Paper 26, p.61-290. -6- �Proceedings of the 51st ILSG Annual Meeting - Part 1 Lake Superiorʼs Oxygen Isotope Record Suggests Overflow to Lake Ojibway between 10,000 and 9,400 CAL BP [~8.9-8.4 14C ka] BRECKENRIDGE, Andy and JOHNSON, Thomas C., Large Lakes Observatory and Dept. of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 Late glacial ostracode and bivalve records from Lakes Huron and Michigan are characterized by extreme δ18O variations, ranging from values that reflect a source that is primarily glacial in origin (~–20 ‰ PDB) to much heavier values characteristic of a regional meteoric source (~–5 ‰ PDB). In contrast, a coeval record from benthic ostracodes (Candona subtriangulata) from a varve sequence in Lake Superior is consistently depleted in 18 O, ranging from –18 to –22 ‰ PDB. Between 10,000 and 9,400 cal BP [8.9-8.4 14C ka], high δ18O values in Huron strongly contrast with much lower values in Superior, which suggests Lake Superior overflow circumvented Lake Huron, and discharged through the Pic-White Otter River Valley en route to Glacial Lake Ojibway. The northern outlet may have opened when lake levels fell from an upper post-Minong water plane at around 10,000 cal BP [~8.9 14C ka]. Thereafter, lake levels probably transgressed towards the post-Minong III shoreline. Because deltaic deposits in the Pic River Valley dated around 9,200 cal BP [8.2 14C ka] record glacial meltwater flow towards Lake Superior (Bajc et al., 1997), northern drainage of Lake Superior must have been blocked by ice advance at around 9,400 cal BP [8.4 14C ka], rather than by differential uplift of a northern outlet. Negative δ18O excursions in Lakes Huron and Michigan between 9,400 and 9,000 cal BP [8.4-8.1 14C ka] record the return of Lake Superior overflow to the upper Great Lakes. In Huron the negative δ18O excursion was previously ascribed to the Late Stanley lowstand (Rea et al., 1994), and in Michigan the event was attributed to Lake Agassiz overflow and labeled ʻA2ʼ (Colman et al., 1994). During this entire period, both Lake Agassiz and glacial meltwater discharged into Lake Superior via the Nipigon inlets. Lake Agassiz and glacial meltwater fluxes into Lake Superior diminished to zero between 9,040 and 8,840 cal BP [~8.1-7.9 14C ka]. References Bajc, A.F., Morgan, A.V., and Warner, B.G. 1997. Age and paleoecological significance of an early Postglacial fossil assemblage near Marathon, Ontario, Canada. Canadian Journal of Earth Sciences. 34, 687-698. Colman, S.M., Keigwin, L.D., and Forester, R.M. 1994. Two episodes of meltwater influx from glacial Lake Agassiz and their climatic contrast. Geology 22, 547-550. Rea, D.K., Moore, T.C., Jr., Anderson, T.W., Lewis, C.F.M., Dobson, D.M., Dettman, D.L., Smith, A.J., Mayer, L.A. 1994. Great Lakes paleohydrology: complex interplay of glacial meltwater, lake levels, and sill depths. Geology 22, 10591062. -7- �Proceedings of the 51st ILSG Annual Meeting - Part 1 Mineralogy of Pegmatites and Spatially Associated Metasomatized Zones, Michels Materials Quarry, Waterloo, WI BUCHOLZ, Thomas W., 1140 12th Street North, Wisconsin Rapids, Wisconsin 54494, FALSTER, Alexander. U. and SIMMONS, Wm. B., Department of Geology and Geophysics, University of New Orleans, New Orleans, Louisiana 70148. The Michels Materials Waterloo Quarry exposes Baraboo Interval quartzites and metapelites that have been intruded by pegmatites of Wolf River age, approximately 1,440 Ma (Aldrich, 1959; cited in Brown, 1986). A pegmatite outcropping nearby in the Crawfish River has been long known, but pegmatite exposures in the quarry itself were first noted during an ILSG field trip in 2001. Samples of pegmatites and associated rocks were collected at that time and during a number of subsequent visits. The pegmatites are highly weathered and in hand specimen consist of kaolinite, quartz and muscovite; virtually all feldspars have altered to kaolinite. The dikes are small, discontinuous, apparently randomly oriented and frequently crosscut bedding in the metasediments. Despite the extensive alteration of feldspars, associated muscovite is quite fresh and a number of accessory phases persist. Small miaroles appear to be present, but are obscured by kaolinite. Most pegmatites exposed to date are in the upper level of the quarry, although recent operations (2004) have exposed one pegmatite in the lower level. Accessory minerals identified in dikes in the upper level of the quarry include dark green gahnite as small aggregates to approximately 2 mm and as well-formed octahedral crystals to approximately 500 µm: MnO content ranges from 0.4 to 1.1 wt. % and FeO content ranges from 1.5 to 2.9 wt. %. Columbite-tantalite group minerals are not uncommon but are inconspicuous due to their small grain size. They form prismatic crystals up to about 900x50 µm. Mn enrichment is pronounced and Ta-enrichment is modest, with compositions falling into the manganocolumbite and manganotantalite fields; there are two distinct composition trends present in these small dikes, one clustering around a Nb-dominant composition and the other clustered around a Ta-dominant composition. TiO2 contents range from 0.4 to 0.9 wt. %. Sn, Bi, and Sb were not detected; the absence of Bi is interesting in light of recent finds described below. Additional minerals in the upper level dikes include apatite, sprays of acicular goethite crystals, ilmenite as crude hexagonal platelets, small grains of a LREE-Ca phosphate (probably monazite-(Ce) or rhabdophane-(Ce)), zircons giving evidence of minor Hf enrichment and a grayitelike Th-phosphate associated with gahnite. Despite the intense alteration of pegmatitic feldspars, associated muscovite is well preserved and retains a substantial Li2O content of up to 0.98 wt %. In September 2004 a somewhat less weathered pegmatite was discovered in the lower level of the quarry. Similar to the upper level dikes, this pegmatite appears to cut across the bedding and terminates before intersecting the upper bench level. Remnants of partially altered feldspars are present in this pegmatite, and the overall mineralogy is somewhat more complex than in the upper level dikes. Some micas in this pegmatite have a pronounced dark pink color and contain substantial Li2O enrichment up to 2.05 wt %. Some additional minerals found include manganotantalite, goethite replacements of pyrite and chalcopyrite crystals, thin tabular crystals of hematite and sparse apatite. Noteworthy are small grains of bismutomicrolite {(Bi,Ca) (Ta,Nb)2 O6 (OH)} to approximately 0.4 mm embedded in feldspar. EMP analysis indicates Ta>Nb, and Bi>Ca+Na. Pale greenish elongated crystals forming small branching groups in vugs and voids, usually along surfaces of quartz grains, are a mixture of kettnerite {(Ca,Bi) (CO3) O, F} and other Bi secondary minerals as indicated by x-ray diffraction study. Kettnerite is a typical secondary Bi mineral probably derived from alteration of a primary Bi mineral such as bismuthinite, although the primary Bi mineral has not yet been found. The Waterloo occurrence represents the first find of gahnite from a pegmatite in the state and the overall composition of these small pegmatites indicates highly peraluminous composition, even though such common peraluminous species such as garnet and tourmaline appear to be absent in them. These are also the first finds of kettnerite and bismutomicrolite for Wisconsin. -8- �Proceedings of the 51st ILSG Annual Meeting - Part 1 Spatially and perhaps genetically related alteration or metasomatic vein-like bodies have been found near the pegmatites. These range from quartz-muscovite±andalusite veins to massive fine-grained lithian muscovite (0.6 wt % Li2O), the two phases interfingering and grading into each other laterally. Considering the close spatial association of these bodies to pegmatites, it seems likely that the fluids responsible for the alteration may have been derived either from the pegmatites or the source igneous body, although mobilization from the metasediments cannot be excluded based on the present evidence. Schorl tourmaline is locally common in the quartz-muscovite+-andalusite veins in small vugs and adjacent pelitic schists. Colors range from black to olive green, brown and pinkish brown. Microprobe analysis indicates FeO contents of 8-9%, reflecting an overall schorl composition, though site occupancy considerations indicate a significant Li content. Hence, as the Fe-content of these tourmaline crystals is only slightly on the schorl side of the line between elbaite and schorl they might be considered elbaitic or lithian schorl. Apatite is common in small vugs near tourmaline, and ranges from dark red tabular hexagonal crystals to short visually zoned yellow-brown hexagonal prisms. Additional accessory minerals noted in vuggy portions of the veins include Ti-oxides, ilmenite-hematite, pyrite, chalcopyrite (both sulfides usually replaced by goethite), calcite and sparse REE-phosphate grains. Accessory minerals noted in the massive fine-grained muscovite-rich portions include ilmenite-hematite grains, enigmatic muscovite replacements of an undetermined mineral, and sparse zircon. The zircons may be detrital and predate metasomatism. Hence it is likely that the highly evolved pegmatites formed in conjunction with regionally widespread Wolf River age magmatism (Medaris et al., 2003). Elevated Ta and Mn levels evidenced in the pegmatites, and B and Li in the metasomatic units are anomalous for Wolf River age intrusions in Wisconsin. However, it appears likely that fluids either from these pegmatites or more directly from the parent intrusion may have had a strong, localized metasomatic effect on adjacent rocks, introducing P, B, Li and perhaps other elements to favorable areas in the host rocks. An intriguing question is what influence assimilation of peraluminous metasediments may have had on the source magma, and subsequently on pegmatite composition. Could this have influenced the mineralogy of the pegmatites and other features, or was the source magma already evolved, having originated by partial melting of different source rocks than typical Wolf River intrusions? References Brown, Bruce A. 1986. Baraboo Interval in Wisconsin, In Proterozoic Baraboo Interval in Wisconsin, Geoscience Wisconsin 10, 1-14. Medaris, L.G. Jr., Singer, B.S., Dott, R H. Jr., Naymark, A., Johnson, C.M. and Schott R.C. 2003. Late Paleoproterozoic Climate, Tectonics, and Metamorphism in the Southern Lake Superior Region and Proto–North America: Evidence from Baraboo Interval Quartzites. Journal of Geology, 111, 243-257. -9- �Proceedings of the 51st ILSG Annual Meeting - Part 1 Geology of Precambrian basement rocks in Iowa and the southern parts of Wisconsin and Minnesota CANNON, W.F., SCHULZ, Klaus J., and DANIELS, David L.,U.S. Geological Survey, Reston, VA, ANDERSON, Raymond, Iowa Geological Survey, Iowa City, IA; CHANDLER, Val, Minnesota Geological Survey, St. Paul, MN, HOLM, Daniel, Kent State University, Kent, OH, SCHNEIDER, David, Ohio University, Athens, OH, and VAN SCHMUS, W.R., Kansas University, Lareence, KS The well-known Precambrian rocks of the Lake Superior region record geologic events in the evolution of Laurentia from 3.6 Ga to 1.1 Ga. Similar rocks extend southward as the crystalline basement beneath Paleozoic and Cretaceous strata in southern Minnesota and Wisconsin and throughout Iowa. Using aeromagnetic and gravity maps of the region, and several hundred basement-penetrating drill holes, we have produced a geologic map and preliminary tectonic interpretation of the Precambrian subcrop for this region (Figure 1). Our interpretation extends the well known Superior Province (Archean) and Penokean orogenic belt (Paleoproterozoic) of the Lake Superior region southward to a prominent geophysical discontinuity, shown clearly by aeromagnetic patterns, named the Spirit Lake-Trempealeau discontinuity (SLTD). This discontinuity separates fundamentally different terranes; rocks to the south are part of the Yavapai Province and those to the north are part of the Penokean Province. Although rocks of Yavapai age, represented by extensive ultra-mature sediments, such as the Baraboo and Sioux Quartzites, and granitic plutons, extend north of the discontinuity, there are as yet no firmly identified Penokean or older rocks south of the discontinuity, although data are sparse. Thus we suggest the possibility that the discontinuity marks the southern limit of the preserved Penokean orogen and its Archean basement. South of the SLTD, rocks at the subcrop include subaerial potassic rhyolite and epizonal granite, formed at about 1.75 Ga, and ultra-mature quartzite, such as the Baraboo Quartzite, which lies unconformably on them. An orogenic complex of gneisses and mafic volcanic rocks, probably basement rocks on which the rhyolites were deposited and from which they formed by partial melting, are inferred from gravity and magnetic patterns to be at subcrop throughout much of this terrane, but its lithology and age are not well constrained. All of these rocks were strongly deformed at about 1.63 Ga during the Mazatzal orogeny, whose northeastern limit may underlie the extreme southeastern part of the map area. The nature of the SLTD is controversial with possibilities including a north-directed Yavapai subduction zone or a major strike-slip fault zone. Major granitic plutons were emplaced into the Yavapai terrane in the interval 1.50-1.43 Ga and some are stitching plutons of the SLTD providing an upper age limit on deformation within it. The final major Precambrian event in the region was formation of the Midcontinent Rift at 1.1 Ga. The rift transects older terranes at a high angle and consists of more than 10 km of flood basalts accumulated in and near a series of grabens, and slightly younger clastic rocks. Its location in the subsurface is clearly shown by very pronounced geophysical anomalies. A major igneous complex in NE Iowa and SE Minnesota is also interpreted to be part of the Midcontinent Rift. - 10 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 ______ _______ _______ 98L0.W _______ 94.0YW 88 OOW SBOW go•do•w EXPLANATION Ma Manson impact structure Red clastic sedimentary rocks Vo Mafic intrusive rocks Penokean Orogen Arthean basement granitic rocks Archean Granite and tonalite Pembine-Wausau Terrane Gnelss of Jeffers Block Mafic volcanic rocks LAng 1 Granite,anorthosite,norite Gneiss of Montevideo Block Mazatzal Orogen Felsic volcanic rocks Gneissof Morton Block Granitic plutons Rhyoliteandgranlte Baraga Group Orogenic complex Late- and post-tectonic granite Flood basalt, minor rtiyolite Anorogenic Plutons Foreland basin Mat Ic volcanic rocks Midcontinent Rift MRs Penokean Orogen (cont.) Yavapai Orogen (cont.) Bimodal volcanic rocks Units of undetermined age ç:: Gneiss — Yavapai Orogen Marshfleld Terrane Mafic gneiss Quartzlte (BarabooSioux, etc) Mt Yr iF Rhyolite and granite ftiOIIl]I]OI Felsic yneiss Iron-formation — Diabase dikes (mostly MCR related) Mafic and untramafic rocks 11111111 Mylonite and sheared granite Mafic volcanic rocks Volcanic rocks Figure 1. Geological Map of Precambrian basement rocks in Iowa and the southern parts of Minnesota and Wisconsin inferred from aeromagnetic and gravity data and drill holes. - 11 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Origin of High-Sulphate Waters of the Hogarth Pit Lake, Steep Rock iron Mine, Atikokan, Ontario CONLY, A.G., and MacDONALD, J.C., Department of Geology, Lakehead University, Thunder Bay, On, P7B 5E1, andrew.conly@lakeheadu.ca Hogarth Pit Lake was formed from the continual flooding of the main open pit iron mine at Steep Rock Lake near Atikokan, Ontario. Hogarth Pit Lake is located in the former middle arm of Steep Rock Lake and was mined for itʼs predominantly goethite iron ores from 1954 to 1977. During this time over 25 million tons of ore were mined [1]. Water within the pit lake is characterized by near neutral pH and extraordinarily high SO42- concentrations (1000 to 3000 ppm; [2], this study). Consequently, the aim of the research is to determine the source(s) of the high SO42- levels. Possible sources of sulphate that are evaluated include: i) oxidation and leaching of finely disseminated sulphides within hanging wall and footwall rock units by groundwater; ii) oxidation and leaching of pyritic portions of the ore zone by groundwater; and, iii) oxidation and leaching of sulphides/sulphates contained in waste rock dumps and tailings piles by surface waters Hogarth Pit Lake is approximately 160 m deep, with a surface area of approximately 100 hectares and is elongated along a north-south axis. The lake is steep sided, well-sheltered by high rock walls. Footwall rocks, east of the Jolliffe ore zone, consist of (in ascending stratigraphic order) the Marmion Geniss Complex, metasedimentary rocks of the Wagita Formation and the Mosher Carbonate [3]. To the west, hanging wall rocks include ultramafic-mafic volcanic and volcaniclastic rocks of the Dismal Ashrock Formation, which is overlain by mafic to felsic volcanic rocks of the Witch Bay Formation [3]. Since cessation of mining activities the water level within the pit continues to increase (~ 3 m/year) from surface runoff and groundwater seepage. Environmental assessment studies completed shortly after closure of the mine indicate that approximately 85% of the water entering the pit is groundwater (R. Bernachez, personal communication, 2003). The lower contribution of surface water is consistent with the limited number of tributaries that currently flow into the pit lake. Water from Hogarth Pit Lake sampled for this study has a pH of ~7.6, contains 1340 to 1381 ppm of SO42, only trace levels (0.1-0.2 ppm) of Fe and Mn, and no detectable deleterious metals (As, Cd, Pb). Pit water has δ13C and δ34S values that range from 0.8 to 0.9‰ and -2.7 to -2.9‰, respectively. Surface waters that are following into the pit are characterized by a wide range in pH (2.8 to 8.1) and SO42- content (734 and 2580 ppm), with δ13C and δ34S values ranging from -20.3 to 1.5‰ and –3.6 to -0.4‰, respectively. The composition of surface and pit waters is comparable with the data of [2], although there is a trend of decreasing SO42- between 1998/99 and 2003. The variation in the chemistry of surface water reflects differences in wall-rock interactions. Surface waters with low pH and δ13C values reflect reactions with footwall and hanging wall volcanic and genesis units and organic matter. Water with near neutral pH values and δ13C values approaching 0‰ indicate buffering by the Mosher Carbonate. Groundwater was not sample as part of this study. However, regional groundwaters in the Atikokan area are generally enriched in Cl- with substantially lower SO42- contents than observed at Hogarth Pit [4]. However, some regional groundwaters have SO42- contents that approach the lower values for surface waters [4]. Tailings Lillill Pyric waste rock Ore zone Hanging wall C Hogarlh pit waters C Hanging wall tributaries Footwall tributaries P 6'$ (permil) Figure 1. Histogram showing the distribution -of12 sulphur isotope data for Hogarth Pit Lake waters The sulphur isotope composition of the pit and tributaries waters (Figure 1) is consistent with oxidation and dissolution of pyrite from the ore zone, waste rock dumps and the hangingwall. The data for the pyritic waste rock dumps is also used to better constrain the composition of pyritic Figure 1. Histogram showing the distribution of sulphur isotope data for Hogarth Pit Lake waters and lithological units �Proceedings of the 51st ILSG Annual Meeting - Part 1 portions of the ore zone, since only one in situ ore sample was obtained for the study. The shift to slightly more depleted δ34S values for the water samples relative to pyrite-bearing rocks is within the range anticipated for pyrite oxidation [5]. However, sulphur isotopes cannot be used in isolation to discriminate potential sources of SO42-. It is necessary to consider both the total sulphur content of the potential source material and water flow pathways. Consequently, the most likely source of SO42- is the oxidation and dissolution of pyrite contained with the Jolliffe ore zone by groundwater. The high bulk sulphur content (3.5-43.8 wt% as FeS2) of pyritic ore and waste rocks indicate that there is an amply supply of pyrite. Conversely, only very minor amounts of SO42- could have been derived from the hanging wall units and tailings, since these lithogies are characterized by low sulphur contents (<2 wt% and < 0.06 wt%, respectively). Furthermore, the substantially higher proportion of groundwater to surface water filling the pit indicates that water-rock interactions with exposed lithologies contribute a subordinate amount of SO42-. References [1] Taylor, B. 1978. Steep Rock The Men and the Mines. Quetico Publishing: Atikokan, Ontario, 114 p. [2] McNaughton, K.A. 2001. The limnology of two proximal pit lakes after twenty years of intense flooding. M.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 85p. [3] Stone, D., Kamineni, D.C. and Jackson, M.C. 1992. Precambrian geology of the Atikokan area, northwestern Ontario. Geological Survey of Canada, Bulletin 405, 106p. [4] Ophori, D.U. 1996. Regional groundwater flow in the Atikokan research area: spatial variable density and viscosity. Atomic Energy of Canada Limited Report AECL-11082, COG-93-184, 44 p. [5] Ohmoto, H. and Rye, R.O. 1979. Isotopes of sulfur and carbon. in Barnes, H.L., ed., Geochemistry of Hydrothermal Ore Deposits, 2nd edition: New York, John Wiley and Sons, p. 509-567. - 13 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Gold Grains, Pathfinder Elements, and Till Clast Composition in a Portion of the Vermilion Greenstone Belt, Northeastern Minnesota DAHL, David A., Lands and Minerals Division, Minnesota Department of Natural Resources, 1525 3rd Avenue East, Hibbing, MN 55746, dave.dahl@dnr.state.mn.us. An orientation project to determine variability of glacial till in the Mud Creek area of northeastern Minnesotaʼs Vermilion Greenstone Belt demonstrates that gold grains and pathfinder elements are present in basal till, and that anomalies stand out in contrast to regional background levels. Clastic and chemical variations within the till sample set are sufficient to consider using the basal till as sampling media for gold dispersal mapping. Of the thirty-two till samples analyzed, four were highly anomalous for gold, with counts of 88-1,282 gold grains per 10 kg of –2mm table sample, and pristine gold grain proportions up to 98%; up to 8,050 ppb gold in HMC (nonmagnetic heavy mineral concentrates); and up to 1,050 ppb gold in the -63µm silt/clay fraction of till. A suit of bedrock grab samples collected as reference mineralization returned assays up to 12,247 ppb Au, and silver concentrations up to 42,500 ppb. Analytical results for the till samples support a hypothesis that clastic dispersal trains of mineralized material exist in tills in the area. Within the project area, particulate gold is more anomalous in basal till samples than in the thin drape of overlying melt-out till. The gold grain counts and gold grain morphology add a transport distance value to chemical measurements of gold in soils and till, and suggest that the gold in the samples is locally derived. Pebble counts and pebble morphology similarly suggest a local derivation for the particulate gold. Analytical results for the present study are comparable to larger, more extensive regional evaluations conducted in neighboring Ontario and further confirm anomalous soil and fine fraction gold values reported in earlier Vermilion Greenstone Belt studies. Analysis of basal till in this portion of the Vermilion Greenstone Belt, particularly for gold grains, offers a capacity to positively detect local, previously unrecognized mineralization both inside and outside of areas of detailed bedrock geologic mapping. - 14 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 The Grenvillian Tomiko quartzites of Ontario: correlatives of the Baraboo quartzites of Wisconsin, the Mazatzal orogen of New Mexico, or unique? Implications for the tectonic architecture of Laurentia in the Great Lakes region EASTON, R.M., Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5, mike.easton@ndm.gov.on.ca The Baraboo and 6 other correlative red quartz arenite sequences in the southern Lake Superior region were deposited between 1710 and 1630 Ma, and locally were affected by regional deformation and metamorphism at about 1630 Ma, as well as by potassium metasomatism at 1465 Ma (Medaris et al., 2003). The Baraboo and related quartz arenites may be correlative with quartz arenites in the Athabasca and Thelon basins (Medaris et al. 2003), indicating widespread sedimentation across much of North America in the late Paleoproterozoic. Key features of the Baraboo and Athabasca basin quartz arenites, include the local presence of well-developed paleosols, a high-degree of supermaturity of the quartz arenites, and chemical index of alteration (CIA) values of 95-99. These three features have been interpreted to indicate that profound chemical weathering occurred across much of North America between 1750 and 1630 Ma (e.g., Medaris et al., 2003). A thick sequence of paragneiss, including quartz arenite and iron formation (hereafter referred to as the Tomiko supracrustal rocks), occurs within the Tomiko terrane of the northern Central Gneiss Belt of the Grenville Province in Ontario. Holmden and Dickin (1995), based on Nd-Sm model age data and regional tectonics, suggested that the Tomiko quartz arenites were possible correlatives of the Baraboo and related quartz arenite sequences. If valid, then this correlation indicates that sedimentation and weathering between 1710 and 1630 Ma occurred east of Lake Superior, and that rocks of the Penokean orogen may underlie large parts of the northern Central Gneiss Belt in Ontario. Testing this correlation has been problematic, as until recently, only limited mapping and geochronological data were available for Tomiko terrane. Mapping during the 2003 field season (Easton, 2003) provides additional constraints for regional correlation, as follows: • Discovery of B-rich (>15% tourmaline) and Mn-rich rocks in association with magnetite-chert iron formation in northern Tomiko terrane, as well as epidote-rich (>20%) rocks in association with rusty gneisses near Crocan Lake and chemically unusual (high TiO2 & P2O5) calc-silicate rocks). • The recognition of possible dacitic metavolcanic rocks and sills interlayered with the Tomiko supracrustal rocks, along with amphibolites that may represent sills or flows. • The possibility that many of the muscovitic and feldspathic units within the Tomiko supracrustal sequence may represent hydrothermally altered rocks (CIA of 60 to 64). • The recognition of possible Archean “basement” rocks infolded with the Tomiko supracrustal rocks. This basement is likely structural, not depositional. • The identification of at least 3 metamorphic events affecting the Tomiko supracrustal rocks, with the intensity of the 2 latest events increasing to the southeast. M2 forms a migmatite front between northern and southern Tomiko terrane, with lower grade rocks to the north. M3 is a regional hydrothermal event, and results in the replacement of blue kyanite and granite leucosome formed during M2 by pale-green kyanite and quartz segregations. • Stratigraphic correlation with metasedimentary rocks of the Huronian Supergroup, exposed immediately to the north, is unlikely, based on geochemistry and Nd/Sm ages. On the basis on the new data, correlation of the Tomiko rocks with the Baraboo and related quartz arenites is still compelling. The Tomiko quartz arenites appear to be mature rather than supermature (CIA of 69 to 78 for quartzites), even though they are associated with metamorphosed volcanic and plutonic rocks and minor amounts of iron formation. It can be argued that kyanite-bearing and muscovite-rich gneisses in Tomiko terrane represent a combination of metamorphosed paleosols and potassium metasomatized quartz-rich sedimentary rocks, respectively. U/Pb zircon ages on detrital zircons from quartz arenite at two localities at different metamorphic grades give a maximum depositional age of 1687 Ma (Krogh, 1989; Easton and Kamo, 2004). Minimum - 15 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 depositional age is 1250±10 Ma, based on the age of grey gneisses interlayered with the quartz arenite; identical within error to the age of several A-type granite plutons that intrude the Tomiko supracrustal rocks (1257+4/-2 and 1244+4/-3 Ma; Easton and Kamo, 2004; Lumbers et al., 1991, respectively). If the correlation with the Baraboo is valid, then the Tomiko supracrustal rocks likely have limited mineral potential. Equally compelling, however, is that the Tomiko supracrustal rocks have many lithologic, stratigraphic and geochronologic similarities to Geon 17 rocks of the Mazatzal orogen exposed in Arizona and New Mexico (Tonto Basin Supergroup and Hondo Group, respectively). For example, the Hondo Group overlies a sequence of older greenstones, and consists of a thin basal unit of quartz pebble conglomerate, over 1 km of quartz arenite, and a capping sequence of muscovite schist and arenite, metarhyolite, phyllite and calc-silicate rocks (Robertson et al., 1993). A manganese-rich horizon, possibly a paleosol, perhaps lateritic, occurs at the top of the greenstones. Furthermore, some of the muscovite schists in the Hondo Group represented hydrothermally altered felsic volcanics. Rocks of the Hondo Group were deposited between 1700 and 1644 Ma (Robertson et al., 1993), and contain detrital zircons ranging in age from 1850 to 1700 Ma. A key difference is that plutons intruded the Hondo Group between 1680 and 1650 Ma, not at circa 1250 Ma. The rock types, stratigraphy, thickness, alteration history and age of the Tonto Basin Supergroup and the Hondo Group is similar to that observed in the Tomiko supracrustal rocks, even though the Mazatzal rocks in New Mexico are now some 3500 km distant. If correlation with the Mazatzal Orogen is valid, then the iron formation in northern Tomiko terrane may be equivalent to the manganese-rich unit at the base of the Tonto Basin Supergroup and the Hondo Group, and may have served as the decollément surface along which northward-directed thrusting occurred. If the Tomiko supracrustal rocks do represent a sliver of the Mazatzal Orogen, which at one time must have stretched from Arizona to Labrador, then the eastern end of the Penokean Orogen has been truncated in Ontario. A third possibility, that the Tomiko quartz arenites were deposited close to 1270 Ma, similar in age to the grey gneiss units interlayered with them, would mean that the sequence is unique in the Lake Superior region, and further begs the question of what happens to the Penokean Orogen east of the Grenville Front? References Easton, R.M. 2003. Reconnaissance study of the geology and mineral potential of the eastern Tomiko terrane, Grenville Province; in Ontario Geological Survey Open File Report 6120, p. 16-1 to 16-25. Easton, R.M. and Kamo, S.L. 2004. The Grenvillian Tomiko quartzites of Ontario: correlative with the Baraboo quartzites of Wisconsin or the Mazatzal orogen of New Mexico? Implications for the tectonic architecture of Laurentia in the Great Lakes region; Geological Society of America, Abstracts with Program, 36, no.5, p.A-459. Holmden, C. and Dickin, A.P. 1995. Paleoproterozoic crustal history of the southwestern Grenville Province; Canadian Journal of Earth Sciences, 32, 472-485. Krogh, T.E. 1989. Provenance and metamorphic ages in the Grenville (NW); in Lithoprobe Abitibi-Grenville Project Workshop, March 1989, p. 5-7. Lumbers, S.B., Wu, T-W, Heaman, L.M., Vertolli, V.M., and MacRae, N.D. 1991. Petrology and age of the A-type Mulock granite batholith, northern Grenville Province, Ontario; Precambrian Research, 53, 199-231. Medaris, L.G., Jr., Singer, B.S., Dott, R.H., Jr., Naymark, A., Johnson, C.M. and Schott, R.C. 2003. Late Paleoproterozoic climate, tectonics, and metamorphism in the southern Lake Superior region and proto-North America: evidence from Baraboo interval quartzites; Journal of Geology, 111, 243-257. Robertson, J.M., Grambling, J.A., Mawer, C.K., Bowring, S.A., Williams, M.L., Bauer, P.W. and Silver, L.T. 1993. Precambrian geology of New Mexico; in Precambrian Conterminous US, Geological Society of America, The Geology of North America, Volume C-2, p. 228-238. - 16 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Stratigraphy of the Mesoproterozoic Sibley Group and Nipigon Sills FRALICK, Philip, METSARANTA, Riku and ROGALA, Becky, Department of Geology, Lakehead University, Thunder Bay, ON, Canada, P7B 5E1 This paper is designed to complement papers on the geochemistry of the Sibley Group (Metsaranta and Fralick, this volume) and Nipigon Sills (Richardson and Hollings, this volume). It will review the existing nomenclature and highlight recent advances made in deciphering the stratigraphy of the Sibley Basin during field investigations forming a portion of the Lake Nipigon Region Geoscience Initiative. Rocks correlated with the Sibley Group are present primarily to the south and west of Lake Nipigon. They are divided into five Formations (Franklin et al., 1980; Cheadle, 1986; Fralick et al., 2000; Rogala, 2003) encompassing, at their thickest, approximately nine hundred meters of stratigraphic section. The basal Pass Lake Formation lies on metamorphosed igneous and sedimentary rocks, except for a limited area in the southwest where it overlies Paleoproterozoic strata of the Rove Formation. Evidence for the existence of small hills and valleys on the paleosurface is common. Age of the Pass Lake Formation is constrained by a Rb-Sr isochron on overlying dolomitic mudstones of 1339±33 Ma (Franklin, 1978), probably representing a diagenetic reset, and detrital zircon geochronology on a Pass Lake sandstone giving a concordant youngest zircon age of approximately 1650 Ma (see Heaman and Easton, this volume). The conglomerates and sandstones of this unit represent a number of depositional environments ranging from Scott-type and South Saskatchewan-type braided streams to cobble beaches, offshore storm sand sheets and possible aeolian dunes. The subaerial environments, where present, are situated at the base of the section, overlain by a flooding event. As the water depth increased the nearshore, storm deposited, sand sheets were succeeded upwards by laminated and massive silt (Rossport Formation). Delta progradation reversed this trend in areas proximal to sediment entry points. Caliche developed in underlying fluvial, off-channel deposits attests to a semi-arid climate and this caused the water body to become saline. Cyclically banded red and cream coloured dolomitic siltstone accumulated as salinity increased. The millimeterto decimeter-scale banding probably reflects periodic movement of the redox front from the water mass to beneath the sediment surface, possibly driven by organic loading. The central portions of thicker, light coloured layers contain bladed crystals and small nodules of gypsum attesting to peak aridity during oxygen minima in the bottom sediments. As the water mass contracted sand, possibly derived from the south, formed extensive sheet deposits in most areas. The strandline is marked by the sporadic development of algal flat deposits with carbonate silt storm layers. Though most of this one to seven meter thick calcareous unit is dominated by smooth mat and breccia small panicles and more classic stromatolites are present. Tepees and gypsum filling stromatactis are also encountered, though more rarely. The upper few centimeters of the carbonates is intensely weathered and commonly overlain by either an excellently developed terra rosa or subareal debris-flows. Regional computer modeling of thickness data indicates that this interval may represent initiation of north-south orientated, half graben formation in the area. The succeeding silt-dominated mudflat assemblage contains abundant nodular and vein gypsum, attesting to the continuation of dry conditions, albeit with a high water table. The transition from the Rossport to Kama Hill Formation is marked by a flooding event and re-establishment of open water conditions, though no evaporite minerals were precipitated from this water mass. Purple shales dominate the lower portion of the succession with thin ripple laminated, fine-grained sandstones, which become more numerous, thicker and coarser grained upward. Trough and hummocky cross-stratified , medium-grained sandstones (Outan Island Formation) appear and then dominate the section, continuing the coarsening upward trend. Slump scars and large areas of chaotic slide-block deposits attest to the periodic failure of, in places, extensive portions of the outbuilding delta complex. The delta top deposits consist of fining and thinning upwards sandstone successions meters to a few tens of meters thick (channels) interbedded with fine-grained sediments (floodplains). The floodplains are mostly composed of friable mudstone with soil horizons representing subareal accumulation but parallel laminated shales deposited in floodplain ponds are also present. The upper deltaic and fluvial deposits of the Outan Island Formation are only present in a core from a hole drilled in Nipigon Bay. - 17 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Detrital zircon geochronology performed on this unit gave a maximum age of deposition of approximately 1470 Ma (see Heaman and Easton, this volume). The Outan Island Formation is erosively truncated and overlain by the Nipigon Bay Formation. Planar cross-sets up to 17 meters thick indicate that the winds which deposited this sea of aeolian sand had a predominant direction from the northeast. The medium-grained sandstones of the Nipigon Bay Formation comprise the uppermost 400 meters of the Sibley Group with conglomerates of the basal Osler Group unconformably overlying them. At approximately 1109 Ma, diabase sills were intruded into the Sibley sedimentary rocks as the Midcontinent Rift established itself in the area. The southernmost region underlying Nipigon Bay has the most complete section of Sibley strata, but only one thin sill near the base of the assemblage. Sibley on the adjacent mainland contains a thick sill in the basal Kama Hill Formation, which is traceable across the southern portion of the basin. Thus, the thick sills are confined to north of Nipigon Bay with a fundamental divide running down this topographic feature. Moving from the southern to central basin a second, lower sill appears in the stratigraphy and sill width increases (toward Muskrat Lake) where their composite thickness is approximately 300 meters. Rossport Formation strata between the two thick sills in the central basin have been extensively thermally metamorphosed reaching temperatures of 600°C locally and 300-400°C tens of meters from the sills. The sills are commonly locally discordant, up- and down-ramping through the stratigraphy. They also appear to be vertically offset by faults, in places defined by surface lineaments. Toward the northern portion of the basin the erosion level cuts downward through the stratigraphy, with only the lower sill and a thin layer of underlying, Sibley-like sediment preserved. In the northernmost region this sill directly overlies a Mesoproterozoic volcanic-intrusive complex. To the east of the basin marginal Black Sturgeon Fault a sill overlies a thin, sporadically present, baked sedimentary layer lying on basement; a scenario similar to the northern area west of the Fault. Generally sill thickness decreases to the west of the central basin and the sills may bifurcate in this direction as well. An anomalous area exists in the western region, northwest of Muskrat Lake where a drillhole encountered a thick (500 m) sequence of diabase. The significance of this occurrence is unknown. Regional sill stratigraphy implies the feeder area was probably in the vicinity of Muskrat Lake and possibly related to the Black Sturgeon Fault system. References Cheadle, B.A., 1986. Alluvial-playa sedimentation in the lower Keweenawan Sibley Group, Thunder Bay District, Ontario. Canadian Journal of Earth Sciences, vol. 23, p. 527-542. Fralick, P.W., Smyk, M. and Mailman, M., 2000. Geology and stratigraphy of the Mesoproterozoic Sibley Group. Institute on Lake Superior Geology, Proceedings Volume46, Part 2: Fieldtrip Guidebook, yellow section. Franklin, J.M., 1978. The Sibley Group, Ontario. In, Rubidium-strontium isochron age studies, Report 2, Geological Survey of Canada, Paper 77-14, p. 31-34. Franklin, J.M., Mcilwaine, W.H., Poulsen, K.H. and Wanless, R.K., 1980. Stratigraphy and depositional setting of the Sibley Group, Thunder Bay District, Ontario, Canada Canadian Journal of Earth Sciences, vol. 17, p. 633-651. Rogala, B., 2003. The Sibley Group: a lithostratigraphic, geochemical, and paleomagnetic study. Unpubl. M.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 254p. - 18 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Mineral Resources for the Future: The Resource Potential of Northern Lake Superior FRANKLIN, James M., Franklin Geosciences Ltd., 24 Commanche Drive, Nepean, Ontario, K2E 6E9 The demand per person in the industrialized nations for primary “old - economy” metals has increased tenfold since 1900. Globalization in recent years has increased competitiveness and access to highly productive regions such as South America, Africa and Oceana, leading to an overall decline in metal prices of 40%. The demand for “new - economy” metals has increased 5 to 100 times since 1985; their prices have increased significantly. In the past five years, industrial expansion in China and India has created an almost unprecedented need for new metal resources. Even with recycling, vast quantities of new resources, particularly for “green” products, must be found, mined and processed in environmentally sustainable ways. The geological attributes of the Northern Lake Superior region provide an outstanding opportunity for exploration for an exceptional range of commodities and ore deposit types. Archean volcano-sedimentary strata with much additional potential include the historically productive Abitibi – Wawa and Wabigoon belts, with world – class volcanogenic massive sulfide deposits (Manitouwadge district), orogenic gold deposits (Geraldton) and a porphyry/epithermal district (Hemlo). Significant pegmatite bodies (Georgia Lake) and Algoma iron formations (Wawa) are also present. Early Proterozoic strata are comprised of an extensional and miogeoclinal sequence that includes the giant Gunflint Superior-type iron formation. .Mid Proterozoic intrusions associated with magmatic plumes (~1500-1600Ma) and the Keweenawan plume-intracratonic rift assemblage (1130-1090Ma) have proven potential for Cu-Ni-PGE deposits, Nb-REE in carbonatite, as well as potential for IOCG (REE-Zr) mineralization. Vein deposits that are contemporaneous with Keweenawan magmatism include Pb-Zn-Ba unconformity-vein deposits, two generations of Ag veins, and unconformity-style uranium occurrences. Epithermal and breccia – pipe copper deposits occur near Batchawana Bay. Redbed copper occurrences are associated with interflow sedimentary strata in the Osler volcanic sequence. Future discoveries require innovative exploration. Quantitative estimation of key geological attributes (e.g. magma fluid contents, paleo- permeability, and fault dynamics) must be applied vigorously in the search for ore. Geophysical techniques normally used for deep imaging of the earth (seismic, magnetotelluric) are being adapted for exploring shallow crystalline terrains. Developing quantitative models of ore forming processes that can be applied at all scales will ensure the supply of metals needed for the rapidly developing nations, and for improved quality of life everywhere. - 19 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Dyke swarms around the Lake Superior Region define ~2 Ga Proterozoic deformation of the Archean Superior Province associated with evolution of the Kapuskasing Zone HALLS, H.C., Department of Geology, University of Toronto, hhalls@utm.utoronto.ca Proterozoic dyke swarms in the Archean Superior Province straddle a ~2 Ga crustal dislocation, the Kapuskasing Zone (KZ), that extends for more than 500 km from James Bay to Lake Superior and divides the Archean Superior Province into eastern and western halves. About ten swarms are present ranging in age from the Matachewan, (2446-2473 Ma) to the Keweenawan (1108-1094 Ma). Nearly all the swarms include at least one reversal of the earthʼs magnetic field. During the Matachewan igneous episode only one reversal, from R to N, occurred at 2446 Ma. Within the KZ only N dykes are found, but outside, R dykes outnumber N by more than 4 to 1. N dykes are relatively common in the focal region of the swarm north of Lake Huron, but gradually disappear to the northwest with only R dykes being found in the Lake Nipigon region. The KZ can be divided up into a number of domains based on Matachewan dyke polarity, the boundaries between R and N domains being major faults that define the crustal uplift along the KZ. N polarity dykes within the KZ represent the roots of R dykes that have been remagnetized at depth before being uplifted. Using the polarity domain concept, major faults have been mapped that define a new segment of the KZ at its southwestern end (the Pineal Lake block), which is offset about 20 km sinistrally from the main Chapleau block to the NE1. The pattern of polarity domains follows exactly that produced by variations in feldspar clouding intensity within the dykes, whereby the clouding, (caused by exsolution of magnetite and other minerals in groundmass feldspars) occurs only in N polarity domains. The secondary magnetite, thought to be caused by slow cooling of the dykes at large crustal depths, increases in concentration with increasing depth. The growth of this magnetite led to the secondary N magnetization. Only a few dykes within the KZ show vestiges of their formerly R polarity1. When Matachewan dyke populations in regions of about 100 km in lateral dimensions were compared, a positive correlation was found 2 between mean Matachewan dyke trend and magnetization declination, a correlation that was absent between dykes within the areas. This result suggested that the variation in mean dyke trend was a product of differential crustal rotation about vertical axes, and therefore that the broad Z-shaped trend of the dykes across the southwestern end of the KZ was the result of the deformation of an originally more linear dyke swarm2. Further work on Matachewan dykes3, and also on younger swarms that have equivalents on both sides of the KZ (2170 Ma Biscotasing4, 2076 Ma Fort Frances/2069 Ma Lac Esprit5), confirm that the western half of the Superior Province has rotated counterclockwise about 20° with respect to the eastern half. More recent work on 2101-2121 Ma Marathon dykes6 suggests that the western half of the Superior Province has not rotated as a single unit because the KZ as a fault zone extends for more than 100 km to the northwest of the main zone of crustal uplift. The age of the rotation is thought to be about 2 Ga old and related to the deformation along the KZ. A possible consequence of the rotation is that a rift in the Superior province opened beneath Hudson Bay and remained a region of crustal weakness to be subsequently exploited in later Phanerozoic subsidence that formed the Hudson Bay basin and associated lowlands4. References [1] Halls, H. C., Zhang, B. 2003. Crustal uplift in the southern Superior Province, Canada, revealed by paleomagnetism. Tectonophysics 362, 123-136. [2] Bates, M. P., Halls, H .C. 1991. Broad-scale Proterozoic deformation of the central Superior Province revealed by paleomagnetism of the 2.45 Ga Matachewan dyke swarm. Canadian Journal of Earth Sciences 28, 1780-1796. [3] Halls, H. C., Stott, G. 2003. Paleomagnetic studies of mafic dykes in the vicinity of Lake Nipigon, northwestern Ontario. - 20 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 in Summary of Fieldwork and Other Activities 2003, Ontario Geological Survey, Open File Report 6120, p. 11-1 to 117. [4] Halls, H.C., Davis, D.W. 2004. Paleomagnetism and U-Pb geochronology of the 2.17 Ga Biscotasing dyke swarm, Ontario, Canada: evidence for vertical-axis crustal rotation across the Kapuskasing Zone. Canadian Journal of Earth Sciences 41, 255-269. [5] Buchan, K.L., Goutier, J., Hamilton, M., Ernst, R E., Mathews, W. 2004. Paleomagnetism of the Lac Esprit dykes and implications for crustal rotation of the Canadian Shield. AGU-CGU Meeting, Montreal [Abstract]. [6] Halls, H.C., Davis, D.W., Stott, G. 2005. Paleomagnetism and U-Pb dating of Proterozoic dykes: a new radiating swarm and an increase in post-Archean crustal rotation westwards from the Kapuskasing Zone, Ontario. GAC-MAC Halifax, Meeting, [Abstract]. - 21 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Mesoproterozoic Diabase Sills of the Nipigon Embayment, northwest Ontario HART, T.R., Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5 (tom.hart@ndm.gov.on.ca) and MacDONALD, C.A., Caracle Creek International Consulting Inc., Sudbury, Ontario, P3E 2V7 The Nipigon Embayment is an approximately 19,000 km2 area of Mesoproterozoic igneous and sedimentary rocks centred on Lake Nipigon, north of Lake Superior. The current extent of the Embayment is defined by the sporadic occurrence of diabase sills suggesting that the Embayment was probably more extensive prior to erosion. A series of shallow dipping, 1110 - 1113 Ma, diabase sills of the Nipigon Sill Complex (Heaman et al., 2005) intrude all Archean and Proterozoic rocks in the Embayment. The Nipigon Sill Complex is composed of at least 3 major, and a series of minor, diabase sills ranging in thickness from a few tens of metres to >200 m. The diabase is commonly sub-ophitic to ophitic, massive, medium- to coarse-grained feldspar and pyroxene with trace olivine and 1 to 2% magnetite. Medium- to coarsegrained diabase forming the majority of the sills should properly be classified as gabbro, but to avoid confusion with other intrusions in the area the diabase classification has been applied to all rocks associated with the sills. The sills are generally shallow dipping occasionally forming broad, up to 6 km wide, saucers with interiors occupied by older rock types (Figure 1). Figure 1. First vertical derivative of the total field magnetic data (Ontario Geological Survey, 2004) showing an inward dipping saucer shaped diabase sill in the southern Black Sturgeon River map area (Hart, 2005). The geometry of these saucers is in part evident from contact relationships observed during bedrock mapping (e.g. Hart, 2005), and from the magnetic patterns on the first vertical derivative of the total field airborne magnetic data (e.g. Ontario Geological Survey, 2004). Modelling a saucer located in the Garden Lake greenstone belt of the Wabigoon Subprovince, west of Lake Nipigon, produced two possible models (J. Rudd, Ontario Geological Survey, personal communication, 2000). However, the model of a shallow dipping thin sill best agreed with field observations, and is the best fit between field observations and geophysical data for similar structures in the Beardmore area (Hart et al., 2000) and in the southern Black Sturgeon River area (Hart, 2005). Similar sill geometry has been suggested for other mafic sill complexes (e.g., Thomson and Hutton, 2004), although the saucers and nested saucers structures in those areas are generally smaller in diameter. One reason for the large scale of the saucers in the Nipigon Embayment may be the more competent nature of the Archean basement rocks compared to the host rocks of other sill complexes. - 22 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 The majority of the sills have relatively uniform mineralogy and geochemistry complicating the correlation of the sills within the Embayment. Minor stratigraphic variations within some sills suggest that they may have formed by injection of multiple pulses of magma. However, a thickening of some sills to 250 and 400 m in thickness in the Muskrat and Gieke lakes area, in combination with geochemical trends, suggests that these thicker sills are single cooling units, and may also indicate proximity to the feeders for the sills. The exception to the uniform geochemistry of the Nipigon sills is the >1120 Ma Inspiration Sill (Heaman and Easton, 2005) in the northwest portion of the Nipigon Embayment, which has higher La/Yb and Zr/Y ratios and a positive magnetic polarity in contrast to the negative polarity of the Nipigon sills (MacDonald, Tremblay and Easton, 2005). Another significant geochemical change occurs around Thunder Bay (Hart, 2004) where the majority of the sills located to the south of the city are part of the 1115±1 Ma Logan Sill Complex (Heaman and Easton, 2005). Similar geochemical variations related to geographic position have been observed in some flood basalt provinces (e.g., Mantovani et al., 1985) suggesting that the Logan and Nipigon sills may be part of a single intrusive event. References Hart, T.R. 2004. Geochemistry of the Proterozoic intrusive rocks of the Nipigon Embayment; abstract in Institute on Lake Superior Geology, Proceedings, 50th Annual Meeting, Duluth, Mn, v.50, pt.1, p.68-69. Hart, T.R. 2005. Precambrian Geology of the South Black Sturgeon River and Seagull Lake Area, Northwestern Ontario; Ontario Geological Survey, Open File Report 6165, 63p. Hart, T.R., terMeer, M. and Jolette, C. 2002. Precambrian Geology of Kitto, Eva, Summers, Dorothea and Sandra Townships, Beardmore Area, Northwest Ontario. Ontario Geological Survey, OFR 6095, 206p. Heaman, L.M. and Easton, R.M. 2005. Proterozoic history of the Lake Nipigon area, Ontario: Constraints from U-Pb zircon and baddeleyite dating; abstract in Institute on Lake Superior Geology, Proceedings, 51ts Annual Meeting, Nipigon, Ontario, v.51, pt.1. Heaman, L.M., Easton, R.M., Hart, T.R., MacDonald, C.A., Fralick, P., and Hollings, P., 2005. Proterozoic history of the Lake Nipigon area, Ontario: Constraints from U-Pb zircon and baddeleyite dating; Canadian Institute of Mining and Metallurgy Annual Meeting, Toronto 2005, Program with Abstracts. MacDonald, C.A., Tremblay, E., and Easton, R.M. 2005. Precambrian Geology of the west-central map area, Nipigon Embayment, northwestern Ontario; Ontario Geological Survey, Open File Report 6164, 48p. Ontario Geological Survey 2004. Ontario airborne geophysical surveys, magnetic and gamma-ray spectrometer data, grid and vector data, ASCII format, Lake Nipigon Embayment Area; Geophysical Data Set 1047a. Thomson K. and Hutton, D. 2004. Geometry and growth of sill complexes: insights using 3D seismic from the North Rockall Trough; Bulletin of Volcanology 66, 364-375. - 23 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Proterozoic history of the Lake Nipigon area, Ontario: Constraints from U-Pb zircon and baddeleyite dating HEAMAN, L.M., Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3 larry.heaman@ualberta.ca and EASTON, R.M., Precambrian Geoscience Section, Ontario Geological Survey, Sudbury, Ontario P3E 6B5 The Lake Nipigon Region Geoscience Initiative, a 2-year, science-focused, Industry-Government-University collaborative geological study, collected new geoscience data in the Lake Nipigon region of Ontario between May 2003 and May 2005. As part of this study, over 40 new U-Pb baddeleyite and zircon age determinations were acquired from predominantly mafic rocks of the Lake Nipigon region. Preliminary results of this study are reported herein. William Logan first described the copper-bearing rocks of the Midcontinent Rift (MCR) in Ontario in 1863. Subsequent studies by many researchers indicate that the MCR in Ontario comprises 3 main tectonic elements: 1) the volcanic and sedimentary rocks in and along the margin of Lake Superior, 2) associated alkalic complexes, 3), the mafic and ultramafic intrusions centred on Lake Nipigon, and whether they represent a “failed arm” of the main rift. Work in Ontario has tended to focus on the Lake Nipigon area, because of accessibility and mineral potential. The best estimate for a maximum depositional age of a greywacke from the upper Rover Formation is provided by a 1777 Ma concordant detrital zircon grain, as the youngest grain in the sample, at 1731 Ma, is 5% discordant. This interpretation is supported by the abundance of grains (n=10) between 1796 to 1777 Ma, and is consistent with U-Pb ash bed ages of 1836±5 and 1832±3 Ma from the basal Rove Formation (Addison et al., 2005). Detrital zircon ages from sandstones of the lower (Pass Lake Formation) and upper (Nipigon Bay Formation) Sibley Group indicate maximum depositional ages of 1634 and 1670 Ma, respectively, with a predominance of Geon 17 and 18, not Archean, detritus. Data from the middle to upper (Outan Island Formation) Sibley Group indicates a maximum depositional age of 1450 Ma, as well as Geon 15 and 17 detritus, but no Geon 18 grains. The Geon 15 detritus may be locally derived, e.g. from the 1547±4 Ma English Bay complex volcanic rocks, the 1590±1 Ma Pillar Lake felsic intrusion, and the 1599±1 Ma Pillar Lake gabbro. In addition, sediment interbedded with flat-lying mafic volcanic rocks south of Armstrong are younger than 1514 Ma, based on detrital zircons in interbedded sediments, but older than overlying, circa 1159 Ma (minimum age of 1120±1 Ma), Inspiration diabase sills. The abundance of Geon 15 ages in the western Nipigon Embayment is impressive, as the period between 1600 and 1520 Ma in eastern North America has been recognized for some time as a period of quiescence throughout Laurentia (Gower et al., 1990). Furthermore, there are no dike swarms or reliable paleopoles for the period 1700 to 1500 Ma. The only other reliable Geon 15 age in North America is the 1576+/-13 Ma upper intercept age from the Priest River metamorphic complex in Idaho, which served as basement to the circa 1450 Ma Belt and Purcell Supergroups (Evans and Fischer, 1986). Gower (1996) suggested that, at least in the eastern Grenville Province, this quiescence might be linked with the development of a passive continental margin, however, such a setting does not explain the felsic magmatism present in the Nipigon area. The new U-Pb data obtained in this study suggest the presence of an earlier period of magnetically normal, localized alkalic (lamprophyre dikes, Queen et al., 1996) and mafic magmatism (Inspiration sills and the Pigeon River dikes (1141±20 Ma)) between 1150 and 1135 Ma in the MCR in Ontario, including the possibility of localized volcanism and sedimentation in the Pillar Lake area south of Armstrong. Baddeleyite data from the 4 ultramafic intrusions in the region shows some scatter within individual - 24 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 intrusions (e.g. from 1113 to 1124 Ma in the Seagull intrusion), which could indicate a protracted (up to 10 m.y.) crystallization history for some of these bodies. In addition, the suite of ultramafic intrusions has a range of ages from circa 1118 to 1107 Ma, however, the aforementioned scatter makes it difficult to ascertain the significance of this range – does it reflect protracted cooling or different emplacement times, or both? A related ultramafic body, the Jackfish Island sill, cuts the English Bay complex and is geochemically similar to the Kitto intrusion, although it appears to be slightly younger in age (1112±3 Ma versus 1118±2 Ma, respectively). In contrast, baddeleyite ages from 4 different Nipigon diabase sills analyzed so far cluster between 1110 and 1114 Ma. Although geochemically distinct from the Nipigon sills (Hart, 2003), a sample from the Logan sill at Mount McKay in Thunder Bay, is similar in age, at 1115±1 Ma. Thus, most mafic and ultramafic rocks in the Lake Nipigon and Superior areas, including the Nipigon and Logan sills, appear to have been emplaced in a short, magnetically reversed, internal between 1115 and 1100 Ma. Emplacement of alkalic intrusions, such as the Coldwell complex, as well as filling of much of the submerged part of the rift in Lake Superior, also occurred in this period. This was followed by a period of magnetically normal, waning mafic and felsic magmatism, between 1096 and 1085 Ma, that is preserved mainly along the Lake Superior shore by units such as the Crystal Lake (1099±1 Ma), Moss Lake (1095±2 Ma) and Blake Lake (1095±2 Ma) gabbros, and the Arrowhead dike (1093±3 Ma). References Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J. Davis, D.W. Kissin, S.A. Fralick, P.W. and Hammond A.L. 2005. Discovery of detrital ejecta from the 1850 Ma Sudbury impact event; Geology, 33, 193-196. Evans, K.V. and Fischer, L.B. 1986. U-Pb geochronology of two augen gneiss terranes, Idaho – new data and tectonic implications; Canadian Journal of Earth Sciences, 23, 1919-1927. Hart, T.R. 2003. Keweenawan mafic and ultramafic intrusive rocks of the Lake Nipigon and Crystal Lake areas, northwest Ontario; Institute on Lake Superior Geology, Proceedings, 49, pt.1, Programs and Abstracts, 21-22. Gower, C.F. 1996. The evolution of the Grenville Province in eastern Labrador Canada; in Precambrian crustal evolution in the North Atlantic Region, Geological Society of London, Special Publication 112, 197-218. Gower, C.F., Ryan, A.B. and Rivers, T. 1990. Mid-ProterozoicLaurentia-Baltica: an overview of its geological evolution and a summary of the contributions made by this volume, in Mid-Proterozoic Laurentia-Baltica, Geological Association of Canada, Special Paper 38, 1-20. Queen, M., Heaman, L.M., Hanes, J.A., Archibald, D.A. and Farrar, E. 1996. 40Ar/39Ar phlogopite and U-Pb perovskite dating of lamprophyre dikes from the eastern Lake Superior region: evidence for a 1.14 Ga magmatic precursor to Midcontinent Rift volcanism; Canadian Journal of Earth Sciences, 33, 958-965. - 25 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Penokean Fold-and-thrust Deformation of the Paleoproterozoic Gunflint Formation near Thunder Bay, Ontario HILL, Mary Louise, Department of Geology, Lakehead University, Thunder Bay ON P7B 5E1 CANADA and SMYK, Mark C., Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 CANADA Flat-lying, Paleoproterozoic Gunflint Formation chemical and clastic sedimentary rocks unconformably overlie Archean basement rocks near Thunder Bay. Folds have long been recognized in the Gunflint Formation near Pass Lake, 40 km northeast of Thunder Bay, but no tectonic explanation has previously been demonstrated. The recent recognition of thrust faults in this area appears to link this folding to Penokean compression. Although Penokean orogenic events (circa 1875 to 1835 Ma) and resultant deformation have been well established south and west of Lake Superior, no structures ascribed to Penokean deformation have ever been described north of Lake Superior. Recent geochronologic data show that the Gunflint Formation (circa 1878 Ma) predates the earliest Penokean thrusting and thrust-loading caused by the collision of the Pembine-Wausau terrane (circa 1860-1850 Ma). This prompted Fralick et al. (2002) to contend that the Gunflint formed in a back-arc extensional setting, rather than in a foredeep (foreland basin) and also suggests that Penokean structures should exist in these rocks. Recent examination of the Gunflint Formation near Pass Lake has led to the recognition of structures typical of fold-and-thrust belt deformation. Discrete bedding-plane faults with locally developed gouge and breccia can be traced laterally into horizontal, hanging-wall ramps with associated fault-bend folding. Fold-and-thrust belt deformation is caused by regional compression. Previous workers had ascribed the folds to syn-sedimentary slumping and Keweenawan diabase sill emplacement and thought that they were attributable to local, rather than regional-scale, deformation. Displacement in fold-and-thrust belts tends to be localized along discrete bedding planes and not easily recognized. This may account for the perceived lack or absence of structures elsewhere in the Gunflint Formation. Penokean structures on the northern side of Lake Superior represent the northward migration of thrust faults into the foreland (passive margin Archean basement + Gunflint Formation) caused by hinterland collision to the south. Future work will focus on recognizing other deformed locales and quantifying fault displacements based on stratigraphic correlation of sedimentary units. References Fralick, P., Davis, D.W. and Kissin, S.A. 2002. The age of the Gunflint Formation, Ontario, Canada: single zircon U–Pb age determinations from reworked volcanic ash; Canadian Journal of Earth Science, v.39, p.1085–1091. - 26 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 The incredible shrinking Penokean orogen: a new look at the accretionary history of the southern Lake Superior region HOLM, D.K., Dept. of Geology, Kent State University, Kent, OH; CANNON, W.F., USGS, Reston, VA; CHANDLER, V.W., MGS, Mnpls, MN; SCHNEIDER, D.A., Ohio University, Athens, OH; SCHULZ, K., USGS, Reston, VA; and VAN SCHMUS, W.R., University of Kansas, Lawrence, KS. The Penokean orogen in the stable continental interior of the upper Great Lakes region records the initiation of rapid late Paleoproterozoic (1.9-1.6 Ga) southward growth of Laurentia. The Penokean orogeny is generally accepted to have been a period of southward subduction from about 1900 to 1850 Ma during which juvenile Paleoproterozoic terranes were accreted to the southern edge of the Superior craton (Archean) and foreland sedimentation and deformation occurred on the craton margin. Historically, juvenile Penokean crust was interpreted to extend southward beneath Paleozoic and Cretaceous strata into southern Iowa, southern Wisconsin, and northern Illinois. However, a growing body of geochronologic data suggests that there may be no rocks older than 1.75 Ga in those areas and that the current Penokean orogen has a much more restricted geographic distribution than heretofore inferred. We have identified a fundamental ENE-trending boundary, the Spirit Lake-Trempealeau discontinuity CB: Cheyenre Belt GFTZ: Great F alIsTectoniozone (SLTD), using a combination of potential field geophysical GLTZ: Great LakesTecton Zore data and drill hole information (see Figure 1), which marks MSM: Mojave-Sonora Megashear NFZ: Niagara F ault Zone the southern limit of Archean and known Penokean rocks. SLTD: Spir Lake-Trempealeau South of this boundary rocks at the subcrop are dominated Dbonnuii 300 km by subaerial potassic rhyolite and epizonal granite, formed at about 1.75 Ga, and ultra-mature quartzite, such as the Baraboo Quartzite, which lies unconformably on them. Gneisses and mafic volcanic rocks, probably basement rocks from which the rhyolites formed by partial melting, are inferred from gravity and magnetic highs to be at subcrop in several areas. Because the SLTD transects geon 18 Penokean structures, it appears to be a post-Penokean feature perhaps marking the northern margin of a Yavapai age accreted terrane. It lies directly above an abrupt offset in Moho depth (deeper to the south) beneath Lake Michigan identified in GLIMPCE deep seismic surveys. The offset was previously interpreted as a north-dipping Penokean subduction zone, but it now seems more likely to be a Yavapai feature. We interpret the SLTD to be a fundamental Yavapai-age Proterozoic boundary, equivalent to the Cheyenne belt paleosubduction zone in N southern Wyoming. The Cheyenne belt juxtaposes geon Figure 1. Map showing major Precambrian crustal 17 Yavapai orogen crust on the south against the Archean Map showing major Precambrian crustal *je provinces age provinces and tectonic boundaries in the western Wyoming craton, and transects geon 18 (Trans-Hudson) and tectonic Ixundaries in thewestem and central and central United Stated United States. structures in southern South Dakota. The interpretation shown here suggests that the Cheyenne suture extends eastward, joins with the Spirit Lake trend in Iowa, and continues east across the Midcontinent Rift as the Trempealeau discontinuity in Wisconsin. Geon 17 plutonic rocks north of the SLTD, and a north-dipping crustal reflection across it, are consistent with intervals of north-directed subduction during Yavapai accretion. Younger, Mazatzal-age compression deformed quartzites that overlie the Yavapai, Marshfield, and Penokean terrane rocks indicating that geon 16 deformation extended far north of the still imprecisely defined Yavapai-Mazatzal terrane boundary. Barring major strike-slip motion, our revised tectonic map suggests progressive accretion of juvenile arc terranes every 100 million years or so during the late Paleoproterozoic (circa 1.85, 1.75, and 1.65 Ga). - 27 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Geologic Implications of Bedrock Mapping in the Ely and Basswood Lake Quadrangles, Northeast Minnesota JIRSA, Mark A. and MILLER, James D., Jr., Minnesota Geological Survey (jirsa001@umn.edu and mille066@umn.edu) Two regional bedrock geologic compilation maps of northeast Minnesota at scale 1:100,000 have recently been completed by the Minnesota Geological Survey (Figure 1), funded in large part by the STATEMAP program of the U.S. Geological Survey. The most current—described here—covers the Ely and U.S. portion of the Basswood Lake 30ʼ x 60ʼ quadrangles (Jirsa and Miller, 2004). The compilation integrates new mapping, unpublished work contained in 7 theses, and the classic Knife Lake maps of Gruner (1941), into a lithostratigraphically consistent, digital (GIS) format. Much of the map depiction of Paleoproterozoic (Animikie Group) and Mesoproterozoic rocks (Duluth Complex and North Shore Volcanic Group) is not changed substantively from recent works by Miller and others (2001). By contrast, the Archean geology is considerably modified from earlier geologic compilations at 1:250,000 scale (Green, 1982). The area encloses the central part of the Boundary Waters Canoe Area Wilderness (Figure 2), and is the first new publication of mapping of that area in many decades. . 9 7?:. -d _______ International Falls -r—,,,•,•.-,,,,,,,•,fl,, - • VERMILION LAKE Quadrangle - . - 92 7, ELY-BASSWOOD LAKE Quadrangles MESOPROTEROZOIC Duluth Complex and North Shore Volcanic Group PALEOPROTEROZOIC Virginia-Thomson Formations Biwabik Iron Formation NEOARCHEAN Granitoid batholiths and plutons I Schist of sedimentary protolith Metavolcanic and metasedimentary rocks Duluth Figure 1. Generalized geologic map of northeastern Minnesota showing the location of recently mapped 30ʼ x 60ʼ quadrangles; Jirsa and Boerboom, 2003, and Jirsa and Miller, 2004. Exposures provide what is likely the most complete view of Archean geology anywhere in Minnesota. The Neoarchean bedrock lies within the Wawa and Quetico subprovinces of the Superior Province (Figure 3). A third, distinctive sequence of rocks—the Knife Lake Group—is interpreted to have been deposited in a complex array of successor basins developed along early-formed faults near the boundary between the two subprovinces. The Knife Lake strata include hornblende-bearing alkalic and calc-alkalic volcanic flows and unusual hornblenderich tuff, conglomerate derived from multiple sources including the 2,689 Ma Saganaga Tonalite, and feldspathic graywacke and mudstone. Much of the temporal distinction between various geologic elements of the Archean bedrock is based on fabrics that resulted from three major phases of deformation, denoted D1, D2, and D3. All three deformation events are the result of north–south- to northwest–southeast-directed compression. The timing of D1 deformation is bracketed between deposition of the volcanic and clastic rocks of the Wawa subprovince at about 2,722 Ma (Peterson and others, 2001), and emplacement of the Saganaga Tonalite at about 2,689 Ma (Corfu and Stott, 1998). Folding in volcanic rocks of the Ely Greenstone attributed to D1 deformation is truncated by Knife Lake strata, indicating that the latter is synchronous with or post-dates deposition and early deformation of the Ely Greenstone. As such, the Knife Lake Group is inferred to be a Timiskaming-type sequence temporally - 28 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 equivalent to the Shebandowan assemblage exposed in adjacent parts of Ontario (Corfu and Stott, 1998). D2 deformation affects all of the Archean supracrustal units and is bracketed by U-Pb dates of intrusions in the Giants Range batholith that place the regional deformation and metamorphic event between about 2,674 Ma and 2,685 Ma (Boerboom and Zartman, 1993). D3 deformation produced faults in the low-grade supracrustal and intrusive rocks of the Wawa subprovince, and folding of granitic and migmatitic rocks in the Quetico subprovince. Saganta Tonallie lUSCr L9' BWCA DULUTH COMPLEX 4sorthosItIc sMn Figure 2. Schematic geologic map of the Ely and Basswood Lake 30ʼ x 60ʼ quadrangles showing major lithologic components, including subdivisions of rock units in the Archean bedrock, Paleoproterozoic rocks of the Mesabi Iron Range, and intrusions of the Mesoproterozoic Duluth Complex. f_i / / South KawlshIw- // BIdEgI C--;' jMASI IRON RANGE - - - - - - /'vr > Partndge F Greenwood , / Beaver I Comple: ,--4' / SHORE Ey-Bassswoad L Th:, SO. x 60 qJ3]. SHEBANOOWAN %. GRtENSIONE BELT _RMPLION (!It!pI@ VERMIUON SHEBANOOWAN T!k!!Q.Lypth&& I(NIFE LME SHEBANOOWN qees GROUP ASSEMBLAGE ft]'Y NEWTOn LAKE GREEN WATERJBURCHELL SobprQWEs M@ISi L—J Ok.-thd€ih-kIIi& Ia 1114 oy& {271&27M} ELYGREENSTONE M) Figure 3. Generalized geologic map showing potential stratigraphic correlation between Archean rocks of the Vermilion district in Minnesota and those of the Shebandowan greenstone belt in Ontario. Shaded areas are lakes. Abbreviated References Boerboom, T.J., and Zartman, R.E. 1993. Canadian Journal of Earth Sciences 30:2510-2522. Corfu, F., and Stott, G.M. 1998. Geological Society of America Bulletin 110:1467-1484. Green, J.C. 1982. Minnesota Geological Survey, Two Harbors Sheet; scale 1:250,000. Gruner, J.W. 1941. Geological Society of America Bulletin 52: 1577-1642. Jirsa, M.A. and Boerboom, T.J. 2003. Minnesota Geological Survey Map M-141, scale 1:100,000. Jirsa, M.A., and Miller, J.D., Jr., 2004, Minnesota Geological Survey Map M-148, scale 1:100,000. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W. and Peterson, D.M. 2001. MGS Map M-119, scale 1:200,000. Peterson, D.P., Gallup, C., Jirsa, M.A. and Davis, D.W. 2001. ILSG, 47th Annual Meeting, Proceedings, p. 77-78. - 29 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Structural State of Plagioclase Phenocrysts in Porphyritic rocks of the Midcontinent Rift, Northeastern Minnesota KNUDSEN, Damion, Department of Geosciences, North Dakota State University, Fargo, ND 58105, damion.knudsen@ndsu.edu; SAINI-EIDUKAT, Bernhardt, Department of Geosciences, North Dakota State University, Fargo, ND 58105, MILLER, James D., Jr., Minnesota Geological Survey, University of Minnesota, c/o Natural Resources Research Inst., 5013 Miller Trunk Hwy, Duluth MN 55811, and DANIELS, Peter, Markstrasse 123, 44803 Bochum, Germany. The structural states of plagioclase phenocrysts from porphyritic rocks of the Midcontinent Rift in northeastern Minnesota were analyzed to determine the origin of the plagioclase and to evaluate possible petrogenetic relationships between rock units. As a general rule, higher states of structural disorder in the crystal lattice of plagioclase reflect higher temperatures of equilibration. The units studied include hypabyssal sills of the Beaver Bay Complex (BBC), anorthositic rocks of the Duluth Complex, and porphyritic basalts of the North Shore Volcanic Group (NSVG). The NSVG is a 7-10 kilometer thick volcanic edifice composed of flood basalts and minor felsic volcanics, intermediate lavas, and interflow sandstones (Green, 1972). The BBC is composed of at least thirteen hypabyssal intrusions, with widely varying compositions, that were emplaced in the medial section of the NSVG (Miller and Chandler, 1997; Miller et al., 2002). The chemistry and texture of plagioclase phenocrysts in some porphyritic NSVG basaltic flows and BBC intrusives suggest they are related to plagioclase-rich magmas (mushes) that are thought to have produced the anorthositic series rocks of the stratigraphically deeper, more plutonic Duluth Complex (Miller and Weiblen, 1990). Plagioclase phenocrysts were separated for XRD using common magnetic and heavy liquid methods. Patterns were collected in 0.02° step scan mode on a Philips Xʼpert PW 3040-MPD diffractometer using 2θ from 18° to 55°, 5 sec/step, and λ =1.54178 Å. Refined unit-cell parameters were used to approximate plagioclase Al, Si distributions using Kroll and Ribbeʼs (1980) γ method, and a derivative method using t10-<t1m> vs. anorthite content (An). An contents were determined by electron microprobe at the Department of Geology and Geophysics, University of Minnesota-Twin Cities. The rock types analyzed for this study are: (a) plagioclase porphyritic, ophitic basalts from near Silver Bay and Croftville, (b) plagioclase porphyritic ferrodiorites of the Cabin Creek and the Leveaux Porphyry intrusions (early intrusions of the BBC), (c) porphyritic leucogabbros and gabbroic anorthosites of the Scott Creek and Katydid Lake intrusions that occur near the transition between the BBC and Duluth Complex (both are thought to represent offshoots of the anorthositic series; Boerboom and Miller, 1994); and (d) a porphyritic diabase dike, possibly an offshoot of the main Scott Creek intrusion. The phenocrysts in these rock types are compared with anorthosite xenoliths occurring in the Beaver River diabase (Knudsen et al., 2005) and with anorthositic series rocks of the Duluth Complex (Bandli and Saini-Eidukat, 2000). XRD results (Figure 1) show the Croftville basalt phenocrysts, along with those of the porphyritic dike, have intermediate to high disorder, plotting near the curve for disordered plagioclase. In contrast, the Silver Bay basalt phenocryst has intermediate to low structure. The Leveaux Porphyry phenocrysts have intermediate structures and plot near Duluth Complex anorthositic series samples, while the Cabin Creek phenocrysts have distinctly more disordered structures. Plagioclase phenocrysts from the Scott Creek leucogabbro and the Katydid Lake gabbroic anorthosite are intermediate to ordered and plot within the field of Duluth Complex anorthositic series samples. Anorthosite xenoliths in the BBC are intermediate to ordered. Plagioclase separated from the matrix of selected samples all have lower An contents than the phenocrysts - 30 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 they host, and show intermediate to high disorder. The relationship of the matrix t10-<t1m> values to those of the phenocrysts they host reverses in a manner similar to that noted by Hoffer (1968) for the Rock Creek porphyritic basalt flow of Idaho. Figure 1. Order distributions of measured plagioclases between the various rock types. A. γ vs. An, after Kroll and Ribbe (1980). B. derivative plot of t10-<t1m> vs. An. Abbreviated References Albers, P. and Miller, J.D., Jr.,2005. (this volume). Bandli, B. and Saini-Eidukat, B. 2000. Proc. ILSG, v. 46, part 1: 4-5. Boerboom, T.J., and Miller, J.D., Jr. 1994. MGS Misc. Map M-81. Knudsen, D., Saini-Eidukat, B., Miller, J.D., Jr., Daniels, P. 2005. GSA–N. Centr, Abstr., Mpls, MN, May 19-20. Green, J.C. 1972. in Geology of Minnesota – A centennial volume: 294-332. Kroll, H. and Ribbe, P.H. 1980. Am. Min. 65: 449-457. Hoffer, J.M. 1968. Am. Min. 53: 908-916. Miller, J.D. Jr. and Chandler, V.W. 1997. in GSA Sp. Pap. 312: 73-96. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M. and Wahl, T.E. 2002. MGS RI 58, 207 p. - 31 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Petrogenesis and PGE mineralization of the Eva-Kitto Intrusion, northern Ontario LAARMAN, J. and HOLLINGS, P., Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, On, P7B 5E1, jlaarman@lakeheadu.ca The Eva-Kitto Intrusion is a Mesoproterozoic semicircular peridotitic ring complex situated in the Nipigon Embayment on the eastern shore of Lake Nipigon near Beardmore, Ontario. The age of the intrusion is 1117.7±1.8 Ma (L.Heaman, University of Alberta, personal communication, 2005) and has been interpreted to be emplaced as the feeder conduit to a more evolved cone sheet during initial asthenospheric upwelling of a mantle plume in the Midcontinent rift system (MCR) (Sutcliffe, 1986). The intrusion cross-cuts both the Archean Southern Sedimentary Belt (SSB) and the Southern Volcanic Belts (SVB) of the Beardmore-Geraldton Greenstone belt, dated at circa 2696-2691Ma, of the Wabigoon Subprovince (Lafrance et al., 2004). Within the SVB are sulphidized magnetite-chert iron formations that run parallel to the strike of the main structural trends of the greenstone belt and are intercalated with mafic volcanic units. In drill core, the intrusion cross-cuts calcareous sandstones and mudstones interpreted to be the Sibley metasediments and is cross-cut by Nipigon gabbro sills which are also found to crop out as horizontal circular ridges around the intrusion. The Eva-Kitto peridotite has high TiO2, K2O and P2O5 and is LREE-enriched similar to the chemistry of the Fe-Ti Osler Volcanics of the MCR (Hart et al., 2002). The sills, on the other hand, have lower TiO2, K2O and P2O5 than the ferropicritic peridotites, and are similar in chemistry to the olivine tholeiitic flood basalts of the MCR. Therefore the Nipigon sills originated from a more fractionated underplated basaltic magma later in the Keewanawan event circa 1109 Ma (Sutcliffe, 1987). j... The petrography and geochemistry of the lithologies within the Eva-Kitto Intrusion are being studied from drill core held by East West Resources Inc. Four holes were drilled by Kennecott Canada Exploration Inc. in 2002 Lake N4Igon based on airborne MegaTEM anomalies surveyed by Fugro Airborne Surveys that same year. Hole EK-2 intersected disseminated po-cpy mineralization and was the only hole to intersect magnetiferous pyritic metasediments of the Archean basement. Assays values up to 0.28%Ni, 0.13%Cu, and 563ppb Pt+Pd in a 1.22m interval have been reported (Rossell, 2003). Lithologies within EK-2 from the top to the end of hole at 345m at depth comprise lherzolite, olivine websterite, sulphidized olivine websterite, vari-textured pyroxenite, spotted pyroxenite, pyroxenite, melanogabbro, magnetiferous pyritic metasediment, and mafic volcanics. EK-2 is the only hole to intersect the more fractionated varitextured pyroxenite to orthocumulate pyroxenite lithologies that contain PGE mineralization. .::L: Figure 1: Geological map ofthe Eva Kitto Ring Complex with its Figure 1. Geological map of the Eva Kitto Ring location circled on the Lake Nipigon inset. The four ER drill holes Complex. four EK drillofthe holes are located at the are located atThe the southern extent peridotitic intrusion. southern end of the peridotitic intrusion Based on preliminary interpretation of the petrography the petrogenesis of the Eva-Kitto Intrusion can be characterized by two magmatic pulses. A first pulse of magma is suggested by the pyroxenite and melanogabbro units of EK-2. The pyroxenite has an orthocumulate texture with cumulate pyroxenes enclosed in a groundmass of lathy plagioclase. The melanogabbro has a poikilitic texture with cumulate pyroxene crystals housed in plagioclase oikocrysts. A second pulse of magma is suggested by the incipient break up of cumulate - 32 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 pyroxene and growth of clusters of secondary pyroxene in the vari-textured and spotted pyroxenites. The patchy texture within these units can be attributed to the churning up of the pyroxenite by a second influx of magma. Lherzolite is the dominant lithology in all the drill holes and may represent the primary cumulate fractionation of an enriched ferropicritic magma (Sutcliffe, 1986). Lherzolite displays a mesocumulate texture of ovoidal cumulate olivine grains and larger cumulate clinopyroxene within poikilitic orthopyroxene and plagioclase. The olivine websterite is darker due to fractionation of pockety pyroxenes with breakdown of olivine. The olivine websterite possibly formed by flow differentiation of a more fractionated magma from the central lherzolite flow zone. Both the lherzolite and olivine websterite contain large amounts of primary biotite minerals. PGE minerals are associated with disseminated sulphides in the sulphidized olivine websterite, vari-textured pyroxenite, pyroxenite and melanogabbro units of drill hole EK-2. Within the olivine websterite and pyroxenite, disseminated pyrrhotite-pentlandite-chalcopyrite sulphides are found interstitial to cumulate olivine and pyroxene minerals. Blebby pyrrhotite-chalcopyrite sulphides, possibly a result of liquid immiscibility, are locally found in the sulphidized olivine websterite/melanogabbro units at the contact with the wallrock metasediments. Ongoing studies are focusing on petrogenesis and mineralization of the intrusion. Scanning electron microscope (SEM) discs made from drill core in Holes EK-1, 3 and 4 are being analysed under the petrographic microscope to compare lithologies in these holes with the fractionated suite of EK-2. The interstitial sulphides will be examined by SEM to identify the PGE minerals. Mineral chemistries within the various lithologies will also be identified using SEM. Bulk rock chemistries, S/Se elemental ratios, trace elements, εNd and Sr patterns, and Ni abundances of olivine will be analysed to further understand the petrogenesis of source magmas, and crustal contamination involved with mineralization. References Hart, T.R., terMeer, M. and Jolette, C. 2002. Precambrian geology of Kitto, Eva, Summers, Dorothea and Sandra townships, northwestern Ontario: Phoenix Bedrock Mapping Project. Ontario Geological Survey, Open File Report 6095, 206p. Lafrance, B., DeWolfe, J.C., Stott, G.M. 2004. A structural reappraisal of the Beardmore-Geraldton Belt at the southern boundary of the Wabigoon subprovince, Ontario, and implications for gold mineralization. Canadian Journal of Earth Sciences 41, 217-235. Rossell, D. 2003. November 2003 Report on Diamond Core Drilling on the Eva Kitto Property. Kennecott Canada Exploration Inc. Sutcliffe, R.H. 1986. Proterozoic rift-related igneous rocks at Lake Nipigon, Ontario. unpublished PhD thesis, University of Western Ontario, London, Ontario, 325p. Sutcliffe, R.H. 1987. Petrology of Middle Proterozoic diabases and picrites from Lake Nipigon, Canada. Contributions to Mineralogy and Petrology, 96, 201-211. - 33 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Geochemistry and Petrography of the Rabbit Islands Breccia, North Central Lake Nipigon LANE, C. J. and HOLLINGS, P., Department of Geology, Lakehead University, 955 Oliver Rd., Thunder Bay, Ontario, P7B 5E1, Canada. cjlane@lakeheadu.ca The Rabbit Islands breccia is a dyke-like intrusion, rich in calcite and dolomite, which intrudes the Nipigon diabase on the southern most island of the Rabbit Islands group in north central Lake Nipigon (Figure 1). Geological relationships indicate that the breccia must represent one of the youngest events associated with the Midcontinent rift. In the past this breccia has been referred to as a carbonatite diatreme (Sutcliffe and Greenwood, 1982), and one of the objectives of this study was to evaluate this model and to compare the breccia to the carbonatites of the Coldwell complex. Whole-rock geochemistry, petrography and scanning electron microscope (SEM) data, indicate that the groundmass material of this breccia is not of carbonatite affinity. The low abundance of trace elements such as Sr, Mn and Ba in SEM semi-quantitative scans of the carbonates and feldspars combined with the low abundances of Sr, Ba, V, and rare earth elements (REEʼs) determined by XRF and ICP-MS, do not satisfy the requirements of Samoilov (1991). Samoilov suggested that in order to be considered a carbonatite the following criteria must be met: enrichment in Sr (>700 ppm), Ba (>250 ppm), V (>20 ppm) and REE (including Y; >500 ppm). The samples analyzed from the Rabbit Islands breccia groundmass contained 191 ppm Sr, 286 ppm V, 115 ppm total REEʼs and low amounts of barium. Consequently, the breccia does not meet the limits set by Samoilov (1991) and therefore is not considered to be of carbonatitic affinity, but rather a carbonate-rich breccia. Evidence for changes in fluid composition has been observed within the groundmass material. For example, in many instances calcite has been shown to brecciate and replace dolomite crystals present in the breccia, however, quartz and feldspar brecciate and replace the dolomite. Other textures, such as calcite replacing feldspar emphasize the fact that there is an overlap of fluid phases, which may be controlled by fluid source or contamination by the (0 •0 (a C 2. a 0= = -, @02 EQ (a e. 0 = o (0 0 -o 'C 2o o = —. == OW0 ao = (a0 (a 0 'C wmz The geochemistry of the clasts and the country rock associated with the breccia has been compared with the different suites of Nipigon diabase: the Inspiration suite, the Jackfish suite, the McIntyre suite and the Main Nipigon suite (MacDonald and Tremblay, 2004; Rabbit Islands Richardson and Hollings, 2004), showing that the breccia intruded into the Main Nipigon suite (Figure 2). Clasts of diabase found within the breccia have undergone extensive alteration. Many of the clasts display consecutive zones of alteration which can be seen in both hand sample and thin section. Elements which have become mobile within the altered clasts, and show variation between each alteration zone include: potassium, calcium, magnesium, strontium, europium, yttrium, lanthanum, and neodymium. Depletion of calcium, strontium, europium, yttrium, lanthanum and neodymium can be seen from the centre of the clast outwards. Whereas, potassium, magnesium and loss on ignition (due to the presence of chlorite) increase from the centre of the clast outwards. Figure 1. Location of the Rabbit Islands (Modified from Hollings et al. 2004). - 34 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Rabbit Islands i><+><i Main Nipigon Suite RbThUNbKTaLaCePrSrNdZrHfSmEuTGdThDyYHoErTmYbALuVSc Figure 2. Primitive mantle multi-element plot to show the similarities between the Rabbit Islands Diabase and the Main Nipigon suite. country rock. References Hollings, P., Fralick, P., Kissin, S. 2004. Geochemistry and Geodynamic Implications of the Mesoproterozoic English Bay Granite-Rhyolite Complex, Northwestern Ontario. Canadian Journal of Earth Sciences, 41, 1329-1338. MacDonald, C.A. and Tremblay, E. 2004. Lake Nipigon Region Geoscience Initiative: Results of bedrock mapping in the northern part of the western Nipigon Embayment, northwestern Ontario, Canada; oral presentation abstract, Institute on Lake Superior Geology, Proceedings, v. 50, pt. 1, Programs and Abstracts p.102-103. Richardson, A.J. and Hollings, P. 2004. Lake Nipigon Region Geoscience Initiative: Geochemistry and Mineralogy of the Nipigon Diabase Sills. Summary of Field Work and Other Activities 2004. Ontario Geological Survey, Open File Report 6145, p.51-1 to 51-5. Samoilov, V.S. 1991. The Main Geochemical Features of Carbonatites. Journal of Geochemical Exploration 40, 251-262. Sutcliffe, R.H. and Greenwood, R.C. 1982. Geology of the Lake Nipigon Area. In Summary of Field Work 1982. Ontario Geological Survey, Miscellaneous Paper 106, 19-23. - 35 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 MetalCORP Ltd. Big Lake Ni-Cu-PGE, Cu-Zn-Ag, and Mo Property MACTAVISH, Allan, MetalCORP Ltd., 309 South Court Street, Thunder Bay, ON, P7B 2Y1, Canada. amactavish@metalcorp.com The Big Lake Property of MetalCORP Ltd. of Thunder Bay, Ontario comprises 33 claims (365 units totaling 5840 hectares) and is located approximately 230 km east-northeast of the City of Thunder Bay and 18 km southeast of the Town of Marathon in Northern Ontario, Canada. Work completed by MetalCORP since early 2004 includs a MegaTEM airborne survey, detailed prospecting (677 samples), linecutting, ground pulse-EM surveys, and 2 phases of diamond drilling, totaling 22 diamond holes (5672 m). This work resulted in the discovery of 4 previously unknown mineralized zones that represent 3 separate and distinct mineralization styles. The mineralized zones include: the J4 and J5 Pt-Pd reefs within the Big Lake Ultramafic Complex; the A2 Ni-Cu Zone within the Gus Creek Mafic Intrusion; and the BL14 Cu-ZnAg Zone within strongly altered mafic metavolcanic flows and associated metasedimentary rocks. The A2 and BL14 zones are not exposed on surface and are buried beneath 10 to 75 m of glacial drift. The property is also host to the historic Playter Cu-Pb-Mo-Ag Prospect which has yet to be evaluated by MetalCORP. The Big Lake Property is located near the southern margins of the eastern portion of the Archean-age Schreiber-Hemlo greenstone belt of the eastern Wawa Subprovince of the Canadian Shield. The greenstone belt is split into distinct eastern and western segments by the 1108 Ma Mesoproterozoic Coldwell Alkalic Complex. The eastern part of the belt is subdivided into the Hemlo-Black River assemblage (2.77 Ma) to the north and the Heron Bay (2.70 Ma) assemblage to the south, both of which are primarily affected by amphibolite-facies regional metamorphism. The western portions of both assemblages are lower in grade and exhibit upper greenschist facies regional metamorphism. The Big Lake Property occurs within the Heron Bay Assemblage which is intruded by the granitic to granodioritic Heron Bay Batholith, the recently recognized mafic to ultramafic Gus Creek Intrusion, the Bellʼs Lake Ultramafic Intrusion, and the Big Lake Ultramafic Complex. The J4 and J5 Pt-Pd Reefs consist of narrow, apparently stratabound intervals hosted within thick peridotite units contained within the upper and central intrusive cycles of the eastern portion of the sill-like Big Lake Ultramafic Complex. The complex is not layered. The two host intrusive cycles dip approximately 43° north and are very similar in their appearance, progression of rock units, and apparent thickness. The observed mineralization consists of low amounts (trace to ~1%) of very finely disseminated pyrrhotite and chalcopyrite within serpentinized to locally talcose, fine-grained peridotite. The J4 Reef varies between 0.58 and 2.11 m in thickness, occurs within the basal peridotite unit of the uppermost intrusive cycle of the Big Lake Complex and is usually directly adjacent to the contact with an overlying feldspathic pyroxenite unit. The J5 Reef is identical in appearance to the J4 Reef, varies between 0.75 and 3.00 m in thickness, and occurs within the basal peridotite of the central intrusive cycle of the complex near, but not adjacent to, the upper contact of the host unit with an overlying feldspathic pyroxenite. The J4 Reef has been traced for 1.60 km and contains up to 0.70 gpt Pt and 0.79 gpt Pd (1.49 gpt PGE)/1.67 m. The J5 reef has been traced for a similar distance and contains up to 0.81 gpt Pt, 0.85 gpt Pd (1.86 gpt 2PGE)/0.75 m. The A2 Ni-Cu Zone occurs near the base of the discordant Gus Creek Mafic Intrusion and consists of disseminated, blebby, and stringered, locally semi-massive pyrrhotite, chalcopyrite, and possibly pentlandite hosted within a 2 to 20 m thick host sequence. The host sequence is a complex interval of variably mineralized (1 to 30% sulphides), varitextured, inclusion-rich, gabbroic to melagabbroic intrusive rocks overlain by unmineralized, medium- to coarse-grained gabbro and quartz leucogabbro and underlain by occasionally mineralized, pyroxenephyric melagabbro and feldspathic pyroxenite. The strongest mineralization occurs near the base of the host sequence, comprises the A2 Ni-Cu Zone, and includes 1.66% Ni and 0.20% Cu/0.30 m, 1.00% Cu and 0.80% - 36 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Ni/0.40 m, 1.40% Cu and 0.27% Ni/0.77 m, and 0.98% Cu and 0.29% Ni/1.40 m. The geometry of the A2 mineralized zone remains uncertain and may be more complex that initially thought, but within the area drilled appears to strike between 125 and 150° and dip southwest at between 40 and 60°. The BL14 Cu-Zn-Ag Zone is located near the east end and stratigraphically below the sill-like Big Lake Ultramafic Complex, approximately 800 metres south of the A2 zone. The zone closely resembles high temperature Cu-rich VMS stringer mineralization and is composed of an intensely biotitized and strongly chloritized breccia containing up to 30% bands, veins, stringers and pods of chalcopyrite and pyrite with up to 5% disseminated to streaked sphalerite and minor galena. The mineralized zone occurs within a strongly to intensely K- and Mg altered, Na2O-depleted package of mafic metavolcanic rocks, minor associated interflow clastic metasedimentary rocks, and deformed bands of chert. Mineralization intersected to date includes 2.56% Cu, 1.00% Zn, 46.0 gpt Ag, 1.60 gpt Au, and 0.10% Pb/0.93 m; 0.80% Cu, 0.50% Zn, and 21.3 gpt Ag/2.62 m; and 0.74% Cu, 0.23% Zn, and 9.0 gpt Ag/0.75 m. - 37 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Mining and Exploration Activity in Northwestern Ontario MAGEE, M. Angelique, Ministry of Northern Development and Mines, Resident Geologist Program, Thunder Bay, Ontario, Canada, P7B 6S7 Northwestern Ontario experienced a significant upswing in mining and mineral exploration in 2004. Six mines produced a total of 1.7 million ounces of gold in 2004, approximately 70% of the Ontario total. Gold producers included: Campbell Mine (Placer Dome Inc.); David Bell Mine (Teck Cominco Limited and Barrick Gold Corporation); Golden Giant Mine (Newmont Canada Limited); Musselwhite Mine (Placer Dome Inc./Kinross Gold Corporation); Red Lake Mine (Goldcorp Inc.); Williams Mine (Teck Cominco Limited and Barrick Gold Corporation). North American Palladium Ltd. produced 308,931 ounces of palladium and 25,128 ounces of platinum at its Lac des Iles Mine and has begun the development of an underground operation below its open pit mine. There are approximately 300 active exploration projects in the northwest, the vast majority of which are focused on gold. Areas receiving the most interest from exploration companies were the Red Lake greenstone belt, Shoal Lake area, Dogpaw Lake area, Shebandowan greenstone belt, Fort Hope greenstone belt, OnamanTashota belt and the Pickle Lake greenstone belt. Elevated mineral commodity prices are contributing to levels of exploration activity in northwestern Ontario not seen since the mid-1980s. - 38 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Preliminary stratigraphy and geochemistry of the Mesoproterozoic Pillar Lake Volcanics, Wabigoon Subprovince, Superior Province, Armstrong, Ontario, Canada MAGEE, M. Angelique, Ministry of Northern Development and Mines, Resident Geologist Program, Thunder Bay, Ontario, Canada, P7B 6S7, HOLLINGS, P. and FRALICK, Philip, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, Canada, P7B 5E1 The Mesoproterozoic Pillar Lake Volcanics (PLV); an approximately 50 m thick series of flat-lying, greenschist facies undeformed massive and pillowed basalt flows; are located 15 km southeast of Armstrong, Ontario. The Pillar Lake Area was originally mapped as part of a geological compilation series with the dominant lithology in the area comprising Nipigon diabase sills formed during the Keweenawan mid-continent rift system of North America (Breaks, 1980; Davies et al., 1970). Mapping completed at a scale of 1: 50,000 by MacDonald et al. in 2003 resulted in the discovery of previously unmapped lithologies which include the Pillar Lake Volcanics (MacDonald, 2004). The PLV are part of a suite of Mesoproterozoic rocks that unconformably overlie the Archean basement of the central Wabigoon Subprovince, Superior Province. The PLV have not yet been successfully dated by geochronological methods. Magnetic remanence studies of the PLV places them at 1140 Ma but there is a significant error associated with this date (C. Hercun, pers. comm., 2005). Fifty detrital zircons from an interflow lithic arenite bounded at its base and top by PLV were dated and the youngest detrital zircon was dated at 1514 Ma (Heaman et al., 2005). The dominant populations of zircons from the interflow lithic arenite fall within 1780 to 1880 Ma and 1900 to 1950 Ma (Heaman et al., 2005). The PLV unconformably overlie a 1599±1 Ma layered gabbroic intrusion but no contact has yet been found except in drill core (Heaman et al., 2005; Middleton, 2005). A nearby 1590±1 Ma syenite may also underlie the PLV (Heaman et al., 2005). The PLV are overlain by Inspiration diabase sills dated at 1120±1 Ma (Heaman et al., 2005). The Inspiration diabase sills are geochemically distinct from the Nipigon sills and hence have been given a new designation. The Inspiration diabase sills extend the timing of North American mid-continent rift-related intrusive activity. Based on these age dates, the PLV are Mesoproterozoic in age, and were erupted between 1514 Ma and 1120 Ma. The PLV have been metamorphosed to greenschist facies and are extensively chloritized with accessory epidote and sericite. The PLV are hematite and actinolite altered with intense alteration found at the top of each flow. The individual massive basalt flows are approximately 0.2 – 1 m thick and each flow is capped with a 5 cm thick flow top breccia with actinolite rims and hematite cores forming the breccia fragments (Figure 1). The pillowed basalt flows tend to be pervasively altered and exhibit autobrecciation throughout the flow layer, perhaps due to higher permeability. Near the base of the volcanic pile, the pillowed basalt flows are rarely unaltered, unbrecciated and preserve primary volcanic features such as pillow selvages, interpillow hyaloclastite, and degassing structures (Figure 2). Associated with the massive and pillowed basalt flows are numerous breccias including an undeformed volcaniclastic breccia that contains fragments of gabbro, amygdaloidal basalt with clay minerals infilling the amygdules, and possibly Figure 1. Flow top breccia capping massive basalt flow with actinolite rims and recessively weathered hematite cores - 39 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 granitoid. Field relationships between the PLV and the breccias are not always discernible due to poor outcrop control, but in several instances the breccias occur as dikes found within the PLV. Based on preliminary geochemistry, the PLV may be the result of a mantle plume and possibly predate the main North American mid-continent rift event. It is hoped that detailed mapping of the PLV in conjunction with additional geochemical analyses will determine the source and cause of the volcanism that resulted in this suite of volcanic rocks. Figure 2. Undeformed chlorite-altered pillowed basalt with preserved selvages and interpillow hyaloclastite. References Breaks, F.W. 1980. Sioux Lookout - Armstrong Sheet, Geological Compilation Series, Ontario Geological Survey, Final Map 2442. Davies, J.C., Pryslak, A.P., Pye, E.P. 1970. Sioux Lookout - Armstrong Sheet, Geological Compilation Series, Ontario Geological Survey, Final Map 2169. Heaman, L.M., Easton, R.M., Hart, T.R., MacDonald, C.A., Fralick, P., and Hollings, P. 2005. Proterozoic history of the Lake Nipigon area, Ontario: Constrains from U-Pb zircon and baddeleyite dating; Canadian Institute of Mining and Metallurgy, 2005 Annual Meeting, April 24-27, Toronto, Ontario. Macdonald, C.A. 2004. Precambrian geology of the south Armstrong-Gull Bay area, Nipigon Embayment, northwestern Ontario; Ontario Geological Survey, Open File Report 6136, 42p. Middleton, R.S. 2005. Diamond Drilling on Red Granite Property, Pillar Lake Sheet, Armstrong, ON, 52I03NW, Resident Geologist Program Thunder Bay North Assessment Files, 55p. - 40 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Sedimentology of the Rove and Virginia Formations and their Tectonic Significance MARIC, Mike and FRALICK, Philip, Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1, philip.fralick@lakeheadu.ca The Paleoproterozoic Rove and Virginia Formations form the upper portion of strata in the Animikie Basin. They are lithostratigraphically and chronostratigraphically correlative units overlying iron formation of the Biwabik and Gunflint Formations. Tuffaceous layers very near the base of the Rove and Virginia Formations gave U-Pb zircon ages of approximately 1835 Ma (Addison et al., 2005), whereas a sandstone sample collected by R.M. Easton (O.G.S.), approximately 400 m higher in the stratigraphy, had a U-Pb minimum detrital zircon age of 1780 Ma (see Heaman and Easton, this volume). These age constraints place sedimentation commencing during the final stages of Penokean igneous activity and stretching over an extended time span. The Rove and Virginia Formations would have been in the appropriate spatial and temporal setting to represent deposition in a foredeep transitional into a foreland basin. This study investigates the internal stratigraphy and sedimentology within the Rove and Virginia to ascertain what the lithofacies present and their stacking order can reveal about the dynamics of the tectonic setting. The data set consists of detailed logging of twelve continuously cored drill holes extending from south of Duluth to south of Thunder Bay. In total 3200 meters of core were examined. This approach is similar to that of Lucente and Morey (1983), though with advances in depositional models and process sedimentology, since their classic work, more information may be able to be extracted. Cl y - § . § - ..— The Rove and Virginia Formations overlie an intensely altered zone in the upper Gunflint and Biwabik Formations. Silicification, formation of neomorphic carbonate spar, pyrite growth in strataform bands and pisoliths all occur in the upper few meters of these underlying units. This is sharply overlain by the basal Rove and Virginia consisting of black, carbonaceous shale with thin interbeds of siltstone and very fine-grained sandstone. Green, friable tuffaceous layers are common in this assemblage, especially in the northern drillholes. - 41 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Approximately five meters above the base the amount of siltstone and sandstone interlayers gradually begins to decrease upward until they are relatively scarce. The next approximately one hundred to one hundred and fifty meters is almost completely dominated by fissile black shale. Microscopic examination reveals the presence of very thin laminae composed of angular silt grains and other microlayers consisting of carbon. The shale dominated succession is interrupted midway towards its top by a siltstone and very fine-grained sandstonerich interval traceable across the basin. This coarser unit is thicker near both the northern and southern basin margins. As well, the black shale dominated interval, in general, contains more coarse-grained interbeds in the southern one third of the basin. A gradational contact exists at the top of the shale assemblage with the overlying sandstone-shale unit. The gradation occurs over eighty meters in the north but thins to the south. The overlying sandstone-shale succession is up to 350 meters thick and consists of a stacked assemblage of over one hundred individually coarsening-upwards parasequences. Internally they consist of graded, commonly massive, finegrained sandstones separated by mmʼs to cmʼs of shale. The shales between parasequences are decimeters to one or two meters thick. This sandstone-shale assemblage fines appreciably to the south. The uppermost unit overlying the graded sandstone packages at approximately 500 meters above the base of the section is dominated by black shale with thin rippled sandstones. Both current and wave ripples are present. The upper Gunflint represents a subareally weathered surface exposed during upbuckling concurrent with the Penokean Orogeny. This is highlighted by the 1878 Ma age of the Upper Gunflint (immediately pre-Penokean, Fralick et al., 2002) and the 1835 Ma age of the Rove (Addison et al., 2005) immediately overlying the weathered surface. Upramped orogenic load probably resulted in resubmergence of the area and a slightly coarser unit was deposited forming the basal Rove and Virginia as the flooding surface migrated across the basin. Eruptions at this time in the core of the Penokean deformed terrain may have supplied the volcanic ash. With increased water depth a sediment starved, condensed sequence developed with anoxia probably caused by high organic loading in the bottom sediments. A slight coarsening of detritus in the southern area probably reflects a source in the Penokean deformation belt, but the general scarcity of sediment influx from this area strongly implies only minor upraising of an orogenic zone during the Penokean. The condensed sequence is further condensed by the lack of biogenic sediments other than carbon resulting in tens of millions of years being represented by only a hundred meters of black shale. Progradation of a turbiditic to shelf system from the north ends the sediment starvation with a flood of detritus from this direction. Detrital zircon geochronology and paleocurrent directions (Morey, 1973) are consistent with derivation of the sediment influx from the TransHudson Orogenic zone to the northwest. References Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W. and Hammond, A.L., 2005. Discovery of distal ejecta from the 1850 Ma Sudbury impact event. Geology, vol. 33, p. 193-196. Fralick, P.W., Davis, D.W. and Kissin, S.A., 2002. The age of the Gunflint Formation, Ontario, Canada: single zircon U-Pb age determinations from reworked volcanic ash. Canadian Journal of Earth Sciences, vol. 39, p. 1085-1091. Lucente, M.E. and Morey, G.B., Stratigraphy and sedimentology of the lower Proterozoic Virginia Formation, northern Minnesota. Minnesota Geological Survey, Report of Investigations 28, 28p. Morey, G.B., 1973. Stratigraphic framework of Middle Precambrian rocks in Minnesota. In, ed G.M. Young, Symposium on Huronian Sedimentation, Geological Association of Canada, Special Paper 12, p. 211-249. - 42 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Riddle of the Sands (Proterozoic) solved by Quartzites at Hamilton Mounds, Central Wisconsin MEDARIS, L.G., Jr and DOTT, R.H., Jr, Department of Geology and Geophysics, University of WisconsinMadison, Madison, WI 53706, medaris@geology.wisc.edu, rdott@geology.wisc. edu Quartzite inliers in central Wisconsin have long been enigmatic with regard to the Proterozoic evolution of the southern Lake Superior region. At Hamilton Mounds the intrusion of quartzite by 1.76 Ga granite has led some to suggest that Baraboo Interval sedimentation (1.75-1.63 Ga) was spatially and temporally associated with Geon 17 magmatism. Re-examination of the Hamilton Mounds area reveals the presence of two different quartzites, an older one intruded by granite, which we designate the Hamilton Mounds Quartzite (HMQ), and a younger one resembling the Baraboo Quartzite, which we call the Seven Sisters Quartzite (SSQ), from the present name of the quarry. Quarrying operations have largely removed the HMQ and intrusive granite, although numerous displaced blocks remain for examination. The HMQ consists of gray, laminated quartzite (Fig. 1), which commonly contains calc-silicate (qtz+ep±am) layers and ellipsoidal domains, perhaps former concretions (Fig. 2; cs, calcsilicate). The HMQ is a metamorphosed immature wacke or arenite, which contains biotite, muscovite, and substantial amounts of microcline (mc) and plagioclase (pl) (Fig. 3). The HMQ is intruded by granite (gr) (Fig. 4), which yields a U-Pb zircon age of 1763 ± 7 Ma (Van Schmus, cited in LaBerge et al., 1991). Contact metamorphic effects of the granite on the HMQ include metasomatic growth of alkali feldspar, but the coarsegrained, poikiloblastic texture of such feldspar (Fig. 5) allows easy distinction between it and fine-grained, recrystallized detrital feldspar (Fig. 3). The SSQ, which crops out in the hills surrounding the quarry, consists of pink, massive to cross-bedded supermature quartzite (Fig. 6), resembling Baraboo Interval quartzites elsewhere, except for a greater degree of recrystallization and the presence of muscovite, rather than pyrophyllite. Such features and extensive brecciation probably reflect the influence of the nearby Wolf River batholith, with which are associated wide-spread Geon - 43 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 14 hydrothermal alteration and K-metasomatism as documented in the Baraboo and Sioux quartzites (Medaris et al., 2003). p r JM1 -- S —S Geon 17 detrital zircons have recently been recognized in a single sample of the “Hamilton Mounds Quartzite” by Van Wyck and Norman (2004). However, the analyzed sample, described as supermature quartzite, was collected several hundred yards from granite and is likely to be from the SSQ, which we correlate with other supermature quartzites of the Baraboo Interval. Thus, it is not surprising that this sample should contain Geon 17 detrital zircons and display a spectrum of detrital zircon ages similar to that from the Baraboo Quartzite. Although the contact between the HMQ and SSQ is not exposed, we interpret the HMQ and intrusive granite to represent basement upon which the SSQ was deposited unconformably. Thus, like the Baraboo Quartzite, the SSQ appears to be younger than, and rests upon, Geon 17 basement. The results of this investigation are consistent with our view of the Baraboo Interval (1.75-1.63 Ga) as representing an episode of extensive weathering and widespread deposition of supermature quartz arenite on a stabilized craton in the interval following cessation of Geon 17 magmatism and prior to Geon 16 Mazatzal deformation (Medaris et al., 2003). Abbreviated References LaBerge, G. L. et al. 1991. U.S. Geol. Surv. Bull. 1904B: B1-B18. Medaris, L. G., Jr et al. 2003. J. Geol. 111: 243-257. Van Wyck, N. and Norman, M. 2004. J. Geol. 112: 305-315. - 44 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Depositional setting of the Pass Lake and Rossport Formations (Sibley Group) inferred from a combined sedimentologic/geochemical approach METSARANTA, R.T. and FRALICK, P.W., Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1, Canada, rtmetsar@lakeheadu.ca The depositional setting of the lower portions of the Mesoproterozoic Sibley Group have been investigated through detailed lithofacies analysis, stable isotope and trace element geochemistry. The portion of the Sibley Group studied is comprised of a lower clastic assemblage, middle mixed carbonate-clastic assemblage and an upper clastic dominated, evaporite rich assemblage. The thickness of each unit varies but is in general about 20-80m. The lower clastic section was deposited in a variety of depositional environments including: poorly developed, coarse-grained alluvial fans; braided, sand rich fluvial systems; fluvial/wave dominated deltaic settings; and nearshore/beach environments. The middle mixed carbonate clastic succession represents lacustrine or shallow marine deposition in a semi-arid environment. The upper red siltstone, evaporite-rich section has been interpreted to represent a saline mudflat environment or sahbka-like setting. Numerous chemical sediment types are preserved within the three distinct successions. Calcrete soils are developed in coarse-grained alluvial fan conglomerates in the lower clastic dominated package as well as higher in the stratigraphy within the upper package. Dolomitic mudstones are found in a cyclic dolomite-siltstone lithofacies association of the middle assemblage. Stromatolitic carbonates are preserved in the transition from the middle to upper succession along with evidence of subaerial exposure of this strand-proximal lithofacies. Sulfate minerals occur as fine nodules within dolomites of the middle mixed carbonate-clastic assemblage and as coarser nodules in the upper evaporitic succession. Carbon and oxygen stable isotopic compositions, Sr isotopic compositions and trace element compositions were determined for various chemical sediment types to study facies (process) dependent and stratigraphic variations in isotopic compositions and to evaluate the degree of post depositional diagenetic alteration. Stable isotopic compositions for dolomitic mudstones range from –3 0/00 (PDB) to about +1 0/00 (PDB) for δ13C and span δ18O compositions from –4 to –8 0/00 (PDB). Stromatolitic carbonates have δ13C from 0 to 2 0/00 and δ18O from –3 to -14 0/00 (PDB). Soil related carbonates vary over δ13C compositions from –1.5 to 1.50/00 (PDB) and δ18O varies from -2 to -6 0/00 (PDB). Sampling through a stratigraphic thickness of about 45m of the middle carbonate clastic succession shows a good correlation between increasing stratigraphic height and heavier stable isotopic composition (Figure 1). Petrographic analysis and trace element geochemistry suggests little diagenetic effect on stable isotopic compositions. A • • • A S l S •• • U• A • • • •A.• . A A. I —a S S S dolomitic mudstone stromatolite soil carbonate • S U 0 1 .4 2 -3 -2 -1 0 81 3c (PDB) dolomitic mudstone 313C (PDB) Figure 1. Stable isotopic compostions of various Sibley Group carbonates (left), and stratigraphic variation in δ13C for the middle dolomite/siltsone lithofacies association (right) - 45 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Sr isotopic compositions were also determined for a variety of carbonate and sulfate samples. Dolomitic mudstones (n=4) display 87Sr/86Sr ratios varying from 0.70846 to 0.71249±1. Soil carbonates appear to have variable compositions from a calcrete low in the stratigraphy at 0.70478±1 (n=1) to weathered stromatolitic material higher in the stratigraphy at a value of 0.70737±1 (n=1). One unaltered stromatolite sample has a value of 0.71234±1. Sulfates (n=8) from the upper evaporitic package vary through a fairly narrow range of compositions between 0.70647±3 to 0.708836±1. A single sample of sulfate from the middle succession in a dolomitic layer yielded a fairly high value of 0.71090±1. Further analyses have been undertaken to strengthen the validity of the apparent facies related variation. Rare earth element geochemistry of carbonate samples was also undertaken using an acetic acid dissolution method to avoid clastic contamination of primary carbonate geochemical signatures. Figure 2 shows Post Archean Australian Shale normalized REE diagram for Sibley Group carbonates. Stromatolites and soil carbonates generally show a distinct negative Ce anomaly, and overall relatively flat slope with a slightly negative HREE slope. Dolomitic mudstones exhibit an enrichment in MREE, with little or no negative Ce anomaly. Positive Gd anomalies are present in most samples of all carbonate types. Stromatolite Together lithofacies analysis and geochemical data provide insight into the depositional environments of the Sibley Group. C and O isotopic compositions fall within the realm of values determined for other Mesoproterozoic carbonate rocks deposited in marine systems. Upward stratigraphic changes in carbon and oxygen isotopic compostions may have been brought about through global scale changes, with a positive shift in C values attributable to increased rates of light (organic carbon) burial. Alternatively the carbon La Ce Pr Nd Sm Eu Tb Gd Dy Ho Er Tm Yb Lu isotopic shift maybe have resulted from processes Dolomitic rnudstone occurring within a restricted lacustrine basin with the upward shift corresponding to increasingly restricted depositional conditions near the top of the sampled succession. 87Sr/86Sr values also lie within the realm of possible seawater Sr values for the Proterozoic. The lowest values in calcrete from near the bottom of the lower succession suggests that the Sr source for these carbonates may have been marine. Relatively high Sr values in the middle succession may imply a lacustrine system. Diagenetic sulfate in the upper succession 0.001 also falls within “marine” values for the Proterozoic La Ce Pr Nd SmEu TbGd Dy Ho Er TmYb Lu and is generally lower than carbonates in the middle Soil Carbonate succession. REE element patterns have a signature consistent with oxygenated seawater; however, little comparative data is available for modern of ancient carbonates deposited in lacustrine or evaporitic environments. 0.001 Pr Nd SmEu Tb Gd Dy Ho Er TmYb Lu Figure 2. Post Archean average shale normalized rare earth element diagrams for various Sibley Group carbonate types. La Ce - 46 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Seagull Intrusion, Ontario: A Unique PGE-Ni-Cu Setting MIDDLETON, R.S., P.Eng. and HEGGIE, G., Thunder Bay, Ontario The Seagull ultramafic (Proterozoic) intrusion is an unique setting when compared to other mafic intrusion in the Lake Superior region and for that matter the rest of the world. A dunite layer exceeding 400 metres in thickness with pristine olivine (unaltered) having a magnesium oxide content exceeding 33% average olivine composition of Fo86 occurs with a 50-100 metre thick gabbro-pyroxenite (lherzolite) basal unit. Overlying is a peridotite 200-250 metres thick and an upper granophyric gabbro-gabbro pyroxenite-gabbro norite (50-100 metres thick). The intrusion has been age dated as 1112 Ma although older dates of 1124 Ma are appearing in other phases (see Heaman and Easton, this volume). The intrusion is adjacent to a failed arm of the Midcontinent Rift that extends north into the Nipigon Embayment and is cut by Nipigon olivine gabbro (1108 Ma) that forms extensive sills in the region. Other olivine-picrite bodies occur in the area such as Eva-Kitto and Hele, but are not known to contain the extensive ultramafic observed at Seagull. Sulphides occur in the basal lherzolite with zoned blebs containing cubanite tops and pyrrhotite-pentlandite bottoms. Assays up to 3.58g Pt +Pd have been obtained with % S and a 1 to 1.1 ratio of Pt and Pd. Nickel values up to 0.25% and Cu up to 0.35% have been observed. Horizons of fine disseminated sulphide occur in the dunite up to 5 metres in thickness containing up to 5.5g PGE, 0.68% Cu and 0.36% Ni in the 470 to 500 metre depth area. Two distinct layers have been identified so far, with highly anomalous zones in between the layers. The horizons are hosted in a medium to coarse dunite above a very coarse dunite. Fine-grained chalcopyrite with minor pyrrhotite-pentlandite occur in these layers. There is no development of feldspar or oxide mineral (magnetite or chromite) layering, however there appears to be a change in the pyroxene composition from both orthopyroxene and clinopyroxene below the mineralized horizons to just clinopyroxene above the mineralized horizon, which occurs in conjunction with a shift to slightly more primitive olivine compositions. Magnetic inversion modeling has been the most useful tool for mapping the shape of the intrusion. Down hole EM has identified conductors within the basal lherzolite which will be followed up for stringer-massive sulphide accumulations. Gravity surveying has helped identify a trough extending east and a scarn zone in the Sibley Group Metasediments. Most of the basement rocks beneath the areas containing values are Archean Quetico Metasediments. Sulphur isotope work suggests the Quetico sulphides acted as a sulphur source. Minerals identified in the basal zone are merenskyite, michnerite, sperrylite, a palladium-copper alloy, native copper, native iron-bismuth, chalcopyrite, cubanite, pyrrhotite and pentlandite. Where the horizons are dominated by bravolite, polarite, sperrylite, telluropalladinite and copper. - 47 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Geology, Geochemistry and PGE Potential of Mafic and Ultramafic Intrusions in Minnesota, excluding the Duluth Complex MILLER, James D. Jr., and JIRSA, Mark A., Minnesota Geological Survey, mille066@umn.edu and jirsa001@umn.edu Exploration for polymetallic deposits, including platinum group elements (PGE), has traditionally focused on mafic intrusions of the Mesoproterozoic Duluth Complex for obvious reasons—the complex is relatively well exposed and contains occurrences of PGE associated with basal Cu-Ni deposits that were discovered decades ago. By contrast, mafic intrusions in the Archean and Paleoproterozoic terranes of the remaining four-fifths of the state have received only cursory investigations by exploration companies and the state. This stems in large part from thick glacial cover over all but the northeast and southeast parts of the state; there is Paleozoic cover in the southeast. The completion of a high-resolution aeromagnetic survey of the state between 1982 and 1990 and the increased demand for PGE has spurred more exploration activity targeting ultramafic and mafic intrusions over the past two decades. Because of the requirement that drill core and related geochemical and geophysical data be turned over to the state after mineral leases expire, a sizeable public database of geologic, geochemical, and geophysical information exists on mafic intrusions in the state. However, this database has not been consistently evaluated for what it implies about the potential for these intrusions to host PGE deposits. Beginning in 2001, a four-year, two-phase study was funded by the state for the Minnesota Geological Survey (MGS) and the Natural Resources Research Institute (NRRI) to inventory the public database and acquire new data for the purpose of evaluating the potential for PGE deposits in mafic intrusions outside the Duluth Complex. Phase 1 of this study, which was completed in July of 2003 and published by the MGS as an interim openfile report, took inventory and digitally compiled basic geologic, lithologic, and geochemical attributes of over 150 individual mafic intrusions studied from field exposures or from drill core. Information was also collected on some of the thousands of diabasic dikes and other geophysical anomalies inferred to be mafic intrusions. The project work conducted on each intrusion varied with the level of detail that existed, but included some combination of geophysical (aeromagnetic and gravity) delineation, outcrop mapping, drill core examination, petrography, and geochemical analysis. This compilation of pre-existing data was augmented with geophysical modeling, relogging of drill core, outcrop mapping and sampling in well exposed areas of north-central Minnesota, petrographic studies of over 100 thin sections from 53 intrusions, and geochemical analyses and assays of 83 samples from 58 intrusions. Also, eight new 40Ar/39Ar analyses of magmatic hornblende and biotite separates were acquired to better constrain the general temporal framework of some intrusions. Phase 2 of this study, which is currently underway, sets out to conduct a more detailed evaluation of the geologic setting, igneous stratigraphy, and geochemistry of 15 specific intrusions for which there is sufficient sampling by drill core. The petrographic and geochemical results of this study phase were not ready at the time of this writing, but will be presented at the meeting. Another component of this study, which is being conducted by the NRRI, is a detailed mapping and chemostratigraphic analysis of the well-exposed Deer Lake layered mafic complex of north-central Minnesota. Based on an evaluation of this database, mafic intrusions can be grouped into several major types according to age and type of host rock, timing of emplacement relative to tectonism and metamorphism, and general lithologic attributes. The attributes of these intrusion types and some examples are summarized in Table 1. Phase 1 of this study acquired 75 new lithogeochemical and assay analyses from more than 50 intrusions. In addition, over 400 publicly available geochemical analyses have been compiled from exploration company files and data previously acquired by various state and federal agencies. For Phase 2, an additional 85 analyses are - 48 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Table 1. Classification of mafic intrusions outside the Duluth Complex in Minnesota. Tecto-metamorphism ithologic attributes Examples (Fig. 1) Subvolcanic maficultramafic sills Intrusion type Age Ar Archean greenstone belts Host rock Pre- to syn-tectonic; low to moderate metamorphic grade Tholeiitic-komatiitic; peridotite, pyroxenite, gabbro, & diorite Deer Lake Complex, Winterfire intr, KIB intrusions, Warroad intrusion Amphibolite sills & dikes Ar Archean highgrade gneiss terrains Pre-tectonic; high metamorphic grade Schistose amphibolite (metagabbro) Small amphibolite bodies in Quetico Subprovince Lamproid dikes & plugs Ar Archean greenstone belts Syn- to post tectonic; low metamorphic grade Hbld-, oxide-, & biobearing ultramafic rocks Lamprophyre plugs in Wawa Subprovince Gabbroic to intermediate intrusions Ar Archean granitegreenstone terrains Post-tectonic; low metamorphic grade Gabbro/diorite, tonalite, monzonite w/mafic enclaves Linden pluton, Oaks intrusion, Grygla pluton Gabbroic anorthosite massifs p< Archean granitegreenstone terrains Syn- to post-tectonic; moderate metamorphic grade Coarse gabbroic anorthosite Mentor intrusive complex Layered gabbronorite intrusions p< Penokean orogen & MN River Valley Subprovince Syn- to post-tectonic; low to moderate metamorphic grade Modally & texturally layered gabbronorite, gabbro, & pyroxenite Lake Washington, LL, BKV & Providence intrs. Pyroxenite & peridotite plugs p< Penokean orogen Post-tectonic; low metamorphic grade Alkaline ultramafic rocks Small plugs in central Minnesota Hornblendic maficintermediate complex p< Penokean orogen Syn- to post-tectonic; low to moderate metamorphic grade Hornblendite, diorite, granodiorite Tibbets Brook intrusion Olivine gabbro intrusions m< Unknown Syn-continental rifting Layered oxide olivine gabbro Fillmore County intrusions currently being acquired from 15 intrusions to more completely characterize the chemostratigraphy of these select intrusions. Currently, the highest PGE values have been found in sulfidic basal contact zones of the Winterfire subvolcanic sills in north-central Minnesota with Pd, Pt, and Au values in 1-3 meter intervals in the range of 5001,000, 100-400, and 50-200 parts per billion, respectively. The discovery of anomalous PGE values is not the primary objective of this study, however. Rather, the goal is to assess the overall potential of particular intrusion types to host either contact-type or reef-type PGE deposits. Working toward this goal, geochemical data will be used to interpret the composition and PGE tenor of the parent magmas, determine the degree to which the intrusions display internal differentiation, and evaluate conditions of sulfide saturation through the intrusions. A draft report on this study will be completed in the summer of 2005 with a final report to follow. In addition to digitally compiling pertinent geologic, geophysical, petrographic, and geochemical data, this report will evaluate the data to assess the PGE potential of the various intrusion types and make recommendations as to what intrusion types merit further study and exploration. - 49 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 New thoughts on old circles: A reexamination of spheroidal Gunflint taxa PLANAVSKY, N.J. and MURPHY, J., Department of Geology, Lawrence University, Appleton WI The first systematic description of the Gunflint biota (Barghoorn and Tyler, 1965) is a seminal work in Precambrian paleontology. The Gunflint biota are represented by an assemblage of incredibly well-preserved microfossils contained within the approximately two billion year old Gunflint Iron Range of the Animikie Basin. Because the Gunflint microfossils were the first definitive and widely publicized evidence of Precambrian life, they have become one of the most famous fossil assemblages in the world. The high level of preservation and diversity of the biota has led to the microfossils becoming a benchmark against which early records of life are compared. The Gunflint and other Animikie basin microfossils are also significant because they provide the most complete record of the biosphere during a dramatic change in Earthʼs history—the initial oxygenation of the atmosphere and shallow seas and the sequestration of the iron from the oceans. The importance of the Animikie microfossils is based upon a clear understanding of the diversity and structure of the Gunflint community. Most Gunflint genera are determined based on obvious morphological characteristics such as having a filamentous or spheroid morphology. However, species boundaries within broad morphological groups such as the spheroids are more difficult to define. Diameter distribution (i.e. unimodal or polymodal) is one factor that has previously been used to determine species composition of a spheroidal population (Strother and Tobin, 1987; Knoll et al., 1978; Knoll and Simonson, 1981). Although the range of species distributions has always been noted, it has been afforded little attention in the evaluation of specific taxon (Knoll et al., 1978). We conducted a study to determine the diameter distribution within a population of the coccoid cyanobacteria Microcystis aeruginosa and a survey of previously recorded diameter ranges for 100 extant coccoid cyanobacteria taxa. Our goal is to determine the validity of Gunflint and other Animkie basin species classifications. Previous studies that have noted a polymodal diameter distribution in a microfossil population have asserted that this indicates the presence of multiple taxa. This was most notably assumed for the genus Huroniospora, one of the most abundant biota of Gunflint. Strother and Tobin (1987) recorded a skewed bimodal size-frequency distribution (modes at 4 and 7 µm) for Huroniospora microfossils found within the Schreiber Beach assemblage. Huroniospora specimens from the Sokoman Iron Formation have a size-frequency distribution with modes at 3, 5, and 10µm (Knoll and Simonson, 1981), in accordance with previous findings from the Schreiber locality. We obtained a diameter distribution for a cultured population (from Lake Winnebago, Appleton, WI) of Microcystis aeruginosa through measurements with an optical micrometer. We made wet mount preparations and recorded the diameter of all specimens within a single optical plane. The diameters were placed into .5μm groups, which is the level of accuracy of the optical micrometer used. In all, 425 individuals were measured. The population showed a distinct bimodal pattern with modes at 6 and 8.5 μm. Although previous algae populations have been shown to have a broadly unimodal diameter distribution, our results show that a monospecific cyanobacteria population can contain a biomodal diameter distribution. Polymodal distributions of cyanobacteria have also been recorded (Prescott, 1951). These results strongly suggest that polymodal diameter distributions with closely spaced modes in a microfossil population, specifically those described by Strother and Tobin (1989) and Knoll and Simonson (1981), in the definitively cyanobacteria taxon of Huroniospora are very unlikely to indicate multiple taxa. At minimum the observations show that a polymodal diameter distribution within a microfossil population cannot be used as a diagnostic indication of multiple taxa. We also conducted an evaluation of the range of diameter distributions in extant cyanobacterial species. Our goal was to see if the reported diameter ranges for fossil taxon are consistent with the ranges seen for extant species. We surveyed 100 marine and freshwater cocciod cyanobacteria species from previous reports. The average diameter distribution was 3.0, with a standard deviation of 3.1. We found a range of 0.2 to 24 μm, with only two species with a reported range greater than 10μm. This observation is relevant because many of the Gunflint taxon have very large ranges that are skewed left. A notable example is Leptoteichos golubicci. The - 50 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 taxon is broadly unimodal with a mean cell diameter of 13.5μm. However, the reported range for the species is 26μm (5-31μm), larger than any of the species we encountered and far above the mean for diameter range. Less than the five percent of Leptoteichos golubicci have a diameter above 24 μm (Knoll et al., 1978). The diameter distribution of and range reported for Leptoteichos golubicci is inconsistent with monospecfic classification of the population. We believe that the cells above 24 microns in the Leptoteichos golubicci population are most likely akinetes, which are large reproductive structures (analogous in many ways to spores) produced by filamentous cyanobacteria. Although akinetes have previously been shown to be present to present in the Gunflint (Licira and Cloud, 1968), no estimate of their abundance was made. Our reinterpretation of the Leptoteichos golubicci population could be used to determine an estimate of akinete abundance. Although the spheroidal Gunflint microfossils are exceptionally well preserved, we need to remember that we are still looking at a collection of circles, from which little morphological variation can be gathered. Therefore information on population structure from modern analogues needs to be used to help ascertain the basic ecology of the Gunflint community. Selected References Braghoorn, E.S. and Tyler, S.A. 1965. Microorganisms form the Gunflint chert. Science, 147, 563-577. Licari, G.R., and Cloud, P.E., Jr., 1968. Reproductive structures and taxonomic affinities of some nannofossils from the Gunflint Iron Formation. Proceedings of the National Academy of Sciences, 59, 1053-1060. Knoll, A.H. Barghoorn, E.S., Awramik, S.W. 1978. New microorganisms from the Aphebeian Gunflint Iron Formation, Ontario. J. Paleontology 52, 976-992. Knoll, A.H. and Simonson, B. 1981. Early Proterozoic microfossils and pencontemporaneous quartz sedimentation in the Sokoman Iron Formation, Canada. Science. 211, 478-480. Prescott, G.W. 1951. Algae of the Western Great Lakes Area. Cranbrook Institute of Science: Bloomfield Hills, Michigan. Strother, P.K. and Tobin, K. 1987. Observations on the genus Huroniospora braghoorn: implications for paleoecology of the gunflint microbiota. Precambrian Research 36, 323-333 - 51 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Geochemical Variation within the Mesoproterozoic Nipigon Diabase Sills RICHARDSON, Adam and HOLLINGS, Pete, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1, Canada, ajrichar@lakeheadu.ca The Nipigon sills are related to the Mesoproterozoic Keweenawan mid-continent (MCR) rift event. Centered on what is now the Lake Superior basin, the igneous activity is interpreted to be the result of a mantle plume interacting with the continent as far to the southwest as Kansas, USA, and to the southeast as far as the Grenville front in southern Ontario, Canada (Davis and Green, 1997). The system operated for approximately 20 Ma producing a volume of intrusive and extrusive igneous rock estimated to be in excess of 1.5 million km3 (Klewin and Shirey, 1992). The Nipigon sills are thick, relatively flat lying hypabyssal, medium-grained subophitic to ophitic olivine gabbronorite with U-Pb ages ranging from 1107 Ma to 1112 Ma (Heaman and Easton, this volume). The goals of this project are to evaluate geochemical variations within the Nipigon sills, compare them to other MCR rocks and generate an emplacement model which identifies and explains any identified intrasill relationships, magma sources. The project area (Figure 1) covers approximately 15,000 km2 of the Nipigon embayment. As part of this project a detailed sampling program was undertaken in the summer of 2003 and 2004 resulting in 250 shoreline samples being collected from Lake Nipigon (Figure 1) and 530 samples from diamond drill core in the southern Lake Nipigon basin near Black Sturgeon and Shillabeer lakes. Selected samples were analysed for major, trace and rare earth elements with a representative subset analysed for Rb-Sr, Sm-Nd and Pb-Pb isotopes. Three distinct sill suites were identified within the basin. Two additional suites have been identified by T. Hart and C.A. MacDonald (Ontario Geological Survey) as part of their mapping projects in the Nipigon Embayment (MacDonald, 2004; Hart and Magyarosi, 2004). The various suites were defined, in part, using rare earth element data (Figure 2). The most spatially abundant suite has been termed the Main Nipigon suite. This suite outcrops over much of the Nipigon embayment having La/Smcn and Gd/Ybcn ratios of 1.4-1.8 and 1.3-1.5 respectively (Figure 2). Mga s s.ys.dns El Meta,oeic • The Main Nipigon and Inspiration suites share similar Gd/Ybcn (Figure 2), TiO2 and MgO values. The Inspiration suite has a higher La/Smcn ratio, but these data lie on a possible trend from the Main Nipigon suite and the whole rock data suggest that the Inspiration and the Main Nipigon suites may be related. OS' Hole CdL3! 9Spl {U Scale 0 102O Km Figure 1. Map showing sample locations, drill holes, geology and project area. - 52 - Other suites in the Nipigon Embayment include the McIntyre and Jackfish suites. These two suites share similar Gd/Ybcn and La/Smcn ratios and lie on a fractionation trend (Figure 2) with the Jackfish �Proceedings of the 51st ILSG Annual Meeting - Part 1 possibly being the source of the McIntyre. The Shillabeer suite is geochemically similar to the Disraeli ultramafic intrusion using Gd/ Ybcn, La/Smcn and whole rock geochemistry suggesting the possibility that this suite is not part of the Nipigon sills but is related to the ultramafic intrusions of the southern Lake Nipigon embayment. 4.5 • • .1;I •a . A U Isotope data (Figure 3) reveals differences in the degree of crustal contamination of the ShiIlaber • Distoeli sill suites. The Main Nipigon and McIntyre •Jockfish suites are the least contaminated (εNd ~0) o o. 1.5 2 2.5 4 4.5 3 3.5 with the Main Nipigon data trending toward La IS men Sibley group sediments suggesting that these Figure 2. Chondrite normalized rare earth data defining sill suites in sediments may be the dominant contaminant. Shillabeer and Inspiration sill suites are the the Nipigon embayment. most contaminated (εNd -5.5 to -7) but do not 0.705 0.110 OIlS 0.720 0.850 0.900 0.950 trend towards Sibley data implying a distinct contaminant. . Moin Nipigon a £Kama Hill • McIntyre .—- 1 p £ £ References Klewin, K.K, and Shirey, S.B. 1992. The igneous petrology and magmatic evolution of the -4 0 Midcontinent rift system. Tectonophysics 213, 33-40. Davis, D.W. and Green, J.C. 1997. Geochronolgy •. •Jadcfih of the North American Midcontinent rift in AMoin Mpon A Shillthter western Lake Superior and implications for its •MCIMyT •Sibky geodynamic evolution. Canadian Journal of Earth Sciences 34, 476-488. 875r/86Sr Hart, T.R. and Magyarosi, Z. 2004. Precambrian Figure 3. Nd and Sm isotope data for the Nipigon sill suites Geology of the northern Black Sturgeon River compared with data from the Sibley sediments and Quetico basement and Disraeli Lake Area, Nipigon Embayment, rocks. northwest Ontario; Ontario Geological Survey Open File Report 6138, 56 p. MacDonald, C.A. 2004. Precambrian geology of the south Armstrong-Gull Bay area, Nipigon Embayment, northwestern Ontario; Ontario Geological Survey, Open File Report 6136, 42p. -'.0 £ •/ - 53 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 The Geology of the Eagle Nickel-Copper Deposit: Marquette County, Michigan ROSSELL, D. M., dean.rossell@kennecott.com, Kennecott Exploration Company, 10861 N. Mavinee Dr. #141, Oro Valley, AZ. 85737 and COOMBES, S., Kennecott Canada Exploration Inc., #354-200 Granville Street, Vancouver, B.C. V6C 1S4 Kennecottʼs discovery of the Eagle nickel-copper deposit in 2002 marked the culmination of more than a decade of exploration work by Kennecott in the Paleoproterozoic in age Baraga sedimentary basin. The discovery hole, YD02-02 completed in July 2002, intersected 84.2m of massive sulfide mineralization averaging 6.3% Ni and 4.0% Cu. The resource estimate for the Eagle deposit at the end of 2003 was 5 million tonnes at 3.68% Ni, 3.06%Cu and 0.1% Co. The Eagle deposit is hosted in the westernmost of two small peridotite bodies historically referred to as the Yellow Dog Peridotite. The Yellow Dog intrusions, which lack penetrative foliations and truncate Penokean tectonic fabrics in the surrounding meta-sediments, are believed to be Keweenawan in age (Klasner et al., 1979). The intrusions are mainly comprised of coarse-grained, variably serpentinized peridotite and feldspathic peridotite. A fine-grained, olivine poor phase is found along the margins of the intrusions and as xenoliths within the peridotite. Possible amygdules in the olivine poor phase(s) suggest a shallow level of intrusion. Three principal types of sulfide mineralization are recognized in the Eagle deposit: disseminated (blebby), semi-massive (matrix) and massive. Although the nickel contents of semi-massive and massive sulfides are relatively uniform through out the deposit, copper contents vary significantly. Platinum group metals (PGM) and gold values are significantly higher in the copper rich massive sulfides. Copper rich veins and disseminations, with significant PGM and gold, in the surrounding meta-sediments may constitute a fourth type of ore. Massive and semi-massive sulfide ore types in the Eagle deposit are irregularly distributed. The contacts between different ore types are sharp and show little evidence of the gradation or layering that might be expected if gravity driven accumulation of sulfides from an overlying, sulfide saturated, silicate magma was the principle mechanism of ore formation. Sequential emplacement of various mixtures of silicate and sulfide magma and cumulus minerals, derived from a lower stratified magma chamber, may provide a better model. Reference Klasner, J.S., Snider, D.W., Cannon, W.F. and Slack, J.F. 1979. The Yellow Dog Peridotite and a possible buried igneous complex of lower Keweenawan age in the northern peninsula of Michigan. Geologic Survey of Michigan DNR report of investigation 24, 31p. - 54 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Mining and Exploration Activity in the Thunder Bay South District SCHNIEDERS, B.R. and SCOTT, J.F., Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 CANADA The Thunder Bay South District covers an area from west of Atikokan, east to White River and north to Armstrong from the United States border. There were four producing precious metal and base metal mines, as well as 15 seasonal amethyst producers in 2004. The three Hemlo gold mines produced 655,000 ounces of gold in 2004, and more than 18 million ounces of gold has been produced from the Hemlo deposit since mining began in 1985. The Lac des Iles mine produced a record 308,931 ounces of palladium from the open pit, as well as platinum, gold, nickel and copper. Underground development was initiated in 2004 and underground mining is scheduled to begin in 2006. Exploration for gold, base metals, platinum group metals, uranium, molybdenum, diamonds and industrial minerals were all active in 2004. Highlights include a new base metal discovery by Freewest Resources Canada Inc. in early 2005. The Sungold Occurrence in the Western Shebandowan greenstone belt (Wawa Subprovince) is located 120 km west of Thunder Bay. Grab samples assayed 33.2 % zinc and 12.5 % Cu, discovered after a V-TEM (Dreamcatcher) and winter beep-mat geophysical surveys were conducted. Gold highlights include the Tower Mountain Property of Valgold Resources Ltd. who conducted more than 10,000 m of diamond drilling with results of up to 164.7 g/t gold over 1.5 m. Further drilling is ongoing. In the Hemlo area Navasota Resources Ltd. intersected 18.28 g/t gold and 47.18 g/t silver over 4 m in diamond drilling. North American Palladium Ltd. continued deep diamond drilling on the High-Grade Offset Zone, including intersecting 5.25 g/t palladium and 0.515 g/t platinum over 3.1 m. Exploration is ongoing on the mine block, and at the Shebandowan project. East West Resource Corporation and Canadian Golden Dragon Resources Ltd. entered into a $7.5 million joint venture with Platinum Group Metals Ltd. on the Seagull-Disraeli property. Several new reef horizons were discovered, assisted by the Ontario Geological Survey, the Lake Nipigon Region Geoscience Initiative and Lakehead University. Assays of up to 7.90 g/t platinum + palladium + gold over 0.44 m have been intersected with a 1:1 Pt:Pd ratio. Rampart Ventures Ltd., in joint venture with New Shoshoni Vcentures Ltd., have staked more than 2000 claims in the Sibley Basin for uranium. Airborne geophysical surveys have been flown and diamond drilling is planned. R ipple Lake Diamonds Inc. discovered three micro-diamonds in a small 3 by 3 m diatreme associated with a calc-alkalic (minette) dike, located on the Trans-Canada Highway approximately 30 km west of Marathon. More than 2200 claim units have been acquired. Exploration is active for a variety of commodities in the Thunder Bay South District. We invite you to come and explore in Northwestern Ontario! - 55 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 PGE and gold potential of the Archean Deer Lake Complex, Minnesota, USA SEVERSON, Mark J., Natural Resources Research Institute, University of Minnesota, Duluth The precious metal potential of the Deer Lake Complex (DLC) has recently been addressed by detailed geologic mapping (2004) and two detailed sampling campaigns (2002 and 2004). The DLC is Archean in age, is located in northeastern Itasca County, Minnesota, and consists of a series of multiple sills, each of which are variably differentiated and steeply inclined. Sampling results, along several traverses across the DLC, suggest that PGE contents are low with maximums of only 84 ppb Pt and 52 ppb Pd. Gold contents, on the other hand, are more encouraging and values up to 2.4 ppm are associated with a shear zone in one of the basal DLC sills. The Deer Lake Complex (DLC) consists of a series of steeply-dipping, tholeiitic, mafic-ultramafic sills that are variably differentiated and complexly interfingered. Where differentiation is complete, the sills exhibit a layering sequence consisting of a basal chilled margin grading upwards through peridotite, pyroxenite, melagabbro, melagabbro-porphyritic gabbro, gabbro, quartz gabbro (or diorite), and in some rare localities, an upper chilled margin. The sills were mostly emplaced into calc-alkaline volcanic and sedimentary rocks of the Joy Lake Volcanic Sequence (Jirsa, 1990). The bottom-most portion of the DLC was initially intruded into highMg tholeiitic basalt and basaltic komatiite; these volcanic rocks are assumed to be the early extrusive equivalent of the DLC (Jirsa, 1990; Englebert and Hauck, 1991). All of the sills are metamorphosed to greenschist facies. The DLC was explored for base metal deposits in the 1970s; however, no significant indications of Ni-Cu mineralization were found. Initial geologic mapping of the DLC (Berkley, 1972; Ripley, 1973) suggested that only 2-3 completely differentiated sills were present, and that repetitions of specific rock types were related to tight isoclinal folds. Later review (Severson, 1987; Jirsa, 1990; this study) suggests that the DLC formed from the emplacement of numerous interrupted magmatic pulses with partial to complete fractional crystallization occurring between the pulses. Thus, not all of the sills contain a peridotite base or grade completely upwards into quartz gabbro. Stratigraphic Order As can be seen on a plot of Pt+Pd versus stratigraphic position (Figure 1), most of the individual DLC sills contain extremely low Pt+Pd contents except for weakly anomalous samples in central portions of several sills. The overall patterns for two of these sills in particular consist of low Pt+Pd values that gradually increase with stratigraphic height to noticeable peaks, followed by dramatic dropoffs to virtually no PGE in the upper portions of the sills. It may be possible that this dramatic changeover in the Pt+Pd content may be related to a Sulfide Saturation??? 7 sulfide saturation event; whereby, PGE was scavenged from the melt. 6 A similar pattern has been reported for the Mesoproterozoic Sonju Lake Sulfide Saturation? intrusion, where a PGE-bearing reef 5 with up to 320 ppb Pd and 66 ppb Pt has been identified by Miller (1999). 4 3 However, detailed follow-up, close2 1 spaced, sampling at one of the “sulfide 0 50 100 150 saturation horizons” during 2004 failed Pt+Pd (ppb) to produce any significant PGE in the DLC. Figure 1. Vertical changes in Pt+Pd (ppb) contents with stratigraphic height in the various sills (numbered in boxes) of the Deer Lake Complex. Shaded areas are peridotite horizons. - 56 - Several anomalous gold values �Proceedings of the 51st ILSG Annual Meeting - Part 1 (>100 ppb) were obtained in samples collected from a drill hole (26508) near the base of the DLC. These values (up to 2,399 ppm Au) are from a well-foliated/sheared gabbro that contains 1-5% pyrite and pyrrhotite, and contains some of the highest Cu values (up to 0.07% only) in the DLC. In summary, the PGE potential of the Deer Lake Complex appears to be low but there is an outside possibility that some of the sills may contain PGE reefs that are associated with sulfide saturation events. On the other hand, Au potential appears to be better, especially in the sheared rocks in drill hole 26508. REFERENCES Berkley, J.L. 1972. The geology of the Deer Lake gabbro-peridotite complex, Itasca County, Minnesota. University of Missouri, M.S. thesis, 107p., 2 pls. Englebert, J.A. and Hauck, S.A. 1991. Bedrock geochemistry of Archean rocks in Minnesota. Natural Resources Research Institute, Technical Report NRRI/TR-91-12, 200 p. Jirsa, M.A. 1990. Bedrock Geologic Map of northeastern Itasca County, Minnesota. Minnesota Geological Survey Miscellaneous Map Series M-68, scale 1:48,000. Miller, J.D. 1999. Geochemical evaluation of platinum group element (PGE) mineralization in the Sonju Lake intrusion, Finland, Minnesota. Minnesota Geological Survey Information Circular 44, 32 p. Ripley, E.M. 1973. The ore petrology and structural geology of the lower Precambrian Deer Lake mafic-ultramafic complex, Effie, Itasca County, Minnesota. Duluth, Minnesota, University of Minnesota Duluth, M.S. thesis, 143p., 2 pls. Severson, M.J. 1986. Summary report on exploration activities in the Deer Lake Area, Itasca County, Minnesota. Santa Fe Minerals Exploration report, Minnesota DNR open files. - 57 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 A view into the Nipigon Embayment: preliminary results of the largest magnetotelluric study ever in Ontario. SHAREEF, S., and CRAVEN, J.A., Geological Survey of Canada, Ottawa, ON K1A 0E9. Over 800 audiomagnetotelluric and combined audiomagnetotelluric and magnetotelluric (A/MT) sites were surveyed along five transects (Figure 1) during the period of late Aug to mid October 2004. The primary goal of the survey was to investigate the deep structures of the region to complement the studies that comprise the Lake Nipigon Regional Geoscience Initiative (LNRGI). The LNRGI was created to provide a comprehensive 4-D geoscience knowledge base in order to facilitate proper land-use decisions and to reduce the risk in mineral exploration of the region. The studies conducted as part of the Initiative are comprised of surface geological and stratigraphic investigation, surface and airborne geophysical surveys and other studies such as geochronological or paleomagnetic studies. Missing from these studies is an investigation of the deep geological structure. The A/MT survey conducted last year was designed to provide subsurface information down to mantle depths and beyond along five major transects. I\ I \. I B38AMTstes orna MT/MJT sites PR OT E H OZO IC Felsic to interrnethale metavolcanic rocks I relsic 0 Intern,edsIe fliwsive rocks —, Coarse dastic Melsedimentaly rocks ARCH EA N Felsic granitic rocks Melsedime ntaiy rocks Massive granodiorite '0 grane 20km Disraeli Lk 3-D • Larger 3-D • Figure 1. Site Locations for 2004 A/MT Nipigon Survey. Possible plan view locations for future 3-D model studies are also shown by red boxes. The magnetotelluric technique is a geophysical technique for imaging the deep structure of the Earth. It images conductors in the Earth and as such will be useful to delineate contacts between crystalline and sedimentary units at depth as they have dramatically different electrical characteristics. The novel feature of A/MT is that it uses the Earthʼs natural magnetic field as a transmitter of energy into the earth. This enables almost an infinite depth of investigation; however in practice the depth of investigation is limited to the asthenosphere. Preliminary results of the Nipigon A/MT survey will be presented with a view to providing a clue to the origin of the enigmatic sedimentary and intrusive units in this area. - 58 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Mineral Deposits and Metallogeny of the Midcontinent Rift in Ontario SMYK, Mark C., Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 CANADA A variety of metallic and non-metallic mineral deposits are associated with Mesoproterozoic (Keweenawan) Midcontinent Rift (MCR)-related magmatism and associated hydrothermal activity in Ontario. Orthomagmatic deposits of copper, nickel and platinum group elements (PGE) are associated with early ultramafic to mafic intrusive rocks. Stratabound, “reef-style” Cu-Ni-PGE mineralization occurs in peridotite in the Seagull layered intrusion; within massive, cumulate Ti-Fe-oxide layers in the border gabbro of the Coldwell Complex; and with Cr-bearing spinel in cyclic anorthosite-gabbro units above the Great Lakes Nickel deposit in the Crystal Lake Gabbro. Varied-textured to pegmatitic gabbro at the base of the Crystal Lake Gabbro hosts the Great Lakes Nickel deposit (~ 41.4 mT @ 0.334% Cu, 0.183% Ni, 0.69 g/t Pd, 0.21 g/t Pt). Similar rocks of the Two Duck Lake gabbro in the Coldwell Complex host the Marathon deposit (24.4 mt @ 1.22 g/t Pd, 0.31 g/t Pt, 0.37% Cu). Late-stage, magmatic deposits of pyrochlore-hosted, U-Nb-mineralization have been investigated in the Prairie Lake Carbonatite Complex and in syenites of the Coldwell Complex (Nb-Zr-Th-U-Ce). Rare metal (Be-Zr-U-Th-) mineralized structures crosscut the Dead Horse Creek carbonatitic diatremes. Epigenetic deposits associated with MCR structures and hydrothermal activity are hosted in a variety Neoarchean, Paleoproterozoic and Mesoproterozoic rocks. Native copper and Cu-sulphides occur in Keweenawan basalt and interflow sedimentary rocks at Mamainse Point (e.g., Coppercorp Mine), on Michipicoten Island (e.g., Quebec Mine) and in Osler Group rocks in western Lake Superior. Hydrothermal fluids ascribed to MCR magmatism are believed to have produced Cu-Mo-Ag-Pb-mineralized breccia pipes in Archean country rocks at the Tribag Mine and Cu-Mo-mineralization at the Jogran porphyry deposit. Copper-mineralized carbonate units of the Mesoproterozoic Sibley Group are found near the contacts with Nipigon diabase sills. Silver-bearing, carbonate-quartz veins near Thunder Bay have been subdivided into two groups: the Mainland Belt (Ag + Zn-Cu-Pb-bearing veins in Paleoproterozoic Animikie Group sedimentary rocks and Logan diabase); and the Island Belt (Ag-Bi-Co-Ni-As-bearing veins in Keweenawan diabase/gabbro; e.g., Silver Islet Mine). Lead-zinc-barite veins occupy structures near the unconformity between Neoarchean basement and Mesoproterozoic Sibley Group sedimentary rocks. Some of these structures also host amethyst deposits. Silver-bearing, lead-zinc veins occur within and proximal to the Coldwell Complex. All of these veins are likely contemporaneous but were generated by different fluid sources. Uraniferous, hematite-rich veins and breccias that cut Neoarchean basement and locally contain Proterozoic xenoliths may also be related to MCR hydrothermal activity, which remobilized uranium from granitic pegmatites in the basement. - 59 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Discrimmination of Archean Terranes in the Sachigo Subprovince and relevance to Volcanogenic Massive Sulphide Exploration STOTT, G. M., Ontario Geological Survey, Sudbury, ON, P3E 6B5 (greg.stott@ndm.gov.on.ca), and RAYNER, N., Geological Survey of Canada, Ottawa, ON Geological and geophysical constraints on the mechanisms for tectonic growth of the Superior Province are progressively summarized in Williams et al., 1992; Stott, 1997; and Percival, 2005, the latter based on Lithoprobe and Natmap research activities in northwestern Ontario and Manitoba since 1997. This increased knowledge permits greater confidence in the subdivision of the Superior Province into terranes and superterranes of contrasting tectonostratigraphic history. Prior to this, for many years, the broad region of Archean crust north of Uchi Subprovince in Ontario was more commonly called the Sachigo Subprovince (e.g., Card and Ciesielski, 1986). A reconnaissance geochronological and Hf isotopic study was initiated on a transect of granitoid and felsic volcanic samples just west of the James Bay Lowlands (Figure 1) to complement the Natmap studies previously conducted near the Ontario - Manitoba border (e.g., Stone, 2005). Thus far, results from U-Pb geochronology in eastern Sachigo Subprovince further confirm that this subprovince can be subdivided into several terranes. Most notably, a broad Neoarchean, Oxford-Stull terrane contains crustal magmatic ages ranging from 2690 to 2730 Ma, and separates two major superterranes to the north and south, each of which contains evidence of > 3.0 Ga history. Since virtually all significant Cu-Zn volcanogenic massive sulphide (VMS) deposits in the Superior Province are Neoarchean in age, the presence of Oxford-Stull terrane bears some importance for exploration. A Neoarchean age of 2737±7 Ma has been determined for a felsic volcanic host of recently discovered VMS deposits in the McFaulds Lake area, inside the edge of the James Bay Lowlands (Figure 2). This argues for the potential of a chain of VMS occurrences to be explored along a Neoarchean tectonic assemblage extending from Kasabonika to the McFaulds Lake area, comparable to the VMS-rich assemblage along the discontinuous Shebandowan-Terrace Bay-Manitouwadge greenstone belt (Figure 2). Assean Lake On Lake Block Bloc SpFit Lake B!ock / / J&nes Bay Lowlands Greenstone Belt a Sedimentary Belt () Pltjtonjc Rocks Figure 1. A subdivision of NW Superior Province into terranes (modified from Stone, 2005). Uncertainty of boundaries increases eastwards reflecting current distribution of geochronological and isotopic data. - 60 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 _______ Kasabonika-MeFaulds greenslone bell I 0 100 I Olher greenstone bells A MFaulds Lake Cu-Zn * Community massive sulpriide deposits (Spider Resou rcesIKWG Resources exploration) 200 K Ia metres 100 200 — Shebandowan-Terrace Bay-Michipicoten band aigreenstone belts Other greenstone bells Kilometres a Cu-Zn massive sulphide deposits Community Figure 2. (a). The discontinuous Neoarchean Kasabonika-McFaulds greenstone belt is the focus of new Cu-Zn VMS exploration, and may resemble (b) the Shebandowan-Terrace BayManitouwadge belt, a bandlike tectonic assemblage, which contains several VMS deposits and mines along its length. References Card, K.D. and Ciesielski, A. 1986. Subdivisions of the Superior Province of the Canadian Shield. Geoscience Canada, 13, 5-13. Percival, J.A. 2005. The Ancient Earth: Development of the Canadian Shield, the oldest parts of North America – building a continental foundation from 4000 to 2500 million years ago. In LITHOPROBE Celebratory Conference Oral and Poster Presentations, R.M. Clowes and C. Li (compilers). Published by the LITHOPROBE Secretariat, University of British Columbia, E-Publication No. 5, v.1. Stone, D. 2005. Geology of the Northern Superior Area, Ontario. Ontario Geological Survey, Open File Report 6140, 94p. Stott, G.M. 1997. The Superior Province, Canada. In Greenstone Belts; Edited by M.J. de Wit and L.D. Ashwal. Oxford Monograph on Geology and Geophysics 35, Oxford Clarendon, p.480-507. Williams, H.R., Stott, G.M. and Thurston, P.C. 1992. Tectonic Evolution of Ontario: Summary and Synthesis, Part 1: Revolution in the Superior Province. In Geology of Ontario; Ontario Geological Survey, Special Volume 4, Part 2, pp.1256-1294. - 61 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Correlation Between Self-Potential and Dowsing (IESG) at the Quincy Mine and at the Calumet and Hecla Mine, Michigan TROW, Jim, Geological Sciences, Michigan State University, emeritus, 540 Lake Avenue #2, Hancock, Michigan 49930, and YOUNG, Charles, T., Geological Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931. The focus of this paper is an 11/24/04 joint traverse over and mapwise perpendicular to the strike of the native copper Calumet Conglomerate Lode at the Hecla #1 Mine, Calumet Michigan, with self-potential (Young) and dowsing (Trow) observations plotted in profile on Figure 1. According to Young (Young and Trow, 2005), “The self-potential apparatus consists of hand-made non-polarizing copper copper-sulfate electrodes, a reel containing AWG 20 stranded, tinned copper wire, patch cords and clip leads and a high-input-impedance digital voltmeter. The reference electrode is installed at a convenient sheltered position along the line where the wire can be tied to a sturdy object. The wire is clipped to the reference electrode and is unwound from the reel to the observation points. The electrodes are installed in shallow holes made with a garden trowel, which is used to remove loose and dry surface material so that the electrode is in contact with moist soil. The repeatability of these self-potential measurements is typically about five millivolts.” Horizontal distances are measured with fiberglass surveyorʼs tape. ! 9 U,. I I .4. •tlfl I 2. . .: I Ic_ — 14• 1994 99 99 —l—m ••91 •U!14944 .9*9 .11 — 191 •tt3flt 1194•t IOU . .tn. T: t 93 9 —Ic 9• 9• 4 .99919,.., ••—1199,• .9ltI. ••!T499 •tC •34 .9 fl I • ni. it/c oIl •!lM •_• 'ull •;3• •Tt •.94 l.!fl.. •4n•9& .4..•.9nulb. .9 9* 9 ' 9 ct .•9t m .mE .9 — •n..Tp •9c! .l l •jj•9] 4!—.. •.—_. •93 •lS.9 •fl9 .9!,; fl .9I •.jfl.9 1.991 •9Iul97_ •3'9n• .9—. .4,3. 9 .9 •..I—_,.9— . . *1 £9 c_fl.... ••..!p •!! 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Figure 1 shows dowsing by Trow with sensitive 3/16”-diameter 6” x 15” iron welding rods, bent into a Ushape, and with less sensitive 6” x 15” L-rods of the same diameter and material. Dowsing with U-rods reset to the standard state after each episode of turning (CCʼ) detects a broader and lower anomaly than Youngʼs –350 mV SP anomaly (AAʼ), but both appear to be distributed symmetrically about the SP negative center, as though dowsing implies an upward-continuation curve for the SP. After all, SP is a conductive method, whereas dowsing is an inductive “airborne” method, involuntarily detected by oneʼs brain (Faraday cage shielding experiments; - 62 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Trow, 1992). BBʼ shows the less sensitive L-rod dowsing results. When favorable weather returns, Trow would like to reverse by traversing SE, to see if the L-rods define the NW edge of the subcrop of the ore body as BBʼ defines its SE edge when traversing NW. DDʼ represents Trowʼs dowsing with the sensitive U-rods, when they are not reset to the standard state after turning, in conformity with most dowsersʼ procedure, which ignores important data compared to CCʼ. They have run out of lever arm (physics terminology). Figure 2 (not shown, please see poster) suggests three hypotheses for a mathematical function to reconcile SP and dowsing: The geologically most plausible is that the square root of the number (x) of Urod convergences over the total –SP anomaly is proportional to the –SP readings of its negative center. An even better fit than Figure 2 results when one considers the total SP relief on Figure 1, from +15 mV at Aʼ to the –350 mV negative center = -365 mV. Calculating for Quincy [(√7/√13)(-365mV)] yields – 268mV, 99% of Youngʼs observed – 270 mV there. Two points, Quincy and C & H, do not define a curve, but they hint at one. Figure 3 (not shown, please see poster) symbolically illustrates (some elements displaced to avoid overlap) characteristic dowsersʼ rod movements, which Trowʼs (1992) experiments with permanent magnets, low-voltage static electric charges, and Faraday cages demonstrated are related to electrostatics, not magnetostatics. REFERENCES Trow, J. 1992. Inductive electrostatic gradiometry (IESG) deciphers Keweenanwan copper plumbing system, Soc. Mining, Metall. And Expl., Phoenix, Preprint 92-32, 22p. Trow, J. 2004. Dowsing employs classical mechanics and static electricity to locate self-potential anomalies inductively and rapidly, 50th Institute on Lake Superior Geology, Duluth, Proceedings, v.50, pt 1., 158-159. Young, C.T. and Trow, J. 2005. Human-sensed fields?: Does dowsing response correlate with self-potential or conductivity anomalies?, 18th Annual Symposium on the Application of Geophysics in Engineering and Environmental Problems, Atlanta, 9p. - 63 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 New age data for the Chocolay Group, Marquette Range Supergroup: implications for the Paleoproterozoic evolution of the Lake Superior and Lake Huron regions VALLINI, Daniela A., School of Earth and Geographical Sciences, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia, CANNON, William F., and SCHULZ, Klaus J., U.S. Geological Survey, 954 National Center, Reston, VA 20192 A geochronological study of the Chocolay Group at the base of the Paleoproterozoic Marquette Range Supergroup in Michigan, Lake Superior region, is attempted for the first time. Age data from detrital zircon grains and hydrothermal xenotime from the basal glaciogenic formation, the Enchantment Lake Formation, and the overlying Sturgeon Quartzite and its equivalent, the Sunday Quartzite, provide maximum and minimum age constraints for the Chocolay Group. The youngest detrital zircon population in the Enchantment Lake Formation is 2317±6 Ma, in the Sturgeon Quartzite it is 2306±9 Ma and in the Sunday Quartzite it is 2647±5 Ma. The oldest hydrothermal xenotime in the Enchantment Lake Formation is 2133±11, in the Sturgeon Quartzite it is 2115±5 Ma and in the Sunday Quartzite it is 2207±5 Ma. The depositional age of the Chocolay Group is constrained to 2300-2200 Ma and the radiometric age data proves it is correlative with part of the Huronian Supergroup in the Lake Huron region, Ontario and reveals that the unconformity which separates the Chocolay Group from the overlying Menominee Group is up to 350 m.y. in duration. The detrital zircon suite in the Sunday Quartzite and part of those in the Enchantment Lake Formation and Sturgeon Quartzite were derived from the underlying Archean basement rocks. The source of the ~2300 Ma detrital zircon populations in the Enchantment Lake Formation and Sturgeon Quartzite remains an enigma as there is no known terrane of this age in the Michigan area. It is speculated that there was once a widespread volcano-sedimentary cover sequence in Michigan which was removed or concealed prior to Chocolay Group deposition. The hydrothermal xenotime ages are coeval with the 2219±4 Ma Nipissing Diabase in the Lake Huron region and the 2200-2100 Ma Kenora-Kabetogama mafic dyke swarm in Minnesota and adjacent Ontario, Lake Superior region. - 64 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Implications of Midcontinent Rift and Oceanic Ridges Analogies and 3DInterpretations of the Subsurface Structure of the Bald Eagle Intrusion in the Duluth Complex and the East Pacific Rise WEIBLEN, Paul, Department of Geology & Geophysics, University of Minnesota, PETERSEN, Dean, Natural Resources Research Institute, University of Minnesota Duluth, and VISLOVA, Tatiana, Department of Geology University of St. Thomas The geometry of the gravity and magnetic anomalies of the Bald Eagle intrusion and the Duluth Complex to the south, as well as the overall Midcontinent Rift, is very similar to the pattern of the seismic reflections profiles of ridge systems (Vislova, 2003). In detail, the geophysical expressions of the Bald Eagle intrusion have the same shape and dimensions as the “bulls eye” pattern of low velocity seismic reflection anomalies along the East Pacific Rise. These anomalies are interpreted to define regions of melt concentrations, i.e., active magma chambers. This suggests that the Bald Eagle intrusion could be a “frozen” dynamic magma chamber. In support of this analogy we note that the magmatic systems of mid-ocean ridges, extensional regimes in back-arc environments, and ophiolites have a common characteristic: the emplacement of magma in extensional environments, and the common products in all four are varieties of layered intrusions, dikes and sills, and overlying volcanic rocks. There is a long history of comparative studies of ophiolites and oceanic magmatic environments (See web sites for ophiolites and the East Pacific Rise). This suggests that it would be profitable to add the studies of the igneous systems of intracontinental rifts to the current interdisciplinary, studies of ophiolites and the oceanic environments. In the accompanying poster session we explore aspects of this approach using 3D modeling of surface mapping, geophysical, and drill hole data of the basal mineralized zone of the Duluth Complex, the Bald Eagle intrusion and seismic reflection data of the East Pacific Rise. Since its discovery in 1961, the Bald Eagle intrusion has posed unresolved questions concerning its origin and magmatic significance (Weiblen, 1965; Weiblen and Morey, 1980; Miller and others, 2002). A number of its characteristics contrast markedly with those of the other mapped intrusions in the Midcontinent Rift: (1) It has a well-defined intrusive contact in anorthositic gabbros around its perimeter except for its southern extension; (2) There is a subtle, but recognizable metamorphic contact effect on the anorthositic gabbros; (3) A primary magmatic foliation is well defined by mineral orientation and discoid segregation of plagioclase from mafic phases; (4) Over 150 measurements of the foliation define a steeply-dipping asymmetric funnel with the foliation paralleling the contact and grading from steep to horizontal inward; (5) The intrusion consists of two cumulus units, a plagioclase-olivine outer cumulate (troctolite) and an inner plagioclase-olivine-clinopyroxene (olivine gabbro) cumulate; and (6) There is only minor (< a few %) intercumulus material in the cumulates which is found as the expected lower temperature minerals in the crystallization sequence, i.e. clinopyroxene and iron oxides in the plagioclase-olivine. Recent petrologic studies include over 2000 electron microprobe analyses and interpretations using the computer-based mineral-melt equilibria routines MELTS and COMAGMAT (Vislova, 2003). These studies indicate that the following scenario could account for the characteristics of the Bald Eagle intrusion listed above. As up-welling magma streams through a dynamic (expanding) funnel-shaped feeder, a constant temperature appropriate to plagioclase-olivine crystallization is maintained by a balance between the heat content of the incoming magma plus the heat of crystallization and the heat loss through the chamber walls. Plagioclase and olivine are left behind and oriented/segregated on the walls of the expanding chamber. At some point conditions change and a temperature appropriate to plagioclase-olivine-clinopyroxene crystallization is established and maintained to form the inner olivine gabbro of the intrusion. This scenario could also produce the plagioclaseolivine-clinopyroxene-iron oxide cumulates found to the south of the mapped expression of the Baled Eagle intrusion. A dynamic, flow-through magmatic system raises questions about mineralization. We note that there are active “smokers” above the proposed magma chambers on the East Pacific Rise. In view of this and assuming - 65 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 the analogy suggested above is appropriate, comparative studies of the smokers and the sulfide mineralization in the intrusions adjacent to the Bald Eagle intrusion may be profitable. Applying this approach, some of the attributes of the Duluth Complex copper-nickel-PGE sulfide deposits resemble those of deposits at Norilʼsk, Russia and Voiseyʼs Bay, Canada that are associated with sulfide mineralization in intrusive feeder zones. The common attributes include occurrence in shallow tholeiitic intrusions associated with plateau basalt volcanism, an external sedimentary source of sulfur, and openness to repeated magma influx and expulsion. A critical attribute of the high-grade Norilʼsk-Talnakh and Voiseyʼs Bay deposits, not yet positively identified in the Duluth Complex deposits, is the location of a magma conduit. A conduit that experienced repeated influxes of magma appears to be key to the formation of high-grade copper-nickel-PGE deposits (Naldrett, 1997). One of the difficulties in evaluating the potential for feeder zone mineralization in the Duluth Complex is determining whether intrusions were fed one-by-one by local magma conduits or by master conduits that sequentially fed several intrusions. Another long-standing question is the source of sulfur. In the case of the Norilʼsk deposits, Jurassic sediments are obvious candidates as are the Virginia and Biwabik Iron formations for the mineralization at the base of the Duluth Complex. However, designation of these sources begs the question of how, when, and where the sulfur is delivered to the magmatic system. This is more straightforward with regard to smokers: circulating hydrothermal solutions deliver sulfur from sea water directly to magma chamber vents. We suggest that consideration of this mechanism in the Midcontinent Rift magmatic systems could provide new insights for exploration targets. Field mapping (Green and others, 1966; Foose and Cooper, 1978) showed the Bald Eagle and the South Kawishiwi intrusions to be in direct contact. Petrologic observations and geophysical interpretations (Chandler, 1990; Chandler and Ferderer, 1989) suggest that the Bald Eagle and South Kawishiwi intrusions were emplaced by successive overplating of magmas from a common feeder centered on the northern Bald Eagle intrusion and extending along the trace of the macrodike that links the Bald Eagle and South Kawishiwi intrusions. Another model relating the emplacement of the two intrusions to a single magma feeder has been presented by Miller and Severson (2002; their Fig. 6.15). This model further speculates that the Bald Eagle intrusion had a more complete differentiation sequence (this contrasts with the dynamic flow model of Vislova (2003) outlined above) that has been eroded away and that this magma system may have fed surface eruptions. At a more detailed level, Peterson (2001) interpreted the copper-PGE mineralization in the Maturi deposit and its extension to the east (Maturi Extension deposit) as indicative of magma input from the northwest via the arcing macrodike that connects the Bald Eagle and South Kawishiwi intrusions. Peterson (2001) envisioned a confined magma flow model that invokes a change from laminar to turbulent flow beneath a pillar of older anorthositic series rocks, increasing the R-factor of the entrained sulfides, and resulting in higher metal contents of the exited (Maturi deposit) and remaining (Maturi Extension deposit) sulfide fraction. If correct, this model predicts that a Voiseyʼs Bay-type copper-nickel-PGE massive sulfide body may exist in an area south of the Spruce Road deposit. References Chandler, V.W. 1990. Geologic interpretation of gravity and magnetic data over the central part of the Duluth Complex, northeastern Minnesota. Economic Geology, v. 85, no. 4, p. 816-829. Chandler, V.W. and Ferderer, R.J., 1989, Copper-nickel mineralization of the Duluth Complex, Minnesota-A gravity and magnetic perspective: Economic Geology 84, 1690-1696. Foose, M.P. and Cooper, R.W. 1978. Preliminary geologic report on the Harris Lake area, northeastern Minnesota. U.S. Geological Survey Open-File Report 78-385, 24p., 1 pl., scale 1:12,000. Green, J.C., Phinney, W.C. and Weiblen, P.W. 1966. Gabbro Lake quadrangle, Lake County, Minnesota. Minnesota Geological Survey Miscellaneous Map M-2, scale 1:31,680. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M. and Wahl, T.E. 2002. Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, 207p. Miller, J.D. and Severson, M.J. 2002. Chapter 6: Geology of the Duluth Complex, in Miller, J.D., Jr., Green, J.C., Severson, - 66 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M. and Wahl, T.E. 2002. Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, 207p. Naldrett, A.J. 1997. Key factors in the genesis of Norilʼsk, Sudbury, Jinchuan, Voiseyʼs Bay and other world-class Ni-CuPGE deposits: Implications for exploration. Australian Journal of Earth Sciences, 44, no. 3, 283-315. Peterson, D.M. 2001. Development of a conceptual model of Cu-Ni-PGE mineralization in a portion of the South Kawishiwi intrusion, Duluth Complex, Minnesota. Society of Economic Geologists, Second Annual PGE Workshop, Sudbury, Ontario, 3 pages. Vislova, Tatiana. 2003. Petrology of the Bald Eagle Intrusion and associated rocks and its relevance to crystallization in dynamic magma chambers in the Midcontinent Rift, unpublished PhD. Thesis, University of Minnesota. Weiblen, Paul W. 1965. A funnel-shaped, gabbro-troctolite intrusion in the Duluth Complex, Lake County Minnesota, unpublished. PhD. Thesis, University of Minnesota. Weiblen, P. W. and Morey, G. B. 1980. A summary of the stratigraphy, petrology, and structure of the Duluth Complex. American Journal of Science, 280A, Part I, 88-133. - 67 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Textural Examination of Kama Point Diabase Sill, Nipigon, Ontario ZIEG, Michael J., FORSHA, Clinton J., and HABARKA, Joseph D., Department of Geography, Geology, and the Environment, Slippery Rock University, 1 Morrow Way, Slippery Rock, PA, 16057, USA, michael. zieg@sru.edu The texture of an igneous rock provides valuable constraints on the cooling history of the magma from which it crystallized. When combined with mineralogical data, the sizes and abundances of crystals in a rock reflect the interactions between the thermal and chemical evolution of the parent magma. In order to avoid uncertainty relating to the absolute magnitude of kinetic parameters such as nucleation and growth rates, this study focuses on textural gradients. These gradients reflect differences in crystallization history and magma composition, and are related to differences in the thermal and compositional state of the magma. The lowermost 35 meters of the 1100 Ma olivine diabase sill at Kama Point, approximately 25 km east of Nipigon, Ontario, has been sampled at ~3 meter intervals from the basal contact upwards. For each sample, textures were quantified using crystal size distributions (CSDs) as well as measurement of mean crystal size. As expected, the CSDs exhibit a clear coarsening-inwards trend through the lowermost 25 meters of the sill. However, above 25 meters, the texture becomes finer-grained. Above this, the coarsening-inwards trend continues, but with a lesser gradient (Figure 1). In this study, analysis of plagioclase CSD data is shown to provide a method for quantitatively evaluating hypotheses for magmatic processes as well as important constraints on crystallization kinetics. CSD Intercept Variation CSD Slope Vailation . . . . . . . 20 . 20 . . . . . . 0 9 10 . . 11 . -14 12 Intercept, In(ni Imm41 -12 -10 -O -S -4 -2 Slope Imm11 Figure 1. Variations in CSD intercept and slope. Intercept is related to the number of crystals per unit volume; slope is related to the mean length of the crystals in the sample. With increasing height above the base of the sill, the number density decreases and the mean length increases. These trends are consistent with increasing cooling durations. These The measured textural gradients have been compared to calculated magmatic cooling durations, based on conductive cooling of an instantaneously injected 200 meter thick diabase sill. The initial conditions for the cooling model are an initial magma temperature of 1200°C and an initial country rock temperature of 100°C; far-field temperature is held at 100°C. The model is used to calculate the length of time required for the magma to cool to a temperature of 1000°C, which is the approximate solidus of the Nipigon diabase. Measured textures were compared to the calculated cooling durations (Figure 2), which yields a bulk growth rate of 1.8×10-10 mm/s (R2 = 0.925). The total number of crystals decreases with cooling duration, consistent with a nucleation rate controlled by the degree of undercooling, which is maximized during rapid cooling. The break in textural gradient at 25 meters is attributed to a recharge event in which a second pulse of - 68 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Mean Length vs Cooling Duration Total Number vs Cooling Duration 5T0 — 4T0 020 E E E 3T0 . C 0,15 a C -I . C 2T0 z 0.10 ITO . . 0 10 20 0 40 10 20 30 40 Cooling Duration yrJ Coo Iiruj Duration [yrj Figure 2. Variations in textural parameters obtained from CSD intercept and slope. Textural parameters are related to calculated cooling durations. 25-m break in texture corresponds to 22-yr cooling duration. magma is injected into the magma chamber, which had at that point crystallized a thickness of 25 meters. This event is believed to have taken place approximately 22-25 years after the initial injection. Cooling of the new batch of magma was slower than cooling in the initial injection because of the cooler, previously crystallized diabase located between the fresh magma and the cold country rock. This new cooling environment qualitatively explains the change in textural gradient above the 25-m discontinuity. Additional modeling will focus on relating this discontinuity to a specific set of recharge conditions. Further field investigation of the sills in the Nipigon area is planned. This work will extend the sampling transect higher into the sill and will include samples from higher in the sill. Goals of this research include a more thorough understanding of the injection history and original dimensions of this intrusion, as well as empirical relationships between intrusion dimensions, initial and boundary conditions, and textural profiles. - 69 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Author Index Addison, W. 1 MacDonald, C.A. 22 Albers, P. 3 MacDonald, J. 12 Anderson, R. 10 MacTavish, A. 36 Blackburn, C. 5 Magee, M.A. Breckenridge, A. 7 Maric, M. Brumpton, G. 1 McNaughton, N. Bucholz, T. 8 Medaris, L. Cannon, W. 10, 27, 64 Metsaranta, R. 17, 45 Chandler, V. 10, 27 Middleton, R. 47 38, 39 41 1 43 Conly, A. 12 Miller, J. Coombes, S. 54 Murphy, J. 50 Craven, J. 58 Petersen, D. 65 Dahl, D. 14 Planavsky, N. 50 Daniels, D 10 Rayner, N. 60 Daniels, P. 30 Richardson, A. 52 Davis, D. 1 Rogala, B. 17 43 Rossell, D. 54 Saini-Eidukat, B. 30 Dott, R. Easton, R.M. 15, 24 Falster, A. 8 Schneider, D. Forsha, C. 68 Schnieders, B. Fralick, P. 1, 17, 39, 41, 45 Schulz, K. 3, 28, 30, 48 10, 27 55 10, 27, 64 Franklin, J. 19 Scott, J. 55 Habarka, J. 68 Severson, M. 56 Halls, H. 20 Shareef, S. 58 Hammond, A. 1 Simmons, W. 8 Hart, T. 22 Smyk, M. Heaman, L. 24 Stott, G. 60 Heggie, G. 47 Trow, J. 62 Hill, M.L. 26 Vallini, D. Hollings, P. 32, 34, 39, 52 Van Schmus, W. 26, 59 1, 64 10, 27 Holm, D. 10, 27 Vislova, T. 65 Jirsa, M. 28, 48 Weiblen, P. 65 Johnson, T. 7 Young, C. 62 Kissin, S. 1 Zieg, M. 68 Knudsen, D. 30 Laarman, J. 32 Lane, C. 34 - 70 - � https://digitalcollections.lakeheadu.ca/files/original/942b8efd07100ae7b5e8a86a8ac19475.pdf 2e91b73833d712df8bd61840ebb527f9 PDF Text Text 51st ANNUAL MEETING Nipigon, Ontario - May 24-28, 2005 INSTITUTE ON LAKE SUPERIOR GEOLOGY Part 2 – Field Trip Guidebook 51st ILSG Nipigon 2005 wwwIakesuperiorgeology.org �51st ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY May 24-28, 2005 Nipigon, Ontario HOSTED BY: Mark Smyk and Pete Hollings Co-Chairs Ontario Geological Survey and Lakehead University Proceedings - Volume 51 Part 2 – Field Trip Guidebook Compiled and edited by Pete Hollings, Lakehead University Cover Photos: Left - basaltic dyke cutting Osler volcanics on Wilson Island, Middle - Aerial view of the Black Sturgeon fault, Right - Ruby Lake near Nipigon, Northern Ontario. �51ST INSTITUTE ON LAKE SUPERIOR GEOLOGY VOLUME 51 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: GEOLOGY AND FOLD MINERALISATION OF THE BEARDMORE-GERALDTON GREENSTONE BELT TRIP 2: QUATERNARY GEOLOGY OF THE BEARDMORE – NIPIGON AREA TRIPS 3 & 6: A STRATIGRAPHIC TRANSECT ACROSS THE NORTHERN FLANK OF THE MIDCONTINENT RIFT NEAR ROSSPORT TRIP 4: GEOLOGY AND RARE ELEMENT PEGMATITES OF THE QUETICO SUBPROVINCE NEAR NIPIGON TRIP 5: GEOLOGY OF THE BLACK STURGEON AREA Reference to material in Part 2 should follow the example below: Hart, T.R., 2005. Geology of the Black Sturgeon Area. In; Hollings, P. (Ed.), Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Part 2 - Field trip guidebook, v.51, part 2, 2-39. Published by the 51st Institute on Lake Superior Geology and distributed by the ILSG Secretary: Pete Hollings - ILSG Secretary Department of Geology Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Email: peter.hollings@lakeheadu.ca ILSG website: www.lakesuperiorgeology.org ISSN 1042-9964 �Proceedings of the 51st ILSG Annual Meeting - Part 2 Table of Contents Introduction, safety considerations and acknowledgements ...............................................1 Fieldtrip 1 - Geology and gold mineralisation of the Beardmore-Geraldton greenstone belt ..............................................................................................................................3 Fieldtrip 2 – Quaternary geology of the Beardmore – Nipigon area ................................41 Fieldtrips 3 and 6 - A stratigraphic transect across the Northern flank of the Midcontinent Rift near Rossport .....................................................................................................57 Fieldtrip 4 - Geology and rare element pegmatites of the Quetico Subprovince near Nipigon .....................................................................................................................71 Fieldtrip 5 - Geology of the Black Sturgeon Area .........................................................85 -i- �Proceedings of the 51st ILSG Annual Meeting - Part 2 Introduction, safety considerations and acknowledgements Pete Hollings Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada and Mark Smyk Resident Geologistʼs Office, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada This volume is intended to serve not only as a guide for 51st ILSG field trip participants but also as a reference for those planning to revisit these areas at a later date. Consequently we have included UTM coordinates in the NAD 83 datum for stops, as well as instructions on how to reach them. As some of the stops are on private and staked land, particularly on Trip 1, please be sure to obtain the land owners’ permission before entering their land. In what is perhaps a first for the Institute this year we are offering a fieldtrip onto Lake Superior. This creates a number of unique safety issues. Please exercise caution when getting in and out of the boats as the outcrops are often extremely slippery. Life jackets must be worn in the boats at all times. If you are planning to revisit these sites please be very careful as Lake Superior is a dangerous lake, waves can often be many metres high and even in mid summer fog can appear very quickly. major highways or busy logging roads. Please take care when crossing or parking along these roads. We would like to thank all the other authors who contributed to this field guide (Peter Barnett, Tom Hart, Phil Fralick and Steve Kissin) and also all those who provided comments and assisted with the running of the field trips themselves. We appreciate the assistance and cooperation of the exploration and mining companies in providing us access and information concerning their properties, particularly David Malouf, Roxmark Mines Limited. The other fieldtrips will be visiting stops along either LAKE r _____9— NIPIGON 0 ta Figure 1. Map showing the location of the five field trips. -1- �Proceedings of the 51st ILSG Annual Meeting - Part 2 -2- �Proceedings of the 51st ILSG Annual Meeting - Part 2 Fieldtrip 1 - Geology and gold mineralisation of the Beardmore-Geraldton greenstone belt Mark Smyk Resident Geologistʼs Office, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada Philip Fralick Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Thomas R. Hart Ontario Geological Survey, Ministry of Northern Development and Mines, Sudbury, Ontario, P3E 6B5, Canada Introduction The Beardmore-Geraldton belt (BGB) is a Neoarchean metavolcanic-metasedimentary terrane at the boundary between the Quetico Subprovince and the eastern Wabigoon Subprovince of the Superior Province (Fig. 1a). The belt can be subdivided into six east-striking sub-belts, all of greenschist facies metamorphic grade (Fig. 1b; Devaney and Williams, 1989). They are: the northern metasedimentary subbelt (NMB), northern volcanic sub-belt (NVB), central metasedimentary sub-belt (CMB), central volcanic sub-belt (CVB), southern metasedimentary sub-belt (SMB) and southern volcanic sub-belt (SVB) (Fig. 2). The northern units are not as laterally continuous as the southern units. Previous studies have shown that although these belts are fault-bounded (Fig. 3), mainly along the southern boundaries of the volcanic sub-belts, they probably reflect an original sedimentary assemblage deposited on a cratonic margin in environments ranging from alluvial fan-braid plain in the NMB, through fan delta-braid delta in the CMB to a submarine fan/ramp in the SMB (Devaney and Fralick, 1985; Barrett and Fralick, 1985; Devaney, 1987; Barrett and Fralick, 1989). Original continuity of this succession prior to tectonic disruption cannot be proven, but is supported by consistent stratigraphic trends and sedimentary structures that mostly young to the north. Although isoclinal folds are present on varying scales, particularly in the southeast part of the terrane near Geraldton, the overall structure of the belt appears to be initially one of stacked, imbricate, internally northward-younging sheets which have been interpreted as the product of accretionary wedge tectonics (Williams, 1986, 1987; Devaney and Williams, 1989; Williams and Stott, V (,n Hudson I et_w. N LEGEND A Paleozoic Cover Prolerozoie Covet Grenville Provincc — •Abitpbi Churchili Province Supenor Province Uciti — Areas: E'iglish River ?ontic Winnipeg LANADA 1 U.SA -Wabigeon' rnIoAea Metasedimentary Belts Cranitic Areas Volcanic Sedimentary Belts Cneissic Terranes -'-—-Quetico kiii Wawa Figure 1a. Location of the Beardmore-Geraldton greenstone belt, Superior Province. -3- �- Proceedings of the 51st ILSG Annual Meeting - Part 2 15 /a..aaa..N?LTnsl r4 I Garuiue—gncnMone suhpnwincc BanMto,e-CcSiion Deli 8GB Aras: I RSy Ltt di*ICL 2 SticN.id.iwan. 3 Manitounfrj 4 tolhioo thNuicl Tnsuc Am" Beardmore-Geraldton Belt belts 0 parsoihests $1 -i Luio 3M IO knu NY iw :2 -r 01 lnieatd II) PROTEINOZOIC 7/4 —— ARCHEAN Da,d.a-G.nIdtoc Bell Eli couuw - kin C rn Md C a_-it ia Pset.poducloIdrrnne; ItAilch. 3 M.ctcodccxbhutt Met.moiplmaad seethtooe & s1flhit ease mdtvokiSv ivck. MaBe In • 2 BaikThdd. • -_ - Real siguiada. number • moqawI saooe. tgdlnc. polymjdk iIIoma*t ta.2à3 — Road and b#way n-ri PWd TrW IbuuaJIe In Sk Geld pefl I' L::: SmedlNemMtmeIndc.rncTocks: o P18nMIS _____ Sn arc cavok reds ___ ______________ Figure 1b. Regional geology and field trip stop locations modified after Lafrance et al. (2004); see text for sub-belt index. 86aow • + * * • * • * • * • • * * • • * • * * • . * • • * •*.,* + * • * * • * 50N • • • * * + ..:wabi9oonsubProvince:.::4:.:.:3> nCCMB t :/. * * Bea rdmore— Geraldton Belt ., 'I, SVB r?ij::::::::::Quetit •:i• _.r.Tir2_!_._&r / I . 10 krii *.] Granitoids Metnodiriiontary rooks I • Felsic 10 irlterrMdiate rrmtavolcanic rocks :E:E:E.j Quetico Subprovince sedimenlary rocks Mafic metavolcanic rocks Figure 2. Generalized geology of the Beardmore-Geraldton belt; see text for sub-belt index. -4- �Proceedings of the 51st ILSG Annual Meeting - Part 2 minor interflow chert-magnetite iron formation. The northern metavolcanic sub-belt (NVB) is subdivided into the northern Bish Bay assemblage (BBA) and the southern Poplar Point assemblage (PPA) (Hart et al., 2002; Fig. 4). The BBA is composed of eaststriking, mafic pillowed to massive flows and rare tuffs resembling the SVB. The PPA consists of northweststriking, intermediate flows, tuff-breccias and tuffs resembling the CVB, with subordinate mafic massive and pillowed flows. Qu.tIo&Wabgoon SVB OVB NVB B,! p LS Z w Figure 4. Schematic cross-section of the southern half of the eastern Wabigoon Subprovince showing imbrication of fault-bounded metavolcanic (VB’s) and metasedimentary (MB’s) sub-belts (modified after Stott and Davis, 1999); S = Southern; C = Central; N = Northern; PLSZ - Paint Lake Shear Zone; B - Blackwater Fault; P - Princess Lake Fault; W - Standingstone - Watson Lake Fault. 1991). Thus, the steeply dipping sheets probably represent the telescoped version of an original cratonic margin assemblage. South of the Beardmore-Geraldton belt, more highly metamorphosed, turbiditic sediments of the submarine margin and trench-fill are present in the Quetico Subprovince (Williams, 1986; Fralick et al., 1993). A number of rock types intrude the supracrustal rocks of the BGB, including a series of mafic to ultramafic, synvolcanic, intermediate to felsic synvolcanic, mafic post-tectonic intrusive rocks and diabase dikes (Hart et al., 2002). The synvolcanic gabbroic rocks form thin sills, subparallel to the strike of the mafic metavolcanic rocks of the SVB and the BBA. A large intrusive body located in the BBA is a composite intrusion with gabbroic and peridotitic phases in the southern and northern parts of the intrusion, respectively. A series of intermediate to felsic dikes and sills ranging from massive granodiorite to quartz-porphyritic, feldspar- N The presence of the Onaman-Tashota volcanic arc terrane (ca. 2740 Ma; Stott and Davis, 1999) to the immediate north, the imbricate thrust structure of the Quetico metasedimentary rocks to the south (Williams, 1986, 1987), and the style of sedimentary depositional systems in the region strongly suggests that the Beardmore-Geraldton belt represents a forearc assemblage or a cross section of a complete island arc system (see “Depositional Environments”) (Fig. 3). The mostly mafic volcanic rocks which form the basement to the sedimentary rocks are probably slivers of oceanic volcanic terranes which were accreted prior to development of the clastic wedge. Similar depositional and tectonic evolutionary trends in the Great Basin of California have been described by Dickinson and Seely (1979). 0 1 Dorothea Sandra 2 Km BBA NVB CSB PPA CVB Lake Nipigon Eva Summers Eva Lake Hwy 11 Hwy 580 SSB SVB Igneous rocks Mafic metavolcanic rocks of the southern metavolcanic sub-belt (SVB) consist of massive and pillowed flows with minor tuffs, lapilli tuffs, and tuff breccias and interflow chert-magnetite iron formations. The central metavolcanic sub-belt (CVB) consists of intermediate massive and pillowed flows with significant tuffs, lapilli tuffs and tuff breccias, and ELMHIRST -RICKABY Camp 72 Road 8..,dmor.-G.raldton R CN Beardmore w rail ay Mafic Intrusive rocks Ultramafic Intrusive rocks Mafic Metavolcanic rocks Intermediate Metavolcanic rocks H Kitto QUETICO SIBLEY Felsic Metavolcanic rocks Metasedimentary rocks Sibley Group sedimentary rocks Post-tectonic Intrusive rocks Synvolcanic mafic intrusive rocks Synvolcanic felsic intrusive rocks Figure 4.of theGeology ofArea,the Beardmore area (HartNVBet al., General geology Beardmore-Geraldton Kitto, Eva, Summers, Dorothea and Sandra townships. Northern Metavolcanic sub-belt, CVB-Central Metavolcanic sub-belt, SVB-Southern Metavolcanic sub-belt, SSBSouthern Sedimentary sub-belt, CSB-Central Sedimentary sub-belt, PPA-Poplar Point Assemblage (part of the NVB), 2002); BBA Bish Bay Assemblage; PPAPoplar Point BBA-Bish Bay Assemblage (part of NVB). Assemblage. -5- �Proceedings of the 51st ILSG Annual Meeting - Part 2 20 Ti/Zr 15 10 5 0 0 5 10 15 La/Yb Figure 4. Zr/Ti versus La/Yb diagram for the metavolcanic rocks of the Beardmore area. Circles - SVB, open and filled triangles - CVB, squares - NVB. Data from Hart et al. (2002) and Tomlinson (1996). porphyritic and feldspar-quartz-porphyritic phases occur within the metavolcanic rocks of the PPA. These units appear to have been emplaced along the regional foliation, although some bodies are subhorizontal in orientation. A feldspar-porphyritic granodiorite dike intrudes the mafic flows of the SVB and resembles the dikes of the PPA. Late, post-tectonic diorite sills predominantly occur within the metasedimentary and metavolcanic rocks along the contact between the SSB and CVB. Additional intrusions located along the northern and southern contacts of the PPA are generally undeformed diorite sills that have chilled contacts with the metasedimentary rocks. A swarm of narrow, generally north-striking diabase dikes intrude the supracrustal rocks and appear to be predominantly Paleoproterozoic in age. A series of Mesoproterozoic diabase sills of the Nipigon Sill Complex intrude all other supracrustal rocks of the BGB. Geochemical groupings in the metavolcanic rocks span the three belts within the western portion of the BGB. The mafic metavolcanic rocks of the SVB and NVB are predominantly basalt with high Ti/ Zr and low Zr/Y, and negative slopes on mantlenormalized, extended element diagrams characteristic of mid-ocean ridge basalt (Fig. 5; MORB; Tomlinson, 1996; Hart et al., 2002). A subset of basalt samples have a mixture of elemental abundances and ratios characteristic of subduction-related volcanic rocks which led Tomlinson (1996) to suggest that these rocks formed in a back-arc basin, similar to the Lau Basin, where there are volcanic rocks with both arc and/or MORB geochemical characteristics. The intermediate metavolcanic rocks are generally confined to the CVB and PPA of the NVB and are predominantly andesite. The andesites have low Ti/Zr, high Zr/Y, and positive slopes with prominent negative niobium and tantalum anomalies on a mantle-normalized, extended element diagram, that are generally characteristic of calcalkaline andesite formed in an island-arc environment (Fig. 5; Tomlinson, 1996; Hart et al., 2002). Two felsic metavolcanic rocks from the CVB and the PPA were analysed by Hart et al. (2002) , and have low (La/Yb)n ratios with moderately high ytterbium values characteristic of FII felsic metavolcanic rocks (Lesher et al., 1986), indicative of a potential to host volcanogenic massive sulphide mineralization. Both samples have elemental abundances and ratios characteristic of active continental margin and/or the destructive plate margin felsic volcanic rocks (e.g., Wood, 1980). A sample of the flow from the PPA has a U/Pb age from zircons of 2724.9 ± 1.1 Ma and the flow from the CVB has a U/Pb age from zircons of 2724.9±1.2 Ma (Hart et al., 2002). The geochemistry of the intrusive rocks of the western portion of the BGB was characterised by Hart et al. (2002). The mafic intrusive rocks have elemental abundances and ratios that generally reflect the composition of the metavolcanic rocks that they intrude. Unfortunately these similarities also mean that the geochemistry does not aid in differentiating coarse-grained, gabbroictextured flows from subvolcanic, gabbroic intrusions. Both the southern mafic and northern ultramafic phases of the composite intrusion in the BBA have similar tholeiitic geochemical characteristics. A few analyses that are available for the intermediate to felsic dikes and bodies indicate that these units have high Zr/Y and (La/Yb)n ratios characteristic of alkalic rocks, which may be interpreted to be the products of either an early Archean, post-tectonic intrusive event or part of a Proterozoic magmatic event related to the initial rifting during formation of the Nipigon Embayment. A number of post-tectonic, mafic intrusions located along the contact between the SSB and the CVB have Ti/Zr, Zr/Y and (La/Yb)n ratios similar to the andesite of the CVB, and generally characteristic of calc-alkaline rocks. The overlap in the geochemistry of these intrusions and the CVB andesite is interpreted to indicate that these two -6- �Proceedings of the 51st ILSG Annual Meeting - Part 2 A B north. . FlAt The turbiditic association of the SMB can be divided into a clastic association and a chemical association, with a high proportion of oxide-facies, banded iron formation (BIF) layers. In the chemical association, clastic interbeds are generally less than several centimetres thick, and range in grain-size from silt to coarse sand. Upward-thickening and -coarsening trends over several metres are locally present, as at Solomon’s Pillars and the Leitch Mine near Beardmore (Fig. 6; Barrett and Fralick, 1985). Within the overall upward trend, oscillations between silts, sands, and iron formation occur. Depending on the relations between these types of beds, four iron formation lithofacies associations (IFLA) can be defined: magn at! le, •d Iat FLAt interlaminated manstit. asp.,. ilbtL and &ate San 2mCn,, E. IFLAc sadtn; siltstone. Slate and mantit IFLAb :—t iItstoe, slot, and • IFLA-a consists of dominantly magnetite-rich sediment with millimetre- to centimetre-scale, graded or ungraded silt interbeds (Fig. 7). Figure 6. Stratigraphic sections of iron formation bearing units. A) Outcrop section measured at the Leitch Mine (for detailed location see Barrett and Fralick, 1985). B) Outcrop section measured at Solomon’s Pillars (for detailed location see Fralick 1987). From Fralick and Barrett (1995). • IFLA-b comprises centimetre-scale graded to sharply bounded silt beds, either contiguous or separated by millimetre-thick laminae of magnetiterich sediment. rock types formed by similar petrogenetic processes, but the occurrence of intrusions in both metavolcanic rocks and metasedimentary rocks indicates that some of these intrusions are younger. • IFLA-c represents sand-rich composite units up to about 1m thick, generally consisting of thin, stacked, ungraded, laminated sand beds. These composite units consist mainly of mediumto coarse-grained sand. They are separated by intervals of IFLA-b up to 15cm thick. Sedimentary rocks Lithofacies Associations The NMB, the northern (uppermost) third of the CMB and the northernmost portion of the SMB are dominated by a conglomeratic assemblage with minor amounts of sandstone. The clast-supported conglomerates are poorly to moderately sorted, and almost always non-graded with a poorly to moderately sorted sand matrix. Bedding is defined by variation in average or maximum clast size between units, but it is commonly indistinct. Scouring is locally preserved, but most other primary features such as imbrication have been destroyed by deformation. Sandstones interbedded with the conglomerates commonly appear massive, but in some outcrops planar lamination and cross-stratification are present. They form lenses in conglomeratic beds; thin, irregular sheets blanketing conglomeratic beds; wedges abutting conglomeratic beds; and thick units separating conglomerates. Clast types in the conglomerates are almost exclusively igneous, representing a suite of rocks similar to those present in the Onaman-Tashota volcanic terrane to the • IFLA-d consists of framework-supported, polymictic conglomeratic beds up to a few metres thick, interbedded with sandstone and minor iron formation, or interbedded with fairly thick iron formation and thin-bedded sands. Conglomerate contain mainly mafic to felsic volcanic and granitic clasts. Although flattened clasts indicate that IFLA-d outcrops are tectonically thinned, their associations are primary, with the conglomeratic units erosively cutting down into BIF-sandstone packages. Transitions between IFLA-a, -b and -c can be gradual or abrupt. Some silt-sand successions containing iron formation exhibit intervals of thicker and wellgraded clastic beds. They form structured sections up to several metres thick within successions that are otherwise generally disorganized. Clastic units in the lower two thirds of the CMB and the SMB are divisible into three lithofacies associations: a thin-bedded, turbidite-dominated -7- �till HUIIIILiIllP EThJliJIIi p p p 11L Proceedings of the 51st ILSG Annual Meeting - Part 2 Figure 7. Photomicrographs of siltstone associated with laminated, magnetite-rich sediment (IFLA-a). Logs to right of photos outline layer contacts and schematically illustrate grainsize variations. Dots and dashes represent siliciclastic and magnetite-rich components, respectively. Scale bars: A.) 5.0 mm; B.) 2.0 mm; C.) 3.0 mm; D.) 2.0 mm. All images are in crossed nicols. From Barrett and Fralick (1985). association (LA2); a medium-bedded, turbiditedominated association (LA3); and a thick-bedded association (LA4). LA2 consists mostly of graded, <10 cm thick siltstone and/or sandstone beds that are either unstructured or thin and fine upwards over 1 to 3 m. LA3 is divisible into two types. LA3a consists of medium- to coarse-grained sandstones with sharp bottom and top contacts. Parallel lamination is present near the tops of some of the otherwise massive beds. These successions are unstructured. LA3b is similar to LA3a except these beds are organized into either upward-thickening or upward-thinning trends. Thick, poorly graded sandstones dominate LA4. The beds usually have a coarse sand or pebbly base, grading into a thick, poorly sorted, massive central area. They are often abruptly capped by thin, fine-grained sandstone. Irregular, erosional bases and scattered rip-up clasts are common. Structured, thinning- and fining-upward sequences, metres to tens of metres thick, are present in the area south of Beardmore. The successions are topped by CDE and/or DE turbidites which are abruptly overlain by massive grain flows/high-density turbidites with internal inverse- to normal-graded, conglomeratic bands. Pebbles present in the conglomerates are mainly felsic igneous rocks (extrusive and intrusive), while rip-up clasts are not the expected mudstone or siltstone, but rather clay- and silt-rich, fine-grained sandstone. Load structures are ubiquitous throughout the area. Commonly, the base of one unit sags into the underlying beds. Locally, multiple internal loads are developed, usually in the B division. These loads sag into the A division, in places extending into the underlying beds. Depositional Environments Deposition of the clastic and chemical sediments in the Beardmore-Geraldton belt occurred within a narrow time frame. Crosscutting felsic intrusive rocks provide a minimum age for the sediments of 2691+3/-2 Ma (Anglin, 1987; Anglin et al., 1988). UPb detrital zircon geochronology on sandstone samples from the region consistently give youngest zircon ages in the range 2701±1 Ma to 2696±2 Ma (Hart et al., 2002; Don Davis, unpublished data, personal communication, 2004). These ages are very similar to -8- �Proceedings of the 51st ILSG Annual Meeting - Part 2 Subprovince on Lake Superior (Purdon, 1995). In addition to geochronology, lithofacies associations and geochemistry indicate that the depositional system probably extended from the cratonic margin (i.e., Wabigoon Subprovince), across the arc-trench gap and trench (i.e., Quetico Subprovince) to the outboard ocean floor (i.e., Wawa Subprovince) immediately prior to the commencement of collision of an island arc system to the south. The NMB is represented by sandstone and conglomerate deposited in a fluvial fan, and/or braid Figure 8a. 2700 Ma paleogeography of the region surrounding plain environment (Devaney and Fralick, 1985). The Beardmore-Geraldton. The island arc in the south represents the Schreiber-Hemlo area of Wawa Subprovince. The trench CMB displays this type of assemblage in its northern system to the north of this arc is now metaturbidites of third, with successions in the southern portion similar the Quetico Subprovince. The sediments feeding from the to those described above for the SMB. Thus, the southern flank of the Wabigoon cratonic arc in the north paleo-shoreline exposed by the present level of erosion formed the Beardmore-Geraldton succession. From Fralick lies within the CMB. All of the sediments in the SMB and Barrett (1991). are marine, except for rare conglomerates at the top ages of detrital zircons from the Quetico Subprovince of the succession and throughout the section in the and a more distal turbiditic assemblage, the McKellar highly deformed Geraldton area. The turbidites were Harbour Formation, outcropping in the northern Wawa deposited in fore-arc environments ranging from deltafront to submarine fan or ramp. Many ______________________ Onommi—Tas bota VoIcoo!c Terrc!t, unstructured sandstone sequences are -. D L ItbfdIfl Aoc!ot!on interpreted as having accumulated on LIt 24 a submarine ramp that was locally Br WI transected by channels feeding deeper fan lobes (Barrett and Fralick, 1989). / An interpretive view of the overall paleoenvironment is shown in Figures 4c' 8a and b. I r - / rTsTL I 0-- (z) c - H--H— zoooe ond — -— -I(onFormacr, 0 !1 seofloor Sediments deposited in the Quetico trench to the south are similar in many respects to the submarine ramp deposits of the adjacent fore-arc basin, but there are differences. AB and ABC Figure 8b. Suggested depositional environments for sedimentary rocks in the Beardmore-Geraldton belt. The subaqueous portion of this diagram is depicting environments mostly present in the Southern Metasedimentary Belt. The Central Metasedimentary Belt represents mainly fan-delta environments, and the Northern Metasedimentary belt a braidplain environment. The entire sedimentary assemblage in all three belts forms a large-scale progradational system. In the hinterland is the Onaman-Tashota volcanic arc terrain which supplied the detritus. This sediment was transported across braidplains to distributary mouth bars. Because of rapid accumulation rates and the tectonically active environment, gravity flows and slumps were frequently triggered. The resultant grain-flows produced the vertically unstructured, poorly graded, mud-poor, medium- to coarse-grained deposits of the ramp succession. More structured sandstone sections formed on locally developed fan lobes and thick grain-flows filled the submarine feeder channels. Iron formation was deposited immediately offshore from the channel mouths on braid deltas during periods of low sediment influx. The meter-scale coarsening-upwards cycles present in the iron formation record the effects of delta lobe outbuilding. Progradational events may result in the stacking of coarsening-upwards iron formation-bearing, subaqueous parasequences or the delta top braid system may erode through the uppermost chemical sediments during maximum outbuilding. The iron formation is limited to the shore-proximal zone indicating a precipitation model where nutrients delivered to the system by rivers caused heightened cyanobacterial growth rates in the near-shore, similar to modern systems. The oxygen produced by photosynthesis caused precipitation of iron oxides in these oxygen oases. From Barrett and Fralick (1989) -9- �Proceedings of the 51st ILSG Annual Meeting - Part 2 turbidites dominate the trench fill, whereas grainflows and A turbidites characterize the more proximal, fore-arc environment. Iron formation is present in the fore-arc basin associated with thinner-bedded, finer-grained sediments deposited in the shallow near-shore portions of the deltas. Iron formation accumulated when transgressions caused intervals of clastic sediment starvation in the proximal pro-delta and distributary bar environments. Iron formation is absent from sediments deposited further offshore in the Beardmore-Geraldton belt and the Quetico trench deposits. The limited distribution of oxide-facies iron formation (occurring only in shore-proximal areas) indicates that biological activity thrived in the nearshore waters where the commonly limiting nutrients, nitrogen and phosphorous, are delivered at higher levels due to river input. It is difficult to prove that iron compound precipitation was driven by free oxygen liberated during photosynthesis by cyanobacteria, but this would be a logical assumption. Rare successions that coarsen and thicken upward may represent limited establishment of discrete fan lobes in the distal fore-arc basin and Quetico trench CVB *CA CYB CA *•J3 OTT The discussion of the structural development is taken largely from recent work and synthesis conducted by Lafrance et al. (2004). Their treatment of the topic is the most recent structural study, outlining three phases of structural overprinting of the volcano-sedimentary assemblage and ascertaining the timing of each. CA I Sediment supplied to the fore-arc basin and the trench was derived from erosion of felsic and intermediate igneous material, with a large contribution probably coming from resedimentation of unconsolidated volcaniclastic material. Clast compositions indicate that the Onaman-Tashota arc terrane to the north was the source (Devaney, 1987; Devaney and Williams, 1989). Grain size and lithofacies trends also strongly support a source to the north. In addition, whole rock geochemistry conducted on sandstone samples from the Beardmore-Geraldton area and the Quetico Subprovince further confirm that the Onaman-Tashota terrane is the only igneous complex in the area which could have supplied the majority of the detritus (Figs. 9 and 10; Fralick and Kronberg, 1997; Fralick, 2003) The ease with which volcaniclastic material can be physically eroded from subaerially erupted debris could have led to short subaerial residency periods, and thus lowered the extent of chemical weathering and production of clays. This is probably the major factor contributing to the rarity of E division fines in the deposits. Structure * OTT 0 floor, but for the most part the trench fill was dominated by overlapping, interfering fans fed by multiple channels extending from the shallow marine portion of the fore-arc basin. One of these channels outcrops south of Beardmore (Stop 1-2). The channel is filled with thick, upward-thinning and -fining successions that grade from conglomerate and coarse sandstone grainflows or turbidites deposited from high density currents near channel bases, to CDE and DE, lowdensity current turbidites that fill abandoned channels near their tops. 2 3 Ti02/(Nb xl,000) Figure 9. Ratio plot of chemically immobile elements in sandstones showing derivation of the sands deposited in the Beardmore-Geraldton terrain and the Quetico Subprovince from the Onaman-Tashota calc-alkaline volcanic arc. From Fralick and Kronberg (1997); Fralick (2003); igneous data from Tomlinson et al. (1993) and Kresz and Zayachivsky (1989). After deposition of the clastic succession the area was tectonised by thrust faulting, regional folding and dextral shearing. Thrust faulting resulted in the regional volcanic and sedimentary packages being vertically juxtaposed in an imbricated thrust stack (Devaney and Williams, 1989). This represents D1 and may be related to uncommon, early, F1 folds. The youngest detrital - 10 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 * on NVB CVB SVB CA 160 Kresz and Zayachivsky, 1989 Tomlinson et al, 1993 mafic to intermediate volcanic Onaman - Tashota Terranc rocks Northern Volcanic Belt Central Volcanic Belt Southern Volcanic Belt Gale - Alkaline o mafic volcanic rocks A intermediate volcanic rocks • intermediate to felsic sub - volcanic intrusive rocks intermediate to felsic intrusive rocks intermediate to mafic intrusive rocks A • *CVB CA Tholeiitic volcanic CYB CA / —— a— and intrusive rocks SVBh 0 ULTRAMAFIC Onaman - Tashp calc - alkaline volcanic and intrusive rocks p I 20 40 60 p p p 00 80 20 p 40 160 p 80 p 200 hO2! Zr Figure 10. A ratio plot similar to Figure 9 using Zr. From Fralick and Kronberg (1997); Fralick (2003); igneous data from Tomlinson et al. (1993) and Kresz and Zayachivsky (1989). zircon recovered from the sedimentary units is 2696±2 Ma (Hart et al., 2002) and this places a maximum age on the thrusting event. The D2 event is recorded by the development of tight to isoclinal folds and a flattening strain fabric responsible for transposed bedding and flattening of clasts and pillows. A homoclinal, north-younging panel of regional extent developed at this time and represents the sheared-off southern limb of a larger syncline. D2 deformation affects altered and gold-mineralized porphyry dykes in the syn-tectonic Croll Lake stock which U-Pb geochronology indicates are 2691+3/-2 Ma (Anglin, 1987; Anglin et al., 1988). An age of 2699±1 Ma for a Au-mineralized feldspar porphyry dyke at the Hard Rock Mine and identical ages of 2690±1 Ma for two phases of the Croll Lake stock put constraints on the timing of major deformation and hydrothermal activity in the belt (Corfu, 2000). The final event, D3, occurred as regional transpression developed in the compressive framework of the area. Vertical bed orientations developed during D2 did not refold but rather were overprinted by a steeply dipping, regional cleavage. Partitioning of the strain, during east-west dextral shear, between less competent argillites and more competent sandstones and porphyries resulted in cleavage refraction near lithologic contacts. The pervasive cleavage developed in the Paint Lake shear zone at this time shows a progressive rotation towards the orientation of the zone (Lafrance et al., 2004; DeWolfe et al., 2000). This is in contrast with the Barton Bay Lithotectonic Zone (BBLZ) where the S2 fabric was reactivated to accommodate the D3 shear. Some folds were generated during this interval but they tend to be smaller Z-folds, overprinting limbs of regional F2 folds. Shear zones active at this time were dextral with nearly horizontal displacements. To sum up the results of Lafrance et al. (2004): • Thrusting along the southern margin of Wabigoon Subprovince in the Beardmore-Geraldton area between 2696 Ma and 2691 Ma resulted in the largescale assemblage of what was to become the belt structure. • The rocks were then modified by a compressive event (D2), probably between 2692 Ma and 2686 Ma, which steepened the beds to a near-vertical position, caused extensive flattening and formed large-scale fold structures. • D3 is more poorly constrained in time but may be synchronous with 2670 to 2650 Ma peak metamorphic conditions in Quetico Subprovince. It is characterized by the development of dextral shear zones and small-scale Z-folding. Gold mineralisation Overview Gold was first discovered in the Beardmore-Geraldton Greenstone Belt in 1925 (Mason and MacConnell, - 11 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Table 1 - Gold mineralisation in the Beardmore-Geraldton area. From Mason and McConnell (1983). Mine Years of production Ore milled (tons) Gold produced (Oz) Average grade (Oz/ton) Silver produced (Oz) Bankfield 1937-42, 1944-47 231,009 66, 417 0.29 7,590 Brengold 1941, 1949 46 134 2.91 Crooked Green Creek 1980-1984 1,455 471 Hard Rock 1938-1951 1,458,375 269,081 0.18 Jellicoe 1939-1941 10,620 4,238 0.40 145 Leitch 1936-1968 920,745 847, 690 0.92 31,802 9,009 Little Long Lac 1934-54, 1956 1,780,516 605,499 0.34 52,750 MacLeod-Cockshutt 1938-1968 10,337,229 1,475,728 0.14 101,388 Magnet Consolidated 1938-43, 1946-52 359,912 152,089 0.42 16,879 Maloney Sturgeon 1937 1 73 73 16 Maylac 1946-1947 1,518 792 0.52 46 Mosher Long Lac 1962-1966 2,710,657 330,265 0.12 34,604 Northern Empire 1934-41, 1949 425,866 149, 493 0.35 19,803 Orphan (Dikdik) 1934-1935 3,525 2,460 0.70 1,558 Sand River 1937-1942 157,870 50,065 0.32 3,628 Sturgeon River 1936-1942 141,123 73,438 0.51 15. 922 Talmora-Long Lac 1942, 1948 6,634 1,417 0.21 36 Tashota-Nipigon 1935, 1938 51,200 12,356 0.24 14, 527 Theresa 1935-38, 1941-1943, 26,120 4,785 0.18 202 69, 120 0.36 1945, 1950-53, 1955 Tombill 1938-42, 1955 190, 622 Total 4,115,611 1983). The Beardmore-Geraldton greenstone belt has produced approximately 4.1 million ounces of gold and over 300,000 ounces of silver. Most of this production came from two distinct gold camps at Beardmore and Geraldton, respectively. The balance of gold production came from several small mines scattered 30 km northeast of Beardmore and one mine south of Longlac. Table 1 (Mason and McConnell, 1983) is a production summary of all past gold producers. Gold in the field trip area occurs dominantly in quartz-carbonate veins and sulphide replacement zones cutting all the major Neoarchean rock types of the area. The vein systems are most typically hosted by: 1) clastic sedimentary rocks; 2) contacts between clastic sedimentary rocks and albite porphyry; 3) interbedded iron formation and clastic sedimentary rocks (Macdonald, 1984); 4) interflow iron formation and mafic metavolcanic rocks; and 5) post-tectonic mafic intrusive rocks (Hart et al., 2002). Structures Related to Mineralization The fine-grained sedimentary rocks in the southern portion of the area are isoclinally folded with a well- 8,595 318,500 developed, sub-vertical, east-striking cleavage. The fold axes vary in trend and plunge: in the west, the fold axes plunge west at moderate angles (Hart et al., 2002; Lafrance et al., 2004); near Jellicoe fold axes are vertical; and as we move progressively to the east the fold axes return to a westward plunge (Kehlenbeck, 1983). Shear discontinuities along axial planar surfaces also become more prevalent to the east (Kehlenbeck, 1983). Earlier work also outlined a later deformational event. It formed chevron folds with cleavage co-planar to the axial surfaces (Kehlenbeck, 1983; Anglin and Franklin, 1985). The second deformational event of Kehlenbeck (1983) is probably spatially and genetically related to regional, east-trending fault zones and is thus at least partially the same event as D3 of Lafrance et al. (2004). Geraldton Camp Gold mineralization in the belt has resulted from the introduction of hydrothermal fluids in zones of high crustal permeability. Permeability was generated by prolonged, multiple periods of deformation which focused not only fluids, but magmatic activity and intrusions. In the Geraldton camp (Fig. 11), a major zone - 12 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 ______ Vv ..VvV . VV - VVVVVV.vVq Vd •''VVVVv.vvvvvvvvv VVVVVVVVy vvvvvyvv i'i. VYVVVvJvvv,..' V-/VQAflqVI ./1IVJ7\#vvvvvvvvvvvvvvvvvv VVVVVVVVV 4vV.#V,..e' :.VQV.vI;--Ivvvvvv-.v., vvv1vvvyy.g Fvv-v vvvvvvv ..rnv--, vv•,v i-# iv., -VJ,V .--.r-ty,v /vVvv vVYgvvvv ,VJVV vvvv vqyyvy.g. 1'. :.v-IV.dVVvVvvvv.,vv..,vv! V vv.vvNv1vvvvv ivvvvvv . V ,.VVv VV/ VVVVVVVVVV Ivivvvv yVjVy1Vv V' .j•. I ;I.vvV;vVvvvvvvv vvvvvjvv Ra,tonliay --IIv,vvvvvv VVVVVVJ I VVVVv . -- IThH S V — Uthotectonjc Zone V 'I GERALOTQn A. - - :: - .... - . -&/ -. 4 + .. — '.IV . I— - — -,.—--._c-_ .- --———-- - . VVv'- .- — — t + 4 ...jCy. .t+ — .—-———.— --- --— —.0 I - _-—: - — - 2 + ÷ 4 4 4 + 4 + + 4 4 + .V.yvVVVVV 4 3 SMile, 6 It iti Porphyfflic lelsic intrusion Equigranular teisic intrusion Matic intruson A S Clastic rnetp$edln,ents iron tormation Gold Occurrences Major producing mines Fault ILITJ Matic to intermeiaIe metavolcamcs Figure 11. Spatial association of gold deposits to the Barton Bay lithotectonic zone north of the Bankfield-Tombill Shear Zone. Modified after Macdonald (1985). of deformation in which the gold mines are located has been alternatively termed: the Bankfield-Tombill Fault Zone (Pye, 1952; Horwood and Pye, 1955); TombillBankfield Deformation Zone (Lafrance et al., 2004); 2 — and Barton Bay Deformation Zone (Williams and Stott, 1991). In acknowledging not only the deformation within this zone, but also its lithologic variety, we will use the term Barton Bay Lithotectonic Zone (BBLZ) (Fig. 12). N Pt (/ —— N _* FFpa,tw Horwood and Pye (1955) noted that the area adjacent to and north of the BBLZ not only has S-folds related to the regional folding pattern but also has Z-folds. The Zfolds in this area only occur along the north side of and proximal to the BBLZ. This led Pye (1952), Horwood Figure 12. Structure and Stop locations within the Barton Bay Lithotectonic Zone. After Lafrance et al. (2004). 'l*ir1JS - 13 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 -- SFABRIC.. north of, and genetically linked to, the Barton Bay Deformation Zone (BBDZ) (Fig. 13). This zone of deformation varies from 1000 to 3000 m in total width (Lavigne, 1983; in press), while the crush zone of the Bankfield-Tombill Fault proper ranges from meters to hundreds of meters in width. The crush zone has been intensely silicified (Pye, 1952), carbonatized (Anglin and Franklin, 1985) and contains minor amounts of gold (Pye, 1952). - — - BANKEICLO — MAGNET MrNE Malic sIotayoicanlcs Malic Introsives _J Mastive Aikose Felsic Dike H Greywacke ore Figure 13. Fault-related fabric development in the Barton Bay Lithotectonic Zone near Geraldton. Modified after Colvine et al. (1984); from Scott (1985). and Pye (1955) and Buck and Williams (1984) to hypothesize that the second deformational event was related to the development of east-trending faults. This was confirmed by Lafrance et al. (2004) (their D3) and the faulting was attributed to transpression causing dextral shear throughout the region. Most mineralized occurrences in the Geraldton camp lie in a zone of deformation to the immediate Numerous Z-folds on various scales were formed in the deformation zone. Auriferous vein systems in the MacLeod-Cockshutt and Hard Rock mines are hosted by one of the Z-folds. This structure plunges shallowly west (Fig. 14; Horwood and Pye, 1955) and is mimicked by minor parasitic folds in the BIF. The parasitic Z-folds are generally less than 5 m in amplitude (Macdonald, 1982, 1983a) and are commonly cut by quartz-carbonate veins subparallel to axial traces. Displacement zones, also subparallel to axial traces, developed at lithologic contacts on isoclinal fold limbs, also host auriferous veins. Veins parallel to axial planes of folds are relatively straight and non-sinuous whereas those orientated at an angle to axial planes of Z-folds are highly ptygmatically folded. Beardmore Camp Gold mineralization in the Beardmore area is hosted by quartz-carbonate and sulphide-bearing quartz veins located in foliation-parallel, carbonatized, brittle-ductile shear zones in either clastic metasedimentary rocks of the SMB or in mafic metavolcanic rocks, proximal to NORTH ZONE F2 S Figure 14. Block diagram of the North Zone at the MacLeod-Cockshutt and Hard Rock mines drawn by Lafrance et al. (2004) using level mine plans published in Horwood and Pye (1955). Diagram shows the overprinting of a S F2 fold by a Z F3 fold on the north limb of the Hard Rock anticline. Ore pods are shown in black. - 14 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 T-e- '293 Figure 15. Map showing the surface trace of mineralized quartz veins on the Sand River - Leitch properties. After Ferguson (1967a). BIF of the SVB (Hart et al., 2002). Gold occurrences are also hosted by quartz veins in post-tectonic intrusive rocks and disseminated sulphides in carbonatized shears in metavolcanic and metasedimentary rocks associated with intrusive rocks. Regional mapping by Hart et al. (2002) and a composite longitudinal section of the Sand River Mine and Leitch Mine vein system based on that of Ferguson (1967a,b; Fig. 15) indicates that: 1) the Leitch Mine No. 2 vein and the Sand River Mine vein are the same vein; 2) the vein is foliation-parallel and is at a low angle to the regional fold axis at a strike of 240° and a dip of 65°; 3) the vein contains multiple ore shoots that are a series of dilatant zones controlled by the intersection of two regional fabrics such that the ore shoots plunge moderately steeply at about 320° and have good down-plunge continuity. The veins are hosted by the ductile-brittle shear system which is part of the Standingstone Lake fault (Hart et al., 2002; a.k.a. Standingstone Lake – Watson Lake fault of Bruce and Laird, 1937; Mackasey, 1975; Shanks, 1993), the western extension of the Oxaline Lake – Watson Lake fault (e.g., Williams, 1986). This fault trends about 075° northeast, and is topographically expressed by a swampy valley that includes Standingstone Lake and its associated tributaries, north of Highway 580, east of the Leitch Mine. The fault is manifested by the development of numerous small, northeast-striking, carbonatized shear zones. Lafrance et al. (2004) defined the structure hosting this Sand River vein as being parallel to the axial plane of this fold and S3. The Standingstone Fault has been interpreted to be a normal, south-side down fault (Carter, 1987) that has experienced multiphase deformation from dip-slip to dextral strike-slip (Williams, 1987). The Northern Empire Mine is hosted by a series of en échelon and composite quartz veins containing differing amounts of pyrite, arsenopyrite, pyrrhotite and chalcopyrite, hosted by carbonatized, sheared metavolcanic rocks and interflow iron formation of the SVB near the Empire fault and the SVB–SMB contact. The quartz veining occurs in brittle features of the deformed iron formation. The Empire Fault trends approximately 060° northeast, is steeply south dipping (but slightly shallower than the average dip of the overturned metavolcanic flows) and is marked by a thin fault gouge of black graphite bordered by a few feet of sheared rock (Benedict and Titcomb, 1948). Previous interpretations (Benedict and Titcomb, 1948; Shanks, 1993) placed the Empire Fault adjacent to the SVB-SMB contact and Shanks (1993) interpreted kinematic indictors to suggest a south side-up sense of motion. Hart et al. (2002) suggested that there are 2 distinct, subparallel faults: the Empire Fault to the east, and the Princess Lake Fault to the west, splaying off the SVB-SMB boundary. Both of these resulted in the formation of zones of brittle deformation in interflow iron formations in mafic metavolcanic rocks of the SVB that host auriferous quartz stringers containing arsenopyrite, pyrite, pyrrhotite and chalcopyrite (Hart et al., 2002). Co-incidence of structures related to the fault separating the BBA and the PPA northwest of - 15 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 _________ Beardmore, and the location of post-tectonic, mafic intrusion appear to have controlled the emplacement of quartz veins hosting gold, copper and/or molybdenum mineralization (Hart et al., 2002). The Tyson #1 gold occurrence (Mason and White, 1986) and the Tyson #2 molybdenum occurrence (Hart et al., 2002; Mackasey 1975) are hosted by quartz veins in the post-tectonic intrusion. Copper mineralization is hosted by sheared, carbonatized lapilli tuff within the fault (Hart et al., 2002). The fault trends approximately 075° to 080°, is marked by intense carbonatization and weak to moderate ankerite and calcite-flooding, and may have controlled the emplacement of the intrusion. Vein Systems Auriferous veins are typically 2 to 10 cm wide and primarily composed of quartz plus ankerite, dolomite and other carbonate species. Pyrite, arsenopyrite, pyrrhotite, sphalerite and chalcopyrite are locally present. The gold occurs as small blebs and fracture fills within individual quartz and sulphide crystals and in voids between crystals. It has also been observed as fine threads extending along cleavage planes in carbonate minerals (Armstrong, 1943). The auriferous veins often show evidence of brecciation and multiple generations of quartz. The brecciated zones are richer in gold that non-brecciated areas (Bruce, 1935, 1937a,b; Horwood and Pye, 1955). Crack-seal-textured ore veins, which predominate at the Leitch and Sand River mines, are indicative of successive periods of dilation and fluid influx. These veins contained minor and varied amounts of pyrite, arsenopyrite, chalcopyrite and tetrahedrite, scheelite ——— a —- - VEPN — Ii] Li .— 1: LY! JHT•::7 Figure 17. Plan of the North Zone (No. 30 Vein system), 250-foot level, Hard Rock Mine. After Horwood and Pye (1955). with septa of sericitic and carbonatized wall rocks. Alteration Halos Substantial alteration halos surround auriferous veins (Fig. 16). Carbonatization (ankeritic dolomite) dominates in host lithologies with low concentrations of iron-bearing minerals (e.g., clastic sedimentary rocks) and fracture zones associated with both major and minor faults. Siltstones have been found to contain up to 20% CO2 (Anglin and Macdonald, 1984). Carbonatization is ubiquitous in the clastic sedimentary rocks at the MacLeod-Cockshutt and Hard Rock mines. However, 300 m along strike away from this structure, carbonate alteration is rare (Macdonald, 1983a). Siderite supercedes ankeritic dolomite as the major carbonate phase within 5 cm of the vein margin. In addition to carbonatization, pyritization occurs in iron-rich rocks (e.g., BIF) adjacent to veins. These pyritic halos not only replace the magnetite/hematite laminae in BIF but also sandstone and siltstone laminae in the Geraldton area. The pyritization generally extends only centimetres to tens of centimetres away 3JN. RELATER MINERALOGY I CLI ——- IJI UAI'ON CL!AST1 4Hr / HOST BOCLM(N &A Lt Figure 16. Host rock, alteration halo and vein mineralogy for veins in iron formation-siliciclastic assemblages. From Scott (1985). - 16 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 1976; Mason and McConnell, 1983; Scott, 1985). Concentrations of gold may reach in excess of 15,000 ppb. In contrast to this, the surrounding, sulphide-free BIF consistently contains less than 20 ppb gold (Anglin and Macdonald, 1984; Fig. 19). E? TREMC CAR 0 OlAt 2 AT ION Generation of Hydrothermal Fluids and Timing of Mineralization S U LPHI OAflOti HALO Figure 18. Schematic representation of vein-related alteration, Hard Rock Mine (Scott, 1985); carbonatized zone is approximately 1 m wide. from individual veins. However, where densely packed systems of veins exist sulphide bodies meters by tens of meters in size have developed (e.g., MacLeodCockshutt-Hard Rock mines; Horwood and Pye, 1955; Figs. 17 and 18). Concentration ofOold ( ppb S r An alternative model to those outlined above is the genesis of the fluids through metamorphic de-watering. Studies conducted by Horwood and Pye (1955) indicated the fluids which deposited the auriferous veins contained Ag, As, Au, C, Ca, Co, Hg, Mg, Pb, S, Si, Te, W, Zn and possibly Fe and Ti. This assemblage is similar to the elements enriched in and near auriferous veins studied by Phillips and Groves (1983), Kerrich and Hodder (1982) and Kerrich (1981). These authors interpreted the fluids which formed those deposits to have been generated during burial and metamorphic de-watering of a volcanic pile. Cuncent,alio'i of Arsi'iic (ppm) Concentration of Arsenic ( ppm) ft Concentration ofOold ( ppb The vast majority of the gold associated with the vein systems in BIF is contained within sulphides in alteration halos. The gold generally occurs as submicroscopic particles and minute blebs within the pyrite (Pye, 1952; Horwood and Pye, 1955; Mackasey, Past studies have suggested the gold-bearing fluids in the Geraldton camp were derived either from a granodioritic body (Croll Lake Stock) lying to the immediate east of the area (Horwood and Pye, 1955; Macdonald, 1983b) or by metamorphic redistribution of gold contained in the banded iron formation (Boyle, 1976; Mackasey, 1975). Arguments can be made against gold originating in the BIF. Sulphide-free BIF in the area generally contains less than 20 ppb gold (Anglin and Macdonald, 1984). This amount is much lower than the average values of 61 ppb Au for highFe tholeiites and 78 ppb Au for volcanic-associated BIF which were obtained from a volcanic pile lying in Superior Province northwest of the BeardmoreGeraldton greenstone belt (Cowan and Crockett, 1980). Obviously, if volcanic and related chemical sedimentary rocks in the map area have similar gold contents (and sparse data indicate they do) they would provide a more adequate source of gold. In addition, alteration halos around the veins indicate the fluids were in marked disequilibrium, both in composition and Eh, with the BIF signifying that they were derived from an external source. Figure 19. Gold and arsenic distribution associated with veins at MacLeod-Cockshutt (MC) and Solomon’s Pillars (SP). From Scott (1985). The spatial association between the east-striking fracture systems and the gold mineralization has led previous authors to believe that the faults and related minor structures served as passageways along which the fluids moved (Horwood and Pye, 1955; Macdonald, 1984; Anglin and Franklin, 1985). Structural control of - 17 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 vein systems, and by inference, fluid migration paths, is common in the Superior Province (Rigg and Helmstaedt, 1981; Fryer et al., 1979; Stott and Schnieders, 1983; Poulsen, 1983; Durocher and Hugon, 1983; Hodgson, 1983). The fluids were probably generated at depth and utilized the permeability developed in deformed zones to gain access to higher crustal levels. Permeable fault zones provide conduits to the fluids allowing them to move upwards towards lower-pressure/temperature zones. Episodic and protracted development of dilatant zones and shearing is indicated by the open spacefilling nature of the veins and their brecciation. Fluids migrated towards dilation zones enveloping fault zones, driven by a pressure gradient and seismic pumping (Sibson et al., 1975). The same model has been put forward to explain structurally controlled gold veins in the western Wabigoon Subprovince (Poulsen, 1983). The development of abundant carbonate and iron sulphide in the reaction halos around veins indicates the fluid could not have been very acid (Krauskoph, 1967; Boyle, 1969) and was probably near neutral (Phillips and Groves, 1983). The formation of massive amounts of carbonate minerals also denotes a very CO2rich fluid. This conforms to data from other studies which advance H2O- CO2-dominated fluids as of major importance in the formation of Archean lode gold deposits (Kerrich, 1981; Kerrich and Hodder, 1982). Fluid inclusion studies of the MacLeod-Cockshutt deposit also indicated a CO2-rich fluid at a temperature in excess of 380°C was involved in forming at least some of the veins (e.g., Macdonald, pers. comm., 1985). Recent research has suggested that major gold deposits may be the result of prolonged mixing of two fluids: a hot, reducing, orogenic fluid that migrates along major regional fault structures; and a colder, oxidized, goldand base metal-enriched, magmatic fluid associated with late, mantle-derived plutons (c.f. Neumayr et al., 2004; Rogers et al., 2004). Gold mineralization in the Beardmore area has not been studied to the same degree as in the Geraldton area, but much of the preceding discussion is probably applicable there as well. However, there are a number of points specific to the Beardmore area. An intermediate to felsic intrusion comparable to the Croll Lake Stock has not been identified. There are a number of posttectonic mafic intrusions, and one of these intrusions along the faulted BBA-PPA boundary hosts gold and molybdenite mineralization (Tyson occurrences) and there is copper mineralization in the adjacent fault (Hart et al., 2002). Mafic intrusions of comparable composition are located to the west of the Sand River Mine, south of the interpreted trace of the Standingstone Lake Fault, but intrusions are not spatially associated with mineralization in either the Leitch or Sand River mines. The Standingstone Lake Fault may be part of the fault system separating the SMB and CVB as it, in part, parallels this contact, but it also is the approximate location of the reversal in top directions in the metasedimentary rocks of the SMB (Hart et al., 2002). Axinite, a borosilicate similar to tourmaline, is located in brittle structures in mafic metavolcanic rocks along strike with the Leitch – Sand River vein to the east, and along the BBA-PPA contact to the north (Hart et al., 2002). Although axinite is not associated with gold mineralization, as in the Chibougamau (Dubé and Guha, 1993) and Cadillac areas (Bardoux et al., 1990), it may be a late-stage mineral associated with an intrusion that may have supplied hydrothermal fluids during the gold-mineralizing event. However, the axinite may be unrelated to the gold mineralizing event and reflecting an unexposed pegmatite or the intrusion of the diabase sill (e.g., Ozaki, 1972). Gold mineralization at the Leitch and MacLeodCockshutt mines, the two largest, past-producing gold mines in the Beardmore–Geraldton Belt, is associated with D3 brittle shear zones and folds, overprinting regional F2 folds (e.g., Lafrance et al., 2004). The plunge of the ore zones is parallel to F3 fold axes and to the intersection of D3 shear zones with F2 and F3 folds. At the deposit scale, the plunge of the ore zones is similar in orientation to the plunge of F3 axes (e.g., North Zone, MacLeod-Cockshutt Mine) and to the intersection of D3 shear zones with F3 folds (e.g., Leitch Mine; Hart et al., 2002; Lafrance et al., 2004) and F2 folds (e.g., F Zone, MacLeod-Cockshutt Mine; Horwood and Pye, 1955). Lafrance et al. (2004) provided an estimate for the age of syn-D3 gold mineralization ranging from 2686 Ma (i.e., the age of the pre-D3 Ottertail granite in the Rainy Lake district) to 2640 Ma (i.e. the minimum age for the formation of D3 structures and high-temperature metamorphism in the Quetico subprovince; Zaleski et al., 1999). On a subprovince scale, regional folds that are cut by D3 dextral shear zones are promising targets for discovering the next generation of large gold deposits (ibid). - 18 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Stops Geraldton gold camp Junction Hwys 11 & 584 Preface 2-1 MacLeod - Cockshutt headframe The Beardmore-Geraldton area has been the subject of numerous field trips and published and unpublished field trip guide books, including those by Mason et al. (1985), Fralick and Barrett (1991) and Williams and Stott (1991). Bear in mind that when visiting active exploration or mine properties, permission must be granted by the property owner. Current ownership information can be obtained from the Resident Geologist’s Office in Thunder Bay. Please exercise caution along highway, road right-of-ways and lake shores. 2-2 Porphyry Hill km Beardmore - South Intersection of Hwy. 11 & CNR (Beardmore) 0 1-3 Blackwater fault/pillowed volcanic rocks 0.0 2-3 Glory Hole BIF Field Trip Road Log Stop Locality 0.0 1.3 Junction Hwys 11 & 584 0.0 Ashmore/Errington boundary 0.5 Mosher Mine turn-off 1.3 2-4 Conglomerate 1.3 2-5 Gabbro 1.6 Gravel Pit turn-off 2.3 Powerline crossing 3.8 Wintering Road turn-off 4.7 Powerline crossing 5.9 McClellan Strip 6.4 Magnet Mine turn-off 8.5 Bankfield Mine turn-off 10.6 Key Lake/Malouf Vein 11.8 Wild Goose Lake Park 27.0 1-2 Quetico metasedimentary rocks 5.7 Jellicoe-Kinghorn Road 1-1 Sibley-Quetico unconformity 13.6 2-8 Southern Sedimentary belt sandstone Jellicoe Post Office Beardmore - West 2-9 Pillowed mafic flows 39.6 47 47.2 Junction of Hwy. 11 & 580 0 East Leitch turn-off 5.8 Junction Hwy 11 & Kinghorn Road 0 Longlac Superior strips 3.7 Ara Lake sign 1.7 72 Road turn-off 5.3 CNR tracks 2.9 7.1 Sturgeon River Bridge 5.3 Sand River Mine turn-off 7.4 6 km Road turn-off 6.0 1-8 Sand River 16 Vein Zone 8.3 2-6 Missing Link extension 7.2 1-7 Leitch BIF 8.4 Missing Link turn-off 7.7 1-6 Eva Creek: conglomerate/BIF 9.3 Missing Link parking spot +0.4 1-5 Eva Creek: pyroclastic rocks 9.6 2-7 Missing Link (NE corner) +0.2 1-9 Leitch mine turn-off Million Dollar Road turn-off 10.4 High Hill Harbour turn-off 12.0 1-4 Poplar Lodge pillowed flows, G & G Goodman driveway (access to lakeshore) 14.0 Stop 1-1 - Archean – Proterozoic Unconformity UTM coordinates - 0423431E 5483881N The angular unconformity between east-trending, steeply dipping Neoarchean Quetico metasedimentary rocks and flat-lying Mesoproterozoic Sibley Group - 19 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Figure 21. Stratigraphic column of Quetico metaturbidites, Stop 1-2 (Fralick et al., 1992) t..& Figure 20. Angular unconformity between steeply dipping Quetico metaturbidites and overlying Sibley Group sandstone-carbonate units, Highway 11 (Stop 1-1). sedimentary rocks is exposed on both sides of Highway 11. The phyllitic Quetico rocks display layer-parallel slickensides (bedding-parallel fault?). This outcrop consists of metasedimentary rocks unconformably overlain by a thin layer of Sibley Group sandstone (Fig. 20). The underlying rocks were folded to a near vertical position during the Kenoran Orogeny, prior to Mesoproterozoic deposition of the Sibley sedimentary units. The Sibley here consists of an upper, 30 cm thick, orthoquartzite separated from the Archean basement by 0 to 30 cm of silicified carbonate. The carbonate occurs in fractures in the substrate up to 60 cm below the paleosurface. This gives the distinct impression that the carbonate is a well-developed, caliche zone. Caliche soils form in semi-arid settings where the evaporation rate exceeds the precipitation rate. Irregular laminations in this layer are up-buckled in places, forming domical structures, probably the result of expansion during crystallization. It is difficult to say much about the overlying orthoquartzite. It is not even known if it is wind- or water-lain. Similar, thin sandstone layers occur both to the east and west of Lake Nipigon, separating basement from Mesoproterozoic (Keweenwan) diabase sills. They are correlated with the Sibley simply on the basis of their probable Mesoproterozoic age. Stop 1-2 - Quetico Metasedimentary Rocks UTM coordinates - 0428530E 5489570N Most of the Quetico consists of an unstructured submarine ramp system (i.e., an environment where large, long-lasting channels do not exist and sediment delivery routes switch rapidly, producing a chaotic structuring to the deposits). This outcrop provides one of the few examples of structured channel outbuilding in this ramp setting. A thinningand fining-upward succession of Bouma ADE, CDE and DE turbidites is sharply overlain by a thick-bedded unit of ABCD turbidites with abundant clay rip-ups and extra-basinal pebbles (Fig. 21). The lower, 8 m-thick, fine-grained assemblage is the upper portion of a subaqueous channel feeding sediment from the Beardmore-Geraldton forearc basin to the Quetico trench. It is Orn erosively overlain by the basal section of the next channel complex. The phyllitic rocks that occur at the northern margin of the Quetico Subprovince south of Beardmore are strongly sheared, exhibiting rust-spot elongation, quartz fibre lineation, and crinkles, some of which can be attributed to non-parallel,cleavage-bedding and cleavage-vein intersections (Williams and Stott, 1991). None of these intersections appear to have reliable or consistent tectonic significance. Stop 1-3 - Blackwater Fault / Pillowed Metavolcanic Rocks UTM coordinates - 0430174E 5493162N Deformed, pillowed mafic metavolcanic rocks of the Southern Volcanic Sub-belt are exposed along the sides of Highway 11 just south of Beardmore. They form the southern margin of the Beardmore-Geraldton greenstone belt and the eastern Wabigoon Subprovince in this area, where it is in fault-bounded contact with Quetico metasedimentary rocks to the south. The Blackwater River Fault (BRF) is a regional-scale structure which separates the two subprovinces. It is likely a dextral, transcurrent fault, similar to other subbelt-bounding structures in the Beardmore-Geraldton belt. It has been traced for over 100 km and underlies the valley occupied by Camproad Creek, a few hundred - 20 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 intermediate to mafic metavolcanic rocks of the Central Volcanic Sub-belt are exposed along the shoreline of Lake Nipigon, just north of Poplar Lodge. They have been described by numerous workers, including Coleman (1907), Laird (1937), O’Brien (1985), Tomlinson (1996) and Hart et al. (2002). Figure 22. Deformed pillow basalt near Blackwater Fault, Highway 11 (Stop #1-3). metres south of this field trip stop. Immediately north of the BRF, Shanks (1993) noted a series of sub-vertical, non-penetrative planar surfaces, interpreted to be either slip surfaces or C-planes. Estimation of the finite strain ellipsoid shape by Williams and Stott (1991) using pillow shapes and selvage thickness indicated plane strain. Localized, small-scale c-s fabrics were noted by Williams and Stott (1991) in schistose selvages, indicating a north over south sense of displacement. Light grey-green weathering basalt flows are foliated at approximately 240°/80° north. They are locally phyllitic, friable and gossanous. Despite flattening and aspect ratios of 3:1 to 10:1, pillows give northward top directions. Pillows range in size from 0.5 to 1.0 m and have recessively weathered selvages (Fig. 22). They contain flattened vesicles near their rims and are locally variolitic. Isolated patches of autoclastic(?) breccia contains 0.10 to 0.25 m-sized, lenticular fragments. The flows are more massive to the south and are crosscut by rusty, “bull-white” quartz veins. Joint surfaces are coated with quartz and calcite. A rusty quartz vein occupying the sheared contact between basalt and sandstone to the north was sampled but returned no gold nor silver. A 3m wide diabase dyke trends ~155° and cuts the supracrustal rocks. Massive, vesicular/amygdaloidal and/or pillowed basalt/andesite, heterolithic breccias, as well as porphyritic and flow-banded units were described by O’Brien (1985). Individual flows in the CVB are commonly a few metres thick, with a few examples that are tens of metres thick with pillows that are generally weakly deformed to undeformed. In this outcrop, the pillows may be greater than 1 m in diameter (Fig. 23). Pillow selvages are in these flows are generally thicker than in the mafic metavolcanic rocks. Internal features include amygdaloidal and vesicular flow laminae textures and feldspar phenocrysts, with some pillows in flows to the south containing concentric flow laminae defined by the alignment of vesicles approximately 1 mm in diameter. Pillow breccias occur as a transition from the pillowed flows to the hyaloclastite-rich, pillow fragment breccias, and are exposed on the western end of the outcrop as it dips into the lake. The pillow breccia portion of the units is generally 1 m wide, with the pillows very abruptly diminishing in size away from the main body of the flow. A good example of this type of breccia is exposed in the bay to the south, ~150m from this location, but is only accessible when the water levels are low. Pillow fragment breccia forms the major part of the unit, and is generally a few metres in width adjacent to the flow front. This transition is visible near the waterline. The breccia thickens to the Srop 1-4 - Poplar Lodge Pillowed Metavolcanic Rocks Private property access to lake shore; please ask permission from cottage owners UTM coordinates - 0421034E 5499814N (northern end of section), 0421089E 5499665N (southern end of section) Weakly deformed, northward-younging, pillowed Figure 23. Pillow basalt, Poplar Lodge, Lake Nipigon (Stop 1-4). - 21 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 west, which may suggest that the direction of flow was towards the west. Fragments generally consist of sub-angular parts of pillows and rounded fragments or amoeboid fragments and small pillows in hyaloclastite matrix. Although the pillow breccias have been classified as volcaniclastic rocks, a portion of the units could be classified as hyaloclastite or rocks that are the result of fragmentation of the magma due to interaction with water (Cas and Wright, 1987; Batiza and White, 2000). There is no depth restriction to the formation of the pillow breccias, as the breccias have been observed in both shallow water and on seafloor sites. Heterolithic breccias consist of predominantly 0.5 to 10 cm sub-angular to angular clasts of intermediate metavolcanic rocks, but minor clasts of feldsparporphyritic, felsic metavolcanic rocks, cherts and mafic metavolcanic rocks overlie pillow breccias in a few locations along the lake shore. Generally, the breccias lack any internal structure. Although some areas appear to have bedding structures, they generally cannot be traced for any distance. The matrix is generally highly chloritized and some of the smaller, more porous clasts may have been totally chloritized; reaction rims along clasts margins may be developed Major element and trace element analysis indicated that the pillowed flows are tholeiitic basalt and andesite (O’Brien, 1985), characteristic of mid-ocean ridge basalt (Tomlinson, 1996; Hart et al., 2002). Stop 1-5 - Eva Creek Pyroclastic and Sedimentary Rocks UTM coordinates - 0424037E 5498780N Dacitic pyroclastic rocks along the southern margin of the CVB are exposed on a small ridge 50 m east of Highway 580 (Fig. 28). A massive, aphyric, monomictic tuff-breccia has an iron-carbonatized matrix and is cut by epidote veins (Fig. 24). It is foliated at 065°/78° south and hosts narrow shear zones with crenulated fabrics and quartz veins. There is a suggestion of flattened pillows at the northern edge of the outcrop. A north-trending diabase dyke cuts the supracrustal rocks. An outcrop on the west side of Highway 580, across from Stop #1-5, exposes typical, medium-bedded turbidites of the SMB. These consist predomintly of A, B, D and very minor E divisions. The minor E division reflects very little clay in the sediment transport system, a logical consequence of sediment feed being mostly derived from active subareal, pyroclastic volcanism to the north in the Onaman-Tashota terrain (Fralick, 2003). The interval of time between eruption, erosion and sedimentation was not sufficient to produce significant amounts of chemical weathering (i.e., clay). The relative lack of C (rippled) divisions is more difficult to account for. One possibility is that higher ocean temperatures resulted in a thinner, laminar sublayer which fine sand was capable of disrupting and, since ripples need a laminar sublayer to form, the C division was suppressed. The non-regular bed thickness variation up through this section reflects rapid changes of the position of sediment feed areas into the subaqueous portions of the basin, resulting in a somewhat chaotic interlayering of thin and thick turbidites. Stop 1-6 - Eva Creek Conglomerate / Banded Iron Formation •L. $ t , , UTM coordinates - 0423925E 5498505N Figure 24. Deformed felsic pyroclastic breccia, Highway 580 (Stop 1-5). Exploration of the local iron ranges began in earnest around 1900 when several claims in what was to become the Sand River and Leitch mines area were staked and surveyed (Laird, 1937). In 1919, the Lake Superior Ore Company was formed to option and drill a portion of the Central range. At this time, the Kokoko camps on claim A.L. 413 were built. Drilling carried on until 1920. Coleman (1907) reported that drilling returned assays in the 40 to 50% Fe range, with negligible amounts of deleterious sulphur and phosphorus. Laird (1937) characterized the iron deposits of the Central range: - 22 - “The beds have been crumpled and intricately folded, �-2. ___2 FIuviaI _________ a Proceedings of the 51st ILSG Annual Meeting - Part 2 ______ •Ly€€d. 40 Dominated .—'--.. .1 .! b SnhiIar. orMagnelite 3 -r and Sirisiclasb. ThinLayered. LI- SiIisidasbc . I 2 0 with Thin MagnetLte t.aminae 20 / No fl(ilrCp Sandstone Lense In Conglomerate 1J1/ IT Trcijh Crot Stfat,flcabnn 0m — Flooding r — Surface Figure 25. Stratigraphy, Eva Creek section (Stop 1-6). vi . ab IF. Im on tSandstV with the result that they have the appearance of great thickness in places. The formation is well-banded and consists of thin layers of red jasper alternating with steelgrey hematite, the whole being irregularly interbedded with dark slaty bands. The jasper-hematite bands are not commonly more than a few feet in width and in places they occur as mere streaks in the slaty material. Magnetite is not a prominent constituent of this band, but in places it is present in quantities sufficient to cause considerable magnetic disturbance.” This is one of the best examples of the stratigraphic position of BIF in the belt. The lower portion of the section consists of interlayered magnetite, siltstone r • • • Figure 26. Conglomerate associated with BIF, Eva Creek (Stop 1-6). and sandstone in various ratios from chemical sediment-dominated to clastic-dominated (Fig. 25). This is erosively truncated by a conglomerate. The clast-supported, polymictic conglomerate (Fig. 26) contains diffuse zones of larger and smaller clasts, lenses of medium-grained sandstone and crescent scours and sand shadows along internal surfaces where sandstone lenses are concentrated. This is overlain by a thick, medium-grained sandstone, which is, in turn, succeeded by a clastic-BIF succession. A coarsegrained sandstone-conglomerate assemblage erosively cuts down into this package, removing the upper metre in places. This outcrop highlights the juxtaposition of BIF and erosively based, fluvial channels. The fluvial system was periodically extending into a nearshore dominated by thin, graded siltstones and sandstones with iron oxide laminae. The depositional architecture is composed of a series of prograding parasequences in the littoral zone with BIF developed on flooding surfaces. Commonly iron-rich BIF directly overlies the flooding surface and is upwardly transitional into more clastic-dominated units. These may be erosively incised by the next channel assemblage. Off-delta, deeper-water turbidite units do not contain BIF. This strongly indicates that BIF deposition was the result of photosynthetic oxidation in the nearshore, producing an oxygenated oasis near nutrient-rich waters emanating from river mouths. - 23 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Stop 1-8 - Sand River Mines Property - #16 Vein system UTM coordinates - 0424339E 5497563N The following history has been modified from a summary reports prepared by Bevan (2004) for Roxmark Mines Limited (who are the current property owners) and Mason and White (1986). Laird (1937) provided the first geological description of the property and its exploration activity. History of the Sand River and Leitch Mines Figure 27. Deformed BIF, Highway 580 (Stop 1-7). Stop 1-7 - Leitch Banded Iron Formation UTM coordinates 0424600E 5497867N This is an excellent exposure of oxide-facies BIF interlayered with siliciclastic layers. Unlike many BIF’S in this region, hematite, not magnetite, dominates this exposure. At first glance the rocks give the impression that there is intense shearing along the cleavage direction disrupting the layering (Fig. 27). Closer examination may lead to a different conclusion. The sharp-sided, clay-poor, medium-grained sandstones form lenses (averaging 1 cm thick), cutting saucershaped scours into the 1 mm-thick, stacked iron oxide laminae. That is, the sandstones are lenticular to flaserbedded. Some lenses appear to be ripple-laminated, although the cleavage cutting through at a similar angle makes identification problematic. This type of siliciclastic depositional system is typical of distal portions of distributary mouth bars in deltaic settings. The interlayering of iron formation indicates relatively rapid precipitation of the chemical sediments in a nearshore environment. The first discovery of gold in the Standingstone River area west of Beardmore was made on what became the Sand River Mine property in June, 1934 by Russell Cryderman (Laird, 1937). The Discovery or Number 1 vein averaged only 13 inches in width, but was remarkably persistent along strike, being exposed in cross-trenches and surface pits over a length of 2100 feet. The vein was traced to the northeast onto the Leitch Mine in 1935 (Fig. 28). Stripping and trenching were carried out by the newly incorporated Sand River Gold Mining Company on the Number 1 vein. Servicing was by two three-compartment shafts, 2950 feet apart in a northeasterly direction: the westerly Sand River shaft to 2656 feet and the Leitch main shaft to 3006 feet, with a winze from the 19th (2875-foot) level to the 30th (4525-foot) level. A road connecting the two properties with Beardmore was constructed in 1936. Production started with milling facilities at 75 tons per day at both properties in 1937. Between 1937 and 1942, Sand River processed 157,870 tons at a recovery grade of 0.32 ounce gold per ton, yielding 50,065 ounces gold, to their 9th (1150-foot) level. Mining was by conventional shrinkage stoping with an approximate width of 3.0 feet. The Sand River operation ceased in 1942. Leitch Mines processed 80 tons per day, milling 176,535 tons at a recovery grade of 0.71 ounce gold per ton in the same period with a high-grade resuing approach. Northern Empire Mines Limited, a subsidiary of Newmont Mining Corp., began milling operations in Beardmore in 1934 with a maximum of 186 tons per day in 1939 and ceased in 1941. They then acquired the Sand River property on a ten-year lease. They drifted out on two of the three un-mined Sand River levels above the diabase sill. Sufficient encouragement led them to collar a winze on the Sand River 11th or 1450-foot level, designed to sink through the diabase. - 24 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 The winze was stopped at 1506 feet when war broke out in Europe. Northern Empire resumed operations at Sand River in 1944 as the Undersill Gold Mining Company. Undersill de-watered and deepened the Sand River shaft through the flat-lying, 600-foot thick diabase sill to 2180 feet. The shaft was bottomed at 2656 feet. They explored the 17th or 2610-foot level with a shaft crosscut of approximately 450 feet and 2340 feet of drifting, of which 1300 feet was eastward to the Leitch boundary, and 1040 feet was westward. The 1947 Leitch Gold Mines Limited Annual Report stated that Northern Empire’s Undersill Gold Mining Company had encountered “good ore” on the Sand River 17th (2610-foot) level, an extension of the highgrade Leitch Number 2 vein. Northern Empire, unable to capitalize on its 1944 to 1947 development with gold at US$35 per ounce, closed down the Undersill Gold Mining operation in 1954 and concentrated development at its Magnet Mine, near Geraldton. Leitch acquired the Sand River property in the same year to protect the depth extension of their steeply westplunging Number 2 vein. Leitch developed the Sand River property by an internal winze at the Leitch Mine from their 19th (2875-foot) level to the 30th (4525foot) level. Leitch’s production from the Sand River - Undersill Gold Mining Company’s holdings totalled 376 283 tons at a recovery grade of 1.11 ounces gold per ton from 1955 to mine closure in 1965. This was an appreciable improvement of grade with depth from 530 112 tons at 0.79 ounce gold per ton from 1938 to 1954 above the 19th level. Environmental requirements were completed on the Leitch Mine and the Sand River property was returned to the original owners in 1968. In 1940, exploration by Halport Gold Mines outlined a parallel vein system, named the #16 vein system. The following table covers the history of this discovery. Historical Summary - #16 Vein System 1940 - Halport Gold Mines located a number of surface veins on Claim AL 415 at the eastern end to the boundary with Sand River Claim TB 12944. 1944 - Leitch acquired the Halport holdings. 1945 - 22 diamond drill holes totalling 16 510 feet were drilled. 1950 - A long crosscut on the 8th level was initiated because surface drilling had indicated ore at this horizon. 1951 - Further crosscutting and some drilling was carried out. 1952 - Further crosscutting and drifting to open up the #16 vein. Drilling of 51 underground holes totalling 11,116 feet, mainly on the 8th level. 1953 - Further drilling and stoping on the #16 vein – production and milling of 3,598 tons grading 0.56 oz. Au/ton. 1969 - Teck Corporation (together with HighlandBell Limited and F.E. Hall) purchased control of the Leitch Gold Mines property. 1971 - The entire Leitch assets were acquired by Teck Corporation. 1946 - Teck Corporation carried out exploration work until 1981 over the Leitch property, including line cutting, VLF-EM magnetometer surveys, 393 holes of overburden drill sampling and 29 short diamond drill holes totalling 3,454 feet and covering a strike length of 1,400 feet. One hole was abandoned. 1987 - Sand River-Cryderman diamond drilling program. 10 holes totalling 2,030 feet were drilled on Claim TB 12944 covering the #16 vein to the east. 1988 - Teck Corporation drilling program in joint venture with San Paulo Explorations Inc. 24 diamond drill holes totalling 22,098 feet, of which three were abandoned, one hit the open stope and one intersected the vertical diabase dyke and was terminated. Airborne and ground geophysical surveys and reverse circulation drilling were also undertaken until 1989. 2003 - Acquisition of Sand River property by Roxmark Mines Limited; Advanced Exploration project initiated. In 2003, Roxmark Mines Limited acquired the Sand River, Leitch and East Leitch properties from Rio Fortuna, Teck Cominco and Kinross, respectively (News Release, Roxmark Mines Limited, November 28, 2003) and began an Advanced Exploration program. In 2004, two vein systems were exposed by trenching, stripping, channel sampling and stockpiling at the East Leitch and Sand River #16 zones, respectively. Stripping of these areas had been recommended in preparation for taking bulk samples (ibid). Geological mapping and sampling was planned to gain a better understanding of the mode of occurrence and identify other possible veins and extensions in the #16 vein system. Further exploration in the form of diamond drilling in five areas was recommended as follow- - 25 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Sand River - Lelich Mines (altar Ferguson (1967a) 0 results confirm the historical sampling data completed by Teck Corporation. Furthermore, the current assay results confirm the historical drill holes that intersected the #16-1 Vein. Recent Roxmark assays of the #16-5 West vein were 1.058 ounces Au per ton over a 26.5 foot strike length. The #16-5 East Vein returned 1.094 ounces Au per ton over a strike length of 35.0 feet. I loOm Geology of the #16 Vein Area Figure 28. Geology of the Sand River-Leitch mines area after Ferguson (1967a), showing Stop locations. up work in preparation for underground exploration and development. Bulk samples will be processed at Roxmark’s 200 ton-per-day Northern Empire Mill in Beardmore. Roxmark reported the results of samples taken from the #16-vein system on the Sand River Mine property (ibid, December 16, 2004). The Company is continuing with its plan for the surface bulk sample program and in particular, the in-fill sampling on the #16-1 and #16-5 veins. Channel samples were cut every three feet. All composite assays used a minimum width of 1.0 feet. The #16-1 West Vein returned an average assay of 1.737 ounces Au per ton over a strike length of 42 feet. The #16-1 East Vein returned 0.347 ounce Au per ton over 37 feet. The recent assay 16 Vein Zone Sand River Mine The Leitch-Sand River area (Figs. 15 and 28), most recently mapped by Hart et al. (2002), is predominantly underlain by clastic metasedimentary rocks of the SMB. Thickly bedded, southward-younging, overturned, feldspathic sandstone and siltstone have been wellexposed by recent stripping operations in the vicinity of the #16 North zone (Fig. 29). Bedding has eastnortheasterly to east-southeasterly orientations with steep, northerly dips. Sub-parallel shear zones are distinguished by well-developed, slaty S3 cleavage, as well as local sericitization and carbonatization. Oblique cleavage – bedding relationships are locally exposed. Cleavage is refracted through interbedded siltstone and sandstone beds (Fig. 30). A number of auriferous quartz veins have been exposed in the immediate area (Figs. 28 & 29). Visible gold occurs within fractures in quartz veins and most commonly is found along thin seams or septa of sericite and chlorite, along with arsenopyrite, pyrite and ankerite in composite, “crack-seal”-textured veins. Tetrahedrite and sphalerite are also reported to occur in these fractures. Pink-beige scheelite is relatively abundant. :i The Leitch Mine No. 2 vein and the Sand River Mine vein are the same vein striking southwest parallel to a N Figure 29. Map of stripping and veins, Sand River 16 Vein zone (Stop 1-8). Figure 30. Refraction of cleavage in sandstone/siltsone, Sand River 16 Vein zone (Stop 1-8). - 26 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 subvertical to vertical tectonic fabric which is subparallel to the original bedding in the clastic metasedimentary rocks (Hart et al., 2002). The vein contains multiple oreshoots that plunge at about 320° in the plane of the 65° northwest dipping vein, and these shoots are controlled by the intersection of the first fabric with a second regional tectonic fabric. Lafrance et al. (2004) defined the strike of the vein as 240° and dip 65°, with plunges to the oreshoots being consistent with the F3 fold axes, parallel to the Standingstone - Watson Lake Fault which was likely developed in two parallel, D3, dextral shear zones. Ore zone plunges are consistent with F3 fold axes. Hart et al. (2002) identified that the potential for additional gold mineralization would be very high in structures subparallel to parallel to the Sand River Mine – Leitch Mine No. 2 vein, such as the No.16 veins that are exposed north of the Sand River Mine shaft. Stop 1-9 - Leitch Mine UTM coordinates - 0425302E 5497484N This stop allows trip participants to scour Leitch Mine dump piles for representative samples of goldbearing quartz veins and clastic sedimentary rocks. Visible gold occurs within fractures in quartz veins and most commonly is found along thin seams or septa of sericite and chlorite, along with arsenopyrite, pyrite and ankerite in composite, “crack-seal”-textured veins. Tetrahedrite and sphalerite are also reported to occur in these fractures. Pink-beige scheelite is relatively abundant. Leitch Mine operated continuously from 1937 to 1965. Mill clean-up ensued between 1966 and 1968. One of the highest-grade gold mines in Canada during its lifetime, the mine yielded 861,981 ounces of fl• gold from 1,022,360 milled tons at a recovered grade of 0.915 ounce gold per ton. 31,802 ounces of silver were also produced at a grade of 0.035 ounce silver per ton. 64.3 tons of tungsten ore were produced at a average grade of 3.95% WO3. Figure 31 gives us a glimpse of the property at the time of its discovery. Day Two - Geraldton Area Past-producing gold mines in the Geraldton camp (~ 3.1 million ounces) are confined to the northern part of the SMB and are localized by the Barton Bay Deformation Zone (BBDZ). The BBDZ (Fig. 12) is a 1 km wide, high-strain zone that extends from the Hard Rock and MacLeod-Cockshutt mines west along Highway 11 to the Bankfield Mine (Pye, 1952; Lavigne, in press; Lafrance et al., 2004). Shear is distributed heterogeneously across the BBDZ. In contrast to the mafic metavolcanic/mafic intrusive rocks to the north, and the monotonous sequence of sandstone to the south, the rocks within the BBDZ (mafic to intermediate intrusive rocks, felsic porphyry dykes and sills, sandstone/pelite, conglomerate and BIF) are complexly intercalated and have high competency contrasts. As noted by Lavigne (in press), only a very few areas in the belt have such lithologic heterogeneity. All of the aforementioned rocks in the BBDZ, except for the conglomerates, host gold. Although Geraldton is often considered to be a BIF-hosted gold camp, it is important to note that less than 30% of the gold came from BIF-hosted ore (Macdonald, 1982). Most of the production came from clastic sedimentary rocks, particularly at or near the contact with feldspar porphyry intrusions. Anglin and Franklin (1985) cited a more consistent spatial relationship between gold mineralization, deformation zones and felsic intrusive rocks. Stop 2-1 - MacLeod-Cockshutt Mine UTM coordinates - 0504200E 5502770N 2 4 Figure 31. No. 1 vein, Leitch Mine, October 1935 (Laird 1937; Stop #1-9). The history of the MacLeod-Cockshutt Mine (Fig. 32) has been summarized by Horwood and Pye (1955) and Mason and White (1985). In 1931, following the discovery of gold by W.W. Smith on Discovery Point, Kenogamisis Lake, F. MacLeod and A. Cockshutt staked the ground adjoining the Hard Rock Gold Mines Limited property to the west. Surface exploration led to the discovery of gold-bearing quartz veins in 1931. The discovery of larger mineralized zones in 1933 - 27 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 'j ____ I Figure 32. Map of the MacLeod-Cockshutt – Hardrock mines area (modified after Ferguson 1967). Pf = albite porphyry; IF = banded iron formation; sandstone is unlabelled and unhatched. led to the organization of a new company, MacLeodCockshutt Gold Mines Limited. In 1934, shaft-sinking began here at the Number 1 shaft; the Number 2 shaft, 600 m to the southeast, was sunk in 1936. The MacLeod-Cockshutt Mine became the fifth producing gold mine in the Little Long Lac area on April 19, 1938 when a mill with a rated capacity of 600 tons per day was brought into operation. In 1967, MacLeodCockshutt Gold Mines Limited, Consolidated Mosher Mines Limited and Hard Rock Gold Mines Limited were amalgamated to form MacLeod Mosher Gold Mines Limited. Underground operations continued until July, 1970. The mine had produced 1,546,980 ounces of gold at an average grade of approximately 0.14 ounces of gold per ton. This total accounts for about half of all the gold produced by the 10 mines in the Geraldton gold camp between 1934 and 1970. In the 1980’s, Lac Minerals Ltd. (now Barrick Gold Corporation) undertook studies of existing underground reserves at the MacLeod-Cockshutt and neighbouring Hardrock mines and carried out lithogeochemical sampling (Gray, 1993). Starting in 1987, Lac conducted ground geophysical surveys, followed by 77 diamond drill holes, totaling approximately 50,000 feet. Targets, especially those with open pit potential, were investigated (e.g., Hardrock D and F; North and South Porphyry; Porphyry Hill zones). In 1992, Asarco Exploration Company of Canada Limited entered into a 5-year earn-in agreement with Lac and in 1993 carried out a program of reverse circulation overburden drilling and diamond drilling, the latter largely focusing on the near-surface portion of the F-zone and targets along the plunging nose of the albite porphyry (ibid). As a result of this work, a geological resource was calculated by Horvath (1993) for the Porphyry Hill, West and East pits: Pit Resource: 1,920,000 tons grading 0.079 ounce gold per ton (with strip ratio, including overburden, of 4.76 to 1) Ramp Resource: 1,160,000 tons grading 0.127 ounce gold per ton Asarco continued their exploration campaign into 1994, completing reverse circulation holes in overburden; sonic holes in tailings; and an additional 40,000 feet of diamond drilling, mostly on the aforementioned targets (Gray, 1994). Cyprus Canada Inc. assumed Asarco’s role in the Lac Minerals agreement in 1996 and drilled 24 holes, leading to the discovery of the B-zone (Mason and White, 1997). The agreement ended in 1997 and the properties have been dormant since then. Lac Minerals Properties Inc. began a rehabilitation program in 1996 that is still active. In the first description of the fledgling Little Long Lac (i.e. Geraldton) camp, Bruce (1935) proposed three types of gold deposits: (1) Shear zones in sedimentary rocks, along which narrow, but closely spaced quartz veins occur in parallel planes; (2) Irregular veins of quartz accompanied by pyrite, filling fractures in iron formation; and (3) Zones of pyritization and silicification in both in sedimentary and intrusive rocks. - 28 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 the porphyry and the adjacent metasedimentary rocks. The second ore type is exemplified by the North, North Limb, South Limb and West zones (Fig. 33). Ore consists of masses, lenses and tongues of almost massive sulphides that replace folded wall rock, particularly laminae of incompetent, sheared sandstone and magnetite beds in banded iron formation. Sulphides, principally pyrite and arsenopyrite, also replace rocks adjacent to steeply dipping, east-trending shear fractures or shallowly west-dipping, north-trending tension fractures. Fractures are commonly healed by late(r)stage quartz. It was reported that replacement-type ore averaged approximately 0.5 ounce gold per ton, while fracture-filling ore typically returned between 1 and 3 ounces gold per ton. p West -. BodieE!\._____ ....-. ... - VS Replacement Systems Albite Porphyry = Iron Sandstone— Argillite 9 6pm Figure 33. Vertical north-south section, MacLeod-Cockshutt Mine. After Horwood and Pye (1955). The third ore type, best exemplified by the Fzone, consists of numerous quartz veins and stringers localized within a wide zone of shearing and fracturing along the northern side of the albite porphyry. The veins and stringers range in width from < 1 to 10 cm. They appear to be largely confined to sandstone and lean BIF nearer to surface and occur in altered quartz diorite at lower levels to the west. Generally, quartz stringers are bordered by buff-coloured, sericitized and carbonatized sandstone with fine- to coarse-grained pyrite and subordinate arsenopyrite. In iron-bearing wall rock, sulphides are more predominant and may Six main ore zones were exploited at the MacLeodCockshutt Mine. Horwood and Pye (1955) classified the MacLeod-Cockshutt orebodies and zones of mineralization into three distinct types: (1) Quartz veins and mineralized zones (2) Irregular, massive sulphide-quartz lenses in a folded series of sandstone and banded iron formation (3) Quartz stringer zones in sandstone and subordinate quartz diorite. The first ore type, exemplified by the South, No. 210 Quartz Vein, No.’s 516, 517 and 519 Drift zones, is manifested by numerous but small lenses of semimassive sulphides (coarse pyrite + arsenopyrite, sphalerite and chalcopyrite) scattered along the contacts of intrusions and metasedimentary rocks; by disseminated, auriferous sulphides in porphyry; and by minor quartz-carbonate veins and stringers cutting both Figure 34. No. 1 Shaft headframe, MacLeod-Cockshutt Mine (Stop 2-1). - 29 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 secondary pyrite in association with iron carbonatization and quartz veining. Transpressive/oblique compression has produced open Z-folds and localized S-folds, with evidence of re-folding and late(r) transposition (Figs. 35 and 36). Consistently shallow (15° to 25°) plunges are observed on fold hinges. Lafrance et al. (2004) noted several, small isoclinal F1folds that were folded by parasitic F2 S-folds, resulting in a strong, rod-like, coaxial lineation that plunges 20° to 40° west, subparallel to the strike of the S2 axial-planar cleavage (100°/85° southwest) of regional folds. Figure 35. Deformed magnetite BIF, MacLeod-Cockshutt Mine (Stop 2-1; coin is 2 cm in diameter). The BIF consists of a finely parallel-laminated sequence of alternating, magnetite-rich layers and silt- to very fine sand-rich layers. The sand layers rarely show grading due to deformation, although some graded examples show increasing magnetite content in their clay-rich tops, which are gradational to the overlying, pure chemical sediment layer. On the microscopic scale, the magnetite layers have small amounts of chert and detrital minerals, but are otherwise mineralogically pure. Although deformation limits the depositional interpretations that can be drawn in this area, other exposures near Beardmore (e.g., Stop 1-6) indicate that the iron-rich sediments accumulated in the shore-proximal positions near the mouths of rivers in braid-delta complexes. Stop 2-2 - Porphyry Hill, Hard Rock Gold Mines Limited 4(j UTM coordinates - 0504810E 5502850N Figure 36. West-plunging folds in BIF, MacLeod-Cockshutt Mine (Stop #2-1). form small replacement lenses. Although the quartz veinlets contain visible gold, much gold is also associated with replacement pyrite and arsenopyrite. The recently exposed outcrop at the base of the No.1 shaft headframe (Fig. 34) displays highly deformed BIF and sandstone, part of a band that extends 600 m east onto the Hard Rock Mine property and hosts the North ore zone there (Stop 2-3). The steeply dipping BIF contains primary magnetite, but displays some In 1935, Hard Rock Gold Mines Limited began surface work and diamond drilling on the “Porphyry Hill” section of the property and discovered goldmineralized zones in and along the northern tongue of porphyry (Horwood and Pye, 1955). The discovery of scattered, finely disseminated pyrite and numerous, small quartz stringers prompted underground followup, resulting in the definition of the No. 1 and No. 2 vein systems. In addition, a small, high-grade shoot outcropped near the western property boundary. The X-vein system was defined underground (3.5 feet wide, 110 feet long, with an average grade of 0.049 ounce gold per ton), within an inclusion of sandstone and lean BIF in albite porphyry (ibid). Asarco’s proposed Porphyry Hill open pit was designed to recover shallow ore from several zones, including the F-zone (sandstone-BIF-hosted; northern porphyry contact-hosted); Porphyry zone (sheared - 30 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Stop 2-3 - Glory Hole BIF, Hard Rock Gold Mines Limited UTM coordinates - 0504862E 5503143N Deformed, gold-bearing BIF is exposed along the edges of a fenced-off, water-filled glory hole, just south of Highway 11 that represents the mined, near-surface portion of the Hard Rock North ore zone. BIF occurs with siltstone/sandstone, porphyry sills and diorite sills and dykes. This orebody consisted of numerous, irregularly shaped replacement lenses of sulphides distributed in an east-trending zone along the northern limb and in the trough of a minor synform having a plunge of approximately 30° west (Horwood and Pye, 1955). The open stope mimics the shape of a F3 Z-fold defined by folded BIF horizons (Macdonald, 1988). Individual lenses were not of great lateral and vertical extent, but had lengths of up to 300 feet and widths up to 30 feet. Collectively, these sulphide lenses formed an ore zone which persisted and plunged onto the Figure 37. Contact between albite porphyry and sandstone/ BIF, Porphyry Hill (Stop 2-2). porphyry-hosted); South Porphyry zone (southern porphyry contact-hosted) and the Southern Iron Formation zone. Horvath’s (1993) geological resource estimate for the Porphyry Hill open pit included an estimated high-grade resource of 1.02 million tons at a grade of 0.100 ounce gold per ton, and a low-grade resource of 0.48 million tons at a grade of 0.034 ounce gold per ton (for a total of 1.5 million tons grading 0.079 ounce gold per ton). Deformed porphyry dykes and sills intruding sandstone and BIF are exposed at this site (Fig. 37). Crowded albite and minor quartz phenocrysts occur in a fine-grained matrix of quartz, feldspar, sericite, calcite and chlorite. An age of 2698.6±1.3 Ma was obtained on a Au-mineralized “crowded albite” feldspar porphyry dyke at the Hard Rock Mine (Corfu, 2000). Figure 38. Oxide-facies banded iron formation (BIF) cut by massive sulphide (MS)-rich alteration envelope surrounding quatz-carbonate vein (QV), Hard Rock Mine (Stop 2-3). MacLeod-Cockshutt property to the west. This is the same BIF that is exposed below the No. 1 headframe (Stop 2-1), approximately 600 m west of this location. The so-called massive sulphide lenses contain up to 65% combined pyrite, arsenopyrite and pyrrhotite, the remainder being quartz. Minor sphalerite, chalcopyrite and galena, as well as accessory ankerite, calcite and tourmaline has also been noted. Replacement, controlled by quartz-filled fractures and bedding planes, is locally pervasive, destroying primary lithologic features (Fig. 38). Pervasive carbonatization accompanies replacement and gold mineralization. Replacement lenses are localized in zones of intense deformation - 31 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 W1iItFigure 39. Pre- and post-rehabilitation views of the MacLeodCockshutt tailings at the junction of Highways 11 and 584, showing new Heritage Interpretative Centre. along contacts between sandstone and BIF, and in the noses of small, asymmetric folds in an orientation consistent with the axial-planar cleavage in these folds (Anglin and Franklin, 1985). North zone ore mined to the mid-1940’s averaged approximately 0.25 ounce gold per ton. Stop (optional) - Geraldton Reclamation Project In the late 1990’s, Barrick Gold Corporation, in conjunction with the Town of Geraldton and government agencies embarked on a tailings reclamation project. This Tourism Development Project led to the development of the Heritage Interpretative Centre and a 9-hole expansion of the Kenogamisis Golf Course. The following description has been gleaned from the website of one of the designers (http://www.gsd. harvard.edu/people/faculty/schwartz/projects.html). In the Geraldton recalmation project, mine tailings were reshaped for both aesthetic and economic reasons (Fig. 39). 14 million tons of tailings from the mines covered 170 acres to a depth of 27 feet. In order to spur economic redevelopment, the Town made the decision to make something of the tailings by improving their appearance and adding opportunities for visitors. Design alternatives involved sculpting the flat pile into compelling sculptural landforms which serve as a dynamic roadway edge and a gateway to the town. Trails invite one to walk, bird watch, mountain bike, snow board, sled or snowmobile. Technical constraints were key to the final form of the earthwork. The different types and sizes of earth-moving equipment and their turning radii provided guidelines for the grading plan. A primary objective in the project was to balance cut and fill, and to maintain a maximum total earth moving of 150 000 cubic meters. Cut is kept to a minimum as arsenic levels are higher toward the bottom of the pile. There is a cap at the bottom of the pile of tailings, and there is a maximum of an additional 5 m that can occur on top. Standing water has been considered as a design element, but the water table has been respected by the re-grading. Storm drainage is maintained and the proposed earthwork does not impede sight lines for traffic safety. Six to twelve inches of peat topsoil were added to disturbed areas to aid in revegetation. A planting plan for the project focuses primarily on native grasses, especially those golden in color. The Geraldton project reveals the power of design to remake a wasteland into a new landscape. Even more than an earthwork, this landform is also a cultural artifact, highlighting the location and role of mining in the life of the town. Stop 2-4 - Conglomerate, Highway 11 UTM coordinates - 0502906E 5502908N Low outcrops on the south side of Highway 11 opposite the turnoff for the Consolidated Mosher Mine display deformed conglomerate, sandstone and BIF of the SMB within the BBLZ. As noted by Williams and Stott (1991), strain is exhibited by differentiated layering in pelitic rocks and elongated clasts in the conglomerates, especially in less-competent, mafic clasts (Fig. 40). Abundant, shallowly west- Figure 40. Sheared conglomerate in BBLZ, Highway 11 (Stop 2-4; Pen is 15 cm long.) - 32 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 plunging, Z-folds with attendant axial-planar fabrics, back-rotated pebbles, pressure fringes and intraclast fractures support the contention that the BBLZ is a major dextral, ductile shear zone (Williams and Stott, 1991). Lafrance et al. (2004) noted that the long axes of deformed granitic clasts in polymictic conglomerate are parallel to the axial plane of F3 folds. S2 strikes 100° in the sandstone matrix away from the clasts, but as it wraps around the clasts, it changes orientation to that of the elongated granitic clasts, resulting in a geometry similar to dextral, asymmetrical strain shadows around rigid objects (ibid). Polymictic, clast-supported, cobble-pebble conglomerate with deformed, lenticular lenses of deformed sandstone is interbedded with thicker, medium-grained sandstone sheets. They collectively represent a gravelly, braided fluvial system with sanddominated channels, separated by gravelly longitudinal bar complexes. The bars were active during flood events with small sand channels within the bars developing as chutes during falling stage. In these types of braided systems, more massive, coarser-grained gravels developed in the bar-head region of the bars. Bar-tails are often composed of interlayered sand and thin gravel layers, dissected by secondary channels. Stop 2-5 - Gabbro, Highway 11 UTM coordinates - 0502530E 5502860N Approximately 300 m west of Stop 2-4, outcrops of deformed gabbro lie on the south side of Highway 11, just east of a small pond. The amphibolitized, medium-grained gabbro is cut by a network of dark green, chloritic zones < 1 cm wide in which hornblende megacrysts are elongated. These anastomosing shear bands separate lozenges of relatively undeformed gabbro, imparting an disconcerting impression of a pillowed basalt flow. With increasing degrees of deformation, the shear bands become sub-parallel, forming a C fabric (Macdonald et al., 1990). In the most deformed rock, a third (C’) fabric develops at 45° to the C fabric. The northwestern orientation of C’ is indicative of dextral shear. Elsewhere on the outcrop, sharply bounded, metre-wide zones of fine-grained greenschist represent mylonitized gabbro (Williams and Stott, 1991). Buck and Williams (1984) considered such variations in the preservation of primary textures, versus the development of shear fabrics, to be a function of the inhomogeneity of strain and not of the protolith. Stop 2-6 - Missing Link Extension, Kinghorn Road UTM coordinates - 0466744E 5511220N The Missing Link Extension occurrence was discovered by Beardmore prospector Myron Nelson in 1986 and optioned to Freewest Resources Inc. in 1990 (Resident Geologist’s Files, Thunder Bay North District, Thunder Bay). It is situated on the west side of the Kinghorn Road, just north of the Paint Lake Deformaion Zone (PLDZ). The PLDZ is a 500 m-wide high-strain zone at or near the northern boundary of the Beardmore-Geraldton belt (Mackasey, 1976; Reilly, 1988; DeWolfe, 2002; Lafrance et al., 2004). The shear zone extends for over 40 km, from Lake Nipigon to the late, sinistral Jellicoe Fault, where it is offset by 500 m. The shear zone can be traced for ~ 9 km on the east side of the fault before it is lost under overburden. At the Missing Link property (1 km to the northeast; Stop 2-7), a northern splay/extension of the PLSZ has been exposed by mechanized stripping. At Paint Lake (DeWolfe, 2002), the PLSZ is parallel to lithological contacts between polymictic conglomerate (NMB) and volcanic rocks (NVB), and forms the boundary with the Onaman–Tashota belt to the north. Shear sense indicators are observed in both conglomerate and volcanic rocks. Several granitic clasts in conglomerate are bounded by dextral, asymmetrical strain shadows filled with quartz fibers. Steeply dipping dextral shear bands cut across S3 (Lafrance et al., 2004). The property straddles intermediate to mafic metavolcanic rocks on the north and clastic metasedimentary rocks to the south, separated by a major, east-striking structure interpreted to be the PLSZ (Resident Geologist’s Files, Thunder Bay North District, Thunder Bay). Stripping and trenching exposed two sub-parallel zones of alteration and mineralization within sheared and isoclinally folded andesite. The orientation of the major fold axis is generally subparallel to the shear fabric, which strikes between 080° and 090° and dips steeply south to vertically. The fold is believed to plunge shallowly to the east. The Main Zone was exposed over a strike length of 40 m and a width of 12 m. Sub-zones were recognized on the basis of variations in the relative amounts of chloritization, carbonatization, silicification, quartz-carbonate veining and sulphide minerals. Finely disseminated arsenopyrite, pyrite and chalcopyrite were noted. Values of up to 0.20 ounce gold per ton were returned - 33 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 variety of surveys were undertaken on the property by Beardmore prospector, Nolan Cox ca. 19861988. Older, undocumented trenches (ca. 1950’s) were discovered during the course of this work. The property straddles intermediate to mafic metavolcanic rocks on the north and clastic metasedimentary rocks to the south, separated by a major structure (at 080°) interpreted to be an extension or splay of the Paint Lake Shear Zone (PLSZ) (Lafrance et al., 2004). At the Missing Link, phyllonites (mafic metavolcanic protolith?) display intense microfolding of S3 on millimetre-scale, producing a fine L3’ crenulation lineation along S3 (ibid) (Figs. 41a,b ). Figure 41a. Crenulated and altered metavolcanic phyllonite with quartz-carbonate veins, Paint Lake Shear Zone, Missing Link property (Stop 2-7). IZ Figure 41b. Crenulated and altered metavolcanic phyllonite, Paint Lake Shear Zone, Missing Link property (Stop 2-7). in sulphide-rich grab samples. Stop 2-7 - Missing Link Property / Paint Lake Shear Zone UTM coordinates - 0467535E 5511479N The Missing Link occurrence was described by Mason et al. (1989). Stripping, prospecting and a DeWolfe et al. (2000) stated that gold mineralization and associated hydrothermal alteration were focused along second- and third-order fault splays off of the PLDZ. Second-order splays generally occur south of the PLDZ, often following lithologic contacts and refracting into an orientation sub-parallel to the PLDZ. Third-order splays are concentrated within the Northern Volcanic Sub-belt, where they are oriented both to the northeast and southeast. Gold mineralization at the Missing Link property is associated with sheared and altered mafic metavolcanic rocks. The deformation zone has been exposed over a width of 60 to 90 m by stripping. Disseminated, foliation-parallel, euhedral pyrite and arsenopyrite within zones of both shearing and quartz-carbonate veining, host gold. Pervasive carbonatization and more localized, erratically distributed silicification and hematitization have also been noted (Mason et al., 1989). Three zones (North Zone; Shaft Pit and Baseline Trench) were sampled by N. Cox. The North Zone strikes 085° and dips 61° south and has returned assay values of up to 0.256 ounce gold per ton over 1.8 m. Grab samples collected by Mason et al. (1989) from all the zones returned up to 0.14 ounce gold per ton. The property was optioned by Homestake Mineral Development Company in 1989, who conducted a number of geochemical, geological and geophysical surveys. Only low gold values (e.g. 2.06 g/t Au over 1.0 m) were returned and in 1990 the property was returned to the vendor (Resident Geologist’s Files, Thunder Bay North District, Thunder Bay). Placer Dome Inc. optioned the property in 1991 and completed just over 1000 m of drilling in four holes. Despite delineating a zone approximately 175 m wide within which intensely sheared and altered rocks occurred, no intersections of economic significance were encountered (ibid). - 34 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Stop 2-8 - Sandstone, Southern Sedimentary Subbelt, Highway 11 UTM coordinates - 0468062E 55005080N Thickly bedded, north-younging, overturned turbidites are exposed on the south side of Highway 11 opposite the Kinghorn Road turn-off . They display a variety of well-preserved sedimentary structures, including graded bedding, ball-and-pillow structures, flames and scours (Fig. 42). Some sandstone beds display a weakly developed cleavage. @) I © 1) 04 0) 0 N) :9 Figure 43. Stratigraphic column of sandstone outcrop (Stop 2-9; scale in metres). N) ri © a 01 ® a G 01 me ICr N) a I) C N) -— 0) 04 0) e melerS 0) ____ __ it — Figure 42. Load casts at base of sandstone (arrows), Highway 11 (Stop 2-8). This section of turbidites contains graded AE and AA beds up to 8m thick (Figs. 43 and 44). Most grading is in the basal few centimeters where the amount of very coarse sand grains decreases rapidly and in a zone approximately three-quarters of the way up through the bed where there is a rapid decrease from medium- to 0 - COG® - Figure 44. Measured sections of the turbidite assemblage in the Southern Metasedimentary Belt between Jelicoe and Geraldton. Note the extreme thickness of some of the beds. These units can be divided into a thin-bedded association, a medium-bedded association and a thick-bedded association. From Barrett and Fralick (1989). - 35 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 fine-grained sand. This results in beds that are quite monotonous, with little change in their grain size through most of their thickness. These sediments were transported by typical, high-density turbidity currents and grainflows which deposit sediment in sourceproximal locations. The fine-grained sand tops on the massive beds were possibly the result of resultant turbulence after the main turbidity current swept through the area. Currents moving water in behind the tail of the turbidity current would sweep sand from the surrounding bottom onto the coarser turbidity current deposits. Stop 2-9 - Pillowed Mafic Metavolcanic Rocks, Highway 11 UTM coordinates - 0460740E 5503325N Pillowed and massive, greenschist-facies, mafic metavolcanic rocks occur on the south side of Highway 11, 200 m west of the Jellicoe post office. They occur in a small, fault-bounded enclave of metavolcanic rocks with clastic metasedimentary rocks of the Southern Sedimentary Sub-belt (Bruce, 1937; Mackasey, 1976). They display a non-penetrative schistosity, on which there is a steeply plunging mineral and stretching lineation. Parallel to the schistosity are shear zones in which a non-penetrative fabric is forming at an angle to the zone, becoming parallel to it and more intense near the axis of the zone, characteristic of S-C fabric relationships (Williams and Stott, 1991). The shear sense based on this local relationship suggests south side-down. In graphitic metapelites and BIF exposed in road cuts 200 m west, the stretching lineation is well-developed. Acknowledgements The authors would like to thank John Mason and Gerry White (Resident Geologist’s Program, OGS, Thunder Bay) for providing property information and historical data. Greg Stott (Precambrian Geoscience, OGS, Sudbury) provided useful structural information and insights on the local tectonic framework. We would also like to acknowledge the support of the Municipality of Greenstone and local exploration companies, especially David Malouf (Roxmark Mines Limited). References Anglin, C.D., 1987. Geology, structure and geochemistry of gold mineralization in the Geraldton area, northwestern Ontario. Unpublished M.Sc. thesis, Memorial University, St Johns, Nfld. Anglin, C.D. and Franklin, J.M. 1985. Gold Mineralization in the Beardmore-Geraldton area of northwestern Ontario: structural considerations and the role of iron formation; in Current Research, Part A, Geological Survey of Canada, Paper 85-1A, p.193-201. Anglin, C.D. and Macdonald, A.J., 1984. Gold mineralization and an iron formation-bearing lithotectonic zone, Beardmore-Geraldton, Ontario; Geological Association of Canada - Mineralogical Association of Canada, Annual meeting, London Ont., Program with Abstracts, p.42. Anglin, C.D., Franklin, J.M., Loveridge, W.D., Hunt, P.A. and Osterberg, S.A.,1988. Use of zircon U-Pb ages of felsic intrusive and extrusive rocks in eastern Wabigoon subprovince, Ontario, to place constraints on base metal and gold mineralization; in Radiogenic age and isotope studies: Report 2, Geological Survey of Canada, Paper 88-2, p.109-115. Armstrong, H.S., 1943. Gold ores of the Little Long Lac area, Ontario; Economic Geology, v.38, p.215-246. Bardoux, M., Jébrak, M., Goulet, M., Morin, D., Giguère, C. and Zadeh, H. 1990. Metallogenesis of the Cadillac shear zone in the McWatters area, Abitibi belt, Canada; abstract in Proceedings, 8th IAGOD Symposium, in conjunction with Mineral deposit modeling international conference, Ottawa, Ontario, p.A260. Barrett, T.J. and Fralick, P.W., 1985. Sediment redeposition in Archean iron formation: examples from the Beardmore-Geraldton greenstone belt, Ontario; Journal of Sedimentary Petrology, vol. 55, p. 205212. Barrett, T.J. and Fralick, P.W., 1989. Turbidites and iron formation, Beardmore-Geraldton, Ontario: application of a combined ramp/fan model to Archean clastic and chemical sedimentation; Sedimentology, v.36, p. 221-234. Batiza, R. and White, J.D.L. 2000. Submarine lavas and hyaloclastite; in Encyclopedia of volcanoes, Academic Press, San Diego, California, p.361-381. Benedict, P.C. and Titcomb, J.A. 1948. Northern Empire Mine; in Structural geology of Canadian ore deposits, Canadian Institute of Mining and Metallurgy, Jubilee Volume, p.389-399. Bevan, P. A. 2004. Qualifying report on the Sand River / Leitch Mines with specific reference to the #16 Vein Systems, Beardmore Area, Northwestern Ontario, Canada; Roxmark Mines Limited, World Wide - 36 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 the Beardmore-Geraldton area of northern Ontario. Unpublished M.Sc. thesis, Lakehead University, Thunder Bay, Ontario, 227p. Website, http://www.roxmark.com, 35p. Boyle, R.W., 1969. Discussion of hydrothermal transport and deposition of gold;. Economic Geology, v.64, p.112-115. Boyle, R.W., 1976. Mineralization processes in Archean greenstone and sedimentary belts; Geological Survey of Canada, Paper 75-15. Boyle, R.W., 1979. The geochemistry of gold and its deposits. Geological Survey of Canada, Bulletin, v.280, 584 p. Bruce, E.L. 1935. Little Long Lac Gold area; Ontario Department of Mines, Forty-fourth Annual Report, v.XLIV, pt.III, 1935, 60p. Bruce, E.L., 1937a. New developments in the Little Long Lac area; Ontario Department of Mines, 45th Ann. Rept., v.45, pt. II, p.118-140. Bruce, E.L. 1937b. The eastern part of the Sturgeon River area (Jellicoe-Sturgeon River section); Ontario Department of Mines, Annual Report, v.45, pt.2, p.159. Bruce, E.L. and Laird, H.C. 1937. The western part of the Sturgeon River area; Ontario Department of Mines, Annual Report 1936, v.45, pt.2, p.60-117. Buck, S., and Williams, H.R., 1984. Structural studies in the Geraldton area; in Summary of Field Work 1984, Ontario Geological Survey, Miscellaneous Paper 119, p.208-211. Carter, M.W. 1987. Geology of McComber and Vincent townships, District of Thunder Bay; Ontario Geological Survey, Open File Report 5648, 144p. Cas, R.A.F. and Wright, J.V. 1987. Volcanic successions: modern and ancient; Allen and Unwin, London, United Kingdom, 528p. Coleman, A.P. 1907. Iron ranges east of Lake Nipigon; Ontario Bureau of Mines, Sixteenth Annual Report, v.XVI, pt.I, p.105-135. Colvine, A.C., Andrews, A.J., Cherry, M.E., Durocher, M.E., Fyon, A.J., Lavigne Jr., M.J., Macdonald, A.J., Marmont, S., Poulsen, K.H., Springer, J.S. and Troop, D.G. 1984. An integrated model for the origin of Archean lode gold deposits; Ontario Geological Survey, Open File Report 5524, 98p. Devaney, J.R. and Fralick, P.W. 1985. Regional sedimentology of the Namewaminikan Group, northern Ontario: Archean fluvial fans, braided rivers, deltas and an aquabasin; in Current Research, Part B, Geological Survey of Canada, Paper 85-1B, p.125-132. Devaney, J.R. and Williams, H.R., 1989. Evolution of an Archean subprovince boundary: A sedimentological and structural study of part of the Wabigoon-Quetico boundary in northern Ontario; Canadian Journal of Earth Sciences, v.26, p.1013-1026. DeWolfe, J.C., Lafrance, B. and Stott, G.M. 2000. Structurally controlled mesothermal gold mineralization in the western Beardmore-Geraldton belt; in Summary of Field Work and Other Activities 2000, Ontario Geological Survey, Open File Report 6032, p. 17-1 to 17-6. Dickinson, W.R. and Seely, D.R., 1979. Structure and stratigraphy of forearc regions; American Association of Petroleum Geology Bulletin, v.63, p.2-31. Dubé, B. and Guha, J. 1993. Factors controlling the occurrence of ferro-axinite within Archean goldcopper-rich quartz veins: Cooke mine, Chibougamau area, Abitibi greenstone belt; The Canadian Mineralogist, v.31, p.905-916. Durocher, M.E., and Hugon, H., 1983. Structural Geology and hydro-thermal alteration in the Flat Lake - Howey Bay deformation zone, Red Lake area; in Summary of Field Work 1983, Ontario Geological Survey, Miscellaneous Paper 116, p.216-219. Ferguson, S.A. 1967a. Leitch Gold Mines Limited: Surface plan of eastern part of property, parts of Eva and Summers townships, District of Thunder Bay; Ontario Department of Mines, Preliminary Map P.484, scale 1:6000. Ferguson, S.A. 1967b. Leitch Gold Mines Limited: Subsurface No.1, 900-foot level / Subsurface No.2, 4525-foot level, parts of Eva and Summers townships, District of Thunder Bay; Ontario Department of Mines, Preliminary Map P.485, scale 1:6000. Corfu. F., 2000. Extraction of Pb with artificially too-old ages during stepwise dissolution experiments on Archean zircon; Lithos, v.53, issue 3-4, p.279-291. Ferguson, S.A., Groen, H.A., and Haynes, R., 1971. Gold Deposits of Ontario, Part I, Districts of Algoma, Cochrane, Kenora, Rainy River, and Thunder Bay; Ontario Ministry of Natural Resources, Mineral Resources Circular, no.13, 315 p. Cowan, P. and Crocket, J.H., 1980. The gold content of interflow metasedimentary rocks from the Red Lake area, Ontario-A preliminary evaluation; Geological Survey of Canada, Paper 80-1B, p.189-133. Fralick, P.W., 2003. Geochemistry of clastic sedimentary rocks: ratio techniques; in Geochemistry of Sediments and Sedimentary Rocks, Geological Association of Canada, Geotext 4. Devaney, J.R., 1987. Sedimentology and stratigraphy of the northern and central metasedimentary belts in Fralick, P. and Barrett, T.J., 1991. Precambrian depositional systems along the southwestern edge of the - 37 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Superior Craton; Geological Association of Canada - Mineralogical Association of Canada - Society of Economic Geologists, Joint Annual Meeting, Toronto ‘91. Field Trip A3: Guidebook, 54 pp. Fralick, P.W. and Barrett, T.J., 1995. Depositional controls on iron formation associations in Canada; in Facies Analysis, Special Publication 22, International Association of Sedimentologists, p.137-156. Fralick, P.W. and Kronberg, B.I., 1997. Geochemical discrimination of clastic sedimentary rock sources; Sedimentary Geology, v.113, p.111-124. Fralick, P.W., Wu, J. and Williams, H.R., 1993. Trench and slope basin deposits in an Archean metasedimentary belt, Superior Province, Canadian Shield; Canadian Journal of Earth Sciences, v.29, p.2551-2557. Fryer, B.J., Kerrich, R., Hutchinson, R.W., Pierce, M.G., and Rodgers, D.S., 1979. Archean precious metal hydrothermal systems, Dome Mine, Abitibi Greenstone belt. I: Patterns of alteration and metal distribution. Canadian Journal of Earth Sciences, v.16, p.421-439. Gray, R.S. 1993. Report of work, Asarco Exploration Company of Canada Limited, Geraldton Project; unpublished OMIP report, Assessment Files, Thunder Bay North District, Thunder Bay, 5p. Gray, R.S. 1994. Geraldton Project, 1994 Exploration program, Asarco Exploration Company of Canada Limited; unpublished OMIP report, Assessment Files, Thunder Bay North District, Thunder Bay, 6p. Hart, T.R., terMeer, M. and Jolette, C. 2002. Precambrian geology of Kitto, Eva, Summers, Dorothea and Sandra townships, northwestern Ontario: Phoenix bedrock mapping project; Ontario Geological Survey, Open File Report 6095, 206p. Geraldton area; in Summary of Field Work 1983, Ontario Geological Survey, Miscellaneous Paper 116, p.201-203. Kerrich, R., 1981. Archean gold-bearing chemical sedimentary rocks and veins: a synthesis of stable isotope and geochemical relations; in Genesis of Archean, Volcanic-Hosted Gold Deposits; Ontario Geological Survey, Miscellaneous Paper 97, p.144167. Kerrich, R., and Allison, I., 1978. Vein geometry and hydrodynamics during Yellowknife mineralization; Canadian Journal of Earth Sciences, v.15, p.16531660. Kerrich, R., and Hodder, R.W., 1982. Archean lode gold and base metal deposits: evidence for metal separation into independent hydro-thermal systems; in Geology of Canadian Gold Deposits, Canadian Institute of Mining and Metallurgy, Special Volume 24, p.144160. Krauskoph, K.B., 1967. Introduction to Geochemistry. McGraw-Hill, New York, 721 p. Kresz, D. and Zayachivsky, B. 1989. Geology of Barbara, Meader and Pifher townships, District of Thunder Bay; Ontario Geological Survey, Report 270, 91p. Lafrance, B., DeWolfe, J.C. and Stott, G.M. 2004. A structural reappraisal of the Beardmore-Geraldton belt at the southern boundary of the Wabigoon Subprovince, Ontario, and implications for gold mineralization; Canadian Journal of Earth Sciences, v.41, p.217235. Laird, H.C. 1937. The western part of the Sturgeon River area (Sturgeon River-Beardmore section); Ontario Department of Mines, Forty-fifth annual report, v. XLV, pt.II, p.60-117. Hodgson, C.J., 1983. The structure and geological development of the Porcupine camp - a re-evalutation; in The Geology of Gold in Ontario, Ontario Geological Survey, Miscellaneous Paper 110, p.211-225. Lavigne, M.J. (in press). Structural geology and gold mineralization in the Geraldton area, BeardmoreGeraldton greenstone belt; Ontario Geological Survey, Open File Report. Hodgson, C.J., and MacGeehan, P.J., 1982. Geological characteristics of gold deposits in the Superior Province of the Canadian Shield; in Geology of Canadian Gold Deposits, Canadian Institute of Mining and Metallurgy, Special Volume 24, p.211229. Lavigne, M.J. 1983. Gold Deposits of the Geraldton area; in Summary of Field Work 1983, Ontario Geological Survey, Miscellaneous Paper 116, p.198-200. Horvath, A.S. 1993. Ore reserve estimate, Geraldton Project, 1993, Asarco Exploration Company of Canada Limited; unpublished OMIP report, Assessment Files, Thunder Bay North District, Thunder Bay, 7p. Horwood, H.C., and Pye, E.G., 1955. Geology of Ashmore Township, Little Long Lac area, Thunder Bay District; Ontario Department of Mines, Annual Report, v.60, pt.5, 105p. Kehlenbeck, M.M., 1983. Structural studies in the Beardmore- Lesher, C.M., Goodwin, A.M., Campbell, I.H. and Gorton, M.P. 1986. Trace-element geochemistry of oreassociated and barren, felsic metavolcanic rocks in the Superior Province, Canada; Canadian Journal of Earth Sciences, v.23, p.222-237. Macdonald, A.J., 1982. The MacLeod-Cockshutt and Hard Rock mines, Geraldton: examples of an iron formation-related gold deposit; in Summary of Field Work 1982, Ontario Geological Survey, Miscellaneous Paper 106, p.188-191. Macdonald, A.J., 1983a. The iron formation-gold association evidence from Geraldton area; in The Geology - 38 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 of Gold in Ontario, Ontario Geological Survey, Miscellaneous Paper 110, p.75-83. Australia; Geological Society of Australia Journal, v.30, p.25-39. Macdonald, A.J., 1983b. A re-appraisal of the Geraldton gold camp; in Summary of Field Work 1983, Ontario Geological Survey, Miscellaneous Paper 116, p.194197. Pirie, J. and Mackasey, W.O. 1978: Preliminary examination of regional metamorphism in parts of Quetico metasedimentary belt, Superior Province, Ontario; in Metamorphism in the Canadian Shield, Geological Survey of Canada, Paper 78-10, p. 37-48. Macdonald, A.J. 1984. Gold mineralization in Ontario, I: The role of banded iron formation; Canadian Institute of Mining and Metallurgy, Special Volume 34, p.412430. MacDonald, R.D. 1940. Geology of the Kenogamisis River area; Ontario Department of Mines, Annual Report 1940, v.49, pt.7, p.1-17. Mackasey, W.O. 1975. Geology of Dorothea, Sandra and Irwin Townships, District of Thunder Bay; Ontario Division of Mines, Geological Report 122, 83p. Mackasey, W.O. 1976. Geology of Walters and Leduc Townships, District of Thunder Bay. Ontario Division of Mines, Geological Report 149. 58p. Mackasey, W.O., Edwards, G.R. and Cape, D.F. 1976. Legault Township, District of Thunder Bay; Ontario Division of Mines, Preliminary Map P.1191, scale 1:15 840. Mason, J.K. and McConnell, C.D. 1983. Gold mineralization in the Beardmore-Geraldton area; in The geology of gold in Ontario, Ontario Geological Survey, Miscellaneous Paper 110, p.84-97. Mason, J.K. and White, G.D. 1986. Gold occurrences, prospects and deposits of the Beardmore-Geraldton area; Ontario Geological Survey, Open File Report 5630, 680p. Mason, J.K. and White, G.D. 1997. Beardmore-Geraldton Resident Geologist’s District 1996; Ontario Geological Survey, Open File Report 5958, p.1-1 to 1-22. Mason, J.K., White, G.D. and McConnell, C. 1985. Field guide to the Beardmore-Geraldton metasedimentary – metavolcanic belt; Ontario Geological Survey, Open File Report 5538, 73p Neumayr, P., Walshe, J., Connors, K., Cox, S., Morrison, R. S., and Stolz, E. 2004. Gold mineralization in the St Ives camp near Kambalda; Geological Survey of Western Australia, Record 2004/16, p.23-45. O’Brien, M.S. 1985. A detailed geological study of the mafic metavolcanic rocks north of Poplar Lodge, Ontario; unpublished H.B.Sc. thesis, Lakehead University, Thunder Bay, Ontario, 76p. Poulsen, K.H., 1983. Structural setting of vein-type gold mineralization in the Mine Centre-Fort Frances Area: implications for the Wabigoon Subprovince; in The Geology of Gold in Ontario, Ontario Geological Survey, Miscellaneous Paper 110, p.174-180. Purdon, R.H. 1995. Lithostratigraphy and provenance of the Neoarchean McKellar Harbour sequence, Superior Province, Ontario, Canada; unpublished M.Sc. thesis, Lakehead University, Thunder Bay, Ontario, 172p. Pye, E.G. 1952. Geology of Errington Township, Little Long Lac area; Ontario Department of Mines, Annual Report, v.60, pt.6, 140 p. Reilly, B.A. 1988. Structural analysis of the Paint Lake Deformation Zone, northern Ontario. M.Sc. thesis, Brock University, St. Catharines, Ontario. Rigg, D.M., and Helmstaedt, H., 1981. Relationships between structure and gold mineralization in Campbell Red Lake and Dickenson Mines, Red Lake Area, Ontario; in Genesis of Archean, Volcanic Hosted Gold Deposits, Ontario Geological Survey, Miscellaneous Paper 97, p.111-127. Rogers J. R., Joyce, K. A., Masterman, G. J., Halley, S. W., and Walshe, J. L. 2004. Kanowna Belle gold mine; Geological Survey of Western Australia, Record 2004/16, p.47-51. Scott, B.M., 1985. A study of the auriferous quartz veins found within the oxide facies banded iron formations of the Beardmore-Geraldton area of northern Ontario; Unpublished H.B.Sc. thesis, Lakehead University, Thunder Bay, Ontario, 96 p. Shanks, W.S. 1993. Geology of Eva and Summer Townships, District of Thunder Bay. Ontario Geological Survey, Open File Report 5821. Sibson, R.H., Moore, J.M., and Rankin, A.H., 1975. Seismic pumping - a hydrothermal fluid transport mechanism; Journal of the Geological Society of London, v.131, p.191-213. Ozaki, M. 1972. Chemical composition and occurrence of axinite; Kumamoto Journal of Scientific Geology, v.9, p.1-34. Stott, G.M., and Davis, D. 1999. Contributions to the tectonostratigraphic analysis of the Onaman–Tashota greenstone belt, eastern Wabigoon Subprovince; in 1999 Western Superior Transect 5th Annual Workshop. Report 70, Lithoprobe Secretariat, The University of British Columbia, Vancouver, B.C., p.122–123. Phillips, G.N., and Groves, D.I., 1983. The nature of Archean gold fluids as deduced from gold deposits in Western Stott, G.M., and Schneiders, B.R., 1983. Gold mineralization in the Shebandowan Belt and in relation to regional - 39 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 deformation patterns; in The Geology of Gold in Ontario, Ontario Geological Survey, Miscellaneous Paper 110, p.181-193. Tomlinson, K.Y. 1996. The geochemistry and tectonic setting of Early Precambrian greenstone belts, northern Ontario, Canada; unpublished Ph.D. thesis, University of Portsmouth, Portsmouth, United Kingdom, 287p. Tomlinson, K.Y., Hall, R.P, Hughes, D.J. and Thurston, P.C. 1993. Mafic metavolcanic rocks of the BeardmoreGeraldton greenstone belt: Preliminary geochemical findings; in Summary of Field Work and Other Activities, Ontario Geological Survey, Miscellaneous Paper 162, p.54-58. Williams, H.R., 1986. Structural studies in the BeardmoreGeraldton belt, northern Ontario; in Summary of Field Work 1986, Ontario Geological Survey, Miscellaneous Paper 130, p.138-146. Williams, H.R., 1987. Structural studies in the BeardmoreGeraldton belt and in the Quetico and Wawa subprovinces; in Summary of Field Work 1987, Ontario Geological Survey, Miscellaneous Paper 137, p.90-92. Williams, H.R. and Stott, G.M., 1991. Subprovince accretion in the southern Superior Province. G.A.C.-M.A.C. annual Meeting, Toronto, Field trip B6, Guidebook, 26 p. Wood, D.A. 1980. The application of a Th–Hf–Ta diagram to problems of tectonomagmatic classification and to establishing the nature of crustal contamination of basaltic lavas of the British Tertiary volcanic province; Earth and Planetary Science Letters, v.50, p.11-30. - 40 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Fieldtrip 2 – Quaternary geology of the Beardmore – Nipigon area Peter J. Barnett Ontario Geological Survey, Ministry of Northern Development and Mines, Sudbury, Ontario, P3E 6B5, Canada Introduction then returns to Nipigon along the same route (Fig. 1). In association with the International Quaternary Association in 1987 (INQUA’87) an excursion quidebook C-12: Quaternary features and scenery along the North Shore of Lake Superior was created (Geddes et al., 1987). Unfortunately the trip was never run. This current field trip and field trip guide utilizes much of the information presented in the INQUA’87 field trip guide particularly as it pertains to the Beardmore – Nipigon area. The route follows the east branch of the Nipigon River, initially across the Nipigon lowlands then along the Pijitawabik Canyon cut through Nipigon Sills and onto more-typical Canadian Shield terrain north of Beardmore. Although this part of Ontario is pivotal to many large-scale glacial histories and theories of the Great Lakes Region, North America and global climate change, very little if any detailed mapping of the Quaternary geology has been done. One exception is the work of Thorleifson and Kristjansson (1993) in the very northern part of the field trip area (scale 1:100,000). In addition, Mollard and Mollard (1981) undertook a terrain analysis (scale 1:100,000) of a large part of the field trip area and produced a map of some of the major landforms and material types that occur there. The field trip area has been displayed on several regional maps including Zoltai (1965a), Sado and Carswell (1987), Barnett et al. (1991) and Sado et al. (1994). In addition, regional studies on selected aspects of interest to Quaternary researchers, particularly associated with ancestral lake features and deposits (e.g., Farrand, 1960; Teller and Thorleifson, 1983, 1987; Teller and Mahnic, 1987) have been done. Zoltai (1965b) described and discussed the formation of some of the major landforms and deposits of the Nipigon area. Lake Nipigon During the trip we will visit several sites and landforms that display clues to the glacial history of the area and hopefully discussions on how they relate, support or conflict, with several of the regional histories proposed in the literature will ensue. The fieldtrip route basically follows Highway 11 north from Nipigon to Beardmore and then northwest on Highway 580 to Poplar Lodge on Lake Nipigon and Figure 1. Location, route and stops. Rectangle is for reference between the various regional figures. - 41 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Regional geological setting During the fieldtrip we will be travelling between Lake Superior and Lake Nipigon following a now abandoned drainage way used during deglaciation between these two lakes. The present drainage divide is located between McKirdy and Shamrock Lake within the Pijitawabik Canyon. Keemle Creek via a series of small lakes drains northward into Pijitawabik Bay of Lake Nipigon (approximately 260 m or 852 ft asl). An unnamed east branch of Cash Creek drains southward from McKirdy Lake into Cash Creek and onto Helen Lake and then into the Nipigon River to Lake Superior (approximately 183 m or 600 ft asl). The main branch of the Nipigon River is the present-day outlet of Lake Nipigon. The fieldtrip area, located on the Canadian Shield, is an irregular, subdued, gently rolling, bedrockdominated terrain underlain by Precambrian igneous and metamorphic rocks (Figs. 2, 3). The area immediately south of Lake Nipigon, however, is not typical shield terrain. It has been referred to as the Nipigon Plain by Bostock (1970) and “is formed on nearly flat-lying Proterozoic gabbro sills and sediments surrounding Lake Nipigon” (Bostock, 1970, p.16). Combined past tectonic activity, differential weathering and glacial/ glacial meltwater erosion of the Proterozoic sills and Sibley Group metasedimentary rock surfaces has produced a unique landscape of buttes and mesas and steep-walled valleys along the margins of the Nipigon Plain. To the east of Lake Nipigon, beyond the extent of the sills, in the area underlain by Archean igneous Nipigon diabase sills Sibley Op. j1 nepheline syenite suite III1 granodiorite to granite 2' granitic rock Metasedinientary rocks Felsic metavolcanic rocks Mafic metavolcanic rocks Figure 2. Fused digital elevation model (DEM) and hillshaded DEM. Rectangle is for reference between the various regional figures. Figure 3. Bedrock geology of the fieldtrip area (OGS 2003). - 42 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 and metamorphic rocks, more typical Shield terrain is prevalent. Farrand (1960) has referred to the southern part of the fieldtrip area as the Nipigon Lowlands. Here, the irregular surface of the Canadian Shield has been subdued and partially infilled by sand, silt and clay of glaciolacustrine origin producing a broad dissected plain. Farrand (1960) recognized at least 4 terraces or steps within this lowland at various elevations (692, 800, 907-912, 950-952 and 980 ft or about 211, 244, 276-278, 290, 299 m) that he related to ancestral lake levels in the Lake Superior basin. Relief over the entire region exceeds 350 m (about 1150 ft). Local relief in areas of Archean rocks is commonly between 15 and 50 m (50 and 165 ft). Along the eroded edges of sills that surround the Nipigon Plain it can exceed 100 m (about 328 ft). Local relief of over 100 m (about 328 ft) also exists along the walls of the Pijitawabik Canyon. Bedrock geology The study area contains rocks of the Superior Craton and the Southern Province (Fig. 3). Major geological subdivisions include the Wabigoon and Quetico subprovinces (Blackburn et al., 1991; Williams, 1991), the Proterozoic Sibley Group and Nipigon Diabase Sills (Sutcliffe, 1991). Metasedimentary and granitic rocks dominate the rocks of the Quetico Subprovince in the southern part of the fieldtrip area. Mafic metavolcanic rocks dominate the Wabigoon Subprovince with granodiorite to granite intrusions forming the inter-greenstone belt parts. Diabase/gabbo in the form of sills and dikes is the dominant rock type in the Nipigon embayment or the north-central part of the fieldtrip area (Fig. 3). Nipigon sills are massive bodies that vary in thickness from 150 to 200 m. Internal textural zoning indicates that the sills are single cooling bodies (Blackburn et al., 1991). The Nipigon sills in the area are very similar texturally and monotonous in appearance. They overlie and interfinger with Sibley Group sedimentary rocks that outcrop or subcrop within the southern part of the Nipigon embayment. The Nipigon sills formed contemporaneous with the initiation of the Lake Superior Midcontinent rift age-dated at approximately 1108 Ma (Blackburn et al., 1991). With the possible exceptions of some lateral and vertical outliers the age of the Sibley is constrained between the youngest detrital zircon in a sample from the Kama Hill Formation (1420 Ma; L. Heaman pers. comm. 2004) and a Rb-Sr isochron from authogenic minerals in the Rossport and Kama Hill Formations (1339 Ma; Franklin, 1978). The Sibley consists of unmetamorphosed red-bed sequences consisting of conglomerates, sandstones and shales. They are believed to have accumulated in a subsiding graben prior to the formation of the midcontinent rift (Sutcliffe, 1991). Greenstone belts east of Lake Nipigon are part of the Eastern Wabigoon Subprovince and have been traced beneath the diabase and sedimentary units (Blackburn, 1991; Smyk et al., 2005). Quaternary geology The surficial (Quaternary) geology of the field trip area south and east of Lake Nipigon has been investigated only at a regional scale despite its importance to the overall understanding of the glacial history of Ontario and in particular to mineral exploration using unconsolidated sediments (drift exploration). Regional maps at various scales that cover the area include Zoltai (1965a), Sado and Carswell (1987), Barnett et al. (1991) and Sado et al. (1994). Zoltai (1965b) described and discussed the formation of some of the major landforms and deposits of the QueticoNipigon area. Mollard and Mollard (1981) undertook a terrain analysis of the study area and produced maps of some of the major landforms and material types that occur there (Fig. 4). Several theses and papers have been written on significant aspects or features of Quaternary age from within the area of interest (Farrand, 1960; Teller and Thorleifson, 1983, 1987; Teller and Mahnic, 1987). Zoltai (1965a, 1965b), Elson (1967), Prest (1970), Dredge and Cowan (1989) Barnett (1991) and Sado et al. (1994) have summarized regional concepts of the glacial history surrounding Lake Nipigon. There is very little agreement as to the details of the sequence and timing of events. The deposits in the area probably represent ice movement during the Michigan Subepisode (Late Wisconsinan), however; it may reflect ice cover during the entire Wisconsin Episode or approximately the last 100,000 years. The Laurentide Ice Sheet flowed southwestward during its maximum extent about 20,000 years - 43 - �r re-advance to produce most of the major moraines in northwestern Ontario. The Marks, Mackenzie and Dog Lake moraines have been suggested to have formed at about the same time, about 10,000 years ago (Burwasser, 1977; Drexler et al., 1983) and are probably correlative to the Grand Marais Moraines that formed during the Marquette advance (Drexler et al., 1983). With further ice margin recession and a suggested major re-advance, the Lac Seul moraine formed west of the fieldtrip area (Prest 1970). Zoltai (1965b) suggested that this ice margin formed the northern part of the Kaiashk Interlobate Moraine (Fig. 5). — — a 4. ,m Proceedings of the 51st ILSG Annual Meeting - Part 2 Prest (1970) suggested that with further ice margin retreat, the Sioux Lookout and Whitewater moraines formed west of Lake Nipigon. These events were followed by a westward advance in the Lake Nipigon basin that produced the Nipigon Moraine (Stop 1) along the west and southern side of the lake (Zoltai, 1965a, b). Prest (1970) suggests that only the northern part of this moraine formed following the Whitewater Moraine and that the southern part is older. Zoltai (1965a, b) identified several other moraines east of the Nipigon Moraine in the Lake Nipigon basin that appear to be younger than the Nipigon Moraine. The Nakina Moraines that are located along the northeastern edge of the Lake Nipigon basin mark the last significant ice margin positions to directly affect the region. To complicate the history even more is the suggestion that glacial Lake Agassiz waters drained catastrophically into the Nipigon/Superior basin several Figure 4. Surficial sediments in the fieldtrip area (modified from Mollard and Mollard, 1981). ago. During deglaciation and possibly during initial ice movement into the area, ice flow direction was controlled on a regional scale by topography. During these times ice flow followed broad topographic lows, especially the Great Lakes basins and most likely Lake Nipigon, producing lobate ice margins that could have possibly behaved somewhat independently (Fig. 5). Prest (1970), following the work of Zoltai (1965b), suggested that an active ice margin in northwestern Ontario fronted glacial Lake Agassiz during ice margin retreat. They (Zoltai, 1965b; Prest, 1970) postulated several alternating stages of ice margin retreat and NbItt 2. . It k BI Ck M@! 3. IC k Mk d Lk gk!& ph d 3.4.7.8 k Kbk LIttbfl 9. k Figure 5. Ice flow direction and ice margins in the Lake Nipigon area (Sado et al., 1994). - 44 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 _________ ________ ______________ _____________ __________ AC4SSIZ EASTERN LAKE SUPERIOR OUTLETS PHASES LAKE MICHIGAN ID LAKE HURON - EARLY NIPISSING (rising): SAULT STRAIT: EARLY NIPISSING 7000 4 SAULT RAPIDS HOUGHTON — (bedrock) CHIPPEWA- STANLEY 8000 8000 I ION PROTO- (toIling) P11<1.— ST MARYS RIVER riS,rI --b (downcutting through drift) KAI. — rising MINONG POST )..::Y.cM!l!f.::: 0 8 I AU TRAIN - • ESCANADA GRAND MARAIS 10,000 PROTOST MARYS -—b RIVER MLNONQI 10,000 CIIIPPEWA / EARLY — — UPPER GROUP STANLEY SHEGUIANDAH SKORAM PAYETTE through 2 ALGONQUIN 1!000 '' 9000 PoRTAGE1jJ::RCuPINE 11,000 Tt?'"-.,, fl750 Figure 6. Chart of ice-sheet fluctuations and glacial lake phases (from Farrand and Drexler, 1985). times during the deglaciation of the Lake Nipigon and Superior basins. Farrand and Drexler (1985) produced a chart summarizing some of the ice sheet fluctuations and their relation to glacial lake phases in northwestern Ontario and the upper Great Lakes basins (Fig. 6). Agassiz drainage into the Nipigon and Superior basins occurred both before and after the Marquette maximum (Farrand and Drexler, 1985). A network of 5 groups of channels (Teller and Thorleifson, 1983, 1987) carried drainage from glacial Lake Agassiz following the Marquette advance (Fig. 7). And “as postulated by Teller and Thorleifson (1983), the outflow through the Nipigon channels must have been catastrophic” (Farrand and Drexler, 1985, p.23). These “catastrophic discharges from Lake Agassiz were responsible for cutting down the Lake Minong sill” (Farrand and Drexler, 1985, p.17). The Nipigon Moraine marks a former ice margin located along the north shore of Lake Superior and has been traced along the southern and western side of Lake Nipigon (Fig. 5). It is commonly associated with ancestral glacial Lake Minong and as such believed Figure 7. Eastern outlets of Lake Agassiz. Outlet number 26 is the Pijitawabik Canyon (from Teller and Thorleifson, 1983). - 45 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 to have formed about 9500 years ago (Farrand and Drexler, 1985). Farrand’s regional work on abandoned shoreline features along the north shore of Lake Superior (Farrand, 1960) is the most comprehensive field study on shorelines to date in the area (Figure 8). Farrand (1960) identified evidence of glacial Lake Minong about 25 km north of the Nipigon Moraine (Stop 3). And recently Slattery (2003) identified deltas that formed along the Nakina Moraines built to Minong III (?) or post-Minong levels at elevations of 360 m in the Longlac area. If catastrophic Agassiz drainage carved the outlet at St Marys River, why do shore features of Minong or fairly high Post Minong levels exist well behind the Nipigon Moraine, a feature that in part post-dates the opening or use of many of the earlier eastern outlets? The proposed eastern outlet events and features were based on several coarse-grained deposits associated with the large deep steep-walled valleys that occur throughout the region (Teller and Thorleifson, 1983, 1987). The Pijitawabik Canyon (Stop 3, Outlet number 26, Fig. 7) was proposed as one of the passageways for this meltwater (Teller and Thorleifson, 1983, 1987) and the Pijitawabik delta (Stop 2) was inferred to have formed during Agassiz drainage events (Geddes et al., 1987). An alternative explanation for the development of most, if not all, the steep-walled valleys (e.g., Pijitawabik Canyon) is that they were initially formed in part by differential erosion along fractures and faults within the diabase/gabbro sills prior to glaciation. The valleys were then subsequently carved by subglacial meltwater floods based on the similarities of this channelled landscape to that of the Channelled Scablands of eastern Washington State (Baker et al., 1987). Many of these valleys contain glacial sediments indicating that the bedrock channels themselves were not carved by postglacial Lake Agassiz drainage events, but already existed. Agassiz outflow may have reoccupied some of these steep-walled channels that were carved by the subglacial meltwater flood(s). The complex and still poorly understood deglacial history of the Nipigon basin does not make drift exploration easy. The events and features discussed above could have resulted in many areas having multiple directions of ice movement due to the shifting of ice centers through time that will require linking evidence of ice movement to specific tills (Fig. 4). Determining the history of meltwater and ancestral lakes in the study area is also very important to interpret the results of drift exploration programs. Studies of tills deposited in the BeardmoreGeraldton area (Thorleifson and Kristjansson, 1993) and in the Hemlo-Matheson-White River area (Geddes and Kristjansson, 1986; Karrow and Geddes, 1987; Hicock, 1988) have brought to light another factor that affects mineral exploration using till (drift exploration) in the fieldtrip area. The till in the Beardmore area appears for the most part to have been deposited subglacially by an actively flowing glacier (ideal for mineral exploration using till). However, where the till is thick it is commonly rich in clasts of Paleozoic carbonate and Proterozoic metasedimentary rocks derived from Hudson Bay Lowlands some 100 km or greater away (Stop 6). The content of carbonate in the till matrix is also high (up to 50%; Thorleifson and Kristjansson, 1993). This “exotic” till tends to grade into locally derived till (Stop 4) at surface in the area surrounding the community of Beardmore. Thorleifson and Kristjansson (1993) attribute the abundance of far-traveled debris to: 1) the high susceptibility of carbonate rocks to erosion 2) low erodibility of granitic rock which outcrop between the Beardmore area and the Hudson Bay Lowlands 3) the short distance of transport over the erodible greenstone belt rocks and 4) a zone of vigorous ice flow (ice streaming). Thorleifson and Kristjansson (1993) suggested that an ice stream would be required to produce and distribute the carbonate-rich or exotic till to the Beardmore-Geraldton area based in part on the regional distribution in a radiating tongue parallel to ice flow. They also suggest that the exotic till was transported beneath the glacier by pervasive shear (deformable bed). They argued that “deposition by lodgement of basal debris seem contradicted by carbonate clast preservation” (Thorleifson and Kristjansson, 1993, p.57). Hicock (1988), however, records evidence within exotic till exposures of discrete not pervasive shear that would suggest lodgement as the process of deposition, although, he also suggested ice streaming to produce the exotic till. - 46 - Geddes and Kristjansson (1986) suggested that �Proceedings of the 51st ILSG Annual Meeting - Part 2 Stops englacial transport of the carbonate debris over long distances was required. Hicock (1986) suggested that englacial load under dynamic ice conditions can be transferred to the glacier bed in the leeside of upland settings. Stop 1 – Nipigon Moraine (J. Nichols Sand and Gravel, Nipigon) UTM coordinates – 0406748E 5430513N Regardless of the mechanism of entrainment, transport and deposition of the carbonate-rich till, it has been suggested that the “standard rules of glacial comminution as applied in mineral exploration may not be applicable” (Karrow and Geddes, 1987, p.368). And that care be taken to recognize and avoid this exotic till when sampling for mineral exploration purposes (Thorleifson and Kristjansson, 1993). Note - permission from the land owner is required for access to this stop. The Nipigon Moraine marks a former ice margin of the Laurentide Ice Sheet that fronted a large glacial lake. It is generally accepted that this lake was glacial Lake Minong and that its formation occurred about 9500 yr BP (Farrand and Drexler, 1985). In the Nipigon area it is a discontinuous ridge that rises more than 40 m above the surrounding lake plains. Elevation at the site is approximately 280 m asl. The ridge rises to the west where it becomes flat-topped at an elevation of about 295 - 296 m asl. The form here does not appear to be primary (i.e. delta) but the result of subsequent erosion and deposition (wave cut and wave built platform). A lower plain occurs to the north of the moraine here at about 290 m asl. These features raise the question of what was the actual water level in the Superior basin during the formation of the Nipigon Moraine. The field trip stops were chosen to give the participants an overview of the Quaternary geology of the area between Lake Superior and Lake Nipigon and to introduce them to some remarkable landforms and scenery that exists in this part of Ontario. For the INQUA’87 fieldtrip guide, Geddes et al. (1987) prepared a figure showing the profiles of water planes and shoreline features recognized by Farrand (1960) in the Lake Superior basin between Thunder Bay and Kama Bay. This figure appears as Figure 9 with a few modifications for reference purposes. An ice marginal feature built along the Nipigon NE SW KILOMETRES NORTH OE AGASSIZ OUTLET -J 200 taJ —300 .J C Iii -250 In 0, —200 I-ISO 400 — L.a 0 a Figure 8. Shoreline-relation diagrams and isobases of glacioisostatic rebound (modified 8. Shoreline-relation and general isobases of from Figure Farrand and Drexler, diagrams 1985). A) glacioisostatic rebound (modified from Farrand and diagram emphasizing glacial retreat (Farrand, Drexler 1985) A) general diagram emphasizing retreat 1969). B) Diagram showingglacial relationships (Farrand 1969) between Lakes Superior and Nipigon (Teller B) diagram showing lakes Superior-Nipigon and relationships Thorleifson, 1983). C) Isobases of (Teller and Thorleifson 1983) C) isobases of rebound glacioisostatic for Washburn glacioisostatic forrebound Washburn and and Minong (Fanand 1960). Minong (Farand, 1960) - 47 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 PROFILES OF FORMER WATER PLANES BETWEEN THUNDER BAY AND NIPIGON (after Parrand 1960) io o I m S 80 60 40 20 0 e I I 50 40 30 20 I I It I I a Ifl •er t 0 S . r —1 S — — — —— 5— - 0 'r-• — =i_— I * • * 0 -. -c r0 I II — • r •—• I S I I 0 •V 0 r m r r 5• 5— • C t 5• - ———... —S •lI* i 00 C I 0 -I In In , S jI 0r I 0 •j_ UGHT0N.- •p_• — 'NIPISSIN0 ALGOMA S .—4.-—--.———— I 3 •___.____1_._. ——4- — . —.—-— •—• • S ION i—-— 55 5 • —e — —;__• S 5•— a, — • -T--I I r m fli I z 0 0 — * S C S In 0 z Figure 9. Profiles of former water planes in the Lake Superior basin from Thunder Bay and Kama Bay (after Farrand, 1960; Geddes et al., 1987). Figure 9. Profiles of former water planes in the Lake Superior basin from Thuder Hay and Kama Bay (after Farrand 1960, Geddes et al. 1987) Moraine south east of Black Sturgeon Lake that has a delta form occurs at an elevation of between 300 and 304 m. This feature would fall near to the isobase passing through Nipigon, essentially making the diagram one isobar off (Fig. 8C). Shore features identified as ice-marginal deltas at Marathon and Wawa, that were reported to be built into glacial Lake Minong (Geddes et al., 1987), may not be deltas at all, but subaqueous fans later planed off by waves of a subsequent lake. This is the case at Wawa (S. Slattery, personal communications, 2003). The sedimentology of the feature at Marathon remains to be studied in detail before such a conclusion can also be made but preliminary observations are not consistent with a deltaic origin. The Jim Nichols Trucking Limited sand and gravel pit provides a look at the internal structures of the sediments making up the Nipigon Moraine. In general he ridge here is made up of a fining upward and outward sequence of stratified sand and gravel. The lower part or core is composed of steeply dipping beds of gravel and sand (Fig. 10). These avalanche or foreset beds probably formed during rapid growth by sediment gravity flow processes. The beds generally dip to the south or across the ridge form. Trough cross-bedded (dunes) sand, pebbly sand and pebble gravel overlie the steeply dipping beds. The aforementioned sediment sequence is common in subaqueous fans (Shaw, 1985). Near the top and along the ice-proximal side (northside) minor flowtill lenses are present. Faults and postdepositional slumping are common along the proximal side of the ridge likely formed following the removal of the glacier support and sediment supply. The sequence is interpreted as an ice marginal subaqueous fan. If so, it only places a minimum (280 m asl) for the lake that existed during its formation. As mentioned previously, the water here during formation could be up to 20 m deep. A shorebluff and bouldery gravel beach bar of a younger lake occurs along the roadway at the entrance of the pit at approximately 250 m asl (Figs. 10 and 11). * Alternative stop - look-out over community of Nipigon from the Nipigon Moraine (UTM coordinates – 0407343E 5430274N). - 48 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 w Profile 260 broad shallow channel /sulting shoreline a £ C' = 240 0 a fining 220 •I.. N 4flntng 260 decrease dip of beds 200 U a i6oo 500 0 C a isào Distance (ni) D N —4 — - H -- im -- -I--'_- B Figure 10. Internal structures of the Nipigon Moraine, Jim Nichols Trucking Ltd sand and gravel pit: A – longitudinal view, B – steeply dipping avalanche bedding, C – faulting and slumping along northern slope of the ridge and D – schematic crosssection of the Nipigon Moraine at this site. Stop 2 – Pijitawabik delta UTM coordinates – 041382E 545403N The route from Stop 1 to Stop 2 crosses the Nipigon lowlands along the east side of Helen Lake. Glaciolacustrine fine-grained rhythmites occur beneath the surface of the plain until just north of Mignet Creek. At the break in slope about 400 m north of the creek, Farrand identified a Dorion shore feature at 800 ft asl or about 244 m (Figs. 11 and 12). Another prominent bluff, a Post-Minong shoreline, occurs at an elevation of between 907 and 912 ft (276-278 m) asl (Farrand, 1960). Farrand (1960, p.194) suggested that the “very extensive, near level surface of sand and some rather heavy gravel” about 25 km north of Nipigon is also a — Like Profile: Lake Superior to Pijitawabik Bay /elth? ]UL}\ B shore bluff Nipigon Moraine shore bluff?—) Pijitawabik Ray V/ H Helen Lake IOD I 0 Approximate utbem limit of the Pthtawabik Canyon ::::: I Distate (m) frail LAe Stwia Figure 11. Profile from Lake Superior to Pijitawabik Bay of Lake Nipigon following the Pijitawabik Canyon. Stop 1 is on the crest of the Nipigon Moraine, Stop 2 at the Pijitawabik delta and stop 3 within the Pijitawabik Canyon. Several features controlled by ancestral water levels in the Superior basin can be seen. - 49 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Figure 12. A fused DEM and hillshade of the Pijitawabik delta and the southern part of the Pijitawabik Canyon. product of glacial Lake Minong (Figs. 11 and 12). This feature has an elevation of 950-952 ft or about 290 m asl. At 980 ft or 299 m asl, Farrand (1960, p.194) identified another possible Minong feature within the Pijitawabik Canyon. Both these features are well inside or north of the Nipigon Moraine. The following quote from Farrand (1960) sums up some of his conclusions as to the origin and relationships that these deposits have with lakes in the Superior and Nipigon basins. “The 980-foot gravel accumulation lies nearly on the isobase of Terrace Bay where the Minong beaches lie at 950 to 1000 feet. Moreover, the abundance of sediment in the Pijitawabik Bay canyon implies the presence of the ice sheet either in Pijitawabik Bay itself or at least in the Lake Nipigon basin”. “These facts lead to the conclusion that the Nipigon basin was ice-filled (or nearly so) in Minong time. As lake levels fell in the Superior basin and the ice sheet retreated from the Lake Nipigon basin these basins apparently contained separate water bodies connected only by the ancestral courses of the Nipigon and Black Sturgeon rivers. In other words, there appears to be no validity in the assumption generally made (Stanley, 1932, p.135; Leverett, 1927, p. l.7) that the Nipigon and Superior basins were occupied at any time by one continuous lake” (Farrand, 1960, p.82). be sound, however convincing evidence that Stanley’s or Leverett’s assumptions had “no validity” was not substantiated. Down-cutting of the delta to the 290 m level may have been caused by a drop in base level as water in the Superior basin lowered in response to erosion at the outlet sill at Sault Ste Marie. The delta may have been fed from the glacier margin that remained within the Pijitawabik Canyon farther up the valley. Alternatively, during basin (Nipigon/Superior) separation, as an outlet stream from the lake Nipigon basin, or possibly built by an Agassiz drainage event through the valley. Stop 3 – Pijitawabik Canyon UTM coordinates – 0420816E 5462100N This stop may be a series of stops along the route The Pijitawabik Canyon is a deep, steep-walled valley cut into Nipigon diabase/gabbro sills. It is about 20 km long with a prominent near right-angle bend about 11 km from its south end. It can be considered to be made up of two segments; a southern segment that is about 2 km wide and a northern segment that is only about 1 km wide (Figs. 13 and 14). Local relief along the valley walls of both segments can exceed 150 m. The initial conclusions made by Farrand appear to - 50 - The valley is controlled by major structures in the �r Proceedings of the 51st ILSG Annual Meeting - Part 2 scJLT Pijitaabik Canyon ProfileA C 0— 285 m 292 m 10000 5000 UsIam(n4 Profile B Profile C 500 500 400 rTh 450 400 i 350 - 350 E30 250 10 E ooo 250 K4onielocs iS 20% 200 3000 4000 5000 20 0 1Q00 Profile D Profile E 450 350- 3000 4000 5000 Profile F - 450- Th 2000 (m) (m) 00 / E 350300 I I 500 1000 (m) 1500 I I 2000 2500 0 lOU 20(X) 310 4000 (m) 1000 2 3000 4 (m) Figure 13. Selected profiles across the Pijitawabik Canyon. bedrock. In addition, the rock characteristics of the Nipigon sills are such that weathering can be intense along fractures, producing grus-like residual soils that are easily removed by erosion. The Pijitawabik delta emerges from the southern end of the canyon and deltaic sediments occur within the canyon walls (Figs. 11, 12 & 13). Remnant terraces of sand and gravel (Figs. 12 & 13) fall from an elevation of about 305 m at the canyon bend to the delta surface at about 299 m at the mouth of the canyon. The ice margin feeding this system must have been just north of the bend as suggested by Farrand (1960), in the vicinity of this stop. The side canyon, Profile D (Fig. 13) is cut down to the level of the terraces only, so major down-cutting here, must have occurred during delta building. Stop 4 – Sand River Property, Roxmark Mines Limited UTM coordinates – 0424364E 5497579N Note - permission from the land owner is required for access to this stop. In 2003, Roxmark Mines Limited acquired the Sand River, Leitch and East Leitch properties from Rio Fortuna, Teck Cominco and Kinross, respectively (News Release, Roxmark Mines Limited, November 28, 2003) and began an advanced exploration program. In 2004, trenching and stripping at Sand River exposed vein systems filled with quartz. Visible gold occurs within fractures in quartz veins and most commonly is found along thin seams or septa of sericite and chlorite, along with arsenopyrite, pyrite and ankerite. Channel samples were cut every three feet. All composite assays used a minimum width of 1.0 feet. The #16-1 West Vein returned an average assay of 1.737 ounces Au per ton over a strike length of 42 feet. The #16-1 East Vein returned 0.347 ounce Au per ton over 37 feet. - Figure 14. View of the Pijitawabik Canyon looking north toward Lake Nipigon (photo courtesy of P. Kor, OMNR). The Sand River area was most recently mapped by Hart et al. (2002) and is underlain predominantly by clastic metasedimentary rocks. Thickly bedded, southward-younging, feldspathic wacke, sandstone, - 51 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 I i. :1 The rock surface has been polished and striated with at least two ice flow directions indicated (Fig. 15). The oldest set is oriented between 265-270°N and the youngest set at 290°N. Minor meltwater erosion forms are also present. The edges of the stripped area and trenches expose up to 2 m of glacial and postglacial sediments. II In one section, an interesting sequence of sediments is exposed. Resting directly on the bedrock surface is a thin (30 to 50 cm) layer of stony, slightly silty, sand diamicton interpreted as subglacial till (Fig. 16). Clast content is high and is composed of many locallyderived lithologies, dominated by metasedimentary rock fragments. The till is typical of areas of thin till within the Canadian Shield allowing for variations in lithology and stoniness. It has less than 2% carbonate content in the silt/clay fraction of the matrix. A sample of till from this layer contained 69% sand, 28% silt and 3% clay-sized particles (2 microns). In comparison a sample of till from Stop 6 contained 45% sand, 49% silt and 6% clay and had a matrix carbonate content of about 27%. Figure 15. Polished bedrock surface containing 2 sets of striations (photo courtesy of M. Smyk, OGS). and argillaceous rocks have been well exposed by the recent stripping operations. Bedding has eastnortheasterly to east-southeasterly orientations with steep, northerly dips. Well-developed, slatey cleavage, as well as local sericitization and carbonatization distinguish sub-parallel shear zones. Oblique cleavage - bedding relationships are locally exposed. Cleavage is refracted through interbedded siltstone and sandstone beds. Overlying the till layer is 30 to 50 cm of rhythmically bedded silt and clay. These sediments were deposited in an ancestral lake located in at least the Lake Nipigon basin during ice marginal recession. A channel sample of the rhythmites contained 7% sand, 77% silt and 16% clay and its carbonate content was about 30%. The rhythmites have been overturned toward the northwest (fold axis ~30°N) and are overlain by a thin layer of diamicton containing sorted lenses and wisps I Figure 16. “Local till” overlain by silt and clay rhythmites at the Sand River Property (photo courtesy of M. Smyk, OGS). Figure 17. Overturned rhythmites and overlying diamicton (photo courtesy of M. Smyk, OGS). - 52 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 till, overlies the bedrock surface. The bedrock surface is striated with the striations oriented at 245°N. The bedrock directly below the till is composed of fine to very fine grained sandstone, 20 to 30 cm thick, resting on Quetico metasedimentary rocks. The Archean - Proterozoic unconformity, an angular unconformity between the flat-lying Sibley Group sedimentary rocks and east-trending steeply dipping Quetico metasedimentary rocks, is exposed on both sides of Highway 11 (UTM 0423431E 5483881N). Figure 18. Transverse dune at stop 5. Inset shows internal structure (photos courtesy of M. Smyk, OGS). of sand and gravelly sand (Fig. 17). The diamicton may represent a minor fluctuation of the ice margin; overriding the glaciolacustrine rhythmites followed by ice marginal debris flow sedimentation (flowtills) or may be debris flows related to slumping or flowing of debris downslope; a post glacial event. The direction of overturning of the rhythmites, however, is similar to the direction of the younger set of striations and perpendicular to the slope direction at the site. Stop 5 – Nipigon dunes UTM coordinates – 0421102E 5497420N At this stop a road cut exposes the internal structures of a transverse dune that probably formed along the shore of an ancestral lake in the Nipigon basin (Fig. 18). Thorleifson and Kristjansson (1983) mapped out a shorebluff several metres above present-day Lake Nipigon. The till fizzes when a weak solution of hydrochloric acid (~10%) is applied. It contains about 27% carbonate (11% calcite and 16% dolomite) in the silt/ clay fraction (Chittick analysis). The till at Sand River (Stop 4) contained less than 2% carbonate. A sample of till from this site contained 45% sand, 49% silt and 6% clay-sized particles (2 microns). In comparison a sample of till from the Sand River Property (Stop 4) contained 69% sand, 28% silt and 3% clay. This exposure of thick, carbonate-rich “exotic till” is one of the furthest west exposures. This exotic till is best exposed farther to the east, between Geraldton and Jellico especially around Wildgoose Lake. It underlies a spectacular streamlined surface (Fig. 19) where “drumlins up to 1 km in length are scattered across the area, but the principal morphological feature on thick till deposits are flutes about 1 km wide and over 20 km long” (Thorleifson and Kristjannsson, 1983, p.54). Zoltai (1965a) noted that the Nipigon Moraine, west of Lake Nipigon, corresponded to the extent of carbonate-rich till and Karrow and Geddes (1986) used the extent of carbonate-rich till to suggest that the The second part of the stop is to a small field of dunes developed on an abandoned lake bottom that presently has an elevation of about 296 m asl. The route follows along the shorebluff mapped by Thorleifson and Kristjansson (1983), then crossing a subtle break in slope at about 178 m asl on our way to the dune field. Stop 6 – Carbonate till UTM coordinates – 0423418E 5483886N In the 7 m exposure on the west side of Highway 11, approximately 6 m of light brownish grey, massive to fissile, slightly silty to silty, very fine to coarse grained sand with pebbles, cobbles and boulders, interpreted as Tn 1 Figure 19. Streamlined till surface near Wildgoose Lake (photo courtesy of OMNR 74-4929-18-156). - 53 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Nipigon Moraine was the correlative of the Chapleau Moraine north of Lake Huron. Stop 7 – Nipigon Lowland Rhythmites UTM coordinates – 0407200E 5437660N This stop is to look at some of the sediments that underlie the Nipigon lowland. At this site, 0.6 to 1 m of apparently massive slightly clayey silt to silt containing freshwater molluscs overlies greater than 14 m of silt and clay rhythmites (Fig. 20). The upper 6 m of the section is well exposed in a drainage ditch just east of Highway 11. The rhythmites are slightly contorted at the top of the unit then become horizontally bedded with depth. Concretions have formed around rootlets below 4.5 m. In the woods to the east, a small abandoned sand and gravel pit occurs at the base of the bedrock-controlled hill and may possibly mark the position of the shore and level of the lake that the molluscs inhabited. Farrand (1960) identified a couple of terraces east of the highway and Helen Lake that he attributed to Nipissing/Algoma. Acknowledgements The writer would like to thank Mr James Nichols, Jim Nichols Trucking Ltd., Nipigon and Mr. Dave Malouf, Roxmark Mines Ltd., for kindly giving permission to visit their properties. Mr Phil Kor (OMNR) and Mark Smyk (OGS) provided most of the photographs used in the fieldguide and Mark kindly provided text on the bedrock geology for several of the field trip stops. Roxanne Corcoran (OGS) produced some of the line illustrations. All of their help is much appreciated. In addition, the indirect contributions of the contributors to the 1987 INQUA fieldtrip guide (Geddes et al., 1987) and Dr. W.R. Farrand (1960) for his work on ancestral lake levels in the Superior basin improved the content of this field guide. This fieldtrip guide was produced with the permission of the Director, Ontario Geological Survey. References Baker, V.R., Greely, R., Komar, P.D., Swanson, D.A., and Waitt Jr., R.B., 1987. Columbia and Snake river plains; in Geomorphic Systems of North America, Geological Society of America, Centennial Special Volume 2, p.403-468. Barnett, P.J. 1991. Quaternary geology of Ontario; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 2, p.1011-1088. Barnett, P.J., Henry, A.P. and Babuin, D. 1991. Quaternary geology of Ontario, west-central sheet; Ontario Geologic Survey, Map 2554, scale 1:1 000 000. Blackburn, C.E., Johns, G.W., Ayer, J.A. and Davis, D.W. 1991. Wabigoon Subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p.303-382. Bostock, H.S. 1970. Physiographic subdivisions of Canada; in Geology and Economic Minerals of Canada, Geological Survey of Canada, Economic Geology Report no.1, p.10-30. Burwasser, G.J. 1979. Quaternary geology of the city of Thunder Bay and vicinity, District of Thunder Bay; Ontario Geological Survey, Geological Report 164, 70p. Dredge, L.A. and Cowan, W.R. 1989. Quaternary geology of the southwestern Canadian Shield; in Quaternary geology of Canada and Greenland, Geological Survey of Canada, Geology of Canada, no.1, p.214-249. Dreimanis, A. 1989. Genetic classification of tills; in Genetic Classification of Glacigenic Deposits, Balkema, Rotterdam, p.17-84. Drexler, C. W., Farrand, W. R. and Hughes, J. D., 1983. Correlation of glacial lakes in the Superior Basin with eastward discharge events from Lake Agassiz; in Glacial Lake Agassiz, J. T. Teller and L. Clayton (eds), Geological Association of Canada Special Paper 26, p.261-290. Figure 20. Example of fresh water molluscs collected from Stop 7. Elson, J.A. 1967. Geology of glacial Lake Agassiz; in Life, land and water, University of Manitoba Press, - 54 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Winnipeg, Manitoba, p.36-95. Farrand, W.R., 1960. Former shorelines in western and northern Lake Superior basin; unpublished Ph.D. thesis, University of Michigan, Ann Arbor, Michigan, 226p. Farrand, W.R. and Drexler, C.W., 1985. Late Wisconsin and Holocene history of the Lake Superior basin; in Quaternary Evolution of the Great Lakes, P.F. Karrow and P.E. Calkin, eds., Geological Association of Canada, Special Paper 30, p.17-32. Franklin, J.M., 1978. The Sibley Group, Ontario. In, Rubidium-strontium isochron age studies, Report 2, Geological Survey of Canada, Paper 77-14, p. 31-34 Geddes, R.S. and Kristjansson, F.J. 1986. Quaternary geology of the Hemlo area: Constraints on mineral exploration; Canadian Geology Journal of the Canadian Institute of Mining and Metallurgy, v.1, no.1, p.5-8. Geddes, R.S., Kristjansson, F.J. and Teller, J.T., 1987. Quaternary features and scenery along the north shore of Lake Superior; International Quaternary Association, INQUA’87, XII International Congress, Excursion Guide Book C-12, 62p. Hart, T.R., terMeer, M. and Jolette, C., 2002. Precambrian geology of Kitto, Eva, Summers, Dorothea and Sandra Townships, northwestern Ontario : Phoenix Bedrock Mapping Project; Ontario Geological Survey, Open File Report 6095, 206p. Hicock, S.R. 1988. Calcareous till facies north of Lake Superior, Ontario: Implications for Laurentide ice streaming; Géographie Physique et Quaternaire, v.42, p.121-135. Karrow, P.F. and Geddes, R.S. 1987. Drift carbonate on the Canadian Shield; Canadian Journal of Earth Sciences, v.24, p.365-369. Leverett, F., 1929. Moraines and shorelines of the Lake Superior basin; United States Geological Survey, Professional Paper, 154-A, 72p. Mollard, D.G. and Mollard, J.D. 1981. Frazer Lake area (NTS 52H/SE), District of Thunder Bay; Ontario Geological Survey, Northern Ontario Engineering Geology Study 42, 32p. Accompanied by Map 5052, scale 1:100 000. Ontario Geological Survey 2003. 1:250,000 scale bedrock geology of Ontario; Ontario Geological Survey, Miscellaneous Release – Data 126. Prest, V.K. 1970. Quaternary geology of Canada; in Geology and economic minerals of Canada, 5th edition Geological Survey of Canada, Economic Geology Report 1, p.676-764. Sado, E.V., Fullerton, D.S. and Farrand, W.R. 1994. Quaternary Geological Map of the Lake Nipigon 40X60 Quadrangle, United States and Canada; U.S. Geological Survey, Miscellaneous Investigations Series, Map I-1420 (NM-16), scale 1:1 000 000. Shaw, J. 1985. Subglacial and ice marginal environments; in Glacial Sedimentary Environments, Society of Economic Paleontologists and Mineralogists, Short Course no. 16, p.7-84. Slattery, S.R., 2003. Sedimentary architecture and Quaternary geology of the Longlac area, northcentral Ontario, Canada; unpublished MSc thesis, Laurentian University, Sudbury, Ontario, 125p. Smyk, M., Fralick, P.W., and Hart, T., 2005. The geology and gold mineralisation of the Bearmore-Geraldton greenstone belt. Institute on Lake Superior Geology 51st Annual Meeting, Proceedings Volume 51, Part 2 – Field trip guide, this volume. Sutcliffe, R.H. 1991. Proterozoic Geology of the Lake Superior Area. in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p.627661. Teller, J.T. and Mahnic, P. 1987. History of sedimentation in the northwestern Lake Superior basin and its relation to Lake Agassiz overflow; Canadian Journal of Earth Sciences, v.25, p1660-1673. Teller, J.T. and Thorleifson, L.H. 1983. The Lake Agassiz– Superior connection; in Glacial Lake Agassiz, Geological Association of Canada, Special Paper 26, p.61-290. Teller, J.T. and Thorleifson, L.H., 1987. Catastrophic flooding into the Great Lakes from Lake Agassiz; in Catastrophic flooding, Allen & Unwin, Boston, Massachusetts, p.121-138. Thorleifson, L.H. and Kristjansson, F.J., 1993. Quaternary geology and drift prospecting, Beardmore-Geraldton area, Ontario, Geological Survey of Canada, Memoir 435, 146p. Williams, H.R. 1991. Quetico Subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p.383-405. Zoltai, S.C. 1965a. Glacial features of the Quetico–Nipigon area, Ontario; Canadian Journal of Earth Sciences, v.2, p.247-269. Zoltai, S.C. 1965b. Surficial geology, Thunder Bay; Ontario Department of Lands and Forests, Map S265, scale 1:506 880. Sado, E.V. and Carswell, B.F. 1987. Surficial geology of northern Ontario; Ontario Geological Survey, Map 2518, scale 1:1 200 000. - 55 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 - 56 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Fieldtrips 3 and 6 - A stratigraphic transect across the Northern flank of the Midcontinent Rift near Rossport Pete Hollings and Philip Fralick Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Safety As this trip will be taking place on Lake Superior it will be weather dependant and could be cancelled or curtailed at very short notice. An alternative trip to the Winston Lake mine will be undertaken in case of bad weather. Please exercise caution when getting in and out of the boats as the outcrops are often extremely slippery. Life jackets must be worn in the boats at all times. It will probably be very cold out on the lake so please dress warmly. Regional geology Archean granites, outcropping along the shoreline near Rossport, are unconformably overlain by strata of the Gunflint Formation (Fig. 1). These sediments were deposited on a south facing shelf at approximately 1878 Ma (Fralick et al., 2002). The Formation consists of a Lower Member composed of basal stromatolitic bioherms overlain by ankeritic, interclastic grainstones. A regressive, karstified surface caps the northern portion of this assemblage (Fralick and Barrett, 1995) and is succeeded by the Upper Member. It begins with a repetition of the underlying lithologies to which, higher in the succession, are added carbonaceous shales, tuffs and rarely mafic volcanic rocks. These chemical and fine-grained siliciclastic sediments record parasequence development on a storm-dominated shelf (Pufahl and Fralick, 2004) forming the relatively stable portion of a back-arc basin (Kissin and Fralick, 1994; Hemming et al., 1995) prior to compressive northward thrusting of the arc at approximately 1860 to 1835 Ma. As the compression of the Penokean Orogeny waned the sea again transgressed over the area depositing black, carbonaceous shales of the Rove Formation. This depositional cycle lasted from 1835 Ma (Kissin et al., 2003; Addison et al., 2005) till less than 1780 Ma ago (new detrital zircon laser ablation ICP MS data). The lower portion of the Gunflint Formation in the Rossport area is poorly exposed. The limited outcrop of the Lower Gunflint is similar to the succession comprising the thin, basal Kakabeka Conglomerate and overlying interclastic grainstones present in exposures to the west near Thunder Bay. Good exposure of the Upper Gunflint exists on Quarry Island and consists of possible basaltic flow rocks with associated stromatolites, overlain by a succession of mediumto coarse-grained, graded, sandstone beds. The geochemistry of the sandstones is similar to Archean rocks to the north indicating probable derivation from this source. Black shales, lithically correlative with the Rove Formation, outcrop on an island approximately 5 km to the west. The shales do not overly the turbidite succession on Quarry Island where arenites of the Sibley Group disconformably rest on an erosion surface at the top of the Gunflint sandstones. The basal unit of the Sibley Group is the Pass Lake Formation. It is composed of the conglomeratic Loon Lake Member and the overlying sandstones of the Fork Bay Member (Cheadle, 1986). Where the basal conglomerates are present they either represent: 1) large channel fills cutting down into sandstones to shales with abundant caliche zones, or; 2) more laterally continuous conglomerates interbedded and overlain by parallel laminated, medium-grained sandstones. The former represent channel gravels in braided fluvial systems and the latter coarse-grained strandline deposits. The overlying Fork Bay sandstones likewise record both braided fluvial deposition and subaqueous, strand proximal sand-sheet development. In addition to upward thinning and fining successions developed during transgressive systems tract formation, other sandstone assemblages thicken and coarsen upwards representing progradational, delta lobe outbuilding. It is not known whether the water body was lacustrine or marine, though preliminary isotopic analyses (C, O, S and Sr) are more in agreement with a hypersaline lacustrine setting for at least the later history of the water mass. This is consistent with Cheadle’s (1986) findings - 57 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 A B Figure 1b Osler Group 1108 Ma Nipigon diabase Stop 8 Sibley Group Lake Superior C 48°30ʼ a Animikie Group Figure 1c 1105 Ma Lake Superior 200 km Rossport N Stop 7 Quarry Island Stop 6 Simpson Island Stop 3 Vein Island Stop 2 Archean rocks a 15 km Stop 5 Stop 4 Stop 1 Channel Island Wilson Island 48°30ʼ Keweenawan intrusive rocks Osler Group volcanics Keweenawan sediments Sibley Group sediments Gunflint Formation Archean basement Copper Island 48°45ʼ 87°30ʼ 87°45ʼ 48°45ʼ 1 km Figure. 1. Map showing the location of the fieldtrip area. B) Regional geology map showing the extent of the Osler Volcanic Group. Age data from Davis and Sutcliffe (1985) and Davis and Green (1997). Modified after Sutcliffe (1986). C) Geological map of the Osler Volcanic Group showing sample locations. Modified after Giguerre (1975). based on regional paleogeography. The increasing salinity of the water resulted in dolomite, minor gypsum and rarer barite and celestite precipitation mixed with mud deposition. A red and green banding developed in this assemblage due to periodic anoxia of the bottom sediments. The final desiccation of the lacustrine basin is recorded by the development of strandline microbial bioherms (stromatolites) which are overlain by either a terra rosa (red, wind-delivered soil) or subareal, intraformational, mass-flow conglomerates. This is succeeded upwards by mudstones with abundant gypsum nodules representing mudflats formed in an arid climatic setting where hypersaline groundwaters precipitated gypsum. Together all these fine-grained sediments comprise the Rossport Formation. It is overlain by the Kama Hill Formation; a coarsening upwards deltaic succession recording flooding of the basin and development of a more humid climate. The age of the Pass Lake and Rossport Formations can be bracketed between laser ablation MS youngest ages on detrital zircons of 1600 Ma and a Rb-Sr isochron of 1339 Ma (Franklin, 1978). Youngest detrital zircons in the Kama Hill Formation are 1420 Ma. The Sibley Group is very well exposed along the shorelines of the islands off Rossport. The basal disconformity can be seen about two thirds of the way up the cliff face on the western side of Quarry Island where it overlies graded sandstone beds of the Gunflint Formation. Blocks of medium-grained sandstone were extracted from the cliff face on the island for use in the construction of buildings in Thunder Bay. These sandstones are medium- and large-scale planar crossstratified and may represent a sandflat composed of transverse bars in a braided stream or small barchan, eolian dunes. Rare pebbles indicate the former may be the case but this evidence is not conclusive. Channel and Copper Islands contain excellent exposures of the lacustrine rocks with outbuilding of channel systems along the paleolake margins. One of the best outcrops of the strandline stromatolitic carbonates occurs on Channel Island and will be visited during this fieldtrip. On Copper Island the Rossport Formation is overlain - 58 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Logan Sills (1109+4/-2 Ma, Davis and Sutcliffe, 1985), the lower portion of the North Shore Volcanics (1108±2 Ma, Davis and Green, 1997), the Swamper Lake Gabbro and Nathan’s Series intrusives (1107 Ma, Paces and Miller, 1993) and the lower portion of the Powder Mill Group (1107±2 Ma, Davis and Green, 1997). These older units outcrop on the rift flanks where erosion has removed the younger rift sequence or, in the case of the The oldest rift-related rocks on which U-Pb age Coldwell and Logan Sills, are intruded into older rocks determinations have been performed lie along the immediately north of the rift. northwestern portion of the rift. These include, from NE The Osler Volcanic Group comprise a ~3km thick to SW, the alkaline intrusives of the Coldwell Complex (1108±1 Ma, Heaman and Machado, 1992), the lower sequence (Cannon et al., 1989) lying unconformably Osler Group volcanics (1108+4/-2 to 1105±2 Ma, above Sibley Group metasediments (Fig. 2). The Davis and Sutcliffe, 1985; Davis and Green, 1997), the volcanic sequence is overlain and intruded by the disconformably by pebbly, fluvial conglomerates of the basal Osler. Thirty kilometers to the west the uppermost unit of the Sibley, the Nipigon Bay Formation, underlies the same disconformity. This highlights the fact that approximately 600 m of erosive downcutting occurred in the Rossport area before the basal Osler was deposited. V 1095 N Copper Harbour Conglmerate Portage lake Volcanics IV Portage Lake IV Volcanics Group 7 V (Group 8) III Group 6 Upper Suite R R Kallander Creek Volcanics Siemens Creek Volcanics Beaver Bay Complex Mostly basalt units Osler Group 1105 Schroeder Basalts V Duluth Complex Great Conglomerate and Group 5 Central Suite III II I Bessemer Quartzite 1110 Lower Suite I Simpson Isl Cgl Nipigon Sills Groups 3,4 Group 2 Group 1 IV IV IV 1100 NE Minnesota SW limb III II I IV North Shore Volcanic Group Copper Harbour Conglomerate 1090 Mamainse Point Michipicoten Island Michipicoten Island Formation Isle Royale 1085 Age (Ma) Isle Royale Black Bay Peninsula Lake Nipigon Upper Michigan NW Wisconsin Elyʼs Peak Basalts I, II, III Nopeming sandstone Archean Basement Figure 2. Schematic correlations of volcanic rocks of the Midcontinent Rift based on the stratigraphic position of distinctive basalt sequences, magnetic polarity and absolute age where possible. Modified after Nicholson et al. (1997). Dashed lines in Upper Michigan section separate lower and upper members of Kallander Creek and Siemens Creek volcanics. Left hand column shows magnetic polarity. Roman numerals I-IV refer to five distinctive laterally extensive basalt compositions identified on the south shore of western Lake Superior. Where equivalent basalt compositions occur in other stratigraphic successions, the appropriate Roman numeral is noted (see Nicholson et al., 1997 for data sources). Shaded regions represent intervals in which contacts are covered or obscured by plutonic rocks. - 59 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 St. Ignace Island Volcanic-Plutonic Complex an intercalated sequence of basaltic rocks and rhyolitic flows (Sutcliffe and Smith, 1988). Detailed descriptions of the Osler Group have been provided by McIlwaine and Wallace (1976) and Lightfoot et al. (1991). Generally the mafic flows of the Osler Group consist of massive to amygdaloidal flows, with locally developed ropey tops and pahoehoe textures (Sutcliffe and Smith, 1988). The flows range in thickness from 5cm to 30m (Lightfoot et al., 1991) with a regional dip of ~6-15° S (Giguerre, 1975; Lightfoot et al. 1991; Hollings et al., 2005). The majority of the exposed section is magnetically reversed with only the upper 100m displaying a normal polarity (Halls, 1974). The contact between the two units is marked by the presence of the Puff Island conglomerate and a discordance between the basalt flows above and below the contact. This has been interpreted as representing a significant break in the eruption history. A felsic porphyry near the base of the Osler Group has yielded an age of 1107.5+4/-2 Ma (Davis and Sutcliffe, 1985) whereas zircons from the Agate Point rhyolite towards the top of the reversely magnetized sequence have yielded an age of 1105±2 Ma (Fig. 1b; Davis and Green, 1997). Within the Osler Group interflow sediments are typically thin and of limited extent. Field descriptions of the sedimentary successions appear in Giguere (1975) and McIlwaine and Wallace (1976). They show that there are two main zones of sedimentary rocks within the Osler Group. One occurs near the base of the volcanic pile. The other is present approximately 2700 meters higher in the succession marking the paleomagnetic reversal. Lightfoot et al. (1991) in a study of the Osler Volcanic Group exposed along the shores of the Black Bay Peninsula to the west of the field trip location proposed that the major and trace element geochemical data could be used to subdivide the flows into an Upper, Central and Lower Suite although the boundaries between the suites were not clear cut. While the geochemical compositions of the Central (750900m) and Upper suites (1900-3000m) overlap their Lower Suite (0-750m) is distinguished by elevated Mg numbers (0.55-0.7 versus 0.3-0.6), lower Al2O3 (8-12 wt% versus 13-17wt%), lower Th/Nb ratios (0.090.70 versus 0.3-0.6) but higher Gd/Ybn ratios (3.5-4.5 versus 1.6-2.6). Nicholson et al. (1997) concluded that there were five geochemically distinct flood-basalt compositions within the Mid-continent rift that are common to most sections and appear in approximately the same stratigraphic order (Fig. 2) They recognized a lower suite in the Siemens Creek Volcanics (Basalt Type 1; Fig. 2), which they suggest is analogous to the Lower Suite of Lightfoot et al. (1991). In the United States this unit is less than 100m thick whereas the Lower Suite of Lightfoot et al. (1991) is ~750m thick. They report a narrow range of εNd(1100) values for the Siemens Creek Volcanics of -0.7 to +0.7. Nicholson et al. (1997) further suggest that there is a broadly recognizable suite of basalts above this (Basalt type II) which includes the upper Siemens Creek Volcanics and in the upper part of the Grand Portage lavas (Fig. 2). The suite is characterized by slight negative Nb anomalies and a range of εNd(1100) values of -1.4 to -6.9. They suggest that this may be analogous to the most primitive members of the Central Suite of Lightfoot et al. (1991). The volcanic flows of the Osler Group on Wilson Island are all basalts or basaltic andesites (SiO2 = 4756 wt%; MgO = 5-16 wt%; Hollings et al., 2005). The basalts are characterized by LREE enrichment (La/Smn = 1.5-3.9) in conjunction with moderately fractionated HREE (Gd/Ybn = 1.5-3.7) and slight positive to moderately negative Nb anomalies (Nb/ Nb* = 0.56-1.13; Hollings et al., 2005; Fig. 3). Major and trace element data show trends of increasing SiO2 and decreasing MgO and display strong positive correlations between La/Smn, Th/La and Th/Nb with height (Fig. 4). This correlation is most pronounced above 400m with samples from the base of the stratigraphy displaying more or less constant values of these ratios (Fig. 4). Measured 143Nd/144Nd ratios for the seven Osler basalts analysed range from 0.5118570.512286 with εNd(t=1106Ma) of +0.3 to -5.3 (Hollings et al., 2005). The high incompatible element abundances, in conjunction with LREE enrichment and strongly fractionated HREE are comparable to modern OIB, albeit at lower absolute abundances (Fig. 3). When compared to other flood basalt sequences the more primitive basalts from this study closely resemble basalts from the Parana-Etendeka flood basalt sequence (Fig. 4; Gibson et al., 2000). The εNd data from the most primitive members of the Osler Group is consistent with an enriched mantle plume rather than a contaminated depleted mantle source, given the lack of trace element evidence for contamination in these samples. Depleted mantle at 1100 Ma would have had a positive εNd perhaps as high as +6 whereas an - 60 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 by an older lithospheric component characterized by pronounced LREE enrichment, high Th abundances but generally unfractionated HREE (Hollings et al., 2005). 100 The sedimentary successions near the base of the Osler Group constitute the Simpson Island Formation and have recently been described in detail by Hollings et al. (2005). They are composed of a Lower Member dominated by trough cross-stratified, medium-grained sandstones directly overlying basement and an Upper Member with a greater variety of siliclastic units. The Lower Member sits on an irregular, erosional surface cut into the underlying quartz arenites of the Nipigon Bay Formation, Sibley Group. A massive pebble-cobble conglomerate overlies the unconformable surface and is in turn overlain by decameter-scale layers of coarse-grained and pebbly sandstone (Fig. 5, Section 1). Sandstones are parallel laminated, commonly have cross-stratified tops, more rarely contain pebbly transverse ribs and chute and pool-like structures. The central portion of the succession is composed of trough cross-stratified, medium-grained sandstone organized into a stacked assemblage of lenses. Pebble stringers and pebbly sandstones commonly occur on the deeper portions of curving set boundaries (Fig. 5, Section 2). Massive pebble-cobble conglomerates sharply overly the sandstone succession (Fig. 5, Sections 2 and 4). The conglomerates contain trough cross-stratified, mediumgrained sandstone lenses; decameter- to meter-scale wedges of planar cross-stratified sandstone and are interbedded with assemblages of trough cross-stratified sandstones up to one meter thick. Another assemblage of trough cross-stratified sandstone, similar to the one in the central portion of the succession, caps the basal sedimentary assemblage (Fig. 5, Sections 3 and 4). Clast lithologies in the pebble-cobble population are dominated by quartz, chert, various types of volcanic 10 Hawaiian OIB Deccan Traps CFB Parana-Etendeka CFB 1 Th Nb La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Al V Sc Rock/Primitive Mantle 100 10 Type 1 Lower Suite 1 Th Nb La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Al V Sc 100 10 Type 2 1 Central Suite Th Nb La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Al V Sc Figure 3. Comparison of primitive mantle normalized plots from the Osler Group with A) Phanerozoic OIB and Continental Flood Basalts (CFB) and B & C) the Lower and Central suites of Lightfoot et al. (1991). From Hollings et al. (2005). enriched plume source would have εNd ~0 (Nicholson and Shirey, 1990; Shirey et al., 1994). Up sequence the basalts are characterized by higher SiO2, Th and La/Smn abundances in conjunction with increasingly negative Nb and Ti anomalies and εNd(1106) values of -4 to -5. This is consistent with contamination of these basalts 900 800 SiO2 MgO Fe2O3 La/Smn Th Gd/Ybn Th/La εNd Th/Nb 700 600 500 400 300 200 100 0 40 50 60 5 10 15 10 15 1 2 3 4 1 2 3 4 1 2 3 4 0.05 0.10 0.15 0.20 0.10 0.15 0.20 -6 -4 -2 0 Figure 4. Geochemical stratigraphy of the Osler Group on Vein and Wilson Islands. The stratigraphic position of the samples has been calculated assuming a dip of 10° parallel to the section. From Hollings et al. (2005). - 61 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 V V V V V V V V V 6 No Longitudinal 24 Complex V V V V V Major 12 V V V V 6 V V V V V V 18 V 3 0 Section 5. V V No O/C V V V V V Nipigon Bay V V V Ripples Pebbles Hummocky Cross-Strat. V V V V V No O/C ? m. m V V V V V Complex No O/C ? m. Sandy Sheetfloods 6 Sheetfloods and Debris flows 3 V V V V V V V V V V Conglomerate Pebbly V. Coarse Medium V. Fine Siltstone Shale Sst. V V V V V V 7 15 V V V 8 m. No O/C 12 21 18 15 12 Bar V 0 Massflow 3 m. No O/C V V V V V V V V V V V V V 5 6 V V V V V V 15 V V V V V V V 10 km Older Units Section Locations Section 7. V V V V V V V V V V Sand Channels with Small Gravel Longitudinal Bars Stacked Channels 9 Sheetflood Sands with Channels 6 Small Stacked Channels Distal 6 V 4 2 1 3 VV V Sedimentary Rocks Igneous Rocks 1 27 Sandy Channels to Distributary Mouth Bar 9 V m Gravel Channels 26 V V V V V Osler Group 24 29 23 V V Section 6. 32 Longitudinal No O/C Trough Cross-Strat. Small Irregular Lenses Paleocurrent Direction V V V V V V V V V V V V V V V V V V V V V V V V V V Bar 0 0 Complex Rhyolite V V V V =318o 15 9 Longitudinal Parallel Lamination V V V V Bar Tail Sand Sheet Longitudinal Bar Nipigon Bay V Sandy Channel 3 Bar Sandy Channel V Section 4. m Basalt INTERFLOW SEDIMENTS m V Channel O/C =268o V 9 3 0 V l e g e n d 18 No 6 V V Sst. V 9 V Major Sandy Channel 0 Bar V m V 3 O/C 15 Section 1. V V No O/C V n = 38 V V 21 o Section 3. m V = 265 27 Section 2. Conglomerate Pebbly V. Coarse Medium V. Fine Siltstone Shale N m V SIMPSON ISLAND FORMATION ( Basal Sediments ) 3 0 Massflow Sheetflood Sands with Channels Figure 5. Sections of sedimentary rocks in the Osler Group. Sections 1 through 4 are the basal sedimentary succession of the Lower Member, Simpson Island Formation, at different locations (see inset map). Sections 5 and 6 are of the Upper Member, Simpson Island Formation, interlayered with basal basalt flows. Section 7 is the sedimentary assemblage near the top of the Osler Group on Puff Island. From Hollings et al. (2005). - 62 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 rocks, red siltstone, metamorphosed granite and at the east end of the outcrop belt, on Copper Island, a higher proportion of unmetamorphosed red granite. Current indicators show flow was to the west, averaging 265°. A sedimentary assemblage also occurs near the upper limit of outcrop of the Osler Formation, at the top of the magnetically reversed interval (Fig. 5, Section 7). These interflow sediments are located on Puff Island and overly a felsic porphyry with a U-Pb age of 1105 Ma (Davis and Green, 1997). They contain: sharp sided assemblages of laterally continuous, pebbly, coarsegrained sandstone beds with caliche horizons which are scoured into by small lenses of conglomerate; large scours filled with trough cross-sets over a meter thick; stacked assemblages of irregular lenses filled by trough cross-stratified, coarse-grained, pebbly sandstone; and, poorly sorted, disorganized, massive boulder-cobble conglomerate. Clasts are all volcanic, ranging from quartz-feldspar porphyries to mafic compositions. Paleocurrents on large-scale sedimentary structures consistently show flow to the southeast. Island. Stop 1 – Osler volcanics, Wilson Island UTM coordinates – 0462794E 5402095N Flows of the Osler Group on Wilson Island are typically >1m thick, frequently amygdaloidal towards the top and bottom of the flow with a massive core and rarely displayed a pahoehoe texture on the flow surface. The basalts are characterized by clinopyroxene and plagioclase phenocrysts in a groundmass of plagioclase, augite and Fe-Ti oxides. Rarely, basalts from the base of the sequence contained pseudomorphed olivine phenocrysts. The basalts have all been subjected to low-grade metamorphism ranging from zeolite to prehnite-pumpellyite facies (McIlwaine and Wallace, 1976). At this stop, approximately 500m above the basal conglomerates, are exposed a sequence of thin rubbly basalt flows ~50cm thick with rare massive flows ~2-4m thick and thin interflow sediments (Fig. 6). Geochemically basalts at this outcrop are similar to the Central Suite of Lightfoot et al. (1991). Sedimentary units are predominantly quartz sandstones with thin shale partings. These are best interpreted as sands washing into small hollows on the surface of the flow units with a mud drape settling out towards the top of the layer. In places units that appear to have been deposited on the surface of basalt flows connect into sub-vertical cooling cracks in the flows. Sediment filled cooling cracks can be up to 2m deep (Fig. 7). cr1 The Simpson Island Formation is composed of a laterally continuous sedimentary succession up to 25 meters thick and discontinuous sedimentary units up to 30 meters thick interlayered with the basal basalt flows. The lowest sedimentary beds fill channelways cut into the underlying sandstones of the Nipigon Bay Formation. The channel fills and overlying sedimentary assemblage represent a braided stream system, similar to the South Saskatchawan model (Miall, 1978), where dunes composed of coarse-grained sand migrated Stop 2 – Osler Volcanics, North end of Wilson down the channels and gravelly longitudinal bars with Island chute channels and bar edge sand wedges form the UTM coordinates – 0461810E 5403438N higher relief areas (Fig. 5). The fluvial interpretation Exposed at this outcrop, are the lower flows of the is consistent with Tanton (1931) and McIlwaine and Osler Volcanic Group ~300m above the conglomerates Wallace (1976). Clast lithologies indicate debris was mainly derived from erosion of local lithologies. C I Stops The trip will depart from the public dock at Rosport and will undertake a traverse through the stratigraphy of the Midcontinent Rift, starting with the youngest rocks of the Osler Volcanic Group, proceeding through the Sibley and Gunflint Formations and finishing with a look at the granites of the Archean basement (Fig. 1). In order to make the most of the calmer weather typical of early mornings we will first travel for approximately Figure 6. Thin interflow sediments between basaltic flows, 30 minutes to the most southerly outcrop on Wilson Stop 1 on Wilson Island. - 63 - �I ') i 1 Proceedings of the 51st ILSG Annual Meeting - Part 2 Figure 8. Pahoehoe texture at Stop 2, Wilson Island. Figure 7. Approximately 2m thick mafic flow cut by sediment filled cooling crack. Stop 1, Wilson Island. of the Upper Simpson Island Formation. The basaltic flows are generally massive, ranging in thickness from 1-3m. Flow tops vary from rubbly to well-developed pahoehoe textures (Fig. 8). The basalts are vesicular and amygdaloidal and in places the vesicles are elongated giving them almost a pipe-like appearance. Geochemically basalts at this outcrop are similar to the Central Suite of Lightfoot et al. (1991). In some areas the basalt flows incorporate xenoliths of a more vesicular material which have both sharp and diffuse contacts (Fig. 9) At the north end of the outcrop the flows are cut by a 2-3m wide mafic dyke. This dyke is geochemically distinct from the flows but comparable to the older diabase intrusions in the vicinity of Lake Nipigon. tel Figure 9. Amygdaloidal xenoliths in basalt flows at Stop 2, Wilson Island. sandstone beds (Figs. 10, 11). A covered interval separates this assemblage from overlying mediumto large-scale, trough cross-stratified, coarse-grained sandstones to conglomerates (Fig. 12). Paleocurrent indicators show flow to the west, though with a higher variance than other sections. Clast lithologies Stop 3 – Upper Simpson Island Formation, Daylight are probably locally derived from both Archean and Point, Wilson Island Proterozoic sources. The fine-grained sandstone near the base of the section represents a wave modified deltaUTM coordinates – 0461450E 5404650N front (ie., a distributary mouth bar of a small delta). A sedimentary assemblage of the Upper Member occurs on Wilson Island, overlying approximately The presence of small, dish-shaped scours suggests a 50 meters of basal basalt. This coarsening upwards shallow water environment with no large channels. The succession has oscillation rippled, very fine-grained upper part of the sequence represents badly organized sandstones at its base (Fig. 5, Section 6). These coarsen river deposits with gravelly, longitudinal bar forms and upwards by the addition of increasing amounts of channels filled with sand. The planar cross sets at the medium-grained, parallel laminated to hummocky base of the cliff were formed by transverse bars, while cross-stratified to oscillation rippled, decimeter-scale the trough cross beds represent migrating dunes. - 64 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 red siltstones outcrop on the shoreline here (Fig. 13). These are interpreted to be part of the cyclic facies (Channel Island Member) of the Rossport Formation as exposed at Kama Hill. The dolomite and minor gypsum indicate a hypersaline environment interpreted to be lacustrine because of the multiple deltas and sand sheets building in from a variety of directions. The cyclic facies is overlain by stromatolites and interbedded thin, sandy, carbonate storm layers (Fig. 14) of the Middlebrun Bay Member. This meter thick Figure 10. Fine-grained red sandstones with thin shale assemblage is similar to Recent sabkha deposits on the partings forming the base of the deltaic deposits at Stop 3. south shore of the Persian Gulf, and in particular the open water ponds on this sabkha where sandy storm layers are well preserved. The upper few centimeters of the stromatolitic unit is altered to a grey-green layer that represents a weathered horizon interpreted to ., 4r 17 ; Figure 11. Oscilation ripples with overlying hummocky, medium-grained sandstone showing the effects of wave reworking on the sediments forming the delta front at Stop 3. Figure 12. Cross-stratified sandstone and massive conglomerate forming the upper portion of the prograding deltaic succession present in the Upper Member of the Simpson Island Formation present at Stop 3. These sediments represent a longitudinal bar-channel complex of a braided stream. Stop 4 – Sibley Group, Mary Ann Bay, Channel Island UTM coordinates – 0462831E 5405801N Note - this is a very small outcrop and access to the rocks may be limited if lake levels are high. Please take care moving from the landing point to the outcrop. A succession of grey dolomites interbedded with Figure 13. Interlayered red siltstones and dolostones (lower unit underlying the more massive strandline carbonate with overlying mass-flow deposits) were deposited in a saline lake away either temporally or spatially from areas of coarse sand influx. The colour banding reflects the position of the redox boundary as the sediments accumulated. The grey layers commonly have slightly higher dolomite contents possibly reflecting higher organic productivity leading to more photosynthetically mitigated carbonate precipitation (higher dolomite content) and heavier organic loading to the sediment (redox boundary moving upward to at or above sediment water interface). Stop 4, Mary Ann Bay. - 65 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 ________ U t1 ii' IIIIII!! CENTIMETRE Figure 14. Stromatolitic layering (smooth mat with small pinnacles) with interbedded coarse silt to very fine-grained sand storm layers (white). This is typical of strandline to sabkha environments and especially open water ponds on the sabkha. Some of the storm layers were remobilized in the form of clastic dykes and sills. www. ]akcu pC 1 recj Ic IgyUIr have formed as the sequence became subaerial and the stromatolites weathered in situ. This weathered zone is traceable throughout the basin with the strandline deposits below it commonly containing well developed tepee structures. The carbonates are overlain by a massflow unit with intraformational clasts of red siltstone, sandstone and dolostone up to boulder size. Although the contact between the carbonates and the mass flow units is locally obscured by the intrusion of a sill, the transition is interpreted to represent a minor time gap based on the weathered zone which expands to thick terra rosa (soil) deposits at other locations. Stop 5 – Gunflint Formation, Quarry Island UTM coordinates – 0462305E 5406730N A succession of sandstones and mafic volcanic rocks outcrop on the south shore of Quarry Island. On the northeastern end of the outcrop area a gabbro, probably related to the Midcontinental Rift, is exposed. Next to this is a small outcrop of stromatolites with a box-like appearance (Fig. 15). The rectangular to square outline of the mounds contrasts with the round to oval appearance of classic stromatolites, though their organic origin is exemplified by the high angle layering, which, when projected into the area now eroded, can be seen to form mounded structures. Areas between the stromatolites are infilled with coarser siliciclastic sandstones and cherty clasts. The next outcrop of Figure 15. Odd shaped structures of probable stromatolitic origin. within the Gunflint Formation. Stop 5, Quarry Island Gunflint volcanic rocks is problematic. Mafic volcanic flow rocks occur interbedded with Upper Gunflint lithologies southwest of Thunder Bay. These are also associated with stromatolites that developed on the firm substrate of the flow tops. Thus, the igneous rocks in the Gunflint assemblage on Quarry Island could be correlative to the other flow rocks, but it is difficult to conclusively show that these rocks are extrusive. Possible flow banding is present, as are areas of finer and coarser material in individual units. The igneous rocks are overlain by medium- to coarse-grained sandstones with bed thicknesses averaging approximately 30 cm. The sandstones are dark in appearance giving the impression they were derived from mafic detritus, but their geochemistry indicates an intermediate source similar in composition to the Archean crust to the north. The layers are excellently graded (Fig. 16) with the only sedimentary structure being sporadically developed parallel lamination. Beds such as these are commonly thought of as typical turbidites and the deposits ascribed to reasonably deep water. However, it must be remembered that graded bedding simply means a decelerating flow deposited the bed, which can occur in any water depth. These beds may be tempastites, ie. beds formed by storm events, in this case in water deeper than storm wave base but certainly not anything approaching abyssal depths. Or they may have formed - 66 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Figure 16. Well graded sandstone layers. Though these are probably turbidites they were not deposited in excessively deep water and may, in fact, represent distal tempestites. Stratigraphic position is problematic as these beds may belong to either the Upper Gunflint or Rove Formations. Stop 5, Quarry Island. from inter- or overflow sediment-water plumes off river mouths, though lack of current reworking of bed tops makes this unlikely. Alternatively they may represent prodelta deposits formed by slumping of the delta front. All of these environments are relatively shallow which agrees with the presence of stromatolites not far stratigraphically below the graded beds. Another Figure 18. A second example of unusual marking observed at Stop 6. The origin of these markings is unclear. Stop 5, Quarry Island interesting point concerning these clastic units is that although such sandstones are common in the upper Rove Formation they are not present in the Gunflint at any other location. Thus, their stratigraphic position is debatable. The third unusual attribute is the presence of difficult to interpret structures on some bedding planes. Series of enechelon small crack-fill like features cut across bedding planes (Fig. 17). In addition a jellyfishlike impression was found on a bedding plane (Fig. 18). This feature had five-fold symmetry, similar to echinoderms, but the age of the rocks and its presence in sandstone leads to the distinct possibility that it was manufactured. Stop 6 – Pass Lake Formation, Quarry Island A 50 meters inland from the last shore outcrop (Stop 5) where the iron train rail lies across the cobble-boulder beach - this stop will be time dependant. This outcrop consists of a cliff-face in sandstones, which were quarried, and the blocks produced used in the construction of buildings in Thunder Bay (Fralick et al., 2000). Here we see the basal sediments of the Sibley Group, the Pass Lake Formation. The Pass Lake forms a diverse group of basal coarse clastic deposits representing environments ranging from braided fluvial through to subaqueous sand sheets. The mediumgrained sandstones present in this cliff are organized into a series of large-scale planar cross-stratified sets with normal to low dip angles. Sorting is fairly good and only one pebble has been found in the succession. Assemblages such as this pose a dilemma in formulating Figure 17. Unusual markings on the bedding planes of the an interpretation of their depositional environment. graded sandstones. Stop 5, Quarry Island. Both aeolian sand dunes and sandflats composed of - 67 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 i —I • , • —l •d• Figure 19. Basal conglomerate of the Gunflint exposed at Gut Point. Photo courtesy of Mark Smyk. transverse bars in braided rivers are capable of creating such an organization of lithofacies. The presence of the pebble is significant as only freak wind-storms, such as tornadoes, can move material of this size, and these do not form sand dunes. So, it is more likely that these deposits are subaqueous but this rests only on a slim piece of evidence. Figure 21. Black chert with gossan within the basal conglomerate of the Gunflint Formation at Gut Point. Photo courtesy of Mark Smyk. is matrix-supported, with subangular to rounded pebbles of white, sugary quartz, lesser cherty and lithic fragments and minor jasper in a medium-grained, sandy matrix (Fig. 20). A black, pyritiferous chert breccia is marked by a conspicuous gossan (Fig. 21). Sulphide mineralization may be related to a persistent, parallel fracture set at 115°. Fractures may host quartz-calcitebarite veins ranging from < 1 to 20 cm wide as well Stop 7 – Basal conglomerate of the Gunflint, Gut as vein breccia. A conjugate fracture set at right angles to the first is locally developed. Silicification adjacent Point to the veins has preserved a thin (1 to 5 cm) veneer of UTM coordinates – 0461610E 5408587N Gunflint from being eroded (especially the carbonate The unconformity and basal Gunflint are exposed units). A 2m wide diabase dyke strikes at 115° through at Gut Point as a thin, discontinuous veneer along the the outcrop. lakeshore on top of Archean basement (Fig. 19). The basement is a medium-grained, equigranular granite, Stop 8 – Archean basement, Selim Point which has been altered (sausseritized/chloritized) UTM coordinates – 0469219E 5409146N beneath the basal Gunflint. The basal conglomerate is up to 30 cm thick and occupies depressions in the paleoFrom the dock in Rosport return to Highway 17 and erosion surface in the basement. The conglomerate head east for ~5 km. Turn right on to Lakeshore Drive just west of Whitesand Provincial Park. Follow the dirt road to a parking spot opposite a small tombola (Fig. 22). The porpyritic granite exposed here is Archean in age and part of the Wawa Subprovince. The area was mapped by Carter (1988) who described the rocks as porphyritic pink, hornblende + biotite alkali feldspar granite, a phase of the Whitesand Lake Batholith. The porphyritic “facies” is surrounded by massive pink and grey phases of alkali feldspar granite that is not exposed at this locality. The batholith is about 8 x 16 km in size and intrudes the Schreiber greenstone belt; no radiometric date has been generated. Feldspar Figure 20. Matrix supported basal conglomerate of the phenocrysts are typically 3-4 cm across (Fig. 23), Gunflint Formation containing rounded pebbles of quartz, subhedral to euhedral and in places appear to display chert and lithic fragments. Photo courtesy of Mark Smyk. localized alignment suggestive of flow banding. - 68 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Morey, G., Sutcliffe, R., and Spencer, C., 1989. The North American Midcontinent Rift beneath lake Superior from GLIMPCE seismic reflection profiling. Tectonics, 8, 305-332. Carter, M.W. 1988. Geology of the Schreiber-Terrace Bay area, District of Thunder Bay; Ontario Geological Survey, Open File Report 5692, 287p. Cheadle, B.A., 1986. alluvial-playa sedimentation in the lower Keweenawan Sibley Group, Thunder Bay District, Ontario. Canadian Journal of Earth Sciences, v. 23, p. 527-542. Davis, D.W., and Green, J.C., 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution. Canadian Journal of Earth Sciences, 34, 476-488. Davis, D.W. and Sutcliffe, R., H., 1985. U-Pb ages from the Nipigon plate and northern Lake Superior. Geological Society of America Bulletin, 96, 1572-1579.Davis, D.W., and Green, J.C., 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution. Canadian Journal of Earth Sciences, 34, 476-488. Figure 22. Porphyritic Archean granite, Selim Point. Fralick, P.W. and Barrett, T.J., 1995. Depositional controls on iron formation associations in Canada. In, ed by A.G. Plint, Sedimentary Facies Analysis, Special Publication of the International Association of Sedimentologists, v. 22, p. 137-156. Fralick, P.W., Kissin, S.A. and Davis , D.W., 2002. The age of the Gunflint Formation, Ontario, Canada: single zircon U-Pb age determinations from reworked volcanic ash. Canadian Journal of Earth Sciences, v. 39, p. 1085-1091. Figure 23. Feldspar phenocrysts in Archean porphyritic granite at Selim Point. Acknowledgements We would like to thank Mark Smyk and John Scott for their help and advice in the preparation of this field guide. In particular Mark Smyk provided text and photographs for Stop 7. References Addison, W.D., Brumpton, G.R., Vallini, D.A., Davis, D.W., Kissin, S., Fralick, P.W., McNaughton, N.J., and Hammond, A., 2005. Discovery of distal ejecta from the 1850 Ma Sudbury impact event. Geology, 33, 193-196. Cannon, W., Green, A., Hutchinson, D., Lee, M., Milkereit, B., Behrendt, J., Halls, H., Green, J., Dickas, A., Fralick, P.W., Smyk, M. and Mailman, M., 2000. Geology and stratigraphy of the Mesoproterozoic Sibley Group. In, ed. by P. Fralick, Fieldtrip Guide Books, Forty-Sixth Annual Meeting, Institute of Lake Superior Geology. p. 7-42. Franklin, J.M., 1978. The Sibley Group, Ontario. In, Rubidium-strontium isochron age studies, Report 2, Geological Survey of Canada, Paper 77-14, p. 31-34. Giguere, J.F., 1975. Geology of St. Ignace Island and adjacent islands, District of Thunder Bay. Ontario Ministry of natural Resources, Geological Report 118, 35p. Halls, H.C., 1974. A paleomagnetic reversal in the Osler Volcanic Group, Northern Lake Superior. Canadian Journal of Earth Sciences, 11, 1200-1207/ Heaman, L.M., and Machado, N., 1992. Timing and origin of the Midcontinent Rift alkaline magmatism, North America: evidence from the Coldwell Complex. Contributions to Mineralogy and Petrology, 110, 289303. - 69 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Hemming, S.R., McLennan, S.M. and Hanson, G.N., 1995. Geochemical and Nd/Pb isotopic evinence for the provinance of the early Proterozoic verginia Formation, Minnisota. Implications for the tectonic setting of the Animikie Basin. Journal of Geology, v. 103, p. 147-168. Hollings, P., Fralick, P. and Cousens, B., 2005. Early History of the Mid-Continental Rift Inferred from Geochemistry and Sedimentology of the Proterozoic Osler Group, Northwestern Ontario. Submitted to GSA Bulletin. Kissin, S.A. and Fralick, P.W., 1994. Early Proterozoic volcanics of the Animikie Group, Ontario and Michigan, and their tectonic significance. Proceedings Institute of Lake Superior Geology, v. 40, p. 18-19. Sutcliffe, R. H., 1986. The petrology, mineral chemistry and tectonics of Proterozoic rift-related igneous rocks at Lake Nipigon, Ontario. Unpublished Ph.D. thesis, University of Western Ontario, London, 325p. Sutcliffe, R.H., and Smith, A.R., 1988. Project number 8717. Geology of the St. Ignace Island volcanic-plutonic complex. Summary of Fieldwork and Other Activities 1988. Ontario Geological Survey Miscellaneous Paper 141, 368-371. Tanton, T.L., 1931. Fort William and Port Arthur, and Thunder Cape map areas, Thunder Bay District, Ontario. Geological survey of Canada Memoir 167, 222p. Kissin, S.A., Vallina, D.A., Addison,W,D. and Brumpton, G.R., 2003. New zircon ages from the Gunflint and Rove Formations, northwestern Ontario. Proceedings Institute of lake Superior Geology, Lightfoot, P., Sutcliffe, R., and Doherty, W., 1991. Crustal contamination identified in Keweenawan Osler Group tholeiites, Ontario: A trace element perspective. Journal of Geology, 99, 739-760. McIlwaine, W.H., and Wallace, H., 1976. Geology of the Black Bay Peninsula Area, District of Thunder Bay, Accompanied by Map 2304, scale 1 inch to 1 mile. Ontario Division of Mines, GR133, 54p. Miall, A.D., 1978. Lithofacies types and vertical profile models in braided river deposits: A summary. In ed. A.D. Miall, Fluvial Sedimentology, Canadian Society of Petroleum Geologists Memoir 5, 597-604. Nicholson, S.W., Shirey, S., Schulz, K., Green. J., 1997. Riftwide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development. Canadian Journal of Earth Sciences, 34, 504-520. Paces, J.B., and Miller, J.D, Jr., 1993. Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota; geochronological insights to physical, petrogenetic, paleomagnetic, and tectonomagnetic processes associated with the 1.1 Ga Midcontinent Rift System. Journal of Geophysical Research, B, Solid Earth and Planets, vol.98, no.8, pp.13,997-14,013. Pufahl, P.K. and Fralick, P.W., 2004. Depositional controls on paleoproterozoic shallow-water iron formation accumulation, Gogebic Range, Wisconsin, U.S.A. Sedimentology, v. 54, p. 791-808. Shirey, S., Lewin, K., Berg, J., and Carlson, R., 1994. Temporal changes in the sources of flood basalts: Isotopic and trace element evidence from the 1100 Ma old Keweenawan Mamainse Point Formation, Ontario, Canada. Geochimica et Cosmochimica Acta, 58, 4475-4490. - 70 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Fieldtrip 4 - Geology and rare element pegmatites of the Quetico Subprovince near Nipigon Mark Smyk Ontario Geological Survey, Resident Geologist’s Program, Thunder Bay, Ontario, Canada Stephen Kissin Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Overview The Quetico Subprovince of the Superior Province is situated between the Wabigoon and Wawa volcanoplutonic subprovinces that bound the Quetico on its northern and southern margins, respectively. This easttrending subprovince has a fairly consistent width of 70 km and consists predominantly of metasedimentary rocks and their migmatitic and anatectic derivatives (Williams, 1991). In general, the boundaries of the Quetico, whether or not they may be primary and/or tectonic, have been mapped as steeply dipping surfaces across which there is commonly a distinct contrast in lithology. In northwestern Ontario, the Quetico has been defined to exclude mafic metavolcanic rocks and derived migmatites; these rocks are instead assigned to the bounding Wawa and Wabigoon subprovinces. In contrast, Seemayer (1992) also described an asymmetric metamorphic grade distribution that had metamorphic grade increasing from south to north across the Quetico, southwest of Lake Nipigon. The southern margin was characterized by greenschist-facies rocks, the central portions were at amphibolite facies and the northern margin displayed an abrupt decrease in grade adjacent to the Wabigoon Subprovince to the north. The Quetico Fault, which is normally situated at the northern margin of the Quetico, lies well within the Quetico southwest of Lake Nipigon (Seemayer, 1992). Seemayer (1992) determined temperatures based on garnet-biotite thermometry ranging from 526°C at the southern margin of the Quetico Subprovince, increasing asymmetrically to a maximum of 714°C, then falling sharply at the northern margin to 517°C. An overview of the lithologic, metamorphic, structural and tectonic characteristics of the Quetico Subprovince has most recently been provided by Easton (2000): “The intensity of metamorphism varies within the subprovince, such that rocks marginal to the subprovince tend to be at lower grade than in the interior. The lowest metamorphic grade is found along the northern boundary with the Wabigoon subprovince (Pirie and Mackasey, 1978). Locally, subgreenschistto greenschist-facies rocks occur along the southern boundary (Borradaile, 1982), but typically, there is a rapid rise in metamorphic grade north of the Wawa subprovince, especially north of Manitouwadge, where a belt of metasedimentary granulites occurs within the Quetico subprovince close to, and parallel with, the northern margin of the Wawa subprovince (Coates, 1968; Williams and Breaks, 1989, 1990; Pan et al., 1994). As a result, grade distribution is asymmetrical, with the maximum in temperature and pressure occurring south of the central Quetico, locally coincident with the southern margin.” 4 Figure 1. Field trip stop locations. - 71 - I �Proceedings of the 51st ILSG Annual Meeting - Part 2 _______ ________ GeneaI geology of the Quetico Subprovince near Nipigon (after Williams 1991) Key to Locations: 7. Jean lake B. Lake HSen II. Georgia Lake 50km ._— Alkalic rrptexes Y to Paleoproterozoic ver and intrusions; Pharierozoic ver Moso Trerd of regional S fabric Sttatigrapliic fadng Fault arrows iricate MusooVe-beaiing granffio raCKs (peralumir,ous). may indude slip sense, baths on upthrow &de. if mown biothe granite S yncline, Massive granadiodte to granite; unsubdivided, mainly biotfte beang antdiie Gneis&c and foliated tonalite wiles Metasedinientary rocks: wacke, paragneiss, granulite; minor mac units Wabigoon, Wawa and Abitibi subprovincos Figure 2. Generalized regional geology (after Williams, 1991). Pressure calculated at near peak temperature was 5 ± 1.5 kbar. Easton (2000) described the regional metamorphic conditions and metamorphic history: “P–T conditions increase from west to east, for example, 500ºC and 2.5 kbar at the Minnesota border west of Thunder Bay (Percival, 1989), to 700–780ºC and 5.4–6.1 kbar adjacent to the Kapuskasing structural zone (Percival and McGrath, 1986; Percival, 1989). Typical conditions in the central region are on the order of 620ºC and 3.3 kbar (Percival, 1989). Granulites north of Manitouwadge yield 680–770ºC and 4.4–6.4 kbar (Pan et al., 1994; Percival, 1989). The regional variation in P–T can be ascribed to a relatively shallow level of erosion in the west (<10 km) and a deeper level in the east (>12 km) (Percival et al., 1985). Rocks located east of the Kapuskasing structural zone are believed to be generally at upper-amphibolite-facies conditions (Williams, 1991).” “Evidence for an earlier, medium-pressure, low-temperature, pre-tectonic or early syntectonic metamorphism comes from four areas within the subprovince. In the Atikokan region, and in northern Minnesota, both at the northern margin of the Quetico subprovince, an early M1 metamorphic peak between D1 and D2 produced Ky–St–Bt assemblages (Ayres, 1978; Tabor et al., 1989). Kyanite inclusions in plagioclase within Grt–Sil–Bt–Pl–Qtz schist near Raith, north of Thunder Bay, have been reported by Percival et al. (1985). Kehlenbeck (1976) also presented textural evidence for a polymetamorphic history along the northern margin of the Quetico subprovince north of Thunder Bay. Again, along the northern margin of the subprovince, in the Beardmore–Geraldton area (Williams, 1989), amphibolite-facies conditions were attained prior to D2 deformation, with the distribution of facies being structurally controlled within thrustbounded panels (Williams, 1991).” “In contrast, the main phase of regional metamorphism (M2), which produced the observed - 72 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 map-pattern (Fig. 2), occurred late syntectonically (Sawyer, 1983; Williams, 1991). The general sequence of isograds, based on the appearance of diagnostic assemblages in pelites, is Chl–Ms–Bt, Grt+And+Sil, Grt+Crd+Sil, in situ granitic leucosome, and Opx (Pirie and Mackasey,1978; Percival and Stern, 1984). The common occurrence of Grt–And in metapelites in the western Quetico subprovince is diagnostic of bathozone 2 (<3.4 kbar; Carmichael, 1978), whereas the presence of Sil–St in the eastern Quetico is diagnostic of bathozones 3 and 4 (3.4–5.5 kbar).” “As noted by Williams (1991), tectonic thickening of the sedimentary pile and intrusion of minor Itype granitic rocks occurred prior to the thermal acme. Most of the large pre- to syntectonic granitic bodies are peraluminous and have sedimentary sources but display little evidence of thermal contact metamorphism; one exception is the South Beatty Lake pluton in the northern Quetico subprovince (Pirie & Mackasey, 1978). Steeply dipping thermal gradients, local increases in temperature around large plutons, and the general association of the highest-grade rocks with abundant generation of leucosome, indicate that the source of heat was a combination of burial, upward magmatic transport, and tectonism.” “In the northern Quetico, M1 metamorphism is estimated to have occurred between 2698 Ma, the maximum age of sedimentation (Davis et al., 1990) and 2688+4 Ma, the age of emplacement of the late syntectonic Blalock pluton (Davis et al., 1990). In the southern Quetico, M1 occurred after 2690 Ma, the maximum age of sedimentation (Zaleski et al., 1999). The timing of M2 metamorphism is less well constrained, and may have been protracted. In the Manitouwadge area, Zaleski et al. (1999) constrained regional D2 deformation to 2680–2677 Ma, and suggested that migmatization in both the northern Wawa and southern Quetico subprovinces occurred after 2679 Ma, broadly coincident with D3 deformation. This inference is consistent with observations elsewhere in the Quetico that contact aureoles around late plutons, dated at 2671+2 to 2665+2 Ma, as well as late granitic pegmatites dated at 2653+4 Ma, overprint regional metamorphic fabrics (Percival and Sullivan, 1988; Percival, 1989). North of Manitouwadge, Pan et al. (1998) reported a U–Pb zircon age of 2666+1 Ma from a granitic pegmatite concordant with respect to the D3 fabric, and suggested that the regional amphibolitefacies metamorphism occurred between 2671 and 2665 Ma, consistent with the ages cited above. The timing of peak granulite-facies metamorphism north of Manitouwadge appears to be some 15 Ma younger, on the basis of U–Pb zircon ages of 2650+ and 2651+3 Ma from a mafic granulite and a tonalitic leucosome, respectively (Pan et al., 1998). Zaleski & van Breemen (1997) reported that titanite ages young with increasing metamorphic grade, ranging from ~2686 Ma in the southern Manitouwadge greenstone belt to ~2640 Ma in the southern Quetico, suggesting that the thermal effects of regional metamorphism may have lasted over ~30 million years, from 2677 to 2640 Ma, in the higher-grade parts of the Wawa and Quetico subprovinces. On the basis of their regional geological and geochronological studies, Zaleski et al. (1999) concluded that “M2 metamorphism occurred after the tectonic juxtaposition of the Quetico and Wawa subprovinces.” This field trip will cover the southern two-thirds of the Quetico Subprovince, from south of Beardmore to Nipigon (Fig. 1). As mentioned above, there is an asymmetric distribution in metamorphic grade, with a gradual progression from greenschist-facies, clastic metasedimentary rocks near the northern contact with the Wabigoon Subprovince; to lower amphibolite-facies schists and gneisses; through to upper amphibolitefacies migmatites and derived granitic rocks near the southern contact with the Wawa Subprovince near Nipigon. Thermal and pressure maximum occurs south of the center of the Quetico. The metamorphic character is of high-temperature/low-pressure (Abukuma-type) metamorphism, associated with the abundant intrusion of granitoid rocks and the regional distribution of migmatites derived from the metasedimentary rocks (Kamineni et al., 1988; Percival and McGrath, 1986; Percival, 1989; Williams, 1989). Peak metamorphic assemblages in the field trip area suggest conditions >650ºC and 5 kbar, corresponding to bathozones 4 to 5 (14 to 15 km deep in the crust) (Easton, 2000; Percival, 1989). The nomenclature of Mehnert (1968) has been used in describing migmatites in this field guide. Overview of the Georgia Lake Pegmatite Field The Georgia Lake pegmatite field, as described by Pye (1965) and Mulligan (1965) represents the largest concentration of rare element mineralization in the - 73 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Table 1. Estimated reserves for the Georgia Lake pegmatite field. Deposit Estimated reserves (tons) Average grade (% Li2O) 759,475 1.65 96,000 1.50 Conway 1,830,000 0.96 Jean Lake No. 4 (Parole) 1,689,000 1.30 261,000 1.03 Nama Creek North Zone 2,784,000 1.11 Nama Creek South Zone 1,508,332 0.96 Jackpot 2,000,000 1.09 750,000 1.38 11,677,807 1.14 Aumacho No. 1 (Brink) Aumacho No. 2 Aumacho No. 3 McVittie Vegan No. 2 (Newkirk) Total Superior Province of Ontario (Breaks et al., 2003a). It comprises 38 rare element occurrences and 10 spodumene-bearing, pegmatite deposits (total resource of 11.7 million tons grading an average of 1.14% Li2O; Pye, 1965; Table 1; Fig. 3) were discovered during an exploration rush in the 1950’s (Pye, 1956). As delineated by Breaks et al. (2003a), the field covers approximately 1200 km2 (32 km by 37 km) and is hosted by upper greenschist- to lower amphibolitefacies, clastic metasedimentary country rocks. Four types of rare element pegmatites (using the classification of Cerný (1991a) have been identified locally; Breaks +:duaoeke aatholth OpSb1::*:..:::.,11rr._ + + L Diebase dikes and Rare V.] (co"pIredfoundbype5entsu'vey) Slb1o GrOtJP. xnsobdivlded ,. be beryp ,.I,wrtonaI,t and m!gn,arnic m.tas.diftl.ntary col -tent endaver GPaOer Lake eatholith Li coumbite-t.ntaILte Lithium Peraluminous S-type graniuc rocks, ssIte,1t pi-nt grantte and pota5sc pegTlaUte r1 clastic maaethn,entary rocks io and medium maçnec oxLdeminera grade metawacke and m*tap.IIte Pet spod petahte spodvnw,.e Maflc to int.rmslate metavorcan,c rocks tour tourmalint bçOOn subprwMce Figure 3. Generalized regional geology (after Breaks et al., 2003a). - 74 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Table 2. Subdivisions of the Georgia Lake pegmatite field, after Breaks et al. (2003a) Pegmatite group Postagoni River Lake Jean Barbara Lake Forgan Lake Type Albite-spodumene Albite-spodumene Albite-spodumene Albite-spodumene Examples McVittie; Dunning; Nama Creek Foster; Giles; Trans Vegan; Newkirk; Point; Jackpot Forgan; Lucky Lake (Lun-Echo) Cosgrave Lake Beryl±petalite sub-type Swanson; MNW Gathering Lake Albite Newly discovered, unnamed biotite-muscovite and muscovite-biotite granite and granodiorite. et al., 2003a): 1) Beryl-type 2) Albite-spodumene-type 3) Complex-type/petalite-subtype 4) Albite-type The Georgia Lake pegmatite field was initially subdivided by Milne (1962) and then Zayachivsky (1985) and Kissin and Zayachivsky (1985) into the Northern, Central and Southern groups. More recent data and mapping conducted by Breaks et al. (2003a) has led to a revised, six-fold subdivision (Table 2). The two pegmatites that will be visited on the field trip, the Dunning pegmatite (Stop 2) and the Foster pegmatite (Stop 4), are examples of the albitespodumene-type. The Glacier Lake Batholith (GLB) is a large mass of medium- to coarse-grained, S-type granite and pegmatitic granite situated immediately south of the northern metasedimentary part of the Quetico Subprovince, extending east of Highway 11 at least 50 km. It dominantly consists of grey, medium- to coarse-grained biotite and muscovite-biotite granite, which locally contains minor tonalitic and migmatitic metasedimentary enclaves. Generally massive, this rock may display a local, weak foliation developed in 1 to 2 cm-thick, shallowly lineated veins of fibrolite, muscovite and quartz that appear to have formed during aluminous hydrothermal alteration controlled by late shearing (Breaks et al., 2003a). Gradation into __ Zayachivsky (1985) divided the felsic plutonic rocks in the Georgia Lake area into three groups (Fig. 4): —— SoRe (1) Two-mica leucogranites, occurring as a large plutonic mass south of the pegmatite field (cf. Glacier Lake Batholith) and as smaller, satellite intrusions; * T—sqs — (2) Kilgour Lake Group granitoids centered around a small gabbroic unit near Kilgour Lake; and I mon I Isiopil — 2t (3) Tonalitic sills, distributed throughout the pegmatite field. / 7 .7./ - [.L Zayachivsky (1985) suggested that the two-mica leucogranites and tonalitic sills were derived from the partial melting of metapelites and metawacke, respectively (i.e. S-type granitoids). The Kilgour Lake rocks were presumed to be the products of the fractional crystallization of a mafic magma generated in the upper mantle/lower crust (i.e. I-type granitoids). Breaks et al. (2003a) described two distinct, fertile, peraluminous, S-type granite plutons (MNW and Barbara Lake stocks) that occur within metasedimentary country rocks. They consist of virtually identical, grey, medium-grained, generally homogeneous, __ __ — — —— Kikwtk. S,S. CR -— MMflOak Figure 4. Plutonic rocks in the Georgia Lake area (after Pye, 1965; Zayachivsky, 1985; Breaks et al., 2003a). - 75 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 consanguineous pods of tourmaline-muscovite potassic pegmatite up to 1 m in diameter occurs locally. There are several episodes of S-type granitic magmatism in the GLB. Undeformed, tourmaline-cordierite or tourmuscovite dikes are the youngest and most chemically evolved units (F. Breaks, Ontario Geological Survey, personal communication, 2004). can be pursued by the identification of dispersion haloes of rare alkali metals (Li, Rb, Cs) in country rocks and some other elements such as Be, Sn and Cu. Hidden pegmatite targets in promising terrain may thus be located. Pye (1965) undertook the first lithogeochemical survey for rare elements in the aforementioned felsic plutonic rocks in the Georgia Lake area. Many anomalies, ranging from 100 to 400 ppm Li, were noted. Uranium-bearing pegmatite dykes are associated with felsic plutonic rocks in this part of the Quetico Subprovince. Several occurrences have been documented by Scott (1987), including some in the vicinity of Nipigon. White, albite-rich, biotitequartz+muscovite+apatite pegmatite dykes up to 30 m wide intrude gneissic and migmatitic rocks. Yellow and yellow green stain (secondary uranophane (Ca(UO2)2Si2O7 · 6H2O?)) characterize many of the uraniferous dykes. Uraninite appears to be the primary uranium-bearing mineral (Franklin, 1978). The Lake Helen occurrence is situated on the western shore of Lake Helen, opposite the roadside rest area on Highway 11 (Stop 7). Trenching and sampling of this occurrence in 1967 by Aggressive Mining Limited yielded values up to 0.135% U3O8 (ibid). Such dykes are considered to be the primary source of uranium that occurs in structures at or near the Archean / Proterozoic unconformity in the Nipigon Basin. Two modes of source fluid derivation may have operated during the emplacement of the Georgia Lake pegmatites (Zayachivsky, 1985). Pegmatites of the Central Group were derived by igneous fractionation of a granitic parent near Barbara Lake and emplaced as low-viscosity, volatile-rich melts in a direction away from the source area. This is supported by a fractionation trend defined most clearly by the concentration of Rb and Cs in perthitic microcline and muscovite across the Central Group. The pegmatites of the Northern Group may have been the result of direct anatexis of lithium-rich Quetico metasedimentary rocks. Breaks et al. (2003a) described the MNW pegmatite (petalite sub-type) as the most evolved of the local pegmatites, based on its geochemistry, mineralogy, including the occurrence of manganotantalite. A general rule for rare element-bearing Ontario pegmatites is that tantalum mineralization is associated with lithium mineralization in the form of spodumene and/or petalite (ibid). Prediction of the source of lithium deposits in “fertile” parental granites is a somewhat uncertain art. The essential parental granite is a siliceous, peralkaline leucogranite, but the lithium content of a prospective parental granite is not highly indicative of the presence of lithium-rich pegmatites (Cerný, 1991a). Data summarized by Stewart (1978) and more recently by Cerný (1991a) indicate that Li-contents in parental granites are typically <100 ppm. Moreover, analyses of bulk compositions of spodumene- and petalitebearing pegmatites yield a closely grouped mean of 1.53 weight % Li2O (Stewart, 1978) and range between 1.5 and 2.0 weight % Li2O (Cerný, 1991b). These data indicate that differences between spodumene- and petalite-bearing pegmatites are caused by pressure and temperature differences during crystallization rather than differences in bulk composition. Cerný (1991b) suggested that exploration for rare-element pegmatites Uraniferous Dykes Stops Field Trip Road Log Stop Locality Lake Jean stops Intersection of Hwy.’s 11 &17 in the Town of Nipigon Take Hwy 11 north Gorge Creek turn-off - head east Reset Postagoni River Bridge 1 Postagoni Stock 2 Dunning pegmatite Powerline crossing 3 Quetico metasedimentary rocks 4 Foster pegmatite Lake Helen stops Intersection of Hwy.’s 11 &17 in the Town of Nipigon Take Hwy 11 north 5 Glacier Lake Batholith - 76 - km 0 37.4 12.4 14.4 18.4 19.1 28 28 0 17.1 �Proceedings of the 51st ILSG Annual Meeting - Part 2 6 7 8 9 10 11 Biotite leucogranite Migmatite - roadside rest area Pull-off area Pegmatitic granite Pegmatitic granite Pegmatites in migmatite Intersection of Hwy.’s 11 &17 in the Town of Nipigon Take Hwy 17 east Migmatites (optional) rocks and display no foliation nor significant internal variation. An U-Pb zircon age of 2684.8 + 2.1 Ma was determined for the Postagoni Stock (D. Davis, University of Toronto, unpublished data). 10.8 7.4 5.9 5.2 4.85 4.6 Stop 2 - Dunning Pegmatite UTM coordinates - 0427823E 5474882N 0 18.4 Stop 1 - Postagoni Stock UTM coordinates - 0425041E 5472869N Large, locally derived boulders of leucotonalite of the Postagoni Stock (a.k.a. Postagoni Lake sill) occur on the eastern side of the forest access road. Pye’s (1965) mapping identified a porphyritic granitic intrusion approximately 1.6 km long and 700 m wide in metasedimentary country rocks. The McVittie pegmatite cuts the stock north of Dive Lake; it was visited during an ILSG field trip in 1990 (Kissin, 1990). Both the McVittie and Dunning (Stop 2) pegmatites are part of the Northern Group (Zayachivsky, 1985) or Postagoni Lake Group (Breaks et al., 2003a), comprising unzoned albite-spodumene-type pegmatites. This small roadside outcrop exposes the Dunning pegmatite dyke, which has an estimated width of 2 to 3 m at this location and intrudes Quetico metasedimentary rocks. Woolverton (1956) noted that pegmatite dykes on the Dunning property were exposed intermittently in a zone extending over 600 m in length. This dyke is part of Zayachivsky’s (1985) Northern Group, which are characterized by medium- to very coarse-grained, perthitic microcline and spodumene in a fine-grained matrix of quartz, muscovite and albite, with trace amounts of apatite and garnet. They are typically unzoned to poorly zoned and commonly contain aplitic stringers, bands and pods, within and parallel to dyke contacts. This dyke is a typical albite-spodumene-type pegmatite (Fig. 5). Green spodumene crystals, up to 4.5 cm long, are aligned perpendicular to the dyke Zayachivsky (1985) described this sill as a coarsegrained tonalite with closely packed crystals of plagioclase up to 1.5 cm long, locally imparting a porphyritic texture. This texture occurs throughout the sill except near its contacts with the metasedimentary country rocks where a several metre-wide chilled margin has developed. The chilled margin texture resembles that of the porphyry dykes described by Pye (1965). A modal analysis of the Postagoni Stock was given by Zayachivsky (1985; Table 3). Table 3. Modal analysis of the Postagoni Stock. Mineral Plagioclase K-feldspar Quartz Muscovite Biotite Epidote Zircon Modal abundance(%) 66.6 5.6 25.7 0.4 1.6 0.1 Trace The Postagoni Stock and the other local tonalitic sills are invariably in sharp contact with the country Figure 5. Drusy spodumene, Dunning pegmatite (Stop 2). - 77 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 ___ ______________________ ____________ -2 _______________ Logging Camp (abandonedi - Sketch Map of the Foster Pegniatite area Amieno Resourc.s Inc. 19B7) STOP 3 nd // / •/./ /// STOP4 of the submarine margin and trench-fill are present in the Quetico (Fralick et al., 1992; Williams, 1989). 10 to 20 cm thick sandstone-siltstone beds strike 100º and likely dip steeply to the south, based on observations in the area by Pye (1965). Graded bedding suggest tops to the south (Fig. 7). Pye (1965) noted that biotitequartz-feldspar schist such as this are most commonly granoblastic; garnet porpyhryoblasts have also been noted. Some lenticular pods of quartzo-feldspathic leucosome are present. Stop 4 - Foster Pegmatite 9 a 100ut1 UTM coordinates - 0436379E 5473665N .---—----—Metasedimentary rocks Figure 6. Sketch map of Foster pegmatite area (Stops 3 & 4). contact. This phenocrystic spodumene typically contains several percent iron and is commonly altered to a dark green to black alteration product of chlorite and sericite (aka “rotten spodumene”). Some altered, brown spodumene is also present, along with quartz, albite (+cleavelandite), beige alkali feldspar, green muscovite and minor oxides. Pye (1965) reported that the lithia contents of the two dykes on the Dunning property were low and of no economic importance. Stop 3 - Quetico Metasedimentary Rocks UTM coordinates - 0436345E 5473931N Medium-grade clastic metasedimentary rocks typical of the north-central part of the Quetico Subprovince are exposed in a small outcrop north of the Foster pegmatite (Fig. 6). South of the Beardmore-Geraldton belt, more highly metamorphosed, turbiditic sediments A series of stripped and washed outcrops reveals a swarm of narrow pegmatite dykes cutting an easttrending sill of massive, biotite tonalite, termed the Parole Lake sill by Zayachivsky (1985; Fig. 8). The Foster deposit was discovered in 1956 and was tested by diamond drilling later that year by Towagmac Exploration Co. Ltd. (Walter, 1957; Pye 1965; Table 4). This work indicated that the lithium deposit was too low-grade and the property was allowed to lapse. Armeno Resources Inc. explored the property (including the neighbouring Lew deposit) from 1986 to 1989, carrying out stripping, geological, magnetometer and electromagnetic surveys, as well as diamond drilling (Assessment Files, Thunder Bay North District, Thunder Bay). The lone drill hole intersected biotite tonalite with numerous quartz-rich, pegmatitic dykes < 1m thick with up to 5% spodumene; no chemical analyses were reported (ibid). The main Foster dyke strikes approximately 085º and dips 80º to 85º south. It is exposed continuously 1 F? 'r--- r-- r - t'r Figure 7. Graded bedding in metaturbidites, Foster area (Stop 3). Figure 8. Pegmatite dykes in tonalite, Foster (Stop 4). - 78 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Table 4. Results of drilling at the Foster deposit (Walter, 1957). Drill Hole Number 7 6 9 Location (feet) 3+50 west 4+50 west 6+50 west 10 7+50 west Core length (feet) 29.0 30.0 31.3 20.0 32.7 over 75 m and averages 9 m in thickness. To the west, it splits into a number of thin, parallel dykes. In the most westerly drill hole (T-10), 7 individual dykes, 0.3 to 2.4 m wide were intersected, separated by sections of host tonalite a few centimetres to a metre wide (Pye, 1965). The deposit contains 10 to 15% spodumene, which is for the most part, unaltered. Similar to other deposits in the Lake Jean Group, crystals are prismatic and perpendicular to dyke contacts and may extend across the entire dyke (ibid). Milne (1962) listed the Foster pegmatite in the Jean Lake sub-group (with the Giles, Camp and Lew pegmatites) on the basis of welldeveloped spodumene orientation and the occurrence of narrow, branching and continuous, longitudinal aplite dykes. Lithia Content (%) 0.26 0.55 1.04 0.58 0.58 Large road cuts on both sides of Highway 11 provide excellent exposures of the Glacier Lake Batholith (GLB). At this location, it consists of white, massive to locally foliated, muscovite < biotite, medium- to coarsegrained granite. Localized pods of tourmaline-biotitemuscovite-potassium feldspar pegmatite occur within the granite. These pods may reach 1 m in diameter and locally contain tourmaline-quartz intergrowths. Numerous, curvate, fibrolite - muscovite + tourmaline veins up to 1 cm thick also occur in the host granite. Purple fluorite is exposed on a fractured outcrop face on the west side of the highway (Fig. 10). Stop 6 - Glacier Lake Batholith Leucogranite/ M igmatite UTM coordinates - 0407163E 5440408N This is another example of the southern margin of the Glacier Lake Batholith where it is in contact with metasedimentary migmatite (Fig. 11). White, biotite-muscovite leucogranite contains rare, bluegreen apatite. Foliation is developed in feldspathic segregations and biotitic seams. Leucogranite dykes and migmatite locally exhibit a lit-par-lit structure. Tight to isoclinal folds have developed in the quartzbiotite-feldspar schist (Fig. 12). All rocks display boudinage and folding. Folded leucosome suggests a Figure 9. Drusy spoduemene, Foster pegmatite (Stop 4). These are classic albite-spodumene-type pegmatites. Fresh green to dark green-black, altered spodumene crystals may reach lengths of 8 cm (Fig. 9). Other constituent minerals include K-feldspar, albite (+ cleavelandite) and quartz, minor apatite and garnet, and tiny, sparsely distributed, black tantalum-niobiumbearing oxide minerals (F. Breaks, OGS, personal communication, 2004). A locally developed border zone (< 5 cm) consists of quartz + feldspar. Stop 5 - Glacier Lake Batholith Leucogranite UTM coordinates - 0409911E 5445897N Figure 10. Fluorite on fracture surface in sericite- and fibrolite-bearing Leucogranite (Stop 5). - 79 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 r '2..—:, 4 A- I.— A Figure 11. Contact between folded metasedimentary migmatite and Leucogranite (Stop 6). Figure 13. Folded metasedimentary schist, incipient leucosome development (Stop 6). an early and protracted deformation history (Fig. 13). Metasedimentary migmatite enclaves are common in the white, S-type pegmatitic granites along the highway. Note that less evolved S-type granite may contain biotite as the only mica. whaleback outcrop behind the highway rest stop/ pull-off area. There is quite a bit of variation in the relative amounts of leucosome and restite (Fig. 14). Boudinaged and pytgmatically folded leucosome pods and veins occur in a quartzo-feldspathic with narrow, biotitic restite septa. Narrow diabase dykes cut the country rocks on the western side of the highway. Stop 8 - Pegmatitic Granite UTM coordinates - 0407272E 5435208N Stop 7 - Migmatite UTM coordinates - 0406986E 5437331N Migmatites are exposed on the shore and islands of Lake Helen and on a large, glacially streamlined, Figure 12. Folded metasedimentary migmatite (Stop 6). This outcrop shows a transition in magmatism from S-type (i.e. Glacier Lake Batholith) to I-type granitoids. Pink, coarse-grained, biotite granite here consists of pink, coarse-grained, perthitic K-feldspar, quartz, coarse-grained biotite, and brown, altered plagioclase (up to 4 cm long). These weakly peraluminous, pegmatitic, biotite granites are relatively primitive and are younger than the white, two-mica leucogranites. Such rocks are probably of I-type origin and typically are metasedimentary enclave-free. Bulk rock levels of Figure 14. Lit-par-lit migmatites with large metasedimentary inclusions (schollen) (Stop 7 area). - 80 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 • [j\ ti'a( Figure 15. Pegmatitic K-spar - quartz – biotite granite (Stop 8). rare-elements are very low: 128 ppm Rb, 2.63 ppm Cs, 1.8 ppm Nb, 0.39 ppm Ta (F. Breaks, OGS, personal communication, 2004). A K-feldspar-megacrystic, pink biotite granite (Fig. 15) with a shallowly east-dipping foliation and metasedimentary xenoliths is intruded by a more massive granite at this locality. The intrusive contact is embayed and scalloped, suggestive of co-mingling magmatic textures (Fig. 16). The megacrystic, foliated granite may represent an earlier, xenolith-bearing phase that was intruded by the massive granite while still relatively warm and plastic. Stop 9 - Pegmatitic Granite UTM coordinates - 0407458E 5434906N feldspar megacrysts may attain lengths of over 60 cm. Stop 10 - Pegmatite Dykes in Migmatite UTM coordinates - 0407587E 5434690N Approximately 200 m south of the massive pink granite, white pegmatite dykes intrude fine-grained metasedimentary schist. This dark, fine-grained, feldspathic, biotite schist displays a well-developed foliation and minor folds, indicative of a long, protracted ductile deformation history, perhaps coeval with dyke emplacement. Note the sub-horizontal mineral lineation. Flattening of feldspars, producing small-scale, augen structures, is also indicative of hightemperature (>500º C) deformation. The boudinaged and annealed, sericite- and biotitebearing dykes range from a few centimetres (lit-parlit structured) to several metres in thickness. They are mineralogically and texturally interesting, containing Lt- ! H -v A large, bare, whaleback outcrop on the east side of the highway consists of coarse-grained to pegmatitic, massive, homogeneous pink granite (i.e. the younger granitic unit at Stop 8; Fig. 17). Crystals or interstitial patches of quartz, biotite and locally sericitized, Kfeldspar average 2 to 3 cm in size (Fig. 18). Individual Figure 17. “Whaleback” outcrop of coarse-grained to pegmatitic granite; note large (>60 cm) feldspar megacryst (circled) (Stop 9). Figure 16. Scalloped contact (arrow) between foliated biotite granite (top) and pegmatitic granite (bottom) (Stop 8). Figure 18. Coarse-grained to pegmatitic granite (Stop 9). - 81 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 both cordierite and quartz-tourmaline intergrowths with alkali (locally sericitized) feldspar. Garnet is conspicuous by its absence. metamorphic rocks. John Scott (Ontario Geological Survey) generated maps for this report and provided information on local uranium mineralization. I 1 Sf C Breaks et al. (2003b) described potassic, biotite pegmatite that grades into a medium-grained, biotite granite near this stop (UTM: Easting 408339; Northing 5433016). The pegmatite contains coarse, euhedral K-spar; quartz; prismatic, medium- to coarse-grained, black tourmaline (schorl-dravite; Fig. 19); and finegrained, green and blue fluor-apatite (0.3 to 0.9 weight % MnO). A biotite-rich, metasomatic contact occurs between the biotite granite and diabase. Graphic, coarse-grained cordierite (< 2 cm) and tourmaline occur in the granite near a diabase contact. Figure 20. Schollen structure in migmatite (Stop 11). References Ayres, L.D., 1969. Metamorphism in the Superior Province of northwestern Ontario and its relationship to crustal development; in Metamorphism in the Canadian Shield; Geological Survey of Canada, Paper 78-10, p.25-36. Ayres, L.D. 1978. Metamorphism in the Superior Province of northwestern Ontario and its relationship to crustal development; in Metamorphism in the Canadian Shield, Geological Survey of Canada, Paper 78-10, p.25-36. Figure 19. Quartz-tourmaline intergrowth in pegmatite dyke (Stop 10). Stop 11 - Migmatites (Optional) UTM coordinates - 5429614N 0425006E This optional stop displays schollen (raft)-structured migmatites in which folded and schlieric, mafic, paleosome xenoliths have been highly deformed and disrupted (Fig. 20). There is a relatively high proportion of quartzo-feldspathic neosome matrix to the xenoliths, suggesting high(er) degrees of partial melting. Patches and dykelets of coarse-grained, biotite-quartz-feldspar neosome contain garnet, cordierite and large (>10 cm), euhedral green apatite crystals. Acknowledgements Dr. Fred Breaks (Ontario Geological Survey) is thanked for the provision of unpublished data on pegmatites and host rocks. Dr. Mary Louise Hill (Lakehead University) provided useful insight into the metamorphic and structural history of the high-grade Borradaile, G.J. 1982. Comparison of Archean structural styles in two belts of the Canadian Superior Province; Precambrian Research 19, p.179-189. Breaks, F.W., Selway, J.B. and Tindle, A.G., 2003a. Fertile peraluminous granites and related rare-element pegmatite mineralization, Barbara-Gathering-Barbaro lakes area, north-central Ontario; Summary of Field Work and Other Activities, 2003, Ontario Geological Survey, Open File Report 6120, p.14-1 to 14-13. Breaks, F.W., Selway, J.B. and Tindle, A.G., 2003b. Fertile peraluminous granites and related rare-element mineralization in pegmatites, Superior Province, northwest and northeast Ontario: Operation Treasure Hunt; , Ontario Geological Survey, Open File Report 6099, 179p. Carmichael, D.M., 1978. Metamorphic bathozones and bathograds: a measure of the depth of postmetamorphic uplift and erosion on the regional scale; American Journal of Science, 278, p.769-797. Cerný, P., 1991a. Rare-element granitic pegmatites, part I. Anatomy and internal evolution of pegmatite deposits; Geoscience Canada, v.18, p.49-67. Cerný, P 1991b. Rare-element granitic pegmatites. Part II: - 82 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Regional to global environments and petrogenesis. Geoscience Canada v.18, p.68-81. north of Manitouwadge, Ontario; Canadian Journal of Earth Sciences, v.31, p.1427-1439. Coates, M.E., 1968. Stevens–Kagiano Lake area; Ontario Department of Mines, Geological Report 68, p. Pan, Y., Fleet, M.E. and Heaman, L.M., 1998. Thermotectonic evolution of an Archean accretionary complex: U–Pb geochronological constraints on granulites from the Quetico subprovince, Ontario, Canada; Precambrian Research,v.92, p.117-128. Davis, D.W., Pezzutto, F. & Ojakangas, R.W., 1990. The age and provenance of metasedimentary rocks in the Quetico Subprovince, Ontario, from single zircon analyses: implications for Archean sedimentation and tectonics in the Superior Province; Earth and Planetary Science Letters,v.99, p.195-205. Easton, R.M. 2000. Metamorphism of the Canadian Shield, Ontario, Canada I: The Superior Province; The Canadian Mineralogist, v.38, p.287-317. Fralick, P., Wu, J., and Williams, H.R., 1992. Trench and slope basin deposits in an Archean metasedimentary belt, Superior Province, Canadian Shield. Canadian Journal of Earth Sciences, 29: 2551–2557. Franklin, J.M., 1978. Uranium mineralization in the Nipigon area, Thunder Bay District, Ontario; Current Research, Part A, Geological Survey of Canada, Paper 78-1A, p.275-282. Kamineni, D.C., Stone, D. and Johnston, P.J. 1988. Metamorphism of Quetico sedimentary rocks near Atikokan, Ontario; in Program with abstracts, Geological Association of Canada-Mineralogical Association of Canada-Canadian Society of Petroleum Geologists Annual Meeting, v.13, p.A63. Percival, J.A and Stern, R.A., 1984. Geological synthesis in the western Superior Province, Ontario; in Current Research, Part A, Geological Survey of Canada, Paper 84-1A, p.397-407. Percival, J.A and Sullivan, R.W., 1988. Age constraints on the evolution of the Quetico belt, Superior Province, Ontario; in Radiogenic Age and Isotopic Studies: Report 2, Geological Survey of Canada, Paper 88-2, p.97-108. Percival, J.A., 1989. A regional perspective of the Quetico metasedimentary belt, Superior Province, Canada; Canadian Journal of Earth Sciences,v.26, p.677-693. Percival, J.A. and McGrath, P.H., 1986. Deep crustal structure and tectonic history of the northern Kapuskasing uplift of Ontario: an integrated petrological-geophysical study; Tectonics,v.5, p.553-572. Percival, J.A., Stern, R.A. and Digel, M.R., 1985. Regional geological synthesis of western Superior Province, Ontario; in Current Research, Part A, Geological Survey of Canada, Paper 85-1A, p.385-397. Kehlenbeck, M.M., 1976. Nature of the Quetico–Wabigoon boundary in the De Courcey – Smiley Lakes area, northwestern Ontario; Canadian Journal of Earth Sciences, v.13, p.737-748. Pirie, J. and Mackasey, W.O., 1978. Preliminary examination of regional metamorphism in parts of Quetico metasedimentary belt, Superior Province, Ontario; Geological Survey of Canada, Paper 78-10, p.37-48. Kissin, S.A., 1990. Granitoid-related mineral deposits of the western Lake Superior region; 36th annual Institute on Lake Superior Geology, Thunder Bay, Field Trip Guidebook, p.52-66. Pye, E.G., 1956. Lithium in northern Ontario; Canadian Mining Journal, v.77, p.73-75. Kissin, S.A. and Zayachivsky, B., 1985. Genesis of pegmatites in the Quetcio gneiss belt of northwestern Ontario: rare-element pegmatites and associated granitoids of the Georgia Lake pegmatite field; in Geoscience Research Grant Program, Summary of Research 1984-1985, Ontario Geological Survey, Miscellaneous Paper 127, p.186-199. Mehnert, K.R., 1968. Migmatites and the origin of granitic rocks. Elsevier. 405p. Milne, V.G., 1962. The petrography and alteration of some spodumene pegmatites near Beardmore, Ontario; unpublished Ph.D. thesis, University of Toronto, Toronto, Ontario, 242p. Mulligan, R., 1965. Geology of Canadian lithium deposits; Geological Survey of Canada; Economic Geology Report 21, 131p. Pan, Y., Fleet, M.E. and Williams, H.R., 1994. Granulitefacies metamorphism in the Quetico subprovince, Pye, E.G., 1965. Geology and lithium deposits of the Georgia Lake area, District of Thunder Bay; Ontario Department of Mines, Report 31, 113p. Sawyer, E.W., 1983. The structural history of a part of the Archean Quetico metasedimentary belt, Superior Province, Canada; Precambrian Research,v.22, p.271-294. Scott, J.F., 1987. Uranium occurrences of the Thunder Bay – Nipigon – Marathon area; Ontario Geological Survey, Open File Report 5634, 158p. Seemayer, B.E., 1992. Variations in metamorphic grade in metapelites in transects across the Quetico Subprovince north of Thunder Bay, Ontario; unpublished M.Sc. thesis, Lakehead University, Thunder Bay, 163p. Stewart, D.B., 1978. Petrogenesis of lithium-rich pegmatites. American Mineralogist, v.63, p.970-980. Tabor, J.R., Hudleston, P.J. and Magloughlin, J., 1989. Metamorphism of the Quetico supracrustals north of - 83 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 the Vermillion granitic complex, northern Minnesota; Geological Association of Canada – Mineralogical Association of Canada, Program with Abstracts, v.14, p.A38. Walter, J.P. 1957. Towagmac Explorations Ltd., Reoprt on exploration program, 1956 season; unpublished report, Assessment Files, Thunder Bay South District, Thunder Bay, 3p. Williams, H.R., 1989. Geological studies in the Wabigoon, Quetico, and Abitibi–Wawa subprovinces, Superior Province of Ontario, with emphasis on the structural development of the Beardmore–Geraldton belt; Ontario Geological Survey, Open File Report 5724, p. Williams, H.R., 1991. Quetico subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, v.1, p.383-403. Williams, H.R. and Breaks, F.W., 1989. Geological studies in the Manitouwadge – Hornepayne area; Summary of Field Work and Other Activities 1989, Ontario Geological Survey, Miscellaneous Paper 146, p.7991. Williams, H.R. and Breaks, F.W., 1990. Geological studies in the Manitouwadge – Hornepayne area; Summary of Field Work and Other Activities 1990, Ontario Geological Survey, Miscellaneous Paper 151, p.4751. Woolverton, R.S. 1956. Report on McVittie lithium option, Noranda Exploration Co. Ltd.; unpublished report, Assessment Files, Thunder Bay South District, Thunder Bay, 6p. Zaleski, E. and van Breemen, O., 1997. Age constraints on plutonism, metamorphism and deformation across the Wawa–Quetico subprovince boundary near the Manitouwadge greenstone belt, northeastern Ontario; Institute on Lake Superior Geology, Program with Abstracts, v.43, p.67-68. Zaleski, E., van Breemen, O. and Peterson, V.L., 1999. Geological evolution of the Manitouwadge greenstone belt and Wawa–Quetico subprovince boundary, Superior Province, Ontario, constrained by U–Pb zircon dates of supracrustal and plutonic rocks; Canadian Journal of Earth Sciences, v.36, p.945-966. Zayachivsky, B., 1985. Granitoids and rare-element pegmatites of the Georgia Lake area, northwestern Ontario; unpublished M.Sc. thesis, Lakehead University, Thunder Bay, Ontario, 234p. - 84 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Fieldtrip 5 - Geology of the Black Sturgeon Area Thomas R. Hart Ontario Geological Survey, Ministry of Northern Development and Mines, Sudbury, Ontario, P3E 6B5, Canada Introduction The Nipigon Embayment is an approximately 19,000 km2 area of Mesoproterozoic igneous and sedimentary rocks centred on Lake Nipigon, north of Lake Superior, approximately 100 km northeast of Thunder Bay, Ontario (Fig. 1). The Embayment is underlain by Archean rocks of the Wabigoon Subprovince to the north, and the Quetico Subprovince to the south. This field trip will be concentrating on the Proterozoic and underlying Archean rocks of the southern portion of the Embayment. u nI_u • nil nuN UU The first geological work in the Lake Nipigon area was conducted by Bell (1870) for the Geological Survey of Canada. Portions of the area were examined by McInnes (1896), Parks (1901), Coleman (1909), and Wilson (1910). Geological mapping by Coates (1972) covered the area south of Lake Nipigon at a scale of 1:63,360. The adjacent area to the south was mapped Figure 1. Mafic and ultramafic intrusions of the Nipigon Embayment. by McIlwaine and Tihor (1975a,b) at 1:15,840 scale. The area surrounding Lake Nipigon, and extending to the southern edge of Black Sturgeon Lake, was mapped at a scale of 1:50 000 by Sutcliffe (1982a,b; 1985a,b,c). Sutcliffe (1986) completed a doctoral thesis on the intrusive rocks of the Lake Nipigon area, including the Nipigon diabase sills and ultramafic intrusions of the Disraeli and Seagull-Leckie Lakes areas. Franklin et al. (1980), Cheadle (1986), and Rogala (2003) have carried out detailed investigations of the stratigraphy of the Sibley Group. Bedrock mapping and research studies were undertaken between 2003 and 2005 as part of the Lake Nipigon Region Geoscience Initiative (LNRGI). The LNRGI is a geoscience-based geological data acquisition and compilation program managed by the Ontario Prospectors Association (OPA) and financially supported by the Northern Ontario Heritage Fund Corporation (NOHFC). The major goals of the initiative are to conduct research and collect new geoscience data that will help to explain the geological history of the area, and identify areas of high mineral potential that will attract exploration to the region. The initiative included bedrock mapping projects (MacDonald, 2004; MacDonald and Tremblay, 2005), airborne magnetic and radiometric geophysical surveys (Ontario Geological Survey, 2004a), ground gravity surveys (Ontario Geological Survey, 2004b), and targeted surficial geochemical and lineament studies (e.g., Barnett, 2004; Dyer, 2004), a paleomagnetic study (e.g., Ernst et al., 2005) and geochronological studies (e.g., Heaman et al., 2005). Mapping of the southern portion of the Nipigon Embayment was completed by the Ontario Geological Survey as part of its commitment of in-kind support to the LNRGI (Hart and Magyarosi, 2004; Hart, 2005a). Lakehead University, as a partner in the Initiative, contributed a detailed sedimentary basin reconstruction (e.g., Metsaranta and Fralick, 2004) and petrochemical characterization of the mafic and ultramafic intrusions (e.g., Heggie and Hollings, 2004; Richardson and Hollings, 2005). - 85 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Mineral exploration and prospecting in the area has been sporadic with the first recorded activity being the exploration for iron ore in the early 1900s (Coates, 1972). Exploration in the early 1900s also resulted in discovery of lead and zinc mineralization to the southwest, and limited production (Shklanka, 1969). The discovery of sporadic copper mineralization near Disraeli Lake in 1965 resulted in a surge in copper and the other base metals in the late 1960s (Coates, 1972). Exploration for uranium has been cyclical, with the greatest activity occurring in the middle to late 1970s coinciding with the discovery of large unconformity related deposits in other locations including the Wollaston Basin of Saskatchewan (Scott, 1987). A uranium occurrence was discovered by R.H. Sutcliffe during the 1981 field season and subsequently acquired and explored by Uranerz Exploration and Mining Corporation Limited until 1985 (Scott, 1987). A number of companies are actively exploring for uranium mineralization at the time this guide was being prepared. Several companies investigated the potential of sedimentary-hosted base metal mineralization in the Sibley Group in the early 1990s, but other than an airborne geophysical survey completed by Cominco in 1993, little of the work was filed for assessment. Exploration for PGE mineralization intensified after the 1998 discovery of mineralization in the Seagull/Leckie Lake intrusion (Osmani and Rees, 1998). Initial results of diamond drilling indicated the presence of a basal pyroxenite hosting up to 1.71 ppm Pt and 1.87 ppm Pd over 2.1 m (Durham, 2000). Resampling completed in early 2004 defined two zones higher in the intrusion with up to 1.52 ppm Pt and 1.78 ppm Pd over 3.0 m and 1.56 ppm Pt and 1.87 ppm Pd over 3 m in the upper and lower zones respectively (East West Resource Corporation, 2004a). Drilling in late 2004 intersected a mineralized zone consisting of 2.38 g/t Pt, 2.65 g/t Pd, 0.19 g/t Au, 0.18 g/t Rh, 0.39 g/t Os, 0.34 g/t Ir and 0.08 g/t Ru (Pd:Pt ratio of 1.11) associated 0.19% Cu and 0.20% Ni over 3.21 metre (Platinum Group Metals Ltd., 2005). General Geology The southern portion of the Nipigon Embayment is underlain by Archean rocks of the Quetico subprovince that are unconformably overlain by Proterozoic sedimentary rocks of the Sibley Group (Fig. 2). Rocks of the Quetico Subprovince and the Sibley Group are io a Kilometres C N!p!g&,] UIt,amafi cIrtr(]iOns s'brey Group eotite Granite [3] Muscovite Gran!Ie FJ Faults Roads Outco Metaseth,]&nta Rocks Transmission Line Gas Pipel,ne Figure 2. Geology of the southern portion of the Nipigon Embayment. - 86 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 intruded by Proterozoic mafic to ultramafic intrusive rocks in the Disraeli and Seagull Lakes, and Hele and Kitto township areas (Fig. 2). Mafic sills of the Nipigon Diabase Sill Complex intrude all other rock types in the map area. Rocks of the Quetico Subprovince consist of biotite and/or andalusite schists that are gradually replaced towards the south by amphibolites (Hart, 2005a). The rocks to the north have a schistosity oriented at approximately 050° to 060°, with near vertical dips. In the south, leucogranites intrude the amphibolite or biotite gneisses and form complex mixtures that have been classified as migmatite. The airborne magnetic survey indicates a regional fabric with about an 080° to 090° orientation in the migmatitic rocks (Ontario Geological Survey, 2004). A number of areas east of the Black Sturgeon River contain a high percentage of coarse grained to pegmatitic irregular bodies and dykes of the muscovite granite suite. Irregular bodies of medium to coarse-grained rocks of the biotite granite suite intrude the amphibolites and leucocratic phases form the leucosome of the migmatitic rocks. Widely spaced, weakly deformed and metamorphosed, north trending diabase dikes intrude the Archean rocks of the Quetico Subprovince. A paleomagnetic study of similar dykes located to the west, in the area of Highway 527, by Ernst et al. (2005) has interpreted these dykes to be part of the early Proterozoic 2121 – 2101 Ma Marathon dyke swarm. The Proterozoic Sibley Group sedimentary rocks unconformably overlie the Archean rocks of the Quetico Subprovince, and are relatively flat-lying, with dips of less than 5° and strikes that are usually difficult to determine. Rocks within this area have been subdivided in to three relatively flat-lying formations, the lower Pass Lake, the middle Rossport, and the upper Kama Hill (e.g., Cheadle, 1986; Fig. 3). The Pass Lake Formation in this area is best observed in drill core, and consists of conglomerates, quartz arenites, and minor dolomites, with rare outcrops of the upper sandstone members. A sandstone of the Pass Lake Formation has a youngest detrital zircon grain with a U-Pb age of 1670 Ma (Heaman et al., 2005). The Rossport Formation consists of mudstone, dolomite and siltstones of the lower Channel Island Member, limestone and stromatolitic limestone of the Middlebrun Bay Member, and predominantly muddy variably carbonatized siltstones and mudstones of the upper Fire Hill Member (see Fig. 3). The Kama Hill Noo Diabass $11 Oonlsx, 1110le1113 S 2005) C,ta,I laed Fom,ton atleasI3silsha,ebeeelifle, d450 Ma Hea!,sa,s $1 at.. 2005) dykes may e*1ea let tholt dlanc.s from the sills .olpresenlirlmap area -1339 M -d€it!!]l dQslt tFrakls]197fl) __— Kane ll FoIrnallo,, mar!csl, one lntades lw el!lts!0 I Pi!a H!ll Mmb. —1ROSSPO,tFOlmalior. SwI. Hale. Dali t,ln,SlOflS. 1106— 1124 Ma (ReAmanetal.2005) Legend Dabs.. s!lI 4e H Mdlethn Say Membr Rosspefl Fomlaijo,, •P PsssLa&sF Ch..,! Id Mrsb., Met JCeflsls!]dMemt.r 1ROSSPOflFO!n,akn,. fl,eH!llMembe[ 0_ Ks H!II Flm!.reks Gs... I Pass Lake Fo,aleUon 2121-2101 MaMaralliorl dib dyks mscoe grane and I lO70Ma O*aman.tal.2005) I blotice andior andalusile ___-I hbls mpl]ibd. blSbs Oa salt.. Figure 3. Generalized stratigraphic column for the southern portion of the Nipigon Embayment. Formation is the uppermost formation of the Sibley Group exposed in the map area and consists of laminated mudstones and siltstones with sandstone prominent in the upper section. A sandstone of the Outan Island Formation, which overlies the Kama Hill Formation has a youngest detrital zircon with a U-Pb age of 1450 Ma (Heaman et al., 2005). A sandstone of the Nipigon Bay Formation, which overlies the Outan Island Formation, has a youngest detrital zircon grain with a U-Pb age of 1657 Ma (Heaman et al., 2005). A Rb/Sr age of 1339 +/- 33 Ma based on analyses of a combination of Rossport and Kama Hill formation samples by Franklin (1978), probably denotes a post-depositional diagenetic event. Cheadle (1986) estimated up to 200 m of topographic relief on the Archean paleosurface at the time of deposition of the Sibley Group. Although the bedding is generally flat-lying on an outcrop scale, these extremes in topographic relief probably controlled sedimentation processes on a local scale complicating stratigraphic correlations (P. Fralick and B. Rogala, Lakehead University, personal communication, 2003). Regionally, the Sibley Group thickens to the south under Lake Superior with two additional formations - 87 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 overlying the Kama Hill formation (Rogala, 2003), and thins to the north and west (e.g., Cheadle, 1986). may be difficult to distinguish from the diabase sills without petrography and lithogeochemistry. Mafic to ultramafic intrusions occur in the Disraeli and Seagull-Leckie Lakes, Hele and Kitto township areas (Fig. 1), and all four are composed of a pyroxene peridotite core with an irregular olivine gabbro zone along the margin (e.g., Hart et al., 2002b; Hart and Magyarosi, 2004). The Seagull Intrusion has been studied in the greatest detail, and petrography indicates that it consists predominately of cumulate textured dunites, lherzolites, and olivine websterite, with minor olivine- hornblende pyroxenite (Heggie and Hollings, 2004). An olivine gabbro forms an irregular border zone to the ultramafic rocks, and is the unit most commonly exposed in outcrop. All of the intrusions have a monzogabbro, or granophyre, unit occurring as irregular pods or bands in the olivine gabbro, but are distinguished from it by the presence of abundant pink feldspar and minor amphibole. An olivine gabbro from the Hele Intrusion has a U-Pb baddeleyite age of 1106+/-1.5 Ma with one fraction yielding a 1119.4 Ma age (L. Heaman, University of Alberta, personal communication, 2004). Samples from the Seagull Intrusion have U-Pb ages from zircon and baddeleyite of between 1105 and 1124 Ma (L. Heaman, personal communication, 2004). Samples from the Disraeli Intrusion have U-Pb ages from baddeleyite and zircon of 1111 Ma (L. Heaman, personal communication, 2004). Samples of an olivine gabbro from the Kitto Intrusion has a U-Pb baddeleyite age of 1117 Ma, but has baddeleyite fractions with a range of ages comparable to the other intrusions (L. Heaman, personal communication, 2004). Diabase sills have chill contacts with olivine gabbro of the Seagull, Hele and Kitto intrusions. A series of shallow-dipping Proterozoic diabase sills intrude all other rock units in the map area. The sills display a range of U-Pb zircon ages from 1110 to 1113 Ma from sills located west of Lake Nipigon (Heaman et al., 2005). The diabase sills have variable internal subdivisions, a generalized section from top to bottom would contain: 1) chill, 2) fine-grained variably amygduloidal, 3) magnetite-rich, 4) medium to coarse-grained, 5) coarse to very coarse-grained, 6) fine-grained, variable amygduloidal, and 7) chill (Hart and Magyarosi, 2004). These subdivisions resemble those proposed by Sutcliffe (1986). Massive, mediumto coarse-grained feldspar and pyroxene diabase is the most common rock type and generally lacks a sub-ophitic or diabasic texture. Although these rocks should properly be classified as gabbro the diabase classification has been applied to all rocks associated with the sills to avoid confusion with other intrusions in the area. The sills range in thickness from less than 5 m to greater than 180 m and there appears to be at least two major sills greater than 100 m in thickness. However, erosion and block faulting hinders the correlation of the sills and thus the determination of the original number of sills. A group of fine grained, massive mafic sills with a higher olivine content may intrude the Sibley Group sedimentary rocks. Some of these sills are spatially associated with the ultramafic intrusions, as in the Moraine Lake and Hele Township areas, and probably represent extensions of the intrusions (Hart, 2005). In the Shillabeer Lake and Kama Hill areas, similar sills are not associated with known ultramafic intrusions (e.g., Richardson and Hollings, 2005; Hart, 2004) and may be comparable to the Jackfish Sills described by MacDonald (2004) in the English Bay area. One of these sills intrudes the Disraeli Intrusion suggesting that some examples of this group post-date the larger ultramafic intrusions (Hart, 2005). This group of sills Quaternary ice-marginal deltaic deposits of the Nipigon Moraine form an extensive cover along the Black Sturgeon River and the area immediately to the west of the river (Barnett, 2004). The rest of the map area further from the river is covered by differing thickness of till that is reported by Barnett (2004) to be mainly locally derived. Regional metamorphism of the Archean rocks of the Quetico Subprovince varies from upper greenschist to lower amphibolite facies, with limited areas of migmatite. Hornfels metamorphism, in zones up to 10 m wide, is variably developed in the Sibley Group in contact with the peridotites, and occasionally the diabase sills. Hematite and specular hematite is associated with some late north-trending fracturing in both Archean and Proterozoic rocks, and appears to be a useful indicator of the north-trending fault systems in this area, where most structures are hidden by talus and glacial deposits (Hart and Magyarosi, 2004). Metasedimentary rocks of the Quetico Subprovince have experienced multiple phases of deformation. Schistosity is well developed along the bedding planes - 88 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 obscuring primary structures but the rocks appear to be the metamorphosed equivalents of the greywackes and siltstones present in the Beardmore-Geraldton greenstone belt (Hart et al., 2002a). North- and northwest-trending regional fault trends have displaced the Proterozoic Sibley Group, and a northeast-trend may only represent minor late movement along previous Archean structures (Hart and Magyarosi, 2004; MacDonald, 2004). The north and northwest trending faults, define the Black Sturgeon Fault Zone, which has been interpreted to be en échelon resulting in the formation of a block faulted asymmetric basin or graben (e.g., Coates, 1972). There appears to have been little lateral displacement based on the correlation of the iron formations across Black Sturgeon Lake in the area north of the current map area (Hart and Magyarosi, 2004). The north trending faults are interpreted to be regionally continuous structures that may correlate with north-trending structures in the Gull Bay area, approximately 150 km to the north. The northwesttrending faults display vertical movement with no apparent horizontal displacement, and may be traced to the west into the adjacent Wabigoon Subprovince. Accurate determination of displacements on these structures is not possible as they have been obscured by talus slopes and glacial deposits. north-trending and centred on Lake Nipigon. The paleomagnetic and geochemical study of the northtrending dykes located to the west of the Nipigon Embayment by Ernst et al. (2005) suggests that they are part of the 2101 to 2121 Ma Marathon dyke swarm, not Keeweenawan. If the identification of these dykes as part of the Marathon swarm is indeed correct, then combined with the lack of a north trending dyke swarm north or east of the Nipigon Embayment, a key element of the failed arm model appears to be absent. The timing of the different faults is not known, and it is probable that they have been reactivated. There have been several proposals made to account for the structural history of the area, with most focussing on the Sibley Group sedimentary rocks. Coates (1972) proposed a half graben with the Black Sturgeon River as the eastern boundary that also served as the main control on the deposition of the Sibley Group. Franklin et al. (1980) proposed that a failed arm of the Mesoproterozoic Keweenawan Midcontinent Rift created a basin, which included the half graben. Sutcliffe (1986) further developed the failed arm model as a major control on the emplacement of the peridotite intrusions and the diabase sills, and highlighted the existence of a subparallel fault system along the Nipigon River to the east. Alternatively, Fralick and Kissin (1995) and Hollings et al. (2004) proposed that the thermal upwelling that formed the English Bay Complex was followed by relaxation and formation of an intracratonic basin in which the Sibley Group was deposited. Timing and magnitude of displacement along faults related to the emplacement of either the ultramafic intrusions or diabase sills is also difficult to determine. The high degree of fracturing and hematite alteration in some of the diamond drill core from the 1106-1124 Ma Seagull Intrusion (Heaman et al., 2005) suggests post emplacement fault activity. Extensive hematite and the occurrence of uranium mineralization in north trending structures has been dated at 1090±20 Ma (Ruzicka and LeCheminant, 1984). Current interpretations suggest that faulting has resulted in displacement of igneous horizons within the Seagull intrusion, but it is just as possible that the intrusion was emplaced into a preexisting structure to produce its current form. Preexisting fault structures probably, in part, controlled the emplacement of the 1111-1113 Ma Nipigon sills (Heaman et al., 2005), as has occurred in other mafic sill complexes (e.g., Leaman, 1975). However, due to the lack of a distinctive marker in the area, it is difficult to determine if the apparently disjointed nature of many of the sills is the result of the sills ramping upward along pre-existing structures and subsequently being eroded away, or if it is a product of late post- The failed arm model predicts the presence of a dike swarm (e.g., Ernst and Buchan, 2003), in this case, The presence of Pass Lake Formation rocks at the base of a number of diamond-drill holes in the map area suggests that the half graben was present before, or was formed during, the deposition of the Sibley Group, sometime between 1670 and 1339 Ma (Heaman et al., 2005; Franklin, 1978). The occurrence of Pass Lake Formation at the base of Moseau Mountain, on the east side of the Black Sturgeon River, suggests that fault activity continued throughout the entire period of sedimentation, or that there has been significant postsedimentation fault re-activation. At this time, there is no definitive means of distinguishing between these models although, the location of Pass Lake Formation rocks at higher elevations to the east could mean that sedimentation originally extended across much of the Nipigon Embayment, consistent with the first model. - 89 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 emplacement fault displacement. 9 Geochemistry 8 Central suite Osler Group 7 Hele Intrusion 6 The Hele Intrusion may be subdivided into olivine gabbro, olivine melagabbro, and peridotite based on their MgO contents, with the peridotite having between 19 - 29 wt.% MgO and the olivine gabbro having MgO contents of less than 11 wt.% MgO. Elemental variations in diamond drill core samples appear to indicate that the Hele Intrusion is composed of more than one pulse of magma, resulting in an interlayering of the peridotite and melagabbro. Pink feldspar may occur as a minor mineral phase in the olivine gabbro, and is a major constituent of the monzogabbro, or granophyre. The presence of pink feldspar usually correlates with higher K2O content that may be up to 7.42 wt.%, with no appreciable change in the Fe2O3 contents compared to the other olivine gabbro. Although the high K2O is possibly related to late stage igneous fractionation, it is considered more likely that these values are a result of assimilation of Sibley Group rocks and that the pink colour is a result of potassic alteration rather than hematization. Total rare earth element (REE) and high field strength (HFSE) concentrations in the peridotite 100 I x-1r o osa S a m p le /p rim itiv e m a n tle DI 1000 10 1 0.5 Th Ta Ce Nd Hf Eu Gd Dy Ho Tm Lu Sc Nb La Pr Zr Sm Ti Tb Y Er Yb V Figure 4. Mantle-normalized extended element diagrams of the olivine gabbro, melagabbro and peridotite of the Hele Intrusion (normalizing factors from Sun and McDonough, 1989). L a /S m Mafic to Ultramafic Intrusive Rocks 5 4 Lower suite Osler Group 3 Upper suite Osler Group 2 1 1.0 1.8 - 2.7 3.5 4.3 5.2 6.0 Gd/Yb Figure 5. Gd/Yb versus La/Sm diagram of the mafic to ultramafic intrusive rocks and the Nipigon diabase sill complex with fields for the volcanic rocks of the Osler Group (after Lightfoot et al., 1991). Legend for symbols, see Figure 4. samples are generally lower than in the olivine gabbro or olivine melagabbro (Fig. 4). All of the rocks have weak negative TiO2 and Zr anomalies on the mantle normalized extended element diagram (see Fig. 4), and these anomalies are more pronounced than negative Nb-Ta anomalies for the Seagull and Disraeli intrusions (Hart and Magyarosi, 2004) although all three intrusion have flattening of the trends between Th and La. All three units have [La/Yb]mn (mantle normalized) and Gd/Yb ratios which are distinctly higher than the diabase sills, and plot toward the high end of the field defined by the Lower Suite of the Osler Group volcanic rocks (Hart, 2005; Fig. 5). Rocks of the Hele Intrusion have a wider range of Gd/Yb ratios than the rocks from the Disraeli and Seagull intrusions, but overlap with the values for the other intrusions. The peridotite and olivine gabbro have REE and HFSE ratios that are comparable to ocean island basalts and most of the rocks from the Disraeli and Seagull intrusions in contrast to the diabase sills (Fig. 6). The peridotite and gabbro are not enriched in nickel, and the peridotites follow an olivine-controlled igneous fractionation trend, which would be consistent with a parental MgO concentration of approximately 13 wt.% (Fig. 7). Both units have a flat positive slope on a primitive mantle normalized PGE diagram with enrichment in Ir and Pt (Fig. 8), compared to the trends for the diabase sills that follow a trend similar to that of - 90 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 (Fig. 9), but the [La/Yb]mn are distinct from those of the rocks of the mafic to ultramafic intrusions. Most of the diabase samples have weak negative TiO2 anomalies, but no apparent Zr anomalies, on the mantle normalized extended element diagram (see Fig. 9), and pronounced negative Nb-Ta anomalies. Some HFSE and REE ratios for the diabase show some similarities to the Central and Upper Suite volcanic rocks of the Osler Group as has been suggested by Sutcliffe (1987) (see Fig. 5). most continental flood basalts (Hart et al., 2002a). 20 Upper Suite Central Suite 10 Upper Crust SZB T h /Ta Logan sills Primitive Mantle The Nipigon diabase sills are not enriched in nickel, and follow an igneous fractionation trend (see Fig. 7). PGE contents of the diabase are similar to the trend followed by most continental flood basalts, and comparable to the PGE contents and trends noted for the diabase sills from the Beardmore area by Hart et al. (2002a; Fig. 10). OIB OPB 1 NMORB Lower Suite MORB 0.5 0.8 1 10 50 La/Yb Figure 6. La/Yb versus Th/Ta diagram of the mafic to ultramafic intrusive rocks and the Nipigon diabase sill complex with fields for the volcanic rocks of the Osler Group and the Logan diabase sills south of Thunder Bay (after Condie et al., 2002). Legend for symbols, see Figure 4. 2000 Within plate basalt Ni (ppm) 1000 Upper crust 100 Mapping and geochemical sampling by Sutcliffe (1986) and Hart et al. (2002a) suggested that the sills do not represent single cooling units. However, the results of detailed sampling of a number of continuous sill sections from diamond drill core suggest that some sills may be single cooling units. Whereas others have chemical variations suggesting a more complex history of emplacement that may include multiple pulses of magma that did not re-equilibrate before cooling. However, the variations are subtle and generally not distinctive enough to be evident in a group of outcrop samples. The diabase sills have HFSE and REE ratios with some similarities to subduction zone basalts and the Central and Upper Suites of the Osler Group (see Fig. 6). 1000 10 0 10 20 30 40 MgO (wt. %) Figure 7. MgO versus nickel diagram of the mafic to ultramafic intrusive rocks and the Nipigon sill complex (after Lightfoot et al., 2001). Legend for symbols, see Figure 4. S a m p le /M a n tle P G E 100 10 1 0.1 0.01 Nipigon Diabase Sill Complex The diabase sills plot along a line indicative of fractionation controlled mainly by plagioclase and clinopyroxene, as suggested by Sutcliffe (1987). The diabase sills display a range in REE and HFSE concentrations that will require further investigation .001 Ni Ir Ru Rh Pt Pd Au Ag Cu Figure 8. Primitive mantle-normalized platinum group element diagrams of the olivine gabbro, melagabbro and peridotite of the Hele Intrusion (normalizing factors from Barnes et al., 1987). Legend for symbols, see Figure 4. - 91 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 and low TiO2 rocks is similar to regional distributions observed in some flood basalt provinces suggesting that the Logan and Nipigon sills may be part of a single intrusive event. S a m p le /p rim itiv e m a n tle 1000 100 10 1 0.5 Th Ta Ce Nd Hf Eu Gd Dy Ho Tm Lu Sc Nb La Pr Zr Sm Ti Tb Y Er Yb V Figure 9. Mantle-normalized extended element diagrams of the Nipigon Diabase Sill Complex (normalizing factors from Sun and McDonough, 1989). Legend for symbols, see Figure 4. 1000 S a m p le /M a n tle P G E 100 10 1 The presence of variants of the sills that have specific geochemical and/or geographic distribution has resulted in the introduction of a variety of new, informal, names for the diabase sills in the past few years, such as the distinction between the Logan sills in the Thunder Bay area and the Nipigon sills around Lake Nipigon. In addition, new sill types, such as the Inspiration sills have also been identified (e.g., MacDonald, 2004; Richardson and Hollings, 2005). This does cause a problem, however, in that the historical term Logan Sills has been used exclusively in the paleomagnetic literature, and in much of the international literature, for all of the diabase sills north of Lake Superior. Until these terminology issues are resolved, it is recommended that the historical nomenclature of Logan Sills should be applied to all the diabase sills in the area north of Lake Superior, with subdivision into the informal terms, Logan sills in the south and Nipigon sills to the north. The Nipigon sills may be further subdivided into several chemical variants, such as the normal Nipigon, Inspiration, and Jackfish-like sills. Clarification of this nomenclature is contingent on further geochemical, petrological and geochronological work. Mineralization 0.1 Platinum Group Elements Hele Intrusion 0.01 .001 Ni Ir Ru Rh Pt Pd Au Ag Cu Figure 10. Primitive mantle-normalized platinum group element diagrams of Nipigon Diabase Sill Complex (normalizing factors from Barnes et al., 1987). Legend for symbols, see Figure 4. The Nipigon diabase sills have lower TiO2, Zr/Y, La/ Yb values than the Logan diabase sills located to the south of Thunder Bay (see Fig. 6) and have a regional distribution that extends south to at least the north side of the City of Thunder Bay (Hart, 2004). Several samples collected from the area south of Thunder Bay have geochemical characteristics comparable to the Nipigon diabase sills (see Fig. 6), but further investigation is required. This regional geographic distribution of high Mineralization has not been located in either of the two diamond drill holes or in outcrop of the Hele Intrusion. The peridotite and olivine gabbro have similar PGE contents with Pt/Pd ratios of 1.03 to 4.63 with the highest values being 75.87 ppb Pt, 50.24 ppb Pd and 24.07 ppb Au in an olivine gabbro (Hart, 2005). The peridotite and olivine gabbro also have similar base metal contents with 18 to 1320 ppm Ni, 17 to 459 ppm Cu, and 72 to 280 ppm Zn. Seagull Intrusion Mineralization in the southern portion of the Nipigon Embayment occurs as three reef style zones hosted by the Seagull Intrusion. The upper zone contains up to 1.52 ppm Pt, 1.78 ppm Pd, 0.34 ppm Os, 0.11 ppm Rh, 0.23 ppm Ir, 0.15 ppm Au, 0.58% Cu, and - 92 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 0.34% Ni over 3 m (East West Resource Corporation 2004a,b). The lower zone contains up to 1.56 ppm Pt, 1.87 ppm Pd, 0.20 ppm Os, 0.10 ppm Rh, 0.18 ppm Ir, 0.15 ppm Au, 0.55% Cu, and 0.35% Ni over 3 m (East West Resource Corporation 2004a,b). The best intersection in the basal zone, close to the intrusion Quetico contact, is 1.71 ppm Pt and 1.87 ppm Pd over 2.1 m associated with 10% disseminated to net textured sulphide minerals including pyrite, pyrrhotite and minor chalcopyrite (Durham, 2000). Diamonddrill holes completed in 2004 intersected a zone near the base of the intrusion consisting of 2.38 g/t Pt, 2.65 g/t Pd, 0.19 g/t Au, 0.18 g/t Rh, 0.39 g/t Os, 0.34 g/t Ir, 0.08 g/t Ru, 0.19% Cu and 0.20% Ni over 3.21 metre (Platinum Group Metals Ltd., 2005). Mineralization in the basal zone is present near the lower contact of the intrusion, and is interpreted to be caused by sulphur saturation of the magma during initial stages of emplacement (Heggie and Hollings, 2004). The upper and lower zones are associated with more primitive whole rock geochemistry and mineralogy possibly reflecting an influx of less evolved magma. The mineralogy has been characterized by Heggie and Hollings (2004) and the following details are from that source. Nickel and copper sulphide minerals occur as disseminated bravoite and chalcopyrite respectively, found interstitial to olivine. Copper is also present as veinlets of native copper cross cutting sulphide minerals. Platinum group minerals are located at the grain boundaries of NiFe sulphide minerals, and are dominated by sperrylite, keithconnite, stibiopalladinite, and a copper palladium alloy. Stops Field Trip Road Log Stop 1a 1b Locality Junction of Highway 585 and Highway 11/17 in the Town of Nipigon Red Rock Black Sturgeon River Rock quarry north of Big Squaw Creek Road cut Big Squaw Creek Road Cut Junction of Highway 11/17 and the Stewart Lake Road Reset Junction of the Stewart Lake Road and Fowlkes Lakes roads km 0 5.5 16 19 19.5 26.5 5.9 Stop 2 3 4 5 6 7a 7b Locality km Reset Hele Intrusion peridotite 2.9 Junction of the Fowlkes Lakes and Driftstone Lake roads 6.3 Reset Hele Intrusion olivine gabbro 3.9 Return to Stewart Lake Road Reset Junction of Stewart Lake and Black Sturgeon roads 3.8 Reset Black Sturgeon Road 3.3 Eagle Mountain 20.3 Junction of Black Sturgeon and Camp 42 roads 22.5 Reset Junction of Camp 42 and Church Road 14.1 Reset Andalusite schist 7 Reset Return to south to junction of Camp 42 and Church Road 7 Reset Road to left 0.75 Granite-diabase contact. South on trail 0.2 Pegmatite 12 Trip starts at the junction of Highway 585 and 11/17 in Nipigon. Proceed southwest on Highway 11/17. 5.5 km - Rossport Formation Oxidized units of the Rossport Formation of the Sibley Group are exposed in the hillside on the east side of the highway. These rocks unconformably overlie Archean rocks of the Quetico Subprovince, and are intruded at the top of the hill by a diabase sill of the Nipigon Sill Complex 16 km - Black Sturgeon River (Photo 1) There are two major regional fault trends, north and northwest, that define the structural zone referred to as the Black Sturgeon Fault Zone. The combination of these two trends has been interpreted to be en échelon resulting in the formation of an asymmetric basin or graben, as originally proposed by Coates (1972). A series of subparallel north-trending faults are interpreted to be regionally continuous structures that can be traced - 93 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 by geophysics and topography for over 150 km north into the area of the English Bay Complex. A series of northwest-trending faults can be traced for similar distances to the west into the Wabigoon Subprovince. Apparent flexures in the north-trending faults probably represent intersections with northwest-trending faults. Vertical displacements on these faults are varied (may be up to 350 m) and are visible along the Black Sturgeon River, interpreted by Coates (1972) to be the halfgraben hinge. There appears to have been little lateral displacement on these faults based on the correlation of the Archean iron formations that extend across Black Sturgeon Lake (Hart and Magyarosi, 2004). Corridors of north-trending faults cut, and may have controlled the emplacement of, the Seagull and Disraeli peridotite intrusions, and possibly the Hele peridotite intrusion. 19 km - turn right onto natural gas pipeline access road on the north side of the highway and immediately turn left and follow trail along the north side of the highway. Proceed 200 m to the edge of the gravel pit * r 4 • 1. 'r 1 *: and park vehicles. Walk for 115 m along trail heading north and skirting the edge of the gravel pit. Stop 1a - Rock Quarry UTM coordinates – 0396320E 5416664N This stop is located on the south side of a rock quarry in a diabase sill which produced fill for highway construction. This outcrop is the upper contact zone of the diabase sill that extends to the south into the Big Squaw Creek road cut. This contact zone is a relatively rare variant of the typical chilled contact consisting of pebble sized, rounded xenoliths of quartz and feldspar in a biotite-rich fine-grained diabase. Contact relationships observed to the south, in the next stop, indicate that the diabase is intruding sedimentary rocks of the Sibley Group. The composition of the country rock probably influences the formation of this relatively rare contact type, as the diabase sills commonly have glassy chilled contacts. This type of contact zone has been interpreted to be a result of the partial melting and assimilation of country rock xenoliths resulting in a quartz-feldspar restite and a biotite-rich diabase. Similar quartz-feldspar xenoliths in a hornfels carbonate-rich sediment are observed along some contacts in the road cut to the south suggesting that the presence of a carbonate-rich unit may be important to the formation of this contact style. However, a similar contact zone is present in a diabase sill intruding rocks of the Quetico Subprovince without any obvious evidence of Sibley sediments, about 10 km southwest of Beardmore, near the Warnesford Quarry —. ' __ 7 4- fl'_. 4. 'C Photo 1. The erosional valley of the Black Sturgeon River following a northwest fault, with the Archean to the right (east) and Hele Intrusion to the left (west). The valley is approximately 800 m wide. Photo courtesy of J. Scott. Stop 1a. Fine-grained, massive, biotitic diabase matrix containing angular to subrounded pebble-sized xenoliths of coarse-grained quartz and white to pinkish feldspar. Markings on scale card are at 1 cm intervals. - 94 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 SeE! •% "tt' Stop 1b. The east block of hornfels units of the Rossport Formation surrounded by diabase sill along the north side of Highway 11/17. on the CNR railway. In other areas, intrusion of diabase and ultramafic sills into carbonate-rich Sibley sediment has resulted in the formation of variolitic sills with a chilled margin composed in part by densely packed fine varioles and a core of widely spaced large varioles. Variolitic textures have been observed southwest of Moraine Lake, in a sill cut by diamond drill hole NI92-7 located west of Disraeli Lake, and in and drill hole HE02-01 below the Hele Intrusion. Return to the highway and turn right. Proceed west for 600 m, driving past the outcrop, located on the north side of the highway, and turn left into the entrance to a private dump to park. Stop 1b - Big Squaw Creek road cut UTM coordinates – 0396114E 5416374N A diabase sill which has intruded and metamorphosed blocks of Rossport Formation mudstone and dolostone is exposed in a rockcut along the north side of Highway 11/17, in east Stirling Township. The eastern block appears to have been lifted, resulting in a westerly dip, with diabase intruding underneath and from the east (see photo above). However, the block is abruptly truncated by a vertical contact to the west, and a less then 25 cm thick sill intrudes along the bedding eastward from this contact. The ambiguity of the injection direction of the diabase may be a result of the rock cut being oblique to the injection direction, and the block may not be totally separated from the surrounding Sibley Group. The western block is composed of sedimentary rocks comparable in composition to those in the east block but has a chaotic structure with no preservation of bedding, and is probably a detached, deformed and metamorphosed xenolith Metamorphism in the sedimentary rocks decreases away from the contact, and the sedimentary rocks close to the contact contain an assemblage of diopside, phlogopite, clinochlore, pargasite, and calcite (Hart, 2005). In some areas along the contact there are irregular zones to isolated pebble sized clasts of quartz and microcline within the calc-silicate hornfels. The quartz and microcline may represent a partial melt of the sedimentary rocks by the diabase, and may be equivalent to the xenoliths observed at the previous stop. A calc-silicate hornfels or skarn (Meinert, 1992) may be developed along the contact between the diabase sills and the Sibley Group sedimentary rocks, but is ubiquitous along the contacts between the peridotites and Sibley Group contacts (Hart, 2005). Where present, the hornfels is varied in width, extending up to 10 m into the sedimentary rocks, although an approximately 60 m thick layer of hornfels occurs in the Rossport Formation between the two diabase sills in diamonddrill hole ST02-01 (Hart and Magyarosi, 2004). The effects of metamorphism are most pronounced in the Rossport Formation, with the mudstone and siltstones beds commonly developing a light green, mauve, reddish gray to light gray banding. Dolomite-rich beds form light green, white to buff marbles dependent on the original mud content. Radial texture in 3 to 8 cm wide zones occur along some sill contacts, with radially oriented amphibole of the 0.2 to 0.5 cm in diameter. - 95 - During his regional study of the Nipigon diabas sills, �Proceedings of the 51st ILSG Annual Meeting - Part 2 Sutcliffe (1986) identified two common assemblages of fine-grained, calc-silicate minerals, calcite-tremoliteforsterite (clinochlore) and calcite-diopside-fosterite at lower and higher metamorphic grades, respectively. The forsterite is altered to serpentine. No dolomite and only minor quartz were observed. Rogala (2001) completed a study of interbedded mudstones and dolomites of the cyclic facies of the Channel Island Member and lower Middlebrun Bay Member of the Rossport Formation, intersected by diamond drill hole NI92-5 located west of Disraeli Lake. These rocks contained a mineral assemblage of pargasite, tremolite, talc, clinochlore, calcite and barite with varying amounts of hematite and apatite, halite, sylvite, gypsum and dolomite. The halite and sylvite were interpreted to be a result of evaporite minerals present in the Sibley Group and the presence of these minerals probably controlled the formation of pargasite (Rogala, 2001). The barite was interpreted to have been introduced into the Sibley Group by later fluid associated with the lead-zinc veining. Rogala (2001) noted that one sill contact had a thin band of pyrite with chalcopyrite, magnetite, ilmenite, galena, and rare gold and silver. High gold has been noted in a sample from the McVicars Resources Muskrat Lake diamond drill hole ML01-3 at 239.8 m and also in samples from along the shore of Lake Nipigon (A. Richardson, MSc student, Lakehead University, personal communication, 2004). Past workers have suggested that the copper mineralization may be related to the intrusion of the diabase or ultramafic intrusions (e.g., Coates, 1972). However, it is not clear if the metals are igneous or sedimentary in origin. The diabase is commonly medium brown to brownish grey, massive, medium- to coarse-grained feldspar and pyroxene with trace to 3% olivine and 1 to 2% magnetite. Medium- to coarse-grained diabase forms the majority of the sills and generally lacks a sub-ophitic or diabasic texture and should properly be classified as gabbro, but to avoid confusion with other intrusions in the area the diabase classification has been applied to all rocks associated with the sills. Portions of this outcrop contain ophitic textures consisting of pyroxene oikocrysts up to 2 cm in diameter, and in diamond drill hole the coarse-grained ophitic textured diabase is often interlayered with sub-ophitic diabase. Calculations by Sutcliffe (1986) suggested that the diabase sills were emplaced at shallow crustal levels, at a lithostatic load of between 0.06 and 0.43 kilobars. The sills were estimated to have crystallized over a temperature range of 1100° to 800°C, adjacent wall rocks were estimated to have reached 665°C, and the metamorphic mineral assemblages suggest temperatures of above 540°C. 26.5 km - west along Highway 11/17, and turn right on to the Stewart Lake Road - reset. 5.9 km follow the Stewart Lake Road for 4.2 km north and then west after a right-angled turn for an additional 1.7 km. Turn right onto the Fowlkes Lakes Road - reset. 2.9 km – north on the Fowlkes Lakes Road to where a skidder trail departs to the west. Stop 2 - Hele Peridotite UTM coordinates – 0386762E 5420119N Park the trucks and proceed by foot along the skidder trail for 500 m to the outcrop. Located in the southeast corner of Hele Township, the Hele Intrusion covers approximately 40 km2 and has a maximum thickness of 130 m, as defined by diamond drilling Peridotite is very poorly exposed in the central portion of the body, east of Driftstone Lake, and is interlayered with olivine gabbro and feldspathic peridotite in diamond drill core. The peridotite is best exposed in an area of clear cut logging located on the northern edge of Stirling Township as a highly weathered and serpentinized rock with a high magnetic susceptibility overlain by a diabase sill. The peridotite is dark green, medium to coarse-grained, massive, variably ophitic, and composed of olivine and pyroxene with variable plagioclase and trace to 1% fine-grained reddish brown mica (Hart, 2005). Numerous, subparallel serpentine and chlorite-rich fractures result in a banded or layered appearance to the otherwise massive peridotite. Contacts between the peridotite, feldspathic peridotite and olivine gabbro observed in diamond drill core are generally gradual transitions in grain size and olivine and plagioclase content. There appears to be more than one band of peridotite interlayered with the melagabbro and feldspathic peridotite suggesting internal subdivisions similar to the Seagull Intrusion. The Hele Intrusion appears to be a relatively flatlying sill intruding the Sibley Group with no known feeder zone, although there are fewer diamond drill holes with which to constrain the geometry of the intrusion. Geophysical inversion modelling of the airborne magnetic data by Desmond Rainsford - 96 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 grained, reddish brown mica is common in the olivine melagabbro in the central areas of the intrusion. An absence or very low abundance of mica and a lower concentration of olivine in the olivine gabbro which forms the northern portion and margins of the intrusion makes it difficult to distinguish these rocks from the olivine-bearing diabase sills. The gabbro contains pods of monzogabbro in outcrop (see photo below) and is interbanded with monzogabbro in the upper portion of the diamond drill holes. - It Stop 2. Example of parallel fractures filled with serpentine and minor biotite in the peridotite of the Disraeli Intrusion. Pencil is approximately 15 cm long. (geophysicist, Ontario Geological Survey) supports this interpretation (Hart, 2005). Inversion modelling of the ground gravity data indicated the presence of an anomaly located along the western edge of the surface expression of the intrusion and extending to the southwest. This anomaly may represent an extension of the ultramafic intrusion, but recent diamond drill results in the area of the Seagull Intrusion intersected a magnetite rich skarn associated with a similar gravity anomaly. The Seagull Intrusion hosts PGE mineralization and differs from the Hele Intrusion in having a lopolithic shape, and intruding along or close to the contact between the Quetico Subprovince and the Sibley Group. The central portion of the intrusion appears to be saucer-shaped, but this may be a product of post-intrusion faulting, and a flat-lying sill along the periphery of the body. An underlying, or peripheral, feeder structure for the Seagull Intrusion has not yet been identified. The monzogabbro, or granophyre, is generally medium to coarse-grained, and massive with a variable amphibole content and distinctive pink feldspar or groundmass that occur in irregular shaped pods up to a metre in diameter. These pods are best exposed along the logging trails in the northeast portion of the intrusion, and are hosted by medium to fine-grained olivine gabbro containing minor pink feldspar. Less commonly, the monzogabbro is fine-grained, and massive reddish pink with minor quartz. This type of monzogabbro is present in irregular pods and dykelets with sharp contacts, with and without the coarser grained monzogabbro, in the western portion of the intrusion. Some of the best examples are in the central part of the intrusion, close to a small exposure of Sibley Group sedimentary rock that is in contact with the upper portion of the intrusion. In diamond drill core, the two types of monzogabbro are generally closely associated and in some examples the finer grained variety is rimmed by the coarser grained rock. The abundance of pink feldspar correlates with elevated K2O values, and has been interpreted to be the result of the assimilation of 6.3 km – junction with the Driftstone Road and turn right - reset. 3.9 km – east along the Driftstone Road. Stop 3 - Hele Intrusion - olivine gabbro UTM coordinates – 0390358E 5424134N The olivine gabbro to olivine melagabbro forms much of the exposed area of the Hele Intrusion. The gabbro is fine to medium-grained, medium to dark green, massive and composed of pyroxene, up to 30% plagioclase, and up to 40% olivine. Trace to 1% fine •i :• Stop 3. Contact between olivine gabbro and an irregular pod of monzogabbro / granophyre, in the western portion of the Hele Intrusion. The pen magnet is approximately 16 cm long. - 97 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Sibley Group sedimentary rocks (Hart and Magyarosi, 2004). The finer grained monzogabbro may be a poorly assimilated sediment xenolith and the coarser grained monzogabbro may be the result of assimilation of the volatile rich sedimentary rocks. Turn vehicles around and return by the same route to the corner of the Driftstone Road and the Stewart Lake Road 10.2 km to the south. Turn right onto the Stewart Lake Road. 3.8 km - Stewart Lake Road ends at the Black Sturgeon Road. Turn right onto the Black Sturgeon Road and drive 3.3 km north. contact of the sill is evident, which appears to be a result of later movement along a northwest-trending structure. Joints are rare in the outcrops forming the interior of the sills, and when present are generally linear in nature. The column-like jointing is probably a result of late stage fracturing of the cooling magma combined with later fault activity rather than a true columnar jointing. 20.3 km – this portion of the Black Sturgeon Road is underlain by units of the Sibley Group and outcrops are rare. Stop 5 - Eagle Mountain UTM coordinates – 0385387E 5433471N Stop at the 20.3 km in the open space on the east side of the road for a view of Eagle Mountain. Stop 4 - Black Sturgeon Road UTM coordinates – 0385313E 5418737N Stop at kilometer 9 of the Black Sturgeon Road and look to the east across the clear cut, a diabase sill intrudes the Rossport Formation of the Sibley Group. Jointing in the diabase sills may be either arcuate or linear. The arcuate joints formed by cooling of the magma vary in strike direction by up to 70° and are commonly observed on horizontal surfaces. Jointing is best exposed in an area to the north, of the clear cut within the 1999 burn. Jointing located close to the sill contacts forms crude polygon joint patterns with “T” or flattened “Y” junctions, and the interiors of the sills lack the “Y” junctions commonly present in polygonal jointed unit (e.g., Aydin and DeGraff, 1988). This lack of “Y” junctions suggests a slow cooling rate which would be in agreement with the 315 to 560 year cooling times for the sills estimated by Sutcliffe (1986). Looking to the northeast, an offset in the basal Eagle Mountain is an approximately 10 km long ridge located west of the Black Sturgeon River and is the first of a series of north-trending fault bounded ridges located further west. The ridges are commonly capped by a diabase sill that has intruded units of the Kama Hill Formation. The lower portions of the ridges are generally poorly exposed but appear to be composed of units of the Rossport Formation. In some areas, as in Eagle Mountain, a second diabase sill intrudes the lower portions of the ridge. There were three diamond drill holes that were completed on the lower slope of Eagle Mountain that provide a stratigraphic section though the lower portion of the Sibley Group. Two of these holes intersected a mafic dyke, which is also exposed in a ravine on the central portion of the ridge. The 5 to 10 m wide fine-grained massive dyke dips approximately 50° south. This dyke appears to be related to a series of fine grained mafic sills with a higher olivine content. Although, these sills Stop 4. Nipigon diabase sill intruding units of the Rossport Formation located along the Black Sturgeon Road. Photo is looking to the southeast. - 98 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 within the wackes (Hart and Magyarosi, 2004). The presence of andalusite, with minor cordierite, suggests a low pressure, high temperature metamorphic event possibly related to the emplacement of the felsic intrusive rocks. The andalusite pseudomorphs are partially replaced by a mica indicating some degree of retrograde metamorphism. Stop 5. Looking north from the top of Eagle Mountain, with units of the Kama Hill Formation intruded, overlain, by a diabase sill. The sill – sediment contact is approximately along the upper tree line in the cliff. are geochemically similar to the ultramafic intrusions, they do not appear to be associated with any known ultramafic intrusion (Hart, 2005; Richardson and Hollings, 2005). 22.5 km – (km 26 on the Black Sturgeon Road) turn right at the load levelers, on to the Camp 42 Road reset. 14.1 km - continue east and then north on the Camp 42 Road until the junction with the Church Road reset. These metasedimentary rocks closely resemble the clastic metasedimentary rocks of the Quetico and Wabigoon subprovinces in the Beardmore area, east of Lake Nipigon (Hart et al., 2002a). For this reason, these rocks are interpreted to have accumulated in a trench environment of an accretionary complex comparable to the depositional environment of the Beardmore-Geraldton greenstone belt (e.g., Williams, 1989). The transition from the southern Wabigoon to Quetico subprovince is located approximately 25 km to the north, where a series of iron formations appear to correlate with the iron formations of the BeardmoreGeraldton belt (e.g., Sutcliffe, 1986). However, the iron formation exposed on the east side of Black Sturgeon Lake (Hart and Magyarosi, 2004) is comparable to the chert-magnetite iron formation in the Mawn Lake area (Coates, 1972) rather than the chert poor units of the Beardmore area. Turn vehicles around 7 km - proceed back along the Church Road to the junction with the Camp 42 Road 7 km – north along the Church Road. Stop 6 - Andalusite schist 0.750 km – trail to left, proceed south up hill along the trail These rocks consist of metamorphosed feldspathic wacke, lithic wacke, and siltstone with metamorphic grade increasing progressively toward the south (Hart 2005). The metawackes and metasiltstones are light grayish brown to medium gray, with 3 to 30 cm thick beds, and a schistosity parallel to bedding resulting in the variable preservation of original textures. These rocks commonly contain 5 to 10% biotite, trace to 1% very fine-grained garnet and, a trace subhedral mineral tentatively identified as cordierite. Specific bands are composed of individual to multiple beds containing 10 to 15% coarse- to very coarse-grained, subhedral to euhedral andalusite. The andalusite occurs disseminated through some beds and occasionally concentrated along parallel bands that appear to represent the original bedding plane. This distribution probably reflects the distribution of clay minerals, with higher Al2O3 content, 0.200 km – park and walk right, to the west, for ~10 m S UTM coordinates – 0388613E 5449432N Stop 6. Coarse-grained andalusite pseudomorphs in metamorphosed feldspathic wackes and siltstones of the Quetico Subprovince. - 99 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Stop 7a - Granite – Diabase Contact UTM coordinates – 0390536 E 5442913N The contact between the leucocratic muscovite granite of the Quetico Subprovince and the Mesoproterozoic diabase sill of the Nipigon Sill Complex is exposed in this flat-lying outcrop. The medium to coarse-grained leucocratic muscovite granite occurs are rounded xenoliths up to 20 cm in length in the chill zone of the diabase sill (see photo below). The sills is composed of a fine to very fine-grained, glomeroporphyritic diabase with polygonal jointing and some area have the glassy chill zone preserved. This outcrop is interpreted to be the upper contact of the diabase sill which is dipping shallowly to the north, and the hill to the southwest is capped to the same diabase sill. This same sill caps the hill above the outcrop at stop 7b. About 20 m to the right or east of the trail, an outcrop of the Quetico Subprovince granite is cut by a northeast-trending, approximately 8 cm wide, aphanitic diabase dyke. Dykes may occur close to the diabase sill contacts generally extending for less than a few metres, but may in some cases extend for a few tens of metres. This dyke is though to be injected from the diabase sill which is located beneath this outcrop. 1.60 km – return to the Camp 42 Road and continue back to the gravel pits. Stop 7b - Pegmatite and Nipigon Sill UTM coordinates – 0389764E 5442111N The west dipping outcrop consists of a muscovite granite dyke intruding biotite schists, and the hill above is capped by a diabase sill. Muscovite granite dykes in this area are generally a few metres in width but may be up to a few tens of metres wide, and are usually irregular in form and orientation although generally following the fabric of the surrounding country rock. This outcrop is composed of pegmatitic quartz, potassium feldspar, and plagioclase with graphic intergrowths of quartz and feldspar. Black, coarse-grained to pegmatitic tourmaline occurs as tabular grains and as graphic intergrowths with quartz. Fine grained subhedral, blue-green apatite occurs as an accessory mineral located along the contacts with the biotite schist country rock. This dyke is one of a number of dykes intruding the metamorphic rocks adjacent to the leucocratic muscovite granite. A body of leucocratic muscovite granite occurs to the north and northwest and is light gray, pinkish gray, to white, massive, and medium to very coarsegrained with occasional pegmatitic sections, composed of quartz and potassium feldspar, and plagioclase with typically less than 5% muscovite (Hart, 2005). Irregular to tabular shaped xenoliths of metasedimentary and schistose rocks up to a few metres in diameter are present through out, but appear to be more common to wards the margins of the body. Numerous muscovite granite dykes intrude the massive body and may constitute up to 30% of the outcrop. These dykes are commonly a few metres in width, but may be up to a The outcrop is located approximately 300 m east of the road, and is accessible by a trail leading from the back of the small gravel pit. Stop 7a. Xenoliths of Quetico granite in the glassy chill zone of a Nipigon diabase sill. Stop 7b. Black tourmaline in the leucocratic muscovite granite dyke. - 100 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 dykes contain intergrowths of tourmaline and quartz, and muscovite and quartz commonly observed in rocks that have been classified as fertile granites, or granites that have the potential to host rare-element mineralization (Breaks et al., 2003). Additional work is on-going to characterise these bodies. The muscovite leucogranite occurs along the regional trend extending from Georgia-Barbara lakes area, about 60 km to the northeast, to the DeCourcey Lake and Onion Lake areas, approximately 45 km to the southwest (Breaks et al., 2003), and this area may represent a similar concentration of fertile granites. Stop 7b. Northeast dipping diabase sill observed to the west of the pegmatitic dyke outcrop. few tens of metres. The granite dykes are composed of pegmatitic quartz, potassium feldspar, and plagioclase and frequently contain graphic intergrowths of quartz and feldspar, black, coarse-grained to pegmatitic, tabular tourmaline usually graphically intergrown with quartz and plumose intergrowths of muscovite-quartz. Trace to 1% fine-grained, disseminated, subhedral, reddish brown garnet, fine grained light green mica or rosy quartz was identified in a few of the dykes. Contact relationships suggest that the leucocratic muscovite granite and pegmatite dykes intruded the country rocks rather than being products of partial melting of the host country rocks. However, the initial mode of formation is not known and these rocks may have formed by partial melting of sedimentary rocks at depth. The pegmatitic, muscovite leucogranite body and Looking west across the valley, a diabase sill can be observed to dip shallowly to the northeast, whereas along the Black Sturgeon River the same sill can be observed to dip shallowly east (see Photo below). The shallow inward dipping orientation of this sill, forming a broad saucer with a width of about 5 km, is also visible in the airborne magnetic pattern (Fig. 11) (Ontario Geological Survey 2004). A number of these saucer-shaped sills, with widths of up to 6 km and interiors occupied by older rock types, are evident from airborne magnetic surveys both in this area and in sills intruding Archean rocks of the Wabigoon Subprovince to the northeast and northwest. Variation in sill orientation over short distances is common in other mafic sill complexes (e.g., Tasmania: Leaman, 1975; Karoo: Chevailler and Woodford, 1999). This change in orientation may be due to a number of factors including a change in orientation of the principal stress direction (Gretener, 1969) during sill emplacement, and the sill may follow pre-existing structures such as faults. There appear to be at least three major sills (100 to ~250 m), and possibly a number of thinner (5 to 25 m) sills in the area, based on a combination of outcrop and diamond drill hole information. However, correlation between outcrops and diamond-drill holes is complicated by the saucer shape of some sills, the presence of block faulting that may post-date sill emplacement, and the lack of distinctive geochemical differences between the sills. 12 km – back to the junction of the Camp 42 and Black Sturgeon roads 26 km – south to Highway 11/17 along the Black Sturgeon Road – bypassing the Stewart Lake Road Figure 11. First vertical derivative of the total field magnetic data the inward dipping saucer shaped diabase sill. Star marks the location of field trip Stop 7. - 101 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 References Office, assessment file 52A15-2.21989. Aydin, A. and DeGraff, J.M., 1988. Evolution of polygonal fracture patterns in lava flows; Science, v. 239, p. 471-476. Barnes, S.J., Boyd, R., Korneliussen, A., Nilsson, L-P., Often, M., Pedersen, R.B. and Robins, B., 1987. 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Sutcliffe, R.H. and Greenwood, R.C., 1985b. Precambrian - 104 - � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2005 Description An account of the resource Institute on Lake Superior Geology. Nipigon, Ontario. May 24-28, 2005. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2005 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/12d45312aebf45dc0fad52fdffb93c7a.pdf 4b91a4fa095c82217c8aef3e3a361ccb PDF Text Text 52nd Annual Meeting Sault Ste Marie, Ontario – May 8 – 12, 2006 Institute on Lake Superior Geology Part 1 – Proceedings and Abstracts Volume 52 part 1 – Proceedings and Abstracts �Proceedings of the 52nd ILSG Annual Meeting – Part 1 52nd Annual Meeting INSTITUTE ON LAKE SUPERIOR GEOLOGY May 8 – 12, 2006 Sault Ste Marie, Ontario Hosted by R. P. Sage and A. C. Wilson Co-chairs Volume 52 Part 1 – Proceedings and Abstracts Edited by A. C. Wilson (Ontario Geological Survey) Cover Photos (clockwise from upper left) – Generalized geology of the Sault Ste Marie, Ontario area (Ontario Geological Survey Map 2543); Stone quarry in nodular anorthosite of the Agnew Lake Intrusion (photo courtesy of M. Easton OGS); Arch Rock, Mackinac Island, Michigan;Gowganda Formation, Hwy. 108 Elliot Lake area; Nicholson outcrop, Arctic Star Diamond Corp. property, Menzies Township; Chippewa Falls unconformity, Hwy.17N. i �Proceedings of the 52nd ILSG Annual Meeting – Part 1 52nd INSTITUTE ON LAKE SUPERIOR GEOLOGY VOLUME 52 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: GLACIAL LAKES ALGONQUIN AND NIPISSING SHORELINE BEDROCK FEATURES: MACKINAC ISLAND, MICHIGAN - FIELD TRIP GUIDEBOOK PART 3: UNUSUAL DIAMOND-BEARING BRECCIAS OF THE WAWA AREA FIELD TRIP GUIDEBOOK PART 4: THE HURONIAN SUPERGROUP BETWEEN SAULT STE MARIE AND ELLIOT LAKE - FIELD TRIP GUIDEBOOK PART 5: KEWEENAWAN ROCKS OF THE MAMAINSE POINT AREA FIELD TRIP GUIDEBOOK PART 6: GEOLOGICAL GUIDEBOOK TO THE PALEOPROTEROZOIC EAST BULL LAKE INTRUSIVE SUITE PLUTONS AT EAST BULL LAKE, AGNEW LAKE AND RIVER VALLEY, ONTARIO - FIELD TRIP GUIDEBOOK Reference to material in Part 1 should follow the example below: Brown, B. A., Czechanski, M. L., Reid, D. D. and Mudrey, M. G. Jr. 2006. New evidence for syn-depositional subsidence in the Middle Ordocivician rocks of southwest Wisconsin; in Wilson A. C. (ed.), Proceedings and Abstracts, Institute on Lake Superior Geology, 52nd Annual Meeting, Sault Ste Marie, Ontario, v. 52 pt 1, p. 7. Published by the 52nd Institute on Lake Superior Geology and distributed by the ILSG Secretary: Pete Hollings - ILSG Secretary Department of Geology - Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Email: peter.hollings@lakeheadu.ca ILSG website: www.lakesuperiorgeology.org ii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Table of Contents Institutes on Lake Superior Geology, 1955-2006 ............................................................. iv Constitution of the Institute on Lake Superior Geology................................................... vi By-Laws of the Institute on Lake Superior Geology ....................................................... vii Membership Criteria for the Institute on Lake Superior Geology.................................. viii Goldich Medal Guidelines ................................................................................................ ix Goldich Medallists ........................................................................................................... xi Goldich Medal Committee ............................................................................................... xi Citation for Goldich Medal Recipient.............................................................................. xii ILSG Student Research Fund ......................................................................................... xiv Eisenbrey Student Travel Awards ....................................................................................xv Eisenbrey Student Travel Award Application ................................................................ xvi Student Paper and Poster Awards .................................................................................. xvii Student Paper and Poster Awards Committee ............................................................... xvii Report of the Chairs of the 51st Annual Meeting ......................................................... xviii Board of Directors.............................................................................................................xx Session Chairs...................................................................................................................xx Local Committee...............................................................................................................xx Banquet Speaker ............................................................................................................. xxi Acknowledgements......................................................................................................... xxi Program.......................................................................................................................... xxii Abstracts .............................................................................................................................1 Author Index .....................................................................................................................72 iii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Institutes on Lake Superior Geology, 1955-2005 # Date Place Chairs 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz & C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims & R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley & E. Mercy 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray 28 1982 International Falls, Minnesota D.L. Southwick 29 1983 Houghton, Michigan T.J. Bornhorst 30 1984 Wausau, Wisconsin G.L. LaBerge 31 1985 Kenora, Ontario C.E. Blackburn iv �Proceedings of the 52nd ILSG Annual Meeting – Part 1 # Date Place Chairs 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey & R.P. Sage 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage & W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller & M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst & R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin & P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey & Jr., B.A. Brown 48 2002 Kenora, Ontario P. Hinz & R.C. Beard 49 2003 Iron Mountain, Michigan L. Woodruff & W.F. Cannon 50 2004 Duluth, Minnesota S. Hauck & M. Severson 51 2005 Nipigon, Ontario M. Smyk & P. Hollings 52 2006 Sault Ste Marie, Ontario R. P. Sage & A. C. Wilson v �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Constitution of the Institute on Lake Superior Geology (Last amended by the Board—May 6, 2004) Article I - Name The name of the organization shall be the “Institute on Lake Superior Geology”. Article II - Objectives The objectives of this organization are: A. To provide a means whereby geologists in the Great Lakes region may exchange ideas and scientific data. B. To promote better understanding of the geology of the Lake Superior region. C. To plan and conduct geological field trips. Article III - Status No part of the income of the organization shall insure to the benefit of any member or individual. In the event of dissolution, the assets of the organization shall be distributed to _________ (some tax free organization). (To avoid Federal and State income taxes, the organization should be not only “scientific” or “educational”, but also “non-profit”) Minn. Stat. Anno. 290.01, subd. 4 Minn. Stat. Anno. 290.05(9) 1954 Internal Revenue Code s.501(c)(3) Article IV - Membership The membership of the organization shall consist of persons who have registered for an annual meeting within the past three years, and those who indicate interest in being a member according to guidelines approved by the Board of Directors. Article V - Meetings The organization shall meet once a year. The place and exact date of each meeting will be designated by the Board of Directors. Article VI - Directors The Board of Directors shall consist of the Chair, Secretary, Treasurer, and the last three past Chairs; but if the board should at any time consist of fewer than six persons, by reason of unwillingness or inability of any of the above persons to serve as directors, the vacancies on the board may be filled by the Chair so as to bring the membership of the board to six members. Article VII - Officers The officers of this organization shall be a Chair, a Secretary and a Treasurer. A. The Chair shall be elected each year by the Board of Directors, who shall give due consideration to the wishes of any group that may be promoting the next annual meeting. His/her term of office as Chair will terminate at the close of the annual meeting over which he/she presides, or when his/her successor shall have been appointed. He/she will then serve for a period of three years as a member of the Board of Directors. B. The Secretary shall be elected at the annual meeting. His/her term of office shall be four vi �Proceedings of the 52nd ILSG Annual Meeting – Part 1 years, or until his/her successor shall have been appointed. C. The Treasurer shall be elected at the annual meeting. His/her term of office shall be four years, or until his/her successor shall have been appointed. The terms of the Secretary and Treasurer shall be staggered so that there will always be a two year overlap between the two. Article VIII - Amendments This constitution may be amended by a majority vote (majority of those voting) of the membership of the organization. By-Laws of the Institute on Lake Superior Geology (Last amended by the Board—May 6, 2004) The by-laws of the Institute on Lake Superior Geology are in revision and will be posted on the ILSG website when completed and approved. Please visit www.lakesuperiorgeology.org vii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Membership Criteria for the Institute on Lake Superior Geology Approved May 8, 1997. Amended by the Board—May 6, 2004 A. Membership in the Institute on Lake Superior Geology requires either participation in Institute activities, or an indication on a regular basis of interest in the Institute. Those individuals registering for an annual meeting will remain as members for 4 years unless: 1) they indicate no further interest in the Institute by responding negatively to the statement on meeting circulars “Remove my name from the mailing list”; or 2) two successive mailings in different years are returned by the postal service as address unknown. B. Those individuals who have not registered for an annual meeting in the past 4 years must indicate an interest in the Institute by postal, electronic, or verbal correspondence with the Secretary at least once every two years. Such individuals will be removed from the membership if they indicate no further interest in the Institute or two successive mailing in different years are returned by the postal service as address unknown. C. The Secretary will maintain a list of current members. The list will include the date of the beginning of continuous membership, dates of returned mail, dates of last contact (expression of interest), and the date membership expires, barring a change of status initiated by the member. Those individuals who have become members of ILSG by Section B will have an expiration date listed at 2 years from the upcoming meeting. For example, a member who expresses interest in September of 1997 (the next annual meeting is May, 1998) will have an expiration date of May, 2000, unless the member contacts the Secretary or attends an annual meeting. D. “Member for Life” status is granted to individuals who have been (nearly) continuous participants of the ILSG meetings for 15 years, Goldich Medal recipients, or those who have served as meeting chairs. This status will be further maintained unless the individuals indicate no further interest in the Institute, or 4 mailings in different years are returned by the postal service as address unknown, or they are deceased. E. All members will be mailed the First Circular for the Annual Meeting and the ILSG Newsletter. The Chair of the annual meeting may opt to send the first circular to additional individuals. All returned mail should be reported to the Secretary. F. The Secretary can designate any individual who is on the ILSG membership list (mailing list) as of January 1, 1997 as a member for life based on participation in ILSG activities. G. Members are strongly encouraged to send address corrections to the Secretary to avoid unintentional lapse of membership. viii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Goldich Medal Guidelines (Adopted by the Board of Directors, 1981; amended 1999) Preamble The Institute on Lake Superior Geology was born in 1955, as documented by the fact that the 27th annual meeting was held in 1981. The Institute’s continuing objectives are to deal with those aspects of geology that are related geographically to Lake Superior; to encourage the discussion of subjects and sponsoring field trips that will bring together geologists from academia, government surveys, and industry; and to maintain an informal but highly effective mode of operation. During the course of its existence, the membership of the Institute (that is, those geologists who indicate an interest in the objectives of the ILSG by attending) has become aware of the fact that certain of their colleagues have made particularly noteworthy and meritorious contributions to the understanding of Lake Superior geology and mineral deposits. The first award was made by ILSG to Sam Goldich in 1979 for his many contributions to the geology of the region extending over about 50 years. Subsequent medallists and this year’s recipient are listed in the table below. Award Guidelines 1) The medal shall be awarded annually by the ILSG Board of Directors to a geologist whose name is associated with a substantial interest in, and contribution to, the geology of the Lake Superior region. 2) The Board of Directors shall appoint the Goldich Medal Committee. The initial appointment will be of three members, one to serve for three years, one for two years, and one for one year. The member with the briefest incumbency shall be chair of the Nominating Committee. After the first year, the Board of Directors shall appoint at each spring meeting one new member who will serve for three years. In his/her third year this member shall be the chair. The Committee membership should reflect the main fields of interest and geographic distribution of ILSG membership. 3) By the end of November, the Goldich Medal Committee shall make its recommendation to the Chair of the Board of Directors, who will then inform the Board of the nominee. 4) The Board of Directors normally will accept the nominee of the Committee, inform the medallist, and have one medal engraved appropriately for presentation at the next meeting of the Institute. 5) It is recommended that the Institute set aside annually from whatever sources, such funds as will be required to support the continuing costs of this award. Nominating Procedures 1) The deadline for nominations is November 1. Nominations shall be taken at any time by the Goldich Medal Committee. Committee members may themselves nominate candidates; however, Board members may not solicit for or support individual nominees. 2) Nominations must be in writing and supported by appropriate documentation such as letters of recommendation, lists of publications, curriculum vita’s, and evidence of contributions to Lake Superior geology and to the Institute. 3) Nominations are not restricted to Institute attendees, but are open to anyone who has ix �Proceedings of the 52nd ILSG Annual Meeting – Part 1 worked on and contributed to the understanding of Lake Superior geology. Selection Guidelines 1) Nominees are to be evaluated on the basis of their contributions to Lake Superior geology (sensu lato) including: a) importance of relevant publications; b) promotion of discovery and utilization of natural resources; c) contributions to understanding of the natural history and environment of the region; d) generation of new ideas and concepts; and e) contributions to the training and education of geoscientists and the public. 2) Nominees are to be evaluated on their contributions to the Institute as demonstrated by attendance at Institute meetings, presentation of talks and posters, and service on Institute boards, committees, and field trips. 3) The relative weights given to each of the foregoing criteria must remain flexible and at the discretion of the Committee members. 4) There are several points to be considered by the Goldich Medal Committee: a) An attempt should be made to maintain a balance of medal recipients from each of the three estates—industry, academia, and government. b) It must be noted that industry geoscientists are at a disadvantage in that much of their work in not published. 5) Lake Superior has two sides, one the U.S., and the other Canada. This is undoubtedly one of the Institute’s great strengths and should be nurtured by equitable recognition of excellence in both countries. x �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Goldich Medallists 1979 Samuel S. Goldich 1993 Donald W. Davis 1980 not awarded 1994 Cedric Iverson 1981 Carl E. Dutton, Jr. 1995 Gene LaBerge 1982 Ralph W. Marsden 1996 David L. Southwick 1983 Burton Boyum 1997 Ronald P. Sage 1984 Richard W. Ojakangas 1998 Zell Peterman 1985 Paul K. Sims 1999 Tsu-Ming Han 1986 G.B. Morey 2000 John C. Green 1987 Henry H. Halls 2001 John S. Klasner 1988 Walter S. White 2002 Ernest K. Lehmann 1989 Jorma Kalliokoski 2003 Klaus J. Schulz 1990 Kenneth C. Card 2004 Paul Wieblen 1991 William Hinze 2005 Mark Smyk 1992 William F. Cannon 2006 Goldich Medal Recipient Michael G. Mudrey Jr Mount Horeb, Wisconsin Goldich Medal Committee Serving through the meeting year shown in parentheses. George Hudak (2006) University of Wisconsin, Oshkosh Tom Hart (2007) Ontario Geological Survey Doug Duskin (2008) Member from Industry xi �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Citation for Goldich Medal Recipient Michael G. Mudrey, Jr. 2006 Goldich Medal Recipient It is my pleasure to acknowledge the many contributions of Mike Mudrey on the occasion of his being awarded the 27th Goldich Medal for “Outstanding Contributions to the Geology of the Lake Superior Region”. Over the past 30 years, Mike has produced an extensive list of papers, maps, and abstracts that have significantly contributed to our understanding of regional geology. He has also compiled an outstanding record of service to the Institute on Lake Superior Geology. Mike Mudrey began his education at South Dakota School of Mines in 1963. In 1964 he transferred to Princeton, where he graduated with an A.B. degree in Geology and Geochemistry in 1967. At Princeton he had the opportunity to study with some of the pioneers of modern geology, and received a strong background in geology and chemistry. He began his association with economic geology working at Homestake in the summer of 1965. After working briefly with vertebrate paleontology in the summer of 1966, he began his long career in the Lake Superior Precambrian working as an assistant to Sam Goldich in the summer of 1967. Mike was a graduate student at SUNY Stony Brook in 1968, then moved to Northern Illinois University, where he completed his M.S. in 1969. His thesis topic was the petrology of the Northern Light Gneiss, completed under Sam Goldich's supervision. During the summers he assisted Sam at the Bureau of Standards and in the field in Minnesota and Ontario. In 1969 Mike moved on to the University of Minnesota, where he graduated with a Ph.D. in geology and analytical chemistry in 1973. His thesis research was a petrologic study of the Pigeon Point Sill, with Paul Weiblen as his advisor. While at Minnesota, Mike worked as a geologist for the Minnesota Geological Survey where he gained additional experience in field mapping and geochemical studies. After graduation, Mike returned to Northern Illinois University to work two years as a Scientist and Project Manager for the Dry Valley Drilling Project of the NSF Antarctic Program. In 1976 Mike joined the Wisconsin Geological and Natural History Survey, were he worked until retirement in 2005. At the WGNHS Mike's first job was to start up a Precambrian mapping program, necessitated by the discovery of volcanogenic massive sulfide deposits in the north. This work led to the first state bedrock map to subdivide the Precambrian, published in 1984. Mike was the driving force behind the long effort to complete gravity and aeromagnetic surveys of the state. Those who have been regulars at the ILSG are familiar with Mike's many contributions to the Precambrian geology of Wisconsin, but working at a small state survey requires one to wear more than one hat. Over his career at WGNHS, Mike ably served as expert on such diverse topics as earthquakes and seismicity, radioactive waste management, mineral and water resources, oil exploration, regional stratigraphy, and radon in the environment. Mike has always had a strong commitment to public service and education as well as scientific research. He was never too busy to answer a question on Wisconsin geology, whether from a legislator or a K-12 science student. In retirement he remains active, continuing to collaborate with his Survey and agency colleagues, and serving as consultant on radon to the Wisconsin Department of Health. xii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Mike has been an active contributor to the Institute on Lake Superior Geology since the early 1970s, when he first met Sam Goldich. He has served as co-chair, field trip chair, board member, member of the Goldich medal committee, Secretary-Treasurer (1990 to 1994), and field trip leader and session chair numerous times. Mike has nearly always contributed an abstract or two, and in the true spirit of Sam, he has never been at a loss for some good critical discussion. It is my pleasure to present the 2006 Goldich Medal to my friend and colleague of many years Mike Mudrey, in recognition of his many contributions to regional geology and service to the Institute. Submitted by B. A. Brown xiii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 ILSG Student Research Fund The 2005 Board of Directors established the ILSG Student Research Fund with US$10,000 from the Institute’s general fund to encourage student research on the geology of the Lake Superior region. A minimum of two awards of US$500 each for research expenses (but not travel expenses) will be made each year. Students are expected to present their research orally or during a poster session at an ILSG meeting. The award winners will also be automatically eligible for the Eisenbrey Travel Awards. To allow the fund to grow, the Fund will receive one-half of any additional proceeds from each annual meeting, after all other commitments and expenses are covered. • • • • • • The Board of Directors will be responsible for selecting a minimum of two awards. The ILSG Treasurer will issue the awards. The ILSG Student Research Fund is available for undergraduate or graduate students working on geology in the Lake Superior region. The applications are due to the ILSG Secretary by August 31st each year. Awards will be made by October 1st of each year. Names of the award recipients will be announced at the next annual meeting and posted on the ILSG website. The proposal application should be at least 500 words, and should have a statement of the research project, background information, a map of the research area, research steps necessary to complete the research, figures (if needed), references, and a list of research expenses. The proposal should also include a proposed end date for the research. The proposal will need to be signed by the researcher’s supervisor. xiv �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Eisenbrey Student Travel Awards The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name “Eisenbrey” was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. “Ned” Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The number of awards and value are determined by the annual Chair in consultation with the Secretary and Treasurer. Recipients will be announced at the annual banquet. The following general criteria will be considered by the annual Chair, who is responsible for the selection: 1) The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. 2) Students who are the senior author on either an oral or poster paper will be given favored consideration. 3) It is desirable for two or more students to jointly request travel assistance. 4) In general, priority will be given to those in the Institute region who are farthest away from the meeting location. 5) Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. The form below is optional. Successful applicants will receive their awards during the meeting. xv �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Eisenbrey Student Travel Award Application Student Name : __________________________________ Address: Date: ____________ __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ email: __________________________________________________________ Educational status: _____________________________________________________ Are you the senior author of an oral presentation or poster? Yes ____ No _____ Will any other students be traveling with you?Yes ____ No _____ If yes, then who? ___________________________________________________ ___________________________________________________ Statement of need (use additional page if necessary): __________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ Signature: ____________________________________________________ Department Head: ____________________________________________________ xvi �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Student Paper and Poster Awards Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper and Poster Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper—last modified by the Board in 2001—follow: 1) The contribution must be demonstrably the work of the student. 2) The student must present the contribution in person. 3) The Student Paper and Poster Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. 4) In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. 5) The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). 6) The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper and Poster Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. 7) The names of award recipients shall be included as part of the annual Chair’s report that appears in the next volume of the Institute. Student papers and posters will be noted on the Program. Student Paper and Poster Awards Committee Dan England - Eveleth Fee Office Inc. John Klasner -Western Illinois University (Retired) Norm Trowell - Ontario Geological Survey xvii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Report of the Chairs of the 51st Annual Meeting REPORT OF THE 51st ANNUAL MEETING OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY Nipigon, Ontario The Ontario Geological Survey and Lakehead University co-hosted the 51st Annual Institute on Lake Superior Geology meeting on May 24-28 in Nipigon, Ontario. The meeting consisted of two days of technical sessions with three pre-meeting and three post-meeting field trips. Ryan Tuomi provided excellent on-site AV assistance that kept the sessions running on schedule and also designed the Meeting web site. Bill Addison, Peter Hinz, Bernie Schnieders, John Scott, Mary Louise Hill and Mike Easton provided invaluable assistance with the field trips. Levina Collins of the Nipigon Economic Development Office acted as a liaison with the Town of Nipigon. Total registration for the meeting was 127 students and professionals. Proceedings Volume 51 was published in two parts: Part 1 – Program and Abstracts, edited by Mike Easton and Pete Hollings, with published abstracts for 26 oral and 14 poster presentations; and Part 2 – Field Trip Guidebook, edited by Pete Hollings. The 51st meeting marked the first time the ILSG Annual Meeting was held in Nipigon, enabling the organization of excellent field trips. On Tuesday, May 24, Tom Hart, Phil Fralick and Mark Smyk co-led a two-day trip to examine the geology and gold mineralization of the BeardmoreGeraldton greenstone belt. The following day, Peter Barnett led a small but dedicated group to view the Quaternary geology of the Beardmore-Nipigon area and, in a first for the Institute; Pete Hollings led a flotilla of small boats out on to Lake Superior to examine the Mesoproterozoic Midcontinent Rift (MCR) stratigraphy near Rossport. On May 28, three trips set out from Nipigon: Pete Hollings and Phil Fralick reprised the Rossport Trip; Tom Hart led a group to examine the geology of the Black Sturgeon area, focusing mainly on the diabase sills and ultramafic intrusions associated with the MCR; and Mark Smyk led a trip to look at the pegmatites and high-grade metamorphic rocks of the Quetico subprovince. One hundred and ten participants attended the Annual Banquet. Dr. Jim Franklin provided the after-dinner presentation, entitled “Mineral Resources for the Future: The Resource Potential of Northern Lake Superior”. Peter Hinz had the privilege of presenting the 2005 Goldich Medal to Co-Chair Mark Smyk of the Ontario Geological Survey. Mark has worked tirelessly for the Institute over the last 17 years and has also made significant contributions to the understanding of Lake Superior geology. The student paper committee (Penelope Morton, Greg Stott and Wally Rayner) were faced with the usual dilemma when it came to picking a winner from the eight talks and two posters. The winners were: 2005 Best Student Paper Awards 1) Daniela Vallini – University of Western Australia ($300, Winner, best oral presentation) 2) Noah Planavsky and Jennifer Murphy – Lawrence University ($200, Winners, best poster presentation) 3) Angelique Magee, OGS/Lakehead University ($100, Honourable mention, oral presentation) In addition, Eisenbrey travel awards in varying amounts were presented to students from: • Lawrence University (Noah Planavsky and Jennifer Murphy); xviii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 • • North Dakota State University (Damion Knudsen); Lakehead University (Adam Richardson, Riku Metsaranta, Dawn-Ann Trebilcock, Chris Lane, Mike Maric, Jordan Laarman, and Bjarne Almqvist). The Institute’s Board of Directors met on May 26, 2005, and brief summary of the meeting follows: 1. 2. 3. 4. 5. 6. 7. Accepted report of the Chairs for the 50th ILSG, Duluth, Minnesota Received, discussed, and accepted 2004-2005 ILSG Financial Summary from ILSG Treasurer Mark Jirsa. Approved Mark Smyk as on-going ILSG Board member Approved 2006 (52nd annual) meeting location—Sault Ste Marie, Ontario, and co-chairs Ron Sage (OGS - retired) and Ann Wilson (OGS). Replaced David Meineke as the “member from industry” on Goldich Committee with Doug Duskin. Amended the Institute’s by-laws in order to qualify for 501c3 status with the IRS. Established the ILSG Student Research Fund with US$10,000 from the Institute’s general fund to encourage student research on the geology of the Lake Superior region. The 51st ILSG meeting was a great success and we would like to thank all the individuals who contributed to this success, including the people and businesses of Nipigon. The following organizations are thanked for their sponsorship of the meeting: Ontario Geological Survey, Lakehead University, Lake Nipigon Region Geoscience Initiative, Ontario Prospectors Association, Canadian Institute of Mining and Metallurgy (Thunder Bay Branch), Northwestern Ontario Prospectors Association, and Chaltrek Geological Supplies Inc. The field trips were well-attended and we would like to extend our thanks to the trip leaders and all those who found themselves with keys to rental cars thrust into their hands at short notice. The Municipality of Greenstone and Roxmark Mines Limited provided generous in-kind support for the BeardmoreGeraldton trip. We would also like to thank all those attendees who pitched in to help move poster boards, chairs and dining tables without having to be asked. The members of the Institute never cease to impress. Both of us were very pleased with the 51st meeting and thankful that it was not marred by accident, injury or inclement weather. We appreciated all the positive feedback from delegates, who enjoyed the small-town setting, meeting venues and varied field trips. Logistical arrangements, although much more daunting in a small community, did not prove to be insurmountable. It bodes well for those considering hosting the Annual Meeting in a smaller town. It was a thoroughly enjoyable and rewarding experience. Respectfully submitted Pete Hollings and Mark Smyk Co-Chairs, 51st ILSG Meeting xix �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Board of Directors Board appointment continues through the close of the meeting year shown in parentheses, or until a successor is selected Ron Sage/Ann Wilson - General Chair 2006 meeting (2009) - Ontario Geological Survey Mark Smyk (2008) - Ontario Geological Survey Steve Hauck (2007) - University of Minnesota, Duluth Mark A. Jirsa - Treasurer (2007) - Minnesota Geological Survey Laurel Woodruff (2006) - U.S. Geological Survey Peter Hollings - Secretary (2006) - Lakehead University, Thunder Bay, Ontario Session Chairs Theodore Bornhorst – Michigan Technological University Peter Hinz – Ontario Geological Survey George Hudak – University of Wisconsin – Oshkosh Helene Lukey – Cleveland Cliffs Inc. Joseph Mancuso – Bowling Green University James Miller – Minnesota Geological Survey - Duluth Richard Ojakangas – University of Minnesota - Duluth Laurel Woodruff – United States Geological Survey Local Committee Co-Chairs R. P. Sage - Ontario Geological Survey (retired), Sault Ste Marie, Ontario Ann Wilson - Ontario Geological Survey, Timmins, Ontario Program and Abstracts Editor Ann Wilson - Ontario Geological Survey, Timmins, Ontario Field Trip Guidebooks Editor R. P. Sage - Ontario Geological Survey (retired), Sault Ste Marie, Ontario Organizing Committee Nora Simm – Chartwells Dining Services Lisa Bagnall - Sault College of Applied Science and Technology Banquet Speaker Dr. Ed Walker, Petrologic Ltd. Exploring for Diamonds in Unconventional Rocks xx �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Acknowledgements Thank you to the following individuals, groups and organizations who contributed to making the 52nd Annual Meeting of the Institute on Lake Superior Geology a success. Ontario Prospectors Association Ministry of Northern Development and Mines – Ontario Geological Survey Minuteman Press - Timmins Volunteer Field Trip Leaders Volunteer Van Drivers xxi �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Program Monday May 8 8:00 a.m. Field Trip 1: Classic Stratigraphy of the Huronian Supergroup – Elliot Lake Transect Leaders: Gerry Bennett (OGS-retired) and Mike Hailstone (OGS) 6:00 p.m. Return to Sault Ste Marie Tuesday May 9 8:00 a.m. Field Trip 1: Classic Stratigraphy of the Huronian Supergroup – Searchmount Transect Leaders: Gerry Bennett (OGS-retired) and Mike Hailstone (OGS) 8:00 a.m. Field Trip 2: Unusual Archean Diamond-bearing rocks of the Wawa Area Leader: Ann Wilson (OGS) 6:00 p.m. Conclusion of Trips 1 and 2 6:00 p.m. - 8.00 p.m. Registration (Sault College)) 6:30 p.m. - 9.00 p.m. Ice Breaker Social (Sault College Cafeteria) and Poster Setup (Sault College) Wednesday May 10 8:00 a.m. - 4:00 p.m. Registration (Sault College) 9:00a.m. - 9:10 a.m. Introductory Remarks – Ron Sage and Ann Wilson, Co-Chairs Technical Session I Session Chairs: Peter Hinz (Ontario Geological Survey), Helene Lukey (Cleveland Cliffs Inc.) 9:10 a.m. Hailstone, Mike An overview of geology of the Sault Ste Marie area 9:35 a.m. 10:00 a.m. Rainbird, Robert H. and Davis, William J. Detrital zircon geochronology of the western Huronian Basin Bennett, Gerry The “Kona Dolomite” of Ontario 10:25 a.m. – 10:45 a.m. Coffee Break and Poster Session Moran, Patrick*, Fralick, Philip and Hollings, Pete Geochemical constraints on the deposition of Mesoarchean banded iron formation at the Musselwhite Mine, North Caribou greenstone belt, Superior Province. 11:10 a.m. Fralick, Philip Iron formation in Neoarchean deltaic successions; Layering styles developed during siliciclastic and chemical sediment deposition, Superior Province, Canada. 11:35 a.m. Jirsa, Mark. A. and Chandler, Val W. Structure of the Biwabik Iron Formation, Mesabi Iron Range, Minnesota 10:45 a.m. 12:00 p.m. – 1:30 p.m. Lunch Break and Poster Session (ILSG Board Meeting by invitation) xxii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Technical Session II Session Chairs: Joseph Mancuso (Bowling Green University), Jim Miller (Minnesota Geological Survey – Duluth) 1:30 p.m. 1:55 p.m. Grabowski, Gary Sampling lamprophyre dikes for diamonds – Discover Abitibi Initiative Shute, Amy* and Hollings, Pete Geology and alteration associated with VMS mineralization in the Hamlin Lake area, Northwestern Ontario 2:20 p.m. – 2:45 p.m. Coffee Break and Poster Session 2:45 p.m. Cannon, William F., Horton, J. Wright Jr., and Kring, David A. The Sudbury impact layer in the Marquette Range Supergroup of Michigan 3:10 p.m. Hollings, Pete and Wyman, Derek Geochemistry of the ~2.7 Ga Blake River Group and Confederation Assemblages: Implications for supra-subduction zone volcanism in the Superior Province 3:35 p.m. Holm, Daniel K., Anderson, R., Boerboom, Terrence J., Cannon, William F., Chandler, Val, Jirsa, Mark, Miller, James, Schneider, D. A., Schultz, Klaus and Van Schmus, W. Randy Continental growth and evolution of the northern interior of the conterminous U. S. 3:55 p.m. Announcements 6:00 p. m. – 7:00 p.m. Cash Bar (Sault College Cafeteria) Annual Banquet and Award Presentation (Sault College Cafeteria) 7:00 p.m. Announcement of 53rd Annual Meeting Location 2006 Goldich Award Presentation to M.G. Mudrey Jr. 2006 Banquet Address - Dr. E. C. Walker Meeting participants not registered for the banquet are welcome to attend the address. Thursday May 11 9:00 a.m. – 10:30 a.m. Registration Technical Session III 9:00a.m. - 9:05 a.m. Announcements Session Chairs: George Hudak (University of Wisconsin-Oshkosh), Laurel Woodruff (United States Geological Survey) 9:05 a.m. 9:30 a.m. Planavsky, Noah*, Knudsen, Andrew and Shapiro, Russell Evidence for widespread distribution of iron dependent metabolisms in Precambrian oceans Waggoner, Thomas D. Sulphur Isotopes from pyrite in the Negaunee Iron Formation xxiii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 9:55 a.m. Mudrey, Michael G. Jr. Statistical analysis of indoor radon data and relationships to geology in Wisconsin 10:20 a.m. – 10:45 a.m. Coffee Break and Poster Session 10:45 a.m. 11:10 a.m. Magee, M. Angelique*, Hollings, Pete and Fralick, Philip W. Geology and geochemistry of the Chimney Lake volcaniclastic breccia near Armstrong, Ontario Miller, James D. Jr. and Peterson, Dean M. The Precambrian Research Center – A new initiative to promote Precambrian field studies at the University of Minnesota Duluth 11:35 a.m. - 1:30 p.m. Lunch Break and Poster Session (Posters removed after lunch) Technical Session IV Session Chairs: Theodore Bornhorst (Michigan Technological University), Richard Ojakangas (University of Minnesota - Duluth) 1:30 p.m. 1:55 p.m. 2:20 p.m. Smyk, Mark C., Hollings, Pete and Heaman, Larry M. Preliminary investigations of the petrology, geochemistry and geochronology of the St. Ignace Island Complex, Midcontinent Rift, northern Lake Superior, Ontario Halls, Henry C., Stott, Greg M., Ernst, R. E., and Davis, Donald W. A Paleoproterozoic mantle plume beneath the Lake Superior region Vallini, Daniela A., Cannon, William F., Schultz, Klaus J., and McNaughton, Neal J. The thermal history of low metamorphic grade Paleoproterozoic metasedimentary rocks of the Penokean orogen, Lake Superior Region: Recognizing a widespread 1786 Ma overprint using xenotime geochronology 2:45 p.m. Presentation of Best Student Paper and Poster Awards and Eisenbrey Awards 3:10 p.m. - 3:35 p.m. Coffee Break NOTE: Asterisk * denotes a student eligible for a Best Student Paper Award 3:30 p.m. Field Trip 6 – Geology of the Paleoproterozoic East Bull Lake Intrusion departs Sault College, Sault Ste Marie for East Bull Lake; overnight at East Bull Lake Lodge 4:00 p. m. Field Trip 4 – Unusual Archean Diamond-bearing rocks of the Wawa Area, participants make their own way to Wawa; overnight in Wawa xxiv �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Poster Presentations Bartingale, R. J.* and Shaw, C. A. Defining flow patterns: Paleomagnetic characteristics of the Wissota Dike Boerboom, T. J. Bedrock geological maps of the Split Rock Point and Two Harbors Northeast 7.5’ quadrangles, north shore of Lake Superior, Minnesota Brown, B. A., Czechanski, M. L., Reid, D. D. and Mudrey, M. G. Jr. New evidence for syn-depositional subsidence in the Middle Ordovician rocks of southwest Wisconsin Buchholz, T.W., Falster, A, U. and Simmons, Wm. B. Some accessory minerals of the Cary Mound granite/granophyre complex, Wood County, Wisconsin Cote, V. The Sault and District Prospectors Association Craddock, J. P., Patel, D., Porter, R., and Wirth, K. Anisotropy of magnetic susceptibility (AMS) analysis of Keweenaw Rift rhyolites, Minnesota Easton, R. M. Complex folding and faulting history in Huronian Supergroup rocks located north of the Murray fault zone, Southern Province, Ontario Gross, A.* and Holm, D. K. Kinematic analysis and monazite geochronology of the Eau Pleine and Niagara shear zones, Wisconsin Hudak, G. J., Hocker-Finamore, S. M. and Heine, J. Field distribution, petrography and lithogeochemistry of epidosites in the vicinities of Fivemile, Needleboy and Sixmile Lakes, Vermilion District, NE Minnesota Jirsa, M. A. New geological mapping of the Mesabi Iron Range Juda, N.*, Wirth, K., Craddock, J., Vervoort, J. and Andring, M. Petrogenesis of a granite xenolith in the 1.1 Ga Midcontinent Rift at Silver Bay, MN Kissin, S. A., Heggie, G. J., Franklin, J. M., Karimzadeh Somarin A. Sulphide saturation mechanisms in gabbroic intrusions in the Nipigon Embayment MacTavish, A. MetalCORP Ltd. Big Lake Cu-Zn-Ag-Au-Co, Ni-Cu-PGE and Mo Property Magee, A. Mining and exploration activity in northwestern Ontario Miller, J. D., Jr. and Severson, M. J. Geology of the Duluth Complex in the four Babbitt 7.5’ quadrangles, northeast Minnesota Mudrey, M. G., Jr. Statistical analysis of indoor radon data and relationships to geology in Wisconsin Peterson, D. M. 3D visualization of mafic intrusions in the Duluth Complex, northeastern Minnesota xxv �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Rousell, D. H. Unresolved problems and the evolution of Sudbury geology Stonier, P.*, Holm, D. K., Medaris, L. G., Jr. and Schneider, D. Characterizing the monazite fingerprint of Paleoproterozoic (Statherian) metasedimentary sequences in central Wisconsin Wirth, K. R., Vervoort, J., Craddock, J. P., Davison, C., Finley-Blasi, L., Kerber, L., Lundquist, R., Vorhies, S. and Walker, E. Source rock ages and patterns of sedimentation in the Lake Superior region: Results of preliminary U-Pb detrital zircon studies NOTE: Asterisk * denotes a student eligible for a Best Student Poster Award Friday May 12 8:00 a.m. Field Trip 3: Keweenawan Rocks of the Point Mamainse Area Leaders: Tom Hart and Anthony Pace (OGS) 8:00 a.m. Field Trip 4: Unusual Archean Diamond-bearing rocks of the Wawa Area Leader: Ann Wilson (OGS) 8:00 a.m. Field Trip 5: Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features – Mackinac Island, Michigan Leader: Ron Sage (retired-OGS) We will be meeting at the Arnold Transit Co. boat dock in St. Ignace MI no later than 9:15 a.m. to catch the 9:30 a.m. ferry 8:00 a. m. Field Trip 6: Geology of the Paleoproterozoic East Bull Lake Intrusion Leaders: Mike Easton (OGS) and R. S. James (Laurentian University) 6.00 p.m. Return of Trips 3 and 6 to Sault Ste Marie, Ontario Field Trip 4 concludes in Wawa Field Trip 5 concludes at ferry dock on Mackinac Island, Michigan xxvi �Proceedings of the 52nd ILSG Annual Meeting – Part 1 DEFINING FLOW PATTERNS: PALEOMAGNETIC CHARACTERISTICS OF THE WISSOTA DIKE. BARTINGALE, R.J. and SHAW, C.A., Department of Geology, University of Wisconsin – Eau Claire, Eau Claire, WI 54702-4004 We analyzed a gabbro dike intruding Precambrian granite below the Lake Wissota Dam in western Wisconsin. Data consisted of alternating field demagnetization and anisotropy of magnetic susceptibility measurements (AMS). Chan (1991) interpreted previous results as consistent with a Keeweenawan age (1.1 Ga) for the dike. However, research done by Macouin et. al. (2003) show similar dikes in the upper midwest and above Lake Superior have been reinterpreted to be related to the 2.07 Ga Kenora-Kabetogama Dike swarm based on moderately SE-plunging paleomagnetic directions. This study was designed to test the age interpretation of the Wissota dike and magma flow patterns. AMS data taken with respect to the major mineral axis indicates a north-east trending, horizontal flow pattern within 4 meters of the north contact and vertical flow in the center. This suggests the concentrations of feldspar phenocrysts on the northern contact were formed near the present level, possibly being fed by the vertical flowing magma. The poles have strong correlation in the center, but weaken within 4 meters of the contact. When fit to a girdle, many samples show a strong foliation. Paleomagnetic poles in several gabbro sites have a characteristic remnant magnetization plunging between 28° and 289° in a WNW direction. Samples have an N-directed overprint we interpret as recent, and record one episode of magnetism. Plotted on an apparent polar wander path for North America, the poles plot near 24° north and 176° west, which is consistent with ages of approximately 1.1 Ga. We conclude that the Wissota dike is probably Keeweenawan in age (Figure 1). References Chan, Lung, 1991, Paleomagnetism of central Wisconsin dike swarm; constraints on thermomechanical model of Midcontinent Rift: Institute on Lake Superior Geology Proceedings and Abstracts, v.37, Part1, p.23. Macouin, M., Valet, J.P., Besse, J., Buchan, K., Ernst, R., LeGoff, M., and Scharer, U., 2003, Low paleointensities recorded in 1 to 2.4 Ga Proterozoic dykes, Superior Province, Canada: Earth and Planetary Science Letters, v. 213, p. 79-95. 1 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Figure 1. Wissota dike virtual geomagnetic poles (VGPs). 2 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 THE "KONA DOLOMITE" OF ONTARIO BENNETT, GERALD, 123 LaSalle Court, Sault Ste Marie, ON The stratigraphic similarity between the Chocolay Group of the Marquette Range Supergroup of Michigan and the lower part of the Cobalt Group of the Huronian Supergroup of Ontario has been recognized for some time. Young (1983) and others accepted the correlation but at that time there were also doubters. Both Chocolay Group and the Cobalt Group lie (at least in part) on Archean basement rocks. The lowermost formations of both groups contain rocks generally considered to be glaciogenic, which are overlain by formations dominated by quartz-arenite. But there the lithologic similarity seemed to end. The Mesnard Formation of the Chocolay Group is overlain by the Kona Formation which contains a thick sequence of dolostone, whereas the (proposed equivalent) Gordon Lake Formation is predominantly a siltstone/sandstone sequence. There have however been reports of thin beds and nodules of dolostone within the Gordon Lake formation by Hoffman et al. (1980) and Jackson (1994). In 1986 Peter Born of the Ontario Geological Survey called the writers attention to a previously unmapped dolostone unit apparently overlying the Lorrain Formation in Fenwick Township, northwest of Sault Ste Marie, Ontario. Subsequent more detailed mapping by the writer revealed that the unit is comprised of at least 10 m of laminated dolostone and chert with clastic dolostone and oolitic dolostone. The writer correlated the dolostone of Fenwick Township with the Gordon Lake Formation of the Huronian Supergroup. Mr. Ken Hatfield, then of Lake Superior State University, pointed out the similarity to the Kona Formation of the Marquette area (Bennett et al., 1989, Born, 1988). No stromatolitic structures comparable to the "big cusp" dolomite of the Kona Formation were noted, but that some thinly laminated units are probably stromatolitic structures or algal mats (Personal communication, Dr. Hans Hoffman, 1990). The occurrence in Fenwick Township is strikingly similar to the dolostone of the Kona Formation of Michigan. Given the recent geochronological studies of Vallini et al. (2005), there is now little doubt that the Cobalt Group of Huronian Supergroup may be correlated with the Chocolay Group of the Marquette Range Supergroup. References Bennett, G., Leahy, E.J, Melisek, J. Born, P. and Hatfield, K. 1989. Sault Ste. Marie Resident Geologists District–1988; in Report of Activities 1988, Resident Geologists, Ontario Geological Survey, Miscellaneous Paper 142, p. 207-217. Born, Peter, 1987. Geology of the Havilland Bay – Goulais Bay Area District of Algoma; Ontario Geological Survey, Open File Report 5602, 114 p with map at a scale of 1:15 840 (1 inch to ¼ mile) Hoffman, H. J., Pearson, D.A.B. and Wilson, B.H., 1980. Stromatolites and fenestral fabric in Early Proterozoic Huronian Supergroup, Ontario; Canadian Journal of Earth Sciences, v.17, p.1351-1357. Jackson, S. L., 1994.Geology of the Aberdeen area; Ontario Geological Survey, Open File Report, 5903, 69p Vallini, Daniela, A., Cannon, William, F, and Schulz, Klaus J., 2005. New age data for the Chocolay Group, Marquette Range Supergroup: Implications for the Paleoproterozoic Evolution the Lake Superior and Lake Huron regions. Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Part I – Proceeding and Abstracts, v.51 part 1 Young, G.M. 1983. Tectono-sedimentary history of early Proterozoic rocks of the northern Great Lakes; in Early Proterozoic Geology of the Great Lakes Region, Geological Society of America Memoir, v.160, p.15-32. 3 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 BEDROCK GEOLOGIC MAPS OF THE SPLIT ROCK POINT AND TWO HARBORS NORTHEAST 7.5-MINUTE QUADRANGLES, NORTH SHORE OF LAKE SUPERIOR, MINNESOTA BOERBOOM, TERRENCE J., Minnesota Geological Survey, boerb001@umn.edu The Minnesota Geological Survey is continuing quadrangle-scale geologic mapping of 7.5' quadrangles adjacent to Lake Superior as part of the U.S. Geological Survey STATEMAP program. This mapping effort has resulted in seven published geologic maps in an area from Duluth to Split Rock Point (Fig. 1A). Work is currently in progress on the Little Marais, Schroeder, and Tofte quadrangles, and Lutsen will be mapped in the coming year. Field mapping is at scale 1:12,000, and map compilations are at 1:24,000. The North Shore is experiencing burgeoning development, creating a growing need for understanding bedrock aquifers and for identifying construction resources. Nearly all water wells near Lake Superior are finished in bedrock aquifers, and saline brines are commonly encountered. Refining the volcanic stratigraphy is the first step in understanding where these brines originate. Identification of intrusive rocks is the first step in locating sources for crushed-rock aggregate. Also, mapping has identified potential sources of paving stone, for which new sources are being pursued by landscaping companies. Thus, the goal of this mapping is to refine the knowledge of the volcanic and intrusive rocks for societal needs, as well as to provide a geologic framework for ongoing studies of the geochemical evolution of the Keweenawan Midcontinent rift system, through collaborative studies with staff from Macalester College. TWO HARBORS NE Prior to this study, no mapping had been done in this quadrangle. Although parts of the quadrangle contain few bedrock outcrops, areas underlain by intrusive rocks are generally well exposed. The newly recognized, informally named, London intrusion is a crudely layered or composite intrusion with a basal laminated ferrogabbro, an intermediate granophyric ophitic gabbro, and a cap of ophitic diabase. Coarse-grained felsicintermediate granophyric rocks form an irregular layer between the ophitic gabbro and the upper ophitic diabase, and also lenses within the other units. Fine-grained ferromonzodiorite occurs locally at the base of the intrusion, likely as a hybridized partial melt of adjacent andesite. Other intrusions include the northward extension of the Silver Creek diabase and Lafayette Bluff diabase, and other poorly exposed units whose distributions are based largely on geophysical data. Sporadically exposed volcanic rocks include the southwestern portion of the Gooseberry River lavas (Green, 2002), here composed of olivine tholeiite in the upper part and transitional basalt to andesite in the lower part, and a newly recognized sequence termed the Gustafson Hill lavas composed of variably porphyritic ferroandesitic to basaltic rocks, separated from the Gooseberry River lavas by the Silver Creek diabase. 4 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 SPLIT ROCK POINT The Split Rock Point quadrangle is dominated by volcanic rocks of the Gooseberry River lavas, with subordinate mafic to felsic intrusions that include the Silver Creek and Beaver River diabase, the Split Rock intrusion, and a narrow multilithic breccia dike (Boerboom, 2004; Boerboom and others, 2004). The upper Gooseberry River lavas include a thick porphyritic basalt flow and a faultsliced sequence of andesite sandwiched between ophitic olivine tholeiite flows. The lower Gooseberry River lavas are poorly exposed, but available outcrops indicate they are composed dominantly of andesitic rocks continuous with those mapped in the Two Harbors Northeast quadrangle. The Split Rock intrusion is a hypabyssal, north–south elongate body with the form of a south-plunging syncline that has a thin, lower mafic phase coeval with the dominant phase of pink, flow-banded, weakly porphyritic felsite that contains scattered but ubiquitous small mafic enclaves. References Boerboom, T.J., 2004, Newly recognized diatreme breccia dikes on Lake Superior near Two Harbors, Minnesota [abs.]: Institute on Lake Superior Geology, 50th Annual Meeting, Duluth, Minn., Proceedings, v. 50, p. 39. Boerboom, T.J., and Green, J.C., 2004, Bedrock geology of the Split Rock Point quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-147, scale 1:24,000. ———2005, Bedrock geology of the Two Harbors NE quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-155, scale 1:24,000. Boerboom, T.J., Green, J.C., and Jirsa, M.A., 2002a, Bedrock geology of the French River and Lakewood quadrangles, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-128, scale 1:24,000. ———2002b, Bedrock geology of the Knife River quadrangle, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map M-129, scale 1:24,000. Boerboom T.J., Green, J.C., and Miller, J.D., Jr., 2003a, Bedrock geologic map of the Castle Danger quadrangle, Lake County Minnesota: Minnesota Geological Survey Miscellaneous Map M-140, scale 1:24,000. ———2003b, Bedrock geologic map of the Two Harbors quadrangle, Lake County Minnesota: Minnesota Geological Survey Miscellaneous Map M-139, scale 1:24,000. Boerboom, T.J., Miller, J.D., Jr., and Green, J.C., 2004, Geologic highlights of new mapping in the southwestern sequence of the North Shore Volcanic Group and Beaver Bay Complex: Institute on Lake Superior Geology, 50th Annual Meeting, Duluth, Minn., Proceedings, v. 50, pt. 2, Field trip guidebook, p. 46-85. Green, J.C., 2002, Volcanic and sedimentary rocks of the Keweenawan Supergroup in northeastern Minnesota, chapter 5 of Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, p. 94-105. 5 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Figure 1. A. Index map showing the location of mapped quadrangles along the North Shore of Lake Superior. M-128–Boerboom and others (2002a); M-129–Boerboom and others (2002b); M-139– Boerboom and others (2003b); M-140–Boerboom and others (2003a); M-147–Boerboom and Green (2004); M-155– Boerboom and Green (2005). Little Marais, Schroeder, and Tofte to be published in 2006; Lutsen in 2007. 6 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 NEW EVIDENCE FOR SYN-DEPOSITIONAL SUBSIDENCE IN THE MIDDLE ORDOVICIAN ROCKS OF SOUTHWEST WISCONSIN BROWN, B.A., CZECHANSKI, M.L. Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison WI 53705 REID, DANIEL D. Wisconsin Dept. Transportation, 3502 Kinsman Blvd., Madison, WI 53704 MUDREY, M.G. JR.† 106 Ravine Road, Mount Horeb, WI 53572 Extensive new rock cuts and exposures were created during the rebuilding of U.S. Highway 151 into a modern 4-lane highway through the Driftless Area of southwest Wisconsin. These cuts, some exceeding 100 feet high, provide a unique cross section of the Middle Ordovician rock of the historic Upper Mississippi Valley Base Metal District of zinc and lead. The new cuts provide a detailed view of the stratigraphy, and expose some structures not previously described in the region. Examples of collapse structures and local block faulting have been recognized throughout the mining district for many years. These structures could be seen in older road cuts, and they were described in many early reports on the mining district. Structures of this type have traditionally been interpreted as pitch-and- flat structures, which resulted from solution and collapse related to the formation of the zinc-lead deposits. The oldest collapse features observed in the new cuts formed in Early Middle Ordovician time, during deposition of the St. Peter Sandstone. The youngest known at this time formed in Late Middle Ordovician time, during deposition of the carbonate of the Galena Formation. These features formed as much as 200 million years earlier than the zinc-lead mineralization, which has been dated as Early Permian in age. The early syn-depositional collapse structures are interpreted to be the result of local collapse of paleokarst features developed in the underlying carbonate of the Early Ordovician Prairie du Chien Group. Extensive karst is known to have formed during the interval of aerial exposure following lithification of the Prairie du Chien rock and prior to deposition of the St. Peter Sandstone. This interval contains a major regional unconformity which marks the SaukTippecanoe sequence boundary throughout the region. Collapse occurred as overlying sediment accumulated and compacted, prior to complete lithification. The role of these syn-depositional features in controlling the path of mineralizing fluids is unknown. The few examples known at this time contain no significant mineralization, although mineralized pitch and flat structures and gash vein lead deposits are known to occur nearby. It is possible that the paleokarst features were small and localized, and were not important as conduits for mineralizing fluids, which migrated along regional tectonic features. In contrast, areas of sulfide mineralization were typically associated with extensive rock alteration and deep weathering, which required modifications to the design of cuts and structures, and use of alternative slope stabilization methods during construction. 7 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 SOME ACCESSORY MINERALS OF THE CARY MOUND GRANITE/GRANOPHYRE COMPLEX, WOOD COUNTY, WISCONSIN. BUCHHOLZ, THOMAS W.†1, FALSTER, ALEXANDER U.2, and SIMMONS, WM. B. 2, 1 1140 12th Street North, Wisconsin Rapids, Wisconsin 54494, 2 Department of Geology and Geophysics, University of New Orleans, New Orleans, Louisiana 70148. Early Proterozoic (1,833 ± 4 Ma) granite, granophyre and comagmatic rhyolite outcrop on Cary Mound in western Wood County, WI, and are exploited in several quarries. All phases of the complex are cut by numerous faults and fractures that served as avenues for fluid transport, resulting in widespread chloritization of the granite and granophyre and development of thin hydrothermal veins. Several studies (Sims, 1990; Bruesewitz and Cordua, 2003) postulate that the complex may be a collapsed caldera and indicate that the complex may be of anorogenic or late orogenic origin. Granophyric phases are locally miarolitic, particularly in the Haske quarry, and host a complex mineralogy ranging from simple magmatic through pegmatitic to hydrothermal mineralization, even though no pegmatites sensu stricto have yet been found on Cary Mound. Miaroles may be either simple vugs lined with crystals of quartz, microcline +biotite, probably formed as a result of local volatile saturation, or may have marginal pegmatitic facies marking the transition from granophyre to miarole, the latter primarily noted in the Haske quarry. In areas where such miaroles are in close proximity, pegmatitic margins may merge and form larger areas of pegmatitic texture. These pegmatitic facies may represent pods of pegmatitic melt generated by fractionation of the crystallizing granophyre; if so, the melt was probably enriched in volatiles and incompatible elements. Typical NYF (niobium-yttrium-fluorine = typical A-type granitic association as opposed to LCT (lithium-cesium-tantalum = typical S/I-type granitic association) pegmatite mineralization is present in these pegmatitic phases; quartz, microcline, fluorite, allanite(Ce) and zircon. Locally ferrocolumbite, samarskite-(Y) and thorite have been identified; a Ti-rich Y-Nb oxide mineral has been noted as well and may be polycrase-(Y), but confirmation is required, and a number of additional phases await further study. Pegmatite-bordered miaroles may be quartz cored or filled with quartz + late chlorite, sulfides, fluorite, siderite and calcite; these are interpreted as products of a pervasive late hydrothermal phase introduced along networks of thin fractures. The most abundant sulfides are pyrite, chalcopyrite and pyrrhotite, though sphalerite, galena, marcasite, arsenopyrite and rarely molybdenite may be present. Unusual acicular sulfide crystal morphologies are sometimes present (Buchholz et al, 1997). Small silvery-colored grains of Cu-Co-Ni sulfides have been noted; no further identification work has been done due to paucity of material. Barite is common in small amounts but is usually inconspicuous. Gypsum of secondary, weathering origin may be locally common in small amounts. Cassiterite is uncommon but has been identified from thin fissures in several sites (Cepress and County quarries), and appears to have formed early, probably as a highertemperature hydrothermal phase perhaps transitional from magmatic/pegmatitic to 8 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 hydrothermal/pneumatolytic. Xenotime-(Y) from a thin fissure in the Haske quarry may be of similar origin. Hydrothermal alteration and chloritization is widespread throughout the complex, and associated mineralized veins and fissures in general reflect the late hydrothermal mineralization noted in miaroles. Although most are rich in chlorite, one vein system in the Cary-Rock Road quarry is mineralized with fine-grained lithian muscovite (1.1 wt. % Li2O) associated with pyrite, chalcopyrite, siderite, apatite and barite. Pyrrhotite is generally absent from vein mineralization, whereas the Fe-S paramorphs pyrite and/or marcasite are usually common. Tiny grains of molybdenite are often common in metasomatized granite/granophyre in the Haske quarry. Sparse millerite (NiS) has recently been identified from a hydrothermal vein in the Haske quarry. Rutile is uncommon but has been noted from the Cepress and Haske quarries. Small late-formed crystals and grains of a LREE-phosphate (probably either monazite-(Ce) or rhabdophane(Ce) have been found on chlorite and pyrite from the Haske and Cepress quarries. The mineralization present in pegmatitic miarole margins may indicate the parent magma had locally evolved to a Nb, Y and F-enriched phase. Abundant fluorite and the existence of lithium-bearing muscovite veins suggest the possibility of pneumatolytic or greisen-type mineralization within the complex. The pervasive chloritic hydrothermal alteration and sulfide mineralization suggest that small vein-type sulfide deposits may be present. However locating these, if they exist, may be challenging due to extensive forests and remnant Cambrian sandstone cover. References Bruesewitz, Jeff and Cordua, W.S., 2003, The Cary Mound Granite: A mineralized collapsed caldera in Wood County, Wisconsin, abstract, Geological Society of America, Abstracts and Programs - North Central Section annual meeting, vol. 35 #2, St. Louis, Mo, p.45 Buchholz, Thomas W., 1997, Apatite and Lithium Bearing Muscovite from Central Wisconsin: Mineral News, June 1997, p.6. Buchholz, T. W., Falster, A. U., and Simmons, Wm. B., 1997, An Unusual Miarolitic Mineral Assemblage From Central Wisconsin, abstract, Rochester Mineralogical Symposium, Program and Abstracts Volume, p. 8. Sims, P.K., 1990, Geologic Map of Precambrian Rocks, Eau Claire and Green Bay 1º x 2º Quads, Central Wisconsin, U.S. Geological Survey Map I-1925 9 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 THE SUDBURY IMPACT LAYER IN THE MARQUETTE RANGE SUPERGROUP OF MICHIGAN CANNON, WILLIAM F.† and HORTON, J. WRIGHT, JR., U.S. Geological Survey, Reston VA 20192 KRING, DAVID A., Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ The 1850 Ma meteorite impact at Sudbury, Ontario created a crater estimated to be 180 km in diameter (Abramov and Kring, 2004). A layer of material formed by the Sudbury impact has been well documented in northwestern Ontario and northern Minnesota (Fig. 1) (Addison and others, 2005). In Michigan, only about 500 km from the center of impact, marine sediments of the Marquette Range Supergroup were being deposited and should record the impact. One possible record of an impact is addition to local sediments of material excavated from the crater. This material may vary from coarse fragments of the target rock from the ejecta curtain to finer particles from the impact-generated dust cloud, including accretionary lapilli and mineral grains bearing shock metamorphic features. At the time of impact at least parts of the Michigan sedimentary basin were in shallow-water suggesting the likelihood of major tsunami-related deposits. We are investigating possible impact generated rocks at five sites in northern Michigan (fig.1), which are at a comparable stratigraphic horizon to the Ontario ejecta and are similar petrographically. All localities are at or within a few hundred meters above the base of the Baraga Group and may record both airborne and tsunami deposition. Baraga Basin- layer 1-15 m thick in lower part of Michigamme Formation. Well developed accretionary lapilli (fig. 2) and planar deformation features (fig. 3). West Dead River- isolated outcrop of bedded to massive lapilli-rich material (fig. 4). Contains angular chert fragments to about 1 m diameter. Strong carbonate replacement. At least 2 m thick and probably at least 100 m above base of Michigamme Formation. East Dead River- bed of breccia about 30 m thick (fig. 5, 6). Sparse lapilli. Crudely graded with coarsest clasts at base. Underlain by banded iron-formation and overlain by black pyritic slate. Contains numerous clasts of chert. About 300 m above base of Michigamme Formation. Marenisco- bed about 2 m thick near base of Copps Formation. Coarse sand to conglomerate containing many clasts of underlying Archean granite. Also contains slabs of chert to about 2 m diameter and quartz grains with possible relict planar deformation features. Strong carbonate replacement. Republic- numerous boulders of lapilli-rich breccia in gravel pit, possibly locally derived. Our study of these localities and the search for additional sites is in its early stages. We hope that calling attention to these likely impact-related rocks will encourage additional searches and discoveries. We suspect that most new “discoveries” will result from recognizing telltale signs of impact processes within already known “unusual” breccias or “volcanic” units within this narrow stratigraphic interval. References Abramov, O, and Kring, D.A., 2004, Numerical modeling of an impact-induced hydrothermal system at the Sudbury crater: J. Geophys. Res., v. 109, p.1-16. Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact: Geology, v. 33, p. 193-19 10 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Figure 1 Location of impact layer sites Figure 4. West Dead River. Bedded accretionary lapilli unit unconformably overlain by massive breccia with angular chert. Card 8 cm long. Figure 2. Baraga Basin. Accretionary lapilli in drill core. Figure 5. East Dead River. Photomicrograph of breccia with “volcanic” shards and rounded quartz grains. Figure 3. Baraga Basin. Quartz grain with two sets of relict planar deformation features. Figure 6 East Dead River. Multi-lithic breccia of chert and a variety of “volcanic” fragments in finer breccia groundmass. 11 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 THE SAULT AND DISTRICT PROSPECTORS ASSOCIATION COTE, VIVIENNE, President, Sault and District Prospectors’ Association, Sault Ste Marie, Ontario The Sault and District Prospectors Association (SDPA) has been in existence since the early 1970’s. The purpose of the association is to promote mineral exploration in the area and raise awareness of the role mineral development plays in the economy of the region and the north in general. Although the group is relatively small it is quite active with speakers from various backgrounds and interests presenting a diverse array of topics at the monthly meetings. The highlight of the year is the SDPA annual field trip. The trips proved so popular that in 2005 a fall field trip was added. The latest trips have included the Archean diamond bearing rocks in the Wawa area, the Keweenawan rocks of the Mamainse Point area, Huronian stratigraphy of the Elliott Lake area as well as the Eagle River Mine in the Wawa district. The poster is a visual overview of the various fieldtrips undertaken by the group against a backdrop of the general geology of the area. A wide variety of participants have attended including prospectors, geologists, students, rock hounds and of course, our mascot, the dog “Chloe”. The SDPA is an associate member of the Ontario Prospectors Association. 12 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 ANISOTROPY OF MAGNETIC SUSCEPTIBILITY (AMS) ANALYSIS OF KEWEENAW RIFT RHYOLITES, MINNESOTA CRADDOCK, JOHN P. and PATEL, DHIREN, Geology Dept., Macalester College, St. Paul, MN 55105; PORTER, RYAN, Geology Dept., Whitman College, Walla Walla, WA; and WIRTH, KARL, Geology Dept., Macalester College, St. Paul, MN 55105 The North Shore Volcanic Group (NSVG) of the 1.1 Ga Midcontinent Rift System (MRS) in Minnesota is dominated by basalt, with approximately 10–25% of the bi-modal igneous suite being composed of felsic flows (rhyolites and icelandites). Several of the rhyolite flows may be rheomorphic ignimbrites due to their vast expanse and presence of tridymite paramorphs and local exposures of flow banding, laminations and lineations (Green and Fitz, 1993). In this study we identified four well-exposed rhyolite flows between Grand Portage and Duluth where the over and underlying mafic flows clearly distinguish the rhyolite flow thickness and outcrop character. Oriented samples were collected from the bottom, middle and top of each of the four flows and oriented cores (or equant cubes) were analyzed at the Institute for Rock Magnetism, University of Minnesota using the “RolyPoly”, which is an alternating current (AC) susceptibility bridge for determining anisotropy of low-field magnetic susceptibility. An alternating current in the external "drive" coils produces an alternating magnetic field in the sample space with a frequency of 680 Hz and amplitude of up to 1 mT. The induced magnetization of a sample is detected by a pair of "pickup" coils, with a sensitivity of 1.2*10-6 SI volume units. For anisotropy determination, a sample is rotated about three orthogonal axes, and susceptibility is measured at 1.8° intervals in each of the three measurement planes. The susceptibility tensor is computed by least squares from the resulting 600 directional measurements. The output is a trend and plunge for each of the principal susceptibility tensors (i.e. Kmax, Kint, Kmin), mean susceptibility, and three axial ratios L=Kmax/Kint, F=Kint/Kmin, and P=Kmax/Kmin (lineation, foliation, and degree of anisotropy respectively). Principal tensors are plotted on lower hemisphere steroenet projections. AMS is, thus, a magnetic proxy for interpreting magmatic flow in the rhyolites. From north to south, we sampled the Kimball Creek (near Hovland, n=56, 366 m thick), Devil Track (near Grand Marais, n=61; 250 m thick), Palisade Head (near Silver Bay, n=62, 100 m thick), and Lakewood (north of Duluth, n=58, 78 m thick) rhyolites. Despite an average anisotropy for the sample suite (n=237) of 7.5%, only the Devils Track (base and top) and Palisade rhyolites preserve a layer-parallel Kmax grouping that is interpretable as rift-normal, suggesting northwestward eruption from the rift axis (Fig. 1). References Green, J.C. and Fitz, T. J. III, 1992, Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent Rift plateau volcanics, Minnesota; petrographic and field recognition: Journal of Volcanology and Geothermal Research, v. 54, 177-196. Rochette, P., Jackson, M., Aubourg, C., 1992, Rock magnetism and the interpretation of anisotropy of magnetic susceptibility: Reviews of Geophysics, v. 30, p. 209-226. 13 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 14 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 COMPLEX FOLDING AND FAULTING HISTORY IN HURONIAN SUPERGROUP ROCKS LOCATED NORTH OF THE MURRAY FAULT ZONE, SOUTHERN PROVINCE, ONTARIO EASTON, R.M., Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5, mike.easton@ndm.gov.on.ca Huronian Supergroup strata located north of the Murray fault zone are generally thought to record a relatively simple structural history of broad folding and faulting, related primarily to distal effects of the ~1835 Ma Penokean orogeny. Detailed mapping by the Ontario Geological Survey in Porter and Vernon townships (Easton 2005, 2006), northeast of Agnew Lake indicates that, at least in the area immediately west of Sudbury, this view is incorrect. At least 2 periods of folding are present, roughly orthogonal to one another - the resulting interference forms a dome and basin pattern (Figure 1). F1 folds Nipissing gabbro intrusions present in the lowermost part of the stratigraphy, whereas Nipissing gabbro appears to be emplaced along fractures related to F2 axial planes. This suggests either multiple periods of gabbro emplacement, or that gabbro emplacement occurred synfolding. In either case, folding cannot be significantly younger than 2210 Ma. The map pattern is also affected by at least 5 major fault sets, 4 of which are post-folding. The earliest faults are north-trending, and juxtapose Archean granitic basement against Huronian Supergroup strata. These faults appear to have been fluid conduits, as indicated by the presence of large quartz vein systems and microbrecciation in Archean basement, and hydrothermal annealing of quartz in sedimentary rocks, adjacent to the faults. Eastnortheast faults also juxtapose Huronian strata against basement rocks, but are post- F1 folding, with both vertical and lateral movement. They may be associated with a set of north to northeast, dominantly normal faults, which may have an older thrust component. Most significant in terms of map pattern, at least in the southern part of the map area closest to the Murray fault system, are east to east-northeast normal faults, across which major changes in stratigraphic level occur. There may be a thrust component to these faults, but if so, it has been obscured by subsequent vertical movement, and the fact that the east to east-northeast faults are the loci for the development of extensive zones of Sudbury breccia. The localization of Sudbury breccia along this fault set suggests that it may have developed at ~1850 Ma, due to the Sudbury impact or the peak of the Penokean orogeny, or both. Finally, significant vertical displacement, occurs along a major set of closely spaced northwest-trending faults. Some of these faults are the loci of Sudbury swarm diabase dikes (~1240 Ma). The dikes are undeformed, which suggest that this fault set formed between 1850 and 1240 Ma. The complex history of the area provides new evidence for an earlier orogenic event in the region (“Blezardian?”), and has major implications for detailed stratigraphic correlation of Huronian Supergroup strata and mineralized Nipissing gabbro intrusions. 15 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Figure 1. Simplified geological map of the northeast shore of Agnew Lake, showing the distribution of fold styles within Porter and southern Vernon townships. The contact between the Mississagi and Bruce formations has been highlighted to illustrate the fold pattern, and units stratigraphically above the Bruce Formation are shown by a pattern. Between the Cameron Creek and Midport faults, the area is dominated by a dome and basin geometry, indicating the presence of two fold generations, with approximately perpendicular axial planes. North of the Midport fault, the early, north-oriented fold style (F1) dominates. Abbreviations: BB = Big Swan basin, CB = Cygnet Lake basin, HB = Hunter basin, PB = Porter basin, SB = Sutherland basin, VS = Vernon syncline. References Easton, R.M. 2005. Geology of Porter and Vernon townships, Southern Province; in Summary of Field Work and Other Activities, 2005, Ontario Geological Survey Open File Report 6172, p.13-1 to 1320. Easton, R.M. 2006a. Geology of Porter and Vernon townships; Ontario Geological Survey, Preliminary Map P.2845. Scale 1:20 000. Colour, with Marginal Notes. 16 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 IRON FORMATION IN NEOARCHEAN DELTAIC SUCCESSIONS: LAYERING STYLES DEVELOPED DURING SILICICLASTIC AND CHEMICAL SEDIMENT DEPOSITION, SUPERIOR PROVINCE, CANADA FRALICK, PHILIP, Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada (philip.fralick@lakeheadu.ca) Neoarchean iron formations (IFs) developed in volcanically quiescent, shallow marine settings consist of magnetite- and/or hematite-rich chemical sediments interbedded with siltstones and slates. The mechanism responsible for depositing such successions contrasts with the two principal models for iron hydroxide or oxyhydroxide precipitation from early Precambrian seawater. The deposition of large Paleoproterozoic iron formations through the mixing of Fe+2 enriched, deep ocean waters with the oxygenated waters on shelves is generally accepted (Cloud 1973, Holland 1973, Pufahl and Fralick 2004). In contrast, many Archean IFs appear to have formed through the venting of hydrothermal fluids associated with volcanically active terrains (Fralick and Barrett 1995). The latter model is not applicable to rocks formed in shallow, volcanically inactive areas and the former has only been applied to Paleoproterozoic shelfs where precipitation was occurring during the oxygenation of the Earth’s atmosphere. The shallow water, Neoarchean iron formations form a unique class of IFs where precipitation was driven by factors other than upwelling or hydrothermal venting. This IF type was examined in the Beardmore-Geraldton area of Wabigoon Subprovince and in the Eagle Island Group of Uchi Subprovince. The shallow water Neoarchean iron formations described here were primarily deposited on flooding surfaces overlying fluvial channel and shore-proximal braid delta deposits. Magnetite and/or hematite laminae are also interbedded with some distributary mouth sediments draping reactivation surfaces on barforms to ripples. Additionally, the iron oxides are present as: disseminated detritus in the upper portion of thin graded siliciclastic layers; intervals of finely parallel laminated hematite and jasper, or magnetite and magnetite+chert, separating clastic layers; and, micro-ripple laminated jasper with hematite drapes. The chemical sediments precipitated in the water column of the nearshore deltaic environment and accumulated during periods of lower current activity and siliciclastic supply. The ferric compounds were redistributed during intervals of river plume outflow, especially accumulating in association with fine-grained detritus in event layers formed where the plume lost contact with the bottom (Fig. 1). Offshore equivalents of these assemblages do not contain IF. The model presented for IF deposition relies on an elevated nutrient flux (N, P) in the near-shore that stimulated microbially induced oxidation of Fe+2. This implies the existence of thriving microbial communities in Neoarchean, near-shore settings; communities of organisms that were able to produce their energy by photosynthesis, oxidize iron either intra- or extra-cellularly, and generate thick successions of IF (Fralick and Pufahl in press). 17 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 References Cloud, P.E., 1973. Paleoecological significance of banded iron-formation. Econ. Geol., v. 68, p. 1135-1143. Fralick, P.W. and Barrett, P.J., 1995. Depositional controls on iron formation associations in Canada; in, G. Plint (ed), Facies Analysis. Int. Ass. of Sedimen., Spec. Pub 22, p. 137-156. Fralick, P.W. and Pufahl, P.K., in press. Iron formation in Neoarchean deltaic successions and the microbially mediated deposition of transgressive systems tracts. Jour. of Sed. Res. Holland, H.D., 1973. The oceans: a possible source of iron in iron formations. Econ. Geol., v. 68, p.1169-1172. Pufahl, P.K. and Fralick, P.F., 2004. Depositional controls on Paleoproterozoic iron formation accumulation, Gogebic Range, Lake Superior region, USA. Sedimentology, v.51, p.791-808. Fig. 1. Schematic representation of the processes responsible for depositing iron formation in shallow Neoarchean settings. 18 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 SAMPLING LAMPROPHYRE DIKES FOR DIAMONDS; DISCOVER ABITIBI INITIATIVE GRABOWSKI, GARY, District Geologist, Ontario Geological Survey, Kirkland Lake, Ontario gary.grabowski@ndm.gov.on.ca The Discover Abitibi Initiative is funded by the private sector and, the federal and provincial governments (respectively Industry Canada through FedNor and the Ontario Ministry of Northern Development and Mines through the Northern Ontario Heritage Fund). The program is designed to stimulate mineral exploration in the Ontario portion of the Abitibi greenstone belt. A project to sample lamprophyre dikes, in the Kirkland Lake – Cobalt area, was approved by the Discover Abitibi program in July, 2004. Forty-five samples, each weighing 24 kg, were submitted to SGS Lakefield Research Ltd. in Lakefield, Ontario for litho-geochemical analysis and diamond extraction, selection and description. Six of the forty-five samples submitted returned diamonds. Samples GGDA0402 and GGDA0432 each returned one microdiamond. Samples GGDA0433, GGDA0435 and GGDA0441 returned 5, 3 and 23 microdiamonds respectively. Sample GGDA0410 contained one 0.011 carat (2.214 mg) macrodiamond. The results of this project demonstrate that diamonds occur in the lamprophyric rock from the Kirkland Lake – Cobalt area. • A 25 kg sample represents about one cubic foot of rock. Although every attempt was made to collect as representative a sample as possible from each exposure, the relatively small volume sampled may have easily missed a diamond. Therefore, sample locations that did not return a diamond should not be considered to barren. • Further study is needed to determine where the diamonds are located within the dikes. Most dikes sampled that returned diamonds contained xenoliths. Spider Resources Ltd. has recently postulated that the diamonds are found in the xenoliths on their Wawa property. • A variety of rock types host lamprophyre dikes and breccia, including all types of metavolcanic and metasedimentary rocks, as well as felsic intrusive rocks including granodiorite, granite and syenite. No preference is apparent for those that contain diamonds. • There are numerous lamprophyre locations that were not sampled in this project. Published Ontario Geological Survey (and its predecessors) can be used to locate these exposures. • The Kirkland Lake – Cobalt area hosts more than 30 kimberlite pipes, over half of which are diamondiferous. There are many targets being tested for potential kimberlite. In May 2005, Tres-Or Resources Ltd. discovered a kimberlite pipe on its Temagami North property, Lapointe 1 target, located 16 km northwest of sample GGDA0402. 19 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 KINEMATIC ANALYSIS AND MONAZITE GEOCHRONOLOGY OF THE EAU PLEINE AND NIAGARA SHEAR ZONES, WISCONSIN GROSS, AMANDA, and HOLM, D.K., Geology, Kent State University, Kent OH, agross4@kent.edu; SCHNEIDER, D.A., Geological Sciences, Ohio University, Athens, OH Introduction. The southern margin of Laurentia experienced several episodes of arc accretion that account for the growth of new continental crust during the late Paleoproterozoic, 1900-1600 Ma. The Niagara fault zone and the Eau Pleine shear zone are structural remnants of an ancient arc-continent collision that occurred during the Penokean orogeny (1870-1830 Ma). As Laurentia continued to grow, these sutures likely persisted as zones of weakness; recent studies have proposed that the Niagara Fault zone may have been reactivated during gneiss dome exhumation (Schneider et al., 2004). Other structural discontinuities in the area show evidence of long-lived reactivation including the Great Lakes tectonic zone to the north. Our purpose is to evaluate the importance of tectonic heredity on the geologic history of the central Penokean orogen. Kinematic analysis. Tectonites from both shear zones contain steep penetrative foliations and dominantly down-dip stretching lineations. Oriented samples of these tectonites exhibit kinematic indicators suggestive of a multi-stage displacement history. Samples of the Niagara Fault zone from Pier’s gorge show spectacular quartz-filled strain shadows (Fig. 1a) that show south-side up relative motion, along with asymmetrical tails on feldspar grains (Fig. 1b) that show south-side down movement. Mesoscopic field indicators from a large outcrop on the north side of Highway 101 preserve definitive south-side down movement as seen in sigma structures and near isoclinal folding of late veins. Oriented samples of the Eau Pleine shear zone at March Rapids display south-side down movement in rotated feldspar grains. Sheared quartzofeldspathic gneiss at the south end of Dancy Quarry exhibits variable grain-size reduction – from coarse gneissic fabric to strongly sheared ultramylonitic fabric all of which are subparallel. The coarse gneiss samples show south-side up movement seen in rotated feldspars. In contrast, the finer-grained mylonites contain bent mica and fish structures which show south-side down relative motion. Multiple episodes of movement are evident from the kinematic work described here. Monazite geochronology. U-Pb dating of monazite has proved useful for determining the timing of formation and reactivation of large shear zones in the western U.S. (McCoy et al. 2005). Initial monazite microprobe work on a single coarse-grained sample of the Eau Pleine shear zone produced a tight cluster of monazite total-Pb ages at ~1846 Ma (Loofboro et al., 2004). This age is consistent with the timing of formation of the shear zone based on cross-cutting relations (Sims et al., 1989). Dating of monazite from the finer-grained ultramylonites is currently in progress. Medium-grained monazite spot dating on two samples of the Niagara fault zone yielded two age populations: the older age of ~1628 Ma is about the same age as the Mazatzal orogeny, whereas the younger ca. 1496 Ma date is about the time that the Wolf River batholith intruded the area (Rose, 2004). Both of these dates, although preliminary, suggest that the Niagara fault zone was influenced by younger tectonic stresses and fluid channelization events. Textural work shows that some monazite grains at the Pier’s gorge site are sheared (Fig. 1c), providing opportunity for constraining maximum age of deformation. Meso- and microscopic kinematic studies combined with total-Pb ages of metamorphic monazite hold great potential for constraining the timing and sense of relative motion along these shear zones. 20 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 21 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 AN OVERVIEW OF GEOLOGY OF THE SAULT STE MARIE AREA HAILSTONE, M. P.GEO, Ministry of Northern Development and Mines, Ontario Geological Survey, Resident Geologist Program, District Geologist, Sault Ste. Marie This presentation will provide an overview of the geology of the Sault Ste. Marie area. The variety of lithologies in the area span the ages from Archean to Paleo- and Mesoproterozoic. The Archean Batchawana Greenstone Belt is part of the Abitibi subprovince and is typical of many Archean Greenstone Belts within the Superior province dominated by metabasalts with intercalated calc alkaline to felsic metavolcanics and metasediments. This belt has been divided into an older Eastern Domain and a Western domain by Grunsky (1991) along a plate-plate collision boundary. The Batchawana Greenstone belt is surrounded on three sides by younger Archean gneisses of the Chapleau, Algoma and Ramsey Lake Gneiss domains. The belt of Paleoproterozoic Huronian rocks between Sudbury and Sault Ste. Marie are part of the Southern structural province. Standard nomenclature for the division of the sedimentary groups within the Huronian utilize a model of four glacial cycles from conglomeratic diamitictite base formations through deep marine formations to deltaic shallow marine formations. Although these environments of deposition work for the sedimentary groups, they mask the genesis of the Huronian basin which is now thought to be an early Proterozoic, active then passive rift system. (Bennett, G. 2006.) Recent studies into the earth’s early atmosphere have revealed that at approximately 2.35 Ga, oxygen made its appearance in the earth’s atmosphere. Huronian sedimentary rocks in the Elliot Lake area preserve that event. Lake Superior is the one of the longest and deepest continental rift systems on the face of the planet and is approximately 1 Ga years old. The rift is also known as the Mid Continental Rift (MCR). In the Mamainse Point Formation Keweenawan subaerial, alkaline basaltic flows, intercalated with conglomerates and intruded by felsic potassic keratophyres of the MCR are exposed on the west side of the Batchawana Greenstone Belt approximately 60 kilometers north of Sault Ste. Marie. Studies of these alkaline basalts demonstrate a geomagnetic reversal separated by a conglomerate unit with the older eastern alkaline basalt flows being reversely polarized. (Hart, T., R. and Pace, A. 2006) References Bennett, G., 2006: The Huronian Supergroup between Sault Ste. Marie and Elliot Lake-Field Trip Guidebook Institute on Lake Superior Geology, 52nd Annual Meeting, Volume 52, Part 4, 65p. Grunsky, E.,C., 1991: Geology of the Batchawana Area, District of Algoma; Ontario Geological Survey, Open File Report 5791, 214p. Hart, T., R. and Pace, A., 2006: Middle Keweenawan Rocks of the Mamainse Point Area - Field Trip Guidebook Institute on Lake Superior Geology, 52nd Annual Meeting, Volume 52, Part 5, 28p. 22 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 A PALEOPROTEROZOIC MANTLE PLUME BENEATH THE LAKE SUPERIOR REGION HALLS, H.C.†, University of Toronto at Mississauga, Mississauga, Ontario L5L IC6, hhalls@utm.utoronto.ca, STOTT , G.M., Ontario Geological Survey, Sudbury, Ontario, ERNST, R.E ., Ernst Geosciences, 43 Margrave Avenue, Ottawa, and DAVIS, D.W., Department of Geology, University of Toronto, Toronto, Ontario. New paleomagnetic and radiometric U-Pb age data on baddeleyite1 show that the 2101 to 2126 Ma Marathon dyke swarm radiates from a region, approximately in east-central Wisconsin (after closure of the 1.0 Ga Mid-Continent Rift), suggesting that these dykes are associated with a possible plume centre that lies off the southern margin of the Superior Province rather than on the northern side within the Hudson bay embayment. The period of magmatic activity includes a reversal of the magnetic field from moderately steep negatively inclined remanences (R polarity) with southeasterly declination to approximately antipodal ones (N polarity)1,2. The Marathon swarm was originally defined on the basis of a set of north-trending dykes in the general area of Marathon, but the new age data show that NE-trending dykes east of Wawa also belong to this swarm. These NE-trending dykes occur within a swarm of NE to ENE-trending “Kapuskasing” dykes that give similar paleomagnetic directions to the Marathon dykes, but with steeper positive and negative inclinations. Outside the Kapuskasing Zone (KZ) both polarities are observed for which positive baked contact tests exist1 . Inside the KZ, only Kapuskasing dykes of R polarity occur. Since feldspar clouding and negative baked contact tests are associated with Kapuskasing dykes lying in the high grade eastern part of the Chapleau Block, thereby demonstrating a secondary, magnetization3, it is possible that all Kapuskasing dykes within the southern KZ have been remagnetized, either during the uplift or as a consequence of slow cooling at depth.. By analogy with the 2.45 Ga Matachewan dykes which show the same phenomenon4, the R magnetization in Kapuskasing dykes is younger than N, which is the same age relation deduced on the basis of U-Pb ages for the Marathon swarm1. The remanence inclination of Kapuskasing dykes, whether R or N, is steeper than the average value for Marathon dykes. A few dykes with comparably steep inclinations are present in the Marathon swarm as originally defined, and one of these dykes (of N polarity) gives a U-Pb age of 2125.7 ±1.2 Ma1. Another, with a relatively shallow N inclination compared to the mean, gives an age of 2121+14/-7 Ma2, so steep inclinations may be a reflection of secular variation rather than apparent polar wander. Geochemically, Kapuskasing dykes cannot be distinguished from Marathon ones, so we provisionally place the Kapuskasing dykes with the Marathon swarm, thus defining a radiating swarm with a fan angle of about 70°, with N to ENE trends. The Fort Frances dyke swarm has R polarity5 and an age of 2076 Ma2 and trends NW, and together with the recently dated 2067± 1 Ma R polarity Franklin dyke6,7 that trends WNW in the Minnesota River valley, forms a broadly radiating swarm that converges to a focal region approximately in central-southern Wisconsin. Taken as a whole, the Marathon and Fort Frances dykes define a radiating swarm with a fan angle of about 140° and a plume centre approximately in Wisconsin. The plume had a life span of about 60 My (from 2126 to 2067 Ma), comparable to that of the older 2.45 Ga Matachewan plume to the east, which had a longevity of at least 50 million years. 23 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Geochemical data from dyke chilled margins show that the Fort Frances dykes have flatter REE patterns compared to Marathon dykes but that one or two dykes within the Marathon and Fort Frances swarms may belong to the other one, which would indicate a radial stress pattern. Alternatively, the noticeable dyke-free gap between the Marathon and Fort Frances swarms may arise if intrusion of the N-NE trending Marathon dykes changed the orientation of the maximum principal stress to favour NW-WNW intrusion of the later Fort Frances dykes. References 1 – Halls, H.C. et al., 2005. Ontario Geological Survey Open File Report 6171, 59 p; 2 - Buchan et al., 1996. Can. J. Earth Sci. 33: 1583-1795; 3 - Halls, H. C. et al., 1994. Can. J. Earth Sci. 31:1182-1196; 4 Halls, H.C. & Zhang, B., 2003. Tectonophysics 362: 123-136; 5 - Halls, H.C. 1986. Can... J. Earth Sci. 23:142-157; 6 – Schmitz, M.D. et al. 2006. GSA Bull. 118: 82-93; 7 - Cavanaugh, M.D. 1983. Unpublished Ph.D.Thesis, University of South Carolina, 79 p. 24 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 GEOCHEMISTRY OF THE ~2.7 GA BLAKE RIVER GROUP AND CONFEDERATION ASSEMBLAGES: IMPLICATIONS FOR SUPRASUBDUCTION ZONE VOLCANISM IN THE SUPERIOR PROVINCE HOLLINGS, PETE† Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1 Canada, peter.hollings@lakeheadu.ca and WYMAN, DEREK School of Geosciences, University of Sydney, NSW, 2006 Australia The broadly coeval Blake River Group (BRG) of the southern Abitibi Belt and the Confederation Assemblage of the Birch-Uchi greenstone belt, have been interpreted as subduction-related volcanic assemblages generated in oceanic and continental margins respectively (Hollings and Kerrich, 2000; Péloquin et al., 1996). Both greenstone terranes contain a range of mafic rocks types (i.e., variably tholeiitic to calc alkaline) and host volcanogenic massive sulfide (VMS) deposits. The Blake River Group is host to numerous VMS deposits, ranging from the Horne Mine (55 Mt massive sulphide ore mined, total tonnage ~144 Mt), to the Quemont Mine along the southern margin of the Sequence (16 Mt), and relatively small ore bodies common in the Noranda Mine Sequence (e.g., 1-5 Mt; Gibson and Watkinson, 1990). The Confederation assemblage is host to the past-producing South Bay VMS mine which produced 1.6 million tons of ore with an average grade of 11% Zn, 2% Cu and 2.12 ounces Ag per ton (Atkinson et al., 1990). Differences in the proportions and types of rocks in the two areas suggest they represent end-members in a range of subduction settings present during the late Archean. The BRG was erupted over a short period between about 2703 - 2698 Ma and is one of the youngest pre-orogenic volcanic suites in the southern Abitibi belt. Plume-associated komatiites are inter-layered with arc-type rocks in the south-central part of the Abitibi Subprovince that contains the BRG. The ~2725-2745 Ma Confederation assemblage was not associated with plume volcanism and was situated at the margin of a proto-continent containing rocks that date back to ~3 Ga. Despite the important differences in their settings, the major element trends of tholeiitic rocks from the two areas resemble each other, and Phanerozoic arcs, more than tholeiites from continental rift settings that may be analogues for some Archean greenstone belts. Rhyolites in both areas are interpreted to be the fractionation products of mantle-derived melts. In addition to documenting variable crustal contamination, the trace elements systematics of the rhyolites provide evidence of zircon fractionation events that occurred without significant changes in major element compositions. These results are probably attributable to extraction of rhyolitic liquids from crystal mush zones that was accompanied by preferential entrainment of zircon crystals, leading to Zr fractionation. The BRG suite includes magmas generated in relict plume asthenosphere but the chemical trends also provide evidence of slab melt metasomatism (Wyman and Hollings, 2005). Primitive rocks in the Confederation assemblage define trace element trends that are analogous to typical modern arcs with no indication that melt-mobilized elements such as Zr and Nb have been introduced in significant amounts. Adakite-like rocks were formed as a result of local events such as arc rifting in the South Bay area, or as an indirect consequence of larger events such as global-scale mantle-plume episodes that 25 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 strongly influenced the southern Abitibi subprovince and the BRG (Wyman and Hollings, 2005). Niobium enriched basalts associated with crustally contaminated rhyolites in the southwest of the South Bay study area are most plausibly linked to rifting of the Uchi Subprovince proto-continent margin. The lack of evidence for HFSE metasomatism in the sources of tholeiitic and calk alkaline mafic rocks, indicates that metasomatism of the sub-arc mantle was dominated by hydrous fluids. Therefore, slab melting occurred not in response to a pervasive steep geotherm but to specific geodynamic events, which in this case were probably linked to the early phases of arc rifting along the continental margin. References Atkinson, B.T., Parker, J.R. and Storey, C.C., 1990, Red Lake Resident Geologist’s District-1990; In Report of Activities 1990, Resident Geologists, Ontario Geological Survey, Miscellaneous Paper 152, 31-66. Gibson, H.L. and Watkinson, D.H., 1990, Volcanogenic massive sulphide deposits of the Noranda shield volcano and cauldron, Quebec. In, Rive, M., Verpaelst, P., Gagnon, Y., Lulin, J.M., Riverin, G. and Simard, A.(eds.), The northwestern Quebec polymetallic belt, The Canadian Institute of Mining and Metallurgy, Special Volume 43, 119-132. Hollings, P. and Kerrich, R., 2000. An Archean arc basalt - Nb-enriched basalt - adakite association: The 2.7 Ga Confederation assemblage of the Birch-Uchi greenstone belt, Superior Province. Contributions to Mineralogy and Petrology 139, 208-226. Peloquin, S., Potvin, R., Paradis, S., Lafleche, M., Verpaelst, P., Gibson, H., 1990. The Blake River Group, Rouyn-Noranda area, Quebec; a stratigraphic synthesis. In: Rive, M., Verpaelst, P., Gagnon, Y., Lulin, J-M., Riverin, G., Simard, A. (Eds.). The northwestern Quebec polymetallic belt; a summary of 60 years of mining exploration. Special Volume - Canadian Institute of Mining and Metallurgy, vol.43, pp.107-118. Wyman, D.A. and Hollings, P., 2005. Late Archean convergent margin volcanism in the Superior Province: A comparison of the Blake River Group and Confederation Assemblage. In: Archean Geodynamics and Environments, AGU Geophysical Monograph Series, 164, 215-237. 26 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 CONTINENTAL GROWTH AND EVOLUTION OF THE NORTHERN INTERIOR OF THE CONTERMINOUS U.S. NICE (Northern Interior Continental Evolution) Working Group HOLM, D.K.† (corresponding author), Kent State University; ANDERSON, R., IGS; BOERBOOM, T.J., MGS; CANNON, W.F., USGS; CHANDLER, V., JIRSA, M. and MILLER, J., MGS; SCHNEIDER, D.A., Ohio University; SCHULZ, K., USGS; VAN SCHMUS, W.R., University of Kansas. www.geo.umn.edu/mgs/index_wNICE.html The Penokean orogeny, long considered the dominant Paleoproterozoic event in the Lake Superior region, has been extrapolated to much of the buried basement of WI, IA, NE, and MI. In contrast, geon 17 crust (Yavapai orogen) is dominant south of the Archean Wyoming craton requiring a 100 m.y. age difference of juvenile crust along strike of the Transcontinental Proterozoic Province. We reconcile this problem with a revised history of the growth and evolution of continental crust in the northern mid-continent based on integration of modern geochronology and regional aeromagnetic data. A new aeromagnetic compilation of the region documents a complex terrane of 3.5-1.0 Ga rocks. In MN, the Archean craton is subdivided by the Great Lakes tectonic zone, a late Neoarchean suture. Paleo-proterozoic rifting of the craton created an irregular continental margin consisting of the Becker embayment and the MRV promontory. Bordering the embayment and onlapping the craton are a Paleoproterozoic fold-thrust belt and foreland basin. Within the Becker embayment are calc-alkaline volcanic and granitoid arc rocks formed by Penokean subduction and suturing of the arc with the craton. A sharp post-Penokean aeromagnetic discontinuity, the Spirit Lake tectonic zone (SLtz), extends eastward from NW Iowa, where it is defined by an abrupt southeastward decrease in the magnetic anomaly, through NC Wisconsin, where it is expressed by a sharp truncation of linear patterns in the Archean gneisses of the Marshfield terrane. The SLtz marks the northern extent of juvenile Yavapai age crust and the southern extent of Archean and Penokean crust. The nature of the SLtz is enigmatic and as yet poorly imaged in the third dimension by geophysical data. However, the preservation of Penokean juvenile crust only in an embayment along the southern rifted margin of the Superior craton suggests the SLtz formed initially as a major strike-slip fault zone responsible for margin truncation. South of the SLtz, the structural grain is subparallel to the SLtz and to axial traces of Mazatzal-age folds (Baraboo syncline). Gneisses and mafic volcanic rocks, probably basement rocks from which 1750 Ma rhyolites formed, are inferred from gravity-magnetic highs to be at subcrop in several areas. The Yavapai terrane is marked by abundant geon 14 granites identifiable by a generally smooth aeromagnetic pattern and low gravitational attraction. Additional high-relief, highintensity circular magnetic anomalies within the Yavapai terrane of Iowa delineate related granites. In southern WI and NE Iowa the aeromagnetic pattern reveals an extensive area of folded basement and Baraboo Interval quartzites beneath thin Paleozoic cover. The YavapaiMazatzal terrane boundary must be southeast of these deformed rocks. This area is bordered to the SE by large irregular shaped magnetic highs making up the Green Island 27 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 plutonic belt (GIPB) along the north edge of the Eastern Granite-Rhyolite province. We interpret the GIPB as having intruded into dominantly Mazatzal age crust. Geochronologic and thermochronologic data corroborate the new tectonic province map and provide important time constraints on the evolution of southern Laurentia during the late Paleoproterozoic. Basement crystallization ages. U-Pb zircon crystallization ages for juvenile crust exposed in WI definitively bracket the Penokean orogeny between 1880 & 1830 Ma. In contrast, U-Pb zircon ages from basement drill hole samples in NE are dominantly 1800 Ma or younger and probably represent eastward extension of the Yavapai age basement of CO. However, very few samples were available from the basement of Iowa, southern MN, SE South Dakota, and southern WI, with the result that many late-20th century interpretations for the buried basement in this region consisted of south-westward extension of the Penokean terrane. For example, the Precambrian basement of NW Iowa was commonly shown as Penokean crust abutting Archean crust, similar to the situation in northern WI along the Niagara Fault Zone. New basement ages appear representative of a growing UPb database for the region south of the SLtz. Key points relevant to this summary are as follows: (a) single-crystal TIMS U-Pb zircon data yield Yavapai-interval crystallization ages (1740 & 1760 Ma); (b) ion-probe analysis of zircons confirm these data; (c) zircons separated from gneissic xenoliths from the Manson impact structure in Iowa confirm the existence of igneous activity ca. 1760 Ma. These data yield no Penokean interval ages. Thus, all presently available U-Pb data support our interpretation that Yavapai orogenic crust extends eastward from NE into Iowa and southern WI, and that Penokean crust may be entirely absent from Iowa and SE Minnesota. Metamorphic & igneous ages. Metamorphism along the southern margin of the Superior province has been historically attributed to Penokean orogenesis. Indeed, a narrow window of amphibolite-facies rocks north of the Niagara Fault zone does record 1.831.80 Ga monazite U-Pb metamorphic ages. Peak metamorphic conditions with attendant magmatism likely mark the culmination of arc accretion. However, the dominant metamorphic and igneous imprint on the Precambrian basement is a regional Yavapai age tectonothermal event dated at ca. 1.76 Ga. Yavapai convergence led to weakening of the mid-crust and generation of the classic gneiss domes now exposed in northern MI and WI. Geon 17 metamorphism extends eastward into the Lake Huron region, where geon 18 metamorphic or magmatic activity is largely absent. Cooling/resetting ages. The results of over 100 modern Ar/Ar mineral ages from basement rocks of the NC midcontinent have allowed detailed characterization of its Proterozoic thermal history. Penokean biotite cooling ages are preserved only in lowgrade arc rocks in EC Minnesota; a few hornblende ages also record Penokean cooling in the metasedimentary rocks of the orogen. Elsewhere throughout the Superior and Huron region, hornblende and mica Ar/Ar ages are predominantly 1.76-1.75 Ga or somewhat younger, reflecting rapid, widespread cooling and orogenic collapse following the aforementioned Yavapai amphibolite-facies metamorphism and magmatism. Geon 17 rapid cooling of Archean and Paleoproterozoic metamorphic rocks of the gneiss dome corridor was caused by their exhumation and regionally was followed by a period of 28 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 tectonic quiescence, crustal stabilization, and deposition of the supermature Baraboo Interval (1730-1630 Ma) quartzites. Across much of the WI bedrock, low-temperature reheating was responsible for resetting mica cooling ages caused by geon 16 Mazatzal collision and foreland deformation. Interestingly, the northern limit of Mazatzal deformation and reheating is approximately located along the Niagara Fault zone in northern WI and upper MI. However, in MN the deformational front must bend south of the MRV promontory as those rocks are not thermally/isotopically reset and are overlain by flat-lying Baraboo Interval quartzites (Sioux quartzite). In the Huron region, Mazatzal heating is recorded only locally along the north shore of Lake Huron. Intrusion of the Wolf River batholith and associated geon 14 A-type plutons across the continental margin had a limited thermal effect on the country rock, in part reflecting their rapid emplacement at shallow levels. However, hydrothermal alteration along the Paleoproterozoic basement/cover contact occurred at considerable distances from the batholith. In summary, the continental interior straddles several terrane boundaries, including the transition from Archean tectosphere to Paleoproterozoic lithosphere. The SLtz is a fundamental Yavapai-age Proterozoic boundary, equivalent to, and possibly a direct extension of the Cheyenne belt suture zone, which also juxtaposes Yavapai orogen crust on the south against the Archean craton, and transects geon 18 (Trans-Hudson) structures in southern South Dakota. The Cheyenne-Spirit Lake structure is a fundamental feature in the evolution of the southern margin of Laurentia, the North American craton. Our new interpretation of the Paleoproterozoic continental growth and evolution of the northern interior of the North American craton suggests greater correspondence to that of the Rocky Mountains than previously thought. Although that region has structurally and magmatically modified during Cenozoic and older tectonism, relatively little tectonism has occurred in the cratonic interior in the last one billion years, providing us a uniquely unaltered perspective into Precambrian evolution of the North American continental lithosphere. 29 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 FIELD DISTRIBUTION, PETROGRAPHY, AND LITHOGEOCHEMISTRY OF EPIDOSITES IN THE VICINITIES OF FIVEMILE, NEEDLEBOY AND SIXMILE LAKES, VERMILION DISTRICT, NE MINNESOTA HUDAK, G. J., HOCKER-FINAMORE, S. M., Department of Geology, University of Wisconsin Oshkosh, Oshkosh, WI 54901, hudak@uwosh.edu HEINE, J., Natural Resources Research Institute, University of Minnesota – Duluth, Duluth, MN 55811 The Lower Member of the Ely Greenstone Formation (LMEG) contains a well-studied, more or less east-west striking, steeply north-dipping and north-facing sequence of Neoarchean submarine volcanic, volcaniclastic, chemical sedimentary, and intrusive strata in the vicinity of Fivemile Lake, Needleboy Lake, and Sixmile Lake in the Vermilion District of northeastern Minnesota. Primary volcanological features of these rocks are generally well-preserved despite syn- to post-volcanic hydrothermal alteration (quartz, epidote, chlorite, sericite, actinolite, albite, iron carbonate, dolomite and calcite) and subsequent greenschist facies metamorphism. Based on field characteristics, the LMEG has been subdivided into an older Fivemile Lake Sequence (FLS) and a younger Central Basalt Sequence (CBS; Peterson and Patelke, 2003). Volcanogenic massive sulfide prospects have been identified in near Fivemile Lake, Skeleton Lake, Needleboy Lake and Eagles Nest Lake #4 (Giagrande, 1981; Peterson and Jirsa, 1999; Hudak and Morton, 1999; Peterson, 2001; Hovis, 2001; Hudak et al., 2002; Hudak et al., 2003). Epidosites (granular to granoblastic, high varience mineral assemblages comprising epidote + quartz ± chlorite ± actinolite) have been identified in several locations within the LMEG. South of Fivemile Lake, epidosites occurs as discrete 0.1-2m diameter round- to lens-shaped pale yellow green masses within a 400m by 500m discordant alteration zone located in an actinolite-epidote-quartz altered diabase dike – sill complex that intrudes the FLS. Petrographic and electron microprobe studies (Hocker et al., 2003) indicate the presence of both pistacite and zoisite within this alteration zone. Epidosites also occur within a 300m by 200m, northeast-trending disconformable alteration zone approximately 500-700m east- southeast of Sixmile Lake. At this location, CBS pillow lavas and lobes are intensely altered to a mineral assemblage composed of pistacite, zoisite, quartz, actinolite, Fe-chlorite, Mg-chlorite, magnetite, chalco-pyrite (locally altered to malachite) and minor sphalerite within approximately 200m of a synvolcanic gabbro sill-dike complex. Isocon analysis (Grant, 1986) has been used to evaluate metasomatism during the genesis of the epidosite alteration zones. Least-altered compositions were selected based on boxplot analysis (Large et al., 2001) and petrographic observations. The LMEG epidosites have specific gravities 5-10% greater than least-altered samples, consistent with observations from epidosites in the Josephine Ophiolite (Harper, 1999). Variation diagrams indicate that Zr and Hf are the least mobile elements during metasomatism in both the diabase intrusion and the CBS lava flows; the best-fit line for these two elements on an isocon diagram defines the isocon. Relative to least-altered diabase, epidosite masses are enriched in Ca, Al, Si, and Sr, and depleted in Fe, K, Na, Mn, Mg, Cu, Zn and Eu. These geochemical variations are consistent with alteration in a high temperature (>350°C), high water:rock ratio reaction zone deep within a synvolcanic submarine 30 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 hydrothermal system (e.g. Harper, 1999) capable of producing volcanogenic massive sulfide mineralization up-section. Variations in chemical behavior within the CBS pillowed flows with increasing distance from the gabbro sill-dike complex suggest that epidosite zones at this locality formed within localized high temperature hydrothermal zones driven by heat derived from the gabbro. Further work is needed to evaluate the extent of epidosites in the CBS, and potential VMS mineralization up-section in the Upper Member of the Ely Greenstone Formation. References Giagrande, P., 1981, Geology and sulphide mineralization of the Skeleton Lake Prospect: unpublished M. S. thesis, University of Minnesota-Duluth, 118 p. Grant, J. A., 1986, The isocon diagram – a simple solution to Gresen’s equation for metasomatic alteration: Economic Geology, v. 81, p. 1976-1982. Harper, G. D., 1999, Structural styles of hydrothermal discharge in ophiolite / sea-floor systems: Reviews in Economic Geology, v. 8, p. 53-73. Hocker, S. M., Hudak, G. J., and Heine, J., 2003, Electron microprobe analysis of alteration mineralogy at the Archean Fivemile Lake volcanic-associated massive sulfide mineral prospect in the Vermilion District of northeastern Minnesota: Natural Resources Research Institute Report of Investigations NRRI/RI-2003/17, 49 p. Hovis, S. T., 2001, Physical volcanology and hydrothermal alteration of the Archean volcanic rocks at the Eagles Nest volcanogenic massive sulfide prospect, northern Minnesota: unpublished M. S. thesis, University of Minnesota – Duluth, Duluth, Minnesota, 137 p. Hudak, G. J., Heine, J., Newkirk, T., Odette, J., and Hauck, S., 2002, Comparative geology, stratigraphy, and lithogeochemistry of the Five Mile Lake, Quartz Hill, and Skeleton Lake VMS occurrences, Vermilion District, NE Minnesota: A report to the Minerals Coordinating Committee, DNR, Minerals Division, State of Minnesota: Natural Resources Research Institute Technical Report NRRI/TR2002/03, 390 pages. Hudak, G. J., Heine, J., Hocker, S. M., and Hauck, S., 2003, Needleboy Lake – Sixmile Lake Geological Mapping Progress Report: June 2003: Natural Resources Research Institute Report of Investigations NRRI/RI-2003/18, 22 p. Hudak, G. J., and Morton, R. L., 1999, Mineral Potential Study, Minnesota Department of Natural Resources Project 326, Bedrock and Glacial Drift Mapping for VMS and Lode Gold Alteration in the Vermilion–Big Fork Greenstone Belt, Part A: Discussion of Lithology, Alteration, and Geochemistry at the Fivemile Lake, Eagles Nest, and Quartz Hill Prospects: Minnesota DNR Division of Minerals Project 326 Report, 136 p. Large, R. R., Gemmell, J. B., Paulick, H., and Huston, D. L., 2001, The alteration box plot: a simple approach to understanding the relationships between alteration mineralogy and lithogeochemistry associated with volcanic-hosted massive sulfide deposits: Economic Geology, v. 96, p. 957-971. Peterson, D. M., 2001, Development of Archean lode-gold and massive sulfide deposit exploration models using geographic information system applications: targeting mineral exploration in northeastern Minnesota from analysis of analogy Canadian Mining Camps: unpublished Ph. D. dissertation, University of Minnesota, Duluth, MN, 503. Peterson, D. M., and Jirsa, M. A., 1999, Bedrock geological map and mineral exploration data, western Vermilion District, St. Louis and Lake Counties, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-98, scale 1:48,000. 31 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 STRUCTURE OF THE BIWABIK IRON FORMATION, MESABI IRON RANGE, MINNESOTA JIRSA, MARK A.†, and CHANDLER, VAL W., Minnesota Geological Survey (jirsa001@umn.edu; chand004@umn.edu) Six years of mapping by the Minnesota Geological Survey along the Mesabi Iron Range generated a variety of new maps, and considerable data regarding the structure of the Paleoproterozoic Biwabik Iron Formation and adjacent units (Fig. 1). Bedrock mapping utilized GIS to integrate data sources that included archived geologic and structure contour maps created by industry and government organizations, test pit and drill hole records, digital bedrock topography, and several iterations of aeromagnetic data. The aeromagnetic data delineate oxidation zones along faults, folds, and fractures in otherwise strongly magnetic iron-formation. These data and field work in more than 400 mines created a mass of structural observations that provide a context for understanding the deformation history of the range. Figure 1. Generalized geologic map of the Mesabi Iron Range showing structures in and along the subcrop of Paleoproterozoic Biwabik Iron Formation (gray). Bedrock north of the Biwabik Iron Formation is largely Archean in age; south of the Biwabik Iron Formation is Paleoproterozoic Virginia Formation bedrock; and to the southeast is the Mesoproterozoic Duluth Complex. The Biwabik Iron Formation is part of the Paleoproterozoic Animikie Group—a sequence of quartzose sandstone, iron-formation, and mudstone—that was deposited unconformably on a relatively stable shelf composed of Archean granite and greenstone. Depositional ages of the Animikie Group vary from 1,878 to 1,777 Ma (Fralick and others, 2002; Addison and others, 2005; Heaman and Easton, 2005). This broad temporal span indicates a protracted history of deposition, and probably also deformation. Much of the Biwabik Iron Formation forms a south-dipping homocline that contains little evidence of disruption, with the exception of locally well developed deformation structures. Although the precise ages of various structural elements on the Mesabi range are nearly impossible to ascertain, a relative chronology has been established from crosscutting relationships. Assigning deformation events to specific structures is probably premature; however, "D0, D1, D2…" nomenclature is applied here to refer to suites of 32 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 apparently related structures. The oldest are those presumably related to soft-sediment deformation (D0), including slumps, sedimentary breccias, and structures that appear to be the result of differential compaction. The earliest "regional" deformation (D1) is manifest in localized, small-scale rotational structures, bedding-parallel slickensides, and larger nappe and sheath folds. The structures commonly lie along boundaries between units having strong rheologic contrast, such as the contacts between thick sequences dominated by mudstone and siliceous grainstone. Nearly all of these structures display asymmetry that indicates south-over-north tectonism. This northward vergence, and the apparent timing relative to later structures, is consistent with compressional deformation—potentially related to the Penokean orogen. One of the long-standing controversies in iron-ore genesis is the question of whether the oxidation and leaching of iron-formation that formed the high-grade hematite ores occurred by supergene or hypogene processes. Although not conclusive, the observation of several early-formed, south-dipping thrust faults with folded, mineralized wall rocks, and bedding-parallel slickensides that host abundant secondary iron and silica implies that at least some of the mineralization was coincident with compressional deformation, perhaps during Penokean orogenesis. This is consistent with the hypogene model proposed by Morey (1999) that attributes oxidation and leaching to ground-water flow driven northward from uplift in the Penokean fold and thrust belt. A second regional suite of structures (D2) is largely extensional. These are monoclines and normal faults that are mutually transgressive; that is, faults that have sympathetically folded wall rocks, and folds that pass laterally to faults. These are some of the major structures along which oxidation and leaching has occurred, and the focus of most hematite ore mining. Veins, vugs, and other secondary mineralization features are abundant. D2 structures likely formed as localized responses to regional tilting. The most recent deformation effects (D3) are trough-like collapse structures, presumably related to post-leaching subsidence. The collapse, and associated oxidation and weathering, are best developed in the uppermost subcrop of iron-formation, implying supergene alteration played a significant role. Thus, the answer to the supergene vs. hypogene debate appears to be that both processes were significant, perhaps at different times. The Virginia horn—where the subcrop extent of iron-formation makes a hookshaped bend—is a complex horst, bounded by faults in the subjacent Archean rocks along which vertical movements have occurred during the entire temporal spectrum from early deposition to latest crustal accommodation. Lacking finite ages, the structures may record Penokean (Geon 18), Yavapai (Geon 17), Mazatzal (Geon 16), and/or Keweenawan (Geon 11) deformation events. The presence of diabase dikes cutting ironformation as far west as Keewatin raises the possibility that at least some of the later phases of deformation on the Mesabi range are related to development of the 1,100 Ma Midcontinent Rift. Funding for Minnesota Geological Survey mapping on the Mesabi range was provided by the Minnesota Legislature on recommendation of the Minerals Coordinating Committee, the Environmental Trust Fund administered by the Legislative Commission on Minnesota Resources, the EDMAP program of the U.S. Geological Survey, and the State Special appropriation to the Minnesota Geological Survey. Products can be digitally downloaded or 33 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 ordered on paper through the Minnesota Geological Survey website (www.geo.umn.edu/mgs). Available products include maps of bedrock topography and depth to bedrock (Miscellaneous maps M-126, M-158), historic 1899 land-surface topography, hydrology, and infrastructure (M-118, M-157), bedrock geology (M-163), and Quaternary geology (M-164). REFERENCES Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology, v. 33, p. 193-196. Fralick, P.W., Davis, D.W., and Kissin, S.A., 2002, The age of the Gunflint Formation, Ontario, Canada: Single zircon U-Pb age determinations from reworded volcanic ash: Canadian Journal of Earth Sciences, v. 39, p. 1085-1091. Heaman, L.M., and Easton, R.M., 2005, Proterozoic history of the Lake Nipigon area, Ontario: Constraints from U-Pb zircon and baddeleyite dating, in Easton, M., and Hollings, P., eds., Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Program and Abstracts, v. 51, pt. 1, p. 24-25. Morey, G.B., 1999, High-grade iron ore deposits of the Mesabi range, Minnesota—product of a continental-scale Proterozoic ground-water flow system: Economic Geology, v. 94, p. 133-142. 34 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 PETROGENESIS OF A GRANITE XENOLITH IN THE 1.1 GA MIDCONTINENT RIFT AT SILVER BAY, MN JUDA, NATALIE, WIRTH, KARL, CRADDOCK, JOHN, Geology Department, Macalester College, St. Paul, MN, 55105; VERVOORT, JEFF, Dept. Geological Sciences, Washington State University, Pullman, WA 99164; ANDRING, MATT, Whitman College, Walla Walla, WA This study examined a well-known granitic xenolith locality within the hypabyssal Beaver Bay Complex of the Midcontinent Rift System (MRS). The xenolith is exposed along the shore of Lake Superior at Silver Bay, MN. Our goal was to constrain the origin of the granite using U-Pb zircon geochronology, whole-rock and trace element geochemistry, and anisotropy of magnetic susceptibility. Previous researchers have interpreted the origin of granite xenoliths contained within MRS rocks as either Archean crustal fragments or MRS felsic plutons. The granite xenolith (~ 50 meters in diameter) occurs within Beaver River diabase, and is crosscut by a mafic dike. The rock consists primarily of quartz, albite, and orthoclase. Granophyric intergrowths of quartz and feldspar are common. In addition, accessory minerals including sphene, apatite, and zircon are present. At the macroscopic level, the xenolith exhibits no indications of magmatic flow or foliation, and our study of anisotropy of magnetic susceptibility (AMS) as a proxy for magmatic flow confirms this (Figure 1). U-Pb analyses of zircons from the xenolith yield an age of 1094 ± 11 Ma on a concordia diagram. This is within error of the age of the youngest dated MRS granophyres. The geochemistry of the granite is similar to other MRS granophyres (e.g., Eagle Mountain, Finland Granite), except that the granite xenolith has higher concentrations of silica and sodium and very low potassium and other alkali elements (e.g., Rb, Ba; Figure 2). The apparent alkali mobility may have resulted from fluid infiltration during late-stage cooling. The compositions of several granitic dikes at Beaver Bay are similar to the Silver Bay xenolith. The Silver Bay xenolith and Beaver Bay dikes share “within plate” and A-type granite major and trace element compositions with MRS granophyres. Geochemical data from granite xenoliths at Split Rock are significantly different from those at Silver Bay and from other MRS granophyres by having volcanic arc characteristics. This suggests that the Split Rock xenoliths might have a different origin from those at Silver Bay. Figure 1. Equal area lower hemisphere projections of AMS results for samples granite xenolith (KP05-45B) and a cross-cutting mafic dike (KP05-45I). 35 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 The Keweenawan age and A-type granite characteristics of the granite xenolith at Silver Bay suggest a greater distribution of MRS granophyres than previously thought. These granophyre bodies may also underlie volcanic flows in the more central portions of the rift. Further isotopic analysis of the granite xenoliths, such as with the Sm-Nd system, would help constrain the petrogenesis of the MRS granophyres. REFERENCES Kennedy, B.C., Wirth, K., Vervoort, J.D. 2000. Petrogenesis of the Midcontinent Rift Granophyric Complexes of Northern Minnesota: Proceedings and Abstracts Institute on Lake Superior Geology, vol. 46, p. 29-30. Miller, J., Chandler, V. W. 1997. Geology, petrology, and tectonic significance of the Beaver Bay Complex, northeastern Minnesota: Geological Society of America Special Paper 312, p. 73-96. Vervoort, J.D., Wirth, K. 2004. Origin of the Rhyolites and Granophyres of the Midcontinent Rift, northeastern Minnesota: Proceedings and Abstracts Institute on Lake Superior Geology, vol.50, p. 158-159. Figure 2. 36 Harker diagrams showing the concentrations of Na2O, K2O, and Al2O3 in the granite xenolith compared to data from other MRS granophyres (Wirth and Vervoort, in prep.). Symbols: filled inverted triangles = Silver Bay granite xenolith; open inverted triangles = Split Rock granite xenoliths; other symbols are MRS granophyre bodies. �Proceedings of the 52nd ILSG Annual Meeting – Part 1 SULPHIDE SATURATION MECHANISMS IN GABBROIC INTRUSIONS IN THE NIPIGON EMBAYMENT KISSIN, S.A., Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1 sakissin@lakeheadu.ca, HEGGIE, G.J., East West Resource Corporation, 1158 Russell Street, Thunder Bay, ON, P7B 5N2, FRANKLIN, J.M., Franklin Geosciences, 24 Comanche Drive, Nepean, ON, K2E 6E9, KARIMZADEH SOMARIN, A., Department of Geology, Faculty of Natural Sciences, University of Tabriz, Tabriz, Iran Interest in the Nipigon Embayment as favourable exploration target for platinum-group element (PGE) deposits was stimulated by the suggestion by Naldrett (1992) that the area is a likely geological setting for the development of nickel-copper-PGE deposits based on criteria established in studies of the Noril’sk deposit in Siberia. Proterozoic gabbroic intrusions in the Nipigon Embayment of northwestern Ontario were studied with the aim of discerning the mechanism of sulphide saturation leading to (PGE) concentrations recently discovered. Two intrusions, the Seagull intrusion (1116.2±9.2 Ma), south of Lake Nipigon, and the Kitto intrusion (1117±1.8 Ma), on the eastern shore of Lake Nipigon, were the subject of the study, as they contain potentially economically significant PGE concentrations. Most of the study was carried out on the Seagull intrusion, as only limited samples were available from the Kitto intrusion. Neither intrusion is well exposed, and most samples were taken from drill-core. Profiles of sulfur, copper, nickel, gold, palladium and platinum as a function of depth in drillholes reveal that sulphur saturation occurred at the base of the Seagull intrusion, where a zone of sulfide mineralisation is developed. However, sulphur saturation was noted at higher levels in the intrusion, notably in the high-grade RGB zone. These observations suggest the operation of different processes in formations of the mineral occurrences – a Noril’sk-type process involving assimilation of sulphur for the basal zone and a reef-type process for the higher zones (Naldrett 1993). Olivine compositions were determined in both intrusions, and in both cases, the compositions indicate that the parental magmas were undersaturated with respect to sulphur. Thus, according to theories of PGE deposit formation, both intrusions have potential for PGE concentration. Contamination of the parental magma, either through assimilation of country rock or magma mixing, has been ascribed a crucial role in the formation of an immiscible melt (Irvine 1975; Naldrett 1989). Neodymium (Nd)-samarium(Sm) isotopes provide a means of testing for contamination. Heggie (2005) reported data on Nd/Sm isotopic studies on Seagull intrusion samples for a range of depths in several drill-holes, as well results from underlying Quetico Subprovince metasedimentary rocks and Sibley Group sedimentary rocks. The calculation of εNd for theses samples yielded values of -0.2 t0 -4.0 (±0.5) for the Seagull intrusion, but -16 to -23 for the Quetico metasediments and a mean of -5 for Sibley Group sediments. Rb/Sr isotopic studies on the same samples were used in a comparison of 143Nd/144Nd vs. 87 Sr/86Sr. Sibley Group sediments differ markedly from the Seagull intrusion in both factors, whereas the Quetico metasediments have lower 143Nd/144Nd ratios and similar 87Sr/86Sr ratios. As the 87Sr/86Sr ratios for the Seagull samples trend to somewhat higher values than those found in the Quetico metasediments, some assimilation of the high 87Sr/86Sr from Sibley material must have occurred. However, since 143Nd/144Nd in the Seagull samples decreases with depth, trending toward lower Quetico metasediment values, assimilation of Quetico material is also likely. 37 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 The source of sulfur in the Seagull intrusion was investigated through the study of sulphur isotopes and selenium/sulfur ratios. Sulphur isotopic compositions of samples from the base of the Seagull intrusion were compared with those from Quetico metapelites and evaporites from the Sibley Group. Sulphides from the Seagull intrusion had ∂34S ranging from –2.3 per mil (‰) to +2.6‰ with a mean value of –0.9‰. Sulphides in Quetico Subprovince metapelites has ∂34S ranging from –2.3‰ to +1.1‰ with a mean value –0.8‰. Finally, Sibley Group evaporites had ∂34S ranging from +7.7‰ to +9.0‰. According to Franklin and Mitchell (1977), these correspond to a mean H2S composition of –4.2‰, based on prior knowledge of temperatures of formation of the sulphate minerals (barite, anhydrite and gypsum) in the Sibley Group. These data provide plausible evidence for incorporation of Quetico sulphide in the sulphide zone at the base of the Seagull intrusion. The source of sulphur in higher zones remains to be determined. Comparison of ∂34S with Se/S x 106 in Seagull sulphides and Quetico metapelites revealed that the Quetico samples lie well outside the region for mantle sulphur, whereas the Seagull samples show considerable scatter in Se/S x 106. Together with their negative ∂34S values, it is evident that assimilation of Quetico sulphide is the explanation for these data. Although the Nipigon Embayment has a number of features that seem to provide for a Noril’sk model setting, continental rifting, evaporites in the section, voluminous basaltic eruption among others, there are problems in its application to the two cases studied here. The Seagull and Kitto intrusions are among the earliest igneous events associated with the Mid-Continent Rift (Davis and Green 1997). Although some contribution from Quetico metapelite sulphide seems likely, the Sibley Group evaporites do not seem to be likely sulphur sources. Rather a reef-type of process seems to be responsible for zones of PGE enrichment at higher levels in both intrusions. References Davis, D.W. and Green, J.C. 1997. Geochronology of the North American Midcontinent Rift in western Lake Superior and implications for its geodynamic evolution. Canadian Journal of Earth Sciences, 34: 476-488. Franklin, J.M., and Mitchell, R.M. 1977. Lead-zinc-barite veins of the Dorion Area, Thunder Bay District, Ontario; Canadian Journal of Earth Sciences, 14: 1963-1979. Heggie, G.J. 2005. Whole rock geochemistry, mineral chemistry, petrology and Pt, Pd mineralization of the Seagull Intrusion, northwestern Ontario; unpublished MSc thesis, Lakehead University, Thunder Bay, Ontario, 156p. Irvine, T.N. 1975. Crystallization sequences of the Muskox intrusion and other layered intrusions-II. Origin of chromitite layers and similar deposits of magmatic ores; Geochimica et Cosmochimica Acta, 39: 991-1020. Naldrett, A.J. 1989. Magmatic sulfide deposits; Oxford University Press, New York, 186p. Naldrett, A.J. 1992. A model for the Ni-Cu-PGE ores of the Noril’sk region and its application to other areas of flood basalt; Economic Geology, 87: 1945-1962. Naldrett, A.J. 1993. Models for the formation of strata-bound concentrations of platinum-group elements in layered intrusions; in Mineral deposit modeling; Geological Association of Canada, Special Paper 40: 373-387. 38 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 MetalCORP LTD. BIG LAKE Cu-Zn-Ag-Au-Co, Ni-Cu-PGE, AND Mo PROPERTY MACTAVISH, ALLAN, MetalCORP Ltd., 309 South Court Street, Thunder Bay, ON, P7B 2Y1, Canada The Big Lake Property of MetalCORP Ltd. of Thunder Bay, Ontario comprises 33 claims (365 units totalling 5840 hectares) and is located approximately 230 km east-northeast of the city of Thunder Bay and 18 km southeast of the town of Marathon in Northern Ontario, Canada. Work completed by MetalCORP since early 2004 includes a MegaTEM airborne time-domain EM and magnetometer survey (2004), a detailed helicopter-borne ATEM III time-domain EM and magnetometer survey, detailed and reconnaissance prospecting (918 samples), linecutting, surface and down-hole pulse-EM surveys, geological mapping, and 3 phases of diamond drilling, totaling 31 diamond holes (8300 m). This work resulted in the discovery of 4 previously unknown mineralized zones that represent 3 separate and distinct mineralization styles. The mineralized zones include: the J4 and J5 Pt-Pd reefs within the Big Lake Ultramafic Complex; the A2 Ni-Cu Zone within the Gus Creek Mafic Intrusion; and the BL14 Cu-Zn-Ag-Au-Co Zone within strongly altered ultramafic metavolcanic flows and associated metasedimentary rocks. The A2 and BL14 zones are not exposed on surface and are buried beneath 10 to 75 m of glacial drift. The property is also host to the historic Playter Mo-Cu-Pb-Ag Prospect which has yet to be fully evaluated by MetalCORP. The Big Lake Property is located near the southern margins of the eastern portion of the Archeanage Schreiber-Hemlo greenstone belt of the eastern Wawa Subprovince of the Canadian Shield. The greenstone belt is split into distinct eastern and western segments by the 1108 Ma Mesoproterozoic Coldwell Alkalic Complex. The eastern part of the belt is subdivided into the Hemlo-Black River assemblage (2.77 Ma) to the north and the Heron Bay (2.70 Ma) assemblage to the south, both of which are primarily affected by amphibolite-facies regional metamorphism. The western portions of both assemblages are lower in grade and exhibit upper greenschist facies regional metamorphism. The Big Lake Property occurs within the Heron Bay Assemblage which is intruded by the granitic to granodioritic Heron Bay Batholith, the recently recognized mafic Gus Creek Intrusion, the Bell’s Lake Ultramafic Intrusion, and the >30 km long Big Lake Ultramafic Complex. The BL14 Cu-Zn-Ag Zone is located stratigraphically below the eastern end of the sill-like Big Lake Ultramafic Complex, approximately 800 metres south of the A2 zone (see below). The south-facing, high temperature, mafic dominated Cu-rich VMS zone is overturned, dips at ~25o to the north, and consists of: 1. A stringer zone of intensely biotitized and strongly chloritized and talcose breccia containing up to 30% bands, veins, stringers and pods of chalcopyrite and pyrite with up to 5% disseminated to streaked sphalerite and minor galena; 2. A semi-massive to near-massive zone of chalcopyrite, pyrrhotite, sphalerite, and minor galena underlain by a strongly to intensely biotitic, chloritic, and talcose footwall alteration zone that locally contains anthophyllite and sillimanite. Trace element lithogeochemistry shows that the footwall metavolcanic rocks contain unusually high amounts of Ni, Cr, TiO2, and Pd that suggests that the host-rocks were originally komatiitic basalts or possibly ferro-picrites. The mineralized zone is capped by relatively unaltered clastic and chemical metasedimentary rocks. The thickest BL14 Zone intersection (DDH BL06-24) to date (April 6, 2006) contained 5.57% Cu, 103.9 grams/tonne (gpt) Ag, 6.74 gpt Au, 1.66% Zn, 39 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 689 ppm Co, and 0.15% Pb over 5.38 m (17.65 ft). This interval included 7.45% Cu, 137.8 gpt Ag, 9.18 gpt Au, 2.24% Zn, 891 ppm Co, and 0.21% Pb over 3.95 m (12.96 ft). Stringer zone mineralization intersected to date contained up to 2.56% Cu, 1.00% Zn, 46.0 gpt Ag, 1.60 gpt Au, and 0.10% Pb over 0.93 m (3.05 ft). The J4 and J5 Pt-Pd Reefs consist of narrow, apparently stratabound intervals hosted within thick peridotite units contained within the upper and central intrusive cycles of the eastern portion of the north-facing, well-differentiated, unlayered, sill-like Big Lake Ultramafic Complex. The ultramafic complex is presently undated; however, it is thought to be younger than most of the supracrustal rocks observed within this portion of the Schreiber-Hemlo Greenstone Belts. Diamond drilling shows that the central portions of the complex dip to the north at ~45o, whereas the eastern portions of the complex dip to the north at ~25o. Geological mapping suggests that the western portions of the complex exhibit a steeper northerly dip. The two host intrusive cycles and are very similar in appearance, progression of rock units, and apparent thickness. The observed mineralization consists of trace amounts of very finely disseminated pyrrhotite and chalcopyrite within serpentinized to locally talcose, fine-grained, pyroxene-oikocrystic peridotite. The J4 Reef varies between 0.58 and 2.11 m in thickness, occurs within the basal peridotite unit of the uppermost (J4) intrusive cycle of the Big Lake Complex and is usually directly adjacent to the contact with an overlying olivine-bearing pyroxenite unit. The J5 Reef is identical in appearance to the J4 Reef, varies between 0.75 and 3.00 m in thickness, and occurs within the basal peridotite of the central (J5) intrusive cycle of the complex near, but not adjacent to, the upper contact of the host unit with an overlying olivine-bearing pyroxenite. The J4 Reef has been traced for 2.20 km (1.37 mi) and contains up to 0.70 gpt Pt and 0.79 gpt Pd (1.49 gpt 2PGE)/1.67 m. The J5 Reef has been traced for a similar distance and contains up to 0.81 gpt Pt, 0.85 gpt Pd (1.86 gpt 2PGE)/0.75 m. It is interesting to note that both reefs were intersected while drill testing the BL14 Zone described above. The A2 Ni-Cu Zone occurs near the base of the discordant Gus Creek Mafic Intrusion (2669.3 ± 1.8 Ma., Jack Satterly Geochronology Laboratory, University of Toronto, 2005) and consists of disseminated, blebby, and stringered, locally semi-massive pyrrhotite, chalcopyrite, and pentlandite hosted within the 2 to 20 m thick, conduit-like, A2 host intrusion breccia sequence. The A2 intrusion breccia is a complex interval of variably mineralized (1 to 30% sulphides), varitextured, inclusion-rich, gabbroic to melagabbroic intrusive rocks overlain by unmineralized, medium- to coarse-grained gabbro and quartz leucogabbro and underlain by occasionally mineralized, pyroxene-phyric melagabbro and feldspathic pyroxenite. Inclusion/fragment types include a variety of gabbros, ultramafic intrusive rocks, and clastic and chemical metasediments. The strongest mineralization occurs near the base of the intrusion breccia, comprises the A2 NiCu Zone, and includes 1.66% Ni and 0.20% Cu/0.30 m, 1.00% Cu and 0.80% Ni/0.40 m, 1.40% Cu and 0.27% Ni/0.77 m, and 0.98% Cu and 0.29% Ni/1.40 m. The geometry of the A2 mineralized zone remains uncertain and may be more complex that initially thought, but within the area drilled appears to strike ~140o and dip southwest between 40 and 60o. It is presently thought that much of the observed sulphide mineralization within the A2 Zone consists of fragments from a high R-factor, massive, Ni-Cu sulphide body located somewhere at depth. 40 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 MINING AND EXPLORATION ACTIVITY IN NORTHWESTERN ONTARIO MAGEE, ANGELIQUE, Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 CANADA Northwestern Ontario continued to see a significant increase in mining and mineral exploration in 2005. Six mines produced a total of 1.5 million ounces of gold in 2005, approximately 70% of Ontario’s total. Gold producers included: Campbell Mine (Placer Dome Inc.); David Bell Mine (Teck Cominco Limited and Barrick Gold Corporation); Golden Giant Mine (Newmont Canada Limited); Musselwhite Mine (Placer Dome Inc./Kinross Gold Corporation); Red Lake Mine (Goldcorp Inc.); and Williams Mine (Teck Cominco Limited and Barrick Gold Corporation). North American Palladium Ltd. produced 177 167 ounces of palladium and 18 833 ounces of platinum at its Lac des Iles Mine and development of the underground operation below its open pit mine continues. The Golden Giant Mine closed its mining operation in December of 2005 and will be decommissioning the mine site in the first half of 2006. There are approximately 400 active exploration projects in the northwest, the vast majority of which are focused on gold. Areas receiving the most interest from exploration companies were the Red Lake greenstone belt, Shoal Lake area, Dogpaw Lake area, Shebandowan greenstone belt, Fort Hope greenstone belt, Onaman-Tashota belt and the Pickle Lake greenstone belt. Elevated mineral commodity prices are contributing to levels of exploration activity in northwestern Ontario not seen since the mid-1980’s. Exploration in northwestern Ontario continues for the following mineral deposit types: diamonds, uranium, copper-nickel-platinum group elements, volcanic hosted massive sulphides, rare earth elements in pegmatites, iron-oxide-copper-gold, coppermolybdenum-gold porphyry, and lode gold. 41 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 GEOLOGY AND GEOCHEMISTRY OF THE CHIMNEY LAKE VOLCANICLASTIC BRECCIA NEAR ARMSTRONG, ONTARIO MAGEE, M. ANGELIQUE† 1, 2, HOLLINGS, PETE 2, FRALICK, PHILIP W. 2 Ontario Geological Survey, Resident Geologist Program, Suite B002, 435 James St. S., Thunder Bay, Ontario, Canada, P7E 6S7, 2 Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, Canada, P7B 5E1 1 The Chimney Lake volcaniclastic breccia (CLVB) is part of a group of Mesoproterozoic rocks that unconformably overlie the Archean basement of the central Wabigoon Subprovince near the northwestern end of Lake Nipigon. Mapping in 2003 resulted in the discovery of a number of previously unmapped, Mesoproterozoic units, including the Badwater layered gabbro intrusion (Middleton 2005), the Pillar Lake volcanic assemblage (PLV), and an undeformed volcaniclastic breccia located on the north shore of Chimney Lake (MacDonald 2004). The Badwater layered gabbro (1599 Ma; Heaman and Easton 2006) intrusion has anorthositic to gabbroic layers. The PLV are a series of flat-lying, greenschist-facies, undeformed, massive and pillowed basalt flows. A robust age date of the PLV has not yet been obtained, but dates obtained by geochronological methods indicate that the PLV were erupted between 1514 Ma and 1120 Ma (Heaman and Easton 2006). The CLVB was originally mapped as a conglomerate within the Pass Lake Formation of the Sibley Group and has not been dated by geochronological methods (MacDonald 2004). Field relationships between the CLVB and surrounding lithologic units, such as the PLV, are not discernible due to poor outcrop control. CLVB contains fragments of gabbro, basalt, amygdaloidal basalt, and fragments tentatively described as granitoid and sedimentary rocks. Fragments range in size from 0.2 cm to 30 cm in diameter. Gabbroic fragments are angular with sharp edges, displaying no evidence of assimilation. Basalt fragments sometimes exhibit assimilation features and have angular to subrounded and ameboidal shapes. Amygdules within basalt fragments are infilled with chlorite and locally clay minerals. Alteration envelopes of chlorite, hematite and clay minerals surround the amygdules. The breccia matrix consists of very fine- to coarse-grained, angular to sub-rounded, hematite- and actinolite-altered volcanic and igneous fragments. Preliminary geochemical results indicate that the CLVB contains fragments of basalt that are similar in composition to the alkaline PLV basalt. The gabbroic clasts vary in composition but it appears that they are similar to the Badwater layered gabbroic intrusion. The Badwater layered gabbroic intrusion and the PLV are geochemically dissimilar. The similarity between the fragment composition in the CLVB and nearby lithologic units suggests a local source. The CLVB could be a reworked autoclastic breccia related to Pillar Lake volcanism, or alternately it may represent a diatreme breccia dike that was emplaced after Pillar Lake volcanism. Ongoing detailed mapping of the volcaniclastic breccia in conjunction with additional geochemical analyses will determine the source and cause of the volcanism that resulted in this breccia unit, as well as other local volcanic rocks. 42 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 REFERENCES Macdonald, C.A. 2004. Precambrian geology of the south Armstrong-Gull Bay area, Nipigon Embayment, northwestern Ontario; Ontario Geological Survey, Open File Report 6136, 42p. Middleton, R.S. 2005. Diamond Drilling on Red Granite Property, Pillar Lake Sheet, Armstrong, ON, 52I03NW, Resident Geologist Program Thunder Bay North Assessment Files, 55p. Heaman, L.M. and Easton, R.M. 2006. Preliminary U/Pb geochronology results: Lake Nipigon Region Geoscience Initiative; Ontario Geological Survey, Miscellaneous Release of Data 191, 86p. 43 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 THE PRECAMBRIAN RESEARCH CENTER—A NEW INITIATIVE TO PROMOTE PRECAMBRIAN FIELD STUDIES AT THE UNIVERSITY OF MINNESOTA DULUTH MILLER, JAMES D., JR.†, Minnesota Geological Survey (mille066@umn.edu) and PETERSON, D.M., Natural Resources Research Institute (dpeters1@nrri.umn.edu) As the minerals industry enters an anticipated period of protracted expansion, a major impediment to this growth is a scarcity of new geoscientists adequately trained in basic field methods. This is especially true in field studies of Precambrian terrains, which host much of the world's ore deposits. The Precambrian Research Center (PRC) is being created at the University of Minnesota Duluth (UMD) with the primary goal of satisfying this new demand for field geologists by providing training and support to upper-level undergraduate students, graduate students, and professional geologists in modern methods of geologic mapping in glaciated Precambrian terrains. A secondary goal of this center is to attract exceptional students, who have an interest in conducting field-oriented thesis research, to the University of Minnesota Duluth's graduate program. Ultimately, it is our hope that the PRC will sustain and enhance the reputation the geology department at UMD has developed over the past 50 years for producing well-trained field geologists. The PRC will have five programmatic components: 1. Summer geology field camp in northeastern Minnesota Beginning in 2007, the PRC will offer a summer field camp focused on the unique aspects of field mapping in Precambrian terrains of the southern Canadian Shield. The field camp will be a course accredited through the College of Science and Engineering at UMD. It will be open to undergraduate and graduate students from throughout the U.S., Canada, and abroad. It will be staffed by 4 to 6 professional field geologists contracted with the PRC. Field camp highlights • Introduction to basic field methods in glaciated Precambrian shield terrains. • Overview of the Precambrian geology of the southern Canadian Shield. • Week-long capstone field mapping projects with small field excursions supervised by professional field geologists; Boundary Waters Canoe Area Wilderness option. • GIS compilation of field data and digital geologic map-making. • Integrating structure, drill core, and geophysics into 3-D geologic interpretations. 2. Graduate assistantships and grants The PRC will offer several yearly research assistantships for students accepted into the UMD graduate program who wish to pursue field-based research projects focused on the Precambrian geology of the Lake Superior region. Small research grants will also be available to undergraduate and graduate students to assist in various aspects of fieldbased studies of Precambrian geology. Undergraduate or graduate students may apply for these grants, but preference will be given to students from UMD and those who have attended the PRC field camp. 3. Professional workshops/field experiences The PRC's goal of providing advanced training to professional geologists, as well as students, will be achieved by sponsoring a regular series of workshops and field 44 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 experiences (at least two per year) on various topics related to field mapping of Precambrian shield geology. In addition, customized geologic mapping experiences for groups of industry geologists and/or the geologic staff of individual companies can be arranged. The PRC will work with UMD to ensure that these programs meet the continuing professional development requirements of geologic licensing boards. Industry members of the PRC will receive registration waivers to these advanced training sessions depending on their level of support. 4. Advanced geology courses at UMD Three new field-based courses will be offered for upper level undergraduates and graduate students within the Department of Geological Sciences. These courses include Advanced Field Methods and Geological Maps; Geology in Three-Dimensions; and Geologic Problem Solving Using Digital Methods. 5. Outreach, field trip offerings, and career planning for students The PRC will offer outreach education to K-12 students and the general public on the geology and mineral resources of the Lake Superior region. It will also offer to lead field trips on the Precambrian geology of the area for UMD students and students visiting the area from other colleges and universities. Finally, the PRC will serve as a clearinghouse for students to find job opportunities in the public or private sector that require field mapping skills. PRC activities and finances will be overseen by the heads/directors of three principal organizing institutions within the University of Minnesota: the Natural Resources Research Institute (NRRI) at UMD, the Department of Geological Sciences at UMD, and the Minnesota Geological Survey (MGS) at the University of Minnesota Twin Cities campus. The NRRI will oversee the business activities of the PRC; the geology department will oversee the PRC's educational programs; and MGS will provide guidance on geologic mapping projects. The PRC will employ 3 to 4 permanent staff and will contract with many field-experienced academic and professional geologists from throughout the Lake Superior region for its various programs and activities. A Board of Advisors consisting of preeminent geologists from industry and academia will be established to provide advice and oversight of PRC activities and programs. Funding for the PRC will come from many sources. Base funding will be sought from the State of Minnesota to support administrative expenses, from the University of Minnesota in the form of tuition deferments, and from students and professional geologists by tuition and fees paid for summer field camp, academic courses, and workshops/field experiences. Funding for particular research projects will be sought from the U.S. Geological Survey through their EDMAP program and from the minerals industry for sponsorship of industry-generated thesis projects. The minerals industry, which stands to be a major beneficiary of the PRC by its professional workshops and by having access to well-trained students, will also be requested to serve as a key benefactor of the PRC. 45 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 GEOLOGY OF THE DULUTH COMPLEX IN THE FOUR BABBITT 7.5' QUADRANGLES, NORTHEAST MINNESOTA MILLER, JAMES D., JR., Minnesota Geological Survey (mille066@umn.edu) and SEVERSON, MARK J., Natural Resources Research Institute (mseverso@nrri.umn.edu) The Minnesota Geological Survey published 1:24,000-scale digital geologic maps of the Babbitt, Babbitt Northeast, Babbitt Southwest, and Babbitt Southeast 7.5' quadrangles in 2005 (Severson and Miller, 2005; Miller and others, 2005; Miller and Severson, 2005; and Miller, 2005, respectively). The area was originally reconnaissance mapped by Bonnichsen (1970a-d). The Precambrian rocks in the four Babbitt quadrangles are best known for hosting the easternmost Mesabi Iron Range taconite mines and some of the Cu-Ni-PGE deposits that occur along the northwestern margin of the Duluth Complex. The maps, which include three cross sections, will be on display as a poster presentation. Trending northeasterly through the map area, the Duluth Complex is in intrusive contact with Paleoproterozoic rocks of the Animikie Group and Archean granitic rocks of the Giants Range batholith. The Animikie Group units include the basal Pokegama Quartzite, the overlying Biwabik Iron Formation, and the Virginia Formation. The Peter Mitchell (Northshore Mining) and Dunka Pit (closed) taconite mines occur in the map area. The northwestern margin of the Duluth Complex is exposed in the Babbitt quadrangles. The Duluth Complex is the largest exposed intrusive component of the Mesoproterozoic (1.1 Ga) Midcontinent Rift. It was emplaced as multiple intrusions into the lower section of comagmatic volcanic rocks of the North Shore Volcanic Group, which is evident from field relationships in the map area. The oldest Mesoproterozoic rock units in the Babbitt quadrangles are several types of mafic hornfels inclusions, which represent thermally metamorphosed remnants of the North Shore Volcanic Group. The most common type is basaltic hornfels, which by the common occurrence of meta-amygdaloidal textures, are clearly thermally metamorphosed mafic volcanic lava flows. Magnetic, nonmagnetic, and plagioclase porphyritic varieties of basalt hornfels are recognized, with the magnetic types usually occupying a stratigraphically lower position. An interesting hornfels type is a well crossbedded mafic hornfels that is interpreted to be a strongly metamorphosed volcanogenic interflow sandstone unit (Patelke, 1996). An enigmatic hornfels type is a medium- to fine-grained, equigranular oxide olivine gabbro that displays a domainal distribution of granular mafic phases possibly representing granoblastic recrystallization of an originally ophitic texture. The thickness, homogeneity, and average medium grain size imply that the unit may be a metamorphosed subvolcanic sill or a large lava flow. The oldest intrusive rocks of the Duluth Complex in the map area are gabbroic to anorthositic rocks of the early anorthositic series. The anorthositic series consists of a structurally complex suite of plagioclase-rich gabbroic rocks that cover large expanses of the upper reaches of the Duluth Complex (Miller and others, 2002). Over most of the map area, anorthositic rock types, with plagioclase ranging from 75 to 95 percent, typically occur as meter- to decameter-sized inclusions in troctolitic rocks. However, in 46 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 the Babbitt Southwest quadrangle, several varieties of anorthositic-series rocks occur over large areas. In addition to standard plagioclase-rich anorthositic-series lithologies, typically with poikilitic to intergranular olivine, these large areas of anorthositic-series rocks also contain a distinct olivine oxide gabbro lithology that only locally is leucocratic. This rock type was referred to as the Powerline gabbro by Bonnichsen (1972), who suggested it was an upper differentiate of the Partridge River intrusion. Paces and Miller (1993) acquired a U-Pb age of 1,098.6 ± 0.5 Ma for this unit, also considering it part of the Partridge River intrusion. However, this mapping has clearly shown this gabbro to be in gradational contact with other anorthositic-series rocks and to be crosscut by troctolitic rocks of the Partridge River intrusion. The main Duluth Complex units in the Babbitt quadrangles are various types of troctolitic (Ol + Pl) cumulates of the layered series. The layered series is the youngest component of the Duluth Complex and is composed of a suite of discrete layered mafic intrusions that show variable degrees of internal differentiation (Miller and others, 2002). The Babbitt quadrangles include parts of four major layered series intrusions: the Partridge River intrusion, the South Kawishiwi intrusion, the Greenwood Lake intrusion, and the Bald Eagle intrusion. Only the Partridge River and the South Kawishiwi intrusions are sufficiently exposed to subdivide their igneous stratigraphies into map units; both intrusions are composed mostly of olivine-plagioclase cumulates, and different map units are distinguished on the basis of subtle differences in the amount of interstitial augite and Fe-Ti oxide, the occurrence of melatroctolitic intervals, and on variable concentrations of anorthositic-series inclusions. The igneous layering in the lower 500 meters of both intrusions is better known because of the high density of exploration drill core. Several of the major Cu-Ni-PGE sulfide deposits that occur along the base of the Duluth Complex in this area were first discovered in the early 1960s. From southwest to northeast, these include the Northmet (formerly Dunka Road), the Mesaba (formerly Babbitt), the Serpentine, the Dunka Pit, and the Birch Lake deposits. Polymet is in the final stages of permitting the Northmet deposit, and if successful, is scheduled to begin development in 2008. Assessment activity on the other deposits has increased as well, particularly on the Birch Lake deposit. The geologic picture portrayed in these maps and cross sections provide new insights and important constraints on models for the emplacement, crystallization, and mineralization histories of the Partridge River and South Kawishiwi intrusions. The potential development of Cu-Ni-PGE deposits will provide further insights into the detailed geology and mineralization of this economically important part of the Duluth Complex. REFERENCES Bonnichsen, B., 1970a, Reconnaissance geologic map of Babbitt quadrangle: Minnesota Geological Survey Open-File Map, scale 1:24,000. ———1970b, Reconnaissance geologic map of Babbitt NE quadrangle: Minnesota Geological Survey Open-File Map, scale 1:24,000. ———1970c, Reconnaissance geologic map of Babbitt SE quadrangle: Minnesota Geological Survey Open-File Map, scale 1:24,000. ———1970d, Reconnaissance geologic map of Babbitt SW quadrangle: Minnesota Geological Survey Open-File Map, scale 1:24,000. ———1972, Southern part of the Duluth Complex, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 361-388. 47 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Miller, J.D., Jr., 2005, Bedrock geology of the Babbitt Southeast quadrangle, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map M-162, scale 1:24,000. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.E., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, 207 p. Miller, J.D., Jr., and Severson, M.J., 2005, Bedrock geology of the Babbitt Southwest quadrangle, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-161, scale 1:24,000. Miller, J.D., Jr., Severson, M.J., and Foose, M.P., 2005, Bedrock geology of the Babbitt Northeast quadrangle, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map M-160, scale 1:24,000. Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic and tectono-magmatic processes associated with the 1.1 Ga Midcontinent rift system: Journal of Geophysical Research, v. 98, no. B8, p. 13,997-14,013. Patelke, R.L., 1996, The Colvin Creek body, a metavolcanic and metasedimentary mafic inclusion in the Keweenawan Duluth Complex, northeastern Minnesota: Duluth, Minn., University of Minnesota Duluth, M.S. thesis, 232 p. Severson, M.J., and Miller, J.D., Jr., 2005, Bedrock geology of the Babbitt quadrangle, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-159, scale 1:24,000. 48 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 GEOCHEMICAL CONSTRAINTS ON THE DEPOSITION OF MESOARCHEAN BANDED IRON FORMATION AT THE MUSSELWHITE MINE, NORTH CARIBOU GREENSTONE BELT, SUPERIOR PROVINCE MORAN, PATRICK†, FRALICK, PHILIP and HOLLINGS, PETE, Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada, P7B 5E1, pcmoran@lakeheadu.ca Iron formations (IFs), chemical sedimentary rocks containing greater than 15% iron (James 1954), can be broadly divided into Superior- and Algoma-type. Superior-type IF are laterally extensive, associated with sedimentary rocks deposited in shallow water settings, and are generally Paleoproterozoic in age. They formed from the upwelling of oxygen-deficient, Fe+2bearing ocean water onto shallow shelves where oxygen was present (Cloud 1973). In contrast, Algoma-type IFs are laterally and vertically limited, associated with volcanic and sedimentary rocks deposited in deep-water, and mostly Archean in age (Gross 1996). They are commonly considered to have formed by precipitation from venting hydrothermal fluids, although shallowwater deposits of Algoma-type are present and probably represent microbially induced precipitation (Fralick, this conference). This study utilized banded chert-magnetite iron formation present in surface exposures at the Musselwhite Mine. The site sampled is an unmineralized area of the gold-bearing horizon. Sixteen samples were collected, from which monomineralic layers were separated and analyzed using XRF and ICP-MS. This is an amphibolite facies Algoma-type IF that overlies a thick mafic metavolcanic succession with apparent conformity. Millimeter to approximately one centimeter thick layers of chert and magnetite alternate in the lower half of the IF. These layers contain very small amounts of siliciclastic material. Higher in the IF there is a gradational increase in the amount of siliciclastic debris intercalated with the chemical sediment layers, until the succession is dominated by siliciclastics. All samples came from the lower, relatively siliciclastic free zone. Concentrations of most major and trace elements are relatively low in the magnetite and chert samples. Exceptions to this are Si in the cherts and Fe, Mn and P in the magnetite layers. The Si, Fe and Mn are self-explanatory. The slightly elevated phosphorous values indicate possible scavenging of PO4-2 from seawater by iron hydroxides or oxyhydroxides during precipitation or microbial activity in the sediment. Trace element abundances normalized to chondrite (Fig.1a,b) indicate the fluid that precipitated the IF was depleted in Ni, Cr, Zn, Co, Cu, Ti and K; and enriched in Sc, Y, W and Cs, relative to chondrites. Figure 1C shows the chert layers have less admixed siliciclastic material and also less Ni, Cr, Zn, Co and Cu, possibly denoting the iron compounds precipitated from higher temperature fluids. Rare earth element plots portray similar patterns to REE plots of recent metalliferous sediment and vent water from the TAG field (Atlantic) and the Atlantis II Deep (Red Sea) (Peter 2003). The geochemical data indicate both the Si- and Fe-rich layers precipitated from vented fluids in an environment where there was sufficient oxygen to form iron hydroxides or oxyhydroxides. References Peter, J.M., 2003, Ancient iron formations: their genesis and use in the exploration for stratiform base metal sulfide deposits, with examples from the Bathurst Mining Camp, in Lentz, D.R., Ed., Geochemistry of Sediments and Sedimentary Rocks: Evolutionary Considerations to Mineral Deposit-Forming Environments: Geological Association of Canada, GeoText 4, p. 145-176 James, H.L., 1954, Sedimentary Facies of iron-formation: Economic Geology, v. 49, p. 29-44 Cloud, P.E., 1973, Paleoecological significance of banded iron-formation: Economic Geology, v. 68, p. 1135-1143 Gross, G.A., 1996, Stratiform Iron: Lake Superior-type iron-formation, Algoma-type iron-formation, Ironstone, in Eckstrand, O.E. Sinclair, W. D., And Thorpe, R.I., Eds., Geology of Canadian Mineral Deposit Types, The Geology of North America, n. 8: Geological Survey of Canada, p. 41-80 49 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Figure 1. (a) Logarithmic plot of trace element values from magnetite layers taken from the BIF at Musselwhite mine. Trace element values normalized to CI Carbonaceous chondrite values of McDonough and Sun (1994). (b) Logarithmic plot of trace element values from chert layers taken from the BIF at Musselwhite mine. Trace element values normalized to CI Carbonaceous chondrite values of McDonough and Sun (1994). (c) Chert values normalized to magnetite values from the present study. (d) REE plot normalized against CI chondrite. 50 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 STATISTICAL ANALYSIS OF INDOOR RADON DATA AND RELATIONSHIPS TO GEOLOGY IN WISCONSIN MUDREY, M.G. JR., 106 Ravine Road, Mount Horeb, WI 53572 (mgmudrey@mhtc.net) In the 1986 with the discovery of exceeding high radon values in Pennsylvania, the US Environmental Protection Agency initiated a nation-wide study to determine the population and geographical risk associated with radon. Because Wisconsin has a strong state radiation protection program, and known occurrences of uranium and other radionuclides, it was one of 10 initial states to be analyzed. Since then, 84,262 residential indoor radon in air covering all areas has been collected. These data are not randomly distributed and reflect collection by interested by home owners. Nearly 50% of the analyses have been accurately located and digitized permitting geologic analysis; the remainder has only zipcode locational information. This study compares the EPA radon survey with the more extensive Wisconsin data base and evaluates the data with respect to various geologic attributes in order to define those areas and geologic units in Wisconsin where radon may pose a higher risk. The original EPA survey of 1194 homes is considered the only statistically useful survey for evaluating average values of indoor radon in Wisconsin: the mean of 3.4, with a Q1 of 1.2 and a Q3 of 4.1. The data are highly skewed. The highest value found in Wisconsin is 938 near Hudson. Elevated indoor radon is found in all areas of Wisconsin as is predicted by geological analysis. Soil derived from granite and carbonate rock that is the principal geological factor leading to elevated indoor radon in Wisconsin. Because radon can migrate only a few meters over the 3.8 day half-life, soil chemistry and soil texture principally influence elevated indoor radon. Because elevated radon is found in all areas of Wisconsin, and because do-ityourself radon testing is inexpensive, it is highly recommended that all houses in Wisconsin should be tested for radon. This study was funded by the U.S. Environmental Protection Agency to the Radiation Protection Section, Wisconsin Division of Public Health, Wisconsin Department of Health and Family Services, and to the Wisconsin Geological and 51 Natural History Survey, University of Wisconsin Extension, while Mudrey was with the WGNHS. �Proceedings of the 52nd ILSG Annual Meeting – Part 1 3D VISUALIZATIONS OF MAFIC INTRUSIONS IN THE DULUTH COMPLEX, NORTHEASTERN MINNESOTA PETERSON, DEAN M., Natural Resources Research Institute, Duluth, MN, dpeters1@nrri.umn.edu) One of the main hallmarks of science is that it allows one to imagine reality. In the geosciences, one of the main realities that geologists try imagine is the geometry and structural juxtaposition of geological units and/or mineralized zones in the subsurface. This is especially true for geologists that work in the mineral exploration and mining industries, earthquake monitoring and hazard assessment agencies, and at contaminated groundwater sites. The advent of 3D geological computer programs has brought about a revolution in the understanding of the Earth in three dimensions. This digital poster presentation using the computer program gOcad (by Earth Decision Science) will display geological features of numerous troctolitic intrusions within the Duluth Complex, and will highlight how such visualizations advance our understanding of geological processes that ultimately led to the formation of billions of tonnes of Cu-Ni-PGE mineralized rocks. An image of the basal surface of a portion of the Partridge River Intrusion is presented in Figure 1. The bowl-shaped depression hosts the Babbitt deposit, currently held by Teck-Cominco. Figure 1. 3D view of the basal surface of a portion of the Partridge River Intrusion. View looking due west and down 10º. Grid lines are UTM coordinates, in meters. 52 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 EVIDENCE FOR WIDESPREAD DISTRIBUTION OF IRON DEPENDENT METABOLISMS IN PRECAMBRIAN OCEANS PLANAVSKY, NOAH †, KNUDSEN, ANDREW, Lawrence University Appleton, WI SHAPIRO, RUSSELL, Gustavus Adolphus College Saint Peter, MN noah.j.planavsky@lawrence.edu Starting with the first detailed descriptions of Precambrian microfossils (Barghorn and Tyler, 1965), the dominant view has been that cyanobacteria were the primary producers in Paleoproterozoic, and more generally in all early Precambrian, ecosystems (Awramik, 1992). An increased understanding about the chemical evolution of the ocean atmosphere system lead some to question the theoretical and observational foundation of this dogma (Blank, 2004) Currently, there are little constraints on many of the basic attributes of most pre1.8ga ecosystems. For instance, the diversity of bacterial metabolism in early environments in still debated and distribution of bacterial metabolisms in early earth’s history is poorly constrained. Although the atmosphere became oxygenated approximately 2.5Ga widespread marine anoxia and sulfate limitation resulted in pre1.8Ga oceans with significant quantities of dissolved iron. There is strong theoretical support from ecological modeling that these iron rich oceans supported an abundant iron dependent microbial community (Konhauser et al., 2002). In our analysis of the Paleoproterozoic Animikie basin we found empirical evidence for the widespread distribution of a microbial community with iron dependent metabolisms that thrived at a characterizeable oxygenic chemocline. There are two distinct stromatolites Animikie Basin; large, hemispheroidal, calcitic, peritidal stromatolites, and iron rich, subtidal, digitate stromatolites. Based on the morphology of the stromatolites, the inferred primary silica composition (Barghoorn and Tyler, 1965), and the microstructure the iron rich stromatolites were proposed as having formed as either sinter deposits or in the hot spring apron (Walter, 1972). The stromatolites proposed to be sinter deposits are defined by the presence of thin laminations (thinner in width than the majority of Gunflint microfossils) with distinct boundaries between the laminations (Hoffman, 1969; Walter, 1972). The spring origin of the stromatolites has been widely accepted, (Sommers and Awramik, 2002; Siminson, 1987) and even popularized to general audiences (Knoll, 2003). Field and microscopy work revealed that the ‘abiogenic’ stromatolites formed under a strong microbial influence. Field observation demonstrated that the stromatolites represent one facies in a well preserved sedimentary package. The iron rich stromatolites also display mesostructural features that cannot be readily explained by abiotic processes. For instance, the columnar sections of the stromatolites display crestal thickening, often with tuffs similar those seen in modern microbial mats and laminations that are far above angle of repose. The presence of alternating residual organic rich and residual organic poor lamination couplets and total organic carbon values of up to 2.5% also strongly suggests microbial mediation. In the least metamorphically altered stromatolites the thinner organic rich laminations are composed predominately of hematite in siliceous cement. The organic poor laminations and the surrounding siliceous material contain very low concentrations of iron. The iron distribution cannot be explained by secondary 53 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 oxidation and therefore necessities a means for iron disproportionation within the organic rich laminae. The rare earth element (REE) pattern and sedimentological characterization suggest that the iron rich stromatolites formed at an oxygenic chemocline. Ce is the only REE that can be oxidized in surface aqueous solutions, which results redox reactions controlling the element’s cycling and abundance. The stromatolites display a negative cerium anomaly, which indicates stromatolite formation in aerobic conditions or at an oxygenic mixing zone. The stromatolites occur at a transition out a zone with regular authogenic iron deposition, which implies formation at an oxygenic chemocline. In modern environments, iron oxidizing β proteobacterium dominate at oxygen mixing zones or in microaerophilic conditions where there are similar or even significantly lower ferric iron concentrations than the predicted (Holland, 1984,) values for the Paleoproterozoic oceans (Emerson and Revsesbach, 1994). Iron oxidizing β proteobacterium induce iron precipitation on average 60 times faster than abiotic reactions providing a means for the observed iron disproportionatation in the organic rich stromatolite laminae. The stromatolites also display a positive Gd anomaly, which serves as independent biogenicity proxy. Abiogenic iron precipitates, because of the lanthanide tetrade effect, display a negative Gd anomaly (Bau, 1999). Microbial communities slightly preferential or proportionately adsorb Gd compared to Tb, and Dy (Anderson and Pedersen, 2003). Modern ocean water has positive Gd anomalies that are mirrored by negative Gd anomalies in the largely abiogenic ferromanganese pavements. The stromatolites have similar or even more pronounced Gd anomalies than modern oceans. The stromatolites positive Gd anomaly necessitates biogenic precipitation of iron oxides within the stromatolites. References: Anderson, C. R. & Pedersen, K. 2003. In situ growth of Gallionella biofilms and partitioning of lanthanides and actinides between biological material and ferric oxyhydroxides. Geobiology 1 (2), 169-178. Barghoorn, E.S. and Tyler S.A., 1965. Microorganisms from the Gunflint Chert. Science 147 (3658), 563577. Bau M., and Dulski P. (1999) Comparing yttrium and rare earths inhydrothermal fluids from the MidAtlantic Ridge: Implications for Yand REE behaviour during near-vent mixing and for the Y/Ho ratio of Proterozoic seawater. Chem. Geol. 155, 77–79. Emerson, D., and N. P. Revsbech. 1994. Investigation of an iron-oxidizing microbial mat community located near Aarhus, Denmark: field studies. Applied Environmental Microbiology 60:4022-4031 Holland, H. D., 1984. The Chemistry of the Atmosphere and Oceans. Wiley, New York. Konhauser, K.O., Hamade, T., Morris, R.C., Ferris, F.G., Southam, G., Raiswell, R., and Canfield, D., 2002. Could bacteria have formed the Precambrian banded iron formations? Geology, 30:1079-1082. Simonson B. M. n, 1985, Sedimentological constraints on the origins of Precambrian iron-formations, GSA Bulletin, 96: 244-252. Walter, M.R. 1972. A hot spring analog for the depositional environment of Precambrian iron formations of the Lake Superior region. Economic Geology, 67: 965-972. 54 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 DETRITAL ZIRCON GEOCHRONOLOGY OF THE WESTERN HURONIAN BASIN RAINBIRD, ROBERT H.† and DAVIS, WILLIAM J. Geological Survey of Canada, 601 Booth St, Ottawa, Ontario, K1A 0E8 rrainbir@nrcan.gc.ca The Paleoproterozoic Huronian basin hosts an up to 12 km-thick succession of mainly siliciclastic sedimentary rocks deposited along the southern margin of the Superior Province (Huronian Supergroup). Paleocurrent data from crossbedding in fluvial sandstones throughout the succession suggest provenance from the west and northwest. U-Pb SHRIMP analysis of detrital zircon from six sandstone samples from the western part of the Huronian basin indicates provenance mainly from Neoarchean sources with prominent modes ca. 2.67 and 2.72 Ga. A sample from the stratigraphically lowest unit (Livingstone Creek Formation-Elliot Lake Group) contains zircon ranging in age from 2.90 - 2.65 Ga, with one ca. 2.50 Ga grain (weighted mean 207Pb/206Pb age of 2497 ± 10 Ma). This grain probably was derived from co-eval volcanic rocks erupted during rifting and initiation of the Huronian basin and provides a maximum age for deposition of the Huronian. A sample from the overlying Matinenda Formation has a unimodal zircon age population at ca. 2.67 Ga. The overlying Mississagi Formation (Hough Lake Group) has a polymodal zircon population varying in age from 3.62 - 2.45 Ga. Given the easterly paleocurrent indicators at the sampling locality, the pre-3.0 Ga grains could have derived from gneisses of the Minnesota River Valley terrane, southwest of the Huronian basin. The two youngest grains from the Mississagi Formation (weighted mean 207Pb/206Pb ages-2445± 9 Ma and 2451 ± 6 Ma), likely were eroded from volcanic rocks (or their intrusive equivalents) in the unconformably underlying Elliot Lake Group. Samples from succeeding thick fluvial quartz arenites of the Serpent (Quirke Lake Group) and Lorrain (Cobalt Group) formations show similar detrital zircon age profiles with a range of ages from 2.88-2.68 Ga, and a significant ca. 2.72 Ga population. A marine quartzarenite from the uppermost unit of the Huronian (Bar River Formation-Cobalt Group) has a generally similar population to that of the Serpent and Lorrain formations but with a broader range of ages, including 3 grains at ca. 2.53 Ga of unknown provenance, and 4 grains at ca. 3.00 Ga. Overall, detrital zircon geochronology and sedimentology of the western Huronian basin is compatible with provenance from the Wawa Subprovince of the western Superior craton with contributions from adjacent older gneissic terranes. 55 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 56 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 UNRESOLVED PROBLEMS AND THE EVOLUTION OF SUDBURY GEOLOGY ROUSELL, D. H. Department of Earth Sciences, Laurentian University, Sudbury, ON, P3E 2S4, drousell@laurenian.ca. Geological events, which shaped the area now occupied by the Sudbury Basin and surrounding footwall, span at least 1470 Ma from 2711 Ma, the minimum age of the Levack Gneiss Complex, to 1238 Ma, the age of olivine diabase dikes. The area has undergone several tectonic, magmatic, metamorphic and mineralization events which have been largely overshadowed by the Sudbury Event at 1850 Ma. Ascribing the Sudbury Event to meteorite impact is an entrenched paradigm; several unresolved problems are largely ignored. The aim of this abstract is to outline the geological evolution of the area and to identify certain outstanding problems. The events which have affected the area may be grouped as follows: doming, Sudbury Event and post Sudbury Event. Early workers recognized that the Paleoproterozoic rocks of the Sudbury Basin were superimposed on an Archean dome, with the NW boundary extending to the Huronian outliers and the other boundaries obscured by later deformation. Evidence for the dome is as follows: 1) in the South and East Ranges the footwall rocks become progressively younger away from the dome; 2) rocks of the Levack Gneiss Complex, metamorphosed to the granulite facies at depths of up to 28 km and uplifted to depths of 5 to 11 km, possibly during emplacement of the Cartier batholith, suggests that the complex cored a paleodome; 3) mafic dikes in the footwall of the North and East Ranges, located between 10 and 15 km from the outer margin of the Sudbury Igneous Complex ( SIC) , are oriented normal to the adjacent margin of the SIC which is consistent with dike emplacement during local magmatic doming; 4) the Nipissing gabbro, which has an affinity for rocks of the Huronian Supergroup, is absent between the NW edge of the SIC and the Huronian outliers, which suggests little or no deposition of Huronian sediments or their complete erosion, implying that the site of the basin was a topographic high; and 5) three felsic plutons , viz., Murray, Creighton and Skead intrude the area of the inferred paleodome. Prior to the Sudbury Event, rocks of the Southern Province and the Huronian outliers were folded about EW- to NE-trending axes and locally about NNE-to NNW-trending axes. The Sudbury Event is ascribed, without question, to meteorite impact by virtually all investigators. The bolide coincidentally struck a local dome which was presumably pregnant with sufficient Ni-Cu-PGE- Zn-Pb-Cu-Ag-Au mineralization to form one of the world’s largest mining camps. The notion that the ores are of cosmic origin has received little support. Features formed by the event include: Footwall Breccia and Sudbury Breccia (SB); shatter cones; planar deformation features; Onaping Formation; and SIC. SB in the granitic rocks of the North Range footwall consists of pseudotachylite, a rock considered to have formed by frictional melting in dry rocks during high-speed slip along large faults concentric with the outer margin of the North Range SIC. Field work has led others to question the existence of the faults. In contrast to the North Range SB, clastic SB occurs in the footwall of the South and East Ranges. It apparently formed by explosive dilation, fluidization and flowage into extension fractures. Clastic SB occurs at Lake Temagami, 80 km NE of the outer margin of the SIC and the most distant locality reported to date. Clasts in breccias surrounding impact craters tend to increase in size with distance from impact. Clasts at Lake Temagami are smaller than those in some bodies near the SIC. Curved and striated fracture surfaces known as shatter cones are supposedly unique to impact craters. At Sudbury, they are scarce in the North Range footwall but are common in the South and East Range footwall where they tend to occur in clusters. Most fractures are in the shape of curved surfaces rather than cones. In attempts to define the size of the Sudbury crater, numerous authors have subscribed to the thesis, first proposed in 1970, that the Huronian outliers 57 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 were preserved in down-faulted blocks formed by the simultaneous collapse of the central uplift and the emplacement of the SIC. Despite convincing evidence that the sediments were deposited in a rim syncline around a domal structure long before the Sudbury Event, relating the preservation of the outliers to impact still persists The origin of the Onaping Formation has been interpreted as follows: volcanic, fall-back breccia from meteorite impact, impact-induced volcanism or hyaloclasis. A discontinuous breccia at the base of the formation contains fragments as much as 79 x 23 km which reflect the lithology of the adjacent country rocks. The clasts were possibly emplaced by sliding down a submarine slope (crater wall?). Above the basal unit the formation consists of a series of breccias. Possibly melt, continuously fed from below, underwent passive to explosive fragmentation and rapid cooling on interaction with water. The “Onaping melt” may represent an impact melt, a hypabyssal intrusion or some combination. Diamonds in the Onaping Formation are referred to, by their discoverers, as “impact diamonds” without even considering alternatives such as a diatreme origin. Based largely on geochemical data, most investigators interpret the SIC to be an impact melt sheet. However, the granophyre, the upper unit of the SIC, intrudes the Onaping Formation. This implies that the SIC crystallized from a magma, possibly impact-triggered. An alternate scenario is that the granophyre represents a later intrusion and that the bulk of the SIC was emplaced before the Onaping, perhaps as a melt sheet. This means that the Onaping “melt” was injected through the SIC before fragmenting into a hyaloclastite. Structural data suggests that the SIC was emplaced in approximately the present disposition of the North Range i.e., dipping inward at 420. Evidence is as follows. Igneous layering in the norite dips less than the dip of the base of the SIC. Folding of a horizontal melt sheet would produce a foliation with a steeper dip than the basal contact and a strain in possible hinge zones such as the lobes located at both ends of the East Range. A plagioclase lineation in the north lobe is orthogonal to the base of the SIC. The lineation is attributed to crystal growth in a magma chamber. Apparently, even minor deformation will destroy the orthogonality. Thus the mineral fabric and the low overall strain in the North Lobe preclude a fold origin for the present shape of the SIC. The Sudbury Event was followed by differentiation of the units of the SIC (1850 Ma), formation of the Ni-Cu-PGE deposits, deposition of the Vermilion, Onwatin and Chelmsford Formations, hydrothermal alteration and formation of Zn-Pb-Cu deposits inside the basin. NW-directed compression and a weaker SW-directed compression (Penokean Orogeny, 1900 to 1700 Ma) folded the rocks of the basin about NW-trending, doubly-plunging fold axes and developed a prominent cleavage in them. Deformation died out to the NW as the North Range Onaping Formation and SIC have undergone only local and mild ductile deformation. In the South Range, the Onaping Formation and the SIC are steepened to the NW and displaced by a SE-dipping zone of reverse shear. On the outcrop-scale, the SIC displays anastomosing conjugate shear zones. A revised model is required which embraces all aspects of Sudbury Geology. Many features are either force-fitted into an impact model or ignored. Some, but not all, elements are better explained by endogenic models such as diatremes and plumes. Because of the time span of Sudbury geology, perhaps too many pieces of the puzzle have been lost, thus precluding an unequivocal history of Sudbury Geology. 58 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 GEOLOGY AND ALTERATION ASSOCIATED WITH VMS MINERALIZATION IN THE HAMLIN LAKE AREA, NORTHWESTERN ONTARIO SHUTE, AMY† and HOLLINGS, PETE, Department of Geology, Lakehead University, 955 Oliver Rd., Thunder Bay, Ontario, P7B 5E1, Canada; alshute@lakeheadu.ca, The Shebandowan Greenstone Belt is located within the Wawa Subprovince of the Superior Province and has been the target of numerous exploration efforts over the last century. The belt is host to many different precious and base metal mineral deposits. Past producers include the Shebandowan Ni-Cu PGE mine, the North Coldstream Cu-AuAg mine, and the Ardeen Au mine, Northern Ontario’s first gold producing mine. With a renewed interest in the belt by many exploration companies, the potential for new discoveries is growing. The Hamlin Lake area is within the Shebandowan Greenstone belt and located approximately 120 km west of Thunder Bay, Ontario. This project will be looking at the alteration and tectonic setting that is associated with the Hamlin Lake volcanogenic massive sulfide (VMS) system. The VMS system was first recognized when massive sulfides were found while surface sampling during the 2005 field season. The mineralization includes pyrite, chalcopyrite and pyrrhotite all at the surface with stringer mineralization found in some areas as well. Trenching and drilling followed during the fall and winter of 2005/06 and revealed significant additional mineralization. The copper values as high as 1.49% have been found in surface samples with 4.88g of gold, and the Ray Smith showing is part of the Hamlin Lake property with copper values as high as 6% and 6.0g of gold. A B Figure 1. (a) Rhyolite showing round amygdules; (b) Felsic fragmental showing partial alignment of the clasts (magnet for scale is approximately 12.0cm long) The Hamlin Lake area consists of four major units; felsic volcanic rocks; mafic volcanic rocks; pink brecciated rock; and banded iron formation. The smallest unit is the banded iron formation, found in one larger outcrop, but also found sporadically throughout the Hamlin Lake area in small (1-2m) lense-like showings. The mineralized pink breccia is the second least abundant unit and is the focus of a currently ongoing drilling project because of its higher grade mineralization. 59 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 The second major unit is the mafic volcanic rocks. This unit consists of a fragmental unit and can vary in groundmass from being chlorite rich to magnetite rich in the field. Geochemically the mafic volcanic rocks have SiO2 values that range from 51.1 to 64.4 wt.%, TiO2 from 0.08 to 0.61 wt.%, and Fe2O3 from 7.0 to 34.3 wt.%. The most abundant rock type and the focus of this study are the felsic volcanic rocks. In the field, the felsic volcanics contain quartz-eyes, amygdules, and fragments with a fine-grained groundmass and overall light grey colour (Fig.1a and b). These rhyolites vary in colour, texture and in geochemistry with preliminary work showing at least two distinct felsic suites when the REE’s are plotted (Fig.2b). The major elements in the felsic volcanic rocks show SiO2 values that range from 76 to 89 wt.%, TiO2 from 0.2 to 0.8 wt.%, and Fe2O3 from 0.9 to 8.1 wt.%. Although the groundmass is different between the felsic and mafic fragmental volcanic rocks, their fragments are both lenticular-shaped and chert-like in appearance. These fragments have yet to be studied to distinguish their origin, but they are likely either pumaceous clasts or fragmented chert layers. 7000 100.00 6000 10.00 4000 REE/PM Ti (ppm) 5000 3000 AS-05-012 AS-05-015 1.00 AS-05-029 AS-05-057 2000 0.10 1000 0 0.01 0 50 100 150 200 250 300 350 400 Th Nb La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Al Zr (ppm) Figure 2. (a) Preliminary Zr vs. Ti (ppm) plot showing two distinct groupings of samples. (b) PM normalized REE plot showing two distinctly different rhyolites suites. Felsic volcanic rocks associated with VMS systems have been the subject of considerable study over the past two decades. Several classifications been created in an attempt to characterize felsic volcanic rocks that are associated with VMS deposits, and that are barren of VMS deposits. Lesher et al. (1986) classified felsic volcanic rocks as being FI, FII and FIII’s with distinctions between their REE patterns, Zr/Y ratios and abundances in high field strength elements. Preliminary work has shown that there are at least two distinct felsic volcanic suites, suite I having flat REE patterns and positive Zr and Hf anomalies and suite II having more fractionated REE and lacking positive anomalies. When comparing Lesher’s classification with the felsic volcanic rocks in the Hamlin Lake area, suite I most closely resemble FII whereas suite II is similar to the FI group. References Lesher, C.M., Goodwin, A.M., Campbell, I.H., and Gorton, M.P., 1986, Trace-element geochemistry of ore associated and barren, felsic metavolcanic rocks in the Superior Province, Canada: Canadian Journal of Earth Sciences, v. 23, p. 222-237. 60 V Sc �Proceedings of the 52nd ILSG Annual Meeting – Part 1 PRELIMINARY INVESTIGATIONS OF THE PETROLOGY, GEOCHEMISTRY AND GEOCHRONOLOGY OF THE ST. IGNACE ISLAND COMPLEX, MIDCONTINENT RIFT, NORTHERN LAKE SUPERIOR, ONTARIO SMYK, MARK C.†, Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 CANADA, HOLLINGS, PETER, Department of Geology, Lakehead University, 955 Oliver Rd., Thunder Bay, Ontario, P7B 5E1, Canada and HEAMAN, LARRY M., Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3. As part of the Lake Nipigon Region Geoscience Initiative, a helicopter traverse was undertaken in 2005 in northern Lake Superior, in order to sample igneous rocks associated with the Mesoproterozoic Midcontinent Rift (MCR), including the St. Ignace Island Complex (SIC; Fig. 1). The SIC intruded the upper portions of MCR-related, Osler Group volcanic rocks (ca.1008 Ma; Davis and Sutcliffe 1985). It consists of a gabbroic to anorthositic ring dyke, which encloses quartz-feldspar porphyritic volcanic rocks (Sutcliffe and Smith 1988; Giguere 1975). Sutcliffe and Smith (1988) described the volcanic component of the SIC as intercalated plagioclase-glomeroporphyritic basaltic rocks, quartz-feldspar-phyric rhyolite flows and fragmental rocks. The pink to grey, rhyolitic rocks in the core of the SIC are dominantly quartz-phyric, with rare pyroxene and feldspar phenocrysts set in a fine-grained to glassy groundmass. They commonly contain wispy to amoeboid, mafic (basaltic?) inclusions, which are typically plagioclasephyric. Geochemically, the quartz-feldspar-phyric rocks from the core of the SIC are dacites and rhyolites (62 to 74 wt% SiO2) with elevated K2O contents (2.3 to 4.8 wt%). The lower silica contents within the core of the complex are apparently associated with small mafic inclusions within the more felsic units. The sampled mafic intrusive rocks from the ring dyke are plagioclase- and pyroxene-phyric, coarse- to fine-grained gabbros to monzogabbros (53 to 58 wt% SiO2). Figure. 1. A) Map of upper Great Lakes showing the location of the study area. B) Regional geology map showing the extent of the exposed portion of the Osler Group and the location of the St. Ignace Island Complex. The SIC samples yielded both zircon and baddeleyite. Subhedral baddeleyite grains from a rhyolite in the core of the SIC yielded a 207Pb/206Pb age of 1107.2±2.4 Ma, whereas zircons recovered from the rhyolite yielded a 207Pb/206Pb age of 1124 Ma. The zircons are large and show signs of resorption; they are also characterized by high Th/U contents that are typical of zircons derived from a mafic source. The fact that this latter age is much older than that of the Osler Group basalts that the SIC has intruded, combined with a lack 61 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 of euhedral zircon and baddeleyite grains, suggests that these grains may be of xenocrystic origin. The presence of baddeleyite in a rhyolite is also somewhat unusual as these are more commonly associated with syenites found in alkalic complexes (e.g. Coldwell Complex). Gabbro from the margin of the SIC yielded a small number of zircon grains with baddeleyite cores. These grains yielded a 207Pb/206Pb age of 1089.2±3.2 Ma. The growth of euhedral zircon on baddeleyite cores is occasionally observed in mafic rocks and is interpreted to indicate increasing silica activity conditions during magma crystallization. This is consistent with field relationships, which suggest that rhyolitic and gabbroic magmas may have intermingled during emplacement. Consequently, the 1089 Ma age may represent the emplacement age of both the rhyolite and the gabbro and suggests that all dates obtained from the rhyolite are xenocrystic. This is consistent with textures observed by Sutcliffe and Smith (1988) who also reported evidence for localized magma mixing and the presence of vesicular basalt fragments in felsic, welded tuffs. This age is similar to that of other MCR-related intrusions in the area (e.g. Crystal Lake, Blake and Moss Lake gabbros; Arrow River dyke) that have intruded Paleoproterozoic rocks, older MCR intrusions and/or Osler Group volcanic rocks during the late stages of MCR magmatism (Heaman and Easton 2006). Further field, petrographic and geochemical studies will seek to better determine the relationships between the various volcanic and intrusive rocks in order to understand the development of the SIC within the Midcontinent Rift. References Davis, D.W. and Sutcliffe, R.H. 1985. U-Pb ages from the Nipigon plate and northern Lake Superior; Geological Society of America Bulletin, v.96, p.1572-1579. Davis, D.W. and Green, J.C., 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution; Canadian Journal of Earth Sciences, 34, p.476-488. Giguere, J.F. 1975. Geology of St. Ignace Island and adjacent islands, District of Thunder Bay; Ontario Division of Mines, Geological Report 118, 35p. Heaman, L.M. and Easton, R.M. 2006. Preliminary U/Pb geochronology results: Lake Nipigon Region Geoscience Initiative; Ontario Geological Survey, Miscellaneous Release of Data 191, 86p. Sutcliffe, R.H. and Smith, A.R. 1988. Geology of the St. Ignace Island volcanic-plutonic complex; Summary of Field Work and Other Activities, Ontario Geological Survey, Miscellaneous Paper 141, p.368-371. 62 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 CHARACTERIZING THE MONAZITE FINGERPRINT OF PALEOPROTEROZOIC (STATHERIAN) METASEDIMENTARY SEQUENCES IN CENTRAL WISCONSIN STONIER, PEGGY, and HOLM, D.K., Kent State University, Kent, OH, peshelma@kent.edu; MEDARIS, L.G., JR, Univ. of Wisconsin-Madison, Madison, WI; SCHNEIDER, D., Ohio University, Athens, OH. Introduction: Supermature siliciclastic rocks of the 1750-1630 Ma Baraboo Interval are widespread in the southern Lake Superior region and signify a period of crustal stability following the Penokean Orogeny and subsequent geon 17 magmatism (Medaris et al., 2003). Although the chronology and tectonic significance of Baraboo Interval sedimentation has been firmly established, certain quartzite inliers in central Wisconsin remain enigmatic. At Hamilton Mounds Baraboo Interval quartzite is reported to be intruded by geon 17 granite (Greenberg, 1986; Van Wyck and Norman, 2004). This proposed cross-cutting relation conflicts with recent detrital zircon age data showing all Baraboo Interval quartzites to be younger than 1750 Ma (Holm et al., 1998; Medaris et al., 2003). Van Wyck and Norman (2004) propose that early onset of Baraboo Interval quartzite deposition was synchronous with magmatism, an interpretation that is unusual for this rock type and in disagreement with deposition on a recently stabilized craton (Dott, 1983). Instead, Medaris et al. (in review) demonstrate that the metasedimentary rocks at Hamilton Mounds consist of two Paleoproterozoic sedimentary sequences: an older meta-arkose intruded by geon 17 granite, and a younger, overlying supermature quartzite, which is likely correlative with Baraboo Interval quartzite elsewhere. Our purpose here is to characterize and date monazite grains in these two units. Recently, studies of detrital zircons in Baraboo Interval rocks have been invaluable for establishing their maximum age and identifying their source terrane. Assessing the monazite "fingerprint" in these rocks may allow us to differentiate between the two depositional interpretations and to better establish sedimentalogical aspects of post-Penokean crustal stabilization. Monazite Textures: The quartzite unit contains only tiny (10-40 micron diameter), rounded to subrounded monazite grains that show simple chemical zonation (rims/cores). Many have cores that are high in both Yttrium and Uranium (Fig. 1a). In contrast, the meta-arkose contains a larger monazite grain-size variance (10-90 micron diameters) and more variable morphology. A few grains are rounded and chemically simple, but many have embayed grain boundaries and are complexly zoned (Fig. 1b). Some grains are irregular and some have a bladed elongate morphology that are concordant to pre-existing textural features (Fig. 1c, d). Monazite Geochronology: Electron microprobe total-Pb analyses of the fine-grained detrital monazite from the quartzite unit yield spot ages ranging from ~2050 Ma to ~1750 Ma, with dominant peaks at ~1800 Ma and ~1860 Ma (Fig. 2a; composite mean age of 1837 ±23 Ma). Similar analyses on the meta-arkose unit yields ages ranging from ~1900 Ma to ~1730 Ma, with a single dominant age peak at 1850 Ma (Fig. 2b; composite mean age of 1849 ±7 Ma). Interpretation: Both rock units at Hamilton Mounds contain abundant Penokean age 63 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 detrital grains, consistent with their having been deposited after the Penokean orogeny. Both rock units also contain some geon 17 ages. The geon 17 ages within the quartzite unit are clearly from detrital grains. However, the geon 17 ages in the meta-arkose are unlikely to be detrital considering that this unit is cut by a ca. 1761 Ma granite dike. We interpret these ages to instead reflect metamorphism associated with dike intrusion. This interpretation is most consistent with the varied monazite morphology in this unit. Our results show that monazite geochronology of metasedimentary units is a powerful tool when combined with detailed in situ textural analysis aided by a comprehensive understanding of the area's geologic context. Figure 1a: Figure 1b: Figure 1c & 1d: In-situ BSE image of meta-Meta-arkose monazite In-situ BSE images of meta-arkose monazite grains quartzite monazite grain grain mapped for Y References Dott, R.H., Jr., 1983. The Proterozoic red quartzite enigma in the north central United States: resolved by plate collision?; Geological Society of America Memoir, v. 160, p. 129-141. Greenberg, J.K., 1986. Magmatism and the Baraboo Interval: breccia metasomatism and intrusion; Geoscience Wisconsin 10, 96-112. Holm, D.K., Schneider, D., and Coath, C., 1998b. Age and deformation of Early Proterozoic quartzites in the southern Lake Superior region: Implications for extent of foreland deformation during final assembly of Laurentia; Geology, v. 26, p. 907-910. Medaris, L.G., Singer, B.S., Dott, R.H., Naymark, A., Johnson, C.M., and Schott, R.C., 2003. Late Paleoproterozoic climate, tectonics, and metamorphism in the southern Lake Superior region and protoNorth America: Evidence from Baraboo interval quartzites; The Journal of Geology, v. 111, p. 243-247. Van Wyck, N., and Norman, M., 2004. Detrital zircon ages from Early Proterozoic quartzites, Wisconsin, support rapid weathering and deposition of mature quartz arenites; The Journal of Geology, v. 112, p. 305315. 64 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 THE THERMAL HISTORY OF LOW METAMORPHIC GRADE PALEOPROTEROZOIC METASEDIMENTARY ROCKS OF THE PENOKEAN OROGEN, LAKE SUPERIOR REGION: RECOGNIZING A WIDESPREAD 1786 MA OVERPRINT USING XENOTIME GEOCHRONOLOGY VALLINI, DANIELA A. University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009 CANNON, WILLIAM F.†, SCHULZ, KLAUS J. U.S. Geological Survey, MS 954, Reston, VA 20192 MCNAUGHTON, NEAL J. University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009 Paleoproterozoic strata in northern Michigan, Wisconsin, and Minnesota were deposited between 2.3 and 1.75 Ga within the Penokean foreland. These strata were metamorphosed by multipleevents, all previously attributed to the Penokean orogeny (1875-1830 Ma). We sampled 10 localities (Fig. 1) in the Marquette Range Supergroup in Michigan and the Animikie, Mille Lacs, and North Range Groups in Minnesota that contain xenotime suitable for in situ SHRIMP U-Pb geochronology and where the metamorphic grade is greenschist to sub-greenschist. The units sampled are Enchantment Lake Formation (sample 1), Sunday Quartzite (sample 5), Ajibik Quartzite (sample 7), and Michigamme Formation (samples 8, 9, 10) in Michigan, and the Mille Lacs Group (sample 2, 3), the Mahnomen Formation (sample 4), and Pokegema Quartzite (sample 6) in Minnesota. Thirty-two U-Pb ages of xenotime in these samples give a population at 1786 ± 4 Ma and 9 ages give a population at 1861 ± 10 Ma. Both populations are contained in samples from the Chocolay Group in Michigan and the Mille Lacs and North Range Groups in Minnesota (Fig. 1) and thus record a region-wide 1860 Ma low-temperature thermal event that is slightly older than the basal units of the Baraga Group in Michigan and the Rove Formation in Minnesota and Ontario. This event coincides with regional uplift that led to the unconformity between the Baraga and Menominee Groups in Michigan, hence xenotime growth must have occurred at shallow depths. Younger units, including the Animikie Group in Minnesota and the Baraga Group in Michigan, record only the 1786 Ma event. Amphibolite-granulite facies rocks within a gneiss dome corridor in the southern part of the foreland, south of our sample sites, show an 1800-1790 Ma monazite population that overprints 1830 Ma Penokean metamorphism (Schneider and others, 2004). These high grade rocks are adjacent to gneiss domes and early geon 17 post-Penokean granite plutons. Our samples are 50 to 150 km away from these features so the 1786 Ma xenotime ages do not appear to reflect local thermal imprints from plutonism and gneiss dome formation. Several sample sites in Michigan are within the low temperature zones of the Republic metamorphic node where metamorphic monazite has been dated at 1760 ± 5 Ma (Rose and others, 2003). Thus, most of our xenotime ages are significantly older than the Republic metamorphism, which does not appear to have been a significant xenotime-forming event at our sample sites. The geographic extent of the 1786 Ma xenotime growth event suggests it was a basin-wide subtle thermal pulse. We suggest two possible causes for this event. First, all of our age localities lie north of a corridor of gneiss domes and granitic plutons that formed in the interval 1800-1765 Ma, during a period of gravitational collapse of overthickened crust of the Penokean orogen (Schneider and others, 2004). This period of 65 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 gravity-driven tectonism and coincident heating may have driven a northward flow of hydrothermal fluids which subtly but pervasively altered the northern parts of the Penokean foreland and resulted in growth of xenotime. Alternatively the xenotime ages may record very distal effects of events within the Yavapai orogen, which truncated the southern part of the Penokean orogen on the south in central Wisconsin and southeastern Minnesota, about 200 km south of our sample sites. This early geon 17 crust-forming event occurred across the central and southwestern U.S. and may, in some as yet poorly understood manner, have caused widespread subtle heating across a broad foreland on its north. References Rose, S., Schneider, D.A., Loofboro, J., and Holm, D.K., 2003, Results and implications of monazite geochronology from the central Penokean orogen, WI & MI (abs): Geological Society of America Abstracts with Programs, v, 35, no.6, p. 505. Schneider, D.A., Holm, D.K., O’Boyle, C.O., Hamilton, M., and Jercinovic, M., 2004, Paleoproterozoic development of a gneiss dome corridor in the southern Lake Superior region, USA: in Whitney, D.L., Teyssier, C., and Siddoway, C.S., eds., Gneiss domes in orogeny: Geological Society of America Special Paper 380, p. 339-357. Figure 1. Map of the western Lake Superior region showing sample locations in relation to major tectonic and stratigraphic units. Inset shows density functions of xenotime ages divided into older and younger stratigraphic groups. 66 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 SULFUR ISOTOPES FROM PYRITE IN THE NEGAUNEE IRON FORMATION WAGGONER, T.D., Negaunee, MI, USA 49866-1007 In 2003 evidence was presented at the ILSG on hydrothermal venting systems preserved in the sediments older than the Negaunee iron formation. Rare earth elements patterns for the hard ore and vent hematite suggested a commonality for the iron source. During the study it was noted that within the hard iron oxide deposits on the Marquette Range there are numerous occurrences of veins, disseminated and massive sulfides. Pyrite is the common sulfide but both chalcopyrite and bornite can be present. Nine pyrite samples associated with hard ores from geographically diverse locations on the Marquette Range were submitted (Geochron Labs) for sulfur isotope analysis. The isotope ratios were determined by using the Canon Diablo troilite (CDT) standard. The physical relation of the oxides to sulfides indicated either a syndeposition or post replacement of the chert and iron oxides by pyrite. Fig. 1 Location of sulfur isotopes from the Marquette Iron Range Sulfur associated with sedimentary processes reflect the composition of biogenic sulfide produced by bacterial reduction of marine sulfate and is likely to result in δ34S values. Sulfur associated with igneous rocks is isotopically similar to that of meteorites and have δ34S values close to 0%o. Further variations are due to complex and interactive chemistry of the fluids and host. 67 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 The values obtained range from + 1%o to +6.8%o with the mean value of 1.9%o The narrow low positive values would suggest that the sulfur is attributable to a hydrothermal source thus supporting the earlier conclusion based on the REE data. Since a hydrothermal component has been shown to exist for portions of the Brockman (Hagemann et al, 1999) Caue (Rosiere et al, 2004) and Carajas (Guedes et al, 2002), it is logical to project an igneous-hydrothermal source be considered for the formation of BIFs in general. It is further suggested that water deposited BIFs could be a natural end product of hydrothermal IOCG type mineralization. Many features (e.g. age, extensional cratonic or continental margin setting, not easily related to igneous activity, mineral assemblage and alteration patterns) common to Iron Oxide deposits (IOCG) are also common to banded iron formations. The existence of end member BIFs in IOCG deposits (e.g. Pilot Knob, MO and Olympic Dam, South Australia) supports the hypothesis. References Guedes, S.C. et al, 2002, Carbonate Alteration Associated with the Carajas High- Grade Hematite Deposits, Brazil. Proceedings: AusIMM Iron Ore 2002, p. 63-66. Hagemann, S.G. et al, 1999, A Hydrothermal Origin for the Giant BIF-Hosted Tom Price Iron Ore Deposit. In: Stanley et al. (eds), Mineral Deposits: Processes to Processing, Balkema, Rotterdam, p. 41-44. Rosiere, C.A. et al, 2004, The Origin of Hematite in High-Grade Iron Ores Based on Infrared Microscopy and Fluid Inclusion Studies: The Example of the Conceicao Mine, Quadrilatero Ferrifero, Economic Geology v. 90 p. 611-624. 68 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 SOURCE ROCK AGES AND PATTERNS OF SEDIMENTATION IN THE LAKE SUPERIOR REGION: RESULTS OF PRELIMINARY U-PB DETRITAL ZIRCON STUDIES WIRTH, K.R.1, VERVOORT, J.2, CRADDOCK, J.P.1, DAVIDSON, C.3, FINLEY-BLASI, L.3, KERBER, L.4, LUNDQUIST, R.3, VORHIES, S.5, WALKER, E.6 1 Geology Department, Macalester College, St. Paul, MN 55105 (wirth@macalester.edu) 2 Department of Geological Sciences, Washington State University, Pullman, WA 99164 3 Department of Geology, Carleton College, Northfield, MN 55057 4 Geology Department, Pomona College, Claremont, CA 91711 5 Department of Geology, Smith College, Northampton, MA 01063 6 Department of Geology, Allegheny College, Meadville, PA 16335 U-Pb age analysis of detrital zircons provides information about source region ages and patterns of sedimentation. Although most commonly applied to orogenic belts and accreted terranes, this technique also has great potential for illuminating the evolution of cratonic regions. Here we report preliminary results of U-Pb analyses of detrital zircons from Paleoproterozoic (Denham Formation, Pokegama Quartzite, Palms Formation, Rove Formation, Thomson Formation), Neoproterozoic (Puckwunge Sandstone, Nopeming Sandstone, Rift Interflow sediments, Fond du Lac Sandstone, and Hinckley Sandstone), and early Paleozoic (St. Peter Sandstone) rocks from Minnesota and Wisconsin. U-Pb analyses of detrital zircons were conducted using laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at Washington State University. Approximately 120 grains were analyzed from each sample. All reported ages are 207 Pb/206Pb ages. Only those grains that are <10% discordant, based on comparison of 206 Pb/238U and 207Pb/206Pb ages, are presented. Arkosic conglomerate and quartz arenite of the Denham Formation are the oldest rocks that we examined in this study. The Denham samples contain zircons with ages between 3.6 and 2.1 Ga, however most grains fall into two age ranges: 3.5 – 3.4 Ga and 2.8 – 2.5 Ga. The youngest grains observed in the Denham Formation are 2.07 Ga. Basal Sandstones of the Animikie (Pokegama) and Marquette (Palms) Supergroups in Minnesota and Wisconsin, respectively, contain mostly Neoarchean zircons with similar age distributions (2.9 to 2.6 Ga). Both formations contain scattered grains of Mesoarchean (Pokegama) and Paleoarchean age (Palms), but neither contains grains with ages < 2.6 Ga. Fine-grained sandstones from upper Rove Formation (NE Minnesota), Thomson Formation (E. Central Minnesota), and Tyler Formation (NW WI) were deposited in a migrating foredeep north of the Penokean orogen. Most zircon grains from these three formations have ages between 2.05 and 1.80 Ga (Fig. 1). All three formations also contain some Paleoproterozoic to Paleoarchean grains, but these ages are relatively few in number. The zircon age histograms also lack the major peak at 2.7 Ga that occurs in 69 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 the basal Pokegama and Palms Formations. Zircons from sandstones immediately below basal volcanics of the Keweenawan Midcontinent Rift (1.1 Ga) of east-central (Nopeming Sandstone) and NE Minnesota (Puckwunge Sandstone) have age distributions that are strikingly different. Zircon ages from the Nopeming Sandstone form three groups: 2.8 – 2.5 Ga, 2.1 - 1.8 Ga, and 1.2 - 1.1 Ga (Fig. 1). A few grains also have ages from 3.3 to 2.8 Ga and 2.4 to 2.2 Ga. Puckwunge zircons have a similar age distribution except that no Mesoproterozoic ages (1.2 – 1.1 Ga) are present. Zircons from interflow sediments of the SW limb of the North Shore Volcanic Group have a dominant age peak at 1.15 to 1.0 Ga, and scattered ages in the range of 2.7 to 2.3 Ga. Neoproterozoic Fond du Lac and Hinckley Sandstones were deposited after the main pulse of rift-related magmatism. Fond du Lac zircons have ages that range from 1.5 to 1.0 Ga; a few grains have older ages at 2.9, 2.5, and 1.9 to 1.6 Ga. Hinckley zircons have similar age distributions, but with many more ages from 3.1 – 2.7 Ga and 2.1 – 1. 5 Ga (Fig. 1). Zircons from the Middle Ordovician St. Peter Sandstone have two age populations: 2.8-2.6 Ga and 1.5-1.0 Ga (Fig. 1). Only three grains have ages between 2.5 and 1.5 Ga. Most of the observed zircon ages can be correlated with known source rock ages in the Lake Superior region (Fig. 1). Some ages, however, have no obvious local sources (e.g., 2.5 – 2.1 Ga, 1.6 – 1.5 Ga, and 1.4 – 1.1 Ga) and must have been derived from more distal sources (Van Wyck and Norman, 2004) or from regional sources with unrecognized multicyclic components. In particular, all Neoproterozoic and Paleozoic sediments that we studied have abundant ages between 1.5 and 1.1 Ga that might have been derived from Grenville sources (e.g., Rainbird et al., 1992; Johnson and Winter, 1999). Figure 1. 70 Histograms of 207Pb/206Pb ages from detrital zircons in Thomson Formation, Nopeming Sandstone, Hinckley Sandstone, and St. Peter Sandstone. Shaded bands indicate possible source region ages in Lake Superior Region. �Proceedings of the 52nd ILSG Annual Meeting – Part 1 References Johnson, C.M. and Winter, B.L, 1999, Provenance analysis of Lower Paleozoic cratonic quartz arenites of the northern Midcontinent region: U-Pb and Sm-Nd isotope geochemistry: Geological Society of America Bulletin, v. 111, 1723-1738. Rainbird, R., Heaman, L., and Young, G., 1992, Sampling Laurentia: Detrital zircon geo-chronology offers evidence for an extensive Neoproterozoic river system originating from the Grenville orogen: Geology, v. 20, p.351-354. Van Wyck, N. and Norman, M., 2004, Detrital zircon ages from Early Proterozoic quartzites, Wisconsin, support rapid weathering and deposition of mature quartz arenites: Journal of Geology, v. 112, p. 305315. 71 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Author Index Magee, M.A. McNaugton, N.J. Medaris, L.G. Jr. Miller, J.D. Jr.27, Moran, P. Mudrey, M.G. Jr. Patel, D. Peterson, D.M. Planavsky, N. Porter, R. Rainbird, R.H. Reid, D.D. Rousell, D.H. Schneider, D.A. Schultz, K.J. Severson, M.J. Shapiro, R. Shaw, C.A. Shute, A. Simmons, W.B. Smyk, M.C. Stonier, P. Stott, G. M. Vallini, D. A. Van Schmus, W.R. Vervoort, J. Vorhies, S. Wagonner, T.D. Walker, E. Wirth, K.R. Wyman, D. Anderson, D.K. 27 Andring, M. 35 Bartingale, R J. 1 Bennett, G. 3 Boerboom, T.J. 4, 27 Brown, B.A. 7 Buchholz, T.W. 8 Cannon, W.F. 10, 27, 65 Chandler, V.W. 27, 32 Cote, V. 12 Craddock, J.P. 13, 35, 69 Czechanski, M.L. 7 Davidson, C. 69 Davis, D.W. 23 Davis, W. J. 55 Easton, R.M. 15 Ernst, R.E. 23 Falster, A.U. 8 Finley-Blasi, L. 69 Fralick, P. 17, 42, 49 Franklin, J.M. 37 Grabowski, G. 19 Gross, A. 20 Hailstone, M. 22 Halls, H.C. 23 Heaman, L.M. 61 Heine, J. 30 Heggie, G.J. 37 Hocker-Finamore, S.M. 30 Hollings, P. 25, 42, 49, 59, 61 Holm, D.K. 20, 27, 63 Horton, J.W. Jr. 10 Hudak, G.J. 30 Jirsa, M.A. 27, 32 Juda, N. 35 Karimzadeh Somarin, A. 37 Kerber, L. 69 Kissin, S.A. 37 Knudsen, A. 53 Kring, D.A. 10 Lunquist, R. 69 MacTavish, A. 39 72 41, 42 65 63 44, 46 49 7, 51 13 44, 52 53 13 55 7 57 20, 27 27, 65 46 53 1 59 8 61 63 23 65 27 35, 69 69 67 69 13, 35, 69 25 � https://digitalcollections.lakeheadu.ca/files/original/2494bf3ae2892ac712fe458477742e86.PDF 843d0608ade7fa8cb4884c9ec370ae36 PDF Text Text Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan i Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan t ) ). By: U Ronald P. Sage, PhD Victoria L. Sage, BSc 2006 -- Field Trip Guidebook, Volume 52, Part 2 Institute on Lake Superior Geology ti. Sault Ste. Marie, Ontario PDF compression, OCR, web-optimization with CVISION's PdfCompressor �ii Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan iii Glacial Lakes Algonquin and Glacial Lakes Algonquin and Nipissing Nipissing Shoreline Bedrock Features: Shoreline Bedrock Features: Mackinac Island, Michigan Mackinac Island, Michigan By: Ronald P. Sage Victoria L. Sage 2006 52nd Field Trip Guidebook for the Institute on Lake Superior Geology Sault Ste. Marie, Ontario By: Ronald P. Sage, PhD Victoria L. Sage, BSc 2006 Field Trip Guidebook, Volume 52, Part 2 Institute on Lake Superior Geology Sault Ste. Marie, Ontario On the cover: Lithograph of Arch Rock on east shore of Mackinac Island. See page 16-17. Colorization: V.Sage. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �iv Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan v Acknowledgements This one-day trip is designed to visit all the prominent rock exposures carved by glacial Lakes Algonquin and Nipissing into the limestones exposed on Mackinac Island. The senior author has more than 30 years of geological field experience working in the Canadian Shield. The bedrock exposures on Mackinac Island are of interest to the author as they relate to the island economy. Victoria L. Sage, BSc, has worked in scientific and technical communication for the medical field. She has provided the computer programming skills required to put this guidebook together and has provided editorial assistance with text and guide format. Both authors have worked on Mackinac Island as Guest Service Representatives for Mackinac State Historic Parks. During this period of employment Mr. Greg Hokans, Marketing; Mr. Phil Porter, Museum Programs and Dr. David Armour, Deputy Director offered encouragement and support to the staff working on the island and assisted in locating various published articles describing the features of Mackinac Island. Mr. Steve Brisson, Curator of Collections, provided copies of most of the historical lithographs of rock formations used in this guide that have made the Island famous. Carol R. Sage, MS, has provided editorial assistance in preparing the Guidebook. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �vi Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan vii Table of Contents Introduction.....................................................................................................................................................1 Silurian–Devonian............................................................................................................................................2 Point aux Chenes Formation.................................................................................................................2 Bois Blanc Formation............................................................................................................................2 Detroit River Formation.......................................................................................................................5 Mackinac Breccia.............................................................................................................................................5 Breccia Origin......................................................................................................................................7 Age of Brecciation.................................................................................................................................7 I. Stop Descriptions................................................................................ 13 Stop 1: Robinson’s Folly............................................................ 14 Stop 2: Arch Rock..................................................................... 17 Stop 3: Eagle Point Cave........................................................... 18 Optional.................................................................................... 18 Stop 4: Pulpit Rock................................................................... 19 Stop 5: Chimney Rock.............................................................. 20 Stop 6: Devil’s Kitchen and Lover’s Leap.................................. 21 Stop 7: Sugar Loaf..................................................................... 22 Stop 8: Skull Cave..................................................................... 24 Stop 9: Crack-in-the-Island and Cave-of-the-Woods................... 24 Additional Suggestions.............................................................. 25 iii r jirA iil!!Y..:. In Recent...............................................................................................................................................................8 Upper Algonquin.................................................................................................................................9 Lower Algonquin..................................................................................................................................9 Submerged (Buried) Stream Valley of the Straits of Mackinac.............................................................. 10 Nipissing Shorelines.............................................................................................................................. 11 Post-Nipissing......................................................................................................................................... 11 Lithograph of ramp into Fort Mackinac and “Gibraltar Rock,” which forms the foundation of the Fort above Lake Huron (Allen 1891, p.185). Gibraltar Rock is part of a sea cliff formed by glacial Lake Nipissing. Bibliography.............................................27 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �viii Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 1 M ackinac Island occurs between Lakes Huron and Michigan and just south of Lake Superior. It lies on the northeast rim of the Michigan Basin, a basin structure that underlies the state of Michigan and portions of the states of Wisconsin, Illinois, Indiana, Ohio and the province of Ontario, Canada (Figures 1 and 2). It is a continental scale basin that is generally poorly exposed except in scattered locations along its rim where the older rocks forming the basin are sometimes exposed. Within central Michigan and in the center of the basin, the rocks exposed along the rim are buried beneath many thousands of feet of younger rocks. Mackinac Island represents one of the best exposures of rocks of Silurian (440-395 million years) and Devonian (395-345 million years) age along the basin perimeter, and these Silurian and Devonian rocks project above the highest water level of the older glacial Lake Algonquin. The effects of coastline erosion are well recorded in the wave cut cliffs and abandoned beaches found on the island. The island offers an excellent opportunity to examine shoreline features related to the glacial Lakes Algonquin and Nipissing formed at the edge of the receding continental ice sheets of 10,000 to 12,000 years ago. Since shoreline features are commonly in unconsolidated Riyer material only the latest events N are recorded in these materi5Mb Boypal Slim Michigan als, the earlier features being SMncn •Mo obliterated by later events. It is SMbb Bno.Bedford SMC ElbwocTh-&a,im anticipated that the actual Mü.ü1t. events are much more complex SMrYc Mtthu than presented here. The SD? Traven. City present island contains a surSüd Thridss SOdr DMrcitR$vw face area of 2,221 acres SDbb Bali Blanc — — (Russell, 1905, p. 56). 0-Sn. MOdSuIc -S — Sand Pdrt St S. — — Oi.ia Erqra&. ajif Sme 0. RId.susid Oc 0? TrSan Cnn £j CrnnbIIOn MILES Figure 1: Bedrock geology of the state of Michigan (Dorr and Eschman, 1970). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �2 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan FNWQOTU — Figure 2: Outline map of the distribution of Silurian age rocks around the Michigan basin (Dorr and Eschman, 1970, p. 104). The line of section passes very close to Mackinac Island near point A. Silurian – Devonian Mackinac Island is composed of rocks of Silurian and Devonian age. The lower Silurian stratigraphy consists of the Point aux Chenes or Salina formation and the upper Devonian consists of the Bois Blanc and Detroit River formations (Figures 3, 4 and 5). Point aux Chenes formation The Silurian Point aux Chenes formation consists of upper beds of variegated shale and thin brown dolomites, which overlie a lowersalt series that can contain 1600 feet of salt in a number of beds (Landes et al., 1945, p. 159). The salt beds are thin along the northeast and northwest flanks of the Michigan Basin and the upper salt beds have been leached from the formation in upper Michigan ( Landes et al., 1945, p. 159160). Landes et al., (1945, p. 161) report that shale beds commonly separate salt beds in the upper half of the formation and dolomite in the lower half. A basal dolomite occurs everywhere below the lowest salt and, where unleached, the Point aux Chenes formation in northeastern Michigan has a thickness of 1175 to 2886 feet (Landes et al, 1945, p.161). Bois Blanc formation Landes et al., (1945, p. 163) report that the Bois Blanc formation contains Onondaga fossils of lower middle Devonian age. The formation consists of limestone and dolomite and ranges from 165 to 1000 feet in thickness in upper Michigan (Landes et al., 1945, p. 165-166). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 3 IEGENO SAEI$?011 lvii. r—I .vp._ L____J —NtS*flI fl SALT SItar Figure 3: Generalized columnar section of the region around the Mackinac Straits (Landes et al., 1945, p. 154). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �4 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan CLASSIFICATION CF ROCKS IN STRAITS THE MACKINAC STRAITS AREA CITY '0. 4 TO. ii:; ROflRC CITY LIMESTONE CflA N OVIA Iii (it______ DUNDEE LIMESTONE T Figure 4: Generalized stratigraphic column at the Straits of Mackinac showing the distribution of Mackinac breccia across the Silurian and Devonian time periods (Sheldon, 1959, p. 12). It C Cr 0 I- 4 a V Eu U I- C, to -J 0015 BLANC FM. PARI( 400' IS. F GARDEN 250•. ST. IGNACE DOLOMITE C, C z C z 'C U 600 —IC POINTE AUX CHENES 511. S IN INDICATE STRATIC ENGADINE DOLOMITE AR MX I V t et. 7 ci 4- ROGERS CITY ORMAIION GARDEN IS. FORMATION DUNDEE FORMATION St IGNACE DOLOMITE DETROIT RIVER GROUP POINTE AUX CHENES C BLANC FORMATION SHALE MACKINAC BRECCIA Figure 5: Generalized surface geologic map of the Mackinac Straits area that displays the distribution of the Mackinac breccia (Sheldon, 1959, p. 3). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 5 Detroit River Formation This formation is not exposed in the area of Mackinac Island. The lower part of the formation consists of limestone and dolomite that is difficult to separate from the Bois Blanc formation (Landes et al., 1945, p. 174). Above the basal limestone of the Detroit River formation, there is an evaporate series consisting of dolomite, anhydrite and salt that in the center of the Michigan Basin may be 1145 feet in thickness (Landes et. al., 1945, p. 174). The evaporate thickness ranges up to 600 feet along the periphery of the basin, but the evaporate series is absent along the rim in the vicinity of Mackinac Island (Landes et al., 1945, p. 174175). Mackinac Breccia Mackinac breccia is the most prominent rock type found on Mackinac Island when one examines the rock formations created by the interaction of glacial Lakes Algonquin and Nipissing. All the rock formations that have received special attention on Mackinac Island are composed of this rock unit. The term “Mackinac breccia” was originally applied by Douglas Houghton, first state geologist for the state of Michigan (Shelden, 1959, p. 19). The Mackinac breccia is an indurated breccia although, in the region of Mackinac Island, non-indurated breccia is the dominate lithology (Landes et al.; 1945, p. 135). Mackinac Island and the St. Ignace peninsula are composed of a non-indurated megabreccia and a small amount of transformational breccia (Landes et al., 1945, p. 134-135). Landes et al. (1945, p. 133) report that the cement to the indurated breccias is carbonate. The indurated Mackinac breccia is easy to recognize in an outcrop by its pock-marked surface that is caused by differential weathering of the breccia fragments. The more soluble fragments are removed leaving large cavities in the breccia units giving rise to the well-developed pock-marked surface. The fragments are angular and up to 10 feet or more in maximum length and consist of a mixture of rock fragments from units higher in the stratigraphy. The resistance to weathering of the indurated breccia suggests that some silica also serves as a cementing agent for the breccia fragments. If all the cement was carbonate, it would be dissolved upon exposure to weathering or solution and the Mackinac breccia would collapse and become indistinguishable from the regional brecciation. Erosion of the regional limestones of Silurian and Devonian age by the glacial Lakes Algonquin and Nipissing has removed the softer, less indurated material enclosing the indurated breccia masses. This has resulted in the prominent rock formations now preserved along the former coastlines of the glacial lakes. Mackinac Island occurs in a broad zone of brecciation found at the northeast corner of the Michigan Basin (Figure 6). Salt decreases in thickness from 1200 feet to 0 feet northwest of Alpena County and north of Cheboygan County, Michigan, and the zone of brecciation corresponds with the disappearance of salt in the stratigraphy. This salt has a blunt edge suggesting this boundary may be a leached rather than a natural, depositional edge (Figure 7) (Landes et. al., 1945, p. 146). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �6 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan c SR*NO I £ L CO 14* KALKAflA TN*YflSE SUROCE LYIDOIG! OF COLLAPSI OF' .COLLAPSt WITH 0 (VIQCHCC OF Figure 6: Map of the Mackinac Straits area showing the region of solution collapse and Mackinac breccia distribution (Landes et al., 1945, p. 175). Figure 7: Isopach map showing combined thickness of Salina salt (Landes et al., 1945, p. 146). The salt disappears abruptly just southeast of Mackinac Island and outcroppings of Mackinac breccia. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 7 Breccia Origin The origin of the Mackinac breccia has been discussed by Stanley (1945), Landes et al. (1945), and Shelden (1959). The most complete discussion of the origin is that of Landes et al. (1945), who completed a detailed summary of all previous work in determining breccia development. Landes et al., (1945, p.142) attribute the development of a modern model for the formation of the Mackinac breccias to Mr. Henry H. Hindshaw, former assistant to the state geologist for New York. A brief summary of the model for the formation of the Mackinac breccia is as follows (Landes et al., 1945, p. 143-145) and has been previously summarized by Shelden (1959). The land surface is floored with rocks of Niagara age (Silurian) and was submerged beneath the Pointe aux Chenes Sea. During this time, several hundred feet of shale, dolomite, salt and gypsum were deposited. Post-deposition, emergence followed along the rim of the Michigan Basin and percolating ground waters leached salt from the rim of the salt-bearing rocks. Caves were produced when the salt in the Pointe aux Chenes was removed from rocks lying above the ground water table. The solution of the salt created caverns that became unstable and collapsed. There were two types of collapse: regional and local. The collapse was probably sudden and the overlying rocks broke into angular fragments of all sizes. This probably created localized sink holes above the collapse, and the larger areas of regionalized collapse created tilted stratigraphy in the region. The period of collapse took place during emergence following Detroit River deposition and preceding Dundee deposition. This collapse took place over a time interval rather than at a specific time. Shelden (1959, p. 23) reports slickensides and normal faults within larger blocks of collapsed rock. The Mackinac breccias occur in columns, and some clasts may represent down drop of 600 to 750 feet (Landes et al., 1945, p. 129). After collapse, erosion of the surface developed a peneplane and surface irregularities (such as sinkholes) were filled. The limestones of the Dundee formation were deposited on this peneplane surface. Percolating ground waters gradually cemented the collapse breccias. Recent emergence has brought the rocks to surface where differential erosion has completed sculpting the breccias into the forms observed on the Island. Age of brecciation Landes et al., (1945, p.137) interpret the age brecciation as likely post-Detroit and pre-Dundee. The brecciation likely took place over an extended period of time. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �8 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan Recent Mackinac Island has been affected by recent glaciation of the Wisconsin period and glacial Lakes Algonquin and Nipissing following retreat of the glaciers. Glacial Lake Algonquin represents an age of 11,000 to 12,000 years ago, and glacial Lake Nipissing represents an age of approximately 4,000 years ago. Viewing of the island from St. Ignace and Mackinaw City presents a profile of an island with a hump in the middle with two relatively flat planes separated by a steep slope (Figures 19 and 20). The present mean level of Lake Huron is 580.37 feet above sea level and the highest point on the island is at Fort Holmes, 900.5 to 904.1 feet above sea level (Stanley, 1945, p. 65, 72-73). The highest level reached by glacial Lake Algonquin is 809 feet above sea level (Taylor, 1915, p. 69), which is the highest peneplane observed in the profile of the island. This former high water level is approximately 229 feet above the present lake level (Taylor, 1915, p. 69). That portion of Mackinac Island lying above this high water mark is referred to as the “Ancient Island”. The “Ancient Island” part of Mackinac Island was glaciated during the Wisconsin period and a few glaciated boulders, cobbles and pebbles displaying glacial striations can be found in the area (Stanley, 1945, p. 13). The ice moved in a southwest direction and left a thin covering of glacial till on the “Ancient Island” (Stanley, 1945, p. 12-13). Glacial ice retreated northeastward in a series of retreats and advances with the retreats exceeding the advances (Landes et al., 1945, p. 10). The process of retreat-and-advance by glacial ice will destroy evidence left in unconsolidated materials from earlier (older) retreats and advances and only the last event is preserved in the geological record. _____________________ U sot SOUTH URE HURON ISO ALSn#01 LIWL soJJlHt4sr NAUlNO LEVEL LAKE NUAON 550 Figure 8: Profile across Mackinac Island showing former shore levels looking east from St. Ignace (Stanley, 1945, p. 22). Glacial LoRe Nipissiog water plane Ancient Island" I Fort Holmes atop GIOCICI Luk. àiqooQuir. wave cut cliff w°ve Lake Nipássing wove duff Glacial UII4 AIUOMU1O watt, PWM Glacial LaKe Nipissing woterplone I I I MdernIoke$evht I Figure 9: Profile of Mackinac Island looking eastward from St. Igance with the various glacial lake levels noted (Dorr and Eschman, 1970, p. 177). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan Upper Algonquin The Lake Algonquin shoreline is a well-developed major feature in the region of the Straits of Mackinac (Landes et al., 1945, p. 11) (Figures 10 and 11). Landes et al. (1945, p. 11-12) indicate that drainage from this glacial lake was eastward towards North Bay and Mattawa, Ontario. But, as the ice sheet withdrew, isostatic rebound caused uplift to the northeast until the waters overflowed to the south at Port Huron and Chicago. This is a three-stage outlet for Lake Algonquin. As isostatic uplift continued in response to the retreating ice sheets, the lake level was stabilized by the outflow through Port Huron and Chicago and the eastward drainage abandoned (two stage outlet) (Landes et al., 1945, p. 12). As uplift continued, the lake level continued to recede and gradually erosion along the Port Huron drainage caused the abandonment of the Chicago outlet (Landes et al., 1945, p. 12). Beach lines associated with the Upper Algonquin of glacial Lake Algonquin lie between 799 and 759 feet above sea level and the type section is the Short Rifle Range lying between Fort Mackinac and Fort Holmes (Stanley, 1945, p. 31-32). Taylor (1915, p. 69) reports that the highest beach level is 809 feet above sea level or approximately 229 feet above the present level of Lake Huron. Glacial straie have not been observed on glacial boulders below the level of 205 feet above lake level (Taylor, 1892, p. 212-213). 9 1' Figure 10: High water level for glacial Lake Algonquin approximately 11,000 years ago. This illustration indicates most of the region was under water. This view is oriented north looking south (Porter and Nelhiebel, 1984, p. 11). Lower Algonquin Below the Upper Algonquin is a zone relatively free of beach development, which is thought to represent a urrLa Iauw SNORE period of relatively rapid falling of the lake level 10.000 Years Ago 116 Abow tflc flunn todsy (Stanley, 1945, p.32). This zone lies between approximately 762 and 635 feet above sea level (Stanley, 1945, Figure 11: Upper glacial Lake Algonquin shoreline approximately 10,000 years ago (Porter and Nelhiebel, 1984, p. 47). p. 32-36). The best-described beach line in the Lower Algonquin is the “Battlefield Beach” at an elevation of 718 feet above present sea level located in the north central part of the island (Stanley, 1945, p. 33-34; Landes et al., 1945, p. 13). Stanley (1945, p. 35-36) and Landes et al. (1945, p. 13-14) describe other locations as examples of beaches within the Lower Algonquin. The Lower Algonquin beaches are characteristically less well developed than the Upper Algonquin. The quick lowering of the lake level is in response to the opening of discharge channels to the east as the ice sheet retreats (Landes et al., 1945, p. 13). The lowering probably extended to much lower levels than present lake levels, and a buried river valley between PDF compression, OCR, web-optimization with CVISION's PdfCompressor �10 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan Lake Huron and Lake Michigan was probably reactivated (Landes et al., 1945, p. 14). At this low water stage the present islands within the Straits did not exist but were part of what is now the mainland. Landes et al. (1945, p. 14) indicate that the lake levels ceased falling when the lowest outlet from the Huron Basin, the Mattawa Valley (east of North Bay), was freed of ice and unobstructed flow eastward was established. The removal of glacial obstruction left only isostatic rebound as a mechanism of changing lake water flow. The Mattawa Valley is located to the east and northeast of Mackinac Island where this continental uplift would be most effective (Landes et al., 1945, p.1 4). Continental uplift to the east generated continually rising lake levels until southern flow was reestablished, and then eastward flow through the Mattawa Valley was terminated (Landes et al., 1945, p. 14). As the lake waters rose, topographic features created in the unconsolidated glacial deposits as the lake level dropped were obliterated or buried beneath the rising waters. Submerged (buried) stream valley of the Straits of Mackinac The presence of a submerged valley in the Straits of Mackinac was disclosed by soundings between 1918 and 1924 (Stanley, 1938, p. 966) (Figure 12). This valley loops around Mackinac Island to the north and is likely the result of Pleistocene or earlier erosion (Stanley, 1938, p. 966, 974). This valley was likely filled with glacial drift during the Wisconsin ice advance and cleaned out during the low water level between the glacial Lakes Algonquin and Nipissing (Stanley, 1938, p. 974). Stanley (1938, p. 966) and Sheldon (1959 p. 59) suggest flow along this valley was towards to the east. This valley lies 150 to 250 feet below the present level of Lake Huron with the greatest depths being recorded through the Straits near Mackinaw City (Stanley, 1938, p. 968). Stanley (1938, p. 968) reports the greatest depth to be 289 feet below present lake level. This valley exceeds 70 miles in length (Stanley, 1938, p. 966). LAKE M'GHIGAN 7 Figure 12: Submerged valley through the Straits of Mackinac (Stanley, 1938, p. 967). At this time, the water level of Lake Huron was approximately 120 feet lower than it is today. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 11 Nipissing Shorelines As isostatic rebound continued, the level of Lake Huron returned to a Scott's Cavg level 50 feet above its present level Croglian Water (Stanley, 1945, p. 39) (Figure 13). Stanley (1945, p. 39) suggests the process of uplift and higher water levels took place slowly over a period of several thousand years and the previous geological features in unconsolidated glacial material were de- Devil's stroyed. Nipissing beaches reached elevations of 629 to 635 feet above sea level (Stanley, 1945, p. 42). At Robinson's rally abmt Laict Huron the time of Nipissing beach developlakE Huron rreseifl ShorelIne ment, 85% of the isostatic rebound had been completed (Shelden, 1959, Figure 13: Modified oblique view of Mackinac Island showing the shoreline of glacial Lake Nipissing with scenic rock formations noted (Porter and Nelhiebel, 1984, p. 13). Friendship p. 59; Landes et al., 1945, p. 14). Nipissing beaches do not completely Altar and Pulpit Rock are the same feature. This shoreline is approximately 4,000 years old. enclose Mackinac Island but obliquely transect the lower Algonquin beaches (Stanley, 1945, p.39, 41). Nipissing beaches are preserved on the northwest and southeast portions of Mackinac Island; the City of Mackinac is built on Nipissing beaches (Landes et al., 1945, p. 15; Stanley, 1945, p. 44; Leverett and Taylor, 1915, p. 452). Post-Nipissing During post-Nipissing time, Lake Huron drains south past Port Huron and is approximately 55 feet lower than the highest level of glacial Lake Nipissing (Sheldon, 1959, p. 63). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �12 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 13 Stop Descriptions T he Mackinac Island field trip is a one-day trip that is designed to visit all the major bedrock outcrops that contribute to the scenery of the island. These outcrops have been described in previous technical and non-technical literature for more than 150 years. These scenic bedrock exposures have contributed much to the island economy over the years. Figure 14 is a simple sketch of Mackinac Island showing the location of the sites on this tour and those geographic features mentioned in this text. The island’s major bedrock outcrops are the direct product of the interaction between glacial lakes and bedrock during the retreat of the Wisconsin ice sheets. Upon arrival, one disembarks on the south side of Mackinac Island where the glacial Lake Nipissing beach deposits are best developed. Fort Mackinac is built on the Lake Nipissing sea cliffs. Below this cliff lie the deposits of glacial Lake Nipissing. These beach deposits are largely obscured in most areas by subsequent construction. The sea cliff below Fort Mackinac has a prominent rock formation called Gibraltar Rock that projects out from the cliff but is not detached. Continued erosion around this prominent rock could have caused its detachment from the sea cliff to create a sea stack (see the Table of Contents for a lithograph of Gibraltar Rock). The cliff face on which Fort Mackinac is built is fenced off from the public and, if time permits, a closer examination can be made after completion of the field trip. The field trip consists of two parts. The first part is a trip around the island on Shoreline Road to examine the erosive activities of glacial Lake Nipissing. The second part is an interior tour of the island to examine the erosive activities of glacial Lake Algonquin. Cave Cake For Fairy Arch hore Former ScoWs Cave Foot N CQve cc the Woods N A 1000 Feet & trn]k HerLot Kitchen Rock Figure 14: Sketch map indicating the sites of rock outcroppings to visit and those geographic features needed to locate them. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �14 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan Stop1: Robinson’s Folly Moving eastward along Shoreline Road (through the settlement) approximately one mile from the landing on Mackinac Island, the first outcrop one encounters is a prominent cliff face of Mackinac breccia known as Robinson’s Folly (Figure 15). This prominent headland is reportedly named after Captain Daniel Robertson of the British 84th Regiment who supposedly built a summer house on the promontory, which subsequently collapsed into Lake Huron (Wood, 1918, p. 584-585). The name “Robinson” is a corruption from the French addressing him as “Robinçon” (Wood, 1918, p. 584-585). Meade (1986 [1897], p. 165-170) presents five stories on the naming of Robinson’s Folly. The reader should refer to Meade’s lengthy discussion of the name if interested in the origin. Robinson’s Folly is 127 feet (Van Fleet, 1970, p. 147; 1882, p. 24) or 128 feet (Winchell, 1861, p. 210) above the present level of Lake Huron. It represents a prominent portion of the sea cliffs formed along the eastern side of Mackinac Island through the erosive action of glacial Lake Nipissing. A short distance north of Robinson’s Folly is a second outcropping of breccia that was the former site of a sea arch known as Fairy Arch (Figure 16). This arch was destroyed during road construction around the island, but a photograph was published by an anonymous source (1899, p. 5). Ak Figure 15: Lithograph of Robinson’s Folly, a glacial Lake Nipissing sea cliff (Woolson, 1894, p. 289). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 15 fr Figure 16: Lithograph of Fairy Arch, a glacial Lake Nipissing shoreline feature (Woolson, 1894, p. 285). This arch was destroyed during road building along the east side of Mackinac Island. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan .1 16 Figure 17: Lithograph of Arch Rock from the shore of Lake Huron (Disturnell, 1875). This is a glacial Lake Nipissing feature. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 17 Stop 2: Arch Rock Arch Rock is located on the east side of Mackinac Island and is the best known rock formation on the island (Figures 17 and 18). The limestone arch is butted against seacliffs of glacial Lake Nipissing to the north and a promontory of Mackinac breccia that projects out from the cliff face but remains attached. The top of the arch is approximately 140 feet above Lake Huron and the top of the buttress that projects out from the cliff is 105 feet above present-day Lake Huron (Winchell, 1861, p. 210; Van Fleet, 1970, p. 147; 1882, p. 24). Arch Rock has a span of 40 to 50 feet (Van Fleet, 1882, p. 20). Figure 19: Lithograph looking down through Arch Rock toward Lake Huron. This view is from the top of the glacial Lake Nipissing sea cliffs (Woolson, 1894, p. 281). Arch Rock is composed of highly fractured limestone. Many of the fractures have been sealed with man-made cement. The northern abutment has been reinforced with man-made cement and rock. In the spring, loose fragments that have fallen from the arch commonly lie below the structure. The highly fractured appearance of the limestone prompted McKenny (1959 [1827], p. 390) and Foster and Whitney (1851, p. 163) to predict a very short life for the arch. In terms of geologic time, this will likely be true. I At the base of the promontory which forms the south abutment to Arch Rock, there is a small arch known as the Sannillac Arch. This arch is named after an Indian warrior named Sannillac, who is the subject of a poem by Henry Whiting that was published in 1831 (Wood, 1918, p. 588-589). In front of Arch Rock on the lake side of the shoreline road there is a large boulder of Mackinac breccia that has broken free of the promontory. This boulder has been referred to as Gitchie Manitou (Stanley, 1945, p. 51). Figure 18: Lithograph of Arch Rock in the moonlight looking up from Lake Huron through the arch that forms part of the glacial Lake Nipissing sea cliffs (Woolson, 1894, p. 279). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �18 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan Stop 3: Eagle Point Cave Eagle Point Cave is located approximately two miles northwest beyond Arch Rock. Take Lake Shore Road to Scott’s Shore Road; turn inland approximately 500 feet to Scott’s Road. The cave is situated approximately 1000 feet south of the junction of Scott’s Shore Road and Scott’s Road on the west side of the Scott’s Road. The site of this cave was once used for some unknown purpose. The remains of concrete steps are found in front of the cave and the floor of the cave has been leveled using logs and dirt fill. Eagle Point Cave occurs in Mackinac breccia and is represented by a large amphitheater-type opening in the breccia. Eagle Point Cave is typical of most of the shoreline features described on the island as caves. These caves are commonly amphitheater-type openings of very limited depth that have resulted from lakeshore weathering-erosion processes. Eagle Point Cave is a glacial Lake Nipissing shoreline feature. Optional After the visit to Eagle Point Cave one can return to the junction of Scott’s Shore Road and Scott’s Road and continue north for 200-300 feet where approximately 100-150 feet west of the road is a small promontory of bedrock in which the former Scott’s Cave was displayed. This cave has either caved-in or become filled-in and relatively little remains to view. The site warrants some restoration effort. Stanley (1945, p. 47) published a good photograph of the cave when it was exposed for viewing. This cave occurred in the same glacial Lake Nipissing shoreline bluff as Eagle Point Cave. One can walk a trail along the top of the bluff from one cave to the other. Scott’s Cave was named after Captain Thomas Scott of the British 53rd Regiment who was stationed at Fort Mackinac in 1787 (Wood, 1918, p.591). Scott’s Road occurs along a flat land surface representing the action of glacial Lake Nipissing. Scott’s Road continues around the northern portion of Mackinac Island but remains relatively unscenic. Return to the Lake Shore Road and continue north around the island to British Landing. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 19 Stop 4: Pulpit Rock Figure 20: Pulpit Rock, a glacial Lake Nipissing shoreline feature on the northwest corner of Mackinac Island. This sea stack occurs 50 to 75 feet west of the wave-cut bench, representing beach erosion of unconsolidated material of glacial Lake Nipissing (Allen, 1891). Pulpit Rock (Figure 7), at present better known as Friendship Altar, is located a short distance northeast of British Landing along the western end of Scott’s Road. The vertical standing rock formation consists of Mackinac breccia standing approximately 10 feet in front of the bluff formed by glacial Lake Nipissing. While the most commonly used name for this rock unit is Friendship Altar, the outcrop is vertical standing like a pulpit and not horizontal lying as an altar would be. The author prefers the term “Pulpit Rock” because it best describes the outcrop shape or form. Wood (1918, p.536-537) mentions that both names, Pulpit Rock and Friendship Altar, have been applied to this outcropping near British Landing. But, he also suggests the term “Pulpit Rock” may have been applied to another exposure known as Vista Rock in the area of Sugar Loaf. Vista Rock, as indicated on the map of Wood (1918), is a poorly exposed outcrop that does not resemble a pulpit and, perhaps, is an outcrop of no particular note. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �20 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan Stop 5: Chimney Rock Continuing along the shoreline road for approximately two miles one passes numerous raised beaches of former glacial Lake Nipissing and of present-day Lake Huron (Figure 21). Bluffs and sea cliffs of the former glacial Lake Nipissing begin to appear south of Heriot Point (Wood, 1918). A prominent vertical rock formation, sometimes referred to as Sunset Rock, occurs approximately ¼ mile from where these bluffs begin to appear along the shoreline road. The name “Chimney Rock” is the original name of this shoreline exposure and the name “Sunset Rock” appears to be of recent origin, perhaps generated by whomever built the platform on the top of the formation. The name “Sunset Rock” appears on this platform outlined by dark pebbles set in concrete used in building the structure. The author uses the term “Chimney Rock” since it is of historical usage and is a very good description of the appearance of the exposure. This feature has only recently become part of the Mackinac State Historical Park holdings and accessible to the public. This was largely due to the efforts of David Armour, PhD, former deputy director of Mackinac State Historic Parks. Van Fleet (1870, p. 144; 1882, p. 24) and Wood (1918, p. 521) have cited Alexander Winchell, professor of geology at the University of MichiFigure 21: Lithograph of Chimney Rock, a glacial Lake Nipissing shoreline gan (Winchell, 1870) and a former state geologist feature. The viewing platform that has been constructed on top of this formation may have damaged the original profile as shown in this lithograph for the State of Michigan (Winchell, 1861), as de- (Woolson, 1894, p. 283). scribing this rock exposure as “one of the most remarkable masses of rock in this or any other state”. The author has failed to identify the original source for this comment. Chimney Rock is a promontory of Mackinac breccia developed by selective erosion and removal of less resistant limestone enveloping the more indurated breccia forming the vertical column of rock making up Chimney Rock by glacial Lake Nipissing. The breccia column remains attached to the headlands, so Chimney Rock is not a sea stack. The top of Chimney Rock was originally 131 feet above the level of Lake Huron (Van Fleet, 1870, p. 147, 1882, p. 24; Winchell, 1861, p. 210). It is unknown whether those that built the platform on Chimney Rock vandalized the upper portion of the chimney-like rock outcrop thereby altering the original profile. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 21 Stop 6: Devil’s Kitchen and Lover’s Leap 1 Continuing along the shoreline road for ½ to ¾ mile, one comes to the Devil’s Kitchen. Devil’s Kitchen represents an amphitheaterlike opening in a large outcrop of Mackinac breccia. This feature is the product of recent interaction of the shoreline of Mackinac Island and the waters of Lake Huron. Earlier erosion by glacial Lake Nipissing may have also had an influence. 4 3: I IL a w Lover’s Leap occurs on the wave-cut bluff behind and slightly west of Devil’s Kitchen. It remains private property and, thus, is unavailable for examination (Figure 22). Lover’s Leap is composed of Mackinac breccia and is comparable to Chimney Rock in appearance and origin. The top of Lover’s Leap is 145 feet above the present level of Lake Huron (Van Fleet, 1870, p. 147; 1882, p. 24; Winchell, 1861, p. 210). - Figure 22: Lithograph of Lover’s Leap, a glacial Lake Nipissing shoreline feature (Woolson, 1894, p. 288). I nner Island Tour Upon completion of the visit at Devil’s Kitchen, one continues back towards the village on Mackinac Island. As we approach the village, turn left onto Mahoney Ave. and continue to Cadotte Ave. Turn left on Cadotte Ave. and travel to Huron Road passed the Grand Hotel. Turn right on Huron Road and continue to the eastern side of Mackinac Island to continue an inner island tour dominated by features related to glacial Lake Algonguin. Huron Road passes along the top of east bluff that represents the effects of glacial Lake Nipissing. Along Huron Road, the first stop will be Robinson’s Folly (Stop 1) where participants can view Lake Huron from the top of the rock formation. From Robinson’s Folly, the group will continue to Arch Rock (Stop 2). The revisit of Stop 2 allows participants to view Arch Rock from above. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �22 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan Stop 7: Sugar Loaf From the revisit of Arch Rock, one continues into the interior of Mackinac Island along Rifle Range Road to Sugar Loaf Road (Figures 23 and 24). Continue along Sugar Loaf Road to Sugar Loaf. Sugar Loaf is a sea stack that is the product of glacial Lake Algonquin. This sea stack is composed of Mackinac breccia and is separated from the headlands of the former glacial Lake Algonquin by approximately 300 feet. The sea cliffs to the west are composed of breccia. The wooden stairs from the former terrace of glacial Lake Algonquin allow great access to the cliffs where the details of the breccia can be closely examined. The stairs continue to the top of the sea cliff known as Point Lookout where an excellent overall view of Sugar Loaf and Lake Huron is possible. The Lake Algonquin terrace around the Sugar Loaf is approximately 140-150 feet above the present Lake Huron. The top of the Sugar Loaf is 855.81 feet above sea level or 274.94 feet above the present level of Lake Huron (Stanley, 1945, p. 65, 72). The base of Sugar Loaf is approximately 134 feet above Lake Huron (Van Fleet, 1870, p. 20-21; 1882, p. 140). Wood (1918, p. 594-595) indicates the Sugar Loaf is 79 feet high using the road as a base. The conical shaped sea stack reportedly received its name from honey bees that constructed a hive in the formation (Wood, 1918, p. 595). t Figure 23: Lithograph of Sugar Loaf, a glacial Lake Algonquin shoreline feature. View is of the west side (Woolson, 1894, p. 287). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 23 Figure 24: Lithograph of Sugar Loaf, a glacial Lake Algonquin shoreline feature. View is of the east side (Woolson, 1894, p. 286). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �24 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan Stop 7: Skull Cave From Sugar Loaf, we continue along Sugar Loaf Road to Rifle Range Road and east to the junction of Rifle Range Road and Garrison Road. Skull Cave is a crumbling sea stack that has a large amphitheater-like opening. This feature is composed of Mackinac breccia and is the result of glacial Lake Algonquin. The sea stack has a summit of 831 feet above sea level, is between 15 and 25 feet high, and is separated from the headland of the former Lake Algonquin sea cliff and bluff by 50 feet (Stanley, 1945, p.26-27). Skull Cave is named after an experience by Alexander Henry, a fur trader who survived the massacre in 1763 of the British garrison at Fort Michilmackinac in Mackinaw City. Henry was sequestered in this cave by Wawatam, a Chippewa Indian, thus saving him from massacre at the Fort. Henry found the cave full of human bones and skulls and was very happy to leave his place of refuge at his earliest opportunity. It is unknown whether the bones are the result of burial or ceremonial sacrifice. Wood (1918, p. 592) stated that Alexander Henry was of the opinion that the bones were from prisoners devoured at war feasts, but there is no evidence as to which interpretation is most probable. Slight variations in this story occur in almost all discussions of Skull Cave. Stop 8: Crack-in-the-Island and Cave-of-the-Woods From Skull Cave, continue north along Garrison Road to State Road and turn left. Continue along this State Road until you reach Island Trail that leads from the road to the west. This trail is accessible with a trail bike but is very rough; caution is recommended. Crack-in-the-Island and Cave-of-the-Woods lie approximately ¼ mile west of State Road and at the edge of Mackinac Island Airport. They are separated by only a few tens of feet. Crack-in-the-Island is a solution crack in the limestone that, at this location, is on the order of one foot wide and 1-2 feet deep. Wood (1918, p. 523) described the crack as a deep fissure several feet wide, but the crack as seen today does not fit that description. Wood (1918) indicates these solution cracks occur in several other places on the island. Cave-in-the Woods is an amphitheater-shaped opening or sea cave in Mackinac breccia formed by glacial Lake Algonquin that sits approximately 140 feet above the level of present-day Lake Huron (Porter and Nelhiebel, 1984, p. 47). Upon completion of the examination of this site, return along the trail to State Road. Even though it is longer in distance, the easiest way back to town and the ferry docks is to continue north along State Road, which rejoins with Garrison Road and continues on towards British Landing. This route is down hill through the center of the island and, then, relatively flat along the perimeter of the island, Lake Shore Road. At British Landing, turn left on to Lake Shore Road and return to the ferry docks, a distance of several miles that passes previous stops illustrating glacial Lake Nipissing shore features. If time permits, it is suggested to tour Fort Mackinac, a Revolutionary War fort. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 25 Additional Suggestions The tour of Mackinac Island is designed to visit all prominent glacial-lake modified bedrock outcroppings on the island in a single day. At the visitor’s discretion and convenience, some outcroppings in and near the city of St. Ignace on the mainland of the Upper Peninsula are worth visiting. St. Anthony’s Rock occurs in a parkette behind the business establishments lining the main street of the city. This is a sea stack of Mackinac breccia formed by glacial Lake Nipissing. It sits several yards in front of a Lake Nipissing cut headland. Castle Rock located just north of St. Ignace is a commercial property that permits tourists of the area to obtain a view from its high vantage point. Castle Rock is a prominent promontory that remains attached to the glacial Lake Nipissing headlands and is, thus, not a true sea stack. Landes et. al. (1945, p. 136) classify Castle Rock as being an indurated transformational breccia, which lies immediately east of megabreccia that is regional in extent. Landes et al. (1945, p. 125) report that the best place to see the various styles of regional brecciation is from the road cut leading to the north entrance of the Mackinac Bridge. Landes et. al. (1945) provides the most complete description of the geology of Mackinac Island and the surrounding area. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �26 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 27 Bibliography Allen, E.A.; 1891. A Jolly trip; or, where we went and what we saw last summer; Central Publishing House, Atlanta, Georgia, 266p. Anonymous, 1899. The Standard Guide, Mackinac Island and Northern Lake Resorts; Foster and Reynolds, 88 p. Disturnell, J., 1875. Island of Mackinac; Philadelphia, 96 p. Dorr Jr., John A. and Eschman, Donald F., 1970. p. 176-217; in Geology of Michigan; University of Michigan Press, Ann Arbor, Michigan; 475 p. Foster, J. W. and Whitney, J.D., 1851, p. 163-166; in Report on the Geology of Lake Superior Land district; A. Boyd Hamilton, Washington D.C., 456p. with illustrations Landes, K. K., Ehlers, G. M. and Stanley, G. M., 1945. Geology of the Mackinac Straits Region and subsurface geology of Northern southern Peninsula; State of Michigan Department of Conservation, Geological Survey Division; pub. 44, Geological Serial 37, 204 p. Leverett, F. and Taylor, F.B., 1915. p. 452-453; in The Pleistocene of Indiana and Michigan and the History of the Great Lakes; United States Geological Survey Monograph, v. 53 McKenny, Thomas L., 1827. Sketches of a tour to the Lakes; Ross and Haines Inc., Minneapolis, Minnesota, 1959, 493 p. Meade, William C., 1897[1986]. Early Mackinac. A sketch Historical and Descriptive with Introductory essay by Larry Massie; Republished 1986 by Avery Color Studios, Au Train, Michigan, 184 p. NOTE: Early Mackinac was first published in 1897 and then revised and published in 1901 and 1912 Porter, Phil and Nelhiebel, Victor, R., 1984. The Wonder of Mackinac, Mackinac Island State Park Commission, Pendall Printing Inc., 52 p. Russell, Israel C., 1905, p. 44-45, 55-57, 102-104; in A geological reconnaissance along the north shore of Lakes Huron and Michigan; Report of the State Board of Geological Survey of Michigan for the year 1904, Wynkoop Hallenbeck Crawford Co., Lansing, Michigan Sheldon, Frances D., 1959. Geology of Mackinac Island and lower and middle Devonian south of the Straits of Mackinac, Michigan; Michigan Basin Geological Society Guide Book; 63 p. Stanley, G.M., 1938. The submerged Valley through Mackinac Straits; Journal of Geology, v. 46, n. 7, p. 966-974 Stanley, George M., 1945. Pre-Historic Mackinac Island; State of Michigan Department of Conservation, Geological Survey Division, Publication 43, Geological Series 36, 74p. Taylor, F.B., 1892. The highest old shoreline on Mackinac Island; American Journal of Science, v. 43, p. 210-218 Taylor, Frank B., 1915. Old shorelines of Mackinac Island and their relations to the Lake History; Geological Society America Bulletin (abst), v. 26, p. 68-70 Van Fleet, J. A., 1870. Old and New Mackinac; Courier Steam Printing-house, Ann Arbor; 176 p. Van Fleet, J. A.; 1882. Mackinaw Region and Adjacent Localities; Lever Print, Detroit, Michigan, 49 p. Winchell, A., 1861. First Biennial Report of the Progress of the Geological Survey of Michigan Geology, Zoology and Botany of the lower Peninsula; Geological survey of Michigan, 339 p. Winchell, Alexander, 1870. p. 247-245; in Sketches of Creation; Harper and Brothers, New York, 459 p. Wood, Edwin O., 1918. Historic Mackinac; The Historical, Picturesque and Legendary Features of the Mackinac Country; The MacMillan Company, New York, 540 p. Woolson, Constance F., 1894. Mackinac, p. 279-291; in Picturesque America, No. 5, March 17, 1894, (previously published 1872), Artist F. T. Woodward; D. Appleton, Publishers PDF compression, OCR, web-optimization with CVISION's PdfCompressor �28 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan References not cited Schoolcraft, Henry R. 1832 [1953]. Narrative Journal of travels through the northwestern regions of the United States extending from Detroit through the great chain of American lakes to the sources of the Mississippi River in the year 1820; Republished Michigan State College Press, 1953, edited by Mentor L. Williams, 520 p. Strickland, W.P. 1860. Old Mackinac or the Fortress of the Lakes; James Challen and Son, Philadelphia, 404 p. PDF compression, OCR, web-optimization with CVISION's PdfCompressor � https://digitalcollections.lakeheadu.ca/files/original/0c4e5c4f4cf1d00d709139b4dc0e22f7.pdf 28c0b52caa344bdb859324c05ee86abc PDF Text Text Unusual -bearing Rocks of Unusual Diamond Diamond-bearing the Wawa Area - - . : - £- a — L . Institute on Lake Superior Geology 52nd Annual Meeting Sault Ste Marie, Ontario Volume Part 3 – Field Trip Guidebook PDF compression, OCR,52web-optimization with CVISION's PdfCompressor �Unusual Archean Diamond-bearing rocks of the Wawa Area by A. C. Wilson Ministry of Northern Development and Mines, Resident Geologist’s Program, Ontario Geological Survey, Timmins, Ontario On the cover (clockwise from top): Giant lower crustal to upper mantle xenoliths, Enigma Property, Menzies Township, Oasis Diamond Corporation Inc.; Diamonds from the Festival Property (photo courtesy of Pele Mountain Resources Inc.); Sandor Diamond Occurrence, Highway 17, Spider Resources Inc. & KWG Resources Inc.; Heterolithic diamond-bearing breccia, Engagement Zone, Northern Sierra Minerals Corporation PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Table of Contents Introduction .................................................................................................................................................... 3 Geological Overview of the Michipicoten Greenstone Belt........................................................................... 3 Quaternary Geology ....................................................................................................................................... 6 Structural Geology of the Diamond-bearing Rocks ....................................................................................... 6 Description of the Diamond-bearing Rocks ................................................................................................... 7 Relationship between diamond content and lithology .................................................................................. 10 Geochronology ............................................................................................................................................. 11 Geochemistry of the Diamond-bearing Rocks ............................................................................................. 11 Diamond characteristics ............................................................................................................................... 11 Origin of the Diamond Deposits................................................................................................................... 12 Field Trip Road Log ..................................................................................................................................... 14 STOP 1 - GQ Diamond Discovery Site ................................................................................................... 14 STOP 2 - Northern Sierra Minerals Corporation Area B......................................................................... 17 STOP 3 - Northern Sierra Minerals Corporation Engagement Zone ....................................................... 21 STOP 4: - Moet Occurrence, Festival Property ....................................................................................... 23 STOP 5: - Sandor Diamond Occurrence.................................................................................................. 25 STOP 6: - Dubreuilville Dike - Xenolith-rich lamprophyre .................................................................... 27 STOP 7: - Monchiquite Dike ................................................................................................................... 28 STOP 8: - Contemplation of the rocks on the fireplace at the Wawa Motor Hotel ................................. 28 Bibliography................................................................................................................................................. 29 Figures 1. Generalized geological map of the Michpicoten greenstone belt ............................................................. 4 2. Composite structural section through the central part of the Michipicoten greenstone belt. .................... 7 3. Detailed geology of the southwestern corner of the Festival Property ..................................................... 8 4. Geological compilation of the GQ Property ........................................................................................... 15 5. Occurrences of diamondiferous bedrock on the GQ Property................................................................ 16 6. Drill hole sections GQ-00-01, GQ-00-02 and GQ-00-03, GQ Property.................................................. 18 7. Northern Sierra Minerals Corporation Area B ........................................................................................ 20 8. Geology and sample locations at the Engagement Zone ......................................................................... 22 9. Simplified cross section through the Engagement Zone looking northwest........................................... 23 10. Detailed geology of the Moet Occurrence.............................................................................................. 24 10. Detailed geology of the Sandor Occurrence ........................................................................................... 26 11. Generalized geology of the Wawa Project ............................................................................................. 26 12. Lower crustal to upper mantle xenolith, Dubreuilville dike ................................................................... 27 Tables 1. Diamond recovery results from 2000 Band-Ore Resources Ltd. diamond-drilling program ................. 17 2. Diamond results from Engagement Zone bulk samples (2001). ............................................................. 21 3. Summary of the diamond results from the 2001-02 sampling of the Moet Occurrence......................... 25 2 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Unusual Diamond-bearing rocks of the Wawa Area Introduction In 1991, local prospector C. “Mickey” Clement recovered at least three alluvial diamonds from the Michipicoten River. Two of the stones were sent to the Department of Mineralogy, Royal Ontario Museum, where they were identified as industrial-grade diamonds with weights of 1.05 and 1.13 carats. Both stones were described as frosted and graphite-inclusion riddled. In 1995, prospector Sandor Surmacz and geologist Marcelle Hauseux recovered diamonds from a bedrock occurrence on the Trans Canada Highway, approximately 20 km north of the town of Wawa. An 18.1-kg bulk sample of a xenolith-rich lamprophyre yielded 1 macrodiamond and 5 microdiamonds. All but one was gem quality. Since then, more than 50 occurrences of diamondiferous bedrock have been reported in an area of approximately 30 km2 in size, centred approximately 20 km north of the town of Wawa. The occurrences are hosted within a sequence of unusual, Archean-aged, heterolithic breccias. Historically, this sequence of rocks has received little exploration interest and was considered to have little economic significance. This field trip will focus on exposures of a series of foliated lamprophyre dikes and associated heterolithic breccias outcropping in Lalibert, Leclaire, Menzies and Musquash townships. This field trip guide represents a summary of information available at the time of writing and should not be considered the final analysis of these rock types. Much more research is required on these rocks. Active exploration and research is still underway on the properties included in this field guide. Given the limitations of time, the field trip will visit only some of the more accessible properties. Bear in mind that when visiting active exploration or mine properties, permission must be granted by the property owner. Current ownership information can be obtained from the Resident Geologist’s Office in Timmins, or the District Geologist’s Office in Sault Ste Marie, Ontario. Geological Overview of the Michipicoten Greenstone Belt The Wawa region lies within the Wawa subprovince of the Canadian Shield. The Michipicoten greenstone belt extends inland for approximately 150 km from the Lake Superior shore and has an average width of 38 km. The greenstone belt consists of supracrustal rocks of Archean age. Younger Archean granitic rocks surround the greenstone belt. Figure 1 shows a generalized geological map of the Michipicoten greenstone belt. The oldest volcanic cycle is approximately 2900 Ma and is of limited distribution. This cycle is best developed in Esquega Township along the southern flank of the supracrustal assemblage. Portions of this metavolcanic cycle extend into eastern McMurray and western Lastheels townships. The base of this volcanic cycle consists of massive to pillowed komatiitic flows intruded by mafic sills. The ultramafic rocks are overlain by intermediate to felsic tuffs capped by thinly bedded chert-magnetite-sulphide iron formation (Judith-Kathleen Iron Formation). Intermediate to felsic metavolcanic tuffs below the Judith iron formation have been dated at 2889 ± 9 Ma (Turek et al. 1992). Overlying the 2900 Ma cycle is a 2750 Ma volcanic cycle. This volcanic cycle is predominately composed of intermediate to felsic tuffs, breccias and flows. Porphyritic and spherulitic flows are not common and most of the intermediate to felsic metavolcanic rock is fragmental. The base of this cycle consists of high magnesium and high iron tholeiitic massive and pillowed flows. It lies conformably atop the JudithKathleen iron formation at the east end of Wawa Lake, but the basal unit is poorly exposed elsewhere. Overlying the mafic metavolcanic rocks is a sequence of heterolithic, intermediate to mafic breccia that has been traced for a distance of over 13 kilometers. This unit is in turn, overlain by a thick section of intermediate to felsic tuffs, breccias and massive flows that reaches a maximum thickness of approximately 2000 m below the Helen Iron Formation. The upper part of the intermediate to felsic metavolcanic rocks has been dated at 2749 ± 2 Ma (Turek et al. 1992). The Michipicoten (Helen) iron formation caps this 3 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �— 10 I —— Lake Superioi Creek Fault an I I I I . \ Diamond Occurr ences in Archean Heterol ithic Breccia Faults Mafic to fic Intrusive Rocks Rocks I Chemical Clastic Metasedi mentary Rocks I Felsic Metavolca nic Rocks Mafic Metavolcai riic Rocks I Carbonatite Intr usive Rocks Alkalic Intrusion S Sanukitoid Suite Intrusions I Granitoids Figure 1. Generalized geological map of the Michipicoten greenstone belt showing some of the diamond occurrences (modified after Stott et al. 2002). N ickenson Lake Stock 1J Lalibert Lec / Discussed in 1 Enigma Prop erty 2 Sandor Occu rrence 3 Cristal Occur rence 4 Engagement Zone 5 GQ Occurren ce 6 Leadbetter P roperty Diamond Pro pe1 rties \\\\ PDF compression, OCR, web-optimization with CVISION's PdfCompressor �this volcanic cycle. It commonly exceeds 100 m in thickness and, in the vicinity of the past-producing Helen Mine; it has been tectonically thickened to over 300 m. This iron formation was the source of virtually all commercial iron ore production in the Wawa area from 1898 to 1998. The youngest metavolcanic rocks in the area are those of the 2700 Ma volcanic cycle. These rocks underlie approximately fifty-percent of the Michipicoten greenstone belt and are found in the central and northern parts of the belt. The basal unit is composed of massive and pillowed mafic amygdaloidal flows that attain a maximum thickness of 5.5 km in the Goudreau area. This unit is overlain by intermediate to felsic metavolcanic rocks, or their stratigraphic equivalent, the Doré metasedimentary rocks. These metavolcanic rocks are typically composed of tuffs and monolithic and heterolithic breccias. Quartz + feldspar crystal tuff and an intermediate tuff from this volcanic cycle returned a U-Pb zircon age of 2701 ± 8 Ma (Turek et al. 1992) and 2701.4 ± 2.1 Ma (Ayer et al. 2003). The intermediate to felsic tuffs interdigitate with clastic metasedimentary sequences that include cross-bedded sandstone and a tonalite cobble conglomerate (Doré conglomerate). Corfu and Sage (1987, 1992) reported an age of 2698 ± 2 Ma for a tonalite clast in the Doré conglomerate and maximum ages of 2680 ± 3 and 2682 ± 3 Ma for sedimentary sequences in northern and central parts of the Michipicoten greenstone belt. Geochronological and structural evidence indicates that sedimentation continued after cycle 3 volcanism and predated a major folding and faulting event. Arias (1996) noted that the rocks comprising cycle 3 in the central part of the Michipicoten greenstone belt are upside down and represent the overturned limb of a belt-scale recumbent nappe fold. This inverted limb has been refolded and imbricated by subsequent southverging thrust faults, which caused local repetition of the stratigraphic sequence (Wilson 2004). Felsic plutonism occurred synchronous with all of the major volcanic cycles and continued after volcanism ceased at Wawa. Plutonic rocks associated with cycle 1 volcanism include the Murray-Algoma porphyry (2881± 6 Ma) and the Regnery biotite granite of the Hawk Lake granitic complex (2888 ± 2 Ma). Both intrusions are situated in Esquega Township. The Jubilee granitic stock, located in McMurray Township, was dated at 2745 ± 3 Ma and is coeval with cycle 2 volcanism. Plutons associated with cycle 3 volcanism range in composition from tonalite through granodiorite and granite and have ages ranging from 2698 to 2693 Ma. These plutons are located south and west of the Michipicoten greenstone belt (Stone and Semenyna 2004). The Kapuskasing Structural Zone extends east from the shore of Lake Superior, northeast through Kapuskasing and into the Hudson Bay Lowland. Local features interpreted to be associated with it include northeast-striking Proterozoic lamprophyre dikes (Sage 1994; Morris 1999). Lamprophyre dikes of middle Proterozoic age are common in the region south of the Wawa-HawkManitowik Lake Fault and rare to non-existent north of the fault. They are carbonate, biotite, and sometimes olivine-rich and usually less than 1.0 m in width. The dikes generally strike northeast. Dikes exposed in McMurray Township commonly have blue to blue-green sodic amphibole developed in their wall rocks. This mineral has been interpreted to be a product of fenitization. These dikes are likely spatially and temporally related to the emplacement of the Keewenawan-age Firesand River Carbonatite (Sage 1994). North of the Wawa-Hawk-Manitowik Lake Fault, in the area extending west from the former Magpie Mine (Leclaire Township), to the east side of the Dickenson Lake Stock (Lalibert Township), there are a series of what appear to be narrow, biotite-amphibole-rich dikes that have been interpreted as Archean lamprophyres. These dikes commonly have large, rounded inclusions (lower crust to upper mantle-derived xenoliths) up to 3.0 m in size, the centres of which are often completely altered to radiating clusters of actinolite crystals. Titanite from the matrix of one of these dikes returned an age of 2703 ± 42 Ma (Sage 2000). The date is interpreted to be a minimum age of intrusion. Subsequent dating of a zircon from a gneissic xenolith from the same dike returned an age of 2684.9 ± 1.4 Ma (Ketchum, Kamo and Davis 2003). 5 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Several narrow (<10 m) kimberlite dikes also intrude the Michipicoten greenstone belt. These dikes are largely restricted to the eastern part of the greenstone belt and are spatially related to the northeast-trending Kapuskasing Structural Zone. Several of these dikes have been dated. Intrusions K115 and K121, located in Isaac Township, returned an average 87Rb/86Sr age of 1097 ± 7 Ma (Kaminsky et al. 2002). A kimberlite dike intersected in Pele Mountain Resources Inc.’s drill hole 97-34, drilled in Riggs Township, returned a 207 Pb/206Pb age of 1197 ± 24 Ma (R. P. Sage, Ontario Geological Survey, unpublished, 2000; Wilson 2004). Quaternary Geology All Quaternary deposits within the Wawa area were deposited during the Late Wisconsin by the Labrador sector of the Laurentide Ice Sheet. Peat, recovered from a bog located within the surface of a terrace associated with the highest glacial lake in the Lake Superior basin, was radiocarbon dated at 9759 ± 170 BP. A caribou antler, recovered from near the surface of the delta of glacial lake Minong III, yielded a radiocarbon date of 8820 ± 80 BP (Morris 2001). Bedrock striae indicate that there were two prominent ice flow directions. The oldest and most pervasive ice flow was south to southwest (159° - 240°). A later, weaker ice flow was to the southwest and west (220° - 290°). The younger set of striae was formed during the latter stages of glaciation as the ice sheet began to thin and bedrock topography began to influence the direction of ice flow. Much of the overburden consists of a thin (< 1 m), discontinuous till veneer draped over the bedrock. Most of the area's thicker surficial deposits are located within bedrock controlled valleys where glaciolacustrine waters from the Lake Superior basin covered the area. In several of these valleys, the glaciofluvial material reaches thicknesses up to 32 m (Morris 2001). Structural Geology of the Diamond-bearing Rocks The greatest concentration of diamond-bearing rocks in the Michipicoten greenstone belt is constrained to a roughly 30 km2 block of land coinciding with a D1 recumbent nappe identified by Arias (1996). The nappe lies immediately north and adjacent to the Kapuskasing Structural Zone and has affected the two youngest rock sequences in the greenstone belt. In cross section, a generalized structural section across the greenstone belt would be upside down and represents the overturned limb of a belt-scale recumbent fold. This inverted limb of the nappe fold has been refolded and imbricated by subsequent south-verging thrust faults. The result is that the geometry of the rocks hosting the diamondiferous bedrock is that of an inverted anticline (Arias and Helmsteadt 1990). The effect is to create a tectonic repetition of the diamond-bearing sequence of rocks. In simplified terms, the rocks hosting the diamonds in the Wawa area can be considered as a single, overturned fold limb that has been faulted along a series of thrust zones. As a result, the diamond-bearing breccias, the lamprophyres associated with them and the surface onto which these metavolcanic rocks were deposited has been repeated at least four times (Walker 2002). The western limit of the nappe is likely the Dickenson Lake fault that passes along the west side of the Dickenson Lake stock. Extrusive lithologies similar to those hosting diamonds were recognized to the east of the stock by Sage (1993) during reconnaissance mapping. To date, no diamondiferous occurrences have been discovered to the west of the Dickenson lake fault. The eastern limit of the nappe is the Marsden Lake fault. Prior to 2002, rocks favourable to hosting diamonds had not been observed east of this structure (Wilson 2004). However, in 2002, Oasis Diamond Exploration Inc. made a discovery of diamonds on the east shore of the Magpie River and in 2004, diamondiferous occurrences of bedrock were found on the west shore of the Magpie River in Chabanel Township. These discoveries suggest that the potential for these host rocks extends farther eastward than previously believed. Figure 2 shows a graphic representation of the nappe structure. 6 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �500 F oslon Figure 2. Composite structural section through the central part of the Michipicoten greenstone belt. Section X-Y from south-central Corbiere Tp. (Josephine Iron Range) to Andre Lake central Corbiere Tp. Section Y-Z is a schematic section from west central Lalibert Tp. The sketch in the lower left explains the present configuration of the belt as a regional nappe fold (F1) refolded about F2. Imbricate thrusts are considered related to Fs (Arias 1996). Description of the Diamond-bearing Rocks The diamond-bearing breccia and associated lamprophyre is broadly distributed throughout Lalibert, Leclaire, Menzies and Musquash townships. On-going mapping by Pele Mountain Resources Inc. on the Festival property and by Nathalie Lefebvre on the GQ Property has helped to refine the classification of these rocks. Systematic exploration and sampling suggest that the individual diamond occurrences are part of a much larger suite of rocks and that diamonds occur primarily within discrete layers at the base of diamond-bearing zones. North-northwest-trending diamond-bearing zones of breccia and lamprophyre are up to 1500 m in length and up to 800 m in width (Pele Mountain Resources Inc., press release, January 18, 2005). The breccia forms thick units (maximum true thickness is approximately 110 m) dipping to the northeast 30°. The lateral extent and thickness of the breccia unit is not well constrained, owing to the large-scale regional folding and thrusting (Lefebvre 2004). Figure 3 provides a detailed map of the southwest corner of the Festival Property showing a recent interpretation of these diamond-bearing zones. The diamond-bearing rocks can be visually subdivided into two classes, lamprophyre (dikes and bodies of indeterminate morphology) and heterolithic or polymict breccias. It is often difficult to differentiate between the two classes since the lamprophyre dikes frequently contain an assortment of inclusions that 7 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �8 PDF compression, OCR, web-optimization with CVISION's PdfCompressor Genesis West II I Geri Mumm ---: I I Rosigtj Bollinger Lake Perth Per Dom .Perlgnon Musquash Tp. Leclaire Tp. it East Perch Salon Salon Cristal Taft • Other • iyric Suite I km • South Zor • North Zor Leger * Diamond I Occurrences F' Figure 3. Detailed geology of the southwestern corner of the Festival Property (modified from Pele Mountain Resources, Inc. press release, January 18, 2005). The units shown in green consist of mafic to intermediate massive and pillowed flows, breccias and tuffs (Vaillancourt 2005b, Sage et al. 1982) Veuv e iot Menzies Tp. —— — — — I_ �give them the appearance of breccia. The lamprophyre dikes cut the breccia units. Both lithologies have been metamorphosed to upper greenschist facies. The breccia primarily consists of angular, pebble-sized, lithic fragments, mainly of volcanic composition, contained within a green to grey fine-grained matrix. The matrix grain size ranges from < 2 mm to 1 mm. At least eleven distinctive types of lithic fragments have been observed in the breccia and the fragments are irregularly distributed throughout the breccia. The clast population is primarily derived from rocks with which the breccia is intercalated. Most typically these clasts are mafic and felsic metavolcanic rocks and intermediate to mafic intrusive rocks. Other clast types include fragments of clast-supported breccia within matrix supported breccia, fragments of earlier matrix-supported breccia with fewer than 5% fragments and coated lithic fragments (Lefebvre 2004). The breccia is characteristically massive, unstratified and poorly sorted with clast size ranging from sand to boulders up to 9 m. Primary sedimentary structures such as bedding and crude grading are rare (Lefebvre 2004). Petrographic work by Lefebvre (2004) on the breccias identified a typical fragmental texture within a mineralogically variable matrix. Typically the groundmass is dominated by actinolite, but chlorite and biotite dominated groundmass are locally predominant. Juvenile magmatic material also was observed as discrete fragments and rims on other clasts in the breccia. Its petrography is distinct from the breccia groundmass. The juvenile magmatic material contains more abundant actinolite grains; fewer epidote and fine-grained plagioclase grains and more fine-grained oligoclase and muscovite; and more oscillatoryzoned hornblende. Most exploration efforts over the past few years have concentrated on the breccias because they are considered to have the best potential for hosting commercial diamond deposits. Over the course of the last few years, explorationists have subdivided the diamond-bearing breccias into three separate facies. These facies are volcanic (pyroclastic), subvolcanic/intrusive breccia and hypabyssal facies. A variable, but distinctive proportion and composition of fragments and/or xenoliths characterizes each facies (Wilson 2004). The volcanic facies contains breccia, lapilli- and ash sized fragments and consists of medium to thickly bedded pyroclastic air-fall deposits. They are characterized by angular to sub-angular Archean supracrustal fragments, some hypabyssal fragments and rare lower crustal to upper mantle xenoliths. The subvolcanic/intrusive breccia facies are the most variable in texture and appearance. The rock is characterized by observed intrusive relationships, a high proportion of fragments and a close proximity to the volcanic facies. The fragment characterization is variable. The facies can contain all or some of the following fragment types: supracrustal fragments, crustal fragments and lower crustal to upper mantle xenoliths. This facies was once included with the lamprophyres but is now interpreted by various exploration companies as a debris flow. The hypabyssal facies hosts variable proportions (<25%), of sub-rounded to rounded mantle xenoliths, as well as minor proportions of gneiss and/or trondhjemite fragments. This facies also was originally classified as lamprophyre but is now considered to be part of the debris flows (Lefebvre et al. 2003). The brecciated unit(s) could also be a single or multiple diatreme(s) localized within Lalibert, Leclaire, Menzies and Musquash townships. A tectonic repetition of the diamond-bearing diatreme(s) has been achieved through the regional deformation events described previously. This regional deformation may help to explain the layered appearance observed at some of the diamond occurrences (J. Ayer, Ontario Geological Survey, personal communication 2006). Lamprophyre occurs as narrow dike-like intrusions or in bodies of indeterminate morphology cross-cutting or intercalated with local country rocks. Dikes range in width from 50 cm to 2 m and may display 2-3 cm offshoots in some locations. Contacts with the host rocks vary from sharp and straight to highly irregular. No variation in grain size within the dikes has been observed and the no variation in colour or mineralogy 9 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �has been observed within country rock adjacent to the dikes. The lamprophyre is fine-grained, grey in colour and contains approximately 5-10% subrounded to subangular fragments. The fragment population is dominated by actinolite-rich monomineralic rocks, or by biotite-rich greenstone and hornblende-rich ultramafic rocks (Lefebvre 2004). Lamprophyre is differentiated from the breccia by: a lower clast content; a predominance of highly altered, coarse-grained actinolite fragments; scarcity of wall rock fragments; the rounded shape of the xenoliths; and the presence of a weaker fabric (Lefebvre 2004). The lamprophyre dikes post-date all other lithologies. The lamprophyre with indeterminate morphology predates most of the host lithologies since fragments of each are found within these lamprophyres. However, in some locations, lamprophyre fragments have been observed within intermediate and mafic intrusive rocks near the contact with lamprophyre (Lefebvre 2004). Fragments within lamprophyre dikes commonly have a biotite-rich rim enclosing the xenoliths and the fragments found in the lamprophyres of indeterminate morphology do not. The lamprophyres of indeterminate morphology also show positive relief of less severely weathered xenoliths more so than the lamprophyre dikes. Lastly, the lamprophyre dikes have a less variable fragment lithology (Lefebvre 2004). The lamprophyre is petrographically distinct from the breccia. It contains a lower abundance of clasts and fewer clast types. Unlike the breccia, the lamprophyre contains no juvenile magmatic material and oscillatory-zoned hornblende grains are rare. Detailed petrographic descriptions of the lamprophyre can be found in Lefebvre (2004). Relationship between diamond content and lithology Microdiamonds have been recovered from a wide array of breccias and lamprophyres in the Wawa area. Between 2003 and 2004, the Ontario Geological Survey (OGS) investigated a number of diamond occurrences during a mapping program conducted in Menzies and Musquash townships. Closer look at locations where bulk samples had been collected suggested that the bulk samples probably included more than one rock type. Diamonds were recovered from each of these bulk sample sites, but it would be difficult to establish from which of the rock types the diamonds were recovered. The OGS undertook a limited sampling program to further investigate the diamond content of specific lithologies (Vaillancourt et al. 2005a). The OGS collected three small samples (95.21 kg total weight) from three different lithological units at the Cristal and Genesis diamond occurrences. These occurrences are on the Festival Diamond Property owned by Pele Mountain Resources Inc. Results from this limited sampling program reinforce the observation that microdiamonds are not restricted to a single unit. Microdiamonds were recovered from heterolithic breccia, both with and without ultramafic magma pockets, and from a fragment-free ultramafic dike. There is the possibility, however, that the diamonds recovered from the ultramafic dike are xenocrysts derived from diamond-bearing host breccia (Vaillancourt et al. 2005a). The OGS concluded that the results from only three samples are not sufficient to draw irrefutable conclusions regarding the location of the microdiamonds. Collection and analysis of more, well constrained samples is necessary to further refine the diamond potential of specific host rocks. In 2005, Spider Resources Inc. and KWG Resources Inc. conducted a similar bedrock sampling program to investigate the diamond-bearing potential of specific lithologies. Separate representative samples (approximate weight 16 kg each) of the matrix, xenoliths and run of mill (ROM) portions of the bedrock were collected from the Wawa Diamond Project and were sent for caustic dissolution. The results are as follows: matrix sample returned 67 diamonds (0.008 total ct), xenolith sample returned 244 diamonds (0.051 total ct) and the ROM sample returned 86 diamonds (0.006 total ct) (Spider Resources Inc., press release, February 20, 2006). 10 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Geochronology In conjunction with recent geological mapping in the area, the Ontario Geological Survey has been conducting geochronological work to help understand the nature and timing of the diamondiferous units and their host rocks within the Michipicoten greenstone belt. A felsic volcanic horizon hosting diamondiferous units returned a 207Pb/206Pb age of 2701.4 ± 2.1 Ma. Maximum 207Pb/206Pb ages of 2685.1 ± 1.0 Ma and 2684.9 ± 1.4 Ma have been returned for diamondiferous lamprophyre dikes cutting the Catfish assemblage (2.7 Ga) intermediate to felsic metavolcanic rocks in Lalibert and Menzies townships (Ayer et al. 2003). A second sample of felsic lapilli tuff, part of the Catfish assemblage, adjacent to the Moet Occurrence contains zircons that returned a 207Pb/206Pb age of 2698.7 ± 1.1 Ma (Vaillancourt et al. 2004). A sample was collected from the diamondiferous breccia at the Moet Occurrence in order to determine the age of brecciation. Five zircons were analyzed. The three oldest ages are 2687 ± 2 Ma, 2683 ± 2 Ma and 2681 ± 2 Ma. The two youngest zircons cave results that precisely overlapped one another at 2679.2 ± 2.1 Ma (Vaillancourt et al. 2004). Since this is the youngest zircon age obtained from the breccia, it represents either the time of cystallization or emplacement of the body if the zircons are magmatic, or a maximum time of emplacement if the zircons are xenocrystic (Vaillancourt et al. 2005a). If the zircons are xenocrystic, this age must still be close to the time of breccia emplacement since a lamprophyre dike from the GQ property returned a date of 2673 ± 8 Ma from titanite (R. P. Sage, Ontario Geological Survey, unpublished 2000). The data indicate that the felsic metavolcanic rocks hosting the diamondiferous breccias are part of the Catfish assemblage. The maximum age for the diamondiferous breccias and the associated dikes is less than 2680 Ma. These absolute age constraints indicate that the breccias are not volcaniclastic units belonging to the Catfish assemblage (Vaillancourt et al. 2005a). Zircons from a sample of felsic lapilli tuff from northwestern Menzies township returned an age of 2736.0 ± 0.8 Ma which is taken to represent the age of eruption and crystallization of the tuff. This age clearly indicates that the volcanic package underlying the iron formation in the western part of Menzies Township is part of the Wawa assemblage (2.75 Ga) and brackets the uppermost part of the assemblage at 2736 Ma (Vaillancourt et al. 2005a). Geochemistry of the Diamond-bearing Rocks Both Williams (2002) and Lefebvre (2004) conclude that the whole rock major element geochemistry is consistent with a calc-alkaline classification for both the lamprophyres and the associated breccia. Both authors also noted that the compositions of chromite in the Wawa metavolcanic rocks are in the range typical for lamprophyres and dissimilar to those in kimberlites and lamproites. Whole rock geochemistry for the diamond-bearing rocks is tabulated in Sage (2000), Williams (2002), Lefebvre (2004), Stone and Semenyna (2004) and Vaillancourt et al. (2005c). Sage and Williams’ work is specific to the diamond-bearing and non diamond-bearing lamprophyres. Work by the other authors relates to both the breccia and the lamprophyres. Whole rock geochemistry for kimberlites of the Wawa area can be found in Kaminsky et al. (2002). Diamond characteristics Lefebvre (2004) undertook a study examining a parcel of 80 macrodiamonds recovered from the volcaniclastic breccia on the GQ Property. Results from this work are summarized below and in De Stefano et al. (2006). Additional work on the morphology of the Wawa diamonds can be found in Stone and Semenyna (2004). Stachel et al. (2004) summarize results of analysis conducted on diamonds from the Genesis and Cristal diamond occurrences presently held by Pele Mountain Resources Inc. 11 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Shape: The diamonds display a highly variable primary growth form. Most of the diamonds are either octahedral aggregates (44% of the population) or single octahedral crystals (26%). Single cubic and cubicoctahedral crystals and their aggregates, as well as macles form the remainder of the population. Fortyeight percent of the diamond population is single crystals and only 28 diamonds could be evaluated for crystal regularity. The majority of the diamonds also display some degree of distortion. Colour and transparency: The diamonds included in this study are classified into colourless, brown, grey, black, yellow and white. No pink, green, violet or blue diamonds were observed. The colour distribution within the population is: colourless (48%), heterogeneous (24%), yellow (11%), black (3%), brown (10%) and grey (3%). The heterogeneity in colour is observed only in aggregates. The diamond population consists of 48% transparent crystals, 25% translucent crystals, 14% opaque crystals and 14% is a combination of opaque and translucent crystals. Transparent crystals are typically colourless and also comprise a few yellow octahedral single crystal and coarse aggregates as well as macles. Translucent crystals comprise all possible primary crystal forms and colours. Opaque crystals are mostly fine-grained aggregates which have black body colouring. Resorption: Generally speaking, the diamonds have experienced low degrees of resorption. Only 21% of the diamond population displays extensive resorption. Some crystals (14%) exhibit non-uniform resorption where one part of the crystal is more strongly resorbed than another. Inclusions: Mineral inclusions were identified in 58% of the diamonds. Both primary and secondary inclusions were observed. The mineralogy of the recovered primary inclusions is listed in descending order of abundance: olivine (Fo92 and Fo89), clinopyroxene (omphacite), plagioclase (albite and An-rich), orthopyroxene (En93) and Fe-Ni sulphide (pentlandite). Cathodoluminescence: The relative abundances of cathodoluminscence (CL) colours for the Wawa diamonds are: orange-red (46%), yellow (28%), orange-green (10%), green (6%), and other non-uniform colours (10%). None of the 69 diamond examined displayed the more common blue CL. Impurities: Fournier Transform Infrared (FTR) spectrometry was used to investigate the nitrogen and aggregation states for 41 diamonds. The majority of the diamonds have low nitrogen contents, < 300 ppm. The diamonds show two modes of nitrogen aggregation suggesting mantle storage at 1100 - 1170° C. Diamonds from the Genesis occurrence are almost exclusively cubes including some fragmented, twinned and moderately resorbed cubes. Most of the crystals contain clouds. Fully transparent stones are dominantly brown although colourless stones also are common; one diamond was yellow in colour. Nitrogen concentrations range from below detection (<10 ppm) to 600 atomic ppm. Nitrogen aggregation is very low (Stachel et al. 2004). Diamonds from the Cristal occurrence range from un-resorbed octahedra to highly resorbed dodecahedra. Octahedral and weakly resorbed octahedral stones dominate the population. About 25% of the population are irregular crystals, macles are common (15%) and about 5% of the diamonds show cubo-octahedral growth. The stones fall into two dominant colour classifications, colourless and a range of brown colouration. Nitrogen contents range from <10 – 560 ppm, but with only one exception nitrogen is ≤170 ppm. Nitrogen aggregation varies between 0 and 97% B-centre. Olivine is the most common mineral inclusion, followed by pyrope garnet and Mg-chromite (Stachel et al. 2004). Origin of the Diamond Deposits Based on published data on the diamond-bearing rocks at Wawa and Cobalt, Wyman et al. (in press) suggest that the tectonic setting of the deposits and nature of the host rocks indicate that the diamonds may be derived from the asthenospheric wedge and subducted slab at shallow depths (100 – 160 km) rather than the deep keels of Archean cratons associated with traditional diamond deposit types. Models of lowtemperature Phanerozoic diamond formation in active subduction zones, or rapid uplift and emplacement of peridotite massif occurrences, can be adapted to the Archean deposits. The stability field of 12 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �diamonds in most Phanerozoic scenarios may be too deep to be accessed by the lamprophyric magmas. Shallow subduction, as proposed for these occurrences of adakitic-type rocks in the Wawa subprovince, could generate two different diamond stability windows at sufficiently shallow depths to account for their presence in lamprophyric magmas. Wyman et al. (in press) states that any tectonic model for these Archean diamond occurrences must address several requirements. These requirements include 1. a deep source for oxidized metasomatic fluids that is activated prior to lamprophyre emplacement 2. a mechanism to isolate this isotopically aged and depleted source for tens or hundreds of millions of years until it is heated in the mantle during orogeny 3. a hybridized mantle source for primitive, hydrous, shoshonitic lamprophyres 4. sustained cold finer effect in the mantle to establish a shallow-mantle diamond stability window Two theories of diamond origin are postulated by De Stefano et al. (2006). Both a cratonic and orogenic model of diamond formation are discussed in an effort to rationalize the observed diamond characteristics. The authors conclude that neither model fully explains all of the observed characteristics. 13 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Field Trip Stops Field Trip Road Log Stop 1 2 3 4 5 6 7 8 Locality Intersection of Hwy 101 and Hwy 17 Take Highway 17 north km 0 Catfish Road forestry road – turn east 19.2 GQ Diamond Discovery Northern Sierra Minerals Corporation Area B Northern Sierra Minerals Corporation Engagement Zone 23.3 23.9 29.1 Return to Highway 17 reset odometer Drive north to access road – turn east Park Walk eastward along trail Moet Occurrence, Festival Property 0 6.2 6.6 7.6 7.6 Return to Highway 17 reset odometer Drive north, park on shoulder of highway Sandor Diamond Occurrence Continue north on Highway 17 to intersection of Highway 519, turn right 0 4.3 4.3 12.6 Safely turn in parking area and return to Intersection of Hwy 17 and 519 reset odometer Drive south on Highway 17, park on shoulder of road Dubreuilville Dike Continue south on Highway 17 Turn left into access road to gravel pit, park Walk south approximately 150 m Monchiquite Dike Drive north on Hwy 17 to Wawa Wawa Motor Hotel 0 3.1 3.1 44.9 45 53.2 STOP 1 - GQ Diamond Discovery Site Northern Sierra Minerals Corporation Area A UTM co-ordinates – 0665570E 5333291N NAD83 Several outcrops of diamondiferous breccia outcrop on the west side of a forestry road. This exposure is an example of the hypabyssal facies of the three identified diamond-bearing units. The rock cut displays the apparently conformable nature of these “lamprophyre” dikes. The most notable features of these outcrops are the actinolite-rich nature of the matrix and the presence of biotite-rich reaction rims around the xenoliths. It is frequently difficult to distinguish these dikes from the mafic to intermediate agglomeratic and tuffaceous host rocks. In the vicinity of the discovery area, located at the south end of Area A, the diamondiferous breccias are arrayed linearly along the logging road where the topography indicates a 5 – 10 m thick, northwest-trending dike (Cavey 2002). A compilation of the geology of the GQ Property is shown in figure 4. Figure 5 provides a compilation of the diamond occurrences on the GQ Property. 14 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �GQ PROPERTY I / I AR! ' I GEOLOGY REVISED FROM CANADA EXPLORATION INC. GABBRO LII METASEDIMENTS GO PROPERlY INTERMEDIATE TO FELSIC METAVOLCANICS L CALCALKALINE IIVB AND LAMPROPIIYRE LOCATIONS ARE APPROXIMATE PROPERTY GEOLOGY MAP Sault Ste. Mane Mining Division Ontario Atter K. KM (2003), Kennecott Figure 4. Geological compilation map of the GQ Property, Musquash Township, Northern Sierra Minerals Corporation (Cavey 2004). 15 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �_____ ____________ iT ___________ a § Drill Hde Occurrences ci In&usive and/or Exinisive flreccias KEY • drilled in 2002 I 2003 KEY D;scovemdprlorto Opthn during a GO PROPERTY COMPILATION MAP drilled in 2000 logging road. 2004 MusquashTownship It Ste. Marie Mirlfng Division Ontario Figure 5. Occurrences of diamondiferous bedrock on the GQ Property, Musquash Township, Northern Sierra Minerals Corporation (Cavey 2004). 16 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �A relatively precise 207Pb/206Pb age of 2674 ± 8 Ma was returned from a sample collected in 2000 by R. P. Sage (R. P. Sage, Ontario Geological Survey, unpublished 2000). The sample analysed was titanite. The titanite grains are considered to be a primary mineral and not xenocrystic. In thin section, the rock is characterized by a green, medium-grained, granoblastic to decussate groundmass of actinolite, biotite, chlorite, plagioclase and accessory minerals. Amphibole or biotite macrocrysts up to 1 mm are common (Stone and Semenyna 2004). Local prospectors T. Nicholson, J. Robert and M. Tremblay made the discovery in the fall of 1999. The first two bedrock samples (63.4 kg and 70.5 kg) collected were processed by Kennecott Canada Exploration Inc. in their Thunder Bay laboratory. According to Kennecott’s report, the 63.4 kg sample yielded 45 diamonds, of which 10 were macro diamonds and 35 were microdiamonds. One of the macro diamonds measured 1.01 mm in one dimension. The 70.5 kg sample yielded 9 microdiamonds. All stones were white in colour and transparent in clarity. Duplicate samples were collected by Band-Ore Resources Ltd. in early 2000 and were processed at SGS Lakefield Research Limited. A 54.6 kg sample yielded 98 microdiamonds. A confirmation sample from the same area yielded 98 microdiamonds from a 54.6 kg sample. In 2000, Band-Ore Resources drilled 3 short holes (75 m total) at the discovery site. Table 1 details the diamond recovery results from the drill program. Only partial intervals from drill hole DDH GQ-00-3 were submitted for microdiamond recovery since portions of the core were used for thin sections, microprobe analysis and display purposes. In total 5 diamonds were recovered from DDH GQ-00-3, including one champagne coloured macro diamond and one white microdiamond from a sample weighing 7.5 kg. Drill sections for DD GQ-00-01 through 03 are shown on figure 6. Table 1. Diamond recovery results from 2000 Band-Ore Resources Ltd. diamond-drilling program Drill Hole DDH GQ-00-1 DDH GQ-00-2 Sample No. Sample Size (kg) No. Macro Diamonds No. Micro Diamonds Sample 1A 63.35 1 Sample 1B 30.17 10 Sample 1C 28.51 6 Sample 2A 37.52 1 30 Sample 2B 30.97 434 Sample 2C 28.51 30 results compiled from Band-Ore Resources Ltd. press releases 2000 To date, the discovery site has yielded 746 diamonds, including 15 macro diamonds, from sample material weighing 785 kg. The largest diamond recovered exceeds 1.0 mm in size and the majority of the stones are gem quality, white, clear and transparent. STOP 2 - Northern Sierra Minerals Corporation Area B UTM co-ordinates – 0665425E 5334748N NAD83 The exposure on the east side of the forestry road provides an excellent exposure of the subvolcanic/intrusive breccia facies. Subrounded to rounded xenoliths dominate the vertical exposure (Figure 7). Field relationships between the intrusive breccia and other heterolithic breccias can be observed in several outcrops along the road. Texturally, the subvolcanic/intrusive facies may resemble both the hypabyssal facies and the intrusive heterolithic breccias. The facies consists of mica and amphibole phenocrysts (<2mm) in a groundmass of mica, actinolitic amphibole and lesser albite, carbonate, sphene and oxides. Alteration includes variably chloritized mica while the other phenocrysts have been extensively altered to varying proportions of mica, albite and actinolitic amphibole. 17 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �z Q 000 / MAFIC VOLCAXICS A S\\IPI I lB -, / MARC VOLCANICS Band-Ore Resources Ltd. GQ PROPERTY Drill Section 3Dm Musquash Township Ste. Marie Mining Division Ontario Drill Hole GQ-OO-O1 Section Looking West Figure 6. Drill hole sections GQ-00-01, GQ-00-02 and GQ-00-03, GQ Property, Musquash Township, Northern Sierra Minerals Corporation (Cavey 2002). 18 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �SAM PLI- XENOLITHIC microdinmonds A (33.81kg) 3 macrodiamonds DIATREME 00) 2A (3 7.52kg) / XYN(YLITIIIC DI.\I I 2B 7 GQ-OO-03 S\\IPLI 2€ 4, Band-Ore Resources Ltd. GQ-OO-02 27m GQ PROPERTY Drill Section Musquash Township Sault Ste. Marie Mining Division Ontario Drill Holes G-OO-2 and GQ-OO-03 Section Looking West Figure 6 cont’d. Drill hole sections GQ-00-01, GQ-00-02 and GQ-00-03, GQ Property, Musquash Township, Northern Sierra Minerals Corporation (Cavey 2002). 19 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �I- - . —I —.-..' - ½ 4 - ( "- Ip .t •q Figure 7. Stop 2 - Northern Sierra Minerals Corporation Area B – rounded lower crustal to upper mantle xenoliths Stone and Semenyna (2004) completed a petrographic examination of one of the ultramafic xenoliths from this site. The sample was dominated by a coarse, radiating to decussate, clear to pale green amphibole of tremolitic to magnesium-rich actinolite composition. Carbonate occurs locally and biotite is concentrated at the rims of the xenolith. Band-Ore Resources Ltd. discovered area B in 2000. Thirty-three (33) reconnaissance samples were collected from this area and a total of 273 diamonds was recovered from 352 kg of material (Cavey 2002). One 24 kg sample returned 126 microdiamonds (1.37 mg total weight). No macro diamonds were recovered from Area B. Band-Ore Resources Ltd. completed only a reconnaissance sampling program, minor stripping and trenching in this area. The Barnett Zone lies approximately 1.6 km to the northwest. It was discovered in the fall of 2001 by Kennecott Canada Exploration Inc. who completed a limited program of mechanical stripping, channel sampling and washing of outcrops over the area. A total of 27 outcrop channel samples (270 kg) were collected. The channel samples returned 330 microdiamonds and 3 macro diamonds. A single 24 kg sample of heterolithic breccia from the Barnett Zone returned 3 macrodiamonds and 123 microdiamonds. A total of 273 diamonds (261 microdiamonds and 12 macrodiamonds) were recovered from 34 samples (352 kg) collected between September 2000 and July 2001 by Kennecott Canada Exploration Inc. in Area B (including the Barnett Zone). 20 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �STOP 3 - Northern Sierra Minerals Corporation Engagement Zone Bulk Sample Site UTM co-ordinates – 0667760E 5336073N NAD83 The main outcrop exposure consists of medium to thickly bedded pyroclastic air-fall deposits and debris flows described as heterolithic tuff-breccias that grade upwards to lapilli-tuff and tuff. The groundmass of the extrusive phase is fine-grained with variable proportions of relatively small (<2mm) altered phenocrysts, including chloritized mica. The groundmass consists primarily of actinolitic amphibole with rare to minor mica or granular albite. The pyroclastic rocks are mineralogically and compositionally similar to intrusive varieties, but have a significantly higher proportion of mica phenocrysts. Both diamond-bearing intrusive and extrusive rocks host significant proportions of fragments derived from the local country rock. Rare to minor, deep crustal and upper mantle xenoliths, such as banded gneiss and extensively altered talcose ultramafic xenoliths are present. Fresh mantle rocks, such as lhertzolite, harzburgite and eclogite have not been identified. The matrix material is typically fine-grained, green, weakly foliated actinolite schist. Albite is present, although it is less abundant (<10%) than in the lamprophyre dikes and implies a more ultramafic composition for the breccia matrix than for the lamprophyre dikes. Titanite is fairly abundant and calcite, epidote, apatite and sulphide minerals occur locally. Macrocrysts of actinolite are commonly observed. Rare macrocrysts of amphibole also are observed and are frequently altered to actinolite. The actinolite macocrysts are probably metamorphic in origin, whereas the amphibole macrocrysts may represent accidental or cognate crystals (Stone and Semenyna 2004). The Engagement Zone has a minimum strike length of 335 m and a horizontal width in excess of 75m. The zone strikes northwesterly and has a shallow dip to the northeast. This zone may represent the southeast extension of Pele Mountain Resources Inc.’s Cristal diamond occurrence located approximately 2 km to the northwest. In 2003, a 0.72 carat macrodiamond was recovered from a bulk sample collected from the Cristal. The geology of the Engagement Zone and sample locations are shown in Figure 8. A simplified cross section of the Engagement Zone is shown in Figure 9. Band-Ore Resources Ltd. discovered this zone in January 2001. A 16 kg sample from a single angular boulder of diatreme breccia returned 128 microdiamonds. Four subsequent samples (96 kg total weight) returned 5045 microdiamonds and 65 macrodiamonds. In 2001 a mini-bulk sample weighing 12.5 tonnes was collected under the supervision of Kennecott Canada Exploration Inc. and shipped to the Saskatchewan Research Council. The largest diamond recovered from this sample was a 0.254 carat, broken, white octahedral stone. Two additional bulk samples were collected in 2003. A 22 tonne sample tested an area where 6 channel samples (63 kg total) recovered 1752 stones. A 20 tonne sample tested an area where 5 channel samples weighing 41.8 kg returned 552 stones. The results from these bulk samples are shown in Table 2. Table 2. Diamond results from Engagement Zone bulk samples (2001) Sample Weight Sieve +1mm Sieve +2mm Sieve +3mm Sieve +5mm Sieve +6mm Total Diamonds Total Carat Weight Engagement Zone East 22.1 tonnes 1 4 4 2 1 12 0.375 Engagement Zone West 20.4 tonnes 2 3 3 8 0.155 Occurrence results compiled from Band-Ore Resources Ltd. press releases 2004 To date, exploration on the Engagement Zone has included an orientation geochemical survey, geological mapping, channel sampling, trenching, bulk sampling and a 9-hole (1775 m) diamond drilling program designed to test the strike continuation of the zone. The drilling program demonstrated that thick diamondiferous breccia deposits can feather out and thin to a few centimeters thickness (Cavey 2003). 21 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Engagement Zone // N 1/ II / APPARENT OUTLINE OF // ii 0 / // MICROS 73 MACROS FROM 481.83 I - excludes the resuj from the Kenneec channel sampling / II V C \ I Mt.51262 12 tonne Exploration Sample hz' - -- I y '.APPROX. DII' - -. O.2o4CaratWhrtsOctahedral / LOG A1FONØ ARE APP ROXIM so icc Engagement Zone • OCCURRENCES OF BRECCIA large angular slabs and/or Duicrop) * SAMPLE PROCESSED FOR MICRODIAMOND RECOVERY (caustic fusion) Figure 8. Stop 5 – Geology and sample locations at the Engagement Zone, GQ Property, Northern Sierra Minerals Corporation (Cavey 2002) 22 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �2001 MINI-DULK SAMPLE A SIRIFFLNG N N VOLCAN N4AFLC VOLCANICS AND INIERELOW N HETEKOLtIHIC MEGALITH VOLCAN[CLAST:C I CC IA — VOLGAN[C •I "NN 105, N I HETEWLFIHIC N N N LOCATIONS SKECCIA ArFROX!MATE "N Figure 9. Simplified cross section through the Engagement Zone looking northwest (Cavey 2004). STOP 4: - Moet Occurrence, Festival Property Pele Mountain Resources Inc. UTM co-ordinates – 0662709E 5338009N NAD83 The Moet occurrence is a large stripped outcrop that extends in a north-south orientation across the forestry road. The outcrop displays all three facies of diamond-bearing bedrock exposed over an area 500 m by 300 m. It is hosted within fine- to medium-grained mafic metavolcanic rocks with closely associated intermediate to felsic metavolcanic rocks and metasediments. The volcanic facies was found concentrated within a series of outcrop exposures along the west side of a 5-8 m north-trending ridge and the subvolcanic/intrusive breccias and hypabyssal rocks are present in several outcrops east of the volcanic facies. The subvolcanic/intrusive breccias are hosted in the metavolcanics and the volcanic facies appears to overlie these rocks, and are in turn overlain by intermediate to felsic metavolcanic rocks. The fragments 23 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �within the volcanic facies consist of country rock fragments with lesser crustal fragments and lower crust to upper mantle xenoliths. The hypabssyal facies displays primarily mantle xenoliths (Walker 2003). A detailed geological map of the Moet locality is shown in Figure 10. ' Stop4 t Moot 5—-/ — I I 1YPE \">" — — — M.lic rock. + an&'o reck, tomainjin mamle xpnnMha cpsTs 0 Figure 10. Detailed geological map of the Moet Occurrence, Festival Property, Pele Mountain Resources Inc. (Kjarsgaard et al. 2003). The access trail to the outcrop passes through a sequence of intermediate to felsic tuffs and tuffaceous breccias. The composition of the fragments and that of the matrix are highly variable from felsic to intermediate and a combination of felsic matrix with intermediate fragments and vice versa is not uncommon. At a distance of approximately 1km, the trail passes through a sequence of mafic to intermediate tuffs and lapilli tuffs (Vaillancourt et al. 2005b). An age date of 2698 ±1 Ma was returned from sample of the felsic metavolcanic rocks at the occurrence. A sample of the breccia returned an age date of 2680 Ma (J. Ayer, Ontario Geological Survey, personal communication, 2004). Discovered by Pele Mountain Resources Inc. in 2001, the occurrence initially gained interest because the breccia has a size distribution of diamonds that includes coarser sized diamonds from relatively small samples. For example, an 8 kg sample collected in 2001 recovered a total of 9 diamonds, 4 of which were in the +600 mesh fraction. Diamonds are consistently recovered from both the volcanic and subvolcanic facies at this showing. A summary of the results of the 2001 and 2002 sampling of the occurrence by Pele Mountain Resources Inc. is found in Table 3. 24 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Table 3. Summary of the diamond results from the 2001-02 sampling of the Moet Occurrence Facies Weight (kg) <425 mesh >425 mesh volcanic volcanic hypabyssal hypabyssal volcanic volcanic hypabyssal subvolcanic breccia 117 16 24 8.6 9.3 14.1 9.3 32 48 2 0 0 7 12 0 30 0 1 0 0 0 5 0 1 total diamonds 48 3 0 0 7 17 0 31 results compiled from Pele Mountain Resources Inc. press releases 2002 Further exploration was conducted at the occurrence in 2003 when DeBeers Canada Exploration Inc. completed a detailed airborne geophysical survey, stripping, mapping and sampling program. A 47.8tonne bulk sample from the site returned 5 diamonds with a total carat weight of 0.13. All diamonds were recovered from the +1 to +3 sieve class screens (Pele Mountain Resources Inc., press release, March 17, 2004). STOP 5: - Sandor Diamond Occurrence Spider Resources Inc. & KWG Resources Inc. UTM co-ordinates – 0659805E 5342191N NAD83 The Sandor occurrence the first confirmed occurrence of diamonds in bedrock in the Wawa area The occurrence is located in a 4 m high road cut on the east side of the Trans Canada Highway (Highway 17). The dike is approximately 5 m wide, steeply dipping and strikes roughly parallel to the regional schistosity at 120°. The dark, greenish-grey rock weathers olive grey, is highly fractured, moderately carbonatized and is non-magnetic. It is composed of up to 40% actinolite replaced mantle xenoliths and supracrustal xenoliths. Towards the margins of the dike xenoliths are less common and the rock grades into an adjacent micaceous dike. Only remnants of the dike remain in situ. The dike is hosted by gabbros and intermediate to felsic crystal tuffs. A (308.6 kg) sample of the dike, collected by Spider Resources Inc. in 1997, returned a total of 97 diamonds comprising 1 commercial stone, 13 macrodiamonds and 83 microdiamonds. A short walk into the forest from the top of the outcrop leads to a larger stripped area where field relationships between the host gabbro and the dike can be observed. A second, xenolith-bearing dike (occurrence LAL-3) is located at the north end of the outcrop. This dike is 2 m wide and closely resembles the Sandor occurrence. A 34.6 kg sample of this dike contained 1 microdiamond. A detailed geological map of the Sandor occurrence is shown in Figure 11. A compilation map of the geology of the Spider Resources Inc. and KWG Resources Inc. property is shown in Figure 12. Using normative mineralogy, Sage (2000) concluded that this dike should be classified as a spessartite. A spessartite is defined as a lamprophyre composed of phenocrysts of green hornblende or clinopyroxene in a groundmass of sodic plagioclase with accessory olivine, biotite, apatite and opaque oxides. Titanite and rutile from the matrix of the Sandor dike returned an age of 2703 ± 42 Ma (Sage 2000). The date is interpreted to be a minimum age of intrusion. Subsequent dating of a zircon from a gneissic xenolith from the Sandor dike returned a 207Pb/206Pb age of 2684.9 ± 1.4 Ma (Ketchum, Kamo and Davis 2003). Spider Resources Inc. and joint venture partner KWG Resources Inc. have taken 3 mini-bulk samples along the 1 km strike extent of the dike since the fall of 2001. The three bulk samples had a combined weight of 7.61 tonnes and returned 11 commercial stones and 9 macro diamonds. These samples were tested only for macro diamond and commercial diamond content (Spider Resources Inc., press releases, February and March 2002). 25 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �0 ' - — M*VSc.iic — tbcoveryOflop ——-Opel— 2__21' S Stops Figure 11. Detailed geology of the Sandor Occurrence, Spider Resources Inc. & KWG Resources Inc. (Kjarsgaard et al. 2003). Figure 12. Generalized geology of Wawa Project, Spider Resources Inc. and KWG Resources Inc. (Spider Resources Inc. and KWG Resources Inc., CD-ROM presentation, update February 24, 2004) 26 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �STOP 6: - Dubreuilville Dike - Xenolith-rich lamprophyre Spider Resources Inc. & KWG Resources Inc. UTM co-ordinates – 0657251E 5347023N NAD83 The dike outcrops in a 1.5 m high exposure on the west side of the Trans-Canada Highway (Highway 17). In the area between the former Magpie Iron Mine in Leclaire Township to approximately the eastern contact of the Dickenson Lake Stock, there are a number of exposures of unusual dike rocks. These rocks are characterized by prominent round, to elliptical inclusions of actinolite or actinolite plus talc. Xenoliths are altered to fine-grained actinolite with or without talc, and some display zoning from talc core to an actinolite rim. The actinolite inclusions may consist of prismatic green crystals as large as 8 cm in length, which may be randomly oriented or radiating inward towards the core. The inclusions containing talc consist of a talc core with the prismatic to acicular actinolite projecting radially inward towards the core. The xenoliths are believed to represent at least two original mafic compositions which are likely to be originally of lower crust origin or deeper. A weakly developed regional schistosity crosses the dike implying an Archean age (Sage 1993, 2000). An example of one of the lower crustal to upper mantle xenoliths is shown in Figure 13. Figure 13. Lower crustal to upper mantle xenolith, Dubreuilville Dike Stop 6, Highway 17 North. This dike may be the one described by Higgins (1986). He reported that the dike consists of 60% euhedral amphibole, 20% biotite replacing amphibole and 15%plagioclase. Minor sphene and opaque minerals are present and chromite is reported from the core of the talc-bearing clasts. The bulk composition of the nodules is reportedly pyroxenite, but Higgins did not indicate whether their source was mantle or crust. The rounded outline of the clasts may reflect magmatic erosion during transport and the present mineralogy is the product of regional metamorphism (Higgins 1986). 27 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �STOP 7: - Monchiquite Dike Michipicoten Post Provincial Park UTM co-ordinates – 0663600E 5309223N NAD83 On the west side of Trans Canada Highway (Highway 17) two, 1-metre wide and one, 3-metre wide monchiquite dikes outcrop at the base of a 3 m high outcrop. The dikes are parallel, strike 060° and crosscut a portion of the Mission Stock. A 4 m wide, east-trending microsyenite dike also can be observed cutting the granodiorite and the monchiquite at this location. The north-northwest-trending Trembley Fault cuts across the north end of the outcrop (Massey 1985). These lamprophyres are most commonly seen in road cuts along the highway and along the shore of Lake Superior. Typically, they are black when fresh and weather to an orange-brown colour. The dikes are usually narrow and are typically less than 1 m wide. Biotite-phyric and olivine-phyric varieties are most common, but pyroxene and feldspar phenocrysts also are observed. Often the dikes show evidence of multiple and composite intrusion, sometimes with tin screens of country rock trapped within the dikes. Lamprophyre dikes south of the Michipicoten River typically have a northeast trend (Massey 1985). Lamprophyre dikes are commonly found cross cutting all lithologies south of the Wawa – Hawk – Manitowik Lakes Fault. Because of their ease of weathering they are infrequently seen in outcrop. However, the dikes are frequently found in underground mine workings. These dikes have a number of macrocrystic resemblances to the dikes seen here. As early as 1927, geologists working in the area had noted a similarity between these lamprophyre dikes and kimberlite (Gledhill 1927). Recent petrographic and mineralogical studies on several dikes in both McMurray and Lendrum townships have suggested that some of the geochemistry falls within the classification of type II kimberlites (orangeites) (Barnett 2001). To date, no diamonds have been recovered from these dikes. These lamprophyre dikes are probably Proterozoic in age and are interpreted to represent Proterozoic alkalic magma emplacement into structures related to the Kapuskasing Structural Zone, perhaps consanguineous with the nearby Firesand Creek carbonatite complex. Rocks from the Firesand Creek carbonatite complex have a U-Pb date of 1078 ± 2.4 Ma (Sage 2000). STOP 8: - Contemplation of the rocks on the fireplace at the Wawa Motor Hotel Acknowledgments The author would like to thank Wayne O’Connor, Northern Sierra Minerals Corporation, Al Shefsky, President, Pele Mountain Resources Inc. and Neil Novak, Vice-President of Exploration, Spider Resources Inc. for permission to access properties described in the field guide. Editorial comments by R. P. Sage also were appreciated during the preparation of the field guide. My thanks also go to Glenn Seim for his technical assistance in the preparation of the guidebook. 28 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Bibliography Arias, Z. G. 1996. Structural evolution of the central Michipicoten greenstone belt, Superior Province, Wawa, Ontario, Canada; unpublished MSc thesis, Queen’s University, Kingston, Ontario, 140p. Arias, Z. G. and Helmstaedt, H. 1990. Structural evolution of the Michipicoten (Wawa) greenstone belt, Superior Province: Evidence for an Archean fold and thrust belt; in Geoscience Research Grant Program, Summary of Research 1989-1990, Ontario Geological Survey, Miscellaneous Paper 150, p. 107-114. Ayer, J. A., Conceição, R. V., Ketchum, J. W. F., Sage, R. P., Semenyna, L. and Wyman, D. A. 2003. The timing and petrogenesis of diamondiferous lamprophyres in the Michipicoten and Abitibi greenstone belts; in Summary of Field Work and Other Activities 2003, Ontario Geological Survey, Open File Report 6120, p. 10-1 to 10-9. Barnett, R. L. 2001. Fletch Kimberlite correspondence, Report of Work, Matchinameigus and Fletch Properties, Dolson & Echum Townships, Sault Ste Marie Mining Division, Northern Ontario; Timmins Resident Geologist’s Office, Dolson Township, assessment file WP–Dolson–4, unpaginated. Cavey, G. 2002. Geological report on the Wawa properties for Band-Ore Resources Ltd. Sault Ste Marie Mining Division, Ontario; unpublished NI 43-101 report, 50p. Cavey, G. 2003. Updated geological report on the Wawa property for Band-Ore Resources Ltd. Sault Ste Marie Mining Division, Ontario; unpublished NI 43-101 report, 35p. Cavey, G. 2004. Summary geological report on the GQ Property for Band-Ore Resources Ltd. Sault Ste Marie Mining Division, Ontario; unpublished NI 43-101 report, 39p. Corfu, F. and Sage R. 1987. A precise U-Pb zircon age for a trondhjemite clast in Doré conglomerate, Wawa, Ontario; in Proceedings and Abstracts, Institute on Lake Superior Geology Annual Meeting, v. 33, p. 18. Corfu, F. and Sage, R. 1992. U-Pb age constraints for deposition of clastic metasedimentary rocks and late-tectonic plutonism, Michipicoten belt, Superior Province; Canadian Journal of Earth Sciences, v. 29, p. 1640-1651. De Stefano, A., Lefebvre, N. and Kopylova, M. 2006. Enigmatic diamonds in Archean calc-alkaline lamprophyres of Wawa, southern Ontario, Canada; Contributions to Mineralogy and Petrology, on-line edition, January 5, 2006. Gledhill, T. L. 1927. Michipicoten Gold Area, District of Algoma; Ontario Department of Mines, Annual Report, 1927, v. 36, pt. 2, p. 1-49. Higgins, M. D. 1986. Nodule-bearing spessartite (lamprophyre) dike near Wawa, Northern Ontario; in Program with Abstracts GACMAC-GCU-AGC-AMC-UCG, vol. 11, p. 81. Kaminsky, F. V., Sablukov, S. M., Sablukova, L. I, Shchukin, V. S. and Canil, D. 2002. Kimberlites from the Wawa area; Canadian Journal of Earth Sciences, v. 39, p. 1819-1838. Ketchum, J., Kamo, S. and Davis, D. 2003. U-Pb ages from the Superior and Grenville Provinces of Ontario; unpublished report of the Jack Satterly Geochronology Laboratory, Toronto. Kjarsgaard, B. A., McClenaghan, M. B., Boucher, D. R. and Kivi, K. 2003. Kimberlites and ultrabasic rocks of the Wawa, Chapleau, Kirkland Lake and Lake Timiskaming Areas; in VIIIth International Kimberlite Conference, Northern Ontario Field Trip Guidebook, B. A. Kjarsgaard (ed) p. 1-37. Lefebvre, N. S. 2004. Petrology, volcanology, and diamonds of Archean calc-alkaline lamprophyres, Wawa, Ontario, Canada; unpublished MSc thesis, The University of British Columbia, Vancouver, British Columbia, 265 p. Lefebvre, N., Kopylova, M., Kivi, K. and Barnett, R. 2003. Diamondiferous volcaniclastic debris flows of Wawa, Ontario Canada; long abstract prepared for VIIIth International Kimberlite Conference, Victoria, British Columbia, Canada. Massey, N. W. D.1985. Geology of the Mishewawa Lake Area, District of Algoma; Ontario Geological Survey, Open File Report 5532, 167p. Morris, T. F. 1999. Overburden as a media for kimberlite, base metal and gold exploration, Wawa region, northeastern Ontario; Geological Association of Canada – Mineralogical Association of Canada, Joint Annual Meeting, GAC-MAC Sudbury 1999, Field Trip B6 guidebook, 67p. Morris, T. F. 2001. Quaternary geology of the Wawa Area, Northeastern Ontario; Ontario Geological Survey, Open File Report 6055, 67p. 29 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Sage, R. P. 1993. Geology of Killins, Knicely and Lalibert townships, District of Algoma; Ontario Geological Survey, Open File Report 5589, 141p. Sage, R. P. 1994. Geology of the Michipicoten greenstone belt; Ontario Geological Survey, Open File Report 5888, 592p. Sage, R. P. 2000. The “Sandor” diamond occurrence, Michipicoten greenstone belt, Wawa, Ontario: A preliminary study; Ontario Geological Survey, Open File Report 6016, 49p. Sage, R. P., England, D., Calvert, T., Oudkerk, G., Worona, R. and Kusciusko, R. 1982. Precambrian geology of Musquash Township, Algoma District; Ontario Geological Survey, Map P.2556, scale 1:15 840. Stachel, T., Blackburn, L., Kurszlaukis, S., Barton, E. and Walker, E. C. 2004. Diamonds from the Cristal and Genesis volcanics, Wawa area, Ontario; in Abstracts of Talks & Posters, 32nd Annual Yellowknife Geoscience Forum, 16-18 November 2004, p. 74-75. Stone, D. and Semenyna, L. 2004. Petrography, chemistry and diamond characteristics of heterolithic breccia and lamprophyre dikes at Wawa, Ontario; Ontario Geological Survey, Open File Report 6134, 39p. Stott, G. M., Ayer, J. A., Wilson A. C. and Grabowski, G. P. B. 2002. Are Neoarchean diamond-bearing breccias in the Wawa area related to late-orogenic alkalic and “sanukitoid” intrusions?; in Summary of Field Work and Other Activities 2002, Ontario Geological Survey, Open File Report 6100, p. 9-1 to 9-10. Turek, A., Sage, R. P. and Van Schmus, W. R. 1992. Advances in the U-Pb ziron geochronology of the Michipicoten greenstone belt near Wawa, Ontario; Canadian Journal of Earth Sciences, v. 27, p. 649-656. Vaillancourt, C., Ayer, J. A., Zubowski, S. M. and Kamo, S. L. 2004. Synthesis and timing of Archean geology and diamond-bearing rocks in the Michipicoten Greenstone Belt: Menzies and Musquash townships; in Summary of Field Work and Other Activities 2004, Ontario Geological Survey, Open File Report 6145, p. 6-1 to 6-9. Vaillancourt, C., Ayer, J. A. and Hamilton, M. A. 2005a. Synthesis of Archean geology and diamond-bearing rocks in the Michipicoten greenstone belt: Results from microdiamond extraction and geochronological analyses; in Summary of Field Work and Other Activities 2005, Ontario Geological Survey, Open File Report 6172, p. 8-1 to 8-11. Vaillancourt, C., Dessureau, G. R. and Zubowski, S. M. 2005b. Precambrian geology of Menzies Township; Ontario Geological Survey, Preliminary Map P.3366, scale 1:20 000 Vaillancourt, C., Zubowski, S. M. and Dessureau, G. R. 2005c. Lithogeochemical data and field photographs for the Wawa area: Menzies and Musquash Townships; Ontario Geological Survey, Miscellaneous Release – Data 151. Walker, E. C. 2002. Diamond deposits of the Festival Property, Wawa, Ontario; Report prepared for Pele Mountain Resources Inc. under National Instrument 43-101, 41p. Walker, E. C. 2003. Diamond deposits of the Festival Property, Wawa, Ontario; Report prepared for Pele Mountain Resources Inc. under National Instrument 43-101, 39p. Williams, F. 2002. Diamonds in late Archean calc-alkaline lamprophyres Ontario, Canada: Origins and implications; unpublished BSc thesis, University of Sydney, Sydney, Australia, 82 p. Wilson, A. C. 2004. Diamond exploration targets, Michipicoten greenstone belt; Canadian Institute of Mining Bulletin, v. 97, no. 1077, p. 41-46. Wyman, D. A., Ayer, J. A., Conceição, R. V., and Sage, R. P. in press. Mantle processes in an Archean orogen: evidence from 2.67 Ga diamond-bearing lamprophyres and xenoliths, Lithos. 30 PDF compression, OCR, web-optimization with CVISION's PdfCompressor � https://digitalcollections.lakeheadu.ca/files/original/97cacb27af6036606c9fbddb715df7e4.PDF 878291d60fa2702c5fc3825c929f6be8 PDF Text Text tFu3 iitiitiuitui blflv..a sri .aridi 7 — - IL - ' r4' -4 V 1. - I -4 'I / Gerald Bennett 2006 Field Trip Guidebook, Volume 52, Part 4 Institute on Lake Superior Geology Sault Ste Marie, Ontario — PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Acknowledgements I would like to acknowledge the rocks of the Huronian Supergroup for providing me with so many years stimulating and rewarding work and for teaching me so much, only to eventually show me how much more there was to learn. I would like to thank my wife and children for their love and support during my long absences while a field geologist for the Ontario Geological Survey. Dr. Ron Sage edited an early version of this manuscript and made many useful suggestions. Cover and page layout by Victoria L. Sage, BSc. Cover Photo: Ripple marks in the lower red quartzite member of the Lorrain Formation at Stop S2.2, Highway 17, west of the Town of Desbarats. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Contents IlSG Field Trip No 1- Day 1 ........................................................................................ 1 Part 1 - Overview of the Huronian Supergroup ....................................................... 1 Introduction ................................................................................................................. 1 The Elliot Lake Group ................................................................................................ 4 The Livingstone Creek Formation ................................................................................ 4 Huronian Magmatism ................................................................................................. 7 Introduction ................................................................................................................... 7 Basement Dikes ............................................................................................................. 7 Layered Gabbro/Anorthosite Complexes ...................................................................... 8 Huronian Volcanic Rocks of the Sudbury Area ............................................................ 8 The Huronian Volcanic Rocks of the Sault Ste. Marie-Elliot Lake area ...................... 8 Sedimentary Rocks Associated with the Thessalon Formation .................................. 14 The Matinenda Formation ........................................................................................... 15 The McKim Formation ................................................................................................ 16 Stratigraphic Relationships within the Elliot Lake Group of the Sault Ste. MarieElliot Lake Area. .................................................................................................... 17 Hough Lake Group .................................................................................................... 17 Introduction ................................................................................................................. 17 Ramsay Lake Formation.............................................................................................. 17 Pecors Formation ......................................................................................................... 18 Mississagi Formation .................................................................................................. 18 Aweres Formation........................................................................................................ 19 Quirke Lake Group ................................................................................................... 19 Bruce Formation .......................................................................................................... 19 Espanola Formation ..................................................................................................... 20 Serpent Formation ....................................................................................................... 20 Cobalt Group ............................................................................................................. 20 Gowganda Formation .................................................................................................. 20 Lorrain Formation ....................................................................................................... 21 Gordon Lake Formation .............................................................................................. 22 Bar River Formation .................................................................................................... 22 Nipissing Intrusions ................................................................................................... 21 Huronian Paleosols and Evidence for the Accumulation of Oxygen in the Huronian Atmospher.............................................................................................23 Regional Tectonic Patterns and Metamorphism .................................................... 25 Tectonic Models for the Development of the Huronian Basin .............................. 29 ILSG Field Trip No. 1 - Day 1 .................................................................................. 29 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �The Elliot Lake Transect ........................................................................................... 29 Stop Descriptions and Road Log .............................................................................. 30 STOP 1.1: McKim Formation and Nipissing Diabase ............................................... 30 STOP 1.2: Radioactive quartz-pebble conglomerate of the Matinenda Formation. .. 30 STOP 1.3: Mississagi Formation, Hough Lake Group. ............................................. 35 STOP 1.4: Nipissing Diabase (Gabbro) altered Mississagi Formation, Bruce Formation. ......................................................................................................................... 35 STOP 1.5: Espanola Formation and Nipissing diabase sills. ..................................... 36 STOP 1.6: Stratified Gowganda Formation. .............................................................. 36 Stop 1.7: Laminated varvite? ..................................................................................... 37 Stop 1.8: Gowganda Formation Serpent disconformity............................................. 38 Stop 1.9: Espanola Limestone Member of the Espanola Formation........................... 38 Stop 1.10: Ramsay Lake Formation overlain by Pecors Formation. .......................... 38 STOP 1.11: Huronian volcanic rocks of the Thessalon Formation (Dollyberry Volcanics). ....................................................................................................................... 38 STOP 1.12. Archean metavolanics with pillow structures. ......................................... 39 STOP 1.13: Bar River Formation. ............................................................................... 40 STOP 1.14: Red beds of the Gordon Lake Formation ................................................ 40 STOP 1.15: Gordon Lake Formation. ......................................................................... 40 STOP 1.16: Lorrain Formation.................................................................................... 40 Highway 556 Transect ............................................................................................... 41 Road Log and stop Descriptions .....................................................................................................40 ILSG Field Trip No. 1 -......................................................................................................................40 Day 2 - Part 1............................................................................................... ...................................40 Huronian Stratigraphy along Highway 556 and correlation with the Chocolay Group of the Marquette area. ............................................................................. 40 STOP 2.1: Island Lake Fault Zone ............................................................................41 STOP 2.2: Gowganda Formation unconformably overlying Conglomerate of the Aweres Formation. ................................................................................................. 45 STOP 2.3: Drop-stones of the Gowganda Formation ............................................... 45 STOP 2.4: Contact between Gowganda Formation and Huronian metabasalt of the Thessalon Formation .............................................................................................. 46 STOP 2.5: Archean granitic rocks and Gowganda Formation. ................................. 46 STOP 2: 6: Jasper Pebble Conglomerate Member of the Lorrain Formation. .......... 46 STOP 2.7: Outcrop of Archean metavolcanic rocks. ................................................ 46 STOP 2.8: Outcrops of Dolostone at the Dolostone Knob. ...................................... 48 STOP 2.9: This stop will be visited as time allows. ................................................... 48 Geological Features and Correlation of Dolostone Unit in Fenwick Township Northeast of Sault Ste Marie ............................................................................... 49 Correlation with the Gordon Lake Formation............................................................. 50 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Correlation with the Chocolay Group of Michigan .................................................... 50 Road Log and Outcrop Descriptions. ...................................................................... 51 STOP S1-1: Lowermost Gordon Lake Formatiom......................................................................51 Supplimentary Road Log for Highway 638 49 STOP 2.9: Dolostone nodules and discontinuous dolostone beds in the Gordon Lake Formation. .............................................................................................................. 52 STOP S1-2: Mudstone overlying matrix-supported conglomerate (diamictite) of the Gowganda Formation. ............................................................................................ 52 STOP S1-3: Dropstones in laminated siltstone and mudstone of the Gowganda Formation. .............................................................................................................. 53 STOP S1-4: Diamictite of the Gowganda Formation. ............................................... 53 STOP S1-5: Jasper pebble conglomerate of the Lorrain Formation (“Pudding Stone”). .............................................................................. 53 Supplimentary Road Log for the area between Sault Ste Marie and Highway 108 ....................................................................................................... 54 STOP S2.1: Sub-Jacobsville unconformity ................................................................ 54 STOP S2.2: Lower red quartzite member of the Lorrain Formation. ......................... 54 STOP S2.3: Purple siltstone member of the Lorrain Formation. ................................ 54 STOP S2.4: Basal Arkose Member of Lorrain Formation. ......................................... 54 STOP S2.5: Bruce Mines Copper Vein. ...................................................................... 54 STOP S2.8: Outcrop of Matinenda Formation on east side of Highway 129. ........... 55 STOP S2.9: Laminated siltstone of the Gowganda Formation with dropstones. ....... 55 STOP S2.10: Rhyolite of the Thessalon Formation. .................................................. 55 STOP S3.1: Mineralized quartz vein breccia. ............................................................ 56 STOP S3.3: McKim Formation - Staurolite (pseudomorph) schist. ........................... 58 STOP S3.2: Pronto Mine Location............................................................................. 58 Publications Cited or Consulted .............................................................................. 59 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �List of Figures Figure 1.1: The Distribution of the Huronian Supergroup in Ontario…………… 1 Figure 1.2: Rock- Time chart for southeast Lake Superior region……………… 2 Figure 1.3: Stratigraphic column in the Elliot Lake area…………………….... 3 Figure 1.4: The cyclicity of Huronian Formations………………………….…… 4 Figure 1.5: Paleocurrent directions in the Huronian Supergroup…………......... 5 Figure 1.6: The distribution of some formations of the Elliot Lake Group…… 6 Figure 1.7: Stratigraphic relationships within the Elliot Lake Group…………… 9 Figure 1.8: Photo of anorthosite of the East Bull Lake Suite, Foul Bight ………… 10 Figure 1.9: Thessalon basalt overlying Livingstone Creek Fm ……………….… 10 Figure 1.10: Internal stratigraphy of the Thessalon Formation………………… 11 . Figure 1.11: Discrimination diagrams for volcanic rocks of the Thessalon Fm…… 13 Figure 1.12: Photo of radioactive quartz-pebble conglomerate and grit overlying the Livingstone Creek Formation……………………………………………… 14 Figure 1.13: Quartz pebble size vs. pyrite size in Matinenda ore beds……..... 15 Figure 1.14: Diamictite of the Bruce Formation, Highway 108…………… 19 Figure 1.15: Concentration ratios for sub-Thessalon, sub-Matinenda and sub-Lorrain Fm. paleosols………………………………………………… 24 Figure 1.16: Geological cross-section of the Blind River and Sudbury-Manitoulin areas. ……………………………………………… 27 Figure 1.17: Metamorphism of the Huronian Supergroup…………………… 28 Figure 1.18: Index maps for geological maps and some areas referred to in the text………………………………………………………… 31 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Figure 1.19: Geological Map of Quirke Lake Syncline ……………………... 33 Figure 1.20: Legend for Figure1.19 and 1.21……………….......................... 32 Figure 1.21: Geological maps of the Flack Lake area………………………... 34 Figure 1.22: Trough cross-bedding in the Mississagi Fm……………………... 33 Figure 1.23: Metamorphosed Espanola Fm………………………………….... 36 Figure 1.24: Stratified conglomerate of the Gowganda Fm. ……………............ 37 Figure 1.25: Dropstone in laminated siltstone of the Pecors Fm………….......... 39 Figure1.26: Pillow Structure in Archean metavolcanics ……………….......... 40 Figure 2.1: Geological map of the Highway 556 transect. ………………....... 42 Figure 2.2: Stratigraphic column for the Highway 556 transect. ………..…..... 43 Figure 2.3: A diagrammatic cross-section along the Highway 556 transect...... 44 Figure 2.4: Gowganda Formation unconformably overlying Aweres Formation. 44 Figure 2.5: Photo of a “drop-pebble” in laminated Gowganda argillite. ……. 44 Figure 2.6: Polymictic conglomerate of the Aweres Formation……………… 45 Figure 2.7: Dolostone of the Kona Formation intruded by a mafic dike. …… 47 Figure 2.8: Dolostone and chert, Fenwick Township………………………..... 47 Figure 2.9: Dolostone nodules in the Gordon Lake Formation ………………… 47 Figure 2.10: Jasper-pebble conglomerate of the Lorrain Fm…………………… 47 Figure 2.11: Map of Dolostone are in Fenwick Township …………………… 49 Figure 2.12: Correlation of the dolostone occurrences of Ontario and the Marquette area, Michigan ……………………………………………... 50 Figure 2.13: Geology of the Pronto Mine……………………………................. 57 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �1 ILSG Field Trip Number 1 - Day 1 Part 1 - Overview of the Huronian Supergroup These first pages are in tended to give participants new to the Huronian Supergroup of Ontario a summary of what we think we knowof these ancient rocks. Much of the material is borrowed from an earlier ILSG guidebook (Bennett et al , 1997). The important ontributions to that guidebook by K. D. Card and Kirsty Tomlinson are acknowledged with thanks. The Huronian Supergroup is one of the Earth’s most studied sequences of rocks. Since the turn of the century the results of hundreds of studies of Huronian rocks have been published in scientific journals and government publications. These studies have led geoscientists, to present evidence for the Earth’s earliest glacial periods, the development of free oxygen in the atmosphere of the early Earth, the deposition of paleoplacer deposits of uranium, and evidence for plate tectonic activity during the Paleoproterozoic. Much of the evidence presented is based on rock exposures, which will be visited during this field trip. The 2219 Ma radiometric age of the Nipissing intrusions places an upper limit on the age of the Huronian Supergroup, while the Copper Cliff Ryolite (2450 Ma) is probably close to the date of initial Huronian deposition in the Sudbury area. The Huronian Supergroup consists of four groups, which in ascending stratigraphic order are: The Elliot Lake Group, Hough Lake Group, PROTEROZOIC Mid-Late Early Archean Hudson Bay Pr ov inc e Introduction nv i lle e North The Huronian Supergroup, of the James Bay Su Southern Tectonic Province of the peri c or Provin Canadian Shield, is a sequence of Range Paleoproterozoic sedimentary and minor Marquette Supergroup re G volcanic rocks lying unconformably 250 km e c Huronian n i v upon Archean rocks of the Superior o Southern Pr Supergroup Province of the Canadian Shield. The Huronian rocks extend eastward from Lake Superior, along the north shore of Figure 1.1: Distribution of Early Proterozoic Rocks Lake Huron to Sudbury and then in the Great Lakes region. northward to the Noranda area of Quebec, a distance of about 450 km Quirke Lake Group and Cobalt Group (Figure (270 miles) (Figures 1.1, 1.2). The Huronian 1.3). Formations of the three upper groups, with Supergroup attains its greatest thickness of the exception of the Serpent Formation of the 12,000 meters (40,000 feet) southeast of Quirke Lake Group, show stratigraphic Sudbury. The sequence thins northward due to continuity, and display a remarkable cyclicity of the wedging out of basal units, thinning of lithological units. Each cycle begins with clastic units, and erosion within the sequence matrix-supported conglomerate (Roscoe, 1969; Frarey and Roscoe, 1970). (diamictite/mixtite), followed by mudstone, gb 2006 (after Jackson, 2001) PDF compression, OCR, web-optimization with CVISION's PdfCompressor �2 G Erosion Erosion LEGEND 200 500 Michigan 600 700 800 900 Erosion 1000 1900 2000 2100 2200 2300 2400 2500 2600 2700 2800 Continental glaciation Radiometric age determination Penokean Orogeny Chocolay Group Folding, faulting, metamorphism Erosion Nipissing Intrusions G G Huronian volcanics G Layered gabbro Dike Swarms Kenoran Orogeny Elliot Lake, Ont. Sault Ste. Marie, Ont. Erosion Free oxygen accumulating in the atmosphere 1800 G G Huronian Supergroup Rifting Erosion folding faulting metamorphism methane-rich atmosphere (no oxygen) 1700 Mainly granite Erosion East Lake Superior, Ont. 1600 Michigan 1500 Marquette Range Supergroup 1400 Thousands of years before the present Proterozoic 1300 Mainly gabbro Keweenawan Mid-continent Rifting Supergroup 1100 1200 Volcanic Rocks Elliot Lake, Ont. 400 Sedimentary Rocks Sault Ste Marie, Ont. 300 East Lake Superior, Ont. Michigan Basin Phanerozoic 100 Archean Pleistocene Glaciation 2900 3000 G. Bennett, 1994-2005 Figure 1.2: A rock-time chart for the southeast Lake Superior region. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �3 Nipissing Diabase (2.22Ga) Gabbro, diabase, granophyre Stop Number Quartz arenite, subarkose minor mudstone, siltstone. 300 m 1.14, 1.15 Gordon Lake Fm. 500 m 1.16 Siltstone, mudstone chert, sandstone Cobalt Group Bar River Fm. 1.13 Quartz arenite, quartzpebble conglomerate Arkose, subarkose Lorrain Fm. 2500 m Mudstone, siltstone 1.6, 1.7 Gowganda Fm. Arkose, siltstone 600 m (top missing) Diamictite, polymictic conglomerate Subarkose, wacke 250m 1.5, 1.9 Espanola Fm. 200 m Diamictite, sandstone siltstone Bruce Fm. 1.4 Dolostone , siltstone, wacke limestone 50 m Subarkose, arkose Mississagi Fm. quartz-pebble 1.3 1.10 450m conglomerate Pecors Fm. Mudstone, siltstone 127 m Ramsay Lake Fm. Diamictite Disconformity Hough Lake Group Serpent Fm. Quirke Lake Group Unconformity 1.8 20 m McKim Fm 1.2 Matinenda Fm 0 - 80 m U, Th 1.11 East Bull Lake Suite 2.48 Ga Gabbro anorthosite Arkose, subarkose quartz-pebble conglomerate Disconformity 180 m Thessalon Fm. Disconformity 0 - 100 m? Livingstone Creek Fm. 0 - 10 m? Archean Basement 1.12 Metavolcanics, metasediments, granitic rocks Stop Numbers Mafic volcanics Minor qtz. peb. cong. Elliot Lake Group U, Th U, Th Mudstone, siltstone wacke 1.1 Subarkose, conglomerate Unconformity The thickness of formations was determined from three drill holes drilled near the transect. gb 1996, 2006 Figure 1.3: A Stratigraphic column for the Elliot Lake transect PDF compression, OCR, web-optimization with CVISION's PdfCompressor �4 siltstone or limestone and capped by a thick sequence of coarse, cross-bedded sandstone( Figure 1.5). Many paleocurrent studies have shown paleocurrent flowed south to southeast, with southeast being the predominant direction (Figure 1.5). The Elliot Lake Group The Elliot Lake Group differs from the overlying Huronian groups in that: (1) Its internal stratigraphy is generally discontinuous and less extensive. (2) It does not show the diamictite-mudstone-sandstone sequence of the overlying groups. (3) It contains the only important uranium deposits and the only volcanic rocks of the supergroup. (4) Most formations have disconformable surfaces. The Livingstone Creek Formation Clast-supported, polymictic conglomerate is the prominent rock type in the lower sections of the Livingstone Creek Formation of most areas. Cobble- to boulder-sized clasts generally of grey granitic rocks and minor mafic plutonic and metamorphic rock clasts are set in a sparse matrix of coarse, grey arkose or arkosic grit. The writer has not observed clasts of Huronian volcanic rocks in these conglomerates. Thin units of cross-bedded, grey arkose are locally interbedded with the conglomerate (Frarey, 1977, Bennett et al., 1991). The granitic mega-clasts of the conglomerate member are predominately pale grey in contrast with the predominantly reddish hues of the underlying Archean basement rocks. A few of the granitic Main environment of deposition COBALT GROUP Main rock types Arkose, subarkose, quartz arenite quartz-pebble conglomerate Disconformity Cycle 2 QUIRKE LAKE GROUP Cycle 3 Bar River Formation Gordon Lake Formation Lorrain Formation Gowganda Formation Serpent Formation Espanola Formation Bruce Formation Fluvial Mudstone, siltstone, carbonate (Espanola Fm.) Marine HOUGH LAKE GROUP Diamictite, mudstone, siltstone clast supported conglomerate Mississagi Formation Pecors Formation Ramsay Lake Formation Elliot Lake Group Cycle 1 Reducing Atmosphere inferred from grey-hued sandstones. Oxidizing atmosphere inferred from presence of red sandstones The conglomerates and sandstones of the Livingstone Creek Formation (Frarey, 1967, 1977) form the lowermost Huronian formation. The Livingstone Creek Formation is at least 1200 feet (400 m) thick in the Sault Ste Marie area and between 300 feet (100 m) and 1000 feet (300 m) thick in the Thessalon area. It consists of two distinctive rock types: an upper, well-sorted, grey sandstone and clast-supported polymictic conglomerate (Bennett et al, 1991), (Figures 1.3, 1.6, 1.7). Glaciogenic Volcanic rocks, marine and fluvial deposits. Paleoplacer uranium deposits Note: Where present, the upper part of the Gowganda Formation is predominantly mudstone. Figure 1.4 The cyclicity of Huronian rocks PDF compression, OCR, web-optimization with CVISION's PdfCompressor �km Elliot Lake Paleocurrents - Matinenta Fm. Lake Huron 50 Huronian Supergroup Figure 1.15: Paleocurrents in the Mississagi and Matinenda Formations N Sault Ste Marie 0 Post Huronian rocks LEGEND Paleocurrents - Mississagi Fm. Archean rocks Some modifications made for display purposes. Data from Long, 1977 and McDowell, 1957. Sudbury Igneous Complex Sudbury 5 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �No subsurface information No subsurface information Murray Fault City of Pecors Lake Elliot Lake Area East Bull Lake Complex No Huronian volcanic rocks (from Drilling Dolleyberry lake area Archean rocks Fault Gabbro, anorthosite East Bull Lake Suite Sandstone , polymictic conglomerate (from outcrop data). Livingstone Creek Fm. The areal extent of some units are exaggerated Crazy Lake Area No Huronian volcanic rocks (from drill hole data) Lake Huron Foul Bight No Huronian volcanic rocks (from drill hole data)) Mafic dikes of Thessalon Fm. intrude Livingstone Creek Fm. East Bull Lake Suite present (From drill hole data) No subsurface information elt eB ak rL No subsurface information Quartz pebble conglomerate at base of Thessalon flows. (locally radioactive) Haughton Tp. area pe Co o Thessalon ) ke ke La La ss en Ba rde rea e a b (A Figure 1.6: The distribution of some formations in the Elliot Lake Group Sault Ste. Marie No subsurface information Duncan Volcanic belt Basalt, andesite, rhyolite etc. (From outcrop and drill hole data) Thessalon Formation Upper Huronian rocks Meso-Proterozoic and Phanerozoic rocks LEGEND 6 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �7 megaclasts in the predominately grey conglomerate near the south end of Pine Ridge Road show the distinct texture of the typical Archean, massive, pink, k-spar megacrystic granite – but with only a hint of the pink color in the phenocrysts. The grey colour of the Livingstone Creek Formation conglomerates appear to be due to the reduction of ferric iron in the feldspars of the granitic clasts, and not a result of differing provenance as some authors have suggested. This conclusion is supported by the writer’s observation that granitic rocks in a “paleosol zone” a few meters to a few tens of meters below the base of the Livingstone Creek Formation commonly display a grey colour as well. conglomerate member occurs as far east as Pecors Lake (Figure 1.6) on the south limb of the Quirke Lake Syncline (Jensen, 1990) and below Huronian volcanic rocks south of Stinson Lake in the Elliot Lake area. The Livingstone Creek Formation has not been recognized east of the Quirke Lake Syncline. The clast size, local source and low stratigraphic position of the Livingstone Creek conglomerates are consistent with deposition on an alluvial fan. The uniform, fine- to medium-grained sand of the trough cross-bedded sandstone member suggests a different, although probably related, more distal depositional environment than that of the conglomerate. The sandstone member may represent deposition by median streams flowing in a fault-bounded valley with walls of Archean rocks partly covered by alluvial fans (Bennett et al., 1991). The well-sorted nature of the sandstone suggests an aeolian component or even aeolian deposition as proposed by Meyer (1983). The grey, sandstone member of the Livingstone Creek Formation may be distinguished from most Huronian sandstones by the uniform grain size (fine- to medium-sand) of the former. Mudstone and pebbly units are lacking within the sandstone member of the Livingstone Creek Formation. Another characteristic feature of the sandstone units is the presence of carbonate along the foreset beds of the commonly Huronian Magmatism well-developed trough cross-beds. Introduction At Maud Lake in Duncan Township near Sault Ste Marie fine to medium-grained, pale-grey quartz-arenite forms the upper few meters of the Livingstone Creek Formation, suggesting a more prolonged period of weathering in that area. Four distinct more-or-less coeval (ca. 2450 Ma), post-Kenoran igneous rock sequences are associated with the Huronian Supergroup: (1) Mafic dike swarms in the basement rocks but not found cutting the Huronian Supergroup. In addition to the well-known exposures in the (2) Igneous complexes of layered Thessalon and Sault Ste Marie areas the gabbro/anorthosite. (3) Mafic to felsic volcanic Livingstone Creek Formation has been flows. (4) 2.48 Ga. felsic plutons in the Sudbury correlated with grey sandstone and conglomerate area (Krogh et al., 1996). near Crazy Lake in Nicholas Township (Bennett, The widespread intrusion of post-Huronian (Ca 1978; Bottrill, 1971) (Figure 1.6). The writer 2200 Ma) Nipissing gabbro/diabase dikes and correlated a basal grey sandstone unit directly sills have yet to be placed in a plate tectonic underlying the Matinenda Formation in context. Haughton Township (Figure 1.6) with the Livingstone Creek Formation (Bennett et Basement Dikes al.1991). An area of clast-supported, grey granite-cobble conglomerate near Samried Lake The north to northwest trending Matachewan-Hearst dike swarm is the second reported by Jackson (2001) is probably an largest dike swarm of the Canadian Shield. The erosional remnant of the Livingstone Creek 2.45 Ga age of the swarm (Heaman, 1988) is Formation. Other probable remnants of the PDF compression, OCR, web-optimization with CVISION's PdfCompressor �8 essentially the same as that of the East Bull Lake thick), and Stobie Formations (1500 m thick) and the felsic, Copper Cliff Formation (760 m Suite and the Huronian volcanic rocks of the thick) . Krogh et al. (1984) obtained and age of Copper Cliff Formation. 2450+25-10 Ma (U-Pb zircon) age for rhyolite of the Layered Gabbro/Anorthosite Complexes Huronian Copper Cliff Formation; the only Huronian supracrustal rock for which there is an Several petrographically distinctive, mafic to absolute age. The volcanic rocks of the Sudbury ultramafic sills and cone-shaped bodies have been recognized between the eastern end of the area show evidence of submarine eruption from fault controlled vents along the edge of a Sudbury Igneous Complex and the nose of the depositional basin into which arkosic sandstones Quirke Lake Syncline. The intrusions are were being transported from the Archean characterized by the presence of anorthositic phases, and locally by a well-developed, primary granitic terrain to the north, while turbidites rhythmic layering of alternating anorthositic and were deposited from sediment from both the volcanic and rejuvenated basement marginal to gabbroic layers. Segregations, dikes and the basin (Card, 1978). sheet-like bodies of granophyre are locally present (Card and Palonen, 1976; Peck et al., The volcanic rocks of the Sudbury differ in 1995). Major intrusions of gabbro-anorthosite terms of internal stratigraphy, overall thickness are found at Agnew Lake (Card and Palonen, and depositional environment from the 1976) and at East Bull Lake (Born and James, Huronian volcanic rocks in the Sault Ste 1978; Kamineni et al., 1984) Marie-Elliot Lake area. Krogh et al. (1984) reported a U-Pb age The Huronian Volcanic Rocks of the Sault 2491+5-5 Ma for zircons from the Ste Marie-Elliot Lake area gabbro-anorthosite intrusion at Agnew Lake and 2480+10-5 for the East Bull Lake Frarey (1967) named the Huronian volcanic gabbro-anorthosite body. The similarity of these rocks in the Thessalon and Sault Ste Marie ages to the 2450+25-10 Ma (U-Pb zircon) age areas that overlie the Livingstone Creek of rhyolite of the Huronian Copper Cliff Formation as the “Thessalon Formation” Formation (Krogh et al., 1984) suggests that (Figures 1.3, 1.6). these gabbro-anorthosite intrusions form part of This writer has examined all known exposures of the early Huronian volcanic events. Bennett et Huronian volcanic rocks as well as all available al. (1991) report two additional bodies between drill-core and drill-hole logs reporting volcanic Blind River and Thessalon. Layered anorthosite rocks in the Elliot Lake–Sault Ste Marie area. It and associated coarse-grained gabbroic rocks was concluded that there is no credible evidence form a sill-like body about 5 km long at Foul for more than one period of Huronian volcanism Bight on the North Shore of Lake Huron in the area of the study and that all the (Figure 1.8). A drill-hole drilled from the ice Huronian volcanic rocks west of the nose of the south of the Town of Thessalon intersected Quirke Lake Syncline are stratigraphically layered gabbroic rocks (Figure 1.6). All correlative with the Thessalon Formation gabbro-anorthosite bodies found to date have (Figure 1.6)(Bennett 1978, Bennett et al 1991). been emplaced in the Archean basement at, or just below, the Archean-Huronian boundary. Unfortunately, none of the many attempts to obtain an absolute age determination from rocks Huronian Volcanic Rocks of the Sudbury of the Thessalon Formation have been Area successful. However, the spatial association of The Huronian bimodal volcanic sequence in the the East Bull Lake intrusions with the Huronian volcanic rocks of the Quirke Lake Syncline Sudbury area has been subdivided in the suggests Huronian volcanism in that area predominately mafic Elsie Mountain (1000 m PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Th,U,py THESSALON AREA py HAUGHTON-OTTER TWPS Feeder dike of Thessalon Fm. Th, U, py Fault U,Th, py U,Th,py ELLIOT LAKE AREA Unconformity or disconformity Quartz-pebble conglomerate Granite, greenstone ARCHEAN BASEMENT Grey arkose, subarkose Polymictic conglomerate LIVINGSTONE CREEK FM. Basalt, andesite, rhyolite flows basalt dike THESSALON FM. CRAZY LAKE AREA NICHOLAS TWP. Paleosol on Livingstone Cr. Fm and basaltic dike (Thessalon Fm.) The thickness of some units is exaggerated purposes of visualization Figure 1.7: Stratigraphic relationships in the Elliot Lake Group. METERS SAULT STE MARIE AREA 1000 LEGEND Arkose, grit quartz-pebble conglomerate MATINENDA FM. ELLIOT LAKE GROUP HOUGH LAKE GROUP 9 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �10 - -4 JjJ622910LJ EW Figure 1.8: Layered anorthositic gabbro. Foul Bight, Lake Huron. Figure 1.9: Stratigraphic relationships at Cullis Lake, Thessalon area. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �11 Thessalon Basalt Upper Island Lake Bass Lake 500 m 1200 m 650-850 m Unit6 Unit 1 Unit 6 Unit 6 Rhyolite Unit 5 Basalt to andesite cycles Mg-lava Radioactive, Quartz-pebble conglomerate Livingstone Creek Fm. Unit 4 Unit 3 Unit 3 Unit 2 Unit 1 Unit 1 Livingstone Creek Fm. I'll Unit 7 ? Livingstone Creek Fm. From Tomllinson, 1996 Figure 1.10: Internal Stratigraphy of the Thessalon Formation in the Sault Ste Marie Area occurred about the same time as the emplacement of the East Bull Lake intrusions (2480 Ma). Given the potential error for the relevant age dates (Krogh et al., 1984), the age of the Thessalon Formation may not differ significantly from that of the Copper Cliff Formation of the Sudbury area (2450 Ma). The maximum thickness of the Thessalon Formation in the Sault Ste Marie area is approximately 650 m (2100 feet) and 820 m (2700 feet) (Frarey, 1977). Diamond drilling has indicated at least 670 m (2200 ft) of Thessalon volcanics under Lake Huron south of the town of Thessalon. The Thessalon Formation may be as much as 1080 m (3500 feet) thick north of Bass Lake in Aberdeen Township. in the Dollyberry Lake, Pecors Lake and Thessalon areas. The lower flow sequences show much lower concentrations of Ni, Cr, and contain higher amounts of Ti and P than the upper flows. In the Dollyberry Lake area the upper basalts appear to be missing. This is assumed to be due to erosion (unpublished analyses in the writers files). Tomlinson (1996), completed a comprehensive analysis the geochemistry of the Huronian volcanic rocks between Sault Ste Marie and Thessalon, concluded that the lavas of the Thessalon Formation are divisible into 7 distinct units based on mapping, petrography and major and trace element geochemistry (Figures 1.10, 1.11). The 7 units were grouped into two “lava In the Sault Ste Marie, Thessalon and Aberdeen series”. The upper lava series (units 6) of Lake areas the Huronian volcanics rocks can be Tomlinson, 1996) is equivalent to the upper tholeiitic basalt sequence of Bennett et al. subdivided into upper tholeiitic basalt unit and lower complex or “mixed member”(Bennett et al (1991). The lower lava series (units 1-5, of Tomlinson, 1996) consists mainly of basaltic 1991) which includes more fractionated rocks andesite with subordinate, local rhyolite, including basaltic andesite, tholeiitic andesite, mugearite, andesite and high magnesium basalt mugearite, hawaiite, rhyolite, basalt. flows; and corresponds to the “diverse member” Magnesium-rich basalts with some of the chemical characteristics of komatiites are present of Bennett et al. (1991). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �12 Bennett et al. (1991) proposed that the upper, basaltic flows of the Thessalon Formation (upper lava series) probably represent part of a continental flood basalt sequence, while the diverse member (lower lava series) appears to have erupted from central vents. The Huronian volcanic rocks of the Quirke Lake Syncline display lithological and geochemical similarities to the lower lava series of the Thessalon Formation west of the Quirke Lake Syncline (G. Bennett, unpublished data). The upper, tholeiitic basalt flows of the Thessalon Formation (upper lava series of Tomlinson, 1996) are almost uniformly greenish-grey fine- to medium-grained tholeiitic metabasalt. The essential minerals are albite, actinolite, chlorite, clinozoisite, epidote and Fe-Ti oxides. Primary clinopyroxene is present in only a few samples of basalt from the Sault Ste Marie area. The andesitic rocks of the lower lava series of are generally darker coloured and commonly contain stilpnomelane and biotite with green pleochroism (Fe+3 rich?) in addition to albite and actinolite (Bennett et al. 1991). Tomlinson (1996) reported the presence of tremolite in some flows. Quartz is a minor component of basaltic and andesitic types. In most areas the metamorphic grade of the Thessalon Formation flows is lower greenschist facies although the presence of albite and primary pyroxene along with elevated soda indicates sub-greenschist grades at the northern end of the Duncan volcanic belt near Sault Ste Marie. “The field and geochemical data indicate that the lavas are comparable to modern day continental basalt and andesites but that their source was similar to that of island arc basalt. Figure 1.11 shows the lavas plotted on a variety of discrimination diagrams where they are most comparable to modern within-plate basalt. Their arc-basalt geochemical characteristics result from their derivation from an upper mantle that had been metasomatically enriched during subduction in the Kenoran orogeny. Units within the sequence were generated from different pulses of magma, each of which stemmed from a different region of the relatively homogenous Huronian mantle. Only the unit 2 lavas require generation from part of the mantle with slightly different characteristics to a source similar to that required to generate island arc basalt ID16 (a primitive basalt used by Tomlinson (1996) in making calculations. gb). Different degrees of partial melting of this source that contains garnet in the residue (at less than 29.8% partial melting) are responsible for the REE characteristics of each unit. In the generation of the lower lava series, different batches of magma underwent crustal contamination to a greater or lesser extent within the lower or upper crust (or both) prior to eruption. The upper lavas series rocks are not contaminated. In all but the unit 2 lavas crystal fractionation occurred both early, as magma ascended to shallower depths, and later, in shallow level sills or magma chambers. Early fractional crystallization was responsible for the low Mg, Ni and Cr values in most units, while later stage crystal fractionation was responsible for the major element and compatible element trends within single units, as shown on bi-element plots”(Tomlinson, 1996). Amygdules of epidote, chlorite, calcite, quartz and stilpnomelane, in complex zonal arrangements, are common. Flattened chlorite-filled amygdules a centimeter or less across is a distinctive feature of most mafic flows of the Thessalon Formation. Pillow structures are rare but were observed in most areas. Scoriaceous flow-tops and crosscutting breccias On the geochemistry and tectonic setting of the are commonly filled with a fine-grained mixture Thessalon volcanic rocks Tomlinson (1996) states: of quartz and grey to red secondary albite. Tomlinson (1996) states: PDF compression, OCR, web-optimization with CVISION's PdfCompressor �13 V - M!1PIU B- pA .100 JO kiP S .1 - MbV : VI VII 3- E 2- Mbk: VII C ki AVB: CD - B D A Figure 1.11: Discrimination diagrams for the Thessalon Formation in the Sault Ste Marie area. From Tomlinson, 1996. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �14 “Various lines of evidence from the Elliot Lake Group as a whole can be used to characterize the Huronian rifting event. Active rifting involves deep mantle upwelling or plume magmatism, which splits the continental crust, whereas passive rifting involves mantle diapirism and adiabatic upwelling induced by differential stresses and stretching in the lithosphere....…..The geochemical evidence from the Thessalon Volcanic Formation indicates that the source of the lavas was metasomatised upper mantle rather than a deep mantle or plume component. Structural subsidence patterns in the Archean basement (Zolnai et al., 1984) are thought to be responsible for lithospheric stretching, in-turn causing mantle upwelling, episodic partial melting and volcanism. This is supported by the presence of syndepositional faults in up to Archean basement. These features indicate that initially volcanism was a consequence of rifting. In active rifts just one uplift and melting event occurs as a plume impacts the lithosphere, but in passive rifts uplift and melting are episodic ........... Multiple erosional surfaces within the Elliot Lake Group indicate that numerous episodes of uplift occurred. Based on these lines of evidence the Huronian rifting event can best be characterized as a typical passive rifting event” (Tomlinson, 1996). Jolly (1987) concluded the Thessalon Formation is a continental flood basalt sequence associated with continental rifting. Sedimentary Rocks Associated with the Thessalon Formation Some early reports refer to the presence quartz-pebble conglomerate within the Livingstone Creek Formation. The writer has examined most of the known occurrences of the Livingstone Creek Formation between Sault Ste Marie and the Quirke Lake Syncline and found no interbedded quartz –pebble conglomerate. However at many locations a thin unit (< 1 m) of radioactive, pyritic, quartz-pebble conglomerate overlain by a few meters of coarse, arkose was found to lie upon the Archean Figure 1.12: Sandstone of the Livingstone Creek Fm. Overlain by basement or directly upon radioactive quartz-pebble conglomerate and grit of the Thessalon the Livingstone Creek Fm. Thessalon Township. Formation where the latter is present. In the absence of the Livingstone Creek Formation the 400 meters of Huronian sedimentary rocks quartz-pebble conglomerate lies directly upon the below the volcanics. G. Bennett (personal basement rocks. In Duncan Township of the communication, 1996) has also described Sault Ste Marie and Thessalon areas this syndepositional features (at 2 locations near quartz-pebble conglomerate-arkose sequence is Elliot Lake and Sault Ste Marie) where found within the lower flows of the Thessalon sedimentary rocks of the Livingstone Creek Formation. (Hay, 1963; Bennett et al., 1978; Formation infill fractures in the underlying Meyer, 1983) (Figures 1.6, 1.7, 1.9, 1.12). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �15 The Matinenda Formation Size of pyrite grains (mm) Bennett et al. (1991) proposed that the conglomerate-arkose units indicate the presence of a disconformity between the Huronian volcanics of the Thessalon Formation and the Livingstone Creek Formation. The wide distribution of these units (Figure 1.6) suggests that they reflect early Huronian erosional period of regional extent. The resistate nature of the mineral assemblage in the quartz-pebble conglomerate (assuming an oxygen deficient atmosphere) points to a period of extreme weathering. Some of this 2.4 quartz-rich regolith may still be visible as a quartz breccia upon the granitic basement west of 1.8 Highway 639 (Stop 1-11of this guide). mudstones of the McKim Formation (Card, 1978). The most abundant rock type of the Matinenda Formation in the Elliot Lake area is generally described as medium- to coarse-grained subarkoses, arkoses and grits consisting of poorly sorted quartz and feldspar grains set in a matrix of sericite and comminuted rock and mineral Correlation Coefficient = 0.93 1.2 The Matinenda Formation of 0.6 the Elliot Lake Group is a sequence of arenites and From Theis, 1976 intercalated quartz-pebble conglomerates which host the 7 14 21 28 35 42 49 56 63 once important uranium Pebble size (mm) deposits of the Elliot Lake Figure 1.13: Quartz pebble size vs. pyrite grain size area. The Matinenda in the ore beds of the Matinenda Formation. Formation is up to 180 m (600 feet) in the Elliot Lake area where it lies on Huronian volcanic rocks and fragments. The ratio of K-spar to sodic feldspar Archean basement (Roscoe, 1969; Robertson, is about 8:1. Minor constituents are pyrite, 1968,1976). In the Thessalon and Sault Ste calcite, chlorite, zircon and rarely, leucoxene Marie areas the Matinenda Formation consists coated iron oxides and monazite. Variable predominantly of fine-to medium-grained, amounts of sericite produce what has various subarkose to subwacke and is probably less than been described as a green, apple green or 50 m (150 ft) thick (Bennett et al., 1978). In greenish-yellow colour. Well-sorted Haughton Township the Matinenda Formation quartz-pebble conglomerate beds, containing lies upon grey sandstones that the writer equates well-rounded pebbles and cobbles of quartz and with the Livingstone Creek Formation (Figure chert, and pebbly subarkose units are scattered 1.7). throughout the coarse subarkose units of the In the Sudbury area clastic units correlated with Matinenda Formation, but are more common the Matinenda Formation are as much as 600 m near the base. (Robertson, 1968; Pienaar, 1963). (2000 ft) thick but they thin rapidly eastward and is intercalated with the mainly metavolcanic Trough cross-bedding, scour and fill structures rocks of the Stobie Formation and the are common in arkosic units (Robertson, 1968; Roscoe, 1969). Paleocurrent studies by PDF compression, OCR, web-optimization with CVISION's PdfCompressor �16 McDowell (1957) and Long (1977) have established a northwest source area for the sediment of the Matinenda Formation (Figure 1.5). Fralick and Miall (1989) suggest the Matinenda Formation was deposited from shallow braided streams flowing down a south dipping paleoslope which underwent tilting to the southeast during deposition. Kimberly et al. (1980) reported that the uraniferous conglomerates contained almost no magnetite-ilmenite and had very high potash/soda ratios. These are also features of Huronian paleosols, and suggest the sediment of the Matinenda Formation was formed by the intense weathering of a granitic source terrain as proposed by Roscoe (1969). Two southeast trending ore zones were recognized since the early days of uranium mining in the Elliot Lake camp. The Nordic zone, east of the City of Elliot Lake is about 1 mile (1.6 km) wide and 3.6 miles (5.6 km) long. The Quirke Zone, in the Quirke Lake area, is about two miles (3.2 km) wide and six miles (9 km) long. Paleotopographic features of the basement are thought to have had a determining influence on the position and orientation of the zones. Ore grade (circa 0.1 % U3O8) conglomerate occurs as persistent lenses with individual units up to 15 feet (4.5 m) thick. The uraniferous quartz-pebble conglomerates are commonly well developed at the base of the Matinenda Formation but also occur within the arkose up to 150 feet (45 m) above the base (Roscoe, 1969, Robertson, 1968). modified paleoplacer origin of the ores as outlined by Roscoe (1969). Advocates of this hypothesis propose that prior to the accumulation of significant free oxygen in the Earth’s atmosphere, southeastward flowing streams carried quartz, pyrite and uraniferous minerals released by the extensive weathering of the Archean granitic terrain, and deposited them as southeast trending units determined by basement topography. Evidence supporting the paleoplacer hypothesis was provided by Theis (1979) who demonstrated that a direct relationship exists between the size of quartz pebbles and pyrite grains (Figure 1.13), and the concentration of many other components of the ore zones. The McKim Formation The McKim Formation is the uppermost formation of the Elliot Lake Group. It is mainly a turbidite sequence of generally dark grey, subarkosic wackes, mudstones, subarkoses, lithwacke and litharenite that extends from the Blind River area to the Grenville Front. Graded beds, parallel laminations, ripple marks, ripple-drift cross-laminations and Bouma cycles, indicative of deposition by submarine turbidity currents are reported (Parvianen, 1973; Card et al., 1977). Robertson (1968) gives a thickness of 0-380 feet (0-100 m) for the McKim Formation on the south limb of the Quirke Lake syncline. It is missing on the north limb. The McKim Formation is thickest in the Sudbury area, where The quartz-pebble conglomerate consists mainly it is up to 2400 meters (8000 feet) thick. of well rounded pale-grey to dark-grey quartz The Murray Fault appears to have exerted an and chert pebbles set in matrix of pyrite with important influence on the deposition of the some quartz/feldspar grit and sericite. Pyrite McKim sediments. North of the Murray Fault commonly forms from about 15% of minable the McKim rarely exceeds a few hundred meters units. Radioactive minerals include uraninite, in thickness, whereas south of the fault the brannerite, and uranothorite (Roscoe, 1968). thickness of the McKim Formations is at least Monazite and zircon are characteristic heavy 2400 meters. Card (1978) noted the change minerals. from laminated siltstone in the west to more The sedimentological and mineralogical features wacke in the east indicated a change from more of the uranium bearing zones of the Elliot Lake distal to proximal facies, in turn suggesting more camp are now generally believed to support a tectonic activity and possibly a source for the PDF compression, OCR, web-optimization with CVISION's PdfCompressor �17 McKim sediments in the east. Fralick and Miall (1989) concluded that the McKim Formation of the Elliot Lake area represents a marine transgression that gradually drowned the Matinenda fluvial plain. extended to the Quirke Lake Syncline as well. The outcrop pattern of the Huronian volcanic rocks on geological maps also suggests that the volcanic rocks are erosional remnants preserved in basement depressions. Stratigraphic Relationships within the Elliot The Thessalon Formation may once have extended beyond the limit suggested from its Lake Group of the Sault Ste Marie-Elliot present outcrop distribution. Its present Lake Area. The stratigraphic relationship between the Matinenda Formation, Thessalon Formation and Livingstone Creek Formation is revealed on a rock face near the northern boundary of Haughton Township about 30 km (18 miles) north of the town of Thessalon (Figures 1.6, 1.7). At this location pyritic quartz-pebble conglomerate of the Matinenda Formation (Chandler, 1976) directly overlies an apple-green paleosol on grey, fine-grained sandstone, which the writer correlated with the Livingstone Creek Formation (Bennett et al.1990). About 600 meters (380 feet) northwest of the above occurrence arkose and quartz-pebble conglomerate of the Matinenda Formation disconformably overlies a steeply dipping, east-striking, mafic dike; the upper few meters of which is a sericite-leucoxene paleosol. The dike intrudes grey sandstone and apple-green paleosol of the Livingstone Creek Formation (Bennett et al. 1990, Bennett et al, 1991; Sutton and Maynard, 1993). Less than 2 km south of the above location Chandler (1976) identified a fault-bounded block of Thessalon Formation volcanic rocks with a minimum thickness of approximately 500 m. The mafic dike referred to above was a feeder for Thessalon flows, since the Thessalon Formation is the only known igneous activity at this stratigraphic position The above observations show clearly and unequivocally that there was a period of volcanic activity, as well as a significant period of erosion, separating the Matinenda Formation and the Livingstone Creek Formation. Since paleosols are well known upon Huronian flows in the Elliot Lake area, the sub-Matinenda unconformity seen in Haughton Township distribution may be but erosional remnants of a once extensive continental flood basalt sequence. This is to be expected on consideration of Macdougall’s (1988) statement “Since many CFB provinces have been uplifted and occur as elevated plateaus, dissection and removal is rapid”. He notes as well that the general scarcity of pre-Cretaceous Continental flood basalt provinces is probably due to their erosion. Hough Lake Group Introduction The Hough Lake Group (Robertson, et al., 1969; Roscoe, 1969) is lowest of the three Huronian groups that display the cyclic deposition of diamictite followed by a mudstone, siltstone, and turbidite or carbonate sequence; and overlain by a cross-bedded arenite unit. Each cycle is generally thought to represent a sequence of glaciogenic - marine - fluvial and/or shallow marine deposition (Figure 1.4). Ramsay Lake Formation The Ramsay Lake Formation is the lowermost formation of the Hough Lake Group and is the oldest of three such conglomerate units that define the base of Hough Lake, Quirke Lake and Cobalt Groups (Roscoe, 1969; Pienaar, 1963) (Figures 1.3, 1.4). The Ramsay Lake Formation is a widespread, relatively thin unit, which is up to 41 m thick in the Bass Lake area (Chandler, 1973). In the Elliot Lake area the thickness of the Ramsay Lake Formation is from 0 to just over 30 m. (Diamond drill logs Assessment files, Sault Ste Marie District Geologist’s Office). The Ramsay PDF compression, OCR, web-optimization with CVISION's PdfCompressor �18 Lake Formation is from 70 to 170 m thick in the common (Card, 1978; Robertson, 1976). The Pecors Formation is the result of transgressive Sudbury-Manitoulin area (Card 1978). units formed in deep water by turbidity currents Matrix supported polymictic conglomerate (Card, 1978). The presence of drop stones is (diamictite) is the most abundant rock type in evidence of a cold paleoclimate, and provides the Ramsay Lake Formation, especially near the supporting evidence for the glaciogenic origin of base. The lowermost few meters of the the underlying Ramsay Lake Formation. formation usually reflects the underlying rock type (Robertson, 1968; Parviainen, 1973). Mississagi Formation Minor amounts of mudstone, wacke and arenite The Mississagi Formation is a thick sequence of are locally present. predominantly grey, arenitic rocks extending Subrounded to well rounded pebbles and throughout most of the length of the Huronian cobbles of grey granitic rocks and angular to outcrop belt. Within the Quirke Lake syncline rounded clasts of very dark green to black the Mississagi Formation is from 344 to 704 m volcanic rocks generally form less than 30 thick. South of the Murray Fault the formation percent of the total volume of the diamictite. is notably thicker, being more than 3000 m in The dark matrix consists of quartz, feldspars, the Sudbury area (Card, 1978; Long, 1978). By chlorite, muscovite-sericite-illite and pyrite far the dominant rock type in the Mississagi (Parvianen, 1973). Formation is moderately well sorted, medium to coarse-grained subarkose and arkose. Small to Although some writers have argued for a debris medium quartz/chert pebble conglomerate is a flow origin, most writers now accept the Ramsay minor component of the formation, but is more Lake Formation as having a significant common in the western and northeastern parts glaciogenic component (Roscoe, 1969; of the Huronian belt. Fine-grained pyrite along Robertson, 1976). Fralick and Maill (1989) forsets commonly results in rusty staining of identified an ice proximal association of pebbly outcrops. Greenish, sericitic units form sandstone and diamictite; subaqueous gravity relatively thin planar-bedded units between flows and ice rainout deposits; and ice-proximal, cross-bedded sandstones. Palonen (1973) fluvial outwash deposits. provided evidence supporting a marine origin for the Mississagi Formation; however, Long (1978) Pecors Formation argued that the abundance of mud-grade matrix The Ramsay Lake Formation is conformably in the immature arenites, and the predominance overlain by a sequence of generally dark, bedded of unimodal paleocurrent directions, and the and laminated wackes, mudstone, siltstones and lack of quartz arenites argued against a marine sandstones of the Pecors Formation (Roscoe, environment for the Mississagi Formation. He 1969). The Pecors Formation is 30 m thick at concluded that the Mississagi Formation was Quirke Lake (Robertson, 1968) but is as much deposited from braided streams with low to as 900 m thick south of the Murray Fault in the intermediate sinuosity and high width to depth Sudbury area (Card, 1978). It was not identified ratios. in the area between Thessalon and Sault Ste Bedding units are commonly about a meter thick Marie (Frarey, 1977). Ripple marks, graded bedding, cross-laminations parallel laminations, but range from a few centimeters to over four meters. Trough cross-stratification and ripple ball and pillow structures, clastic dikes and cross-stratification are common sedimentary slumpage features have been reported in the structures (Long, 1978). Individual formation. The basal part of the formation is commonly laminated resembling varves and, in cross-stratified beds may show grain size gradation (McDowell, 1957). places, has dropstones (Robertson, 1968; Parvianen, 1973). Partial Bouma sequences are PDF compression, OCR, web-optimization with CVISION's PdfCompressor �19 Long (1978) measured over 2500 cross-stratified units in the Mississagi Formation (Figure 1.5). Two major stream systems were recognized: a stream system flowing southeast to easterly from the Sault Ste Marie area joined a stream system flowing southwestward from the Cobalt Plain to form southward flowing system southwest of the Sudbury area. These observations suggest that the area now occupied by the Sudbury Igneous Complex was elevated during the period of Mississagi deposition(Long, 1978). Aweres Formation fan system that extended in a more or less northeast direction beyond the present northern limit of the groups below the Cobalt Group. The Mississagi Formation may represent a more distal depositional environment than that of the Aweres Formation. Quirke Lake Group Bruce Formation The Bruce Formation extends from the Garden River Indian Reserve near Sault Ste Marie to about 70 km northeast of Sudbury. It consists mainly of matrix supported and minor clast supported conglomerate. Pebbly wacke, arkose, wacke and siltstone are locally present. Robertson (1968) reports the Bruce Formation is from 79 m (260 feet) to 12 m (40 feet) thick in the Elliot Lake area. It is 26 m (85 feet) to 37 m (120 feet) thick under the greater part of the Quirke Lake Figure 1.14: Diamictite of the Bruce Fm. Syncline. Highway 108, Elliot Lake area. In the Sault Ste Marie area, the 1700 m thick sequence of conglomerate and sandstone of the Aweres Formation (McConnell ,1927) unconformably overlies the mafic volcanics of the Thessalon Formation (Figures 2.1, 2.2). The internal stratigraphy and lithologies of the Aweres Formation is consistent with deposition in an alluvial fan environment. The base of the formation is almost entirely of mafic volcanic clasts while higher levels show a progressive increase in granitic components. The uppermost rocks of the Aweres Formation south of Aweres Lake are mainly arkose with thin pebble conglomerate beds. The lithological variation with stratigraphic height indicates the continual erosion of an uplifted fault-bounded plateau of Huronian volcanic rocks. Pebble- to boulder-sized, angular to subrounded clasts generally consist of pale-grey granitic rocks, Archean supracrustal rocks and fine-grained mafic clasts. The upper parts of the formation may contain up to 5% carbonate (Robertson, 1968). The Bruce Formation is generally interpreted as a tillite with minor beds and lenses of glacially derived sandstone. Dropstones have been observed in laminated units (Robertson, 1968). Casshyap (1969) concluded the formation was The distinct lithology of the Aweres Formation deposited from terrestrial wet-base glaciers. prevents its direct correlation with other However Sims et al. (1981) proposed that the Huronian rocks. The upper surface is partly fault Bruce Formation is an accumulation of debris bounded but is unconformably overlain by the flows released as a result of normal faulting, a Gowganda Formation on Highway 556 (Figure sudden increase in paleoslope and a sudden 2.3). It is possible that the Aweres Formation is increase in water depth. an erosional remnant of a once extensive alluvial PDF compression, OCR, web-optimization with CVISION's PdfCompressor �20 Espanola Formation The Espanola Formation is the only widespread carbonate unit of the Huronian Supergroup. It is a present from the Sault Ste Marie area to the Maple Mountain area about 70 km northeast of Sudbury. Its widespread distribution and distinctive lithology make it the most useful stratigraphic marker unit of the Huronian Supergroup. In the Elliot Lake area the Espanola Formation can be subdivided into three parts: a lower limestone member (the Bruce Limestone), a middle siltstone, wake, arenite member (the Espanola Greywacke) and an upper dolomite member (The Espanola Limestone) (Robertson, 1968). The latter generally contains 3 - 4% total iron which gives a distinctly brownish colour to weathered surfaces. All 3 members are generally thinly bedded to laminated. The threefold subdivision is not so well developed south of the Murray Fault (Young, 1982). Contacts between members tend to be gradational. Intraformational breccias, mud cracks, ripple-marks, flame structures and ball-and-pillow structures are common sedimentary features. Hofmann et al. (1980) described stromatolites in the Espanola Formation on Quirke Lake. These features suggest deposition in a quiet shallow water environment with carbonate deposition being interrupted by influx of fine-grained sediment. Young (1973) proposed that the relatively sharp change from the diamictites of the Bruce Formation suggests warm climatic conditions following a glacial advance. However, the recognition of carbonate deposition at high latitudes (Williams, 1975) and the association of detrital uranium with intense chemical weathering (Maynard et al, 1991) adds uncertainty to attempts to model Huronian paleoclimate. activity preceding deposition of the Gowganda Formation of the Cobalt Group. Reported thickness estimates of the Serpent Formation range from 150 up to 1500 m (Bennett et al., 1991). Robertson (1968) states that nowhere in the Blind River-Elliot Lake area is there any evidence that the total thickness of the Serpent Formation has been preserved. The Serpent Formation is mainly fine to medium-grained, quartz arenite and arkose. Conglomeratic units have been noted, especially near the base of the Formation. Carbonate is a significant component near the base of the formation in the Elliot Lake area (Robertson, 1968). Planar cross-bedding, festoon cross-bedding, rip-up clasts, fine-laminations and mud cracks have been reported. Long (1976) proposed that the Serpent Formation was deposited in a distal braided stream environment with calcareous units representing a sabkha environment. Young (1982) noted that the presence of very large cross-beds and a bi-modal size distribution suggest aeolian processes may have been active at least locally. Cobalt Group Gowganda Formation The Gowganda Formation is a complex sequence of conglomerates, sandstones, siltstones and mudstones, which comprise the lowermost formation of the Cobalt Group. Its thickness ranges from 1070 m in the Sault Ste Marie area; from 970 to 1150 m around Whitefish Falls, on the North Shore of Lake Huron; and from 950 to 2700 m near Sudbury. Near Dunlop Lake, in the Elliot Lake area, the Gowganda Formation is about 600 m thick. Matrix-supported conglomerates are common, especially in the lower parts of the formation. However, these are commonly intercalated with clast-supported conglomerates, sandstone, Serpent Formation wacke units. Laminated mudstones and siltstone are especially prominent in the upper parts of The Serpent Formation is found throughout the Gowganda Formation. Many occurrences of much of the Huronian belt; however, it is locally ice-rafted drop-stones have been reported in removed by erosion during a period of tectonic laminated mudstone/siltstone units. Individual PDF compression, OCR, web-optimization with CVISION's PdfCompressor �21 units are generally relatively thin and discontinuous making subdivision of the Gowganda formation difficult except in well-exposed areas. Most granitic clasts of Gowganda conglomerates have a distinctly pinkish or reddish hue, in comparison to the grey, granitic clasts in the matrix-supported conglomerates of the stratigraphically inferior Ramsay Lake and Bruce Formations. Also, pink and red hued sandstones first make their appearance in the Gowganda Formation. Roscoe (1969) pointed out the appearance of red coloration (i.e. ferric iron) just above the basal units if the Gowganda Formation, and argued that it represents the appearance of free oxygen in the Earth’s atmosphere, and a change from the previously reducing atmospheric conditions that allowed the accumulation of readily oxidized minerals such as pyrite and uraninite in a fluvial environment. Lorrain Formation The Lorrain Formation is generally well exposed throughout most of the Huronian belt, where it commonly forms a background to some of the most scenic views in northern Ontario. It is overwhelmingly an arenitic sequence, with local siltstone units present in lower parts of the section. The formation is up to 2500 m thick near Sault Ste Marie and in the LaCloche Syncline, southwest of Sudbury. It is up to 2300 m thick in the Cobalt Embayment. In general, the lower part of the Lorrain Formation is dominated by pink, arkosic sandstones; the middle by hematite-rich subarkose and quartz-arenite; and the upper portion by pale grey to white mature, quartz-arenite. A distinctive jasper-pebble conglomerate found in the Sault Ste Marie area is a popular decorative stone, known locally as “pudding stone”. The presence of aluminous minerals is a characteristic feature of the upper, quartz-arenites of the Lorrain Formation. Diaspore and kaolinite The depositional environment of the are common in the quartz-arenite of the Sault Ste diamictites in the Gowganda Formation have Marie area and north of Elliot Lake (Wood, 1973) been the subject of discussion since Coleman while kyanite, andalusite and kaolinite are present (1905) proposed a glacial origin for the matrix as metamorphosed equivalents in the LaCloche supported conglomerates. Many later writers Lake-Kilarney area (Card, 1978). Young (1973) including Ovenshine (1965); Casshyap (1969); and Wood (1973) interpret the presence of Lindsay (1971); Young and Nesbitt (1985), diaspore and kaolinite as the result of and others have also supported a glacial, post-depositional in-situ alteration of feldspar glacial-marine or glaciolacustrine origin for the under hot and humid climatic conditions. Gowganda Formation. The presence of abundant detrital hematite in the Card (1968) concluded that, although Lorrain Formation and the occurrence of glaciation may have supplied coarse detritus to thorium-rich, monazite-bearing, quartz-pebble the basin initially, debris flows and turbidity conglomerate north of Elliot Lake has been currents, released by vertical tectonic interpreted by Frarey and Roscoe (1970) as movement, may better explain the thickness indicating an oxidizing environment. variations, rock associations and distribution he observed it the Sudbury Manitoulin area. Planar and trough cross-bedding are common, as Roscoe (1969) also emphasized that glaciation are ripple marks and other primary depositional is only one of the several processes responsible structures. There is no consensus as to the for the deposition of Gowganda sediments. depositional environment of the Lorrain PDF compression, OCR, web-optimization with CVISION's PdfCompressor �22 Formation. Most of the sedimentary structures of the Lorrain Formation can be found in either shallow marine or fluvial environments. Wood (1973), Young (1973) and Frarey (1977) favored a fluviatile model while Pettijohn (1970) supported a marine environment for the Lorrain Formation. Card (1976) proposed that the Lorrain Formation represents near-shore coastal shelf deposition during episodic marine transgression and regression. Gordon Lake Formation The Gordon Lake Formation displays a gradational contact with the underlying Lorrain Formation. It is made up predominantly of variegated mudstone, siltstone, chert and minor fine-grained sandstone. Robertson (1986) subdivided the Gordon Lake Formation of the Flack Lake area into a lower member of reddish arenite, siltstone, and chert with anhydrite and gypsum nodules; a middle member of green siltstone and mudstone; and an upper reddish mudstone, siltstone and chert. Abundant sedimentary features include small-scale cross-bedding, ripple marks and desiccation cracks. Some of the features of the Gordon Lake Formation are unique for the Huronian Supergroup. Wood (1973) noted on the abundance of feldspar in the formation, a marked contrast to rocks of the immediately underlying Lorrain Formation. He also described hematite ooliths and the abundance of grains in the 0.02 to 0.05 mm range, a relatively uncommon grain size in sedimentary rocks. Since this size is found in loess deposits, Wood (1973) proposed that the quartz silt of the Gordon Lake Formation was formed by glacial action and carried by the wind and deposited in a tidal flat environment. Bar River Formation The Bar River Formation is the uppermost formation of the Huronian Supergroup. It is characterized by quartz-arenite with minor ferruginous arenite and siltstone. It is approximately 300 m thick in the Flack Lake area, north of Elliot Lake. Wright and Rust (1985) concluded that the Bar River Formation was deposited in a tidal environment. Nipissing Intrusions Dikes, sills, and cone sheets of gabbro, diabase and granophyre, commonly referred to, as “Nipissing diabase” are the most widespread igneous rocks associated with the Huronian Supergroup. Baddelyite from Nipissing gabbro sills in the Gowganda area has been dated at 2219 Ma; the minimum age for the Huronian Supergroup (Corfu and Andrews, 1986). Buchan and Card (1985) report that paleomagnetic data suggests at least two periods of Nipissing intrusive activity. Olivine-bearing hypersthene gabbro, gabbro, feldspathic pyroxenite, two-pyroxene quartz gabbro, hornblende gabbro, granophyric gabbro and granophyre have been identified in Nipissing intrusions. Many of the Nipissing sills are characterized by chilled margins 50 cm to 5 m wide, overlain by 10-20 m of quartz gabbro, then 100-500 m of hypersthene-poor gabbro-norite and vari-textured diabase (Lightfoot and Naldredt, 1996) Nipissing intrusions are widely and evenly distributed throughout the Huronian belt but, with few exceptions, are not recognized within the Archean terrain. Individual intrusions may be up to several hundred meters thick and extend over an area of several hundred square kilometers. The form and orientation of Nipissing intrusions indicate that their emplacement may be controlled by older faults, folds and competency of the enclosing rocks (Card and Pattison, 1973). Lightfoot and Naldredt (1996) discuss the geochemical characteristics of the Nipissing magmas and the potential for platinum group metal deposits. They conclude that the Nipissing magmas were emplaced into the Huronian sedimentary sequence over a period of less than 10 million years. Parental magmas of remarkably uniform composition underwent PDF compression, OCR, web-optimization with CVISION's PdfCompressor �23 in-situ contamination and differentiation within the intrusions Lightfoot and Naldredt (1996). A spatial association between Nipissing intrusions and both magmatic and vein-type mineralization have long been recognized. Huronian Paleosols and Evidence for the Accumulation of Oxygen in the Huronian Atmosphere It has long been recognized that the study of Huronian paleosols (ancient soil profiles) could provide information pertaining to the development the Earth’s atmosphere and climate during the Proterozoic. Since iron is much less soluble in the ferric state than when in the ferrous state, the behavior of iron in paleosols should provide some indication of the oxygen partial pressure of the environment. Many of the best descriptions of Precambrian paleosols have been from those associated Huronian unconformities (Gall, 1992). Grandstaff et al. (1986) identified 8 features of paleosols; most of which have been described in Huronian paleosols. These features are: 1. Stratiform 2. Relatively thin (<20 m) Roscoe, 1970; Gay and Grandstaff, 1980; Kimberly et al, 1984; G-Farrow and Mossman, 1988; Prasad and Roscoe, 1991, Sutton and Maynard, 1992, 1993). Bennett et al. (1991) have proposed that there are three disconformities or unconformities within the Elliot Lake Group, which have, potential for paleosol development (Figure 1.7). These are in descending stratigraphic order the sub-Matinenda disconformity, the sub-Thessalon Formation disconformity, and sub-Livingstone Creek Formation unconformity. The sub-Livingstone Creek Formation unconformity is the basal unconformity of the Huronian Supergroup and is the only entirely sub-Huronian unconformity (Figure 1.3, 1.7). This unconformity is exposed in the Thessalon area, where the upper few meters of the Archean granitic rocks can be seen to progress from angular, slightly rotated blocks, separated by grey grit and fine-grained sandstone, upward, to more rounded boulders with a higher proportion of finer clastic material (Collins, 1925). This zone may be termed a “paleo-regolith”, since there is little or no obvious development of the yellow, sericitic paleosol commonly found in the younger, sub-Matinenda paleosols. 3. Transitional lower boundary-sharp upper boundary Prasad and Roscoe (1996) described two paleosols in the same drill core from the Denison Mine at Elliot Lake. A paleosol was found 4. Colour variations above Huronian volcanic rocks and another, less 5. Destruction of primary rock textures well-developed paleosol, was found upon accompanied by the development of soil textures Archean tonalite below a short section of quartz-pebble conglomerate and grit (Prasad, 6. Destruction of primary minerals with personal communication, 1997) below the 9 m formation of clay minerals or metamorphic thick volcanic unit (Figure 1.15). equivalents 7. Dikes of material from overlying sediment washed down into desiccation cracks in the soil 8. Rip-up clasts of overlying sediments Well-preserved paleosols below the Matinenda Formation in the Elliot Lake area have been described by many workers (Roscoe, 1969; Pienaar, 1963; Robertson, 1968; Frarey and The best developed, and most studied, Huronian paleosols have been found directly below the Matinenda Formation. On mafic rocks, the sub-Matinenda paleosols can generally be recognized by the presence of an upper, distinctly apple-green to yellowish, sericitic zone which grades downward, over few centimeters to several meters, to an underlying, PDF compression, OCR, web-optimization with CVISION's PdfCompressor �24 granitic rocks is generally lacking or relatively thin. black, fine-grained, chlorite-rich eluvial zone up to several meters thick. Abundant pseudomorphs of titanium oxides after ilmenite are a feature of paleosols on mafic igneous rocks. Rip-up clast of sericitic paleosol (the “argillite scraps” of mine geologists) are commonly found in the lower few meters of the overlying Matinenda Formation. Prasad and Roscoe (1996) report significant amounts of carbonate and pyrite in sub-Matinenda paleosols in the Elliot Lake area. Paleosol Denison Drill hole S-62 Sub-Matinenda Formation Paleosol K K 0 C CO2 Sub-Matinenda Paleosol -178 Sub-Lorrain Formation Paleosol Massive Basalt -1336 1850 -1950 Fe2 Fe3 Ca Mg Fe2 K Fe3 Paleosol [( Na liii Ville Marie granite Transition zone -899 500 Metabasalt-increasingly weathered upward Fet Mg Ca Sub-Thessalon Fm. Paleosol Chlorite, sericite rich bands in paleosol "Argillite" (paleosol) NaCa Mg Fe2 Fe3 Coglomerate Grit 0 Tonalite - increasingly weathered upward cm Hematitc breccia Lorrain Formation Sub-Matinenda paleosols commonly show the pronounced loss of soda found in most paleosols. Lime, and magnesia are also depleted, but there is generally a large increase in potash content (Gay and Grandstaff, 1980) (Figure 1.15). In most cases, iron and manganese are depleted in upper parts of the paleosol. This is held to provide good evidence of weathering in a reducing environment. However, Gay and Grandstaff (1980) noted an upward increase in The uppermost sections of sub-Matinenda Formation paleosols on Archean granitic rocks total iron in paleosol from the Pronto Mine area. After Prasad and Roscoe(1996), Rainbird et al. (1990). Figure 1.15: A comparison of sub-Matinenda Fm. and sub-Lorrain Fm. Paleosols and arkosic sedimentary rocks, is generally apple-green to yellowish rock composed mainly of quartz and sericite (Robertson, 1968; Gay and Grandstaff (1980); Sutton and Maynard (1992). Where the texture of the protolith is well preserved, but the original mineralogy is replaced, the paleosol may be termed a saprolith (Rainbird et al, 1990). The chlorite-rich eluvial zone of paleosols on They concluded this upward increase in iron content indicated the presence of free oxygen in early Huronian atmosphere although at approximately 1% of the present level (Gay and Grandstaff, 1980). They suggested that the loss of iron shown in most Huronian paleosols could be due to local reducing environments. Some writers have concluded that the increase in potash (as sericite) in Huronian paleosols is largely due largely to diagenetic and PDF compression, OCR, web-optimization with CVISION's PdfCompressor �25 metamorphic processes which may mask the environmental and hydrologic conditions operative during paleosol development (Gay and Grandstaff (1980); G-Farrow and Mossman (1988). purple siltstone member of the Lorrain Formation near Sault Ste Marie (Frarey, 1977). Not all workers accept the above explanation for the preservation of uraninite and pyrite, and the observed change of colour with stratigraphy. The mineralogy and geochemistry of Sub-Lorrain Ohmoto (1996) has stated “ the loss of total iron in paleosols of all ages is not due to a Formation paleosols have been described by Rainbird et al. (1990) and Sutton and Maynard reducing atmosphere but to the reductive (1992). In contrast to the older, sub-Matinenda dissolution of ferric hydroxides under an oxic paleosols. Sub-Lorrain paleosols commonly show atmosphere”. an enrichment of Fe+3 at the expense of Fe+2 Regional Tectonic Patterns and without a significant loss of total iron. Hematite Metamorphism is a common mineral in the upper parts of sub-Lorrain paleosols, in contrast to the presence Major structures of the Huronian belt follow two of pyrite in sub-Matinenda paleosols. In this main trends: 1) west-northwest trending folds respect the sub-Lorrain paleosols resemble many and faults of the Sault Ste Marie-Elliot Lake post-Huronian paleosols and are consistent with area; and 2) east to northeast striking folds and weathering in an oxidizing atmosphere (Prasad faults of the Sudbury-Manitoulin area (Figure and Roscoe, 1996; Rainbird et al., 1990). 1.16). These two structural orientations are associated with differing fold styles, Since pyrite and uraninite are unstable under metamorphic grade and metamorphic fabric. oxidizing conditions, the abundance of detrital Changes are most notable across the east-west pyrite and uraninite in the paleoplacer uranium faults of the Murray fault system. This northeast ore zones in the Matinenda Formation provide striking fault zone (Figure 1.16) is the most good evidence for an oxygen deficient significant structural feature of the Huronian atmosphere during weathering, transport and belt. Since many formations show a significant deposition of early Huronian sediments. increase in thickness south of the fault, it is In marked contrast to the common red beds of generally interpreted as an inverted growth fault. younger clastic sequences, sandstones (and most (i.e. an early listric normal fault, active during granitic clasts in conglomerates) below the sedimention; which, during a later compressive Cobalt Group are almost all drab coloured in regime, was converted to a thrust or reverse spite of the abundance of red and pink granitic fault (Card, 1978; Jackson (2001). rocks in the continental source area (Roscoe The rocks of the Huronian Supergroup have 1969, 1973). Frarey and Roscoe (1970) noted been subjected to several deformational events. the above and proposed that the drab colour of This is particularly evident south of the Murray lower Huronian clastic rocks is due to the lack of Fault zone. In the Whitefish Falls area (south of free oxygen in the atmosphere during the period the Murray Fault) Young and Nesbitt (1985) of deposition of lower Huronian rocks. conclude that some large-scale folding is related Red hued, hematite bearing rocks, which Roscoe to syn-depositional and/or post-depositional (1969) proposed mark the presence of an deformation of unconsolidated sediment. Early oxidizing atmosphere, make an appearance with syndepositional deformation is also indicated by the Gowganda Formation of the Cobalt Group, an important unconformity below the and are important in parts of the Lorrain and Gowganda Formation of the Cobalt Group; and Gordon Lake Formations of that group. What the presence of ragged, slumped contacts and may be the first true red-beds are found in the large slump blocks along major faults (Card, 1978; Young, 1983). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �26 Convincing evidence of at least one important pre-Nipissing (2.2 Ga) deformational event, assigned to the Blezardian Orogeny (Stockwell, 1982), is provided by the observation that Nipissing bodies in the Sudbury-Whitefish Falls area transects axial surfaces of major folds (Card, 1978). Jackson (2001) did not observe such relationships in the areas north of the Murray Fault. However, Jackson (2001) points out, as earlier noted by Robertson (1964), that the tendency of Nipissing dikes to occupy structures parallel to the axial plane of the Chiblow Anticline, suggests pre-Nipissing folding. Jackson (2001) also noted evidence of pre-Nipissing faults north of the Murray Fault zone. Following emplacement of the Nipissing intrusions but prior to the emplacement of the Sudbury Igneous Complex there was further deformation and regional metamorphism. Rb-Sr isotope studies of Huronian metasediments indicate that prograde metamorphism occurred at about 1.90-1.85 Ga (Fairbairn et al., 1969). This event is probably correlative with the Penokean Orogeny of Michigan-Minnesota (Sims et al., 1989). After the emplacement of the SIC and the deposition of the Whitewater Group, there is evidence of further deformation and low-grade metamorphism, followed by intrusion of granite plutons at about 1.75 and 1.5 Ga. The post-SIC deformation and retrograde metamorphism was found mainly south of the Murray Fault, especially in the area between the SIC and the Grenville Front. Shanks and Schwerdtner(1991) report that this deformation is characterized by south-dipping thrust faults or deformation zones with northeast-trending foliation and southeast-plunging lineation. The last recognized deformational feature affecting Huronian rocks is the movement along faults, which post-date the diabase dikes of the 1.25 Ga Sudbury swarm (Jackson, 2001). A study of magnetic fabrics, strain patterns, and microstructures in granitoid rocks of the Creighton and Murray plutons and their Huronian host rocks by Riller (1996) have provided information on the pre-2.2 Ga “Blezardian Orogeny” (Stockwell, 1982). Riller concluded that major folding and amphibolite facies regional metamorphism in the Sudbury area was coeval with the emplacement of the Creighton Pluton and Murray Plutons, which were dated at 2.3 Ga by Frarey et al (1982) and Krogh et al. (1984) and more recently at 2477+/- 9 Ma by Krogh et al. (1996). Large-scale dome-and-basin structures in Archean basement and Huronian cover rocks represent major south-vergent nappes. The Sudbury Structure, a deformed relic of an astrobleme, is superimposed on a major antiformal dome cored by Archean granulite and granitoid rocks and flanked by overturned Huronian strata on the south and several remnant rim synclines of Huronian rocks on the north (Riller, 1996; Bennett et al, 1997). In the Sault Ste Marie-Elliot Lake area fault and fold structures trend in general west-northwest to northwest direction. Folds are generally upright, open concentric structures with gentle, variably plunging hinges (Figure 1.16). There is only a weak development of minor tectonic structures. The metamorphic grade is subgreenschist (Card, 1978b). The major structural features of the Elliot Lake area include a gently south-dipping homocline south of the Flack Lake Fault, the open fold of the Quirke Lake Syncline and the Chiblow Anticline between the Quirke Lake Syncline and the Murray Fault. Jackson (2001) found no evidence of detachment at or near the basement-cover interface in the Elliot Lake area. He also proposed that the “inverted growth-fault” model as applied by Zolnai et al. (1984) to structural-stratigraphic relationships in the Huronian may, in some cases at least, be interpreted as thrust faults with flats following depositional boundaries, and ramps that cut up through the stratigraphic section. Given the data available, neither model could be rejected for major northwest trending faults in the Sault Ste Marie area (Jackson, 2001). Jackson (1997) points out that the curvature of the Flack Lake PDF compression, OCR, web-optimization with CVISION's PdfCompressor �27 M q V ±PLfl2I 4011f4' Mall uqicc1G blOCK roK6 gOflil ?b04 fl CCLGUA!IIG bLOAIUCG) QOMU4JLOMU ?!qG c°p°ii CLOnb L0CK2 EQflI4 (Op?GLAGq — bk6-COPOU 43LOflb LOCK2 qsLIAGq 410w VLCIJ6OU pO2GWGU4 I2lauq ELOU4 IGC4OU!C LOCI(2 (3flbGLiOL bLOAIUCa) E0fl14 wE2 VU4!Cl!LJ& bIG—CopoK Eafl14 (4LOC6OpJ6 OU 2flL4CC6 4L0C60P16 22 wee IU E r000iIou 241nc4n16 El r1uuaw6q 5 U0141J 04 Figure 1.16: Main structural features of the Huronian Supergroup. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �28 Fault is in the opposite direction to that expected if it is a thrust fault as proposed by Zolnai et al. (1984). In the Sault Ste Marie area the presence of trust faults, as suggested by out-of-sequence units, has been confirmed, in at least one location, by diamond drilling (Assessment files, Sault Ste Marie Resident Geologist’s Office). The Murray fault zone separates the moderately developed. More than one age of major and minor structures can be discerned south of the Murray Fault (Jackson, 2001). Metamorphism south of the Murray fault ranges from low greenschist to low amphibolite facies (Figure 1.17). Rocks of higher metamorphic grade occur in two zones or nodes, one along the Murray Fault zone and another northwest of the Grenville Front. Both zones coincide with major r.ø.uq 4dCIIU LJ rUM 1 w!qqç dLG6L)UCIJ!24 WQ OLGUAIIIG LLOU4 EIUIC IU4LUUIOUU bLOA!UCG Figure J1.17: Metamorphism of the Huronian Supergroup deformed, low metamorphic grade rocks to the north from the multi-deformed, higher-grade rocks of the Sudbury-Manitoulin area to the south. In the Sudbury-Manitoulin area is characterized by open to sub-isoclinal, flattened buckle folds with upright to northward overturned axial surfaces. Elongate domes and basin are formed by reversals in plunge. Penetrative axial place cleavage and steeply plunging rodding or mineral lineations are well anticlinoria, although in detail, metamorphic isograds transect fold axes (Jackson, 2001). Higher-grade metamorphic nodes do not coincide with the few granitic intrusions that intrude the Huronian rocks south of the Murray fault. The 1.850-1.900 Ga age of metamorphism is much younger than the Creighton-Murray granite (2.45 Ga) and older than the PDF compression, OCR, web-optimization with CVISION's PdfCompressor �29 1.700-1.750 Ga of the Chief Lake and Cutler granites. Jackson (2001) considers the origin of the high-grade staurolite-biotite assemblages of the McKim Formation in the hanging wall of the Murray Fault as one of the most enigmatic aspects of the tectonic history of the Southern Province. He concludes that geobarometry indicates a relatively low- pressure metamorphism (2-3 kbar) at high temperature. These conditions differ significantly from the 6-7 kbar pressures estimated for the Penokean metamorphism in Minnesota as determined by Holm and Silverstone (1990). He concluded that the high-temperature metamorphism was at or below pressure corresponding to the thickness of the Huronian rock column, thereby precluding crustal thickening as the origin of the metamorphism. Jackson (2001) concluded that a high heat flow regime as developed in areas of crustal extension and related mantle upwelling Jackson (2001). Such a model is compatible with Card’s (1964) view that the high-grade metamorphism may be the result of rapid, focused heat flow. Tectonic Models for the Development of the Huronian Basin There have been various tectonic models proposed for the early development and later deformation of the Huronian basin. Many reconstructions are essentially modifications of the model put forth by Deitz and Holden(1966) which stated that the Huronian Supergroup represents a rift and passive margin sequence that was compressed, partly tectonically buried and metamorphosed during a collision with the Superior craton and another mass which overrode its southern edge. Zolnai et al. (1984), Bennett et al. (1991) accept the essential aspects of the Dietz and Holden (1966) model. They propose that rifting and continental break-up was coeval with Huronian volcanism (2.45 Ga) and that the much later deformation was equivalent to the Penokean Orogeny (1.860-1.835 Ga). This model does not attempt to account for the multiple deformation events affecting Huronian rocks and the origin of the Nipissing magmatic event. Young (1982) proposed that the Huronian Supergroup was deposited in an aulocogen, an easterly trending fault bounded trough, which opened towards an ocean in the area now occupied by the Grenville Province. Sims et al. (1980, 1981) concluded that the Huronian Supergroup, the Marquette Range Supergroup and Animikie rocks were deposited as intra-continental, fault controlled basins developed along a major, late Archean structure, the Great Lakes Tectonic Zone. More recently, Roscoe and Card (1992), noting the close stratigraphic correlation between the Early Proterozoic sequences of the Wyoming craton and the Huronian Supergroup, proposed that the Superior and Wyoming cratons are rifted portions of what was once a single continental land mass. They suggest the direction of the Matachewan-Hearst dike swarm (2.45 Ga) indicates an east-west tensional regime, which resulted in, a Huronian basin elongated in a north-south direction. On this larger craton the Huronian sediment was deposited in a southward deepening intracratonic basin. Roscoe and Card (1992) propose that it was during the Nipissing igneous event (2.2 Ga) that successful rifting of the Superior Province took place with the eventual drifting of part of the missing Superior Province to its present location as Wyoming craton. They attribute pre-Nipissing folding to the Blezardian Orogeny of Stockwell (1982) and the later more important deformation to be coeval with the Penokean Orogeny of Michigan, Wisconsin and Minnesota (Roscoe and Card, 1992). Jackson (2001) supports the model of Roscoe and Card (1992) since the high heat flow, which he considers necessary to give the observed features of the high-grade metamorphic rocks, would be a necessary effect of mantle upwelling during continental break-up. He also interprets some of the early high-strain deformation as being consistent with a Nipissing age break-up of the Superior craton. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �30 ILSG Field Trip No. 1 - Day 1 The Elliot Lake Transect On the first day of this field trip we will examine many of the excellent rock exposures of one of the Earth’s most well-known and most completely studied stratigraphic sequences. The rocks of the Elliot Lake transect display clearly visible evidence of the nature of the climate and atmosphere of the Earth more than 2 billion years – of course not all Earth scientists agree as to what this evidence tells us. Stop Descriptions and Road Log From Sault Ste Marie, Ontario proceed east on Highway 17 for 162 km (101 mi) to the intersection of Highway 17 and Highway 108, then 29 km (18 mi) north on Highway 108 to Elliot Lake. At Elliot Lake continue north past Hillside Drive South and the first few stoplights to Hillside Drive North. Turn left (west) onto Hillside Drive North. Continue west on Hillside Drive North for about 1 km (0.6 mi) to Spruce Street. Turn right (North) on Spruce Street to Valley Crescent continue on Valley Crescent to Balsam Place and stop at the cull-de-sac. When collecting samples please exercise care to avoid damaging any vehicles parked nearby. cul-de-sac. Note the deflection of the axial plane cleavage in the mudstone units. The movement of adjacent beds inferred from the deflection of the cleavage indicates the south limb of a syncline. A gabbroic dike is separated from the sedimentary rocks by a zone of sheared and fractured rocks. The McKim Formation is missing on the north limb of the syncline. Return to Hillside Drive North and continue west for about 1 km to Spine Road. Turn right (west) onto Spine Road to Lawerence Avenue at the west end of Spine Road. 14.9 km = 9.3 mi. 17-370462E, 5137991N STOP 1.2: Radioactive quartz-pebble conglomerate of the Matinenda Formation. The low outcrops on north side of Spine Road are grey, buff and dark-grey arkose and radioactive, pyritic, quartz-pebble conglomerate of the Matinenda Formation. Pebble units are about 20-30 cm thick and dip about 10 degrees to the north. Pebbles in this outcrop are about 1-2 cm across and are generally much smaller than the typical pebbles in the ore zones of the Elliot Lake mines. Ruzicka and LeCheminant, (1984) report the radioactive conglomerate contains “rare-earth-element-bearing uranothorite (?), large zircons, a Ti-U-Si-Fe phase (brannerite?), 17 – 372751E, 5138695N (All coordinates in chalcopyrite and chromite. The distribution of the Elliot Lake are within UTM zone 17; NAD radioactive minerals in the conglomerate 83 which is essentially equivalent to NAD displays layering thus indicating a detrital WGS84). origin of these grains” STOP 1.1: McKim Formation and Nipissing Diabase The dark-grey areas in the radioactive beds are due to the presence of minor amounts Outcrops are argillite/slate and grey sandstone radioactive carbon generally known in the of the Mckim Formation on the east side of the Elliot Lake area as “thucolite” but also referred PDF compression, OCR, web-optimization with CVISION's PdfCompressor �North 2.1 Hy. 17 2.7 Hy. 556 Sault Ste. Marie Area of Figure 2.1 2.8 2.9 1.12 Hy. 17 from drill hole Hy. 128 LAKE HURON Thessalon Bruce Mines Hy. 638 Foul Bight Highway 556 transect Elliot Lake transect Field trip stop Faults Cutler Granite Haughton Twp. area. Gordon Lake area Hy. 17 Hy. 556 Fenwick Township Hy. 546 Area of Figure 1.21 Hy. 108 50 km City of Elliot Lake cline Hy. 108 Area of Figure 1.19 e syn e Lak Quirk Hy. 639 Flack Lake area Archean rocks East Bull Lake Suite Gabbro anorthosite Elliot Lake, Hough Lake and Quirke Lake Groups Cobalt Group Gordon Lake Fm. Huronian Supergroup Murray Fault LEGEND Figure 1.18: Index map for included geological maps and some areas mentioned in the text. Lake Superior Paleozoic and Keweenawan rocks 31 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �32 LEGEND POST-HURONIAN MAFIC INTRUSIVE ROCKS NIPISSING DIABASE HOUGH LAKE GROUP Subarkose, arkose, conglomerate Diabase, gabbro, metagabbro, granophyre 16 HURONIAN SUPERGROUP COBALT GROUP Bar River Formation 15 Pecors Formation (4) Argillite siltstone 8 Ramsay Lake Formation Quartz arenite Gordon Lake Formation 14 Siltstone, argillite, subarkose Lorrain Formation 13 Quartz arenite, subarkose, arkose, conglomerate, thorium-bearing conglomerate Gowganda Formation 12 Conglomerate, argillite, wacke, subarkose, siltstone DISCONFORMITY QUIRKE LAKE GROUP Serpent Formation 11 Subarkose, conglomerate Espanola Formation 10 Limestone, dolomite, calcareous siltstone Bruce Formation Diamictite Not Shown Mississagi Formation Fault 3 9 7 Diamictite ELLIOT LAKE GROUP McKim Formation 6 Siltstone, argillite, wacke Matinenda Formation 5 Subarkose, arkose, conglomerate, uranium-bearing conglomerate DISCONFORMITY Thessalon Formation 4 Basalt, andesite, mugearite-hawaiite, minor basal quartz-pebble conglomerate 4 DISCONFORMITY Livingstone Creek Formation 3 Sandstone, polymictic conglomerate UNCONFORMITY ARCHEAN 2 ARCHEAN Plutonic granitic rocks, gneisses, INTRUSIVE CONTACT Mine (Past Producer) 1.2 Field trip stop 1 Felsic to mafic metavolcanic rocks, metasedimentary rocks Sources of Information Giblin and Leahy(1979) Figure 1.20: Legend for figures 1.19 and 1.21. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �7 8 9 10 2 12 12 7 8 12 9 5 8 7 10 12 14 2 13 9 16 9 16 5 2 5 Elliot Lake 1.2 10 12 13 11 4 6 1.1 1.3 2 2 16 12 City of Elliot Lake 1.4 1.5 1.6 Hy 108 1.11 Hy 639 9 16 9 12 1.7 1.8 1.9 9 10 2 16 5 8 16 6 11 5 12 11 Qirke Lake 12 7 6 1.10 16 10 2 Figure 1.19: Geological map of the Quirke Lake Syncline. 5 9 10 11 Dunlop Lake 4 9 2 7 4 1 11 4 9 7 10 2 2 14 1 11 10 3 rs 1 co Pe 16 12 ke 6 12 1.1 La 9 16 2 16 5 10 2 1.1 9 16 0 1 9 1 9 2 2 9 16 3 2 1 6 1 N 1 4 miles km 6 2 Whiskey Lake 2 33 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �2 12 13 2 13 12 12 12 3 14 Hy 546 13 12 16 12 9 13 1.14 Ten Mile Lake 4 9 4 3 Flack Lake 1.15 Hy 639 16 11 15 10 14 1.16 12 2 14 2 15 1.13 14 9 15 Hy 639 1.12 13 Mount Lake Figure 1.21: A geological map of the Flack Lake area. 10 14 16 16 12 2 14 1.11 16 1 16 151 16 2 14 1 16 Rawhide Lake 4 10 Semiwite L Lake 16 13 12 2 2 0 16 1 3 2 2 13 16 12 2 1 2 North 4 miles km 6 2 34 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �35 to a hydrocarbon kerogen. Ruzicka and LeCheminant, (1983) note that several generations of carbon occur in the conglomerates of the Matinenda Formation. The earliest generation occurs as layers concordant with the bedding or a component of the matrix and appears to have been deposited in areas of quiescent sedimentation during the last phase of an upward fining sedimentary cycle. Diamond drilling has indicated that there are no ore-grade units in this area. Grab samples collected by the author in 1982 returned up to 0.80 lbs U3O8 /ton and 0.78 lbs ThO2/ton. A continuous chip sample returned 0.31 lbs U3O8/ton and 0.53 lbs ThO2/ton. Return to Highway 108 and proceed north on Highway 108. Reset odometer at Stanleigh Road. About 1.5 km north of Hillside Drive North). 1.0 km = 0.63 mi. 17 - 371717E, 5140426N STOP 1.3: Mississagi Formation, Hough Lake Group. One-meter thick beds of grey subarkose of the Mississagi Formation on the west side of the highway display the rusty staining on the face of the outcrop reflecting the minor pyrite content along the foreset beds of trough cross-beds (Figure 1.22). The paleocurrent direction (from the west) can be best observed on the upper surface of the outcrop. – Please Figure 1.22: Trough cross-bedding in the Mississagi Fm. exercise caution when walking on smooth, wet rock surfaces - The grey Stop 1.3. colour of these sandstones and the presence of apparently detrital pyrite Later generations are probably remobilized is held by most geoscientists to indicate of the phases of the first generation of carbon. The very low partial pressure of free oxygen of the carbonaceous matter in the Elliot Lake ores is atmosphere during the deposition of the comparable in occurrence and composition to Mississagi Formation. similar hydrocarbon in the Witwatersrand gold 1.4 km - 0.9 mi. 17-371652E, 5140948N reefs; interestingly Ruzicka and LeCheminant (1984) report elevated gold content (1000 STOP 1.4: Nipissing Diabase (Gabbro) 2000 ppb) in the carbonaceous matter of the altered Mississagi Formation, Bruce Elliot Lake ore beds. The radioactive carbon at Formation. this site is reported to be auriferous, although A sill-like body of Nipissing gabbro/diabase is the gold content is not available. exposed on the east side of Highway 108. In 1955 Rio Algom Mines Limited completed a Rhythmic, compositional layering is visible on diamond drill hole about 30 m south of this the vertical face of the road-cut. Near the north location. The logs of this hole indicate that the end of the Nipissing outcrop face note the radioactive beds exposed here are about 35 relatively planar, striated, surface is truncated by meters above the Archean basement rocks. more irregular surface that has been interpreted PDF compression, OCR, web-optimization with CVISION's PdfCompressor �36 as the result of erosion by high-pressure, waterborne sediment, presumably as a result of melting of an adjacent Pleistocene ice sheet. North of gabbro sill, the upper portion of the Mississagi Formation is exposed along the east side of the highway. The distinctly pinkish hue of the sandstone is probably due to the formation of albite by hydrothermal fluids activated by the adjacent intrusion. Further evidence of hydrothermal activity is seen by dark-green to black chlorite deposited along fractures. below the north-dipping diabase sill near the north end of the exposure, where it occurs as sub-parallel groups of pale-grey to white prismatic crystals about one mm wide and up to a centimeter long. X-ray diffraction analysis has determined that the pink coating on joint surface is apophyllite (KFCa4[Si8O20]8H20) an uncommon mineral, sometimes found in amygdules in basalts, but is also associated with calc-silicates (Figure 1.23). Young (1991) states Continue northward on foot for a few tens of meters. Here (17-371668, 5141245) the Mississagi Formation is overlain by diamictite of the Bruce Formation at the base of the Quirke Lake Group. The dispersed megaclasts of the Bruce Formation are predominantly grey granitic rocks with smaller mafic clasts of predominantly Huronian volcanic rocks of the Thessalon Formation. The abundant matrix of the conglomerate is dark-grey to black. Sand-sized quartz grains have a glassy, black appearance, a reflection of the dark matrix behind the Figure 1.23: Espanola Formation with calc-silicate minerals. Stop 1.5 clear quartz. There is no evidence of significant disconformity at the base of the Bruce Formation. that the small scale thrust faults and folds in the limestone on the west side of the highway is Proceed by vehicle to near the top of the hill. probably the result of slumping during early 1.8 km = 1.4 mi. 17-371621E, 5142684N tectonic activity. STOP 1.5: Espanola Formation and Nipissing diabase sills. The upper, ferruginous dolostone-bearing member of the Espanola Formation and the The base of the Espanola Formation is a green, overlying Serpent Formation are not present at this location but are well represented on the laminated unit about a meter or so thick. Laminated silty limestones and minor thin, chert north limb of the syncline. Since the Serpent beds of the Bruce Limestone member, Espanola Formation is missing we can infer that the ferruginous dolostone was removed during a Formation, overlie this unit. At this location the proximity of Nipissing gabbro intrusions has period of pre-Gowganda erosion. led to the development of calc-silicate minerals Continuing north the basal units of the including: grossularite garnet, idocrase Gowganda Formation are visible in road-cuts (vesuvianite), diopside, and wollastonite typical along the highway. of a skarn (Robertson, 1968). Wollastonite 2.0-2.5 km = 1.3-1.6 mile. Gowganda (identified by X-ray diffraction) is found just Formation diamictite. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �37 Megaclasts of pink granite, grey granite and granitic gneiss and mafic rocks are widely distributed in a dark green matrix. Most geologists now consider at least some of the diamictites in the Gowganda Formation to be tillites, although a debris-flow origin, either glaciogenic or as submarine debris flows is a more reasonable interpretation at specific localities. Roscoe, (1969) places the appearance of free oxygen in the atmosphere (“oxyatmoversion”) as coinciding with the appearance of the reddish hue of hematite just above the base of the Gowganda Formation a short distance north of STOP 1-5. (Figure 1.24). The mineral assemblage and metamorphic grade of the few of the mafic clasts examined by the writer many years ago indicated that the clasts were probably from Huronian basaltic flows. 4.7 km. = 2.9 mi. Pink sandstones and diamictite of the Gowganda Formation. 6.7 km = 4.2 mi. Diamictite with large boulder, Gowganda Formation. t —-.1 3.8 km. = 2.4 mi. 17- 371841E, 5143180N STOP 1.6: Stratified Gowganda Formation. Park just south of the rock-cut on east side of the highway. This is an impressive exposure though a stratified sequence of diamictites, Figure 1.24: Crudely stratified conglomerate of the clast-supported conglomerates and sandstones of the Gowganda Formation. Gowganda Fm. with mafic volcanic clasts. Stop 6. The base of the sequence at the extreme southern end of the exposure on the west 8.2 km. = 5.0 mi. Stanrock Mine Road. Reset side of Highway 108 consists of massive odometer. Turn east onto Stanrock Road diamictite overlain by a thin unit of laminated Optional Stop mudstone-siltstone with a few small dropstones. 2.2 km = 1.4 mi. 17-374911E, 5147007N About a meter of stratified Gowganda overlies the siltstone. Distinct beds of diamictite, Stop 1.7: Laminated varvite? sandstone, pebbly sandstone and clast supported Laminated siltstone/mudstone of the Gowganda polymictic conglomerate are present. Some Formation on the north side of the road. This conglomerate units display normal and reverse unit has been interpreted as varves found as grading suggestive of debris flows. The rocks deposits from Pleistocene glacial lakes. Return to displayed here may be interpreted to represent a Highway 108. depositional environment proximal to retreating glacial margin. Set odometer to 0 at Stanleigh Road and Highway 108. Note the predominance of red and pink granitic clasts, in marked contrast to the pale grey clasts 3 km = 1.9 mi. Denison Mine Road - Turn east. of the Bruce and Livingstone Creek Formations Reset odometer to 000 km. seen earlier. There is also a significant 1.5 km. = mi. 17 -375102E, 5150499N proportion of black pebble to cobble-sized clasts PDF compression, OCR, web-optimization with CVISION's PdfCompressor �38 Stop 1.8: Gowganda Formation Serpent disconformity Stop 1.10: Ramsay Lake Formation overlain by Pecors Formation. The Serpent Formation is not present in the south limb of the Quirke Lake Syncline and the Blind River Sault Ste Marie area where it was probably removed during a period of pre-Gowganda erosion. At this location, on the south side of the road, well-sorted sandstone of the Serpent Formation is overlain by polymictic conglomerate of the Gowganda Formation. The contact is sharp but irregular. Evidence of a sub-Gowganda disconformity at this location is given by the presence of pebble and cobbles of the Serpent Formation near the base of the Gowganda Formation. Diamictites of the Ramsay Lake Formation contain cobbles of grey granitic rocks, mafic clasts of Huronian volcanic rocks and Archean felsic volcanic clasts in an abundant dark-grey to black sandy matrix. The Ramsay Lake Formation is overlain by dark laminated siltstone and mudstone (argillite) of the Pecors Formation. The latter contains a few dropstones (Figure 1.25). Note: the Matinenda Formation of the Elliot Lake Group, expected between the basement and the Ramsay Lake Formation, is truncated by the Ramsay Lake Formation in this area. The Matinenda Formation does occur in the mine workings down-dip from this location. Return to Highway 108. Reset odometer at Highway 108 and Denison Mine Road. Continue east to the end of the Panel Mine road. This is the rehabilitated area of the former 0.5 km = 0.3 mi. Disseminated carbonate in Panel Mine. There is little evidence of the sandstone of the Serpent Formation on east side uranium mine and mill complex that was on this of Highway 108. site until 1993. 1.1 km = 0.68 mi. Road to Quirke Lake and Return to Highway 108. Reset Odometer. former Panel Mine. Reset odometer to 0. Turn Continue north on Highway 108. Tailings dam east onto Panel Mine road. of the Quirke Mine is visible west of the 1.5 km = 0.9 mi. 17-375258E, 51511331N Highway. Stop 1.9: Espanola Limestone Member of the Espanola Formation. Rock-cut in ferruginous dolomite and siltstone of the Espanola Limestone Member of the Espanola Formation. The Espanola Limestone Member is the uppermost of the thee members of the Espanola Formation recognized in the Elliot Lake area (Robertson, 1968). It is characterized by intercalated siltstone and reddish-brown weathering, ferruginous dolostone beds containing 3-4% FeO. Intraformational breccia, ripple marks, small-scale cross-bedding, and various soft sediment features are present but faintly visible on the south outcrop. Near the east end of the outcrop a grey clastic dike crosses the stratification at a high angle. 4.1 km = 2.5 mi. 17 – 377379E, 5152019N Highway 108 ends and Highway 639 begins. 1.0 km = 0.6 mi. Diamictite of the Bruce Formation is exposed west of the Highway. 1.5 km = 0.9 mi. Outcrops of Mississagi Formation are exposed along Highway 639. Note the yellowish colour characteristic of the Mississagi Formation where it lies directly on the Archean granitic basement (Robertson, 1968). 2.5 km = 1.6 mi. 17 – 371508E, 5152302N STOP 1.11: Huronian volcanic rocks of the Thessalon Formation (Dollyberry belt). A gated road leads west from Highway 639 leads to one of the Quirke Mine tailings dams. Park near the gate and walk a short distance along a rough road from the gate to the base of the tailings dam. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �39 Note the very dark-green to black, flattened, chlorite amygdules characteristic of the Huronian mafic volcanic rocks between Sault Ste. Marie and the Elliot Lake area. Cross the stream and proceeding northward a short distance along a rough road to the crest to the low hill. The Huronian volcanics at this location (17 – 371428E, 5152342N) include transitional observed identical scattered, quartz pebbles were found along the contact of the Huronian volcanic rocks where they overlie Archean mafic volcanic rocks in the haulage drift of the Stanleigh Mine on the south limb of the Quirke Lake Syncline. The writer proposes that these quartz cobbles are lag deposits left behind while the finer sediment was washed off the surface. A few kilometers west of this location, occurrences of this conglomerate unit contain more rounded quartz, and are commonly are overlain by a thin arkosic unit. Quartz-pebble conglomerate, which locally contain significant pyrite as well as uranium, occur sporadically at the base of the Huronian volcanic sequence of the Quirke Lake Syncline and westward to the Sault Ste. Marie area. The conglomerate at this location occupies the same stratigraphic position as the radioactive, quartz-pebble conglomerate. Figure 1.25: Dropstone in laminated argillite of the Pecors Formation. Stop 10. alkalic types, hawaiite and mugearite (unpublished analyses in the authors files). The eastward trending, south dipping unconformity between the Archean granitic basement rocks and Huronian volcanic is visible near the crest of the hill. There appears to be no visible paleosol development at this location. Near the west end of the outcrop, a thin, quartz-pebble conglomerate/breccia unit, comprised mainly of angular, quartz-clasts, overlies the granitic rocks at the base of the volcanic unit. Scattered, isolated, mainly cobble-sized clasts of quartz can also be found along the unconformity. Some visitors to this site have proposed that the contact between the Archean and Huronian volcanics is not an unconformity, but a fault contact. However, during a visit to the Stanleigh Mine in 1990, the writer along with Rob Henderson, Mine Geologist and Dr. Larry Jensen of the Ontario Geological Survey Return to vehicle, set odometer to 0 and continue north on Highway 639. 0.2 km = 0.1 mi. Archean outcrops cut by diabase outcrops along the highway. 6.8 km = 4.3 mi. 17 – 368530E, 515773N STOP 1.12. Archean metavolanics with pillow structures. Archean mafic metavolcanics displaying well-developed pillow structures are exposed, on north sloping outcrop, the east side of highway (Figure 1.25). Pillows are deformed, but facing directions can easily be determined. Small amygdules are concentrated near the upper surface of many pillows. 7.7 km =4.9 mi. Flack Lake fault occupies a valley near this point. 8.2 km = 5.2 mi. Outcrops of hematite-stained sandstone of the Bar River Formation. 8.8 km = 5.6 mi. Entrance to Mississagi Provincial Park. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �40 9.1 km = 5.7 mi. Pale grey sandstone of the Bar River Formation with herringbone cross-bedding. to suggest that these features are the result of the transportation of consolidated desiccation fracture fillings. 9.8 km = 6.2 mi. Christman Lake. 13.6 km = 8.6 mi. 17 – 364395E, 5162758N 17 – 366863E, 5160228N STOP 1.14: Red beds of the Gordon Lake Formation 10.2 km - 6.5 mi. Laminated, maroon buff siltstone and chert with reduction spots in the upper part of the Gordon Lake Formation. This is another red-bed occurrence within the Cobalt Group. 17 –366742E, 5160652N STOP 1.13: Bar River Formation. 14.9 km = 9.4 mi. 17 – 363062E, 5163194N STOP 1.15: Gordon Lake Formation. — - 'U: __t - - -. Siltstones and sandstones of the Gordon Lake Formation display ripple marks, desiccation cracks, cross bedding and a late cleavage . Note: the presence of pyrite in contrast to hematitic nature of the Gordon Lake Formation near the top of the formation. 19.3 km = 12.20 mi. 17 – 361771E, 5166967N STOP 1.16: Lorrain Formation White to pale pink quartz arenite of the upper portion of the Lorrain Formation is exposed on the east side of the highway. Figure 1.26: Pillow structures in mafic metavolanics. Stop 1.12. 21.0 km = 13.3 mi. At Little White River Road (Highway 546) – End of Field Trip Return to Sault Ste Marie. Sandstones of the Bar River Formation display ripple marks, mud cracks and sinuous structures, 22.8 km 17.8 mi. Junction with Little White which have been described as possible worm River road (Highway 546.) casts. Comparison with desiccation structures in End of Day one field trip. the Gordon Lake Formation led Young (1969) PDF compression, OCR, web-optimization with CVISION's PdfCompressor �41 ILSG Field Trip No. 1 Day 2 - Part 1 Huronian Stratigraphy along Highway 556 and correlation with the Chocolay Group of the Marquette area. On Day 2 we will examine some of the Huronian rocks of the area north of Sault Ste Marie where we will see important differences in the stratigraphy and structural relationships when compared to the sequence in the Quirke Lake Syncline of Day 1. The transect along Highway 556 will show that all three formations of the Quirke Lake Group are missing and correlation with formations of the Hough Lake Group is problematical. Also, there is a significant unconformity at the base of the Gowganda Formation. We will spend part of Day 2 examining a dolostone unit in Fenwick Township near the north shore of Goulais Bay, and a dolostone-bearing unit in the Gordon Lake Formation, southeast of Sault Ste Marie. Those familiar with the Kona Formation of the Chocolay Group in the Marquette Range Supergoup will notice the correspondence in lithology and stratigraphy. Road Log and Stop Descriptions (Refer to Figures 2.1 and 2.2) Island Lake fault in this area are locally chloritized and albitized micro-breccia. The lack of such alteration in the Huronian rocks south of the fault suggests that the alteration pre-dates Huronian sedimentation (Delio Tortosa personal communication, 2005). IMPORTANT: When observing roadside rock-cuts, be especially careful to park your vehicle well away from the pavement and exercise the necessary caution when crossing STOP 2.1: Island Lake Fault Zone the road. Where stops are located on or near blind turns, park in a safe place and walk to the Rocks of the Island Lake Fault Zone separating rock exposure. the Archean rocks on the north from Huronian rocks on the south are exposed along Highway GPS datum is NAD 83 =WGS 84. UTM 638 near Lower Island Lake. Rocks within the Zone17. fault zone are highly sheared and brecciated. Pink rock in the shear zone is a Keweenawan 000 km- Second Line and Highway 17 N. felsic intrusion (felsite). Continuing north on Highway 17. 13.3 km. = 8.2 mi. Intersection of Highway 17 6.4 km. = 4.9 mi. - Railroad crossing and Highway 566. Reset odometer to 0. 8.1 km. = 5.0 mi. 16-709788E, 5175568N Proceed east on Highway 556. 5.6 km. =3.5mi. Fractured altered Archean rocks. The Archean granitic rocks north of the Park on shoulder just west the outcrops. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �42 1a 6 1a 1a Hy 556 2.7 9 7 9 9 2.4 2.3 6 2.6 7 1a 2.2 2b 6 7 2.5 ACR 2b Hy 552 2.1 4 2a 2a o -50 25 Jarvis Lake 2b 20 o 4 5 2a Reserve Lake o -45 5 15 Aweres Lake N 3 -45 Hy 5565 Island Lake Upper Island Lake LEGEND 0 2 1 Keweenawan Supergroup 9 1 Jacobsville Fm. Red Sandstone, conglomerate 2 Gabbro, diabase Huronian Supergroup Mafic flows Livingstone Creek Fm 3 Sandstone, conglomerate Archean 2b. Granitic gneiss 2a. Massive granitic rocks 2a, 2b 7 Lorrain Fm. Quartz arenite, quartz-pebble conglomerate. 6 1 Mafic metavolcanics Gowganda Fm Mudstone, siltstone, diamictite 5 km Thessalon Fm 4 Nipissing 8 3 miles Aweres Fm Sandstone, polymictic conglomerate 2.4 Field trip stop. (Day2) Fault (inferred or assumed) Figure 2.1: Geology of part of Highway 566. (Day 2) From Bennett et al, 1975 Frarey, 1977, unpublished maps and notes of P.E. Giblin and E. J. Leahy. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Jacobsville Fm. Red sandstone, conglomerate Gordon Lake Fm. Lorrain Fm. Keweenawan Mudstone, sandstone, minor dolostone in the Gordon Lake area and Fenwick Twp. only. Quartz arenite, quartzpebble conglomerate Cobalt Group (2.21 Ga) Gabbro, diabase, granophyre 43 Mudstone, siltstone Nipissing Diaabse Gowganda Fm Arkose, siltstone Diamictite, polymictic conglomerate Unconformity Hough Lake Group ? Mainly arkose, subarkose & pebble conglomerate Aweres Fm. ( = Ramsay Lake / Mississagi Fm. ? ) Mainly subarkose, granite & volc. cobble conglomerate Thessalon Fm. U, Th Disconformity Metabasalt, basaltic andesite Minor qtz. peb. cong. near base Disconformity Livingstone Creek Fm. Archean Subarkose, conglomerate Unconformity Elliot Lake Group Mainly volcanic cobble conglomerate Metavolcanics, metasediments, granitic rocks Figure 2.2: A stratigraphic column for the Highway 556 transect PDF compression, OCR, web-optimization with CVISION's PdfCompressor �44 Symbols Fault Unconformity Gowganda Formation Stop 2.7 Archean metabasalt Belleview Fault Stop 2.5 Stop 2.6 Gowganda Fm. Stop 2.2 Stop 2.4 Stop 2.3 Stop 6 Lorrain Formation Archean Archean Metabasalt syenite Nipissing Gabbro Island Lake Fault Aweres Formation Huronian Metabasalt Stop 2.1 Hydrothermally altered Archean granitic rocks Livingstone Creek Fm. (Not exposed on Highway 556) Figure 2.3: A diagramatic cross-section along the Highway 556 transect. I' i1'jq G. Bennett, 2005 Figure 2.4: Gowganda Formation. (Behind figure) unconformably overlying conglomerate of the Aweres Formation. Stop 2.2. Figure 2.5: Small dropstone in laminated siltstone of the Gowganda Formation. Stop 2.3. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �45 STOP 2.2: Gowganda Formation unconformably overlying Conglomerate of the Aweres Formation. On south side of highway an almost black, volcanic pebble to cobble, clast-supported conglomerate of the Aweres Formation is overlain by massive, grey, pebbly wacke of the Gowganda Formation. The contact is abrupt and visible in outcrops along both sides of the The Aweres Formation seems to have been deposited as alluvial fans along the valley walls of a down-dropping fault block (rift valley?). The surrounding plateau of Huronian volcanic rocks were eventually eroded to the underlying Archean basement. The Aweres Formation is probably stratigraphically equivalent to the Mississagi Formation. At the north boundary of the Garden River Indian Reserve, about 12 km southeast of this area, the writer noted a southward transition from Aweres-type conglomerate to the sandstones of the Mississagi Formation (Bennett and Sawiuk, 1979). 9.2 km = 5.6 mi. 16 - 710458E; 5176230N STOP 2.3: Drop-stones of the Gowganda Formation The vertical face of an outcrop of laminated argillite of the Gowganda Formation on the Figure 2.6: Polymictic conglomerate of the Aweres south side of the highway contains a few Formation, Aweres Twp. pebble-size dropstones. (Figure 2.5). The juxtaposition of pebble to boulder-sized clasts, normally indicative of a high-energy highway at this location (Figure 2.4). The entire environment, with laminated mudstone of a low Quirke Lake Group is missing in this area energy environment is unusual. Dropstones are (Figures 1.3, 2.2). It is possible that tectonic the important indicators of paloclimate because, activity north of this area, such as that which in Precambrian rocks, dropstones can only be produced the Kapuskasing structure may be responsible for the uplift and erosion of some of explained as a rain of coarse sediment from floating ice onto silt and mud of a relatively still the sub-Gowganda rocks in this area. lake bottom or sea floor. It is the presence of dropstones at within stratified, fine-grained The lithology of the Aweres Formation seen metasediment of Gowganda Formation (also the here is identical to the lowermost part of the Aweres Formation as seen directly overlying the Bruce and Ramsay Lake Formations) which points to the glaciogenic origin of Huronian Thessalon Formation in the Jarvis Township a matrix supported conglomerates. A very few kilometres southeast of this point. The proportion of volcanic clasts within the Aweres impressive occurrence of dropstones is found in formation decreases as the proportion of granitic a cliff of Gowganda Formation about 50 meters clasts and sandstone increases with stratigraphic west of Highway 129 about 30 km north of the intersection of Highway 129 and Highway17. height (Figure 1.14) (Bennett et al., 1975). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �46 9.5 km. = 6.9 mi. 16 - 710962 E, 5176196 N granitic rocks. About 15 meters of overburden separates the granitic rocks from an outcrop of STOP 2.4: Contact between Gowganda grey pebbly wacke assigned to the Gowganda Formation and Huronian metabasalt of the Formation. The east end of the outcrop (nearest the syenitic rocks) contains angular Thessalon Formation blocks of reddish the syenite/syenodiorite suggesting an unconformable relationship Park on south shoulder just west of outcrop. Pebbly wacke and matrix supported, polymictic rather than a fault. The unconformity crosses pre-Gowganda rocks that dip from 10o to 30o conglomerate with abundant granite clasts west. (Gowganda Formation) show an abrupt contact with the dark-grey Huronian basalt 12.6 km. - 7.9 mi - Quartzite of the Lorrain flows to the east. The contact between the Formation. Fractured reddish quartzite volcanics and the Gowganda conglomerate strikes 135 dips 65 W. Faint bedding (?) in Gowganda Formation is essentially parallel to 14.9 km. = 9.2 mi. 16- 714951E, 5177887N the contact with the volcanic rocks. There is minor fracturing parallel to the contact in the STOP 2: 6: Jasper Pebble Conglomerate Member of the Lorrain Formation. volcanics but no significant deformation is noticeable in the conglomerate near the This striking rock is a pebbly sandstone and contact. The writer (eventually) concluded quartz pebble conglomerate known locally as that the contact is an unconformity (Figure “puddingstone”. The abundant pebbles of 2.3). variously coloured jasper presumably had a provenance in Archean iron formation, which The Huronian volcanics (Thessalon Formation) contains clots of altered plagioclase is well represented in the Superior Province to the north. This member is in about the crystals (glomeroporphyritic texture). Whole rock chemical analyses of the basaltic flows in stratigraphic middle of the Lorrain Formation this area show the elevated soda similar to that (Frarey 1977). At this point the dip of the bedding here is vertical or nearly so. A possible of spilite. Albite, clinozoizite, actinolite and some (original) clinopyroxene are the essential axial plane cleavage is visible, suggesting the rocks lie on the south limb of an anticlinal minerals. structure. 10.8 km. - Railroad trestle. 16.4 km.= 10.25 mi - Northland Lake Road. 11.3 km. = 7.2 mi. 16- 712648E, 5176124N. Sharp Corner – watch for traffic. You may park 17.6 km. = 10.9 mi (Optional) on the level area south of the Highway if your vehicle has a relatively high road clearance. STOP 2.7: Outcrop of Archean metavolcanic rocks. STOP 2.5: Archean granitic rocks and Gowganda Formation. The easternmost, high rock-cut is of pink, coarse-grained Archean syenite to syenodiorite. Two mafic dikes intrude the Fine to medium-grained amphibolite is part of the hornfels zone near the granitic plutons. End of road log for Highway 556. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Figure 2.7: Dolostone of the Kona Formation. Highway 63, Marquette area, MI. Figure 2.8: Dolostone and chert. Fenwick Township, ON. 47 0 .:Y3 Figure 2.10: Jasper-pebble conglomerate member of the Lorrain Formation. Stop 2.6. Figure 2.9: Dolostone nodules and lenses (dark areas) in the Gordon Lake Formation. Near Highway 638, Gordon Lake area, ON. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �48 Return to Highway 17. At intersection of Highway 17 and Highway 556 reset odometer to 0. Continue north on Highway 17 to Highway 552 (about 13 km). Turn left (west) onto Highway 552. 5.5 km = 3.4 mi. Goulais Mission Road. Turn right onto Goulais Mission Road. 11.4 km = 7.1 mi. Private road of Case Construction Company. Permission must be obtained from Case Construction before continuing. We will continue to the end of the road as indicated by a locked gate. A short walk (> 100m) in a southwest direction will bring us to a small hill we refer to as the “dolostone knob”. STOP 2.8: Outcrops of Dolostone at the Dolostone Knob. Thinly bedded and laminated dolostone and chert are found along the steep sides of an outcrop knob. Some laminated silty sections may be stromatolitic bedding. STOP 2.9: This stop will be visited as time allows. This stop is located near Highway 638. Directions and a stop description are included in the supplementary road log for Highway 638. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �49 Geological Features and Correlation of a Dolostone Unit in Fenwick Township, Northeast of Sault Ste Marie, Ontario millimeters to at least 1 m thick. The metachert beds have a similar thickness range but form less than 25 percent of the visible section (Figure 2.8). No clearly identifiable stromatolitic structures were observed but irregular and wavy, closely spaced laminae of brownish chert, suggestive of algal mats, were noted. Pale grey, clastic dolostone and oolitic dolostone appear to be the main rock types in the upper part of the carbonate unit. A few lenses, up to 50 cm wide, of coarse, pink barite are found within dolostone on the south side of the knob. Born (1987) described, a previously unrecognized occurrence of dolostone and associated clastic rocks in Fenwick Township, approximately 20 miles (30 km) northwest of Sault Ste. In the spring and fall of 1988 the writer mapped the known extent of the dolostone, and some of the surrounding rocks, was mapped by at a scale of about 1:8000 (Bennett et al, 1990). Mr. Paul Morra of Sault Ste Marie provided a stadia survey of selected areas. 1 1 1 1 1 1 1 1 1 3 Gravel Pit 1 4c 1 4c 70 4c,b 3 3 4c,b 1 1 1 3 3 1 1 75 4b 3 4b 75 4b 3 3 Dolostone knob 4b 3 4b 4a 35 65 4b 3 No outcrop 4c 2 3 Sand and gravel 2 Mi ss North 0 100 200 Meters 300 Legend Gordon Lake Fm 4a. Red Sandstone, siltstone 2 2 3 4c 4b 4b 4b 70 io n 4a Ro ad 4b. Dolostone, chert 4c. Siltstone, mudstone Lorrain Fm 3. Quartz arenite Outcrop with code Fault Gowganda Fm 2. Mudstone, sandstone Archean Figure 2.11: Geology of dolostone occurrence in Fenwick Township. 1. Metavolcanics Geology by G. Bennett, 1988 The general geological setting of the dolostone units in Fenwick Township is shown on Figure 2.11. The best exposures of the dolostone unit are found in the area indicated on the figure as the “dolostone knob”. In the area of the dolostone knob, the lowermost third of the dolostone sequence consists mainly of very fine-grained, pale pink to reddish pink dolostone with intercalated grey to pink metachert. Individual dolostone beds are from a few At the western side of the knob, the clastic dolostone is directly overlain by deep-red sandstone, and red to pink laminated siltstone. The dolostone unit is underlain by green and maroon siltstone and argillite. The southern contact between the Lorrain quartz arenites and the dolostone is not exposed but lies within an east-trending, narrow, linear, PDF compression, OCR, web-optimization with CVISION's PdfCompressor �50 recessive zone about 10 m wide, and is inferred to be a fault. lithological and stratigraphic similarities of the Chocolay Group (and equivalent groups further west) and the Cobalt Group have been pointed Correlation with the Gordon Lake out by many authors (Figure 2.12). At the time, Formation this correlation was not widely accepted because previous radiometric dating of rocks considered Red siltstone and sandstone, purple and green to underlie the Chocolay Group was seen as siltstones, chert and dolostone of Fenwick providing evidence that the Chocolay Group is Township have been assigned to the Gordon significantly younger than the Huronian Lake Formation of the Huronian Supergroup Supergroup of Ontario (Young, 1983). (Bennett et al, 1990). This correlation is However, Vallini et al (2005) have successfully indicated by the juxtaposition of the dated hydrothermal xenotime in the Kona dolostone and associated rocks with the Formation and detrital zircons in the underlying Lorrain Formation (Although this is likely a Mesnard Formation thereby constraining the fault contact). The reddish hues of the associated sandstones Gordon Lake area Flack Lake Area Marquette Area Fenwick Twp indicate they are Sault Ste Marie Bar River Fm. Wewe Slate Ontario stratigraphically superior Kona Gordon Lake Fm. Cu Cu Fault Fm. to the Serpent Formation Mesnard Fm. Lorrain Fm Lorrain Fm (Roscoe (1969). Most Gowganda Enchantment L. Fm. Fm importantly, nodules and Gowganda Fm. Archean Archean continuous beds of 250 km dolostone have been Lower Huronian Groups Dolostone identified in the Gordon Lake Formation of the Diamictite 130 km Gordon Lake area SIltstone, sandstone (Jackson, 2001; Hofmann Slate, argillite et al 1980). Quartz arenite Figure 2.12: Proposed stratigraphic relationships between the During a discussion (in Lower Proterozoic rocks of Ontario and Michigan. 1988) of the problematic correlation of the timing of deposition of the Chocolay Group dolostone units with Mr. Ken Hatfield of between 2300-2200 Ma thus proving a Lake Superior State University, the latter correlation with the Huronian Supergroup of suggested a correlation with the Kona Ontario. Dolomite of the Marquette, Michigan area (Figure 2.7). There seems litte doubt now that the Cobalt Correlation with the Chocolay Group of Group rocks of the Huronian Supergroup are Michigan stratigraphically equivalent to the Chocolay Group of the Marquette Range Supergroup of There has been a long-standing debate as to Michigan and to correlative units further west. stratigraphic and temporal equivalence of the Huronian rocks of Ontario and the Middle Precambrian rocks of the Lake Superior region of Michigan (Young, 1983) The PDF compression, OCR, web-optimization with CVISION's PdfCompressor �51 Supplimentary Road Log for Highway 638 Highway 638 extends north from Highway 17 at A few small, ground-level rock exposures on the Echo Bay and continues in a broad loop to rejoin north side of the road near the western limit of Highway 17 at Bruce Mines (Figure 1.18). the northern rock-cut exhibit ancient mud-cracks as polygonal patterns of apple-green Only STOP 2-9 of this road log is part of the mudstone and pink sandstone. Day 2 sequence. Stops preceded by “S” are supplementary or optional stops. A few of these Ripple-marks and laminated, red siltstone can be stops may be made if time allows. The seen at about midpoint on the north outcrop. remaining outcrop descriptions are abstracted Small-scale (less than 20 cm) cross-bedding is from a previous guidebook (Bennett et al, 1997) well developed in pale grey and pink sandstone for the general interest of the reader. on the eastern portion of the southern road-cut but can be observed in most of the sandstone of the road-cut. Road Log and Outcrop Descriptions. 0 km = 0 mi. The Husky Service Station on Highway 17 near the eastern limits of the City of Sault Ste. Marie. 17.8 km =11.1 mi. Junction of Highway 638 and Highway 17 in the Town of Echo Bay. Reset odometer. Continue along Highway 638. 5.4 km = 3.4 mi. Outcrops of pink weathering sandstone of the Lorrain Formation. 10.6 km = 6.6 mi. Large road cuts on both sides of highway. The west limit of the outcrop area is at NAD 83 (WGS 84) 17-273278E, 5015292N. STOP S1-1: Lowermost Gordon Lake Formation The sandstones and minor mudstone exposed in the rock cuts are either the lower units of the Gordon Lake Formation or the uppermost rocks of the underlying Lorrain Formation. A large portion the sandstone is “bleached” almost white in contrast to the pink colour dominant in much of the outcrop. Note that the boundary between the pink and pale-grey sandstone clearly crosses the stratification in the rocks. That the bleaching (reduction) is the later event can be seen near the eastern limit of the road-cut. There the pale-grey sandstone is cut by dark red, hematite staining for several meters on both sides of steeply dipping, northeast striking fractures in the sandstone. A wide zone of reduction can be seen on both sides of the oxidized fractures. A hypothesis consistent with these observations is that chemically active fluids passing upward though the fractures first reduced the red hematite in the pink feldspar of the sandstone over a broad area along the fracture and in doing so became enriched in ferrous iron. Some time later, the system became oxidizing resulting in the precipitation of the much less soluble ferric iron as hematite adjacent to the fracture zone. These later fluids may have been oxygenated groundwater percolating downward from the surface encountering the ascending Fe+2 bearing reducing fluids that had scavenged iron from the underlying rocks. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �52 A mafic dike about 10 cm wide intrudes the sandstone near the fracture zone. The dike occupies a fracture with a strike (direction) and dip similar to that of the hematite-filled (reddened) fractures. This suggests that hot magma was in the vicinity when the fractures existed and thus may have provided the thermal energy to drive the fluids through the fracture system. 10.9 km = 6.7 mi. East end of outcrop at 17-273416E, 5150243N. 11.6 km = 7.2 mi 17 – 274165E, 5150594N STOP 2.9: Dolostone nodules and discontinuous dolostone beds in the Gordon Lake Formation. NOTE: The outcrop at this location is on private property. To protect the rights of the property owner the exact location of the outcrop will not be provided here. Outcrops of buff coloured siltstone and sandstone contain deeply recessed, discontinuous beds and scattered dark-brown to black dolostone. Jackson (1994) states: “Buff brown to black weathering siliceous dolostone nodules are common throughout the lowermost Gordon Lake Formation. The nodules are commonly amoeboid and concentrically zoned with carbonate-rich core and dark hematitic-rich rinds. A large number of the amoeboid forms are, however, asymmetrically zoned with the hematitic rind being concentrated, or better developed, towards the stratigraphic top of the amoeboid forms suggesting at least local upward flow of fluids. In red to deep maroon siltstones, the carbonate-rich nodules are very pale pink to white, reflecting the widespread reduction and/or removal of iron from these volumes of the rock” Return to Highway 638. 23.2 km = 14.4 MI. Village of Leeburn Corner and general store. 25.0 km = 15.3 mi. East side of road. Dark grey-green mudstone and siltstone of the Gowganda Formation. Thin-bedded to very thick bedded with some thin beds of pink siltstone. 25.3 km = 15.8 mi. Dark grey siltstone with smooth, striated outcrop surface due to most recent (Pleistocene) glaciation. 26.4 km = 16.4 mi. Nipissing granophyre and diabase, fractured and cut by calcite and iron carbonate veinlets. 30.7 km = 19.1 mi. Stop Sign at the village of Ophir. 32.7 km = 19.7 mi. West side of highway. STOP S1-2: Mudstone overlying matrix-supported conglomerate (diamictite) of the Gowganda Formation. Matrix supported conglomerate consist of pebbles and bounders dispersed in a fine matrix so that the pebble and boulder sized clasts are supported by the matrix, not each other. Such conglomerates cannot be formed by the transport and deposition of clay to boulder-sized clasts by water or wind. Matrix supported conglomerates usually imply deposition from mudflows or directly from ice as glacial till. 32.9 km = 20.4 mi. West side of Highway. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �53 STOP S1-3: Dropstones in laminated siltstone and mudstone e of the Gowganda Formation. STOP S1-5: Jasper pebble conglomerate of the Lorrain Formation (“Pudding Stone”). A few granitic pebbles lie within dark grey laminated siltstone. It is generally agreed that the only process that can allow the juxtaposition of pebbles or boulder-sized clasts in stratified, fine-grained sediment of is the release of stones by melting icebergs onto the sea bed or lake bottom. Outcrops of pale-grey quartz arenite and jasper-quartz pebble conglomerate are found at east side of the highway. It is assumed that the jasper pebbles were derived from Archean banded iron formation, which is well represented in the Batchawana greenstone belt about 130 km northwest of these outcrops. The jasper-pebble conglomerate member (Frarey, 1977) of the Lorrain Formation is a minor but persistent unit. It has been found near the middle of the Lorrain Formation from the Sault Ste Marie area in the west to the Quebec border in the east. Although the high concentration of red, maroon and pink jasper presents a striking appearance it has a limited commercial use as a decorative stone because of its extreme hardness. 33.6 km = 20.9 mi. Large gravel pit with high rock face on east side of the highway. A short unsurfaced road leads to the gravel pit. STOP S1-4: Diamictite of the Gowganda Formation. The high rock face is worn smooth by rushing water charged with abrasive sand etc. Vertical channels in the rock face suggest the erosive effect of sand-charged water under the extreme pressure of an overlying ice sheet. The northern section of the outcrop is matrix-supported conglomerate of the Gowganda Formation. A diabase dike (probably of Nipissing age) intrudes the southern part of the outcrop. The dike is about 20 meters wide (estimated). 42.8 km =26.6 Outcrops of Lorrain Formation sandstone on west side of highway. At least two faults show hematization in the fracture zone. A pebble unit is seen to be displaced along one of the fault. 44.1 km = 27.4 mi. Bridge at Rydal Bank. 36.2 km = 22.5 miles Rock Lake Road. 52.7 km = 32.7 mi Junction of Highway 628 and Highway 17 at Bruce Mines. 39.8 km = 24.7 mi. End of road log for Highway 638. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �54 Supplimentary Road Log for parts of Highway 17 and Highway 129 between Sault Ste Marie and Highway 108 Most of the outcrops included in this section are not included in the ILSG 2006 field trip. Stops indicated as supplementary are preceded by the letter “S”. They are included to allow those interested to visit them if traveling in the area at some future date. UTM coordinates have been provided for some locations. Note: the differing ripple directions and local interference ripples. This impressive outcrop is known locally as the ”Ripple Rock”. 26.8 km = 16.6 mi. STOP S2.3: Purple siltstone member of the Lorrain Formation. Dark red to purple siltstone and sandstone Proceed east along Highway 17 from Sault Ste. constitute true red-beds in the Huronian (Frarey, 1977). These hematite-rich rocks Marie. display the reduction along fractures typical of The road log starts at the intersection of red beds, and a good indicator of deposition in Highway 638 and Highway 17 East in the an oxidizing environment. The bedding is Town of Echo Bay faint, but locally distinct, striking 140o and dipping 10o south. 0.0 km = 0.0 miles - Highway 638 at Echo Bay - continue east on Highway 17. 27.7 km = 17.2 mi. Lake Huron Drive in the town of Desbarats. 8.8 km = 5.5 miles - At sign for Calabogie Road UTM 16- 725322E,5144204 N,. 28.0 km = 17.4 mi. 17- 5136434N, 275510E, STOP S2.1: Sub-Jacobsville unconformity STOP S2.4: Basal Arkose Member of Lorrain Formation. Outcrop on west side of Highway 17. The most southerly outcrop is white and grey quartzite (quartz arenite) of the white orthoquartzite member (Frarey, 1977) of the Lorrain Formation quartzite is overlain by a sedimentary breccia of the Keweenawan, Jacobsville Formation. The conglomerate is comprised of angular clasts of Lorrain quartzite up to 60 cm across in a red siltstone matrix. The contact is irregular. No stratification was observed. Large outcrops of pink, medium-grained, rather massive arkose beds with hematite-rich spots about one centimeter across. Pale laminations in outcrop on south side of the road indicate shallow dips of about 10o west. The unit is about 1700 feet thick in this area (Frarey, 1977). 40.5 km = 25.0 mi. STOP S2.5: Bruce Mines Copper Vein. 25.0 km = 15.5 mi. In a rock-cut on the north side of Highway 17, in the town of Bruce Mines, is a 3 m wide STOP S2.2: Lower red quartzite member quartz vein in Nipissing diabase. The vein contains small amounts of chalcopyrite, of the Lorrain Formation. chalcocite, bornite and malachite. It is one of On the north side of Highway 17 steeply the few remaining remnants of the copper dipping beds of the lower red siltstone member mineralization that once made this town of the Lorrain Formation reveal well preserved known throughout the mining world. Mining oscillation ripples an a large south facing began here in 1846 and continued to 1875, surface. making Bruce Mines the first mining town in Canada. Please do not take samples from this outcrop. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �55 About 75 meters north of this outcrop there is a reconstruction of a copper mining operation of that period built on one of the original mine workings, known as the “Simpson Shaft”. 42.6 km = 26.5 mi. Entrance to the quarry of Ontario Trap Rock (R.W. Tomlinson Limited). The quarrying of trap rock (Nipissing diabase) at Bruce Mines resumed in 1990. A predecessor near Lake Huron operated during the First World War. chlorite. The relatively small, monomineralic chlorite amygdules (“chlorite buttons”) are characteristic of the mafic volcanics of the Thessalon Formation between Sault Ste. Marie and Elliot Lake. Glacial striae and chatter marks are well developed on the outcrop surface. Proceeding north on Highway 129. Reset odometer to 00 at Intersection of Highway 17 and Highway 129. 45.3 km = 28.1 mi. 17-289512E, 5129999N 1.0 km = 0.62 mi. STOP S2.6: Low outcrops of Mississagi Formation. STOP S2.8: Outcrop of Matinenda Formation on east side of Highway 129. Medium-sand to grit-sized, grey, subarkose with thin quartz-pebble beds and prominent planar cross-bedding are well exposed along the north side of the highway. Rusty staining on the outcrop reflects minor detrital (?) pyrite along the foreset beds. Black chert pebbles in pebbly beds, minor pyrite and meter scale bedding are common features of the Mississagi Formation. Roscoe, (1969) concluded that the drab colour and presence of detrital pyrite in Huronian sandstones and conglomerates is evidence of the reducing nature of the early Huronian atmosphere. 45.4 km = 28.2 mi. Waltonen Road near east end of the outcrop area. This fine-grained, pale pink to greenish grey, subarkose which directly overlie the Thessalon Formation in the Thessalon area has been correlated with the Matinenda Formation of the Elliot Lake Group (Bennett et al., 1991). Faint outlines of trough cross-bedding, visible on the upper surface of the outcrop, indicate paleocurrent directions from the northwest. The greenish (sericitic) units and the trough cross-beds are typical of the Matinenda Formation, but the fine grain size is not. At Thessalon Point about 4 km southwest of this location these rocks contain thin quartz pebble conglomerate units, but the pyrite and radioactivity, characteristic of the Matinenda Formation of the Elliot Lake area, is lacking. 60.6 km = 37.6 mi. 17 - 303650E, 5126650N 23.0 km = 14.9 miles The village of Warncliffe Intersection of Highway 17 and Highway 129. 29.1 km = 18.4 miles north end of Appleby Lake. Stop S2.7: Thessalon Formation tholeiitic basalt. Dark green, fine to medium grained basalt is exposed on the east side of Highway 129 just north of the intersection with Highway 17. This tholeiitic basalt forms the top of the Thessalon Formation in the Thessalon area and is typical of the thicker tholiitic sequences which make up the upper parts of the Thessalon Formation in the Sault Ste. Marie and Aberdeen Township areas. The rocks consist of albite, chlorite, epidote, clinozoisite, leucoxene and minor quartz and oxides. The metabasalt contains amygdules of albite, quartz, epidote, calcite and 30.3 km = 19.4 miles UTM 17, 5 145 009N, 320 904E STOP S2.9: Laminated siltstone of the Gowganda Formation with dropstones. West of Highway 129. Proceed up steep slope to cliff face located a few tens of meters into the bush. Vertical joint faces in laminated siltstones-mudstone of the Gowganda Formation form an impressive cliff just west of Highway 129. The bedding comprises regular and remarkably continuous mudstone-siltstone PDF compression, OCR, web-optimization with CVISION's PdfCompressor �56 couplets from 0.5 to 2 cm thick. The grey siltstone unit at the base of each couplet grades up to a darker mudstone which is in sharp contact with the siltstone base of the overlying couplet. The sequence closely resembles varves of Pleistocene glacial lakes. A few pink, discontinuous, fine, sandstone beds are also present. Widely scattered trough the sequence are pebble to boulder-sized “drop stones”, predominantly of Archean granitic rocks. In the absence of volcanic activity within the Gowganda Formation, the only plausible source of the drop stones is the presence of floating ice which released its rock load during melting. This outcrop, and many others of a less spectacular nature, provide some of the best evidence for a cool climate during Gowganda deposition more than 2 Ga.. Return to Highway 17 Reset Odometer. 3 km = 1.9 Pine Ridge Road Proceeding east on Highway 17 from Pine Ridge Road Reset odometer to 0. 13.5 km = 8.4 mi. Melwell Road 14.1 km = 8.7 mi. 17-321874E, 5129338N – Outcrop with prominent white quartz veins near crest of hill . STOP S3.1: Mineralized quartz vein breccia. Quartz-carbonate veins and vein breccia cut pebbly siltstone of the Gowganda Formation Church and Young, (1972) interpret the on the south side of Highway 17. The vein laminated unit as a varved sequence. They and mineral assemblage also note the presence of small sedimentary (quartz-carbonate-pyriteclasts, lithologically similar to the enclosing rocks. They interpret the clasts of sedimentary chalcopyrite-hematite) are typical of many in rocks as having been derived from the ablation such occurrences the area between Sault Ste. of sediment on the upper surface of the glacier. Marie and Sudbury. The extensive brecciation suggests surface venting with resulting 30.9 km = 19.5 miles. Intersection with hydrothermal boiling, adiabatic cooling with Highway 554. Return to Thessalon along coincident brecciation and mineral deposition. Highway 129. Some sections of the vein system show well-developed mineral zoning and pre-vein 30.9 km = 19.5 miles. Intersection of alteration. Minerals identified include: quartz, Highway 17 and Highway 129 at Thessalon. iron-carbonate (ankerite?), pink calcite, Reset odometer to 0.0 chalcopyrite and specular hematite. The lack 7.3 km - 4.6 miles. 17 - 306085E, 5126967N. of pyrite and the late deposition of hematite, indicates a sulfur deficient hydrothermal fluid. Green Lane. Cross the railway tracks (with care) and immediately turn left then south to short section of unpaved road between private 47.6 km = 29.6 mi. Town of Blind River properties. Outcrops of Thessalon Formation 66.7 km = 41.4 mi. Pronto Road. 17 – felsic metavoclanics lie at the end of the road. 367425E, 5117509N STOP S2.10: Rhyolite of the Thessalon Formation. Access to granitic paleosol and a remnant of radioactive quartz-pebble conglomerate of the former Pronto Mine. Volcanics are fine grained, grey to pink and maroon rhyolite consisting of a fine grained mosaic of albite, k-feldspar, and quartz with At the time of writing (2006) the Pronto Mine minor green pleochroic biotite. Amygdules are site is currently not open to the public. Those filled with quartz, biotite and stilpnomelane. planning to visit this site should contact the Collapsed vesicules are also present. Local iron staining and darker chloritic areas indicate the presence of iron sulfides. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �57 A S3.2 Former mine site B Pr on to Th ru st LEGEND Fa ul Diabase t HURONIAN Mine workings Higher Formations Matinenda Fm. Ore bed Paleosol ARCHEAN After Robertson (1970). Granitic rocks N A S3.2 Former mine site Since rehabilitated rust Fault nto Th Pro Hy dro El ec tri c Line Pond Pond B nto Pro d Roa l Fau r r ay Mu t ay ghw 17 Hi Lake Huron 1 km After Robertson (1970). HURONIAN SYMBOLS Formations above the Matinenda Matinenda Formatioin Pater Formation ARCHEAN Granitic rocks S3.2 Field trip stop Fault Projection of original ore body Figure 2.13. Plan and section of the geology of the (former) Pronto Mine area. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �58 displayed as dikes and irregular bodies of aplite and pegmatite, can be recognized in the paleosol (saprolith). Continuing up the southward sloping outcrop the yellowish colour of the paleosol can be seen to take on a pinkish and reddish hue normal to the Archean granitic rocks of the area. Continuing north Turn north onto the Pronto road and continue and west to near the highest point on the until prevented by a gravel barrier from outcrop area one can discern the faint outline proceeding further. Continue on foot past the of core-stones in the original granitic rocks. barrier to the west end f a beaver pond just north of the road. Proceed along a trail across 11.6 km = 7.2 mi Highway 108 to Elliot Lake the west outlet of the pond, then around the Return to Highway 17. Continue east along north end of the pond and up a steep hill. Turn left (to the north) on a gravel road at the Highway 17 to Highway 108 to Elliot Lake. top of the hill. Continue to an area of outcrop Continue eastward for a few hundred meters and gravel which marks the former open stope past Highway 108 to an outcrop just south of of the Pronto mine at 17-367804, 5119019. If Highway 17. not accompanied by a guide, please do not Continue for about 100 m past the intersection attempt to access this site without a GPS of Highway 17 and Highway 108. receiver or compass as well as suitable footwear and clothing. Low outcrops a few tens of meters south of Highway 17. (Refer to Figure XXX). District Geologist office of the Ministry of Northern Development and Mines at Sault Ste Marie, Ontario (705 - 945-6931) to learn of the site status and the phone number of a contact person with the authority to grant permission to access the property. STOP S3.2: Pronto Mine Location This property is currently (2006) undergoing restoration to a condition as closely as possible to it’s original state. STOP S3.3: McKim Formation Staurolite (pseudomorph) schist. This outcrop, which lies a few hundred meters south of the Murray Fault Zone, displays the The Pronto Mine of Rio Algom Limited, the higher metamorphic grade and deformation first operating uranium mine of the Algoma common to rocks south of the fault. At this District, produced 2.1 million tones of ore location argillaceous units of the McKim between 1955 and 1960. (Figure 2.13. Formation contain pale prismatic crystals of staurolite that have undergone retrograde This is one of the few locations where metamorphism to sericite-quartz aggregates. A exposures of uraniferous quartz pebble conglomerate of the Matinenda Formation can few pseudomorphs show the characteristic cruciform twinning of staurolite. The growth of be found on surface in the Elliot Lake-Blind large staurolite crystals in the originally more River area. In the depression south of the argillaceous upper portions of the beds has road, a few remnants of rusty-weathering resulted in “reverse graded bedding” in a few outcrops of radioactive, pyritic, quartz-pebble places. conglomerate of the Matinenda Formation mark the base of the ore bed of the Pronto Return to the intersection of Highway 17 and Mine. The conglomerate lies directly upon Highway 108. Proceed north along Highway yellowish, sericitic paleosol developed on 108 to the Town of Elliot Lake. Archean granitic basement. Visitors are requested not to take samples of the Continue north on Highway 108 to the City of conglomerate. Elliot Lake. Proceeding in a general northerly direction, some features of the original granitic rocks, PDF compression, OCR, web-optimization with CVISION's PdfCompressor �59 Publications Cited or Consulted Bennett, G. 1978. Huronian volcanism, districts of Algoma and Sudbury; in Summary of Field Work, 1978; Ontario Geological Survey, Miscellaneous Paper 82, p.105-111. Bennett, G. 1982. Geology of the Two Horse Lake Area, District of Algoma; Ontario Geological Survey, Geological Report 210, 63p. Bennett, G., Hillier, R. D., Nentwich, F., Dupuis, C. P. and Pucovsky, M.., 1975. Jarvis Lake-Garden River area, District of Algoma, Ontario Division of Mines; Preliminary Geological map, P.1064, 1 inch to ¼ mile or 1:15,840. Bennett, G and Sawiuk, Myron. 1979, Jarvis Lake-Garden River area (Southern Part), District of Algoma, Ontario Division of Mines (Now Ontario Geological Survey); Preliminary Geological map, P.2241, 1 inch to ¼ mile or 1:15,840. Born, Peter, 1987. Geology of the Havilland Bay – Goulais Bay Area District of Algoma; Ontario Geological Survey, Open File Report 5602, 114 p with map at a scale of 1:15 840 (1 inch to ¼ mile) Born, P. and James, R.S. 1978. Geology of the East Bull Lake anorthosite intrusion, District of Algoma, Ontario; in Proceedings and Abstracts, Geological Association of CanadaMineralogical Association of Canada, Joint Annual Meeting, v.3, p.369. Born, P., Worona, R., and Stephenson C., 1986. Precambrian Geology of the Haviland-Goulais Area. District of Algoma; Ontario Geological Survey, Map P2959. Geological Series-Preliminary Map. Scale 1:15 840 or 1 inch to ¼ mile. Geology 1985. Bottrill, T. J., 1971. Uraniferous conglomerates of the Canadian Shield; in Report of Activities, Part A: April to October, 1970, Geological Bennett, G., Leahy, E. J, Melisek, J. Born, P. and Survey of Canada, Paper 71-7, p.77-83. Hatfield, K. 1989. Sault Ste. Marie Resident Buchan, K. L. and Card, K.D., 1985. Preliminary Geologists District—1988; in Report of comparison of petrographic and paleomagnetic Activities 1988, Resident Geologists, Ontario Geological Survey, Miscellaneous Paper 142, p. characteristics of Nipissing diabase intrusions in northern Ontario; in current Research, Part A, 207-217. Geological Survey of Canada, Paper 85-1A, Bennett, G., Leahy, E. J. and Walmsley, J. 1990. p.131-140. The Sault Ste. Marie Resident Geologist’s District-1989; in Report of Activities, Regional Cannon,W. F., 1962. Plutonic evolution of the and Resident Geologists, Ontario Geological Cutler area, Ontario; unpublished PhD thesis, Syracuse University, Syracuse, New York, 105p. Survey, Miscellaneous Paper 147, p.205-215. Bennett, G., Dressler, B.O. and Robertson, J.A. Card, K. D., 1964. Metamorphism in the Agnew 1991. The Huronian Supergroup and associated Lake area, District of Sudbury, Ontario, Canada; intrusive rocks; in Geology of Ontario, Ontario Geological Society of America Bulletin, v.75, no. Geological Survey, Special Volume 4, Part 1, p. 10, p.1011-1030. 549-592. Bennett, G., Card K. D. and Tomlinson, K. Y. 1997. The Huronian Supergroup between Sault Ste Marie and Elliot Lake; Institute on Lake Superior Geology, 43rd Annual Meeting, May 6, 11, 1997, Sudbury, Ontario. Card, K. D., 1976. Geology of the MacGregor Bay – Bay of Island Area, Districts of Sudbury and Manitoulin, Ontario Division of Mines Geoscience Report, 131, 63p. Card, K. D., 1978. Geology of the Sudbury-Manitoulin area, districts of Sudbury PDF compression, OCR, web-optimization with CVISION's PdfCompressor �60 and Manitoulin; Ontario Geological Survey, Report 166, 238p. Card, K. D., Church, W.R., Franklin, J.M., Frarey, M.J., Robertson, J.A., West, G.F. and Young, G.M. 1972. The Southern Province; in Variations in Tectonic Styles in Canada, Geological Association of Canada, Special Paper 11, p.335-380. Card, K. D., Innes, D.G. and Debicki, R.L., 1977. Stratigraphy, sedimentology, and petrology of the Huronian Supergroup in the Sudbury-Espanola Area; Ontario Division of Mines, Geoscience Study 16, 99 p. Chandler, F.W., Young, G.M. and Wood, J., 1969. Diaspore in early Proterozoic quartzite (Lorrain Formation) of Ontario; Canadian Journal of Earth Sciences, v.6, p.337-340. Church, W. R. and Young, G. M., 1972. Precambrian geology of the southern Canadian Shield with emphasis on the lower Proterozoic (Huronian) of the North Shore of Lake Huron; International Geological Congress 24th, Montreal, Guidebook to field excursion A36-C36, 65 p. Coleman, A. P., 1905. The Lower Huronian ice age; Journal of Geology, v.16, no. 2, p.149-158. Card, K. D. and Pattison, E. F., 1973. Nipissing diabase of the Southern Province; in Huronian Collins, W. H., 1925. North shore of Lake Huron; Geological Survey of Canada, Memoir Stratigraphy and Sedimentation, Geological 143, 160p. Association of Canada, Special Paper 12, p.7-30. Corfu, F., and Andrews, A., 1986. A U-Pb age Card, K. D. and Palonen, P.A., 1976. Geology for mineralized Nipissing diabase, Gowganda, Ontario; Canadian Journal of Earth Sciences, of the Dunlop-Shakespeare area, District of v.23, p.107-112. Sudbury; Ontario Division of Mines, Geoscience Report 139, 52p. Dietz, R. S. and Holden, J.C, 1966. Card, K. D. and Jackson, S.L., 1995. Tectonics Miogeosynclines in space and time; Journal of Geology, 74, p. 566-583. and metallogeny of the Early Proterozoic Huronian fold belt and the Sudbury Structure Dyer, B. D., Krumbein, W.E. and Mossman, of the Canadian Shield; Field Trip Guidebook, D.J., 1988. Nature and origin of stratiform Precambrian 95, Geological Survey of Canada kerogen seams in lower Proterozoic Open File 3139. Witwatersrand-type placers-the case for Casshyap, S. M., 1969. Petrology of the Bruce biogenicity; Geomicrobiology Journal, v.6, p.33-47. and Gowganda formations and its bearing on Huronian Sedimentation in the Eisbacher, G. H., and Bielenstein, H.U.,1969. Espanola-Willisville area, Ontario, Canada; The Flack Lake Depression, Elliot Lake area, Paeleogeography, Paleoclimatology and Ontario (41J/10); in Report of Activities, Part Paleoecology, v.6, p5-36. B, Geological Survey of Canada, paper 69-1B, Chandler, F. W., 1973. Geology of McMahon p.58-60. and Morin Townships, District of Algoma, Fahrig, W. F., 1987. The tectonic setting of Ontario Division of Mines, Geoscience report continental mafic dike swarms: failed arms and 112, 77p. early passive margins; in Mafic Dike Swarms, Geological Association of Canada, Special Chandler, F. W., 1976. Geology of the Paper 34, p.331-348. Saunders Lake area, District of Algoma, Ontario Division of Mines, Geoscience report Fralick, P. W. and Miall, A.D., 1989. 155, 46p. Sedimentology of the lower Huronian Supergroup (early Proterozoic), Elliot Lake PDF compression, OCR, web-optimization with CVISION's PdfCompressor �61 area, Ontario, Canada; Sedimentary Geology, v.63, p.127-153. Frarey, M J., 1967. Three new Huronian names; Geological Survey of Canada, Paper 67-6, 3p. Frarey, M. J., 1977. Geology of the Huronian belt between Sault Ste. Marie and Blind River, Ontario; Geological Survey of Canada, Memoir 383, 87p. Frarey, M. J. and Roscoe, S.M., 1970. The Huronian Supergroup north of Lake Huron; in Symposium on Basins and Geosynclines of the Canadian Shield; Geological Survey of Canada, 70-40, p. 143-158. Frarey, M. J., Loveridge, W.D. and Sullivan, R.W., 1982. A U-Pb zircon age for the Creighton granite, Ontario; in Rb-Sr and U-Pb Isotopic Age Studies, Report 5, Current Research, Part C, Geological Survey of Canada, Paper 82-1C, p.129-132. Frarey, M. J. and Krogh, T.E., 1986. 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Lithophile elements in Huronian low-Ti continental tholeiites from Canada and evolution of the Precambrian mantle; Earth and Planetary Science Letters, v.85, p.401-415. Junnila, W. T., 1987. A bibliography of the Huronian Supergroup; 1921-1987; Ontario Geological Survey, Open File Report 5651, 71p. igneous province; Ontario Geological Survey Study 58, 80p. Lindsay, D.A., 1971. Glacial marine sediments in Precambrian Gowganda Formation at Whitefish Falls, Ontario; Paeleogeography, Paleoclimatology and Paleoecology, v.9, p7-25. Long, D. G. F., 1976. Stratigraphy and sedimentology of the Huronian (Lower Aphebian) Mississagi and Serpent Formations; Kamineni, D. C., McCrank, G.F.D., Stone, D., unpublished PhD thesis, University of Western Ejeckman, R.B., Flindall, R. and Sikorsky, R., Ontario, London, Ontario, 291 p. 1984. Geology of the central plateau of the Long, D. G. F., 1978. 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The sedimentology of the Huronian Ramsay Lake and Bruce formations, north shore of Lake Huron; unpublished PhD thesis, University of Western Ontario, London, Ontario. Peck, D. C., James, R. S., and Chubb, P. T. , Previc, S. A. and Keays, R. R., 1995. Geology, metallogeny and petrogenesis of the East Bull Lake Intrusion, Ontario; Ontario Geological Survey, Open File Report 5923, 117p. Pettijohn, F. J., 1970. The Canadian Shield: a status report; in Symposium on Basins and Geosynclines of the Canadian Shield, Geological Survey of Canada, Paper 70-40, p.329355. Pettijohn, F. J., 1975, Sedimentary Rocks, Harper and Rowe, 628p. Pienaar, P. J., 1963. Stratigraphy, petrography and genesis of the Elliot Group, Blind River, Ontario, including the uraniferous conglomerate; Geological Survey of Canada, Bulletin 83, 140p. Prasad, N. and Roscoe, S M. ,1991. Profiles of altered zones at ca 2.45 Ga unconformaties beneath Huronian strata, Elliot Lake Ontario: evidence for early Aphebian weathering under anoxic conditions; in Current Pretorius, D.A. 1981. Gold and uranium in quartz-pebble conglomerate; Economic Geology, 75th Anniversary Volume, p.117-138. Rainbird, R. H., Nesbit, H. W. and Donaldson, J. A., 1990. Formation and diagenesis of sub-Huronian saprolith: comparison with a modern weathering profile; Journal of Geology, 98, p. 801-822. Rice, R. J., 1991. Regional sedimentology and paleoplacer gold potential of the Lorrain Formation, Huronian Supergroup, in the Cobalt plain; Ontario Geological Survey, Open File Report 5761. Riller, U. P. ,1996. Tectonometamorphic episodes affecting the southern footwall of the Sudbury Basin and their significance for the origin of the Sudbury Igneous Complex, Central Ontario, Canada; unpublished Ph.D. thesis, University of Toronto, 135 p. Robertson, J. A., 1961. Geology of townships 143 and 144; Ontario Department of Mines, Geological Report 4, 66p. Robertson, J. A., 1962. Geology of Townships 137 and 138; Ontario Department of Mines, Geological Report 10, 94p. Robertson, J. A.,1968. Geology of Township 149 and Township 150, District of Algoma; Ontario Department of Mines, Geological Report 57, 162p. Robertson, J. A., 1970. Geology of the Spragge area, District of Algoma; Ontario Department of Mines, Geological Report Number 76, 109p. Robertson, J. A., 1976. The Blind River uranium deposits; the ores and their setting, Ontario Division of Mines, Miscellaneous Paper 65, 45p. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �64 Robertson, J. A., 1986. Huronian Geology and the Blind River (Elliot Lake) uranium deposits, 7-31, the Pronto Mine, in Uranium Deposits of Canada, Canadian Institute of Mining and Metallurgy, Special Paper 33, p. 46-43. Sutton, S.J. and Maynard, J.B. 1992. Multiple alteration events in the history of a sub-Huronian regolith at Lauzon Bay, Ontario, Canada; Canadian Journal of Earth Sciences, vol. 29, p. 432-445. Robertson, J. A., Frarey, M .J. and Card, K. D., 1969. The Federal-Provincial Committee on Huronian Stratigraphy: Progress Report; Canadian Journal of Earth Sciences, v.6, p.335-336. Sutton, S.J. and Maynard, J.B. 1993. Sediment and basalt hosted regoliths in the Huronian supergroup: role of parent lithology in middle Precambrian weathering profiles; Canadian Journal of Earth Sciences, vol. 30, p. 60-76. Roscoe, S. M., 1969. Huronian rocks and uraniferous conglomerates; Geological Survey of Canada, Paper 68-40, 205p. Theis, N.J. 1979. Uranium-bearing and associated minerals and their geochemical and sedimentological context, Elliot Lake, Ontario; Geological Survey of Canada, Bulletin 304, 50p. Roscoe, S. M., 1981. Temporal and other factors affecting deposition of uraniferous conglomerates; in Genesis of Uranium- and Gold-Bearing Precambrian Quartz-Pebble Conglomerates; United States Geological Survey, Professional Paper 1161-A-BB, p.W1W7. Tomlinson, K.Y. (1996). The geochemistry and tectonic setting of early Precambrian greenstone belts, Northern Ontario, Canada; unpublished Ph.D. Thesis, University of Portsmouth, 278p. Vallini, Daniela, A., Connon, William, F, and Schulz, Kalus J., 2005. New age data for the Chocolay Group, Marquette Range Supergroup: Implications for the Paleoproterozoic Evolution the Lake Superior Ruzicka, V. and LeCheminant, G. M., 1984. and Lake Huron regions. Institute on Lake Uranium deposit research 1983; in Geological Superior Geology Proceedings, 51st Annual Survey of Canada paper 84-1A, p. 39-44. Meeting, Nipigon, Ontatio, Part I – Proceeding Shanks, W. S. and Schwerdtner, W. M., 1991. and Abstracts, v.51 part 1 Structural analysis of the central and Williams, G. E. 1975. Late Precambrian glacial southwestern Sudbury Structure, Southern climate and the Earth’s obliquity; Geological Province, Canadian Shield; Canadian Journal Magazine, v112, p.441-465. of Earth Sciences, vol. 28, p. 411-430. Williamson, M. and Keen, C.E., 1995. How Sims. P. K., Card, K. D. and Lumbers, S. active are passive margins? Modern analogues B.,1981. Evolution of early Proterozoic basins of magmatism in rifts; the Canadian of the Great Lakes Region; in Proterozoic Mineralogist, vol. 33, p. 943-944. Basins of Canada, Geological Survey of Wright, D. J. and Rust, B. R., 1985. Canada, paper 81-10, p. 379-397. Preliminary report on the stratigraphy and Stockwell, C. H.,1982. Proposals for time sedimentology of the Bar River Formation; in classification and correlation of Precambrian Geoscience Research Grant Program, rocks in Canada and adjacent areas of the Summary of Research 1994-1995, Ontario Canadian Shield: part 1- a time classification Geological Survey, Miscellaneous Paper, 127, of Precambrian rocks and events; Geological p. 119-123. Survey of Canada Paper 80-19, p690-698. Roscoe, S.M. and Card, K.D., 1992. Early Proterozoic tectonics and metallogeny of the Lake Huron region of the Canadian Shield; Precambrian Research, V. 58, 9. 99-119. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �65 Wood, J., 1973.Stratigraphy and sedimentation in Upper Huronian rocks of the Rawhide Lake-Flack Lake area; in Huronian Stratigraphy and Sedimention, Geological Association of Canada Special Paper 12, p73-95. Young, G. M., 1983. Tectono-sedimentary history of early Proterozoic rocks of the northern Great Lakes; in Early Proterozoic Geology of the Great Lakes Region, Geological Society of America Memoir, v.160, p.15-32. Young, G. M., 1973. Tillites and aluminous quartzites as possible time markers for Middle Precambrian (Aphebian) rocks of North America; in Huronian Stratigraphy and Sedimentation, edited by G. M. Young; Geological Association of Canada Special Paper 12, p. 97-127. Young, G. M. and Nesbitt, H. W., 1985. The Gowganda Formation in the southern part of the Huronian outcrop belt, Ontario, Canada; Precambrian Research, v.29, p.265-301. Young, G. M., 1991. Stratigraphy, sedimentology and tectonic setting of the Huronian Supergroup; Field Trip B5:Guidebook; Young, G. M., 1982. Field excursion guide book; Geological Association of Canada and excursion 13B: Depositional environments and Mineralogical Association of Canada, Society of Economic Geologists Joint Annual Meeting, tectonic setting of the early Proterozoic Huronian Supergroup; International Association Toronto 1991., 34 p. of Sedimentologists, Eleventh International Zolnai, A. I., Price, R. A. and Helmstaedt, H., Congress on Sedimentology, McMaster 1984. Regional cross-section of the Southern University, Hamilton, Ontario, Canada. Province adjacent to Lake Huron, Ontario: implications for the tectonic significance of the Murray fault zone; Canadian Journal of Earth Sciences, v.21, p.447-456. PDF compression, OCR, web-optimization with CVISION's PdfCompressor � https://digitalcollections.lakeheadu.ca/files/original/7f3aee4f6a39e32e2c43f4b4a0ddd107.pdf 0babd4791a930a720e5fac1812b24e76 PDF Text Text Keweenawan Rocks of the Mamainse Point Area Field Guide for the 52nd Annual Institute on Lake Superior Geology Vol. 52, Part 5 U . -J • S U By: Thomas R. Hart, Ontario Geological Survey, Ministry of Northern Development and Mines Antonio Pace, Resident Geologist Program, Sault Ste. Marie District, Ministry of Northern Development and Mines PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Contents Page INTRODUCTION ............................................................................................................ 1 REGIONAL GEOLOGY.................................................................................................. 1 GENERAL GEOLOGY.................................................................................................... 2 Mafic Volcanic Rocks........................................................................................... 3 Felsic Rocks .......................................................................................................... 4 Clastic Sedimentary Rocks ................................................................................... 5 Younger Clastic Sedimentary Rocks .................................................................... 6 ALTERATION ................................................................................................................. 7 STRUCTURAL GEOLOGY ............................................................................................ 7 LITHOGEOCHEMISTRY ............................................................................................... 9 ECONOMIC GEOLOGY................................................................................................. 10 Mamainse Mine .................................................................................................... 10 Coppercorp Mine .................................................................................................. 11 GEOCHRONOLOGY ...................................................................................................... 15 PALEOMAGNETISM ..................................................................................................... 15 REFERENCES ................................................................................................................. 15 FIELD TRIP GUIDE ........................................................................................................ 18 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide 1989; Nicholson et al. 1997). Although there is only very limited geochronology in the MPF, recent work in the Lake Nipigon area indicates that rift related magmatic activity began at ~1114 Ma (Heaman and Easton 2005) which extends the period of magmatic activity from the ~22 Ma by Davis and Green (1997) to ~28 Ma. This extended period of time is much longer than the 1 to 5 Ma of volcanic activity proposed for more Phanerozoic continental flood basalt provinces by Jerram and Widdowson (2005), but is comparable to the time periods proposed for Archean age magmatic events possibly related to mantle plumes. This trip will hipiccten provide an opportunity to reIs Ia rid examine many of the same Ontado stops described by Giblin (1974) and Annells (1973). MIDDLE KEWEENAWAN ROCKS OF THE MAMAINSE POINT AREA INTRODUCTION Mid-Keweenawan mafic volcanic flows, felsic intrusive to extrusive rocks, and clastic sedimentary rocks of the Mamainse Point Formation (MPF) are located about 64 km north of Sault Ste. Marie, along the east shore of Lake Superior (Fig. 1). This area was the subject of a Nipigon Emb ayment—.5. M anitoba Say -Jr to z Minnesota ir. - REGIONAL GEOLOGY The ~1.1 Ga Midcontinent Rift extends for over 2000 km and is interpreted to be an aborted Wisconsin continental rift (Fig. 1) (e.g. Van Schmus and Hinze, 'C 1985). Seismic profiling of Michigan Lake Superior indicates that -J the rift consists of a series of a series of asymmetric grabens 'S.. separated by accommodation zones filled with up to 30 km Inferred extent of Keweenawan and sediments of volcanic and sedimentary Iowa rocks (e.g. Cannon et al., Upper sediments 1989). Initiation of rifting has KeweenaA'an igneous been related to a mantle KeweenaNan extrusive rocks plume, or hot spot (e.g. sediments Kansas Hutchinson et al. 1990; Figure 1. The Midcontinent Rift System and some of the Nicholson and Shirey 1990), major regional geological features from Lightfoot et al. which resulted in the production of < 1 500 000 (1999). km3 of volcanic and intrusive rocks (Klewin and Shirey 1992). Rift related volcanic and field trip by Giblin (1974) during the Annual sedimentary rocks of the Keweenawan Meeting of the Institute of Lake Superior Supergroup are generally exposed along the Geology. Since that time, there has been various margins of the rift, with the greatest volumes studies completed on the MPF (e.g. Massey 1980; exposed west of Lake Superior (Fig. 2) Klewin and Berg 1991; Shirey et al. 1994; (Nicholson et al. 1997). Limited exposures are Lightfoot et al. 1999; Walker et al. 2002) and on present along the east shore of Lake Superior at the rocks in other parts of the Keweenawan Mamainse Point, Cape Gargantua south of Wawa, Midcontinent Rift (e.g. Green 1983; Cannon et al. and Michipicoten Island with a more extensive Penr Ia \VVVV. S v'.vv 1 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide 1114 r 1 Seagull intrusion 1117 Ma Kltto Intrusion 1114 r 1 Disraeli intrusion Ca. 1,1 Ga Pigeon River Arr: call 1540 Ma English eay Grante ca. :.i GaKenora-Fortrrances GENERAL GEOLOGY The Keweenawan rocks are underlain by Archean rocks of the Batchawana Greenstone Belt that consists of mafic to intermediate metavolcanic and minor felsic metavolcanic rocks, and Algoman-type iron formation (Giblin 1974). The Archean rocks strike east, have been metamorphosed to amphibolite facies, and deformed resulting in northeast 01.12 TtCtOflICZOflt trending isoclinal North Bhoro folds and penetrative Pr fabric with steep dips. MW. MichipicoMn Mand Faue The Mamainse Point Formation (MPF) unconformably Archean overlies the Archean rocks, and consists of a sequence * alkalic complexes — International boundary estimated to be Faults between 4200 and ..A.._A.. Thrustfauks 6000 m thick Dikes consisting of subaerial mafic flows, intercalated clastic sedimentary and felsic igneous rocks (Fig. 4 and 5). The mafic flows can be subdivided into lower olivine- and plagioclasebearing basalts and upper plagioclase-bearing basalts (Annells 1973), which corresponds to a major change in the chemistry of the basalts (e.g. Klewin and Berg 1991; Lightfoot et al. 1999). Clastic sedimentary rocks consist of predominately conglomerates, and the 550m thick Great Conglomerate horizon marks the break between the lower and upper basalts. Minor felsites, quartz porphyry and flowNCF KF OF Paleozoic Illinois and Michigan basins lesoproterozoic Midcontinent Rift CIa stic sedimentary rocks AnorogenicfPost-orogenic suites Sibley Group-sandstone, - >134 Ga) Granitoid rocks (1.48.1.77 Ga) Paleoproterozoic Penokean Orogen and Related Rocks Granitoid and volcanic arc rocks (1 .89 - 1.84 G a) Intrusive rocks (1 11-1 Group Marquette Sup ergroup (ca, 2.1 1,85 Ga) Figure 2. Geological map of the Lake Superior basin after Lightfoot et al. (1999). and modified based on the recent mapping in the Nipigon Embayment (Hart and MacDonald in press). sequence of volcanic rocks present on Simpson and St. Ignace islands and Black Peninsula near Nipigon (Fig. 2). A rift-wide correlation of the volcanic rocks was presented by Nicholson et al. (1997) (Fig. 3), but the position of the MPF could only be estimated using paleomagnetic data due to a lack of detailed geochronology. C FauC —— 2 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide — (b) (a) a (d) (c) n Upper Michigan NWWisocnsin I 085 —0.- NE MInnesota OWlimb NE Minnesota NElirtib (e) Isle Royale Black Bay Peninsula a LakeNipigon o Lake Silo,. Trspsk*, 3 Man,ainse Point Michipicolen Island echipicoten and Forniator logo 1095 1100—s1105 1110 —0APPROX. THICKNESS OF VOLC. SECTION: Isle Royele: Oaler Group: Figure 3. Schematic correlation of MRS volcanic rocks in western and eastern Lake Superior from Nicholson et al. 1997. 3.5 lust 3 lust Midi. Island: Manieinse Pt: Along the north side, the Mamainse Point Formation is unconformably overlain by the Mica Bay Formation which is considered to be the equivalent of the Freda Formation south of Lake Superior (Fig. 4) (e.g. Annells 1973; Giblin and Armsburst 1969). To the south, the Mamainse Point Formation is in fault contact with a red sandstone interpreted to be part of the Jacobsville Formation. Paleomagnetic age estimates by Halls and Pesonen (1982) suggest that both of these units are late Keweenawan. 4 lan 5 km Mafic Volcanic Rocks The MPF consists of 300 to 350 individual mafic flows that commonly range in thickness from 1.5 to 9.0 m, with some flows being up to 30m thick and other as thin as 0.15 m (Annells 1973). Many of the flows have upper and lower vesicular zones, with the lower zone vesicles often pipe-like and bent in the direction of flow. Most of the flows have ropy pahoehoe surfaces, but some have clinkery scoriaceous flow tops, and Annells (1973) noted that the olivinerich flows commonly have scoriaceous rather than pahoehoe flow tops. Prismatic jointing is also common in the olivine-rich flows, but also observed in the finer grained flows. The mafic flows were subdivided into a Lower and Upper Division by Annells (1973) with the polymicitic conglomerates of the Great Conglomerate forming the break between the divisions (Fig.5). The Lower Division, located along the north side of the MPF, is about 1700 m thick and consists of about 20% flows with olivine phenocrysts and 80% flows with plagioclase and minor olivine and pyroxene phenocrysts. Two volcanic conglomerates and two thin felsic horizons are intercalated with the flows. The Upper Division, located to the south, consists of banded rhyolites occur as intrusive and possibly extrusive units within the MPF. Felsic dykes, porphyries, and breccias also intrude the Archean rocks to the east, and are considered to be related to the felsic volcanic and intrusive rocks occurring within the MPF. Some of the intrusions host mineralization, with the best examples being the ~1055 Ma breccia pipe hosting Cu-Au-Ag at the past-producing Tribag Mine (Wanless et al. 1968) and several Cu-Mo prospects at the Jogran Porphyry (Tortosa and Moss 2004). The volcanic rocks generally strike to north to northwest with homoclinal dips of 15o-60o to the southwest and are cut by northwest- and northeast-trending faults. The mafic flows of the Alona Bay area form an about 1300 m thick sequence of basaltic flows with less lithological variation than the MPF, and lacking intercalated sedimentary and felsic igneous rocks (Annells 1973). Located approximately 4 km to the north of the MPF, the Alona Bay flows consist of olivine- and plagioclase-bearing basalts that have been proposed to be equivalent to the lower division of the MPF (Walker et al. 2002; Annells 1973). 3 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide 2 ki Iometree KINCAID + RYAN about 65 km to the north. These flows also resemble a plagioclase-rich ponded flow in the Osler Volcanic Group volcanic rocks which is exposed on the southeast corner of St. Ignace Island, in the Thunder Bay area (Sutcliffe and Smith 1988). The top of that flow has a texture resembling the glomeroporphyritic “Daisey Stone” while the core of the flow consists of plates of feldspar up to 5 cm in width. Felsic Rocks Generally rhyolitic in composition, the felsic igneous rocks in the MPF have been classified as felsite, quartz porphyry and flowbanded rhyolite. Many of these rocks occur as plugs, Cong dykes, and sheets intruding Basalt the mafic flows commonly Baserrent with auto-brecciated zones Ro Ia centimeters to tens of centimeters in width containing clasts of basalt and Dip orb felsite and occasional agglomeratic zones suggesting there may be eroded rhyolitic domes (e.g. Annells 1973; Giblin 1974). The depth of intrusion is not known, or whether some of these units may have formed high level cryptodomes that may have breached surface. These rocks are commonly reddish brown to pink to grey-white, and vary from fine-grained and massive to quartz and/or feldspar porphyritic, with the groundmass and massive portions composed of quartz, altered feldspar, mica, chlorite, and epidote. Some units may be flow banded with the best exposed examples located along the margins of one body exposed at Cottrell Bay (Fig, 3). This flow banding shows tight asymmetric isoclinal folds with bands that are generally parallel to the contact. The quartz porphyries commonly form small plugs or thin intrusive sheets with paramorphed b-quartz Figure 4. Geological map of the Mamainse Point Formation. Based on OGS Map 2251 (Giblin and Armburst 1969) and modified after Lightfoot et al. (1999). about 3000m of fine-grained to aphanitic, subophitic to ophitic flows. There are a number of clastic sedimentary and felsic horizons intercalated with the Upper Division flows. The plagioclase glomeroporphyritic “daisy stone” flow is a distinctive unit towards the base of the Lower Division and contains radiating calcic plagioclase laths, up to 5 cm across (Annells 1973). Annells (1973) also described a flow in the Mamainse Point area, near the base of the Upper Division, as containing a 3.25 m zone of large euhedral plagioclase up to 10 cm long. Annells (1973) also notes that there is a flow with similar textures in the Cape Gargantua sequence 4 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide it P Mafic Intrusive Rocks Mafic intrusive rocks are relatively rare in the MPF and typically consist of narrow diabase and fine-grained dykes cutting the flows. Annells (1973) describes the dykes as typically less than 3 m, east or north striking with steep dips, and varying degrees of alteration. No connection was observed between the dykes and flows. I rhyolite magmaltsm Diab ass V rhyolite magmaltsm to ophitic tholeiite rhyolite magmatism Diab ass to fine-grained aphyric tholeiite — VI Cong 10 m crate A — rhyolite magmatism Ophftio basalt III — rhyolite magmatism Oltvine-phyric basalt basalt Fbgioclase-oliv basalt basalt spherulite daisystone 1 Oltvine-phyric pioritic basalt Figure 5. Petrographic and geochemical variations through the Mamainse Point Formation with the stratigraphic subdivisions of Annells (1973), Massey (1983) and Klewin and Berg (1991) modified by Lightfoot et al. (1999). Magnetic polarity is based on Halls and Pesonen (1982). phenocrysts up to 8 mm in size and altered euhedral feldspar (Annells 1973). In one location Annells (1973) described a composite sheet of felsic and mafic material which was interpreted to indicate that the felsic and mafic lavas were erupted simultaneously. Felsite dykes cut, and are cut by, structures hosting the copper mineralization at the Coppercorp and Mamainse mines, and are considered to have been emplaced contemporaneously with fault movement, brecciation and sulphide deposition (Heslop 1970). Keweenawan quartz-feldspar porphyries and breccia pipes hosting copper and molybdenum mineralization intrude the Archean rocks to the east of the MPF (e.g. Tribag Mine), and may be related to the felsite dykes. Clastic Sedimentary Rocks Clastic sedimentary rocks occur at a number of levels within the stratigraphy of the MPF and consist of volcanic conglomerates in the Lower Division and polymicitic conglomerates in the Upper Division (Fig. 5). The volcanic conglomerate was described by Annells (1973) as being composed of poorly sorted, angular to subangular clasts of fine- to coarse-grained Keweenawan basalt in a silty basaltic matrix. Some of the clasts are irregular, rounded, amoeboid shaped which was interpreted to be a result of deposition while still hot. Rare red siltstone, but no basement rocks or felsic volcanic, clasts were observed. It was noted that the horizons become finer grained and well laminated upwards. There are a number of horizons of clastic sedimentary rocks intercalated with the mafic flows of the Upper Division, but the largest volume occurs in the Great Conglomerate which is located approximately in the middle of the Formation between the Lower and Upper Divisions. The Great Conglomerate is a sequence of predominantly polymictic boulder conglomerates containing thin lenses of red to grey sandstone, siltstone, shale and granite, and is interbedded with two basalt flows of the Upper Division. The base of the conglomerate is described by Annells (1973) as a thin layer of red sandstone, overlying a scoriaceous flow top. The conglomerates are poorly sorted, with indistinct bedding containing well rounded to sub-rounded, pebble to boulder sized clasts of granite and basalt, with minor amounts of diabase, felsite, and vein quartz. Conglomerates exposed along strike to the south of the highway occur as 10 to 20 cm thick, graded beds with some beds displaying weakly developed cross bedding suggesting that the trough in which the Great Conglomerate was deposited is deeper to the north-northwest. Some of the well laminated sandstone lenses in the 5 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide with fresh feldspar. Plagioclase is variably zeolitised and some have patches of albite and epidote. Augite is partially altered to clinoamphibole and chlorite. Opaque minerals are commonly hematized, with titanium-rich phases altered to leucoxene and sphene. In the felsic rocks, the feldspar is kaolinized and quartz fills cavities. The felsic rocks at Cotrell Cove are mottled light reddish brown to beige, and also contain irregular patches or funnel-shaped features. The mottling resembles the variations observed in the oxidized Sibley Group sedimentary rocks of the Nipigon area, suggesting that this variation may be in part a result of possibly deuteric reduction of an originally oxidized rock. However, the funnel-shaped features may be a result of alteration during degassing of the flows. All vesicles within the mafic flows are always filled, and the composition of the amygdules is variable. Annells (1973) describes some fillings as an outer zone of chlorite with a chalcedony rim, and a core of colourless quartz and zeolite but some have yellow epidote, colourless prehnite and calcite. Similar mineralogy may also occur as fracture fillings. Lightfoot et al. (1999) suggest that there was a stratigraphic variation with zeolites, including heulandite and stilbite, common in the Upper Division flows and prehnite and pumpellyite common in the Lower Division flows. Epidote alteration is also more common in the Lower Division flows (e.g. Annells 1973) and, along with hematite and specular hematite, in major crosscutting veins and fissures associated with mineralization at the Coppercorp and Mamainse Mines (e.g. Richards and Spooner, 1989). Calcite is also present in fractures cutting the clastic sedimentary units interbedded with the Lower Division and in the Great Conglomerate. highway section are cross-bedded. Annells (1973) suggested that the conglomerates were deposited in an alluvial fan environment. The presence of granite clasts suggests a reduction in the rate of, or possible hiatus in, volcanism so that material could be transported from the adjacent Archean terrains or a more extended period of time than previous interpreted. Regionally, the Great Conglomerate resembles the conglomerate exposed on Puff Island, near the top of the exposed portion of the Osler Group which is located near the top of the ~3 km of exposed volcanic rocks. Younger Clastic Sedimentary Rocks The Mica Bay Formation is an approximately 61 m thick sequence of clastic sedimentary rocks unconformably overlying the volcanic rocks of the MPF along the north side of the formation (Giblin 1974). An up to 30 cm thick, polymictic, matrix supported conglomerate forms the basal member of the formation. Most of the formation is composed of grey-brown siltstones, arkoses, and minor immature quartz pebble conglomerates with siltstones forming approximately 70% of the section. Giblin (1974) interpreted these rocks to have been deposited in a shallow water environment based on the presence of ripple marks, graded bedding, cross-lamination, flame structures, and ball-and-pillow structures with flute casts and clastic dikes indicating current flow towards the north. The Mica Bay Formation is considered to be the equivalent of the Freda Formation (e.g. Annells 1973; Giblin and Armburst 1969). To the south, the Mamainse Point Formation is in fault contact with sandstones displaying the typical red and white mottling of the Jacobsville Formation. The Jacobsville Formation is interpreted to form the floor of much of the east part of Lake Superior (e.g. Giblin 1974). Paleomagnetic age estimates by Halls and Pesonen (1982) suggest that both of these younger sedimentary units are late Keweenawan. STRUCTURAL GEOLOGY The flows of the MPF vary in strike from south to north from northwest to north with dips decreasing from east to west from 55o to 15o west defining a broad antiform that gently plunges to the west (Annells 1973). In the area of Pancake Point and Cottrell Cove, the strike of the flows is highly variable and some flows appear to be overturned which has been interpreted to be a ALTERATION All of the mafic flows appear to have undergone low-grade hydrothermal metamorphism, and some alteration appears to be deuteric. Annells (1973) noted that olivine is replaced by saponite and hematite even in basalts 6 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide Major faults (upthrown - side indicated) I - L 0 100km Cross-structure faults Extent of MCR rocks the flows. These fractures are up to 30 cm wide and commonly filled with silt and other material similar to the matrix of the conglomerates. Annells (1973) speculated that these fracture fillings were clastic dykes formed as a result of minor adjustment of the flows during deposition. There are 2 major regional faults, Mamainse Point and Montreal River faults, that bound the MPF to the north and south (Fig. 4 and 6) (Manson and Halls 1997). To the south, the northeast-trending Mamainse Point Fault juxtaposes the flows of the MPF against red sandstones of the Jacobsville Formation. This fault is a re-activated ductile shear zone that forms the southern margin of the Batchawana greenstone belt to the east and may correlate with faults in northern Michigan and Wisconsin. To the north, the Montreal River Fault forms the northern boundary of the MPF and is the western boundary of the Abitibi and Wawa subprovinces, and the southwest extension of the Ivanhoe Lake fault which is the eastern boundary of the Kapuskasing Structural Zone. This fault may correspond to the Keweenaw Fault to the west (Fig. 6). Both of these faults have been interpreted to be major reverse faults associated with late compression related to the Grenville Orogen in the late Keweenawan. Two major faults, Mamainse Lake and Hibbard Bay faults, that transect the MPF and offset or truncate stratigraphy (Fig. 4 and 6). The Mamainse Lake Fault trends northeast, has a variable sinistral displacement, and appears to Figure 6. Major faults of the Lake Superior portion of the Midcontinent Rift. From Manson and Halls (1997) result of the intrusion of a number of felsic bodies. Rocks of the Mica Bay Formation, that overlie the flows of the MPF to the north, strike 065o, are gently folded, and dip 15o to 30o north or south (Giblin 1974). The Jacobsville sandstones, which overlie the flow to the south, strike 335o to 320o and dip 15o to 25o west. The Alona Bay flows strike north and dip 45o to 49o west with the basal unit unconformably overlying Archean felsic plutonic rocks (Annells 1973). This section terminates against a northwest-trending fault filled by a diabase dyke. The orientation of these flows is comparable to the northern flows of the MPF. A large number of northeast- and northwest-trending normal faults with limited vertical displacements cut the flows of the MPF (Annells 1973). However, Annells (1973) considered the ~6000 m of stratigraphy within the MPF to be in part a result of possible fault repetition. A series of apparently radially distributed faults, with a focal point roughly near the Coppercorp Mine, offset stratigraphy (Tortosa and Moss 2004). The focal point corresponds to a magnetic high and an adjacent large felsite body located about 4 km east of the Coppercorp Mine. A large number of fractures of variable orientation and no discernable displacement cut 7 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide converge with the Mamainse Point Fault under Pancake Bay. The Hibbard Bay Fault is northwest-trending, subparallel to the rift axis located to the west under Lake Superior, and truncates stratigraphy at an acute angle. Cu in some flows; other flows have high La/Sm and SiO2 suggesting crustal contamination. Height in section (m) LITHOGEOCHEMISTRY Geochemical studies by Massey (1980) and Klewin and Berg (1991) have characterised the flows of Mamainse Point Formation, and have identified geochemical variations correlating with stratigraphic position within in the MPF. Massey (1983) identified five major groups of flows with the break between the Lower and Upper Division occurring stratigraphically at the break between Groups III and IV, across the Great Conglomerate (Fig. 5). Klewin and Berg (1991) subdivided the MPF into seven groups, with Groups 1 to 5 corresponding to Groups I to III of Massey (1983) (Fig. 5). Lightfoot et al. (1999) proposed a subdivision of the groups of Klewin and Berg (1991) (Fig. 5). The Lower Division contains olivine-rich flows (Annells 1973), and some of these flows were identified by Klewin and Berg (1991) to be picritic in composition. Based on the presence of skeletal olivine and the fact that the flow composition did not fall along an olivine control line, Berg and Klewin (1990) concluded that these flows were not olivine cumulates but rather formed as a result of melting at deeper levels in the mantle. Overall the Lower Division flows, Groups 1 to 5, consist of basal picrites to picritic basalts overlain by the “daisy stone”, and then a series of tholeiitic to high Mg-tholeiitic basalts. Some of the main characteristics of the different groups were summarized by Lightfoot et al. (1999) (Fig. 5, 7, and 8a, b): Figure 7. Variations in selected elements with stratigraphic position after Klewin and Berg (1991) and arrows indicating the general direction of geochemical evolution. GC - Great conglomerate; BCC - basalt clast conglomerate. Sample position in metres above the base (from Lightfoot et al. 1999). Most of the variation in the Upper Division flows is a difference in the absolute trace element abundances rather than incompatible element ratios. Some of the main characteristics of the different groups were summarized by Lightfoot et al. (1999) (Fig. 5, 7, and 8a, b): Group 6 - high Yb (2.3-5.7 ppm), highest TiO2 (1.8-3.4 wt.%) and Mg-number (0.30-0.48); subgroups 6a, 6b, and 6c defined by varying Ce, Yb, and Zr contents and may reflect interdigitate throughout the stratigraphy, possibly in part due to local fault repetition Group 7 – more primitive than Group 6 with elevated Ni (>75 ppm), low Cu (dominantly less than 100ppm), and TiO2 (~0.9 wt.%); subgroup 7a has elevated Mg-number (0.62-0.70), low TiO2 (0.9-1.2 wt.%), and low incompatible elements abundances (e.g. Ce=15-20 ppm); subgroup 7b has lower Mg-number (dominantly 0.52-0.60), Group 1 and 2a - high Mg-numbers (0.640.70), TiO2 (1.1-2.0 wt.%) and Gd/Yb, with low CaO and Al2O3. Groups 2b and 3 - lower MgO, TiO2, and Gd/Yb compared with Groups 1 and 2a, but high La/Sm, Al2O3, and CaO. Group 4 – higher MgO (9.0-10.5 wt.%) but low P, Zr and Hf compared to other groups Group 5 - quite low TiO2 (<1.5 wt.%), a wide range of MgO and La/Sm with low Gd/Yb, with very low Ni and moderate 8 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide but higher TiO2 (1.3-1.9 wt.%) and incompatible element abundances (Ce=20-25 ppm). Group 8 – are interspersed with Groups 6 and 7; elevated incompatible elements and LREE (e.g. Ce: 120-130ppm) and high TiO2 (3.6-3.7 wt.%) Group 9 - elevated TiO2, Ce, Yb, Zr, La/Sm, LILE, and low Mg-number, Al2O3, and Ni 1973; Lightfoot et al. 1999). These rocks have Zr/Y ratios of 1.9 to 7.3, 10 to 53 ppm Y, (La/Yb)cn ratios of 2.67 to 8.92 with (Yb)cn values of 4.55 to 25. They also have moderately positive Nb+Ta, and negative P, Ti, and Eu on a chrondrite normalized extended element diagram (Lightfoot et al. 1999). Lightfoot et al. (1999) compared these rocks to sialic volcanic rocks found in other CFB such as the Salsette Island suite north of Bombay, India, in the Deccan Trap, and based on this similarity suggested that these rocks formed by a similar mechanism of partial melting of basaltic crust. These rocks could also be classified as FII type felsic volcanic rocks which suggests that they may have the potential to host volcancogenic massive sulphides mineralization, and that they formed by partial melting of a basaltic source at depths >10 km (e.g. Hart et al. 2004). The model proposed by Klewin and Berg (1991) to explain the upward geochemical variations in the MPF was partial melting of different sources at decreasing depths followed by fractional crystallization replaced by periodic replenishment, tapping, and fractionation in magma chambers combined with assimilation and fractional crystallization, and finally simple fractional crystallization at two broad levels in the crust. This model was modified by Shirey et al. (1994) based on isotopic data to reflect interaction with old subcontinental lithospheric mantle in the lower flows, crustal assimilation in the middle flows, and a mixture of plume and depleted mantle comparable to Phanerozoic mid-ocean ridge basalt (MORB). A study of the picritic basalts by Shirey (1997) suggested a mixed source of enriched mantle plume and unradiogenic continental lithosphere, and possible involvement of recycled slab from late Archean subduction. Lightfoot et al. (1999) compared the MPF with the Osler Group volcanic rocks and observed that Groups 1 to 3c, except for the possible lithospheric interaction, resemble but are not identical to the Lower Formation of the Osler Group. Picritic basalts are also present in the Lower Formation but are not the lowermost flows and are underlain by tholeiites. The Osler Group lacks flows comparable to Group 5. Groups 6 and 7 broadly resemble the least contaminated basalts of the Central formation of the Osler Group. 4.6 Gd/Yb I I Osler Group 'Upper Formation 'Central Form A Lower Fom,ation Mamainse Point Group 09 • 7a-d • Ga-c 0 5ac ft 2o A l,2a,2c, 1000 La/Lu Figure 8a. Variation in La/Sm versus Gd/Yb with symbols representing samples from different groups in the MPF and Osler Group volcanic rocks; Figure 8b. Variation in Ce/Nb versus La/Lu in MPF and Osler Group flows. (from Lightfoot et al. 1999) The felsic rocks have SiO2 of 72-81 wt.% with moderate Al2O3 (11-15 wt.%) (e.g. Annells, 9 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide There are only a couple of picritic basalt flows in the Osler Group, and although they are located near the base of the Group but are not the lowest flows (Hart 2002). ECONOMIC GEOLOGY Copper deposits of Keweenawan age are of economic importance in the area and consist of 3 types: (i) fissure filling carbonate vein breccias which carry chalcocite, bornite, chalcopyrite, specular hematite, and very minor amounts of native copper (ii) porphyry copper type deposits, in which chalcopyrite, chalcocite, molybdenite and pyrite are disseminated in quartzfeldspar porphyry (iii) breccia pipe deposits in which wallrock fragments are set in a matrix of quartz, carbonate and minor fluorite. Chalcopyrite and pyrite, with minor molybdenite, sphalerite, galena, tetrahedrite, stibnite and scheelite occur in the matrix of the breccia. Photo a: Mamainse copper mill (Coleman 1899) The Tribag Mine is the best example of the breccia pipe deposits but will not be visited during this field trip. The deposit is located 16.1 km north of Batchawana Bay, and past production included copper and minor gold and silver from a breccia pipe during the period 1967 to 1974. Milling rate at these mines averaged 400 to 500 tons per day of ore grading approximate 2% Cu with minor values in gold and silver. Photo b: Shaft and engine house at Mamainse (Coleman 1899) of 1890, the deepest workings were on the 300foot horizon at the Copper Creek shaft. There is no record that any significant amount of copper was produced until recent years when two mines, the Coppercorp and the Tribag came into production in the Batchawana Bay area. There are three shafts at the Mamainse Mine with 4 levels established. Work began by the Lake Superior Native Copper Company in 1882. The shafts are about 300 m apart on the vein. Amongst the companies engaged in early exploration were the Montreal Mining Company in 1856-57, the Ontario Mineral Lands Company in 1871, the Silver Islet Consolidated Mining and Lands Company in 1882-84, the Canada Lands Purchase Company in 1890, and the Nipigon Mining Company, probably about 1892. From 1906 to 1908, the property was optioned by the Mamainse Mine The existence of copper at Mamainse Point was known in very remote times and the socalled “Indian diggings” near Hibbard Bay attest to the mining activities of the natives in this region. The similarity of the geology at Mamainse Point to that of the Michigan copper district attracted the attention of mining interests when the Michigan camp was being developed. There are sketchy records of intermittent mining activity from 1842 until 1894 (Photo a and b). During this period shafts were put down at several places in the area: the best known developments are those at the Mamainse mine and the Copper Creek and Silver Creek shafts. According to the report of the Royal Commission 10 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide mineralization consists of disseminated native copper in amygdules, and veins and vein hosted copper sulphides that occur in fault-related breccia zones that transect flood basalts, conglomerates, and felsic intrusive and volcanic rocks of the Keweenawan-age Mamainse Point Formation (Fig. 11). Mineralization consists of chalcocite with minor malachite and chalcopyrite associated with pyrite and hematite. Regional westward folding of the Mamainse Point Formation combined with possible contemporaneous radial faulting may have provided the structural conduits for the mineralizing fluids in the Coppercorp Mine and elsewhere on the property (Fig. 12). The presence of a high area of magnetic intensity in the focal area of the radial fault system combined with associated felsic intrusive and extrusive activity in the lower volcanic sequence suggests the presence of a volcanic or intrusive centre in the area. Calumet and Hecla Mining Company of Michigan. Mineralization at the Mamainse Mine is hosted by a quartz-carbonate vein / fracture fill, striking 334o dipping 50o east and varying from 0.46 to 4.0 m wide. Tracable for 457 m, the vein and an associated felsite intrusion pinch and swell along strike in the northwest-trending fracture which cross-cuts the north striking mafic volcanic flows at an oblique angle. Mineralization in these veins consists of hematite and chalcocite in veins with minor chalcopyrite and native copper along with malachite and azurite as weathering products. The structure associated with the mineralization at the Mamainse Mine is similar to the main NNW trending, east dipping structures seen at the Coppercorp (Tortosa and Moss 2004). The main vein can be examined along the shoreline (see Stop 6), and it was along this vein where three shafts were sunk to 18.3, 85.3 and 97.6 m with four levels established. The vein cuts flows of the Upper Division and hosts minor native copper and finely disseminated chalcocite in cross fractures. The basalts are thin flows of diabasic and ophitic olivine tholeiite with amygdules containing quartz, agate, calcite, copper sulphides and native copper. Annells (1973) reported that specks of native copper are common in the vesicles and flow tops of the Upper Division flows and that a 67 kg piece of native copper was discovered during highway construction in the area south of the Mamainse Mine. History Exploration on the property probably began at the same time as the work on the nearby Mamainse Mine, and much of the early work appears to have concentrated on the native copper mineralization in amygdules and fractures (Table 1). Between 1948 and 1952, work by Macassa Mines, and C.C. Houston and Associates examined the old copper showings and drilled 10,180 m outlining several mineralized zones, including the C Zone, D Zone, SB Zone and Silver Creek Zone (Fig. 11 and 13). Table 1: History of Ownership of Montreal Mining Sand Bay Location Years Ownership Coppercorp Mine The Coppercorp Mine is a past producer located in the Mamainse Point area, about 80 km north of Sault Ste. Marie, Ontario. The mine produced 1.021 million tons grading 1.16% Cu, along with 237,603 oz Ag and 1,964 oz Au from a number of veins between 1965 and 1972 (Tortosa and Moss 2004). Nikos Explorations Ltd. currently has an agreement to acquire Amerigo Resources Ltd. interest in the property. The following information has been extracted from the technical report of Nikos Explorations Ltd. completed by Tortosa and Moss (2004). The Coppercorp deposit is hosted by volcanic and intrusive rocks situated on the eastern edge of the Midproterozoic Midcontinental Rift system. Copper 1856-1857 1871 1882-1884 1890 1892 1906-1908 1948 1951 1955 1964 2002 Montreal Mining Co. Ontario Mineral Lands Co. Silver Islet Consolidated Mining and Lands Co. Canada Lands Purchase Synd. Nipigon Mining Co. Calumet and Hecla Co. Macassa Mines Ltd. C.C. Huston and Associates Coppercorp Ltd. leased by Vauze Mines Ltd North Canadian Enterprises Ltd. Terry Nicholson & William Gibbs from Tortosa and Moss (2004) 11 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide Figure 10: Detailed Geology around the Coppercorp Mine area showing the location of some of the surface and projected mineralized zones (Giblin, 1973). Blue outline shows the C Zone, L Zone, Lutz Vein and Mamainse Vein. J — — — Oldest Figure 11: Distribution of faults and simplified regional geology of the Mamainse area with the outline of the Coppercorp Property shown in yellow (after Giblin, 1973; Richards, 1995) from Tortosa and Moss (2004). Figure 12: Mineralized structures in the Coppercorp deposit (Heslop, 1970) from Tortosa and Moss (2004) 12 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide Coppercorp Ltd. was a new company created in 1954 to sink a shaft, to 168 m with levels at 76, 114, and 152 m (Tortosa and Moss 2004). During the underground development, 4,267 m of lateral development were completed and 60,000 tons of ore were stockpiled. Operations ceased in 1957 due to falling copper prices. Vauze Mines Ltd. (controlled by Sheridan Geophysics Limited) leased the mine in 1963 and completed additional drilling along with a surface exploration program which included geophysical surveys and geological and geochemical examinations. In 1965, a decision was made to bring the Coppercorp deposit into production and the original shaft was dewatered and deepened to 192 m. The operation produced 500 tons per day processed into a 50% copper concentrate at a recovery rate of <90%. The Coppercorp Mine produced 1, 021,358 tons grading 1.16% Cu plus approximately 237,603 ounces of silver and 1,964 ounces of gold from such veins between 1965 and 1972. Some of the available historical statistics on underground development, drilling, pre-production ore reserve estimates and production figures are provided in Table 2. Recent exploration on portions of the property was completed by J.F. Paquette and Cominco Ltd.. J.F. Paquette held a property covering the Lutz vein and L zone in 1991-92, and a self-potential survey along with prospecting and sampling programs were conducted (Fig. 11). Cominco Ltd. optioned this property in 1993 and completed geological mapping, surficial geochemistry, electromagnetic (UTEM) and magnetic surveys. Mineralization Copper mineralization occurs as disseminated native copper in amygdule and veins, and vein-hosted copper sulphide deposits. Production on the property has concentrated on the copper sulphides mineralization. The copper sulphide veins occur in faultrelated breccia zones that generally display gradual transitions from high grade sulphide veins to barren oxide cement. The copper sulphides are dominantly chalcocite, with lesser chalcopyrite and bornite, rarely native copper and are usually accompanied by specular hematite. Massive chalcocite veins, 20 to 25 cm wide, were found at numerous localities within the deposit. Richards (1985) recognized four stages of mineralization, which were: 1) pyrite-chalcopyrite, 2) chalcopyrite-bornite, 3). chalcocite-hematite, 4). native copper, native silver, copper arsenides, malachite and hematite. The third stage was the most important source of copper producing rich veins of chalcocite and replacing earlier sulphides. The veins and breccias consist of quartz and carbonate with subordinate laumontite and fluorite. Large vugs of varying size are lined with quartz, calcite, and sulphides and were commonly found throughout the deposit, suggesting a shallow ‘open space filling’ type of mineralizing process (Heslop, 1970). The wallrock is commonly chloritized and sericitized and may contain epidote. The faults hosting mineralization cut Keweenawan basalt flows and conglomerates. The width of the fault zones varies along strike from shears less than 1 m to disrupted lenses up to 12 m across (Richards, 1985). The faults have two orientations, northeast and northwest. Northnortheast trending faults dip 60o to 65o east and host the Copper Creek Zone, Silver Creek Zone and `G`, `H`, and `F` Zones. North-northwest Table 2: Historical statistics on underground development and drilling at the Coppercorp Mine (Tortosa and Moss 2004). ______________________________ _______________________ ________ ltI,naitlo. Sourer FsplunIIun Activist I Indcrgruund flnckçrnenl DnAuig. 34.8*2 [ccl SMS1R (1150X57 Cronuculs 3,62* (cci L)nllang StutTacc. 16,000 [cci ShOOK 000152 20,000 eel j • SMDR 000152 - Iable 4 H,stcrral Pre'Production Ore Reserve Eshma4cC at the Coppertoip Muir Mi•erslIrS Zone Ij,toemitiss Source Tore Maceve Rationale C Zone red C7.apa Soisth" 400000 ions® 23% Cu ShOOK 000852; Cuppntcoqi L Silta Creek ScoUt Zone 490,000 ices a I 9%Cu SB and Silsti Creek North 630,000 Ions Tolal Ore Runurce 1,540000 loot I I I%Cu I 965 SMOR '100852, Coppeccoep SMOR 000852, Coppccccep Report lee 1983 • Cu SMOR 000*52, Cnppeeroqi Repoil (or 1965, Northern MIner 1961 Iv,, Year •J9570 1965 1966 1967 Au(Os) US$2 2*519 3*6 115,848 1146,601 149691 Mills 1971 tT.thr" Ag(ck) ('uØI*) 411000 - 1 I 146,441 142,9*6 — 1969 1970 (Source 5M014000*52L toot Hoisted 3,716.325 290 I - 268 33,622 3,175,730 231 1,785 33,570 3,109,73* t(pj,1181 141,955 111,811 83.5/') 156111 84.492 , ni on 440 '1 I 7 23,782,128 13 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide trending faults that dip 50o to 70o east and almost parallel the volcanic and sedimentary rocks (Fig. 12 and 13). These faults are the most productive structures, hosting the C Zone, SB Zone, D Zone and B Zone. Where northwest faults intersect the Great Conglomerate, the fault narrows and the sulphide mineral content decreases possibly due to the lower competency of the conglomerate compared to the mafic volcanic rocks (Heslop 1970). Mineralized structures also cut, and are cut by, felsite and diabase dykes. Both the diabase and felsite intrusions are considered to have been emplaced contemporaneously with fault movement, brecciation and sulphide deposition. Heslop (1970) defined four major stages of fault development based on the crosscutting relationships of the faults (Table 3) with an apparent younging in the faults from south to north, but mineralogical changes in the ore or other characteristics associated with this relative structural timing have not been documented. Some zones, including the C, SB, and L zones, Lutz and Mamainse veins, display an apparent stratigraphic control. Mineralization in these zones occurs primarily where the structure is hosted by basalts of the upper section of the Mamainse Point Formation, 75 to 150 m above the Great Conglomerate (Fig. 11). Table 4: Copper deposits in the Mamainse PointBatchawana Area (Tortosa and Moss 2004) Strike Dip SB Zone Copper Creek Zone C Zone Silver Creek Zone D Zone B Zone F Zone G Zone H Zone 342-335 020 East 55-60 E 1 2 345 010 55-68 E 50-65 E 3 4 300 345 030 020 020 45 NE East Southeast East East 4 4 4 4 4 Production Rarnts 1965to in 102 M cons 3 4 Copperguatlz YCLO iBSZco 1884 3 3 2 Ratio I%7ro1973 2% Co 4CM tons oboes 300ns 125M tons (410.13% I Deposit Typo Coppacorp CoppoT. Mancciose Thus8 qon coin Sours . a • I 16% Co ii I6S%Cu Pipes Jlrrson lien cIa &sss8rrccso 3 CootcdoO4%MoS2 F Wnsltrrrcaa F 0 RI tons a 0.6 to I IO%WO, Jogs00 porphyry I8Mtomta#0.19% N6 SoorcesiRoport, 1%?; 2 Moore 1426, 3 I Cuond080%Mo5 1 M&R. t989, 4 SMDR 000852 Deposit Model Tortosa and Moss (2004) have proposed that the Coppercorp deposit is an iron-oxide copper-gold (IOCG) type deposit similar to Olympic Dam based on: 1. A continental rift-related tectonic setting on the eastern margin of the Midcontinent Rift system. 2. The Keweenawan basalts represent a significant volume of potential copper source rocks with an estimated thickness of 4,300 to 6,000 m. 3. The presence of a massive magnetite vein grading 3.9% Cu over 1.05 m at the Jogran porphyry and fluorite associated with the Breton Breccia at Tribag and with Coppercorp ore. 4. The presence of numerous faults some of which are splays off major crustal faults including the Mamainse Point Fault to the south of the property. 5. The apparent high level emplacement of the felsic intrusives 6. The presence of dilational sites along active structures. 7. The presence of a high temperature saline brine (350o to 450oC) 15-20 eq. wt.% CaCl2 believed to be magmatic in origin, and a lower temperature fluid (<100o to 350oC, 0 to 15 eq. wt. %) believed to be a mixture of magmatic and meteoric fluid (Richards, 1985). 8. The occurrence of widespread Cu mineralization in the area as both low tonnage medium grade deposits (e.g. Coppercorp) and high tonnage low grade Table 3: Relative age of fault zones based on crosscutting relationships with 1= oldest and 4=youngest (Heslop, 1970) from Tortosa and Moss (2004). Mineralized (Fault) Zone Production Yonn Deposit Relative Age There are several other deposits in the Batchawana area and these are summarized in Table 4. 14 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide PALEOMAGNETISM Paleomagnetic studies of Keweenawan rocks of the Midcontinent Rift (e.g. Portage Lake volcanic rocks, Michigan; North Shore volcanic and intrusive rocks, Minnesota; various dykes south of Thunder Bay; Logan sills, Thunder Bay; Osler Group volcanic rocks, Nipigon; Cape Gargantua volcanic rocks, Wawa) indicate that the older rocks have reverse magnetic polarity and the younger rocks have normal polarity (e.g. Halls and Pesonen 1982; Nicholson et al. 1997). The volcanic rocks of the MPF have more complex pattern with 2 zones of reversed polarity overlain by zones of normal polarity with the polarity changes coinciding with the base of clastic sedimentary units (Fig. 5) (Palmer 1970). It was originally proposed that this repetition was the result of a strike fault located at the boundary between the lower normal and upper reverse zones, but the lithogeochemical stratigraphy indicates another explanation is required. deposits (e.g. East Breccia zone of Tribag mines). 9. The presence of a broad, regional aeromagnetic anomaly over the property and the presence of several gravity anomalies. 10. The production of limited amounts of gold and silver along with the copper at the Coppercorp Mine and the anomalous concentrations of gold and silver found in the outlying copper occurrences. GEOCHRONOLOGY There is a limited amount of geochronology completed in the Mamainse Point area and correlations with other areas in the Midcontinent Rift are often based on paleomagnetic data (e.g. Nicholson et al, 1997). A number of K/Ar ages have been determined but are regarded as minimum ages due to the probability of Ar loss (Wanless et al, 1966, 1967, 1968). These ages include the basalt exposed at Chippewa Falls (Stop 11), south of Pancake Bay and the main portion of the Mamainse Point Formation, which have an age of 915 +/-140 Ma, and samples of the Tribag Mine breccia pipe which returned ages of 785 +/- 103 Ma to 1055 +/- 35 Ma. A more accurate age of volcanism is the 1070 +/- 50 Ma Rb/Sr age for a felsites in the Mamainse Point area (Van Schmus, 1971). A U/Pb age of 1096.2 +/- 1.9 Ma for a felsic unit in the area of the volcanic conglomerate of the Lower Division is very similar to the Rb/Sr age but younger than the 1105 +/-4 Ma felsic volcanism in the Osler Group (Davis et al. 1995). A Re-Os age of 1128 +/- 54 Ma for the basal picrite volcanic rocks (Shirey 1997) probably represents a maximum age for the Mamainse Point Formation, and is comparable to the 1124 to 1114 Ma age for the ultramafic Seagull Intrusion of the Lake Nipigon area (Heaman and Easton 2005). However, if the picritic flows of Mamainse Point are equivalent to the Seagull Intrusion, the basal portion of the MPF is much older previous interpretations, and 1096 Ma felsic age of Davis et al. (1995) means that the sediments of the Great Conglomerate may represent a longer hiatus in volcanic activity. REFERENCES Annells, R. N. 1973. Proterozoic Flood Basalts of Eastern Lake Superior: the Keweenawan Volcanic Rocks of the Mamainse Point Area, Ontario; Geological Survey of Canada, Paper 72-10. 51 p. Cannon, W.F., Green, A.G., Hutchinson, D.R., 1989. The North American Midcontinent rift beneath Lake Superior from GLIMPCE seismic reflection profiling. Tectonics, v. 8, p. 305-322. Coleman, P.A., 1899. Copper Regions of the Upper Lakes, Report of the Bureau of Mines Vol. VIII Part 2, p.152 Davis, D.W. and Green, J.C. 1997. Geochronology of the North American Midcontinent Rift in western Lake Superior and implications for its geodynamic evolution; Canadian Journal of Earth Sciences, v.33, p.476-488. Davis, D.W., Green, J.C., and Manson, M., 1995. Geochronology of the 1.1. Ga North American Mid-continent Rift; Institute of Lake Superior Geology, 41st Annual Meeting Proceedings Volume 41, Part 1: Abstracts, p. 9-10. Giblin, P. E. 1974. Middle Keweenawan Rocks of the Batachawana-Mamainse Point Area. Institute of Lake Superior Geology 20th Annual Meeting, Nipigon, Ontario, Program, Abstracts, and Field Guides, v. 20, p. 39-67. Giblin, P. E., and Leahy, E. J. 1967. Sault Ste. Marie - Elliot Lake Sheet, Districts of Algoma, Manitoulin, and Sudbury. Ontario Department of Mines, Geological Compilation Series, Map 2108. Scale 1 inch to 4 miles. Giblin, P.E. and Armsburst, G.A. 1969. Batchawana, Ontario Department of Mines. Map 2251, scale 1:63 360. 15 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide Gmitro, T.T., 1990. Chemostratigraphy and petrogenesis of Keweenawan lavas at Alona Bay, Ontario. unpub. M.S. thesis, Northern Illinois University, DeKalb, Ill. Green, J.C., 1983. Geological and geochemical evidence for the nature and development of the Middle Proterozoic (Keweenawan) midcontinent rift of North America: Tectonophysics, v. 94, p.413-437. Halls, H.C. and Pesonen, L.J., 1982. Paleomagnetism of Keweenawan rocks; in Geology and Tectonics of the Lake Superior Basin; ed. R.J. Wold and W.J. Hinze; Geological Society of America, Memoir 156. p.173-201. Hart, T.R. 2002. Proterozoic Volcanic and Intrusive Whole Rock Geochemical Data Associated with the Keweenawan Midcontinent Rift, Lake Superior Area, Ontario; Ontario Geological Survey, Miscellaneous Release Data 114. Hart, T.R. and MacDonald, C.A., submitted, Proterozoic and Archean Geology of the Nipigon Embayment: Implications for Emplacement of the diabase sills and PGE-enriched mafic to ultramafic intrusions; Canadian Journal of Earth Sciences, Hart, T.R., Gibson, H.L., and Lesher, C.M., 2004. Trace Element Geochemistry and Petrogenesis of Felsic Volcanic Rocks Associated with Volcanogenic Massive Cu-Zn-Pb Sulfide Deposits; Economic Geology, v. 99, p. 1003-1013. Heaman, L.M., and Easton, R.M., 2005. Proterozoic history of the Lake Nipigon area, Ontario: Constraints from U-Pb zircon and baddeleyite dating. ; Institute of Lake Superior Geology, 51st Annual Meeting, Nipigon, Ontario, Part 1 – Proceeding and Abstracts, v. 51, part 1, 24-25. Heslop, J.B., 1970, Geology, Mineralogy and textural relationships of the Coppercorp Deposit, Mamainse Point area, Ontario. unpublished M.Sc. Thesis, Department of Geology, Carleton University, Ottawa, 95 p. Hutchinson, D.R., White, R.W., Cannon, W.F., and Schulz, K.J., 1990. Keweenaw hot spot: geophysical evidence for a 1.1 Ga mantle plume beneath the Midcontinent rift system in western Lake Superior; Journal of Geophysical Research, v. 95, p. 10 86910 884. Jerram, D.A. and Widdowson, M. 2005. The anatomy of Continental Flood Basalt Provinces: geological constraints on the processes and products of flood volcanism; Lithos, v. 79, p. 385– 405 Klewin, K.W. and Berg, J.H. 1990. Geochemistry of the Mamainse Point volcanics, Ontario, and implications for the Keweenawan paleomagnetic record; Canadian Journal of Earth Sciences, v. 27, p. 1194-1199. Klewin, K.W., and Berg, J.H. 1991. Petrology of the Keweenawan Mamainse Point Lavas, Ontario: Petrogenesis and continental rift evolution. Journal of Geophysical Research, v. 96, p. 457-474. Klewin, K.W. and Shirey, S.B. 1992. The igneous petrology and magmatic evolution of the Midcontinent rift system; Tectonophysics, v. 213, p.33-40. Lightfoot, P.C., Sage R.P., Doherty W., Naldrett A.J., and Sutcliffe, R.H. 1999. Mineral potential of Proterozoic Keweenawan intrusions: implications of major and trace element geochemical data from bimodal felsic and volcanic sequences of Mamainse Point and the Black Bay Peninsula, Ontario; Ontario Geological Survey OFR 5998, 91p. Manson, M.L. and Halls, H.C. 1997. Proterozoic reactivation of the southern Superior Province and its role in the evolution of the Midcontinent Rift. Canadian Journal of Earth Sciences, v. 34, p. 562-575. Massey, N.W.D. (1980). The geochemistry of some Keweenawan metabasites from Mamainse Point, Ontario. Ph.D. thesis. McMaster University, Hamilton, Ontario. 352p. Massey, N.W.D. 1983. Magma genesis in a late Proterozoic proto-oceanic rift: REE and other trace-element data from the Keweenawan Mamainse Point Formation, Ontario, Canada. Precambrian Research, v. 21, p. 81-100. Nicholson, S.W., and Shirey, S.B., 1990. Evidence for a Precambrian mantle plume: a Sr, Nd, and Pb isotopic study of the Midcontinent Rift System in the Lake Superior region; Journal of Geophysical Research, v. 95, p. 10 851-10 868. Nicholson, S.W., Shirey, S.B., Schulz, K. and Green, J.C., 1997. Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development. Canadian Journal of Earth Sciences, v. 34, p. 504-520. Palmer, H. C.1970. Paleomagnetism and Correlation of some Middle Keweenawan Rocks, Lake Superior. Canadian Journal of Earth Sciences, v. 7, p. 14101436. Ojakangas, R.W., and Morey, G.B. 1982. Keweenawan sedimentary rocks of the Lake Superior region: a summary. In Geology and tectonics of the Lake Superior basin. Edited by R.J. Wold and W.J. Hinze. Geological Society of America, Memoir 156, pp. 157–164. Richards, J.P., 1985, A fluid inclusion and stable isotope study of Keweenawan fissure-vein hosted copper sulphide mineralization, Mamainse Point, Ontario. Unpublished M.Sc. thesis, Department of Geology, University of Toronto, 290p. Richards, J.P. and Spooner, E.T.C. 1989. Evidence for Cu(Ag) Mineralization by Magmatic-Meteoric Mixing in Keweenawan Fissure Veins, Mamainse Point, Ontario. Economic Geology, v. 84, pp. 360385. Shirey, S.B., 1997. Re-Os isotopic compositions of Midcontinent rift system picrites: implications for plume – lithosphere interaction and enriched mantle sources. Canadian Journal of Earth Sciences, v. 34, p. 489-503. Shirey, S.B., Klewin, K.W., Berg, J.H. and Carlson, R.W., 1994.Temporal changes in the sources of flood basalts: Isotopic and trace element evidence from the 1100 Ma old Keewenawan Mamainse Point Formation, Ontario, Canada; Geochemica et Cosmochimica Acta, v. 58, pp. 4475-4490. 16 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide Sutcliffe, R.H. and Smith, A.R. 1988. Project Number 87-17. Geology of the St. Ignace Island Volcanic-Plutonic Complex; in Summary of Field Work and Other Activities, Ontario Geological Survey Miscellaneous Paper 141, p. 368-371. Tortosa, D. and Moss, R. 2004. Geology and Exploration of the Coppercorp Property, Sault Ste. Marie Mining Division, Ontario; March 23, 2004, prepared for: Nikos Explorations Ltd., 158p.; filed with SEDAR May 11, 2004; available from http://nikosexplorations.com/_resources/Coppercor p2004Report.PDF Van Schmus, W. R. 1971. Rb-Sr Age of Middle Keweenawan Rocks, Mamainse Point and Vicinity, Ontario, Canada. Geological Society of America, Bulletin 82, p. 3221-3226. Van Schmus, W.R. and Hinze, W.J. 1985. The mid-continent rift system. Annual Review of Earth and Planetary Sciences, v. 13, p.345-384. Walker, J.A., Gmitro T.T. and Berg J.H. 2002. Chemostratigraphy of the Neoproterozoic Alona Bay lavas, Ontario; Canadian Journal of Earth Sciences, v. 39, p.1127-1142 Wanless, R. K. , Stevens, R.D., Lachance, G.R., and Rimsaite, J. Y. H. 1966. Age Determinations and Geological Studies, K-Ar Isotopic Ages, Report 6. Geological Survey of Canada, Paper 65-17, p. 58. Wanless, R. K., Stevens, R. D., Lachance, G. R. and Edmonds, C. M. 1967. Age Determinations and Geological Studies, K-Ar Isotopic Ages, Report 7. Geological Survey of Canada, Paper 66-17, p. 8485. Wanless, R. K., Stevens, R. D., Lachance, G. R. and Edmonds, C. M.1968. Age Determinations and Geological Studies, K-Ar Isotopic Ages, Report 8. Geological Survey of Canada, Paper 67-2, Part A, p. 95-96. 17 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide ITINERARY Stop Locality Km (south) Km (north) 1 Alona Bay 0 105.7 2 Unconformity: Mica Bay Formation Mamainse Point Formation 8.0 97.7 3 Glomeroporphyritic Basalt "Daisy Stone" 8.5 97.2 4 Ropy Flow Top 9.2 96.5 5 Volcanic Conglomerate 10.7 or 11.3 10.7 95.0 6 Mamainse Mine 14.2 91.5 entrance to road 15.0 90.7 7 Ubetuwantit (Coppercorp Property) 2.3 8 Coppercorp Mine 4.6 9 Interbedded Conglomerate and Basalt 21.8 83.9 10a Domeykite Occurrence 25.4 80.3 10b Pseudotechylite 25.9 79.8 10c Cottrell Cove Felsic Intrusion 26.1 79.6 Pancake Provincial Park 32.9 72.8 Pancake River 34.6 71.1 Mamainse Point Fault (approx.) 35.5 70.2 Road to the Tribag Mine Site 40.1 65.6 Batchawana River 45.3 60.4 Chippewa Falls 54 51.7 Keweenawan basalt flows 65.2 40.5 Jacobsville Formation sandstone 69.0 36.7 Intersection at Highway 17 and 556 Water Tower Inn 105.7 0 11 the 1840's. A sample of pitchblende was described by Le Conte in the American Journal of Science in 1847 with the location given as a vein 5.1 cm wide located about 112.6 km north of Sault Ste. Marie, but subsequent papers gave the location as between 64.4 to 144.8 km. In 1948, Robert Campbell of Toronto found pitchblende at the northwest end of Theano Point, which can be seen north across the bay. Subsequently prospectors found several other pitchblende deposits inland, to the north and east. Point Aux Mines on the south side of Alona Bay, was the site of the earliest organized mining venture in Ontario. In 1772-73, a mining company formed by Alexander Henry, the noted fur-trader, worked a copper deposit and shipped a small amount of ore to England. Theano Point to the north and Point Aux Mines to the west southwest consists of Archean felsic plutonic rocks cut by diabase dikes. The Alona Bay volcanic rocks are an approximately 1200 m thick, southward younging sequence of Keweenawan basalt flows unconformably overlying the Archean rocks (Annells 1973; Gmitro 1990). These flows are unconformably overlain by siltstones and sandstones similar to the Mica Bay Formation, and possibly comparable to the post-magmatic Freda Sandstone (e.g. Annells 1973; Ojakangas and Morey 1982). The Alona Bay flows have been correlated with the Lower Division flows of the Mamainse Point Formation (MPF) (Walker et al. 2002; Annells 1973). The Alona Bay flows are described by Annells (1973) and Gmitro (1990) as being composed of about 107 mafic flows averaging 6.8 m in thickness. Most of the flows have pahoehoe surfaces and vesicular-amygdular flow tops, and pipe vesicles are relatively common just above flow bases. Gmitro (1990) subdivided these flows into olivine phyric, plagioclase phyric, olivineplagioclase phyric, and aphyric groups. Two basic dikes cut the section, one about 500 m from the base of the section and one at the top of the section in fault contact with Archean rocks. Clastic dikes are common throughout the Alona Bay section. All of the Alona Bay lava flows have undergone some low-grade burial metamorphism to prehnite-pumpellyite facies and deuteric alteration, This field guide is an amalgamation of the field trip guides of Annells (1973) and Giblin (1974). All coordinates are in UTM (Universal Transverse Mercator), Zone 16, with a NAD83 Canada datum. STOP 1 Alona Bay Scenic Lookout UTM coordinates - 673636E 5219651N The following historical description is from Giblin (1974). An historical plaque commemorates the first discovery of uranium in Canada, made nearby in 18 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide _______ General Geology and Field Trip Stop Locations Map-Batchawana-Mamainse Point Area N 'b in ,C ii Stop Stop Stop Stop Stop lOb Stop I 200 0 200 400 Meters Legend Jacobsville Formation Archean Rocks Granitic Sandstone, Siltstone Mets Mafic Intrusive ROCkS 1 Feisk Diabase, Gabbro Keweenawan Felsic Intrusive and Volcanic ROCkS Felsite, quartz-feldspar porphyry Cong Iomer2te Mat ic r Basalt Figure 14. Field trip stops geology from Giblin and Leahy (1967) 19 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide shallowly west dipping outcrop located along the edge of the water. similar to that observed in the MPF (e.g. Massey 1980). Walker et al. (2002) identified 4 geochemical groups based on decreasing TiO2 Ni and MgO contents that reflect an upwards progression through the volcanic sequence. Although proposed to be equivalents to the flows of the Lower Division of the MPF, the majority of the Alona Bay flows have MgO concentrations between 6 and 10 wt.%, and none have MgO > 15.0 wt.%. BE CAUTIOUS AS THE OUTCROP WILL BE SLIPPERY IF WET The underlying Keweenawan basalts of the MPF strike N 30° W and dip 45° to 50° W. The basalts are thin olivine tholeiitic flows near the base of the MPF (Annells 1973), with upper vesicular zones exposed in this outcrop. Unconformably overlying the basalts is an up to 30 cm thick polymictic, matrix supported conglomerate of the Mica Bay Formation (Photo 1a). Very little of the basal conglomerate remains, please do not collect samples or deface the exposure The Mica Bay Formation has a total thickness of approximately 61 m, but only the lower 18.3 m are exposed along the beach. This section includes the basal conglomerate, which is overlain by grey-brown siltstones, arkoses, and minor immature quartz pebble conglomerates. Siltstones constitute approximately 70% of the section. Sedimentary structures include ripple marks, graded bedding, cross-lamination, flame structures, and ball-and-pillow structures indicative of a shallow water environment (Photo 1b and 1c). In other exposures, north of the section examined, flute casts and clastic dikes indicate a northerly direction of current flow. The sedimentary rocks strike N 65° E, and in the section examined, dip 15° N. Steeply-dipping faults cut the sedimentary rocks with offsets of up to 1.8 m. These rocks lie on the south limb of a regional syncline, with the north limb exposed between the creek and the small granite headland at the north end of the beach. The north limb is in fault contact with the underlying Archean granite, and a thin basal breccia occurs on the granite north of the fault. Similar basal breccias are found overlying the granite, at several points along the shore of Mica Bay. The age and correlation of these rocks are uncertain. The unconformity observed at this stop indicates that they are post-Middle Keweenawan, but they are lithologically and structurally different than the rocks of the Jacobsville Formation. The Mica Bay Formation appears to underlie the Jacobsville, which forms nearby islands in Lake Superior and much of the floor of Proceed for ~8 km to the south 0 – 2 km: Alona Bay flows 2 – 6.6 km: Archean felsic plutonic rocks and diabase dykes unconformably underlying the Alona Bay and MPF 6.6 – 8 km: Mica Bay Formation STOP 2 Unconformity: Mica Bay Formation and Mamainse Point Formation UTM coordinates – 673691E 521891N Park in the east side of the highway on a portion of the old highway roadbed. BE CAUTIOUS AS THE HIGHWAY IS BUSY AND HEAVILY TRAVELLED BY TRACTOR TRAILERS Cross the highway and proceed north to the culvert where the stream can be followed down to the beach. BE CAUTIOUS AS THE MUDDY SILTSTONES CAN BE SLIPPERY, ESPECIALLY IF WET Fine-grained siltstone and graphitic siltstone with trace sulphides are exposed in the section cut by the stream. Proceed about 200 m to the south along the cobble beach, passing exposures of the Mica Bay Formation. An angular unconformity between the clastic sedimentary rocks of the Mica Bay Formation and the underlying Keweenawan basalt of the Mamainse Point Volcanic Group is exposed in a 20 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide the eastern part of the lake. The age of the Jacobsville Formation is uncertain and has been variously assigned to the Lower and Middle Cambrian, and to the Upper Keweenawan. Thus, the Mica Bay Formation may be either Upper Keweenawan or Lower Cambrian. A thinner section of similar clastic sedimentary rocks occurs in Alona Bay, 6.4 km north, and may also be part of the Mica Bay Formation. Proceed south for ~1.0 km STOP 3 BE CAUTIOUS AS THE HIGHWAY IS BUSY AND HEAVILY TRAVELLED BY TRACTOR TRAILERS Glomeroporphyritic "Daisy Stone" flows of the Lower Division of the MPF STOP 4 Ropy Flow Top UTM coordinates – 673127E 5217816N Park in the drive about 200 m south of the rock cut Basalt UTM coordinates – 673437E 5218459N Cross the highway and walk north along the shoulder of the highway to the south side of the rockcut at the top of the hill. Proceed south along the highway for about 400 m from the same parking site as used for Stop 2. BE CAUTIOUS AS THE HIGHWAY IS BUSY AND HEAVILY TRAVELLED BY TRACTOR TRAILERS A ropy flow top is exposed on the south face of the outcrop located on the east side of the highway (Photo 3). The pahoehoe marks the top of a 6.5 m thick olivine tholeiite flow (Annells, 1973). Proceeding to the north through the rock cut, there are several flows with thin basal pipe vesicle zones, thicker amygdaloidal zones in the upper parts of the flows, and ropy flow tops. The vesicles and amygdules are filled with calcite, celadonite, and a green mica. About 150 m further north, on the east side of the highway, a northwest trending, 26o E dipping diabase dyke crosscuts the flows located near the top of the olivine phyric group (Annells, 1973). Scoriaceous flowtops probably on olivine-rich flows, are reported to occur in the west side of the rock cut in the area of the dyke. The distinctive glomeroporphyritic “daisy stone flows” are located interbedded with olivine tholeiite flows of the Lower Division of Annells (1973), towards the north side of the MPF. The flow is exposed in the rock cut located on the east side of the highway, and can also be observed along the shoreline to the northwest. This flow strikes north to northwest and dips about 50° W which is steeper than the dips present in subsequent stops higher in the sequence. Giblin (1974) reported that the flow can be traced intermittently along strike for 11.3 km, and a new exposure located about 4.3 km south of this location provides an excellent exposure of these flows. This flow varies from massive, fine-grained basalt to glomeroporphyritic, with radiating calcic plagioclase laths, up to 5 cm across (Photo 2). Annells (1973) reported that the massive portions of the flows are non-porphyritic containing abundant pseudomorphs after small olivine crystals. The plagioclase is variably epidotized or hematitized in different portions of the flow. A plagioclase-rich ponded flow in the Osler Volcanic Group volcanic rocks is exposed on the southeast corner of St. Ignace Island, in the Thunder Bay area (Sutcliffe and Smith 1988). The top of that flow has a texture resembling the glomeroporphyritic “daisy stone” texture. Proceed south for 1.5 km 0 – 1.0 km flows of the Lower Division of the MPF 1.0 – 1.5 km clastic sediments (volcanic conglomerate horizon) STOP 5 Volcanic Conglomerate UTM coordinates – 670790E 5214845N Parking is along the shoulder of the road. 21 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide Photo 1a: Polymictic conglomerate of the Mica Bay Formation unconformably overlying a MPF flow. Photo 2: Glomeroporphyritic Basalt (daisy stone) with a distinctive radiating calcic-plagioclase in a fine grained to aphanitic basaltic matrix (new location to the south of the highway). Photo 1b: Ball and Pillow Structures within the siltstone of the Mica Bay Formation. Photo 3: Ropy flow top on the south face of the outcrop, east of the highway. Photo 1c: Cross-laminations in the siltstones and arkoses of the Mica Bay Formation. Photo 4: Mamainse Vein looking east. Fracture fill carbonate-quartz vein hosted by the basaltic flows of the Upper Division. 22 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide BE CAUTIOUS AS THE HIGHWAY IS BUSY AND HEAVILY TRAVELLED BY TRACTOR TRAILERS mining operation, containing abundant specular hematite with lesser chalcocite. The area is underlain by thin, dark green, amygduloidal olivine tholeiite flows (Annells 1973), that strike north and dip 30o to 40o west. Amygdules filled with quartz, agate, calcite, chlorite and / or epidote, and carbonate and quartz-carbonate fracture fillings are common in the flows. The Mamainse Vein is described as striking o 335 and dipping 50o east, similar to the C Zone on the Coppercorp property (Tortosa and Moss 2004). The vein was traced for 457 m, pinching and swelling along strike varying in width from 0.46 to 4.0 m. This vein appears to be exposed along the shore to the south, and there are a number of smaller fractures containing mineralization including one north of the parking site. Interbedded with the basalt flows in the Mamainse Point Group are volcanic conglomerates composed entirely of basalt clasts. The conglomerate was described by Annells (1973) as poorly sorted, angular to subrounded clasts in a matrix of fine basaltic debris with some silty material. The clasts are massive to vesicular, up to 0.30 m in diameter, and some clasts are irregular shaped which Annells (1973) interpreted to be a result of emplacement while still semisolidified. Annells (1973) also reported that this unit has a fine-grained, well laminated upper zone resembling a hyaloclastite or peperite. Although there are clasts and lenses of occasional crossbedded red-brown silt there are no clasts of Archean basement rocks. The conglomerate is cut by carbonate veins which carry minor amounts of quartz and laumontite. BE CAUTIOUS AS THE OUTCROP WILL BE SLIPPERY IF WET Proceed south for 3.7 km Approximately 60 m south of the parking area on the lake shore, a west northwest trending fracture hosts an about 10 cm wide felsite dyke and the quartz-carbonate Mamainse vein fracture filling (Photo 4). Very fine fractures, filled with native copper and calcite, are oriented at right angles to the main northwest-trending fracture. The carbonate-quartz fracture filling and veining contains disseminated chalcocite, chalcopyrite, lesser native copper, with malachite and azurite weathering products, and associated specular hematite. North of the parking area, a subparallel 5 to 10 cm wide fracture filled with carbonate and quartz also hosts chalcocite and chalcopyrite, with malachite and azurite. Along the shore to the north, and in the rock cut on the east side of the highway, a polymictic conglomerate is interbedded with the flows. This conglomerate is located stratigraphically above the “Great Conglomerate” (Giblin and Armburst 1969). 0 – 1.8 km: Flows of the upper portion of the Lower Division 600 m: distinctive hematite alteration along fractures in the flows - east side of the highway 1.8 – 2.6 km: clastic sediment, mainly polymicitic conglomerates, of the “Great Conglomerate” 2.6 – STOP 6 3.5 km: mafic flows and intercalated conglomerates of the Upper Division of the MPF Mamainse Mine UTM coordinates – 669939E 5214040N Exit to the west onto the bush road, proceed about 100 m to the second opening and park. The Mamainse Mine consists of 3 shafts that were put into production between 1842 and 1894, along with a stamp mill, service facilities and a village were constructed by the Lake Superior Native Copper Company. The area to the south of the parking site consists of rubble from the Proceed south for ~875 m Turn east on the bush road Proceed south for 2.3 km 23 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide STOP 7 Ubetuwantit Property) Vein (Coppercorp Proceed south for ~5.8 km UTM coordinates – 670432E 5211228N 0 – 5.8 km: flows and intercalated clastic sediments and felsic rocks of the lower portion of the Upper Division, MPF Park the vehicles along the bush road and proceed about 300 m to the east along the trail to the pits. This vein is exposed in two shallow pits on the west side of the powerline. Sampling by Nikos Explorations (Tortosa and Moss 2004) indicates that the 5 to 10 centimetre wide vein is composed of quartz and carbonate hosted by fracturing and minor brecciation within a basalt flow. The veins contain chalcocite and chalcopyrite with abundant specular hematite. The flow is amygduloidal with calcite veinlets and trace malachite staining. Some portions of the flow are pervasively hematitized with up to 1% specular hematite. 4.3 km: south entrance to the Coppercorp Mine. STOP 9 UTM coordinates – 66836E7 5207904N Park in the area near the top of the rise on the west side of the highway BE CAUTIOUS AS THE HIGHWAY IS BUSY AND HEAVILY TRAVELLED BY TRACTOR TRAILERS Continue south for ~ 2.3 km Walk north along the highway to the rock cut and cross with caution. Passing through the area of the main shaft and waste pile for the Coppercorp Mine. STOP 8 Interbedded Conglomerate and basalt At this location, two polymictic conglomerates are separated by a basalt flow. There are a number of conglomerate horizons interbedded with the flows the MPF, with thicker concentration occurring in the “Great Conglomerate” located to the north and stratigraphically below this stop, and this conglomerate is typical of the polymicitic conglomerates of the MPF (e.g. Annells 1973; Giblin 1974). The conglomerates generally contain subrounded, up to 0.61 m clasts of predominantly Archean granitic rocks, with minor amounts of Archean mafic metavolcanic rocks, iron formation, and Keweenawan basalt (Fig. 5). In some locations, well laminated to cross-bedded sandstone is interbedded with the conglomerates (Annells 1973). The basalt flow separating the conglomerate beds was described by Giblin (1974) as having a narrow amygdaloidal zone, with a few pipe vesicles, at the base, rapidly coarsening upwards through the flow, and a thicker amygdaloidal zone near the flow top. Minor faulting, ranging from a few to 30 cm, occurred at the base of the flow, and has been filled by carbonate. Carbonate occurs in fractures cutting the conglomerate and also in the matrix. Coppercorp Mine UTM coordinates – 671103E 5209365N In the open cut, the C Zone can be observed in a north-northwest trending fault. This is one of a set of north-trending, generally 50o-70o east dipping, faults that are almost parallel to the strike of the flows. The veins and breccia fillings consist of quartz and carbonate with subordinate laumontite and fluorite. The width of the fault zones varies along strike from shears less than 1 metre to disrupted lenses up to 12 metres across (Richards, 1985). The wallrock is commonly chloritized and sericitized and may contain epidote. Mineralization consists of predominantly copper sulphides, chalcocite with lesser chalcopyrite and bornite, usually accompanied by specular hematite. Large vugs of varying size are lined with quartz, calcite and sulphides and were commonly found throughout the deposit. Samples may be obtained from the blocks piled to the south of the open cut. The mine may be exited either by returning north for ~4.6 km along the bush road to the highway, or if the roads are passable proceed south and west for ~2.3 km to the highway Proceed south for ~3.6 km. 24 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide felsic magma, perhaps concentrated in late stage magmatic fluids migrating along zones of shearing and deformation during the last stages of crystallization of the magma. 0 – 3.6 km: flows and intercalated clastic sediments and felsic rocks of the Upper Division ~2.6 km: hyaloclastite intercalated with the massive flows – west side of highway STOP 10a Return to the vehicles Domeykite Occurrence Proceed south for ~800 m UTM coordinates – 669783E 5204844N STOP 10b Park in the lane on the west side of the highway, south of the outcrop Cottrell Cove Felsic Intrusion UTM coordinates – 670390E 5204502N Park in the bush road on the east side of the highway and proceed about 60 m north to the outcrop on the west side. BE CAUTIOUS AS THE HIGHWAY IS BUSY AND HEAVILY TRAVELLED BY TRACTOR TRAILERS BE CAUTIOUS AS THE HIGHWAY IS BUSY AND HEAVILY TRAVELLED BY TRACTOR TRAILERS Walk north to the large sloping outcrop located on the east side of the highway and cross the highway with caution. The rock cut consists of a rock of rhyolitic composition (Annells 1973; Lightfoot et al. 1999), and is part of an approximately 250 m wide body that includes the pseudotachylite stop to the north (Stop 10c). The domeykite outcrop has been interpreted to be the northwest branch of this unit (Giblin and Armburst, 1969). This unit is also exposed along the shore of Cottrell Cove to the west. The felsic unit is fine-grained, reddish brown, and varies from massive in the core to quartz porphyritic and flow banded towards the margins. The flow bands forms tight asymmetric isoclinal folds that are generally parallel to the contacts (Photo 6a). The contact consists of a breccia ranging from centimetres to tens of centimetres, and containing subangular to subrounded, pebble to cobble sized fragments of this unit, basalt, and granite in a felsic matrix (Fig. 6b). Dykes of felsite breccia reportedly intrude the adjacent basalts (Giblin 1974). The massive core contains light brown to beige lenses and irregular patches or funnels Domeykite (Cu3As) is isometric, hextetrahedral, gray to yellow-brown to white, with a brittle fracture and habits that include botryoidal, reniform, and massive uniformly indistinguishable crystals. It has a hardness of 3 to 3.5, metallic luster, and a black gray streak (http://webmineral.com /data/Domeykite.shtml). Domeykite mineralization is restricted to a pink to red, fine-grained felsic rock of rhyolitic composition that dips from 30o to 40o northwest with an overall thickness from 10 to 15m. The lowermost 3 to 4 m of the unit is strongly flow banded with tight asymmetric isoclinal folds. The lower contact with the basalt is sharp and has an aphanitic, 2 to 10 cm chilled zone that contains subangular to subrounded fragments of basalt. Spherical calcite-filled amygdules occur within the felsic unit for up to a few centimetres from the basalt. The basalt is a medium-grained, diabasic, dark green amygduloidal olivine tholeiite. This felsic unit is interpreted to be an intrusion into the older basaltic flows (e.g. Annells 1973). Domeykite is a copper arsenide concentrated in green to grey patches best developed in the flow banded portions of the lower and central parts of the unit. The domeykite has not been observed in the basalts stratigraphically above or below. The restriction of the copper arsenide mineralization to the felsic unit suggests that the mineralization may be genetically related to the 25 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide Photo 5: Polymicitic conglomerate containing clasts of Archean felsic plutonic rocks intercalated with basaltic flows. Photo 6c: Alteration along vertical fractures in rhyolite possibly related to magmatic degassing. Photo 6a: Flow-banded rhyolite near the contact, with pink, grey to white alteration. Photo 7: Pseudotachylite consisting of an aphanitic, matrix containing fragments of quartz, felsite, and basalt — -— -k.- i Photo 6b: Autobrecciated felsite in contact with diabasic basalt. Photo 8: Keweenawan basalt unconformity overlying Archean felsic plutonic rocks. (http:// www.start.ca /users/ mharris/ waterfalls/ chippewa-falls.html) 26 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide are set in a matrix of finely comminuted fragments of the above-noted materials and larger, irregular grains of carbonate and chlorite. An overall pinkish colour was interpreted to be a result of finely disseminated hematite. Giblin (1974) tentatively interpreted the pseudotachylite to be a result of gas escaping from the underlying felsic magma streamed upwards, under high pressure, through fractures in the overlying rocks. Fragments of the wallrocks, detached by erosive power of the gas and structural dislocation, were transported in a fluidized gas-solid system, which further eroded the wallrocks and the entrained particles. The fluidized material filled the fractures, and upon the eventual decrease in gas pressure, remained as veins of very fine-grained rock powder. (Photo 6c). Giblin (1974) reported that this change was a result of alteration and extensive development of kaolin. The orientation of the funnel suggests that this alteration may be a result of post-depositional degassing and possibly related to the pseudotachylite of the previous stop. This irregular colour variation also resembles variations observed in the oxidized Sibley Group sedimentary rocks of the Nipigon area, suggesting that some of this variation may be in part a result of reduction of an originally oxidized rock. These felsic units or felsites have been interpreted to be intrusive in nature (e.g. Annells 1973; Giblin 1974) based primarily on the brecciated nature of the exposed contacts. The depth of intrusion is not known, or whether some of these units may have formed high level cryptodomes that may have breached surface. The only known geochronology for the Mamainse Point area is a U/Pb zircon age of 1096+/-2.1 Ma (D. Davis, personal communication, 2005) for a unit with similar characteristics located north of the Mamainse Mine. However, the contact relationships at that location are ambiguous. Proceed south for ~27.7 km 6.6 km: Pancake Bay Provincial Park UTM coordinates – 675727E 5204458N 8.3 km: Pancake River UTM coordinates – 678422E 5203459N Proceed for ~200 m north ~8.8 km: Mamainse Point Fault separates the MPF to the north from the Jacobsville Formation sediments to the south. outcrop located on the west side STOP 10c Pseudotachylite 8.8 – 21.8 km: Jacobsville Formation UTM coordinates-670190 5204595 ~13.8 km: the road to the Tribag Mine exits on the east side of the highway Please do not collect samples from the outcrop, as there are abundant fragments in the rubble used as fill for the road bed. ~19.0 km: Batchawana River UTM coordinates – 688231E 5200663N The pseudotachylite was originally described by Giblin (1974) and occurs as narrow branching veins, ranging in thickness from a few centimetres to thin films on fracture surfaces, cutting both the massive fine-grained felsic rock and the wallrock basalts (Photo 7). Veins are composed of a dark grey, black, to dark brown, aphanitic, matrix, that often breaks with a sub-conchoidal fracture, containing fragments of quartz, felsite, and basalt. Thin-sections material show the veins to be a microbreccia consisting of angular fragments of quartz (free of strain shadows), feldspar, devitrified glass, quartz-feldspar porphyry with a matrix of devitrified glass, opaque minerals, felsite, and basalt (Giblin 1974). These fragments ~21.8 – 27.7 km: Archean felsic plutonic rocks The highway crosses over the Harmony River The entrance to the parking lot is to the south side of the river on the east side of the highway. STOP 11 Chippewa Falls UTM coordinates – 695982E 5200414N 27 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide A monument in the small roadside park marks the approximate mid-point of the Trans-Canada Highway. Follow the trails for approximately 90 m to the top of the lower falls, where there is an excellent exposure of Mamainse Point flows unconformably overlying Archean trondhjemite and diabase dykes. The best exposure is on the small island which is accessible only during periods of low water (Fig. 8). The contact on the island and the east side of the river can be viewed from the trail. Giblin (1974) described the unconformity as generally 10° - 30° southwest dipping, but is nearly vertical on the east bank of the river. The basalt flow has a chilled lower contact with rare pipe vesicles, and near the east side of the island, a few pillows ranging in length from 0.3 to 1.2 m occur in the lower 3 m of the flow. These are the only pillows found to date in the Keweenawan lavas of the east shore of Lake Superior. 25.0 – 32..1 km: Jacobsville Formation in the Goulais River valley. 32.1 – 45.9 km: Archean granitic gneisses and migmatites cut by diabase dikes with Huronian sediments to the east along Highway 550. 45.9 - 51.7 km: Jacobsville Formation to the Water Tower Inn Proceed 51.7 km south to Sault Ste. Marie 0 – 9.8 km: Archean felsic plutonic rocks 9.8 - 13.1km: Keweenawan mafic flows 11.2 km: Optional Stop, Keweenawan Basalt Optional Stop – Keweenawan Basalt UTM coordinates –700261E 5192354N Good cross-sections of the basaltic flows are exposed in the rock cut on the eats and southeast side of the highway, and include thin basal amygdaloidal zones, thicker upper amygdaloidal zones, and common reddish, ropy flow tops. 13.1 – 15.2 km: Jacobsville Formation 15.2 – 17.4 km: Archean felsic plutonic rocks 17.4 – 19.9 km Archean felsic to intermediate metavolcanic rocks with the hill top to the west being a Nipissing sill. 19.9 – 25.0 km: sedimentary rocks of the Gowganda Formation of the Southern Province, intruded by gabbro sills of the Nipissing diabase to the northeast. 28 PDF compression, OCR, web-optimization with CVISION's PdfCompressor � https://digitalcollections.lakeheadu.ca/files/original/22ea3b680000e76ba99ce28da77c8e43.PDF 384ef9ee0a2ad25d4df809f0ebf0283f PDF Text Text Ontario Ontario Geological Survey Open File Report 6135 Geological Guidebook to the Paleoproterozoic East Bull Lake Intrusive Suite Plutons at East Bull Lake, Agnew Lake and River Valley, Ontario 2004 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �ONTARIO GEOLOGICAL SURVEY Open File Report 6135 Geological Guidebook to the Paleoproterozoic East Bull Lake Intrusive Suite Plutons at East Bull Lake, Agnew Lake and River Valley, Ontario by R.M. Easton, L.S. Jobin--Bevans and R.S. James 2004 Parts of this publication may be quoted if credit is given. It is recommended that reference to this publication be made in the following form: Easton, R.M., Jobin--Bevans, L.S. and James, R.S. 2004. Geological guidebook to the Paleoproterozoic East Bull Lake intrusive suite plutons at East Bull Lake, Agnew Lake and River Valley, Ontario; Ontario Geological Survey, Open File Report 6135, 84p. e Queen’s Printer for Ontario, 2004 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �e Queen’s Printer for Ontario, 2004. Open File Reports of the Ontario Geological Survey are available for viewing at the Mines Library in Sudbury, at the Mines and Minerals Information Centre in Toronto, and at the regional Mines and Minerals office whose district includes the area covered by the report (see below). Copies can be purchased at Publication Sales and the office whose district includes the area covered by the report. Although a particular report may not be in stock at locations other than the Publication Sales office in Sudbury, they can generally be obtained within 3 working days. All telephone, fax, mail and e-mail orders should be directed to the Publication Sales office in Sudbury. Use of VISA or MasterCard ensures the fastest possible service. Cheques or money orders should be made payable to the Minister of Finance. Mines and Minerals Information Centre (MMIC) Macdonald Block, Room M2-17 900 Bay St. Toronto, Ontario M7A 1C3 Tel: (416) 314-3800 Mines Library 933 Ramsey Lake Road, Level A3 Sudbury, Ontario P3E 6B5 Tel: (705) 670-5615 Publication Sales 933 Ramsey Lake Rd., Level A3 Sudbury, Ontario P3E 6B5 Tel: (705) 670-5691(local) 1-888-415-9845(toll-free) (705) 670-5770 pubsales@ndm.gov.on.ca Fax: E-mail: Regional Mines and Minerals Offices: Kenora - Suite 104, 810 Robertson St., Kenora P9N 4J2 Kirkland Lake - 10 Government Rd. E., Kirkland Lake P2N 1A8 Red Lake - Box 324, Ontario Government Building, Red Lake P0V 2M0 Sault Ste. Marie - 70 Foster Dr., Ste. 200, Sault Ste. Marie P6A 6V8 Southern Ontario - P.O. Bag Service 43, 126 Old Troy Rd., Tweed K0K 3J0 Sudbury - Level B3, 933 Ramsey Lake Rd., Sudbury P3E 6B5 Thunder Bay - Suite B002, 435 James St. S., Thunder Bay P7E 6S7 Timmins - Ontario Government Complex, P.O. Bag 3060, Hwy. 101 East, South Porcupine P0N 1H0 Toronto - MMIC, Macdonald Block, Room M2-17, 900 Bay St., Toronto M7A 1C3 This report has not received a technical edit. Discrepancies may occur for which the Ontario Ministry of Northern Development and Mines does not assume any liability. Source references are included in the report and users are urged to verify critical information. Recommendations and statements of opinions expressed are those of the author or authors and are not to be construed as statements of government policy. If you wish to reproduce any of the text, tables or illustrations in this report, please write for permission to the Team Leader, Publication Services, Ministry of Northern Development and Mines, 933 Ramsey Lake Road, Level B4, Sudbury, Ontario P3E 6B5. Cette publication est disponible en anglais seulement. Parts of this report may be quoted if credit is given. It is recommended that reference be made in the following form: Easton, R.M., Jobin-- Bevans, L.S. and James, R.S. 2004. Geological guidebook to the Paleoproterozoic East Bull Lake intrusive suite plutons at East Bull Lake, Agnew Lake and River Valley, Ontario; Ontario Geological Survey, Open File Report 6135, 84p. iii PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Contents Abstract ...........................................................................................................................................................xiii Introduction........................................................................................................................................................ 1 Safety......................................................................................................................................................... 1 Acknowledgements ................................................................................................................................... 2 East Bull Lake Intrusive Suite............................................................................................................................ 2 Nomenclature ............................................................................................................................................ 2 Regional Geological Setting...................................................................................................................... 3 Overview of East Bull Lake Intrusive Suite .............................................................................................. 9 Depth and Mechanism of Emplacement.................................................................................................. 10 Magma Composition and its Relationship to Mineralization .................................................................. 10 Orthopyroxene Hornblendite Bodies....................................................................................................... 12 Geochemistry .................................................................................................................................. 14 Field Trip Guidebook....................................................................................................................................... 16 East Bull Lake Intrusion—Overview ...................................................................................................... 16 Country Rocks, Structure and Geometry of the Intrusion............................................................... 16 Stratigraphy and Petrography.......................................................................................................... 17 Marginal Series ...................................................................................................................... 18 Lower Series........................................................................................................................... 18 Main Series............................................................................................................................. 18 Upper Series ........................................................................................................................... 20 Geochemistry .................................................................................................................................. 21 Mineralization................................................................................................................................. 26 Road Log, Day 1, East Bull Lake Intrusion............................................................................................. 27 Stop 1. Paleoproterozoic Metagabbro............................................................................................. 28 Stop 2. Neoarchean Parisien Lake Syenite ..................................................................................... 28 Stop 3. Southern Margin of the East Bull Lake Intrusion ............................................................... 28 Stop 4. Country Rocks .................................................................................................................... 29 Stop 5. Rhythmically Layered Zone ............................................................................................... 29 Stop 6. Folson Lake Deformation Zone.......................................................................................... 30 Stop 7. Dendritic Texture in the Varitextured Gabbronorite Zone ................................................. 30 Stop 8. Layered Gabbronorite Zone................................................................................................ 30 Stop 9. Olivine Gabbronorite Zone................................................................................................. 31 Stop 10. Rhythmically Layered Zone, East Bull Lake Intrusion .................................................... 31 Stop 11. Moon Lake Traverse......................................................................................................... 31 Stop 12A. Parisien Lake Traverse................................................................................................... 33 Stop 12B. Parisien Lake Traverse................................................................................................... 34 Agnew Lake Intrusion—Overview.......................................................................................................... 35 Country Rocks, Structure, and Geometry of the Intrusion.............................................................. 35 Stratigraphy and Petrography.......................................................................................................... 35 Marginal Series ...................................................................................................................... 35 Lower Series........................................................................................................................... 37 Upper Series ........................................................................................................................... 39 Geochemistry .................................................................................................................................. 39 v PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Mineralization................................................................................................................................. 40 Road Log, Day 2, Agnew Lake Intrusion................................................................................................ 42 Stop 1. Inclusion-Bearing Unit ....................................................................................................... 43 Stop 2. Nodular Anorthosite Unit, Marginal Leucogabbronorite Zone .......................................... 44 Stop 3. Inclusion-Bearing Unit, Lower Gabbronorite Zone............................................................ 44 Stop 4. Inclusion-Bearing Unit, Lower Gabbronorite Zone............................................................ 46 Stop 5. Sudbury-Type Breccia........................................................................................................ 47 Stop 6. Varitextured Unit, Marginal Leucogabbronorite Zone ....................................................... 47 Stop 7. Dendrite Unit, Lower Gabbronorite Zone .......................................................................... 48 River Valley Intrusion, Overview............................................................................................................ 49 Country Rocks, Structure and Geometry ........................................................................................ 49 Structural State and Degree of Preservation........................................................................... 50 Contacts.................................................................................................................................. 52 Stratigraphy and Petrography.......................................................................................................... 55 North of the Sturgeon River ................................................................................................... 55 Marginal Zone ............................................................................................................... 55 Inclusion and/or Fragment-Bearing Zone ...................................................................... 56 Olivine Gabbronorite Zone............................................................................................ 57 Gabbronorite Zone......................................................................................................... 57 Leucogabbronorite Zone................................................................................................ 57 South of the Sturgeon River ................................................................................................... 57 Stratigraphic Comparison with other East Bull Lake Suite Intrusions ................................... 59 Geochemistry .................................................................................................................................. 60 Road Log, Day 3, River Valley Intrusion................................................................................................ 64 Stop 1. Shear-Zone Hosted Orthopyroxene Hornblendite Body..................................................... 64 Stop 2. Rocks of the Crerar Gneiss Association ............................................................................. 65 Optional Stop. Crerar Gneiss Association ...................................................................................... 65 Optional Stop. Red Cedar Lake Gneiss........................................................................................... 65 Stop 3. Varied Degrees of Preservation in Rocks of the River Valley Intrusion ............................ 66 Stop 4. Autolith Fragments and Layering, Layered Gabbronorite Zone......................................... 66 Stop 5. Layered Olivine Gabbronorite............................................................................................ 67 Stop 6. Footwall Alkali Feldspar Granite and Sudbury Diabase Dike............................................ 67 Stop 7. Azen Creek Copper-Nickel-PGE Occurrence .................................................................... 68 Optional Stop. Huronian Metavolcanic Rocks................................................................................ 69 Optional Stop. Variably Preserved Nipissing Gabbro .................................................................... 69 Stop 8. Mylonitized Anorthosite of the River Valley Intrusion ...................................................... 70 Optional Stop. Mississagi Formation and Mylonitic Contact with the River Valley Intrusion....... 70 Stop 9. Dana Lake Copper-Nickel-PGE Occurrence ...................................................................... 71 Stop 9A. L6+00N, Contact environment................................................................................ 74 Stop 9B. L7+00N, Contact environment................................................................................ 74 Stop 9C. Road Zone ............................................................................................................... 74 Stop 9D. Central Zone............................................................................................................ 75 Stop 9E. Trench Zone............................................................................................................. 76 Stop 9F. South Zone............................................................................................................... 76 References........................................................................................................................................................ 77 vii PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �FIGURES 1. Distribution of U-Pb zircon ages from Neoarchean and Paleoproterozoic rocks in the Sudbury area, as well as the distribution of East Bull Lake intrusive suite bodies................................................................. 4 2. Geologic map of East Bull Lake Intrusion .................................................................................................. 16 3. Stratigraphic sections for the East Bull Lake and Agnew Lake intrusions.................................................. 17 4. Pearce-element ratio diagrams for the East Bull Lake, Agnew Lake and River Valley intrusions ............. 22 5. Chondrite-normalized rare earth element plots for the East Bull Lake and Agnew Lake intrusions........... 23 6. Geological map of the Moon Lake area, East Bull Lake Intrusion, showing the route of the traverse undertaken during Stop 11........................................................................................................................... 32 7. Geology of the Agnew Lake Intrusion ........................................................................................................ 36 8. Map showing the degree of primary textural preservation within rocks of Crerar and Dana townships..... 51 9. Interpreted stratigraphic sequence and distribution of primary mineral phases and compositions within the River Valley intrusion north of the Sturgeon River..................................................................................... 56 10. Generalized cross-sections for the Street metagabbro and the River Valley, East Bull Lake and Agnew Lake intrusions of the East Bull Lake intrusive suite .................................................................................. 59 11. Chondrite-normalized rare earth element plots for the River Valley intrusion for rocks north of the Sturgeon River............................................................................................................................................. 61 12. Map of the Dana North property of Pacific North West Capital Corporation, showing the location of stops 9A through 9F, as well as the extent of the drilling program undertaken between 1999 and 2002............. 73 PHOTOS Frontispiece. Typical copper-nickel-PGE mineralization from the River Valley intrusion ................................. xv 1. Orthopyroxene phenocrysts in matrix-rich orthopyroxene hornblendite in Henry Township..................... 13 2. Igneous layering in the East Bull Lake Intrusion ........................................................................................ 19 3. Dendritic texture within the Upper Series of the East Bull Lake Intrusion ................................................. 20 4. Centimetre- to decametre-scale igneous layering within rocks of the Agnew Lake Intrusion .................... 38 5. Dendrite texture developed with the Lower Gabbronorite Zone of the Agnew Lake Intrusion .................. 38 6. Stop 1, Day 2. Large pyroxenite pod hosted by rocks of the Inclusion-Bearing unit, which at this locality are hosted in a predominantly leucogabbronoritic matrix ........................................................................... 43 Stop 2, Day 2. a) Stone quarry in Nodular Anorthosite unit. b) Close-up of nodular anorthosite, showing closely packed plagioclase glomerophenocrysts with minor interstitial, altered, pyroxene ........................ 45 Nodular anorthosite from the Inclusion-Bearing Gabbronorite Zone, Lower Series, of the Agnew Lake Intrusion ...................................................................................................................................................... 46 Diabase dike containing large, rounded plagioclase crystals, cutting across rocks of the Inclusion-Bearing Gabbronorite Zone, Lower Series, of the Agnew Lake Intrusion................................................................ 47 7. 8. 9. ix PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PHOTOS Cont'd 10. Progressive textural changes. a) Igneous-texture in medium-grained norite of the River Valley intrusion, Crerar Township. b) Partly recrystallized rock of the River Valley intrusion, preserving igneous-texture. c) Recrystallized but massive leucogabbronorite from leucogabbronorite zone of the River Valley intrusion, Crerar Township. d) Typical undeformed but recrystallized leucogabbronorite from the Positano Quarry, Loughrin Township ......................................................................................................... 52 11. Layering in the River Valley intrusion. a) Igneous layering within the norite zone. b) Igneous layering near the top of the anorthosite zone and the base of the olivine gabbronorite zone. c) Layering within leucogabbronoritic rocks immediately above the heterogeneous basal zone. d) Large-scale igneous layering within leucogabbronoritic rocks exposed in the Dana Quarry....................................................... 53 12. a) Typical leucogabbroic gneiss of an East Bull Lake intrusive suite intrusion in Henry and Loughrin townships. b) Protomylonitic to mylonitic textures present in a high-strain zone in the River Valley intrusion. c) Strain gradient within rocks of the River Valley intrusion, northern Crerar Township. d) Epidote-clot developed in leucogabbro gneiss of the River Valley intrusion, Crerar Township................. 54 13. Photographs illustrating the textural variation observed in the Inclusion-Bearing unit in the Dana North area .............................................................................................................................................................. 75 TABLES 1. Summary of stratigraphic, mineralogical and geochemical data for the East Bull Lake intrusive suite...... 5 2. Summary of geochronology on rocks of the East Bull Lake intrusive suite................................................ 7 3. Timing of major geological events and summary of age constraints on the main rock units present in the study area..................................................................................................................................................... 8 Representative and average anhydrous whole-rock geochemical analyses from the East Bull Lake and Agnew Lake intrusions................................................................................................................................ 24 Representative chemical analyses from the River Valley intrusion ............................................................ 62 4. 5. xi PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Abstract Intrusions of the ~2480 million-year-old East Bull Lake intrusive suite occur as an east-northeast-trending belt along the boundary of the Archean Superior and the Proterozoic Southern provinces in Ontario, Canada. The East Bull Lake intrusive suite is part of a regional Paleoproterozoic, bimodal magmatic event resulting from a mantle-plume driven, intracontinental rifting event that formed a major basin, filled by sedimentary and igneous rocks of the Huronian Supergroup. Intrusions of similar age and composition in Finland and Wyoming were once contiguous with the East Bull Lake intrusive suite prior to tectonic dispersion during the Proterozoic. Field and geochemical evidence indicates that the 3 largest intrusions (East Bull, Agnew, River Valley) crystallized from similar, low-Ti, high-Al, PGE-rich, tholeiitic parent magmas that originated in deeper, more mafic chambers. The primary magmas were likely second-stage melts derived from a sublithospheric depleted mantle source modified by Neoarchean subduction and accretion events. Petrologically, the intrusions reflect plagioclase-dominated fractional crystallization that generated a pronounced Fe-enrichment trend in the residual magmas. Olivine and orthopyroxene occur as cumulus phases in only the interpreted lower parts of the stratigraphy. Rhythmically and irregularly modally layered leucogabbronorite and gabbronorite make up much of the middle part of the stratigraphy. Ferrogabbro and ferrosyenite occur only in the uppermost Agnew Lake Intrusion. A marginal facies, comprising brecciated and, locally, thermally recrystallized and/or partially melted footwall rocks, is common and is testimony to a high-energy flow regime during initial emplacement. This marginal unit typically grades into a heterolithic, inclusion-rich gabbronorite with erratically distributed leucocratic to melanocratic autoliths and footwall xenoliths and eventually into a thick interval of undifferentiated, plagioclase-rich cumulates, locally with spectacular glomerocrystic textures. Disseminated, bleb and interstitial magmatic chalcopyrite-pyrrhotite-pentlandite mineralization (1 to 5% sulphide minerals, up to 10 g/t Pt+Pd) occurs in autolith-rich gabbronorite breccia within 5 to 50 m of the contact of the intrusions. The mineralization is commonly spatially associated with pyroxenite cumulates and autoliths that are otherwise poorly represented in the stratigraphy. Vigorous convection and explosive breccia-producing emplacement of sulphide-saturated magma formed PGE-rich zones at the margins (sidewall or floor?). The magmatic sulphide zones are overprinted and enclosed by a broader envelope of metamorphic and/or hydrothermal sulphide mineralization of similar mineralogy (± pyrite) that extends into adjacent leucogabbronoritic to anorthositic units, and less so into the country rocks. The hydrothermally enriched zones contain 2 to 10% sulphide minerals with 1 to 10 g/t Pt+Pd, and represent the main exploration target. The high-grade zones are enclosed by broader, lower grade mineralization with Pt+Pd levels in the background range of 20 to 50 ppb. Day long field excursions to each of the East Bull Lake, Agnew Lake, and River Valley intrusions are also included in this report. The combination of road accessible and walk-in stops is designed to illustrate important aspects of the geology, stratigraphy and mineralization found in each of the intrusions. xiii PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �S Frontispiece. Typical copper-nickel-PGE mineralization from the River Valley intrusion. a) Typical outcrop exposure of mineralized olivine gabbronorite from the River Valley intrusion in the vicinity of the Jackson’s Flats occurrence. Note sulphide “burns” to the right of the 8.5 cm long scale card. b) Drill core from drill hole DL-14 in the Dana North area (Stop 9C, Day 3), showing the typical low tenor of sulphide and the presence of blue-grey quartz. A sample like this typically contains greater than 2 g/t of Pt and Pd mineralization. c) Close-up of mineralization from the Inclusion-Bearing unit at Dana South (Stop 9F, Day 3). All photographs courtesy of S. Jobin-Bevans. xv PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Geological Guidebook to the Paleoproterozoic East Bull Lake Intrusive Suite Plutons at East Bull Lake, Agnew Lake and River Valley, Ontario R.M. Easton1, L.S. Jobin-Bevans2 and R.S. James3 Ontario Geological Survey Open File Report 6135 2004 1 Precambrian Geoscience Section, Ontario Geological Survey Ministry of Northern Development and Mines, Sudbury, Ontario, Canada P3E 6B5 2 Caracle Creek International Consulting Inc., Sudbury, Ontario, Canada P3E 2V7 3 Department of Earth Sciences, Laurentian University, Sudbury, Ontario, Canada P3E 2C6 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Introduction Since the summer of 1998, mafic intrusions of the East Bull Lake intrusive suite, specifically the East Bull Lake, Agnew Lake and the River Valley intrusions, have been the subject of ongoing copper-nickelplatinum group element (PGE) mineral exploration. This increased economic interest in these intrusions has led to renewed interest in the geology, stratigraphy and geochemistry of the East Bull Lake intrusive suite, as summarized by James et al. (2002a, 2002b) and Easton (2003). After some 3 decades of study there are only a very few field trip guides to any of these intrusions. Peck et al. (2000) and Brisbin et al. (2001) produced guidebooks for the East Bull Lake Intrusion for PGE Exploration Workshops at Laurentian University in 2000 and 2001. A field guide for the Agnew Lake Intrusion has never been prepared. Hrominchuk and Jobin-Bevans (2000) prepared a guidebook for the River Valley intrusion to compliment the first PGE Exploration Workshop at Laurentian University in 2000. Earlier guides by Lumbers (1978) and Davidson (1986) included a few stops in the River Valley intrusion, but were prepared prior to detailed mapping of the intrusion. The geology of the East Bull Lake suite intrusions is a significant topic in understanding the earliest evolution of the Southern Province in central Ontario. There are now numerous professional publications that describe and discuss the geology, geochemistry and mineral deposit potential of these intrusions (Table 1, and references in James et al. 2002a, 2002b). This integrated field guidebook to the East Bull Lake, Agnew Lake and River Valley intrusions is intended to compliment those studies. The field trip stops have been selected to show typical stratigraphic units in each intusion, as well as the geology of mineralized zones near the contacts of all 3 intrusions. The stops are also designed to compare and contrast the geological characteristics of the 3 major intrusions. The field trip uses a combination of road accessible outcrops, as well as some stops that require some short traverses through the bush, generally utilizing trails of varied condition. Although most of the stops can be accessed using a two-wheel drive vehicle, use of a vehicle with high ground clearance and fourwheel drive capability is recommended for both the Agnew Lake and River Valley intrusion stops, especially in the spring and fall. The structure of the guidebook is as follows: following the introduction, a brief overview of the East Bull Lake intrusive suite is provided. This is followed by descriptions of each one-day field trip to the East Bull Lake, Agnew Lake and River Valley intrusions. These descriptions include an overview of the geology, stratigraphy, geochemistry and mineralization in each intrusion, as well as a detailed road log and stop description. Day 1, the East Bull Lake Intrusion, was prepared mainly by R.S. James. Day 2, the Agnew Lake Intrusion, was prepared mainly by S. Jobin-Bevans and R.S. James. Day 3, the River Valley intrusion, was prepared mainly by R.M. Easton and S. Jobin-Bevans. The guidebook was compiled and edited by R.M. Easton. SAFETY For users of this guidebook, please bear in mind that some of the stops listed in this guidebook involve hiking in the bush. Therefore, standard bush safety practices should be followed by users of this guidebook. Such practices include travelling in pairs; advising others of your starting time and location and your expected return time; carrying sufficient water for the trip; being prepared for sudden changes in the weather; and carrying the appropriate emergency and safety gear. Most of the trip routes are on Crown land, but access is on or near private property for some routes. As in all such situations, please respect the property rights of others, so that future access for other geologists is not adversely affected. 1 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �ACKNOWLEDGEMENTS The authors acknowledge the contributions of information kindly made available to them by Mustang Minerals Corporation with regard to access and information on the geology and mineralization of the East Bull Lake Intrusion. Pacific North West Capital Corporation and its partner Anglo American Platinum Corporation Limited are also thanked for similar information on the Agnew Lake and River Valley intrusions. Some of the material on the River Valley intrusion is related to work conducted as part of Ontario Geological Survey bedrock mapping projects PU99-007, 99-019, 01-009 and 02-008, as described in Easton (2003). This guidebook was prepared initially for use with a field trip held in conjunction with the Geological Association of Canada–Mineralogical Association of Canada joint annual meeting in St. Catharines, May 12 to 14, 2004. East Bull Lake Intrusive Suite NOMENCLATURE The term East Bull Lake intrusive suite was only recently introduced by Easton (1999), and is similar to the term, East Bull Lake suite, used by Vogel et al. (1999). Prior to 1991, rocks of this suite were generally only described as individual intrusions, or as gabbro-anorthosite bodies that were intrusive into the lower part of the Elliot Lake Group (e.g., Bennett, Dressler and Robertson 1991). Prevec et al. (1995) referred to rocks of the East Bull Lake intrusive suite as “trans-Huronian” intrusions. Keays et al. (1995) and Peck et al. (1995) grouped them as part of the Huronian–Nipissing Magmatic Province, Domain or Belt. Nomenclature of individual intrusions within the East Bull Lake intrusive suite has been inconsistent over the years; however, James et al. (2002a) attempted to standardize the nomenclature for the intrusions within the suite, and to make this terminology consistent with the North American Stratigraphic Code (NACSN 1983). For example, the intrusion located west of Agnew Lake has been termed the Shakespeare–Dunlop intrusion, the Shakespeare–Dunlop gabbro-anorthosite intrusion, the Agnew Lake Intrusion and the Agnew intrusion, which was standardized in James et al. (2002a) as the Agnew Lake Intrusion. The standardization of nomenclature recommended by James et al. (2002a) is used herein, and is summarized in Table 1. Table 1 also includes the location of reference sections for the better-studied intrusions, details on contact relationships, and brief descriptions of the various intrusions. Recent studies on the East Bull Lake intrusive suite have used a three-tiered classification scheme to describe the rocks of the suite (e.g., Chubb 1994; Vogel 1996; James et al. 2002a), which has been used in this report. The first-tier differentiates rock types based on colour index and variation in modal abundance of mafic minerals and plagioclase. The second-tier groups individual rock layers into mappable series, zones, subzones and units (Irvine 1982). It should be noted that units outlined by this second-tier classification are not formal stratigraphic units as defined by the North American Stratigraphic Code (NACSN 1983), although some units, in particular the zones, are roughly equivalent to lithodemic units. The third-tier is based on petrography or distinctive textures, but also uses the IUGS classification scheme for plutonic rocks (Le Maitre 1989) where primary mineralogy or whole-rock geochemistry is present. Most primary igneous mafic minerals are replaced by amphibole, chlorite and epidote group minerals (e.g., Kamineni et al. 1985; Kamineni 1986), which reflects regional metamorphism in East Bull Lake intrusive suite bodies in the Southern Province. Plagioclase may be unaltered or partially to completely 2 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �saussuritized. Rocks of the East Bull Lake intrusive suite within the Grenville Province can range from almost wholly unaltered to completely recrystallized to an upper amphibolite facies, amphiboleplagioclase assemblage (e.g., Easton and Hrominchuk 1999; Tettelaar 2000; Easton 2002). In either geological province where metamorphic alteration is pervasive, rock nomenclature is based on geochemically determined CIPW-normative mineralogy combined with petrographic data. Unless otherwise stated, all UTM co-ordinates are in Zone 17, datum NAD 83. Unless otherwise noted, all analytical work in this report was completed by the Geoscience Laboratories, Geoservices Centre, Sudbury. REGIONAL GEOLOGICAL SETTING The Paleoproterozoic East Bull Lake intrusive suite (Easton 1999; James et al. 2002a) consists of several bodies of dominantly gabbronorite to gabbroic anorthosite that occur in both the Southern and Grenville provinces between Elliot Lake and the Ottawa River (Figure 1). The 3 largest bodies are the East Bull Lake and Agnew Lake intrusions (Southern Province) and the River Valley intrusion (Grenville Province) (see Table 1). All were emplaced between ~2491 and 2475 Ma (Table 2). Smaller bodies include the intrusions in Drury, Falconbridge, May, Street and Wisner townships (see Table 1, see Figure 1). Most bodies of the East Bull Lake intrusive suite were emplaced into Archean rocks present in either the Superior or Grenville provinces. Table 3 summarizes the main geological events that have occurred in this part of the Canadian Shield during the Precambrian. The distribution of the bodies of the East Bull Lake intrusive suite approximates the base of the Huronian Supergroup in the Southern Province (see Figure 1). Contact relationships between rocks of the East Bull Lake intrusive suite and the Huronian Supergroup are either faulted or equivocal. Consequently, it is not known for certain if the East Bull Lake intrusive suite intruded the Huronian Supergroup, or was unconformably overlain by it, or both (e.g., Card 1978; McCrank et al. 1989; Peck, James and Chubb 1993). Emplacement of the bodies of the East Bull Lake intrusive suite, subsequent eruption of volcanic rocks of the Huronian Supergroup, and formation of the depositional basin filled by Huronian Supergroup sediments is attributed by most authors (e.g., Card et al. 1972; Young 1983; Fahrig 1987; Hoffman 1989; Bennett, Dressler and Robertson 1991; Heaman 1997; Card and Poulsen 2000; Ernst and Buchan 2001) to an intracontinental rifting event resulting from a mantle-plume centred near Sudbury. A suite of igneous rocks (hereafter called the “rifting suite”) records the initial trace of this plume-induced rifting event; from oldest to youngest, they include: 1. The East Bull Lake intrusive suite (~2490 to 2470 Ma). 2. Elliot Lake Group metavolcanic and minor plutonic rocks, for example, the Elsie Mountain, Stobie and Copper Cliff formations, May Township metavolcanic rocks, and minor synvolcanic mafic intrusions (~2490 to 2450 Ma). Metavolcanic rocks of the Elliot Lake Group constitute the lowermost of 4 stratigraphic groups in the Huronian Supergroup. 3. Matachewan (2473 Ma) and Hearst (2446 Ma) mafic dike swarms (Heaman 1997). 4. Granitic intrusions within the Elliot Lake Group and the Superior Province (~2385 to 2460 Ma), including the Creighton and Murray plutons and the Street Township granitic plutons that may be coeval with Elliot Lake Group felsic volcanism. 3 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x < x 2473 >Matachewan x x x x x x Hearst x x x x -x Province x x X X x X x x x x x X x x x /x x )< / x x xI x A x,'x x I x X )< x X x x X x X x X x x 2681 ±13 x x X / x + )( + )< x + < > x + + ++ Province i- + > + + )< ) x X x +25 + -r —In. - + + + + + + + + + + + + + + - + + - + + + + + + +++ + + + + + + + + + + + + + + + + + + 2475±2+ + • •+ + + - + + - Lake Nipissing + 1+ East Bull Lake intrusion tonalite gneiss Agnew Lake intrusion Archean basement Falconbridge Twp. intrusion ® MayTwp. River Valley intrusion ® Levackgneiss Sudbury Igneous Complex intrusion plutonism + metamorphism Copper Cliff rhyolite 8 (fl\ Drury Twp. '—' intrusion 1 Temagami Island Gabbro + + Flett Twp. intrusions Grenville Province rocks Wisnerlwp. Huronian metasedimentary rocks intrusion Huronian volcanic rocks southern Wanapitei complex Street Twp. intrusion EBLI suite rocks x Superior Province rocks Figure 1. Distribution of U-Pb zircon ages from Neoarchean and Paleoproterozoic rocks in the Sudbury area, as well as the distribution of East Bull Lake intrusive suite bodies. Dashed circle indicates 125 km radius from a point central to the Sudbury Structure; most intrusions lie within this radius. Sources used: 1, 2, 8, 9) Krogh, Davis and Corfu 1984; 4) Wodicka and Card 1995; Krogh, Davis and Corfu 1984; 5) Chen, Krogh and Lumbers 1995; 6 and 10) Prevec 1993; 7) Heaman in Easton, Davidson and Murphy 1999; below and above circle 16, Corfu and Easton (2000). Age of Hearst and Matachewan dikes from Heaman (1997). Abbreviations: GFTZ = Grenville Front tectonic zone; EBLI = East Bull Lake intrusive suite. Current geochronological data suggests 2 main pulses in magmatism. 1. A predominantly mafic pulse at ~2475 Ma, represented by the East Bull Lake intrusive suite, Matachewan dikes, and the mafic part of the Elliot Lake Group. 2. A felsic-dominated pulse at ~2450 Ma, represented by felsic metavolcanic rocks of the Elliot Lake Group, related granitic intrusions, and Hearst dikes. The rifting event produced an east-trending, southward-deepening basin that likely resulted in formation of oceanic crust (e.g., Card et al. 1972; Young 1983; Fahrig 1987; Hoffman 1989; Bennett, Dressler and Robertson 1991). The geometry of the Matachewan and Hearst dikes and their distribution with respect to the metavolcanic rocks of the Elliot Lake Group is consistent with the north-northwest trend of the dikes reflecting the orientation of a failed arm of the rift (e.g., Fahrig 1987; Ernst and Buchan 2001). If this geometry is correct, then the east-trending East Bull Lake intrusive suite bodies not only occur along the continent-side of the rift basin, but also within a 125 km radius of a plume centered on Sudbury (see Figure 1). At present, it is uncertain if the rocks of East Bull Lake intrusive suite occur directly along a cryptic structure, or structures, related to this rift basin, or if the present distribution of the bodies reflects some other emplacement mechanism (James et al. 2002a). 4 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �5 PDF compression, OCR, web-optimization with CVISION's PdfCompressor Size (km2) Dominant Rock Types 15 ~5 May Twp. or Salmay Lake Wisner Twp. 43 East Bull Lake, includes outliers at Splake and Tee Lakes 1-2 8-10 Drury Twp. or Worthington Falconbridge Twp. or Norduna 50 Agnew Lake or Shakespeare –Dunlop Leucogabbronorite Leucogabbro, gabbro, melanogabbro. Leucogabbronorite gabbronorite. Leucogabbronorite gabbronorite, olivine gabbronorite. Leucogabbro, gabbro. Leucogabbronorite gabbronorite, olivine gabbronorite. Southern Province hosted intrusions Intrusion Shock metamorphosed, upper greenschist? Lower to upper greenschist Shock metamorphosed, upper greenschist Lower to upper greenschist Shock metamorphosed, locally upper greenschist Upper greenschist to lower amphibolite Metamorphic Grade n = 8 major and trace element analyses, REE and isotope data also available n = ~15 major and trace element analyses, REE and isotope data also available n = ~5 major and trace element analyses, REE and isotope data also available n = ~200 major and trace element analyses, REE and isotope data also available n = ~100 major and trace element analyses, REE and isotope data also available Geochemisty 5 No data No data Plag = An60-80 Rhythmically layered zone: ol = Fo59-65, opx = En64-66Fs30-32Wo4, cpx = En41-45Fs15-16 Wo39-44 Olivine gabbronorite zone: ol = Fo65-74, opx = En67-706Fs26-29Wo4, cpx = En41-43Fs14-15 Wo43-44 Plag = An60-76 No data Plag = An2-79 Primary Mineral Chemistry Unknown Contacts typically zones rich in Sudbury breccia. Intrusion in contact aureole of the Sudbury Igneous Complex. Melanogabbro better preserved than leucocratic rocks. Ramsey-Algoma granitoid complex rocks in footwall, diking and brecciation in footwall at contacts, to east, possible unconformable contact with Huronian Sgp. metasedimentary rocks. Contacts typically zones rich in Sudbury breccia. Intrusion in contact aureole of the Sudbury Igneous Complex. Parisien Lake syenite, Whiskey Lake greenstone belt, RamseyAlgoma granitoid complex rock, diking and brecciation in footwall at contacts, possible unconformable contact with Huronian Supergroup metasediments to the north. Contact relationships Unknown Unknown Not defined Not defined Not defined Not defined ~850 m Unknown Not defined Not defined Location of Reference Sections, UTM Zone 17, NAD 83 Unknown at least 1000m, as much as 2100 m in some interpretations Stratigraphic Thickness Table 1. Summary of stratigraphic, mineralogical and geochemical data for the East Bull Lake intrusive suite (modified from James et al. 2002a; Easton 2003). Vogel, James and Keays 1998 Cape 1973, Robertson 1976, Prevec 1993 DeGagne 1982, Prevec 1993 Born 1979; James and Born 1985; McCrank et al. 1989; Peck, James and Chubb 1993; Peck et al. 1995; Prevec 1993, Vogel , James and Keays 1998 Brons 1984; Prevec 1993 Vogel 1996; Vogel, James and Keays 1998; Vogel et al. 1999 References �6 PDF compression, OCR, web-optimization with CVISION's PdfCompressor Size (km2) Dominant Rock Types ~25 ~5 ~5 Contact Third Concession ~100 Loughrin Lake River Valley Gneissic leucogabbronorite, gabbronorite, associated orthopyroxene hornblendite. Gneissic, leucogabbronorite, gabbronorite, associated orthopyroxene hornblendite. Gneissic leucogabbronorite, gabbronorite, associated orthopyroxene hornblendite. Leucogabbronorite gabbronorite, olivine gabbronorite. Grenville Province hosted intrusions Intrusion Table 1. continued Upper amphibolite Upper amphibolite Upper amphibolite Greenschist to upper amphibolite, with some areas of primary mineralogy preserved, mainly in Dana Township. Metamorphic Grade n = 1 major and trace element and REE analyses No data n = 12 major and trace element analyses, REE data also available n = ~70 major and trace element analyses, REE data also available Geochemisty 6 No data No data No data Marginal zone: Plag = An79 (core); An68, (intercumulate) Olivine gabbronorite zone: ol = Fo74 plag = An70-75 opx = En60-80 Primary Mineral Chemistry Primary Contacts: Dana Twp.: Sharp contact with paragneiss or alkali feldspar granite, no dikes in footwall, marginal zone contains footwall xenoliths. Fault contacts with Huronian Sgp. metasedimentary and metavolcanic rocks, equivocal contact with Nipissing gabbro. Crerar Twp: Sharp contact with alkali feldspar granite, alteration of granite in footwall, fine-grained phase at contact, no dikes in footwall, marginal zone contains footwall xenoliths. Hosted by Crerar gneiss association. North contact mainly tectonic. South contact sharp, with scattered orthopyroxene hornblendite bodies. Hosted by Street gneiss association. North contact locally cut by Geon 24 to 23 felsic intrusions. South contact sharp, with scattered orthopyroxene hornblendite bodies. Hosted by Street and Crerar gneiss association. Same as Contact intrusion. Both contacts locally intrude migmatitic garnet amphibolite. Hosted by Street gneiss association. Contact relationships ~500 m less than 500 m ~500 to 1000m ~1000 m Stratigraphic Thickness Not defined Not defined Marginal zone: 555310E, 5172625N; 556610E, 5174225N to 556510E, 5171225N; 558760E, 5167825N to 558510E, 5167475N. Olivine Gabbronorite zone: 559260E, 516825N; 563260E, 5166225N; 560410E, 5167825N. Gabbronorite zone: 558510E, 5165575N Not defined Location of Reference Sections, UTM Zone 17, NAD 83 Easton 2003 Easton 2003 Easton 2003 Easton and Hrominchuk 1999; Hrominchuk 1999, 2000; Easton 2003 References �7 PDF compression, OCR, web-optimization with CVISION's PdfCompressor ~5 ~10 ~5 Street metagabbro Red Deer Lake southern Wanapitei complex Gneissic leucogabbro, gabbro, norite. Gneissic leucogabbronorite, gabbronorite. Leucogabbronorite gabbronorite, norite, associated orthopyroxene hornblendite. Dominant Rock Types Upper amphibolite Upper amphibolite Upper amphibolite Metamorphic Grade n = 8 major element analyses n = 1 major element analysis n = 25 major, trace element and REE analyses Geochemisty Coronitic olivine present No data Coronitic orthopyroxene present Primary Mineral Chemistry Unknown Relative Ages River Valley Street Twp. River Valley Drury Twp. Falconbridge Twp. Intrusion Absolute Ages Agnew Lake East Bull Lake Rb/Sr whole-rock >2446±3 >2460±20, 2475+25/-10 Ashwal and Wooden (1989) Pb/Pb whole-rock Nd/Sm whole-rock and mineral 2562±165 2377±68 2165±130 2185±105, 1960±100 2468±5 U/Pb baddeleyite and zircon U/Pb zircon 7 Heaman (1997) Corfu and Easton (2000), Easton and Hrominchuk (1999) Corfu and Easton (2000) Krogh, Davis and Corfu (1984) Krogh, Davis and Corfu (1984) McCrank et al. (1989) Kamineni (1986) McCrank et al. (1989) McCrank et al. (1989) Prevec (1993) Prevec (1993) Prevec (1993) Heaman, unpublished in Easton, Davidson and Murphy (1999) Ashwal and Wooden (1989) Ashwal and Wooden (1989) U/Pb zircon U/Pb baddeleyite and zircon Nd/Sm whole-rock and mineral Ar/Ar hornblende K/Ar whole rock K/Ar whole rock U/Pb zircon U/Pb zircon U/Pb zircon U/Pb baddeleyite and zircon 2491±5 2480+10/-5 2472±76 1859±5 1725±18 1155±14 1855±10 2441±3 ~1850 2475+2/-1 U/Pb zircon Reference Method Age (in Ma) Unknown Unknown less than 500 m Stratigraphic Thickness Not defined Not defined Location of Reference Sections, UTM Zone 17, NAD 83 530510E, 5155225 to 5155725N Rousell and Trevisiol 1988 Lumbers 1975 Easton 1999, 2003; Easton and Murphy 2002 References Dikes of the Matachewan dike swarm cut the River Valley intrusion. Felsic dikes likely related to felsic intrusions coeval with Huronian Stobie Formation cut the River Valley intrusion. age from orthopyroxene hornblendite body thought to be genetically related to East Bull Lake intrusive suite. n=4, second age is resetting event. lower intercept, dates Sudbury event. from layered gabbronorite zone, either in Erana or Dana quarries, north of Sturgeon River. n=6, igneous textured samples only. n=4, igneous textured samples only, second age is resetting event. from alkali feldspar granite that may not be part of the intrusion. from olivine gabbronorite zone, few grain, hence high error. from olivine gabbronorite zone. no details on location. from olivine gabbronorite zone. from olivine gabbronorite zone. shocked zircons, dates Sudbury event. Comment Basal contact intrudes migmatitic garnet amphibolite, ~2475 Ma granites contain fragments of intrusion. Hosted by Street gneiss association. Unknown Contact relationships Table 2. Summary of geochronology on rocks of the East Bull Lake intrusive suite (from Easton 2003). Size (km2) Intrusion Table 1. continued �Table 3. Timing of major geological events and summary of age constraints on the main rock units present in the study area (from Easton 2003). Event and/or Map Unit Age Constraint (in Ma) Comment and/or Source Grenville dike swarm 586±4 Kamo, Krogh and Kumarapeli (1995) Pegmatite vein emplacement 989±2 Corfu and Easton (2000) Age of peak metamorphism in the hanging wall of the Grenville Front tectonic zone 1000 to 990 Corfu and Easton (2000) Age of peak Grenvillian metamorphism in Crerar and Dana townships 1040 and 1030 Sudbury dike swarm 1238±4 emplaced in or along northwest-trending faults in the Southern Province. Deformed and metamorphosed within the Grenville Province. Krogh et al. (1987). Killarney magmatic belt second-stage magmatism, coincident with magmatism in the Eastern Granite Rhyolite Province and in the Central Gneiss Belt 1471±3 van Breemen and Davidson (1988) Regional albitization metasomatic event 1701±4 U/Pb monazite, Schandl, Gorton and Davis (1994); fluid focussed along northwesttrending faults Killarney magmatic belt volcanism and high-level plutonism 1740, 1747±3, 1749±12 van Breemen and Davidson (1988); Sullivan and Davidson (1993); Davidson and van Breemen (1994) Northwest-trending regional faults Pre-1700, post-1850 Penokean Orogeny (folding and metamorphism of Huronian Supergroup rocks) ~1835 Peak metamorphism. Holm et al. (2001). Impact event and formation of Sudbury breccia 1850±1 Krogh, Davis and Corfu (1984) Thrust faulting post-F2 pre-regional faulting Sudbury breccia localized along these faults, suggesting they are pre-Sudbury Structure F2 folding post-2200, pre-1700, pre 1850? Pre-regional faulting, Nipissing sills folded, relationship to Sudbury Structure uncertain F1 folding Pre-2200 Emplacement of Nipissing gabbro sills 2219±4 to 2210±4 Huronian sedimentation >2220 but <2460 Huronian felsic volcanism and related plutonic rocks ~2477 to 2375 (2450±25, 2460±20, 2477±9, 2415±5, 2376±2) Emplacement of East Bull Lake intrusive suite rocks 2475±2 see also Table 2 Emplacement of orthopyroxene hornblendite bodies 2468±5 Emplacement of alkali feldspar granite bodies in Crerar and Dana townships 2640 High-grade Archean metamorphism and migmatization 2647±4 Emplacement of megacrystic granodiorite bodies in Crerar gneiss association 2660 Emplacement ages of Archean units in the Sudbury area 2711±7 to 2642±1 this study; see also Carr et al. (2000) Faults cut Sudbury Structure Nipissing sills in Street Township appear unaffected by this folding Corfu and Andrews (1986); Noble and Lightfoot (1992) Krogh, Davis and Corfu (1984), Corfu and Easton (2000), Krogh, Kamo and Bohor (1996), Smith (2002) Heaman (geochronologist, University of Alberta, personal communication, 1999) Corfu and Easton (2000) Bodies intrude Crerar and Pardo gneiss, this study Krogh, Davis and Corfu (1984); Wodicka and Card (1995) this study Krogh, Davis and Corfu (1984); Wodicka and Card (1995); Chen, Krogh and Lumbers (1995); Meldrum et al. (1997) 8 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Within the Southern Province, rocks of the East Bull Lake intrusive suite are affected by folding and upper greenschist to lower amphibolite facies metamorphism associated with the Penokean Orogeny, likely around 1835 Ma (Holm et al. 2001; see also Table 3). Within the Grenville Province, rocks of the East Bull Lake intrusive suite may have been locally affected by metamorphism and deformation at ~1700 Ma and ~1450 Ma, in addition to the main pulse of Grenville orogenesis at ~1070 to 1040 Ma (Corfu and Easton 2000). OVERVIEW OF EAST BULL LAKE INTRUSIVE SUITE Table 1 lists all known major members of the East Bull Lake intrusive suite along with notable features common to all and a listing of all studies performed to date. The distribution of the bodies is illustrated in Figure 1. Table 2 summarizes the geochronology of the intrusive suite. The 3 largest intrusions are the East Bull Lake, Agnew Lake, and River Valley intrusions, all of which have been mapped in the last 25 years at either 1:10 000 or 1:20 000 scale in whole or in part (Born 1979; Kamineni et al. 1984; Chubb 1994; Peck et al. 1995; Vogel 1996; Easton and Hrominchuk 2001a, 2001b), with accompanying petrological, geochemical or economic studies (James and Born 1985; McCrank et al. 1989; Chubb 1994; Peck, James and Chubb 1993; Peck et al.1995; Vogel 1996; Vogel, James and Keays 1998; Vogel et al. 1998, 1999; Easton and Hrominchuk 1999; Hrominchuk 1999, 2000; Peck et al. 2001; James et al. 2002a, 2002b). The preserved size of the intrusions varies from 1 to ~100 km2 (Table 1). The most completely preserved body is the Agnew Lake Intrusion with approximately 2 km of stratigraphic section; both the East Bull Lake Intrusion and River Valley intrusion contain roughly 1 km of section. The East Bull Lake, Agnew Lake and River Valley intrusions are sporadically layered at centimetre- to decametre-scale throughout their stratigraphy. Leucogabbronorite and gabbronorite are the dominant rock types in the lower and middle parts of most bodies of the East Bull Lake intrusive suite. Olivine gabbronorites and leucogabbronorites are common in the Main Series of the East Bull Lake intrusion and the Lower Series of the Agnew Lake Intrusion. Melagabbronorites and troctolites are common in the lower parts of the River Valley intrusion. Ferrogabbros, ferrosyenites and alkali granites form the top 150 m of the Agnew Lake Intrusion. The significant volume of melanocratic norites and troctolites recognized in the River Valley Intrusion is not present in those intrusions west of the Grenville Front, and may indicate that the former represent a deeper part of the stratigraphy of this intrusive suite. The crystallization order of primocryst phases in most of the intrusions is plagioclase, olivine, orthopyroxene, clinopyroxene and titanomagnetite. The attitude of metre-scale phase layering and stratigraphic units in the East Bull Lake and Agnew Lake intrusions suggest that they represent lopoliths joined by dike-like units (James et al. 2002a). Original geometry of the bodies present in the Grenville Province cannot be reliably ascertained, although the northeast part of the River Valley intrusion appears to be a shallowly-dipping (10 to 30°) sheet. Without exception, breccias containing footwall and mafic cognate xenoliths in a gabbronorite matrix occur at the base of all intrusions, where the footwall contact is preserved. The thickness and abundance of these breccia units varies considerably. Also near the contact, footwall breccias and zones of extensive footwall dikes may be present. Disseminated chalcopyrite and pyrrhotite, typically in modal amounts from 0.5 to 2.0%, occur in the matrix of the marginal and brecciated rocks. This sulphide mineralization commonly contains between 1 and 5 g/t of combined Pt+Pd+Au, and is the focus for current mineral exploration (James et al. 2002a, 2002b). 9 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �DEPTH AND MECHANISM OF EMPLACEMENT Researchers studying East Bull Lake suite intrusions in the Southern Province have suggested shallow depths of emplacement (<8 km), possibly at the contact between the Archean basement and overlying supracrustal rocks of the Huronian Supergroup and its precursors (e.g., Card 1978; Peck et al. 1995; Vogel et al. 1998, 1999). In contrast, researchers studying the River Valley intrusion have favoured moderate to deeper depths of emplacement (8 to 12 km), either at the boundary between Archean plutonic rocks and greenstone belts or at the boundary between gneisses and a plutonic rock-dominated midcrustal layer (e.g., Easton 2000; J.L. Hrominchuk, personal communication, 2000). Easton (2002) went further, suggesting that several depths of emplacement are recorded within the East Bull Lake intrusive suite, based on the fact that the intrusive bodies display subtly different features depending on which country rock gneiss association they are in contact with. Two models have been proposed with respect to the emplacement mechanism of the East Bull Lake intrusive suite; both are discussed in James et al. (2002a), and are summarized here. The first model has been proposed specifically for the area extending from Elliot Lake to Sudbury. Based on geologic reconstruction and “unfolding” of the effects of Penokean deformation on the East Bull Lake and the Agnew Lake intrusions, and their similar stratigraphy, it has been proposed that the middle and upper parts of both these intrusions were part of an extensive, subhorizontal sheet. This sheet may have been approximately 2 km thick, representing the upper 75% of a lopolith that had horizontal dimensions of 30 to 50 km in a northerly direction and greater than 100 km in an easterly direction (James et al. 2002a; Vogel et al. 1999). In this model the lower and mineralized sections of the intrusions occupied rift faultrelated structures or embayments that controlled magma intrusion, replenishment, and mineralization, and thus, may not have been as widespread as the main mass of the lopolith. Furthermore, the mineralized zones would not have been connected. This model has been expanded to include the possibility that the East Bull Lake, Agnew Lake, and River Valley intrusions, as well as the intervening smaller intrusions (see Figure 1, Table 1), are the remnants of one large interconnected Great Dike-like lopolith that can be traced for at least 300 km across central Ontario (see Figure 1) (James et al. 2002a). If so, then small size is not a concern in terms of modelling the potential size of mineralized sections, and the area of potential interest for exploration is much larger. The second model proposes that the East Bull Lake intrusive suite was emplaced as a group of smaller, separate bodies, rather than as one or two large, now dismembered, sill complexes (James et al. 2002a). One consequence of this model is that the various bodies could have been emplaced at different crustal levels. This model would also provide an explanation for the differences in stratigraphy between the various intrusions, as not every intrusion would differentiate to the same degree. Finally, an important implication of this model is that mineralization style and grade will vary between intrusions. MAGMA COMPOSITION AND ITS RELATIONSHIP TO MINERALIZATION Detailed studies (Peck et al. 1995, 2001; Vogel et al., 1999; James et al. 2002a, 2002b) demonstrate that the magma(s) that formed the East Bull Lake and the Agnew Lake intrusions were high-Al, low-Ti (Al2O3 ~17.5 wt %; TiO2 ~0.5 wt %) tholeiites. In these 2 intrusions, the main fractionation trend formed by leucogabbronorite and norites is interrupted by emplacement of more primitive (less fractionated), olivine-rich rocks (the Olivine Gabbronorite zones) which form a distinctive geochemical pattern not easily attributed to the same magma source. 10 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �For the River Valley intrusion, at least north of the Sturgeon River, its stratigraphy, including the Olivine Gabbronorite, Gabbronorite and Leucogabbronorite zones, could be attributed to a magnesian variant (8 to 10 % MgO) of the high-Al, low-Ti, tholeiite (James et al. 2002a). Such magma would have olivine followed by plagioclase on the liquidus, which satisfies the petrographic data for these rocks. The magma giving rise to the sporadically mineralized orthopyroxene-phyric norite and gabbronorite that is the main component of the Marginal Zone of the River Valley intrusion is part of an earlier magmatic sequence: its pyroxene-phyric, olivine-absent character and disseminated PGE mineralization distinguish it from norites and gabbronorites higher in the stratigraphy (James et al. 2002a). The magmatic evolution of River Valley intrusion, however, is not as well constrained as it is for the East Bull Lake and Agnew Lake intrusions. As noted by Hrominchuk in James et al. (2002a), a unit of massive, medium-grained, orthopyroxene-phyric norite with a boninitic composition (MgO ~11.5 wt %, Al2O3 ~13.5 wt. %, SiO2 ~51.5 wt %), located near the southeast margin of the River Valley intrusion, could represent the parent magma of the intrusion. Furthermore, Hrominchuk argued that the Olivine Gabbronorite, Gabbronorite and Leucogabbronorite zones were derived from this boninitic parental magma by a combination of fractional crystallization and mixing of variably fractionated magma batches within the magma chamber. With the available analytical data, it is equally plausible, however, that the Olivine Gabbronorite, Gabbronorite and Leucogabbronorite zones of the River Valley intrusion formed from a high-Al, low-Ti magma similar to that which produced the East Bull Lake and the Agnew Lake intrusions. The boninitic magma may have only been injected near the base of the intrusion to form the Marginal and Inclusion and/or Fragment-bearing zones (James et al. 2002a). An argument against a boninitic parental magma is that such magmas are currently found only in ocean island arc settings (e.g., Bonin islands, Japan, Papau New Guinea), unlike the inferred continentalrift plume-setting for the East Bull Lake intrusive suite. Nonetheless, boninites are thought to form from a relatively high degree of partial melting of previously melted metasomatized lithospheric mantle (Hamlyn et al. 1985). Metasomatism is generally thought to occur from fluids derived from dehydration of a subducted slab of oceanic crust (Hickey and Frey 1982) and can produce light rare earth element (LREE) enrichment in the resulting melt. Alternatively, crustal contamination of a second-stage mantle melt can produce LREE enrichment in an otherwise primitive melt (Vogel et al. 1999). Vogel et al. (1999) found crustal contamination to be an unlikely mechanism for the uniformly elevated LREE in the Agnew Lake Intrusion, and suggested that the parent magma must have come from a partial mantle melt enriched by LREE-rich fluids. In the case of the River Valley intrusion, the LREE and trace element patterns are much less uniform and contamination is an important process, especially in the Marginal and Inclusion and/or Fragment-bearing zones (Hrominchuk 2000), which is one reason to favour a boninitic parental magma (James et al. 2002a). Regardless, both the proposed high-Al, low-Ti and boninitic parental magmas are second-stage, mantlederived magmas as described by (Hamlyn and Keays 1986). Hamlyn and Keays (1986) argued that second-stage magmas are critical to the formation of PGE deposits, a view also favoured in James et al. (2002a, 2002b). These fertile magmas arise from previously depleted mantle, typically in areas underlying Precambrian cratons or island arcs. Both the interpreted high-Al and boninitic parental magmas of the East Bull Lake intrusive suite are PGE fertile (Pt+Pd ~20 to 50 ppb) and sulphur-poor (~100 to 200 ppm), and together with their continental rift-margin tectonic setting, are good examples of second-stage magmas. Their low sulphur-content likely reflects prior extraction of MORB-type basalt liquids from the mantle, which removed most of the mantle sulphide phase, but left a small amount of a PGE-rich residual sulphide phase (Keays 1995). Pd and Se data from barren cumulates and possible feeder dikes from the East Bull Lake Intrusion (Peck et al. 2001) illustrates the PGE-rich nature of the parental magmas compared to first stage (MORB-type) magmas derived from fertile mantle, which have significantly higher S(Se) and lower PGE/S(Se) ratios. 11 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �In terms of crust-mantle scale processes, James et al. (2002a) suggested that fractional crystallization of a mantle-derived, second-stage magma in the lower to middle crust generated PGE-enriched, sulphurundersaturated leuconorite or norite magmas in the upper part of a zoned ultramafic to mafic magma chamber. During ascent and intrusion into the upper crust, this buoyant residual magma (parent to much of the marginal parts of the East Bull Lake and Agnew Lake intrusions) became sulphur-saturated, and apparently escaped from the more primitive subchamber without the loss of the sulphide phase. In this model, rocks of the more primitive Olivine Gabbronorite zone in the East Bull Lake and Agnew Lake intrusions represent influxes of magma from the more mafic parts of the same magmatic system. They should be relatively barren in PGE, except where mixing with fractionated liquids from the high-Al magmas may have generated reef-type deposits. Similar layered olivine-rich rocks in the River Valley intrusion appear to be part of a fractionating system that forms norites and leucogabbros in the upper part of this body (James et al. 2002a). Detailed studies on PGE mineralization have been done only on the East Bull Lake Intrusion. Many fine-grained gabbros in the Marginal Series and feeder-type plagioclase-phyric dikes in the footwall, have 20 to 100 ppb Pt+Pd (Pd/Pt = 2:1 to 1:1) and contain small (<1%) amounts of fine-grained disseminated sulphide minerals (pyrite ± pyrrhotite ± chalcopyrite). These PGE and sulphide contents are interpreted as evidence that early magma pulses were sulphur-saturated when they entered the magma chamber (James et al. 2002a). The PGE values in these rocks far exceed those expected for basaltic liquids, even those derived from depleted mantle sources (Hamlyn and Keays 1986; Keays 1995), which is further evidence for sulphur saturation. Vigorous convection within the magma chamber, as evidenced by the Inclusion-bearing and Anorthosite zones at the East Bull Lake Intrusion, produced high silicate/sulphide liquid mass ratios that allowed growing sulphide liquid droplets to attain the observed high PGE contents (1 to 10 g/t) (James et al. 2002a). ORTHOPYROXENE HORNBLENDITE BODIES Bodies of orthopyroxene hornblendite consist of orthopyroxene (bronzite) phenocrysts, ranging from 0.5 to 5 cm in length, hosted in a fine-grained amphibole-plagioclase matrix (Photo 1). These bodies occur throughout Awrey, Dryden, Henry, Loughrin and Street townships (Easton 1998, 1999, 2002, 2003; Easton and Murphy 2002; Davidson 1998). Large glacial erratics of orthopyroxene hornblendite, some up to 6 m across, are common in Henry, Loughrin and Street townships. The erratics are generally found within several hundred metres of observed bodies in outcrop, or in areas where bodies may be hidden beneath overburden. In most of these aforementioned townships, the orthopyroxene hornblendite bodies are found in close proximity to mafic intrusive rocks of the East Bull Lake intrusive suite. Zircon from an orthopyroxene hornblendite body in Street Township has been dated at 2468±5 Ma (Corfu and Easton 2000), similar in age to other intrusive rocks of the East Bull Lake intrusive suite (James et al. 2002a). Even though some orthopyroxene hornblendite bodies occur within 100 m of the Grenville Front (Easton and Murphy 2002; Hubbard 1998), equivalent rocks to these bodies have not been reported in conjunction with East Bull Lake intrusive suite bodies located within the Southern Province. No orthopyroxene hornblendite bodies have been found in association with the River Valley intrusion, although some ultramafic layers present within the River Valley intrusion in Crerar Township are chemically similar to rocks of the orthopyroxene hornblendite suite, even though they are texturally and mineralogically distinct. Easton (2002) suggested that the orthopyroxene hornblendite bodies are restricted to country rocks of the Street gneiss association, although some bodies do occur in association with other East Bull Lake intrusive suite rocks along the boundary between the Street and Crerar gneiss associations. This would explain the absence of these rocks in the Southern Province and in association with the River Valley intrusion, as well as their abundance in Awrey, Dryden, Henry, Loughrin and Street townships, all of which are underlain by rocks of the Street gneiss association. 12 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �—I-', Photo 1. Orthopyroxene phenocrysts in matrix-rich orthopyroxene hornblendite in Henry Township. Hammer handle is 30 cm long. UTM 547314E 5158815N. Easton and Murphy (2002) suggested that these bodies exhibited only 2 types of contact relationships; however, work by Easton (2003) suggests that there might be 4 types of contact relationships. These contact types are summarized below: 1. The first type occur near sharp, possibly modified igneous contacts between metagabbroic and metaleucogabbroic rocks of the East Bull Lake intrusive suite and their host gneisses (Street gneiss association). Bodies of this type appear to have been emplaced into pre-existing embayments along the metagabbro-gneiss contact. All of the Street Township bodies are of this type. 2. The second type occurs within deformation zones characterized by flattened gneissic rocks (e.g., the body exposed on the south side of Highway 17 in Awrey Township, Stop 1, Day 3). 3. Some bodies in Loughrin Township, show a linear distribution and may represent boudinaged dikes. It is possible that types 2 and 3 are related, with type 2 representing a more deformed version of type 3. 4. Many of the larger bodies found in Henry and Loughrin townships, are not clearly associated with the contact of any of the East Bull Lake intrusive suite bodies. Some are located over 1 km from the contact. The field term for these rocks, orthopyroxene hornblendite, is based on the dominant mineral phases observed in hand sample. Some of the better-preserved bodies contain relict olivine grains. Mesonorm calculations suggest a metamorphic mineralogy dominated by pyroxene and amphibole consistent with the observed mineralogy in the field. CIPW normative calculations for these rocks suggest a primary mineral assemblage dominated by olivine (4 to 22%), hypersthene (32 to 62%), diopside (3 to 22%) and calcic plagioclase (13 to 21%) (Easton and Murphy 2002). The CIPW normative mineralogy suggests 13 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �that these rocks were originally olivine gabbronorites or olivine websterites, depending on whether they contained moderate or only minor amounts of plagioclase, respectively. Orthopyroxene hornblendite bodies located near the Grenville Front contain euhedral bronzite grains up to 5 cm long. The bronzite grains become subrounded to rounded and overall grain size decreases with increasing distance from the Grenville Front. Similar observations can be observed from the margin to the core in some of the smaller hornblendite bodies. This change in bronzite form with increasing distance into the Grenville Province can be attributed to greater degrees of metamorphic recrystallization toward the interior of the Grenville Province. Mineralogically, the orthopyroxene crystals in the bodies are complex and represent primary phenocrysts that recrystallized during regional metamorphism. Evidence that they were primary phenocrysts comes from the presence of zircon and chromite inclusions within the bronzite grains. Both types of inclusions contain well-developed cores and rims. In the case of the zircon grains, subrounded cores are surrounded by euhedral fractured rims. The cores yielded the aforementioned U/Pb age of 2468±5 Ma, whereas the rims yielded an age of 1471±10 Ma (Corfu and Easton 2000). Chromite cores are rimmed by iron chromite that typically forms only during regional metamorphism (Deer, Howie and Zussman 1966). Small euhedral zircons are also present in the fine-grained amphibole matrix and yielded an age of 1052±19Ma (Corfu and Easton 2000). Mineral chemistry data on the matrix amphiboles indicates that they are magnesium hornblende, tremolite, cummingtonite and magnesium cummingtonite. The amphiboles occur both as individual grains and complexly exsolved and intergrown grains. Olivine grains from a body located near the Grenville Front have compositions of Fo73. Orthopyroxene compositions lie in the bronzite field and are magnesium-rich (En75-80) (Easton and Murphy 2002; Hubbard 1998). Geochemistry Analyses from 11 separate bodies show only a limited compositional range in major element chemistry (Easton 2003). The SiO2 content of the unaltered orthopyroxene hornblendites ranges from 46.0 to 50.5 weight percent which is unusually high given the corresponding MgO content of 21.0 to 27.0 weight percent. On a Jensen diagram the data from the orthopyroxene hornblendites plot as a cluster in the peridotitic komatiite field although there is no evidence to suggest that these rocks were extrusive. Plots of Ti versus Cr and Cr-Ni are consistent with these rocks being crystallized from layered mafic sills. Trace element data from orthopyroxene hornblendite samples plot in 2 distinct groups on chondritenormalized rare earth element (REE) diagrams (Easton 2003). One group has a relatively flat, 10 times chondrite pattern with a negative Eu anomaly and total REE less than 35 ppm, whereas the second group has moderate light REE enrichment (to 60 times chondrite), no Eu anomaly, and total REE greater than 80 ppm. The REE content and high silica content of the orthopyroxene hornblendites suggests that they do not represent the most primitive rocks of the East Bull Lake intrusive suite. The first group, characterized by low total REE contents and magnesium contents greater than 20 weight percent, occur as isolated bodies, near, but not intimately associated with other East Bull Lake intrusive suite rocks. In contrast, the second group with the higher REE contents and lower magnesium contents (<19.0 wt. %) occur at primary intrusive contacts, or as layers within East Bull Lake intrusive suite bodies. For example, hornblendite samples from the Street metagabbro plot in the LREE enriched group, as do olivine gabbronorites and pyroxenites from the River Valley intrusion. It can be argued that the two groups are related to one another, with the lower magnesium group being a more fractionated differentiate of the magma that produces these unusual rocks. 14 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �All unaltered orthopyroxene hornblendite samples contain from 2900 to 4700 ppm Cr and 750 to 1000 ppm Ni. Copper content is generally low (Easton 2003). Chromium is present as chromium-spinel, iron-chromite and chromite and is concentrated within olivine and orthopyroxene crystals. Platinum group element content in all of the analyzed orthopyroxene hornblendite samples is low (generally at or below detection limits), regardless of degree of alteration. The orthopyroxene hornblendites differ from other rocks of the East Bull Lake intrusive suite in their high chromium and nickel contents, as listed above. One explanation for this difference is that precipitation and removal of pyroxene from the primary magma (possibly by crystal settling) increases the silica content of the melt. The increase in silica would move the composition of the melt away from the olivine-chromite cotectic into the field of chromite and lead to the co-precipitation of bronzite and chromite. This process, in conjunction with olivine precipitation, would effectively strip the primary magma of much of its chromium and nickel, leaving rocks of the East Bull Lake intrusive suite with low chromium and nickel contents. Chromium numbers of the orthopyroxene hornblendites are 20 to 38 and magnesium numbers are 78 to 82. The bodies have chemical affinities to high-aluminium chromitite from ophiolitic complexes, most notably, chromium number less than 50, TiO2 greater than 0.2 weight percent, CaO greater than 1.5 weight percent, and Al2O3 greater than 1.2 weight percent. High aluminium chromitites are found in back-arc spreading and rifting tectonic environments that are consistent with previous suggestions of a rift-setting for the lower Huronian Supergroup (e.g., Bennett, Dressler and Robertson 1991). 15 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Field Trip Guidebook EAST BULL LAKE INTRUSION—OVERVIEW Country Rocks, Structure and Geometry of the Intrusion The East Bull Lake Intrusion, located roughly 80 km west of Sudbury (see Figure 1), is the type example of the East Bull Lake intrusive suite. The intrusion is hosted by a Neoarchean granite-greenstone belt to the southwest, but principally by tonalitic gneiss and syenite (James and Born 1985). The geometry of the intrusion (Figure 2) has been interpreted as primary (Peck, James and Chubb 1993; Born 1979) or as a result of polyphase open folding related to Penokean deformation (Vogel, James and Keays 1998). The East Bull Lake Intrusion consists of 2 interconnected magma chambers that locally exceed 1 km in thickness (see Figure 2). The eastern end of the East Bull Lake Intrusion is connected to the western end of the Agnew Lake Intrusion by the Streich dike (see Figure 2). The dike-like conduit that connects the eastern and western lobes of the East Bull Lake Intrusion (see Figure 2) consists of the lowermost stratigraphic units of the intrusion, and may represent part of the feeder dike to the magma chambers. These stratigraphically lowest parts of the intrusion are significant, as they are proximal to mineralized zones and feeder dikes that are largely obscured by the main mass of the lopolith. An unknown thickness of the upper part of the intrusion apparently has been eroded away. The East Bull Lake Intrusion was metamorphosed at the greenschist to lower amphibolite facies (Card 1978; Kamineni 1986; Chubb 1994). Igneous textures are typically well preserved throughout the intrusion, although primary pyroxene and olivine are commonly pseudomorphed by secondary amphiboles. Primary mineralogy has been deduced mainly from detailed textural and geochemical studies (e.g., James and Born 1985; Chubb 1994). EAST BULL LAKE SCALE 1 20000 0 05 1 2 25 Kflometres Figure 2. Geologic map of East Bull Lake Intrusion. Numbered units within the intrusion correspond to stratigraphic zones shown in Figure 3. Units 1 to 3 are Archean strata; units 14 to 17 are post-intrusion Paleoproterozoic units; for details see Peck et al. (1995). 16 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �w C a >< 12 Agnew Intrusion .2 (U 5: Transition unit ö o 0 a E .2 ö 1= _2000 _1 800 _1 600 (0 w w a 0) An<3 Ferrosyenite unit Fe-Ti Oxide Zone Leucogabbro unit G) a Porphyritic unit w 0. 0. Upper .1400 Gabbronorite ( Zone _1200 Pod-bearing unit Porphyritic unit East Bull Lake Intrusion Ce w Mixed unit Dendrite unit Layered unit Massive Gabbronorite Zone _1 000 Olivine Gabbronorite•- _800 Layered Gabbronorite Zone Olivine Gabbronorite Zone — Dendrite unit Rhythmically layered Zone Lower Gabbronorlte Zone Van-textured Gabbrononite Massive unit Leucogabbronorite Zone 0 Anorthosite Zone ow .E 0)01 Inclusion-bearing Zone Gabbronorite Zone Border Zone _600 Inclusion-bearing Gabbronorite Zone Marginal Leucogabbronorite Zone 5 Marginal Gabbronorite Zone Figure 3. Stratigraphic sections for the East Bull Lake and Agnew Lake intrusions. Mineral distribution and plagioclase compositions are from Vogel et al. (1999). Unit numbers correlate with geologic maps shown in Figures 2 and 6a. Stratigraphy and Petrography The type stratigraphic section for the East Bull Lake Intrusion is shown in Figure 3. Three mappable subdivisions are recognized: the Lower, Main and Upper Series. 17 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �MARGINAL SERIES The contact between the East Bull Lake Intrusion and Archean basement is represented by the Border and Gabbronorite zones of the Marginal Series. The Border Zone is commonly several tens of metres thick, and comprises varied proportions of gabbroic and anorthositic veins and locally derived basement xenoliths. The relative proportion of footwall xenoliths and gabbroic veins and the size of the basement xenoliths increase away from the intrusion and into the undisturbed basement. The basement xenoliths are commonly thermally recrystallized to hornfels and locally display evidence of in situ melting. Adjacent gabbroic veins commonly contain several percent quartz and/or patchy, granophyric-textured leucotonalite and leucogranite. The Border Zone is locally separated from the main mass of the intrusion by the Gabbronorite Zone, a 1 to 50 m thick texturally and modally banded gabbronorite. Chubb (1994) suggested that the Gabbronorite Zone, as exposed in the conduit linking the western and eastern lobes of the East Bull Lake Intrusion, probably developed from late injections of mafic magma that were unable to penetrate the overlying Lower Series rocks. Elsewhere in the East Bull Lake Intrusion, the Gabbronorite Zone is clearly gradational into the overlying Inclusion-Bearing Zone and underlying Border Zone, and likely represents an irregular chilled margin. LOWER SERIES The Lower Series hosts most of the known PGE mineralization in the East Bull Lake Intrusion, and comprises a lower inclusion-rich unit (Inclusion-Bearing Zone) and an upper anorthositic unit (Anorthosite Zone). The Inclusion-Bearing Zone consists of a matrix of medium- to coarse-grained gabbronorite and/or leucogabbronorite and a heterolithic inclusion suite. The inclusions typically represent only 1 to 10% of the unit and include basement xenoliths, anorthosite and leucogabbro autoliths and discontinuous, podiform pyroxenite and melagabbro bands and pods. The Anorthosite Zone consists of monotonous, coarse-grained anorthosite and leucogabbronorite. Locally, these anorthositic rocks are brecciated by crosscutting gabbroic vein networks. Spectacular “nodular” (glomerocrystic) textures occur in anorthositic rocks in the lower parts of the Anorthosite Zone. These occurrences are largely restricted to the conduit linking the west and east lobes and to the easternmost part of the east lobe. The nodular textures consist of tightly packed centimetre-size spheroidal aggregates of coarse-grained plagioclase in a matrix of coarse-grained pyroxenite and/or finer grained gabbronorite. The textures are believed to have formed due to enhanced cooling rates in the narrow parts of the intrusion (Chubb et al. 1995). MAIN SERIES The Main Series incorporates the Leucogabbronorite Zone, the Rhythmically Layered Zone and the Olivine Gabbronorite Zone (see Figure 3). Metre-scale, isomodal, phase layering becomes increasingly better developed upward through the Main Series (Photo 2). Igneous layering becomes better developed upward through the Main Series. In the Leucogabbronorite Zone, most of the layering reflects modal variations of plagioclase and pyroxene. Phase layering in the Rhythmically Layered Zone is typically caused by variations in olivine and plagioclase and like the underlying zone, overall, these rocks are leucocratic. The Olivine Gabbronorite Zone is distinct in having a high proportion of olivine-rich layers throughout its 50 to 70 m stratigraphy, relative to the underlying layered zones. Throughout the Main Series, modally graded layers typically become more plagioclase-rich upward. They are believed to have formed by separation of high-temperature, buoyant, cumulus plagioclase from dense, Fe- and Mg-rich residual magma ± olivine. 18 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �—. .' 4 •! .,. Photo 2. Igneous layering in the East Bull Lake Intrusion. a) Well-developed layering between plagioclase-rich and plagioclasepoor units. Hammer handle is approximately 30 cm long. b) More typical, subtle layering within gabbronoritic rocks of the Rhythmically Layered Zone, Main Series. 19 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �UPPER SERIES The most chemically evolved rock types in the East Bull Lake Intrusion occur in the Upper Series, but they are less evolved than rocks present in the Upper Series of the Agnew Lake Intrusion (see Figure 3). Accordingly, it is believed that the uppermost parts of the East Bull Lake Intrusion have been eroded away. The Upper Series comprises the Layered Gabbronorite Zone and the Massive Gabbronorite Zone. The Layered Gabbronorite Zone consists of planar or disrupted, irregular, texturally and modally layered gabbronorite. The Massive Gabbronorite Zone comprises planar, fine- to medium-grained gabbronorite and ferrogabbronorite (Peck et al. 1995). Varitextured gabbronorite occurs throughout the East Bull Lake Intrusion, forming irregular, metresized veins, pods and discontinuous layers that commonly display dendritic textures and have highly varied grain sizes (Photo 3). The thickest band occurs in the Layered Gabbronorite Zone to the west of Moon Lake (see Figure 2). Born (1979) mapped these rocks as a stratigraphic unit in the central plateau area of the East Bull Lake Intrusion. In the eastern part of the East Bull Lake Intrusion, these same rocks are not apparently stratigraphically controlled (Chubb 1994) leading to the schematic interpretation in Figure 3 of these rocks transecting stratigraphy. At the Agnew Lake Intrusion these same rocks form a distinctive stratigraphic unit as indicated in the stratigraphic section in Figure 3 (Vogel 1996). The crystallization order throughout the East Bull Lake Intrusion is plagioclase ± olivine → orthopyroxene → clinopyroxene + magnetite → sodic myrmekite + apatite. Core plagioclase compositions vary from An70-80 in the Anorthosite Zone to near An60 at the top of the stratigraphy. Analyses of preserved igneous plagioclase, olivine and 2 pyroxenes in small areas of the Rhythmically Layered and the Olivine Gabbronorite zones (see Table 1) show Fo60-74 compositions for primocryst olivine. This suggests that the Olivine Gabbronorite Zone formed from a more primitive magma composition than the underlying Rhythmically Layered Zone. Where present, cumulus olivine appears to have co-precipitated with plagioclase and is commonly mantled and partly resorbed by orthopyroxene. Orthopyroxene includes hypersthene and iron-rich bronzite, whereas the dominant clinopyroxene is augite. Disseminated, chromium-rich spinel and clotty, coarse-grained magnetite locally occur within the Gabbronorite Zone (Marginal Series) in the southwestern part of the East Bull Lake Intrusion. '. Photo 3. Dendritic texture within the Upper Series of the East Bull Lake Intrusion. a) Frond-like pyroxene dendrites. Hammer handle is approximately 30 cm long. b) Large, pyroxene-dendrites present just to the right of the 5.5 cm lens cap. 20 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Geochemistry A comprehensive geochemical database for the East Bull Lake Intrusion can be extracted from Peck et al. (1995) and Chubb (1994), which includes several hundred, whole-rock major and minor element analyses and selected trace and rare-earth element analyses. Additional, relevant geochemical data for the East Bull Lake Intrusion is contained in several Atomic Energy of Canada Ltd. publications (e.g., Ejeckam et al. 1990) and previous academic studies (Born 1979; James and Born 1985). Selected representative and average geochemical compositions for the major stratigraphic units in the East Bull Lake Intrusion are presented in Table 4 and illustrated in Figures 4 and 5. Included in Table 4 are 2 estimates of the parent liquid composition for the majority of the rocks in the East Bull Lake Intrusion. These are represented by a probable feeder dike (sample 91DCP291; Chubb 1994) from the eastern lobe of the East Bull Lake Intrusion, and a weighted bulk composition for the entire intrusion (Average WEBLI). The bulk composition was determined using the calculated volumes of all of the major stratigraphic units and their average chemical compositions. Both estimates give similar, Al-rich, low-Ti leucogabbroic magma compositions. The East Bull Lake Intrusion is dominated by rocks having leucogabbronorite compositions that are olivine-normative in parts of the Lower Series and much of the Main Series. Detailed geochemical studies of surface and diamond-drill core samples from both the eastern and western lobes of the intrusion (e.g., James and Born 1985; Ejeckam et al. 1990; Chubb 1994; Peck et al. 1995) have shown that welldefined chemical fractionation trends are absent through the East Bull Lake Intrusion stratigraphy. This likely reflects the fact that 1. the early crystallization of the parent magmas was dominated by plagioclase with intermediate to calcic compositions that, as calculated by James et al. (2002a), were near neutrally buoyant in their Al-rich parent magmas. 2. the chamber was fed by multiple injections of chemically similar magma that became mixed with the resident magma, thereby precluding the development of chemical fractionation trends. Nonetheless, some geochemical trends are present in the intrusion, a few of which are illustrated on Pearce-element ratio diagrams (see Figure 4) and chondrite-normalized rare-earth element plots (see Figure 5). In Figure 4a, Olivine Gabbronorite Zone rocks from the East Bull Lake and Agnew Lake intrusions (olivine-plagioclase cumulates) clearly form a more olivine-controlled group than do the rest of the samples from these intrusions. These data support the interpretation that intrusion of the Olivine Gabbronorite Zone magma represents a separate and more primitive magma pulse into the chamber. Figure 4b shows that plagioclase is the only significant calcium-bearing phase involved in the fractionation of the East Bull Lake Intrusion magma. This is consistent with petrography and rules out clinopyroxene as a mineral phase influencing fractional crystallization for this intrusion. In both Figures 4a and 4b, Upper Series rocks plot closer to the origin of the diagram reflecting their more fractionated compositions. Many of the mafic veins in the Border Zone are quartz normative and display field and geochemical evidence of significant assimilation of basement-derived, siliceous partial melts. Nevertheless, there is no evidence for significant contamination of the main body of magma in the East Bull Lake Intrusion through partial or total assimilation of country rock inclusions. The majority of the rocks in the lower parts of the East Bull Lake Intrusion are enriched in Al and depleted in incompatible trace elements in comparison to the proposed parent magma estimates (see Table 4, Figures 4a and 4b). The Main Series contains, on average, the most magnesian rocks in the East Bull Lake Intrusion, with olivine-plagioclase cumulates occurring throughout the Olivine Gabbronorite Zone and in parts of the underlying Rhythmically Layered Zone. The Upper Series contains the most evolved rocks in the East Bull Lake Intrusion, and on most chemical fractionation plots, fall at the evolved end of the fractionation trends (e.g., see Figure 4b). 21 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �a) 300 0I X Opx Cpx 250 • Lower and Main Series A Upper Series, EBLI X OGNZ, EBLI • Marginal and Lower Series, AGI 0 Upper Series, AGI X OGNZ,AGI • Feeder Dyke, EBLI • Streich Dyke, AGI ——Linear (OGNZ, EBLI) Linear (Lower and Main Series, EBLI( Plag .X 200 • . •U 0, XX. U- U U • 150 y=O.3085x-O.1328 R2= 0.7853 • 100 • U U 0 0 0 100 200 400 300 500 600 S11T1 140 120 80 60 20 200 100 300 Al/Ti Figure 4. Pearce-element ratio diagrams for the East Bull Lake, Agnew Lake and River Valley intrusions. a) Fe+Mg/Ti versus Si/Ti plot for East Bull Lake and Agnew Lake intrusions. b) Ca/Ti versus Al/Ti plot for East Bull Lake and Agnew Lake intrusions. c) Ca/Ti versus Al/Ti plot for the River Valley intrusion. In figures 4a and 4b, solid symbols are for lower and middle portions of the stratigraphy, with open symbols for the upper part. The Olivine Gabbronorite Zone is shown separately as are the estimated parent magma compositions for the 2 intrusions. Solid lines are fitted curves for the East Bull Lake Intrusion, as examples. Mineral vectors indicate how the data should vary if trends are due to that phase only. The data in figures 4a and 4b show that the 2 intrusions exhibit identical chemical variations. In Figure 4a, Olivine Gabbronorite Zone rocks are clearly more mafic (primitive) in composition than the remainder of each intrusion which fall on a single trend with a much shallower (plagioclase-enriched) slope. The positions of the parent magma composition are consistent with the 2 groups being derived from cogenetic magmas. In Figure 4b, data for all samples fall on a linear trend with a slope in the range 0.40 to 0.45 indicating that calcic plagioclase feldspar mainly controls this trend, and that clinopyroxene is not a major modal phase in these rocks. In the lower left of 4a, samples of Upper Series rocks deviate from the trend near the origin of the diagram because they have primocryst ilmenomagnetite and thereby, Ti is no longer a conserved element. In figure 4c, rocks from the River Valley intrusion display a similar plagioclase-controlled trend, but with a greater proportion of samples lying closer to the origin, reflecting a slightly more mafic bulk composition of the samples. 22 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �1000 a) C C 0 C.) 0 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu —4-- Ferrosyenite Subzone (7b) b) —0— Leucogabbro Subzone (7a) —U— Porphyritic unit (6c) —X— Olivine Gabbronorite Zone (5) unit (4b) —0—Inclusion-bearing and Massive units (3+4a) —4— Marginal Leucogabbronorite Zone (2) 100 ci) •0 C 0 0 C.) 0 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Figure 5. Chondrite-normalized rare earth element plots for the East Bull Lake and Agnew Lake intrusions. a) East Bull Lake Intrusion. b) Agnew Lake Intrusion. Data are from Peck et al. (1995) and Vogel (1996). Normalizing values are from Sun and McDonough (1989). In both diagrams, all data are the average for 2 or more samples. “Feeder dike” in Figure 5a and “Streich dike” in 5b are estimates of parent magma compositions to the East Bull Lake Intrusion and the Agnew Lake Intrusion, respectively. In 5a, note the U-shaped boninite-like pattern for pyroxenite layers from the Inclusion-Bearing Zone. Figure 5a shows that all major rock types in the intrusion are enriched in light REE; this is typical for the East Bull Lake intrusive suite as a whole. The plagioclase-rich and cumulate character of rocks from the Lower and the Main Series in the East Bull Lake Intrusion is illustrated by their depleted total REE relative to the parent magma composition (“feeder dike”), and strong to moderate Eu anomaly. 23 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Table 4. Representative and average anhydrous whole-rock geochemical analyses from the East Bull Lake and Agnew Lake intrusions (from James et al. 2002a). Analysis Series Zone Rock Type N SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Mg # Cr Ni Co Sc V Cu Zn Rb Ba Sr Nb Zr Y Analysis Series Zone Rock Type 1 Basement Footwall Archean tonalite 11 69.2 0.09 17.5 1.70 0.02 0.44 2.89 6.94 1.21 0.04 33.75 17 61.8 0.54 17.8 4.19 0.09 1.16 2.48 5.36 6.22 0.27 35.46 1 50.9 0.09 25.7 2.74 0.04 2.24 13.3 4.28 0.56 0.08 61.82 1 52.9 0.09 22.2 4.34 0.08 4.88 11.1 3.80 0.52 0.08 69.02 2 51.8 0.21 13.8 9.72 0.20 11.9 10.4 1.71 0.22 0.08 70.86 6 Marginal Border Quartz-rich gabbronorite vein 8 56.1 0.50 17.1 8.17 0.13 5.33 9.13 2.72 0.74 0.09 56.38 <10 <5 20 1.8 16 12 31 23 784 378 8.8 74.6 13.6 <10 8 7 4.0 53 17 77 201 1252 723 32.4 550.6 41.6 38 51 28 <1.0 32 377 31 27 225 607 4.9 37.8 7.6 113 107 41 <1.0 39 88 50 24 195 675 4.1 37.5 7.3 682 392 61 <1.0 131 922 112 20 103 259 2.6 24.4 8.6 138 192 42 15.2 153 214 70 27 370 367 7.0 73.7 15.9 12 Lower Anorthosite Anorthosite 14 Main LGN Leucogabbro norite 42 50.5 0.34 20.3 8.09 0.13 6.46 11.1 2.44 0.60 0.05 61.28 72 49.6 0.42 21.0 8.28 0.12 6.16 11.1 2.58 0.59 0.06 59.55 25 46.9 0.30 14.6 13.4 0.18 14.5 8.19 1.56 0.27 0.04 68.22 155 194 41 17.2 100 95 71 26 166 271 6.2 46.9 11.1 81 195 40 17.2 127 80 72 24 201 295 6.0 48.6 12.1 230 505 90 13.1 104 91 91 15 118 183 6.1 41.5 14.4 9 Lower IBZ Pyroxenite pods, EEBLI 2 Basement Footwall Parisien Lake syenite 10 Lower IBZ Pyroxenite layers, WEBLI 3 Marginal Border Anorthosite vein 11 Lower IBZ Chlorite schist, WEBLI 4 Marginal Border Leucogabbro vein 5 Marginal Border Melagabbro norite vein N SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Mg # 10 53.8 0.38 6.28 13.5 0.23 17.0 8.12 0.52 0.11 0.05 71.37 2 52.71 0.17 4.59 14.15 0.21 23.76 4.29 0.05 0.06 0.01 76.88 1 38.55 0.12 16.96 20.74 0.26 17.99 4.77 0.56 0.03 0.02 63.21 88 49.7 0.36 21.9 7.67 0.12 5.50 11.6 2.52 0.56 0.05 58.71 13 Lower Anorthosite Matrix to nodular anorthosite 10 49.2 0.84 13.9 13.1 0.19 10.0 10.8 1.49 0.31 0.12 60.19 Cr Ni Co Sc V Cu Zn Rb Ba Sr Nb Zr Y 1360 383 87 <1.0 196 80 116 15 51 31 6.7 57.7 17.3 87 403 59 47 152 1010 95 <2 <3 4 <1.0 11 2 998 2670 152 12 100 2000 365 <2 8 59 <1.0 21 3 116 289 40 14.1 120 338 69 21 177 300 6.9 40.5 11.2 289 332 96 <1.0 296 72 97 19 76 117 9.9 70.4 24.3 7 Marginal GN Gabbronorite EEBLI 8 Lower IBZ Matrix, EEBLI 4 50.7 0.56 15.0 13.6 0.19 7.55 9.63 2.21 0.52 0.06 52.38 9 51.5 0.57 15.3 12.2 0.19 7.67 9.79 2.24 0.53 0.06 55.41 264 201 79 39.1 340 188 99 19 113 333 4.1 62.7 16.7 210 244 43 19.4 127 209 67 16 140 253 <1.0 34.9 7.8 15 Main RL Leucogabbro norite 16 Main OGN Olivine gabbronorite 24 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Table 4. continued. Analysis Series Zone Rock Type N SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Mg # Cr Ni Co Sc V Cu Zn Rb Ba Sr Nb Zr Y 17 Main LGN Gabbronorite, Folson Lake 18 Main LGN Gabbronorite, Bull Lake 191 Upper GN Varitextured gabbronorite 20 Upper GN Massive gabbronorite 21 Dikes 22 Dikes Plagioclasephyric dikes, WEBLI 8 51.3 0.80 15.0 12.7 0.20 6.36 10.7 2.04 0.73 0.10 49.74 89 85 47 37.3 250 119 97 35 224 230 <1.0 67.9 18.8 24 51.2 0.52 18.1 10.0 0.16 6.24 10.6 2.37 0.76 0.07 55.25 30 52.2 0.69 18.1 10.5 0.16 4.79 9.73 2.67 1.07 0.09 47.48 29 51.7 0.65 15.6 11.5 0.18 6.81 10.4 2.32 0.71 0.08 53.90 32 52.9 0.90 15.3 13.3 0.22 5.59 7.79 2.80 1.13 0.12 45.46 Gabbronorite dike, Folson Lake 6 51.0 0.80 15.8 12.7 0.19 6.30 10.2 2.17 0.74 0.11 49.60 138 121 43 25.3 167 90 85 27 258 261 6.3 60.2 14.2 72 92 37 25.6 200 182 85 42 328 255 <1.0 66.8 15.9 184 113 51 30.6 220 121 90 27 245 223 6.4 68.3 16.6 63 67 43 32.1 264 138 125 46 392 193 7.4 100.8 20.5 136 116 45 30.8 235 138 97 34 260 210 <1.0 80.5 18.5 23 Dikes Aphyric diabase dikes, WEBLI 24 Parent Average WEBLI 25 Parent 11 41.0 1.22 14.1 15.2 0.22 5.37 9.01 2.86 0.89 0.16 41.13 342 50.2 0.45 19.5 9.31 0.14 6.60 10.6 2.44 0.65 0.06 58.41 Feeder 91DCP291, Streich dike 1 50.8 0.54 19.6 9.25 0.16 5.47 10.8 2.96 0.30 0.07 53.94 69 65 44 33.7 310 153 120 44 312 239 <1.0 116.6 32.6 125 224 45 18.9 142 179 80 26 211 267 6.5 52.5 13.1 153 140 33 <1.0 171 66 70 8 93 261 2.5 65.0 14.6 Notes: Major element oxides are in wt. %, trace element data are in ppm; Mg number = atomic Mg/Mg + Fe, where Fe = total Fe expressed as ferrous iron. All samples from Peck et al. (1995), except analyses 7, 8, and 13, which are from Chubb (1994). Abbreviations: EEBLI = eastern East Bull Lake Intrusion; GN = Gabbronorite Zone; IBZ = Inclusion-Bearing Zone; LGN = Layered Gabbronorite Zone; OGN = Olivine Gabbronorite Zone; RL = Rhythmically Layered Zone; WEBLI = western East Bull Lake Intrusion. Thermodynamic modelling of the crystallization of the East Bull Lake Intrusion based on the “feeder dike” parent magma (see Table 4), using the program COMAGMAT (Ariskan et al. 1993), suggests that plagioclase was the sole liquidus mineral over the temperature range 1300 to 1200°C, followed by olivine and then orthopyroxene (1176°C) (James et al. 2002a). The most robust model is in agreement with the predictions made by James and Born (1985), using Pearce-element ratio diagrams, who indicated that plagioclase accounts for nearly 85% of the phase volume of the fractionating assemblage in the Lower Series and 77% in the Upper Series. Orthopyroxene and clinopyroxene represent only about 20% of the phase volume throughout most of the East Bull Lake Intrusion, and olivine control is only evident in the middle to upper parts of the Main Series. The average Mg-number for the parent magmas is about 73, and the average, theoretical, plagioclase composition is about An73. Our calculations also suggest that most of the pyroxenite pods in the Inclusion-Bearing Zone represent dense, fractionated, Mg- and Feenriched residual magma that originated in enclosing or overlying strata. Their REE-enriched character (see Figure 5a), relative to the main mass of Anorthosite Zone rocks, is consistent with this interpretation. There are, however, discontinuous, metre-scale, cumulate layers in the PGE-rich, basal part of the Inclusion-Bearing Zone along the southwest margin of East Bull Lake Intrusion (Bullfrog zone, Mustang Minerals Corporation) that have U-shaped REE profiles (see Figure 5a) indicating that they formed from boninitic magma. The origin and distribution of these atypical rocks requires further study, as they suggest that 2 magma types were present in the early (mineralized) stages of formation of this intrusion. 25 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Mineralization Mineral exploration in the East Bull Lake Intrusion began with prospectors who gained access along newly completed logging roads in the early 20th century. They focused their attention on gold and sulphide mineral prospects. Several companies, including Noranda Mines Ltd., El Pen-Rey Oil-Gas and Mines Ltd., Sylvanite Gold Mines Ltd. and the Mining Corporation of Canada Ltd., investigated the base metal potential of previously discovered sulphide mineral occurrences in the 1950s and 1960s. The PGE potential of the East Bull Lake Intrusion was first recognized by independent prospectors E. Gallo, M. Hauseux and S. Surmacz, as well as by BP Minerals. Their observations were corroborated by an extensive study of the mineral potential of the East Bull Lake Intrusion carried out by researchers from Laurentian University and the Ontario Geological Survey in the early 1990s (Chubb 1994; Peck, James and Chubb 1993; Peck et al. 1995). Both Inco and WMC International undertook small exploration programs on the East Bull Lake Intrusion, but sustained exploration did not take place due to low palladium prices and the unfamiliar mineralization style. Renewed exploration of the East Bull Lake Intrusion began in 1998, led by Mustang Minerals Corporation, whose property covers about 90% of the intrusion. The current exploration target is a bulk, near surface, low grade (e.g., 2.5 g/t Pd+Pt) PGEcopper-nickel resource suitable for open pit mining (Brisbin et al. 2001). The geological environments for PGE-copper-nickel mineralization in the East Bull Lake Intrusion were first described in detail by Peck, James and Chubb (1993). They recommended the non-genetic term “contact sulphide mineralization” (now “contact-type”) be used to describe all of the PGE-rich disseminated sulphide mineralization occurring in the Marginal and Lower Series of the intrusion. Sulphide mineralization is present throughout the Lower and underlying Marginal Series but is best developed within the Inclusion-Bearing Zone, within a few tens of metres of the footwall or sidewall contact. Recent diamond drilling by Mustang Minerals Corporation and Freewest Resources Canada Inc. has proven that this zone is typically 20 to 50 m thick and, despite the nugget-type distribution of the sulphide minerals, consistently displays an average sulphide mineral content of 1 to 2%. The sulphide minerals typically consist of approximately equal amounts of coarse chalcopyrite and pyrrhotite, and subordinate, finer-grained pentlandite. Where their original textures are preserved, the sulphide minerals in the Inclusion-Bearing Zone form coarse-grained blebs up to 3 cm in diameter. Sulphide minerals (<1 to 10%) also commonly occur in the Border Zone and the Gabbronorite Zone of the Marginal Series, but these occurrences are enriched in pyrrhotite and/or pyrite relative to the chalcopyrite-rich mineralization in the Inclusion-Bearing Zone. Sulphide minerals rarely occur in the Main and Upper Series and, where present, are principally composed of pyrrhotite and appear to have formed as late-crystallizing grains from trapped intercumulus liquids. In Olivine Gabbronorite Zone samples, however, disseminated finegrained chalcopyrite and magnetite have been reported in the most altered rocks (James et al. 2002a) and may account for Pd+Pt+Au assays in the 0.5 to 1.0 g/t range from this unit (Peck et al. 1995, 2001). Disseminated to massive sulphide minerals occur within the Parisien Lake deformation zone (PLDZ on Figure 2), a major east-trending structure. Most of the known mineralization is exposed in trenches located about 1 km east of East Bull Lake (see Figure 2). The sulphide minerals occur in metre-size, ovoid zones that are elongate parallel to the strike of the primary shear fabric in the Parisien Lake deformation zone. The sulphide minerals consist of pyrrhotite and lesser chalcopyrite, pyrite and pentlandite and are intergrown with quartz, amphibole, chlorite and magnetite. Field and geochemical data suggest that hydrous fluids permeating the Parisien Lake deformation zone leached chalcophile metals from pre-existing, magmatic sulphide mineralization in the deformed Lower Series rocks and redeposited them within dilatant zones. The geochemistry and genesis of the contact-type PGE-copper-nickel mineralization in the East Bull Lake Intrusion is discussed in detail by Peck et al. (2001). Based on all available data, the mineralization 26 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �is characterized by highly varied whole-rock PGE grades that reflect the erratic distribution of the sulphide minerals on a hand sample scale. Diamond drilling confirms, however, that the average PGE grade of the Inclusion-Bearing Zone in the southwestern part of the East Bull Lake Intrusion is roughly 0.5 to 2.5 ppm combined Pd+Pt+Au. Analyses from grab samples range from typical values of 1 to 2 ppm to 43 ppm combined Pd+Pt+Au (sample collected by Freewest Resources Canada Inc. from a trench on their Folson Lake property). Data presented by Peck et al. (1995) show that 20% of all of the reconnaissance samples collected from the Lower Series in the western lobe of the intrusion contain greater than 1 ppm combined Pd+Pt+Au, with an average Pd content of approximately 200 ppb. These values represent highly anomalous background PGE contents in comparison to most other mafic or ultramafic intrusions. For comparison, barren norites, pyroxenites and harzburgites from the Lower, Critical and Main Zones of the Bushveld Complex have values ranging from 13 to 478 ppb Pt+Pd (Barnes and Maier 1999). Average Pd/Pt ratios are highest in the Anorthosite Zone (3.2 to 4.3) and lowest in the pyroxenitic pods of the Inclusion-Bearing Zone (1.56) and the Border Zone (2.0). Average Cu:Ni ratios range from 0.6 in samples from the matrix in Inclusion-Bearing Zone to 4.8 in the pyroxenitic pods; however, most samples have values between 2 and 3. Border Zone sulphide occurrences have the lowest average PGE grades (at similar total sulphide mineral abundance), but show a considerable range in abundance. Average S/Se ratios for the PGE-rich sulphide mineralization range from 1240 to 2840, which are within the empirically-determined range for mantle-derived sulphide mineralization (Eckstrand et al. 1989), and are indicative of a magmatic origin for the contact-type PGE mineralization. Copper displays a strong positive correlation with both S and Se. Pd and Pt display significant, positive, correlation with each other, with Cu and, to a lesser degree, with S. As is typical of most mafic-hosted magmatic sulphide mineralization, the contact-type sulphide mineralization in the East Bull Lake Intrusion shows strong enrichment of the low-melting point PGE (e.g., Pd, Pt) relative to the refractory PGE (Ir, Rh, Ru) on a chondrite-normalized chalcophile metal plot (James et al. 2002a). The structurally-controlled sulphide mineralization from the Parisien Lake deformation zone show lower PGE contents per unit sulphide in comparison to the contact-type mineralization. The former sulphide mineralization type also have much higher Pd:Ir ratios, a feature consistent with the known differences in degree of mobility of Pd versus Ir during hydrothermal alteration of mafic and ultramafic rocks (Keays et al. 1982). ROAD LOG, DAY 1, EAST BULL LAKE INTRUSION Note: Highway 533 is used regularly by logging trucks. Consequently, extreme caution should be taken when parking vehicles on the shoulder and when examining outcrops along Highway 533. Geological map reference: Chubb, Hannila and Peck (1994). 0.0 km Start at the junction of Highway 69S and the Highway 17W bypass in Sudbury. Set odometer to zero. Drive west on Highway 17 to Massey. 0.4 km Junction of Highway 17W bypass and Regional Road 80 (Long Lake Road), continue west on Highway 17 bypass. 70.9 km Junction of Highway 17 and 6, just north of Espanola. Continue west on Highway 17. 97.0 km Junction of Highway 17 and 553 in Massey. Turn right (north) onto Highway 553 toward East Bull Lake. 119.9 km Pull over on shoulder of Highway 533 at the top of a high ridge. Examine cleaned outcrops on both sides of the highway. 27 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Stop 1. Paleoproterozoic Metagabbro The high ridge consists of a linear(?) body of massive, medium-grained, metagabbro which may represent part of a feeder system to other East Bull Lake intrusive suite sills that have been eroded from the surrounding terrain. Prevec (1993) suggested that the rocks from this stop have a suitable parent composition for the East Bull Lake Intrusion. Alternatively, these rocks could be part of a large Matachewan diabase dike, but unlike most of the Matachewan dikes, it is not plagioclase-phyric. Return to vehicles and continue north on Highway 533. UTM 408177E, 5133860N. 122.4 km Power line crosses the Highway, continue north. 124.8 km Approximate southern margin of the Neoarchean Parisien Lake syenite (2665±2; Krogh, Davis and Corfu 1984). 125.3 km Junction of Highway 533 and logging road to the east. Park vehicles and walk a short distance east along the logging road to examine outcrops on the south and north sides of the logging road. Stop 2. Neoarchean Parisien Lake Syenite At this locality we see small amounts of coarse-grained syenite showing a well-developed igneous flow fabric on a local scale, as well as more abundant exposures of massive, medium-grained syenite. Regional mapping indicates that several types of medium-grained syenite intrude the coarser-grained variety; the latter is the more abundant rock type in the body. The Neoarchean age of the syenite is distinct from the 2470 to 2490 Ma age range of the East Bull Lake intrusive suite (see Table 2). These rocks form the footwall to the East Bull Lake Intrusion at Stop 12b. Return to vehicles and continue north on Highway 533. 126.0 km Low outcrops on the east side of Highway 533 consist of a plagioclase-phyric diabase dike typical of the Matachewan dike swarm. The dike contains greenish altered plagioclase phenocrysts in a fine-grained metadiabase matrix. Mafic diabase dikes, some plagioclasephyric, but many not, intrude both the Archean basement and the Parisien Lake syenite. Many of the same dikes also intrude the East Bull Lake Intrusion. A few very large dikes do not cut the intrusion, and are interpreted to be potential feeders to the sills to the East Bull Lake intrusive suite bodies in this area and elsewhere in the Southern Province. Typically, the dikes strike to the northwest, but they also strike west to west-northwest when they are proximal to the Southern–Superior province boundary. 126.5 km Outcrop of coarse-grained syenite, in the central part of the Parsien Lake syenite. The outcrop is intruded on its south side by a 10 to 20 m wide, northwest-trending, Matachewan diabase dike. 127.6 km Pull off to shoulder just past the curve in road where the highway passes between 2 large outcrop ridges. Leave vehicles and work your way up to the top of the large ridge on the west side of the road. Stop 3. Southern Margin of the East Bull Lake Intrusion This area of the East Bull Lake Intrusion forms a high ridge of very coarse-grained, massive varitextured leucogabbronorite. At its southern extremity, it is inclusion-bearing and sparsely mineralized. The outcrop area represents both the Inclusion-Bearing and Anorthosite zones, of the Lower Series, inward of 28 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �the intrusion contact. At this locality, the contact with the Parisien Lake syenite is sharp and most likely is a fault. Return to vehicles and continue north on Highway 533. 128.0 km Folson Lake deformation zone. At this location Highway 553 is “offset” to the east due to this major shear zone that strikes northwest across the entire intrusion (see Figure 2). 128.7 km Pull off to the shoulder and examine low outcrops on both sides of the road exposed on a curve as it passes over a small rise. Stop 4. Country Rocks These outcrops consist of country rocks of the Neoarchean Ramsay–Algoma gneiss complex. The Ramsay–Algoma gneiss complex (Jackson and Fyon 1991) is intruded by both the Parisien Lake syenite and the East Bull Lake Intrusion. Typically these gneisses are mixtures of strongly foliated granitic gneiss and metatextite to diatextite that enclose strongly deformed enclaves of mafic gneiss that likely represent remnants of greenstone belt metavolcanic rocks and/or diabase dikes. UTM 408300E, 5141425N. 128.8 km Parisien Lake road leads to mineralized zones along southern contact of the East Bull Lake Intrusion. 129.3 km Junction to the west between Highway 533 and the AECL (Atomic Energy of Canada Limited) road. The AECL road leads to Moon Lake, Central Plateau and the Folson Lake deformation zone. 129.8 km Outcrops present in this sandpit, 30 m east of Highway 533, expose metre-scale isomodal phase layering in the Rhythmically Layered Zone, Main Series, East Bull Lake Intrusion. The layering consists of thin leucogabbronorite layers (with scarce olivine primocrysts) that pinch and swell in thickness and continuity, and metre-thick isomodal layeres of olivine melagabbronorite. Boundaries between the layers are sharp (similar to Photo 2a). Outcrops at the road at this locality show graded layers, as well as layers with more gradational boundaries (similar to Photo 2b). UTM 409022E, 5142098N. 130.5 km East Bull Lake Lodge just to the west of Highway 533. Lodge is owned by Gerry Vauteur. 130.7 km Pull over and park where space is available. Examine vertical faces in outcrop on the west side of the road. Stop 5. Rhythmically Layered Zone This stop is located within the Rhythmically Layered Zone of the Main Series of the East Bull Lake Intrusion. On the west side of the road, vertical faces in outcrop exhibit several 0.3 to 1 m thick layers that dip west into the outcrop at 20 to 30°. The layers contain primocrysts of olivine and plagioclase enclosed in a matrix of orthopyroxene and clinopyroxene. Typically within the East Bull Lake Intrusion, all of the mafic minerals are replaced by secondary amphiboles, but plagioclase is commonly preserved. In this outcrop, however, well-formed outlines of olivine are easily observed; their proportion changes abruptly across layer boundaries. Layer compositions vary from olivine leucogabbronorite to olivine melagabbonorite; in all cases the layer boundaries are typically sharp and evidence of grading is very sparse. In this outcrop, evidence of a gravity-driven convection system is negligible. Return to vehicles, turn around and retrace route south to the junction of Highway 533 and the AECL road. Turn on to the AECL route and follow it to the end. Park vehicles in the pullout area (UTM 409405E, 5142725N), and walk south along the trail to a series of stripped outcrops. 29 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Stop 6. Folson Lake Deformation Zone Be careful of your footing on these smooth outcrops if the surfaces are wet. At this locality the Folson Lake deformation zone intersects the Layered Gabbronorite Zone of the Upper Series of the East Bull Lake Intrusion (see Figure 2). Kamineni et al. (1984) refer to this altered gabbronorite that extends as a narrow zone along the northwest-trending Folson Lake fault as the “Stockwork Gabbro”. It contains a complex network of at least 2 generations of quartz veins as well as feldspar veins (both plagioclase and adularia); that together comprise up to 30 volume per cent of the rock. Diabase dikes transect the altered gabbro stockwork both parallel and perpendicular to the northwest-striking zone of deformation and faulting. Petrological studies of the gabbros in this and related areas by Kamineni et al. (1985) indicate that the regional greenschist facies to epidote-amphibolite facies mineralogy is altered on a local scale to minerals characteristic of prehnite-pumpellyite facies and zeolite facies (e.g., laumontite, analcite, calcite). UTM location at parking area is 406330E, 5142108N. Return to vehicles. 0.0 km Reset odometer to zero and head east toward Highway 533. 0.8 km Park at edge of road by large outcrop. Stop 7. Dendritic Texture in the Varitextured Gabbronorite Zone This outcrop is located in the Varitextured Gabbronorite Zone, Upper Series, East Bull Lake Intrusion. Pods and lenses, 0.5 to 1.0 m in maximum dimension, of coarse-grained, pegmatitic and dendritic gabbronorite occur sporadically in the massive, medium-grained gabbronorite host rock. The rapid textural variation is diagnostic of this zone, as is the appearance of pyroxene dendrites forming frond-like feather patterns in a plagioclase matrix (see Photos 3a, 3b). Rarely, the feldspar also exhibits this growth pattern. At this locality the dendritic pegmatites occur as pods; elsewhere (e.g., Agnew Lake Intrusion) they have been observed in layer-like zones, dendrites pointing upward, reminiscent of crescumulate textures. In the East Bull Lake Intrusion, rocks hosting these textural varieties of gabbronorite have been mapped by Born (1979) as a conformable stratigraphic zone of the Upper Series of the intrusion. More recently, D Peck and P. Chubb have interpreted these rocks as late diapiric melts that were intrusive into the pre-existing stratigraphy. UTM 406755E, 5142610N. Return to vehicles, continue east. 1.1 km Park at edge of road by large outcrop. Stop 8. Layered Gabbronorite Zone This outcrop exposes delicate small-scale, centimetre- to decimetre-scale layering in the Layered Gabbronorite Zone, Upper Series. Unlike the layering observed so far today, there is the suggestion here of current action and erosion of earlier formed cumulates suggestive of either active convection or slumping and/or soft sediment deformation of units. Rock compositions in the lower part of the outcrop are medium-grained gabbronorite, whereas at the top of the outcrop it is a coarse-grained leucogabbronorite. UTM 406900E, 5142790N. Return to vehicles, continue east. 2.2 km Park at edge of road in pullout area. To reach this stop, head downhill toward the north end of Moon Lake. Cross the beaver dam at the end of the lake and follow the slope roughly ~200 m to the south to an area where scattered low-lying outcrops occur on the west side of Moon Lake at the base of the central plateau. This traverse should not be made alone and should be only conducted using appropriate footwear and field equipment. 30 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Stop 9. Olivine Gabbronorite Zone This stop is located in the Olivine Gabbronorite Zone, within the uppermost part of the Main Series of the East Bull Lake Intrusion. Rocks of this zone occur as metre-scale layered sequences with the more mafic olivine-rich cumulates forming the dominant rock types interspersed with thin, leucocratic plagioclasedominant layers. In these rocks olivine, plagioclase and occasionally orthopyroxene are primocryst phases. Olivine and plagioclase compositions are more primitive than in zones above and below this sequence. The Olivine Gabbronorite Zone is interpreted to represent an influx of more primitive magma into the magma chamber which interrupted the fractional crystallization process that links the Rhythmically Layered Zone of the Main Series with the Layered Gabbronorite Zone of the Upper Series. UTM 407370E, 5142942N. Return to vehicles, continue east. 2.6 km Park alongside the road where possible. Stop 10 is located on an outcrop about 20 m west of the AECL road. Stop 10. Rhythmically Layered Zone, East Bull Lake Intrusion This outcrop shows metre-scale layering in rocks of the Rhythmically Layered Zone, Main Series, that have a typical shallow dip to the west. At the road, these rocks are cut by a 5 to 10 m wide, northwesttrending diabase dike. As previously noted, such dikes are abundant throughout the intrusion. UTM 407585E, 5142708N. Return to vehicles, continue east. 3.3 km Park on the west side of the road in small pullout area. This stop is a kilometre-long walk through the units that form the margin (bottom or sidewall) of the East Bull Lake Intrusion in the Moon Lake area. The trace of the traverse is shown in the accompanying map of this area (Figure 6) that was prepared by D. Peck and P. Chubb. This traverse should not be made alone and should be only conducted using appropriate footwear and field equipment. Stop 11. Moon Lake Traverse Stop 11 is a traverse through the Lower Series and the Border Zone of the intrusion, which are both areas where disseminated copper-PGE mineralization is common. The Border Zone (see Figure 6, units 4a to 4e) represents footwall and/or sidewall lithologies to the main intrusion. It consists of Archean gneissic and metaplutonic rocks that are intruded by, or which contain, irregular to linear zones of anorthosite, leucogabbnorite and gabbronorite that appear to be related to the East Bull Lake Intrusion. The Border Zone is in abrupt contact with the Lower Series, which at this locality consists of the Inclusion-Bearing Zone (see Figure 6, units 6c and 6d) and the Anorthosite Zone (see Figure 6, unit 7a and 7b). Sulphide mineralization occurs in units 6c and 6d (Inclusion-Bearing Zone) proximal to the margin of the intrusion; leucogabbronorites in these units are typically very coarse grained, commonly varitextured, variably inclusion bearing, and show no evidence of layering. Xenoliths and autoliths vary from abundant to scarce to absent; they are typically 1 to 10 cm in maximum dimension. Autoliths vary in composition from pyroxenite to anorthosite. 31 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �LEGEND Aphyric and PlagioclasePhyric Diabase Dikes GABBROIC ANORTHOSITE SUBZONE 1 rn-Scale Layered Unit Leacogabbronorite ANORTHOSITE SUBZONE C Leacogabbrononte to Pyroxenite (pods) BORDER ZONE to Mediurn-Grained Gabbronorite and Anorthosite Camulaten Gabbroic and Anorthositic Veins and Anorthositic Veins and Granitoids and Anorthositic Veins, Sabordinate Granitoids ARCH EAN GRANITOIDS Tonalite, Tonalitic Gneiss and Granite SYMBOLS Geological Boandary Observed Geological Boundary Approximate Fault Traverse Route C:.) Outcrop Main Zone ot PGE-Cu-Ni Mineralization Figure 6. Geological map of the Moon Lake area, East Bull Lake Intrusion, showing the route of the traverse (heavy line) undertaken during Stop 11. Shaded areas indicate areas of disseminated sulphide mineralization. D.C. Peck and P.C. Chubb originally prepared the map in 1991 for the Department of Earth Sciences, Laurentian University. Inset map in the upper right indicates in detail the route of the traverse, as well as outcrop areas crossed during the traverse. Numerical rock codes in the legend correspond to those used in Figures 2 and 3, and on the map of Chubb, Hannila and Peck (1994). Note that UTM coordinates in this figure are in Zone 17, datum NAD 27. Trace to 3% disseminated chalcopyrite, pyrrhotite ± pentlandite and pyrite ocurs in these units. Typically the sulphide minerals are medium- to coarse-grained aggregates in an epidote (saussurite)-rich zone of alteration in plagioclase, and also as much finer grained xenoblasts in the same silicate minerals. The sulphide mineral assemblage appears to have both a magmatic and metamorphic and/or hydrothermal paragenesis. In these rocks Cu+Ni = 0.1 to 0.5 wt %, with Cu>Ni, and Pd+Pt = 0.5 to 10g/t and Pd/Pt = 1 to 5. The Anorthosite Zone (see Figure 6, units 7a and 7b) consists of coarse- to very coarse-grained massive anorthosite and leucogabbronorite. Altered pyroxenes locally occur as coarse-grained interstitial anhedral to subhedral crystals and also as 1 to 10 cm zones (aggregates of crystals) that are interpreted to have formed from fractionated interstitial liquids due to prolonged early crystallization of plagioclase feldspar. As illustrated in the inset map in Figure 6, the traverse route encounters the following sequence 32 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �of rock types: units 7a and 7b (Anorthosite Zone); unit 6c (Inclusion-Bearing Zone), unit 4a (Border Zone), units 6d and 6c (Inclusion-Bearing Zone). The shaded area in Figure 6 shows the general distribution of the PGE mineralization, which may be associated with disseminated sulphide minerals. Some more specific data on the mineralization in the Moon Lake area is given below, summarized from detailed information presented in Peck et al. (1995, Appendix 5, Part A). Border Zone (unit 4) rocks related to the East Bull Lake Intrusion include gabbro, quartz gabbro and leucogabbro. Unlike the sulphide mineral assemblage in the Lower Series of the intrusion, pyrite is particularly abundant. In this zone Pd+Pt normally does not exceed 700 ppb with Pd/Pt ≈ 2. Cu+Ni is in the range 80 to 1300 ppm and typically Cu ≈ Ni. Mineralization in the Inclusion-Bearing Zone shows a range in Pd+Pt = 150 to 3000 ppb and Pd/Pt = 2 to 6; with Cu+Ni = 700 to 7000 ppm and Cu/Ni = 2 to 4. The data that is reported by Peck et al. (1995) indicates that the range and ratios for the PGE, Cu and Ni for the Anorthosite Zone are very similar to those for the Inclusion-Bearing Zone. Unpublished semiquantitative data collected by D. Peck and B. Jago on the platinum group minerals in 2 samples from the Inclusion-Bearing Zone in the Moon Lake area indicate that the PGE in the samples comes from 1 to 10 micron-size grains of sperrylite, moncheite, Bi-kotulskite and platarsite/hollingsworthite. These phases are normally found at grain boundaries of one or more of chalcopyrite, actinolite, epidote, albite or quartz, or within the silicate minerals in fractures or cleavage planes and much less commonly in chalcopyrite. Return to vehicles, continue east on AECL road to Highway 533. 4.0 km Junction of the AECL road and Highway 533. Turn south (right) onto Highway 553. 4.5 km Junction, Highway 533 and Parisien Lake trail on the east side of the highway. The trail is poorly marked and partly overgrown by saplings. To reach the key outcrop areas of Stops 12A and 12B, walk east on the trail for 20 to 30 minutes to a stripped north-trending ouctrop area (Stop 12A). Following examination of these stripped outcrops, continue east to a large east-trending stripped area (Stop 12B). The 2 stripped areas expose rocks of the Marginal and Lower Series, and associated mineralization along the south side of the East Bull Lake Intrusion, respectively. This traverse should not be made alone, and should be only conducted using appropriate footwear and field equipment. Stop 12A. Parisien Lake Traverse This series of north-trending, stripped outcrops lies perpendicular to the contact of the East Bull Lake Intrusion, and consists of rocks of the Border Zone (footwall lithologies) and mineralized leucocratic rocks of the Lower Series. Walking north from the southernmost outcrops in the Border Zone, one observes layered Archean metasedimentary gneisses intruded by Parisien Lake syenite. Both rock types are in sharp contact on the north side of the outcrop with a Matachewan plagioclase-phyric diabase dike; an alternative interpretation is that the diabase represents a unit of the Gabbronorite Zone in the Marginal Series. The north side of the diabase is in sharp fault (?) contact with a 3 to 5 m thick, lens-shaped zone of sulphide-rich metapyroxenite to peridotite. The silicate minerals in this zone consist of pyroxene and/or olivine replaced by medium-grained calcic amphibole (tremolite?) enclosed and intergrown with fine- to medium-grained magnetite-chalcopyrite-pentlandite. The opaque-rich material in the zone is typically a foliated, medium- to coarse-grained, calcic amphibole-magnetite-pyrite-pyrrhotite assemblage. To the north of this mafic autolithic zone is a 20 m thick unit of varitextured leucogabbronorite which has abundant disseminated interstitial magmatic pyrrhotite-chalcopyrite-pentlandite and minor pyrite mineralization. Locally the silicate mineral assemblage has been altered to rusty zones of fine- to medium-grained biotite-actinolite-chlorite-feldspar and up to 5% idioblastic tourmaline (no sulphide 33 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �minerals). Platinum group element content in grab samples range from 1.4 to 2.4 g/t; channel samples have analysed 0.84 g/t over 21 m, including 1.36g/t over 6 m. This rock becomes “flinty” in the northernmost part of the stripped outcrop, probably reflecting mylonite development proximal to the Parisien Lake deformation zone. Continue to walk east along trail. Roughly 3 minutes to the east, you will encounter a long series of stripped outcrop that are oriented subparallel to the trail. Stop 12B. Parisien Lake Traverse The dominant rock type is coarse-grained leucogabbronorite, with lesser amounts of melagabbronorite, at the west and north ends of this large stripped outcrop. Some of the melagabbronorite could be interpreted as disrupted dikes. In a few places discontinuous, sinuous, steeply dipping, centimetre- to metre-scale layering is observed in the leucogabbronorites. The orientation of the layering is subparallel to the margin of the intrusion. Both rock types host disseminated sulphide minerals. Convoluted boundaries with reaction zones indicate interaction between the leucogabbronorite and the melagabbronorite, suggesting that magma mixing occurred between these phases. At the east end of the stripped outcrop area, leucogabbronorite is in sharp contact with Parisien Lake syenite. This contact represents the boundary between the Lower and Marginal Series of the intrusion. UTM co-ordinates at the east end of the stripped area are 409788E, 5141490N. This concludes Day 1. Retrace route back to Highway 533. 34 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �AGNEW LAKE INTRUSION—OVERVIEW Country Rocks, Structure, and Geometry of the Intrusion This section is largely based on data reported in Vogel (1996), Vogel, James and Keays (1998) and Vogel et al. (1999). The main mass of the Agnew Lake Intrusion is located on the west side of Agnew Lake, roughly 65 km southwest of Sudbury (see Figure 1). It is similar in age and size to the East Bull Lake Intrusion (2491 versus 2480 Ma and 50 km2 versus 43 km2, respectively, see Table 2). It is linked to the East Bull Lake Intrusion, on its northwest side, by the Streich dike, a 200 to 300 m wide composite body with a strike length of approximately 10 km. Pink to white, coarse-grained granites and orthogneisses of the Neoarchean Ramsay–Algoma gneiss complex (Jackson and Fyon 1991) form the footwall rocks to the intrusion around its north, west, and south sides (Figure 7a). There are only rare occurrences of footwall breccia zones at the intrusion's contact with the granitic rocks equivalent to the Border Zone of the East Bull Lake Intrusion. More commonly there is an abundance of felsic dikes (thermal aureole partial melts) intruding older granitic basement rock, particularly where mafic (feeder) dikes are abundant near the intrusion contact. The east margin of the intrusion is in fault contact with, or is apparently unconformably overlain by, McKim Formation metasedimentary rocks of the Huronian Supergroup (see Figure 7a). The longitudinal axis of the Agnew Lake Intrusion is 110 to 120° (see Figures 7a and 7b), which is parallel to the axis of the East Bull Lake Intrusion, the Streich dike and Matachewan diabase dikes in the southernmost Superior Province (Halls and Bates 1990). This trend is thought to reflect the orientation of the rift structure that permitted magma intrusion. The attitude of igneous layering in the intrusion is illustrated in Figure 7b. It shows that the body has an asymmetric (synclinal) shape with the axis of the structure plunging gently northeast. Its axial plane is parallel to that of Penokean folds in younger Huronian Supergroup strata. This fact and its high angle of intersection with the longitudinal axis of the intrusion and Streich dike, suggests that the present geometry of the Agnew Lake Intrusion is due to Penokean deformation of a lopolith. Stratigraphy and Petrography Figure 3 presents a composite stratigraphic section for the Agnew Lake Intrusion. It consists of a Marginal, Lower and Upper Series which correlate, as illustrated, with the Lower, Main and Upper Series of the East Bull Lake Intrusion. Mafic mineralogy is replaced by calcic-amphiboles formed during lower amphibolite facies, Penokean-age metamorphism. Compositions of plagioclase cores commonly appear to be near original magmatic values. Primary igneous textures are preserved, but less clearly than in the East Bull Lake Intrusion. MARGINAL SERIES The Marginal Gabbronorite Zone (unit 1, see Figures 3 and 7a) is characterized by a massive, mediumgrained, commonly olivine-normative, gabbronorite. It occurs as units up to 200 m thick with chilled outer contacts against the footwall and encloses large in situ remnants of granitic footwall. Sills or dikes, 1 to 20 m thick with chilled margins, commonly occur at the intrusion contact, and may be related to young dike sequences that transect the entire intrusion. 35 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Figure 7. Geology of the Agnew Lake Intrusion. a) Geologic map showing main stratigraphic units; unit numbers correspond to those in Figure 3 and in the text. b) Structural map showing attitude of centimetre- and metre-scale layering. The strike and plunge of the axial plane of the synclinal, lopolithic structure is the same as that in the Huronian Supergroup supracrustal rocks on the east side of Agnew Lake. Both figures modified from Vogel (1996). 36 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �The Marginal Leucogabbronorite Zone (unit 2, see Figures 3 and 7a) averages 200 m thick and occurs along most margins of the intrusion, where it either overlies the younger, Marginal Gabbronorite Zone or is in direct contact with the footwall. It consists of 3 distinctive units, namely, the Varitextured, Mottled and Nodular units. The Varitextured Unit occurs in the west and south margins of the intrusion, where it is dominated by leucocratic gabbronorites; mafic layers or lenses are common as are felsic differentiates. Grain size is highly varied on a centimetre-scale, from medium-grained to pegmatitic. These rocks are very similar to varitextured gabbronorite pods in the Anorthosite Zone in the East Bull Lake Intrusion. The equivalent, or somewhat younger, unit in the north-central margin of the Agnew Lake Intrusion is the Mottled Unit, which is characterized by large amphibole oikocrysts (after pyroxene) in a plagioclase-rich matrix. As with the Varitextured Unit, smaller but significant gabbronorite, melanogabbronorite and anorthosite also occur. Layering in the Mottled Unit is irregular and discontinuous. Of great significance in both units is the presence of centimetre- to metre-scale inclusions and pods of melanogabbronorite, footwall granite and, rarely, massive quartz where the unit is adjacent to the footwall. Disseminated PGE-rich chalcopyrite and pyrrhotite are common in the inclusion-bearing parts. The Nodular Unit forms large lens-like areas in the northwest part of the intrusion. It is distinguished by the dominance (70 to 90 modal percent) of 6 cm diameter, well-sorted and closely packed plagioclase nodules in a coarse-grained amphibolitized pyroxenitic matrix. Disseminated PGEbearing sulphide minerals sporadically occur in the matrix. Whole-rock Mg-number indicates this unit is the most fractionated of the Marginal Series. All of the units in this series have significant proportions of rock with normative olivine in the 1 to 10% range and supporting textural evidence for the presence of modal olivine. Plagioclase core compositions range from An77 to An32. LOWER SERIES The base of this series is the Inclusion-Bearing Gabbronorite Zone (unit 3, see Figures 3 and 7a). The zone is about 180 m thick and hosts footwall and cognate xenoliths; associated copper-nickel-PGE mineralization; and is limited in outcrop to the northwest margin of the Agnew Lake Intrusion where it leads to the Streich dike. Inclusions of Marginal Series rocks are common—especially of the Nodular Unit—in a gabbronorite matrix (An79-52). Similar rocks form the Inclusion-Bearing Zone at the base of the Lower Series in the eastern lobe of the East Bull Lake Intrusion (Chubb 1994). The overlying 350 to 400 m of the Lower Series is represented by the Lower Gabbronorite Zone, consisting of a Massive (4a) and a Lower Layered Unit (4b) (Photo 4), that are separated by a younger conformable phase of the Dendrite Unit (8) (Photo 5). The Massive Unit consists of massive, medium- to coarse-grained gabbronorite and leucogabbronorite (An79-61) and rare melanocratic inclusions. Unit 4b (see Figures 3 and 7a) is a layered (centimetre- to metre-scale), medium- to coarse-grained gabbronorite or leucogabbronorite (An65-53). Rocks in both units are commonly olivine-normative. These 2 units are similar to rocks in the Anorthosite and Rhythmically Layered zones, respectively, in the East Bull Lake Intrusion. Dendrite Unit rocks are coarse-grained to pegmatitic gabbronorites, commonly weakly layered, and typically varitextured, with a diagnostic feature being the common occurrence of delicate, branching, pyroxene crystals several centimetres long with interstitial plagioclase, titanomagnetite and granophyre. This unit also occurs higher in the stratigraphy and formed from a more evolved magma than its host rocks. 37 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Photo 4. Centimetre- to decametre-scale igneous layering within rocks of the Agnew Lake Intrusion. V Photo 5. Dendrite texture developed with the Lower Gabbronorite Zone of the Agnew Lake Intrusion. Compare with Photo 3a and 3b. Similar rocks will be observed at Stop 7, Day 2. 38 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �UPPER SERIES Most of the rocks in this series have not been recognized in other intrusions of the East Bull Lake intrusive suite; either they were never deposited or they were eroded away. The Olivine Gabbronorite Zone (unit 5, see Figures 3 and 7a) is notably more mafic than other units in the Agnew Lake Intrusion. It consists of centimetre- to metre-scale, layered olivine gabbronorite and leucogabbronorite. Disseminated sulphide minerals are common and PGE contents can be as high as those present in the inclusion-bearing rocks in the Marginal and Lower Series. Similar strata, 50 to 60 m thick, occur at the same stratigraphic level in the East Bull Lake Intrusion. The Upper Gabbronorite Zone (units 6a to 6e, see Figure 3) forms about 1000 m of the stratigraphy of the Agnew Lake Intrusion. Vogel, James and Keays (1998) and Vogel et al. (1999) divided it into 5 units. Olivine is absent from rocks in the Upper Gabbronorite Zone. Plagioclase and orthopyroxene are the primocryst (cumulate) phases, with plagioclase compositions being mostly in the range An65-40. The upper Layered (6a) and Mixed units (6b) underlie the Porphyritic Unit (6c), which forms the main mass of this part of the stratigraphy (see Figures 3 and 7a). The Porphyritic Unit is a dominantly quartz-normative rock unit characterized by varied amounts of plagioclase phenocrysts and glomerophenocrysts set in a gabbronorite matrix. Diffuse, macrorhythmic, decametre-scale layering of plagioclase-phyric leucogabbronorite and nonporphyritic gabbronorite is typical of this unit. The Fe-Ti Oxide Zone (units 7a and 7b, see Figure 3) forms the upper 200 to 400 m of the Agnew Lake Intrusion; its base is marked by the first appearance of titanomagnetite primocrysts. Primary igneous plagioclase is totally altered in this zone and magmatic textures are very poorly preserved. The Leucogabbro Unit (unit 7a, see Figure 3) forms the lower part of this zone. It consists of massive, coarsegrained leucogabbro (normative clinopyroxene greater than orthopyroxene) that has a distinctive clotty texture as a result of metamorphic biotite growth around altered primocryst titanomagnetite. Metamorphic garnet occurs sporadically in these iron-rich rocks. Of particular interest is the presence of disseminated sulphide minerals (with anomalous PGE), most commonly at the top of the unit. The basal contact of the overlying Ferrosyenite Unit (unit 7b, see Figure 3) is not exposed, but has been drilled by New Millenium Minerals Corporation (which became Platinum Group Metals Limited in October 2001 as the result of a merger). In this unit, ferrosyenite grades upward into alkali feldspar granite. The ferrosyenite consists of relatively scarce phenocrysts of albite (An3) and as much as 25 modal percent primocryst magnetite in a recrystallized feldspar-quartz matrix. The magnetite-rich phase of this unit is steel-grey, massive and fine grained. The overlying granite is a white, recrystallized quartz-feldspar assemblage with less than 5% magnetite. Normative calculations suggest that the igneous assemblage had as much as 30% potassium feldspar. Geochemistry A comprehensive geochemical database for the Agnew Lake Intrusion can be extracted from Vogel (1996) and a representative suite is presented in Vogel et al. (1999). Representative analyses for units from the Upper Series (not represented in the East Bull Lake Intrusion) and the Streich dike are presented in Table 4. The latter is an estimate of the parent magma composition for all units in the intrusion except for the Olivine Gabbronorite Zone (see Figures 3 and 7a). As with the East Bull Lake Intrusion, it is a high-Al, low-Ti, tholeiitic composition. Rocks in the Marginal and Lower Series of the Agnew Lake Intrusion (except for unit 8, the Dendrite Unit) are olivine-normative. Olivine leucogabbronorite compositions dominate in the Marginal Series (Mg-number 65-76), whereas olivine gabbronorite is dominant in the Lower Series (Mg-number 39 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �55-74). In the lower two-thirds of the Upper Series, gabbronorite and their leucocratic equivalents (Mgnumber 63-31) dominate. Ferrogabbro, ferrosyenite, and alkali granite compositions (Mg-number 46-12) characterize the Fe-Ti- Oxide Zone. Detailed stratigraphic studies in the Marginal and Lower Series show that these rocks formed in an open system where frequent magma pulses produced a rather monotonous sequence of mafic rocks with thin irregular intervals of fractionated material (Vogel 1996). Pearceelement ratio plots in Figures 4a and 4b show that the main chemical variation is due to plagioclase and olivine for the Marginal Series, and plagioclase, olivine and orthopyroxene for the Lower Series. All units in the Agnew Lake Intrusion show flat heavy rare earth element (HREE) patterns (10 to 40 times chondrite) and light REE enrichment (6 to 100 times chondrite for La) (see Figure 5b). REE element concentrations for the Marginal (unit 2) and Lower Series (units 3, 4a, 4b) are the most primitive of the suite (see Figure 5b). Both show positive Eu anomalies and both are more primitive in composition than the Streich dike parent magma composition. Vogel et al. (1999) illustrated, using incompatible elements such as Y, Zr, and Nb, that rocks from the Upper Series of the Agnew Lake Intrusion ultimately fractionate to form iron-rich differentiates, namely, the Fe-Ti Oxide Zone of associated ferrogabbros and oxide-rich ferrosyenites. There is no physical or geochemical evidence that these differentiates are due to magmatic assimilation of country rock. Rather, this part of the intrusion, at least in its upper portion, has apparently become a closed system, i.e., Skaergaard-like in terms of magmatic process (McBirney 1996). Pearce-element ratio diagrams (see Figure 4) and petrography indicate that plagioclase and orthopyroxene are the major phases that account for the chemical variation of rocks in the Upper Gabbronorite Zone. REE data for this zone (unit 6c) are consistent with it being less primitive than the underlying Marginal and Lower Series rocks (see Figure 5b). Rocks in the Leucogabbro subzone (unit 7a) near the top of the stratigraphy are classified as iron-rich gabbronorites by Vogel (1996). Here titanomagnetite ± clinopyroxene join plagioclase and replace orthopyroxene as the dominant mafic phases. Titanium is not a conserved element in rocks of the Fe-Ti Oxide Zone; consequently, as seen in Figure 4b, rocks from this zone are distinct from all previously described units, and their trend can not be ascribed to fractionation by a specific phase or phases. The REE pattern for this subzone (see Figure 5b) mimics the previous patterns but has substantially higher total REEs, consistent with these rocks being comagmatic with the underlying part of the Upper Series. In the Ferrosyenite subzone (unit 7b) at the roof of the intrusion, titanium is not conserved in the Pearce-element ratio plots. The trend of the data for these rocks extends from near the origin parallel to the Al/Ti axis, reflecting the varied proportion of alkali feldspar in these calcium-poor rocks. The slope of the data in Figure 5b for this subzone, and their increased total REE abundance is consistent with it being a fractionated part of the intrusive sequence. The negative Eu anomaly is due to the absence of plagioclase. Although rocks of the Olivine Gabbronorite Zone occur in the central part of the Agnew Lake Intrusion stratigraphy (see Figure 3), the trend they define in the Pearce-element ratio plot (see Figure 4a) shows that they are, on average, more mafic than strata above and below. Their trend intersects that defined by the rest of the rocks in the Agnew Lake Intrusion near the estimated parent magma composition (Streich dike) indicating that the 2 groups are comagmatic. These data are consistent with previously discussed mineralogical data. In Figure 5b, the REE pattern for rocks from the Olivine Gabbronorite Zone has a slope that is similar to other Agnew Lake Intrusion rocks, supporting the comagmatic nature of the 2 groups. The low total REE abundance is indicative of the relatively primitive and mafic character of this zone. Mineralization Finely disseminated and erratically distributed, blebby (up to 3 cm in diameter) chalcopyrite and pyrrhotite, with minor to trace pentlandite, in modal amounts from 0.5 to 2.0%, typically characterize 40 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �mineralized localities in the Agnew Lake Intrusion. Typically Cu is greater than Ni and combined Cu+Ni is approximately 1 to 1.5 wt %. In a high proportion of sulphide-rich samples, values of Pt+Pd = 0.5 to 3.0 g/t are not uncommon and values as high as 10 g/t do occur, with Pd/Pt typically 1 to 3 (data from Assessment File Reports filed by BP Resources Canada Limited, Resident Geologist’s office, Sudbury). Although there is often a positive correlation between PGE, Cu and total sulphide contents, some of the highest PGE samples have less than 100 ppm S. As with the East Bull Lake Intrusion, the greatest sulphide mineralization and highest PGE values have been found in inclusion-bearing units, with the sulphide minerals normally occurring in the matrix. Mineralization occurs in the Marginal Leucogabbronorite Zone at the stratigraphic base of the Agnew Lake Intrusion, specifically along its west margin. The varitextured Leucogabbronorite unit (unit 2a, see Figure 7a) is highly prospective within 50 to 100 m of the contact, as is the Mottled Leucogabbronorite (unit 2b) along the north margin of the body. The rocks that host this mineralization are very similar in mineralogy, texture and stratigraphic position to those that host sulphide mineralization in the Anorthosite Zone in the western East Bull Lake Intrusion (i.e., Moon Lake area). Mineralized, fragment-rich rocks also occur in the Inclusion-Bearing Gabbronorite Zone (unit 3, see Figures 3 and 7a), which forms the lowest subdivision of the Lower Series. The mineralogy and textural distribution of the silicate and sulphide mineral phases is the same as in the Marginal Series. These inclusion-bearing rocks intrude units of the Marginal Series, in particular the Nodular Unit, and clearly represent a major phase of magma injection into the magma chamber. The similarity in rock types, mineralization and field relationships with those at the east and west ends of the eastern lobe of the East Bull Lake Intrusion, suggest that the Lower Series, Inclusion-Bearing Zone, in the Agnew Lake Intrusion is correlative with the Lower Series, Inclusion-Beaing zone, in the East Bull Lake Intrusion (see Figure 3). A third significant mineralized zone occurs near the top of the intrusion in the uppermost part of the Leucogabbro subzone and immediately below the Ferrosyenite subzone (see Figure 3). The mineralization was predicted to be present by Vogel (1996), forming as a result of closed-system fractionation of a sulphur-undersaturated, PGE-rich magma. This type of occurrence is characterized by disseminated chalcopyrite and pyrrhotite, in amounts of 1 to 2 modal percent, that increase in abundance toward the contact with the overlying Ferrosyenite subzone. Mapping and trenching by New Millenium Minerals Corporation has identified several occurrences of this type of mineralization near this contact along the eastern side of Agnew Lake. A fourth environment merits mention although mineralization has yet to be recognized. The Olivine Gabbronorite Zone at the base of the Upper Series is believed to represent an influx of primitive magma (Mg-number ~70) into a chamber dominated by fractionated liquids (Mg-number ~55). Thus, the opportunity may have existed for the formation of reef-type mineralization due to magma mixing. This idea is supported by the presence of Pt+Pd values as high as 0.5 to 1.0 g/t in rocks of the same type and stratigraphic level in the East Bull Lake Intrusion (Peck et al. 2001). Vogel (1996) analyzed 22 barren samples from the Agnew Lake Intrusion, sampling all stratigraphic units shown in Figure 3. Since the PGEs partition strongly into sulphide liquid, their abundance in a silicate rock is an excellent indicator of the status of sulphur-saturation of the magma from which the sample crystallized (Hamlyn and Keays 1986; Vogel and Keays 1997). In the barren samples, Pt varies from 0.7 to 62 ppb, with most samples in the 10 to 30 ppb range; and Pd varies from 0.2 to 58 ppb, with most samples ranging from 10 to 40 ppb. These data mimic those for the East Bull Lake Intrusion (Peck et al. 1995) and are very similar to most, non-reef, silicate mineral assemblages from the Lower, Critical and lower Main Zones of the Bushveld Complex (Barnes and Maier 1999). When plotted against stratigraphic position, Pt and Pd data show peaks: 1. at the base of the Marginal Series 2. at the base of the Inclusion-Bearing Zone, Lower Series 41 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �3. near the estimated position of the Olivine Gabbronorite Zone, and 4. at the base and the top of the Leucogabbro subzone near the roof of the intrusion. All correspond to areas of known or suspected mineralization. Mantle-normalized PGE distribution for barren rocks from the Agnew Lake Intrusion is similar to that for rocks of the East Bull Lake Intrusion. The Pt-Pd-Au-Cu part of the figure is elevated (1 to 10 times mantle abundance except for Au) relative to the Ni-Ir-Ru part (<0.01 to 0.5 times mantle values) (James et al. 2002a). The anomalous PGE concentrations, together with typically very low sulphur contents (≤150 ppm), of barren samples from the Agnew Lake Intrusion indicate that the main mass of Agnew Lake Intrusion magma did not experience sulphide saturation prior to emplacement into the chamber. All samples plot in the sulphur-undersaturated area in the Cu-Pd diagram of Vogel and Keays (1997). Peach, Mathez and Keays (1990) estimated that saturation for mantle-derived magmas is about 800 ppm, well above what is observed in these rocks. Peck et al. (2001) argue that contact-type mineralization in the Marginal and Lower Series of the East Bull Lake Intrusion formed from magmas that were sulphur-saturated prior to intrusion. Mineralization was further upgraded in PGE content by prolonged high temperature mixing of very small volumes of sulphide phases with high volumes of silicate liquid upon entering the magma chamber. The complex, chaotic, varitextured nature of the rocks that host mineralization in the Marginal and Lower Series of the Agnew Lake Intrusion, and the evidence that the magma system was “open” during formation of this part of the intrusion is consistent with the model of Peck et al. (2001). The mechanism to explain mineralization near the roof of the intrusion, i.e., sulphide saturation due to fractionation in a closed system, is also consistent with a PGE-rich, sulphur-undersaturated, parent magma. It also allows for the possibility of reef-type mineralization at the base of the Upper Series due to magma mixing between the sulphur-undersaturated, parent magma and the sulphur-saturated, fractionated magma present within the evolving magma chamber (e.g., the Olivine Gabbronorite Zone, unit 5, see Figure 3.) ROAD LOG, DAY 2, AGNEW LAKE INTRUSION Note: Most of the roads used during day 2 are used regularly by logging trucks. Consequently, extreme caution should be taken when parking vehicles on the shoulder and when examining outcrops during day 2. Geological map reference: Vogel (1996), Card and Palonen (1976). 0.0 km Start at the junction of Highway 69S and the Highway 17W bypass. Set odometer to zero. Head west on Highway 17 to Massey. 0.4 km Junction of Highway 17W bypass and Regional Road 80 (Long Lake Road). Continue west on Highway 17 bypass. 70.9 km Junction of Highway 17 and 6, just north of Espanola. Continue west on Highway 17. 76.4 km Junction of Highway 17 and Agnew Lake Lodge Road on the west side of the hamlet of Webbwood. Turn right (north), and follow Agnew Lake Lodge Road north to the intersection with West Branch Road. 82.4 km Turn left onto West Branch Road and drive north for 17.0 km to the intersection with Power Line Road. 42 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �99.4 km Junction between West Branch Road and Power Line Road, turn left and drive west on Power Line Road. 100.8 km Junction between Power Line Road and Quarry Road. Park vehicles and walk south and west from junction onto a large outcrop. Stop 1. Inclusion-Bearing Unit UTM 4226691E, 5137144N. This outcrop is within the Inclusion-Bearing unit, which occurs at the base of the Lower Gabbronorite Zone (Unit 3, Vogel (1996) map, outcrop #153). On the west side of this outcrop the Inclusion-Bearing unit is in contact with the rocks of the Neoarchean Ramsay–Algoma gneiss complex. This stop shows the following features. 3 1. Overall the unit consists of metre-scale, pod-like autoliths of nodular anorthosite and pyroxenite in a medium grained, massive leucogabbronorite to melagabbronorite matrix (Photo 6). The outcrop is cut by plagioclase-phyric Matachewan diabase dikes with chilled margins. The dikes strike 340°, and exhibit fractures and veins that trend 250° and which show sinstral displacement. Photo 6. Stop 1, Day 2. Large pyroxenite pod in centre of photograph is hosted by rocks of the Inclusion-Bearing unit, shown in the foreground, which at this locality are hosted in a predominantly leucogabbronoritic matrix. Scale card near contact between the pod and the leucogabbronorite is 8.5 cm long. 43 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �2. PGE concentration for disseminated chalcopyrite-pyrrhotite-pentlandite mineralization averages 107 ppb Pd+Pt based on 723 analyses; the range is 1 to 1873 ppb, Pd/Pt = 2.3 and Cu/Ni = 2.1. 3. In part of the outcrop, pyroxenite (assemblage blue-green hornblende+quartz+magnetite in one sample) occurs as discontinuous lenses, or pods, that are 30 to 40 m in length and 5 m in width, strike 290° (see Photo 6), and which contain trace disseminated copper-nickel-PGE mineralization. 4. Assay data for the pyroxenite pod shown in Photo 6. Sample 154547, at the edge of pyroxenite pod has Pd+Pt = 264 ppb, Pd/Pt = 4.6, Ni+Cu = 969ppm, Cu/Ni = 4.2, S = 1800 ppm. Sample 154533 has Pd+Pt = 37 ppb, Pd/Pt = 1.5; Cu+Ni = 220 ppm, Cu/Ni = 1.3, S = 200ppm. Samples 154607 and 154739, middle of pyroxenite pod, have PGE at or below detection limits. 5. Granitic xenoliths are common in the Inclusion-Bearing unit at the contact between the intrusion and the older granitic gneisses at the west margin of the outcrop. Return to the Power Line Road and walk north for about 200 m along the Quarry Road to Stop 2. Stop 2. Nodular Anorthosite Unit, Marginal Leucogabbronorite Zone The quarry outcrop (Photo 7a) is an excellent example of the Nodular unit, the youngest unit of the Marginal Leucogabbronorite Zone, Agnew Lake Intrusion (Unit 2c, Vogel (1996) map, outcrop #126). This is the only area of the intrusion where this unit is observed and its age relationship relative to the previous outcrop area is clear as it occurs as autoliths in the Inclusion-Bearing unit. It occurs in a similar stratigraphic and marginal position at the east margin and the central narrow part of the East Bull Lake Intrusion (Nodular Anorthosite unit, Anorthosite Zone; Chubb, Hannila and Peck (1994)). At this locality the principal rock type is a very coarse-grained, nodular, glomeroporphyritic anorthosite (Photo 7b). It consists of glomerophenocrysts of plagioclase (1 to 14 cm in diameter) that form 80 to 90% of the rock and interstitial altered pyroxene. One to 3% disseminated chalcopyrite and pyrrhotite occur in the matrix to the nodules. Peck et al. (1995, Appendix 5, Part A) reported Pd+Pt in the range 35 to 470 ppb, Pd>Pt, Cu+Ni = 244 to 2138 ppm and Cu>Ni from similar sulphide mineral-bearing rocks from the East Bull Lake Intrusion. Chubb et al. (1995) suggested that the nodules formed due to elevated degrees of undercooling within dike-like constrictions at the margin of the intrusion. Nodule formation may have resulted from feldspar growth on pre-existing plagioclase phenocrysts to form primary nodules, followed by agglomeration processes forming larger secondary and tertiary nodules. Northwest-trending plagioclase-phyric Matachewan diabase dikes intrude the Nodular unit in this outcrop (see Photo 7a). UTM 426718E, 5137364N. Return along the Quarry Road for about 30 m and walk onto the outcrop immediately east of the road. Stop 3. Inclusion-Bearing Unit, Lower Gabbronorite Zone In walking from Stop 2 to Stop 3 you have traversed into the Inclusion-Bearing unit of the Lower Gabbronorite Zone (Vogel (1996) map, outcrops #192 to 195). The main phase of the Inclusion-Bearing unit consists of metre-scale zones or autoliths of nodular anorthosite enclosed in a coarse-grained matrix 44 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Photo 7. Stop 2, Day 2. a) Stone quarry in Nodular Anorthosite unit. Author R.S. James is standing on nodular anorthosite; dark rocks in foreground belong to a Matachewan swarm diabase dike that intrudes the Nodular Anorthosite unit. b) Close-up of nodular anorthosite, showing closely packed plagioclase glomerophenocrysts with minor interstitial, altered, pyroxene. Scale card is 8.5 cm long. 45 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �of gabbronorite to leucogabbronorite. Two several metre-wide Matachewan diabase dikes, striking 290°, intrude the unit. A younger, Mesoproterozoic, 10 to 20 cm wide Sudbury swarm diabase dike (~1240 Ma) strikes 300° and intrudes all rock units. The Sudbury dike has narrow chilled margins, cooling fractures perpendicular to its margin, and contains trace disseminated sulphide minerals. It also contains small pyroxenitic and granitic xenoliths, and weathers a rusty brown colour, in contrast to the older Matachewan swarm dikes. At this locality Pd+Pt are as high as 2155 ppb and average 178 ppb (based on 154 analyses), with Pd/Pt = 3.1 and Cu/Ni = 1.4. UTM 426870E, 5137333N. Return to vehicles, and head east from the Quarry Road–Power Line Road junction. Reset odometer to zero. 0.2 km Park by side of road and walk to outcrop beside the road. (Vogel (1996) map, outcrop #215.) Stop 4. Inclusion-Bearing Unit, Lower Gabbronorite Zone This is an additional stripped outcrop area illustrating the Inclusion-Bearing unit. It contains autoliths of pyroxenite and nodular anorthosite in a matrix of leuco- to melagabbronorite (Photo 8). Disseminated sulphide minerals occur in the matrix assemblage and not in the autoliths and range from 1 to 5 modal percent. Assay values for this mineralization are Pd+Pt average 180 ppb (based on 451 analyses), ranging from 2 to 2012 ppb, with Pd/Pt = 3.1 and Cu/Ni = 1.5. Note the 2 to 5 cm wide plagioclase-phyric dike that is locally choked with euhedral plagioclase phenocrysts where the dike width narrows (Photo 9). UTM 427074E, 5137348N. Return to vehicles and head east on Power Line Road. 0.6 km Park by side of road and examine outcrop on the north side of the road (Vogel (1996) map, outcrops #216 and 217). Photo 8. Nodular anorthosite from the Inclusion-Bearing Gabbronorite Zone, Lower Series, of the Agnew Lake Intrusion, roughly 200 m south of the nodular anorthosite quarry, in the vicinity of Stop 4, Day 2. Note the pyroxenite fragment in the centre of the photograph. 46 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Photo 9. Diabase dike containing large, rounded plagioclase crystals, similar to the nodules of the Nodular Anorthosite unit, cutting across rocks of the Inclusion-Bearing Gabbronorite Zone, Lower Series, of the Agnew Lake Intrusion. Stop 5. Sudbury-Type Breccia In this outcrop, Sudbury-type breccia cuts through a Matachewan diabase dike that strikes 230°. The dike intruded rocks of the Ramsay–Algoma gneiss complex. The breccia consists of xenoliths of plagioclase, diabase, and granitic material, ranging from millimetre- to centimetre-sized, in a foliated, fine-grained matrix that, in part, has a distinctive flow-like fabric. Chubb et al. (1995) provide detailed descriptions of several similar dikes and argue that they represent pseudotachlyte that formed as a result of the Sudbury impact event at 1850 Ma. UTM 427504E, 5137368N. Return to vehicles and head east on Power Line Road. 1.4 km Junction with West Branch Road, turn right and head south on West Branch Road. 7.1 km Pull over and park vehicles, examine outcrops on both sides of the road (Unit 2a, Vogel (1996) map, outcrops #326 (east) and #329 (west side of road)). Stop 6. Varitextured Unit, Marginal Leucogabbronorite Zone This outcrop lies within the Varitextured Unit of the Marginal Leucogabbronorite Zone, Marginal Series. Both outcrop areas (AZ 1 – east side of road, and AZ 2 – west side of road) consist dominantly of gabbronorite to melagabbronorite and minor leucogabbronorite. Inclusions of pyroxenite and scarce nodular anorthosite are scattered throughout the matrix assemblage, indicating that at least part of the 47 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �outcrop area may belong to the Inclusion-Bearing unit. Disseminated sulphide mineralization occurs mainly in the gabbronorite matrix. Over 200 analyses from the 2 outcrop areas average 232 ppb Pd+Pt with individual analyses as high as 2375 ppb; Pd/Pt is about 2 as is Cu/Ni. Later quartz veins strike at 220° and are offset by fractures and/or faults that strike 290°. UTM 425257E, 5133932N. Return to vehicles and continue south on West Branch Road. 8.0 km Junction with logging road. Turn left (east) and drive east on the logging road for ~200 m to red flagging on road side, located at UTM 425917E, 5133294N. Follow the flagged line through bush, walking north at about 334° for approximately 440 m. On the route, you will cross Gridline 3-5, 275E of Pacific North West Capital Corporation. This traverse should not be made alone, and should be only conducted using appropriate footwear and field equipment. Stop 7. Dendrite Unit, Lower Gabbronorite Zone UTM 425744E, 5133665N. The outcrop is the bottom of a cliff face that shows extensive excellent exposure of the dendritic unit of the Lower Gabbronorite Zone. This unit is recognized as a distinct stratigraphic unit in the Agnew Lake Intrusion where it is well-exposed along the west and southwest margin of the plateau area that forms the central part of the intrusion. The textures and mineralogy are the same as observed the previous day at East Bull Lake Intrusion (Stop 7, Day 1), but here the dendritic texture is much more extensively developed. The texture occurs either as large irregular masses, sometimes almost layer-like, or as pod-like zones, particularly where this unit is gradational into the overlying layered gabbronorites. At this locality the dendritic habit of the altered pyroxenes can be rapidly replaced by coarse-grained gabbroic pegmatite assemblages, so that the rocks become quite varitextured. Examine the outcrop at the base of the cliff face and then work your way up the ridge, all the time observing the variation in rock types or the lack of variation. Having climbed the cliff face we will traverse east through the Dendritic Unit and into the overlying Layered unit of the Lower Gabbronorite Zone. In doing so we are walking through outcrop #333 into #334 (Vogel (1996) map). At the top of the cliff face (in outcrop #333) the dendritic gabbronorite is more typically restricted to local pod-like areas and enclosed in a coarse-grained to almost pegmatitic gabbroic rock. In the western part of outcrop #334 metre-scale isomodal phase layering is present, identical in general features to that observed in the East Bull Lake Intrusion. At this locality it dips approximately 30° to the east. Locally, pods of dendritic gabbronorite are observed in the Layered unit. The stratigraphic units that overlie the Layered unit to the east exhibit decametre or thicker scale layering. As a result, shallow dipping surfaces consisting of individual layers form extensive outcrops of apparently only one rock type. Return to the vehicles by walking southward (~160°) across the top of the plateau to intersect the logging road, then follow the road southwest to the parked vehicles at the base of the plateau. This ends Day 2. Retrace route to West Branch Road, Agnew Lake Lodge Road, and Highway 17. 48 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �RIVER VALLEY INTRUSION, OVERVIEW Country Rocks, Structure and Geometry The River Valley intrusion is located about 65 km east-northeast of Sudbury (see Figure 1) near the community of River Valley. The intrusion underlies parts of Crerar, Dana, Henry, Janes, McWilliams and Pardo townships, but is best exposed in Dana Township, where it locally contains well preserved primary mineralogy and textures (Hrominchuk 1999, 2000; Easton and Hrominchuk 1999, 2001a, 2001b; Easton 2001, 2003). Although recently mapped (Easton and Hrominchuk 1999, 2001a, 2001b; Easton 2003; Easton and terMeer 2004; Hrominchuk 1999, 2000), studies of the stratigraphy, geochemistry and mineralization present within the River Valley intrusion are limited in comparison to similar studies on the East Bull Lake and Agnew Lake intrusions. Of all the East Bull Lake intrusive suite bodies studied to date, the River Valley intrusion exhibits the clearest relationships with other members of the Paleoproterozoic rifting suite described earlier. It is cut by mafic dikes geochemically correlated with the Matachewan and Hearst dike swarms, as well as by felsic intrusive rocks coeval with the Huronian Supergroup metavolcanic rocks (Easton and Hrominchuk 1999, 2001b). Furthermore, in Street Township, granitic rocks dated at 2460±20 Ma, contain inclusions of East Bull Lake intrusive suite leucogabbroic and anorthositic rocks (Easton and Murphy 2002). Easton (2003) has reported similar inclusions in granitic plutons in Henry and Loughrin townships. Unfortunately, the relationship between the River Valley intrusion and the Huronian Supergroup is obscured by tectonism along the Grenville Front. Along the Grenville Front, the River Valley intrusion is either in thrust contact with quartzite of the Mississagi Formation (Davidson 1986) or is in an unknown contact relationship with mafic and felsic metavolcanic rocks of the lower Huronian Supergroup (Easton and Hrominchuk 1999). On the basis of geochemical correlation, however, these metavolcanic rocks are likely ~2460 Ma in age (Easton and Hrominchuk 1999), and consequently, most likely were deposited after emplacement of the River Valley intrusion. The River Valley intrusion is also cut by olivine diabase dikes of the 1238 Ma Sudbury dike swarm (Easton 2000) and the Grenville dike swarm (590 Ma). Intrusive contacts between the River Valley intrusion and the Archean basement are exposed throughout the eastern half of the intrusion, and are generally sharp. Zones of intense diking, equivalent to the Border Zone in the East Bull Lake Intrusion, have not been observed. In fact, in both Dana and Crerar townships, Archean para- and orthogneisses adjacent to the intrusion are devoid of any mafic dikes, apart from a few Matachewan–Hearst swarm diabase dikes. In some parts of Dana Township, a fine- to medium-grained gabbro is present at the contact, and may represent a chilled facies of the intrusion. In many areas, however, an inclusion and/or fragment-bearing facies occurs proximal to the contact, making it difficult to ascertain the true extent and significance of this finer grained facies. As in the case of the East Bull Lake Intrusion, a “syenite” (actually an alkali feldspar granite) body in Dana Township was originally interpreted as a felsic differentiate of the River Valley intrusion (e.g., Card and Lumbers 1975). On the basis of contact relationships and geochemistry, Easton (2000) interpreted it as Archean in age, an interpretation that has been recently confirmed by U/Pb zircon dating, which yielded a preliminary age of ~2660 Ma (Easton and Kamo 2003). 49 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Previous maps of the River Valley intrusion (e.g., Lumbers 1973; Card and Lumbers 1975) indicated that the intrusion extended west into Henry and Loughrin townships as a series of “tails” surrounded by gneissic country rocks. This map pattern was interpreted to be the result of either 1. a pattern of complex interference folding during Penokean and Grenvillian orogenesis, or 2. a system of feeders to the main body of the intrusion located in Crerar and Dana townships. Furthermore, it was also speculated that other isolated bodies of East Bull Lake intrusive suite rocks in the Grenville Province, west of the River Valley intrusion (e.g., Red Deer Lake, Street Twp., southern Wanapitei complex, Table 1), represented parts of the River Valley intrusion now isolated from the main mass through erosion and tectonism. Work conducted by Easton (2003) suggests otherwise. East Bull Lake intrusive suite rocks west of Crerar Township are different in form, stratigraphy, and composition, and are intruded into distinctively different country rocks than those found in the River Valley intrusion. Consequently, East Bull Lake intrusive suite rocks located west of Crerar Township likely represent part of one or more intrusions that were emplaced separately from the River Valley intrusion. STRUCTURAL STATE AND DEGREE OF PRESERVATION Figure 8 outlines the general structural state of the River Valley intrusion and its country rocks in Dana and Crerar townships, as well as the degree of textural preservation within the intrusion and the country rocks. Six main preservation regimes can be recognized, which are described by Easton (2003), these form the basis of the divisions shown in Figure 8. The Sturgeon River, and the Neoproterozoic to early Paleozoic fault that extends along it, is an important structural feature within the River Valley intrusion (see Figure 8). North of the Sturgeon River, the River Valley intrusion contains large areas of preserved or partly preserved primary mineralogy. Deformation is concentrated along discrete shear zones, both vertical and subhorizontal, that cut the intrusion (Easton and Hrominchuk 1999, 2001b; Hrominchuk 2000). Grenvillian deformation is largely concentrated near the Grenville Front, along the west side of the intrusion (see Figure 8). North of the Sturgeon River, Grenvillian metamorphism results in the incipient development of corona textures in rocks of the River Valley intrusion and the Sudbury diabase dike swarm, with metamorphic conditions estimated at 5 to 7 kb and 625°C (Tettelaar 2000; Easton and Hrominchuk 1999). An empirical observation is that rocks of leucogabbroic composition are most commonly recrystallized, whereas orthopyroxene-bearing rocks are generally more resistant to metamorphic recrystallization. The geometry of the intrusion north of the Sturgeon River appears to be largely sheet-like, with igneous layering generally dipping shallowly (20 to 30°) to the south and southeast. This has resulted in exposure of the basal contact (margin or sidewall?) of the River Valley intrusion for 8 to 10 km along its north and east margins, providing an extensive target area for mineral exploration. In contrast, south of the Sturgeon River, pockets of preserved mineralogy and texture (Photo 10) are less abundant, occurring mainly in eastern Crerar Township (see Figure 8). Igneous layering at several scales is preserved within this area of the intrusion (Photo 11) (Easton 2003). Where present, igneous layering dips moderately (40° to 60°), and stratigraphy is apparently folded (Easton and Hrominchuk 2001a). This folding results in a variety of trends for the basal, mineralized contact of the intrusion, making mineral exploration more difficult than in the north. Penetrative fabrics of Grenvillian-age are common (Photo 12), with much of the River Valley intrusion consisting of gneissic rocks of gabbroic and leucogabbroic composition. Grenvillian metamorphic effects result in more complete development of corona textures in rocks of both the River Valley intrusion and the Sudbury diabase dike swarm in Crerar Township, with metamorphic conditions estimated at 7 to 9 kb and 650°C (Tettelaar 2000; Easton and Hrominchuk 1999). 50 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �5174225 N C) drift covered Structural and Preservation Regimes Regime I Regime2a Regime 2b Regime2c Regime3a Regime 3b Regime4a Regime 4b Regime 5 Regime 6 • Regime 7 ,,' geological contact road ,)( railway (abandoned) ,V township boundary 5153225 N Figure 8. Map showing the degree of primary textural preservation within rocks of Crerar and Dana townships (from Easton 2003). 51 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Photo 10. Progressive textural changes. a) Igneous-texture in medium-grained norite from leucogabbronorite zone of the River Valley intrusion, Crerar Township. Note dark colour of feldspars. UTM 556115E 5171466N. b) Partly recrystallized rock of the River Valley intrusion, preserving igneous-texture in medium-grained gabbronorite from leucogabbronorite zone of the River Valley intrusion, Crerar Township. Mafic minerals have thick, green-weathering amphibole coronas rimming dark orthopyroxene grains. UTM 556886E 5161009N. c) Recrystallized but massive leucogabbronorite from leucogabbronorite zone of the River Valley intrusion, Crerar Township. This rock no longer preserves primary mineralogy, but is relatively undeformed. Note white colour of feldspars, and lack of well defined crystal shapes. Mafic mineral clots are aggregates of fine-grained amphibole. UTM 557400E 5160225N. d) Typical undeformed but recrystallized leucogabbronorite from the Positano Quarry, East Bull Lake intrusive suite, Loughrin Township. UTM 547000E 5160600N. Hammer handle in all photos is 30 cm long. CONTACTS Copper, nickel and platinum group element exploration in the River Valley intrusion has focussed on its contacts, as outlined by Lumbers (1973). This is only an effective exploration tool if the contacts of the intrusion are primary and not tectonic. Easton (2003) noted 3 main contact types that are present between the River Valley intrusion and other rock units in the map area: 1. preserved intrusive contacts, 2. deformed and disrupted intrusive contacts, and 3. wholly tectonic (fault) contacts. Near the northwest edge of Dana Township, the western contact of the intrusion is tectonic, as originally described by Davidson (1986). The northern and eastern contacts of the intrusion are primary, although they are locally cut, with minimal lateral offset, by mylonite of the Southern–Grenville province boundary zone. Mineralization in northwestern Dana Township (Dana North and South zones of Pacific North West Capital Corporation) is spatially associated with preserved intrusive contacts. Much of the 52 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Photo 11. a) Igneous layering within the norite zone, River Valley intrusion, Crerar Township. UTM 556769E 5159980N. b) Igneous layering within the River Valley intrusion in Crerar Township, near the top of the anorthosite zone and the base of the olivine gabbronorite zone. Dark layers are melanorites, some of which are olivine-bearing. UTM 557390E 5160360N. c) Layering within leucogabbronoritic rocks immediately above the heterogeneous basal zone of River Valley intrusion, Crerar Township. UTM 557585E 5159590N. d) Large-scale igneous layering within leucogabbronoritic rocks of the River Valley intrusion exposed in the Dana Quarry, Dana Township. UTM 558630E 5165385N. Hammer handle in all photos is 30 cm long. northern contact of the intrusion in Dana Township is likely primary, and both the Lismer’s Ridge and Azen Creek zones occur near this contact. It should be noted that based on surface exposure, the Azen Creek zone is apparently located near, but not at the contact. Recent drilling by Pacific North West Capital Corporation has shown that the contact in the Azen Creek zone dips generally shallowly to the south and in fact the Azen Creek zone is proximal to the contact – the difference is that the mineralization is hosted in “inclusion-bearing” or fragment-bearing units rather than the breccias observed in the Dana North area. Also, recent drilling at Jackson’s Flats shows the contact is oriented in a northwest direction and is steeply dipping, with mineralization occurring in fragment-bearing units rather than “breccia”, much like the situation at the Azen Creek zone. In Crerar Township, a primary intrusive contact is preserved in the area that includes the Tomrose occurrence, and Crerar 2 to 4. To date, most of the mineralization identified in Crerar Township lies near this contact. 53 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �a. rN Photo 12. a) Typical leucogabbroic gneiss of an East Bull Lake intrusive suite intrusion in Henry and Loughrin townships. Note the lack of a prominent leucosome phase. UTM 548443E 5159789N. b) Protomylonitic to mylonitic textures present in a highstrain zone in the River Valley intrusion, Crerar Township. UTM 559751E 5163890N. c) Strain gradient within rocks of the River Valley intrusion, northern Crerar Township. Foliated to gneissic leucogabbro on top, straight and irregularly layered gneiss on bottom. UTM 557225E 5162512N. d) Epidote-clot developed just right of the hammer handle in leucogabbro gneiss of the River Valley intrusion, Crerar Township. Such epidote clots are common in gneissic leucogabbro throughout the study area. UTM 555495E 5163580N. Hammer handle in all photos is 30 cm long. In contrast, most of the southern contact of the intrusion in Crerar Township, although poorly exposed, is a major shear zone, which is discordant to internal contacts in the intrusion. It is likely that the eastern contact of the River Valley intrusion north of the Sturgeon River, is also a tectonic contact. Examples of tectonized intrusive contacts may be present in 3 areas. 1. The western contact of the intrusion along Highway 805, particularly in the southern half of Dana Township. However, it is possible that this contact represents an upper, rather than a basal, contact of the intrusion. 2. The other 2 examples occur in Crerar Township, but represent areas where zones of mafic straight gneisses are in contact with other highly deformed gneisses or different units within the intrusion. Similar rocks occur just south of Glen Afton, south of the Sturgeon River; however, to date no mineralization has been reported from the latter area. 54 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Hrominchuk (1999, 2000) recognized 2 types of mafic rock associations near areas of primary contacts. These are a marginal zone and an inclusion-bearing zone. There is considerable variation in thickness of both these zones along the primary contacts of the intrusion, ranging from 50 m thick to almost 500 m thick in the Dana North area. To date, mineralized zones along the contact occur where these units are relatively thick (e.g., Dana North, Varley, Azen, Tomrose). Stratigraphy and Petrography Although historically referred to as an anorthosite to anorthositic gabbro intrusion, the River Valley intrusion is dominated (~60% of surface area) by gabbro, norite, gabbronorite, leucogabbronorite and leuconorite, with some units containing modal olivine. True anorthosite makes up less than 10% of the surface area of the intrusion, but even in these areas, the distribution of mafic minerals is irregular, so that the areas of anorthositic rock are closer to noritic anorthosite or gabbroic anorthosite in composition. Noritic and gabbroic anorthosite probably underlie about 30% of the surface area of the intrusion. Observed stratigraphy in the intrusion differs north and south of the Sturgeon River. NORTH OF THE STURGEON RIVER Low regional dip and poor exposure in key areas in Dana Township has hampered stratigraphic analysis, despite the textural and mineralogical preservation of the rocks. Despite these difficulties, Hrominchuk (2000), as presented in Figure 9, developed an interpretive stratigraphic section for the River Valley intrusion located north of the Sturgeon River. The intrusion north of the Sturgeon River has been subdivided into 5 zones (Hrominchuk 2000; James et al. 2002a), as follows: 1. Marginal zone 2. Inclusion and/or Fragment-Bearing zone 3. layered Olivine Gabbronorite zone 4. Layered Gabbronorite zone, and 5. massive, locally varitextured, Leucogabbronorite zone. Marginal Zone The Marginal zone is characterized by a fine- to medium-grained, black plagioclase-phyric, quartzphenocryst-bearing, gabbronorite that is partly chilled and is typically strongly mineralized at the contact with granitic plutonic and gneissic rocks. This contact is commonly sheared and normally consists of a complex chaotic mixture of footwall blocks and fine- to medium-grained gabbronoritic rocks. Because rocks immediately adjacent to the contact usually contain large blocks and screens of footwall material, exact definition of the contact in some areas is problematic. The Marginal zone consists mostly of medium-grained norite to gabbronorite and is typically well preserved. In thin section, the rock consists of dark-brown to tan cumulate (An79 core; An74 rim) to intercumulate (An68) plagioclase, woody-brown cumulate orthopyroxene (En44-76), interstitial clinopyroxene, with accessory magnetite, quartz, biotite, sulphide minerals, and calcic myrmekite. Fragments are commonly quartzofeldspathic gneiss, alkalifeldspar granite and amphibolite gneiss. Granitic fragments contain quartz with a small amount of alkali feldspar and biotite or amphibole. Quartzofeldspathic gneiss fragments commonly have a preserved gneissic texture and are mostly quartz with a small amount of biotite. Amphibolite gneiss fragments are a fine-grained, commonly foliated, matte of blue-green amphibole and epidote group minerals. 55 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �______ 0) tic. U C.0.2 a -s 900— vv vvv vv vvv vv a a u 9 a ._a I En% Fo°k An°/o 800— v v v a I I I I Leucogebbronorite . zone 56-63 700— v"v"v vvv vv vvv 56-69 I I - - - - - - t - 600— : I 500— 57-66 5875 Gebbronorite zone : I ' • . : 300— . olivine Gebbronorite zone 200— -. . es;-:->: Inclusion and/or Fragment Bearing -zone Ivierginel zone 67-78 72-76 ' . 170-76 ' I • • 68-73 1 68 155-69 I • 39-81 I :44-76 1 1 : 1 • • I 1 :I 1 1 1 I I (meters) Primocryst mineral Interstitial mineral Figure 9. Interpreted stratigraphic sequence and distribution of primary mineral phases and compositions within the River Valley intrusion north of the Sturgeon River. From Easton (2003). Inclusion and/or Fragment-Bearing Zone This zone is commonly in gradational contact with the underlying Marginal zone, but its appearance is marked by a sharp increase in the size and amount of felsic fragments and the appearance of compositionally and texturally varied mafic to ultramafic inclusions or xenoliths. Inclusions consist of 3 main types: alkali feldspar granite and other granitic rocks, amphibolite, and olivine-rich rocks (troctolites, peridotites). Granitic, gneissic and amphibolite fragments are similar to those described above for the Marginal zone, but they are commonly surrounded by pods or lenses of a very coarsegrained to pegmatitic assemblage of biotite, muscovite, amphibole, plagioclase, quartz and alkali feldspar (Hrominchuk 2000). It is important to note, however, that in all of the mineralized areas explored by Pacifc North West Capital Corporation to date, there have been less than 1% exotic fragments in the Breccia and Inclusion-Bearing units, which is their equivalent to this zone. Autoliths are abundant (typically up to 90%) and locally (e.g., outcrop scale) may constitute up to 100% of the rock. Troctolite or peridotite autoliths are similar in all respects to the olivine norite to melatroctolite and peridotite from the Olivine Gabbronorite zone described below; most are olivine-bearing gabbronorite to peridotite in composition, with subordinate leucogabbronorite. They are usually well rounded and weakly to very strongly mineralized. The host matrix to the autoliths and fragments is typically a fine- to mediumgrained, green to black gabbro or olivine-bearing gabbro to gabbronorite, often with disseminated to blebby sulphide mineralization. 56 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Olivine Gabbronorite Zone The Olivine Gabbronorite zone is composed of 0.3 to 1.5 m thick layers that are olivine-rich at the base, and erosive into underlying layers, and which grade upward into olivine gabbronorite or troctolite to slightly more plagioclase-rich end members. Layers are deeply weathered due to the presence of serpentine minerals and fine-grained magnetite after olivine. This zone is well exposed in only a few places, but is normally 50 to 150 m thick, and grades into the overlying, olivine-poor layered Gabbronorite zone. The Olivine Gabbronorite zone contains a wide range of rock types; however, olivine is present in most, with troctolite and olivine norite predominant. Olivine is generally medium- to coarsegrained, Mg-rich (Fo72-76), coronitic, and variably serpentinized. Plagioclase is calcic (An72-76), darkcolored, and inclusion rich. Orthopyroxene (En70-76) occurs in 2 forms, as euhedral woody prisms, and as large (up to 10 cm) oikocrysts enclosing olivine and plagioclase. Clinopyroxene is found only as interstitial anhedral crystals. Gabbronorite Zone The Gabbronorite zone is composed of metre-scale, modally layered rocks with a maximum continuous thickness of about 200 m. This represents a minimum stratigraphic thickness, since faults or zones of intense deformation always truncate the top of the section. The unit typically consists of alternating metre-thick layers of norite and gabbronorite (±olivine) and leucogabbronorite. Locally, this unit is very coarse-grained, with zones rich in felsic fragments surrounded by pegmatitic pods containing quartz, epidote, magnetite, alkali feldspar, plagioclase, amphibole, pyroxene, biotite and garnet. The Gabbronorite zone consists of layers that begin as olivine-bearing norite and fractionate to gabbronorite. Coarse-grained black norite dominates; it consists of large (2 cm) orthopyroxene (En57-66) and brown plagioclase (up to 3 cm)(An58-75) primocrysts, interstitial clinopyroxene and inverted pigeonite, and rare olivine. Metamorphosed equivalents contain altered plagioclase, amphiboles (fine-grained actinolite and coarse-grained hornblende), small garnets and quartz. Leucogabbronorite Zone This zone consists of non-layered, massive, medium- to coarse-grained rocks ranging in composition from leucogabbronorite to anorthosite. It forms the highest stratigraphic levels of the intrusion (generally the tops of topographic highs) in Dana Township. They are commonly highly deformed or faulted; a complete section of these rocks has not yet been found in Dana Township; however, a more complete section may be present in Crerar Township (Easton and Hrominchuk 1999). These rocks consist mainly of dark plagioclase (An56-63) with subordinate interstitial and primocryst clinopyroxene and inverted pigeonite (En56-69). Where metamorphosed, the more leucocratic rocks form very white-weathering rocks with small pods of amphibole-rich material dispersed throughout. SOUTH OF THE STURGEON RIVER As previously noted, establishing a stratigraphic column for the River Valley intrusion has been difficult, despite the state of preservation of the rocks. South of the Sturgeon River, steeper regional dips have made the task somewhat easier; however, the section there may contain more layer-parallel faulting, and units are only well preserved in the southwestern part of the intrusion. Easton and Hrominchuk (1999) originally suggested that a section up to 3.5 km thick could be identified; however, this interpretation assumed that the units were dipping uniformly to the west at roughly 50°, that the section was not folded, 57 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �and that there were no layer-parallel faults present. Further analysis indicated that the section was indeed folded (Easton and Hrominchuk 2001a). Unlike the area north of the Sturgeon River, this section has not been formally divided into stratigraphic zones; for reference, however, informal zone names are given in italics to facilitate comparison with the Dana Township section. The Crerar Township section has a maximum thickness of 1.4 km, and from base upward, consists of: • 150 to 200 m of gabbro and melanogabbro, forming a heterogeneous Marginal zone that contains fragment-rich zones, disrupted layering, and considerable diking. Sulphide minerals are common in this zone. This zone grades upward into leucogabbro and anorthositic gabbro. This zone corresponds roughly to the Marginal and Inclusion and/or Fragment-Bearing zones present in the intrusion north of the Sturgeon River. • 400 to 500 m of gabbroic anorthosite to anorthosite (Anorthosite zone and thin Olivine Gabbronorite zone). Melanogabbro and olivine melanogabbronorite layers up to 2 m thick occur near the top of this zone. This zone apparently does not have an equivalent unit north of he Sturgeon River, but a lowermost gabbroic anorthosite to anorthosite is present in most other East Bull Lake suite intrusions (Figure 10). It is possible that the melanocratic layers in the upper part of this zone correspond to the Olivine Gabbronorite zone present north of the Sturgeon River. • 350 m of norite and gabbronorite (Gabbronorite zone), commonly layered on the scale of centimetres to decametres, with layers having pyroxenite bases and gabbroic anorthosite tops (Photo 10b). Layering on the scale of metres to decametres is also inferred to be present in this zone. This zone may be equivalent to the Gabbronorite Zone present north of the Sturgeon River. • 200 m of leucogabbro to gabbroic anorthosite (Leucogabbronorite zone). This zone may be equivalent to the Leucogabbronorite Zone present north of the Sturgeon River. • 200 m or more of oxide-bearing leucogabbro to gabbroic anorthosite (Oxide-Bearing Leucogabbronorite zone). A regional magnetic anomaly is associated with this unit. Rocks within this zone show a marked decrease in Mg-number (less than 50) compared to other stratigraphic units in the River Valley intrusion (typically greater than 57). • The top of the section is not observed due to intense deformation in the rocks above this stratigraphic level. The most significant difference between the Dana and Crerar township sections is the presence of a thick unit of leucogabbro to anorthosite between the Marginal zone and the Olivine Gabbronorite and Gabbronorite zones in the Crerar Township section. In one locality, a few outcrops of nodular anorthosite are present near the base of the leucogabbro to anorthosite unit. At another locality, this unit contains a decametre-thick layer of megacrystic anorthosite, with 1 to 5 cm equant, blocky plagioclase crystals partly separated by clotty, coarse-grained and weakly poikilitic, amphibolitized pyroxene. In addition, the varitextured to massive uppermost Oxide-Bearing Leucogabbronorite zone is not observed in Dana Township, and is the only unit that contains both Mg-number and normative An less than 55. 58 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �____________ ______________ ___________ __________ , +. __________ __________ well mineralized zones Fe-Ti oxide zone e layered zones C leucogabbronorite C gabbronorite, norite Agnew Lake Intrusion EliEl gabbroic anorthosite a ] melatroctoiite, 0 livine nonte and 2250 gabbronorite 2000 olivine gabbronorite inciusion - bearing zone 1750 border zone Upper Series 1500 River Valley Intrusion (south) East Bull Lake Intrusion / V VVVV — 1250 River Valley Intrusion 1 (noah) Norite-gebbro1 norite zone Upper Series Street Intrusion 1000 750 — norite zone1 Main Olivine 500 Series zone £ s SASS a a aS 1 Inclusion! Fragment Bearing zone tLeU000abbro Contact 250 H ii —in— ni Lower -S-if. 0 Marginal Series (metres) Figure 10. Generalized cross-sections for the Street metagabbro and the River Valley, East Bull Lake and Agnew Lake intrusions of the East Bull Lake intrusive suite. East Bull Lake section modified from Peck et al. (1995), Agnew Lake Intrusion section modified from Vogel (1996), River Valley data from Easton (2003). STRATIGRAPHIC COMPARISON WITH OTHER EAST BULL LAKE SUITE INTRUSIONS As shown in Figure 10, the composite stratigraphic section from the best preserved enclave of the River Valley intrusion in Crerar Township contains elements common to both the northern River Valley, the East Bull Lake, and the Agnew Lake intrusions. If the Crerar Township stratigraphy is a true section through the intrusion, then it is similar in thickness to other East Bull Lake intrusions (see Table 1). The exception is the Agnew Lake Intrusion (∼2.3 km thick), which unlike the other intrusions, includes felsic differentiates near the top. The Crerar Township section also suggests that the River Valley intrusion locally includes an anorthositic zone near the base of the body, similar to most other East Bull Lake intrusions (see Figure 10). 59 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Geochemistry Geochemical data from the intrusion can be found in Easton and Hrominchuk (2002) and Easton (2003). Overall, the chemistry of the rocks are similar to that of other intrusions of the East Bull Lake suite. Silica is generally less than 50 weight percent. Vogel et al. (1999) noted that in the case of the Agnew Lake Intrusion, SiO2 less than 50 weight percent is characteristic of the stratigraphically lower part of the intrusion. The same effect may be present in the River Valley intrusion, as most of the samples reported to date are from the lower part of the intrusion (below the Oxide-Bearing Leucogabbronorite zone). A few samples, located stratigraphically higher in the River Valley intrusion in Crerar Township show a marked decrease in Mg-number (from typically greater than 57 to less than 43). These rocks are generally of gabbroic anorthosite composition and oxide mineral-bearing. Representative analyses of each stratigraphic unit present in the River Valley intrusion north of the Sturgeon River are listed in Table 5, as well as in Hrominchuk (2000) and James et al. (2002a). Rocks of the Marginal zone are CIPW quartz-normative; Olivine Gabbronorite zone rocks are olivine-normative, whereas most rocks in the upper units are only silica saturated. Samples from the Marginal zone (see Table 5, analyses 1 and 2) show much higher Ti, Fe, K, Na, P, Ba, Zr and Y contents than do most other River Valley intrusion samples. Hrominchuk (2000) interpreted these elemental variations as a possible contamination signature. The high concentration of Ti and Fe in some footwall phases (see Table 5, analysis 12), relative to typical River Valley intrusion rocks, is consistent with some of the Ti and Fe contents in sample CZ7-1000 resulting from wall rock assimilation (James et al. 2002a). The matrix of the Inclusion and/or Fragment-Bearing zone is high in Mg, Ca, Al, and Fe, consistent with its plagioclase, orthopyroxene and olivine-dominated mineralogy. The fragments, inclusions and xenoliths of this zone are extremely varied in composition, partially resorbed, and commonly mineralized. The felsic fragments show similarities to the footwall alkali feldspar granite (James et al. 2002a). Mafic inclusions are similar to the rocks in the lowest part of the layered Olivine Gabbronorite zone. The Olivine Gabbronorite zone is characterized by rocks high in MgO (15 to 24 wt.%) and low in SiO2 (38 to 45 wt.%), with anomalous Ni contents attributed to the predominance of primocryst olivine (see Table 5, analyses 5 and 6). Rocks from the layered Gabbronorite zone show gradual fractionation to more Si-, Al-, Ca-, Na-, K-rich and Mg-poor compositions (see Table 5, analysis 9), a trend also observed in the uppermost stratigraphic unit. A sample of magnetite gabbro from the uppermost part of the stratigraphy in Crerar Township (Easton 2003) may be similar to the lowermost ferrogabbros from the Agnew Lake Intrusion, and is the most evolved sample found to date in the River Valley intrusion. Pearce element ratio diagrams for the River Valley intrusion (James et al. 2002) and detailed petrography indicate that major and trace element trends are controlled by plagioclase, olivine and orthopyroxene, with very little contribution from clinopyroxene (James et al. 2002a). Samples from the Olivine Gabbronorite zone show a variation from olivine + plagioclase near the base to plagioclase + orthopyroxene controlled geochemistry near the top of the zone (James et al. 2002a). Plagioclase is commonly a phenocryst phase within marginal and fine-grained rocks indicating very early crystallization, and most likely quickly joined by olivine as the first cumulate phases. Fractionation and cooling of the magma caused orthopyroxene to replace olivine as a cumulate phase during crystallization of the marginal rocks and the Olivine Gabbronorite zone. This is indicated by orthopyroxene rims on olivine and large orthopyroxene oikocrysts enclosing olivine and plagioclase near the top of this zone (James et al. 2002a). Rocks above this point show trends that indicate fractionation of magma due to plagioclase and orthopyroxene toward more Fe- and alkali-rich compositions. Chondrite-normalized rare earth element patterns for samples north and south of the Sturgeon River are shown in Figure 11. Most samples show total REE contents and patterns similar to those found in the 60 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �East Bull Lake and the Agnew Lake intrusions. In Figure 11, REE patterns for the proposed Dana Township stratigraphy show a slight fractionation trend with stratigraphic height and the influence of cumulate plagioclase (reflected in the magnitude of the negative Eu anomaly). The contaminated contact sample shows significant light REE enrichment over other marginal samples and rocks of all other zones. There are 2 groups of melanocratic rocks (Easton 2003). One with patterns and REE contents similar to other River Valley intrusion rocks; the other with high total REE and La/Yb. This second group has higher normative-orthopyroxene and lower-normative olivine contents than the first. A similar two-fold grouping occurs in the orthopyroxene hornblendite samples, perhaps suggesting a genetic linkage between the River Valley intrusion and these bodies. A sample of a primitive dike or layer from the Marginal zone exhibits a flat (~10 times chondrite) to slightly U-shaped pattern (see Figure 11), and is also characterized by relatively high silica and MgO, and low Ti and alkalis. It may represent a boninite-like parent magma composition for the River Valley intrusion (James et al. 2002a). The significance of such a parental magma has already been discussed in the section “Magma Composition and its Relationship to Mineralization”. —4— Contaminated Contact —0—Marginal Zone —A—Inclusion and\or Fragment Bearing Zone —X—Olivine Gabbronorite Zone —— Gabbronorite Zone —0-— Leucogabbronorite Zone —0-— Primitive dike/layer La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Figure 11. Chondrite-normalized rare earth element plots for the River Valley intrusion for rocks north of the Sturgeon River. Sample from the “primitive dike or layer” may represent the parent magma composition to the mineralized parts of the intrusion in Dana Township. 61 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Table 5. Representative chemical analyses from the River Valley intrusion (data from Easton and Hrominchuk 2002). Analysis Sample No. 1 CZ7-500 2 CZ7-1000 3 99RME-2242 4 99RME-2243 Zone Rock Type MZ fg gabbrodiabase MZ fg gabbrodiabase IFBZ Stop 7, cg troctolite inclusion 559150 5167678 IFBZ Stop 7, mg olivine gabbronorite 559149 5167677 Easting Northing wt. % SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 CO2 S LOI Total Mg # Ppm Cr Ni Co Sc V Cu Zn Rb Ba Sr Nb Zr Y Zr/Y Ppb Au Pt Pd Pt/Pd Total (Au+Pt+Pd) 558431 5168104 558431 5168604 5 99RME2371A OLGN cg troctolite 559597 5166341 6 JLHS1-S3 7 JLHS2-S1 8 JLHS2-S4 OLGN cg melatroctolite GN cg gabbronorite GN cg norite 559657 5166341 558643 5165245 558644 5165393 52.51 0.93 14.17 13.40 N/A 0.17 5.16 8.79 2.16 0.85 0.09 0.26 0.024 1.36 99.59 40.9 51.85 0.91 14.36 13.09 N/A 0.18 5.48 8.96 2.25 0.94 0.10 0.22 <0.005 1.60 99.72 43.0 44.14 0.16 12.79 2.57 13.21 0.20 15.67 7.18 1.18 0.20 0.01 <0.03 0.67 0.01 97.32 62.1 48.42 0.21 18.81 1.51 8.38 0.14 10.16 9.82 1.87 0.30 0.03 00.07 0.18 0.25 99.90 62.9 43.32 0.10 18.30 9.34 N/A 0.12 14.06 8.37 1.34 0.28 0.03 0..22 <0.005 4.51 99.77 73.0 38.30 0.14 7.85 15.10 N/A 0.20 23.72 3.70 0.70 0.12 0.04 0..22 <0.005 10.12 99.99 73.9 50.21 0.13 19.95 8.23 N/A 0.13 6.90 10.32 2.09 0.21 0.03 0..18 <0.005 1.99 100.19 60.2 50.81 0.18 18.43 8.92 N/A 0.14 7.46 11.19 1.87 0.20 0.03 0.15 <0.005 0.43 99.66 60.1 90 57 50 29 284 171 109 37 341 255 2.9 86 16 5.5 115 75 39 30 287 401 108 46 251 193 0.9 80 18 4.3 203 1840 119 15 93 4512 122 3 81 151 0.4 12 4 3.0 250 623 65 14 91 1320 84 5 110 219 0.7 21 6 3.5 136 595 78 11 43 16 92 4 93 235 0.3 7 4 1.7 159 1360 155 17 48 162 111 1 79 94 0.9 17 8 2.2 73 97 41 20 71 18 56 16 139 238 0.2 3 0.7 4.7 90 92 38 28 109 94 66 10 121 220 0.2 4 2 2.4 <5 18 19 0.95 36 6 18 25 0.72 49 145 576 1980 0.30 2701 59 106 293 0.36 458 <5 <8 <8 <5 10 11 0.95 21 <5 <8 19 <5 11 29 0.38 40 <21 19 Abbreviations: cg = coarse-grained, fg = fine-grained; mg = medium-grained; N/A = not analyzed; IBZ = inclusion-bearing zone, GN = gabbronorite zone, LGN = leucogabbronorite zone, MZ = marginal zone, OLGN = olivine gabbronorite zone. Easting and Northings are in UTM Zone 17, datum NAD 83. 62 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Table 5. continued. Analysis Sample No. Zone Rock Type Easting Northing wt. % SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 CO2 S LOI Total Mg # Ppm Cr Ni Co Sc V Cu Zn Rb Ba Sr Nb Zr Y Zr/Y Ppb Au Pt Pd Pt/Pd Total (Au+Pt+Pd) 9 99RME-2116 LGN cg gabbronoritic anorthosite 556346 5166811 10 99RME-2244 LGN cg gabbronoritic anorthosite 558522 5165429 11 99RME-2139 Footwall Stop 6, alkalifeldspar granite 559785 5167228 12 99RME-0375 Footwall biotite diorite gneiss 49.59 0.16 25.52 1.99 2.55 0.07 3.17 13.65 2.41 0.30 0.01 0.03 <0.01 0.98 100.40 54.2 51.07 0.10 22.84 1.08 4.47 0.11 5.38 10.91 3.03 0.22 0.01 0.08 <0.01 0.72 99.94 61.6 72.26 0.10 15.21 0.16 0.44 0.01 0.28 1.14 4.02 5.52 0.03 0.20 <0.01 0.61 99.78 46.1 55.29 1.57 13.8 3.37 10.02 0.17 3.35 6.73 2.58 1.94 0.19 0.11 0.04 0.76 99.77 31.4 63.76 0.58 15.62 2.09 3.11 0.06 3.08 3.23 3.63 2.97 0.13 0.12 0.09 1.18 99.44 52.4 50.36 1.15 14.07 3.09 11.11 0.22 5.89 10.02 1.97 0.68 0.12 <0.03 0.12 0.68 99.36 43.0 45.50 2.97 15.60 2.16 13.60 0.23 5.21 8.21 3.26 1.45 0.75 0.12 0.12 0.09 99.03 37.4 138 44 13 13 75 46 36 4 85 258 0.4 9 5 1.8 86 83 28 11 55 18 51 3 89 246 0.1 3 2 1.5 20 <5 <5 <1 11 <5 15 106 1748 950 1.2 93 1 93 30 57 41 26 330 92 142 75 593 157 12.5 166 47 3.5 183 64 19 9 17 10 94 139 670 464 6.8 117 8 14.6 90 81 57 37 319 278 67 13 91 145 4.8 92 31 3.0 37 68 57 26 204 71 190 41 673 250 21.6 348 59 5.9 5 15 19 0.79 39 <5 <8 <8 <5 <8 <8 <5 <8 <8 <21 7 38 72 0.53 117 <5 <8 <8 <21 9 6 14 0.43 29 558780 5159433 13 99RME-0025 Footwall Migmatitic metwacke, Pardo gneiss 560005 5173855 <21 14 99RME-0031 Dike Matachewan diabase dike 15 99RME-0137 Dike Sudbury diabase dike 559913 5174428 561255 5169258 <21 Abbreviations: cg = coarse-grained, fg = fine-grained; mg = medium-grained; N/A = not analyzed; IBZ = inclusion-bearing zone, GN = gabbronorite zone, LGN = leucogabbronorite zone, MZ = marginal zone, OLGN = olivine gabbronorite zone. Easting and Northings are in UTM Zone 17, datum NAD 83. 63 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �ROAD LOG, DAY 3, RIVER VALLEY INTRUSION Note: Highway 805 is used regularly by logging trucks. Consequently, extreme caution should be taken when parking vehicles on the shoulder of the highway and when examining outcrops located along Highway 805. Geological map reference: Lumbers (1973); Card and Lumbers (1975); Easton and Hrominchuk (2001a, 2001b), Easton (2001). 0.0 km Start at the junction of Highway 69S and the Highway 17 southeast bypass. Get on the bypass heading east toward Coniston and North Bay. 11.8 km Junction Highway 17 and southeast bypass, turn right onto Highway 17 and head east toward North Bay. 20.0 km Bridge over the Wanapitei River. The river lies along the trace of the Grenville Front. 30.0 km Pull off onto a pullout located on the south side of Highway 17. Stop 1. Shear-Zone Hosted Orthopyroxene Hornblendite Body Examine the outcrop and large blasted boulders present on the west side of the pullout. They belong to an orthopyroxene hornblendite body that is present within a high-strain zone that extends subparallel to the highway. Examples of these highly strained felsic gneisses can be examined in outcrops at the base of the hill east of the pullout. The top of the ridge south of the road and above the stop consists of layered leucogabbronorite of the East Bull Lake intrusive suite. Thus, although proximal to rocks of the suite here, the orthopyroxenite body is not directly in contact with the suite, although that relationship has been observed elsewhere in Street and Awrey townships (Easton and Murphy 2002). Note the large, equant, orthopyroxene crystals, and the fine-grained amphibole matrix. Return to vehicles and head east on Highway 17. 60.6 km Junction of Highway 17 and 539 in Warren. Turn north onto Highway 539, reset odometer to zero. 0.0 km Junction, Highway 17 and 539 in Warren, turn north onto Highway 539 and proceed toward River Valley. After heading 200 m north on Highway 539, turn right, then after going 100 m east, turn left and continue north on Highway 539. 4.8 km Coloured Aggregates Limited/Crea-Mac quarry operation is located east of the highway. This operation is one of several quarries in Ratter Township which produces pink aggregate from syenite pegmatite of probable Archean age that occur within this part of the Grenville Province. 8.3 km Junction Highway 539 and Kipling Road West, continue north on Highway 539. 12.5 km Outcrops on both sides of the road expose typical rocks of the Crerar gneiss association, which consists of granodiorite, tonalite, and amphibolite rocks (2680 to 2660 Ma), commonly migmatized. Tight to isoclinal folds are common in the outcrop on the east side of the road. 14.5 km Pull over onto right shoulder and park by outcrops at crest of hill. 64 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Stop 2. Rocks of the Crerar Gneiss Association This outcrop illustrates typical mafic and intermediate composition gneisses of the Crerar gneiss association (Easton 2003), including tonalite, 2 phases of granodiorite, amphibolite layers and pods, and minor calc-silicate rock. Rocks of the Crerar gneiss association form the country rocks to the southern margin of the River Valley intrusion. A sample from this outcrop yielded a Nd/Sm model age of 2710 Ma (Dickin 1998). Return to vehicles and continue northward on Highway 539. 21.6 Park on shoulder of road by large, isolated outcrop knob on east side of the highway. Optional Stop. Crerar Gneiss Association UTM 561947E, 5157087N. The outcrop on the east side of the road exposes a megacrystic granodiorite, which is a late intrusion into the Crerar gneiss association. This rock cuts earlier migmatitic fabrics, but is itself locally strongly deformed. A sample from this outcrop has yielded a U/Pb zircon age of 2663±4 Ma, with titanite dating the waning of Grenville metamorphism at 974±12 Ma (Easton and Kamo 2003). Return to vehicles and continue northward on Highway 539. 23.1 km Bridge over the Sturgeon River. 23.6 km Junction Highway 539, which heads east toward Field, and Highway 539A, which heads west-northwest toward River Valley. Turn left. Immediately pull over and park for optional stop. Optional Stop. Red Cedar Lake Gneiss This is Stop 2.3 of Lumbers (1978). Outcrops north of the highway at and just west of the junction expose typical granular gneiss of the Red Cedar Lake gneiss. The Red Cedar Lake gneiss is a widespread unit that extends from its western terminus here, at the Sturgeon River, east to the Ottawa River, a distance of over 80 km. It is thought to be a diatextite derived from Archean metasedimentary and intermediate composition intrusive rocks (Easton 2003), and forms the country rock along the eastern side of the River Valley intrusion north of the Sturgeon River. Much of the texture in the gneiss is probably Archean, rather than Grenvillian, in age. Return to vehicles and continue northward on Highway 539. 24.9 km Dupras Store in "downtown" River Valley. 26.2 km Bridge across the Temagami River. 27.6 km Junction Highway 539A and Erana Mines Road. The Erana Mines Road leads to Erana building stone quarry and Crea-Mac aggregate quarry, both are hosted in well-preserved rocks of the Gabbronorite Zone of the River Valley intrusion. 29.5 km End Highway 539A, start Highway 805. End of continuously paved road. 29.55 to 29.9 Pull over and park by the shoulder at roughly 29.55 km. The 350 m long section of outcrop on the north side of the road constitutes Stop 3. 65 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Stop 3. Varied Degrees of Preservation in Rocks of the River Valley Intrusion This is Stop 2.4 of Lumbers (1978). UTM at the west end of the outcrop is 559691E, 5163308N. This 350 m long section along the north side of the highway exposes rocks of the River Valley intrusion exhibiting a wide variety of textures and deformation. The east end of the outcrop consists of dark-grey to grey weathering gabbronorite and leucogabbronorite, with well-preserved plagioclase laths and only incipient corona development around mafic minerals. As one proceeds east, the rocks become paler, largely due to whitening of the plagioclase grains during recrystallization. Note the presence of many small, thin, anastomosing shear zones. Heading west, the outcrop becomes a black and white leucogabbronorite, then a foliated equivalent of the same, finally becoming protomylonitic to gneissic at the west end of the outcrop where the road curves slightly. The variation seen in this outcrop is typical of the variation in the degree of preservation and deformation observed throughout the River Valley intrusion north of the Sturgeon River. In general, well-preserved, little-deformed igneous rocks form the core of topographic highs, whereas valleys and low areas are generally underlain by gneissic rocks similar to those observed in the west end of this outcrop. South of the Sturgeon River, recrystallized to gneissic varieties of gabbronorite and leucogabbronorite predominate. 30.2 km Junction with Giroux road. Turn right (north) and proceed north on gravel road. Note that the road is gated. Seek permission (and key) for entry from Mr. Giroux, who lives in the house just east of the gate. 31.3 km Road crosses Azen Creek, a major north-northeast-trending deformation zone within the River Valley intrusion. Most of the copper-nickel-PGE mineralization in the River Valley intrusion lies to the west of the Azen Creek zone, even though similar rock units occur along the contact of the intrusion to the east of the Azen Creek shear zone. 31.8 km Junction, stay to the left and follow side road roughly 600 m to the first of several quarry pits present in the Dana quarry. Park at base of hill by first large cliff face, just past the large quarry building. Proceed uphill along the sideroad to the west (right) to large open area of outcrop. Stop 4. Autolith Fragments and Layering, Layered Gabbronorite Zone This is the southernmost quarry face of a large building stone quarry, which has been exploited at various times over the last 30 years. Stone from this quarry face was used as floor tiles in the Willet Green Miller Centre, Ministry of Northern Developments and Mines, at Laurentian University in Sudbury. The rocks exposed here span nearly the entire Layered Gabbronorite zone, ranging from olivine-bearing norite in the south part of the exposure to a pigeonite gabbro in the northwest part. The dominant rock type here is a very coarse-grained, black norite to gabbronorite with varied amounts of oxide minerals in addition to metamorphic amphibole and garnet. Layering is almost horizontal in this area (~15 to 25°). Layering is subtle, and commonly disrupted or convoluted. Pods, dikes and irregular layers of pegmatitic and granophyric material are found in the more central region of this outcrop area. Pegmatite and granophyre commonly occur together and in association with mafic cognate inclusions and felsic fragments. The development of very coarse grains of pyroxene, plagioclase, amphibole and magnetite around these inclusions led Hrominchuk (2000) to conclude that these pods and dikes were not formed simply by differentiating magma. He suggested that they may have formed by contamination of a relatively unfractionated magma (olivine norite) by cooler, compositionally different, footwall and cognate xenoliths. These pegmatitic patches are very similar to those developed in the Inclusion-Bearing zone, which we will see subsequently at Stop 7. 66 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �If one continues to follow the road north, it passes by an abandoned quarry pit at 400 m and ends above another 20 m high quarry face at 600 m. In this quarry, subtle, shallow dipping (~15 to 25°) layering is present (see Photo 11d) as are pegmatitic veins and pods. Unlike the main stop, mafic cognate inclusions and disrupted layering are not observed. Turn vehicles around and retrace road back to main gravel road. 32.5 km Junction, continue north on main road. 34.4 km Pull off onto small road to the left (west) of the road and park. Follow old skidder trail through the bush west for approximately 200 m to the top of a cliff face, then work your way down safely along the side of the cliff. Watch for loose rocks on the slope and falling rocks. Stop 5. Layered Olivine Gabbronorite This stop displays olivine-rich rocks of the Olivine Gabbronorite zone, which may be stratigraphically equivalent to the Olivine Gabbronorite zones in both the East Bull Lake and Agnew Lake intrusions (see Table 5, analyses 5 and 6). The sequence of olivine-bearing rocks here is roughly 30 m thick. The section grades upward from melatroctolite to orthopyroxene oikocrystic melatroctolite near the base to a clinopyroxene-bearing orthopyroxene melatroctolite near the top. Layering is well developed in the lower portion of the cliff face and contains erosive scours that grade from plagioclase-poor olivine cumulates to more plagioclase-rich troctolites. All olivine in the section is partially to extensively serpentinized, creating a fractured and rubbly appearance. Plagioclase in this zone is generally interstitial with some euhedral primocrysts. Mineral compositions obtained by Hrominchuk (2000) are plagioclase, An70-76; olivine, Fo72; orthopyroxene forms oikocrystic or euhedral primocrysts at En70 and clinopyroxene is either absent or interstitial. Low-temperature alteration of the olivine-rich rocks produces serpentine, carbonate and magnetite. Return to vehicles, continue north on gravel road. 35.5 km Junction with side road to Azen Creek occurrence to the left (west), continue on main road. 36.3 km Park vehicles and walk south to clean outcrop exposures. Stop 6. Footwall Alkali Feldspar Granite and Sudbury Diabase Dike This quick stop shows a few outcrops of a weakly foliated alkali granite, which is the main footwall rock type to the River Valley intrusion north of the Sturgeon River in Dana Township. This granite cuts the migmatitic, metasedimentary-derived Pardo gneiss. Lumbers (1973) classified these granitoid rocks as monzonite to quartz syenite; however, geochemistry (Easton 2003; see also Table 5, analysis 11) suggests that these rocks are more appropriately termed alkali feldspar granite using the normative classification of Streckeisen (1976). The alkali feldspar granites are peraluminous, with low total REE (less than 50 ppm), positive Eu anomalies (Eu/Eu* 7.9), and are corundum-normative, suggesting an S-type (sedimentaryderived) affinity. Breaks and Moore (1992) reported similar REE patterns from their Type 2 phase of the Neoarchean, peraluminous, S-type Ghost Lake batholith in northwest Ontario, and suggested that the unusual chemistry of these rocks resulted from feldspar accumulation. U/Pb zircon geochronology from this outcrop yielded an age of 2660 Ma (Easton and Kamo 2003). Although we are at least 300 m north of the contact with the River Valley intrusion at this point, lead loss related to emplacement of the intrusion at 2475 Ma has affected this granite, and prevents determination of a more precise age. Titanite from this outcrop gives an age of 966±6Ma (Easton and Kamo 2003), confirming that these rocks, as well as the River Valley intrusion, have been affected by Grenvillian metamorphism. As was the case for the Parisien Lake syenite adjacent to the East Bull Lake Intrusion, this granite was originally thought to be a felsic differentiate that was coeval with the River Valley intrusion (e.g., Card and Lumbers 1975). 67 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Also present in the outcrop is a Sudbury swarm diabase dike. In the Southern Province, the Sudbury swarm dikes trend northwesterly, and consist of titaniferous (2.5 to 3.5 wt. % TiO2; see also Table 5, analysis 15) olivine-biotite-magnetite-gabbros. In Dana Township, Sudbury swarm dikes generally show northwest or northerly trends, and can be traced for several hundreds of metres along strike. Dikes range in size from a few centimetres in width to greater than 100 m wide. Olivine in the Sudbury swarm dikes north of the Sturgeon River commonly show incipient corona formation, similar to that observed in the River Valley intrusion, suggesting that coronas in both rock types developed in response to Grenvillian metamorphism. Sudbury swarm dikes south of the Sturgeon River are more extensively metamorphosed, exhibiting nearly complete replacement of olivine by orthopyroxene, spinel and garnet, and dike segments are shorter and more randomly orientated. UTM 559785E, 5167228N. Return to vehicles, turn around and retrace route. 37.1 km Sideroad to Azen Creek occurrence, turn right (west) and proceed roughly 500 metres to stripped outcrops. Stop 7. Azen Creek Copper-Nickel-PGE Occurrence The Azen Creek zone was one of the initial discovery sites for PGE mineralization in the River Valley intrusion in the fall of 1998. Its location along the north contact of the intrusion, some 6 km from Dana Lake, was the first indication that PGE mineralization might occur discontinuously to continuously over a considerable strike length. The stripped area south of the road contains inclusions of medium-grained gabbronorite, troctolite, amphibolite, alkali feldspar granite and other felsic material within a mediumgrained, grey-green olivine gabbronorite. Inclusions are more abundant on the west side of the stripped area, and vary from centimetre- to metre-scale. Mineralization consists of finely disseminated chalcopyrite and pyrrhotite in the matrix, as well as some sulphide-rich inclusions. Hrominchuk and Jobin-Bevans (2000) suggested that the inclusion-bearing zone might be intrusive into the contact zone of the intrusion, roughly coincident with the boundary between the fine-grained contact zone rocks and overlying olivine-rich cumulates of the Olivine Gabbronorite zone (e.g., Stop 5). The stripped area north of the road contains less mineralization, but shows a variety of pegmatitic veins and pods. The pegmatitic pods are commonly cored by inclusions of felsic material, likely incorporated from the footwall of the intrusion. Return to vehicles, turn around, and retrace route to Highway 805. 0.0 km Junction with Highway 805, reset odometer to zero. Proceed west on Highway 805. 0.3 km Highway bends south, side road heads west to Upper Canada Stone Company and Giroux gravel pit. Note piles of crushed, black aggregate derived from the well-preserved gabbronorite of the River Valley intrusion. From this point westward, the road parallels the Sturgeon River on the south, as well as winding its way back and forth across an abandoned Canadian National Railways right-of-way. 1.4 to 1.1 km High ridges to the north of the road are well preserved gabbronorite of the River Valley intrusion. 5.4 km Outcrop ridge north of the road and the right-of-way consists of gabbronorite of the River Valley intrusion which contains 2 fabrics, the later fabric being parallel to the Sturgeon River. Rocks similar to these were previously mapped as being deformed equivalents of Nipissing gabbro (Dressler 1979), however, chemically and petrographically they belong to the River Valley intrusion. 68 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �6.8 km Road turns north at Glen Afton. Glen Afton lies near the eastern margin of the 1 to 2 km wide boundary zone that marks the Grenville–Southern Province boundary in this area. For the next 20 km, Highway 805 weaves back and forth across this boundary zone, exposing zones of intense mylonization separated by lenses of relatively well-preserved rocks of the Huronian Supergroup, the River Valley intrusion, and the Nipissing intrusive suite. Note that Highway 805 was re-aligned between 1988 and 1989; consequently, users of Davidson's (1986) guidebook for the Grenville Front in this area are cautioned that the distances and stop locations listed in that guide are not always easily determined. Where possible, this guide notes the location of some of Davidson's stops. 7.0 km Park vehicles just past old railroad right-of-way and examine outcrops on west side of Highway. Optional Stop. Huronian Metavolcanic Rocks This is Stop 2.6 of Lumbers (1978). UTM for the south outcrop 554000E, 5166220N; for the north outcrop 554005E 5166267N. The outcrop just north and west of the abandoned right-of-way consists of medium-layered dacite, which may be part of the Stobie Formation of the Huronian Supergroup. The low outcrop beside the road (100 m to the north) consists of felsic to intermediate pyroclastic rocks assigned to the Huronian Supergroup. These outcrops represent the easternmost extent of Huronian felsic metavolcanic rocks that have been recognized to date. It is difficult to differentiate fine-grained metavolcanic rocks from mylonitic rocks along the Grenville Front, due to the intensity of the deformation. Return to vehicles and continue north on Highway 805. 8.0-8.2 km Outcrops west of the highway consist of fine-grained amphibolite, possibly representing Huronian Supergroup metavolcanic rocks, whereas outcrops east of the highway, generally covered by vegetation, consist of deformed leucogabbronorite of the River Valley intrusion. 9.5 km Low outcrops of gneissic leucogabbronorite of the River Valley intrusion occur on both sides of the road. 9.8 km ATV trail to Lismer's Ridge area on property of Pacific North West Capital Corporation on the right (east). 10.1 to 10.6 km Outcrops in this stretch of road, particularly those on the north side of the road, represent an optional stop. Optional Stop. Variably Preserved Nipissing Gabbro This location approximates Stop 2.7 of Lumbers (1978). The highway transects a body of Nipissing gabbro that has intruded the River Valley intrusion. Outcrop exposures on the north side of the road, in the vicinity of UTM 554777E 5168511N, exhibit varied degrees of preservation of the Nipissing rocks, similar to that observed in rocks of the River Valley intrusion. Dark green to almost black-weathering rocks are Nipissing gabbro retaining primary orthopyroxene and minor olivine grains, whereas greenweathering outcrops consist of typical, greenschist-grade, amphibolitized Nipissing gabbro, typical of what most of these rocks look like in the Southern Province. Note that plagioclase is much less abundant in the Nipissing rocks than in the River Valley intrusion. Return to vehicles and continue north on Highway 805. 10.7 km Outcrops north of road consist of protomylonitic gabbronorite of the River Valley intrusion and ultramylonite of unknown protolith. 69 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �10.9 to 11.1 km Outcrops east of the road consist of foliated to protomylonitic leucogabbronorite of the River Valley intrusion, intruded by deformed, pink felsite veins that are chemically similar to Huronian Supergroup felsic metavolcanic rocks (Easton 2003). UTM 554415E, 5168855N. 11.2 km Outcrop east of the road consists of varitextured gabbro of the River Valley intrusion. UTM 554500E, 5169105N. 11.8 km Park vehicles and examine outcrops on east side of road. Stop 8. Mylonitized Anorthosite of the River Valley Intrusion This is Stop R6a of Davidson (1986) and Stop 2.8 of Lumbers (1978). Outcrops west of the road consist of ultramylonite derived from rocks of the River Valley intrusion. Please note a few large blocks in the area, in which mylonitic texture is developed in rocks that compositionally are anorthosite. Return to vehicles and continue north on Highway 805. 12.0-12.2 km Road extends uphill and around a curve, passing through outcrops of greenish weathering Nipissing gabbro. This curve is the location of stop R5 of Davidson (1986). 12.3 km Outcrops on both sides of the road consist of black, flinty, ultramylonite of unknown protolith, but which may be derived, in part, from Huronian felsic metavolcanic rocks. This outcrop approximates stop R4 of Davidson (1986). 12.5 km Outcrop west of road consists of boulder conglomerate, likely belonging to the Gowganda Formation, in contact with weathered granite of unknown age. Outcrops east of the road are a black, flinty ultramylonite (UTM 554057E, 5170254N). The section of road between kilometre 11.8 and 12.5 illustrates the rapid transition between weakly deformed and intensely deformed rocks within the Southern–Grenville province boundary. Metasedimentary and metavolcanic rocks of the Huronian Supergroup occur in close proximity to rocks of the River Valley intrusion in this area, but deformation has obscured primary contacts. 13.3 km Outcrops on curve consist of felsite and black, flinty ultramylonite. 15.1 km Pull over and park, examine outcrops on the east side of Highway 805. Optional Stop. Mississagi Formation and Mylonitic Contact with the River Valley Intrusion This is Stop R3a and R3c of Davidson (1986). Outcrops on the east side of Highway 805 consist of deformed quartz arenite of the Mississagi Formation. UTM of road outcrop is 554413E, 5172600N. Proceed east through the bush for approximately 100 m to a cliff face. Black, flinty ultramylonite occurs at the base of the cliff, above quartz arenite of the Mississagi Formation. The mylonite is graditional upward, over about 1 to 3 m, into foliated leucogabbronorite of the River Valley intrusion. The Dana North zone, at the next field trip stop, occurs about 50 m stratigraphically above this location, and about 400 m east of this stop. Return to vehicles and continue north on Highway 805. 15.5 km Turnoff on right (east) leads to Pacific North West Capital Corporation Dana North exploration area. 16.3 km Junction, continue south roughly 500 m on road and park in flat area opposite large stripped outcrop. 70 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Stop 9. Dana Lake Copper-Nickel-PGE Occurrence Prospectors L. Luhta, R. Bailey and R. Orchard first reported promising platinum-group element assay values from rocks of the River Valley intrusion near Dana Lake and Azen Creek in November 1998. Subsequently, Pacific North West Capital Corporation optioned and staked most of the north contact of the River Valley intrusion in Dana Township (inset map in Figure 12). Mustang Minerals Corporation optioned and staked most of the River Valley intrusion in Crerar Township, as well as part of the north contact near the Dana–McWilliams Township boundary. As of this writing, Pacific North West Capital Corporation has identified a dozen exploration targets along this northern contact. These are, from northwest to southeast, the Pardo, Dana North, Dana South, Banshee, Lismer's Ridge (Lismer's North and Lismer's South), Macdonald, Varley, Azen, Jackson's Flats, and Razor zones. The Thomson zone is located more centrally in the intrusion. Between 2000 and spring 2003, Pacific North West Capital Corporation, in conjunction with their partner, Anglo American Platinum Corporation Limited, had completed 5 diamond-drill programs. Approximately 288 diamond-drill holes totalling 58 000 m have been completed on their River Valley intrusion property in Dana Township. Diamond drilling was focussed almost entirely on the Dana North (see Figure 12) and Lismer's Ridge targets. A 6th diamond-drill program, aiming for a potential total of another 40 000 m of diamond-drill core, is underway, also focussing mainly on the Dana North and Lismer's Ridge zones (Pacific North West Capital Corporation, Press Release, April 10, 2003; http://www.pfncapital.com/s/NewsReleases). The scope of these diamond drilling programs is too large to effectively summarize herein; consequently, the reader is referred to the comprehensive assessment file reports (Resident Geologist’s office, Sudbury) related to these programs. Pacific North West Capital Corporation has released 2 Mineral Resource estimates for the Dana North zone, based on the Phase 1 to 5 drilling programs. In both cases, Derry, Michener, Booth and Wahl Consultants Limited conducted the resource studies. The first in situ resource estimate (Pacific North West Capital Corporation, Press Release, October 16, 2001) was a total measured, identified and inferred resource of 593 000 ounces palladium, platinum and gold at Dana North, Dana South and Lismer's Ridge. This estimate used a 0.7 g/t Pt+Pd cut-off grade, and can also be expressed as 12.7 million tonnes at 1.46 g/t Pt+Pd+Au. This can be broken down into 7.74 million tonnes at 1.60 g/t Pt+Pd+Au at Dana North and South, and 4.97 million tonnes at 1.24 g/t Pt+Pd+Au at Lismer's Ridge. The second Mineral Resource estimate (Pacific North West Capital Corporation, Press Release, October 17, 2002) was a total measured and indicated resource of 825 900 ounces palladium, platinum and gold. There were inferred resources of 200 600 ounces palladium, platinum and gold, yielding a total of 1 026 500 ounces at Dana North and Lismer's Ridge. This estimate used a 0.7 g/t Pt+Pd cut-off grade, and can also be expressed as 18.1 million tonnes of measured and indicated at 1.36 g/t Pt+Pd and 5.4 million tonnes inferred at 1.11 g/t Pt+Pd. Roughly 60% of this resource is in the Dana North area. A description of the Dana North zone has been published by S. Jobin-Bevans, Projects Manager for Pacific North West Capital Corporation, in James et al. (2002b). The description below, in a slightly smaller font, is derived from that published report. The distribution of the marginal Inclusion-bearing/Breccia zone (roughly equivalent to the Inclusion- and/or Fragment-bearing zone of Hrominchuk (2000) and James et al. (2002a)), the loci of mineralization, is in abrupt, intrusive contact to the east with the Neoarchean-age Pardo gneiss, and to the west these zones are replaced by a weakly layered to massive leucogabbro-gabbro-melagabbro sequence. Small pod-like, alkalic intrusions of unknown age are observed at two places along the intrusive contact where they displace the Inclusionbearing/Breccia Zone. As well, faults related to the Grenville Front and a 1238 Ma Sudbury swarm olivinemagnetite gabbro dyke cut or displace this zone. Silicate assemblages that host the mineralization in this part of the 71 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �River Valley intrusion exhibit upper greenschist facies mineralogy in contrast to the upper amphibolite facies and preserved magmatic assemblages that dominate the remainder of the intrusion. The stratigraphy of the Inclusionbearing/Breccia Zone is normally ~100 m wide in plan view. From the footwall Pardo gneiss, westward into the intrusion, the sequence and character of the distinguishable units are: 1. Footwall Breccia Unit: typically 5 to 15 m wide, but may be absent. It consists of partly rounded to angular, centimetre- to decimetre-size fragments of country rock (~75%; Pardo gneiss, Archean gabbro, diabase, diorite, minor Huronian Supergroup metasedimentary rocks) and River Valley intrusion material (~25%; chilled gabbro and medium-grained melagabbro) in a matrix of finer grained rock of similar composition and (or) an aplitic to granitic matrix. A narrow zone of migmatite at the contact of the intrusion is probably due to contact metamorphism, and granitic veins can be traced from this unit into the footwall at Lismer’s Ridge. Sulphide minerals are dominantly pyrite and pyrrhotite with local areas of trace to 1 volume percent chalcopyrite + pyrrhotite. Platinum group element concentrations are normally less than 25 ppb. 2. Boundary Unit: 5 to 20 m wide, but may be absent. It contains partly rounded to subangular, centimetre- and decimetre-size fragments of country rock (typically 10-25%), and cognate xenoliths of melagabbro, gabbro and less commonly leucogabbro to anorthosite in a matrix of gabbro to melagabbro ± aplite/granite, as in the Footwall Breccia. Sulphide minerals are mainly pyrite and pyrrhotite, locally up to 3 volume percent chalcopyrite + pyrrhotite occur; platinum group element contents are typically <75 ppb with local concentrations greater than 1000 ppb. 3. Breccia Unit: 20 m wide to greater than 100 m. It contains as much as 95% dominantly cognate xenoliths of gabbro to melagabbro and subordinate leucogabbro in a medium-grained matrix of similar composition; fragments are partly rounded to round probably due to partial assimilation, and centimetre to decimetre in size. Those greater than a metre are mainly footwall compositions (including Huronian Supergroup metasedimentary rocks) and tend to be larger with increasing proximity to the intrusive contact. Sulphide minerals (1 to 5 volume percent pyrrhotite + chalcopyrite) occur as both bleb and disseminated types; platinum group element contents are highly varied, but most values range from 500-6000 ppb with local concentrations greater than 10,000 ppb. 4. Inclusion-bearing Unit: 10 to 50 m wide. It contains >90% autoliths of leucogabbro, subordinate gabbro and less melagabbro in a matrix of either medium-grained leucogabbro or gabbro; the leucogabbro xenoliths are subangular to partly rounded, dominantly decimetre to metre in scale, and appear to be stoped inclusions from the adjacent (overlying) Leucogabbronorite zone. Sulphide minerals include trace to 3 volume percent pyrrhotite + chalcopyrite; platinum group element contents range from 100-500 ppb with local concentrations greater than 1000 ppb. Interestingly, the Breccia Unit, which shows the highest and most persistent sulphide-associated platinum group element mineralization, has the smallest proportion of footwall inclusions (<1%); perhaps an indication that chemical contamination from footwall lithologies is not a major controlling factor on mineralization. Fine-grained gabbro and diabase dikes cut all of the above units as well as the Leucogabbronorite zone in the main part of the intrusion. These dikes are metamorphosed at a grade similar to the intrusion in the Dana North area and are distinct from younger dikes of the Sudbury swarm. Drill hole data suggest that the dip of the contact of the Inclusion-bearing/Breccia zone with the footwall is at least 45° to 65° and toward the intrusion, whereas the apparent dip of the surface between the Inclusion-bearing Unit and Breccia Unit is steeper (~70° ) and toward the intrusive contact. The attitude of metre-scale layering in the Leucogabbronorite zone adjacent to the mineralized Inclusion-bearing/Breccia zone is poorly constrained but is estimated to be near-vertical (~70° to 90° west and east) and possibly shallowing (i.e. less than 70° west dip) westward into the intrusion. S/Se ratios for 13 of 15 samples (mineralized and barren) from the Dana North and Lismer’s Ridge areas have values from 500 to 2120, which are well within the magmatic range indicted by the Merensky and J-M reefs, and Konttijarvi-Portimo contact-type mineralization (i.e., 700-800 S/Se). Footwall rocks have low Pt, Pd and Au, and Pd/Pt and Cu/Ni are less than 1; these metal values indicate that the Archean footwall rocks are not genetically related to the platinum group element mineralizing event(s). Estimates of the metal values for barren magma that forms large parts of the intrusion or which are feeders, have less than 100 ppm Cu and anomalous Pt + Pd 72 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �concentrations averaging 32 to 35 ppb, but their metal ratios, Pd/Pt and Cu/Ni, are both less than 1, which is unlike mineralized samples (see below). Analytical data for sulphide-bearing felsic and mafic dikes and the Boundary, Breccia and Inclusion-bearing Units all show moderate to high average Pt+Pd (peaking in the Breccia Unit, as expected), and all show Pd/Pt and Cu/Ni ratios based on metal averages in the range 1.1-4.8, distinct from the low sulphide, Cu-poor assemblages. The mineralized zones at the Bull Frog Zone of the East Bull Lake Intrusion show the same geochemical features. River Valley Project -2004 • Dr,II Hole Locetono EHilIholeTrecen — — - lntrno,oe Content — — - Faults — Roads Trenches Lnke Figure 12. Map of the Dana North property of Pacific North West Capital Corporation, showing the location of stops 9A through 9F, as well as the extent of the drilling program undertaken between 1999 and 2002. 73 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Stop 9 is located on the Pacific North West Capital Corporation property, and consists of an examination of several outcrop areas that were exposed between 1999 and 2001 (see Figure 12). These outcrops provide an excellent opportunity to examine the contact of the River Valley intrusion, the mineralized zones and the character of the inclusion-bearing unit (Photo 13). Stop 9, Day 3, makes for an interesting comparison with Stop 11, Day 1, the traverse across the East Bull Lake mineralized contact in the Moon Lake area. STOP 9A. L6+00N, CONTACT ENVIRONMENT The line 6+00N clearing to the west of the road was completed in May 2000 in order to expose and sample the up-dip geology and mineralization intersected in diamond-drill hole RV00-08. This clearing provides excellent exposure of the stratigraphy in the contact environment. Walk to the western edge of the cleared area, and then walk back past diamond-drill hole collar RV00-08 toward the road. This will bring you from massive gabbronorite through the inclusion-bearing zone at the west end of the clearing through to the footwall Archean migmatite, paragneiss and gabbroic rocks at the east end of the clearing. The highest assay value from samples collected in this area is 10.1 g/t Au+Pd+Pt (sample L6N-09). Note that the footwall gneisses are pale green and washed out, reflecting a regional chloritic alteration zone, first noted by Lumbers (1973), which affects all rock units in northwest Dana Township and southwest Pardo Township. Walk north and across the access road for about 100 m to the next clearing to the east that intersects the road, this is grid line 7+00N. STOP 9B. L7+00N, CONTACT ENVIRONMENT The line 7+00N clearing was completed in May to June 2000 in order to expose and sample an extensive induced polarization chargeability anomaly outlined in this area. This clearing offers excellent exposure of the stratigraphy in the contact environment including numerous complex structural features that crosscut both country rocks and intrusive rocks. As on line 6+00N, walk to the western edge of the cleared area, then walk back toward the road. Diamond-drill hole collars RV00-23 and 24 are located at the north edge of the clearing, on your left. Pay special attention to the extensive east-northeast trending mylonite zone that cuts across the exposure. The highest assay value from samples collected in this area is 8.6 g/t Au+Pd+Pt (sample L7N-164). Walk south along the road for about 150 m to north end of large stripped outcrop ridge. Start by the rusty weathering part of outcrop closest to the road. STOP 9C. ROAD ZONE The Road zone exposes several different rock types of the River Valley intrusion, including massive gabbroic anorthosite and gabbro that “overlie” the inclusion-bearing and breccia units. A large, faulted felsic dike cuts the breccia zone and appears to originate from the footwall contact that is exposed toward the north end of the clearing. This area is one of the original discovery zones from the fall of 1998, with the remnants of the original exposed pit located at the extreme northeast corner of the clearing, alongside the road. Another cleared area, referred to as the Road zone east, is located immediately east of the road. The highest assay value from samples collected in this area is 13.4 g/t Au+Pd+Pt (sample RZ-361). 74 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Follow the ATV trail situated on the east side of the road by the Road zone and walk southeast. On the way you will pass by North zones 1, 2 and 3 on your left. After about 200 m (about grid line 3+00N), follow a drill road north (toward the left) for about 100 m (near diamond-drill hole collar RV00-05). The Central zone is located on a north-trending outcrop ridge. STOP 9D. CENTRAL ZONE The Central zone exposes the inclusion-bearing unit (northwest part of clearing) (Photo 13b, 13d) and the mafic breccia unit (southeast part of clearing). Note the large fragments in the outcrop at the northeast edge of the clearing and note the leucocratic nature of the matrix (see Photo 13b) for comparison to the melanocratic matrix you will see in the South zone (see Photo 13c). The highest assay value for samples collected from this zone is 9.1 g/t Au+Pd+Pt+Rh (sample CZ-54). r Retrace your route back to the ATV trail and continue south for roughly 250 m. • .-.. ..•, • Photo 13. Photographs illustrating the textural variation observed in the Inclusion-Bearing unit in the Dana North area (Day 3, Stop 9). a) Large, leucogabbronorite fragment hosted in a gabbronorite to pyroxenite matrix. Scale card is approximately 8.5 cm long. b) Gabbronorite to pyroxenite fragments hosted in a leucogabbronorite matrix in stripped outcrop at the central zone (stop 9D, Day 3). c) Mafic fragment and matrix-dominated portion of the Inclusion-Bearing unit at Dana South (stop 9F, Day 3). Hammer handle is approximately 35 cm long. d) Gabbronorite to pyroxenite fragments hosted in a leucogabbronorite matrix in stripped outcrop at the Central zone (stop 9D, Day 3). Marker is approximately 14 cm long. 75 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �STOP 9E. TRENCH ZONE The Trench zone is one of the original discovery areas from the fall of 1998, with one of the original trenches (circa 1969) still visible along the west edge of the clearing. This zone exposes the mafic breccia unit, which is best developed along the western edge of the clearing (Photo 13c). Note the “shotgunpattern” of sulphide mineralization “bleeding” in outcrop. A plagioclase-phyric dike (Matachewan swarm?) occurs in the sheared contact with footwall migmatitic paragneiss (altered Pardo gneiss) and is in sharp contact with rocks of the intrusion along the southern edge of the clearing. Samples from the dike have analyzed up to 818 ppb Au+Pd+Pt+Rh and 448 ppm Cu (sample TZ-03); finely disseminated sulphide mineralization is visible. Take time to look at the “fault breccia” located at the east edge of the clearing, just north of the dike. What is this breccia reminiscent of? The highest assay value from samples collected from this zone is 13.2 g/t Au+Pd+Pt+Rh (sample TZ-07). Keep walking south along the trail down the hill from the Trench zone and cross the stream (beaver dam) that connects Platadium Pond on the right with Dana Lake on the left. A worn bush trail will take you to the South zone clearing. Total distance from the Road zone to the South zone is about 650 m. STOP 9F. SOUTH ZONE The South zone is one of the original discovery areas from the fall of 1998. Initially, several small pits along the eastern and northwestern edges of the clearing were the only exposed areas of mineralized outcrop. It is in this area that one grab sample assayed greater than 12 g/t Au+Pd+Pt. This clearing exposes the mafic mineralized breccia unit along the east half of the outcrop and the Inclusion-Bearing zone along the western half. Note the presence of several northeast-trending diabase dikes that cut across stratigraphy. The highest assay value to date from samples collected in this area is 16.4 g/t Au+Pd+Pt+Rh (sample SZ-219). Note the presence of blue-grey quartz grains in the mafic rocks and its general association with sulphide mineralization, particularly in the area of the large blast pit. This ends Day 3. Return to vehicles and retrace route to Highway 805, head south on Highway 805 to River Valley, Warren and Highway 17. 76 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �References Ariskin, A.A., Frankel, M.Ya., Barmina, G.S., and Nielsen, R. 1993. COMAGMAT: A Fortran program to model magma differentiation processes; Computers and Geoscience, v.19, p.1155-1170. Ashwal, L.D. and Wooden, J.L. 1989. River Valley pluton, Ontario: A late-Archean/early Proterozoic anorthositic intrusion in the Grenville Province; Geochimica et Cosmochimica Acta, v.53, p.633- 641. Barnes, S.-J. and Maier, W.D. 1999. 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Kamineni, D.C., McCrank, G.F.D., Stone, D., Ejeckam, R.B., Flindall, R. and Sikorsky, R. 1984. Geology of the central plateau of the East Bull Lake pluton, northeastern Ontario; in Current Research, Part B, Geological Survey of Canada, Paper 84-1B, p.75-83. 80 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Kamineni, D.C., McCrank, G.F.D., Stone, D., Ejeckam, R.B. and Sikorsky, R. 1985. A preliminary report of alteration and fracture-filling mineralogy in the East Bull Lake pluton, District of Algoma, Ontario; in Current Research, Part B, Geological Survey of Canada, Paper 85-1B, p.81-88. Kamineni, D.C. 1986. A petrochemical study of calcic amphiboles from the East Bull Lake anorthosite-gabbro layered complex, District of Algoma, Ontario; Contributions to Mineralogy and Petrology, v.93, p.471-481. Kamo, S.L., Krogh, T.E. and Kumarapeli, P.S. 1995. Age of the Grenville dyke swarm, Ontario-Quebec: Implications for the timing of Iapetan Rifting; Canadian Journal of Earth Sciences, v.32, p.273-280. Keays, R.R. 1995. The role of komatiitic and picritic magmatism and S-saturation in the formation of ore deposits; Lithos, v.34, p.1-18. Keays, R.R., Nickel, E.H., Groves, D.I. and McGoldrick, P.J. 1982. Iridium and palladium as discriminants of volcanic-exhalative, hydrothermal and magmatic nickel sulfide mineralization; Economic Geology, v.77, p.1535-1547. Keays, R.R., Vogel, D.C., James, R.S., Peck, D.C., Lightfoot, P.C. and Prevec, S.A. 1995. Metallogenic potential of the Huronian-Nipissing magmatic province; The Canadian Mineralogist, v.33, p.932-933. Krogh, T.E., Davis, D.W. and Corfu, F. 1984. Precise U-Pb zircon and baddelyite ages for the Sudbury Structure; in Geology and Ore Deposits of the Sudbury Structure, Ontario Geological Survey, Special Volume 1, p.431-446. Krogh, T.E., Corfu, F., Davis, D.W., Dunning, G.R., Heaman, L.M., Kamo, S.L., Machado, N., Greenough, J.D. and Nakamura, E. 1987. Precise U-Pb isotopic ages of diabase dykes and mafic to ultramafic rocks using trace amounts of baddeleyite and zircon; in Mafic Dyke Swarms, Geological Association of Canada, Special Paper 34, p.147-152. Krogh, T.E., Kamo, S.L., and Bohor, B.F. 1996. Shock metamorphosed zircons with correlated U-Pb discordance and melt rocks with concordant protolith ages indicate an impact origin for the Sudbury structure; in Earth Processes: Reading the Isotopic Code, American Geophysical Union, Geophysical Monograph 95, p.343-353. Le Maitre, R.W. 1989. A classification of igenous rocks and glossary of terms; Blackwell Scientific Publications, Oxford, United Kingdom, 193p. Lumbers, S.B. 1973. Geological series, River Valley area, districts of Nipissing and Sudbury; Ontario Division of Mines, Preliminary Map, P.844, scale 1:63 360. Lumbers, S.B. 1975. Geology of the Burwash area, Districts of Nipissing, Parry Sound, and Sudbury; Ontario Division of Mines, Geological Report 116, 158p. Lumbers, S.B. 1978. Geology of the Grenville Front Tectonic Zone in Ontario; in Toronto ’78, Field Trips Guidebook, Geological Society of America–Geological Association of Canada–Mineralogical Association of Canada, p.347-361. McBirney, A.R. 1996. The Skeargaard Intrusion; in Layered Intrusions, Elsevier, New York, Developments in Petrology, v.15, p.147-180. McCrank, G.F.D., Kamineni, D.C., Ejeckam, R.B. and Sikorsky, R. 1989. Geology of the East Bull Lake gabbroanorthosite pluton, Algoma District, Ontario; Canadian Journal of Earth Sciences, v.26, p.357-375. Meldrum, A., Abdel-Rahman, A.-F.M., Martin, R.F. and Wodicka, N. 1997. The nature, age and petrogenesis of the Cartier Batholith, northern flank of the Sudbury Structure, Ontario, Canada; Precambrian Research, v.82, p.265-285. 81 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �NACSN. 1983. North American Stratigraphic Code, 1983; Association of Petroleum Geologists, Bulletin, v.67, p.841-875. Noble, S.R. and Lightfoot, P.C. 1992. U-Pb baddeleyite ages of the Kerns and Triangle Mountain intrusions, Nipissing diabase, Ontario; Canadian Journal of Earth Sciences, v.29, p.1424-1429. Peach, C.L., Mathez, E.A. and Keays, R.R. 1990. Sulfide melt-silicate melt distribution coefficients for noble metals and other chalcophile elements as deduced from MORB: Implications for partial melting; Geochimica et Cosmochimica Acta, v.54, p.3379-3389. Peck, D.C., James, R. and Chubb, P. 1993. Geological environments for PGE-Cu-Ni mineralization in the East Bull Lake Gabbro-Anorthosite Intrusion, Ontario; Exploration and Mining Geology, v.2, p.85-104. Peck, D.C., James, R.S. and Chubb, P.T. and Keays, R.R. 1995. Geology, metallogeny and petrogenensis of the East Bull Lake intrusion, Ontario; Ontario Geological Survey, Open File Report 5923, 117p. Peck, D.C., Keays, R.R., James, R.S., Chubb, P.T. and Reeves, S.J. 2001. Controls on the formation of contact-type PGE mineralization in the East Bull Lake Intrusion; Economic Geology, v.96, p.559-581. Peck, D.C., Barrie, T., Turmel, R., James, R., Wood, P., Larrivierre, J. and Lapierre, K. 2000. Geology of the East Bull Lake intrusion and its contact-type PGE-Cu-Ni mineralization; Laurentian University SEG Student Chapter PGM Exploration Short Course Field Trip Guidebook, November 3, 2000, 38p. Prevec, S.A. 1993. An isotopic, geochemical and petrographic investigation of the genesis of early Proterozoic mafic intrusions and associated volcanics near Sudbury, Ontario; unpublished PhD thesis, University of Alberta, Edmonton, Alberta, 223p. Prevec, S.A., James, R.S., Keays, R.R. and Vogel, D.C. 1995. Constraints on the genesis of Huronian magmatism in the Sudbury area from radiogenic isotopic and geochemical evidence; The Canadian Mineralogist, v.33, p.930932. Robertson, J.A. 1976. Geology of the Massey area, districts of Algoma, Manitoulin and Sudbury; Ontario Department of Mines, Geological Report 136, 130p. Rousell, D.H. and Trevisiol, D.D. 1988. Geology of the mineralized zone of the Wanapitei complex, Grenville Front, Ontario; Mineralium Deposita, v.23, p.138-149. Schandl, E.S., Gorton, M.P. and Davis, D.W. 1994. Albitization at 1700+/-2 Ma in the Sudbury–Wanapitei Lake area, Ontario; Canadian Journal of Earth Sciences, v.31, p.597-607. Smith, M.D. 2002. The timing and petrogenesis of the Creighton pluton, Ontario: an example of felsic magmatism associated with Matachewan igneous events; unpublished MSc thesis, University of Alberta, Edmonton, Alberta, 123p. Streckeisen, A. 1976. To each plutonic rock its proper name; Earth-Science Reviews, v.12, p. 1-33. Sullivan, R.W. and Davidson, A. 1993. Monazite age of 1747 Ma confirms post-Penokean age of the Eden Lake Complex, Southern Province, Ontario; in Radiogenic Age and Isotopic Studies: Report 7, Geological Survey of Canada Paper 93-2, p.45-48. Sun, S.S, and McDonough, W F. 1989. Chemical and isotopic systematics of oceanic basalts: Implications for mantle compositions and processes; in Magmatism in ocean basins, Geological Society of London, Special Publication 42, p.313-345. 82 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Tettelaar, T. 2000. Petrography and geothermobarometry of variably metamorphosed and deformed leucogabbronorites of the River Valley intrusion, Grenville Province, Ontario; unpublished BSc thesis, Carleton University, Ottawa, Ontario, 59p. van Breemen, O. and Davidson, A. 1988. Northeast extension of Proterozoic terranes of mid-continental North America; Geological Society of America Bulletin, v.100, p.630–638. Vogel, D.C. 1996. The geology and geochemistry of the Agnew intrusion: implications for the petrogenesis of early Huronian mafic igneous rocks in central Ontario, Canada; unpublished PhD thesis, University of Melbourne, v.1, 292p. Vogel, D.C. and Keays, R.R. 1997. Stratigraphic correlation and structure of the East Bull Lake intrusions: implications for the evolution of the Paleoproterozoic Huronian rift zone in Ontario; Geological Association of Canada, Program with Abstracts, v.22, p.A-154. Vogel, D.C., James, R.S. and Keays, R.R. 1998. The early tectono-magmatic evolution of the Southern Province: implications from the Agnew Intrusion, central Ontario, Canada; Canadian Journal of Earth Sciences, v.35, p.854-870. Vogel, D.C., Vuollo, J.I., Alapieti, T.T. and James, R.S. 1998. Tectonic, stratigraphic, and geochemical comparisons between ca. 2600-2440 Ma mafic igeneous events in the Canadian and Fennoscandian Shields; Precambrian Research, v.92, p.89-116. Vogel, D.C., Keays, R.R., James, R.S. and Reeves, S.J. 1999. The geochemistry and petrogenesis of the Agnew Intrusion, Canada: a product of S-undersaturated, high-Al and low-Ti tholeiitic magmas; Journal of Petrology, v.40, p.423-450. Wodicka, N. and Card, K.D. 1995. Late Archean history of the Levack gneiss complex, southern Superior Province, Sudbury, Ontario: New evidence from U-Pb geochronology; Precambrian ’95, Program with Abstracts, p.191. Young, G.M. 1983. Tectono-sedimentary history of early Proterozoic rocks of the northern Great Lakes area; in Early Proterozoic Geology of the Great Lakes Region, Geological Society of America, Memoir 160, p.15-32. 83 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Metric Conversion Table Conversion from SI to Imperial Conversion from Imperial to SI SI Unit Multiplied by Gives Imperial Unit 1 mm 1 cm 1m 1m 1 km 0.039 37 0.393 70 3.280 84 0.049 709 0.621 371 LENGTH inches 1 inch 25.4 inches 1 inch 2.54 feet 1 foot 0.304 8 chains 1 chain 20.116 8 miles (statute) 1 mile (statute) 1.609 344 mm cm m m km 1 cm@ 1 m@ 1 km@ 1 ha 0.155 0 10.763 9 0.386 10 2.471 054 AREA square inches 1 square inch square feet 1 square foot square miles 1 square mile acres 1 acre 6.451 6 0.092 903 04 2.589 988 0.404 685 6 cm@ m@ km@ ha 1 cm# 1 m# 1 m# 0.061 023 35.314 7 1.307 951 VOLUME cubic inches 1 cubic inch cubic feet 1 cubic foot cubic yards 1 cubic yard 16.387 064 0.028 316 85 0.764 554 86 cm# m# m# CAPACITY 1 pint 1 quart 1 gallon Multiplied by 1L 1L 1L 1.759 755 0.879 877 0.219 969 pints quarts gallons 1g 1g 1 kg 1 kg 1t 1 kg 1t 0.035 273 962 0.032 150 747 2.204 622 6 0.001 102 3 1.102 311 3 0.000 984 21 0.984 206 5 MASS ounces (avdp) 1 ounce (avdp) 28.349 523 ounces (troy) 1 ounce (troy) 31.103 476 8 pounds (avdp) 1 pound (avdp) 0.453 592 37 tons (short) 1 ton (short) 907.184 74 tons (short) 1 ton (short) 0.907 184 74 tons (long) 1 ton (long) 1016.046 908 8 tons (long) 1 ton (long) 1.016 046 90 1 g/t 0.029 166 6 1 g/t 0.583 333 33 CONCENTRATION ounce (troy)/ 1 ounce (troy)/ ton (short) ton (short) pennyweights/ 1 pennyweight/ ton (short) ton (short) Gives 0.568 261 1.136 522 4.546 090 L L L g g kg kg t kg t 34.285 714 2 g/t 1.714 285 7 g/t OTHER USEFUL CONVERSION FACTORS 1 ounce (troy) per ton (short) 1 gram per ton (short) 1 ounce (troy) per ton (short) 1 pennyweight per ton (short) Multiplied by 31.103 477 grams per ton (short) 0.032 151 ounces (troy) per ton (short) 20.0 pennyweights per ton (short) 0.05 ounces (troy) per ton (short) Note: Conversion factors which are in bold type are exact. The conversion factors have been taken from or have been derived from factors given in the Metric Practice Guide for the Canadian Mining and Metallurgical Industries, published by the Mining Association of Canada in co-operation with the Coal Association of Canada. 84 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �ISSN 0826--9580 ISBN 0--7794--5906--7 PDF compression, OCR, web-optimization with CVISION's PdfCompressor � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2006 Description An account of the resource Institute on Lake Superior Geology. Sault Ste Marie, Ontario. May 8-12, 2006. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2006 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/30b3fdc2335802234c13370066ca4fc8.pdf d8abacfe9bcc9e8d2baa6532033dc6cb PDF Text Text 53 ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY LUTSEN, MINNESOTA MAY 8 — 13,2007 PROCEEDINGS VOLUME 53 PART 1- PROGRAM AND ABSTRACTS �INSTITUTE ON LAKE SUPERIOR GEOLOGY 53RD ANNUAL MEETING MAY 8-13, 2007 LUTSEN, MINNESOTA HOSTED BY: LAUREL G. WOODRUFF AND JAMES D. MILLER, JR. Co-Chairs U.S. Geological Survey Minnesota Geological Survey Volume 53 Part 1 – Proceedings and Abstracts Edited by Laurel Woodruff, U.S. Geological Survey Cover Photos: Top - Lake Superior shoreline at Lutsen Resort; Right - Grand Portage National Monument stockade and Hay Bay; Left - High Falls on Pigeon River; Bottom - Suzie Islands (photographs courtesy of Bill Cannon) �53RD INSTITUTE ON LAKE SUPERIOR GEOLOGY VOLUME 53 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: IGNEOUS STRATIGRAPHY OF THE POPLAR LAKE INTRUSION (FORMERLY NATHAN’S LAYERED SERIES) TRIP 2: GEOLOGIC AND CULTURAL HISTORY OF GRAND PORTAGE NATIONAL MONUMENT TRIP 3: MIDCONTINENT RIFT-RELATED MAFIC INTRUSIONS NORTH OF THE INTERNATIONAL BORDER TRIP 4: GEOLOGY AND CU-NI-PGES MINERALIZATION – NICKEL LAKE MACRODIKE, SOUTH KAWISHIWI INTRUSION TRIP 5: GEOLOGY OF THE NORTH SHORE FROM LITTLE MARAIS TO GRAND MARAIS TRIP 6: GEOLOGY OF THE GUNFLINT TRAIL Reference to material in Part 1 should follow the example below: Hart, T.R. and MacDonald, C.A., 2007, Emplacement of the Nipigon Sill Complex and mafic to ultramafic intrusions of the Nipigon Embayment [abstract]; Institute on Lake Superior Geology Proceedings, 53rd Annual Meeting, Lutsen, MN, v. 53, part 1, p. 36-37. Published by the 53rd Institute on Lake Superior Geology and distributed by the ILSG Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca ILSG website: http://www.lakesuperiorgeology.org ISSN 1042-9964 ii �TABLE OF CONTENTS PROCEEDINGS VOLUME 53 PART 1—PROGRAM AND ABSTRACTS Institutes on Lake Superior Geology, 1955-2007.............................................................. iv Goldich Medal Committee................................................................................................. vi Past Goldich Medalists ...................................................................................................... vi Citation for 2007 Goldich Medal Recipient ..................................................................... vii Sam Goldich and the Goldich Medal............................................................................... viii ILSG Student Research Fund ..............................................................................................x Student Paper Awards........................................................................................................ xi Eisenbrey Student Travel Awards .................................................................................... xii Report of the Chair of the 52nd Annual Meeting ............................................................ xiii 2007 Board of Directors ....................................................................................................xv 2007 Session Chairs...........................................................................................................xv 2007 Student Paper Awards Committee ............................................................................xv 2007 Local Committees .....................................................................................................xv 2007 Banquet Speaker ..................................................................................................... xvi Program........................................................................................................................... xvii Poster Presentations ....................................................................................................... xxiii Abstracts ..........................................................................................................................xxv iii �PREVIOUS INSTITUTES ON LAKE SUPERIOR GEOLOGY ILSG YEAR PLACE CHAIRS 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz and C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims and R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley and E. Mercy 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray 28 1982 International Falls, Minnesota D.L. Southwick 29 1983 Houghton, Michigan T.J. Bornhorst iv �30 1984 Wausau, Wisconsin G.L. La Berge 31 1985 Kenora, Ontario C.E. Blackburn 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey and R.P. Sage 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage and W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller, Jr. and M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst and R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin and P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey, Jr. and B.A. Brown 48 2002 Kenora, Ontario P. Hinz and R.C. Beard 49 2003 Iron Mountain, Michigan L.G. Woodruff and W.F. Cannon 50 2004 Duluth, Minnesota S.A. Hauck and M. Severson 51 2005 Nipigon, Ontario P. Hollings and M.C. Smyk 52 2006 Sault Ste. Marie, Ontario R.P. Sage and A.C. Wilson 53 2007 Lutsen, Minnesota L.G. Woodruff and J.D. Miller, Jr. v �PAST GOLDICH MEDALISTS 1979 Samuel S. Goldich 1993 Donald W. Davis 1980 not awarded 1994 Cedric Iverson 1981 Carl E. Dutton, Jr. 1995 Gene La Berge 1982 Ralph W. Marsden 1996 David L. Southwick 1983 Burton Boyum 1997 Ronald P. Sage 1984 Richard W. Ojakangas 1998 Zell Peterman 1985 Paul K. Sims 1999 Tsu-Ming Han 1986 G.B. Morey 2000 John C. Green 1987 Henry H. Halls 2001 John S. Klasner 1988 Walter S. White 2002 Ernest K. Lehmann 1989 Jorma Kalliokoski 2003 Klaus J. Schulz 1990 Kenneth C. Card 2004 Paul Weiblen 1991 William Hinze 2005 Mark Smyk 1992 William F. Cannon 2006 Michael G. Mudrey 2007 GOLDICH MEDAL RECIPIENT Joseph Mancuso Bowling Green State University Bowling Green, Ohio GOLDICH MEDAL COMMITTEE Serving for the meeting year shown in parentheses Tom Hart (2004-2007) Doug Duskin (2005-2008) Richard Ojakangas (2006-2009) vi Government representative Industry representative Academic representative �CITATION FOR GOLDICH MEDAL RECIPIENT Joseph Mancuso, 2007 Goldich Medal Recipient Membership in the Institute of Lake Superior Geology is composed of government personnel who produce maps and other essential services; exploration geologists who walk the bush looking for ore deposits, and university professor who teach geology to future government personnel, exploration geologists, and professor. Membership also includes students who are learning the dimensions of the profession. In addition to teaching, Joe was an active exploration geologist during the 40 years he taught at Bowling Green State University. Joe has been an active member of ILSG for over 50 years. He attended the first meeting when he was an undergraduate student at Carleton College. During the past 50 years he has contributed more than 20 oral presentations and posters to ILSG meetings, plus 42 published papers and abstracts on the geology of the Lake Superior region. He also direct 39 MS theses on the geology of the Lake Superior region, as well as conducting annual field trips for his students to examine Precambrian geology in the field and in mines in Michigan, Wisconsin, and Ontario. Joe has produced some outstanding students. One brought two gold mines into production; another induced Freeport to drill Grasberg Mountain (4100 meters elevation) in Irian Jaya, Indonesia. Grasberg is now the largest single gold producer in the world in terms of ounces and revenue per year. It is also a high profitable porphyry copper mine (225,000 tons per day milled) that has produced the cash to Freeport to purchase PhelpsDodge. Another student recognized the alteration associated with a blind VMS deposit in Manitoba and directed a successful drilling program. He was recognized for this achievement by being named "Prospector of the Year" by the Prospectors and Developers Association of Canada. Another student collected the grab samples on the J-M platinum/palladium layer in the Stillwater layered Igneous Complex in Montana. This layer now supports two mines: Stillwater and Boulder. Finally, two of his students did the grab and development sampling at the Jarrett Canyon gold mine prior to being brought into production. Joe, you earned the Goldich award. Submitted by Ron Seavoy vii �SAM GOLDICH AND THE GOLDICH MEDAL Sam Goldich received an AB from the University of Minnesota in 1929, a M.A. from Syracuse University in 1930, and a Ph.D. from Minnesota in 1936. During World War II Sam worked for the U.S. Geological Survey in mineral exploration. In 1948, Sam returned to the University of Minnesota, and became Professor and Director of the Rock Analysis Laboratory the following year. He rejoined the U.S. Geological Survey in 1959 and was appointed as the first Branch Chief of the Branch of Isotope Geology. Sam returned to academia in 1964 when he went to Pennsylvania State University. He left PSU in 1965 and moved to the State University of New York at Stony Brook, where he stayed for 3 years. Restless yet again, he moved to Northern Illinois University in 1968 where he was a professor until his retirement in 1977. Sam’s final move was to Denver where he became an emeritus at the Colorado School of Mines. Sam died in 2000, less than a month before his 92nd birthday. In the late 1970’s, Geological Society of America Special Paper 182, which included seminal geochronological studies by Sam Goldich and coworkers on the Archean rocks of the Minnesota River Valley, was nearing completion. At this time various ILSG regulars began discussing the possibility of recognizing Sam for his pioneering work on the resolution of age relationships and thus the geology of Precambrian rocks in the Lake Superior region. Three members, R.W. Ojakangas, J.O. Kalliokoski and G.B. Morey, presented the idea to the ILSG Board of Directors in 1978. The Board approved the creation of an award, provided funding could be obtained. It was suggested that collecting one or two dollars at registration for a dedicated account would provide resources for striking the medal. A general request was made to the ILSG membership for donations and Sam himself offered a challenge grant to match the contributions. In total $4,000 was collected and thus began the work of creating the Goldich Medal. The initial Goldich Award was presented to Sam by G.B. Morey in 1979 and consisted of a large paper proclamation. For the actual medal, G.B. Morey consulted with the foundry on production details, while Dick Ojakangas and Jorma Kalliokoski worked on the design of the award, suggesting that it be given for “outstanding contributions to the geology of the Lake Superior region.” Simultaneously, a committee of J.O. Kalliokosi, W.F. Cannon, M.M Kehlenbeck, G.B. Morey, and G. Mursky developed the Award Guidelines that were approved by the ILSG Board. By 1981 all the elements of the Goldich Award had come together, and the second recipient, Carl E. Dutton, Jr., received viii �the Goldich Medal for 50 years of significant contributions to the understanding of the geology of the Lake Superior region. Since the beginning, the Awards Committee has consisted of individuals representing industry, government and academia, with each member of the Committee serving for three years. The medal is now awarded every year at the annual ILSG meeting. Reference: Morey, G.B. and Hanson, G.N. (editors). 1980. Selected studies of Archean gneisses and Lower Proterozoic rocks, southern Canadian Shield. Geological Society of America, Special Paper 182, 175 p. Prepared by various Goldich Medal Awardees, 2007 INSTITUTE ON LAKE SUPERIOR GEOLOGY GOLDICH MEDAL ix �ILSG STUDENT RESEARCH FUND The 2005 Board of Directors established the ILSG Student Research Funs with $10,000 US from the Institute’s general fund to encourage student research on the geology of the Lake Superior region. A minimum of two awards of $500 US each for research expenses (but not travel expenses) will be made each year. Students are expected to present their research orally or during a poster session at an ILSG meeting. The award winners will also be automatically eligible for the Eisenbrey Travel Awards. To allow the fund to grow, the Fund will receive one-half of any additional proceeds from each annual meeting, after all other commitments and expenses are covered. • The ILSG Board of Directors will be responsible for selecting a minimum of two awards each year. The ILSG Treasurer will issue the awards. • The ILSG Student Research Fund is available for undergraduate or graduate students working on geology in the Lake Superior region. • The applications are due to the ILSG Secretary by August 31st of each year. Awards will be made by October 1st of each year. • Names of the award recipients will be announced at the next annual meeting and posted on the ILSG website. • The proposal application should be at least 500 words, and should have a statement of the research project, background information, a map of the research area, research steps necessary to complete the research, figures (if needed) , references, and a list of research expenses. The proposal should also include a proposed end date for the research. • The proposal will need to be signed by researcher’s supervisor. In 2006 the ILSG Board of Governors awarded the first three $500 awards from the Student Research Fund. This year awards were made to: Cole Edwards (University of Wisconsin - Oshkosh) - Controls on the formation of the earliest marine phosphate deposits, Marquette Supergroup, Michigan Noah Planavsky (Rosenstiel School of Marine and Atmospheric Sciences Marine Geology and Geophysics, Miami) - Iron isotopes as oceanographic tracers in Animikie Basin Iron Formations Michael Taylor (University of Minnesota - Duluth) - Pleistocene glaciation as a mechanism for emplacement of high-salinity groundwater at anomalously shallow depths in the Lake Superior basin x �STUDENT PAPER AWARDS Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper—last modified by the Board in 2001—follow: • The contribution must be demonstrably the work of the student. • The student must present the contribution in-person. • The Student Paper and Poster Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. • In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. • The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). • The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper and Poster Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. • The names of award recipients shall be included as part of the annual Chair's report that appears in the next volume of the Institute. Student papers are noted on the Program. In 2006 the ILSG Student Paper Committee presented four awards from the ILSG Student Paper Fund. The awards were made to: Patrick Moran -- Lakehead University, Thunder Bay, ON (best oral presentation) -- $250 Noah Planavsky -- Lawrence University, Appleton, WI (oral presentation, honorable mention) -- $50 Amanda Gross -- Kent State University, Kent, OH (best poster) -- $250 Ryan Bartingale -- University of Wisconsin - Eau Claire, Eau Claire, WI (poster, honorable mention) -- $50 xi �EISENBREY STUDENT TRAVEL AWARDS The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name "Eisenbrey" was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. "Ned" Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The annual Chair in consultation with the Secretary-Treasurer determines the number of awards and value. Recipients will be announced at the annual banquet. The student travel award application is available on the ILSG website. The following general criteria will be considered by the annual Chair, who is responsible for the selection: • The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. • Students who are the senior author on either an oral or poster paper will be given favored consideration. • It is desirable for two or more students to jointly request travel assistance. • In general, priority will be given to those in the Institute region who are farthest away from the meeting location. • Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. • Successful applicants will receive their awards during the meeting. In 2006 the ILSG awarded six travel awards from the ILSG Eisenbrey Student Travel Fund. The awards were made to: Peggy Stonier -- Kent State University, Kent, Ohio Amanda Gross -- Kent State University, Kent, Ohio Jenny Murphy -- Lawrence University, Appleton WI Noah Planavsky -- Lawrence University, Appleton WI Clare Stielstra -- Lawrence University, Appleton WI Davis Voights -- Lawrence University, Appleton WI xii �REPORT OF THE CHAIRS OF THE 52ND ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY SAULT STE MARIE, ONTARIO The Ontario Geological Survey hosted the 52nd Annual Institute on Lake Superior Geology meeting on May 10-11 2006 in Sault Ste Marie, Ontario. The meeting consisted of two days of technical sessions with two pre-meeting and four post-meeting field trips. Pete Hollings and Mike Mudrey provided invaluable technical assistance with the web site and Anthony Pace provided on site assistance with the audio-visual component of the program. Gerry Bennett, Mike Easton, Mike Hailstone and Anthony Pace provided assistance with field trip preparation. Vivienne Coté and Roger Poulin (Ontario Prospectors Association) and Marc Gaudreau and Anthony Pace (Ontario Geological Survey) provided aid with the loan, retrieval, delivery and set-up of the poster boards for the poster session. Lisa Bagnall, Scheduling Office, was the on site co-ordinator with the Sault College of Applied Arts and Technology. Total registration for the meeting was 110 students and professionals. Proceedings Volume 52 was published in six parts: Part 1 – Proceedings and Abstracts, edited by A. C. Wilson contained abstracts for 19 oral presentations and 21 posters; Part 2 – Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan – Field Trip Guidebook; Part 3 – Unusual Diamond-bearing breccias of the Wawa Area – Field Trip Guidebook; Part 4 – The Huronian Supergroup between Sault Ste Marie and Elliot Lake – Field Trip Guidebook; Part 5 – Keweenawan Rocks of the Mamainse Point Area – Field Trip Guidebook and Part 6 – Geological Guidebook to the Paleoproterozoic East Bull Lake Intrusive Suite Plutons at East Bull Lake, Agnew Lake and River Valley, Ontario – Field Trip Guidebook. All guidebook volumes were edited by R. P. Sage. Last held in Sault Ste Marie, Ontario in 1974, a return visit to the area allowed participants to take part in a smorgasbord of new field trips. On Monday, May 8, Gerry Bennett, assisted by Mike Hailstone, led a two-day field trip to examine the classic stratigraphy of the Huronian Supergroup in the Elliot Lake and Searchmont areas. The next day, Ann Wilson introduced a rain-soaked but enthusiastic crowd of participants to the unusual diamond-bearing breccias of the Wawa area. Following the conference four field trips were held: Ann Wilson reprised the Wawa field trip; Tom Hart and Anthony Pace’s trip examined the Keweenawan rocks along the Lake Superior shoreline; Ron Sage led a small group of bicyclists to Mackinac Island to examine bedrock exposures there; and Mike Easton and Dick James (Laurentian University) took a group east of the Sault to investigate a suite of intrusive rocks along the boundary between the Archean Superior and the Proterozoic Southern provinces. Eighty-five participants attended the Annual Banquet. Dr. Ed Walker gave the after dinner key note address providing his insight into the “Emplacement and Geochemistry of Archean-aged diamondiferous rocks of the Wawa Area”. Jim Miller presented the 2006 Goldich Medal to Mike Mudrey for his 30 years of contributions to the understanding of the regional geology and for his service to the Institute. The student paper committee consisted of Dan England, John Klasner and Norm Trowell all of whom conscientiously deliberated over the four oral presentations and four poster presentations. The winners were: xiii �2006 Best Student Paper Awards 1) Patrick Moran – Lakehead University ($250, winner best oral presentation) 2) Amanda Gross – Kent State University ($250, winner best poster presentation) 3) Noah Planavsky – Lawrence University ($50, honourable mention oral presentation) 4) Ryan Bartingale – University of Wisconsin ($50, honourable mention poster presentation) In addition, Eisenbrey travel awards were presented to: 1) 2) 3) 4) 5) 6) Noah Planavsky – Lawrence University ($150) Jenny Murphy – Lawrence University ($150) Clare Stielstra – Lawrence University ($150) Davis Voights – Lawrence University ($150) Penny Stonier – Kent State University ($200) Amanda Gross – Kent State University ($200) The Institute’s Board of Directors met on May 10, 2006 and a brief summary of the meeting follows. 1. Accepted the Report of the Chairs for the 51st ILSG from Mark Smyk and Pete Hollings, and the minutes of the last Board meeting, May 26, 2006 from Pete Hollings. 2. Received, discussed and accepted the 2005-06 ILSG Financial Summary from ILSG Treasurer, Mark Jirsa. 3. Approved Ann Wilson as the on-going board member. 4. Approved 2007 (53rd annual) meeting location as Lutsen, Minnesota with Laurel Woodruff as chair. 5. Replaced George Hudak, academic member, on the Goldich Committee with Dick Ojakangas. The 52nd meeting was not without its challenges, including the last minute resolution of a strike at Ontario’s Colleges. However, no challenges were insurmountable and the meeting and field trips went off without a hitch. We thank all individuals who contributed to the success of the meeting, including the staff of Sault College, the bus drivers from AJ Bus Lines Limited and Reid Bus Line, volunteer field trip leaders and drivers and the Ontario Geological Survey. The field trips were astonishingly well attended and we are grateful for the kind feedback that we received from the field trip participants. Both of us were extremely satisfied with the 52nd meeting. We appreciated all of the positive feedback from the delegates who enjoyed a return visit to the Sault. As always, we wouldn’t have succeeded without the continuing support of the ILSG members. Respectfully submitted Ron Sage and Ann Wilson Co-Chairs, 52nd ILSG Meeting xiv �2007 BOARD OF DIRECTORS Board appointment continues through the close of the last meeting year, or until a successor is selected Laurel Woodruff and Jim Miller, Co-Chairs 53rd meeting (2010) U.S. Geological Survey, St. Paul, MN Minnesota Geological Survey, Duluth, MN Ann Wilson (2009) Ontario Geological Survey, South Porcupine, ON Mark Smyk (2008) Ministry of Northern Development and Mines, Thunder Bay, ON Steve Hauck (2007) Natural Resources Research Institute, University of Minnesota – Duluth, Duluth, MN Peter Hollings – Secretary (2007) Lakehead University, Thunder Bay, ON Mark A. Jirsa – Treasurer (2009) Minnesota Geological Survey, St. Paul, MN 2007 SESSION CHAIRS Ted Bornhorst, Michigan Technological Survey, Houghton, MI Mary Louise Hill, Lakehead University, Thunder Bay, ON Gordon Medaris, Jr., University of Wisconsin, Madison, WI Klaus Schulz, U.S. Geological Survey, Reston, VA Thomas Waggoner, retired, Cliffs Mining Services Co., Ishpeming, MI Ann Wilson, Ontario Geological Survey, South Porcupine, ON 2007 STUDENT PAPER COMMITTEE Marcia Bjornerud (Chair), Lawrence University, Appleton, WI Daniela Vallini, Woodside Energy, Ltd., Perth, Western Australia Graham Wilson, Magma Metals, Campbellford, ON 2007 LOCAL COMMITTEES General Co-Chairs Laurel G. Woodruff – U.S. Geological Survey, St. Paul, MN James D. Miller, Jr. – Minnesota Geological Survey, St. Paul, MN Program and Abstracts Editor Laurel G. Woodruff -- U.S. Geological Survey, St. Paul, MN Field Trip Guidebook Editor James D. Miller, Jr. – Minnesota Geological Survey, St. Paul, MN Acting Local Committee, Lutsen, MN Gretchen Klasner – Marquette, MI xv �2007 BANQUET SPEAKER Don Hunter PolyMet Mining, Inc. Hoyt Lakes, MN PolyMet’s NorthMet Project – the long road from exploration to production xvi �PROGRAM xvii �TUESDAY MAY 8, 2007 8:00 a.m. FIELD TRIP 1: IGNEOUS STRATIGRAPHY OF THE POPLAR LAKE INTRUSION (FORMERLY NATHAN’S LAYERED SERIES) Jim Miller, Minnesota Geological Survey Eric Jerde, Morehead State University WEDNESDAY MAY 9, 2007 8:00 a.m. FIELD TRIP 2: GEOLOGIC AND CULTURAL HISTORY OF GRAND PORTAGE NATIONAL MONUMENT Bill Cannon, U.S. Geological Survey Brian Phillips, Lakehead University David Cooper, National Park Service 8:00 a.m. FIELD TRIP 3: MIDCONTINENT RIFT-RELATED INTRUSIONS NORTH OF THE INTERNATIONAL BORDER Mark Smyk, Ministry of Northern Development and Mines Peter Hollings, Lakehead University 6:00 p.m. Return of Trips 1, 2 and 3 4:00 p.m. - 8:00 p.m. Registration 7:00 p.m. - 9:00 p.m. Ice Breaker Social and Poster Setup THURSDAY MAY 10, 2007 Note: Asterisk * denotes a student eligible for Best Student Paper Award Presenter underlined 8:00 a.m. - 9:00 a.m. REGISTRATION 8:25 a.m. INTRODUCTORY REMARKS Laurel Woodruff and Jim Miller, Co-Chairs, 2007 ILSG TECHNICAL SESSION I Session Chair: Klaus Schulz, U.S. Geological Survey 8:30 a.m. Addison, William D., Cannon, William F. and Brumpton, Gregory R. How to identify Sudbury impact ejecta in the Lake Superior Region 8:50 a.m. Cannon, William F. and Addison, William D. The Sudbury impact layer in the Lake Superior iron ranges: A time-line from the heavens 9:10 a.m. Burton, Justin and Fralick, Philip Deposition and cementation of Paleoproterozoic Gunflint Formation carbonate: Implications for early hydrosphere chemistry xviii �9:30 a.m. Planavsky, Noah* and Murphy, Jennifer Rare earth element patterns in Steep Rock Carbonates 9:50 a.m. - 12:00 p.m. COFFEE BREAK AND EXTENDED POSTER SESSION 12:00 p.m. Lunch Break – 2007 ILSG Board Meeting (by invitation) TECHNICAL SESSION II Session Chairs: Mary Louise Hill, Lakehead University Gordon Medaris, Jr., University of Wisconsin, Madison 1:30 p.m. Green, John C. and Slade, Andrew A State Scientific and Natural Area and a North Shore non-profit association 1:50 p.m. Boerboom, Terry J. Newly recognized thick interflow sandstones in the upper northeast limb of the North Shore Volcanic Group, Minnesota 2:10 p.m. Mudrey, Michael G., Jr. and Wooden, Joseph L. A chemical and Sr isotopic study of the Pigeon Point Sill, Cook County, Minnesota 2:30 p.m. Bjornerud, Marcia Evidence for paleoseismic events during closure of the Midcontinent Rift, Atkins Lake-Marenisco Fault, N. Wisconsin 2:50 p.m. Hudak, George J., Hoffman, Adam T., Peterson, Dean M. and Heine, John Recent developments understanding the volcanic, magmatic, tectonic, and metallogenic evolution of the Ely Greenstone Formation, Vermilion District, NE Minnesota 3:10 p.m. COFFEE BREAK AND POSTER SESSION 3:30 p.m. Puumala, Mark A. New insights into the metallogeny of the eastern portion of the Archean Uchi Domain, Superior Province, Ontario 3:50 p.m. Alexander, Malcolm* and Mitchell, Roger H. Rare metal mineralization in pegmatites of the Coldwell Alkaline Complex 4:10 p.m. Good, David and Walford, Phillip PGE-rich mineralization at the Marathon Deposit, Coldwell Alkaline complex, Ontario xix �4:30 p.m. Brown, Alex C. Deposition of native copper lodes on the Keweenaw Peninsula, northern Michigan, from a gravity-driven evolved meteoric brine 6:00 p.m. ICE BREAKER – MIXER – CASH BAR 7:00 p.m. ANNUAL BANQUET AND AWARD PRESENTATION • Announcement of 54th Annual Meeting Location • 2007 Goldich Award Presentation to Joseph Mancuso • 2007 Banquet Address Meeting participants who are not registered for the banquet are welcome to the banquet address FRIDAY MAY 11, 2007 8:25 a.m. INTRODUCTORY REMARKS Laurel Woodruff and Jim Miller, Co-Chairs TECHNICAL SESSION III Session Chairs: Thomas Waggoner, retired, Cliffs Mining Services Co. Ann Wilson, Ontario Geological Survey 8:30 p.m. Student, James J., Wark, David A., Mutchler, Scott R. and Bodnar, Robert J. Thermal evolution of Proterozoic (>1Ga) rhyolite magma based on analysis of melt inclusions and trace elements in quartz from the Keweenaw Peninsula of Michigan 8:50 p.m. Hart, Tom R. and MacDonald, Carol Anne Emplacement of the Nipigon Sill Complex and mafic to ultramafic intrusions of the Nipigon Embayment 9:10 p.m. Hollings, Pete, Smyk, Mark C. and Hart, Tom Geochemistry of Midcontinent Rift-related mafic dykes and sills near Thunder Bay: new insights into geographic distribution and the geochemical affinities of Nipigon and Logan sills and Pigeon River and other dykes 9:30 p.m. Forsha, Clint J.* and Zieg, Michael J. Textural stratigraphy of Nipigon diabase sills: A tool for correlation and petrologic interpretation 9:50 p.m. Zieg, Michael J. and Forsha, Clint J. A comparison of textural profiles in diabase sills from the Midcontinent and Transantactic rift 10:10 a.m. COFFEE BREAK AND POSTER SESSION xx �10:30 p.m. Cockerton, Sarah, Conly, Andrew G. and Lee, Peter An experimental water-rock interaction study into the origin of high-sulfate waters associated with the Steep Rock Mines, Atikokan, Ontario 10:50 p.m. Stevens, Larissa B.* and Fralick, Philip Investigation of ferromanganese nodule precipitation and arsenic uptake in modern lacustrine biochemical sediments 11:10 a.m. Rodengen, Tommy*, Theissen, Kevin and Sugita, Sugita Elemental and isotopic shallow lake proxies of landscape changes in the Prairie Pothole region of Minnesota 11:30 a.m. Diedrich, Tamara* and Sharp, Thomas G. The effect of H2O on olivine to ringwoodite transformation: Implications for subduction zone dynamics and the deep earth water cycle 11:50 a.m. Thorliefson, Harvey Potential use of the Midcontinent Rift for CO2 sequestration 12:10 p.m. LUNCH BREAK TECHNICAL SESSION IV Session Chair: Ted Bornhorst, Michigan Technological University 1:40 a.m. McSwiggen, Peter L. Trace element analyses: Avoiding data distortion 2:00 p.m. Waggoner, Tom D. Definition of the Proterozoic terrain under the Paleozoic – Central U.P., Michigan 2:00 p.m. Oreskovich, Julie Documenting underground mine workings on the Mesabi Iron range in GIS format 2:40 p.m. Metsaranta, Riku T., Fralick, Philip W. and Bowdidge, Colin Metamorphosed halite-dominated evaporates of the Lower Sibley Group 3:00 p.m. Presentation of Student Paper Awards Marcia Bjornerud, Lawrence University: Student Paper Committee 3:10 a.m. FINAL COFFEE BREAK AND POSTER SESSION – POSTERS TO BE REMOVED AFTER THE BREAK xxi �SATURDAY MAY 12, 2007 8:00 a.m. FIELD TRIP 4: GEOLOGY AND CU-NI-PGES MINERALIZATION – NICKEL LAKE MACRODIKE, SOUTH KAWISHIWI INTRUSION Dean Peterson, Natural Resources Research Institute Paul Albers, Duluth Metals 8:00 a.m. FIELD TRIP 5: GEOLOGY OF THE NORTH SHORE FROM LITTLE MARAIS TO GRAND MARAIS Terry Boerboom, Minnesota Geological Survey John Green, University of Minnesota - Duluth Jim Miller, Minnesota Geological Survey 8:00 a.m. FIELD TRIP 6: GEOLOGY ALONG THE GUNFLINT TRAIL Mark Jirsa, Minnesota Geological Survey Paul Weiblen, University of Minnesota – Twin Cities 6:00 p.m. Return of Trips 5 & 6 SUNDAY MAY 13, 2007 6:00 p.m. Return of Trip 4 xxii �POSTER PRESENTATIONS Behling, Stuart J. Mercury in fish: a geologist’s perspective Boerboom, Terry J., Green, John C., Albers, Paul and Miller, James D., Jr. Bedrock geologic map of the Little Marais, Schroeder, and Tofte 7.5 minute quadrangles, North Shore of Lake Superior, Minnesota Boisjoli, Troy* and Flood, Timothy P. Petrographic differentiation of the five phases of the Lower Cretaceous Star Kimberlite, Saskatchewan, Canada Buchholz, Thomas W., Falster, Alexander U. and Simmons, William B. A novel locality for pseudobrookite – the Nine Mile-Pluton, Marathon County, Wisconsin Costello, Daniel E.*, Flood, Timothy P. and Thole, Jeffrey T. Origin of a mafic pegmatite within the Duluth Complex, Northern Minnesota Dean, J. Frederick and Phillips, Brian A.M. Distribution of certain ichthyofauna in relation to eastern outlets of Lake Agassiz, with emphasis on the Gunflint-Arrow Lakes corridor and the Keating Complex Elsenheimer, Don, Frey, Barry A. and Hudak, Joe N. Gold mineralization in the Virginia Horn Greenstone Terrain, St. Louis County, Minnesota: A prospect re-visited Hogan, Amanda*, Jacobs, Travis* and Theissen, Kevin Lacustrine sedimentary organic matter proxies of recent lake state changes and climatic conditions on Christina and Morrison Lakes of western Minnesota Hudak, Joe N. and Frey, Barry A. Evaluation of mineral exploration drill cuttings in the Rice River area, east central Minnesota Jasinevicius, Renata R.* and Gordon, Elizabeth A. Paleoenvironmental interpretation of a Lower Paleozoic stromatolite reef, northeastern Wisconsin Jirsa, Mark A., Boerboom, Terry J., Chandler, Val W., Lively, Richard S., Miller, James D., Jr., Mossler, John H., Runkel, Anthony C., Setterholm, Dale R. and Wahl, Timothy E. Proposed new bedrock geologic map of Minnesota Medaris, L. Gordon, Jr., Fournelle, John H. and Guggenheim, Stephen J. An occurrence of agrellite in the Wausau Alkaline Igneous Complex, Marathon County, Wisconsin xxiii �Nicholas, Sarah L.*, Wirth, Karl R., Engstrom, Jennifer and Lapakko, Kim A. Investigations of sulfide minerals leached in the presence of alkaline solids Quigley, Patrick O.* Michigan kimberlites revisited: New mineral, chemical and petrographic analyses Rymaszewski, Jody A.*, Friedrich, Jason L. and Czeck, Dyanna M. Bedrock fractures in southeastern Wisconsin: Paleostress estimates and relationships to the Waukesha Fault Saxton, Samantha* and Cordu, William Precambrian geology of the Opelt Quarry, Neillsville, Wisconsin Severson, Mark J. and Heine, John Revised stratigraphy of the Biwabik Iron Formation, Mesabi Range, Minnesota – developing the “Rosetta Stone” Stott, Greg, Corkery, Tim, LeClair, Alain, Boily, Michel and Percival, John A revised terrane map for the Superior Province as interpreted from aeromagnetic data Taylor, Michael L.* and Swenson, John B. Pleistocene glaciation as a mechanism for emplacement of high-salinity groundwater at anomalously shallow depths in the Lake Superior basin Theriault, Stephanie A.* and Hickson, Thomas A. Possible influence of a buried fault on elevated indoor radon levels, south Washington County, Minnesota Thorliefson, L. Harvey, Harris, Kenneth L., Hobbs, Howard C., Jennings, Carrie E., Knaeble, Alan R., Lively, Richard S., Lusardi, Barbara A. and Meyer, Gary N. Till geochemical and indicator mineral reconnaissance of Minnesota xxiv �ABSTRACTS xxv ��HOW TO IDENTIFY SUDBURY IMPACT EJECTA IN THE LAKE SUPERIOR REGION Addison, William D., R.R. 2, Kakabeka Falls, Ontario, POT 1W0, Canada Cannon, William F., U.S. Geological Survey, MS 954, Reston VA 20192, USA Brumpton, Gregory R., 211 Henry St., Thunder Bay, Ontario, P7E 4Y7, Canada For at least 15 years geologists looked for ejecta from the 1850 Ma Sudbury impact in sedimentary rocks of the Lake Superior region. Addison and others (2005) first reported its occurrence but more recent work has shown that “volcaniclastic breccia” or “submarine slump deposits” described and mapped as early as the 1940’s are, in fact, ejecta deposits and other impact-related rocks (Cannon and Addison, 2007). To date, Sudbury ejecta deposits have been identified in six areas in the western Lake Superior region and provide a precise regional stratigraphic marker (Cannon and Addison, 2007). It seems nearly certain that other sites will be discovered. Toward that end we pass on some of our observations and experience in the commonly frustrating search for the impact layer in hopes that others will make additional discoveries of this important chrono-stratigraphic marker bed. Ejecta, the material thrown upward and outward from the Sudbury impact, arrived in the Lake Superior region in two ways. First, was coarse to fine material, primarily melted and solid target rock, that followed a ballistic trajectory to about five times the final crater radius (5r), most recently estimated at 130 km (Spray and others, 2004). Thus, ballistic ejecta should not be expected beyond 650 km from the impact center. Five of the six known occurrences lie within the 5r distance. At those sites ballistic ejecta arrived with horizontal velocities as high as a few km/sec, and were capable of causing extreme disruption and erosion of surface sediments. The result was breccia deposits of ejecta and local country rock. Ejecta inside the 5r ballistic zone include devitrified glass in vesicular blobs (both streamlined and irregular shapes) and microtektites and spherules (frozen melt droplets), both singly and in clusters. Other features include accretionary lapilli, and angular grains of quartz, a small percentage of which contain relict planar deformation features, the only proof that the ejecta are of impact origin (French, 1998). Second, a huge super-heated expanding cloud of vapor, sub-mm melt droplets, and sub-mm rock shards reached hundreds of km above Earth. Condensed and solidified melt droplets and tiny rock shards settled over a period of days to more than a year covering all of Earth. These ejecta do not substantially alter the surface on which they are deposited and do not form the breccias typical of ballistic ejecta. Microtektites and spherules and quartz and feldspar grains, some showing shock metamorphic features, from the cloud have been found at sites near Hibbing, Minnesota, beyond the range of ballistic ejecta. Finally, most of the ballistic deposits and perhaps some of the earlier vapor cloud deposits have been reworked by impact-induced tsunamis resulting in polymict breccias of local rocks and ejecta (Cannon and Addison, 2007). 1 �So, how and where should you look for a Sudbury ejecta deposit and how will you recognize it? Known sites are near the top of the principal iron-bearing units (Gunflint, Biwabik, Ironwood, etc.) in the region to about 300 m farther up section and new discoveries will most likely be within or not far from this interval. Within the 5r zone, easily seen macroscopic features include breccia, especially black chert breccia, mm-cm size blobs of devitrified vesicular glass, and clusters or beds of accretionary lapilli. Smaller-scale features include 0.5-2 mm gray ovoid, round, or irregular microtektites and spherules that may also be replaced by green clay minerals. Similar sized rounded quartz and feldspar grains occur along with sub-mm angular quartz and feldspar grains, a few of which contain one or more sets of thin parallel lamellae, the diagnostic shock deformation features. Note that these lamellae are very rare and commonly require hours or days of careful petrographic examination under high magnification to find. Beyond the 650 km ballistic zone, represented by only the sites near Hibbing, MN, there are no characteristic macroscopic features. Ejecta at the top of the Biwabik Iron-formation are within a siliceous, light gray coarsely recrystallized carbonate layer. Within the light carbonate there is a darker gray reticulated pattern giving a vague appearance of 2-4 mm circles. Thin sections from the reticulated zone reveal microtektites, spherules and vesicular glass. Rounded quartz grains are present as are angular quartz and feldspar. Shocked quartz and feldspar grains with the relict lamellae are exceedingly rare. All known localities are in pristine to weakly metamorphosed rocks. Unfortunately, the considerable regions of higher grade metamorphism in the Lake Superior region may never yield any of the features described here, particularly the diagnostic shock lamellae in quartz, because of thorough recrystallization. Certainly this is true at Peter Mitchell Mine, Babbitt, MN. Searches in weakly metamorphosed sequences are most likely to succeed. References: Addison, W. D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology, v. 33, p. 193-196. Cannon, W. F. and Addison, W.D., 2007, The Sudbury impact layer in the Lake Superior iron ranges: a time-line from the heavens: Institute on Lake Superior Geology Proceedings, Vol. 53, Part 1. French, B.M., 1998, Traces of catastrophe: Lunar Planetary Inst. Contrib. 954, 120 p. Spray, J. G., Butler, H. R., and Thompson, L. M., 2004, Tectonic influences on the morphometry of the Sudbury impact structure: Implications for terrestrial cratering and modeling: Meteoritics & Planetary Sci., V. 39, p. 287-301. 2 �Rare Metal Mineralization in Pegmatites of the Coldwell Alkaline Complex Malcolm Alexander and Roger H. Mitchell Department of Geology, Lakehead University, Thunder bay, Ontario ABSTRACT The Coldwell Complex (northwestern Ontario), is a multiphase alkaline intrusion that is host to rare earth element, actinide and other high field strength element mineralization. Preliminary studies have shown that these minerals are concentrated in pegmatites associated with Center 1 ferrorichterite-ferroaugite syenites. These syenites exhibit the geochemical characteristics of A-type granitoid magmatism. The focus of this study is the distribution, nature and petrochemical controls of rare metal mineralization within these pegmatitic bodies. Within the Center 1 formation there are two subunits; the border gabbro, a massive to layered intrusion ranging in composition from wehrlite to diorite and locally to anorthosite; and oversaturated massive to locally layered ferroaugite syenites. Both subunits host pegmatitic quartz bearing residua characterized by cumulus perthitic alkali feldspar, hedenbergite-aegirine pyroxenes, with interstitial quartz, calcite, and calcic-tosodic-calcic-to-sodic amphiboles (Fig. 1). The pegmatitic residua are of the niobiumyttrium-fluorine (NYF) type, and characterized by the enrichment of Nb-Ti-Ta oxides, with niobium predominant over tantalum. Back-scattered electron petrography and quantitative X-ray spectrometry have been used to characterize the mineral paragenesis. Pegmatitic syenitic residua emplaced in, but not derived from, the border gabbro contain a wide range of rare element minerals which include: chevkinite; fergusonite; monazite; allanite; kainosite; xenotime; REE fluorocarbonates: bastnaesite, synchysite and parisite. Other rare element enriched minerals include fersmite, apatite, zirconolite, U-Th-Sipyrochlore, Nb-rutile and Nb-bearing ilmenite. Early-formed rare element minerals such as chevkinite and pyrochlore are commonly replaced by complex aggregates of laterforming phases. REE-fluorocarbonates commonly exhibit syntaxial intergrowths. Other ferroactinolite-quartz bearing pegmatitic residua, occurring in the upper series of a differentiated aenigmatite-ferroaugite syenite unit, contain a more limited range of rare element minerals, including zircon, xenotime, monazite and the fluorocarbonates together with REE bearing apatite and pyrochlore. The differences in rare element mineralization between the two pegmatites examined to date suggest that they were derived from different batches of ferroaugite syenite magma. Initial data indicate that different pegmatitic residua in the Center 1 subunits also evolved differently. Intensive parameters have been estimated using amphibole mineralogy and Fe-Ti-oxide compositions, as seen in Figure 2. These parameters indicate the pegmatites intruding the border gabbro and syenite residua had equilibration temperatures of 450 ºC at oxygen fugacities of +0.5-1.0 and +2.0-2.2, relative to QFM, respectively. 3 �Calcic Amphiboles 1.0 Tremolite 0.8 Actinolite Magnesiohornblende Magnesiotschmakite 0.6 0.4 0.2 FerroFerrohornblende Ferrotschermakite actinolite Mg/(Mg + Fe2+) Mg/(Mg + Fe2+) 1.0 Sodic-Calcic Amphiboles 0.0 0.8 Richerite Magnesiokatotophorite Magnesiotaramite 0.6 0.4 Ferro0.2 richterite Katophorite Taramite 0.0 8.0 7.5 7.0 6.5 6.0 5.5 8.0 Si in Formula 7.5 7.0 6.5 6.0 5.5 Si in Formula A B Figure 1: Stability fields of calcic (A) and sodic-calcic (B) amphiboles. Each point represents a mean value of 5 to 9 point analyses. Circles represent the residua injected into the border gabbro, while triangles represent residua injected into the saturated ferroaugite syenite. A B Figure 2: Oxygen fugacity diagrams, using the quartz-fayalite-magnetite (QFM) oxygen buffer. Diagram A contains two points, the right hand value is representative of the ferroaugite syenite and the left hand value is representative of the pegmatitic residua (Mitchell and Platt, 1977). Diagram B is a Fe – Ti oxide geothermometer and oxygen barometer. ILM is given in mol% ilmenite, USP is mol% ulvöspinel. Triangles are saturated ferroaugite syenite residua, circles are border gabbro residua. Adapted from Ghiorso and Sack (1991). REFERENCES Ghiorso, M.S. and Sack, O.S. 1991, Fe – Ti oxide geothermometry: thermodynamic formulation and the estimation of intensive variables in silicic magmas: Contributions to Mineralogy and Petrology, v.108, p. 485. Mitchell, R.H. and Platt, R.G. 1977, Mafic mineralogy of ferroaugite syenite from the Coldwell Alkaline Complex, Ontario, Canada: Journal of Petrology, v. 19, p. 627. 4 �MERCURY IN FISH: A GEOLOGIST’S PERSPECTIVE BEHLING, Stuart James, Retired, Superior National Forest Geologist 4712 Anderson Rd., Duluth, MN 55811 USA stu-ball@mchsi.com Mercury contamination of fish has been a health concern in Northeastern Minnesota since testing began over 30 years ago. In spite of this concern, less than 15% of the lakes in this area have been tested. The data show that fish mercury contamination varies considerably between lakes but no pattern of distribution has been recognized. The data also show that fish mercury contamination varies between species and increases with length of fish. For example, walleyes are more contaminated than northern pike of the same size, from the same lake, and longer fish have more mercury than shorter ones. However, there is not enough data available from most lakes to plot these contamination trends. A model that could predict fish mercury contamination levels by species and length of fish would be a great asset in understanding the extent and distribution of fish mercury in this area. In this study, Northeastern Minnesota was divided into groups of geologically related lakes termed lake provinces. Ten lake provinces were identified in two distinct geomorphic areas; seven are in an area of bedrock controlled terrain and three are in an area of glacial deposition. In the bedrock controlled terrain, the size shape and orientation of the lakes are directly related to structural or compositional weakness in the underlying bedrock. Weathering and glacial erosion exploited these weaknesses to produce distinct patterns of lakes that were used to identify the provinces. In the areas of glacial deposition, the lake provinces were associated with the deposits left by three distinct glacial advances. When fish mercury was plotted against length of fish by province, the correlation coefficients increased significantly. The R squared values for the provinces ranged from .58 to .81 as compared to an R squared value of .32 for all samples from the study area plotted collectively. A map showing the distribution of mercury contamination in walleyes and northern pike was produced using the results from this ongoing study. 5 �EVIDENCE FOR PALEOSEISMIC EVENTS DURING CLOSURE OF THE MIDCONTINENT RIFT, ATKINS LAKE-MARENISCO FAULT, N. WISCONSIN Bjornerud, Marcia, Geology Department, Lawrence University, 115 S. Drew St. Appleton, Wisconsin 54911 USA. bjornerm@lawrence.edu The closure, or inversion, of the Midcontinent Rift at ca. 1.0 Ga remains one of the least well understood chapters in the Precambrian tectonic history of the Lake Superior region. Rift-bounding normal faults were reactivated as major reverse faults, some with estimated net reverse displacements of several kilometers. One such structure is the north-dipping Atkins Lake-Marenisco fault in Bayfield, Ashland and Iron Counties of northern Wisconsin. According to the interpretation of Cannon et al. (1993), the Atkins Lake-Marenisco fault is a crustal-scale listric fault zone along which lower Proterozoic metasedimentary rocks and middle-Proterozoic rift-related volcanic rocks have been thrust southward over Archean gneisses (the Proterozoic sequences had originally occupied a structurally lower position in the axial graben of the rift). The Atkins Lake-Marenisco Fault is well exposed near the falls of the Marengo River in eastern Bayfield County. There the fault zone is ca. 100 m wide and cuts through the Archean (ca. 2.7 Ga) Puritan Batholith, a granitic body with a weak gneissic fabric. In a downstream transect beginning just below the falls, the rocks record extreme strain gradients. Over a distance of < 20 m, pristine gneiss gives way first to cataclasite and then to dark, extremely fine-grained phyllonites, all heavily veined with K-feldspar, chlorite and zeolite. In thin section, cataclastic textures can be seen to overprint quasiductilely deformed feldspars, indicating that displacement along the fault zone may have begun at temperatures close to 450°C and continued as the rocks cooled. Most of the cataclasites have a crudely fractal (log-linear) clast size distribution, with the average grain size decreasing progressively with strain intensity, as expected when cataclasis is the dominant deformation mechanism. Samples from one outcrop near the base of Marengo Falls, however, have textures suggestive of devitrified pseudotachylyte. Dark-colored veins and lenses, now chloritized, contain relatively large (mm-sized) clasts of feldspar, but finer-grained clasts are notably absent. Many of the 6 �larger clasts, moreover, show unusual microstructural features. Some have embayments and concave edges – shapes not typical of ordinary cataclasites, in which grains are commonly equant and polygonal to rounded. Some of the clasts also display mosaicism, a texture that can form as a result of rapid volumetric changes. The bimodal grain size distribution and embayed and mosaicized grains could be explained by the presence of a seismically generated frictional melt, which could have preferentially assimilated the finest grains, partly resorbed larger ones, and created a short-lived pressure pulse as a result of expansion upon melting. If this interpretation is correct, we should begin to consider the role seismicity may have played in facilitating transient fluid flow and consequent mineralization during the closure of the Midcontinent Rift. Reference cited: Cannon, W., Z. Peterman, and P.K. Sims, 1993. Crustal-scale thrusting and origin of the Montreal River monocline: A 35-km thick cross section of the Midcontinent Rift in Northern Michigan and Wisconsin. Tectonics v. 12, p. 728-744. 7 �NEWLY RECOGNIZED THICK INTERFLOW SANDSTONES IN THE UPPER NORTHEAST LIMB OF THE NORTH SHORE VOLCANIC GROUP, MINNESOTA BOERBOOM, Terrence J., Minnesota Geological Survey, boerb001@umn.edu New mapping, funded in part by the USGS STATEMAP program, has identified several major units of interflow sandstone between lava flows in the upper Northeast Sequence of the North Shore Volcanic Group (NSVG), which are as much as 200-300 feet (60-90 meters) thick. The recognition of these sandstones approximately triples the total thickness of known interflow sandstones in that area. These sandstones were intersected in several water well cutting sets collected by McKeever Well Drilling of Schroeder, Minnesota. Although the wells intersect only partial sections of the sandstone units, these intersections project into prominent northeast-trending topographic lows which correspond with linear low aeromagnetic anomalies. These lie between and parallel to ridges held up by basalt that correspond to linear positive magnetic anomalies. The eastern extent of these sandstones has yet to be determined, but will be examined in the coming field season. The sandstones identified in water well cuttings are pinkish-tan, medium-grained, and similar in composition and appearance to the well-known Cutface Creek sandstone (Fig. 1). In the northern-most sandstone, a well just south of the Monker Lake diabase (Fig. 1) intersected 80 feet (25 meters) of sandstone above the Devil’s Track rhyolite. However, a well located 8 miles along strike to the southwest intersected only 10 feet of sandstone, below basalt and above the Devil’s Track rhyolite, indicating either that it pinches out to the southwest or that the thickness varies due to deposition on an irregular surface. The middle sandstone unit shown on Fig. 1 was intersected in only one well, which penetrated 15 feet (5 meters) of sandstone beneath basalt. The color, composition, and grain size is consistent with it being the uppermost part of a thick interflow sediment, quite distinct from the typical thin interflow sediments that are found throughout lava flows in the NSVG. The southern-most of the three sandstone units is inferred entirely on the basis of topographic and aeromagnetic features that mimic those of the middle unit. Sandstone of substantial thickness was also intersected in water wells near the top of Eagle Mountain at the Lutsen Ski Resort (Fig. 1), which penetrated 15–60 feet (4-20 meters) of tan, medium-grained sandstone. This sandstone lies below the Leveaux ferrodiorite and above the Eagle Mountain basalt, and likely acted as an anisotropic plane of weakness that controlled emplacement of the Leveaux ferrodiorite sill. Its irregular thickness may be the product of having been variably cut out by the intrusion. Two thick interflow sandstone units – the Cut Face and Indian Camp Creek sandstones, had long been recognized in the upper NE limb of the NSVG from outcrops near Lake Superior. Both of these are soft and easily eroded relative to the surrounding volcanic rocks, hence are exposed only near the shore in actively eroding stream channels. Based on inferences from paleocurrent measurements, the interflow sandstones in the NSVG were deposited in two separate depositional basins that correspond to the SW and NE limbs of the NSVG (Jirsa, 1984). Sedimentary rocks between the SW and NE limbs, in the area from Illgen City to Lutsen (Fig. 1), are composed mainly of conglomerate and fragmental volcanic rocks, in contrast to sandstone that dominates on either side. This area coincides with a negative gravity anomaly known as “White’s Ridge” (White, 1966), that represents a block of uplifted crust (Boerboom, 1994). To the southwest of White’s ridge (SW Limb NSVG), the stratigraphically lower lavas dip more steeply towards the center of the rift than higher lavas, implying greater subsidence towards the rift axis during volcanism. To the NE of White’s ridge (NE Limb NSVG), the volcanic rocks throughout the stratigraphy show no appreciable change in dip, implying that subsidence during volcanism was uniform, and that tilting towards the rift axis occurred after volcanism. (Green, 1972). The 250 foot (76 meter) thick Cut Face Creek sandstone, exposed at Terrace Point (Fig. 1), is dominantly a reddish-brown, medium-grained, well-sorted, lithic arkose, but nearly 30% of the section is composed of thinly bedded, graded layers of fine-grained sand, silt, and clay, with lithologic and 8 �sedimentary features indicative of deposition into a fluvial-lacustrine environment, such as streams flowing into ponded water (Jirsa, 1984). The lacustrine aspect is speculated to be caused by blockage of flowing water against an obstruction, perhaps related to deformation and magmatism along White’s ridge. The recognition of several thick interflow sandstones indicates that clastic sedimentation in the NE limb of the NSVG played a more significant role than in the SW limb, but whether this is a result of a higher rate of clastic input, more efficient trapping of sediment against a topographic impediment such as White’s ridge, a higher rate of subsidence, or longer periods of volcanic quiescence is not clear. The inferred fluvial-lacustrine depositional environment of the Cut Face Creek sandstone implies that ponding and sediment-trapping processes were more operative than in the SW limb, where dominantly fluvial sedimentary features indicate sediment flow-through, and deposition closer to the axis of the rift. From a practical standpoint, these sandstones may be important aquifers, however, little is presently known about their water-yielding characteristics. Boerboom, T. J., 1994, Archean crustal xenoliths in a Keweenawan hypabyssal sill, northeastern Minnesota. White was right!: Institute on Lake Superior Geology 40th Annual Meeting, Houghton, MI: Proceedings v. 40, pt. 1 Abstracts, p. 5-6. Green, J.C., 1972, North Shore Volcanic Group, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A Centennial Volume, p. 294-332 Jirsa, M.A., 1984, Interflow sedimentary rocks in the Keweenawan North Shore Volcanic Group, northeastern Minnesota: Minnesota Geological Survey Report of Investigations 30, 20 p. White, W.S., 1966, Tectonics of the Keweenawan basin, western Lake Superior region: U.S. Geological Survey Professional Paper 524-E, 23 p. 9 �BEDROCK GEOLOGIC MAP OF THE LITTLE MARAIS, SCHROEDER, AND TOFTE 7.5MINUTE QUADRANGLES, NORTH SHORE OF LAKE SUPERIOR, MINNESOTA BOERBOOM, Terrence J., Minnesota Geological Survey, boerb001@umn.edu GREEN, John C., University of Minnesota-Duluth, jgreen@d.umn.edu ALBERS, Paul, Duluth Metals, palbers@duluthmetals.com MILLER, James D., Jr., Minnesota Geological Survey, mille066@umn.edu The Minnesota Geological Survey is continuing to map the bedrock geology of 7.5’ quadrangles near Lake Superior as part of the USGS STATEMAP program, resulting to date in ten published 1:24,000 scale maps from Duluth to Tofte, in addition to 10 quadrangles already published under the former USGS COGEOMAP program. The Lutsen quadrangle will be published in July 2007; the Deer Yard Lake and Good Harbor Bay quadrangles will be mapped in the 2007 and published in 2008 (Fig. 1A). All the maps in this series are available as printed maps, or as PDF and ArcView export files at the MGS website (http://www.geo.umn.edu/mgs/). Field trip 5 of this meeting will begin in the Little Marais quadrangle at the southwest, and end in the Good Harbor Bay quadrangle at the northeast. With the exception of a strip near the shore of Lake Superior (Green, 1992), parts of the Little Marais quadrangle that were mapped but not published, and thesis work by Albers (UMD M.S. 2007), much of the area covered by these maps had never been mapped. The combined area of these three maps straddles the transition across the uppermost Southwest to the uppermost Northeast sequences of the North Shore Volcanic Group (NSVG), as well as the SchroederLutsen sequence, which unconformably overlies the Southwest and Northeast sequences (Fig. 1B). Together, these maps cover the stratigraphically highest portion of the NSVG in Minnesota. In addition, components of the Beaver Bay Complex, including multiple phases of the Beaver River diabase, the Blesner Lake diorite, and the Leveaux ferrodiorite are present in the area of these maps. The new mapping has delineated a series of dominantly felsic to intermediate composition volcanic rocks (rhyolite, icelandite, and andesite) beneath the Schroeder-Lutsen basalts, informally termed the Onion River lavas, which include some tholeiitic basalt flows. The strike of these flows at a high angle to that of the overlying Schroeder-Lutsen basalts, which are nearly parallel to the Lake Superior shoreline (Fig 1B). The basal contact of the Schroeder-Lutsen sequence is repeated by several shore-parallel reverse faults. Poorly exposed units of rhyolite, volcanic breccia, and conglomeratic to sandy sedimentary rocks, which are likely components of older volcanic and interflow sedimentary rocks such as the Onion River lavas, are exposed within the Schroeder-Lutsen sequence by normal and reverse faults. The Carlton quarry lavas (Fig. 1B) form a northwest-dipping group of lavas composed of rhyolite at the base, andesite, and ophitic basalt at the top. The position of these lavas relative to the surrounding lavas is poorly constrained due to lack of outcrop, but they are assumed to be in fault contact, given the anomalous northwest dip. The rhyolite here has been dated at 1094.3±2.0, the youngest age obtained to date from the NSVG in Minnesota, but the surrounding volcanic rocks have not been dated. Intrusive rocks of the Beaver Bay Complex are most abundant and diverse in the Little Marais quadrangle, where they are composed of multiple phases of the Beaver River diabase and the slightly older, multi-phase Blesner Lake diorite. Elsewhere, the Beaver River diabase is restricted to erosional remnants of a larger, generally southeast-dipping sheet that commonly contains large anorthosite inclusions, such as at Carlton Peak. The porphyritic Leveaux ferrodiorite occurs mainly in the Tofte quadrangle as a subvolcanic sill emplaced into volcanic rocks, and as inclusions in the Beaver River diabase. 10 �REFERENCES Green, J.C., 1992, Geologic map of the North Shore of Lake Superior, Lake and Cook Counties, Minnesota: Part 1. Little Marais to Tofte: Minnesota Geological Survey Miscellaneous Map M-71, scale 1:24,000. Figure 1. A. Index map showing the location of mapped 7.5’ quadrangles along the North Shore of Lake Superior. M numbers refer to MGS Miscellaneous maps. Visit the MGS website for references to individual maps. Lutsen will be published in 2007; Deer Yard Lake and Good Harbor Bay in 2008. B. Index map showing the locations of the major units mentioned in the abstract. 11 �PETROGRAPHIC DIFFERENTIATION OF THE FIVE PHASES OF THE LOWER CRETACEOUS STAR KIMBERLITE, SASKATCHEWAN, CANADA BOISJOLI, Troy and FLOOD, Timothy P., Department of Geology, Saint Norbert College, DePere, WI 54115; troy.boisjoli@snc.edu Geologic Setting The Star Kimberlite in the Fort a la Corne region, east-central Saskatchewan, is a large lithologically complex structure that preserves both intra-crater and extra-crater kimberlite (Zonneveld et al. 2004). The Star Kimberlite occurs on the eastern rim of the North American Platform. The oldest rocks in the area are believed to be part of the Archean Sask craton, which are overlain by other Precambrian rocks similar to those exposed to the north in the Glennie Domain. These Precambrian rocks are successively overlain by 300-500m of mixed siliciclasticcarbonate sediments that range in age from Cambrian to Devonian; these sediments are overlain by approximately 150m of Lower Cretaceous sediments; which are overlain by 85 to 115m of glacial deposits. The Star Kimberlite itself is intermittent and interstratified with Lower Cretaceous rocks that include the Cantuar Formation inferred as coastal plain facies, the Pense Formation inferred as shoreface facies, and the Joli Fou and Viking Formations inferred to be proximal offshore to shoreface facies. Geology of the Study Area Five different phases of the Star Kimberlite have been recognized (Zonneveld et al. 2004). In stratigraphic order from oldest to youngest they are; Cantuar Kimberlite (CK), Pense Kimberlite (PK), Early Joli Fou Kimberlite (EJFK), Middle Joli Fou Kimberlite (MJFK) and Late Joli Fou Kimberlite (LJFK). The purpose of this study was to determine if petrography and geochemistry could be used as tools to differentiate these different phases of the Star Kimberlite. The CK is the oldest kimberlite in the Star body. In drill core, kimberlite intersections in this unit range in thicknesses from 0.3 to 11.8m and are bedded at the 0.2 to 9.0m scale. The beds within the kimberlites have sharp contacts and are usually massive or fining upwards. The CK is interpreted as medial to distal primary pyroclastic airfall material, with some reworked sedimentary equivalents. The location of the feeder vent for this event has not been determined. The PK overlies CK and drill core intersections range in thickness from approximately 2 to 20m. The individual beds are highly variable, with massive, normal graded and reverse graded beds. Sharp contacts are typical and in some beds the upper 0.1 to 1m show evidence of marine reworking. PK is interpreted as proximal to distal, primary volcaniclastic material. The EJFK overlies the PK and is volumetrically the most important phase of the Star Kimberlite. The EJFK deposits are 1 to 25 m thick, with bedding ranging from 1 to 10 m. Individual beds have sharp to diffuse contacts and are typically massive or normally bedded. Reverse bedding is rare. The EJFK is interpreted as primary subaerial and distal marine fall material. The MJFK overlies the EJFK and until recently was lumped with the upper portion of the EJFK (Kjarsgaard 2007). 12 �The LJFK overlies the MJFK The thickness of this unit is unknown but bedding within the unit occurs on a scale of 0.5 to 30m, with beds of 10 to 20m the most common. The LJFK is interpreted primarily as pyroclastic subaerial fall material to pyroclastic flow material (Zonneveld et al. 2004). Analysis Petrographic analysis was performed on 23 thins sections using an average of 600 points per thin section. For all units, altered olivine comprised from 40-60% of the sections. Juvenile clasts comprised from 5-30% of the sections and opaques ranged from 1-4%. Late alteration calcite and serpentine accounted for between 0.5-6% of the sections. Quantitative petrographic analysis was not useful for distinguishing the different phases of the Star Kimberlite. An alternate petrographic method based on textural and mineralogical criteria of the juvenile clasts (Webb 2006) was performed. In this analysis, ten criteria are the basis for distinguishing various mechanisms of emplacement for kimberlites. These criteria include; sphericity, roundness, irregularity, internal structure, vesicularity, clast-host relationship, primary phenocrysts, groundmass crystallinity, mineralogy and size range/modal average. A preliminary compilation of these criteria suggest that some units are distinguishable from other units relative to emplacement mechanisms. For example, vesicularity in the CK is high relative to other phases and consistent with the original interpretation as distal primary pyroclastic airfall. Geochemical analysis was preformed on the primary olivine clasts using an SEM-EDS. The initial results of this analysis were inconclusive due the highly altered nature of the grains but work is on going. References: Kjarsgaard, B.A., 2007, personal communication. Webb, K.J., 2006, Juvenile clasts in kimberlites: Standardized comprehensive description towards unraveling emplacement mechanisms: in Kimberlite Emplacement Workshop, DeBeers Canada Inc, p. 1-5. Zonneveld, J.P., Kjarsgaard, B.A., Harvey. S.E., Heaman, L.M., McNeil. D.H., and Marcia. K.Y., 2004, Sedimentologic and stratigraphic constraints on emplacement of the Star Kimberlite, east-central Saskatchewan: Lithose 76 115-138. 1: Juvenile clast with primary olivine and vesicles infilled with calcite. Field of view 10mm. 13 �DEPOSITION OF NATIVE COPPER LODES ON THE KEWEENAW PENINSULA, NORTHERN MICHIGAN, FROM A GRAVITY-DRIVEN EVOLVED METEORIC BRINE BROWN, Alex C., Dept. of Civil, Geol. and Min. Eng., École Polytechnique de Montréal P.O. Box 6079, Sta. Centre-Ville, Montreal, QC, Canada H3C 3A7, acbrown@polymtl.ca The Keweenaw Peninsula of northern Michigan is the site of famous native copper lodes hosted by flow-top breccias and amygdaloidal portions of flood basalts and interbedded conglomerates of the Portage Lake Volcanics (PLV). This mineralization is generally attributed to an up-dip flow of a cupriferous metamorphogenic fluid generated during deep burial metamorphism of the host strata in axial portions of the Midcontinent Rift (Stoiber & Davidson 1959; White 1968; Jolly 1974; Livnat et al. 1983). In an alternative model proposed here, copper is leached from immature rift sediments and porous portions of the PLV by deeply circulating, gravity-driven meteoric water. The meteoric water is initially cool, fresh, slightly acid and oxygen-rich. At depth, it becomes warmer, highly saline by the assimilation of evaporites or evaporitic brine, near-neutral in pH by equilibration with silicates and carbonates, and oxygen-deficient by reddening of its aquifers (hematitization of ferrous iron in abundant labile mafic mineral constituents of the rift sediments and mafic constituents of the PLV basalts). At first, the Eh of the brine evolves progressively toward moderately oxidizing conditions at which it becomes capable of leaching and transporting trace amounts of copper from its aquifers. Metamorphogenic water and copper may have been incorporated during circulation at deep burial levels. Highland recharge eventually drives the hybrid cupriferous brine up-dip along PLV aquifers. With continued hematitization of its aquifers, the brine becomes highly reducing by oxygen depletion, and native copper is deposited (in the absence of sulfide). Coincidentally, the same highly reducing conditions favor the solution of hematite, a feature readily visible as locally bleached rock (hematitic pigment removed) where native copper mineralization occurs on the Keweenaw Peninsula. This conceptual model provides 1) an explanation for the highly saline brine needed to mobilize copper, and 2) a means to drive dense brines up-dip from axial portions of the rift basin. It also explains the pervasive hematitic reddening of the PLV, and ties the timing of copper leaching to that diagenetic reddening event. The overall timing of native copper mineralization may be linked to the Grenvillian compressional event which tilted PLV strata to deep levels along the rift axis and probably raised highlands around the rift basin. References: Brown, A.C. (2006) Genesis of native copper lodes in the Keweenaw district, northern Michigan: a hybrid evolved meteoric and metamorphogenic model: Economic Geology, v. 101, p. 1437-1444. Jolly, W.T. (1974) Behavior of Cu, Zn and Ni during prehnite-pumpellyite rank metamorphism of the Keweenawan basalts, northern Michigan: Economic Geology, v. 69, p. 1118-1125. Livnat, A., Kelly, W.C., Essene, E.J., and Rye, R.O. (1983) P-T-X conditions of sub-greenschist burial metamorphism and copper mineralization, Keweenaw Peninsula, northern Michigan: Geological Society of America Abstracts, v. 15, p. 629. Stoiber, R.E. and Davidson, E.S. (1959) Amygdule mineral zoning in the Portage Lake Lava Series, Michigan copper district; Part I, Part II: Economic Geology, v. 54, p. 1250-1460, 1444-1460. White, W.S. (1968) The native-copper deposits of northern Michigan, in Ridge, J.D., ed., Ore Deposits of the United States, 1933-1967: Amer. Inst. of Mining, Metallurgical and Petroleum Eng., GratonSales Volume, p. 303-326. 14 �p. 2 Fig. 1 Schematic illustration of the geologic setting of native copper lodes (from Brown 2006) NW tion a r d Hy / ll y it e al t Ba s l Cg 0 Cu S an H2 O H o 2+ Cu Conditions favorable for significant copper solubilities in redbeds Cu S Cu 2O an H2 O H o Cu H 2 Meteoric Water 2CuCl 3 + CuCl 2 CuO Ground Water o Cu Conditions favorable for significant copper solubilities in redbeds CuO Cu 2S dj O2 O Cu 2O Ground Water 2 Cu 2S 5 6 ppm O H2 2 O Cu 2S -0.4 25 C Cl= 0.5 M Contours of copper solubility 64 ppm Cu4 (OH)6Cl 2 -4 10 (6 ppm) -3 10 mg/l (64 ppm) Cl=0.5 M Meteoric Water Water dj ult o s 2 3 oC 25 25 -4 C a =10 mg/l 2CuCl 3 + CuCl 2 Eh (V) lt sa Ba Cu4 (OH)6Cl 2 o 25 C +0.4 t B) System Cu-O-H-Cl A) System Cu-O-H-S-Cl 2+ Cu l sa Ba lt sa Ba n w Fa ratio d y h De een a pe Pum ns it ion Tr a e t o id Ke w Fig. 2 Eh-pH stability diagrams (below) for A) the transport of copper, and B) the deposition of native copper (from Brown 2006) +0.8 In In Flow-Top Breccias Conglomerates and Amygdaloids Pr esent-day Er osion Sur face Ep Contours of copper solubility SE Native Copper Lodes Cu 2S 7 pH 9 7 5 3 11 pH 9 11 I ON IPITAT PREC Fig. 3 Schematic illustration of the deep flow of meteoric and metamorphogenic waters, to form native copper lodes (from Brown 2006) Hi g h la nd s Evapo rites Meteoric water o +Cl - Cu 2- - CuCl 3 + CuCl2 Progressive reddening of aquifers and leaching of copper Assimilation of metamorphogenic water and copper Mantle heat 15 Deposition of native copper �A NOVEL LOCALITY FOR PSEUDOBROOKITE - THE NINE MILE PLUTON, MARATHON COUNTY, WISCONSIN T.W. Buchholz1, A.U. Falster2, Wm. B. Simmons2. 11140 12th St. N., Wisconsin Rapids, WI 54494; 2Department of Earth and Environmental Sciences, University of New Orleans, New Orleans, LA 70148. Pseudobrookite, (Fe3+, Fe2+)2(Ti,Fe3+)O5, is a typical pneumatolytic mineral found in lithophysae in extrusive Ti-rich rocks such as andesite and rhyolites (Anthony et al., 1997). Recent work in the Ladick Trucking & Excavating weathered granite quarry has exposed a novel occurrence of pseudobrookite within the Proterozoic Nine Mile Pluton, the youngest and most silicic of four plutons comprising the Wausau Complex. The Ladick quarry is located approximately ¼ mile east of STH 107 in the southwest portion of the Nine Mile Pluton. Black to dark grey elongated crystals of pseudobrookite up to approximately 1.5 mm in length and clustered in radiating sheaves and as single crystals were found in an alteration assemblage associated with a thin quartz-pyritefluorite vein. The material was recovered from a pile of shot rock derived from blasting large boulders removed from the grus in the northern portion of the operation. The vein was emplaced in a fracture cutting the granite, and it appears fluids circulating along the fracture preferentially removed quartz and perhaps other minerals from adjacent granite to a distance of up to about 12 cm from the fracture. Subsequently K-feldspar, biotite, minor quartz and a series of associated minerals described below were deposited. EMP analysis of clean grains of pseudobrookite shows no significant components other than Ti and Fe and minor amounts of Mn and Nb, and XRD analysis yielded excellent structural agreement. Morphology is that of ideal, short-prismatic pseudobrookite. Associated minerals other than K-feldspar, biotite and quartz include ilmenite, rutile, anatase, sphalerite, chalcopyrite, pyrite, arsenopyrite or marcasite (now represented by goethite pseudomorphs), molybdenite, cassiterite, fluorapatite, fluorite, monazite, zircon and a LREE-carbonate mineral, probably bastnaesite-(Ce). Secondary minerals include small but attractive sprays of gypsum crystals; the Ca was probably derived from weathering fluorite, and the S from weathering sulfides. Interestingly, the cassiterite crystals often contain inclusions of fluorapatite and monazite; all monazite identified so far has been as inclusions in cassiterite. The monazite and fluorapatite inclusions may represent an earlier, otherwise transient paragenesis that was preserved by inclusion in cassiterite; the LREE released by breakdown of early-formed monazite may have contributed to the later formation of the LREE-carbonate mineral. Although pseudobrookite typically is found in lithophysae in certain eruptive rocks and this would seem to be a very different environment, the Nine Mile pseudobrookite probably formed in an environment that was not inconsistent with its usual environment of formation. As indicated by the association with cassiterite, molybdenite (this is the first report of molybdenite for the Nine Mile granite), fluorite and other Ti-bearing oxides the conditions were probably high-temperature, volatile- and Ti-rich. Conditions were likely close to a pneumatolytic environment – consistent in these respects with typical 16 �pseudobrookite occurrences. The principal variant may have been ambient pressure, although it is believed that the Nine Mile granite was intruded at a shallow, albeit as yet undefined level in the crust. Hence the environment of formation probably had at least some features in common with the usual volatile-rich, high temperature andesites and rhyolites that are the normal hosts for pseudobrookite. Reference cited: Anthony, J.W., Bideaux, R.A., Bladh, K.W. and Nichols, Monte C., 1997, Handbook of Mineralogy, V. III, p. 453. 17 �Deposition and Cementation of Paleoproterozoic Gunflint Formation Carbonate: Implications for Early Hydrosphere Chemistry Justin Burton and Philip Fralick, Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada, philip.fralick@lakeheadu.ca The Gunflint Formation extends 190 km towards the northeast from an intrusive contact with the Duluth Complex. Surface exposures form a narrow band of outcrops with basal siliciclastics and underlying Neoarchean rocks to the west and overlying black shales of the conformable Rove Formation to the east. Erosional remnants 98 km northeast of Thunder Bay at Schreiber, Ontario, suggest a pre-erosional continuation of the chemical sedimentary rocks to the east. Rock comprising the Gunflint Formation primarily consists of an assemblage of chemical sediments including chert, iron oxides and carbonates. Volcanogenic shales and similar debris within the chemical sediments is also present. In 1956 the Formation was divided by Goodwin into six major sedimentary facies representing two depositional cycles and four members: 1) Basal Conglomerate, 2) Lower Gunflint, 3) Upper Gunflint, and 4) Upper Limestone. These units were deposited on a wave and tide influenced broad shelf deepening to the south (Fralick and Barrett 1995, Pufahl and Fralick 2004). Storm events eroded the fine-grained chemical sediments abrading them into sand-sized fragments that accumulated as cross-stratified layers and lenses (Fralick 1988). In this study we examine the limestone unit at the top of the Gunflint Formation and compare its petrology and geochemistry to ferronian dolomitic (ankeritic) grainstones common in the shoreproximal exposures near Thunder Bay. The stratigraphy of the limestones is consistent in the outcrops studied, even though some are tens of kilometers apart. The unit is conformable with underlying silicified ankeritic grainstones. The boundary between the two units forms a replacement front in places, with microscopic examination clearly revealing replacement of calcite by ankerite. The limestone consists of a very coarse sand to granule grainstone layer overlain by cabbage sized and shaped stromatolites. Layers of both fine-grained and very coarsegrained calcareous sand are banked up against and overlie the stromatolites. This assemblage is less than one meter thick. It is sharply overlain by a pebble to boulder conglomerate with some tabular clasts up to four meters long. Fragments of the underlying ankerite and limestone layers form most of the clasts. Ankerites from lower in the succession are composed of neomorphic spar with none of the original texture preserved. Geochemically they have high concentrations of Ca, Fe, Mg and Mn with relatively low abundances of most other elements. Their REE curves are LREE enriched with positive La and, to a lesser extent, Ce anomalies. Eu is flat to positively enriched. The limestones have clasts composed of Fe-rich chlorite, consistently with V contents of 1-2 percent. They are surrounded by blocky calcite cement that in places overgrows the grains. Samples from higher in the grainstone assemblage have more pronounced dissolution pathways along calcite crystal boundaries. These dissolution channels are filled with either calcite or quartz. The stromatolites have alternating layers of Fe-rich chlorite and blocky calcite cement similar in composition to the adjacent grainstones. The grains in some grainstones are pieces of stromatolite. The limestones are very geochemically different from the ankerites. The limestones have higher values of most elements, but especially U (5 to 10 times more abundant), REE’s (10 to 100 times more abundant) and V (100 times more abundant). Their REE curves are LREE enriched and show distinct Ce and Eu depletion. 18 �New U-Pb geochronological data (Don Davis, Pers. Comm) indicates that the pebble to boulder conglomerate is the tsunami deposit that resulted from the Sudbury impact. This reinforces previous interpretations that a significant nondepositonal interval with subaerial exposure existed between deposition of the Gunflint and Rove Formations in the study area (Addison et al. 2005). The blocky calcite cements and Fe chlorites would have formed in the phreatic zone during this interval. The extreme V-U enrichments indicate oxidized fluids leaching V and U from the subaerial environment and precipitating these redox sensitive elements upon encountering the organic-rich sediments. The negative Ce anomalies of the REE curves for the calcite cements agree with this interpretation. The ankeritic sediments deposited in the marine environment have REE abundances typical of precipitation from oxygen deficient water with abundant dissolved Fe and Mn. This data points to an imbalance in oxygen levels at this time with a relatively oxic atmosphere and anoxic oceans. This agrees with findings from other geochemical investigations of the Gunflint and Rove Formations (Poulton et al. 2004). Addison, W. et al., 2005. Geology, vol. 33, p. 193-196. Fralick,P.F. 1988. Memoir 13, Can. Soc. of Petroleum Geologists, p. 24-29. Fralick, P.W. and Barrett, T.J., 1995. Spec. Publ.22, Inter. Ass. of Sedimentologists, p. 137-156. Poulton, S.W., Fralick, P.W. and Canfield, D.E., 2005. Nature, vol. 431, p. 173-177. Pufahl, P.K. and Fralick, P.W., 2004. Sedimentology, vol. 54, p. 791-808. Figure 1. Archean mudstone normalized REE curves for ankeritic samples (lower 5 curves) and limestone samples (upper 3 curves). 19 �THE SUDBURY IMPACT LAYER IN THE LAKE SUPERIOR IRON RANGES: A TIME-LINE FROM THE HEAVENS Cannon, William F., U.S. Geological Survey, MS 954, Reston VA 20192 Addison, William D., R.R. 2, Kakabeka Falls, Ontario, POT 1W0, Canada A large meteorite impact near Sudbury, Ontario at 1,850 Ma produced a geologically instantaneous regional, and probably global, catastrophe leaving a unique imprint within the classic iron ranges of the Lake Superior region 500 to 900 kilometers from the impact site. We have now identified the impact layer in or near five of the iron ranges. The layer is a unique and ultra-precise time line, having formed over a time span of days or less across the region, and allows an unequivocal temporal correlation of the stratigraphic units with which it is interbedded. The impact layer consists of various rock types and may have formed by processes including direct deposition of airborne ejecta either from a high energy ejecta curtain traveling at ballistic velocities, or from a turbulent cloud of finer ejecta. More commonly the layer consists of polymict marine breccias of reworked ejecta and local sediments possibly formed by 1) “ballistic erosion” and mixing of sediment caused by high energy ejecta deposition, 2) erosion and redeposition caused by mega-tsunamis generated by the impact, 3) resurge of ocean water toward the “hole in the bottom of the sea”, or possibly 4) submarine slumping triggered by impact generated earthquakes. Regardless of diverse lithology and depositional processes, occurrences of the impact layer are linked by the presence of quartz grains exhibiting relict shock metamorphic features, which form only under the extreme pressures produced by hypervelocity impacts. 20 �The following are brief summaries of the characteristics each site: Gunflint- At top of Gunflint Iron-formation; nearly pure ejecta deposits of devitrified vesicular glass, microtectites, and spherules; accretionary lapilli and shocked quartz; apparently not reworked by tsunamis. Thunder Bay- Tsunami debrisite at top of Gunflint Iron-formation containing reworked, locally derived breccia fragments plus ejecta similar to the Gunflint ejecta; shocked quartz known from one locality. Mesabi- Poorly known unit at top of Biwabik Iron-formation; contains devitrified spherules, microtectites, and vesicular glass; shock metamorphosed quartz. Puritan- Known from a few thin sections of drill core from 1960’s study. “Volcaniclastic breccia” in Tyler Formation about 100 m above Ironwood Ironformation; rare shock metamorphosed quartz. Marenisco- Possible tsunami deposit at base of Copps Formation lying on Archean granite. Shock metamorphic features not well documented at this time. Huron River- Breccia and fine “volcaniclastic rocks”; near base of Michigamme Formation a few meters above contact with Archean granite. Abundant shock metamorphosed quartz grains. Connors Creek- Breccia, greywacke, and accretionary lapilli beds; erosional contact on slightly ferruginous chert about 150 m above base of Michigamme Formation; shock metamorphosed quartz. McClure- Breccia and greywacke about 300 m above base of Michigamme Formation; erosional contact on carbonate iron-formation; shock metamorphosed quartz; grades up to black slate. Iron River- Breccia and greywacke forming lower part of Hiawatha Graywacke; shock metamorphosed quartz; known best from thin section collection from 1940’s; erosional contact on Riverton Iron-formation. The diagram below shows the precise temporal correlation of stratigraphic units in the five areas where we have identified the Sudbury impact layer. . 21 �AN EXPERIMENTAL WATER-ROCK INTERACTION STUDY INTO THE ORIGIN OF HIGH-SULFATE WATERS ASSOCIATED WITH THE STEEP ROCK IRON MINES, ATIKOKAN, ONTARIO. COCKERTON, S., and CONLY, A.G., Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1 andrew.conly@lakeheadu.ca LEE, P., Department of Biology, Lakehead University, Thunder Bay, ON, P7B 5E1 Continual flooding of the two primary open pits of the Steep Rock iron mines near Atikokan, Ontario, has lead to the formation of Hogarth and Caland pit lakes, which are located in the former middle and eastern arms of Steep Rock Lake, respectively. At present both lakes are ~200 m deep and display distinct differences in water chemistry [1]. Hogarth pit lake is characterized by non-stratified to weakly stratified and oxygenated water column (4-12 mg/L O2), near neutral pH (6.4-8.0), extraordinarily high SO42- concentrations (1200-2000 mg/L), lower alkalinity (50-125 mg/L), increased hardness (1200-1800 mg CaCO3/L) and is chronically toxic to aquatic fauna. On the other hand, Caland pit lake is well stratified with an upper oxygenated fresh water lens (~20 m in depth and 4-12 mg/L O2) that is not toxic, which overlies an anoxic (<1 mg/L O2) and moderately saline (200-500 mg/L SO42-) water column. The pH of Caland pit water is similar to that of Hogarth, but is more alkaline (100-200 mg/L) and has a lower hardness (400-1000 mgCaCO3/L). The concentration of metals in both lakes is negligible. With the pits being filled at a rate of ~3 m/year [2] and ground water presumed to be the largest input of water, the differences in water chemistry between the two pit lakes likely reflect differences in the nature of water-rock interactions between ground water and bedrock. Previous stable sulfur isotope studies have concluded that the dissolved sulfate of the pit waters is derived from oxidation and leaching of pyrite-bearing units of the Jolliffe ore zone by either ground water or due to pit water-wall rock interaction [1,3]. The geology of two pits is largely similar and includes [4,5]: 1) footwall rocks that consist of the Marmion Geniss Complex, Wagita Formation and Mosher Carbonate; 2) the Joliffe ore zone, which is comprised of the Manganiferous Paint, Goethite and Pyrite members; and, 3) hanging wall rocks that include the Dismal Ashrock Formation and the Witch Bay Formation. In order to better determine the nature of ground water-rock interactions a series of batchreaction column leaching experiments were performed. A series of four PVC columns (4 inches in diameter and 24 inches in length) were loaded with crushed (1-2 mm) pyrite and goethitehematite waste rock, representing the Goethite and Pyrite members. Two of the columns also contained 2 inches of crushed Mosher Carbonate at the base. Columns were filled with Atikokan ground water ensuring that the crush rock was completely submerged. In addition, another three columns were loaded with crushed goethite-hematite tailings (jit reject), goethite-hematite waste rock and pyritic waste rock, and filled with rain water in order to assess the contribution provided from surface water. Approximately 30 mL of water was extracted and filtered at 3 hr, 24 hr, 3 days, 5 days, 7 days and then weekly for a total of 14 weeks. Cations and metals were determined by ICP-AES, anions (SO42-, Cl-) were determined by ion chromatography, and pH was measured using an Accumet AB15 pH meter. Results from the column leaching experiments indicate that the high SO42-(aq) concentration of the pit lakes could only be produced from ground water oxidation of pyritic portions of the Joliffe ore zone (Fig. 1a). 22 �Figure 1. Normalized plots, to the average composition of Hogarth pit lake, showing the (A) relative concentration of pH, anions and cations from goethite-hematite and pyrite waste rock experiments, and (B) results of mixing between leachates derived from goethite-hematite and pyrite waste rock. To generate the near neutral pH of either pit lake requires subsequent interaction with Mosher Carbonate (Fig. 1a). Figure 1b shows the results of mixing the leachates derived from goethitehematite and pyrite waste rocks. Consequently, the pit waters are likely produced from the mixing of ground waters derived from both iron oxide and pyritic portions of the ore zone that were subsequently buffered by carbonate wall rock.. However, discrepancies between experimental waters and measured Hogarth pit lake water (e.g., Mg) likely reflect interaction of pit lake water with other wall rock lithologies and/or other ground water contributions. Experiments are currently underway to assess both the interaction of ground water with different lithologies and the effect of varying water-rock ratios. Initial results from these experiments will be presented at the meeting. Notwithstanding these discrepancies, the results from batchreaction column experiments supports stable isotope evidence [1,3] that the sulfate in the two lakes is derived from ground water leaching of pyritic portions of the Joliffe ore zone. In addition, the differences in water chemistry between Hogarth and Caland pit lakes primarily reflects differences in the relative pyrite content of the Joliffe ore zone, with the ore zone beneath Hogarth pit lake containing a higher abundance of pyrite. References: [1] Goold, A., Conly, A.G., and Lee, P., 2006. Integrated uses of water chemistry, stable isotope geochemistry and tie studies in identification of metal toxicity in pit lakes: A case study at the Steep Rock iron mine, Atikokan, Ontario: GAC-MAC Program with Abstracts, v. 31, p. 57-58. [2] McNaughton, K.A., 2001. The limnology of two proximal pit lakes after twenty years of intense flooding. M.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 85 p. [3] Conly, A.G. and MacDonald, J.C., 2005. An Investigation into Potential Sulphate Sources of the Hogarth Pit Lake, Steep Rock Iron Mine, Atikokan. ILSG 51st Annual Meeting, Proceedings Volume 51. [4] Joliffe, A.W. 1966. Stratigraphy of the Steeprock Group, Steep Rock Lake, Ontario. GAC Special Paper No. 3, p. 75-96. [5] Stone, D., Kamineni, D.C., and Jackson, M.C., 1992. Precambrian geology of the Atikokan area, northwestern Ontario. Geological Survey of Canada, v. 405, 106 p. 23 �ORIGIN OF A MAFIC PEGMATITE WITHIN THE DULUTH COMPLEX, NORTHERN MINNESOTA COSTELLO, Daniel E.1, FLOOD, Timothy P.1, and THOLE, Jeffrey T.2 Department of Geology, Saint Norbert College, De Pere, WI 54115, dan.costello@snc.edu, 2 Department of Geology, Macalester College, St Paul, MN 55105 1 Geologic Setting The Duluth Complex of Northern Minnesota is a layered igneous intrusion that formed due to rifting of continental North America approximately 1.1 billion years ago. The Complex is composed of numerous layered plutonic bodies, which formed through multiple intrusions of mafic to felsic tholeiitic magmas. These layered series are typically thick and exhibit foliation and layering that dips to the southeast, towards the axis of the Keweenawan Rift (Miller et. al. 2002). This study focuses on units within the South Kawishiwi intrusion (SKI). Near the base of this intrusion, two similar troctolitic units are separated by a pegmatitic unit. The petrogenetic relationship between the overlying and underlying troctolitic units is uncertain, as is the origin of the pegmatite. This project aims to determine the origin of and relationship between these three units using petrographic and geochemical data. Igneous Stratigraphy The pegmatitic unit, hereafter referred to as the PEG, was first noted by Foose and Weiblen (1984). This unit is used as a marker bed to differentiate the surrounding troctolites: The rock below the PEG is classified as the ultramafic three unit (U3), and the rock above the PEG is classified as part of the ultramafic two unit (U2). In drillholes where there is no PEG present, it is impossible to differentiate between these two troctolitic units. The U3 is the lowermost ultramafic unit within the SKI. The thickness of this unit averages approximately 100 feet, and ranges from 3 to 410 feet. (Severson 1994). The U3 is characterized by massive oxide horizons as well as mineralization of sulfides and platinum group elements. The PEG overlies the U3 unit and has an average thickness of approximately 95 feet. The pegmatite is used as a marker boundary between a sulfide-rich zone below and a sulfidepoor zone above (Foose and Weiblen 1986). The pegmatite is not always present in drill core. The U2 overlies the PEG and is petrologically similar to the U3 unit, with the exception that the U2 lacks massive oxides and sulfide-rich horizons (Severson 1994). This unit averages about 90 feet in thickness. Analysis Petrographic study revealed visible variations among the three units. The U2 and U3 appear to be similar; however, multiple differences occur within the pegmatite. The presence of minerals such as amphibole and biotite in the PEG suggests that this unit formed at a later stage of crystallization and/or within higher P(H20) conditions than its surrounding units. The PEG also contains significant amounts of a plagioclase symplectite texture (Figure 1). This texture may indicate the presence of a late stage magmatic fluid which would promote the growth of a pegmatitic unit. This symplectitic texture is rare below the pegmatitic unit, but is present in small amounts in all units above the PEG (Severson 1994). 24 �Geochemical studies focused on the changing concentrations of elements, notably the different behaviors of compatible and incompatible trace elements. From the U3 to the U2, not including the PEG, the concentrations of both Ni and Co decrease, suggesting olivine fractionation. In a similar fashion, the concentration of Ba increases from the U3 to the U2, suggesting plagioclase fractionation. This is consistent with a model of Ba as an incompatible element during early stages of crystallization. Preliminary mineral chemistry was obtained on plagioclase and olivine grains from all units. The two troctolitic units contain olivine of Fo63-65 and plagioclase of composition An75-77. In comparison, the pegmatite contains olivine with Fo47-60 and plagioclase with An59-63. These results are consistent with a model of fractional crystallization between the U3 and the U2 and with elevated P(H20) conditions within the pegmatite. Using the preliminary results outlined above, we infer that the three units are syngenetic and the U2 and U3 units formed through fractional crystallization. The pegmatite formed as a water rich phase concentrated within the magma chamber, with some water being derived from the footwall. As normal fractionation occurred from the U3 to the U2, a concentration of water resulted in the formation of a pegmatitic horizon between these two units. References: Foose, M.P. and Weiblen, P.W., 1986, The physical and chemical setting and textural and compositional characteristics of sulfide ores from the South Kawishiwi Intrusion, Duluth Complex, Minnesota, USA: in 27th Int. Geol. Congress (Moscow), Special Copper Symposium: Springer-Verlag, New York, p. 8-24. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, 207 p. Severson, M.J., 1994, Igneous stratigraphy of the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/TR 93/34, 210 p. Figure 1: Plagioclase symplectite texture found in the pegmatitic unit. Field of view 1.5mm 25 �Distribution of Certain Ichthyofauna in relation to Eastern Outlets of Lake Agassiz, with emphasis on the Gunflint-Arrow Lakes Corridor and the Keating Complex J. Frederick Dean and Brian A. M. Phillips, Lakehead University, Thunder Bay, ON Canada. Based on a retrodeformed elevation model (Phillips, 1997) of the Upper Herman stage of Lake Agassiz, Phillips and Hill (2004) proposed that the lake discharged via the Gunflint-Arrow Lakes Corridor into Lake Superior down the Arrow valley. Fisheries work in Minnesota and Ontario reveals a distribution of fish species that corroborates a flow of Agassiz water into the Arrow, Whitefish and lower Kaministiquia watersheds. There are two sub-species of the temperate Johnny Darter (Etheostoma nigrum eulepis and Etheostoma nigrum nigrum), a small benthic fish unable to migrate up rapids or falls. The former, the earlier of the two, is found along the trend of the Herman shoreline and into the Gunflint-Arrow Lakes Corridor (Figure 1). The later sub-species relates to the Campbell shoreline, appearing in the Matawin and Shebandowan rivers. Intergrades developed where the two sub-species mixed (Underhill, 1963; Schmidt, 1993; Momot and Stephenson, 2007). An Arctic marine glacial relict, Deepwater Sculpin also inhabits Gunflint-Arrow Corridor lakes. Dean (2006, pers. comm.) found Pontoporeia affinis (Lindstrom), another Arctic marine relict amphipod, in bottom waters of Arrow Lake. Both migrated along the northern, icebound margin of Lake Agassiz, as flow was funneled eastwards through the corridor. The topographically rugged west-east trend of the borderlands, the “Keating Complex”, likely caused ice margin retreat to be intricate in detail (Figure 2). During and after the Steep Rock phase of Rainy Lobe retreat (circa 11,000 B.P.), Dean suggests successive spillways directed Agassiz waters towards Lake Superior. While Superior Lobe ice blocked the ArrowPigeon valley, water ran through the Stump and Swamp valleys and along the northern edge of coastal ice, first exiting via the Brule and Kimball valleys. Underscored is the potential for people to have migrated eastwards into the corridor at this time. Further ice retreat later deflected flow down the Whitefish-Kaministiquia valleys, before the Matawin exit opened. Momot, W.T. and Stephenson S.A., 2007, Atlas of the Distribution of Fishes within the Canadian Tributaries of Western Lake Superior: Lakehead University, Thunder Bay, 383p. Phillips, B.A.M., 1997, Retro-deformation of the D.E.M. of Northern Minnesota as a Tool for Reconstruction of Former Glacial Lakes, in Proceedings of the 9th Minnesota GIS/LIS Consortium Conference, St. Cloud, Mn. Phillips, B.A.M. and Hill C.L., 2004, Deglaciation History and Geomorphological Character of the Region Between the Agassiz and Superior Basins, Associated with the ‘Interlakes Composite' of Minnesota and Ontario, in L.J. Jackson and A. Hinshelwood, Eds., The Late Palaeo-Indian Great Lakes - Geological and Archaeological Investigations of Late Pleistocene and early Holocene Environments, Mercury Series, Archaeology Paper 165, Canadian Museum of Civilization, Chapter 10, p. 275-301. Schmidt, K.P., 1993, Minnesota Parks Fish Species Lists: http://www.nativefish.org, 67p. Underhill, J.L., 1963, Distribution in Minnesota of the Sub-species of Percid Fish Etheostoma nigrum, and their Intergrades: American Midland Naturalist, v.70, p. 470-478. 26 2 �Figure 1 – Distribution of Johnny Darters and Flow from Lake Agassiz to Lake Superior. Figure 2 – Flow from Lake Agassiz to Lake Superior and the Keating Complex, MN-ON. 27 3 �The effect of H2O on olivine to ringwoodite transformation: Implications for subduction zone geodynamics and the deep earth water cycle Tamara Diedrich * † Thomas G. Sharp School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287 As the most abundant minerals in the earth’s upper mantle and transition zone, phase transformations between olivine and its high-pressure polymorphs, wadsleyite and ringwoodite, have major consequences for mantle geodynamics. Based on the thermal profile of subducting lithospheric slabs, it has been proposed that metastable olivine could persist into the mantle transition zone. Its presence would decrease negative buoyancy of the slab and slow subduction. In addition, the rapid transformation of metastable olivine has been identified as a potential initiating mechanism for deep-focus earthquakes. Several lines of evidence suggest that mantle olivine may contain significant H2O in the form of hydroxyl point defects. While previous studies have shown that H2O enhances olivine to wadsleyite transformation rates, under sufficiently cold conditions, olivine would transform directly to ringwoodite. Our research measures the effect of H2O on olivine to ringwoodite transformation rates and shows that even modest hydration of olivine results in near equilibrium transformation of olivine under subduction zone conditions and no metastable wedge of olivine below 400 km. For this study, a novel experimental approach was developed that consisted of two stages: hydration of mantle derived olivine in a piston-cylinder apparatus, and partial transformation of * Present address: Economic Geology Group, Natural Resources Research Institute, University of Minnesota, 5013 Miller Trunk Hwy, Duluth, MN 55811 † Corresponding author e-mail: tdiedric@nrri.umn.edu 28 �these spheroids in a multi-anvil device. After hydration, the olivine contained 298 (±29) ppm by weight H2O. The multi-anvil experiments were conducted for varying run durations at 18 GPa and 700°C, 900°C, and 1100°C. The partially transformed samples were thin sectioned and the ringwoodite rim thicknesses were plotted as a function of run duration. Both growth rates and activation enthalpy for growth were determined. Water appears to promote olivine to ringwoodite transformation by enhancing growth rates, decreasing activation enthalpy for growth, and facilitating the dissipation of transformation stress. Growth rates for the hydrated olivine are almost two orders of magnitude faster than for anhydrous olivine at 1100 °C and about four orders faster at 700°C. At 700°C, growth is fast enough to be observed on an experimental timescale; this is significant because kinetic parameters from previous studies suggest that, at 500°C, anhydrous olivine does not transform on a geologic timescale. The addition of water also enables ringwoodite growth rates to remain constant throughout transformation. The large volume decrease associated with olivine to ringwoodite transformation, if not accompanied by sufficient plastic deformation, causes transformation stress. As transformation progresses, this stress results in the build-up of strain energy, which slows growth by counteracting the driving force for transformation. Hydrolytic weakening of the olivine and/or ringwoodite facilitates the dissipation of transformation stress so that growth rates can remain constant throughout transformation. Using our kinetic parameters for hydrated olivine transformation, the addition of as little as 289 ppm by weight H2O effectively eliminates metastable olivine from the transition zone. Alternatively, if the presence of metastable olivine is confirmed, it must contain significantly less than 298 ppm H2O, placing a constraint on models of the deep earth water cycle. 29 �GOLD MINERALIZATION IN THE VIRGINIA HORN GREENSTONE TERRAIN, ST. LOUIS COUNTY, MINNESOTA: A PROSPECT RE-VISITED Elsenheimer, D.1, Frey, B. 2, and Hudak, J. 2, 1Minnesota Department of Natural Resources, 500 Lafayette Rd., St. Paul, MN 55155, 2Minnesota Department of Natural Resources, 1525 Third Ave. E, Hibbing, MN, 55746, donald.elsenheimer@dnr.state.mn.us Petrologic, geochemical and fluid inclusion data from archived drill core have been combined with a digital compilation of historic exploration data and new maps to reassess the gold mineralization potential of the Virginia Horn Greenstone Terrain, a fifty square kilometer exposure of Wawa subprovince rocks located in St. Louis County, Minnesota. The Virginia Horn Greenstone Terrain consists of a folded and faulted metamorphosed assemblage of Archean volcanic and sedimentary strata, intruded by a quartzofeldspathic porphyry (QFP) and overlain by a conglomerate sequence that was interpreted by Jirsa and Boerboom (2003) as a Timiskiming-type package correlative with parts of the Knife Lake and Shebadowan Groups (Figure 1). Metamorphic grade varies from prehnite-pumpellyite to middle greenschist facies. Visible gold within QFP outcrops were first identified in the 1930’s and associated with quartz veining, carbonate-sericite alteration and major shear zones. Historic mineral leasing and gold exploration activity involved the completion of more than fifty diamond drill core borings, mostly within or near the highly-altered and sheared QFP (Figure 2). Assay results within portions of drill core were significant, but too inconsistent to be considered economic. There has been no drilling within the Virginia Horn since 1991, and all mineral leases held on state-owned or managed lands (which cover more than 50% of the terrain) have been allowed to expire. Completion of subsequent Shebandowan 25km Moss Lake geologic mapping and analog modeling, Ardeen combined with a resurgence of priceSHEBANDOWAN Thunder sensitive gold exploration activity GREENSTONE BELTBay prompted the Minnesota Department of Mud Creek Shear Zone Natural Resources (MnDNR) to reassess Prospect Knife Lake the potential for economic gold deposits Timiskaming sequences Ely within the Virginia Horn. A digital Plutonic rocks Virginia Horn compilation was created of historic Greenstone Greenstone Belt mineral lease records and subsequent Gold Deposit After Jirsa and Boerboom (2003) Gold Occurrence petrologic and geochemical analyses on drill core housed in the MnDNR’s drill core repository. This compilation includes the conversion of Manitoba hard-copy sample location reports and laboratory reports into GIS-friendly maps, shape files and data tables. Drill core Ontario samples were reviewed to fill identified data gaps and better correlate drill core logs prepared by different companies, and new petrologic, geochemical and fluid inclusion samples were Minnesota collected to better constrain the conditions and timing of gold mineralization. Figure 1 30 �Historic exploration efforts focused on a lode gold mineralization model for the QFP. The depth and density of drill cores completed using this model were not sufficient to rule out the possibility of economic lode gold deposits within the QFP. That said, reinterpretation of the Virginia Horn as a Timiskiming-type greenstone terrain would support new exploration efforts beyond the QFP, and into both the overlying basal conglomerate and associated country rock, particularly where major shearing and alteration is found. Figure 2 # Virginia ### # # # # # # ## # ### ### # # # # # ## # #### ###### # ## Paleoproterozoic Animiki Group # # # ## Eveleth # Exploratory Drill Hole Virginia Fm Biwabik Iron Formation Pokegama Quartzite Neo-Archean Giants Range # Batholith Monzodiorite Tonalite Conglomerate Archean Greenstone Quartz Feldspar Porphyry Argillite/Greywacke Mafic intrusives Mafic volcanics 0 miles 5 Support for new drilling within relatively unexplored portions of the Virginia Horn comes from a new visible gold occurrence (Figure 3), discovered during re-logging and examination of drill core Figure 3 DML-3. This bore hole intersected a major fault and was completed within a meta-argillite unit located approximately 8,000 feet from the QFP. The visible gold occurred in a quartz-calcite vein with local minor pyrite. The quartz vein, at 207.5 to 209 feet, had a broken ribbon texture parallel to the vein margins. The vein was oriented about 5 degrees to the core axis. Other quartz-calcite (and Field of view = 1mm minor pyrite) veins were more numerous, but the internal texture was one of crackled quartz with infilling calcite. Prior orientation of the vein during this stage of deformation may have created the different styles. Gold grain counts and pathfinder element concentrations in glacial till collected from locations down ice from both the QFP and this new visible gold occurrence will be used to further constrain competing gold mineralization models. 31 �Textural Stratigraphy of Nipigon Diabase Sills: A Tool for Correlation and Petrologic Interpretations FORSHA, Clinton J. and ZIEG, Michael J., Department of Geography, Geology, and the Environment, Slippery Rock University, 1 Morrow Way, Slippery Rock, PA 16057, USA, cjf8856@sru.edu The relationship between texture and position in an igneous intrusion can be used to interpret the injection dynamics of a sill. In addition, because texture is a strong function of cooling history, textures can be used to determine position in the sill, when the upper and lower contacts are not exposed. The primary purpose of this study is to examine the textures variations in sections through a diabase sill at multiple locations in the Nipigon area, in an attempt to correlate the profiles and construct a composite section. Previous investigations of the Nipigon sills, 1.1-Ga olivine tholeiites, have demonstrated, on the basis of mineralogical variations (e.g., Sutcliffe, 1989), and textural anomalies (Zieg and Forsha, this volume), that the sills were filled via a series of injection pulses rather than in a single, instantaneous injection. This episodic filling history impacted the modal mineralogy and textural variations in the lower contact zone, and is clearly discernible using crystal size distribution (CSD) variations. In order to test the use of textural variations for stratigraphic correlation in igneous intrusions, three sample profiles, from different heights within the sill, were investigated. The Kama Point section includes the lowermost 35 m of the sill, and exhibits strong internal discontinuities related to reinjection events. The Moseau section, which has a total thickness of ~45 m, but does not include an upper or lower contact, is significantly coarser-grained than the Kama Point section and contains no significant textural anomalies. The Palisades section, which has a thickness of ~40 m but does not include an upper or lower contact, does not follow same coarsening-upwards trend as the Kama Point and Moseau sections. The rocks in this section display a slight coarsening toward the center of the section, and appear to become slightly finer-grained in the upper part. With the coarse nature of the Palisades section and its gradual change from coarsening to fining, it is believed that this section is located in the center of the sill. As an application of this method, we combine the different sections into a composite profile through the lower half of the sill. This technique is useful in areas such as Nipigon, where total relief is less than the thickness of individual sills. Used together with mineralogical and chemical constraints, textural stratigraphy is a powerful tool for comparing and interpreting the histories of igneous intrusions. References Sutcliffe, R.H. 1989. Mineral variation in Proterozoic diabase sills and dykes at Lake Nipigon, Ontario. Canadian Mineralogist, 27: 67-79. 32 �PGE-rich mineralization at the Marathon Deposit, Coldwell Alkaline Complex, Ontario David Good and Phillip Walford, Marathon PGM Inc. Mineralization at the Marathon deposit occurs within the Two Duck Lake gabbro, a late-stage gabbroic subunit of the Eastern Gabbro located along the eastern margin of the Coldwell alkaline complex. To date, 70 million tonnes of in-pit resource including 750,000 tonnes of PGE-rich ore have been outlined. PGE-rich ore occurs within a mineralized envelope that is stratigraphically above relatively Curich ore in the Malachite and Southern Resource areas and has been intersected in 64 drill holes over a strike length of 1 km. The highest grade intersection is 106 ppm Pt+Pd+Au and 0.02% Cu over 2m and the highest value intersection is 55 ppm Pt+Pd+Au and 0.59% Cu over 8m. The 64 drill hole intersections were subdivided into a subset of 28 intersections based on a cut-off value of 3ppm Pt+Pd and a minimum thickness of 4 m (2 samples). The average grade and thickness of this subset is 9.38 ppm Pt+Pd+Au and 0.21% Cu over 7.56m. The Pd/Pt ratio is 3.25+/-0.81. The host Two Duck Lake gabbro consists of coarse grained to pegmatitic gabbro with subhedral plagioclase and olivine with ophitic-textured clinopyroxene and magnetite, and minor apatite and biotite. Xenoliths of fine-to medium-grained gabbro are common. Alteration or secondary minerals such as chlorite, sericite, serpentine and calcite occur locally and typically make up less than 5% of the rock. Sulphide mineralization occurs as disseminated chalcopyrite and minor pyrrhotite within coarse grained to pegmatitic olivine gabbro. A detailed study of platinum-group minerals in samples from the Malachite zone determined that approximately 70% of the observed individual grains were closely associated with chalcopyrite or bornite and that less than 7% were associated with secondary minerals such as chlorite or serpentine (Liferovich, 2007). Previous examination of metal-metal distribution diagrams for the Main zone showed strong inter-element correlations between Pd-Pt, Pd-Rh, Pd-Cu and Pd-Au (Good and Crocket, 1994). A comparison of PGE-rich ore to that of the Main zone shows two interesting relationships. First, the trend for Pd-Pt and Pd-Rh shown in the Main zone is continuous with that for the high grade ore. Second, there is no correlation between Pd and Cu in the high grade ore. The Au data are less decisive and somewhat scattered but seem to show behaviour similar to that for Pd. A magmatic model for partitioning of PGE and Cu by sulphide segregation and variations in the R-factor as described for mineralization in the Partridge River Intrusion (Theriault et al, 2000) is somewhat consistent with the data. A simple calculation using a concentration factor of 1000 for the mineralized zones suggests a magma column that is at least 10,000 m thick is required to produce a 10 m intersection with 10 ppm Pd, but the host Two Duck Lake gabbro is only 200 to 400 m thick. 33 �Thériault, R. D., Barnes, S.-J. and Severson, M. J., 2000, Origin of Cu-Ni-PGE Sulfide Mineralization in the Partridge River Intrusion, Duluth Complex, Minnesota: Economic Geology, v. 95, p. 929-943 Liferovich, R., 2007, Mineralogy of PGE, Gold and Silver in the Malachite ore zone, Two Duck Lake deposit, Marathon, NW Ontario, Internal Company report. Good, D.J., and Crocket, J.H., 1994, Genesis of the Marathon Cu-platinum-group element deposit, Port Coldwell alkalic complex, Ontario: A mid-continent rift-related magmatic sulfide deposit: Economic Geology, v. 89, p.131 –149 34 �Sugarloaf – A State Scientific and Natural Area and a North Shore Non-Profit Association John C. Green Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 Andrew Slade Sugarloaf: The North Shore Stewardship Association, 6008 London Road, Duluth, MN 55802 The 7-acre Sugarloaf Point Scientific and Natural Area (SNA) in southwesternmost Cook County, Minnesota, was designated in 1992 to preserve and protect the geological and ecological features of this bit of the North Shore, especially the basalt lava-flows and the dynamic beaches. Despite its history as a logging-era brownfield, it preserves excellent and accessible evidence for the character of both the 1.1 Ga Midcontinent Rift volcanism and Holocene postglacial processes, still active today. The bedrock consists of olivine tholeiite flows, part of the Schroeder basalts, near the top of the North Shore Volcanic Group (Miller et al., 2001; Miller et al., 2006). Features such as ophitic and amygdaloidal textures, columnar jointing, pipe amygdules, segregation veins, ropy surfaces, vesicle cylinders, and clastic dikes are well exposed (Green, 1989). Surficial features include an abandoned strandline and the overall tombolo form, with modern beaches linking the rocky point to the mainland. Beach clasts imply derivation from Rainy Lobe as well as Superior Lobe glacial deposits, along with local bedrock. The SNA, as well as 27 surrounding acres, is managed by a nonprofit group known as Sugarloaf, the North Shore Stewardship Association. The Association has built a log interpretive center, and has accomplished considerable environmental restoration on this heavily impacted site. Major restoration projects include excavation and replanting, with locally-grown native vegetation, of a rare coastal wetland between the berms of the tombolo. Sugarloaf hosts many popular interpretive programs, including day-long workshops on various aspects of North Shore natural history. It works with landowners and public land managers throughout the North Shore on both landscape restoration and interpretation. A major current initiative is the opening of the North Shore Interpretive Center at the University of Minnesota Duluth’s historic “Limno Lab” building at the Lester River in northeastern Duluth. Exhibits there will introduce visitors to North Shore natural and settlement history and UMD based research on the North Shore and Lake Superior. References Cited Green, J. C., 1989, Physical volcanology of mid-Proterozoic plateau lavas: The Keweenawan North Shore Volcanic Group, Minnesota: Geological Society of America Bulletin, v. 101, p. 486-500. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-119, scale 1:200,000. Miller, J.D., Jr., Green, J.C., and Jerde, E.A., 2006, Bedrock geologic map of the Little Marais quadrangle, Lake and Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-172, scale 1:24,000. 35 �Emplacement of the Nipigon Sill Complex and Mafic to Ultramafic Intrusions of the Nipigon Embayment T.R. Hart and C.A. MacDonald Precambrian Geoscience Section, Ontario Geological Survey (tom.hart@ontario.ca) (caroleanne.macdonald@ontario.ca) The Nipigon Embayment consists of Proterozoic sedimentary and intrusive rocks underlain by Archean rocks of the English River, Wabigoon, and Quetico subprovinces. Recent geological mapping suggests that the Nipigon Embayment is underlain by a series of north-, northwest- and northeast-trending faults (e.g. Hart 2005; MacDonald et al. 2005). The north-trending faults can be traced for over 150 km, and commonly have apparent sinuous traces resulting from an intersection with north- and northwest-trending faults. A number of the northwest-trending faults control the roughly circular-shape of diabase sills as in the Kopka River (Sutcliffe 1986). The east- to northeast-trending faults include deformation zones exposed in surrounding greenstone belts, as well as greenstone belt and subprovince boundaries. Some faults, and particularly the intersections of a number of faults, appear to have been the locus for magmatic events active at a number of geological periods. A number of 2697-2686 Ma late- to post-tectonic mafic to ultramafic intrusions (Heaman and Easton 2006) occur across the central Wabigoon Subprovince from Lac des Iles north to Awkward Lake in the Obonga Lake greenstone belt near major east- to northeast-trending structures. The ~1540 Ma English Bay Complex and ~1595 Ma Badwater Intrusion (Heaman and Easton 2006) are located near the junction of the east-trending Obonga Lake mylonite zone / Humboldt Bay high-strain zone and a number of north-trending faults traceable south to Black Sturgeon Lake. These intrusions may represent magmatism related to formation of the Nipigon Embayment. In the same area, the 1159±33 Ma Inspiration diabase sill and 1112 Ma ultramafic Jackfish sill (Heaman and Easton 2006) may represent early stages of the MRS. To the south, the 1109 Ma Disraeli, 1113 Ma Seagull, and 1107 Ma Hele ultramafic intrusions are located near the intersection of northand northwest-trending faults with the Quetico-Wabigoon subprovince boundary (QWSB), also represent early MRS magmatic events. The 1117 Ma mafic to ultramafic Kitto Intrusion (Heaman and Easton 2006) is located near the intersection of the QWSB, and the north-trending Nipigon River fault. Many of the diabase sills have morphologies controlled in part by these three faults directions, particularly the roughly circular sills exposed in the Kopka and Garden PROTEROZOIC ARCHEAN Volcanic rocks Osler Group (1109 Ma) Lake areas. Granitic rocks Nipigon Sills (1112 Ma) Franklin et al. (1980) suggested that Mafic and ultramafic Ultramafic intrusions Ultramafic intrusions intrusive rocks the Embayment represented a failed Sibley Group Metasedimentary and arm of the Midcontinent Rift System Metavolcanic rocks English Bay Complex Sanukitoid intrusions 36 �(MRS). Alternatively, the Embayment could represent a structurally controlled basin formed by subsidence following an anorogenic thermal upwelling event represented by the ~1540 Ma English Bay Complex (Hollings et al. 2004). Studies completed by Rogala et al. (2005) indicate that deposition of the lower portions of the Sibley Group sedimentary rocks (>1339 Ma; Franklin 1978) pre-dated the formation of the ~1100 Ma MRS by ~200 Ma. The presence of sills rather than dykes and apparent lack of a dyke swarm common in many failed rift arms (Ernst et al. 2006) suggest that the Embayment was not extensional during the MRS event. A uniform lithogeochemistry for the diabase sills suggests a single magma source (e.g. Hollings et al. in press). Limited anisotropy of magnetic susceptibility data (Middleton et al. 2004) indicates westerly flow directions in the western Embayment suggesting a source, or sources, near the north-trending faults in the Black Sturgeon Lake area. Manson and Halls (1997) proposed that the geometry of the MRS could be the result of the interaction between the MRS and a series of faults within the bounding Archean rocks. The faults in Nipigon Embayment could be part of this series of faults and partially explain the lack of a classic failed arm. Some of these faults could have been deep structures providing pathways for the emplacement of a variety of magmas at different periods. The timing and magnitude of displacement along these faults is difficult to determine, but a high degree of fracturing and hematite alteration in diamond drill core from the 1112 Ma Seagull Intrusion (Heaman and Easton 2006) suggests multiple periods of fault activity. Late stage activity may be related to a 1090+/-20 Ma uranium mineralization (Ruzicka and LeCheminant 1984) located along a north-trending structure east of Black Sturgeon Lake. This younger age is closer to many of the events in the late Keweenawan, including the 1091+/-4.5 Ma Blake Gabbro (Heaman and Easton 2006), and suggests fault activity in the Nipigon Embayment for a period of up to 20 Ma. REFERENCES Ernst, R.E., Buchan, K.L., Heaman, L.M., Hart, T.R., and Morgan, J. 2006. Multidisciplinary study of N to NNE trending dykes in the region west of the Nipigon Embayment: Lake Nipigon Region Geoscience Initiative; Ont. Geol. Sur., Misc. Release Data, MRD 194. Franklin, J.M. 1978. The Sibley Group, Ontario; in Rubidium-strontium isochron age studies, report 2; ed. R.K. Wanless and W.D. Loveridge; Geol. Sur. Canada, Paper 77-14: 31-34. Franklin, J.M., McIlwaine, W.H., Poulsen, K.H. and Wanless, R.K. 1980. Stratigraphy and depositional setting of the Sibley Group, Thunder Bay District, Ontario, Canada; Can. J. Earth Sci., 17: 633-651. Hart, T.R. 2005. Precambrian geology of the southern Black Sturgeon River and Seagull Lake area, Nipigon Embayment, northwestern Ontario; Ont. Geol. Sur., Open File Report 6165, 63p. Heaman, L.M. and Easton, R.M. 2006. Preliminary U/Pb Geochronology Results: Lake Nipigon Region Geoscience Inititative; Ont.Geol. Sur., Misc.Release—Data, MRD191 Hollings, P., Hart, T.R., Richardson, A., and MacDonald, C.A. in press. Geochemistry of the Mesoproterozoic Intrusive Rocks of the Nipigon Embayment, Northwestern Ontario: Evaluating the earliest phases of rift development; Can. J. Earth Sci. Hollings, P., Fralick, P. and Kissin, S. 2004. Geochemistry and geodynamic implications of the Mesoproterozoic English Bay granite-rhyolite complex, northwestern Ontario; Can. J. Earth Sci., 41: 1329-1338. MacDonald, C.A., Tremblay, E. and Easton, R.M. 2005. Precambrian geology of the west-central map area, Nipigon Embayment, northwestern Ontario: L.N.R.G.I.; Ont. Geol. Sur., Open File Report 6164, 49p. Manson, M.L. and Halls, H.C. 1997. Proterozoic reactivation of the southern Superior Province and its role in the evolution of the Midcontinent Rift. Can. J. of Earth Sci., 34: 562-575. Middleton, R. S., G. J. Borradaile, D. Baker, and K. Lucas, 2004. Proterozoic diabase sills of northern Ontario: Magnetic properties and history; J. of Geop. Res., 109, B02103, doi:10.1029/2003JB002581. Rogala, B., Fralick, P.W. and Metsaranta, R. 2005. Stratigraphy and Sedimentology of the Mesoproterozoic Sibley Group and Related Igneous Intrusions, Northwestern Ontario: Lake Nipigon Region Geoscience Initiative; Ont. Geol. Sur., Open File Report 6174, 87p. Ruzicka, V. and LeCheminant, G.M., 1984. Uranium deposit research, 1983; in Current Research, Part A, Geol. Sur. Canada, Paper 84-1a: 39-51. Sutcliffe, R.H. 1986. Proterozoic rift related igneous rocks at Lake Nipigon, Ontario; unpublished PhD thesis, University of Western Ontario, London, Ontario, 325p. 37 �Lacustrine Sedimentary Organic Matter Proxies of Recent Lake State Changes and Climatic Conditions in Christina and Morrison Lakes of Western Minnesota Amanda Hogan, Travis Jacobs, and Kevin Theissen Department of Geology, University of St. Thomas, 2115 Summit Ave., St. Paul, MN 55105 Organic matter in shallow lake sediments reveal past states of the lake, indicating dryer or wetter periods as well as periods which were dominated by algal or vascular plants. We have analyzed lake sediment cores dating approximately back to the year 1400 and have correlated stable isotopic and elemental data to support recent lake transitions. In the summer of 2006, we collected sediment cores from Lake Christina and Morrison Lake, shallow lakes (< 4 m max depth) in western Minnesota’s Prairie Pothole Region. We took samples from the cores at 1 cm intervals and prepared them to be sent to the Stanford University Stable Isotope laboratory where they were analyzed for stable isotopic (δ13C and δ15N) and elemental (C/N, TOC, TN) values. Finally, over the last two months, the data have been analyzed and correlated to a sedimentation-rate based time scale to determine changes in ecological status of the lakes as well as past changes of precipitation in the areas surrounding them. Here we report preliminary results for Lake Christina. C/N ratios (8 to10) and δ13C values (-24 to-19 per mil) show a history where the lake has been dominated by lacustrine algae (Figure 1). The range of these values is small, and while they fluctuate, the levels never reach those of a lake dominated by vascular plants (1). These small changes indicate slight changes in organic matter production and sourcing. Based on our preliminary age estimates for Lake Christina, the values for δ15N show a steady increase from the year 1680 to 1900 where they begin to decline sharply over the next 100 years (Figure 2). The low values in the years 1400-1700 might indicate a wetter climatic period where the lake level was higher and algal productivity in the lake was greater (1). Precipitation slightly decreased over the next 300 years causing upward shifts in the value of δ15N as well as a decrease in %CaCO3. This drier period was possibly windier, causing more sediment particles to be blown into the lake leading to more turbulent waters with large amounts of mixing (3). It is likely that land use changes had an impact on the overall carbon isotopic values of the sediments. Values of δ13C show a sharp decline around 1900, which corresponds to a time when most of this area was farmland rather than grassland. This might signal a change to a more algal dominated lake at this time. However, around the year 1940, δ15N values begin to decline again indicating a wetter period. In the years since 1938, more wet years are on record than in the years prior. The decline in δ15N could be attributed to the wetter climatic conditions in the area over the last 60 years (1). 38 �Lake Christina d13C vs. C:N Ratio -19.50 -20.00 -20.50 -21.00 -21.50 -22.00 -22.50 -23.00 -23.50 -24.00 7.50 8.00 8.50 9.00 9.50 10.00 C:N Ratio Figure 1. Lake Christina d15N vs. Time 2.5 2 1.5 1 0.5 0 -0.5 -1 -1.5 -2 -2.5 1400 1500 1600 1700 1800 1900 2000 2100 Time (years) Figure 2. References: (1) Meyers, P.A., and Lallier-Verges, E., 1999, Lacustrine sedimentary organic matter records of Late Quaternary Paleoclimates. Journal of Paleolimnology, 21, 345-372. (2) Pratt, L.M., Comer, J.B., and Brassell, S.C., 1992, Geochemistry of Organic Matter in Sediments and Sedimentary Rocks, 29-72. (3) Xu, H., Ai, L., Tan, L., An, Z., 2006, Stable isotopes in bulk carbonates and organic matter in recent sediments of Lake Qinghai and their climatic implications. Chemical Geology, 235, 262-275. 39 �Geochemistry of Midcontinent Rift-related mafic dykes and sills near Thunder Bay: New insights into geographic distribution and the geochemical affinities of Nipigon and Logan sills and Pigeon River and other dykes HOLLINGS, Pete, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada, SMYK, Mark C., Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 Canada, HART, Thomas, Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, ON, P3E 6B5, Canada Recent geochronological studies (Heaman et al., 2007) have shown that the diabase sills and mafic/ultramafic intrusions in the vicinity of Lake Nipigon are among the oldest expression of igneous activity associated with the Midcontinent Rift (~1106 to 1115 Ma). Diabase sills, extending from the vicinity of Thunder Bay to east of Lake Nipigon, represent the northern expression of the Midcontinent Rift (MCR), and have previously been collectively referred to as the Logan sills (Stockwell et al. 1972). However recent work suggests a geochemical difference between the sills to the north and south of Thunder Bay (Hart 2003; Hart et al. 2005). It has been proposed that the term Logan Igneous Suite, which would fall within the Midcontinent Rift Intrusive Supersuite (Miller et al. 2002), should be applied to all the diabase sills in the area north of Lake Superior, with subdivision into more informal categories (i.e. Nipigon sills for the sills north of Thunder Bay, and Logan sills for those to the south) (Hollings et al., 2007). In 2006 a sampling traverse was undertaken through Thunder Bay in order to help determine the boundary between these two suites. The large geochemical data set for the Nipigon sills obtained as part of the Lake Nipigon Region Geoscience Initiative (Hollings et al., 2007) and a number of recently analyzed, east-northeast- to northeast-trending dykes south of Thunder Bay provide a means of comparing the sills near Thunder Bay. The sills and dykes sampled for this study are all characterized by LREE and enrichment and negative Nb anomalies typical of the majority of the local intrusive rocks associated with the MCR (Hollings et al., 2007). The majority of these sills fall are geochemically similar to the Nipigon sills. However, two ultramafic sills are geochemically similar to the Logan sill suite (i.e. with higher TiO2 and Gd/Ybn; Fig. 1). This suggests that the boundary between the Nipigon and Logan sills lies southwest of Thunder Bay. Data from other dykes south of Thunder Bay (L. Hulbert and R. Ernst, Geological Survey of Canada, pers. comm., 2006) show that the geochemistry of the Pigeon River dykes (Arrow River and Rita Bolduc occurrences) is similar to the range of data from other dykes of the Pigeon River swarm sampled on Lake Superior (Cloud Bay and Jarvis Point; Fig. 1). The Pigeon River dyke swarm geochemistry more closely resembles that of the sills of the Nipigon suite (Hollings et al., 2007) than that of the ultramafic intrusions or the Logan sills (Fig. 1). In contrast, the Mt. Mollie dyke appears to be transitional between Nipigon sills and Inspiration sills. Geochemical trends in individual dykes suggest fractionation or assimilation/contamination but data is currently limited. A sample taken from a Pigeon River dyke near its chilled contact with Rove Formation on Jarvis Point (JP-3) shows elevated Th and LREE compared to the dyke interior (JP-1, -2, -4; Fig. 2). A granophyric phase of the Mt. Mollie dyke on the southern end of Victoria Island (VI-9) displays similar, but elevated, REE compared to the main gabbroic part of the dyke. The granophyre also shows a marked depletion in Al and also in Ti and V, perhaps due to the relative lack of oxides in this late-stage differentiate. Additional isotopic and 40 �geochronological studies will be required in order to further investigate the relationships between these MCR-related intrusions. Figure 1: Major and trace element variation diagrams illustrating the geochemical affinities of the dykes and sills in around Thunder Bay. Data from Logan sills are from Hart (2002), Nipigon data are from Hollings et al. (2007). Date for dykes south of Thunder Bay are from and L. Hulbert and R. Ernst (Geological Survey of Canada, pers. comm.. 2006) from samples collected by M. Smyk and J. Scott (Ontario Geological Survey). Figure 2: Primitive mantle normalised diagram comparing geochemistry of dykes south of Thunder Bay. References Heaman, L.M., Easton, R.M., and Hart, T.R, 2006. Further Refinement to the Timing of Mesoproterozoic Magmatism, Lake Nipigon Region, Ontario. Canadian Journal of Earth Sciences, in press. Hart, T.R. 2003. Keweenawan mafic and ultramafic intrusive rocks of the Lake Nipigon and Crystal Lake areas, northwestern Ontario; ILSG, Proceedings Volume 49, Part 1-Programs and Abstracts: 21-22. Hart, T.R. 2005. South Black Sturgeon River–Seagull Lake Area, Nipigon Embayment, Northwest Ontario: Lithogeochemical, Assay and Compilation Data. Ontario Geological Survey, Miscellaneous Release of Data 147. Hollings, P., Hart, T.,Richardson, A. and MacDonald, C.A., 2007. Geochemistry of the Mesoproterozoic Intrusive Rocks of the Nipigon Embayment, Northwestern Ontario: Evaluating the Earliest Phases of Rift Development. Canadian Journal of Earth Sciences, in press. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E. 2002. Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58. Stockwell, C.H., McGlynn, J.C., Emslie, R.F., Sanford, B.V., Norris, A.W., Donaldson, J.A., Fahrig, W.F., and Currie, K.L. 1972. Geology of the Canadian Shield. In Geology and economic minerals of Canada. Edited by R. Douglas. Geological Survey of Canada Economic Geology Report 1: 838. 41 �RECENT DEVELOPMENTS UNDERSTANDING THE VOLCANIC, MAGMATIC, TECTONIC AND METALLOGENIC EVOLUTION OF THE ELY GREENSTONE FORMATION, VERMILION DISTRICT, NE MINNESOTA HUDAK, G. J., Department of Geology, University of Wisconsin Oshkosh, Oshkosh, WI 54901, hudak@uwosh.edu HOFFMAN, A. T., Department of Geology, University of Minnesota – Duluth, Duluth, MN 55812 PETERSON, D.M., HEINE, J., Natural Resources Research Institute, University of Minnesota – Duluth, Duluth, MN 55811 The Ely Greenstone Formation comprises a steeply north- to southwest-dipping sequence of Neoarchean supracrustal and associated intrusive rocks associated with the Tower-Soudan anticline in the Vermilion District of northeastern Minnesota. Three distinctive units comprise this formation. The Lower Member of the Ely Greenstone (LMEG) is composed of calc-alkalic and tholeiitic basalt and basalt- andesite lava flows and tuffs with subordinate felsic lava flows, tuffs, epiclastic rocks and iron formations (Schulz, 1980; Southwick et al., 1998; Hudak et al., 2002, Hoffman, in press). The LMEG has been subdivided into the older Fivemile Lake Sequence (FLS) and the younger Central Basalt Sequence (CBS; Peterson and Patelke, 2003). The Soudan Iron Formation Member (SMEG) comprises Algoma-type interlayered cherty iron formation, basalt lava flows, epiclastic rocks and felsic tuffs (Peterson and Patelke, 2003; Hoffman, in press). The Upper Member of the Ely Greenstone Formation (UMEG) is composed of a monotonous sequence of poorly vesiculated tholeiitic basalt lava flows and localized Algoma-type iron formation lenses (Schulz, 1980; Southwick et al., 1998). The UMEG is commonly interstratified with the Lake Vermilion Formation (LVF:Schulz, 1980; Southwick et al., 1998), which is composed of greywacke, slate, conglomerate, and dacite tuff, as well as subaerial to submarine dacite to trachyandesite lava flows, tuffs, and associated intrusions (the informally named Gafvert Lake Sequence (GLS)); locally, however, LVF unconformably overlies LMEG and SMEG strata (Southwick et al., 1998; Peterson and Patelke, 2003). Schulz (1980) interpreted volcanological and sedimentary textures to indicate a change from a subaerial / shallow subaqueous setting to a deeper subaqueous environment during the temporal genesis of the Ely Greenstone Formation. Southwick et al. indicate a sharp transition from arc-associated volcanism (LMEG) to MORB-like volcanism (UMEG) occurs abruptly at the top of the SMEG. Recent volcanological reconstructions between the Soudan Mine and Armstrong Lake provide further evidence for the interpretations of Schulz (1980). The initial deepening of the submarine volcanic setting occurred during the development of the LMEG (Peterson et al., 2005; Hoffman, in press). Abundant primary mafic and felsic volcaniclastic strata, highly vesicular basalt to basalt-andesite pillow lavas and sheet flows, multiple selvege pillows, and epithermallike zinc stringer mineralization that characterize the FLS indicate a depositional setting in a shallow subaqueous environment (Hudak et al., 2002; Hoffman, in press). The overlying CBS comprises exceptionally well-preserved, single-selvege pillow lavas and associated sheet flows with sparse (generally <5%) vesicularity which are interstratified with subordinate banded iron formation and mafic tuff (resedimented basalt hyaloclastite) interpreted to have formed in a deep subaqueous setting (Peterson and Patelke, 2003; Peterson et al., 2005). The SMEG contains 42 �finely laminated chemical sediments and resedimented tuffs, as well as sparsely vesicular mafic lava flows suggestive of deposition within a deep subaqueous setting (Peterson and Patelke, 2003; Hoffman, in press). This deeper water setting appears to have persisted through the development of the UMEG, and at least locally during the development of the LVF. New major- and trace element data indicate the lithogeochemistry of volcanic rocks in the LMEG is more complicated than previously recognized. Arc-related basalts and basaltic andesites and FI- and FII-type rhyodacites and rhyolites characterize the FLS. In the CBS, arcassociated basalts and basaltic andesites transition up-section into E-MORB, OIB and back-arc basin-like basalts which are associated with FIII-type felsic volcanic rocks (Hudak et al., in prep. Hoffman, in press). The UMEG is characterized by MORB compositions that may also be the product of back-arc spreading (Southwick et al., 1998). A model encompassing initial arc development followed by back-arc development and rifting during the CBS, with subsequent development of the SMEG, appears to be most consistent with the observed volcanological and lithogeochemical characteristics in this part of the Vermilion District. Iron formations within the SMEG occur immediately up-section from the arc – back-arc transition, a stratigraphic position shown in many studies to have high prospectivity for hosting volcanogenic massive sulfide orebodies. References Hoffman, A. T., in press, Lithostratigraphy, Hydrothermal Alteration, and Lithogeochemistry of Neoarchean Rocks in the Lower and Soudan Members of the Ely Greenstone Formation, Vermilion District, NE Minnesota: Implications for Volcanogenic Massive Sulfide Deposits: Unpublished M. S. thesis, University of Minnesota – Duluth. Hudak, G. J., Heine, J., Newkirk, T., Odette, J., and Hauck, S., 2002, Comparative geology, stratigraphy, and lithogeochemistry of the Five Mile Lake, Quartz Hill, and Skeleton Lake VMS occurrences, Vermilion District, NE Minnesota: A report to the Minerals Coordinating Committee, DNR, Minerals Division, State of Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2002/03, 390 pages. Hudak, G. J., Heine, J., Newkirk, T. T., Hocker, S. M., and Hauck, S., in prep., Comparative Geology, Stratigraphy, and Lithogeochemistry of the Needleboy Lake – Six Mile Lake Area, Vermilion District, NE Minnesota: Natural Resources Research Institute Geological Report of Investigation. Peterson, D. M., Jirsa, M. A., and Hudak, G. J., 2005, Architecture of an Archean greenstone belt: stratigraphy, structure, and mineralization; in Robinson, L, ed., 2005, Field trip guidebook for selected geology in Minnesota and Wisconsin: Minnesota Geological Survey Guidebook 21, p. 154-180. Peterson, D. M., and Patelke, R. L., 2003, National Underground Science and Engineering Laboratory (NUSEL). Geological site investigation for the Soudan Mine, NE Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2003, 88p. Schulz, K. J., 1980, The magmatic evolution of the Vermilion Greenstone Belt, NE Minnesota: Precambrian Research, v. 11, p. 215-245. Southwick, D. L., Boerboom, T. J., and Jirsa, M. A., 1998, Geological setting and descriptive geochemistry of Archean supracrustal rocks and hypabyssal rocks, Soudan-Bigfork area, northern Minnesota: implications for metallic mineral exploration: Minnesota Geological Survey Report of Investigations 51, 69 p. 43 �EVALUATION OF MINERAL EXPLORATION DRILL CUTTINGS IN THE RICE RIVER AREA, EAST CENTERAL MINNESOTA Hudak, J. N. and Frey, B. A., Minnesota Department of Natural Resources, Lands and Minerals Division, 1525 3rd Ave. East, Hibbing, MN 55746 The Rice River area is located at the east end of the Cuyuna South Range, near the town of Aitkin. From 1949 to 1952, M.A. Hanna drilled 334 reverse circulation drill holes along approximately 12 km strike length of an iron formation related magnetic anomaly, to search for naturally enriched iron ore. Current DNR work in the Rice River area involves relogging of core and cuttings, the analysis of 325 samples for indications of base and precious metals, and the recovery of more accurate drill hole locations. The mid-Proterozoic basement rock does not crop out in the area, and drill cuttings and core are currently the only available bedrock samples. Recent replotting of geochemical lake sediment survey data revealed clustered metal anomalies in the Cuyuna South Range area. The Rice River area drill holes have been predominantly analyzed for iron and manganese and not fully evaluated for mineral potential. The main goal of the current evaluation is to see if there is any evidence of those anomalies in the Rice River area. Plausible ore deposit models for the area include SEDEX and iron-oxide-copper-gold. The magnetic iron anomaly is linear except for the large s-shaped fold, which is spatially associated with stratified sulfide occurrences and large quartz veins. Pyrite is often remobilized within shear zones of the graphitic argillite associated with this area. Veinlets with small amounts of bornite also occur scattered within the Rice River area. Currently, 60 of the 325 samples collected have been analyzed yielding anomalous levels of copper (2200 ppm) and antimony (6.75 ppm). A report of the work and findings are scheduled for release on June 30, 2007. 44 �PALEOENVIRONMENTAL INTERPRETATION OF A LOWER PALEOZOIC STROMATOLITE REEF, NORTHEASTERN WISCONSIN JASINEVICIUS, Renata R., GORDON, Elizabeth A., St. Norbert College Geology Department, De Pere, WI 54115, elizabeth.gordon@snc.edu INTRODUCTION Strata recently exposed in Schaal Quarry near Gillett, Wisconsin, provide an exceptional opportunity to examine the facies architecture of a lower Paleozoic stromatolite reef. The threedimensional nature of the reef, and its stratigraphy, are the focus of this study. The dolomitic rocks yield conodonts of early Ordovician age (Miller, 2006) and these strata are therefore assigned to the Oneota Formation. The Oneota dolomite overlies an unconformity, considered by many to be of regional extent. At this quarry, sandstones of uncertain Cambrian(?) age lie beneath the unconformity. Bedrock exposures in northeastern Wisconsin are extremely rare, due to widespread glacial deposits. This site therefore provides an important new opportunity to study Cambro-Ordovician boundary strata along the eastern margin of the Wisconsin Arch. RESULTS The Oneota Formation contains diverse stromatolite morphologies. At this locality, four stromatolite types occur in regular stratigraphic order: laterally linked hemispheroids, cabbageshapes, isolated small domes, and larger elliptical compound domes. In cross section, the larger stromatolites appear to form a mound- or ridge-shaped structure. The following paragraphs offer a summary and interpretation of the major sedimentary features associated with these stromatolites. The base of the Oneota in Schaal Quarry is dominated by an intraclastic conglomerate of variable thickness which overlies an unconformity. Desiccation cracks on the base of the conglomerate record subaerial exposure of the underlying units. Clast types include red shale, dolostone, sandy to silty dolomite pebbles and cobble to boulder-sized silcrete. The matrix is a dolomitic sandstone, and the unit grades upwards to sandy and silty dolomites. The silty dolomites contain discontinuous layers with microbial-like structures and laminated mats. These layers are interbedded with cross- and planar laminated, flaser- to wavy-bedded grainstones, and rare desiccation cracks. Grains types are primarily coated quartz sand and minor dolomite intraclasts. Upsection, discontinuous horizons of laterally linked hemispheroids (LLHS; Logan, 1964) mark the first visibly distinctive stromatolites, which predominate several horizons. In places they are vertically stacked, forming beds up to 20 cm thick. Individual hemispheres are round to elliptical with variable dimensions. LLHS are buried by ripple cross- and planar-laminated grainstones, grading to wackestones. Clay and glauconite locally contribute a greenish color to these rocks. 45 �Farther upsection, a fine-grained silty dolomite contains chert-lined vugs interpreted to be molds of anhydrite. These structures suggest hypersaline conditions. The overlying grainstones contain abundant crescent-shaped fossils of unknown origin, similar to “chitons” reported by Raasch (1952). Gastropod molds are present but rare. Conodonts include “Teridontus sp., Acanthodus uncinatus, Aloxoconus sp., Cordylodus lindstromi, and are assigned to the interval from the uppermost Cordylodus lindstromi Zone, the Iapetognathus Zone, and into the lower part of the Cordylodus angulatus Zone” (Miller, 2006), indicating an early (but not earliest) Ordovician age. Continuing upsection, there is a general upwards increase in bed thickness, accompanied by a decrease in coated quartz sand grains, and an increase in peloids and sparry cored ooids. Local grapestone intraclasts are also present. Within this interval cabbage-shaped stromatolites (CS) form discontinuous patches along one horizon. They are widely associated with chert. One wall reveals a curious relationship: adjacent stromatolite heads successively increase in size, suggesting a mound-like geometry of this cluster. CS are overlain by cross-bedded, oolitic peloidal grainstones, indicating burial by subaqueous dunes. Near the top of the quarry, thicker cross-bedded grainstones enclose nearly circular, isolated stromatolite domes. The domes appear to be concentrated in a mound or ridge. Higher still, the top surface of the quarry exposes large elliptical domes. A visible change in stromatolite synoptic relief indicates increasing water depth with time (Gerdes and Krumbein, 1994). The elliptical domes are clustered locally with subparallel long axes. The axes are aligned with paleocurrent azimuths measured from adjacent crossbeds and parting lineations. This association suggests stromatolite morphology was controlled in part by subaqueous currents. The predominant paleocurrent direction is approximately perpendicular to the inferred paleoshoreline. This implies subaqueous currents were tidal in origin and that stromatolite domes developed in subtidal channels. Collectively, these sedimentary features record evolution of a reef during marine transgressions. Interpreted paleoenvironments reflect tidal flat to subtidal environments under extreme conditions. REFERENCES Gerdes, G. and Krumbein, W., 1994, Peritidal stromatolites; in Phanerozoic Stromatolites II, ed. Bertrand-Sarfati, J. and Monty, C.: 114-17. Logan, B.W., Rezak, R., and Ginsburg, R.N.; 1964, Classification & environmental significance of stromatolites; Journal of Geology 72: 68-83. Miller, James F., 2006, Southwest Missouri State University; personal communication. Raasch, G. O., 1952, Oneota Formation, Stoddard Quadrangle, Wisconsin; Illinois Academy of Science Transactions, V. 15 p. 85-95. 46 �PROPOSED NEW BEDROCK GEOLOGIC MAP OF MINNESOTA JIRSA, Mark A. (jirsa001@umn.edu), BOERBOOM, T.J., CHANDLER, V.W., LIVELY, R.S., MILLER, J.D., Jr., MOSSLER, J.H., RUNKEL, A.C., SETTERHOLM, D.R., and WAHL, T.E, Minnesota Geological Survey (www.geo.umn.edu/mgs) Having conducted detailed geologic mapping in many areas of the state, and reprocessed state-wide geophysical data, the Minnesota Geological Survey (MGS) is well positioned to produce a new interpretation of bedrock geology. We are preparing to construct a state-wide bedrock geologic map— one that will be comprehensive, up to date, digital, and multi-layered. In the process, we also intend to upgrade digital bedrock topography and depth to bedrock maps and make them consistent with the new geologic framework (for preliminary imagery, see MGS website, Open-File 06-02). Together, these maps will provide the context necessary for various minerals and water-related applications in progress and under consideration. They will also establish the understanding of bedrock geology and crustal structure needed to address a number of impending, high-profile regional and national science initiatives, including EARTHSCOPE (www.earthscope.org). At this writing, the work of compiling hundreds of archival maps is underway. The existing state-wide bedrock map (MGS map S-20), completed in 2000, was reasonably detailed and accurate for its time; however, more than 50 new bedrock geologic maps have been created in the intervening 7 years since its production. Furthermore, the earlier map was compiled at a generalized 1:1,000,000 scale. As such, it portrays contacts that locally are inconsistent with high-resolution geophysical imagery, and it is 2-dimensional—in the sense that it depicts only the uppermost bedrock units. By contrast, the new map will be compiled at more detailed scales overall (compilation scale 1:100,000; publication scales 1:500,000 and 1:1,000,000), and geologic contacts will be reconciled with reprocessed geophysical data. It will also contain thematic layers, including trajectories of diabasic dikes, generalized metamorphic grades, and themes depicting Mesozoic, Paleozoic, Mesoproterozoic, Paleoproterozoic, and Archean geology. The Quaternary layer will be represented at this stage by revised depth to bedrock imagery. This layering will give users the opportunity to consider the third dimension by permitting removal of geologically younger layers to reveal older ones, to the extent that this can be done with some certainty. Perhaps more important than the printed maps and printable pdfs of archived maps, is the aim of producing a digital platform that will allow web-based user navigation from compilations to the most recent, accurate, and detailed mapping. Associated digital files will be searchable on attributes of lithology, age, and other themes. Mapping will be integrated with broader digital efforts using compilation formats consistent with those developed nationally (e.g., USGS) and globally (e.g., OneGeology.org). The project requires compilation of the best available maps—both digital and analog—together with new work designed to fill voids, augment existing map imagery, and evaluate the nature of buried geologic terranes. In addition, a small program of high-precision geochronology will acquire ages for a hand-full of critical samples. The results will be a seamless state-wide geologic interpretation, a webbased project that links this image to original mapping, and a digital cache of associated files. The poster presented here for general discussion depicts an initial effort to bring together the various digital map coverages. 47 �TRACE ELEMENT ANALYSES: AVOIDING DATA DISTORTION McSWIGGEN, Peter L., McSwiggen and Associates, P.A., 2855 Anthony Lane South, Suite B1, St. Anthony, MN (PMcS@McSwiggen.com) Electron microprobes have been used for trace element analyses for decades. This is because they can analyze minerals in situ, the size of the analytical area is very small, and the cost per analysis is very low. None-the-less, there is an ongoing push to measure trace elements in ever-lower concentrations. Over the years, improvements in both the electronics and the spectrometers of microprobes have made this possible. However, how the data is collected and how it is subsequently processed can mean the difference between meaningless numbers and real insights into the geologic question being investigated. Typically, people collecting trace element data want to know the minimum detection limit (MDL) of the method being used. Secondly, they want to know what should they do with the data that fall below the MDL. Should those values be set to zero? Should those values be set to the MDL? Should all of the data that fall below the MDL be discarded? If the set of data is going to be used to determine an average concentration for a rock body or a mineral type, all three options are wrong. In most cases involving trace element analyses, the calculated minimum detection limit for a single analysis should be ignored. Often, analysts will attempt to adjust their analytical conditions to get the lowest calculated minimum detection limit. However this can result in erroneous data. The MDL can be lowered using a number of strategies – for example, by counting for a longer period of time or by using a higher beam current to produce a higher count rate, both will lower the MDL. However both of these strategies can work against the overall objective. A higher beam current can damage the mineral being analyzed and therefore completely throw-off the results. Similarly, extremely long counting times can mask electronic instability in the instrument, thereby producing erroneous results. A far better strategy would be to collect a series of analyses, using more reasonable counting times and beam currents, and determine an average value from that set of data. This would minimize the effect of beam damage and would both monitor and correct for any electronic drift in the instrument; thus yielding a true detection limit that is much lower than that from the individual analyses. If one is using a set of analyses to determine an average composition, the calculated MDL becomes a meaningless value. Figure 1 shows a set of analyses run on a glass standard with a reported Ni content of 0.046 wt% using only a two second counting time per analysis. The average for this set of data was 0.053 wt%, a difference of only 0.007 wt% from the reported value. However, the calculated MDL for a single analysis was 0.25 wt%. Therefore, if all of the data that fell below the MDL was discarded, most of the data would have been thrown out, and those remaining values would have produced a completely erroneous composition of 0.275 wt% (Fig. 1). 48 �Not only should the data that fall below the MDL not be discarded, results that are below zero should also NOT be discarded. These negative values are just as important in determining the true average, as are the positive values. Each analysis has a certain error component associated with it. That error may be positive or negative. For trace element analyses, a negative error added to the real value, may result in a measured value less than zero. To get the true value, you need to average all of the data. By discarding only values on the negative side of the histogram, the data set becomes biased to a higher average value (Fig. 2). This is a big problem, because most commercial analytical instruments handle their data in this manner. Figure 1. Histogram showing multiple analyses of the NIST standard SRM-610. Two second counting times were used, resulting in a calculated MDL of 0.25 wt%, for a single analysis. The calculated average composition is 0.053 wt%, which agrees closely with the reported composition of 0.046 wt%. Figure 2. Shown is the same data set used in Figure 1, however all of the analyses that reported a negative concentration were set to zero. This is the typical procedure used by commercial analytical equipment. However, this method produces a distortion of the results. The average has been shifted from 0.053 wt% to 0.064 wt%, and this will distort any other statistics generated from the data. 49 �AN OCCURRENCE OF AGRELLITE IN THE WAUSAU ALKALINE IGNEOUS COMPLEX, MARATHON COUNTY, WISCONSIN MEDARIS, L. G. Jr and FOURNELLE, J. H., Dept. of Geology and Geophysics, Univ. of Wisconsin-Madison, Madison, WI 53706, medaris@geology.wisc.edu, johnf@geology.wisc.edu; GUGGENHEIM, S., Dept. of Earth and Environmental Sciences, Univ. of Illinois at Chicago, Chicago, IL 60607, xtal@uic.edu Agrellite is a rare, triclinic sodium-calcium fluorosilicate that occurs in alkaline (agpaitic) igneous complexes. Agrellite was first described from metamorphosed nepheline syenite in the Kipawa Complex, Quebec (Gittins et al., 1976), and its crystal structure was determined by Ghose and Wan (1979). To date, the only other reported occurrence is in the Khibina alkaline complex, Russia (Khomyakov, 1995). During construction of Interstate Highway 39 in northcentral Wisconsin, an agrellite-bearing dike was exposed in a roadcut, which was subsequently covered. The agrellite-bearing dike is located in the northern margin of Center 1 in the Wausau Complex (Fig. 1), where it cuts syenite and monzodiorite. White to light gray, tabular crystals of agrellite, ranging up to 15 mm in length, are concentrated along the center of the dike, where they are associated with quartz, perthitic alkali feldspar, plagioclase, aegirine-augite, miserite, apatite, fluorite, and locally eudialyte (Fig. 2). Fig. 1 Simplified map of igneous complexes near Wausau and the agrellite locality (modified from LaBerge and Myers, 1983) Fig. 2 Photomicrograph under crossed polarizers; a, agrellite; p, pyroxene (aegirine-augite); mf, finegrained miserite and feldspar 50 �Agrellite is biaxial, with 2VX = 55º, nX = 1.570(1), nY = 1.581(1), nZ = 1.584(1), and nZ - nX = 0.014. Cleavage is excellent on {110} and {110} and poor on {010}. The optic directions, Z and Y, are nearly parallel to the crystallographic axes, c and b, respectively, and the angle between the {110}and {110} cleavages is 40º (Fig. 3). Agrellite is triclinic, space group P1, and has the following cell parameters: a = 7.757(7) Å, b = 18.90(3) Å, c = 6.975(6) Å, α = 90.00(8)º, β = 116.81(6)º, γ = 94.4(1)º, V = 909.5(1) Å3, and Z = 4. Fig. 3 Sterographic projection of optical and crystallographic elements of agrellite The theoretical end-member formula for agrellite is NaCa2Si4O10F. Naturally occurring agrellite lies close to this composition, with the main deviations arising from the substitution of minor amounts of REE, Mn, and Sr for Ca (Table 1). Agrellite from the Wausau Complex is similar compositionally to that from the Kipawa Complex, except for having lower contents of REE and higher contents of MnO and SrO. The phase equilibrium conditions for agrellite are presently unknown, but the crystallization of agrellite, rather than wollastonite, in alkaline igneous complexes is likely promoted by high alkaline contents, i.e., (Na + K) / Al > 1, and high fluorine activity. The coexistence of agrellite with quartz in the Wausau Complex and with nepheline in the Kipawa Complex demonstrates that agrellite is stable in both silicasaturated and silica-undersaturated environments. Table 1 Composition of Agrellite SiO2 ZrO2 REE* MnO CaO SrO BaO Na2O K2O F -O=F Total 1 59.7 na 1.18 0.79 24.8 2.91 0.15 7.39 0.07 4.55 101.5 1.9 99.6 2 58.8 0.18 3.84 0.25 25.7 0.16 0.06 7.90 0.22 4.45 101.6 1.9 99.7 3 60.9 28.4 7.86 4.82 102.03 2.03 100.00 na, not analyzed 1 Wausau Complex, EMP analysis, UW-Madison 2 Kipawa Complex, Gittins et al., 1976 3 Theoretical end-member composition * Total of REE oxides References Ghose, S.G. and Wan, C (1979) Amer. Mineral. 64, 563-572. Gittins, J. et al. (1976) Canadian Mineral. 14, 120-126. Khomyakov, A.P. (1995) Mineralogy of Hyperagpaitic Alkaline Rocks: Oxford, 223 pp. LaBerge, G.L. and Myers, P.E. (1983) Wisc. Geol. Nat. Hist. Survey, Inf. Circ. 45, 88 pp. 51 �Metamorphosed, halite-dominated evaporites of the Lower Sibley Group Metsaranta, R.T.1, Fralick, P.W.2 and Bowdidge, C.1 1 RPT Uranium Corp. 537 Hilldale Rd., Thunder Bay, Ontario, P7B5N1, Canada 2 Department of Geology, Lakehead University, 955 Oliver Rd., Thunder Bay, Ontario, P7B 5E1, Canada. This paper describes newly discovered sedimentary halite deposits of Mesoproterozoic age from the Lower Sibley Group. The deposits were intersected in two closely spaced diamond drill holes, BSW-06-1a and BSW-06-4, drilled by RPT Uranium Corporation during the summer of 2006. The drill holes are located in the central portion of the preserved Sibley Basin, immediately west of the Black Sturgeon Fault, a major structure controlling the outcrop pattern of the Sibley Group. In the area, the Sibley Group is intruded by two thick Nipigon diabase sills that have produced significant contact metamophic effects. The evaporites occur in an approximately 50m thick section of Sibley Group rocks preserved between the lower diabase sill and Archean basement rocks. The basic stratigraphy beneath the lower diabase sill consists of from, bottom to top: a lower, well-cemented, sandstone dominated unit 14m thick, an interbedded halitemudstone unit 5m thick, an upper almost totally unconsolidated, partially halite cemented sandstone unit approximately 10-15m thick and an upper unit of interbedded calcareous mudstones and siltstones 20m thick. Lithostratigraphically, the lower sandstone unit can be correlated with the Pass Lake Formation while calcareous mudstones and siltstones in the upper portion of the section likely correlate with the lower Rossport Formation. Halite occurs in several forms within the section. First, it occurs as relatively large angular nodules within sandstones in the top few metres of the lower sandstone dominated interval (Fig. 1a). Second, it occurs as massive relatively pure beds 1-25cm in thickness within the interbedded halite-mudstone interval (Fig. 1b and c). Third, it occurs in irregular mixtures with clastic material, where clastic material is found as inclusions within halite and/or halite occurs in nodular form within the clastic sediment (Fig. 1.d). Finally, it occurs as cement and isolated nodular material in clastic sediments of the upper unit (Fig. 1e and f). Mineralogically, the composition of the evaporites is dominated by halite. Based on xray diffraction and reflectance spectroscopy gypsum is entirely absent from salt samples analyzed but there may be trace sylvite in some cases. Interbedded clastic materials commonly consist of mineral assemblages containing tremolite-actinolite, calcite, phlogopite, and serpentine group minerals consistent with contact metamorphism of carbonate-rich lithologies. Some samples contain clay minerals vermiculite, saponite and allophane which likely resulted from post metamorphic alteration of the contact metamorphic mineral assemblage. Sedimentologic and geochemical analysis of these unique deposits is currently underway. 52 �A B C D E F Figure 1. Various halite facies in the BSW-06-01a and BSW-06-4. A) Angular halite nodules within quartz-rich sandstone. B) Interbedded halite and mudstone, showing cyclically interbedded nature of halite beds (dark) with mudstone (light). C) Detail of a relatively pure, massive halite bed. D) Irregularly interbedded clastic material and halite. E) Very poorly consolidated halite-cemented sandstone. F) Photomicrograph of a halite cemented quartz sandstone. 53 �A CHEMICAL AND SR ISOTOPIC STUDY OF THE PIGEON POINT SILL, COOK COUNTY MINNESOTA M.G. Mudrey, Jr., 106 Ravine Road, Mount Horeb, WI 53572, mgmudrey@mhtc.net Isotopic and trace element by J.L. Wooden, then of LOCKHEED, NASA-Johnson Space Center, Houston, TX) In 1980, Wooden and Mudrey presented some isotopic and trace element data for the Keweenawan troctolite sill on Pigeon Point, Minnesota, the type locality of the mineral pigeonite. Wooden’s data were collected at the Johnson Space Center, Houston Texas, under exceedingly controlled laboratory conditions, comparable to lunar specimens at that time. The data have never been published, and interpretation has been limited because of other obligations of Mudrey and Wooden. Major elements were analyzed by a combination of colormetric, gravimetric and other classical techniques by S.S. Goldich and colleagues at Northern Illinois University and Pennsylvania State University. The Pigeon Point sill is an approximately 120 meter thick. differentiated troctolite to olivine gabbro intrusion of Keweenawan age situated on the Pigeon River between Minnesota and Ontario. The sill is of petrologic interest because of the association of three rock types troctonite/olivine gabbro, a ferro granodiorite, and a granophyre, whose genetic relationships have been debated for many years. Processes that have been invoked to explain the basaltic magma composition and significant abundance of granophyre (30 percent of sill thickness) on Pigeon Point include: Crystal Fractionation (Grout,1928); Magma Mixing Assimilation(Daly, 1917); Partial Melting (Bayley, 1893); Hydrothermal Alteration (Bastin, 1938); and Assimilation-Differentiation (Mudrey, 1973). The basal, chilled troctolite (PP-219-3) is a high alumina olivine tholeiite with a light enriched REE pattern (normalized La=19, Yb=6.8) having a positive Eu anomaly (Eu/*Eu=1.22), and for many years was viewed as a possibility as a parental magma for some parts of the Keweenawan. While this sample may approximate a parental liquid composition, an origin involving plagioclase and olivine accumulation is also possible. Rb-Sr isotopic data for 5 olivine gabbro and ferrogranodiorite samples form an approximate 1.429 Ga (initial 87Sr/86Sr = 0.70417). isochron. This "age" is similar to Rb-Sr ages determined for the RoveA Formation that the Pigeon Point sill intrudes. Compositional data for the sill suggests that selective contamination by alkalies and radiogenic Sr from the Rove Formation has occurred. The Sr isotopic data of the granophyre does not follow the pattern of the other samples, and its Rb/Sr ratio of 1.64 allow model ages of 1.11 to 1.15 Ga (probably representative of the crystal1ization age) to be calculated when the other sill samples are used for initial ratio control (I=0.7076 - 0.7046). (Best model age 1.132 Ga, 87Sr/86Sr = 0.70549). Petrologic, and trace element data argue for magma ascent from the upper mantle through Archean basement with low degrees of partial melting of potassic/alkaline constituents adjacent to the conduit followed by differentiation at an intermediate reservoir with ascent and emplacement at Pigeon Point at shallow depths (vaiolitic cavities, 1000 m). Local partial melting 54 �of overlying Rove Formation and stoping of quartzite within the Rove Formation account for the significant volume of granophyric rock. Bastin, E.S., 1938, Hydrothermal alteration in rocks of Pigeon Point, Minnesota, Journal of Geology, v. 46, p. 1058-1072. Bayley, W.S. 1893, The eruptive and s3edimentary rocks on Pigeon Point, Minnesota and their contact phenomena: U.S. Geological Survey Bulletin 109, 121 p. Daly, R.A., 1917, The geology of Pigeon Point, Minnesota: American Journal of Science, Series 4, v. 43, p. 423-448. Grout, F.F., 1928, Anorthosites and granite as differentials of a diabase sill on Pigeon Point, Minnesota, Geological Society of America Bulletin v. 39, p. 555-577. Mudrey, M.G., Jr., 1973, Structure and petrology of the sill on Pigeon Point, Cook County, Minnesota: University of Minnesota PhD dissertation, 310 p. Wooden, J.L., and Mudrey, M.G., 1980, A Chemical and Sr Isotopic Study of the Pigeon Point Sill, Cook County Minnesota (abs): EOS (Transactions AGU) v. 61, no 48 p. 1193. 55 �INVESTIGATIONS OF SULFIDE MINERALS LEACHED IN THE PRESENCE OF ALKALINE SOLIDS Nicholas, S.L. and Wirth, K.R., Geology Dept., Macalester College, St. Paul, MN 55105 Engstrom, J. and Lapakko, K.A., MN-DNR, Div. of Lands and Minerals, St. Paul, MN 55155 The oxidation of sulfide minerals present in mine waste exposed to the atmosphere can lead to the generation of acidic drainage. We used scanning electron microscopy and energy dispersive spectrometry (SEM-EDS) to examine unreacted rock and eight remnant solids. The solids were leached for periods of 117 to 899 weeks in laboratory experiments conducted by the Minnesota Department of Natural Resources (MN-DNR) on finely-ground (<0.149 mm) Duluth Complex rock (75 g) (Lapakko et al. 1997; 2000). This rock, which contains approximately four percent pyrrhotite (Fe1-xS) and lesser amounts of chalcopyrite-cubanite, pyrite, and pentlandite, was mixed with different amounts of rotary kiln (RK) fines or limestone. The rock samples were rinsed weekly with 200 mL of distilled deionized water, and the resultant drainage was analyzed regularly to assess acid-producing and acid-neutralizing reactions of each mixture and the controls. The effects of chemical reactions on water quality were documented and evaluated in detail, but little was known about the effects on the solidphase sulfide minerals. A working hypothesis was that oxidation of sulfide minerals occurred from the outside of the grain inward, leaving a sulfur-depleted rind around the grain. This oxidation rind might retard diffusion of sulfur from the center of the grain into solution. We identified the presence or absence of residual alkaline solids using a binocular microscope. SEM was used to describe the degree of sulfide oxidation, using the relative proportions of sulfur and oxygen determined by EDS. The extent of oxidation was described as the fraction of total analyzed sulfide area that was oxidized to a specific degree. Limestone grains persist in the limestone-loaded reactors, whereas no alkaline grains were detected in the reactors loaded with RK fines. Oxidation of sulfide grains begins along the edges, cracks, and partings of grains, eventually forming irregular rinds of sulfur-bearing iron oxide (Figures 1 and 2). Less of the pyrrhotite area was oxidized in samples with an alkaline addition than in the control (Figure 3), and reactors with limestone additions had smaller extents of oxidized areas than did the reactors loaded with RK fines. Comparison of the extent of pyrrhotite oxidation in two reactors with the same loading that were leached for different lengths of time (Figure 3, reactors loaded with 4.7g RK fines) indicates that the rate of pyrrhotite oxidation is greater under acidic conditions, a result that is consistent with leachate water chemistry. The four treated reactors that generated no acidic drainage were among the five least oxidized. References Lapakko, K.A., D.A. Antonson, and J. Wagner 1997. Mixing of limestone with finely-crushed acid-producing rock. Proc. 4th Int. Conf. on Acid Rock Drainage, Vol. 3, Vancouver, B.C., p. 1345-1360. Lapakko, K.A., D.A. Antonson, and J. Wagner 2000. Mixing of rotary-kiln fines with finegrained acid-producing rock. Proc. 5th Int. Conf. on Acid Rock Drainage. SME, Littleton, CO. p. 901-910. 56 �Figure 1: Backscattered electron (BSE) image of pyrrhotite from reactor (0.8 g RK fines addition) showing complex oxidation (darker regions) along grain edge and fractures (scale = 50 µm). Figure 2: BSE image of pyrrhotite with a rind of sulfur-bearing iron oxide from reactor with 7.9 g limestone addition. Shattering of grain occurred during sample preparation (scale = 50 µm). Figure 3: Extent of pyrrhotite oxidation as a function of reactor conditions and water quality data. The degree of oxidation increases from unoxidized pyrrhotite (unleached original) to sulfur-bearing iron oxide (>95% of control reactor). Weeks pH<6 is the number of weeks of leaching under acidic (pH<6) conditions. Alkaline Addition (g) is the total amount of Rotary Kiln Fines or limestone added to a reactor. Total S Leached (%) is the total sulfur measured in the reactor leachate expressed as a percentage of the initial sulfur present in the solid sample prior to leaching. 57 �DOCUMENTING UNDERGROUND MINE WORKINGS ON THE MESABI IRON RANGE IN GIS FORMAT ORESKOVICH, JULIE A., Natural Resources Research Institute, Duluth, MN 55811 (joreskov@nrri.umn.edu) Seventy years of mining natural iron ore by underground methods on the Mesabi Iron Range of northeastern Minnesota has resulted in an extensive network of subsurface voids along much of the Range. Subsidence resulting from these voids is manifested today as sinkhole depressions, pools, ponds, lakes and, most notably, road and structure damage. These voids impact the area hydrology and pose unknown risks to existing and potential surface developments, as well as to present-day mining operations. Because towns grew up adjacent to the early mines, today’s Range cities are often located, at least in part, on top of the Biwabik Iron Formation (BIF). Underground workings faded from the collective memory as generations passed and open pit mining became the norm, with an occasional jolt to the public consciousness caused by a dropped highway lane, a shifted building, or a newly opened hole in the ground. However, with prospects for several major developments on the Range, along with an expected population increase, planning groups such as the Central Iron Range Initiative (CIRI) have recognized the need to factor the underground mine workings into planning and design work in the region. The Central Iron Range Sanitary Sewer District (CIRSSD) has secured funding for the Minnesota Department of Natural Resources (MN DNR) to research the underground mines of the central Mesabi Range. CIRSSD’s study area extends from the Itasca county line on the west (T 57 N, R 21 W) to midway between the cities of Buhl and Mountain Iron on the east (T 58 N, R 19 W), following the trend of the Biwabik Iron Formation. Nearly 150 natural ore mines (underground, open pit, or both) operated in this area. Mining of natural iron ore on the Mesabi Range began with underground workings in the early 1890’s. The last of the underground mines, the Godfrey Mine, closed in 1963. The history of each mine in the study area can be traced through mining directories dating back to 1922. Data, including land parcels, operators, dates of operation, type of operations, fee interests, and lessees, is captured and entered into an Access database. Mine maps are collected from numerous sources, including mining companies, fee representatives, the MN Department of Revenue, the MN DNR map collection, and the archives of the Iron Range Resource Center (IRRC) located at Ironworld in Chisholm, MN. These maps are digitally scanned, and then rectified and digitized in ArcGIS. A unique USX aperture card collection housed at the IRRC contains annual level maps for Oliver Iron Mining Company operations dating back to 1903, enabling 3-D renderings of these mines. The end product of this research will be a GIS database that will be made accessible to counties, local communities, development organizations and mines for planning purposes. 58 �Rare Earth Element Patterns in Steep Rock Carbonates Noah Planavsky: Rosenstiel School of Marine and Atmospheric Sciences, 4600 Rickenbacker Causeway Miami FL 33149. nplanavsky@rsmas.miami.edu. Jennifer Murphy: Eastern Mineral Resources, USGS. Reston, VA 20194. jmurphy@usgs.gov One of the most dramatic changes in Earth’s history is the shift from a reducing to an oxidizing atmosphere. Multiple lines of evidence suggest the atmosphere first became oxygenated around 2.4 billion years ago. The evolution of oxygenic photosynthesis is ultimately responsible for the Earth’s oxygenation. The timing of the emergence of the earliest oxygenic photosynthetic organisms, likely cyanobacteria, is typically thought to predate the rise the atmospheric oxygen. Thus there exists the possibility for redox disequilibrium between ocean atmospheric systems; local areas of high photosynthetic productivity could induce local oxidizing conditions under an overall reducing atmosphere. Rare earth element (REE) trends can be used to explore paleoredox conditions. Modern oxygenated oceans display a strong negative cerium anomaly when normalized to shale composites, reprehensive of bulk crustal values, (Byrne and Sholkovitz, 1996) while anoxic or microaerophilic conditions display a positive or no cerium anomaly (shale normalized). The negative cerium anomaly is largely due to the oxidation of Ce3+ to Ce4+ and its preferential removal from the water column onto Mn-Fe oxide particles or as precipitate. In suboxic and anoxic waters Ce anomalies are smaller due to reductive dissolution and redox cycling of settling Mn- and Fe-rich particles within the suboxic and anoxic zones, respectively. Microbial induced carbonate precipitates and carbonate cements have been shown to faithfully record seawater REE values in modern reefs and can be used as a paleoredox proxy (Kamber and Webb, 2001). REE provide a unique tool at basin scale redox cycling in ancient environments. The approximately 2.9 Ga Steep Rock Group is one of the oldest extensive carbonate successions with a low metamorphic grade and a high state of preservation. The Steep Rock Group, therefore, is an ideal place to explore the redox state of mid-Archean marine environments. Stromatolites and abiogenic carbonate cements from various depositional settings do not contain negative cerium anomalies. The trace element systematics, traditional stable isotopes, and carbonate textures indicate exceptional preservation and rule out the possibility of a diagenetic overprint. Since Ce is preferentially released during anoxic diagenesis (Byrne and Sholkovitz, 1996), the cerium anomalies observed in the siliceous stromatolites could not have formed during early anoxic diagenesis. Thus despite apparently high biological productivity, noted by the abundance of stromatolites and organic matter, the Steep basin was anoxic or suboxic. The lack of negative cerium anomalies in previously investigated early Archean stromatolites has been used to argue for absence of cyanobacteria (e.g., Tice and Lowe, 2005). Basic modeling of reductant and oxidant fluxes at the basin scale, rather than the more commonly modeled global 59 �scale, provides atmospheric scenarios in which oxygenic photosynthesizing communities could exist in reducing conditions. References: Byrne R. H. and Sholkovitz E. R. 1996. Marine chemistry and geochemistry of the lanthanides. In Handbook on the Physics and Chemistry of Rare Earths (eds. K. A. Gschneidner, Jr. and L. Eyring), Vol. 23, pp. 497–593. Elsevier, Amsterdam. Kamber, B.S. and Webb, G.E. 2001. The geochemistry of late Archaean microbial carbonate: Implications for ocean chemistry and continental erosion history. Geochimica et Cosmochimica Acta. 65: 2509-2525. Tice, M.M. and Lowe, D.R. 2006. Hydrogen-based carbon fixation in the earliest known photosynthetic organisms: Geology, 34: 37-40. 60 �New Insights into the Metallogeny of the Eastern Portion of the Archean Uchi Domain, Superior Province, Ontario PUUMALA, Mark A., Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James Street South, Thunder Bay, ON P7E 6S7 Canada In 2005, the Ontario Geological Survey began the Far North Geological Mapping Initiative (FNGMI). The goal of this program is to obtain a better understanding of the geological history and mineral resource potential of the portions of Ontario that are located north of 51ºN latitude. The Far North is a remote area that has a much smaller historical geological database than the southern portions of Ontario. Nevertheless, this region does have a long history of mineral exploration that has resulted in the collection of much valuable information about the nature of economic mineralization. These data are currently being compiled to evaluate metallogenic patterns that can in turn be used to better understand the regional tectonic framework and economic mineral distribution. Figure 1. Map illustrating Far North Mineral Deposit Compilation study area. To date, geological data have been compiled for mineral occurrences located within the eastern portions of the Archean Uchi Domain of the Sachigo Superterrane (Stott and Rainsford, 2006), of the Superior Province (Figure 1). The majority of the mineral deposits in this area are located within or immediately adjacent to supracrustal rocks of the Miminiska-Fort Hope, Pickle Lake, Lake St. Joseph, Meen-Dempster and Lang Lake greenstone belts of Stott and Corfu (1991). A wide variety of mineral deposit types have been identified in the study area, including: vein and replacement gold; polymetallic vein; mafic to ultramafic intrusion-hosted coppernickel-platinum group elements (PGE); rare-element-bearing pegmatite; volcanogenic massive sulphide (VMS) copper-zinc; intrusive porphyry-related copper-molybdenum-gold; and Algomatype banded iron formation. Although the individual greenstone belts listed above are useful geographic subdivisions, it is perhaps more useful to ascribe styles of mineralization to individual assemblages that were deposited during a discrete interval of time in a common depositional or tectonic setting (Stott 61 �and Rainsford 2006). The relationships between mineral occurrences and the individual tectonic assemblages have been used to obtain information regarding the possible locations and timing of significant metallogenic events in the study area. Preliminary observations suggest the following sequence of metallogenic events: 1. Mafic to ultramafic intrusion-hosted Cu-Ni-PGE mineralization in rocks of the >2860 Ma Pickle Crow assemblage in the Pickle Lake and Miminiska-Fort Hope belts; 2. VMS-type mineralization associated with rocks of the 2825-2842 Ma Kaminiskag (former Woman) Assemblage in the Pickle Lake and Meen-Dempster belts; 3. Later VMS-type mineralization associated with rocks of the <2744 Ma Confederation Assemblage in the Lang Lake, Meen-Dempster, and Pickle Lake greenstone belts, and with rocks of the <2723 Ma St. Joseph Assemblage in the Miminiska-Fort Hope Greenstone Belt; 4. Copper-nickel-PGE mineralization associated with late-tectonic mafic (commonly anorthositic) intrusions located near the southern boundary of the Miminiska-Fort Hope Greenstone Belt, and in the Lang Lake Greenstone Belt near the Bear Head Fault Zone; and 5. Structurally controlled gold mineralization that was likely to have been associated with the 2.72-2.70 Ma collision between the Uchi Domain and the Winnipeg River Terrane (Percival et al. 2006). In the Pickle Lake Greenstone Belt, this event post-dates Confederation Assemblage volcanism (>2739 Ma) and pre-dates the emplacement of the post-tectonic 2697-2716 Ma Hooker-Burkoski stock (Young et al. 2006). The metallogeny of other deposit types in the study area is currently more difficult to interpret due to the small number of known occurrences and/or a lack of geological data. However, as additional information is collected through FNGMI mapping projects, it is anticipated that the tectonic events associated with porphyry copper mineralization observed in the Lang Lake Belt and polymetallic veins found in the North Bamaji Lake area, for example, can be determined. References Percival, J.A., Sanborn-Barrie, M., Skulski, T., Stott, G.M., Helmstaedt, H., and White, D.J. 2006. Tectonic evolution of the western Superior Province from NATMAP and Lithoprobe Studies; Canadian Journal of Earth Sciences, Vol. 43, p. 1085-1117. Stott, G.M. and Corfu, F. 1991. Uchi Subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p. 145-236. Stott, G.M. and Rainsford, D.R.B. 2006. The Precambrian geology underlying the James Bay and Hudson Bay lowlands as interpreted from aeromagnetic data and a revised terrane map for Northwestern Ontario; in Summary of Field Work and Other Activities 2006, Ontario Geological Survey, open File Report 6192, p. 13-1 to 13-10. Young, M.D., McNicoll, V., Helmstaedt, H., Skulski, T., and Percival, J.A. 2006. Pickle Lake revisited: New structural, geochronological and geochemical constraints on greenstone belt assembly, western Superior Province, Canada; Canadian Journal of Earth Sciences, Vol. 43, p. 821-857. 62 �MICHIGAN KIMBERLITES REVISITED: NEW MINERAL, CHEMICAL AND PETROGRAPHIC ANALYSES Quigley, P. O., Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55414, quig0026@umn.edu Over twenty kimberlite pipes have been reported in the Western Upper Peninsula of Michigan (Jarvis, 1993). Most of the known activity in assessing these pipes took place during the 1980’s, and therefore did not include classification according to currently available mineral chemical and petrographic methods. Drill core samples held by the Michigan DNR from several pipes across the Michigan kimberlite field therefore are presently being analyzed in order to provide a further assessment of diamond grade prospects based on mineral chemistry, as well as an updated petrographic classification. Data collection consisting of electron microprobe analyses of garnet, ilmenite, and pyroxene from heavy mineral concentrates have been completed for three of the kimberlites, and have shown a high degree of variability. Two of the intrusions appear to have sampled a portion of the mantle within the diamond stability field, due to the presence of harzburgitic garnets, and other favorable compositions, according to the Grütter et al. (2004) classification scheme. Overall diamond grade is, however, inferred to likely be economically insignificant as the proportion of diamond indicator garnets to overall garnets is moderate to low. Analyses currently in progress will further examine degree of variability in mineral chemical analyses among these kimberlite intrusions, as well as examination of polished thin sections to permit use of textural features to categorize the intrusions as crater, diatreme, or hypabyssal facies kimberlite. REFERENCES Grütter, S. H., Gurney, J.J., Menzies, H.A., and Winter, F., 2004. An updated classification scheme for mantle-derived garnet, for use by diamond explorers. J. Barry Hawthorne Volume Proc. 8th Int. Kimb. Conf., pp 841-857. Jarvis, William, 1993. Michigan Kimberlites: An Update. Abstract at Prospectors and Developers Association of Canada 61st Annual Meeting (Paper M-10). 63 �Elemental and Isotopic Shallow Lake Proxies of Landscape Changes in the Prairie Pothole Region of Minnesota Tommy Rodengen1, Kevin Theissen1, and Shinya Sugita2 1 Department of Geology, University of St. Thomas, 2115 Summit Ave, St. Paul, MN 55105 2 Department of Ecology, Evolution, and Behavior, University of Minnesota, 1987 Upper Buford Circle, St. Paul, MN 55108 Investigations of the manipulation of the landscape around lakes have shown to have profound impact on their sedimentary records (1). We have explored lake sediment cores with records spanning the last two centuries and correlated changes in proxy data to large shifts in landscapes around lakes in Western Minnesota. In February 2006 we collected sediment cores from Lee, Bore, 8-Mile and Sweet lakes in the Prairie Pothole region of Minnesota. Cores were sampled for organic matter (SOM) at 1 cm intervals, dried, ground, weighed and treated with sulfurous acid (2). These samples were sent to the Stanford University Stable Isotope laboratory and analyzed for elemental (TOC, TN, C/N) and stable isotopic (δ13C and δ15N) values. Lastly, pollen was extracted from sediments using standard palynological methods (3). Over the past 6 months I have analyzed a set of aerial photographs spanning 1938-present for the region of interest. δ13C (-29 to -22 per mil) and C/N (5-17) values reflect a dominant algal source in the sediments but also show times of mixing and terrestrial sourcing across the lake core record (4). These values suggest changes in productivity and/or changes in sourcing organic matter. We used sharp increases in the abundance of ragweed (ambrosia) as a marker of the cultural horizon (~1850 A.D.) (5). We believe the sharp changes in N isotopes indicate land use change by humans and the addition of fertilizers. This is illustrated by a large δ15N spike between 39cm and 21cm depth (Figure 1), which if you assume a linear rate of sedimentation (.224 cm/yr) is between ~1832 A.D. and ~1912 A.D. According to our earliest records of aerial photography these years were a time of extreme drought and farming of fertile lake soil was common, directly introducing high levels of nitrogen to the sedimentological record. The importance of human manipulation of the landscape, specifically synthetic fertilizer, appears to have a clear effect on the sedimentological record found in these lakes. References: (1) Lamb, H.F., Damblon, R.W., Maxted, R.W., 1991, Human Impact on the Vegetation of the Middle Atlas, Morocco, During the Last 5000 Years. J. Biogeography, 18, 1-14. (2) Verardo, D.J., Froelich, P.N., McIntyre, A., 1990, Determination of organic carbon and nitrogen in marine sediments using Carlo-Erba NA1500. Deep-Sea Res., 37, 157165. (3) Faegri, K. Iversen, J., 1989. Textbook of Pollen Analysis. 4th ed. J. Wiley & Sons, Chichester. 328pp. 64 �(4) Meyers, P.A., and Lallier-Verges, E., 1999, Lacustrine sedimentary or Late Quaternanry paleoclimates. J. Paleolimnology, 21, 345-372. (5) Jacobson, G.L. and Grimm, E.C., 1986. A numerical analysis of Holocene forest and prairie vegetation in central Minnesota. Ecology, 67, 958-966. Figure 1: 8-Mile lake δ15N values 8-Mile Lake 0 10 Depth (cm) 20 30 40 50 0.00 60 1.00 2.00 3.00 4.00 5.00 6.00 δ N 15 65 7.00 8.00 9.00 10.00 �BEDROCK FRACTURES IN SOUTHEASTERN WISCONSIN: PALEOSTRESS ESTIMATES AND RELATIONSHIPS TO THE WAUKESHA FAULT RYMASZEWSKI, Jody A., FRIEDRICH, Jason L., and CZECK, Dyanna M. Department of Geosciences, University of Wisconsin – Milwaukee, P.O. Box 413, Milwaukee, WI 53201 jody@uwm.edu INTRODUCTION The Waukesha Fault is an enigmatic structure cutting through Silurian dolomite in southeastern Wisconsin. It is a normal fault, oriented ~N40E, 60SE with an apparent offset of 10 m (Mikulic and Mikulic, 1977; Sverdrup et al., 1997). It is only known to outcrop at the Waukesha Lime and Stone Co. quarry in Waukesha, Wisconsin. Detailed gravity surveys in the region reveal the lateral and vertical extent of the fault (Sverdrup et al., 1997; Skalbeck et al., 2006). The fault trace extends NE to the town of Port Washington, Wisconsin on the shore of Lake Michigan (Sverdrup et al., 1997) and can be traced to depths greater than 600m (Skalbeck et al., 2006), showing that it is a major feature of an otherwise undeformed region. The goals of our study are to 1) estimate the paleostress orientations that formed the Waukesha Fault, 2) measure the orientations of nearby small-scale fractures, 3) estimate the paleostress orientations that formed the small-scale fractures, and 4) compare the paleostress orientations for all the structures. METHODOLOGY The one known exposure of the Waukesha Fault is currently not available for active study. Therefore, we conducted paleostress orientation estimates on the Waukesha Fault, assuming its geometry matched that predicted by Sverdrup et al., 1997. We measured the orientations of 158 fractures in Silurian bedrock at the Lannon Stone Products quarry (located in Lannon, Wisconsin: ~11km NE of the known Waukesha Fault outcrop and ~3 km NW of the Waukesha Fault trace) during field seasons from 2004-2006. We also measured the orientations of 38 fractures in Devonian bedrock at the Harrington Beach State Park abandoned quarry and beach outcrops (located ~ 10km NE of Port Washington, and ~2.5 km NW of the Waukesha Fault trace). We used standard stereonet procedures to calculate the orientations of the paleostresses likely to have formed the Waukesha fault and the fractures at each of the field locations. RESULTS Based on the assumed geometry of the Waukesha Fault, σ1, the maximum principal stress was oriented vertically, σ2, the intermediate principal stress, was oriented (plunge/trend) 0°/040 σ3, the minimum principal stress, was oriented 0°/130. The Lannon quarry contains fractures with no discernable offset, and faults with small-scale (maximum ~4 cm, mostly normal sense) offsets. Some rare fractures have preserved ridge and groove lineations or plumose features, allowing classification as shear fractures or extensional fractures, respectively. The fractures at the top of the quarry have a random distribution. The small faults found at the low-mid levels of the quarry exhibit an approximate N32E/56SE 66 �orientation. Most of the fractures with no apparent offset at mid and lower levels have the approximate orientation N30E/65S; a second group has approximate orientation of N45W/ steepsubvertical. Cross-cutting relationships between the two sets are inconclusive. Two prominent fracture sets formed at Harrington Beach State Park. The approximate orientations of these two sets are 1) N74E/subvertical and 2) N25W/ subvertical. Unfortunately, the textures of most fracture surfaces are too weathered to determine whether the fractures are extensional or shear fractures. Thus, the paleostress estimates must be partly based on the relative geometries. The small faults at the Lannon quarry have estimated paleostress orientations: σ1 = vertical; σ2 = 0°/040, σ3 = 0°/130. Most of the fractures at the Lannon quarry have a similar orientation (but often with slightly steeper dips) to the small faults. Therefore, we interpret that most of these fractures are small extensional or shear fractures with the same paleostress orientations as the small faults. The second group of fractures at the Lannon quarry could either be extensional or shear fractures, but lack of any offset supports the likelihood that they are extensional fractures. If so, the paleostress orientations are σ1 = vertical; σ2 = 0°/315, σ3 = 0°/225. The fractures at Harrington Beach could either be two distinct sets of extensional fractures or a conjugate set of shear fractures. If they are extensional fractures, the paleostress orientations are 1) σ1 = vertical; σ2 = 0°/074, σ3 = 0°/164 and 2) σ1 = vertical; σ2 = 0°/335, σ3 = 0°/245. If they are shear fractures, the paleostress orientations are σ1 = 0°/114 σ2 = vertical, σ3 = 0°/024. CONCLUSIONS While the paleostresses on all fractures could not be conclusively determined, the paleostress orientations inferred from most of the geometries of the small-scale features in the Silurian bedrock at Lannon are consistent with those estimated for the Waukesha Fault. However, the paleostress orientations of the Devonian rocks at Harrington Beach are inconsistent with those estimated for the Waukesha Fault. Therefore, it seems likely that 1) the timing of the Waukesha Fault may be bracketed by the deposition of the Silurian rocks at Lannon and the Devonian rocks at Harrington Beach, or 2) the deformation associated with the Waukesha Fault encompassed a broader region in the south. Further study of fractures in the region is required to test these hypotheses. REFERENCES Mikulic, D.G., Mikulic, J.L., 1977. History of geologic work in the Silurian and Devonian of southeastern Wisconsin: Guidebook 41st annual tri-state field conference, A19-A27. Skalbeck, J.D., Couch, J.N., Helgesen, R.S., Swosinski, D.S., 2006. Coupled modeling of gravity and aeromagnetic data to estimate subsurface basement topography in southeastern Wisconsin. Geoscience Wisconsin 17, 53-64. Sverdrup, K.A., Kean, W.F., Herb, S., Brukardt, S.A., Friedel, R.J., 1997. Gravity signature of the Waukesha Fault, southeastern Wisconsin. Geoscience Wisconsin 16, 47-54. 67 �Precambrian Geology of the Opelt Quarry, Neillsville, Wisconsin SAXTON, Samantha and CORDUA, William, Department of Plant and Earth Science, University of Wisconsin- River Falls, River Falls, WI 54022, samantha.saxton@uwrf.edu, william.s.cordua@uwrf.edu Near Neillsville, Clark County, Wisconsin, a quarry owned and operated by Opelt Sand and Gravel exposes a complex group of rocks dominated by amphibolites but including biotite gneisses, granitic to dioritic plutons, intrusive breccias and chlorite-rich shear zones. Given the quarry's location south of the Eau Plaine shear zone, it gives an important fresh exposure of the rocks of the Marshfield Terrain in central Wisconsin. We did field reconnaissance at the quarry combined with thin section, geochemical and structural analyses to study the metamorphic history. Published research for the surrounding area suggests the Opelt rocks are associated with an accreting micro-continent, which formed the foundation for later deformation during the Penokean orogeny (1880 - 1830 mya) and perhaps younger metamorphic events (Holm and Schneider, 2007). Previous work performed by Maass (1983) on rocks within 10 miles of the quarry using U-Pb zircon dating gives ages in a range of 1875 to 1820 million years old. A study by Sims and Peterman (1980) using a Rb-Sr whole-rock technique places them at roughly 1885 +/- 65 million years, correlating with the Penokean Orogeny. Thin section analysis showed the common mineral assemblage in the gneisses and amphibolites to be a combination of plagioclase, biotite, hornblende, and clinozoisite, with several minor minerals such as calcite, chlorite, and epidote among others. Within the granitic samples, quartz, plagioclase, and clinozoisite become the dominant minerals, with the same range of minor minerals. Veins of quartz and calcite are found cross-cutting the granites. Replacement textures were evident from the thin sections. Amphibolite samples display replacement rims or coronas of titanite surrounding the opaque ilmenite (Figure 1). We also see replacement relationships between several sets of minerals: primarily clinozoisite, muscovite, epidote and/or calcite replacing plagioclase feldspar, and chlorite replacing biotite. These two suites of minerals most likely represent a retrograde metamorphic event. Figure 1: Amphibolite showing titanite replacing the opaque mineral, ilmenite, seen in the middle of the rings. Geochemical analysis using discrimination diagrams derived from Pearce et al. (1982) and Gomez-Pugnaire et al. (2003) proved to be helpful in defining the protolith of the assorted rocks. A discrimination diagram for granitic rocks, based on the ratio of Rb versus Y + Nb, allocates samples to three origins- anorogenic, oceanic ridge, and island arc. Our data points, two granites and a diorite, indicate within a reasonable margin of error, a calc-alkaline island arc origin for the plutonic rocks found in the quarry. In addition, 68 �several diagrams from Pearce (1982) show the igneous rocks that were protoliths for the amphibolites to be tholeiitic volcanic or island arc basalts. Maass (1983) describes several folding events within in nearby rocks: F-1- isoclinal folds, to F-3 - very open to tight folds, as well as a mineral lineation associated with the F-3 folding. He also proposes that the plutonic activity of the area seems to have started at the latter part of the F-1 folding and continued past the end of the F-3 event. (Maass, 1983) Many of these same features can be seen in the Opelt Quarry. Field, thin section, and geochemical data suggest the following sequence of events in the Opelt rocks: 1. Amphibolite protolith = tholeiitic island arc basalts 2. Penokean Orogeny ~ Metamorphic Event #1 - Amphibolite Facies (Hb, plag) Occurrences F-1 Foliation F-2 Veins and Granitic Intrusions ~ Metamorphic Event #2 - Greenschist Facies (Chl, epi) Occurrences F-3 Foliation ~ Formation of Shear Zones ~ Veins- quartz, calcite, epidote ~ Multiple series of faulting 3. Deposition of Younger Sediments/ Later Glacial Sediments The later folding and retrograde metamorphism events could be late stage Penokean orogeny, or could be related to the Yavapai (1800-1750 mya) and Mazatzal (1650 mya) events that took place post-Penokean Orogeny (Holm and Schneider, 2007). Many structures and features originally attributed to the Penokean Orogeny now may actually be an overprint from later Yavapai and Mazatzal events (Cannon and Schulz, 2007). Radiometric work and further geochemical analysis will help resolve these questions. Cannon, W. and Schulz, K., 2007, The Penokean Orogeny in the Lake Superior Region (abst.): The Geological Society of America Abstracts with Programs, 41st Annual Meeting, Lawrence, KS, v. 39, p. 76. Gomez-Pugnaire, M.T., Azor, A., Fernandez-Soler, J.M. and V. Lopez Sanchez- Vizcaino, 2003, The amphibolites from the Ossa-Morena/Central Iberian Variscan suture (Southwestern Iberian Massif): geochemistry and tectonic interpretation: Lithos, v. 68, p. 23-42. Holm, D. and Schneider, D., 2007, Metamorphic Record of the Yavapai and Mazatzal accretion in the upper Great Lakes region, U.S. and Canada (abst.): The Geological Society of America Abstracts with Programs, 41st Annual Meeting, Lawrence, KS, v. 39, p. 76-77. Maass, R.S., 1983, Early Proterozoic tectonic style in central Wisconsin: Geological Society of America Memoir, v. 160, p. 85-95. Pearce, J.A., 1982, Trace element characteristics of lavas from destructive plate boundaries, In: Thorpe, R.S. (ed.) Andesite, p. 525-548. Sims, P.K. and Peterman, Z.E., 1983, Evolution of Penokean foldbelt, Lake Superior region, and its tectonic environment: Geological Society of America Memoir, v. 160, p. 3-14. 69 �Revised Stratigraphy of the Biwabik Iron Formation, Mesabi Range, Minnesota – Developing the “Rosetta Stone” SEVERSON, Mark, J. and HEINE, John, J., Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN 55811 The NRRI is currently evaluating the possibility of using specific waste rock products from taconite mining as aggregate materials. Paramount to defining specific horizons with good aggregate potential, a better understanding of the stratigraphy of the iron-formation has been accomplished through detailed logging of drill holes and detailed in-pit mapping (where core are unavailable). To date, 130 holes have been logged from seven areas that include: CliffsErie site (old Erie/LTV mine), Biwabik area (Mittal Steel’s planned “east reserve”), Laurentian Mine, United Taconite, Minntac, Hibtac, and the oxidized taconite of the Coleraine area. In each of these areas, the stratigraphy, as determined by bedding types in each of the holes, is compared to the stratigraphy as defined by each of the mining companies. Through this approach, a new picture of the iron-formation layering is evolving and for the first time it has become possible to correlate the mining units of one mine to those in adjacent mines - even though each of the mines uses a different stratigraphic terminology. In essence, this work has set forth the beginnings of a “Rosetta Stone” whereby ore and waste horizons can be better understood. Hopefully, the “Rosetta Stone” can eventually be used to aid in determining the depositional environments of internal units within the iron-formation. All descriptions of the Proterozoic iron-formations consistently attribute thin-bedded ironformations as reflecting deposition in a deep water environment, and granular, variably-bedded iron-formations as reflecting deposition in shallower water. Overall, the iron-formation has been divided into four members consisting of Lower and Upper Cherty members (regressive sequences), and Lower and Upper Slaty members (transgressive sequences). Using these simple concepts, coupled with other bedding types as seen in drill core, several unique relationships have been defined along the length of the Mesabi Range that include: • The Lower Cherty is the most consistent member of the iron-formation and consists of a single shallowing-upward parasequence consisting of thin-bedded iron-formation at the base grading upwards through sequences of regular-bedded, wavy-bedded, irregular-bedded, and thick-bedded granular iron-formation units. • The top of the Lower Cherty consists of an iron-poor, variably-bedded, granular chert that is currently being used as road aggregate, e.g., the “Mesabi Select” material from United Taconite’s Thunderbird North mine. This unit is inferred to have been deposited in shallow water near the edge of the shelf. • The base of the Lower Slaty is the most persistent marker bed in the iron-formation and at most locales consists of carbonaceous mudstone referred to as the “Intermediate Slate.” Tuffaceous units are reported to be present at the base of the Lower Slaty but have yet to be encountered in the drill holes looked at in this study. • For the most part, the Lower Slaty consists of a thick sequence of thin-bedded, ironcarbonate and iron-silicate iron-formation that was deposited in deep water. However, in the “east reserve” area, lenses of granular iron-formation (deposited in shallow water and similar to the “Mesabi Select” unit of the Lower Cherty) are commonly interbedded with the thin-bedded rocks of the Lower Slaty. Internal structures within these lenses 70 �• • suggest that they formed from slumpage of shelf material into the basin; possibly related to large storm events or earthquakes during periods of tectonic instability. The top of the Lower Slaty member is perhaps the most poorly defined as it is transitional into the overlying Upper Cherty member. This is most evident in the Virginia Horn area where both slaty and cherty iron-formation types are common and alternate at all scales. This has lead to stratigraphic inconsistencies wherein the actual division between the Upper Cherty and Lower Slaty members varies drastically from mine to mine. Recent stratigraphic correlations suggest that the upper portion of the Lower Slaty contains several laterally-restricted tongues/channels of Upper Cherty-type beds that have locally been referred to as “interbedded cherts” or IBCs. These IBCs appear to be related to several regressive/transgressive events and represent several small parasequences at the top of the Lower Slaty. The Upper Cherty member is profoundly different from the Lower Cherty in that it does not display a consistent stratigraphy nor is a shallowing-upward sequence evident. Furthermore, the Upper Chery is commonly “oolitic.” These relationships suggest that the Upper and Lower Cherty members were deposited in distinctly different depositional environments. While development of the “Rosetta Stone” adequately displays the various differences of the Biwabik Iron Formation along the entire length of the Mesabi Range, only continued logging of holes will aid in exhibiting the 3D nature of the iron-formation. When that point is reached, a more complete analysis of the actual depositional environments can be ascertained. 71 �Investigation of Ferromanganese Nodule Precipitation and Arsenic Uptake in Modern Lacustrine Biochemical Sediments Stevens, Larissa B. and Fralick, Philip, Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada, (larissa.stevens@lakeheadu.ca) This study was conducted to evaluate the environmental setting, geochemistry and formational processes involved in the precipitation of lacustrine ferromanganese nodules, and to investigate the mechanisms involved in arsenic uptake in these deposits. The research was conducted on precipitates in Lake Charlotte, Nova Scotia. However, the findings are applicable to the Lake Superior region as ferromanganese deposits have been found in a number of temperate North American lakes and may be present in many more. Ferromanganese deposits in the form of nodules, coated sand and layers in the sediment are present in all the Great Lakes (Sly and Thomas, 1974; Callender, 1970). Mothersill and Shegelski (1973) studied iron- and manganese-rich layers in Lake Superior near Thunder Bay. Ferromanganese concretions have also been found in inland lakes of the Lake Superior Region including: Lake Schebandowan, Ontario (Carpenter et al., 1972), Trout Lake, Wisconsin (Twenhofel et al., 1945) and the Minnesota Lakes (Zumberge, 1952). The layering styles of ferromanganese nodules can be used to infer the approximate position of the redox boundary where precipitation is occurring. The concretions found in Lake Charlotte, NS, developed on the bottom in a shallow lacustrine setting. These deposits consist of 1 cm thick crusts divisible into 1) a lower zone of iron precipitates cementing siliciclastic sands; 2) thin laterally continuous laminae of manganiferous and ferric precipitates; 3) micro-stromatolites composed of manganiferous and ferric precipitates and 4) grumulous and clotted textured manganiferous and aluminous precipitates. These deposits formed by the interaction of oxygenated lake water with diffuse reduced groundwater seeps on the sandy bottom. The development of microbial mats and presence of stromatolites and clotted precipitates indicates biochemical processes played a role in the formation of the crusts. The ferromanganese nodules are accumulating arsenic, with concentrations ranging from ~ 670 to 1400 ppm. Elevated arsenic concentrations were also found in groundwater samples suggesting that the arsenic is entering the lake from the groundwater seeps. Microbiological experiments investigating bacterial involvement in arsenic coprecipitation have found that no arsenic oxidizers were present. However, iron oxidizers are. This suggests a possible mechanism whereby: the iron oxyhydroxide precipitates were formed, at least in part, by the iron oxidizers in the microbial mats on the bottom. As the iron oxyhydroxide precipitated from the diffuse groundwater flow entering the oxygenated lake it scavenged the arsenic that was also being transported in solution. 72 �References Sly, P.G. and Thomas, R.L., 1974. Review of geological research as it relates to an understanding of Great Lakes limnology. J. Fish. Res. Bd Can., 31: 795-825. Callender E., 1970. The economic potential of ferromanganese nodules in the Great Lakes. Proc. 6th Forum Geol. Ind. Miner. Michigan Geol. Surv. Misc., 1: 55-65. Mothersill, J.S. and Shegelski, R.J., 1973. The formation of iron and manganese rich layers in the Holocene sediments of Thunder Bay, Lake Superior. Can. J. Earth Sci., 10: 571-576. Carpenter, R., Johnson, H.P. and Twiss, E.S., 1972. Thermomagnetic behavior of manganese nodules. J. Geophys. Res., 77: 7163-7174. Twenhofel, W.H., McKelvey, V.E., and Feray, D.E., 1945. Sediments of Trout Lake, Wisconsin. Bull. Geol. Soc. Am., 56: 1099-1142. Zumberg, J.H., 1952, The lakes of Minnesota, their origin and classification. Bull. Minn. Geol. Surv., 35: 1-90. A C B D A) Picture of a typical ferromanganese nodule overgrowing a cobble. The small domes are stromatolites; B) Cross section of a nodule with a stromatolitic zone at the top; C) Reflected light photomicrograph of stromatolitic layering in a nodule. The round white areas are sand grains; D) Reflected light photomicrograph of the internal layering of a nodule. 73 �A REVISED TERRANE MAP FOR THE SUPERIOR PROVINCE AS INTERPRETED FROM AEROMAGNETIC DATA STOTT, Greg, Ontario Geological Survey, Sudbury, ON, P3E 6B5, greg.stott@ontario.ca CORKERY, Tim, Manitoba Geological Survey, Winnipeg, MB LECLAIR, Alain, Géologie Québec, Québec, QC BOILY, Michel, GÉON, Montréal, QC PERCIVAL, John, Geological Survey of Canada, Ottawa, ON The Superior Province has in the past been subdivided into “Subprovinces” characterized largely by contrasting lithology (e.g., Card and Ciesielski 1986). With an increasingly refined understanding of the tectonic assembly of the Superior Province through a progression of orogenies (Stott and Corfu 1988; Percival et al. 2006), supported by geochronological and SmNd isotopic data (e.g., Tomlinson et al. 2004; Rayner and Stott 2005), we can identify individual terranes and associated domains. Recent aeromagnetic interpretation of the Precambrian crust underlying the James Bay and Hudson Bay lowlands (Stott and Rainsford 2006), permits more confident terrane correlations across James Bay. Figure 1 illustrates a new map of the Superior Province in Canada with its principal tectonic subdivisions as currently used in Manitoba, Ontario and Quebec. Archean terranes are crustal blocks with geological histories substantially different from adjacent blocks, although parts of the late (Neoarchean) history of adjacent terranes typically overlap. They include one or more tectonic assemblages and plutonic suites, and at least parts of the terrane boundaries are clearly marked by major faults. Those regions with more complex Meso- to Neoarchean orogenic histories are superterranes thought to comprise at least two terranes. Domains are marginal parts of terranes or superterranes that are thought to have been added autochthonously or for which sparse evidence exists (e.g., Marmion domain) to treat them as separate terranes. Card, K.D. and Ciesielski, A. 1986. DNAG #1. Subdivisions of the Superior Province of the Canadian Shield; Geoscience Canada, 13, 5-13. Percival, J.A., Sanborn-Barrie, M., Skulski, T., Stott, G.M., Helmstaedt, H. and White, D.J. 2006. Tectonic evolution of the western Superior Province from NATMAP and Lithoprobe studies; Can. J. Earth Sci., 43, 10851117. Rayner, N. and Stott, G.M. 2005. Discrimination of Archean domains in the Sachigo Subprovince: a progress report on the geochronology; Ontario Geological Survey, Open File Report 6172, 10-1 to 10-21. Stott, G.M. and Corfu, F. 1988. Whither the Kenoran Orogeny?; Geological Association of Canada/Mineralogical Association of Canada Annual Meeting, Program with Abstracts, 13, A120. Stott, G.M. and Rainsford, D.R.B. 2006. The Precambrian geology underlying the James Bay and Hudson Bay lowlands as interpreted from aeromagnetic data and a revised terrane map for northwestern Ontario; Ontario Geological Survey, Open File Report 6192, 13-1 to 13-10. Tomlinson, K.Y., Stott, G.M., Percival, J.A. and Stone, D. 2004. Basement terrane correlations and crustal recycling in the western Superior Province: Nd isotopic character of granitoid and felsic volcanic rocks in the Wabigoon subprovince, N. Ontario, Canada; Precambrian Research, 132, 245-274. 74 �Figure 1 (next page). This map displays a composite of an aeromagnetic base and bedrock geology of the northern portion of the Archean Superior Province across Manitoba, Ontario and Quebec with terrane and domain boundaries. Names of superterranes, terranes and domains are as currently used in each of the provinces. Some features to note: - The older, Mesoarchean North Caribou terrane forms a core flanked to the north and south by outward additions of Meso- to Neoarchean crust across Uchi and Island Lake domains, which merge eastward where the North Caribou terrane pinches out. Uchi domain and Oxford-Stull domain merge under the James Bay Lowland; - A large indentor-like feature underlies the James Bay Lowland, flanked by faults; - English River and Quetico metasedimentary domains are separated east of the Wabigoon Subprovince by a ridge of felsic plutonic rocks, with high magnetic field intensity, extending westward from the Opatica gneisses of Québec. The Opatica terrane might extend to the Winnipeg River terrane where the latter is interpreted to underlie the northern part of the eastern Wabigoon Subprovince; - The Trans-Hudson Orogen underlies the northern half of the Hudson Bay Lowland; - Apparent reworked Archean Northern Superior superterrane crust under Hudson Bay Lowland within the Trans-Hudson Orogen; - Broad areas of pronounced aeromagnetic field intensity characterize large parts of the Northern Superior superterrane under the Hudson Bay Lowland in Ontario and correspondingly resemble the Pikwitonei gneisses, including the Assean and Split Lake blocks on strike in Manitoba. - Potential correlation of Northern Superior superterrane with the 3.8 Ga Nuvvuagittuq greenstone belt (David et al. 2002) in Tikkerutuk domain and the >3.5 Ga Assean gneisses (Böhm et al. 2000). - Areas interpreted to be part of the Paleoproterozoic Sutton Inlier are outlined where exposed. The Sutton “Inlier”, of shallow water platform sedimentary units and overlying gabbro sill, does not appear to be one large area but a set of inliers and should in future be referred to as the Sutton Inliers. The eastern Sutton Inliers with subsurface extensions are shallowly dipping, northward concave and crescent-shaped, based on aeromagnetic patterns, which correspond closely with the distribution of outcrops observed by Bostock (1970). They resemble “klippe” that might have been transported a short distance southwards. Folded strata related to the Sutton Inliers are shown aeromagnetically to extend discontinuously northwards towards the Hudson Bay coast upon apparently reworked Archean crust within the Trans-Hudson Orogen. The TransHudson Orogen, including areas of reworked Archean crust, appears to underlie the northern half of the Hudson Bay Lowland, based on interpretation of aeromagnetic images. 75 �76 Figure 1 Ibis map tJisiavs tha tectonic subdivision o!Lbe Archean Superior Province across Ntarutotm. Ontario and Quebec on a composite aeromaurtetic and hcdrocL aeolog\ hase Names arc as currently used or proposed In each of the pio inces- The northerriniost edge ol the Superior Ioeince in Maiinotxs and Ontario is locaIk o erprintcd hs the Paleoprocerozoic Trans-I [udson Orogeir �THERMAL EVOLUTION OF PROTEROZOIC (>1Ga) RHYOLITE MAGMA BASED ON ANALYSIS OF MELT INCLUSIONS AND TRACE ELEMENTS IN QUARTZ FROM THE KEWEENAW PENINSULA OF MICHIGAN STUDENT, James J., Central Michigan University, 314 Brooks Hall, Mount Pleasant, MI 48859, stude1jj@cmich.edu, WARK, D.A., Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180, MUTCHLER, S.R., Department of Geological Sciences, Virginia Tech, Blacksburg, VA 24061, and Bodnar, R.J., Department of Geological Sciences, Virginia Tech, Blacksburg, VA 24061. Melt inclusions (MI) form when small droplets (less than a few hundred microns in diameter) of magma become trapped inside growing crystals in magma. MI provide the best source of information concerning the chemical and physical evolution of a magma chamber. Glassbearing MI are common in quartz and zircon from porphyritic rhyolite occurrences in the Midcontinent Rift System, including rhyolite flows in the North Shore Volcanic Group, the Porcupine Volcanics, and the Portage Lake Volcanics on the Keweenaw Peninsula of Michigan. Porphyritic rhyolite cobbles are abundant in the Allouez conglomerate within the Portage Lake Volcanics, and the characteristics and geochemistry of MI and quartz phenocrysts contained in this conglomerate are described below. The MI have been categorized based on their phase assemblages and preservation style. Type 1 MI contain clear glass and a shrinkage bubble, Type 2 contain clear glass, a shrinkage bubble and 1 or more, coarser grained (> 3 um) crystals, and Type 3 MI are totally devitrified or otherwise breached. The MI range in size from 1 to over 200 um in diameter, and they typically have a negative hexagonal bi-pyramidal morphology. Cathodoluminescence (CL) observations of the quartz phenocrysts (n=42) reveal several periods of new quartz growth and dissolution on what we tentatively interpret to be nonmagmatic cores (Fig. 1A). In eight quartz phenocrysts from four texturally different porphyritic rhyolite cobbles, Ti (measured by EPMA) increases from about 50 to 220 ppm towards the rim in the magmatic growth zones, corresponding to a temperature increase from 675 to 850 °C, estimated using the TitaniQ method of Wark and Watson (2006) with a fixed TiO2 activity equal to 1. Zircon microphenocrysts and MI are distributed throughout the magmatic portions of the quartz phenocrysts. Type 1 and Type 2 MI were analyzed by EPMA and LA-ICPMS for major elements and 13 trace elements, including Zr. Zircon saturation temperatures calculated using the model described by Watson and Harrison (1983) based on Zr and major oxide concentrations in MI (n=11) from one cobble range from 670 to 866 °C and agree well with the Ti in quartz geothermometry from the same cobble (Fig. 1B). Petrographic observations and chemical analyses indicate that many (if not most) of the type 1 and 2 MI have remained chemically closed systems for over 1 Ga. Zoned calcite (imaged by CL) associated with native copper that is related to later non-magmatic mineralization has selectively replaced some of the feldspar in rhyolite cobbles from the Allouez conglomerate, and random fractures that are partially filled with calcite and secondary planes of fluid inclusions cut across the quartz phenocrysts and some Type 3 MI. The well-preserved growth zoning recorded by CL imaging of quartz is consistent with the preservation of other 77 �chemical characteristics in the MI. Based on both textural and chemical evidence, zircon and quartz remained saturated in the magma while temperatures increased by more than 175 °C sometime prior to eruption. Blundy et al., (2006) and Wark et al., (2007) recently presented evidence that the magma temperature increased during eruption at Mount Saint Helens WA and prior to the eruption of the Bishop Tuff at Long Valley CA. Quartz-bearing porphyritic rhyolite from the Keweenaw shows a similar thermal trend. The thermal and chemical evolution of the Midcontinent Rift System rhyolite magmas may be better constrained utilizing the chemical record preserved in MI, quartz, and zircon. Figure 1. A) CL image of a typical quartz phenocryst from porphyritic rhyolite cobbles of the Allouez conglomerate. Light and dark regions in the quartz correspond to high and low Ti concentrations, respectively. The dark region in the core has very low Ti concentrations (15 ppm) and may be “non-magmatic” based on TitaniQ temperature estimates (567 °C). B) Calculated zircon saturation temperatures from MI (left) and TitaniQ temperature estimates for quartz growth (right) for one Allouez conglomerate rhyolite cobble. References Blundy, J., Cashman, K., and Humphreys, M. (2006): Magma heating by decompressiondriven crystallization beneath andesite volcanoes. Nature, 442, 76-80. Wark, D.A, and Watson, E.B, (2006): TitaniQ: A Titanium-in-Quartz geothermometer. Contributions to Mineralogy and Petrology, 152, 743-754. Wark, D.A., Hildreth, W., Spear, F.S., Cherniak, D.J., And Watson, E.B. (2007): Preeruption recharge of the Bishop magma system. Geology, 34, 235-238. Watson, E.B, and Harrison, T.M., (1983): Zircon saturation revisited: temperature and composition effects in a variety of crustal magma types. Earth Planet. Sci. Lett., 64, 295-304. 78 �Pleistocene glaciation as a mechanism for emplacement of high-salinity groundwater at anomalously shallow depths in the Lake Superior basin MICHAEL L. TAYLOR and JOHN B. SWENSON Department of Geological Sciences, University of MinnesotaDuluth, Duluth, Minnesota Along the shores of Lake Superior, in northeast Minnesota and northern Wisconsin, highly saline groundwater exists at anomalously shallow depths of 30 meters or less, contaminating domestic wells and community water supplies. These saline waters, which produce significant hydrogeologic problems for determining locations for domestic water sources, appear geochemically similar to high salinity groundwater (Na-Ca-Cl brines) commonly found at depths greater than one kilometer on the Canadian Shield (Frape and Fritz, 1982). Currently there is little research that attempts to explain this phenomenon of anomalously shallow, highly saline groundwater from a hydrodynamic perspective. Most work completed thus far has been from a geochemical perspective. One recent hydrodynamic model suggests that glaciation acted as a mechanism for flushing deep brines closer to the surface: During the Pleistocene, the Superior Lobe of the Laurentide ice sheet repeatedly occupied the Lake Superior basin. The temperate (wet-based) lobe created a strong hydraulic-head gradient across the sedimentary and volcanic rock package beneath the ice. High hydraulic head existed down the axis of the lobe and decreased towards the margins. In response to this head gradient, meltwater at the base of the glacier may have been driven deep into the underlying rock package beneath the lobe axis, where it could mix with and flush brines to the basin margins. As the Superior Lobe retreated, the head gradient was removed and the high-salinity groundwater displaced to the lobe margins began to migrate (back-flush) to its original position deep within the basin. This study develops further this model and attempts to test it via a combination of mathematical modeling and geochemical fingerprinting. Mathematical modeling comprises the development of a relatively simple model of groundwater flow and solute transport to test basic physical plausibility of the ‘flushing’ model. The flushing model predicts that samples of groundwater should represent a combination of parent brine formed deep within the rock package underlying the Lake Superior basin, isotopically light, subglacial Pleistocene recharge, and modern meteoric recharge. Geochemically fingerprinting saline groundwater samples will test the model predictions which involved: (1) field work to sample wells in the Lake Superior basin, with emphasis on the south shore, where data are sparse, (2) stable isotope and major element analyses of the fluids, and (3) stable isotope and major-element analyses of the sedimentary rocks that comprise the Mid-continent Rift System. Reference: Frappe, S.K. and Fritz, P., 1982. The Chemistry and Isotopic Composition of Saline Groundwaters from the Sudbury Basin, Ontario. Canadian Journal of Earth Sciences, v. 19, p. 645-661. 79 �Possible influence of a buried fault on elevated indoor radon levels, south Washington County, Minnesota Stephanie A. Theriault and Dr. Thomas A. Hickson Geology Department, University of St. Thomas, St. Paul, Minnesota 55105 Correlation between high indoor radon levels and a local fault in south Washington County were found in a 2004-2005 study done by the Geology Department at the University of St. Thomas and the Washington County Public Health and Environment Department. The location of the fault was important to this study because radon characteristically can travel farther along leaky faults, which could explain the unusually high levels of radon in this specific area. However, the Minnesota Geological Survey revised their mapping shortly after the 2004-2005 study and did not include the specific fault on their new map. This research focuses on determining if there is evidence for the proposed fault. Stratigraphic offset, as seen in well logs, aid in determining the possible presence of the fault. Three cross sections were created across the proposed fault from well log data. Offset was found both in the Prairie du Chien and Jordan units across the proposed fault. Structure contour maps of the top of the Prairie du Chien and Jordan units also support the presence of this fault. In conclusion, evidence provided by the well logs from the study area suggests the possible presence of a fault and that it could be related to the unusually high indoor radon levels encountered in this area. 80 �POTENTIAL USE OF THE MIDCONTINENT RIFT FOR CO2 SEQUESTRATION Thorleifson, L. H., Minnesota Geological Survey, 2642 University Ave West Saint Paul, Minnesota 55114-1057 Increasing concern about climate change and reliance on fossil fuels necessitate the exploration of carbon capture and geologic CO2 sequestration as a technology to reduce CO2 emissions. Characterization of the Midcontinent Rift, a southwestward extension of the Lake Superior basin that presents the best sequestration opportunity in the immediate region, is needed to permit a full feasibility assessment of this technology in the region. Unlike better known rocks in oil or coal producing regions, we have little information on the Rift, yet geologic factors suggest that it may be a potential target. A recent meeting therefore was held to develop a shared understanding of current knowledge on the potential opportunities and tradeoffs for deep geologic sequestration of CO2 in Minnesota, Iowa, and western Wisconsin, and to facilitate collaboration between stakeholders. At the meeting, held on March 21, 2007, over forty representatives from upper Midwest governmental, community, academic, and industry groups met to discuss prospects for geologic CO2 sequestration in the region. At the meeting, Brad Crabtree, of the Great Plains Institute (GPI), described work done by the Coal Gasification Workgroup to build consensus on a long-term vision for low-carbon utilization of coal. In May 2007, GPI will release a 50-year regional energy roadmap including a chapter on the role of advanced coal technologies with geologic carbon capture and sequestration (CCS). Bill Grant of the Izaak Walton League outlined a bill currently before the Minnesota Legislature (H.F. 1666) that would provide funding for assessment of both geologic and terrestrial carbon sequestration in Minnesota. If enacted, this bill would provide funds to the Minnesota Geologic Survey and other agencies to begin geologic assessment of the Midcontinent Rift along with steps toward analysis of required policy and technology. Elizabeth Wilson of the University of Minnesota Hubert Humphrey Institute provided an overview of policy considerations for a carbon-managed energy system. Carbon Capture and Sequestration (CCS) is currently in commercial operation at several sites around the world, and is the focus of a major research effort by the U.S. Department of Energy (DOE). Technology alone, however, will not be enough, as deployment will need to be embedded in a wider societal dialogue. States will play a key role in property rights and liability issues, as well as in managing potential risks to public safety, health and the environment. Wilson drew the attention of meeting participants to the Intergovernmental Panel on Climate Change report on Carbon dioxide Capture and Storage as an important reference. Ed Steadman of the Energy & Environmental Research Center (EERC) at the University of North Dakota described the work of the Plains CO2 Reduction Partnership (PCOR), one of seven regional partnerships under the U.S. Department of Energy’s Regional Carbon Sequestration Partnership Program. PCORP has completed Phase I and Phase II research, including assessment and ranking of sequestration opportunities, outreach materials, and a PCOR Partnership Regional Atlas. PCOR is now preparing to conduct a number of CCS pilot projects 81 �in the Williston Basin under Phase III. Ed discussed the Midcontinent rift as part of his broad presentation, both in terms of its potential and the lack of available information on the topic. Ray Anderson of the Iowa Geologic Survey gave a presentation on the geology of the Midcontinent Rift. He described how the billion-year-old rift can be clearly seen across the region as a gravity and magnetic anomaly, while rocks of the rift are only exposed in the Lake Superior Area. The limited additional information available about rift formations comes from deep test borings done for petroleum exploration in Iowa and Wisconsin in the mid 1980s, along with a few shallower drillholes in Minnesota. He indicated that available data indicate that the rift is composed of a central volcanic block, flanked by sedimentary basins up to 5 miles in thickness. Smaller sedimentary basins sit atop the volcanic block in several areas. Geologic sequestration requires porous rock formations at a depth of at least 2,500 feet or roughly a km, overlain by impermeable rock formations to trap the carbon dioxide. Preliminary indications are that the Midcontinent Rift formations may have the necessary characteristics, but much more geologic assessment is required. A breakout on policy and technology recommended that we pursue coordinated policy and regulatory development among neighboring states to pursue early development of a carbon management infrastructure, linking jurisdictions, power plants, coal resources, enhanced oil recovery (EOR) options, along with other sequestration options. This is presently emerging among Illinois Basin states, but could be expanded to other neighbors and even beyond the Illinois Basin. Once initial geological results are available, we will need to conduct comprehensive analysis of different energy futures. This analysis will examine the policy, economic and environmental implications of a) importing electricity from out of state, b) producing electricity in-state but exporting produced CO2 through pipelines, and c) producing electricity and sequestering CO2 within the state. Such an analysis will allow for the advantages and tradeoffs of different energy futures to be considered. The geology breakout recommended that geologic assessment be conducted in phases. Phase One, at a cost of about $0.1M, would assemble existing information, and could include new modeling based on existing seismic data, in order to lay the groundwork for a more rigorous geologic assessment of the CCS potential in the Midcontinent Rift. Bill H.F. 1666, currently before the Minnesota Legislature, could potentially fund Phase One. An important purpose of a Phase I study would be to inform a decision on whether it is warranted to move on to a phase II study. A Phase Two would be a geologic assessment based on new geophysical surveys and about 3 to 6 boreholes, at a cost of perhaps $5M to $10M, perhaps through public funding or a public-private partnership. The objective of Phase Two would be to determine whether Geologic Sequestration potential exists in the Midcontinent Rift. If the results of Phase Two are positive, Phase Three would expand the geological assessment to estimate the geologic storage capacity of the Rift, and begin characterizing the most promising sites. References: • • http://www.geo.umn.edu/mgs/co2_seq.htm http://arch.rivm.nl/env/int/ipcc/pages_media/SRCCS-final/IPCCSpecialReportonCarbondioxideCaptureandStorage.htm 82 �TILL GEOCHEMICAL AND INDICATOR MINERAL RECONNAISSANCE OF MINNESOTA Thorleifson, L. H., K. L. Harris, H. C. Hobbs, C. E. Jennings, A. R. Knaeble, R. S. Lively, B. A. Lusardi, G. N. Meyer, Minnesota Geological Survey, 2642 University Ave West Saint Paul, Minnesota 55114-1057 As a cooperative project of the Minnesota Geological Survey and industry, the entire State of Minnesota and adjacent regions was sampled for till geochemistry and indicator minerals at a 30km spacing during summer 2004. Within target cells, each a quarter-degree latitude by a halfdegree longitude, till from between about 1 and 2 m depth was sampled by filling a 15 liter plastic pail. At a few sites, vertical profiles were collected. In addition, three transects to the north were sampled, to help identify sediments derived by long-distance glacial transport, to obtain reference samples from the Thompson nickel belt, and also to extend sampling to the limit of Hudson Bay-derived carbonate-bearing sediments, to permit comparison to Minnesota carbonate-bearing sediments. Three control samples anomalous in kimberlite indicator minerals from Kirkland Lake, Ontario, were also obtained. The resulting batch consisted of 250 samples covering Minnesota and adjacent areas, 20 samples from Canada, and the three standards. Upon completion of the sampling, the samples were randomized, given numeric laboratory identifications, and shipped to a processing lab, where four quarter-liter splits, two for fine fraction geochemistry, one for texture, and one for an archive were removed. The remaining 14 liters were disaggregated, screened at 2 mm, and the gravel was retained for lithological analysis. The <2 mm fraction was then processed for gold grains, a ferromagnetic heavy mineral concentrate, and a nonferromagnetic heavy mineral concentrate that supported subsequent analysis for precious metal, base metal, and gemstone indicator mineral counts, indicator mineral chemistry, bulk mineralogy counts, and heavy mineral geochemistry. The resulting data are now a significant new information resource with respect to environmental geochemistry topics such as understanding the distribution of deleterious elements in food and water, while providing insights into transport history and composition of the sediments that make up our soil parent materials. Many variables provide insights into regional geology, and reflect known mineral deposits. Some of the data seem to provide faint insights into what may be mineralization that was not previously recognized, such as various base-metal and precious-metal-related elements that show patterns of varying clarity over portions of the state. None of these patterns, however, are obvious discoveries of something that was previously unknown, at the current stage of interpretation. With respect to kimberlite indicator minerals, however, there are two noteworthy patterns, including a few Cr-pyrope garnets in an area from the Twin Cities to southwestern Minnesota (Figure 1), as well as Mg-ilmenites and high-chrome Cr-diopsides in the far northcentral part of the State. Sample spacing in the thin sediments of northeastern Minnesota was not adequate to fully test for the presence of sources such as potential single kimberlite pipes, although samples at a closer spacing are presently being processed by Natural Resources Research Institute to address this point. The current results are, however, faint but clear indications of kimberlite indicator minerals sources that are not unlike several of the patterns that have been found, for example, in Canada, where some of such patterns have eventually resulted in kimberlite discoveries. In the case of the Minnesota results, the data may indicate sources 83 �within the state, or quite possibly could be manifestations of long distance glacial sediment transport, possibly from known or unknown sources in neighboring states or in Canada. In summary, the results are a highly significant step forward in mapping our geochemical landscape, in clarifying mineral potential, in provision of reference data useful to environmental protection, public health, and exploration, and in supporting follow-up with respect to potential mineralization. Figure 1: Example of results from the statewide survey Reference: Thorleifson, L. H., K. L. Harris, H. C. Hobbs, C. E. Jennings, A. R. Knaeble, R. S. Lively, B. A. Lusardi, G. N. Meyer, 2007, Till geochemical and indicator mineral reconnaissance of Minnesota. Minnesota Geological Survey Open File Report OFR-07-01, 512 p., 15 pdf digital files, 5 digital images 84 �DEFINITION OF THE PROTEROZOIC TERRAIN UNDER THE PALEOZOIC-CENTRAL U.P., MICHIGAN T.D. WAGGONER 141 CHIPPEWA, NEGAUNEE, MI 49866 E-mail: thomaswaggonergeo@hotmail.com As soon as it was recognized in the 1880’s that the eastern Menominee range was covered by Paleozoic sediments east of Waucedah, exploration has sought to identify iron mineralization under cover in the central Upper Peninsula. For over 100 years work has progressed to identify the sources of the magnetic anomalies that are found west of the NW-SE rift hinge line and the western limit of the Paleozoic cover. Fig. 1 Magnetic highs under the Paleozoic cover of the central U.P. The amplitude of the airborne total intensity magnetic anomalies range from 3,000 to 23,000 gammas. The intensity is generally dependent on both the amount of magnetite and depth of burial. Airborne and ground magnetics along with ground gravity defined target areas that have since been drilled in the search for iron ore through the 1970’s. Most of the major anomalies are caused by steeply dipping banded iron formations (bifs) similar to the Proterozoic equivalents that outcrop west of the covered area. The primary iron oxide minerals are magnetite, specularite. These are accompanied by iron carbonate silicates, carbonate and chert. Supergene oxidation and enrichment is present in about half of these deposits. Paleozoic 85 �cover ranges from inches to at lease 1800 feet. Associated rocks include slates, quartzites and schists and in one case gneiss. Some of these rocks have been intruded by diabase and granite dikes. The east-west structural attitude of the anomalies and bifs are consistent with outcrop areas to the west. Metamorphic rank from chlorite through garnet is present as noted by the gangue mineralogy associated with the bifs. There have been geophysical data bases generated for diamond exploration that would be useful in further defining the buried Proterozoic terrain. Although economic deposits of bif are probably not present, there is a distinct possibility that IOCG type deposits, feeders for the water deposited bifs are present. Further geophysical surveys like airborne gravity could help provide better definition. 86 �Detrital Zircon Provenance and Structural Geology of the Hamilton Mounds Inlier, Central Wisconsin Jakob Wartman, John P. Craddock, Karl R. Wirth, Geology Department, Macalester College, St. Paul, MN 55105; e-mail: craddock@macalester.edu Gordon Medaris, Jr., Dept. of Geol. & Geophysics, UW-Madison, Madison, WI 53706 Jeff D. Vervoort, Dept. of Geology, Washington State Univ., Pullman, WA 99164 Cam Davidson, Geology Department, Carleton College, Northfield, MN 55057 Hamilton Mounds is one of a series of small inliers in Adams County, WI that are composed of Proterozoic quartzites (Oslander, 1931). Exposures in the Seven Sisters quarry at Hamilton Mounds reveal the presence of a lower, folded metasedimentary sequence, the Hamilton Mounds meta-arkose (HMM), that is intruded by granite (1762 ± 7 Ma [2σ], TIMS-zircons; Medaris et al., 2007). The overlying Seven Sisters orthoquartzite (SSQ) is also folded and contains detrital zircon and monazite that suggest correlation with Baraboo-equivalent quartzites (see Table 1). Finite strains in the Seven Sisters quartzite are also consistent with the regional, NW-SE shortening Mazatzal orogen strain pattern (Craddock and McKiernan, 2007). Both sedimentary units at Hamilton Mounds are crosscut by breccia zones associated with Wolf River batholith intrusion at 1470 Ma. The HMM and SSQ are overlain unconformably by Cambrian sandstones (see Brown and Greenberg, 1986). Detrital zircons are abundant in the Hamilton Mounds meta-arkose. We analyzed 120 grains of which 68 yielded nearly concordant (<10% discordant) results (Figure 1). The youngest zircon age we observed is 1750 ± 16 Ma (2σ), the approximate age of the crosscutting granite dike. The zircon population includes large numbers of post-Penokean (1775 Ma) to Penokean (1800-1875 Ma) grains, and lesser amounts of Archean (25003500 Ma) grains. These data suggest that the Hamilton Mound sediments were sourced Table 1: Summary of Geochronology for Proterozoic Quartzites n= Quartzite Barron 6* Sioux 9* 9* Flambeau McCaslin 2 Hamilton Mounds HMQ1 (SSQ) 69 HM-SSQ (m) 11* HM-Basal (m) 10* HM-Basal (z) 68 Baraboo Basal 7 1250 m up 7 BARQ2 47 BARQ1 100 Method SHRIMP SHRIMP SHRIMP TIMS Youngest Age 1751 Ma 1730 1714 1773 Range 1751-1180 Ma 1730-1850 1714-1880 1773-1775 Reference Holm et al., 1998 Holm et al., 1998 Holm et al., 1998 Van Wyck, 1995 LA-ICPMS Electron Probe Electron Probe LA-ICPMS 1772 1750 1720 1783 1772-3566 1750-2165 1720-2120 1783-3456 Van Wyck & Norman Medaris et al., 2007 Medaris et al., 2007 This Study TIMS TIMS LA-ICPMS LA-ICPMS 1691 1844 1716 1724 1691-1865 1844-2588 1716-3200 1724-3100 Medaris et al., 2003 Van Wyck, 1995 Van Wyck & Norman Van Wyck & Norman [Key: All ages are 207Pb/206 Pb. HM is Hamilton Mound, with upper Seven Sisters Quartzite (SSQ) and older, basal arkose (HM-Basal). Detrital monazite (m) and zircons (z) are indicated. Monazite ages (*) are from multiple analyses: Barron (6 grains, 28 spots); Sioux (9/19); Flambeau (9/16); HM-SSQ (11/83) and HM-Basal (10/154)]. Younger HMM monazite ages (<1760 Ma) are from non-detrital grains. 87 �from the north, including the Penokee Mountains, Archean granite-greenstone terranes, and the Marshfield (or Watersmeet) Archean gneiss terranes. This is broadly consistent with detrital zircon results for other contemporaneous units (e.g., Virginia, Rove and Michigamme Fms.; see Wirth et al., 2006) along the Penokean margin. The Hamilton Mound arkose was deformed in the Yavapai orogeny, then eroded and buried by quartz arenites of the Baraboo interval (1750-1630 Ma). 12 10 8 6 4 2 0 1500 2000 2500 3000 3500 Age (Ma) Figure 1. Histogram of detrital zircon ages from basal Hamilton Mound arkosic quartzite. References Cited Brown, B.A. and Greenberg, J.K., 1987, Proterozoic quartzite at Hamilton Mound, central Wisconsin: Geological Society America Centennial Field Guide—North Central Section (DNAG), p. 195-98. Craddock, J. P. and McKiernan, A.W., 2007, Finite strain gradient in Baraboo-interval quartzites, Wisconsin and Minnesota, USA: Prec. Res. Sp. Vol. Holm, D., Schneider, D., and Coath, C.D., 1998, Age and deformation of Early Proterozoic quartzites in the southern Lake Superior region: Implications for extent of foreland deformation during final assembly of Laurentia. Geology, v. 26, p. 907-910. Medaris, L.G., Singer, B.S., Dott, R.H., Naymark, A., Johnson, C.M., Schott, R.C., 2003. Late Paleoproterozoic climate and tectonics in the southern Lake Superior region and Proto-North America: Evidence from Baraboo interval quartzites: J. Geol., v. 111, p. 243-257. Medaris, L.G., Van Schmus, W.R., Loofboro, J., Stonier, P.J., Zhang, X., Holm, D.K., Singer, B.S., Dott, R.J., Jr., in press, Two Paleoproterozoic (Statherian) siliciclastic metasedimentary sequences in central Wisconsin: the end of the Penokean orogeny and cratonic stabilization of the southern Lake Superior region: Precambrian Research. Ostrander, A.R., 1931, Geology and structure of Hamilton Mounds, Adams County, Wisconsin: unpublished M.S. thesis, Univ. of Wisconsin, Madison, 27 p. Van Wyck, N., 1995, Major and trace element, common Pb, Sm-Nd, and zircon geochronology constraints on petrogenesis and tectonic setting of pre- and early Proterozoic rocks in Wisconsin. unpublished Ph.D. thesis, Univ. of Wisconsin, Madison, 47-280. Van Wyck, N. and Norman, M., 2004, Detrital zircon ages from early Proterozoic quartzites, Wisconsin, support rapid weathering and deposition of mature quartz arenites: J. Geol. 112, p. 305-315. Wirth, K.R., Vervoort, J., Craddock, J.P., Davidson, C., Finley-Blasi, L., Kerber, L., Lundquist, R., Vorhies, S., and Walker, E., 2006, Source rock ages and patterns of sedimentation in the Lake Superior region: results of preliminary U-Pb detrital zircon studies: 52nd Institute on Lake Superior Geology, p. 69-71. 88 �A Comparison of Textural Profiles in Diabase Sills from the Midcontinent and Transantarctic Rifts ZIEG, Michael J. and FORSHA, Clinton J., Department of Geography, Geology, and the Environment, Slippery Rock University, 1 Morrow Way, Slippery Rock, PA 16057, USA, michael.zieg@sru.edu The 176-Ma Ferrar Dolerite of the Transantarctic Rift (TAR) and the 1.1-Ga Nipigon Diabase of the Midcontinent Rift (MCR) represent magmatic activity during the early stages of continental rifting. Both of these systems include large sills intruded at or near the unconformity between basement rock and overlying sedimentary sequences (the Beacon Group and Sibley Group). Understanding the emplacement history of these sill complexes is critical to understanding the thermal and mechanical properties of the developing rifts. As in the Nipigon area, there are very few feeder dikes exposed in the Ferrar system. Thus, it has been suggested that a significant proportion of the magma transport in the TAR occurred within the sills themselves. Hart and Macdonald (2005) and Sutcliffe (1987, 1989) have suggested, and this work provides additional evidence, that the Nipigon sills experienced multiple injections of magma, which may be an indication that the sills acted as conduits for the distribution of magma through the system. Focused magma flow results in a very different thermal environment than episodic, discrete injection events, and therefore it is very important to understand the nature of the injection history when evaluating the tectonic development of a magmatically active region. In this study, we focus on the application of textural stratigraphy within mafic sills to the understanding of sill emplacement history. In particular, textural profiles from Nipigon sills are compared to profiles from Ferrar sills. These comparisons illustrate the textural signatures of singular, pulsed, and flow-differentiated injections. While the Nipigon sill from Kama Point exhibits a strong reinjection signature, the reinjection signatures in the Antarctic sills are less clear. The significance of different injection histories is that, while a network of sills and dikes emplaced via single injection events can efficiently distribute heat through a large crustal volume, reinjection and/or extended flow through a sill or dike can produce higher temperatures, though in a more localized volume. References Hart, T.R., and McDonald, C.A. 2005. Mesoproterozoic diabase sills of the Nipigon Embayment, northwest Ontario. In Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Vol. 51, Part 1, pp. 22-23. Sutcliffe, R.H. 1987. Petrology of middle Proterozoic diabases and picrites from Lake Nipigon, Canada. Contributions to Mineralogy and Petrology, 96: 201-211. Sutcliffe, R.H. 1989. Mineral variation in Proterozoic diabase sills and dykes at Lake Nipigon, Ontario. Canadian Mineralogist, 27: 67-79. 89 � https://digitalcollections.lakeheadu.ca/files/original/22fb6182a89be392ecbc6d2c2bc1ab99.pdf 29c15d555e9c2109dc1f30c0cb447f7b PDF Text Text 53RD ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY LUTSEN, MINNESOTA MAY 8 – 13, 2007 PROCEEDINGS VOLUME 53 PART 2 – FIELD TRIP GUIDEBOOK �INSTITUTE ON LAKE SUPERIOR GEOLOGY LUTSEN, MINNESOTA MAY 8 – 13, 2007 HOSTED BY: LAUREL G. WOODRUFF, U.S. GEOLOGICAL SURVEY MEETING CHAIRPERSON JAMES D. MILLER, JR., MINNESOTA GEOLOGICAL SURVEY FIELD TRIP COORDINATOR PROCEEDINGS VOLUME 53 Part 2 Field Guidebook Compiled by James D. Miller, Jr. (MGS) and Dean M. Peterson (NRRI) -i- �- ii - �Table of Contents Proceedings Volume 53 Part 2 – Field Trips 1) IGNEOUS STRATIGRAPHY OF THE POPLAR LAKE INTRUSION 1 (FORMERLY NATHAN’S LAYERED SERIES) Leaders: J. Miller & E. Jerde 2) GEOLOGIC AND CULTURAL HISTORY OF THE GRAND PORTAGE NATIONAL MONUMENT 23 Leaders: B. Cannon, D. Cooper, & B. Phillips 3) MIDCONTINENT RIFT-RELATED MAFIC INTRUSIONS NORTH OF THE INTERNATIONAL BORDER 53 Leaders: M. Smyk & P. Hollings 4) GEOLOGY OF THE NICKEL LAKE MACRODIKE AND ITS ASSOCIATION WITH CU-NI-PGE MINERALIZATION IN THE NORTHERN SOUTH KAWISHIWI INTRUSION, DULUTH COMPLEX, NORTHEASTERN MINNESOTA 81 Leaders: D. Peterson & P. Albers 5) GEOLOGIC HIGHLIGHTS OF NEW MAPPING IN THE UPPER SOUTHWESTERN TO NORTHEASTERN SEQUENCE OF THE NORTH SHORE VOLCANIC GROUP AND BEAVER BAY COMPLEX 109 Leaders: T. Boerboom, J. Miller, & J. Green 6) GEOLOGY ALONG THE GUNFLINT TRAIL 143 Leaders: M. Jirsa & P. Weiblen Cover Illustrations (clockwise from upper left) • Canoeing on Poplar Lake (photo by Jim Miller) • Beach at Grand Portage (photo by Bill Cannon) • 3-D model of the South Kawishiwi and Bald Eagle intrusions (from Dean Peterson) • Stromatolitic Gunflint Iron-formation, Magnetic Rock Trail (photo by Jim Miller) • North shore view from Sugar Loaf Point (photo by Jim Miller) • Logan Sills in Rove Formation (photo by Mark Smyk) - iii - �- iv - �-v- �53rd Annual Institute on Lake Superior Geology FIELD TRIP 1 IGNEOUS STRATIGRAPHY OF THE POPLAR LAKE INTRUSION (formerly Nathan’s Layered Series) James D. Miller, Jr. Minnesota Geological Survey & Department of Geological Sciences University of Minnesota Duluth Duluth, Minnesota and Eric A. Jerde Department of Physical Sciences Morehead State University Morehead, Kentucky ILSG07 1 Trip 1 �Introduction The Duluth Complex is the largest exposed intrusive component of the Mesoproterozoic Midcontinent Rift, covering an area of over 5,000 km2 in northeastern Minnesota (Fig. 1-1). The Complex comprises a more or less continuous mass of mafic to felsic plutonic rocks that extends in an arcuate fashion from Duluth to nearly Grand Portage. These rocks were emplaced as sheet-like intrusions into the lower section of a comagmatic volcanic edifice (the North Shore Volcanic Group), which itself was erupted onto a peneplained surface composed of Paleoproterozoic sedimentary rocks and Neoarchean granite-greenstone assemblages. The Duluth Complex is generally subdivide into four major series on the basis of intrusive age, dominant lithology, internal structure, and structural position within the complex (Miller and others, 2002). In order of younging, these are: Felsic Series – massive granophyric granite and smaller amounts of intermediate rock that occurs as a semicontinous mass of intrusions strung along the eastern and central roof zone of the complex and was emplaced during early stage magmatism (~1108 Ma). Early Gabbro Series – layered sequences of dominantly gabbroic cumulates that occurs along the northeastern contact of the Duluth Complex and also was emplaced during early stage magmatism (~1108 Ma) Anorthositic Series – a structurally complex suite of foliated, but rarely layered, plagioclase-rich gabbroic cumulates that was emplaced throughout the complex during main stage magmatism (~ 1099 Ma). Layered Series – a suite of stratiform troctolitic to ferrogabboic cumulates that comprise at least 11 variably-differentiated mafic layered intrusions and occurs mostly along the base of the Duluth Complex. These intrusions were emplaced during main stage magmatism, but generally after the anorthositic series. The Poplar Lake Intrusion, which is the focus of this field trip, and the Crocodile Lake intrusion to the east comprise the only two known intrusions in the Early Gabbro Series. The early emplacement of these intrusions was first implied by cross-cutting field relations mapped by Nathan (1969), who recognized that the Poplar Lake intrusion units are cut on their west end by rocks now recognized as belonging to the Layered and Anorthositic Series. An early intrusive age was also implied by their reversed magnetic polarity (Beck, 1970), which is similar to the lowermost lavas of the North Shore Volcanic Group (Green, 1972). U-Pb ages of zircons from the base of the Poplar Lake intrusion confirm its early emplacement by yielding an age of 1106.9±0.6 Ma (Paces and Miller, 1993). This age is similar to lower reversed-polarity lavas of the Midcontinent Rift (1109-1107 Ma, Davis and Green, 1997) and to the reversed polarity Logan Sills (1108(+4/-2) Ma; Davis and Sutcliffe, 1985), which occur in the footwall of the Poplar Lake Intrusion (Fig. 1-1). ILSG07 2 Trip 1 �Figure 1-1. Generalized geology of northeastern Minnesota showing the locations of the Poplar Lake and Crocodile Lake intrusions, which comprise the Early Gabbro Series of the Duluth Complex. Previous Studies The geological survey of Minnesota (1872-1901), directed by N.H. Winchell and assisted by U.S. Grant, H.V. Winchell, and A. H. Elftman, visited the Gunflint Trail area many times over the latter years of the survey. A geologic map of Cook County (Fig. 1-2), as well as more detailed (1”= 2mi) plates of the Gunflint Lake and Rove Lake areas were published in volume 4 of the Final Report (Winchell, 1899). These maps do not distinguish the various types of gabbro, but they do accurately portray the basal contact of the gabbro with the Rove formation and gabbro-granophyre contact just south of Winchell Lake. Frank Grout and coworkers conducted reconnaissance field mapping in Cook County in a period between 1913 and 1953. This work was compiled soon after Grout’s death in 1958 in MGS Bulletin 39 (Grout, Sharp, and Schwartz, 1959). In township maps of the Poplar Lake area (Fig. 1-3), Grout distinguished gabbro, oxide-rich gabbro, anorthositic gabbro, intermediate ILSG07 3 Trip 1 �N Figure 1-2. Geologic map of Cook County by U.S. Grant from the Minnesota Geological Survey Final Report vol. 4 (Winchell, 1899). EWLAMA lION F.' IF I I I I..I! .i!! I!. I IF - if K U *S I •'I(iI,IIr.X'.- I,..XIV — (..,!III'Ii!nIK,F,l'I',iI.1,,r,r INIFrIlI. RIIIIKFIK'FFI II.PI.Iii.u,I.p*lt,\IIlIil,OtNI!rII,.IIInug,, I Figure 1-3. Reconnaissance township geologic maps of the Poplar Lake area from Grout, Sharp, and Schwartz (1959) ILSG07 4 Trip 1 �intrusive rocks and granophyre and noted the moderate southernly dip of layering and foliation. The oxide-rich intervals of the Poplar Lake intrusion were previously cited by Broderick (1917) and by Grout (1950) as potential titanium and iron ore resource. By far the most complete study of the Poplar Lake intrusion comes from the PhD dissertation work of Harold Nathan (1969). Nathan mapped the bedrock geology of the Duluth Complex in the Gunflint Lake, South Lake, and Hungry Jack Lake and followed this up with a very thorough petrographic study. Reconnaissance geologic maps of these quadrangles were published nearly a decade later (Morey and Nathan, 1977; Mathez, Nathan, and Morey, 1977; and Morey and Nathan, 1978). Phinney (1972) briefly summarized Nathan’s work for the MGS Centennial Volume in a section describing the northern prong of the Duluth Complex (Figure 1-4). In that same volume, Davidson (1972) first adopts the name “Nathan’s layered series”. Later, Weiblen and Morey (1980) referred to the intrusion as the “Layered Series of Nathan” and Weiblen (1982) used the term “Nathan’s Layered Series”. In the new geologic map compilation of the Duluth Complex (Miller and others, 2001) and the companion report (Miller and others, 2002), the informal name of Nathan’s Layered Series was replaced by the Poplar Lake intrusion to acknowledge the location where most of the lithologies recognized by Nathan (1969) are exposed and accessible. 9P00' 9045 9045 9O3O' C) SO!09y 07 fljfl7flfl7 LOIW, oorn Lose. EXPLANATION Hugry Jack Lake quadrangles r0m Nalhon. (969) Remainder (ran, 6.-our 010 orhrs (see ak I 2 3M!I,) ljt7IQlfljn aIte,otinn ci 71 by 4J J IA (mone ,hcei—liIc. nçI.niyn or Al 2 0 _5 IlifT fl Mac IJ F,GlkicIusicns ai P LIfT] °(::?rr iL±J cn4 HI Figure 1-4. Generalized geology of the Poplar Lake intrusion as portrayed by Phinney (1972) based on mapping by Nathan (1969) and Davidson (1972). Except for limited reconnaissance mapping and geochemical sampling by Jerde (2001), little new mapping or petrologic research has been devoted to this very interesting early gabbroic intrusion of the Duluth Complex. Therefore, the igneous stratigraphy and unit descriptions given for this field trip through the lower half of the intrusion will largely follow Nathan’s (1969) conventions. One notable exception is that whereas Nathan (1969) recommended naming rocks by listing the minerals present ILSG07 5 Trip 1 �in the rock in decreasing order of abundance (e.g., a plagioclase-augite-olivine rock), we have adopted more conventional modal rock name nomenclature (e.g., an olivine gabbro; see Fig. 1-5 for the modal classification scheme used here). Also, we will point out where our petrographic analysis of samples from certain field stop locations differ from Nathan’s descriptions. Mela - Pl < 60-50% Leuco - Pl > 75% Gabbroic Rocks Plagioclase = 30-80/85% OLIVINE TROCTOLITE 3:1 AUGITE TROCTOLITE OLIVINE GABBRONORITE OXIDE TROCTOLITE OXIDE-OLIVINE GABBRONORITE OXIDE-OLIVINE NORITE 1:1 OLIVINE GABBRO OLIVINE NORITE OXIDE-OLIVINE GABBRO 1:3 NORITE OXIDE GABBRONORITE FE-TI OXIDE OXIDE-RICH ROCKS 1:3 1:1 OXIDE GABBRO 3:1 3:1 GABBRONORITE LOW-CA PYROXENE GABBRO HIGH-CA PYROXENE Figure 1-5. Modal classification for gabbroic rocks (Pl 30-80/85%) used in this guide. Prefixes melaor leuco- (e.g. melatroctolite, olivine leucogabbro) are used when plagioclase abundance is <60 to 65% or >75%, respectively. Taken from Miller and others, 2002. Igneous Stratigraphy and Petrologic Interpretations of the Poplar Lake Intrusion Nathan (1969) distinguished 27 different units in his mapping of the three quadrangles that include the Poplar Lake intrusion (Fig. 1-4). He labeled these alphabetically from A through AA in order of decreasing age. Nathan interpreted the relative ages of the units on the basis of fine-grained margins, cross-cutting intrusive relationships, inclusions, and/or thermal effects. The various map units Nathan (1969) indentified within the Poplar Lake intrusion can be generally grouped into 6 major lithologies: 1) oxide-rich, gabbro to olivine gabbro: F, G, H, T, U, V 2) olivine gabbro to troctolite: P, Q 3) gabbronorite to olivine gabbronorite: A, B, M 4) anorthositic rock types: J, some M, some Q, S 5) intermediate to felsic rock types: K, O, AA, Maa 6) fine-grained mafic rocks (inclusions?): C, D, E, N, X, Y ILSG07 6 Trip 1 �Davidson conducted reconnaissance mapping in quadrangles to the south of the three that Nathan (1969) mapped and had some different interpretations of the upper units of the Poplar Lake Intrusion (Davidson, 1972, 1977a, 1977b; Davidson and Burnell, 1977). Davidson (1972) interpreted the extensive mass of granophyre at the top of the Poplar Lake intrusion, which Nathan (1969) designated as unit AA (Fig. 1-4), to be a separate intrusion that belongs to the felsic series of the Duluth Complex. Like Nathan (1969), he interpreted the massive granophyre to be one of the youngest intrusion of the Duluth Complex based on observations that granophyre dikes and irregular masses commonly cut most other rock types. Nathan’s unit Maa, which underlies the granophyre mass and is composed of a highly altered assemblage of intermediate rock types, is interpreted to be an altered and contaminated phase of unit M caused by the late intrusion of granophyre (unit AA) (Fig. 1-4). Davidson (1972, 1977a, 1977b; Davidson and Burnell, 1977) interpreted these intermediate rocks (unit grd) to be marginal phases of the granophyre. Another reinterpretation of Nathan’s (1969) mapping by Davidson (1972) and others (Morey and Nathan, 1978; Morey and others, 1981) is that Nathan’s unit R is actually part of the Tuscarora Intrusion and thus belongs to the layered series (or the troctolite-olivine gabbro series as Davidson (1972) called it). In the most recent map compilation of the Duluth Complex (M-119; Miller and others, 2001), several reinterpretations were made of both Nathan’s (1969) and Davidson’s (1972) map interpretations. One change is the suggestion that Nathan’s unit S, a gabbroic anorthosite that cross cuts lower units of the intrusion, is actually part of the younger anorthositic series. More significantly, whereas both workers considered the granophyre mass, now termed the Misquah Hills granophyre, to be one of the youngest intrusions is the Duluth Complex, it is now considered to be one of the oldest. Major evidence for this comes from U-Pb age dating, which indicates a crystallization age of 1106 ± 6 Ma (Vervoort and Wirth, 2004). Consequently, the intermediate rocks of Nathan’s unit Maa and Davidson’s unit grd, which underlie the granophyre, are now seen to represent hybridization between partially melted granophyre and underlying mafic magmas of the Poplar Lake intrusion. Miller and others (2002) suggested that the granophyre acted as a low-density trap, which caused the Poplar Lake intrusion to underplate and partially melt the felsic mass. Figure 1-6. Diagrammatic cross-section through the central part of the Poplar Lake intrusion. Figure from Phinney (1972, Fig. V-33) based on data from Nathan (1969). ILSG07 7 Trip 1 �Examining the stacking of Nathan’s (1969) units, it is immediately clear that he interpreted the emplacement of the Poplar Lake intrusion to be a complex multistage process (Figs. 1-6 and 1.7). He envisioned various sill-like intrusions to have been multiply emplaced at seemingly random levels in the intrusion. Nathan (1969) interpreted most intrusive units to have crystallized in place with little evidence of fractional crystallization. This rather unconventional interpretation of the igneous stratigraphy of a mafic layered intrusion compels some skepticism as to Nathan’s model. For example, the sulfide-mineralized heterogeneous basal unit (F), commonly the earliest formed rock type in other Duluth Complex layered intrusions, is thought to have been emplaced after considerable volumes of A and B were emplaced. Many of the fine-grained gabbro units have limited areal extent and appear similar to hornfels basalt inclusions commonly found throughout other Duluth Complex intrusions. Moreover, the anorthositic units are mostly seen as late, however, anorthositic rock are typically some of the earliest intrusive phases throughout most of the Duluth Complex. Nevertheless, until a rigorous follow-up field and petrologic of this intrusion is conducted, Nathan’s multiple emplacement hypothesis stands as the current model. F [11 kHIll Mesonroterozoic POPLAR LAKE INTRUSION '' (Units based on Nathan (1969)) prismatic 01; gradational into dp 01 gabbro- fine?, foliated, dp intergran, 0l:subpoik-subprism — dm dj dg F2 df dc db Gabbronorite- coarse,oph. intergran, commonly leucocratic Gabbronoritic Anorthosite - coarse, ophitic-subophitic Olivine oxide gabbro- crs, foliated, intergran,0l:spoik-sprism; dg'- oxide-rich (>10%) zone Olivine oxide gabbrovan-textured, locally sulfidic Mafic hornfels- fine, granoblastic; basalt inclusions? Gabbronorite- medium, fOliated, Opx:poikilitic-subpoikilitic rc1:i: Gabbronoritic Troctolite- — -.- 1-' Troctolite- medium, foliated, medium, fOliated, intergranular, Opx:poikilitic — —— LOGAN INTRUSIONS Diabase- fine-coarse, suboph- VJTLT.. + HI—I LrIrIai+t — 500 Ii iitrrm- i— -+ r-n.r4 - Fr4tI--I P 0 IHi-iH4:frHiLrT Hji500 1000 Meters H - d rri dt intergranular, locally PI-phyric Paleoproterozoic ROVE FORMATION 1 Argillite/Graywacke- thin-to thick-bedded; locally hornflsic Figure 1-7. Geologic map of the field trip area based on Morey and Nathan (1977). Unit labels after Nathan (1969), but standard modal rock names (see Fig. 1-5) interpreted from Morey and Nathan’s (1977) descriptions. Locations of more detailed maps of stop locations (Figs. 1-8, 1-9 and 1-14) are shown by inset boxes. ILSG07 8 Trip 1 �FIELD TRIP 1- DAY 1 Field Stop Descriptions Upon arriving at Rockwood Lodge and unpacking gear, we will set out to investigate roadcuts along a one-mile stretch of the Gunflint Trail for the rest of the morning. STOP 1-1. BASAL MARGIN UNITS – F, C, G Location: Roadcuts along Gunflint Trail between entrance and exit of Cook County Road 82. South Lake 7.5' quadrangle (T64N, R2W, Sec. 1). UTMs: start – 683540E 5326180N; end – 684950E 5325380N Description: Roadcuts along this section of the Gunflint Trail expose some of the rock types composing the marginal zone of Nathan's layered series (Fig. 1-8). Beginning from the junction of the Gunflint Trail with County Road 92, we will proceed east along the north side of the Trail back toward the Rockwood Lodge entrance. Mesoproterozoic POPLAR LAKE INTRUSION (Units based on Nathan (1969)) dp dg 01 gabbro- fine?, foliated, intergran, Ol:subpoik-subprism Olivine oxide gabbro- cr5, foliated, intergran,Ol:spoik-sprism; —-- df dc dg'- oxide-rich (>10%) zone Olivine oxide gabbrovan-textured, locally sulfidic Mafic hornfels- fine, granoblastic; basalt inclusions? LOGAN INTRUSIONS Diabase- fine-coarse, subophIi intergranular, locally PI-phynic Paleoproterozoic ROVE FORMATION 1 Argillite/Graywacke- thin- to thick-bedded; locally hornflsic Figure 1-8. Geology of the Gunflint Trail area northwest of the Rockwood Lodge (based on Morey and Nathan, 1977). Roadcuts A – E are described in the text for Stop 1-1. Roadcuts A – These exposures show a more homogeneous and sulfide poor phase of Unit F. The rock is a coarse-grained ophitic oxide olivine gabbro. Similar exposures occur in the roadcuts on the south side of the Trail, west Co Rd 92 junction. This is the location of geochronology sample NLS-5,which yielded a U-Pb zircon age of 1106.9±0.8 Ma (Paces and Miller, 1993). Roadcuts B – This series of a low, deeply weathered roadcuts and low pavements are more typical of unit F. About 150 m from the junction is of coarse-grained, decussate, ophitic, biotitic oxideolivine gabbro. This noncumulate, vari-textured (taxitic) gabbro forms the lowermost unit of ILSG07 9 Trip 1 �Nathan's (1969) layered series (his unit F). Locally, Cu-Fe sulfide is present and gives rise to the very rusty appearance of the outcrop. Sulfide mineralization in the lower contact zone is characteristic of many parts of the Duluth Complex (Bonnichsen, 1972; Miller and others, 2002). As is interpreted from the Cu-Ni-PGE sulfide deposits of the NW Duluth Complex, the probable source of the sulfur are the shale and graywacke of the Paleoproterozoic Rove Formation, which forms the footwall of the gabbro. The lack of significant sulfide mineralization here compared to the NW area may indicate a lower sulfur content to the sediments or depletion of sulfur by earlier intrusion of the Logan Sills. Roadcut C – At this prominent roadcut, a very fine grained gabbro is exposed. It has irregular areas of coarser gabbro scattered throughout and some pockets of sulfide. This is Nathan's unit C, which he suggested could be recrystallized inclusions of Keweenawan lavas, Logan sills, or more likely, the chilled contact of a precursor intrusion (his units A and B). In thin section, the rock is a strongly recrystallized, poikiloblastic gabbronorite with small (<5 mm) high-density oikocrysts of augite and hypersthene enclosing granular plagioclase. Because the texture of Logan sills remains unaffected as the contact with the layered series approached (Nathan, 1969), it is unlikely that this body is such an inclusion. Rather, the zones rich in ovoid clots of coarse feldspar are similar in appearance to what have been interpreted elsewhere in the complex as recrystallized amygdules (Bonnichsen, 1972), and this inclusion is most likely a basalt flow. The abundance of these types of inclusions and the relative lack of quartz- or cordierite-bearing inclusions of Rove Formation suggest that the focus of emplacement of Nathan's layered series was at the horizontal to shallow-dipping discontinuity between Early Proterozoic sedimentary rocks and Keweenawan lava flows. Roadcuts D and E About 700 m farther east past a bend in the road, another deeply weathered roadcut exposes a medium- to coarse-grained, moderately laminated oxide troctolite with ophitic augite. This belongs to Nathan's unit G which he characterized as "coarse-grained, olivine-plagioclase and augite-plagioclase rocks with strongly foliated plagioclase and abundant tironals (Fe-Ti oxides) . . . bioite is conspicuous . . . density graded layering is prominent" (Nathan, 1969, p. 68). Although they form structurally lowest cumulates in the layered series, Nathan interpreted this unit to be younger than structurally higher troctolitic to gabbronoritic units on the basis of a complex sequence of discordant relationships (Units A and B, Fig. 3.21). The validity of Nathan's intrusive stratigraphy clearly needs to be tested. --Return to Rockwood Lodge for lunch and prepare for afternoon canoe trip.-- WARNING: The ice just went out (we hope) on the lake and so the water is just above freezing. We will take some time over the lunch hour to instruct everyone on basic canoe safety and what to do in the event that someone dumps in the water. We have BWCAexperienced assistants on the trip and we will bring special equipment to guard against hypothermia. Despite these precautions, the best preventative is using extreme care and common sense. ILSG07 10 Trip 1 �it-' '} POPLAR LAKE INTRUSION Units based on Nathan (1969) dq d Troctolite medium, foliated, prismatic 01; gradational into dp 01 gabbro- fine?, foliated, intergran, 0l:subpoik-subprism Gabbronoritic Anorthosite - coarse, ophitic-subophitic dg Olivine oxide gabbro- crs, fOliated, intergran,0l:spoik-sprism; dgt oxide-rich (>10%) zone df dc —db - da Olivine oxide gabbro van-textured, locally sulfidic Mafic hornfels - fine, granoblastic; basalt inclusions? Gabbronorite- medium, fOliated, Opx:poikilitic-subpoikilitic Gabbronoritic Troctolite medium, fOliated, intengranular, Opx:poikilitic Figure 1-9. Geologic map of the central Poplar Lake area showing field stop locations for the afternoon of Day 1. Dashed line shows canoe route. Geology after Morey and Nathan (1977). STOP 1-2. UNIT G’ – OLIVINE OXIDE GABBRONORITE Location: Float by shoreline ledges on island about 300m due south of Rockwood Lodge. UTMs: 684800E 5325000N Description: These lichen-covered, grussy outcrop ledges are composed of medium-grained, wellfoliated, oxide gabbro typical of Unit G’, an oxide-rich (locally >20%) interval within Unit G. Paddling to the east, a prominent outcrop can be seen at the western point of a peninsula. The well-developed foliation gives rise to a sheet jointing that dips about 20° to the south. --Canoe around the east end of this island then south into a narrow channel. Head WSW to eastern point of another island which has a sizeable dock for disembarking. -- STOP 1-3. UNIT G – OXIDE OLIVINE GABBRONORITE Location: Private residence and dock at east point of island; please request permission from owners to view exposure. UTMs: 684905E 5325355N Description: The main rock type exposed over this outcrop point is a medium coarse-grained, foliated, intergranular, oxide olivine gabbronorite. This exposure is typical of Unit G. One curious feature is the occurrence of a 20- to 30-cm-thick, fine grained sill of a similar mineralogy ILSG07 11 Trip 1 �and texture exposed in the cliff face back from the boat dock. The lower contact is rather sharp but the upper contact is gradational (Fig. 1-10A). Throughout the sill and especially near the upper contact, coarse crystals and crystal clusters of pyroxene and plagioclase occur (Fig. 1-10B). Petrographic observations across this sill show that it differs from the coarser host in several subtle ways. Olivine in the fine-grained gabbro is commonly subpoikilitic compared to granular olivine in the host gabbro. Low-Ca pyroxene is less abundant in the fine-grained gabbro (5% compared to 20%) and is dominantly hypersthene rather than inverted pigeonite. Perhaps this represents a new pulse of magma into the chamber or it marks venting event from the magma chamber causing decompression quenching of a water-saturated evolved magma. The presence of up to 2% biotite and the generally similar mineralogy and texture of the fine-grained and medium-grained gabbros suggest the latter explanation is more likely. A. B. Figure 1-10. A) Fine-grained gabbro sill in medium-grained gabbro host; B) Close-up of piece of float from the sill show dispersed coarse plagioclase and pyroxenes in the fine matrix. --Canoe about 400m due south to northernmost point of eastern end of peninsula. Limited landing spots, so this stop is optional,-- STOP 1-4. UNIT G OR P?- LAYERED OLIVINE GABBRO (Optional) Location: Point of land in southwestern Poplar Lake near the far north central part of Sec. 12. UTMs: 684785E 5324469N Description: Nathan implies by his map and unit designation that this as a thin inclusion? of unit G enclosed between younger units P and J (Fig. 1-9). The exposure here is a medium- to medium fine-grained, moderately layered, well-foliated, intergranular oxide olivine gabbro. A thin section sample from here shows a medium fine-grained oxide olivine gabbro that has a very granular (granoblastic?) texture and is lacking in low-Ca pyroxene. This rock has similarities to Nathan’s Unit P by being fine, olivine-rich, and oxide poor. The lack of Opx makes it distinct from either Unit P (commonly contains some poikilitic hypersthene) and especially Unit G (typically contains significant amounts of inverted pigeonite). --Canoe about 550m to the east-southeast to island. Land canoes against outcrops on the southeast and southwest sides of the island.,-- ILSG07 12 Trip 1 �STOP 1-5. UNIT J – OLIVINE OXIDE LEUCOGABBRO Location: Island in southern part of Poplar Lake. UTMs: 685210E 5324200N Description: Exposures on the south side of the island are composed of coarse-grained, foliated, ophitic to intergranular olivine oxide leucogabbro, which Nathan classifies as Unit J. Plagioclase ranges from 70 to 85% of the rock and is the only consistently cumulus phase. Augite and Fe-Ti oxide are the dominant mafic minerals and vary from anhedral granular to poikilitic. Minor olivine (3-5%) is also variable in texture from subprismatic (Fig. 1-11A) to subpoikilitic. Layers and pods rich in granular Fe-Ti oxides and pyroxene are evident on the southwestern corner of the island (Fig. 1-11B). In poor exposures on the north side of the island (downsection), granular (cumulus) augite, olivine, and oxide are present and the rock seems less leucocratic. A. B. B Figure 1-11. A) Common texture of leucogabbro with granular mafic phases (Ol, Aug, FeOx). B) Massive Fe-Ti oxide layer in leucogabbro. -- Paddle ~175m to the southeast to small island with campsite. Land canoes against outcrops on the south side or in cove on east side of the island. Welcome to Happy Island!-- STOP 1-6. UNIT P – OLIVINE GABBRO Location: Island in southern part of Poplar Lake. UTMs: 685310E 5324050N Description: “Happy Island” is composed of two east-west oriented outcrop areas. Exposures on the south side of the island are composed of medium-grained, well-foliated, intergranular olivine gabbro with variable abundances of Fe-Ti oxide (1-7%). Olivine occurs as small (<1cm), subpoikilitic clots or as prisms 0.5 to 3 cm long (~1:10 aspect ratios; Fig. 1-12) that tend to be concentrated in thin layers. This rock type is consistent with Nathan's description of unit P. Morey and Nathan’s (1977) map shows a contact between unit P to the south and unit Q, a troctolite, to the north running through the middle of the island. However, outcrop exposures and petrographic observations of two samples from the northern end show the rock to be mostly an oxide olivine gabbro like Unit P. The only significant differences are a slightly coarser grain size, subtle textural and modal layering, and a slightly higher concentration in inverted pigeonite. Nothing on the north half looks like an augite-poor troctolite, which characterizes Unit Q (See Stop 1-9). ILSG07 13 Trip 1 �A. B. Figure 1-12. Textures of olivine found in Unit P: A) 5mm subpoikilitic clots (Stop 1-7); B) 1cm long prismatic olivine (Stop 1-6) -- Paddle ~200m to the east-southeast to the south side of an E-W island. Land canoes against outcrops on the south-central of the island. -- STOP 1-7. UNIT P – OLIVINE GABBRO Location: Island in southern part of Poplar Lake. UTMs: 685532E 5323990N Description: Island is composed of two outcrop areas on the south side of the island, between which access is difficult. Both exposure areas are composed of medium-grained, well-laminated, intergranular olivine gabbro (PAOf cumulate) as observed on the south side of the island at Stop 3-3. Locally, 2-5-cm clots of coarse pyroxene and oxide are present and elsewhere are pods (inclusions?) of leucogabbro (Unit J?). Again olivine occurs as small subpoikilitic clots or as prisms up to 3 cm long (Fig. 1-12). Because prismatic olivine tends to be concentrated in thin layers along which joints tend to develop, this texture is preferentially displayed on the outcrop face. Note that on the plane of layering, prismatic olivine is commonly oriented parallel to the strike of layering. This observation and the delicate prismatic habit of the olivine argues against crystal settling by density currents. -- Canoe ~250m to the southwest to steep cliffs lining the bay leading the portage to Lizz Lake. Float along the cliffs and then land canoes at the portage. -- STOP 1-8. UNIT J –LEUCOGABBRONORITE-GABBROIC ANORTHOSITE Location: Bay leading to portage (to Lizz Lake) at south shore of Poplar Lake. UTMs: 685460E 5323700N Description: Exposed in sheer cliffs along the west side of the bay leading to the portage to Lizz Lake is leucogabbroic rock typical of Unit J. Lichen and moss cover make it difficult to see much of the rock. A thin section of a sample collected here shows the rock to be a medium coarse-grained ophitic leucogabbronorite. In some places , pockmarked surfaces are evident and indicate the presence of large oikocrysts of poikilitic olivine. This texture is very well ILSG07 14 Trip 1 �displayed in the exposure on the west side of the portage trail (Fig. 1-13). Here the olivine oikocrysts are up to 5 cm across. Ophitic augite is also evident here in oikocrysts up to 10 cm across. Interestingly, this rock type is very similar to the dominant lithology of the Anorthositic Series of the Duluth Complex. It would worthwhile determining the magnetic polarity of this rock type to see if it has a typical Poplar Lake reversed polarity, or is if is normal like all known Anorthositic Series rocks. Figure 1-13. Mottled surface of ophitic olivine gabbroic anorthosite at Stop 1-8. Clots are formed from both olivine and augite oikocrysts. -- Canoe ~600m to the northeast past the eastern point of island where Stop 1-7 is located. Continue northeast to middle point of large island. Not sure how many canoes can land here so this is an optional stop.. -- STOP 1-9. UNIT Q –TROCTOLITE (OPTIONAL) Location: Middle point of west side of large island in southern Portage Lake. . UTMs: 685850E 5324150N Description: According to a thin section sample of this outcrop that shows a medium-grained, well foliated troctolite with small olivine grains, this site is more typical of Nathan’s Unit Q. -- Canoe northwest back to Rockwood Lodge (~1.6 kilometers). End of Day 1.— ILSG07 15 Trip 1 �FIELD TRIP 1- DAY 2 Depart from Rockwood Lodge and return to portage to Lizz Lake (Stop 1-8). Portage canoes to Lizz Lake landing then double back on portage trail to view first stop of the day. Mesoproterozoic POPLAR LAKE INTRUSION (Units based on Nathan (1969)) 01 gabbro- fine?, foliated, dp intergran, oI:subpoik-subprism F • Gabbronorite- coars%oph- - dni - - intergan, cornnoniy eucocratic Gabbronoritic Anorthosite dj — - db cia - coarse, ophitic-subophitic Gabbronorite- medium, bliated, Opx:pcikilitic-subpoikilitic Gabbroncritic Troctolitemedium, Ibliated, intergranular, Opx:poikiiitic Figure 1-14. Geologic map of the Lizz and Caribou Lakes area showing field stop locations for Day 2. Dashed line shows canoe route. Geology after Morey and Nathan (1977). STOP 1-10. UNITS J AND B AND THE LIZZ LAKE FAULT Location: Lizz Lake portage trail just 50m south of junction with Banadad trail. UTMs: 685500E 5323600N Description: Exposed on the east side of the trail is a medium coarse-grained ophitic oxide gabbronoritic anorthosite typical of leucocratic Unit J. About 10m to the west of the trail is a poorly exposed ledge of medium-grained, foliated, gabbronorite with poikilitic hypersthene. The rock contains no olivine or Fe-Ti oxide. This rock type is typical of Nathan’s Unit B. The juxtaposition of these two distinct rock types across this gap is the best evidence of right lateral offset across a fault that Morey and Nathan (1977) inferred through here and down the elongate trend of Lizz Lake (Fig. 1-14). The Lizz Lake fault is the only fault that Nathan (1969) identified in the interior of the Poplar Lake Intrusion. -- Canoe a short distance to the outcrop island about 100 m to the southeast of the portage-- ILSG07 16 Trip 1 �STOP 1-11. UNIT J? – GABBRONORITIC ANORTHOSITE Location: Island with abundant outcrop on north end of Lizz Lake. UTMs: 685685E 5323490N Description: Good exposures over most of this island show the rock to be a medium coarsegrained, poorly foliated, ophitic, gabbronoritic anorthosite similar to the Unit J outcrop at Stop 1-10. However, Nathan has this exposure mapped within Unit B. This may indicate that the Unit J-Unit A/B contact is farther south on the east side of the Lizz Lake fault. Alternatively, perhaps this is an inclusion in Unit B though this would violate Nathan’s interpretation of Unit B being an early phase of the Poplar Lake intrusion. More mapping in this area is needed to resolve this. -- Float along a series of cliff faces on the western shore of Lizz Lake to view Stop 12 exposures-- STOP 1-12. UNIT B – GABBRONORITE Location: Series of outcrop ledges along the northwestern shoreline of Lizz Lake. UTMs: 685715E 5323345N Description: Although largely obscured by lichen, the ledges along this shoreline display a homogeneous, medium-grained, well-foliated gabbronorite with Opx > Cpx. Orthopyroxene (hypersthene and inverted pigeonite) is typically subpoikilitic to poikilitic, whereas clinopyroxene (augite) is typically anhedral granular (Fig, 1-15A). In one well exposed location beneath a tree tip-up, a subtle modal layering was recognized (Fig. 1-15B). A. B. Figure 1-15. A) Texture of the gabbronorite typical of Unit B. B) Subtle modal layering common observed under a tip-up at Stop 1-12. -- Canoe to the south end of Lizz Lake to portage to Caribou Lake. From the portage landing, head west about 8m to a rock cliff -- ILSG07 17 Trip 1 �STOP 1-13. UNITS B AND X? – GABBRONORITE AND FINE GABBRO Location: Outcrop ledge southwestern corner of Lizz Lake near portage to Caribou Lake. UTMs: 686195E 5322605N Description: Most of this exposure is a very fine-grained mafic rock in sharp contact with a medium-grained gabbro. Thin sections of the fine-grained rock shows it to be a feltytextured, oxide gabbronorite and the medium-grained gabbro is a well foliated, intergranular gabbronorite with poikilitic olivine. Nathan mapped this area as largely unit B with an occurrence of Unit X, which he defines as a fine-grained Pl-porphyritic oxide gabbro. By its unit letter designation, Nathan interpreted this as young intrusive unit. The medium gabbronorite is similar to typical unit B, except for the poikilitic olivine. Perhaps it is a hybrid between Units B and A. The fine-grained rock being an intrusion into B would be consistent with its felty texture. One would expect the rock to display a granoblastic texture if it was a hornfels inclusion. We will investigate the exposure to ascertain whether the fine-grained rock is an intrusion or an inclusion. -- Take the portage to Caribou Lake. Canoe 180m south to a campsite fronted by a large outcrop pavement-- STOP 1-14. UNIT A – OLIVINE GABBRONORITE Location: Outcrop pavement at campsite on south shore of northeast arm of Caribou Lake. UTMs: 685960E 5322155N Description: Very homogeneous exposure of medium-grained, foliated, olivine gabbronorite with poikilitic inverted pigeonite. Some of the inverted pigeonite oikiocrysts can be up to 30 cm across. This exposure is identified as Unit A by Morey and Nathan (1977), which they describe as being a fine-grained troctolite with up to 9% inverted pigeonite and 3% augite. The thin section observed from this outcrop contains 8% augite and 12% inverted pigeonite as well as 20% olivine. Given that unit A occurs at many statigraphic levels in the upper part of the Poplar Lake intrusion, it is perhaps not surprising that it is variable in mode, -- Canoe ~1200 m to the southeast arm Caribou Lake to outcrop-fronted campsite on northern shore-- STOP 1-15. UNIT M – AUGITE NORITE Location: Outcrop pavement at campsite on north shore of southeastern arm of Caribou Lake. UTMs: 686347E 5321875N Description: Homogeneous outcrop of medium-grained, well-foliated, intergranular augite norite. Thin section show that 25% of rock is anhedral to subprismatic granular inverted pigeonite with anhedral granular augite comprising about 10% of the rock. The prismatic inverted pigeonite weathers brown which makes it appear similar to olivine in outcrop (Fig. 1-16). The rock is devoid of olivine and contains only trace Fe-Ti oxide. ILSG07 18 Trip 1 �Nathan and Morey (1977) describe unit M as a coarse-grained, poorly foliated, gabbronorite with average mafic mineral concentrations of up to 2% oxide, 13% inverted pigeonite, and 24% augite. Figure 1-16. Texture of the prismatic augite norite at Stop 1-15. -- Canoe ~400 m south to the portage to Horseshoe Lake -- STOP 1-16. UNIT M – GABBRONORITIC ANORTHOSITE Location: Small outcrop at portage landing to Horseshoe Lake in southeastern Caribou Lake. UTMs: 686375E 5321490N Description: A small outcrop at portage landing appears to be an example of what Nathan cites as the “plagioclase-rich facies” of Unit M, which typically contains coarsely poikilitic pyroxene (Morey and Nathan, 1977). A thin section of this rock shows it to be medium coarse-grained, poorly foliated, ophitic gabbronoritic anorthosite. -- Canoe ~1500 m to a campsite on a peninsular point in western Caribou Lake -- ILSG07 19 Trip 1 �STOP 1-17. UNIT M OR A? – OLIVINE GABBRONORITE Location: Outcrop-fronted campground on peninsula in western Caribou Lake. UTMs: 685030E 5322070N Description: Nathan includes this exposure within Unit M, but the rock is clearly olivine-bearing and is more similar to Unit A lithologies. A thin section from this location reveals a mediumgrained, foliated, ophitic olivine gabbronorite with about 17% granular olivine, 13% poikilitic inverted pigeonite, and 4% subophitic augite. Basically, it is similar to the olivine gabbronorite observed at Stop 1-14, which is mapped as Unit A. Morey and Nathan (1977) place the Unit M – Unit A contact just north of this peninsula (Fig. 1-14), suggesting that it should be farther south. -- Canoe north around island and then east; continue past small peninsula on north shore to campsite; total distance approximately 800m. -- STOP 1-18. UNIT M OR B – LAYERED GABBRONORITE Location: Outcrop pavement at BWCA campsite on the north shore of Caribou Lake. UTMs: 685575E 5322400N Description: Generally a medium coarse-grained, subophitic to intergranular leucogabbronorite (Fig. 1-17A) locally with cm-scale mesocratic to melanocratic layers (Fig. 1-17B). This exposure is situated right at a contact between Unit B and an irregular body of Unit M. Given its coarser grain size and generally leucocratic composition, it seems closer to Unit M. A. B. Figure 1-17. A) Texture of the leucogabbronorite at Stop 1-18. B) Cm-scale wavy modal layering displayed at Stop 1-18. -- Continue along north shore to Lizz Lake portage. Return to Rockwood Lodge. – END OF FIELD TRIP 1 ILSG07 20 Trip 1 �References Beck, M.E., 1970, Paleomagnetism of Keweenawan intrusive rocks, Minnesota. Journal of Geophysical Research, v. 75, p. 4985-4996. Bonnichsen, B., 1972, Southern part of the Duluth Complex. In: Sims, P.K. & Morey, G.B. (eds.) Geology of Minnesota - A centennial volume. Minnesota Geological Survey, p. 361-388 Broderick, T.M., 1917, The relationship of the titaniferous magnetites of northeastern Minnesota to the Duluth Gabbro. Economic Geology, v. 12, p. 663-696 Davidson, D.M., Jr., 1972. Eastern part of Duluth Complex. In: Sims, P.K. & Morey, G.B. (eds.) Geology of Minnesota - A Centennial Volume. Minnesota Geological Survey, p. 354-360 Davidson, D.M, Jr., 1977a, Reconnaissance geologic map of the Eagle Mountain quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-28, scale 1:24,000 Davidson, D.M, Jr., 1977b, Reconnaissance geologic map of the Lima Mountain quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-32, scale 1:24,000 Davidson, D.M, Jr., and Burnell, J.R., Jr., 1977, Reconnaissance geologic map of the Brule Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-29, scale 1:24,000 Davis, D.W., and Green, J.C., 1997, Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution: Canadian Journal of Earth Science, v. 34, p. 476-488 Davis, D.W., and Sutcliffe, R.H., 1985, U-Pb ages from the Nipigon plate and northern Lake Superior: Geological Society of America Bulletin 96, p. 1572-1579 Green, J.C., 1972, North Shore Volcanic Group, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota—A centennial volume. Minnesota Geological Survey, p. 294-332. Grout, F.F., 1950, The titaniferous magnetites of Minnesota: Department of Iron Range Resources and Rehabilitation, 117 p. Grout, F.F., Sharp, R.P., and Schwartz, G.M, 1959, The geology of Cook County Minnesota. Minnesota Geological Survey Bulletin, v. 39, 163 p. Jerde, E.A. , 2001, The early gabbroic series of the Midcontinent Rift system: Continued assessment magmatic origins. 47th Annual Institute on Lake Superior Geology, Madison, WI, p. 36-37. Mathez, E.A., Nathan, H.D., Morey, G.B., 1977, Geologic map of the Hungry Jack Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-39, scale 1:24,000 Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Miscellaneous Map Series, M-119, scale 1:200,000 Miller, J.D. Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.E., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, 207p. w/ CD-ROM Morey, G.B., and Nathan, H.D., 1977, Geologic map of the South Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-38, scale 1:24,000 ILSG07 21 Trip 1 �Morey, G.B. and Nathan, H.D., 1978, Geologic map of the Gunflint Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-42, scale 1:24,000 Morey, G.B., Weiblen, P.W., Papike, J.J., and Anderson, D.H., 1981, Geologic map of the Long Island Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-46, scale 1:24,000 Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: geochonological insights to physical, petrogenetic, paleomagnetic and tectono-magmatic processes associated with the 1.1 Ga Midcontinent rift system: Journal of Geophysical Research, v. 98, no.B8, p. 13,997-14,013. Phinney, W.C., 1972, Northern prong. In Sims, P.K., and Morey, G.B., eds., Geology of Minnesota— A centennial volume. Minnesota Geological Survey, p. 346-353 Vervoort, J.D. and Wirth, K.R., 2004, Origins of the rhyolites and granophyres of the Midcontinent Rift, northeast Minnesota. 50th Annual Institute on Lake Superior Geology, Duluth, MN, p. 158-159. Weiblen, P.W. and Morey, G.B., 1980, A summary of the stratigraphy, petrology, and structure of the Duluth Complex. American Journal of Science, v. 280-A, 88-133. Weiblen, P.W., 1982. Keweenawan intrusive igneous rocks. In: Wold, R.J. & Hinze, W.J. (eds.) Geology and tectonics of the Lake Superior Basin. Geological Society of America Memoir 156, 57-82. Winchell, N.H., 1899, The Geology of Minnesota. Geological and Natural History Survey of Minnesota, Final Report v. 4, 354p. w/ 100 plates. ILSG07 22 Trip 1 �53rd Annual Institute on Lake Superior Geology FIELD TRIP 2 GEOLOGIC AND CULTURAL HISTORY OF THE GRAND PORTAGE NATIONAL MONUMENT William F. Cannon U.S. Geological Survey David J. Cooper U.S. National Park Service, Grand Portage National Monument Brian A.M. Phillips Lakehead University, emeritus Engraving of Grand Portage area made from field sketch done in 1849 by survey party from U.S. General Land Office. Illustration is included in report by David Dale Owen in 1852. Engraving shows physiography of the area as well as geologic cross sections. ILSG07 23 Trip 2 �GEOLOGIC AND CULTURAL HISTORY OF THE GRAND PORTAGE NATIONAL MONUMENT William F. Cannon, U.S. Geological Survey Brian A.M. Phillips, Lakehead University, emeritus Including a section on cultural history excerpted from a report by Douglas Birk, Senior Archeologist emeritus, Institute for Minnesota Archeology Introduction The Grand Portage National Monument, maintained by the U.S. National Park Service, consists of a restoration of an 18th century fort and trading post on the shore of Lake Superior and the 8.5 mile portage trail extending northwestward from the lake shore to the Pigeon River. The unrestored Fort Charlotte lies at the northwest end of the trail, on the south bank of the Pigeon River, at the head of a long series of rapids and waterfalls that made this long portage necessary in order to continue westward canoe travel from Lake Superior. The portage trail was the principal highway to the west for 18th century explorers, settlers, and fur traders who crossed the portage from Lake Superior to the Pigeon River, then into the westward draining Rainy River system, and eventually deep into the northern Great Plains and Rocky Mountains. It was over this trail that the wealth of furs was brought on their way to Montreal and eventually to Europe. Those who crossed the trail were largely responsible for the exploration and development of the Canadian west and northern U. S. Great Plains. The early fur traders, the Voyageurs, made their annual trek over the portage in late summer and then began their long canoe journey to the west to trade with native peoples for furs over the ensuing winter. They returned over the portage the following summer to the trading post to deliver their bounty of furs to representatives of the North West Company (NWC) who annually traveled from Montreal to the Grand Portage trading post. The geology of the Monument and surrounding countryside consists of: 1) the Rove Formation, a Paleoproterozic clastic sequence of shale and sandstone, 2) mafic dikes and flows formed in the Mesoproterozoic Midcontinent Rift, and 3) glacial and post-glacial deposits produced by the last tongue of Pleistocene glaciers that occupied the western Lake Superior basin and a variety of lake shoreline features formed by a series of post-glacial lakes that inundated the area shortly after deglaciation. During this trip we will tour the restored fort and trading post, discuss the European and Native American history of the site, and examine bedrock and glacial to post-glacial geologic features of the area, particularly as they affected the human history through the unique set of physiographic features that they produced. Figure 2.1 is a map of the Monument showing the location of restored buildings on Grand Portage Bay, and three geologic field trip stops. ILSG07 24 Trip 2 �Figure 2.1. Map of the Grand Portage region showing the location of the National Monument and portage trail. Geologic field trip stops are also shown. Cultural history (This account of cultural history is excerpted and slightly modified from Douglas Birk, 2005, National Register of Historic Places, Grand Portage) Grand Portage derives its name and much of its historical significance from a formidable portage, more than eight miles long, which links the Great Lakes with an interconnected series of inland waterways to the northwest. Key to the development of the North American fur trade, the Grand Portage gave Canadian explorers access to the northern Plains and to travel routes extending to the Arctic and Pacific Oceans. In the late eighteenth century the portage became a hub of seasonal coalescence and transshipment activities that linked suppliers in far-off places like England, the West Indies, and Brazil with a network of trading posts set in mosaic Native hunting territories throughout western Canada. The setting of Grand Portage, on the west side of Lake Superior and near the mid-point of a 3,000-mile commercial canoe transportation system, enhanced its role in channeling, shaping, and controlling the northwestern Montreal fur trade. Inspired by these qualities, one historian has declared that Grand Portage was once “probably the single most important fur trade location in the world.” North American Indian peoples knew the use of canoes and portaging thousands of years before the arrival of Europeans, and they initially opened the Grand Portage route. French colonials traveling west from Montreal first arrived at Lake Superior early in the seventeenth century. Having learned of the Grand Portage from Indian informants by at least 1722, the French began crossing it in 1731 to advance their affairs and alliances in more remote areas to the Northwest. From then through the mid-1800s, the portage attracted a colorful succession of French, British, and American fur traders. Between 1780 and 1802, at the height of the commercial traffic, Grand Portage was a western headquarters and regional trade center of the North West Company. It was also the scene of an annual summer gathering that made it one of the busiest nodes of human activity in the midcontinent. By promoting the mercantile interests of France and Great Britain, the Grand Portage facilitated the spread of European colonialism and the development of the nations of North America. In the nineteenth century Grand Portage also played an important part in settling the disputed boundary between Minnesota and Ontario, which is to say between Canada and the contiguous United States in the region west of Lake Superior. ILSG07 25 Trip 2 �The most enduring human presence at Grand Portage is that of aboriginal North Americans, including, most recently, the Ojibwe, Algonkian-speaking peoples with Woodland cultural traditions. Details surrounding the emergence of the Ojibwe people or the Ojibwe identity are often debated. Some traditional accounts indicate that, in the seventeenth and eighteenth centuries, the Saulteur (Ojibwe) of the eastern Lake Superior region extended their territorial prerogatives westward with the advancing fur trade. A group since known as the Grand Portage Ojibwe established ties to Grand Portage by the 1730s. They were active in that region throughout the fur trade era, and they remain a vital community today. All landscapes, resources, sites, properties, themes, and contexts that are now part of the Grand Portage National Monument story are in some way linked to the Grand Portage Ojibwe. The National Monument was formed through their invitation and cooperation, and efforts to maintain and interpret historic properties and traditions at Grand Portage have long been assisted by that local band or by individual members of the band. French and later British traders commonly entered the Great Lakes-Northwest trade by traveling west from Montreal. The use of Indian guides, birchbark canoes, and a vast network of established Indian canoe routes and portages assisted their movements (Morse, 1969; Birk, 1994). From the beginning, the fur trade built on entrenched Indian exchange practices while catering to Indian preferences and needs (e.g., White, 1982, 1987). Over time, as the business of accruing and transporting goods grew more complex and exchange frontiers expanded westward, the systems of trade were institutionalized. Certain individuals, like voyageurs, became specialists in the trade hierarchy just as certain places gained wider distinction as nodes or corridors of commerce. The most influential settlements were those that played strategic roles in the flow of workers, provisions, merchandise, and information (e.g., Hirth, 1976). Grand Portage became such a place in the eighteenth century. Through regular use, the Grand Portage emerged as a hub for local trade and as a transshipment center--a gateway community that linked markets and linear transportation lanes in the east with branching trade routes and mosaic trade districts to the northwest (Birk, 1984). Between 1731 and 1804 tons of supplies and furs were shuttled over the portage; some, in and out of the warehouses eventually built at either end of the trail. After 1760 the portage became a general rendezvous. It was a veritable beehive of activity during summers, but in winters the outposts there were comparatively quiet and staffed only by small crews engaged in local trading and facilities management (Gilman, 1992). At the height of the trade, around 1800, Grand Portage was the western headquarters of the North West Company (NWC) and the rival XY Company (XYC), two of the largest commercial establishments in North America. When the NWC and XYC moved their operations north to Kaministikwia (later Fort William, Ontario) at the start of the nineteenth century, Grand Portage lost its identity and its status as trade center, abruptly becoming remote to the main channels of trade and communication and less important to the outside world. Grand Portage is a place of majestic scale and natural beauty with many prominent landmarks. The lakefront area faces Grand Portage Bay, a horseshoe-shaped inlet that forms the deepest natural indentation on the Minnesota coast of Lake Superior (Schwartz, 1928). Flanking the northeast side of the bay is the rolling ridge of Hat Point (Pointe au Chapeaux), backed farther inland by the promontory of Mount Josephine, the top of which towers nearly 750-feet above the surface of the lake. The opposite, southwest edge of the bay is framed by the sloping headland of Raspberry Point (Pointe à la Framboise). About a mile offshore, in the center of the bay and partly shielding the bay from the big lake, is Grand Portage Island (also known as Pete’s Island, Sheep Island, Isle aux Mouton, etc.) (Gates, 1965; Grout and others, 1959). Near the lakefront within the Grand Portage National Monument is an imposing hill known as Mount Rose. The rounded and rocky summit of the hill rises several hundred feet above the bay and overlooks all of the historic properties clustered along the shoreline there (Winchell, 1899; Thompson, 1969; Gilman, 1992), as well as the mouth of Grand Portage Creek, a stream channel that drains from the interior highlands and skirts the base of Mount Rose before entering Lake Superior. Mount Rose once ILSG07 26 Trip 2 �served as a vantage to watch for approaching watercraft, the earliest such documented use perhaps occurring in 1767 (Carver, 1956). The Grand Portage is the first leg of a remarkable inland canoe route that passes from Lake Superior through a chain of lakes and rivers along the present international boundary between Ontario and Minnesota. That mainline route, sometimes called the “Voyageur’s Highway,” links with a vast network of other branching canoe trails in the hinterlands beyond (Morse, 1969). Few viable inland water routes emanate from the west side of Lake Superior, and the Grand Portage-Pigeon River route early proved to be the most direct and efficient gateway to the “border lakes” region and the far Northwest (Gilman, 1992). The Pigeon River is a relatively short stream whose headwaters at the rim of the Superior basin are only about 50 miles west of Grand Portage Bay and enters Lake Superior about five miles to the northeast of Grand Portage. The river is navigable by canoe for much of its length above Fort Charlotte. For 20 miles below Fort Charlotte, however, the Pigeon River is impassable. That stretch follows an eastward course through a sinuated series of rapids, canyons, and falls before reaching the big lake. The terrain on the Canadian side of the lower Pigeon River is too rugged and the distance between the lake and navigable parts of the river too great for portaging to be practical there (Schwartz, 1928; Buck, 1931; Burpee, 1931; Morse, 1969, Birk, 1998). The ascent from Lake Superior through the Grand Portage and up the Pigeon River to the height-of-land separating the Laurentian and Hudsonian basins is the steepest part of the entire historic canoe route that once ran between Montreal and Athabasca (Morse, 1969; Birk, 1998). Prior to the time the NWC made improvements to the trail, heavily outfitted canoe brigades could take up to ten days to move from Lake Superior to the Pigeon River (Wallace, 1934). During the heyday of the fur trade, with the NWC installed at Grand Portage, the same carry might take from five to seven days (Gilman, 1992). Some men loaded with packs or kegs weighing 180 pounds or more were able to make a round trip on the portage within six hours (Buck, 1931). Because porters often moved at a trot and took short rests en route, their actual progress was about as expedient as that of an unencumbered walker, who, under optimal conditions and without rests, might cross the portage in 2.5 hours (Thompson, 1969). Early fur traders tried using horses and oxen on the portage but under normal conditions in warmer seasons of the year, the trail was best suited for human porters (Burpee, 1931; Thompson, 1969; Gilman, 1992). Animal-drawn carts or sleds were more commonly used during the “off season” (other than summers) or during the nineteenth century, when traffic on the portage was greatly reduced. Diminished forest cover along some parts of the portage in the 1800s opened extensive views of the surrounding terrain and perhaps changed the surface environment of the trail (e.g., Winchell, 1899). In 1858, for example, the trail was said to be dry and in good condition, making it passable for oxen teams (Hind in Dawson, 1968). The Fort Charlotte site complex involves the remains of fur trade facilities that once stood along the Pigeon River at the head of the portage on either side of the mouth of Snow Creek. Evidence suggests that the NWC conducted operations along the river north of the creek (between the creek and the whitewater rapids), while the XYC and perhaps other earlier firms occupied an area along the river south of the creek. To reach these various facilities the portage trail probably forked somewhere east of the Pigeon River so that one branch of the trail led to the south side of the creek and the other continued along the north side. The main trail, the one most used and over the greatest length of time, was that on the north side. Underwater archeological investigations conducted at the main Pigeon River landing suggest that, through about 1780 or so, the main portage intersected the river very close to the mouth of Snow Creek. With the formation and growth of the NWC, the portage landing migrated downstream, first to the place where the NWC built a canoe landing and quay or “dock” (Winchell, 1899), and later to where the trail meets the river today, at a point about midway between Snow Creek and the head of the whitewater rapids (Birk, 1975; Birk and Wheeler, 1976; Wheeler and others, 1975; Birk, 1979). Modern recreational use of the landing, along with floating ice, fluctuating water levels, river currents, and log drives on the ILSG07 27 Trip 2 �Pigeon River, may have damaged early landing structures or associated offshore archeological deposits at Fort Charlotte (Birk, 1975). French traders frequented the portage between 1731 and 1760 (Nute; 1944; Woolworth and Wooworth, 1982) and perhaps earlier (e.g., Burpee, 1931), yet suspected French-era deposits or structures at Grand Portage Bay have eluded archeological discovery. Surprisingly little information about French activities at Grand Portage is found in available written records either. Nevertheless, some scholars suggest that French traders erected buildings at Grand Portage before 1760 (e.g., Warren, 1957; Buck, 1931; Babcock, 1940; Nute, 1944; Woolworth, 1967; Woolworth and Woolworth, 1982), that they had facilities at both ends of the portage as early as 1732 (Woolworth and Woolworth, 1982), or that they maintained structures at each end of the portage from ca. 1732 through 1760 (Woolworth and Woolworth, 1982). An unsubstantiated “French post” at the Pigeon River landing even has an assigned cultural resource inventory number (CR-105) (Woolworth and Woolworth, 1982). Other researchers are skeptical of such varied interpretations (e.g., Burpee, 1931; Thompson, 1969; Gilman, 1992). The apparent lack of Frenchcolonial materials on the lakefront at Grand Portage Bay west of Grand Portage Creek hints that the greatest potential for such materials there may lie in areas east of the creek (Brown, 1937). To date, the only documented French colonial archeological loci are those identified during field investigations at the Pigeon River landing (Birk and Wheeler, 1976) and at the suspected Parting Trees pose on the Grand Portage trail (Douglas A. Birk, personal observation). Barring future archival discoveries, most questions regarding French presence at Grand Portage may only be answered through further archeological inquiry. Figure 2.2. Artist’s conception of North West Company Depot at Grand Portage The earliest known trading houses at Grand Portage Bay were those established by British traders in about 1768. The houses were built west of Grand Portage Creek at the place later developed for the NWC depot (Nute, 1940; 1944). Other trade facilities were later constructed along the lakefront and, by 1793, the NWC alone was said to have sixteen buildings within its fort. The NWC also claimed dominion over adjoining parcels at the bay and may have opened facilities at the Pigeon River at the time of the its formation (1783). Rival traders soon followed the NWC’s example by building a “hangard or store” on the Pigeon River in 1785 (Gates, 1965; Thompson, 1969). Being more abundant and pervasive, material ILSG07 28 Trip 2 �evidence of British presence at Grand Portage has proved easier to find, identify, and interpret than that relating to earlier French colonial operations. Some features noted or implied in early historical records have yet to be identified through archeology. For example, the NWC had horses, cattle, hogs, and sheep at Grand Portage (Thompson, 1969; Gilman, 1992), which likely necessitated the use of barns, stables, pens, or corrals. The central clearing at Grand Portage Bay could have been used for pasturage, and grazing livestock there may actually have given the NWC a reason and excuse to fence the perimeters of that opening. To overwinter livestock required labor-intensive preparations. Hay was said to be abundant at Grand Portage in the late 1700s, but it had to be cut and stacked by hand. At the same time, cold damp fog, along with high levels of ground moisture and lingering winters, often caused harvested hay to rot (e.g., Thompson, 1969). Gardening was similarly impaired by weather and soil conditions. Potatoes were the only food crop typically grown with any success (e.g., Thompson, 1969; Woolworth and Woolworth, 1982). The size, location, and arrangement of the NWC’s barns, corrals, pastures, and gardens are presently unknown. Grand Portage National Monument Grand Portage was designated a National Historic Site in 1951. In 1958 Grand Portage National Monument was established to commemorate and preserve a premier site and route of the 18th century fur trade that led to pioneering international commerce and exploration in North America as well as cultural contact between Ojibwe and other Native societies and the North West Company partners, clerks, and canoe-men. The Monument was also established to work with the Grand Portage Band in preserving and interpreting the heritage and lifeways of the Ojibwe people. Grand Portage National Monument is a homeplace of tribal and family history and cultural persistence. Grand Portage National Monument contains reconstructed buildings and well-preserved archeological remains of several fur trading posts instrumental in the exploration of the West and in the economic history of the United States and Canada. The national monument contains the entire length of the portage that marked the entrance into the interior of western Canada. The national monument is significant because of the fundamental interrelationship of Ojibwe heritage and fur trade history. Grand Portage National Monument protects, commemorates, and interprets a reconstructed fur depot of the North West Company, a rendezvous site for international commerce and canoe route for transcontinental exploration, Native heritage, natural scene, and history of cross cultural contact and accommodation between traders, Ojibwe, and other participants in the fur trade. Today nearly 90,000 visitors pass through the reconstructed stockade and related buildings each year. Three reconstructed historical buildings currently serve these visitors. Based on archeological excavations and research from 1938 through the present, the reconstructed buildings have been furnished in the period of the late 1700’s. Technology from the time can be viewed in the Ojibwe Village and Voyageur Encampment staffed by National Park Rangers. The Great Hall and kitchen buildings contain reproduction furnishings similar to those used by late 18th century North West Company business partners, clerks, voyageur guides and Ojibwe families. In a room of the Great Hall from June until September, Ojibwe artisans create and sell beaded designs and birchbark basketry popular to Indian people of the Lake Superior region. From late May until early September interpretive programs are presented around the historical site. Subjects include historic cooking and baking, Ojibwe craft demonstrations, historic black powder musket firings, lever fur press operation, historic gardens and conducted walking tours. An annual highlight is Rendezvous Days, held the second week of August, during which the annual meeting of company representatives from Montreal, fur traders and local native people is recreated. Geologic history The geology of Grand Portage is discussed in regard to Precambrian bedrock features, a relatively uncomplicated story based on both Paleoproterozoic and Mesoproterozoic rock units, and Pleistocene and ILSG07 29 Trip 2 �Holocene history representing a dynamic story of geologically rapid landscape change caused by postglacial changes in the levels of ancestral Lake Superior. These latter changes overlapped with early human occupation of the area and undoubtedly had a major influence on human activity. The most recent geologic investigations of the monument were done in 2001 and funded by the National Park Service. Results are in administrative reports to NPS: (1) the bedrock geology was studied by W.F. Cannon and Laurel G. Woodruff of the USGS; (2) glacial and post-glacial geology was surveyed by Brian A.M. Phillips of Lakehead University. Much of the following descriptions are taken from those reports. Bedrock geology The bedrock geology of extreme northeastern Minnesota, including the Grand Portage National Monument, consists of Precambrian rocks of two separate ages. The bedrock geology of the Grand Portage region is shown in Figure 2.3. Most of the area is underlain by sedimentary rocks of the Rove Formation, formed during Paleoproterozoic time, approximately 1,850 million years ago. These rocks have been slightly tilted toward the southeast but otherwise are remarkably little altered for rocks of such antiquity. A second period of rock formation occurred during Mesoproterozoic time, approximately 1,100 million years ago. At that time diabase dikes were emplaced into the Rove Formation. These dikes are of two distinct generations separated by a short time interval and exhibit contrasting magnetic polarity. The oldest are known as the Grand Portage dike swarm and are generally east-trending features no more than a few tens of feet thick. Slightly younger dikes of the Pigeon River swarm are much more voluminous and support the higher ridges of the region such as Mt. Rose and Mt. Josephine. The dikes form a roughly orthogonal array of NE-trending and NW-trending bodies. Figure 2.3. Bedrock geologic map of the Grand Portage area in northeastern Minnesota. Geology extracted from digital files by Miller and others, 2001 The essentials of the geologic relationships were established by the earliest geological reconnaissance of the region conducted by J.G Norwood in 1849 (Owen, 1852). An etching from Owen’s report (see title ILSG07 30 Trip 2 �page) illustrates in a schematic manner the geology of the Grand Portage area. It shows large diabase bodies that form both sills, as on Pigeon Point, and dikes, as on Hat Point. “Slates” of the Rove Formation in Grand Portage Bay are cut by thin diabase dikes. There are three principal references on the geology of the region. First, Grout and Schwartz (1933) studied the Rove Formation and diabase intrusions and presented the first detailed maps of the Grand Portage area. In addition to a compilation of the regional geology, they presented maps of individual townships showing the geology and the location of bedrock exposures observed during their studies. Later, Grout, Sharp, and Schwartz (1959) published a report on the geology of Cook County, which provided some updated information on the Grand Portage area. Finally, in 1969, Morey published a detailed examination of the Rove Formation in northeastern Minnesota and adjacent parts of Ontario. This publication provides the best description and interpretation of the geologic history of the Rove and includes some specific observations within the Monument, particularly at Mt. Rose. Rove Formation The report by Morey (1969) provides an excellent description of the Rove Formation. The abstract of that report is presented below as a summary of the regional character of the Rove and some of our specific observations within the Monument follow. According to Morey (1969, p. 1-2): “The Middle Precambrian Rove Formation, the upper part of the Animikie Group, is estimated to be at least 3,200 feet thick and is exposed between northwestern Cook County, Minnesota and the Thunder Bay district, Ontario. It is a sequence of graywacke, argillite, locally abundant intraformational conglomerate, quartzite, and carbonate rocks. The Formation was deposited sometime between 2.0 and 1.7 b.y. ago in a northeast-trending basin, the configuration of which was controlled by a pre-existing structural grain. Detailed mapping in the 7 ½ minute South Lake quadrangle combined with a field and laboratory study of approximately 150 other scattered stratigraphic sections provide a basis for the recognition of three informal lithic units. From oldest to youngest these are: (1) lower argillite, 400 feet thick; (2) transitional beds of interbedded argillite and greywacke, 70 to 100 feet thick: and (3) thin-bedded greywacke, as much as 2,700 feet thick. It is concluded that the argillite and associated greywacke-sandstone and graywacke-siltstone units were deposited in moderately deep, quiet water. Repeated greywacke sedimentation units indicate sediment transport and deposition by turbidity currents. A sedimentation unit reconstructed from composite sections consists of (1) a basal conglomeratic graywacke, (2) a structureless unit which grades indistinctly into (3) a graded graywacke that is overlain by (4) a laminated graywacke, which may be modified by (5) small-scale cross-bedding, or (6) contorted bedding. Any one or several of these may be absent, but the units are always overlain by (7) an argillite. Post-depositional soft-sediment structures such as load casts, flame structures, clastic dikes, bed pullaparts, overfolds, and micro-faults indicate rapid deposition of Rove sediments, active bottom currents, and post-depositional deformation, implying a significant paleoslope. A detailed analysis of paleocurrent directional indicators such as groove casts, flute casts, dendritic ridges, and cross-bedding shows that the turbidity currents had a southerly trend about perpendicular to the axis of the Rove basin. However, ripple marks, winnowed lag deposits at the tops of many greywacke beds, and possibly some festoon-type cross- bedding show that the turbidites were later modified by bottom currents that trended southwesterly or parallel to the axis of the basin. ILSG07 31 Trip 2 �The heavy minerals of the Rove are characterized by epidote-group minerals, apatite, sphene, and tourmaline, and are typical of older Precambrian igneous rocks now exposed north of the present Rove outcrop area. Thin-section and x-ray analyses of 200 samples show that the graywackes consist of angular, poorly sorted grains of clastic quartz and plagioclase (An10-An20) embedded in an argillaceous matrix that now consists of quartz, chlorite, and muscovite. The fine-grained fissile argillite and mudstone have the same mineralogy and microtextures as the greywacke. Erosion subsequent to pre-Keweenawan tilting removed an unknown amount of the formation prior to deposition of Lower Keweenawan sedimentary rocks. The intrusion of Middle Keweenawan mafic igneous rocks caused local metamorphism of the Rove Formation to a variety of mineral assemblages now assigned to the pyroxene- and hornblende-hornfels facies, but the remainder of the formation is essentially unmetamorphosed.” An additional study of the Rove has been reported recently (Maric and Fralick, 2005) and presents a somewhat different picture from Morey’s 1969 description. Based on examination of a series of drill holes near Thunder Bay, they defined an internal stratigraphy consisting of: 1) a 150 m-thick basal unit of carbonaceous shale with lesser siltstone and fine-grained sandstone beds, which grades upward into, 2) a unit about 350 m thick containing more than 100 individually coarsening upward beds of sandstone with interlayered shale. This sequence is overlain by the uppermost preserved unit of 3) black shale with thin rippled sandstone beds. They interpreted the lower carbon-rich sediments to have been deposited in a sediment-starved anoxic basin in which very slow sedimentation rates resulted in accumulation of only 100 m of black shale during tens of millions of years of sedimentation. The upper coarser-grained units were deposited by a southward prograding turbiditic to shelf system. Most, if not all, of the Rove Formation exposed in the Grand Portage Monument is part of the upper thinbedded greywacke unit described by Morey (1969). Most exposures consist of flaggy, fine- to mediumgrained greywacke, with minor interbeds of argillite. The unit tends to split into slabs 1-2 inches thick parallel to indistinct bedding. The extensive outcrops and roadcuts on the north side of Highway 61 where the portage trail crosses the highway (Fig. 2.1, Stop 2.3) are typical of many of the other exposures along the trail to the north. The exposures on Mt. Rose (Stop 2) differ in that bedding of the greywacke is somewhat thinner and more distinct. Features such as cross-bedding and contorted bedding are present, although rare. An additional significant difference is the abundance of units of fine-grained fissile argillite as much as several tens of feet thick. The Rove Formation immediately adjacent to the Mt. Rose diabase dike is strongly metamorphosed. Original sedimentary textures are largely obliterated and metamorphic minerals, including biotite and clots of a mineral preliminarily identified as cordierite, have formed in the original greywacke matrix. Over the past few years, the age of the Rove Formation has been clarified by radiometric dating and the results provide a more precise age estimate than the 300 m.y. span indicated by Morey’s study in the 1960’s. Thin volcanic layers near the base of the Rove Formation near Thunder Bay, Ontario have been dated by the U-Pb zircon technique and indicate that deposition began somewhat before 1835 Ma. The dated volcanic layer is about 6m above the base of the formation and has an age of 1836+/- 8 Ma (Addison and others, 2005). The base of the Rove in that same area lies on a layer of ejecta believed to have originated from the major 1850 Ma meteorite impact at Sudbury, Ontario (Addison and others, 2005), further helping to bracket the age of the basal beds of the formation between 1850 and 1836 Ma. An additional age constraint comes from radiometric dates on detrital zircons from a bed about 400 m above the base and near the top of the preserved part of the Rove about 15 km north of Grand Portage. Concordant ages of individual grains are as young as 1777 Ma, providing a maximum age for deposition of the middle part of the Rove Formation (figure 2.4) (Heamon and Easton, 2006). A minimum age is not well constrained at this time. ILSG07 32 Trip 2 �Figure 2.4. Age spectrum for detrital zircons in the Rove Formation presented by Heamon and Easton (2006, figure 33). Spectrum indicates that a majority of detrital zircons were derived from a source younger than rocks of the Penokean orogeny to the south, and have come from a terrane with an abundance of zircons with ages of 1850 to 1750 Ma. These new age data require a rethinking of traditional interpretations of the Rove, both in terms of stratigraphic correlations and tectonic setting of deposition. The Rove Formation has been long considered to be the stratigraphic equivalent of major turbidite sequences to the south, such as the Thomson Formation in east-central Minnesota and the Michigamme Formation in northern Michigan. Recent discovery of the Sudbury ejecta layer in Michigan near the base of the Michigamme Formation confirms that the onset of sedimentation was nearly synchronous across the region at 1850 Ma. However, the duration of sedimentation between Michigan and northern Minnestoa/Ontario appears to be drastically different. The 1777 Ma concordant detrital zircons from the Rove are from a bed about 400 m above the base of the formation. Although this is near the top of the preserved formation in the sample area, a total thickness nearly 1000 m was estimated for the Rove (Morey, 1969). Therefore, sedimentation may have lasted well beyond the 1777 Ma maximum age. In Michigan, it is well established that the Michigamme Formation was deposited, deeply buried by tectonic overthrusting, metamorphosed, and intruded by posttectonic granite by 1835 Ma (Schneider and others, 2002; Schneider and others, 2004), which is the approximate age of the volcanic layers near the base of the Rove. Thus, much of the thick clastic sequence of the Rove appears to be entirely younger than the Michigamme Formation. The Rove has commonly been considered to have been deposited in the foreland sedimentary basin north of the 18901835 Ma Penokean orogen, but it now appears that sedimentation was almost entirely after the Penokean deformation ceased in Michigan and Wisconsin. The detrital zircon suite in the Rove, as shown in figure 4, consists dominantly of grains in the 1850-1750 Ma age range rather than Archean zircons, indicating that a source area with rocks of that age was well established and contributing the majority of detritus to the basin during at least the younger part of Rove deposition. Detrital zircon analyses of the Thomson and Rove Formations in Minnesota yielded somewhat similar results (Kerber, 2006). Although no zircons younger than about 1800 Ma were found in either formation based on analyses of a single sample from ILSG07 33 Trip 2 �each, the great preponderance of zircons are in the range 2000-1800 Ma and Archean zircons are scarce). Thus, based on presently available data, it appears that the preponderance of detritus contained in the vast turbidite deposits of the Rove, Virginia, Thomson, Tyler, and Michigamme Formations across the western Lake Superior region was derived from a source terrane(s) with ages in the approximate range 2000-1750 Ma. A widely accepted model for this turbidite deposition has been a tectonic foreland in which subsidence was being driven by northward overthrusting of arc rocks of the Wisconsin magmatic terranes, now exposed in northern Wisconsin, and that these rocks provided much of the detritus of which the turbidites are composed. This model is still acceptable for the southern parts of the basin, but complications arise in applying it to our current understanding of the Rove Formation. Relationships in Michigan and Wisconsin indicate that tectonic loading by overthrusting onto the craton margin was completed by 1835 Ma (Schneider and others, 2002). However, deposition of the basal units of the Rove was just beginning at that time. Deposition of the Rove continued for at least 60 million years after that time, long after the end of the Penokean orogeny, and basin subsidence of about 1 km occurred during that period. What was the driving mechanism for that subsidence? An additional enigma is provided by a wealth of paleocurrent data from the Rove which indicates a consistent southerly sediment transport (Morey, 1969). This direction is inconsistent with a sediment source from the Wisconsin magmatic terranes. The preponderance of Paleoproterozoic detrital zircons suggests that much of the Rove detritus was derived from a Paleoproterozoic orogen north of the Lake Superior region, perhaps the Trans-Hudson orogen in part, which contains rock units as young as 1775 Ma. Thus the Rove Formation no longer fits well into the model of Penokean foreland sedimentation because the time of sedimentation and sediment source directions are inconsistent with that model. The nature and cause of the Rove basin is a newly emerging topic for further study. Because so much of the Rove sedimentation is significantly younger than the Penokean orogeny and paleocurrent data indicate that the Penokean orogen is not the source for the Paleoproterozoic-aged detritus that seems to make up most of the Rove, it is necessary to divorce the Rove, and perhaps at least part of the correlative Virginia Formation, from the suite of features ascribed to the Penokean orogeny, and seek a cause in younger orogenic events. It appears, based on the most recent data, that the basal parts of the Rove Formation composed of black shale deposited in a sediment-starved basin may be a product of Penokean foreland basin development. However, the remaining coarser clastic deposits of the upper part of the Rove reflect a reactivation of the basin during a time spanning the 1775 Ma maximum age of the middle part of the formation. A recent reinterpretation of the tectonic assembly of the north-central part of the US (NICE Working Group, in press) indicates that the mid-geon 17 Yavapai orogeny extends eastward from its originally defined extent in the southwestern US into the southern part of the Lake Superior region. A belt of gneiss domes, granite plutons, and metamorphic nodes also developed across central Minnesota and northern Michigan at that time (Schneider and others, 2004; Holm and others, 2005). Thus, it has been established recently that major mid-geon 17 tectonic activity occurred only about 200 km south of the Rove basin. Is this tectonic episode responsible for reactivation of the older Penokean foreland basin and deposition of the Rove Formation? Although these Yavapai-age events might in some poorly understood manner have caused basin subsidence and also provide a source of detritus of the same age as that established for the Rove, the enigma of southerly-directed sediment transport in the Rove of material of that age remains unresolved. Grand Portage diabase dikes A set of thin diabase dikes was described by Green and others (1987) that they named the Grand Portage dike swarm. These dikes are present throughout the Grand Portage region and intrude the Rove Formation as well as the Middle Proterozoic volcanic rocks exposed south of Grand Portage. More than 50 of these dikes have been identified, some of which are shown on maps in Green and others (1987) and ILSG07 34 Trip 2 �in the digital compilation by Miller and others (2001). The dikes have a relatively uniform trend from N65o – 90o E. The Grand Portage dikes tend to be quite strongly magnetized, but acquired their magnetic properties as they cooled during a period when the earth’s magnetic field was inverted. This reversed magnetic polarity interferes with the present earth’s magnetic field resulting in distinctive linear negative anomalies on magnetic maps. This characteristic makes them quite easy to identify, even in areas where they are not exposed. A combination of surface exposures and magnetic expression has allowed us to identify 11 new dikes of the Grand Portage swarm that cross the monument. The most prominent of these are a set of five dikes that are exposed along the east side of Mt Rose (one is just south of the Monument boundary, the other four are on U.S. Park Service property). Each dike stands as a low to prominent ridge as much as 10 m wide as a result of differential erosion between the relatively hard diabase and much softer Rove Formation. One of these dikes shown in Figure 2.5a is probably the dike that inspired the somewhat fanciful etching that appeared in Owen’s 1852 report (Figure 2.5b). Figure 2.5. a- Diabase dike of Grand Portage swarm at south edge of Monument property. b- Etching from Owen (1852) based on field sketch, probably of the dike in a. ILSG07 35 Trip 2 �The magnetic profile shown in Figure 2.6 illustrates the sharp negative magnetic anomalies caused by these dikes. Three other similar anomalies are shown on a magnetic profile along the Grand Portage trail. Although dikes are not exposed at the surface in these areas they are inferred with confidence to occur beneath the surficial cover material. Two other dikes were identified in outcrop just north of Highway 61 but their magnetic expression is less distinct. Figure 2.6. Magnetic profile measured at 100 foot spacing along roadway northward from south boundary of Monument. The four sharp negative anomalies are caused by the four Grand Portage dikes exposed along the roadway. Pigeon River diabase dikes The Pigeon River diabase dike swarm (Green and others, 1987) is compositionally similar to the Grand Portage dikes but is slightly younger and volumetrically much more prominent. The Pigeon River dikes trend mostly about N65oE and less commonly about N40oW. Because of their hard, massive nature they form the prominent, steep-sided ridges of the region, including Mt. Rose. Because they have normal magnetic polarity, these dikes tend to produce positive anomalies on magnetic maps, but they are apparently somewhat variably magnetized so cannot be recognized in magnetic data as readily as the Grand Portage dikes. Absolute ages of Grand Portage and Pigeon River dikes The Grand Portage and Pigeon River diabase dikes have not been dated directly, but their age can be closely approximated because of their relationships with other nearby units that have been dated. A regional correlation of rock units related to the Midcontinent Rift and a summary of radiometric ages was published by Nicholson and others (1997). The Grand Portage dikes are known to cut the Hovland and Grand Portage basalts south of the Monument. Those basalts were erupted about 1,110 to 1,105 million years ago. The shift of the earth’s magnetic field from reversed to normal polarity occurred about 1,100 years ago. The intrusion of the Grand Portage dikes, therefore, must have occurred after eruption of the Grand Portage basalts, but before the magnetic field reversal in the interval 1,105 to 1,100 million years ILSG07 36 Trip 2 �ago. The Pigeon River dikes, which have normal magnetic polarity, are younger than 1,100 million years. They may be related to the Lutsen basalts that were erupted about 1095 to 1090 million years ago. Glacial and post-glacial geology The effect of ice Because ice advances reached as far south as Illinois, Iowa, Nebraska, and Kansas numerous times during the Pleistocene it is safe to assume that the Grand Portage area has been affected by the passage of several ice lobes over the last 500,000 years. The general direction of ice movement was from the north and northeast, and present-day landscapes in the borderlands first were sculpted of sub-glacial processes of plucking, abrasion and the high pressure flow of sub-glacial meltwater. Next, the processes of deglaciation resulted in deposition of ablation tills and a variety of fluvioglacial landforms (eskers, kames, ice-contact deltas) and sediments (sorted and unsorted silts, sands and gravels). Meltwater, temporarily ponded between the waning ice margin and enclosing topography, formed short-lived pro-glacial lakes that changed in area and depth as the configuration of the ice margin changed. Many of these lakes existed long enough to deposit substantial layers of lacustrine clays, and many ended their existence by draining catastrophically down spillways exhumed by the melting ice. Inundation of the land recently revealed by ice melt was common as various lake bodies occupied the Lake Superior basin at different times and water levels, depending upon the rate of water supply and the loss through such outlets as were available. In the Grand Portage area, the present topography is the product of the effects of glacial ice. Rock types more resistant to glacial processes remain upstanding and less resistant rock materials have been more deeply eroded and removed. However, ice must be considered as a very viscous liquid, not only exerting its influence on the underlying topography, but also being influenced by that topography. Though ice crossed the area from generally north to south, the already upstanding diabase ridges that characterize the area will have deflected more recent ice events. As shown in R.P. Sharp’s map (Fig. 2.7), glacial striae, the grooves ground into solid rock by the scraping action of sub-glacial debris, are found sub-parallel to the ridges rather than orthogonal to them (as in R5E) (Sharp, 1953). The most effective ice erosion is performed early in ice advance. Later, under increasing thickness of ice, erosion of the underlying ground surface all but ceases. Shearing takes place between upper layers of ice sliding over lower layers of ice that are trapped within the topographic depressions. This trapped ice becomes largely stagnant and therefore ineffective at erosion. The topography of the Grand Portage area suggests that this scenario is quite likely to have taken place. ILSG07 37 Trip 2 �Figure 2.7. Part of R.P. Sharp’s map of Cook County, MN (Sharp, 1953) showing glacial striae directions (arrows). Sub-glacial water flow It is likely that the topographic differentiation between the hard intrusive rocks and the less resistant sedimentary rocks began in pre-glacial time by the time-honored agents of runoff, wind and chemical weathering. At this time too, the conspicuous breaks in the ridge features are likely to have first been formed. A number of major faults, along which detectable vertical movement has taken place, cut across the area (Fig. 2.3), but many unmapped smaller faults and shatter zones also are present. These linear weaknesses in the hard rock materials are the first to be exploited by the agents of denudation, and once initiated, processes such as running water become focused in these locations, exacerbating the erosional effects. The role of sub-glacial water flow was particularly important. Unrecognized by earlier workers studying glacial geomorphology, water flowing beneath and within ice is often under considerable hydraulic pressure. Unaffected by gravity, such flow can run counter to the slope or topographic grain of the underlying surface. The force of water, carrying subglacial debris and running at the interface of the ice and ground surface, is a highly effective erosive tool. Scouring of rock surfaces and the erosion of steep sided channels into the rock surface (called tunnel valleys) can take place. In post-glacial landscape glacial meltwater flow in these valleys might be seen to run uphill, cross divides or run contrary to the drainage pattern. Some are used by present drainage, but many remain dry or hold lakes and bogs. Associated with these sub-glacial features are eskers, which represent a final stage of deposition within the tunnels. These eskers might run along the valleys in places or climb over the margins onto ground alongside the main channel. ILSG07 38 Trip 2 �Isostatic uplift Although the mechanism for post-glacial isostatic uplift or rebound remains inadequately explained by geophysics, the effect of this mechanism can readily be observed and measured. The process began as ice margins began to wane and withdraw, and continues to the present day. Greater uplift occurs where greater depression was made by a greater weight of ice. As a result, models for post-glacial rebound see greater and longer continuing uplift towards the center of the former ice mass. Figure 2.8. Shoreline diagram for the Grand Portage area showing present elevations of former lake shores. Along the Minnesota north shore the axis of uplift is sub-parallel to the shore, and currently, uplift at the Pigeon River is about 250 feet greater than at Duluth. Consequently, an initially horizontal line, such as a lake shoreline, is now tilted such that its elevation rises over this range between Duluth and the Pigeon River. The sequence of lakes that occupied the Superior Basin as ice withdrew and isostatic uplift occurred resulted in a staircase of shorelines found along the north shore, diverging to the northeast (Fig. 2.8). Thus in Cook County, the vertical distance between these former shoreline features is greatest at Grand Portage. A more detailed account of pre-historic water levels can be found in Phillips (2001, pp.828). Evidence of former lake margins Typically, the presence of a lake margin, sustained for a period of time sufficient for waves to do work, is represented either by an erosional form such as a bluff, or by a depositional form such as a beach. In addition, rivers entering the lake body tend to deposit fluvial sediments in the form of deltas, or at least a series of river mouth bars that characteristically curve out of the river mouth and along the shore. Both features mark the position at which rivers formerly met with the shore of the lake. ILSG07 39 Trip 2 �Streams along the Minnesota north shore have had to respond to steady isostatic uplift and falling water levels. Typically, this has resulted in the lengthening of stream courses and the deep incision of their beds as they struggle to meet the lakeshore at grade. Not surprisingly, the north shore abounds in gorges, waterfalls and rapids as outcrops of more resistant rock inhibit equal downcutting along the stream course. Another characteristic feature that results from isostatic rebound is the building of deltas at successively lower elevations as the stream course extends, each delta in part built from the material eroded from an earlier one as a result of incision into the former delta surface. The Grand Portage Trail crosses a number of these features, each denoting a period of lake level stability lengthy enough for a defined landform to be formed (Figs. 2.9, 2.10). The features have significance for finding Paleo-Indian artifacts, since it is established that early people tended to favor shoreline and river mouth locations for their temporary habitation and activities. The succession of shorelines on the Grand Portage Trail As Marquette ice, the youngest glacial lobe in the western Superior basin, melted back and exhumed the basin successively from southwest to northeast, the highest post-Marquette shorelines are those of Lake Duluth, the earliest phase of which was restricted to the Duluth area. When ice still covered the Thunder Bay region, Lake Duluth first extended across the international border into the southern part of the Nor’wester mountains. At Grand Portage, the high Duluth level was at 1340 feet and although Mt. Maud was now part of the mainland, no part of the Grand Portage Trail was above water. Figure 2.9. Idealized former water levels at Grand Portage. The gradual withdrawal of ice uncovers recently glaciated ground. If, as in this case, the ground is lower than the ice marginal (pro-glacial) lake, it is immediately inundated and protected from exposure to weathering and erosion, even obtaining a modest cover of pro-glacial lake (glaciolacustrine) clays or silts if under water long enough for offshore sediment to accumulate. ILSG07 40 Trip 2 �Portions of the Grand Portage Trail first appeared above water during the later Low Lake Duluth phase (1290 feet). A good portion of the Trail north of Old Hwy 61 lay above water, including the uppermost part of the narrow ridge along which the Trail runs for a mile or more. This part of the Trail, having been inundated only briefly, is not overlain by lacustrine sediments. The thin till is wave washed, and bare rock is exposed in many places. A pattern can be seen emerging here. High areas, subject to inundation for a lesser period, are less likely to be covered by lacustrine sediments. Lower areas, subject to a long period under water may accumulate lacustrine sediments, though only if a source of such sediment is available. To a large extent, what happens is related to the degree of shelter or exposure to wave action. This will vary locally as the water level lowers. Sheltered areas may show little modification of the existing sediments, but exposed areas are likely to show winnowing of the glacial sediments, removal of finer particles and a lag of larger clasts left on the surface. Sufficient wave action may have time to sort and transport sediment, forming recognizable beach or bar features where longshore transport is consistent. Extreme wave action may have the energy to erode and form soft-sediment bluffs, or even undercut and fashion solid rock into wave-abraded forms. In summary, as the already complicated topography of the Grand Portage Trail area slowly emerged, an ever-changing pattern of wave refraction around the shoals, islands and peninsulas led to a localized sculpting of the topography. At the Lake Highbridge level (1200 feet), the present Grand Portage Trail was divided into two clearly different portions. The Trail above Old Hwy 61 was above water, while that below the highway was still inundated. This dual character is still very visible today. The character also occurs into the High Lake Washburn phase (1110 feet), with the steep section of the Trail immediately above Old Hwy 61 forming a fairly stable promontory, down which the water level fell in stages (Figure 2.10). Meanwhile, Mt. Josephine emerged, as did higher parts of the narrow diabase ridges, the links gradually joining as water level fell. During the High Lake Washburn phase (Fig. 2.10A), two ‘gaps’ were noticeably still inundated. The eastern one marks the line of the Grand Portage fault, and the Grand Portage Trail passes through the western one. At this time, the Poplar Creek basin had become significantly protected from wave action, though, as in present Lake Superior, water flow through the two gaps was likely intensified by seiche and meteorological events, leading to possible scouring and some transfer of sediment to one or both sides of the ‘channel’. The lakeward side of the emerging ridges probably experienced the full force of Lake Superior wave action, so the transfer was likely to have been mostly towards the Poplar Creek basin. Within the relatively calm waters of the basin, lacustrine sediment would have accumulated liberally. It is during the High Lake Manitou level (1020 feet) (Fig. 2.10B) that the ‘gaps’ became dry. On both sides of the gaps, some crossing feature such as a beach or a bar was likely, though if earlier scoured to bedrock, little sediment might have been available to form these. If soft sediments formed a shoreline feature in such an emerged gap, it would be named a ‘tombolo’, in that it joined mainland to a former island. This is a feature only occasionally seen on the shore of present Lake Superior. During the High Lake Manitou level, the basin of the Grand Portage Creek also became a more sheltered reentrant, and the upper part of the creek course probably flowed. At the subsequent High Lake Beaver Bay level (920 feet), a distinct Grand Portage Bay emerged and Grand Portage Creek was a sheltered river mouth extending far up valley. This character was less pronounced by the time of Mid Lake Beaver Bay (845 feet) (Fig. 2.10C), when the summit of Mt. Rose appeared as a small island, but the potential for river mouth habitation by Paleo-Indian people is good during the five Beaver Bay lake stages. As Mt. Rose became a larger island during the Lower Lake Beaver Bay stage (775 feet), it probably protected the river mouth and the potential for habitation was sustained. At this time, wave refraction around Mt. Rose would have been quite complex and the area immediately behind the steep island would have been very protected. Lake Minong, dated as about 9,500 BP, is notable as the first of the succession of lakes to occupy the whole of the ice-free Superior basin, and its shoreline can be traced around the present lake. Lake ILSG07 41 Trip 2 �Minong time is also notable in the Minnesota-Ontario borderlands as a period in which evidence of PaleoIndian activity became more pronounced than before. In the Grand Portage area, the Minong shoreline occurs at 715 feet. A defined bluff forms a deep ‘V’ shaped reentrant into the valley of the Grand Portage Creek, which the Grand Portage Trail now surmounts. At this time, Mt. Rose ceased to be an island and further contributed to the sheltered nature of Grand Portage Bay (Fig. 2.10D). The potential for PaleoIndian occupation around this sheltered river mouth is high. Figure 2.10. Reconstruction of former shorelines at five levels of post-glacial lakes in the Grand Portage region. ILSG07 42 Trip 2 �The Post-Minong period was a time when pro-glacial Lake Agassiz flowed through a series of eastern outlets into the Nipigon basin, and thence into Lake Superior and the lower Great Lakes. At least five episodes of catastrophic discharge have been recognized (Teller and Thorliefson, 1983). Each had the potential to briefly raise the level of the Post-Minong lakes, and erode the glacial sediments that swathed the St. Mary’s river outlet to Lake Huron. Ultimately, the outlet was eroded to bedrock and the Lake Houghton phase was established as the lowest of the succession of post-glacial lakes in the Superior basin. Water level declined to a low position of 582 feet in Grand Portage Bay, placing the shoreline adjacent to Grand Portage Island, which separated two shallow bays. Little if anything remains of this portion of history, because there followed a steady rise of water level in the Superior basin as the St. Mary’s outlet was isostatically raised. This transgression drowned the recently exposed Post-Minong surface and culminated in the Nipissing shoreline, which in places truncates and reoccupies older shoreline features. In the Grand Portage area the Nipissing rose to 636 feet, preserving the Minong and the upper PostMinong shorelines (Fig. 2.10E). Near Grand Marais, the Nipissing inherited the older Minong bluff, refreshing it, and further south the Minong was destroyed and drowned by the Nipissing transgression. Subsequently, water level fell to the Lake Algoma level (621 feet) and the Sault level (610 feet) before declining to present lake level (602 feet). Features formed by these more recent water levels run through the area of the Stockade, though more recent shore erosion has removed the lowest. In summary, the present Grand Portage Trail traverses a landscape that has been fashioned first by ancient tectonic activity that emplaced the pattern of dikes, which are responsible for the basic topographic character of the area. Secondly, multiple glaciations have ground across the ridges and basins, steepening and smoothing rock surfaces, and depositing varying depths of sandy, coarse tills, particularly in lower areas. Thirdly, a succession of post-glacial lakes has fully submerged the course of the Trail. As the water level gradually declined, the Trail topography progressively emerged. These lakes have left some evidence of their brief periods of stillstand in the form of wave-washed sediments, erosional bluffs and constructional beaches and bars. They have also deposited a variable thickness of lacustrine clays and silts, particularly within the basins and valleys of the bedrock topography. The geological underpinnings of human history of the Grand Portage region Early human activity in any region is strongly influenced by physical barriers to transit and by the easiest solutions to the impediments that those barriers represent. Geological events that occurred from as little as ten thousand years ago to as much as 1.1 billion years ago largely predetermined the course of European history in the Grand Portage region because they controlled the physiography and the unique physiographic characteristics of Grand Portage that made it the obvious location for the “highway to the west”. These events not only dictated that the portage was necessary, but determined the best route for the portage, and, on a broader scale, were responsible for the enormous advantage to westward travel inherent in making the portage. European exploration and settlement of the continental interior was strongly guided by the relatively easy water access provided by the Great Lakes as far west as western Lake Superior. Access farther west into the deep interior of the northern Great Plains, Canadian prairies, and beyond was facilitated by additional relatively easy water routes up the Pigeon River, over the continental divide, and through the Rainy River system westward to major north-flowing drainages and their tributaries. The shortest distance from Lake Superior to waters navigable westward by canoe is over the Grand Portage. This short route to the west was known and used by Native Americans well before the first Europeans arrived in the early 1600’s. ILSG07 43 Trip 2 �But the stretch of high ground from the lakeshore to the upper reaches of the Pigeon River was the most prominent physical barrier along the entire westward route from Montreal to the Rocky Mountains. In fact, the length and difficulty of the portage lead to a mutiny among the first westward-bound French exploration party that arrived at the site in 1731. The portage was necessary because of a long series of rapids and falls along the lower reaches of the Pigeon River which falls about 660 feet from the upper end of the portage down to the level of Lake Superior. But a remarkable feature of this area is that once past the 8.5 mile portage, the way west was through the relatively flat water of the upper Pigeon River and a series of interconnected lakes. Within only about 40 miles from the west end of the portage one leaves the Lake Superior drainage, crosses the continental divide, and enters the Rainy Lake watershed which drains ultimately to Hudson Bay. Thus, three regional features: 1) Lake Superior and the easy route that it provides for water-borne commerce from the east, particularly Montreal, 2) the steep gradient of the Pigeon River in its lower reaches that made canoe transit impossible, and 3) the very small size of the Lake Superior watershed which, once exited, provided essentially unlimited canoe access to the interior of North America, all played fundamental roles in making the Grand Portage the most favored and easiest route for westward travel. The present basin of Lake Superior is the exhumed sediment-filled central valley of an ancient rift that formed 1.1 billion years ago. At that time a great rift valley, known as the Midcontinent rift, began to split the North American continent. The rift was filled first with tens of kilometers of volcanic rocks, mostly basalt such as the Grand Portage basalts exposed just south of the Monument, and finally by many kilometers of sediments, now mostly red sandstones. Figure 11 shows the trace of the axis of the Midcontinent rift which faithfully follows the southward concave shape of the present lake. The sandstones in the central part of the rift presented an easily erodible substrate for glacial scouring in contrast to the much more durable igneous rocks along the rift flanks, which today stand as the prominent ridges and uplands of the region. As a result, repeated glaciations during the Pleistocene gouged the deep valley now occupied by Lake Superior. Thus, Lake Superior and its favorable transportation route owe their existence to these two ancient events of rifting and glaciation. But both of these events combined to create the unfavorable situation of the extensive series of rapids and waterfalls along the lower reaches of the Pigeon River, which required the arduous “grand portage” to circumvent them. The Pigeon, along with all other major rivers along the northwest shore of Lake Superior, reach the lake over a series of rapids and falls that extend nearly to the lake shore. This is in contrast to the more typical morphology of rivers which commonly have extended reaches of slack water upstream from their terminus and thus provide waterborne access well inland. It is again the repeated glacial scouring along the ancient rift axis that created this situation. Numerous lobes of ice flowed southwestward through the western Lake Superior basin and oversteepened the topographic gradient near the present lake shore. This steep gradient extends well below the present lake level into a bedrock trough whose bottom is more than 1500 feet below the lake and about 1000 feet below sea level. In the relatively short period since the last glaciation, about 10,000 years, the rivers have not had adequate time to modify this glacially induced gradient and thus plunge down this steep topography until they intersect the modern level of Lake Superior making upstream canoe travel from the river mouths impossible. A still more fundamental geologic phenomenon is responsible for the highly unusual situation of a remarkably small watershed surrounding Lake Superior thus allowing a short distance of transit from the lake to outward flowing drainages. Modern interpretations of the geodynamic causes of the Midcontinent rift suggest that the region was underlain by a newly developing plume of upwelling mantle which caused both intense basaltic volcanism along the rift and the active splitting of the continent (Hutchinson and others, 1990; Cannon and Hinze, 1992). As the new plume waned in intensity, welding of cooling mantle material to the base of the crust, a so called rift pillow, resulted in an unusually thick crust in the Lake Superior region. Interpretation of seismic refraction surveys shows that present day crust beneath the lake is as much as 55 km thick in contrast to a typical thickness of about 40 km in surrounding regions (Halls, ILSG07 44 Trip 2 �1982). The isostatic effect of this thick crust is to maintain the Lake Superior region as a subtle topographic high (Allen and others, 1992). Examination of the continental scale drainage patterns (Fig. 11) shows that the Lake Superior watershed is anomalously small in relation to the size of the lake, and that major river systems drain radially away from the lake as a result of this isostatic bulge. Thus Lake Superior is, in a sense, a hub from which numerous possible outward draining water routes are available once the Superior divide is crossed. The Grand Portage is the shortest of these available routes. On a smaller scale, geologic features also played a significant role in determining the course of the portage and both its upper and lower terminus. Grand Portage Bay is the most sheltered harbor anywhere along the northwest shore of Lake Superior and thus was a naturally favorable location for developing the trading post at the western terminus of the Great Lakes transportation route. The bay is underlain by relatively non-resistant sedimentary rocks of the Rove Formation which make up most of the low country in the region. The sheltering headlands are formed from resistant igneous rocks. To the northeast, Hat Point and the high ridge extending inland is a reflection of the Mt. Josephine dike of the Pigeon River swarm. To the southwest, basalt flows of the Grand Portage Volcanics support the topographic high that forms the southern shore of the harbor. These same volcanic rocks also underlie Grand Portage Island that lies across the mouth of the bay and provides additional shelter. In order to traverse westward from Grand Portage Bay a way needed to found through the imposing steep-sided northeast-trending ridges that are supported by the resistant diabase of the Pigeon River dikes. A gap through these ridges is now followed by Grand Portage Creek, which flows along a fault trace (see Fig. 2.3). Although this fault has only minor offset, it apparently created weakened rocks that were either eroded by pre-glacial streams or were plucked by glacial ice or subglacial streams. In any case, this topographic gap is readily seen from Lake Superior and likely attracted the earliest humans in the area as a relatively easy course to the west. At the western terminus of the portage, local geology also played a role in determining the most favorable location at which canoes could be easily embarked and disembarked. Fort Charlotte was established at a point where a diabase dike, probably of the Pigeon River swarm, intrudes the Rove Formation and trends across the Pigeon River. This resistant dike forms a small topographic rib which creates a small rapid on the river that could not be crossed by canoes and, perhaps more significantly, impounded an upstream reach of still and unusually deep water creating a small natural harbor. These features illustrate the importance of the geologic history of the region and its influence on present physiography. It was both local and regional physiography that dictated the course of human occupation and exploitation of the Grand Portage region and its enormous significance in the 18th century history of the northern part of the continent. This “geological predetermination” of human events serves as a prime example of the interaction between the physical challenges encountered by early human exploration and settlement of a region and the ingenuity and industry with which they adapted to those challenges and opportunities. ILSG07 45 Trip 2 �Figure 2.11. Map showing the Lake Superior watershed (stippled), the axis of the Midcontinent Rift (short dashed line), and regional drainage patterns, simplified from Allen and others (1992). Arrows indicate direction of drainage and illustrate the central position of Lake Superior relative to the outward radial continental drainage pattern. Reference circles of 250 km and 500 km radius from western Lake Superior, the presumed central point of the mantle plume responsible for the rift. ILSG07 46 Trip 2 �GEOLOGICAL STOPS The following are brief descriptions of specific geological features that can be observed in several different areas of the Grand Portage National Monument. Locations of stops are shown in Figure 2.1. Stop 2.1. Stockade to Visitor Center: Grand Portage diabase dikes, Rove Formation, and glacial lake features. Shoreline features of some of the lower post-glacial lakes are visible near the Stockade. A distinct feature that runs along the shore zone and through the Stockade is the gentle bluff of the Sault stage (610 feet). To the east of the Grand Portage Creek, the bluff curves lakeward on Premiers Point, and runs northeast through the picnic area, parallel to the shore until rising erosional bluffs of Lake Superior truncate and eliminate it. Offshore, a lake floor of large boulders that fringes the east side of the sandy creek channel indicates the former extension of Premiers Point. Just above the Sault shoreline lies another curving bluff, the Algoma (621 feet). To the west of Grand Portage Creek Creek, the Sault bluff runs through the Stockade enclosure. The Great Hall building is constructed on the bluff, such that the lakeward side lies at the foot and the inland side lies on the top. The bluff forms a marked step through the Ojibwe Village exhibit to the west of the Stockade and is then truncated again by rising erosional bluffs near the Visitor Center. Behind the Canoe Warehouse in the Ojibwe Village exhibit, a small section of Algoma bluff has survived the disturbance of road construction. As you walk the roadway between the stockade and visitor center (under construction in 2007) you will cross four diabase dikes of the Grand Portage swarm (Green and others, 1987). A fifth dike crops out just south of the Monument boundary and is especially prominent as a free-standing wall of diabase about 20 feet tall, a classic geomorphic dike form. Figure 5 compares a recent photograph of this dike with an etching from Owen’s 1852 General Land Office report. All of the dikes form linear ridges up to a few tens of feet wide as a result of being much more resistant to erosion than the Rove shale into which they are intruded. The topography created by the dikes was probably emphasized by wave erosion along the glacial Lake Nipissing shore when water level was a few tens of feet higher than present Lake Superior. Pocket beaches were probably formed on the Rove Formation between the resistant dikes although talus from the steep south flank of Mt. Rose now covers these areas. Individual dikes here are 10-20 feet thick, fine grained, massive, and commonly show chilled contacts against the Rove Formation. The dikes were formed during early stages of development of the Midcontinent rift. In addition to the Rove, they also intrude volcanic flows of the Grand Portage Volcanics to the southwest of the Monument. The dikes are rather stongly magnetized and have a reversed remanent polarity. This makes them easy to find and identify by magnetic surveying because they appear as linear negative anomalies. A magnetic profile measured along the road with a proton precession magnetometer in 2001 shows four distinct magnetic lows that correspond to the four dikes that cross the road (Fig. 2.6). A profile along the portage trail, also done in 2001, found several additional dikes that had not been know previously. The dikes can be dated rather precisely by a combination of geologic relationships and magnetic polarity. They cut the Grand Portage Volcanics that were extruded early in the history of the Midcontinent rift and acquired their remanent magnetization before the magnetic reversal, which is recognized widely in igneous rocks of the Midcontiinent rift, at about 1,100 Ma. Stop 2.2. Rove Formation and Pigeon River diabase on Mt. Rose ILSG07 47 Trip 2 �The Mt. Rose Trail provides a well exposed section of the Rove Formation. The summit is underlain by a diabase dike of the Pigeon River swarm and a contact metamorphic halo in the Rove near the diabase contact. The following descriptions are keyed to numbered placards along the trail. Much of the eastfacing flank of Mt. Rose, over which the trail climbs, consists of steep bluffs of bedrock exposures of the Rove Formation. The steepness is probably due to wave erosion during higher level stands of post-glacial lakes. The Nipissing transgression reoccupied the base of Mt. Rose around 636 feet, with wave action vigorously cutting a bluff seen from Stop 2.1 on the Mt. Rose Nature Trail. The base of the bluff is now mostly covered by more recent talus. Cut in shale, no abrasion notch is expected, but an angular undercut can be seen in places to the south of the Trail. The Trail follows a bench at approximately 678 feet between lookout Stops 5 and 6 that faces Lake Superior and which would have experienced strong wave action at the post-Minong water levels. Large accumulations of blocky talus below a sharp bluff between Mt. Rose Nature Trail Stops 7 and 10 represent the work of waves during the period of Lake Minong (715 feet). In several places, a convincing undercut is seen, though the base of the rock bluff is almost everywhere covered by more recent talus accumulation. At 775 feet, at the time of Lake Low Beaver Bay, Mt Rose was an island separated from the mainland by a shallow strait. On the Mt. Rose Nature Trail, there are one or two rock benches that could have been wave modified by this level of Beaver Bay, but the strata outcrops in nearly horizontal form and it is not possible to distinguish between wave formed features and structural control. The Rove Formation along the trail is mostly flaggy sandstone in which prominent bedding partings emphasize bedding at inch-scale. Cross bedding is seen rarely and a few beds show soft-sediment slump structures. The sandy units are separated by thinner units of finer-grained fissile argillite. Beds are nearly flat lying with dips of only a few degrees toward the southwest. Within 10 or 20 feet of the contact with the diabase, the Rove is distinctly baked in the contact aureole. Closest to the contact, bedding is largely obliterated and the sediments are recrystallized to a hornfels containing biotite and clots of a secondary mineral in the matrix that is tentatively identified in thin section as cordierite. The diabase is mostly medium-grained and massive. The nearly vertical southeastern contact can be seen near the top of steep bluff as well as in pavement outcrops near the summit. The rock seen here is typical of the Pigeon River diabase dikes in the region in that it forms dikes hundreds of feet thick, has normal magnetic polarity and forms the prominent high, steep-sided ridges that are the most distinctive elements of the local topography. Stop 2.3. Rove Formation and Grand Portage dikes along the Beaver Bay shoreline. At this stop, both bedrock and post-glacial lake features are displayed. On the north side of Highway 61 at the trail crossing there are extensive exposures of the Rove Formation and two diabase dikes of the Grand Portage swarm. The unusual amount of bedrock exposure is probably a result of wave washing along the coastline of Beaver Bay stage lakes. This shoreline erosion stripped the cover of glacial till or lake sediment that probably originally mantled the bedrock, as it does over most of the surrounding area. The Rove in this area is fine-grained gray sandstone that breaks into approximately inch-thick flaggy slabs. Bedding dips about 5 degrees to the southeast, typical of the Rove over most of the region. Vertical diabase dikes of the Grand Portage swarm can be traced across the outcrop area and into the roadcut exposures along Highway 61. Just north of the bedrock outcrop, the Grand Portage Trail follows a terrace along the 816-foot contour, the inner edge of which is bounded by an indistinct bluff at 828 feet. The surface of the Trail is ILSG07 48 Trip 2 �noticeably roughened with large boulders, suggesting scour and winnowing of the finer sediments. It seems likely that this remnant terrace represents the floor immediately within the river mouth as it was in Mid Beaver Bay time (Fig. 2.12). As it is traced up valley, the bluff rises to 831 feet and 834 feet as would be expected within the river mouth. The Trail drops down a bluff from the 816 foot level into a small boggy embayment. This represents the shore of the 4th level of Beaver Bay (810 feet, between Mid and Low) and is a strong feature. The Trail rises briefly again to the former terrace (now 822 feet) and then descends into a broad embayment, backed by the continuation of the same 4th level of Beaver Bay before rising again to the terrace, (now 834 feet). A tributary coming in from the east breaks the bluff. Figure 2.12. Sketch map of the area along Grand Portage Trail north of Highway 61 showing outcrop of the Rove Formation and the trace of the bluff of the Beaver Bay stage shoreline. References Addison, W.D., Brumpton, G.R., Vallini, D.A, McNaughton, N.J., Davis, D.W., Kissin, S. A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology, V., 33, p. 193-196. Allen, D.J., Hinze, W.J., and Cannon, W.F., 1992, Drainage, topographic, and gravity anomalies in the Lake Superior region: evidence for a 1110 Ma mantle plume: Geophysical Research Letters, v. 19, p 2199-2122. Babcock, Willoughby M. Jr. 1940, Re-building the Grand Portage Stockade: Some Problems in Historical Reconstruction: The Museum News, December 15, 1940, pages 6-8. Birk, Douglas A.,1975, Recent Underwater Recoveries at Fort Charlotte, Grand Portage National Monument, Minnesota: The International Journal of Nautical Archeology and Underwater Exploration 4(1):73-84. Birk, Douglas A., 1979, Whitewater Archeology. The Minnesota Volunteer 42(244):12-19 ILSG07 49 Trip 2 �Birk, Douglas A., 1984, John Sayer and the Fond du Lac Trade: The North West Company in Minnesota and Wisconsin Rendezvous: in Thomas C. Buckley, editor, Selected Papers of the Fourth North American Fur Trade Conference, 1981, pp. 51-61. St. Paul, North American Fur Trade Conference. Birk, Douglas A., 1994, When Rivers Were Roads. Deciphering the Role of Canoe Portages in the Western Lake Superior Fur Trade: The Fur Trade Revisited. Selected Papers of the Sixth North American Fur Trade Conference, Mackinac Island, Michigan, 1991, 359-76. Jennifer Brown, W. Eccles, and Donald Heldman, eds., East Lansing, Michigan State University Press. Birk, Douglas A.1998,The Hudson Bay Trail: A Study of Nineteenth Century Travel Routes Between Grand Portage, Minnesota, and Fort William, Ontario: Institute for Minnesota Archaeology Reports of Investigations, Number 466. Minneapolis. Birk, Douglas, 2005, National Register of Historic Places, Grand Portage. Birk, Douglas A., and Robert C. Wheeler, 1976, Fort Charlotte Underwater Archeology Project: Geographic Research Reports 1975, 791-99. Washington: National Geographic Society. National Brown, Ralph D., 1937, Recent Excavations at Grand Portage: Minnesota History, v, 18, p.456-458. Buck, Solon J., 1931, The Story of the Grand Portage: Minneapolis: Private printing. Burpee, Lawrence J., 1931, Grand Portage: Minnesota History, v. 12, p. 359-377. Cannon, W.F. and Hinze, W.J, 1992, Speculations on the origin of the North American Midcontinent rift: Teconophysics, v. 213, p. 49-55. Carver, Jonathan, 1956, Travels Through the Interior Parts of North America, in the Years 1766, 1767, and 1768: Reprint. Minneapolis: Ross and Haines. Dawson, Samuel J., 1968, Report on the Exploration of the Country Between Lake Superior and the Red River Settlement, and Between the Latter Place and the Assiniboine and Saskatchewan: Appendix No. 36. Reprint of 1859 edition. New York: Greenwood Press. Gates, Charles M. (editor), 1965, Five Fur Traders of the Northwest: St. Paul, Minnesota Historical Society. Gilman, Carolyn, 1992, The Grand Portage Story: St. Paul, Minnesota Historical Society Press. Green, J.C., Bornhorst, T.J., Chandler, V.W., Mudrey, M.G., Jr., Myers, P.E., Pesonen, L.J., and Wilband, J.T., 1987, Keweenawan dykes of the Lake Superior region: evidence for evolution of the Middle Proterozoic Midcontinent rift of North America: Geologic Association of Canada Special Paper 34, p. 289-302. Grout, F.F., and Schwartz, G.M., 1933, The geology of the Rove Formation and associated intrusives in northeastern Minnesota: Minnesota Geological Survey Bulletin 24, 103 p. Grout, Frank F., Robert P. Sharp, and George M. Schwartz, 1959, The Geology of Cook County Minnesota: Minnesota Geological Survey, Bulletin 39. Halls, H. C, 1982, Crustal thickness in the Lake Superior region: in Wold, R. J, and Hinze, W.J., eds., Geology and tectonics of the Lake Superior basin, Geological Society of America Memoir 156, p. 239-244. Heamon, L.M., and Easton, R.M., 2006, Preliminary U/Pb geochronology results: Lake Nipigon Region Geoscience Initiative: Ontario Geological Survey Miscellaneous Release-data 191, 78 p. Hirth, Kenneth G., 1976, Interregional Trade and the Formation of Prehistoric Gateway Communities: American Antiquity, v. 43, p. 35-45. Holm, D.K., VanSchmus, W.R., MacNeill, L.C., Boerboom, T.J., Schweitzer, D, and Schneider, D, 2005, U-Pb zircon geochronology of the Paleoproterozoic plutons from the northern mid-continent, USA: evidence for subduction flip and continued convergence after the geon 18 Penokean orogeny: Geological Society of America Bulletin, v. 117, p. 259-275. Hutchinson, D.R., White, R.S., Cannon, W.F., and Schulz, K.J., 1990, Keweenaw hotsopot: geophysical evidence for a 1.1 Ga mantle plume beneath the Midcontinent rift system: Journal of Geophysical Research, v. 95, p. 10869-10884. ILSG07 50 Trip 2 �Kerber, L., 2006, Minimum age and provenance of the correlated Thomson and Rove Formations of eastern Minnesota: 19th Annual Keck Symposium, p. 142-146. Maric, M., and Fralick, P., 2005, Sedimentology of the Rove and Virginia Formations and their tectonic significance: Proceedings of the 51st Institute on Lake Superior Geology, v. 51, part 1, Proceedings with Abstracts, p. 41-42. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-119, digital or print on demand, nominal scale 1:200,000. Morey, G.B., 1969, The geology of the Middle Precambrian Rove Formation in northeastern Minnesota: Minnesota Geological Survey, Special Publication Series, SP-7, 62 P. Morse, Eric W., 1969, Fur Trade Canoe Routes of Canada/ Then and Now: Queen's Printer, Ottawa. NICE working group, in press, Reinterpretation of Paleoproterozoic accretionary boundaries of the north-central United States based on a new aeromagnetic-geologic compilation: Precambrian Research. Nicholson, S.W., Shirey, S.B., Schulz, K.J., and Green, John C., 1997, Rift-wide correlation of the 1.1 Ga Midcontinent rift sywtem basalts: implications for multiple mantle sources during rift development: Canadian Journal of Earth Science, v. 34, p. 504-520. Nute, Grace Lee (editor), 1940, A British Legal Case and Old Grand Portage: Minnesota History, v. 21, p. 117-148. Nute, Grace, 1944, Lake Superior: The American Lakes Series, Milo M. Quaife, editor, Indianapolis, Bobbs-Merrill Company Owen, D.D., 1852, Report of a geological survey of Wisconsin, Iowa, and Minnesota and incidentally of a portion of Nebraska Territory: Report to U.S. General Land Office, 623 p. Phillips, B.A.M., 2001, Water Level History and Shoreline Change – Grand Portage National Monument, MN: A Report for the U.S. National Park Service, Grand Portage National Monument, 88 pages. Schneider, D.A., Bickford, M.E., Cannon, W.F., Schulz, K.J., and Hamilton, M.A., 2002, Age of volcanic rocks and syndepositional iron formations, Marquette Range Supergroup: implications for the tectonic setting of Paleoproterozoic iron formations of the Lake Superior region: Canadian Journal of Earth Sciences, v. 39, p. 999-1012. Schneider, D.A., Holm, D.K., O’Boyle, C., Hamilton, M., and Jercinovic, M., 2004, Paleoproterozoic development of a gneiss dome corridor in the southern Lake Superior region, USA: in Whitney, D.L., and Siddoway, C.S., eds., Gneiss domes in orogeny, Geological Society of America Special Paper 380, p. 339-357. Schwartz, George M., 1928, The Topography and Geology of the Grand Portage: Minnesota History, v. 9, p.26-30. Sharp, R.P. 1953, Glacial features of Cook County, Minnesota: American Journal of Science, v. 251, p. 855-883. Teller, J.T., and Thorleifson, L.H., 1983, The Lake Agassiz-Lake Superior connection: in. Teller. J.T, and Clayton, L., eds., Glacial Lake Agassiz: Geological Association of Canada Special Paper 26. pp. 261-290. Thompson, Erwin N., 1969, Grand Portage. A History of the Sites, People, and Fur Trade: U. S. Department of the Interior. Washington: National Park Service. Wallace, W. Stewart (editor), 1934, Documents Relating to the North West Company: Reprint edition (1968). New York: Greenwood Press. Warren, William W.,1957, History of the Ojibway Nation: Minneapolis: Ross and Haines. Wheeler, Robert C., Kenyon, Walter A., Woolworth, Alan R., and. Birk, Douglas A.,1975, Voices From the Rapids. An Underwater Search for Fur Trade Artifacts, 1960-73: Minnesota Historical Archeology Series No. 3, St. Paul, Minnesota Historical Society. White, Bruce M., 1982, Give Us a Little Milk. The Social and Cultural Meanings of Gift Giving in the Lake Superior Fur Trade: Minnesota History, v. 48, p. 60-71. ILSG07 51 Trip 2 �White, Bruce M., 1987, A Skilled Game of Exchange. Ojibway Fur Trade Protocol: Minnesota History, v. 50, p. 229-240. Winchell, Newton H. (editor), 1899, The Geology of Minnesota: Geological and Natural History Survey of Minnesota, Volume 4 of the Final Report, St. Paul: Pioneer Press Company. Woolworth, Alan R., 1967, Archeological Excavations at Grand Portage: An 18th Century Fur Trade Metropolis: The Minnesota Archaeologist, v. 29, p.3-17. Woolworth, Alan R. and Nancy L. Woolworth, 1982, Grand Portage National Monument. An Historical Overview and An Inventory of Its Cultural Resources: Two Volumes. Unpublished, typewritten report prepared for the National Park Service. St. Paul: Minnesota Historical Society. ILSG07 52 Trip 2 �53rd Annual Institute on Lake Superior Geology FIELD TRIP 3 MIDCONTINENT RIFT-RELATED MAFIC INTRUSIONS NORTH OF THE INTERNATIONAL BORDER Mark Smyk Ontario Geological Survey, Ministry of Northern Development and Mines Suite B002, 435 James St. South Thunder Bay, ON P7E 6S7 Canada Peter Hollings Department of Geology, Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Frontispiece: View looking southwest from the top of ridge (Crystal Lake Gabbro) at the Great Lakes Nickel property. Mesas are underlain by Logan diabase sills which have intruded Rove Formation sedimentary rocks. ILSG07 53 Trip 3 �INTRODUCTION This field trip covers an area that has been the focus of much research and economic interest. The guidebook has benefited from recent geochronologic and geochemical studies conducted as part of the Lake Nipigon Region Geoscience Initiative. These studies have elucidated the nature of magmatism in the northern part of the Midcontinent Rift (MCR) and have been used to augment and refine our previous understanding of these magmatic events. This trip focuses on a variety of mafic intrusions associated with the MCR north of the International Border near Pigeon River. These intrusions represent changes in the nature of early to mid-stage MCR magmatism over a span of ~20 million years. They include Logan sills, Pigeon River and other dykes and the layered Crystal Lake Gabbro, host to the Great Lakes Nickel coppernickel deposit. Contacts with Paleoproterozoic Rove Formation sedimentary rocks are well-exposed in this area and illustrate some of the mechanisms of dyke/sill emplacement, as well as magma-wallrock interactions. These interactions may play an important role in local metallogenesis. This guide book builds upon those previously written and compiled by Franklin and Kustra (1972), Miller and Smyk (1995), Parker et al. (2001) and Miller et al. (2002). Bear in mind that when visiting exploration or private properties, permission must be granted by the property owner. Current ownership information can be obtained from the Resident Geologist’s Office, Ontario Geological Survey, in Thunder Bay. Please exercise caution along highway right-of-ways, cliffs and mine workings. REGIONAL GEOLOGY Situated within the Southern Province of the Canadian Shield, the field trip area is dominantly underlain by Paleoproterozoic Rove Formation clastic sedimentary rocks (Animikie Group) which have been intruded by MCR mafic intrusions (Figure 3-1). Previous mapping has been conducted by Tanton (1936a,b), Geul (1970, 1973) and Smith and Sutcliffe (1989). Detailed mapping, geophysical surveys and diamond drilling undertaken by exploration companies have provided additional detail and much-needed information about sub-surface geology. This area is a rugged, upland area of diabase-capped mesas and ridges that occupies a 70 km by 30 km, northeast-trending topographic feature between Thunder Bay and the Minnesota border, termed the “Logan Basin” by North (2000). Logan Sills underlie mesas that commonly rise 150 m above valleys underlain by deeply eroded, flat-lying, Rove Formation sedimentary rocks (see Frontispiece). Northwest of the Logan Basin, Archean granitoid rocks of the Superior Province form low, rolling hills. Southeast of the Logan Basin the topography is dominated by northeast-trending, linear ridges underlain by Pigeon River dykes. Animikie Group At approximately 2.45 Ga a rift to passive margin developed along the southern edge of the Superior Province when a land-mass to the south separated (Fralick and Miall 1989). Later closure of the resultant ocean led to the deposition of the Animikie and North Range Groups as a backarc basin developed (Pufahl and Fralick 1995, 2004: Hemming et al. 1995; Van Wyck and Johnson 1997; Pufahl et al. 2000), which collision later transformed into a foreland setting (Hoffman 1987; Southwick and Morey 1991; Hemming et al. 1995; Ojakangas et al. 2001; Maric and Fralick 2005). ILSG07 54 Trip 3 �Figure 3-1: General geology of the field trip area (after Smith and Sutcliffe 1987) ILSG07 55 Trip 3 �The Animikie Group in the study area is represented by the Rove Formation, which overlies the Gunflint formation and has an approximate thickness of 500 to 600 m south and east of Thunder Bay, thickening to the south. Rocks of the Rove Formation are flat-lying or dip gently to the southeast. Recent work by Amurawaiye (2001) and Maric and Fralick (2005) described a submarine ramp system in which the movement of coarse sediments into the deeper parts of the basin was mainly through the action of lowand high-density turbidity currents. Fair-weather and storm-generated currents dominated depositional activity at the edge of the basin. Mud and ash particles accumulated during tranquil periods (ibid). Amurawaiye (2001) has stated that approximately 70% of the Rove Formation locally consists of organic shale whose hydrocarbon contents have now been degraded. The lower 100 to 150 m of the Rove Formation and the correlative Virginia Formation in Minnesota consist of alternating shale-siltstone and black, pyritiferous shale successions, probably reflecting fluctuations in sea level (Maric and Fralick 2005). These successions, and especially the upper black shale, likely represent a major condensed interval deposited in water ~100 to 200 m deep. Lucente and Morey (1983) ascribed sedimentation of this interval to pelagic rainout of fine-grained sediment from dilute suspension or hemipelagic processes involving diffuse turbidity currents. The presence of abundant, sub-millimeter rip-up intraclasts also denotes the operation of sporadic bottom currents (Maric and Fralick 2005). Tidal deposits present in correlative rocks to the south of Lake Superior (Ojakangas et al. 2001) confirm open connection to the ocean. Above the upper, pure black shale interval, graded finegrained sandstones are organized into a coarsening-upward succession approximately 100 m thick that is transitional into 400 m of medium-grained, sandstone-dominated, stacked parasequences (Maric and Fralick 2005). This is overlain by lenticular to wavy bedded sandstones and shales with both wave and current ripples. The coarsening-upward to sandstone-dominated portion of the Virginia and Rove Formations has been interpreted as a submarine fan (Lucente and Morey 1983, Maric and Fralick 2005) with the uppermost ripple laminated succession representing progradation of distal distributary mouth bars of a delta (Maric and Fralick 2005). A sandstone sample from the submarine fan portion of the succession yielded a youngest U-Pb detrital zircon age of approximately 1780 Ma (Heaman and Easton 2006). The predominantly Paleoproterozoic zircon population and paleocurrents indicating sediment derivation from the north (Morey 1973), strongly suggest the Trans-Hudson Orogen was the source of the detritus. Keweenawan Supergroup Mesoproterozoic intrusive, volcanic and minor sedimentary rocks associated with the MCR collectively constitute the Keweenawan Supergroup. On the northern margin of the MCR, Keweenawan rocks include a variety of intrusive rocks and Osler Group volcanic rocks which represent some of the earliest magmatism in the MCR. As shown in Table 3-1, ages range from ca. 1140 Ma (Heaman and Easton 2006) to ages younger than the magnetic polarity reversal that occurred between 1105 and 1102 Ma (Davis and Green 1997). A tabulated synopsis is provided below; bolded units occur within the field trip area. Most mafic and ultramafic rocks in the Lake Nipigon and northern Lake Superior areas, including the Nipigon and Logan sills, appear to have been emplaced in a short, magnetically reversed, interval between ca. 1115 and 1100 Ma (Heaman and Easton 2006). Emplacement of alkalic intrusions, such as the 1108 Ma Coldwell Complex (Heaman and Machado 1992), and filling of much of the submerged part of the rift in Lake Superior, also occurred in this period. This was followed by a period of magnetically normal, waning mafic and felsic magmatism, between 1096 and 1085 Ma, that is preserved mainly along the Lake Superior shore by units such as the Crystal Lake (1099±1 Ma), Moss Lake (1095±2 Ma) and Blake (1095±2 Ma) gabbros, and a Pigeon River dyke near Arrow River (1093±3 Ma; Heaman and Easton 2006). ILSG07 56 Trip 3 �Table 3-1: Geochronology data of MCR-related rocks in Northwestern Ontario. Lithologic Unit St. Ignace Island Complex gabbro Arrow River Dyke Pigeon River Dyke Blake Gabbro Moss Lake Gabbro Crystal Lake Gabbro Pine Point – Mt. Mollie dyke Osler Group rhyolite (central suite) Osler Group rhyolite (lower suite) St. Ignace Island Complex Rhyolite Coldwell Complex Locality / Age (Ma) St. Ignace Island / 1089.2 +3.2 Arrow River / 1078 + 3 Rita Bolduc / 1141±20 Blake Township / 1091.0 + 4.5 Black Bay Peninsula / 1094.7 ±3.1 Great Lakes Nickel / 1099.6 + 1.2 n/a Agate Point / 1105±2 Reference(s) Smyk et al. (2006) Heaman and Easton (2006) Heaman and Easton (2007) Heaman and Easton (2006) Heaman and Easton (2006) Heaman and Easton (2006) Black Bay Peninsula / 1107.4 +4/-2 St. Ignace Island / 1107.2 ± 2.4 Davis and Sutcliffe (1985) Smyk et al. (2006) Coldwell Complex / 1108 + 1 Logan Sills Nipigon Sills Ultramafic Intrusions Pigeon River Inspiration Sill Marathon lamprophyre dykes Mt. McKay / 1115 + 1 Nipigon Embayment / 1114-1110 Nipigon Embayment / 1124-1113 Crooks Twp. / 1141 + 20 Lake Nipigon / 1141 + 20 McKellar Harbour / 1145 +15/-10 Heaman and Machado (1987) Heaman and Easton (2006) Heaman and Easton (2006) Heaman and Easton (2006) Heaman and Easton (2006) Heaman and Easton (2006) Queen et al. (1996) Davis and Green (1997) Hypabyssal Mafic Rocks Diabase sills, extending from the vicinity of Thunder Bay to east of Lake Nipigon, represent the northern remnants of the Midcontinent Rift, and have previously been referred to as the Logan sills (Stockwell et al. 1972), however recent work suggests a geochemical difference between the sills to the north and south of the City of Thunder Bay (Hart 2003; Hart et al. 2005b). Hollings et al. (2007) proposed that the term Logan Igneous Suite, which would fall within the Midcontinent Rift Intrusive Supersuite of Miller et al. (2002), should be applied to all the diabase sills in the area north of Lake Superior, with subdivision into the informal terms, Nipigon sills for the sills north of Thunder Bay, and Logan sills to the south. Logan sills generally consist of fine- to coarse-grained, ophitic to intergranular, quartz tholeiitic diabase/gabbro (Smith and Sutcliffe 1987; Geul 1970, 1973). Coarse-grained, intergranular gabbro, locally rich in granophyric mesostasis, is common in the interior of the thicker sills. The upper sections of the diabase sills are commonly plagioclase-porphyritic, containing as much as 60% phenocrysts. Chilled margin and bulk compositions are iron-rich, quartz-tholeiitic basalt. Compositional and textural variation in sills has been noted by North (2000) and Beskar (2001) in Blake Township, where varitextured, "taxitic" gabbro has been described. Logan sills are recognized by their reversed magnetic polarity and generally take the form of columnarjointed, thick sheets and sills whose geometry is strongly controlled by the subhorizontal bedding of the country rock. They form conspicuous erosional remnants that create mesa and cuesta topography. From the international boundary area to Thunder Bay as many as six diabase sheets were emplaced nearly conformably into Animikie sedimentary rocks (Weiblen et al., 1972; Smith and Sutcliffe 1987, 1989). Diamond drilling has also shown that stacked sills exist in the subsurface. For example, Dumont Nickel Inc. reported intersecting 14 gabbroic sills in a 705 m deep drill hole in central Pardee Township (Assessment Files, Thunder Bay South Resident Geologist’s District, Thunder Bay). North of the border, ILSG07 57 Trip 3 �Smith and Sutcliffe (1989) reported sills up to 50 m thick, whereas in Minnesota, Jones (1984) studied four sills ranging from 50 to 160 m in maximum thickness. Rare exposures of feeder dykes to sills and preserved sill terminations have been noted (e.g., Stop 3-3). The textural stratigraphy, which varies from a lower, ophitic zone to an upper pegmatitic zone, indicates that in most cases, the sills cooled as single units, probably over a period of 200 to 500 years (Smith and Sutcliffe 1989). Chilled contact zones are developed against sedimentary country rocks; sedimentary xenoliths are rare. Pigeon River dykes trend east-northeast to northeast and dip steeply to the southeast (Geul 1970, 1973; Smith and Sutcliffe 1989). Displacement and warping of the Rove Formation is evident along many of the dikes. Composite intrusions are noted in several dykes (add reference). Dyke widths average between 50 and 70 m, but may be as much as 150 m across in Ontario (Smith and Sutcliffe 1987) and 500 m in Minnesota (Green et al. 1987). Forming northeast-trending, linear ridges, dykes can be traced semicontinuously for 15 km along strike. As noted by many workers, some dykes clearly crosscut Logan sills. However, Geul (1973) and Smith and Sutcliffe (1987) noted that others display somewhat ambiguous crosscutting relationships. In these latter cases, dykes may appear to merge with sills, suggesting that they were contemporaneous or that sills impeded the upward migration of the dykes. The presence of multiple sets of horizontal columnar jointing suggests the development of multiple or composite dykes. The dykes typically consist of ophitic diabase that may be plagioclase-porphyritic. A typical, nonporphyritic olivine diabase consists of approximately 60% plagioclase (zoned labradorite; An 55-70), 20% augite + hypersthene, up to 15% olivine and up to 5% magnetite, ilmeno-magnetite and sulphides (Geul 1970, 1973). Average whole rock compositions of Pigeon River dikes are moderately evolved (mg# = 52) olivine tholeiitic basalt. A 15-km long, northwest-trending diabase dike, termed the Arrow River dyke by Smith and Sutcliffe (1989) crosscuts Pigeon River dikes in Ontario. This dike and two shorter, similarly oriented dikes are composed of intergranular, quartz diabase that is commonly plagioclase-phyric. They are collectively known as the Arrow River dykes, despite the fact that they occur well north of the Arrow River. The authors recommend that they be named Cloud River dykes.) Confusion has arisen because the term “Arrow River dyke” has also been used colloquially to denote a prominent Pigeon River dyke that occurs between the Arrow River and Pigeon River in southern Devon Township (i.e. Stop 3-1). The youngest intrusions in the area tend to be more irregularly shaped and internally zoned. One of these is the Crystal Lake gabbro, which is Y-shaped in plan view, with a west-northwest-striking limb 5km long and an east-northeast-striking, southern limb 2.75 km long (Figure 3-2). Internal layering and foliation suggest that the surface geometry of the northern limb may result from the tilting of a canoeshaped body, open on its western end (Smith and Sutcliffe 1987, 1989). The intrusion was subdivided into three major zones by Reeve (1969) and Geul (1970) and has been further subdivided by Smith and Sutcliffe (1987, 1989) and Cogulu (1990) into four major, roughly equivalent, lithologic zones: (1) an upper zone (60 to 80 m) of sulfide-barren troctolite, olivine gabbro and anorthositic gabbro; (2) a middle zone (30 to 42 m) of cyclic, layered anorthositic and olivine gabbro, Cr-spinel- bearing anorthosite, and olivine gabbro (Figures 3-3 and 3-4); (3) a lower, unlayered zone (50 m) of vari-textured gabbro and leucotroctolite, which hosts the bulk of the Cu-Ni-sulfide deposit; and (4) a basal zone (1 to 7 m) of fine-grained, chilled melagabbro and hornfelsed country-rock xenoliths. ILSG07 58 Trip 3 �Table 3-2: Geochemical analyses of select intrusive rocks from the Border Area Intrusive Unit: Source: Logan Sills Grand Portage dikes Mt Josephine Pigeon River dikes A B C D C Chilled Margin Bulk Comp Avg. of 17 dikes Composite dike Average of 14 dikes SiO2 49 50.1 52.5 47.9 48.9 TiO2 3.4 3.6 2.48 3.66 1.65 A12O3 13.1 13.1 13.4 11.8 16.3 FeOt 15.6 14.3 13.3 16.2 11.5 MnO 0.22 0.16 0.19 0.24 0.18 MgO 5.6 3.9 3.68 4.04 6.46 CaO 7.45 7.2 6.72 8.9 10.21 Na2O 2.52 3.4 3.17 2.63 2.42 K2O 1.16 1.5 1.79 1.29 0.52 P2O5 0.38 0.43 0.48 0.53 0.18 Volatiles 0.92 2.5 ----- ----- ----- Total 99.35 100.19 99.11 99.6 99.8 mg# 38.4 32.2 34.1 33 50.1 Cr 50 --- 44 50 100 Ni 80 --- 61 22 136 Rb --- --- 55 28 15.7 Sr 700 --- 420 359 280 Zr 250 --- 281 280 --- Hf --- --- 9 --- 2.94 Description: Trace Elements (ppm) Table from Miller and Smyk (1995); Sources: (A) Geul (1970), Table 3, Sample 2; (B) Jones (1984), Table A-I, Sill A; (C) Green et al. (1987), Table II; (D) Green (1986), Appendix E, Sample GP-60. ILSG07 59 Trip 3 �Figure 3-2: General geology of the Crystal Lake Gabbro after Smith and Sutcliffe (1987) ILSG07 60 Trip 3 �Figure 3-3: Polished slab of massive Crspinel layers in contact with medium-grained gabbro, Great Lakes Nickel property (Stop 3-4). White bar is 4 cm long. Figure 3-4: Igneous layering in the Crystal Lake Gabbro, exposed in the upper cliff section on the Great Lakes Nickel property (Stop 3-4). This is part of the middle zone of the intrusion, characterized by cyclic, layered anorthositic and olivine gabbro, Crspinel-bearing anorthosite, and olivine gabbro. ILSG07 61 Trip 3 �Projecting east of the Crystal Lake gabbro and arcing to the northeast to form a string of islands in Lake Superior is a major composite dike called the Pine River-Mount Mollie dyke (Geul 1973). Smith and Sutcliffe (1989) described this 35 km-long body as consisting of gabbroic, dioritic and granophyric rocks. Gabbroic rocks in the margins of the dike commonly have modal layering and sulphide mineralization similar to that in the Crystal Lake gabbro. Inward from the gabbro, quartz-bearing diorite cores much of the dike. In the central and western portions, granophyre occupies the core of the dike and shows contact relationships with diorite that indicates mixing of felsic and mafic liquids (Smith and Sutcliffe 1987, 1989). As described by Geul (1973), the Pine River-Mount Mollie Gabbro comprises two segments of an eaststriking intrusion which appears to postdate the Pigeon River dykes. The poorly exposed Pine River segment apparently connects to the eastern end of the Crystal Lake Gabbro. Magnetometer survey data and drilling indicate that the Pine River and Mount Mollie segments are connected beneath deep overburden. Field relationships and diamond drilling show that the dip of the gabbro contacts varies from near vertical to 35º north, indicating that the body has the shape of a tilted, truncated cone in crosssection. The exposed width ranges from approximately 60 m (true thickness) to 300 m (apparent thickness), depending upon the attitudes of the contacts. The gabbro has a distinct mottled appearance and is further distinguished from most other diabases in the area by larger grain size, leucogabbroic composition, and degree of mineralization. Diamond drilling logs summarized by Geul (1973) defined a crude zonation in the gabbro, based on variations in grain size, feldspar (i.e. anorthosite) and sulphide content. Thin section petrography by Geul (1973) indicated that the gabbro ranges in composition from anorthositic gabbro to olivine gabbro and quartz gabbro. Deuteric alteration is slight to intense and consists of secondary amphibole, biotite, saussurite, sericite, and hematite and appears to be closely related to the presence of an intergranular granophyric phase. Bodies of granophyre cut the gabbro in several locations and appear to represent the differentiated end product of the gabbroic magma. The gabbro is composed essentially of pyroxene and zoned labradorite. DISCUSSION OF GEOCHEMISTRY As part of the Lake Nipigon Region Geoscience Initiative whole rock analyses were performed on a number of the intrusions south of Thunder Bay; these data can be combined with published (Cogulu 1990), unpublished data for some of the dykes in the region (Larry Hulbert, Geological Survey of Canada, personal communication, 2006) and data from exploration projects (e.g., Rosatelli 2002), in order to assess variations in geochemistry. On a primitive mantle-normalized diagram, the samples of Crystal Lake gabbro and Pigeon River dykes show similar patterns (Figure 3-5). Pigeon River dykes from the Arrow River (Stop 3-1) and Rita Bolduc occurrence (Stop 3-2) are similar to the range of data from other dykes of the Pigeon River swarm sampled in Lake Superior (Victoria Island, Cloud Bay and Jarvis Point (Figure 3-6; Larry Hulbert, Geological Survey of Canada, personal communication, 2006). However, samples of the Pigeon River dyke swarm from Lake Superior are characterized by higher Th abundances than those from the study area, resulting in more pronounced negative Nb anomalies. The only published analysis for the Arrow Dyke swarm (Rosatelli 2002) is indistinguishable from the Pigeon River swarm. By contrast, samples from the Mount Mollie dyke are characterized by elevated La/Smn ratios, higher Th contents and more pronounced Nb anomalies (Figure 3-6). This may be a reflection of greater crustal contamination during emplacement or possibly a distinct mantle source. Regardless, this geochemistry provides another means for distinguishing these dykes. ILSG07 62 Trip 3 �When the data from the dyke swarms are compared to the regional data set generated for the sills and intrusions of the Lake Nipigon embayment (Hollings et al. 2007) it can be seen that Pigeon River dyke swarm geochemistry more closely resembles that of the sills of the Nipigon suite than that of the ultramafic intrusions or the Logan sills (Figure 3-6). By contrast, the Mt Mollie dyke geochemistry appears to be transitional between that of the Nipigon sills and Inspiration sills. Additional isotopic and geochronological studies will be required in order to further investigate the relationships between these MCR-related intrusions. Figure 3-5: Primitive mantle-normalized plots for mafic intrusive rocks in the field trip area. Fields from data contributed by L. Hulbert, Geological Survey of Canada, personal communication (2006). Normalizing values from Sun and McDonough (1989). CONTACT METAMORPHISM There is a remarkable range in the reported intensity and nature of contact metamorphic effects in Rove sedimentary rocks at diabase dyke and sill contacts, owing mainly to the subjectivity of the mapper and the exposures in question. Geul (1973) noted that sedimentary hornfelsic rocks are restricted to a narrow zone of baking between 2 to 10 cm wide at diabase dike contacts. Metamorphosed siltstone displays two stages: first: slight recrystallization of biotite aggregates in an incipient hornfelsic texture; and second, a more complete recrystallization of biotite, surrounded by pale sericitic aggregates, set in a quartzofeldspathic matrix. Franklin (1970) suggested that contact effects existed up to 8 m from sill contacts and possibly up to 23 m. They were manifested as microporphyroblasts of mica and chlorite (“spotted alteration”), graphite destruction and the conversion of pyrite to pyrrhotite. Geul (1973) noted that minute particles (< 0.01 mm) of oxide and sulphide minerals are locally abundant in the contact zone. Rove Formation sedimentary rocks may be deformed along dyke contacts. As noted by Geul (1973) beds appear to dip toward the dykes or are “up-dragged” along dyke contacts (e.g. Stop 3-2). Up-dragging of Rove Formation rocks was also noted along the southern contact of an east-striking Pigeon River dyke at Arrow Rapids by Geul (1970). Deformed and fractured sedimentary rocks have been noted near sill terminations (e.g. Stop 3-3). Narrow, parallel, tension gashes filled with quartzo-feldspathic leucosome/neosome occur in metatectic, deformed siltstones at Stop 3-2 (Figure 3-8). ILSG07 63 Trip 3 �Figure 3-6: Discrimination diagrams for mafic and ultramafic intrusions near Thunder Bay. Data are from Hollings et al. (2007) and Hart (2002). ILSG07 64 Trip 3 �MINERAL EXPLORATION Active prospecting was first carried out during the latter part of the nineteenth century, following the discovery of silver vein deposits along the shoreline and islands of Lake Superior (Island Silver Belt) and to the northwest (Mainland Silver Belt; Oja 1967). Small silver mining operations took place on Spar and Jarvis Islands and at the Prince Mine. Barite was produced from veins on McKellar Island and Jarvis Island. Silver-bearing veins are in or near crustal-scale, extensional listric faults that formed during MCR extension and are spatially associated with MCR-related mafic intrusions (Franklin et al. 1986). Copper-nickel-mineralized boulders were found in 1903 by J. A. McCuaig in Pardee Township (Geul 1970). This discovery led to a concerted exploration effort to find their source (Geul 1970, 1973). Local copper-nickel occurrences were soon described by Tanton (1935, 1937). United States Smelting, Refining and Mining Company conducted exploration in 1936, and were followed by Frobisher Exploration Company in 1942. In the summer of 1952, J.S. Brodie and T.W. Page examined a large outcrop of gabbroic rocks, 6 km northeast of the original float discovery on the property now held by the Great Lakes Nickel Corporation Limited. Following this discovery, several blocks of ground were staked in Crooks Township by Whitegate Mining Company Limited, J. S. Strickland, and others, in an effort to cover the interpreted eastward extension of the favourable host rock. In 1967, Canadian Exploration Limited investigated the Strickland-Whitegate claims in southwestern Crooks Township and Anaconda American Brass Limited prospected the Pine River-Mount Mollie area as part of a regional reconnaissance program. In 1968, Phelps Dodge Corporation of Canada Limited explored and drilled claims in the Pine RiverMount Mollie area of Crooks Township. In 1989, International Platinum Corp. drilled a 3592 m hole in central Crooks Township. A renewed interest in platinum group elements in the late 1990s led to a reevaluation of the area’s potential and a flurry of exploration and drilling by companies like BHP Billiton World Expl. Inc., Falconbridge Ltd., McVicar Minerals Ltd., North Atlantic Nickel Corp. and Dumont Nickel Corp. A Noril’sk-style model was employed, as had been earlier suggested by Lightfoot and Lavigne (1995) and others in promoting the area’s mineral potential. ILSG07 65 Trip 3 �Figure 3-7: Columnar joints in the Crystal Lake Gabbro, exposed above the upper adit on the Great Lakes Nickel property (Stop 3-4). The adit opening (lower left) is approximately 3m wide. Figure 3-8: Narrow, parallel tension gashed filled with quartzo-feldspathic leucosome/neosome in metatectic, deformed siltstone near contact with Pigeon River dyke, Stop 3-2 ILSG07 66 Trip 3 �FIELD TRIP STOPS: ROAD LOG AND DESCRIPTIONS STOP NAME Middle Falls Dyke Arrow River Dyke STOP NUMBER LANDMARK (0.0 km) DISTANCE (km) Reset Trip Odometer Visitor Centre @ Border 0.0 / travel north Junction with Hwy. 593 2.3 / turn left, travel west Middle Falls 4.5 Junction with Old Border Road 10.9 / turn left, travel west 500 m Junction with Logging Road 11.4 / turn right, travel northwest 3.8 km Arrow River bridge 14.4 Optional Reset Trip Odometer McCuaig Float Occurrence ILSG07 Junction of Hwy. 593 and Old Border Road 0.0/ left turn, travel north Junction with Hwy. 597 (Pardee Road) 1.7 (on right) 3.8 / 150 m north of hwy. (flagged trail) Optional Reset Trip Odometer Rita Bolduc Occurrence / Pigeon River Dyke 15.2 3-1 Junction of Hwy's. 593 and 61 NORTHING EASTING (NAD 83) 5321108 304872 5324134 296694 5325546 299009 0.0 / turn left, travel north 4.4 3-2 67 5324701 310563 Trip 3 �Mount Mollie Lookout Great Lakes Nickel Property 6.6 (gated, on left) Pine River Bridge 6.75 Junction with Memory Road 6.9 / turn right, travel east 3.1 km Trail to Mount Mollie lookout 10.0 / trail on east side of road Follow trail ~300 east to top of ridge Optional Reset Odometer Rove Formation Quarry Junction with Great Lakes Nickel Road Return to Highway 61; turn left, travel south Travel south 300 m to Great Lakes Nickel Road (gated, on right) Junction of Hwy. 61 with Great Lakes Nickel Road 0.0 / travel west 5325444 313843 3-3 4.7 (south side of road) 5328687 306401 3-4 7.5 (western end of ridge; cliff face) 5328392 303612 STOP (OPTIONAL): Middle Falls Location: South side of Highway 593, 4.5 km west of Visitor Centre at Border on Highway 61 (UTM 304872E / 5321108 N; NAD83) (N.B. On older maps, this location is referred to as Little Falls.) Description: This roadside pull-off offers a picturesque view of Middle Falls, an 18 m high waterfall underlain by a resistant, west-northwest-trending, diabase dyke cutting Rove Formation clastic sedimentary rocks. The dyke is exposed in a roadcut on the highway opposite the pull-off area. It is cut by a number of parallel, rusty carbonate veins. STOP 3-1: Pigeon River Dyke (a.k.a. the “Arrow River Dyke”) Location: Road cut, 15.2 km west of Visitor Centre at Border on Highway 61, via Old Border Road (@ 10.9 km) and logging road (@ 11.4 km) (UTM 296694E / 5324134N; NAD83) ILSG07 68 Trip 3 �Description: This stop is situated on top of a prominent northeast-trending ridge which extends for over 20 km and rises up to 150 m above the surrounding countryside. The ridge is underlain by a Pigeon River dyke which, owing to its location, has been colloquially referred to as “the Arrow River dyke”, although it is not part of the northwest-trending Arrow River dyke swarm of Smith and Sutcliffe (1989). The exposed dyke consists generally of massive fine- to medium-grained diabase. Conspicuous, pyroxene-phyric patches have diffuse to indistinct contacts with the finer-grained diabase, suggesting that they may represent fluid-rich pockets or cognate xenoliths. (Similar textures are noted at Stop 3-2 as well.) A very small amount (~50 grains) of tiny baddeleyite fragments and blades were recovered from a sample of diabase at this location (Heaman and Easton 2006). A larger number of 40–100 μm colourless resorbed zircons were also recovered, but these were interpreted to be of xenocrystic origin. The U/Pb results are from one small multi-grain baddeleyite fraction. This baddeleyite fraction has a low uranium content (188 ppm U) and a low Th/U ratio (0.022); the latter is typical for magmatic baddeleyite. This baddeleyite analysis displays slight reverse discordance (–1.4%) and has a 206Pb / 238U age of 1092.7±2.6 Ma, which is interpreted as a good estimate for the minimum age of the Arrow River diabase dike (ibid). STOP (OPTIONAL): McCuaig Float Occurrence Location: North side of Highway 593, 3.8 km west of junction of Highway 593 and Old Border Road / 1.7 km west of junction of Highways 593 and 597 (Pardee Road) (UTM 299009E / 5325546 N; NAD83) Description: This stop comprises a number of pits and trenches developed in a hummocky, boulderstrewn hillside. Copper-nickel-mineralized boulders were found in 1903 by J. A. McCuaig (Geul 1970). In 1936, a magnetometer survey was conducted by United States Refining and Mining Company. In 1942, Frobisher Exploration Company Limited completed a second magnetometer survey. In 1949-1950, J. A. McCuaig (original discoverer’s son) carried out some minor trenching and sampling and drilled a 200 foot subvertical hole which reportedly showed the mineralized rock was in the form of float boulders. A small bulk sample was tested by Falconbridge Nickel Mines Limited. In 1967, Hollinger Consolidated Gold Mines Limited carried out geophysical surveys. Hanna Gold Mines Limited completed a vertical hole nearby. In 1990, Fleck Resources Ltd. carried out magnetometer and geochemical surveys along with limited prospecting, trenching and sampling just north of this occurrence (Resident Geologist’s Files, Thunder Bay South District, Thunder Bay). Tanton (1935, 1937), McCuaig (1950) and Geul (1970) have described mineralized, vari-textured diabase, gabbroic anorthosite and norite. Interstitial to blebby pyrite, pyrrhotite, chalcopyrite and pentlandite characterize the sulphide mineralization. Assay results are shown below: Reference Cu (%) Ni (%) Co (ppm) Pd (ppb) Pt (ppb) Geul (1970) 0.23 .018 n/a Trace n/a Resident Geologist Files, OGS 1.76 0.72 190 n/a 610 Tanton (1935) 0.62 0.34 n/a n/a 5140 ILSG07 69 Trip 3 �STOP 3-2: Rita Bolduc Occurrence (Pigeon River Dyke) Location: Highway 61, 4.4 km north of the junction between Highways 593 and 61; (UTM 310563E / 5324701N; NAD83) Description: This highway roadcut (Figure 3-9) exposes the southern contact and an almost complete cross-section of a sulphide-mineralized Pigeon River dyke. The property was explored with a self potential survey and trenching in 1957 (Geul 1973). It was part of a property that was acquired and explored by Falconbridge Limited and McVicar Minerals. The main sulphide showing is situated along the south edge of a Pigeon River olivine diabase dike, carrying disseminated to massive coppernickel-sulphide mineralization in a fracture zone, up to 35 feet in width and extending for a distance of at least 100 feet along the dike (Geul 1973). A grab sample of massive to disseminated pentlandite, pyrrhotite with minor chalcopyrite returned 0.70% Cu and 0.35% Ni and trace amounts of Pd (ibid). Net-textured pyrrhotite and chalcopyrite blebs occur locally. Grab samples collected from sulphide-mineralized sections of the dyke on the west side of the highway yielded the following results: Sample 01-BRB-01 01-BRB-02 01-BRB-03 Au (ppb) Cu (ppm) Ni (ppm) Pd (ppb) Pt (ppb) 1442 707 28.03 21.84 14.29 880 505 17.55 13.90 25.02 6517 2664 23.16 n.d. (Resident Geologist's Files, Thunder Bay South District, Thunder Bay) Hornfelsed, rusty weathering, Rove Formation sedimentary rocks are exposed along the southern dyke margin. They are fissile and display an orthogonal joint pattern. Beds have been up-folded along the contact, suggesting either dip-slip fault motion and/or high magma flow rates. Similar “up-drag” features were noted on the southern contact of the Arrow River dyke at Arrow Rapids by Geul (1970), also in association with a copper-nickel sulphide-mineralized fault/fracture zone. Narrow, parallel tension gashed filled with quartzo-feldspathic leucosome/neosome occur in metatectic, deformed siltstone; infilled fractures may produce leucosome dykes (Figure 3-8). They are oriented parallel to the dyke contact and occur several metres away from the contact. Rare, small (< 1 cm diameter), round chloritic patches may represent incipient “spotted” alteration, similar to that described by Franklin (1970). Geochronologic sampling by Heaman and Easton (2006) at this location showed that the least magnetic fraction contained a large amount of colourless euhedral apatite, pyrite and a small amount of baddeleyite and zircon. A very small amount of tiny tan baddeleyite blades and fragments was recovered and a number of composite baddeleyite/zircon grains were identified. Although not that common, primary igneous baddeleyite overgrown by magmatic zircon has been reported previously and is consistent with the phase relationship for crystallization of these two minerals in a mafic magma where the silica activity is progressively increasing throughout crystallization. The four zircon analyses have varied uranium and thorium contents (163–1104 ppm and 34–237 ppm, respectively), but have high Th/U (1.17–1.81), consistent with primary magmatic zircon crystallizing from a mafic magma.The four zircon analyses are discordant (8.7–11.7%) and have 207Pb/206Pb ages that vary between 1050 and 1090 Ma. The baddeleyite fraction (#5) is least discordant (3.8%) and contains relatively low uranium (213 ppm) for baddeleyite. The 1133.8 Ma 207Pb/206Pb age obtained for this baddeleyite fraction is interpreted as a minimum estimate for the age of this dike. A Model 1 regression treatment of three zircon (#1, #2, #3) and the baddeleyite (#5) yields and upper intercept age of 1141±20 Ma, which is interpreted as the best estimate for the emplacement of the Pigeon River dike (Heaman and Easton 2006). ILSG07 70 Trip 3 �Figure 3-9: Sketch map of the road cut exposing a Pigeon River dyke on the eastern side of Highway 61 at Stop 3-2. Geology modified after Peck and Rosatelli (personal communication, 2001) in Parker et al.(2001). Geochemical sample locations are shown as numbered black dots. STOP (OPTIONAL): Mount Mollie Lookout Location: Approximately 300 m north along old road bed, 3.1 km east of Highway 61 on Memory Road. The junction of Memory Road and Highway 61 is 6.9 km north of the junction of Highways 593 and 61, just north of the Pine River bridge (UTM 313843E / 5325444N; NAD83) Please be advised that this stop is located on private land. Permission must be granted by the land owner. Contact the Resident Geologist’s Office in Thunder Bay for further information. Description: This stop, atop a prominent northeast-trending ridge, affords a panoramic view of Pine Bay, Lake Superior and the surrounding, ridge-dominated topography. This and other nearby ridges are underlain by ophitic Pigeon River diabase dykes. Pigeon Point, Minnesota (type locality for pigeonite) can be seen on the horizon to the south. Northwest-trending ridges, underlain by Arrow River dykes, extend inland from south of Big Trout Bay, just north of this location. A long (>1 km) and challenging trail extends north and east from this vantage point to copper-nickel-mineralized occurrences in the Pine River-Mount Mollie dyke (ca. UTM 314748E / 5325640N; NAD27), which is approximately 60 m wide at that location. Heslop (1968) noted blebs of pyrrhotite-pentlanditechalcopyrite and magnetite-ilmenite intergrowths at this occurrence. Supergene(?) native copper has also been noted there. ILSG07 71 Trip 3 �STOP 3-3: Rove Formation Quarry, Great Lakes Nickel Limited Road Location: 4.7 km west of Highway 61 on the unmarked Great Lakes Nickel Road (private access). The GLNR extends west from Highway 61 at a point just south of the Pine River bridge, 6.6 km north of the junction of Highways 61 and 593 (UTM 306401E, 5328687N; NAD83). Description: A dormant shale quarry on the south side of the Great Lakes Nickel Limited access road provides a representative section of nearly flat-lying Rove Formation turbidites, diabase dykes and a sill. The quarry face is capped by a 1.1 m thick wacke bed that is underlain by a 1.1 m thick, conformable diabase sill in the main part of the quarry. Approximately 12 m of thinly to thickly bedded wacke and shale are exposed below the sill. The wackes are typically massive, and display sharp, planar contacts with the recessively weathered shale units. Scours of underlying shale are locally developed. Some of the shale interbeds display sediment loading features and distinctive sole features and are locally flaser-bedded. Another, 22 cm wide diabase dyke intrudes the sedimentary rocks east of the sill/dyke exposure, strikes ~060º and dips ~80º east. The columnar-jointed diabase sill is fine- to medium-grained and has conspicuous, grey chilled margins. It pinches out on the eastern end of the quarry face where it apparently “steps down” through the sedimentary rocks and merges with a near-vertically dipping diabase (feeder?) dyke. This dyke is weakly magnetic, strikes 030º and has an exposed thickness of approximately 4m. A sample collected near the chilled margin returned the following analysis: Sample Al2O3 CaO Fe2O3 K2O LOI MgO MnO Na2O P2O5 SiO2 TiO2 TOTAL 01-BGL-01 14.51 9.15 13.92 0.78 1.08 5.31 0.2 2.69 0.15 51.78 1.52 101.1 (Resident Geologist’s Files, Thunder Bay South District, Thunder Bay) In contrast a sample of the upper sill was considerably more silica rich (GL-1; SiO2 = 72 wt%) and is characterized by elevated La/Smn ratios (3.2) and Th contents (7 ppm vs. <1.3 in other dykes and sills). This likely reflects localized assimilation of Rove wacke by the sill during emplacement. A thin veneer of the hornfelsed sedimentary host rocks is locally preserved against the chilled dyke margin. The diabase is plagioclase-phyric and contains small sub-rounded shale xenoliths. A tendril of diabase extends west from the dyke (below the sill) and forms a bulbous, pillow-like termination with radial cooling fractures. The adjacent shale is chaotically folded and crenulated, suggesting that shearing accompanied intrusion. Two metres further west, an isolated ovoid (45 cm by 75 cm) “blob” of diabase intrudes the shale. Similar fingering termination features were described at the periphery of sills by Pollard et al. (1975). At finger terminations, the host rock is wedged aside and compacted and along the edges of the fingers, buckling and shearing of strata are common. Pollard et al. (1975) ascribed finger formation to the instability of the advancing interface between a viscous magma and a more viscous host rock. Antonellini and Cambray (1992) also studied a possible structural control on local sill emplacement. A block of wacke was selected at this location for a detrital zircon study (Heaman and Easton 2006). The detrital zircon ages form two distinct age clusters, with the majority of grains having 207 Pb/206Pb ages in the 1780–1880 Ma range. A second population consists of Archean grains, most of which are in the 2600–2700 Ma range. The oldest grain has a 207Pb/206Pb age of 2914 Ma. The youngest grain, with a 206Pb/238U age of 1731 Ma places a maximum age on the depositional age of this wacke. This grain is 5% discordant, consequently, a better estimate for a maximum depositional age is provided by a 1777 Ma concordant detrital zircon grain. This interpretation is supported by the abundance of grains (n=10) between 1796 and 1777 Ma, and is consistent with U/Pb ash bed ages of 1836±5 and 1832±3 Ma from the basal Rove Formation (Addison et al. 2005). The section exposed in the quarry walls represents a portion of the middle of the almost kilometer-thick Rove Formation (P. Fralick, Lakehead University, personal communication, 2007). The lower, underlying Rove is dominated by approximately 120 m of carbonaceous shale. This gradationally coarsens and thickens upwards through 100 m by the addition of thin sandstone ILSG07 72 Trip 3 �turbidites, culminating in beds similar to the ones exposed in the quarry. Proceeding upwards, above the rocks exposed in this section, ripples become more abundant in the sandstones until the assemblage is dominated by lenticular to flaser-bedded, wave- and current-rippled units. The Rove Formation represents a period of sediment starvation followed by progradation of a prodeltaic turbidite apron and distal delta top distributary mouth bar complex. Sections in cored drillholes, equivalent to the exposed strata here, are composed of parasequences that coarsen and thicken upwards over meters to a few tens of meters by the addition of thicker sandstone beds. These represent prograding turbidite lobes that are rapidly abandoned as the subaqueous feeder channel switches position. A period of sediment starvation follows with mud accumulation. This is ended by the gradational outbuilding of another lobe as the feeder channel moves closer to this area again. The thicker shales overlying the sand-rich tops of the parasequences are carbonaceous and pyritic, compared to the thinner shales forming the tops of individual turbidites. This denotes that the clays forming the thicker shales separating lobe outbuilding periods were deposited slowly allowing the remains of microbes to become common in the sediment. During diagenesis the microbes degraded, forming strongly reducing conditions that led to formation of diagenetic pyrite in the sediment. In contrast, the more rapid deposition of clay between turbidity current events during lobe outbuilding produced lower concentrations of carbon and much less pyrite. The difference between the two types of shale can easily be seen due to staining of the pyrite-rich rock during oxidation. Complete Bouma Sequences are rarely exhibited by the turbidites as the rippled C-division is commonly not present. However, flute marks produced by scouring of the viscous turbidity current into the muddy bottom and subsequent filling with sand, are ubiquitous (Figure 3-12). Commonly, in other turbiditic formations, flute marks are randomly developed on the bedding surface, but here they are in places arranged geometrically in diagonally offset rows. They also take on unusual elongate shapes, more akin to groove marks produced by something dragging across the bottom. The cause of the abnormal patterns and shapes is not known. Figure 3-10: View, looking south, of the eastern end of the dormant quarry in Rove Formation sedimentary rocks on the Great Lakes Nickel road, Stop 3-3. A diabase feeder dyke (left-center db) branches out into two sills: a main one, near the top of the quarry face (top-center db) and midway up the face, where it fingers and terminates (white box corresponds to Figure 3-11). The white dashed line separates oxidized, pyritic shale below and relatively pyrite-free shale above. ILSG07 73 Trip 3 �Figure 3-11: Diabase (db) “finger and blob” in Rove Formation wacke, Stop 33. The diabase was injected from a feeder dyke to the left of the photograph. A propagation fracture, with attendant folding and shearing, is denoted by the dashed line. Figure 3-12: Casts of flute marks on the bottom of a sandstone layer, Stop 3-3. Some have the classic “V” shape with their deepest area at the apex of the V. The V opens in the down-current direction. Others are drawn out and appear to have migrated down-current as they cut, producing a groove rather than a V. ILSG07 74 Trip 3 �STOP 3-4: Crystal Lake Gabbro / Great Lakes Nickel Property Location: 7.5 km west of Highway 61 on the unmarked Great Lakes Nickel Road (private access). The GLNR extends west from Highway 61 at a point just south of the Pine River bridge, 6.6 km north of the junction of Highways 61 and 593 (UTM 303612E, 5328392N; NAD83). Description: A switch-back road extends from the base of a 140 m high ridge to the lower adit, excavated at the contact between Rove Formation country rocks and the base of the Crystal Lake gabbro. An overgrown trail continues upward to the upper adit in “taxitic”, varied-texture gabbro and talus boulders. The upper portions of the intrusion, including layered rocks, are visible from this vantage point but are inaccessible for safety reasons. A stockpile of mineralized gabbro is available for sampling on the road at the base of the ridge. A second dump, further up the road, consists mainly of Rove Formation shales and argillites with abundant zeolite and carbonate veins. The discovery of copper- and nickel-mineralized float boulders in this area early in this century led to a concerted exploration effort to find their source. United States Smelting, Refining and Mining Company conducted exploration in 1936, and were followed by Frobisher Exploration Company in 1942. In the summer of 1952, J.S. Brodie and T.W. Page examined a large outcrop of gabbroic rocks, 6 km northeast of the original float discovery. Prospecting soon indicated a Cu-NiPt-mineralized zone and the ground was staked for Mattawin Gold Mines. Due to lack of operating capital, the Mattawin property was optioned to Falconbridge Nickel Mines in late 1952. A trench was subsequently excavated to test the lower gabbro contact. Although trench samples returned anomalous copper and nickel values, they did not warrant further investigation and the option was allowed to lapse. Additional work by J.S. Brodie drew attention to the area north of the trench. The property was optioned to R. Barker and W. Dawidowich in 1954. Six diamond drill holes, totalling 1058 m, were completed. One 55-foot section returned 0.54% Cu and 0.18% Ni with some precious metal values. Mogul Mining Corporation Limited held the property from 1954 to 1957 and drilled seven holes, totalling 1693 m. Intersections of the mineralized zone averaged 9 to 12m and assayed 0.9% combined Cu and Ni. Late in 1964, Great Lakes Nickel Corporation Limited acquired the property and initiated their exploration program in June, 1965. From 1965 to 1970, 47 803 m of surface drilling was completed; 19 underground holes, totalling 392 m, were also drilled. Underground drilling was conducted from a newly constructed, 37 m long adit, driven into the base of the hillside. In addition, Thunder Bay Nickel Mining Corporation Limited drilled 16 holes, totalling 13 579 m, on the down-plunge extension of the deposit, 2.5 km east of the main workings. In 1972, access and development work was undertaken by Great Lakes Nickel to further test the deposit. This work included the excavation and driving of a 522 m development portal and drift and over 12 000 m of surface and underground drilling. Plant site surveys, bulk sampling, metallurgical and feasibility tests were also conducted, financed largely by the Swedish company, Boliden Aktiebolag. By 1974, plans were made to mine the deposit at an initial rate of 1.8 million tons per year (subsequently increased to 2.5 million tons per year). Up to that point, about $10 M (Canadian) had been spent on the property on 58 689 m of surface drilling, 26 182 m of underground drilling and the driving of the adit, which eventually reached a length of 1041 m. This work had outlined a deposit of 32.8 million tons grading 0.36% Cu and 0.20% Ni, with a further potential reserve of 40 million tons of about the same grade. However, cost escalations, high interest rates and uncertain metal prices forced suspension of mine development in October, 1974. Rising interest in platinum-group elements in the mid-1980’s prompted Fleck Resources Ltd. to re-evaluate the deposit whose reserves then stood at 45.6 million tons at a grade of 0.334% Cu and 0.183% Ni. Between September, 1986 and February, 1987, Fleck completed geological mapping and sampling, as well as the re-logging and assaying of more than 9144 m of drill core. Six holes were drilled to test the deposit for its PGE potential. Sampling by Fleck returned the following assays on a 3.7 million ton portion of the deposit: 0.006 oz./ton Pt, ILSG07 75 Trip 3 �0.57% Cu, 0.027 oz./ton Pd, 0.264% Ni, 0.003oz./ton Au, 0.016% Co, 0.04 oz./ton Ag. Based on February, 1987 metal prices, this material was valued at $34.29 (Can.) per ton (Fleck Resources Ltd., Annual Report, 1987). Great Lakes Nickel has undertaken a comprehensive, due-diligence review of the property (Hubacheck 2001) in order to generate a valuation opinion and summary for potential investors or joint-venture partners. The geology of the Crystal Lake gabbro has been described by Geul (1970), Reeve (1969), MacRae and Reeve (1968), Whittaker (1986) and Smith and Sutcliffe (1987, 1989). It intrudes Rove Formation argillaceous and arenaceous rocks. The intrusion was subdivided into three major zones by Reeve (1969) and Geul (1970) and has been further subdivided by Smith and Sutcliffe (1987, 1989) and Cogulu (1990) into four major, roughly equivalent, lithologic zones: (1) an upper zone (60 to 80 m) of sulphide-barren, troctolite, olivine gabbro and anorthositic gabbro; (2) a middle zone (30 to 42 m) of cyclic, layered anorthositic and olivine gabbro, Cr-spinel- bearing anorthosite and olivine gabbro (Figure 3-4); (3) a lower, unlayered zone (50 m) of vari-textured gabbro and leucotroctolite, which hosts the bulk of the Cu-Ni-sulphide deposit; and (4) a basal zone (1 to 7 m) of fine-grained, chilled melagabbro and hornfelsed country rock xenoliths. Orthocumulate rocks predominate in the Crystal Lake intrusion; adcumulates occur only in the cyclic, layered rocks, in association with Cr-spinel mineralization (Cogulu 1990). Plagioclase, with lesser olivine and Cr-spinel (Figure 3-3), is the main cumulus mineral, while clinopyroxene, magnetite and Cu-Ni-sulphides are intercumulus (Cogulu 1990; Smith and Sutcliffe 1987, 1989). Total REE, LREE/HREE ratios, and sulphide content decrease from the basal zone to the cyclic sequence, while the modal proportion of olivine increases (ibid). Smith and Sutcliffe (1987, 1989) have suggested the following crystallization sequence: Cr-spinel → olivine → plagioclase → clinopyroxene → magnetite → apatite. Biotite is epitaxial to olivine and augite, and it may also rim intercumulus sulfides and oxides (Cogulu 1990). Sulphides are disseminated, interstitial and included grains and droplets. Pyrrhotite, chalcopyrite, cubanite and pentlandite are the main sulphides. Accessory minerals include violarite, troilite, niccolite, maucherite, native bismuth, mackinawite, bornite, millerite, nickeloan pyrite, sphalerite and marcasite (Cogulu 1993a). Adcumulate and orthocumulate Cr-spinels have been subdivided by Cogulu (1993b) into compositionally and texturally distinct groups with complex re-equilibration histories. Heterogeneity between Cr-spinels was also noted by Whittaker (1986). Local sulphur sources exist in local sedimentary rocks, as well as in the Archean basement. Data from the Duluth Complex indicated similar sulphur-isotopic signatures exist between basement sedimentary rocks and Ni-Cu-sulphides (Mainwaring and Naldrett 1974). Similar S-isotopic signatures also exist between Crystal Lake gabbro and Rove Formation shales (J. Franklin, personal communication, 1999.) Franklin (1970; personal communication, 2001) has also noted contact effects in Rove shales below diabase sills which include the conversion of pyrite to pyrrhotite, the loss of carbon in the spotted contact aureole and the rearrangement of clay mineral structures. Eckstrand and Cogulu (1986) presented Se/S data which supported the contention that sulphur in the Ni-Cu-sulphide minerals at Crystal Lake was derived from the barren, Se-poor, sedimentary sulphides of the Rove Formation. Sulphide concentrates from the Rove Formation and the lower parts of the intrusion (which host the sulphide deposit) yielded lower Se/S ratios than those gleaned from the Crspinel-bearing, cyclic layered zone whose Se/S ratios correspond closely to chondritic values (~200 to 600 x 10-6), implying that the sulphur was likely mantle-derived. ILSG07 76 Trip 3 �Stratigraphic Unit Cyclic Layered Zone Lower Unlayered Zone Basal Contact Zone Rove Formation (Footwall) (Se/S) x 10-6 213 - 687 70 - 151 28 - 114 2 – 118 (Eckstrand and Cogulu 1986) A grab sample of non-magnetic, massive subhedral pyrite with minor, interstitial, fine-grained silicates taken from the lower adit area, near the basal contact of the Crystal Lake Gabbro, returned 116 ppb Au, 688 ppb Pd, 14 ppb Pt, 13494 ppm Cu, 4756 ppm Ni and 255 ppm Zn (Resident Geologist's Files, Thunder Bay). ACKNOWLEDGEMENTS The trip leaders wish to thank the following individuals and companies for their assistance in the preparation of this field trip guide book and the field trip. Phil Fralick (Lakehead University) contributed sections on the Animikie Group and wrote the Rove Formation description for Stop 3-3. John Scott (Resident Geologist’s Program, Ontario Geological Survey) provided logistical support. The authors have benefited from discussions with Tom Hart and Mike Easton (Precambrian Geoscience Section, OGS), Larry Heaman (University of Alberta) and Jim Miller (Minnesota Geological Survey / University of Minnesota Duluth). The ongoing cooperation of Great Lakes Nickel Limited and other exploration companies is also appreciated. REFERENCES Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W. and Hammond, A.L. 2005. Discovery of distal ejecta from the 1850 Ma Sudbury impact event; Geology, v.33, p.193-196. Amurawaiye, O. 2001. The Paleoproterozoic Rove Formation of northwestern Ontario: A turbidite-dominated shelf sequence; unpublished H.B.Sc thesis, Lakehead University, Thunder Bay, Ontario, 44p. Antonellini, M.A. and Cambray, F.W. 1992. Relations between sill intrusions and bedding-parallel extensional shear zones in the Mid-continent Rift of the Lake Superior region: Tectonophysics, v. 212, p. 331-349. Beskar, S. 2001. The Blake gabbro: A taxitic-textured gabbro sill south of Thunder Bay, Ontario; 47th Institute on Lake Superior Geology, Annual Meeting, Madison, Wisconsin, May 9-12, 2001, Proceedings Volume 47, Part 1, p.1 Cogulu, E.H. 1990. Mineralogical and petrological studies of the Crystal Lake intrusion, Thunder Bay, Ontario: Geological Survey of Canada, Open File 2277, 15 p. plus figures and tables. -----. 1993a. Mineralogy and chemical variations of sulphides from the Crystal Lake intrusion, Thunder Bay, Ontario; Geological Survey of Canada, Open File 2749, 21 p. plus figures and tables. -----. 1993b. Factors controlling postcumulus compositional changes of chrome-spinels in the Crystal Lake intrusion, Thunder Bay, Ontario; Geological Survey of Canada, Open File 2748, 28 p. plus figures and tables. Davis, D.W. and Green, J.C. 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution; Canadian Journal of Earth Sciences, v.34, p.476-488. Davis, D.W. and Sutcliffe, R.H. 1985. U-Pb ages from the Nipigon Plate and northern Lake Superior; Bulletin of the Geological Society of America, v. 96, p. 1572-1579. ILSG07 77 Trip 3 �Eckstrand, O.R. and Cogulu, E. 1986. Se/S evidence relating to genesis of sulphides in the Crystal Lake gabbro, Thunder Bay Ontario; Geological Association of Canada-Mineralogical Association of Canada-Canadian Geophysical Union, Joint Annual Meeting, Ottawa, Ontario, Program with Abstracts, p.66. Fralick, P.W. and Miall, A.D., 1989. Sedimentology of the Lower Huronian Supergroup (Early Proterozoic), Elliot Lake, Ontario, Canada. Sedimentary Geology, v. 63, p. 127-153. Franklin, J.M. 1970. Metallogeny of the Proterozoic rocks of the Thunder Bay District, Ontario; unpublished Ph.D. thesis, University of Western Ontario, London, 317p. Franklin, J.M. and Kustra, C.R. 1972. The Proterozoic rocks of the Lake Superior area, northwestern Ontario; in Field Excursion C34: The Precambrian rocks of the Atikokan-Thunder Bay-Marathon area, 24th International Geological Congress, Guidebook, p.20-46. Franklin, J.M., Kissin, S.A., Smyk, M.C., and Scott, S.D. 1986. Silver deposits associated with the Proterozoic rocks of the Thunder Bay District, Ontario; Canadian Journal of Earth Sciences, v.23, p.1576-1591. Geul, J.J.C. 1970. Geology of Devon and Pardee Townships and the Stuart Location; Ontario Department of Mines, Geological Report 87, 52 p. -----. 1973. Geology of Crooks Township, Jarvis and Prince Locations, and Offshore Islands, District of Thunder Bay; Ontario Department of Mines, Geological Report 102, 46 p. Green, J.C. 1986. Lithogeochemistry of Keweenawan igneous rocks; Minnesota Department of Natural Resources, Division of Minerals, St. Paul, Project 241-4, 94p. Green, J.C., Bornhorst, T.J., Chandler, V.W. et al. 1987. Keweenawan dikes of the Lake Superior region: evidence for evolution of the middle Proterozoic Midcontinent Rift of North America; in Mafic dike swarms, H.C. halls and W.F. Fahrig, eds., Geological Association of Canada, Special Paper 34, p.289-302. Hart, T.R. 2002. Proterozoic volcanic and intrusive whole rock geochemical data associated with the Keweenawan Midcontinent Rift, Lake Superior area, Ontario; Ontario Geological Survey, Miscellaneous Release of Data, MRD 114. -----. 2003. Keweenawan mafic and ultramafic intrusive rocks of the Lake Nipigon and Crystal Lake areas, northwestern Ontario; 49th Institute on Lake Superior Geology, Proceedings volume 49, Part 1, Programs and abstracts, p.21-22. Hart, T.R., MacDonald, C.A., Hollings, P., and Richardson, A., 2005. Proterozoic intrusive rocks of the Nipigon Embayment and Midcontinent Rift. In, T.O. Tormanen and T.T Alapieti, 10th International platinum Symposium Extended Abstracts, Geology Survey of Finland, 365-368. Heaman, L.M. and Easton, R.M. 2006. Preliminary U/Pb geochronology results: Lake Nipigon Region Geoscience Initiative. Ontario Geological Survey, Miscellaneous Release-Data 191, 79p. -----. 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon Region, Ontario. Canadian Journal of Earth Sciences, in press. Heaman, L.M. and Machado, N. 1987. Isotope geochemistry of the Coldwell alkaline complex: 1. U-Pb studies on accessory minerals; Geological Association of Canada-Mineralogical Association of Canada, Joint Annual Meeting, Saskatoon, Saskatchewan, Program with abstracts, p.54. Hemming, S.R., McLennan, S.M. and Hanson, G.N., 1995. Geochemical and Nd/Pb isotopic evidence for the provenance of the Early Proterozoic Virginia Formation, Minnesota. Implications for tectonic setting of the Animikie Basin. Journal of Geology, v. 103, p. 147-168. Heslop, J.B. 1968. Mineralogy and textural relationships of the Mount Mollie sulphides, Pine Bay area, Thunder Bay District, Ontario. Unpublished B.Sc. thesis, University of Western Ontario, London, Ontario. Hoffman, P.F., 1987. Early Proterozoic foredeeps, foredeep magmatism and Superior-type iron-formations of the Canadian shield. In, ed. Kroner, A., Proterozoic Lithospheric Evolution, American Geophysical Union Series, v. 17, p. 85-98. Hollings, P., Hart, T., Richardson, A., and MacDonald, C.A. 2007. Geochemistry of the Midproterozoic intrusive rocks of the Nipigon Embayment. Canadian Journal of Earth Sciences, in press. ILSG07 78 Trip 3 �Hubacheck, P. 2001. Great Lakes Nickel copper-nickel-PGE project: Executive summary; unpublished report, Resident Geologist's Files, Thunder Bay, 7p. Jones, N.W. 1984. Petrology of some Logan diabase sills, Cook County, Minnesota; Minnesota Geological Survey, Report of Investigations 29, 40p. Lightfoot, P.C. and Lavigne, Jr., M.J. 1995. Nickel, copper, and platinum group element mineralization in Keweenawan intrusive rocks: new targets in the Keweenawan of the Thunder Bay region, northwestern Ontario; Ontario Geological Survey, Open File Report 5928, 32p. Lucente, M.E. and Morey, G.B., 1983. Stratigraphy and sedimentology of the lower Proterozoic Verginia Formation, northern Minnesota. Minnesota Geological Survey Report of Investigations 28, 28 p. MacRae, N.D. and Reeve, E.J. 1968. Differentiation sequence of the Great Lakes Nickel intrusion: 14th annual Institute on Lake Superior Geology, Superior, Wisconsin, Program with Abstracts, p.28. Mainwaring, P.R. and Naldrett, A.J. 1974. Genesis of Cu-Ni sulfides in the Duluth Complex; in Society of Economic Geologists, Annual Meeting, Economic Geology, v.69, no.7, p.1183-1184. Maric, M. and Fralick, P.W., 2005. Sedimentology of the Rove and Virginia Formations and their tectonic significance. Institute on Lake Superior Geology, v. 51, p. 41-42. McCuaig, J.A. 1950. A copper-nickel occurrence in Pardee Township, Thunder Bay District; unpublished M.Sc. thesis, McGill University, Montreal. Quebec, Miller, J.D. and Smyk, M.C. 1995. Gabbroic intrusions of the International Boundary area; in Field trip guidebook for the geology and ore deposits of the Midcontinent Rift in the Lake Superior region; International Geological Correlation Program Project 336, Minnesota Geological Survey, Guidebook 20, p.171-181. Miller, J.D., Smyk, M.C., Severson, M.J., Lavigne, M.J. and Middleton, R.S. 2002. PGE occurrences in mafic intrusions around western Lake Superior, USA and Canada; 9th International Platinum Symposium, Field Trip Guidebook, 135p. Morey, G.B. 1973. Stratigraphic framework of middle Proterozoic rocks in Minnesota. In, ed. G.M. Young, Huronian Stratigraphy and Sedimentation. Geological Association of Canada Special Paper 12, p. 211-249. North, J. 2000. Nature and distribution of Logan diabase sills and gabbro channels in the Keweenawan rift near Thunder Bay, Ontario: Brief comparison to Noril'sk; Abstract, 46th Institute on Lake Superior Geology, Annual Meeting, Thunder Bay, Ontario, Proceedings Volume 46, Part 1 (2000). Oja, R. 1967. Geochemical investigations of the Thunder Bay silver area; in Proceedings, Symposium on Geochemical Prospecting, Ottawa, 1966, Geological Survey of Canada, Paper 66-54, p.211-221. Ojakangas, R.W., Morey, G.B. and Southwick, D.L., 2001. Palaeoproterozoic basin development and sedimentation in the Lake Superior region, North America; Sedimentary Geology, v. 141, p. 319-341. Parker, D.P. (ed.), Middleton, B., Schnieders, B.R., Smyk, M.C. and Scott, J.F. 2001. Intrusions of the Nipigon Basin; Superior PGE 2001, Canadian Institute of Mining and Metallurgy, Geological Society Field Conference, Thunder Bay, September 16-19, 2001, Field Trip Guidebook, 43p. Pollard, D.D., Muller, O.H. and Dockstader, D.R. 1975. The form and growth of fingered sheet intrusions; Geological Society of America Bulletin, v.86, p.351-363. Pufahl, P.K. and Fralick, P.W., 1995. Paleogeographic reconstruction of the Gunflint-Mesabi-Cuyuna depositional system: a basin analysis approach; 41st Institute on Lake Superior Geology, v. 41, Proceedings with abstracts, p.59-60. Pufahl, P. and Fralick, P. 2000. Depositional environments of the Paleoproterozoic Gunflint Formation; 46th Institute on Lake Superior Geology, v.46, pt.2, Proceedings with abstracts. Pufahl, P.K. and Fralick, P.W., 2004. Depositional controls on Paleoproterozoic iron formation accumulation, Gogebic Range, Lake Superior region, USA.. Sedimentology, v. 51, p. 791-808. Pufahl, P.K., Fralick, P.W. and Scott, J., 2000. Depositional environments of the Palaeoproterozoic Gunflint Formation; 46th Institute on Lake Superior Geology, v.46, pt.2, Field Trip Guide, 46 p. ILSG07 79 Trip 3 �Queen, M., Heaman, L.M., Hanes, J.A., Archibald, D.A. and Farrar, E. 1996. 40Ar/39Ar phlogopite and U-Pb perovskite dating of lamprophyre dykes from the eastern Lake Superior region: Evidence for a 1.14 Ga magmatic precursor to Midcontinent Rift volcanism; Canadian Journal of Earth Sciences, v.33, p.958-965. Reeve, E.J. 1969. Petrology and mineralogy of a gabbroic intrusion in Pardee Township near Port Arthur, Ontario: unpublished M.Sc. thesis, University of Wisconsin, Milwaukee, 79 p. Rosatelli, M.P., 2002. Assessment report on the 2002 lithogeochemical rock sampling program, Pigeon River block. McVicar Minerals Ltd., BHP Billiton World Exploration Inc.. and Falconbridge Limited; Assessment Files, Thunder Bay South District, Thunder Bay, FN 2.24485, 36p. Smith, A.R. and Sutcliffe, R.H. 1987. Keweenawan intrusive rocks of the Thunder Bay area: in Summary of Field Work 1987, Ontario Geological Survey, Miscellaneous Paper 137, p. 248-255. -----. 1989. Precambrian geology of Keweenawan intrusive rocks in the Crystal Lake-Pigeon River area: Ontario Geological Survey, Map P.3139, scale 1:50 000. Smyk, M.C., Hollings P. and Heaman, L.M. 2006. Preliminary investigations of the petrology, geochemistry and geochronology of the St. Ignace Island Complex, Midcontinent Rift, northern Lake Superior, Ontario; Institute on Lake Superior Geology, 52nd Annual Meeting, Sault Ste. Marie, ON, Program with Abstracts, v. 52, p.6162. Southwick, D.L. and Morey, G.B., 1991. Tectonic imbrication and foredeep development in the Penokean Orogen, east-central Minnesota - an interpretation based on regional geophysics and the results of test-drilling. United States Geological Survey Bulletin 1904-C, 17 p. Stockwell, C.H., McGlynn, J.C., Emslie, R F., Sanford, B.V., Norris, A.W., Donaldson, J.A., Fahrig, W.F. and Currie K L. 1972. Geology of the Canadian Shield, in Geology and Economic Minerals of Canada, edited by R.J.W. Douglas, Geological Survey of Canada, Economic Geology Report 1, 838 p. Sun, S.S. and McDonough, W.F. 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes; in Saunders, A.D. and Norry, M.J. (Eds.), Magmatism in ocean basins, Geological Society of London Special Publication 42, p.313-345. Sutcliffe, R.H. 1989. Mineral variation in Proterozoic diabase sills and dykes at Lake Nipigon, Ontario; Canadian Mineralogist, v.27, p.67-79. Tanton, T.L. 1935. Copper-nickel mineral occurrences in Pigeon River area, Ontario; Geological Survey of Canada, Paper 35-1, 11p. -----. 1936a. Pigeon River area, Thunder Bay District; Geological Survey of Canada, Sheet 1, Map 354A, scale 1:63 360. -----. 1936b. Pigeon River area, Thunder Bay District; Geological Survey of Canada, Sheet 2, Map 355A, scale 1:63 360. -----. 1937. Copper-nickel occurrences in Pigeon River area, Ontario. The Precambrian, May, 1937, p.18-19. Van Wyck, N. and Johnson, C.M. 1997. Common lead, Sm-Nd, and U-Pb constraints on petrogenesis, crustal architecture and tectonic setting of the Penokean Orogen (Paleoproterozoic) in Wisconsin, U.S.A. Geological Society of America Bulletin, v. 109, p. 799-808. Weiblen, P.W., Mathez, E.A. and Morey, G.B. 1972. Logan intrusions; in Sims, P.K. and Morey, G.B. eds., Geology of Minnesota: A centennial volume; Minnesota Geological Survey, p.394-410. Whittaker, P.J. 1986. Chromite deposits in Ontario: Ontario Geological Survey, Study 55, 97 p. ILSG07 80 Trip 3 �53rd Annual Institute on Lake Superior Geology FIELD TRIP 4 GEOLOGY OF THE NICKEL LAKE MACRODIKE AND ITS ASSOCIATION WITH CU-NI-PGE MINERALIZATION IN THE NORTHERN SOUTH KAWISHIWI INTRUSION, DULUTH COMPLEX, NORTHEASTERN MINNESOTA Dean M. Peterson University of Minnesota Duluth, Natural Resources Research Institute Paul B. Albers Duluth Metals Limited, Ely, Minnesota “Imagination is more important than knowledge” Albert Einstein INTRODUCTION Mineral deposits are concentrations of specific ore-minerals that society utilizes in immeasurable ways. They have formed from Archean times up to the present, and vary greatly in commodity, mineralogy, alteration, trace element signature, geophysical properties, and grade-tonnage. Individual ore deposits are always unique, and this uniqueness arises from two main sources: 1) fundamental differences in geologic processes and environments; and 2) local, site-specific, geologic variations and associated bounding geometries. The seemingly limitless number of permutations of these (and other) features of mineral deposits defies imagination. Geologists employed in the search for ore deposits have, over the last century, developed the intellectual concept of Ore Deposit Models, which seek to organize many of ILSG07 81 Trip 4 �these variables, lump individual mineral deposits into classes, and establish criteria to aid in mineral exploration (Peterson, 2001a). Such models may be strictly empirical – a collection of observable facts associated with the occurrence of certain metals in economic proportions – or genetic – which attempts to describe the physical and chemical processes responsible for the development of an ore deposit and its related empirical features. Advances in our collective understanding of ore deposit types is nonlinear, rather it occurs in fits and starts. Take, for example, the great advance in knowledge – and thus mineral exploration models and exploration success – for porphyry copper deposits in the 1960s and 1970s (Lowell and Guilbert, 1970), volcanogenic massive sulfide deposits in the 1970s and 1980s (Franklin et al., 1981), and mesothermal lode gold deposits in the 1980s and 1990s (Hodgson, 1993). These advances were the product of many factors, which conceivably the most important was collaborative ore deposit research between the mineral industry, academia, government agencies, and research organizations. We are currently in the midst of a similar great advance in knowledge of magmatic Ni-Cu-PGE deposits (Arndt et al., 2006; Barnes and Lightfoot, 2006; Eckstrand and Hulbert, in prep.). Perhaps the most important impetus for this current revolution was the fall of the Iron Curtain, which opened the door in the early 1990s to some of the world’s greatest magmatic Ni-Cu-PGE deposits, i.e., the Noril’sk-Talnakh deposits in Russia and Jinchuan deposit in China, to economic geologists trained in open western societies (especially Canada). In addition, the 1993 discovery of the magmatic Ni-Cu deposit at Voisey’s Bay, Labrador, Canada, and its inferred origin as a magmatic feeder dike, has brought about a revolution in our collective understanding of the origin of magmatic Ni-Cu±PGE deposits. This revolution can simple be stated as “find the magmatic feeder dike and/or channelized magma flow zones” to the orebearing mafic/ultramafic intrusion(s). A magmatic conduit that experienced repeated influxes of magma appears to be the key to the formation of high-grade, world-class, Ni-Cu-PGE deposits (Naldrett, 1997). This field trip is truly the result of over two decades of dedicated Duluth Complex research, almost entirely funded by the State of Minnesota and the University of Minnesota, by geologists of the University of Minnesota Duluth’s (UMD), Natural Resources Research Institute (NRRI). Steve Hauck, director of the NRRI’s Economic Geology Group (EGG), is here thanked for his decades of dedicated research and seeing through that the EGG remains financially solvent through a never-ending series of budgetary crises. As well, the NRRI’s Mark Severson – who has logged well over 900,000 feet of Duluth Complex drill core – has without a doubt the more knowledge than anyone on the geology of the mineralized zones within the deposits of the Duluth Complex. The authors of this guidebook have simply built upon this intellectual capital in new ways, including geochemical and 3-D modeling of drill core data, and especially the most basic geologic endeavor, detailed field mapping. FIELD TRIP TENETS Many basic attributes of the Duluth Complex Cu-Ni-PGE sulfide deposits resemble those of deposits at Noril’sk, Russia, Jinchuan, China, and Voisey’s Bay, Canada that are associated with sulfide mineralization in intrusive feeder zones. Such attributes include shallow tholeiitic intrusions associated with plateau basalt volcanism, external sedimentary sources of sulfur, and openness to repeated magma influx and expulsion. However, the biggest difference between these world-class magmatic ore systems and the deposits of the Duluth Complex is the lack of significant nickel-rich (Ni>Cu) massive sulfide orebodies at Duluth. A critical attribute of the high-grade Noril’sk–Talnakh and Voisey’s Bay deposits, not previously positively identified in the Duluth Complex, is the location of a magmatic conduit, i.e., the feeder zone. Several geologists have previously identified possible conduits that may have fed the Partridge River (PRI) and South Kawishiwi (SKI) intrusions of the Duluth Complex. Severson and Zanko (unpub. data) suggest that the Grano fault might mark a possible feeder zone for the Local Boy ore zone at the northeast ILSG07 82 Trip 4 �end of the PRI. Thériault et al. (2000) postulated that a PRI conduit was present somewhere between the Wetlegs and Dunka Road deposits. Another possible PRI feeder zone may have been along the prolongation of the Siphon fault, which is a Paleoproterozoic growth fault (Graber, 1993) that may have been reactivated during emplacement of the Duluth Complex (Severson and Hauck, 1997). Several authors have suggested the presence of a sub-vertical magmatic feeder beneath the Bald Eagle intrusion (BEI) based on field relations (Weiblen and Morey, 1980) and geophysical attributes (Chandler, 1990). Peterson (2001b) interpreted the systematic variation in Cu-Ni-PGE mineralization in the Maturi deposit and its extension east to Maturi Extension, as indicative of magma input from the east-northeast via an arcing macrodike (herein first termed the Nickel Lake Macrodike (NLM)) that connects the deep-seated source of the BEI and the SKI. Subsequent mapping and research (Peterson, 2002a-f, 2006a, Peterson et al., 2004, Peterson and Hauck, 2005) has built on this model, which recently culminated with the publication of a new detailed bedrock geology map of the area (Peterson et al., 2006). If correct, this model predicts that a Voisey’s Bay-type Ni-Cu-PGE massive sulfide body may exist at depth in the area where the NLM meets the SKI, south-southeast of the Spruce Road deposit (see Figs. 4.2 and 4.3). This field trip will investigate evidence that the NLM is a feeder to the SKI, and thus is one of the principal conduits (only?) that brought Cu-Ni-PGE from the Earth’s mantle and/or lower crustal magmatic staging chambers into the Earth’s upper crust via the NLM into the SKI. This will be accomplished through scientific discussions (hopefully heated) on numerous outcrops (Day 1) as well as visualization of the geology in 3-D and displays of recently drilled cores by Duluth Metals Limited (Day 2). The fundamental tenet of this field trip is to convey to the participants the notion that science gives one the ability to imagine reality. Herein, science is geologic research of the Duluth Complex (geologic mapping, drill core logging, geochemical studies, and exploration drilling), and reality is new understanding how this magmatic system concentrated and enriched known and potential concentrations of Ni-Cu-PGE at the base of the NLM in the field trip locale as well as adjacent areas of the SKI. The field trip leaders ask the participants (and others who may use the guidebook in the future) to use their imaginations throughout the field trip (or subsequent field excursions) to think about a few basic known and possible realities of the Duluth Complex: 1) The Duluth Complex is perhaps the world’s largest untapped resource of Ni-Cu-PGE, with multibillion tons of geologic resources estimated to be worth >1 trillion dollars, (Peterson, 2006c); 2) The general geologic setting of the deposits in the Duluth Complex is similar to other world class Ni-Cu-PGE mining camps hosted by rocks in rift settings (Noril’sk-Talnakh, Jinchuan); 3) Overall, the ratio of Ni to Cu in Duluth Complex deposits average 1:3; 4) Worldwide, the ratio of Ni to Cu in similar deposits averages between 1:1 to 2:1; 5) By analogy, there seems to be an enormous mass of missing Ni-rich mineralization; 6) At depth, the Nickel Lake Macrodike may host a large percentage of this missing Ni in the SKI. MAGMATIC NI-CU-PGE ORE DEPOSIT MODEL The basic starting point to begin our quest to understand the significance (both geologic and economic) of the Nickel Lake Macrodike is a quick review of the magmatic Ni-Cu-PGE ore deposit model. There are literally hundreds of recent publications in the geological literature that deal with this important class of ore deposit, and readers interested in this topic may find that Barnes and Lightfoot (2006), Arndt et al. (2006), Eckstrand and Hulbert (in prep.), Naldrett, (1989, 1997, 1999), Naldrett et al. (2000), Li and Naldrett (1999), Lightfoot et al. (1994) and references therein are excellent reviews that describe magmatic Ni-Cu-PGE deposits in general, and major deposits in particular. ILSG07 83 Trip 4 �Magmatic Ni-Cu-PGE sulfide deposit occur as sulfide concentrations associated with a variety of mafic to ultramafic rocks in four major geological settings: 1) rifts and continental flood basalt settings (Noril’skTalnakh, Russia; Duluth Complex, Minnesota; Jinchuan, China); 2) meteorite impacts (Sudbury, Ontario, the only mining camp in this class); 3) komatiite lava flows and related intrusions (Thompson, Manitoba; Raglan, Québec; Kambalda, Australia; Pechenga, Russia); and 4) a variety of miscellaneous tholeiitic intrusions (Voisey’s Bay, Labrador; Lynn Lake, Manitoba). The ores are enriched in sulfur, iron, nickel, copper, cobalt, and the platinum group elements (Pt, Pd, Rh, Ru, Ir, and Os) and may contain minor Ag, As, Au, Bi, Hg, Pb, Sb, Se, Te, and Zn. Grade-tonnage diagrams for magmatic Ni-Cu-PGE sulfide deposits/camps are presented in Figure 4.1 which highlights the major camps/deposits listed above. Figure 4.1. Tonnage and Ni grades of magmatic Ni-Cu sulfide deposits; B. Tonnages and Cu grades of magmatic Ni-Cu sulfide deposits. Inclined contours show quantities of contained metals (tonnes) in each figure. Figure modified from Eckstrand and Hulbert, in prep. ILSG07 84 Trip 4 �The basic intrinsic geological features characteristic of a vast majority of magmatic Ni-Cu-PGE sulfide deposits include: (1) olivine-rich magmas; (2) proximity to a major crustal fault; (3) sulfide-bearing country rocks; (4) chalcophile element depletion in related intrusive or extrusive rocks; (5) field and/or geochemical evidence of interaction between the magma and the country rocks; and (6) presence of, or proximity to, a magma conduit (Naldrett, 1999). Fundamental geologic processes and constraints that together leads to the formation of these deposits include: 1) Deposits form as the result of segregation and concentration of droplets of liquid sulfide from mafic or ultramafic magmas; 2) Chalcophile elements from the silicate melt partition into the droplets as a result of turbulent magma flow; 3) An appropriate physical environment is required so that the sulfide liquid mixes with enough magma to become adequately enriched in chalcophile metals; 4) Sulfides must cluster in a restricted locality, generally due to the influence of gravity, so that the resulting metal concentration is of ore grade; 5) Massive sulfide concentrations form a high-temperature monosulfide solid solution (MSS); 6) As the MSS cools, it exsolves minerals and fractionates; a. Forms a solid cumulate mass of pyrrhotite-rich massive sulfide (enriched in Fe, Ni, Co, Ir, Ru, Rh). b. Forms a liquid residuum that is enriched in Cu, Pd, Pt, Au, and other minor elements, including As, Bi, Te, Sb (which will crystallize later into minerals as the system cools). 7) For some time, the Cu-Pd-Pt residual liquid can move and form high grade ore shoots/deposits a. i.e., footwall veins in Sudbury can be 30 wt. % Cu and multi-ounce/ton Pt + Pd. Possibly the greatest recent advance in understanding Ni-Cu-PGE sulfide deposits has been the appreciation of coherent and compelling scientific arguments that have shown that magma dynamics play a key role in the concentration and metal enrichment of sulfide minerals in these deposits (Naldrett, 1997). These arguments build on the long held notion that sulfide-rich orebodies achieve their concentrations mainly through the settling of sulfide droplets in magmas due to the effects of gravity. The new fundamental tenet has been the realization that decreases in the flow rate of magmas, principally due to the geometry and obstructions in magmatic conduits, is the major factor in settling entrained sulfide droplets and forming sulfide-rich orebodies (Eckstrand and Hulbert, in prep.). These geometries include, for example, widened parts of magmatic feeder dikes, the lowest zones of undulating basal zones of conduits, and the location where such dikes enter larger magma chambers. Geologists that are engaged in Ni-Cu-PGE exploration projects should use their imaginations to envision magma dynamics like a person fly-fishing along a stream, and search for deposits behind the large boulders and in the slow-moving pools below rapids where the big rainbow and brown trout are most likely to be found. REGIONAL GEOLOGIC SETTING, DULUTH COMPLEX The Duluth Complex and associated intrusions of Keweenawan age (~1.1 billion) in northeastern Minnesota constitute one of the largest mafic intrusive complexes in the world, second only to the Bushveld Complex of South Africa (Miller et al., 2002). These rocks cover a 5,700 square kilometer arcuate area associated with the two strongest gravity anomalies (+50 and +70 milligals) in North America, that imply intrusive roots more than 13 kilometers deep (Allen and others, 1997). The comagmatic flood basalts and intrusive rocks underlying most of northeastern Minnesota were emplaced during the development of the Mesoproterozoic Midcontinent rift, which can be traced geophysically from exposures in the Lake Superior region along a 2,000 kilometer-long, segmented, arcuate path to ILSG07 85 Trip 4 �Kansas and Lower Michigan. The Duluth Complex is defined as the more or less continuous mass of mafic to felsic plutonic rocks that extends for >275 kilometers in an arcuate fashion from Duluth nearly to Grand Portage (Fig. 4.2). It is bounded by a footwall of Paleoproterozoic sedimentary rocks and Archean granite-greenstone terranes (Peterson and Severson, 2002), and a hanging wall largely of comagmatic, rift related flood basalts and hypabyssal intrusions of the Beaver Bay Complex (Fig. 4.2). In genetic terms, the Duluth Complex is composed of multiple discrete intrusions of mafic to felsic tholeiitic magmas that were episodically emplaced into the base of a comagmatic volcanic edifice between 1108 and 1098 Ma. Figure 4.2. Generalized geologic map of northeastern Minnesota. Highlighted intrusions include the Bald Eagle (BEI) and South Kawishiwi (SKI) intrusions, as well as the linking Nickel Lake Macrodike (NLM) (modified from Miller et al., 2002). The geology of the Duluth Complex and adjacent areas has recently been described in two major publications by the Minnesota Geological Survey (MGS). These include a 1:200,000 scale regional bedrock geological map of northeastern Minnesota (Miller et al., 2001), and a comprehensive written description of the geology depicted on this map (Miller et al, 2002), commonly referred to as the “bible” by geologists working on Duluth Complex geology. Readers’ interested in more detailed descriptions of the geologic setting of the Duluth Complex should begin their quest for knowledge by downloading these publications from the MGS website (ftp://mgssun6.mngs.umn.edu/pub2/). Within the nearly continuous mass of intrusive igneous rock forming the Duluth Complex, four general rock series are distinguished on the basis of age, dominant lithology, internal structure, and structural position within the complex. ILSG07 86 Trip 4 �Felsic series—Massive granophyric granite and smaller amounts of intermediate rock that occur as a semicontinuous mass of intrusions strung along the eastern and central roof zone of the complex, emplaced during an early stage magmatism (~1108 Ma). Early gabbro series—Layered sequences of dominantly gabbroic cumulates that occur along the northeastern contact of the Duluth Complex, emplaced during early stage magmatism (~1108 Ma). Anorthositic series—A structurally complex suite of foliated, but rarely layered, plagioclase-rich gabbroic cumulates emplaced throughout the complex during main stage magmatism (~1099 Ma). Layered series—A suite of stratiform troctolitic intrusions that comprises at least 11 variably differentiated mafic layered intrusions that occur mostly along the base of the Duluth Complex. These intrusions were emplaced shortly after the Anorthositic series (~1099 Ma). This field trip will investigate rocks of the Layered Series – the SKI, NLM, and by implication the BEI – and Anorthositic series rocks in outcrops within and along the margins of the NLM. It is hoped that discussions on the outcrop (Day 1), coupled with examinations of selected sections of mineralized Duluth Metals Limited drill core from the SKI, and visualization of the geology in a 3-D presentation (Day 2) will bring new insight to the geology and mineral potential of the field trip area to the participants. Prior to these investigations, the quick descriptions of the local geology, basal contact-associated styles of CuNi-PGE mineralization, and calculated Cu-Ni grade-tonnage geologic resources within this area that follow will give the field trip participants a better appreciation of the significance of the NLM and its inferred potential for hosting great quantities of Ni-rich sulfide mineralization at depth. LOCAL GEOLOGIC SETTING Robust field and geophysical data suggest that the emplacements of the BEI and SKI may be closely linked (Weiblen and Morey, 1980). At the northern margin of the BEI, a macrodike of well-foliated troctolite (herein termed NLM) arcs northwest to southwest and merges with the middle of the northern SKI (Fig. 4.3). Green et al., (1966) mapped the macrodike as part of the SKI, but its composition is very similar to the troctolitic phase of the BEI. Peterson (2001b) proposed a model for the mineralization in the SKI whereby the initial emplacement of the intrusion was formed by sulfide-contaminated magmas that emerged from the NLM and flowed southwest between a footwall of Archean granite and a hanging wall of Anorthositic series rocks. Miller et al. (2002) present an important interpretive model for the emplacement of the BEI and SKI via a common feeder system, as well as depict the origin of sulfur saturation of basal SKI magmas (Fig. 4.4) due to contamination from Paleoproterozoic sedimentary rocks (Ripley, 1986). Peterson et al. (2006) completed detailed geological mapping along the western end of the NLM and adjacent SKI, and have defined distinct mappable units in both of these troctolitic bodies. BALD EAGLE INTRUSION The BEI is a large (4.5 to 16.5 km x 31 km) troctolitic to gabbroic body that was emplaced partially within Anorthositic series rocks, the SKI, and the Greenwood Lake Intrusion (Fig. 4.2). Weiblen (1965) mapped the well-exposed northern portion of the intrusion and showed that it consists of an outer zone of troctolite and an inner zone of olivine gabbro. In the poorly exposed southwestern portions of the intrusion, field mapping by Green et al., (1966) and Foose and Cooper (1978) showed the BEI and SKI in direct conformable contact. Steep foliation and modal layering (Weiblen, 1965; Green et al., 1966) integrated with a distinct gravity anomaly over the northern BEI imply that the northern part of this intrusion is funnel shaped and necks down to a steep feeder dike. Weiblen and Morey (1980) interpreted the limited cryptic variation (Weiblen, 1965), the steep dip of lamination and layering, and adcumulate nature of the BEI as indicative of its being an open conduit to higher intrusions and perhaps volcanic flows. ILSG07 87 Trip 4 �Figure 4.3. Simplified views of an integrated 3-D model of the BEI, NLM, and SKI of the Duluth Complex. A. plan view and B. view to the southwest. Model surfaces built from drill hole piercing points, detailed geological mapping, and interpretation of gravity and aeromagnetic data. ILSG07 88 Trip 4 �Petrologic observations and geophysical interpretations (Chandler, 1990; Chandler and Ferderer, 1989) suggest that the BEI and SKI were emplaced by successive overplating of magmas from a common feeder centered on the northern BEI and extending along the trace of the NLM that links the BEI and SKI. A model that depicts the origin of the BEI and SKI via a common dynamic feeder by Miller et al. (2002) is presented in Figure 4.4. In a related analogy, Cartwright and Møller-Hansen (2006) have shown that interconnected sill complexes transect the middle to upper crust over a vertical distance of 8-12 km offshore of Norway. The geometry of the gravity and magnetic anomalies of the BEI, as well as the overall Midcontinent Rift is very similar to the pattern of the seismic reflections profiles of active ridge systems (Vislova, 2003). In detail, the geophysical expressions of the BEI have the same shape and dimensions as the “bulls’ eye” pattern of low velocity seismic reflection anomalies along the East Pacific Rise. These anomalies are interpreted to define regions of melt concentrations, i.e., active magma chambers. These data suggest that the BEI could be a “frozen” dynamic magma chamber (Weiblen et al., 2005; Peterson and Hauck, 2005). The BEI has posed unresolved questions concerning its origin and magmatic significance since its discovery in 1961 (Weiblen, 1965, Weiblen and Morey, 1980, Miller et al., 2002). A number of its characteristics contrast markedly with those of the other mapped intrusions in the Midcontinent Rift: 1) it has a well-defined intrusive contact in Anorthositic series rocks around its northern perimeter; 2) there is a subtle, but recognizable metamorphic contact effect on these anorthositic gabbros; 3) a primary magmatic foliation is well defined by mineral orientation and discoid segregation of plagioclase from mafic phases; 4) foliation measurements define a steeply-dipping asymmetric funnel with the foliation paralleling the contact and grading from steep to horizontal inward; 5) the intrusion consists of two cumulus units, an outer troctolite and inner olivine gabbro; and 6) there is only minor (< a few %) intercumulus material in the cumulates, i.e., clinopyroxene and iron oxides (Weiblen et al., 2005). SOUTH KAWISHIWI INTRUSION The South Kawishiwi intrusion (SKI), together with the similar sized Partridge River intrusion (PRI) immediately to the south, are most renown for hosting the largest tonnage of Cu-Ni sulfide mineralization in the world (Naldrett, 1997). The realization that the SKI hosts vast quantities of Cu-Ni mineralization over 50 years ago has lead to the publication of numerous geologic maps, (Green et al., 1966; Bonnichsen, 1974; Foose and Cooper, 1974; Miller et al., 2001; Peterson, 2002e, f; Peterson et al., 2004; Peterson, 2006b; Peterson et al., 2006), articles (Bonnichsen et al., 1980; Weiblen and Morey, 1980; Ripley, 1986; Chandler and Ferderer, 1989; Lee and Ripley, 1996; Hauck et al., 1997; Peterson, 2001b) theses (Weiblen, 1965; Vislova, 2003; Marma, 2003), and reports (Phinney, 1969; Phinney, 1972; Listerude and Meineke, 1977, Morey and Cooper, 1977; Foose, 1984; Dahlberg, 1987; Dahlberg et al., 1989; Kuhns et al., 1990; Severson, 1994; Zanko et al., 1994; Hauck et al., 1997; Peterson, 1997; Peterson, 2001c; Miller et al., 2002; Peterson, 2002d; Patelke, 2003; Severson and Hauck, 2003). The SKI is shallow dipping (~20º to the east-southeast) sill-like intrusion dominantly composed of troctolitic cumulates that are exposed in an 8- x 32-kilometer arcuate band along the northwestern margin of the Duluth Complex (Fig. 4.2). Footwall rocks include the Paleoproterozoic Virginia Formation in the Serpentine and Dunka Pit deposits, the Paleoproterozoic Biwabik Iron Formation in the Dunka Pit and Birch Lake deposits, and the Archean Giants Range batholith from the northern Birch Lake deposit north to the Spruce Road deposit (see Fig. 4.3 for deposit locations). The presence of shallow-dipping Biwabik Iron Formation inclusions as far north as the Spruce Road deposit indicates that the majority of Paleoproterozoic units were assimilated and removed from the footwall during emplacement of the SKI, leaving the Giants Range batholith as the dominant footwall rock type. Alternately, the Virginia and Biwabik Iron Formations may simply have been largely eroded prior to the development of the Mid Continent Rift. Also present as inclusions in the SKI are mafic volcanic hornfels (North Shore Volcanic Group), quartz sandstone hornfels (either the Puckwunge or Nopeming sandstones), and anorthosite (of ILSG07 89 Trip 4 �the Anorthosite series). Anorthositic series rocks abut the SKI on the northeast – and enclose an interpreted SKI feeder dike (the NLM) that extends farther northeast – the PRI forms the southern sidewall of the SKI, and the BEI and Anorthositic series rocks overlie the SKI to the east (Fig. 4.2). Figure 4.4. Interpretive model which depicts the emplacement and mineralization mechanisms of the South Kawishiwi and Bald Eagle intrusions via a common feeder system. A) Intrusion of plagioclase crystal mushes into volcanic rocks to create Anorthositic series rocks; B) Intrusion of the SKI below Anorthositic series rocks, with the early basal units reaching sulfur saturation via contamination from Paleoproterozoic sedimentary rocks; and C) Intrusion of the BEI above Anorthositic series rocks. Modified from figure 6.15 of Miller et al. (2002). ILSG07 90 Trip 4 �On the regional Duluth Complex map of Miller et al. (2001), the SKI is subdivided into five major map units. These are, from the base upward, 1. Heterogeneous sulfide-bearing troctolite, gabbro, and norite with localized hornfels inclusions; 2. A thick unit of subophitic to ophitic augite troctolite; 3. Discontinuous and localized layers of poikilitic leucotroctolite; 4. A thick homogeneous sequence of ophitic troctolite; and 5. A thick uppermost sequence of homogeneous troctolite that contains numerous anorthositic layers. Severson (1994) and Zanko et al. (1994) further subdivided the SKI into 17 different lithostratigraphic units that are present in over 180 drill holes over a strike length of 31 kilometers. Sulfide mineralization is confined to the BH, BAN, UW, and U3 units near the base of the intrusion, and to a lesser extent the U1, U2, and PEG units. Major marker horizons that are correlated in drill holes include three horizons with abundant cyclic ultramafic layers (U1, U2, and U3 units) and a pegmatite-bearing unit (PEG unit) that was initially recognized by Foose (1984). The understanding of the significance of a large anorthositic inclusion (Fig. 4.3), originally intersected in six deep drill holes east of the Maturi deposit, and its role in magma dynamics of the SKI has been a key feature in the development of an exploration model for Duluth Metals Limited’s Maturi Extension deposit (Peterson, 2001c). Basal SKI Mineralization Basal mineralization within the South Kawishiwi intrusion has traditionally been divided into five distinct deposits: 1) Serpentine, 2) Dunka Pit, 3) Birch Lake, 4) Maturi, and 5) Spruce Road. Recent drilling by Duluth Metals Limited has confirmed the existence on an additional economically significant deposit east of Maturi first envisioned by Peterson (2001c), named the Maturi Extension deposit (Fig. 4.3). Although the style of mineralization in all of the deposits is dominated by disseminated Cu-Ni sulfides, differences occur between the deposits in igneous stratigraphy, sulfide mineralogy, Cu-Ni and PGE grade, mineralization thickness, and contained tonnes. In addition to mineralization spatially associated with the base of the intrusion, a distinct zone of Cu and PGE-enriched mineralization occurs thousands of feet above the base of the intrusion within linear zones in the South Filson Creek deposit (Kuhns et al., 1990). Compilation and analysis of drill hole assay data by Peterson (2001, 2002a-d) has led to new understanding of two distinct styles of mineralization associated with the base of the SKI. These distinctive styles of mineralization are spatially coherent, i.e., the boundaries between them are linear (Fig. 4.5), and Peterson (2001b, c) informally termed them open and confined, which are described in more detail below. "Open" - vertically extensive (can be > 450 meters) mineralization with low - high Cu-Ni grade and low Au+PGE grades. Cu-Ni grades commonly increase towards the basal contact although mineralized zones are typically erratic in their spatial extent and grade. Restricted zones of massive sulfide locally occur at, and/or immediately below, the basal contact. The erratic pattern of mineralization in part mirrors the lithologic heterogeneity of the basal units and may reflect repeated input of small pulses of barren and sulfur-contaminated magma. Examples of this "Open" style include the Spruce Road, Serpentine, and Dunka Pit deposits. The Serpentine deposit is unique within this group as it contains significant tonnage of pyrrhotite-rich massive sulfide at the basal contact that is associated with an immediate footwall sulfide source (Zanko et al., 1994). "Confined" - vertically restricted (< 150 meters) mineralization with moderate - high Cu-Ni grades and moderate to very high (locally) Au+PGE grades. Cu-Ni grades typically are the highest near the top of the mineralized zone (upper BH into U3) and gradually decrease with depth toward the basal contact. Only limited zones of massive sulfide occurring at, and/or immediately below, the basal contact have been identified. For example, the upper portion of the mineralized zone within the Maturi deposit (which averages ~150 feet thick) commonly exhibits copper values nearing 1.0% ILSG07 91 Trip 4 �that decrease to ~0.25% at the basal contact. The spatial continuity of both the igneous stratigraphy and Cu-Ni-PGE grades of this style of mineralization point toward larger sustained inputs of magma (and/or more turbulent input of magma, thus higher fractionation of base- and preciousmetals into the sulfide fraction) than the "Open" style. Examples of the "Confined" style include the Maturi, Maturi Extension, and the Birch Lake deposits. Figure 4.5. Location map of Open and Confined styles of mineralization in the South Kawishiwi Intrusion. Recently completed (Peterson, 2002a, 2006a, b, c) and ongoing research on the distribution of Cu-NiPGE in the SKI integrates detailed geological mapping, drill hole logging, assay compilation, gradetonnage calculation, recalculation of assay data to 100% sulfide, and 3-D visualization in order to imagine the coupled magmatic dynamics and mineralization history of the area. The most profound difference in the Open and Confined styles of Cu-Ni-(PGE) mineralization is perhaps best revealed in plots of assay data that have been recalculated to 100% sulfide compositions (Kerr, 2001, Naldrett et al., 2000). Such plots give one clues to the complicated history of immiscible sulfide droplets within SKI magmas, and could be used to calculate R-factors (herein the mass transfer of chalcophile elements between immiscible sulfide and silicate liquids in magmas) of the mineralization systems. As well, discrimination of different styles of mineralization and their internal metal budgets lets one come to grips with item 3 of the previously described Ni-Cu-PGE ore deposit model, that states, “An appropriate physical environment is required so that the sulfide liquid mixes with enough magma to become adequately enriched in chalcophile metals”. Geochemical plots of drill core assay data recalculated to 100% sulfide for the Open and Confined styles of mineralization are presented in Figure 4.6, and reveal profound differences in the metal budgets of the sulfide mineralization. Any genetic ore deposit model and/or mineral exploration model used to search for additional mineralization in the SKI must attempt to explain these differences in context with the overall NLM-SKI magmatic system, i.e. its timing, geometry, and magmatic plumbing. ILSG07 92 Trip 4 �Figure 4.6. Geochemical plots utilizing recalculation of assay data to 100% sulfide (Kerr, 2001) to discriminate variations of metals in sulfide minerals in the two major styles of basal contact associated mineralization in the SKI. A) Open-style PGE+Au vs. Cu plot, B) Open Style Cu/Pd ratio vs. Pd plot, C) Confined-style PGE+Au vs. Cu plot, D) Confined-style Cu/Pd ratio vs. Pd plot. Recent research in developing Cu and Ni grade maps (Peterson, 2002d) for deposits in the SKI, coupled with the realization that there are two distinct styles of basal-contact associated mineralization, has lead to the publication of inferred grade-tonnage estimates of Open and Confined styles of mineralization (Peterson, 2002c). Grade and tonnage data categorized into the Open and Confined styles of Cu-Ni mineralization within the South Kawishiwi intrusion are given in Table 4.1, which used all publicly available assay data in the year 2001. These mineral resource estimates have been calculated by the senior author (Peterson) following as much as possible the definitions and guidelines adopted by the Canadian Institute of Mining, Metallurgy, Petroleum (CIM "Standards on Mineral Resources and Reserves") in August 2000 (Postle et al.). Due to the fact that they rely on historic data from a variety of different sources, the level of data verification expected by the guidelines was not possible in all cases. Inherent uncertainties in the estimation and accuracy of these mineral resource estimates are a function of the quantity and quality of the available drill hole assay data and the quality of the methods used to determine them, which for the Spruce Road deposit are outlined in Peterson (2002d). It is hoped that the data presented so far in this field guide, once integrated together in one’s mind, leads to the conclusion that understanding the geologic history of the NLM may lead to profound advances in our understanding of Cu-Ni-PGE mineralization in the SKI. Such understanding may ultimately lead to the discovery of the “Missing Nickel” in the Duluth Complex Cu-Ni-PGE deposits, thus redefining the district as the Duluth Complex Ni-Cu-PGE deposits. ILSG07 93 Trip 4 �Table 4.1. Cumulative inferred mineral resource estimates for the Open and Confined styles of mineralization in the South Kawishiwi intrusion. Open style modeled to 1500 Ft. (457m.) and Confined style modeled to 500 Ft. (152m.) above base. Copper Cutoff 1.05 1.00 0.95 0.90 0.85 0.80 0.75 0.70 0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.01 Open Style Mineralization Cu % 1.043 1.029 0.969 0.933 0.924 0.844 0.818 0.758 0.685 0.644 0.597 0.547 0.502 0.461 0.421 0.374 0.329 0.286 0.242 0.201 0.156 0.107 Ni % 0.559 0.415 0.388 0.324 0.327 0.313 0.304 0.290 0.246 0.227 0.210 0.194 0.184 0.171 0.158 0.143 0.129 0.114 0.099 0.083 0.065 0.045 Tonnes 114,000 229,000 800,000 1,715,000 1,943,000 4,572,000 6,058,000 11,773,000 33,262,000 61,608,000 122,874,000 245,862,000 448,519,000 747,645,000 1,154,558,000 1,862,655,000 2,894,340,000 4,343,339,000 6,371,617,000 8,950,828,000 12,606,986,000 19,087,646,000 Confined Style Mineralization Cu % 1.035 1.010 0.989 0.962 0.936 0.893 0.852 0.802 0.748 0.693 0.641 0.590 0.553 0.517 0.482 0.446 0.408 0.369 0.329 0.283 0.219 0.131 Ni % 0.233 0.222 0.227 0.233 0.238 0.244 0.243 0.239 0.232 0.222 0.211 0.197 0.187 0.176 0.165 0.153 0.141 0.128 0.115 0.099 0.078 0.047 Tonnes 3,200,000 24,232,000 37,034,000 52,579,000 69,038,000 100,585,000 139,905,000 206,200,000 316,386,000 505,669,000 785,936,000 1,230,797,000 1,667,886,000 2,181,785,000 2,754,206,000 3,428,584,000 4,254,298,000 5,230,432,000 6,406,364,000 7,998,811,000 10,943,215,000 18,987,747,000 Note: 1) Cutoff grade intervals calculated from summation of modeled data (see Peterson, 2002d). For example, the 0.55 Cu cutoff includes all gridded data that falls between 0.575 and 0.525 wt. % Cu. 2) Cu%, Ni%, and tonnes represent cumulative addition of data from the Cu cutoff value and all data of higher grade. 3) Low Cu cutoff data dominantly represents barren rock within the mineralized zone of the Open style and barren rock above the mineralized zone within the Confined style. 4) Tonnes rounded to the nearest 1,000. 5) All intervals not assayed were assigned Cu and Ni values of 0.00 wt% and integrated into the model. NICKEL LAKE MACRODIKE Detailed geological mapping at a scale of 1:5,000 by the authors and Chris White, a Masters Candidate in the Department of Geological Sciences at the University of Minnesota Duluth, was completed in the late summer and fall of 2006, and published at a scale of 1:10,000 by the NRRI (Peterson et al., 2006). This map, available online at http://www.nrri.umn.edu/egg/REPORTS/MAP200604/MAP200604.html is the foundation upon which the field component of this trip will be based. During the course of this mapping, approximately 1,000 outcrops along nearly 100 kilometers of field traverses were examined to identify and confirm the internal lithologic variability, contact relationships, and structure of the western extent of the NLM, the adjacent SKI, and bounding rocks of the Anorthositic series. The authors wish to acknowledge Dr. Paul Weiblen (emeritus professor of geology at the University of Minnesota) for his keen insight on the geology of the area and Dr. George Hudak and undergraduate student Jeremiah ILSG07 94 Trip 4 �Gowey of the University of Wisconsin Oshkosh for assistance in mapping outcrops around and south of Omaday Lake. As well, a one day field excursion to Nickel Lake with Dave Peck (Anglo American), Harry Noyes (Encampment Resources), and Theodore DeMatties (consulting geologist) prior to the mapping campaign developed new insight on identifying dynamic magmatic systems in the field to the senior author (Peterson). Additional reconnaissance mapping in early November by Dean Peterson was completed to field check compiled outcrop locations depicted on the 1957 INCO map of the Spruce Road Deposit and the 1968 Hanna Mining map of the South Filson Creek deposit (both of which are publicly available in the DNR archive at Hibbing, Minnesota). The reconnaissance mapping confirmed the location of gossanous Cu-Ni bearing INCO outcrops and reconfirmed the outstanding field mapping of all types of Duluth Complex rocks by Hanna Mining Company geologists of the late 1960s. The NLM is a northwest to southwest-trending (Fig. 4.3), steeply dipping, asymmetric troctolitic and gabbroic intrusion interpreted to be a feeder dike for the northern portions of the SKI. The macrodike is interpreted to be located within a major rift-parallel normal fault (down to the southeast) now obscured by intrusion of NLM igneous rocks. Regional southward tilting (based on the deep level of erosion of the northern Bald Eagle Intrusion directly east of this area) leads to the interpretation that the southwest end of the NLM (near Omaday Lake) is structurally higher than the northeastern portion of the dike, and represents the location where magma flow changed from dike-like to sill-like, as it exited the dike – thus the magma velocity slowed – and entered the growing SKI magma chamber. Excellent potential exists for Ni-Cu rich massive sulfide at the basal contact where the dike enters the SKI (Section 31, T62N, R10W). The dike is composed of three main units: 1) inclusion-rich, locally sulfide-bearing, heterogeneous troctolite (unit N-Th); 2) layered troctolite, melatroctolite, and dunite (unit N-Tl); and 3) a late, crosscutting, coarse-grained to pegmatitic oxide-rich, olivine-gabbro to melagabbro (unit N-xG). Description of NLM Map Units The basis for all of the field aspects of this field trip is the aforementioned recently published bedrock geologic map of the NLM (Peterson et al., 2006). As well, the description of “Field Trip Stops” to follow differ from most geology field trip guidebooks in that we are simply going to take some walks in the bush, mostly along logging roads and snowmobile trails, look at numerous outcrops of the NML and adjacent rocks, and discuss the geology. We are NOT GOING TO SPECIFIC OUTCROPS to try to make a case for our interpretations; instead we urge you to use your imagination while we look at outcrops and add to the conversation. Instead of writing detailed descriptions of specific outcrops we’ll visit during the “stops” in the field trip, the authors have instead decided to simply copy verbatim the Description of Map Units from the map NRRI/MAP-2006-04 (Peterson et al., 2006) below, and use this as a reference during the field trip. The rocks of the NLM include, generally from youngest to oldest: Oxide Gabbro (N-xG) - Dark-grey, coarse-grained to pegmatitic, recessive weathered, oxide-rich (magnetite and ilmenite), olivine-gabbro to melagabbro. Contains small inclusions of anorthosite (unit N-Ai), basalt (unit N-Bi), and troctolite (unit N-Th). Interpreted to be the youngest phase of the dike based on inclusion types and cross-cutting relationships. Layered Troctolite to Dunite (N-Tl) - Grey to black, medium-grained, well-layered troctolite, melatroctolite, and dunite. Lamination of plagioclase and olivine parallel to modal layering is commonly observed as well as igneous scours and crossbedding. Small inclusions of anorthosite (unit N-Ai) and basalt (unit N-Bi) rare. Layering possibly developed as the up-welling magma streams through the dynamic (expanding) feeder dike. A constant temperature appropriate to plagioclase-olivine crystallization is maintained by a balance between the heat content of the incoming magma plus the heat of crystallization and the heat loss through the chamber walls. ILSG07 95 Trip 4 �Plagioclase and olivine are left behind and oriented/segregated on the walls of the expanding chamber (Weiblen, pers. comm.). Sulfide noted locally in outcrop in the SE corner of Section 31. Heterogeneous Troctolite (N-Th) - Light to dark grey, medium- to coarse-grained, inclusion-rich, heterogeneous troctolitic rocks with local igneous scour structures. Unit composed of intermixed troctolite, anorthositic-troctolite, melatroctolite, and gabbroic phases surrounding numerous local country rock (unit N-Ai) and exotic (units N-IF and N-Bi) inclusions that are elongate parallel to the macrodike, as well as hosts the sulfide-bearing unit N-Ts. Interpreted to be the initial highly dynamic magmatic phase of the dike that carried exotic inclusions from deep in the crust to their present level. Sulfide-Bearing Troctolite (N-Ts) - Rusty weathered, medium- to coarse-grained, sulfide-bearing, heterogeneous troctolitic and gabbroic rocks. Generally forms recessive weathering, Fe-stained, gossanous outcrops near, but not at, the northern margin of the macrodike. The current extent of this unit on the map is confined to those areas with outcrop, which in reality may be much more extensive as these outcrops generally end along linear swampy areas. Anorthosite Inclusion (N-Ai) - Light-grey, medium- to coarse-grained troctolitic-anorthosite, commonly with 1-2 cm poikilitic olivine pits. The large anorthosite inclusions at the southwest end of the dike (around Omaday Lake) are interpreted to represent a "logjam" of blocks that quit moving due to the decreased speed of the macrodike magmas as they entered the South Kawishiwi Intrusion magma chamber. Includes blocks within the SKI adjacent to the Nickel Lake macrodike. Basaltic Hornfels Inclusion (N-Bi) - Grey, fine-grained, steeply-dipping to vertical, granoblastic, locally magnetic, massive to amygdaloidal basaltic hornfels. Includes a highly magnetic block within the SKI adjacent to the Nickel Lake macrodike. Biwabik Iron Formation Inclusion (N-IF) - Well-bedded, steeply-dipping, recrystallized (layered magnetite and pyroxenite) iron-formation commonly with disseminated Cu-Ni sulfides. Forms an intense localized positive magnetic anomaly. ------------------------------------------------------------------------ Anorthositic Series - Subsuite of the Duluth Complex composed predominantly of plagioclase cumulates displaying complex internal structure and lacking obvious signs of in situ differentiation. Occurs throughout the Duluth Complex as anorthosite, troctolitic-anorthosite, and gabbroic-anorthosite, commonly poikilitic. Anorthositic Rocks Undivided (A-tA) - Mixed group of anorthositic cumulates occurring as large sill-like masses and as inclusions within troctolitic cumulates. Common rock types include troctoliticanorthosite, leucotroctolite, anorthosite, and olivine-bearing gabbroic-anorthosite. Olivine ranges from 2 to 15 percent in mode and from granular to poikilitic in texture, with oikocrysts ranging from 1 to 3 centimeters in diameter. Plagioclase mode ranges from 75 to 95 percent and varies from being non-foliated to well-foliated. Inclusions range in size from a few centimeters to elongate bodies hundreds of meters long that are parallel to foliation in the enclosing troctolite. DAY 1, FIELD TRIP STOPS Traverse #1 The location of the first field trip traverse (#1) is given in Figure 4.7. This walk in the bush begins within numerous outcrops of the bounding Anorthositic series rocks (map unit A-tA) on the northern margin of the NLM. The trail will take us southwest into inclusion-rich heterogeneous troctolitic rocks (map unit N-Th) and into southwestern most zone of known Cu-Ni-(PGE) mineralization in the NLM (map unit NTs), which in this location is associated with a large, sub-vertical inclusion of Biwabik Iron Formation (map unit N-IF). One must try to imagine from where such an inclusion came from (see Figure 4.4), how ILSG07 96 Trip 4 �it relates to the model that the NML is a feeder dike to the SKI, why it and other large mapped inclusions in the NML (Fig. 4.7) are concentrated at the southwestern end of the NML (magma velocity). Think back to the previously described fundamental tenet of Ni-Cu-PGE magmatic sulfide deposits, “… realization that decreases in the flow rate of magmas, principally due to the geometry of the conduit, is a major factor in settling entrained sulfide droplets and forming sulfide-rich ore bodies…”. Figure 4.7. Bedrock geology map of portions of the southwestern end of the Nickel Lake Macrodike in Sections 29 and 30, T62N, R10W. Dark lines are superimposed locations of field trip traverse #1, and traverse #2 along logging roads and snowmobile trails. Dashed lines represent short traverses through the bush to additional known outcrops. Traverse #2 The location of the second field trip traverse (#2) is presented in Figure 4.7. This walk to the southeast along a logging road/snowmobile trail begins in coarse-grained, oxide-rich olivine gabbro to melagabbro of map unit N-xG. Contact relationships between the N-xG and layered troctolitic to dunitic rocks (map unit N-Tl) can be complex, and we’ll investigate these relationships early on the traverse. The bulk of this traverse will be through the widest section (>650 m) of unit N-Tl that has been mapped in the NLM. Careful attention should be directed towards igneous textures in the N-Tl, and what they imply to magmatic processes (expanding dike with time, modal layering, pasting plagioclase and olivine phenocrysts on dike walls, solidification fronts, etc…). Optional traverses shown include a walk to ILSG07 97 Trip 4 �outcrops of the very large (~ 1.5 km long) basalt inclusion (map unit N-Bi), and to exceptional exposures of layered troctolite and dunite around a small beaver pond. Traverse #3 The location of the third field trip traverse (#3) is presented in Figure 4.8. This walk to the northnortheast of Nickel Lake will traverse through most of the main units identified in the NLM, including map units N-Th, N-xG, N-Tl, N-Ts, N-Bi, and N-Ai. An important walk through the bush (optional traverse on Figure 4.8) will visit several Cu-Ni-(PGE) mineralized outcrops near the northwestern margin of the NLM, and allow us to view some spectacular exposures, around the margin of a drained beaver pond, of the Anorthositic series rocks immediately northwest of the NLM. The authors cannot speak to strongly on the importance of finding these types of exposures (totally free of lichen, moss, trees, etc…), early in a mapping program, as they provide proxies for subsequent mapping of outcrops deep in the bush, where trees, shrubs, shade, forest litter, dirt, and black flies partially obscure exposures and/or ones willingness to observe them. Figure 4.8. Bedrock geology map of portions of the Nickel Lake Macrodike in the vicinity of Nickel Lake, Section 29, T62N, R10W. Dark line in the center of the image is the superimposed location of field trip traverse #3 along a logging road. Dashed line in the northwest quadrant represents a short traverse through the bush into the Cu-Ni-PGE mineralized northern zone of the NML. Note that beaver dams along the main traverse may cause very wet and/or impassable conditions. ILSG07 98 Trip 4 �Field relationships that offer evidence that a dike-like mafic to ultramafic intrusion is a conduit through which magma ascended upwards in the Earth’s crust all lead back to the fundamental tenets of the Ni-CuPGE ore deposit model. Such relationships provide evidence that the rocks formed in a dynamic, sulfidebearing magmatic system (once again, think like a person fly fishing a trout stream) that include: 1) early phases should be inclusion-rich (some of which should be from a deeper crustal level) and form as the igneous conduit breeches upwards into the Earth’s crust; 2) imbrication and/or elongation of entrained country rock inclusions parallel to igneous foliation; 3) igneous scour structures; 4) prominent steeplydipping igneous foliation and localized disruption due to magmatic injection; 5) cross-bedding of modal layering; 6) evidence of sulfide mineralization; and 7) evidence that that magma velocity varies. One of the authors’ goals of the field trip is to expand this list based on conversations on the outcrop with the participants. Outcrop photographs given in Figure 4.9 show a few of these lines of evidence for the dynamic nature of the NLM. Figure 4.9. Photographs of selected outcrops that give us clues to the dynamic nature of the magmatic processes which ultimately led to the formation of the NLM, and by interpretation, the northern portion of the SKI and its associated Cu-Ni-PGE mineralization. A) Scour structure in map unit N-Th. B) Disrupted igneous foliation along the southern margin of map unit N-Tl. C) Cross-bedding of troctolitic rocks in map unit NTl. D) Cu-Ni sulfide mineralization in map unit N-Ts. ILSG07 99 Trip 4 �DAY 2, 3-D VISUALIZATION AND DRILL CORE DISPLAYS Day two of this field trip will be spent at Duluth Metals Limited’s field office and drill core logging facility in Ely, Minnesota. The day will be split up into two principal activities: 1) 3-D visualization of subsurface geological features of the Nickel Lake macrodike and South Kawishiwi intrusion utilizing the computer program gOcad (geologic object computer aided design) and a Geowall (bring your camera because we’ll all be wearing those funny looking 3D glasses), and 2) examination of selected core intervals from a number of holes drilled by Duluth Metals Limited over the last year. 3-D GEOLOGICAL MODELING AND VISUALIZATION The science of geology uses a variety of tools to study the earth. However, the basis for every type of geologic study is fundamentally rooted in observations made of rocks in their natural habitat – “in the field”. Geologists that do not have an intimate appreciation of the power and fundamental nature of field geology cannot, in turn, appreciate coherent and compelling field-based scientific arguments from which all other geologic interpretations grow. This basic tenet may never be truer than for geologists engaged in mineral exploration. The advance in computer technology over the last twenty years has revolutionized all aspects of our lives. One such advance in geology has been the development of sophisticated 3-D geological software that, if used correctly, i.e., created and maintained by geologists who understand the rocks, can be an outstanding tool for letting geologists interpret data (field observations, drill hole data, etc...) into the subsurface where direct observation is impossible. The first author of this guidebook (Peterson) integrates many types of geological data into the computer program gOcad®, which is perhaps the most sophisticated 3-D geological modeling software available (see websites http://www.gocad.org/www/ and http://www.earthdecision.com/). 3-D models of active mines and/or advanced exploration projects benefit from continuous validation and upgrading of the underlying database, as well as the production of regional geological syntheses, integrating new geological, geophysical, geochemical, geotechnical, and geohydrological models into a single platform. The natural outgrowth of 3-D geological models is their extrapolation away from areas with large amounts of data to more remote areas with less and/or no data, and can assist in the definition of new exploration targets. For geologic teams working in a mineral exploration setting, the main advantages of using integrated 3-D geological models are to (from Fallara et al., 2006): 1) Share the information a. Conveys the data and their interpretation in an immersive format b. Avoids the loss of knowledge and interpretation which may be, i. Distributed within various locations within the company ii. Filed in disorganized ways iii. Never filed and existing only in a geologists memory 2) Be a catalyst in the development of geologic knowledge a. Easily integrates data in a common format b. Preserves data that is easily shared, seen, and analyzed 3) Hastens problem solving throughout the company a. Adaptable to team-driven resolution b. Easy access to geologists and management c. Allows for the shared comprehension of the data ILSG07 100 Trip 4 �4) Accelerates the process of data integration and interpretation a. Define potential exploration targets with reduced uncertainty b. Focuses work on interpretation c. Optimizes data subsets 5) Direct access to data and manipulation within the gOcad® software a. Integrated 3-D querying within the geological model b. Saves time and money, rapid validation of data, and uncertainty resolution A 3-D display of the first author’s gOcad modeling of geologic data from the Duluth Complex in general, and the SKI, NLM, and BEI in particular, will be presented in Duluth Metals Limited’s drill core logging facility at the start of Day 2 of this field trip. A simple screen dump out of gOcad of some of this data is presented in Figure 4.10. Figure 4.10. Print screen image of the SKI and NLM gOcad model (Peterson, unpublished data). DULUTH METALS LIMITED DRILL CORE DISPLAYS Recent drilling (Fig. 4.11) and core logging by Duluth Metals Limited east of the Maturi deposit in 2006 and 2007 has confirmed numerous stratigraphic units described by Severson (1994), especially the sulfide-bearing PEG, U3, BH, BAN, and upper GRB units. ILSG07 101 Trip 4 �Figure 11. Property map of Duluth Metals Limited. Included are historic (black) and recent (red) drill holes. The numerous correlative units of the SKI from historic and recent drill holes include the following: Main AGT – Thick zone of homogenous augite troctolite to olivine gabbro. AT&T – Thick zone of homogeneous medium-grained anorthositic troctolite with troctolites. AT(T) – Homogeneous anorthositic troctolite grading to troctolite and lesser amounts augite troctolite. T-AGT – Homogeneous medium-grained troctolite and augite troctolite. Pic – Thin horizon of medium-grained picrite that is rarely developed, and thus usually uncorrelative. Upper Gabbro – Heterogeneous, medium- to coarse-grained, oxide-rich gabbro and olivine gabbro. AN Group – Combination of coarse-grained homogenous anorthosite, troctolitic anorthosite, anorthositic troctolite, and medium-grained homogeneous gabbro, and olivine gabbro of the Anorthositic series of the Duluth Complex. Basalt Inclusion – Fine-grained, typically magnetic, basalt hornfels with sharp external contacts and locally preserved stretched amygdules. U2 – Ultramafic horizon of locally serpentinized, medium-grained picrite and troctolite with sharp contacts. PEG – Heterogeneous pegmatoidal and pegmatitic troctolite and anorthositic troctolite with minor amounts of augite troctolite, olivine gabbro, and troctolitic anorthosite. The PEG unit typically is weakly sulfide-bearing with sulfide-barren rocks above. ILSG07 102 Trip 4 �U3 – Ultramafic zone of locally serpentinized, sulfide-bearing, medium-grained dunite, picrite, and troctolite with sharp contacts. This unit is the PGE-dominate horizon at the Birch Lake deposit. BH – Basal heterogeneous zone that is the main sulfide-bearing unit of the SKI. Consists of fine- to medium- to coarse-grained and pegmatitic troctolite and to a lesser extent anorthositic troctolite and augite troctolite. BAN – Basal augite troctolite and norite which is the contaminated zone between the BH and underlying GRB. Consists of fine- to medium-grained, sulfide-bearing augite troctolite and norite. GRB – A variety of rocks from the footwall Archean Giants Range Batholith. Rock types include medium- to coarse-grained hornblendite, diorite, and porphyritic or nonporphyritic monzonite to quartz monzodiorite. Sulfide mineralization is usually restricted to the upper 20 feet of the GRB, but can exist in small concentrations over a hundred feet into the granitic footwall. Simplified drill hole logs portrayed as stratigraphic columns from two holes recently drilled in the eastern (MEX-07) and western (MEX-08) portions of the Maturi Extension deposit are presented in Figure 4.12. In addition, reported Cu-Ni-PGE grades from Duluth Metals Limited for these holes are given in Table 4.2. The stratigraphy observed in MEX-07 is typical of the eastern exploration area and the deep holes in this region (>3,000 feet) that are situated below a very large pillar/xenolith of Anorthosite series rocks (see Fig. 4.3). Channelized magma flow out of the NLM and under this Anorthosite block (think of eddies due to overlying friction as a fisherman would) has been interpreted by Peterson (2001b, c) as one of the driving mechanisms for enhanced turbulent flow of sulfide-bearing magmas, thus forming one of the elements of the Ni-Cu-PGE ore deposit model, “an appropriate physical environment is required so that the sulfide liquid mixes with enough magma to become adequately enriched in chalcophile metals”. The stratigraphy observed in MEX-08 correlates with many of the drill holes in the western exploration area (as well as Franconia Minerals’, Maturi deposit) immediately to the west of the Anorthosite series pillar/xenolith. Mineralization in both of these holes consist of chalcopyrite-dominated disseminated sulfides (typically 2-5% total sulfide) restricted to a confined zone directly above the footwall and contain moderate to high Cu, Ni, and PGE concentrations. If one would very simply (unlike the data depicted in Figure 4.6, which used the strict rules of Kerr, 2001) recalculate these reported grades (Table 4.2) to 100% sulfide, i.e. as a specific gravity concentrator would do as a first step at an active mine, then one would have to multiply the assay data by a factor between 20 to 50 (5% to 2% total sulfide minerals in the assayed drill hole intervals) to imagine the true reality that is the enrichment of Cu-Ni-PGE in the sulfide minerals of the Maturi Extension deposit. Such recalculations lead to grades (at 100% sulfide) of ~33% Cu, ~9% Ni, and ~11 – >82 g/t PGE+Au for the sulfide fraction of the Maturi Extension deposit, which point to these preliminary conclusions: 1) The extensive sulfide mineralization at the Maturi Extension deposit (as well as the Maturi, and Birch Lake deposits, i.e., the Confined mineralization of Peterson 2001, 2002a-d) is similar at a recalculated 100% sulfide grade to the metal budget of the fractionated residuum of a MSS; 2) One can imagine such mineralization (the Confined-style of Peterson 2002a) initially forming from incorporation of the Cu-PGE residuum of a fractionating Ni-rich MSS at the junction of the NLM and SKI into magmas streaming out of the NLM and into the SKI. 3) A seemingly robust geologic model of a dynamic magmatic system that incorporates the BEI, NLM, and the SKI exists herein that vector towards an exploration target for a Ni-rich massive sulfide body (the “missing Ni-rich MSS”) at the junction of the NLM and the SKI. ILSG07 103 Trip 4 �Table 2. Reported mineralized intervals for holes MEX-07 and MEX-08 drilled by Duluth Metals Limited on the Maturi Extension property in 2006. MEX-08 Overall including including From (Ft) 2032 2052 2202 To (Ft) 2236.5 2142 2236.5 Total (Ft) 204.5 90 34.5 Cu % 0.69 0.81 0.89 Ni % 0.22 0.24 0.28 Pd (g/t) 0.313 0.365 0.352 Pt (g/t) 0.121 0.147 0.12 Au (g/t) 0.108 0.056 0.096 MEX-07 Overall including including From (Ft) 2543 2608 2628 To (Ft) 2798 2673 2663 Total (Ft) 255 65 35 Cu % 0.429 0.824 0.898 Ni % 0.129 0.275 0.310 Pd (g/t) 0.370 0.906 1.004 Pt (g/t) 0.183 0.444 0.472 Au (g/t) 0.077 0.173 0.177 Figure 4.12. Stratigraphic sections of holes MEX-07 and MEX-08 drilled by Duluth Metals Limited on the Maturi Extension property in 2006. 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Lightfoot, P.C., Naldrett, A.J., Gorbachev, N.S., Fedorenko, V.A., Hawkesworth, C.J., Hergt, J., and Doherty, W., 1994, Chemostratigraphy of Siberian trap lavas, Noril’sk District: Implications for the source of flood basalt magmas and their associated Ni-Cu mineralization, in Lightfoot, P.C., and Naldrett, A.J., eds., Proceedings of the Sudbury–Noril’sk symposium: Ontario Geological Survey, Special Volume 5, p. 283312. Listerud, W.H., and Meineke, D.G., 1977, Mineral resources of a portion of the Duluth Complex and adjacent rocks in St. Louis and Lake Counties, northeastern Minnesota: Hibbing, Minnesota Department of Natural Resources, Division of Minerals, Report 93, 74 p. Lowell, J.D. and Guilbert, J.M., 1970, Lateral and vertical alteration-mineralization zoning in porphyry ore deposits: Economic Geology, v. 65, p. 373-408. Marma, J.C., 2003, Magmatic and hydrothermal PGE mineralization of the Birch Lake Cu-Ni-PGE deposit in the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: Unpublished M.S. thesis, University of Wisconsin: condensed version, University of Minnesota Duluth, Natural Resources Research Institute, Technical Report, NRRI/TR-2003/39, 112 p. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Survey, Miscellaneous Map Series, Map M-119, scale 1:200,000. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey, Report of Investigations 58, 207 p. 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Postle, J., Bernie-Haystead, B., Clow, G., Hora, D., Vallée, M., and Jensen, M., 2000, CIM standards on mineral resources and reserves, definitions and guidelines: Canadian Institute of Mining, Metallurgy and Petroleum, 26 p. Ripley, E.M., 1986, Origin and concentration mechanisms of copper and nickel in Duluth Complex sulfide zones—a dilemma: Economic Geology, v. 81, no. 4, p. 974-978. Severson, M.J., 1994, Igneous stratigraphy of the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-93/34, 210 p., 15 pls. Severson, M.J., and Hauck, S.A., 1997, Igneous stratigraphy and mineralization in the basal portion of the Partridge River intrusion, Duluth Complex, Allen quadrangle, Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-97/19, 102 p., 4 pls. Severson, M.J., and Hauck, S.A., 2003, Platinum-group elements (PGEs) and platinum-group minerals (PGMs) in the Duluth Complex: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report, NRRI/TR-2003/37, 296 p., 1 CD. Thériault, R.D., Barnes, S.J., and Severson, M.J., 2000, Origin of Cu-Ni-PGE sulfide mineralization in the Partridge River intrusion, Duluth Complex, Minnesota: Economic Geology, v. 95, p. 929-944. Vislova, T., 2003, Petrology of the Bald Eagle intrusion and associated rocks and its relevance to crystallization in dynamic magma chambers in the Midcontinent Rift: Unpublished Ph.D. Thesis, University of Minnesota. Weiblen, P. W., Morey, G. B., 1980, A summary of the stratigraphy, petrology, and structure of the Duluth Complex: American Journal of Science, vol. 280A, Part I, p 88-133. Weiblen, P., Peterson, D.M., and Vislova, T., 2005, Implications of Midcontinent Rift and oceanic ridges analogies and 3-D interpretations of the subsurface structure of the Bald Eagle intrusion in the Duluth Complex and the East Pacific Rise: Institute on Lake Superior Geology, 51st Annual Meeting, Sault Ste Marie, Ontario, v. 51, 3 p. Weiblen, Paul W., 1965, A funnel-shaped, gabbro-troctolite intrusion in the Duluth Complex, Lake County Minnesota: Unpublished Ph.D. Thesis, University of Minnesota, 161 p. Zanko, L.M., Severson, M.J., and Ripley, E.M., 1994, Geology and mineralization of the Serpentine copper-nickel deposit: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-93/52, 90 p., 3 pls. ILSG07 108 Trip 4 �53rd Annual Institute on Lake Superior Geology FIELD TRIP 5 Geologic Highlights of New Mapping in the Upper Southwestern to Northeastern Sequence of the North Shore Volcanic Group and Beaver Bay Complex Terrence J. Boerboom Minnesota Geological Survey University of Minnnesota James D. Miller, Jr. Minnesota Geological Survey & Department of Geological Sciences University of Minnesota Duluth John C. Green Department of Geological Sciences University of Minnesota Duluth View northeast across Pork Bay on the North Shore ILSG07 109 Trip 5 �INTRODUCTION The Minnesota Geological Survey is continuing to map the Keweenawan bedrock geology of 1:24,000scale quadrangles near the North Shore of Lake Superior, in northeastern Minnesota. The first phase of mapping from 1985-1993, partially supported by the USGS COGEOMAP program, focused on the Beaver Bay Complex in the central part of the region (Fig. 5-1; Miller, 1988; Miller and others, 1989, 1993, 1994; Boerboom and Miller, 1994). More recent mapping, supported by the USGS STATEMAP program, has focused on quadrangles that intersect the shoreline, where the bedrock is predominantly volcanic rocks. This mapping phase started at the outskirts of Duluth in 2001, and to date has proceeded to the Lutsen quadrangle, having skipped past the COGEOMAP quadrangles. Ten quadrangle-scale geologic maps have been published to date from this work (Boerboom and others, 2002a, 2002b, 2003a, 2003b; 2006; 2007; Boerboom and Green, 2004; 2005, 2006; Miller and others, 2006), and at least two more are planned (Fig. 5-1). Figure 5-1. North Shore 7.5' quadrangles mapped by the MGS through the USGS-sponsored COGEOMAP and STATEMAP programs. M-numbers refer to MGS Miscellaneous Map Series. A trip similar to this, which covered the Upper Southwest sequence of the North Shore Volcanic Group (NSVG), was conducted for the 2004 Institute on Lake Superior Geology meeting held in Duluth. The trip for this year’s meeting will cross the transition from the uppermost Southwest limb to the uppermost Northeast limb, as well as the basal Schroeder-Lutsen sequence of the NSVG (Fig. 5-2). ILSG07 110 Trip 5 �Figure 5-2. Simplified geology of northeastern Minnesota showing the major sequences of the North Shore Volcanic Group (taken from Fig. 5.2 of Miller and others, 2002). VOLCANIC AND SEDIMENTARY ROCKS OF THE NORTH SHORE VOLCANIC GROUP This overview is a slightly modified excerpt from the MGS Report of Investigations 58, Chapter 5: "Volcanic and sedimentary rocks of the Keweenawan Supergroup in Northeastern Minnesota" by John Green (Miller and others, 2002). Magmatic activity related to the 1.1 Ga Midcontinent Rift produced a more than 10 kilometer thick edifice of lava flows and subvolcanic intrusions that are exposed along Minnesota's north shore of Lake Superior. The lava flows and minor sedimentary rocks are referred to as the North Shore Volcanic ILSG07 111 Trip 5 �Group (Goldich and others, 1961) and the intrusive rocks are variably assigned to the Duluth Complex, the Beaver Bay Complex, and miscellaneous intrusions of the Midcontinent Rift Intrusive Supersuite. Although previous publications have subdivided the North Shore Volcanic Group into informal volcanic suites and distinctive flows (Green, 1972, 1982), the 1:200,000-scale map (M-119) produced by Miller and others (2001) and accompanying report (Miller et al., 2002) represents the first time that the NSVG has been subdivided into coherent lithostratigraphic units on a geologic map. A brief description of the North Shore Volcanic Group and associated interflow sedimentary rocks of the Keweenawan Supergroup is given here. Rock classification, recognition, and textures As a coherent tholeiitic compositional suite, the volcanic rocks of the North Shore Volcanic Group can be described using only a few rock names (Fig. 5-3). The most primitive rocks are olivine tholeiites, which form an iron-enrichment trend with further evolution. They display ophitic textures and pahoehoe surfaces nearly everywhere. Most olivine tholeiites are aphyric, but those that are porphyritic contain dominantly plagioclase phenocrysts, less commonly olivine. Transitional basalts contain somewhat higher alkalies and other incompatible elements than the olivine tholeiites, but generally not enough to classify them as alkalic. Their texture is typically intergranular and fine- to medium-grained. Porphyritic varieties generally contain small phenocrysts of plagioclase, olivine, clinopyroxene, and magnetite. The reversed-polarity Hovland lavas, however, are characterized by transitional basalts (grading to basaltic andesites) that contain abundant, large, tabular, plagioclase phenocrysts. Transitional basalt flow surfaces are generally smooth (pahoehoe), although a few show breccia tops. The basaltic andesites and andesites (greater than 52 percent SiO2) are tholeiitic, rather than calcalkaline; they show iron enrichment and contain only anhydrous ferromagnesian minerals. These rocks are typically fine-grained and intergranular to felty or pilotaxitic, and many contain small phenocrysts of plagioclase, olivine, clinopyroxene, and magnetite. The andesites generally weather to a red-brown color, and have flow-brecciated (aa) tops but not bases. Many flows that contain 50 to 55 percent silica show millimeter-scale oxidation lamination (Green, 1989) parallel to the base. A few highly iron-enriched flows, separable only by chemical analysis, can be called ferroandesites. Figure 5-3. AFM compositional diagram for the lavas of the North Shore Volcanic Group (modified from Green, 1982). The boundary between tholeiitic and calcalkalic rocks is modified from Irvine and Baragar (1971). FeO* = FeO + 0.9Fe2O3. ILSG07 112 Trip 5 �Carmichael (1964) first used the name "icelandite" for rocks intermediate in character between andesites and rhyolites in the Tertiary lavas of eastern Iceland. They might be considered the tholeiitic equivalent of calc-alkaline dacite in orogenic suites. Other examples of these rocks have been described from the Galapagos (McBirney and Williams, 1969) and the Miocene of Nevada–Oregon (Wallace and others, 1980). Very large flows of similar composition in the Etendeka volcanics of Namibia have been referred to as quartz latites by Milner and others (1992). Icelandites in the North Shore Volcanic Group (Green and Fitz, 1993) are characterized chemically by SiO2 contents ranging from 60 to 68 percent, high FeO* (averaging 7 percent), K2O + Na2O values between 6.5 and 9 percent, a potassium/sodium atomic ratio of about 0.9, and an Mg number [Mg/(Mg + Fe) atomic] averaging 0.14. Petrographically, North Shore Volcanic Group icelandites grade continuously from the andesites to somewhat paler colors (brown or tan), but have a similar phenocryst assemblage. Quartz and alkali feldspar are common in the groundmass but never occur as phenocrysts. Flowtop features (crusty to coarsely brecciated) indicate that the icelandites were erupted as lavas. The rhyolites have higher silica and total alkali contents, and lower FeO than the icelandites. They are generally light gray, pink, or red. Several are very thick, extensive, and voluminous (up to several hundred cubic kilometers; Green and Fitz, 1993). Although most rhyolites are porphyritic (phenocrysts of quartz, alkali feldspar ± plagioclase, and altered fayalite ± ferroaugite), some lack quartz and alkali feldspar phenocrysts, and rare flows are aphyric. Groundmass textures are fine-grained holocrystalline, typically with a meshwork of platy quartz paramorphs after primary tridymite, which may show a flow structure. A "snowflake" texture is common, in which poikilitic quartz patches (coalesced ex-tridymite grains) enclose small, dusty alkali-feldspar grains. In quartz-phyric flows, these poikilitic quartz patches are in optical continuity with adjacent quartz phenocrysts (Green, 1990). Outcrop-scale flow structure, including folding, is common near flow tops and bases. In some flows, distinct fiamme, deformed to varying degrees, are recognizable. These imply an explosive eruption that produced a pyroclastic flow, which welded and remobilized to produce a rheoignimbrite. Structure and Lithostratigraphy As stated above, the Midcontinent Rift volcanic rocks and interbedded redbeds in northeastern Minnesota comprise the North Shore Volcanic Group (Goldich and others, 1961; Green, 1972, 1977, 1982; Basaltic Volcanism Study Project, 1981). In general, these rocks form an arcuate stack that is slightly tilted toward the southeast and forms the roof rocks into and under which the Duluth Complex and associated hypabyssal intrusions were emplaced. At the southwest end of the North Shore Volcanic Group near Duluth, the volcanic rocks strike north with a 10º to 20º easterly dip; at the northeast end at Grand Portage, the flows strike east–west with a 10º southerly dip (Fig. 5-2). Thus, traveling northeast along the Lake Superior shore northeast from Duluth, and southwest from Grand Portage, one encounters successively higher flows in the volcanic stratigraphy until the Tofte–Lutsen area in southern Cook County, where the highest exposed flows crop out. Exposure is generally excellent along the eroding lakeshore, and along the lower, high-gradient stretches of tributary streams, providing good control on the stratigraphy. A stack of volcanic rocks approximately 9.7-kilometers-thick has been measured in the “southwest limb” (Table 5-1), and another stack of volcanic rocks about 7.2-kilometers-thick has been measured in the “northeast limb” (Table 5-2, Miller and others, 2002). This implies nearly continuous subsidence during the rifting process. The difference in stratigraphic thickness between the two limbs reflects major complications in the central area, which appears not to have subsided at the same rate as in the limbs, and into which many of the subvolcanic intrusions were emplaced. Except for the capping Schroeder–Lutsen basalt sequence, no stratigraphic unit can be traced from one limb to the other; each limb has its own stratigraphic column. To aid in the correlation of intrusive and extrusive rock units throughout the Midcontinent Rift system, their paleomagnetic polarity has been used. Nearly all of the igneous and sedimentary rocks associated with the Midcontinent Rift were formed either during an earlier, reversed-polarity interval or a succeeding normal-polarity interval. Thus, in each limb of the North Shore Volcanic Group, the lower stratigraphic ILSG07 113 Trip 5 �units show reversed polarity, and the upper sequences show normal polarity. This polarity reversal forms the basis for distinguishing upper and lower sequences in the northeast and southwest limbs. U-Pb zircon dates demonstrate that the reversed-polarity magmatism occurred mainly in the time interval from 1108 to 1107 Ma; whereas, around the Lake Superior basin, normal-polarity magmatism occurred mainly in the interval from 1099 to 1094 Ma (for example Davis and Paces, 1990; Paces and Miller, 1993; Davis and Green, 1997). These two pulses were separated by a magmatically inactive time (at least in the upper crust), which appears to be expressed as a slight unconformity in the volcanic sequence on the north shore. However, because intrusions subsequently penetrated along this horizon in the North Shore Volcanic Group, this unconformity has not been recognized in outcrop. The Duluth Complex separates the upper and lower sequences of the southwestern limb (Fig. 5-2, Table 5-1). One other significant gap in the stratigraphic continuity of the North Shore Volcanic Group occurs near the stratigraphic top, where the Schroeder–Lutsen sequence overlies the upper units of the northeast and southwest sequences (Fig. 5-2, Tables 5-1 and 5-2). In the northeast limb, southwest of Grand Marais (Fig. 5-3), the Schroeder–Lutsen sequence unconformably overlies the Good Harbor Bay lavas, which are now recognized to include the Terrace Point basalt, as well as the Cut Face Creek Sandstone and the Good Harbor Bay andesites. The Terrace Point basalt contains a granite xenolith dated at 1096.7±0.9 Ma and thus must be younger than that. In the southwest limb, the Schroeder–Lutsen sequence overlies andesites and icelandites of the Onion River lavas (Boerboom and Green, 2006; Boerboom and others, 2006) as well as a sandstone and conglomerate unit (the Little Marais conglomerate) that in turn rests in sharp angular unconformity atop structurally disturbed flows of the Belmore Bay lavas near Little Marais. The Onion River lavas form a coherent package of dominantly intermediate-composition lavas that have only recently been mapped in detail, and their stratigraphic thickness is not taken into account in Table 5.1. They are located in approximately the same stratigraphic position as the Belmore Bay lavas, and may be continuous with them, but their relationship has been obscured by emplacement of the Beaver Bay Complex. Furthermore, the gently dipping Schroeder basalts have not been penetrated by the abundant hypabyssal intrusions of the Beaver Bay Complex that complicate the underlying volcanic sequence in this mid-shore area (Green, 1992; Miller and others, 1993). Attempts to date the Schroeder–Lutsen sequence have been unsuccessful to this point. The five major lithostratigraphic sequences comprising the two limbs of the North Shore Volcanic Group are further subdivided into informal lithostratigraphic units (Table 5-1 and 5-2). Some of these units are individual flows of distinctive lithology and/or substantial thickness and lateral extent. Most are suites of lava flows that have distinct lithologic characteristics or that are separated by intrusions. Some lava formations contain distinct flows or sedimentary rock units within an otherwise homogeneous package of lavas. Such units are given informal names (such as the Silver Beaver rhyolite within the Baptism River lavas, Manitou transitional basalt within the Schroeder basalts, and Indian Camp sandstone within the Lutsen basalts). See Chapter 1 of RI58 (Miller and others, 2002) for more details on stratigraphic nomenclature of Keweenwan rock in northeastern Minnesota. ILSG07 114 Trip 5 �Table 5-1. Generalized stratigraphy of the southwest limb of the North Shore Volcanic Group showing U/Pb ages (Davis and Green, 1997; Green and others, 2001). Positions of intrusions denote approximate stratigraphic level affected and not age of emplacement. Thickness(m) Lithostratigraphic units Lithologic character U/Pb age 9735 Total section basalt, andesite, and rhyolite flows 150 Carlton Quarry lavas (fault 1094.3±2.0 bounded) Schroeder–Lutsen sequence (normal polarity) 945 ophitic olivine tholeiite basalt flows; 900 Schroeder basalts <45 Little Marais conglomerate includes Manitou transitional basalt and Pork Bay breccia polymict volcanic conglomerate and sandstone angular unconformity 8275 Upper southwest sequence (normal polarity) 565 Bell Harbor lavas 100 Palisade Head rhyolite mostly quartz tholeiite basalt and basaltic andesite flows gray-pink, porphyritic rhyolite flow Beaver Bay Complex 700 Baptism River lavas 20 Silver Bay porphyritic basalt 730 Gooseberry River basalts 1096.6±1.7 ~1096 mixed lavas, mostly basalt; includes 165-meter-thick Silver Beaver rhyolite ophitic basalt flow with abundant large plagioclase phenocrysts mixed basalt flows, mostly ophitic Lafayette Bluff, Silver Creek diabase intrusions 315 Two Harbors basalts 550 Larsmont basalts mixed aphyric basalt flows; quartz tholeiite flows at base ophitic olivine tholeiite flows Stony Point–Knife Island diabase sheet 1500 Sucker River basalts mixed basalt flows, mostly ophitic 1350 Lakewood lavas mostly basalt flows; rhyolite, icelandite, and ferroandesite at base Lester River diabase sill 1285 mixed basalt, andesite, icelandite, and rhyolite flows Lakeside lavas 1098.4±1.9 Endion diabase sill 1160 Leif Erickson Park lavas ~1099 Duluth Complex 370 mixed basalts, andesites Lower southwest sequence (reversed polarity) 370 Ely's Peak basalts >8 Nopeming Sandstone porphyritic, diabasic, and ophitic basalts; pillowed and pyx-phyric basal flow white to tan quartzite and conglomerate angular unconformity Thomson Formation (Paleoproterozoic) ILSG07 115 Trip 5 �Table 5-2. Generalized stratigraphy of the northeast limb of the North Shore Volcanic Group showing U/Pb ages (Davis and Green, 1997; Green and others, 2001). Positions of intrusions denote approximate stratigraphic level affected and not age of emplacement. Thickness(m) ages 7359 325 Lithostratigraphic units Lithologic character U/Pb Total section Schroeder–Lutsen sequence (normal polarity) olivine tholeiite basalts, mostly Lutsen basalts ophitic; includes Indian Camp sandstone angular unconformity 3998 Upper northeast sequence (normal polarity) 130 Terrace Point basalt (within upper Good Harbor Bay andesites) 100 Cut Face Creek sandstone 131 Good Harbor Bay andesites ophitic, olivine tholeiite basalt younger than 1096.7±0.8 red, laminated, ripple-marked sandstone brown, porphyritic basaltic andesites Beaver Bay Complex – Beaver River diabase and Leveaux ferrodiorite brown, columnar-jointed basalt flow 122 Breakwater basalt 348 Grand Marais felsites pink to gray porphyritic rhyolite and felsite 335 Croftville basalts 250 70 Devil’s Track rhyolite Maple Hill rhyolite intergranular basalt and andesite flows, thick interflow sandstone aphyric, intergranular rhyolite flow porphyritic rhyolite flow 274 Red Cliff basalts 366 539 Kimball Creek rhyolite Marr Island lavas 198 235 900 Naniboujou basalts Devil’s Kettle rhyolite Brule River lavas ophitic olivine tholeiite flows, some plagioclase-phyric porphyritic rheoignimbrite mixed basalt, tholeiitic andesite, and icelandite flows intergranular basalt flows porphyritic lava flow interbedded basalt and rhyolite flows 1097.7±1.7 1100.2±1.9 Brule Lake-Hovland gabbro 3036 BASE 60 Lower northeast sequence (reversed polarity) 1932 Hovland lavas 67 92 945 Red Rock rhyolite Deronda Bay andesite Grand Portage lavas 60 Puckwunge Sandstone mostly plagioclase-phyric basalt flows, some rhyolite and andesite red/tan, porphyritic rhyolite tan/brown, porphyritic andesite basalt lava flows, pillowed at base 1107.7±1.9 1107.9±1.8 tan/white, cross-bedded quartz sandstone slight angular unconformity Rove Formation (Paleoproterozoic) ILSG07 116 Trip 5 �Physical Volcanology The volcanic rocks of the Midcontinent Rift, including the North Shore Volcanic Group, represent one of the world’s oldest and best-preserved examples of plateau lavas. However, they contain a greater thickness of flows and, in the North Shore Volcanic Group, a higher proportion of evolved compositions than typical plateau lavas. They are similar physically and chemically to the Tertiary lavas that make up eastern Iceland (Sigvaldason, 1974; Walker, 1974; Green, 1977; Wood, 1978), and they formed similarly over a plume at another major rift. The rocks also resemble the late Tertiary and Quaternary volcanic rocks of the southern Snake River Plain, Idaho and southeastern Oregon, because of their interbedded basalts and large rhyolites (for example Bonnichsen and Kauffman, 1987; Link and Hackett, 1988; Reidel and Hooper, 1989; Manley, 1996). The basalts range in character from typical flood flows as voluminous as tens of cubic kilometers to more modest, “plains-type” flows (Greeley, 1982) and thin flow units less than a meter thick. At Duluth in the southwest limb, and Grand Portage in the northeast limb, the lowest flows in the volcanic sequences are pillowed, and thus inferred to have erupted subaqueously; however, nearly all of the other flows were erupted subaerially. The flows show different physical characteristics, closely tied to their chemical compositions and viscosities (Green, 1977, 1989; Green and Fitz, 1993). Olivine tholeiites, which dominate the North Shore Volcanic Group, all have pahoehoe surfaces, with or without ropy structures. Other physical characteristics of the various rock types were previously discussed in the “Rock classification, recognition, and textures” section of this chapter. All of the flows ranging in composition from basalts to icelandites were erupted as lavas. The rhyolites are notable in their abundance relative to other plateau-lava sequences, their size (up to several hundred cubic kilometers), and extent (Green and Fitz, 1993). Several rhyolites show textural evidence of rheomorphic flow after eruption as ash-flow tuffs, though some were lavas. One of the largest, the Devil Track rhyolite in Cook County, which is as thick as 250 meters and can be traced for 40 kilometers along strike, has ambiguous features that make its mode of eruption difficult to discern; it may be a lava flow. Nearly all the icelandites and rhyolites show evidence of an unusually high temperature of eruption, such as magmatically crystallized groundmass tridymite. The evidently low viscosity of these large rhyolites is attributed to their high temperature, high iron and fluorine contents, and low oxidation state (Green and Fitz, 1993). Geochemistry and Chemostratigraphy The North Shore Volcanic Group constitutes a subalkalic, tholeiitic suite that ranges continuously from rather primitive olivine tholeiite to rhyolite, and shows a strong iron-enrichment trend (Fig. 5-3; also Basaltic Volcanism Study Project, 1981; Green, 1982; Brannon, 1984). However, relative abundances are strongly bimodal; basalts are greatly predominant, but rhyolites make up 10 to 25 percent of the section. The basalts show trace element and isotopic evidence of derivation mostly from a mantle plume (Nicholson and others, 1997), whereas most of the rhyolites include major contributions from partial melting of the Archean basement (Vervoort and Green, 1997). The most common basalt type, ophitic olivine tholeiite, is generally aluminum-rich (16 to 18 percent Al2O3). The most primitive flows have Mg numbers of about 0.65 to 0.68. The basal few flows in both limbs of the North Shore Volcanic Group have a unique geochemical and petrographic character. Typically they contain augite phenocrysts, are aluminum-poor, and are rich in both compatible (chromium and nickel) and incompatible elements (titanium, phosphorus, and lanthanum) with steep chondrite-normalized La/Yb ratios. This suggests derivation by relatively smallfraction melting of the initial plume head (Nicholson and others, 1991, 1997; Green, 1995). In general, there is little stratigraphic regularity of compositional change within the North Shore Volcanic Group, with the following exceptions. In the middle of the upper southwest sequence, there is a marked upward progression toward more primitive compositions through a 3.4-kilometer section from rhyolite east of the Lester River into a thick group of primitive olivine tholeiites in the Knife River–Two Harbors area (Brannon, 1984). This includes the Lakewood lavas, the Sucker River basalts, and the Larsmont basalts. In contrast, in the lower northeast sequence, the approximately 1-kilometer-thick basal ILSG07 117 Trip 5 �Grand Portage lavas progress upsection from basalt to increasingly evolved compositions, ending with Red Rock rhyolite (Green, 1995). As mentioned above, the Schroeder–Lutsen sequence, the youngest in the North Shore Volcanic Group, is composed almost entirely of olivine tholeiites. All of the North Shore Volcanic Group has been affected to some degree by hydrothermal/burial metamorphism. The more permeable (fractured, vesicular) tops and bases of the flows have undergone considerable mineralogical change (deposition of amygdule minerals, alteration of primary minerals), but in many cases the massive flow interiors are remarkably little-altered. Where alteration has approached equilibrium, mineral assemblages range from lower greenschist facies at the base of the North Shore Volcanic Group to zeolite facies at the top (Schmidt, 1993; Schmidt and Robinson, 1997). Interflow Sedimentary Rocks Clastic redbed strata occur at many horizons within the North Shore Volcanic Group (Jirsa, 1984). They are lenticular and range in thickness from a few centimeters to about 100 meters. As these rocks are relatively soft and erodable compared to the adjacent volcanic flows, they are mostly covered and are exposed only along actively eroding sites such as streambeds and the lakeshore. However several interflow sandstones have recently been recognized from water well cutting samples (Boerboom and others, 2007). Overall, the sedimentary rocks are predominantly red to brown, well sorted sandstone, with minor conglomerate, siltstone, and shale. Conglomerate beds are most abundant in the midshore area from Little Marais to Lutsen, either on or in close proximity to a prominent gravity low that is at a right angle to the North Shore. This gravity low is thought to reflect an uplifted crustal block (White, 1966), and intrusive rocks along this trend are known to contain large xenoliths of Archean crust (Boerboom, 1994). Although the relative timing and effects of uplift on the adjacent volcanic rocks are not fully understood, the association of interflow conglomeratic rocks with this structure implies that uplift may have been active during volcanism. Compositionally, these redbeds are mainly immature lithic arkose and feldspathic lithic arenite (see Fig. 5.3 in Miller and others, 2002). The angular to subrounded clasts are mainly plagioclase, mafic to felsic volcanic rock fragments, clinopyroxene, and Fe-Ti oxides; devitrified or replaced volcanic ash particles and shards are present in a few beds. Quartz is uncommon to absent. The framework grains have been variably cemented with hematite, calcite, prehnite, and a variety of zeolites, depending on the local hydrothermal/burial/contact metamorphic conditions. In some places hydrothermal minerals have replaced many or most of the clasts. A few of these redbed units have thicknesses in excess of 25 meters. These include a cross-bedded sandstone in Leif Erickson Park in Duluth (35 meters), which disconformably overlies an eroded basalt flow; the Little Marais conglomerate (and sandstone) exposed in the Manitou River area near Little Marais (as thick as 45 meters); the Indian Camp sandstone (68 meters) northeast of Lutsen; the Cut Face Creek sandstone southwest of Grand Marais (100 meters); and other newly recognized units to the north of and lower in the stratigraphy than the Cut Face Creek sandstone (See Boerboom – abstract volume for this meeting). The Cut Face Creek sandstone can be traced in outcrop for at least 4 kilometers along strike, and topographic and aeromagnetic data indicate the newly-recognized sandstones to the north extend for at least 10-15 kilometers along strike. Of these, the Little Marais conglomerate occurs at the base of the Schroeder–Lutsen basalt sequence. The Cut Face Creek sandstone underlies the Terrace Point basalt, which is now recognized to occur in the upper part of the Good Harbor Bay sequence (Boerboom and Green, 2007), and the sandstones intersected by well drilling to the north occur between basaltic lava flows tentatively grouped with the Croftville basalts. The sandstone in these units is typically planar- or cross-bedded, and some beds are ripple-marked or mud-cracked. The rocks are inferred to be dominantly fluvial, deposited by moderate-gradient, east- to southwest-flowing streams from sources nearly entirely within the subsiding Midcontinent rift basin (Jirsa, 1984). Many flow-top breccias of andesite and basalt with aa structure contain laminated red sandstone as a matrix because sand filtered down from the flow surface. Similarly, red, ILSG07 118 Trip 5 �laminated sandstone and siltstone form clastic dikes or crevice-fillings a few centimeters wide in the upper parts of some lava flows. GEOLOGY OF THE BEAVER BAY COMPLEX The Beaver Bay Complex (BBC) is a hypabyssal, multiple-intrusive igneous complex that was emplaced into the upper part of the NSVG over a 600-km2 area in northeastern Minnesota (Fig. 5-4). Much of this area was the focus of detailed bedrock mapping by the Minnesota Geological Survey between 1985 and 1994 (Miller, 1988; Miller and others, 1989, 1993, 1994; Boerboom and Miller, 1994). Recent and ongoing mapping in the eastern BBC (Albers, 2006; Boerboom and others, 2006) will better established the geology of this part of the complex. Three general areas of the BBC, southern, northern, and eastern, are distinguished on the basis of distinctive rock types and intrusion form (Miller and Chandler, 1997). The relationship of BBC intrusions to other subvolcanic intrusions within the NSVG (Fig. 5-4) is unclear, because of poor exposure to the southwest and insufficient mapping to the northeast. Within the mapped area of the BBC, thirteen intrusive units have been identified that represent at least six major intrusive events (Miller and Chandler, 1997). Most intrusive activity forming the BBC occurred around 1096 Ma based on U-Pb dates of 1095.8±1.2 Ma for a Silver Bay intrusion, the youngest unit of the BBC, and Figure 5-4: Geology of the southern and northern Beaver Bay Complex (after Miller and others, 2001). Units labels are: nsl - NSVG Schroeder basalts; nsb - NSVG basaltic volcanics; nsf - NSVG felsic volcanics; asa - anorthositic series of the Duluth Complex; wlg - Whitefish Lake granophyre; slid Shoepack Lake inclusion-rich diorite; ccp - Cabin Creek porphyritic diorite; hct - Houghtaling Creek trocolite; blg - Blesner Lake gabbro; llg - Lax Lake gabbro; fg - Finland granophyre and qtz ferromonzonite; slg - Sonju Lake troctolite and gabbro; lvp – Leveaux porphyritic ferrodiorite; brd Beaver River diabase; sbi - Silver Bay intrusions. See map M-119 for more details. Locations of field trip stops are shown by black dots with corresponding stop number in white. ILSG07 119 Trip 5 �1096.1±0.8 Ma for the Sonju Lake intrusion (Paces and Miller, 1993). Whether activity overlapped the main stage of Duluth Complex magmatism at 1099 Ma is unknown, because attempts to date the oldest component of the BBC were not successful (Paces and Miller, 1993). The boundary between the BBC and Duluth Complex is generally marked by a northeast-trending keel-shaped intrusion in the northern BBC (Houghtaling Creek troctolite – hct on Fig. 5-4) that separates largely dike and sill intrusions of the BBC to the southeast from massive granophyric granite and extensive areas of structurally complex gabbroic anorthosite to the northwest that are typical of the roof zone of the Duluth Complex. The range of BBC parent magma compositions is similar to the olivine tholeiite and transitional basalt compositions that dominate the NSVG (Fig. 5-5A). Moreover, like the NSVG, the sequence of intrusion of BBC magmas generally involved progressively more primitive compositions. Compositional variations within the various intrusive units developed as a result of in situ magmatic differentiation (Fig. 5-5B), assimilation of footwall rocks, and/or composite intrusions of evolved magma from deeper staging chambers (Fig. 5-5C). The tightly clustered trend of BBC parent magma compositions evident on an AFM diagram (Fig. 5-5A) and the systematic variation of other elemental abundances suggest that all mafic BBC magmas evolved from a common olivine tholeiitic primary magma type. Such a primary composition, which is approximated by the most primitive, high-Al olivine tholeiites of the NSVG, is thought to have given rise to most MCR magmas, especially in later stages of magmatism (Green, 1983; Figure 5-5: AFM diagrams of Beaver Bay Complex (BBC) intrusions. A) Plot of estimated parental magma compositions to various BBC intrusions (see Miller and Chandler, 1997 for unit descriptions and details) compared to major NSVG lava compositions: OT - olivine tholeiite, TB - transitional basalt, A - andesite, FA - ferroandesite, I - icelandite, R - rhyolite (after Green, 1983). NSVG-pot is a primitive NSVG olivine tholeiite composition. The composition of the Leveaux porphyry is indicated by the label – lp. B) Calculated liquid line of descent of the Sonju Lake intrusion through troctolitic (slt), gabbroic (slg-slmd) and monzodiorite (slmd) intervals of the layered sequence. Also plotted are whole rock compositions of Finland granite (frg) and quartz ferromonzodiorite (frpm). C) Whole rock composition plots of Beaver River diabase (brd; ophitic margins and coarse subophitic interiors distinguished), Silver Bay intrusions (sbi; coarse marginal facies, layered ferrogabbroic cumulate interiors, and granophyric compositions distinguished), and composite intrusions from the northern BBC (nbbc). Sonju Lake intrusion differentiation trend (dashed line) is also shown. ILSG07 120 Trip 5 �Miller and Weiblen, 1990; Klewin and Shirey, 1992). That even the most primitive of the BBC intrusions, the Beaver River diabase, is significantly evolved from a primitive olivine tholeiite composition (Fig. 5.5C) indicates that all BBC parent magmas were generated in turn by magmatic differentiation of such a primary composition in deeper staging chambers. Petrologic modeling of some BBC intrusions and other hypabyssal bodies that intruded the NSVG (Jerde, 1991) suggests that most magmas experienced multistage, polybaric fractionation between their extraction from the mantle and their subvolcanic emplacement. Although the available radiometric ages do not indicate an overlap of magmatic activity between the BBC and the Duluth Complex (Paces and Miller, 1993), additional dates and more detailed petrologic studies may show that some Duluth Complex intrusions acted as the final levels of staging and differentiation of some BBC-bound magmas. The focus of emplacement of BBC intrusions appears to have migrated toward the rift axis and toward higher stratigraphic levels with time, perhaps reflecting plate drift and thickening of the volcanic pile. Over the exposed extent of the BBC, intrusion shapes appear to have been controlled by a shallow crustal ridge which trends northwest across the BBC. The presence of this buried crustal ridge is indicated by a pronounced saddle in the gravity high over northeastern Minnesota (Chandler, 1990) and the presence of Archean granitic gneiss, biotite schist, metagraywacke and granodiorite inclusions in early intrusions over the gravity low (Boerboom, 1994). The broad network of dikes and sheets characterizing the southern BBC becomes tightly focused into a narrow zone of subparallel dikes in the northern BBC which is situated over the gravity minimum (Fig.5-4). The eastern BBC opens up again into thick sheet intrusions. Based on geologic, geochronologic, geophysical, and geochemical evidence, Miller and Chandler (1997) suggested that the BBC, particularly the youngest Beaver River diabase dike and sheet network, acted as a magma conduit and structural boundary to the formation and infilling of the western end of the Portage Lake Volcanic basin during the main to late stages of rift volcanism and graben formation. On the North Shore, these volcanic rocks are represented by the Schroeder-Lutsen basalts (Fig. 5-4). For more detailed information on these and other units of the Beaver Bay Complex see reports by Miller and Chandler (1997) and Miller and others (2002, Chapter 7) and the 1:24,000 bedrock geologic maps of the area (Miller, 1988; Miller and others, 1989, 1993a, 1994; Boerboom and Miller, 1994; Boerboom and others, 2006) This trip will have stops in the Beaver River diabase and the Leveaux porphyritic ferrodiorite. The Beaver River diabase typically forms prominent, lumpy topographic highs due in part to the large, resistant inclusions of anorthosite contained in it (e.g. Carlton Peak, stop 4B). It occurs as a series of interconnected set of dikes and sills composed predominantly of ophitic olivine gabbro. The northern and western limit is a dike that is sharply defined by outcrop and aeromagnetic data, and the area below this arc is a series of large, bifurcating dikes and thick, gently southeast-dipping sheets that are likely erosional remnants of a former semi-continuous sill (Figure 5-4). The upper and lower margins of the larger diabase sheets commonly contain inclusions of nearly pure anorthosite such as at Carlton Peak, and less commonly, large inclusions of granophyric granite. The Leveaux porphyritic diorite (LPD) is a semi-continuous, hypabyssal sill, at least 90 meters thick, that intruded into upper NSVG sequence rocks and was later intruded by the Beaver River diabase (Albers, 2006). The LPD makes up five prominent, cuesta-like ridges that trend parallel to Lake Superior over a 20 km distance and numerous smaller knob-like outcrops that occur as large inclusions within the younger Beaver River diabase. Examples of the Leveaux cuestas are Moose and Eagle Mountains, which can be seen uphill from the Lutsen Resort where this meeting is being held. Recent detailed bedrock geologic mapping by Albers (2006) and Boerboom and others (2006) has subdivided the LPD into four units: the Upper Contact Zone, the Porphyritic Zone, the Aphyric Zone, and the Sparsely Porphyritic Unit. The Upper Contact Zone is a thin (~10m), sparsely porphyritic (~5%), moderately vesicular, and locally amygdaloidal fine-grained ferromonzodiorite that forms the upper part of the LPD sill and is often not exposed due to erosion. The Porphyritic Zone is a thick (40-60m), fine- to ILSG07 121 Trip 5 �medium fine-grained ferromonzodiorite with 30-40% plagioclase megacrysts that composes the midsection of the LPD sill. The Aphyric Zone is a thin (4-19m), fine-grained ferromonzodiorite with rare plagioclase megacrysts (<3%) that occurs at the base of the sill. The Porphyritic-Aphyric Transition (PAT) is a gradational interval that separates the porphyritic and aphyric zones over a thickness of 10-40 cm. The Sparsely Porphyritic Ferrodiorite unit, which contains 5-20 % plagioclase megacrysts, occurs as outliers to the main LPD ridge. and may represent feeder dikes the LPD sill. The orientation of the PAT and sheet joints from the aphyric section indicate a gentle (10-20 degrees) southeast dip. Stops 6 and 9 will examine the porphyritic and upper contact zones of the Leveaux ferrodiorite. FIELD TRIP 5 STOP DESCRIPTIONS The general location of all stops are shown on Figure 5-6. A more detailed location map is included with each stop description, showing a portion of the appropriate 1:24,000 quadrangle map. Also listed are UTM coordinates (nad ’83), township-range-section, and the 7.5’ quadrangle name. Figure 5-6. Simple map showing relative locations of field trip stops, listed below. Stop 1. Kennedy Landing/Bell Harbor – conglomerate below SLB Stop 2 Caribou River - Pork Bay breccia and overlying basalt Stop 3. Sugar Loaf Cove – Schroeder-Lutsen sequence basalt Stop 4. Carlton Peak – Rhyolite and anorthosite Stop 5. Tofte Town Park – Schroeder-Lutsen sequence Stop 6. Springdale Hill – Porphyritic phases of Leveaux diorite Stop 7. Base of Schroeder-Lutsen sequence Stop 8. Onion River – Good Harbor Bay andesites Stop 9. Oberg Mountain -Leveaux porphyritic diorite and shoreline view Stop 10. Conglomerate Stop 11. Cut Face Creek – Terrace Point basalt, sandstone, and Good Harbor Bay andesite ILSG07 122 Trip 5 �---Leaving Lutsen Resort, head southwest on Hwy 61. Proceed approximately 26.5 miles to overpass over prominent creek. We will take the creek bed down to the shore and return to the road by private driveway. This is private property, so permission to access the shore is required. .--- Stop 5-1. Unconformity Conglomerate at Kennedy Landing/Bell Harbor. UTM coordinates (nad ’83) 640454E, 5274226N T.57N., R.6W., Sec.30, Finland quadrangle Highlights: ophitic basalt (Schroeder-type), polymict conglomerate, angular unconformity, felsic lithic tuff, agate-bearing quartz tholeiite flow, normal faults, interflow basalt clast conglomerates. U, 0 t z p.-, Stop 1 Description: From the cliff face at the west end of this bay, several volcanic and structural features are visible as noted in Figure 5-7. Exposed in the cliff face is an ophitic basalt which is the basal flow of the Schroeder-Lutsen sequence (OB). Underlying this basalt is a polymict volcanogenic conglomerate (PCg). Various types of basalt dominate this conglomerate, but it also contains clasts of felsite, granophyre, lithic tuff, and autolithic . Figure 5-7. Plan view of the geology exposed at Stop 5-1. Unit abbreviations are: OB – ophitic basalt, PCg – polymict conglomerate, B1-3 – basalt flows, FLT – felsic lithic tuff, BC1-2 – basalt clast conglomerate. Inset show fault relationships between OB and PCg units in western cove. ILSG07 123 Trip 5 �shale fragments. In the back wall of the cove between the ophitic basalt ledge and the point of conglomerate, two splayed faults are exposed (Fig. 5-7, inset). On the northeast side of the point of polymict conglomerate is in sharp angular contact with a thin sheet of basaltic andesite (B1) and and underlying a felsic lithic tuff (FLT) with imbricated clasts of porphyritic rhyolite. The contact between the polymict conglomerate and the basalt/tuff appears to be an unconformable erosional surface, but a fault contact cannot be discounted. . Continuing northeast, this basalt/lithic tuff sequence also abruptly give way across a small gap (fault?) to a basalt clast conglomerate (BC1). Near the outlet of a stream, the BC1 conglomerate grades downsection to a basalt breccia flow top of quartz tholeiite basalt flow (B2). This flow is colored a deep army green and contains abundant vugs and amygdules of agate and amethyst. Northeast of the point formed by flow B2, another basalt clast conglomerate (BC2) caps the upper part of the next underlying lava flow (B3). Discussion: The polymict conglomerate, which is termed the Little Marais conglomerate (Miller et al., 1993; Miller et al., 2006), is interpreted to occupy an angular unconformity that separates the SchroederLutsen sequence from the Upper Southwestern sequence of the NSVG. It can be traced at least 10 km to the northeast into the Little Marais quadrangle. Farther to the east, this unconformity develops a conformable relationship with the underlying volcanics and is marked by a thick sandstone unit (Cutface Creek Sandstone) (see Stop 5-11). ---Return northeast on Highway 61. Approximately 7.5 miles after crossing the Caribou River, turn left into parking area. Take the Superior Hiking trail path upstream about 300m to low-lying outcrops on the rivers edge .--- Stop 5-2. Pork Bay Breccia and Manitou River basalt. UTM coordinates (nad ’83) Area A-648250E, 5258850N; Area B-648480E, 5258950N T.58N., R.6W., Sec. 36, Little Marais quadrangle Highlights: Volcaniclastic breccia (Pork Bay breccia), thin olivine tholeiite flows, transitional basalt (Manitou River basalt) A Description: Location A- Exposed in low-lying outcrops on the south bank of the river is a brick red, polymict, matrix-supported, volcaniclastic breccia – the Pork Bay breccia. It is composed of subangular to subrounded volcanogenic clasts up to 40 cm across in an unbedded, sandy matrix (Fig. 5-8A). Clasts are composed dominantly of massive to scoriaceous and oxidized to fresh basalt and minor amounts (<5%) of other riftrelated rock types (e.g., gabbro, felsite, volcanic sandstone). The matrix consists of granule to medium sand-sized volcanic fragments cemented by calcite and zeolite. B Stop 2 Location B – Progressing upstream about 200 m to where the river make a sharp turn to the left (north), units overlying the Pork Bay Breccia are exposed (Fig. 5.8B). In the streambed are several thin (<1m) amygdaloidal olivine tholeiite flows oriented N50E/~20°SE. Exposures of volcaniclastic breccia occur ILSG07 124 Trip 5 �adjacent to the thin basalts, but contacts are not exposed. At the base of the cliff face on the southern shore are poor exposures of the breccia which presumably overlie the thin basalts. The overlying this breccia and exposed over most of the 8m-high cliff face is a massive, slightly Pl-porphyritic transitional basalt – the Manitou River basalt. At the basal contact, this flow is strongly chilled possibly irregular pockets of glass. We will not have time to continue upstream to Caribou Falls, however, a later visit is encouraged. The falls is interpreted to represent a major fault scarp that juxtaposes the Pork Bay breccia (downstream) with olivine tholeiitic basalts of the Schroeder-Lutsen sequence. Despite ample exposure in the falls area, a well-identified fault zone could not be identified leaving open the possibility that the contact may be steep lithologic contact rather than a structural one. MRB PBB OT Figure 5-8. A) Close-up photo of the polymict clasts in the Pork Bay breccia; whitish interstitial mineral is mostly calcite. B) Exposures in southern stream bank at Location B – OT – thin olivine tholeiite flows in streambed, PPB – breccia exposed in lower cliff face, MRB – massive, transitional basalt exposed in upper cliff face. Discussion: The origin of the Pork Bay Breccia is enigmatic. Various ideas put forth to explain its formation include that it is a lahar or some type of mass wasting deposit, that it represents a collapse breccia within a caldera, and that it represents deposits of an explosive cinder cone. None of these completely explain the salient attributes of the unit or its relationship with adjacent units. These attributes include: the wide variety of volcanic, intrusive and sedimentary clast types, the subrounded nature of the clasts, the porous sandy matrix, the lack of bedding, the thickness of the breccia (up to 60 m), the occurrence of variable thicknesses of tholeiitic basalts (2- 100m) between the breccia and the Manitou River basalt, and the occurrence of basalt flows within the upper parts of the breccia (as seen at location B). Given these attributes, we will discuss possible modes of origin on the outcrop. ---Continue northeast on Highway 61. Approximately 2.7 miles from the Caribou River, turn right into Sugar Loaf Cove Interpretive Center (look for a brown highway sign). Park in the lot and follow trails down to Lake Superior shore.--- ILSG07 125 Trip 5 �Stop 5-3. Olivine tholeiitic basalt flows at Sugarloaf Cove. UTM coordinates (nad ’83) 652044E, 5261120N T.58N., R.5W., Sec. 29, Little Marais quadrangle No hammers please! Note: This stop is on the property of the Sugarloaf Cove Interpretive Center Association and a State of Minnesota Scientific and Natural Area. It has been established to preserve the natural features of the beach and point for all to enjoy. Please leave your hammers behind and take only pictures, not samples. Highlights: Morphological features of olivine tholeiitic basalt flows. Description: The lava flows exposed at and near this stop are typical of the Schroeder-Lutsen basalts. Flows exhibit features characteristic of fluid, low-viscosity pahoehoe olivine tholeiites (Fig. 5-9). Columnar joints and sandstone-filled clastic dikes (sand-filled cracks) are common in the upper part of the massive flow interiors. Figure 5-9. Idealized internal structure of olivine tholeiite-composition lava flows. From Green, 1989. Six lava flows are exposed on the point (Fig 5-10A). The about 10 m of the upper part of flow 1, an ophitic basalt, is exposed on the southwest end of the point. Flows 2 and 3 are 1- to 2-meters thick and display bent pipe amygdules and oxidized ropey flow tops (Fig. 5-10B). Flow 4 is 2-3 meters thick and displays well developed columnar joints, ropey flow tops, and abundant clastic dikes at its eastern extent. Flows 5 and 6 define a trough structure atop flow 4 in the midsection of the point. At the northeast end, flow 5 occurs as two thin (~1m) flows and over 5 meters of ophitic basalt of represent the lower part of flow 6. ILSG07 126 Trip 5 �A. B. 4 3 35 2 Figure 5-10. A) Sketch of distribution and orientation of six flows on Sugarloaf Cove Point. B) Photo of thin lava flow 2-4 at the west end of the point. Discussion: These olivine tholeiite basalt flows are typical of the Schroeder-Lutsen sequence, the uppermost sequence of the North Shore Volcanic Group. In this area the Schroeder-Lutsen basalts unconformably overlie a thick sequence of mafic to felsic volcanic rocks, informally termed the Cross River lavas (Boerboom and Green, 2006), that strike approximately N55E and dip 10-20° SE, in contrast with the nearly shore-parallel strike of the Schroeder-Lutsen basalts (see Fig. 5-15, stop 7). The occurrence of these olivine tholeiite flows at the top of the NSVG highlights the fact that the NSVG generally becomes more primitive (less evolved) up section. ---Return to Highway 61 and drive NE approximately 7 miles to Temperance River. Turn left on small gravel road 0.8 miles NE of Temperance River across from National Forest Service Sign. Drive 0.95 miles up hill, over outcrops in road bed (Schroeder basalt) and park; Stop 4A outcrops just east of road.-- Stop 5-4A. Carlton Peak rhyolites. UTM coordinates (nad ’83) 660737E, 5271317N T.59N., R.4W., Sec. 20, Tofte quadrangle Highlights: Aphyric and porphyritic rhyolite flows. Description: There are several low outcrops in this area of rhyolite (at the south edge) and porphyritic rhyolite (at the north edge). There are also outcrops of andesite to the northeast of the rhyolite, and of ophitic basalt to the north of that. The porphyritic rhyolite is grayish-pink, with 58% phenocrysts of K-feldspar, quartz, oxidized iron silicate minerals, and magnetite. The aphyric rhyolite is light gray to purplish-red, with rare small phenocrysts of quartz, K-feldspar, altered mafic silicates, and magnetite. Small vuggy quartz-lined amygdules and tension gashes are common throughout. Discussion: The rhyolites here are part of the Carlton quarry sequence, which includes ophitic basalt at the top, ILSG07 127 Trip 5 �andesite, porphyritic rhyolite, and aphyric rhyolite at the base. This sequence has been intruded by the Beaver River diabase (stop 4B). This entire set of flows dips moderately northwest, and is presumably fault-bounded. The porphyritic rhyolite has an age of 1,094±2.0 Ma (Green and others, 2001), the youngest age obtained to date from Keweenawan rocks in Minnesota, and is comparable in age and stratigraphic position to rhyolite in the Porcupine Volcanics in Michigan and to the upper part of the Portage Lake Volcanics (Green, 2002 and references therein). The position of the Carlton Quarry lavas within the NSVG is somewhat enigmatic, given the age, and fortuitous gaps in outcrop preclude full understanding of the relationships of these lavas to the rest of the NSVG. ---Continue on up the gravel road to a large flat area below the main quarry face, or turn left on the way and wind your way up to the quarry wall. If you go up there, BE CAREFUL!--- Stop 5-4B. Carlton Peak anorthosite and Beaver River diabase. UTM coordinates (nad ’83) 661309E, 5272060N T.59N., R.4W., Sec. 20, Tofte quadrangle Highlights: Carlton Peak anorthosite, Beaver River diabase (BRD). Description: In the quarry staging area are multiple large blocks of pale green to locally purple, coarsegrained anorthosite, and blocks of weathered, dark green, Beaver River diabase. The anorthosite contains scattered 2-3 cm diameter dark clots of altered poikilitic olivine, but otherwise is composed of nearly 100% fresh, glassy plagioclase. The anorthosite is well exposed in natural outcrops at the top of Carlton Peak, which requires a moderately strenuous hike of approximately one mile on the Superior Hiking Trail, accessed by driving 2.8 miles north of Tofte on County Highway 2 (Caribou Trail) to the well-marked Superior Hiking Trail parking lot. An alternative short hike to the top of Britton Peak, located just east of and above the parking lot, leads to an inclusion of the same type of anorthosite, and a clear view of Carlton Peak. Other places on the shore where these anorthosite inclusions can be easily seen are at Split Rock Lighthouse State Park (see guidebooks by Miller and others, 1987, Stop 1 and Boerboom and others, 2004, Stop 2-8), and Tettegouche State Park. The easiest place of all is a newly-constructed road cut on Highway 61 in Silver Bay, which has clean exposures of anorthosite inclusions within the BRD. Discussion: Carlton Peak is a prominent topographic knob formed by the Carlton Peak anorthosite, which occurs as large inclusions in the BRD. Here and at many other places along the North Shore, anorthosite forms rounded, bare knobs because it is very massive, contains few fractures, and is more resistant to chemical weathering than the surrounding diabase (Fig. 5-11). Individual anorthosite inclusions at Carlton Peak range up to 1100 feet in diameter, and are among the largest of the known anorthosite inclusions in the BRD. The anorthosite inclusions throughout the BRD are round to angular in shape and vary in size from individual plagioclase crystals to large inclusions such as at Carlton Peak, and some are brecciated and recrystallized (Morrison and others, 1983). They are all composed of 90-99% course-grained bytownite to labradorite, and minor Mg-rich olivine, orthopyroxene, and less commonly clinopyroxene. The inclusions are most common near the bottoms of BRD sills, and in sharp contact with the BRD, but the BRD is not chilled against them. Anorthosite inclusions of this type are different from anorthositic-series rocks of the Duluth Complex, but there are similar inclusions of anorthosite within the anorthositic series. The highly disordered structural state of plagioclase and the absence of chilled margins in the BRD against the anorthosite indicate these inclusions were derived from a lower to mid-crustal source (Miller and Weiblen (1990). ILSG07 128 Trip 5 �Figure 5-11. Google earth oblique image of Carlton Peak anorthosite, looking northeast up the shore. From this perspective it appears to be a large slab-shaped inclusion. Isotopic and trace-element compositions of the anorthosite xenoliths imply that they may be preKeweenawan in age (Morrison and others, 1983), but the data are ambiguous. As an alternative, Miller and Weiblen (1990) suggest a crystal-mush theory in which significant proportions of Keweenawan anorthosite may have been generated by flotation of plagioclase under high pressure in the deep crust prior to Beaver Bay Complex magmatism at 1,096 Ma. Plagioclase cumulates formed under deep crustal conditions would probably be distinctive in character and composition, compared to anorthosite cumulates formed in the shallow crust. The ambiguous isotopic compositions of the inclusions may indicate that the anorthosites formed from Keweenawan magmas that were contaminated by older crust, rather than older anorthosites being contaminated by interaction with Keweenawan magmas. The Beaver River diabase (BRD) forms an extensive network of dikes and sills, and is one of the youngest intrusive phases of the Beaver Bay Complex. The BRD extends from Split Rock Point to just west of Grand Marais, in a large arc that is approximately 57 miles (90 km) long (Figure 5-4). Most of the diabase is fine-grained ophitic olivine gabbro, but the centers of some of the thicker sills (up to 150 meters) grade into coarse-grained subophitic to intergranular gabbro. Historic sidelight: In 1902, anorthosite from an inclusion similar to this, in present-day Split Rock Lighthouse State Park, was quarried by a group who mistakenly thought the anorthosite was corundum. In fact there is actually a high point of land on Lake Superior in the park, held up by anorthosite, known as Corundum Point. Another quarry site was near the Baptism River at Illgen City. Their plan was to sell the “corundum’ on the east coast, but they had to abandon that plan and turn to other ventures. This company went on to become the Minnesota Mining and Manufacturing Company, now simply 3M! ---Drive back to Highway 61 and head northeast towards Tofte for 0.8 miles, then veer right onto Tofte Park Road (24). Take first private drive to the right and head to shore OR continue to Tofte town park and boat access on right after about ¼ mile. From boat landing, head southwest about 1 km along semicontinuous outcrop of tholeiitic lava flows (~7 flows). Stop is 250m southwest of old boathouse.--- ILSG07 129 Trip 5 �Stop 5-5. Flow features and feeder dikes in Schroeder basalts UTM coordinates (nad ’83) 661800E, 5270200N T.59N., R.4W., Sec. 28, Tofte quadrangle Highlights: Ophitic olivine tholeiite basalt flows, columnar jointing, amygdaloidal feeder dikes. Description: Accessing the shore at the old boathouse (662000E, 52700350N), a smooth billowing flow contact dipping gently to the northeast is exposed at the boat ramp. Heading southwest about 30 meters, a north-trending 0.5m-wide feeder dike is exposed in the amygdaloidal flow. The dike is zoned in the degree of oxidation and in the concentration of vesicles and amygdules (Fig. 512A). Boat Landing Boat House Stop 5 Continuing about 130 meters to the southwest over the polygonally-jointed surface of an ophitic basalt flow, a domal flow contact with a thin flow is encountered. Passing across this dome back onto the ophitic basalt, one can see a feeder dike breaking through the amygdaloidal top of the underlying flow and feeding the ophitic basalt. The feeder dike is lined with pipe amygdules that are curved upward (Fig. 5-12B). A . B .. Figure 5-12. A) Feeder dike with zonation in oxidation and amygdule concentration. B) Feeder dike with upturned pipe amygdules. ILSG07 130 Trip 5 �---Continue northeast on Tofte Park Road to Highway 61. Turn right (northeast) and go three miles to Leveaux Ridge road on the left, across from the Chateau Leveaux. Go up Leveaux Ridge Road a short distance (~0.25 mi)., then turn right on Overlook Trail. Head uphill(~0.3 mi), park where road curves to west near driveway on right (21 Overlook Tr.). --- Stop 5-6. Porphyritic Phases of Leveaux Porphyritic Diorite (LPD) UTM coordinates (nad ’83) 665850E, 5274260N T.59N., R.4W., Sec. 14, Tofte quadrangle andesite Stop 6 - :i-çl " - H- ' -- Schroeder- Highlights: Amygdaloidal olivine tholeiite basalt of Schroeder Lutsen sequence, inferred fault zone, amygdaloidal, sparely porphyritic upper zone and porphyritic zones of the Leveaux porphyritic sill. Description: Exposures in a low ledge and the drainage ditch south of the driveway are moderately amygdaloidal intergranular basaltic andesite. Several mineralized northeast-trending shear zones cut the rock. On the opposite side of the driveway is a 5’ roadcut of sparsely plagioclase porphyritic and amygdaloidal, fine-grained ferrodiorite. Centimeter-sized plagioclase phenocrysts compose about 5% of the rock as does a similar abundance of quartz amygdules (Fig. 5-13A). This rock type continues along this roadcut to the next driveway (#33). Opposite the bridge across a substantial creek are pavement outcrops of strongly porphyritic (30-40%) ferrodiorite which are typical of the main Porphyritic Zone of the LPD sill (Fig. 5-13B) Figure 5-13. A) Outcrop appearance of sparsely porphyritic and amygdaloidal ferrodiorite typical of the Upper Zone of the Leveaux ferrodiorite. B) Outcrop appearance of strongly porphyritic ferrodiorite typical of the main Porphyritic Zone of the Leveaux ferrodiorite. Discussion: This exposure of the Leveaux porphyritic diorite sill is the only location where the upper contact zone is preserved. Along most of the cuesta mountain exposures, the upper part of the sill is ILSG07 131 Trip 5 �eroded into the porphyritic zone (see Stop 8). The vesicular/amygdaloidal character of the upper contact zone speaks to the shallow level of emplacement of the LPD into the volcanic edifice. The sparsely porphyritic nature of this zone is interpreted by Albers (2006) to have been caused by flow differentiation of phenocrysts out of the upper contact zone during emplacement and subsequent rapid crystallization. Also, this inferred fault is a major fault zone which can be traced for 10’s of kilometers subparallel to the shore. ---Go back to Highway 61 and continue NE 0.16 miles to roadcuts on uphill side of Highway. Park at edge of highway. WATCH OUT FOR HEAVY TRAFFIC!. --- Stop 5-7. Base of Schroeder-Lutsen Sequence and top of underlying andesites UTM coordinates (nad ’83) 666381E, 5274345N T.59N., R.4W., Sec. 13 Tofte quadrangle Highlights: Contact between base of Schroeder-Lutsen basalt sequence and underlying Good Harbor Bay andesites. This stop will be at a roadcut, but if permission can be obtained a better outcrop may be visited down on the lake shore. The description below is of the lakeshore outcrop, but most of the features in it can be see on the roadcut. Description: Upper part of andesites is composed of amygdaloidal andesite fragments up to 50 cm in size, surrounded by reddish-brown sandstone. The andesite clasts are angular, and vary in shape from equant to slab-like; the latter commonly oriented near vertical. Sand makes up 3040% of the volume of the upper 3 meters of the breccia, and the breccia is capped by a cross-bedded sandstone wedge that is cut off by erosion on a cliff corner (Fig. 5-14). Figure 5-14. Photographs of base of Schroeder basalt and underlying fragmental andesite, from outcrop on shoreline below stop 7. Hammer is approximately 45 cm long. ILSG07 132 Trip 5 �The base of the Schroeder basalt flow undulates slightly and contains pipe amygdules. In places, if the light is right, one can make out vague, bulbous lobe-like forms approximately 30 cm in diameter at the base of the flow. Discussion: There are two more exposures between here and Lutsen similar to this one, all characterized by large, boulder-sized and blocky amygdaloidal andesite clasts surrounded by sandstone, with a thin, discontinuous sandstone cap. These are also similar to andesite exposed beneath the Little Marais conglomerate, approximately 15 miles to the southwest (Fig. 5-15; Miller and others, 2006). The Little Marais Conglomerate is at least 60 feet thick, and composed mostly of sandstone, but contains some silt/shale beds, as well as large rounded boulders of basalt near the base and large angular blocks of basalt higher in the section that are ‘floating’ in sandstone. In all cases the underlying fragmental andesites are unusually thick, with substantially larger clasts than typical andesitic flow-top breccias. Despite the fact that the Schroeder-Lutsen sequence is clearly unconformable to the underlying rocks, as made evident by the discordant strike directions between the two (Fig. 5-15), the andesite flow tops below are all fragmental, at least in the available exposures. The upper surface of the Good Harbor Bay andesites may have been a slightly eroded surface made up of clinkery lava flows with an irregular topography, covered by sedimentary rocks of variable thickness. Local fault escarpments that formed prior to eruption of the Schroeder-Lutsen basalts may account for the unusually thick Little Marais conglomerate, with blocks of basalt fallen off the fault escarpment into the sedimentary pile. Figure 5-15. Simplified geologic map of a strip along Lake Superior from near Little Marais to Lutsen showing locations where unconformity beneath the SchroederLutsen basalts is exposed. ILSG07 133 Trip 5 �---DIRECTIONS: Continue northeast on Highway 61 about 0.7 miles. Just past the Onion River, pull into the Ray Berglund State Scenic Memorial Wayside. Park there and walk SW along highway to Onion River. An excerpt from MN Dept. of Transportation website: “The Onion River derived its name from a Paul Bunyan legend. The legend says that wild onions grew in such abundance in the area, that Paul Bunyan's loggers shed tears while cutting lumber”.--- Stop 5-8. Morphology of andesite lava flows in the Onion River valley. UTM coordinates (nad ’83) 667525E, 5275197N T.59N., R.4W., Sec. 12 Tofte quadrangle *The extent to which these flows can be seen is dependent upon the water level in the Onion River. If the river is high, some outcrops can be accessed on the highway and on the shoreline near the river mouth. Highlights: Andesite flow features, Terrace Point basalt, minor faults. Description: This stop will go through a multiple series of andesitic lava flows which exhibit classic flow features (Fig. 516). The andesite is porphyritic (plagioclase, clinopyroxene, some magnetite), with a matrix of fine felty plagioclase, interstitial K-feldspar, granular clinopyroxene, and blocky Fe-Ti oxides. A sand-filled flow-top breccia that caps the first flow in the river series is exposed on the roadcut just SW of the Onion River and on the shore at the river mouth. The base of the same flow, which exhibits classic andesite oxidation-lamination texture, from which the attitude of the flow can be reliably measured, is exposed in the river bed below the highway. A 3 meter wide brittle fault (attitude N45E, 85°N), with reverse sense of movement (north side down approximately 2 meters) is exposed about 200 feet north of the highway on the east bank. The fault zone contains brecciated andesite fragments surrounded by coarse white calcite and pinkish-orange laumontite. This fault projects back southwest across the river to a till-covered portion of the slope. Next to the fault, upstream, is another thick sand-filled flow-top breccia at the top of the next underlying flow. Proceed upstream past a total of five andesite flows. The sixth flow is the western end of the Terrace Point basalt flow (see also Stop 11 and Fig. 5-18). Although the top of the flow is fragmental similar to the andesite flows downstream, it is internally ophitic (very rare for an NSVG andesite), and contains rare fresh and glassy, blocky plagioclase phenocrysts. The next flow below this ophitic basalt is probably icelandite in composition, but lacking chemistry has been lumped with the andesites. After a series of andesite and icelandite flows, the river makes a sharp bend to the east where it intersects a prominent fault marked with slickenside surfaces that forms a high escarpment on the south side of the river. Above this is a narrow canyon that can be traversed in late summer when the water level is lowest, through a series of thin, irregular olivine tholeiite basalt flows. Traverse upstream until a snowmobile trail bridge is encountered; there is a small trail along the east side of the river that goes south from the snowmobile trail back to the parking area. Discussion: These flows were original termed the Onion River lavas by Boerboom and others (2006), with all the lava flows from Highway 61 to beyond the snowmobile bridge lumped together. Subsequent mapping to the east (Boerboom and others, 2007) has shown that the Onion River lavas are actually the western end of the Good Harbor Bay andesites. To the east the Terrace Point basalt is underlain by the Cut Face Creek sandstone, but that apparently pinches out and is no longer present here. ILSG07 134 Trip 5 �Figure 5-16. Idealized flow structure of quartz tholeiite to andesitecomposition lava flows. From Green, 1989. Figure 5-17. Geologic map of the Onion River lavas. ---Continue 0.4 miles northeast on Highway 61 to the Onion River Road. Take the Onion R. Rd 1.7 miles to parking area for the Superior Hiking Trail. Take Oberg Mtn trail about 0.6mi (~1000m) to the southwest overlook . --ILSG07 135 Trip 5 �Stop 5-9. Porphyritic Zone of Leveaux Porphyritic Diorite (optional stop) UTM coordinates (nad ’83) 666960E, 5276900N T.59N., R.4W., Sec. 1 Tofte and Honeymoon quadrangles Highlights: Porphyritic Zone of the LPD, vista of Leveaux Mountain and shoreline. Description: At the southwestern crest of Oberg Mtn, exposures of typical porphyritic ferrodiorite of the upper Porphyritic Zone of the LPD are exposed. The weathering of the outcrop accentuates the porphyritic texture of the white plagioclase against a brownish matrix. From this vantage point, the southeast-dipping sill can be seen in the topographic expression of Leveaux Mountain visible across the Onion River valley to the southwest. Farther southwest just off shore, Gull and Bear Islands can be seen (on a clear day) near the Taconite Harbor power plant, about 10 miles away. These islands are formed by outliers of LPD and represent the southernmost exposures of the Leveaux porphyritic diorite sill. ---DIRECTION: Continue east on Highway 61 for about 2.5 miles to the Poplar River. 600 feet past the Poplar River, veer right onto County Road 35 (gravel). Drive east on this road 1 mile and park at pull off where road joins back onto Highway 61. Walk straight down to lake through the woods to small cove.--- Stop 5-10. Interflow conglomerate. UTM coordinates (nad ’83) 673942E, 5279298N T.63N., R.3W., Sec. 24 Lutsen quadrangle Highlights: Interflow conglomerate, flow contact. Description: Small, 2 meter thick exposure of reddishbrown, poorly bedded, moderately well sorted and rounded, cobble conglomerate exposed below a thick flow of Lutsen basalt. The conglomerate is exposed as a small erosional window through the Lutsen basalts. Cobbles in the conglomerate are mostly less than 5 cm in diameter, in a matrix of coarse gritty sand and white calcite cement. In order of abundance, the clasts are composed of ambiguous brown, fine-grained mafic to intermediate volcanic rocks, flow-banded rhyolite, amygdaloidal basalt possibly similar to the adjacent Lutsen basalt, reworked sandstone, and rare agate. In thin section many of the clasts are composed entirely of a felty-matted, gray-birefringent mineral, presumably some type of secondary zeolite; others are replaced by ILSG07 136 Trip 5 �calcite in irregular alteration zones and clots. Less altered clasts exhibit relict flow banding typical of NSVG rhyolites, and relict textures typical of NSVG mafic to intermediate volcanic rocks. The contact with the overlying flow of dark green, ophitic Lutsen basalt is very sharp and straight, measured at N70E, 15° SE, typical for the NSVG. Discussion: Further northeast up the shore there are some reddish-brown, fine-grained, interflow sandstones contained entirely within the Lutsen basalts, as scattered 2-3 meter thick and one 75 meter thick bed. Although the conglomerate at this stop is likely not more than a few meters thick and may similarly be contained entirely within the Lutsen basalts, it does not outcrop in nearby stream exposures where anticipated if it were a conformable layer in the Lutsen basalts. More likely, this exposure is a window eroded through the base of the Schroeder-Lutsen sequence, and the conglomerate marks the unconformity between the Good Harbor Bay andesites and the Lutsen basalts (see Fig. 5-15, stop 7). The conglomerate contains few if any clasts derived from the texturally distinct Lutsen basalts, but rather is composed predominantly of felsic-intermediate composition volcanic rocks, basalt unlike Lutsen basalt, and some reworked sandstone. Thus the source for the conglomerate is apparently dominated by older volcanic and sedimentary rocks that are similar in composition to those observed inland from the Schroeder-Lutsen sequence. The conglomerate may represent local deposition along a minor fault escarpment, or a proximal-facies high-energy fluvial deposit filling topographic depressions on the andesites. ---Drive northeast on Highway 61 approximately 13 miles to Historical Marker/Geologic Marker at top of steep hill (Terrace Point) at curve in road. Park at wayside, stop is across Highway at large roadcut. BE CAREFUL CROSSING HIGHWAY.--- Stop 5-11A. Cut Face Creek Roadcut UTM coordinates (nad ’83) 691780E, 5289170N T.61N., R.1W., Sec. 34 Good Harbor Bay quadrangle Highlights: Thick interflow sandstone, deformation features in sandstone at base of flow, shale rip-up chips, dessication cracks, Terrace Point basalt flow, fragmental basalt intruded and overrun by basalt flow. Description: In high roadcut on north side of highway is an obvious contact between the Cut Face Creek Sandstone and the overlying Terrace Point basalt flow. The sandstone overlies the Good Harbor Bay andesites (stop 11b). Sandstone –Jirsa (1984) measured approximately 73 meters of sandstone and 3 meters of shale in this section of the Cut Face Creek sandstone, but reported overall that nearly 30 percent of it is composed of thinly bedded, graded layers of fine-grained sand, silt, and clay. He reports both symmetrical and asymmetrical ripple marks, and bimodal paleocurrent distribution, and concluded that the Cut Face Creek sandstone was deposited in a fluvial-lacustrine environment. Unlike nearly all the other exposed interflow sandstones within the NSVG, planar cross-bedding is predominant, but some trough cross beds are present near the top of the section. Other features that may be visible in ILSG07 137 Trip 5 �the sandstone are dessication cracks filled with sandstone or coarse pink zeolite minerals, and rip-up textures. The base of the sandstone is exposed to the north in the Cut Face Creek valley, where it overlies the Good Harbor Bay andesite (see stop 11B). Basalt – The Terrace Point basalt, which overlies the sandstone, is a major ridge-forming unit from here to the southwest, forming ‘sawtooth mountains’. It is a distinctive flow characterized by a dark green color, white thomsonite amygdules, uniform 3-4mm ophitic texture, and scattered small glassy plagioclase phenocrysts. In general the contact with the sandstone is sharp and straight, but in places the sandstone has been slightly deformed by the basalt flow. At the south end of the outcrop is a unit of scoriaceous, fragmental basalt that is intruded and by the Terrace Point flow. Similar rock types and relationships have been observed to the southwest, also near the flow base. The breccia contains 1-100 cm angular basalt fragments that are both massive and amygdaloidal, and scattered large blocks of massive basalt. At this locality and others, sub-volcanic dikes of Terrace Point basalt that intrude the fragmental basalt are slightly chilled, and contain small amygdules stretched parallel to, and columnar joints perpendicular to, curvilinear dike margins. The fragmental unit is interpreted as a cinder cone or lahar-type deposit that was subsequently intruded and overrun by the thick Terrace Point basalt flow. Figure 5-18. Simplified geologic map showing distribution of interflow sandstones and major volcanic units in the Lutsen to Terrace Point area. Discussion: The Terrace Point basalt flow has been considered (e.g., Green, 1972) to be the basal flow of the Schroeder-Lutsen sequence. However, recent detailed mapping to the southwest (e.g. Boerboom and Green, 2006, Boerboom and others, 2006 and 2007) has shown that the Terrace Point basalt is actually contained within the Good Harbor Bay lavas. On the Onion River continuous exposures show that the top of the Terrace Point basalt is fragmental and overlain by more andesite (Stop 8), and is clearly an ILSG07 138 Trip 5 �integral part of the flow sequence. Also, the base of the Schroeder-Lutsen sequence overlies both the Terrace Point basalt and andesites in the Good Harbor Bay lavas (Figure 5-15, stop 7). Thus, based on new mapping, the Terrace Point flow and the Good Harbor Bay andesites are now considered to be part of a continuous series termed the Good Harbor Bay lavas. The Cut Face Creek Sandstone was considered to represent clastic deposition during a hiatus in volcanism prior to eruption of the Terrace Point/Schroeder-Lutsen basalts. It was also considered to be somewhat unique in that it was one of the few thick sandstone units in the northeast limb of the North Shore Volcanic Group, along with the 68 meter thick Indian Camp sandstone (which is within the Schroeder-Lutsen basalts). However, recent remapping has shown that there are likely at least three more substantially thick sandstone units that occur within lower series of lava flows to the north of the Good Harbor Bay lavas (Fig. 5-18; Boerboom and others, 2007). Thus, it is now recognized that the Cutface Creek and Indian Camp sandstones, exposed because of more active erosion near the Lake Superior coast, are only one part of a larger set of thick sandstone units, some of which are located over 5 miles inland. The recognition of thick interflow sandstones throughout several different series of lava flows at different stratigraphic heights implies active sedimentation during a prolonged period of volcanism. Fragmental flow tops in the Good Harbor Bay Lavas contain abundant sandstone infillings, and thin, discontinuous, layers and crack fillings of sandstone are common in the Schroeder-Lutsen basalts. Elevated levels of clastic deposition during active volcanism may account for the relative abundance of sand at the tops of the lava flows, and the thick interflow sandstones may have formed during periods of volcanic quiescence, but continued basin subsidence. Available data seem to indicate that the thicker sandstones may vary in thickness along strike, consistent with deposition onto an irregular lava surface. ---Drive northeast on Highway 61 approximately ¼ of a mile to the Cut Face Creek wayside located to the right on the lake shore, just past Cut Face Creek. Park at wayside. Either cross highway to small roadcuts across from wayside (WATCH TRAFFIC), or if the creek is low walk back on beach to Cut Face Creek and proceed upstream in creek valley.--Stop 5-11B. Good Harbor Bay andesite UTM coordinates (nad ’83) 691953E, 5289526N T.61N., R.1W., Sec. 34 Good Harbor Bay quadrangle Highlights: Good Harbor Bay andesites, Cut Face Creek Sandstone Description: Small outcrop directly across highway from parking area and to northeast, and outcrops at the mouth of Cut Face Creek, are fine-grained, brownish-gray, sparsely porphyritic andesite. These are part of the Good Harbor Bay andesites, a series of andesitic flows that stretches from near Grand Marais to Tofte. The andesite here is overlain by the Cut Face Creek Sandstone, and the contact is exposed in numerous locations in the meandering Cut Face Creek valley. The basal contact appears to be sub-conformable with the andesite on a local scale, but on a regional scale the upper andesite surface probably undulates. Small pebbles of massive to amygdaloidal andesite are common in the lower 3 meters of the sandstone, typically in 3-25 cm thick planar cross-bedded pebbly beds. The upper part of the andesite typically contains stretched amygdules, and has cracks filled with sandstone (Fig. 5-19). Like the exposures in the Onion River (Stop 9), the andesites here have rubbly, sand-filled flow top breccias with fragments of stretched amygdaloidal andesite. Figure 5-19. Photograph of base of Cut Face Creek sandstone (white arrow), showing clastic dike (black arrow) filling a crack in underlying andesite. Hammer in circle is 45 cm long. ILSG07 139 Trip 5 �REFERENCES Albers, P.A., 2006, The geology and petrology of the Leveaux Porphyritic Diorite Intrusion: Investigating possible magmatic relationships to the anorthositic series of the Duluth Complex, Cook County, Minnesota. Unpublished MS thesis, University of Minnesota Duluth, 121 p. w/ map. Basaltic volcanism study project (BVSP), 1981, Pre-Tertiary continental flood basalts: in Basaltic Volcanism on the Terrestrial Planets, NY, Pergamon Press, p. 30-77. Boerboom, T. J., 1994, Archean crustal xenoliths in a Keweenawan hypabyssal sill, northeastern Minnesota. White was right!: Institute on Lake Superior Geology 40th Annual Meeting, Houghton, MI: Proceedings v. 40, pt. 1 Abstracts, p. 5-6. Boerboom, T. J., Green, J.C., and Albers, P., 2007, Bedrock geologic map of the Lutsen quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-1XX, scale 1:24,000. (in press). Boerboom, T. J., and Green, J. C., 2006, Bedrock geology of the Schroeder quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-170, scale 1:24,000. Boerboom, T. J., Green, J. C., Albers, P., and Miller, James D., Jr., 2006, Bedrock geology of the Tofte quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-171, scale 1:24,000. Boerboom, T. J., and Green, J. C., 2005, Bedrock geology of the Two Harbors NE quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-155, scale 1:24,000. Boerboom, T. J., and Green, J. C., 2004, Bedrock geology of the Split Rock Point quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-147, scale 1:24,000. Boerboom, T.J., Green, J.C., and Jirsa, M.A., 2002a, Bedrock Geology of the French River and Lakewood 7.5’ quadrangles, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, Map M-128, scale 1:24,000. Boerboom, T.J., Green, J.C., and Jirsa, M.A., 2002b, Bedrock Geology of the Knife River 7.5’ quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, Map M-129, scale 1:24,000. Boerboom, T.J., Miller, J.D., Jr., and Green, J.C., 2004, Field trip 2: Geologic highlights of new mapping in the southwestern sequence of the North Shore Volcanic Group and in the Beaver Bay Complex: Institute on Lake Superior Geology, 50th Annual Meeting, Duluth, Minn., Proceedings, pt. 2, Field Trip Guidebook, p. 45-85. Boerboom, Terrence J., Miller, James D. Jr., and Green, John C., 2003a, Bedrock geology of the Two Harbors 7.5’ quadrangle, Minnesota Geological Survey Miscellaneous Map Series Map M-139, scale 1:24,000. Boerboom, Terrence J., Miller, James D. Jr., and Green, John C., 2003b, Bedrock geology of the Castle Danger 7.5’ quadrangle, Minnesota Geological Survey Miscellaneous Map Series Map M-140, scale 1:24,000. Boerboom, T.J., and Miller, J.D., 1994, Bedrock geologic map of the Silver Island Lake, Wilson Lake, and western Toohey Lake Quadrangles, Lake and Cook Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-81, scale 1:24,000. Bonnichsen, B., and Kauffman, 1987, Physical features of rhyolite lava flows in the Snake River Plain Volcanic province, southwestern Idaho: in The Emplacement of Silicic Domes and Lava Flows, J.H. Fink, ed., p. 119145. Brannon, J.C., 1984, Geochemistry of successive lava flows of the Keweenawan North Shore Volcanic Group: unpublished PhD. Thesis, Washington University, St. Louis, 212 p. Carmichael, I.S.E., 1964, The petrology of Thingmuli, a Tertiary volcano in eastern Iceland” Journal of Petrology, v. 5, p. 435-460. Davis, D.W., and Green, J.C., 1997, Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution: Canadian Journal of Earth Science, v. 34, p. 476-488. Davis, D.W., and Paces, J.B., 1990, Time resolution of geologic events on the Keweenaw Peninsula and implications for development of the midcontinent Rift system: Earth and Planetary Science Letters, v. 97, p. 5464. Goldich, S.S., Nier, A.O., Baadsgaard, H., Hoffman, J.H., and Krueger, H.W., 1961, The Precambrian geology and geochronology of Minnesota: Minnesota Geological Survey Bulletin 41, 193 p. Greeley, R., 1982, The Snake River Plain, Idaho: Representative of a new category of volcanism: Journal of Geophysical Research, v. 87, N. B4, P. 2705-2712. Green, J.C., 2002, Volcanic and sedimentary rocks of the Keweenawan Supergroup in northeastern Minnesota, in Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., and Wahl, T.E., Geology and ILSG07 140 Trip 5 �mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, p. 94-102. Green, J.C., 1995, Chemostratigraphy at the fringe of the Midcontinent Rift System: The northeast limb of the North Shore volcanic Group, Minnesota (ext. abs.): in Proceedings, Petrology and Metallogeny of Volcanic and Intrusive Rocks of the Midcontinent Rift System: 1995 IGCP Project 336 Field Conference and Symposium, p. 55-56. Green, J.C., 1992, Geologic map of the north shore of Lake Superior, Lake and Cook Counties, Minnesota: Part 1. Little Marais to Tofte: Minnesota Geological Survey Miscellaneous Map Series M-71, scale 1:24,000. Green, J.C., 1990, Primary tridymite crystallization and inferences from tridymite and quartz textures in high-T Keweenawan rhyolites and granophyres, Minnesota (abs.): Geological Society of America, Abstracts with Programs, v. 22, p. A289-290. Green, J. C., 1989, Physical volcanology of mid-Proterozoic plateau lavas: The Keweenawan North Shore Volcanic Group, Minnesota: Geological Society of America Bulletin, v. 101, p. 486-500, 21 figs., 2 tables. Green, J. C., 1983, Geologic and geochemical evidence for the nature and development of the middle Proterozoic (Keweenawan) Midcontinent Rift of North America. Tectonics, v. 94, p. 413-437. Green, J.C., 1982, Geologic map Atlas of Minnesota, Two Harbors Sheet: Minnesota Geological Survey, scale 1:250,000. Green, J.C., 1977, Keweenawan plateau volcanism in the Lake Superior region, in Baragar, W.R.A., Coleman, L. C., and Hall, J.M., eds., Geological Association of Canada Special Paper 16, p. 407-422. Green, J.C., 1972, North Shore Volcanic Group, in Sims, P.K., and Morey, G.B., eds., Geology of MinnesotaA centennial volume: Minnesota Geological Survey, p. 294-332. Green, J. C., Davis, D.W., and Schmitz, M.D., 2001, Three new zircon dates for the Midcontinent Rift, North Shore, Minnesota: More data, more questions: Institute on Lake Superior Geology, 47th Annual Meeting, Madison, Wis., Proceedings, pt. 1, Programs and Abstracts, p. 29. Green, J.C., and Fitz, T.J. III, 1993, Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent Rift plateau volcanics, Minnesota: petrographic and field recognition: Journal of Volcanology and Geothermal Research, v. 54, p. 177-196. Irvine, T.N., and Baragar, W.R.A., 1971, A guide to the chemical classifications of the common volcanic rocks: Canadian Journal of Earth Sciences, v. 8, no. 5, p. 523-548 Jerde, E.A., 1991, Geochemistry and petrology of hypabyssal rocks associated with the Midcontinent rift, northeastern Minnesota: unpublished Ph.D. Thesis, University of California, Loa Angeles, 305. pp. Jirsa, M.A., 1984, Interflow sedimentary rocks in the Keweenawan North Shore Volcanic Group, northeastern Minnesota: Minnesota Geological Survey Report of Investigations 30, 20 p. Klewin, K.W., and Shirey, S.B., 1992, The igneous petrology and magmatic evolution of the Midcontinent Rift System: Tectonophysics, v. 213, p. 33-40. Link, P. K. and Hackett, W. R., editors, 1988, Guidebook to the Geology of Central and Southern Idaho. Idaho Geological Survey, Bulletin 27, 319 p. Manley, C.R., 1996, Physical volcanology of a voluminous rhyolite lava flow: The Badlands lava, Owyhee Plateau, southwestern Idaho: Journal of Volcanology and Geothermal Research, v. 71, p. 129-153 McBirney, A.R., and Williams, H., 1969, Geology and petrology of the Galapagos Islands: Geological Society of America Memoir 118, 197 p. Miller, J.D., Jr., 1988, Geologic map of the Split Rock Point NE and Silver Bay quadrangles, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-68, scale 1:24,000. Miller, J.D., Jr., Boerboom, T.J., and Jerde, E.A., 1994, Bedrock geologic map of the Cabin Lake and Cramer 7.5’ quadrangles, Lake and Cook Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M82, scale 1:24,000. Miller, J.D., Jr., and Chandler, V.W., 1997, Geology, petrology, and tectonic significance of the Beaver Bay Complex, northeastern Minnesota: in Ojakangas, R.J., Dickas, A.B., Green, J.C., (eds.) Middle Proterozoic to Cambrian Rifting, Central North America”: geological Society of America Special Paper 312, p. 73-96. Miller, J.D., Jr., Green, J.C., Boerboom, T.J., and Chandler, V.W., 1993, Geology of the Doyle Lake and Finland quadrangles, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-72, scale 1:24,000. Miller, J.D., Jr., Green, J.C., and Boerboom, T.J., 1989, Geology of the Illgen City quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-69, scale 1:24,000. ILSG07 141 Trip 5 �Miller, J. D., Jr., Green, J.C., and Jerde, E.A., 2006, Bedrock geology of the Little Marais quadrangle, Lake and Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-172, scale 1:24,000. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-119, scale 1:200,000. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., ad Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, 207 p. Miller, J. D., Jr., and Weiblen, P.W., 1990, Anorthositic rocks of the the Duluth Complex: Examples of rocks formed from plagioclase crystal mush: Journal of Petrology, v. 31, p. 295-339. Miller, J.D., Jr., Weiblen, P.W., and Green, J.C., 1987, Roadlog and stop descriptions for the Beaver Bay Complex, in Balaban, N.H., ed., Field trip guidebook for selected areas in Precambrian geology of northeastern Minnesota: Minnesota Geological Survey Guidebook 17, p. 55-60. Milner, S.C., Duncan, A.R., and Ewart, A., 1992, Quartz latite rheoignimbrites flows of the Etendeka Formation, northwestern Namibia: Bulletin of Volcanology, v. 54, p. 200-219. Morrison, D.A., Ashwal, L.D., Phinney, W.C., Shih, C., and Wooden, J.L., 1983, Pre-Keweenawan anorthosite inclusions in the Keweenawan Beaver Bay and Duluth Complexes, northeastern Minnesota: Geological Survey of America Bulletin, v. 94, no. 2, p. 206-221. Nicholson, S. W., Shirey, S. B., and Green, J. C., 1991, Regional Nd and Pb isotopic variations among the earliest Midcontinent Rift basalts in western Lake Superior (abstr). In Programs with Abstracts, GAC/MAC Annual Meeting, Toronto. Nicholson, S.W., Shirey, S.B., Schulz, K.J., and Green, J.C., 1997, Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development Canadian Journal of Earth Science, v. 34, p. 504-520. Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: geochronological insights to physical, petrogenetic, paleomagnetic and tectonomagmatic processes associated with the 1.1 Ga Midcontinent rift system: Journal of Geophysical Research, v. 98, np. B8, p. 13,997-14,013. Reidel, S. R. and Hooper, P. R., 1989, Volcanism and Tectonism in the Columbia River Flood-basalt Province. Geological Society of America Special Paper 239, 386 p Sigvaldson, G.E., 1974, Basalts from the centre of the assumed Icelandic mantle plume: journal of Petrology, v. 15, p. 497-524. Schmidt, S. Th., 1993, Regional and local patterns of low-grade metamorphism in the North Shore Volcanic Group, Minnesota, USA: Journal of Metamorphic Geology, v. 11, p. 401-414. Schmidt, S. Th., and Robinson, D., 1997, Metamorphic grade and porosity and permeability controls on mafic phyllosilicate distributions in a regional zeolite to greenschist facies transition on the North Shore Volcanic Group, Minnesota” Geological Society of America Bulletin, v. 109, NO. 6, p. 683-697. Vervoort, J.D., and Green, J.C., 1997, Origin of evolved magmas in the Midcontinent Rift System, northeast Minnesota: Nd-isotope evidence for melting of Archean crust” Canadian Journal Earth Sciences, v. 34, p. 521535. Walker, G.P.L., 1974, The structure of eastern Iceland. In Kristiansson, L., ed., Geodynamics of Iceland and the North Atlantic Area, D. Reidel Publishing Co., Dordrecht, p. 177-188 Wallace, A.B., Drexter, J.W., Grant, N.K., and Noble, D.C., 1980, Icelandite and aenigmatite-bearing pantellerite from the McDermitt caldera complex, Nevada-Oregon: Geology, v. 8, p. 380-384. White, W.S., 1966, Tectonics of the Keweenawan basin, western Lake Superior region. U.S. Geological Survey Professional Paper 524-E, 23p. Wood, D.A., 1978, Major and trace element variations in the Tertiary lavas of eastern Iceland and their significance with respect to the Iceland geochemical anomaly: Journal of Petrology, v. 19, p. 393-436. ILSG07 142 Trip 5 �53rd Annual Institute on Lake Superior Geology FIELD TRIP 6 GEOLOGY ALONG THE GUNFLINT TRAIL Mark A. Jirsa Minnesota Geological Survey University of Minnesota and Paul W. Weiblen Department of Geology and Geophysics University of Minnesota ILSG07 143 Trip 6 �Figure 6.1. Regional geologic map of northeastern Minnesota showing the location of Gunflint Trail (modified from Jirsa and Miller, 2005). Inset box outlines Figure 6.4. Field trip area lies at the junction of Neoarchean, Paleoproterozoic, and Mesoproterozoic terranes. INTRODUCTION The Gunflint Trail provides access to the Boundary Waters Canoe Area Wilderness (BWCAW), which lies a short distance off either side of the road. The western end of the trail also transects a nearly complete geologic section of Archean, Paleoproterozoic, and Mesoproterozoic rocks (Fig. 6.1), and a diversity of well displayed contact relationships. In this area, the Archean greenstone-granite terrane of the Wawa subprovince of Superior Province is represented by a succession of metavolcanic rocks (ca 2700 Ma) intruded by the Saganaga Tonalite (ca 2689 Ma). The Archean rocks are unconformably overlain by Paleoproterozoic sedimentary strata of the Animikie Group (ca 1870-1830 Ma), which includes the Gunflint Iron Formation. Mesoproterozoic rifting is manifest in hypabyssal dikes and sills of the Logan intrusions (ca 1115 Ma), and several phases of the Duluth Complex (ca 1100 Ma), emplaced into both Archean and Paleoproterozoic rocks. Despite this exceptionally well exposed record of Precambrian history, little geologic mapping has been conducted in this region since the 1970's. As a result, this field guide relies heavily on prior mapping (Grout, 1929; Mathez and others, 1977; Morey and Nathan 1977; 1978; Morey and others 1981) and field trip guides (e.g., Weiblen and others, 1971; Miller ILSG07 144 Trip 6 �and others, 1987). Although the individual stops are different, much of this guide is modified only slightly from earlier publications, with permission from the authors. Mapping in this region by the Minnesota Geological Survey has resumed, after a 30+ year hiatus. The new mapping will benefit from geological techniques and insights acquired during the intervening decades, particularly those resulting from the work of Canadian geologists. Furthermore, exposures in some areas are more accessible due to road building and recent forest fires. Regarding the latter, many parts of the BWCAW and adjacent lands experienced a megastorm in 1999 that resulted in large tracts of downed and tangled trees. Controlled burns and wild fires in some of the devastated areas—including the 2006 Cavity Lake fire—laid bare many outcrops that once were deeply concealed in forest, lichen, and moss. GEOLOGIC SETTING Neoarchean The oldest rocks exposed in the region are Neoarchean metavolcanic and metasedimentary rocks— probably equivalent to, but not demonstrably continuous with, the Ely Greenstone. Although the supracrustal successions are dissected by faults and intrusions, some correlation can also be made with adjacent terrane in Ontario (Fig. 6.2). Most recent mapping in this terrane was conducted by Jirsa and Miller (2004) in the 1:100,000-scale, Ely-Basswood Lake map sheet that lies just west of the Gunflint Trail. A mapping thesis by Vervoort (1987) delineated geology in the JAP Lake area, which lies between that map sheet and the Gunflint Trail exposures. Rocks of the terrane are divided into a number of faultbounded segments, each having distinct geologic characteristics that cannot be easily correlated from place to place. The Gunflint Trail area exposes the eastern end of what Gruner (1941) referred to as the Gabimichigami segment. Supracrustal rocks in this segment include variably pillowed, hypabyssal mafic metavolcanic flows (Stop 6-1), and andesitic pyroclastic rocks. The supracrustal rocks are cut by hornblende-phyric intrusions that are similar in composition to the associated andesite, and on this basis, inferred to be more or less contemporaneous with them (Feirn, 1977). Based on stratigraphic facing directions established from pillowed metabasalt flows, the supracrustal sequence forms an east-trending and steeply south dipping and younging homocline. The Saganaga Tonalite was emplaced along the northern edge of the supracrustal succession. The metamorphic grade of greenstone along this contact is locally increased from greenschist facies that is typical of much of the belt, to amphibolite grade, and foliation is well-developed. The western edge of the Gabimichigami segment is terminated by a northnortheast-trending fault that juxtaposed greenstone against sedimentary and volcanic rocks of the Knife Lake Group. The Knife Lake Group includes the Ogishkemunce conglomerate that contains detrital clasts of the Saganaga Tonalite. This distinctive sequence of conglomerate, sandstone, and alkalic rocks is interpreted to have been deposited in a complex array of successor basins developed along earlyformed faults (Jirsa and Miller, 2005) at some time after emplacement of the Saganaga Tonalite. ILSG07 145 Trip 6 �Figure 6.2. Geologic setting of Archean bedrock in northeastern Minnesota and adjacent Ontario (modified from Jirsa and Miller, 2005). Inset map shows location relative to subprovinces of the Superior Province. Lacking detailed geochronologic data for this immediate area, much of the temporal distinction between various geological elements of the Neoarchean bedrock is based on the correlation of fabrics that resulted from three major phases of deformation, denoted D1, D2, and D3, with U-Pb zircon dates acquired elsewhere. All three deformation events are the result of N-S- to NW-SE-directed compression. The timing of D1 deformation is bracketed between deposition of the volcanic and clastic rocks of the Wawa subprovince at about 2722 Ma (Peterson and others, 2001), and emplacement of the Saganaga Tonalite at about 2689 Ma (Corfu and Stott, 1998). Folds in the Ely Greenstone and related rocks attributed to D1 deformation are truncated by faults associated with Knife Lake strata, indicating that the latter is synchronous with or post-dates deposition and early deformation of the Ely. As such, the Knife Lake Group is inferred to be a Timiskaming-type sequence temporally equivalent to the Shebandowan assemblage exposed in adjacent parts of Ontario (Corfu and Stott, 1998). D2 deformation effected all of the Archean supracrustal units and can be crudely bracketed by U-Pb dates of intrusions in the Giants Range batholith to the southwest that place the regional deformation and metamorphic event between about 2674 Ma and 2685 Ma (Boerboom and Zartman, 1993). D3 deformation produced faults in the low grade supracrustal and intrusive rocks of the Wawa subprovince, and folds and metamorphic fabric in granitic and migmatitic rocks of the Quetico subprovince to the north. The batholith generally referred to as the Saganaga Tonalite was emplaced into metavolcanic and gneissic granitoid country rocks. Igneous fabric is best developed in the border zones; but overall, the intrusion is moderately foliated and has a shallow eastward lineation. U-Pb age dates include 2689±1 Ma for the tonalite; whereas, 2 phases of the Northern Light Gneiss, which lies in contact with the Saganaga along its northeastern boundary in Ontario, are dated at 2707±2 Ma and 2750±2 Ma (Corfu and Stott, 1998). On the basis of geochemistry, Vervoort (1987) infers that the Saganaga batholith represents the magma source for dacitic lavas that overlie mafic and ultramafic rocks in the JAP Lake area. The Saganaga batholith was divided by Grout (1929) into a medium- to coarse-grained tonalitic phase that makes up the majority of the batholith (Stops 6-2 and 6-3), and a more mafic border phase (Stops 6-4 and 6-5). Grout (1929, p.568) defined this border zone approximately one mile in width along the south, east, and south sides of the batholith. In the Gunflint Trail area, the border phase is more irregular in ILSG07 146 Trip 6 �distribution and consists of strongly foliated, medium to coarse-grained, quartz-bearing hornblende diorite to granodiorite that generally lacks quartz eyes. The border phase is also marked by a wide variety of inclusions. Grout (1929) suggested that some of the inclusions in the Saganaga batholith are the products of igneous processes that ranged from assimilation to the mixing of mafic and felsic magmas. Alternatively, Hanson (1972) suggested that some of the included mafic material could be a product of assimilation of greenstone country rocks during pluton emplacement. Field evidence showing the presence of nearly all gradations of inclusions between mafic and felsic magmatic-looking components, and the presence of quartz phenocrysts in some mafic inclusions, supports the former interpretation for many of the inclusions. The results of analyses of two mafic inclusions acquired recently by the authors are summarized in Figures 6.3A, B, and C. These data indicate that the mafic inclusions (Fig. 6.3D) have the common chemical characteristics of the sanukitoid suite of Archean granitoid rocks. Thus, we infer that the inclusions studied are autoliths derived from a separate, primitive sanukitoid-magma. These data support Grout's earlier interpretations. This is not to say that country rock inclusions are not present—some fine-grained, dark-green, and variably cleaved inclusions do occur very near the contact with enclosing metavolcanic rocks (Stop 6-6a). To date there has been no systematic study of the mafic components of the Saganaga batholith. As pointed out by Didier and Barbarin (1991) in the conclusion of their multi-authored volume, further study of enclaves will contribute to the advancement of the overall petrogenesis of granites. The Saganaga batholith provides ample opportunity for such studies. Figure 6.3. Geochemical comparison of Saganaga batholith, mafic inclusions, and related rocks. A. SiO2-MgO plot. Abbreviations and data sources: TTG=Compositional field of the Archean TonaliteTrondhjemite-Granodiorite suite; S=Compositional field of primitive Archean sanukitoid samples from the southwestern Superior Province (Stern and others, 1989). Open circles: 1 Ely greenstone; 2 and 3 mafic inclusions at Stop 6-5; 4 border phase of the Saganaga Tonalite at Stop 6-5; 5, 6 Ely greenstone; 7-10 tonalite; 11 granodiorite. Analyses 1-4 were acquired for this study (on-file with Minnesota Geological Survey); analyses 5-11 are from Arth and Hanson, 1975, Table 2, p. 331, analyses 1-2, 19-22, and 24 respectively. Analyses 12-14 are from Stern and others, 1989, Table 1 p. 1695, samples 1-3. Crosses: analyses of the Roaring River Complex, Superior Province, Canada from Stern and Hanson (1991, Table 2, p. 220). They have demonstrated that incremental equilibrium crystallization of a melt ILSG07 147 Trip 6 �with the composition of analysis 15 (sample #7, op cit) can produce the three members of the TTG suite indicated. The two analyses of mafic inclusions at Stop 6-5 (#'s 2 and 3) plot in the primitive sanukitoid field, and their chemistry is similar to other chemical characteristics of the sanukitoid suite (Figs. 6.3B and C). These data imply that the mafic inclusions must either be deformed xenoliths of a sanukitoid body or segregations from a consanguineous melt. B. Chemical characteristics of selected sanukitoid suite rocks (data from Stern and others, 1989) and mafic enclaves in the Saganaga batholith (data from this study). Solid squares, dashed line = range of high-silica sanukitoid; Solid diamonds, dashed line = range of low-silica sanukitoid; Solid triangles = average of high- and low-silica sanukitoid; Pluses = high-silica mafic inclusion (#2 from Fig. 6.3A), in the Saganaga batholith; Crosses = low-silica mafic inclusion (#3) in the Saganaga batholith. The analyses are normalized to an average of high and low values for each chemical characteristic of several sanukitoid analyses. Data for the mafic inclusions lie well within the range of the sanukitoid values, except for Cr in mafic inclusion #3, which plots just above the high range of the sanukitoid rocks, and Ba and Sr for both mafic inclusions that plot slightly lower than the low range of the sanukitoid rocks. ILSG07 148 Trip 6 �C. REE analyses. Open squares = Saganaga mafic enclave from this study (#2 from Fig. 6.3A), Solid squares = Saganaga mafic enclave from this study (#3), Open diamonds = Saganaga tonalite this study (#4), Solid diamonds = Saganaga tonalite of Arth and Hanson, 1975, sample 19, Open circles = Saganaga tonalite of Arth and Hanson, 1975, sample 20. The REE data for the mafic enclaves in the Saganaga batholith are distinct from that of the tonalite samples, and lie roughly within the sanukitoid field (shaded) as defined by Stern and others (1989, Fig. 10, p.1702). These data support the conclusion from the chemical characteristics (Fig.6.3B), and the SiO2-MgO (Fig. 6.3A). D. Photographs of mafic inclusions analyzed for this study. Both inclusions occur in the strongly foliated, granodioritic border phase of Saganaga batholith at Stop 6-5. LEFT=inclusion #2, material for analyses shown on Fig. 6.3A-C was taken from circled (snow-filled) drill hole. RIGHT=Sample 3: material for analyses taken from bottom of sample below X. ILSG07 149 Trip 6 �Paleoproterozoic Animikie Group Sedimentary rocks of the Paleoproterozoic Animikie Group are exposed in an east-trending belt that extends from Thunder Bay on Lake Superior to a point 12 miles (19km) west of Gunflint Trail, where the belt is truncated by the Mesoproterozoic Duluth Complex (Fig. 6.4). The Animikie Group in this area consists of basal conglomerate and iron-bearing strata of the Gunflint Iron Formation, overlain by slate and siltstone of the Rove Formation. The sequence is broadly correlative with Animikie strata exposed to the southwest along the Mesabi Iron Range. The rocks form a homocline that dips gently southward, except where deformed by folding, faulting, and emplacement of Mesoproterozoic intrusions. For example, local folding, inferred to be associated with emplacement of the Logan intrusions, may be the product of magmatic shouldering into the relatively ductile sedimentary country rocks. Sequence "inflation" by the Logan sills may explain the observation that the dip of Paleoproterozoic rocks increases from 10 degrees away from the contact with Duluth Complex, to 60 degrees locally near it. Faults are present locally, but few have displacements greater than 50 feet. A notable exception is the block-shaped Lookout fault (Fig. 6.5) that lies adjacent to the Gunflint trail. As much as 200 feet of uplift on the west and south is speculated (Morey and others, 1981). In addition, the dip of Animikie strata on the west side of the fault is much steeper than that on the east, which explains in part the difference in the widths of map units apparent from the geologic map. Much of the complex-looking fold pattern east of the fault is likely an artifact of complicated surface topography and shallowly dipping units. The Lookout fault displaces both Animikie rocks and the Logan intrusions, but the timing of offset relative to emplacement of the Duluth Complex is unclear. Figure 6.4. Geologic map along the northwestern end of the Gunflint Trail, modified from published quadrangle maps (Morey and Nathan, 1977; 1978; and Morey and others, 1981), and mapping by Grout (1929). The locations of stops 6-2, 6-3, and 6-4 are shown—other field trip stops lie within the outline corresponding to Figure 6.5. Across the border in Canada, the Gunflint Iron Formation is essentially unmetamorphosed and consists of quartz, hematite, iron carbonates, greenalite, and magnetite. A thorough review and sedimentalogical interpretation of these rocks can be found in Pufahl and Fralick (2000), Fralick and others (2002), and references therein. In Minnesota, the original mineralogic character of the ironformation was modified by contact metamorphism adjacent to Mesoproterozoic intrusions, and many of the fine-scale textural and mineralogical attributes used to subdivide the iron-formation in Canada have ILSG07 150 Trip 6 �been destroyed. Consequently, a four-fold nomenclatural scheme that emphasizes various bedding aspects that are readily visible has been used for the Gunflint since the early work of Broderick (1920). The scheme was originally devised for the Biwabik Iron Formation on the Mesabi Iron Range the Mesabi Iron Range. It defines four informal members—Lower cherty, Lower slaty, Upper cherty, and Upper slaty. These terms emphasize the relative proportions of strata dominated by thickly bedded and granular (cherty) beds consisting of rounded sand-size intraclasts, vs. thinly bedded and very fine-grained (slaty) beds. The Lower and Upper members are grouped on the published geologic map that shows ironformation (Morey and others, 1981) and on Figure 6.5. The Lower cherty member ranges in thickness from 15 to 45 ft (4.5-13.5m). A conglomerate containing clasts of Archean granite and greenstone set in a matrix of feldspathic quartzite or stromatolite-bearing chert occurs locally (Stop 6-6b). The basal conglomerate is absent in most of the area, and a persistent thick-bedded to massive, magnetite-rich unit 5-15 ft (1.5-4.5m) thick (Stop 6-6c) composes the base of the Lower cherty member and unconformably overlies Archean rocks. The magnetite-rich layer is in turn overlain by a thick-bedded, chert-rich, magnetite-poor unit about 15 ft (4.5m) thick. The overlying Lower slaty member is 80-95 ft (24-29m) thick. The lowermost 10 ft (3m) is a black, thinly bedded, nearly magnetite-free argillite composed predominantly of volcanically derived material. This may equate with the Intermediate slate unit known on the Mesabi range. Massive and cherty beds that resemble the upper part of the Lower cherty member occur above the argillite, but they pass abruptly upward into a sequence of thick, chert- and silicate-rich beds with sparse magnetite intercalated with intervals of thinly laminated silicate-rich beds. The uppermost 50 ft (15m) of the Lower slaty member consists of thinly bedded to laminated slate containing various silicates and 20-35 percent magnetite (Stop 6-7a). The Upper cherty member is approximately 50 ft (15m) thick. The gradational contact between the Lower slaty and the Upper cherty members is marked by the appearance of irregularly bedded to lenticular chert-rich layers and by thin irregular layers consisting almost entirely of magnetite. The upper part of the Upper cherty member is characterized by several thick units of granular chert that contain microbial structures, intraformational conglomerate, and abundant magnetite. (Stop 6-7c). The Upper slaty member is approximately 150 ft (45m) thick. Thick lenticular beds of chert containing disseminated magnetite occur in the lower few meters, but most of the member consists of thin-bedded to laminated, silicate-rich units that are intercalated with intervals of thinly laminated graphitic argillite and centimeter-thick beds of relatively pure chert. The upper 10 ft (3m) are nearly magnetite-free and consist of limestone and chert interbedded with argillite (Stops 6-8a, 6-8b). The Rove Formation gradationally overlies the Gunflint Iron Formation and is at least 3,200 ft (970m) thick, although estimates of thickness are complicated by multiple Mesoproterozoic intrusions and likely fault repetition. It can be divided into a lower argillaceous unit, a transitional sequence, and an upper thin-bedded graywacke-rich unit (Morey, 1969). The lower argillite consists of intercalated argillaceous siltstone, silty argillite, and carbonaceous argillite (Stop 6-9). The transitional sequence separates dominantly argillaceous rocks below from an interval containing intercalated beds of coarseand fine-grained graywacke. The sandy beds contain many primary sedimentary structures indicative of deposition by turbidity currents that flowed dominantly southward. Contact metamorphic effects on the Gunflint Iron Formation were studied in detail by Floran and Papike (1975, 1978), and metamorphism of the Rove Formation was described by Labotka and others (1981). The sizes of the metamorphic aureoles adjacent to the Logan intrusions (as much as 33 ft (10m) wide) and their mineral assemblages are directly related to the thicknesses of the sills. Adjacent to the Duluth Complex, three metamorphic zones have been distinguished: 1) An outer zone of slightly metamorphosed iron-formation consisting of quartz, iron carbonate, minnesotaite, stilpnomelane, and hematite partially replaced by magnetite. 2) A 1.2 mile (2km) wide intermediate zone of moderately metamorphosed iron-formation containing grunerite-cummingtonite, hornblende, and actinolite, as well as quartz and finegrained magnetite. ILSG07 151 Trip 6 �3) A proximal zone of highly metamorphosed iron-formation occurring within 0.3 miles (0.5km) of the contact with the Duluth Complex and composed of quartz, magnetite, iron-rich pyroxenes, and fayalite. By contrast with the iron-formation, contact-related recrystallization of the Rove Formation is much less pronounced, even very near the Duluth Complex. The aureole is marked by a complex mixture of rock types suggestive of partial melting within a meter or so of the contact. In this contact zone, argillaceous rocks retain large-scale layering, but have a vague granoblastic texture. Individual layers contain cordierite and hypersthene locally, with minor biotite, ilmenite, plagioclase, K-feldspar, and olivine. At varying distances away from the contact, but generally no more than 100 ft (30m) away, biotite is the only well-developed metamorphic mineral in the pelitic rocks. Calcareous beds near the contact contain minor grossular garnet. A U-Pb age of 1878 ± 1 for the Gunflint Iron Formation was determined by Fralick and others (2002) from reworked volcaniclastic zircons in Ontario exposures. Subsequent geochronologic study of the Rove and equivalent Virginia Formations has considerably complicated and protracted the timeline of Animikie Group deposition. For example, dates by Addison and others (2005) of detrital zircons near the base of the Rove Formation yielded ages of 1827 ± 8 and 1836 ± 5 Ma, and 1832± 3 from a similar stratigraphic position in the Virginia Formation. All of these samples were acquired from several meters above what Addison and others infer is an ejecta layer derived from the Sudbury impact event (ca 1850). Recently acquired detrital zircon dates by Heaman and Easton (2005) indicate that upper parts of the Rove Formation are as young as 1777 Ma. Wirth and others (2006) report ca 1790 Ma detrital zircons in the Rove and equivalent Thomson Formations. Collectively, these data imply that deposition of strata assigned to the Animikie Group spanned some 100 million years, and hints at the likelihood of several major unconformities. The tectonic and depositional setting of the Gunflint and other semi-contemporaneous Lake Superior iron-formations remains somewhat contentious (summarized for all iron-formations in Clout and Simonson, 2005). Modern interpretations include deposition in one or more foredeep basins developed cratonward of crustal loading during Penokean orogenesis (Morey and Southwick, 1995). More recently, Pufahl and Fralick (2004) have inferred from sedimentalogical evidence that deposition occurred along a south-sloping continental margin undergoing subsidence in a back-arc extensional setting. Geochemical evidence for within-plate signatures of volcanic rocks associated with iron-formation in Michigan suggested to Schulz and Cannon (in press) a similar interpretation of back-arc spreading. From this and geochronologic work, they infer that deposition occurred in a broad foreland having local extensional basins hosting iron-formation. Following iron-formation deposition, turbidites of the Rove Formation and equivalent strata were deposited in a rapidly subsiding basin, with subsidence presumably driven by crustal loading by the northward migrating fold and thrust belt. A hiatal boundary between ironformation and overlying turbidites identified in Ontario and Minnesota (Addison and others, 2005), and Michigan (Schulz and Cannon, in press) roughly coincides with the ca 1850 Sudbury impact event. Mafic dikes Mafic dikes emplaced into the Saganaga Tonalite are prominent on aeromagnetic maps as two sets of positive linear anomalies trending northward and eastward. Exposures mapped along the north-trending magnetic trajectories (Fig. 6.4) show that the western of the two is diabasic, and the eastern consists of lamprophyre. The latter was the subject of work by O'Brien (1982) who mapped the 70 foot thick dike, reprocessed aeromagnetic data, and conducted petrologic work. He classifies the lamprophyre dike as camptonite, defined by its content of kaersutite (calcic amphibole), titanbiotite, and titanaugite phenocrysts in a plagioclase-rich groundmass. The precise age of these dikes is unclear, but a Paleoproterozoic age is likely. K-Ar dates of biotite by Goldich and others (1961) yielded an age of about 1750 Ma, which they attributed to late thermal resetting during alteration of the dikes. Petrographic and geochemical analyses by O'Brien indicate that much of the alteration was deuteric—having formed during cooling of the dike. On this basis, O'Brien speculated that the K-Ar date may be an accurate estimate for emplacement. A second set of aeromagnetic anomalies—trending east and slightly less pronounced than ILSG07 152 Trip 6 �the north-trending ones—have not been investigated in detail. The roadcut at Stop 6-4 exposes diabase inferred to lie along one of the anomalies. Mesoproterozoic Mesoproterozoic mafic intrusions comprise the remaining exposures in the Gunflint Trail area (Figures 6.4 and 6.5). The rocks represent the early magmatic stages of the Midcontinent Rift system. The apparently earliest of these are diabasic sills and dikes emplaced into the Animikie strata and collectively referred to as the Logan intrusions. A baddeleyite age of 1115 ± 1 Ma is reported from a sample of a Logan sill near Thunder Bay (Heaman and Easton, 2005). The sills are intruded, with slight angular discordance, by medium to coarse-grained gabbro and troctolite of the Poplar Lake and Tuscarora intrusions of the Duluth Complex. The Poplar Lake is part of the early gabbroic series of the complex. A basal gabbroic unit of the intrusion yielded a date of 1106.9 ±.8 (Miller and Severson, 2002). Field relationships indicate that the Poplar Lake is intruded by the Tuscarora intrusion, which is considered to be part of the layered series of Duluth Complex. Logan Intrusions The Logan intrusions are exposed along a series of prominent, east-trending ridges formed by the differential erosion of diabase sills and sedimentary rocks, particularly the Rove Formation. Individual sills are as much as 1100 ft (33m) thick, and can be traced along strike for several kilometers. Branching and merging of individual sills is common, and many sills thicken and thin down-dip. Some sills terminate against joints and inferred faults. Locally, fractures in the Rove Formation are occupied by thin dikes, which give a box-work configuration to the hypabyssal intrusions. Rock types include aphyric basalt, fine- to medium-grained diabase with ophitic clinopyroxene enclosing plagioclase, plagioclase cumulates, and granophyre (Jones, 1984). Chilled margins form sharp contacts with, and locally contain inclusions of, the country rocks (Stop 6-10). Diabase coarsens to medium-grained near the center of individual sills, and clinopyroxene is ophitic throughout. Minor differentiation is manifest in accumulations of plagioclase or granophyric intergrowths (quartz, sodic plagioclase, and orthoclase) in upper parts of sills. The configuration of Logan intrusions closely follows that of the gently folded Animikie Group strata, implying that minor ductile deformation occurred during the Mesoproterozoic. Duluth Complex The Duluth Complex is a sequence of generally discordant plutonic rocks consisting of many separate intrusions. Two of these intrusions, the Poplar Lake and Tuscarora, are exposed along the Gunflint Trail. The Poplar Lake intrusion, formerly referred to as Nathan's layered series, consists of interlayered gabbroic cumulates, with minor amounts of troctolitic and anorthositic cumulates. Rocks of the Poplar Lake have reversed magnetic polarity, and thus are broadly correlative with lower lavas of the North Shore Volcanic Group and with the Logan intrusions. As originally defined by Nathan (1969), the Poplar Lake intrusion is composed of at least 27 sheet-like units of mafic cumulates and intermediate to felsic rocks. More detail about the Poplar Lake intrusion can be found in this volume (Miller and Jerde, 2007; Field Trip #1). The Tuscarora intrusion irregularly cuts across the layered gabbro of the Poplar Lake intrusion (Morey and Nathan, 1978). The basal part of the Tuscarora intrusion consists of a fine-grained, augite-poikilitic, olivine gabbro (unit ttf on Fig. 6.5) (Stops 6-11 and 6-12). Within 0.3 mi (0.5km) of the basal contact, fine-grained troctolite coarsens to medium-grained (unit ttm). The troctolite units consist of 65-70 percent plagioclase and 10-15 percent olivine. Relative amounts of poikilitic augite and irontitanium oxides vary locally. Orthopyroxene mantles olivine and occurs in symplectic intergrowth with plagioclase. Biotite is locally present in association with iron-titanium oxides. Modal layering is well developed and generally concordant with unit boundaries that dip gently to the south—typically more shallowly dipping than the subjacent Animikie Group strata. The ttp and ttf units commonly contain chalcopyrite, pyrrhotite, and minor pentlandite interstitial to plagioclase and olivine. The sulfide concentrations are subeconomic, but locally form mappable zones (Stop 6-12). ILSG07 153 Trip 6 �FIELD TRIP 6 Stop Descriptions NOTES: 1) This group will not visit all of the stops described below. A number of stop descriptions are included in this guide to provide context and for future visits to the region. 2) Small maps showing location of individual stops are modified from USGS 7.5-minute (1:24,000scale) topographic maps, which are named below. 3) All UTM coordinates are given in NAD 83. Figure 6.5. Geologic map and schematic cross-section (A-A'), showing field trip stops 6-1 and 6-5 through 6-12 (geology modified from Morey and others, 1981). ILSG07 154 Trip 6 �STOP 6-1 Neoarchean pillowed basalt and basal Gunflint Iron Formation Location: T. 65 N., R. 4 W., sec. 28, NW, NE; Kekekabic Trail to Lookout (Fig. 6.5) Long Island Lake quadrangle 6-1a = So-called "Paulson Mine" at UTM 660,504E/5,328,673N (NAD83) 6-1b = Lookout at UTM 660,857E/5,328,364N [Requires 2 mile hike] Description: Follow the Kekekabic hiking trail from the parking lot, westward approximately 0.6 mi. to the junction with a north-bound trail that leads to the lookout. The main Kekekabic trail parallels the base of the Gunflint Iron Formation, and the north-facing slope immediately south of the trail contains exposures of the Lower slaty member. The iron-formation has been strongly metamorphosed in this area and now consists of various assemblages of quartz-grunerite-fayalite-magnetite and quartz-cummingtonitegrunerite-pyroxene-magnetite. 6-1a. Continuing westward about 800 ft from the trail junction, several test pits can be seen, including one that is fenced and labeled "Paulson Mine 1893," but probably isn't (the mine apparently lies some distance farther west). This and other test pits are developed in the Lower cherty member of ironformation, and waste piles contain abundant pyrrhotite, other sulfide minerals, and magnetite. 6-1b. Returning to the junction with the north-bound trail and following it up-slope, there are numerous exposures of Neoarchean metabasalt. Several small outliers of iron-formation containing iron-silicates and magnetite can be found along the route, implying that this south-facing surface is very near the unconformable contact between Archean and Paleoproterozoic rocks. About 120 feet north of the junction, a ledge of fine-grained diabase crosses the trail. Preliminary analyses (this study) indicate that it has a komatiitic composition. Intrusions of this composition are also found in the Newton Lake Formation, some 30 miles to the southwest, and high-Mg tholeiitic basalt flows and pyroxenitic to peridotitic sills were described by Vervoort (1987) in the JAP Lake area two miles to the west. At and near the lookout, metabasalt flows with pillow structures and autobreccia fabrics are exposed. Judged from pillows—which have been moderately flattened by regional D2 deformation—bedding trends eastnortheastward, is steeply south dipping and stratigraphic facing is southward. ILSG07 155 Trip 6 �STOP 6-2 Felsic phase of the Neoarchean Saganaga Tonalite cut by a small mafic dike. Location: T. 66 N., R. 4 W., sec. 31, NW, SW; Campsite #18 (Fig. 6.4) Munker Island quadrangle UTM: 656,333E/5,335,730N Description: Most of the outcrops in this area consist of gray, massive to trachytoid-foliated, medium to coarse-grained tonalite, having plagioclase in much greater abundance than microcline (Fig. 6.6). Large quartz phenocrysts, or eyes, as much as 1 cm in diameter are characteristic of this phase, which is typical of 90% of the batholith. Quartz eyes are polycrystalline aggregates, in which each crystal has a different optical orientation. Quartz also occurs as an interstitial mineral to subhedral plagioclase (An20-28). Small amounts of microcline occur as antipathetic exsolution in plagioclase, as rims on plagioclase, and as small interstitial grains. Hornblende is the dominant ferromagnesian mineral, together with minor amounts of augite, biotite, epidote, and chlorite. Figure 6.6. Texture typical of the Saganaga Tonalite, including lineated quartz "eyes" (medium gray). The small dike of aphanitic mafic rock in this exposure has not been sampled, but is inferred to be related to larger north-trending diabasic and lamprophyric dikes that form prominent northtrending anomalies on aeromagnetic maps. The pronounced foliation in the Saganaga Tonalite was inferred by Grout (1933) as a primary flow fabric (trachytoid). The tonalite is inferred to have been emplaced into Archean metavolcanic rocks shortly after early (D1) deformation, based a U-Pb date of 2689±1 in Canadian exposures (Corfu and Stott, 1998). As such, it experienced major regional metamorphism and transpression associated with D2 deformation. As with many large plutons in such terranes, the debate remains unresolved about whether the fabrics are wholly magmatic, wholly tectonic, or some hybrid of the two. ILSG07 156 Trip 6 �STOP 6-3 Neoarchean Saganaga Tonalite with rounded dioritic to granodioritic inclusions Location: T. 66 N., R. 4 W., sec. 31, SW, NW; Campsite #13 (Fig. 6.4) Munker Island quadrangle UTM: 656,219E/5,336,079N Description: Equigranular tonalite with characteristic quartz eyes, containing a wide variety of inclusions. The term "inclusion" has a tortured usage—we prefer to use the term to apply to material that has a contrasting composition or appearance from its host, regardless of origin, size, shape, degree of assimilation, or extent of equilibration with the enclosing host magma. Inclusions may represent xenolithic blocks of county rock incorporated into the Saganaga Tonalite, or cognate phases of the intrusion (autoliths). The inclusions found here have not been studied in detail. They present a unique opportunity for further petrologic and mineral chemistry research to explore their complex origin. The shape of inclusions at this stop is related to either magmatic or tectonic processes. In either case, one must keep in mind that 2dimensional shapes are dependent on the orientation of exposure-surface relative to that of the inclusions. This exposure, for example, provides only one surface—it is impossible to say from this whether the inclusions are discoid, sub-spherical, or egg-shaped. STOP 6-4 Granodioritic phase of Neoarchean Saganaga Tonalite with inclusions; cut by diabase dike Location: T. 66 N., R. 4 W., sec. 32, near center; Roadcut on both sides of Gunflint Trail (Fig. 6.4) Conners quadrangle UTM: 658,575E/5,335,837N Description: Exposures on the west side of the road consist of pinkish to gray hornblende granodiorite to granite inferred to be a border phase of the Saganaga Tonalite and containing inclusions that are both more felsic and more mafic than the enclosing rock (Fig. 6.7). For example, the large angular block shown in Fig. B ILSG07 157 Trip 6 �consists of quartz-eye-bearing tonalite, much like rock that is typical of the main phase of the intrusion. Foliation is poorly developed and likely magmatic in origin. This exposure demonstrates that the Saganaga is a composite batholith that, despite its apparent homogeneity, consists of quite varied magmatic phases, particularly near its border. Tonalite on the east side of the road is cut by a fine-grained diabasic dike several meters in width. Although this location lies some distance east of the prominent north-trending aeromagnetic trends associated with dikes that are described in the text above, the dikes presumably are related. Figure 6.7. Granodioritic phase of Saganaga batholith, containing many and varied inclusions. Large, lighter colored block in center of exposure is inferred to be an autolith of quartz-eye-bearing tonalite similar to the major phase of the batholith. STOP 6-5 Border phase Neoarchean Saganaga Tonalite with flattened inclusions and well-developed foliation in contact zone with Neoarchean metabasalt Location: T. 65 N., R. 4 W., sec. 22, SE, NE, SW; Roadcut on east side of Gunflint Trail (Fig. 6.5) Long Island Lake quadrangle UTM: 661,834E/5,329,257N Description: This location exposes the border phase of the Saganaga batholith, characterized by a granodioritic composition, general lack of quartz eyes, and an abundance of dioritic inclusions consisting of varied proportions of hornblende, pyroxene, biotite, plagioclase, and minor quartz (Fig. 6.8). The irregular ILSG07 158 Trip 6 �ovoid and discoid shape of inclusions is oriented subparallel to well developed, steeply dipping and easttrending foliation. Hornblende crystals and aggregates define a prominent lineation plunging shallowly to the east. Petrology indicates that much of this fabric appears magmatic, yet foliation may be a hybrid of approximately coaxial magmatic flow and regional tectonic deformation (D2). This is typical of the border zone of the intrusion against Archean metabasaltic country rocks, which presumably lie in the low ground just to the south. A preliminary comparative geochemical study, summarized in Fig. 6.3 above, indicates that the mafic inclusions are not partially assimilated, recrystallized, and tectonically deformed country rock volcanic xenoliths as implied by some earlier workers. They have the common chemical characteristics of the sanukitoid suite of Archean granitoid rocks. Thus, we infer that the inclusions studied are autoliths derived from a separate, primitive sanukitoid-magma. Figure 6.8. Granodioritic border phase of Saganaga Tonalite containing mafic inclusions (see discussion associated with Fig. 6.3). STOP 6-6 Contact zone of Neoarchean Saganaga Tonalite and metabasalt; unconformably overlain by Paleoproterozoic conglomerate of the Lower cherty member of Gunflint Iron Formation Location: 3 exposures, all in T. 65 N., R. 4 W., sec. 22, NW, SW, SE; Bush path off Gunflint Trail [individual UTM coordinates given below] (Fig. 6.5) Long Island Lake quadrangle ILSG07 159 Trip 6 �Description: 6-6a [UTM: 661,925E/5,329,065N] Archean metavolcanic rocks containing abundant granitic sheets and dikes, presumably related to border phases of Saganaga Tonalite (Fig. 6.9). The boundary between tonalite and metabasalt has been mapped in many places as a fault (Weiblen and others, 1971). These exposures do not preclude that possibility, but they imply that passive emplacement of the intrusion has also occurred, at least locally. Figure 6.9. Outcrop of border zone of Saganaga Tonalite. Dark gray material probably represents various phases of the intrusion; lighter gray, wedge-shaped area in the foreground is inferred to be metavolcanic country rock. 6-6b [UTM: 661,965E/5,329,062N] Conglomerate developed at the gently southward dipping unconformity between Neoarchean intrusive and metavolcanic rocks and the overlying basal part of the Paleoproterozoic Animikie Group. The unit, regionally known as the Kakabeka Conglomerate, is present only locally on the western end of the Gunflint. In most places, the Lower cherty member of ironformation lies directly on eroded Archean surfaces. This small outcrop is one of the few places where the conglomerate is exposed and accessible along the contact. The conglomerate is greenish gray, poorly bedded, and contains subangular to subrounded fragments of Saganaga Tonalite and related granitoid rocks, metabasalt, and quartz, in a granular siliceous matrix. 6-6c [UTM: 661,965E/5,329,037N] Walking southward from the basal conglomerate is a low "step-up" onto southward dipping strata of the Lower cherty member of Gunflint Iron Formation. ILSG07 160 Trip 6 �STOP 6-7 Stromatolitic cherty Gunflint Iron Formation Location: 3 exposures, all in T. 65 N., R. 4 W., sec. 22 [specific locations given below], Magnetic Trail (Fig. 6.5) Long Island Lake quadrangle CAUTION and ADVICE: This is a fairly long hike, approximately 1 mile round-trip; please be prepared with water and other field needs. Although this is not in the BWCAW, it does lie within Superior National Forest and is frequented by hikers. For this reason, and to preserve scientific value, please be respectful in matters of hammering and sampling. Description: 6-7a [Sec. 22 SW, SE; UTM: 662,034E/5,328,8895N] Thin-bedded, fine-grained, chert-amphibolemagnetite-bearing strata assigned to the upper part of the Lower slaty member of Gunflint Iron Formation. Beds strike ENE and dip generally less than 8 degrees southward. 6-7b [Sec. 22 SE, SE; UTM: 662,401E/5,329,093N] Near the edge of this large controlled burn area are several exposures of stromatolitic iron-formation—inferred to be stratigraphically near the base of the Upper cherty member of the Gunflint Iron Formation. Irregular domal and laminar forms are present. Some float blocks of basal conglomerate can be found here, presumably glacially transported southward from the basal unconformity (Fig. 6.10). Figure 6.10. Boulder of basal conglomeratic unit of Gunflint Iron Formation. ILSG07 161 Trip 6 �6-7c [Sec. 22 NE, SE; UTM: 662,583E/5,329,257N] Inferred to be in the Upper cherty member of Gunflint Iron Formation. Crest of ridge exposes abundant 3-dimensional views of stromatolites, intraformational conglomerate, and stromatolite "hash," all in a peloidal to ooidal, cherty grainstone matrix. Given the apparent mineralogic replacement and moderate metamorphic grade, little of the original carbon-based material is likely present. Despite this, examples of nearly all morphological forms of stromatolites can be found, including columnar, domal, and laminar (Fig. 6.11). Figure 6.11. Cherty, stromatolitic Gunflint Iron Formation. A=Oblique surface of small columnar stromatolites; B=Horizontal surface of irregular domal stromatolites; C=Vertical section of laminar stromatolites; D="Stromatolite hash." ILSG07 162 Trip 6 �STOP 6-8 Upper member Gunflint Iron Formation Location: 2 exposures in T. 65 N., R. 4 W., sec. 26 SW, NE; Gravel pit north of Gunflint Trail (Fig. 6.5) Long Island Lake quadrangle Description: 6-8a [UTM: 663,754E/5,328,212N] Exposed here is a dip-slope composed primarily of granular (cherty) and laminated (slaty) strata of the uppermost Gunflint Iron Formation. The slope defines the southern limb of a large, east-plunging anticline, outlined by interdigitated iron-formation and nearly concordant Logan sills (Fig. 6.5). The gentle dip of this limb illustrates the observation that low angle folding and moderate-relief topography are responsible for the complex map pattern here on the east side of the Lookout fault. The bedding surface is marked by what we tentatively infer to be syneresis cracks. The cracks, now filled with quartz, occur both concentrically and radially around a central, apparently raised core within a single granular layer of siliceous iron-formation (Fig. 6.12). Syneresis cracks are defined generally as shrinkage cracks formed by dewatering in a gel or colloidal suspension. They differ from septarian cracks that may develop in a similar way, in that the latter typically occur in concretions. Surprisingly diverse interpretations can be found in the literature about syneresis cracks (summarized in Pratt, 2001). There is, however, general agreement that they represent localized tensional failure during sediment dewatering. The explanation for localized semi-brittle response to what likely were formation-wide stresses—caused by compaction or vibration due to syn-sedimentary earthquakes—is more contentious. It has been ascribed variously to the localization of cements, locally increased pore pressure, or zones of granular sediment made coherent by "microbial glue." It is interesting to note that syneresis structures are more prevalent in Precambrian and Cambrian rocks than younger ones. This may be due in part to more uniform organic bonding of clays in younger strata, which reduced the occurrence of stress-localization. Figure 6.12. Syneresis cracks on eroded bedding surface of thinly bedded Gunflint Iron Formation. 6-8b [UTM: 663,637E/5,328,055N] Metamorphosed carbonate-rich iron-formation. This small exposure is likely an outcrop, based on the observation that carbonate strata are seen elsewhere in the uppermost part of the Gunflint, and it is properly positioned relative to stratigraphy as depicted on the geologic map (Morey and others, 1981). The rock appears to be a metamorphosed breccia containing fragments of thinly layered carbonate. Mineralogy is not well known, as the exposure was only recently discovered. Based on published studies (Floran and Papike, 1975) it may contain some combination of ferrohypersthene, fayalite (±quartz), grunerite-cummingtonite and perhaps garnet (Fig. 6.13). ILSG07 163 Trip 6 �Figure 6.13. Uppermost carbonate facies of Gunflint Iron Formation. Large, light-colored crystals may be grunerite-cummingtonite; dark spots appear to be garnet. STOP 6-9 Paleoproterozoic slate of the Rove Formation Location: T. 65 N., R. 4 W., sec. 25 NE, NE; Road cut on County Road #46 about 0.5 mi. N of Gunflint Trail (Fig. 6.5) Long Island Lake quadrangle UTM: 666,750E/5,328,753N Description: The small roadside outcrop exposes slate that is typical of the argillaceous parts of the Rove Formation. Bedding strikes eastward and dips gently to the south. ILSG07 164 Trip 6 �STOP 6-10 Mesoproterozoic Logan Intrusion Location: T. 65 N., R. 4 W., sec. 24 SE, SE; Bush trail off County Road #46 about 0.8 miles north of Gunflint Trail (Fig. 6.5) Long Island Lake quadrangle UTM: 665,674E/5,329,111N Description: Most of the Logan intrusions consist of medium-grained diabase, as seen along the trail leading to this outcrop. By contrast, this exposure—inferred to lie near the roof of the sill—consists of deformed and partially assimilated xenolithic fragments of Rove Formation slate and siltstone in white-weathering, feldspathic, locally porphyritic and granophyric rock (Fig. 6.14). Figure 6.14. Irregular border phase at upper part of one of the Logan Intrusions, containing irregular inclusions of Rove Formation slate country rock. ILSG07 165 Trip 6 �STOP 6-11 Mesoproterozoic Tuscarora intrusion of Duluth Complex—atypical border phase Location: T. 65 N., R. 4 W., sec. 27 SW, SE; Roadcut on Cross Lake road (CR#47) (Fig. 6.5) Long Island Lake quadrangle UTM: 662,074E/5,327,457N Description: A confusing exposure of the lower units (ttp, ttf) of the Tuscarora intrusion. The outcrop consists of intergranular to ophitic gabbro and augite troctolite, with pods and veinlets of coarse mafic pegmatite and shear bands containing sulfide mineralization. Spheroidal weathering produced "core stones" locally. STOP 6-12 Mesoproterozoic Tuscarora intrusion of Duluth Complex Location: T. 65 N., R. 3 W., sec. 30 SW, SE; roadcut on Gunflint Trail east of CR#50. (Fig. 6.5) Long Island Lake quadrangle UTM: 666,638E/5,327,433N Description: Just south of the parking pull-off is the rather poorly exposed intrusive contact between Rove Formation and units ttp and ttf of the Tuscorara Intrusion (Fig. 6.5 explanation; Morey and others, 1981). Unit ttf is a typical example of Cu-sulfide mineralized augite troctolite that is found at the base of the Duluth Complex here and in the Hoyt Lakes-Kawishiwi area to the southwest. It contains disseminated pyrite, pyrrhotite, and chalcopyrite. In the 1970's, International Nickel Company (INCO) drilled 7 holes in the basal Duluth Complex (Tuscarora and western Poplar Lake intrusions) to evaluate potential for Cu-Ni mining. All of these holes lie along the basal part of the intrusion within a few miles east and west of this stop. The archived drill cores were studied by Mogessie (1976) and Mogessie and others (1976). ILSG07 166 Trip 6 �REFERENCES Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N. J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology: 33:193196. Arth, J.G., and Hanson, G.N., 1975, Geochemistry and origin of the early Precambrian crust of northeastern Minnesota: Geochim. et Cosmo. Acta, 39:325-362, Table 2, p. 331-332. Boerboom, T.J., and Zartman, R.E., 1993, Geology, geochemistry, and geochronology of the central Giants Range batholith, northeastern Minnesota: Canadian Journal of Earth Sciences, 30:2510-2522 Broderick, T.M., 1920, Economic geology and stratigraphy in the Gunflint iron district, Minnesota: Economic Geology 15:422-452. Clout, J.M.F., and Simonson, B.M., 2005, Precambrian iron formation-hosted iron ore deposits: in Hedenquist, J.W., Thompson, J.F.H., Goldfarb, R.J., and Richards, J.P., Economic Geology 100th Anniversary Volume, p.643-679. Corfu, F., and Stott, G.M., 1998, Shebandowan greenstone belt, western Superior Province: U-Pb ages, tectonic implications, and correlations: Geological Society of America Bulletin 110:1467-1484 Didier, J., and Barbarin, B., 1991, Enclaves and granite petrology: Elsevier, 625p, reference to p. 549. Feirn, W.C., 1977, The geology of the early Precambrian rocks of the Jasper Lake area, Cook County, northeastern Minnesota: Unpublished MS thesis, University of Minnesota, Duluth, 146p. Floran, R.J., and Papike, J.J., 1975, Petrology of the low-grade rocks of the Gunflint iron-formation, OntarioMinnesota: GSA Bulletin 86:1169-1190. Floran, R.J., and Papike, J.J., 1978, Mineralogy and petrology of the Gunflint Iron Formation, Minnesota-Ontario: Journ. Petrology, 19:215-288. Fralick, P.W., Davis, D.W., and Kissin, S.A., 2002, The age of the Gunflint Formation, Ontario, Canada: single zircon U-Pb age determinations from reworked volcanic ask: Can. J. Earth Sci., 39:1085-1091. Goldich, S.S., Nier, A.O., Baadsgaard, H., Hoffman, J.H.M and Drueger, H.W., 1961, The Precambrian geology and geochronology of Minnesota: Minnesota Geological Survey Bulletin 41, p. 57. Grout, F.F., 1929, The Saganaga Granite of Minnesota-Ontario: Journal of Geology 37:562-591. Grout, F.F., 1933, Structural features of the Saganaga granite of Minnesota-Ontario: Report of XVI International Geological Congress, Washington, p. 255-270. Gruner, J.W., 1941, Structural geology of the Knife Lake area of northeastern Minnesota: Geological Society of America Bulletin 52:1577-1642. Hanson, G.N., 1972, Saganaga batholith: in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 102-107. Heaman, L.M., and Easton, R.M., 2005, Proterozoic history of the Lake Nipigon area, Ontario: Constraints from UPb zircon and baddeleyite dating: in Easton, M., and Hollings, P., eds., Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Proceedings and Abstracts, v. 51, part 1, p. 24-25. Jirsa, M.A., and Miller, J.D., Jr., 2004, Bedrock geology of the Ely and Basswood Lake (U.S. portion) 30’ x 60’ quadrangles, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-148, scale 1:100,000. Jirsa, M.A., and Miller, J.D., Jr., 2005, Geologic implications of bedrock mapping in the Ely and Basswood Lake quadrangles, northeast Minnesota: in Easton, M., and Hollings, P. (eds.), Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Proceedings and Abstracts, v. 51, Part 1, p. 28-29. Jirsa, M.A., and Miller, J.D., Jr., 2005, Classic Precambrian geology of northeast Minnesota: Field Trip 2 in Robinson, L., ed., Field trip guidebook for selected geology in Minnesota and Wisconsin: Minnesota Geological Survey Guidebook Series 21, p. 8-33. Jones, N.W., 1984, Petrology of some Logan sills, Cook County, Minnesota: Minnesota Geological Survey Report of Investigations 29, 40p. Lobatka, T.C., Papike, J.J., Vaniman, D.T., and Morey, G.B., 1981, Petrology of contact metamorphosed argillite from the Rove Formation, Gunflint Trail, Minnesota: Am. Mineralogist, 60:70-86. Mathez, E.A., Nathan, H.D., and Morey, G.B., 1977, Reconnaissance geology of the Hungry Jack Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Misc. Map Series M-39, scale 1:24,000. Miller, J.D., Jr., and Jerde, E.A., 2007, Poplar Lake intrusion: this volume. ILSG07 167 Trip 6 �Miller, J.D., Jr., Morey, G.B., and Weiblen, P.W., 1987, Seagull Lake-Gunflint Lake area: A classical Precambrian stratigraphic sequence in northeastern Minnesota: in Biggs, D.L. ed., North-Central Section of the Geological Society of America Centennial Field Guide Volume 3, p.47-52. MillerJ.D., Jr., and Severson, M.J., 2002, Geology of the Duluth Complex: in Miller, J.D., Jr., Green, J.C., Severson, J.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, p. 106-143. Mogessie, A., 1976 Petrologic study of copper-nickel mineralization in the Tuscarora intrusion Duluth Complex, northeastern Minnesota: M.S. thesis, University of Minnesota, Twin Cities, 137p. Mogessie, A., Stumpfl, E.F., and Weiblen, P.W., 1976, The role of fluids in the formation of platinum-group minerals, Duluth Complex, Minnesota: Mineralogic, textural, and chemical evidence: Econ. Geol. 86:15061518. Morey, G.B., 1969, The geology of the Middle Precambrian Rove Formation in northeastern Minnesota: MGS Special Publication 7, 62p. Morey, G.B., 1972, Gunflint Range, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 218-225. Morey, G.B., and Nathan, H.D., 1978, Geologic map of the Gunflint Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Misc. Map M-42, scale 1:24,000. Morey, G.B., and Nathan, H.D., 1977, Reconnaissance geologic map of the South Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Misc. Map M-38, scale 1:24,000. Morey, G.B., and Southwick, D.L., 1995, Allostratigraphic relationships of Early Proterozoic iron-formations in the Lake Superior region: Econ. Geol. 90:1983-1993. Morey, G.B., Weiblen, P.W., Papike, J.J., and Anderson, D.H., 1981, Geologic map of the Long Island Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Misc. Map M-46, scale 1:24,000. O'Brien, H.E., 1982, Petrology, geochemistry, and magnetics of a lamprophyre dike intruded into the Saganaga Tonalite batholith, northeastern Minnesota: Senior Thesis, University of Minnesota, Twin Cities, 59p. Peterson, D.P., Gallup, C., Jirsa, M.A., and Davis, D.W., 2001, Correlation of Archean assemblages across the U.S.Canadian border: Phase I geochronology, (abstract): Institute on Lake Superior Geology Proceedings, 47th Annual Meeting, Madison, WI, 2001, Part 1, p.77-78. Pratt, B.R., 2001, Syneresis cracks: subaqueous shrinkage in argillaceous sediments caused by earthquake-induced dewatering: Sedimentary Geol., 117:1-10. Pufahl, P.K., and Fralick, P.W., 2000, Depositional environments of the Paleoproterozoic Gunflint Formation: in Institute on Lake Superior Geology Proceedings, 46th Annual Meeting, Thunder Bay, Ontario, Part 2 Field Trip Guidebook, v. 51. Pufahl, P.K., and Fralick, P.W., 2004, Depositional controls on Paleoproterozoic iron formation accumulation, Gogebic Range, Lake Superior region, USA: Sedimentology 51:791-808. Schulz, K.J., and Cannon, W.F., 2007 (in press), The Penokean Orogeny in the Lake Superior region: Precambrian Research. Stern, R.A., and Hanson, G.N., 1991, Archean high-Mg granodiorite: A derivative of light rare earth elementenriched monzodiorite of mantle origin: J. Petrol. 32:201-238, Table 2, p. 220. Stern, R.A., Hanson, G.N., and Shirey, S.B., 1989, Petrogenesis of mantle-derived, LILE-enriched Archean monzodiorites and trachyandesites (sanukitoids) in southwestern Superior Province: Can. J. Earth Sci., 26:1688-1712; Table 1 p. 1695. Vervoort, J.D., 1987, Petrology and geochemistry of the Archean of the JAP Lake area, northeastern Minnesota: M.S. Thesis, University of Minnesota-Duluth, 193 p. Weiblen, P.W., Morey, G.B., and Mudrey, M.G., 1971, Guide to the Precambrian rocks of northwestern Cook County as exposed along the Gunflint Trail: in Davidson, D.M., Darby, D.G., Green, J.C., and Grant, J.A., eds., Technical Sessions, Abstracts and Field Guides, 17th Annual Institute on Lake Superior Geology, Annual meeting, Duluth, Minnesota, p.97-127. Wirth, K.R., Vervoort, J., Craddock, J.P., Davidson, C., Finley-Blasi, L., Kerber, L., Lundquist, R., Vorhies, S., and Walker, E., 2006, Source rock ages and patterns of sedimentation in the Lake Superior Region: Results of preliminary U-Bp detrital zircon studies: in Wilson, A.C., (ed.), Proceedings and Abstracts, Institute on Lake Superior Geology Proceedings, 52nd Annual Meeting, Sault Ste Marie, Ontario, v. 51, Part 1, p. 69-71. ILSG07 168 Trip 6 � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2007 Description An account of the resource Institute on Lake Superior Geology. Lutsen, Minnesota. May 8-13, 2007. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2007 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/749c5c6ee43eb071b1c6eea81168ff01.pdf 7cea60901a11884b9ca5c94d5c9eda94 PDF Text Text Institute on Lake Superior Geology Special Publication #1 Field Trip Guidebook for the Slate Islands, Ontario Pete Hollings, Mark Smyk, Bill Addison & Phil Fralick �Institute on Lake Superior Geology Special Publication #1 Field Trip Guidebook for the Slate Islands, Ontario Pete Hollings Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Mark Smyk Resident Geologist’s Office, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada Bill Addison R.R. 2, Kakabeka Falls, Ontario, P0T 1W0, Canada Phil Fralick Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Cover Photos: Top - Shatter cone clast in impact breccia; Middle - Interflow sandstone unit in Paleoproterozoic basalts; Bottom - West coast of Patterson Island. �Institute on Lake Superior Geology Special Publication #1 Field Trip Guidebook for the Slate Islands, Ontario Reference to material in this volume should follow the example below: Hollings, P., Smyk, M., Addison, B. and Fralick, P., 2006. Field trip guidebook for the Slate Islands. Institute on Lake Superior Geology, Special Publication 1, p. 21. Published by the Institute on Lake Superior Geology and distributed by the ILSG Secretary: Pete Hollings - ILSG Secretary Department of Geology Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Email: peter.hollings@lakeheadu.ca ILSG website: www.lakesuperiorgeology.org ISSN 1042-9964 �ILSG Special Publication #1 - The Slate Islands Table of Contents Introduction..........................................................................................................................1 Safety Considerations..........................................................................................................1 Acknowledgements..............................................................................................................1 Regional Geology................................................................................................................2 One Archipelago, Two Possible Origins..............................................................................5 The Case for an Extraterrestrial Impact Origin for the Slate Islands Structure.........5 The Case for a Cryptoexplosion Origin for the Slate Islands Structure...................10 The Debate...............................................................................................................11 Economic Geology.............................................................................................................12 Stops...................................................................................................................................13 Stop A – “Honeymoon Bay” near Cove Island........................................................13 Stop B – Sunday Harbour.........................................................................................14 Stop C – Horace Cove..............................................................................................15 Stop D – Western shore of Patterson Island.............................................................16 Stop E – McGreevy Harbour....................................................................................18 References..........................................................................................................................20 -i- �ILSG Special Publication #1 - The Slate Islands Introduction Safety Considerations This volume is intended to serve not only as a guide for participants during the August 2006 field trip to the Slate Islands, but also as a reference for those planning to revisit the area at a later date. Consequently we have included UTM coordinates (NAD 83 datum) for stops. The Slate Islands are covered by southern boreal forest with some shoreline arctic-alpine disjunct flora and is protected as a Natural Environment Provincial Park with no visitor facilities. Rock collecting and sampling is prohibited throughout the entire archipelago unless a permit is first obtained from the Ministry of Natural Resources: Slate Islands Provincial Park Ministry of Natural Resources P.O. Box 970 Nipigon, ON P0T 2J0 A field trip to the Slate Islands creates a number of unique safety issues. Please exercise caution when getting in and out of the boats, as the outcrops are often sharp and extremely slippery. Personal flotation devices should be worn in the boats at all times. If you are planning to revisit these sites please be very careful. Lake Superior is a cold, dangerous lake; waves can often be metres high and even in mid-summer fog can appear very quickly. A GPS system, compass and maps should be utilized. We strongly encourage you to charter a large boat from the mainland rather than trying to make the trip to the islands yourself. Phone: (807) 825-3403 This is the first time a publication has been produced for a field trip that is not directly associated with an ILSG Annual Meeting. However, the location of the Slate Islands dictates that field trips to the islands are best made later in the summer when weather and lake conditions are more conducive to travel. Acknowledgements We would like to thank all those who provided comments on this guide and assisted with the running of the field trips, particularly Doug Caldwell and John Scott. Woodland Caribou on the Slate Islands -1- �ILSG Special Publication #1 - The Slate Islands Regional Geology The Slate Islands comprise a 7 km-wide archipelago of 17 islands located in northern Lake Superior approximately 12 km southeast of Terrace Bay (Fig. 1). The geology of the islands has been mapped by Coleman (1901), Parsons (1918) and by Sage (1975, 1991). The islands comprise both Archean and Proterozoic rocks. The Archean rocks are part of the Schreiber-Hemlo greenstone belt (Wawa Subprovince). Paleoproterozoic sequences include the Gunflint and Rove Formations of the Animikie Group. Mesoproterozoic Keweenawan basalts are interpreted to be an extension of the Osler Group of the Midcontinent Rift (Sage, 1991). Sage (1978, 1991) mapped greenschist facies Archean metavolcanic rocks and subvolcanic intrusive rocks ranging in composition from calc-alkaline dacite to tholeiitic basalt. The Archean supracrustal rocks consist of coarse felsic pyroclastic units, felsic to mafic tuffs, feldspar-phyric flows and amygdaloidal, pillowed and variolitic mafic flows with thin interbeds of argillite and siltstone (Fig. 2; Sage, 1991). On the basis of pillow facing directions Sage proposed that an antlicinal structure crosses the centre of Mortimer Island. The pillowed flows are most common on Mortimer and Delaute islands (Fig. 2); pillows are typically bun- to mattress-shaped and up to 2m across (Sage, 1991). In places on Mortimer Island the massive and pillowed basalts grade into flow breccias. Volcanic and intrusive rocks of more felsic compositions are found on Patterson, Dupuis, Spar and Leadman islands (Fig. 2) and have been interpreted by Sage (1991) to be highly sheared, amygdaloidal and porphyritic carbonatised sequences. Archean metasedimentary rocks are relatively rare and are predominantly volcaniclastic as they appear to interfinger with the volcanic flows (Sage, 1991). The Archean mafic volcanic rocks from the Slate Islands are can be subdivided into two distinct geochemical suites. One suite is characterized by flat primitive mantle-normalized patterns typical of tholeiitic rocks found in modern oceanic plateaus whereas the second, more abundant, suite is characterized by weakly LREE-enriched patterns with minor negative Nb anomalies, characteristic of rocks formed in an island arc setting (Fig. 3; P. Hollings, unpublished data). Similar assemblages have been reported in the Schreiber-Hemlo greenstone belt (Polat et al., 1998). Figure 1. Map showing the location of the Slate Islands. -2- �Figure 2. Geological map of the Slate Islands. Modified after Sage (1991). -3- 1 km Patterson Island Mortimer Island D Horace Cove C Edmonds Island McColl Island 87°00’ Sunday Harbour B McGreevy E Harbour Bowes Island 87°00’ A1 Dupuis Island Delaute Island A2 Cove Island Field trip stop 84°40’ Mafic volcanic rocks Felsic volcanic rocks Metasedimentary rocks Mafic intrusive rocks Felsic intrusive rocks Archean Animikie group Osler group Diabase dikes Breccias Post-Archean N ILSG Special Publication #1 - The Slate Islands 84°40’ �ILSG Special Publication #1 - The Slate Islands Figure 3. Primitive mantle normalised diagram showing representative samples of the two geochemical suites recognised amongst the Archean mafic volcanic rocks of the Slate Islands. ST33 = arc-type, ST38 = plateau-type. Normalising values from Sun and McDonough (1989). On the western shore of Patterson Island, Sage (1991) reported an ~20m thickness of iron formation of the Gunflint Formation lying unconformably on the Archean basement and below Mesoproterozoic Keweenawan basalts (Fig. 2). The lowermost three metres of the sequence consists of interbedded jaspilitic chert, hematite and carbonate overlain by a sequence of hematitic chert. The argillites are generally massive and only locally display well-developed bedding. Recent re-examination of outcrops on eastern Mortimer Island and Delaute Island mapped as Paleoproterozoic Rove Formation clastic sedimentary rocks, has resulted in them being reinterpreted as Archean metasedimentary rocks analogous to the McKellar Harbour turbidite sequence on the mainland and this is now reflected on Figure 2. Keweenawan basalts unconformably overlie the Gunflint rocks (Fig. 2) and form an ~120m thick flow sequence that dips ~80° at its base and diminishing to ~25° towards its top (Sage, 1991). This implies some degree of block rotation. Twenty-two individual flows can be recognized within the upper portion of the sequence. Interflow contacts are typically sharp and often marked by thin interflow sedimentary units (Sage, 1991). The basalts are typically vesicular and amygdaloidal, and in places show poorly developed ropy flow tops. The feldspar and pyroxene-phyric basalt flows are incipiently to completely altered, to sericite, carbonate and calcite (P. Hollings, unpublished data). However, even the relatively unaltered samples are significantly more altered than Osler basalts in the vicinity of Rossport (Hollings et al., 2006). Red, medium-grained, well-sorted, arkosic sandstone interflow units consist of sub-rounded to sub-angular grains of predominantly quartz, plagioclase, Kfeldspar, volcanic rock fragments and amphibole. The feldspars, especially the K-feldspar, are commonly intensely weathered (seriticised). Most grains have very fine-grained hematitic coatings. The sandstone is relatively matrix-poor with an earlier phase of radiating chalcedony and quartz fans to drusy cement overgrown by a later stage of blocky carbonate, void-filling cements. Halls (1974) proposed that the paleomagnetic signature of the basalts was comparable to the lower portions of the Osler volcanic group in northwestern Lake Superior. The aforementioned rocks are also intruded by a number of Keweenawan dikes and breccia bodies which commonly occupy and obscure lithologic contacts (Sage, 1991). Hinze et al. (1966), on the basis of aeromagnetic data, interpreted the presence of two major faults, which intersected to the south of the Slate Islands (Fig. 4). Sage (1991) has proposed that the onshore extension of the Figure 4. Faults of eastern Lake Superior with inferred directions of movement from Sage (1991). Modified from Hinze et al. (1966). -4- �ILSG Special Publication #1 - The Slate Islands northeast-trending Big Bay-Ashburton Bay Fault may be related to northeast-trending structures associated with the Mesoproterozoic Midcontinent Rift-related alkalic carbonatitic complexes at Deadhorse Creek and Prairie Lake. Sage (1991) reported that, in the vicinity of Patterson Island, breccia dikes cut and enclose blocks of lamprophyre with carbonatite affinity that have yielded a K-Ar age of ~300 Ma. Recently this unit has been dated using the U-Pb method, yielding a Keweenawan age of ~1100 Ma (L. Heaman, University of Alberta, personal communication, 1994, referenced in Dressler et al., 1999) suggesting the young K-Ar ages are likely the result of resetting. The breccias have been used to both argue for and against a meteor impact theory for the Slate Islands and are discussed further below. One Archipelago, Two Possible Origins In addition to the complex bedrock geology, the islands have also been the focus of interest and debate because they are considered by some to represent the “best-preserved, medium-sized, meteor impact structure on Earth” (V. Sharpton, Lunar and Planetary Institute, NASA, pers. comm. 1995). However, this theory is not universally accepted and Sage (1991, 1999) has proposed an endogenous cryptoexplosion process for formation of the islands. The Case for an Extraterrestrial Impact Origin for the Slate Islands Structure Before discussing the evidence for an impact, it is worth outlining the basic dynamics of a hypervelocity extraterrestrial impact. The continuous process that occurs during an impact is more readily understood if it is dealt with in stages (summarized by French, 1998, and outlined in particular for the Slate Islands by Dressler et al., 1998). 1) Contact/Compression Phase. As the impactor hits, hypervelocity shock waves are generated in both the impactor and the target rocks which forces the bedrock downwards and outwards, instantaneously vapourizing the impactor and the target rock near the point of impact, while further away the target melts as the shock pressures attenuate. 2) Decompression/Excavation Phase. The shock wave is immediately followed by a rarefaction or tensional wave, decompressing the remaining rock and allowing it to relax, opening fractures large and small, driving material downward, outward and upward, excavating an extremely short-lived, steep-sided, unstable transient crater. 3) Central Peak Formation. If the impactor and consequent forces are large enough, the unloading of deep bedrock by the removal of overlying rock plus the decompression following the shock wave, results in material in the bottom of the crater rebounding upward into a central peak within the transient crater (as when a drop of water hits the calm surface of a pond). 4) Transient Crater Collapse and Formation of the Final Crater. As the transient crater reaches its maximum size, the fractured and faulted oversteepened walls begin collapsing into the crater in a rush, meeting and mixing with the likewise collapsing central uplift, before settling into an approximation of the final crater form. The entire process from first contact by the impactor to this stage has not lasted much more than 5-10 minutes in most craters, perhaps 15 minutes in the very largest craters a couple of hundred of kilometres in diameter. 5) Long Term Adjustment. Then begins a long process of adjustment, lasting decades to many millennia, depending on many things, but primarily crater size. During this final stage, loose debris continues settling, aided by tremors as stresses are released, hydrothermal activity begins in medium-sized to large craters, cooling continues, and finally, consolidation and lithification of breccias takes place. A number of authors have proposed that the Slate Islands have preserved the site of a meteor impact (Halls, 1975, 1976; Robertson and Grieve, 1976; Halls and Grieve, 1976; Halls and Stesky, 1978; Dressler et al., 1995, 1998, 1999). The islands themselves have been identified as the central uplift of a mediumsized impact structure, which, bathymetry suggests, is surrounded by a submerged annular trough ringed by a ridge 30 to 32 km in diameter (Halls and Grieve, 1976; Dressler et al., 1995), representing the suggested final crater diameter. A crater this size implies an ~1.5 km diameter impactor with an arrival velocity of ~ 15 km/s. This circular feature was also transected and confirmed by the Great Lakes International Multidisciplinary Program of Crustal Evolution (GLIMPCE) seismic reflection line (Fig. 5; Mariano and Hinze, 1994). According to Dressler et al. (1998) almost all the rocks of the archipelago are somewhat brecciated and -5- �ILSG Special Publication #1 - The Slate Islands Figure 5. Reprocessed northern part of GLIMPCE Line A (courtesy of B. Milkereit, Geological Survey of Canada, 1994, published in Dressler et al., 1999). Left vertical axis is seconds of two-way time, right vertical axis is approximate depth in kiometres and horizontal axis shows shot points. X is the westward projection of the approximate centre of the central uplift. The distance from the centre of the central uplift (approximate geographic centre of the archipelago) to R is 15-16 km. R lies approximately where the rim of the structure is placed based on bathymetry. The strong reflections at 0.5 s may represent arenites of the Jacobsville Formation and not multiple reflections of the lake bottom which is at relatively shallow depth in the area investigated here. AB: Keweenawan basalt; BC: Jacobsville Formation. From Dressler et al. (1999). they propose that the bedrock can be considered a megabreccia, although the detailed mapping of Sage (1991) showed good structural coherence across the islands. For Sage (1999) this structural coherence between the blocks and with rocks on the mainland argues for an endogenous origin for the breccias on the islands. All local rocks have been intruded by a network of anastamosing breccia bodies ranging in colour from brick-red to greenish grey. The breccias consist of sharply angular to sub-rounded fragments up to four metres across derived from local Precambrian rocks. The breccias have been ascribed to both endogenous intrusive activity (e.g., Sage, 1991) and meteor impact (e.g., Sharpton et al., 1996) and have been used to argue both for and against the impact theory on the Slate Islands. The age of the Slate Islands structures and breccia bodies is poorly constrained (Table 1). Grieve et al. (1995) proposed an age of <350 Ma based on similarities in the erosional level between the Slate Islands and the ~350 Ma Charlevoix structure in Quebec. Sharpton et al. (1996) have proposed an age of 500-800 Ma based on the presence of clasts of the Mesoproterozoic Jacobsville sandstone (southern shore of Lake Superior, Michigan) and absence of any Devonian or Ordovician carbonates. However, the Slate Islands sandstone clasts are similar to sandstone interflow units found within the Osler basalts on the western shore of Patterson Island and, thus, may not be Jacobsville sandstone. More recent Ar-Ar age determinations on impact-generated pseuodotachylites have yielded spectra consistent with an age of ~450 Ma (Fig. 6; Sharpton et al., 1997; Dressler et al., 1999). Features that have been used in support of an impact event include dikes of clastic-matrix breccia Figure 6. 40Ar-39Ar release spectra. Samples 95SL103 and 95SL13:3e: dark gray, inclusion bearing “impact melts” (Keweenawan basalt). Sample 94B1D2, inclusion-poor pseudotachylite. From Dressler et al. (1999). -6- �ILSG Special Publication #1 - The Slate Islands Table 1. Stratigraphic age constraints on the Slate Islands impact. The age of the Jacobsville is ~1100 Ma, however these clasts may be interflow sandstones from within the Osler-like volcanic flows (see text). From Dressler et al. (1999). (Halls and Grieve, 1976; Sage, 1991), which include; pseudotachylites, polymictic allogenic breccias and monomictic autoclastic breccias (Sharpton et al., 1996) concentrated on the eastern shore of Patterson Island as well as Mortimer, Dupuis and Delaute islands. Shatter cones occur throughout the islands but are most obvious in the Keweenawan basalts. They are interpreted to have formed from the passage of a high-pressure shock wave (Dietz, 1964). They are characterized by a surface decorated with linear ridges and grooves (horsetail striations) that radiate from the apex of the cone. On the Slate Islands most shatter cones range from 2cm to ~30cm long; those in the Keweenawan rocks are 10 to 30 cm long (Sharpton et al., 1996). In addition Sharpton et al. (1996) reported a number of “mega cones” at least 10m long (and possibly up to 20m) in McGreevy Harbour (Fig. 2). As with the breccias the origin of the cones themselves remains controversial (Sharpton et al., 1996). Dressler et al. (1999) has suggested that the shatter cones formed during the compressional phase of the impact (Fig. 7) and indicate a minimum shock pressure in the target rocks of 3 GPa. Sage (1991) observed that shatter cones were most extensive close to breccia outcrops and used this to argue that the explosive emplacement of diatreme dikes was responsible for their formation. However, more detailed work (Sharpton et al., 1996) indicated that the shatter cones are ubiquitous on the islands. Dressler et al. (1995, 1999) have reinterpreted Figure 7. Formation of the Slate Islands impact structure. A) preimpact target; B) contact and compression; C) excavation; D) central uplift; E) central uplift collapse and modification; F) final structure; G) present structure, black areas indicate impact melt overlain by allogenic breccias (assumed, not shown in DF). a, Proterozoic and younger supracrustal rocks: Deformed Archean greenstone assemblage (assumed in annular trough); b, Mafic metavolcanics, minor metasediments and intrusive rocks; c, Intermediate and felsic metavolcanics, minor metasediments and intrusive rocks. From Dressler et al. (1999). -7- �ILSG Special Publication #1 - The Slate Islands Sage’s diatremes as impact breccia bodies (e.g., Bunte breccia, suevite). Microscopic planar deformation features (PDFs) in quartz and feldspar have been observed in rocks from the Slate Islands by Halls and Grieve (1976), Sage (1991) and Dressler et al. (1994). These planar lamellae are shock induced micro-melt zones <2-3 μm wide along crystallographic axes. They first appear in quartz at pressures of ~8 GPa along the {0001} and {1011} axes and at ~10 GPa they begin to appear along the {1013} axis (French, 1998). PDFs are considered diagnostic of the extremely intense shock waves produced during hypervelocity impacts. However, a single set of PDFs can easily be confused with Bohm lamellae and other planar features, and thus, two or more criss-crossing sets of PDFs along different measured crystallographic axes are the preferred diagnostic features. Crisscrossing sets of PDFs are seen both in the Slate Islands host rocks and in breccia components. In a detailed study of these features Dressler et al. (1998) showed a zone of maximum shock intensity on Patterson Island (Fig. 8), suggesting the location of the impact was slightly west of the center of Patterson Island. types cross-cut other breccias, plus features such as PDFs, allowed them to hypothesize when various features formed during the impact process. It is within this context that Dressler and Sharpton (1997) place their interpretations of the breccias (Figs. 7, 8 & 9; Table 2). The breccias identified by the authors include: A detailed study of the breccias on the Slate Islands has been undertaken by Dressler and Sharpton (1997) who have estimated that breccias make up ~15 to 25% of the Islands’ rocks. Interpretation of which breccia • Pseudotachylites which are thought to have formed as a result of brittle-or brittle-ductile seismic faulting and instantaneous melting due to the passage of the hypersonic shock wave during the compressional phase of the impact event. Pseudotachylites are relatively rare in the archipelago and occur as small veins and dikes. The early formation of these pseudotachylites is supported by the presence of clasts of pseudotachylite in the breccias. • Polymictic clastic matrix breccias are the most abundant breccia type on the islands but are more common on Patterson Island than on the outlying islands. The breccias contain a wide variety of clasts from all host lithologies, that are angular to sub-rounded, and range in size from <1mm to several metres. These are interpreted to have formed when decompression allowed opening of fractures within the crater walls and floor (Dressler et al., 1999) excavating the crater to a depth of ~1.5 km in approximately 1 minute. Figure 8. Sketch map of Slate Islands impact structure, located in northern Lake Superior. Dashed lines show concentric trends of coast lines and structural elements indicating crater center on western side of Patterson Island (approximate location is shown by cross). Previous estimates of crater center, based on shatter cone orientations (Stesky and Halls, 1983) or shock isobars deduced from planar deformation features in quartz (Grieve and Robertson, 1976), are shown as filled circles. Shatter-coned outcrops are shown as small unfilled circles. Filled diamond shows location of >10m shatter cone. Map is adapted from Sharpton et al. (1996). Shaded field is the area of highest shock values from Dressler et al. (1998). -8- �ILSG Special Publication #1 - The Slate Islands Table 2. Slate Islands impact breccias. From Dressler and Sharpton (1997). Figure 9. Section across the Slate Islands complex impact structure showing distribution of breccias investigated. Minor polymictic clastic matrix breccias are also present further away from the centre of the structure than shown here. Profile is based on bathymetric information from around the archipelago and on topographic maps of the islands. From Dressler and Sharpton (1997). -9- �ILSG Special Publication #1 - The Slate Islands The presence of Proterozoic clasts in breccias dominated by Archean material has been used to argue for downward movement and mixing of clasts over distances possibly as much as 5 km (Dressler and Sharpton, 1997). • Allogenic breccia deposits containing altered glass fragments (suevites) or with no glass fragments (Bunte breccia) are present on Dupuis and Patterson islands and have been used to argue for a shallower erosion level for the archipelago (Sharpton and Dressler, 1996). The Bunte breccias are interpreted to have formed either as fall-back deposits in the crater or as ground-surge deposits. The breccias contain mainly Proterozoic clasts, supporting their origin as fall-back deposits (Dressler et al., 1999). Dressler et al. (1999) have also reported the presence of suevite breccias and use the absence of aerodynamically shaped glass fragments to argue that they are also fall-back breccias. • Monomictic, autochthonous breccias are found on Mortimer Island and a number of the small outlying islands. The breccias comprise angular, densely packed fragments typically up to 20 cm in size within a matrix of similar clastic rock powder. These are interpreted to have formed late in the impact process during the crater modification phase (Fig. 7) as huge blocks of rock slumping off the transient crater walls ground together during their slide into the crater over several minutes (Dressler et al., 1999). The breccias are often autoclastic with transitional borders with their host rocks (Dressler and Sharpton, 1997). Halls and Grieve (1976) and Grieve and Robertson (1976) proposed that the Slate Islands represented uplifted basement that preserved breccias injected during impact into the crater subfloor (~0.5 to 1.5 km below the central peak). However, Sharpton et al. (1996) have suggested that the allogenic and autoclastic breccias indicate that the present exposure surface is only a few hundred metres below the original ground surface. This issue is not without controversy and is discussed further in Halls (1997), Grieve and Robertson (1997) and Sharpton and Dressler (1997). cone orientations and shock barometry (Stesky and Halls, 1983; Grieve and Roberston, 1976) suggested that it was closer to the center of Patterson Island (Fig. 8). However, given that both locations are within 1.5 km of each other and given that the impacting body was estimated to have a diameter of ~1.5 km, none of the proposed locations should be considered definitive (Sharpton et al., 1997). The Case for a Cryptoexplosion Origin for the Slate Islands Structure The title of Sage’s (1999) paper clearly stated his case: “The Slate Islands: A Uniquely Sited Cryptoexplosion Structure”. He noted that the Slate Islands are situated on or near the Proterozoic-Archean boundary and at the intersection of two major inferred faults, the Big Bay-Ashburton Bay Fault (or accommodation zone) and the Michipicoten Fault. He also noted that the Slate Islands lie on the flank of the Midcontinent Rift where crustal thickness reaches 50 km or more, and on a topographic ridge, extending southwest to Superior Shoals and northeast to the mainland, which bisects this thick crust. Perhaps most importantly to Sage, the Slate Islands are close to the Port Coldwell Alkalic Rock Complex, the Kilalla Lake Alkalic Rock Complex, Prairie Lake Carbonatite, Deadhorse Creek Diatremes and McKellar Creek Diatremes (Sage, 1999), all of Keweenawan age. Sage (1991) argued that “the possibility of a meteorite impact at this precise location – on a ridge traversing the Lake Superior Basin, on the nose of an Archean fold structure, at the precise location of the Proterozoic-Archean contact, at the precise location of two intersecting regional faults, and at the precise location of highly volatile alkalic magmatism – is too incredible to accept (Sage, 1991, p.56)”. Sage (1978) presented a number of geological observations favouring a non-impact origin many of which were elaborated upon in Sage (1991). These included: Current interpretations based on topographic and structural trends place the crater center in the westcentral part of Patterson Island (Fig. 8; Sharpton et al., 1996) whereas earlier interpretations based on shatter - 10 - • Clast size sorting in the diatremes from finegrained at the margins to coarse-grained at the center is typical of laminar flow (Sage, 1978) and more likely to occur in a diatreme than by downward intrusion. • Orientation of shatter cones was not consistent with a central impact structure. • Contact metamorphic effects between the breccia and alkalic diabase indicates that hydrothermal �ILSG Special Publication #1 - The Slate Islands activity accompanied breccia emplacement. • The presence or absence of an igneous matrix in breccia dikes on the islands and the mainland does not preclude an igneous origin. The Debate The debate – cryptoexplosion vs impact – over the origin of the Slate Islands structure largely mirrors (in a more genteel way) the vigorous debate which began when Dietz (1964) proposed that the Sudbury Structure was due to an impact. When Alvarez et al. (1980) proposed that the K/T extinction was caused by an impact, the debate became a nasty scientific controversy. Now, some 26 years later, the debates over Sudbury and the Chicxulub-K/T extinction crater are resolved in favour of an impact origin for both. The debate is summarized by Powell (1998) in an interesting, very readable popular book, “Night Comes to the Cretaceous”. To reiterate Sage’s statement, “the possibility of a meteorite impact at this precise location – on a ridge traversing the Lake Superior Basin, on the nose of an Archean fold structure, at the precise location of the Proterozoic-Archean contact, at the precise location of two intersecting regional faults, and at the precise location of highly volatile alkalic magmatism – is too incredible to accept” (Sage, 1991, p.56). Halls (1979) counters that the absence of complex overlapping shatter cone sets argues against the multiple emplacement events proposed by Sage (1978). On a larger scale Halls (1978) argued that the regional faults proposed by Sage (1978) are only inferred from geophysical data and the magnetic anomalies may also delineate the unfaulted margin of the Keweenawan basin. Halls (1979) also provided alternative explanations for the apparent coincidences suggested by Sage, observing that the lower and upper Precambrian contact predates the shock event and cannot be used to argue either for or against. The Slate Islands debate centres mainly on the interpretation of three sets of features: the breccias, shatter cones, and planar deformation features (PDFs). Sage (1991) has proposed that the breccias originated from the forcible emplacement of volatilerich magmas formed at depths > 35 km which have risen to a shallower level and exsolved a gas phase. The higher volatile contents of clasts and matrix have been argued to support this model. However, arguing against an origin at depths of ~35 km is the absence of deep-seated or magmatic material in the breccias (Robertson and Grieve, 1979). Another breccia problem is explaining how dikes containing upper level Paleoproterozoic fragments were emplaced into lower level Archean rock. Halls and Grieve (1976) were the first to suggest a downward injection of breccias into fractures (during the crater modification stage following the passage of the initial shock wave) as a result of an impact event (Robertson and Grieve, 1979). However, Sage (1991) has countered that the presence of stratigraphically high level clasts at depth could also be explained by collapsing fluid columns after the emplacement of diatremes. Today, the various breccia types (pseudotachylites, polymictic, allogenic, and monomictic autochthonous), their relationships within each other, and their locations within the Slate Islands structure seem to be best explained by their production during various phases of the impact process (Sharpton and Dressler, 1997; Dressler and Sharpton, 1997; Dressler et al., 1999). Sage (1991) has also advocated that the forceful emplacement of diatremes formed at depths > 35 km could account for the shock features – shatter cones and PDFs – preserved on the islands. Sage (1999) provides a number of examples of other occurrences of planar deformation lamellae and shock textures that may have been produced by kimberlite emplacement, however, he acknowledges that these features have also been interpreted as having been formed by impact events. Robertson and Grieve (1979) observed that shatter cone formation is a function of lithology as well as shock pressure and the fissile Archean metavolcanics would display more poorly developed cones than the structurally isotropic Keweenawan flows. The distribution of microscopic shock effects has been recorded by Grieve and Robertson (1976) who showed that the intensity of these features increase in a consistent fashion from the coast inward to the proposed impact centre (see Fig. 8). Roberston and Grieve (1979) also argued that diatreme emplacement is normally considered to be a process of “drilling and venting by gas streaming” rather than by violent explosions and that the pressures induced by this process are unlikely to exceed 1.5 GPa, whereas pressures of 2-6 GPa are required to generate shatter cones (French, 1998). Halls (1979) took issue with Sage’s measurements of shatter cones suggesting that he did not use the correct measurement procedure and did not properly correct - 11 - �ILSG Special Publication #1 - The Slate Islands his data and that stated that there is no convincing spatial correlation between shatter cones and breccia dikes, and furthermore, the breccias contain shatterconed clasts (Halls and Grieve, 1976). sampled by Resident Geologist staff in 1995 near Cove Island returned 604 ppm Zn, <100 ppm Cu and 157 ppm Pb (Resident Geologist’s Files, Thunder Bay South District, Thunder Bay). PDFs seem beyond debate, so long as they are really PDFs (and not Böhm lamellae), based upon measured widths and spacing of lines and particularly based upon their alignment along measured crystallographic axes. French (1998) has summarized a large body of laboratory experimental evidence and field evidence on PDFs, as do Dressler et al. (1998). Dressler et al. (1998) measured a large number of PDF orientations in Slate Islands quartz crystals. They found PDFs aligned along many different axes, notably the {1013} and {1012} axes, indicative of shock pressures as high as 18 GPa, pressures equivalent to those many hundreds of kilometers depth within Earth. Such observations cannot be explained by diatremes. A little discussed problem for the impact hypothesis is the apparent absence of evidence for an impact on the mainland north shore of Lake Superior, only 15 km from the proposed impact centre, which supposedly produced a crater ~15-16 km in radius. Thus, on both side of the debate, problems still have to be resolved, but the majority of the evidence described from the Slate Islands Structure is best explained by an impact. Economic Geology A synopsis of the mineral exploration history and mineralization is provided by Sage (1991). Two styles of mineralization in the Archean metavolcanic rocks have garnered the most exploration interest: 1) lode gold; and 2) volcanogenic massive sulphide copperzinc. Gold is associated with quartz-carbonate veins in deformed and altered (Fe-carbonate, sericite, chlorite, tourmaline) rocks. More than 20 occurrences of visible gold in float boulders of quartz vein material have been recorded on the islands (Resident Geologist’s Files, Thunder Bay South District, Thunder Bay). Visible gold has also been noted in-situ near Horace Cove (aka St. Mary’s Bay). Massive sulphides occur in felsic metavolcanic rocks or as fragments in pyroclastic rocks. Massive pyrite sampled by Sage (1991) returned 0.11% Cu and 0.28% Zn. A sulphide-facies banded iron formation - 12 - �ILSG Special Publication #1 - The Slate Islands Stops are cut by a number of grey, heterolithic breccia dikes The field trip stops are labeled with letters rather than numbered in sequence as access to all stops, particularly on the outer shores of the islands, is weather-dependent. Many of the stops require some wading in order to reach all the outcrops so a change of footwear is recommended. Please take care when getting in and out of boats as outcrops are usually extremely slippery. with 0.5 to 5 cm, angular to sub-rounded clasts (Figs. 11 & 12). Narrow (<1 cm) dikelets may extend into the wall rock from the parent breccia dike. Stop A – “Honeymoon Bay” near Cove Island UTM coordinates – 0502004E 5386377N This small bay near Cove Island (Fig. 2) provides exposures of strongly sheared Archean felsic metavolcanic rocks at its northeast end (Stop A1). These metavolcanic rocks are phyllitic, displaying a pronounced west-trending, steeply dipping foliation with minor folds and kink bands. They are intruded by a 1 m wide, Paleoproterozoic diabase dike that zigzags across the outcrop (Fig. 10). A small (50 cm) wide breccia dike crosscuts the metavolcanic rocks. Metavolcanic rocks along the western shore of the bay Figure 12. Heterolithic breccia dyke at Stop A1. From this point groups will be shuttled out to a small island to the east of Honeymoon Bay (Stop A2; Fig. 2; UTM coordinates 0502426E 5386437N). This island consists of heterolithic breccia with clasts over 2 m across (Fig. 13). From the top of the island it is possible to look down upon a series of anastamosing dark grey breccia dikes under the water in the bay (Fig. Figure 10. Diabase dyke intruding Archean felsic metavolcanic rocks at Stop A1. Figure 11. Breccia dyke intruding Archean felsic metavolcanic rocks at Stop A1. Dykelets marked by arrows. Figure 13. Typical breccia exposed on the breccia island at Stop A2. - 13 - �ILSG Special Publication #1 - The Slate Islands from larger, parent dikes. Stop B – Sunday Harbour UTM coordinates – 0500801E 5386425N Figure 14. Anastamosing breccia dykes at Stop A2. 14) and across to a second island where an ~2 m wide, recessively weathered clast of reddish metavolcanic rock is clearly visible. You will need to get your feet wet to fully appreciate this outcrop. The majority of clasts typically range between <1 to 10 cm in size. They are derived from Archean metavolcanic rocks, Mesoproterozoic diabase and a variety of nondescript, fine-grained, variably altered rocks of indeterminate origin. Ragged, injected bodies of breccia may extend The beach consists of reworked glaciofluvial sediments characterized by a variety of locally derived and exotic rounded cobbles and boulders. Most of these are felsic plutonic and mafic metavolcanic rocks of the Schreiber-Hemlo greenstone belt. The exotic clasts are best exemplified by what have been termed “omars” (Prest, 1990), glacial erratics of massive, dark siliceous greywacke that contain light-toned (generally buff-weathering) calcareous concretions which are typically subspherical and weather recessively (Fig. 15). Omars, which commonly occur in and on eskers and outwash, but which also may be found in till and lacustrine deposits, are inferred to have been derived from the Omarolluk Formation of the Belcher Group in southeastern Hudson Bay (Prest et al., 2000). Most of the erratics were dispersed northwestward and westward across the Hudson Bay Paleozoic Basin by Labrador Sector ice, followed by westward and southwestward movement of ice across the Paleozoic and Archean terrain of northern Ontario, northern Manitoba and the upper Midwestern United States. A series of breccias are exposed on the eastern shore of Sunday Harbour. Dressler et al. (1999) reported the presence of two allogenic breccias at this outcrop: a Bunte Breccia is reported from the southern portion of the outcrop and a suevite breccia to the north. The southern end of the outcrop is a heterolithic grey breccia with clasts up to 50cm wide. It also contains clasts with well-developed shatter cones (Fig. 16). The grey breccia contains conspicuous reddish metavolcanic clasts and rare mafic to ultramafic clasts up to 50 cm across. At this location, the breccia is quite friable and easily dislodged from outcrop faces. Narrow diabase dikes with quartz-filled tension gashes intrude the metavolcanic rocks and are exposed just offshore in shallow water. Foliation orientations in the metavolcanic rocks are variable, suggesting either large-scale folding or rotation of large blocks of country rocks. Reddish alteration zones appear as dike-like bodies, cutting the grey breccia in places. Figure 15. Omars from the beach at Sunday Harbour (Stop B). - 14 - �ILSG Special Publication #1 - The Slate Islands chalcopyrite and hematite are noted. Grab sampling of vein material by Resident Geologist’s Program staff returned up to 3.95 ounces Au per ton and 0.2 ounce Ag per ton (Resident Geologist’s Files, Thunder Bay South District). The style of mineralization, alteration and deformation resembles that at Heron Bay, on the mainland shore of Lake Superior, approximately 50 km east of this location. The following synopsis of gold exploration at Horace Cove (aka St. Mary’s Bay) was modified from that of Sage (1991). Parsons (1918) concluded that the gold showing on the northwest corner of Horace Cove was the most promising of the known gold occurrences. From 1960 to 1963 Kimberly-Clark Pulp and Paper Company Limited conducted a mineral exploration program of the islands to test two gold showings. The main gold showing (St. Mary’s Bay occurrence) is on the northwestern corner of Horace Cove and the second occurrence (Cosen’s Showing) lies 240 m to the northeast. Figure 16. Shatter cone clast in breccia at Sunday Harbour (Stop B). Stop C – Horace Cove UTM coordinates – 0497386E 5387206N Shoreline outcrops at this location expose pervasively Fe-carbonatized and sericitized, schistose Archean metavolcanic rocks (Fig. 17). A strong, westtrending and steeply dipping foliation has resulted in the development of fissile, phyllitic rocks that also contain quartz, chlorite, green mica (chromian muscovite) and pyrite. Hydrothermal alteration and deformation preclude definitive recognition of the protolith. Sage (1991) has noted schistose basaltic to andesitic rocks, as well as dacitic to rhyolitic flows in the vicinity. Quartz- and feldspar-phyric units and sections containing quartz blebs (amygdules?) are also noted. Thin section analysis of this quartz-sericite schist by Nichols (1963) revealed a fine-grained groundmass of quartz blebs and scaly intergrowths of sericite that hosts siderite euhedra, altered albite and prochlorite and quartz amygdules. Along the shoreline, quartz-carbonate veins, with which most of the gold is associated, occupy a 050°trending fracture set. They range up to approximately 8 cm in width and are locally folded. Visible gold, pyrite, In 1960 Kimberly-Clark contracted an aeromagnetic and electromagnetic survey of the island. In 1961 and 1962 trenching, bulldozing, stripping, sampling and geologic mapping was done by the company over both the St. Mary’s Bay zone and Cosen’s showing. At St. Mary’s Bay bulldozer stripping to depths of 1.6 to 2.0 m exposed an area of approximately 18,900 m2; at Cosen’s showing 240 m to the north, approximately 5350 m2 of similar stripping was completed (G.E. Parsons, consulting geologist, personal communication, 1976). In 1963, Kimberly-Clark formed the Slate Island Mining Company Limited (The Northern Miner, September 19, 1963). Kimberly-Clark held a 50% interest, Junior Frood Mines Limited 25%, Upper Canada Mines Limited 12.5% and Cadamet Mines Limited 12.5% in Figure 17. Intensely sheared Archean metavolcanic rocks and folded quartz-carbonate vein at Horace Cove (Stop C). - 15 - �ILSG Special Publication #1 - The Slate Islands this new company (Financial Post Survey of Mines, 1964, p.169). In 1963, this company completed 20 diamond drill holes, totalling an estimated 1974 m on the St. Mary’s Bay zone (G.L. Puttock, personal communication, 1974). This work disclosed variable, but locally very high-grade, gold mineralization in quartz veins of short strike length and over narrow widths of 2 to 10 cm. Mineral exploration of the islands ceased with the termination of the efforts of KimberlyClark. In April 1973, finding the islands of no further use to them, Kimberly-Clark transferred its rights to the islands back to the Crown. Subsequently, in September 1973 the islands were removed from staking. The gold-bearing, quartz-carbonate veins of St. Mary’s Bay zone and Cosen’s showing display a strong southwesterly strike. Since the host rocks of the veins are folded into a northwest-trending sequence, these veins are approximately normal to stratigraphy as was observed in several places along the eastern shore of Patterson Island. Some evidence for shear folding of the quartz veins is indicated at the St. Mary’s Bay zone by the irregular “sawtooth” pattern of some of the veins. Nichols (1963) suggested that gold-bearing quartz veins on Patterson Island occurred in the nose of a fold and occupied shear and tension fractures. Based on samples and descriptions by G.E. Parsons (consulting geologist for Kimberly-Clark Pulp and Paper Company Limited, personal communication, 1974), gold locally occurs in three ways. These are: (1) in association with pyrite within the quartzcarbonate veins; (2) as flakes and thin sheets along the flanks of the quartz-carbonate veins; and (3) as thin sheets or flakes along schistosity planes of the rocks enclosing the quartz-carbonate veins. Sampling by Sage (1991) of various quartz veins returned nil to insignificant gold values except for the St. Mary’s Bay zone, where assays of 0.5 ounce Au per ton over widths of 2 to 3 cm were obtained. The quartz veins vary from tabular, lensoid, clearly defined veins to irregular anastomosing structures with no clearly discernible attitude. An average of 98 clearly defined veins gave an average width of 10.7 cm and a length of 5.5 m (Sage, 1991). Reddish-brown, coarse-grained carbonate is an ubiquitous, accessory to dominant mineral and pyrite is common to abundant. Rarely, black needle-like crystals of tourmaline were noted. A contoured stereonet plot of 167 quartz vein attitudes indicated a rather broad spread of attitudes with one and possibly two maxima. The strongest maximum defines a 070°-trending vein set dipping approximately 60° southeast. The second maximum defines a 035°trending, vertically dipping vein set. The intersection of these two trends would define a lineation striking 210°, plunging about 14° southwest. Brummer (1962) delineated an area of sericite schist and shearing extending for 300 m north-south and 570 m east-west at the northern end of Horace Cove. Pyroclastic rocks, diorites and porphyritic metavolcanic rocks were also noted. Three steeply dipping to vertical vein sets were identified: a major set at 035°; and minor sets at 063° and 050° to 060°. The 40 veins that had been discovered at that point ranged in strike length between 16 and 60 m and in width from 0.5 to 20 cm, averaging 5 cm. Brummer (1962) noted that approximately 80% of the gold occurred as this films along the outer vein margins. The altered wall rock was not sampled for assay; Nichols (1963) noted an absence of gold in wallrock. Stop D – Western shore of Patterson Island UTM coordinates – ca. 495965E 5387400N This location on the western shore of Patterson Island (Fig. 2) is a microcosm of Slate Islands geology, in that a variety of rock types and geologic features are exposed. The southernmost outcrops (UTM coordinates - 495965E 5387324N) are sheared Archean metavolcanic rocks which are unconformably overlain by hematite-jasper banded iron formation and ferruginous shales of the Paleoproterozoic Gunflint Formation (Fig. 18). Figure 18. Unconformable contact between Keweenawan basalts (left) and Animikie Gunflint Formation (right). - 16 - �ILSG Special Publication #1 - The Slate Islands The base of the Gunflint outcrop consists, very approximately, of 20m of interlayered grainstone and slaty iron formation. Diabase and red breccia dikes have obscured the lower contact of the Gunflint Formation. The grainstone layers are 1 to 30 cm thick and are composed of intraclasts of chert and jasper. Clast sizes range up to that of small pebbles but are dominated by medium-to coarse-grained sand. The rock has a pervasive quartz cement. The slaty iron formation layers form bundles less than one to several centimetrers thick. Individual layers are millimeter to sub-millimeter in thickness. They are composed of magnetite mixed with what is probably siliciclastic clay and silt. This unit is overlain by approximately 7 m of just the slaty iron formation. This denotes a rapid change from shallower, storm-dominated, bottom to deeper, more quiescent conditions, a trend similar to the Gogebic iron formation successions described from Wisconsin (Pufahl and Fralick, 2004). The exposed upper part of the Gunflint section dips approximately 20° to 30° to the north. The upper contact of the Gunflint with overlying Mesoproterozoic (Keweenawan) flood basalts is also obscured by diabase and breccia bodies. Small, delicate shatter cones (< 5 cm long) are developed in the argillaceous portions of the sedimentary rocks (UTM coordinates - 495964E 5387369N; Fig. 19). Figure 20. Amygdaloidal Keweenawan basalt. Stop D on Patterson Island. approximately 1m thick interflow sandstone unit can be accessed by wading across the small bay. Mediumgrained, interflow red sandstones form successions up to a couple of metres thick. Bed thicknesses vary from a few centimeters to approximately 1 m. The sandstones are massive; sedimentary structures, aside from upper flow regime parallel laminations, are not A series of north-striking basalt flows ranging from 1 to 2 m thick outcrop along the shoreline. Sage (1991) has noted 22 separate flows in this section. The basalts are vesicular and amygdaloidal and dip approximately 20° to 60° to the west. Pipe amygdules occur near flow bases; coalescing amygdules may form flow-parallel lenses and bands (Fig. 20). Ropy flow tops, characteristic of pahoehoe lava, are locally preserved (Fig. 21). An Figure 21. Pahoehoe texture developed on basalt flow tops at Stop D on Patterson Island. Figure 19. Shatter cones in the Gunflint Formation at Stop D on Patterson Island. well preserved. This may be the result of fluid escape, especially during heating by overlying basalt flows. Trough-like structures in the top of one bed overlain by basalt flows may represent gouge marks where blocks of solidified lava caught up in the overriding basalt flow has been dragged through the unlithified sand in a manner analogous to glacial striae (Figs. 22 & 23). The channels are oriented in an east-west direction, perpendicular to the strike of the flows. The interflow sandstone is thicker than interflow sedimentary rocks in Osler basalts on Wilson Island (Hollings and Fralick, 2005). In addition to the thick interflow unit, thin layers of baked interflow mudstone can be seen within and - 17 - �ILSG Special Publication #1 - The Slate Islands Figure 24. Shattercones in Keweenawan diabase, Stop D Patterson Island. Figure 22. Keel marks left in the upper surface of an interflow sandstone unit at Stop D, Patterson Island. Flow direction is parallel to black arrows. between the basalt flows. Shatter cones are particularly well-developed in the basalt flows and in the boulders and blocks that litter the beach (Fig. 24). In places the shatter cones exceed 20 cm in length. Stop E – McGreevy Harbour UTM coordinates – 500825E 5390752 N Dressler et al. (1999) have interpreted the structures preserved in the Archean felsic volcanic rocks at this site as large shatter cones, the largest being ~10m high (Fig. 25). To the west of the larger shatter cone a partial cone may be preserved that would imply a total length on the order of 20m. It is difficult to disembark at this site and equally hard to clamber on the steep talus cascading into the water. The scale of these large features is better appreciated from 15 to 20 m offshore. Figure 23. Close-up of keel marks left in the upper surface of an interflow sandstone unit at Stop D, Patterson Island. - 18 - �ILSG Special Publication #1 - The Slate Islands Figure 25. Large shatter cone visible in the cliff side in McGreevy Harbour (Stop E). - 19 - �ILSG Special Publication #1 - The Slate Islands References Alvarez, L.W., Alvarez< W., Asaro, F., andMichel, H.V. 1980. Extraterrestrial cause for the CretaceousTertiary extinction. Science, 269, 1095-1108. Brummer, J.J. 1962. Gold-bearing veins, St. Mary’s Bay, Patterson Island, Lake Superior, Ontario; unpublished report, Resident Geologist’s Files, Thunder Bay South District, 5p. Cannon, W.F., Green, A.G., Hutchinson, D.R., Lee, M., Milkereit, B., Behrendt, J.C., Halls, H.C., Green, J.C., Dickas, A.B., Morey, G.B., Sutcliffe, R. and Spencer, C., 1989. The North American Midcontinent Rift Beneath Lake Superior from GLIMPCE Seismic Reflection Profiling; Tectonics, v. 8, n. 2, p. 305332. Grieve, R. A. F., Rupert, J., Smith, J., and Therriault, A., 1995, The record of terrestrial impact cratering: GSA Today, v. 5, p. 189, 194–196. Halls, H.C., 1974. A Keweenawan sequence from the Slate Islands, Northern Lake Superior. Institute on Lake Superior Geology, 20th Annual Meeting, Sault Ste. Marie, Ontario, p.14. Halls, H.C., 1975. Shock-induced remanent magnetization in Late Precambrian rocks from Lake Superior. Nature, 255, 692-695. Halls, H.C., 1976. The Slate Islands: The Central Uplift of a meteorite impact crater. Institute on Lake Superior Geology, 22nd Annual Meeting, St Paul, Minnesota, p.27 Card, K.D., Sanford, B., and Davidson, A., 1994. Bedrock geology, Lake Superior, Ontario, USA. Natural Resources Canada, Map NL-16/17-G. Halls, H.C., 1979. Diatremes and shock features in Precambrian rocks of the Slate Islands, northeastern Lake Superior: Discussion. Geological Society of America Bulletin, 90, 1084-1086. Coleman, A.P., 1901. The Slate Islands in Iron Ranges of Northwestern Ontario, Ontario Bureau of Mines, v. 11, 552-555. Halls, H.C., 1997. New constraints on the Slate Islands impact structure: Comments and Reply. Geology, 25, 666. Dietz, R. S., 1964, Sudbury structure as an astrobleme: Journal of Geology, v. 72, p. 412–434. Halls, H. C., and Grieve, R. A. F., 1976, The Slate Islands: A probable complex meteorite impact structure in Lake Superior: Canadian Journal of Earth Sciences, v. 13, p. 1301–1309. Dressler, B.O. and Sharpton, V.L., 1997. Breccia formation at a complex impact crater: Slate Islands, Lake Superior, Ontario, Canada. Tectonophysics, 275, 285-311. Dressler, B.O., Sharpton, V.L., Schuraytz, B. and Scott, J., 1994. Bunte breccia, impact melt and suevite at teh SLate Islands impact structure, Ontario. Ontario Geological Survey, Miscellaneous Paper 163, 59-61. Dressler, B.O., Sharpton, V.L., Schnieders, B. and Scott, J., 1995. New observations at the Slate Islands impact structure, Lake Superior. Ontario Geological Survey, Miscellaneous Publication 164: 53-61. Halls, H. C., and Stesky, R.M., 1978. Paleomagnetic and shatter cone measurements from the Slate Islands, Northern Lake Superior. Canadian Geophysical Union, 5th Annual Meeting, London, Ontario, p.34. Hinze, W.J., O’Hara, W.N., Trow, J.W. and Secor, G.B., 1966. Aeromagnetic studies of eastern Lake Superior, in Steinhart, J.S., ed., The earth beneath the continents. American Geophysical Union, Geophysical Monograph, 10, 95-110. Dressler, B.O., Sharpton, V.L. and Schuraytz, B.C., 1998. Shock metamorphism and shock barometry at a complex impact structure: Slate Islands, Canada. Contributions to Mineralogy and Petrology, 130, 275-287. Hollings, P., and Fralick, P., 2005. A stratigraphic transect across the northern flank of the Midcontinent Rift near Rossport. In; Hollings, P. (Ed.), Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Part 2 - Field trip guidebook, v.51, part 2, 57-70. Dressler, B.O., Sharpton, V.L. and Copeland, P., 1999. Slate Islands, Lake Superior, Canada: A mid-size, complex impact structure. Geological Society of America, Special Paper 339, 109-124. Hollings, P., Fralick, P. and Cousens, B., 2006. Geochemistry and sedimentology of the Osler Formation: Evaluating rifting in the Proterozoic. Canadian Journal of Earth Sciences, in press. French, B.M., 1998. Traces of catastrophe. Lunar and Planetary Institute, Contrib. 954, 120 pp. Mariano, J., and Hinze, W. J., 1994, Structural interpretation of the Midcontinental rift in eastern Lake Superior from seismic reflection and potential-field studies: Canadian Journal of Earth Sciences, v. 31, p. 619– 628. Grieve, R. A. F., and Robertson, P. B., 1976, Variations in shock deformation at the Slate Islands impact structure, Lake Superior, Canada: Contributions to Mineralogy and Petrology, v. 58, p. 37–49. Grieve, R. A. F., and Robertson, P. B., 1997. New constraints on the Slate Islands impact structure: Comments and Reply. Geology, 25, 666-667. Nichols, L.C. 1963. Gold occurrences on the Slate Islands of Lake Superior; unpublished B.Sc. thesis, Queen’s University, Kingston, Ontario, 61p. Norris, A.W. and Sanford, B.V., 1969. Paleozoic and - 20 - �ILSG Special Publication #1 - The Slate Islands Mesozoic geology of the Hudson Bay lowlands. In Hood, P.J., ed., Earth science symposium on the Hudson Bay. Geological Survey of Canada, Paper 68-53, 169-205. Parsons, A.L., 1918. Slate Islands, Lake Superior. Ontario Bureau of Mines, Annual Report, 27, 155-167. Polat, A., Kerrich, R., and Wyman, D., 1998. The late Archean Schreiber-Hemlo and White River-Dayohessarah greenstone belts, Superior Provnce: Collages of oceanic plateaus, oceanic arcs, and subductionaccretion complexes. Tectonophysics, 289, 295-326. Powell, J.L.,. 1998. Night comes to the Cretaceous. Harcourt Brace and Company, New York, 250 pp. Prest, V.K. 1990. Laurentide ice-flow patterns: A historical review, and implications of the dispersal of Belcher Island erratics; Géographie physique et Quaternaire, v.44, p.113-136. Sharpton, V., Dressler, B., Herrick, R., Schnieders, B., Scott, J., 1996. New constraints on the Slate Islands impact structure, Ontrio Canada. Geology, 24, 851-854. Sharpton, V.L., Copeland, P., Dressler, B.O. and Spell, T.L., 1997. New age constraints on the Slate Islands impact structure, Lake Superior, Canada. Lunar and Planetary Science Conference XXVIII, 1287-1288. Stesky, R. M., and Halls, H. C., 1983, Structural analysis of shatter cones from the Slate Islands, northern Lake Superior: Canadian Journal of Earth Sciences, v. 20, p. 1–18. Sun S. S., and McDonough., W.F., 1989. Chemical and isotopic systematics of oceanic basalts: Implications for mantle composition and processes. In Magmatism in the ocean basins. Edited by A.D. Saunders, and M.J. Norry. Geological Society Special Publication. 42: 313-345. Prest, V.K., Donaldson, J.A. and Mooers, H.D. 2000. The Omar story: The role of Omars in assessing glacial history of west-central North America; Géographie physique et Quaternaire, v.54, no.3, p.257-270. Pufahl, P., and Fralick, P., 2004. Depositional controls on Paleoproterozoic iron formation accumulation, Gogebic Range, Lake Superior Region, USA. Sedimentology, 51, 791-808. Roberston, P.B. and Grieve, R.A.E., 1976. Comparison of the distribution of shock metamorphism at Charlevoix, P.Q., and Slate Islands, Ontario. Geological Association of Canada – Mineralogical Association of Canada, Annual Meeting, Program with Abstracts, v. 1, p.42. Roberston, P.B. and Grieve, R.A.E., 1979. Diatremes and shock features in Precambrian rocks of the Slate Islands, northeastern Lake Superior: Discussion. Geological Society of America Bulletin, 90, 10871088. Sage, R.P., 1975. Slate Islands. Ontario Division of Mines, Ministry of Natural Resources, Map P. 997, scale 1” to 0.25 miles. Sage, R.P., 1978. Diatremes and shock features in Precambrian rocks of the Slate Islands, northeastern Lake Superior. Geological Society of America Bulletin, 89, 1529-1540. Sage, R. P., 1991, Precambrian geology, Slate Islands: Ontario Geological Survey Report 264, 111 p. Sage, R.P., 1999. The Slate Islands: A uniquely sited cryptoexplosion structure. In Summary of Field work and other activities 1999, Ontario Geological Survey, Open File Report 6000, 28-1 to 28-13. Sharpton, V. and Dressler, B., 1997. New constraints on the Slate Islands impact structure: Comments and Reply. Geology, 25, 668-669. - 21 - � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Special Publication #1: Field Trip Guidebook for the Slate Islands, Ontario Description An account of the resource Institute on Lake Superior Geology: Special Publication #1: Field Trip Guidebook for the Slate Islands, Ontario Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2006 Contributor An entity responsible for making contributions to the resource Pete Hollings Mark Smyk Bill Addison Phil Fralick Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/10e97ee6c7c1c33da71620302b21e77e.pdf 3a06ee87c9410827142e7614a85509e2 PDF Text Text INSTITUTE ON LAKE SUPERIOR GEOLOGY 54TH ANNUAL MEETING MAY 6-10, 2008 MARQUETTE, MICHIGAN HOSTED BY: Michigan Technological University THEODORE J. BORNHORST AND JOHN S. KLASNER Co-Chairs Proceedings Volume 54 Part 1 – Program and Abstracts EDITED BY THEODORE J. BORNHORST AND GEORGE W. ROBINSON A. E. SEAMAN MINERAL MUSEUM, MICHIGAN TECHNOLOGICAL UNIVERSITY Cover Photos: Lake Superior region minerals in the collection of the A.E. Seaman Mineral Museum Top Left Clockwise: amethyst – Thunder Bay, Ontario; gold – Balmertown, Ontario; silver – Silver Islet, Ontario; copper – Phoenix, MI; goethite – Ishpeming, MI; gypsum – Crystal Falls, MI; chalcocite – Ladysmith, WI; rhodochrosite – Montreal, WI (photographs by George Robinson and John Jaszczak) �54TH INSTITUTE ON LAKE SUPERIOR GEOLOGY PROCEEDINGS VOLUME 54 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: BANDED IRON FORMATION OF THE MARQUETTE DISTRICT TRIP 2: ARCHEAN-PALEOPROTEROZOIC UNCONFORMITY AT SILVER LAKE—SEISMITES FROM THE SUDBURY IMPACT? TRIP 3: GEOLOGY OF THE BACK FORTY PROJECT TRIPS 4 AND 8: GEOLOGY OF THE EAGLE PROJECT. TRIP 5: THE SUDBURY IMPACT LAYER AT THE MCCLURE LOCALITY TRIP 6: SUSTAINABLE RECOVERY OF IRON FROM THE MARQUETTE DISTRICT TRIP 7: GEOLOGY OF THE KEWEENAWAN BIC INTRUSION Reference to material in Part 1 should follow the example below: Cannon, W.F. and Schulz, K.J., 2008, Unusual features along the Archean/Paleoproterozoic unconformity at Silver Lake, Michigan—seismites from the Sudbury impact [abstract]: Institute on Lake Superior Geology Proceedings, 54th Annual Meeting, Marquette, MI, v. 54, part 1, p. 10-11. Published by the 54th Institute on Lake Superior Geology and distributed by the ILSG Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca ILSG website: http://www.lakesuperiorgeology.org ISSN 1042-9964 ii �TABLE OF CONTENTS PROCEEDINGS VOLUME 54 PART 1— PROGRAM AND ABSTRACTS Institutes on Lake Superior Geology, 1955-2008........................................................................... iv Sam Goldich and the Goldich Medal................................................................................. vi Goldich Medal - Past Medalists and Committee ............................................................. viii Citation for 2008 Goldich Medal Recipient....................................................................... ix ILSG Student Research Fund............................................................................................. xi Student Paper Awards....................................................................................................... xii Eisenbrey Student Travel Awards ................................................................................... xiii Report of the Chair of the 53rd Annual Meeting............................................................. xiv 2008 Board of Directors..................................................................................................................... xvii 2008 Session Chairs........................................................................................................ xvii 2008 Student Paper Awards Committee ......................................................................... xvii 2008 Local Committees .................................................................................................. xvii Special Recognition ....................................................................................................... xviii 2008 Banquet Speaker ..................................................................................................... xix Program............................................................................................................................ xxi Abstracts ........................................................................................................................ xxix iii �PREVIOUS INSTITUTES ON LAKE SUPERIOR GEOLOGY ILSG YEAR PLACE CHAIRS 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz and C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims and R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley and E. Mercy 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray iv �28 1982 International Falls, Minnesota D.L. Southwick 29 1983 Houghton, Michigan T.J. Bornhorst 30 1984 Wausau, Wisconsin G.L. La Berge 31 1985 Kenora, Ontario C.E. Blackburn 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey and R.P. Sage 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage and W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller, Jr. and M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst and R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin and P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey, Jr. and B.A. Brown 48 2002 Kenora, Ontario P. Hinz and R.C. Beard 49 2003 Iron Mountain, Michigan L.G. Woodruff and W.F. Cannon 50 2004 Duluth, Minnesota S.A. Hauck and M. Severson 51 2005 Nipigon, Ontario P. Hollings and M.C. Smyk 52 2006 Sault Ste. Marie, Ontario R.P. Sage and A.C. Wilson 53 2007 Lutsen, Minnesota L.G. Woodruff and J.D. Miller, Jr. 54 2008 T.J. Bornhorst and J.S. Klasner Marquette, Michigan v �SAM GOLDICH AND THE GOLDICH MEDAL Sam Goldich received an AB from the University of Minnesota in 1929, a M.A. from Syracuse University in 1930, and a Ph.D. from the University of Minnesota in 1936. During World War II Sam worked for the U.S. Geological Survey in mineral exploration. In 1948, Sam returned to the University of Minnesota, and became Professor and Director of the Rock Analysis Laboratory the following year. He rejoined the U.S. Geological Survey in 1959 and was appointed as the first Branch Chief of the Branch of Isotope Geology. Sam returned to academia in 1964 when he went to Pennsylvania State University. He left PSU in 1965 and moved to the State University of New York at Stony Brook, where he stayed for 3 years. Restless yet again, he moved to Northern Illinois University in 1968 where he was a professor until his retirement in 1977. Sam’s final move was to Denver where he became an emeritus at the Colorado School of Mines. Sam died in 2000, less than a month before his 92nd birthday. In the late 1970’s, Geological Society of America Special Paper 182, which included seminal geochronological studies by Sam Goldich and coworkers on the Archean rocks of the Minnesota River Valley, was nearing completion. At this time various ILSG regulars began discussing the possibility of recognizing Sam for his pioneering work on the resolution of age relationships and thus the geology of Precambrian rocks in the Lake Superior region. Three members, R.W. Ojakangas, J.O. Kalliokoski and G.B. Morey, presented the idea to the ILSG Board of Directors in 1978. The Board approved the creation of an award, provided funding could be obtained. It was suggested that collecting one or two dollars at registration for a dedicated account would provide resources for striking the medal. A general request was made to the ILSG membership for donations and Sam himself offered a challenge grant to match the contributions. In total $4,000 was collected and thus began the work of creating the Goldich Medal. The initial Goldich Award was presented to Sam by G.B. Morey in 1979 and consisted of a large paper proclamation. For the actual medal, G.B. Morey consulted with the foundry on production details, while Dick Ojakangas and Jorma Kalliokoski worked on the design of the award, suggesting that it be given for “outstanding contributions to the geology of the Lake Superior region.” Simultaneously, a committee of J.O. Kalliokosi, W.F. Cannon, M.M Kehlenbeck, G.B. Morey, and G. Mursky developed the Award Guidelines that were approved by the ILSG Board. By 1981 all the elements of the Goldich Award had come together, and the vi �second recipient, Carl E. Dutton, Jr., received the Goldich Medal for 50 years of significant contributions to the understanding of the geology of the Lake Superior region. Since the beginning, the Awards Committee has consisted of individuals representing industry, government and academia, with each member of the Committee serving for three years. The medal is now awarded every year at the annual ILSG meeting. Reference: Morey, G.B. and Hanson, G.N. (editors). 1980. Selected studies of Archean gneisses and Lower Proterozoic rocks, southern Canadian Shield. Geological Society of America, Special Paper 182, 175 p. Prepared by various Goldich Medal Awardees, 2007 INSTITUTE ON LAKE SUPERIOR GEOLOGY GOLDICH MEDAL vii �PAST GOLDICH MEDALISTS 1979 Samuel S. Goldich 1994 Cedric Iverson 1980 not awarded 1995 Gene La Berge 1981 Carl E. Dutton, Jr. 1996 David L. Southwick 1982 Ralph W. Marsden 1997 1983 Burton Boyum 1998 Zell Peterman 1984 Richard W. Ojakangas 1999 Tsu-Ming Han 1985 Paul K. Sims 2000 John C. Green 1986 G.B. Morey 2001 John S. Klasner 1987 Henry H. Halls 2002 Ernest K. Lehmann 1988 Walter S. White 2003 Klaus J. Schulz 1989 Jorma Kalliokoski 2004 Paul Weiblen 1990 Kenneth C. Card 2005 Mark Smyk 1991 William Hinze 2006 Michael G. Mudrey 1992 William F. Cannon 1993 Donald W. Davis 2007 Joseph Mancuso Ronald P. Sage 2008 GOLDICH MEDAL RECIPIENT Theodore J. Bornhorst Michigan Technological University Houghton, Michigan GOLDICH MEDAL COMMITTEE Serving for the meeting year shown in parentheses Doug Duskin (2005-2008) Richard Ojakangas (2006-2009) Terry Boerboom (2007-2010) Industry representative Academic representative Government representative viii �CITATION FOR GOLDICH MEDAL RECIPIENT Theodore J. Bornhorst, 2008 Goldich Medal Recipient It is my great pleasure to introduce Professor Ted Bornhorst as the recipient of the 2008 Goldich Medal. Ted is being honored with this award for his many contributions to the understanding of Lake Superior geology through his research and teaching and, most significantly, for his unparalleled service to the Institute over the past 25 years. Ted began (and will likely finish) his geological career at Michigan Technological University. He received his B.S. from Michigan Tech in 1974 and actually presented his first ILSG talk at that year’s meeting in Sault St. Marie on the topic of his senior research project the geology and geochemistry of the Fish Cove rhyolite in Keweenaw County. Ted headed to the University of New Mexico to complete his M.S. (1976) and Ph.D. (1980) degrees in economic geology only to return to Michigan Tech as an assistant professor in 1981. Ted received a full professor appointment in 1993 and was appointed director of the world class A.E. Seaman Mineral Museum at Michigan Tech in 2003. During his tenure at Michigan Tech, Ted has taught classes, advised students, and conducted research on a variety of topics and in a range of locations (e.g., gold in Finland, rhyolites in New Mexico, quaternary volcanic ashes off the coast of Guatemala). But by far, the main emphasis of his academic pursuits has been the Precambrian geology and mineral deposits of Upper Michigan. A true indication of his passion for Lake Superior geology is the fact that Ted has directed a five-week intensive Precambrian field course in the western UP almost every summer since arriving at Michigan Tech in 1981. This course has been taken by over 500 students. I would be willing to bet that the approximately 7000 students who have taken the introductory geology course from Professor Bornhorst since 1989 have probably received a hefty dose of Lake Superior geology, as well. Ted has advised (to completion) 4 Ph.D. dissertations, 24 M.S. theses, and 29 senior research projects, almost all dealing with Lake Superior geology. In addition to being the advisor and mentor for scores of Michigan Tech geology students, many of whom still do research and mineral exploration work in the Lake Superior region, Ted also serves the general public and the geological community by being a member of the State of Michigan's Mineral Well Advisory Committee, representing the western Upper Peninsula. And because of his knowledge of UP geology and mineral deposits, Ted has been appointed to many state committees and boards looking into rules and regulations for non-ferrous mineral exploration and mining in the state. Ted’s research interests in the Lake Superior region have focused on three general areas: the Cu deposits of the Keweenwaw Peninsula, the geology and mineral potential of the Ishpeming greenstone belt, and the petrology of granitoids in the Archean Northern Complex. Ted has been first or second author on over 50 journal publications. Of these, 19 were on topics dealing with Lake Superior geology. He has authored 16 field trip guides and 15 maps and technical reports on the geology and ore deposits of Upper Michigan. ix �Beyond this impressive resume of academic contributions to the geology of the Lake Superior region, perhaps the main reason that Ted is so deserving of the Goldich award is his unwavering commitment and very real service to the ILSG. Ted’s impressive list of contributions to the institute include: • Presenting 10 abstracts and coauthoring many more of his student’s presentations. • Organizing, writing guidebooks for, and leading 6 field trips; most impressive of these has been the Field Guide to the Geology of the Keweenaw Peninsula, by Bornhorst, Rose, and Paces. This now classic guidebook, which was originally published in 1983 and republished in 1994, has sold over 5,000 copies to geologists and non-geologists alike. • Serving as chairman or co-chairman of 4 annual meetings: 1983 (Houghton), 1994 (Houghton), 1999 (Marquette), and now 2008 (Marquette). • Serving as the Institute’s first webmaster beginning in 1995 and maintaining the ILSG webpage until 2004, when the position passed over to the Secretary-Treasurer. It is the exceptional contributions and service given by individuals like Ted Bornhorst that has sustained the ILSG over these past 54 years. As token of the Institute’s appreciation, it is altogether fitting and appropriate that Ted join the ranks of other prominent Lake Superior geologists who have been honored with this prestigious award. I know I speak for all the institute members when I say “thank you, Ted, for all you have given the Institute.” Jim Miller University of Minnesota Duluth x �ILSG STUDENT RESEARCH FUND The 2005 Board of Directors established the ILSG Student Research Fund with $10,000 US from the Institute’s general fund to encourage student research on the geology of the Lake Superior region. A minimum of two awards of $500 US each for research expenses (but not travel expenses) will be made each year. Students are expected to present their research orally or during a poster session at an ILSG meeting. The award winners will also be automatically eligible for the Eisenbrey Travel Awards. To allow the fund to grow, the Fund will receive one-half of any additional proceeds from each annual meeting, after all other commitments and expenses are covered. • The ILSG Board of Directors will be responsible for selecting a minimum of two awards each year. The ILSG Treasurer will issue the awards. • The ILSG Student Research Fund is available for undergraduate or graduate students working on geology in the Lake Superior region. • The applications are due to the ILSG Secretary by August 31st of each year. Awards will be made by October 1st of each year. • Names of the award recipients will be announced at the next annual meeting and posted on the ILSG website. • The proposal application should be at least 500 words, and should have a statement of the research project, background information, a map of the research area, research steps necessary to complete the research, figures (if needed) , references, and a list of research expenses. The proposal should also include a proposed end date for the research. • The proposal will need to be signed by researcher’s supervisor. In 2006 and 2007 the ILSG Board of Governors awarded three $500 awards from the Student Research Fund. Cole Edwards (University of Wisconsin - Oshkosh) - Controls on the formation of the earliest marine phosphate deposits, Marquette Supergroup, Michigan Noah Planavsky (Rosenstiel School of Marine and Atmospheric Sciences Marine Geology and Geophysics, Miami) - Iron isotopes as oceanographic tracers in Animikie Basin Iron Formations Michael Taylor (University of Minnesota - Duluth) - Pleistocene glaciation as a mechanism for emplacement of high-salinity groundwater at anomalously shallow depths in the Lake Superior basin xi �STUDENT PAPER AWARDS Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper—last modified by the Board in 2001—follow: • The contribution must be demonstrably the work of the student. • The student must present the contribution in-person. • The Student Paper and Poster Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. • In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. • The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). • The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper and Poster Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. • The names of award recipients shall be included as part of the annual Chair's report that appears in the next volume of the Institute. Student papers are noted on the Program. In 2007 the ILSG Student Paper Committee presented four awards from the ILSG Student Paper Fund. Two first place Best Student Paper awards: ($200 each) were given to: Sarah Nicholas (Macalester College) for her poster titled: Investigations of sulfide minerals leached in the presence of alkaline solids Noah Planavsky (Rosentiel School of Marine and Atmospheric Science) for his talk titled: Rare earth element patterns in Steep Rock Carbonates. Two Honorable Mention Best Student Paper awards ($100 each) were given to: Troy Boisjoli (St. Norbert College) Larissa Stevens (Lakehead University) xii �EISENBREY STUDENT TRAVEL AWARDS The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name "Eisenbrey" was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. "Ned" Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The annual Chair in consultation with the Secretary-Treasurer determines the number of awards and value. Recipients will be announced at the annual banquet. The student travel award application is available on the ILSG website. The following general criteria will be considered by the annual Chair, who is responsible for the selection: • The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. • Students who are the senior author on either an oral or poster paper will be given favored consideration. • It is desirable for two or more students to jointly request travel assistance. • In general, priority will be given to those in the Institute region who are farthest away from the meeting location. • Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. • Successful applicants will receive their awards during the meeting. In 2007 the ILSG awarded 15 travel awards from the ILSG Eisenbrey Student Travel Fund. The awards were made to: Malcolm Alexander – Lakehead University Troy Boisjoli – St. Norbert College Dan Costello – St. Norbert College Clinton Forsha – Slippery Rock University Amanda Hogan – St. Thomas University Carissa Isaac – Lakehead University Travis Jacob – St. Thomas University Renata Jasinevicius – St. Norbert College Noah Planavsky – Rosentiel School of Marine and Atmospheric Science Patrick Quigley – University of Minnesota, Duluth Tommy Rodengen – St. Thomas University and Jody Rymaszewski – University of Wisconsin-Milwaukee Larissa Stevens – Lakehead University Michael Taylor – University of Minnesota, Duluth Stephanie Theriault – St. Thomas University xiii �REPORT OF THE CHAIRS OF THE 53RD ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY LUTSEN, MINNESOTA The U.S. Geological Survey with assistance from the Minnesota Geological Survey hosted the 53rd Annual Institute on Lake Superior Geology on May 8 – 13, 2007 at Lutsen Resort in Lutsen, Minnesota. The meeting consisted of two days of technical sessions with pre- and post-technical session field trips. Bill Cannon and Klaus Schulz helped with pre-meeting logistics. Gretchen Klasner provided valuable logistical assistance on-site at Lutsen Resort during the technical sessions. Connie Dicken and Bill Addison were media czars for the technical sessions, keeping all presentations on track with fewer glitches than normal. Pre-meeting registration was 114 students and professionals with an additional 69 on-site registrations, for a total of 183 registrants, an excellent turn-out. Proceedings Volume 53 was published in two parts. Part I – Program and Abstracts, edited by Laurel Woodruff, contains 49 published abstracts for 28 oral and 21 poster presentations; Part 2 – Field Trip Guidebook, edited by Jim Miller, contains descriptions of six field trips, three premeeting and three post-meeting. The 53rd ILSG marked the first time in its long history that an ILSG meeting was held in this part of Minnesota. Field trips visited areas new to the ILSG, which resulted in an excellent subscription for all the trips. On Tuesday, May 8, Jim Miller and Eric Jerde led an intrepid group of 8 participants on a two-day field trip into the Boundary Waters Canoe Area Wilderness to examine the mafic igneous rocks of the Poplar Lake Intrusion. Based out of the Rockwood Lodge off the Gunflint Trail, the trip required canoeing across Poplar Lake and several lakes farther into the BWCAW, only two weeks after ice-out! Thanks to warm weather, the couple of “soggy mishaps” served only as fodder for the Homer Award (way to go Steve!). On Wednesday David Cooper, from the National Park Service, Bill Cannon, and Brian Phillips guided 18 participants on a trip through the Grand Portage National Monument, looking at both the geological and cultural history of the area. Another trip on Wednesday into southern Ontario to look at Midcontinent Rift-related intrusions north of the international border involved 31 participants and was lead by Mark Smyk and Pete Hollings. A misstep on the outcrop on that trip resulted in an injury to former Goldich Medal recipient and long-time ILSG stalwart Dick Ojakangas, who unfortunately had to miss the rest of the meeting as he returned to Duluth for treatment. Throughout the meeting, the 60 people who had signed up for Mark Jirsa’s and Paul Weiblen’s Gunflint field trip were closely tracking the Ham Lake fire as it burned across the northern end of the Gunflint Trail. (The Poplar Lake Trip was far enough south of the fire to proceed, though the smoke plume was clearly visible). A final decision by the Forest Service closed the upper part of the Gunflint Trail on Friday and put the kibosh on any hope of running even a modified Gunflint Trail trip. Instead, Mark and Paul, with generous assistance from Bill Addison and Phil Fralick, quickly reconfigured the trip for a visit to the Thunder Bay area to see similar rocks as well as spectacular outcrops of Sudbury impact-related rocks. All of us involved appreciate the forbearance of seasoned and new ILSG attendees who were remarkably sanguine about the travails of the Gunflint Trail trip. Most of the people originally signed up for the Gunflint Trip xiv �stayed on for the substitute Thunder Bay trip, and we heard of no one being disappointed. One consolation from the cancellation is the Gunflint’s future potential as a field trip area for ILSG and enhanced exposure of rocks in burned areas, including newly discovered Sudbury ejecta layers found by Mark Jirsa while sneaking past the DO NOT CROSS signs on the Trail. Also on Saturday, Terry Boerboom, John Green and Jim Miller led 16 participants on a field trip along the spectacular shoreline geology of the North Shore from Little Marais to Grand Marias to highlight some recent detailed mapping. A very timely and popular two-day trip lead by Dean Peterson and Paul Albers (of Duluth Metals) set off for Ely on Saturday with a group of 20 participants. The trip looked at the geology and Cu-Ni-PGE mineralization of the Nickel Lake macrodike and South Kawishiwi Intrusion of the Duluth Complex. One hundred and fifty participants attended the banquet on Thursday night, even without the prospect of dessert. This year’s banquet speaker was Don Hunter, who is a Project Manager for the PolyMet Mining Corporation’s NorthMet copper-nickel project near Babbitt, MN. Mr. Hunter drew on his long international career in the mining industry for his talk on the ins-andouts of mineral exploration around the globe. The Powerpoint presentation of the informal ‘Homer’ award suffered from severe homerism itself due to an uncooperative projector, but eventually Steve Kissin was honored for his canoeing acumen. As always, a highlight of the banquet was the presentation of the 2007 Goldich medal to Joe Mancuso, retired professor from Bowling Green University in Bowling Green, OH. The medal was presented to Joe by Ron Seavoy, who ended his citation with “Joe, you deserve this,” a sentiment echoed by all who know Joe and his long-term contributions to the geology of the Lake Superior region. The student paper committee had its usual difficult job this year of selecting among 16 student oral and poster presentations. This year’s committee was Marcia Bjornerud from Lawrence University, Daniela Vallini of Woodside Energy, Ltd., and Graham Wilson from Magma Metals. In the end, two first place Best Student Paper awards ($200 each) were given to Sarah Nicholas (Macalester College) for her poster titled: Investigations of sulfide minerals leached in the presence of alkaline solids, and Noah Planavsky (Rosentiel School of Marine and Atmospheric Science) for his talk titled: Rare earth element patterns in Steep Rock Carbonates. In recognition of the excellent student presentations, two additional students were chosen for Honorable Mention ($100 each) – Troy Boisjoli (St. Norbert College) and Larissa Stevens (Lakehead University). Eisenbrey Student Travel Grants were given to 15 students: Patrick Quigley and Michael Taylor – University of Minnesota, Duluth; Malcolm Alexander, Carissa Isaac, and Larissa Stevens – Lakehead University; Troy Boisjoli, Dan Costello and Renata Jasinevicius, – St. Norbert College; Clinton Forsha – Slippery Rock University; Amanda Hogan, Travis Jacob, Tommy Rodengen, and Stephanie Theriault – St. Thomas University; Jody Rymaszewski – University of Wisconsin-Milwaukee; and Noah Planavsky – Rosentiel School of Marine and Atmospheric Science. All awards were presented at the conclusion of the technical sessions. Two of the presentations at the meeting were made by recipients of the 2006 ILSG Student Research Fund, Michael Taylor and Noah Planavsky. xv �The Institute’s Board of Directors met on May 8, 2007 and a brief overview of the meeting is provided below: 1. Accepted the Report of the Chair for the 52nd ILSG from Ron Sage and minutes of last Board meeting from ILSG secretary, Pete Hollings. 2. Accepted the 2006-2007 ILSG Financial Summary from ILSG treasurer, Mark Jirsa. 3. Approved one co-chair from the 53rd meeting, Jim Miller, as on-going board member. 4. Nominated Terry Boerboom of the Minnesota Geological Survey to replace Tom Hart on the Goldich Committee, a position that Terry later graciously accepted. 5. Approved Marquette, Michigan as the location for the 2008 (54th annual) ILSG and co-chairs Ted Bornhorst and John Klasner. 6. Discussed the continued scanning of the ILSG publications and posting of those electronic publications on the ILSG website The 53rd ILSG meeting was a great success and we wish to thank all the people who contributed to that success. The staff of Lutsen Resort was professional and responsive to the needs of a large group. The setting on the shore of Lake Superior was stunning, and the weather was perfect (fireweather, apparently). The field trips this year had a large number of participants, and thanks are due to field trip leaders, van and bus drivers, and everyone else who stepped up when needed to drive, sort lunches, lay out core, or keep the crowds moving. As always, everyone who attended the 53rd ILSG was willing to help as necessary or adapt to any situation that developed. The meeting this year was well attended and we are heartened by the excellent student participation and attendance, a trend we hope continues. Jim and I were very pleased with the outcomes of the 53rd ILSG and hope that others agree that the meeting was a big success. Chairing a meeting requires a lot of organization and planning with a significant time commitment, and we thank our respective organizations for their recognition of the importance of the ILSG. We also thank the ILSG community and members who make the experiences of the co-chairs almost fun, especially once the meeting is over and we encourage others to take on the task. Laurel Woodruff and Jim Miller Co-Chairs, 53rd Institute on Lake Superior Geology xvi �2008 BOARD OF DIRECTORS Board appointment continues through the close of the last meeting year, or until a successor is selected Theodore J. Bornhorst and John S. Klasner, Co-Chairs 54th meeting (2011) Michigan Technological University, MI and retired Western Illinois University, IL Jim Miller (2010) University of Minnesota Duluth, MN Ann Wilson (2009) Ontario Geological Survey, South Porcupine, ON Mark Smyk (2008) Ministry of Northern Development and Mines, Thunder Bay, ON Peter Hollings – Secretary (2008) Lakehead University, Thunder Bay, ON Mark A. Jirsa – Treasurer (2009) Minnesota Geological Survey, St. Paul, MN 2008 SESSION CHAIRS Marcia Bjornerud, Lawrence University, Appleton, WI Milt Gere, Michigan DNR, Lansing, MI Elizabeth A. Gordon, St. Norbert College, De Pere, WI Joe Maki, Michigan DEQ, Office of Geological Survey, Gwinn, MI Penny Morton, University Minnesota, Duluth, MN Klaus Schulz, U.S. Geological Survey, Reston, VA Glenn Scott, Cliffs Mining Service Company George J. Hudak, University of Wisconsin, Oshkosh, WI 2008 STUDENT PAPER COMMITTEE Allan Blaske (Chair), STS, Lansing, MI Melanie Humphrey, Michigan DEQ, Office of Geological Survey, Gwinn, MI Laurel Woodruff, U.S. Geological Survey, Mounds View, MN 2008 COMMITTEES General Co-Chairs Theodore J. Bornhorst – Michigan Technological University, Houghton, MI John S. Klasner – Retired Western Illinois University, Macomb, Illinois Program and Abstracts Editors Theodore J. Bornhorst and George W. Robinson Michigan Technological University Field Trip Guidebook Editors John S. Klasner – Retired Western Illinois University Theodore J. Bornhorst – Michigan Technological University Local Registration Gretchen Klasner – Marquette, MI Registration Darlene M. Comfort – Michigan Technological University xvii �SPECIAL RECOGNITION The Co-Chairs of the 54th Annual Institute on Lake Superior Geology wish to give special recognition to several corporations for their generous financial support. Kennecott Minerals Company A Member of the Rio Tinto Group Eagle Project 1004 Harbor Hills Dr Marquette, MI 49855 Cliffs Mining Services Company Subsidiary of Cleveland-Cliffs Inc. 550 E. Division St. Ishpeming, MI 49849 Aquila Resources Inc. U.S. Office: Suite 310 - 314 W. Superior Street Duluth, MN 55802 STS 401 S. Washington Square, Suite 103 Lansing, MI 48933 1-800-959-4261 www.sts.aecom.com Other STS offices in the Great Lakes region: Marquette, Michigan; Green Bay, Wisconsin; Minneapolis, Minnesota xviii �2008 BANQUET SPEAKER Jon Cherry General Manager Kennecott Minerals - Eagle Project Marquette, MI The Kennecott Eagle Project xix �xx �PROGRAM xxi �TUESDAY MAY 6, 2008 8:00 a.m. FIELD TRIP 1: BANDED IRON FORMATION OF THE MARQUETTE DISTRICT Tom Waggoner, retired, Cliffs Mining Services Company WEDNESDAY MAY 7, 2008 8:00 a.m. FIELD TRIP 1 CONTINUED: BANDED IRON FORMATION OF THE MARQUETTE DISTRICT Tom Waggoner, retired, Cliffs Mining Services Company 8:00 a.m. FIELD TRIP 2: ARCHEAN-PALEOPROTEROZOIC UNCONFORMITY AT SILVER LAKE—SEISMITES FROM THE SUDBURY IMPACT? Bill Cannon, U.S. Geological Survey 8:00 a.m. FIELD TRIP 3: GEOLOGY OF THE BACK FORTY PROJECT Tom Quigley and Bob Mahin, Aquila Resources Inc. 8:00 a.m. FIELD TRIP 4: GEOLOGY OF THE EAGLE PROJECT Andrew Ware and Jon Cherry , Kennecott Minerals Inc. Xin Ding, Indiana University 6:00 p.m. Return of Trips 1, 2 3, and 4 4:00 p.m. - 10:00 p.m. Registration at Ramada Inn 7:00 p.m. - 10:00 p.m. Ice Breaker Social and Poster Session THURSDAY MAY 8, 2008 Note: Asterisk * denotes a student eligible for Best Student Paper Award Presenter underlined 8:00 a.m. - 12:00 noon REGISTRATION 8:30 a.m. INTRODUCTORY REMARKS Theodore J. Bornhorst and John S. Klasner, Co-Chairs, 2008 ILSG xxii �TECHNICAL SESSION I Session Chairs: Milt Gere, Michigan DNR, Lansing, MI Penny Morton, University Minnesota, Duluth, MN 8:40 a.m. Peter Hollings and Mark Smyk Whatever Happened to the Logan Sills? Ongoing Research into the Geochemistry of Midcontinent Rift-related Mafic Intrusive Rocks South of Thunder Bay 9:00 a.m. Curtis D. Williams*, Edward M. Ripley, and Chusi Li The Effect of Magmatic Volatile Phase Separation Linked to Intrusion of the Duluth Complex: Solution to Anomalous Os Isotopic Compositions of the Virginia Formation? 9:20 a.m. Duncan J. Bain* The Shakespeare Cu-Ni-PGE Deposit: Evidence for a Two-Stage Emplacement Mechanism 9:40 a.m. Xin Ding*, Edward M. Ripely and Li C Geochemical and Stable Isotope Studies of Hydrothermal Alteration Associated with the Eagle Deposit, Northern Michigan 10:00 a.m. COFFEE BREAK AND POSTER SESSION 10:20 a.m. Natalie J. Pietrzak*, Norm Duke, Glenn Scott and Helen Lukey A Study of the Paragenetic Stages of Mineral Growth in Complex Iron Ores at the Tilden Mine and Development of a Mine Scale Model for Application to Ore Treatment Methods 10:40 a.m. Muatala H. Muvi-Tjikalepo, Theodore J. Bornhorst, George W. Robinson and W.C. Williams Multi-element Geochemical Signature of Copper Mineralization at the White Pine Mine, Midcontinent Rift System, Western Upper Peninsula, Michigan 11:00 a.m. Natalie King* Using Mineralization to Evaluate Small-Scale Controls on Shale Permeability in the Nonesuch Formation 11:20 a.m. Mary Louise Hill and Andrew Cheatle Iron-formation-hosted gold in the Superior Province of Northwestern Ontario 11:40 a.m. Lunch Break – 2008 ILSG Board Meeting (by invitation) xxiii �TECHNICAL SESSION II Session Chairs: Klaus Schulz, U.S. Geological Survey Glenn Scott, Cliffs Mining Service Company 1:00 p.m. T.W. Buchholz, A.U. Falster and Wm. B. Simmons Observations on Lanthanide Fractionation in the Wausau Complex, Marathon County, Wisconsin 1:20 p.m. G.J. Baldwin*, P.C. Thurston, B.S. Kamber, and M.G. Houle The Deloro-Tisdale SIZ of the Abitibi Greenstone Belt: An Example from McArthur Township, Ontario 1:40 p.m. Patrick Moran*, Philip Fralick, Mary Louise Hill, and Peter Hollings Geochemistry of Sedimentary Rocks associated with the Musselwhite Gold Deposit, Northwestern Ontario 2:00 p.m. Carrissa Isaac* and Pete Hollings Stable Isotope Geochemistry of the Musselwhite Au Mine, N. Ontario: Implications for Mineralization 2:20 p.m. COFFEE BREAK AND POSTER SESSION 2:20 p.m. Norman Duke Evidence for Reactivating Archean Structural Breaks During Paleoproterozoic Rift Sedimentation and Subsequent Accretionary History “Once a Fault Always a Fault” 3:00 p.m. Susan M. Karberg* Structural and Kinematic Analysis of the Mud Creek Shear Zone, Northeastern Minnesota; Implications for Archean (2.7 Ga) Tectonics 3:20 p.m Emerald J. Erickson* and Vicki Hansen Structural and Kinematic Analysis of the Archean Shagawa Lake Shear Zone, Superior Province, Northeastern Minnesota 3:40 p.m. Sally Goodman* Structural and Kinematic Analysis of the Kawishiwi Shear Zone, Superior Province: Insight on Granite-Greenstone Terrain Tectonics and Archean Crustal Evolution 6:00 p.m. ICE BREAKER – MIXER – CASH BAR 7:00 p.m. ANNUAL BANQUET AND AWARD PRESENTATION • Announcement of 55th Annual Meeting Location • 2008 Goldich Award Presentation to Ted Bornhorst • 2008 Banquet Address by Jon Cherry, Kennecott Minerals Company All registered participants are welcome to the banquet address xxiv �FRIDAY MAY 9, 2008 8:50 a.m. INTRODUCTORY REMARKS Theodore J. Bornhorst and John S. Klasner, Co-Chairs, 2008 ILSG TECHNICAL SESSION III Session Chairs: Elizabeth A. Gordon, St. Norbert College, De Pere, WI George J. Hudak, University of Wisconsin, Oshkosh, WI 9:00 a.m. M.G. Mudrey Jr., Peter Hollings, Lura E. Joseph, Mark Jirsa and Jo Kalliokoski On-line Electronic Access to Institute on Lake Superior Geology Publications 9:20 a.m. A.E. Hanson and B.A. Frey MN DNR Drill Core Evaluation Project—The Application of an XRF to Elucidate Gold Mineralization in the Vermilion Greenstone 9:40 a.m. E.M. Fein*, E.C. Ferré and D.K. Holm Flow Fabric Determination of Two Mesoproterozoic Midcontinent Rift Dike Swarms, Northeastern Minnesota 10:00 a.m. Val W. Chandler and Richard S. Lively Upgrade of Aeromagnetic Data at the Minnesota Geological Survey 10:20 a.m. COFFEE BREAK AND LAST POSTER SESSION 10:40 a.m. Lauri J. Pesonen Keweenawan Apparent Polar Wander Path: New Observations , New Ideas 11:00 a.m. James D. Miller, Dean M. Peterson, and George J. Hudak The Inaugural Season of the Precambrian Field Camp at the University of Minnesota Duluth 11:20 a.m. Rodney Johnson, Robert Seasor, and Tom Suszek An Archean-aged PGE-bearing Intrusion, Baraga County, Michigan 11:40 a.m. R.M. Easton and L.M. Heaman Detrital Zircon Geochronology of Hurnoian Supergroup Sandstones located within the Vernon Structure, North of Espanola, Ontario 12:00 p.m. Presentation of Student Awards Student Travel Awards Best Student Paper Awards 12:00 p.m. LUNCH BREAK xxv �TECHNICAL SESSION IV Session Chairs: Marcia Bjornerud, Lawrence University, Appleton, WI Joe Maki, Michigan DEQ, Office of Geological Survey, Gwinn, MI 1:00 p.m. L.G. Medaris Jr. and R.H. Dott Jr. The Seeley Slate and Baraboo Interval Sedimentation 1:20 p.m. Jared D. Lubben Stratigies for Drilling Unconsolidated Material and Historic Underground Mine Workings: Examples from Hibbing Taconite Company’s 2007 Diamond Drilling Campaign 1:40 p.m. W.F. Cannon and K.J. Schulz Unusual Features Along the Archean/Paleoproterozoic Unconformity at Silver Lake, Michigan—Seismites from the Sudbury Impact 2:00 p.m. K.J. Schulz and W.F. Cannon Geochemistry of the Sudbury Impact Layer, Northern Michigan: Implications for the Nature of the Source Materials 2:40 p.m Mark A. Jirsa, Paul W. Weiblen, Tatiana Vislova and Peter L. McSwiggen Sudbury Impactite Layer Near Gunflint Lake, NE Minnesota 3:00 p.m. FIELD TRIP 5: THE SUDBURY IMPACT LAYER AT THE MCCLURE LOCALITY Bill Cannon, U.S. Geological Survey 6:00 p.m. Return of Trip 5 SATURDAY MAY 10, 2008 8:00 a.m. FIELD TRIP 6: SUSTAINABLE RECOVERY OF IRON FROM THE MARQUETTE DISTRICT Glenn Scott, Helene Lukey, Al Strandlie, and CCI/CCMO staff Cleveland Cliffs Inc. 8:00 a.m. FIELD TRIP 7: GEOLOGY OF THE KEWEENAWAN BIC INTRUSION Dean Rossell, Kennecott Minerals Inc. 8:00 a.m. FIELD TRIP 8: GEOLOGY OF THE EAGLE PROJECT Andrew Ware and Jon Cherry , Kennecott Minerals Inc. Xin Ding, Indiana University 6:00 p.m. Return of Trips 6, 7, & 8 54TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY ENDS xxvi �POSTER PRESENTATIONS Terra N. Anderson* and Dyanna M. Czeck Quartz Fabrics and Quantified Strains During Transpressional Deformation in the Seine Metaconglomerates Terrence J. Boerboom, John C. Green and Paul Albers Bedrock Geologic Map of the Lutsen Quadrangle, North Shore of Lake Superior, Minnesota Elizabeth Drommerhausen* and Steven Losh Properties of Fluid Involved in Formation of Natural Ore in the Mesabi Iron Range, Minnesota R.M. Easton, M.G. Houlé, D. Rowell and N.F. Trowell Toward a Common Map Legend for Ontario Shelby J. Frost*, Natalie A. Juda, and Jim Miller Student Capstone Map from the UMD Precambrian Research Center’s Field Camp: Bedrock Geology Map of Homer Lake and Adjacent Areas, Cook County, Minnesota Lynn Galston*, Karen G. Havholm and Stephen T. Hasiotis Reinterpretation of the Trace Fossil-Bearing Devils Island Sandstone, Keweenawan Rift, Northern Wisconsin Mark A. Jirsa, Edward Starns, Daniel E. Costello, Benedek Gal, Steven A. Hoaglund and Amanda J. Putz “Capstone” Geologic Mapping Near Gabimichigami Lake, Boundary Waters Canoe Area Wilderness, By Students of the Precambrian Research Center’s 2007 Field Camp J.N. Koester*, J.W. Goodge, and V.L. Hansen Structural and Metamorphic History of the Burntside Lake Shear Zone with Possible Implications for Archean Granite-Greenstone Formation Helene M. Lukey Mineral Zonation and Stratigraphy of the Tilden Hematite Deposit S. Moosavi*, T.K. Johnson, C. Wendland, A. Anderson, and G.J. Hudak Bedrock Geology of the Footwall to the Soudan Iron Formation South of Twin Lakes, St. Louis County, Northeastern Minnesota Northern Michigan Geologic Repository Association xxvii �Richard Patelke PolyMet Mining: NorthMet Cu-Ni-Co-PGE Project, Hoyt Lakes, Minnesota Dean Peterson Geological Map of the Northern South Kawishiwi Intrusion and Surrounding Areas, Duluth Complex: St. Louis and Lake Counties, Northeastern Minnesota A.K. Sartorelli*, A. Anderson and G.J. Hudak Pillow Morphological Studies Southwest of Fivemile Lake, Vermillion District, NE Minnesota K.J. Schulz, S.W. Nicholson and W.R. Van Schmus Penokean Massive Sulfide Deposits: Age, Geochemistry, and Paleotectonic Setting Tharalson, E., Sweet, G., Boisjoli, T., Lentz, B., Fellows, T., Peterson, D. Geological Map of the Nickel Lake Macrodike and Northern Bald Eagle Intrusion: Lake County, Northeastern Minnesota xxviii �ABSTRACTS xxix �xxx �Quartz fabrics and quantified strains during transpressional deformation in the Seine Metaconglomerates Anderson, Terra N., and Czeck, Dyanna M. Department of Geosciences, University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, WI 53201; tna@uwm.edu. INTRODUCTION Previous studies have documented quartz fabrics in the field (e.g. Bestmann et al., 2004; Lisle, 1985; Stipp et al., 2002) and the lab (e.g. Heilbronner and Tullis, 2002; Hirth and Tullis, 1992; Mainprice and Paterson, 1984). However, these previous studies have not been conducted in areas with deformation under natural conditions where strain can be quantified and deformation mechanisms have been determined. In our study, we link quartz shape and crystallographic fabrics within a deformed metaconglomerate across a strain gradient. Fissler (2006) conducted strain analysis on the Seine Metaconglomerates from the Rainy Lake region in northwestern Ontario. She determined strain independently for various lithological groupings of clasts and determined various degrees of strain types and magnitudes throughout the region. Using her strain results as a framework for our fabric analysis, we use petrographic and Electron Backscattered Diffraction (EBSD) techniques to document the quartz grain fabrics throughout the region. METHODOLOGY Fissler (2006) classified twenty-six outcrops of the Seine Metaconglomerates as low, medium, or high strain. Oriented quartzite clasts and quartz veins were collected from several locations with different strain magnitudes. The quartzite clasts experienced the complete deformation history of the conglomerate whereas the quartz veins, having been intruded during the deformation, only exhibit evidence of later deformation stages. The quartzite clasts and quartz veins have a simple mineralogy that is readily compared to numerous results from deformation experiments. Thin sections were made from quartzite and quartz vein samples with the x-direction parallel to lineation and the y-direction perpendicular to foliation. Using a SEM-EBSD system and CHANNEL5 software produced by HKL Technology, an automatic map of the thin section was used to determine the crystallographic orientations of quartz grains. The map was constructed to maximize the area analyzed on the thin section with one measurement taken from each grain. The step size was determined by the average grain size and ranged from 150 to 200 microns. The data were entered into PFch5, a pole figure-plotting program developed by Mainprice (2005). Microstructures in the Seine have been studied in detail by Czeck (2001) and Fissler (2006). Both found evidence for very similar deformation mechanisms at all strain magnitudes in a range of clast types. Rocks were primarily deformed by diffusive mass transfer processes with some dislocation and micro-fracturing processes in certain phases. Using a petrographic microscope, we conducted additional microstructural analyses to infer deformation mechanisms within the quartz samples because the previous studies did not focus on quartz. PRELIMINARY RESULTS Petrographic analyses indicate the following microstructures. In low-moderate strained quartzite clasts, grain size is bimodal with larger grains 20 to 40 μm and smaller grains around 5 μm in diameter. Grain boundaries are irregular and show slight preferred orientation parallel to lineation, forming a shape preferred orientation (SPO). Within the grains, undulose extinction is common and some subgrains exist. Some recrystallized grains, primarily as “necklaces” around the larger quartz grains, can be observed at the edges of larger quartz grains. We interpret that the quartz underwent deformation primarily by dislocation creep with subgrain 1 �formation and recrystallization recovery mechanisms. Late stage fractures occurred which cut the main fabric and allowed fluid movement resulting in veins filled with calcite and some minor amounts of mica. In highly strained quartzite clasts, grain size is uniform ranging from 5 to 10 μm. The round, equant grains have regular smooth boundaries that often intersect at triple junctions. Internally, the grains often have undulose extinction throughout, and some subgrains are observed. Calcite fills in fractures that are oblique to lineation. The grain size near these zones is smaller, less than 5 μm in diameter. We interpret that the quartz grains are deforming primarily by dislocation creep, and that the reduced grain size is due to extensive recrystallization. The number and size of late stage fractures increased from the low strained examples. The fractures cut the main fabric and filled with calcite and more mica relative to the low strain clasts. In low-moderate strained quartz veins, grain sizes vary greatly from 20 to 500 μm. Grain boundaries are irregular and show a slight SPO subparallel to lineation. Some ~5 μm recrystallized grains are observed at the edges of the larger quartz grains. These are seen primarily as “necklaces” around the larger quartz grain edges. In moderate-highly strained quartz veins, grain size is fairly uniform ranging from 20-40 μm in diameter. Some larger grains that are 500 μm in diameter exist, often with “necklace” recrystallization of quartz and calcite around them. Boundaries are irregular and larger grains have undulose extinction. Calcite fills in fractures that are oblique to lineation. Preliminary results of the EBSD analyses show increase cluster of crystallographic preferred orientation (CPO) in the c-axis of quartz within quartzite clasts with increasing strain magnitudes. Similarly, in most cases there was an increase in CPO of quartz c-axes within quartz veins when strain magnitudes increased. These observations are consistent with deformation by dislocation creep with recovery primarily by recystallization. One interesting feature that stands out when comparing the microstructural and EBSD studies is that the SPO of quartz grains decreases with increasing strain, and the CPO of quartz grains increases with increasing strain. The increasing CPO without a corresponding increase in SPO is consistent with deformation by dislocation creep with recrystallization. We interpret the different microstructural observations between clasts of low to high strain to be consistent with dislocation creep that operated to a variety of strain magnitudes. REFERENCES Bestmann, M., Prior, D.J., Veltkamp, K.T.A., 2004. Development of single-crystal σ-shape quartz porphyroclasts by dissolution-precipitation creep in a calcite marble shear zone. Journal of Structural Geology, 26: 869-883. Czeck, D.M., 2001. Strain analysis, rheological constraints, and tectonic model for an Archean polymictic conglomerate : Superior Province, Ontario, Canada. Thesis, University of Minnesota. Fissler, D.A., 2006. A Quantitative Analysis of Strain in the Seine River Metaconglomerates, Rainy Lake Region, Northwestern Ontario, Canada. M.S. Thesis, University of Wisconsin-Milwaukee. Heilbronner, R., Tullis, J., 2002. The effect of static annealing on microstructures and crystallographic preferred orientations of quartzites experimentally deformed in axial compression and shear. Deformation Mechanisms, Rheology and Tectonics: Current Status and Future Perspectives; Geological Society of London, Special Publications, 200: 191-218. Hirth, G., Tullis, J., 1992. Dislocation creep regimes in quartz aggregates. Journal of Structural Geology, 14: 145-159. Lisle, R.J., 1985. The effect of composition and strain on quartz-fabric intensity in pebbles from a deformed conglomerate. Geologische Rundschau, 74: 657-663. Mainprice, D. “PFch5” Petrophysical Software, Unicef Careware, 2005. <http://www.gm.univ- montp2.fr/PERSO/mainprice/index.html> Mainprice, D.H., Paterson, M.S., 1984. Experimental studies of the role of water in the plasticity of quartzites. Journal of Geophysical Research, 89: 4257-4269. Stipp, M., Stunitz, H., Heilbronner, R., Schmid, S.M., 2002. The eastern Tonale fault zone: a ‘natural laboratory’ for crystal plastic deformation of quartz over a temperature range from 250 to 700°C. Journal of Structural Geology, 24: 1861-1884. 2 �THE SHAKESPEARE CU-NI-PGE DEPOSIT: EVIDENCE FOR A TWO-STAGE EMPLACEMENT MECHANISM Duncan J. Bain, B.Sc., P.Geo., Department of Earth Sciences, University of Western Ontario, London, ON, N6A 5B7, dbain3@uwo.ca The Shakespeare Cu-Ni-PGE Project occurs in Shakespeare and Baldwin townships, located 70 km west-southwest of Sudbury, Ontario, Canada and is held by Ursa Major Minerals Incorporated. The property contains probable reserves of 7.3 million tonnes of 0.37% Ni, 0.39% Cu, 0.024% Co, 0.37 g/tonne Pt, 0.40 g/tonne Pd and 0.20 g/tonne Au and an indicated resource of 12.0 million tonnes of 0.35% Ni, 0.36% Cu, 0.02% Co, 0.34 g/tonne Pt, 0.38 g/tonne Pd and 0.19 g/tonne Au. It varies in width from 24 to 61 m and has an exposed strike length of 600 m but is open along strike and at depth. This mineralized zone occurs at the stratigraphic top of a Nipissing gabbro sill injected along the contact between metamorphosed hangingwall Mississagi Formation quartz arenites and footwall Pecors Formation wackes, both of the Paleoproterozoic age Hough Lake Group of the lower Huronian Supergroup. These units form the bulk of the Southern Structural Province. The deposit strikes ENE and dips steeply to the north. It consists of blebs, stringers and disseminated grains of pyrrhotite, chalcopyrite, pyrite, cobaltite and pentlandite within a melanogabbro to quartz diorite sill. Several mafic magmatic events overlap with deposition of the Huronian sediments. The 2490 Ma East Bull Lake Layered Intrusive Suite underlies the Huronian Supergroup and is cut by the 2473 Ma Matachewan and 2446 Ma Hearst dyke swarms. Bimodal mafic to felsic volcanism (Elsie Mountain, Stobie, Copper Cliff and Salmay Lake formations) and granite intrusions (Creighton and Murray) occurred between 2450 Ma and 2380 Ma. The Nipissing gabbro sills intruded Huronian sediments between 2220 and 2210 Ma. These magmatic events indicate a continental margin or intracratonic rifting environment, probably initiated by a mantle plume head. This re-activated the long-lived Great Lakes Tectonic Zone of Neoarchean age. The ~ 1870 to 1850 Ma Penokean Orogeny collisional event overprints the Southern Province, and late thermal nodes relate to 1750 to 1730 Ma intrusions such as the Cutler Granite. In the Shakespeare area the thermal effects imposed upper greenschist to lower amphibolite facies metamorphism. Post-Penokean mylonitic shearing re-activated regional scale northeast-trending Penokean structures such as the Murray and Hunter Lake faults. The East Bull Agnew Lake Intrusion occurs directly west of the Shakespeare deposit. Previous mapping by the Ontario Geological Survey shows the Nipissing gabbro at Shakespeare as a feeder system cutting up through both Archean basement rocks and Agnew Lake Intrusion. The 2003/2004 field seasons were spent detailed mapping and sampling the Shakespeare and Nipissing gabbro sills and Huronian sedimentary rocks. Examination of the deposit area and property grids to the ENE revealed “windows” of melanogabbro along a 10 km strike length under a thick cap of Mississagi Formation quartz arenites. This suggests the Shakespeare sill overlies much of the Nipissing sill throughout the immediate area. Locally the main Nipissing sill is a saucer shaped body dipping ~ 20o NNW but in the area of the deposit it and the overlying Shakespeare units dip 40o to 60o NNW. Cumulate layering of sulphides at the base of the Shakespeare melanogabbro indicates postmineralization tilting. Lidar imagery shows numerous property scale structures with a NNE to NE trend associated with the Hunter Lake Fault system; at least two are mylonite shears, Stumpy Bay and Spanish River. The thin 24 to 61 m Shakespeare sill overlies the thick 350 to 600 m Nipissing sill and is differentiated from basal melanogabbro through to upper quartz diorite, and melanogabbro is the main host for Shakespeare mineralization. 3 �Hand samples show color and texture differences between the melanogabbro at the base of the Shakespeare sill and the gabbro of the underlying Nipissing sill. Thin and polished section petrography of 45 samples of core and surface rock were studied to establish the degree of metamorphic overprinting. Most of the primary minerals, essentially plagioclase and augite, have been moderately to very strongly altered to chlorite, epidote, actinolitic hornblende and biotite, confirming the medium-grade regional metamorphism reported by others. Primary igneous textures are recognizable, a confirmation of essentially only thermal metamorphic effects. Apatite is a common trace mineral in thin section and accounts for the elevated P2O5 content. Polished sections showed fine blebs of pyrrhotite-chalcopyrite, with rare pentlandite. Minor amounts of ilmenite rim magnetite, accounting for a low TiO2 to FeO* ratio. However, both FeO* and TiO2 as well as V are higher in Shakespeare melanogabbro than in Nipissing gabbro, which indicates a Shakespeare melt more evolved than the main Nipissing magma. Major, minor, trace and REE geochemical studies were carried out to compare Shakespeare melanogabbro to the main Nipissing gabbro. Melanogabbro ranged down to 47.01% SiO2 whereas Nipissing gabbro is no lower than 50.23% and averages 52% SiO2. Al2O3 and CaO are also lower in melanogabbro, consistent with its lower plagioclase content. A lower Mg# indicates that melanogabbro is more differentiated than Nipissing gabbro. Rb/Sr ratios, Y and Zr are elevated in Shakespeare compared to average Nipissing gabbro values, reflecting contamination by continental crust. Chondrite-normalized REEs from Shakespeare melanogabbro and Nipissing gabbro show similar patterns but with higher abundances in Shakespeare, again indicating the Shakespeare melt is more evolved. No Eu anomaly is present in any of these rocks and Shakespeare and Nipissing are probably separate melt batches from the same chamber with Shakespeare more evolved than Nipissing. Ni-Cu-Co are erratically anomalous in both sill types. Agnew Lake mineralization has much lower Cu:Ni and Au/Pt+Pd ratios than the Shakespeare/Nipissing mineralization. These markedly differing metal signatures show the Shakespeare deposit cannot be derived by any direct remobilization process from Agnew mineralization that may have been encountered in the feeder system. The Shakespeare sill originated from the evolved melt capping the Nipissing magma chamber. The initial heave of this melt was intruded along the contact between hangingwall Mississagi quartz arenites and footwall Pecors wackes. Continental contamination, including sulphur from Pecors and silica mixing from Mississagi, lowered the temperature of sulphur saturation within a sulphurenriched Shakespeare melt, causing the rapid precipitation of Ni-Cu-Co sulpides and PGEs to the chamber floor. The large main heave of Nipissing melt was injected soon after, underplating the Shakespeare sill. Post-deposition extensional faulting caused the mineralized Shakespeare and Nipissing sills to be tilted along an ENE hinge line to produce the present day form of the deposit. 4 �The Deloro-Tisdale SIZ of the Abitibi greenstone belt: an example from McArthur Township, Ontario G.J. Baldwin1, P.C. Thurston1, B.S. Kamber1, M.G. Houle2 1 Dept. of Earth Sciences, Laurentian University, Sudbury, ON P3E 2C6 2 Ontario Geological Survey, Sudbury, ON P3E 6B5 The NeoArchean Abitibi Greenstone Belt encompasses 6 geochronologically resolvable tectonostratigraphic assemblages consisting of volcanic and minor sedimentary rocks. The assemblages were autochthonously deposited between 2750 Ma and 2696 Ma. Evidence for autochthonous emplacement is, for example, evident from zircon xenocrysts inherited from the 27302724 Ma Deloro assemblage in the 2710-2704 Tisdale assemblage, which are in direct, stratigraphic contact.. In the field, there is no evidence for an erosional unconformity between these units, but instead a series of sedimentary interface zones (SIZ’s) consisting primarily of Banded Iron Formation (BIF) and other related sedimentary rocks. South of Timmins, ON, in the Bartlett Dome area (McArthur, Bartlett, and English Townships) three of these SIZ’s can be observed in the Deloro assemblage. The uppermost SIZ follows the contact between the Deloro and Tisdale assemblages; however the best exposed and geochronologically constrained is the middle SIZ, particularly in McArthur Township. A nearly complete section of the ~50 m thick middle SIZ reveals 6 distinct lithologies from base to top; a substrate of felsic volcanic rocks; basal sulphide facies BIF; silicate facies- BIF; oxide facies BIF; debris flow bearing BIF; and oxide-sulphide facies BIF, with some of these lithologies recurring higher in stratigraphy.. Stratigraphy and primary structures represent features characteristic of submarine unconformities (Shanmugam, 1988), such as heterolithic debris flows, syn-sedimentary deformation features and cherty hardgrounds. Tabular chert breccia units were observed in stratigraphic association with debris flows, and chert-wacke units indicating synsedimentary production of the breccia units and early silicification prior to the debris flow event. Petrographic observations strongly suggest that many of the cherts within the BIF may in fact be silicified ash, an assertion that is being tested using REE+Y geochemistry. These geochemical studies will also aid in distinguishing between hydrothermal vs. seawater origin of Si in the formation of the BIF in the SIZ. U-Pb zircon dating of rhyolites immediately above and below the SIZ are used to help to constrain the deposition period. The underlying rhyolite yielded an age of 2728.1 +/- 1.6 Ma while the overlying rhyolite returned a slightly less precise age of 2724.5 +/- 2.1 Ma. The deposition of this particular SIZ could thus have lasted for a period of 1-6 million years. Taken at face value, the dates indicate a depositional period of 3.6 Ma and hence extremely slow sedimentation rates, or prolonged periods of water-rock interaction during the period of deposition. Despite the potential overlap in these dates, the overlying U-Pb age is similar to ages for felsic rocks overlying iron formation, further north in Deloro Twp (2724 +/- 3.7 Ma), as well as in the correlative Swayze greenstone belt (2724 =/2 Ma) (van Breeman et al, 2006), indicating a regional pattern. Further study is required at other localities along this and other SIZ’s in the Bartlett Dome area to confirm the interpretation that these SIZ’s represent a major gap in volcanism, possibly across the Abitibi greenstone belt. Shanmugan, G. 1988, Origin, recognition, and importance of erosional unconformities in sedimentary basins in Kleispehn, K.L. and Paola C., eds., New Perspectives in Basin Analysis (Frontiers in Sedimentary Geology): New York, Springer-Verlag, p. 84-108. van Breeman, O., Heather, K.B., and Ayer, J.A. 2006, U-Pb geochronology of the NeoArchean Swayze sector of the southern Abitibi greenstone belt. Geological Survey of Canada, Current Research 2006-F1, p 1-32. 5 �BEDROCK GEOLOGIC MAP OF THE LUTSEN QUADRANGLE, NORTH SHORE OF LAKE SUPERIOR, MINNESOTA BOERBOOM, Terrence J., Minnesota Geological Survey, boerb001@umn.edu GREEN, John C., University of Minnesota-Duluth, jgreen@d.umn.edu ALBERS, Paul, Duluth Metals, palbers@duluthmetals.com The Minnesota Geological Survey is continuing to map the bedrock geology of 7.5’ quadrangles near Lake Superior as part of the USGS STATEMAP program, resulting to date in eleven published 1:24,000 scale maps from Duluth to Lutsen, in addition to 10 quadrangles already published under the former USGS COGEOMAP program. The Deer Yard Lake and Good Harbor Bay quadrangles will be published in July 2008 (Fig. 1A). All the maps in this series are available as printed maps, or as PDF and Arcview export files at the MGS website (http://www.geo.umn.edu/mgs/). With the exception of local thesis mapping by Albers (2006) and a regional-scale compilation map (Map M-119; Miller and others, 2001), the Lutsen quadrangle had not been published, and had been incompletely mapped. Approximately 30 sets of water well cutting samples, collected by Mckeever Well Drilling of Little Marais, Minnesota, were examined as part of the mapping effort. The area of this map lies in the uppermost portion of the Northeast sequence of the North Shore Volcanic Group (NSVG), and also the unconformably overlying Schroeder-Lutsen sequence (Fig. 1B). In addition, components of the Beaver Bay Complex, including the Leveaux ferrodiorite, the Lake Clara and Monker Lake diabases, and multiple phases of the Beaver River diabase are present in the map area. The new mapping has refined the volcanic stratigraphy of the NSVG in this area and has identified several previously unrecognized units and stratigraphic relationships. In particular it has been demonstrated that the Terrace Point basalt flow, which was formerly considered to be the lowermost unit of the Schroeder-Lutsen sequence, is instead part of the underlying, slightly older lava flows. In addition, three thick sandstone units were identified. One of these, the Eagle Mountain sandstone, is intersected in several water wells, and ranges from 6 to 18 meters in thickness; however much of the original stratigraphic thickness may have been removed by intrusion of the overlying Leveaux ferrodiorite sill, which apparently utilized the sandstone as a plane of weakness during emplacement. The other sandstone units are largely extrapolated from the north and east, where they have been identified in outcrops or water well cuttings as units up to 100 meters thick; they are extrapolated into this map area on the basis of linear topographic depressions and coincident linear negative aeromagnetic anomalies. Intrusive rocks of the Beaver Bay Complex are composed mainly of the Beaver River diabase, the porphyritic Leveaux ferrodiorite, and a newly-recognized ‘Silver-Bay’ type intrusion composed of a lower cumulate ferrogabbro and upper pyroxene-quartz ferromonzonite. The Leveaux ferrodiorite is a subvolcanic sill emplaced into volcanic rocks, and is intruded by the Beaver River diabase. The Monker Lake diabase dike (250 – 300 meters thick) is poorly exposed in the Lutsen quadrangle, but is easily demarcated by a sharp, linear positive aeromagnetic anomaly. The Lake Clara diabase, in the northwest corner of the map, is poorly constrained due to relatively poor outcrop and an indistinct aeromagnetic signature. This diabase locally forms a shallow-dipping sill capped by a thin quartz ferromonzodiorite which contains mafic enclaves interpreted as the product of melting of, and contamination by, the overlying rhyolite. References Albers, P.B., 2006, The geology and petrology of the Leveaux porphyritic dioritic intrusion: Investigating possible magmatic relationships to the anorthositic series of the Duluth Complex, Cook County, Minnesota: Duluth, Minn., University of Minnesota Duluth M.S. thesis, 206 p. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Map Series Miscellaneous Map M-119, sheet 1 of 2, scale 1:200,000. 6 �Figure 1. A. Index map showing the location of mapped 7.5’ quadrangles along the North Shore of Lake Superior. M numbers refer to MGS Miscellaneous maps. B. Index map showing the locations of the major units mentioned in the abstract. 7 �OBSERVATIONS ON LANTHANIDE FRACTIONATION IN THE WAUSAU COMPLEX, MARATHON COUNTY, WI. T.W. Buchholz1, A.U. Falster2, Wm. B. Simmons2. 11140 12th St. N., Wisconsin Rapids, WI 54494; 2Department of Earth and Environmental Sciences, University of New Orleans, New Orleans, LA 70148. The Wausau Complex is composed of a series of four anorogenic intrusions ranging in age from 1565 Ma +3-5 for the alkalic Stettin pluton (Van Wyck, 1994) to 1505.9 ± 2.7 Ma for the Nine Mile Granite (Dewane & Van Schmus, 2007). The complex is usually considered to be a precursor to the younger Wolf River Batholith. Recent work on various sites in the Stettin and Nine Mile plutons has revealed interesting trends in lanthanide fractionation. While most REE bearing minerals are not overly unusual in their compositions, exceptions exist and may be clues to processes involved in the formation of these plutons. Minerals bearing the lanthanide elements, more commonly referred to as the Rare Earth Elements (REE), are relatively common in the Stettin and Nine Mile intrusions of the Wausau Complex (Falster et al, 2000). In general, minerals bearing REE elements will tend to accumulate either the light rare earth elements (LREE) or the heavy rare earth elements (HREE), and not both. LREE rich minerals are usually Ce-dominant, and HREE minerals are usually Y-dominant. Significant enrichments in other LREE elements are unusual, and in HREE elements even less common. LREE fluocarbonates, usually bastnaesite or related species, are usually A-site Ce-dominant with lesser levels of La, Nd and other LREE. However, it is not particularly uncommon to observe A-site Nd-dominance, and less commonly La-dominance in bastnaesite-group minerals from the Wausau Complex. This has particularly been noted in a small pegmatite designated the Ravine Pegmatite on the west side of the old Dehnel quarry in the Stettin intrusion, in a pneumatolytic-like assemblage in the Ladick quarry, Nine Mile pluton, and in a late-stage miarolitic phase in the County Materials quarry, Nine Mile pluton. LREE phosphates (typically monazite-(Ce)) are also usually A-site Ce-dominant. However, monazite-(Nd) has been noted in a small pegmatite exposed in a utility excavation on Evergreen Rd, and in mine dump material from the Summit Lake thorium prospect, both in the Stettin pluton. Additionally, minute crystals of rhabdophane-(Ce), rhabdophane-(Nd) and rhabdophane-(La) have been noted as late-formed minerals in a small miarolitic body briefly exposed in Wimmer Pit #3 in the Nine Mile pluton Rhabdophane is a hydrated LREE phosphate typically formed as a late-stage mineral in pegmatites, and may also form under weathering conditions. LREE-bearing members of the pyrochlore group associated with bastnaesite species in the Ravine Pegmatite in the Dehnel pit show LREE enrichment tendencies similar to trends in the associated bastnaesite-group minerals, as does closely associated fersmite (first report of this mineral for Wisconsin). 8 �It is less common to observe enrichment of specific HREE relative to Y, and no instances have yet been noted in the Wausau Complex of HREE minerals which are not Y-dominant. However, strong enrichment in Dy and Yb was observed in xenotime-(Y) intergrown with monazite-(Nd) in a small pegmatite exposed by utility work on Evergreen Drive in the Stettin pluton. Concentrations of Dy2O3 are consistently near 12 wt %, a level that has only been reported previously from the Annie #3 claim, Manitoba. Lesser degrees of Dy and Yb enrichment have also been noted in xenotime-(Y) from border phases of the Dehnel ravine pegmatite, Stettin Pluton, in xenotime-(Y) from a greisen assemblage in the Maguire Pit associated with cassiterite, ferberite/huebnerite and topaz in the Nine Mile pluton, and in xenotime-(Y) from a pegmatite exposed in the Red Rock North pit off of Spring Brook Rd, also in the Nine Mile pluton. Euxenite-(Y) associated with xenotime from the Red Rock North pegmatite shows a similar HREE enrichment pattern, and is also enriched in Ta and somewhat in Ti as well, approaching though not reaching the compositional field of tanteuxenite. Possible causes of fluctuations in relative abundances of LREE and HREE vary. Variations in LREE dominance in an oxidizing environment may result from oxidation of Ce3+ to Ce4+, facilitating retention of Ce4+ in solution and/or removal of Ce4+ from the system. Causes of variations in enrichment of various HREE elements is less certain; some authorities have suggested high levels of F may facilitate enrichment in even-numbered HREE (Dy, Er, Yb) relative to Y. Since F in the form of fluorite and F-rich micas is generally abundant in the Wausau Complex, why is such enrichment uncommon? References: Dewane, T. J., Van Schmus, W. R. (2007): U-Pb geochronology of the Wolf River batholith, north-central Wisconsin: Evidence for successive magmatism between 1484 Ma and 1468 Ma. Precambrian Research, V. 157, pp. 215-234. Falster, Alexander U., Simmons, Wm. B., Webber, Karen L., Buchholz, Tom (2000): Pegmatites and Pegmatite minerals of the Wausau Complex, Marathon County, Wisconsin. Memorie della Società Italiana di Scienze Naturali e del Museo Civico di Storia Naturale di Milano, V. XXX, pp. 13-28. Van Wyck, N. (1994): The Wolf River A-type Magmatic Event in Wisconsin: U/Pb and Sm/Nd Constraints on Timing and Petrogenesis. Institute on Lake Superior Geology, 40th Annual Meeting, Part 1, Program and Abstracts, p. 81-82 9 �Unusual features along the Archean/Paleoproterozoic unconformity at Silver Lake, Michigan—seismites from the Sudbury impact W.F. Cannon and K.J. Schulz, U.S. Geological Survey, Reston VA At Silver Lake, 50 km northwest of Marquette, Michigan, rocks along the unconformity between Archean crystalline rocks and the basal beds of Paleoproterozoic sedimentary rocks show an unusual type of deformation. About a meter of the Paleoproterozoic sediments, part of the Michigamme Formation, are preserved. They consist mostly of laminated mudstones and fine sandstone, with thin lenses of basal conglomerate and breccia. The Archean rocks are mostly granite with lesser metavolcanic rocks. Deformation in Archean rocks was brittle, varying from intense brecciation to small offsets between meter-scale joint blocks; in Paleoproterozoic sediments relict soft-sediment deformation indicates liquefaction and flow as well as injection as sediment dikes into Archean rocks and breccia zones. Fracturing and brecciation of the Archean rocks and consequent injection of dikes of the overlying Michigamme sediments occurs to some extent in all exposures. In places Archean granitic rocks are thoroughly broken into angular fragments ranging down to centimeter-scale or smaller (Figure 1). These fragments are now suspended in a matrix of clastic material derived from the overlying sedimentary beds. Brecciation and dike injection extends for at least 3-4 meters below the unconformity (Figure 2). Dikes with diverse orientations range from nearly a meter to only a few millimeters thick. In some exposures joint blocks of Archean basement rocks, some up to tens of meters long, show meter-scale displacements relative to each other. These displacements resulted in reshaping of the original nearly flat unconformity surface and molded the overlying sediments to this new shape to form an unusual array of small folds with very diverse orientations (Figure 3). We suggest that these unusual features are near-surface manifestations of an earthquake of about M10.5 on the Richter scale that was generated by the Sudbury impact, centered about 500 kilometers to the east, and record the passage of unprecedentedly intense seismic waves. The Sudbury ejecta layer is well documented in the lower part of the Michigamme Formation about 3 km from Silver Lake. We have tentatively identified ejecta particles within some of the sediment dikes indicating that the rocks at Silver Lake were very near the earth’s surface at the time of impact. Part of the energy released by giant impacts such as Sudbury is transmitted as seismic waves more intense than any generated by terrestrial events. The energy arrives first as a compressional wave which imparts energy to the rocks and is followed by the complementary rarefaction wave. During this rarefaction, rocks near the earth’s surface may respond by shattering and upward displacement creating open spaces into which overlying sediments can be injected. We interpret the breccia zones and abundant sediment dikes within the Archean rocks at Silver Lake to record this explosive rarefaction and dilation. The diversely oriented dikes occur in an abundance that requires substantial dilation and volume increase of the upper parts of the basement rocks to create the space now occupied by the dikes. Seismogenic disruption may be an important process in the formation of the Sudbury ejecta horizon and may be the cause of autochthonous breccias at the base of the ejecta-bearing layer at many other recently discovered localities. 10 �11 �UPGRADE OF AEROMAGNETIC DATA AT THE MINNESOTA GEOLOGICAL SURVEY CHANDLER, Val W., and LIVELY, Richard S., Minnesota Geological Survey, 2642 University Ave., St. Paul, MN 55114, chand004@umn.edu Between 2005 and 2007 the Minnesota Geological Survey (MGS) conducted an upgrade of the existing aeromagnetic data for Minnesota. These aeromagnetic data were acquired between 1979 and 1991 as part of a statewide survey, and data processing was limited by the computer capabilities of the time. Since then, significant technological improvements allow efficient handling of massive databases and post-processing tasks, such as line leveling and gridding, to be accomplished with small desktop computer systems. This project to upgrade the MGS aeromagnetic database was designed to reprocess and regrid the data using up-to-date software and hardware. As an added benefit access to the data was improved by making all the upgrades available via the internet. Considerable effort was directed towards recovering line data that were missing from the primary digital archive, a CD produced in 1992 by the National Geophysical Data Center of the National Oceanic and Atmospheric Administration. Approximately 2,700 line kilometers of missing data were fully recovered from digital media that were archived at the MGS and U.S. Geological Survey. Residual magnetic anomaly values for an additional 1,038 line kilometers were approximately recovered by digitizing the position of photo-spotted points and interpolating 100-meter-spaced magnetic profiles from the original state aeromagnetic grid (213.36 meter interval). Efforts were also made to mitigate line-leveling errors in the data, which locally caused striping artifacts, especially in derivative-enhanced grids. Selective leveling, using either statistical iteration or manual-editing, was applied to a total of 229 flight-lines and 101 tie lines, equating to approximately 21,000 line kilometers of adjusted data. In addition, approximately 522,933 line kilometers of data, nearly all of the available line data, were subsequently adjusted by micro-leveling. The revised line data were used to generate new aeromagnetic grids for Minnesota with a grid interval of 100 meters. To provide a regional context and minimize boundary discontinuities, gridded aeromagnetic data from areas adjoining Minnesota were also incorporated. To enhance the utility of the data for geologic mapping, filtered grids were produced, including aeromagnetic data reduced to vertical polarization (reduced to pole or RTP), the first vertical derivative of the RTP data (Fig. 1), and the second vertical derivative of the RTP data. The revised line data and the new statewide grids are downloadable from the MGS website at http://www.geo.umn.edu/mgs/magnetics.htm. Additional information available from this site includes copies of flight logs from the original surveys, and grid-based estimates of depth to magnetic basement derived using Euler’s equation. These products represent a major step forward in providing complete and up-to-date aeromagnetic coverage for Minnesota, and should serve the needs of the exploration and scientific communities in the region for many years to come. Acknowledgements: This study was supported by the Minnesota Minerals Coordinating Committee. Acquisition of most of the aeromagnetic data was supported by the Legislative Commission on Minnesota Resources. Additional data were contributed by the U.S. Geological Survey, U.S. Steel Corporation and the Geological Survey of Canada. David Dahl of the Minnesota Department of Natural Resources converted original line data to the NAD83 datum, and edited these data for repeats and other spurious features prior to this project. Robert Kuchs and Patricia Hill of the U.S. Geological Survey were instrumental in retrieving data originally acquired by their agency. Processing of the aeromagnetic data in this project was conducted using the OASIS/MONTAJ software system of Geosoft. 12 �Figure 1. First Vertical derivative of the revised aeromagnetic data reduced to pole. 13 �Geochemical and stable isotope studies of hydrothermal alteration associated with the Eagle deposit, northern Michigan Ding, Xin, Ripley, Edward M., Li C Department of Geological Sciences, Indiana University, 1001 East 10th Street, Bloomington, IN 47405 The Eagle Ni-Cu-PGE deposit occurs in olivine-rich melagabbro, melatroctolite, and feldspathic peridotite units that were emplaced during the early stage of development of the Midcontinent Rift. U-Pb dating of baddeleyite from a feldspathic peridotite indicates a crystallization age of 1107.2±5.7 Ma. Mineralization is characterized by the presence of two semi-massive sulfide zones in olivine-rich rocks that are linked by a zone of massive sulfides. Pyrrhotite, pentlandite, and chalcopyrite comprise the bulk of the sulfide mineral assemblage. Hydrothermal alteration in the igneous rocks is locally intense. Phase relations involving serpentine, chlorite, actinolite, prehnite and pumpellyite suggest the temperatures of alteration did not exceed 300℃. Olivine in the intrusive rocks has been variably converted to mixtures of serpentine and fine-grained magnetite (Fig.1). Interstitial pyroxene and plagioclase have been altered to mixtures of serpentine, chlorite, amphibole, and lesser amounts of talc. Disseminated sulfide minerals are locally replaced by magnetite, serpentine, and chlorite. Serpentine shows a broad and distinctive compositional spectrum, with variations linked to the parent mineral which is replaced. Serpentine which replaces olivine is enriched in Mg and depleted in Al and Fe relative to serpentine that replaces pyroxene and sulfide minerals (Fig.2). Aluminum is thought to be derived from clinopyroxene, and the relatively high Al content of serpentine associated with sulfide minerals suggests that Al was a mobile element during low-temperature hydrothermal alteration. δD value of serpentine range from -85 to -109, suggesting equilibration with fluids of δD values between -58 and 82 at 300℃. The δD values suggest that evolved, long path-length meteoric water characterized by a low time integrated water/rock ratio was responsible for the alteration. Oxygen isotopic analyses of serpentine are in progress; results will aid in further constraining fluid sources. 14 �Figure 1 Photomicrographs of serpentine replacing olivine, clinopyroxene and sulfide in Eagle deposit 15 �Properties of fluid involved in formation of natural ore in the Mesabi Iron Range, Minnesota Elizabeth Drommerhausen, Steven Losh; Dept of Chemistry and Geology, TN 242, Minnesota State University, Mankato MN 56001 Natural ore in the Mesabi Range has long been thought to have been formed by downward percolation of meteoric water, which dissolved non-oxide minerals (chiefly chert) from the Biwabik iron formation and oxidized, hydrated, and concentrated the remaining iron oxides. However, some investigators (e.g., Gruner, 1946) have suspected a hydrothermal origin for some if not all of the fluid that interacted with the iron formation. Morey (1999) hypothesized that the fluids responsible for formation of natural ore were driven northward and upward along an aquifer from the Penokean-aged collision zone ~100 miles south of the Mesabi Iron Range. We are evaluating the origin and the fate of fluids involved with leaching silica and producing natural ore by fluid inclusion and cathodoluminescence techniques. In the Hibbtac pit, brecciated quartz veins lie within and next to minor high-angle fault zones that experienced movement due to volume loss that accompanied chert dissolution from iron formation; oxidation and dissolution generally took place in the hangingwalls of the faults. Because the quartz is largely confined to faults that moved as a result of dissolution and the quartz is brecciated by movement on those faults, the quartz we sampled was very likely precipitated from the same fluid that dissolved chert from the iron formation. Thus temperatures and salinities of fluid inclusions hosted by this quartz may thus constrain the source of the fluid involved with formation of natural ore. We have also examined fluid inclusions in quartz from a 6-inch thick bedding-parallel quartz + bladed hematite vein from the LTV Number 6 pit near Aurora. The iron formation is unoxidized and gently-dipping, and bedding is slickensided, implying low-angle fault movement. Other veins in slickensided, unoxidized iron formation at this location contain carbonate minerals, but the vein we examined did not. Preliminary fluid inclusion data from two Hibbtac quartz vein samples from one high-angle fault indicates that the fluid that dissolved silica from the iron formation. Fluid inclusions in these samples are primary, simple liquid + vapor inclusions exhibiting the same liquid:vapor ratio throughout. They are not necked or otherwise deformed. The fluid was characterized by a wide range of temperatures as indicated by homogenization temperatures (Thomog): two vein samples from one fault give Thomog of 60º to 122º C (n=6). Salinities, determined from ice melting temperatures, range from 4.5% to 19 wt% NaCl equivalent. Fluid inclusion from the quartz + hematite vein at the LTV 6 mine have similar characteristics. Fluid inclusions from a second fault at the Hibbtac mine display significantly higher homogenization temperatures, from 164º to 240º C (n=5). Clearly, nearly all of these fluids are much hotter and have much higher salinity than expected for meteoric waters that percolated a short distance downward from the paleo-surface, even during a period of warm climate. Their characteristics are those of a relatively deeply-sourced fluid. 16 �The range in homogenization temperatures, which can vary by tens of degrees over a distance of millimeters, implies mixing of relatively warm and cool fluids during precipitation of quartz. Fluid mixing and/or episodic pulsing may manifest as alternating domains of contrasting cathodoluminescence. Cold cathodoluminescence reveals some growth zoning in quartz from the LTV Pit 6 samples. The preliminary data in hand support the notion that the rocks presently exposed in the Mesabi Iron Range were infiltrated by warm, saline fluids that are quite similar at two locations, and in two different stages of quartz paragenesis, that are about 50 miles apart. We are continuing to obtain more data from already-sampled localities and to expand the number of sample localities throughout the Mesabi Iron Range to evaluate the nature and variability, in space in time, of properties of fluid that interacted with rocks of the Biwabik Iron Formation. Gruner, J., 1946, The Mineralogy and Geology of the Taconites and Iron Ores of the Mesabi Range, Minnesota; St. Paul, Office of the Commissioner of the Iron Range Resources and Rehabilitation, 127 pp. Morey, G., High-grade iron ore deposits of the Mesabi Range, Minnesota – Product of a continental-scale Proterozoic groundwater flow system; Econ. Geol. v. 94, pp. 133-141 17 �EVIDENCE FOR REACTIVATING ARCHEAN STRUCTURAL BREAKS DURING PALEOPROTEROZOIC RIFT SEDIMENTATION AND SUBSEQUENT ACCRETIONARY HISTORY “ONCE A FAULT ALWAYS A FAULT” Norman Duke, Department of Earth Sciences, University of Western Ontario, London, ON, N6A 5B7, nduke@uwo.ca Isolated remnants of Paleoproterozoic cover sequences are occasionally preserved along structural breaks within Archean basement, even well removed from the major tracts of sedimentary cover. Excellent examples occur in the northeast Churchill Province where Hurwitz outliers are preserved along Pykes Break on the northern margin of the Rankin Greenstone Belt as well as along similar basement faults occurring within the Kaminak Greenstone Belt. Outliers of Huronian occur along segments of the Kirkland-LarderCadillac-Bousquet break and the Cobalt Plate is transected by the Temagami-Ville MarieBelleterre break in the southern Superior Province. A prime example from the Penokean Fold Belt is the outlier of Adjibik Quartzite preserved at Ropes, well interior to the Dead RiverIshpeming Greenstone Belt north of the Marquette Trough. That the Negaunee Iron Formation buttresses against the Southern Gneiss Complex, a wholey different Archean terrane-type compared to the bordering greenstones to the north, indicates reactivation of Archean subprovince boundaries played an important role in forming the major depositional troughs. The origin of the Archean breaks stem from the terminal Neoarchean cratonization event, i.e. complex fault zones accommodating the unroofing of the voluminous granite plutons and bounding local depocenters of orogenic ash tuff and polymictic conglomerate (see Figure A). Subsequent reactivation occurred during Paleoproterozoic rifting, accommodating the deposition of thick siliciclastic cover successions (see Figure B). Closing of rift basins during the Hudsonian/Penokean collisional events re-established the Archean basement to pre-rift thicknesses by reverse shear and lead to open folding and upward increasing tectonic telescoping within the overlying sedimentary cover successions (see Figure C). As a result, Proterozoic sedimentary outliers are preserved along reactivated Archean basement break environments, while abrupt stratigraphic/structural buttressing is commonplace along the boundaries of the major sedimentary troughs (see Figure D). Break reactivation well accounts for Proterozoic metamorphic overprinting of Archean lode gold mineralization, and may even have played a pivotal role in the late oxidation of the Negaunee siderite ores marginal to the Southern Shear, the southern boundary to the Marquette Trough. Strong brittle fracturing with associated chlorite that overprints the Southern Shear suggests this particular boundary fault had a protracted history culminating in normal displacement late in the Penokean. Could the oxidation of the Negaunee siderite bands to form supergene magnetite-martite-platey hematite ores relate to meteoric ingress as late as 1730 Ma? The primary lessons to be gleaned from the geological evidence for repeated displacements are: “once a fault, always a fault”; that these structures accommodated opposing normal/reverse displacements over time; and that they remained a focus for very different fluid types over their protracted geological history. 18 �A B C D Figures: A - 2695-2675 Ma Neoarchean Orogenic Break. B - 2450-2200 Ma Paleoproterozoic Rift. C - 1850 Ma Penokean Collision. D - Post-Penokean Thermal Domes. 19 �Toward a Common Map Legend for Ontario R.M. Easton, M.G. Houlé, D. Rowell, and N.F. Trowell, Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, 7th Floor, Sudbury, ON P3E 6B5, Canada, mike.easton@ontario.ca Ontario encompasses over 1 million km2, 3 major Precambrian geological provinces (Superior, Southern, Grenville) and a variety of younger Paleozoic rocks and Quaternary sediments. The Ontario Geological Survey (OGS) typically produces maps at several scales: detailed (1:20 000), regional (1:50 000 and 1:250 000), or provincial (1:1 000 000). Historically, creation of map legends has been individualistic, dependent on both the mapping geologist and the character and scale of the map area. With the advent of increasing interjurisdictional collaborative mapping projects, digital collection of field data, and a focus on compilation maps, there is an increasing need for the OGS to develop a common map legend for all OGS Precambrian geology maps. Creating a common map legend is more challenging than simply creating a standard scheme for rock nomenclature, although the latter is a necessary prerequisite. A map legend must also be able to reflect relative age relationships, formal stratigraphic nomenclature where established, be applicable to areas of well known and poorly known geology, yet be independent of map scale. Furthermore, the methodology used must be relatively easy for the end user to understand and, for hard-copy maps, must contain as few characters as possible in order to avoid clutter and be comprehensible. This presentation presents a first attempt by the OGS to create a common map legend. At its essence is a basic 3 or 4 letter alphanumeric code: legend/colour code_rock class_root rock name[±key modifier]. For example, 5Ig[bi] is a biotite granite, assigned colour code of unit 5. Comments on the merits and drawbacks of this scheme are welcomed by the authors. During the course of developing a common map legend, specific problems were encountered with respect to existing nomenclature schemes for metamorphic, altered, and/or deformed rocks (a.k.a. composite-origin rocks) rocks. For example, is composition critical to the name (e.g., marble); or texture (e.g., schist); or protolith (e.g., metabasalt); or mineralogy (e.g., diopside marble); or a combination of factors (e.g., kyanite metapelite)? Unfortunately, all recent rock nomenclature schemes for compositeorigin rocks (e.g., Fettes and Desmons 2007) have focused on naming such rocks at the hand sample level, not at a macroscopic scale. To address this problem, a hierarchical nomenclature scheme, is proposed, as outlined in brief in the table below. Type Metamorphic Alteration Structural Subtype Root Name (examples) Structural Compositional schist, gneiss, granolite marble, amphibolite, pelite breccia, agmatite migmatite silicification fenite, skarn cataclastite and related rocks mylonite and related rocks Textural Hybrid Hydrothermal Metasomatic Brittle Ductile Impact Optional Modifier (textural, mineralogical, descriptive) kyanite schist calcitic marble fault gouge melt, breccia References Fettes, D. and Desmons, J. 2007. Metamorphic Rocks: A Classification and Glossary of terms, Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Metamorphic Rocks; Cambridge University Press, Cambridge, UK, 244p. 20 �Detrital zircon geochronology of Hurnoian Supergroup sandstones located within the Vernon structure, north of Espanola, Ontario R.M. Easton, Precambrian Geoscience Section, Ontario Geological Survey, Sudbury, Ontario P3E 6B5 mike.easton@ontario.ca and L.M. Heaman, Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3 larry.heaman@ualberta.ca Located north of Espanola, Ontario, the Vernon structure is a former depositional graben which preserves a near complete stratigraphic section through the Huronian Supergroup; from its contact with Neoarchean basement to the Lorrain Formation (upper Cobalt Group) (Easton 2005, 2006). The graben is bounded by Neoarchean monzogranitic rocks of the Birch Lake granite, part of the southern Abitibi subprovince, which has yielded a U-Pb zircon age of 2651±1 Ma (Kamo 2006). The depositional setting of the Huronian Supergroup in the Vernon structure provides an opportunity to investigate the influence of local versus regional provenance sources on the composition of Huronian Supergroup metasandstones. For comparison, Rainbird and Davis (2006) provide regional data on the detrital zircon geochronology of Huronian Supergroup metasandstones from the Elliot Lake and Cobalt areas. Samples were collected from 3 sandstone-dominated formations in the Vernon structure, from stratigraphically lowest to highest these are the Matinenda, Mississagi and Serpent formations. U-Pb zircon geochronology was conducted using laser-ablation analyses at the University of Alberta; 45 grains were analyzed from the Matinenda sample, and 36 grains each from the Mississagi and Serpent samples. The Matinenda Formation sample was collected from the base of the formation where it overlies a package of Huronian Supergroup mafic metavolcanic and volcaniclastic rocks in Baldwin Township. Most of the near concordant grains in the sample are Neoarchean, and define 2 groupings; 1) 2600 to 2690 Ma (n=18), and 2) 2690 to 2740 Ma (n=8). The latter age range is representative of the age of volcanism and plutonism in the Abitibi greenstone belt. The younger age range images emplacement ages representative of plutonic rocks of the southern Abitibi subprovince. In addition, 4 near concordant grains gave an average 207Pb/206Pb age of 2455±9 Ma, which is interpreted as representing the age of locally-sourced Huronian metavolcanic material. Rainbird and Davis (2006) document a similar population distribution of Neoarchean grains in their Matinenda Formation sample, but they did not find any Paleoproterozoic grains. The Mississagi Formation sample was collected from the uppermost part of the formation in Porter Township. This sample has an SiO2 content of 93 wt.%, and a Chemical Index of Alteration (CIA) value of 65. Again, the most concordant grains defined 2 groups, with 10 grains in the range from 2600 to 2690 Ma, and 5 grains in the 2690 to 2740 Ma interval. This sample also contained 4 grains in the range 2900 to 3000 Ma and 1 grain at 3320 Ma. This distribution is similar to that reported by Rainbird and Davis (2006), who also reported a population of very old grains, which only occurred in the Mississagi Formation. Roughly half of the analyzed grains in the Mississagi Formation sample were >10% discordant, in contrast to the other two samples, where significant discordance was much less common. The Serpent Formation sample was collected from the middle of the formation in Porter Township. This sample has a SiO2 content of 81.3 wt. %, a NaO2 content of 5.0 wt. % and a CIA value of 82; the high sodium content is typical of the formation. Although the two Neoarchean age groups are also present in this sample, the younger group (2600 to 2690 Ma) is dominant (22 grains), with only 2 grains present from the 2690 to 2740 Ma group, and 2 grains ranging from 2740 to 2790 Ma. This result differs from Rainbird and Davis (2006), who found the 2690 to 2740 Ma range dominant, as well as the presence of a sizable Geon 28 population. The dominance of the younger Neoarchean grains in the Serpent Formation in the Vernon structure may indicate that the sandstone there is sourced more locally than the Elliot Lake sample studied by Rainbird and Davis (2006). 21 �The geochemistry of the metasandstones provides limited provenance information. The Huronian metavolcanic rocks in the Vernon area lack any diagnostic trace element signatures (such as high Cr contents) that could be detected in the sandstones. The Birch Lake granite is characterized by high SiO2 (70-74 wt.%) and elevated thorium contents (20-50 ppm), and the metasandstones contain between 15 and 140 ppm thorium, with low uranium. Thus, plutons similar in composition to the Birch Lake granite could have provided a source for both quartz and thorium for the sandstones in the Vernon structure. Probably the most significant result from this study is the abundance of grains in the interval from 2600 to 2690 Ma (mostly 2660 to 2680 Ma) in all 3 samples, which appear to increase in abundance up section. If the sandstones were sourced solely from the northern Abitibi subprovince, the dominant population would be expected to range from 2690 to 2740 Ma. The limited geochronology available for intrusive rocks in the southern Abitibi subprovince range in age from 2660 to 2680 Ma (e.g., Prevec 1993, Krogh et al. 1984), suggesting that a significant portion of the detrital zircons in the sandstones are derived locally from the southern Abitibi subprovince. A titanite grain from the Matinenda Formation sample yielded a Pb207/Pb206 age of 2139±15 Ma, similar to regional Rb/Sr ages reported from Nipissing gabbro intrusions in the Southern Province (e.g., 2110±80 Ma, adjusted to current decay constants, Van Schmus 1965), perhaps reflecting postemplacement cooling of Nipissing gabbro intrusions that were emplaced into the Huronian Supergroup at circa 2215 Ma, or a regional metamorphic event. Attempts to date the timing of regional metamorphism using staurolite grains present in lower amphibolite facies metapelitic rocks of the McKim Formation, collected from classic localities near Agnew Lake, were inconclusive, due to low U contents in the staurolite grains coupled with the effects of metamorphic retrogression on the grains. References Easton, R.M. 2005. Geology of Porter and Vernon townships, Southern Province; in Summary of Field Work and Other Activities, 2005, Ontario Geological Survey Open File Report 6172, p.13-1 to 13-20. Easton, R.M. 2006. Geology and mineral potential of Southern Province rocks in Baldwin Township; in Summary of Field Work and Other Activities, 2006, Ontario Geological Survey Open File Report 6192, p.14-1 to 14-21. Kamo, S. 2006. Report on U-Pb geochronological data from the southern Abitibi Subprovince, Bannockburn– Montrose and Vernon townships, and the Grenville Front region, Thistle–Sisk townships, Ontario; internal U/Pb age report from the Jack Satterly Geochronology Laboratory for the Ontario Geological Survey, Department of Geology, University of Toronto, 22 Russell Street, Toronto, Ontario M5S 3B1, 20p. Krogh, T.E., Davis, D.W., and Corfu, F. 1984: Precise U-Pb zircon and baddeleyite ages for the Sudbury Structure; in Geology and Ore Deposits of the Sudbury Structure; Ontario Geological Survey Special Volume 1, p. 431-446. Prevec, S.A. 1993. An isotopic, geochemical and petrographic investigation of the genesis of early Proterozoic mafic intrusions and associated volcanics near Sudbury, Ontario; unpublished Ph.D. thesis, University of Alberta, Edmonton, Alberta, 223p. Rainbird, R.H. and Davis, W.J. 2006. Detrital zircon geochronology of the western Huronian Basin; Institute on Lake Superior Geology, Proceedings, 52, pt.1, Programs and Abstracts, 55-56. Van Schmus, W.R. 1965. The geochronology of the Blind River-Bruce Mines area, Ontario; Journal of Geology, v. 73, p. 755-780. 22 �Structural and kinematic analysis of the Archean Shagawa Lake shear zone, Superior Province, northeastern Minnesota Emerald J. Erickson., Dept. of Geology, Univ. of Minnesota-Duluth, eric1757@d.umn.edu Vicki Hansen., Dept of Geology, Univ. of Minnesota-Duluth, vhansen@d.umn.edu Granite-greenstone terrains exist in all Archean (3.8-2.5 Ga) cratons and are important to understanding Archean geologic processes. The debate about the formation of Archean granite-greenstone terrains features two hypotheses: 1) sagduction and diapirism caused by crustal scale density instabilities that led to sinking of greenstone basins and rise of granitic bodies and 2) arc-terrane accretion via plate tectonics processes that led to interlayered greenstone basins and volcanic arc plutons. The Superior Province in northeastern Minnesota is an Archean granite-greenstone terrain which is historically explained by arc-terrane accretion. The evolution of shear zones within greenstone terrains can provide critical insight between these hypotheses. The steeply dipping, east striking Shagawa Lake shear zone, northeastern Minnesota, lies within a greenstone belt bordered to the north and south by granite complexes. The Shagawa Lake shear zone lies within a greenstone belt consisting of Newton Lake formation (metadiabase and greenstone), Knife Lake group (felsic volcanic tuff and metasediments) and Ely Greenstone (greenstone and iron formation). The Shagawa Lake shear zone is bordered to the north by the Vermilion Granitic Complex and to the south by the Giants Range Batholith. I undertook a structural and kinematic study to understand the deformation within the Shagawa Lake shear zone. This work focuses on a ~18 km long and ~3-4 km wide portion of the shear zone (Fig 1.). I compiled structural data from published Minnesota Geological Survey maps and collected structural data and oriented samples for microstructural analysis along three north-south transects. The Shagawa Lake shear zone foliation averages 064o, 79oS. Elongation lineations are dominantly steeply plunging (90o ±30o) with subsidiary shallowly plunging lineations (15o ±10o). The average lineation is 143o, 84o. Locally dip-slip and strike-slip elongation lineations exist in the same rock. Shallow strike-slip lineations are few and overprint the penetrative elongation lineations. In the case of both strike-slip and dip-slip lineations, sections cut normal to foliation and parallel to lineation show fabric asymmetry, whereas sections cut normal to foliation and lineation display fabric symmetry. These relations are consistent with non-coaxial shear. Shear sense interpretations within the motion plane, normal to foliation and parallel to lineation, proved challenging. Of the 47 thin sections I was able to, with varying degrees of confidence, interpret 39. Microstructures within the Shagawa Lake shear zone 1) in the west, from north to south transition approximately from north-side-up to south-side-up, 2) towards the east both north-side up and south-side-up shear are recorded across the shear zone, 3) both right-lateral and left-lateral strike-slip shear occurs throughout and 4) zones of localized right-lateral and left-lateral strike-slip are not penetrative and probably postdate the later dipslip elongation lineation. The structural patterns within the Shagawa Lake shear zone are consistent with the rising of the northern and southern granitic complexes and concurrent sinking of greenstone basins forming north-side-up and south-side-up dip-slip displacement within the greenstones and volcanic sediments. 23 �24 �Flow fabric determination of two Mesoproterozoic midcontinent rift dike swarms, northeastern Minnesota Fein, E. M., (Department of Geology, Kent State University, Kent, OH 44242, efein@kent.edu), Ferré, E.C. (Department of Geology, Southern Illinois University, Carbondale, IL, 62901), & Holm, D. K. (Department of Geology, Kent State University, Kent, OH 44242) The planar geometry of mafic dikes provides an important constraint on the direction of magma flow, and their fast cooling preserves laminar flow fabrics of viscous magma, which aligns rigid grains (e.g., Cañón-Tapia, 2004). The aim of this study is to document regional-scale igneous flow patterns in two Mid-continent Rift System (MRS) dike swarms and to locate vent sources feeding these intrusions by obtaining a reliable measure of magmatic fabrics using Anisotropy of Magnetic Susceptibility (AMS). Previous thin section petrofabric analyses of aligned plagioclase phenocrysts suggested primarily lateral flow in both swarms, albeit from a relatively confined data set (Reichhoff, 1987). Another objective therefore is to assess the potential importance of lateral mass transfer in the process of intracratonic rifting and crustal evolution. The Carlton County (CC) and Duluth dike swarms, located in and around Duluth, MN, are geographically proximal but distinct in age, strike pattern, and chemical composition. The reversely polarized (older) CC dikes intrude Paleoproterozoic metagreywackes, and are subparallel in mapview. The younger, normally polarized Duluth dikes intrude MRS volcanics, and strike somewhat irregularly. The mean orientation measured for the CC swarm is N28°E, 89°NW. Duluth swarm’s mean orientation again strikes N28°E, although more variably, with shallower and more diverse dip directions ranging from ca. 70°NW to 70°SE (planes, Fig. 1). The dike geochemistry is consistent with a mantle plume source with some input from partial melting of the continental lithosphere (Seifert & Olmsted, 2004). Both dike swarms are basalt to basaltic andesite in composition. The CC dikes are predominantly quartz tholeiite, whereas the Duluth dikes are predominantly olivine tholeiite, with major primary minerals in both swarms including: plagioclase, olivine, augite, magnetite, and ilmenite (Reichhoff, 1987). The dikes in both swarms appear massive in outcrop and hand sample and no flow structures such as stretched vesicles were observed, making more traditional, macroscopic flow fabric measurement impossible. AMS, defined by preferred orientations of both crystallographic axes and shape preferred orientations of individual mineral grains and grain clusters, provides for delineation of rock fabrics in such macroscopically isotropic rocks (Rochette et al., 1992). The AMS technique is powerful and can measure the orientations of hundreds of grains in minutes, compared to hours of measurement using other approaches (Borradaile & Henry, 1997). This technique has been successfully applied to magma flow in dikes in studies since Knight & Walker (1988) empirically showed that AMS fabric is equal to magma transport direction in Hawaiian dikes. Since then, a basic understanding of the technique has been developed, and AMS is now accepted as a robust geologic tool for defining rock fabrics (Cañón-Tapia, 2004). We collected two oriented samples (at differing distances from the dike margins) from 26 dikes (13 from each swarm). To date, 21 oriented samples from 19 dikes have been analyzed. For each sample site an average of 16 cubes were analyzed, with a minimum of 9 per site. AMS measure-ment of each cube produces a magnetic susceptibility ellipsoid, represented by a second-order tensor describing the length and orientation of three principle axes, K1 > K2 > K3. K1 gives the average magnetic lineation, used as a proxy for magmatic flow fabric. K1 and K2 determine the plane of average magnetic foliation (Rochette et al., 1992; Ferré et al., 2004). Measurements were made using a Kappabridge KLY4-S susceptibility bridge at field intensity 300 A/m. The AMS results have a bulk magnetic susceptibility of about 1-3 × 10-2 SI, meaning that the magnetic signal is likely dominated by the fabric of ferromagnetic phases. The AMS signal is robust as the bulk magnetic susceptibility is well within the instrument sensitivity (2×10-8 SI at 300 A/m; Pokorný et al., 2004). The degree of magnetic anisotropy is on average 1.04 with a standard 25 �deviation of 0.03 for both swarms. Site means for 12 dikes in the CC swarm and 9 dikes in the Duluth swarm are plotted (Fig. 1). The open triangles indicate the swarm mean of the K1 site means, yielding average magnetic lineation orientations for each swarm. Results for the CC swarm cluster relatively consistently and include a magnetic lineation that is sub-vertical. In contrast, results for the Duluth swarm are complex and give poor statistics; however overall the Figure 1. Site averaged AMS results for all data collected to date in lower hemisphere stereoplots for each swarm (CC left; Duluth right): K1=squares; K2=triangles; K3=circles. Mean dike orientation for each swarm is also plotted. magnetic lineations are sub-horizontal or inclined. The preliminary AMS measurements suggest differing magmatic emplacement directions in the two dike swarms. The CC swarm was emplaced predominantly vertically, and the Duluth dike swarm displays more variable, oblique emplacement patterns, which may be influenced by inverse magnetic fabrics from single-domain magnetite in the Duluth swarm rocks. To independently corroborate these results, image analysis will be performed on three mutually perpendicular, oriented thin sections of a number of our samples using a software package to quantify mineral alignment as a proxy for magma flow. Borradaile & Henry, 1997, Tectonic applications of magnetic susceptibility and its anisotropy, Earth-Science Reviews, 42, 49-93. Cañón-Tapia, E., 2004, Anisotropy of magnetic susceptibility of lava flows and dykes: A historical account, from: MartínHernández, F., Lüneberg, C.M., Aubourg, C., & Jackson, M. (eds.) Magnetic Fabric: Methods and Applications, Geological Society, London, Special Publications, 238, 205-225. Ferré, E.C., Martín-Hernández, F., Teyssier, C., & Jackson, M., 2004, Paramagnetic and ferromagnetic anisotropy of magnetic susceptibility in migmatites: Measurements in high and low fields and kinematic implications, Geophysics Journal International, 157, 1119-1129. Knight, M.D. & Walker, G.P.L., 1988, Magma flow directions in Dikes of the Koolau Complex, Oahu, Determined from Magnetic Fabric Studies, Journal of Geophysical Research, v. 93, p. 4301-4319. Pokorný, J., Suza, P., & Hrouda, F., 2004, Anisotropy of magnetic susceptibility of rocks measured in weak magnetic fields using the KLY-4S Kappabridge, in: Martín-Hernández, Lüneberg, Aubourg, & Jackson, M. (eds.) Magnetic Fabric: Methods and Applications, Geol. Soc. London Spec. Pub 238, 69-76. Reichhoff, J., 1987, Two Keweenawan basaltic dike swarms in the Duluth area, Minnesota, Unpublished M.S. Thesis, University of Minnesota Duluth. Rochette, P., Jackson, M., & Aubourg, C., 1992, Rock magnetism and the interpretation of anisotropy of magnetic susceptibility, Reviews of Geophysics, v. 30, p. 209-226. Seifert, K.E. & Olmsted, J.F., 2004, Geochemistry of North Shore Hypabyssal dikes and sills in the Midcontinent Rift of Minnesota: an example – the 47th Avenue sill, Can. J. Earth Science, 41, 829-842. This research is supported by ILSG, GSA, Sigma Xi, and Kent State University. 26 �Student Capstone Map from the UMD Precambrian Research Center’s Field Camp: Bedrock Geology Map of Homer Lake and Adjacent Areas, Cook County, Minnesota Shelby J. Frost1, Natalie A. Juda2, and Jim Miller3 1) Winona State University, Winona, Minnesota; 2) Macalester College, St. Paul Minnesota (presently at Golden Chalice Resources, Timmins Ontario); 3) Department of Geological Sciences, University of Minnesota Duluth One of the four capstone mapping projects conducted during the fifth week of UMD’s Precambrian Research Center’s 2007 summer field camp focused on an incompletely mapped Keweenawan gabbroic complex in far northeastern Minnesota (Fig. 1). This extensive gabbroic complex, named the Brule Lake/Hovland gabbro (BLHg) by Miller et al. (2001), occurs as an east-west trending, semicontinuous intrusive unit composed of an unknown number of separate intrusions. The structural position of the BLHg complex is generally above the mafic and felsic intrusions of the Duluth Complex and reversed-polarity lavas of the NSVG and below the Eagle Mountain and Pine Mountain granophyre bodies. Vervoort et al. (2007) reported U-Pb ages for the Eagle Mtn and Pine Mtn granophyres of 1098.6±3.6 Ma and 1095.3 ±3.8 Ma, respectively. Gradational contacts between the BLHg and the overlying granophyres suggest that the BLHg may have a similar age of emplacement. These possible ages and the high structural position of the BLHg are consistent with this intrusive complex being more similar to the Beaver Bay Complex (~1096 Ma) than to the deeper-seated and older Duluth Complex (1099-1108Ma, Paces and Miller, 1993). The western extent of the BLHg, which is where our mapping was focused (Fig. 1), was first reconnaissance mapped by Grout in the 1930’s and 40’s. Most of his mapping was published in MGS Bulletin 39 (Grout et al., 1959) as a series of small-scale (~1:100,000) township maps that show the generalized outcrop distribution of various rock types. In the Homer Lake area (T63N, R3W), Grout et al. (1959, Figure XXIV) documented the occurrence of locally foliated and layered gabbro, oxide gabbro, anorthositic gabbro, intermediate rocks, basalt, Figure 1. Generalized geology of northeastern Minnesota and hornfels. Davidson and showing the location of the field area. Intrusive units Brunell (1977) published mentioned in the text are labeled. another reconnaissance map of the area that focused mainly on the geology of the Brule Lake area, to the north of Homer Lake. Curiously, the geology of the Homer Lake area shown on the 1:24,000 scale Davidson and Burnell (1977) map does not show the various rock types that Grout et al. (1959) identified ringing Homer Lake. In fact, only one outcrop is shown in the Homer Lake area, which proved to misidentified. This suggests that these Davidson and Burnell did not actually map in this area. 27 �During the capstone week, we worked out of a base camp on Homer Lake just outside the Boundary Waters Canoe Area Wilderness (BWCAW) and made daily forays into the field by canoe. We completed five full days of field mapping as a single party of three. Jim Miller had also done two days of mapping in the previous summer to preview the area for the capstone project. Most mapping concentrated on the shoreline exposures of Homer, Axe and Whack Lakes. During the field work, over 110 outcrop stations were observed, the orientation of 50 structures were measured, and 41 handsamples were collected (petrographic study of these samples are planned for this spring). The field data were used to interpret a map area of about 3 miles, east to west, by 1 mile, north to south. In the reconnaissance map of Davidson and Brunell (1977), which was directly integrated into the regional Duluth Complex map (Miller et al., 2001), this area was interpreted to be underlain largely by gabbroic anorthosite related to the anorthositic series of the Duluth Complex. They show olivine gabbro in the northern part. Our mapping showed there to be few if any anorthositic series lithologies in the area, except for small inclusions. Rather, we found the area to be dominated by variably foliated oxide gabbro that dips moderately (15° - 35°) to the south . The most prominent and traceable feature in the area is sheet of hornfels basalt overlain by hornfels cross-bedded, volcanogenic sandstone. This volcanic-sedimentary unit can be traced along the north shore of Homer Lake for over two miles of strike length and forms a 100m thick screen that separates two distinct conformable gabbro sequence. The intrusive rocks north of (underlying) the screen is composed of mafic to intermediate rock types grouped as the Axe Lake gabbroic sequence. The gradual progression of mafic (diabase and oxide gabbro) to intermediate (ferromonzodiorite to granophyric gabbro) rock type from north to south (upsection) implies that this may represent a differentiated sequence formed by in situ fractional crystallization. South of (overlying) the volcanic screen is a sequence of oxide gabbro to gabbronorite that alternate between coarse-grained, poorly foliated and medium-grained, well foliated textures. This is informally termed the Homer Lake gabbroic sequence. What is perhaps the most interesting result of our mapping is that, while it shows Davidson and Brunell’s (1977) interpretation to be incorrect, it actually verifies the reconnaissance mapping of Grout et al. (1959). The main criteria for selecting a particular area for a capstone mapping project for the Precambrian field camp is that it be a generally a poorly mapped, but well exposed area, which previous reconnaissance mapping has shown to have a diverse and interesting geology. The Brule Lake/Hovland gabbro is one of the last poorly mapped parts of the greater Duluth Complex. Because of its location in and near the BWCAW, the BLHg will likely only become well mapped by academic-based mapping. To tease out the detailed geology of the BLHg, we plan to incrementally expand the map coverage of this intrusive complex over the coming years of the Precambrian field camp. Next year, we plan to work north and west of the current map area. References Davidson, D.M, Jr., and Burnell, J.R., Jr., 1977, Reconnaissance geologic map of the Brule Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-29, scale 1:24,000 Grout, F.F., Sharp, R.P., and Schwartz, G.M, 1959, The geology of Cook County Minnesota. Minnesota Geological Survey Bulletin, v. 39, 163 p. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Miscellaneous Map Series, M-119, scale 1:200,000 Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: geochonological insights to physical, petrogenetic, paleomagnetic and tectonomagmatic processes associated with the 1.1 Ga Midcontinent rift system: Journal of Geophysical Research, v. 98, no.B8, p. 13,997-14,013. Vervoort, J.D., Wirth K., Kennedy, B., Sandland , T., Harpp, K.S., 2007, The magmatic evolution of the Midcontinent rift: New geochronologic and geochemical evidence from felsic magmatism: Precambrian Research 157, p. 235–268. 28 �REINTERPRETATION OF THE TRACE FOSSIL-BEARING DEVILS ISLAND SANDSTONE, KEWEENAWAN RIFT, NORTHERN WISCONSIN GALSTON, Lynn, Geology, University of Wisconsin - Eau Claire, 105 Garfield Avenue, Eau Claire, WI 54702, galstolm@uwec.edu and HAVHOLM, Karen G., Department of Geology, University of Wisconsin-Eau Claire, Eau Claire, WI 54702 and HASIOTIS, Stephen T., Department of Geology, University of Kansas, Lawrence, KS, 66045 The Devils Island Sandstone is a fine-grained quartz arenite that is a late-stage Keweenawan rift-fill deposit. The unit is exposed on the coast of Lake Superior creating cliffs and sea caves on Sand and Devils Islands as well as the shoreline between Menard Road and Sand Point. There are also limited exposures on the Brule, Iron, and Siskiwit Rivers. Both the Devils Island Sandstone and the Outcrop locations, modified from Adamson, 1997 stratigraphically equivalent Hinckley Sandstone were interpreted as a shallow lacustrine environment (Tryhorn and Ojakangas, 1972; Adamson, 1997). Identification of adhesion structures in the Hinckley Sandstone, however, indicates an environment that experienced subaerial exposure. Further studies of sedimentary structures in the Hinckley Sandstone indicate a depositional environment ranging from fluvial to eolian dune and interdune (Johnson, Beaster, Kohn and Havholm, 2001). This revised interpretation of the Hinckley Sandstone prompted re-evaluation of the Devils Island Sandstone. Outcrops of the Devils Island Sandstone have been studied in some detail. A boat was used to access many of the outcrops, and stratigraphic sections were measured and described. Four out of five facies identified are similar to facies within the Hinckley Sandstone. Tangential cross-strata indicate a predominantly eolian dune environment with localized subaqueous features. Cross-strata dip primarily to the northeast, east and southeast. Trough cross-strata with rip-up clasts and coarser sand grains, including granules and pebbles in some localities, suggest a fluvial environment. Trough orientations indicate paleocurrent directions primarily to the northeastern quadrant. Sandy planar bedding contains sedimentary structures reflecting an environment of deposition that varied from dry (wind-ripple strata) to damp (adhesion structures) to wet (subaqueous ripple forms). Convolute bedding, including fluidescape structures and cm- to dm-scale faults as well as folds, appears to have resulted from rapid deposition onto a saturated substrate. These facies suggest a fluvial and dune-interdune environment. The fifth facies—silty planar bedding—contains laterally continuous, thinly laminated fining-upward sequences containing mm- to cm-scale silt-sand couplets with rippled to irregular silty capping laminae and, locally, tool marks and horizons with mudcracks. This facies indicates a predominantly subaqueous environment with fluctuating energy levels, possibly lacustrine. 29 �At one locality within the silty planar facies trace fossils are present including 1) curved to sinuous and Y-branching, cm-wide, meandering traces, and 2) sub-mm to mm-scale sinuous trails that overlap and cross over larger traces. Identification of trace fossils within the Devils Island Sandstone could serve either to constrain its age to Late Proterozoic, rather than Mid-Proterozoic, or provide rare evidence of Mid-Proterozoic multicellular life. REFERENCES Adamson, K.F., 1997, Petrology, Stratigraphy and Sedimentation of the middle Proterozoic Bayfield Group, northwestern Wisconsin. Unpublished Masters Thesis, University of Minnesota, 203pp. Johnson, A.D., Beaster, K.F., Kohn, J.D. and Havholm, K.G., 2001, Braided-stream/eolian environment of Proterozoic Hinckley Sandstone, Keweenawan rift, east-central Minnesota. Geological Society of America Abstracts with Programs, v. 33, p. A229. Tryhorn, A.D., and Ojakangas, R.W., 1972, Sedimentation and petrology of the upper Precambrian Hinckley Sandstone of east-central Minnesota. In P.K. Sims and G.B. Moey, eds., Geology of Minnesota: a Centennial Volume, Minnesota Geological Survey, Minneapolis, p. 431-435. 30 �Structural and Kinematic analysis of the Kawishiwi shear zone, Superior Province: Insight on granite-greenstone terrain tectonics and Archean crustal evolution Sally Goodman, University of MN Duluth, Dept. of Geological Sciences, good0491@d.umn.edu Two conflicting hypotheses have been invoked to explain the formation of Archean (~2.7 Ga) granite-greenstone terrains: 1) granite-greenstone terrains are collapsed back-arc or sutured forearc basins of subduction-driven accreted terranes, or 2) granite-greenstone terrains formed by density-driven sagduction of an insulating greenstone cover and concurrent rising of granite diapirs. Granite-greenstone terrains commonly preserve shear zones, which may reveal a history of Archean crustal evolution and tectonic processes. A structural and kinematic analysis of the Kawishiwi Shear Zone (KSZ), within the Vermillion District, Superior Province, was conducted to evaluate the tectonic processes of granite-greenstone formation. The KSZ, broadly bounded by the Vermillion Granitic Complex to the north and the Giants Range Batholith to the south. forms one of several east-striking, steeply dipping shear zones in the Vermillion District. Metamorphism of regional rocks is generally in greenschist and amphibolite facies. The ~30km KSZ length is truncated by younger (1.1 Ga) Keeweenawan intrusive rocks in the southeast and the north-striking Waasa fault in the west. Locally, the KSZ lies within Ely Greenstone; Knife Lake Group graywackes, tuffs, and conglomerates; and the Giants Range Batholith to the south and eastern extent. In order to understand the KSZ kinematic and structural evolution, major data collection and analysis included: 1) metamorphic foliation, elongation lineation and pitch domains from field mapping the eastern 24km x 10km KSZ and also existing geologic maps, 2) outcrop-scale and microstructural kinematic analysis from oriented rock samples, quartz c-axis petrographic fabric analysis, and 3) general metamorphic history from lithology and thin section petrography. The data show that the KSZ is ~3-5 km wide and steeply dipping; it has an average foliation of 253, 89 , and a regionally dominant steeply plunging mineral elongation lineation with some gently plunging lineations (fig, A). Pitch values are bimodal; pitch domains show a distributed region of high pitch values and a narrow zone of low pitch values along strike of the KSZ (fig, B). The pillowed Ely Greenstone commonly does not have a foliation or lineation, likely due to strain partitioning around pillows. Of 55 oriented thin sections, 21 had discernible asymmetric fabrics. Asymmetric fabrics occur in foliation perpendicular, lineation parallel planes and symmetric fabrics occur in foliation perpendicular, lineation perpendicular planes; therefore, mineral elongation lineations are parallel to the motion plane or vorticity normal section, which is consistent with noncoaxial shear with lineation forming parallel to shearing. It follows, in the regions with high pitch values, the displacement is vertical to oblique and shear recorded was both north-side-up and southside-up: along the narrow zone with shallow pitch, displacement is consistently dextral strike-slip. The structural and kinematic results point to two displacement events in the KSZ: one of distributed ductile dip-slip shear (both north- and south-side-up), and a subsequent, more focused dextral strike-slip shear. The distributed and dominant dip-slip shear favors the sagduction-diapirism hypothesis whereby a high Archean geotherm dictates widespread density-driven ductile deformation of rising granites and sinking greenstone/sediment packages. Green, J.C., Phinney, W.C., Weibler, P.W., 1966. Geologic Map of Gabbro Lake Quadrangle, Lake County, MN. MNGS: M-2, scale 1:31,680. Green, J.C. and Schulz, K.J., 1982. Geologic Map of the Ely Quadrangle, St. Louis and Lake Counties, Minnesota. MNGS, M-50, scale 1:24,000. Jirsa, M.A. and Miller Jr., J.D., 2004. Bedrock Geology of the Ely and Basswood Quadrangles, Northeast Minnesota. MNGS, M-148, scale 1:100,000. 31 �32 �MN DNR DRILL CORE EVALUATION PROJECT – THE APPLICATION OF AN XRF TO ELUCIDATE GOLD MINERALIZATION IN THE VERMILION GREENSTONE Hanson, A.E. and Frey, B.A., Minnesota Department of Natural Resources, Lands and Minerals Division, 1525 3rd Ave. E. Hibbing, MN 55746 The 2.7 Ga Vermilion Greenstone Belt is located in the southern extensions of the Wawa Subprovince, within the Superior Province of the Canadian Shield. It has a diverse setting that consists of volcanic-dominated lithologies including calc-alkaline and tholeiitic basalt flows, dacitic tuffs and banded iron formations (Larson 2004). This project focuses on a 90 square mile area of the Vermilion district from Tower, MN to just east of Ely, MN. As in the Canadian equivalents, there is evidence of lode gold deposits and volcanogenic massive sulphide deposits (Williams et al., 1991), however none of these is currently being mined in Minnesota. The main focus of this project is to look for gold mineralization potential. This includes the establishment of gold mineralization indicators, and mineralization patterns. To date, more than thirty drill holes have been logged, with additional samples taken for analyses. As the cores were being logged, the Innov-X Vacuum Portable X-Ray Fluorescence spectrometer* (XRF) was used to complement our other activities. The XRF provides us with real-time chemistry data and allows for quick correlation between chemistry, as well as the visible and invisible, core or cuttings features. The XRF input was also useful for indicating sample intervals for further chemistry work. The goal of this project focuses on sharing the information we obtain about the gold mineralization. The information will be provided in a digital format online, including GIS maps. There are some characteristics of the Innov-X XRF that we have to consider carefully, many of which involve interference. The XRF analyzes for multiple elements in one measurement. Hardware, software, and physics constraints require that trade-offs occur when obtaining results, however. Examples of interference include sample matrix effect, chemical matrix effect, instrument resolution, and calibration. The sample matrix effect takes the heterogeneity and/or homogeneity of the sample into account. The window through which the X-Rays are emitted is about ½ cm2, allowing only a small area to be analyzed. When we are examining drill core, this may be an advantage or disadvantage, depending on the sample characteristics. For example, we can analyze a small, unidentified grain, or do numerous analyses on an altered area leading up to a vein in order to see the chemical changes. This may be a disadvantage if we are trying to obtain a general composition of the matrix of the core. The XRF has been subject to chemical matrix effects, which occur as XRay absorption and enhancement phenomena, both of which involve iron in this case. We may have had lower than actual amounts of copper and elevated amounts of chromium due to the presence of iron. The effect of instrument resolution manifests when two elements have similar energy peaks, so it is especially prevalent with light elements. We cannot use the XRF for exact results because it is hard to recognize when this effect occurs. Calibration is extremely important, and can be manipulated in various ways to obtain various results, such as detection of lighter elements, or more accurate readings for specific elements. IF all samples have similar compositions, then calibration can be tweaked to optimize accuracy. This option is not available for a project such as ours with a wide range 33 �of greenstone belt rock compositions. Our XRF is calibrated for the optimized analysis of 20 transition metals specifically. The adjustable sampling time interval can be increased to increase sensitivity and lower the detection limit, but this also reduces the numbers of samples analyzed. The use of standards is important, and their general composition should approximate that of the samples, if possible. The software outputs analytical values for respective elements along with a “+/-“ value. The latter value will vary as the entire sample composition varies. Analytical values will not be produced unless analytical values are approximately 3 times the “+/-“ value, or more. The XRF may have a 1 ppm detection limit for gold, but it will not produce analytical values for a “+/-“ value of 3ppm, unless the sample actually has about 10 ppm Au. A lack of analytical values does not preclude gold amounts of interest. Overall, the XRF is a very useful real-time guide, but we will not use it as a precise quantitative instrument. Interesting Results. The XRF was first used with our previous work in the Virginia Horn near Gilbert, MN. In that case, we found elongate as opposed to equigranular arsenopyrite crystals, within the altered dacitic porphyry, which was found to correlate strongly with elevated gold amounts. The approximate ½ cm2 XRF sampling area is ideal for looking at the detailed mineral association related to gold mineralization. While working in the Vermilion area near Ely at the Raspberry Prospect, XRF analyses indicated that galena or rutile were the associations of choice for gold. Other areas of elevated base metals (such as molybdenum, copper, or zinc) lacked gold. Gold was also not arsenopyrite associated. XRF analyses also indicate the apparent regional tendency for fractures and shears to be associated with anomalous mercury. The XRF does not replace geologic observations, or laboratory assay and analyses, but it offers us one more portable tool in our arsenal. *The MN DNR does not endorse this product. The manufacturer’s name appears herein solely because it is considered essential to this report. References Larson, P.C., 2004, Regional Till Sampling of the Western Vermilion Greenstone Belt, Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Technical Report NRRI/TR-2004/23, 33p., 1 plate. Williams, H.R., Stott, G.M., Heather, K.B., Muir, T.L., and Sage, R.P., 1991, Wawa Subprovince, in Thurston, P.C., Williams, H.R., Sutcliffe, R.H., and Stott, G.M., eds., Geology of Ontario: Ontario Geological Survey Special Volume 4, Part 1, p. 485-539. 34 �IRON-FORMATION-HOSTED GOLD IN THE SUPERIOR PROVINCE OF NORTHWESTERN ONTARIO HILL, MARY LOUISE, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, Canada P7B 5E1 and CHEATLE, ANDREW, Chief Geologist, Musselwhite Mine, Box 7500, Thunder Bay, ON, Canada P7B 6S8 At Musselwhite Mine and in the Beardmore-Geraldton area, Archean iron formation hosts significant gold mineralization. Both Musselwhite and Beardmore-Geraldton lie within metavolcanic and metasedimentary belts of the Superior Province. In both locations, the iron formation is complexly folded and metamorphosed to the upper greenschist or lower amphibolite facies. These deposits can be compared to Archean iron-formation-hosted gold deposits that were mined at Homestake Mine, ND and Lupin Mine, NWT. What is the significance of iron formation as a host to gold mineralization in these deposits? Deposit-scale analysis at Musselwhite demonstrates that gold mineralization was synchronous with amphibolite facies metamorphism and deformation. The deposit is giant in scale (more than 2 million ounces mined to date and 2 million ounces remaining in mineral reserve) with an average grade of approximately 5.5 g/t. Gold occurs primarily in a semi-pelitic unit within the iron formation, with the richest ore typically found at intersections of narrow high-strain shear zones with this particular lithology. Heterogeneity of metamorphism and deformation within the iron formation appears to be a key factor in mineralization. Strain incompatibilities caused by competency contrasts at various scales have produced enough transient permeability to allow localized fluid migration and mineralization. In the Beardmore-Geraldton belt a network of anastomosing shear zones separates regional-scale lithons of metasedimentary and metavolcanic rock metamorphosed to the greenschist facies. The shear zones are steeply dipping with nearly horizontal transcurrent displacements. Here again, gold mineralization is linked to high-strain deformation zones. At this grade of metamorphism, iron formation has deformed in a more ductile manner than adjacent metavolcanic lithologies, and folding is common. Competency contrasts between lithologies are more pronounced during deformation at these temperatures, and this is significant to mineralization potential. Historic mining in this area has produced over 4 million ounces of gold in total, but no single mine has been a giant producer. Relationships seen at Musselwhite and other iron-formation-hosted gold deposits could have important implications for exploration in this area. The most significant contribution of iron formation to gold mineralization in Archean metavolcanic and metasedimentary belts may be the heterogeneities in deformation produced during synchronous metamorphism and deformation of contrasting lithologies in brittle-ductile shear zones. 35 �Whatever happened to the Logan sills? Ongoing research into the geochemistry of Midcontinent Rift-related mafic intrusive rocks south of Thunder Bay HOLLINGS, Pete, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada, SMYK, Mark C., Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 Canada. Until recently, the Mesoproterozoic Midcontinent Rift (MCR)-related gabbroic sill complexes around Thunder Bay and the Nipigon Embayment were described as the Logan Sills and considered to be a relatively homogeneous package of rocks (Stockwell et al., 1972). Recent work as part of the Lake Nipigon Region Geoscience Initiative has shown that the sills in the vicinity of Lake Nipigon can be subdivided into a number of discrete units with distinct geochemical and isotopic characteristics (Hollings et al., 2007a,b; Heaman et al., 2007). None of these sills were thought to have been part of the Logan Sill suite, which was restricted to the area south of Thunder Bay (Hollings et al., 2007a). As part of an ongoing investigation of the MCR-related rocks south of Thunder Bay, samples of gabbroic sills and dykes were collected and analysed in order to further characterize these spatially distinct suites. The sills share a number of common characteristics. Like the sills of the Nipigon embayment, the majority of sills from this study are characterized by negative Nb anomalies, consistent with pervasive crustal contamination at depth. The 2007 reconnaissance sampling shows that the southern sills can be divided into at least two, and possibly three, distinct suites. A mafic to ultramafic sill was identified in the Riverdale area in 2006 (Hollings et al., 2007c) and characterized by elevated Gd/Ybn ratios (Fig. 1), comparable to the Hele and Disraeli intrusions of the Nipigon embayment. Present levels of sampling suggest that this “Riverdale sill” is restricted to a relatively small area. If the samples are from a single sill, then it must be at least 60 m thick. At one location, the Riverdale sill is cut by a ~1 m wide vertical dyke that is geochemically similar to the Nipigon sills. Figure 1: Major and trace element variation diagrams illustrating the geochemical affinities of the sills south of Thunder Bay. Data from Logan sills are from Hart (2003, 2005). Nipigon data are from Hollings et al. (2007a). Data for dykes south of Thunder Bay are from L. Hulbert and R. Ernst (Geological Survey of Canada, pers. comm. 2006) from samples collected by M. Smyk and J. Scott (Ontario Geological Survey), Hollings et al. (2007c) and this study. 36 �A second sill suite is characterized by Gd/Ybn ratios between 2 and 2.5, comparable to the Jackfish and McIntyre sills of the Nipigon Embayment. However, this suite, like the Logan sills is characterized by somewhat higher La/Smn ratios (Fig. 1). The elevated TiO2 values of this sill are also consistent with the values reported for the Logan sills (Fig. 1). The third suite is geochemically similar to the main Nipigon suite of the Nipigon Embayment (Fig. 1). It is characterized by lower La/Smn ratios and lower TiO2 values for a given content of MgO. A single sample within this suite, collected from a ~1m thick sill that crops out in a package of Rove shale lies within the field of the Inspiration sills and may represent a distinct suite but additional sampling will be required to validate this. This sampling, planned for 2008, will also provide more comprehensive coverage of all the mafic intrusions south of Thunder Bay. This will help to elucidate boundaries between sill suites and individual intrusions and provide insight into mafic magmatism along the northwestern margin of the MCR. References Hart, T.R. 2003. Keweenawan mafic and ultramafic intrusive rocks of the Lake Nipigon and Crystal Lake areas, northwestern Ontario; ILSG, Proceedings Volume 49, Part 1-Programs and Abstracts: 21-22. Hart, T.R. 2005. South Black Sturgeon River–Seagull Lake Area, Nipigon Embayment, Northwest Ontario: Lithogeochemical, Assay and Compilation Data. Ontario Geological Survey, Miscellaneous Release of Data 147. Heaman, L.M., Easton, M., Hart, T.R., Hollings, P., MacDonald, C.A., and Smyk, M., 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon Region, Ontario. Canadian Journal of Earth Sciences, 44, 1055-1086. Hollings, P., Hart, T., Richardson, A., and MacDonald, C.A., 2007c. Geochemistry of the Midproterozoic intrusive rocks of the Nipigon Embayment, Northwestern Ontario. Canadian Journal of Earth Sciences, 44, 1087-1110. Hollings, P., Richardson, A., Creaser, R., and Franklin, J., 2007b. Radiogenic isotope characteristics of the Midproterozoic intrusive rocks of the Nipigon Embayment, Northwestern Ontario. Canadian Journal of Earth Sciences, 44, 1111-1129. Hollings, P., Smyk, M., and Hart, T., 2007c. Geochemistry of Midcontinent Rift-related mafic dykes and sills near Thunder Bay: New insights into geographic distribution and the geochemical affinities of Nipigon and Logan sills and Pigeon River and other dykes. In Woodruff, L (ed.), Proceedings and Abstracts, Institute on Lake Superior Geology 53rd Annual Meeting, Proceedings Volume 53, Part 1 – Program and Abstracts, 40-41. Stockwell, C.H., McGlynn, J.C., Emslie, R.F., Sanford, B.V., Norris, A.W., Donaldson, J.A., Fahrig, W.F., and Currie, K.L. 1972. Geology of the Canadian Shield. In Geology and economic minerals of Canada. Edited by R. Douglas. Geological Survey of Canada Economic Geology Report 1: 838. 37 �STABLE ISOTOPE GEOCHEMISTRY OF THE MUSSELWHITE AU MINE, N. ONTARIO: IMPLICATIONS FOR MINERALIZATION ISAAC, Carissa, Lakehead University, Thunder Bay, Ontario cisaac@lakeheadu.ca and HOLLINGS, Pete, Department of Geology, Lakehead University peter.hollings@lakeheadu.ca The Musselwhite mine is located on the south shore of Opapimiskan Lake, approximately 720 km north of Thunder Bay. Hosted in the ~2.8Ga North Caribou Lake greenstone belt of the Sachigo Superterrane, Superior Province, the Musselwhite mine is classified as a shear hosted orogenic gold deposit, with an estimated reserve of 1.78 million ounces Au (Goldcorp, 2008). The amphibolite grade metabasalts and komatiitic metabasalts that host the mine have elevated gold concentrations with the highest gold grades hosted in iron formation contained within the volcanic pile(Hall and Rigg, 1986). Oxygen isotope signatures from quartz samples from Musselwhite range from +12 to +14.7 per mil. These values are consistent with previous work by Otto (2002) and indicate fluid compositions that fall within both the magmatic and metamorphic range. The large overlap between the magmatic and metamorphic fluid oxygen isotope signatures have led researchers to use nitrogen isotopes to investigate the source of gold bearing fluids in orogenic gold deposits (Pitcairn et al., 2005; Jia, 2000). Nitrogen substitutes for K+ as NH4+ in potassic minerals and has the advantage over oxygen and hydrogen isotopes in that it is present at low abundances in rocks. This reduces the effects of re-equilibration during successive thermal events such as metamorphism. Twenty biotite and 30 quartz samples have been analyzed for δ15N, δ18O and δD from the Musselwhite mine as well as 12 biotite samples from the surrounding granitoid rocks of the North Caribou Lake Greenstone Belt. Nitrogen isotopes in biotite from the Musselwhite mine are characterized by a δ15N range from 1.3 to 11.1 per mil, whereas biotite samples from granites and metasedimentary rocks adjacent to the deposit have a δ15N range of -6.9 to +6.1 per mil. The range of nitrogen isotopes at Musselwhite suggests mixing between magmatic and metamorphic fluids (Figure 1). Oxygen and hydrogen isotopic ranges for the granitoid plutonic rocks are +2.0 to +4.0 per mil and -59 to -80 per mil respectively; these values are typical of felsic plutonic rocks (Taylor, 1974). Oxygen and hydrogen isotopes of biotite samples from the mine range from +7.7 to +9.6 per mil for oxygen and -85 to -103 per mil for hydrogen. The anomalously low δD signature could be influenced by: 1) the iron content of the biotites, 2) degassing of the deposit at hypabyssal levels, 3) biologic activity. In conclusion, the stable isotopic data generated for the Musselwhite Mine suggest the possibility for fluid mixing and indicate that magmatic fluids may have had a role in transporting gold. References: Goldcorp, 2008. Goldcorp, inc. website – Musselwhite. http://www.goldcorp.com/operations/musselwhite/ Hall, R. S. and Rigg, D. M., 1986. Geology of the West Anticline Zone, Musselwhite Prospect, Opapimiskan Lake, Ontario, Canada. In Macdonald, A. J. Gold '86; an international symposium on the geology of gold deposits, pp. 124 -136. Hill, M.L.; Cheatle, A.; Liefrovich, R., 2006. Musselwhite Mine: an orogenic gold deposit in the Western Superior Province; In Programs with Abstracts, Geological Association of Canada Vol. 31, pp. 67. Jia, Y., 2000. Giant quartz vein systems in accretionary orogenic belts; the evidence for a metamorphic fluid origin from delta (super 15) N and delta (super 13) C studies,: Earth and Planetary Science Letters, December 30, 2000, Vol. 184, Issue 1, pp. 211-224. 38 �Otto, A. 2002. Ore forming processes in the BIF-hosted gold deposit Musselwhite mine, Ontario, Canada. M.Sc. thesis (unpublished) Freiberg University of Mining and Technology, Germany, 86 pp. Pitcairn I. K.; Teagle, Damon A. H.; Kerrich, R.; Craw, D.; Brewer, T. S.2005. The behavior of nitrogen and nitrogen isotopes during metamorphism and mineralization; evidence from the Otago and Alpine schists, New Zealand , Pitcairn, I. K. In: Earth and Planetary Science Letters, April 30, 2005, Vol. 233, Issue 1-2, pp.229-246 Taylor, H. P., Jr. 1974. The Application of Oxygen and Hydrogen Isotope Studies to Problems of Hydrothermal Alteration and Ore Deposition; Economic Geology and the Bulletin of the Society of Economic Geologists, Vol. 69, Issue 6, pp.843-883 Figure 1: Plot of Nitrogen isotope date for the Musselwhite mine. Stars represent samples from Musselwhite Mine and hexagons represent regional granitoid samples. Fields for comparative data sets from Jia, 2000; Gsed = greenschist metasedimentary rocks, LG = lower Greenschist rocks, MG UG = middle-upper Greenschist rocks, GAT = Greenschist-Amphibolite Transition zone, LA = lower Amphibolite rocks, UAsed = upper Amphibolite metasedimentary rocks. 39 �“CAPSTONE” GEOLOGIC MAPPING NEAR GABIMICHIGAMI LAKE, BOUNDARY WATERS CANOE AREA WILDERNESS, BY STUDENTS OF THE PRECAMBRIAN RESEARCH CENTER’S 2007 FIELD CAMP Jirsa, Mark A., (jirsa001@umn.edu); STARNS, Edward, Costello, Daniel E., Gal, Benedek, Hoaglund, Steven A., and Putz, Amanda J. The Precambrian Research Center—a branch of the University of Minnesota, Duluth—conducted its first season of field camp in 2007. After 5 weeks of field training, students were assigned “Capstone Projects” that provide an opportunity to create new geologic maps in areas of poorly understood geology. The four students listed above worked with the first and second authors to map in an area of the 2006 Cavity Lake forest fire in the northeastern part of the Boundary Waters Canoe Area Wilderness. The fire was the delayed result of a mega-storm in 1999 that flattened thousands of acres, reducing the woods to a tangle of downed trees and stubby new growth. The fire greatly improved access, and the subsequent rains cleaned outcrops to reveal details about the bedrock geology not previously visible. Gabimichigami Lake straddles the contact between Neoarchean and Mesoproterozoic rocks. Specifically, the mapping occurred along the north, west and east shores of the lake in parts of the U.S.G.S. Gillis Lake and Ogishkemuncie 7.5-minute quadrangles. Previous mapping was regional in scale; and although fairly accurate, it lacks detail and is quite dated (Gruner, 1941). Students were divided into two teams, each tackling a different aspect of the geology. One team (Costello and Gal) mapped the basal contact zone of the Tuscarora Intrusion, a component of the Mesoproterozoic Duluth Complex; the other team (Hoaglund and Putz) mapped the structure and stratigraphy of Neoarchean volcanogenic sedimentary rocks that form the intrusion’s footwall. The map and geologic descriptions presented here are based solely on the observations from several days of field work. Although some samples were taken, no petrographic or geochemical data were acquired. The Neoarchean rocks consist of volcanic, volcaniclastic, and intrusive rocks informally called the Jasper Lake sequence. Regionally, the sequence consists of hornblende trachyandesitic flows and volcanogenic sedimentary strata derived from them, all intruded by dikes of similar composition. Though folded, the sequence appears to be generally southwestward-younging. It grades irregularly up-section from pyroclastic flows and breccia, to a thick package of poorly stratified volcanic conglomerate, to interbedded conglomerate, sandstone, and mudstone. Individual units of volcanic conglomerate are clastsupported, poorly sorted, and locally contain an abundance of one textural and compositional variant of trachyandesite over another. Taken together, the monolithic composition, generally poor sorting, disorganized bedding, and presence of dikes of similar composition imply that the poorly stratified conglomeratic units represent volcanic debris flows. The conglomeratic package grades irregularly to the south and west into a package of interbedded, moderately well-sorted conglomerate and volcanic sandstone, which grades further to predominantly volcanic graywacke, having graded bedding and mudstone drapes indicative of turbidite deposition. Mapping at Gabimichigami Lake was largely within the latter, presumably water-lain part of the sequence. Bedding dips steeply, and locally is overturned slightly to the south. Stratigraphic younging deduced from graded beds defines tight to open folds having northwest-trending axes, and plunges that range from northwest to southeast. No axial-planar cleavage was observed. Conglomerate layers in this package are white to brownish-gray, and clast-supported. Clasts are moderately to well rounded, moderately well sorted, and range from 1 cm to 20 cm in diameter. Clasts consist of porphyritic to aphyric hornblende dacite to trachyandesite, similar to the volcanic rocks near Jasper Lake. The presence of rare clasts of medium- to coarse-grained rocks having the same hornblende-rich composition may indicate erosion from hypabyssal intrusions in the otherwise largely volcanic source region. Graywacke is gray to light gray, fine- to medium-grained, and contains abundant feldspar and hornblende. Graded units of sandstone, siltstone, and mudstone are common. The well 40 �developed graded bedding, flame structures, ball and pillow structures, and mudstone rip-ups indicate that sediment was deposited by turbidity currents. Metamorphic grade in the Jasper Lake sequence appears to be low greenschist facies, except in the narrow contact aureole of the Tuscarora intrusion where biotite, staurolite, and possibly andalusite occur in metapelitic beds. Quartz-chlorite veins having variable trends are common near shear zones and fold axes. . The Mesoproterozoic intrusive rocks of the Tuscarora Intrusion are divided here into 4 units; a basal contaminated zone, a heterogeneous unit, the main augite troctolite, and a northwest-trending troctolite dike. With the exception of the dike, unit contacts, magmatic foliation, and oriented xenolithic inclusions strike northeast and dip shallowly to the southeast. The basal contaminated zone contains abundant inclusions of varied size and composition, including footwall metasedimentary rocks, in a fine-grained granodioritic, noritic, or possibly ultramafic matrix. Disseminated biotite is present near the footwall, and decreases in abundance stratigraphically upward. Mineral lineation and igneous layering are present locally. The basal contact is transitional, magmatically and structurally disturbed and brecciated, and is marked by local intrusions of fine-grained felsic material emplaced into the metasedimentary footwall rocks. The heterogeneous unit consists of fine- to coarse-grained, pegmatite-rich, subophitic augitetroctolite, mela-troctolite, and poikilitic anorthositic troctolite. The texture of the rocks is variable and taxitic. The contact with the underlying contaminated zone is abrupt. The main augite troctolite is medium- to coarse-grained, modally layered, and subophitic. It locally contains pegmatitic pods that are particularly abundant in the lower parts of the unit. Inclusions of poikilitic, troctolitic anorthosite are also present. Disturbed layers of pegmatite and varitextured rock, locally enriched in specific mineral phases (olivine, plagioclase, pyroxene and oxides) are common near the inclusions. The contact with the underlying heterogeneous unit is transitional. The troctolite dike consists of fine- to medium-grained, homogeneous, poikilitic troctolite and subophitic augite-troctolite. Trachytoid texture and weak modal layering occur locally near and parallel to the subvertical contacts. The unit contains small inclusions of poikilitic troctolitic anorthosite, which may have been derived from the Anorthositic Series rocks of the Duluth Complex. A preliminary geologic map of the Cavity Lake fire area that incorporates the student work described here will be available in July, 2008. This map will provide details about both the distribution and relative temporal framework of the Precambrian bedrock. Students enrolled in the 2008 field camp will continue mapping in the region. In addition, one student from the 2007 camp, Dan Costello, will begin thesis field work in the Tuscarora Intrusion during 2008. A final map of the area will be published after completion of these efforts. Support for mapping in the Cavity Lake fire area by the first and second authors is provided by the U.S. Geological Survey’s 2007 State Geologic Mapping Element (STATEMAP) of the National Geologic Mapping Program, and the State Special Appropriation to the Minnesota Geological Survey. REFERENCE Gruner, J.W., 1941, Structural geology of the Knife Lake area of northeastern Minnesota: Geological Society of America Bulletin 52:1577-1642 41 �SUDBURY IMPACTITE LAYER NEAR GUNFLINT LAKE, NE MINNESOTA Jirsa1, Mark A., Weiblen2, Paul W., Vislova3, Tatiana, and McSwiggen4, Peter L. (1 Minnesota Geological Survey (jirsa001@umn.edu); 2 University of Minnesota-Twin Cities; 3 SUNY College-Oneota; 4 McSwiggen and Associates, St. Paul) We present here a preliminary, largely macroscopic description of rocks inferred to contain ejecta from the ca. 1850 Ma Sudbury meteorite impact event—following the discovery of similar deposits in Thunder Bay, Ontario and Michigan that are well documented by Addison and others (2005), Cannon and Addison (2007), and Pufahl and others, (2007). The Gunflint Lake exposures lie some 480 miles (770 km) west of Sudbury, making this one of the most distant sites known to contain what is considered “proximal ejecta” from the impact. It should be noted at the onset, that only a small portion of the material described here can be considered true ejecta—the great majority of the 7 meter-thick deposit is breccia that consists of thoroughly disheveled fragments that appear to have been derived from subjacent strata. Like the deposits near Thunder Bay, the breccia is sandwiched between Gunflint Iron Formation and sedimentary strata of what has traditionally been assigned to Rove Formation. Unlike deposits near Thunder Bay, the breccia lies within the metamorphic aureole of the Tuscarora and Poplar Lake Intrusions of the Duluth Complex (ca. 1100 Ma), and is intruded by diabasic sills of the Logan Intrusions (ca. 1115 Ma; Heaman and Easton, 2005). Pervasive carbonate mineralization and metamorphism has overprinted and obscured much of the original, delicate mineralogic features, but macroscopic textures and geochemical content that convey information about protolith and depositional mechanisms are preserved. The study of these rocks underway—in the context of the other ejecta sites—will provide a more detailed geologic history of the impact event. Figure 1—Composite and schematic sketch showing stratigraphy of Gunflint Lake impactite adjacent rocks. Approximate ages cited references. Symbols in breccia are broadly representative of fragment type and relative size: closed polygons=chert-rich formation; lines= layered, carbonate-rich formation; solid circles= and from ironironlapilli. To date, the Gunflint Lake breccia has been mapped within the broadly-folded, shallowly south-dipping sequence of Paleoproterozoic strata over a strikedistance of about 1.5 mi (2.4 km). The thickness, internal stratigraphy, and position relative to Logan Intrusions vary considerably along strike; however, the composite section shown in Figure 1 represents the gross sedimentalogical characteristics that are present in many exposures. Most of the deposit consists of very poorly sorted breccia containing blocks and slabs as large as 3 meters having nearly 42 �random orientations. The breccia is crudely graded; largest blocks occur near the base, and smaller and more rounded fragments occur up-section. The contact with underlying iron-formation is abrupt and clearly unconformable, but there is surprisingly little relief on the surface at outcrop scale. The ironformation footwall is not particularly fractured, nor is it ductily deformed: implying that neither seismic brecciation nor soft-sediment deformation, respectively, played a significant role in fragmentation. Most fragments appear to have been derived from the underlying iron-formation, though alteration and metamorphism likely destroyed some of the recognition criteria for smaller clasts in the “matrix” of large breccia blocks. The upper part of the breccia contains small to large blocks locally infilled with concentrically zoned, spherical structures as large as 1.5 cm. that are interpreted to be accretionary lapilli. The lapilli are the only fragments in the deposit presently considered to be part of the impact ejecta blanket—no grains of shocked quartz, microtektites, or other exotic clasts have been identified. A unit of bedded, and locally graded lapillistone and microbreccia caps the megabreccia in many locations. The unit contains both intact and abraded lapilli. It appears to have formed within channel-like depressions on the irregular surface of underlying breccia. The deposit is overlain by interbedded mudstone, limestone, and graded turbidite layers of the basal Rove Formation. The breccia marks an abrupt shift in depositional facies from iron- and silica-rich to iron- and silicapoor sediments, implying a catastrophic origin. Any interpretations at this time are tentative; however, the combination of fragment size, random orientation, chaotic distribution, and apparently local derivation counter-indicates deposition by common mechanisms such as sea cliff erosion or gravity-driven debris flow. Deposition may have been the result of impact-generated tsunamis, or be more directly linked to the turbulent impact ejecta plume. The deposit resembles those described in Thunder Bay and Michigan, to which a similar depositional history has been ascribed. Crude grading in the Gunflint megabreccia implies deposition by a single, intense, and waning event. The cap of bedded lapillistone and microbreccia may be the product of channelized back-flow following tsunami run-up, reworking by secondary or tertiary tsunami waves, or settling during collapse of the ejecta plume. Water depth at the time of impact is problematic—clearly the underlying iron-formation was deposited in fairly deep water, as was the overlying Rove Formation. However, the chaotic breccia and abundance of carbonate mineralization may indicate subaerial exposure during part of the depositional history. The presence of accretionary lapilli in only the upper part of the deposit is also problematic. In the tsunami scenario, this observation could reflect hydraulic sorting that precluded lapilli deposition during wave run-up, and instead selectively washed lapilli onto the brecciated landscape as the wave (s) receded. Many other questions remain, including the nature of the substrate at the time of breccia deposition, the content of exotic (ejecta) fragments, the precise mineralogic and geochemical composition, and the origin, paragenesis, and significance of carbonate. Work to address some of these questions and integrate the results into broader interpretations of the Sudbury impact event is underway. REFERENCES Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology 33:193196. Cannon, W.F. and Addison, W.D., 2007, The Sudbury Impact layer in the Lake Superior iron ranges: A time-line from the heavens: ILSG Proceedings, 53rd Annual Meeting, v. 53, Part 1, p. 20-21. Heaman, L.M., and Easton, R.M., 2005, Proterozoic history of the Lake Nipigon area, Ontario: Constraints from UPb zircon and baddeleyite dating: in Easton, M., and Hollings, P., eds., ILSG Proceedings, 51st Annual Meeting, Nipigon, Ontario, Proceedings and Abstracts, v. 51, Part 1, p. 24-25. Pufahl, P.K., Hiatt, E.E., Stanley, C.R., Morrow, J.R., Nelson, G.J., and Edwards, C.T., 2007, Physical and chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan: Geology 35:827-830. 43 �AN ARCHEAN-AGED PGE-BEARING INTRUSION, BARAGA COUNTY, MICHIGAN JOHNSON, R., SEASOR, R. and SUSZEK, T., Rod Johnson and Associates, Inc., 1550 Baldwin Avenue, Negaunee, MI 49866 This paper describes the discovery, geology and exploration of an Archean-aged, PGE-bearing, ultramafic intrusion discovered in Baraga County, Michigan and unofficially named by the authors the Kortio Lake Intrusion (KLI). During 1997 a program was undertaken to determine if an analog to the Lac des Isle PGE deposit of northwestern Ontario, Canada might exist on the southern side of the Mid-continent Rift within the Northern Complex granite-greenstone terraine of the Upper Peninsula of Michigan. Initial inspection was concentrated upon known appinitic intrusions in northern Marquette County. Lack of any evidence of mineralization within the surface exposures of these bodies led to the decision to move the search to the far western boundary of the granite-greenstone terraine with the intent of prospecting the entire area, from western Baraga County to eastern Marquette County. Coarse-grained hornblendite intrusions were identified in an area previously believed to be underlain by amphibolites. Also discovered were glacial float boulders of coarse-grained hornblendite containing several percent pyrite and chalcopyrite. The outcrops that were the source of the glacial float were located. These outcrops exhibited easily identified copper staining, as malachite, along with varying amounts of pyrite, chalcopyrite and nickel-bearing sulfides. The northern edge of KLI is located immediately west and southwest of Kortio Lake (aka Little Summit Lake), 3 kilometers (2 miles ) south of Herman, Baraga County, Michigan A review of available geochemical and airborne magnetic surveys further supported the interpretation that the area had potential for hosting PGE and copper nickel mineralization. Reconnaissance mapping of the surrounding area indicated the potential for an intrusion of appreciable size. It was determined that the complex was approximately 4,300 meters (14,000 feet) long, in a northsouth direction and in excess of 3,200 meters (10,500 feet) wide, east to west. Detailed geologic mapping and outcrop prospecting and sampling was undertaken. That effort defined a hornblendite intrusion that has a surface expression of 4,300 meters (14,000 feet) long and a range in width of from 1,600 meters (5,250 feet) to 300 meters (1000 feet). Several areas of mineralization were identified in the prospecting and sampling phase. Field relationships indicate that the KLI is a late Archean-aged intrusive that is truncated, on its southern end, by the Great Lake Tectonic Zone (GLTZ). Mapping indicated that the hornblendite intrusion had been intruded by a later, but nearly contemporaneous, fine to medium grained diorite. The contact zone between the hornblendite and diorite ranges from sharp to gradational and numerous locations exhibit areas where the diorite contains variable-sized, autoliths of hornblendite. The KLI is intruded by dikes of probable Archean age (Metachewan?) and numerous east-west trending Keweenawan age diabase dikes. Also noted are narrow granitic pegmatite dikes and small intruded plugs ranging from granitic to dioritic in composition. The intrusion is also crossed by numerous shear zones and faults with unknown, but probable minor displacement. The structural relationships have yet to be determined. 44 �The hornblendite intrusion is a highly variable rock, both in texture and in content of primary and accessory minerals. Textures range in grain size from fine-grained, nearly aphanitic, to pegmatitic and include equigranular, porphyritic and breccia phases. Coarse-grained zones have hornblende crystals that range up to 4 cm in size. The coarse-grained zones generally contain an appreciable amount of biotite. Other primary accessory minerals of note occur within distinct zones. There are surface exposures of coarse-grained hornblendite that contain up to 25% of coarse-grained, interstitial carbonates. Other exposures of the coarse-grained hornblendite commonly contain visible apatite. Some areas of the hornblendite contain appreciable amounts of magnetite. Mineralization in outcrop and glacial float consists of pyrite and chalcopyrite. Grain size of the sulfides ranges from microscopic to blebs in excess of 2.5 cm in size. Analysis of the mineralized samples indicated the presence of nickel, platinum, palladium and gold in amounts that are well above background levels. The distribution of mineralization indicates magmatic segregation into layers. Although minor mineralization has been found within the diorite intrusion, and within veins and shear zones that cut it, the bulk of the mineralization is found within the hornblendite and that rock has been the focus of the exploration efforts. During 2004 a geochemical survey utilizing a field-portable X-ray fluorescence analyzer yielded metal contents in outcrop of up to 7% copper, 0.3% nickel, 0.6% chromium and 0.1% cobalt. Select hand samples have yielded up to 500 parts per billion of combined platinum group metals by fire assay. The project moved forward with an airborne survey in 2005 and a drilling project in 2006. During the summer of 2005 a helicopter-borne time domain EM and aeromagnetic survey was flown with excellent results in outlining the extent of the KLI with magnetics and the identification of numerous E-M conductors within the intrusive body. A total of 450 line kilometers were completed. During the summer of 2006 six diamond drill holes totaling 941 meters (3,088 feet), from three locations, were completed in the area of the original discovery of PGE mineralization. A seventh hole was drilled, with a total length of 365 meters (1,197 feet), to test an area of very high magnetism. Results of the diamond drilling confirmed the presence of copper and nickel mineralization that contains varying anomalous amounts of platinum, palladium and gold. PGE minerals have been identified by electron microscopy. The assay results indicated the potential for greater concentrations, at economic levels, within the body. 45 �Structural and Kinematic Analysis of the Mud Creek Shear Zone, Northeastern Minnesota; Implications for Archean (2.7 Ga) Tectonics Karberg, Susan M., University of Minnesota – Duluth, Duluth, MN 55812, karbe002@d.umn.edu Granite-greenstone terrains preserve large tracts of stable Archean continental crust and as such they may hold the only tangible record of Archean tectonic processes. Granite-greenstone terrains include granitoid bodies surrounded by layered, folded, felsic to ultramafic volcanic and volcaniclastic greenstone belts, commonly cut by shear zones or faults. Researchers suggest two hypotheses for the formation of Archean continental crust: (1) sagduction/diapirism, in which crustal density instabilities cause greenstone sequences to sink into basins as granitoid bodies rise (Anheusser et al., 1969), and (2) volcanic arc accretion, in which modern-style plate tectonic processes drive the accretion and imbrication of island arcs and associated basins. Although the Superior Province is widely accepted as a result of transpressional volcanic arc terrane accretion, no robust evidence exists that dismisses the sagduction/diapirism hypothesis and there is a lack of strong arguments for the plate tectonics processes of volcanic arc terrane accretion. The Archean age Mud Creek shear zone, located between two Archean granitic bodies, offers an excellent location to examine the sagduction/diapirism and arc accretion hypotheses. The Mud Creek shear zone lies within a greenstone belt between the Vermilion Granitic Complex to the north and the Giants Range Batholith to the south. The supracrustal rocks between the two granitic bodies consist of basalt flows, tuff, iron formation, greywacke, slate, conglomerate, felsic lava and pyroclastics flows (Peterson and Jirsa, 1999). The Mud Creek shear zone defined on recent geologic maps is an area approximately 14 km in length and 1 km wide, however foliation and elongation lineation occur throughout the15 km by 7 km study area that encloses the shear zone. Therefore the Mud Creek Shear zone in this study is considered the larger deformed area. The goal of this study is to examine foliation, elongation lineation, and microstructural kinematic evidence to understand the process or processes which controlled the deformation of the Mud Creek shear zone. I compiled existing structural data including bedding, foliation and lineation orientations from geologic maps, collected additional new foliation and elongation lineation orientations, and oriented samples for microstructural analysis. Foliation orientations are consistently near vertical, ranging from steeply north-dipping to steeply-south dipping, trending northeast-southwest. Generally, bedding parallels foliation. Pillow basalts are not foliated, but pillow rims record evidence of shear, most likely due to strain partitioning around the strong pillow cores. Motion direction is unknown due to a general lack of the third dimension, but most pillows appear flattened into pancakelike shapes which are elongate in map view in the same orientation as other foliated rock types. In some locations outside of the study area pillows are stretched out into vertical pencil-like columns which parallel the vertical elongation lineation seen throughout the Mud Creek shear zone. Foliation trend lines are drawn in figure 1. The pattern shows the continuity of foliation orientation throughout the Mud Creek shear zone, dominantly northeastsouthwest. Elongation lineation occurs in all rock types, with the exclusion of some pillowed basalt. Pillow basalt may be too strong and strain is likely to be partitioned around pillows, leaving little to no record of visible elongation lineations. Orientations of elongation lineations range from shallow strike-slip to steep dip-slip. Shallow lineations are rare and occur as an overprint to dip-slip lineations or in confined zones. Vertical lineations (90º ± 10º) are dominant throughout foliated rock types, and also observed on the compositional layering of banded-iron-formation. Figure 1 contains a map of elongation lineation pitch showing the dominantly vertical aspect of displacement, but also shows pockets or zones of moderately east, moderately west, and shallow strike-slip displacement. Microstructural analysis reveals interesting spatial patterns (figure 2): 1) south-side-up displacement is dominant in the northwest portion of the study area, 2) north-side-up displacement is dominant in the southeast portion of the study area, and 3) a central zone records both displacement domains, including individual samples that record both domains. Individual samples that record both domains have a penetrative fabric (S-C foliations) that indicates south-side-up displacement, and a less penetrative fabric confined to small zones recording north-side-up displacement (S-C foliations). Results from microstructural analysis support the sagduction/diapirism hypothesis, with rise of the southern granitic complex, followed by rise of the northern granitic complex. Cross-cutting relationships indicate later strikeparallel dextral displacement occurred along narrow zones (<1km wide) of previously formed foliation planes. Other samples record shallow to moderate elongation lineation orientation that record sinistral displacement confined to the northwest corner of the study area. 46 �References: Anhaeusser, C.R., Mason, R., Viljoen, M.J., Viljoen, R.P., 1969. A reappraisal of some aspects of Precambrian shield geology. GSA Bulletin 80, pp.2175-2200. Peterson, D.M. and Jirsa, M.A., 1999. Bedrock geologic map and mineral exploration data, western Vermilion district, St Louis and Lake counties, Northeastern Minnesota. University of Minnesota. Figure 1. Schematic map of the Mud Creek shear zone with foliation trends (grey) and elongation lineation pitch zones (colors). Figure 2. Sample kinematics within the Mud Creek shear zone. D = down, U = up. Includes interpretive cartoon cross section from NW-SE. 47 �Using mineralization to evaluate small-scale controls on shale permeability in the Nonesuch Formation Natalie King Department of Geosciences, Colorado State University, Fort Collins, CO 80523-1482 Mineralization in the Nonesuch Formation, which hosts the White Pine stratiform copper deposit, is being used to evaluate controls on paleopermeability. Relatively little work has been done on characterizing the controls on permeability in shales, despite their importance in fluid migration and accumulation in sedimentary basins. This study will improve understanding of controls on shale permeability that affect both the formation of sediment-hosted ore deposits and the integrity of petroleum top seals. Cores through the Nonesuch Fm, three from northern Wisconsin, outside the main stratiform copper mineralization, and three from the upper peninsula of Michigan within the zone of mineralization of the White Pine stratiform copper deposit, were logged noting stratigraphy, color, texture, mineralogy, grain size, organic content, laminations, bedding, and other sedimentary structures. Four representative cores, two from the mineralized zone and two from the unmineralized zone, were sampled for petrographic and whole rock geochemical analyses. Initial results from geochemical analyses, indicate three geochemically distinct groups in the Michigan cores; above the ore zone, well-mineralized ore horizons, and poorly mineralized ore horizons. The poorly mineralized horizons show a positive Cu to S trend that increases downward in the lower stratigraphic sections in the Nonesuch Fm. Above the ore zone, As concentrations are slightly higher than in the ore zone. Preliminary point counting data show a positive correlation between organic content and Cu concentrations. These data support fluid movement through the lower Nonesuch Fm that in turn indicates that there was paleopermeability in the lower Nonesuch Fm. Further work will examine small-scale controls such as grain and organic orientation, grain size, and mineral content to characterize these paleofluid pathways and establish small-scale controls on shale permeability. 48 �Structural and metamorphic history of the Burntside Lake shear zone with possible implications for Archean granite-greenstone formation J.N., Koester, J.W., Goodge, V.L. Hansen, Department of Geological Sciences, University of Minnesota-Duluth, Duluth MN 55812, koest036@d.umn.edu There is much debate as to the origin of Archean granite-greenstone terranes. In an effort to understand the genesis of these ancient terranes, it is helpful to focus on the boundaries between greenstone and granite bodies. The Burntside Lake shear zone represents a good location to study the relative importance of vertical and horizontal displacement because it lies at the boundary between a greenstone terrane, the Newton Lake formation, and a granite body, the Vermilion granitic complex. In an effort to better understand the origin of granite-greenstone terranes in this area, my study addressed the structural and kinematic evolution of the shear zone and used metamorphic petrology to evaluate crustal displacements across the zone. The Burntside Lake shear zone lies at the boundary of the Wawa and Quetico subprovinces of the Superior Province. The Superior Province represents one of the largest tracts of Archean crust preserved in the rock record. The classic interpretation of the Superior Province is that it involved a process similar to island-arc accretionary tectonics occurring today (1-7). However, other Archean granite-greenstone terranes are interpreted as being born of saguduction/diapirism, in which greenstone units blanket underlying granites to induce a density instability. Due to this instability, granite bodies will rise while greenstone units sink into inter-granitoid basins (8-9). Through further study of structural boundaries in granite-greenstone terranes we can hope to learn more about processes active in the Archean. Fieldwork was conducted along an 8 km length of the Burntside Lake shear zone which stretches 50 km from the Vermilion fault in Minnesota northeastward to Ontario. The shear zone strikes between N30E to N50E. During fieldwork, orientation data were collected from both foliations and lineations to aid in understanding the structural regime recorded in rocks of the study area. In addition to these fabric data, 106 samples were collected for petrographic study as hand samples and thin sections. Of these, 19 were selected from different field locations to be cut into thin sections. Thin sections were used to study microfabrics and metamorphic mineral assemblages. My study of the Burntside Lake shear zone focuses on the interaction between a granitoid body and a greenstone body. The shear zone is characterized by vertical to nearly vertical south-dipping foliation orientations, only a down-dip lineation was observed (Fig. 1). Dominant rock types collected in the field are biotite schists and chloritic greenstones. On the thin section scale, foliations vary from planar to anastomosing. Foliation is defined by the alignment of biotite, chlorite and hornblende. Elongate needles of hornblende, biotite, and chlorite define lineations. However, in a few samples, quartz is elongated as well. S-C’ fabrics in biotite and chlorite were used to determine sense of shear, along with sigma grains when applicable. These microfabrics were poorly preserved and in some thin sections completely ambiguous, resulting in very few samples having definitive petrofabrics with which to interpret shear sense. However, north-side-up shear sense dominate rocks in the field area. Mineral assemblages present in the field area indicate a low to intermediate metamorphic grade associated with the shear zone. On the north side of the zone, peak mineral assemblages include garnet-biotite and hornblende-plagioclase. To the south of the fault, chlorite is the dominant peak metamorphic mineral. Using electron microprobe analysis to obtain mineral chemistry, I was able to determine metamorphic temperatures in the shear zone for eight samples. To the north of the fault, garnet-biotite pairs record a temperature of about 550 °C, whereas hornblende-plagioclase records an average temperature of 700 °C. The temperatures recorded by these two geothermometers show a 150 °C difference. This difference may be real, or may be an artifact of the calibrations chosen. Additionally, the plagioclase grains analyzed may record igneous temperatures or some intermediary temperature that 49 �represents neither a purely igneous or metamorphic temperature. To the south, chlorite compositions record temperatures between 150 – 330 °C. In conclusion, the Burntside Lake shear zone represents a nearly vertical to vertical zone of deformation approximately 1.5 km wide. Through kinematic analysis of microfabrics the zone appears to have a north-side-up sense of shear (please see caveats noted above), as interpreted from thin sections. Rocks to the north side of the shear zone were metamorphosed in the upper greenschist to lower amphibolite facies, whereas the south side of the shear zone records lower greenschist-facies. Thus, metamorphism recorded on both sides of the fault agrees with a north-side-up sense of shear, determined from microstructures. Further work on must be done to allow for a greater understanding of the complex history recorded in the rocks defining the Burntside Lake shear zone. Specifically, work to further constrain pressure/temperature, and kinematics including timing between the conflicting senses of shear apparent in some of the samples utilized in this study. And lastly, a comparison study of the many shear zones in the area to understand any relation/timing relationships that may exist. Figure 1: Equal area stereoplot of foliation orientations from the Burntside Lake Shear zone (compilation of my orientations as well as those complied by 10-11) References: (1) Card, K.D, 1990, P. Research, 48, 99-156. (2) Cawood, P.A., Kroner, A., Pisarevsky, S., 2006, GSA Today, 16, 7, 4-11. (3) Jirsa, M.A., Southwick, D.L., Boerboom, T.J., 1992, Can. J. E. Sci., v. 29, p. 2146-2155. (4) Tabor, J.R., Hudleston, P.J., 1991, Can. J. E. Sci., 28, 292-307. (5) Bauer, R.L., Bidwell, M.E., 1990, Can. J. E. Sci., 27, 1521-1535. (6) Hudleston, P.J., Schulta-El, D., Southwick, D.L., 1988, , Can. J. E. Sci., 25, 1060-1068. (7) Bauer, R.L., Hudleston, P.J., Southwick, D.L., 1992, Can. J. E. Sci., 29, 208 2103. (8)McGregor, A.M., 1951, Trans Geol. Soc. S. Africa, 54, 27-70. (9) Rey, P.F., Philippot, P., Thebaud, N., 2003, P. Research, 127, 43-60. (10) Green, J.C., Schulz, K.J., 1983, MGS Univ. MN. (11) Sims, P.K., Mudrey, M.G., Jr., 1978, MGS Univ. MN. 50 �Strategies for Drilling Unconsolidated Material and Historic Underground Mine Workings: Examples from Hibbing Taconite Company’s 2007 Diamond Drilling Campaign Jared D. Lubben1,2 1 Hibbing Taconite Company, P.O. Box 589, Hibbing, MN 55746 2 Cliffs Technology Group, 550 E. Division Street, Ishpeming, MI 49849 As mining efforts at Hibbing Taconite advance north-eastward along the strike of the Biwabik Iron Formation, diamond drilling information is critical for evaluating the potential of new mine areas. To the east of Hibbing Taconite’s current open pit mine operations, significant obstacles related to prominent rock stockpiles, strong oxidation trends, and historic underground mine workings are encountered during diamond drilling. These problematic zones typically overlie ore-strata and are encountered in almost every drill hole. Consequently, new ideas and drilling strategies are required to penetrate these zones. Over the summer of 2007, Hibbing Taconite completed its annual diamond drilling program within areas hosting the prominent historic underground mining operations. The focus of this report is two-fold: Different lines of evidence used for drill hole targeting and positioning will be explained while various drilling methods and strategies used to reach target depths for drill holes under the 2007 program will be discussed. 51 �Mineral Zonation and Stratigraphy of the Tilden Hematite Deposit Helene M. Lukey, Senior Staff Geologist. Cliffs Technology Group, Cliffs Mining Service Company, Ishpeming, MI 49849 Hypogene alteration zones have been described as a precursor to high-grade hematite deposits in Australia, Brazil and elsewhere. This alteration described as being below or distal and is typically magnetite-carbonate-silicates reflecting Mg-Fe metasomatism. This paper describes the mineral zonation and possible structural control on mineralogic variation within the Tilden Mine. In contrast to the high-grade deposits, these variations are not peripheral to the ore bodies but are the ore. The deposit occurs within the Negaunee Iron Formation, along the southern margin of the Paleoproterozoic Marquette Range Supergroup basin. The regional structure is a fault bounded west plunging syncline within the crustal scale Great Lakes Tectonic Zone, which forms the boundary between Archean granite-greenstone and gneissic terranes. Local structure consists of the second order anticlines and synclines with variable northwest and southwest plunges. The interaction of sedimentation and diagenesis within growth fault controlled basins, along with a metamorphic and/or hydrothermal overprint has resulted in a complex series of ore types. For modeling purposes, the deposit has been divided into geologic domains based on both lithology and metallurgical response in the laboratory and in the plant. Mineralogic variation is recognized within individual mineral grains; at blast pattern (10 to 15 meter) and development drill hole (100 meter) scales. A simplified stratigraphic column consists of the Main Pit Carbonate domain (martite-magnetitevarious carbonates±silicates) in the core of the Main Pit anticline overlain by the Martite domain (martite+local goethite) and the West Hematite domain (microplaty hematite+goethite). The Clastic domains consist of quartz-rich lenses with variable matrix material within a variety of iron formation types. Although variable, the domain stratigraphic thicknesses are 100 to 200 meters. These compositional and associated textural variations affect plant efficiency and product quality, and are critical factors for ore reserve modeling, ore control and mine planning decisions. 52 �THE SEELEY SLATE AND BARABOO INTERVAL SEDIMENTATION MEDARIS, L. G. Jr and DOTT, R. H. Jr., Dept. of Geology and Geophysics, University of Wisconsin-Madison, Madison, WI 53706, medaris@geology.wisc.edu The Baraboo Interval represents a major episode of sedimentation between 1700 and 1630 Ma in the Lake Superior region. In the Baraboo Range, the type area, the Baraboo Quartzite is overlain conformably by ~120 m of gray slate, the Seeley Slate, which in turn is overlain by ~300 m of dolomite and discontinuous iron formation, the Freedom Formation. These two formations occur only in the subsurface in the hinge of the Baraboo syncline and have been described previously from exposures in underground iron mines, now abandoned, and drill cores by Weidman (1904) and from drill cores by Leith (1935), Schmidt (1951), and Geiger (1986). Because information on the Baraboo Interval comes largely from the prominent Baraboo Quartzite and correlative quartzites, we have undertaken a new study of the Seeley Slate to gain further insight into the nature of Baraboo Interval weathering and sedimentation. During exploration for iron ore in 1907, the Oliver Iron Mining Company obtained numerous drill cores from the Freedom Formation and Seeley Slate in the western part of the Baraboo Range along the axis of the Baraboo syncline. Access to these drill cores was provided by the WGNHS, and samples of gray slate, shown by the driller logs as occurring beneath iron formation, were selected from nine drill cores for detailed investigation. The Seeley is a gray slate with cleavage discordant to stratification in most cores. At least a small percentage of quartz silt is dispersed among phyllosilicate minerals even in the most homogeneous appearing intervals, which range up to several centimeters in thickness. Lamination visible in most cores is defined by lighter-colored concentrations of quartz silt, which vary in thickness from 0.5 to about 1 cm. Rare, coarser laminae have quartz grains up to 0.2 mm in diameter. A small percentage of silty laminae show discernible graded bedding (e.g. Fig. 1, core 3691-2). Soft sediment deformation by depositional loading is evident in core 3601-6; some other laminae were contorted slightly by cross-cutting cleavage (core 4045-2). Although most deformation was ductile, brittle cracking is evident in a few silty laminae (core 3691-2). As determined by petrographic examination and XRD and EMP analyses, the Seeley slate consists predominantly of quartz and 2M1 muscovite, with subordinate rutile, pyrite, chlorite [repidolite, (Mg3.5-2.9Al2.82.9Fe5.7-6.2)12(Si5.3Al2.7)8O20(OH)16], and 53 �tourmaline [schorl, ( 0.2-0.3Na0.8-0.7)(Fe1.5-1.8Mg1.5-1.2)3Al6Si6(BO3)3(OH)4]. Locally, siderite [(Fe0.86Mn0.01Mg0.05Ca0.08)CO3] occurs with quartz and muscovite in fine laminae. The Seeley slate has an unusual composition for shale; four samples of typical slate are virtually devoid of CaO and Na2O, contain 4.0 - 6.8 wt% FeO, 1.9 - 2.0 wt% MgO, 0.60 - 0.69 wt% TiO2, and plot near the compo-sition of muscovite in an Al2O3-CaO+Na2O-K2O molar projection (Fig. 2). The composition of the Seeley slate may have resulted from extensive weathering of granite (Fig. 2). Although the Seeley Slate is relatively mature, having a CIA index of 70 to 74, it is much less mature than argillite and metapelite in the Baraboo Quartzite. The Seeley Slate contrasts markedly with the underlying and overlying formations. The red Baraboo Quartzite below was deposited initially by braided streams followed by shallow marine tidal and wave processes. The inferred marine transgression continued during Seeley deposition in a deeper, quieter setting on a marine shelf. Occasional density currents carried quartz silt and rare sand into the otherwise mud-dominated environment. The overlying Freedom Formation was probably deposited in the same setting, which was now starved of clastic input so that dolomite and iron-rich facies could accumulate chemically. Baraboo Interval strata accumulated as a sedimentary wedge on the southern passive margin of a Proto-North American craton. With a total thickness of about 1500 m and extending at least 1000 km east-west, they comprise a significant stratigraphic package, which includes one of the youngest known Banded Iron Formations in the world. The red color due to hematite cement in the Baraboo and correlative Sioux and Barron Quartzites indicates an oxygenic atmosphere, and the extreme compositional maturity of both the quartzites and their interstratified pelitic layers resulted from intense chemical weathering of the source terrain of these clastic sediments. In contrast, the gray color, mineralogy, and bulk composition of the Seeley Slate imply deposition of less intensely weathered detritus in a reducing environment as compared to the Baraboo Quartzite,. References Geiger, C.A. (1986) Geosci. Wis. 10, 28-36 Leith, A. (1935) Kansas Geol. Soc. Guidebook, 9th Ann. Field Conf., 320-322 Schmidt, R.G. (1951) M.S. thesis, Univ.Wis. – Madison, 40 pp. Weidman, S. (1904) WGNHS Bull. 13, 190 pp. 54 �The Inaugural Season of the Precambrian Field Camp at the University of Minnesota Duluth Miller, James D., Dept. of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 (mille066@umn.edu) Peterson, Dean M., Natural Resources Research Institute at UMD, 5013 Miller Trunk Hwy., Duluth, MN 55811 (dpeters1@nrri.umn.edu), and Hudak, George J., Department of Geology, University of Wisconsin Oshkosh, Oshkosh, WI 54901(hudak@uwosh.edu) This past summer, the Precambrian Research Center at UMD ran the first season of a one-of-a-kind field camp. The camp teaches students mapping techniques that are best suited to field studies of Precambrian rocks of the Canadian Shield. By all measures, the camp was a rousing success. The Precambrian field camp was attended by 15 students - four from UMD and the remaining mainly from schools throughout Minnesota and Wisconsin. Jim Miller (MGS), Dean Peterson (NRRI) and George Hudak (UW-Oshkosh) served as full-time instructors, with assistance on various projects by Terry Boerboom (MGS), Val Chandler (MGS), John Goodge (UMD), Mark Jirsa (MGS), Howard Mooers (UMD), Mark Severson (NRRI), and Nigel Wattrus (UMD). The six credit course ran for six weeks from mid-July to late August. Although students worked six days a week, commonly 10-hour days, and often under very hot and humid conditions, they consistently kept a positive attitude throughout. The first two weeks of the camp were run out of UMD. Daily mapping exercises included 1) structural and outcrop mapping at Thompson Dam, 2) an introduction to geophysical field methods (gravity, ground magnetics, seismic), 3) mapping mafic cumulate rocks of the Duluth Complex at Spirit Mountain, and 4) mapping intrusive and volcanic rocks along the North Shore near Tettegouche State Park. The latter exercise included a day of canoe mapping along Lake Superior. For weeks 3 and 4, we moved the camp to Vermilion Community College in Ely. There students started with a stratigraphic correlation project along the Biwabik Iron-formation, that included core logging and measuring section in one of the taconite mine pits. Next, students conducted a two-day project of mapping along the basal mineralized contact of the Duluth Complex. At the end of the third week, we arranged an evening of mapping by lamplight along the main adit of the Soudan Iron Mine. During week 4, students conducted a multifaceted mapping project on greenstone belt geology and overlying glacial geology in the Fivemile Lake area west of Ely. The true highlights of the summer were the capstone mapping projects during Week 5. Here, students chose field mapping projects that entailed detailed bedrock mapping in previously unmapped wilderness areas, most in the BWCA. George Hudak mentored three students on mapping greenstone belt geology in the Twin Lakes area west of Ely. Mark Jirsa led four students into the BWCA off the Gunflint Trail, which had been intensely burned over in the Cavity Lake Fire of July 2006. Jim Miller worked with two students in the eastern prong of the Duluth Complex in the Homer Lake area off the Caribou Trail. Dean Peterson supervised a group of five students in mapping project of a gabbroic macrodike in the Nickel Lake-Gabbro Lake area of the BWCA. Upon returning to UMD for the final week, students worked on digitally compiling their capstone mapping data into ArcView and then creating geologic maps in Illustrator. These maps are on display in this year’s poster session. 55 �The true measure of the success of the camp is how the student shave taken advantage of this experience to better their academic and career goals. Of the 15 students, five have secured jobs with exploration companies, six are pursuing Master’s degrees (four at UMD, two at Lakehead University in Thunder Bay), three are currently finishing their undergraduate degrees with one planning to attend UMD next term. Many of the students have expressed to us how beneficial this camp has been to them, not only in teaching them particular field methods best suited to Precambrian terranes, but moreso in giving them the confidence to work with this type of geology, which we all know can be challenging. Being the only open field camp in the US that focuses on the unique attributes of Precambrian shield geology, we believe that UMD’s Precambrian field camp fills a very important niche in geological field education. Moreover, we hope this camp helps to reverse the alarming degradation of field geology as a primary component of geological education in many US schools. The art and science of observing and interpreting rocks in the field is still at the core of all geological endeavors and deserves a preeminent place in all geology curriculum. It is our mission to provide students with the tools and techniques that will start them on a life-long path of looking at rocks in the field, especially those of the Precambrian. To see photos from last year’s camp or download the geologic maps that were produced from the capstone projects and are on display at this meeting, please visit the PRC website (www.d.umn.edu/prc). 56 �BEDROCK GEOLOGY OF THE FOOTWALL TO THE SOUDAN IRON FORMATION SOUTH OF TWIN LAKES, ST. LOUIS COUNTY, NORTHEASTERN MINNESOTA MOOSAVI, S., JOHNSON, T. K., WENDLAND, C., ANDERSON, A., HUDAK, G. J., Precambrian Research Center, University of Minnesota – Duluth, Duluth, MN 55811, hudak@uwosh.edu The Ely Greenstone Formation of the Vermilion District of northeastern Minnesota is made up of a steeply north- to southwest-dipping sequence of Neoarchean supracrustal and associated intrusive rocks folded about the Tower-Soudan anticline. Recent mapping in the Vermilion District has concentrated in the area between the Soudan Mine (to the west) and Armstrong Lake (to the east; Hudak et al., 2002; Peterson and Patelke, 2003; Heine, 2005; Hoffman, 2007; Hudak et al., in prep.). The Lower Member of the Ely Greenstone (EGLM) is composed of submarine calc-alkalic and tholeiitic basalt and basalt-andesite sheet lava flows, pillow lava flows and pillow breccias with associated basalt to andesite tuffs and lapilli tuffs, along with subordinate rhyodacitic to rhyolitic lava flows, lapilli tuffs and tuffs, with minor intercollated epiclastic rocks and iron formations (Schulz, 1980; Southwick et al., 1998; Hudak et al., 2002, Hoffman, 2007). The EGLM has been subdivided into the older, largely shallow submarine Fivemile Lake Sequence (FLS) and the younger, deep submarine Central Basalt Sequence (CBS; Peterson and Patelke, 2003). The Soudan Iron Formation Member (EGSM) comprises Algoma-type cherty iron formation, massive to pillowed basalt lava flows, polymict epiclastic rocks and minor associated felsic tuffs (Peterson and Patelke, 2003). The Soudan Member is overlain by the Upper Member of the Ely Greenstone (EGUM). The EGUM is composed of poorly vesiculated tholeiitic basalt lava flows locally interlayered with Algomatype iron formation lenses (Schulz, 1980; Southwick et al., 1998). Near Tower and Soudan, the EGUM is commonly interstratified with the Lake Vermilion Formation (LVF: greywacke, slate, conglomerate, and dacite tuff), and the Gafvert Lake Sequence (GLS: subaerial to submarine dacite to trachyandesite lava flows, tuffs, and intrusions; locally, the LVF unconformably overlies EGLM and EGSM strata (Southwick et al., 1998; Peterson and Patelke, 2003). Detailed (1:5000 scale) geological mapping (Moosavi et al., 2007) occurred in the eastern part of the Vermilion District during August, 2007 as one of four capstone projects associated with the Precambrian Research Center field school. The mapping extended from about 1.5km south of Twin Lakes to the southern shoreline of Twin Lakes, and had the goal of increasing our understanding of the stratigraphy, hydrothermal alteration, and structural geology in this poorly understood part of the Vermilion District. The steeply north-dipping, east-west striking stratigraphic sequence is locally cut by east- east-northeast-trending, typically dextral shear zones composed of chlorite schist. The southwestern part of the field area comprises sparsely to moderately vesicular basalt and andesite lava flows which are correlative with the Eagle’s Nest Basalts (Jirsa et al., 2001). These lava flows are overlain by what we interpret as the CBS, which comprises a 1.0 to 1.5km thick sequence of exceptionally well-preserved sparsely- to moderately vesicular pillowed and massive basalt lava flows and associated hyaloclastite that are locally interlayered with 10-50m thick rhyodacite to rhyolite tuffs and lapilli tuffs. Locally, 2040m thick polymict breccias occur near the top of the CBS. These are compositionally and 57 �texturally similar to breccias observed by Hudak et al. (in prep.) near the top of the CBS south of Sixmile Lake. Overlying interbedded Algoma-type iron formations, felsic tuffs and felsic lava flows are correlative with the EGSM. A sharp contact exists between the EGSM and stratigraphically overlying dacite to rhyodacite tuffs which we interpret as the GLS. This supracrustal sequence is intruded by fine- to coarse-grained, ophitic gabbro sills. These sills are correlative with gabbro sills mapped in the Soudan Mine area (Peterson and Patelke, 2003) and near Needleboy and Sixmile Lakes (Hudak et al., in prep.), and may represent hypabyssal intrusions which fed the overlying EGUM basalt lava flows. Synvolcanic hydrothermal alteration south of Twin Lakes is variable. Basalt and andesite volcanic rocks are generally moderately altered to an epidote-chlorite-actinolite-quartz alteration assemblage. Felsic rocks are commonly sericite-altered. Near the top of the CBS, mafic to intermediate volcanic rocks are strongly altered to an epidote-quartz ± garnet (andradite) assemblage similar to that observed in the uppermost 50m of the CBS in the vicinity of Sixmile Lake (Hudak et al., in prep.). References Heine, J., 2005, Gafvert Lake Reconnaissance Mapping Project: Natural Resources Research Institute Technical Report NRRI/TR-2005/20, 12 p. Hoffman, A. T., 2007, Lithostratigraphy, Hydrothermal Alteration, and Lithogeochemistry of Neoarchean Rocks in the Lower and Soudan Members of the Ely Greenstone Formation, Vermilion District, NE Minnesota: Implications for Volcanogenic Massive Sulfide Deposits: Unpublished M. S. thesis, University of Minnesota – Duluth, 295 p.. Hudak, G. J., Heine, J., Newkirk, T., Odette, J., and Hauck, S., 2002, Comparative geology, stratigraphy, and lithogeochemistry of the Five Mile Lake, Quartz Hill, and Skeleton Lake VMS occurrences, Vermilion District, NE Minnesota: A report to the Minerals Coordinating Committee, DNR, Minerals Division, State of Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2002/03, 390 pages. Hudak, G. J., Heine, J., Newkirk, T. T., Hocker, S. M., and Hauck, S., in prep., Comparative Geology, Stratigraphy, and Lithogeochemistry of the Needleboy Lake – Six Mile Lake Area, Vermilion District, NE Minnesota: Natural Resources Research Institute Geological Report of Investigation. Jirsa, M. A., Boerboom, T. J., and Peterson, D. M., 2001, Bedrock Geological Map of the Eagles Nest Quadrangle, St. Louis County, Minnesota: Minnesota Geological Survey, Miscellaneous Map Series M-114, 1:24000 scale. Moosavi, S., Johnson, T., Wendland, C., Anderson, A., and Hudak, G., 2007, Bedrock Geology Map of the Footwall to the Soudan Iron Formation South of Twin Lakes, St. Louis County, Northeastern Minnesota: Precambrian Research Center, Geological Map Series Map 20074, Natural Resources Research Institute, University of Minnesota-Duluth. Peterson, D. M., and Patelke, R. L., 2003, National Underground Science and Engineering Laboratory (NUSEL). Geological site investigation for the Soudan Mine, NE Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2003, 88p. Schulz, K. J., 1980, The magmatic evolution of the Vermilion Greenstone Belt, NE Minnesota: Precambrian Research, v. 11, p. 215-245. Southwick, D. L., Boerboom, T. J., and Jirsa, M. A., 1998, Geological setting and descriptive geochemistry of Archean supracrustal rocks and hypabyssal rocks, Soudan-Bigfork area, northern Minnesota: implications for metallic mineral exploration: Minnesota Geological Survey Report of Investigations 51, 69 p. 58 �GEOCHEMSTRY OF SEDIMENTARY ROCKS ASSOCIATED WITH THE MUSSELWHITE GOLD DEPOSIT, NORTHWESTERN ONTARIO MORAN, PATRICK; FRALICK, PHILIP; HILL, MARY LOUISE, HOLLINGS, PETE, Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada, P7B 5E1, pcmoran@lakeheadu.ca The Musselwhite gold deposit, 100% owned and operated by Goldcorp Inc., has cumulatively produced in excess of 2 million ounces since 1997 and has a projected mine life through 2013. It is situated in the North Caribou Lake metavolcanic/metasedimentary belt in the central northwestern portion of Superior Province, approximately 430 km northwest of Thunder Bay. The belt occurs along the contact between the North Caribou Terrane and the Island Lake Domain, with a large, crustal-scale deformation zone forming its eastern margin. The Musselwhite gold deposit is hosted by amphibolite grade rocks dominated by banded iron formation (BIF). This study primarily focuses on the Northern Iron Formation (NIF) metasedimentary assemblage, host to the majority of gold mineralization at Musselwhite Stratigraphic and geochemical analyses suggest that the lithologies of the NIF assemblage were deposited on Mesoarchaen mafic to ultramafic volcanic rocks forming the ocean-floor. The NIF assemblage and another iron formation lower in the stratigraphy, the Southern Iron Formation (SIF), record hydrothermal regimes associated with, and interrupted by, eruptive volcanic activity. The stratigraphically lowest lithologies in the NIF assemblage, meta-argillite, quartzgrunerite BIF, and magnetite-dominant BIF, were deposited in deep, calm water, in association with venting hydrothermal fluids. These ancient chemical sediments are analogous in geochemistry to modern day deposits in places such as the Red Sea and East Pacific Rise. Differing Eu content between chert and magnetite layers indicate that rhythmically changing temperature variations drove the hydrothermal system, imparting the banded nature. The quite chemically pure chert and magnetite layers of the lower portion of the NIF assemblage contrast with silicate-dominant banded iron formation; the silicate-dominant BIF increases in importance stratigraphically upwards. It represents a decreasing hydrothermal system and/or an increase in the rate of clastic sedimentation. Hornblende-garnet and biotite-garnet schists were formed by metamorphism of mudstones composed of eroded material. The sediment that formed the hornblende-garnet schist is the same sediment that composes the siliciclastic component of the silicate-dominant NIF. Similarly the biotite-garnet schist represents a mudstone, but unlike the hornblende-garnet schist, it is primarily derived from intermediate to felsic source rocks. Lastly the garnet-quartzite represents metamorphosed sandstone eroded from the same intermediate to felsic igneous source rocks as the biotite-garnet schist. Just as there is an overall increase in clastic content upwards through the approximately 30 m thick sedimentary succession there is also a change from more mafic sourced debris to a more intermediate/felsic source. The majority of samples collected from Musselwhite did not experience significant remobilization of typically immobile elements. This is indicated by the relatively linear geochemical ratios between the immobile elements in question (Al2O3, TiO2, Zr, U, Th, etc). 59 �Even elements that are commonly more mobile (K2O, Na2O, etc) appear to have remained relatively immobile at Musselwhite. The only samples that show significant geochemical change are from shear zones. The gold mineralization is primarily associated with shear zones within the siliciclastic-rich, upper NIF assemblage, where pyrrhotite (possibly originally pyrite) replaced iron oxides and iron silicates. This indicates that the control on areas of gold mineralization was a combination of: 1) the presence of structural zones allowing gold-bearing fluids to move through the NIF, which could act as a geochemical trap for gold; and 2) structural conditions in the siliciclastic-rich NIF that favoured hydrothermal fluid involvement with this unit. 60 �ON-LINE ELECTRONIC ACCESS TO INSTITUTE ON LAKE SUPERIOR GEOLOGY PUBLICATIONS M.G. Mudrey, Jr., 106 Ravine Road, Mount Horeb, WI 53572 USA (mgmudrey@mhtc.net) Peter Hollings§ Department of Geology, Lakehead University, Thunder Bay, ON P7B 5E1,Canada (peter.hollings@lakeheadu.ca) (corresponding author) Lura E. Joseph, Geology Library, 223 Natural History Building, 1301 W Green, Urbana, IL 61801, USA (luraj@uiuc.edu) Mark Jirsa, Minnesota Geological Survey, 2642 University Ave., St. Paul, MN 5514 USA 55114 (jirsa001@umn.edu) Jo Kalliokoski, 1010 7th Ave., Houghton, MI 49931 Authors are Present and Emeriti Secretaries/Treasurers of the Institute during the last 25 years and one librarian (Joseph) who helped the Institute to compile, index and create search mechanisms for its varied publications In the past 54 years, over 16 gigabytes of abstract, field trip guides and supplemental information from more than 100 individual documents have been produced as part of the annual proceedings of the Institute on Lake Superior Geology (ILSG). These include comprehensive volumes on mineral properties, combined abstract and field guides, and field guides published by sponsoring organizations. During the past three years students at Lakehead University and other volunteers have scanned and digitized these volumes. Use of optical character recognition (OCR) software has allowed the Institute to make the digital files searchable. By use of proprietary software from Cvision®Technologies, Inc, this quantity of information was compressed in PDF format to less than 600 megabytes, approximately 3600 compression ratio. The documents can be opened and read directly in Adobe Reader® and other software with read PDF compatibilities. The documents can be directly accessed for download at no cost at http://www.lakesuperiorgeology.org/ Work on the scanning consisted of direct scanning to PDF and scanning to tiff format images. PDF and tiff images were cleaned, and where necessary (particularly with older, yellowed, and mimeographed documents) extensively enhanced to improve readability. Where necessary, colored maps and illustrations were treated separately. The documents were then compressed. Compression to 5 megabytes was achieved from a 1.5 gigabyte original scan. To accompany the online collection of ILSG publications, a searchable index was created by Joseph - geology librarian at University of Illinois, Urbana-Champaign (UIUC) http://search.grainger.uiuc.edu/ilsg/ Work on the database project progressed in the following steps during summer and fall of 2007: 61 �• A bibliography was created from information available from GeoRef®. The references were subsequently parsed and imported into an Excel® database. Many entries were missing from GeoRef®, and gaps were filled in from the collection at UIUC, from PDFs as they were added to the ILSG web site, and via Interlibrary Loan. Each reference was checked for accuracy, and edited for consistency of the source field information for each volume. In addition, information was added to fields that are unlikely to be included in commercial indexes, such as type (oral presentation, poster, guidebook, address, or other), meeting location, and meeting date, and the speakers and titles of presentations for which abstracts are unavailable. • The completed Excel® database was imported into an Access database and loaded on a library server at UIUC. • A search engine that had been created in-house to search other library databases was modified to search the ILSG database. The Access® database can be easily updated. At some time in the future, key words could also be added to the database to enhance search results; however that would be a major project. There are currently over 2400 references in the index. The database can be searched by entering search terms into any or all of the following fields: author, title, source (including volume number), publication year, meeting location and/or meeting dates. In addition, there is a drop-down menu to search a type of reference (oral presentation, poster, guidebook, and so forth). The results can be sorted by author, publication year, type, meeting location, or meeting date. The default sort is by author. The default for type is “any.” If no search terms are entered, the results will include all references, one hundred at a time. This is useful for browsing. Each of the search fields can be searched for either “all of the terms” or “any of the terms.” Searching of phrases is beyond the capability of this search engine. Wildcards (truncation symbols such as *) are not used; however, truncation is automatic: that is, a partial word will return results. For example, “Eidu” in the author field will return references by Eidukat, and “mineral” in the title field will return references with “mineral,” “minerals,” “mineralization,” and so forth. Case (upper or lower) is not recognized, so is not important. The database format and search capabilities will be demonstrated in a poster presentation. A complete list containing such information as author, editor, chairperson, and sponsoring organization is maintained by: MTU Archives & Copper Country Historical Collections J. Robert Van Pelt Library Michigan Technological University Houghton MI 49931 Phone: 906-487-2505 e-mail: copper@mtu.edu Photocopies of most back volumes can be ordered from the MTU Archives at the prevailing copy rate. 62 �Multi-element Geochemical Signature of Copper Mineralization at the White Pine Mine, Midcontinent Rift System, Western Upper Peninsula, Michigan MUVI-TJIKALEPO, Muatala H., BORNHORST, Theodore J., and ROBINSON, George W., A. E. Seaman Mineral Museum, Michigan Technological University, Houghton, MI 49931 and WILLIAMS, W.C., Phoenix, AZ 85048 The lower beds of the Nonesuch Shale and uppermost beds of the Copper Harbor Conglomerate host a giant stratiform copper deposit at the White Pine Mine. The ore body yielded roughly 2.0 billion kilograms of copper and 50 million ounces of silver from 198 million tons of ore from 1953 to 1996 (an average grade of 1.14 % copper and 0.25 ounces of silver per ton) (Johnson et al., 1995). Whereas the White Pine deposit has been the subject of both basic and in-depth geologic studies, heretofore there has not been a multi-element geochemical investigation. For this study, samples were selected from drill core to avoid the problems of geochemical mobility in surface exposures. A systematic vertical section through the ore body was obtained from 5 holes at the White Pine Mine. The holes were from both the center and edge of each deposit. A total of 387 samples (includes areas not discussed here) were analyzed for 64 elements by Activation Laboratories in Toronto, Canada. Copper ore samples are defined here as those samples with 0.2 % Cu or more. At the White Pine Mine, the weighted average for the samples used in this study have a copper content of 1.22 % Cu which is similar to the average grade in mined ore of 1.14 % Cu. The geochemical character of the White Pine copper ore is given in Table 1. Several elements (Cu, Ag, As, Hg, Ba, Br, Ca, Cd, Fe, Hg, K, Mn, Na, Ni, Pb, Re, S, V, Zn, U) were selected for more in-depth study (MuviTjikalepo, 2007). At the White Pine Mine, distribution of Ag, Hg, and Re closely parallel that of Cu. Arsenic does not appear to correlate well with the vertical changes in the amount of Cu. The vertical distributions of Pb and Cd appear to correlate well with each other, but not with Cu. The vertical variation of Zn shows no significant correlation to that of Pb-Cd. The contents of Zn and Ni appear to be closely related to one another and slightly resemble Cu. The vertical variations of Ba and Br appear to be irregular and/or nearly constant throughout and show no significant correlation to the distribution of Cu. The geochemical profiles of Ca and Fe appear to show an antithetic relationship with each other and appear unrelated to the distribution of Cu. The Mn concentration is more or less constant throughout the vertical ore column, and shows a weak relationship with Cu and a moderate connection with Ca. The vertical distribution of S displays a clear relationship with that of Cu. The geochemical profiles of both K and Na show no significant resemblance to the geochemical profiles of Cu, and their vertical distributions appear more or less homogenous throughout the ore column. The geochemical profiles of U closely parallel those of Cu, except in the uppermost part of the mineralized column where the U content appears to rise slightly with a corresponding slight drop in Cu. The V content on the other hand appears to be largely constant. In comparison to the Kupferschiefer ore horizon and the ore shale of the Zambian Copperbelt, the White Pine occurrence is geochemically dominated by Cu and Ag. In ore shale of the Zambian Copperbelt the metal association is Cu and Co. The association of metals in the Kupferschiefer shale and Zambian Copperbelt shale are relatively more polymetallic than White Pine. Acknowledgements This study was partially funded by Phelps Dodge Exploration (currently Freeport-McMoRan Copper & Gold), a Fulbright Fellowship to Muvi-Tjikalepo, and Michigan Technological University. The drill core from the White Pine Mine was obtained by Bornhorst during the time of mine closure through the generous assistance of Dr. Rod Johnson. 63 �Table 1: Weighted average values of elemental concentrations in copper ore from the White Pine Mine. Copper ore is defined as those samples with a minimum copper of 0.2 %. Weighted average determined by weighting using the total thickness of the interval sampled in the core. For the White Pine Mine, 49 samples covered 69 ft of core defined as ore. The number of digits listed is not necessarily significant on the basis of accuracy or precision. The QA/QC program included blind duplicate samples, a blind “blank” sample run with each batch, blind geochemical reference samples, laboratory duplicate samples, and laboratory geochemical reference standards. Raw and QA/QC data are given in Muvi-Tjikalepo (2007). Cu % 1.22 Al % Ag ppm As ppm Au ppb Ba ppm Be ppm Bi ppm Br ppm Ca % Cd ppm Ce ppm Co ppm Cr ppm Cs ppm Dy ppm Er ppm Eu ppm Fe % Ga ppm Gd ppm Ge ppm Hf ppm Hg ppb Ho ppm In ppm Ir ppb K% La ppm Li ppm Lu ppm Mg % 5.54 10.9 5.6 <2 621 2 0.2 4.5 2.10 7.8 52 33 79 6.1 6.6 3.9 1.63 4.58 22.2 7.0 0.3 6.1 92 1.3 <0.1 <5 2.14 25.7 43.7 0.5 2.16 Mn ppm Mo ppm Na % Nb ppm Nd ppm Ni ppm P% Pb ppm Pr ppm Rb ppm Re ppm S% Sb ppm Sc ppm Se ppm Sm ppm Sn ppm Sr ppm Ta ppm Tb ppm Te ppm Th ppm Ti % Tl ppm Tm ppm U ppm V ppm W ppm Y ppm Yb ppm Zn ppm Zr ppm 1216 1.1 1.47 16.3 31 51 0.08 11 7.8 46.6 0.15 0.32 0.6 17.0 1.3 7.2 3 120 0.9 1.1 0.3 6.8 1.77 0.82 0.46 2.4 133 <1 30.6 3.5 117 237 References Cited Johnson, R.C., Andrews, R.A., Nelson, W.S., Suszek, T., and Sikkila, K. 1995. Geology and mineralization of the White Pine copper deposits: unpublished Copper Range Company Report. Muvi-Tjikalepo, M.H. 2007. Stratigraphy and trace element distribution in the lower Nonesuch Formation of the Michigan segment of the North American Mid-continent Rift System, Gogebic-Ontonagon Counties, Michigan: M.S. Thesis, Michigan Technological University, 133 p. 64 �Northern Michigan Geologic Repository Association The geological repository facilities in Marquette are full-to-overflowing. There is no room for new cores or samples. Furthermore, the state cannot provide permanent, ongoing funding. The Northern Michigan Geologic Repository Association (NMGRA) is being created as a notfor-profit-organization to provide support for repository efforts in the Northern Peninsula to collect geologic cores, samples, specimens, and associated documents relating to the geology and house, materials collected in the State of Michigan. These materials will be catalogued and preserved for charitable, educational, scientific, and literary purposes. NWGRA will work in cooperation with scientific, professional, educational, governmental, social and philanthropic organizations to advance mutual concerns and activities. As a not-for-profit-organization, your contributions will be tax deductible. 65 �PolyMet Mining: NorthMet Cu-Ni-Co-PGE Project, Hoyt Lakes, Minnesota, USA Richard Patelke, Project Geologist, PolyMet Mining LOCATION: NorthMet is 70 miles north of Duluth in the heart of the world-class Mesabi Iron Range District of northeastern Minnesota. It is one of at least ten deposits in the region. STORY: Drilling for Cu-Ni-Co-PGE began with US Steel in 1969, with work by PolyMet since 1998. There are 371 drill holes over 285,757 feet with 34,186 multi-element assays. The Project includes the former LTV Steel Mining Company taconite iron ore concentrator, the “Erie plant,” idle since 2001 and now wholly owned by PolyMet. The Erie plant comprises a 100,000 tpd concentrator and associated facilities, such as a tailings basin with 28 years of capacity at PolyMet's intended initial production of 32,000 tpd. Also included are a rail fleet, shops, office buildings, and ready access to power and water. The plant is ready to run NorthMet ore after simple refurbishment, installation of flotation equipment, and construction of the hydromet facility. A bankable feasibility study was completed in 2006, permits and financing are expected in late 2008, with construction commencing immediately thereafter, concentrate production will begin in 2009, followed closely by hydrometallurgical processing, and ultimately copper metal production along with Ni-Co hydroxide and PGE concentrate. GEOLOGY: The NorthMet deposit is a large, disseminated sulfide, Cu-Ni-Co-PGE ore body in the Keweenawan Duluth Complex. It is hosted in grossly layered troctolitic rocks overlying metamorphosed greywackes. There are seven igneous stratigraphic units divided and recognized by texture and basal ultramafics. All intrusive and country rock units dip gently to the southeast. DEPOSIT: The main ore zone is in the basal igneous stratigraphic unit with local extension of mineralization into the overlying unit. A secondary ore zone (the "Magenta Zone") in the upper units in the western part of the deposit crosses stratigraphy. The Magenta Zone is copper and PGE-rich and sulfur poor relative to the rest of NorthMet. Mineralization is chalcopyrite, cubanite, pyrrhotite, and pentlandite. PGE are correlated with copper. The deposit is open along strike and down-dip, with continuing drilling expected to add resource in both directions. NorthMet has a resource (2007) of 638 million tons measured and indicated, 252 million tons inferred, and reserves of 275 million tons proven and probable at a grade of 0.28% Cu, 0.08% Ni, 0.008% Co, and 0.337 g/tonne Pt + Pd + Au. DISPLAY: Four core boxes displaying stratigraphic and mineralization suites, poster with crosssections, maps, and photos. 66 �Keweenawan apparent polar wander path: new observations, new ideas L.J. Pesonen Department of Physics, Laboratory for Solid Earth Geophysics, PB 64, 00014 University of Helsinki, Finland (lauri.pesonen@helsinki.fi) The Keweenawan apw track (the “Logan Loop“) of Laurentia has been considered to be the best documented Precambrian apw-sequence in the world. However, even after nearly 50 years of work, the cause of the pole motion is still debated. Here we present new observations extracted from the literature. We focus on two cases, (i) the Logan intrusions and (ii) the Mamainse Point volcanics. These units are of prime importance since, unlike many other Keweenawan units, they record two to three successive reversals (at least the latter case) thus being crucial in testing the various models of the Logan Loop. Mamainse Point Volcanics. Palmer (1970) pointed out that in the Mamainse Point section there are four “magnetostratigraphic” units, which are from oldest to youngest: R2-N2-R1-N1. Of the reversals the oldest one (R2-N2) is asymmetric in a similar manner as in most Keweenawan units: R inclinations are steep upward whereas N inclinations are moderate shallow downward. Recently, Swanson-Hysell et al. (2006) have given new evidences that the older reversal is not asymmetric if one considers the data in time progression (flow by flow) and not as mean values. Following this principle, they interpret the asymmetry as a rapid motion of Laurentia from high to nearly equatorial latitudes. This idea cast doubt that the asymmetry is due to a hypothetical non-dipole field as suggested by Pesonen and Nevanlinna (1981). There are some support for their idea. First, a close look of the Fig. 19 of Palmer (1970) shows also a streaking in reversed polarity (R2) inclinations, from steep values to shallow ones. Unfortunately, we don’t know if Palmer´s data correspond the stratigraphic order. There are more examples where time progression of the pole can be notified along the western arm of the Logan Loop, such as in Osler volcanics, where the pole of the lower R units is distinctly older than that of the upper R units (Fig. 1). Also, the pole of Logan R sills seem to be older than that of dykes (which cut the sills; Fig. 1). However, there are also contrasting evidences. For example, the N-poles of the Portage Lake lava sequence, when plotted in stratigraphic order (Books, 1972), do not plot along the track but make an oval across it, resembling typical secular variation pattern (Donadini 2007). Secondly, there are also some evidences of “back-and-forth” movement of the pole along the apw-track. Another problem is the age of the Mamainse Point volcanics, since according to Davis et al. (1995) the upper part of the lower R2 unit is ca. 1096 Ma, some 15-20 Ma younger than other R polarity poles, such as the Logan sills (ca. 1110 Ma). The only way to reconcile this is that the dated flow is an intrusive (sill) but even in this case, the pole will make back-and-forth movement along the track which is against the plate motion model. We have proposed that the major part of the loop, the asymmetry, and the back-and-forth movement can all be explained in terms of the fluctuating non-dipole field (Pesonen et al., 2006). Pigeon River dykes. Hollings et al. (2007) have pointed out that in the Pigeon River area (Ontario), there are three cross-cutting dyke swarms. Recently, one dyke from the the Pigeon River swarm has yielded an U-Pb age of 1141±20 Ma (Heaman et al., 2007) which is markedly older than the generally accepted age of the Keweenawan igneous units (1115-1087 Ma). In addition, Hollings et al. (2007) point out that geochemically the Thunder Bay (and south of it) sills are distinct from Lake Nipigon sills. These results call for a relook at the paleomagnetic data base if there are supporting evidences for their observations. Fig. 1 shows the poles from three, supposed to be nearly coeval (1115-1087 Ma), Keweenawan igneous areas in the northern part of the Lake Superior. These are Pigeon River (triangles), Thunder Bay (squares) and Lake Nipigon (circles). The dual polarity poles are derived of dykes (small symbol) and sills (large symbol), respectively. All the rock units have been paleomagnetically studied by three to six authors (Dubois, Robertson&Fahrig, Pesonen, Palmer, Halls and Stott, Middleton et al.; see references) thus allowing a consistency check to be made. The result is somewhat surpising: in the case of 67 �N-polarity data, there are no marked differences between poles from the three areas and the consistency check is excellent. In the case of R-polarity data, there is a slight tendency that the Nipigon sills and dykes are the oldest (in the sense of apw; Fig. 1), the Thunder Bay sills and dykes are next and the Pigeon River sills are the youngest units. Most important, the pole of the Pigeon River R-polarity dykes, recently dated at 1141±20 Ma, differ from other R poles. Unfortunately, data from only three dykes are so far available but the consistency between the two studies (Robertson and Fahrig, 1971; Pesonen, 1979) is good. It is noteworthy that this R polarity pole, if indeed of 1141 Ma old, cannot be matched with the coeval 1141 Ma Abitibi poles (either N or R). If the Pigeon River R-pole stands in future studies (more data urgently needed), the new pattern of Keweenawan-Abitibi poles does not allow a simple western arm of the Logan Loop to be drawn but requires, a more complex apwp. Alternatively, the complex pattern of poles may reflect a rapidly oscillating non-dipole field prevailing during the (oldest) R polarity epoch. The R2-N1 asymmetry in the Coldwell Complex (1108 Ma), and the new, surprisingly “young” age, 1104 Ma, for an R-polarity Nipigon sill with steep upward inclination (Halls and Stott, 2005; Fig. 1) requires also a back-and-forth movement of the pole along the apw-track, in support of an fluctuating non-dipole field model. The new poles from the coeval dykes from Central Arizona, being 14o apart from the Lake Superior area (Fig. 1) can also be interpreted in terms of an oscillating non-dipole field, although they are not yet dated with modern standards (Donadini, 2007). Figure 1. Paleomagnetic poles of Keweenawan intrusions from Pigeon River (PR), Lake Nipigon (NP) and Thunder Bay (TB) areas. Large (small) symbols denote sills (dykes) and open (closed) symbols denote R (N) polarities, respectively. Some key ages are added. Also shown are the ca. 1140 Ma old poles from the Abitibi dykes, Ontario (both polarities) and new results from the Central Arizona dykes (both polarities). The dotted swathe is the western arm of the Logan Loop. References Books, K.G. (1972). USGS Surv.Prof. Pap., 760, 42 p. Davis, D. et al., 1995. ILSG, 41, 9-10. Donadini, F. (2007). PhD thesis, Univ. of Helsinki, 188 p. DuBois, P.M., 1962. Geol.Surv. Can. Bull. 71, 1-75. Halls,H.C.&Pesonen, L.J (1982). GSA Mem., 156, 173-201. Halls,H.C&Stott, G (2005). OGS Open file report, 617, 52 p. Heaman, L. M. et al. (2007). CJES, 44, 1-32. Hollings, P. et al. (2007). CJES, 44, 389-412. Middleton, R.S. et al. (2004). JGR-B, 109, 2103, doi:10/1029 Palmer, H.C. (1970). CJES, 7, 1410-1436. Pesonen, L.J., 1979. Bull. Geol. Soc. Finl. 51, 27-44. Pesonen, L.J.&Nevanlinna, H., (1981). Nature, 294, 436-439. Robertson, W.A.&Fahrig, W., 1971. CJES, 8, 1355-1372. Swanson-Hysell, N. et al. (2006). GSA Abtracts, 38, 398 68 �GEOLOGICAL MAP OF THE NORTHERN SOUTH KAWISHIWI INTRUSION AND SURROUNDING AREAS, DULUTH COMPLEX: ST. LOUIS AND LAKE COUNTIES, NORTHEASTERN MINNESOTA PETERSON, Dean (Natural Resources Research Institute, University of Minnesota Duluth) The recent boom in metal prices has brought about a worldwide resurgence in the exploration for virtually all mineral commodities, and the Lake Superior district has certainly seen its share of these exploration dollars. In the Duluth Complex of northeastern Minnesota, five companies are actively working on Cu-Ni-PGE properties in the Partridge River (Polymet Mining, TeckCominco) and South Kawishiwi (Franconia Minerals, Duluth Metals Limited, and Encampment Minerals) intrusions. Such mineral exploration relies on published geological information (maps, reports) from geological surveys and/or academic organizations. The publication of a new bedrock geological map of the northern South Kawishiwi intrusion (SKI) and adjacent areas has been one of the author’s research projects over the last several years. Such work, if it is completed correctly, adds important knowledge that can be directly integrated into the detailed databases that the companies maintain for their properties. The caveat (if it is completed correctly) relates to how seamless the published data can be incorporated into exploration programs, resource calculations, and mine plans. For this project, geologic units that the companies use to define the igneous stratigraphy of the SKI (based on ~700 drilled within the map sheet) have been used to define the map units of the basal zone of the SKI. The author’s mapping has evolved from a geological mapping study of a small area (to understand magma inflow into the SKI) into a comprehensive geologic mapping and compilation project (~105,000 acres) to answer some of the fundamental questions on the origin of the extensive known and undiscovered Cu-Ni-PGE mineralization in the northern portion of the SKI. Such an increase in scope is needed due to the economic significance of the published resource estimates (>$140 billion) from this area. To date, over 15,000 outcrops, 1,400 structural measurements, and 12,500,000 meters of elevated contour lines have been integrated into the comprehensive GIS database. The new map area includes the geology from each of the seven major lithologic units in the area: the footwall Late Archean Giants Range batholith and Paleoproterozoic Biwabik Iron and Virginia Formations; the enclosing Mesoproterozoic Anorthositic Series rocks; and the Troctolitic Series Bald Eagle (BEI) and SKI intrusions and the arcuate Nickel Lake Macrodike that links the BEI and SKI. A new insight of this work has been the recognition that the northern SKI is not a shallowly dipping sill but rather a southwest trending funnel-like body. Such an interpretation leads to the conclusion that the eastern contact of the SKI, which previously was interpreted as the top of the intrusion, is a basal contact, and thus has great potential for hosting Cu-Ni-PGE mineralization at depth. The map presented in the 2008 ILSG poster session should be viewed as an update, as there are still several large areas that are yet to be mapped in detail. 69 �References Foose, M.P., and Cooper, R.W., 1978, Preliminary geologic map of the Harris Lake area, northeastern Minnesota: U.S. Geological Survey Open-File Report 78-385, 24 p., 1 plate, scale 1:12,000. Green, J.C., Phinney, W.C., and Weiblen, P.W., 1966, Geologic map of Gabbro Lake quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-2, scale 31,680. Miller, J.D., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Geological map of the Duluth Complex and related rocks, Northeastern Minnesota: Minnesota Geological Survey, Miscellaneous Map M119, scale 1:200,000. Miller, J.D., Jr., Severson, M.J. and Foose, M.P., 2005, Bedrock geology of the Babbitt Northeast quadrangle, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map M-160, scale 1:24,000. Peterson, D.M., Patelke, R.L., and Severson, M.J., 2004, Bedrock geology map and Cu-Ni mineralization data for the basal contact of the Duluth Complex west of Birch Lake, St. Louis and Lake Counties, Northeastern Minnesota: Natural Resources Research Institute, Map Series NRRI/MAP-2004-02, scale 1:10,000. Peterson, D.M., Albers, P.B., and White, C.R., 2006, Bedrock Geology of the Nickel Lake Macrodike and Adjacent Areas: Lake County, Northeastern Minnesota: Natural Resources Research Institute, Map Series NRRI/MAP-2006-04, scale 1:10,000. Peterson, D.M., 2006, Digital Base for Geological Mapping within the Northern South Kawishiwi Intrusion: Lake and St. Louis Counties, Northeastern Minnesota: Natural Resources Research Institute, Map Series NRRI/MAP-2006-01, scale 1:20,000. Phinney, W.C., 1967, Reconnaissance geologic map of part of the Kangas Bay quadrangle: Minnesota Geological Survey file map, scale 1:24,000. Tharalson, E., Sweet, G., Boisjoli, T., Lentz, B., Fellows, T., and Peterson, D., 2007, Geological Map of the Nickel Lake Macrodike and Northern Bald Eagle Intrusion, Lake County, Northeastern Minnesota: Precambrian Research Center Map Series PRC/MAP-2007-01, scale 1:10,000. 70 �A Study of the Paragenetic Stages of Mineral Growth in Complex Iron Ores at the Tilden Mine and Development of a Mine Scale Model for Application to Ore Treatment Methods Natalie J. Pietrzak1, Norm Duke1, Glenn Scott2 and Helen Lukey2 1 2 Earth Sciences Department, University of Western Ontario, London, Ontario N6A 5B7 Canada Cliffs Technology Group, Cliffs Mining Services Company, Ishpeming, 49849, Michigan, USA. The Tilden Mine in Negaunee, Michigan is owned and operated by the Cleveland-Cliffs mining company. The Tilden operation is mining a segment of the Negaunee Iron Formation that is stratigraphically located within the Proterozoic aged Marquette Range Supergroup (Lougheed, 1983). Cleveland-Cliffs currently experiences a reduction in pellet quality when processing ore mined from various locations within the Tilden pit. From a geological prospective, factors that can effect the plant operation and pellet quality are related to mineralogy and mineral textures. In previous work, the Tilden ore body has been divided into a series of geologic domains based on lithology, structure and metallurgy. Five principle domains have been outlined using the major metadiabase horizons as markers: 1) Northwest Domain; 2) CDIII-West Pit Domain; 3) Main Pit Domain; 4) Intrusive Domains and; 5) Main Pit Footwall Domain. These domains are then further subdivided based on ore type, ore lithology and metallurgy (Lukey et al., 2007). Focus of this Phd thesis is to characterize the mineralogical domains at pit scale. With the results of this thesis, the Tilden staff can determine the mineralogical variations that affect their pellet production quality. The main objectives of this project include: 1) to determine ore composition and textures of the metallurgically classified domains and; 2) characterize the mineralogical variations and separate primary, metamorphic and/or hydrothermal stages. Lougheed, M.S., 1983, Origin of Precambrian Iron-Formations in the Lake Superior Region: Geological Society of America Bulletin, v. 94, p. 325-340. Lukey, H.M., Johnson, R.C., and Scott, G.W., 2007, Mineral Zonation and Stratigraphy of the Tilden Haematite Deposit, Marquette Range, Michigan, USA: Iron Ore Conference. 71 �VERMILION DISTRICT, NE MINNESOTA SARTORELLI, A. K., Department of Geology, University of Wisconsin Oshkosh, Oshkosh, WI 54901, sartoa61@uwosh.edu ANDERSON, A., Precambrian Research Center, University of Minnesota – Duluth, Duluth, MN 55811 HUDAK, G. J., Department of Geology, University of Wisconsin Oshkosh, Oshkosh, WI 54901 Locating volcanic vents can be problematic because many of the physical features that characterize active volcanism (eruptions, volcanoes, hydrothermal and fumarolic activity) are no longer present. Ancient submarine lava flows appear to have formed with similar morphologies to modern submarine lava flows (Jones, 1968; Moore, 1975; Dimroth, et al., 1978; Cas and Wright, 1987; Walker, 1992; Kennish and Lutz, 1998; Batiza and White, 2000); therefore, detailed mapping and facies analysis are essential tools for evaluating: 1) proximity to ancient vent sites; 2) the water depth in which ancient submarine lava flows formed; and 3) the paleotopography associated with the genesis of ancient lava flows. Evaluating such features is necessary to accurately reconstruct ancient volcanic environments (Dimroth et al., 1978; Walker, 1992), and can provide important information useful for locating natural resources in ancient volcanic rocks (Cas, 1992; Gibson et al., 1999; Franklin et al., 2005). Detailed (1:500 to 1:5000 scale) volcanic facies mapping was performed in the Fivemile Lake Sequence of the Lower Member of the Neoarchean Ely Greenstone Formation (EGLM; Peterson and Patelke, 2003) immediately west of Fivemile Lake in the Vermilion District of northeastern Minnesota during August and September, 2007. The purpose of this mapping (which was performed as part of an Undergraduate Student – Faculty Collaborative Research Grant from the University of Wisconsin Oshkosh) was threefold: 1) to further document the distribution of various flow morphologies (sheet lavas, pillow lavas) in the EGLM; 2) to utilize facies analysis in an attempt to determine ancient volcanic vent locations in the EGLM; and 3) to further evaluate the hypothesis that increased thicknesses of hyaloclastite deposits (glassy fragmental rocks) associated with ancient submarine lava flows appear to correlate directly with increasing proximity to ancient vent sites and anomalous base metal mineralization (Newkirk et al., 2001; Hudak et al., 2002a). The mapping included making detailed observations, descriptions, and measurements of pillow morphology, pillow strike, pillow dip and topping directions, pillow bud orientations, pillow dimensions (maximum, minimum, and average pillow horizontal and vertical dimensions), and thickness of interpillow hyaloclastite deposits. Petrographic studies were performed after the mapping to further evaluate the mineralogical and textural characteristics of these extremely well preserved ancient pillow lavas. Our detailed mapping indicates that the region immediately south and west of Fivemile Lake is composed of pillow lavas which were erupted from two separate pillow volcanoes. Pillow budding directions indicate that the oldest pillows were formed in a shallow submarine environment and flowed east from a volcanic vent or vents located west of Fivemile Lake, whereas younger pillow flows flowed west from a shallow submarine volcanic vent or vents located east of the study area. Based on previous studies by Newkirk et al. (2001) and Hudak et al. (2002), this vent or vents may have been located along a north-northeast-trending synvolcanic structure immediately east and northeast of Fivemile Lake. It appears that our 72 �detailed pillow measurements are able to distinguish the products of overlapping pillow volcanoes that were active approximately 2.7 billion years ago. References Batiza, R., and White, J. D. L., 2000. Submarine Lavas and Hyaloclastite: in Sigurdsson, H. (editor in chief), Encyclopedia of Volcanoes: Academic Press, San Diego, p. 361-381. Cas, R. A. F., 1992. Submarine volcanism: eruption styles, products, and relevance to understanding the host-rock successions to volcanic-hosted massive sulfide deposits: Economic Geology, v. 87, p. 511-541. Cas, R. A. F., and Wright, J. V., 1987. Volcanic Successions: Chapman and Hall, London, 528 p. Dimroth, E., Cousineau, P., Leduc, M., and Sanschagrin, Y., 1978. Structure and organization of Archean subaqueous basalt flows, Rouyn-Noranda area, Quebec, Canada: Canadian Journal of Earth Science, v. 15, p. 902-918. Franklin, J. M., Gibson, H. L., Jonasson, I. R., and Galley, A. G., 2005. Volcanogenic massive sulfide deposits: Society of Economic Geologists 100th Anniversary Volume, p. 523-560. Gibson, H. L., Morton, R. L., and Faculty Member, 1999. Submarine volcanic processes, deposits, and environments favorable for the location of volcanic-associated massive sulfide deposits: in Barrie, C. T. and Hannington, M. D., 1999, Volcanic-Associated Massive Sulfide Deposits: Processes and Examples in Modern and Ancient Settings, Reviews in Economic Geology, v. 8, p. 13-51. Hudak, G. J., Heine, J., Newkirk, T. T., Odette, J., and Hauck, S., 2002a. Comparative Geology, Stratigraphy, and Lithogeochemistry of the Fivemile Lake, Quartz Hill, and Skeleton Lake VMS Occurrences, Vermilion District, NE Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2002/03, 390 p. Jones, J. G., 1968. Pillow lava and pahoehoe: Journal of Geology, v. 76, p. 485-488. Kennish, M. J., and Lutz, R. A., 1998. Morphology and distribution of lava flows on mid-ocean ridges: a review: Earth Science Reviews, v. 43, p. 63-90. Moore, J. G., 1975. Mechanism of formation of pillow lava: American Scientist, v. 63, p. 269277. Newkirk, T. T., Faculty Member, and Hauck, S. A., 2001. Preliminary lava flow morphology studies at the Fivemile Lake VMS prospect, Vermilion District, NE Minnesota: implications for volcanic processes, volcanic paleoenvironments, and VMS exploration: Geological Society of America Abstracts and Programs, v. 33, no. 6, p. A-398. Peterson, D. M., and Patelke, R. L., 2003. National Underground Science and Engineering Laboratory (NUSEL): Geological Site Investigation for the Soudan Mine, NE Minnesota: NRRI Technical Report NRRI/TR-2003/29, 88p. Walker, G. P. L., 1992. Morphometric study of pillow size spectrum among pillow lavas: Bulletin of Volcanology, v. 54, p. 459-474. 73 �Geochemistry of the Sudbury Impact Layer, Northern Michigan: Implications for the Nature of the Source Materials K.J. Schulz and W.F. Cannon, U.S. Geological Survey, Reston, VA 20192 (kschulz@usgs.gov, wcannon@usgs.gov) We have analyzed selected materials from the Sudbury impact layer in Northern Michigan for major and trace elements including rare earth elements (REE). Samples include accretionary lapilli, black, possibly devitrified glass fragments, vitric-rich breccia, and matrix from chert breccia. Major element concentrations are variable in these samples; however, most element concentrations vary inversely with increasing SiO2 content. Matrix from a chert breccia at the base of the Hiawatha Graywacke in the Iron River-Crystal Falls district has the lowest SiO2 (61.1 %) and highest FeOt (24.5 %) and P2O5 (1.44 %) contents that probably reflect the presence of a significant component of the Riverton Iron-Formation that directly underlies the breccia. Two black, possibly devitrified glass fragments from drill core at L’Anse have the highest SiO2 contents (~ 95 %) and concomitantly low concentrations of all other major elements. The other samples show a range of SiO2 content from about 71 to 82 %. A distinctive characteristic of all but one of these impact layer samples is very low CaO (after removal of secondary carbonate) and Na2O contents (Fig. 1a). The trace element concentrations of the impact layer samples in Michigan also tend to decrease in abundance with increasing SiO2 content. However, the samples mostly have similar ratios of relatively immobile trace elements (e.g., Zr/TiO2, Zr/Nb, Th/Hf, Sc/Yb). Chondritenormalized REE patterns are similar for all impact layer samples and are characterized by enriched light-REE ([La/Yb]n = ~8 to 20) and no to moderate negative Eu anomalies (Eu/Eu* = 0.65 – 0.94). Chondrite-normalized extended trace element patterns also are similar for most of these impact layer samples and have prominent negative anomalies for Ba, Nb, Ta, Sr, and Ti, and no to negative P anomalies (Fig. 1b). The low Sr abundance complements the low CaO and Na2O content and suggests a very low plagioclase component in the impact layer samples. All of the samples have a relatively high U/Th ratio (0.46-7.43; mean = 1.94 versus 0.26 for average upper crust) that probably reflects secondary redistribution of uranium. In addition, the accretionary lapilli are anomalously enriched in Y, and two samples also are enriched in V and Cr. These enrichments may reflect secondary carbonate complexing, precipitation from seawater under appropriate redox conditions and/or prior enrichment of source materials. The most ironrich sample is also anomalously enriched in V. The compositional similarity of the accretionary lapilli and vitric-rich breccia samples (Fig. 1) suggests that the composition of the impact layer materials primarily reflects that of the source materials at the site of impact. Compared to the Onaping Formation, which represents the impact-related fallback breccia at Sudbury (Ames et al., 2002), the impact layer samples in Michigan have higher SiO2, FeOt and K2O contents, and lower abundances of the other major elements particularly CaO and Na2O (Fig. 1a). The abundances of most trace elements also are generally lower in the Michigan impact layer samples, but their chondrite normalized patterns are very similar to those of the Onaping Formation with the exception of prominent depletions of Ba, Sr and Ti (Fig. 1b). The low abundances of Ca, Na, Sr, and Ba in the Michigan impact layer samples are interpreted as a primary feature reflective of the original source material. Such low abundances are not common of most crustal rocks, but are characteristic of sediments derived from intensely 74 �weathered terranes in which Ca, Na, and Sr are selectively leached from weathering profiles. This suggests that the dominant source material for the impact layer was most likely sediments derived from an intensely weathered terrane. Carbonaceous shales in the Onaping are not depleted in Ca, Na, Sr and Ba, but shales in the Iron River-Crystal Falls district of Michigan are characterized by low abundances of these elements and have overall trace element patterns similar to those of the Sudbury impact layer samples (Fig. 1b). We suggest that similar sediments also were present in the Sudbury area at the time of impact and form the dominant component of the material ejected distally from the impact site. This conclusion is supported by the results of impact modeling studies that predict that the upper crustal portion of the shock-melted target rock volume at Sudbury would be ejected away from the crater area and produce lithological and chemical variations with radial distance from the crater in which deep basement components would dominate near the crater (i.e., Sudbury Igneous Complex) and upper crustal sedimentary components would increasingly dominate farther from the crater center. _______________________________________________________________________ Ames, D.E., Golightly, J.P., Lightfoot, P.C., and Gibson, H.I., 2002, Vitric compositions in the Onaping Formation and their relationship to the Sudbury Igneous Complex, Sudbury structure: Economic Geology, v. 97, p. 1,541-1,562. Figure 1. a) CaO-Na2O-K2O diagram for the Sudbury impact layer samples, Michigan and the Onaping Formation, Ontario, and b) chondrite-normalized extended trace element patterns for accretionary lapilli and vitric-rich breccia samples, Michigan. Also shown in 1b are fields for samples from the Onaping Formation, Ontario (Ames et al., 2002) (gray field) and shales from the Iron River-Crystal Falls (IR-CF) basin, Michigan (Schulz, unpublished data) (stippled field). 75 �Penokean Massive Sulfide Deposits: Age, Geochemistry, and Paleotectonic Setting K.J. Schulz1, S.W. Nicholson1, and W.R. Van Schmus2 1 U.S. Geological Survey, Reston, VA 20192 (kschulz@usgs.gov) (swnich@usgs.gov) 2 University of Kansas, Lawrence, KS 66045 (rvschmus@ku.edu) The Paleoproterozoic volcanic terrane that extends across northern Wisconsin eastward into northern Michigan hosts a number of copper-zinc±lead±gold massive sulfide deposits including Crandon and the recently discovered Back Forty deposit. These deposits collectively contain more than 100 million tons of identified base- and precious-metal mineralization and represent a large but still mostly undeveloped resource. The age of the volcanic terrane in northern Wisconsin is constrained between about 1890 and 1860 Ma by the dating of mostly intrusive rocks within the belt (Sims et al., 1989); however, published ages have not been available for the massive sulfide deposits. We have analyzed zircons from two samples from the Back Forty deposit, Menominee County, Michigan; a quartz porphyry from the footwall and a feldspar porphyry that intrudes the deposit. Zircon fractions from the two samples cluster within error along a common cord and regression through the origin defines a U/Pb age of 1874 ± 4 Ma (Fig. 1). This age is within error of an unpublished U/Pb zircon age (~1870 Ma) determined for the Lynne deposit in Wisconsin (R. Thorpe, personal communication to T. DeMatties, 1995) and firmly establishes that massive sulfide formation within the Wisconsin volcanic terrane was contemporaneous with iron formation deposition in the foreland basins in Michigan and Minnesota. To expand on our previous geochemical studies of the volcanic rocks hosting the Bend and Pelican massive sulfide deposits in northern Wisconsin, we have now analyzed volcanic rocks from the Flambeau, Thornapple (Eisenbrey), Lynne, Ritchie Creek, and Back Forty deposits. These results show that the felsic volcanic rocks hosting the Bend, Pelican, Ritchie Creek and Back Forty deposits, located in the central and eastern parts of the terrane, have similar orogenic calc-alkaline compositions and are characterized by strong enrichments in highly incompatible trace elements including Th (Fig. 2) and light REE ([La/Yb]n ~ 6-15). In contrast, the felsic volcanic rocks hosting the Flambeau and Thornapple deposits, both located in the western part of the volcanic terrane, are much less enriched in Th (Fig. 2) and other incompatible trace elements ([La/Yb]n ~ 3-6). The volcanic rocks hosting the Lynne deposit have compositions that are mostly transitional between the other two groups (Fig. 2). The enriched Th and light REE of the Bend, Pelican, Ritchie Creek and Back Forty felsic volcanic rocks are similar to calc-alkaline felsic volcanic rocks in continental back-arc and intra-arc rifts such as the Taupo volcanic zone in North Island, New Zealand; whereas, the compositions of the Flambeau and Thornapple felsic volcanic rocks are comparable to felsic rocks in oceanic (Kermadec) and evolved island arc (Kuroko) back-arc basins (Fig. 2). The enriched trace element chemistry of the volcanic rocks in the central and eastern portion of the volcanic terrane in Wisconsin and Michigan suggests that continental basement is more widespread in the terrane than previously recognized. Paleozoic massive sulfide deposits associated with similar enriched calc-alkaline felsic volcanic rocks that formed in extensional continental arc settings are generally characterized by a significant lead content (e.g., Bathurst deposits; Yang and Scott, 2003). In contrast, the Penokean massive sulfide deposits are generally lead-poor, with the 76 �exception of the Lynne deposit. The reason for lead-poor nature of the Penokean deposits remains a topic of investigation. References cited: Sims, P.K., Van Schmus, W.R., Schulz, K.J., and Peterman, Z.E., 1989, Tectono-stratigraphic evolution of the Early Proterozoic Wisconsin magmatic terranes of the Penokean Orogen: Canadian Journal of Earth Sciences, v. 26, p. 2,145-2,158. Yang, K., and Scott, S.D., 2003, Geochemical relationships of felsic magmas to ore metals in massive sulfide deposits of the Bathurst Mining Camp, Iberian Pyrite Belt, Hokuroku District, and the Abitibi Belt: Economic Geology Monograph 11, p. 457-478 Figure 1. Concordia diagram for zircon fractions from footwall quartz porphyry (108415) and cross cutting feldspar porphyry (108408), Back Forty massive sulfide deposit, Menominee County, Michigan. Figure 2. Th vs Hf diagram for felsic volcanic rocks hosting Penokean massive sulfide deposits. Also shown are fields for more recent felsic volcanic rocks from extensional back-arc settings ranging from oceanic (Kermadec, SW Pacific) to evolved island arc (Kuroko, Japan) to continental (Taupo, NZ and Bathurst, NB). 77 �GEOLOGICAL MAP OF THE NICKEL LAKE MACRODIKE AND NORTHERN BALD EAGLE INTRUSION: LAKE COUNTY, NORTHEASTERN MINNESOTA THARALSON, Erik (Department of Geological Sciences, University of Minnesota Duluth, Minnesota)1 SWEET, Gabriel (Department of Geology, Macalester College, Minnesota)2 BOISJOLI, Troy (Department of Geology, St. Norbert College, Wisconsin)3 LENTZ, Brian (Department of Geoscience, Winona State University, Minnesota)4 FELLOWS, Tyler (Department of Geosciences, University of Wisconsin-Milwaukee, Wisconsin) PETERSON, Dean (Natural Resources Research Institute, University of Minnesota Duluth) 1 – Currently employed by Encampment Minerals, Ely Minnesota 2 – Currently MS student, Lakehead University, Thunder Bay, Ontario 3 – Currently employed by Cameco Mining, Saskatchewan 4 – Currently employed by Golden Chalice Resources, Timmins, Ontario The Precambrian Research Center’s (PRC) 2007 Precambrian field camp culminated in the mapping and publication of a series of bedrock geological maps from four areas in northeastern Minnesota. These “Capstone” projects were completed in areas of Minnesota where either previous mapping was only reconnaissance scale or in historically previously mapped areas (> 40 years ago) that are adjacent to ongoing research mapping projects. The new bedrock geology map of the Nickel Lake Macrodike and northern Bald Eagle intrusion of the Duluth Complex (presented in the poster session) is the result of seven days of field mapping by the authors in 2007. This map was created by PRC Field Camp students under the supervision and guidance of Dr. Dean Peterson, NRRI Senior Research Associate. The purpose of this map is to aid understanding the nature of the Nickel Lake Macrodike and its pertinence to Cu-Ni-PGE mineralization in the South Kawishiwi Intrusion (Peterson and Albers, 2007). Additionally, this map will extend the range of Dr. Peterson’s ongoing project to map in the northern half of the SKI. Access to the 641 outcrops mapped for this project was provided by extensive canoe shoreline mapping and traverses in the bush (approximately 300 kilometers of total mapping traverses. Mapping was completed at 1:5,000 and 1:10,000 scales and reduced for this map to 1:10,000. The new geologic information was integrated with previous data compiled from Green et al., (1966) and Peterson et. al., (2006) to form the foundation of the final published map. References Green, J.C., Phinney, W.C., and Weiblen, P.W., 1966, Geologic map of Gabbro Lake quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-2, scale 1: 31,680. Peterson, D.M. and Albers, P.B., 2007, Geology of the Nickel Lake Macrodike and its association with Cu-Ni-PGE mineralization in the northern South Kawishiwi Intrusion, Duluth Complex, northeastern Minnesota: Institute on Lake Superior Geology, 53rd Annual Meeting, Trip #4, Field Trip Guidebook, Lutsen, Minnesota, Volume 53. Peterson, D.M., Albers, P.B., and White, C.R., 2006, Bedrock Geology of the Nickel Lake Macrodike and Adjacent Areas: Lake County, Northeastern Minnesota: Natural Resources Research Institute, Map Series NRRI/MAP-200604, scale 1:10,000. 78 �The Effect of Magmatic Volatile Phase Separation Linked to Intrusion of the Duluth Complex: Solution to Anomalous Os Isotopic Compositions of the Virginia Formation? WILLIAMS, Curtis D., RIPLEY, Edward M., LI, Chusi, Department of Geological Sciences, Indiana University, 1001 East 10th Street, Bloomington, IN 47405, cuwillia@indiana.edu Both pyrite- and pyrrhotite-bearing pelitic rocks of the 1.85 Ga Virginia Formation show Re and Os isotopic ratios consistent with a mixing event at ~1.1 Ga. Initial 187Os/188Os ratios suggest that mixing must have involved a component with a near-chondritic Os isotopic ratio. The data can be simply interpreted to reflect the interaction of the Virginia Formation with mantle-derived (chondritic) magmas of the Duluth Complex. However, the Virginia Formation rocks in question occur one and one half kilometers or more from the Duluth Complex, are only slightly recrystallized and have never been in direct contact with magma. Fluid inclusions in layer-parallel quartz veins hosted by the Virginia Formation are characterized by pressure corrected (2 kb) homogenization temperatures between 500 and 600˚C. Together with oxygen isotope values of the quartz (5 ‰-6 ‰), the data suggest that magma-derived fluids may have been responsible for the Re-Os isotopic disturbance. In order to further constrain this premise we have undertaken LA-ICP-MS analysis of individual fluid inclusions and have evaluated the expected compositional variations of fluid derived from a mafic magma. We have utilized the program MELTS (Ghiorso and Sack, 1995) to evaluate the compositional variations expected in derivative melts and the program MagmaticVolatile-Phase (Candela and Piccoli, 1995) to estimate concentrations and ratios of elements in a fluid produced from H2O-saturated magma. A high-Al, olivine tholeiite was used as the parental magma, with initial H2O content varied between 0.2 and 2.0 wt%. At 75% fractional crystallization of a parental melt with 0.2 wt% H2O, the fluid produced is very Fe-rich (~4.9 wt%) with Fe≥Na>Ca>K>Mn≥Mg (assuming 1:1 Na melt-fluid partitioning). Fluids produced from 75% fractional crystallization and up to 1.0 wt% initial H2O show less pronounced differences between elements, with 1.2 wt% Fe, 1.1 wt% Na, 0.7 wt% Ca, 0.48 wt% K, 0.12 wt% Mn and 10ppm Mg. Fluids produced during equilibrium crystallization are also high in Fe, but Na and Ca display similar concentrations, and Mg concentrations for all starting H2O concentrations exceed 0.5 wt%. Relative concentrations also vary with Mg≥K>Mn. Between 1.0 and 1.2 wt% initial H2O, the derivative melts become saturated in H2O at variable crystallization intervals (85-51%, respectively) and exsolve a volatile phase. As in the case at 75% fractional crystallization, Fe is the dominant cation in these fluids, followed in order of decreasing concentration by Ca, K, Mn and Mg. As initial H2O is increased to 2.0 wt%, Fe concentrations rises (~2.2 wt%) such that Fe>>Ca>K>Mn>Mg. Modification of the starting melt composition was also varied via assimilation of Virginia Formation (melt minimum Qz(50)-Ab(24)-Mu(24)-H2O(2)). The assimilation of 10% Virginia Formation leads to the production of a volatile phase with less Fe (~3 wt%), but higher Ca (~1.1 wt%) and K (0.47 wt%). Both fractional and equilibrium crystallization of the contaminated 79 �starting composition show decreases between element concentrations as H2O content of the starting melt increases. To evaluate the initial Cl/H2O ratio of a melt, the program Magmatic-Volatile-Phase was used which incorporates this parameter along with the evolution of H2O in the melt. These models suggest a parental melt with 1.0 to 2.0 wt% H2O(i) will produce a Fe-rich vapor similar to that modeled indirectly by MELTS between 1.0 and 1.2 initial wt% H2O, with the exception of lower Ca concentrations. Our fluid inclusion analyses show Fe concentrations in excess of 1.3 wt%, while Na and K reach 0.16 wt% and 0.04 wt% respectively. Mg concentrations peak around 1700 ppm, while Ca is low or below detection limit. Compositions of the fluid inclusions are very similar to modeled results, which are consistent with the premise that interaction with magmatically-derived fluids was responsible for the anomalous Os isotopic signatures recorded in sulfide-bearing rocks of the Virginia Formation. 80 � https://digitalcollections.lakeheadu.ca/files/original/b76c560400c357210d234ab2d255788e.pdf d289755b7798e12879921a791be977d5 PDF Text Text INSTITUTE ON LAKE SUPERIOR GEOLOGY 54TH ANNUAL MEETING MAY 6-10, 2008 MARQUETTE, MICHIGAN HOSTED BY: Michigan Technological University THEODORE J. BORNHORST AND JOHN S. KLASNER Co-Chairs Proceedings Volume 54 Part 2 – FieldTrip Guidebook EDITED BY THEODORE J. BORNHORST AND JOHN S. KLASNER A. E. SEAMAN MINERAL MUSEUM, MICHIGAN TECHNOLOGICAL UNIVERSITY AND RETIRED WESTERN ILLINOIS UNIVERSITY Cover Photo: Brecciated banded iron formation from Ishpeming, MI. This photograph is similar to Plate XXV in Van Hise, Bayley, and Smyth, 1897, U.S. Geological Survey Monograph 28 “The Marquette Iron-Bearing District of Michigan.” (photograph by Tom Waggoner) i �54TH INSTITUTE ON LAKE SUPERIOR GEOLOGY PROCEEDINGS VOLUME 54 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: BANDED IRON FORMATION OF THE MARQUETTE DISTRICT TRIP 2: ARCHEAN-PALEOPROTEROZOIC UNCONFORMITY AT SILVER LAKE—SEISMITES FROM THE SUDBURY IMPACT? TRIP 3: GEOLOGY OF THE BACK FORTY PROJECT TRIPS 4 AND 8: GEOLOGY OF THE EAGLE PROJECT TRIP 5: THE SUDBURY IMPACT LAYER AT THE MCCLURE LOCALITY TRIP 6: SUSTAINABLE RECOVERY OF IRON FROM THE MARQUETTE DISTRICT TRIP 7: GEOLOGY OF THE KEWEENAWAN BIC INTRUSION Published by the 54th Institute on Lake Superior Geology and distributed by the ILSG Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca ILSG website: http://www.lakesuperiorgeology.org ISSN 1042-9964 ii �TABLE OF CONTENTS PROCEEDINGS VOLUME 54 PART 2— FIELD TRIP GUIDEBOOK TRIP 1: BANDED IRON FORMATION OF THE MARQUETTE DISTRICT .............................................1 TRIP 2: ARCHEAN-PALEOPROTEROZOIC UNCONFORMITY AT SILVER LAKE ............................45 —SEISMITES FROM THE SUDBURY IMPACT? TRIP 3: GEOLOGY OF THE BACK FORTY PROJECT ..........................................................................65 TRIP 4 AND 8: GEOLOGY OF THE EAGLE PROJECT ..........................................................................87 TRIP 5: THE SUDBURY IMPACT LAYER AT THE MCCLURE LOCALITY......................................115 TRIP 6: SUSTAINABLE RECOVERY OF IRON FROM THE MARQUETTE DISTRICT .....................127 TRIP 7: GEOLOGY OF THE KEWEENAWAN BIC INTRUSION ........................................................181 iii �iv �54th Annual Institute on Lake Superior Geology Field Trip 1 BANDED IRON FORMATION OF THE MARQUETTE DISTRICT Tom Waggoner Consulting Geologist retired, Cliffs Mining Services Company The absence of evidence is not Evidence of absence Carl Sagan 1 �BANDED IRON FORMATION OF THE MARQUETTE DISTRICT This two day field trip within Marquette County will concentrate on the distinct types of iron deposits and clues to their formation. The trip will include both active and idle mining sites and outcrop locations never included in any prior field trip of the Marquette iron range. ClevelandCliffs will open their mines for the group. This field trip guide will include a narration of the overall Proterozoic geology of the Marquette Range. All Figures and Tables denoted with capital letters are part of the text. All figures denoted by a small “f” are found on the accompanying CD where full color will enhance their value in the discussion The CD also has plan geology maps of each of the stops and a photo gallery of old mining pictures taken throughout the history of mining on the range. Trip stops will include examples of banded magnetite, hematite, carbonate, silicates, clastics), hard ores (microplaty, specularite, magnetite) and supergene oxidation with enrichment. We will visit several examples of silica/hematite vents along with examples of hydrothermal alteration. We will also examine the late non ferrous metal overprint on the Negaunee iron formation. Acknowledgements I would like to thank Cleveland-Cliffs Inc. for reproducing the plan geology map of the Marquette range and for permission to attend their operations on the range. Appreciation is also expressed to all of Cliff’s personnel including Glenn Scott, Helene Lukey and Al Strandlie who helped on the field trip and in moving the drill core for examination during each day’s lunch session. USX is also acknowledged for allowing access to their Champion property The old photos presented in the CD have been graciously contributed by Cleveland-Cliffs, Jack Deo of Marquette, the Michigan Iron Industry Museum and the State Archives of Michigan. Both the Iron Industry Museum and the Cliffs Shaft Museum are acknowledged for hosting a lunch session on each of the field days. Thanks also go to the City of Negaunee for hydraulically cleaning outcrops in the area of the Jackson Mine. I am also indebted to John and Gretchen Klasner for their patient editing of the field trip guide. MARQUETTE RANGE SUPERGROUP Paleoproterozoic strata of the Marquette Range Supergroup (MRS) lie within the Marquette and Republic troughs that formed within the Archean basement (see plan geology foldout map). The three Groups (i.e. Chocolay, Menominee and Baraga) are related to plate tectonic activity associated with the Penokean and younger plate tectonic events (Schulz, 2007). They were drapfolded and faulted into the underlying troughs in the basement forming the gently west or northwest-plunging Marquette and Republic synclines respectively. The Chocolay Group includes a basal conglomerate overlain by quartzites, carbonates and slates. The Menominee group contains alternating slates and quartzites with a limited banded iron formation overlain by the economically significant Negaunee iron formation. The stratigraphic column includes the Hemlock Volcanic formation from the Amasa Oval where an overlying iron formation was emplaced at the close of volcanic activity. An unconformity separates the iron formation from the overlying Baraga Group. 2 �Chocolay Group Enchantment Lake Formation This group was named by Gair and Thaden (1968) for a sequence of conglomerate, greywacke and slate found at the eastern end of the Marquette Syncline. It lies on Archean basement or Mesnard quartzite. It has been described as lenticular by Gair (1975) who felt it was missing over topographic highs in the older terrain. The thickness of the basal unit varies from 0 to 600 feet. The unit correlates with the Fern Creek found in the Menominee and Felch Districts. In the Marquette area of the Animikie basin the earliest unit is a fairly thin sequence of conglomerate with minor sandstone and shale. The conglomerate contains clasts of the underlying local Archean terrain indicating limited movement of the clasts. Interpreted dropstone evidence convinced Pettijohn (1943), Gair (1975) and Ojakangas (2001) to conclude the unit was deposited in a glacial environment. However, Bayley et al., (1966), Gair and Thaden (1968) and LaRue (1980) favored an alluvial fan depositional environment. Van Hise and Lieth (1911) noted that the composition of the conglomerate is a function of the underlying rock upon which it rests. The conglomerate in section 22, T. 46 N., R. 26 W. consists of salmon colored cobbles or boulders (fig. 1). In the SE, NE, section 22, T. 47 N., R. 26 W. abundant octahedral and platy hematite replacing fine detrital grains found between the pebbles (fig. 2). Gair (1975, p.17 & 18) noted elevated potassium and sodium values for the slate component indicating that fluid influx caused the alteration. Mesnard Quartzite This unit was named by Van Hise and Bayley (1892) for the prominent outcrop in the Harvey area. The massive white vitreous quartzite attains a maximum thickness of 500 feet. It contains numerous ripple marks (fig. 3) and clasts and in places exhibits cross bedding. It is present only in the eastern half of the Marquette trough and has not been identified west of section 31, T. 48 N., R. 26 W. The predominant color of the well sorted quartz (+90% silica) is white. Numerous quartz veins and veinlets are common and some contain specular hematite. The unit is believed to be equivalent to the Sunday quartzite on the Gogebic Range, the Sturgeon quartzite on the Menominee Range and the Pokegama quartzite on the Mesabi Range. Kona Dolomite The major Kona outcrop area is on the eastern end of the Marquette syncline. The unit is not present in the central and west portions of the trough either due to faulting or non deposition. The Kona formation consists of dolomite (48%), argillite (42%) and quartzite (10%). The argillite was originally siltstone, mudstone or clay stone. Thickness of the unit varies between 0 and 800 meters. Pastel colors of cream, buff, pink, orange, salmon, tan, maroon, and purple characterize most of the carbonate series. The argillite is gray, green, tan, orange and chocolate brown. The quartzite exhibits shades of white to pink (fig. 4) to red. Minor brown chert beds are also observed. Bedding thickness varies from one centimeter to five centimeters. Some dolomites exhibit oolites. Quartz grains both in the dolomite and quartzite average .05 mm while 3 �the grain size of the dolomite averages .3 mm with some dolomite beds containing a uniform coarse crystal size that averages .25 cm. The Kona has been correlated with the Randville dolomite on the Menomonee Range and the Amasa uplift area. It is also correlated with the Bad River dolomite on the Gogebic Range based on its relative position in the stratigraphic column and the absence of dolomites in general in the younger sediments throughout the entire Lake Superior area. The Kona has many similarities to portions of the Lorraine formation north and east of Sault St. Marie, Ont. Taylor (1972) (Figure 1) identified eleven units that make up the Kona formation. His work helped to identify the distinctive members that in turn helped to resolve the structural movement along faults. Detrital quartz appears to originate from the south. The source direction of detritals for the younger iron formations is again shown to originate also from the south. Ripple marks (fig. 5) and other sedimentary features suggest a shallow or shore line environment. Bladed or fibrous rosettes of chert, irregular nodular quartz and cubic to rectangular bright red dolomite crystals indicate replacement of gypsum, anhydrite (fig. 6) and halite (fig 7). These occur in both the dolomite and pelitic sequences. 4 �Styolites can commonly be found between sedimentary units. Episodic super saturation could have caused the precipitation of the salts. There is abundant evidence of early life preserved in the Kona primarily in the form of stromatolites. These can be very large bioherms of the colleria type (fig. 8) or small thin undulating or corrugated mats (fig. 9) that have been replaced by silica and tend to weather out into thin wafer zones in the dolomite. Sharks Bay in Western Australia currently contains sub tidal stromatolites that grow under hyper saline conditions. Detail mapping utilizing known stratigraphy has confirmed a number of east-west striking high angle (75o to the north) normal faults with north side down parallel to the main syncline axis. The eastern terminus of the Marquette trough in the Harvey area appears to be a north-south flexure with the Proterozoic sequence being present to the east of Harvey under Lake Superior. 5 �Airborne magnetic data suggest a synclinal structure plunging to the east under the lake to the Keweenawan Rift hinge line east of the City of Marquette. Neither soft sediment slumping nor tectonic folding is prevalent in the Kona. Metamorphism, where it exists, is the low greenschist facies and lies within the chlorite zone (James, 1955). Intraformational brecciation is common (fig. 10) and may reflect a local tectonic event (Bayley et al, 1966) or could be due to a collapse in a supratidal environment (Larue, 1981). Isopach maps of six Kona members (Taylor, 1972) clearly illustrate a general thickening of the trough centering on section 6, T.47 N., R. 25 W. By the time the Negaunee iron formation was formed the largest basinal down warping had shifted southwestward to section 19, T. 47 N., R. 26 W. The general plunge of the Marquette syncline is to the west. The north side of the syncline exhibits a steep dip to the south with only minor north-south fault offsets. The south and southeastern areas exhibit domal flexures and complex fault offsets. A number of near-vertical, basic, fine-grained dikes (fig. 11) cut the Kona. Most are now primarily chlorite in composition. In addition there are abundant veinlets and joint fillings composed of quartz, carbonate, microcline and tourmaline. The Kona hosts strata bound copper sulfide resources, which have attracted exploration interest dating from 1888. Copper sulfides are found as disseminated grains, grain aggregates and shear zone fillings (fig. 12 & 13). Both vertical and lateral sulfide zonation have been identified. Over a 10 mile distance the sulfide assemblage shows a decrease in copper from east to west (Table 1). Table 1 Copper Assemblage of the Kona Dolomite from West to East WEST pyrite--------pyrite---------chalcopyrite-----bornite-------chalcocite chalcopyrite bornite chalcocite pyrite pyrite pyrite EAST Taylor (1972) suggested the copper was deposited in a Sabka environment involving connate waters that carried and deposited sulfides in a reducing environment. Since there is no evidence of a reducing environment existing at anytime, this is not a particularly appropriate process for the deposition of the Kona sulfides. An alternate source could be progressive hydrothermal precipitation much like that which occurred at White Pine. However, the only other alteration is silica flooding which does not follow the sulfides. The silica does carry specular and micaceous hematite. A resource of half a billion short tons of argillite/quartzite containing about 1% copper has been indicated. In addition to the copper mineralization there are concentrations of microplaty to specular hematite (fig. 14), the significance of which will be further discussed in the section covering the Negaunee, iron formation. The abundance of carbonate, algal features, psuedomorph quart/chert after evaporates, weathering, ripple marks and mud casts suggest a shallow lagoonal to open tidal environment with hypersaline water subject to intermittent subaerial exposure. Further, a warm to temperate climate is most likely to have accompanied this environment. 6 �Wewe Slate The Wewe occurs only on the eastern end of the Marquette Syncline and does not have an equivalent anywhere else in the Paleoproterozoic Lake Superior Basin. The unit is characterized as a green-gray-black, fine grained, faintly-laminated or banded fissile argillite. The major components are quartz, sericite and chlorite. The thickness ranges between 400 and 3000 feet. Some zones contain 2-5% pyrite as disseminations, cubes or concretions and contain the occasional calcite veins. Outcrops are too limited to define the internal stratigraphy but existing diamond drill holes would provide an excellent basis for developing the detailed stratigraphy. Menominee Group Ajibik Quartzite The Ajibik is a generally white vitreous quartzite with some graywacke components. It lies uncomfortably over the Wewe and is in turn conformably overlain by the Siamo slate. A basal conglomerate occurs in many places around the Wewe Hills uplift. Gair (1975) noted in section 23, T. 47 N., R. 26 W. that the basal conglomerate contains jasperoid (silicified Kona dolomite) and clasts of andalusite/chloritoid schist suggesting some of the hydrothermal alteration occurred prior to formation of the conglomerate. The north end of the quartz/hematite stockworks transects the Ajibik and caused brecciation in the Ajibik. It suggests the linear stockworks zone was relatively long lived starting before the Ajibik and lasting until after the Negaunee Iron formation. The thickness of the Ajibik ranges from 450 to 600 feet. Siamo Slate Rock types included in this formation are slate with lesser feldspathic quartzite and graywacke. Both chlorite and sericite are locally abundant. The first iron formation that occurs in the Marquette trough is the Goose Lake member which is up to 100 feet in thickness. It can be traced via outcrop and magnetics for at least 5 miles on the south and east side of the trough. Gair (1975) identified several lithologic sub units within the Siamo: lower laminated slate, Goose Lake iron member, middle slate, quartzite/graywacke and supper slate with chert bands near the top. The transition to the overlying Negaunee is gradual. The Goose Lake member was named by Tyler and Twenhofel (1952) for the banded-laminated iron formation which is primarily siderite, chert with minor magnetite, chlorite and stilpnomelane. Phosphorus values are four times higher than those found in the Negaunee. The Siamo ranges in thickness from 1000 to 3100 feet. Outcrops of the Goose Lake can be found along the shoulders of M-35 just northeast of the Empire Mine in sections 15, 16 and 20 T. 47 N., R. 26 W. 7 �Negaunee Iron Formation The Negaunee iron formation has been the focus of economic exploitation since its discovery in 1844. Early mining concentrated on the hydrothermal hematite and magnetite ore (hard ores) that outcrop at surface on topographical highs. Later during the 1800s shallow supergene enriched hematite (soft ores) deposits were brought into production via underground mining methods. By the middle of the 1900smost of these deposits were depleted. Throughout the period of hard and soft ore production, significant tonnages of banded iron formation were mined and shipped as siliceous ore that was used in the furnace operation to provide adequate slag production. During the 1950s concentration schemes were applied to the banded iron formation to produce an iron concentrate/pellet with relatively low silica and superior physical and chemical characteristics that improve furnace productivity. The Negaunee iron formation is one of many iron formations that occurs in the much larger Paleoproterozoic Animikie basin and whose remnants can be found around Lake Superior in Wisconsin, Minnesota, Michigan and Ontario. The collective iron deposits became known as the Lake Superior Type (LST) as opposed to the smaller Algoma Type. A comparison of LST and Algoma Type features of banded iron formations (bifs) (Table 2) would suggest more difference than actually exists. The argument that LST lack an igneous association is not valid. The Animikie basin contains some very extensive extrusive rocks in the form of the Clarksburg, Hemlock and Emperor volcanics. Similarly, bifs in both Western Australia and S. Africa contain many tuff intervals (called shales) within the iron formation (LaBerge, 1966, 1966). Table 2 Features Common to LST vs Algoma Type BIF LST low igneous association 1.8-2.4 b.y. rifting environment low S, Na, K, Al, P higher CO2 Algoma direct igneous association +2.5 b.y. volcanic arc environment higher S, Na, K, AL, P lower CO2 8 �More recent exploration worldwide has identified large iron formations in the Sokoman in Labrador, the Hamersley and Nubbaru in Australia, the Minas Gerais and Carajas in Brazil, the Kuruman/Griqualand deposits in South Africa and the Krivoy Rog in the Ukraine. It has long been debated as to whether the Negaunee is a time equivalent to the Biwabic, Gunflint, Ironwood, Fence, Riverton and Vulcan. Recent work on defining the existence and position of the Sudbury ejecta (Cannon, 2007) has established that these iron formations are essentially time equivalents. It also indicates the iron formation is older than 1850 Ma and slightly younger than 1875 Ma. Unlike the Gogebic and Mesabi Ranges where the stratigraphic units within the iron formation are traceable over many miles, the Negaunee is extremely variable over very short distances indicating variation in iron source, transportation, environment of deposition, diagenetic alteration and the degree of metamorphism. A general stratigraphic correlation (Figure 2) can be made by the commonality of certain features (Waggoner, 1972). For example the predominately clastic horizon present in the southeast corner of the Marquette trough and the Palmer fault block makes good marker horizon for the southeast end of the syncline. The original mineralogy of LST and Algoma banded iron formations are quite simple in that they contain silica in the form of chert and iron in the form of carbonate, oxides, silicates and sulfides. They also contain ubiquitous apatite and can also be associated with fine to medium clastic components. Common silicates present on the eastern portion of the Marquette trough are stilpnomelane and minnesotaite. Based on the presence of greenalite on the very western part of the Biwabic range where the metamorphic effect is non existent, it can be speculated that greenalite could be a precursor to either or both these silicates. Heat or pressure can transform 9 �the low metamorphic silicates to either cummingtonite or grunerite. Further heat and/or pressure can result in these silicates becoming amphiboles and pyroxenes. Indeed, in the vicinity of the large diabase sills on the eastern end of the Range the low grade silicates have been converted to coarse grunerite. Sodium rich zones in the Negaunee contain Riebeckite (fig. 15) and Acmite (fig. 16). Sodium rich minerals are present in LST bifs worldwide. Crocidolite and riebeckite are ubiquous to the iron formation in Western Australia (fig. 17) and South Africa (fig. 18). Influx of sodium into the early sediments translates into thee minerals during diagensis or low rank metamorphism. Calcium sulfate in the form of gypsum is quite common on the eastern end of the Marquette trough and eastern end of the Palmer Fault block. It can be present in both oxidized bif or as the soft ore matrix. Its presence is detrimental to ore quality and successful beneficiation applications. Quartz veins with micaceous hematite (fig. 19) are believed to have been emplaced during the formation of the hydrothermal hard ore. Cobbles of this material are included in the basal Goodrich conglomerate. The four major iron minerals we find today in the field do not necessarily reflect the original minerals. Banded iron formations on the Marquette Range have undergone diagenetic alteration. The majority of the magnetite (Han, 1971) and martite ore was originally carbonate silicate chert. Remnants of this lithology can be found at both the Empire and Tilden deposits. Han (1982) has shown that the core of most magnetite grains were originally very fine hematite that was altered to magnetite under a reducing diagenetic environment. The magnetite carbonate horizon at the Empire Mine was the exception as there was never any primary hematite and the magnetite formed was due to selective volume for volume replacement of iron carbonate. The seed hematite is found through the major bifs in the world (Han, 1988). The bulk of the magnetite, formed as a replacement to the iron carbonate, is fine grained (less than 50 microns) and exhibits a sooty gray black color. The carbonate is generally a mixture of siderite, ankerite, iron dolomite and calcite with variable amounts of Ca, Mg and Mn. With an abundance of Fe++ available much of the iron carbonate has been converted to magnetite with or without a fine hematite precursor seed core. Most of the iron silicates we see in the iron formation are either due to conversion of preexisting silicates/carbonate during digenesis ore metamorphic processes. Sulfides are conspicuous by their general absence from both the stratigraphic column and specifically in the Negaunee iron formation. However, at some places where hard ore is found, sulfides do occur and can be locally abundant. Pyrite and chalocopyrite predominate but bornite and pyrrhotite have been noted. A suite of sulfides representing nine geographical locations was analyzed for sulfur isotope values (Waggoner, 2006). Sulfur isotope values ranged from .02 to 6.8 %o indicating they most likely were hydrothermal in origin and support the hydrothermal origin of hard ores in general. Jasper may form by replacement of the gray/white chert with fine hematite resulting in orange jasper like that found at the Milwaukee-Davis south of Negaunee (fig. 20). This replacement method usually leaves “islands” of unaffected chert suggesting the replacement. Most of the bright red jasper on the eastern portion of the Marquette trough shows multi hued layers but they do not show any indication of replacement like cross cutting veins and gradation to gray/white chert. The bright jasper could be primary at a vent site that in turn grades laterally into normal color chert and carbonate as opposed to replacement of the chert carbonate. The presence of fine microplaty hematite as veins and diffusions (fig. 21) in the jasper shows definite signs of cross 10 �cutting and replacing the jasper. Replacement of the jasper proceeds from simple fracture and vein filling through brecciation and silica removal resulting in a high grade hard hematite ore. This sequence of replacement can be illustrated by a sequence of samples showing the stages of replacement (fig. 22-26). There is only a little existing evidence of life forms in the Negaunee unlike the Gunflint and Biwabic where there is evidence of extensive reef building. Mancuso, et al (1971) described some physical features that could be algal mats. Lougheed, et al (1973) described hematite framboid psuedomorph after pyrite that could have formed from a decaying biomass of some life form. Han et al (1992) described floating algae (fig. 27) that were preserved in the slaty layers of the magnetite-carbonate-silicate horizon of the Negaunee mined at the Empire Mine. Evidence suggests various life forms thrived in the environment associated with the generation of banded iron formations. Many workers have suggested life forms are integral to the precipitation of the iron minerals in bifs, however, preservation of the delicate banding of the chert in all color forms suggest life was not a major presence at the time of formation. Moreover life forms are not the only ways the banded iron formations could have formed. A combination of particulate matter and hydro gels can also be hypothesized. Iron formations have rationally been viewed as an entirely chemical precipitate. Since 1977 we now know sulfides can form instantly upon exiting vents (black smokers) as can calcium and silica (white smokers). It is quite possible that iron in the form of hematite and magnetite (temperature sensitive) can form in this same manner and accumulate as discrete bands. Both hydrothermal hematite and magnetite exhibit a metallic luster unlike hematite and magnetite generated by diagenetic replacement or supergene oxidation/enrichment seen on the eastern Marquette Range. James (1955) indicated primary hematite and magnetite are fairly inured to conversion to other forms during either diagensis or metamorphism other than expand in crystal size. In addition to diagensis and metamorphism there is supergene oxidation and enrichment of carbonate, silicates and magnetite to hematite, goethite and martite (James, 1953). Near surface oxidation and enrichment make a good case for top down oxidation by meteoric waters, possibly warmed by the exothermic reaction of oxidation of magnetite. The presence of certain clays (Bailey, 1960) can not easily be explained without a hydrothermal input. An example is the existence of dickite and high chrome nontronite (fig. 29) which does not generally form from weathering. The Negaunee iron formation on the northeast corner of the Marquette trough was originally iron carbonate silicate chert that has undergone supergene oxidation and enrichment to ore grade (fig.30 and 31). On the Marquette Range syncline axis form loci for ore formation floored by impervious slate or intrusives. Supergene ores generally form in structural lows where the underlying rock, in this case slate, is relatively impervious (Figure 3). In some instances basic dikes or faults can further constrain the formation of ore. On the Marquette Iron Range the bulk of supergene ore has been extracted by underground methods resulting in extensive caving of the surface due to poor structural integrity of the overlying rock. 11 �The chemistry of the various ore types shows the iron is usually increased during diagensis (Table 3). Table 3 MARQUETTE RANGE IRON ORE CHEMISTRY Carb Empire* Method Ore Type sol. Fe silica alum. phos lime magnes. Mn H2O s Silicous Richmond OP OP carb chert hem-mart. 30.5 39.5 28.9 37.3 1.37 0.98 0.1 0.042 1.29 0.23 4.6 0.37 0.6 0.09 0.7 3.22 0.018 Hard Ore Cliffs Sft Republic OP/UG fine hem. 61 8.61 1.24 0.163 0.07 1.21 0.029 UG spec. 64.3 5.35 1.38 0.074 0.46 0.36 0.5 0.5 0.01 Soft Ore Athens Salisbury UG UG earthy hem. 52.9 51.5 6.04 7.83 2.56 1.15 0.114 0.091 1.14 0.7 0.73 0.46 0.43 0.45 12.5 12.5 0.011 0.016 * Source-USGS PP 769 Source of others American Iron Ore Association Annual Analysis Book 12 �13 �Magnetite replacement of carbonate increases the iron while removing Ca, Mg and Mn. Oxidation of the ferrous iron to martite again increases the iron. Supergene oxidation and enrichment of the carbonate silicate chert increases the iron and reduces the other elements. Under some circumstances the manganese is also enriched as evidenced at the South Jackson and Lucy Mines in Negaunee. The uniformity in the chemistry for the Marquette Range soft ores (Table 4) suggests similar process acting on a common carbonate silicate chert protore. Table 4 Marquette Range Soft Ore Partial Chemistry Mine Ore Type Method Element Fe Silica Al2O3 Phos Lime Mag. Mn H2O Sul. Tracy* Salisbury* underground 54.1 6.57 2.24 0.08 0.29 9.45 0.315 underground Maas* Negaunee* underground underground 51.5 7.83 1.15 0.091 0.7 0.46 0.45 12.5 0.016 52.4 7.1 2 0.089 1.29 0.25 0.23 11.2 0.011 52 6.86 2.39 0.083 0.42 0.36 0.2 12.2 0.014 Athens* underground 52.9 6.04 2.56 0.114 1.14 0.73 0.43 12.5 0.011 *Data Source AIOA book Tracy--1962, Salisbury--1916, Maas--1916 Negaunee--1916, Athens--1930. From east to west there is a progressive change in the metamorphic mineral assemblage as indicated by James (1955). This is based primarily on the altered mineral assemblages in the iron formation. The associated stratigraphy both above and below the Negaunee do not necessarily show the same degree of alteration suggesting that much of the metamorphic effect is due to the late hydrothermal imprint of hematite and magnetite replacing earlier iron formation. Baraga Group Goodrich Quartzite The basal portions of the Goodrich in a few areas contain a unique conglomerate consisting of pebble of white vein quartz, jaspilite, oolitic jasper and hard hematite fragments. In some areas the interstices of the conglomerate are filled with hydrothermally emplaced hematite. Zones which have exhibited a high iron grade have been mined as iron ore (fig. 31). The Hard Ore Mine in Ishpeming and the Republic Mine have had considerable production from such deposits. 14 �The Goodrich Mine production came entirely from within the conglomerate. The schistose hematite pebbles where the hematite is arranged parallel to the chert banding now shows a chaotic arrangement indicating the schistose nature was present before being included in the conglomerate forming event. The presence of filled voids and replacement of fine detrital grains with hematite indicates the hydrothermal process continued throughout the period of conglomerate formation. The presence of the conglomerate would suggest that a period of tectonic change (Figure 4) preceded the formation of the conglomerate; an event that may have indicated a change from extensional basin development to one of compression, closing off the main iron mineralization and radically altering the nature of subsequent sedimentation. Figure 4 Possible Paleoproterozoic depositional environment for the basal Goodrich conglomerate that was facilitated by faulting. Modified after Grenne, et al, 1990 The remainder of the Goodrich is composed of a fine white quartzite with several minor argillite units. Ojakangas (1994) determined the material for the quartzite in eastern Baraga and western Marquette Counties came from the west-northwest and south east. The overall Goodrich varies from 300 to 1400 feet in thickness. 15 �Michigamme Formation This formation has an aggregate thickness ranging from 11,000 to 20,000 feet. Paleocurrent data recorded for the graywackes (Ojakangas, 1994) indicate the primary direction of sedimentation is from the southwest with lesser influx from the north. Lower Slate Member The slate unit is characterized by the presence of significant graphite (Fig. 32) and ultra-fine pyrite. The amorphous graphite as measured by the carbon content ranging from 4% to 20% as determined by Kramer (1987): Cannon et al. (1972) estimated the maximum thickness in the trough to be about 1500 feet and becoming non existent west of the Greenwood Quadrangle. In places the lower contact is phosphatic (fig. 33) and contains fluorapatite crystals as groups in a matrix or as fine phosphatic pebbles. Phosphate enrichment has been reported in nine separate areas with most sites occurring in the Dead River Basin located north of the Marquette trough. A major outcrop in the NE, section 15, T. 49 N., R. 28 W. contains a 15 meter thick areas of channel filled conglomerate. The sandy matrix contains abundant flattened and elongate pebbles of quartzite and black phosphatic slate. Drill hole and channel samples show the rock contains 15+% P2O5 as fluorapatite. The phosphatic pebbles weather quickly to produce negative relief on exposed surfaces. The US Bureau of Mines and Institute on Mineral Research at Michigan Technological University have been able to produce a suitable flotation weight recovery and grade on the limited resource. Greenwood Iron Formation Member The Greenwood was named by Swanson et al. (1930) for a laminated magnetite bearing argillite found in the lower Michigamme formation. It is present from West Ishpeming west to Humboldt. It varies from 600 feet thickness on the east end to over 1200 feet between the communities of Clarksburg and Humboldt. The laminated or bedded rock contains hornblende, biotite/chlorite along with grunerite, magnetite and quartz. Classic iron formation chert is missing and instead higher alumna, alkalies and calcium oxide are present representing clastic (or possibly pyroclastics) input. Clastic dilution is believed responsible for the lower than normal magnetite content. Chemical data is shown in Table 6. 16 �Table 6 Chemical Analysis Greenwood Iron Formation SiO2 Al2O3 Fe2O3 FeO MgO CaO Na2O K2O H2O+ H2OTiO2 P2O5 MnO CO2 Total 62.34% 6.65 6.22 15.76 2.30 1.26 .39 1.28 1.82 .07 .47 .22 .49 .07 Cannon, 1972, p. 90 N=4 99.34% Physical features of the magnetite suggest that it is primary (or possibly diagenetic) and not of clastic origin. The presence of the Greenwood in the same area of the Clarksburg volcanics found directly above is strong associative evidence that each emanated from the same vent area found near the community of Humboldt. Clarksburg Volcanic Member The Clarkburg is a member of the Michigamme Formation (James, 1958) and is composed of mafic pyroclastics, primarily tuffs (fig. 34) and agglomerated (fig. 35) with minor argillite and iron formation. It is estimated to be 2000 feet thick near the community of Clarksburg. Evidence of the center of extrusive activity around this community is present as a feeder zone that outcrops in section 18, T. 47 N., R. 28 W. The chemical composition indicates most rocks fall in the alkaline olivine basalt range according to Cannon et al. (1972). Much of the Clarksburg displays up to 20% iron calcite replacing fragments of devitrified glass shards and plagioclase. The member does not occur on the north limb of the syncline. Middle Graywacke/Slate Member Bijiki Iron Formation Member The iron formation can be characterized as a silicate sulfide chert unit with minor magnetite. It also contains the asbestos form of cummingtonite in the vicinity of the Peshekee River and US 41. 17 �The unit varies from 100 to 200 feet in thickness. Mining from the unit has produced over 4 million long tons of iron ore, primarily limonite (fig. 36) west of Lake Michigamme. Upper Slate Member The Upper slate is most prevalent in Baraga County where it consists of gray to black slate, impure quartzite and greywacke. The lower segments are carbonaceous with fine pyrite. Fairly abundant concretions (fig. 37) occur within the slate. The concretions are generally rich in carbonate (Table 7) as indicated by partial chemical analysis. They are believed to have formed diagenetically by replacing both the quartz and feldspar. Henrickson (1956) has used the mineralogy of the concretions to define the relative regional metamorphic rank. The chemistry of the slate itself is shown in Table 8. Table 7 Chemistry of Selected Upper Michigamme Concretions Metamorphic Zone Oxide Analysis* Chlorite Biotite N=2 N=1 SiO2 29.7% Al2O3 6.7 Fe2O3 .9 FeO 2.7 MnO .3 MgO 1.7 CaO 29.2 TiO2 .6 *Modified after Henrickson, 1956 64.8 11.5 2.1 4.0 2.5 7.2 .6 Garnet N=1 61.8 14.6 1.1 8.8 3.8 .7 .8 Staurolite N=1 60.2 12.7 2.0 3.4 .2 2.3 11.6 .5 chlorite samples: section 2, T 46 N., R. 37 W. biotite sample: section 19, T. 47 N., R. 32 W. garnet sample: section 19, T. 47 N., R. 32 W. staurolite sample: section 30, T. 48 N., R. 30 W. Table 8 Chemistry of the Upper Michigamme Slate* SiO2 Al2O3 Fe2O3 FeO MnO MgO CaO TiO2 P2O5 Carbon Sulfur 62.3% 15.3 1.1 6.2 .1 3.0 .5 .7 .1 3.0 .3 18 Modified after Henrickson, 1956, p. 71, N=25 �Near the Marquette/Baraga County line metamorphism of the Michigamme has resulted in quartz-bitotite garnet and staurolite schists (fig. 38) The Michigamme in Iron County hosts antahraxolite (fig. 39) as reported by James et al (1968) and Mancuso (1983, 1989) indicating biological activity. Paleoprotozoic Igneous Activity The Hemlock volcanic sequence centered on the Amasa Oval southwest of the end of the Marquette trough in Iron County is primarily basalt/andesite extrusives with lesser rhyolite extrusives. The volcanic pile exceeds 10,000 feet in thickness on the western flank of the Amasa Oval. The Fence River/Amasa iron formation found directly on top of the volcanics is equivalent with the Negaunee. The Hemlock volcanic event is believed to have provided the minerals and heat responsible for the bif. Schneider (2002) established a date of 1.87 Ga. for the rhyolite portion of the Hemlock. The Hemlock has been intruded by the large Kieranen sills which attain a thickness of 6000 feet. The Western Sill has undergone differentiation resulting in a basal peridotite grading upward into a gabbro complete with disseminated Cu+Ni sulfides to a top granophyre component complete with titaniferous magnetite. The Hemlock thins rapidly eastward and pinches out east of the Wilson Creek anticline and is not present in the south flank of the Republic Trough. Within the Negaunee there are many sills and dikes. Numerous small sheared chlorite dikes transect the iron formation, often along faults. The chemistry of the sheared dike shows low calcium and alkalies compared to the diabase sills (Table 9) suggesting a different source for the igneous activity. Supergene oxidation of the dikes is conspicuous by the presence of earthy red hematite. Where the oxidized dikes are in proximity to the soft ores they are difficult to distinguish save for the remnant cleavage of the original rock. The sills exhibit a diabasic texture and chemistry. Gair et al. (1970) and Simmons (1972) have described variations of gabbro, syenite and granophyres as minor and local variations. Diabase sills are resistant to erosion and form prominent ridges with iron formation occupying the erosional valleys. The USGS has named a number of the largest sills with extended surface exposure, some as long as 7 miles and a thickness of 900 feet. The varying texture and chemistry were not part of the naming process. In the Summit Mountain sill, in section 24, T. 47 N., R. 27 W. at least three separate intrusive events were recognized by contacts, textural variations and alterations. The complexity of episodic and closely timed events has yet to be satisfactorily resolved. All the diabasic minerals have been altered. Gair offered a (1975, p 122) description of the alteration observed. “Plagioclase is saussuritic and albitic, particularly in the large bodies, or is variably replaced by carbonate, chert, chlorite, sericite, biotite and clay minerals; in one place, replacement is by an unusual mixture and of biotite and epidote. Original pyroxene commonly is replaced by pale-green tremolite-actinolite and minor chlorite or biotite, or entirely by chlorite-biotite.” 19 �Sill and dike contacts generally exhibit a chill margin in the diabase while within the iron formation the carbonate was quickly altered to fine magnetite that was not affected by later diagenetic crystal overgrowth noted in the rest of the iron formation. Gair (1975) noted that intrusive dikes and sills were more altered when in contact with banded iron formation than with slates or quartzites. The iron silicates, minnesotaite and stilpnomelane are elevated to grunerite/cummingtonite at many intrusive contacts. The coarse diabase was found to contain partially assimilated xenoliths of fine grained chlorite dike. Both the field relations and analytical data indicate two intrusive events with the sheared chlorite preceding the massive intrusives. Epidote which is ubiquitous to the none sheared crystalline intrusive is absent from the smaller chlorite dikes. Within the Marquette trough the Clarksburg volcanic sequence found in the lower Michigamme strata is most likely to have provided the diabase found as dikes and sills in the underlying Archean, Chocolay group and Menominee group of the Marquette iron range. The volcanic center occurred on the south side of the trough in the vicinity of Clarksburg. A vent in section 18, T. 47 N., R. 28 W. coupled with an abundance of intrusive bodies and the thickest sequence would suggest this area was the center of greatest volcanic activity. The extrusive extends to Lake Michigamme on the west and to West Ishpeming on the east. They do not occur in the stratigraphic sequence on the north limb of the trough. Tuffs and agglomerates are common on the margins. The chemistry of the sills generally matches (Table 9) those of the Clarksburg diabase and basalts. Amygdaloidal basalts were recognized in the Negaunee at the Greenwood Mine (Cannon, 1974) and in the northwestern portion of the Palmer quadrangle (Gair, 1975). Amygdaloidal basalt is sandwiched between a footwall slate unit in section 27, T. 47 N., R. 27 W., suggesting that there may be syn-depositional origin for at least some igneous rocks. 20 �Table 9 Partial Chemical Analysis of Paleoprotozoic Intrusive RocksMarquette Range, MI 1 Oxides SiO2 Al2O3 Fe Fe2O3 FeO CaO MgO K2O Na2O TiO2 MnO P2O5 N= V Ni Cu Cr Co 3 14.41 11.5 18.57 14.4 0.34 7.74 0.7 0.51 1.68 8 283 103 184 60 43 Sample # 1 2 3 4 5 6 7 8 9 10 39.7 18.35 17.43 16.31 16.74 0.48 8.33 1.8 0.14 0.26 12 2 4 5 47.26 13.44 11.85 7.9 8.13 0.31 9.35 tr 0.48 1.33 6 43.11 48.13 14.8 16.31 12.02 8.67 1.89 1.75 13.76 9.57 0.41 1.34 8.77 2.68 11.62 7.47 2.5 0.64 0.4 2.15 2.05 2.5 3.08 0.96 0.41 0.17 0.41 0.17 3 1 1 6 Trace Metals in ppm 259 90 40 219 7 13.79 8.54 7.43 5.53 3.97 4.48 1.95 4 8 9 10 46.06 14.41 8.91 1.85 9.92 8.99 7.16 0.71 2.12 1.37 0.22 0.22 2 44.97 15.34 8.8 1.51 9.96 7.29 5.91 1.53 4.49 1.96 0.29 0.29 2 49.1 17 9.07 2.4 9.5 10 6.9 0.6 2.6 1.4 0.3 0.3 252 94 129 223 57 References Chlorite dikes at the Empire Mine. USGS PP 76- p. 123 Gair and Han 1975 Sheared un-oxidized chlorite dikes at the Tilden Mine, Sec 25 & 26 Sheared oxidized chlorite dikes at the Tilden Mine, Sec. 25 & 26 A 20 foot thick chlorite dike at the Tracy Mine, Sec. 8, 47-26 Mathias 1958 Clarksburg tuff, Cannon GQ-1168 p. 8 table 1 1974 Large meta-diabase sills T. 47N., R 26 & 27 W., Mathias table 3, 1958 Fine-medium unoxidized meta-diabase, Tilden Mine Meta-diabase Clarksburg Quadrangle, Cannon USGS Map GQ-1168, table 1 p.8 1974 Metabasalt Clarksburg formation. Cannon USGS Map GQ-1168, table 1, p.8, 1974 Typical basalt The latest intrusive event is marked by a few scattered quartz, ankerite and micaceous veins that cut both chlorite and meta-diabase dikes but do not cut the Keweenawan dikes. The large micaceous hematite crystals show the occasional striations that resemble twinning. The quartz is milk white and the carbonate is cream white to light brown in color. None of the veins persist over significant distances. 21 �Keweenawan Diabase Dikes (1108 Ma) Reversely polarized Keweenawan diabase dikes are present throughout the Marquette iron range. They exhibit an east-west trend with a near vertical dip. One narrow dike has been traced over a three mile strike length and down dip over 1200 feet. The diabase ranges in width from 6 inches to 80 feet and locally bifurcates over short strike distances. Where fresh the diabase is a dark green to black and with a clear dibasic texture. Where the diabase cuts iron formation the diabasic texture is obliterated by complete alteration to clay minerals. Where the Keweenawan dikes cut older sills and dikes in the iron formation, the diabase dikes appear fresh and unaltered suggesting contact with the evolving oxidation of the iron formation facilitated the dike alteration. The altered color is generally olive green with occasional oxidation of the contact zone to a slightly yellow color. Altered dikes are quite susceptible to weathering. Newly exposed surfaces usually disintegrate within two weeks. Rapid weathering explains why the dikes have not been observed in outcrop. Where the Keweenawan dikes have been observed in underground workings they are always argillized where they cut enriched natural ore bodies. Table 10 Selected Partial Analysis of Keweenawan Diabase Dikes, Marquette Iron Range, MI Sample I.D. T25-7-732 T26-75-1119 EM-1165-2 EM-1340-1 Coordinates NA NA 18,740S, 3,450W**** 20.570S. 2,370W NENW Sec.31 47-26 Description altered diabase altered diabase altered diabase altered diabase unaltered diabase OV-1 T25-7-732 T26-75-1119 EM-1165-2 EM-1340-1 OV- FeO* 9.08 21.09 25.46 16.08 14.66 SiO2** Al2O3 57.2 16.24 42.26 14.26 28.88 16.36 23.06 20.14 60.1 13.58 T25-7-732 T26-75-1119 EM-1165-2 Em-1340-1 OV1 P2O5 0.042 0.029 0.024 0.048 0.014 TiO2 0.57 0.85 0.3 0.57 0.24 Per Cent CaO MgO*** 1.55 3 2.6 7.5 0.62 7.8 1.07 6.3 8.64 3.3 MnO 0.01 0.02 0.06 0.01 0.02 K2O 0.24 0.6 0.1 1.01 0.06 Na2O 0.84 1.46 1.11 1 2.88 Cu 153 430 64 316 324 Cr 122 65 29 115 70 Co 37 60 50 50 70 1 ppm Ba 967 1660 1950 1180 1250 V 575 499 281 641 409 Ni 49 55 19 48 60 Sample Legend T = Tilden Mine DDH-Sec.-hole#-footage in hole Em = Empire Mine bench sample-footage OV-1 = Isabella dike in sec. 31, T. 47 N., R.26 W. * Soluble iron calculated as FeO ** Titration analysis *** Wet Chem. **** Mine triangulation coordinates ***** 1978 Barringer 42 element analysis-Co@, H2O and S not determined 22 �Although they do not control enriched ore concentrations like the older intrusives, they were definitely altered by the solutions acting in the porous oxidized and enriched zones. Gair (1975) suggested the solutions responsible for oxidation of the iron formation were active after Keweenawan time and were a causative effect in the alteration of the Keweenawan dikes. Table 10 shows the chemical variation found within the altered dikes. Alteration of the Keweenawan dikes resulted in a pronounced reduction of silica and calcium where the dike was in contact with ferrous iron formations (i.e. Empire) and to a lesser extent in a ferric environment (i.e. Tilden) Recent dating of the Eagle nickel-copper deposit indicates that it is 1107+-3.7 Ma. and establishes a good age for the general Keweenawan diabase event. Paleomagnetic data and chemistry of the abundant east-west Keweenawan dikes are similar to the Logan sills of the North Shore Volcanic group that have an age of 1108 matching the recent Eagle number. Northwest faults offset the Keweenawan dikes and indicate that the age of the offset is younger than 1108 Ma. The dikes are devoid of any metamorphic fabric. Structure Schulz (2007) summarized the Penokean Orogeny as starting with the collision of oceanic, island-arc terrain (the Pembine-Wausau terrain of Wisconsin) with the Archean Superior craton to the north. Upon collision the direction of subduction flipped to the north, creating extensive back-arc extension. This resulted in the development of the Animikie basin into which a thick group of sedimentary and volcanic rocks representing the Menominee Group were deposited. The accumulation included the Negaunee iron formation and its equivalents in other iron districts. The north-directed subduction brought the Marshfield terrain toward the Superior craton. The Pembine/Wausau island arc was thrust north onto the Superior craton resulting in subsidence of a foreland basin into which the Baraga group of sedimentary rocks, including turbidities, and volcanics were deposited. This took place between 1850 and 1835 Ma. Metamorphism occurred during folding, thrusting and tectonic thickening resulting in a metamorphic gradation of existing rocks accumulated during the Penokean Orogeny. The end of the Penokean was marked by emplacement of a number of plutons in the metamorphosed terrene. A north-verging fold-thrust system in the early Paleoproterozoic continental foreland in northern Michigan has been recognized by many workers (Cannon, 1973 and Klasner, 1978, 1991). Initial deformation caused thin skinned shortening of the Paleoproterozoic strata along decollements. This deformation also formed north-verging structures along with south-dipping foliation. Subsequent deformation involved block uplift of the Archean basement and formation of structures such as the Marquette and Republic troughs and possible development of Archean gneiss domes. One other structural event was active from early Chocolay sedimentation up through the end of Menominee sedimentation. A significant down warping occurred in the vicinity of section 6, T. 23 �47 N., R. 25 W. for the Chocolay sequence (Taylor, 1972) and migrated to the southwest to roughly the current position of sec. 19 T. 47 N., R. 26 W. where the Negaunee iron formation attains a thickness of over 1000 meters. The Wewe slate and Siamo slate, including the Goose Lake iron formation occur only in the eastern portion of the trough in the same down warping progression event. It is possible that a detailed study of the stratigraphy of either or both the Wewe and Siamo would reveal a similar thickening along the same NE-SW trend line. The north and south margins of the Marquette trough are bounded generally by high angle faults. The east-west Palmer fault, forming the south boundary of the Tilden ore body, dips uniformly at 55 degrees to the north. FIELD TRIP STOPS 1 THROUGH 8 IRON ORE HISTORY Although iron ore was discovered in outcrop in the vicinity of Negaunee in 1844, production was slow to materialize due to the poor to non existent transportation system. Early ore production went to a number of local charcoal furnaces that produced pig iron using charcoal produced from the local forests. As railroads were built and a system of boat locks installed at Sault Ste Marie in 1855 to accommodate large lake vessels, production of raw iron ore increased and moved to the lower lake steel mills. Initial iron ore mining concentrated on open pit hard ores that formed topographic highs. Discovery of soft enriched supergene ore on the eastern end of the range followed along with the application of underground extraction starting in 1880. These two types of ore were augmented by siliceous ore which was slightly enriched banded iron formation used for furnace slagging. By the 1950’s high grade ore reserves were almost depleted on most of the iron ranges in North America. This spurred the advent of beneficiating techniques to upgrade the banded iron formation into a high grade concentrate that could be pelletized for superior furnace operation. On the Marquette Range the first concentrating/pellet operation was the Humboldt Mine opened in 1954 followed by the Republic Mine opened in 1956. Both used an anionic hot oil flotation system to concentrate the specular hematite. The Empire Mine was opened in 1964 and used magnetite separation. The Tilden Mine followed in 1974 using a cationic silica flotation to concentrate the hematite. It should be mentioned that the soft ores from the Mather A underground mine were pelletized up until the mine closed in 1978. Thomas A. Edison invested the proceeds from the sale of his electric business to GE into iron mining and beneficiating schemes to upgrade magnetite banded iron formation to produce a salable concentrate product. In 1888 Edison provided the financial backing to Walter Mallory to construct a crushing/grinding/electro magnetic separation operation at Humboldt utilizing ore from the Sampson Mine (later to be included as part of the Humboldt Mine in 1952). The Edison Iron concentrating company began operation in 1889 and produced 893 long tons of magnetite concentrate up until a fire destroyed the mill in 1890. The mill was not rebuilt and Edison turned his attention to Fe-oxide deposits such as the magnetite in gneiss at Ogdensburg, New Jersey. He essentially went broke on his mining and concentrating activities 24 �partly because concentrate (small size) was not the preferred furnace feed. He was an inventor and his electro magnetic separation and roll crushing are integral to North American iron ore operations today. The last operating underground hard ore mines were the Cliffs Shaft and Champion Mine and they ceased operation in 1967 and 1968 respectively. The last underground soft ore mine on the range was the Mather B which ceased operation in 1972. The last siliceous mine was the Old Tilden Mine that shipped its last loads in 1973. The Humboldt ceased production in 1974 due to exhaustion of the ore while the Republic Mine shut down in 1981 due to difficult economic circumstances. Each field trip stop is identified by a latitude, longitude and elevation that can be used with a GPS to locate the outcrop on the ground. Stop 1 - Wewe Hills Vents section 21-23, 47-26 A- 46o 27’ 13.22” N, 87o 32’ 38.96” W, +1503 B- 46o 27’ 16.07” N, 87o 32’ 47.17” W, +1505 C- 46o 27’ 41.99” N, 87o 33’ 8.23” W, +1514 Stops 1 A-C are located on a continuous silicified/hematite zone adjacent to a fault, both of which trend N 30o W and is at least 6000 feet long. The stop exhibits similar features found in a number of other localities on the eastern portion of the Marquette Range. Most occurrences contain quartz and hematite stockworks that were emplaced by episodic pulses of silica usually followed by the emplacement of hematite/jasper. The hematite habit varies from botryoidal to microplaty through specularite to very coarse micaceous hematite. Rare earth analysis of the hematite shows high concentrations of LREE relative to HREE (Waggoner, 2003). The REE pattern of the vent hematite is mirrored by the hard ore hematite mined from the upper portion of the Negaunee Iron formation. 25 �Figure 1-1 Wewe Hills Stockworks. Stops A, B and C There has been considerable discussion in regard to the proper placement of these rocks within the stratigraphic section. Van Hise (1897), Van Hise & Leith (1911) and Gair (1975) thought the major slate lithology represented the Wewe slate. Taylor (1972) was able to identify the silicified rock in sections 23 & 24 as the big cusp algal dolomite member of the Kona formation making it more likely that the rocks directly above the basal conglomerate in sections 21 & 22 also belong to the Kona formation. Further east in section 23, directly above the silicified Kona, much of the slate has been hydrothermally altered. Gair (1975) has described chloritoid, andalusite (fig. 40), pyrophyllite and cordierite associated with sericite, chlorite and quartz alteration. In the venting process the elements of iron, silica, potassium, sodium, calcium manganese and aluminum are vented in solution or as fine particulates into a sea water environment. Settling and precipitation resulted in the formation of a banded sequence rich in one or more of the elements as carbonates, oxides or silicates. These same elements when confined to an intrusive or replacement environment resulted in zoned alteration of the host rock. The configuration of the venting in the Wewe Hills suggests the plumbing was linear that allowed for variable volume of discharge along its entire length. On the Marquette Range circumstantial evidence suggests the zone of weakness was present on or near the south side of the trough. Certainly the presence of the Clarksburg and several other Michigamme units only on the south side of the trough points to a weak zone in this location through which the fluids vented. 26 �During the venting process elements of iron, silica, potassium, sodium, calcium, manganese and aluminum are vented in solution or as particulates into a seawater environment. Differential settling and precipitation resulted in the formation of a banded sequence rich in one or more of the elements. These same elements when confined to an intrusive or replacement environment in rock result in zoned alteration. The configuration of the stockwork zone in the Wewe Hills suggests the plumbing was linear and allowed for variable volumes of discharge along the entire length. On the Marquette Range circumstantial evidence suggests the zone of weakness was present on or near the south edge of the trough. Clear timber cutting has aided visual inspection of silicified dolomite/slate units similar to those found to the east of Goose Lake Stop 1A. This outcrop consists of silicified slate and dolomite belonging to the undifferentiated Chocolay Group. The stockworks quartz (fig. 41) trends similar to that of the outcrop. Cross cutting hematite veins (fig. 42) with disseminated wall rock alteration can be distinguished by the red color. The dolomite contains breccia units where the silicified dolomite contains fine specular infill that in some areas resemble a crackly breccia. The breccia contains coarse quartz veins with minor micaceous hematite. Looking east across the valley the large outcrop is an orange to green boulder conglomerate (fig. 43) forming the basal Chocolay Group. Some parts of the conglomerate in sections 21 & 22 contain specularite and martite (fig. 44) replacing the fine fractions of the matrix in the conglomerate usually near shears and joints that acted as a fluid conduit. Stop 1B. The early exploration for iron ore resulted in the excavation of the shallow test pits where high concentrations of iron were encountered . The three to four foot near vertical vein of hematite jasper breccia was tested for size and grade at this location. The episodic injection/dilation caused openings to form into which fine hematite was precipitated as botryoidal hematite (fig. 45). This will be one of two places for Stop 1 where samples can be collected. Stop 1C. This is another early exploration where shallow excavation was used to examine both grade and size of the hematite concentration. The outcrop next to the road is an excellent example of the quartz stockworks overprinted by microplaty hematite and jasper veins. The red oxidation present in the gray green slate is defiantly later than the foliation and as such may be a later event not related to the quartz/hematite which occurred before the end of Goodrich time. This feature is mentioned because this kind of oxide staining at the Wernecke deposit in the NW Territories is believed to have occurred with the metallic oxide alteration. 27 �First day lunch stop will be at the Iron Industry Museum For any of the following Tilden and Empire ore types we are unable to access due to ongoing operations, core and samples will be on display at this lunch break. Core: magnetite silicate iron formation-Empire Mine magnetite carbonate iron formation-Empire Mine carbonate chert iron formation-Empire Mine clastic magnetite silicate chert-Empire Mine ultrafine magnetite at diabase contact soft hematite ore-Cascade Range “blue steel” ore-Cascade Range supergene enrichment-Cascade Range earthy hematite (like Mather ores)- Cascade Range goethite chert-Tilden Mine leopard goethite-Tilden Mine martite clastics-Tilden Mine magnetite carbonate chert-Tilden Mine martite hematitic chert-Tilden Mine martite chert-Tilden Mine Stop 2 - Tilden & Empire Mines section 18, 19, 20, T. 47 N., R. 26 W. and sections 22-27, T. 47 N., R. 27 W. The advanced development of these mines can afford many excellent features of the Negaunee Iron formation on the eastern end of the Range. The Negaunee is over 3000 feet thick in section 19. It has been divided into a lower transition unit that contains fine clastic material, a magnetite silicate horizon (fig. 46), a magnetite carbonate horizon (fig. 47), a clastic horizon (fig. 48) and finally an upper undifferentiated unit. All horizons have provided some ore but the largest sources are the magnetite silicate and magnetite carbonate horizons located in the center of the pit (see Table 2). The protore on the eastern end of the range was a fine iron carbonate silicate chert (fig. 49). Remnants of this type of material occur on the hanging wall of the Empire pit and the SW extension of the Empire pit, the deep portions of the Tilden pit and the Tilden CD III extension adjacent to the Keweenawan dike. Both mines exhibit good exposures of the clastic component of the iron formation. Good examples of the martite after magnetite are found in the Tilden ore horizon. 28 �Stop 3 - Marquette County Rd. 480 46*29”89.49”N 87o 35’ 51, 17”W +1391 Figure 3-1 County Road 480 road cut This stop illustrates the original protore to be found on the eastern end of the Marquette trough. The outcrop in the road cut is a gray to buff iron carbonate with minor silicates and some magnetite alternating with chert. Surface oxidation is very thin and tends to develop along exposed surfaces and bedding planes. This type of iron formation is very susceptible to supergene oxidation and enrichment. Stop 4 - North Jackson Mine 46o 29’ 53.98” N, 87o 37’ 20.48W +1428 The Jackson Iron Mine was opened in 1848. It provided iron ore for local pig iron furnaces and was the source of ore that fed the furnaces at Fayette down on the shore of Lake Michigan on the Stonington Peninsula. The site has been off limits for fifty year due to the underground Mather B operations which recovered the soft ore along the footwall of the Negaunee iron formation. 29 �The City of Negaunee purchased the lands recently and has opened part of it for a heritage park. This action allows our group to visit a classic mining site and examine the detail geology. We will enter the very eastern portion of the old North Jackson mine where both hard hematite and soft ore were mined during the middle of the 1800s. Figure 4-1 North Jackson Mine The access cut passes through soft ore on the south wall while the north wall is formed by a vertical sheared chlorite dike possibly of Clarksburg age. The cut opens into a two level pit where the lower level is under water. On the southeast part of the pit is a rubble pile of very large blocks of jaspilite that illustrates hematite replacing jasper. The majority of the blocks appear to be breccia with cross cutting veins of hematite and occasional magnetite. One large boulder has been cleared of organic material and should afford a good photographic opportunity. On the west wall is an exploration adit (screen covered) and the south pillar shows a vuggy metallic hematite with clay filling (probably kaolin). This showing illustrates that the 1.8 million year old hard hematite was further leached by the supergene process with resulting dissolution of the remaining jasper and formation of the classic porous soft ore texture. 30 �Stop 5 - Jasper Knob, Ishpeming 46o 29’ 12.43” N, 87o 39’ 15.01” W +1607’ The outcrop represents the very upper portion of the Negaunee iron formation that has undergone the initial stages of hydrothermal replacement of the jasper by microplaty hematite. The laminated jasper shows only minor disturbance including brecciation. Most of the brecciation is intraformational bedding breccias (or conglomerates). The jasper bands contain less than 0.5% iron oxide. Figure 5-1 Jasper Knob Some workers suggested oxidation and replacement of the siderite chert by jasper which in turn was replaced by hematite while others suggest a primary origin for the jasper and some of the banded hematite. Replacement of pre-existing chert/oxides by jasper (fig. 50) at both Tilden and Empire have left evidence of the process. Within the fine jasper in Ishpeming relic transitions are not in evidence. Note the undisturbed fine bedding in the jasper similar to both white and gray chert. If there was abundant fauna present, it would have disturbed the delicate banding or at least have left some evidence of its physical activity. To complicate matters supergene oxidation can produce a brick orange chert like that present at the Milwaukee-Davis Mine in Negaunee. The process usually leaves “islands” of uncolored chert (fig. 50a) Fold axis are horizontal to gently plunging to the west. Look carefully for isolated granular or oolitic zones. 31 �Stop 6 - Saginaw Mine 46o 27’ 26.48” N, 87o 43’ 43.36” W, +1648’ The Saginaw Mine is an interesting property due to the diverse types of banded iron formation to be found there. This is a transition area within the Negaunee iron formation on the Marquette range. To the east most of the iron formation was originally carbonate replaced by magnetite or oxidized to hematite and enriched. Figure 6-1. Saginaw Mine To the west the Negaunee is primarily banded specularite chert or banded magnetite silicate chert. To the east of the Saginaw the hard ore hematite is microplaty hematite. The lean magnetite silicate chert on the Saginaw property is very coarse and contrasts with the much finer martite goethite chert up section to the north underlying the jasper-hard ore deposit that was mined from 1872 to 1891 and produced 451,000 long tons. At the Saginaw the Negaunee begins a significant reduction in thickness to the west that may be either a function deposition or due to faulting (Simmons, 1972). Midway up the Negaunee section is a lean coarse magnetite cummingtonite chert iron formation overlain by earthy hematite-goethite chert iron formation (fig. 51) as a result of supergene oxidation of an original carbonate silicate chert iron formation. The grain size is much finer than the underlying 32 �magnetite horizon. Overlying the hematite-goethite are intermittent zones of jasper, microplaty and specular hematite. The high grade hard ore both at the top of the Negaunee and in the basal Goodrich was the target of mining at the Saginaw. Above the contact of the Negaunee with the Goodrich is a thick sequence of conglomerate consisting of jasper, microplaty hematite and minor vein quartz. The schistose nature of the hematite in the jaspilite pebbles and cobbles is parallel to bedding and occurs in random orientation in the conglomerate indicating the schistosity was present prior to incorporation of the pebble and cobbles in the conglomerate. The magnetite cummingtonite chert is fairly lean. Both the ground residual gravity and the total field magnetics indicate this unit forms a north facing thickening that could either be a fold or vent mound where coarse magnetite and iron silicates quickly accumulated in a pile. At the Little Commonwealth deposit in Florence Co., WI a small bif separates an up-slope quartzite from a down-slope slate indicating the iron formation acted as a barrier to sedimentation. This suggests positive relief to its surroundings. The Commonwealth also contains irregular shapes of silica globs that look like fiamme features. These features, however, are not present at the Saginaw. Looking northwest along the power line clearing an outcrop can be seen about half a mile to the northwest where the power line turns north. This is one of the best outcrops (fig. 52) of Goodrich conglomerate on the range. It contains cobble of microplaty hematite, banded jasper, oolitic jasper and white vein quartz. From this vantage point there is a large boulder of the Goodrich conglomerate that illustrates many of the features of the conglomerate portion of the Goodrich. Several hundred of feet into the woods line west of the turn in the power line is the Goodrich Mine where 50,000 long tons of enriched Goodrich conglomerate (fig. 53) was mined for iron from 1873-1882. The contact between the earthy hematite chert and the Goodrich conglomerate can be seen in the adit separated by a fault breccia (fig. 54). The basal Goodrich is an unconformity by definition but the same hydrothermal event responsible for hard ore formation was active after the conglomerate accumulated as evidenced by the hematite present in the interstices between pebble and cobbles. The movement that resulted in the accumulation of coarse clasts did no stop the hydrothermal process. The microplaty/specularite conglomerate juxtaposition to the earthy hematite suggests the specularite is primary and not a metamorphic product of preexisting earthy hematite. Although not apparent in outcrop the property contains a swarm of basic dikes and sills as determined from diamond drilling. Based on dump material at least one oxidized Keweenawan dike transects the property. From the rusty zones in outcrop very minor sulfides are present in the magnetite cummingtonite chert iron formation. Second day lunch stop at Cliffs Shaft Mine The Cliffs Shaft Mine Operated from 1848 to 1967 shipping 29 million long tons of high grade microplaty hematite with minor magnetite and siderite. The mine was accessed by a vertical shaft (Koepe hoist) to the 15th level-1250 feet below surface. The two cement obelisk shaped shafts were constructed in 1919. Ore was mined by room and pillar methods (see old photos on the CD). Copper sulfide veins are common throughout the ore body (fig. 55). The drainage ditch leading to the sump on the 10th level was lined by a thick layer of black copper oxide that assayed over 3% Cu. Samples of the hard ore can be found on the surface of the stocking area just west of Euclid St. (west of the shaft area). 33 �During lunch the field trip attendees will be able to view the Hawes mineral collection and examine examples of the following: Core Goodrich conglomerate – Saginaw area Soft supergene hematite-goethite – Saginaw area Goodrich quartzite contact with hard spec. ore – Humboldt Mine Samples Goodrich conglomerate ore-Republic Mine Goodrich conglomerate polished slab – Goodrich Mine Coarse magnetite – Champion Mine Coarse specularite – Champion Mine Greisen with molybdenite - Champion Mine Sulfides associated with hard ore – various sources Copper sulfide – Cliffs Shaft Stop 7 - Kloman Exploration, Republic Mine 46o 24’ 23.46”N 87o 59’ 10.94”W +1517 Figure 7-1 Kloman/Republic Mine The following production history for the Republic area was tabulated: Republic underground 1872-1926, 8.6 MLT (60-65% Fe) Republic open pit 1956-1981, 62 MLT from 145 MLT of specular hematite Kloman/Columbia underground 1873-1883, 95,000 LT 34 �The Republic ore outcropped in a bluff on a bend in the Michigamme River that corresponds to the mapped geology (Cannon, 1975). Initially mined from shallow pits, most production came from a number of shafts including the Pascoe (see old photo gallery on the CD) which was inclined 48 degrees down four thousand feet of the fold axis that bottomed 2900 vertical feet below the river. After being idle for many years the deposit was reactivated as an open pit to produce crude suitable for concentration. The crude feed had the following chemistry: Sol. Fe Silica Al2O3 MgO 38% 42.5% .72% .9% CaO Na2O K2O P .53% .03% .04% .033% The ore mined at the Republic Mine for taconite feed (1954-1981) originated from a specular jaspilite (fig. 56) found above the metadiabase sills and below the Goodrich quartzite on the southeastern end of the trough. The basal Goodrich is a conglomerate rich in specular fragments that were rich enough to constitute ore. The specularite generally contains less the 0.2% TiO2 but in the conglomerate area values of 0.50 to 1.65% TiO2 as rutile associated with hematite. The high grade specular hematite and magnetite occur at the top of the Negaunee as irregular replacement bodies. The magnetite may indeed be post tectonic as suggested by Cannon (1973). The banded iron formation consisting of specular hematite with minor magnetite does not show cross cutting veins or disturbances to the bedding, suggesting the specular chert is a primary facies generated when the hydrothermal hematite crystallized upon release into an aqueous environment and quickly settled as distinct bands. As seen in the Saginaw Mine area the presence of coarse specularite is not a metamorphic product of a preexisting oxide form because it coexists with soft oxide hematite and goethite. Examination of the magnetite silicate horizon of the lower Negaunee iron formation exhibits a uniform banding (fig. 57), unlike the Champion area where the magnetite silicate chert has undergone hydrothermal alteration as indicated by brecciation and clear evidence of solution replacement features corresponding to the change in mineralogy. Conceptually the magnetite silicate could have resulted from the same diagenetic replacement of the original carbonate silicate chert and subsequent metamorphism resulting in conversion of the low grade silicates to cummingtonite and grunerite with enhanced size to the magnetite. It could also be primary mineralogy enhanced by metamorphic overgrowth. The stratigraphy in the Republic trough consists of the undifferentiated Siamo and Ajibik formations, Negaunee iron formation, Goodrich conglomerate/quartzite and Michigamme slate (schist). A number of diabase sills parallel to the iron formation bedding have undergone the same folding flexure in creating the keel of the trough. The tight compressional folding that thickened the iron formation in the keel of the trough did not markedly impact the vertical and thinner iron formation on the north side of the fold. The keel plunges 48 degrees to the northwest. An excellent exposure of the upper portion of the Negaunee Iron formation can be found just north of the old pit on the Kloman property. Cannon (1972) provide a good stratigraphic section for this outcrop (Figure 7-2). The high grade lens was mined just at the Goodrich contact (fenced area). The iron formation consists of alternating bands of specularite 35 �and magnetite grading down section to more magnetite silicate chert. Compare the stratigraphy of Figure 2 with the stratigraphy shown for the Republic Mine and note the change in mineralogy over a relatively short distance. The Republic trough area was flown for gravity using Falcon method several years ago and a drilling program was conducted on a gravity target located several hundred feet east of Highway 95 and south of the Michigamme River. An economic target was not located. Klasner et al. (1974) using gravity indicted the depth of the Republic trough on Highway 95 to be 1,424 meters. A resource of 120 million long tons of oxide iron remains within the pit outline. 36 �Figure 7-2 Stratigraphic section-Kloman Mine. After Cannon (1972) 37 �Stop 8 - Champion Mine #7 46o 30’ 27.59”N, 87o 59’ 11.51”W, +1705’ Figure 8-1 Champion Mine The Champion Mine provides an excellent example of coarse specularite and magnetite hard ore similar to that found in the Humboldt, the Greenwood and the Republic Mines. Even though Babcock (1966, 1974) identified over 70 minerals at Champion, only the most common are available. The Champion Mine was operated spasmodically by multiple operators for over 100 years (1867-1967) and produced 6.5 million long tons of combined coarse specular hematite (fig. 58) and magnetite (fig. 59). The mine was accessed from seven shafts numbered progressively from east to west (see fig. 60 longitudinal section). The ore body is irregular, thin and dips 78 degrees to the north-northwest. The spindle shaped ore shoots rake steeply to the southwest. The lower level (26th) is at 2100 feet below the collar of shaft #7. Generally specular hematite occurs near or at the hanging wall Goodrich quartzite contact. In some cases iron ore has replaced some the overlying quartzite. Magnetite is more common on the footwall side and with increased depth. The iron formation is approximately 400 feet thick throughout the mine. Oxygen isotope studies have indicated iron oxides formed at 400-500oC. 38 �The original mineral composition of the banded iron formation is impossible to determine but we know the magnetite-silicate-chert was subjected to alteration that started with the formation of sericite and chlorite themselves replaced by specularite and magnetite respectively. Some of the magnetite postdates the specularite. Tourmaline and quartz followed while the last major addition included: quartz, jacobsite, manganese iron silicates, gold, pyrite (fig. 61), chalcopyite, bismuthinite, molybdenite and scheelite. Bodwell (1972) indicated the late mineral stage was prevalent between the #5 and #7 shafts associated with multiple quartz veining in massive magnetite. Alteration minerals are many but only a few are readily recognizable. Among them are: chloritoid, andalusite (fig. 62), andradite (fig. 63 ), garnet, tourmaline (fig. 64), sericite and chlorite. The manganese addition can be found in the minerals: jacobsite, mn-chloritoid, spassertine, kutnohorite, mn-cummingtonite, pyrophanite and mn-actinolite. L. Babcock’s studies of the mineralogy, using samples collected in the mine and from the dumps, reported the presence of gold. One 2 foot intersect in a diamond drill hole assayed 0.198 ounces per ton. Dump samples consisting of garnet-chloritoid-sulfides and massive magnetite with sulfides can assay 1-2 ppm gold. In some parts of the mine greisen has developed that contain visible molybdenite (fig. 65) that assays above 1% Mo. Subsequent workers have compared the Champion FeOx gold occurrence to the Tennant Creek and Starra deposits found in Australia. Taken by itself it could be just an anomaly, but coupled with widespread late sulfides associated with range wide hard ore, elevated hard ore REE values, the presence of copper and tungsten found on the south limb of the Republic trough and the presence of ferrites would suggest the iron formation and subsequent hydrothermal alteration are distal end products of a possible remote Iron Oxide Copper Gold (IOCG) feeder system (s). The high level metamorphic assemblage found at the Champion Mine is not seen in the surrounding sedimentary or volcanic rocks indicating the high grade metamorphic rank is site specific and not regional in nature. Bibliography for Banded Iron Formations of the Marquette District-2008 ILSG Field Trip Babcock, L.L., 1966, The Manganese-Bearing Silicate Minerals of Champion Mine Champion, Michigan, MS thesis, MTU, 70 p. Babcock, L.L., 1974, Mineralogy, Geochemistry, and Genesis of the MagnetiteJacobsite Mineral Series and Manganese-Ferrite-Bearing Iron-Formation From the Champion, Mine, Champion, Michigan, PhD thesis, MTU. Babcock, L.L., 1974, Mineralogy, Geochemistry, and Genesis of ManganeseFerrite-Bearing Negaunee Iron-Formation form Champion Mine, Champion, Michigan, pt II of PhD thesis, MTU. Bailey, S.W, and Tyler, S.A., 1960, Clay Minerals Associated with the Lake Superior Iron Ores, Econ. Geol. v. 55, p. 150-175. Bayley, R. W., and Tyler, S.A., 1966, Geology of the Menominee Iron-Bearing District,Dickinson County, Michigan and Florence and Marinette Counties, Wisconsin, USGS Prof. Paper 513, 96p. 39 �Bodwell, W.A., 1972, Geologic Compilation and Non-ferrous Metal Potential Precambrian Section, Northern Michigan, MS thesis, MTU. Cannon, W. F., and Klasner, J.S., 1972, Guide to Penokean Deformational style and Regional Metamorphism of the Western Marquette Range, Michigan, 18th Annual Institute on Lake Superior Geology, p. B1-B38. Cannon, W.F., 1972, Geology of the Greenwood 7 ½ Minute Quadrangle, Marquette County, Michigan, USGS unpublished Prof. Paper, 191p. Cannon, W. F., 1973, Penokean Orogeny in Northern Michigan, GAC Spec. Paper No. 12, p. 251-271. Cannon, W. F., 1973, High Grade Magnetite Deposits at Republic, Michigan: Their Bearing on the Genesis of the Marquette Range Hard Ore, 19th ILSG. Cannon, W. F., 1974, Bedrock Geologic Map of the Greenwood Quadrangle, Marquette, USGS Map GQ-1168. Cannon, W. F., 1975, Bedrock Geologic Map of the Republic Quadrangle, Marquette County, Michigan, USGS Map I-862. Cannon, W. F., and Addison, W.D., 2007, The Sudbury Impact Layer in the Lake Superior Iron Ranges: A Time-Line from the Heavens, 53rd ILSG. Gair, J.E., and Thaden, R.E., 1968, Geology of the Marquette and Sands Quadrangles, Marquette County, Michigan, USGS PP 397, 77 p. Gair, J.E., 1970, Metadiabase Sills in Negaunee Iron-Formation South of Negaunee, Michigan, USGS Bull. 1324A, p. 24-30. Gair, J.E., 1975, Bedrock Geology and Ore Deposits of the Palmer Quadrangle, Marquette County, Michigan, and Reflection on the Empire Miner by T.M. Han, USGS PP 769, 159p. Grenne, T., and Vokes, F.M., 1990, Sea-floor sulfides at the Hoydal Volcanogenic Deposit, Central Norwegian Caledonides, Econ. Geol. v. 85, p. 344-359. Han, T.M., 1971, Diagenetic-Metamorphic Replacement Features in the Negaunee Formation of the Marquette Iron Range, Lake Superior District, Soc. Mining Geology, Japan, Spec. Issue 3, p. 430-438. Han, T.M., 1982, Iron-Formation of Precambrian Age: Hematite-Magnetite Relationships in Some Proterozoic Iron Deposits-A Microscopic Observation, In Ore Genesis-The State of the Art, ed. Amstutz, G.C., et al, p.452-459. 40 �Han, T.M., 1988, Origin of Magnetite in Precambrian Iron-Formations of Low Metamorphic Grade, Proceedings of the Seventh Quadrennial IAGOD Symposium, p. 641-656. Han, T.M., and Runnegar, B., 1992, Megascopic Eukaryotic Algae from the 2.1 Billion-Year-Old Negaunee Iron-Formation, Michigan, Science v. 257, p. 232-235. Henrickson, E.L., 1956, A Study of the Metamorphism of the Upper Huronian Rocks of the Western Portion of the Marquette District, U.of Minn. PhD thesis James, H.L., 1953, Origin of the Soft Iron Ores of Michigan, Econ. Geol. v 48, p. 726-728. James, H.L., 1954, Sedimentary Facies of Iron-Formation, Econ. Geol. v. 49 p. 235-293. James, H.L., 1955, Zones of Regional Metamorphism in the Precambrian of Northern Michigan: Geol Soc of America Bull, v. 66, p. 1455-1488. James, H.L., 1958, Stratigraphy of pre-Keweenawan Rocks in Parts of Northern Michigan, USGS PP 314-C, p. 27-44. James, H.L., Dutton C.E., Pettijohn, F.A., and Wier, K.L., 1968, Geology and Ore Deposits of the Iron River-Crystal Falls District, Iron County, Michigan, USGS PP 570, 134p. Klasner, J.S. and Cannon, W.F., 1974, Geologic Intrepretation of Gravity Profiles in the Western Marquette District, Northern Michigan, GSA Bull. v. 85, p. 213-218. Klasner, J.S., and Cannon, W.F., 1978, Bedrock Geologic Map of the Southern Part of the Michigamme and Three Lakes Quadrangles, Marquette and Baraga Counties, Michigan, USGS Map I-1078. Klasner, J.S., Ojakangas, R.W., Schulz, K.J., and LaBerge, G.L., 1991, Nature and Style of Early Proterozoic Deformation in the Foreland of the Penokean Orogen, Michigan, USGS Bull 1904-K, 22 p. Kramer, R.S., Hwang, J.Y. and Johnson, A.M., 1987, A Mineralogical and Chemical Study of the Graphitic Lower Slate Member of the Michigamme Formation, Marquette and Baraga Counties, Michigan, Mich. DNR OFR, 97p. Larue, D.K., and Sloss, L.L., 1980, Early Proterozoic Sedimentary Basins of the Lake Superior Region, GSA Bull. v. 91, pt. I, p. 450-452, pt. II, p. 1836-1874. 41 �Larue, D.K., 1981, The Chocolay Group, Lake Superior Region, U.S.A.: Sedmentologic Evidence for Deposition in Basinal and Platform Settings on an Early Proterozoic Craton, GSA Bull. v. 92, p. 417-435. LaBerge, G.L., 1966, Altered Pyroclastic Rocks in Iron-Formation in the Hamersley Range, Western Australia, Econ. Geol. v. 61, p. 147-151. LaBerge, G.L., 1966, Altered Pyroclastic Rocks in South African Iron-Formation, Econ. Geol. v. 61, p. 572-581. Lougheed, M.S., and Mancuso, J.J., 1973, Hematite Framboids in the Negaunee Iron Formation Michigan: Evidence for their Biogenic Origin, Econ. Geol. v. 68, p. 202-209. Mancuso, J.J. and Lougheed, M.S. and Wygant, T., 1971, Possible Biogenic Structures from the Precambrian Negaunee (Iron) Formation, Marquette Range, Michigan, American Jour. of Sci. v. 271, p. 181-186. Mancuso, J.J., Kneller, W.A. and Quick, J.A., 1989, Precambrian Vein Pyrobitumen: Evidence for Petroleum Generation and Migration 2 Ga Ago, Precambrian Research v. 44, p. 13; 7-146. Mathias, D.L., 1959, The Basic Igneous Rocks of the Eastern Marquette Range, Michigan, PhD, Columbia Univ., 134p. Ojakangas, R.W., 1994, Sedimentary and Provenance of the Early Proterozoic Michigamme Formation and Goodrich Quartzite, Northern Michigan-Regional Stratigraphic Implications and Suggested Correlations, USGS Bull. 1904-R, 31p. Ojakangas, R.W., 2001, Paleoproterozoic Basin Development and Sedimentation In the Lake Superior Region, North America, Sedimentary Geol. v. 141-142 p. 319-341. Pettijohn, F. J., 1943, Basal Huronian Conglomerates of Menominee and Calumet Districts, Michigan, Jour. of Geol. v. 51, p. 387-397. Schneider, D.A., Bickford, M.E., Cannon, W.F., Schulz, K. and Hamilton, M., 2002, Age of Volcanic Rocks and Syndepositional Iron Formations, Marquette Range Supergroup: Implications for the Tectonic Setting of Paleoproterozoic Iron Formations of the Lake Superior Region, Can. Jour. Earth Sci. v. 39, p. 999-1012. Schulz, K.J., and Cannon, W.F., 2007, The Penokean Orogeny in the Lake Superior Region, Precambrian Research v. 157, p.4-25. 42 �Simmon, G.C., 1972, Metadiabase Sills in Negaunee iron-formation Near National Mine, Michigan, USGS Bull. 1394A, p. A70-A75. Simmons, G.C., 1974, Bedrock Geologic Map of the Ishpeming Quadrangle, Marquette County, Michigan, USGS Map GQ-1130. Swanson, C.O., 1930, Report on a Portion of the Marquette Range Covered by the Michigan Geological Survey in 1929, Michigan Geological Survey in 1929, 15 p. Taylor, G.L., 1972, Stratigraphy, Sedimentology and Sulfide Mineralization of the Kona Dolomite, PhD, MTU, 112p. Tyler, S.A. and Twenhofel, W.H., 1952, Sedimentation and Stratigraphy of the Huronian of Upper Michigan, American Jour. of Sci., v. 250, Part I, p. 1-27 and Part II, p. 118-151. Van Hise, C.R., and Bayley, W.S., 1897, Marquette Iron-Bearing District of Michigan, USGS Monograph 28, 608 p. Van Hise, C.R., and Leith, C.K., 1911, The Geology of the Lake Superior Region, USGS Monograph 52, 641p. Waggoner, T.D., 1972, Stratigraphy, A tool in the Economic Development of the Marquette Iron Range, AIME meeting, Houghton, MI. Waggoner, T. D., 2003, A Hydrothermal Component o Iron Formations-A Marquette Range Perspective, 49th ILSG. Waggoner, T. D., 2006, Sulfur Isotopes from Pyrite in the Negaunee Iron Formation, 52nd ILSG. 43 �44 �45 �Introduction The Silver Lake area (Figure 2.1) lies along the north margin of the Dead River Basin, a structural outlier of Paleoproterozoic strata surrounded by Archean crystalline rocks. Silver Lake, a natural water body, was enhanced by an impoundment constructed in 1910 and served as a storage basin for downstream hydroelectric generation along the Dead River. Small outcrops along the north shore of the enhanced lake showed a variety of interesting and puzzling features at the Archean-Paleoproterozoic unconformity (Klasner and others, 1979). In May 2003, after very heavy rains, a segment of an earthen dam failed, resulting in catastrophic flooding downstream and a drop of the lake to near the original natural level. The current lake level is 25 to 30 feet below the former impounded level and about 1,000 acres of the previous lakebed are now dry land. This has resulted in reemergence of numerous outcrops on the former lake floor. These outcrops are along the Archean-Paleoproterozoic unconformity and are the focus of this trip. Because reconstruction of the dam and subsequent reflooding of the basin are planned, there is only a narrow time window in which to observe and study these unique features. Figure 2.1. Map of the Marquette region showing the location of Silver Lake and the driving route north from U.S. Highway 41 in Ishpeming. General Geology The Silver Lake area lies on the northern flank of the Dead River basin, which is a structural basin filled with Paleoproterozoic sedimentary rocks and surrounded by Neoarchean crystalline rocks of diverse lithology. The geology of the basin was mapped in detail during the 1970’s (Puffett, 1974; Clark and others, 1975; Klasner and others, 1979). The Paleoproterozoic rocks consist entirely of various informal units of the Michigamme Formation, a part of the Baraga Group. The Michigamme is volumetrically dominated by a thick succession of turbidites, which form the upper part of the formation. The lower units, however, including those seen at Silver 46 �Lake, consist of quartzite and conglomerate, laminated argillite, carbonaceous shale, and lean iron-formation or ferruginous chert. Recent studies also identified a layer of ejecta-bearing rocks in the lower part of the formation, which has been correlated with the Sudbury impact event (Cannon and others, 2006a, b; Cannon and Addison, 2007a, b; Kring and others, 2006; Pufahl and others, 2007). Figure 2.2. Map of the Silver Lake field trip area showing localities (1 through 17) described in this guide. General geologic relationships are generalized from Klasner and others (1979). The heavy dashed line is an unmaintained logging road drivable in some seasons by the stout of heart. This trip will hike the road from the point indicated. The extent of Silver Lake as shown is that prior to the 2003 dam failure. It’s configuration in 2008 is much smaller than shown. The structure of the Dead River Basin basin is complex as a result of both Penokean and Yavapai deformation. During the Penokean orogeny, between 1850 and 1830 Ma, thin-skinned deformation produced folds and slatey cleavage that are best developed in the upper part of the Michigamme. None of the structures seen at Silver Lake can be definitively assigned to the Penokean orogeny. The present structural basin is a result of differential movement between fault-bounded blocks of Archean rocks and the molding of Proterozic strata around the fault blocks. This deformation has long been interpreted to be a late phase of the Penokean orogeny, 47 �but recent geochronological data suggest that it is younger and equivalent in age to the Yavapai orogeny at appxoximately 1775-1750 Ma (see recent summaries by Holm and others, 2007; Schulz and Cannon, 2007). A low-temperature regional hydrothermal event has been documented to have occurred at nearly this same time during which xenotime cements formed in the basal Michigamme Formation. Such secondary xenotime is well developed at Silver Lake and material collected from Locality 16, described below, yielded a xenotime crystallization age of approximately 1785 Ma (Vallini and others, 2007). The geology in the area of this field trip is shown in Figure 2.2. Archean rocks, a diverse suite of metavolcanic and granitic rocks, form the prominent uplands whereas the Michigamme Formation underlies the lowlands. The very steep hillsides reflect the extreme contrast in erosional resistance of these two units. The area is divided into two structural panels by a prominent fault along the northeastern shoreline of Silver Lake. Northeast of the fault, Archean rocks are relatively uplifted and tilted toward the northeast. Along the steep hillside descending onto the Mulligan Plains there are sporadic exposures of the basal beds of the Michigamme Formation which dip 40° northeast and thus indicate the amount of rotation. Foliation in outcrops along the fault is nearly vertical. Southwest of the fault, including the area that is the focus of this trip, a block of Archean rocks has been uplifted and tilted slightly toward the southwest so that the unconformity between it and the Michigamme Formation forms a gently southeast-dipping surface along the lakeshore in the northwestern part of Figure 2.2. Draining of the lake exposed extensive new outcrops, such as shown in Figure 2.3, that consist of Archean rocks, mostly massive to foliated granite, and the basal beds of the Michigamme Formation. The outcrop surface closely mimics the unconformity surface so that discontinuous patches of the Michigamme are preserved in declivities on that surface. Lithology of basal Proterozoic beds. The maximum thickness of the preserved Paleoproterozoic strata at the field trip stops is only about one meter. A variety of rock types from conglomerate to fine-grained laminated sedimentary rocks are present and the rock type at any particular locality may reflect the micro-topography along the surface at the time of deposition. A few lenses of pebble conglomerate appear to be somewhat mature and consist of rounded and obviously waterworked debris, including rounded quartz pebbles. These may be lenses of wave-washed gravel that accumulated in depressions on the Archean surface during the earliest phase of marine transgression. More typically, basal beds are breccia consisting almost entirely of angular fragments of rock types contained in the immediately underlying Archean basement. They may be a residuum of physically weathered basement rock that experienced little or no wave action. Laminated fine-grained sedimentary rocks also are widespread and occur both above the basal conglomerate lenses or lie directly on the Archean basement where basal conglomerate is absent. 48 �Figure 2.3. Area near Location 17 showing the newly exposed outcrops of the former lake bottom. The land surface very closely mimics the unconformity between Archean granitic rocks and basal beds of the Michigamme Formation. Hundreds of individual vestiges of the basal sediments dot the surface of the granite. A significant aspect of the lowermost sediments is phosphatic material that occurs as masses of nearly pure carbonate flourapatite. These masses are typically from a few to as much as 10 cm in diameter and some have shapes and internal structures suggestive of stromatolitic growth. These were first described by Cannon and Klasner (1976) along with numerous other occurrences of phosphatic material within basal Baraga Group rocks in the Marquette area. A good example of these is shown in Figure 2.4A where stromatolite-like growths of apatite have repeatedly developed in the lowermost few centimeters of the Michigamme Formation immediately adjacent to the unconformity with Archean rocks and occur in three or four individual layers separated by fine-grained clastic sedimentary rocks. In this particular case, microtopography along the unconformity appears to have localized growth on a banded quartz vein which stood in relief above the surrounding granite. Phosphate masses grew on the relatively steep surface of the quartz vein, a situation apparently in some way physically favorable for phosphate accumulation. Other phosphate masses, such as shown in Figure 2.7B, occur directly on the unconformity with Archean granite. Some of this phosphatic material was also reworked into overlying conglomerate lenses. 49 �Figure 2.4. A- Masses of carbonate flourapatite (dark areas of negative relief just above unconformity) that appear to have grown in successive layers along steeply dipping microtopography on the unconformity. Host beds are laminated argillite. The immediately underlying rock is a banded quartz vein that cuts Archean granitic rocks. This feature can be seen at Locality 11. B- Small fold of Michigamme argillite between blocks of Archean granite. A vestige of the unconformity is seen at point A where the argillite was deposited in flatlying bedns on the Archean. The space not occupied by the fold was created by lateral opening of a gap between granite blocks B and C and slumping of the soft sediments into the new space. Other joints in the granite (D) are filled with injected sediments. Feature can be seen at Locality 16. 50 �Structure The excellent exposures created by the draining of Silver Lake reveal a set of unusual, intriguing, and puzzling small-scale structures along the unconformity. The gross structure is simple. The unconformity seen at localities 9 through 17 is gently inclined toward the south and southeast as a result of rotation of the Archean basement rocks, probably in the time interval 1775-1750 Ma. There are no penetrative fabrics within the Archean rocks that can be ascribed to this period of deformation; rather the Archean appears to have moved as a series of rigid blocks separated by faults and the overlying Paleoproterozoic strata moved passively with them. A variety of small-scale structures are also well exposed and appear to record an unusual structural event that we propose might have been a powerful earthquake caused by the giant Sudbury impact event. At several nearby localities within the Dead River Basin, a layer of ejecta-bearing breccia, interpreted to have been formed by the Sudbury impact is within the lower part of the Michigamme Formation (Cannon and others, 2006a, b; Kring and others, 2006; Cannon and Addison, 2007a, b; Pufahl and others, 2007). There is clearly a close temporal correspondence with the basal Michigamme beds exposed at Silver Lake and the time of the impact. In fact, our preliminary petrographic study of these rocks, discussed more fully below, found features that might be directly caused by the impact. Two types of structures are present: 1) drapes of Michigamme sediments around Archean blocks that have undergone small displacements relative to each other, and injection of sediments into joints within the Archean basement; 2) intense brecciation of the Archean rocks, soft-sediment flow of the basal Michigamme and intermixing of the two rock types. The first type of features, drapes of sediment around Archean blocks, is best seen at localities 14 through 17. Figure 2.4B shows the essential characteristics of this type of deformation. Finegrained laminated sediments were deposited unconformably on Archean granite in essentially flat-lying beds. This unconformiable surface is widely preserved (such as at A in Figure 2.4B). The tight syncline shown in figure 2.4B formed as these flat-lying sediments slumped and flowed into an open space created as blocks of granite (B and C) separated laterally. Numerous joint surfaces (D in Figure 2.4B) are also filled with sediments which apparently flowed into open spaces during this same event. Individual granite blocks ranged up to several meters or tens of meters in diameter and experienced relative displacements up to several meters. The result is a complex unconformity surface with structural relief of meters and complexly folded basal Michigamme strata. The second type of feature, brecciation of basement rocks and soft-sediment flow of the basal Michigamme, is very well displayed at localities 9 through 13 and illustrated in Figure 2.6 (Locality 9). All stages of brecciation of basement rocks are preserved, ranging from small movements on joint surfaces and infilling of the spaces thus created by sediments (Figure 2.9; Locality 13) to intense dismemberment of the Archean rocks into angular fragments which are intermixed with a matrix of clastic sediments (Figure 2.8; Locality 13). In the less brecciated granitic basement, clastic dikes are very common and range in thickness from nearly a meter to a few millimeters. The wider dikes commonly have an internal lamination (Figure 2.7A for example; Locality 11) indicating that the sediment fill was caused by an injection of originally overlying laminated sediments rather than an infiltration of individual clastic particles into open 51 �space. In places (locality 13 for example) a remarkable intersecting array of clastic dikes extends at least several meters below the unconformity showing that sediments were able to completely infiltrate a joint system well below the unconformity. Such features imply that a period of dilation affected the Archean rocks during which overlying soft sediments were injected into all available open spaces. Sudbury Seismites? Could the array of unusual features seen at Silver Lake have been caused by intense seismic shaking, and could that shaking have been caused by the giant impact at Sudbury? Giant impacts do generate exceptionally powerful earthquakes. For instance the Chicxulub impact in Mexico has been variously estimated to have generated a quake of M 10 to13 on the Richter scale, significantly more powerful than the largest known terrestrially generated earthquake. The Sudbury impact was a somewhat larger event. It too should be expected to have generated an earthquake of nearly unprecedented energy and to have left a unique imprint on rocks of the region. A calculation of the seismic intensity from the Sudbury impact using the on-line Earth Impact Effects Program (Marcus and others, 2004) indicates an intensity of 10.5 on the Richter scale, greater than any earthquake in recorded history, and a Mercalli Scale Intensity of X at Silver Lake (nearly total destruction of man-made structures in the modern sense). The Chicxulub impact has been shown to have produced seismic disturbance of sediments well over a thousand kilometers from the impact site (Norris and others, 2000; Terry and others, 2001). Thus it seems likely that the Silver Lake area, only about 500 km from Sudbury, was well within the range of significant seismic disturbance from the Sudbury impact. The intense shattering of Archean basement rocks and contemporaneous flow of overlying soft sediments are features that could have been caused by the passage of an impact-generated shock wave and consequent shaking. The complex array of sedimentary dikes that cut the Archean require a period of dilation during which fractures in the granite were opened and then filled by the injection of overlying soft sediments. Such features can form during passage of a seismic wave in which the leading edge of the wave is compressional and is followed by a dilational wave (Melosh, 1989). During this instantaneous dilation the Archean rocks may have expanded and formed open spaces along fractures. Overlying sediments would have been injected into the newly created space. Similar features have been reported from the Locke impact structure in Sweden (Sturkell and Ormo, 1997) where sediment dikes cut shattered granitic rocks surrounding this small Ordovician crater. Some of the material at Silver Lake is also similar to “clastic Sudbury breccia” (Rousell and others, 2003) that is found as much as several tens of kilometers outside of the present Sudbury Basin. A final piece of evidence that suggests a possible link to the Sudbury impact is possible impactrelated grains that have been found in some of the clastic dikes. Although our petrographic examination is very preliminary at this point, we have observed numerous millimeter-scale round to ovoid grains consisting of very fine-grained brownish clay. Many have abundant shrinkage cracks (Figure 2.5). These are very similar in appearance to grains that occur in some phases of the Sudbury impact layer at nearby occurrences and have been interpreted as altered microtectites formed from impact-generated vapor. These same rocks also contain a sparse collection of quartz grains that have planar features that may be shock-induced planar 52 �deformation features (Figure 2.5D), but we have not yet found truly definitive shock features. Nevertheless, there is at least suggestive evidence that ejecta material from the Sudbury impact was incorporated into the clastic injections. The distance which the Sudbury layer lies above the stratigraphy exposed at Silver Lake is unknown but it could have been very small. The nearest known exposure, Connors Creek, is only about 3 km to the south. There the Sudbury layer is about 150 meters above the Archean unconformity. Regional relationships suggest that underlying strata thin to the north and the layer may be very close to the level exposed at Silver Lake. Thus, we propose a model in which Sudbury ejecta material arrived at Silver Lake essentially contemporaneously with severe seismic shaking and deformation of Archean rocks that were overlain by only a meter or two of Paleoproterozoic sediments at the time. Both the sediments and ejecta particles were emplaced as sediment dikes in newly opened fractures. Figure 2.5. Photomicrographs of clastic material from sediment dikes injected into the Archean basement rocks. A, B- Broken and distorted spherules of aphanitic brown clay in siliceous finegrained matrix containing abundant quartz sand grains. These spherules are very similar in appearance to spherules within Sudbury ejecta deposits nearby. C- intact spherule of aphanaitic clay. D- quartz grain cut by planar features the may be shock induced planar deformation features. FIELD TRIP LOCALITIES The localities of principal interest for this trip are numbers 9 through 17 as shown on Figure 2.2. The easiest walking route to these localities is along the old shoreline on the northeast side of Silver Lake. Along this route numerous outcrops (localities 1 through 8) lie along the fault that juxtaposes Archean basement rocks and the Michigamme Formation. The route crosses the fault several times so exposures of both the Archean basement and Michigamme Formation can be examined. These eight localities are described briefly and localities 9 through 17 are described in 53 �more detail. Latitude and longitude values are given for each locality to assist in GPS navigation to them. In general, the photographs shown in Figures 2.6 through 2.11 are taken within a few tens of feet of the GPS locations. Locality 1.-- (46.6603, -87.8134) Small outcrop of sheared quartz and plagioclase phyric rhyolite; part of Archean basement. Locality 2.-- (46.6614, -87.8149) Long outcrop (300 ft) of massive to weakly foliated Archean biotite amphibolite. Foliation is irregular. Some features may be relict pillow selvages or cryptic pillows. The southeasternmost part of the outcrop is highly sheared, probably by movement on the fault. Locality 3.-- (46.6625, -87.8180) Michigamme Formation. Rusty-weathering, dark gray to black slate. A steep uniform cleavage is very well developed. In places bedding laminations from ½ to 1 inch thick are parallel or subparallel to cleavage. Locality 4.-- (46.6632, -87.8192) Michigamme Formation similar to Locality 3, except there are beds of coarser, more massive greywacke toward the north side of the outcrop. Locality 5.-- (46.6642, -87.8213) Michigamme Formation similar to locality 3. Locality 6. -- (46.6656, -87.8234) Long outcrop (300 feet) of highly sheared mafic volcanic rock (Archean). Way point is at northwest end of outcrop. Locality 7.-- (46.6667, -87.8273) Michigamme Formation as at Locality 3. Locality 8.-- (46.6674, -87.8282) Sheared Archean amphibolite with lesser felsic layers. Coarse amphibole crystals in places. Layering is parallel to nearly vertical shear foliation. Locality 9.-- (46.6652, -87.8319) After leaving locality 8 and turning southwest the route passes onto the Archean basement block south of the fault that has been tilted gently to the southeast. The unconformity with the base of the Michigamme Formation is well exposed here as a surface that dips about 15° to the southeast. The Archean rocks are sheared felsic metavolcanics with nearly vertical compositional layering and shear foliation (Figure 2.6A). They are overlain by a layer of breccia only an inch or two thick. Clasts are mostly very angular and appear to be in very large part of the same lithology as the immediately underlying Archean rocks (Figure 2.6B). Laminated gray argillite overlies the basal breccia. Only the lowest foot or two of this unit is exposed here. The relationships shown here leave no doubt that the intense penetrative deformation in the Archean rocks entirely pre-dates deposition of the Michigamme Formation. 54 �Figure 2.6. Features seen at Locality 9. A- unconformity between sheared Archean felsic volcanic rocks below and gray laminated argillite of the Michigamme Formation above. The base of the Michigamme Formation consists of a few inches of conglomerate with very angular fragments, mostly with lithology identical to the immediately underlying Archean rocks. The surface is cut obliquely through the strata which exaggerates the apparent thickness of the basal conglomerate bed. B- closeup view of the basal conglomerate showing the extreme angularity of most clasts and the essentially unsorted nature of the bed. Locality 10.-- (46.6649, -87.8333) Rocks here are entirely Archean and are mostly laminated metasedimentary rocks in beds ½ to 1 inch thick. Beds are highly folded and fold axes plunge about 55° to the southeast. The north edge of the outcrop is a sheared mafic rock, possibly a dike intruded into the metasedimentary unit. 55 �Locality 11 (46.6649, -87.8338) and Locality 12 (46.6642, -87.8337).-- These two localities are on the north and south ends respectively of a large outcrop area. Near locality 11, on the north part of the outcrop, a thin skin of basal conglomerate lies on the Archean basement. In places the Archean foliated granite is broken into large blocks and open spaces between the blocks are filled with sediment and small angular granite fragments. Lamination in the sediments is preserved in part and was deformed against the granite blocks (Figure 2.7A). At several places along the unconformity, light gray aphanitic masses of carbonate flourapatite lie directly on the granite (Figure 2.7B). Toward the south end of the outcrop, near Locality 12, one to two feet of breccia forms a layer down the east side of the exposure. This differs from most of the other nearby basal conglomerates in having a quartz-chlorite matrix and a diversity of lithic fragments of Archean rocks, including chert and quartz pebbles. Is some of this material Subury ejecta? The southern end of the outcrop is Archean granite. Locality 13.-- (46.6636, -87.8345) The outcrops here show the best examples of intensely brecciated Archean rocks, soft-sediment deformation and flow of Michigamme argillite, and intense development of sediment dikes in the Archean basement. On the northside of the outcrop the Archean rocks are brecciated to highly variable degrees. In places blocks of the Archean have moved apart on joint surfaces to create open space that was filled by clastic sediments. All variations can be seen from this relatively mild deformation to complete shattering of the Archean into centimeter-scale angular fragments that are suspended in a clastic matrix (Figure 2.8B). In some cases adjacent fragments can be fitted together to reconstruct the pre-brecciation geometry indicating that fragments have not moved far during the brecciation process. The unconformity is also well exposed here. Rather than a basal conglomerate, the base of the Michigamme is laminated argillite. Black to pinkish gray color banding emphasizes the bedding and readily shows intense soft-sediment deformation features (Figure 2.8A). There is an intermixing of the Michigamme argillite and Archean granite; granite fragments are incorporated into the basal foot of the sediments and masses of the argillite occur within the upper foot or two of the breccia. We interpret these relationships to indicate that the brecciation and soft-sediment deformation occurred simultaneously in response to the same seismic event. The south side of the outcrop provides a cross section of the upper 3-4 meters of the Archean basement granite below the unconformity. A remarkable array of sedimentary dikes is seen here at scales varying from about 0.5 m to a few millimeters (Figure 2.9). Virtually every joint surface appears to have a least a thin film of sedimentary material along it. The larger dikes generally have an internal layering shown by variations in grain size that is parallel to the dike margins. Such features indicate that the sedimentary material was forcefully injected into the joints rather than accumulating by settling of grains into open spaces. This in turn implies that there was a dilational event that simultaneously opened all of the fractures in this geometrically diverse fracture system to allow injection of the sediments. 56 �Figure 2.7. Features seen at Locality 11. A- Fractured Archean granite with gray laminated argillite injected between granite blocks. Numerous fragments of the country rock granite are incorporated into the argillite. B- View looking down on the surface of the unconformity. Archean rock is foliated granite. Several masses of carbonate flourapatite are directly on the unconformity surface and are overlain by gray argillite. 57 �Figure 2.8. Features seen at Locality 13. A- intensely brecciated granite. Angular granite clasts are suspended in a clastic sedimentary matrix. Overlying banded argillite is intensely deformed by soft-sediment flow. Note fragments of granite intermixed with basal beds of the argillite, and masses of argillite scattered through the granite breccia. B- brecciated granite showing varying degrees of fragmentation. Note rounded clast to right of scale with only slight separation of fragments and fractures filled with clastic sediment. Elsewhere angular clasts of various sizes are suspended in a clastic matrix. 58 �Figure 2.9. Features seen at locality 13. Views of the south side of the outcrop at various scales. The surface of the unconformity exposed on the north side of the outcrop projects to just above the top of the outcrop in A. Note the abundance if sedimentary dikes (darker), throughout the massive Archean granite. Dikes vary from about 2 feet wide (near top of outcrop in A) to paper thin seams (D). Locality 14.-- (46.6632, -87.8351) At this locality we begin to see a transition in the type of deformation from the brecciation and dike injection to the north to differential movement of larger Archean blocks, ranging in size up to tens of meters, and molding of soft sediments around these blocks. Although sediment dikes and brecciation are still fairly well developed here, there are also several examples of folds in the Michigamme where the sediments have been molded around or compressed between joint blocks of Archean granite. Note that where the basal Michigamme sediments are tightly folded the foliation in the adjacent Archean rocks is unaffected by the folds indicating that the Archean rocks moved as rigid blocks and the Michigamme was molded to the new shape of the top of the Archean. These features are very well exposed at Localities 16 and 17 to the south. Locality 15.-- (46.6630, -87.8355) This outcrop is entirely Archean rocks, mostly massive granite. On the north end of the outcrop there are many thin sediment dikes but their abundance diminishes to the south. Locality 16 (46.6613, -87.8365) and Locality 17 (46.6613, -87.8398).—Beginning in the vicinity of Locality 16 and continuing westward to Locality 17, the western limit of good 59 �exposures, there are a multitude of small-scale folds in the basal beds of the Michigamme Formation. These are mostly synclinal features with diverse orientations and plunges (Figures 2.10 and 2.11). They appear to have formed as the soft sediments were molded around blocks of Archean rocks as those blocks were structurally rearranged. Note numerous instances where tight folds in the Michigamme have no expression in the immediately adjacent Archean rocks, showing that the folds have formed in response to the newly acquired shape of the unconformity surface on top of the Archean by draping over that surface, or in some instances by being injected into open joints. Figure 2.10. Features seen at Locality 16. Examples of small folds in the basal beds of the Michigamme Formation caused by molding the sediments to the shape of blocks of Archean granite. These fine-grained sediments were no doubt deposited in flat-lying beds on a horizontal surface but were later distorted to their present configuration as blocks of Archean rocks were displaced relative to each other. 60 �Figure 2.11. Features seem between Localitites 16 and 17. A- synclinal folds of Michigamme Formation argillite with variable plunges formed between blocks of Archean granite. BFractures in granite filled with fine-grained sediment. C- fractured boulder with laminated argillite compressed into the open fracture. D- undulating unconformity surface on top of Archean granite with basal Michigamme sediments draped over it. References Cannon, W.F., and Addison, W.A., 2007a, Distal ejecta from the 1850 Ma Sudbury impact in the Lake Superior iron ranges: Geological Society of America Abstracts with Programs, v 38, p. 58. Cannon, W.F., and Addison, W.D., 2007b, The Sudbury impact layer in the Lake Superior iron ranges: a time-line from the heavens: Institute on Lake Superior Geology 53rd Annual Meeting, v. 53, p. 20-21. Cannon, W.F., Horton, J.W. Jr., Kring, D.A., 2006a, Discovery of the Sudbury impact layer in Michigan and its potential significance: Geological Society of America Abstracts with Programs, v. 38, no. 7, p.58. Cannon, W.F., Horton, J.W Jr.., Kring, D.A., 2006b, The Sudbury impact layer in the Marquette Range Supergroup of Michigan: Institute on Lake Superior Geology 52nd Annual Meeting, v. 52, p. 10-11. 61 �Cannon, W.F. and Klasner, J.S., 1976, Phosphorite and other apatite-bearing sedimentary rocks in the Precambrian of Northern Michigan: U.S. Geological Survey Circular 746, 6 p. Clark, L.D., Cannon, W.F., and Klasner, J.S., 1975, Bedrock geologic map of the Negaunee SW Quadrangle, Marquette County, Michigan: U.S. Geological Survey Geological Quadrangle Map GQ-1226, scale 1:24,000. Holm, D.K., Schneider, D.A., Rose, S., Mancuso, C., McKenzie, M., Foland, K.A., and Hodges, K.V., 2007, Proterozoic metamorphism and cooling in the southern Lake Superior region, North America and its bearing on crustal evolution: Precambrian Research, v. 157, p. 106-126. Klasner, J.S., Cannon, W.F., and Brock, M, 1979, Bedrock geologic map of parts of Baraga, Dead River and Clark Creek basins, Marquette County, Michigan: U.S. Geological Survey Open File map 79-135, scale 1:62,000. Kring, D.A., Horton, J.W., Jr., and Cannon, W.F., 2006, Discovery of the Sudbury impact layer in Michigan, USA: Meteoritics and Planetary Science, v. 41, supplement, p. A100. Marcus, R, Melosh, H.J., and Collins, G., 2004, Earth Impact Effects Program: http://www.lpl.arizona.edu/impacteffects/ Melosh, H.J., 1989, Impact cratering: a geologic process: Oxford University Press, New York. Norris, R.D., Firth, J., Blusztajn, J.S., and Ravizza, G., 2000, Mass failure of the North Atlantic margin triggered by the Cretaceous-Paleogene bolide impact: Geology, v. 28, p. 1119-1122. Pufahl, P.K., Hiatt, E.E., Stanley, C.R., Morrow, J.R., Nelson, G.J., and Edwards, C.T., 2007, Physical and chemical evidence for the 1850 Ma Sudbury impact event in the Baraga Group, Michigan: Geology, v., 35, p. 827-830. Puffett, W.P., 1974, Geology of the Negaunee Quadrangle, Marquette County, Michigan: U.S. Geological Survey Professional Paper 788, 53 p. Rousell, D.H., Fedorowich, J.S., and Dressler, B.O., 2003, Sudbury breccia (Canada): a product of the 1850 Ma Sudbury event and host to footwall Cu-Ni-PGE deposits: Earth-Science Reviews, v. 60, p. 147-174. Schulz, K.J., and Cannon, W.F., 2007, The Penokean orogeny in the Lake Superior region: Precambrian Research, v. 157, p. 4-25. Sturkell, E.F.F., and Ormo, J., 1997, Impact-related clastic injections in the marine Ordovician Lockne impact structure, central Sweden: Sedimentology, v. 44, p. 793-804. Terry, D.O., Chamberain, J.A., Stoffer, P.W., Messina, P., and Jannett, P.A., 2001, Marine Cretaceous-Tertiary boundary section in southwestern South Dakota: Geology, v. 29, p. 10551058. 62 �Vallini, D.A., Cannon, W.F., Schulz, K.J., and McNaughton, N.J., 2007, Thermal history of low metamorphic grade Paleoproterozoic sedimentary rocks of the Penokean orogen, Lake Superior region: evidence for a widespread 1786 Ma overprint based on xenotime geochronology: Precambrian Research, v. 157, p. 169-187. 63 �64 �54th Annual Institute on Lake Superior Geology Field Trip 3 GEOLOGY OF THE BACK FORTY PROJECT Tom Quigley Bob Mahin Aquila Field Office Geologic Staff 65 �54th Annual Institute on Lake Superior Geology  FIELD TRIP # 3    Back  Forty Geology and Mineralization  Tom Quigley   Bob Mahin    Aquila Field Office Geologic Staff      Gossan mineralization exposed at surface at the Back Forty project site     66 �Introduction  The Back Forty Volcanogenic Massive Sulfide (VMS) deposit located alongside the Menominee River in  the Upper Peninsula of Michigan is the most recent deposit of this type found in the Early Proterozoic  aged Penokean Volcanic Belt (PVB) which trends east west through Wisconsin and extends into the  Upper Peninsula of Michigan.  Numerous massive sulfide occurrences and several significant deposits  were discovered as a result of protracted exploration efforts focused on the Wisconsin portion of the  PVB during the 1960’s, 1970’s, and early 1980”s including the Crandon deposit (61 million tonnes 5.6%  Zn), Flambeau (5.8 million tonnes 4% Cu) and Lynne (6.1 million tonnes 8.7% Zn).    The Back Forty was discovered in 2002, and has a resource (current as of April 2007) of 6.6 million  tonnes with 5.3% Zn, 2.3 grams/tonne (g/t) Au, 29 g/t Ag, and 0.5% Cu in the measured and indicated  category, and an additional 1.75 million tonnes of 2.6% Zn, 2.8 g/t Au, and 32 g/t Ag in the inferred  category, making it the 2nd largest deposit found in the PVB to date and placing it in the upper 30th  percentile in size of VMS deposits worldwide.  The April 2007 resource was calculated on the basis of 151 diamond drill holes (35,000 meters).  Since  then an additional 150 holes (30,000 meters) of drilling has been completed in anticipation of a new  resource calculation, preliminary mine design, metallurgical testing, and pre feasibility studies planned  for 2008 and 2009.  Regional Geologic Setting of the Back Forty VMS Deposit  The Back Forty VMS deposit is one of a number of similar deposits located within the Ladysmith‐ Rhinelander volcanic complex in northern Wisconsin and western Michigan. The complex lies within the  Early Proterozoic Penokean volcanic belt (PVB), also known as the Wisconsin Magmatic Terrain, on the  western edge of the Paleozoic Michigan Basin (Figures 1 and 2).  Figure 1    67 �Figure 2.  Location of Back Forty project and other major VMS deposits      The PVB is characterized by volcanic island‐arc‐basin assemblages containing abundant calc‐alkaline  metavolcanic units, intrusive rocks and lesser amounts of sedimentary rocks, and is in structural contact  to the north, along the Niagara Fault zone, with a back arc basin sedimentary terrain containing  subordinate interbedded tholeiitic metavolcanic rocks and major Superior‐type, oxide‐facies iron  formations. This supracrustal sequence appears to correlate with the Marquette Range Supergroup in  Michigan (Dematties 2004).  The Back Forty project is located at the eastern edge of the PVB where the older volcanic supracrustal  rocks of the belt are covered by Paleozoic sedimentary rocks of the Michigan Basin.  Local Geologic Setting   Figure 3 shows the interpreted bedrock geology of the Back Forty area derived from published geologic  maps, airborne and ground geophysical data and sparse outcrops.  68 �    Figure 3.  Bedrock Geology of the Back Forty Project area.    Back Forty mineralization is hosted by dominantly felsic volcanic rocks which appear to be spatially and  possibly genetically related to a large intrusive complex of granite, tonalite and more mafic phases  exposed sporadically in Wisconsin and interpreted to extend into Michigan based on gravity and  magnetic data.  This central complex of felsic intrusive and volcanic rocks is flanked on the north and  69 �south by more mafic volcanic sequences as well as argillites and fine grained tuffaceous rocks to the  south.    Deposit Scale Geology  Geology of the host rocks  Back Forty mineralization consists of massive, semi massive, and stringer sulfide mineralization as well  as precious metal zones with sparse sulfides, developed within a highly altered sequence of rhyolite  breccias and pyroclastic rocks cut by dikes, sills and irregular intrusions of porphyritic dacite and  rhyodacite.  Late mafic dikes and at least one dioritic to gabbroic intrusive intrude the felsic sequence.   Figure 4 shows the bedrock geology of the immediate deposit area.                                    Figure 4 .  Bedrock geology of the Back Forty deposit.  Rhyolitic rocks comprise the main host for the massive sulfide mineralization and consist of three  chemically distinct sequences of rhyolite breccias, pyroclastic rocks and thin interbedded tuffaceous  rocks.  A well developed sequence of finely bedded tuffaceous sediments, including a cherty exhalative  70 �horizon, occurs at the break between the middle and upper rhyolite sequences.  Younger dacitic quartz,  feldspar porphyries intrude the entire sequence including the massive sulfides.    Structurally, this rhyolite sequence and associated massive sulfide mineralization has been deformed  into an asymmetric, moderately plunging (300 west‐southwest) anticlinal fold characterized by a gently  dipping north limb (30o northwest), a steeply dipping and sheared south limb (70o southeast).   The hinge  of the fold and associated massive sulfide mineralization have been breached by erosion in the vicinity  of the East Zone or near or at the fold’s subcrop to the east.  Folding has produced an axial planar  schistosity and faulting has offset lithologies and created zones of weakness for younger intrusive rocks.  Three chemically distinct (but not always visually distinct) rhyolite sequences have been identified in the  immediate area of mineralization.  The units are identified as Rhyolite 1, 2 and 3 from oldest to  youngest,  as illustrated in the generalized stratigraphic column shown in figure 5.    Figure 5.  Rhyolite stratigraphic section with major massive sulfide zones.  71 �Figure 6 shows two discrimination plots used to characterize Back Forty host rocks.  The largely rhyolitic  composition of the volcanic rocks is illustrated in the SiO2 vs Zr/TiO2 plot, and the immobile element plot  of Zr/TiO2 vs Al2O3/TiO2 clearly distinguishes the three rhyolites.    WR plot   Figure 6.  Discrimination diagrams for Back Forty host rocks.  72 �All three rhyolite types exhibit intense leaching of Na2O (feldspar breakdown) and concomitant increase  in K2O (sericitization) as a result of intense hydrothermal alteration.  Altered host rocks form  assemblages of quartz – sericite – pyrite throughout the drilled section hosting the massive sulfide  mineralization and throughout an extensive area surrounding the known mineralization.  The degree  and extent of this alteration is evidence for a large and long lived hydrothermal system and suggests the  potential for additional mineralization in the area.  Mineralization  Mineralization at the Back Forty project consists of base metal massive sulfide, semi massive sulfide and  stringer sulfide mineralization as well as precious metal (gold and silver) mineralization.  Base metal mineralization Massive, semi massive, and associated stringer sulfide mineralization occurs in at least 3 stratigraphic  intervals roughly 100 m apart within the altered rhyolite sequence (figure 5), originally occupying  horizons separating the major rhyolite eruptive events. Subsequent folding, shearing, faulting and  emplacement of younger intrusive rocks has complicated and disrupted this primary stratigraphy.  Massive sulfides are dominantly of the zinc‐rich variety although copper‐rich zones occur in some lenses  and copper‐rich stringer mineralization is locally common.  The felsic‐dominant volcanic stratigraphy  that is host to the Back Forty mineralization point to a bimodal‐felsic or a Kuroko‐style of mineralization,  defined by having >50% felsic volcanic rocks, and <15% siliciclastic rocks in the host stratigraphic  succession (Barrie, 2007).     Zinc‐rich massive sulfides at Back Forty consist of medium to coarse grained aggregates of pyrite,  sphalerite, and lesser chalcopyrite and galena, with varying amounts of silver and gold.  Pyrite is the  dominant gangue mineral with minor amounts of pyrrhotite and aresenopyrite.  Galena attains  potentially recoverable amounts in one of the zinc‐rich massive sulfides (the Tuff Zone), which also  contains elevated silver values relative to the other horizons.    Two massive sulfide lenses come to surface and have been intensely oxidized to form iron oxide‐rich  gossans which cap fresh massive sulfide.  The gossan mineralization consists  principally of botryoidal,  colliform and brecciated hematite and goethite, with lesser amounts of the minerals found in the  primary massive sulfide that have undergone partial to near complete replacement by the oxides. Minor  to trace amounts of bornite, gold, argentite, diaphorite, acanthite, ramdohrite (Ag3Pb6Sb11S24), Ni‐ skutterudite, eugenite, meneghinite, clausthalite (PbSe), cassiterite, and other trace phases are present  (Barrie, 2007).  Gold and electrum are present at grain boundaries of other minerals, and within colloidal  hematite.  Although recoverable amounts of copper are present in the Pinwheel gossan, base metal  tenors in gossan are generally very low, with gold and silver locally attaining very high grades.     Copper‐rich massive sulfides are limited in extent relative to the zinc‐rich variety, and are composed of  mainly pyrite and chalcopyrite, with some supergene enrichment to bornite in near surface zones  underlying gossan.  Like the zinc‐rich massive sulfides copper‐rich zones contain varying amounts of gold  and silver.  Textures in the massive sulfide lenses are massive to bedded with extremely variable bedding attitudes,  indicating post depositional deformation and remobilization.  73 �Stringer sulfide mineralization normally consists of cross cutting veins and fracture fillings of pyrite with  varying amounts of chalcopyrite and gold, and normally underlies massive mineralization but may be  laterally correlative with some massive lenses.  To better understand the geometry and stratigraphic position of the Back Forty massive sulfides, several  views and cross sections are presented below.  In plan view (Figure 4) the massive sulfide lenses occupy  the hinge and north and south limbs of the folded stratigraphy.  Figure 7 shows a longitudinal 3D section  of the mineralized zones viewed from the north, and  figures 8, 9, and 10 are cross sections (locations  shown on figure 4) which further illustrate the morphology of the massive lenses and host rocks.  Approx. 100 meters       Figure 7.  Three dimensional view of Back Forty sulfide and gossan mineralization.              74 �Figure 8.  Cross section of the near surface East Zone.  For legend see figure 4.  Scale is in meters.                    75 �Figure 9.  Cross section through the  Main Zone (hinge area and south limb) and Tuff Zone.  For legend  see figure 4.  Black diamonds represent gold intercepts.                    76 �Figure 10.  Cross section through the Pinwheel and 90 Gold Zone.  For legend see figure 4.              77 �Metal distribution is variable within massive sulfides and associated host rocks.  Figures 11 and 12  illustrate the typical patterns of base and precious metals in massive sulfides, stringer zones, altered  host rocks and younger intrusive porphyry.    Figure 11.  Metal distribution, Main Zone (Hinge area) massive sulfide and stringer zone.  This pattern  of metal distribution is typical of the Main Zone with zinc grades increasing towards the bottom of the  massive sulfide with strong and consistent gold mineralization in the underlying stringer sulfides.   Note also the gold values associated with the intrusive quartz – feldspar ‐ porphyry (QFP).  Au g//t Ag g/t 5 10 50 100 Zn % 5 10 15 Cu % Pb % 1 23 1 23 Massive Sulfide 65.7 m Au g/t Ag g/t Zn % 1.6 21 6.9 Cu % 0.3 Stringer Sulfides 31 m Au g/t Ag g/t Zn % Cu % 10.0 65 0.27 0.2   78 �Figure 12.  Metal distribution associated with Tuff Zone massive sulfide mineralization.  Note the  widespread gold and silver values in the tuffaceous sediment sequence hosting this massive sulfide  lens.  Siliceous Tuffaceous Sediments 65.1 m Au g/t Ag g/t Zn % Cu % 1.5 64.4 4.1 0.04 Massive Sulfide 9.5 m Au g/t Ag g/t Zn % Cu % Pb % 1.1 152.5 20.9 0.06 5.3   79 �Precious metal mineralization at Back Forty  Gold and silver occurs in all types of sulfide mineralization (massive sulfides, stringer zones), as well as  gossans, altered rhyolite host rocks, and younger intrusive porphyries which cut the host strigraphy and  sulfide mineralization.  Gold in massive sulfides and associated stringer zones is of variable grade, and although it can attain  very high grades locally (> 20 g/t), the average for all massive sulfides is 2.3 g/t.  A breakdown of metal  grades by individual sulfide zone is shown in figure 13.  Gold in sulfide mineralization is usually fine  grained and is closely associated with chalcopyrite and as native gold and electrum within pyrite and  along pyrite grain boundaries.      3D Model of Massive Sulfide and Gossan 16   Figure 13.  Tonnage and grade for individual sulfide zones (April 2007 NI‐43‐101 Resource)    Gossan mineralization derived from the oxidation of massive sulfide shows strong enrichment in gold  overlying the East Zone and gold and silver overlying the Pinwheel, as illustrated by the resource  numbers in Figure 13.  Fine grained, free gold in hematite is common in the gossan.  Silver occurs in  mercury silver minerals, eugenite (Ag11Hg2) and luanheite (Ag3Hg), and in acanthite (AgS), and locally as  coarse native silver.    80 �Not shown in figure 13 is precious metal mineralization associated with rocks surrounding massive  sulfide mineralization.  Numerous gold and silver intercepts in lithologies peripheral to the massive  sulfides and stringers prompted follow up drilling in 2007 and 2008 to target this style of mineralization.  As a result of this drilling, two zones of gold and silver mineralization – the 90 Gold Zone hosted by  siliceous sediments, and the PM Gold Zone hosted by quartz feldspar porphyry have been identified  (figure 14).  Both gold zones contain fine grained gold disseminated in silicified host rock with small  amounts of galena, aresenopyrite, chalcopyrite and pyrite.  Figure 14.  Plan view of massive sulfide lenses and gold zones        Expansion of all mineralized zones is ongoing at the project with 3 drills active.  A new resource estimate  is currently being prepared and preliminary mine planning, metallurgical studies, and environmental  baseline work is underway.    81 �82 �  FIELD TRIP DESCRIPTION AND STOPS  The field trip will concentrate on local and deposit geology to be illustrated by inspection of outcrops in  the vicinity of the Back Forty mineralization, drill core, and a review of technical data developed from  geophysical studies and drilling.  Stop 1   Field Office:   The Aquila field office is located directly adjacent to the Back Forty massive  sulfide discovery.  Field trip participants will assemble at the field office for an overview of the project  setting.  Stop 2  Porphyry Outcrops: From the field office, proceed across the River Road to outcrops of  quartz‐feldspar porphyry.  These outcrops are  a couple of hundred meters north of the main  mineralization but are typical of this rock type which occurs as dikes, sills and irregular intrusions  throughout the host rhyolite sequence.  Compositionally they are dacitic to rhyodacitic and typically fine  to medium grained with a dark ground mass of chlorite, biotite, amphibole, sericite and 5mm to 1 cm  phenocrysts of feldspar and lesser quartz.  These units are variably altered and locally gold mineralized –  especially along margins where they intrude rhyolite with heavy or massive sulfide.  Where they are  mineralized they are normally silicified with destruction of phenocrysts, and contain minor amounts of  chalcopyrite, sphalerite, galena and aresenopyrite and occasionally visible gold.    Stop 3  Rhyolite 2 Outcrops: Abundant outcrops of altered rhyolite occur in a broad area north  of the known massive sulfide mineralization, and are likely hanging wall (Rhyolite 2) to the Main Zone  (Hinge, South Limb and East Zone) massive sulfides, occurring on the northwest dipping, north limb of  the west south west plunging, asymmetric fold.  These outcrops contain quartz phenocrysts in a fine  matrix of sericite and quartz which is typical of Rhyolites 1 and 2, and are considered to be favorable  host rocks for massive sulfide mineralization.  They contain abundant disseminated pyrite and only  rarely sphalerite or other base metal sulfides and are considered part of the large hydrothermally  altered halo to the massive sulfide system.  The only obvious texture is a steeply dipping planar fabric –  probably an axially planar cleavage related to the fold system.  Stop 4 Pinwheel Gossan: Gossan outcrops here are the only exposure of massive sulfide  (formerly) at the project, and are completely to partially oxidized rocks composed principally of  botryoidal, colliform and brecciated hematite and goethite, clays, and chlorite, with lesser amounts of  the minerals found in the primary massive sulfide that have undergone partial to near complete  replacement by the oxides. Minor to trace amounts of bornite, gold, argentite, diaphorite, acanthite,  ramdohrite (Ag3Pb6Sb11S24), Ni‐skutterudite, eugenite, meneghinite, clausthalite (PbSe), cassiterite, and  other trace phases are present. Gold and electrum are present at grain boundaries of other minerals,  and within colloidal hematite.  These outcrops represent the up dip extension of the Pinwheel massive  sulfide, near the axis of the fold, where the Pinwheel Zone has been breached by erosion.  The Pinwheel  represents a stratigraphically higher sulfide horizon than the Main Zone massive sulfides which are  located about 100 meters below the outcrops of gossan.  This area of the Pinwheel gossan contains  significant magnetite and the resulting ground magnetic response clearly defines this portion of the  gossan.  Other parts of this gossan as well as the entire East Zone gossan however, are totally non  magnetic.    83 �                                    Reflected light (RL) and transmitted light (TL) photomicrographs of gossan samples. Top left: East  zone gossan: LK‐76 10.1‐10.85 ‐ gold in colloidal hematite;   0.8 mm; RL.  Top right: top of 90 zone: LK‐99 32.‐33.5 ‐ tiny gold‐electrum granules with pyrrhotite,  galena (grey) and acicular ramdohrite (grey) in vein in hematite matrix. 1.3 mm; RL. Middle left  and right: Pinwheel gossan: LK‐130P 10.2‐11.08 ‐zoned dolomite filling void between colloidal cpy  rimming UM9. 5.8 mm, TL (left) and RL (right). Bottom left and right: Pinwheel gossan: LK‐130P 14‐ 15.3: ‐mercury‐silver aggregates rimming an unidentified mineral and against zoned carbonate. 1.5  mm TL (left) and RL (right).  From Barrie (2007).    84 �Stop 5  Rhyolite 3 Outcrops:  Outcrops of rhyolite 3 are exposed south of the fold axis on the  south limb of the fold.  Unlike rhyolites 1 and 2, Rhyolite 3 is a non porphyritic rhyolite (usually) and  chemically distinct from the other rhyolites.  This unit is also highly altered – sericite, chlorite, silica, and  pyrite, and in drill core contains very distinctive round or ovoid, dark chloritic alteration spots.  It also  contains appreciable pyrrhotite and has a resulting positive magnetic signature.  The stratigraphic  position of this unit, although shown as the stratigraphically highest rhyolite (figure 5), is actually  uncertain.  It has not been identified on the north limb of the fold, and may represent a younger  intrusive unit into the mineralized rhyolite sequence.   Stop 6  Water Well Location:  This is the site of the original water well which encountered  massive sulfides.  The well was drilled by Kleiman Pump and Well from Iron Mountain Michigan.  The  drillers recognized heavy sulfides in the cuttings and subsequently contacted geologist Richard Lassin  who analyzed them and confirmed high (10%) zinc values.  Lassin and Kleiman also identified the gossan  outcrops and correctly speculated that the water well intercept represented the down dip, unoxidized  Pinwheel massive sulfide.  After contacting and partnering with Minerals Processing Corp. – a privately  held Michigan exploration company – a gravity survey was conducted down the River Rd. between the  gossan outcrops and water well, as well as on State of Michigan owned minerals to the east.  Both  surveys detected strong gravity responses.  A coincident electromagnetic response on the state ground,  prompted the initial diamond drilling program.  This is the site of the discovery hole, in what is now the  East Zone massive sulfide.  Stop  7  Field Office:  Drill core of representative mineralization and host rocks and other technical  information will be on display.  Stop 8  Daggett Core Facility:  More core will be on display with assay and other information at  the core storage facility in Daggett Michigan.    References  Barrie, C.T.,  2007  Petrography and mineral chemistry of the Back 40 VMS deposit, Menominee County,  Michigan: Initial Observations, Technical report prepared for Aquila Resources Inc., 27p    Dematties, T. A.,  2007, An evaluation of the Back Forty volcanogenic massive sulfide (VMS) deposit,  Menominee County, Michigan, U.S.A.:  Technical report prepared for Aquila Resources Inc., 52p    85 �86 �54th Annual Institute on Lake Superior Geology Field Trips 4 and 8 GEOLOGY OF THE EAGLE PROJECT Andrew Ware, Kennecott Minerals Company Jon Cherry, Kennecott Minerals Company Xin Ding, Indiana University 87 �Proterozoic high-MgO basaltic magmatism in the Midcontinent Rift system, northern Michigan: Precise baddeleyite U-Pb age and petrogenesis of the Eagle sulfide-bearing mafic-ultramafic intrusion Introduction The Eagle sulfide-bearing intrusion, first drilled in 2001 by Kennecott Mineral Company, consists of both disseminated and massive sulfides in most parts of the intrusion. The Eagle intrusion is part of the intrusive-extrusive association in the 1100 Ma Midcontinent Rift System (MRS). Major exposures of the volcanic rocks occur along the shores of Lake Superior, but not in the Eagle area. Instead, equivalent mafic dikes are abundant in the Eagle area. The styles of sulfide mineralization in the Eagle intrusion differ significantly from those associated with Duluth and Mellen Complex, the principle exposed plutonic rocks of the rift. In the Duluth Complex sulfide mineralization is restricted to the basal contact zones whereas Eagle sulfide mineralization is distributed throughout the host intrusion. The Cu, Ni, and PGE tenor of the sulfide ores from Eagle are also much higher. These features, together with higher olivine abundance and a lack of layering in the Eagle intrusion suggest that the Eagle intrusion may represent a dynamic magma conduit similar to the feeder dyke of the Voisey’s Bay Ni-Cu sulfide deposit in Labrador. The summary below represents our current understanding of the Eagle system based on the preliminary results of a collaborative study with Kennecott Minerals Company. Geological backgroud The Eagle Ni-Cu sulfide deposit occurs in the Baraga basin in northern Michigan (Fig.1). The Baraga basin was intruded by the Mesoproterozoic Baraga-Marquette dike swarm, which is considered to be related to the early stage basaltic magmatism in the MRS (Wilband and Wasuwanich, 1981, Green et al., 1987). Sulfide mineralization occurs in two intrusions referred to as the Eagle and East Eagle deposits. The western intrusion, which hosts the Eagle deposit, is ~480 m in length and 100 m wide near the 88 �surface. It narrows to ~10 m at the depth of ~340 m. The eastern intrusion is located 650 m to the east of the Eagle deposit. Fig.1 Map of the Lake Superior region showing major exposure of volcanic and plutonic rocks associated with the Midcontinent rift (after Davis and Green, 1997; Nicholson et al., 1997). The exposed volcanic rocks of the MRS are located around Lake Superior in southern Ontario, northern Minnesota, northern Wisconsin and Michigan. Volcanic rocks are also found in deep drill cores as far south as Kansas. Most of the volcanic rocks are thoeliitic in nature, with smaller amounts of intermediate and rhyolitic rocks (e.g. Nicholson et al., 1997). The principal intrusive rocks of the MRS are the Duluth Complex in Minnesota and the Mellen Complex in Wisconsin, both of which contain low-grade Ni-Cu sulfide mineralization. The Duluth Complex and associated subvolcanic intrusions comprise a large (5,000 km2) intrusive complex that represents a significant low-grade, but high tonnage, resource. The smaller Mellen Complex emplaced near the base of the Keweenawan volcanic section along the southeastern flank of Lake Superior, also 89 �contains low-grade mineralization. U-Pb dating of zircons from various intrusions in the Duluth Complex provides an age of 1099 Ma (Paces and Miller, 1993) and correlates with Keweenawan high Al olivine tholeiite basalts of the North Shore volcanic group (Chalokwu et al., 1996). The Mellen Complex was emplaced at 1102 Ma, and has been correlated with the Kallander Creek Volcanics of the Powder Mill group (Zartman et al., 1997). Fig.2 Stratigraphic diagram illustrates country rock around Eagle deposit, including Arhcean gneiss and Proterozoic sediment. The Eagle intrusion intruded Early Proterozoic sedimentary rocks of the Marquette Range Supergroup in the Baraga Basin (Fig.2). The Marquette Range Supergroup is divided into the Chocolay, Menominee, and Baraga Groups. The Baraga Group is thought to be contemporaneous with the Proterozoic Animikie Group in Minnesota (Ojakangas et al., 2001). In the Baraga Basin, the Baraga Group sediments are low-grade metamorphosed marine sediments that contain disseminated pyrite, pyrrhotite, or both. The Baraga basin is bounded to the north and south by Late Archean gneiss and 90 �granitoids, and to the east and southeast by Late Archean, low-grade metamorphosed volcanic and sedimentary rocks. The lowest member of the Baraga Group is the Goodrich Quartzite, which is overlain by a chert carbonate member. The chert carbonate member is overlain by the Michigamme Formation. Kennecott geologists informally divide the Michigamme Formation into three members: the Lower Slate, Upper Greywacke, and Fossum Creek. Sulfide and graphite-rich horizons are present in the Lower Slate and Lower Fossum Creek units. The sulfide assemblages are pyrrhotite-dominant, with lesser amounts of pyrite, chalcopyrite, and pentlandite. Lithology and sulfide mineralization of the Eagle intrusion The Eagle intrusion comprises feldspathic peridotite, gabbronorite, melatroctolite, melagabbro and olivine gabbro (Fig.3 and Fig.4). The basal contact occurs as an elongated feeder, which is composed of melatroctolite, which ranges in dip from steeply southeast to vertical. The melatroctolite is not restricted to the basal contact of the steeply dipping feeder, but can occur higher where it turns into a flat lying sheet. The melatroctolite is also discontinuously underlain by a thin (~25 m thick) olivine gabbro. In the central part of the Eagle intrusion, thick melatroctolite encloses ~60 m of melagabbro. However, there is no thermal contact or chilled margin between melatroctolite and melagabbro. In the upper part of the intrusion, the melagabbro is overlain by feldspathic peridotite. Gabbronorite occurs as enlongated lenses (a few meters thick), along the contact between feldspathic peridotite and melatrocolite. In general, lithological units of the Eagle intrusion show a broad range of orientations. Most strike east-southeast parallel to the trend of the Eagle intrusion and have flat to moderate dips to both north and south. 91 �Fig.3 Long section through the Eagle intrusion showing its stratigraph Fig.4 Section 431470E showing the stratigraphy of the Eagle intrusion 92 �Fig.5 Block diagram illustrating ore body distribution Three distinct types of sulfide mineralization occur at the Eagle deposit (Fig.5). They are described as disseminated, semi-massive and massive sulfide. Finely disseminated sulfide minerals can be found in most portions of the intrusion. The ore reserve is comprised of two semi-massive sulfide zones that are linked by a zone of massive sulfides. The mineralogy is typical of magmatic sulfides, and consists of pyrrhotite, chalcopyrite, pentlandite, and cubanite. The average grade of semi-massive sulfide ores are 2.1% Ni, 2.2% Cu, 0.5 g/t Pt and 0.3 g/t Pd. The average grade of massive sulfide ore is 6.1% Ni, 4.2% Cu, 1.1 g/t Pt and 0.8 g/t Pd. Petrography Modal proportions of minerals in rock samples from the Eagle intrusion have been estimated by point-counting. The results are shown in Fig.6. Olivine occurs as cumulus phases and pyroxene and 93 �plagioclase occur as interstitial phases in olivine-rich samples. But pyroxene and plagioclase occur as cumulus phases in olivine-poor or olivine-free samples. The percentage of granular, cumulus olivine grains increase from melagabbro, to melatroctolite, to feldspathic peridotite. Spinel occurs as inclusions in olivine suggesting that it is also an early cumulus phase. Minor amounts of amphibole and biotite occurs as interstitial phases in all samples. Fig.6 Modal proportions of the main rock types in Eagle intrusion plotted in Olivine-pyroxne-plagioclase phase diagram constructed after Morse (1980). Feldspathic peridotite (Fig.7a) consists of 50-65% cumulus olivine (average grain size < 5 mm), forming as large crystals. Intercumulus pyroxene (20-30%) commonly forms okiocrysts (3-5 mm). Intercumulus plagioclase (15-25%) occurs typically as euhedral to subhedral grains of variable size. Spinel (< 2%) occurs as inclusions in olivine and poikilitic pyroxene and plagioclase. Melatroctolite (Fig.7b) consists of 40-50% cumulus olivine (3~5 mm) occuring as medium to large elliptical grains, or as inclusions in pyroxene and plagioclase. Pyroxene (20-35%) occurs as euhedral to subhedral, intercumulus grains of variable size and plagioclase (20-30%) occurs as tabular, randomly 94 �oriented grains in the intercumulus space. Minor spinel inclusions are present in olivine, pyroxene and plagioclase. Fig.7 Photomicrographs showing typical textures of the main rock types. Photos (a) Feldspathic peridotite (b) melatrocotlite (c) melagabbro (d) gabbronorite (e) olivine gabbro 95 �Melagabbro (Fig.7c) consists of 30-40% cumulus olivine (0.5~3 mm), 30-45% cumulus pyroxene and 25-40% cumulus plagioclase. Olivine occurs as small as inclusions in pyroxene and plagioclase or relatively large crystals intergrown with pyroxene and plagioclase. Olivine-free gabbronorite (Fig.7d) is composed of euhedral pyroxene and plagioclase, showing preferred orientation. Pyroxene is present as large crystals and plagioclase normally occurs as euhedral to subhedral tabular crystals. Olivine gabbro (Fig.7e) consists of 20-40% cumulus olivine (< 2 mm), 40-50% cumulus pyroxene (< 1.5 mm) and 20-40% cumulus plagioclase (< 1.5 mm). Unlike other units, the olivine gabbro unit has very low sulfide concentration but high percentages of ilmenite and hematite. Baddeleyite U-Pb dating Fig.8 U-Pb isotopic data of baddeleyite from Eagle intrusion Results of U-Pb isotopic analysis for four abraded baddeleyite crystals and one unbraded zircon crystal from feldspathic peridotite are listed in Table 1 and illustrated in Fig. 8. The four baddeleyite fractions are concordant, but the zircon grain is discordant. The zircon grain defines a 207Pb/206Pb age of 96 �2623.3 Ma, which is consistent with the age of Archean basement of the Baraga basin. The weighted average of the 207Pb/206Pb ages for the four baddeleyite fractions is 1107.3±3.7 Ma. All baddeleyite fractions together yield a concordant age of 1107.2±5.7 Ma. The Eagle intrusion is now recognized as the second oldest intrusion in the southern part of the MRS and correlates with the eruption of the Siemens Creek volcanic suit and Mamainse Point volcanic suit (Fig.9). Fig.9 Chronostratigraphic correlation diagram for volcanic and plutonic rocks in western and eastern Lake Superior (after Davis and Green, 1997; Nicholson et al., 1997) Stratigraphic variations of olivine composition, whole rock Zr/Y and La/Yb ratios and δ34S The variation in the compositions of olivine in different rock types of the Eagle intrusion has been examined. The Fo contents of olivine in the sulfide-poor samples from the Eagle intrusion vary between 85 to 76 mod%. The contents of Ni in olivine are from 1,300 to 1,400 ppm. Compared to olivine from the olivine gabbro unit, olivines from other rock units are significantly depleted in Ni and exhibit a positive Fo-Ni correlation that is characteristic of fractional crystallization. In drill core 03EA034, the Fo contents of olivine decrease progressively with height in the melagabbro unit (Fig.10), stay relatively constant in the overlying melatroctolite unit, and reverse in the feldspathic peridotite unit. 97 �Fig.10 Stratigraphic variations of olivine composition, Mg# of clinopyroxene, plagioclase An number, incompatible element ratios, and S isotope in drill core 03EA034 Fig.11 Stratigraphic variations of olivine composition, incompatible element ratios, and S isotope in drill core YD0106 98 �The melagabbro unit is characterized by relatively low Zr/Y and La/Yb and elevated δ34S values. The melatroctolite unit has similar δ34S but distinctly higher Zr/Y and La/Yb ratios than the melagabbro. The Zr/Y and La/Yb ratios of the peridotite unit are similar to the underlying melatroctolite unit but the peridotite has distinctly lower δ34S values. In the stratigraphic diagram (Fig.10 and Fig.11), δ34S values are consistent from melagabbro through melatroctolite before a successive decrease towards the top of feldspathic peridotite. Controls on whole rock compositions In the plots of MgO versus FeO and MgO versus Al2O3 (Fig.12), the compositions of rocks from the Eagle intrusion are controlled by abundances of olivine and trapped liquid. Figure 13 illustrate chondrite-normalized trace element patterns for the Eagle intrusion and country rock. The slopes of trace element for Archean gneiss and Proterozoic sedimentary rock are much steeper than those of the Eagle intrusion. Feldspathic peridotite, melatroctolite, melagabbro and gabbronorite units generally have similar trace element slopes. The olivine gabbro unit has higher trace element abundances than the other rock types in the Eagle intrusion. All rock samples from the Eagle intrusion exhibit a negative Nb anomaly which is characteristic of crustal contamination. Fig.12 Variations of major elements, olivine, clinopyroxene, and plagioclase in Eagle intrusion 99 �Fig.13 Trace element abundance patterns for samples from (a) Eagle intrusion and (b) country rock, normalized to chondrite (values from McDonough and Sun, 1995) Fig.14 Frequency diagram illustrating Sulfur isotopic values of (a) feldspathic peridotite, melatrocotlite and melagabbrounits and (b) semi-massive and massive sulfides δ34S values of sulfide minerals from the Eagle intrusion vary between 1.0‰ and 4.3‰. The feldspathic peridotite and olivine gabbro samples have δ34S <3‰. Elevated δ34S values ranging from 3.6‰ to 4.3‰ are present in the melagabbro and melatroctolite units. Semi-massive and massive sulfide samples also have elevated δ34S values. Discussion Multiple pulses of magma and genetic relations The negative Fo-Ni correlation of olivine in melagabbro, melatroctolite, and feldspathic 100 �Fig.15 Plots of δ34S versus incompatible element ratios and olivine composition peridotite units is consistent with fractional crystallization. However, these different rock units have different La/Yb ratios and/or δ34S values (Fig.15). These variations are likely related to different degrees or different types of crustal contamination. The lack of systematic variations in La/Yb ratios and δ34S values with rock type (Fig.10) suggests that in situ contamination is not the main reason for the variations. It is more likely that those variations resulted from contamination at depth. The olivine gabbro unit has distinctly higher Ni content, which requires a Ni undepleted magma, unlike the depleted magma that formed other units. All these data suggest that at least three parental magmas were involved in the development of the Eagle intrusion: a Ni-undepleted magma, a Ni-depleted magma with δ34S <3‰, and a Ni-depleted magma with δ34S >3‰. They are likely related to each other by a differentiation process in a staging chamber such as: olivine crystallization, sulfide segregation or lack of sulfide segregation, or variation in crustal contamination history. Based on S field relations, it appears that Ni undepleted magma 101 �for the olivine gabbro unit intruded first, followed by the Ni-depleted magma with δ34S <3‰ to form the feldsapthic peridotite unit, and finally by Ni-depleted magma with δ34S >3‰ to form the melatroctolite and melagabbro units. Variations of La/Yb in each of these magmas suggest that additional, variable contamination took place during magma ascent and emplacement. Sulfide saturation and concentration The association of Ni depletion in olivine with elevated δ34S values in coexisting sulfides is consistent with the interpretation that sulfide saturation was caused by the addition of crustal S. However, some samples with Ni depleted olivine do not have elevated δ34S values. This may be due to variable δ34S value in the contaminant, or contamination with S-poor country rocks. The abundance of sulfide in the Eagle intrusion far exceeds the cotectic ratio during olivine crystallization. Some mechanism of sulfide concentration was required during magma emplacement. We envision that immiscible sulfide liquid droplets were carried along with olivine crystals by magma from a staging chamber. They settled out at the entrance of the subvertical feeder to the Eagle chamber due to a sudden decrease in velocity. In this model, the Eagle intrusion was a wider part of a dynamic conduit system that fed magma to overlying dykes or sills. Parental magma characteristics The FeO/MgO ratio of a parental magma can be estimated by using KD=(FeO/MgO)olivine/(FeO/MgO)liquid=0.3 (Roeder and Emslie, 1970). The calculated FeO/MgO for most primitive olivine from the Eagle intrusion is 1.04, which is similar to the values of picritic basalts in the LSCV suite and Group 1 of Mamainse Point. The Al2O3 contents of the liquids in equilibrium with spinels in the intrusion estimated using the relation of (Al2O3)spinel = 0.035(Al2O3)2.42 (Al2O3 in wt.%) by Maurel and Maurel (1982) are from 8.41 to 102 �Fig.16 Modeling curves of olivine fractionation with variable initial Ni contents Fig.17 Trace element abundance patterns for average Eagle intrusions, and volcanic rocks (data from Shirey et al., 1994). Model trace element compositions calculated from group 1 of Mamainse Point. 10.87 wt%. These values are also similar to those of picritic basalts in the LSCV suite and Group 1 of Mamainse Point. These similarities permit us to use the average compositions of picritic basalts to simulate fractional crystallization for the Eagle intrusion using the MELTS program by Ghiorso and Sack (1995). The results for olivine and trace element is shown in Fig.16 and Fig.17, modeled trends match the observed values well. 103 �Summary Mineral chemistry, whole-rock composition, and S isotopes indicate that the Eagle intrusion formed by multiple pulses of magma. The different magma pulses are different in the degrees of fractionation and type of contamination. Age correlation and phase relations suggest that the parental magma of the Eagle intrusion is similar to pictitic basalts found in the Lower Siemens Creek volcanic suite and the group 1 basalts at Mamainse Point, which both erupted during the early development of the Midcontinent rift system. The results of numerical modeling using the MELTS program indicate that the average picritic basalt can produce mineral assemblages and mineral compositions similar to those observed in the Eagle intrusion. Our current understanding of sulfide mineralization in the Eagle intrusion is that a mantlederived, high-MgO basaltic magma rose to a staging magma chamber, crystallized olivine and segregated immiscible sulfide droplets due to contamination with sulfide-bearing country rocks. The olivine- and sulfide- charged magma was then pushed up to a higher level at Eagle by new surges of magma into the staging chamber. Olivine and immiscible sulfide droplets became concentrated in the wider part of the Eagle conduit system where the silicate liquid continued to ascend. This process may have been repeated at least twice. References Chalokwu, C.I., Ariskin A.A. and Koptev-Dvornikov E.V. (1996) Magma dynamics at the base of an evolving mafic magma chamber: Incompatible element evidence from the Partridge River intrusion, Duluth Complex, Minnesota, USA. Geochim Cosmoch. Acta 60, 4997-5011. Green, J.C., Bornhorst, T.J., Chandler, V.W., Mudrey, M.G., Myers, P.E., Pesonen, L.J., and Wilband, J.T. (1987) Keweenawan dykes of the Lake Superior region: evidence for evolution of the Middle Proterzoic Midcontinent rift of North American. In Halls, H.C. and Fahrig, W.F., eds., Mafic dyke swarms: Geological Association of Canada Speical paper 34, 289-302. Maurel, C. and Maurel, P., (1982) Etude expérimentale de la solubilité du chrome dans les bains silicatés basiques et sa distribution entre liquide et minéraux coexistants: conditions d’existence du spinelle chromifére, Bulletin Minéralogie 105, 197–202. Nicholson, S.W., Shirey, S.B., Schulz, K.J., and Green, J.C. (1997) Rift-wide correlation of 1.1 Ga MRS basalts: implications for multiple mantle sources during rift development. Can. J. Earth Sci 34, 504520. Ojakangas, R.W., Morey, G.B., and Green, J.C. (2001) The Mesoproterzoic mid-continent rift system, Lake Superior region, USA. Sedimentary Geology 141-142, 421-442. 104 �Paces, J.B., and Miller, J.D. (1993) Precise U-Pb ages of Duluth Complex and related mafic intrusions, Northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic, and tectonomagmatic processes associated with the 1.1 Ga mid-continent rift system. Journal of Geophysical Research 98, 997-14013. Wilband, J.T. and Wasuwanich, P. 1981. Models of basalt petrogenesis: Lower Keweenawan diabase dikes and middle Keweenawan Portage Lake Lavas, upper Michigan. Contrib. Mineral. Petrol. 75, 395-406. Zartman, R.E., Nicholson, S.W., Cannon, W.F. and Morey, G.B. (1997) U-Th-Pb zircon ages of some Keweenawan supergroup rocks from the south shore of Lake Superior. Can. J. Earth Sci 34, 549-561. 105 �Eagle Project Area Quaternary Geology and Hydrostratigraphy* With the exception of the Peridotite outcrops in the Project area, the bedrock surface across the Plains is mantled by unconsolidated glacial deposits from the Quaternary period continental glaciation of the region. This surface forms the base of the Quaternary deposits. Hydrologically, this surface is considered to create a boundary to the movement of groundwater within the unconsolidated materials. The observed thickness of Quaternary deposits ranges from 0 ft (at the Peridotite outcrops) to greater than 200 ft. The deposit thickens in all directions away from the Peridotite outcrops, with the greatest thickness observed east and west of the Project area. The Quaternary deposits that define the Plains then thin toward the north and south, terminating at a boundary that is approximately coincident to the boundaries of the Baraga Basin metasedimentary rocks adjacent to the Archean bedrock formations that outcrop north and south of the Plains. Surficial geology is illustrated in Figure 1. A general hydrostratigraphic correlation nomenclature system was developed for the EBS and is summarized below. Surface Soil Layer A surface soil layer (black color with organic material and tree litter) was identified at most drilling locations. This layer is generally less than 1 ft thick (and mapped regionally as 0-2 in. thick on the Plains) and is classified as a sandy organic soil). Thin surficial layers of peat have also been identified in the area directly overlying the Eagle deposit ore body. Outwash and Beach Deposits (A Zone) The outwash and beach deposits are comprised of well-sorted, stratified fine- to medium-grained sand, with some gravel and minor quantities of silt and clay (less than 10%). The sand fraction of this material appears to be predominantly rounded quartz with trace to minor amounts of angular and sometimes platy mafic or fine-grained sedimentary rock grains. The unsaturated portion of this deposit is typically red to reddish brown and the saturated portion is brown. These surficial deposits are mapped regionally as having very rapid water infiltration rate characteristics (greater than 10 in./hr) (Twenter 1981). An unconfined water table defined as the A zone hydrostratigraphic unit occurs in the saturated portion of this deposit. The unsaturated zone is very thin in the southern portion of the Plains, where a large wetland complex exists. The unsaturated portion of the A zone then thickens significantly towards the northern edge of the Plains (up to 100 feet thick at wellQAL009 A/D northeast of the Peridotite outcrop). Generally a fining downward sequence is found in the A zone, with the fine sand fraction increasing with depth. A Zone groundwater elevation contours are shown in Figure 2. 106 �Transitional Deposit (B Zone) A gradational contact exists between the A zone outwash sand and a deeper transitional zone that contains a mix of fine sand, silt and clay, and typically continues to fine downward to predominantly silt and clay. While the A zone outwash and this transitional deposit may both be derived from melt water processes and could be lumped as outwash, the grain size characteristic change from predominantly sand to predominantly silt and clay. This transition is considered significant to primary conditions affecting groundwater flow as it indicates a decrease in permeability of the Quaternary formation from the coarse grained material to the fine-grained material. Directly above the Eagle deposit, the A zone coarse-grained materials are very thin (generally less than 5 ft in thickness) and the B zone fine-grained deposits form the bulk of the Quaternary deposits. As a result this area contains much more poorly drained surface soil and wetlands. Lacustrine Deposit (C Zone) A laterally extensive, massive clay deposit was identified in samples from most borings, and is found to be thickest south of the Peridotite outcrops, and thinnest north of the outcrops towards the north terrace. The clay deposit is easily recognized in soil sample cores as lean clay with medium to high plasticity. In some core samples it appears to be a massive deposit, while in other locations it contains thinly laminated and stratified layers of silt and clay. A sharp contact is typically observed at both the top and bottom of this deposit. On average the deposit contains 98% silt and clay. This deposit is defined as the C zone hydrostratigraphic unit. The clay deposit identified in soil borings ranged in thickness from 7-63 ft, thickest and most consistent in its elevation in the south/southeast part of the Plains (from locations QAL005A/D to QAL010A) and thinnest and less continuous towards the north and northeast, where this unit eventually pinches out near the edge of the north terrace. The pinch-out of the transitional and lacustrine deposits of the B zone near the north terrace is consistent with the glacial depositional model, as the transitional unit would be expected to pinch out at the edge of the moraine. This areal distribution pattern indicates that the fine-grained deposits were formed in ponded water between the bedrock highlands south of the Plains and glacial ice to the north, also consistent with the depositional model proposed by Segerstrom (1964). Outwash/Ablation Till (D Zone) A deposit of coarser-grained material was encountered beneath the C zone lacustrine deposit at most drilling locations. Samples from this deposit are predominantly fine- to medium-grained sand and are similar to samples of A zone material. This material appears to be outwash deposited prior to the glaciallake period on the Plains. This deposit is defined as the D zone hydrostratigraphic unit. Greater heterogeneity in grain size characteristics was observed within the D zone compared to the A zone. At 2 locations (QAL004A/D and QAL005A/D) south and southwest of the Peridotite outcrop, the D zone contains a layer with significant amounts of gneiss and granitoid cobble and 107 �gravel-sized outwash material indicative of high flow velocity glacial drainage channel deposits. At other locations (QAL001A/D, QAL002A/D and the base of QAL004A/D), the D zone contains a relatively high percentage of fine sand and silt, and generally becomes increasingly finer-grained toward its base. The finer grained portion is possibly derived from direct ice melt or sublimation (ablation till), since the base of this zone is most often identified in contact with a basal till deposit, described below. This outwash deposit is also discontinuous, interrupted by shallow bedrock and pinched out between the fine-grained units above and below. This deposit was not encountered beneath the C zone at well nests QAL006A/B and QAL010A. This deposit appears to be confined or partially confined, except at location QAL009A/D where the overlying C zone clay is absent. As a result of the pinch-out of the B and C zones in close proximity to the northern edge of the Plains, the A and D zone aquifers at this location become a single unconfined system. D Zone groundwater elevation contours are shown in figure 3. Basal Till (E Zone) Poorly-sorted basal till consisting of boulder- to sandy-sized clasts in a fine grained matrix is the lower most Quaternary deposit material identified in samples from all but one boring (QAL004A/D). This unit is substantially thicker east (QAL009A/D), west (QAL007A/D) and southeast (QAL010A) of the Project. Bedrock is encountered at greater depths at these locations, indicating that earlier glacial moraine deposition occurred in the bedrock valleys. Boulders are commonly present along the north terrace Lower Outwash Units (F Zone) At 2 locations (QAL007A/D and QAL010A), lower outwash deposits were found interlayered with E zone till. Representative samples of the lower outwash material are predominantly fine- to medium-grained sand. In QAL010A these units were found to be dry. The interlayered nature of the till and lower outwash units indicates fluctuations in glacial advances and retreats during earlier glacial depositional sequences. This lower outwash deposit is defined as the F zone hydrostratigraphic unit. *Text and figures extracted form Internal Company Report by Wiitala, D. et al. North Jackson Company. “Kennecott Minerals Company Eagle Project” Comprehensive Summary of Hydrologic Reports”. Feb, 2006 References Segerstrom, K. 1964. Negaunee Moraine and the Capture of the Yellow Dog River, Marquette County, Michigan. U.S. Geological Survey Professional Paper 501-C, pages C126-C129. Twenter, F. R. 1981. Geology and Hydrology for Environmental Planning in Marquette County, Michigan. U.S. Geological Survey Water Resources Investigations Report 80-90, Prepared in cooperation with the Michigan Department of Natural Resources, 44 pages. 108 �Figure 1. Regional Quaternary Geology. 109 �Figure 2. A Zone Groundwater Elevations (Summer Base Flow, August 2005) 110 �Figure 3. D Zone Groundwater elevation Contours (Summer base flow August 2005) 111 �Eagle Project - Field Trip Stops There are three field trip stops on the Eagle tour. Exposure in the Yellow Dog Plains is limited to two outcrops of ultramafic intrusives. Stop 4-1) Yellow Dog Peridotite – Eastern Outcrop. (UTM coordinates 432 440E 5 177 380N _ North side of County Road AAA) Travel north-west from Marquette on County Road 550. Turn on to County road 510 and then on to County Road AAA. Total distance from the 550/510 intersection is approximately 12 miles. The AAA turn will be flagged. Use precaution on the 510/AAA roads as logging trucks use these narrow roads for access. The main outcrop forms the western end of an inverted fin shaped intrusion that plunges to the east. Drilling has intersected feldspathic peridotite to a depth 720m below surface on the eastern end. In outcrop, both the eastern and western peridotites have distinctive, reddish brown, pitted weathered surfaces with rare bright red patches indicating oxidized pyrrhotite blebs. Weathering rinds are typically less than a centimeter thick, and relatively fresh looking sulfides can be seen within a few millimeters of the surface. Two primary lithologies, peridotite and pyroxenite, are recognized within both intrusions. Serpentinization of olivines, uralization of pyroxenes and chloritization of amphiboles are noted in thin section work. Stop 4- 2) Yellow Dog Peridotite – East end of Host Intrusive Complex for the Eagle Deposit. (UTM 431 720E 5 177 580N – South side of the County Road AAA) The eastern exposure is of an olivine gabbro. This particular phase represents one of the more primitive melts as defined by sulphur isotope data. Rare disseminated sulphide mineralisation can be observed in the outcrop. Stop 4-3) Kennecott Eagle Mineral Co Core storage Facilities 200 Echelon Drive, Negaunee. Turn Right of Highway 41 at the TV6 Studios. The Michigan State Police post is located opposite the turn off on the south side of the highway. Drive North for 200 yards and turn west through a set of large gates. The core storage buildings are located on the left hand side of the road. The turn off form the highway is located approximately 3 miles east of Negaunee. (Do not use MapQuest Directions). A review of core from the Eagle Project and The BIC project will be available for review. 112 �Eagle – Baraga Basin Exploration History The Baraga Basin region has until recently been subject to only sporadic exploration efforts. The earliest historical accounts of exploration in the basin date back to the mid- 1800’s when a group of investors tried to develop slate quarries along Slate River. Little documented exploration work took place in the Baraga basin between 1910 and 1950. During the 1950’s Jones and Laughlin conducted an exploration program along the northern portion of the East branch of the Huron River, investigating uranium-silver mercury mineralization associated with a graphitic shear exposed in the river. During the 1960’s and 1970’s, various interests conducted exploration programs on Ford mineral lands in the Baraga Basin and the western portion of the Marquette Trough. The programs were primarily focused on uranium and zinc. The U.S. Department of Energy provided funding to drill a number of deep holes in the Baraga Basin during the 1970’s presumably to provide stratigraphic information for the uranium exploration effort. Concurrently, the USGS began a bedrock-mapping program of the basin, focusing primarily on exposures in rivers, which produced an open file outcrop map with no report (Cannon, 1977). In 1976, Michigan Technological University drilled a 31-meter hole on the east end of the Yellow Dog (East Eagle) outcrop. The hole bottomed in coarse-grained peridotite with only traces of sulfide. In 1979, the Michigan DNR, in conjunction with the USGS, published a report on the Yellow Dog peridotite describing the results of geochemical, petrographic and geophysical studies of the peridotite (Klasner and others, 1979). The authors concluded that the anomalous sulfur and copper contents of the outcropping peridotite indicate a significant potential for copper-nickel ore deposits. Kennecott Exploration started working in the region in 1991 and actively explored for sedex zinc deposits through 1994. During the course of mapping, float boulders of peridotite with sulphides were discovered that indicated the potential for magmatic Ni-Cu sulphide mineralization. Kennecott partially shifted to magmatic nickel exploration in 1995 and drilled four holes to test the Yellow Dog peridotite (East Eagle). One hole (YD95-2) intersected 10 meters of moderate to heavy disseminated sulfide mineralization along the southern contact. Two more angle holes (YD95-3 and YD95-4) collared on the east end of the Yellow Dog East outcrop demonstrated the peridotite widened to the east but only intersected a meter or two of weak sulfide mineralization along the north and south contacts. The Michigan program was put on hold in the summer of 1996 and the Crystal Falls Office was closed as Rio Tinto reorganized the newly merged CRA, Kennecott and Rio Tinto exploration groups. The land position around Eagle was reduced to a core group of private and state leases in 1997 and 1998. Interest in the project was regenerated in 2000 through the persistent efforts of Kennecott geologist Dean Rossell who recognized the potential for the region to host significant nickel mineralization in light of recent published papers on Norils’k and Voisey’s Bay. The current nickel exploration program was started late in 2000. Drilling at East Eagle in July 2001 intersected 30 meters of disseminated, net textured and massive sulfides averaging 1.03% Ni and 0.75% Cu (YD01-01) and one of three holes on the east end of Eagle intersected 85 meters of disseminated sulphides averaging 0.6% Ni and 0.5%Cu (YD01-06). 113 �2002 drilling at Eagle targeted the center of a magnetic anomaly defined by ground surveys in 2001. The first hole, YD02-02, intersected 84.2 meters of massive pyrrhotite-pentlanditechalcopyrite averaging 6.3% Ni and 4.0% Cu, firmly establishing the presence of economic grade and width mineralization at Eagle. Subsequent definition drilling continued through the summer and fall of 2002 and has continued through to the present. References: Cannon, W.F., 1977, Bedrock geology in parts of the Baraga, Dead River, and Clark Creek Basins, Marquette and Baraga Counties, Michigan: U.S. Geological Survey Open-File report 77-467, scale 1:62,500. Klasner, J.S., Snider, D.W., Cannon, W.F., and Slack, J.F, 1979. The Yellow Dog Peridotite and a possible buried igneous complex of lower Keweenawan age in the northern peninsula of Michigan. State of Michigan, Dept. of Natural Resources, Geological Survey Division; Report of Investigation 24, 31 pp. 114 �115 �Introduction Note: As this guidebook is being prepared a substantial road construction project is underway to realign County Road 510 through the area of interest. Specifically, the road is being moved several hundred feet to the west of the location shown in Figures 2 and 3 and will connect to a new bridge being constructed over the Dead River. The descriptions and outcrop locations shown in this guide are those that existed through late 2007. When the field trip is conducted in May of 2008 the outcrop and access situation may be somewhat altered. The outcrops to the west of 510 are nearly all on land owned by Marquette County and are publicly accessible. Please observe private property boundaries to the far west and south of this area. The outcrops east of 510 are on private property to which the owners have granted access for scientific examination and reasonable sampling for research purposes. A set of outcrops near County Road 510, about 5 miles northwest of Marquette, Michigan (Figure 5.1) provides a complete section through the layer of debris deposited as a result of the giant impact at Sudbury, Ontario, which occurred about 500 km to the east at 1850 Ma . The Sudbury layer here is a breccia and sandstone unit about 40 m thick, which lies on banded ironformation and is overlain by pyritic black slate. Outcrops include: 1) the basal contact of the layer that consists of large rip-up clasts of the underlying iron-formation; 2) exposures of matrixsupported breccias in which most large fragments are chert, but many smaller fragments are impact glasses; 3) an upper massive sandstone with minor chert clasts and glass particles; and 4) the upper contact with black slate. The McClure site is the best-exposed section of the Sudbury layer currently known in Michigan and also is the thickest. In addition it is the closest exposure to the impact site at Sudbury. Because there are no preserved rocks of 1850 Ma age between here and Sudbury, the McClure site contains the most proximal ejecta that is likely to be found. General geology The McClure site is in the Dead River Basin, a structural outlier of Paleoproterozoic strata surrounded by Neoarchean crystalline rocks. The strata consist entirely of various informal units of the Michigamme Formation. The Sudbury layer at McClure was mapped as a chert conglomerate by W.P. Puffett (1974) who provided a detailed outcrop map of the immediate site as well as a 1:24,000 scale map of the Negaunee Quadrangle on which the layer was shown as a map unit. The unit was extended further west into the adjacent Negaunee SW quadrangle by Clark and others (1975). 116 �Figure 5.1. Map showing location of the McClure site. The Sudbury layer at McClure lies within a north-facing monoclinal succession of sedimentary rocks, all informal members of the Michigamme Formation, a part of the Baraga Group, which lies unconformably on Neoarchean granitic rock (Figure 5.2). The Michigamme Formation consists of a basal unit of quartzite and conglomerate, probably equivalent to the Goodrich Quartzite of the Marquette Range. The unit is about 60 meters thick and grades upward into a 150-200 meter-thick unit of impure quartzite and argillite. A 60 meter-thick unit of banded chert-hematite-goethite iron-formation overlies the impure quartzite and is the unit on which the Sudbury impact layer was deposited. Overlying the Sudbury layer with an apparent gradational contact is pyritic black slate. Thus the Sudbury layer at the McClure site lies about 250-300 meters above the base of the Baraga Group. This field trip will examine a set of outcrops that exposes a cross section of the impact layer as well as the upper and lower contacts with the adjacent stratigraphic units (Figure 5.3). Description of the Sudbury impact layer The rock layer here referred to as the Sudbury impact layer has been studied and described for nearly a century, but only in 2006 was it documented to be an impact-related unit. The most complete previous description was by Puffett (1974) who mapped and described the unit as a chert conglomerate containing many fragments of volcanic rocks. He interpreted it to have originated “during a period of volcanism in which thick tuff deposits accumulated, then was disturbed by landslides or other gravity-activated mechanisms that dumped material into the site of deposition.” Puffet clearly recognized the essentially instantaneous deposition of this massive, 117 �graded unit and the unusual mixture of volcanic fragments, chert clasts, and quartz sand grains, and called upon a reasonable combination of terrestrial geologic processes to have formed it. As an historic note, my former colleague, Willard Puffet, showed these outcrops to me in September 1967 on one of my first days of employment with the USGS in the Marquette Field Office. He asked if I could help explain these unusual features. Fortunately, he didn’t give me a deadline. Our current interpretation of the nature and origin of the Sudbury layer at the McClure site is based on examination of the outcrops and standard thin section petrography of a suite of samples collected at a regular interval across the unit. The definitive microscopic evidence for a link between the breccia bed at McClure and a major impact is the documentation of shock metamorphic features within it. A small percentage of the quartz grains within the breccia matrix contain relict planar deformation features (pdf’s) indicative of the extreme pressures generated instantaneously during a hypervelocity impact (Figure 5.4 A,B). There are no terrestrial processes capable of generating pressures remotely within the range needed to form these distinctive features. Figure 4 illustrates two examples of quartz grains with two sets of relict pdf’s. These planar features were originally lamellae of impact-generated glass resulting from breakdown of the quartz lattice along preferred crystallographic planes by extreme shock pressures. Over time the glass has recrystallized to quartz, but has left behind planes rich in inclusions, relict pdf’s, that mark the original shock lamellae. At the McClure locality the identification of true pdf’s is complicated by the occurrence of extraordinarily abundant Bohm lamellae, features produced by terrestrial deformation (sometimes referred to as metamorphic deformation lamellae). Apparently the temperature and pressure of deformation was optimum for development of these lamellae. Like pdf’s they occur as parallel lamellae within quartz grains (Figure 5.4 C,D) and can be difficult to distinguish with certainty from pdf’s. The most characteristic Bohm lamellae are thin planar features in which the quartz lattice has been slightly distorted so that the lamellae have extinction angles that vary by a few degrees from the host grain (seen best in Figure 5.4 C). Bohm lamellae are commonly somewhat curviplanar in contrast to unvaryingly planar pdf’s, and also commonly develop at approximately right angles to boundaries between individual crystallographic domains within strained quartz grains (also seen well in Figure 5.4 C). Bohm lamellae are also common in quartz grains in the underlying quartzite and greywacke so seem clearly to have developed in situ during deformation of the host rocks and are not related to the Sudbury impact. 118 �Figure 5.2. Geologic map of the area near the McClure site. Modified from Puffett (1974). 119 �Figure 5.3. Detailed map of the McClure site. Modified from Puffett (1974). Note that the location of County Road 510 is that prior to relocation in 2007-08. The new road is not shown. A cross section of the unit and modal compositions are shown in Figure 5.5. The most distinctive feature of the layer is the coarse chert breccia that makes up approximately the lower half of the unit. The breccia grades upward both in size and abundance of clasts, mostly chert. The basal unit is a framework of chert slabs up to a meter long surrounded by a matrix largely of clastic material and subordinate altered glass particles (Fig 5.6A). This grades up into matrix supported breccia (Figure 5.6 B,C,D,E) in which accretionary lapilli occur sparsely (Figure 5.6C). Clasts generally show little or no preferred orientation, but locally (Figure 5.6D) are well aligned. Most large clasts are chert, at least partly derived from the underlying iron-formation, but some phases have abundant exotic fragments, apparently volcanic rocks (Figure 5.6D). In some outcrops, many chert clasts have an alteration rim (Figure 5.6E) suggesting reaction between the clast and matrix. As shown by Figure 5, the breccia matrix has relatively constant composition expressed as the percentage of clastic quartz sand grains, altered glass particles, and fine groundmass. Glass particles account for 35-40% of the matrix. The glass particles are now mostly chlorite (Figure 5.7 A, B, C, E) in which relict vesicles are common. Many vesicles are flattened indicating considerable post-depositional distortion. Some particles have a complex intermixing of compositions (Figure 5.7A), possibly a result of immiscible melts. Other rock types, such as the quartzite clast in Figure 5.7F, are rare. The groundmass in the breccia matrix is aphanitic, 120 �apparently of felsic composition, and clouded with uniformly distributed opaque grains. Its nature is not clear at this point in our studies. Figure 5.4. A and B- quartz grains with two intersection sets of relict planar deformation features expressed by abundant fine inclusions. These are definitive indicators of intense shock pressures. C and D- Quartz grains containing Bohm lamellae showing slight variations in extinction angles from host grains, curvilinear nature, and right angle intersections with boundaries of deformation zones in strained grains (best seen in C). The upper part of the layer, beginning about 25 meters above the base, is a massive dark gray to black impure sandstone. Angular chert pebbles are sparse and much less abundant than in the underlying breccia. Glass particles are common but less abundant than in the breccia and average about 20% of the rock (Figure 5.7D). Rounded to subangular quartz grains are more abundant than in the underlying breccia and make up roughly 35% of the sandstone. The groundmass also differs from that of the breccia and appears to be fine clastic particles with a wide range of grain size in contrast to the very uniform groundmass of the breccia. The contact between the lower breccia and upper sandstone appears to be gradational over several meters. 121 �Figure 5. 5. Cross section of the Sudbury impact layer at McClure showing variations in the modal composition of the matrix of the lower breccia and the upper sandstone. The upper contact of the Sudbury layer with overlying black slate can be seen in very small exposures in the bed of the intermittent stream that is subparallel to Co. Rd. 510. When the stream is flowing, these outcrops are largely below the shallow water. The contact appears to be gradational over a meter or two in which fine-grained sandstone gives way to laminated carbonaceous slate. Interpretation Several features of the Sudbury impact layer at the McClure site provide clues to the processes responsible for its deposition: 1) Shock metamorphic features provide verification that it contains ejecta from a major extraterrestrial impact. Independent age constraints place the time of deposition within a roughly 40 million year time window that includes the 1850 Ma Sudbury event. No other major impact events of that age are known in the region, so a link to the Sudbury impact is deemed very likely. 2) The massive, graded and poorly sorted nature of the deposit and complete lack of internal bedding or laminations suggest the entire 40 m thickness records a single depositional event. 3) The high energy deposition indicated for the Sudbury layer is in sharp contrast to the very low energy environments indicated for the underlying even-bedded iron-formation and overlying laminated black slate. So deposition appears to be a unique instantaneous event. Both the underlying and overlying units were deposited in a marine setting with water depths greater than the depth of wave action. 4) The abundance of rounded quartz and chert sand grains throughout the unit indicates that, in addition to material ejecta from the crater at Sudbury, the unit contains a substantial component 122 �of material that was acquired by erosion of surficial materials that existed between Sudbury and the McClure site. 5) The abundance of altered particles of glass, a very high percentage of which are highly vesicular and of mafic composition, and have complex delicate shapes suggests that the particles were not derived by erosion of older volcanic rocks, which would have produced a variety of textures and compositions, but rather formed from solidification of highly gas-charged impactgenerated melts and acquired their present shapes in situ. 6) The very coarse breccia at the base of the unit, consisting of meter-scale slabs of the underlying iron-formation, indicates that the onset of deposition was a very high-energy event. Although studies of the Sudbury layer here are still in the early stages, a preliminary interpretation is presented based on current observations. Deposition began in relatively deep quiet water on a substrate of banded iron-formation. The basal beds are a result of highly energetic disruption of the iron-formation and may have been produced either by erosion caused by a fast-moving mass of ejecta or by seismic disruption of the surface sediments moments before the arrival of ejecta. The seismic shock wave generated by the impact would have arrived here within a minute or two after the impact, whereas the first ejecta may have arrived a few minutes later. Spaces between iron-formation slabs are filled with a mixture of clastic grains, particles of altered glass, and sparse accretionary lapilli indicating that the ejecta arrived while there was open space between the slabs. The remainder of the unit at McClure may record deposition from a single turbidity flow. Numerous numeric models of giant impacts have been published in recent years and all predict a rapid expansion of an ejecta cloud or ejecta curtain consisting of solid rock, impact melt, and vapor. Horizontal velocities of thousands of kilometers per hour are predicted. As this high velocity mass returns to the Earth’s surface, it continues to move at high velocities as a ground surge. This surging mass is capable of eroding and transporting surficial material and eventually incorporating it into hybrid deposits consisting both of ejecta and the eroded surficial materials. The mixture of ejecta material at McClure with quartz and chert sand and larger rock fragments, largely chert derived from the nearby iron-formation, suggests that a ground surge played a significant role in its formation. A less certain aspect of the interpretation is how a ground surge would have interacted with the ocean water that covered the area at the time. Did the surge ride atop the water column and eventually sink through it as it lost velocity, or did the entire water column become part of the surge. If the basal breccia is a result of erosion by the surge, then the water column must have been incorporated into the surge. If the breccia is a result of seismic disruption and later infiltration by ejecta an ocean-overriding mechanism is possible. Much additional work is required to understand the intriguing features so well exposed at the McClure locality. 123 �Figure 5.6. A- Coarse breccia at base of Sudbury layer containing meter-scale slabs of the inderlying iron formation. B- Typical lower breccia containing chert fragments supported in a matrix of sand-sized quartz grains and fragments of altered glass. C- Accretion lapilli in matrix of lower breccia. D- Elongated chert fragments showing preferred orientation. E- Chert fragment in lower breccia showing alteration rim. F- Lower breccia with an unusually high abundance of exotic (non-chert fragments. Coin is US penny in A-D. 124 �Figure 5.7. Photomicrographs of samples from the McClure site. A-Complex glass particle from lower breccia with two distinct compositions, possible immiscible melts. B- Complex fiamme of highly vesicular glass, now largely chlorite (trending upper right to lower left). C- Lower breccia matrix with numerous flattened particles of vesicular glass, now largely chlorite. D- Upper sandstone with numerous particles of altered glass, now largely chlorite; E- Vesicular glass particle in lower breccia and aphanitic matrix. F- Grain of quartzite in matrix of lower breccia. Note the abundance of rounded quartz grains in all samples. 125 �References Clark, L.D., Cannon, W.F., and Klasner, J.S., 1975, Bedrock geologic map of the Negaunee SW Quadrangle, Marquette County, Michigan: U.S. Geological Survey Geological Quadrangle Map GQ-1226, scale 1:24,000. Puffett, W.P., 1974, Geology of the Negaunee Quadrangle, Marquette County, Michigan: U.S. Geological Survey Professional Paper 788, 53 p. 126 �54th Annual Institute on Lake Superior Geology Field Trip 6 SUSTAINABLE RECOVERY OF IRON FROM THE MARQUETTE DISTRICT Glenn Scott, Helene Lukey, Al Strandlie, and CCI/CCMO staff Cleveland Cliffs Inc. 127 �Geology, Ore Processing and Reclamation at the Cleveland-Cliffs Michigan Iron Mines Ishpeming, Michigan 54th Annual Meeting of the Institute on Lake Superior Geology May 10, 2008 WELCOME 128 �Welcome to Cleveland-Cliffs Michigan Operations! In the early discussions of field trips for this conference, Ted Bornhorst suggested the inclusion of the processing plants in addition to the “standard” geologic tour. As we talked, this expanded into something with a decidedly broader scope to include a wide range of environmental quality and reclamation topics. Therefore, in contrast to the Institute field trip in 1999, this excursion does not focus on the geologic details of the Negaunee iron formation and the ore. Instead, the attention will be on the mining, processing and associated environmental aspects through to closure. Cleveland-Cliffs is proud of the environmental quality efforts at the Michigan Operations and we are looking forward to this opportunity to host the Institute. Please remember safety at all times by wearing your protective equipment, watching for hazards and paying attention to the suggestions of the guides We would like to thank Cleveland-Cliffs Inc and Cleveland-Cliffs Michigan Operations for supporting this visit. Special thanks to John Klasner for time and effort as editor. Field guides are: Helene Lukey Al Strandlie Al Koski Keith Kramer Karla Brudi John Meier Enjoy the tour – Glenn Scott FIELD TRIP STOPS NOTE: Unfortunately, CCI policy requests that visitors do not take photographs. 129 �Empire Pit Service building Pit operations for both the Empire and Tilden Mines are coordinated from the Empire pit service building. Empire overlook - Al Strandlie The view is to the north with the Empire Main Pit syncline plunging to the west. The CDV pit is to the northwest with the depleted CDI and CDII pits further north. Attachment 6-A Hematite overlook - Helene Lukey From the southwest end of the Tilden hematite pit, the mining operation and major geologic features can be seen (weather permitting). The view is to the east with the fault contact between the iron formation and the Archean to the south, with the martite and carbonate ore operations below. The large hill to the northeast shows a cross section of the intrusives on the north limb of the anticline. To the north, the “Slot” leads along strike to CDIII and the magnetite deposit. Attachment 6-A Ore Stockpiles If the pit access is such that only drive by viewings of the ore sites are possible, samples of the primary ore types can obtained at the stockpile area. Rock Stockpile reclamation - Al Koski As part of the reclamation plan, the rock (waste) stockpiles are being vegetated on an ongoing basis. The results can be seen in several areas; time and access will determine the exact locations. Attachment 6-B Tilden Plant - Keith Kramer The processes are described in the Attachment 6-C. The plant metallurgists and operators will lead the tour. 130 �Empire Tailings Basin - Gary Goodman The rejected material from the concentrating process is pumped to the tailing storage basins. Construction, maintenance and water balance will be discussed by Cliffs personnel as illustrated in Attachment 6-D. Access will be determined by weather and road conditions. Republic Wetlands Preserve - John Meier The Republic Wetlands Preserve is portion of Cleveland-Cliffs mitigation of impacts of the mining operations. The tour will be led by a representative of Cliffs Technology Group. The preserve is described in Attachment 6-E. Access will be determined by weather and road conditions. 131 �INTRODUCTION Cleveland-Cliffs Inc has been active on the Marquette Range since 1847 and has operated a series of underground and surface mines. Production in the early years was of from high grade natural ores but since 1967 production has been from low grade iron formation as pellets. The Marquette Range production began in 1846 on natural ores and pellet production began in 1956 (Boyum, 1979). Total production of the now depleted natural ore was over 300 million tons and pellets exceed 500 million tons. Pellet production has come primarily from the now exhausted Humboldt and Republic Mines and the presently operating Empire and Tilden Properties. On this tour, we will first visit the Empire and Tilden pits to observe the mining operations and examine the iron formation. We will then tour the Tilden concentrator and pellet plant followed with a visit to the active Empire tailings basin. Time and access permitting, we can view and discuss the reclamation of the rock stockpiles. The trip will conclude at the Republic Mine to view the reclaimed plant site and tailings area and to compare these to the active operations. AS WE WILL BE IN OR AROUND ACTIVE WORKING AREAS, PLEASE BE AWARE OF MOBILE EQUIPMENT AND OF THE POSSIBLITY OF SLIPS AND TRIPS. OVERVIEW The Tilden and Empire Mines are operated by Cleveland Cliffs and are located in the Upper Peninsula of Michigan, about 30 kilometers from the shore of Lake Superior (Figure 6-1). In the Lake Superior region, Tilden is unique in that the principle production (75%) is from a hematite deposit. The flotation process is complicated and can be sensitive to variations in mineralogy, chemistry and morphology of the iron and gangue minerals. The flotation ores are typically referred to as ‘hematite’. The actual minerals present and concentrated are hematite (both as martite and microplaty), magnetite, goethite/limonite and various carbonates including siderite, ankerite and dolomite. The common gangue minerals are quartz, chlorite and clays. Phosphorous occurs as apatite. Magnetite mineralogy is simpler as nonmagnetic species are (mostly) rejected in the concentrating process. Gangue minerals are quartz, hematite and carbonates. The 35% crude iron is upgraded to 65% before pelletizing. Annual production capacity is 8 million tons of pellets from 20 million tons of crude ore. Total production to date is 394 million tons of ore and 149 million pellet tons; published reserves are 717 million tons of ore and 260 million tons of pellets. Empire processes only magnetite with a capacity of about 5 millions tons of pellets per year. Total production to date is 777 million tons of ore and 220 million tons of pellets. 132 �Primary ore and waste parameters are crude to pellet weight recovery; concentrate chemistry (silica, phosphorous) and crude iron. These data are based on rather involved laboratory tests (Table 6-I) which may not directly reflect the plant response. REGIONAL GEOLOGIC SETTING The regional structures (Figure 6-2) are the Niagara Fault Zone, the collision zone between the Wisconsin Magmatic Terrane and the Superior craton (Schneider et al, 2002), and the Great Lakes Tectonic Zone, which forms the boundary between Archean granite-greenstone and gneissic terranes (Sims et al, 1980). In the Marquette Range area, deformation along the Great Lakes Tectonic Zone evolved from extension and deposition (Schneider et al, 2002) to closure and transpressional deformation and basin inversion (Cambray, 2002). The resulting faultbounded shallow west plunging asymmetric syncline contains a series of second-order growth fault basins that define the detailed stratigraphic variations. The Paleoproterozoic rocks in Michigan are termed the Marquette Range Supergroup (Cannon and Gair, 1970) and consist of three fining upward sequences (Map in Pocket). The lower portion has been correlated with the upper part of the Ontario Huronian (~2.2 Ga) and the upper parts, which contain the 1875 Ma iron formations, with the Mesabi Range of Minnesota. Simplistically, the sequence is from the Chocolay Group shelf facies quartzites and dolomite to the Menominee Group with argillites and the major iron formations to turbidites, greywacke and shale along with minor iron formation in the Baraga Group (Figure 6-3). Mafic igneous rocks with a continental tholeiite geochemical signature (Schulz, 1983) are present in the Menominee and Baraga Groups. Basal quartzites in each sequence are used as local structural and stratigraphic marker horizons. Metamorphic grades vary from sillimanite in the west to chlorite in the east and at the Tilden Mine (James, 1955) Negaunee Iron Formation and equivalents hosted the majority of the natural ore deposits and all of the concentrating grade production in Michigan. In the Marquette trough, the Negaunee reaches a thickness of 1300 meters without including the mafic igneous horizons. Due to the lack of correlative iron formation horizons, the igneous rocks, termed sills locally, are used for structural markers. There appears to be a poorly defined change from dominantly carbonate-chert on the north to magnetite-hematite-chert on the south (Waggoner, 2007). LOCAL GEOLOGY The Tilden and Empire Mines are located on the southern margin of the trough and are in fault contact with the Archean gneiss terrane (Attachment 6-A, Figure 6-3and 6-4). Local structure consists of upright to steeply inclined second order anticlines and synclines with low angle northwest and southwest plunges (Cambray 2002, Webster, 1999). At Tilden, due to the lack of clear marker horizons and rapid facies changes within the iron formation, igneous horizons are used for stratigraphic and structural correlation (Lukey, Johnson and Scott, 2007). At Empire, stratigraphy is determined by the igneous horizons and by the proportions of carbonates, silicates and clastics in the iron formation (Nordstrom, 1997; Han, 1975). See Figures 6-4 through 6-9 for mine geology. 133 �GEOLOGIC DOMAINS The domains are defined by geologic and metallurgical consistency (Table II, III) and are the basis for the resource modeling (Scott and Lukey, 1999; Nordstrom, 1999). Magnetite deposits are less variable (or perhaps the process is more forgiving). As there is no type example of iron formation within the mine, it is problematic if the mineralogic and textural variations reflect deposition in growth fault basins, diagenesis or hypogene events. IGNEOUS ROCKS Two ages of mafic rocks occur in the mine, the synsedimentary sills and associated dikes and a dike series of Keweenawan (~1000 Ma) related to the Midcontinent Rift. The older series vary from fine porphyritic to diabasic/ophitic and typically display chlorite-carbonate alteration assemblages, particularly in deformation zones. The younger series are typically unaltered diabase. The iron formation is variably altered along the intrusive contacts with the type and extent of alteration dependent on the thickness of the intrusive and the composition of the iron formation. FOLDING/FAULTING The major structures are the large (100s meters) scale Tilden Main pit anticline and Empire Main pit syncline; the fault that marks the contact of the Southern Complex and the iron formation; and the CDIII syncline. Smaller features (Figure 6-4) are the Section 20, CDI, II and V mining areas. Present geometry of these features is related to transpression during basin closure (Webster, 1999). The fault, initially a basin margin listric normal fault, was reactivated and is now a reverse fault that dips about 65° north (Cambray, 2002). At blast pattern level, faults and folds at the 1-20 meter scale tend to follow the trends seen in the larger structures. These features, while of relatively small amplitudes, can be significant in the detail block modeling and ore type boundaries. 134 �MINING AND PROCESSING The mining, concentrating and pelletizing processes are described in some detail in Attachment 6-B. The sequence begins with a scheduled mine plan and blast design. Holes are drilled, sampled and blasted. Broken ore is loaded into trucks to be transported to either the crusher and waste is taken to a rock stockpile. After initial crushing, the ore enters a series of autogeneous mills where it is ground to ~80% -31 microns (face powder) before the iron minerals are separated from the gangue. The magnetite process relies primarily on mechanical separation using the magnetic properties of the minerals. Hematite is processed by flotation and relies on chemical reagent selectivity. The tailings are pumped to the tailings basins and the water returned to the process (Attachment 6-D). Pelletizing is essentially the same in either ore type. The concentrate is “rolled’ into “green balls’ which are fired in the kilns to harden them for shipment. The kilns are heated by a combination of coal and natural gas. ENVIRONENTAL QUALITY AND RECLAMATION During operations, discharges of materials that might be harmful to the environment are monitored and prevented. This includes discharges into the water and air such as trace chemicals and particulate matter from stacks, tailings and roads. As stockpiles are completed the reclamation process is begun with introduced vegetation (Koski, 2007, Attachment 6-B). An example of closure reclamation will be seen at Republic Mine (Attachment 6-E). References Boyum, BH, 1979. The Marquette district of Michigan, 2nd edition, The Cleveland-Cliffs Iron Company: Ishpeming Cambray, FW, 2002. The evolution of a Paleoproterozoic plate margin, Northern Michigan, field trip guide for the Great Lakes Section–SEPM 32nd annual fall field conference Cannon, WF and Gair, JE. 1970, A revision of stratigraphic nomenclature of middle Precambrian rocks in northern Michigan, Geological Society of America Bulletin, 81: 2834-2846 Han, TM. 1975, Lithology, Stratigraphy and Petrology of Iron Formation at the Empire Mine, in Gair, JE, Bedrock Geology and Ore Deposits of the Palmer Quadrangle, Marquette County, Michigan, USGS PP 769, pp. 76-106 James, HL. 1955, Zones of regional metamorphism in the Precambrian of northern Michigan, Geological Society of America Bulletin, 66: 1455-1488 135 �Koski, AE. 2007, Reclamation of waste stockpiles at Cleveland-Cliffs Michigan Operations, n National Meeting of the American Society of Mining and Reclamation, June 2-7, 2007. Published by ASMR, 3134 Montavesta Rd., Lexington, KY 40502. Lukey, HM, Johnson, RC and Scott, GW. 2007, Mineral Zonation and Stratigraphy of the Tilden Hematite Deposit, Marquette Range, Michigan, USA, in Proceedings Iron Ore Conference2007, pp. 123130 (The Australian Institute of Mining and Metallurgy: Melbourne). Nordstrom, PM. 1999, Geologic field trip to the Empire Mine, in Institute on Lake Superior Geology Proceedings, 45th Annual Meeting, Marquette, MI, v. 45, part 2, p.129-134 Sims, PK, Card, KD, Morey, GB and Peterman, ZE. 1980, The Great Lakes tectonic zone – A major crustal structure in central North America, Geological Society of America Bulletin, Part 1, 91:690-698 Schneider, DA, Bickford, ME, Cannon, WF, Schultz, KJ and Hamilton, MA. 2002, Age of volcanic rocks and syndepositional iron formations, Marquette Range Supergroup: implications for the tectonic setting of Paleoproterozoic iron formation of the Lake Superior region, Can. J. Earth Sci. 39:999-1012 Schulz, KJ. 1983, Geochemistry of the volcanic rocks of northeastern Wisconsin [abs.], in Proceeding of the 29th Annual Institute on Lake Superior Geology, Michigan Technological University, Houghton Scott, GW and Lukey, HM. 1999, Geologic field trip to the Tilden Mine, in Institute on Lake Superior Geology Proceedings, 45th Annual Meeting, Marquette, MI, v. 45, part 2, p.114-128 Waggoner, TD. 2007, Personal communication 18 January Webster, CL. 1999, Structural analysis of a ductile shear zone within the Marquette Iron Range, Upper Peninsula, Michigan, MS thesis (unpublished), Michigan State University, East Lansing 136 �Table 6- I Glossary of Terms and abbreviations used at the mine and plant Natural Weight Recovery – The amount of material recovered from the material fed into the concentrator circuit. In other words, it’s the tons of concentrate made (measured as filter cake) from tons of crude ore used (measured by #3 belt scale) Metallurgical Weight Recovery (Met. Wt. Rec.) – Calculated by comparing the iron losses (as tailings) with the iron content of the crude ore fed into the concentrator (i.e., the head Fe). The formula used for this calculation is called the iron balance formula, or sometimes called the concentration formula. %Wt. Rec. = (Head Fe – Tail Fe) --------------------------- x 100 (Grade – Tail) Grade - Also called the concentrate grade, is a chemical measurement (assay) of the total iron oxide of the concentrate. Iron oxide is found in iron minerals such as hematite (Fe2O3), magnetite (Fe3O4), goethite (Fe2O3*OF) and iron carbonate (FeCO3). Concentrate Silica Grade - The chemical measurement (assay) of the % SiO2 in the concentrate. When a lower concentrate silica grade is achieved, the losses in iron units (tailings) increases. Head Grade - The assayed iron content of the crude ore fed into the concentrator circuit. Iron Recovery (Fe Rec.) – A calculation of the efficiency of the concentrator’s ability to recover the iron available. This is calculated by comparing the Met. Wt. Recovery, at some iron grade, with the head Fe of the crude. For example, %Fe Rec. = (Met. Wt. Rec. x Grade % Fe) --------------------------(Head % Fe) 137 �Percent Magnetic Iron Recovery (% Mag Fe Rec.) – The calculation of the efficiency of recovering the magnetic iron that was in the feed (crude ore). The Met. Wt. Rec., at some iron grade is compared with the magnetic potential (i.e. head) o the crude ore. For example, %Mag Fe Rec. = (Met. Wt. Rec. x Grade % Fe) --------------------------(Head Mag % Fe) Tailings - The product lost in the process. Tailings always include iron, because iron is always associated with many other minerals (silica, phosphate, carbonate, etc. or may be a liberation issue). Flot (flotation) – Flot ores are the martite, hematite, goethite, clastics and carbonates that are treated by selective chemical processes to achieve Fe and silica grade. The final stage of the magnetite process is flotation to achieve target silica grade. WIF (waste iron formation) – Iron formation that due to low weight recovery and/or high silica cannot be treated in the plant to produce economic concentrate. Rarely, phosphorous levels are too high to be treated. Magnetic Iron - The percent of the crude iron that is concentrated in the Davis Magnetic Tube Test (DMTT). %MagFe = DMTT Wt. Rec. x DMTT Grade The assumption is that all of this occurs as magnetite. However, in the Tilden ores an appreciable amount of hematite is locked up with the magnetite and is carried into the DMTT concentrate. This tends to over estimate the magnetic Fe content by 1-2% points and therefore overestimate the weight recovery. Satmagan - The Satmagan magnetic iron content is measured using susceptibility and is the actual magnetite content of the crude or concentrate. Domain – The deposit is divided into volumes of material with similar metallurgical response. These are usually stratigraphic horizons but may be fault bounded or nonconformable alteration/oxidation zones. The domains are the basis of the economic and planning models. 138 �139 �140 �141 �142 �143 �144 �145 �146 �147 �148 �149 �150 �151 �152 �153 �154 �155 �156 �157 �158 �159 �160 �161 �162 �163 �164 �165 �166 �167 �168 �169 �170 �171 �172 �173 �174 �175 �176 �177 �178 �179 �180 �54th Annual Institute on Lake Superior Geology Field Trip 7 GEOLOGY OF THE KEWEENAWAN BIC INTRUSION Dean Rossell Kennecott Minerals Company 181 �The Geology and Geologic Setting of the BIC Cu-Ni-PGE Prospect, Baraga County, Michigan U.S.A. Introduction The BIC mafic/ultramafic intrusion is located in Baraga County, Michigan, approximately 8 km southeast of the town of L’anse, Michigan. The roughly 1.1 km by 0.4 km, oval shaped intrusion forms a prominent hill with good exposures of the principle units that comprise the intrusion. The BIC intrusion has not been dated yet. However, based primarily on compositional similarities, Kennecott geologists believe it is similar in age to the mafic/ultramafic intrusion that hosts the Eagle Cu-Ni-PGE deposit, located ~35km to the east (fig 1), which has been recently dated at 1107.2+/- 5.7ma (Ding, 2007) The BIC intrusion has been the target of periodic exploration by Kennecott Exploration Company since the first discovery of Cu-Ni-PGE mineralized boulders near the intrusion in the mid-1990’s. The first drill hole into the intrusion, in 1995, was positioned at the south edge of the intrusion. The hole (BIC95-1, fig. 3) intersected ~3 m of disseminated sulfide mineralization in olivine melagabbro at the base of the intrusion, averaging 0.43%Cu, 0.32%Ni, 0.325ppm Pt and 0.345ppm Pd. Figure 1) Geology map of the northern portion of the Upper Peninsula of Michigan showing the location of the Baraga Basin and the BIC intrusion. Modified from Gregg (1993) 182 �No significant Cu-Ni-PGE resource has been identified at the BIC prospect yet. However, a drill hole completed by Kennecott Minerals Company in 2006 (07BIC-007), intersected 16.47m averaging 0.88%Cu, 1.00%Ni, 0.679ppm Pt, 0.991ppm Pd and 0.104ppm Au . This interval included a 2.8m interval with bands of massive sulfide, located in the meta-sediments immediately below the base of the intrusion, which averaged 1.66%Cu, 4.23%Ni, 1.383ppm Pt and 2.521ppm Pd. The metal tenor of the massive sulfide bands is comparable to some of the massive sulfides in the Eagle deposit. This could suggest that there is still some potential for a high grade massive sulfide body in the less explored portions of the BIC intrusion. Previous Geologic Studies No detailed geology map covers the area immediately around the BIC intrusion. The geology shown in Figure 2 is, in part, modified from data included in the USGS 1:62,500 scale open file geology map of the Precambrian geology of the Dead River, Clark Creek and Baraga Basins (Cannon, 1977). The area in figure 2 is also covered by the Iron River 1º x 2º quadrangle (Cannon, 1986). Geology in the Taylor Mine area (fig. 2) is compiled and modified from detailed mapping by Klasner (1972) and Klasner and others (1991). Ojakangas (1991) discussed stratigraphic correlations of Paleoproterozoic rocks in the area shown in figure 2. Gregg (1991) and Klasner and others (1991) described Penokean age deformation in the same area. The Archean geology to the southeast of the BIC intrusion is described in an unpublished master’s thesis by Turner (1979). A review of the Paleoproterozoic stratigraphy in the Baraga Basin, including the Taylor mine area, was recently undertaken by Gabe Nelson as part of a Masters thesis at Acadia University under Pier Pufal. The above data sources were supplemented by periodic reconnaissance mapping by me during the period 1999-1996. This work was augmented by regional geophysical studies and drilling programs carried out by personnel of Kennecott Exploration Company, Kennecott Minerals Company and various contractors. The more detailed geologic data from the BIC area is compiled from work by me, other Kennecott Exploration and Kennecott Minerals geologists, contract geologists and reports on petrography completed for Kennecott by Barnett (1995), Hauck (2001) and Johnson (2007). Regional Setting The BIC intrusion cuts Paleoproterozoic sediments in the southwestern portion of the Baraga Paleoproterozoic sedimentary basin (fig 1). The Baraga basin is bounded to the north and south, and underlain by Archean crystalline rocks. The Baraga basin merges with the Paleoproterozoic sediments of the Marquette Syncline southwest of the BIC intrusion (fig 1). The Archean, Paleoproterozoic and Mesoproterozoic geology is briefly summarized below. Archean The Archean terrane to the immediate south of the BIC intrusion (fig.2) is comprised largely of coarse grained, felsic gneiss and lesser amphibolite intruded by a variety of small mafic to ultramafic intrusions. Although there has been little mapping to confirm it, the gneissic rocks are most likely a continuation of the gneiss, intrusions and lower metamorphic grade supracrustal 183 �rocks (Marquette Greenstone Belt) that collectively comprise the Northern Complex (fig 1) to the east. A tonalitic intrusion dated at 2703 Ma and a rhyolite dated at 2780 Ma (Sims, 1993), are the only available age dates from the Northern Complex. Paleoproterozoic The recent discovery of the Sudbury ejecta horizon in the Baraga Basin (see below) constrains the bulk of Paleoproterozoic sedimentation to post 1850ma. Gregg (1993) divided the Baraga basin into two principle structural domains; the northern Huron River parautochthon and the southern allochthonous Falls River slice. Gregg proposed the boundary between the terrranes, which is marked by an abrupt change in structural style, is a south dipping thrust fault that he named the Falls River Thrust (fig. 2). Paleoproterozoic sediments to the north of the Falls River Thrust are characterized by weakly asymmetrical, relatively open folds with shallow axial plunges to the northwest or southeast. A single, southwest dipping, axial planar foliation is evident in most pelitic and siltstone horizons. Immediately south of the Falls River Thrust, folds are tight to isoclinal, generally overturned and often recumbent. In the Falls River slice, larger scale folds are overprinted by a second generation of folds with an associated crenulating foliation that is particularly evident in pelitic sediments. Boudinaged and folded quartz veins and lenses are prevalent in coarser-grained metagreywacke beds in the Falls River slice. Klasner and others (1991) mapped a thrust fault in the Komtie Lake area, south of the BIC intusion (fig. 2). They reported that a vertical exploration drill hole, located on the south side of Komtie Lake, penetrated 30 m of Archean gneiss followed by 3 m of mylonite before intersecting 45 m of Paleoproterozoic sediments. They proposed an approximately east-west striking and south dipping thrust fault that brought Archean gneiss over a thin veneer of the basal Paleoproterozoic sediments. They extended the fault westward to include strongly foliated rocks exposed along Plumbago Creek (fig 2). I extended the Komtie Lake thrust fault further to the northeast in figure 2, to an area where magnetic anomalies originating in the Paleoproterozoic sediments appear to continue under exposures of Archean gneiss. This extension has not been confirmed by mapping. Exposures of pelitic rocks in the immediate area of the Taylor mine (stop 3, fig. 2) generally lack the prominent crenulating cleavage seen in pelitic rocks exposed all along Taylor Creek further to the north (stop 4, fig. 2). Drill hole T-5, a 68.5 m deep vertical exploration hole collared northeast of the Taylor mine pit (fig. 2), bottomed in mylonitic rock. I propose that there is another generally east-west striking thrust fault north of drill hole T-5, separating the overriding Taylor Mine slice from the more deformed rocks of the Falls River Slice. Alternatively, the fault could be the westward continuation of the Komtie Lake thrust fault. Historically, deformation of the Paleoproterozoic sediments in the western portion of the Upper Peninsula has been attributed to a series of collisional events between 1888 Ma and 1830 Ma that collectively make up the Penokean orogeny (Schultz and Cannon, 2007). However, Schultz and Cannon (2007) point out that there is evidence of vertical faulting and uplift that significantly post date1830 Ma. They concluded that this younger deformation cannot be attributed to the Penokean orogeny and that it is more likely of Yavapai age. 184 �Mesoproterozoic Mesoproterozoic flood basalts associated with the Keweenaw Flood basalt Province are exposed along the length of the Keweenaw Peninsula and 30km southwest of the BIC intrusion at Silver Mountain, Michigan. The Keweenaw Flood Basalt province represents the exposed portion of the Midcontinent Rift system in the Lake Superior region. The Midcontinent Rift forms a prominent gravity anomaly that can be traced from the Lake Superior region southwest into central Kansas, and southeastward into southern Michigan. The total length of the geophysical feature is in excess of 2000 km (Hinze and others, 1997). Seismic data indicates the rift below Lake Superior is filled with more than 25km of volcanics buried beneath a total thickness of up to 8km of rift filling sediments (Bornhorst and others, 1994). The estimated volume of magmatic rocks associated with the rift is greater than 2 million cubic kilometers (Cannon, 1992). The Keweenaw Flood Basalt province was formed over an approximately 23 million year period, from ~1111 Ma. to ~1089 Ma. Volcanism was bimodal, but with preserved basaltic rocks much more abundant than rhyolitic rocks. Volcanism occurred in two distinct phases, with an approximately 5 million-year hiatus between phases (Miller, 1996). In Michigan and Wisconsin, the early phase volcanics are comprised of the Sieman’s Creek formation and volcanics of the Powdermill group (Wiband and Wasuwanich, 1980). The Portage Lake volcanics comprise the younger phase. The early phase volcanics are primarily reversely polarized. The Portage Lake volcanics are normally polarized. A mantle plume model has been widely evoked to explain the staged evolution and large volume of magmatic products associated with the Midcontinent Rift (Nicholson, 1997). Red bed sandstones (Jacobsville Sandstone) shed off the horst block formed during inversion of the Midcontinent Rift, cover Paleoproterozoic sediments west of BIC (fig. 2). Rift inversion may have begun as early as 1080 Ma and was completed by about 1040 Ma (Cannon, 1994). The probable cause of compression was continental collision in the Grenville province (Cannon, 1994). Paleoproterozoic Stratigraphy Archean rocks are either unconformably overlain by, or in fault contact with, Paleoproterozoic meta-sediments along the southern margin of the Baraga Basin. Ojakangas (1994) has correlated sediments in the Baraga Basin and western Marquette trough with the Baraga Group, the youngest of the three dominantly clastic sedimentary groups that comprise the Marquette Range Supergroup. He concluded, on the basis of paleocurrents, paleogeographic setting and isotopic data that the best tectonic model for Baraga Group sedimentation is a northward migrating foreland basin. Quartzites at the base of the Paleoproterozoic sedimentary sequence in the Baraga basin north of the Falls River thrust and in the Canyon Falls area (stop 1-fig. 2) are correlated with the Goodrich formation by Ojakangas (1994). The basal quartzites at both these localities appear to rest unconformably on Archean basement. The quartzites range from thickly to thinly bedded, with locally well developed planar and trough cross bedding. Quartzites in the Baraga basin are typically arkosic with conglomerate lenses. Ojakangas (1994) proposed that the Goodrich 185 �quartzites were deposited in a tidal environment. In the Baraga Basin, the Goodrich formation ranges in thickness from less than a meter in the eastern portion of the basin, to approximately 40 m in the western portion of the basin (Nelson, 2006). I interpret widely scattered outcrops of similar appearing quartzite exposed along the margins of the Archean to the south and east of the BIC intrusion as equivalents of the Goodrich quartzite described above. However, in most places they appear to be in fault contact with the Archean. Klasner and others (1991) interpreted strongly foliated, quartz rich schists along the north side of Plumbago Creek in the Taylor mine area (fig. 2) as mylonitic textured Archean gneiss. I have examined some of these outcrops and feel they could, in part, be strongly foliated arkosic Goodrich quartzite. The proximity of the sheared “quartzite” with iron formation exposed along the banks of Plumbago Creek has potential stratigraphic implications in the Taylor mine area. The Goodrich formation is overlain by the Michigamme formation, the uppermost formally recognized formation in the Baraga Group. Leith, et al (1935) divided the Michigamme formation into three principle members which, in ascending order are: the Lower Slate member, the Bijiki iron formation, and the Upper Slate member. Kennecott geologists have generally used this nomenclature for describing stratigraphic relationships in the Baraga Basin. However, in the western portion of the Baraga Basin, the Goodrich formation quartzites are immediately overlain by a thin interval (typically less than 20m thick) of inter-bedded chert and iron rich carbonate. Ojakangas (1994) suggested that this cherty horizon may be the equivalent of the Bijiki iron formation and that the Lower Slate member is missing in parts of the Baraga basin. However, Kennecott geologists believe this is a separate unit below the Lower Slate member and informally refer to it as the Chert Carbonate member. That informal designation is used in the rest of this field guide and in figure 2. William Cannon (personal communication) has identified layers with accretionary lapilli, pumice grains and, at one location, quartz grains, with shock lamellae from bedrock exposures and core samples of the Chert Carbonate member in the Baraga Basin. Cannon has proposed that these are ejecta from the 1850 Ma Sudbury impact event and correlated them with other ejecta horizons previously identified in Ontario and Minnesota (Addison et al, 2005). Kennecott drill hole 07BIC-033, the deepest hole completed at the BIC prospect, intersected intervals with probable accretionary lapilli and pumice fragments (Cannon, personal communication) in cherty rocks starting at a depth of 586 m. The likely presence of the Sudbury ejecta layer in the BIC drill hole provides confidence that the more deformed rocks in the southwestern portion of the Baraga basin (south of the L’anse thrust fault in figure 2) are stratigraphically correlative with the rocks in the northern portions of the Baraga Basin. The Chert Carbonate member and Sudbury ejecta layer is overlain by dominantly black to dark gray, thinly bedded, meta-siltstone and pelite in the Baraga Basin. The pelitic rocks are often graphitic and sulfide rich and contain only minor intervals of fine-grained greywacke. As mentioned above, Kennecott geologists believe this is the Lower Slate member of the Michigamme formation. This siltstone-pelite dominated interval increases from 20-90 m in the northern part of the Baraga Basin to thicknesses I speculate might be greater than 200 m in the vicinity of the BIC intrusion. However, structural complexities and insufficient drilling make 186 �accurate determinations of the thickness of this sequence currently impossible in much of the southern portion of the Baraga Basin. In the Taylor mine area (stop 3-fig.2) the Lower Slate member is overlain by the Bijiki iron formation. The Bijiki iron formation is primarily comprised of thinly bedded, black and white chert with lesser siltstone, iron carbonate and iron oxides (Ojakangas, 1994). In the immediate Taylor mine area the Bijiki iron formation ranges from 20-80m in thickness (Ford Motor Company reports). A Kennecott Exploration drill hole, ALB95-3, located approximately 2.7km west of the Taylor mine (fig. 2), intersected 280 m of banded iron formation, with lesser intervals of graphitic slate, starting at a depth of 110 m and continuing to the bottom of the hole. Bedding angles to core, along with the lack of any compelling evidence of fold or fault repetition, suggest that this is likely to be close to a true thickness. A second hole, ALB95-2, collared 1.1 km further to the west, intersected 194 m of iron formation. Both holes were terminated while still in iron formation so the total thickness of iron formation at this location is unknown. Kennecott geologists believe the iron formation in both holes is the Bijiki indicating a rapid westward thickening of the unit. This thicker part of the Bijiki is within a rhomb shaped magnetic and gravity high. The rapid westward thickening of the iron formation, and shape of the coincident geophysical anomalies, might be evidence of a fault bounded, second order basin that formed during deposition of the Lower Slate and Bijiki iron formation. The BIC intrusion cross cuts an approximately 15km long linear magnetic anomaly. Drilling and mapping by Kennecott geologists has confirmed that the linear magnetic anomaly is caused by abundant pyrrhotite in graphitic sediments. The sediments contain numerous thin bands of contorted quartz and 0.5-1cm thick bands and lenses of semi-massive pyrrhotite and pyrite with minor sphalerite and chalcopyrite. The ratio of pyrrhotite and pyrite varies considerably along strike, and within a drill intersection, significantly affecting its magnetic susceptibility. Similar sulfide rich sediments are seen immediately below the Bijiki iron formation at the Taylor mine and in a 25-35m interval immediately above the Bijiki iron formation in drill holes ALB95-2 and ALB95-3 (pyrite rich in hole ALB95-3 and pyrrhotite rich in hole ALB95-2). The author proposes that these sulfide rich, variably magnetic sediments are the continuation of the Bijiki iron formation member northward into the BIC area. However, this important marker horizon has not been identified anywhere else in the northern part of the Baraga basin. The Bijiki member is overlain by the Upper Slate member in the Taylor mine and BIC prospect areas. The Upper Slate member contains a significant percentage of greywacke inter-bedded with siltstone and pelite distinguishing it from the Lower Slate member. Ojakangas (1994) reported that greywacke beds made up 18% of a measured section in the Silver River north of the BIC intrusion. The greywacke beds are commonly graded and contain rip ups and other features indicative of deposition by turbidity currents. Baraga-Marquette Dyke Swarm The Baraga-Marquette dyke swarm is comprised of more than 150 diabase dykes (Green and others, 1987). The primarily east-west trending dikes form a belt that extends from the northern edge of the Baraga basin at least 75 km southward into southern Marquette County. Although 187 �most dykes in the swarm are less than 30 m thick, individual dykes are up to 185 m thick and can be traced for up to 59 km (Green et al., 1987). The majority of the known dykes are reversely polarized, forming prominent magnetic linear anomalies on magnetic maps. None of the diabase dykes have been dated. However, the measured diabase dyke paleomagnetic pole position in the Marquette area is virtually identical to that of reversely magnetized intrusions from the Thunder Bay area (Wilband and Wasuwanich, 1980). Sutcliff (1987) reported an age of 1109ma for the reversely polarized Logan sills in the Thunder Bay area. The dykes typically have subophitic to diabasic textures and contain 50-70% plagioclase, 3050% clinopyroxene and 1% or less olivine and Fe-Ti oxides. Most dykes are relatively fresh with little sign of alteration (Wilband and Wasuwanich, 1980). Most of the reversely polarized dykes have high TiO2 (3-5%), P2O5 (0.30-0.55%) and <15% Al2O3 (Wilband and Wasuwanich, 1980).The dykes also typically have high Cu (300-500ppm) and low Ni (<100ppm) contents (Kennecott data). Interestingly, no reversely polarized dykes are evident in magnetic data sets north of the Falls River thrust fault (fig. 2). This might suggest that the fault played some role in localizing the reversely polarized dykes of the Baraga-Marquette dyke swarm. The BIC Intrusion The BIC intrusion is located about 35km southwest of Eagle and 8km southeast of the town of L’anse, Michigan. The intrusion forms a prominent hill approximately 1100m long by 400m wide. Mapping, geophysics and drilling indicate the intrusion has roughly the same dimensions as the hill at bedrock surface (fig. 3). Although not well constrained along much of the intrusion, based on the drilling completed, the intrusion appears to be generally V shaped in cross section. Drilling and mapping in the eastern portion of the intrusion suggest the southern margin of the intrusion dips moderately to the north (fig. 4). Knowledge of the northern contact is limited, but it appears to be steeply, south dipping. A much smaller, shallow bowl shaped intrusion, referred to as Little BIC, was located just to the northwest of the BIC intrusion during 2006 drilling (fig. 3). The smaller intrusion is comprised mostly of relatively olivine rich lithologies very similar to those seen along the base of the main BIC intrusion. This smaller intrusion could be a fault offset of the larger BIC intrusion, or possibly a separate intrusion. The best mineralized intersections in drilling completed through 2007 have primarily come from this smaller intrusion. Unlike the intrusion hosting the Eagle ore body, the BIC intrusion is distinctly layered. Core logging, thin section work and very limited geochemistry show that the BIC intrusion can be subdivided into three principal units; an upper coarse-grained gabbro, a middle unit comprised of fine-grained gabbro and feldspathic clinopyroxenite, and a lower unit of feldspathic wehrlite and olivine melagabbro. All three units thicken toward the center of the intrusion and thin toward the margins. The following descriptions of the units are summarized from core logs and observations of outcrops and hand samples. Most of the descriptive mineralogy is taken from unpublished 188 �petrography reports prepared for Kennecott Exploration and Kennecott Minerals by Rod Johnson (2007) Steve Hauck (2002), and Bob Barnett (1995). Upper Unit - Gabbro The upper gabbro is the thinnest unit with no drill intersections exceeding 75 m (no upper contact has been located so this is only a minimum total thickness). It is exposed in a few scattered locations on the top of the hill. The best exposures are along the drill roads on top of the hill in the eastern portion of the Intrusion. The upper gabbro is an altered, medium to coarse-grained, oxide gabbro with 55% lath like plagioclase and 35% prismatic or granular clinopyroxene. The gabbro contains up to several percent titanomagnetite, minor apatite and trace olivine. The upper gabbro is moderately to strongly magnetic. Strong alignment of plagioclase laths, which can be up to 2cm in length, and prismatic clinopyroxnene creates a foliation in the gabbro in places. In other places, the crystals radiate, creating a stellate pattern. Small patches of granophyre are present in drill core and outcrop. The upper gabbro is moderately to intensely altered with plagioclase variably altered to sericite and clinopyroxene altered to amphibole and chlorite. Very fine grained hematite coats some plagioclase giving it a pinkish color and titanomagnetite is altered to martite and maghemite. Pyrite occurs as disseminations and rare veins (Hauck, 2002). Football size and shape pods of strong light green, epidote rich rock are common in outcrop and drill core of the upper gabbro. The pods, which have sharp contacts, can form up to 5% of some outcrops. The shape, size and distribution of the pods suggests that they might be preferentially altered xenoliths or autoliths. Middle Unit-Gabbro/Clinopyroxenite The middle unit is comprised of gabbro and clinopyroxenite which forms 3-10m high cliffs around the perimeter of the hill. The middle unit is by far the best exposed unit at the BIC prospect. Intersections in drill core of the middle unit reach 100m in drill holes in the eastern half of the intrusion but it appears to thin to the west. The unit is comprised of fine-grained, equigranular gabbro and feldspathic clinopyroxenite. The upper few meters of the unit is a fine-grained, strongly magnetic equigranular, oxide rich, cumulate textured gabbro with 40-50% granular clinopyroxene and 20-50% granular titanomagnetite and minor ilmenite. Plagiclase content varies, but is typically less than 40% in this oxide rich part. Biotite and amphibole are minor components in the upper portion of the unit. This magnetite rich interval is present in most holes and creates a distinctive spike in magnetic susceptibility profiles in most BIC drill holes (a magnetic profile is shown for hole BIC02-02 in figure 4) Magnetite content decreases rapidly with depth in the middle unit and most of the unit below the first few meters is weakly to non-magnetic. Clinopyroxene content increases downward and in the eastern portion of the intrusion much of the lower part of the middle unit is fine-grained, cumulate textured, feldspathic clinopyroxenite. The presence of cumulate clinopyroxenite is suspected in the western portion of the intrusion but not yet confirmed by thin section work. Alteration is similar to that seen in the upper gabbro with plagioclase largely altered to sericite, carbonate and actinolite and pyroxene is variably altered to chlorite, carbonate and amphibole. 189 �Fine-grained, disseminated chalcopyrite and trace bornite is found through out the unit, generally in trace amounts, but locally up to 0.5%. Minor pyrite and sphalerite are present in western outcrops of the middle unit, in addition to chalcopyrite. Lower Unit- Wehrlite/Olivine Melagabbro Unlike the upper two units, which contain only very rare olivine and orthopyroxene, the lower unit is relatively olivine rich and has up to 5% orthopyroxene in some thin sections. The lower unit is poorly exposed, with just a few outcropings along the south side and none on the north side. The unit is best exposed on the west end of the hill. Drilling indicates it is the thickest of the three units and has a thickness of greater than 200 m in drill hole BIC02-02 (fig 4). The upper portion of the lower unit is comprised of fine grained, moderately magnetic, feldspathic wehrlite and olivine melagabbro with 35-60% cumulate olivine, 10-20% clinopyroxene, 10-34% plagioclase and minor sulfide. Clinopyroxene is either granular or poikolitic on olivine and plagioclase is poikolitic on both olivine and clinopyroxene. Titanium rich phlogopite and amphibole are also minor (1-2%) primary mineral phases. Chromite occurs as grains within olivine and minor titanomagnetite and ilmenite occur as single or composite grains, often subpoikolitic on clinopyroxene. Barnett (1995) reported olivine compositions for outcrop samples of the lower unit that ranged from fo76 to 83. These values closely overlap with the range of fo76 to 85 reported for olivine melagabbro at the Eagle deposit (Ding, 2008). In most holes, olivine content decrease with depth in the lower unit, while clinopyroxene, plagioclase and sulfide increase. In the eastern portion of the intrusion, this change in mineralogy is accompanied by an increase in grain size in the lower 50m of the intrusion. Alteration is moderate to severe in the lower unit with olivine partially to completely altered to either iddingsite or serpentine and fine-grained magnetite. Both plagioclase and clinopyroxene are variably altered to chlorite and carbonate. The alteration tends to turn everything green in the most altered samples, often making visual determination of the primary mineralogy difficult in hand and core samples. Contact metamorphic Aureole Meta-sedimentary rocks peripheral to the BIC intrusion show the effects of low pressure contact metamorphism. Johnson (2007) studied thin sections cut from drill core samples of metasediments peripheral to the BIC intrusion. He divided metamorphic assemblages in the metasediments into a proximal granoblastic hornfels, a more distal porphroblastic spotted hornfels, and a regional green schist assemblage. Within two to three meters of the contact of the intrusion, primary structures and foliations in the meta-sediments are very poorly preserved. The regional metamorphic assemblage is overprinted by a granoblastic assemblage of cordierite, quartz, biotite, vesuvianite and sphene +/- andalusite, sillimanite, kspar and plagioclase. Scattered small pods and veins of coarser grained k-spar and quartz within the granoblastic hornfels suggest localized partial melting of the meta-sediments in close proximity to the intrusion. The granoblastic hornfels grades outward into spotted hornfels which in some drill holes can be recognized in the meta-sediments10 to 15m from the contact with the intrusion. The spotted hornfels is characterized by the growth of small (<0.5 mm) porphyroblasts in phyllosilicate rich 190 �beds. Johnson (2007) reported cordierite, andalusite and sillimanite as the principal prophyroblasts in the spotted hornfels. Johnson also reported that much of the high temperature metamorphic assemblage has been overprinted by a retrograde assemblage with porphyroblasts replaced by chlorite and white mica and biotite by chlorite. Mineralization Three types of sulfide mineralization related to the BIC intrusion have been recognized: disseminated chalcopyrite-pyrite mineralization in the middle unit, copper and PGE rich disseminated sulfide mineralization in the lower unit and thin bands of “Eagle like” massive sulfide in the hornfels beneath the intrusion. However, exploration work completed to date at BIC has not yet identified any significant Cu-Ni-PGE resource. Fine-grained chalcopyrite with trace pyrite, sphalerite and rare bornite is disseminated throughout the middle unit. Limited sampling of this interval in drill hole BIC01-01 gave Cu values up to 0.16% over 1.5 m. However, Ni values were all below 500ppm and Pt and Pd values were all at, or below, the detection limits (Kennecott Exploration data). Disseminated sulfides are erratically distributed throughout the lower unit In the BIC intrusion. However, sulfide abundance seldom exceeds 5% in most of the drill tested portions of the intrusion. The greatest abundance of sulfide is typically located within a 3-4m interval 1-2m above the base of the intrusion. In the Little BIC intrusion, the abundance of disseminated sulfides reaches 10% over short intervals. Continuous intervals with >4% disseminated sulfides exceeding 20 m have been intersected in some drill holes at Little BIC. Sulfides in the lower unit are comprised of irregularly shaped, composite grains of pyrrhotite, chalcopyrite and pentlandite that are subpoikolitic on olivine, clinopyroxene, plagioclase, amphibole, ilmenite and titanomagnetite (Hauck, 2002). Cubanite occurs both as lamellae in chalcopyrite and as irregular grains. Recalculating the metal contents of disseminated sulfides to 100% sulfide, BIC and Little BIC disseminated sulfide metal tenors in the lower unit average 12.77% Cu, 5.88% Ni, 10.5ppm Pt and 12.91ppm Pd (avg. 109 samples with 0.9-10% S). In contrast, disseminated sulfides in the Eagle deposit recalculated to 100% sulfide average 6.24% Cu, 6.39% Ni, 1.5ppm Pt and 0.9ppm Pd (avg. 2350 samples with 0.9-10% S). The significantly higher Cu:Ni ratio and greater PGE content of BIC disseminated sulfides compared to Eagle disseminated sulfides suggest a greater silicate melt to sulfide melt ratio (R factor) at BIC. Thin (<1m) bands of massive sulfide occur in the hornfels within a few meters of the base of the Little BIC intrusion, and in a few holes in the western portion of the BIC intrusion. Two samples of massive sulfide from hole 06BIC-007 (Little BIC intrusion- fig.3), selected to maximize sulfide content, averaged 2.72% Cu, 6.02% Ni, 1.8ppm Pt and 3.1ppm Pd (avg. 35.8% S). The significantly lower Cu and PGE tenors of the massive sulfides hosted in the meta-sediments suggests that they were not directly formed by gravitational settling of the overlying disseminated sulfides. Interestingly, the massive sulfides at BIC have metal tenors and Cu:Ni ratios very similar to Cu poor massive sulfides at the Eagle deposit. 191 �Acknowledgements I would like to thank Kennecott Exploration Company and Kennecott Minerals Company for granting permission to prepare this field guide. I would also like to thank the rest of the Kennecott North American Ni exploration team and the Kennecott Minerals exploration staff for the hard work they have put in over the years on the Michigan program including the BIC project. In particular, I would like to acknowledge the many contributions made by Andrew Ware and Steve Coombes. I would also like to thank Karen Rossell and Andrew Ware for reviewing the field guide. References Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology, v. 33 pp. 193-196. Barnett, R.L., 1995, BIC Gabbro: Kennecott internal company report. Bornhorst, T.J., and Rose, W.I., 1994. Self-guided geological field trip to the Keweenaw Peninsula, Michigan. Institute on Lake Superior Geology, Proceedings 40th annual meeting, Houghton, Michigan. vol. 40, part 2, 185p. Corfu, F., and Lightfoot, P.C., 1996, U-Pb geochronology of the Sublayer environment, Sudbury Igneous Complex, Ontario: Economic Geology and the Bulletin of the Society of Economic Geologists, v. 91 pp. 1263-1269. Cannon, W.F., 1977, Bedrock geology in parts of the Baraga, Dead River, and Clark Creek Basins, Marquette and Baraga Counties, Michigan: U.S. Geological Survey Open-File report 77-467, scale 1:62,500. Cannon, W.F., 1986, Bedrock geologic map of the Iron River 1x2 degree quadrangle, Michigan and Wisconsin: U.S. Geological Survey Misc. Investigations Series Map I-1360-B, scale 1:250,000. Ding, X., 2007, Geology of the Eagle nickel deposit: Kennecott internal company report. Green, J.C., Bornhorst, T.J., Chandler, V.W., Mudrey, M.G., Myers, P.E., Pesonen, L.J., and Wilband, J.T., 1987. Keweenawan dykes of the Lake Superior region: evidence for evolution of the Middle Proterozoic Midcontinent rift of North America. In `Halls, H.C. and Fahrig, W.F., eds., Mafic dyke swarms: Geological Association of Canada Special Paper 34, pp.-289-302. Gregg, W.G., 1993, Structural geology of parautochthonous and allochthonous terranes of the Penokean Orogeny in Upper Michigan-comparisons with northern Appalachian tectonics: U.S.Geological Survey Bulletin, 1904-Q, 28p. Hauck, S.A., 2002, Petrographic descriptions of samples from drill holes BIC-2 and RLP-95-1, Northern Michigan: Kennecott internal company report. Hinze, W.J., Allen, D.J., Braile, L.W., Mariano, J., 1997. The Midcontinent rift system: a major Proterozoic continental rift. In Middle Proterozoic to Cambrian Rifting. Eds. Ojakangas, R.W., Dickas, A.B, and Green, J.C., Geological Soc. of America Special Paper 312, pp. 7-35. Johnson, R.C., 2007, Petrography and petrology of selected igneous and meta-sedimentary drill core samples: Kennecott internal company report. 192 �Klasner, J.S., 1972, Style and sequence of deformation and associated metamorphism due to the Penokean orogeny in the Western Marquette Range, Northern Michigan: Houghton, Mich., Michigan Technological University Ph. D. dissertation, 132p. Klasner, J.S., Ojakangas, R.W., Schultz, K.J. and Laberge, G.L., 1991, Nature and style of deformation in the foreland of the Early Proterozoic Penokean Orogen, Northern Michigan: U.S. Geological Survey Bulletin. 1904-K, 22p. Miller, J.D., 1996. The latent magmatic stage of the Midcontinent rift: a period of magmatic underplating and melting of the lower crust (abst.); Institute on Lake Superior Geology, Proceedings 42nd annual meeting, Cable Wisconsin, vol. 42, part 1, p. 33 Nelson, G., 2006, Sedimentologic characteristics of the Baraga Basin: Kennecott internal company report Ojakangas, 1994, Sedimentology and Provenance of the Early Proterozoic Michigamme formation and Goodrich quartzite, Northern Michigan – regional stratigraphic implications and suggested correlations: U.S. Geological Survey Bulletin 1904-R, 31p. Turner, Thomas R.,1973, The geology of the northern complex near Herman, Michigan thesis (M.S.)--Michigan Technological University, 1973. Sutcliff, R.H., 1987. Petrology of Middle Proterozoic diabases and picrites from Lake Nipigon, Canada:. Contrib. Mineral Petrology, vol.96, pp.201-211. Sims,P.K., 1996, Early Proterozoic Penokean orogen, in Sims, P.K. and Carter, L.M.H., eds. Archean and Proterozoic geology of the Lake Superior region, U.S.A., 1993, U.S. Geological Survey Professional Paper 1556,p.28-51. Wilband, J.T. and Wasuwanich, P., 1980, Models of basalt petrogenesis: lower Keweenawan diabase dikes and middle Keweenawan Portage Lake lavas, Upper Michigan: Contrib. Mineral. Petrol., v.75, pp.395-406. 193 �Field Trip Stops The first four stops on this trip are intended to highlight the variety of sediments that comprise the Paleoproterozoic Baraga Group in the vicinity of the BIC intrusion. They also provide an opportunity to see and discuss some of the structural complexity in this area. At stops 5 and 6 we’ll examine exposures of the BIC intrusion. Stop 7 will be at the Kennecott Minerals Company core shed near Negaunee, Michigan. Here we’ll have an opportunity to look at drill core form the BIC intrusion including mineralized intervals that are not exposed in the field. The location of field trip stops 1-6 are shown on figure 2. The locations of stops 5 and 6 are also shown on the more detailed BIC geology map. GPS coordinate locations provided for the stops are in UTM (Universal Transverse Mercator), zone 16. The datum is Nad 83. All of the field trip stops, except stop 1, are in areas of privately owned surface. Permission from the surface owners is required before accessing these areas. Some of the stops are along rivers and streams with high, often slippery banks and with potentially poor footing. Caution should be used in walking around these areas. Steep, cliff like outcrops are present in the vicinity of Stop 6, they provide great views but please stay well back from the edges. Stop 7-1 Canyon Falls on the Sturgeon River (UTM coordinates 386938E 5164275N) Good exposures of the Goodrich formation quartzites are exposed along the Sturgeon River at this location. To access the area, park at the Sturgeon River roadside park on the west side of US Highway 41 and follow the marked hiking trial south about 600m to the falls overlook. This area was a stop on a previous ILSG field trip led by Bill Cannon and John Klasner in 1972. The following stop description is an excerpt from that field guide. “This stop illustrates an anomalous structural style in that the rocks are relatively nonfolded as compared with the deformation style of nearby Precambrian X metasedimentary rocks, Here the quartzites, composed of quartz grains in a clay matrix with chlorite porphyroblasts, show very gentle N 70º W trending monoclinal folds. Ripple marks and sole marks are common on bedding surfaces. The more argillaceous layers show the development of a N 70º W cleavage” Ojakangas (1994) has correlated the thinly layered quartzite at this location with the Goodrich formation. 194 �Stop 7-2 Conglomerates on top of the Bijiki iron formation near the Taylor Mine. (UTM coordinates 388973E 5168500N) The stop is at rubble (subcrop) along the north side of a small drainage into Ogemaw Creek about 30m southeast of Old Hwy 41 (note: Old hwy 41 from the turn off of US highway 41 to the Taylor mine turnoff is a poorly maintained road that is often rutted and muddy and occasionally flooded. Klasner (1972) mapped a horizon of poorly exposed conglomerate and greywacke along the top of the Bijiki banded iron formation at this location. The reddish sandstone contains scattered matrix supported clasts of chert up to 10cm across. Drilling by Kennecott a few km to west of this location suggests that the Bijiki iron formation rapidly increases in thickness to the west. Perhaps, these conglomerates are additional evidence of a higher energy environment associated with the formation of a fault controlled sub-basin to the west. Stop 7- 3 Taylor mine site (UTM coordinates ~ 389660E 5169000N) The Taylor Mine site can be accessed by walking east from old hwy 41 along the old Taylor mine road. A trail to the north, along an old rail grade just before the old Taylor mine pit, leads to several good bedrock exposures. The Taylor Iron Co. shipped 32,970 tons of iron ore from the Taylor mine between 1880 and 1883 (Lake Superior Iron Ore Association, 1952). The property was explored by Ford Motor Company for iron ore during the 1950’s and 1960’s. Additional drilling was carried out on the property in the 1970’s as part of a regional uranium exploration program. John Klasner (1972) produced a detailed map of the mine area as part of his Ph.D. dissertation at Michigan Technological University. Kennecott acquired mineral title to the property as part of the purchase of all of the Ford Motor Company mineral title holdings in the Upper Peninsula. The mine site provides good exposures of the Lower Slate and Bijiki members of the Michigamme formation and diabase dykes of the Baraga-Marquette dyke swarm. Well exposed folds also contrast with the very weakly folded quartzite at stop 1. Klasner (1972) describes the folds at the Taylor mine as “asymmetric with slight overturning to the north and a recognizable S1 axial plane foliation. The folds have an amplitude of 400 feet (122 m) and a period of 600 feet (183 m). Minor folds are superimposed on the larger folds” 195 �Stop 7-4 Taylor Creek (optional) (UTM coordinates 390436E 5170300N) Good exposures of probable Upper Slate member of the Michigamme formation are found downstream along Taylor Creek from where old hwy 41 crosses it. However, in many places the banks of Taylor Creek are very steep and rocky. Access to this stop will depend on how high spring run off water level is. The banks of Taylor Creek at this stop are steep and the footing can be poor. Use caution when climbing down to view the exposures along the creek. Taylor Creek is within the Falls River slice, the allocthon proposed by Gregg (1993) south of the Falls River thrust fault (see fig. 2). Deformation evident in the bedrock exposures along Taylor Creek is different than that seen at either the Taylor mine or further north in the Baraga basin. In Taylor creek, small scale folds, where visible, are often nearly recumbent. In pelitic horizons, S1 foliations typically dip gently southward and are affected by a well developed crenulating cleavage associated with a second generation of folds. Stop 7-5 Exposures of the Lower and Middle Units on the west end of the BIC intrusion (UTM coordinates 396027E 5174514N) The west end of the BIC intrusion is accessible by hiking eastward from the Indian road along a series of old logging trails. The best exposures are located just below the top of the hill. The surface and mineral title are held by Kennecott Minerals Company at this stop and permission is required to access the area. At this stop, a natural flat terrace on the west facing slope of the prominent hill held up by the BIC intrusion, marks the unexposed contact between the Lower and Middle units of the BIC intrusion. Outcrops down slope from the terrace are comprised of rocks that range in composition from feldspathic werhlite to olivine melagabbro. They contain minor disseminate pyrrhotite, chalcopyrite and pentlandite. Nearly complete replacement of plagioclase by secondary minerals makes accurate determinations of modes very difficult in most hand samples of this unit. The Lower Unit of the BIC intrusion is compositionally similar to the olivine rich melagabbro that hosts much of the mineralization at the Eagle Ni-Cu-PGE deposit in the eastern end of the Baraga basin. Exposures upslope from the terrace are of equigranular, locally ophitic textured gabbros of the Middle unit. Unlike the Lower unit, neither olivine nor orthopyroxene appear to be present in the Middle unit. Minor pyrite and chalcopyrite are found as disseminations through out the unit. Hematite locally coats plagioclase giving it a pinkish hue. The contact between the olivine rich Lower unit and the olivine free Middle unit is relatively sharp. It is currently unclear if the change represents closed system fractionation or multiple pulses of different magmas. There is currently no recognized analog for the BIC intrusion Middle or Upper units at Eagle. More detailed descriptions of the units at BIC can be found in the first part of the guide. 196 �Stop 7-6 Upper Unit exposures on the east end of the BIC intrusion. (UTM coordinates 397013E 5174477N) The east end of the BIC intrusion is accessible by a series of logging and drill roads starting off the Silver River road north of the intrusion. The last part of the road to the top of the hill is typically deeply rutted and often not drivable. Walking the last part is recommended. Permission from Kennecott Minerals Company is required before accessing this stop. Glaciated exposures of the medium to coarse-grained oxide gabbro that comprise the Upper unit of the BIC intrusion are present in, and alongside the drill road going up the eastern end of the hill. Exposures of the gabbro near the top of the hill contain football size and shape patches with intense epidote alteration. The boundaries of the intensely altered rock are very sharp. It is currently uncertain if these are intensely altered xenoliths or cross sections of sub-parallel “pipe like” zones of hydrothermal alteration. Stop 7-7 Kennecott Minerals Company core shed. The Kennecott core shed is located 2.6 miles east of the town of Negaunee. Turn north off of US Highway 41 at the blue TV 6 building (across from the Michigan Police post) on to the old airport road. Follow the road around the curve to the west and proceed through the gate. The core buildings are the long sheds on the south side of the road just past the gate. Core from the BIC and Little BIC intrusion will available for viewing and discussion. 197 �198 �Figure 3) Geology map of the BIC intrusion showing the location of field trip stops 7-5 and 7-6. Figure 4) BIC intrusion cross-section A to A’ 199 � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2008 Description An account of the resource Institute on Lake Superior Geology. Marquette, Michigan. May 6-10, 2008. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2008 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/049e0430282e1e4f5060cb8ecfd9dcd4.pdf 9b5e2fadcebef50097dce270ceefa13e PDF Text Text INSTITUTE ON LAKE SUPERIOR GEOLOGY 55TH ANNUAL MEETING MAY 5-10, 2009 ELY, MINNESOTA ORGANIZED BY: THE PRECAMBRIAN RESEARCH CENTER UNIVERSITY OF MINNESOTA DULUTH JAMES D. MILLER, JR., GEORGE J. HUDAK, AND DEAN M. PETERSON Co-Chairs Proceedings Volume 55 Part 1 – Program and Abstracts Edited by George J. Hudak, University of Wisconsin-Oshkosh and Precambrian Research Center, University of Minnesota Duluth Cover Photos: Various photos from Precambrian Research Center field areas over the past two years. Top row from left to right: Sunset over Ima Lake (BWCA), mapping anorthositic troctolite east of Jordan Lake (BWCA), moonrise over Ima Lake (BWCA). Center Row, left to right: Island within Twin Lakes (Superior National Forest), mapping north of Ima Lake. Bottom Row, left to right: Neoarchean pillow lavas in Ely, Soudan Member of the Ely Greenstone Formation east of Soudan Mine, and intermixed coarse-grained troctolite and medium-grained ophitic anorthositic olivine gabbro southwest of Alworth Lake (BWCA). Photos courtesy of George Hudak, Jim Miller, Dean Peterson and Eric Stifter. �55TH INSTITUTE ON LAKE SUPERIOR GEOLOGY PROCEEDINGS VOLUME 55 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: CU-NI DEPOSITS OF THE DULUTH COMPLEX TRIP 2: GLACIAL GEOLOGY OF THE VERMILION MORAINE TRIP 3: TOUR OF THE SOUDAN IRON MINE AND PHYSICS LAB TRIP 4: PIONEER MINE CANOE EXCURSION TRIP 5: GEOLOGY AND METAMORPHISM OF THE MESABI RANGE TRIP 6: GEOLOGY OF THE LAKE ONE TROCTOLITE BY CANOE TRIP 7: ARCHITECTURE OF AN ARCHEAN GREENSTONE BELT Reference to material in Part 1 should follow the example below: Boerboom, T. J., and Green, J. C., 2009, Bedrock geological map of the Deer Yard Lake and Good Harbor Bay Quadrangles, north shore of Lake Superior, Minnesota [abstract]: Institute on Lake Superior Geology Proceedings, 55th Annual Meeting, Ely, MN, v. 55, part 1, p. 4-5. Published by the 55th Institute on Lake Superior Geology and distributed by the ILSG Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca ILSG website: http://www.lakesuperiorgeology.org ISSN 1042-9964 ii �TABLE OF CONTENTS PROCEEDINGS VOLUME 55 PART 1— PROGRAM AND ABSTRACTS Previous Institutes on Lake Superior Geology, 1955-2009 ......................................................... iv Sam Goldich and the Goldich Medal ................................................................................. vi Past Goldich Medalists and the 2009 Goldich Medal Recipient ..................................... viii Goldich Medal Committee............................................................................................... viii Citation for 2009 Goldich Medal Recipient....................................................................... ix ILSG Student Research Fund............................................................................................. xi Student Paper Awards ....................................................................................................... xii Eisenbrey Student Travel Awards ................................................................................... xiii Report of the Chair of the 54rd Annual Meeting ............................................................. xiv 2009 Board of Directors..................................................................................................................... xvii 2009 Session Chairs ........................................................................................................ xvii 2009 Student Paper Awards Committee ......................................................................... xvii 2009 Local Committees .................................................................................................. xvii 2009 Meeting Sponsors.................................................................................................. xviii 2009 ILSG/Eisenbray Funds ............................................................................................ xix 2009 Banquet Speaker ..................................................................................................... xix Program ............................................................................................................................ xxi Abstracts ........................................................................................................................ xxix iii �PREVIOUS INSTITUTES ON LAKE SUPERIOR GEOLOGY, 1955-2009 ILSG YEAR PLACE CHAIRS 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz and C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims and R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley and E. Mercy 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray iv �28 1982 International Falls, Minnesota D.L. Southwick 29 1983 Houghton, Michigan T.J. Bornhorst 30 1984 Wausau, Wisconsin G.L. La Berge 31 1985 Kenora, Ontario C.E. Blackburn 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey and R.P. Sage 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage and W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller, Jr. and M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst and R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin and P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey, Jr. and B.A. Brown 48 2002 Kenora, Ontario P. Hinz and R.C. Beard 49 2003 Iron Mountain, Michigan L.G. Woodruff and W.F. Cannon 50 2004 Duluth, Minnesota S.A. Hauck and M. Severson 51 2005 Nipigon, Ontario P. Hollings and M.C. Smyk 52 2006 Sault Ste. Marie, Ontario R.P. Sage and A.C. Wilson 53 2007 Lutsen, Minnesota L.G. Woodruff and J.D. Miller, Jr. 54 2008 Marquette, Michigan T.J. Bornhorst and J.S. Klasner 55 2009 Ely, Minnesota J.D. Miller, Jr., G.J. Hudak, D.M. Peterson v �SAM GOLDICH AND THE GOLDICH MEDAL Sam Goldich received an AB from the University of Minnesota in 1929, a M.A. from Syracuse University in 1930, and a Ph.D. from the University of Minnesota in 1936. During World War II Sam worked for the U.S. Geological Survey in mineral exploration. In 1948, Sam returned to the University of Minnesota, and became Professor and Director of the Rock Analysis Laboratory the following year. He rejoined the U.S. Geological Survey in 1959 and was appointed as the first Branch Chief of the Branch of Isotope Geology. Sam returned to academia in 1964 when he went to Pennsylvania State University. He left PSU in 1965 and moved to the State University of New York at Stony Brook, where he stayed for 3 years. Restless yet again, he moved to Northern Illinois University in 1968 where he was a professor until his retirement in 1977. Sam’s final move was to Denver where he became an emeritus at the Colorado School of Mines. Sam died in 2000, less than a month before his 92nd birthday. In the late 1970’s, Geological Society of America Special Paper 182, which included seminal geochronological studies by Sam Goldich and coworkers on the Archean rocks of the Minnesota River Valley, was nearing completion. At this time various ILSG regulars began discussing the possibility of recognizing Sam for his pioneering work on the resolution of age relationships and thus the geology of Precambrian rocks in the Lake Superior region. Three members, R.W. Ojakangas, J.O. Kalliokoski and G.B. Morey, presented the idea to the ILSG Board of Directors in 1978. The Board approved the creation of an award, provided funding could be obtained. It was suggested that collecting one or two dollars at registration for a dedicated account would provide resources for striking the medal. A general request was made to the ILSG membership for donations and Sam himself offered a challenge grant to match the contributions. In total $4,000 was collected and thus began the work of creating the Goldich Medal. The initial Goldich Award was presented to Sam by G.B. Morey in 1979 and consisted of a large paper proclamation. For the actual medal, G.B. Morey consulted with the foundry on production details, while Dick Ojakangas and Jorma Kalliokoski worked on the design of the award, suggesting that it be given for “outstanding contributions to the geology of the Lake Superior region.” Simultaneously, a committee of J.O. Kalliokosi, W.F. Cannon, M.M Kehlenbeck, G.B. Morey, and G. Mursky developed the Award Guidelines that were approved by the ILSG Board. By 1981 all the elements of the Goldich Award had come together, and the vi �second recipient, Carl E. Dutton, Jr., received the Goldich Medal for 50 years of significant contributions to the understanding of the geology of the Lake Superior region. Since the beginning, the Awards Committee has consisted of individuals representing industry, government and academia, with each member of the Committee serving for three years. The medal is now awarded every year at the annual ILSG meeting. Reference: Morey, G.B. and Hanson, G.N. (editors). 1980. Selected studies of Archean gneisses and Lower Proterozoic rocks, southern Canadian Shield. Geological Society of America, Special Paper 182, 175 p. Prepared by various Goldich Medal Awardees, 2007 INSTITUTE ON LAKE SUPERIOR GEOLOGY GOLDICH MEDAL vii �PAST GOLDICH MEDALISTS 1979 Samuel S. Goldich 1994 Cedric Iverson 1980 not awarded 1995 Gene La Berge 1981 Carl E. Dutton, Jr. 1996 David L. Southwick 1982 Ralph W. Marsden 1997 1983 Burton Boyum 1998 Zell Peterman 1984 Richard W. Ojakangas 1999 Tsu-Ming Han 1985 Paul K. Sims 2000 John C. Green 1986 G.B. Morey 2001 John S. Klasner 1987 Henry H. Halls 2002 Ernest K. Lehmann 1988 Walter S. White 2003 Klaus J. Schulz 1989 Jorma Kalliokoski 2004 Paul Weiblen 1990 Kenneth C. Card 2005 Mark Smyk 1991 William Hinze 2006 Michael G. Mudrey 1992 William F. Cannon 1993 Donald W. Davis 2007 Joseph Mancuso 2008 Theodore J. Bornhorst Ronald P. Sage 2009 GOLDICH MEDAL RECIPIENT L. Gordon Medaris, Jr. University of Wisconsin Madison, Wisconsin GOLDICH MEDAL COMMITTEE Serving for the meeting year shown in parentheses Richard Ojakangas (2006-2009) Terry Boerboom (2007-2010) Allan MacTavish (2008-2011) Academic representative Government representative Industry representative viii �CITATION FOR GOLDICH MEDAL RECIPIENT L. Gordon Medaris, Jr. - 2009 Goldich Medal Recipient Gordon Medaris’s many and diverse contributions to the geology of the Lake Superior region, as well as his long-continued participation in ILSG over the past five decades, are appropriately recognized by the Goldich Medal. Medaris’s broad interests make him difficult to pigeonhole. He is best known internationally as an igneous and metamorphic petrologist who has emphasized the study of eclogites and orogenic peridotites of the North American Cordillera, the Caledonides of Scandinavia, the Variscides of the central European Bohemian Massif and the Variscides of the southern Carpathians. He has also studied mantle xenoliths from California, central Europe and the Middle East. Gordon’s European contributions have been recognized with two awards from Charles University of Prague, the Gold Medal of Science in 1998 and the Boricky Medal in 2006. It is fair to say that Gordon has developed a Bohemian love affair. Better known to us is Gordon’s research in the Lake Superior region that began in the 1970s with a ground breaking study of the Wolf River batholith of east-central Wisconsin, which is part of a continental-scale Geon 14 magmatic event. That work was initiated in collaboration with Randy Van Schmus and Phillip Banks and was continued and expanded by his student J. Lawford Anderson. Next, Medaris studied with geochemist Robert Cullers the rare earth elements of the Seabrook Lake carbonatite and cogenetic alkaline rocks. By the 1980s, we find Gordon, Van Schmus, and student Randy Maass publishing syntheses of Penokean deformation and metamorphism across Wisconsin and adjacent areas. In 1983, Gordon was the principal convener and editor for an international symposium on Proterozoic Geology, which resulted in two GSA Memoirs, Number 160 being The Early Proterozoic Geology of the Lake Superior Region. Gordon, Dave Moecher, and others then studied the metamorphic conditions of Sam Goldich’s favorite high-grade gneisses in the Minnesota River Valley. Since retiring in 1998, Medaris has redoubled his research efforts in the Lake Superior region with collaborations that culminated in 2003 in the benchmark Journal of Geology article about the age, composition, and metamorphism of Baraboo Interval rocks and their tectonic significance. Gordon’s discovery of a paleosol beneath the Baraboo Quartzite and the previous recognition of paleosols beneath the Barron and Sioux Quartzites have important paleoclimatic implications, which he has discussed. He also has helped archaeologists to resolve pipestone artifact provenance by characterizing two distinct mineral assemblages in pipestone quarries -hematite-quartz-kaolinite in the Barron and hematite-muscovite-pyrophyllite-diaspore in the Sioux and Baraboo Quartzites. Six journal articles have appeared from these Baraboo Interval investigations and we are still counting. ix �Besides the full-length publications alluded to above, Gordon has contributed talks at no less than 21 ILSG meetings beginning in 1973. He also has been a major organizer of three different ILSG field trip guidebooks (1973, 1986, and 2001) and all of us have seen him on many other ILSG field trips. As many of you know, Gordon is a superb field and laboratory petrologist and mineralogist. Like Sam Goldich, he is gifted with the vision to spot a significant problem, to work out a research strategy, and to pursue it by whatever techniques are needed to answer critical questions. Gordon likes collaborative research, so does not hesitate to recruit colleagues from any specialty to work with him. All of us who have had the privilege to work with Gordon appreciate his vision and encouragement in these joint efforts. He never tries to dominate and is quick to give encouragement and credit, all with a wonderful quiet dignity. I know that I speak for many other co-workers in thanking Gordon for sharing the pleasure of his collaborations. For his many, varied and fundamental contributions to our knowledge of Lake Superior Geology and for his stimulation of the efforts of others, Randy Van Schmus, Daniel Holm, and Brad Singer join me in presenting L. Gordon Medaris, Jr. as the 2009 recipient of the Goldich Medal. R. H. Dott, Jr. April, 2009 x �ILSG STUDENT RESEARCH FUND The 2005 Board of Directors established the ILSG Student Research Fund with $10,000 US from the Institute’s general fund to encourage student research on the geology of the Lake Superior region. A minimum of two awards of $500 US each for research expenses (but not travel expenses) will be made each year. Students are expected to present their research orally or during a poster session at an ILSG meeting. The award winners will also be automatically eligible for the Eisenbrey Travel Awards. To allow the fund to grow, the Fund will receive one-half of any additional proceeds from each annual meeting, after all other commitments and expenses are covered. • The ILSG Board of Directors will be responsible for selecting a minimum of two awards each year. The ILSG Treasurer will issue the awards. • The ILSG Student Research Fund is available for undergraduate or graduate students working on geology in the Lake Superior region. • The applications are due to the ILSG Secretary by August 31st of each year. Awards will be made by October 1st of each year. • Names of the award recipients will be announced at the next annual meeting and posted on the ILSG website. • The proposal application should be at least 500 words, and should have a statement of the research project, background information, a map of the research area, research steps necessary to complete the research, figures (if needed) , references, and a list of research expenses. The proposal should also include a proposed end date for the research. • The proposal will need to be signed by researcher’s supervisor. In 2008 the ILSG Board of Governors awarded four $500 awards from the Student Research Fund. Dan Costello (University of –Minnesota - Duluth) - Geology of the Tuscarora Intrusion, northeastern Minnesota, and its relationship to the Anorthositic Series of the Duluth Complex James Hiller (California State University Chico) - Detailed petrographic analysis of anthraxolite morphology in the Biwabik Iron-Formation, northern Minnesota Angela Hull (Kent State University) – Preliminary results of 40Ar/39Ar thermochronology from the Central Yavapai Province, U. S. Midcontinent Andrew Jansen (University of Wisconsin Oshkosh) - Lithogeochemical evaluation of Neoarchean mafic volcanic rocks comprising the footwall of the Soudan Member of the Ely Greenstone Formation, northeastern Minnesota xi �STUDENT PAPER AWARDS Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper—last modified by the Board in 2001—follow: • The contribution must be demonstrably the work of the student. • The student must present the contribution in-person. • The Student Paper and Poster Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. • In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. • The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). • The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper and Poster Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. • The names of award recipients shall be included as part of the annual Chair's report that appears in the next volume of the Institute. Student papers are noted on the Program. In 2008 the ILSG Student Paper Committee presented three awards from the ILSG Student Paper Fund. Each of the following recipients received a $200 award: Elizabeth Drommerhausen (Minnesota State University) for her poster titled: Properties of fluid involved in formation of natural ores in the Mesabi Iron Range, Minnesota Carissa Isaac (Lakehead University) for her talk titled: Stable isotope geochemistry of the Musselwhite Au Mine, north Ontario: Implications for mineralization Natalie King (Colorado State University) for her talk titled: Using mineralization to evaluate small-scale controls on shale permeability in the Nonesuch Formation xii �EISENBREY STUDENT TRAVEL AWARDS The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name "Eisenbrey" was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. "Ned" Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The annual Chair in consultation with the Secretary-Treasurer determines the number of awards and value. Recipients will be announced at the annual banquet. The student travel award application is available on the ILSG website. The following general criteria will be considered by the annual Chair, who is responsible for the selection: • The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. • Students who are the senior author on either an oral or poster paper will be given favored consideration. • It is desirable for two or more students to jointly request travel assistance. • In general, priority will be given to those in the Institute region who are farthest away from the meeting location. • Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. • Successful applicants will receive their awards during the meeting. In 2008 the ILSG awarded 11 travel awards from the ILSG Eisenbrey Student Travel Fund. The awards were made to: Terra Anderson – University of Wisconsin Milwaukee Ding Xin – Indiana University Elizabeth Drommerhausen – Minnesota State University Emerald Erickson – University of Minnesota - Duluth Elizabeth Fein – Kent State University Shelby Frost – Winona State University Lynn Galston – University of Wisconsin – Eau Claire Sally Goodman – University of Minnesota - Duluth Susan Karberg – University of Minnesota - Duluth Natalie King – Colorado State University Curtis Williams – Indiana University xiii �REPORT OF THE CHAIRS OF THE 54TH ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY MARQUETTE, MICHIGAN The A.E. Seaman Mineral Museum of Michigan Technological University hosted the 54th Annual Institute on Lake Superior Geology on May 6 – 10, 2008 at the Ramada Inn in Marquette, Michigan. The Marquette/Ishpeming area has been the site of the ILSG annual meeting a total of 5 times out of 54. There were a total of 230 registrants for the meeting. Of these 37 were students and 215 pre-registered. The number of registrants for the meeting exceeded expectations. The Proceedings Volume 54 was published in two parts. Part 1 – Program and Abstracts, edited by Theodore J. Bornhorst and George W. Robinson. Part 1 contains 45 published abstracts for 29 oral and 16 poster presentations. The cover of Part 1 was highlighted by photos of Lake Superior minerals in the collection of the A.E. Seaman Mineral Museum and was designed by George Robinson. Part 2 – Field Trip Guidebook was edited by Theodore J. Bornhorst and John S. Klasner. Part 2 contains guides of seven field trips. The cover of Part 2 was highlighted by a photo of brecciated banded iron formation from Ishpeming, MI similar to one in Van Hise, Bayley and Smyth, 1897 and was provided by Tom Waggoner. Field trips were a dominant part of the 54th ILSG, consistent with ILSG tradition. There were four pre-meeting, one “syn-meeting” and three post-meeting trips. Participation in the field trips was excellent. Most were full to capacity and one even had names on a waiting list. This first of the pre-meeting trips, was a two day trip on Tuesday and Wednesday, May 6 and 7. Tom Waggoner led this trip of 27 participants to examine banded iron formation of the Marquette District. In addition to the printed field trip guide in the Proceedings Volume, all registrants received supplemental material related to this field trip on a CD and a colored map of the Marquette District. Cliffs Mining Services Company is thanked for providing financial support to distribute colored maps to all registrants and for access to the operating iron mines. On Wednesday, May 7, there were three concurrent one day field trips. Bill Cannon and Klaus Schulz lead a trip to inspect the Archean-Paleoproterozoic unconformity at Silver Lake and possible seismites from the Sudbury Impact. This was a once in a lifetime opportunity since beginning in 1910 these outcrops have been under the waters of Silver Lake, a natural body of water enhanced for hydroelectric generation. In May 2003 an earthen dam failed and exposed the outcrops making the trip possible for the 25 participants. The reconstruction of the dam will likely be completed in a few years, so once again the outcrops will be underwater. Tom Quigley and Bob Mahin of Aquila Resources Inc. led a field trip focused on the geology of the Back Forty project south of Marquette in Menominee County for 43 participants. Aquila Resources Inc. is thanked for providing this field trip for ILSG and for financial support for color printing of the Field Trip Guidebook. Andrew Ware, Jon Cherry, and Xin Ding led a field trip that concentrated on the geology of the Eagle Project. Kennecott Minerals Company is thanked for generously providing this field trip not just once before the technical sessions, but twice, before and after the technical sessions for a total of 87 participants. Kennecott also provided financial support for color printing of the Field Trip Guidebook. The Sudbury impact layer is a topic of high interest. Since a single locality with outcrops of this layer was available near Marquette, the xiv �organizers sought to have an abbreviated “syn-meeting” field trip. Bill Cannon, graciously agree to lead yet another field trip for the 54th ILSG. Thus, immediately following the technical sessions on Friday May 9 there was a 3 hour field trip to the McClure locality to examine the Sudbury impact layer. This trip was so popular with registrants, that in addition to the 52 participants, there was a waiting list and multiple people were not able to participate. There were three post-meeting field trips on Saturday May 10. One was a repeat of the pre-meeting Eagle trip. Glenn Scott, Helen Lukey, Al Strandlie and CCI/CCMO staff led a trip focused on sustainable recovery of iron from the Marquette District. This environmentally oriented trip had 13 participants. A color version of the printed field trip guide was provided to all participants on CD. Cliffs Mining Services Company is thanked for providing this field trip for the 54th ILSG. Dean Rossell led a trip to study the geology of the Keweenawan BIC intrusion. This trip, like the other trips, was well attended with 38 participants. Kennecott Minerals Company is thanked for making this trip possible for the 54th ILSG participants. The two days of technical sessions were held at the Ramada Inn of Marquette. The eight session chairs helped keep the presentations on track. There were the normal technical glitches. The student paper committee once again had a difficult job of selecting among 18 student oral and poster presentations. The committee awarded three Best Student Paper awards with a cash prize of $200 each: Elizabeth Drommerhausen (Minnesota State University) for her poster presentation titled: Properties of fluid involved in formation of natural ore in the Mesabi Iron Range, Minnesota, Carissa Isaac (Lakehead University) for her oral presentation titled: Stable isotope geochemistry of the Musselwhite Au Mine, north Ontario: Implications for mineralization, and Natalie King (Colorado State University) for her oral presentation titled: Using mineralization to evaluate small-scale controls on shale permeability in the Nonesuch Formation. One hundred and sixty-three participants attended the banquet on Thursday night. The 2008 banquet speaker was Jon Cherry, General Manager of Kennecott Minerals – Eagle Project. Jon brought participants up-to-date on the Kennecott Eagle Project with a well received Powerpoint presentation. A highlight of the banquet for me (Ted Bornhorst, C0-chair) was the presentation of the 2008 Goldich medal. Ted Bornhorst, Michigan Technological University was presented the medal by Jim Miller. Jim cited Ted for his contributions to Lake Superior geology and his service to ILSG. The Institute’s Board of Directors met on May 8, 2008. A brief overview of the meeting is provided below: 1. Accepted the Report of the Chair for the 53th ILSG from Laurel Woodruff and Jim Miller. 2. Accepted the minutes of last Board meeting from ILSG secretary Pete Hollings. 3. Accepted the 2007-2008 ILSG Financial Summary from ILSG treasurer, Mark Jirsa 4. Accepted the motion to reappoint Pete Hollings as Secretary of the ILSG, and Mark Jirsa as the Treasurer of the ILSG. In keeping the ILSG constitution, the motion by the board to reappoint Hollings and Jirsa was brought forward to the membership of ILSG at the annual banquet. The membership passed the motion unanimously. 5. Ted Bornhorst, agreed to serve as on-going ILSG Board Member. xv �6. Nominated Al MacTavish of Magma Metals (Canada) Ltd. to replace Doug Duskin as the “industry member” on the Goldich Committee. The Board approved MacTavish as the new Goldich Committee member with a term of 3 years. 7. Received from Mike Mudrey a progress report on scanning initiative. 8. Discussed changes as proposed by Chair Bornhorst to the membership criteria as posted on the web site. A previous Board made email as the only contact to determine membership in ILSG. The proposed revisions returned the criteria to postal address and made “Member for Life” status truly member for life. Motion to accept the changes proposed by Bornhorst by Jim Miller, second by Mark Smyk, passed unanimously. 9. Approved Ely, Minnesota at the site for the 55th annual ILSG meeting with co-chairs Jim Miller, George Hudak, and Dean Peterson of the Precambrian Research Center. 10. The Board once again discussed special awards for contributions to ILSG. The Board agreed that the only award from ILSG will be the Goldich Award. Chairs of individual meetings can consider special awards or recognition of individuals, but only with prior consent of the Board. The Co-chairs, Ted and John, thank the participants, field trip leaders, and presenters for without you there would be no ILSG. And we also thank others, not already cited above, who played a role in this years meeting: Gretchen Klasner was invaluable to the success of the meeting as she did all of the on-site registration. The staff of Ramada Inn was professional and did an excellent job of responding to last minute requests. Undergraduate geo majors from Michigan Tech were drivers of vans for the field trips: Carla Alonso, Austin Andres, James Julip, Matt Laird, Eric Murray. Darlene Comfort, A.E. Seaman Mineral Museum, did a great job of keeping track of all of the registration details. This was a major effort for her! John and I hope that you agree that the 54th ILSG was a real success. Attendance in general and for the field trips was above our initial expectations. We are gratified that for all of the positive comments provided by many of you, thanks as it does make a difference. We are both happy to have the 54th annual ILSG in our past. Yes, being Chair of the annual meeting is a lot of work and added stress. But, it is worth it and we encourage others to try it out!! ILSG is a great professional organization with a long and rich history. We look forward to seeing you at the 55th ILSG and many more. Ted Bornhorst and John Klasner Co-Chairs, 54th Institute on Lake Superior Geology xvi �2009 BOARD OF DIRECTORS Board appointment continues through the close of the last meeting year, or until a successor is selected Jim Miller, Co-Chair, 55th Meeting (2010; joined board after 2007 meeting) University of Minnesota Duluth, MN / PRC, University of Minnesota Duluth, Duluth MN George J. Hudak, Co-Chair 55th Meeting (will continue on board until 2012) Univ. of Wisconsin Oshkosh / PRC, University of Minnesota Duluth, MN Dean M. Peterson, Co-Chair, 55th Meeting Duluth Metals Limited/ PRC, University of Minnesota Duluth, MN Theodore J. Bornhorst (2011) Michigan Technological University, MI Ann Wilson (2009) Ontario Geological Survey, South Porcupine, ON Peter Hollings – Secretary (2011) Lakehead University, Thunder Bay, ON Mark A. Jirsa – Treasurer (2011) Minnesota Geological Survey, St. Paul, MN 2009 SESSION CHAIRS Meghan Blair, Barr Engineering, Duluth, MN Dyanna Czeck, University of Wisconsin - Milwaukee Dave Dahl, Minnesota Department of Natural Resources, Hibbing, MN Dan England, Eveleth Fee Office, Inc., Eveleth, MN Mary Louise Hill, Lakehead University, Thunder Bay, ON Phillip Larson, Cliffs Natural Resources, Eveleth, MN Allan MacTavish, Magma Metals (Canada) Ltd., Thunder Bay, ON Greg Stott, Ontario Geological Survey, Sudbury, ON 2009 STUDENT PAPER AWARDS COMMITTEE Thomas Fitz (Chair), Northland College, Ashland, WI Dorothy Campbell, Ontario Geological Survey, Thunder Bay, ON John Gartner, Prime Meridian Resources Corp., Iron River, MI 2009 LOCAL COMMITTEES General Meeting Planning and Promotion James D. Miller, Jr., University of Minnesota Duluth Program and Abstracts Editor and Student Awards George Hudak, University of Wisconsin Oshkosh Field Trip Guidebook Editor Dean M. Peterson, Duluth Metals Limited Registration Julie Ann Heinz – Natural Resources Research Institute xvii �2009 MEETING SPONSORS The organizers wish to acknowledge and thank several companies and organizations who have contributed financial support to various components the meeting. Welcoming Reception Sponsor Guidebook Sponsor Student Sponsors American Institute of Professional Geologists – Minnesota Chapter Brooke Fahrendrog Angela Hull Kevin Kane Aaron Rowland Jeff Bruesewitz Kyle Makovsky University of Wisconsin - Eau Claire Kent State University Grand Valley State University University of Wisconsin - Eau Claire University of Wisconsin - Eau Claire Minnesota State University Minnesota Mineral Resource Education Foundation Kevin Kane Aaron Rowland Andrew Jansen Ryan Dayton Shelby Frost Tom Johnson Grand Valley State University University of Wisconsin - Eau Claire University of Wisconsin - Oshkosh University of Minnesota Duluth University of Minnesota Duluth University of Minnesota Duluth Northland Securities Brooke Fahrendrog Andrew Jansen Jessica Gary Steve Hoaglund Levi Markwood University of Wisconsin - Eau Claire University of Wisconsin - Oshkosh University of Minnesota Duluth University of Minnesota Duluth Slippery Rock University  Mesabi Range Geological Society Ryan Dayton Shelby Frost Tom Johnson Jessica Gary Steve Hoaglund Dan Costello Cara Leitheiser University of Minnesota Duluth University of Minnesota Duluth University of Minnesota Duluth University of Minnesota Duluth University of Minnesota Duluth University of Minnesota Duluth University of Minnesota Duluth  xviii �2009 ILSG/EISENBRAY FUNDS Part of the registration costs for the 2009 meeting went toward establishing a fund to provide travel support for students. Eisenbray funds, totalling $1000, will be specifically distributed to the four recipients of 2009 ILSG Research Grants (* below). Another $1900 will be distributed to the other students listed below. Dan Costello* Michael DeAngelis Adam Fage Nathan Forslund Benedek Gal James Hiller* Angela Hull* Andrew Jansen* Maura Kolb Cara Leitheiser Natalie Pietrzak Victoria Stinson University of Minnesota Duluth University of Tennesse - Knoxville Lakehead University Lakehead University Eotvos Lorand University California State University Chico Kent State University University of Wisconsin -Oshkosh Lakehead University University of Minnesota Duluth University of Western Ontario Lakehead University 2009 BANQUET SPEAKER The Deep Underground Sky Dr. Marvin Marshak Professor of Physics/Director of Undergraduate Research at the University of Minnesota Founder of the University of Minnesota Underground Research Laboratory at the Soudan Mine xix �xx �PROGRAM xxi �TUESDAY MAY 5, 2009 8:00 a.m. FIELD TRIP 1: CU-NI DEPOSITS OF THE DULUTH COMPLEX Rich Patelke – Polymet Mining Mark Severson – Natural Resources Research Institute, Univ. of Minnesota Duluth Dean Peterson – Duluth Metals Ltd. Tim Jefferson – Teck American Ernie Lehmann – Franconia Minerals 6:00 p.m. RETURN OF FIELD TRIP 1 TO GRAND ELY LODGE WEDNESDAY MAY 6, 2009 8:00 a.m. FIELD TRIP 1 (CONTINUED): CU-NI DEPOSITS OF THE DULUTH COMPLEX 8:00 a.m. FIELD TRIP 2: GLACIAL GEOLOGY OF THE VERMILION MORAINE Phil Larson – Cliffs Natural Resources Howard Mooers – Department of Geological Sciences, University of Minnesota Duluth 6:00 p.m. RETURN OF FIELD TRIPS 1 AND 2 TO GRAND ELY LODGE 4:00 p.m. - 10:00 p.m. REGISTRATION AT GRAND ELY LODGE 7:00 p.m. - 10:00 p.m. ICE BREAKER AND POSTER SESSION THURSDAY MAY 7, 2009 8:00 a.m. - 12:00 noon REGISTRATION 8:45 a.m. INTRODUCTORY REMARKS Jim Miller, George Hudak, and Dean Peterson, 2009 ILSG Co-Chairs 8:55 a.m. REMEMBERANCE OF JOE MANCUSO (1934-2009) xxii �TECHNICAL SESSION I Session Chairs: Mary Louise Hill, Lakehead University Phillip Larson, Cliffs Natural Resources 9:00 a.m. Medaris, L. G., Jr., Jicha, B. S., Dott, R. H. Jr., and Singer, B. S. A 1465 Ma 40Ar/39Ar age for the Seeley Slate: implications for metamorphism and deformation in the Baraboo Range, WI 9:20 a.m. Addison, W. D., Brumpton, G. R., Fralick, P. W., and Kissin, S. A. The complex Gunflint-Rove Formations boundary at Thunder Bay, Ontario: Two disconformities and a base surge debrisite 9:40 a.m. Cannon, W. F., and Schulz, K. J. Reconstructing the Penokean Foreland Basin using the Timeline of the 1850 Ma Sudbury Impact Layer 10:00 a.m. COFFEE BREAK AND POSTER SESSION 10:40 a.m. Peitrzak, N. J.*, Duke, N., Scott, G., and Lukey, H. Ore Textures and Mineral Chemistry within the Oxide-Carbonate-Silicate Flotation Ores at the Cliffs Natural Resources’ Tilden Mine, Michigan 11:00 a.m. Walsh, James F. Hydrostratigraphy of the Biwabik Iron Formation – Implications for Current Groundwater Flow Patterns and Past Genesis of Natural Ore Bodies 11:20 a.m. Lunch Break (2009 ILSG Board Meeting - by invitation) _____________________________ TECHNICAL SESSION II Session Chairs: Dyanna Czeck, University of Wisconsin Milwaukee Greg Stott, Ontario Geological Survey 1:00 p.m. Gilbert, H. P. Bird River Belt in Southeastern Manitoba – A Nearchean Volcanic Arc in the Western Superior Province 1:20 p.m. Forslund, N. R.*, Hill, M. L., and Middleton, R. S. Alteration in the Southern Felsic Volcanics at Marshall Lake, Northwestern Ontario xxiii �1:40 p.m. Jirsa, M. A., and Driese, S. G. Neoarchean Weathering and Atmospheric pO2 Inferred from Paleosaprolite between Granite-Greenstone and Superjacent Conglomerate in the Boundary Waters Canoe Area, NE Minnesota 2:00 p.m. COFFEE BREAK AND POSTER SESSION 2:40 p.m. Stinson, V. R.*, Kolb, M. J.*, and Hill, M. L. Metamorphism and Deformation at Musselwhite Mine 3:00 p.m. Wendland, C., Fralick, P., and Hollings, P. Diamondiferous Mass-Flow and Placer Deposits Forming a Neoarchean Fan Delta, Wawa Area, Superior Province 3:20 p.m. Mudrey, M. G. Goldich Award Winners Who Have Passed On _____________________________ 6:00 p.m. ICE BREAKER – MIXER – CASH BAR 7:00 p.m. ANNUAL BANQUET AND AWARD PRESENTATION • Announcement of 56th Annual Meeting Location • 2009 Goldich Award Presentation to L. Gordon Medaris, Jr. • 2009 Banquet Address by Dr. Marvin Marshak, University of Minnesota All registered participants are welcome to the banquet address FRIDAY MAY 8, 2009 8:30 a.m. INTRODUCTORY REMARKS Jim Miller, George Hudak, and Dean Peterson, Co-Chairs, 2008 ILSG TECHNICAL SESSION III Session Chairs: Allan MacTavish, Magma Metals (Canada) Limited Dave Dahl, Minnesota Department of Natural Resources 8:40 a.m. Hansen, E., Reimink, J., and Harlov, D. Titanite, Pseudorutile, and REE-Minerals in the Allouez Conglomerate, Keweenaw Peninsula, Michigan 9:00 a.m. Hollings, P., Smyk, M. C., Halls, H., and Heaman, L. Mesoproterozoic Midcontinent Rift-Related mafic intrusions in Northwestern Ontario: continuing geochemical, paleomagnetic, petrographic, and geochronological studies xxiv �9:20 a.m. Chandler, V. W. Magnetic Anomalies from Pleistocene Sources in the Western Lake Superior Region: The Edge of Insanity or a Promising Threshold? 9:40 a.m. Verburg, R., and Dunlavy, P. Mine Water Quality Prediction and Environmentally-Responsible Mining -Yes We Can! 10:00 a.m. COFFEE BREAK AND LAST POSTER SESSION 10:40 a.m. Schulz, K. J., and Nicholson, S. W. Geochemistry of Midcontinent Rift-related Dikes and Mafic-Ultramafic Intrusions in the Baraga Basin, northern Michigan: Implications for the Nature of Rift Magmatism and Ni-Cu-PGE Mineralization 11:00 a.m. Watkins, K. P. Magma Conduit Hosted Platinum-Palladium-Copper-Nickel Mineralization at the Thunder Bay North Project, Northwest Ontario: Discovery, Exploration, Geology, and Resource Potential 11:20 a.m. Gál, B.*, Peterson, D. M., ad Molnár, F. Magmatic vs. Hydrothermal Processes in the South Filson Creek Mineralization, South Kawishiwi Intrusion, Duluth Complex 11:40 a.m. Peterson, D. M. The Nokomis Cu-Ni-PGE Deposit, Duluth Complex, Minnesota 12:00 p.m. LUNCH BREAK AND MEETING OF THE STUDENT PAPER COMMITTEE _____________________________ TECHNICAL SESSION IV Session Chairs: Mehgan Blair, Barr Engineering Co. Dan England, Eveleth Fee Office, Inc. 1:00 p.m. PRESENTATION OF THE STUDENT TRAVEL AND BEST PAPER AWARDS 1:20 p.m. Thorleifson, H. Options for Geologic Sequestration of Carbon in the Upper Midwest: Mineral Carbonation and Deep Injection 1:40 p.m. Arends, H., Johnson, R., Hanson, K., Friedrich, H., and Kostka, S. Structuring, Gathering, and Distributing Geological Data for Public Use 2:00 p.m. Miller, J. D, Carranza-Torres, C., Davis, R., and Hendrickson, D. New Educational Initiatives at the University of Minnesota Duluth: Preparing Students for Future Jobs in the Mining and Minerals Exploration Industries xxv �2:20 p.m COMPLETION OF 2009 ILSG TECHNICAL SESSIONS AND FINAL STATEMENTS BY THE 2009 ILSG CO-CHAIRS _____________________________ 3:00 p.m. FIELD TRIP 3: TOUR OF THE SOUDAN IRON MINE AND PHYSICS LAB Dean Peterson – Duluth Metals Ltd. James Pointer – Minnesota Department of Natural Resources, Parks and Recreation Marvin Marshak – Department of Physics, University of Minnesota 3:00 p.m. FIELD TRIP 4: PIONEER MINE CANOE EXCURSION Mark Jirsa – Minnesota Geological Survey 6:30 p.m. RETURN OF FIELD TRIPS 3 AND 4 TO GRAND ELY LODGE SATURDAY MAY 9, 2009 8:00 a.m. FIELD TRIP 5: GEOLOGY AND METAMORPHISM OF THE EASTERN MESABI RANGE Dick Ojakangas – Department of Geological Sciences, Univ. of Minnesota Duluth Mark Severson – Natural Resources Research Institute, Univ. of Minnesota Duluth Doug Halverson, Jeff Bird, Tom Campbell, Jarred Lubben, and Peter Jongewaard – Cliffs Natural Resources William Everett – Mesabi Nugget 8:00 a.m. FIELD TRIP 6: GEOLOGY OF THE LAKE ONE TROCTOLITE BY CANOE Jim Miller – Department of Geological Sciences, University of Minnesota Duluth 8:00 a.m. FIELD TRIP 7: ARCHITECTURE OF AN ARCHEAN GREENSTONE BELT Dean Peterson – Duluth Metals Ltd. Mark Jirsa – Minnesota Geological Survey George Hudak – Department of Geology, University of Wisconsin Oshkosh 6:00 p.m. RETURN OF TRIPS 5 AND 7 TO GRAND ELY LODGE SUNDAY, MAY 10, 2009 8:00 a.m. FIELD TRIP 6: GEOLOGY OF THE LAKE ONE TROCTOLITE BY CANOE (CONTINUED) 8:00 a.m. FIELD TRIP 7: ARCHITECTURE OF AN ARCHEAN GREENSTONE BELT (CONTINUED) 4:00 p.m. RETURN OF TRIPS 6 AND 7 TO GRAND ELY LODGE _____________________________________________ 55TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY ENDS xxvi �POSTER PRESENTATIONS Boerboom, T. J. and Green, J. C. Bedrock Geologic Map of the Deer Yard Lake and Good Harbor Bay Quadrangles, North Shore of Lake Superior, Minnesota Bruesewitz, J.* and Cameron, B. Geochemical and New SHRIMP-RG Zircon Age Constraints of the Cary Mound Granite, Wood County, Wisconsin Coleman, J., and Chiriboga, E. Methods for Estimation of Indirect Hydrologic Impacts on Wetland Plant Communities at Potential Hard Rock Mine Sites Costello, D. E.*, Miller, J. D. Jr., and Jirsa, M.A. Geology of the Tuscarora Intrusion, Northeastern Minnesota and its Relationship to the Anorthositic Series of the Duluth Complex Dayton, R. N.*, Miller, J. D. Jr., and Vervoort, J. D. Quantifying Assimilation vs. Fractional Crystallization using Sm-Nd, Lu-Hf and Pb Isotope Systems: The Geochemical Evolution of the Sonju Lake Intrusion, Finland, MN Diedrich, T., Brecke, D., Schreiber, M., and Zanko, L. Taconite-Derived Mineral Dust in Population Centers on the Mesabi Iron Range: Tracking Mineral Fibers from Ore to Air Fage, A.* and Hollings, P. Geology and Geochemistry of the Fearless-Python Property, Schreiber-Hemlo Greenstone Belt, Ontario Frey, B. A. Vermilion Greenstone Gold – New Data, Northeastern Minnesota Gary, J. L.*, Wattrus, N. J., Colman, S. M., and Voytek, E. B. Characterizing the Discharge Features of Glacial Lake Agassiz During the Post-Marquette Period Using Marine Seismic-Reflection Methods Gere, M. A., and Hoane, T. B. 2009 Update: Leasing State of Michigan Lands for Metallic and Nonmetallic Minerals Hage, M. M.*, and Fedo, C. M. Geochemistry and Petrology of Gunflint Iron Formation, Gunflint Trail, Minnesota Hauck, S. A., Heine, J. J., and Thorleifson, L. H. A Follow-up Glacial Till Indicator Mineral Survey in Minnesota: What Does It Indicate About Exploration for Diamonds And Other Mineral Deposits xxvii �Hefferan, K. P., and Heywood, N. C. Developing a 21st Century Geoscience Major: Melding the Old with the New Hiller, J. A.* and Shapiro, R. S. Detailed Petrographic Analysis of Anthraxolite Morphology in the Biwabik IronFormation, Northern Minnesota Hull, A.*, Holm, D., and Schneider, D. Preliminary Results of 40Ar/39Ar Thermochronology from the Central Yavapai Province, U. S. Midcontinent Jansen, A.C.*, Hudak, G. J., Heine, J. J., and Peterson, D. M. Lithogeochemical Evaluation of Neoarchean Mafic Volcanic Rocks Comprising the Footwall of the Soudan Member of the Ely Greenstone Formation, Northeastern Minnesota Jirsa, M., Cowan, H.*, Kowalik, J.*, and Niedermiller, J.* Geologic Mapping of Neoarchean Rocks Near Paulsen Lake, Boundary Waters Canoe Area Wilderness, by Students of the Precambrian Research Center’s 2008 Field Camp Johnson, T. K.*, Hansen, V. L., Hudak, G. J., and Peterson, D. M. Structural, Kinematic, and Lithogeochemical Investigation of the Murray Shear Zone, Northeast Minnesota Makovsky,K.*, and Losh, S. Fluid Movement through the Mesabi Iron Range, Minnesota Markwood, L. W.*, and Zieg, M. J. Interpretations of the Emplacement and Cooling History of a Thin Diabase Sill, Nipigon, Ontario Medaris, L. G. Jr., and Fournelle, J. H. Metamorphic Pseudorutile in the Seeley Slate, Baraboo Range, Wisconsin Meineke, D. G., and Djerlev, H. Geology and Magnetic Taconite Resources of Western Gogebic Iron Range, Wisconsin Shapiro, R.S. Alteration of Stromatolite Biosignatures in the Biwabik Iron-formation: Relevance to Astrobiololgy Stifter, E.*, Wartman, J.*, Gibbons, J.*, Kane, K.*, Murphy, L.*, Carlson, A.*, Mason, T.*, Hudak, G. and Peterson, D. Bedrock Geologic Map of the Disappointment and Ima Lakes Area, Lake County, Northeastern Minnesota xxviii �ABSTRACTS xxix �xxx �THE COMPLEX GUNFLINT-ROVE FORMATIONS BOUNDARY AT THUNDER BAY, ONTARIO: TWO DISCONFORMITIES AND A BASE SURGE DEBRISITE William D. Addison, Gregory R. Brumpton,2 Philip W. Fralick,3 Stephen A. Kissin3 1 R.R. 2, Kakabeka Falls, P0T 1W0, Canada (baddison@tbaytel.net). 2 211 Henry St., Thunder Bay, P7E 4Y7, Canada. 3Department of Geology, Lakehead University, Thunder Bay, P7B 5E1, Canada. Eight subaerially exposed chaotic debrisites containing ejecta from the 1850 Ma (Krogh et al., 1984) Sudbury impact event have been discovered in and near the City of Thunder Bay, Ontario. Ejecta features include planar deformation features (PDF) in quartz grains, vesicular devitrified glass clasts (DVG) and accretionary lapilli (Addison et al., 2005). Megascopic to microscopic Gunflint breccia clasts and ejecta are minor components embedded in an often recrystallized dominantly carbonate matrix. Seven sites are erosionally truncated. Only the Terry Fox site shows a complete profile extending from the Gunflint Formation up through the debrisite and into the overlying Rove Formation. The study area has had a complex history, summarized as follows. The Upper Gunflint Formation exhibits an ocean regression assemblage, terminating in stromatolites at most study localities, disconformably overlain by the debrisites. We postulate that the regression was completed at some unknown time before the 1850 Ma Sudbury impact event, leaving a subaerially exposed carbonate landscape. A sporadic, 0.3-1.2 m thick, iron-rich alteration profile found 0-1 m below the debrisite base at most sites may be evidence of a paleosol. If further work supports this hypothesis, it would confirm that the study area was subaerial prior to impact. Approximately two minutes after the Sudbury impact began, violent earthquakes fractured and delaminated lithified portions of the Upper Gunflint Formation, as evidenced by still in situ fractured rock at the Hwy 588 and GTP sites and by numerous mostly sharply angular sub-millimeter to meter size Gunflint clasts within the debrisite. The earthquakes were followed by massive base surges which stripped all unlithified material down to bedrock and ripped up and entrained most of the earthquake fractured Upper Gunflint Formation rock. The base surges then contained the following mixture of features in order of volume percent: 1) clasts of fractured carbonate in the silt to coarse sand size range; 2) ripped up clasts of Gunflint fractured chert, chert-carbonate and stromatolites; 3) ejecta consisting primarily of DVG, and much lesser volumes of accretionary lapilli, tektites and microtektites, quartz and feldspar grains, some of which show planar features and PDFs; and 4) small clasts of uncertain origin. The travel distance for most Gunflint chert and chert-carbonate clasts was relatively short as most are very angular. Slightly rounded chert-carbonate clasts are less common and probably travelled only slightly further from their source than the angular ones. No clasts show weathering rinds. The base surges contained sufficient water vapor that accretionary lapilli were able to form. Some accretionary lapilli passed through zones with varying water vapor concentrations allowing them to accumulate alternating coarser-grained layers and finergrained layers. 1 �The debrisites deposited by these base surges are chaotic and show significant changes in clast sizes and composition over meter scale and even centimeter scale distance within the deposits. The one exception to this chaos is a clear upward fining of Gunflint clasts within the debrisite, which can probably be attributed to the limited lifting and transporting power of a gas-supported fluidized flow like a base surge. These observations are consistent with base surge deposits described from the Chicxulub, Houghton and Ries impact events. The DVG shows varying degree of vesicle collapse ranging from none (round vesicles) to partial collapse (ovoid vesicles) to totally flattened vesicles. This suggests that DVG clasts arrived at varying temperatures and plasticities and had time to deform before cooling was complete. Had DVG landed in water, quenching would have been nearly instantaneous and all vesicles would have been round. We initially interpreted these debrisites as tsunami deposits. If tsunamis were ever involved, we would expect some sorting of DVG, with more of the least dense clasts (most vesicles) being deposited towards the top of the debrisites and with denser DVG clasts (few or no vesicles) being more common towards the base of the debrisite. That is not the case. The debrisites were then subaerially exposed for 15-18 Ma (Addison et al, 2005). It is improbable that an unlithified deposit could survive exposed for this long. The underlying Upper Gunflint Formation and overlying Rove Formation hint at a possible preservation mechanism. Volcanic tuffs are common in both the Upper Gunflint Formation and especially in the lower Rove Formation where there are seven tuff layers per meter on top of the debrisite. If similar tuffs were deposited on top of the debrisite during the period of subaerial exposure they may have provided sufficient protection to allow survival of some of the debrisite. Anastomosing chert and agate within the debrisite and centimeter-scale agate stalactites in debrisite vugs suggest that silica was leached from such tuffs and was redeposited within the debrisite until ocean transgression and deposition of the Rove Formation protected it until today. Subsequently ocean transgression deposited about one meter of carbonate before deposition of the lower Rove Formation organic-rich muds and interlayered volcanic ash. This abrupt transgression marks a disconformity at the debrisite-Rove Formation boundary. Large scale carbonate replacement and recrystallization during diagenesis destroyed or partially obscured many ejecta and non-ejecta features in the debrisite. Silica replacement did the same to a lesser extent. These observations and interpretations add heretofore unknown detail to what happened at the Gunflint-Rove boundary. Will similar sequences be discovered elsewhere in the Lake Superior region? References Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology, v. 33, p. 193-196. Krogh, T. E., Davis, D. W., and Corfu, F., 1984, Precise U-Pb zircon and baddeleyite ages for the Sudbury area in Pye et al.., eds., The Geology and Ore Deposits of the Sudbury Structure: Ontario Geological Survey Special Volume 1, p. 431-446. 2 �STRUCTURING, GATHERING AND DISTRIBUTING GEOLOGIC DATA FOR PUBLIC USE Heather Arends, Minnesota Department of Natural Resources, Aggregate Resource Mapping Program, St. Paul MN, USA. E-mail: Heather.Arends@dnr.state.mn.us, Tel: 1 651 259-5376; Fax: 1 651 259-5939 Renee Johnson, Kevin Hanson, Hannah Friedrich, and Steven Kostka, Minnesota Department of Natural Resources, Aggregate Resource Mapping Program, St. Paul MN, USA The primary audience using aggregate resource maps and data are non-geologists, local units of government, and the general public. With increasing use of geospatial information and web-based mapping software, like Google Earth©, users expect information that is Internet accessible, interactive, easily compiled, and well documented. In addition, toolsets available for geologic mapping are evolving with the advancement of GIS (Geographic Information Systems) and GPS (Global Positioning Systems). With this evolution, governments and publicly financed institutions have a new responsibility to produce geospatial information. Over the past two years, the Aggregate Mapping Resource Program modified aspects of their data management, mapping methodology, and distribution of data to meet the growing demand of digital data. Internally, benefits of these changes include eliminating data entry redundancies, streamlining documentation, and an overall accelerated rate of mapping. Currently three geologists in our DNR program gather, enter, and produce geologic data. Databases are needed to ensure standards between geologists while accommodating different mapping styles and geologic settings. Standardization includes determined attribute widths and names, field order, information stored as text or numbers, and how people look at data versus how data is queried. Considering the pros and cons of database software, we prefer Microsoft Access© to ESRI© geodatabases for several reasons: the ability to develop one to many relationships, create new attributes on the fly, and programming flexibility. Data standardization expedites writing metadata and allows for the compilation of different project data with no additional processing for the user. Advancements in our methods of data collection include using a tablet computer with GIS software in conjunction with GPS in the field. Previous mapping methods consist of recording observations on a USGS (United States Geologic Survey) 24K quadrangle maps and/or within a fieldbook. Geologists then digitize location information and re-enter field descriptions into a database. Consequently, the same data is recorded twice and transcriptional error is potentially introduced into the dataset. Using a field computer and GPS provides better location accuracy, tracking capabilities, and eliminates redundant data entry processes. Furthermore, consulting data sets, such as aerial photography, parcel and ownership data, high-resolution elevation models, historical maps, and water well stratigraphy, in the field while simultaneously making observations provide additional benefits to the geologists. Various GIS software packages were tested. In our determination, the most stable configuration combines ArcGIS 9.3© with the GPS tacking toolbar for ArcGIS 9.2/9.3©. 3 �To improve distributing data, a DNR developed, web-based map server geospatially displays information used to map aggregate resources, aggregate resource data, base maps, and links to data documentation. The map server allows users to interact with GIS data without the having to download, install, and learn a new software program. By interacting with the data, a greater level of transparency exists on how different geologic units are delineated and classified given the available information at a single time, which reinforces the relationship between geology and distribution of mineral resources. 4 �BEDROCK GEOLOGIC MAP OF THE DEER YARD LAKE AND GOOD HARBOR BAY QUADRANGLES, NORTH SHORE OF LAKE SUPERIOR, MINNESOTA BOERBOOM, Terrence J., Minnesota Geological Survey, boerb001@umn.edu GREEN, John C., University of Minnesota-Duluth, jgreen@d.umn.edu The Minnesota Geological Survey is continuing to map the bedrock geology of 7.5’ quadrangles near Lake Superior as part of the USGS STATEMAP program, resulting to date in thirteen published 1:24,000 scale maps from Duluth to Grand Marais, in addition to 10 quadrangles already published under the former USGS COGEOMAP program. The Deer Yard Lake and Good Harbor Bay quadrangles are the most recent of these geologic maps (Fig. 1A). All maps in this series are available as printed maps, or as PDF and Arcview export files at the MGS website (http://www.geo.umn.edu/mgs/). Outcrop mapping was augmented by some 50 sets of water well cutting samples, collected at 10 foot intervals by Mckeever Well Drilling of Little Marais, Minnesota. These provided a crucial glimpse of the volcanic stratigraphy in the third dimension as well as information where the bedrock is poorly exposed. The area of this map lies near the top of the 7-10 km thick North Shore Volcanic Group (NSVG), and crosses the boundary between the upper portion of the Northeast sequence and the slightly discordant overlying Schroeder-Lutsen sequence (Fig. 1B). In addition there are several thick sandstone units, as well as components of the Beaver Bay Complex (Leveaux ferrodiorite, Murphy Mountain diabase, and Beaver River diabase) present in the map area. In keeping with prior work, the NSVG is subdivided into informal lithostratigraphic packages separated by major compositional changes, by intrusions or faults across which correlation is tenuous, or where thick flows or flow sequences form mappable units. The informal lithostratigraphic packages shown on this map include the Lutsen basalts, the Good Harbor Bay lavas (which include the Good Harbor Bay andesites, the Cutface Creek sandstone, and the Terrace Point basalt flow of Green, 2002), the Breakwater basalt, the Grand Marais felsites (rhyolite and icelandite), the Cascade River basalt, and the Croftville lavas (which includes the Pincushion Mountain trachybasalt of Green, 2002). The volcanic units immediately overlying (southeast of) the Leveaux ferrodiorite are poorly exposed and not named. The new mapping has refined the volcanic stratigraphy of the NSVG in this area and has continued previously mapped units, in particular thick interflow sandstone units, for several kilometers along strike from where mapped prior to the west (Boerboom and Green, 2007). Four thick sandstone units were identified, one within the Lutsen basalts (Indian Camp Sandstone), one below the Terrace Point flow (Cut Face Creek sandstone), and two other unnamed units identified mainly in water well samples. The Indian Camp and Cut Face Creek sandstones were already well known from exposures near the shore. Examinations of water well samples show that the Indian Camp sandstone is up to 170 feet thick and the Cut Face Creek sandstone more than 250 feet thick. The northern-most sandstone units were identified only in scattered water well cutting samples, but they correspond well with linear topographic depressions and coincident linear negative aeromagnetic anomalies, and can be confidently extended for some distance beyond the well intercepts (Boerboom, 2007). The Terrace Point basalt is a thick ophitic flow with scattered tabular phenocrysts and rare large megacrysts of glassy plagioclase, and contains rare but locally abundant xenoliths of granite, anorthosite, porphyritic ferrodiorite, rhyolite, conglomerate, andesite, and basalt, most of which are only tens of centimeters in size. However, near the base of the flow in the Cascade River the basalt contains a 60 m-diameter xenolith of coarsegrained biotite granite which yields a U-Pb zircon age of 1096.7±0.8 Ma (Green and others, 2001) as well as xenoliths of thermally metamorphosed sedimentary rocks, porphyritic ferrodiorite that matches the distinct texture of the Leveaux porphyry, and other volcanic rock types. The basalt here contains local hybrid pods contaminated by felsic material melted from the xenoliths, and exhibits a strong vertical to irregular flow banding. Overall, these features indicate this may be a feeder zone to the overlying Terrace Point basalt flow, a unique feature rarely found in the NSVG. The types of xenoliths and the plagioclase phenocrysts in the ophitic Terrace Point basalt are similar to those in the Beaver River diabase, leading to the speculation that it may be the extrusive equivalent to the diabase, which is also known from mapping to the southeast to intrude the Leveaux porphyry. Intrusive rocks of the Beaver Bay Complex in this map area consist of the Beaver River and Murphy Mountain diabase, and the eastern-most occurrence of the porphyritic Leveaux ferrodiorite. The latter is 5 �inferred from map distribution and measurements of aligned feldspar phenocrysts to form a southeast-dipping, funnel-shaped, subvolcanic sill-like intrusion. References Boerboom, T.J., 2007, Newly recognized thick interflow sandstones in the upper northeast limb of the North Shore Volcanic Group, Minnesota: Institute on Lake Superior Geology 53rd Annual Meeting, Lutsen, MN: Proceedings v. 53, pt. 1 – Programs and Abstracts, p. 8-9. Boerboom, T.J., and Green, J.C., 2007, Bedrock geology of the Lutsen quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous map M-174, scale 1:24,000. Green, J.C., 2002, Volcanic and sedimentary rocks of the Keweenawan Supergroup in northeastern Minnesota, in Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., and Wahl, T.E., Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, p. 94-102. Green, J.C., Davis, D.W., and Schmitz, M.D., 2001, Three new zircon dates for the Midcontinent rift, North Shore, Minnesota: More data, more questions: Institute on Lake Superior Geology 47th Annual Meeting, Madison, WI: Proceedings v. 47, pt. 1 – Programs and Abstracts, p. 28. Figure 1. A. Index map showing the location of mapped 7.5’ quadrangles along the North Shore of Lake Superior. M numbers refer to MGS Miscellaneous maps. B. Index map showing the locations of the major intrusions and volcanic sequences in part of northeastern Minnesota. 6 �GEOCHEMICAL AND NEW SHRIMP-RG ZIRCON AGE CONSTRAINTS ON THE CARY MOUND GRANITE, WOOD COUNTY, WISCONSIN Bruesewitz, Jeffrey1, and Cameron, Barry2, University of Wisconsin Milwaukee, Milwaukee, WI, 53201; bruesew3@uwm.edu1, bcameron@uwm.edu2 The Cary Mound granite is a late to post Penokean granite suite approximately 10 miles south of Marshfield, WI, and would be included with the 1835 Ma alkali-feldspar granite suite of Sims et al. (1989). Included within the suite are the alkali-feldspar granophyric granite, coeval rhyolite, diorite and mafic enclaves assumed coeval with the granite/rhyolite, and lamprophyre of uncertain younger age based on crosscutting relationships. Updated SHRIMP-RG zircon dates from USGS-Menlo Park have been obtained. The Cary Mound samples are characterized by euhedral zircon with homogenous cores with textural evidence for minor recrystallization surrounded by oscillatory zoned rims (Fig. 1). There is no difference in ages obtained for core versus rim domains. Using ten analyses from sample CMG-04 (county highway department quarry) give a concordia age of 1826 ± 9 Ma. Twelve analyses from sample CMG-15 (Haske quarry) give a Concordia age of 1827 ± 5 Ma (Fig. 2). These ages are statistically the same as the 1833 ± 4 Ma date reported by Sims et al. (1989). Previous interpretations of the 1835 Ma alkali feldspar granite indicate that it is most likely the result of crustal melting of a thickened post Penokean crust. The presence of significant amounts of diorite crosscutting and intermingled with the granite and mafic enclaves of basaltic nature that are related to the diorite indicate melting was more complex than a simple batch melt of thickened crust. The granite/rhyolite is likely a product of partial melting from a feldspar-rich continental crust as indicated by the strong depletions in Sr and Ti. The high concentrations of MgO, Ni, Cr, Zn, and V would be indicative of a mantle source for the mafic enclave with the diorite forming by fractionation from the parent basalt. The lamprophyre is characterized by an orthoclase and anorthoclase groundmass with abundant phlogopite phenocrysts and euhedral pseudomorphs of amphibole that have been replaced by montmorillonite. Using the classification of Rock (1991) the lamprophyre best falls into the calc-alkaline field and is termed a minette. It is enriched in Ba and Sr as is typical for lamprophyres. The lamprophyre is also enriched in the rare earth elements (REE’s), especially the LightREE’s. 7 �Figure 1. Representative cathodoluminescence images of zircons from samples (A) CMG05-04 and (B) CMG05-15. Ellipses indicate individual analysis spots for the sensitive high-resolution ion microprobe - reverse geometry (SHRIMP-RG). Each spot is labeled by grain number, alysis number (e.g. 4.1) and the corresponding 207Pb/206Pb age (± 1 σ Ma). Figure 2. Tera-Wasserburg plots of sensitive high-resolution ion microprobe – reverse geometry (SHRIMP-RG) U-Pb data of zircon for samples (A) CMG-05-04 and (B) CMG-0515. The data are presented as 1 σ error ellipses uncorrected for common Pb. References Rock, N. M. S., 1991, Lamprophyres: New York, Van Nostrand Reinhold, 285 p. Sims, P. K., Van Schmus, W. R., Schulz, K. J., Peterman, Z. E., 1989, Tectonostratigraphic evolution of the Early Proterozoic Wisconsin magmatic terranes of the Penokean Orogen. Can. J. Earth Sci. Vol 26, p. 2145-2158 8 �RECONSTRUCTING THE PENOKEAN FORELAND BASIN USING THE TIMELINE OF THE 1850 MA SUDBURY IMPACT LAYER Cannon, W. F., and Schulz, K.J., U.S. Geological Survey, MS 954, Reston, VA 20192 wcannon@usgs.gov, kschulz@usgs.gov The evolution of foreland basins, which are linear sedimentary basins formed on continental margins during terrane accretion, is highly variable both in time and space. Where terranes override thinned continental margins, a deep basin or foredeep forms near the margin and passes landward successively to a shallower outer slope, forebulge , and shelf. As accretion proceeds, these zones migrate toward the continent and are superimposed on previous sedimentary successions. Geologic and geochronologic studies (see Schulz and Cannon, 2007, for a review) have confirmed that the Paleoproterozoic Animikie Group and Marquette Range Supergroup in the Lake Superior region record the complex history of such a basin, the Penokean foreland. Unraveling the history of this complex basin is hampered by paucity of precise time lines. Only a few volcanic layers have been precisely dated and these are insufficient to reconstruct the basin history in more than a general manner. However, recently a bed of ejecta-bearing breccia that was deposited instantaneously across the Lake Superior region has been recognized as being related to the 1850 Ma meteorite impact at Sudbury, Ontario (Addison et al., 2005; Jirsa et al., 2008; Cannon et al., 2009). The Sudbury impact layer (SIL) can be traced regionally and provides a unique opportunity to reconstruct the Penokean foreland basin when the Penokean orogen was in a transformative state from a period of mild extension to the earliest stages of thrusting of volcanic arcs onto the continental margin (Schulz and Cannon, 2007). At the time of deposition of the SIL, the northern part of the Penokean basin near Thunder Bay was very shallow to subaerial as indicated by the occurrence of algal stromatolites beneath the SIL and local evidence of subaerial weathering. This shoaling of the basin probably accompanied the arrival of the forebulge. To the southwest conditions were different. At Gunflint Lake and along the Mesabi Range, the SIL lies conformably on the Gunflint and Biwabik Iron Formations for 250 km along strike. In this area, the stratigraphic succession records a progressively deepening basin with the upper cherty member of the Biwabik, a shallow-water, partly stromatolitc unit, overlain by deeper-water carbonate and silicate iron-formation of the upper slatey member. The SIL was deposited on the upper slatey member and was succeeded by black shale of the Virginia and Rove Formations. The shallow-water deposits of the upper cherty member may record the passage of the forebulge slightly before it arrived at Thunder Bay; the upper slatey member may mark the ensuing submergence on the outer slope prior to 1850 Ma. Farther south, deposition of the major iron-formations of the Gogebic and Marquette Ranges ended well before 1850 Ma. By then, both ranges had been uplifted, eroded, and resubmerged, recording passage of the forebulge (outer arch) and submergence onto the outer slope. As much as 500 m of clastic sediments, largely reduced-facies black shale, that represent temporal equivalents of the iron-formations of the Gunflint and Mesabi Ranges, covered the iron-formations of the Gogebic and Marquette Ranges by 1850 Ma. Just north of the Marquette Range, the SIL lies on a chert-carbonate unit of the Michigamme Formation, which is the southern temporal equivalent of the Gunflint Formation. Thus, at 1850 Ma iron- 9 �formations were being deposited on distal (shoreward) parts of the outer slope while black shales were being deposited on more proximal, deeper parts of the slope. Figure 1. Reconstruction of the Lake Superior region at 1850 Ma based on lithofacies immediately below the Sudbury impact layer. Area as shown has been foreshortened by 30 km across the Midcontinent Rift to restore relations prior to Mesoproterozoic extension. In the southernmost part of the basin, deep-water iron-formations of the Iron RiverCrystal Falls district occur directly beneath the SIL indicating that an additional ironformation facies was deposited deep within the axial zone of the basin near the advancing overthrusting arc terrane. Thus, the reduced shale facies deposited on the deeper portions of the outer slope passed southward to at least a brief period of ferruginous chemical sedimentation in the sediment-starved axial zone of the foreland basin. References Addison, W.A., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology, v. 33, p. 193-196. Cannon, W.F., Schulz, K.J., Horton, J.R., Jr., and Kring, D.A., 2009, The Sudbury impact layer in the Paleoproterozoic iron ranges of northern Michigan: Geological Society of America Bulletin, v. 121, in press Jirsa, M.A., Weiblen, P.W., Vislova, T., and McSwiggen, P.L., 2008, Sudbury impact layer near Gunflint Lake, NE Minnesota: Institute on Lake Superior Geology Proceedings, v. 54, p. 4243. Schulz, K.J., and Cannon W.F., 2007, The Penokean orogeny in the Lake Superior region: Precambrian Research, v. 157, p. 4-25. 10 �MAGNETIC ANOMALIES FROM PLEISTOCENE SOURCES IN THE WESTERN LAKE SUPERIOR REGION: THE EDGE OF INSANITY OR A PROMISING THRESHOLD? CHANDLER, Val W., Minnesota Geological Survey, 2642 University Ave., St. Paul, MN 55114, chand004@umn.edu Over the last few decades high-quality aeromagnetic data in Minnesota, Wisconsin and Lake Superior has been crucial to interpreting the geology of the Precambrian bedrock, which lies concealed beneath a nearly continuous cover of Pleistocene glacial deposits. It has been generally assumed that Pleistocene deposits are non-magnetic, and thereby “transparent” to the magnetic data, but it is now appears that weak anomalies, generally on the order a few nanoTeslas to a few 10’s of nanoTeslas, are associated with the Pleistocene deposits themselves. Detection of these weak magnetic anomalies, which is usually most effective using derivative-enhanced data, requires that the underlying bedrock be non-magnetic, thereby providing a sufficiently quiet magnetic anomaly background. These quiet background conditions are best developed over thick basins of nonmagnetic sedimentary rocks, such as the Keweenawan sandstone sequences of the Mesoproterozoic Midcontinent Rift System and the slate-greywacke sequences of the Paleoproterozoic Animikie basin and associated outliers. The outlines of these basins are shown in Figure1, along with the interpreted traces of Pleistocene-related anomalies. These weak anomalies have straight to wandering forms that are reminiscent of stream channels, and they appear to be most closely associated with glacial deposits of the Rainy and Superior Lobes, both of which passed over magnetite-enriched bedrock at short distances up-ice. Magnetic susceptibility determinations of the glacial deposits, based either on model studies or on direct measurements of till and outwash samples, indicate that moderate values, generally in the 0.0025-0.0050 SI range, are common. Some of these weak anomalies can be directly related to topographic features, such as the Toimi drumlins of northeastern Minnesota and the Wadena drumlins of west-central Minnesota (Figure 1), but most show little or no correspondence to surface features. The causes of many of these weak anomalies remain unknown, but recent investigations indicate that at least some are related to bedrock valleys that are filled with relatively magnetic glacial materials. In east-central Minnesota these anomalies have been particularly useful in tracing buried valleys in areas with sparse drill-hole control. Weak anomalies in western Lake Superior have been used to trace several deep bedrock channels that may have developed as tunnel valleys beneath Superior Lobe ice. Although little geologic control exists in the Animikie basin, many stream-like anomalies parallel the expected east-west structural grain for the bedrock, and could therefore reflect bedrockcontrolled valleys. Although these channel-like anomalies are geophysical oddities that are somewhat restricted in their occurrence, they have proven to be pertinent to hydrogeologic and Pleistocene studies in the region, and further investigations are warranted. Aeromagnetic data used in this study were acquired with support form the U. S. Geological Survey, the Geological Survey of Canada, and the Minnesota Legislature through the Legislative Commission on Minnesota Resources,. Ship-borne magnetic data from Lake Superior were acquired by the Minnesota Geological Survey, in cooperation with the Large Lakes Observatory. Interpretive work supported by the Minnesota Geological Survey through the State Special Appropriation, the County Geologic Atlas Program, and appropriations from the Minnesota Minerals Coordinating Committee. 11 �Figure 1. Map showing the traces of magnetic anomalies that are interpreted to reflect Pleistocene deposits. Lighter lines designate anomalies that are related to drumlin fields. Stippled areas outline basins of Paleoproterozoic and Mesoproterozoic sedimentary rocks described in text. 12 �METHODS FOR ESTIMATION OF INDIRECT HYDROLOGIC IMPACTS ON WETLAND PLANT COMMUNITIES AT POTENTIAL HARD ROCK MINE SITES Coleman, J. and Chiriboga, E., Great Lakes Indian Fish and Wildlife Commission, Odanah, WI 54861, jcolema1@wisc.edu In areas of mineral development where wetlands are common it is frequently necessary to predict how mine development may affect wetlands through direct and indirect impacts. Direct impacts to wetlands are usually identified as filling or removal of wetlands during mine development, facilities construction, and waste storage or disposal. Indirect impacts to wetlands are less clearly defined but can result from, among other factors, modifications in physical hydrology. Developments in modeling of physical hydrology of ground and surface waters and a better understanding of how wetland communities are tied to physical hydrology allow for estimation of the indirect impacts of mineral development on wetland communities. As part of evaluation of several proposed mine projects, tools for evaluation of mine induced changes to surface and groundwater have been explored. These tools, primarily various surface and ground water models, integrate site specific geologic and hydraulic data into a framework that is based on the current understanding of how waters interact with the land surface and shallow and deep geology. Given an adequate understanding of a site's soils and geology these modeling tools can predict how modifications to the landscape through mining, facilities construction, or waste disposal may effect the level and rate of flow of waters in wetlands. Although these modeling approaches are fairly mature, their success depends on the type and quality of data available on which to base the analysis. Some of the most critical pieces of information for such modeling are the character of bedrock fracturing, the character of surficial materials, and hydraulic links between bedrock fractures and surficial materials. Existing drilling programs for mineral exploration can be adapted to provide some of this information by the analysis of fracture patterns and orientation in core and the retention of data on surficial materials that are penetrated prior to bedrock entry. The methods for integrating geologic and hydrologic information into ground and surface water models have been in use for many years. On the other hand, methods to predict the effect that a change in physical hydrology has on wetland plant communities has, up to this point, been less clearly defined. As part of the effort to evaluate the potential impacts of the proposed Crandon Mine in Wisconsin, methods were developed for identifying the sensitivity of plant communities to water level changes. These methods, while still being refined, present an opportunity to bridge the gap between expected changes in physical hydrology and effects on wetland plant communities. 13 �GEOLOGY OF THE TUSCARORA INTRUSION, NORTHEASTERN MINNESOTA AND ITS RELATIONSHIP TO THE ANORTHOSITIC SERIES OF THE DULUTH COMPLEX COSTELLO, DANIEL E.*, MILLER, JAMES D., Jr., Department of Geological Sciences, University of Minnesota-Duluth, Duluth, MN 55812 (coste082@d.umn.edu), and JIRSA, M.A., Minnesota Geological Survey, University of Minnesota, St. Paul,MN 55455 The petrogenetic relationship between the layered series and anorthositic series of the Duluth Complex is not well understood. The Tuscarora Intrusion, located in the northeastern portion of the complex, is one of the best examples of this ambiguous relationship. Previous work in the Tuscarora within the Long Island Lake quadrangle (Morey et al. 1981) has described the troctolitic and anorthositic lithologies as interlayered on the scale of centimeters to meters. This observation is unique among the layered series of the Duluth Complex. This project seeks to take advantage of recent wildfires within the area to study the relationship between the Tuscarora Intrusion and the Anorthositic Series within the Gillis Lake quadrangle, through field mapping, petrographic observations, and geochemical analyses. The layered series occurs as a number of discrete mafic layered intrusions at the base and mid-levels of the complex, all of which are overlain by a structurally complex cap of anorthositic gabbros of the anorthositic series and granophyric rocks of the felsic series. Field relationships observed over many decades of study throughout the Duluth Complex typically show anorthositic series rock types as inclusions within layered series rocks or show layered series rock intrusive into anorthositic rocks. These observations, along with the very distinctive lithologies and internal structures of the two series, had long been interpreted to suggest that the anorthositic series was significantly older than the layered series (Miller and Weiblen 1990). However, U-Pb zircon ages show that the two units are essentially the same age (1099 Ma +/-0.5 Ma; Paces and Miller 1993). These age data imply not only that the two main stage rock series of the Duluth are approximately the same age, but also that they may be comagmatic or at least part of the same magmatic event. This possibility of a closer genetic relationship between the two series is actually supported by many gradational to ambiguous relationships between the two series, which in the past had been largely ignored as inconsequential anomalies (Paces and Miller, 1993). The Tuscarora Intrusion is one of the best examples of this ambiguous relationship, as described by Morey et al.(1981). This study found no direct evidence for interlayering between the troctolitic and anorthositic lithologies. Rather, the anorthositic series were found to be elongate inclusions within the troctolitic rocks of the Tuscarora Intrusion. The anorthositic inclusions are concentrated in the upper portion of the Tuscarora, as described below. Geochemical analyses of samples collected from both series show similar mineral chemistries and trace element behavior trends. This is interpreted to suggest that the two lithologies are closely related and may be part of the same magmatic event, even though they are not interlayered. These results agree with other studies where anorthositic rocks have been found as inclusions within the layered series. An unexpected discovery of this project is that the Tuscarora Intrusion can be divided into two distinct lithologic zones, based on modal mineralogy, textural patterns, internal structure, and inclusion type and amount (Fig. 1). Moreover, each zone can be divided into a couple of distinctive units. The lower zone (LZ) is somewhat heterogeneous, with compositions ranging from olivine gabbro to augite troctolite. A thin basal unit (Tbh) is very taxitic, with local biotite and orthopyroxene suggestive of footwall contamination. The augite troctolite unit (Tat) contains welldeveloped but variably oriented foliation and modal layering. Some of this structural variability may be due in part to the presence of very large (up to 100s of meters across) mafic hornfels inclusions (unit Thf, fig. 1), which occur throughout the Tat unit. These inclusions are interpreted to have been 14 �derived from the North Shore Volcanic Group, into which the Tuscarora and other Duluth Complex intrusions were emplaced. In contrast, the upper zone (UZ) is much more homogenous and consistently troctolitic. It can be subdivided into a thin melatroctolite basal unit (Tmt or Tum) that grades upwards into a finegrained, well-foliated troctolite to leucotroctolite (unit Ttr or Tut). This unit contains an abundance of anorthositic-type inclusions as described above, presumed to have been derived from the overlying anorthositic series (unit Aau). In addition, a large inclusion of the adjacent Poplar Lake inclusion (unit Pgb) has been identified by previous mapping (Morey 1981). A troctolitic dike (unit Ttd) has been mapped in the western portion of the map area extending from the upper zone, through the lower zone and into the footwall sedimentary rocks. This dike is approximately 80 meters across and contains several small inclusions of poikilitic troctolitic anorthosite. Based on results from field mapping, petrographic observations, and geochemical studies, the two zones of the Tuscarora Intrusion are interpreted to represent successive injections of melt within a single magma chamber. The Lower Zone was emplaced first, and encountered NSVG lithologies located between the recently formed Anorthositic Series and footwall rocks. This unit began to crystallize, followed by the introduction of a replenished melt to the chamber. The Anorthositic Series served as a hanging wall to this newly emplaced magma, and was incorporated as elongate inclusions in the roof portion of the crystallizing Upper Zone. This poster presentation focuses on the geologic map produced as part of this study. Support for mapping was provided by the Educational Component of the National Cooperative Geologic Mapping Program (EDMAP) of the United States Geological Survey. Geochemical studies were supported by grants from the Institute of Lake Superior Geology and from the Dept. of Geological Sciences and the Precambrian Research Center at UMD. References Miller, J.D., Jr., and Weiblen, P.W., 1990, Anorthositic rocks of the Duluth Complex: Examples of rocks formed from plagioclase crystal mush: Journal of Petrology, v. 31, p. 295–339. Morey, G.B., Weiblen, P.W., Papike, J.J., and Anderson, D.H., 1981, Geologic map of the Long Island Lake quadrangle, Cook County, Minnesota: MN Geol. Surv. Misc. Map Series, M-46, scale 1:24,000 Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: New insights for physical, petrogenetic, paleomagnetic and tectono-magmatic processes associated with the 1.1 Ga Midcontinent Rift system. Journal of Geophysical Research, v. 98, no. B8, p. 13,997-14, 013. Pgb Figure 1: Generalized bedrock geologic map of the Tuscarora Intrusion and related rocks within the Gabimichigami and Gillis Lake quadrangles. Unit descriptions are provided in text. 15 �QUANTIFYING ASSIMILATION VS. FRACTIONAL CRYSTALLIZATION USING Sm-Nd, Lu-Hf AND Pb ISOTOPE SYSTEMS: THE GEOCHEMICAL EVOLUTION OF THE SONJU LAKE INTRUSION, FINLAND, MN DAYTON, R. N. and MILLER, J.D. Jr., Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 VERVOORT, J. D., Dept. of Geology, Washington State Univ., Pullman, WA 99164 The Sonju Lake Intrusion (SLI), located within the Beaver Bay Complex near Finland, MN, is the most completely differentiated intrusion related to the 1.1Ga Midcontinent Rift System (Miller and Ripley, 1996). The Finland granite, which is composed of micrographically-textured leucogranite to ferromonzonite, forms the hanging wall of the SLI. The SLI exhibits a cumulate stratigraphy consistent with closed system differentiation of tholeiitic magma by fractional crystallization (Stevenson, 1974; Miller and Ripley, 1996). Field relationships from outcrop and drill core through the SLI and the overlying Finland granite show a cyclic to irregularly gradational contact between the two bodies. This relationship, the smooth compositional variations across the contact and the parallel zonation of the two subunits of the Finland granite with the strike of the mafic cumulates of the SLI are consistent with the Finland granite being a late stage felsic differentiate of the SLI (Miller and Ripley, 1996). However, geophysical modeling of gravity and aeromagnetic data implies a volume of granite that approaches that of the SLI and therefore exceeds the volume of felsic material that could be accounted for by differentiation of a mafic body the size of the SLI (Miller et al., 1990). Miller and Ripley (1996) suggested that the earlier emplacement of the Finland granite acted as a density barrier to the upward movement of the mafic SLI magma. Underplating of the hot mafic SLI magma would be expected to lead to melting in the lower portions of the Finland granite. This poses a fundamental question of what portion of the apparent differentiation of the SLI is related to fractional crystallization, and how much is related to assimilation of a felsic partial melt from the granite. Major and minor element whole rock data are inadequate to the task of distinguishing these processes because the Finland Granite geochemically resembles an upper differentiate of the SLI. However, a radiogenic isotope study of these two systems has the potential to address the question, as reconnaissance isotopic data show that the granite has a radiogenic isotope signature that is distinct from the mafic rocks of the SLI (Vervoort, Unpublished data). To evaluate the roles of fractional crystallization and assimilation in the crystallization of the SLI, a total of 21 samples were collected from outcrop and drill core for analysis for Sm-Nd, HfLu, and Pb isotopes. The analyses were conducted at the radiogenic isotope facility at Washington State University using a Finnegan Neptune MC-ICPMS. Pb isotope compositions were analyzed for all 21 samples and 16 samples were chosen to measure Sm-Nd and Lu-Hf isotope compositions. These data were combined with Nd data from 8 samples in a previous reconnaissance study (Vervoort, 1996, unpublished data) to profile of the isotopic variation through the Sonju Lake intrusion and up into the overlying Finland Granite. The sample locations are shown on Figure 1. All of the samples collected from the exposed eastern area of the SLI and Finland Granite (Fig. 1) show initial epsilon Nd values for the SLI that are consistent with other uncontaminated, mantle-derived mafic volcanic rocks of the rift (epsilon Nd 0 ± 2, Vervoort et al., 2007). Samples from the Finland Granite yield moderately radiogenic initial epsilon Nd values of ≈ -3.5. Only minor contamination effects are evident in the uppermost SLI cumulates. However, a surprising result came from six samples collected from a drill core (SLI-1) that penetrates the transition zone between the SLI and the granite about 7 km to the west of the exposure area (Fig. 1). These samples, taken mostly from the well-foliated apatite ferrodiorite cumulates of the slad unit (Fig. 1), yield the most radiogenic initial epsilon Nd values, ranging from -4.1 to -5.2. This may imply that the Finland granite has isotopic heterogeneities, which has been shown by Beard (2008) to be possible in magmatic systems formed by partial melting. Several follow up samples were submitted to better 16 �understand this discrepancy. We hope to have these results and our best interpretation of these data available at the time of our presentation. References Beard, James S., 2008. Crystal-melt separation and the development of isotopic heterogeneities in hybrid magmas Journal of Petrology (May 2008), 49(5):1027-1041 Miller, J.D., Jr, Schaap, B.D., and Chandler, V.W., 1990, The Sonju Lake intrusion and associated Keweenawan rocks: Geochemical and geophysical evidence of petrogenetic relationships. 36th Annual Institute on Lake Superior Geology, p. 66–68. Miller, J.D., Jr., Green, J.C., Chandler, V.W., and Boerboom, T.J., 1993, Geologic map of the Finland and Doyle Lake quadrangles, Lake County, Minnesota. Minnesota Geological Survey Miscellaneous Map Series M-72, 1:24,000 scale. Miller, J.D., Jr., and Ripley, E.M., 1996, Layered intrusions of the Duluth Complex, Minnesota, USA. in Cawthorne, R.G. (ed.):Layered Intrusions: Amsterdam, Elsevier, p. 257-301. Stevenson, R.J., 1974. A mafic layered intrusion of Keweenawan age near Finland, Minnesota. M.S. Thesis, University of Minnesota, Duluth, 160 pp. Vervoort, J.D., Wirth, K., Kennedy, B., Sandland, T. and Harpp, K.S., The magmatic evolution of the Midcontinent rift: new geochronologic and geochemical evidence from felsic magmatism, Precambrian Research 157 (1–4) (2007), pp. 235–268 17 �TACONITE-DERIVED MINERAL DUST IN POPULATION CENTERS ON MESABI IRON RANGE: TRACKING MINERAL FIBERS FROM ORE TO AIR Tamara Diedrich, Devon Brecke, Megan Schreiber, Larry Zanko, Natural Resources Research Institute, University of Minnesota Duluth In an effort to address long-standing questions regarding the impact of dust derived from mining taconite on human health, the University of Minnesota is conducting multiple complementary health-related studies, including an exposure assessment, epidemiology studies, and exposure characterization research. As one of these studies, NRRI performing a detailed characterization of the dust that is produced from mining and processing Biwabik Iron Formation ore, with emphasis on any mineral fibers present and the elongated mineral particles that are produced from mining and processing activities. The characterization of mineral fibers and elongated particles begins with the examination of fibrous minerals in situ from thin sections of metamorphosed and unmetamorphosed ironformation (fig.). We are also looking at the crushed material corresponding to these thin sections (fig.), and the particulate matter present in the air of the taconite operations where it is mined and processed. Finally, we are conducting a three-year long, field-based study of taconite-derived mineral dust present in the air of communities directly surrounding the taconite operations of the Mesabi Iron Range. The community air sampling will result in long-term average mineral fiber concentrations in ambient air at these locations; characterization of any mineral fibers that are found using metrics relevant to their impact on human health (aerodynamic diameter, dimension, mineralogy, and chemistry); and average total particulate matter and its size distribution at these locations. Retrospective observations will be made using dated lake sediment cores from the region. Particulate matter (in ambient community air, within taconite operations, and aerosolized crushed material) is being collected using a Micro-Orifice Uniform-Deposit Impactor and on a total filter. These samples are then analyzed by a gravimetric method, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. Figure: BSE SEM image of Minnesotaite needles in thin section of unmetamorphosed Biwabik Iron Formation (left); SEI SEM image of Minnesotaite particle liberated from crushing that same rock (right). 18 �GEOLOGY AND GEOCHEMISTRY OF THE FEARLESS-PYTHON PROPERTY, SCHREIBER-HEMLO GREENSTONE BELT, ONTARIO FAGE, Adam and HOLLINGS, Pete, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada The Fearless-Python property, owned by Metalcorp Limited, is located approximately 50 kilometres east of the town of Marathon, Ontario and is situated within the Schrieber-Hemlo greenstone belt of the Wawa subprovince of the Archean Superior Province. Fearless-Python has been extensively explored over the past 40 years, mainly being examined for Hemlo-type gold deposits. The property is sandwiched between the Cedar Lake Pluton (2688 – 2687 Ma, Corfu and Muir, 1989) to the north and the Pukaskwa Batholith (2719 and 2688 Ma, Corfu and Muir, 1989) to the south. The geology is dominated by generally east-west trending and northerly dipping metasedimentary rocks with mafic to intermediate metavolcanic rocks and minor felsic metavolcanic rocks. High level intrusive dykes and Proterozoic diabase dykes crosscut all lithologies (Thompson and Paakki, 2001). The entire Schreiber-Hemlo Greenstone belt has been affected by lower to mid amphibolite facies metamorphism (Pan and Fleet, 1993). The geology of the property is favorable for several deposit types, including; 1) the Gouda shear zone which hosts gold mineralization lies within the southern portion of the property and the major structural trend which is host to the Hemlo gold deposit, directly to the east, is also present within the property. The highest gold values are located within the Gouda Lake Horizon which occurs at the base of a well-developed, potassically altered quartz eye sericite schist hosting gold as well as disseminated and semi-massive to massive sulphides consisting of pyrite, pyrrhotite, sphalerite and lesser galena 2) VMS mineralization has been recognised at a number of locations within the property. Significant occurences of massive sulphides are present at two locations on the property, as well as several smaller zones of anomalous Zn, Cu, Pb values. 3) Molybdenite occurs as coarse aggregates in crowded feldspar porphyry and granite pegmatite dykes in the Duck Lake area north of the Gouda shear zone. 4) Possible Outokumpu-type Ni-Co-Zn-Cu mineralization has also been reported at two locations on the property. Prelimary analysis of trace element geochemistry shows that the metavolcanic rocks range from calc-alkaline to tholeiitic in composition. Primitive mantle normalized plots shown in Figure 1 indicate island arc and MORB-like affinities for the metavolcanic rocks. Felsic metavolcanic rocks have MgO contents of 0.2-1.4 weight percent, SiO2 of 68.3 weight percent and La/Sm, Gd/Yb ratios of 2.11-5.17 and 2.19-5.33 respectively. Intermediate and mafic metavolcanic rocks have MgO contents of 3.5-8.9 weight percent, SiO2 of 47.4-58.2 weight percent and La/Sm, Gd/Yb ratios of 0.64-3.31 and 1.04-4.72 respectively. Spatial relationships suggest that there may be a genetic relationship between the different deposit types at the Fearless-Python property. Hemlo-type gold and VMS mineralization occur together in the Gouda Lake Horizon. Molybdenite occurs in close proximity to the north of the Gouda shear zone. The CADI zone, which is a nickel prospect, occurs along strike to the west of the Gouda shear zone. Additional work in the summer of 2009 will 19 �utilise drill core and surface mapping to explore these relationships and test the model that the multiple mineralization events are genetically related. Figure 1. Representative primitive mantle normalised diagram for volcanic rocks of the Fearless-Python property (normalising values from Sun and McDonough, 1989). References Corfu, F. and Muir, T.L., 1989. The Hemlo-Heron Bay greenstone belt and Hemlo Au-Mo deposit, Superior Province, Ontario, Canada. I: Sequence of igneous activity determined by zircon U-Pb geochronology. Chemical Geology. 79:183-200. Fleet, M.E. and Pan, Y.,, 1991. Metamorphic Petrology of the White River Gold Prospect, Hemlo Area. Ontario Geological Survey, Grant 305, Final Report. 47 pp. Sun, S.-s., and McDonough, W.F. 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In Magmatism in the ocean basins. Geological Society, Special Publication No.42: 313-345. Thompson M. and Paakki, J., 2001. Assessment Report on the 2000 Exploration Program on the White River Property, Bomby, Brothers and Laberge Townships, Ontario. Teck Exploration Ltd. Report No. 1340. 20 �ALTERATION IN THE SOUTHERN FELSIC VOLCANICS AT MARSHALL LAKE, NORTHWESTERN ONTARIO NATHAN R. FORSLUND1,2 (nforslun@lakeheadu.ca), MARY LOUISE HILL1 and ROBERT S. MIDDLETON2 1 Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada. 2 East West Resource Corporation, Thunder Bay, Ontario, Canada. The Marshall Lake property is a copper-zinc-rich volcanic-hosted massive sulfide (VHMS) deposit located approximately 255km northeast of the city of Thunder Bay, Ontario. The study area consists of a series of Archean rocks including metavolcanics that range in composition from mafic to felsic, and metasedimentary units, both clastic and chemical. The Summit Lake pluton is another significant lithology that may have played a role in driving the ore-bearing fluids, since it is penecontemporaneous with the surrounding metavolcanics. In the past, most work in the area has concentrated on the northern part of the sequence where most of the mineralization is known to occur. In 2006, East West Resource Corporation acquired the property, and since this time there has been an increased effort to understand the deposit as a whole; not only the area proximal to the mineralization, but also the distal rocks to the south. The alteration and subsequent metamorphism at Marshall Lake are typical for a bimodal mafic VHMS type deposit. The metamorphic assemblages present in the southern felsic metavolcanics represent depletion in sodium and silica and enrichment in magnesium and potassium when compared with an unaltered rock of similar composition. Most of the mapped area, especially along the contact between the metavolcanics and metasedimentary rocks, are dominated by the metamorphic assemblage garnet-amphibole. The intensity of this alteration decreases with distance from this contact. In the genesis of a VHMS deposit two types of fluids can contribute to the alteration geochemistry: the ore-bearing fluids that penetrate the siliceous cap rock underneath the precipitation site, and the convecting seawater that enters through fractures in the seafloor. The latter fluid type would result in the enrichment in potassium and magnesium that is seen. The evidence seen in the field, through petrography, and in the geochemistry is suggestive that seawater had more effect on the alteration of the southern volcanics than the ore-bearing fluids, however a background signature from the ore-bearing fluids is still present as can be seen with the depletion of sodium and silica. If this is indeed the case then it would indicate that the metasedimentary units to the south of the metavolcanics, consisting of banded iron formation within clastic sediments, may have been the paleoseafloor during the genesis of the deposit, and the volcanics to the north would have underlain these sediments. The sequence would then be regionally topping to the northeast, which agrees with the few structural measurements that are available. This would have some economic significance as well, since it may warrant exploration for the presence of a lead-zinc-rich horizon within (or in close proximity to) the banded iron 21 �formation, since lead and zinc would precipitate out at the top of the stratigraphic pile. Geophysical surveys used in the past to identify targets would have been difficult to interpret, since the iron formation itself is such a strong conductor and has such a high magnetic susceptibility. 22 �VERMILION GREENSTONE GOLD - NEW DATA, NORTHEASTERN MINNESOTA Frey, B.A., Minnesota DNR – Lands & Minerals Division, 1525 3rd Avenue East, Hibbing, Minnesota, 55746 Barry.Frey@dnr.state.mn.us A petrologic and geochemical reexamination of thirty archived sets of drill samples from the Vermilion Greenstone Belt in Northeastern Minnesota has revealed previously unrecognized gold-bearing intervals and mineral associations supporting new or additional gold mineralization models with individual prospects. Gold concentrations as high as 148ppm were observed during the collection of 3,772 semi-quantitative XRF analyses. The presence of acicular sodic-rich amphiboles in a sequence with gold-bearing chert-graphite-sulfide layers suggests that gold mineralization may be associated with sodic metasomatism. The Vermilion Greenstone Belt has long been associated with gold. In 1865, a “gold rush” occurred in the area around Lake Vermilion. Appreciable gold was not found at the time, but it did lead to the discovery of direct shipping iron ore at Soudan, Minnesota and eventually Ely, Minnesota. The Soudan Mine and the five major mines in Ely produced about 100 million tons or ore. Exploration also led to the developing of numerous smaller mines in the greenstone. Many of these did not even warrant a rail spur, but the ensuing exploration activities and relatively good outcrop exposure encouraged future work for other metals, including gold. Most Vermilion greenstone rocks have been subjected to greenschist facies metamorphism. Outcrop exposure is variable, but good compared with most of Minnesota. Past sampling, maps, and reports have been produced by the Minnesota Geological Survey, the Natural Resource Research Institute, and the Minnesota DNR. Exploration by at least fifteen exploration companies have produced historic drill hole samples, geochemistry, geophysical work and other data A known, widespread presence of anomalous gold within the Vermilion “greenstone” is one fruit of these efforts. Our work has included drill sample logging; semi-quantitative, real-time “hand-held” XRF chemistry; and limited assay and microprobe work in order to better elucidate the varied gold occurrences. The .76 cm2 XRF sample size complements the visual descriptions, and the grain size of most rocks. The sample size provides direct elemental associations with gold associated with discrete mineral grains. XRF traverses are also useful for zoning associated with veins and alteration fronts. The context of the sampling size, however, must be maintained when examining this data. Besides providing more detailed element associations with each prospect and encountered mineralization type, the XRF has also allowed for better physical placement of gold mineralization found within anomalous previous assays. Visible clues may be established since gold mineralization may be otherwise hidden. XRF element associations have been found with a summary in Table 1. Several gold mineralization types occur. Note, however, that the values for Foss Lake and Eagles Nest Prospects are based on a smaller number of semi-quantitative XRF readings with anomalous measurable gold. Also, the rutile association of the “Raspberry” is from visible logging. The rutile is found in close proximity to XRF 23 �anomalous gold, but not as intimately as with galena. All the prospects probably have several geochemical processes involved in determining the final gold locations and associations. Table1 - XRF Element Associations Prospect Raspberry Au Mineralization Type Intrusion hosted Semiquantitative XRF Value Au Association Pb (galena), Quartz veins, To 101 ppm Au Rutile(?); Fe, As, Mn, Cr, Se, Sn Raspberry Shear zone related (remobilization?) To 67 ppm Au Pb (galena), Quartz veins, Rutile?; Pb, Ag, Se Foss Lake Algoma BIF related Au To 28 ppm Au Fe oxide to Sulfide-graphite transition; As, Ba, Pb Shear zone related Pyrite; Mn, Sr, Ba, Mo?, Cu? Eagles Nest Shear Murray Shear To 4 ppm Au Volcanic Hosted Massive Sulfide? To 148 ppm Au Sphalerite, Cr, Zn, Sb, Cd, Hg Foss Lake Prospect drill core. The first Foss Lake Prospect drill core examined (DDH# 6314-36-1) contained a sequence of iron formation within basalts. Previously unknown gold was found within disturbed chert and graphite interlaminations at the broad transition from oxide-silicate-carbonate BIF to sulfide-graphite BIF. Away from the transition, the sulfide iron formation was heavily assayed with minimal Au. The elevated XRF gold was associated with elevated As, Cu, Co, Pb, and Mo. Chert layers within the sequence were locally noticeable because of a slight bluish cast exhibited in their appearance. Hand lens examination showed the presence of pale acicular fibers in and on the margins chert. Microscope examination indicated the minerals were amphiboles. Very fine grains were also scattered throughout the chert. The bluish minerals appear to be sodic amphiboles, probably crossite, and indicate probable sodic metasomatism. The nature of the association with the gold mineralization is unknown. A “dacitic” volcaniclast also had elevated gold. Minor dolostone and ultramafics also occurred within this drill core. 24 �MAGMATIC VS. HYDROTHERMAL PROCESSES IN THE SOUTH FILSON CREEK MINERALIZATION,SOUTH KAWISHIWI INTRUSION, DULUTH, COMPLEX BENEDEK GÁL, Eötvös Loránd University, Budapest, galbenedek@yahoo.com DEAN M. PETERSON, Natural Resources Research Institute, UMD, dpeterson@duluthmetals.com FERENC MOLNÁR, Eötvös Loránd University, Budapest, molnar@abyss.elte.hu The South Filson Creek (SFC) deposit (located in Sections 25 and 36, Township 62 North, Range 11 West) occurs above the basal units of the South Kawishiwi Intrusion (SKI), and represents a unique geological setting of Cu-Ni-PGE mineralization within the Duluth Complex (DC). It is located in an approximate stratigraphic high of 1000 m above the basal contact unlike all other known ore occurrences in the DC. Researchers have traditionally referred to this area as location of „structurally controlled” mineralization as signs of hydrothermal alteration has been described in previous papers (Kuhns et al. 1990, Severson & Hauck, 2003), however this attribute has to be revised in some points. The nature of hydrothermal processes overprinting the primary magmatic features and their significance in ore-genesis have been characterized in details during our current studies. Extensive field mapping has revealed mineralization both in the Layered Series troctolites and in the Anorthositic Series in the SFC. Sulfides in the Layered series appear as disseminated fine-grained patches and pockets in an area of approx. ¼ square kilometer only. This type of mineralization was in the main focus of exploration so far. Pyrrhotite, chalcopyrite, pentlandite and cubanite are the main ore-forming sulfide minerals with subordinate amount of other copper-bearing sulfides, presumably secondary in origin (bornite, covellite, talnakhite). Sulfides form interstitial blobs between cumulus silicates, fine-grained disseminations and microscopic veinlets. At least 5 different platinum group minerals have been distinguished in the samples, associated both with magmatic sulfides and secondary hydrothermal alteration products. Highest metal values for this type of mineralization were 1.25 wt% Cu and 0.2 wt% Ni and 2.4 ppm Pd, 1.2 ppm Pt and 0.4 ppm Au. Brittle lineaments in the area do not affect the distribution of sulfides. The magmatic mineralization in the troctolites of the SFC area are similar to the „confined”- style of mineralization (Peterson 2001, 2002). Mineralization in the Anorthositic Series is most likely hydrothermal in origin as its occurrence shows strong correlation to brittle structures and associated secondary alteration products. It shows only elevated copper-values (but still not as high as in Layered Series) up to 0.2 wt% Cu and some silver enrichment (up to 3 ppm Ag) but no PGE or Ni showings. Based on petrographic work, three types of hydrothermal alteration were possible to distinguish in the SFC area: • Alteration products of the first event can only be found in the Layered Series rocks. Serpentinization of olivine, chloritization of mafic minerals, albitization of plagioclase and several secondary sulfide minerals (bornite, covelline, talnakhite) are the product of this alteration event. Redistribution of PGEs have likely occurred due to the elevated Cl-content and salinity of the migrating fluids, however transportation of PGEs on bigger distances did not happen. Magmatic fluids containing high chlorine concentrations most likely were segregated from 25 �• • the crystallizing troctolitic melt, which process was documented in the Fenrichment and Cl-depletion trend of apatite in pegmatoidal samples and the presence of highly saline fluid inclusions in apatite. The second hydrothermal event can be observed both in the Layered Series and in the anorthosites and was most likely responsible for the formation of the mineralization in the Anorthositic Series rocks. Alteration products are chlorite, fibrous green amfiboles, sericite, prehnite, pumpellyite, carbonate and pyrite. Temperature of the fluids (based on chlorite thermometry and paragenesis) was between 250 and 350°C, pH was near neutral. The fluid was not capable of mobilizing precious metals. The third event is marked by rusty joints throughout the whole area. The joints are filled with rusty, serpentine-like material but alteration is not extensive around them and they do not have any significance regarding mineralization. References Kuhns, M.J.P., Hauck, S.A., and Barnes, R.J. (1990): Origin and occurrence of platinum group elements, gold and silver in the South Filson Creek copper-nickel mineral deposit, Lake County, Minnesota: Duluth – University of Minnesota, Natural Resources Research Institute, Technical Report, NRRI/GMIN-TR-89-15, 60 p. Peterson, D. M. (2001): Development of a conceptual model of Cu-Ni-PGE mineralization in a portion of the South Kawishiwi Intrusion, Duluth Complex – Minnesota; Society of Economic Geologists, 2nd Annual PGE Workshop, Sudbury, Ontario, p.3 Peterson, D. M. (2002): Cu-Ni-PGE Mineralization in the South Kawishiwi Intrusion, Northeastern Minnesota; Variation due to Magmatic Processes – Institute on Lake Superior Geology, 48th Annual Meeting, Thunder Bay, Ontario, Proceedings vol. 48. Severson, M. J. & Hauck, S. A. (2003): Platinum group elements (PGEs) and platinum group minerals (PGMs) in the Duluth Complex – Natural Resources Research Institute Technical Report NRRI/TR-2003/37, p. 296. 26 �CHARACTERIZING THE DISCHARGE FEATURES OF GLACIAL LAKE AGASSIZ DURING THE POST-MARQUETTE PERIOD USING MARINE SEISMIC-REFLECTION METHODS J.L. Gary, N.J. Wattrus, S.M. Colman, and E.B. Voytek, Large Lakes Observatory & Department of Geological Sciences, University of Minnesota – Duluth, Duluth, MN 55812 Glacial Lake Agassiz was the largest of the North American glacial margin lakes. Over its 4,000 year existence, Lake Agassiz varied substantially in aerial extent and volume. This variability was a function of the fluctuating retreat pattern of the Laurentide Ice Sheet’s southwestern margin, differential isostatic rebound of the North American crust, the topography of the land exposed by the retreating ice, and erosion of the various outlet channels draining the lake. These factors combined to form a history of Lake Agassiz punctuated by sudden and sometimes catastrophic rerouting of its drainage from one outlet channel to another (Teller, 2001). The amount and routing of Lake Agassiz discharge has become controversial. However, extensive onshore observations of Glacial Lake Agassiz discharge features have firmly established that northwestern Lake Superior was a major drainage route following the retreat of the Marquette glacial advance ca. 9,500 years 14C BP (Clark et al, 2001; Teller et al, 2002). We describe a high-resolution single channel seismic reflection dataset collected with a small airgun that we acquired to test our hypothesis that this drainage event (corresponding to the Nipigon Phase of Lake Agassiz) left diagnostic stratigraphic and geomorphic signatures beneath Lake Superior. The unique bathymetry of northwestern Lake Superior, where water depth plunges off Nipigon and Black Bays, makes this location ideal for the identification and characterization of the Post-Marquette depositional features. The steep and sudden dropoff from the shallow water bays into the deep offshore waters of the lake would have caused the high-velocity floods to slow and drop much of the sediment they were carrying. Our results confirm the existence of these sediment packages, which are now buried below a thin blanket of Holocene sediment. They form wedges of sediment that are thickest (some over 70 m thick) in the deep water area adjacent to the flood outlet. The apron of sediment thins lakeward and shore-parallel away from the outlet. The seismic character of the basal units of the apron, proximal to the outlet, is chaotic and only very weakly stratified suggesting that these deposits represent coarse sediment laid down during the initial stages of the flood when flow was presumably at its peak. These sediments are overlain and draped by a weakly stratified package that is more widely developed (extending lakeward beyond the bounds of our survey). We interpret this unit, which becomes more stratified and thinner lakeward, to represent the fine grained sediment associated with the latter stages of the flood when flow had eased. 27 �References Clark, P.U., Marshall, S.J., Clarke, G.K.C., Hostetler, S.W., Licciardi, J.M., and Teller, J.T., 2001, Freshwater forcing of abrupt climate change during the last glaciation: Science, v. 293, p. 283-287. Teller, J.T., 2001, Formation of large beaches in an area of rapid differential isostatic rebound: the three-outlet control of Lake Agassiz: Quaternary Science Reviews, v. 20, p. 1649-1659. Teller, J.T., Leverington, D.W., and Mann, J.D., 2002, Freshwater outbursts to the oceans from glacial Lake Agassiz and their role in climate change during the last deglaciation: Quaternary Science Reviews, v. 21, p. 879-887. 28 �2009 Update: Leasing State of Michigan Lands for Metallic and Nonmetallic Minerals Milton A. Gere, Jr. and Thomas B. Hoane, Michigan Department of Natural Resources, Forest, Mineral and Fire Management, P.O. Box 30452, Lansing, MI 48909-7952 The Department of Natural Resources offers leasing programs for state-owned mineral lands for the exploration and development of oil and gas, underground gas storage and metallic and nonmetallic minerals. 2009 Information on State of Michigan Metallic and Nonmetallic Mineral Leasing Programs: State Ownerships - Three major categories Fee - own both surface and mineral rights (4.0 million acres) Surface - own surface rights only (547,458 acres) Minerals - own severed mineral rights only (2.3 million acres) The state also owns 25 million acres of Great Lakes Bottomlands, which includes the mineral rights; however these are not open to exploration or development. Leases for Metallic and Nonmetallic Minerals – There were 45,240 acres under 200 State Metallic Mineral Leases and 3,688 acres under 48 State Nonmetallic Minerals Leases at the end of FY 2008. Leasing Process - Nominations – Sealed-bid Auctions - Direct Leases – Fees, other requirements: Lands nominated for leasing are field reviewed by Department of Natural Resources (DNR) foresters, wildlife biologists and fisheries biologists. Input is requested from other specialists. Lands are classified in four categories: 1) Leaseable; 2) Leaseable with Restrictions, 3) Leaseable, nondevelopment, and 4) Nonleaseable. Public notice is given, public input is requested, and any private surface owners are notified. Before classified lands are leased, final approval must be received from the DNR, and two state approval boards. Performance bonds and insurances are required. A mining permit and other permits, such as air quality, water discharge, and others, may be required by the Michigan Department of Environmental Quality (DEQ) and possibly other agencies prior to mining. Metallic Mineral Leases - Exploration and potential development - for any metallic mineral commodity found. Nomination fee is $300.00 for up to 640 acres within four contiguous sections. Lease terms are 10 years, with possible extension, or held while producing. The Direct Lease process is currently used. Upon leasing - Bonus fee (one-time) $3 per acre. Annual rental of $3 per acre increases to $6 on sixth year. (February 2009 rates, subject to revision) Approved exploration plans and surface use permits required for intrusive exploration. Approval to produce and approved mining and reclamation plan required. Production royalty rate is a percentage of selling price, by commodity as listed in the lease. Royalty may be renegotiated at a later date. See lease document on website for details. Nonmetallic Mineral Leases -- May be nominated by state or individual, currently there is no nomination fee. A. Production of a known nonmetallic commodity from a known location. 1. Sealed Bid Lease Auction - Usually on commodity-specific lease document. Fixed Annual Minimum Royalty. Bid on Royalty Rate per ton, subject to revision every three years based on U.S. Producers Price Index changes. Lease time terms vary, subject to possible extension. 2. Direct Leases --a. To County Road Commissions for sand and gravel from known locations at fixed CRC Royalty Rates, subject to revision every 3 years based on U.S. Producers Price Index b. To adjacent operations or private surface owners, in some cases. Commodity, Royalty rate and other terms negotiated. B. Exploration and potential development for any nonmetallic mineral commodity found, nominations may be for up to 640 acres within four contiguous sections. Sealed Bid Lease Auction --- Bid on the one-time Bonus rate per acre. Royalty rate fixed 29 �by percentage of selling price of commodity(s) produced, as listed in the Lease. Rate percentage may be renegotiated at a later date. C. Specific Commodity Exploration Lease ..Will vary with item. A lease for Potash is currently being developed and a sealed bid lease sale is expected to be held in late spring/early summer, 2009 --------------------------------------------------------------------------------------------------------------------------------------Note --Leases may also require various Surface Use Permits and Fees. Any Intrusive Exploration Activities require an approved Exploration Plan. Various State, local and Federal regulations may apply. Website Information - Go to the DNR Website --- www.michigan.gov/dnr Choose Doing Business with DNR - (on Left side) Open and scroll down to Minerals and open Metallic Minerals (M.M.) – Information, Procedure (policy), Rules, M.M. Lease Document. http://www.michigan.gov/dnr/0,1607,7-153-10368_11800_46635---,00.html Nonmetallic Minerals (N.M.) – Information, Procedure (policy), Rules, General N.M. Lease Document, Sand & Gravel Lease Document. http://www.michigan.gov/dnr/0,1607,7-153-10368_11800_46635---,00.html Choose Publications & Maps (on Left side), Choose On-line Maps ---open and view http://www.michigan.gov/dnr/0,1607,7-153-10371_14793---,00.html Land & Mineral Ownership (Shown as entire 40 acre blocks, number in upper right corner indicates how much land State owns and what type (surface, mineral, surface & mineral, mix.) http://www.michigan.gov/dnr/1,1607,7-153-10371_14793-31345--,00.html Mineral Leases (Left County List – County map with Lease numbers. Right County List List of Lease numbers and Lessee names for the County). http://www.michigan.gov/dnr/1,1607,7-153-10371_14793-30992--,00.html --------------------------------------------------------------------------------------------------------------------------------------Questions? Wish to discuss our Mineral Leasing programs? Contact us: Michigan DNR-- Forest, Mineral and Fire Management -- Mineral and Land Management Section, P.O. Box 30452, Lansing, MI, U.S.A., 48909-7952 Phone: 517-373-7663 Fax: 517-373-2443 Milt Gere, Geologist … Phone: 517-335-3249 E-mail: gerem@michigan.gov Tom Hoane, Geologist … Phone: 517-241-3769 E-mail: hoanet@michigan.gov --------------------------------------------------------------------------------------------------------------------------------------More info. … .For additional information about Michigan’s minerals, geology, Geological Sample and Drill Core Repository and required permits related to mining, etc. …Contact: Michigan Department of Environmental Quality, Office of Geological Survey. Lansing, MI: 517- 241-1515 Gwinn, MI: 906-346-8300 Website – www.michigan.gov/deq or www.michigan.gov/deqogs Choose Land (on Left side), Choose Geology in Michigan, Geological Mapping, or Gas, Oil, and Minerals, etc. (on Left side). A Public Benefit from the leasing and production of state-owned oil & gas and minerals --Most monies collected in State of Michigan Mineral Lease fees, rentals and royalties go to the Michigan Natural Resources Trust Fund (MNRTF) or the Fish and Game Fund, dependent upon origin of the State land ownership, about a 90/10 split. Local and State governmental units may apply to the MNRTF for grants for the purchase and development of public recreational properties. In Fiscal Year 2008, the State’s income from the leasing and production of state-owned minerals and oil and gas was approximately $104 million. About 90 percent, nearly $94 million was placed into the MNRTF. “Explore Michigan’s Minerals” 30 �BIRD RIVER BELT IN SOUTHEASTERN MANITOBA – A NEOARCHEAN VOLCANIC ARC IN THE WESTERN SUPERIOR PROVINCE GILBERT, H.P. Manitoba Geological Survey (360-1395 Ellice Ave., Winnipeg MB R3G 3P2). paul.gilbert@gov.mb.ca The Neoarchean Bird River Belt (BRB) in southeastern Manitoba is currently the focus of geological and geochemical investigations that have led to a revised interpretation of its tectonic setting and geological history. The BRB is part of a 150 km long, east-trending supracrustal belt that extends from Manitoba eastwards as far as Separation Lake in Ontario. It is located in the Bird River Subprovince within the southwestern Superior Province and occurs in a transitional oceanic–continental-margin setting between flanking older cratonic blocks — the 3.0-2.87 Ga North Caribou Superterrane to the north and the 3.4-2.8 Ga Winnipeg River Subprovince to the south (Percival et al., 2006). The predominant 2.724 Ga arc-type volcanic rocks of the BRB are compositionally and stratigraphically distinct from flanking, mid-ocean-ridge basalt (MORB)–type basaltic sequences that are probably relatively older than the arc-type rocks and may be associated with early arc rifting in a backarc setting (Gilbert et al., 2008). The MORB and arc-type volcanism together spanned at least 20 Ma; a mafic-ultramafic intrusion in the north part of the belt (2.745 Ga Bird River Sill) postdates the MORB-type volcanism but was emplaced prior to the arc-type volcanism. The arc-type volcanic rocks are divided into ‘north’ and ‘south’ structural panels that are each characterized by geochemically and stratigraphically distinct volcano-sedimentary sequences. The north panel rocks are akin to modern subduction-related rocks at active continental margins, whereas the sequence in the south panel documents incipient rifting in an extensional tectonic regime. Subsequent to arc volcanism, orogenic sedimentation (2.71– 2.70 Ga) resulted in the deposition of turbidites (Booster Lake Formation) and fluvialalluvial deposits (Flanders Lake Formation). Detrital zircon data indicate these orogenic sedimentary rocks may be stratigraphically equivalent to epiclastic rocks and metamorphic derivatives in the English River Subprovince, northeast of the BRB. Base-metal mineralization prospects in the BRB include both magmatic types and stratigraphically associated occurrences of probable hydrothermal origin (Gilbert, 2008). The Bird River Sill hosts base-metal and platinum-group-element (PGE) mineralization; elsewhere, base-metal mineralization commonly occurs at lithological contacts within the volcano-sedimentary sequences. The BRB also contains the TANCO mine at Bernic Lake, wholly owned by the Cabot Corporation. The mine produces Ta, Li and Cs from pegmatite and accounts for approximately 80% of global reserves of Cs. References Gilbert, H.P., 2008: Stratigraphic investigations in the Bird River greenstone belt, Manitoba (part of NTS 52L5, 6); in Report of Activities 2008, Manitoba Science, Technology, Energy and Mines, Manitoba Geological Survey, p.121-138. 31 �Gilbert, H.P., Davis, D.W., Duguet, M., Kremer, P.D., Mealin, C.A. and MacDonald, J. 2008: Geology of the Bird River Belt, southeastern Manitoba (parts of NTS 52L5, 6); Manitoba Science, Technology, Energy and Mines, Manitoba Geological Survey, Geoscientific Map MAP2008-1, scale 1:50 000 (plus notes and appendix). Percival, J.A., McNicoll V. and Bailes, A.H. 2006: Strike-slip juxtaposition of ca. 2.72 Ga juvenile arc and >2.98 Ga continent margin sequences and its implications for Archean terrane accretion, western Superior Province, Canada; Canadian Journal of Earth Sciences, v. 43, p. 895–927. 32 �GEOCHEMISTRY AND PETROLOGY OF GUNFLINT IRON FORMATION, GUNFLINT TRAIL, MINNESOTA HAGE, Melissa M.1 and FEDO, Christopher M.1 (1) Earth and Planetary Sciences, Univ. of Tennessee, Knoxville, TN 37996, mhage@utk.edu Near the NW shore of Lake Superior, along the Minnesota-Ontario boarder, the Gunflint Trail provides access to Paleoproterozic sedimentary rocks of the Animikie Group (ca. 18701830 Ma; Fralick et al., 2002), including the Gunflint Iron Formation. Animikie Group rocks crop out along a NE-SW-trending outcrop belt that extends approximately 175 km from ThunderBay, Ontario, where the unit is unmetamorphosed, to ~19 km west of the Gunflint Trail in northern Minnesota, where it is truncated by the Mesoproterozoic Duluth Complex. Metamorphic grade increases to upper amphibolite facies (Floran and Papike, 1978; Jisra andWeiblen, 2007). Although focusing research on metamorphosed Gunflint banded iron formation (BIF) might seem unusual from a sedimentalogical perspective when unmetamorphosed equivalents exist, many Archean BIFs the world over are highly metamorphosed, making an examination of the Gunflint Formation in a locale that has been metamorphosed very appropriate for making comparisons between BIFs of varying age. An ~ 8 m section of BIF from the lower slaty unit of the Lower Gunflint Formation was measured and samples representative of all the major lithologies were collected and facies logged. At field scale, the main lithologies include finely banded magnetite-quartz BIF and coarsely banded magnetite-quartz BIF, with less common centimeter-scale chert-dominated layers containing rip-up clasts of magnetite-rich layers. In thin section, samples range between Fe-silicate and oxide facies BIF, and contain varying amounts of quartz, magnetite, and amphibole. When quartz is present, it is as equant grains with 120º interlocking grain boundaries, which suggests recrystallization, and range in size from ~50 μm to 500 μm. If magnetite is present, it is typically anhedral, ranges in size from ~ 10 μm to 200 μm, and occurs either has disseminated grains between quartz or amphibole grains or as distinct bands. Amphiboles are found in all samples and typically occur as a clotted mass of fine (< 50 μm) equant grains or needles with a few larger (~ 200 μm) grains also present. Similar to the magnetite, the amphiboles occur either as disseminated grains and needles in-between quartz and magnetite grains, or as distinct layers. Some samples also contain trace amounts of very small (5-10 μm) grains of apatite. Samples from the measured stratigraphic section were also analyzed for bulk major-, trace- and rare earth element geochemistry. The major element chemistry of the Gunflint BIF is typical of other BIFs and is dominated by SiO2 (~ 36 to 82 wt%) and Fe2O3(T) (~ 16 to 57 wt%), with much lesser amounts of CaO (~ 0.6 to 3.0 wt%), MgO (~ 0.5 to 5 wt%), MnO (~ 0.1 to 0.5 wt%), Al2O3 (~0.3 to 1.7 wt%), Na2O (~ 0.1 to 0.2 wt%), K2O (~ 0.1 to 0.3 wt%), and P2O5 (~0.1 wt%). Although no evidence for clastic contamination was observed in outcrop or thin section scale, the geochemistry indicates slightly elevated abundances, relative to other very pure chemical sediments, of trace elements typically related to clastic detritus, such as Sc (1 to 3 ppm), Th (0.19 to 1.03 ppm), Hf (0.1 to 0.7 ppm), Zr (3 to 27 ppm), and Rb (1 to 35 ppm). Other evidence for aluminosilicate contamination is the lower Fe2O3/Pr ratios (30 to 76) relative to pure BIF (130 to 317) (Bau and Dulski, 1996). It has been suggested that Precambrian BIFs free from clastic contamination display a similar REE signature, regardless of provenance, age, and metamorphic grade: HREE enrichment, positive LaSN, EuSN, GdSN, and YSN anomalies, negative CeSN anomaly, (La/Sm)CN > 1, (Sm/Yb)SN < 1, and (Eu/Sm)SN >1 (Bau and Dulski, 1996; Bolhar et al., 2004). The PAASnormalized REE + Y (REYSN) plots of preliminary metamorphosed Gunflint samples show 33 �positive Ce, Eu, and Y anomalies (~1.1, 1.5 and 33, respectively) with an overall background slope that reflects depletion of the light REE and enrichment of heavy REE (Figure 1). Typically, seawater, and thus BIF, has a strong negative Ce anomaly, however the composition of chemical sediments also reflects the local redox conditions and is strongly influenced by post-depositional changes, which suggests that the Ce anomalies in these samples may not be primary (Derry and Jacobsen, 1990; Rollinson, 1993). The Eu anomalies are only weakly positive, ranging from 1.31.7, suggesting only minor hydrothermal input into the depositional basin (Klein, 2005). This is not unexpected as the size of the positive EuSN anomaly becomes smaller with decreasing age of deposition (Derry and Jacobsen, 1990; Bau and Dulski, 1996). Only one sample has a positive LaSN anomaly (1.01), with the others being only slightly negative (~0.8), and none of the samples have a positive GdSN anomaly (range from 0.03 to 0.25). Other BIF “fingerprints” are found in the amphibolite-grade Gunflint samples analyzed here, with (La/Sm)CN values ranging from ~2.7 to 3.5, (Sm/Yb)SN values ranging from ~0.7 to 1.1, and (Eu/Sm)SN values ranging from ~1.4 to 1.9. The one exception is the sample that has a (Sm/Yb)SN value slightly greater than 1 (1.14). In conclusion, preliminary analyses of metamorphosed Gunflint BIF geochemistry suggests that BIFs are capable of retaining near original compositions through diagenesis and metamorphism. This coincides with the findings of Frost et al., (2007) which found that Fe isotope heterogenetites in BIF are preserved during diagenesis and metamorphism. However, some of the traditional signatures used to fingerprint BIF, such as a negative Ce anomaly, require further investigation to test their veracity. Figure 1. PAAS-normalized REE + Y diagram comparing compositions from three preliminary samples collected from the amphibolite grade Gunflint Iron Formation, along the Gunflint Trail, Minnesota with BIF from amphibolite grade Isua BIF. Note positive Ce, Eu, and Y anomalies in the Gunflint samples. References Bau, M. and Möller, P., 1993, Geochimica et Cosmochimica Acta, 57, 2239-2249. Bohlar et al., 2004, Earth and Planetary Science Letters, 222, 43-60. Derry, L. and Jacobsen, S., 1990, Geochimica et Cosmochimica Acta, 54, 2965-2977. Floran, R. and Papike, J., 1978, Journal of Petrology, 19, 215-288. Fralick, P., Davis, D., and Kissen, S., 2002, Canadian Journal of Earth Science, 39, 1085-1091. Frost, D. et al., 2007, Contributions to Mineralogy and Petrology, 153, 211-235. Jisra, M. and Weiblen, P., 2007, 53rd Annual Institute on Lake Superior Geology Field Trip Guide 6: Geology along the Gunflint Trail. Klein, C., 2005, American Mineralogist, 90, 1473-1499. Rollinson, H., 1993, Using geochemical data: evaluation, presentation, interpretation, 133-142. 34 �TITANITE, PSEUDORUTILE, AND REE-MINERALS IN THE ALLOUEZ CONGLOMERATE, KEWEENAW PENINSULA, MICHIGAN Edward Hansen1, Jesse Reimink1, Daniel Harlov2 1 2 Geological and Environmental Sciences, Hope College, Holland, Michigan, 49423 GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany The Allouez Conglomerate is an interflow sedimentary unit in the Portage Lake Volcanic Series that has undergone low grade metamorphism, copper mineralization and supergene alteration (Bornhorst, T.J., personal communication, 2008). Mineral associations and textures in 6 samples were investigated with the JEOL LV-4500 scanning electron microscope in the Department of Geophysical Sciences, University of Chicago and selected minerals were analyzed with the CAMECA SX-100 electron microprobe at the GeoForschungsZentrum, Potsdam. Titanite occurs in lamellae within oxide grains, in composite grains with hematite and/or magnetite (Fig.1), and in small independent grains in epidote-rich domains. It is frequently associated with a Ti-Fe oxide with an average Fe/Ti ration of 2/3 (Fig. 2) and electron microprobe totals of ~ 93%. This appears to be the oxihydroxide pseudorutile that commonly forms as an intermediate product, together with rutile or anatase, during weathering or diagenetic alteration of ilmenite (Schroeder et al.,2004). In the Allouez Conglomerate pseudorutile occurs in three different associations: 1. pseudorutile + titanite + TiO2, 2. pseudorutile + titanite and 3. pseudorutile + TiO2. These mineral associations are most easily explained by a model in which ilmenite is first altered to pseudorutile + TiO2 followed by the formation of titanite by the reaction: TiO2 + CaCO3 + SiO2 → CaTiSiO + CO2. Calculations done with the Perple_X program suggest an upper limit of 0.015 – 0.002 for xCO2 in the fluid phase during the formation of titanite at temperatures of 240 – 320 oC and inferred pressures of 150 MPa (Livnat et al. 1983) during metamorphism of epidote-bearing assemblages in the Portage Lake Series. Pure CaTiSiO5 – Fe oxide assemblages in very low-grade rocks require relatively CO2-poor, oxidizing conditions. Low-grade titanite also frequently contains significant amounts of CaAlSiO4(OH) (Enami et al., 1993) which may increase its stability. Aluminum concentrations in titanite from the Allouez conglomerate range from near 0 to 30% of the Ti site (Fig. 3). The wide range in Al values indicates disequilibrium. There is a strong correlation between Al and F concentrations and stochiometry (Fig. 3), indicates that between 50 and 100% of the Al takes the form of a CaAlSiO4F component. This suggests the presence of F in the metamorphic fluid phase. In medium to high-grade rocks titanite, together with allanite, can play an important role in the REE and Th budget. However both REE and Th were below electron microprobe detection limits in titanite from the Allouez conglomerate. Bright rims and patches evident on epidote grains in BSE images (Fig. 4) indicate an enrichment in LREE elements relatively late in the growth of the epidote. While cores of epidote grains generally contain no detectable LREE bright rims range up to 0.22 REE per 8 cations (Fig.5): close to the boundary between REE-rich epidote and allanite. The other major host for REE elements appears to be synchysite ((REE)CaF(CO3)2) which occurs as acicular crystals associated with calcite in veins and amygdules. REFERENCES Enami, M., Suzuki, K., Liou, J.G., Bird, D.K., 1993. Al–Fe3+ and F–OH substitutions in titanite and constraints on their P–T dependence. European Journal of Mineralogy 5, 219– 231. Livnat, A., Kelly, W.C., Essene, E.J., and Rye, R.O., 1983, P-T-X conditions of sub-greenschist burial metamorphism and copper mineralization, Keweenaw Peninsula, northern Michigan [abs.]: Geological Society of America Abstracts, v. 15, p. 629. Schroeder P.A., Pruett, R.J., and Melear, N.D. (2004) Crystal chemical changes in an oxidative weathering front in a Georgia kaolin deposit. Clay and Clay Minerals 52, 211-220. 35 �36 �A FOLLOW-UP GLACIAL TILL INDICATOR MINERAL SURVEY IN MINNESOTA: WHAT DOES IT INDICATE ABOUT EXPLORATION FOR DIAMONDS AND OTHER MINERAL DEPOSITS? Hauck, S.A., Heine, J.J. – Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Hwy., Duluth, MN 55811-1442 shauck@nrri.umn.edu and jheine@nrri.umn.edu, and Thorleifson, L.H. – Minnesota Geological Survey, University of Minnesota, Twin Cities, 2642 University Ave., St. Paul, MN 55144-4057, thorleif@umn.edu. Why should there be diamondiferous kimberlites in Minnesota? Minnesota has the following attributes that meet the requirements to host diamondiferous kimberlites; 1) an Archean-aged Superior Craton root that underlies 2/3s of MN, including a 3.2-3.8b.y cratonic fragment in SW MN that is required to produce diamonds (Helmstaedt, 2006), and diamondiferous kimberlites that have been found elsewhere within the Superior Craton in Ontario and Michigan; 2) major crustal structures cross-cut MN’s cratonic root, e.g., Vermilion Fault Zone, Great Lakes Tectonic Zone, Quetico Fault, etc., which are excellent kimberlite exploration areas; 3) KenoraKabetogama and Keweenawan dike swarms that intersect these and other structures and could have provided pathways for kimberlite emplacement, e.g., kimberlites found in the Kyle Lake and Attawapiskat kimberlite clusters in N. ONT.; 4) an Archean terrain with calc-alkaline lamprophyres and ultramafic volcanics that are time-equivalent with the Michipicoten greenstone belt, i.e., Wawa area, in ONT that has diamondiferous calc-alkaline lamprophyres/volcanics associated with diamondiferous heterolithic breccias and conglomerates. A diamondiferous ultramafic pyroclastic unit (Grassy Ultramafic Pyroclastic, MetalCORP Ltd.) occurs just north of the Minnesota border in Ontario (Ont. Geol. Survey, 2008). Figure 1. Location of various indicator mineral surveys. Since 2004, samples from the B- or C-horizon soils of various glacial tills throughout MN were collected for indicator minerals and/or related geochemistry, including pristine (1,189gr.), modified (321gr.), and reshaped (1,606gr.) gold grains (Heine et al., 2008). The six indicator mineral and geochemical sampling campaigns included: 1) one statewide C-horizon survey (WMC-MGS; Thorleifson et al., 2007); 2) two regional C-horizon surveys (NRRI-MGS, 2006-2007, in prep.; Heine et al., 2008, 2009); 3) two local B-horizon surveys (MnDNR; Dahl, 2005; Elsenheimer, 2006, respectively); and 4) Larsen (2004) reported on the -63µ geochemistry from Chorizon tills in the western Vermilion District in NE MN. Also, Martin (1995) reported on a select number of kimberlitic indicator minerals in MN, and additional till geochemistry on buried tills (Martin et al., 1988, 1989, 1991). As a follow-up to the 2004 WMC-MGS glacial till sampling program (30 km spacing) in MN, a jointly funded Minerals Coordinating Committee (MCC) and Permanent University Trust Fund (PUTF) project collected glacial till samples on a10 km spacing in NE and E-central MN. The WMC-MGS survey found carbonate clasts and carbonate matrix material throughout most of the State, except in the area of the MCC-PUTF-funded survey (Fig. 1). This thin drift area was deemed an area where closer spacing would be required in this heterogeneous area. The WMC-MGS survey collected 270 till samples, and the follow-up survey collected 79 samples. A 3rd survey collected 42 older till samples. The rationale for this survey was based upon a number of Cr-pyrope samples located during the initial WMC-MGS survey (Fig. 2). These samples were collected from rotosonic drill core in older glacial tills and exposures of Superior tills. Much of the previous sampling was in Des Moines tills. Also, unanalyzed indicator minerals from a previous Manitoba survey were analyzed using PUTF funds to understand the provenance of the G2, G7, G9, and G11 garnets found in southern MN (Figs. 1, 2; Thorleifson et al., 2009) and Mg-chromites and Mg-ilmenites found near MN northern border (Fig. 2). Data from the Manitoba Geological Survey indicator database will be combined with these data to better understand the provenance of MN Des Moines till samples. 37 �The data from these surveys indicate: 1) there is an anomalous gold, plus As, zone south of the Vermilion Fault in NE MN; 2) an anomalous area west of the basal contact of the Duluth Complex that extends into the Western Vermilion District, and is anomalous in Au, As, Ag, Pt, Pd, Cu, Co, Ni, chromite, gahnite, etc. (Fig. 2); 3) several concentrations of Crdiopsides in the 1st survey and 2nd survey that may relate to older tills; 4) the anomalous garnets in the 1st survey area are probably related to: a) reworked older till in MN; or b) they were transported from Manitoba; and 5) a combination of Mn-epidotes, gahnites, corundum, tourmalines, , and geochemical Cu, Co, and Ag anomalies in the old Figure 2. Results of indicator mineral and geochemistry surveys in MN. tills suggest a relationship the Wisconsin VMS belt and/or its MN extension. References Dahl, D.A., 2005, Results of glacial till sampling in the Vermilion greenstone belt, NE MN: MN Dept. Nat. Res., Div. Lands and Minerals, Project 365, 79 p. Elsenheimer, D., 2006, Results of glacial till sampling in the Virginia Horn Greenstone Belt, St. Louis County, Minnesota; St. Paul, MN Dept. Nat. Res., Div. Lands and Minerals, Project 370, Open-File Report, February 2008. Heine, J., Hauck, S., Thorleifson, H., Dahl, D., and Martin, D., 2008, Distribution of gold grains in Minnesota till: University of Minnesota Duluth, Natural Resources Research Institute, NRRI Poster-2008/02. Heine, J., Hauck, S., and Thorleifson, H., 2009, Selected indicator mineral and till chemistry results from multiple till surveys in MN: University of Minnesota Duluth, Natural Resources Research Institute, NRRI Poster-2009/01. Helmstaedt, H.H, 2006, From cratons to carats: Relationships between lithosphere-forming events and diamond growth episodes: Prospectors and Developers Association of Canada, PowerPoint presentation with voice over, CD #1. Larsen, P.C., 2004, Regional till geochemistry survey of the western Vermilion greenstone Belt, Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Technical Report NRRI/TR-2004/23, 33 p. Martin, D., 1995, A limited survey of selected kimberlite indicator minerals from glaciofluvial sediments across Minnesota: MN Dept. Nat. Res., Div. Minerals, Report 314, 29 p. Martin D.P., Dahl, D.A., Cartwright, D.F., and Meyer, G.N., 1991, Regional survey of buried glacial drift, saprolite, and Precambrian bedrock in Lake of the Woods County, MN: MN Dept. Nat. Res., Div. Minerals Report 280, 75 p. Martin, D.P., Meyer, G.N., Cartwright, D.F., Lawler, T.L., Pasitka, J.T., Jirsa, M.A., Boerboom, T.J., and Streitz, A.R., 1989, Regional geochemical survey of glacial drift drill samples over Archean granite-greenstone terrane in the Effie area, northeastern MN: MN Dept. Nat. Res., Div. of Minerals, Report 263, 2 vols., v. 1 , 59 p., v. 2, 323 p. Martin, D. P., Meyer, G. N., Lawler, T. L., Chandler, V. W., and Malmquist, K. L., 1988, Regional survey of buried glacial drift geochemistry over Archean terrane in northern MN: MN Dept. Nat. Res., Div. of Minerals, Report 252, Part I, 74 p., Part II, 386 p. Ontario Geological Survey, 2009, Diamonds 2009; Handout, Prospectors and Developers of Canada Association meeting, Toronto, Canada, March 1-3, 2009, 13 p. Thorleifson, L.H., Harris, K.L., Hobbs, H.C., Jennings, C.E., Knaeble, A.R., Lively, R.S., Lusardi, B.A., and Meyer, G.N., 2007, Till geochemical and indicator mineral reconnaissance of Minnesota: MN Geol. Surv. OFR-0701. Thorleifson, L.H., Matile, G.L.D., Keller, G.R., and Hauck, S.A., 2009, Till geochemical and indicator mineral reconnaissance of southeastern Manitoba (west half of NTS 52E AND 52L and all of 62H and 62I): final results: Manitoba Geological Survey, Open File Report 2009-13, 6 p., plus Access Database and plates. 38 �Developing a 21st Century Geoscience Major: Melding the Old with the New HEFFERAN, Kevin P. (kheffera@uwsp.edu) and HEYWOOD, Neil C. (nheywood@uwsp.edu), Department of Geography and Geology, University of Wisconsin-Stevens Point, Stevens Point, WI 54481 Many Geology Programs are facing uncertain futures due to budgetary constraints and poor communication of the role Geoscientists play in our world. According to the U.S. Bureau of Labor (http://www.bls.gov/oco/ocos288.htm), Geoscience—which incorporates fields such as geology, geography, biology, chemistry, physics, climatology and oceanography—is anticipating over 20% job growth during the next 15 years as existing workers retire, and energy and water resource needs expand. In response to this need, the University of Wisconsin-Stevens Point (UWSP) Department of Geography and Geology recently received a $1.7 million dollar grant to develop a GIS (Geographic Information Systems) Center for geospatial studies. This GIS Center will provide a means for undergraduate students to apply GIS, remote sensing, and other geospatial techniques to address local, regional, and global issues. UWSP’s Department of Geography & Geology has also implemented a new Geoscience Major for the Spring 2009 semester. Our goal is to retain fundamental elements of traditional geology programs and to incorporate high-technology geospatial skills applicable to the 21st Century workforce. Fundamental geology courses in physical geology, Earth history, Earth materials, structural geology, and sedimentary geology are coupled with remote sensing, GIS, environmental, and hydrogeology course offerings. Perhaps most important of all is a 3-credit field component within the U.S.A. that represents the keystone experience for Geoscience majors. Field research is a key training element for geoscientists, and is essential for understanding Earth processes. With respect to pedagogical developments, we present the recently-published Physical Geography Laboratory Manual (Lemke, Ritter and Heywood, 2008) and the imminent Earth Materials textbook (Hefferan and O’Brian 2010). Both texts expose students to the question, “What does contemporary society need, and expect, from Geoscientists?” We believe the new UWSP Major encompasses three critical elements to a contemporary undergraduate Geoscience Program: 1) fundamental Geoscience courses coupled with computer-based instruction; 2) active field experience; and 3) pedagogical innovations that reflect and adapt to the evolving field of Geoscience. Communication between the general public, industry and scientists are critical for successful Geoscience programs. At this interactive poster, we would appreciate a lively dialogue with other Geoscience faculty, students and professionals to learn of their approaches to future needs and demands. This is a Poster Presentation by Two UWSP Faculty Members 39 �DETAILED PETROGRAPHIC ANALYSIS OF ANTHRAXOLITE MORPHOLOGY IN THE BIWABIK IRON- FORMATION, NORTHERN MINNESOTA HILLER, James A. and SHAPIRO, Russell S. Dept. of Geological and Environmental Sciences, California State University, Chico; Chico, CA 95929 jamesashland@gmail.com Anthraxolite is a pyrobitumin composed of ~95% carbon (Morey 1994). It is widely believed to be the product of metamorphosed petroleum having had nearly all of its volatiles driven off. In the Biwabik Iron-Formation in northern Minnesota, anthraxolite has been found in a variety of locations but is generally constrained between the Intermediate Slate and the stromatolite layers of the Lower Cherty member (Morey 1994). This has led some to believe that the anthraxolite was sourced from the overlying Intermediate Slate, interpreted as a carbon-rich ash layer. The goal of this research is to study the morphology of anthraxolite at varying depths to provide a clearer explanation and understanding of the pattern of migration. This is accomplished by analyzing thin-sections at varying depth from core. The anthraxolite in core MGS-5 is located from 947'-995' (Severson 2005) and is found between the Intermediate Slate and the Lower Cherty stromatolites, consistent with Morey's (1994) observations. Five thin sections were made from several depths in order to better understand the events leading to the present distribution and morphology of the anthraxolite: 950', where thin sections were made parallel and perpendicular to bedding; 963', 988.5' and 995' where thin sections were made perpendicular to bedding. At 950', the anthraxolite ranges from 69-235 µm across (averaging 110 µm), is almost exclusively in the carbonate veins, and fills the void space between crystals. The anthraxolite is also found as rounded blebs when viewed parallel to bedding, with both concave and convex surfaces. Where it is found outside of the vein fill, the blebs range from 7-67 µm with an average diameter of 20 µm. These fragments are highly fractured and appear to be cutting the cherty matrix. At 963' the anthraxolite is in chert, between and overprinting 97-345 µm diameter grains of calcite. Here, the crescent-shaped blebs open in the upward direction with the upside fractured and the down-side cutting across older minerals. The crescents range in size from 268-843 µm in diameter and are spherical to crescent-shaped. The anthraxolite is also found in both cherty matrix and carbonate veins as spherical blebs. This distribution and morphology suggest movement as a liquid. Anthraxolite is also found as irregularly fractured grains suggesting movement after solidification. The crosscutting relationships associated with the anthraxolite in the Biwabik Iron- Formation is evidence of a complex history leading to its present location and morphology. There is, however, very little evidence of the original source of the anthraxolite despite previous hypotheses. Proposed future work includes analyzing pure anthraxolite samples that have already been isolated, using catalytic hydropyrolysis (HyPy) to drive off volatiles not removed during the solvent extraction. HyPy allows for more product to be analyzed by gas chromatography mass spectrometry (GC-MS) without damaging the larger organic ring structures (Marshall et al. 2007). The HyPy analysis should allow better identification of possible biomarkers and will significantly aid in the determination of a source. 40 �References Marshall, C.P., Love, G.D., Snape, C.E., Hill, A.C., Allwood, A.C., Walter, M.R., Van Kranendonk, M.J., Bowden, S.A., Sylva, S.P., Summons, R.E., 2007. Structural characterization of kerogen in 3.4 Ga Archaean cherts from the Pilbara Craton, Western Australia. Precambrian Research 155, 1–23. Morey, G. B., 1994, Anthraxolite in the Early Proterozoic Biwabik Iron Formation, Mesabi Range, northern Minnesota in Southwick, D. L. [editor] Short contributions to the geology of Minnesota. Minnesota Geological Survey, St. Paul, MN, United States), Report of Investigations 1994, 39–47. Severson, M. J., 2005. Preliminary correlation of submembers within the Biwabik Iron Formation as deciphered from geological descriptions obtained from various iron ore mines and other sources on the Mesabi range of Minnesota. Natural Resources Research Institute, Duluth, Minnesota, NRRI/MAP-2005-01 (draft). 41 �MESOPROTEROZOIC MIDCONTINENT RIFT-RELATED MAFIC INTRUSIONS IN NORTHWESTERN ONTARIO: CONTINUING GEOCHEMICAL, PALEOMAGNETIC, PETROGRAPHIC AND GEOCHRONOLOGIC STUDIES HOLLINGS, Pete, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada, SMYK, Mark C., Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 Canada, HALLS, Henry, Department of Geology, University of Toronto at Mississauga, Ontario L5L 1C6, HEAMAN, Larry, Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada During the summer of 2008 a detailed sampling program of the Midcontinent Rift-related intrusions was initiated around Thunder Bay. This sampling built upon previous work undertaken between 1999 and 2007 by M. Smyk and/or P. Hollings (Hollings et al, 2007a; Hollings and Smyk 2008) who collected and analyzed over 100 samples. Over 150 additional samples were collected during the 2008 field season for geochemistry, petrography, geochronology, radiogenic isotopes and paleomagnetism studies. The whole rock geochemistry of the gabbroic sills to the south of Thunder Bay supports earlier observations by Hollings and Smyk (2008) that Logan-type sills predominate in this area, with the exception of the recently recognised Riverdale sill. Logan sills are typically composed of sub-ophitic gabbro and are distinguished geochemically by elevated Gd/Ybn (ca. 2.0 to 2.7) and La/Smn (2.0 to 2.5), as well as elevated TiO2 (ca. 3.0 to 4.5) as compared to Nipigon sills (Hollings et al. 2007b). Gabbro dykes containing anorthositic gabbro blocks crosscut Logan sills but share a similar rare earth chemistry. In contrast, the majority of the dykes analysed are geochemically comparable to the Nipigon sills suggesting that they do not represent feeders for the Logan sill complexes. However, limited data suggest that there is no significant geochemical difference between the Pigeon River, Cloud River and Mt. Mollie dyke suites, despite their different orientations and reported ages. Preliminary results suggest the presence of two other distinct dyke suites: one north-trending suite which intruded Rove Formation rocks southwest of Thunder Bay and a west-northwest-trending suite sampled on the Sibley Peninsula which intruded Sibley Group sedimentary rocks. Samples for paleomagnetic study were collected at four sites (Smyk et al. 2008). 1) The Riverdale sill and a geochemically distinct dyke which intruded both sill and Rove Formation sedimentary rocks exhibit reversed magnetic polarity. Directions obtained from the sill and the dyke are indistinguishable. 2) A 40 m wide, northeast-trending Pigeon River dyke near Arrow River (1078 + 3 Ma; Heaman et al. 2007) yielded a normal polarity. 3) An 85 m wide, northeast-trending Pigeon River dyke (Rita Bolduc locality; 1141 + 20 Ma, Heaman et al. 2007). Like the dyke near Arrow River, this dyke also exhibits normal magnetic polarity. Site directions in these two Pigeon River dykes are not significantly different at the 95% confidence level. 4) A 100 m wide, northwest-trending Cloud River dyke in Crooks Township yielded a reversed polarity. 42 �Samples were also collected for geochronologic study. Samples taken from the Riverdale sill yielded no dateable material and baddeleyite ages for the Cloud River dyke are pending. Preliminary data from baddeleyite from the Mt. Mollie dyke indicate an age of 1109.3 ± 6.3 Ma. This is older than the age of 1099.6 + 1.2 Ma (Heaman et al. 2007) that has been reported for the Crystal Lake gabbro with which the Mt Mollie dyke has been traditionally associated (Smith and Sutcliffe 1987). Additional geochronologic and radiogenic isotope data are pending. An enigmatic volcanic unit mapped by Tanton (1936) in central Devon Township was also sampled in 2008. Initially mapped as Rove Formation basalt, the unit consists of massive to columnar-jointed basaltic andesite flows and perhaps subvolcanic sills amongst Rove Formation clastic sedimentary rocks. Flows exhibit vesicular and amygdaloidal textures and locally have ropy tops. Although the rare earth element geochemistry of this volcanic unit is similar to that of the Riverdale sill, its magnesium content is lower. The remarkably coherent rare earth element geochemistry of this volcanic unit has been used to discriminate it from nearby Logan sills. References Davis, D.W. and Green, J.C. 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution; Canadian Journal of Earth Sciences, v.34, p.476-488. Heaman, L.M., Easton, M., Hart, T.R., Hollings, P., MacDonald, C.A. and Smyk, M. 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario; Canadian Journal of Earth Sciences, v.44, p.1055-1086. Hollings, P. and Smyk, M, 2008. Whatever happened to the Logan sills? Ongoing research into the geochemistry of Midcontinent Rift-related mafic intrusive rocks south of Thunder Bay; in Institute on Lake Superior Geology, Proceedings, v.54, part 1, p.36-37. Hollings, P., Smyk, M., and Hart, T., 2007a. Geochemistry of Midcontinent Rift-related mafic dykes and sills near Thunder Bay: New insights into geographic distribution and the geochemical affinities of Nipigon and Logan sills and Pigeon River and other dykes; in Institute on Lake Superior Geology 53rd Annual Meeting, Proceedings, v.53, part 1, p.40-41. Hollings, P., Hart, T., Richardson, A., and MacDonald, C.A., 2007b. Geochemistry of the Midproterozoic intrusive rocks of the Nipigon Embayment, Northwestern Ontario. Canadian Journal of Earth Sciences, v.44, p.1087-1110. Smith, A.R. and Sutcliffe, R.H. 1987. Keweenawan intrusive rocks of the Thunder Bay area; in Summary of Field Work 1987, Ontario Geological Survey, Miscellaneous Paper 137, p. 248255. Smyk, M., Hollings, P., Halls, H., and Easton R.M., 2008. Project Unit 08-021. Mesoproterozoic Midcontinent Rift-Related Mafic Intrusions near Thunder Bay: Geological, Paleomagnetic, Geochemical and Geochronological Studies. Summary of Field Work and Other Activities 2008, Ontario Geological Survey, Open File Report 6226, p. 18-1 to 18-6. Tanton, T.L. 1936. Pigeon River area; Canada Department of Mines, Geological Survey, Map 354A, scale 1:63 360. 43 �PRELIMINARY RESULTS OF 40AR/39AR THERMOCHRONOLOGY FROM THE CENTRAL YAVAPAI PROVINCE, U.S. MIDCONTINENT Angie Hull, Daniel Holm, Dept. of Geology, Kent State University, Kent, OH 44242; David Schneider, Dept. of Geological Sciences, Univ. of Ottawa, Ontario, Canada Much is known about the Precambrian U.S. tectonic and crustal evolution in regions where bedrock is exposed for direct observation (i.e. Rocky Mountains, Lake Superior region; Karlstrom & Keller, 2005; Holm et al., 2007). However, Phanerozoic sedimentary cover overlying the central Yavapai Province (YP), between the Rockies and the Great Lakes, limits our knowledge of that important region. Ar-Ar thermochronology from the upper Great Lakes region and western U.S. has proven critical for assessing Proterozoic tectono-metamorphic overprinting and cooling during and following growth and stabilization of southern Laurentia. Here we present the first results of Ar/Ar thermochronology on Paleoproterozoic basement drill core rocks in easternmost Colorado, Nebraska, and southern South Dakota (see figure). South Dakota. Hornblende from medium grained metabasalt in south central SD (SDBE) yields a complicated age spectra with a total gas age of 2268 Ma and a preferred age of 2449 Ma from selected steps constituting 31% of the gas released. Further east, a garnet rich gneiss (Woon W-1) yields a biotite plateau age of 1869 Ma (7 steps; 86% of total gas released). In easternmost SD, biotite from a granite gneiss (SDGT) yielded a plateau age of 1728 Ma (5 increments; 59% of total gas released). Southern Nebraska. Muscovite from a granite (NBDU) yields a plateau age of 1251 Ma (12 steps; 70% of the gas released). Farther north, biotite from a granite gneiss (NBCS-2) yields a plateau age of 1267 Ma (5 increments; 51% of the gas released). In the central region of southern NE, hornblende from a sheared tonalite (NBBF-1) yields a complicated age spectra with a total gas age of 1468 Ma and a preferred age of 1487 Ma from a large step constituting 36% of the total gas released. Nearby, biotite from a granite gneiss (NBDA-2) yields a plateau age 1222 Ma (6 increments; 75% of the total gas). In the southeast corner of NB, both biotite and muscovite from a granite (NBPN-1) were analyzed. The biotite yielded a slight saddle-shaped spectra, indicating possible excess argon, with a total gas age of 1138 Ma; muscovite yielded a plateau age of 1200 Ma (11 increments; 69% of the gas released). Slightly northeast of this locale, biotite from a granite yields a complex spectra and a total gas age of 1231 Ma. Eastern Colorado. Biotite from a deformed gabbro in Kit Carson Co. yielded a plateau age of 1238 Ma (7 increments; 85% of the total gas). In the north, the 1.86 Ga biotite age is consistent with rapid cooling after peak Trans-Hudson/ Penokean metamorphism. The 1728 Ma biotite age (SDGT) is similar to abundant 1750-1720 Ma hornblende/ biotite ages reported in east-central Minnesota, suggesting that geon 17 metamorphic affects extend westward into SD. Most surprising is the 2449 Ma hornblende age, which suggests the presence of Archean crust within the Proterozoic mobile belts of southern Laurentia. In the south, our results demonstrate the YP experienced the affects of younger, Proterozoic events largely unfelt throughout neighboring regions. Mica ages are consistently young across a >700 km swath of southern NE and eastern CO. Similarly young ages are reported from only a handful of the hundreds of samples dated in the Rockies and the upper Great Lakes region. Because Proterozoic crust in the southern Lake Superior were virtually unaffected thermally by 1.1 Ga rifting, we consider it unlikely that our 1130-1260 Ma mica ages represent widespread partial resetting during rifting. In CO and NM, similarly young mica ages are interpreted as partially reset by Tertiary igneous activity or representing deeper, and therefore more slowly cooled crustal levels (Shaw et al., 2005). However, both interpretations seem unlikely for our study area. We tentatively suggest that a regional thermal 44 �event possibly related to Neoproterozoic deformation may be responsible for the anomalously young Ar/Ar ages reported here. Holm, D.K., Schneider, D., and Chandler, V.W., 2007a, Proterozoic tectonic and crustal evolution of the Upper Great Lakes region, North America, Precambrian Research, v. 157. Karlstrom, K.E., Keller, G.R. (Eds.), 2005, Rocky Mountain Region: An Evolving Lithosphere. Am. Geophys. Union, Geophysical Monograph 54, pp. 421-441. Shaw, C.A., Heizler, M.T., and Karlstrom, K.E., 2005, 40Ar/39Ar Thermochronologic record of 1.45-1.35 Ga intracontinental tectonism in the southern Rocky Mountains: Interplay of conductive and advective heating with intracontinental deformation, in The Rocky Mountain Region: An Evolving Lithosphere Geophysical Monograph, the American Geophysical Union, Series 154, p. 163-184. Van Schmus, W.R., Schneider, D.A., Holm, D.K., Dodson, S., and Nelson, B.K., 2007, New insights into the southern margin of the Archean-Proterozoic boundary in the north-central United States based on U-Pb, Sm-Nd, and Ar-Ar geochronology:Precambrian Research, v. 157, p. 80-105. 45 �LITHOGEOCHEMICAL EVALUATION OF NEOARCHEAN MAFIC VOLCANIC ROCKS COMPRISING THE FOOTWALL OF THE SOUDAN MEMBER OF THE ELY GREENSTONE FORMATION, NORTHEASTERN MINNESOTA JANSEN, A.C., HUDAK, G. J., Department of Geology, University of Wisconsin Oshkosh, Oshkosh, WI 54901, jansea73@uwosh.edu HEINE, J. J., PETERSON, D.M., Natural Resources Research Institute, University of Minnesota - Duluth, Duluth, MN 55811 The Ely Greenstone Formation (EG) comprises a steeply north- to southwest-dipping, north – to southwest younging sequence of Neoarchean supracrustal and associated intrusive rocks that are warped about the Tower-Soudan anticline in the Vermilion District of northeastern Minnesota. The EG has historically been broken up into three members. Upsection, these are: a) the Lower Member of the Ely Greenstone Formation (LMEG, composed of calc-alkaline to tholeiitic basalt and basalt andesite lava flows and tuffs with subordinate felsic lava flows, volcaniclastic and epiclastic rocks, and iron formations); b) the Soudan Iron Formation Member (SMEG, composed of Algoma-type banded iron formations, basalt lava flows, epiclastic rocks and minor felsic lava flows and tuffs); and c) the Upper Member of the Soudan Iron Formation (UMEG, composed of a monotonous sequence of tholeiitic basalt lava flows and local Algoma-type iron formation lenses (Schulz, 1980; Southwick et al., 1998; Hudak et al., 2002; Peterson, 2001; Peterson and Patelke, 2003; Hudak et al., 2007). Schulz (1980) and Hudak et al. (2007) interpret volcanic textures, sedimentary textures, and lithological characteristics to indicate a transition from a subaerial/shallow subaqueous setting to a deeper subaqueous environment during the temporal genesis of the EG. Southwick et al. (1998) indicate that a sharp transition from basaltic and basalticandesitic rocks with arc-like geochemical signatures in the LMEG, to basaltic rocks with MORB-like geochemical signatures in the UMEG, occurs abruptly at the top of the SMEG. More recent studies in the vicinities of Fivemile-Needleboy-Sixmile Lakes (Hudak et al., 2007) and Armstrong Lake (Jirsa et al., 2001) indicate that the transition from arc-like magmatism to MORB-like magmatism is more complicated than previously thought, with MORB-like basalts first occurring in the uppermost parts of the LMEG approximately 50100 meters into the footwall of the SMEG (Hudak et al., 2007), as well as locally within the SMEG (Jirsa et al. 2001). A model encompassing initial volcanic arc development followed by back-arc rifting immediately prior to the deposition of the SMEG has been proposed (Hudak et al., 2007). The purpose of this investigation was to evaluate, on a more regional scale than had been previously performed, the lithogeochemistry of mafic and intermediate volcanic rocks that occur in LMEG in the footwall to the SMEG. In addition to field and lithogeochemical data from the Fivemile-Needleboy-Sixmile Lake areas (Hudak et al., 2007) and Armstrong Lake areas (Jirsa et al., 2001), detailed mapping (1:2500-1:5000 scale) and sampling was performed south of Twin Lakes (on the northeastern part of the Vermilion District) as well as in the vicinity of Putnam Lake (in the southwestern part of the Vermilion District). Following field mapping, petrographic and lithogeochemical studies were performed. Petrographic work was utilized to distinguish between fin-grained massive flows and diabase sills and 46 �dikes. Whole rock major and trace element lithogeochemical analyses utilizing a wide variety of analytical methods (ICP/MS, instrumental neutron activation analysis, coulometry, and gravimetric) were employed. In the Twin Lakes area, mafic volcanic rocks with arc-like lithogeochemical signatures transition up-section into mafic volcanic rocks with MORB-like lithogeochemical signatures consistent with back-arc basin basalts in the immediate footwall (<100 meters) to the SMEG. The same lithogeochemical transition has also been documented in the immediate footwall rocks to the SMEG in the vicinity of Putnam Lake. Our results allow us to, for the first time, to document a change from arc-like to MORB-like (back-arc basin consistent) magmatism in the immediate footwall rocks to the SMEG on a regional basis. The development of a back-arc basin in a volcanic arc not only enables the occurrence of MORB-like volcanism, but also is commonly associated with the development of vigorous, regional hydrothermal activity which can produce chemical sedimentary rocks (e.g Algoma-type iron formations) as well as volcanogenic massive sulfide deposits (Franklin et al., 2005; Piercey et al., 2004). It is therefore not suprising that the SMEG occupies a position immediately up-section from the arc- to MORB lithogeochemical transition within the EG. The presence of this transition, apparently across the entire strike length of the SMEG may be explained if the current erosional surface is sub-parallel to a rift structure which occurred within the proposed back arc basin. References Hoffman, A. T., 2007. Lithostratigraphy, Hydrothermal Alteration, and Lithogeochemistry of Neoarchean Rocks in the Lower and Soudan Members of the Ely Greenstone Formation, Vermilion District, NE Minnesota: Implications for Volcanogenic Massive Sulfide Deposits; unpublished M.S. thesis, University of Minnesota-Duluth, 295 p. Franklin, J. M., Gibson, H. L., Jonasson, I. R. and Galley, A. G., 2005. Volcanogenic massive sulfide deposits: Society of Economic Geologists 100th Anniversary Volume, p. 523-560. Hudak, G. J., Hoffman, A. T., Peterson, D. M., and Heine, J., 2007. Recent developments understanding the volcanic, magmatic, tectonic, and metallogenic evolution of the Ely Greenstone Formation, Vermilion District, NE Minnesota: Institute on Lake Superior Geology, Proceedings Volume 53, Part 1, Proceedings and Abstracts, p. 42-43. Jirsa, M. A., Boerboom, T. J., and Peterson, D. M., 2001. Bedrock geologic map of the Eagles Nest Quadrangle, St. Louis County, Minnesota: Minnesota Geological Survey, Miscellaneous Map Series Map M-114, 1:24000 scale. Peterson, D. M., and Patelke, R. L., 2003. National Underground Science and Engineering Laboratory (NUSEL): Geological Site Investigation for the Soudan Mine, NE Minnesota: NRRI Technical Report NRRI/TR-2003/29, 88p. Piercey, S. J., Murphy, D. C., Mortenson, J. K., and Creaser, R. A., 2004. Mid-Paleozoic initiation of the northern Cordilleran marginal backarc basin: geologic, geochemical, and neodymium isotope evidence from the oldest mafic magmatic rocks in the Yukon-Tanana Terrane, Finlayson Lake District, Southeast Yukon, Canada: Geological Society of America Bulletin, v. 116, no.9/10, p. 1087-1106. Schulz, K. J., 1980, The magmatic evolution of the Vermilion Greenstone Belt, NE Minnesota: Precambrian Research, v. 11, p. 215-245. Southwick, D. L., Boerboom, T. J., and Jirsa, M. A., 1998, Geological setting and descriptive geochemistry of Archean supracrustal rocks and hypabyssal rocks, Soudan-Bigfork area, northern Minnesota: implications for metallic mineral exploration: Minnesota Geological Survey Report of Investigations 51, 69 p. 47 �GEOLOGIC MAPPING OF NEOARCHEAN ROCKS NEAR PAULSEN LAKE, BOUNDARY WATERS CANOE AREA WILDERNESS, BY STUDENTS OF THE PRECAMBRIAN RESEARCH CENTER’S 2008 FIELD CAMP Mark Jirsa, (jirsa001@umn.edu); Hugh Cowan, Jacqueline Kowalik, and John Niedermiller The Precambrian Research Center—a branch of the University of Minnesota, Duluth—conducted its second season of field camp in 2008. After 5 weeks of field training, students were assigned “Capstone Projects” that provide an opportunity to create new geologic maps in areas of poorly understood geology. Junior authors listed above are students who mapped Neoarchean bedrock in the Paulsen Lake area, which was burned by the 2006 Cavity Lake forest fire in the northeastern part of the BWCAW. The fire greatly improved access to and visibility of geologic features, allowing detailed mapping of rock units and complex contact relationships. The 2008 map area (Fig. 1) lies in the northeastern part of the U.S.G.S. Gillis Lake 7.5-minute quadrangle. Previous mapping was fairly detailed (Vervoort, 1987), but contacts were not well constrained. The map and geologic descriptions presented here are based on literature review and the observations from several days of field work— no petrographic or geochemical data were acquired. Figure 1. Simplified geology of the Cavity Lake fire area (fire=dotted outline), showing location of detailed mapping described here. Small inset map shows location within the Wawa subprovince of Superior Province. Geology modified from Jirsa and Starns, 2008. 48 �The principal Neoarchean rock units (from oldest to youngest) in the 2008 map area are: Paulson Lake sequence—a vertically dipping basement package of back-arc or ocean floor origin, consisting of pillowed and massive basalt flows, and hypabyssal, mafic-ultramafic sills. The age of the sequence is unknown. Well developed spherulitic and spinifex textures, and the presence of peridotite sills imply correlation with the largely ultramafic Newton Lake Formation, which lies to the west and southwest. Saganaga Tonalite (2689 Ma; Corfu and Stott, 1998)—coarse-grained, polyphase intrusion and apophosial dikes that intrude and cut out the base of the Paulsen Lake sequence. The tonalite has a distinctive texture marked by the abundance of quartz "eyes" and presence of hornblende. Jasper Lake sequence—hornblende- and pyroxene-porphyritic, dacitic to trachyandesitic volcanic and volcaniclastic rocks, cut by similarly porphyritic hypabyssal intrusions. Geochemical data and mineralogic similarity indicate that the Saganaga Tonalite may represent the magma chamber from which these rocks erupted. Bedding in supracrustal rocks dips steeply and stratigraphic younging is generally southward, as deduced from pillow morphology in flows and graded bedding and scour structures in conglomeratic rocks. Angular relationships of bedding imply that volcanic conglomerate units of the younger Jasper Lake sequence lie unconformably on the Paulsen Lake mafic-ultramafic rocks. Metamorphic grade appears to be low greenschist facies, except immediately adjacent to the Saganaga Tonalite where amphibolite facies assemblages are well developed and the rocks contain cleavage and a strong, shallow east-plunging mineral lineation. The boundary between rocks of contrasting metamorphic grades is a fault. We infer considerable uplift on the north side immediately adjacent to the Saganaga Tonalite. Conglomerate layers in the Jasper Lake sequence are white to brownish-gray, and clastsupported. Clasts are moderately to well rounded, moderately well sorted, and range from 1 cm to 20 cm in diameter. Clasts consist of porphyritic to aphyric hornblende dacite to trachyandesite, and some fragments have textures similar to phases of the Saganaga Tonalite—supporting the inference that it represents magmatic source to the Jasper Lake volcaniclastic strata. . A preliminary geologic map of the Cavity Lake fire area, created prior to the student work, was published as an open-file map (Jirsa and Starns, 2008). A final map that incorporates student work and recent thesis mapping will be published in coming months. Support was provided by the U.S. Geological Survey’s 2007 State Geologic Mapping Element (STATEMAP) of the National Geologic Mapping Program, the Precambrian Research Center (2007 and 2008 capstone projects), and the State Special Appropriation to the Minnesota Geological Survey. REFERENCES Corfu, F., and Stott, G.M., 1998, Shebandowan greenstone belt, western Superior Province: U-Pb ages, tectonic implications, and correlations: GSA Bull. 110:1467-1484. Jirsa, M.A., and Starns, E., 2008, Preliminary bedrock geologic map of the Cavity Lake fire area, parts of the Ester Lake, Gillis Lake, Munker Island, and Ogishkemuncie Lake quadrangles, northeastern Minnesota: Minnesota Geological Survey Open-File Report OF-08-05, scale 1:24,000. Vervoort, J.D., 1987, Petrology and geochemistry of the Archean of the JAP Lake area, northeastern Minnesota: M.S. Thesis, University of Minnesota-Duluth, 193 p. 49 �NEOARCHEAN WEATHERING AND ATMOSPHERIC pO2 INFERRED FROM PALEOSAPROLITE BETWEEN GRANITE-GREENSTONE AND SUPERJACENT CONGLOMERATE IN THE BOUNDARY WATERS CANOE AREA, NE MINNESOTA Jirsa, Mark A., Minnesota Geological Survey (www.geo.umn.edu/mgs); and Driese, Steven G., Department of Geology, Baylor University, Waco, Texas Neoarchean rocks in parts of the Boundary Waters Canoe Area are exceptionally well exposed after recent forest fires that were the largest in Minnesota since 1894. New mapping in the burns reveals considerable detail, particularly about complex contact relationships (Jirsa and Starns, 2008). For example, the contacts between the Ogishkemuncie conglomerate and the volcano-plutonic country rocks from which it was derived are both faults and unconformities. Where not faulted, the contacts are marked locally by paleosaprolite developed in both the 2.689 Ga Saganaga Tonalite (Corfu and Stott, 1998), and the adjacent ca. 2.7 Ga metavolcanic rocks that it intruded. Petrographic and geochemical analyses of 2 suites of samples taken across the paleosaprolitic contact zones will constrain our understanding of environmental conditions during the Neoarchean. The Neoarchean rocks (from oldest to youngest) are: Paulson Lake sequence—a vertically dipping package of back-arc or ocean floor origin, consisting of pillowed basaltic to komatiitic flows, hypabyssal sills, and rare tuff, chert, and distal turbidite. Saganaga Tonalite—coarse-grained batholith and dikes that cut the Paulsen Lake sequence and have distinctive textures marked by abundant quartz "eyes" and hornblende. Jasper Lake sequence—hornblende- and pyroxene-porphyritic, dacitic to trachyandesitic, volcanic and volcaniclastic rocks cut by similarly porphyritic hypabyssal intrusions. Ogishkemuncie conglomerate—conglomerate and sandstone containing readily recognizable fragments of all the rock sequences described above. Sedimentary structures indicate deposition in coalescing alluvial fans, with fluvial transport locally into standing water. The strata are similar in many respects to other Timiskaming-type assemblages in the Superior Province that are inferred to represent deposition in successor-basins (e.g., Jirsa, 2000). Characteristics of the inferred 2.7 Ga paleosaprolite satisfy most of the 5 diagnostic criteria proposed by Rye and Holland (1998) for identification of pre-land plant paleosols. The unconformity dips 30-60o, in contrast to the vertical dip of underlying country rock. Macroscopic and microscopic features of the paleosaprolite include alteration, destruction of igneous texture, and diminution of feldspar expressed in increased content of quartz eyes in paleoweathered tonalite (Fig. 1); and oxidation, microfracturing, conversion of Fe-Mg minerals to chlorite, and semi-ductile (“soft”) deformation in metavolcanic rocks. The effects differ from those of tectonic cataclasis primarily in the irregularity of altered zones, locally producing equant paleocorestones of tonalite, with annealed concentric exfoliation structures. Figure 1. Photomicrographs showing textures of A. comparatively fresh Saganaga Tonalite 55 m below Ogishkemuncie conglomerate; B. weathered tonalite 15 m below conglomerate; and C. conglomerate 7 m above tonalite (note rounded to angular quartz and rock fragments). All in plane light; scale-bar 2 mm. 50 �Although primary paleosaprolite microfabrics and mineralogy of weathered granitic rocks are modified by low-to medium-grade metamorphism that is typical here, whole-rock geochemical patterns related to paleoweathering are commonly well-preserved, except for general K2O increases related to metasomatism (Driese et al., 2007; Driese and Medaris, 2008). Calculations of elemental gains and losses relative to fresh parent material for both the Saganaga Tonalite and the adjacent metavolcanic rocks will permit estimation of Neoarchean paleoatmospheric pO2, based on theoretical differences in the ratio of O2 demand to CO2 demand during weathering of granitic vs. mafic rocks, as well as Fe gains and losses (Pinto and Holland, 1988). Paleoatmospheric pO2 is inferred to have been quite low at 2.7 Ga, well before the circa 2.2 Ga “Great Oxidation Event” of Rye and Holland (1998), as illustrated in Figure 2. Additional information about pCO2 can also be extracted using mass-balance geochemical methods of Sheldon (2006). Analytical work is underway. Figure 2. Comparison of 2.7 Ga Ogishkemuncie paleosaprolite with other paleosols. (after Rye and Holland, 1998) REFERENCES Corfu, F., and Stott, G.M., 1998, Shebandowan greenstone belt, western Superior Province: U-Pb ages, tectonic implications, and correlations: GSA Bull 110:1467-1484. Driese, S.G., and Medaris, L.G., 2008, Evidence for biological and hydrological controls on the development of a Paleoproterozoic paleoweathering profile in the Baraboo Range, Wisconsin, USA: Journal of Sed. Res. 78: 443-457. Driese, S.G., Medaris, L.G., Ren, M., Runkel, A.C., and Langford, R.P., 2007, Differentiating pedogenesis from diagenesis in early terrestrial paleoweathering surfaces formed on granitic composition parent materials: Journal of Geology 115: 387-406. Jirsa, M.A., 2000, The Midway sequence: a Timiskaming-type, pull-apart basin deposit in the western Wawa subprovince, Minnesota: Can. Journal of Earth Sci. 37:1-15. Jirsa, M.A., and Starns, E., 2008, Preliminary bedrock geologic map of the Cavity Lake fire area, parts of the Ester Lake, Gillis Lake, Munker Island, and Ogishkemuncie Lake quadrangles, northeastern Minnesota: Minnesota Geological Survey Open-File Report OF-08-05, scale 1:24,000. Pinto, J.P., and Holland, H.D., 1988, Paleosols and the evolution of the atmosphere; Part II, in Reinhardt, J. and Sigleo, W.R. (eds.), Paleosols and Weathering through Geologic Time: GSA Special Paper 216: 21-34. Rye, R., and Holland, H.D., 1998, Paleosols and the evolution of atmospheric oxygen, a critical review: Am. Journal of Sci. 298:621-672. Sheldon, N.D., 2006, Precambrian paleosols and atmospheric CO2 levels: Precambrian Research 147:148-155. 51 �STRUCTURAL, KINEMATIC, AND LITHOGEOCHEMICAL INVESTIGATION OF THE MURRAY SHEAR ZONE, NORTHEAST MINNESOTA JOHNSON, Tom K. (University of Minnesota-Duluth, joh04310@d.umn.edu); HANSEN, Vicki L. (University of Minnesota-Duluth, vhansen@d.umn.edu); HUDAK, George J. (University of Wisconsin-Oshkosh, hudak@uwosh.edu); PETERSON, Dean M. (Natural Resources Research Institute, Duluth, MN, dpeters1@nrri.umn.edu) Archean (3.8-2.5 Ga) cratons host gold-bearing quartz vein systems in zones of inhomogeneous structural architecture. In the Superior craton, the largest and most goldproductive zones of multifarious structure include the Porcupine-Destor and Larder LakeCadillac breaks of the Abitibi Greenstone Belt, Canada. Previous geologic mapping and mineral exploration in southern reaches of the Superior craton of northeastern Minnesota revealed anomalous quartz vein-hosted gold in the Murray Shear Zone (Peterson and Patelke, 2003). This investigation of the Murray Shear Zone attempts to better understand unique characteristics shared with gold-rich belts in Canada, and the paucity of developed gold districts in northeastern Minnesota, through research into structural architecture, crustal kinematics, and geochemistry. The Murray Shear Zone cuts an arcuate succession of rocks comprising the Lower and Soudan Members of the Ely Greenstone, and the Lake Vermilion Formation. The 19-km shear zone strikes east-west, extending from the Tower-Soudan area to the Giants Range Batholith to the east. Investigation uncovers microstructural evidence for exclusively dipslip shear, parallel to steep-plunging mineral lineations (089/68) within steeply dipping foliation. Strain appears to be partitioned along east-striking, steeply-dipping metamorphic foliation planes that describe an anastomozing network. In the western portion of the study area curvilinear splays of focused strain fabric diverge to 4 km in width (map view) from 0.4 km width to the east. Strain heterogeneities exist within the Murray Shear Zone. Metamorphic foliations diverge around coherent lithologic blocks devoid of penetrative foliation. Brittle deformation features overprint ductile features in the form of extensional quartz veins that cut metamorphic foliations at high angles. Interpretations of instantaneous principal stress orientations attempt to correlate rheological behavior of rocks with field observations for a conceptual gold model. Strain asymmetry and steep planar and linear fabric coexist with quartz vein systems that host gold. Structurally-hosted gold materializes from: 1) differential stresses in the crust; 2) metamorphic devolatilization; and 3) corresponding fluid pressure fluctuations (deviatoric stresses) at the brittle-ductile transition. Geochemical data show zones of hydrothermal alteration enclosing gold-bearing quartz veins of anomalous gold mineralization in regionally-prevalent greenschist grade to locally amphibolite grade metamorphism. Alteration envelopes approaching the veins include chlorite schist, carbonate-chlorite schist, and carbonate-sericite ± green mica schist. Correspondingly, mass balance analysis utilizing the isocon method (Grant, 1986) indicates gains in Fe, Mg, Mn, Ca, Co, CO2, Cr, K, Ni, V, Sr, Sn, Rb, Ba, and Eu, and losses in Cu, Er, 52 �Na, SiO2, and Zn. Silica leaching occurs adjacent to gold-bearing quartz veins in a matter similar to that illustrated at the Yellowknife gold deposit, Northwest Territories, Canada (Boyle, 1955). References Peterson, D.M., Patelke, R.L., 2003, National underground science and engineering laboratory (NUSEL): geological site investigation for the Soudan Mine, northeastern Minnesota. Economic Geology Group, National Resources Research Institute, University of Minnesota Duluth: Technical Report NRRI/TR-2003/29. Grant, J.A., 1986, The isocon diagram—a simple solution to Gresens’ equation for metasomatic alteration: Econ. Geol. 81:1976-1982. Boyle, R.W., 1955, The geochemistry and origin of the gold-bearing quartz veins and lenses of the Yellowknife greenstone belt: Econ. Geol. 50:51-66. 53 �FLUID MOVEMENT THROUGH THE MESABI IRON RANGE, MINNESOTA Kyle Makovsky, Steven Losh; Dept of Chemistry and Geology, FH 145, Minnesota State University, Mankato MN 56001 The Mesabi Iron Range in northern Minnesota has been an important contributor of iron necessary for products that we use every day. The iron-rich sedimentary rocks were initially deposited in a shallow sea about 1.8 billion years ago. Later, fluids flowed through the rocks dissolving everything but the iron oxides and concentrating them into high-grade ore. Previous work has described these fluids as meteoric waters that have percolated downward through the rocks from the surface. G.B. Morey (1999) has stated that these fluids were driven up through the rocks due to the Penokean Orogeny. To determine the source of fluids associated with high-grade (natural) ore, we have sampled veins in low to high angle faults exposed in the Hibbtac, Thunderbird (UTac), and LTV #6 pits. Brecciated quartz cemented by quartz-hematite is found primarily in the high angle faults of this region. The sampled high-angle faults locally define boundaries between unoxidized and oxidized ore and are thought to have served as conduits for fluids that dissolved chert from taconite. Some of the movement on these faults was associated with collapse due to chert dissolution. Thus the quartz in these faults may have been precipitated from the same fluid that was responsible for leaching the taconite. The properties of the fluid that affected these rocks are being determined using cathodoluminescence and fluid inclusion techniques. Microscope examination of the iron ores revealed that the early-formed iron-rich mineral greenalite was replaced by other ironrich minerals, minnesotaite and stilpnomelane during diagenesis. Samples taken from the Thunderbird and Hibbtac mines have been analyzed using cathodoluminescence. Quartz in the veins and in the iron formation display growth banding produced by pulses of fluid moving through the rock and precipitating minerals episodically. Along with growth banding, differences in the vein quartz and matrix quartz due to fluid interactions with the rock can be seen. The source of fluids has been studied by analysis of fluid inclusions, which can give both the homogenization temperature and the salinity of fluids that are trapped in minerals. Fluids that ascended through the rocks would be expected to have a relatively high temperature and salinity, whereas meteoric fluids that descended from the surface would be expected to have much lower temperatures and salinities. One sample from a fault associated with high-grade iron ore in the Thunderbird mine near Eveleth showed high homogenization temperature values, 85.8-141.5°C (mean 125°C, n=26) and a high weight % NaCl equivalent (mean 3.98 weight %, n=12), suggesting the fluids that precipitated quartz in the faults ascended from depth. These values largely overlap temperatures and salinities of fluid inclusions from quartz breccia in faults in the Hibbtac Mine, as well as in bedding-parallel veins in the LTV6 samples. All of this data supports the idea of fluids rising from depth, not percolating downward. 54 �Geochemical data will also be presented for rocks in veins and in traverses from oxidized, leached iron formation into unoxidized iron formation to determine the nature and effects of the fluids responsible for the high-grade ore. References Morey, G., High-grade iron ore deposits of the Mesabi Range, Minnesota – Product of a continental-scale Proterozoic groundwater flow system; Econ. Geol. v. 94, pp. 133-141. 55 �INTERPRETATIONS OF THE EMPLACEMENT AND COOLING HISTORY OF A THIN DIABASE SILL, NIPIGON, ONTARIO MARKWOOD, Levi W. and ZIEG, Michael J., Department of Geography, Geology, and the Environment, Slippery Rock University, Slippery Rock, PA, 16057. lwm9100@sru.edu, michael.zieg@sru.edu Thin dikes and sills are often produced by single instantaneous injection events (e.g., Gray, 1978). Textural and compositional evidence of this type of formation typically includes the lack of internal chills or accumulation of phenocrysts into distinctive horizons. However, complex injection histories are being recognized in an increasing number of intrusions (e.g., White, 2007). In this study, we examine a thin diabase sill from the Nipigon embayment, Ontario. This sill is located beneath the main sill at Kama Hill, east of Nipigon. Textural and mineralogical evidence suggest that it was emplaced in a single injection, and thus represents an important end-member style of sill formation. Petrographic examination of the rocks in this sill focused on opaque oxides, as they can be characterized easily using automated image processing techniques. Digital photomosaics (transmitted, plane-polarized light) were prepared from thin sections of 18 samples spanning the 1.25 m thick sill. Randomly oriented test lines were used to determine mean crystal length using a method reviewed in Higgins (2006): Lmean= VV/PL, where VV is the modal fraction of the mineral of interest, as determined by both automated image analysis and by point counting, and PL is the number of crystals intersected along the test line. This analysis reveals smooth variations in grain size with distance, from smaller grains at the chilled margin to larger grains in the center of the sill, indicating that the magma cooled as a single unit. Unit cooling is consistent with emplacement of magma as a single injection event. The modal abundance of opaque oxide minerals in the sill was determined by digital threshholding and by point counting (N~1200). The abundance of oxides throughout the sill is statistically uniform. This lack of significant variation in mineral abundances is further evidence supporting unit emplacement: multiple injections would likely have disrupted the solidification fronts established in the cooling magma and resulted in identifiable mineralogical discontinuities. Using a numerical cooling model, we can extract crystallization kinetics (growth rates) from the grain size data, and predict texture variations in a variety of scenarios, in particular predicting the magnitude of the “chilling” signature in the case of reinjection events. Based on mineralogy and texture, the sill in this study was almost certainly emplaced in a single injection event. Using this as a baseline, we can identify complex injection histories as departures from the textural profile observed here. This makes it useful as a null hypothesis for testing formation history: unless textures depart significantly from those presented here, we must assume that the intrusion was formed in a single injection. Such textural criteria are particularly important when the magma composition remained constant through several injections, in which case the recognition of multiple injection events would not be reflected in the modal mineralogy of the rocks. References Gray, N.H., 1978. Crystal growth and nucleation in flash-injected diabase dikes. Canadian Journal of Earth Sciences, 15, 1904-1923. Higgins, M.D., 2006. Quantitative textural measurements in igneous and metamorphic petrology. Cambridge University Press, Cambridge, UK. 265 pp. White, C.M., 2007. The Graveyard Point Intrusion: an example of extreme differentiation of Snake River Plain basalt in a shallow crustal pluton. Journal of Petrology, 38, 303-325. 56 �57 �58 �59 �60 �GEOLOGY AND MAGNETIC TACONITE RESOURCES OF WESTERN GOGEBIC IRON RANGE, WISCONSIN Meineke, David G. (david.meineke@GlobalMineralsEng.com) and Djerlev, Henry (henry.djerlev@GlobalMineralsEng.com), LaPointe Iron Company, 3920 13th Avenue East, Hibbing, Minnesota 55746 The Gogebic Iron Range is 60 miles long, extending from the western Upper Peninsula of Michigan into northeastern Wisconsin. From 1886 to 1964 over 300 million tons of high grade, direct-shipping, natural iron ore was mined from the Gogebic Iron Range, largely by underground mining in Michigan and Wisconsin. The iron in these ores occurred primarily in hematite, goethite and limonite. Only one operation, the Berkshire Mine, in 1922-1924 mined and processed magnetite ores (4,000 tons) that required concentration to make a marketable grade product. The Berkshire operation, along with those early magnetic taconite operations in Minnesota and Michigan, marked the beginning of the Lake Superior taconite industry which now produces nearly all of the iron ore mined in the United States for the U. S. steel industry. The geology of the western Gogebic Iron Range has most recently been described by Cannon, LaBerge, Klasner and Schulz (2008). The iron ore-bearing members are part of the Paleoproterozoic Ironwood Iron Formation. The direct-shipping natural iron ores were mined along 25 miles of strike from Upson, Wisconsin, to Wakefield, Michigan. The natural ores were likely formed by circulating groundwater that oxidized the iron minerals and removed silica. A large magnetic taconite resource has been identified on the western Gogebic Iron Range from Upson to Mineral Lake (21 miles) in Wisconsin and a smaller, less defined magnetic taconite resource east of Wakefield, Michigan, west and east, respectively, of the 25 miles of the Gogebic Iron Range where natural iron ores were mined from Wakefield to Upson. The Ironwood Iron Formation underwent Mesoproterozoic deformation which tilted the strata 40° to 90° north (Cannon, LaBerge, Klasner and Schulz, 2008). For the 21 miles from Upson to Mineral Lake, we estimate an average dip of 65°, with the thickness of the potentially economic magnetic taconite 400 to 560 feet thick. The potentially economic magnetic taconite occurs in most of the lowest member (Plymouth) of the Ironwood Iron Formation and in the upper part of the Yale and lower part of the Norrie members, both of which occur stratigraphically above the Plymouth member, respectively. Diabase and gabbro dikes and sills have intruded the Ironwood Iron Formation in the western Gogebic Iron Range. LaPointe Iron Company has estimated the Wisconsin magnetic taconite resource located in the 21 miles from Upson to Mineral Lake to be 2.1 billion tons at a 1:1 maximum strip ratio which, based on Davis Tube magnetic concentrates from exploration drilling, could produce over 600 million tons of concentrate with greater than 65% iron and an average strip ratio of 0.40. Marsden (1978), in a study conducted for the U. S. Bureau of Mines using 1970’s technologies, costs and product prices, estimated that 3.7 billion tons of magnetic taconite could be profitably extracted over the same 21 miles. References Cannon, W.F., LaBerge, G.L., Klasner, J.S., and Schulz, K.J., 2008, The Gogebic Iron Range—A sample of the northern margin of the Penokean fold and thrust belt: U.S. Geological Survey Professional Paper 1730, 44 p. 61 �Marsden, R.W., 1978, Iron ore reserves of Wisconsin—A minerals availability system report, in Proceedings, American Institute of Mining Engineers, 51st annual meeting, Minnesota Section, Duluth, Minn., Jan. 11-13, 1978: Duluth, Minn., University of Minnesota, American Institute of Mining Engineers, no. 39, p. 24-1 to 24-28. 62 �NEW EDUCATIONAL INITIATIVES AT THE UNIVERSITY OF MINNESOTA DULUTH: PREPARING STUDENTS FOR FUTURE JOBS IN THE MINING AND MINERALS EXPLORATION INDUSTRIES James D. Miller Jr., Dept. of Geological Sciences, University of Minnesota Duluth mille066@umn.edu (218-726-6582) Carlos Carranza-Torres, Dept. of Civil Engineering, University of Minnesota Duluth carranza@d.umn.edu (218-726-7842) Richard Davis, Dept. of Chemical Engineering, University of Minnesota Duluth rdavis@d.umn.edu (218-726-6162) David Hendrickson, Coleraine Minerals Research Laboratory, Natural Resources Research Institute, University of Minnesota Duluth, dhendric@nrri.umn.edu (218-2454204) The departments of Geological Sciences, Civil Engineering and Chemical Engineering along with the Natural Resources Research Institute (NRRI) at the University of Minnesota Duluth are collaborating to develop new courses and new degree options that will prepare students for professional jobs in mining and minerals exploration industries, both locally and globally. These new course offerings and degree options are expected to be in place by the fall term of 2010 or soon thereafter. The Department of Geological Sciences will offer a BS degree with a mining and mineral exploration emphasis. To qualify for this option, students will be required to take the standard coursework required for a B.S. in geology (including two semesters each of calculus, chemistry, and physics and a six-week summer field camp), along with seven other required courses. These additional courses are Geologic Maps, Engineering Geology, Probability and Statistics, Economic Geology, Minerals Exploration, Mine Design and Operation, and Mineral Processing. Course proposals for the last three courses will be submitted this summer. The Geological Sciences department is also looking into developing a five-year professional masters degree in mining and exploration geology. Preliminary ideas for this degree program are that it will require advanced graduate level courses in geology, mining, exploration, and business and an internship with a mining or exploration company. The Civil Engineering department at UMD, which was initiated in 2008, has developed plans to offer a BS in Civil Engineering with a mining engineering minor. The mining minor will require students to take courses in soil mechanics, rock mechanics, engineering geology, mine safety, mine design and operation, excavation design, and mineral processing. Soil and rock mechanics are currently offered and the others will be developed in the coming year or two as faculty personnel allow. The Department of Chemical Engineering intends to offer a minor in Mineral and Material Process Engineering that builds on the department’s history of activity in education and research in this area. In developing this minor, the Chemical Engineering department is looking to partner with the Natural Resources Research Institute to offer courses in mineral processing and extractive metallurgy. 63 �Involving the experienced staff and research facilities of the NRRI in both teaching and research is a key component to these initiatives. The NRRI Coleraine Minerals Research Laboratory can provide a great resource for teaching and conducting applied research into mineral processing and extractive metallurgy. In addition, the economic geology group at the NRRI may be tapped for their experience in mineral exploration and ore deposits. In addition to teaching and research collaboration between our three academic departments and the NRRI, we are also hoping to tap into the practical expertise of local mining and exploration company personnel. We consider these course additions and curriculum modifications as a modest first step toward addressing the severe manpower and talent shortages that existed in the mining and minerals exploration industries prior to the recent economic downturn. When the recovery comes, we expect that these curricula changes will put UMD in a position to readily supply the exploration and mining industries with the well-trained geologists and engineers they will desperately need once again. We especially want to be the source of human capital for the venerable iron ore industry and the nascent base metal mining industry in northern Minnesota. Both of these industries can provide well paying and fulfilling jobs for decades to come. To fully serve the needs of the local, as well as the global mining industry, we are considering a grander plan that we are tentatively calling the Center for Mining and Mineral Exploration at UMD. The center would include a well integrated and more robust curriculum among our three departments and would seek support for basic and applied collaborative research by faculty and student for the benefit of the mining and exploration industries, especially locally. Of course, to fully realize this plan will require the support of the mining and exploration industries in the form of endowed faculty positions, funds to support faculty and student research, and in-kind contributions from local mining experts in teaching and research. So while these admittedly grandiose plans will have to at least wait for the mining industry and the rest of the economy to recover, the curriculum and program changes outline here will implemented regardless. Hopefully, these changes will be well received by the industry and will lead to support for our larger goal in the future. 64 �THE NOKOMIS CU-NI-PGE DEPOSIT, DULUTH COMPLEX, MINNESOTA Peterson, Dean M., Duluth Metals Corporation, 306 West Superior Street, Suite 407, Duluth, MN 55802. dpeterson@duluthmetals.com Duluth Metals Limited’s Nokomis deposit is the most recently discovered Cu-Ni-PGE deposit in the 1.1 Ga. Duluth Complex, Minnesota. The deposit was discovered utilizing a genetic ore deposit model that identified and back-tracked channelized magma flow within the South Kawishiwi intrusion (SKI). The model led to exploratory drilling in 2006, deposit discovery and initial resource estimation in 2007, and significant resource expansion in 2008, all in a period of 18 months. The deposit’s updated 2008 NI 43-101 compliant Resource Estimate, based on 108 holes drilled by Duluth Metals and 52 historic drill holes on and off the property, contains 449 million tonnes of Indicated Resources grading 0.624% copper, 0.199% nickel, and 0.600 grams per tonne of total precious metals (TPM = Platinum+Palladium+Gold), and an additional 284 million tonnes of Inferred Resources grading 0.627% copper, 0.194% nickel, and 0.718 grams per tonne of TPM. The combined Indicated and Inferred Resources contain approximately 10 billion lbs Cu, 3.1 billion lbs Ni, 165 million lbs Co, 4 million ounces Pt, 9 million ounces Pd, and 2 million ounces of Au. Within these NI 43-101 resources are large tonnages of higher grade material, and the company has commenced an internal research program to identify the geologic controls on the formation nickel-rich and PGE-rich mineralization in the SKI, as well as copper-PGE rich mineralization in the footwall Archean rocks. To date, Duluth Metals has drilled more than 500,000 Ft. (~152,000 m) of core in 154 holes into the deposit, and has only drilled about half of the property. The ore deposit model was developed in cooperation with researchers from the Natural Resources Research Institute of the University of Minnesota, Duluth. A fundamental aspect of the ever-developing ore deposit model is an understanding of the initial conditions of the magmatic system – its crystallinity, sulfur capacity, geochemistry, and geometry – and how the sulfur saturated SKI magma lived, worked, and died. Such understanding includes the realization that the magma was a crystal-liquid (silicate and sulfide liquids) slurry and the identification of magma channelways and sub-channels and their associated thermal anomalies. In addition, the SKI magmas locally melted the footwall granitoid rocks, and such melts have been incorporated into the sulfide-bearing troctolitic melts of the SKI. In the end, hard work and intellectual geologic thought has been used to identify one of the world’s largest resources of Cu-Ni-PGEs. 65 �66 �67 �68 �69 �ALTERATION OF STROMATOLITE BIOSIGNATURES IN THE BIWABIK IRONFORMATION: RELEVANCE TO ASTROBIOLOGY Russell S. Shapiro, Department of Geological and Environmental Studies, California State University, Chico, California, 95929 rsshapiro@csuchico.edu Stromatolites, the macrofossil evidence of microbial activity, are an important potential biosignature in the search for life on early Earth and in extraterrestrial missions, yet the taphonomic effect of metamorphism is poorly known. While broad regional metamorphism related to convergent tectonics may be largely restricted to post-Hadean Earth, alteration from volcanism, heated and reducing fluids, and impacts is quite common throughout the solar system. The present study describes and quantifies the effect of contact metamorphism on a biogenic stromatolite bed. While the specific conditions of this example may not be widely applicable in studies of current astrobiology targets, the pathways and resultant changes will serve as a valuable analog for developing tools for extraterrestrial stromatolite recognition. The stromatolites are constrained to two, meter-scale sequences in the Biwabik Iron-Formation of the Mesabi range on the western margin of the Animikie Basin. The Biwabik records shallow marine sedimentation during the Paleoproterozoic before and during a major collision, the Penokean Orogeny (Ojankangas et al., 2001). Deformation related to the Penokean orogeny and subsequent events lasted for nearly 30 million years based on faulting and synorogenic intrusions (see summary in Schulz and Cannon, 2007). However, it is assumed that no major mineralogical effects in the study area resulted from this event. The next major phase of alteration was related to mid-continental rifting at 1,100 Ma. Though ultimately a failed rift, conspicuous volumes of basalt flows and subjacent gabbro, troctolite, granodiorite and anorthosite formed along the rift. The contact aureole extends for approximately five kilometers in the Mesabi range with temperatures near the contact in excess of 850 degrees C (Hyslop et al., 2008). Mineralogic changes were detailed by French (1968), Loughheed (1983), Floran and Papike (1978), Frost et al. (2007), Hyslop et al. (2008) and others, defining isograds within the contact aureole. Stromatolites were compared from the each of the two Biwabik beds from both outside and inside the contact aureole. Petrographic thin- and thick-sections were studied with standard transmitted and reflected light microscopy. The unmetamorphosed representative samples came from core at U.S. Steel Minntac (basal stromatolite layer) and Minnesota Geological Survey deep core 2 (upper stromatolite layer from the Upper Cherty member). Samples from within the contact aureole were collected from taconite mine exposures in Polymet (formerly Cliffs-Erie / LTV) Area 5 (basal) and Northshore block 20 (upper stromatolites). Based on isograds presented in French (1968) and modified by Frost et al. (2007), the Area 5 locality is between zones 5 and 6 and Northshore 20 is between zones 7 and 8. Zone 5 records formation of ferrohypersthene with graphite, zone 6 is defined by hedenbergite, zone 7 by fayalite, and zone 8 by orthopyroxene. Results Minntac (basal stromatolites, outside aureole)—The stromatolites in this least altered location are composed of sideritic laminae. The laminae are defined by 0.5 mm thick bundles composed of bands averaging 25 μm thick of organics and hematite. Lenses of microquartz occur in shelter porosity. Granules between stromatolite columns are composed of quartz with thin magnetite rims. Filamentous microfossils comparable in size and density to Gunflintia minuta from the Gunflint IronFormation are rarely preserved in hematite. Early diagenetic features include rare stilpnomelane and ankerite either as late-stage spar or replacing late-stage silica cement. MGS-2 (upper stromatolites, outside aureole)—These stromatolites are composed of very fine (10-20 μm) laminae and ministromatolites. The thin, dark laminae are defined by organics. Granules are rare and composed of the iron phyllosilicate greenalite. Most grains are ooids rim-replaced by 70 �subhedral magnetite. Early marine cements are preserved by quartz though most of the stromatolites are neomorphosed into mosaic microquartz. Diagenesis is recognized by blocky euhedral ankerite that crosses fabrics. PolyMet 5 (basal stromatolites, within aureole)—The thin-sections show prevalent destruction of laminae, replaced by microquartz. Secondary removal of microquartz is defined by intrusions of magnetite and calcite. Where preserved, the dark laminae are leached, leaving 1-2 μm thick bands of iron oxides. Veins through the stromatolites are filled with magnetite, calcite, and pumpellyite. Radiating rosettes of either grunerite or minnesotaite are found within laminae and among basal quartz sand grains. Granules of epidote occur at the base of stromatolite columns. Northshore 20 (upper stromatolites, within aureole)—This location is closest to the gabbro. The stromatolites are still recognizeable with laminae defined by single crystals of magnetite preserved within mosaic microquartz. Crystals of microquartz average 40 μm across. Fabric-destructive, larger subhedral magnetite ~7-20 μm across, occurs throughout the stromatolite. Alteration History 1) Stromatolites formed under normal marine conditions as alternating thin laminae of sideritic mud and organic-rich, hematite layers. Greenalite granules washed in from deeper waters. 2) Early marine silica formed as rim and shelter porosity cements. 3) Burial diagenesis led to reduction of hematite, formation of magnetite rims on grains and in stromatolitic laminae. Some destruction of laminae through replacement by microquartz. Ankerite formed as small crystals irregardless of fabric. Growth of stilpnomelane. No increase in crystal sizes. 4) Intrusion of gabbro led to further reduction and loss of hematite and remaining iron-carbonate. Laminae now composed of single crystal-thick bands of magnetite. Complete replacement by microquartz. Formation of fabric-destructive calcite, usually in association with magnetite. Pumpellyite formed in veins associated with calcite and magnetite. References Schulz, K. J., and Cannon, W. F., 2007, The Penokean orogeny in the Lake Superior region: Precambrian Research, 1578:4—25. Okjakangas, R. W., Morey, G. B., and Southwick, D. L., 2001, Paleoproterozoic basin development and sedimentation in the Lake Superior region, North America. Sedimentary Geology, 141142:319—341. Lougheed, M. S., 1983, Origin of Precambrian iron-formations in the Lake Superior region, Bulletin of the Geological Society of America, 94:325—340. French, B. M., 1968, Progressive contact metamorphism of the Biwabik Iron-formation, Mesabi Range, Minnesota: Minnesota Geological Survey Bulletin 45, 103 p. Floran, R. J., and Papike, J. J., 1978, Mineralogy and petrology of the Gunflint Iron Formation, Minnesota-Ontario: correlation of compositional and assemblage variations at low to moderate grade: Journal of Petrology, 19:215—288. Frost, C. D., von Blanckenburg, F., Schoenberg, R., Frost, B. R., and Swapp, S. M., 2007, Preservation of Fe isotope heterogeneities during diagenesis and metamorphism of banded iron formation: Contributions to Mineralogy and Petrology, 153:211—235. Hyslop, E. V., Valley, J. W., Johnson, C. M., and Beard, B. L., 2008, The effects of metamorphism on O and Fe isotope compositions in the Biwabik Iron Formation, northern Minnesota: Contributions to Mineralogy and Petrology, 155:313—328. 71 �BEDROCK GEOLOGIC MAP OF THE DISAPPOINTMENT AND IMA LAKES AREA, BOUNDARY WATERS CANOE AREA, LAKE COUNTY, NE MINNESOTA Stifter, E., Wartman, J., Gibbons, J., Kane, K., Murphy, L., Carlson, A., Mason, T., Hudak, G., and Peterson, D., Precambrian Research Center, University of Minnesota Duluth, 229 Heller Hall, 1114 Kirby Drive, Duluth, MN 55812, ecstift@yahoo.com The “capstone” project for the Precambrian Research Center summer field camp encompasses one week of detailed field mapping in small groups with faculty from the summer field camp. During the fifth and sixth weeks of the 2008 field camp, seven students mapped in the vicinities of Disappointment and Ima Lakes (located within the Boundary Waters Canoe Area Wilderness) under the direction of PRC Faculty Dr. Dean Peterson and Dr. George Hudak. Geological maps of the area were originally published in reports by Van Hise (1901) and Gruner (1941). The purpose of this capstone mapping project was 1) to better understand the nature of the contact between the Mesoproterozoic Duluth Complex and Neoarchean supracrustal strata; 2) to better understand the compositional make up of the Duluth Complex in this area; and 3) to better understand the stratigraphic and structural characteristics of the Neoarchean rocks in this locale. The final map was published at 1:10000 scale and covered an area of approximately 15 square miles. Prior to mapping, detailed field mapping sheets were constructed. Field mapping sheets were produced at 1:5000 scale. One side of each field mapping sheet consisted of digitized topographic maps, topographic contours and bathymetric contours (digitized in 3D). The second side of each field mapping sheet consisted of an air photo and aeromagnetic data (Chandler, 1991). Mapping was primarily done from canoe, but several difficult traverses through blow-down were also accomplished. For example, there were several traverses completed by faculty and students where our feet never touched the ground for several hundred meters, as we were climbing over dead and fallen trees. Each night, each field party copied their field data on to a master field map, so that by the end of the week, the field map was essentially completed. During the sixth week of the field camp, students digitally produced the field map utilizing a wide variety of software (including ArcView, AutoCad, Surfer, and Adobe Illustrator). Neoarchean strata (~2.72-2.67Ga) in the study area varied from steeply southwest-dipping to steeply northeast-dipping. The base of the stratigraphic section is interpreted to comprise the Knife Lake Group, and is composed, from oldest to youngest units, of a) massive and pillowed basalt lava flows ; b) interbedded rhyodacitic to dacitic tuff/ lapilli tuff and polymict lapilli tuff/tuff-breccia deposits; c) andesitic lapilli tuff/tuff- breccias; d) interbedded mustone, chert, and Algoma-type oxide-facies banded iron formation; and e) interbedded mustones and greywackes with minor Algoma-type oxide-facies banded iron formation. Subsequent regional D2 deformation led to the development of west-northwest – eastsoutheast-trending zones of chlorite schist that are up to 50 meters thick and that can be followed along strike for 500-800 meters. Timiskiming-type metasedimentary strata composed of polymict conglomerates and conglomeratic sandstones occur north of the chlorite schist zones, and are believed to comprise the Ogishke conglomerate (Jirsa and Miller, 2004). Locally, Neoarchean intrusive rocks are locally present and include 72 �synvolcanic diabase dikes, and post-volcanic quartz-feldspar porphyry dikes and diorite stocks. The Mesoproterozoic (~1.1 Ga) Duluth Complex in the vicinity of Ima Lake is composed of early Anorthositic Series rocks (anorthosite and anorthositic gabbro) that were subsequently intruded by the new, informally named Ima Lake Intrusion. The Ima Lake Intrusion is broadly layered, and comprises a) a basal unit composed primarily of oxide- and sulfidebearing gabbros with local zones comprising sulfide-bearing norite; b) an extensive unit of augite troctolite; c) and an upper zone composed of anorthositic troctolite. The sulfidebearing gabbroic base of this intrusion is akin to Early Gabbro Series intrusions to the eastnortheast and differs markedly to the classic sulfide-bearing troctolitic intrusions of the Troctolite Series to the southwest (ie., the South Kawishiwi and Partridge River intrusions). A two kilometer long by up to 500 meter thick zone of pyroxene hornfels occurs along the southwestern margin of the Ima Lake Intrusion, suggesting that Neoarchean supracrustal strata were thermally metamorphosed during the emplacement of the Duluth Complex. This detailed mapping project, combined with analysis of aeromagnetic data, has resulted in the relocation of the contact between the Duluth Complex and the adjacent Neoarchean supracrustal strata approximately 1.6 kilometers from previous mapping (Miller et al., 2001). References Chandler, V. W., 1991. Aeromagnetic anomaly map of Minnesota: Minnesota Geological Survey State Map Series S-17, scale 1:500,000. Gruner, J. W., 1941. Structural Geology of the Knife Lake Area of Northeastern Minnesota: Geological Society of America Bulletin, v. 52, p. 1577-1642. Jirsa, M.D., and Miller, J. D., 2004. Bedrock geology of the Ely and Basswood Lake 30’ x 60’ quadrangles, northeast Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-148, scale 1:100,000. Miller, J. D. Jr., Green, J. C., Severson, M. J., Chandler, V. W., and Peterson, D. M., 2001. Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-119, scale 1:200,000, two sheets. Van Hise, C. R., 1901. The iron-ore deposits of the Lake Superior region: 21st Annual Report of the U. S. Geological Survey, Part III. 73 �METAMORPHISM AND DEFORMATION AT MUSSELWHITE MINE Stinson, Victoria R. vrstinso@lakeheadu.ca, Kolb, Maura J., and Hill, Mary Louise, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, Canada P7B 5E1 Musselwhite Mine is a shear-zone-hosted orogenic gold deposit located within the Superior Province in Northwestern Ontario, 480 km north of Thunder Bay. An Archean metamorphosed and deformed banded iron formation is host to the economic gold mineralization. Metamorphism and deformation are contemporaneous with gold mineralization at Musselwhite Mine. Most of the gold mineralization is within garnet-grunerite-biotite schist. Associated garnet-staurolite-biotite schist and sillimanite-garnet-staurolite-biotite schist indicate metamorphism within the staurolite or sillimanite zone of the amphibolite facies. True amphibolites are absent within the mine due to dominance of metasedimentary lithologies. Grunerite schist and biotite-grunerite schist, often logged as mafic volcanics, are interpreted as metasedimentary. As expected at amphibolite facies metamorphic conditions the rocks have mainly undergone ductile deformation. At the microscopic scale, strain is heterogeneous across compositional bands within the metamorphosed banded iron formation. Quartz bands have relatively larger grains with undulose extinction and irregular grain shapes typical of grain boundary migration recrystallization. Adjacent to grunerite, quartz is finer grained, implying some difference in strain rate or deformation mechanisms along phase boundaries. Within iron-rich bands, grunerite, biotite, and garnet are products of metamorphic reaction. Finegrained biotite and grunerite define the foliation. Strain partitioning between these iron-rich bands and the quartz bands is likely. Although most minerals exhibit evidence of ductile deformation mechanisms at these metamorphic conditions, garnet does not. The garnet crystals have undergone brittle deformation. This small amount of brittle deformation in otherwise ductile conditions may create temporary porosity in a region which otherwise has none. This seems to be important to gold mineralization because gold is found in and around areas where both brittle and ductile deformation structures are present. 74 �OPTIONS FOR GEOLOGIC SEQUESTRATION OF CARBON IN THE UPPER MIDWEST: MINERAL CARBONATION AND DEEP INJECTION Harvey Thorleifson, Minnesota Geological Survey, 2642 University Ave W, St Paul, MN 551141057 USA; Telephone 612-627-4780 ext 224; Fax 612-627-4778; thorleif@umn.edu Increasing concern about climate change has necessitated assessment of ways to reduce emissions, while increasing our preparedness to adapt. Emissions reductions can be achieved by reducing combustion of fossil fuels, by reducing other activity that generates greenhouse gases, and by increasing carbon storage in vegetation and soils. In addition, the technology to capture CO2 from sources such as electrical generating stations and ethanol plants is available, allowing geologic sequestration through methods such as deep injection or mineral carbonation to return carbon to the geosphere. Options within Minnesota therefore are being assessed, as an alterative to eventual transportation of CO2 by pipeline to a jurisdiction such as North Dakota or Illinois. In relation to deep injection, the most prospective rocks in Minnesota at the 1 km depth required to maintain CO2 in a liquid-like state are sedimentary basins of the Midcontinent Rift, present in two north-south belts on either side of the Twin Cities, running from Pine County and Washington County, south to Iowa. Currently available data, however, indicate that there is a very low probability of success in confirming suitable geologic conditions in these rocks, due to likely lack of adequate porosity and permeability, as well as inadequate seal integrity - while at the same time, it is recognized that drilling may be required to adequately clarify options prior to major expenditures on other options (Thorleifson, 2008). Another geologic technique is mineral carbonation, in which CO2 is reacted with olivine-rich material from mining, producing mineral products for disposal or use in construction (Metz et al., 2005). A clear advantage of this method is the lack of risk due to leakage. Minnesota may eventually be well positioned to utilize the mineral carbonation method of geologic carbon sequestration, given the presence of large tonnages of appropriate rock material near Duluth that may be mined for copper, nickel, and platinum group elements, in proximity to well developed infrastructure. In this method, CO2 is reacted with minerals such as olivine, yielding carbonate and quartz. Should these deposits go into production, it is possible that the slurry of minerals produced as a waste product from the mines could be suitable for mineral carbonation of CO2, and in a future carbon-trading scenario, these mines could obtain significant revenue by selling carbon credits. The principal constraint to mineral carbonation at present, however, appears to be cost. According to the Intergovernmental Panel on Climate Change (IPCC), costs for deep injection of CO2 into saline formations are estimated at 0.5 to 8 US$/tCO2, while their estimate for mineral carbonation is 50 to 100 US$/tCO2 (Metz et al., 2007). Furthermore, although the process has been demonstrated experimentally, it has not been tested at a scale that approximates field conditions. Nevertheless, there could be developments in the method, and there could be circumstances in which a particularly favorable mineral carbonation opportunity could coincide with constraints to other sequestration options, such as transportation, thus possibly making mineral carbonation a conceivable option. Minnesota agencies therefore are preparing to conduct an analysis of the mineral carbonation option, to place available information into a Minnesota context, largely by modeling a scenario related to Duluth-region mining in order to evaluate the magnitude of this potential opportunity. A needed aspect of the analysis will be an approximation of the foreseen waste rock composition and production rates at the proposed and contemplated mines, based on literature, analogy to mines elsewhere, and data made available by the project proponents. The analysis will seek to identify the amount of CO2 that could be stored, given well-outlined assumptions, resulting in an estimate of the potential that can readily be updated as needed information is enhanced. This information will enable 75 �Minnesota public and private agencies to be aware of the likelihood that mineral carbonation could be an option for the state, the approximate magnitude of the potential in terms of the amount of carbon that could be sequestered at what rate and cost, and the factors that will govern whether implementation can be anticipated. By doing so, an activity that could possibly bring significant economic benefits to the state in association with anticipated climate change policy initiatives will be clarified. Should the literature review phase be encouraging, further analysis of mineral processing considerations would be called for, to be conducted by parties with the required expertise. References Metz, B., O. Davidson, , H. de Coninck, M. Loos, and L. Meyer, eds., 2005, IPCC Special Report on Carbon Dioxide Capture and Storage, Cambridge University Press, 431 p. Metz, B., O. R. Davidson, P. R. Bosch, R. Dave, L.A. Meyer, eds., 2007: Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, 851 pp Thorleifson, L. H., ed., 2008, Potential capacity for geologic carbon sequestration in the Midcontinent Rift System in Minnesota, Minnesota Geological Survey Open File Report OFR-08-01, 138 p. 76 �MINE WATER QUALITY PREDICTION AND ENVIRONMENTALLYRESPONSIBLE MINING – YES WE CAN! Rens Verburg Golder Associates, Redmond, WA E-mail: rverburg@golder.com, Tel: 425 883 0777; fax: 425 882 5498 Patrick Dunlavy Golder Associates, Roseville, MN E-mail: pdunlavy@golder.com, Tel: 651 697 9737; fax: 651 697 9735 Minnesota is entering a new era of mine development as economic prospects associated with the Duluth Complex are being pursued with considerable vigor. Equally vigorous has been the opposition to such developments, fueled in large part by the concerns related to potential environmental impacts. Coupled with the notion that prediction of such impacts is considered by some to be exceedingly difficult and frequently flawed, it would seem that exploitation of the Duluth Complex faces an uphill battle. In reality, prediction and prevention of environmental impacts has made significant strides in the last decade. Current available guidance on sample collection, geochemical characterization methods, interpretation of results, water quality prediction, and evaluation of potential environmental impacts is based on best practices that have met with demonstrated success. In particular, the most recent guidance, the Global Acid Rock Drainage Guide (GARD Guide), developed by the International Network for Acid Prevention (INAP, 2009), a consortium of mining companies dedicated to reducing liabilities associated with sulfide mine materials, contains a wealth of information in this regard, and presents a roadmap to evaluate and manage environmental issues related to generation of acid rock drainage and metal leaching (ARD/ML). Our general understanding of geologic materials, mine wastes, and hydrogeochemical factors that govern mine water quality continues to advance through the implementation of laboratory and field experiments. Similarly, ongoing characterization and monitoring of mine facilities allows for development of improved scaling factors needed to extrapolate results from smaller-scale tests to an operational level. Further, the necessary tools required for geochemical, hydrological, and hydrogeological modeling in support of water quality prediction exist. It should further be noted that the use of more sophisticated tools does not necessarily equate to more accurate and precise water quality predictions. The nature and sophistication of a prediction effort should vary depending on the desired outcome. For instance, a prediction exercise aimed at answering a “yes/no” question (for example: will the water quality criterion for constituent X be exceeded?) requires less a priori understanding of the system being evaluated, in which case the use of a relatively basic prediction approach may suffice. In contrast, when a more quantitative answer is required (for example: what is the expected concentration of constituent X), the complexity of the prediction effort may be quite significant, requiring both a detailed conceptualization of the system being modeled as well as use of advanced modeling codes. Therefore, tools should be selected that suit the need of a particular application and are compatible with the range and quality of the input data. Also regulatory expectations with respect to water quality prediction should become more realistic. It should be recognized that massive sampling and testing sampling campaigns coupled with extensive, expensive, and frequently impenetrable water quality modeling are not always a guarantee for more precise and accurate water quality predictions, but in fact may provide a false sense of security. Instead, use of simpler and more robust approaches, that take into account accumulated knowledge complemented by a targeted verification campaign, may be just as reliable and useful for decision making. A good example of such a robust approach is the use of geo-environmental models. Geo-environmental models provide a very useful way to interpret and summarize the environmental signatures of mining and mineral deposits in a systematic geologic context (Plumlee, 1999), and can, therefore be used to anticipate water qualities and potential environmental problems at future mines, operating mines, and orphan sites. 77 �Figure 1 shows ranges of pH and trace metals concentrations for mine water discharging from two types of ores of interest to Minnesota: banded iron formation ores (i.e., the taconite ores), and magmatic sulfide deposits such as the Duluth Complex. Without having collected a single sample, stakeholders can readily identify potential water quality issues associated with the two different ore types. Through a limited sampling and characterization program, these water quality ranges can be refined and verified. Information presented in Figure 1 can also be used to identify and evaluate prevention and mitigation alternatives. Although some site-specific testing will be needed, such a program can focus from the outset on appropriate and relevant methods, bounded by pragmatic constraints. Figure 1 identifies that exploitation of the Duluth Complex may generate mine water that is of different quality than the taconite operations have traditionally produced. However, the modern mining industry is committed to and capable of minimizing environmental impacts. Kennecott’s Flambeau Mine in Wisconsin is a good example of such an approach. Although, due to the nature of the ore deposit, potential water qualities at this site were predicted to be much poorer than expected from the Duluth Complex, proper waste management and reclamation have lead to successful prevention of water quality impacts while the site has been restored to beneficial use References INAP (2009). The Global Acid Rock Drainage Guide http://www480.pair.com/aturner/gardwiki/index.php/Main_Page Plumlee, G.S. (1999). The Environmental Geology of Mineral Deposits. In: The Environmental Geochemistry of Mineral Deposits, Part A: Processes, Techniques and Health Issues (Eds.: Plumlee, G.S., and M.J. Logsdon). Reviews in Economic Geology Vol 6A. Society of Economic Geologists, Inc. Figure 1. Water quality ranges for magmatic sulfide deposits and banded iron formation 78 �HYDROSTRATIGRAPHY OF THE BIWABIK IRON FORMATION – IMPLICATIONS FOR CURRENT GROUNDWATER FLOW PATTERNS AND PAST GENESIS OF NATURAL ORE BODIES Walsh, James F., Minnesota Department of Health, james.f.walsh@state.mn.us Recent well logging studies conducted by the Minnesota Geological Survey and the Minnesota Department of Health on the west-central Mesabi Iron Range suggest that groundwater flow within the Biwabik Iron Formation is influenced by internal stratigraphic contacts. Groundwater flow appears to be focused in distinct zones within the Upper and Lower Cherty members. The strongest evidence for flow appears near the top of these members, close to the contact with overlying slaty strata. The Upper Cherty flow zone is characterized by groundwater that is young in recharge age, warm and high in total dissolved solids relative to the Lower Cherty flow zone. These findings have several implications regarding mine dewatering and other environmental considerations. In addition, they may reflect on the origins of the natural ore bodies that were exploited in the early years of mining on the Mesabi Range. These data support a model whereby vertical movement of oxidizing fluids along faults and fractures was accompanied by lateral flow along contacts between slaty and cherty strata. 79 �MAGMA CONDUIT HOSTED PLATINUM-PALLADIUM-COPPER-NICKEL MINERALIZATION AT THE THUNDER BAY NORTH PROJECT, NORTHWEST ONTARIO: DISCOVERY, EXPLORATION, GEOLOGY AND RESOURCE POTENTIAL Keith P. Watkins, Magma Metals Limited, Level 3, 18 Richardson Street, West Perth, Western Australia WA6005, Australia, keith.watkins@magmametals.com.au Glacially transported Pt-Pd-Cu-Ni mineralized peridotite boulders were found on the west shore of Current Lake, approximately 50 km northeast of Thunder Bay, by two geologists prospecting through the area in 2001. Magma Metals Limited optioned the claims in 2005 and found a larger occurrence of mineralized boulders on the east shore of the lake in mid2006. Magma drilled its first hole, TBND001, under these boulders in December 2006 and intersected 10.5 m @ 2.69g/t Pt+Pd, 0.45% Cu & 0.34% Ni from 72 m down-hole. Subsequent geophysical surveys and over 45,000 m of drilling have delineated a large mineralized intrusive complex within a 50 km2 area. The complex forms a network of maficultramafic magma conduits associated with the Keweenawan-age Midcontinent Rift. The mafic-ultramafic magmas were emplaced within late-Archean granitoids and metasediments of the Quetico subprovince of the Superior craton. Most of the exploration work has focused on the Current Lake Intrusive Complex (CLIC), the 5 km-long easternmost conduit, which is described here. Several intrusive phases have been identified and there are strong structural controls on the emplacement of the conduits. The earliest intrusive phase in the CLIC is a minor pegmatoid unit which intrudes flat structures. This is followed by gabbro phases which have assimilated much country rock and show a variety of textures and compositions; these intrude flat and steep structures. Finally, several pulses of sulphide-rich melagabbro to peridotite intrude the earlier phases and form the main conduits. The dominant style of mineralization is disseminated sulphide, principally pyrrhotite and chalcopyrite. Thick zones have been intersected in systematic drilling over a strike length of 3 km in the CLIC, some of these zones are relatively high-grade, e.g. 61.7 m @ 2.87g/t Pt, 2.74g/t Pd, 0.66% Cu & 0.38% Ni from 29.3 m down-hole, including 35.5 m @ 4.52g/t Pt, 4.31g/t Pd, 1.04% Cu & 0.57% Ni in drill-hole TBND061. Zones of net-textured (semimassive) sulphide also occur. Some narrow intervals of high-grade massive sulphide mineralization have also been intersected near the base of the conduit, including 0.4 m @ 13.80g/t Pt, 10.75g/t Pd, 3.70% Cu & 2.91% Ni from 315.15 m down-hole in drill-hole BL08-61. There is generally good correlation between pyrrhotite-chalcopyrite abundance and grade of mineralization. Sulphide tenors (estimated grades in 100% sulphide) are consistently in the ranges 3-4% Ni, 6-8% Cu, 24-38g/t Pt and 22-37g/t Pd. There is excellent correlation between Pt, Pd, Cu and Ni in the melagabbro-peridotite indicating a pristine magmatic system with little alteration or re-distribution of metals. Preliminary petrochemical analysis indicates the mineralized melagabbro-peridotite had a tholeitiic parent magma (~6% MgO) and there is evidence of homogenized crustal contamination in addition to localized marginal contamination. 80 �Recent reconnaissance drilling in other parts of the magma conduit network to the west of the CLIC has confirmed the potential for further mineralization outside of the area currently being systematically drilled. There is potential for an aggregate multi-million ounce resource of platinum-group metals with substantial copper and nickel credits within the magma conduit network. There is also potential for associated Ni-Cu massive sulphide deposits. A major drilling program is in progress to define initial resources within the northwestern and central parts of the CLIC over a strike-length of 3 km. 81 �DIAMONDIFEROUS MASS-FLOW AND PLACER DEPOSITS NEOARCHEAN FAN DELTA, WAWA AREA, SUPERIOR PROVINCE FORMING A WENDLAND, Corey, FRALICK, Philip, and Hollings, Peter, Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada, P7B 5E1, philip.fralick@lakeheadu.ca Diamond bearing Neoarchean metaconglomerates are present in the Michipicoten Greenstone Belt, Wawa-Abitibi Subprovince, near Wawa, Ontario. They form a portion of the Dore Metasediments in the Arliss Lake Subbasin, and unconformably overlie a succession of mafic metabasalts. The conglomerates are transitional to the south into argillite and are overlain by argillite. This in turn is conformably overlain by metabasalts. The conglomeratic succession under study here has a maximum thickness of 454 meters and is confined in what appears to be a deformed paleovalley at the base of the sedimentary succession. It pinches out against basement on its northern margin and is terminated by a fault on its southern margin, where it is 200 meters thick. Both matrix supported and clast supported beds characterize the unit. Dominant clast types include basalt, rhyolite, gabbro, diorite and sandstone. Clasts are generally angular to sub-rounded, and more rarely rounded to well-rounded. Matrix compositions and textures are variable from mud- and silt-sized material dominating within the matrix-supported, cobble to boulder conglomerates, to fine-grained to very coarse-grained feldspathic, quartz-rich and mafic sands within the clast-supported, cobble to pebble conglomerates. Boulder conglomerates are poorly sorted, while cobble to pebble conglomerates are poorly to moderately-well sorted with a decrease in clast size generally corresponding to better sorting. The unit can be divided into two main lithofacies associations. The dominant lithofacies within Association 1 is massive, matrix-supported cobble to boulder conglomerate, characterized by an abundance of mud-sized matrix material that is disorganized to swirlly. This is interbedded with massive to crudely horizontally layered, and more rarely trough cross-stratified, cobble to pebble conglomerate and minor horizontally layered coarsegrained sandstone. Contacts between these lithofacies are commonly planar and rarely erosive. This facies association is most prevalent near the base of the succession in the center of the apparent paleovalley. Many of the features of the mud-rich conglomerates suggest weakly sheared and highly viscous debris-flows. The poor-sorting, high mud matrix content, angularity of clasts and lack of sedimentary structures implies that few grain-to-grain bedload collisions occurred (weakly sheared) and the transportation mechanism was incapable of winnowing fine sediments and sorting the clasts. Highly viscous debris-flows are characterized by a muddy matrix and often occur in the proximal reaches of alluvial fans, whereas sandy matrix is typical of less viscous, distal-fan debris-flows. Some units containing disorganized clast fabrics may have formed from non-sheared (high strength) plug flow, or only weakly sheared, high viscosity flow on the upper fan. The trough cross-stratified and horizontally layered conglomerates interlayered with the massive conglomerates are the product of active bedload traction transport that was more efficient at winnowing fine sediment and sorting clasts. The sporadically developed erosive bases exhibited by these units and their upward fining is typical of turbulent fluidal flow or heavily sediment laden stream flow following a debris-flow event. Where the stratified conglomerates exhibit more extensive erosional bases they probably represent later erosive reworking of debris-flow material by stream activity. The horizontally stratified sandstones present capping the layered conglomerates were deposited by waning flow resulting in cessation of movement of clasts, falling of sand from the saltation and suspension populations into the traction population, and its deposition. Composite units with thick, crude to distinct internal layering or with the presence of thin, discontinuous sandy zones may result from rapidly surging flows. Lithofacies Association 2 occurs in the middle and upper portions of the unit. It is composed of interbedding of massive, clast-supported conglomerate and horizontally laminated and trough crossstratified sandstone. Other minor lithofacies that occur locally are horizontally layered and trough 82 �cross-stratified conglomerate; planar cross-stratified, ripple laminated and scour-fill sandstones: and horizontally laminated and massive mudstones. The major difference between the massive conglomerates of this Association and Association 1 is that the former is generally finer-grained, dominated by cobbles and pebbles, is clast supported and has a coarse-grained sand to granule matrix. This association was deposited by traction currents in braided fluvial channels of the Scott type. These channels were dominated by gravel longitudinal bars with sandy lenses formed by the infilling of chute channels and scour hollows during falling stage and low water. Sand and gravel bar edge sand wedges were also present in the system and major channels were probably mostly dominated by gravel, as more extensive sandstone successions, typical of sandy large channels, are rare. Association 1 was the product of proximal alluvial fan debris-flows that would have occurred on a fairly steep gradient allowing for such rapidly surging flows to result in an increase in sediment instability causing the coarse-grained sediment and mud-charged debris-flows to move down the alluvial fan-delta. As the gradient decreased the highly viscous debris-flows would deposit almost instantaneously resulting in the immature nature of the fabric and large boulder sized clasts being left in suspension in the muddy, swirlly textured matrix. Once the initial surge of the debris-flow had been deposited the fluvial activity that remained would, on occasion, be strong enough to actively transport bedload material depositing stratified gravels and sands as the stage fell. Further down fan was subjected to less debris-flow activity and it developed an extensive network of gravelly braided channels and gravel bars. It is likely that the upper fan-delta was dominated by off channel processes, whereas in the mid-fan-delta, where the main channel was no longer entrenched, water and sediment was delivered to the fan’s surface on a more consistent basis leading to a better developed fluvial network. A transgressive event resulted in a rapid drowning of the fan delta and deposition of finegrained sediment. Whole-rock geochemistry was conducted on samples of metasedimentary rock from the unit. Ratio plots utilizing immobile elements clearly indicate that the sediment was mostly derived from mafic rock mixed with an ultramafic igneous source. A minor number of samples had a significant felsic source component. The CIA values for the sandstones indicate most samples have undergone a moderate amount of weathering with some reflecting fairly intense weathering. Elements typically contained in heavy minerals, and therefore enriched in placer deposits, (i.e. Zr, Ti, Nb, Y, REEs, Cr and Ni) do not show any systematic enrichment in the sandstones sampled that cannot be accounted for by the composition of the source rocks. This strongly implies that preferential heavy mineral enrichment did not occur in the sandstones. The conglomerates are still under investigation. Diamond concentrations are correlated with increased amounts of Ni, Cr, Co, Ti, Fe and Mg in the samples. When the sandstone samples were plotted on a TiO2/Zr, Nb/Y diagram they defined a more alkalic trend than the felsic to mafic volcanic rocks in the area. This is in agreement with an alkalic ultramafic source rock for the diamonds. The source could either be ultramafic, diamond-bearing lamprophyre dikes present in the area or as yet undiscovered or eroded kimberlites. Whatever the source, a prolific amount of it must have been exposed on surface during formation of the fan delta. 83 �84 � https://digitalcollections.lakeheadu.ca/files/original/d69a6dae6ff77240acdeb4f1d620f717.pdf 86b54caaca65e5dfc5993a8fdcf2341f PDF Text Text 55TH ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY ELY, MINNESOTA, MAY 5 – 12, 2009 PROCEEDINGS VOLUME 55 PART 2 – FIELD TRIP GUIDEBOOK �INSTITUTE ON LAKE SUPERIOR GEOLOGY 55TH ANNUAL MEETING MAY 5-10, 2009 ELY, MINNESOTA HOSTED BY: Precambrian Research Center, UMD JAMES D. MILLER, GEORGE J. HUDAK AND DEAN M. PETERSON Co-Chairs Proceedings Volume 55 Part 2 – FieldTrip Guidebook EDITED BY DEAN M. PETERSON DULUTH METALS LIMITED AND THE PRECAMBRIAN RESEARCH CENTER Cover Photo: Photographs taken in recent years during geological mapping, ore deposit research, fishing for dinner, and educational activities for undergraduate geology students in northeastern Minnesota. (photographs by Dean Peterson) i �55TH INSTITUTE ON LAKE SUPERIOR GEOLOGY PROCEEDINGS VOLUME 55 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: CU-NI DEPOSITS OF THE DULUTH COMPLEX TRIP 2: GLACIAL GEOLOGY OF THE VERMILION MORAINE TRIP 3: SOUDAN IRON MINE AND PHYSICS LAB TOUR TRIP 4: PIONEER MINE CANOE EXCURSION TRIP 5: GEOLOGY & METAMORPHISM OF THE EASTERN MESABI RANGE TRIP 6: GEOLOGY OF THE LAKE ONE TROCTOLITE BY CANOE TRIP 7: ARCHITECTURE OF AN ARCHEAN GREENSTONE BELT Published by the 55th Institute on Lake Superior Geology and distributed by the ILSG Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca ILSG website: http://www.lakesuperiorgeology.org ISSN 1042-9964 ii �TABLE OF CONTENTS PROCEEDINGS VOLUME 55 PART 2— FIELD TRIP GUIDEBOOK TRIP 1: CU-NI DEPOSITS OF THE DULUTH COMPLEX ............................................................ 1 TRIP 2: GLACIAL GEOLOGY OF THE VERMILION MORAINE ............................................ 81 TRIP 3: SOUDAN IRON MINE AND PHYSICS LAB TOUR ..................................................... 100 TRIP 4: PIONEER MINE CANOE EXCURSION............................................................................. 110 TRIP 5: GEOLOGY & METAMORPHISM OF THE EASTERN MESABI RANGE ........... 116 TRIP 6: GEOLOGY OF THE LAKE ONE TROCTOLITE BY CANOE .................................. 156 TRIP 7: ARCHITECTURE OF AN ARCHEAN GREENSTONE BELT ................................... 178 The editor (Dean M. Peterson) wishes to thank the field trip guidebook authors for their contributions to the 55th Annual ILSG conference Field Trip Guidebook tome. This annual meeting is especially built around excellent field trips for ILSG participants that mineral industry companies and local geologists in the Lake Superior region provide to the Institute. I believe that this field guide continues that tradition, and will be useful for professional geologists and students of geology for many decades to come. The editor also wishes to tell the authors of the guidebook that some of the text and figures have been edited for clarity (which I guess is the role of a compiler and editor), and I hope that such editing better conveys the message that the written descriptions and especially the rocks in the Lake Superior District are trying to tell us. The institute wishes to thank Duluth Metals for their $2,000 contribution to the 55th ILSG Field Trip Guidebook. These monies have been used to pay for the numerous color figures and thus lower the cost to participants of the printed version. iii �55th Annual Institute on Lake Superior Geology Field Trip 1 CU-NI-PGE DEPOSITS OF THE DULUTH COMPLEX GEOLOGY AND DEVELOPMENT Rich Patelke (PolyMet Mining) Dean Peterson (Duluth Metals Ltd. & PRC) Mark Severson (Natural Resources Research Institute) Tim Jefferson (Teck American) Ernie Lehmann (Franconia Minerals) 1 �COPPER-NICKEL-PGE PROJECTS GEOLOGY PROGRESS AND DEVELOPMENT INSTITUTE ON LAE SUPERIOR GEOLOGY, ELY MINNESOTA, MAY 2009 Field Trip Coordinator: Dean Peterson Field Guide Compiled By: Richard Patelke Written Contributions By: Richard Patelke-PolyMet Steve Geerts-PolyMet Mark Severson-Natural Resources Research Institute Tim Jefferson-Teck American Dean Peterson-Duluth Metals Richard Routledge-Scott Wilson RPA for Franconia Presenters of Introductory Talks: Kevin Pylka-PolyMet Dean Peterson-Duluth Metals William Brice-Franconia Paul MacRobbie-Teck American Mark Severson-Natural Resources Research Institute Richard Patelke-PolyMet Field Trip Leaders: Richard Patelke PolyMet Mark Severson-Natural Resources Research Institute Tim Jefferson-Teck American Ernie Lehman-Franconia Stuart Behling-Franconia Dean Peterson-Duluth Metals 2 �PART 1: INTRODUCTION, DEPOSITS, AND REGIONAL GEOLOGY OF THE FIELD TRIP AREA By: Richard Patelke, Mark Severson, Tim Jefferson, and Dean Peterson PURPOSE OF TRIP Over time there have been many trips to drill sites, core sheds and outcrops of the mineralized parts of the Duluth Complex. As the current projects move toward development it becomes appropriate to visit the sites, assess the development potential, and see how that potential has changed over time. We will visit core displays for four of these projects, PolyMet’s NorthMet project, Teck American’s Mesaba deposit, Franconia’s Birch Lake deposit, and Duluth Metals Nokomis deposit. PolyMet is in the permitting process with state and federal agencies; Franconia and Duluth Metals have commissioned scoping studies on their deposits and are on the cusp of proceeding to development. Teck does their assessment work internally and is taking a different and less public route through economic review and development. These write-ups are not core specific and are intended to give a sense of the general geology of these projects. Besides the four companies whose projects we will visit, Encampment Resources, Cardero Resources, Prime Meridian Resources, and Kennecott have been active in the area. Technical reports from most of these companies are also available on “SEDAR” at http://www.sedar.com. These reports will give good background on the reserve and resource calculations, as well as development planning. Throughout these descriptions, the term “PGE” means platinum group elements (mostly platinum and palladium) and gold. Since these deposits were first known in the 1950's and 1960's, market and technological changes have made older assessments of the projects obsolete. The big market change has been the rise of PGE as important products, providing about a fourth of the revenue for these projects. Technological change has included: the ability to routinely and inexpensively analyze for PGE; improvements in flotation processes that allow the capture of almost all sulfide (minimizing sulfur in tails as well as improving recoveries), allow for the production of saleable split (i.e., separate copper and nickel) concentrates; and the refinement of hydrometallurgical processes that are able to cleanly recover not just copper and nickel, but PGE, gold, cobalt, and silver. Better flotation and the polymetallic nature of these deposits have pushed the ore vs. waste cut-off much lower than in historic estimates. The change in environmental aspects of these projects due to the recent market and technological changes cannot be overstated. Much of the research on these issues over the years has assumed metal and sulfur contents in waste rock and tailings far above expected ore versus waste cut-off grades for these deposits today, when hydrometallurgical processes are considered. EXPLORATION AND DEVELOPMENT BACKGROUND Large resources of low-grade copper-nickel sulfide ore that locally contain PGE concentrations are well documented by drilling in the basal zones of the Partridge River and South Kawishiwi intrusions. At least eleven occurrences of significant mineralization have been delineated in the basal 300 to 1000 feet of both intrusions. Of these eleven occurrences, three projects are currently undergoing deposit definition drilling, including the Mesaba (Teck American), Birch Lake (Franconia Minerals), and Nokomis (Duluth Metals Ltd.) deposits. A fourth project, the NorthMet deposit (PolyMet Mining) is currently undergoing environmental review and mine permitting. Overall, the copper-nickel mineralization consists predominantly of disseminated sulfides that collectively constitute over 4.4 billion tons of material 3 �averaging 0.66% Cu and 0.20% Ni at a 0.5% Cu cut-off, according to an earlier study by Listerud and Meineke (1977). Serious exploration for Cu-Ni deposits at the base of the Duluth Complex began in 1948, about 8 miles to the southeast of Ely, MN, when strongly mineralized rocks were uncovered in an excavation used to source road material for Spruce Road. Local prospector Fred S. Childers of Ely noted copper stains in the material and he, along with Roger V. Whiteside of Duluth, began searching along the basal contact in the vicinity of the Kawishiwi River. In 1951, they diamond drilled a 188 foot deep hole and intersected mineralized gabbro that averaged 0.36% Cu and 0.13% Ni. In 1952, both Bear Creek Mining Company (BMC) and the International Nickel Company (INCO) began intensive exploration efforts along a 38 mile-long zone that coincided with the basal contact. INCO eventually picked up the Childers-Whiteside properties (Spruce Road and Maturi deposits); whereas, BMC concentrated most of their effort near the town of Babbitt which resulted in the discoveries of the Babbitt (formerly called Minnamax and now known as Mesaba) and Serpentine deposits. By 1960, these exploration efforts indicated that very large tonnages of disseminated Cu-Ni mineralization were present; however, the low-grade nature of the deposits and the unavailability of state-owned mineral lands at the time led to suspension of activities. In 1966, state mineral leases were offered by the Minnesota Department of Natural Resources (DNR) and were awarded to successful bidders, resulting in renewed exploration activity (including the return of BMC and INCO). Since 1966, over 20 companies have been actively involved in exploration for Cu-Ni and Fe-Ti-V deposits along the basal contact of the Complex. Over 2,000 holes, totaling over 2 million feet of core, have been drilled. Exploration efforts during this period also defined several more deposits including: Dunka Road (now NorthMet) and Wyman Creek (United States Steel Corp.), Birch Lake (Duval Corporation and Newmont Mining), South Filson Creek (Hanna Mining), Dunka Pit (Erie Mining, BMC, and Exxon), and Wetlegs (BMC and Exxon). AMAX Exploration Inc. leased the Babbitt deposit from BMC and renamed it the Minnamax deposit in 1973. During mid to late 1970s, the Spruce Road and Minnamax deposits came closest to development. Mining plans were submitted, test shafts were sunk (one each at the Maturi and Minnamax/Mesaba deposits), surface bulk samples were collected from three sites, and various land-use and water-use permits were requested from State and Federal agencies. In 1974, the Minnesota Environmental Quality Board required that a regional Environmental Impact Statement (EIS) be conducted prior to acceptance of any site-specific EIS mining-related proposals. The DNR discontinued lease sales of State lands (1974-1982) until completion of the regional EIS. However, by the time the regional EIS was submitted in 1979, development of the Cu-Ni deposits was put on hold by the most of the mining companies involved due to weakened copper and nickel markets and the inability to make marketable (i.e., "smeltable") separate copper and nickel concentrates. Amax abandoned their plans to develop an underground high grade ore zone within the Minnamax/Mesaba deposit (known as the Local Boy ore body) in late 1982. Then starts the “PGE” era. During the early period of drilling (prior to 1980), all of the exploration companies recognized that the Cu-Ni deposits had some potential for hosting PGEs. Based on very limited sampling, the companies assumed that the typical Cu-Ni ore contained no more that a few hundred parts per billion (ppb) combined platinum and palladium. In 1985, the DNR and Minerals Resource Research Center (MRRC of the U of M) conducted a geochemical evaluation of portions of a Duval drill hole (DU-15), from the Birch Lake area, and found significant values of up to 9 parts per million (ppm) combined Pt and Pd (Sabelin and Iwasaki, 1985, 1986). This was at a time when demand for these elements was increasing due to their use in automotive catalysts. A short time later, Morton and Hauck (1987) compiled all of the known PGE data for the Complex and reported the presence of anomalous PGE values, often associated with high Cu values, at several other Cu-Ni deposits. These discoveries sparked renewed interest in the Cu-Ni deposits as potential polymetallic deposits (Miller et al., 2002; and references therein). E.K. Lehman and Associates of Minnesota obtained mineral leases from the state of Minnesota and began drilling wedges off the discovery hole (DU-15W) in the Birch 4 �Lake area. These Lehmann leases were later incorporated into Franconia Minerals holdings. Additional drill holes were sampled and analyzed for PGEs by several other companies throughout the Duluth Complex, and as a result, significant PGEs were found at many deposits. The occurrences of PGE mineralization for each deposit will be more thoroughly discussed later in this guidebook. Enter the “Hydromet” era. Early development of the deposits was hampered both by state leasing issues, complex metallurgy that resulted in an inability at the time to make marketable separate Cu and Ni concentrates, and by general environmental concerns regarding sulfide mining and conventional pyrometallurgical processes. In the mid to late 1990s, the potential of developing the Cu-Ni deposits using hydrometallurgical techniques once again sparked renewed activity in the Duluth Complex. PolyMet plans to use the PlatSol technique, developed and patented by SGS Lakefield on NorthMet ores, to recover Cu, Ni, Co, and PGE at the NorthMet deposit (Dunka Road). Teck American is conducting tests on Mesaba ore to utilize its patented Cominco Engineering Services Laboratory (CESL) process to recover metals. Both Duluth Metals and Franconia have PlatSol licenses. The PlatSol and CESL processes are of similar concept, both utilizing an autoclave (pressure oxidation) process wherein sulfides are converted to sulfates and metals put into, and then recovered from, solution. Thus the sulfur air emissions of conventional smelting are eliminated and an inert and potentially marketable by-product of gypsum (calcium sulfate) is produced. Other residues from the processes are easily isolated for landfill disposal or other containment. REGIONAL GEOLOGIC SETTING, DULUTH COMPLEX The Duluth Complex and associated intrusions of Keweenawan age (~1.1 billion years) in northeastern Minnesota constitute one of the largest mafic intrusive complexes in the world, second only to the Bushveld Complex of South Africa (Miller et al., 2002). These rocks cover a 2,200 square mile (5,700 square km) arcuate area associated with the two strongest gravity anomalies (+50 and +70 milligals) in North America, that imply intrusive roots more than 8 miles (13 km) deep (Allen and others, 1997). The comagmatic flood basalts and intrusive rocks underlying much of northeastern Minnesota were emplaced during the development of the Mesoproterozoic Midcontinent rift, which can be traced geophysically from exposures in the Lake Superior region along a 1250 mile (2,000 km) long, segmented, arcuate path to Kansas and Lower Michigan. The Duluth Complex is defined as the more or less continuous mass of mafic to felsic plutonic rocks that extends for >170 miles (275 km) in an arcuate fashion from Duluth nearly to Grand Portage (Fig. 1-1). It is bounded by a footwall of Paleoproterozoic sedimentary rocks and Archean granite-greenstone terranes (Peterson and Severson, 2002), and a hanging wall largely of comagmatic, rift related flood basalts and hypabyssal intrusions of the Beaver Bay Complex (Fig. 1-1). In genetic terms, the Duluth Complex is composed of multiple discrete intrusions of mafic to felsic tholeiitic magmas that were episodically emplaced into the base of a comagmatic volcanic edifice between 1108 and 1098 Ma. The geology of the Duluth Complex and adjacent areas has recently been described in two major publications by the Minnesota Geological Survey (MGS). These include a 1:200,000 scale regional bedrock geological map of northeastern Minnesota (Miller et al., 2001), and a comprehensive written description of the geology depicted on this map (Miller et al., 2002), commonly referred to as the “bible” by geologists working on Duluth Complex geology. Readers’ interested in more detailed descriptions of the geologic setting of the Duluth Complex should begin their quest for knowledge by downloading these publications from the MGS website (ftp://mgssun6.mngs.umn.edu/pub2/). Within the nearly continuous mass of intrusive igneous rock forming the Duluth Complex, four general rock series are distinguished on the basis of age, dominant lithology, internal structure, and structural position within the complex. 5 �Felsic series—Massive granophyric granite and smaller amounts of intermediate rock that occur as a semicontinuous mass of intrusions strung along the eastern and central roof zone of the complex, emplaced during an early stage magmatism (~1108 Ma). Early gabbro series—Layered sequences of dominantly gabbroic cumulates that occur along the northeastern contact of the Duluth Complex, emplaced during early stage magmatism (~1108 Ma). Anorthositic series—A structurally complex suite of foliated, but rarely layered, plagioclase-rich gabbroic cumulates emplaced throughout the complex during main stage magmatism (~1099 Ma). Layered series—A suite of stratiform troctolitic intrusions that comprises at least 11 variably differentiated mafic layered intrusions that occur mostly along the base of the Duluth Complex. These intrusions were emplaced shortly after the Anorthositic series (~1099 Ma). Figure 1-1. Generalized geologic map of northeastern Minnesota. (modified from Miller et al., 2002). LOCAL GEOLOGIC SETTING-PARTRIDGE RIVER, SOUTH KAWISHIWI, AND BATHTUB INTRUSIONS The four deposits under review for this trip are located in three of the oldest intrusions in the Complex. NorthMet and parts of the Mesaba deposit in the Partridge River intrusion, Parts of Mesaba in the newly defined Bathtub intrusion, and Birch Lake and Nokomis in the South Kawishiwi intrusion (Fig. 1-2). 6 �Figure 1-2. Location of Cu-Ni±PGE sulfide deposits, Fe-Ti±V oxide deposits (Oxide-bearing Ultramafic Intrusion - OUI), and other exploration areas along the western edge/base of the Duluth Complex. Note that the NorthMet deposit was referred to as the Dunka Road deposit and the Mesaba deposit was referred to as the Babbitt deposit; the most recent names for these two deposits are used in this guidebook. 7 �Partridge River intrusion The Partridge River intrusion (PRI) consists mainly of troctolitic cumulates, dips gently to the southeast, and is exposed in an arc-shaped area that extends from the Water Hen deposit, on the southwest, to the southern edge of the Mesaba/Babbitt deposit, on the northeast (Fig. 1-3). Footwall rocks include the Paleoproterozoic Virginia Formation and locally the Biwabik Iron Formation. The basal 3000 ft. (900 meters) are known in great detail from studies of abundant drill core (Severson and Hauck, 1990; Severson, 1988) and are subdivided into seven or more units that can be traced over a strike-length of 15 miles (24 kilometers). The units of the Partridge River intrusion (PRI) are recently described in Miller and Severson (2002) and are depicted in Figure 1-3. At the base of the PRI is Unit I which consists of a suite of heterogeneoustextured troctolitic rocks that contain the vast majority of disseminated sulfide-mineralized zones. The top of Unit I is characterized by a fairly persistent ultramafic horizon, which in actuality is at the base of Unit II. Within Unit I are several laterally-discontinuous ultramafic horizons and abundant footwall sedimentary inclusions of the Virginia Formation. Noritic rocks are common at the basal contact and adjacent to the inclusions. Unit II consists of more homogenous-textured troctolitic rocks with minor sulfide-bearing zones. However, at the Wetlegs deposit, both Units I and II contain abundant laterallydiscontinuous ultramafic horizons, interbedded with troctolitic rocks that are collectively referred to as the Wetlegs Layered Interval (Fig. 1-3). Unit III is a major marker bed throughout much of the PRI (Wetlegs to Mesaba deposits - Fig. 1-3) in that it is characterized by a poikilitic leucotroctolite with olivine oikocyrsts that are randomly dispersed throughout the rock giving it a mottled appearance. This mottled-appearance, and the relatively finegrained nature of Unit III, give it a distinct appearance in drill core and it is easily identified. Unit III pinches out to the west of the Wetlegs deposit and is present on only the southern fringe of the Mesaba deposit. The rapid pinch-out of Unit III to the north within the Mesaba deposit appears to be related to emplacement of a distinctly different sub-intrusion herein referred to as the Bathtub intrusion (see discussion below). Figure 1-3. Generalized stratigraphy of the basal zone of the Partridge River intrusion (modified from Severson, 1994). Roman numerals (I through VIII) denote igneous units in the Partridge River intrusion; BT1 and BT4 denote igneous units in the Bathtub intrusion; and OUI denotes Oxide-bearing Ultramafic Intrusions. 8 �Overlying Unit III in the PRI are units IV through VIII. Unit IV varies from a troctolite to augite troctolite, often contains an ultramafic base, and commonly grades upward into Unit V which is coarsergrained and varies from a troctolite to troctolitic anorthosite. Units VI and VII, and additional units above VII, are generally homogenous-textured troctolitic to anorthositic troctolitic rocks; each with a persistent ultramafic base that record magma injection events. Bathtub intrusion The Bathtub intrusion (BTI) is wholly contained in the central portion of the Mesaba (Babbitt) deposit. It has recently been singled out as a separate intrusion to explain the abrupt change from typical Partridge River intrusion stratigraphy in the southern part of the deposit to a completely different stratigraphy, to the north, in the remainder of the deposit. There are three structural features that are pertinent to understanding the intrusive history of the BTI that include (Fig. 1-4): 1) an east-west trending paired syncline and anticline in the footwall rocks referred to as the Bathtub Syncline and Local Boy Anticline; 2) a zone that is closely associated with the Local Boy Anticline, referred to as “The Hidden Rise,” that separates the PRI and BTI; and 3) a north-trending zone fault zone, referred to as the Grano Fault, that has been postulated to have been the feeder zone for the BTI and footwall-injected massive sulfides of the Local Boy ore zone. The “Hidden Rise” is a loosely-defined zone wherein scattered hornfels inclusions, and associated noritic rocks, are fairly common. When viewed collectively, the inclusions in “The Hidden Rise” define an eastwest trending “ridge” that is roughly positioned at the contact between the PRI and BTI. Thus, “The Hidden Rise” is used to both define this hornfels-bearing “ridge” and to artistically, and conveniently, divide the BTI from the PRI. The morphology of this feature suggests that it may have originally served as the floor and/or north edge of an earlier intruded PRI and later served as a wall along the south edge of the BTI as it was emplaced. The BTI has been subdivided into two main units, BT1 and BT4, each of which contain several internal subunits (Fig. 1-4). In the vicinity of the Bathtub Syncline, ultramafic layers and modally-bedded rocks are extremely common within the BT4 Unit and have been collectively referred to as the Bathtub Layered Interval (BTLI). Figure 1-4. Schematic “type-section” cross-section, looking east, through the Mesaba deposit that crudely displays the spatial distribution of most of the igneous units in the Bathtub intrusion and pertinent structural features. Note that not all of the PRI units are shown on the right side of the figure. Cu-rich massive sulfides are locally present at the Mesaba deposit in a small zone referred to as the Local Boy ore zone. Local Boy is positioned along the crest of the Local Boy Anticline, in close to proximity to “The Hidden Rise,” and just west of the Grano Fault. Most of the massive sulfides are associated with 9 �either hornfelsed sedimentary inclusions above the basal contact or with footwall rocks below the contact while the interfingering intrusive rocks are relatively barren of massive sulfides (Severson and Barnes, 1991). This suggests that the massive sulfide ores were not formed by the gravitational settling of sulfides, but rather, the ores formed by injection of an immiscible sulfide melt into structurally prepared areas within the footwall rocks along the Local Boy anticline in a vein-like setting. A possible feeder vent for the sulfide injection event may have been the Grano Fault, which was repeatedly reactivated during emplacement of the Complex. West-directed increases in Cu and PGE, associated with the massive sulfides at Local Boy, suggest that the immiscible sulfide melt fractionally crystallized and became progressively enriched in Cu and PGE as it was deposited in an east-to-west direction. Partridge River and Bathtub intrusion footwall rocks Because the footwall at NorthMet and Mesaba is so similar, following is a generic description appropriate to both deposits. The drilled footwall rock types at Mesaba and NorthMet consist mainly of the Virginia Formation and Biwabik Iron Formation. Both are Paleoproterozoic in age (approximately 1.9-1.8 Ga) and are the two upper units of the Animikie Group . Any discussion on these two formations must include a description of their type-section on the Mesabi Range, as well as, a description of them as related to the metamorphism and partial melting that was produced during emplacement of the Complex. Lying beneath the Biwabik Iron Formation, but encountered only in a few drill holes are the Paleoproterozoic Pokegama quartzite (also of the Animike Group), along with granites and gneisses of the Archean Giant’s Range Batholith. Biwabik Iron Formation The Biwabik Iron Formation (BIF) exposed on the nearby Mesabi Range has typically been subdivided into four informal lithostratigraphic members (Wolff, 1917) that are, from the bottom up: Lower Cherty, Lower Slaty, Upper Cherty, and Upper Slaty. Diamond drill holes at Mesaba and NorthMet generally pierce the top submembers of the Upper Slaty, and end in submember C or D: Submember A is comprised of a pale to white chert and marble, submember B is characterized by alternating bands of green diopside and chert, and submember C is a green to gray thin-bedded rock consisting of chertfayalite-ferrohyperstene with wispy black bands of magnetite (taconite). The upper contact of the BIF is gradational with the overlying Virginia Formation. Virginia Formation Below the PRI and BTI The Virginia Formation is a thick sequence of argillite, siltstone, and graywacke at the top of the Animikie Group. On the basis of lithotypes present in five drill holes, Lucente and Morey (1983) divided the Virginia Formation into two informal members – a lower argillaceous lithosome and an upper silty and sandy lithosome. The lower lithosome is approximately 600 feet (180 meters) thick and contains common intervals wherein black, thin-bedded, carbonaceous argillite is the dominant rock type (Lucente and Morey, 1983); visible sulfides are locally present. The lower lithesome is the unit which underlies the Mesaba deposit, and the base of the Virginia Formation is what is most commonly intersected below the Bathtub intrusive, prior to reaching the Biwabik Iron Formation. In close proximity to the Duluth Complex, the Virginia Formation is described as being a hornfels that, as defined by Turner (1968), is a nonfoliated rock composed of a mosaic of equidimensional grains. In reality, many of the “hornfels” textures exhibited by the metamorphosed Virginia Formation do not meet this criterion, but as the term has been widely used in descriptions of the Virginia Formation in the vicinity of the Complex, it is retained in this guidebook. Mineral assemblages in the hornfels, in both the footwall and in inclusions within the Complex, consist of varying mixtures of cordierite, quartz, K-spar, and biotite with lesser amounts of chlorite, muscovite, plagioclase, orthopyroxene, and minor graphite and sulfides. 10 �The effects of partial melting are profound and portions of the hornfelsed Virginia Formation no longer even remotely resemble a sedimentary rock. Severson et al. (1994a) subdivided the hornfelsed Virginia Formation, in both the footwall and in inclusions within the Duluth Complex, into at least five informal units based largely on metamorphic attributes, which are each related to varying degrees of partial melting. These members, and a pre-Duluth Complex sill, are described below and are schematically portrayed in Figure 1-5 - although in real occurrence, this idealized metamorphic progression is more erratic, often with rapid lateral and vertical changes between the four metamorphic units discussed below. Figure 1-5. Schematic cross-section showing the general relationships of the metamorphosed footwall rocks beneath the Duluth Complex at the Mesaba, NorthMet, Wetlegs, and Serpentine deposits. Cordieritic hornfels Directly beneath the basal contact of the Duluth Complex, the adjacent Virginia Formation typically consists of massive/nonfoliated, cordierite-rich hornfels that display a bluish-gray color in drill core. The rock is generally fine-grained, granoblastic, and biotite-poor (due to loss of water into the Complex) and locally may contain porphyroblastic and/or poikiloblastic cordierite. Original bedding planes are preserved in some localities, but mostly the bedding planes have been obliterated by contact metamorphism. Recrystallized unit (RXTAL) Beneath the cordieritic “capping” the next metamorphic variant of the Virginia Formation nearest to the Duluth Complex is a rock that is referred to as the RXTAL unit. The RXTAL unit is properly classed as a diatexite and is characterized by fine- to medium-grained cordierite, plagioclase, biotite, quartz, and Kspar with lesser amounts of Opx and opaques. Bedding planes of the original argillaceous rocks are obliterated and what remains is a massive recrystallized rock with decussate biotite that contains enclaves (blocks and folded boudins) of more structurally competent calc-silicate hornfels and thin-bedded siltstone Disrupted unit (DISRUPT) With increased distance from the Complex, the RXTAL unit progressively grades into the DISRUPT unit which is a thin-bedded rock that is visibly deformed and underwent less degrees of partial melting. 11 �Textures that characterize the DISRUPT unit are bedding planes that are extremely chaotic and random in orientation due to pervasive small-scale folding, faulting, and brecciation. Superimposed on this chaotic pattern are abundant zones of leucocratic partial melts that are also chaotic and folded. The rock consists of varying amounts of quartz, cordierite, K-spar, biotite, plagioclase, and muscovite with leucosome veins and patches containing quartz, K-spar (microperthite), plagioclase, and muscovite (Duchesne, 2004). The DISRUPT unit is properly classed as a metatexite. Graphitic argillite and Bedded Pyrrhotite (BDD PO) units Carbonaceous argillite of the lower lithosome of the Virginia Formation is commonly preserved as either the BDD PO unit, or graphitic argillite, in close proximity to the Duluth Complex. This rock commonly contains over 5% disseminated pyrrhotite and/or extremely thin-bedded pyrrhotite laminae (hairlinethick), and variable amounts of graphite, staurolite(?) and sillimanite. Wherever the unit contains conspicuous and regularly-spaced laminae of pyrrhotite (0.5-3.0 mm thick at 1-20 mm spacings) it is informally referred to as the bedded pyrrhotite unit (BDD PO unit). VirgSill The VirgSill is generally present in the bottom 0.5-130 feet of the Virginia Formation, and as local apophyses into the top of the Biwabik Iron Formation. The VirgSill was intruded along the contact between the Virginia Formation and Biwabik Iron Formation and exhibits a granoblastic texture indicating that it was metamorphosed by the Duluth Complex (and thus the VirgSill is pre-Duluth Complex in age). On this basis, the VirgSill is inferred to be equivalent to the Logan sills (circa 1,109 Ma); as is another sill, the BIFSill, in the C submember of the Biwabik Iron Formation (Hauck et al., 1997). However, the VirgSill and BIFSill are different chemical entities (the VirgSill is much more Crenriched), and thus, these two sills may be related to at least two different intrusive events. When present, the VirgSill ranges from a few centimeters to several meters thick. Identification of the VirgSill in drill core is hampered by the fine-grained granoblastic texture that makes it difficult to distinguish from the enclosing hornfelsed Virginia Formation rocks; both were metamorphosed by the Duluth Complex. The VirgSill is subdivided into two textural varieties (Severson et al., 1994a; Park et al., 1999): 1. MG unit – fine-grained, massive, gray-colored unit (massive gray unit or MG unit) that appears to be a border phase or chill zone - albeit quite thick at some localities (up to 200 feet-thick in drill hole B1-264). In some drill holes the entire interval of the VirgSill consists of the MG unit. 2. Coarser-grained interior – medium- to coarse-grained, green- to brown-colored, olivine- and hornblende-bearing interior of the sill that is easily identified as an intrusive rock in drill core. The coarse-grained interior is not always present, and when present, may be up to 80 feet thick, and occurs as either a single lense within a thick MG unit or as several vertically-stacked lenses within the MG unit. Both the MG unit and coarser-grained interior of the VirgSill contain variable amounts of plagioclase, olivine, hornblende, clinopyroxene, orthopyroxene, and biotite with local sulfides (pyrrhotite, chalcopyrite, and bornite). The presence and metamorphic effect of this sill has caused an “armoring” wherein previously metamorphosed Virginia Formation has resisted assimilation by the Duluth Complex. Across much of the lower parts of the NorthMet and Mesaba deposits there is a persistent thin (<10ft.) rind of Virginia Formation associated with this sill. South Kawishiwi intrusion The South Kawishiwi intrusion (SKI) consists mainly of troctolitic cumulates and dips gently to the southeast. The SKI is exposed in an arc-shaped area that extends from the Serpentine deposit, on the southwest, to the Spruce Road deposit, on the northeast (Fig. 1-2). Footwall rocks include the Paleoproterozoic Virginia Formation, Biwabik Iron Formation and Archean Giants Range Batholith, the latter is the dominant footwall rock type. The presence of Biwabik Iron Formation as inclusions, from the 12 �Birch Lake deposit to as far north as the Spruce Road deposit, indicates that the majority of Paleoproterozoic units were assimilated and removed from the footwall during emplacement of the South Kawishiwi intrusion (Severson et al., 2002). The basal stratigraphic section is known in great detail from studies of abundant drill core and is subdivided into 17 different units (Fig. 1-6) that are present over a strike-length of 19 miles (31 kilometers). The lowermost units are unevenly distributed along the strike length of the intrusion in a “compartmentalized” fashion, suggesting a complicated intrusive history (Miller and Severson, 2002). A few salient features to keep in mind regarding the igneous stratigraphy of the SKI include: • The vast majority of sulfide mineralization is confined to the BH (Basal Heterogeneous Unit), BAN (Basal Augite Troctolite and Norite Unit), UW (Updip Wedge Unit), and U3 (Ultramafic 3 Unit); • Major marker beds include three horizons that contain abundant cyclic ultramafic layers (U1, U2, and U3 Units) and a pegmatite-bearing unit (PEG Unit - originally recognized by Foose, 1984). The U1, U2 and U3 Units represent periods of rapid and continuous magma replenishment that crystallized more primitive ultramafic layers before mixing with the resident magma (Severson et al., 2002); • The U3 Unit is unique in that it contains several massive oxide pods (titanomagnetite-rich), as well as, recognizable inclusions of bedded Biwabik Iron Formation. The spatial correspondence between the U3 Unit and footwall iron-formation suggests that most of the massive oxide pods are iron-rich “restite” produced by assimilation and partial melting of the iron-formation (Muhich, 1993; Severson, 1994; Severson et al., 2002); • The U3 Unit contains the vast majority of high PGE values, especially within the Birch Lake area and possibly at the Nokomis deposit. However, high PGE values are also present in the PEG Unit (Birch Lake area and Nokomis deposit), the top of the BH Unit (Maturi deposit and Nokomis deposit), and in very locally in troctolitic rocks situated well above the basal contact (South Filson Creek deposit); and • A large inclusion/pillar of anorthosite is present at the Nokomis deposit. This pillar, and possible proximity to a vent area and magma flow paths (see discussion for Nokomis deposit) are the inferred reasons for high PGE values at the Nokomis deposit. Figure 1-6. Generalized stratigraphy of the basal zone of the South Kawishiwi intrusion (modified from Severson, 1994; and included in Miller and Severson, 2002). The lowermost igneous units are: BAN = Basal Augite Troctolite and Norite; BH = Basal Heterogeneous; U3 = Ultramafic 3; PEG = Pegmatitic unit of Foose (1984); U2 = Ultramafic 2; U1 = Ultramafic 2; AT-T = Anorthositic Troctolite to Troctolite; UW = Updip Wedge; Main AGT = Main Augite Troctolite. 13 �REGIONAL ECONOMIC GEOLOGY While Minnesota is home to the United States iron ore industry, development of its known non-ferrous deposits has been hampered by industry downturns, remoteness from the rest of the base metals industry, a complex land situation, and a (wrongly) perceived environmental risk. The state has done much to support non-ferrous exploration, in particular maintaining an impressive drill core and data library in Hibbing, and sponsoring extensive mapping and sampling projects. Research arms of the MDNR and the University have contributed much knowledge to mineral processing methods for these ores. In general this work has not been well publicized. This short discussion on regional mineral potential focuses on rocks in the north part of the state, but there has been recent exploration for nickel and diamonds throughout the state. For detailed discussion of potential in Archean rocks see Peterson (2001a) and for Duluth Complex rocks see Miller et al., 2002. Mesabi Range Iron Mines-Status There are six operating iron mines employing about 3,500 people in the region. They all produce taconite pellets from low grade magnetic ore. Four are captive to steel companies, two produce pellets for market, generally through long term contracts. Product shipping is largely by rail to one of four ports on Lake Superior, then by boat to mills on the lower lakes. About thirty eight million tons of pellets were produced in 2007. Total material movement (ore and stripping) was on the order of two hundred million tons. The one-hundred plus year history of this world class mining district means that there is an extensive developed infrastructure and service industry for these mines. Two iron related projects are in development in 2009: Mesabi Nugget, located near the PolyMet plant site, will initially use purchased concentrate to produce iron nuggets suitable for electric furnace production of steel, the company has submitted plans to the state to re-open some of the LTV pits and produce their own concentrate; and Essar Steel (Minnesota Steel Industries, formerly Minnesota Iron & Steel) plans to re-open a closed taconite mine (Butler Taconite) at the western end of the Mesabi Range and produce direct reduced iron from taconite pellets on site. That direct reduced iron will in turn be used to make steel slab for shipment. This will be the only mine to steel, single-site production facility in North America. Both of these projects are under construction. Other Regional Economic Geology Two broad age groups dominate the other rocks with mineral potential in northern Minnesota. Archean rocks represent possible hosts for lode gold, Volcanogenic Massive Sulfide (VMS), and diamond prospects. North of the Duluth Complex is extensive terrane of exposed Archean rock, similar to that in Ontario (Wawa and Quetico subprovinces). Over the years various prospects for gold and base metals have been delineated, but follow up work has been sporadic and generally short lived. These prospects include, for gold: Raspberry, Murray Shear zone, Spaulding Bay, Mud Lake, Pac Man Pond, and Section 6, investigated by Goldfields, Newmont, Kerr McGee, and Noranda among others. For base metals the deposits include: Clear Lake, Skeleton Lake, Fivemile Lake, and Purvis Road worked by the above companies as well as Exxon, Teck, Rendrag, and Lehmann. A 2001 PhD. thesis (Peterson, 2001a) is an excellent review of the Archean mineral potential of the region. That report makes detailed analytical comparisons between producing gold and base metals camps in Canada and prospects in Minnesota. Diamond work in Archean age rock of Minnesota includes Exmin (DeBeers), WMC, and others; as well as the Minnesota Geologic Survey. Proterozoic rocks in northern Minnesota include Animikian Basin (Paleoproterozoic, ~1.8 Ga) sedimentary rocks (Pokegama Quartzite, Biwabik Iron-Formation, and the Virginia Formation). Besides current iron ore production, past exploration has focused mostly on zinc and other base metals in the Virginia. Anomalous mineralization (sphalerite, molybdenite) has been found in the Virginia, but no prospects have been defined. 14 �Titanium and Other Oxide Mineral Potential in the Duluth Complex There are at least four titanium deposits within 12 miles (19 kilometers) of PolyMet’s Hoyt Lakes plant site (out of 13 known titanium deposits in the area). They are all located in the Duluth Complex and locally called “OUI’s” for “Oxide Ultramafic Intrusions”. These are titanium rich plugs which cross-cut the rocks of the Complex. All are greatly undersampled, especially for PGE and other oxides besides titanium (chromium, vanadium, etc.) Bulk samples have been processed from two of these titanium prospects. The titanium in these deposits is in magnesium-rich ilmenite which is not easily processed by current commercial methods. BHP, Coleraine Minerals Research Laboratory of the NRRI, and others have done extensive process testing towards adding value to these prospects. All 13 have been drilled, but generally not to a point where a legitimate (i.e., NI43-101 compliant) resource can be declared. Two OUI’s with reasonable historical resource estimates are Longnose, with 50 million tons averaging 21% TiO2 based on 11 drill holes and Water Hen, with 62 million tons averaging 14% TiO2, based on 37 drill holes. At present, Cardero Resources has leased Longnose, as well as the Section 34 deposit about halfway between Duluth and the Iron Range. This group of deposits represents great potential for undiscovered copper-nickel-PGE, silver, as well as oxide rich “plug-like” intrusions with known titanium resources and possibilities for chromium, vanadium, etc. It is obviously unknown what is undiscovered, but historic exploration on the Duluth Complex and associated rocks has focused almost exclusively on the large copper-nickel deposits along the northwestern contact with virtually no work done on the interior of the Complex. There is a large collection of core available for all of these projects, and in general a good collection of related data stored at the Minnesota Department of Natural Resources in Hibbing. NRRI has logged most of this core, and consolidated whatever assay data is available (Patelke, 2003). 15 �PART 2: POLYMET NORTHMET DEPOSIT By: Richard Patelke, Steve Geerts, Mark Severson NORTHMET PROJECT SUMMARY NorthMet, located in the Partridge River intrusion of the Duluth Complex, is a large, disseminated sulfide deposit in heterogeneous troctolitic rocks associated with the 1,100 million year old Mid-Continent rift. Metals of interest are copper, nickel, cobalt, platinum, palladium, and gold. The majority of the metals are concentrated in four sulfide minerals: chalcopyrite, cubanite, pentlandite, and pyrrhotite, with platinum, palladium and gold also found in bismuthides, tellurides, and alloys. NorthMet is one of eleven coppernickel-PGE deposits along the northern margin of the Complex (PGE: platinum, palladium, gold). All of these share grossly similar geologic settings–disseminated sulfides with minor local massive sulfides in heterogeneous rocks forming the basal unit of the Duluth Complex along the contact with older rocks. The deposit is on the southern flank of the Mesabi Iron Range, which is host to six large operating taconite mines, the closest of which is less than two miles (3.2 km) north of the planned NorthMet pits (Figure 1-7). Ore from NorthMet will be processed at a rate of 32,000 short tons per day through the former LTV Steel Mining Company iron ore concentration plant (“Erie Plant) with new facilities for processing of the NorthMet copper-nickel-PGE concentrates through a hydrometallurgical method day to produce copper metal and various hydroxide and concentrate products of nickel-cobalt-PGE (Figure 1-8). EXPLORATION and DEVELOPMENT There have been four major drilling programs since 1969, re-sampling for PGE began in 1989, three PolyMet joint ventures were pursued and dissolved in the 1990's, processing technology was developed in the late 1990's, the former LTV Steel Mining Company concentrator and other property was optioned in 2003, and the metallurgical process was refined in 2005-2008. Drilling programs have been conducted by United States Steel (USS, 1969-1974) and PolyMet Mining Inc. (Reverse Circulation or “RC” drilling and core drilling in 1998-2000 & two phases of core drilling in 2005 and 2007), plus two (actually two pairs of twins) holes by NERCO Minerals Company in 1991. This drilling encompasses 285,756 feet over 371 holes as of May 2008. Over 35,973 acceptable assays have been taken from this drilling (216,344 feet assayed). Table 1-1 gives a breakdown of years, footages, and number of assays for all project drilling. United States Steel (USS) began core drilling at NorthMet (as the Dunka Road project) in 1969. Drilling targeted a conductor that turned out to be in the footwall metasedimentary rocks, but the first drill hole hit massive sulfide in the Duluth Complex. Drilling continued over five years for 112 holes with 133,716 feet of intercept. The working assumption was to mine the deposit from underground, sampling was limited to the most continuous zones with strong visible copper-nickel mineralization, and only about 2,200 samples representing about 22,000 feet were taken. USS assayed only for copper, nickel, sulfur, and iron. PGE presence was known from sampling on concentrates, but the economics of PGE recovery were apparently not pursued. Project work stopped while apparently incomplete and was not restarted. USS did not do much follow-up, but kept their land ownership, core, pulps, coarse rejects, and records for the project. In the mid 1980's the Minnesota Department of Natural Resources (MDNR) began sampling various historic drill core intervals in the Duluth Complex for PGE and got some good, but localized, results. In 1989 Fleck Resources (Fleck) leased the Dunka Road property from USS and began a program of re-assaying USS pulps and coarse rejects with a much more extensive multi-element suite, as well as 16 �adding in some new samples from existing core through cooperative work with the Natural Resources Research Institute (NRRI), associated with the University of Minnesota Duluth. The results were very positive in showing elevated PGE values in the deposit and confirming the previous copper-nickel assays. Fleck partnered with NERCO in 1991 for some bulk sample work, mine plans, environmental reviews etc., done through Fluor Daniel Wright engineers, but the partnership was eventually dissolved. In 1995 Fleck joined with Argosy Mining Corp. to do more work on the project, again with no major progress towards production. In June 1998, Fleck became PolyMet and focused their resources on Dunka Road, which was renamed NorthMet. Without partners, except for a brief venture with North Mining (North), PolyMet drilled and sampled 87 holes in 1998-2001, and sent two large bulk metallurgical samples to Lakefield Laboratories (now SGS) in Lakefield, Ontario for development and refinement of the PlatSol hydrometallurgical process and began some environmental background work. In the summer of 2000, North was taken over by Rio Tinto. The joint venture agreement was terminated by PolyMet upon consideration that NorthMet appeared to be a low priority to Rio Tinto. The main concern was that other partnership opportunities might be missed during the time that Rio Tinto assessed and prioritized the ongoing North projects. However, much of the North funding was already in place and was used to partially finance the 2001 pre-feasibility study. After release of the pre-feasibility study (2001), a brief hiatus, and a major re-evaluation of how the project should proceed, PolyMet became active again in 2003 with new management and a new development plan. This plan involves integrating the former LTV Steel Mining Company iron ore concentration plant (“Erie Plant) with new facilities for processing of the NorthMet copper-nickel-PGE concentrates through a hydrometallurgical method at rate of 32,000 short tons of ore per day to produce copper metal and various hydroxide and concentrate products of nickel-cobalt-PGE. Geologic work towards this end began in 2004 and first focused on a careful and total re-compilation of the historic NorthMet project drill hole related data. This effort organized and verified all drilling metadata, location, downhole survey, lithology, and assay data, and cataloged all paper (and digital) records for the project. Of note is that this resulted in an increase in the number of acceptable assays from 12,000 to around 17,200 and an improved geologic picture from careful consolidation of existing records. This work was used as background for a revised resource estimate in January 2005 and planning of a drill program for 2005. The 2005 program entailed drilling and sampling 109 holes (77,000 feet), collection of a forty ton metallurgical bulk sample for pilot scale testwork, geotechnical (oriented core) drilling, in-fill sampling of previously drilled core, and extensive collection of waste characterization data. The 2005 drilling program added 13,450 multi-element assay records to the existing database. A PolyMet report covers the details of historic drilling and assaying (Patelke & Geerts, 2006). 17 �Figure 1-7. PolyMet NorthMet project site. 18 �Figure 1-8. Detail of Erie Plant site showing existing facility and new construction. 19 �Drilling in 2007 for 24,530 feet with 3,546 assays concentrated on defining mineralization in the upper units in the west part of the deposit (the “Magenta Zone”). This drilling and the subsequent re-modeling of the deposit turned about 50 million tons of material previously classed as waste to ore. There is also over 34,000 feet of hydrogeology drilling and “stratigraphic holes” (drilling by other companies not done as part of the NorthMet project). No assays are in use from these 44 holes which are used for geologic control. Approximately 89.5% of Unit 1 and about 57% of the upper units have been sampled across the deposit. The sampled percentages are higher in the anticipated area of mining. Table 1-1. Total drilling and assaying for NorthMet project Company Drilling years Assaying years No. of drill holes Total footage for group No. of assay intervals used in “accepted values” tables Assayed footage used in final database Assay Laboratories US Steel 19691974 1969-1974, 1989-1991, 1999-2001, 2005-2006, 2008 112 133,716 11,259 73,303 USS, ACME, ALS-Chemex NERCO 1991 1991 2 (4) 842 165 822 ACME PolyMet reverse circulation drilling 19982000 1998-2000 52 24,650 4,765 23,767 ACME PolyMet core drilling 19992000 2000-2001, few in 32 22,156 4,058 20,727 ALS-Chemex PolyMet RC drilling deepened with AQ core tail 2000 2000 3 2,696 524 2,610 ALS-Chemex PolyMet core drilling 2005 2005-2006 109 77,166 11,656 71,896 ALS-Chemex PolyMet core drilling 2007 2007 61 24,530 3,456 23,310 ALS-Chemex Totals for Exploration Drilling: 371 285,756 35,973 216,344 1970's? none 6 9,647 none none INCO 1956 none 3 2,015 none none Humble Oil / Exxon 19681969 none 3 9,912 none none Bear Creek / AMAX 19671977 none 11 8,893 none none PolyMet / Barr Engineering (hydrologic testing) 20052007 none 21+ 3,459+ none none US Steel stratigraphic holes 20 �Table 1-2. Large metallurgical samples collected at NorthMet Bulk Sample Year Tons Location of sample USS Bulk sample pit No. 1 1971 Unknown, but small Pit in center of property USS Bulk sample pit No. 2 1971 300 Pit at east end of property USS Bulk sample pit No. 3 1971 20 Pit at east end of property NERCO PQ drill core 1991 Estimated at 4.5 tons or less by drill core size One PQ drill hole from each end of property Argosy Mining 1995 Unknown, but small Composited from USS coarse rejects PolyMet RC drill cuttings 1998 26 One composite, mostly from what is now considered east part of 10 year pits PolyMet RC drill cuttings 2000 33 One composite, mostly from what is now considered east part of 10 year pits PolyMet 4 inch and PQ core and coarse reject 2005 10.5, 21.5, and 10.7 Three composites from within ten year pits across property PolyMet coarse reject 2006 4.2 and 4.94 One composite from 10 year east pit, one from 20 year pit across property PolyMet ¼ core from 2005 and 2007 Drilling 2007 500 kg One composite, from east and west pit areas PolyMet ¼ core from 2005 and 2007 Drilling 2008 4.44 One composite, from east and west pit areas PolyMet ¼ core from 2005 and 2007 Drilling 2008 4.48 One composite, from east and west pit areas Sampling in Unit 1 (the main mineralized zone) is now mostly continuous through the zone for all generations of drilling. The PolyMet RC and core holes have continuous sample through the upper waste zones (which do have some intercepts of economic mineralization). Work in 2005 through 2008 essentially completed the sampling of historic USS core within the area likely to be mined. This broad sampling limits the possibility of location bias in the sample set. While not all of the USS core has been sampled, there is no known unsampled mineralized core. There have been numerous bulk samples taken at NorthMet (Table 1-2). Samples have been representative by Unit and rock type. Agreement between calculated grades (based on core sampling) and analyzed grades of final sample has been excellent. Earlier bulk samples represented the first ten years of production, more recent samples used material from across the deposit. Each bulk sample has built upon the previous, and work has progressed to the point where PolyMet has confirmed the ability to make separate, saleable, copper and nickel concentrates. This will allow the company to develop cash flow from sales much earlier in production while completing construction of the hydrometallurgical facility. The planned hydrometallurgical process (PlatSol) was developed on NorthMet ores. The process uses pressure oxidation (225°C, over 30 atmospheres) in the presence of chloride to capture all base and precious metals in the concentrate. Hydromet process recoveries are all over 98%. Other geologic data collected includes: recovery and RQD measurements on all core, over 7,000 specific gravity measurements, over 900 whole rock analyses, over 300 Rare Earth Element packages and a large amount of microprobe data collected for waste characterization purposes. 21 �GEOLOGY OF THE NORTHMET DEPOSIT NorthMet consists of seven igneous units that dip southeast, with most economic sulfide mineralization in the top parts of the lowermost unit (Unit 1). The following is a summarized description of the geology of the deposit, based on observations from drill core and limited outcrop mapping. Quaternary Geology In general the Quaternary geology of the region is a thin (0-30 feet or 0-10 meters., but locally thicker) blanket of glacial deposits including till, lacustrine materials, and outwash. Low spots are usually peat bog or open wetland. Topography is subdued and drainage is poor. Site specific geologic studies of the drift have not been done, though a series of geophysical soundings were carried out in 2006 to better define drift thickness outside the area to be mined (Ikola, 2006). Lehr and Hobbs (1992) mapped the area as part of the Wampus Lake Moraine. Minnesota Geologic Survey map 164 (Jennings and Reynolds, 2005, includes GIS database) categorizes all drift materials as Rainy Lobe till and re-sedimented glacial deposits, overlain locally by post glacial peat. Test pits for preliminary PolyMet engineering studies and informal observations of sumps and other small excavations bear this out. Most areas consist of unsorted sand / silt / clay with cobbles and boulders. Boulders on surface can be greater than 10 feet in size and there may be a boulder lag horizon just below the ground surface in some areas. As measured from drill holes, thickness of the drift ranges from 0 to 50 feet (mostly less than 20 feet) and averages about 12 feet. The 2006 geophysical soundings measured thicknesses up to 60 feet past the western margins of the drilled area. Structural Geology The general structure of the NorthMet deposit, as defined by igneous contact dips, foliation in serpentinized zones, bedding trends in the Biwabik Iron Formation (BIF) and in the Virginia Formation, is dominated by an overall dip ranging from 15-25° to the southeast, striking about N56°E. Dips in the seven igneous units are grossly similar, but dips of the mineralized zone are up to 60° in the east pit area. Dips in both the Animikian and the Duluth Complex rocks can be attributed to crustal loading, associated with the input of large volumes of magma originating from the Mid-continent Rift System (Sims and Morey, 1972). Numerous faults have been proposed across the NorthMet Deposit, based largely on reconciling dips in the footwall rocks. Unfortunately, not enough evidence has been established through drilling to indicate with certainty the exact location of major offsets or faulting within the igneous rock units or the footwall rocks on a hole-to-hole basis. This definition difficulty is compounded by the fact that over time the fault representations have been extended vertically from ground surface to footwall, though many were originally thought to only show offset in the footwall, or were based solely on limited outcrop evidence. Clearly however, offset or faulting exists, at least within the footwall rocks, due to substantial offsets in the BIF (assuming an average 20° dip) as evidenced between drill holes portrayed in cross-sections. Many of these same offsets can be correlated in adjacent cross-sections. Fault zones are apparent in drill core and show up as brecciated intervals (up to several feet thick), including gouge mineralization (clay, calcite, quartz, etc.), slickensides on serpentinized fracture faces, and/or severely broken (rubble) core. However, the exact location of all faults/offsets at the NorthMet Deposit on a hole to hole basis has only been approximated, due to the sparse structural information as so far provided by drilling. Extensive angle drilling in 2005 and 2007 (142 of 170 holes) brought no great clarity to this issue (virtually all previous drilling was vertical). The current geological model and working cross-sections are therefore constructed with minimal faulting influence, especially within the igneous rock units of the Partridge River intrusion, until more evidence clarifies this issue. 22 �Logging and Mapping Units A summary of the general stratigraphy of the NorthMet Deposit is outlined below. Rock units and formations are listed in descending order, as would be observed from top to bottom in drill hole. NorthMet units are labeled as Units 1 through 7 (Units I through VII in Severson’s terminology), bottom to top. Unit 3 is probably the oldest, the intrusion sequence of the other units is not clear. The broad picture is of a regular stratigraphy of troctolitic to anorthositic rock units, dipping southeast at 20° to 25°, with basal ultramafic units defining the boundaries of some of these units. The basal ultramafic zones tend to have diffuse tops, sharp bases, and are commonly serpentinized and foliated. Geologists have generally picked the unit boundaries at the base of these ultramafics though there are local exceptions. Economic sulfide mineralization is ubiquitous in the basal igneous unit (Unit 1) and is locally present, but restricted, in the upper units. Rock Type and Unit Classification Igneous rock types in the Complex are classified at NorthMet by visually estimating the modal percentages of plagioclase, olivine, and pyroxene, using a rock classification scheme (Figure 1-9) modified from Phinney (1972). Due to subtle changes in the percentages of these minerals, a variation in the defined rock types within the rock units may be present from interval to interval or hole to hole. This is especially true for Unit 1. Figure 1-9. Modified Phinney (1972) diagram for rock type classification Unit definitions are based on: overall texture of a rocktype package; mineralogy; sulfide content; and context with respect to bounding surfaces (i.e., ultramafic horizons, oxide-rich horizons). Unit definitions are not always immediately clear in logging, but usually clarified when drill holes are plotted on crosssections. In other words, to correctly identify a particular stratigraphic unit, the context of the units directly above and below should also be considered. Based on drill hole logging, the generalized rock type distribution at NorthMet is about 83% troctolitic, 6% anorthositic, 4% ultramafic, 4% sedimentary inclusions, 2% noritic and gabbroic rocks, and minor pegmatite, breccia, basalt inclusions, and others. Unit Definitions and Descriptions Descriptions of the general igneous Stratigraphy for the NorthMet deposit is described below and presented in a stratigraphic column in Figure 1-9. 23 �Figure 1-9. Generalized stratigraphic column for NorthMet units (modified after Geerts, 1994) Unit 7 Unit 7 (Figures 1-9, 1-10, and 1-11) is the uppermost unit intersected in drill holes at the NorthMet Deposit. It consists predominantly of homogeneous, coarse-grained anorthositic troctolite and troctolitic anorthosite, characterized by a continuous basal ultramafic subunit that averages 20 ft. thick. The ultramafic consists of fine- to medium-grained melatroctolite to peridotite and minor dunite. The average thickness of Unit 7 is unknown due to erosion removing the upper parts. Unit 7 is generally not mineralized. Unit 6 Very similar to Unit 7, Unit 6 is composed of homogeneous, fine- to coarse-grained, troctolitic anorthosite to troctolite. It averages 400 ft. thick and has a continuous basal ultramafic subunit that averages 15 ft. thick. Overall, sulfide mineralization is minimal, although a number of drill holes in the southwestern portion of the NorthMet Deposit contain significant sulfides and associated elevated PGEs 24 �(Geerts 1991, 1994). Sulfides within Unit 6 generally occur as disseminated chalcopyrite/cubanite with minimal pyrrhotite. This mineralized occurrence, the “Magenta Zone”, transitions into Units 3, 4, and 5, and is discussed in greater detail below. Unit 5 Unit 5 exhibits an average thickness of 250 ft. and is composed primarily of homogeneous, equigranulartextured, coarse-grained anorthositic troctolite. Anorthositic troctolite is the predominant rock type, but can locally grade into troctolite and augite troctolite towards the base of the unit. The lower contact of Unit 5 is gradational and lacks any ultramafic subunit, therefore the transition into Unit 4 is a somewhat arbitrary pick. Due to the ambiguity of this contact, thicknesses of both units vary dramatically. However, when Units 5 and 4 are combined, the thickness is fairly consistent deposit-wide. Aside from Magenta Zone mineralization in the west, Unit 5 is not mineralized. Unit 4 Being somewhat more mafic than Unit 5, Unit 4 is characterized by homogeneous, coarse-grained, ophitic augite troctolite with some anorthosite troctolitic. Unit 4 averages about 250 ft. thick. At its base, Unit 4 may contain a local thin (usually no more than 6 inch) ultramafic layer or oxide-rich zone. The lower contact with Unit 3 is generally sharp. Unit 4 is rarely mineralized outside the Magenta Zone. Unit 3 Unit 3 is used as the major “marker bed” in determining stratigraphic position in the PRI. It is composed of fine- to medium-grained, poikilitic and/or ophitic, troctolitic anorthosite to anorthositic troctolite. Characteristic poikilitic olivine gives the rock an overall mottled appearance. On average Unit 3 is 300 ft. thick. As with Units 4 and 5, the thickness of Units 2 and 3 tend to be highly variable, whereas if combined into one unit, it is more consistent deposit-wide (though not as consistent as Units 4 & 5). Unit 2 Unit 2 is characterized by homogeneous, medium- to coarse-grained troctolite and augite troctolite with a consistent basal ultramafic subunit. The continuity of the basal ultramafic subunit, in addition to the relatively uniform grain size and homogeneity of the troctolite, makes this unit distinguishable from Units 1 and 3. Unit 2 has an average thickness of 100 ft. The ultramafic subunit at the base of Unit 2 is the lowermost continuous basal ultramafic horizon at the NorthMet Deposit, averages 25 ft. thick, and is composed of melatroctolite to peridotite and minor dunite. In some ways the characteristics of Unit 2 and how it fits into the stratigraphy are ambiguous. It can be interpreted as the lower part of Unit 3, the upper part of Unit 1, or a separate unit. Based on continuity of the ultramafic boundary it seems to be a lower, more mafic, counterpart to Unit 3 or a separate unit. However, even though Unit 2 has been historically described as barren, in the western part of the deposit it appears to have mineralization grossly continuous with that at the top of Unit 1. The general lack of footwall inclusions would argue against Unit 2 being older than Unit 1. Unit 1 Of the seven igneous rock units represented within the NorthMet Deposit, Unit 1 is the only unit that contains significant deposit-wide sulfide mineralization. Sulfides occur primarily as disseminated interstitial grains between a dominant silicate framework and are chalcopyrite > pyrrhotite > cubanite >pentlandite. Unit 1 is also the most complex unit, with internal ultramafic subunits, increasing and decreasing quantities of mineralization, complex textural relations and varying grain sizes, and abundant sedimentary inclusions. It averages 450 ft. thick, but is locally 1,000 feet thick and is characterized lithologically by fine- to coarse-grained heterogeneous rock ranging from anorthositic troctolite (more abundant in the upper half of Unit 1) to augite troctolite with lesser amounts of gabbro-norite and norite (becoming increasingly more abundant towards the basal contact) and numerous sedimentary inclusions. 25 �By far the dominant rock type in Unit 1 is medium-grained ophitic augite troctolite, but the textures can vary wildly. Two internal ultramafic subunits occur in drill holes in the southwest, and have an average thickness of 10 ft. Footwall rocks are covered in the Partridge River intrusion description. Inclusions Two broad populations of inclusions occur at NorthMet: hanging wall metabasalts (Keweenawan) and footwall metasedimentary rocks. Basalts are fine-grained, generally gabbroic, with no apparent relation to any mineralization. Footwall inclusions may carry substantial sulfide (pyrrhotite) and often appear to contribute to the local sulfur content. Footwall inclusions are all Virginia Formation, no iron-formation, Pokegama Quartzite, or older granitic rock has been recognized as an inclusion at NorthMet. Sedimentary inclusions make up about 4% of the logged rocktypes, and basalt inclusions sum to less than 1% of the drilling footage. Inclusions and Timing Generally, hanging wall inclusions are restricted to Unit 3 and the units above, while footwall inclusions are most abundant in Unit 1. This zoned distribution of inclusions indicates that one possible scenario for order of intrusion is that Unit 3 intruded first, created space between the basalt and the Virginia Formation, then portions of the hanging wall basalts collapsed into the Unit 3, but for some reason Unit 3 was not able to dissagregate or assimilate much of the footwall rock (due to temperature, viscosity of magma or ductility of the footwall). Unit 1 however, intruded between Unit 3 and the footwall and was able to assimilate large portions of the footwall and thus contaminate itself with both sulfur and silica. In this scenario Unit 2 is intruded after Unit 1, between Units 1 and 3, as Unit 2 has limited footwall inclusions. Unit 3's intrusion would have separated the footwall and Unit 1 from later Units 4 through 7, which never reacted with the footwall at the NorthMet site. Therefore, any footwall inclusions seen in Units 4 through 7 (and probably those seen in Unit 2) can be interpreted as being carried in from some other part of the magmatic system. Note that basalt overlies and is in direct contact with the Virginia Formation at the Wetlegs deposit to the west of NorthMet, implying that the starting conditions for this chain of events are plausible. Other Igneous Units Quadrangle scale outcrop mapping indicates that other igneous stratigraphic units are present above Unit 7. These units are similar to Units 6 and 7 in that they consist of homogeneous-textured troctolitic rocks with basal ultramafic members. There are minor, unmineralized, pre-Complex sills in both the Virginia Formation and Biwabik Iron Formation at NorthMet (VirgSill and BIF Sill in footwall descriptions above). In neither case is there any apparent relation to Duluth Complex mineralization. Early sills in the Virginia probably metamorphosed the Virginia, forming a zone that resisted assimilation during later intrusion of the Complex–hence leading to the thin “rind” of metamorphosed Virginia on top of the BIF seen in the deeper downdip drill holes at NorthMet. Alteration The vast majority of rock within the NorthMet Deposit would be considered fresh and is unaltered or only weakly altered. Types of alteration most commonly observed in NorthMet rocks are serpentinization / chloritization of olivine, sericitization and saussuritization of plagioclase, and uralitization of pyroxenes. Most alteration is related to close proximity of fractures and/or joints that cross-cut the troctolitic rocks. Likewise, on a microscopic level the center of alteration is focused around microfractures. This pattern suggests that both fracturing and accompanying alteration of the rock occur as a result of the migration of late-stage deuteric fluids during the cooling phase. The vast majority of sulfide mineralization is 26 �independent of alteration. Nickel in Silicates (Lab Assay Nickel vs. Recoverable Nickel) It has been characteristic of NorthMet and other Duluth Complex deposits to show lower nickel recoveries in process test work than would be expected from laboratory assays on drill core. Generally there is a loss of about 25-35% of the nickel compared to drill core assays when concentrating sulfides. From previous work, it is known that small amounts of unrecoverable nickel occur as a magnesium-ironnickel silicate [(Mg,Fe,Ni)2 SiO4] that is tied up in the mineral olivine, which is one of three significant gangue minerals that occur across the NorthMet deposit. Testwork has shown that most of the very small amount of nickel contained in silicates would not be recovered during the autoclaving process proposed. For example, mineralogical studies show that approximately 25% to 35% of the rock in NorthMet is composed of olivine. Previous microprobe study, plus work by PolyMet in 2006, has shown an average of about 0.10% nickel in olivine. The approximate nickel grade of the PolyMet metallurgical bulk samples is 0.10%. Because the average nickel in the olivine is the same as the average nickel in the bulk samples, the unrecoverable nickel in the olivine would be expected to reduce nickel recovery by the amount of olivine in the bulk sample - 25% to 35%. Nickel recoveries on the six PolyMet metallurgical bulk samples have ranged from 69% to 77%. This is in line with an approximate 25% to 35% loss of nickel to silicate. 27 �Figure 1-10. Geologic map of NorthMet Deposit, all units dip southeast, Magenta Zone is projected upward, does not actually subcrop 28 �Figure 1-11. Cross section 35700 at west end of property and 45600 at east end. Purple shading indicated ore zones, bar graphs along holes indicate grades expressed as dollar values, where red = $7.42 cut-off to average grade (~$14.39), and purple shows above average grade, blue are zones of potential lean ore should metals prices rise. 29 �ECONOMIC MINERALIZATION The majority of economic mineralization (copper, nickel, cobalt, platinum, palladium, and gold) at NorthMet occurs in the upper parts of basal Unit 1, with copper and nickel in chalcopyrite, cubanite, and pentlandite, all in the presence of pyrrhotite. Cobalt is contained in sulfides. Platinum, palladium, and gold, while showing good correlation with sulfur and the other metals, are also in a variety of tellurides, bismuthides, and alloys, as well as associated with the major and minor sulfides. Table 1-3 shows correlation of metals values in drill core data. Table 1-3. Simple correlation ® table for economic metals and sulfur Cu % Cu % Ni % S% Pt ppb Pd ppb Au ppb Pt+Pd+Au Co ppm Zn ppm Ni % S% Pt ppb Pd ppb Au ppb Pt+Pd+Au Co ppm Zn ppm 1.000 0.860 1.000 0.541 0.572 1.000 0.568 0.508 0.195 1.000 0.750 0.635 0.292 0.673 1.000 0.591 0.472 0.250 0.482 0.699 1.000 0.760 0.645 0.292 0.778 0.983 0.755 1.000 0.544 0.704 0.621 0.217 0.281 0.241 0.288 1.000 -0.021 -0.004 0.286 -0.041 -0.037 -0.017 -0.039 0.093 1 The simple correlation table above (number of samples=19,516) shows the strong relation of copper, nickel, and palladium, and a somewhat surprising relation of cobalt to sulfur. Zinc’s low factor is probably related to its multiple origins as either magmatic or derived from assimilation of footwall rock, hence representing two populations of data. The sulfur vs. metal correlation is probably greatly affected by iron, the presence of which is not shown here, but is in excess in all rocks. Grades are highest at the top of Unit 1 and fade going down hole. Grades appear to be higher down-dip though this may be an artifact of less dense sampling. There is a smaller zone of economic mineralization (about 50 million tons) at the western end of the property in the upper units, known as the “Magenta Zone.” This zone is generally copper and PGE-rich (sulfur-poor relative to metals) and of “average” reserve grade. The minerals of interest from a waste characterization perspective are the same as above, but pyrrhotite is expected to be the main mineral affecting water quality in regards to waste rock, though the traces of chalcopyrite, cubanite and pentlandite will require study for waste rock storage. Trace pyrite and pyrrhotite are the main sulfide minerals found in the tailings. Pyrite is largely from joint faces and other secondary sources-it is rarely seen in polished section or core. Most sulfide mineralization at NorthMet is of a distant source (but sedimentary?), some is locally modified by sulfur derived from footwall metasedimentary rocks (Virginia Formation). Minor veins and other crosscutting relations indicate some movement of sulfides within the deposit, but there is no evidence recognized for large scale relocation of sulfides, nor any macroscopic evidence for any hydrothermal event that may have remobilized PGE’s or sulfides. Virtually all sulfide mineralization at NorthMet moved in with magmatic pulses, and metal enrichment of the magma happened in a deeper chamber. Therefore, the main controls on the location of mineralization within the deposit may be the specific magmatic pulse or pulses making up the individual units. While textures in Unit 1 are described as heterogeneous, there is also a broad homogeneity in regards to mineral 30 �occurrence, mineral chemistry, whole rock and REE chemistry, and gross rock type that all reinforce the view of a large system of magma pulses replenishing the resident magma at the NorthMet site. The exception to this is that some sulfur, particularly in Unit 1, was derived locally from assimilation of footwall rocks (evidenced by high pyrrhotite content nearer footwall inclusions). The main effect of this assimilation has been to dilute the sulfide grade with additional pyrrhotite in Unit 1, rather than this sulfur scavenging more base metals from the magma. RESOURCE The PolyMet resource and reserve (Table 1.4) models have been done in cooperation with several consultants, most recently PEG Mining of Toronto. PolyMet supplies the geologic solids model, database, and block model geometry. Geostatistics and population of the block model, and hence the resource estimate, are done in consultation, with finalized resource block models then sent forward to engineers for reserve calculation and mine planning. Resource geologic modeling treats the NorthMet deposit as five separate domains: 1. Virginia Formation footwall rocks; 2. a domain including the upper, higher grade parts of Unit 1, locally merged with the higher grade zones at the base of Unit 2; 3. the remainder (lower part) of Unit 1; 4. the Magenta zone in Units 3, 4, 5, & 6 in the western part of the deposit; 5. and the remaining, less mineralized, parts of Units 2 through 7. Unit 1 is mineralized throughout the deposit area, with other units (2 through 6) showing some economic mineralization in the western and central parts of the deposit, but essentially no continuous zones in the east. There is no known economic mineralization in the footwall rocks. Deposit wide, Unit 1 has the highest grades near its top. Though grades vary, Unit 1 is also mineralized to the east of the deposit, down-dip (south) to depths of at least 2,500 feet, and past the limits of expected pit development in the west. The development of waste rock stockpiles over these areas in the east and south is not expected to encumber any material that could reasonably be classed as ore because the upper units are barren and the Unit 1 mineralization is from 1,700 to over 2,500 feet below ground surface. For modeling purposes, Unit 1 is bounded by both “hard” and “soft” geologic surfaces. A “hard” boundary is one where the interpolation of drill hole data into the block model does not cross geological surfaces, a soft boundary is one where interpolation crosses geological boundaries. The top of Unit 1 (i.e., the ultramafic at the base of Unit 2) is a soft boundary for mineralization estimation as the mineralized domain model crosses from Unit 1 into Unit 2. The base of Unit 1, where it contacts the Virginia Formation, is a hard boundary for estimation and metals values, with virtually all sulfide in the Virginia Formation below as pyrrhotite. No data from Unit 1 is used in estimating grades in the Virginia Formation, or vice versa. In the up-dip, west half of the deposit there is an arbitrary and diffuse geologic boundary within Unit 1 that vanishes to the east. This is roughly equal to the top of a petrological contamination zone where large quantities of the footwall metasedimentary rocks have been assimilated. This zone is informally called the “front” or “norite zone” by PolyMet geologists. Precious metals values drop off in this zone and pyrrhotite becomes the dominant sulfide. Moderate copper values may persist below this line, but this is essentially a lower physical limit to combined polymetallic grades above the likely project cut-offs. 31 �Table 1-4. NorthMet resource and reserve values. Work done by Wardrop Engineering 2007. Cut-off based on “Net Metals Value” per ton, accounting for grade, average flotation and hydromet recovery, realization costs, metal prices, and other factors. See Desaultels and Patelke, 2008 for resource calculation details. Enough reserve has been shown for 24 years of production. RESERVES-2007 Cut-off Million Cu Ni Co Pt Pd Au value Tons % % ppm ppb ppb ppb Proven Probable Proven and Probable $7.42 $7.42 $7.42 118.1 156.5 274.6 0.30 0.27 0.28 0.09 0.08 0.08 75 72 73 75 75 75 275 248 260 38 37 37 Cut-off value Million Tons Cu % Ni % Co ppm Pt ppb Pd ppb Au ppb $7.42 $7.42 $7.42 $7.42 202.5 491.7 694.2 229.7 0.285 0.256 0.265 0.273 0.083 0.075 0.077 0.079 74 70 71 56 71 66 68 73 258 231 239 263 36 34 35 37 RESOURCES-2007 Measured Indicated Measured & Indicated Inferred ASSUMPTIONS Metal and Units Assumed Metal Price Average % recovery, as used in DFS Cu % Ni % Co ppm Pt ppb Pd ppb Au ppb $1.25 lb 92.33 $5.60 lb 70.34 $15.25 lb 40.75 $800 oz 75.74 $210 oz 72.69 $400 oz 67.04 In the center of the deposit the highest, near surface, Unit 1 grades transition into the middle of the unit, while in the east, mineralization is strong and vertically persistent throughout the unit. The top of the merged Unit 1 and Unit 2 mineralized domain (domain 1) forms a hard boundary that, combined with the bedrock ledge (depth to bedrock) surface, forms the bottom and top estimation boundaries for the upper units (exclusive of the “Magenta Zone”, which is internal to this domain). There is no conclusive relation between specific Unit 1 specific rock type and presence or grade of mineralization except that noritic rocks are generally of lower grade. Units 2 and 3: These units are treated as one unit in the geologic model, with PolyMet geologists considering them as a single package grading from an ultramafic base to an anorthositic top for modelling purposes. The thickness of the package stays relatively constant, though the thickness of the two individual units varies, primarily due to Unit 2 locally thinning. While generally barren, Unit 2 has mineralization at its base in the western half of the deposit. These zones may not be strictly equivalent to Unit 1 type mineralization. Copper and nickel values are lower, as is pyrrhotite, but behavior of other metals is inconsistent, with PGE (Pt + Pd +Au) content varying locally relative to nearby grades at the top of Unit 1. Above the basal zone of Unit 2 it is usually barren, mediumgrained, and homogenous in texture. Average PGE in Unit 2 is slightly above that of Unit 1. Unit 3 shows mineralization in the west, in the middle of the unit and near the top. This occurrence is merged into the Magenta Zone. Units 4 and 5 are also modeled as a geologic package. There is no compelling geologic reason to fully separate these units, the boundary between them being an arbitrary pick based on overall changes in texture from homogenous to heterogeneous, grain size, and plagioclase content, but without a well defined 32 �bounding horizon. The top boundary of Unit 5 is the basal ultramafic of Unit 6, which is an unused hard boundary in grade modelling. The bottom boundary of Unit 4 is a discontinuous ultramafic horizon. There are also discontinuous oxide-rich zones along the contact between Units 3 and 4. Metals and sulfur grades in Unit 4 are proportional to Unit 1, but consistently lower. Unit 4 has few high copper or sulfur assay intervals. There is some near surface mineralization, modelled as a part of the Magenta Zone, described below. Otherwise there is only low grade, discontinuous material at the base. Unit 6 and Unit 7: These units are very similar in nature. Both are homogenous anorthositic troctolites with well defined ultramafic bases. No top for Unit 7 has been seen in drill hole. Units 3, 4, 5 and 6 host a zone of mineralization, modeled as the Magenta Zone. Unit 6 material was described by Geerts (1994) as the “Magenta Horizon” when originally found in six drill holes. Further drilling has extended these copper rich, sulfur poor zones (of moderate overall grade) into more than fifty drill holes in Units 3, 4, 5, and 6. The zone transitions across the ultramafic base of Unit 6 and into Units 3, 4 and 5, (i.e., does cross the igneous stratigraphy) which is problematic if the emplacement model of these units representing individual pulses of magma is correct. There is no gross evidence for this mineralization being hydrothermal, which could cross boundaries, but would presumably alter large masses of rock. Unit 7 has a few good assay intercepts, but no apparent continuity for sulfides. Table 1-5. Average values for assays by unit after removal of the less than 0.05% copper intervals (drill core samples). Unsampled zones not accounted for here. Data complete through 2006. Cu % Ni % S % Pt+Pd+Au Co Cu+Ni Cu/Ni Cu/S ppb ppm % Total % of unit sampled Average sample length-feet Unit 1 0.3 0.09 0.83 349 76 0.39 3.35 Unit 2 0.2 0.07 0.39 365 73 0.27 2.74 0.43 90 5.3 0.61 80 5.6 Unit 3 0.19 0.05 0.5 286 62 0.25 3.19 0.53 71 7.2 Unit 4 0.21 0.06 0.58 269 66 0.28 3.40 0.44 51 7.6 Unit 5 0.27 0.07 0.54 398 65 0.35 3.64 0.54 41 7.8 Unit 6 0.33 0.08 0.48 532 69 0.41 3.74 0.69 27 7.2 Unit 7 0.2 0.06 0.32 330 83 0.26 3.60 0.72 11 8.4 Copper, nickel, and sulfur values in Table 1-5 are calculated after removing samples with less than 0.05% copper. Samples removed are generally those collected for waste characterization purposes, many well outside the expected mining area, and these low values can somewhat obscure the ore chemistry / mineralogy relations in the “ore.” Ratios are calculated on all raw data, not on the copper-nickel-sulfur values shown here. • No lateral or vertical zonation has been recognized in sulfide or silicate mineral Chemistry; • Gatehouse (North Mining) did report some geochemical cyclicity in unit 1, but this has not been revisited with the larger data set; • Poor assay grades in the noritic rocks are related to footwall assimilation and contamination, otherwise there is little connection between grades and specific rock type. About 83% of the igneous rocks at NorthMet are troctolites, 6% anorthositic rocks, 4% ultramafic rocks, and 4% footwall inclusions. The remainder are norites, gabbros, and other; • Within Unit 1 copper:sulfur ratio tends to be highest at top, then diminishes with depth, following the pattern of PGE’s; 33 �• • • The upper units have higher copper:sulfur ratios than Unit 1 (i.e., more chalcopyrite rich), but lower overall copper values; Ratio of PGE to copper is lowest in Unit 1, but Unit 1 has greatest quantities of both; Chalcopyrite is the dominant sulfide in the upper units regardless of total sulfur content; Sulfide (Ore) Mineral Proportions Various metallurgical test programs have been conducted on NorthMet ores since the 1970's. Reported sulfide mineral proportions have not been entirely consistent between these tests. Table 1-6 shows well characterized sulfide mineral proportions for waste rock from studies done by PolyMet in 2006 and results from various previous studies. Sulfide mineralogy within the NorthMet Deposit has been described in detail through petrographic observations and microprobe analysis. Approximately 95-98% of all sulfide mineralization consists of 4 predominant species, in decreasing order of abundance: chalcopyrite (cp) > pyrrhotite (po) > cubanite (cb) > pentlandite (pn). In general, Po:Cp+Cb ratios increase towards the basal contact or in proximity to sedimentary inclusions. Likewise, Cp:Cb ratios increase with increased distance away from the footwall rocks. In core logging and other work, chalcopyrite is often not distinguished from cubanite. The results show a fairly wide range of values, which may not be totally representative of the deposit as a whole. It is important to note that these discrepancies may be the results of differences in composites, mixed intervals from multiple units including both waste rock and ore, or variations in petrography or laboratory procedures. Some of the composite samples submitted for metallurgical studies were prepared from relatively limited representative core (NERCO samples from 2 drill hole locations), while others were prepared from multiple locations evenly distributed across the deposit. Table 1-6. Sulfide Average Percentage (recalculated to 100 % Sulfide) SGS Lakefield 1991 (NERCO L1) Chalcopyrite % 44 Cubanite % 12 Cp:Cb ratio 4:1 Pyrrhotite % 2 Pentlandite % 9 SGS Lakefield 1991 (NERCO H1) 36 18 2:1 3 8 SGS Lakefield 2000 (PolyMet Conc.) 32 14 2:1 27 7 SGS Lakefield 2005 (Comp. 1) 37 9 4:1 38 16 SGS Lakefield 2005 (Comp. 2) 42 7 6:1 36 15 SGS Lakefield 2005 (Comp. 3) 36 7 5:1 41 16 - - - - - 54 15 4:1 21 3 METALLURGICAL STUDIES: INDEPENDENT STUDIES: Geerts 1994 (Unit 1 – Ore) - - - - - Unit 6 76 6 12:1 5 4 Unit 5 55 3 17:1 17 5 Unit 4 41 5 8:1 32 16 Unit 3 48 6 9:1 35 5 Unit 2 52 13 4:1 24 9 Unit 1 39 7 6:1 44 9 POLYMET WASTE ROCK STUDY 2006: Mining Mining (Figure 1-12) at NorthMet will begin with contractor clearing and overburden stripping of the pit and stockpile areas. Engineered stockpile bases and liner systems must be in place before mining begins, 34 �as does the overall water collection system for treatment and pumping to the tailings basin. Ore and waste production will start in the east pit, with production from the west pit ramping up soon afterwards. Up through about year 11 or 12 production from both pits will be equal until the east pit is mined out. At that point, backfilling of the east pit will begin, with the ultimate goal of constructing a wetland in that pit. The central pit area will be mined last. Ore will be moved at a rate of 32,000 tons per day. Waste to ore strip ratio will be about 1.46:1. Ore will be moved by truck to the “superpocket” and loaded to 100 ton capacity side dump rail cars by pan feeder. There will be twenty trains per day of 16 cars each. Trains will go to the crushers via the existing rail line parallel to the Dunka Road and a short run of new track through the former LTV pits. The equipment fleet is expected to be eight 240 ton haul trucks, two electric hydraulic shovels, one large wheel loader, two rotary blasthole drills, plus bulldozers, rubber tired dozers, road graders, water trucks, and smaller fleet vehicles. GPS control for dispatch is planned for all equipment, with high precision GPS planned for the shovels and drills. High precision GPS may also be installed on the haul trucks. Bench heights will be forty feet. Ore blast patterns will have blasthole burden and spacing of 25' by 28', and the holes will have 5 ' of subdrilling and 17' of stemming. Waste rock blast patterns will have blasthole burden and spacing of 29' by 34', 6' of subdrilling and 20' of stemming. ANFO and emulsion blends will be the explosives used. Ore and waste categorization (“ore control”) will be by assay of core and / or blast holes and careful pit mapping. Waste material will be sorted to stockpiles, and stockpile liners will be built, according to the sulfur and metals content of the waste rock. Over 80% of the waste rock will never produce acid drainage, and about 4% will produce acid drainage within one year. Safe final disposition of this rock will be a permit condition, but as yet the state has not concluded what that condition will be. PolyMet has proposed to place some under capping systems, and to place some underwater. Figure 1-12. Layout of mine site showing 20 year pits, stockpiles for various waste categories, haul roads, railroad, pipelines, lease boundary, and loadout. 35 �PolyMet Process Plant The Plant Site consists of three areas: ● Process Plant - discussed in this section – beneficiation and hydrometallurgical processing facility including Tailings Basin and Hydrometallurgical Residue Facility ● Area 1 Shop – mine mobile equipment major repair facility ● Area 2 Shop – base of mine and railroad operations, locomotive service facility, rail car maintenance facility The Process Plant design is based on key parameters determined by the characteristics of the deposit to be mined and the beneficiation and hydrometallurgical processes that will extract the metals from the ore. On average, 32,000 short tons of ore will be processed each day. This results in annual metal production of 38,821 short tons of copper, 9,037 short tons of nickel, 400 short tons of cobalt, 22,184 ounces of platinum, 87,129 ounces of palladium and 13,824 ounces of gold. The process plant is located at the LTVSMC plant site. The entire Process Plant is in an area that was previously disturbed by mining operations. The Beneficiation Plant will use the Coarse Crusher Building, Fine Crusher Building and Concentrator Building that were part of the LTVSMC taconite plant. The Hydrometallurgical Plant will be located in the three new buildings constructed at the LTVSMC site - the Hydrometallurgical Facility, the Cu Solvent Extraction Building and the Cu Electrowinning Tank House. The Beneficiation Plant consists of the following processing steps: ● Ore Crushing – reducing 48” ore from the mine to 2.5” in the Coarse Crusher Building and from 2.5” to 0.315” in the Fine Crusher Building ● Ore Grinding – reducing the 0.315” ore from the Fine Crusher Building to 120 microns in the Concentrator Building – Flotation – separating the ore into ƒ Concentrate containing the minerals with the metals to be extracted in the Hydrometallurgical Plant ƒ Tailings containing the rock in the ore that is not valuable to be pumped to the Tailings Basin ● Concentrate Regrinding – reducing the size of the concentrate particles from 120 micron to 15 micron The flotation process has been designed to recover virtually all of the sulfide minerals to the concentrate and minimize the amount of sulfide minerals remaining with the tailings. This process has been tested with ore samples representing the NorthMet ore in a pilot plant set up to represent the proposed grinding and flotation process. The flotation tailings generated by this work has been subjected to rigorous waste characterization. Collectively this has demonstrated that the flotation tailings can be placed in the LTVSMC Tailings Basin. The Hydrometallurgical Plant consists of the following processing steps: ● High Pressure Oxidation Autoclave. – Oxidize the copper, nickel, cobalt, iron sulfides and the gold and platinum group minerals (AuPGMs) contained in the concentrate to liberate the contained metal and drive those metals into a leach solution. – Generate a leach residue containing the portions of the concentrate that did not dissolve. ● AuPGM Precipitation – precipitate the AuPGMs from the leach solution and filter the precipitated solids to be sold as a product for further refining. 36 �● ● ● ● ● ● ● ● Solution Neutralization – neutralize the leach solution (minus AuPGMs) by adding limestone and generating a solution neutralization residue. Copper Solvent Extraction – separate the leach solution (minus AuPGMs) into two components. – A strip solution containing the copper. – A raffinate solution containing the non copper portions of the leach solution plus residual copper. Copper Electrowinning – transfer the copper from the strip solution onto plates as a very pure final product. Raffinate Neutralization with Iron and Aluminum Removal. – Neutralize the raffinate solution (minus AuPGMs and copper) by adding limestone and generating a raffinate neutralization residue. – Precipitate iron and aluminum (which behave as impurities in downstream processing) with the raffinate neutralization residue. Residual Copper Removal – precipitate residual copper from raffinate solution and recycle. Mixed Hydroxide Precipitation – precipitate the nickel, cobalt and zinc from the raffinate solution and filter the precipitated solids to be sold as a product for further refining. Magnesium Removal – precipitate magnesium from the raffinate solution and pump the precipitated solids to the Hydrometallurgical Residue Facility along with the leach residue, solution neutralization residue, and raffinate neutralization residue. These residues are not hazardous waste individually or combined. The total annual residue generation is 794,000 short tons. The Hydrometallurgical Residue Facility will be located within the LTVSMC Tailings Basin. The facility will have engineered lined containment areas designed for the residues to be disposed. Water management at the Plant Site will: ● Segregate water that has contacted hydrometallurgical residues in the Hydrometallurgical Plant and the lined Hydrometallurgical Residue Facility and recycle it to the Hydrometallurgical Plant. ● Collect water that has seeped from the Tailings Basin and return it to the Tailings Basin for reuse. ● Use treated water from the Mine Site (via the Tailings Basin) as make-up water. ● Supplement the Mine Site make-up water with new water from Colby Lake. The result is that there is no surface discharge of process water at the Plant Site and that the requirement for make-up water via water appropriation from Colby Lake is minimized. The Plant Site already has required service infrastructure available: ● County Road 666 ends at the Main Gate for the industrial area that included the NorthMet Process Plant, Area 1 Shop and Area 2 Shop. ● The Canadian National Railroad serves the industrial area that included the NorthMet Process Plant, and existing PolyMet track connects to the Area 1 Shop and the Area 2 Shop. ● Three Minnesota Power 138Kv transmission lines serve the NorthMet substation. ● Access to railroad line from mine to plant site The closure plan for the Plant Site consists of standard mineland reclamation practices and landfill closure practices (for the Hydrometallurgical Residue Facility). Original watersheds will be restored to the extent practicable. Structures will be removed and the structure areas covered with soil and revegetated. 37 �PART 3: DUNKA ROAD FIELD TRIP STOPS By: Mark Severson INTRODUCTION In transiting from the PolyMet plant site to Babbitt, we will follow the Dunka Road, a private mine road originally used as a service road for the LTV railroad and to access the Dunka Pit iron mine east of the Northshore mine. The road is now controlled by PolyMet, Cliffs Erie, and Minnesota Power. Access is restricted. Note that UTM locations given for the field trip stops (Fig. 1-13) are estimates. NORTHMET AND MESABA DEPOSITS As the bus proceeds to Babbitt, we will make a few stops, time and weather permitting, and traverse across the southernmost limits of the NorthMet deposit and the Mesaba deposit. There is very little to see in the way of mineralized exposures for either of these deposits. However, one point to keep in mind is the immense size of these low-grade deposits. The NorthMet deposit extends for about 3 miles (4.8 km) along the road, and the Mesaba deposit extends for another 2.5 miles (4 km) along the road. The deposits are separated by undrilled and untested area along the Partridge River that is about 1 mile 1.6 km) wide. Most of the undrilled area is within the PolyMet lease area. Figure 1-13. Locations of field trip stops along Dunka Road. LTV Mine Just outside the town of Hoyt Lakes, the bus will enter and proceed east through the now inactive LTV taconite mine. Founded in 1957 by the Erie Mining Company, this mine was the second taconite production facility on the Mesabi Iron Range. Ore was mined from a series of pits in the Paleoproterozoic Biwabik Iron Formation. At least four horizons were mined: one from the Upper Slaty Member (submembers D and E); two from the Upper Cherty Member (submembers G-H-I-J, and submembers LM); and one from the Lower Cherty Member (submembers S-T-U). Crude ore, which contains about 30% iron, was typically crushed and ground to a powder-like consistency and passed through several stages of magnetic separation and silica flotation to form a magnetite concentrate with 67% iron content. It was then mixed with a bentonite binder and rolled in balling drums to produce “green pellets” (or balls about the size of a marble). The pellets were then conveyed to a furnace where they were heat-hardened at about 2,400° Fahrenheit (1,300° C). After the pellets were 38 �hardened they were able to be easily handled and transported by rail to the Taconite Harbor Pier, and shipped via the Great Lakes to iron mills. In 1973, a record high of 13,104,000 tons of pellets was shipped from Taconite Harbor. In its last years of full production, the LTV Mining Company produced about 8 million tons of taconite pellets annually. In May of 2000, the LTV Steel Corporation announced its intent to close the mine and facilities, including a power plant on Lake Superior. Over 1,400 people were laid off in January 2001. The mine, facilities, and tailings basin were acquired by Cliffs Management Services in late 2001. PolyMet bought the concentrator and tailings basin from Cliffs for use in processing Cu-Ni material from the NorthMet Deposit. As the bus progresses down the Dunka Road (a private mine-related road) through the mine area note the undulating nature of bedding planes in the iron-formation (LTV Area 3 where iron ore was mined from the Lower Cherty member). This area is located near the base of the Biwabik Iron Formation and it does not take much imagination to envision deposition of the gently undulating beds in shallow waters along the shores of the Animikie Basin. To the north (left side of bus), the iron-formation conformably overlies beach sands of the Pokegama Quartzite (bottom of the Animikie sediment package). To the south, the iron-formation is conformably overlain by deep water sediments and turbidites of the Virginia Formation (top of the Animikie sediments). After going through Area 3 the road takes a sharp jog to the south (right). At this locale, the surface trace Siphon Fault approximates the road and iron-formation is present to the west of the road and the Virginia Formation is present across the swamp on the east side of the road. Just before the road curves back to the east to climb a low hill, take the immediate left and go down a secondary mining road for about 500 feet (150 meters). Proceed to the north on foot along a flagged trail to the next stop. STOP 1-1: Virginia Formation near Siphon fault No hammering please! Location: Cliffs-Erie site, T. 59 N., R. 14 W., sec. 26, SE of SE of NE Allen quadrangle; UTM: 569,505E/5,271,610N (NAD 83) Description: This is the only natural exposure of the Virginia Formation on the Mesabi Iron Range. Unfortunately, it is only a few feet thick. A total of 1,443 feet (440 meters) of the formation is present in drill cores from holes drilled south of the range. Note the graded beds, mud chips, concretions, and loading at the bases of these beds. The bedding is near vertical in this location due to proximity to the north-trending Siphon fault—an inferred growth fault (Graber, 1993) wherein the iron-formation decreases in thickness to the east (across the fault) by about 100 feet (30 meters). STOP 1-2: Wetlegs Cu-Ni Prospect / Partridge River Intrusion Location: Rail tracks just south of Dunka Road T. 59 N., R. 13 W., sec. 18, NE of SE of SE Allen quadrangle; UTM: 572,72E/5,271,178N (NAD 83) A west to east traverse along the LTV railroad tracks will be conducted at this field stop. The traverse, approximately 0.5 miles (0.8 km) long, will start at the basal contact and progress upwards through the igneous stratigraphy of the Partridge River intrusion. Outcrops of Units I and III will be viewed. A generalized geologic map of the Wetlegs prospect is shown in Figure 1.13. Detailed descriptions of the railroad cuts are listed in Table 1-7 below. 39 �Figure 1-13. Geology of the Wetlegs Cu-Ni deposit area and the location of Stop 1-2 (Severson and Miller, 1999). 40 �Table 1.7. Wetlegs Prospect outcrop descriptions of Stop 1-2. Feet (W to E) Description of Outcrops 0 ft. Railroad culvert – Longnose Creek 100-110 ft. (left side of tracks) Sulfide/gossan-cemented till consisting mostly of large boulders of a pyrrhotite- and graphite-rich member of the Virginia Formation (the BDD PO unit). Note the rounded granite cobbles beneath the BDD PO boulders. Drilling indicates that the BDD PO subcrops very close to this location. 337-355 ft. (left side of tracks) Very fine-grained (chilled) gabbronorite at basal contact with 60% plagioclase, 30% orthopyroxene, 5 % clinopyroxene, trace to 2 % biotite, and 3 % oxides. The outcrop contains well assimilated “streaks” of Virginia Formation inclusions (very hard to see recently). 400-1025 ft. (both sides of tracks) Unit I – taxitic medium- to coarse-grained ophitic augite troctolite (POcf) to olivine gabbro (PcOf) with patches and lenses of augite-rich pegmatite. The outcrops are sulfide-bearing with the sulfides unevenly distributed along patches, spots, lenses, and joint faces (at numerous orientations!). Trace amounts to 5% uralite due to pervasive deuteric alteration is present. 1245-1355 ft. (two outcrops on north side of tracks) Unit I – taxitic, pervasively uralitized, medium- to coarse-grained ophitic olivine gabbro (PcOf) to augite troctolite (POcf)with trace to 1% sulfides. Also present within these outcrops are very coarse-grained anorthosite inclusions that vary from 10 cm across to 3x5 meter blocks. The edges of the inclusions are sharp, and straight to highly lobate. Within the inclusions, the plagioclase foliation is subvertical and highly variable. 1385-1685 ft. (many outcrops on both sides of tracks) Unit I – taxitic, medium- to coarse-grained ophitic augitic troctolite (POcf) with abundant irregular pegmatitic patches and lenses. The pegmatites contain variable amounts of saussurized plagioclase, clinopyroxene, oxides, biotite, and uralite, with minor quartz, kfeldspar, graphic granite, and sulfides. Most of the sulfides are either within or adjacent to the pegmatites. Uralite is common throughout the outcrops as pervasive replacement products in irregular patches and along joints. 2355-2480 ft. (four outcrops on both sides of the tracks) Unit III – mottled, poikilitic/ophitic troctolite (Po(cf)) to augite troctolite (Pocf). This is a major marker bed due to the presence of medium- to high-density olivine oikocrysts up to 10 cm across. This unit is easily recognized in drill core due to the mottled texture, and the relatively finer-grained plagioclase (1-5 mm). STOP 1-3: Dunka Railroad Junction-PolyMet Discussion Stop Location: Where rail line splits along Dunka Road, T. 59 N., R. 13 W., sec. 13, NW of NE Babbitt SW quadrangle; UTM: 578,358E/5,273,795N (NAD 83) Weather permitting we will stop here to look across a clear cut where some of the PolyMet facility will be located, talk about development plans, note Dunka Junction as a landmark on most regional maps and air photos, and look at outcrops of the upper units of the PRI (Unit VIII or 8?), barren of sulfides, but very typical. Some basalt inclusions are in the flat outcrop to the south of the road. 41 �STOP 1-4: Dunka Railroad Hornfels and Late OUIs Location: Abandoned rail grade, south and parallel to the Dunka Road T. 60 N., R. 12 W., sec. 33, NE of SW, Babbitt SE quadrangle; UTM: 584,808E/5,276,122N (NAD 83) The Dunka Railroad hornfels is a large mafic volcanic inclusion located near the eastern margin of the Partridge River intrusion and just south of the Mesaba deposit. Outcrops and drill hole information indicates that the inclusion is about 900 x 1,500 meters across. The hornfels is a fine-grained, granoblastic to poikiloblastic basaltic hornfels that contains variable amounts of plagioclase, augite, hypersthene, inverted pigeonite, and olivine. Both massive and meta-amygdaloidal varieties are present. In addition to the basaltic hornfels, several small bodies of late intrusive OUIs (Oxide Ultramafic Intrusions-cross-cutting plugs of oxide-rich rock) are also present in the railroad cuts. The abundance of OUI at this locality is related to proximity to the north-trending Grano Fault, which may have served as a feeder vent for the massive sulfides at the Local Boy area of the Babbitt/Mesaba deposit. These OUIs, and a wide variety of granitic rocks, occur as lenses and bodies that cut the troctolitic rocks of the Partridge River intrusion. They are extremely common in drill holes, within a 1,300- 1,900 foot (400-580 meter) wide zone, on the west side of the Grano Fault. The OUIs at this locality are characterized by medium- to coarse-grained clinopyroxenite with 75-80% augite, 0-15% olivine, <5% interstitial plagioclase, and 515% oxides (ilmenite is dominant). Contact relationships with the basaltic hornfels are sharp but highly irregular and lobate. Granitic dikes and veins, present within a NNE-trending zone, are also evident in the exposures. They are rarely observed in close proximity to the OUI, but where they are, the dikes crosscut the OUI. Four sites, shown on Figure 1-14, can be observed at this locality. Figure 1-14. Geology of the Dunka Railroad hornfels and the location of Stop 1-4. (From Miller, Severson, and Foose, 2002). 42 �PART 4: MESABA DEPOSIT – TECK AMERICAN INCORPORATED By: Tim Jefferson and Mark Severson BACKGROUND Previous exploration and development work The Mesaba deposit was first discovered along the base of the Duluth Complex in 1958 by Bear Creek Mining Company (BMC) which targeted a geophysical anomaly. Between 1958 and 1960 BMC completed 55 shallow drill holes for 43,000 feet (13,952 meters) on their discovery located 5 miles (8 km) south of Babbitt. BMC renewed drilling activities in 1967-1971 completing 149 additional holes. Drill hole B1105 intersected substantial amounts of semi massive to massive sulfide mineralization between 1,400 and 1,800 feet (425 and 550 meters) below surface in footwall rock. Subsequent drilling defined a high grade zone appropriately named the Local Boy deposit after BCM geologist Stuart Behling, the “local boy,” who encouraged BMC to continue drilling this site. Bear Creek defined the overall tonnage and grade for the deposit to be 851 million tons (772 million tonnes) grading 0.46% Cu, 0.12%Ni at 0.25% Cu cut-off. In late 1973, AMAX Exploration, Inc. agreed to take over BMC’s state and private leases. During the next four years (1974-1978) AMAX continued drilling over the entire deposit (completing 228 drill holes), and evaluated whether an underground operation was feasible (Watowich, 1978). In particular their focus was drawn to the Local Boy ore body, and following successful permitting, they sank a shaft in 1976-1977. Four drifts totaling 3,800 feet (1,160 meters) were developed and 218 underground holes were completed. This detailed definition resulted in an overall underground resource of 364 million tons (330.2 million tonnes) averaging 0.84% Cu and 0.19% Ni , with a Local Boy-only resource of 5 million tons (4.54 million tonnes) grading 1.89% Cu, 0.36% Ni. Both underground resources were estimated based on a 0.60% Cu cut-off. Due to weakening copper and nickel markets and the inability to produce separate high grade Cu and Ni concentrates, Amax abandoned their plans to develop the deposit in late 1981. Rhude and Fryberger obtained leases and evaluated the Local Boy deposit circa 1990. Arimetco Inc, picked up the Babbitt deposit leases, renamed the Mesaba deposit, and evaluated the property circa 1994-1996. They did not complete any drilling but collected two bulk samples for metallurgical test work. Arimetco upgraded the resource estimate to 3,300 million tons (2,993.7 million tonnes) grading 0.46% Cu, 0.12% Ni, cut-off 0.38% Cu (Miller et al, 2002). Arimetco Inc. declared bankruptcy in late 1996. Present exploration and development work Teck American Incorporated picked up a package of state and private leases covering the Mesaba deposit in 1997. In 2001 the evaluation of the resource included collection of a 5,511 ton (5,000 tonnes) bulk sample. Sulfide concentrates were made from this bulk sample and tested at their Cominco Engineering Services Lab near Vancouver, BC, using their patented CESL hydrometallurgical pressure oxidation process. New definition drilling began in the fall of 2007 and continued until June, 2008 for a total of 67,430 feet (20,560 meters) in 64 drill holes (Fig. 1.15). This drilling was concentrated on the western portion of the deposit to complete a 400 foot (120 meter) grid infill program. In addition to this work, a new 4,409 ton (4,000 tonnes) bulk sample was mined in the fall of 2008. Sulfide concentrates from this ore were made at NRRI’s Coleraine Mineral Research Laboratory. The concentrates were shipped to the CESL facilities for a new round of hydrometallugical tests, including the successful recovery of an intermediate mixed nickel-cobalt hydroxide. While Teck has not released a new reserve estimate pending completion of additional definition drilling, they are continuing their evaluation of the deposit on several fronts, including baseline environmental studies, geophysical surveys, and flotation and ore beneficiation / 43 �recovery test work at their Applied Research and Technology division at Trail, BC, along with other engineering studies. Figure 1-15. Drill hole location map for Mesaba Deposit. Grid north is about 33° west of north. Recent re-logging of historic holes at the Mesaba deposit, in addition to information gained from logging of holes completed in 2007-2008 by Teck American geologists and Mark Severson (NRRI), indicates that the deposit is primarily hosted by a previously unrecognized intrusion within the Duluth Complex (Severson and Hauck, 2008). It is believed that this intrusion, informally named the Bathtub intrusion (BTI), lies between the Partridge River intrusion (PRI), to the south, and the South Kawishiwi intrusion (SKI) to the north and east. This intrusion is believed to have been fed by a vent in the Grano Fault area on the east side of the Mesaba deposit. The BTI is believed to pre-date the South Kawishiwi intrusion, and is coeval in age to the Partridge River Intrusion. It is further believed, based on drill hole evidence, that igneous units of the PRI and BTI overlap and co-mingle, with the upper units of the PRI overlying BTI units. Supporting evidence for this new interpretation is based on igneous units that are unique to either the PRI or BTI, and different styles of sulfide mineralization between the two. The following geologic discussion is largely based on the work of Severson and Hauck (2008) but is condensed and summarized. The reader is referred to the regional geology section for further description of the Partridge River and South Kawishiwi intrusions. 44 �Figure 1-16. Preliminary geologic map of the Mesaba deposit showing major geologic units of the Bathtub, Partridge River, and South Kawishiwi intrusions. Major structural features associated with the deposit are also shown. Outline of ore deposit and Teck property boundaries are indicated. 45 �GEOLOGIC SETTING Footwall Rocks As there is great commonality between the footwall rocks at the NorthMet Deposit and the Mesaba Deposit, they are discussed in the regional geology section for the PRI Structure There are three structural features that are pertinent to understanding the intrusive history of the BTI that include (Fig. 1.16): 1. an east-west trending paired syncline and anticline in the footwall rocks referred to as the Bathtub Syncline and Local Boy Anticline; 2. a zone that is closely associated with the Local Boy Anticline, referred to as “The Hidden Rise,” that separates the PRI and BTI; and 3. a north-trending fault zone, referred to as the Grano Fault, that has been postulated to have been the feeder zone for the BTI and footwall-injected massive sulfides of the Local Boy ore zone. The paired Local Boy Anticline and Bathtub Syncline have been determined to be pre-complex, and pre-or syn-deposition of the BIF and Virginia formations by the following evidence: 1) BIF submembers A and B are notably thin or absent along the trough of the syncline. There is always a thin layer of Virginia Formation between the BIF and the overlying Bathtub intrusion in the synclinal trough (indicting that the Duluth Complex has not assimilated BIF A and B in the syncline). 2) The VirgSill also is thickest along the axis of the anticline and limbs of the syncline, and thins or is absent in the trough of the syncline. This attests to a more open structural setting along anticlinal folding for injection of the sill. It is thus interpreted that the thinning of BIF within the trough is due to lack of deposition of these more calcareous units in deeper water (Severson et al., 1994a). The “Hidden Rise” is a loosely-defined zone wherein scattered hornfels inclusions of footwall Virginia Formation, and associated noritic rocks, are fairly common. When viewed collectively, the inclusions in “The Hidden Rise” define an east-west trending “ridge” that is roughly positioned at the contact between the PRI and BTI. Thus, “The Hidden Rise” is used to both define this hornfels-bearing “ridge” and to artistically, and conveniently, divide the BTI from the PRI. The morphology of this feature suggests that it may have originally served as the floor and/or north edge of an earlier intruded PRI and later served as a wall along the south edge of the BTI as it was emplaced. Along the far eastern edge of the Mesaba deposit is the north-trending Grano Fault, so named for the abundant and sometimes voluminous amounts of late granitoid and oxide rich pyroxenitic lenses (OUIs) associated with the fault zone (Severson, 1994). The late intrusive lenses are interpreted to have vertical configurations. They were injected along subsidiary fault zones parallel to, and immediately west of, the Grano Fault. The late intrusives cut the troctolitic rocks and thus, demonstrate that the fault was active during and after emplacement of the PRI, BTI and SKI and may represent a rift related transform fault. Other features that are associated with the Grano Fault include: • a steep drop in the basal contact (down to the east); • abundant pre-Complex sills are common within the Biwabik Iron Formation in a limited area at the Serpentine deposit - the localized increase in the sills outlines the fault trace at Serpentine and suggests that the fault was activated prior to emplacement of the SKI and PRI (see also Zanko et al., 1994); and • a well-defined topographical lineament occurs along the trace of the fault to the south of the Mesaba deposit. Along the eastern edge of Mesaba, a buried valley, defined by contouring the top 46 �of the ledge (Plate IV, Severson et al., 1994b), is also present on the northern extension of the same topographical lineament. BATHTUB INTRUSION The newly named Bathtub intrusion (BTI) is wholly contained in the central portion of the Mesaba (Babbitt) deposit. The BTI has recently been singled out as a separate intrusion to explain the abrupt change from typical Partridge River intrusion (PRI) stratigraphy, in the southern part of the deposit, to a completely different stratigraphy to the north in the remainder of the deposit. The BTI has been divided into two major units, BT1 and BT4, each of which contain several subunits. These units, in addition to footwall rocks, and structural features, are portrayed in Fig. 1.17. Figure 1-17. Schematic “type-section” cross-section, looking east, through the Mesaba deposit that crudely displays the spatial distribution of most of the igneous units in the Bathtub intrusion. Note that not all of the PRI units are shown on the right side of the figure. BT1 Unit The lowermost unit of the BTI is referred to as the BT1 Unit. It is very similar to Unit I of the nearby PRI in that it is heterogeneous-textured at all scales, contains abundant hornfels inclusions near the basal contact, and is the main sulfide-bearing unit at Mesaba. However, there are some important differences between Units I and BT1 that include: • Augite troctolite is the dominant rock type in the bottom half of BT1 (as is also the case for Unit I) but in many of the cross-sections the entire up-dip portion of BT1 consists of augite troctolite; • Massive sulfide occurrences are more common near the basal contact in the BT1 than in Unit I (excluding the unique Local Boy ore zone) indicating that sulfide settling may have been a more important mineralization mechanism in the BTI; • Coarse- to very coarse-grained disseminated sulfides (up to several centimeters across) are exceedingly common in the lowermost portions of BT1; whereas, this same relationship is not so obvious in Unit I – this again implies the importance of a sulfide settling origin, and; • Ultramafic horizons and patches are very common in portions of the BT1; whereas, similar ultramafic horizons are not as common in Unit I of the PRI. The BT1 Unit has been further subdivided into several internal subunits that are discussed below. BT1-a 47 �This subunit of the BT1 is a heterogeneous-textured augite troctolite grading to olivine gabbro. The BT1-a subunit is more common in the bottom half of the BT1 Unit and increases up dip (to the north) at the expense of most other subunits of the BT1. BT1-c At the base of the BT1 there is significant silica contamination of the magma, due to assimilation of the footwall rocks, and noritic rocks (norite to gabbro norite), with common hornfels inclusions, are the dominant rock types with lesser amounts of augite troctolite. The BT1-c subunit spatially occurs as a rind or coating along the basal contact of the BTI. BT1-uz Wherever olivine-rich ultramafic rocks are common over appreciable intervals in the BT1 Unit this subunit is used to designate ultramafic zones. The morphology of the ultramafic rocks in these zones ranges from well-defined layers to zones where irregular ultramafic patches are presumably peppered throughout a troctolitic host rock. BT1-at This subunit of the BT1 is used to denote areas where anorthositic troctolite is the dominant rock type. The BT1-at zone is located at the very top of the BT1 Unit in the cross-sections of this report. It is a small unit that is locally present in only the central portion of the Bathtub ore zone. BT-sli A few holes in the extreme western end of the BTI exhibit well-defined modally-bedded rocks consisting of alternating troctolitic and olivine-enriched ultramafic rocks. These intervals are designated as BT-sli for the Bathtub Side Layered Interval (Figures 8 and 9a). The BT-sli subunit occurs about in the center of BT1 unit where it comes in close proximity to “The Hidden Rise.” While the BT-sli can be readily mapped out and correlated between cross-sections, it is difficult to tell if this subunit is a downward continuation of the BTLI or "± Picrite" (as depicted in Fig. 1.17). BT4 Unit The uppermost unit of the BTI is referred to as the BT4 Unit. It was originally correlated with Unit IV of the PRI. However, the BT4 Unit is distinctly different from Unit IV in that the BT4 Unit at Mesaba is: • heterogeneous-textured at all scales and composed of many alternating rock types; • sulfide-bearing whereas Unit IV is mostly sulfide-barren – the sulfides in BT4 are generally, finergrained and generally of lower ore grade tenor in comparison to sulfide-bearing zones in the underlying BT1 Unit; • floored by a semi-persistent ultramafic layer termed the "± Picrite" (see discussion below) in the central portion of the Bathtub ore zone; and • ultramafic layers and modally-bedded zones, termed the Bathtub Layered Interval (BTLI), are common in the central portion of the Bathtub ore zone. The BT4 Unit as been further subdivided into several internal subunits based on the presence of a dominant rock type. The various subdivisions of the BT4 Unit are briefly discussed below. BT4-a This subunit of the BT4 on the cross-sections denotes areas where heterogeneous-textured augite troctolite is the dominant rock type. BT4-at This subunit of the BT4 is used to denote areas where anorthositic troctolite is the dominant rock type. Thick zones of BT4-at are common to some cross-sections through the Mesaba deposit and show relatively good correlation and predictability with similar zones in adjacent cross-sections.. "± Picrite" 48 �At the base of BT4 is a semi-persistent olivine-enriched ultramafic horizon referred to as the "± Picrite." It is present in about 70% of the drill holes in the BTI-portion of the Mesaba deposit. The "± Picrite" is generally absent in the up dip direction (to the north) and is variably present to the south in the contact zone between the PRI and BTI. Where present, the "± Picrite" is about 1-15 feet thick, but exceptions are locally present. In some areas, the "± Picrite" consists of several stacked ultramafic horizons, or modal beds, that are interlayered with troctolitic rocks, and thus, the zone represents a collection of several cyclic layers. In other areas of the Mesaba deposit, the "± Picrite" is not always easily singled out as it occurs in close proximity to a downward thickening BTLI with similar ultramafic layers and modal beds. Therefore, in some instances it is difficult to pick the "± Picrite" out of a myriad of ultramafic horizons associated with either the BTLI or BT-sli. Bathtub Layered Interval (BTLI) In the vicinity of the Bathtub Syncline, ultramafic layers are extremely common within the BT4 Unit. The ultramafic layers may represent repetitious cyclic layers and can be correlated in drill holes as an overall rock package. This package of abundant cyclic layers, present in the BT4 Unit, is referred to as the Bathtub Layered Interval (BTLI). In the eastern half of the Mesaba deposit the BTLI appears to be present in a subhorizontal saucer-shaped morphology. Conversely, in the western half of the deposit, the BTLI is confined to one or two cylinder-shaped zones, albeit with irregular edges, that are positioned in close proximity to “The Hidden Rise.” Overall, the ultramafic rock types of the BTLI are characterized by alternating assemblages of either/or: melatroctolite (picrite), feldspathic peridotite, peridotite, dunite (minor), olivine-rich troctolite, and troctolite with modal beds of olivine-rich layers. One or more of these rock types may be stacked above the other in no particular order, and the thickness of this assortment may be highly variable between drill holes. The number of individual ultramafic layers present within the BTLI for any particular drill hole varies drastically. In some holes, over 75 individual ultramafic layers and modal beds are intersected, whereas in other holes only a few scattered ultramafic beds are encountered. The range in thickness for each of the individual ultramafic beds also shows considerable variation, ranging from a few inches to over tens of feet thick. Although the BTLI can be correlated as a package of alternating troctolitic and ultramafic layers, each of the individual ultramafic layers cannot be correlated on a hole by hole basis. This situation indicates that the ultramafic layers either: 1) commonly bifurcate - thick ultramafic layers may divide into many thin ultramafic layers; 2) some may actually represent dike-like features (filter pressed?); 3) some may pinch out or have very limited spatial extent due to localized crystallization or other deposition-related origins; or 4) combinations of the above. Gradational tops and sharp bases are commonly present, indicating that crystal settling may have been important (this is especially true in the eastern half of the Mesaba deposit). However, the reverse (gradational bottoms and sharp tops) is also locally present. In addition, the inclination of contacts and modal bedding associated with the ultramafic layers are highly variable, ranging from 5°-80° (with localized overturned beds). This variation in inclinations can even be present in even a single drill hole. For the most part, the bedding and contact inclinations in the BTLI are steeper higher up in the drill hole and gradually shallow with depth. The shallow to steep angles exhibited by the BTLI may reflect that the ultramafic layers originated via a variety of mechanisms that include: 1) crystal settling to form subhorizontal layers (dominant in the eastern half of the deposit); 2) filter-pressing to form localized dikelike morphologies; 3) slumpage and folding of the beds took place before they were fully crystallized to form highly irregular and overturned beds; 4) compaction differences took place during lithostatic loading of the crystal pile to form steep and irregular beds; 5) cooling and crystallization took place along, and parallel to, the southern wall of the BTI (up against “The Hidden Rise”); or 6) combinations of all of these mechanisms. Whatever their origin, the steep beds displayed by the BTLI in the western half of the Mesaba deposit are inordinately associated with “The Hidden Rise.” 49 �PARTRIDGE RIVER INTRUSIVE (PRI) AT MESABA Many of the igneous rock units that are present at the nearby NorthMet deposit are also present along the southern edge of the Mesaba deposit and are believed to represent units of the Partridge River Intrusion (see previous geologic setting discussion). Additionally, Units IV through VI of the PRI appear to extend northward and overlie the heterogeneous-textured BT4 Unit. This relationship, also depicted in Figure 1.17, suggests that the BTI was eventually over-ridden/overlain by the upper units of the PRI. The overall timing of emplacement for the PRI versus the BTI is unknown but correlations in the cross-sections crudely suggest the following: • Units I through III were intruded first along the southern edge of the Mesaba deposit with a vent area located somewhere to the southwest. “The Hidden Rise” generally marks the northern extent of this intrusive activity and originally formed as part of the floor to these units. Unit III may have been intruded as thin lenses across and north of “The Hidden Rise” – this may explain the local presence of Unit III-like inclusions in the BTI. • Concurrent with or after the above activity, the BT1 Unit was intruded from a vent area located somewhere to the east, possibly from the Grano Fault area. “The Hidden Rise” formed the southern wall of this particular magma chamber. • The BT4 Unit was intruded into the same magma chamber but was emplaced above the BT1 Unit. • Concurrent with or after the above activity, Units IV through VII+ of the PRI were intruded from a vent area located somewhere to the southeast. These upper units were emplaced over the BT4 Unit. MINERALIZATION AT MESABA The Mesaba deposit is characterized by disseminated sulfide mineralization, which occurs most commonly as intercumulus accumulations of chalcopyrite, cubanite, and pyrrhotite. Pentlandite crystals are less commonly identified megascopically. Additionally, less common occurrences of talnakite and bornite have been noted. Short intercepts of semi-massive to massive sulfide mineralization are often encountered in drill core in the Virginia footwall rocks immediately adjacent to noritic intrusive rock. Sulfur isotope analyses have indicated that the source of the sulfur used in the formation of the sulfides of the Mesaba deposit is the pelitic sediments of the Virginia Formation (Ripley, 1986). The model of sulfide deposition entails turbulent injection of units of the BTI wherein immiscible sulfide droplets coalesce within the silicate melts and attract the chalcophile elements (chiefly copper and nickel) through magma mixing. Thus, the most contaminated magma (from assimilation of footwall Virginia formation) hosts basal sulfides that contain excess sulfur relative to intrusive units higher above the footwall. The sulfide content of the rock increases, often dramatically as the footwall is approached. This sulfide content increase is accompanied by the increasing presence of pyrrhotite and a subsequent change in the copper bearing sulfides (cubanite is dominant over chalcopyrite). The disseminated mineralization is generally composed of 1-4% sulfides, but can reach upwards of 8-12 % sulfides as the footwall is approached. In addition to the interstitial disseminated sulfide and semi-massive to massive sulfide mineralization, sulfides may locally occur as clots up to several cm in diameter, and are seen occasionally as chalcopyrite rich vein fillings indicative of a late sulfide-rich fluid origin. The most important mineralized zone at Mesaba is the basal zone, starting at the footwall Virginia Formation contact, that commonly ranges between 200 and 400 feet thick (60 and 125 meters thick). Higher up in the intrusive package, often overlapping the BT1-BT4 unit boundary, is a second zone of disseminated sulfide mineralization which is more erratic and discontinuous in nature. 50 �The Mesaba deposit (hosted by the BTI) displays significant differences with the nearby NorthMet deposit (hosted by the PRI). At NorthMet, the ore zone lower in the deposit is more stratiform and near the top of PRI Unit I, while at Mesaba the main mineralized zone starts immediately at the footwall contact zone. As noted in earlier discussions, units of the BTI are more erratic and chaotic than those of the adjoining PRI intrusion. This is also true of the sulfide distribution which is often locally quite chaotic, and variable but overall the basal zone is tied together by adjacent drill holes to define a strongly mineralized ore body of considerable extent. The footprint of the Mesaba deposit is an oblong to arcuate shape, 3,000 by 13,000 feet (925 by 4,000 meters) in approximate dimension, cropping out to surface on the northern/up-dip side and extending to approximately 1,650 feet (500 meters) below surface in the southern/downdip direction. The strongest basal mineralization is often localized within the Bathtub Syncline. Here, concentration of sulfides by gravitational settling into the footwall depression has likely occurred. Teck American has not released any new reserve/resource estimates, and the reader is referred to historic reserve numbers as reported at the start of the Mesaba section. Three geologic cross sections from the western half of the Mesaba deposit depicting composited Cu-Ni grades from historic and recent drilling are displayed below (Figures 1.18, 1.19, and 1.20). PGE Mineralization at Mesaba Platinum group metal mineralization (PGE) occurs at Mesaba and the other Duluth Complex copper-nickel deposits. Along with analyzing all new drill core for precious metals (Pt-Pd-Au), Teck American is in the process of cataloging and analyzing pulps from historic drill holes (BMC, AMAX) for precious metals at this time as well. Analytical results of this data collection (in progress) have not been released to the public. As a general statement, the western side of the Mesaba deposit (hosted by the BTI) generally contains very low PGE values, while there are occurrences of anomalous PGE values in the eastern half of the Mesaba deposit. It is postulated that the higher values to the east may be dependent on a more proximal distance from the hypothesized vent area of the BTI. Analytical results of limited analysis of historic pulps for PGEs from the Local Boy ore body and pulps from historic drill holes intersecting upper portions of Unit I of the PRI in the southern portions of the Mesaba lease have been previously published and are summarized below: Local Boy PGE Occurrences Numerous anomalous PGE and precious metal values are confirmed to be present within the massive sulfide ores (Severson and Barnes, 1991; Hauck and Severson, 2000). Maximum values include: Pd – 11,100 ppb, Pt – 8,300 ppb, Au – 13,100 ppb, and Ag – 62 ppm (note that these values are present in sampled intervals that range from 5 feet to 15 feet thick). The majority of the anomalous PGE values are spatially distributed along the axis of the Local Boy anticline with the highest Cu and PGE values occurring in the west half of Local Boy. The Grano Fault may have served as a feeder zone to the massive sulfides that were injected into the footwall rocks along the Local Boy Anticline as an immiscible sulfide melt. This melt fractionally crystallized in an east-to-west direction and progressively became enriched in PGE towards the west (see discussion below). PGE Mineralization at the Top of PRI Unit I (along the Southern Margin of the Mesaba Deposit Drill holes along the southern margin of the Mesaba deposit intersect an igneous stratigraphic section similar to the section present at the nearby NorthMet deposit. Limited sampling for PGE at the top of Unit I (the equivalent of the Red Horizon of Geerts (1991, 1994) at NorthMet) has taken place in a few holes at the Mesaba deposit (Severson and Hauck, 2003). For the most part, the Pd contents at the top of Unit I in the sampled holes are similar to Pd contents of the Red Horizon at the NorthMet deposit. Publicly available data indicates a maximum of 1,267 ppb Pd is present at the top of Unit I in the southern portion of the Mesaba deposit (Severson and Hauck, 2008). 51 �MASSIVE SULFIDES AT THE LOCAL BOY ORE ZONE OF THE MESABA DEPOSIT Cu-rich massive sulfides near the basal contact of the Complex are locally present at the Mesaba deposit in a small zone referred to as the Local Boy ore zone. In 1976, AMAX Inc. completed a 1,700-foot-deep exploratory shaft (Minnamax shaft) down to massive sulfides of the Local Boy ore zone and, in 1977, completed four drifts (A, B, C, and D; Figures 1.21 through 1.24). Underground Fan drilling (217 holes) was completed in 1978 to further define the massive sulfide distribution. Sulfide minerals include pyrrhotite, pentlandite, chalcopyrite, talnakhite, cubanite, maucherite (nickel arsenide), sphalerite, bornite and late mackinawite, chalcocite, covellite, godlevskite, and native silver. A more detailed description of these minerals, along with microprobe compositions, microphotographs, and possible paragenetic sequence, are presented in Severson and Barnes (1991). The Grano Fault may have served as a feeder zone to both the BTI and to massive sulfides of the Local Boy ore zone. Severson and Hauck (2003) speculated that magma that issued from the Grano Fault may have been initially enriched in PGE, forming the PGE-enriched massive sulfides at the base of the PRI in the Local Boy ore zone, but as the magma intruded in an east-to-west direction [to form the BTI] it became progressively impoverished with respect to PGE in such a manner that rocks at the extreme western end of the Bathtub ore zone contain very little PGE. Footwall Structures in the Local Boy Ore Zone Several investigators have recognized that pre-existing structural conditions in the footwall rocks strongly influenced the basal contact of the Duluth Complex (Mancuso and Dolence, 1970; Watowich, 1978; Holst et al., 1986; Martineau, 1989; Severson and Barnes, 1991). Major irregularities in the basal contact are generally related to folds in the underlying country rock indicating that intrusion proceeded more or less along bedding planes in the footwall rocks (Holst et al., 1986). This is readily expressed by a major eastwest -trending trough and ridge in the basal contact at Mesaba that coincides exactly with a synclineanticline that is defined by the top of the Biwabik Iron Formation (BIF). The thickness of preserved Virginia Formation between the Complex and the BIF is variable due to the amount of material assimilated by the Complex. The Local Boy ore zone is also situated over this anticlinal ridge. The majority of massive sulfide ore zones, hosted mainly by the Virginia Formation (Severson and Barnes, 1991), are broadly coincident with the axis of the anticline. The contoured top of the BIF in the Local Boy area is shown in Figure 1.21. Similar anticline geometries are also present for the basal contact as shown in Figure 1.21. All the data indicate that an EW-trending anticline is the major structural feature present within the footwall rocks of the Local Boy area. 52 �Figure 1-18. Cross-section 68+00W through the west end of the Mesaba deposit showing grades of significant intervals. Drill holes MB-08-38 from this cross-section will be on display at Teck’s core shack. 53 �Figure 1-19. Cross-section 44+00W through the west end of the Mesaba deposit showing grades of significant intervals. Drill holes MB-07-15 and MB-08-37 from this cross-section will be on display at Teck’s core shack. 54 �Figure 1-20. Cross-section 36+00W through the west end of the Mesaba deposit showing grades of significant intervals. Drill hole MB-08-36 from this cross-section will be on display at Teck’s core shack. 55 �Figure 1-21. Contoured top of the Biwabik Iron Formation at Local Boy (left), and the contoured top of the basal contact between the footwall Virginia Formation and the Partridge River intrusion at Local Boy (right). The spacing of the contours in Figure 1.21 suggests that the anticline is asymmetrical with a steeper flank to the immediate south of the anticlinal crest. Also, fault zones in drill core, as well as recognizable fault offsets of correlative units, are most commonly present on the south flank of the anticline. Taken collectively, all these data suggest that additional structural features, in the form of increased faulting and shearing, are more important on the south flank of the anticline in the Local Boy area. The northeasttrending Kulas Fault is also shown in the Figure 1.21. This fault was initially mapped by Jim Kulas in the underground drifts at Local Boy. The cross-sections included in Severson and Hauck (2008) also recognized the fault, with an offset of 10-20 feet, and named the fault after Kulas. Mineralization Trends in the Massive Sulfide at the Local Boy Ore Zone The vast majority of massive sulfides at Local Boy are contained within the Paleoproterozoic Virginia Formation. Even though the massive sulfides straddle the basal contact, most of the massive sulfides are associated with either hornfelsed sedimentary inclusions above the contact or with footwall rocks below the contact while the interfingering intrusive rocks are relatively barren of massive sulfides (Severson and Barnes, 1991). This suggests that the massive sulfide ores were not formed in this area by the gravitational settling of sulfides, but rather, the ores formed by injection of an immiscible sulfide melt into structurally prepared areas within the footwall rocks along the Local Boy anticline in a vein-like setting. A similar mechanism is proposed for the Norilsk-Talnakh deposits in Russia. Even though the basal contact of the Complex with the Virginia Formation is highly undulatory, the massive sulfides exhibit a definite top and bottom. The ore is distributed such that most of it is contained within a zone between 20 feet and 300 feet above the top of the Biwabik Iron Formation. The geologic constraint for the bottom of the ore zone generally corresponds to the top of the VirgSill. The constraints for the upper portion of the ore zone are unknown and may have been obliterated during emplacement of 56 �the Complex. Figure 1.22 is an attempt to show, in a plan view, where massive sulfide zones are present. Also shown in the figure are the different massive sulfide types (ranging from pyrrhotite-dominant to Curich) relative to structural features. The relationships shown in Figure 1.22 indicate that: 1) semicontinuous massive sulfide zones are present, mainly to the south of the Kulas Fault; and most important 2) the massive sulfides show a progressive change in an east-to-west direction from Cu-poor massive sulfides to Cu-rich massive sulfides in the vicinity of the Local Boy anticline. These relationships suggest that the injected immiscible sulfide melt underwent fractional crystallization and progressively became more Cu and PGE enriched as it moved through the footwall rocks in an east-to-west direction. Figure 1-22. Potential distribution of semi-massive to massive sulfide types (Cu-poor versus Cu-rich) at the Local Boy ore zone (left); and an isopach map of the cumulative thickness of the massive sulfide zones at the Local Boy ore zone (right). Note that the massive sulfides are not present as a continuous blanket, but rather, as one or more stacked disjointed/separated multiple horizons near the basal contact. A possible feeder vent for the sulfide injection event may have been the Grano Fault, which was repeatedly reactivated during emplacement of the Complex. Other data that indicates that the Grano Fault was a potential feeder vent include: 1) the massive sulfides are more common, and thicker (Figure 1.22), close to the Grano Fault (feeder) and along the axis of the Local Boy anticline (structurally-prepared site); 2) the VirgSill rarely contains significant amounts of disseminated sulfides – except near the Grano Fault; and 3) the Biwabik Iron Formation rarely contains sulfides – except near the Grano Fault. In summary, the massive sulfides at the Local Boy ore zone are interpreted to be structurally controlled in that they are situated along the axis of the Local Boy anticline. The massive sulfides are Cu-rich (5-25% Cu) and are almost exclusively hosted by the Virginia Formation. Sulfide textures suggest that the massive sulfides were injected as an immiscible sulfide melt into the footwall rocks. The overall pattern of sulfide types and PGE contents suggest that the sulfides formed via a process of fractional crystallization of an immiscible sulfide melt as it migrated into the footwall rocks. The Grano Fault is inferred to represent the potential feeder zone in this scenario. 57 �PART 5: FRANCONIA BIRCH LAKE PROJECT Extracted from Routledge, 2004; some contribution by Mark Severson Birch Lake Deposit A few widely-spaced holes were drilled in the Birch Lake area by Duval Corporation in the 1970s. Some Cu-Ni mineralization was intersected at great depths in many of these holes; however, exploration companies were not impressed with the Cu-Ni grades and largely abandoned the area - the potential for PGE mineralization was never considered. In the mid-1980s, the MDNR and Mineral Resources Research Center (MRRC) conducted analyses of iron-rich intervals in the basal portion of drill hole Du15. PGE values as high as 9,123 ppm Pd+Pd, associated with high Cr2O3 contents (5.3%) were documented (Sabelin and Iwasaki, 1985; 1986). This discovery marked the start of serious PGE exploration in the Duluth Complex and a multitude of holes and wedges have since been drilled at Birch Lake. Property Geology The Birch Lake property is entirely underlain by the South Kawishiwi intrusion (SKI) that is itself bordered on the southwest by the Partridge River body and on the southeast by the Bald Eagle pluton. The SKI extends approximately 25 miles (40 km) northeast-southwest and is up to 4.5 miles (7 km) wide. The footwall Middle Precambrian metasediments outcrop less than a kilometer west of the property boundary in the area of the Birch Lake deposit, and are best exposed in the Dunka pit area. Figure 1.23 shows the property geology. On the Birch Lake property, the SKI ranges in vertical thickness from 1150 feet to 4,420 feet as interpreted from 34 pilot hole drill intercepts of the footwall metasedimentary rocks or Giants Range monzonite. The lithology and igneous stratigraphy of the SKI (Fig. 1-6) been has been simplified to seven principal units found on the property as summarized in Table 1.8. The Cu-Ni bearing sulfides and associated PGE mineralization at Birch Lake occur consistently in the upper portion of the “U3” ultramafic unit below its contact with pegmatitic phases of sulfide-barren, hanging wall troctolites, gabbros, and anorthosites. The U3 unit is mainly troctolite phases with compositions ranging to anorthosite. It is variable in modal mineralogy, composition and texture over short distances but can be distinguished by the presence of sulfides, cumulus olivine with interstitial plagioclase, and olivine rich ultramafic intervals (dunites, melatroctolites, picrites) ranging from less than 1 foot to tens of feet (0.3 m to tens of meters). Late granitic and felsic dikes cut the SKI and U3 unit. The U3 unit has been intersected at depths as shallow as 1,125 feet (343 m) and up to 3000 feet (915 m) deep in 31 pilot holes on the property. Intercept thicknesses of the U3 range from 48 feet to 375 feet (14.6 m to 114.3 m) and vertical thickness averages 175 feet (53 m) based on statistics for 31 pilot holes. Strong PGE enrichment is associated with late stage Cu-Ni sulfides, particularly with chalcopyrite, talnakhite, and bornite where they occur as replacement mineralization. Weaker PGEs are found where chalcopyrite and pyrrhotite are the primary sulfides. Saussauritization and serpentinization are common deuteric alteration in the U3 unit. Retrograde alteration and schistosity accompany the east west and northwest faults. The Complex has not been significantly deformed since magma consolidation, but it has been subjected to displacements along reactivated basement faults as well as cross faults. Mapped structures are mostly sub-vertical north-northeasterly faults and fault zones that are evident as linears on airphotos and topographic maps. Rowell (2002) believes that these faults have been active pre-, syn- and postemplacement of the SKI and offset the mineralized U3 unit. Where exposed in parts of the SKI and footwall rocks, movement on these faults ranges from 10 feet to 400 feet (3 m to 120 m). 58 �Table 1.8. Stratigraphic Section for South Kawishiwi intrusion at Birch Lake. Unit Thickness Remarks Hanging Wall Rocks AT&T Main AGT AT-T Thick Averages 275 m 21 m to 365 m; average 115 m Thin picrite units at top or middle of unit Cu-Ni-PGE Mineralized Units PEG U3 BH BAN 3 m to 80 m; averages 28 m 1 m to 125 m; averages 30 m Extremely variable 3m to 115 m; averages 38 m Sulfides first appear near base Up to 20 ; Cu-Ni-PGEs-oxides enriched Primary host for Cu-Ni sulfides in intrusion 120 m Contact metamorphosed to pyroxene hornfels Melted and recrystallized Footwall Rocks: BIF GRB West-northwest trending faults cut the northeasterly faults and show left lateral displacements in the south portion of the property and right lateral offsets under Birch Lake (Rowell, 2001). These late faults have vertical displacements in the order of 10m and may be akin to transform faults that accompany rifting elsewhere. The Bob Bay Fault zone trends north through Bob Bay but northeasterly south and north of Bob Bay. The faulting appears to have influenced the localization of sulfides and higher PGEs and it effectively cuts off the Birch Lake deposit on the west. Drill holes in this fault zone commonly intersect massive and disseminated sulfides in the footwall rocks and/or felsic dikes that cross-cut the intrusion. The sense of displacement between holes 88-1 and DU-15 is in the order of 200 to 300 feet (60 m to 90 m) down on the east side. A regional west-northwest fault diminishes mineralization and metal grades along its length and divides the deposit into a north, main segment and a small south segment. The spatial distribution of faults on the property is such that the Birch Lake deposit is displaced laterally and vertically by the sub-vertical fault sets. However, at the relatively wide spacing of the mostly vertical drill holes that test the deposit, the location of most of the interpreted faults is not known with the degree of confidence generally required for reserve estimation and mine planning. Deposit Types The Duluth Complex hosts four types of magmatic mineralization at or near its footwall: • Large, low grade disseminated Cu-Ni sulfide deposits that are locally enriched in PGEs. • Localized high grade zones of massive Ni-Cu sulfides, which maybe enriched in PGEs. • Disseminated, PGE enriched, Cu-Ni sulfides associated with specific types of phase-layer transitions, and in this sense are stratabound deposits. • Titanium and vanadium oxides-rich ultramafic plugs that are, in some cases, potentially deposits. The Birch Lake deposit is an example of the third- type of mineralization: 1% to 5% disseminated copper and nickel sulfides bearing significant palladium, platinum, and gold values with lesser silver, cobalt and rhodium. The mineralization is stratabound in that it is consistently associated with the top of the U3 unit and pegmatite marker horizon that is traceable hole to hole as the deposit hanging wall. Palladium and platinum enrichment and the Pt:Pd ratio is greater near top of the U3 unit; copper and nickel grades are variable as is the Ni:Cu ratio. The base of the deposit within the U3 unit is determined by assay cut-off. 59 �Figure 1-23. Birch Lake property geology (from Routledge, 2004). Mineralization Information on mineralization has been obtained from core logging by personnel of the BBJV, its partners and State geologists, laboratory analysis and the detailed mineralogical investigation of five core samples from five drill holes. The latter consisted of reflecting light microscope and scanning electron microscope 60 �study of polished thin sections prepared from heavy minerals concentrated by heavy liquid separation of crushed and ground core (Cabri, 2002). These samples had relatively high PGE grades. Detailed petrography and electron microprobe work on drill core from four holes has also been done by the UMN and the NRRI (Marma et al., 2002). Sulfides are disseminated interstitially in the rock matrix and mirror the size of rock forming mineral grains: coarser sulfides with coarse grained to pegmatoid fabrics, finer sulfides with medium grained rocks. The sulfides occur: • intergrown as eutectic and replacement textures • as triple point exsolution between rock mineral grains • intergrown with silicates • rarely as sulfide seams or veinlets Microscope study by Cabri (2002) has identified the major ore minerals to be chalcopyrite and undefined members of the chalcopyrite family, possibly one or more of talnakhite, mooihoekite, putoranite and haycockite; the oxide minerals chromian spinel, ilmenite, magnetite, chromite and native copper and troilite. Common minerals found are the copper sulfides bornite, chalcocite, and cubanite as well as nickel sulfide minerals heazlewoodite and pentlandite. Trace amounts identified are altaite, digenite, frobergite, galena, mackinawite, millerite, sphalerite and nine different PGM-bearing minerals, native silver, silver telluride and alloys of silver and gold. Cobalt is analyzed from pentlandite up to 2.12 wt%. Iron sulfide gangue is pyrrhotite and troilite. The PGEs occur as various fine grained Pd tellurides with other Pt, Os, Ru, Au, Ag, Te and B minerals. Ninety percent of the PGMs are associated with copper sulfides (Cabri, 2002) as discrete grains attached to sulfides, as sulfide inclusions and at the margins between sulfides and gangue silicates. The PGMs may form halos around, or be included in, interstitial copper sulfides, pyroxenes, secondary amphiboles and biotite. PGEs are also remobilized in chlorite, serpentine or secondary magnetite. High PGE values were first analyzed from an interval characterized by poikilitic chromite in plagioclase feldspar known as " 2 in one texture". Pd minerals occur at twice the frequency of Pt minerals and this is reflected in drill core analyses. Native silver is generally occluded in sulfides with Ag and Au-Ag alloys found as discrete grains and inclusions. Geologic controls on spatial distribution of metals and mineralization are not fully understood (Lehmann, 2002b). Lehmann (2002a) suggests that partially crystallized ultramafic magma was injected into the crystallizing SKI from vents associated with faults at the base of the Duluth Complex. This produced thick localized piles of dunites and melatroctolites in the SKI along the margin of the Duluth basin. Metal enriched residual fluids introduced with the ultramafic magma migrated upwards and laterally to be trapped and precipitate sulfide mineralization where permeability was disrupted horizontally, such as elevation changes and changes in magma composition. An initial pulse of mineralization introduced widespread chalcopyrite and cubanite and pyrrhotite with low, but anomalous, PGE values at the base of the Complex. A subsequent more localized mineralization event precipitated sulphur deficient copperiron sulfides and troilite accompanied by low grade PGEs that were trapped stratigraphically higher in the SKI at the pegmatite hanging wall unit at the Birch Lake deposit. PGEs and copper may have been redistributed by cooler, late stage, magmatic fluids that caused the deuteric alteration of the ultramafic host rocks. Further complicating this picture, the presence of chlorine-rich drops (formed by a deliquescent process) on the drill core suggests that a hydrothermal model of concentrating the PGEs could also be invoked. 61 �PART 6: DULUTH METALS DRILL CORE REVIEW Dean M. Peterson - Duluth Metals Limited INTRODUCTION AND COMPANY HISTORY Wallbridge Mining Company Limited began active mineral exploration in the Duluth Complex in 1998 when it reviewed data on the adjacent Maturi and Spruce Road deposits. The company completed limited field work including geological mapping, geophysical surveys and a three hole drill program. A review of existing data allowed an estimation of an underground Inferred Resource of copper, nickel, cobalt and PGE of the Maturi deposit and its extension to the east. Based on this initial work, the company elected to proceed to acquire properties immediately to the east of the Maturi Deposit, previously referred to as the Maturi Extension Property but currently referred to as the Nokomis Property. Duluth Metals was incorporated under the laws of the State of Delaware on January 18, 2000 under the name of Wallbridge America Corporation. During 2000 and 2001 Duluth Metals acquired four state leases and two federal prospecting permits within the Nokomis Property for future exploration. The initial exploration work in 2001 and 2002 included contracting a 3D computer model of the geology (completed by individuals at the Natural Resources Research Institute or NRRI) and reviewing the exploration potential of Duluth Metals' holdings on the basis of this new 3D geological model. Resampling was done on drill core from seven holes on the property and one hole immediately adjacent to the property stored at the core storage facility in Hibbing, Minnesota. Results of this new geochemical data provided the foundation of the initial magma flow model for the deposit (Peterson, 2001b). During 2003 and 2004 Duluth Metals conducted no significant exploration and merely maintained its properties in good standing. Increased development activity in the area and increasing commodity prices in 2005 resulted in Wallbridge activating Duluth Metals. Also in that year Wallbridge concluded that the most effective way to finance the exploration and possible development of these properties was to "spin off" Duluth Metals by vending it into a free standing company to be run independently. In early 2006 a fully equipped field office was set up in Ely, Minnesota located approximately 15 minutes drive from the project site. Facilities including three bedroom accommodation, computerized data handling, core analysis and storage have been organized at the Ely field office. A seven hole drill program completed in the spring of 2006 extended the mineralization to the east into the property acquired in January 2006 from a private party. The results of the drilling also confirmed the grade and consistency of the mineralization located beneath a very large anorthosite block in the middle of the Nokomis Property. Through sound geologic analysis of the public domain data and the fortuitous open properties available between the known Maturi and Spruce Road deposits, the foundation for a world-class Cu-Ni-PGE discovery was put into place. However, it requires more than a bit of well positioned property to make a world-class discovery. The favorable alignment of world metals demand, increasing metals prices, new hydrometallurgical extractive technologies, and a bit of luck provided the strong market conditions facilitating the successful Initial Public Offering (IPO) of Duluth Metals stock in October of 2006. The financial underpinnings of the IPO provided the fuel for a systematic exploration drilling of the property. This approximately 500,000 foot drill program was incredibly successful, with every one of the 155 drill holes intersecting Cu-Ni-PGE mineralization. This extensive drill program provided the cornerstone of the development of the giant Nokomis Cu-Ni-PGE deposit. In tandem with the drilling efforts were property acquisition efforts (still ongoing), resulting in the purchase of private surface lands, leasing of additional State lands, and the optioning of the Dunka Pit property as a potential process and tailings site (Fig. 1-24). 62 �Figure 1-24. Simplified terrain and geology map with overlays of identified Cu-Ni-PGE & TiO2 deposits and Duluth Metals properties. 63 �NOKOMIS DEPOSIT MILESTONES With the success of drilling efforts (Fig. 1-25) came National Instrument 43-101 (NI 43-101) compliant resource calculations, showing that within the Duluth Complex the Nokomis stands out with its continuity of grade and overall dimensions. NI 43-101 is a mineral resource classification scheme used for the public disclosure of information relating to mineral properties owned by, or explored by, companies which report these results on stock exchanges within Canada. Figure 1-25. Defined resource blocks categorized as Indicated and Inferred, Duluth Metals drill holes, and the location of selected drill holes on display for this field trip (ILSG, 2009). With the positive resource picture, metallurgical testing was warranted on the Nokomis data and a study was contracted with SGS Lakefield. Here again the Nokomis deposit proved itself by demonstrating exceptionally high recoveries of Cu, Ni, and precious metals (Table 1-9). These high recoveries are above typical metal recoveries of conventional smelting, therefore a technology agreement was signed by Duluth Metals for the use of the patented Platsol process. Table 1-9. Nokomis metallurgical test results, completed by SGS Lakefield. Metal / Recovery Cu Ni Pt Pd Au Flotation / Concentration Recovery 95.3% 72.4% 86.0% 87.0% 73.0% Hydrometallurgical (Platsol) Recovery 99.6% 99.2% 97.6% 98.1% 84.1% Combined Recovery 94.9% 71.2% 83.9% 85.4% 61.3% 64 �With the positive metallurgical testing, two scoping studies were completed on the Nokomis Project by the consulting firm Scott Wilson Roscoe Postle Associates Incorporated (SWRPA) resulting in very positive outcomes. Based on indicated and inferred resources, the Nokomis Deposit ranks as one of the largest combined base and precious metal resources discovered in North America in decades. For comparison, the Nokomis deposit contains a higher than average grade for deposits within the Duluth Complex. Specifically, the Nokomis Deposit contains significantly higher grade zones of copper, nickel, platinum, palladium, gold and silver zones in large coherent portions of the deposit. One of the main objectives of the Company is to quantify these higher grade zones and appraise the potentially positive impact on future mining scenarios. An updated interim resource estimate was received on June 4, 2008 from SWRPA based on drilling completed through April 2008 (Fig 1-25). The increased resource estimate update at a 1% copper equivalent cut-off and various copper cut-off grades is shown in Table 1-10. Based on SWRPA review of metal prices, process recoveries, refining costs and underground mine operating costs likely to apply at the Nokomis deposit site, the 1.0% copper equivalent cut-off grade is reasonable for the statement of indicated and inferred resources at this time. Table 1-10. Indicated and inferred resource estimate for the Nokomis Deposit. Cut-off 1.0% CuEq 0.5% Cu 0.6% Cu 0.7% Cu 0.8% Cu Cut-off Grade 1.0% CuEq 0.5% Cu 0.6% Cu 0.7% Cu 0.8% Cu Tonnes (000's) 449,413 376,306 247,149 112,035 41,078 Tonnes (000's) 284,230 236,102 155,743 72,418 33,292 Cu % Ni % 0.624 0.658 0.714 0.794 0.883 0.199 0.206 0.216 0.233 0.255 Cu % Ni % 0.627 0.667 0.725 0.817 0.900 0.194 0.198 0.201 0.214 0.215 Indicated Resources Co Au Pt % g/t g/t 0.010 0.011 0.011 0.011 0.011 0.084 0.090 0.103 0.123 0.152 0.159 0.172 0.199 0.240 0.293 Inferred Resources Co Au Pt % g/t g/t 0.01 0.01 0.01 0.01 0.01 Note: 1. 2. 0.096 0.105 0.120 0.144 0.173 0.191 0.210 0.239 0.280 0.320 Pd g/t TPM g/t CuEq % 0.358 0.390 0.452 0.549 0.679 0.600 0.653 0.753 0.912 1.124 1.46 1.54 1.66 1.86 2.09 Pd g/t TPM g/t CuEq % 0.431 0.475 0.542 0.640 0.739 0.718 0.790 0.902 1.065 1.231 1.50 1.58 1.69 1.88 2.03 CIM definitions were followed for Mineral Resource estimation and classification. Mineral Resources are estimated at a zone definition (wireframe) cut-off grade of approximately 1.0% Cu equivalent grade (CuEq). 3. The approximately 1.0% CuEq cut-off grade includes all material in the wireframed zones. 4. Bulk density is 3.01 t/m3 5. Resources were estimated to a maximum depth of approximately 1,350 m. 6. Copper equivalent (CuEq%) is based on Net Smelter Return Factors as determined for the Preliminary Economic Assessment by Scott Wilson RPA dated January 18, 2008. 7. Metal Prices used were $1.75/lb copper, $7.00/lb nickel, $10.00/lb Co, $600/oz Au, $1100/oz Pt and $350/oz Pd. 8. Copper equivalent (CuEq%) = Cu% + 3.03 x Ni% + 0.63 x Co% + 0.30 x Au g/t + 0.76 x Pt g/t + 0.24 x Pd g/t based on expected metal prices and process recovery and refining charges. 9. TPM is Au g/t + Pt g/t + Pd g/t. 10. Co, Au, Pt, Pd grades, that are lacking in historic drill holes, have been entered in the resource database based on regression of assay grades from DML drill hole assays 65 �The indicated and inferred resource estimates for base and precious metal content in the Nokomis deposit truly are enormous (Table 1-11). It is perhaps more revealing to compare these Nokomis numbers on the world stage of magmatic Cu-Ni-PGE sulfide deposits. This comparison (Fig. 1-26) is based on two types of data for this class of ore deposit. Historic metal contents for mining camps and individual deposits are taken from Anthony Naldrett’s book titled “Magmatic Sulphide Deposits: Geology, geochemistry, and exploration” published in 2004. Publically reported metal contents for these same mining camps and deposits, as well as the Nokomis deposit, are from SEC filings, NI 43-101 reports, and related audits available online. Review of this figure clearly displays the size and importance of the Nokomis deposit on the world stage. For example, a simple comparison of the publically reported data for the Sudbury mining camp versus Nokomis shows that the Nokomis deposit contains more copper and PGE than is left in all of the active and developing mines in Sudbury. Precious Base Table 1-11. Calculated base and precious metal content of the Nokomis Deposit. Metal Indicated Inferred Copper 6.18 Billion lbs. 3.93 Billion lbs. Nickel 1.97 Billion lbs. 1.21 Billion lbs. Cobalt 103.00 Million lbs. 62.80 Million lbs. Platinum 2.30 Million ozs. 1.75 Million ozs. Palladium 5.17 Million ozs. 3.94 Million ozs. Gold 1.21 Million ozs. 0.88 Million ozs. TPM (Pt+Pd+Au) 8.68 Million ozs. 6.57 Million ozs. Figure 1-26. Chart of the contained base and precious metals in world class Cu-Ni-PGE mining camps and deposits. 66 �On January 12, 2009, Duluth Metals announced the receipt of a new independent NI 43-101 Preliminary Assessment ("PA" or Scoping Study) on its Nokomis Project from SWRPA. This report (available online at SEDAR) provides an updated preliminary assessment of the Nokomis Project, based on the June 2008 mineral resource estimate. This PA is based on an expanded 40,000 tonne per day ("tpd") production rate scenario which doubles the January 2008 PA production rate. The report confirms positive economics for the Nokomis Deposit even at today's lower metal prices with the potential to be one of the world's low cost copper-nickel producers (Table 1-12). Table 1-12. Nokomis production rate scenarios. Base Case1 Scenarios Production Rate2 @ 20,000 tpd Production Rate3 @ 40,000 tpd Undiscounted Net Present Value $4.328 Billion $8.214 Billion Net Present Value @ 10% $792 Million $1.598 Billion Average Annual Cash Flow $205 Million $434 Million Internal Rate of Return (IRR) 21.0% 23.0% Capital Cost $795 Million $1.332 Billion Payback Period 4 years 4 years Annual Metal Production 102.1 million lbs Cu 23.8 million lbs Ni 121,000 ozs TPM 181.7 million lbs Cu 42.3 million lbs Ni 251,000 ozs TPM Note, all monetary units are in $US: 1 Scott Wilson RPA: Base Case Prices of $1.75/lb Cu; $7.00/lb Ni; $10.00/lb Co; $1,100/oz Pt; $350/oz Pd; $600/oz Au January 18, 2008 Scott Wilson RPA Preliminary Assessment on the Nokomis Project, Minnesota, U.S.A. 3 January 8, 2009 Scott Wilson RPA Preliminary Assessment on the Nokomis Project, Minnesota, U.S.A. 2 The new PA for the Nokomis Project envisions and includes costing for a fully integrated mining and processing facility that encompasses the following general process flow: Ore production rate of 40,000 tonnes per day (14 million tonnes per year); Underground mining by blasthole open stoping with partially recoverable pillars; Underground access by shaft and ramp; Underground ore handling by conveyor systems; Underground primary crushing, further crushing and grinding on surface; Transfer from mine to concentrator via 10 km slurry line; Agitated holding tanks at mill with a minimum capacity of 11,000 m3; Flotation concentration, producing a bulk copper-nickel-cobalt-PGM-gold concentrate; Hydrometallurgical processing using PLATSOL™ process; Production of saleable copper and nickel metal via standard electrowinning and production of cobalt and PGM-gold products to shipped to refineries for final processing to metal; and a brownfields tailings disposal facility within three kilometers of the processing site. Table 1-13 shows unit cash costs (operating and capital), calculated for either nickel or copper. This measure provides a means to compare costs in dollars per pound of metal to individual metal prices (also in dollars per pound of metal). SWRPA notes that the calculation is net of byproduct credits, which in this case amount to approximately 60% of revenue. Since no single metal is a dominant contributor, unit cash costs calculated in this manner appear very low, or negative in most cases. 67 �Table 1-13. Unit cash costing chart for evaluating the Nokomis deposit as a Copper or Nickel mine with all other metals as byproducts. Assumptions Low Case Base Case Market Case Key Variable - Cu Price ($/lb) Key Variable - Ni Price ($/lb) Capital Cost, including Contingency (Billion $) Conceptual Projected Mine Life $1.55 /lb. $4.90 /lb. $1.332 22 years $1.75 /lb. $7.00 /lb. $1.332 22 years $3.31 /lb. $12.70 /lb. $1.332 22 years $(0.09) /lb. $0.42 /lb. $0.32 /lb. $(0.72) /lb. $0.42 /lb. $(0.30) /lb. $(2.36) /lb. $0.42 /lb. $(1.94) /lb. $(2.15) /lb. $1.79 /lb. $(0.36) /lb. $(3.61) /lb. $1.79 /lb. $(1.82) /lb. $(11.62) /lb. $1.79 /lb. $(9.83) /lb. Nokomis as a Copper Mine Unit Cash Costs - Operating ($/lb Cu) Unit Cash Costs - Capital ($/lb Cu) Total Unit Cash Costs - Operating + Capital ($/lb Cu) Nokomis as a Nickel Mine Unit Cash Costs - Operating ($/lb Ni) Unit Cash Costs - Capital ($/lb Ni) Total Unit Cash Costs - Operating + Capital ($/lb Ni) Low Case - reflecting recent low metal prices of 1.55/lb Cu; $4.90/lb Ni; $10.00/lb Co; $795/oz Pt; $295/oz Pd; $600/oz Au. Base Case - same base case prices used in the previous Technical Report (January 2008 PA), based on long-term average price forecasts of the past several years. ($1.75/lb Cu; $7.00/lb Ni; $10.00/lb Co; $1,100/oz Pt; $350/oz Pd; $600/oz Au). Market Case - prices as of January 13, 2008*, for direct comparison to the previously-reported 20,000 tpd scenario contained in the January 2008 PA. ENVIRONMENTAL STUDIES With the scoping studies completed, Duluth Metals initiated baseline and preliminary Environmental Studies. Duluth Metals recognizes environmental guardianship as an important corporate priority, and is working closely with State and Federal agencies to establish policies, programs and practices for conducting its business in an environmentally sound manner. Our company realizes its long-term success depends on the well being of the environment and the community as a whole. Duluth Metals goal is to not only remain in compliance with all State and Federal regulations governing their operations but to exceed them. Duluth Metals has an environmental policy under which it has made a number of commitments consistent with responsible environmental stewardship. The Company is committed to: 1. 2. 3. 4. 5. 6. 7. 8. 9. Retain qualified, professional employees and consultants to design and implement their field programs; Develop, design and operate facilities in a socially and environmentally friendly manner in order to mitigate environmental impacts; Support research to advance understanding of industry's impact on the environment and to reduce harmful effects through improved practices and technologies; Contribute to the dissemination of environmentally sound technology and management methods; Work with government and the public to develop effective, efficient, and equitable measures to protect the environment based on sound science; Require contractors to comply with our Company's environmental policy requirements; Encourage dialogue on environmental issues with employees and the concerned public groups and to be responsive to their concerns; Ensure that all employees understand and are able to fulfill their environmental responsibilities; and, Reclaim work sites in accordance with site specific criteria in a planned and timely manner. 68 �The numerous studies need to be completed (over a number of years) in order for Duluth Metals to gain the right to mine the deposit in an environmentally sound manner is perhaps best viewed in a graphical form (Fig. 1-27). This chart clearly displays the complex interplay of geology, geoengineering, metallurgy, mine planning and design, and extensive environmental studies that must mesh together into a coherent plan to put the Nokomis Deposit into production. Duluth Metals has initiated baseline environmental and engineering studies in support of the Nokomis environmental review and permitting process. Examples of baseline environmental studies completed include: Wetlands Delineations and Functional Assessments; Stream Morphology Assessments; Wildlife Habitat Surveys; Sensitive Plant Species Surveys; and, Phase 1a Cultural Resources Searches. In addition, several preliminary environmental engineering studies have been completed to help with siting potential facilities. Figure 1-27. The environmental review and permit process utilized in Minnesota for the Cu-Ni-PGE projects in the Duluth Complex. Dark grey boxes represents work that the company has to complete, light grey boxes are completed by the state and federal agencies, and white boxes represent times the public has input into the process. GEOLOGY OF THE NOKOMIS DEPOSIT Duluth Metals Limited’s Nokomis deposit is the most recently discovered Cu-Ni-PGE deposit in the 1.1 Ga. Duluth Complex of northeastern Minnesota. The deposit is located near the north end of the South Kawishiwi intrusion (SKI) west-southwest of the junction of the Nickel Lake Macrodike (NLM) and the SKI (Peterson et al., 2006; Peterson and Albers, 2007; Tharlason et al., 2007; Peterson, 2008; White, in prep; Gal, 2008). The deposit was discovered utilizing a genetic ore deposit model that identified channelized magma flow within the SKI under a large xenolith/pillar of anorthosite. The model led to exploratory drilling in 2006, deposit discovery and initial resource estimation in 2007, and significant resource expansion in 2008, all in a period of 18 months. The regional scale magma flow model that is being used by Duluth Metals to interpret the origin of the Nokomis Deposit is presented in Figure 1-28. Duluth Metals has come to the realization that the initial basaltic composition SKI magmas that ultimately created the Nokomis deposit intruded as sulfide-bearing, crystal-laden slurries (olivine and plagioclase crystals). Therefore the company has reinterpreted the regional basal stratigraphy (PEG, U3, BH, BAN) of the SKI (Fig. 1-6) at Nokomis into the Basal Mineralized Zone, or BMZ. The company believes that 69 �the geometry of the system (sill-like sub-horizontal intrusion) led to crystal sorting and melting the footwall granitic rocks to create the heterogeneous lithologies of the BMZ (Fig. 1-29). Figure 1-28. Regional scale magmatic flow model for the northern SKI. Modified after Peterson, 2008. Figure 1-29. Simplified crystal-liquid slurry model for the SKI in the Nokomis area. 70 �The SKI is a shallow dipping (~24º east-southeast) sill-like troctolitic intrusion exposed in an 8- x 32kilometer arcuate band along the northwestern margin of the Duluth Complex. Lithologic units within the Nokomis deposit include Mesoproterozoic rocks of the SKI and Anorthositic Series of the Duluth Complex as well as basalt xenoliths of the North Shore Volcanic Group. At Nokomis, SKI magmas intruded between hangingwall anorthositic rocks and footwall granitic rocks of the Neoarchean Giants Range batholith (Fig. 1-29). Brief descriptions of the map units that Duluth Metals recognizes on the property are given in Table 1-14. Table 1-14. Lithostratigraphic units within the Nokomis deposit. Duluth Complex and related rocks (1.1 Ga.) SKI Anorthositic troctolite to troctolite (ATA Series) - Medium to coarse-grained, homogeneous, wellfoliated and locally layered anorthositic troctolite, troctolite, and ophitic troctolitic rocks. In the field, this unit is commonly referred to as the “sea of troctolite”. Augite-bearing troctolite (Main AGT) - Heterogeneous, coarse-grained, subophitic to ophitic, poorly foliated augite troctolite characterized by scattered augite-rich pegmatitic clots and patches. Commonly capped by hanging wall inclusions (HB & Ai) and interpreted to be the solidified basaltic liquid that carries the crystals and sulfides of the BMZ. Xenoliths in the SKI Sulfide-bearing troctolite (BMZ) - Heterogeneous, sulfide-bearing, vari-textured troctolite, augite troctolite, anorthositic troctolite, and olivine gabbro with 0.5 - 5% disseminated chalcopyrite, cubanite, pentlandite and pyrrhotite. Anorthosite (AN-G & Ai) - Undifferentiated Anorthositic Series inclusions. Includes well-foliated anorthosite, troctolitic-anorthosite, poikilitic troctolitic anorthosite, gabbroic anorthosite, and rarely gabbro and troctolite. Inclusions range from a few cm’s to elongate bodies measured in km’s. Anorthositic gabbro to gabbro (Upper Gabbro) - Mixed group of Anorthositic Series rocks that occur in the central portion of the map area. Includes well-foliated anorthositic gabbro, gabbro, anorthosite, hornfelsed basalt, and augite troctolite. Basaltic hornfels (Upper Basalt, HB) - Fine-grained, granoblastic to poikiloblastic basaltic hornfels; consists of variable amounts of plagioclase, augite, olivine, hypersthene, and inverted pigeonite. Commonly associated with Anorthosite xenoliths (unit Ai). Footwall Giants Range Batholith (2.68 Ga.) Porphyritic quartz monzonite (GRB) - Pink, coarse-grained, hornblende-phyric, quartz monzonite with large (1-2 cm) orthoclase phenocrysts. Also contains irregular zones of aplite and supracrustal xenoliths. Strongly recryatallized and partially melted locally anong the contact with the SKI. Two detailed geologic cross sections through the Nokomis Deposit are presented in Figure 1-30. These sections display the continuity of the basal mineralization as well as the differences in the hangingwall stratigraphy from west to east through the deposit (see Fig. 1-6). In the east, the deposit is located under an extremely thick (>1000m) pillar of Anorthosite Series rocks, and in essence the basal SKI can be viewed as a thin sill-like body. To the west, the anorthosite pillar ends and the immediate hangingwall rocks to the deposit are troctolites of the Main AGT unit. We interpret that the Main AGT as the 71 �solidified troctolite melt (see Fig. 1-29) that carried the crystals and sulfide droplets of the magmatic slurry. Figure 1-30. Geologic cross sections through the Nokomis Deposit. 72 �NOKOMIS ORE DEPOSIT MODEL Duluth Meals is utilizing advanced geological modeling and analysis of the Nokomis deposit to identify and characterize higher grade copper, nickel, platinum, palladium and gold zones within the deposit. This same modeling defines contiguous, large tonnage, higher grade zones that will be important in the evaluation of future mining scenarios. The geologic modeling also provides an innovative way to understand the origin of, and the controls on, grade distribution within the deposit. As well, the model will assist on directional search parameters and variography of subsequent block models and grade tonnage estimates. Duluth Metals is commissioning a new 43-101 resource estimate which will include 155 drill holes and 63 wedges as part of its higher grade zone targeting effort. The results of our models show four distinct types of higher-grade Cu-Ni-PGE mineralization within the Nokomis Deposit. These four distinct types of mineralization are briefly described below and are presented graphically in Figure 1-31. 1) PGE- rich disseminated mineralization on the Eastern side of the Nokomis Property. This area is known as the Eastern High Grade Corridor (21 drill holes as press released October 27, 2008). This large coherent zone of significantly higher grade PGE mineralization is the result of the initial constriction of the magma channel beneath a large block of older Anorthositic Series rocks within the Nokomis Deposit. Such a blockage impedes the flow of crystals and sulfide droplets. However, the liquid portion of the magma continues to flow along with its dissolved Platinum Group Elements ("PGE"). Once these dissolved and flowing PGE's come in contact with a stuck sulfide droplet, they dissolve into the sulfide droplets and increase the PGE tenor of the sulfide. 2) Ni-Co enriched semi-massive sulfides at, or immediately below the base of the magma channel. This mineralization is believed to have formed by continuous flushing of hot magma through the channelway which melted footwall granitic rocks. These granitic melts were incorporated into the magma and induced the formation of new sulfide minerals beneath the crystal slurry. These new sulfides scrubbed the melt of nickel and cobalt and settled to the bottom of the system. 3) Cu-PGE enriched disseminated mineralization deep in the footwall below the magma channel. The ever-deepening (into the melting footwall) magma channel induced and pinned hydrothermal convection cells in the footwall beneath the magma channel. The water in the system was released from the footwall granitoids by thermal metamorphism and circulated Cu-PGE downwards beneath the channelway. 4) Cu-PGE enriched disseminated mineralization at the top of the mineralized zone (Top-Loaded). To the sides of the magma channelway, the sulfide-bearing crystal-liquid slurry was intruded as batches of magma out and to the sides of the channel. Since these highly crystalline magmas are distal to the very hot main magma channel, they solidified quickly into sulfide-bearing troctolite. Once the silicates (olivine+plagioclase) are solid, buoyant fractionated sulfide (Cu-PGE) liquid and magmatic waters moved upwards through the solidifying crystal pile and precipitated Cu-PGE sulfide and hydrous silicate minerals beneath the overlying crystal-poor, liquid silicate magma. Company sponsored and internal research on many aspects of the mineralization within the Nokomis Deposit is continuing. Models based on normative mineralogical compositions have shown that the composition of Cu-Ni-PGE mineralization reflects local segregation and fractionation of the sulfide liquid as well as processes involved in the transportation upwards and/or downwards of the evolved sulfide liquid through the solidified host rock (Figs. 1-32 and 1-33). Further details of segregation-fractionationinfiltration and transportation processes of evolved Cu-Ni-PGE sulfide liquids/fluids are currently being explored by Duluth Metals. Such studies include detailed petrography, fluid inclusion studies, researchgrade total PGE analyses, whole-rock analyses, and stable isotope geochemical studies. 73 �Figure 1-31. The integrated ore deposit model for the Nokomis deposit. Figure 1-32. Copper-Nickel assays (A), normalized sulfide mineralogy calculated at 100% sulfide (B), and sulfide fractionation model for Nokomis drill hole MEX-084. 74 �Figure 1-33. Copper-Nickel assays (A), normalized sulfide mineralogy calculated at 100% sulfide (B), and sulfide fractionation model for Nokomis drill hole MEX-109. DRILL CORE DISPLAY Intervals from four drill holes will be on display in the Duluth Metals drill core logging shed in Ely Minnesota. These holes were chosen to display typical aspects of the four styles of mineralization outlined above that occur throughout the deposit. The locations of the four holes (MEX-084, MEX-109, MEX-141, and MEX-154) are given in Figure 1.25 and press released assay data for these holes are given in Table 1-15. The only drill hole on display that has exceptional textures and/or Cu-Ni-PGE grades for the defined style of mineralization (Fig. 1-31) is MEX-084, which is perhaps the finest example of mineralization in the footwall Giants Range batholith ever drilled in the Duluth Complex. The other drill holes (MEX-109, MEX-141, MEX-154), display typical textures and Cu-Ni-PGE grades (Table 1-15) within the Nokomis Deposit. 75 �We at Duluth metals invite you to examine these drill hole intervals and think about the physical and chemical processes that came together to create truly one of the world’s largest resources of Cu-Ni-PGE (Fig. 1-26). Herein at the Nokomis deposit, a channelized and crystal-laden (plagioclase and olivine phenocrysts) basaltic melt containing sulfide droplets intruding sub-horizontally as a sill-like body between a granitoid footwall and anorthosite hangingwall. The physical magmatic processes that the system underwent imparted discrete thermal and chemical processes onto the system. Many of these combined processes can be seen as snapshots in time and place in the drill hole intervals presented. Table 1-15. Composite assays from the four selected Duluth Metals drill holes on display. Details of the Interval From (ft) To (ft) Int. (ft) Cu (%) Ni (%) Au (g/t) Pt (g/t) Pd (g/t) TPM (g/t) Ag (g/t) MEX-141 Drill hole in the magma channel in the PGE-rich Eastern High Grade Zone 0.3% Cu cut-off 4032.0 4202.0 170.0 0.779 0.246 0.131 0.296 0.718 1.146 2.981 0.5% Cu cut-off 4032.0 4192.0 160.0 0.809 0.251 0.136 0.309 0.747 1.192 3.152 including 4147.0 4182.0 35.0 1.201 0.436 0.158 0.352 0.957 1.468 3.907 including 4177.0 4182.0 5.0 2.050 1.280 0.134 0.700 2.080 2.914 5.300 MEX-84 Drill hole beneath the magma channel with abundant footwall mineralization 0.3% Cu cut-off 2849.7 3115.4 265.7 0.691 0.250 0.097 0.184 0.423 0.704 2.452 0.5% Cu cut-off 2859.6 3110.4 250.8 0.705 0.258 0.100 0.187 0.433 0.720 2.515 including 3041.5 3110.4 68.9 1.033 0.270 0.146 0.286 0.637 1.068 4.082 including 3041.5 3071.0 29.5 1.209 0.367 0.188 0.334 0.670 1.192 4.559 MEX-109 Drill hole into Top-Loaded mineralization in the center of the property 0.3% Cu cut-off 3483.0 3668.0 185.0 0.760 0.227 0.130 0.260 0.602 0.991 2.903 0.5% Cu cut-off 3493.0 3648.0 155.0 0.828 0.247 0.143 0.286 0.660 1.089 3.150 including 3498.0 3538.0 40.0 0.955 0.278 0.213 0.414 0.901 1.527 3.863 MEX-154 Drill hole into thick Top-Loaded mineralization in the Western portion of the property 0.3% Cu cut-off 1627.0 1937.0 310.0 0.609 0.179 0.065 0.117 0.260 0.443 2.408 0.5% Cu cut-off 1627.0 1797.0 170.0 0.690 0.196 0.073 0.129 0.287 0.490 2.757 including 1667.0 1682.0 15.0 1.020 0.289 0.156 0.188 0.446 0.790 3.833 0.5% Cu cut-off 1882.0 1937.0 55.0 0.582 0.166 0.072 0.120 0.268 0.460 2.141 76 �REFERENCES Desaultels, P., and Patelke, R., 2008, Updated Technical Report on the NorthMet Deposit, Minnesota, USA, by Wardrop Engineering, (Toronto) for PolyMet, 109 p. 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Graber, R., 1993, LTV Steel Mining Company, (abst.), Institute on Lake Super Geology, Geology and Taconite Mines of the Mesabi Range, Field Trip Guide, pages 39-45. Hauck, S.A., Severson, M.S., Zanko, L.M., Barnes, S.-J., Morton, P., Aliminas, H.V., Foord, E.E., and Dahlberg, E.H., 1997, An overview of the geology and oxide, sulfide, and platinum-group element mineralization along the western and northern contacts of the Duluth Complex, in Ojakangas, R.W., Dickas, A.B., and Green, J.C., eds., Middle Proterozoic to Cambrian rifting, central North America: Geological Society of America Special Paper 312, p. 137-185. Holst, T.B., Mullenmeister, E.E., Chandler, V.W., Green, J.C., and Weiblen, P.W., 1986, Relationship of structural geology to the Duluth Complex to economic mineralization: Minnesota Department of Natural Resources, Division of Minerals Report 241-2. 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Lehr, J.D., and Hobbs, H.C., 1992, Glacial geology of the Laurentian divide area, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Field Trip guidebook series, 18, Prepared for the 39th Midwest Friends of the Pleistocene Field Trip, Biwabik, Minnesota, 1992, 73 p., map, 1:250,000. Listerud, W.H., and Meineke, D.G.., 1977, Mineral resources of a portion of the Duluth Complex and adjacent rocks in St. Louis and Lake Counties, northeastern Minnesota: Minnesota Department of Natural Resources, Division of Minerals, Report 93, 74p. Lucente, M.E., and Morey, G.B., 1983, Stratigraphy and Sedimentology of the Lower Proterozoic Virginia Formation, northern Minnesota: Minnesota Geological Survey Report of Investigations 28, 28 p. Mancuso, J.D., and Dolence, J.D., 1970, Structure of the Duluth Gabbro Complex in the Babbitt area, Minnesota (abs.): 16th Annual Institute on Lake Superior Geology, Thunder Bay, Ontario, p. 27. Marma, J.C., Brown, P.E., and Hauck, S.A., 2002, Magmatic and hydrothermal PGE mineralization of the Birch Lake Cu-Ni-PGE deposit in the South Kawishiwi intrusion, Duluth Complex, northeast Minnesota: Paper No. 52-2 presented at the 2002 Annual GSA meeting, Denver, October 27-30, 2002. 77 �Martineau, M. P., 1989, Empirically derived controls on Cu-Ni mineralization: A comparison between fertile and barren gabbros in the Duluth Complex, Minnesota, U.S.A.: in Prendergast, M. D., and Jones, M. J., eds., Magmatic Sulphides - The Zimbabwe Volume: Inst. Min. Metall., London, p. 117-137. Miller, J.D., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Geological map of the Duluth Complex and related rocks, Northeastern Minnesota; Minnesota Geological Survey, Miscellaneous Map M119, scale 1:200,000. Miller, J.D., Jr. and Severson, M.J., 2002, Geology of the Duluth Complex in Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002a, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations RI-58, p. 106-143. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002a, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations RI-58, 207 p. Miller, J.D., Jr., Severson, M.J., and Foose, M.P., 2002, Geologic map of the Babbitt NE 7.5’ quadrangle, Lake and St. Louis Counties, Minnesota. Minnesota Geological Survey Open-file Map, 1:24,000 scale. Muhich, T.G., 1993, Movement of titanium across the Duluth Complex–Biwabik Iron Formation contact at Dunka Pit, Mesabi Iron Range, northeastern Minnesota: Duluth, Minn., University of Minnesota Duluth, M.S. thesis, 154 p. Naldrett, A.J., 2004, Magmatic Sulphide Deposits: Geology, geochemistry, and exploration: Springer-Verlag, Berlin, Germany, 727 pages, ISBN 3–540–22317–7. Park, Y.R., Ripley, E.M., Severson, M.J., and Hauck, S.A., 1999, Stable isotopic studies of mafic sills and Proterozoic metasedimentary rocks located beneath the Duluth Complex, Minnesota: Geochimica et Cosmochimica Acta, v. 63, no. 5, p. 657-674. Patelke, R.L., 2003, Exploration drill hole lithology, geologic unit, copper-nickel assay, and location database for the Keweenawan Duluth Complex, northeastern Minnesota: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report, NRRI/TR-2003/21, 96 p. Patelke, R.L., and Geerts, S.D.M., 2006, PolyMet NorthMet Drill Hole / Geological Database Recompilation: Location, Downhole Survey, Assay, Lithology, Geotechnical Data, and Related Information, 2004 to 2006, PolyMet Mining Inc. Internal report, 86 p. Peterson, D.M. and Albers, P.B., 2007, South Kawishiwi Intrusion Cu-Ni-PGE mineralization in association with the Nickel Lake Macrodike, Institute on Lake Superior Geology, 53rd Annual Meeting, Field Trip Guidebook, Lutsen, Minnesota, Volume 53. Peterson, D.M., 2001a, Development of a conceptual model of Cu-Ni-PGE mineralization in a portion of the South Kawishiwi Intrusion, Duluth Complex, Minnesota: Laurentian University – Society of Economic Geologists, Second Annual PGE Workshop, Sudbury, Ontario. Peterson, D.M., 2001b, Copper-Nickel-PGE mineral potential of the eastward extension of the Maturi Cu-Ni deposit, Duluth Complex, Lake County, Minnesota; Natural Resources Research Institute, Confidential Report of Investigations NRRI/RI-2001-02, 29 pages, 15 plates, 1 CD-rom. Peterson, D.M. and Severson, M.J., 2002, Chapter 4, Archean and Paleoproterozoic rocks forming the footwall of the Duluth Complex, in Geology and mineral potential of the Duluth Complex and related intrusions of northeastern Minnesota, Minnesota Geological Survey, Report of Investigations 58, pp. 76-93. Peterson, D.M., 2008, Bedrock geologic map of the Duluth Complex in the northern South Kawishiwi intrusion and surrounding area, Lake and St. Louis Counties, Minnesota: Natural Resources Research Institute, Map Series NRRI/MAP-2008-01, scale 1:20,000. Peterson, D.M., Albers, P.B., and White, C.R., 2006, Bedrock geology of the Nickel Lake Macrodike and adjacent areas: Lake County, Northeastern Minnesota: Natural Resources Research Institute, Map Series NRRI/MAP-2006-04, scale 1:10,000. 78 �Phinney, W.C., 1972, Duluth Complex, history and nomenclature, in Sims, P. K., and Morey, G. B., eds., Geology of Minnesota: A Centennial Volume: Minn. Geol. Survey, pp. 333-334. Ripley, E.M., 1986, Origin and concentration mechanisms of copper and nickel in Duluth Complex sulfide zones – a dilemma: Economic Geology, v. 81, p. 974-978. Routledge, R.E., 2004, Review of the mineral resources of the Birch Lake Property, Minnesota, U.S.A. prepared for Franconia Minerals Corporation: Roscoe Postle Associates Inc. NI 43-101 technical report filed on SEDAR, January 22, 2004, 92 p. Sabelin, T., and Iwasaki, I., 1985, Metallurgical evaluation of chromium-bearing drill core samples from the Duluth Complex (Contract report of Minnesota Department of Natural Resources, Division of Minerals): Minerals Resources Research Center, University of Minnesota, Minneapolis, MN, 58 p. Sabelin, T., and Iwasaki, I., 1986, Evaluation of platinum group metal occurrence in Duval 15 drill core from the Duluth Complex: Internal report, Minerals Resources Research Center, University of Minnesota, Minneapolis, MN, 23 p. Severson, M.J., 1988, Geology and structure of a portion of the Partridge River Intrusion, northeastern Minnesota: Natural Resources Research Institute, Univ. Minn., Duluth, Tech. Rept., NRRI/GMIN-TR-88-08, 78 p. Severson, M.J., 1994, Igneous stratigraphy of the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/TR 93/34, 210 p. (with plates) Severson, M.J., and Barnes, R.J., 1991, Geology, mineralization and geostatistics of the Minnamax/Babbitt Cu-Ni deposit (Local Boy area), Minnesota, Part II: Mineralization and geostatistics: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report, NRRI/TR-91/13b, 216 p. Severson, M.J., and Hauck, S.A., 1990, Geology, geochemistry, and stratigraphy of a portion of the Partridge River intrusion: Natural Resources Research Institute, University of Minnesota-Duluth, Technical Report, NRRI/GMIN-TR-89-11, 236p. (with plates). Severson, M.J., and Hauck, S.A., 2003, Platinum group elements (PGEs) and platinum group minerals (PGMs) in the Duluth Complex: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-2003/37, 296 p. Severson, M.J., and Hauck, S.A., 2008, Finish logging of Duluth Complex drill core (And a reinterpretation of the geology at the Mesaba (Babbitt) deposit): University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-2008/17, 68 p. Severson, M.J., and Miller, J.D., Jr., 1999, Bedrock geologic map of Allen quadrangle, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-91, scale 1:24,000 Severson, M.J., Miller, J.D., Jr., Peterson, D.M., Green, J.C., and Hauck, S.A., 2002, Mineral potential of the Duluth Complex and related intrusions in Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002a, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations RI-58, p. 164-200. Severson, M.J., Patelke, R.L., Hauck, S.A., and Zanko, L.M., 1994a, The Babbitt copper-nickel deposit, Part B: Structural datums: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/TR-94/21b, 48 p. Severson, M.J., Patelke, R.L., Hauck, S.A., and Zanko, L.M., 1994b, The Babbitt copper-nickel deposit, Part C: Igneous geology, footwall lithologies, and cross-sections: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/TR-94/21c, 79 p. Sims, P.K., and Morey, G.B., eds. 1972, Geology of Minnesota: A Centennial Volume, Minnesota Geological Survey. Tharalson, E., Sweet, G., Boisjoli, T., Lentz, B., Fellows, T., and Peterson, D., 2007, Geological Map of the Nickel Lake Macrodike and Northern Bald Eagle Intrusion, Lake County, Northeastern Minnesota: Precambrian Research Center Map Series PRC/MAP-2007-01, scale 1:10,000. 79 �Turner, F.J., 1968, Metamorphic Petrology – mineralogical and field aspects: McGraw-Hill Book Company, New York, 403 p. Watowich, S.N., 1978, A preliminary geological view of the Minnamax copper-nickel deposit in the Duluth Gabbro at the Minnamax project: in Graven, L.K., compiler, Productivity in Lake Superior Mining, 39th Proc., Annual Mining Symposium, University of Minnesota, Minneapolis, p. 19.1-19.11. White, C., in prep, The Nokomis Deposit, a Masters of Geology thesis: University of Minnesota, Duluth. Wolff, J.F., 1917, Recent geologic developments on the Mesabi range, Minnesota: American Institute of Mining and Metallurgical Engineers Transactions, v. 56, p. 142-169. Zanko, L.M., Severson, M.J., and Ripley, E.M., 1994, Geology and mineralization of the Serpentine copper-nickel deposit, Duluth Complex, Minnesota: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report, NRRI/GMIN-TR-93-52, 90p. 80 �55th Annual Institute on Lake Superior Geology Field Trip 2 GLACIAL GEOLOGY OF THE VERMILION MORAINE 3D interpretation of the location of the Laurentide ice sheet in the Soudan Mine area at the time of the development of the Vermilion Moraine (modified from Peterson and Patelke, 2003). Phil Larson (Cliffs Natural Resources) Howard Mooers (Department of Geological Sciences, UMD) 81 �FIELD TRIP 2 GLACIAL GEOLOGY OF THE VERMILION GREENSTONE BELT Phillip C. Larson (Cliffs Natural Resources) Howard D. Mooers (Department of Geological Sciences, University of Minnesota-Duluth) INTRODUCTION This field trip provides an introduction to the glacial geology and glacigenic sedimentary environments of the Vermilion district of northeastern Minnesota. The bedrock geology and topography of this area are characteristic of millions of square km of glaciated Precambrian shield terranes in Canada, Scandinavia, and the United States. Overlying bedrock in the belt is an assemblage of glacigenic sediments characteristic of a great portion of the glaciated shields on earth. In recent years, these characteristics have allowed the glaciated Vermilion greenstone belt to serve as a virtual laboratory for investigating the nature and scale glacial erosion and entrainment, and transport processes. Metavolcanic and metasedimentary rocks of the Vermilion district are also notably prospective for volcanogenic massive sulphide and shear-hosted lode gold deposits (Peterson and Jirsa 1999; Peterson, 2001; Peterson and Patelke 2004). Mineralized clasts and geochemical anomalies related to such deposits are evident in glacigenic deposits overlying and down-ice of the greenstone belt. The presence of anomalous mineralization, combined with the well-described nature of glacigenic sediment dispersal, in the district provides a unique opportunity to demonstrate the techniques by which till geochemical and mineralogical data can be incorporated into an integrated mineral exploration program. This field trip visits a selection of sites that illustrate the range of glacigenic sediment types present in this typical glaciated shield terrane, as well as their compositional heterogeneity. A second theme of the trip is the collection and manipulation of the types of data useful to mineral exploration in glaciated terranes. REGIONAL BEDROCK GEOLOGY (The following overview of regional bedrock geology is largely taken from Peterson, Jirsa, and Hudak (2009), included in this guidebook.) Supracrustal rocks in the Vermilion district consist of volcanic-dominated stratigraphic sequences of the Wawa subprovince of the Superior Province of the Canadian Shield (Fig. 2-1). Rocks of the Wawa subprovince in northern Minnesota are divided on the basis of stratigraphic and structural setting into: (1) the Soudan belt, to the south, and (2) the Newton belt, to the north (Jirsa et al., 1992; Southwick et al., 1998). The two belts are fault-bounded, and the relationship between stratigraphic units within each belt is largely conformable, although faults obscure contacts locally. The boundary between these contrasting structural panels occurs along the Mud Creek shear zone (Hudleston et al., 1988), small segments of the Vermilion and Wolf Lake faults (Sims and Southwick, 1985), and the Bear River fault (Jirsa et al., 1992). The Soudan belt contains large, broad folds involving calc-alkalic and tholeiitic volcanic strata overlain by, and locally interdigitated with, turbiditic rocks. In contrast, the Newton belt consists of elongate, northeast-trending, and mostly northward-younging volcanic and volcaniclastic sequences. Volcanic rocks of the Newton belt differ from those of the Soudan belt in containing locally abundant komatiitic flows and peridotitic sills. 82 �To the south, supracrustal rocks of the Soudan belt are intruded by syn- to post-tectonic granitoid rocks of the Giants Range batholith. At the northern edge of the Vermilion district, the Vermilion fault marks the boundary with granite, granite-rich migmatites, migmatites, paragneisses, and biotite schists of the Quetico subprovince of the Superior Province of the Canadian Shield (Jirsa and Boerboom, 2003). Figure 2-1. Bedrock geology of the Vermilion district, modified from Morey and Meints, 2000. GLACIAL HISTORY OF THE VERMILION DISTRICT In common with most of the high latitude regions of North America, the Vermilion district was repeatedly glaciated during the ice ages of the Pleistocene Epoch. Glacigenic sediments and landforms in the Vermilion district are associated with the Rainy Lobe of the Laurentide ice sheet. While there are a number of possible definitions of what constitutes the Rainy Lobe – sedimentological, textural, compositional, and association with particular geomorphic features – a definition rooted in glacial dynamics perhaps works best. In this sense, the Rainy Lobe refers to that portion of the Laurentide ice sheet lying northwest of Lake Superior (occupied by the Superior Lobe), and east of the Winnipeg Basin and Red River Valley (occupied by the Red River Lobe). In common, Rainy Lobe landforms and glacigenic sediments reflect a general northeast to southwest ice flow direction, and a Labradoran (northeastern) sediment provenance. Preglacial Environment In common with much the Canadian Shield, glacial erosion has nearly completely stripped preglacial regolith from bedrock in the Vermilion district. However, preglacial saprolites are a common occurrence 83 �underlying glacigenic sediments in central and western Minnesota; the nearest such occurrences are exposed in open pit mines of the Mesabi Range, on the south flank of the Giant’s Range. Massive iron oxide mineralization exposed in surface workings of the Consolidated Vermilion mine (near Stops 2-6 and 2-7) contains voids lined by botryoidal goethite. The botryoidal goethite occurs as dripstone-like structures, suggesting that it precipitated in open spaces in the vadose zone. Inasmuch as the water table of the preglacial peneplained Canadian Shield was likely similar to the modern, this occurrence supports the hypothesis that little erosion of unweathered bedrock was accomplished by multiple glaciations through the course of the Pleistocene. The topography observed in the Vermilion district today therefore likely represents that of the base of the preglacial weathering regolith. Pre-Late Wisconsinan Throughout the field trip area, no till or glaciofluvial sediments older than those deposited during the Late Wisconsinan retreat of the Laurentide ice sheet have been described. However, circumstantial evidence of the past existence of pre-Late Wisconsinan glacigenic sediments are present, and pre-Late Wisconsinan glacigenic sediments are preserved a few 10s of km to the south. Winter (1971) and Winter and others (1973) described a dark-colored, sandy-silty calcareous till in exposures in open pit mines on the Mesabi Iron Range. Inasmuch as this till, where present, occurred immediately above bedrock, they referred to it as the “basal till”. Stark (1977) and Lehr and Hobbs (1992) described occurrences of Winter’s basal till in exposures in the Dunka Mine, approximately 20 km south of the area encompassed by the current field trip. The matrix of Winter’s basal till is calcareous, and the pebble fraction contains carbonate clasts in addition to the granitic and metamorphic lithologies typical of Rainy Lobe tills. A northeast-southwest pebble fabric in Winter’s basal till strongly supports a northeastern provenance for this till. In this case, carbonate in pebbles and till matrix is derived from Paleozoic carbonates in the Hudson Bay Lowlands. Surficial Rainy Lobe tills in the region are typically non-calcareous, leading some workers to suggest that calcareous tills in the region must have a Keewatin (northwestern) provenance. However, it is more likely that calcareous Labradoran (northeastern) provenance tills are the norm in northeastern Minnesota, with the relatively thin, noncalcareous tills deposited during the Late Wisconsin the exception (Mooers and Lehr 1997, Larson and Mooers 2005, Larson and Mooers 2008). Mooers and Lehr (1997) and Larson and Mooers (2008) noted the closed associated between carbonate and distinctive clasts of Paleoproterozoic greywacke in HBL provenance tills. In many cases, the greywacke clasts are able to survive englacial comminution and weathering better than the carbonates, and only greywacke clasts remain. The presence of greywacke clasts in till is therefore strong evidence that the material forming the till was once calcareous. The presence of calcareous Labradoran provenance tills to the southwest (Mooers and Lehr 1997), and to the northeast (northwestern Ontario) (Karrow and Geddes 1987), suggests that a continuous sheet of calcareous till once extended across the Vermilion greenstone belt. However, by Late Wisconsinan time, much of this calcareous till had been remobilized and removed from the Canadian Shield, including the field trip area (Larson and Mooers 2008). Late Wisconsinan By no earlier than 15 ka, the Rainy and Superior Lobes began to retreat from the St. Croix Moraine in central Minnesota (Clayton and Moran 1982). By the time the St. Louis Sublobe advanced from the northwest into the basins of Glacial Lake Upham I and Aitkin I, the southern margin of the LIS was roughly coincident with the Giant’s Range (Fig. 2-2). 84 �Figure 2-2. Surficial geology of the Vermilion district. Prior to retreating to the Giant’s Range and the Laurentian divide, the topography generally sloped away from the margin of the LIS. Consequently, meltwater tended to be efficiently channeled away from the ice margin. However, once the margin retreated to the north of the height of land, meltwater ponded in front of the ice sheet, forming proglacial lakes, with the surface elevation of these lakes controlled by the elevation of the outlet spilling over the height of land. The first proglacial lake formed north of the Giant’s Range – Glacial Lake Norwood – was the first in a succession of lakes and lake stages that characterized the margin of the LIS until the final drainage of Glacial Lake Ojibwe. The retreat of the LIS margin north of the height-of-land marked a momentous change in the nature of sediment and landforms associated with the ice margin. South of the height-of-land, ice margin sedimentlandform assemblages are dominantly hummocky moraines composed of till and outwash channels, fans, and plains composed of glaciofluvial sediment. North of the height-of-land, ice margin sedimentlandform assemblages are dominantly sharp-crested moraines composed of glaciofluvial sediments and proglacial plains composed of glaciolacustrine sediment. Glacial Lake Norwood Once the margin of the LIS retreated north of the Giant’s Range, meltwater ponded to form Glacial Lake Norwood (GLN) (Winchell 1901), which drained to the south through the Embarrass Gap. The initial level of GLN was approximately 1475 feet, indicating that the area south and west of Big Rice Lake was 85 �still occupied by ice (Lehr and Hobbs 1992). This stage of the lake likely corresponds to the ice marginal position marked by the Big Rice moraine. Retreat of the LIS from the Big Rice moraine to the Wahlsten moraine roughly coincided with a drop in the level of GLN to 1450 feet (Lehr and Hobbs 1992). The drop in lake level was triggered in part by downcutting of the outlet through the Embarrass Gap. Further downcutting of the Embarrass Gap outlet was prevented since the outlet was draining into Glacial Lake Upham II at the time, also at roughly 1450’ elevation. Continued outflow through the Embarrass Gap indicates the presence of glacial ice in the area north of Nashwauk and Hibbing. GLN persisted at the 1450 foot level during the retreat and re-advance of the LIS to the Vermilion moraine. As ice and debris covered ice melted north of the Giant’s Range the lake expanded to the west. This larger lake is referred to as Glacial Lake Koochiching (GLK) (Hobbs 1983), in reference to its being fronted by Keewatin (northwestern) provenance ice in southern Koochiching County. The highest level of GLK (Mizpah stage) formed strandlines at 1430 feet elevation as far west as southwestern Koochiching County (Hobbs 1983). Lowering of the level of Glacial Lake Upham II allowed continued downcutting of the Embarrass Gap outlet to 1430 feet, and later to 1400 feet (Lehr and Hobbs 1992). Melting of stagnant ice and debris-cored ice in northeastern Itasca County ultimately opened a new outlet to GLN/GLK southward through the Prairie River. Opening and downcutting of the Prairie River outlet rapidly lowered GLK to about 1350 feet elevation (Gemmell stage) (Hobbs 1983). The LIS probably did not retreat from the Vermilion moraine prior to opening of the Prairie River outlet, as glaciolacustrine sediments are not found north of the moraine. Persistence of a proglacial lake draining through the Embarrass Gap resulted in much of the area between the Vermilion moraine and the Giant’s Range being wave-washed, or mantled by glaciolacustrine sediment. Big Rice moraine The first moraine to the north of the Giant’s Range is the Big Rice moraine. The moraine orientation is roughly east-west, indicating ice flow was from north to south (180°). The LIS margin was for the most part subaerial while this moraine formed, as GLN was of limited extent. Consequently, most of this moraine is comprised of hummocky meltout till. Björck (1990) radiocarbon dated basal organic sediment from Heikkila Lake at 12,100±150 ka (LU-2556) (14.1±0.5 ka cal yr bp). Heikkila Lake is a kettle lake hosted in the moraine, so this date represents a minimum age on moraine stabilization. Wahlsten moraine The second moraine to the north of the Giant’s Range is the Wahlsten moraine. Similar to the Big Rice moraine, the Wahlsten moraine’s orientation is roughly east-west, indicating continued ice flow to the south (180°). The Wahlsten moraine is generally lower in elevation is lower than the Big Rice, so a significantly greater portion of the LIS margin fronted GLN. To the west, where the ice margin fronted the lake, the moraine is composed of coalescing ice-contact deltas. Here the moraine has a sharp, symmetrical cross sectional profile, and is composed predominantly of glaciofluvial sands and gravels. To the east, the moraine is composed of hummocky meltout till, fronted by proglacial outwash fans. Basal organic radiocarbon dates from Lempia Lake, a kettle lake immediately south of the moraine, are 12,050±240 ka (Lu-2555) and 11,550±550 ka (Lu-2502) (14.1±0.7 ka cal yr bp) (Björck 1990). As with the Heikkila Lake date, this like represents a minimum age of landscape stabilization. Vermilion moraine The Vermilion moraine is the third prominent moraine to the north of the Giant’s Range. Unlike the Big Rice and Wahlsten moraines, Vermilion moraine orientation is roughly west-northwest to east-southeast. 86 �This change in moraine orientation is reflected in a change in ice flow direction, to approximately 210°. Reorientation of the ice flow direction was possibly triggered in part by faster recession of the LIS from the Wahlsten moraine in the area fronted by GLN. Asymmetric retreat and re-advance to the Vermilion moraine resulted in a slight inflection in the ice margin orientation in the vicinity of Bear Head Lake. This inflection in turn produced a slight trough in the surface of the LIS, extending in the up-ice direction from Bear Head Lake, including past the general vicinity of Ely. This slight trough in the LIS resulted in enormous volumes of surface meltwater and glacial debris, captured from a disproportionately large area of the ice sheet, being channeled to a single discharge point. A large sediment fan, composed of sandy, pebbly, cobble, and boulder gravel was deposited in front of the Vermilion moraine (Lehr and Hobbs 1992), in part by evacuating stagnant ice north of the Wahlsten moraine, and a segment of the Wahlsten moraine itself. West of Soudan, the LIS margin fronted GLN, and the moraine is composed predominantly of coalescing ice-contact deltas. In this area, the moraine has a sharp, symmetrical cross-sectional profile., and is composed primarily of glaciofluvial sands and gravels. East of Soudan, the LIS margin was subaerial, and the moraine is composed of hummocky meltout till and associated small, proglacial outwash fans. The age of the Vermilion moraine is poorly constrained, however a basal radiocarbon date on woody material from a kettle lake in the Vermilion moraine southeast of Ely is 12,000±85 ka (ETH-29845) (13.8± 0.1 ka cal yr bp) (Lowell and others, in press), placing a minimum age on moraine stabilization. The next prominent moraine set formed by the LIS after the Vermilion greenstone belt are the EagleFinalyson-Steep Rock moraines of northwestern Ontario, 100 km to the northeast (Zoltai 1965). Björck (1985) dated obtained a bulk sediment radiocarbon date of 11,100±110 ka (WIS-1375) (13.0±0.2 ka cal yr BP) from a lake on the down-ice of these moraines, suggesting the area to the north of the Vermilion moraine was deglaciated by that time. GLACIGENIC SEDIMENT IN THE VERMILION DISTRICT In the Vermilion district, Rainy Lobe tills are pebble- and boulder-rich, with a sandy matrix. The lithologic and geochemical composition of tills and associated glaciofluvial sediments is highly heterogeneous, reflecting the similarly heterogeneous regional bedrock composition. Tills generally lack a significant component of clay-size grains, or clay minerals. The coarse-grained nature of tills, and lack of clay-size material, result in high permeability and poor compaction – even in lodgment tills. Spatially, the distribution of glacigenic sediment is quite variable, with till, glaciofluvial, and even glaciolacustrine sediment widely distributed through the region. Sediment cover is patchy, with abundant bedrock outcrop, particularly in the areas north of the Vermilion moraine and above the level of Glacial Lake Norwood. Although glacigenic sediment may locally be several meters thick, it typically averages ~1 m. Rainy Lobe provenance tills in the Vermilion district are essentially non-calcareous, and have a very low acid buffering capacity. This, combined with their high permeability, results in deep weathering, particularly of labile sulphide minerals. The spruce, fir, and pine forests common to the area favor formation of podzolic soils. Most soil profiles evidence an Of horizon overlying a bleached Ae horizon. Even the thickest soil profiles in till lack a true C horizon, with most displaying weak iron oxide staining (presumably due to weathering of iron sulphide minerals). In many cases, a C horizon is completely absent, with a vibrant red Bf horizon resting directly on bedrock. When sampling till for geochemical 87 �analysis, the weathered C horizon is the preferred sampling media. Where the C horizon is absent or inaccessible, care is taken to sample the Bf horizon. Erosion rates Larson and Mooers (2004) estimated a bedrock erosion and entrainment rate of ~10 mm·a-1 during the period the Laurentide ice sheet was actively eroding, entraining, and transporting sediment to the Vermilion moraine, a rate comparable to that of modern temperate glaciers in Alaska (Hallet et al. 2006). Assuming active erosion and entrainment occurred at that rate over a modest mean flowline length of 10 km, as much as 300 tons of debris per meter length per year were delivered to the Vermilion moraine. This debris delivery rate is sufficient to construct the observed Vermilion moraine in as little as 100 years. Clearly, such high erosion and entrainment, and sediment delivery rates could not have acted over either a large area of the ice sheet’s bed, or over a long period of time. This provides strong support to the hypothesis that till and glaciofluvial sediment deposits and landforms associated with the Vermilion moraine formed over a relatively short period of the history of the Laurentide ice sheet – likely only a few centuries. In this sense, surficial sediments in the VGB and elsewhere on glaciated shield terranes represent a snapshot in time in the long history of both the Pleistocene CHARACTERIZATION OF GLACIGENIC SEDIMENTS FOR DRIFT EXPLORATION Incorporation of geochemical and mineralogical data collected from glacigenic sediments into an integrated mineral exploration program is a key to successful exploration in glaciated terranes. In common with surficial sediment sampling programs in other geomorphic environments, the three critical questions an explorer seeks to answer over the course of a drift exploration program are: • • • What is the significance of a particular anomaly? How far has the material been transported? What direction was the material transported? The answers to these questions can be more systematically determined in a glaciated terrane than in any other surficial environment. In this sense glaciation, far from obscuring bedrock mineralization, provides an important tool for detecting and vectoring to bedrock mineralization. Mean transport length Till in the VGB is characterized by extreme compositional variability, reflecting the composition of local bedrock. A useful concept for quantifying this variability is the concept of a mean transport length – the distance from within which 50% of the material in the till has been derived. Mean transport length has been characterized for the VGB on two different size fractions of till. This is accomplished by plotting the concentration of a till component derived from a known source (indicator) against the distance along a glacial flow line down-ice of the source. In the case of the VGB, the extreme contrast in composition between granitoid and high-grade metamorphic rocks of the Vermilion Batholith to the north of the Vermilion fault, and the greenschist grade metavolcanic and metasedimentary rocks of the VGB to the south of the Vermilion fault provides a widespread, but easily recognizable, indicator in the form of the lithic and geochemical components derived from the Vermilion Batholith. In particular, potassium feldspar-bearing coarse-grained granitoid clasts in tills from the VGB are uniquely derived from north of the Vermilion fault. In addition, rocks of the Vermilion Batholith are characterized by relatively high potassium content (~3.4% K2O) relative to those of the VGB (~1.7% K2O). The concentration of both indicators in till decrease systematically with distance from the Vermilion fault; this decrease can be mathematically approximated using the equation: 88 �ciT >0 = ciT =0 e − aT , (1) Where ci is the indicator concentration, T is the transport length, and a is a dimensionless constant. Solving for T at the point at which ciT>0 = 0.5ciT=0 gives an estimate of the transport length from within which 50% of the material in the till is derived. In the case of K2O in the -63 micron fraction of till, this relationship is: K 2 O = 1.73 + 1.66e −0.225T , (2) where T is transport length in km, indicating a mean transport length for the -63 micron fraction of ~3.0 km (Fig. 2-3) (Larson 2004). (The factor 1.73 is the apparent mean K2O concentration in weight percent of rocks of the greenstone belt.) 4.5 4.0 K2O (wt %) 3.5 3.0 2.5 2.0 1.5 1.0 -2000 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 Transport Length (m) Figure 2-3. K2O concentration in the -63 micron fraction of tills in the VGB, plotted against distance in the ice-flow direction from the Vermilion fault. (Larson 2004.) 0.7 y = 0.592e-0.270x Granitoid fraction 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -2 0 2 4 6 8 10 12 14 Transport Length (km) Figure 2-4. Granitoid concentration in the 2-4 mm fraction of tills in the VGB, plotted against distance in the iceflow direction from the Vermilion fault. (unpublished data from 2008 PRC Field Camp students) 89 �In the case of granitoid clasts in the 2-4 mm fraction of till: c gran = 0.592e −0.270T , (3) where the background granitoid fraction of tills in the greenstone belt is 0.0, indicating a mean transport length for 2-4 mm pebbles of ~2.6 km (Fig. 2-4) (unpublished data from 2008 PRC field camp students). These two independently derived estimates provide a robust assessment of the mean transport length of till-forming material in the Vermilion district. Ice flow direction Measurements of striae and grooves in bedrock indicate preservation of a number of predominant orientations, corresponding to a number of paleo ice flow directions. Between the Vermilion and Wahlsten moraines, striae cluster between 180-185°, roughly normal to the east-west orientation of the Wahlsten moraine. Immediately south and to the north of the Vermilion moraine, striae cluster between 200-225°, with a predominant mode between 210-215°; this orientation is roughly normal to the WNW to ESE orientation of the Vermilion moraine. Scattered throughout the area, a very few striae have been measured oriented ~240°; where present, these striae orientations are overprinted by the 180-185° and 210-215° striae sets (Fig. 2-5). Figure 2-5. Rose diagram of ice flow indicator (striae and grooves) orientations in the Vermilion district. The multiple ice flow directions recorded in striae and moraine orientations are interpreted to record changing ice flow directions during retreat of the Rainy Lobe. The older 240° orientation is interpreted to represent ice flow direction when the area was completely covered by the Laurentide ice sheet, and the ice margin was located a significant distance to the southwest. The 180° orientation reflects the ice flow direction when the ice margin was located at the Big Rice and Wahlsten moraines, while the 210° orientation reflects the ice flow direction during the readvance to the Vermilion moraine. North of the Vermilion moraine, striae in the 200-225° cluster varies. At a given outcrop, or cluster of outcrops, individual striae orientations typically vary over a range of ~5°. However, over length scales of ~1 km, striae cluster orientations may vary up to ~20°. This may be the product of spatial and temporal variation in erosion rates as ice flow direction changed during retreat and re-advance of the Rainy Lobe to the Vermilion moraine. 90 �Mineral endowment Considered at a sufficiently large scale, the composition of till broadly reflects that of the substrate underlying the ice sheet. In the case of a system where mean transport length is relatively short, till composition reflects that of the local bedrock. A sufficiently large sample of till compositional data from such an area therefore represents a statistically meaningful characterization of bedrock composition. In addition to providing important information regarding threshold values for background and anomalous concentrations, comparison of a sample population from a particular area to a reference population may provide information regarding the relative endowment of elements of economic interest. Rencz and others (2006) present a useful compendium of the statistical characteristics of a number of geochemical variables for the -63 micron fraction of tills in Canada. They included in excess of 13,000 till samples compiled from over 50 separate surveys. Bajc (1999, 2000) collected nearly 700 -63 micron till geochemical samples from the Shebandowan greenstone belt, a Wawa subprovince supracrustal sequence located 150 km northeast of the Vermilion district. Larson (2004) reported 150 -63 micron till geochemical analyses collected in the Vermilion district. Comparison of these three data sets provides an assessment of the relative thresholds for background and anomalous geochemical values of tills in the Vermilion district. 1.00 Percentile rank 0.95 0.90 Shebandow an Vermilion 0.85 Canada 0.80 0.75 10 100 1000 Ni (ppm ) -63 m icron till Figure 2-6. Relative abundance of nickel in the -63 micron fraction of till. Data from Rencz and others (2006), Bajc (1999,2000), and Larson (2004). Nickel displays high abundance in the fine fraction of Vermilion district tills relative to both the larger Canadian data set (Rencz et al., 2006) and the Shebandowan greenstone belt (Bajc 1999, 2000) (Fig. 2-6). However, in both the Canadian and Shebandowan datasets, the 98th percentile values are higher than in the Vermilion dataset. This suggests that while tills in the Vermilion district are characterized by high background nickel values, no truly anomalous samples indicative of potential economic nickel mineralization have been collected. Copper displays a slightly elevated abundance in the fine fraction of Vermilion district tills relative to both the larger Canadian data set (Rencz et al., 2006) and the Shebandowan greenstone belt (Bajc 1999, 2000) (Fig. 2-7). In both the Canadian and Shebandowan datasets, only the maximum values are higher than in the Vermilion dataset. This suggests that tills in both the Vermilion district and Shebandowan belt are characterized by similar background values somewhat higher than Canada as a whole. However, the highest copper values in the Vermilion district are lower than both other data sets, suggesting somewhat lower potential for economic copper mineralization than in the Shebandowan belt, or Canada as a whole. 91 �Zinc displays high abundance in the fine fraction of Vermilion district tills relative to both the larger Canadian data set (Rencz et al., 2006) and the Shebandowan greenstone belt (Bajc 1999, 2000) (Fig. 2-7). In both the Canadian and Shebandowan datasets, only the maximum values are higher than in the Vermilion district. This suggests that tills in the Vermilion district belt are characterized by background zinc values higher than the Shebandowan belt and Canada as a whole. The significantly higher 98th percentile zinc values in the Vermilion district are significantly higher than for both other data sets, suggesting higher potential for economic zinc mineralization than in the Shebandowan belt, or Canada as a whole. Figure 2-7. Relative abundance of copper (left) and zinc (right) in the -63 micron fraction of till. Data from Rencz and others (2006), Bajc (1999,2000), and Larson (2004). Integrating ice flow directions, mean transport length, and geochemical data: The Probability Window Integration of the three parameters can serve as a powerful tool for vectoring to economic mineralization in glaciated terrane. When a mineralized clast or anomalous geochemical sample is found in till or other glacigenic sediment, the only statement that can be said with any confidence about its origin is that it did not come from bedrock underlying the sample site. However, careful observation of ice flow direction indicators, calculation of mean transport length, and assessment of the significance of anomalies can be used to constrain the potential source to a probability window. The probability window (a polygon) is constructed by extending side lines in the general up-ice direction of a sample location. The exact orientations of the lines are opposite the limits of the potential range of ice flow directions determined for the area in question. The up-ice limit to the probability window is imposed by constructing a third line at a distance equal to the mean transport length. The polygon thus defined represents an area within which there is theoretically a 50% probability of the anomalous sample originating (Fig. 2-8). Clearly, given the inherently heterogeneous nature of till composition, relatively low confidence can be placed any single such determination of a potential source area for an anomalous sample. However, multiple probability windows calculated from multiple anomalous samples, augmented by iterative sampling, offer to provide increasing confidence in identifying potential source areas. This information, when integrated with geologic and geophysical data, can provide a powerful tool for vectoring toward economic mineralization in glaciated terrane. 92 �Figure 2-8. Illustration of the use of probability windows to constrain the potential source area of mineralized clasts or anomalous till geochemical samples(data from Larson 2004). FIELD TRIP STOPS Note: All location coordinates are given as UTM Zone 15, NAD 83 datum STOP 2-1. Hidden Valley Recreation Area Location: 588490E, 5306530N Ice-contact glaciofluvial deposits. The Hidden Valley exposure reveals poorly sorted coarse gravel glaciofluvial sediments of the Rainy lobe. In this vicinity, the lithologic composition of the sediment reflects a predominance of granitoid rocks eroded from the Quetico subprovince of the Canadian Shield. This deposit is part of a broad zone of voluminous glaciofluvial deposits extending northeast from Bear Head Lake, likely deposited in a subtle trough in the surface of the ice sheet extending up-ice of an inflection point in the orientation of the ice margin. The enormous quantity of glaciofluvial sediment deposited on this trend is a reflection of the enormous surface meltwater discharge captured by the trough in the ice sheet. STOP 2-2. Wolf Creek Road gravel pit Location: 570310E, 5304700N Ice-contact glaciofluvial deposits. The Wolf Creek gravel pit reveals 40 feet of glaciofluvial sediment deposited subglacially as an esker. On the flanks of the esker, a stony diamict is exposed; this may be a till deposited by the LIS during the re-advance to the Vermilion moraine. This locality lies essentially on the Vermilion fault, and the 93 �lithologic composition of the sediment is overwhelmingly dominated by granitoid and high-grade metasedimentary rocks of the Vermilion Granitic Complex. This lithologic composition represents the composition of sediment in transport by glacier ice as the Rainy Lobe begins flowing across greenschistgrade metasedimentary and metavolcanic rocks of the greenstone belt. The concentration of greenstone lithologies in tills down-ice of this locality provides a quantitative assessment of the rate at which greenstone rocks are eroded and entrained by the Rainy Lobe. STOP 2-3. Vermilion moraine (subaqueous segment) In this vicinity, the Vermilion moraine is characterized by a proximal (northeast) ice-contact scarp and a distal subaqueously-deposited outwash apron. The crest of moraine is generally ~1450’ elevation, corresponding to the level of Glacial Lake Norwood. The bulk of the moraine at this location is composed of well-sorted sand and gravel, suggesting much of the sediment was deposited in water, either directly from supra- and englacial meltwater, or from resedimented ice-contact debris. Pinching and swelling of the moraine along it length reflects the spacing of meltwater drainage channels on the ice surface. Locally, where the crest of the moraine exceeds 1450’ elevation, sediment at the moraine crest is composed of diamict, presumably meltout till, formed by release of englacial debris on the surface of the ice sheet. This suggests as significant fraction of the sediment delivered to the ice margin was englacial debris. STOP 2-3A. Forest Lane Resort Location: 548090E, 5298730N Ice-proximal scarp of moraine. The area on the up-ice side of the scarp is characterized by an abundance of large boulders. Large boulders are common in deposits of the Rainy Lobe, and result from the isotropic nature of the granitoid bedrock and widely spaced fractures, which favors glacial quarrying of large boulders. The boulders were of sufficient size that meltwater draining from the ice surface was unable to transport them beyond the ice margin. STOP 2-3B. Holmes Excavating Gravel Pit Location: 547250E, 5298480N Ice-distal slope of moraine. At this locality, the Vermilion moraine is composed of subaqueously-deposited sand and gravel representing a system of prograding deltas along the ice margin. Sediment size range from silt to coarse sand and gravel indicating a variety of depositional energies, and the sediments are often highly deformed. The deformation suggests that the deposition was rapid. Loading was apparently fast enough that finer-grained sediments were not efficiently dewatered whereas coarser sediments dewatered effectively. The strength of soft sediment is a function of water content; consequently, deformation of the coarser gravel lenses tends to be brittle in character (Fig. 2-9), while in the finer grained sediment deformation tends to be ductile in character. Deformation of the sediments ranges from small-scale faults and folds to large-scale (10s of meters) folds and detachment structures (Fig. 2-10). 94 �Figure 2-9. View of brittle deformation features in sand and gravel of Vermilion moraine, Holmes Excavating Gravel Pit. (photo from fall of 2002) Note that the coarser gravel beds behave as rigid blocks, while the interbedded silts and sands deform in a ductile fashion. Figure 2-10. View of nappe structure and recumbent fold nose in sand and gravel of Vermilion moraine, Holmes Excavating Gravel Pit. (photo from fall of 2002). 95 �STOP 2-4. Breitung Township gravel pit Location: 558020E, 5295840N Vermilion moraine at strandline of Glacial Lake Norwood. At this location on the Vermilion moraine, the margin of the ice sheet was positioned roughly at the level of Glacial Lake Norwood. Rather than building a steep-crested moraine, sediment delivered to the margin by meltwater formed a prograding flat-topped fan delta. Visible in the walls of the pit are channel scourand-fill structures. Not coincidentally, this particular fan delta is located at the discharge point of an esker system traceable for several km up-ice (see Stop 2-8). STOP 2-5. Murray Road Location: 562380E, 5294540N Vermilion moraine (subaerial segment) At this location on the Vermilion moraine, the ice margin was position well above the level of Glacial Lake Norwood, and a relatively steep subaerial moraine formed. Here the moraine is composed of a complex association of sand, gravel, and meltout till. Large boulders are common in the moraine. One boulder in particular is a felsic tuff, correlative with felsic Metavolcanic bedrock mapped only a few km up-ice. On subaerial segments such as this, the moraine is fronted by small outwash fans and aprons on the distal side. STOP 2-6. Mud Creek Road Location 563680E, 5303070N Till on bedrock. This exposure provides an opportunity to examine basal till of the Rainy lobe. It is difficult to determine the nature of the till deposition, however, the sediments and the local geomorphic expression suggest a lodgment origin. The lithologic composition of the till is a direct reflection of the local bedrock, and the till has a sandy loam texture with a slightly higher proportion of silt than typical exposures. STOP 2-7. Mud Creek Road Location: 563970E, 5302860N Thick till. At this location, a relatively thick sequence of till is exposed in a roadcut. Its position in the lee side of an east-west trending bedrock ridge may be responsible for its preservation. The lithologic composition of the till varies along the exposure and directly reflects the lithology of local bedrock immediately beneath the till. At the eastern end of the exposure, the till is composed of a predominance of iron formation, incorporated from bedrock immediately underlying the till. Also at this location, a single clast of Hudson Bay-provenance Paleoproterozoic greywacke was recovered. Its occurrence lends further support to the idea that a continuous sheet of carbonate-greywacke bearing drift at one time extended over the Vermilion district. It also suggests that a drift sheet deposited early in the Wisconsinan glaciation was largely removed by remobilization later in the glacial cycle. 96 �STOP 2-8. Mud Creek Road Location: 564020E, 5302360N Esker. This gravel pit contains the remnants of an esker; the same esker system responsible for deposition of the fan delta at Stop 2. This esker system, like most of those associated with the Vermilion Moraine, is discontinuous. Scour and polish are visible on striated bedrock, suggesting sediment-laden meltwater may have been eroding bedrock. STOP 2-9. Trygg Road gravel pit Location 567060E, 5300080N Sulphide mineralized clasts in glacigenic sediment. The Trygg Road pit exposes glaciofluvial sediment and basal till of the Rainy Lobe, and lays approximately 5 kilometers downglacier from the Vermilion fault. This gravel pit is notable in that the till and glaciofluvial sediments exposed here contain an abundance of sulphide mineralized metasedimentary clasts, ranging in size up to bouders. These clasts are likely derived from cherty metasediments hosted by the Gafvert Lake felsic complex. Both the size of the largest mineralized clasts, as well as their abundance, reflect their relative short transport distance from their bedrock source, perhaps no more than 1 km up-ice. Therefore this pit offers an opportunity to visualize clearly the rapid rate of entrainment of local lithologies and the concept of calculation of the mean transport length from the ratio of locally derived lithologies to those derived north of the Vermillion fault. REFERENCES Bajc, A.F. 1999. Results of regional humus and till sampling in the eastern part of the Shebandowan greenstone belt, northwestern Ontario. Ontario Geological Survey Open File Report 5993, 92 p. Bajc, A.F. 2000. Results of regional till sampling in the western part of the Shebandowan greenstone belt, northwestern Ontario. Ontario Geological Survey Open File Report 6012, 82p. Björck, S., 1985, Deglaciation chronology and revegetation in northwestern Ontario, Canadian Journal of Earth Sciences 22: 850-871. Björck, S., 1990, Late Wisconsin history north of the Giants Range, northern Minnesota, inferred from complex stratigraphy. Quaternary Research 33: 18-36. Clayton, L., and Moran, S. R. 1982. Chronology of late Wisconsinan glaciation in middle North America. Quaternary Science Reviews 1: 55–82. Jirsa, M.A., and Boerboom, T.J. 2003. Bedrock geology of the Vermilion Lake 30' x 60' quadrangle, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map M-141, scale 1:100,000. Hallet, B., Hunter, L., and Bogen, J. 1996. Rates of erosion and sediment evacuation by glaciers: a review of field data and their implications. Global and Planetary Change 12: 213–235. Hobbs, H.C. 1983. Drainage relationship of glacial Lake Aitkin and Upham and early Lake Agassiz in northeastern Minnesota. In: J. T. Teller and L. Clayton, Editors, Glacial Lake Agassiz, Geological Association of Canada Special Paper 23, pp. 245–259. Hudleston, P.J., Schultz-Ela, D.D., and Southwick, D.L. 1988. Transpression in an Archean greenstone belt, northern Minnesota. Canadian Journal of Earth Sciences 25: 1060-1068. 97 �Jirsa, M.A., Southwick, D.L., and Boerboom, T.J. 1992. Structural evolution of Archean rocks in the western Wawa subprovince, Minnesota: Refolding of pre-cleavage nappes during D2 transpression. Canadian Journal of Earth Sciences 29: 2146-2155. Jirsa, M.A., and Boerboom, T.J., comps. 2003. Bedrock geology of the Vermilion Lake 30’ X 60’ quadrangle, northeastern Minnesota. Minnesota Geological Survey Miscellaneous Map M-141, scale 1:100,000. Karrow, P.F. and Geddes, R.S. 1987. Drift carbonate on the Canadian Shield. Canadian Journal of Earth Sciences 24: 365-369. Larson, P.C. 2004. Regional Till Sampling of the Western Vermilion Greenstone Belt, Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Technical Report NRRI/TR-2004/23, 33 p., 1 plate. Larson, P.C., and Mooers, H.D. 2004. Glacial indicator dispersal processes: a conceptual model. Boreas 33: 238249. Larson, P.C. and Mooers, H.D. 2005. Comment on ‘‘Subglacial erosion and englacial sediment transport modeled for North American ice sheets’’ by D.H.D. Hildes, G.K.C. Clarke, G.E. Flowers, S.J. Marshall. Quaternary Science Reviews 25: 233-234. Larson and Mooers. 2008. Control on Laurentide ice sheet dynamics by unconsolidated sediment in the Hudson Bay lowlands. Geol. Assoc. Canada Min. Assoc. Canada Joint Annual Meeting Abstracts 33: 92-93. Lehr, J.D. and Hobbs, H.C. 1992. Field trip guidebook for the glacial geology of the Laurentian Divide area, St. Louis and Lake Counties, Minnesota. Prepared for the 39th Midwest Friends of the Pleistocene Field trip, Biwabik, Minnesota. University of Minnesota, Minnesota Geological Survey, St. Paul, Minnesota, Guidebook Series No. 18, 73 p. Lowell, T.V. and Fisher, T.G. In press. Radiocarbon deglaciation chronology of the Thunder Bay, Ontario, area and implications for ice sheet retreat patterns. Quaternary Science Reviews. Mooers, H. D., and Lehr, J. D. 1997. Terrestrial record of Laurentide Ice Sheet reorganization during Heinrich events. Geology 25: 987-990. Morey, G.B., and Meints, J., comps. 2000. Geologic map of Minnesota, bedrock geology(3rd edition): Minnesota Geological Survey State Map Series S-20, scale 1:1,000,000. Peterson, D. M., 2001, Development of Archean lode-gold and massive sulfide deposit exploration models using geographic information system applications: targeting mineral exploration in northeastern Minnesota from analysis of analog Canadian mining camps: unpublished Ph. D. dissertation, University of Minnesota, Duluth, Minnesota, 503 p. Peterson, D.M., and Jirsa, M.A., comps. 1999. Bedrock geologic map and mineral exploration data, western Vermilion district, St. Louis and Lake Counties, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map M-98, scale 1:48,000. Peterson, D. M., and Patelke, R. L., 2003, National Underground Science and Engineering Laboratory (NUSEL): Geological site investigation for the Soudan Mine, Northeastern Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2003/29, 88 p. Peterson, D.M., and Patelke, R.L. 2004. Bedrock geology and lode gold prospect data map of the Mud Creek Road area, northern St. Louis County, Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Map NRRI/MAP-2004/01, scale 1:12,000. Rencz, A.N., Garrett, R.G., Adcock, S.W., and Bonham-Carter, G.F. 2006. Geochemical Background in soil and till. Geological Survey of Canada Open File 5084. Sims, P.K., and Southwick, D.L. 1985. Geologic map of Archean rocks, western Vermilion district, northern Minnesota: U.S. Geological Survey Miscellaneous Investigations Map I-1527, scale 1:48,000. Southwick, D.L., Boerboom, T.J., and Jirsa, M.A. 1998. Geologic setting and descriptive geochemistry of Archean supracrustal and hypabyssal rocks, Soudan–Bigfork area, northern Minnesota: Implications for metallic mineral exploration: Minnesota Geological Survey Report of Investigations 51, 69 p. 98 �Stark, J.R., 1977, Surficial geology and groundwater geology of the Babbitt-Kawishiwi area, northeastern Minnesota, with planning implications [M.S. thesis]: University of Wisconsin, Madison, 104 p. Winchell, N.H. 1901. Glacial lakes of Minnesota, Geological Society of America Bulletin 12: 109-127. Winter, T.C. 1971. Sequence of glaciation in the Mesabi-Vermilion iron range area, northeastern Minnesota: U.S. Geological Survey Professional Paper 750-C, p. 82-88. Winter, T.C., Cotter, R.D., and Young, H.L. 1973. Petrography and stratigraphy of the glacial drift, MesabiVermilion iron range area, northeastern Minnesota: U.S. Geological Survey Bulletin 1331-C, 41 p. Zoltai, S.C. 1965. Glacial features of the Quetico-Nipigon Area, Ontario. Canadian Journal of Earth Sciences 2: 247–269. 99 �55th Annual Institute on Lake Superior Geology Field Trip 3 SOUDAN IRON MINE AND PHYSICS LAB Rail depot at the Soudan Mine circa 1925, © Minnesota Historical Society/CORBIS. Dean Peterson (Duluth Metals Ltd. & PRC) James Pointer (MN Dept of Natural Resources, Parks and Trails) Marvin Marshak (Department of Physics, Univ. of Minnesota) 100 �FIELD TRIP 3 SOUDAN IRON MINE AND PHYSICS LAB Dean Peterson (Duluth Metals Ltd. & PRC) James Pointer (MN Dept of Natural Resources, Parks and Trails) Marvin Marshak (Department of Physics, Univ. of Minnesota) INTRODUCTION The Soudan Mine was the first iron ore mine in Minnesota when it opened in 1882. Since then the Minnesota Iron Range’s have contributed billions of tons of iron ore to the industrialization of America and the world. Iron mining at Soudan began in surface pits but due to the rock formation, the miners quickly went underground. By 1892 the entire mining operation was underground. The mine remained open for 80 years, but technology changes caused the mine to cease production in 1962. The Soudan Mine, a registered National Historic Landmark, is currently owned and operated by the State of Minnesota, Department of Natural Resources, Division of State Parks and Trails. Each year approximately 30,000 visitors go underground at Soudan, mostly during the summer season from Memorial Day weekend through the end of September and the first three weekends in October. This figure includes over 5,000 K-12, college, and other formal groups that visit the mine throughout the year. In February 2004, Dr. Michael Turner, Assistant Director of the National Science Foundation (NSF), announced the NSF’s intent to pursue a three-phase process leading to the establishment of a Deep Underground Science and Engineering Laboratory (DUSEL) in the United States (see Bachall et al., 2001). DUSEL would provide a program and one or more locations to study the deep (to depths of ~2.5 km) underground geological and biological environment and use the isolation of that environment to frontier physical science and engineering initiatives. The goals of NSF’s deep underground geological and biological science initiative were outlined in the 2003 document “EARTHLAB: A Subterranean Laboratory and Observatory to Study Microbial Life, Fluid Flow, and Rock Deformation” (McPherson et al., 2003). The Soudan Mine in northeastern Minnesota was one of eight sites under consideration by the NSF for hosting DUSEL under the leadership of Dr. Marvin Marshak of the University of Minnesota (Marshak et al., 2003, 2005, 2007) Although the University of Minnesota’s three proposals for the Soudan Mine ultimately did not win the competition for hosting the DUSEL, the collaboration between the universities’ School of Physics & Astronomy in Minneapolis and the Natural Resources Research Institute in Duluth laid the foundation for this field trip. SOUDAN UNDERGROUND MINE STATE PARK The promise of gold brought the early settlers to the Vermilion Iron Range in 1865. These early pioneers discovered economical quantities of iron ore while prospecting. One of these prospectors was George Stuntz, a government surveyor. Stuntz discovered the existence of commercial value iron ore on the South shore of what is now known as Stuntz Bay on Lake Vermilion. From 1865-75 Stuntz made many trips back to the Lake Vermilion area collecting numerous samples. Some of these specimens were shown to George C. Stone, a Duluth banker and Minnesota legislator. He became interested in the prospect of iron mining on the Vermilion. George Stone convinced Charlemagne Tower, a lawyer in Pennsylvania, that the mining operation would be a good business venture. At the time a new steel making process, known as the Bessemer process, was being introduced in the United States, creating a need for high grade iron ore. Tower dispatched Professor Albert Chester, a renowned geologist, to the Lake Vermilion region 101 �as head of an exploring expedition. Chester returned with a detailed account of the mineral lands. In his report Chester mentioned large quantities of iron ore and little or no gold, thus ending Minnesota's brief and dramatic gold rush. When the mine opened in 1882, the miners used a rock quarrying method of mining. The basic tools were hand drills, pry bars, black powder and bare hands. The railroad finally reached Tower in 1884, allowing the first shipment of ore from the Soudan Mine to be made on July 31st. The ore was railed to Two Harbors, Minnesota, where it was shipped to the steel mills in the East. As the ore was removed and the mine pits were sunk deeper into the hill, it became necessary to send the miners, equipment, and supplies down ropes hung over the side of the pit. Like all mining, the work was dangerous. Masses of earth and rock kept caving in, which resulted in heavy losses of lives and halts in production. These tragic incidents prompted the decision to work the mine by underground methods. By 1892, the entire operation had been moved underground. Of the thirteen shafts on property, eventually only two shafts remained: the No. 8, which is in use today for tours, and the Alaska shaft, which is about 1700 feet to the east. The No. 8 reaches a depth of almost 2,400 feet with horizontal drifts extending more than three-fourths of a mile east and west to various ore bodies. It connects with the Alaska shaft on a variety of levels. The year 1924, marked the end of an era for the Soudan Mine, because the mine was changed from a "mule and man-powered mine" to a modern electrified operation. At this time a new head frame and hoisting system were constructed, which is the equipment that is in use today. After World War II, the Soudan Mine, like other underground operations on the Vermilion Range, started to slow down. The 27th level was the last to be opened for mining. The level was never fully developed because of the advancement of the new taconite process and changes in smelting methods detracted from the usefulness of Soudan ores. In the early 1960's, increasing costs of production began to reduce profits. Soon the Vermilion ores could not compete economically with the newly developed pelletized taconite ores of the giant Mesabi Range, just to the south. The Soudan Mine ceased operations on December 15, 1962. The last shipment from the stockpile was made in August 1963. The United States Steel Corporation generously donated the mine to the State of Minnesota for creation of the Soudan Underground Mine State Park. The Department of Natural Resources, Parks and Trails Division, administers the park. The park is a testimonial to the men who labored for us all. The Park maintains the entire underground tour route, which includes the No. 8 shaft, the west section of the 27th level, the physics lab, and the rescue chamber. In addition there are pumping stations at the 12th, 22nd and 27th levels, and a reservoir for a sprinkler system on the 25th level. The remaining areas of the mine are monitored and research is conducted in a variety of these areas. Researchers are taking water samples, soil samples, and gas samples in hopes of learning more about their properties. Bat research is also conducted in the mine, since the mine is a large hibernacula for bats. The State Park also has a number of historic surface buildings, many of which are open for public visitation. These buildings include the “dry house”, the engine house, the drill shop, and the crusher. The historic “dry house” includes exhibit space and a gathering area for the tours. The State Park’s Historic Underground Mine Tour leads visitors through the world of underground mining. Visitors don hard hats and enter a "cage" for the descent into the mine. The typical 90-minute mine tour takes visitors nearly half a mile down into the Earth to the 27th level of the mine. Once underground, a 3/4 mile train ride takes visitors to the Montana ore body, the last and deepest area mined at Soudan. Public tours run daily from Memorial Day weekend through the end of September and the first three weekends in October. The park offers group tours to schools, colleges, organizations and 102 �businesses. In addition to the traditional underground mine tour, the Park is working to offer a geology tour for groups that includes a walking tour of the 27th level drift. The historic underground mine tour was made ADA accessible in 2008, with the addition of ramps and a new elevator. There is a charge for the underground mine tours. SOUDAN UNDERGROUND LABORATORY The University of Minnesota has operated the Soudan Underground Laboratory since 1980. Soudan’s existing laboratories are 710 m deep and include two caverns (MINOS and Soudan 2), each 15 m wide by 15 to 16 m high. About 250 scientists and engineers and a support staff of 9 members now work on experiments at the Soudan Laboratory. Neutrinos and WIMPS Neutrinos are elementary particles that often travel close to the speed of light, lack an electric charge, are able to pass through ordinary matter almost undisturbed and are thus extremely difficult to detect. Neutrinos have a minuscule, but nonzero mass. They are usually denoted by the Greek letter ν (nu). Neutrinos are created as a result of certain types of radioactive decay or nuclear reactions such as those that take place in the Sun, in nuclear reactors, or when cosmic rays hit atoms. There are three types, or "flavors", of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos; each type also has an antimatter partner, called an antineutrino. Electron neutrinos or antineutrinos are generated whenever neutrons change into protons or vice versa, the two forms of beta decay. Interactions involving neutrinos are generally mediated by the weak force. Most neutrinos passing through the Earth emanate from the Sun, and more than 50 trillion solar electron neutrinos pass through the human body every second. Observations of the large-scale structure of the universe show that matter is aggregated into very large structures that would not have time to have form under the force of their own self-gravitation. It is generally believed that some form of missing mass is responsible for increasing the gravitational force at these scales, although this mass has not been directly observed. This is a problem; normal matter in space will heat up until it gives off light, so if this missing mass exists, it is generally assumed to be in a form that is not commonly observed on earth. A number of proposed candidates for the missing mass have been put forward over time. Early candidates included heavy baryons that would have had to be created in the big bang, but more recent work on nucleosynthesis seems to have ruled most of these out. Another candidate are new types of particles known as weakly interacting massive particles, or "WIMP"s. As the name implies, WIMPs interact weakly with normal matter, which explains why they are not easily visible. Detecting WIMPs thus presents a problem; if the WIMPs are very weakly interacting, detecting them will be extremely difficult. Detectors like CDMS at Soudan and similar experiments measure huge numbers of interactions within their detector volume in order to find the extremely rare WIMP events. MINOS Experiment The Main Injector Neutrino Oscillation Search (MINOS) Far Detector Laboratory is 82 m long and houses the 5,500 tonne MINOS Far Detector. The MINOS detector at Soudan is the target of the 735 km Fermilab Neutrinos at the Main Injector (NuMI) beamline. The $125 million NuMI beam first produced neutrinos in January 2005. A deeper understanding of lepton physics is the goal of a multi-decade program using the NuMI beam line, the MINOS Detector to measure 2-3 neutrino oscillations, the NOνA Off-Axis Detector to study 1-3 neutrino oscillations and a Proton Driver, to significantly upgrade the NuMI intensity. MINOS is a particle physics experiment designed to study the phenomena of neutrino oscillations, first discovered by Super-Kamiokande experiment in 1998. Neutrinos produced at Fermilab by the NuMI (Neutrinos at Main Injector) beamline are observed at two detectors, one very close to where the beam is 103 �produced (the near detector), and another much larger detector 735 km away in northern Minnesota (the far detector). Both MINOS detectors are steel-scintillator sampling calorimeters made out of alternating planes of magnetized steel and plastic scintillator. The magnetic field causes the path of a muon produced in a muon neutrino interaction to bend, making it possible to separate neutrino and antineutrino interactions. This feature of the MINOS detectors allows MINOS to search for CPT violation with atmospheric neutrinos and anti-neutrinos. The MINOS experiment started detecting neutrinos from the NuMI beam in February 2005. On March 30, 2006, the MINOS collaboration announced that the analysis of the initial data, collected in 2005, is consistent with neutrino oscillations, with the oscillation parameters which are consistent with Super-K measurements. The NuMI beam is the only long baseline neutrino beam in the United States and one of only three such beams in the world. NuMI and the CERN-Gran Sasso beam are both >700 km in length. The K2K and T2K beams in Japan are less than 300 km long. NuMI has the highest flux of any of the three beams at the energy required for the first maximum of 2-3 neutrino oscillations. The NuMI beam represents a substantial functioning asset to neutrino physics in both the United States and the world. To produce the NuMI beamline, 120 GeV Main Injector proton pulses hit a water-cooled graphite target. The resulting interactions of protons with the target material produce pions and kaons, which are focused by a system of magnetic horns. The neutrinos from subsequent decays of pions and kaons form the neutrino beam. Most of these are muon neutrinos, with a small electron neutrino contamination. Neutrino interactions in the near detector are used to measure the initial neutrino flux and energy spectrum. Because they are so weakly interacting, the vast majority of the neutrinos travel through the near detector and the 735 km of rock, then through the far detector and off into space. On the way toward Soudan, about half of the muon neutrinos oscillate into other flavors. CDMS II Experiment The Cryogenic Dark Matter Search (CDMS), housed in the 70 meter long Soudan 2 laboratory, is a series of experiments designed to directly detect particle dark matter in the form of WIMPs. Using an array of semiconductor detectors (silicon and germanium) at near absolute zero temperatures, CDMS has set the most sensitive limits to date on the interactions of WIMP dark matter with terrestrial materials. The first experiment, CDMS 1, was run in a tunnel under the Stanford University campus. The current dark matter search experiment at Soudan (CDMS 2) has been recently upgraded with almost double the detector mass and is now called SUPER CDMS. NOνA Experiment The University of Minnesota will receive a grant of $40.1 million for the NOνA neutrino experiment construction. Funds are part of $1.2 billion from Recovery Act to be disbursed by Department of Energy’s Office of Science. The NOvA funding for Minnesota will generate an estimated 60-plus construction jobs and procurements for concrete, steel, road-building materials and mechanical and electrical equipment from U.S. firms. The NOνA Experiment will construct a detector optimized for electron neutrino detection in the existing NuMI neutrino beam. The primary goal of the experiment is to search for evidence of muon to electron neutrino oscillations. This oscillation, if it occurs, holds the key to many of the unanswered questions in neutrino oscillation physics. The NOνA far detector will be located in northern Minnesota between Orr and International Falls. The groundbreaking for construction will take place on May 1, 2009. The NOνA detectors will be constructed from liquid scintillator contained inside extruded PVC modules. The far detector will have a total mass of 15 kilotons and be 15.7 meters wide, 15.7 meters tall, and 78 meters long. A smaller copy of the far detector will be constructed in the NuMI beam on the Fermilab site to measure the neutrino event rates prior to oscillation. 104 �SOUDAN GEOLOGY The Soudan mine is located in the Neoarchean (~2.7 Ga) Vermilion Greenstone Belt of the Wawa subprovince of the Superior Province of the Canadian Shield. Supracrustal rocks in the greenstone belt consist of volcanic-dominated stratigraphic sequences that are divided on the basis of stratigraphic and structural setting into: (1) the southern Soudan belt and (2) the northern Newton belt (Jirsa et al., 1992; Southwick et al., 1998). The boundary between these contrasting structural panels can be traced geophysically across the width of Minnesota, and was designated informally as the Leech Lake structural discontinuity (Jirsa et al., 1992). In the Soudan Mine area, this discontinuity occurs along the Mud Creek shear zone (Hudleston et al., 1988), immediately north of the Soudan Mine (Fig. 3-1). The Soudan belt, which hosts the Soudan Mine contains large, broad folds involving calc-alkalic and tholeiitic volcanic strata overlain by, and locally interdigitated with, turbiditic rocks. In contrast, the Newton belt consists of elongate, northeast-trending, and mostly northward-younging volcanic and volcaniclastic sequences that locally contain komatiitic flows and peridotitic sills. The two belts are faultbounded, and the relationship between stratigraphic units within each belt is largely conformable, although faults obscure contacts locally. To the east, the Soudan belt is continuous with the Saganagons assemblage in Ontario and terminates against the Saganaga pluton and Northern Light Gneiss. The Newton belt extends discontinuously eastward into the Shebandowan District of Ontario to form the Greenwater and Burchell assemblages. Intrusive rocks in both belts vary from gabbroic and felsic porphyries demonstrably related to volcanism, to large plutons emplaced posttectonically. Both districts contain unconformable, Timiskaming-type sequences composed of calcalkalic volcanic rocks, conglomerates, and finer grained sedimentary rocks. A simplified regional geological map of the Neoarchean terranes of northeastern Minnesota and adjacent Ontario is presented in Figure 3-1. Figure 3-1. Simplified correlation map of Neo-Archean assemblages across the U.S. - Canadian border, modified from Peterson et al., 2001). Inset shows major subprovinces of the southwestern Superior Province. 105 �Local Geological Setting A detailed geological study of the Soudan DUSEL area was recently completed by Peterson and Patelke (2003) and is available online at http://www.nrri.umn.edu/egg/REPORTS/TR200329/TR200329.html. Detailed field mapping ranging from 1:100 to 1:2000 scale (Peterson and Patelke, 2003; Hudak et al., 2002, 2003), petrographic studies (Hudak et al., 2002, 2003), whole rock lithogeochemical studies (Hudak et al., 2002, 2003), and electron microprobe investigations (Hocker et al., 2003) have led to a more complete understanding of the tectonic environment, stratigraphy, structure, hydrothermal processes and mineralization episodes associated with the diverse strata in the Soudan Mine vicinity. The local geological setting has been subdivided into three stratigraphic sequences (Fig. 3-2). The older Fivemile Lake Sequence is composed dominantly of coherent and volcaniclastic, dominantly calcalkaline to transitional basalts and andesites, minor intercalated coherent and volcaniclastic rhyodacite and rhyolite, associated chemical exhalites (including subeconomic VMS horizons) and epiclastic strata. Facies analysis indicates the FLS formed within a relatively shallow submarine arc environment. The younger Central Basalt Sequence (CBS) comprises an extremely texturally well-preserved sequence of sparsely amygdaloidal calc-alkaline, transitional, and tholeiitic basalt and andesite pillowed and massive lava flows; in-situ and resedimented rhyodacite to rhyolite lava flows, lava domes, and tuffs; as well as volcanic-derived mudstones, sandstones and breccias intercalated with chert, exhalites, and Algoma-type iron formations. Facies analysis indicates the CBS formed in a deeper water submarine environment which, based on lithogeochemical data, may be transitional between a volcanic arc and the opening of a back-arc rift. Overlying the CBS are sedimentary-dominated rocks of the Upper Sequence (US), which in the Soudan DUSEL area consists dominantly of Algoma-type iron-formation; dacitic epiclastic rocks, crystal tuff and tuff breccia; mixed basaltic rocks and iron-formation; and greywacke. The contact between the CBS and the overlying US is transitional over approximately 100-150 meters, and marks a stratigraphically upward increase in the proportion of chemical sediments and decrease in volcanic rocks. The supracrustal strata in the immediate area have been intruded by numerous synvolcanic – diorite and gabbro sills, diabase and feldspar-porpyhry dikes – and post-tectonic – lamprophyre dikes – intrusions. A simplified geologic map of the Soudan DUSEL area is presented in Figure 3-2. Periods of generally N-S directed compression resulted in three Neoarchean deformation events in the Soudan area. The earliest deformation (D1) produced the Tower-Soudan Anticline (Fig. 3-1), which is a west-plunging anticline within which the axis and plunge changes orientation along strike from nearly vertical in basalts to shallow NE plunging in the western sedimentary rocks. Axial-planar cleavage associated with this early fold typically is lacking, although Bauer (1985), Hooper and Ojakangas (1971), Hudleston (1976), and Jirsa et al. (1992) have described early cleavage (S1) locally, and Peterson and Patelke (2003) have described D1 flexural-slip shear zones immediately north of the Soudan Mine. A second deformation (D2) associated with synchronous regional metamorphism resulted in foliation development and shear zones having largely dextral asymmetry. D2 is constrained in the Vermilion greenstone belt to the time period 2674 to 2685 Ma (Boerboom and Zartman, 1993). The relationship between S2 fabric and shear structures indicates that most shearing occurred relatively late in the D2 event (Jirsa et al., 1992). Major shearing that produced the Mud Creek and related shear zones, e.g., the Murray, Mine Trend, and Linking shear zones in the Soudan Mine area (Fig. 3-2), is attributed to the late stages of D2 dextral transpression. Structures related to the third deformation (D3) include abundant NEand NW-trending faults (Fig. 3-2). 106 �Figure 3-2. Simplified bedrock geologic map and stratigraphic section of the Soudan Mine area, Sections 26 and 27, T62N, R15W. Modified from Peterson and Patelke, 2003. 107 �SOUDAN FIELD TRIP The short time-frame of the 2009 ILSG field trip to the Soudan Mine only allows for quick tours of the facility and the scientific treasures it holds. The excursion begins at the surface of the Soudan Mine and will include a quick overview of the local geology based on geologic maps and observations of the region from the head frame vista. The three minute trip underground in the inclined shaft will take us 2,341 feet underground. We will gather in the main hall of the MINOS cavern for brief PowerPoint presentations about the Soudan Mine from our field trip leaders. The presentations will pertain to the following topics: the State Park (James Pointer), geology of the area (Dean Peterson), and the various physics experiments (Marvin Marshak). The group will be split in half and rotate through a tour of the Underground Physics Laboratory and a historic mine tour, which takes us via a 3/4 mile train ride to the Montana ore body, the last and deepest area mined at Soudan. REFERENCES Bahcall, J., Barish, B., Calaprice, F., Conrad, J., Doe, P.J., Gaisser, T., Haxton, W., Lesko, K.T., Marshak, M., and Robinson, K., 2001, Underground Science: University of Washington Institute for Nuclear Theory report, 36 p. Bauer, R.L., 1985, Correlation of early recumbent and younger upright folding across the boundary between an Archean gneiss belt and greenstone terrane, northeastern Minnesota: Geology, v. 13, p. 657-660. Boerboom, T.J., and Zartman, R.E., 1993, Geology, geochemistry, and geochronology of the central Giants Range batholith, northeastern Minnesota: Canadian Journal of Earth Science, v. 30, p. 2510-2522. Hocker, S.M., Hudak, G.J., and Heine, J., 2003, Electron microprobe analysis of alteration mineralogy at the Archean Five Mile Lake volcanic-associated massive sulfide mineral prospect in the Vermilion District of NE Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Report of Investigation NRRI/RI-2003/17, 49 p. Hooper, P., and Ojakangas, R., 1971, Multiple deformation in the Vermilion district, Minnesota: Canadian Journal of Earth Sciences, v. 8, p. 423-434. Hudak, G.J., Heine, J., Newkirk, T., Hocker, S., and Hauck, S., 2003, Comparative geology, stratigraphy, and lithogeochemistry of the Needleboy Lake – Six Mile Lake area, Vermilion District, NE Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Report of Investigation, NRRI/RI-2003/18, 22 p. Hudak, G.J., Heine, J., Newkirk, T., Odette, J., and Hauck, S., 2002, Comparative geology, stratigraphy, and lithogeochemistry of the Five Mile Lake, Quartz Hill, and Skeleton Lake VMS occurrences, Vermilion District, NE Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Technical Report, NRRI/TR-2002/03, 390 p. Hudleston, P.J. 1976, Early deformational history of Archean rocks in the Vermilion district, northeastern Minnesota: Canadian Journal of Earth Sciences, v. 13, p. 579-592. Hudleston, P.J., Schultz-Ela, D., and Southwick, D.L., 1988, Transpression in an Archean greenstone belt, northern Minnesota: Canadian Journal of Earth Sciences, v. 25, p. 1,060-1,068. Jirsa, M.A., Southwick, D.L., and Boerboom, T.J., 1992, Structural evolution of Archean rocks in the western Wawa subprovince, Minnesota: Refolding of pre-cleavage nappes during D2 transpression: Canadian Journal of earth Sciences, v. 29, p. 2,146-2,155. Marshak, M.L., Cushman, P.B., Heller, K., and Peterson, E.A., 2003, SOUDAN: A Proposal for a National Underground Science and Engineering Laboratory (NUSEL): National Science Foundation, submitted proposal, 146 p. 108 �Marshak, M.L., Peterson, D.M., Peterson, E., Kieft, T.L., and Cushman, P.B., 2005, Site and Conceptual Design for the Soudan Deep Underground Science & Engineering Laboratory (DUSEL), a proposal submitted to the National Science Foundation, 15 pages. Marshak, M.L., Peterson, D.M., Peterson, E., Alexander, C.A., and Cushman, P.B., 2007, Frontiers of Underground Science, a proposal submitted to the National Science Foundation, 100 pages. McPherson, B.J., and the EarthLab Steering Committee, 2003, EarthLab: A subterranean laboratory and observatory to study microbial life, fluid flow, and rock deformation: Geosciences Professional Services, Inc., 62 p. Peterson, D.M., Gallup, C., Jirsa, M.A., and Davis, D.W., 2001, Correlation of Archean assemblages across the U.S. - Canadian border; Phase I geochronology, abstract and oral presentation, Institute on Lake Superior Geology, 47th Annual Meeting, Thunder Bay, Ontario, v. 47, p. 77-78. Peterson, D.M and Patelke, R.L., 2003, National Underground Science and Engineering Laboratory NUSEL); Geological site investigation for the Soudan Mine, northeastern Minnesota: Natural Resources Research Institute, Technical Report NRRI/TR-2003/29, 97 p., 3 plates, 1 CD-rom. Southwick, D.L., Boerboom, T.J., and Jirsa, M.A., 1998, Geologic setting and descriptive geochemistry of Archean supracrustal and hypabyssal rocks, Soudan-Bigfork area, northern Minnesota: Implications for metallic mineral exploration: Minnesota Geological Survey, Report of Investigations 51, 69 p. 109 �55th Annual Institute on Lake Superior Geology Field Trip 4 PIONEER MINE CANOE EXCURSION Sibley Mine headframe, Ely Minnesota 1905, Minnesota Historical Society. Mark Jirsa (Minnesota Geological Survey) Mike Hillman (Ely Historian) 110 �FIELD TRIP 4 PIONEER MINE (Miners Lake) CANOE EXCURSION Mark Jirsa—Minnesota Geological Survey, Mike Hillman—Ely historian Figure 4-1. General geology of Archean rocks in the western half of the Vermilion district (after Clements, 1903). Black= iron-formation; diagonal rule=metasedimentary rocks; white=metavolcanic rocks (and lakes); plus symbol=intrusions. MINERS LAKE (information from Minnesota Department of Natural Resources website) Miners Lake was formed by the flooding of the open pit portion of a series of mostly underground iron mines after mining was discontinued in 1967. The lake has a maximum depth of about 140 feet. The flooded basin of Miners Lake includes the formerly mined area and a shallow bay to the east that was not mined. Brook and Rainbow trout are stocked at the rate of about 3000/year. Bass, pan fish, and white suckers are also present. The 2009 fishing season starts May 9. INTRODUCTION The Pioneer Mine was one of the richest underground mines on the Vermilion Iron Range from its opening in 1888, until closure in 1967. Mining extracted crystalline hematite from jaspilitic ironformation, and the underground workings eventually attained a depth of more than 1500 feet. The ironformation occurs as discontinuous layers and lenses within the Neoarchean Ely Greenstone along the Vermilion Iron Range or Vermilion district, as it was known (Fig. 4.1). The geology at Pioneer Mine is much like that at other mines of the district, including the Soudan Mine about 20 miles to the west. The Pioneer Mine and the adjacent Chandler, Sibley, Savoy, and Zenith mines (Fig. 4.2) lie within a complex isoclinal fold—known as the Ely Trough—having steeply dipping limbs. The brief geologic description presented here is based on historical records, primarily from mining company geologists of what was then the Oliver Iron Mining Division, U.S. Steel Corporation. This trip starts with a visit to the Pioneer Mine Dry with local historian “Iron-Mike” Hillman. We then embark on a short canoe trip down the axis of the trough, exploring exposures along the mine walls. 111 �Steeply dipping, pillowed metabasalt flows, lean and brecciated iron-formation, and a complex array of intrusions are visible from the gunnels. Figure 4-2. Geology of Ely trough mines (from Reid, 1956). EXPOSURES No descriptions of individual field “stops” are given here—instead, participants are encouraged to paddle along at a safe distance from the shoreline and observe, using the following general information as a guide. Bedding trends more or less parallel to the long axis (E-W) of the lake and dips steeply. Exposures at the west end of the lake show layered jaspilitic and slaty iron-formation capping pillowed, fragmental, and amygdaloidal metabasalt; overlain to the north by massive metabasalt. This is the west edge of the Chandler Mine. The south shore presents dip-slope exposures of pillowed metabasalt, mafic intrusions, and lenses and layers of lean iron-formation, all trending subparallel to the shoreline. These exposures lie within the Chandler and Pioneer Mines. Stratigraphic younging based on pillow morphology is northward, consistent with the interpretation that this shoreline exposes the south limb of the Ely trough. Easternmost exposures the north shore are within the Zenith Mine, and show liesegangbanded, altered (leached, oxidized) iron-formation. CAUTION should be taken to avoid disturbing the metastable wall rocks! 112 �LOCAL MINING HISTORY Time-line for development of the Vermilion Iron Range (modified from Skillings, 2004) 1865 Iron ore discovered at Soudan by George R. Stuntz 1883 Iron ore discovered at Ely by H.R. Harvey 1884 Completion of Duluth and Iron Range Railroad line to Tower 1884 First shipment of ore from Soudan mine 1888 First shipment from Chandler and Pioneer Mines 1901 Largest annual shipment of >2 million tons 1967 Last shipment from Vermilion (at Pioneer Mine—289,000 tons of gravity concentrate) 1884-1967 Total iron ore shipped from Vermilion = 103,752,604 tons Last iron ore reserve estimate calculated in 1985 for the Vermilion Iron Range = 6,237,076 tons. Ely Trough mining and production (Skillings, 2004): Mine Name Operation Yrs. ~Total (million tons) Chandler N 1891-1942 9 Chandler S 1888-1957 2 Pioneer 1888-1967 41 Zenith 1892-1964 21 Sibley 1899-1954 10 Savoy 1899-1916 2 GEOLOGY The main rock types in the mine, as described in previous literature (Reid, 1956) include: Greenstone (pillowed and massive metabasalt, minor hyaloclastite and mafic tuff) Porphyry intrusions (plagioclase and quartz phenocrysts) Basaltic intrusions (metagabbro, metadiabase) Jaspilite Slaty iron-formation Graphitic and sericitic schist “Paint rock” (an alteration product, apparently of any of the rocks types above) The large deposit of iron ore at Ely is found in the basal part of a trough-like lens of iron-formation about 1.75 miles long and .25 miles wide at the surface. Mines within the trough structure included the Chandler North and South, Pioneer, Zenith, Sibley, and Savoy. The structure is blunt U shape at the west end (Chandler) and narrows to the east. The ore body plunges eastward from the Chandler into the Pioneer mine, and is abruptly split by a broad mass of paint rock. The ore continues along both N and S limbs of the trough, but the north limb ore pinches out eastward into the Zenith. Most of the Zenith, and all of the Sibley and Savoy ore bodies lie within the south limb of the trough, and the north limb is presumably truncated by a fault. The ore is fragmental hematite, cemented to varying degrees that increase with depth by secondary hematite. The limbs of the trough, as defined by iron-formation, converge at about 1500 depth, and the ore-bearing structure is less than 500 feet wide. The trough roughly parallels the east-northeast-trending regional strike of the Vermilion district. Its axis is warped, with a steep north to vertical dip on the west, and a steep southerly dip on the east. Footwall rocks surrounding the trough consist of metabasalt of the Ely Greenstone and several types of intrusive rocks. Least-altered metabasalt exposed outside of the mines consists of actinolite, epidote, chlorite, quartz, and albite. 113 �Near the ore bodies, the rocks contain a more pronounced cleavage and consist of chlorite, fine-grained muscovite (sericite), and quartz. The trough interior is composed largely of jaspilite (alternating layers of hematite and chert), with lesser amounts of argillaceous iron-formation, mafic sill-like intrusions, irregular plagioclase-quartz porphyries, and lenses of graphitic and sericitic schist. Felsic porphyry intrusions were encountered during mining, and mafic sills and dikes were reported to cut iron-formation locally. Both of these rock types are common in the regional geology. The precise age of folding responsible for the Ely trough is unknown. Jirsa and Miller (2004) speculate that the Ely trough and other similarly trending structures were deformed during D1, much like the deformation that formed the Tower-Soudan anticline to the southwest. This is largely based on the observation that fold axes near Ely are truncated by D2 structures related to deformation along the Knife Lake-Burntside Lake trend. ORE DEPOSITS The major hematite ore bodies occur near the base of the trough. It was considered a “rubble ore,” because the bulk of it consisted of small angular fragments of hard blue hematite. The ore was relatively soft in the upper parts of the mine, but hardness increased with depth as it became partially to completely cemented by secondary hematite. In many places the ore was as hard as the “lump” ores at Soudan. The high grade ore bodies are more or less conformable with layering in jaspilite and greenstone, implying that the ores occupy the stratigraphic position of lean iron-formation. The ores contained 55-65 percent iron, low silica, P, Mn, and Al2O3. An increase in pyrite content with depth was observed, and carbonate minerals increased eastward within the trough. Alteration is more intense than at any other deposits in the Vermilion district. The alteration includes oxidation and leaching, and involves the loss of silica, lime, magnesia, soda, and potash, and the addition of iron and oxygen. “Paint rock” is the ultimate extent of that alteration, resulting locally in material consisting almost entirely of kaolin and ferric oxide. The origin of hematite ores on the Vermilion Range is controversial. speculated historically: Three scenarios have been 1) Leaching of silica from the primary iron-formation resulted in the formation of a residual hematite body, which was later compacted, hardened, and fractured by deformation and metamorphism: 2) Post-metamorphic leaching of silica and introduction of iron by downward-moving surface water; 3) Post-metamorphic leaching of silica and simultaneous introduction of iron by rising heated waters A detailed study of mineralogy and structure at the Zenith Mine, which lies just east of the Pioneer, produced the following conclusions (Machamer, 1968): 1) The ore deposits are post-metamorphic; 2) They formed by the replacement of silica in iron-formation by iron oxides; 3) The replacement resulted from rising hydrothermal fluids, most likely generated from higher metamorphic grades at depth. REGIONAL MINING HISTORY Although the 6 mines described here were by far the most productive, at least 12 iron mines were explored and developed in the area from Ely; westward to Armstrong Lake and Mud Creek Road; and eastward along the Fernberg Road (Stenlund, 1988). Only 3 of these mines actually shipped ore, and their life-spans were short due to high costs in this rugged terrane and low tonnage. The local mining and 114 �logging history is displayed in the Vermilion Interpretive and History Museum of the Ely-Winton Historical Society, housed on the campus of Vermilion Community College. Overall the Soudan iron-formation member and other iron-formations in the Ely Greenstone consist of lens shaped bodies interleaved with volcanic strata. The most productive mines occur at Tower-Soudan and Ely. It is interesting to note that these are the two localities along the Vermilion range where ironformation is “doubled” by folding. At the least, the folding consolidated otherwise attenuated iron-rich units. It is also likely that deformation played an important role in the alteration of the iron-bearing units to high-grade hematite deposits. REFERENCES Clements, J.M., 1903, The Vermilion Iron-bearing district of Minnesota: USGS Monograph 45, 463p. Jirsa, M.A., and Miller, J.D., Jr., 2004, Bedrock geology of the Ely and Basswood Lake (U.S. portion) 30’ x 60’ quadrangles, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-148, scale 1:100,000. Machamer, J.F., 1968, Geology and origin of the iron ore deposits of the Zenith Mine, Vermilion District, Minnesota: Minnesota Geological Survey Special Publication SP-2, 56p. Reid, I.L., 1956, Geology of the Ely Trough: in Schwartz, ed., Guidebook for Field Trips, Geological Society of America Guidebook Series No.1 Precambrian of northeastern Minnesota, p.135-148. Skillings Mining Review, 2004 Minnesota Mining Directory; Westmorelandflint Publishing, Duluth, Minnesota. Stenlund, Milt, 1988, Ghost Mines of the Ely Area: Ely-Winton Historical Society, Ely, MN, 44p. 115 �55th Annual Institute on Lake Superior Geology Field Trip 5 GEOLOGY & METAMORPHISM OF THE EASTERN MESABI RANGE Dick Ojakangas (Dept. of Geological Sciences, UMD) Mark Severson (Natural Resources Research Institute) Doug Halverson (Cliffs Natural Resources) Jeff Bird (Cliffs Natural Resources) Tom Campbell (Cliffs Natural Resources) Jared Lubben (Cliffs Natural Resources) Peter Jongewaard (Cliffs Natural Resources) William Everett (Mesabi Nugget) 116 �FIELD TRIP 5 GEOLOGY AND METAMORPHISM OF THE EASTERN MESABI IRON RANGE Richard W. Ojakangas, Professor Emeritus, University of Minnesota Duluth Mark J. Severson, Natural Resources Research Institute, Thomas J. Campbell, Cliffs Technical Group, Cliffs Natural Resources Jared D. Lubben, Cliffs Technical Group, Cliffs Natural Resources Peter K. Jongewaard, Cliffs Technical Group, Cliffs Natural Resources Douglas G. Halverson, Northshore Mining/Cliffs Natural Resources Jeff Bird, Northshore Mining/Cliffs Natural Resources William Everett, Mesabi Mining INTRODUCTION Iron-formation was described as early as 1866 by Henry Eames on what was to become the Mesabi Iron Range. Several attempts were made by individuals to find ore on the Mesabi range on their way north to the iron mines of the Vermilion range (Soudan to Ely, Minnesota); however, it was not until November 16, 1890 that the first rich iron ore on the Mesabi range was discovered by the Merritt brothers near what is now Mountain Iron, Minnesota. In 1892, the first shipment from this mine was 4,245 tons of ore (White, 1954). Exploration for iron ore ensued and within the next few years, most of the productive parts of the Mesabi Range were discovered. The Mesabi Iron Range is the largest iron range in the United States and is one of the largest in the world. It is 0.25 to 3.0 miles wide and 120 miles long (Fig. 5-1). The Biwabik Iron Formation, as thick as 750 feet, in general dips gently to the southeast at an angle of about 7° to 15°. The iron-formation, called taconite, typically contains 30 to 40 percent iron and 40 to 50 percent SiO2, plus other components (Morey, 1992). In numerous places along the length of the range, silica was leached out, thereby enriching the iron content to over 55 percent. These pockets became the high-grade natural ore mines; there were more than 500 individual mines prior to merging into larger mines as the ore between adjacent properties was removed. These were very important in making the United States an industrial giant, and were instrumental in providing raw material for World Wars I and II. As the high-grade ore was depleted, the taconite process was developed. In 1967, taconite production exceeded natural ore production. Currently, six taconite plants are in production (Fig. 5-2). The name of Biwabik Iron Formation was chosen by Van Hise and Leith (1901, p. 356), "…because the word Biwabik is the Chippewa word for a piece or fragment of iron." The word taconite is also used in discussions pertaining to hard, unoxidized portions of the iron-formation. H.V. Winchell (1882, p. 135) originally called portions of the Biwabik Iron Formation "taconyte" because he thought the rocks correlated with lower Cambrian rocks in the Taconic Mountains in northern New England. Since that time, many geologists have used taconite in their descriptions of the iron-formation and it has thus become firmly established. Perhaps a more proper definition for taconite is an economic term for ironformation from which iron can be profitably extracted after fine-grinding, followed by magnetic separation and pelletizing (Morey, 1993). 117 �Figure 5-1: Generalized map of the Mesabi Iron Range (cross-hatched). Note the Duluth Complex (Keweenawan, 1.1 Ga) on the east side. REGIONAL GEOLOGY The peneplaned Archean craton in the Lake Superior region formed a platform upon which a Paleoproterozoic continental margin assemblage was deposited in Minnesota, Michigan, and Wisconsin. Extension resulted in localized rifts that received thicker accumulations of sediments and volcanic rocks than did adjacent parts of the platform. Seas transgressed onto the continent one or more times and an ocean basin opened south of present-day Lake Superior. Island arcs that formed during southward subduction collided with the craton margin to the north as the ocean basin closed. A remnant of this oceanic crust is poorly preserved as a dismembered ophiolite sequence in Wisconsin (Schulz, 1987, 2003). The arc volcanics are preserved as the Wisconsin magmatic terranes. The collision resulted in a fold-and-thrust belt known as the Penokean orogen. To the north of the fold-and-thrust belt, a northwardmigrating foreland basin—the Animikie basin—developed as the stacked thrusts weighed down the crust (Fig. 5-3). Thick turbidite successions were deposited along the basin axis, and terrigenous clastics and Lake Superior-type iron-formation were deposited on the shelf along the northern margin (the foreland or peripheral bulge) of the basin. See Ojakangas and others (2001) and Severson and others (2003) for more detailed summaries on Paleoproterozoic basin development in the Lake Superior region. The development of the Midcontinent Rift System at 1.1 Ga severed the basin into northwestern and southeastern segments (Fig. 5-3). If the Midcontinent Rift System rocks are removed from the geologic map, the different portions of the Animikie basin become contiguous and the fold-and-thrust belt rocks of Minnesota, Wisconsin, and Michigan become continuous (Fig. 5-4). Figure 5-5 is an interpretive cross-section of the Animikie basin during its formative stages, with sediments derived from the Archean basement to the north and from the fold-and-thrust belt to the south. The Paleoproterozoic supracrustal rocks in the northwestern segment, including east-central and northeastern Minnesota and the adjoining part of Ontario, are for the most part poorly exposed. However, 118 �mining of iron ore on the Mesabi and Cuyuna ranges and continued mining of taconite on the Mesabi Range have resulted in excellent artificial exposures and an abundance of drill hole information. Geophysical surveys and stratigraphic test drilling by the Minnesota Geological Survey have also been major sources of information (for example Southwick and others, 1988). Figure 5-2: Generalized map of the Mesabi Iron Range. Inset A - Aerial distribution of taconite pits and cities. Inset B - Longitudinal section of the Biwabik Iron Formation showing: average thickness of the iron-formation at each taconite operation (along with the thickness of the various submembers at each operation), and mined taconite intervals (as black columns adjacent to the sections). From Severson and others (in prep.) Figure 5-3: Generalized geologic map showing the distribution of Precambrian rocks and structural elements of the Lake Superior region, modified from Ojakangas 1994 and references therein (from Ojakangas and others, 2001). 119 �Figure 5-4: Schematic hypothesized paleogeography at the time of sedimentation of the Paleoproterozoic Animikie Group turbidites that overlie shelf deposits in the Animikie basin. The rocks of the 1,100 Ma Midcontinent Rift System have been removed from the map, and Michigan and Wisconsin are thus positioned 60 miles closer to Minnesota-Ontario than they were after the formation of the Midcontinent Rift System. Arrows denote generalized transportation directions of sediment from major source areas. Compare with Fig. 5-3. Modified from Ojakangas (1994) and references included therein (from Ojakangas, et al., 2001). Figure 5-5: Schematic cross-section depicting deposition of the Animikie Group turbidites that overlie shelf deposits in the Animikie basin, with sediment derived from both the north south. The southern area, the fold-andthrust belt, comprises a complex assemblage including: 1) accreted Paleoproterozoic volcanic and plutonic rocks and volcanic rocks of the Wisconsin magmatic terranes; 2) accreted Archean miniplate terranes; 3) older Paleoproterozoic passive-margin sedimentary rocks and volcanic rocks produced during initial rifting of the continental margin, both scraped off the southward-subducting Archean Superior craton; and 4) recycled initial foredeep deposits, possibly including basal shallow water sandstones deposited in the transgressing sea of the northward-migrating foreland basin. The peripheral bulge comprises a source-rock assemblage of Archean granitic rocks and Archean volcanic-sedimentary (greenstone) belts. Scale is approximate. Compare with Fig. 5-4. Modified from Ojakangas (1994) and references included therein (from Ojakangas and others, 2001). 120 �Animikie Group The Paleoproterozoic Animikie Group unconformably overlies the Mille Lacs and North Range Groups to the south and the Archean basement to the north (see Fig. 5-6; Southwick and Morey, 1991). Magnetic data show North Range structures are present beneath Animikie strata to the east of the exposed North Range Group (Chandler, 1993). The group consists of three conformable major formations on both the Mesabi and Gunflint ranges. The respective units on the two ranges are the Pokegama Formation and the Kakabeka Quartzite (the lowest units), the Biwabik and Gunflint Iron Formations (the middle units) and the Virginia and Rove Formations (the upper units, composed of graywacke and shale). The Thomson Formation in the northern part of east-central Minnesota is correlative with the Virginia and Rove Formations. The Biwabik and Gunflint Iron Formations are on strike with each other and were probably continuous prior to the intrusion of the Duluth Complex at about 1,100 Ma. In the model presented here, the Animikie Group in Minnesota and Ontario on the Mesabi and Gunflint ranges and the Baraga Group of Michigan and Wisconsin on the Gogebic Range were both deposited in the Animikie foreland basin. The basal units comprised of siliciclastic sediment derived from the Archean basement, and the overlying iron-formation, were deposited in a shallow sea on the northern edge (the peripheral bulge or foreland) of the northward-migrating Animikie basin (for example Ojakangas, 1994). Additional details are provided below in the section titled "Environments of deposition, Animikie Group." The siliciclastic and iron-formation units are exposed on the Gogebic Range of northern Michigan and Wisconsin (the Palms Quartzite and Ironwood Iron Formation), on the Mesabi Range of northern Minnesota (the Pokegama Formation and the Biwabik Iron Formation), and on the Gunflint Range of northeast Minnesota and Ontario (the Kakabeka Quartzite and the Gunflint Iron Formation), and are lithostratigraphic equivalents. They probably were continuous from south to north prior to development of the Midcontinent Rift System in Mesoproterozoic time. A consequence of this model is that they are diachronous, with the units in Michigan and Wisconsin (located about 60 miles to the south of the Mesabi Range during deposition) thus somewhat older than those in Minnesota and Ontario. The thickest and uppermost units in the basin, essentially lithostratigraphic correlatives but probably differing somewhat in age, are the Michigamme, Tyler, and Copps Formations of the southeastern segment and the Thomson, Virginia, and Rove Formations of the northwestern segment. These are typical turbidite-shale (flysch) sequences, with graded beds and intercalated muddy "rain-out" sediment (Fig. 5-6). Figure 5-6: Generalized correlation chart of Paleoproterozoic strata in the Lake Superior region. Note that recently obtained age dates are shown for the Gunflint Iron formation, Mahnomen Formation, Hemlock Volcanics, and the Rove and Virginia Formations (sources listed in the references). The position of the Sudbury layer is from Cannon and Addision (2007). 121 �Ages Along the Mesabi Range, the Pokegama Formation rests unconformably on diabase dikes of the Kenora– Kabetogama dike swarm that give a Rb-Sr isochron age of 2,125 ± 45 Ma (Southwick and Day, 1983; Beck, 1988), and this provides a maximum age for deposition of the Pokegama Formation. A minimum age of 1,930 ± 25 Ma (Pb/Pb) for the Pokegama Formation was obtained by Hemming and others (1990) from quartz veins that cut the Pokegama Formation. A U/Pb age on euhedral zircons from an ash layer in the upper Gunflint Iron Formation of Ontario is 1,878 ± 2 Ma (Fralick and Kissin, 1998; Fralick and others, 2002). A similar age of 1,874 ± 9 Ma was obtained on zircon from rhyolite in the Hemlock Formation that is adjacent to (and is possibly interlayered with) the Negaunee Iron Formation in the Marquette Range Supergroup of Michigan (Schneider and others, 2002). A zircon age from an ash layer near the base of the Virginia Formation is 1,850 Ma (Hemming and others, 1996), and an age of 1,821 ± 16 Ma has been obtained from an ash layer in the Rove Formation about 70 meters above the Gunflint Iron Formation (Kissin and others, 2003). Several of these ages are shown on Figure 5-6. It has been recently postulated that deposition of the iron-formations in the Lake Superior region was affected by the Sudbury impact event at 1850 Ma (Cannon and Addison, 2007; Jirsa, 2008). Field and petrographic evidence for this event are clear in the Gunflint Iron Formation (Addison and others 2005; Jirsa, 2008) and the iron-formations of Michigan (Cannon and Addison, 2007). However, an indication of this event in the top of the Biwabik Iron Formation is more subtle (Addison and others, 2007), possibly due to deeper water conditions that are recorded in the presently mined and drilled stratigraphic section on the Mesabi Range. Pokegama Formation This formation has long been called the Pokegama Quartzite, but because it contains appreciable argillite and siltstone, the name Pokegama Formation is more appropriate. It has been studied by several workers since it was named by Winchell (1893) for exposures at the western end of the Mesabi Iron Range. Much of the previous work has been summarized by Morey (1972, 1973, 2003). Few natural exposures exist, as thick glacial drift generally covers the formation. Outcrops, road cuts, and mine cuts occur at a few places along the length of the range, but most exposures are in the central portion of the range. A few drill holes have penetrated the entire formation. One is located just south of Eveleth (T. 57 N., R. 17 W., sec. 5, NE, NE) and another is southwest of Mountain Iron (T. 58 N., R. 18 W., sec. 8, SE, SE); the thicknesses are 167 feet and 85 feet, respectively (Fig. 5-7). Other drill cores, some recently rediscovered and some recently drilled, have not yet been studied in detail. Numerous drill holes have penetrated only the upper few feet of the formation, as the drilling was generally undertaken in relation to iron ore exploration and development. The Pokegama Formation is thin at the eastern end of the range and thickens to the western end where it may be more than 300 feet thick. The formation is composed of three main rock types—argillite, siltstone, and quartzite. The quartzite is generally silica-cemented quartz sandstone, and is therefore an orthoquartzite rather than a metaquartzite. Morey (2003) determined that mineralogical changes in the Pokegama Formation and the Biwabik Iron Formation are the result of diagenesis rather than metamorphism, except at the eastern end of the range adjacent to the Duluth Complex. These three rock types make up three gradational members—lower, middle, and upper—respectively, as shown in Figure 6-7. Minor thin conglomerates occur at the base of the formation, and seem to represent a weathered residuum on the surface of Archean rocks, perhaps reworked by fluvial processes. The Pokegama Formation unconformably overlies Archean metavolcanic, metasedimentary, and plutonic rocks. There may be as much as 100 feet of relief on the Archean surface (Grout and Broderick, 1919), 122 �but the surface was, nevertheless, essentially a peneplain. Some Archean "knobs" were islands when the Pokegama Formation was being deposited, and are present in the wooded areas between Eveleth and Virginia where they have been re-exhumed. The Pokegama Formation–Biwabik Iron Formation contact is gradational, with some cherty horizons in the upper Pokegama Formation and some sand grains of quartz in the lowest bed of the Biwabik Iron Formation. Various geologists have placed the contact at different stratigraphic levels. Figure 5-7: Measured sections from two drill holes that penetrate the entire Pokegama Formation. Dark shading represents shale, thin blank units represent siltstone, the slanted pattern represents sandstone and siltstone, and the dotted pattern represents sandstone. Modified from Ojakangas (1983). Biwabik Iron Formation This is one of the world's major iron-formations, and the largest in the United States. The formation is 200 to 750 feet thick and consists of four divisions as defined by Wolff (1917). These lithostratigraphic units, now informal members, are from the bottom up, the Lower Cherty, the Lower Slaty, the Upper Cherty, and the Upper Slaty (these are miners' terms, and do not indicate metamorphism; Fig. 5-2). The cherty members are dominantly granular (sand-textured), thick-bedded (several inches to a few feet), and are largely composed of chert and iron oxides. The slaty members are dominantly fine-grained (mudtextured), thin-bedded (less than 1 inch), and composed mostly of iron silicate and iron carbonate with local chert beds. However, these two rock types are interbedded on all scales and are generally gradational. They contain about the same high quantities of silica, 42 to 47 percent (Morey, 1992). The Lower Slaty member is not present at the far western end of the range. There are some diagnostic marker units within the formation. Two stromatolite-bearing intervals several feet thick are present, one at the base of the Lower Cherty member and the other in the middle of the Upper Cherty member. The black "Intermediate Slate" at the base of the Lower Slaty member is reportedly an ash-fall tuff containing about 4 to 5.5 percent aluminum oxide (Morey, 1992). At the top of 123 �the Upper Slaty member are several feet of limestone and dolomite. Most of these marker units, which are prominent in the eastern and central parts of the range, pinch out to zero in the vicinity of Hibbing, about 60 miles from the west end of the range (Severson and others, in press). Virginia Formation There are rare exposures of the Virginia Formation in mines at the east end of the Mesabi Range where it has been metamorphosed by the mafic intrusions of the Mesoproterozoic Duluth Complex. Several holes drilled south of the range to study the underlying iron-formation have been drilled through the Pleistocene cover and have intersected as much as 1,443 feet of the preserved lower part of the formation (Lucente and Morey, 1983). The lower portion of the formation in the drill holes is dominantly black shale. The upper portion of the drill core, while still dominantly shale, contains beds of siltstone and fine-grained feldspathic graywacke comprising thickening- and coarsening-upward turbidite sequences. Ash-fall tuff, cherty sideritic ironformation, chert, and limestone are minor rock types low in the formation. The contact with the underlying Biwabik Iron Formation is gradational. The clastic rocks were largely derived from the Archean rocks to the north, with some contributions from lower Proterozoic rocks to the south (Lucente and Morey, 1983). The Virginia Formation is correlated with the Thomson Formation (Morey and Ojakangas, 1970) that is exposed 60 miles to the south in the vicinity of Carlton and Cloquet, Minnesota, and also with the Rove Formation in northeast Minnesota and adjacent Ontario (Morey, 1967). ENVIRONMENTS OF DEPOSITION, ANIMIKIE GROUP The Pokegama Formation is interpreted to have been deposited in a tidally influenced shallow marine setting near the shoreline, having received clastics from the Archean basement to the north (Ojakangas, 1983). In this model of a transgressing sea, the lower (argillaceous) member was deposited at the shoreline in the upper tidal flat, the middle member of intercalated argillaceous and silty sediment was deposited seaward in the middle tidal flat, and the upper member of quartz sand was deposited still further seaward in a lower tidal flat/subtidal environment. This is illustrated in Figure 5-8. Walther's Law is applicable here, with the vertical facies showing the relationships of the lateral facies. The lowermost Pokegama Formation contains siltstone beds that contain alternating thicker and thinner laminae that have been interpreted as evidence of the diurnal inequality, and are being investigated further for possible clues to the Paleoproterozoic lunar orbit (Ojakangas, 1996). The Biwabik Iron Formation is interpreted to have been deposited seaward of the Pokegama Formation on a shallow marine, tidally dominated shelf (Fig. 5-8). Precipitation of iron minerals including iron carbonate, iron silicate, chert, and perhaps some hematite, occurred on the outer shelf in waters below wave base, giving rise to the mud-textured (slaty) iron-formation. These minerals were likely related to upwelling waters from the deeper part of the basin. 124 �Figure 5-8: Sedimentation model showing lateral relationships of the siliciclastic tidal facies of the Pokegama Formation, the two main facies of the Biwabik Iron Formation, and the Virginia Formation (on the slope?). Thicknesses and geography are not to scale; modified from Ojakangas (1983). The two sand-textured members (Lower Cherty and Upper Cherty) formed in a shallow-water, highenergy environment, as indicated by stromatolites, cross-bedding, and rounded (locally oolitic) grains of iron minerals and chert. Shoreward-moving tidal currents (flood tides) and/or storms may have disrupted the mud-textured sediment (precipitates) and transported sand-sized aggregates into shallower water where they were altered by seafloor processes and early diagenetic processes. Thus these granules are interpreted as "intraclasts" derived from within the basin. Shallow channels up to a mile wide and tens of feet deep were cut into the Lower Slaty member and filled with sand-textured grains of iron minerals and chert in the Virginia horn area. These grains apparently were derived from shallow water and carried seaward into the deeper water environment in which the iron minerals were precipitating. Ebb-flow tidal currents, or offshore flowing storm-generated currents, are interpreted as the erosion and transportation agent. A plot of 102 cross-bed measurements in the Minorca Mine (Fig. 5-9) on the northeast edge of the Virginia horn shows 90 percent of the readings making a very prominent mode to the north–northeast and a minor, broader mode to the south (Fig. 5-9). This distribution is interpreted as the product of a strong flood tide toward the paleogeographically determined northern shoreline and a much weaker ebb tide. A study of the orientations of stromatolite mounds in the stromatolite horizon within the Upper Cherty member was conducted by Boerst (1999). His map is presented in Figure 5-10. A paleocurrent plot of mound elongation (Fig. 5-10) is interpreted as the result of shore-normal tidal currents and shore-parallel longshore currents in shallow water. The repetition of the cherty and slaty members has long been interpreted as the result of transgression and regression (White, 1954). The Virginia Formation was deposited seaward of the iron-formation, probably in a slope-type environment (Fig. 6-8) where episodic turbidity currents deposited graded beds. Some volcanic ash falls 125 �evidently settled into the basin forming graded beds with a totally volcanic composition. The dominance of black, fissile shale suggests the "raining out" of clay (such as settling through the water column) and deposition in deep, anoxic water below the wave base. Minor, thin, sandstone lenses were deposited by bottom currents (Lucente and Morey, 1983). N 102 Figure 5-9: A. Paleocurrent rose diagram of 102 cross-bed measurements from the Lower Cherty member (LC-4 submember) in the Minorca Mine. Inset B - Photo of cross-bedding in the Minorca Mine. Inset C - Photo of herringbone cross-beds in the Minorca Mine 126 �Figure 5-10: Mapped stromatolite mounds in the algal submember (I submember) in the Upper Cherty of the LTV 2E pit. The rose diagram represents the elongation of the mounds, with each elongate mound plotted on both sides of the rose diagram. From unpublished work by Kevin Boerst (1999). MINERALOGY Details on the origin of the primary iron minerals of the Biwabik Iron Formation, or any BIF, are exceedingly complex and controversial and are beyond the scope of this guidebook. In brief, Eh and pH are major controls on the stability of the various iron minerals and possible colloidal precipitates in the depositional environment and subsequent diagenetic environment. A variety of primary chemical precipitates for iron-formation in general have been postulated by an assortment of authors and include siderite, iron hydroxide/oxyhydroxide, iron silicates (Konhauser et al., 2002; Rajan et al., 1996), nontronite and iron oxides (Hiemenz, 1997), and colloidal iron silicates (Lascelles, 2007). Metamorphic effects, especially in the easternmost Mesabi Range, modified the mineralogy through progressive devolatilization reactions and modification or near obliteration of primary textures, especially as the contact of the Duluth Complex is approached. Recrystallization and replacement of primary iron minerals and granules during diagenesis and metamorphism has been extensive, and consisted of a number of discrete events. Earlier work on the oxidized taconites of the western Mesabi Range was accomplished by Bleifuss (1964). He showed that late hematite was developed by the oxidation and pseudomorphic replacement of magnetite octahedra, that layers of goethite were precipitated from solutions likely derived from the oxidation of siderite, and that some goethite formed by the oxidation of acicular iron silicate minerals. Additional work was done by Ojakangas in Zanko and others (2003). All of the magnetite grains are euhedral and are interpreted as late diagenetic in origin. Some of the hematite inclusions and crystals in magnetite are similar to those illustrated by Han (1982). He proposed that much of the magnetite formed by the replacement of, and overgrowth on, pre-existing hematite that served as nuclei. Han further suggested that ionic diffusion of ferrous iron was a key process in the formation of the magnetite. Organic carbon may have acted as a reductant in this process. Some of the magnetite may also originate from replacement of precursor siderite as observed in many examples in drill core and polished thin section from the western and central Mesabi Range. The genesis of the high-grade (natural) ore bodies that occur as pockets along fault zones in the Biwabik Iron Formation has long been debated. It is clear that a major hydrologic event removed 40 to 60 percent 127 �of the silica and oxidized the iron minerals to hematite and goethite. However, it is unknown whether these fluids were descending, cool, meteoric waters or ascending hydrothermal waters related to igneous activity. Did this event occur during the Cretaceous (the age of conglomerates that contain clasts of highgrade hematite), or prior to that time? Morey (1999) provided an excellent review of the arguments. He then proposed that a large-scale, topography-driven, hydrothermal ground-water system moved waters northward, during the Paleoproterozic, through the sands of the underlying Pokegama Formation, from the vicinity of the regional Penokean orogenic uplift in northern Wisconsin and east-central Minnesota. Graber and Strandlie (1999) questioned this concept and pointed out that the lack of metamorphosed natural ore bodies in the eastern Mesabi Range proves that they were formed long after emplacement of the Duluth Complex rather than during the Paleoproterozoic. The mineralogy of the Eastern Mesabi Range is reviewed in detail by Gundersen and Schwartz (1962) and that of the Dunka Pit area by Bonnichsen (1975). The following discussion serves to compliment the work of these researchers with work performed to date by Cliffs Natural Resources geologists in the area of Northshore Mining’s Peter Mitchell Mine. In contrast to iron formation of the Western Mesabi Range, which is dominated by greenschist metamorphic facies assemblages, rocks of the eastern Mesabi have been subjected to middle greenschist through upper amphibolite facies metamorphic conditions. Hornblende and pyroxene hornfels facies are also observed at the contact of iron formation with the Duluth Complex. Progressive effects of contact metamorphism toward the contact of the Duluth Complex have produced distinct mineral assemblages that document changing metamorphic conditions and concomitant changes in mineralogy. These changes are readily apparent in the highly reactive mineral assemblages of the Biwabik Iron Formation. The Biwabik Iron Formation in the vicinity of the Peter Mitchell Mine consists of higher grade metamorphic assemblages that include magnetite, iron-rich chain silicates, and quartz. Minerals found to occur in the Biwabik Iron Formation from the Peter Mitchell Mine and Dunka Pit are given in Table 5-1 below. The overlying pelites of the Virginia Formation are generally quartz-muscovite-biotite-microcline-plagioclase+/-chlorite schist with local zones of carbonaceous+/pyrite+/-pyrrhotite phyllite. Minerals found in the Virginia Formation in the area of the Peter Mitchell Mine are provided in Table 5-2. Excluding Cu-Ni PGE mineralization, the mineralogy of gabbroic rocks of the Duluth Complex in this area is relatively simple: labradorite, anorthite, augite, olivine group minerals, with accessory magnetite, ilmenite, and apatite. Table 5-1. Minerals found in the Biwabik Iron Formation in the area of the Peter Mitchell Mine and Dunka Pit. Amphibole Group Grunerite Ferro-actinolite (minor) Ferrohornblende (minor) Ferro-pigeonite Cummingtonite Pyroxene Group Hedenbergite (minor) Ferrohypersthene (minor) Ferroselite Diopside Olivine Group Fayalite Other Silicates Quartz Oxides/ Hydroxides Magnetite Carbonates Phosphates Siderite Hydroxylapatite Ferroan Clinochlore Hematite (minor) Ankerite Pyrite Biotite Goethite Rhodocrosite (minor) “Anthraxolite” Oligoclase Ilmenite Ferroan Dolomite Ferroan Kutnahorite Calcite Andesine Almandite Andradite Cordierite Sekaninaite? Wollastonite Epidote Titanite 128 Other Minerals Pyrrhotite �Table 5-2: Mineralogy of the Virginia Formation in the vicinity of the Peter Mitchell Mine. Silicates Muscovite Biotite Quartz Microcline Anorthite Oligoclase Andesine Anthophyllite Cordierite Staurolite Wollastonite Ferroan Clinochlore Andalusite Sillimanite Almandine Epidote Titanite Other Minerals Ilmenite Pyrrhotite Pyrite Chalcopyrite Rutile Ankerite Calcite Graphite/Carbonaceous material Quartz is the most abundant mineral in the Biwabik Iron Formation and constitutes layers of recrystallized chert. It also occurs pervasively in the matrix of the iron formation admixed with varying amounts of other fine-grained minerals such as magnetite, and minerals of the amphibole group. Quartz grains are equant to irregular and granoblastic. Fayalite also occurs as equant, granoblastic grains with quartz along with fine-grained magnetite, especially in the M, N, and O submembers (see later discussions on submembers of the iron-formation). In addition to quartz, fayalite is commonly associated with ferrohypersthene, cummingtonite, grunerite, magnetite, and rarely cordierite. Sekaninaite, the Feanalogue of cordierite, has been tentatively identified in one sample by x-ray diffraction methods. Magnetite is the second most abundant mineral in the Biwabik Iron Formation and is the ore mineral of interest in taconite units of the entire Mesabi Range. The occurrence of magnetite in the Eastern Mesabi Range is described in detail by Gundersen and Schwartz (1962) and Bonnichsen (1975) and will be only briefly reviewed here. In the Peter Mitchell Mine and at Dunka Pit, magnetite is abundant in the F through O taconite submembers where it constitutes ore. Equant magnetite grains range from submillimeter to locally 5 mm in size and are invariably euhedral due to their high placement in the idioblastic series due to their high force of crystallization, similar to garnet group minerals. Magnetite occurs in distinct laminae and in thin to thick, planar to wavy layers of variable continuity that consist of profusely intergrown grains with minor amounts of fine-grained quartz. Many magnetite-rich layers in the Upper Cherty member exhibit parting surfaces containing coarse-grained aggregates of magnetite crystals. Magnetite also is found as fine- to coarse-grained isolated clusters or “granules” in recrystallized chert, and as fine-grained replacement of algal stromatolites (I submember) and intraformational conglomerate clasts. Magnetite-rich layers alternate with quartz-rich (recrystallized chert) layers and amphibole-rich (generally cummingtonite-grunerite) layers. Poikiloblastic textures of magnetite are only seen upon microscopic examination of grains in polished thin section and reveals poikiloblasts enclosing quartz with very minor amounts of amphibole group minerals and fayalite. Minerals of the amphibole group are found throughout the formation with grunerite and cummingtonite most prevalent. These amphiboles are very fine- to very coarse-grained, subhedral, prismatic, isolated 129 �crystals or occur as intergrown crystals forming divergent fascicles, rosettes and radial, acicular aggregates in the matrix or in distinct bands of variable thickness. Poikiloblastic crystals of cummingtonite-grunerite can be observed macroscopically and microscopically to enclose grains of quartz and magnetite. Ferrohornblende and ferroactinolite, although relatively abundant, are difficult to distinguish from minerals of the cummingtonite-grunerite series macroscopically. Cummingtonite, grunerite, ferrohornblende, and ferroactinolite have all been observed replacing retrograded pyroxenes. Minerals of the pyroxene group are dominated by ferrohypersthene and hedenbergite in most of the submembers. Bonnichsen (1975) reports ferro-pigeonite and diopside locally at Dunka Pit. All of the pyroxenes in iron formation are medium- to very-coarse-grained, subhedral, variably poikiloblastic and exhibit replacement by amphiboles locally. Pyroxenes increase in both modal abundance and grain size as the contact with the Duluth Complex is approached (west-northwest to east-southeast across the Peter Mitchell Mine). Bonnichsen (1975), and Cliffs Natural Resources observations, show that ferrohypersthene crystals are disproportionately large compared to other mineral species when the mineral is found in abundance. Ferrohypersthene crystals, from 2-7 cm, are not uncommon in close proximity to the Duluth Complex and can be coarsely poikiloblastic containing quartz, magnetite, fayalite, hydroxylapatite, and amphiboles. Growth of these pyroxenes is not encumbered by previously existing or slower growing crystals. Almandine garnet occurs locally in the Biwabik Iron Formation, largely restricted to the Lower Slaty and Lower Cherty members, as 2 mm to 2 cm, red-brown subhedral to euhedral crystals generally associated with amphiboles, biotite, and ferroan clinochlore. Andradite garnet is commonly found in the A and G submember, in similar habit to almandine, at the Peter Mitchell Mine. Most of this garnet is a constituent of the silicate-rich bands, but it is also locally found in late-stage calcite-quartz veins with diopside and ferrohornblende. Layer silicates are subhedral and more common in the “slaty” layers imparting a schistosity. Oligoclase, andesine, rare carbonate group minerals, ilmenite, epidote, and hydroxylapatite are generally very finegrained and comprise matrix constituents. METAMORPHISM Contact metamorphism has affected the Biwabik Iron Formation and Virginia Formation disproportionately; assemblages in the iron-formation are very reactive and sensitive to more subtle changes in P-T compared to the Virginia Formation. The pelitic Virginia Formation lacks diverse mineral assemblages and records few reactions because many of the minerals are stable over large ranges in temperature and fH2O. In contrast, the Biwabik Iron Formation preserves prograde metamorphic reactions several kilometers west and north of the contact with the Duluth Complex; whereas, contact effects within the Virginia Formation, including partial melting, are limited to within 200 meters from the Duluth Complex. A very visual and pronounced example of the contact effects of the Duluth Complex is observed in the A submember at the top of the Biwabik Iron Formation. The A was originally a thin, “dirty” limestone-dolomite layer that was drastically transformed mineralogically into an off-white and dark-speckled rock with an assemblage containing typical calc-silicate contact metamorphic minerals that include wollastonite, diopside, andradite, ferrohornblende, titanite, andesine, epidote, fluorapatite, and recrystallized calcite and dolomite. Profound thermal effects of the Duluth Complex on adjacent metasediments are largely due to its high emplacement temperature. Chalokwu and others (1993) estimated emplacement temperatures of the Partridge River intrusion of the Duluth Complex to be approximately 1,150°C. Klein (1973), and subsequent researchers, have noted that olivine-bearing assemblages in iron-formation have largely been described from contact metamorphic occurrences involving high temperature intrusions. 130 �Taconite units of the Biwabik are characterized by regular interlayering of magnetite-rich and quartz-rich layers ranging from <1cm up to 12cm thick, locally. Outcrops farthest from the Duluth Complex display prominent banding with sharp contacts between magnetite, quartz, and quartz-amphibole layers. Boudining of quartz-rich layers, re-healed structural discontinuities, and intraformational conglomerate clasts are observed locally within taconite submembers. The degree of metamorphism and deformation increases with proximity to the Complex. Outcrops nearest the Compex lack pronounced banding compared to exposures that are more distal in the aureole. Exposures in close proximity to the Complex exhibit larger grain size, increased reaction band thickness, and more diverse mineral assemblages that can include quartz, magnetite, ferrohypersthene, hedenbergite, ferroselite, ferrohornblende, ferroactinolite, cummingtonite, grunerite, oligoclase-andesine, ilmenite, titanite, and pyrrhotite. Magnetite grain size is essentially unaffected by increased metamorphic grade, but does exhibit coarsening locally near the contact with the Duluth Complex along with increased Ti content. In the relative close proximity to the Duluth Complex, French (1968) delineated four zones of progressively metamorphosed iron-formation (Fig. 5-11) as follows. Zone 1 = “Unaltered” taconite with quartz, magnetite, hematite, siderite, ankerite, talc, chamosite, greenalite, minnesotaite, and stilpnomelane. Contrary to French (1968) and McSwiggen and Morey (2008), “unmetamorphosed iron-formation” in the Mesabi Range does not exist. All rocks have been subjected to diagenetic effects as well as burial and/or regional metamorphism, not to mention contact metamorphic effects due to the Duluth Complex in the Eastern Mesabi Range. Zone 2 = Transitional taconite with the above minerals but with extensive replacement by quartz and ankerite. French (Fig. 23; 1968) portrays zone 2 as occurring 2.8-10.0 miles from the Duluth Complex but notes that the first widespread metamorphic affects to the iron-formation occur three miles from the Duluth Complex. Zone 3 = Moderately metamorphosed taconite marked by the appearance of iron-rich amphibole (grunerite) at the expense of original iron carbonates (ankerite and siderite) and iron silicates (minnesotaite and stilpnomelane). Zone 3 is present 1.7-3.0 miles from the contact with the Duluth Complex and the temperature of formation was probably 300-400° C. Zone 4 = Highly metamorphosed taconite (sillimanite grade) is well expressed by the appearance of iron pyroxenes and complete recrystallization of the taconite. The taconite consists of quartz, iron amphiboles (cummingtonite and blue-green hornblende), iron pyroxenes (hedenbergite, and ferrohypersthene), and magnetite, with variable amounts of fayalite, garnet, and calcite. Zone 4 occurs within 1.7 miles of the contact with a minimum temperature of formation around 600° C (as indicated by the presence of wollastonite in the A submember). Morey and others (1972) agreed with French (1968) but show the divisions between the four zones to be positioned much closer to the Duluth Complex (see Fig. 5-11). Griffin and Morey (1969) found that hedenbergite is the dominant pyroxene phase in the Zone 3 / Zone 4 transition. Frost and others (2007) have since revised and expanded these zones/isograds to specifically include the formation of ferrohypersthene with crystalline graphite, hedenbergite, fayalite, and orthopyroxene. The reader is referred to this reference for details as well as an extensive list of possible metamorphic reactions in the Biwabik Iron Formation. 131 �#3 R.15W. re y R.14W. French #2 30 36 12 7 13 18 8 4 9 Aurora 27 31 #4 35 34 33 32 2 1 10 11 4 5 6 13 24 25 18 8 17 C h t lu Du le p om x 36 3 Trend of Biwabik Iron Formation Fault 9 8 12 7 Hoyt Lakes 17 14 7 Mo r ey 26 Drill Hole 17700 Biwab ik 11 12 35 Biwabik 10 28 29 31 4 5 23 22 36 30 French #2 Metamorphic zones of French (1968) 22 R.16W. R.15W. R.14W. Mo r ey #3 one mile T.58N. Mo 5 6 #2 Mo 1 Mo r ey #1 re y T.59N. 2 25 34 3 3 26 21 35 34 25 ault on F 33 32 20 19 27 28 29 24 14 26 27 19 24 6 Siph French #1 23 15 16 33 32 31 1 11 17 18 22 21 36 2 13 20 28 23 T.60N. French #4 French #3 22 T.59N. T.60N. R.13W. Metamorphic zones of Morey and others (1972) Figure 5-11: Approximate boundaries of four zones of metamorphosed iron-formation after French (1968) and Morey and others (1972). French (1968) found that metamorphism of the iron-formation was largely isochemical with a progressive loss of H2O and CO2 towards the Duluth Complex; with no change in the siderite and ankerite compositions towards the Complex. Possible reactions in Zone 4 (French, 1968) include: 1. 2. 3. 4. 5. hedenbergite formed by grunerite+calcite=hedenbergite+quartz+H2O+CO2; hedenbergite formed by ankerite+quartz=hedenbergite+CO2; ferrohypersthene formed by grunerite=ferrohypersthene+quartz+H2O; fayalite formed by grunerite=fayalite+quartz+H2O; and fayalite formed by magnetite+quartz=fayalite+O2. Bonnichsen (1968) felt that the metamorphism at Dunka Pit was also isochemical. He suggested that all of the CO2 was driven from the iron-formation; whereas, a portion of the H2O remained behind, although a mechanism of this partial segregation of volatiles was not proposed. Bonnichsen (1968) indicated this water reacted with the various pyroxenes and fayalite to form hydrous minerals, mainly amphiboles, until the supply of H2O was exhausted. However, textural evidence indicating recrystallization of siderite, and some greenalite, directly to hedenbergite and fayalite suggests the overstepping of intermediate minnesotaite and grunerite producing reactions. This indicates a steep thermal gradient over a protracted time interval during emplacement of the Duluth Complex and the local occurrence of pigeonite indicates temperatures of at least 800°C (Vocke, 1981). A rapid rise in temperature during emplacement of the Duluth Complex and overstepping of lower temperature metamorphic reactions would result in a virtually complete devolatilization of the iron formation. Dehydration and decarbonation reactions during prograde metamorphism of iron formation are responsible for a 10-50% volume reduction of the original lithology, depending on the original bulk chemistry (Floran, 1975; Caddey and others., 1990). Volume loss in the Biwabik Iron Formation due to metamorphic devolatilization has enhanced the thinning of this unit eastward across the Mesabi Range. 132 �Closer to the Duluth Complex, Bonnichsen (1968) found that metamorphic grade of the Biwabik Iron Formation reached pyroxene hornfels facies in the Dunka Pit area. At this locale, the iron-formation is positioned ≤¼ mile from the Duluth Complex. It is mainly composed of recrystallized quartz, magnetite, and orthopyroxene – the orthopyroxene is mostly ferrohypersthene with inverted pigeonite (characterized by ferrohypersthene with hedenbergite lamellae). Also present are lesser and variable amounts of Ca pyroxene (hedenbergite, diopside), fayalite, cummingtonite, talc, and hornblende±actinolite (mostly formed during retrograde metamorphism). Bonnichsen (1968) describe poikilitic textured minerals (ferrohypersthene, fayalite, and cummingtonite) in the iron-formation enclosing quartz and magnetite. Perry and Bonnichsen (1966) determined the maximum temperature of metamorphism at Dunka Pit, as determined by O18/O16 ratios of quartz and magnetite, was estimated to be 700-750° C. This was confirmed by Hyslop and others (2008) in their recent O and Fe isotope study of the Biwabik Iron Formation. They showed that isotopically determined temperatures decreased from about 700°C at the contact of the Duluth Complex, to 375°C at a distance of 2.6 km from the contact. Pigeonite in ironformation in contact with the Partridge River intrusion of the Duluth Complex suggests peak metamorphic temperatures in excess of 825°C, as determined by Chalokwu and others (1993). At the Wyman Creek Cu-Ni deposit, Perry and Bonnichsen (1971) estimated a temperature range of 400-650° C for metamorphism of the iron-formation in drill hole #17700 (Fig. 5-11), in which the top of the ironformation is located 525 feet below the Duluth Complex. A more recent study by Muhich (1993) at Dunka Pit, found that where the Biwabik Iron Formation is in direct contact with the Duluth Complex (the intervening Virginia Formation is absent), metamorphism was not isochemical and that some metasomatic transfer of elements took place in a thin zone that spans the contact. This transfer is illustrated by significant gains in titanium in the iron-formation. Overall, within a 25 foot wide zone on one side of the contact, Muhich (1993) found that the iron-formation showed gains in TiO2, V, Al2O3, CaO, Na2O, K2O, Ba, Rb, Sr, MgO, Cu, Ni, and H20; and a loss of SiO2 and P2O5. On the other side of the contact, also in a 25 foot wide zone, the Duluth Complex showed gains in K2O, Rb, S, Fe2O3, and H2O; with a loss of TiO2 and MnO. A temperature of metamorphism for the iron-formation, as determined from coexisting titanomagnetite/ilmenite pairs, was found to be in the range of 651-689° C. PRODUCTION FIGURES—IRON ORE AND TACONITE The annual amounts of direct-shipped ore and taconite produced from the Mesabi Iron Range are shown in Figure 5-12. Production and shipping of direct ore started in 1892 and rose steadily until 1953 when a maximum 76 million tons were produced in one year (note the precipitous drop in direct ore production corresponding to the Great Depression). At around 1955, there was a dramatic decrease in the amount of direct ore as the various mines became depleted. This also corresponds to the initial start-up of taconite mining, using a concentrating and pelletizing method developed by E.W. Davis of the University of Minnesota. Reserve Mining opened the first taconite operations in 1955 (Peter Mitchell Mine) and was shortly followed by Erie Mining in 1957 (the old LTV site). Six more taconite operations were added in the 1960s, and by 1967, annual taconite production exceeded direct ore production. The mid-1980s marked a serious depression in the iron ore and steel industry that resulted in the closure of one operation (Butler Taconite) and the bankruptcies of two other taconite producers including Reserve Mining Company – the former operator of the Peter Mitchell Mine. The Peter Mitchell Mine reopened as Cyprus Northshore Mining in 1989. It was subsequently purchased by Cleveland-Cliffs in 1994 and has been operated as Northshore Mining since that time. More recently, LTV Steel and Eveleth Taconite have closed; Evtac has since reopened as United Taconite. 133 �Figure 5-12: Annual production figures for direct ore (includes all forms of direct ore) and taconite for the period 1892-2008. Data and graph from James Sellner, Minnesota Department of Natural Resources, Lands and Minerals Division, Hibbing, MN WHAT'S IN A NAME? (THOSE CONFUSING IRON-FORMATION SUBMEMBERS) The four-fold stratigraphy of Lower and Upper Cherty and Lower and Upper Slaty members (Wolff, 1917) is still used at each of the currently operating (and inactive) taconite mines on the Mesabi Iron Range. However, each of the mining companies further subdivides the Biwabik Iron Formation into several submembers based on bedding types (Fig. 5-13) and mineral assemblages. It is at this point that the Biwabik Iron Formation stratigraphy becomes very complicated and at times confusing. This is mainly due to the following reasons: • • • • There are localized lateral facies changes between mines (and even within a single mine). Some mines reconcile these differences by splitting out numerous submembers (each with a distinct bedding type, texture, ore grade, and/or mineral assemblage), whereas other mines lump many of these same differences within a single submember. There are significant lateral facies changes over several miles between mines. For example, a particular horizon may be massive-bedded at one location but is regular-bedded a few miles away. This is particularly troublesome within the Upper Cherty member in the western two thirds of the Mesabi range. Not all mines use the same numbering system—some use abbreviations (for example LC—Lower Cherty member) followed by a number (as in LC-5 at the top of the Lower Cherty member). However, other mines use an alphabet system, devised by Gundersen and Schwartz (1962), starting with the A submember at the top of the Upper Slaty member (in this system the top of the Lower Cherty member corresponds to the R submember). And further still, another mine refers to the Lower Cherty member as the number 1 unit and subdivides it into eight submembers, with 1-8 at the top of the Lower Cherty member. Some mines label downward in their numbering system, whereas other mines label upward in their numbering system. 134 �Figure 5-13: Textural characteristics of the Biwabik Iron Formation (from Severson and others, in prep. – modified from Pfleider and others, 1968). The submember nomenclature that is used at each of the mines is summarized in Figures 5-15 through 518. It can readily be seen on these summary charts that submember names change nomenclature from one mine to the next. This is because there are few good marker horizons within the Biwabik Iron Formation, and even these can exhibit gradual lateral facies changes or pinch-and-swell relationships to each other. A few of the potential marker horizons within the Biwabik Iron Formation are presented below. • Top contact of the Biwabik Iron Formation with the Virginia Formation—In the eastern half of the Mesabi Iron Range a carbonate horizon is present at the very top of the Upper Slaty member and the contact between the Biwabik Iron Formation and Virginia Formation is easily recognized (Gruner, 1924). However, to the west of Hibbing, the carbonate layer is absent and lenses of thin-bedded iron carbonate iron-formation are present in the Virginia Formation, and the top of the Biwabik Iron Formation is not easily discerned. • Algal horizon (submember I) – A thin unit containing algal stromatolites and jasper-bearing intraformational conglomerate is present near the top of the Upper Cherty member. This submember is easily recognized but is not present west of Hibbing. • Lower Cherty member - The Lower Cherty member is remarkably homogeneous over most of the Mesabi Range. The major units that are present, depicted in Figure 5-15, range from thin-bedded rocks at the base (Basal Red Unit of Figure 5-15) that is overlain successively by Regular-Bedded, Wavy-Bedded, and Irregular-Bedded & Mottled units respectively. Each of these units have been recently correlated in over 380 dill holes along the Mesabi Range and are so named for the bedding type that is dominant (Severson and others, in prep.). These major units (left side of Figure 5-15) are suggestive of a transgressive period at the beginning of iron-formation deposition (Basal Red Unit) followed by a regressive period wherein the overlying units were deposited in progressively shallower water. As can also be seen in Figure 5-15, each of these units has been called a plethora of submember names by the each of the various taconite mines. 135 �• Base of the Biwabik Iron Formation—The base of the Lower Cherty member is generally characterized by thin-bedded iron-formation (also called the Basal Red Unit - Fig. 5-15) with localized algal stromatolite and basal conglomerate horizons. However, at many localities the base of the Biwabik Iron Formation exhibits a gradational contact with the underlying Pokegama Formation. In the Virginia horn area, the base of the Biwabik Iron Formation contains an ironbearing sandstone (White, 1954) that some mines include with the iron-formation, whereas others lump this type of material with the Pokegama Formation. Figure 5-14: Correlation chart of submembers at each of the mines/areas within the Biwabik Iron Formation (correlations from Severson and others, in prep., except for the Evtac and Butler mines). All columns are hung on the base of the Lower Slaty member. Bars to the left of the columns indicate mined taconite ore zones. Note that correlations at the Thunderbird South Mine (Evtac) and the MSI mine (MSI-Mesabi Steel and Iron) are tenuous. From the above description it is evident that there are few good marker horizons within the upper portions of the iron-formation. Even the top and bottom contacts of the iron-formation are gradational and subject to various interpretations. The "Intermediate Slate," the algal horizon in the Upper Cherty member, and most of the units in the Lower Cherty member are the only easily recognizable marker units. However, even using these horizons as markers, one can see from the correlation charts that there are problems. Clearly, much additional work needs to be done to understand how submembers at one mine correlate with submembers at an adjacent mine. These types of studies could inevitably be important in determining why ore grades, and waste rock characteristics, change between mines and even within a 136 �single mine. Such studies have been initiated by geologists at the Natural Resources Research Institute (Severson and others, in prep.) and over 380 drill holes have been logged and correlated in an area extending from Biwabik to Coleraine, MN. W E Correlation Chart of Lower Cherty submembers at various taconite mines/areas on the Mesabi Range Red bars represent portions of the Lower Cherty that are mined at the various taconite operations. No Scale Implied Lower Cherty thins to the west and is not recognized in Cass County LC-3 Ox Bold Striped LC-4A-upper Variably-bdd, Mott Reg/Irreg-bdd, halos LC-4A-lower Wavy-bdd Wavy-Bdd Unit LC-4B Wavy-bdd w/thick wavy LC-3 Wavy-bdd FeSil LC-4C Wispy/Wavy S&P 1-6 Wavy-bdd w/thick wavy LC-5A Local Thin-bdd Unit not present LC-5B 1-4 1-3 8-3 LC-6 1-2 LC-2 Basal Red Unit LC-1B Basal Contact Units LC-1A no submember designation approx. 300' 280' Avg. thickness of Lower Cherty (in feet) - in the mine handouts rarely drilled LC-5B BoldStriped Mesabi Select Laurentian LC-8 Mesabi Select LC-7 Bold Striped BoldStriped LC-5A Irreg/Congl FeSil (mott) LC-5A Irreg/Congl FeSil (mott) LC-6 Mott LC-4 Wavy-bdd LC-4 Wavy-bdd LC-5 Wavy-bdd no halos LC-3 Wispy Wavy S&P LC-1 bottom of LC-1 or top of basal units algal/congl/basal Ss 250' BoldStriped LC-4 LC-4 3-37' S T halo distribution unknown R through W R halo distribution unknown no halos @ depth LC-3 LC-3 U LC-2 LC-2 V LC-1 LC-1 W 225' 190' 110' LC-1 waste bottom of LC-1 or top of basal units algal/congl/basal Ss rarely drilled "Footwall IF" rarely drilled no submember designation approx. 250' 200' 200' Mesabi Select LC-5A Mott LC-4 Wavy-bdd S&P Northshore LC-5B LC-5A Mott LC-2 ore bottom of 1-0 BoldStriped Mesabi Select Cliffs/Erie (old LTV) LC-3 waste LC-2 LC-1 McKinley/ East Reserve LC-5B incr Si LC-3 Wispy Wavy S&P LC-2 1-0 "Basal Red" rarely drilled LC-5B Utac ? 1-5 Wispy/Wavy S&P not present Reg-Bdd Unit Minntac East Pit 1-7 Wavy-bdd Transition Zone Wk Wavy/Reg-bdd Minntac West Pit Mesabi Select 1-8 Med/Reg-bdd (mott) LC-4A-middle LC-4 Wavy-bdd (w/halos unless otherwise noted) LC-2 oxidized LC-5 Thick/Reg-bdd (Mott) oxidized oxidized Irreg-Bdd & Mott Unit (FeSil) Hibtac LC-1 Ox Mesabi Select Mesabi Select Unit Bold Striped Unit Keetac Siphon Fault Generalized Units Coleraine (OxTac) Figure 5-15: Correlation chart of Lower Cherty submembers at the various taconite mines/areas along the Mesabi Range. Contacts between all submembers are transitional/gradational! Note that the MSI mine area (old Butler mine) is not portrayed on this figure; however, the submembers are similar to those at Keetac. “S&P”= salt-andpepper texture due to disseminated magnetite in chert. From Severson and others (in prep.). W E Correlation Chart of Lower Slaty submembers at the various taconite mines/areas on the Mesabi Range Red bars represent portions of the Lower Slaty that are mined at the various taconite operations. No Scale Implied! Generalized Units Coleraine (OxTac) Keetac Minntac West Pit Hibtac Minntac East Pit Utac Laurentian McKinley/ East Reserve Cliffs/Erie Northshore, & Dunka Pit considered to be ore all the way to top of Upper Cherty Alternate Contact for top of Lower Slaty probably similar to contact of Gruner (1924) and recognized by Severson and others (in prep.) Uppermost Thin-bedded UC-3 LS-10 Middle IBC Middle Thin-bedded 90-115' Possible top of Lower Slaty at Utac 10-65' No Lower Slaty west of Grand Rapids probably similar to contact of White, 1954 10-80' LS Traditional Contact for top of Lower Slaty 15-20' 5-25' gap 25' LS LS all or bottom half Intermediate Slate Total Thickness of Thin-bedded rocks (including IBCs) to bottom of Lower Slaty member LS-9 "Interbedded Cherts" (IBCs) Reg-bedded w/wavy-bedded zones "Intermediate Slate" UC-3 (bottom) UC-2 LUC-2 40-80' LUC-1 Thin-bdd zone observed in hole MGS-2 80-110' UC-1 Traditional top of Lower Slaty 30-90' 85-130' 110-145' LS LS-6 LS-2 LS-6 LS 50-85' P P (mixed thin-bdd, "Mesabi Select equivalent" & slate) 15-20' 8-45' 0-26' LS-1 3' 1-10' 15-20'gap <1.5' locally present 1-2' Thin-bedded iron-formation UC-3A UC-3 UC-2 UC-1 LUC-3 LS-7 25-45' algal layer UC-3 (middle) 100-140' LS-8 LS-7 Traditional Contact for top of Lower Slaty 145-200' LS-9 215'approx. UC-4 LS-10 90-115' LS-8 Lower IBC Lowermost Thin-bdd UC-13 UC-12 UC-11 UC-13 UC-12 UC-11 Upper IBC 230-260' 145-240' 145-200' <6' locally present LS 10-20' Q 25' Q locally present Chalcedonic Chert ("Flint") at Base and top of Lower Slaty Conglomerate Algal columns and/or jasper-bearing conglomerate Paint Rock "Mesabi Select equivalent" (Similar to Mesabi Select Unit but in Lower Slaty !) NOTE: Specific correlations of the IBCs are tenuous as they occur as large lenses in the thin-bedded rocks and are not necessarily laterally-continuous from one mine to the next. No Scale Implied! Figure 5-16: Correlation chart of Lower Slaty submembers at the various taconite mines/areas along the Mesabi Range. Note the three optional contacts for the top of the Lower Slaty. From Severson and others (in prep.). 137 �W E Correlation Chart of Upper Cherty submembers at various taconite mines/areas on the Mesabi Range Red bars represent portions of the Upper Cherty that are mined at the various taconite operations. (dashed red bars represent portions of the Upper Cherty that are possible taconite ore zones) 25'/15 150'/11 145'/12 Generalized Units Minntac West Pit Minntac East Pit Utac Average Thickness of Upper Cherty 140'/15' 125'/10 100'/7 (130-160') (100-160') (90-100') (130-190' downdip) Upper Cherty merges with Lower Cherty at Grand Rapids; both pinch out in Range 26 (White, 1954) (in feet/no. of holes) (120-165') (3-40') (195-100') (Thickness Range) (in feet) downdip 190' Conglomerate Lower Reg-bdd Unit Algal/ conglomerate UC-2 plus Thick-bdd and mottled submarine valley? Thinbedded Upper Cherty Alternatingbedded based on oxidation textures Wavybedded Algal Units UC-3 UC-8 Upper Reg-bdd Unit ? Upper Alt-bdd Unit Algal Unit UC-16 UC-15 UC-15 Bottom Alt-bdd Unit UC-1 UC-16 UC-14 UC-14 UC-7 UC-6 UC-5 upper pit limit ? UC-3 (upper 1/3) Reg/Medbedded 105' Cliffs/Erie (old LTV) Northshore approx. 215' approx. 140' McKinley/ East Reserve Laurentian ? based mostly on units intersected in MGS-2 Hibtac Drastic change in bedding types of Upper Cherty in vicinity of Biwabik, MN Keetac Coleraine (OxTac) G Reg/Wavy H thin Wavy I algal I algal J Thin Wavy J Thick-bdd K Wavy/Mott K Wavy-Bdd w/congl L wide Wavy L Wavy-bdd FeSil Thick-bdd M Reg M Reg-bdd w/ congl N Wavy N Fayalite-Qtz rock low Mgt content O Fayalite-Qtz rock Mgt granules Thick/Irreg-bdd algal Wavy-bdd H algal ? ? O ? Reg-bdd Eastern Mesabi Range No Scale Implied Strongly oxidized - may not always be present wide Wavy Wavy-bdd another 2' thick algal about horizon 5-8 feet below base of Upper Cherty Figure 5-17: Correlation chart of Upper Cherty submembers at various taconite mines/areas along the Mesabi range. Note that the Upper Cherty thins drastically in the central portion of the Keetac area in a valley-like morphology. Note also the distinct change in bedding types that constitute the Upper Cherty in the eastern Mesabi Range and the two possible algal units. Upper Cherty submembers at Coleraine are based solely on oxidation characteristics that have been described in Zanko and others (2003). Submembers at the Cliffs-Erie site, and Northshore and Dunka Pit mines, are described by Gundersen and Schwartz (1962). Modified from Severson and others (in prep.). W E Correlation Chart of Upper Slaty submembers at various taconite mines/areas on the Mesabi Range Generalized Units Average Thickness of Upper Slaty - in feet/no. of holes (Thickness Range/ no. of holes) Minntac West Pit Minntac East Pit up to 260' 115'/10 100'/3 80'/4 (30' to West) (153' @ west end) (98-104'/3) (64-102'/4) Coleraine (OxTac) 25'/9 Keetac (13-33'/9) Hibtac (145 & 205') (89-153'/10) 2 holes - east end Utac Laurentian Cliffs/Erie (old LTV) McKinley/ East Reserve Northshore 120' 93' in MGS-2 (74' &134') in 2 holes Virginia Formation top not drilled not exposed Not Reg-Bedded Unit Alt-Bedded Unit Thin-Bedded Unit Chalcedonic Chert ("flint") unit D very s in o rille d bu sim ne ho ilar le to u (MG t ... nits S-2) at C a liffs re -Eri e A Dolomite/ Limestone Unit valley fill material? Upper Slaty thickens and is mixed with voluminous argillite to the west (White, 1954) - difficult to pick top contact!! Red bars represent portions of the Upper Slaty that are mined at the various taconite operations. B A Dolomite/Marble Vague-bdd B Chert & Diopside Thin-bdd C Thin-bdd D Wavy-bdd E F G Mass-bdd w/ syneresis cracks Dolomite/Marble C D Thin & Wavy E Mass-bdd w/ syneresis cracks F Thin/Alt-bdd Thin & Wavy-bdd Mass-bdd Eastern Mesabi Range No Scale Implied Figure 5-18: Correlation chart of Upper Slaty units and submembers at the various taconite mines/areas along the Mesabi Range. Note that the Upper Slaty exhibits drastic changes in thickness across the Mesabi Range. Submembers at the Cliffs-Erie site, and Northshore and Dunka Pit mines, have been described by Gundersen and Schwartz (1962). Modified from Severson and others (in prep.). 138 �Cliffs-Erie Site (old LTV) Stratigraphy modified from Gundersen and Schwartz (1962) and stratigraphic column in Morey (1993) Stratigraphic Columns for taconite mines on the east end of the Mesabi Range Total iron-fm thickness = 350-470 feet (Grout and Broderick, 1919) Modified from Gundersen and Schwartz, 1962 Virgina Fm Total iron-fm thickness = 350-470 feet (Grout and Broderick, 1919) 35-55' Limestone A Vague BDD, B Chert + Fe Sil C Upper Slaty Virginia Formation Virgina Fm Thin BDD Keweenawan? Sill delaminated A TAC ORE 25-55' D Thin&wavy BDD +/- chert pebbles E Mass w/syneresis cracks, High Phos (columns hung on base of Lower Slaty) Northshore 3-6' A Chert/marble 13-20' B Chert + Diop 26-43' 35-50' C Dunka Pit Thin BDD Keweenawan Sill Thin BDD F +/- gran jasp beds modified from Bonnichsen, 1968 Total iron-fm thickness = 175-300 feet 8-10' D Wavy BDD 2-6' E Sept. cracks 13-20' F Thin/Wavy-bdd w/ Sept. cracks 25-34' G Med-bdd Mgt-rich Beds w/Irreg-bdd zones <5' Algal congl 8-12' H Reg-bdd 3-10 I Algal/congl 15-24' Wavy-bdd w/ thick cs-grn J Mgt beds 30-47' 15-40' Reg BDD w/ wavy-straight BDD Algal 20-35' O wide spaced wavy beds 85-130' P Thin BDD Non-mag Lower Slaty Aurora Sill (also intrudes submembers I through O) B Chert + Diop ? C Thin/Vague-bdd Wavy-bdd thin-spaced F Reg/Med-bdd (slumped), Garnet Alt-bdd w/dk grn G hedenbergite beds (garnet) 30-60' L Wavy-bdd thick bnds @ irreg spacings 9-20' Reg-bdd M w/fayalite 2-5' N Fayalite-qtz 6-17' O Thick-bdd 60' P Vague/Thin-bdd green w/minor Mgt bands Q 10-20' S&P R Thick-bdd, mott 11' S TAC ORE Lower Cherty T Vague, wavy BDD +/- mott U Thin-reg BDD Thick + thin BDD 25-45' Thin BDD V + Arg + jasp W 3' V Wavy BDD +/- mott Thin-reg (Hem) Algal / congl MGT granules Pokegama Fm O Thin-bdd sim ilar to P but better-defined bedding & w/m ore mgt Vague/Thin-bdd P Green (FeSil) wk-mod mag 50-70' Lower Cherty Granule/qtzose Congl Mass/Vague-bdd P Green (FeSil) nil-wk mag Q Thin-bdd & black w/ sulfides & (garnet) Lower Slaty Lower Cherty Pokegama Fm 50' Submember Name: Upper Cherty 3 UC-3 Bedding Descriptions (see abbreviations) 0 Upper Cherty M Straight/Wavy-bdd Average Thickness (in feet) Vertical Scale in feet 50 25 K Wavy-bdd (congl) GraphiticArgillaceous IF R Thick BDD 8' S ? BDD/MGT rich 5' T Granules 10' U Reg BDD? Reg/Mass-bdd w/ H pinch & swell beds I Intraform. Congl Mott, Mixed bdd = J Thin/Med-bdd K' Wavy-bdd (congl) 3-35' 10-20' Q carbonaceous 2' 5-15' Low MGT content The most metamorph & reconstituted 45-95' Upper Slaty E Mass-bdd (no sept. cracks) K 26' "Intermediate Slate" Keweenawan Sill C Thin/Vague-bdd D Thin/Lenticular-bdd 20-40' Wavy BDD w/ intraform rip ups L M SIPHON FAULT TAC ORE 40-80' Keweenawan? Sill TACONITE ORE K wide spaced thin wavy beds T ACONITE ORE Upper Cherty Keweenawan Sill iron-formation metamorphosed by Duluth Complex and thusly contains superimposed metamorphic textures A Chert/marble 20-40' I J Wavy BDD, mott Virg. Fm delaminated A 25-85' H Thin wavy BDD Duluth Compex 20-40' TAC ORE 40-75' Green, wavy reg G BDD Variable!! R thru V can't be distinguished Columns denote submembers of the Biwabik Iron Formation as defined by this investigation coupled with individual taconite mine descriptions along the length of the Mesabi Range. Figure 5-19: Submember nomenclature for the Biwabik Iron Formation as used at the various taconite mines and idled mine sites that will be visited on this field trip. Modified from Plate II in Severson and others (in prep.). Mined horizons are shown by a bar on the side of the column. 139 �Figure 5-20: Bedrock geology at the Mesaba Nugget project. Note the locations of two cross-sections (shown in yellow) that are portrayed in Figures 5-21 and 5-22. Figure 5-21: Cross-section A-A’ at the Mesabi Nugget project. Figure 5-22: Cross-section F-F’ at the Mesabi Nugget project. 140 �Figure 5-23: Modeled geologic section of Northshore Block 21. Diamond drill hole traces and relative unit thickness are illustrated on the section. The general geology of rocks seen at stops 5-6 through 5-11 is shown. Figure 5-24: Modeled geologic section of Northshore Block 1E. Submember thickness is illustrated on the section. The overlying Virginia Formation (VF) and intruding Duluth Complex (GB) are viewed along the south end of the section. 141 �FIELD TRIP STOPS Figure 5-25: Location of field trip stops in the eastern Mesabi Iron Range of northeastern Minnesota. Dark black lines outline the margins of the Biwabik Iron Formation. DIRECTIONS: Drive south from Ely on highway 21 through Babbitt (take a right at the T intersection at Babbitt) and Embarrass to the Four Corners intersection. Take a left at the intersection and go south on 135 about 5.3 miles to a gated entrance for PolyMet Mining and Mesabi Nugget (also known as the Cliffs-Erie site or old LTV mine). Continue down this private drive about one mile to the construction entrance to the Mesabi Nugget plant and turn right – proceed down this road and check in at the guard shack. STOP 5-1: Mesabi Nugget Project Site Location: T. 59 N., R. 15 W., Sections 13 and 24, Allen quadrangle Description: Steel Dynamics Inc. and Kobe Steel Ltd. have jointly partnered in the Mesabi Nugget Project to construct and operate the world’s first large scale Rotary Hearth Furnace (RHF) on the Mesabi Iron Range. The Mesabi Nugget Project is under construction on a portion of the former Erie Mining Company / LTV Steel Mining Company property. The construction site is located on the footwall of the old Area 1 mine site. The ITmk3 RHF technology was developed by Kobe Steel and will be used to produce a domestic supply of high-quality iron nuggets for Steel Dynamics’ electric arc furnace facility in Butler, Indiana. The technology will combine iron concentrates and non-coking coal into a simple onestep process that reduces and melts the iron concentrate within the rotary furnace, producing a 97 percent pure iron nugget. The second phase of the project involves permitting and reactivation of several taconite pits adjacent to the construction site to extract iron ore for the nugget process. The Mesabi Iron Range was initially identified by the first Minnesota State Geologist, Henry Eames, in 1866 at the northeast end of Embarrass Lake, roughly two miles to the west of this project site, at a point where the Biwabik Iron Formation intersected the Embarrass River. The eastern end of the Biwabik Iron Formation was first mapped from Embarrass Lake to Birch Lake by the Minnesota Geological Survey during the years of 1879 and 1881. By 1884 the Duluth & Iron Range Railroad had been built to provide access to the newly discovered Vermilion Iron Range at the southeastern end of Lake Vermilion. The railroad provided access for iron ore shipments from Tower to Lake Superior. The original rail route runs north–south along the eastern boundary of the Mesabi Nugget project site. The Village of Mesaba, shown in Figure 5-21, was the first incorporated community on the Mesabi Range. The village’s rail station, 142 �Mesaba Junction, provided a departure point for the early mineral exploration and new arrivals to the region. Today the rail access into the Mesabi Nugget plant intersects the Canadian National rail line near the site of the original Mesaba Junction. The structural geology and outline of the four informal members of the Biwabik Iron Formation are shown in Figure 5-21 overlying a current aerial photograph of the project area. Four mining areas of the former Erie/LTVSMC comprise the Mesabi Nugget project area: Area 1 to the north, Area 6 & 9 to the west, and Area 2wx to the south. The western mining area is dominated by the northwest striking Donora fault and several northeast trending faults. To the south the Aurora Sill was intruded into the lower portion of the Upper Cherty member and caused a pronounced steepening of the dip in Area 6, as shown in Figure 5-22. The structural complexity of the area has resulted in a diverse and long history of both natural ore and taconite mining. By the early 1900’s there were eight underground and open-pit mining operations within the boundary of the Mesabi Nugget project. The early mining operations extracted natural ore from oxidized zones in the Upper Cherty member associated with the fault structures of the region. The prominent mining operations are shown in Figure 5-21. Figure 5-23 is a cross-section through the Area 2wx mining area. It identifies the gently sloping Biwabik Iron Formation, the Wentworth Fault, a diabase dike intersecting Area 2wx, and the Duluth Complex to the south with associated metamorphic isograds. In the early 1950’s Pickands Mather & Co. constructed and operated a taconite pilot plant adjacent to the present Area 9 pit on the western edge of the project area. During the ensuing years Erie Mining Company was constructed to utilize the extensive ore reserves of high grade taconite of the eastern end of the Mesabi Iron Range. Figure 5-19 is a generalized geologic column of the Biwabik Iron Formation in the Aurora area. NEXT: Return to private drive and proceed approximately 2.5 miles east to the guard shack near the office buildings of PolyMet Mining Corp. After receiving permission to enter the property, go straight and follow this road about 2.3 miles to a T-intersection with Dunka Road (another private company road). Continue north (left) at this intersection and drive about 3.5 miles along various mining company roads to the Pit 5E location. The stratigraphic nomenclature for the Biwabik Iron Formation at the Cliffs-Erie site is presented in Figure 5-19. STOP 5-2: Lower Cherty and Lower Slaty members Location: Pit 5E, Cliffs-Erie site, T. 60 N., R. 14 W., sec. 36, SE, SE Allen quadrangle; UTM: 571,040E/5,275,876N (NAD-83) Description: At this inactive mine pit the entire stratigraphic section of the Lower Cherty and Lower Slaty members can be viewed. Also present at this site are localized exposures of the underlying Pokegama Formation and granitic rocks of the Archean Giants Range Batholith. Both are exposed in the floor of the mine where they are present as several small domal features giving the overall impression of an egg-carton morphology. The Giants Range Batholith at this site is characterized by a porphyritic quartz monzonite that contains an unusual amount of chlorite. The Lower Cherty member at this site is about 80 feet thick. Most of the units are regular- to mediumbedded and constitute taconite ore. At the bottom of the mine bench, the W submember that is characterized by interfingering conglomerate, algal stromatolite horizons, and pale gray chalcedonic chert bands with quartz-filled syneresis cracks; all three can occur in direct contact with either the underlying Pokegama Formation or granitic rocks of the Giants Range Batholith. The stromatolites are recognized by bright red jasper columns. The conglomerate contains a variety of iron-formation clasts (cherty and 143 �thin-bedded iron carbonate) in a chalcedonic matrix with detrital quartz grains. Above the W submember is the V submember which is typically thin-bedded, locally contains chalcedonic chert bands (up to a few inches thick), and appears to be less than one foot thick. The V submember is not exposed in the mine walls (covered by mining rock debris) but can be found as loose pieces on the mine floor in close proximity the first stromatolite horizon that will be visited. These pieces are typically thin-bedded, nonmagnetic, and exhibit fine-scale cross-beds that could be likened to hummocky cross-stratification (HCS). At the bottom of the mine face is a Regular-Bedded Unit that is correlative with the U submember. A one foot thick imbricated conglomerate is present about three feet above the stromatolite horizon in the U submember. This unit transitions upward into a Wavy-Bedded Unit (the T submember), which in turn, grades upward into the Mottled Unit (submember S). Both are similar in that they exhibit magnetite-rich wavy beds that terminate, bifurcate, and pinch and swell in a semi-random manner. The S submember contains conspicuous iron carbonate mottles (ankerite?) that are pink and <1 cm across. The WavyBedded Unit (submember T) at this locality is identical to all of the other wavy-bedded units at all of the taconite mines in the Mesabi Range. Within the T submember the magnetite is in: 1. the wavy beds, 2. disseminated throughout the cherty bands, and 3. within mottles that are generally less than 1 centimeter in diameter and cored by iron-carbonate. The T submember is about 50 feet-thick and is easily recognized in drill core due to the wavy beds and a salt-and-pepper texture that is defined by disseminated magnetite. Cross-beds are observed in some of the fallen blocks that are associated with this unit. Crossbedding measurements taken elsewhere (Ojakangas and others, 2005), although not definitive, are suggestive of a tidally-influenced marine environment. This, coupled with Walther's law of succession of sedimentary facies (the facies observed vertically are also similarly related laterally), places the deposition of the iron-formation seaward of the Pokegama Formation. The Lower Slaty member at this site is 80 feet thick and consists entirely of thin-bedded, weakly to moderately magnetic rock. At the base of the Lower Slaty member is the "Intermediate Slate" which is characterized by thin-bedded, black, organic-rich mudstone that is about five feet thick at this locality. The "Intermediate Slate" is extremely fissile and locally exhibits bright, shiny graphitic surfaces with bedding-parallel slickensides. Pyrite is common to this submember and is present as both disseminated fine- to medium-grained cubes and as thin disks (marcasite/pyrrhotite) along bedding planes. All of the Lower Slaty member constitutes waste rock at this mine. Note that the name "slate" has been applied to all thin-bedded rocks in the Biwabik Iron Formation, but this term is a misnomer, because these rocks are essentially unmetamorphosed and do not have the cleavage of a true slate but merely a parting parallel to bedding (White, 1954). Morey (1992, 1993) reported that the "Intermediate Slate" possibly contains an ash-fall component. It has the highest Al2O3 content of any analyzed sample of the Biwabik Iron Formation. NEXT: From Stop 5-2, return to the south to the intersection with Dunka Road. Take a left and head east along Dunka Road about 1.8 miles. While continuing to the next stop the bus will go through Pit 3 where ore was mined from the Lower Cherty member. Note that the undulating floor of this pit was exposed during striping by following the bedding trends of one or two bed forms. Imagine that this roll-and-swell topography may mimic the original bedding surface of the unlithified iron-formation as it was deposited in shallow water. After going through Pit 3 the road takes a sharp jog to the south. At this locale, the surface trace Siphon Fault is present along the east side of the road. Iron-formation is present to the west of the road and the Virginia Formation is present across the swamp on the east side of the road. Just before Dunka Road curves back to the east take the immediate left and go down a secondary mining road for about 500 feet. Proceed to the north on foot along a flagged trail to the next stop. 144 �STOP 5-3: Virginia Formation near Siphon fault No hammering please! Location: Cliffs-Erie site, T. 59 N., R. 14 W., sec. 26, SE, SE, NE Allen quadrangle; UTM: 569,505E/5,271,610N (NAD 83) Description: This is the only natural exposure of the Virginia Formation on the Mesabi Iron Range. Unfortunately, it is only a few feet thick. A total of 1,443 feet of the formation is present in drill cores from holes drilled south of the range. Note the graded beds, mud chips, concretions, and loading at the bases of these beds. The bedding is near vertical in this location due to proximity to the north-trending Siphon fault—an inferred growth fault (Graber, 1993) wherein the iron-formation decreases in thickness to the east (across the fault) by about 100 feet. NEXT: From Stop 5-3, proceed back to, and angle across Dunka Road to a gated entrance (not locked). Go down this secondary mine road about 0.5 miles. Park and walk about 300 feet to the south. STOP 5-4: Algal submember (I submember) near the top of the Upper Cherty member Location: Pit 2E, Cliffs-Erie site, T. 59 N., R. 14 W., sec. 23, N 1/2, NW Access to this site is via Dunka Road, which is a private mining company road. Allen quadrangle; UTM: 568,202E/5,2706,25N (NAD 83) Description: Algal structures were first described by Leith (1903) as "contorted bedding." Grout and Broderick (1919) were the first who assigned an organic origin to them. The algal submember within the Upper Cherty member consists of mounds of fossilized algal colonies that are separated by jasper-bearing intraformational conglomerate; the overall thickness of this unit is 2 to 20 feet. This horizon occurs only in the eastern half of the range (not present west of Hibbing). However, there is some ambiguity in the positioning of this unit within the Upper Cherty member. In the Virginia Horn area, this horizon is positioned near the base of the Upper Cherty and can be traced upwards in a series of drill holes (Severson and others, in prep.) to a position near the top of the Upper Cherty near Hibbing (see Fig. 517). To the east of the Virginia Horn, the horizon can be traced at the base of the Upper Cherty to a position near Biwabik, MN, where two algal horizons are reported (in drill hole MGS-2 and in the old LTV submember nomenclature). These two horizons occur in the bottom and top of the Upper Cherty member. It is difficult to tell which of these algal horizons is the one that was traced eastward from the Virginia Horn. The lower algal horizon pinches out further east and the upper horizon can be traced as far east as Dunka Pit. The algal horizon that will be visited on this trip occurs near the top of the Upper Cherty member. This locality is an excellent place to view a nearly horizontal portion of the iron-formation that contains abundant individual mounds of algal stromatolites. Stripping of glacial overburden in this area once revealed a dip slope the size of a football field that contained stromatolite mounds (Graber, 1993). Figure 5-10 illustrates a large portion of that exposure that has since been mined. The present stop is at an area located several hundred feet west of that site. Internally, the mounds are characterized by many individual, columnar, finger-like structures that are convex upward. The mounds protrude up through a thin veneer of the overlying thin- to wavy-bedded H submember. Measurements on a nearby mine face in this horizon showed that all the columnar stromatolites were inclined at 30° to the vertical; unfortunately, that site has also been removed by mining. At the extreme eastern edge of this exposure are the J and H submembers. Both of these submembers 145 �contain rare anthraxolite, which is an organic bitumen containing 95 percent or more carbon that is black with a vitreous luster and conchoidal fracture and resembles obsidian (Morey, 1994). Morey (1994) reported that anthraxolite is present throughout the iron-formation but is most common beneath the carbon-rich Intermediate Slate. Furthermore, he suggested it formed via a mechanism of concentrating carbon from a mass-kill phenomenon, followed by later migration of a carbon-rich liquid to form the anthraxolite. NEXT: Return to Dunka Road and proceed eastward about 14.6 miles (through a locked gate about midway down the road) to the entrance and guard shack of the Peter Mitchell Mine. After receiving permission to enter the property, proceed down the road to the offices of Northshore Mining. The next ten stops are on the mine property. The stratigraphic nomenclature of submembers in the Peter Mitchell Mine is portrayed on Figure 5-20. STOP 5-5: Submembers I and G (and lunch) Location: Peter Mitchell Mine, Block 35, T.60N, R.13W., Section 33 Babbitt quadrangle; UTM 574,773E/5,276,403 (NAD-83) Description: This site was chosen as an excellent area to collect unique garnets, up to several inches across, from the G submember (located at the top of the mine bench) and to take another look at the I submember (exposed at the base of the bench near the water). At this locale, the I submember is 3-5 feet thick and consists almost entirely of intraformational conglomerate, often with bright red jasper clasts, and rare algal stromatolites. The G submember contains coarse-grained, brown to brown-red, euhedral andradite garnet. The garnets occur as single crystals and as clusters of crystals. Both exhibit dodecahedral forms – especially when associated with patches that are rich in calcite, quartz, and actinolite. NEXT: The following series of stops offer a review of several submemers of the Upper Slaty and Upper Cherty members. The samples examined to date from these stops exhibit middle to upper greenschist facies metamorphism based on the identified gangue mineralogy. Thin sections reveal gangue mineralogy mostly consisting of quartz-cummingtonite-grunerite. Reflected light studies of ore from these stops indicate a wide range of magnetite grain size and liberation characteristics, which are important parameters for ore grading and classifying. It is important to note that rocks from most of these stops are lower metamorphic grade when compared to the metamorphic grades in the extreme northeast end of the Peter Mitchell Mine and Dunka Pit. STOP 5-6: Submembers K and L Location: Peter Mitchell Mine, Block 21, T. 60 N., R. 13 W., sec. 26, NE, NW Babbitt quadrangle; 578,337E/5,278,665N (NAD-83) Description: Both the K and L submembers of the Upper Cherty member constitute taconite ore. Both are wavy-bedded and contain isolated black chert intraclasts. The K submember is slightly different in that the wavy beds are thinner and more closely-spaced than the wavy beds of the L submember. Petrographic review of samples from the L submember at this stop indicate abundant subhedral magnetite grains (5-15 microns) mainly confined to well-defined bands parallel to bedding. Further contributing to the high-grade nature of this submember are coarse assemblages (granules and pebbles) of similar subhedral magnetite grains disseminated within the cherty beds of the rock. X-ray analysis of a sample from this location indicates gangue mineralogy of quartz, cummingtonite, and grunerite. 146 �Magnetite grains in the overlying K submember typically occur as granules and assemblages of euhedral grains ranging up 200 microns in diameter. The assemblages are randomly distributed within the matrix; however, diffuse bands of magnetite assemblages are observed. X-ray analyses of these samples indicate a gangue mineral suite of quartz, cummingtonite, and grunerite that is consistent with the suite identified for submember L. The magnetite textures in both submembers observed at this location can differ greatly from textures to the east where coarser assemblages are commonly observed closer to the Duluth Complex. These changes in texture can affect magnetite grade and the liberation/grindability performance of individual ores. STOP 5-7: Submembers I, J, and K (all constitute taconite ore) Location: Peter Mitchell Mine, Block 21, T.60N., R.13W., section 24, S ½, NE Babbitt Quadrangle; UTM 578,505E/5,278,416N (NAD-83) Description: Submember K (described above), as well as, submembers J and I, are viewed at this site. The I submember is present at the top of the mine bench but can be readily spotted in loose pieces in the floor of the mine where it is distinguished by: 1. pink coloration; 2. conglomeratic nature; and 3. the presence of ribbon-like stringers of quartz veins that parallel bedding. Metamorphism is higher in this area relative to the previous stops and the I submember does not display the striking algal features, but rather a more “washed-out” appearance. The J submember is typically regular-bedded to weakly wavybedded with close-spaced magnetite-rich beds. A northeast-trending fault is present in this area and is characterized by chloritic breccia. Some weak oxidation (brown-stained zones) is associated with the fault; however, these zones are still strongly magnetic. STOP 5-8: Submembers G, H, and I (all constitute taconite ore) Location: Peter Mitchell Mine, Block 21, T.60N., R.13W, Section 24, N ½, NE Babbitt Quadrangle; UTM 578,617E/5,278,464N (NAD-83) Stay away from the steep wall (loose blocks above)! Description: Submember H at this location displays thin-bedded to weakly wavy-bedded quartz taconite with abundant magnetite. Along with large accumulations of magnetite, a gangue assemblage of quartzgrunerite-ferroactinolite is also displayed within the exposure. The top of the H can host a thin (~1 foot) intraformational conglomerate that separates the H and G submembers (not seen at this location). This feature can be used to map the contact between the Upper Slaty and Upper Cherty members. Note that Gundersen and Schwartz (1962) interpret the G submember as occurring in bottom of the Upper Slaty; whereas the G submember is considered to be at the top of the Upper Cherty at the Cliffs-Erie site and Dunka Pit mine (Figure 5-19). The G submember differs from H in that it is wavy-bedded and contains lensoidal chert bands that often exhibit bulb-shaped pinch-and-swell margins. Reflected light analysis of this exposure displays well-defined bands of coarse magnetite (>50 microns), which also follow the wavy bedded nature of the iron formation typically associated with the H submember. The I submember is subtly noticed as you walk to the north and displays several examples of jasper-bearing algal quartz taconite. Magnetite grains range between fine (<5 microns) and coarse (> 50 microns) mostly occurring as disseminations within the matrix of the unit. Both submembers are typically classified as ore at the Peter Mitchell Mine. Within the G submember, medium to coarse-grained andradite is observed. Bedding in the G submember 147 �is significantly thicker than other submembers at the Peter Mitchell Mine and shows well-developed pinch and swell characteristics. STOP 5-9: Submembers F (waste at this site but ore to the east) and G (taconite ore) Location: Peter Mitchell Mine, Block 21, T.60N., R.13W, Section 24, NE, NE Babbitt Quadrangle; UTM 578,818E/5,278,516N (NAD-83) Description: The F and G submembers of the geologic section at the Peter Mitchell mine can be viewed at this stop. The G submember is described above. The F submember typically consists of alternating magnetite-rich, thin-bedded sets, and thicker-bedded cherty sets (alternating bedding of Figure 5-13). Locally present in the F submember are small septaria-like structures, or syneresis cracks, that consist of whitish quartz-filled subvertical circular and radial fractures in the granular cherty layers. As can be seen at this location, contacts between submembers are typically gradational and are best determined in drill core where changes in bedding characteristics and mineralogy can be readily observed. The F submember can contain appreciable magnetite; however, it is typically classified as waste due to the fine grain size making it difficult to liberate the iron. Magnetite grains observed under reflected light range between <5 microns to 50+ microns in submember F while analysis of submember G identifies a coarser magnetite grain size range of 10 – 50+ microns occurring in easy-to-liberate assemblages. Further micro-examination of the F and G at this location confirm the presence of quartz and ferroactinolite, while common gangue minerals can also include the andradite, cummingtonite, and hedenbergite. At this location in particular, thin zones of cummingtonite crystal growth interrupt the thicker cherty beds within the iron formation. STOP 5-10: Submembers C and D (waste) Location: Peter Mitchell Mine, Block 21, T.60N., R.13W, Section 24, S1/2, NE Babbitt Quadrangle; UTM 578,681E/5,278,133N (NAD-83) Description: The contact between submembers C and D of the Upper Slaty member can be viewed at this location. Both the C and D submembers are thin-bedded units of the Upper Slaty member; however, the D submember is different in that it contains slightly thicker beds and lenses of chert. A 20-foot-thick Keweenawan sill is also present near this stop (see Stop 5-11) and is positioned towards the top of the C submember. Petrographic review of samples from this area display minor magnetite with a gangue mineralogy of cummingtonite-grunerite series amphiboles as well as cordierite and ferroactinolite. The C submember also contains magnetite, fayalite, ferrohypersthene, and chert. X-ray analysis of submember D at this location indicates gangue mineralogy of quartz, cummingtonite, and hedenbergite. Magnetite, although scarce in the C submember, is fine-grained (5-10 microns) and confined to fractures and open-spaces within the matrix of the recrystallized slaty beds. Magnetite grains identified within the D submember are very-fine grained (~5 microns) and disseminated within the quartz-rich matrix of the rock. The C and D submembers are typically categorized as waste based mostly on low magnetic iron values, however, high phosphorous concentrations measured within both submembers also limits them as waste. 148 �STOP 5-11: Keweenawan Sill in the C submember Location: Peter Mitchell Mine, Block 21, T.60N., R.13W, Section 24, N1/2, SE Babbitt Quadrangle; UTM 578,702E/5,278,080N (NAD-83) Description: A 2- to 18-foot-thick sill is present towards the top of the C submember across most of the strike length of the Peter Mitchell Mine and Dunka Pit. A sill with identical chemistry and textures is present within the J submember much further to the west in Pit 2E at the Cliffs-Erie Site (Severson and Hauck, 1997). The sill at this locality, informally referred to as the BIFSill or C-Sill, is generally fine- to medium-grained with locally very coarse-grained plagioclase phenocrysts and vertical columnar jointing. A granoblastic texture is evident in thin-section, indicating that the sill was emplaced in the early Keweenawan and was later metamorphosed by intrusion of the Duluth Complex. Hauck and others (1997) noted that the BIFSill is chemically similar to the Logan sills to the northeast in the Rove Formation, and have informally called this sill a "Logan-type" sill. X-ray diffraction data collected from samples of the sill at this site yield a mineral assemblage of quartzalbite-actinolite-antigorite-illite with magnetite. In reflected light, magnetite is abundant and occurs as randomly oriented lath-shaped crystals believed to represent the pseudomorphic replacement of preexisting ilmenite crystals by magnetite. STOP 5-12: Keweenawan Sill (in the Virginia Formation) and A and B submembers Location: Peter Mitchell Mine, Block 11, T.60N., R.12W, Section 19, SW, SE Babbitt Quadrangle; UTM 581,691E/5,278,881N (NAD-83) Description: At the very top of the Biwabik Iron Formation is a 2- to 6-foot-thick chert and marble unit (A submember) that corresponds to the carbonate horizon that is present in only the eastern half of the Mesabi Range. This unit is locally absent in some areas (non-depositional unconformity) and extremely thick in other areas. The B submember is characterized by alternating chert and diopside bands up to one foot thick; marble layers are locally present. In some areas, pink granophyric veins locally cut the B submember. These veins exhibit pinch-and-swell relationships in that the veins thicken within the diopside bands and pinch in the chert bands. The mineralogy of the marble beds represents an assemblage of calcite-ankerite-wollastonite-quartz, while the B submember consists of a diopside-ankerite-wollastonite-magnetite mineral assemblage. Magnetite grains are scarce within these beds and are very fine grained when present. At the very base of the Virginia Formation is a 2- to 100-foot-thick sill, informally called the VirgSill, that consists of a fine-grained, granoblastic rock with varying amounts of plagioclase, clinopyroxene, orthopyroxene, hornblende, olivine, and biotite. The informal term of "Cr-bearing sill" was first used by Hauck and others (1997) to highlight the relatively high chromium contents (600 to 1,200 parts per million) that are characteristic of this sill. This sill exhibits two varieties: 1) a fine-grained, massive, gray-colored unit (this exposure) that is extremely difficult to distinguish from the hornfelsed Virginia Formation in drill core, and 2) a medium- to coarse-grained unit that is olivine- and/or hornblende-rich and is easily recognized. A Keweenawan age is inferred for the VirgSill, but this sill is decidedly different than the BIFSill and age dates are needed for both of these sills. STOP 5-13: Duluth Complex and metamorphosed Virginia Formation (optional) Location: Peter Mitchell Mine, Block 0, T.60N., R.12W, Section 16, SW, SW 149 �Babbitt Quadrangle; UTM 584,266E/5,280,874N (NAD-83) Description: In close proximity to the Duluth Complex, the well-bedded sediments of the Virginia Formation are typically transformed into a rock that at first appearance looks like an intrusive rock due to the presence of randomly oriented biotite. This rock is informally referred to as the "recrystallized unit," but is more properly classed as a diatexite (Sawyer, 1999). During emplacement of the Duluth Complex, the sediments of the Virginia Formation were heated, generating 20 to 40 percent pervasive partial melts, that enabled these rocks to literally flow in response to stresses that were applied during emplacement. All bedding planes are obliterated and what remains is a medium-grained recrystallized rock that contains plagioclase, cordierite, orthopyroxene, and randomly oriented biotite. Within this recrystallized matrix are blocks/boudins of more structurally competent siltstone and calc-silicate hornfels (originally limey layers). Also at this locality are gabbroic rocks at the basal contact of the South Kawishiwi intrusion. The rocks exposed at this stop consist of weakly to moderately mineralized, fine- to medium-grained, ophitic augite troctolite to olivine gabbro. Copper-nickel values are unknown for this exposure. As you walk to this site you will notice numerous blasted blocks of the C submember with shiny dark green faces along bedding planes and on joint faces. This coating is an iron silicate known as hisingerite (Fe3+2Si2O5(OH)4·2H2O). Also present are blasted blocks with coarse-grained magnetite, and coarsegrained pyroxene that has grown along bedding planes during metamorphism by the Duluth Complex. STOP 5-14: Submembers F-P (Blasted Rock) Location: Peter Mitchell Mine, Block 3E-4E, T.60N., R.12W., Section 9,SW, SE Babbitt Quadrangle; UTM 5282276N, 585022E (NAD 83) Description: This stop was chosen to show the highly metamorphosed nature of the iron formation in close proximity to the Duluth Complex. The iron formation in this area of the pit contains abundant coarse grained iron pyroxenes, iron amphiboles, fine grained fayalite, garnets, and coarse grained granular magnetite. The F and G submembers in this area of the pit are hard to distinguish in drill core and in broken form because the original bedding textures, primarily in the F, have been overprinted or altered by recrystallization of the iron formation. In situ, and in blasted form, the presence of red to brown-red garnet, which is abundant in the G, is the best indicator to use in distinguishing between the two submembers. The I submember in this area is a 1’ to 2.5’ thick quartzite conglomerate retaining no stromatolite textures due to overprinting caused by the intense recrystallization of the chert. In the J through M submembers little has changed with the banding characteristics of the magnetite allowing the submembers to be readily identified in the field. The biggest change seen in the J through M submembers, other than the increase in grain size, is the overall decrease in thickness of each unit. This change in thickness may in part be explained by a decrease in the average thickness seen in the chert bands separating the magnetite bands. This gives each submember an overall condensed appearance as the magnetite bands become more closely spaced. The O and P submembers can be tough to identify in drill core and in the field due to their rather massive appearance. The best indicator to distinguish the two units is the abundant disseminated granular magnetite which occurs in the O. NEXT: Leave the Peter Mitchell Mine and return to Dunka Road. Take a left after the guard shack and cross the Tomahawk Trail. Continue north to a locked gate (permission is needed to enter) and follow several mining roads to the next stop. 150 �STOP 5-15: Dunka Pit (Optional) Location: Dunka Pit T. 61 N., R. 12 W., section. 35, SE, NW Babbitt NE quadrangle; UTM 587,337E/5,286,348N (NAD-83) Description: At the extreme north end of the pit, the iron-formation is in fault contact with the Duluth Complex. The fault is near-vertical, north-trending, and juxtaposes iron-formation, west of the fault, against gabbroic rocks of the South Kawishiwi intrusion (SKI) to the east of the fault. The age of the fault is not easily determined, but faulting appears to have been initiated before emplacement of the SKI. In the immediate area of the fault, a westward change in the character of the iron-formation is obvious over a 20-25 foot wide zone. This change consists of well-bedded iron-formation grading into a banded plagioclase-magnetite rock, that in turn, grades into a troctolitic rock with magnetite lenses. Emplacement of the SKI induced upper-greenschist to middle-amphibolite facies metamorphism of the iron-formation resulting in coarse grain sizes that can be viewed within both ore and gangue minerals. To the immediate west of the fault, the iron-formation contains high amounts of titanium due to the metasomatic transfer of Ti across the fault (Muhich, 1993). Due to its high titanium content, which is detrimental to making steel, the iron-formation immediately adjacent to the fault was discarded during mining. END OF TRIP (return to Ely) REFERENCES Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1840 Ma Sudbury impact event: Geology 33, p. 193-196. Addison, W.D., Cannon, W.F., and Brumpton, G.R., 2007, How to identify Sudbury impact ejecta in the Lake Superior Region: Institute of Lake Superior Geology, 53rd Annual Meeting, May 8-13, 2007, Lutsen, Minnesota, v. 53, Part 1-Proceedings and Abstracts, p. 1-2. Beck, J.W., 1988, Implications for Early Proterozoic tectonics and the origin of continental flood basalts, based on combined trace element and neodymium/strontium isotopic studies of mafic igneous rocks of the Penokean Lake Superior belt, Minnesota, Wisconsin, and Michigan: Minneapolis, University of Minnesota, Ph.D. dissertation, 262 p. Bleifuss, R.L., 1964, Mineralogy of oxidized taconites of the western Mesabi and its influence on metallurgical processes: Transactions of AIME, v. 229, p. 235-244. Boerst, K., 1999, Stromatolites in the LTV 2E pit, Mesabi range, northeastern Minnesota: Unpublished Undergraduate Research Opportunity Project, College of Science and Engineering Office, University of Minnesota Duluth, 4 p. Bonnichsen, B., 1968, General geology and petrology of the metamorphosed Biwabik Iron Formation, Dunka River area, Minnesota: Unpublished Ph.D. dissertation, University of Minnesota, Minneapolis, 240 p. Bonnichsen, B., 1975, Geology of the Biwabik Iron Formation, Dunka River area, Minnesota: Economic Geology, v. 70, no. 2, p. 319-340. Caddey, S. W., Bachman, R. L., Campbell, T. J., Reid, R. R., and Otto, R. P., 1991, The Homestake Gold Mine, an Early Proterozoic iron-formation-hosted gold deposit, Lawrence County, South Dakota. U. S. Geological Survey Bulletin 1857-J. 151 �Cannon, W.F. and Addison, W.D., 2007, The Sudbury Impact layer in the Lake Superior iron ranges: A time-line from the heavens: Institute of Lake Superior Geology, 53rd Annual Meeting, May 8-13, 2007, Lutsen, Minnesota, v. 53, Part 1-Proceedings and Abstracts, p. 20-21. Chalokwu, C. I., Grant, N. K., Ariskin, A. A., and Barmina, G. S., 1993, Simulation of primary phase relations and mineral compositions in the Partridge River intrusion, Duluth Complex, Minnesota: implications for the parent magma composition. Contrib. Mineral. Petrol., v. 114, p. 539-549. Chandler, V.W., 1993, Geophysical characteristics, in Sims, P.K., ed., The Lake Superior region and Trans-Hudson Orogen, Precambrian: Conterminous US: Geologic Society of America: The Geology of North America, v. C-2, p. 81-89. Fralick, P.W., Davis, D.W., and Kissin, S.A., 2002, The age of the Gunflint Formation, Ontario, Canada: Single zircon U-Pb age determinations from reworked volcanic ash: Canadian Journal of Earth Sciences, v. 39, p. 1089-1091. Fralick, P.W., and Kissin, S.A., 1998, The age and provenance of the Gunflint lapilli tuff [abs.]: Institute on Lake Superior Geology, 44th Annual Meeting, Minneapolis, Minn., Proceedings, v. 44, pt. 1, p. 66-67. French, B.M., 1968, Progressive contact metamorphism of the Biwabik Iron Formation, Mesabi Range, Minnesota: Minnesota Geological Survey Bulletin 45, 103 p. Frost, C. D., von Blackenburg, F., Schoenberg, R., Frost, B. R., and Swapp, S. 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Gruner, J.W., 1924, Contributions to the geology of the Mesabi range: With special reference to the magnetites of the iron-bearing formation west of Mesaba: Minnesota Geological Survey Bulletin 19, 71 p. ———1946, The mineralogy and geology of the taconites and iron ores of the Mesabi range, Minnesota: Office of the Commissioner Iron Range Resources and Rehabilitation, St. Paul, in cooperation with the Minnesota Geological Survey, 127 p. Gundersen, J.N., and Schwartz, G.M., 1962, The geology of the metamorphosed Biwabik Iron Formation, eastern Mesabi district, Minnesota: Minnesota Geological Survey Bulletin 43, 139 p. Han, T.M., 1982, Iron formations of Precambrian age: Hematite-magnetite relationships in some Proterozoic iron deposits—microscopic observations, in Amstutz, G.C., El Goresy, A., Frenzel, G., Kluth, C., Moh, G., Wausehkuhn, A., and Zimmerman, R.A., eds., Ore genesis: The state of the art: New York, SpringerVerlag, p. 451-459. 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Rajan, S., Mackenzie, F. T., and Glenn, C. R., A thermodynamic model for water column precipitation of siderite in the Plio-Pleistocen Black Sea. Amer. Jour. Sci., v. 296, p. 506-548. Sawyer, E.W., 1999, Criteria for the recognition of partial melting: Physics and Chemistry of the Earth, v. 24, no. 3, p. 269-279. Schneider, D.A., Bickford, M.E., Cannon, W.F., Schulz, K.J., and Hamilton, M.A., 2002, Age of volcanic rocks and syndepositional iron formations, Marquette Range Supergroup: Implications for tectonic setting of Paleoproterozoic iron formations of the Lake Superior Region: Canadian Journal of Earth Sciences, v. 39, p. 999-1012. Schulz, K.J., 1987, An Early Proterozoic ophiolite in the Penokean orogen [abs]: Geologic Association of Canada Program Abstracts 12, p. 87. ———2003, A Paleoproterozoic suprasubduction zone ophiolite-island arc complex in northeastern Wisconsin [abs]: Institute on Lake Superior Geology, 49th Annual Meeting, Iron Mountain, Mich., Proceedings, v. 49, pt. 1, p. 71-72. 154 �Severson, M.J., and Hauck, S.A., 1997, Igneous stratigraphy and mineralization in the basal portion of the Partridge River intrusion, Duluth Complex, Allen Quadrangle, Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-97/19. Severson, M.J., Heine, J.J. and Patelke, M.M., in prep., Geologic and stratigraphic controls of the Biwabik Iron Formation and the aggregate potential of the Mesabi Iron Range: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-2009/09. Severson, M.J., Zanko, L.M., Hauck, S.A., and Oreskovich, J.A., 2003, Geology and SEDEX potential of Early Proterozoic rocks, east-central Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-2003/35, 160 p. Southwick, D.L., and Day, W.C., 1983, Geology and petrology of Proterozoic mafic dikes, north-central Minnesota and western Ontario: Canadian Journal of Earth Sciences, v. 20, p. 622-638. Southwick, D.L., and Morey, G.B., 1991, Tectonic imbrication and foredeep development of the Penokean orogen, east-central Minnesota—an interpretation based on regional geophysics and the results of test-drilling: U.S. Geological Survey Bulletin 1904, 17 p. Southwick, D.L., Morey, G.B., and McSwiggen, P.L., 1988, Geologic map (scale 1:250,000) of the Penokean orogen, central and eastern Minnesota, and accompanying text: Minnesota Geological Survey Report of Investigations 37, 25 p. Vallinni, D.A., McNaughton, N.J., Rasmussen, B., Fletcher, I., and Griffin, B.J., 2003, Using xenotime U-Pb geochronology to unravel the history of Proterozoic sedimentary basins: a study in Western Australia and the Lake Superior Region [abs.]: Institute on Lake Superior Geology, 49th Annual Meeting, Iron Mountain, MI, Proceedings, v. 49, pt. 1, p. 79-80. Van Hise, C.R., and Leith, C.K., 1901, The Mesabi district: U.S. Geological Survey Annual Report, v. 21, pt. 3, p. 351-370. Vocke, C. M., 1981, T-fO2 conditions of the metamorphism of the Stillwater Iron-Formation, Montana. M.S. Thesis. State University of New York, Stony Brook. White, D.A., 1954, The stratigraphy and structure of the Mesabi range, Minnesota: Minnesota Geological Survey Bulletin 38, 92 p. Winchell, N.H., 1882, The Potsdam sandstone: Minnesota Geological and Natural History Survey Annual Report, v. 10, p. 123-136. ———1893, Twentieth annual report for the year 1891: Minnesota Geological and Natural History Survey, 344 p. Wolff, J.F., 1917, Recent geologic developments on the Mesabi Iron Range, Minnesota: American Institute of Mining and Metallurgical Engineers, Transactions, v. 56, p. 229-257. Zanko, L.M., Severson, M.J., Oreskovich, J.A., Heine, J.H., Hauck, S.A., and Ojakangas, R.W., 2003, Oxidized taconite geological resources for a portion of the western Mesabi Range (west half of the Arcturus Mine to the east half of the Canisteo Mine), Itasca County, Minnesota—a GIS-based resource analysis for land-use planning: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-2001/40, 85 p. 155 �55th Annual Institute on Lake Superior Geology Field Trip 6 GEOLOGY OF THE LAKE ONE TROCTOLITE BY CANOE Jim Miller (Department of Geological Sciences, UMD) 156 �FIELD TRIP 6 GEOLOGY OF THE LAKE ONE TROCTOLITE (BY CANOE) NORTHWESTERN DULUTH COMPLEX Jim Miller (University of Minnesota Duluth) INTRODUCTION The Duluth Complex and associated Keweenawan intrusions in northeastern Minnesota constitute one of the largest mafic intrusive complexes in the world, second only to the Bushveld Complex of South Africa. 2 These rocks cover an arcuate area of over 5,000 km (Fig. 6-1) and give rise to two strong gravity anomalies (+50 & +70 mgal) that imply intrusive roots to more than 13 kilometers depth (Allen and others, 1997). The intrusive rocks of northeastern Minnesota were emplaced into a comagmatic volcanic edifice during formation of the Midcontinent Rift between 1108 and 1095 Ma (Paces and Miller, 1993). The Duluth Complex is physically defined as a more or less continuous mass of mafic to felsic plutonic rocks that extends in an arcuate fashion from Duluth to nearly Grand Portage (Fig. 6-1). It is bounded by a footwall of predominantly Paleoproterozoic and Archean rocks, a hanging wall of largely mafic volcanic rocks and hypabyssal intrusions, and internally, it contains scattered bodies of strongly recrystallized mafic volcanic and sedimentary hornfels. Defining the Duluth Complex more genetically, it is composed of multiple discrete intrusions of mafic to felsic tholeiitic magmas that were episodically emplaced into the base of a comagmatic volcanic edifice in two general stages - an early stage at about 1108 Ma and a main stage at 1099 Ma. Within the Duluth Complex, four general rock series are distinguished on the basis of age, dominant lithology, internal structure, and structural position. Felsic Series – massive granophyric granite and smaller amounts of intermediate rock that occurs as a semi-continuous mass of intrusions strung along the eastern and central roof zone of the complex and was emplaced during early stage magmatism (~1108 Ma). Early Gabbro Series – layered sequences of dominantly gabbroic cumulates that occur in two major intrusions along the northeastern contact of the Duluth Complex and were also emplaced during early stage magmatism (~1108 Ma) Anorthositic Series – a structurally complex suite of foliated, but rarely layered, plagioclase-rich gabbroic cumulates that was emplaced throughout the complex during main stage magmatism (~ 1099 Ma). Layered Series – a suite of stratiform troctolitic to ferrogabbroic cumulates that comprise at least 11 variably-differentiated mafic layered intrusions and occurs mostly along the base of the Duluth Complex. These intrusions were emplaced during main stage magmatism, but generally after the anorthositic series. The Lake One troctolite refers to layered series rocks that occur along the base of the Duluth Complex in the Lake One area (Fig. 6-1). In contrast to most other layered series bodies, it is not designated as an “intrusion” because its eastern extent is not known because of inadequate mapping in the heart of the Boundary Waters Canoe Area (BWCA). This trip will focus on three general aspects of the geology of the Lake One Troctolite: 1) the contact relationships between the Lake One troctolite and Archean rocks of the footwall and anorthositic series of the hanging wall; 2) the general lithostratigraphic variations in through the troctolite and 3) the characteristics of large volcanic hornfels inclusions within the troctolite. The descriptions of the Lake 157 �One troctolite that follow are based largely on field mapping conducted in 1981, 1982, and 1984 as part of my PhD dissertation (Miller,1986). Lake One Troctolite Figure 6-1. Generalized geology of the Duluth Complex and related Keweenawan rocks of northeastern Minnesota highlighting the location of the Lake One Troctolite. Other layered series intrusions are: TI – Tuscarora intrusion, WLI – Wilder Lake intrusion, SKI – South Kawishiwi intrusion, BEI – Bald Eagle intrusion, PRI – Partridge River intrusion, GLI – Greenwood Lake intrusion, OSI – Osier Lake intrusion, WMI – Western Margin intrusion, BLI – Boulder Lake intrusion, DLS – Layered Series at Duluth. Felsic Series intrusions are: CL – Cucumber Lake granophyre, MH – Misquah Hills granophyre, BL - Beth Lake granophyre (and Wine Lake monzodiorite), WL – Whitefish Lake granophyre, I – Isabella granophyre, MW - Mt. Weber granophyre, and FB – Fairbanks/Brimson granophyres. Geology modified from MGS Misc. Map M-119 (Miller et al., 2001). PREVIOUS STUDIES OF THE LAKE ONE TROCTOLITE The geology of the northwestern Duluth Complex was first examined during the early state geological survey conducted from 1875 – 1901. In the fourth volume of the final report, Winchell (1899) accurately showed the contact between gabbroic rocks and older greenstone, metasedimentary rocks, and granite in the Lake One-Snowbank Lake area (Chapter XXII, Winchell, 1899, p. 420). After the discovery of Cu- 158 �Ni mineralization in the Spruce Road area of the basal Duluth Complex in 1948, exploration activity ensued all along the basal contact of the complex, including the Lake One area. William Phinney (University of Minnesota), who was responsible for mapping most of the Duluth Complex in the Gabbro Lake 15’ quadrangle in the early 1960’s (Green et al., 1966), continued reconnaissance mapping to the east in the late 1960’s into what is now the BWCA. Although geologic mapping in this area was conducted entirely by canoe, Phinney + was able to cover considerable ground with the aid of floatplane access to areas deep within the wilderness. (The present strict restrictions on motorized vehicles within the BWCA, which was established in 1978, make such easy access impossible today.) From 1966 to 1970, Phinney reconnaissance mapped parts of nine 7.5’quadrangles in that four year period: Snowbank Lake, Quagda Lake, Isabella Lake, and Lake Insula (all part of the Forest Center 15’ quadrangle), Alice Lake, Lake Polly, Ensign Lake East, and Kekekabic Lake (Fig. 6-2). Unfortunately, no published or open file maps were produced from this effort. Rather, the only documentation of this mapping is a small figure included in Phinney’s (1972) article on the northwestern part of the Duluth Complex contained in the Geology of Minnesota Centennial Volume (Fig. 6-2). Snowbank Lake  7.5’ quadrangle  Figure 6-2. Generalized geology of the northwestern part of the Duluth Complex from detailed mapping in the Gabbro Lake 15’ quadrangle (Green et al., 1966) and reconnaissance mapping elsewhere. Figure from Phinney (1972, figure V-26). Detailed mapping the southern half of the Snowbank Lake quadrangle (NW quad of the Forest Center 15’) was conducted by the author for his PhD dissertation (Miller, 1986). Although most of the mapping focused on the anorthositic series rocks in the quadrangle, the troctolitic rocks exposed between the basal contact and the anorthositic series hanging wall in the Lake One and Lake Two areas was also mapped in detail. This mapping showed the presence of a multiply emplaced suite of troctolitic cumulates centered 159 �on Lake One, which were referred to as the Lake One troctolite in the thesis and in a subsequent paper written from the thesis research on the anorthositic series rocks (Miller and Weiblen, 1990). Although generally troctolitic rocks extend along the basal contact from the Lake One area to nearly the Gunflint Trail, it is not clear whether this is one continuous intrusion or several discrete bodies. ROCK NOMENCLATURE Rock names used in this guidebook will follow the modal rock classification scheme recommended by Miller, Severson, and Green (2002) and shown in Figure 6-3. The modal rock names will also commonly be modified with textural terms that indicate the general grain size and the intergrowth relationship between clinopyroxene and plagioclase, namely ophitic (pyroxene oikocrysts enclose many plagioclase chadacrysts), subophitic (pyroxene encloses only a few plagioclase), and intergranular (pyroxene and plagioclase are both anhedral to subhedral granular). The rock terms may also be modified to indicate the presence of minor or accessory mineral phases (e.g., biotitic, apatitic, olivine-bearing, …). Figure 6-3. Modal classification scheme for gabbroic rocks of the Duluth Complex based on modal proportions of plagioclase (Pl), olivine (Ol), and clinopyroxene + Fe-Ti oxide (Cpx+Ox) (from Miller et al., 2002). Rock type abbreviations at the plagioclase-rich end are: LT – leucotroctolite, ALT – augite leucotroctolite, OLG – olivine leucogabbro, LG – leucogabbro, GA – gabbroic anorthosite, OGA – olivine gabbroic anorthosite, TA – troctolitic anorthosite, A – anorthosite. An alternative, short-hand rock classification scheme is used here that incorporates both the mode and textures of the major minerals into a rock code. Basically, the scheme lists the minerals above 2 modal % in decreasing order of abundance. Uppercase abbreviations indicate a granular habit and lowercase abbreviations indicate an intergranular habit. This scheme is outlined in Table 6-1. Although based on non-genetic descriptions of mode and texture, one can interpret the rock code in terms of a cumulate rock 160 �type whereby granular minerals may be inferred to be cumulus and intergranular minerals may be interpreted to be intercumulus. Table 6-1. “Cumulate” Classification Scheme Granular (Cumulus) / Interstitial (Intercumulus) Mineral Codes PP*/P/p- plagioclase F/f - Fe-Ti oxide O/o - olivine A/a - apatite C/c - clinopyroxene (augite) −/b −/α −/g - biotite I/i - inverted pigeonite H/h - hypersthene, bronzite - amphibole - granophyre * used with anorthositic group rocks Cumulate Code Translation of Some Common Rock Types in the Duluth Complex Ophitic augite troctolite POcf Augite-bearing oxide troctolite POFc Olivine gabbroic anorthosite with poikilitic olivine PPoc Ophitic olivine gabbro PcOf Biotitic, feldspathic dunite with poikilitic plagioclase Opb Intergranular, apatitic oxide olivine gabbro PCFOA Ophitic biotitic augite leucotroctolite POcb ARCHEAN FOOTWALL ROCKS The footwall of the Lake One troctolite in the Snowbank Lake quadrangle is composed of a variety of Archean rocks including granite, greenstone, and metasedimentary rocks (Gruner, 1941; Sims, 1985; Jirsa and Miller, 2004). In the Lake One area, the basal contact projects across the northern channel of Lake One where it outflows into the Kawishiwi River (Fig. 6-4). Along this stretch, calc-alkaline to tholeiitic metavolcanic and metavolcaniclastic rocks with a strong east-west penetrative foliation occur. These units have been interpreted to belong to the upper member of the Ely Greenstone (units Aeu and Aec, Fig. 6-4). Younger metasedimentary rocks including conglomerates, arkosic sandstones and siltstone of the Knife Lake Group lie unconformable on the Ely Greenstone units (unit Akc, Fig. 6-4) and are interpreted to be Timiskiming-equivalent sediments (Jirsa and Miller, 2004). Another major feature of the footwall is the east-west-trending North Kawishiwi fault, which is a major splay of the regional transpressional Vermilion Fault system. The fault projects at an acute angle into the base of the Duluth Complex in the northern Lake One area (Fig. 6-4). That the fault appears to merge into the basal contact and does not obviously cut the Duluth Complex suggests that it may have been reactivated during emplacement of the Duluth Complex and thus controlled the shape of the basal contact. 161 �Figure 6-4. Geology of the Lake One troctolite in part of the Snowbank Lake quadrangle (after Miller, 1986). Geology of the Archean footwall taken from Jirsa and Miller (2004). The red line shows the approximate profile line along which the stratigraphic column in Figure 6-6 is constructed. ANORTHOSITIC SERIES The hanging wall of the Lake One troctolite is composed of various plagioclase-rich rock types of the anorthositic series. The contact between the troctolitic rocks and the anorthositic series is very irregular in shape and in character (Fig. 6-4). In the Snowbank Lake quadrangle, the contact curves southeast to Rock Island Lake and the western shore of Lake Three and then irregularly back to the north through Lake Two. From Lake Two, it curves to the east and more or less parallels the basal contact. Despite the irregular shape of the contact, the troctolitic rocks consistently strike to the northeast (i.e., roughly parallel to the basal contact) and dip to the southeast, except in the Rock Island Lake area where the dip locally reverses to the north to define a synform/antiform structure (Fig. 6-4). Where the contact between the Lake One troctolite and the anorthositic series is structurally disconformable, the change in lithology is abrupt but the two rocks are rarely observed to be in contact. This suggests possible fault contacts, especially in places where the troctolite strikes at high angles to the inferred contact. However, in a few places where the contact can be approximated to within a several meters, troctolitic rocks are observed to 162 �be somewhat finer grained, nonfoliated and hosting scattered anorthosite inclusions, thus implying an intrusive contact. In one area of Lake Two, contacts are marked by zones of troctolite with abundant anorthosite inclusions (unit Dmx, Fig. 6-4). And in another area of Lake Two where the internal structure of the troctolite is semi-conformable to the contact, a more gradational contact relationship is observed. The anorthositic series of the Duluth Complex is composed of a structurally complex suite of medium– to coarse-grained, variably foliated, plagioclase-rich cumulates that occurs over the entire breadth of the complex from the western edge of the Poplar Lake intrusion of the early gabbro series to the southern terminus of the Duluth Complex at Duluth (Fig. 6-1). Over the entire exposure area of the anorthositic series, its lithologic character, internal structural, and contact relationships with adjacent rocks remain fundamentally constant. This is also true of its age of emplacement as two anorthositic series rocks from different parts of the complex have nearly identical U-Pb ages - 1099.1±0.6 Ma and 1099.3±0.6 Ma (Paces and Miller, 1993). The average rock type of the anorthositic series is an altered, coarse-grained, moderately foliated, ophitic olivine leucogabbro composed of about 80% plagioclase. With the exception of some occurrences of granular olivine in sub-cotectic abundance, subhedral to euhedral plagioclase is the only cumulus phase in these rocks. An average rock type is somewhat misleading, however, since lithologic variability on a small (outcrop) scale is a ubiquitous feature of the anorthositic series (Miller and Weiblen, 1990). The range of the anorthositic series rock types results from variations in plagioclase mode from 70-99%; mafic mineral proportions; olivine texture from granular to poikilitic (oikocrysts up to 15 cm across); grain size from medium to very coarse; and abundance of felsic mesostasis up to 20%. Moreover, moderate to nearly complete hydrothermal alteration of olivine, pyroxene and, to a lesser degree, plagioclase is evident in many anorthositic series rocks. Despite variations in texture and modal mineralogy and the complex zoning of individual crystals, one consistent feature of anorthositic series rocks is the constancy of average plagioclase compositions (An65-62; Miller and Weiblen, 1990). This constant An content contrasts markedly with considerable variability in the mg# of mafic silicates and defines a distinct trend of An-Fo variation compared to layered series rocks, which more closely resemble normal gabbroic differentiation trends (Fig. 6-5). The An-Fo variations of anorthositic series rocks are similar in trend to anorthositic rocks of the Middle Banded Zone of the Stillwater Complex, though the latter have overall greater Fo and An contents (MBZ, Fig. 6-5). In general, mineralogical and chemical variations indicative of in situ fractional crystallization are generally lacking in anorthositic series rocks (Miller and Weiblen, 1990). Complex internal structure is perhaps the most cogent feature of the anorthositic series in understanding its formation. Although rarely layered, anorthositic series rocks typically have some degree of plagioclase alignment. The attitude of this foliation, however, is extremely variable on an outcrop scale. Another structural complication is presented by changes in rock type that occur across sharp (but unchilled) to gradational boundaries. Discrete inclusions of one type of anorthositic rock (typically a more leucocratic composition) in another are common (Taylor, 1964; Miller and Weiblen, 1990). In the Snowbank Lake quadrangle, at least three intrusive episodes in the formation of anorthositic series rocks can be distinguished, with each successive episode producing less leucocratic compositions (Miller, 1986; Miller and Weiblen, 1990). Because of these outcrop-scale structural complexities, it is difficult to divide the anorthositic series into lithologic subunits. What is clear however, is that the anorthositic series was emplaced in multiple intrusive events. Nearly all workers who have investigated the anorthositic series have come to the common conclusion that these rocks formed from the emplacment and static crystallization of plagioclase crystal mushes (Grout ,1918; Phinney, 1969; Davidson, 1972; Miller and Weiblen, 1990 ). Miller and Weiblen (1990) cited field, petrographic, and mineral chemical evidence to support the interpretation that the anorthositic 163 �series was formed from multiple intrusions of variably differentiated basaltic magma physically enriched in intratelluric plagioclase of a near constant composition. However, because of their high viscosity, these mushes did not experience significant differentiation after being emplaced into the Duluth Complex. Rather, differentiation and the physical enrichment of plagioclase in anorthositic series parent magma/mushes was accomplished in a deeper staging chamber. Under higher pressures of the lower crust, plagioclase would be bouyant in basaltic magmas. Anorthositic Series Figure 6-5. Fo-An variations of coexisting olivine and plagioclase in gabbroic and anorthositic rocks from the Duluth Complex and other mafic layered intrusions (Skaergaard and Stillwater). Plotted are the average (symbols) and range (lines) of Fo-An values determined from microprobe analyses of 21 anorthositic series rocks from Snowbank Lake quadrangle. (see Miller and Weiblen, 1990) Most studies of the Duluth Complex interpreted the unique attributes of the anorthositic series as indicating an origin that was early, similarly unique, and unrelated to that of the layered series. In most areas of the complex, inclusions of anorthositic series rocks within the layered series abound as we will see in the Lake One area. In contrast, inclusions of troctolite have not been reported within anorthositic series rocks. These observations and a well-documented chilled margin of the layered series against the anorthositic series at Duluth (Taylor, 1964) seemed to imply that, in fact, anorthositic rocks were considerably older (and colder) than the layered series (Miller and Weiblen, 1990). However, this interpretation, which came to develop the status of a paradigm (Miller, 1992), was challenged by precise U-Pb ages that showed the anorthositic series and layered series to be essentially the same age of 1099 Ma (Paces and Miller, 1993). 164 �Miller and Severson (2002) suggested that the anorthositic series and layered series were likely tapped in succession from a lower crustal chamber that was initially highly charged with suspended plagioclase. Whereas early extractions from this deep staging chamber was highly enriched in plagioclase, such crystal suspensions were progressively flushed out, resulting in successively less crystal-rich magmas. This model seems best able to explain the lithologic and structural attributes and contact relationships of the Duluth Complex at Duluth and along the northwestern contact area. Moreover, with the 1099 Ma age of the Duluth Complex approximately marking the transition from the latent stage to the main magmatic stage of Midcontinent Rift evolution, this model fits well with the latent stage being a period of extensive underplating of the crust (Miller and Vervoort, 1996; Vervoort et al., 2007). LITHOSTRATIGRAPHY OF THE LAKE ONE TROCTOLITE The Lake One troctolite is composed of a 1.5 kilometer-thick stratiform sequence of troctolitic cumulates exposed between the basal contact and the axis of the synformal structure in the vicinity of Rock Island Lake (Fig. 6-4). Within this stratigraphic section, seven stratiform map units can be distinguished (Figs. 6-4 and 6-6). Figure 6-6 shows the general modal variations across this stratigraphic section as well as map units originally defined by Miller (1986) and their equivalents in Figure 6-4. The basal unit (D1cz) is a texturally and modally heterogeneous (i.e. taxitic) unit that occurs in the lower 50 meters of the intrusion. Modal mineralogy varies from troctolite to olivine oxide gabbro that locally contains signficant amounts of orthopyroxene and biotite. This is commonly interpreted as indicating contamination from the siliceous and volatile-rich footwall (Severson, 1994; Miller and Severson, 2002). Modal layering and igneous foliation is variably developed. This basal contact zone is similar to contact zones of other layered series intrusions in terms of its heterogeneity, however, it is notably poor in Cu-Ni sulfide which is the hallmark of the contact zones on the Partridge River and South Kawishiwi intrusions to the southwest (Fig. 6-1). Between 50 and 300 m above the basal contact, the heterogeneous nature of the contact zone gives way to a more homogeneously textured zone of medium-grained ophitic olivine gabbro to augite troctolite (PcOf to POcf cumulates; unit D1og). The rocks are typically moderately to well foliated and show subtle modal layering. Biotite and orthopyroxene gradually decrease in abundance through the unit. The contact between the D1og and D1at unit is chosen where the augite to olivine ratio consistently drops below 1:2 and isomodal olivine layering becomes intermittently developed. This transition give rises to a monontonous sequence of augite troctolite to troctolite. The largest unit of the Lake One troctolite, D1at, is a consistently medium-grained, moderately to well foliated, ophitic troctolite to augite troctolite (PO, POcf) that is locally well layered. It is also rich in mafic hornfels inclusion as will be described below. Near the top of the unit exposed at the southeast end of Lake One, this unit becomes more consistently layered, enriched in ophitic augite, and coarser grained (Fig. 6-6). This augite troctolite abruptly gives way to a narrow interval of medium-grained troctolite with augite troctolite inclusions and then to rhythmically layered troctolite and melatroctolite (PO/OP). This unit is labeled the D1t’ unit (Figs. 6-4 and 6-6). The abrupt transition between the D1at and D1t’ units seems best explained as marking major recharge event of more primitive magma into the Lake One magma chamber. This D1t’ troctolite unit (D1t’, Fig. 6-4) grades upward into a medium-grained, but coarsely ophitic augite troctolite (POcf) containing augite oikocrysts up to 15 centimeters across (unit D1at’). Together, the D1t’ and D1at’ units are about 150 m and may represent a differentiation cycle. The augite troctolite of the D1at’ unit then abruptly gives way to a homogeneous, well foliated, locally oxide-bearing troctolite (POF-PO cumulate; unit D1t”), which likely represents another recharge event. . Unlike the D1t’ unit, 165 �this troctolite unit is rarely layered in the Lake Two area. The oxide troctolite quickly grades into a medium-grained ophitic augite-bearing troctolite to form the uppermost unit of the Lake One troctolite (unit D1at”). Both the D1t” and D1at” units contain anorthositic series inclusions. Again the area in southern Lake Two is so rich in anorthositic inclusions that it is distinguished as a separate map unit (Dmx). Foliation and intermittent layering in the D1ta” unit defines a synformal structure north of Rock Island Lake that trends to the NE (Fig. 6-6). The oxide troctolite that occurs on the south limb of this synformal structure is presumably the D1t” unit reappearing, however in this case, it displays a very well developed modal layering. Farther southeast of Rock Island ake, the troctolite returns to a southeasterly dip, thus defining an anticlinal structure before descending below the anorthositic series cap. Figure 6-6. Idealized stratigraphic variation in modal mineralogy through the Lake One troctolite approximately along the red profile line shown in Fig. 6-4 (modified from Miller, 1986). Spikes between Plag and Ol fields schematically portrays intervals of olivine isomodal layering. Units defined by Miller (1986) are correlated with map units shown in Figure 6-4. Arrows show the stratigraphic intervals formed by three successive intrusive pulses and subsequent crystallization that are interpreted to have created the intrusion. The monotonous troctolitic composition of the Lake One troctolite is typical of other layered series intrusions comprising the northern and northwestern margin of the Duluth Complex - specifically the Tuscarora Intrusion (Morey et al., 1981; Costello et al., 2009); the South Kawishiwi intrusion (Phinney, 1972; Severson, 1994; Miller and Severson, 2002), and the Partridge River Intrusion (Severson and Hauck, 1990; Miller and Severson, 2002) (Fig. 6-1). Moreover, the cyclical attributes of the Lake One troctolite wherein augite troctolite abruptly gives way to well layered oxide-bearing troctolite/ melatroctolite is commonly observed in these other intrusions. These troctolite-dominated intrusions contrast with other well-differentiated systems like the Layered Series at Duluth and the Greenwood Lake intrusion. (Miller and Ripley, 1996). The igneous stratigraphies of these layered series intrusions 166 �demonstrate a general sequence of cumulus mineral crystallization that is typical of complete fractional crystallization of tholeiitic magmas (Wager and Brown, 1968): Ol(±CrSp) —> Pl+Ol —> Pl+Cpx+Ox(±Ol±Opx)—> Pl+Cpx+Ox+Ap(±Ol), That the Lake One troctolite and its kindred intrusions only display the first part of this crystallization sequence is best explained as indicating that these were systems open to repeated and significant recharge of primitive magma. The cyclical changes in mode and texture in the Lake One troctolite, along with a limited range in mineral compositions (Miller, 1986), suggest that a least three major episodes of magma emplacement were responsible for its formation. INCLUSIONS AND STYLE OF EMPLACEMENT Two general types of inclusions occur in the Lake One troctolite – mafic hornfels inclusions and various types of anorthositic inclusions. Anorthositic series inclusions were described above. I will concentrate on the mafic hornfels inclusions here and discuss what the distribution of the inclusions implies about the emplacement of the Lake One troctolite. The mafic hornfels inclusions range from meters to kilometers in size, but tend to be elongate in the plane of foliation and layering in the troctolite (unit Nmh. Fig. 6-4). The hornfels displays a fine-grained granoblastic texture and mineralogically ranges from troctolite to olivine gabbro. Oxide and orthopyroxene are also locally observed, but no systematic petrographic study of the inclusions has been conducted to determine the most common mineralogic types. Some inclusions contain well distributed coarse gabbroic knots that have been interpreted elsewhere (Miller and Severson, 2002) to represent meta-amygdules (some even show pipe amygdule shapes). Similar inclusions occur throughout the lower sections of layered series intrusions and are generally interpreted to be thermally metamorphosed basalt flows from the lower section of the North Shore Volcanic Group (NSVG). The largest inclusion (or group of inclusions) in the map area occurs between the northeastern part of Lake One and the north arm of Lake Two (Fig. 6-4). Troctolitic rocks in the vicinity of inclusions are coarser and enriched in augite and iron oxide to create olivine gabbro to augite troctolite (mapped as unit D1og). Perhaps this gabbroic aureole formed from volatiles and partial melts driven off the large inclusion. Alternatively, perhaps it represents a fragment that was delaminated from a position originally closer to the lower contact. Interestingly, mafic hornfels inclusions of significant size are almost exclusively contained within the lower two cycles of the troctolite (up to unit D1at’). The only exception to this is the mafic hornfels that will be investigated at Stop 6-13. Also, although anorthositic inclusions occur throughout the Lake One troctolite, large anorthositic series inclusions occur exclusively in the upper cycle (D1t” and D1at”). This strongly suggests that the anorthositic series was emplaced not directly at the unconformity between the Archean and basal lava flow, but instead some unknown distance up into the volcanic pile of NSVG. Interpreting the stratigraphy of the three successive cycles of the Lake One troctolite as indicating emplacement from bottom to top, implies that the first injection of Lake One troctolite magma was at the Archean-volcanic unconformity. This large scale intrusion effectively delaminated the basal lava flows that remained between the Archean footwall and the anorthositic series. The second recharge event was also was largely into the basaltic flows, but the third cycle was emplaced into the base of the anorthositic series. Recent mapping in the Tuscarora intrusion to the east shows a similar relationship (Costello et al., 2009). Two general troctolitic zones, comparable to the Lake One cycles, can be distinguished in the Tuscarora 167 �with the lower one rich in mafic hornfels inclusions and the upper zone exclusively containing anorthositic series inclusions. This provides supporting evidence for the Lake One troctolite being the western extension of the Tuscarora intrusion. STOP DESCRIPTIONS DAY 1 GEOLOGY OF THE LOWER LAKE ONE TROCTOLITE Figure 6-7. Geologic map of the lower Lake One troctolite showing the field stop locations for Day 1. Red dashed line shows the travel route to and from the Kawishiwi Lodge (KW). 168 �Departing from the Kawishiwi Lodge, we will canoe west into the Kawishiwi River channel to the basal contact of the L1T. The geology between the lodge area and the BWCA canoe landing is dominantly metasedimentary rocks (conglomerates, arkosic sandstone and siltstone) of the Knife Lake Group. Downstream of the canoe landing, metavolcanics and metavolaniclastics of the Upper Ely Greenstone predominate. STOP 6-1: Contact of Lake One Troctolite (D1cz) and Archean Metavolcaniclastic Rocks (Aec) Location: NW bank of Kawishiwi River (UTM83: 613225/5309950) Exposed in outcrop forming the point to the southwest is a medium- to coarse-grained, poorly to moderately foliated, subophitic biotitic oxide olivine gabbro (PcOfb) with local patches of pegmatitic gabbro. An irregularly-shaped, meter-sized inclusion of gabbroic anorthosite is exposed in this area. Back to the northeast and inland about 5 meters is a ledge outcrop of a very dense, metasedimentary hornfels. Continuing NE back toward the shore, is more olivine gabbro. Here, the vari-textured nature of the basal contact zone is more evident with grain size ranging from fine to coarse. An interesting feature exposed here are spaced stringers of coarser gabbro at right angles to the moderately developed foliation. Perhaps this represents vapor streaming out of the footwall. At the far northeast end of the outcrop, the contact with metasedimentary hornfels is inferred within a meter of exposure. Canoe SW to the portage. Cross over the portage and leave canoes. We will walk to the SW to view outcrops STOP 6-2: Augite Troctolite of the Lower D1og Unit Location: NW bank of Kawishiwi River at SW end of portage (UTM83: 612927/5309765) Exposed in a pavement outcrop SW of the portage trailhead is a homogeneous medium- to medium coarse-grained, moderately to poorly foliated augite troctolite (POcf). This rock and more gabbroic varieties are typical of the D1og unit. Augite and oxide comprise about 8% of the rock here, but gradually increase in abundance to the NW to about 15%. About 1-2% biotite is also present. Follow the north shore of the channel to the next portage. STOP 6-3: Contact of Lake One Troctolite (D1cz) and Archean Metavolcaniclastic Rocks (Aec) Location: NW bank of Kawishiwi River at E end of portage (UTM83: A – 612560/5309866; B 612658/5309802) Area A - Exposed in along the north bank of the rapids to the west of the portage is a strongly foliated, well-bedded metavolcaniclastic. Foliation and bedding are oriented ~N75E/80SE. The metasedimentary rock is cut by abundant quartz and chlorite veins. Granoblastic texture is locally evident. Area B - Exposed in outcrop forming the point to southeast of the portage landing is a medium coarsegrained, poorly to moderately foliated, biotitic ophitic oxide olivine gabbro (PcOf) to augite troctolite (POcf). High density oxide oikocrysts (<1 cm) stand out in relief and augite oikiocrysts are 1-2 cm across. This rock is unusually homogeneous for being so close to the basal contact. Note also that it is barren of sulfide. Canoe SW to the portage at the west end of the bay. Cross over the portage and continue up the meandering channel about 600 m to an island in the channel. 169 �STOP 6-4: Oxide Troctolite in the Lower D1at Unit Location: Island in river channel (UTM83: 612348/5309210) The exposures on this island are medium-grained, moderately foliated, augite-bearing oxide troctolite (POfc). Subpoikilitic to anhedral granular oxide is about as abundant as ophitic augite (each about 5%). Typically the augite:oxide ratio is 2-3:1. The greater abundance of oxide and its partially granular habit suggest that it is cumulus in part. Analyses of these granular oxide elsewhere indicate that they are complex Cr-Mg-Al-Ti spinels. Note that the rock is subtly plagioclase porphyritic. Also, subtle olivine layering is locally evident, especially on the NW corner of the island. All of these characteristics are common in rocks of the D1at unit. Canoe S to the portage. Store canoes here while we explore the outcrop along the trail and exposures to the east. STOP 6-5: Oxide-rich Melagabbro Intrusion/Inclusion? in the D1ta Unit Location: North end of portage to Lake One (UTM83: 612387/5309040) Outcrops on the west side of the portage trail are altered medium-grained augite troctolite. The mafic phases have been replaced by actinolite and chlorite and plagioclase is partially sericitized. The source of this alteration be related to the rock seen to the east along the river channel. Here we find a coarsegrained to pegmatitic amphibole-oxide-plagioclase rock. Unfortunately, a thin section was not made of this rock to determine whether the green amphibole is primary (hornblende?) or secondary (actinolite?) after pyroxene. Moreover, it is not clear whether this rock is an inclusion (perhaps of recrystallized iron formation) or is an oxide ultramafic intrusion (OUI’s of Severson and Hauck, 1990). The alteration near the portage trail and the abundance of chloritic veins in the area suggest it may be a volatile-rich intrusion. However, the distortion of foliation to a dip of 70° in troctolite on the opposite shore suggests that it might be an inclusion. If the latter, it further implies that Paleoproterozoic strata existed on top of the Archean basement in this area when the Duluth Complex intruded. The last exposure of Biwabik Iron-formation observed in the footwall of the Duluth Complex occurs 30 km to the SE in the Dunka Pit area (Fig. 6-1). Portage canoes to S end of the portage trail. Store canoes here while we explore the outcrop in the campsite area to the west. STOP 6-6: Intermittently Layered Oxide Troctolite (D1ta Unit) Location: Campsite west of south end of portage trail near Lake One Dam (UTM83: 612340/5308833) This stop is mostly intended as a lunch stop. The pavement outcrops at the campsite are medium-grained, layered and moderately foliated oxide troctolite (POFc). Granular (cumulus?) iron oxide up to 3 modal% is present in greater abundance than interstitial augite (1-2%). Olivine-rich layers are common throughout the outcrop. Canoe from portage past island to the S, then head E to SW end of large island. STOP 6-7: Mafic Hornfels (Nmh) in the D1ta Unit Location: Large island in central area of Lake One (UTM83: 612820/5308566) Exposures on the SW end of this island show a mafic hornfels with a generally troctolitic modal composition. Locally, patches and stringers of coarse altered (amphibolitized) gabbro define crude layering that trends NE, roughly parallel to foliation and layering in the bounding troctolite. These 170 �gabbroic areas are interpreted to be metamorphosed amygdaloidal zones of the original basalt flows. As is typical of mafic hornfels, they are cut by a network of chlorite-quartz veins. Canoe into the northern basin of Lake One and head east to the outlet for the north arm of Lake Two. STOP 6-8: Large Mafic Hornfels inclusion (Nmh) in Olivine Gabbro (D1og) Location: Large outcrop knob north of outlet for the north arm of Lake Two (UTM83: 615090/5309395) Exposures in the lower part of the slope are medium- to medium coarse-grained, non-foliated, subophitic olivine oxide gabbro. Progressing NE up the large knob, the gabbro coarsens and then a complex contact with mafic hornfels is encountered. The mafic hornfels is generally fine-grained, granoblastic and varies in mineralogy from olivine gabbro to troctolite. Crs clots of gabbro (meta-amygdules?) are dispersed throughout and the inclusion is cut by abundant chlorite veins. The inclusion on this knob is about 30 meters across, but it is probably part of a high concentration of hornfels inclusions in this area (greatly simplified in Fig. 6-7). Northeast of the inclusion is more olivine oxide gabbro that locally displays a plagioclase porphyritic texture. It is not clear whether this olivine oxide gabbro is a contamination halo affecting troctolitic rocks around the inclusions or whether it is “attached” to the hornfels that collectively comprise a larger inclusion mass included in the D1at unit. Canoe west back into the northern basin of Lake One to an island near the entrance to the northern arm. STOP 6-9: Well-layered Troctolite of the D1at unit Location: Island near the entrance to the northern arm of Lake One (UTM83: 613938/5309517) Exposed along a NE –trending ridge on northern side of this island is medium-grained, well-foliated augite-poor troctolite (PO) that commonly displays modal layering. Olivine enriched layers are 2-10 cm thick and contain 40-45% olivine compared to normal troctolite with 25-30% olivine. Low angle crossbedding is noted locally. In one location, a small (<1 m) anorthositic inclusion is present. A similarly well layered troctolite is exposed on the northwestern projection of the island. Canoe into the northern arm to exposure on E side of channel just before it opens into the northern bay. STOP 6-10: Rhythmically-layered Augite Troctolite of the D1og unit Location: Cliff face on east bank of channel (UTM83: 614164/5310251) Exposed in the cliff face is a rhythmic layering in a medium-grained, poorly foliated, biotitic ophitic augite troctolite (POcfb). The layering is differentially weathered such that the more plagioclase-rich intervals stand out in relief. The dip of the layering is about 40° to the southeast. Some of the layering seems to be graded with olivine-rich intervals having sharp bases and gradational upper parts that grade to normal augite troctolite. Some subtle cross-bedding may also be recognized. Below this layered interval, the rock becomes considerably more gabbroic and vari-textured typical of the contact zone (D1cz). If time permits, we may investigate the contact zone again which is exposed on the west side of the channel. It is similar to the relationships seen at stop 6-1. 171 �DAY 2 GEOLOGY OF THE UPPER LAKE ONE TROCTOLITE Departing from the Kawishiwi Lodge, we will canoe south down the northern arm of Lake One and into the southern basin of Lake One. There we head east toward to the portage to Lake Two. Figure 6-8. Geologic map of the upper Lake One troctolite showing the field stop locations for Day 2. Red dashed line shows the travel route. 172 �STOP 6-11: Autolithic Inclusions near the D1at-D1t’ Contact Location: Small pavement outcrop at NE shore of southern basin of Lake One west of portage entry to Lake Two (UTM83: 613670/5307863) In this small outcrop, four different blocks of troctolitic rock types are exposed in sharp contact with one another as shown in the sketch in Figure 6-9. Based on the cross-cutting relationships of foliation by sharp contacts, rock type 1 is the youngest and rock type 2 is the next oldest. Just to the east of this outcrop, a well layered oxide troctolite of the D1t’ unit occurs that is interpreted to indicate an new magma recharge cycle. Perhaps this auto-brecciation represents the structural disruption that accompanied this recharge event. Figure 6-9. Sketch of contact relations among troctolitic rock types exposed at Stop 6-11 (from Miller field notes, Station 450, Aug. 8, 1981) Head east to the portage to Lake Two. Store canoes and investigate outcrops in landing area. STOP 6-12: Layered Troctolite of the D1t’ Unit Location: Outcrop at portage landing and in stream to the north of the portage. (UTM83: 613925/5307821) The small outcrop at the landing is a medium-grained, augite-poor troctolite (PO) with very subhedral granular olivine. Progressing into the stream, the troctolite here shows rhythmic to intermittent isomodal olivine layering. Olivine layers range from 2 to 20 cm thick. Upstream, the rock gradually increases in augite and oxide mode. At the SE end of the portage, the rock is an augite troctolite (POcf) containing inclusions of mafic hornfels. Head across the two small portages to Lake Two. The next stop will be at the SE end of the second portage. STOP 6-13: Meta-amygdaloidal Mafic Hornfels Inclusions in Oxide Troctolite of the D1t” Unit Location: Outcrop at portage landing and in stream to the north. (UTM83: 614126/5307391) 173 �Pavement outcrops at the landing into Lake Two show a complex mix of mafic hornfels inclusions that locally display coarse clots and stringers of gabbro (meta-amygdules) hosted in a medium-grained, augitebearing oxide troctolite (POFc). An anorthositic inclusion also is exposed in this outcrop. In the troctolite, oxide occurs as small subhedral grains and is equal to more abundant than augite. The troctolite is near the base of the D1t” unit. These mafic hornfels represent the highest stratigraphic occurrence of volcanic inclusions in the Lake One troctolite. Head south from the portage to the SW arm of Lake Two and to the portage to Rock Island Lake. Cross the portage and canoe into the east corner of Rock Island Lak.e STOP 6-14: Northwest-dipping Layered Troctolite of the D1t” Unit Location: Outcrop at the east end of Rock Island Lake. (UTM83: 614038/5305437) Rhythmic isomodal olivine layering is exposed throughout the eastern end of Rock Island Lake. Olivine layers range are 1-10 cm thick. Small granular (cumulus) oxides are present in both the melatroctolite layers and the intervening troctolite. Pyroxene is rare throughout (<1%). The northwesterly dip of the layering here defines a synform structure in this area. The cause of this deformation is not clear (sag over a feeder?), but it likely occurred when the rocks were still semi-molten or at least at a high temperature since no deformation features are noted on a handsample or thin section scale. Canoe along SW along shoreline of Rock Island Lake to bay at south end. STOP 6-15: Anorthositic Series (Das) Location: Sloping pavement outcrop on east side of bay. (UTM83: 613847/5305200) Three different anorthositic rock types are displayed on this outcrop. At the north end is a medium coarsegrained poikilitic olivine gabbroic anorthosite with 5-8 cm oikocrysts of olivine. Across an obscured, but presumably sharp contact is a coarse grained troctolitic anorthosite with large anhedral granular olivine, ophitic augite, and subpoikilitic oxide collectively totaling about 10% of the rock. Across a clearly sharp contact is a medium-grained leucotroctolite containing 15-20 % olivine and very little augite or oxide. This latter rock contains inclusions of the other varieties and is thus the younger intrusion. The outcrop scale variability in anorthositic rock types and the sharp unchilled contact relationships displayed here are typical of the anorthositic series. Canoe and portage back into Lake Two. Head to the campsite on the eastern point of land that separates the SW arm of Lake Two from the main part of the lake. STOP 6-16: Mixed Troctolitic and Anorthositic Rocks of the Dmx Unit Location: Campsite in SW Lake Two (UTM83: 614250/5306703) Exposed over the clean pavement surface at this campsite is a complex mix of anorthositic series rock types as inclusions in a fine- to medium-grained matrix. The anorthositic inclusions range from cm to tens of meters across and show a range of mineralogic and textural varieties typical of the anorthositic series. The intervening troctolite appears to be of two generations, an early finer grained troctolite that is locally Pl-phyric and a later medium-grained type. The contacts between the troctolite and anorthositic inclusions range from sharp to gradational over a several centimeters. This suggest that some of the anorthositic inclusions may have been partially molten when intruded by the troctolite and subsequently became disaggregated. A meter-sized mafic hornfels inclusion is exposed in one area of the outcrop. 174 �Head east through the cluster of islands in southern Lake Two to an island in the east central part of the lake. STOP 6-17: Mixed Troctolitic and Anorthositic Rocks of the Dmx Unit Location: Island in east central Lake Two (UTM83: 615570/5307100) Exposures around the margins of this small island provide another view of the scale and complexity of mixing between anorthositic and troctolitic rock types in the Dmx unit. Anorthositic varieties, which tend to be medium- to coarse-grained, include poikilitic olivine gabbroic anorthosite, granular troctolitic anorthosite, leucotroctolite and nearly pure anorthosite. The troctolite is variable in texture and mode as it locally seems to become hybridized with the anorthositic inclusions. Head north to the entrance to the northern arm of Lake Two. STOP 6-18: Contact between Augite Troctolite (D1ta’) and Oxide Troctolite (D1t”) Location: Point on east side of entrance to north arm of Lake Two (UTM83: 615488/5307646) Shoreline outcrops here display medium-grained, moderately foliated oxide troctolite containing up to 3% anhedral granular to subpoikilitic oxide (POF-POf). Some subtle olivine layering is also evident. Progressing to the north, this rock abruptly transitions in to a coarsely ophitic augite troctolite with pyroxene oikocrysts up to 4 cm diameter (POcf). This transition is taken to define the D1ta’-D1t” contact and is interpreted to represent the third major recharge event that created the Lake One troctolite. Canoeing into north arm of Lake Two to the next stop, we can observe several interesting features in the outcrops lining the western side of the channel. Specific points of interest are labeled on Fig. 6-8. These include: O (615456/5307950) – coarsely ophitic augite troctolite w/ 5cm oikocrysts; also gabbroic pegmatite P (615473/5308241) – gabbroic pegmatite cutting augite troctolite. L (615596/5308493) – modal layering dipping S (sheet jointing dipping N); this is in the D1t’ unit STOP 6-19: Augite Troctolite of the Upper D1ta Unit Location: Sloping pavement at campsite on east side of channel (UTM83: 615488/5307646) Pavement outcrops here are medium-grained, moderately foliated, ophitic augite troctolite (POcf) corresponding to the upper part of the D1ta unit. Augite oikocrysts are present, but do not stand out as much as to the south at the previous stop (6-18). Head north to the landing adjacent to the old dam. Investigate outcrop to the north. STOP 6-20: Olivine Oxide Gabbro Aureole (D1og) Location: Landing on east side channel at northern end of the north arm of Lake Two (UTM83: 615572/5308640) Outcrop ledges along the east bank of the channel show a vari-textured (medium – pegmatitic), subophitic olivine? oxide gabbro similar to that seen yesterday at Stop 6-8. Some streaky textural layering is evident in some exposures. The occurrence of gabbro here demonstrates that the gabbro forms a complete 175 �aureole around the mafic hornfels block (or blocks) that are abundant in this area. The gabbroic pegmatite cutting troctolitic rocks upsection from this (as seen in the channel exposures to the south) may also have emanated from the gabbroic aureole. If so, this would argue for the aureole having developed by contamination of volatile-rich partial melts driven from the hornfels inclusion(s). Return to Kawishiwi Lodge. End of field trip. REFERENCES Allen, D.J., Hinze, W.J., Dickas, A.B., and Mudrey, M.G., Jr., 1997, Integrated geophysical modeling of the North American Midcontinent Rift System: New interpretations for western Lake Superior, northwestern Wisconsin, and eastern Minnesota. In: Ojakangas, R.J., Dickas, A.B., Green, J.C., (eds.) Middle Proterozoic to Cambrian Rifting, Central North America: Geological Society of America Special Paper 312, p.47-72. Costello, D. E, Miller, J.D., Jr., and Jirsa, M.A., 2009, Geology of the Tuscarora intrusion, northeastern Minnesota and its relationship to the anorthositic series of the Duluth Complex. 55th Annual Institute on Lake Superior Geology, Program and Abstracts vol. 55, p 14-15. Davidson, D.M., Jr., 1972. Eastern part of Duluth Complex. In: Sims, P.K. & Morey, G.B. (eds.) Geology of Minnesota - A Centennial Volume. Minnesota Geological Survey, p. 354-360 Green, J.C., Phinney, W.C., & Weiblen, P.W., 1966. Gabbro Lake quadrangle, Lake County, Minnesota. Minnesota Geological Survey Miscellaneous Map M-2, scale 1:24,000. Grout, F.F., 1918, Internal structures of igneous rocks; their significance and origin with special reference to the Duluth Gabbro. Journal of Geology 26, 439-458. Gruner, J.W., 1941, Structural geology of the Knife Lake area of northeastern Minnesota: Geological Society of America Bulletin, v. 52, p. 1577-1642. Jirsa, M.A., and Miller, J.D., Jr., 2004, Bedrock geology of the Ely and Basswood Lake 30’ x 60’ quadrangles, northeast Minnesota. Minnesota Geological Survey Miscellaneous Map M-148, scale 1:100,000. Miller, J.D., Jr., 1986, The geology and petrology of anorthositic rocks in the Duluth Complex, Snowbank Lake quadrangle, northeastern Minnesota. unpublished Ph.D. dissertation, University of Minnesota, 280 p. Miller, J.D., Jr., 1992, The need for a new paradigm regarding the petrogenesis of the Duluth Complex. 38th Annual Institute on Lake Superior Geology,p. 65-67. Miller, J.D., Jr., and Ripley, E.M., 1996, Layered intrusions of the Duluth Complex, Minnesota, USA. In Cawthorne, R.G., ed., Layered Intrusions: Amsterdam, Elsevier Science, p. 257-301. Miller, J.D., Jr. and Severson, M.J., 2002, Chapter 6. Geology of the Duluth Complex. In Miller et al., Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, 207p. Miller, J.D., Jr., and Vervoort, J.D., 1996, The latent magmatic stage of the Midcontinent Rift: a period of magmatic underplating and melting of the lower crust. 42nd Annual Institute on Lake Superior Geology, Proceedings Volume, Part I – Program and Abstracts, p. 33-35. Miller, J.D., Jr. and Weiblen, P.W., 1990, Anorthositic rocks of the Duluth Complex: Examples of rocks formed from plagioclase crystal mush. Journal of Petrology 31, p. 295-339 Miller, J.D. Jr., Green, J.C., and Severson, M.J., 2002, Chapter 1. Terminology, nomenclature, and classification of Keweenawan igneous rocks of northeastern Minnesota. In Miller et al., Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, 207p. 176 �Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Miscellaneous Map Series, M-119, scale 1:200,000 Morey, G.B., Weiblen, P.W., Papike, J.J., and Anderson, D.H., 1981, Geologic map of the Long Island Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-46, scale 1:24,000 Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: geochonological insights to physical, petrogenetic, paleomagnetic and tectonomagmatic processes associated with the 1.1 Ga Midcontinent rift system: Journal of Geophysical Research, v. 98, no.B8, p. 13,997-14,013. Phinney, W.C., 1969, The Duluth Complex in the Gabbro Lake quadrangle, Minnesota. Minnesota Geological Survey Report of Investigation 9, 20 p. Phinney, W.C., 1972, Northwestern part of Duluth Complex. In: Sims, P.K. & Morey, G.B. (eds.) Geology of Minnesota -A centennial volume. Minnesota Geological Survey, p. 335-345 Severson, M.J., 1994, Igneous stratigraphy of the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/TR 93/34, 210 p. (with plates) Severson, M.J., and Hauck, S.A., 1990, Geology, geochemistry, and stratigraphy of a portion of the Partridge River intrusion: Natural Resources Research Institute, University of Minnesota-Duluth, Technical Report, NRRI/GMIN-TR-89-11, 236p. (with plates). Sims, P.K., 1985, Generalized bedrock geologic map of the west-central Vermilion district, northern Minnesota. U.S. Geological Survey Miscellaneous Investigations Map I-1529, scale 1:48,000. Taylor, R. B., 1964. Geology of the Duluth Gabbro Complex near Duluth, Minnesota. Minnesota Geological Survey Bulletin 44, 63 pp. Vervoort et al., K. Wirth, B. Kennedy, T. Sandland and K.S. Harpp, 2007, The magmatic evolution of the Midcontinent rift: new geochronologic and geochemical evidence from felsic magmatism, Precambrian Research 157 (1–4) (2007), pp. 235–268 Wager, L.R. and Brown, G.M., 1968. Layered Igneous Rocks. San Francisco: W.H. Freeman, 588 pp. Winchell, N.H., 1899, The Geology of Minnesota. Geological and Natural History Survey of Minnesota, Final Report v. 4, 354p. w/ 100 plates. 177 �55th Annual Institute on Lake Superior Geology Field Trip 7 ARCHITECTURE OF AN ARCHEAN GREENSTONE BELT: STRATIGRAPHY, STRUCTURE, MINERALIZATION Graded bedding in greywacke of the Lake Vermilion Formation, Pike River dam (stop 7-14) "In the region occupied by these rocks are found immense bodies of magnetitic and hematitic iron ore. In the talcose slates and primitive schistose rock are veins of quartz, carrying auriferous and argentiferous sulphides of iron and copper." Henry H. Eames, 1866 Dean Peterson (Duluth Metals Ltd. & PRC) Mark Jirsa (Minnesota Geological Survey) George Hudak (Dept. of Geology, UW-Oshkosh & PRC) 178 �FIELD TRIP 7 ARCHITECTURE OF AN ARCHEAN GREENSTONE BELT: STRATIGRAPHY, STRUCTURE, AND MINERALIZATION Dean M. Peterson (Duluth Metals Limited & PRC) Mark A. Jirsa (Minnesota Geological Survey) George J. Hudak (University of Wisconsin Oshkosh) INTRODUCTION Archean greenstone belts are one of the world's premier geological settings for hosting a variety of economically important mineral deposit types. These deposits include high-grade iron ore, lode-gold, volcanogenic massive sulfide, komatiite associated nickel, magnesite, and a number of others. The origin of these deposits are intrinsically linked to the architecture of the greenstone belt, namely the interrelationships between stratigraphy, structural setting, and multiple generations of hydrothermal fluids. The Vermilion district of northeastern Minnesota contains one of the classic granite-greenstone terranes in the United States. This district comprises the south-central part of the Wawa subprovince of the Superior Province of the Canadian Shield, and has been broadly correlated with the Saganagons Assemblage of the Wawa subprovince in northwestern Ontario (Peterson et al., 2001; Peterson and Patelke, 2003). In Canada, the Wawa subprovince hosts numerous lode gold (e.g. the Hemlo and Renabie districts) and volcanic-hosted massive sulfide (VMS) ore bodies (e.g. the Winston Lake, Willroy, Big Nama Creek, Willecho, and Geco deposits; Fyon et al., 1992). The Vermilion district is well known for its numerous, previously mined, massive hematitic iron ore deposits. These iron deposits were discovered in the early 1880s, and virtually all-subsequent exploration efforts in the region were targeted on similar iron-formation hosted hematite deposits. However, the discovery of world-class ore deposits in Ontario (the Kidd Creek VMS deposit in 1964 and the Hemlo gold deposit in 1980) led to short periods of both base metal and gold mineral exploration in the Vermilion district. To date, no lode gold and/or VMS ore bodies have been discovered in the Vermilion district, although abundant evidence exists that future exploration may result in the discovery of economically important deposits. GREENSTONE BELTS A strong debate continues on the origin, development, and architecture of Archean greenstone belts, particularly with regard to the roles of subduction, plume magmatism, rifting, diapirism and autochthonous vs allochthonous development (e.g. de Wit, 1998; Hamilton, 2003). Studies in the Superior and Slave Provinces of Canada indicate that strongly contrasting tectonic styles may have been in operation at the same time. For example at ca. 2.7 Ga, large diapiric batholiths and synclinal greenstone keels may suggest that diapirism was an important tectonic process in the Slave Province (Bleeker, 2002), whereas Stott (1997) suggests that the linear distribution of belts suggests that accretionary tectonics (i.e. plate tectonics) may have dominated in the Superior Province. Neither theory precludes the other, and in developing models for Archean tectonic evolution, no one model will be equally applicable to all areas. Hoffman (1990) presents a model of greenstone belt formation via arc-trench progradation as an application of the principle of lateral and temporal equivalence, also known to sedimentologists as "Walther's Law". In this model (Fig.7-1), accretion of the overriding plate in a subduction zone involves scraping of material off the downgoing plate and arc magmatism. The off-scraped material consists of sediment and the tops of igneous bathymetric highs (ie. island arcs, remnant arcs, seamounts, oceanic island chains, submarine plateaus, fracture zones and microcontinents). 179 �Figure 7-1. Prograding arc-trench model for the generation of granite-greenstone terranes. On the left is an idealized cross-section of an arc-trench system: AS, asthenosphere; CUP, cumulate underplating; LM, lithospheric mantle; MA, magmatic arc; MM, magma melting and mixing; PM, zone of partial melting; TR, subduction trench. Fore-arc accretion is achieved by scraping of sediment and topographic highs off of the downgoing plate. Examples of bathymetric highs that could be scraped off and eventually form greenstone belts are depicted on the right. Modified from Hoffman, 1990. We hope to portray our present understanding of the Vermilion district in the context of its geological architecture—highlighting the interrelationships between stratigraphy, structure, and mineralization. The trip will revisit many outcrops on which so many historical discussions, many of them heated, pertaining to the regional geologic setting of the Archean occurred. Many of the stop descriptions in this field trip have been modified from the Field Trip Guidebook of the 50th Annual Meeting of the Institute on Lake Superior Geology, which was held in Duluth in May of 2004. In particular, volcanology and hydrothermal alteration within the Lower Ely Greenstone was described in detail by Hudak et al. (2004), gold mineralization north of the Mud Creek shear zone was described in detail by Peterson and Patelke (2004b), and classic outcrops in northeastern Minnesota were described by Jirsa et al (2004). Readers of this guidebook should review these documents for detailed descriptions. REGIONAL GEOLOGICAL SETTING Supracrustal rocks in the Vermilion district consist of volcanic-dominated stratigraphic sequences of the Wawa subprovince of the Superior Province of the Canadian Shield. Rocks of the Wawa subprovince in northern Minnesota are divided on the basis of stratigraphic and structural setting into: (1) the Soudan belt, to the south, and (2) the Newton belt, to the north (Jirsa et al., 1992; Southwick et al., 1998). The boundary between these contrasting structural panels can be traced geophysically across the width of Minnesota, and was designated informally as the Leech Lake structural discontinuity (Jirsa et al., 1992). In the region west and north of the Soudan Mine, the Leech Lake structural discontinuity occurs along the Mud Creek shear zone (Hudleston et al., 1988), small segments of the Vermilion and Wolf Lake faults (Sims and Southwick, 1985), and the Bear River fault (Jirsa et al., 1992). The Soudan belt contains large, broad folds involving calc-alkalic and tholeiitic volcanic strata overlain by, and locally interdigitated with, turbiditic rocks. In contrast, the Newton belt consists of elongate, northeast-trending, and mostly northward-younging volcanic and volcaniclastic sequences. Volcanic 180 �rocks of the Newton belt differ from those of the Soudan belt in containing locally abundant komatiitic flows and peridotitic sills. The two belts are fault-bounded, and the relationship between stratigraphic units within each belt is largely conformable, although faults obscure contacts locally. In its eastern extension, the Soudan belt is continuous with the Saganagons assemblage in Ontario and terminates against the Saganaga pluton and Northern Light Gneiss. The Newton belt extends discontinuously eastward into the Shebandowan District of Ontario to form the Greenwater and Burchell assemblages. Intrusive rocks in both belts vary from gabbroic and felsic porphyries demonstrably related to volcanism, to large plutons emplaced post-tectonically. Both districts contain unconformable, Timiskaming-type sequences composed of calc-alkalic volcanic rocks, conglomerates, and finer grained sedimentary rocks. A simplified regional geological map of the Neo-Archean terranes of northeastern Minnesota and adjacent Ontario is presented in Figure 7-2. Figure 7-2. Simplified correlation map of Neoarchean assemblages across the U.S. - Canada border (modified from Peterson et al., 2001). Inset illustrates major subprovinces of the southwestern Superior Province. Lithostratigraphic units in the western Vermilion district include: (1) the Lower member, Soudan Iron Formation member, and Upper member (Upper Ely) of the Ely Greenstone, the Lake Vermilion Formation (including the informally named Britt and Gafvert Lake sequences), and the Knife Lake Group of the Soudan belt; (2) the Bass Lake sequence (Peterson and Jirsa, 1999) and the Newton Lake Formation of the Newton belt; and, (3) syn- to post-tectonic granitoid intrusions of the Giants Range batholith, and a suite of post-tectonic alkalic stocks and plutons. Contacts between the different units are typically conformable, although considerable overlap in time and space is documented between volcanic and sedimentary sequences (Southwick, 1993). Rock types associated with the lithostratigraphic units in the area are presented in Table 7-1. 181 �Table 7-1. Lithostratigraphic units within the western Vermilion district (from Peterson and Jirsa, 1999). Intrusive Rocks Late Intrusions Vermilion Granitic Complex Plutons and stocks of syenite, monzonite, diorite, and lamprophyre Granite, schist, amphibolite, and schist-rich migmatite Giants Range Batholith Granite, granodiorite, monzodiorite, and schist-rich migmatite Supracrustal Rocks Newton Belt Newton Lake Formation Tholeiitic and komatiitic basalt flows, intrusions, and clastic strata Bass Lake Sequence Tholeiitic basalt lava flows, iron-formation, and felsic porphyries Soudan Belt Knife Lake Group Lake Vermilion Formation Graywacke, slate, conglomerate, & sheared equivalents Graywacke, slate, dacitic tuff, and minor conglomerate Gafvert Lake Sequence Dacitic to trachyandesitic lava flows, tuffs, and intrusions Britt Sequence Tholeiitic basalt lava flows Upper Ely Greenstone Tholeiitic basalt lava flows and iron-formation Soudan Iron Formation Layered cherty iron-formation, epiclastic rocks, and tuff Lower Ely Greenstone Calc-alkalic & tholeiitic basalt-rhyolite lava flows, tuffs, epiclastic rocks and minor iron-formations STRUCTURAL GEOLOGY Periods of generally N-S directed compression resulted in three major regional deformation events in the Neoarchean terranes of northern Minnesota. The earliest deformation event (D1) produced broad, locally recumbent folds within the Soudan belt and major fault zones throughout the region. In the Newton belt, D1 was accommodated by thrust imbrication of large crustal blocks, resulting in mainly northward stratigraphic facing. Field relationships indicate that uplift, faulting, and the deposition of Timiskamingtype clastic sedimentary sequences in local fault-bounded basins occurred late in D1 deformation (Jirsa, 2000). A large, map-scale structure related to D1 deformation in the western Vermilion district is the Tower-Soudan Anticline, which is a west-plunging anticline within which the axis and plunge changes orientation along strike from nearly vertical in basalts to shallow NE plunging in the western sedimentary rocks. Axial-planar cleavage associated with this early fold typically is lacking, although Bauer (1985), Hooper and Ojakangas (1971), Hudleston (1976), and Jirsa et al. (1992) have described early cleavage (S1) locally. A second deformation event (D2) associated with synchronous regional metamorphism resulted in foliation development and structures having largely dextral asymmetry. D2 is constrained in the Vermilion district to the time period 2674 to 2685 Ma (Boerboom and Zartman, 1993), and between about 2680 and 2685 Ma in the Shebandowan (Corfu and Stott, 1998). Because D2 deformation affected all of the supracrustal rocks in the area and is reasonably constrained by geochronology, the regional foliation (S2) can be used in the field to temporally relate other structural, intrusive, and deformation events. The relationship between S2 fabric and shear structures indicates that most shearing occurred relatively late in the D2 event. Major shearing that produced the Mud Creek and related shear zones is attributed to the late stages of D2 dextral transpression. Structures related to the third deformation event (D3) include abundant NE- and NW-trending faults that dissect the stratigraphic assemblages. Named structures related to D3 include the NE-trending Waasa and 182 �Camp Rivard faults east of the Soudan Mine area, and the WNW-trending, crustal-scale Vermilion and related faults that form the Wawa-Quetico Subprovince boundary. ECONOMIC GEOLOGY Since the mid-1860’s, numerous mineral exploration programs have been conducted in the Vermilion district. Most of these exploration programs focused on identifying minable deposits of massive hematitic iron ores, such as those mined between 1883 and 1962 in the Soudan Iron Formation at the Soudan Mine. During the 1980s and early 1990s, subeconomic lode gold mineralization was discovered in close proximity to the east-west-trending Murray shear zone, which dissects the Lower Ely, and in close proximity to the Mud Creek shear zone, which separates the Soudan belt from the Newton belt to the north. Four VMS prospects occur within the Lower member of the Ely Greenstone, and occur in close proximity up-section from a semiconformable quartz-epidote alteration zone that extends for at least 19km along strike in the north limb of the Tower-Soudan Anticline (Peterson, 2001). These VMS prospects include the Skeleton Lake prospect (drilled by Exxon, 1972), the Eagles Nest prospect (drilled by Newmont, 1988), the Fivemile Lake prospect (drilled by Teck, 1994), and the Purvis Road prospect (drilled by Rendrag, 1999). Recent studies of these three types of mineral deposits in the Vermilion district link stratigraphy and structure, and thus help unravel the architecture of the greenstone belt. Brief descriptions of hematite, VMS, and lode gold mineralization in the field trip area are presented below. Origin of Massive Hematite from Algoma-Type Iron Formation Most iron ores mined today comprise the iron oxide minerals magnetite, Fe3O4 (72% Fe); hematite, Fe2O3 (70% Fe); goethite, Fe2O3(s) * H2O, (63% Fe); and limonite, a mixture of hydrated iron oxides (up to 60% Fe). The world’s most important iron ore resources occur in iron-rich sedimentary rocks (20% - 40% Fe) known as banded iron-formations (BIFs), which occur on all continents and are almost exclusively of Precambrian age. In many iron-mining districts, e.g., the Mesabi Range of northern Minnesota, the BIFs are mined as iron ore with the iron content concentrated into pellets (~65% Fe) in large on site facilities. In other districts, e.g., the historic Vermilion Range of the Soudan Mine area; Hamersley District, Western Australia, the BIFs are the source rocks for large, natural high-grade concentrations of iron that typically occur as bodies of massive hematite and/or hematite-goethite with >60% Fe. The origin of these important natural concentrations of iron minerals remains highly debated. The Fe atoms in hematite are all Fe3+, whereas in magnetite they are comprised of two Fe3+ and one Fe2+ atoms. Therefore, the transformations of magnetite to hematite, or hematite to magnetite, in Fe-conservative systems is always a redox reaction, with the Fe2+ atoms in magnetite oxidized to Fe3+ atoms, or the Fe3+ atoms in hematite reduced to Fe2+ atoms, by reactions such as: 2Fe3O4(mt) + 1/2O2(g) → 3Fe2O3(hm) (1) and 3Fe2O3(hm) + H2(g) → 2Fe3O4(mt) + H2O (2) In the past, the study of the transformation of magnetite to hematite (1) and conversely hematite to magnetite (2) in natural systems has largely focused on these reactions, which require either an oxidizing or reducing agent and an occurrence under specific redox environments. Since virtually all of the known concentrations of high-grade iron ores are hematite-dominant, the exploration for such deposits has concentrated on supergene enrichment of magnetite-rich BIFs. In this model, magnetite-rich BIFs are uplifted and subjected to weathering under oxygenated conditions to form goethite-rich ores; and are subsequently buried and metamorphosed to hematite-rich ores (Morris, 1985). Problems with this model have recently been discussed by Ohmoto (2003) for the Tom Price Mine of the Hamersley district, and are applicable to the origin of the massive hematite ores of the Soudan Mine. Ohmoto (2003) has proposed an alternative mechanism for the transformation of magnetite-rich iron-formations to massive hematite ores by the acid-base reaction: Fe3O4(mt) + 2H+ ↔ Fe2O3(hm) + Fe2+ + H2O (3) 183 �Similar to most acid-base reactions, reaction (3) would be most efficient at high temperatures, and such hydrothermal fluids are capable of leaching silica as well as Fe2+ from magnetite. In addition, the conversion of magnetite to hematite by reaction (3) produces a volume decrease of 32%, greatly increasing permeability of the rocks, which would facilitate further water-rock reactions and enhance conversion of banded chert-magnetite to massive hematite. VMS-Associated Volcanic and Hydrothermal Alteration Processes, Lower Ely Greenstone The Lower Ely Greenstone is composed of calc-alkalic and tholeiitic basalt and basalt-andesite, lava flows, tuffs, and lapilli tuffs with subordinate felsic lava flows, tuffs, epiclastic rocks, and iron formations (Schulz, 1980; Southwick et al., 1998; Jirsa et al., 2001; Hudak et al., 2002a, 2002b; Hoffman, 2007; Jansen et al., in press). The Lower Ely Greenstone has been subdivided into the older Fivemile Lake Sequence and the younger Central Basalt Sequence (Peterson and Patelke, 2003). The Central Basalt Sequence is correlative with the Armstrong Lake Basalts (Jirsa et al., 2001). The Fivemile Lake Sequence is characterized by abundant primary mafic and felsic volcaniclastic strata, highly amygdaloidal basalt and basalt-andesite pillow lavas and sheet flows, multiple selvege pillows, and epithermal-like zinc stringer mineralization that, taken together, indicate a shallow subaqueous depositional setting (Hudak et al., 2002a; Hudak et al., 2002b; Hudak et al., 2007; Hoffman, 2007). The overlying Central Basalt Sequence/Armstrong Lake Basalts comprise exceptionally well-preserved, generally sparsely amygdaloidal sheet flows and single selvege pillow lavas which are locally interstratified with subordinate banded iron formation and chert exhalite horizons, mafic tuff (interpreted primarily to be resedimented hyaloclastite), lithic wackes and polymict breccias that have been interpreted to have formed in a deep subaqueous setting (Peterson and Patelke, 2003; Peterson et al., 2005). Major and trace element data (Hudak et al., 2007; Hoffman, 2007; Jansen, in press) indicate that the lithogeochemistry of volcanic rocks in the Lower Ely Greenstone is more complicated than previously thought (e.g. Southwick et al., 1998). In the Fivemile Lake Sequence, basalt and basalt-andesite flows and volcaniclastic rocks illustrate arc-like lithogeochemical characteristics, and rhyodacites and rhyolites have FI- and FII chemistries. In the Central Basalt Sequence, basalt and basalt-andesites with arc-like lithogeochemical signatures transition up-section into basalts with MORB-like trace element patterns and rhyodacites and rhyolites illustrate FI, FII, and FIII lithogeochemical characteristics. Hudak et al. (2007), Hoffman (2007) and Jansen et al. (in press) have proposed a tectonic model for the development of the Lower Ely which encompasses initial arc development followed by back-arc rifting and associated MORB-type and FIII-type volcanism immediately prior to the deposition of the overlying Soudan Iron Formation. This model appears to be most consistent with the observed stratigraphic, volcanological, and lithogeochemical characteristics of the Lower Ely, as well as studies of both ancient and modern back-arc basins which illustrate that the development of back-arc basins in oceanic volcanic arc settings are commonly associated with the presence of vigorous submarine hydrothermal activity which may produce Algoma-type iron formations and/or volcanogenic massive sulfide deposits (Franklin et al., 2005; Piercey et al., 2004). Geological mapping by Peterson (2001) has indicated the presence of a regional semiconformable quartzepidote alteration zone extending for at least 19 km along strike within the Lower Ely along the north limb of the Tower-Soudan anticline. This type of alteration is a common feature in many Archean VMS camps (e.g. Noranda (Gibson, 1989) and Snow Lake (Skirrow and Franklin, 1994)), and is attributed to silica- and calcium-dumping that occurs in the deep, sub-seafloor as downwelling hydrothermal fluids are heated to temperatures in excess of 350°C (Franklin, 1986; Franklin, 1993; Franklin et al., 2005). Semiconformable alteration zones associated with VMS systems are generally much larger in area than their associated mineralization, and therefore provide exploration geologists regional areas in which to concentrate more detailed, follow-up field mapping, geochemical studies, and geophysical surveys for identifying VMS targets. 184 �The composition and distribution of hydrothermal alteration mineral assemblages in the Lower Ely is similar to that described in major lava-flow dominated VMS mining districts worldwide (e.g. the Noranda Camp, Quebec; Morton and Franklin, 1987; Franklin, 1996; Gibson et al., 1999; Hudak and Morton, 1999; Peterson, 2001; Hudak et al., 2002a,b). Results of recent studies in the Vermilion district (Hudak et al., 2002a; Hudak et al., 2002b; Hocker et al., 2003; Hudak et al., 2004; Hudak et al., 2006; Hoffman, 2007) indicate that not only are the compositions and geometries of the regional alteration mineral assemblages identical to those present in many lava flow dominated massive sulfide mining districts, but that detailed alteration mineral chemistries (Hocker et al., 2003) are also consistent with those associated with the VMS ore deposits in these mining camps. These two observations suggest that the processes that formed the alteration mineral assemblages in the Lower Ely were similar to those that formed equivalent alteration zones in well-established VMS mining camps. A general genetic model for the formation of VMS deposits and associated hydrothermal alteration zone, as recently presented by Franklin et al. (1998), requires convective metalliferous hydrothermal fluid generation in the sub-seafloor environment via heating of down-welling seawater and leaching of metals from the enclosing volcanic and sedimentary strata (Fig. 7-3). The size of a convective hydrothermal system is a function of the abundance of heat in the upper two kilometers of the sub-seafloor crust (Franklin, 1996; Franklin et al., 1998). The intrusion of hypabyssal synvolcanic dikes and/or sills into the shallow sub-seafloor may vigorously enhance the dynamics of convective hydrothermal cells (Campbell et al., 1981). On reaching a critical reaction temperature of ~ 350°C, sustained acid pH in the hydrothermal fluid (evolved fluid) is achieved, and metals are leached from the rocks into the evolved fluid via primary mineral breakdown by calcium metasomatism, silicification, and hydrolysis reactions (Seyfried et al., 1999). In basalt-dominated systems (such as that in the Lower Ely), leaching-related alteration of mafic "source" zones (lower semi-conformable alteration) forms a mineral assemblage composed of albite-epidote-zoisite/clinozoisite-actinolite-quartz. These zones are variably metal-depleted, and are characterized by patchy silicification and epidotization associated with areas metasomatically enriched in silica and calcium. In lava flow-dominated stratigraphic sequences, regionally confined discordant “pipe-like”, and more regionally extensive “semiconformable” alteration zones are present (Morton and Franklin, 1987). The “pipe-like” semi-conformable alteration zones are closely associated with zones of cross-stratal permeability (e.g. synvolcanic fault zones), and are characterized by well-defined vertically extensive alteration zones containing anomalous abundances of sericite, chlorite (both Fe- and Mg-rich varieties), actinolite/ferroactinolite, quartz, pyrite, and locally, chalcopyrite and/or pyrrhotite. Semiconformable alteration zones extend for several kilometers to tens of kilometers in the rocks stratigraphically beneath and adjacent to VMS mineralized horizons (Santaguida et al., 2002a; Santaguida et al., 2002b). In maficdominated volcanic environments, such alteration typically is associated with regional zones of spilitization (an alteration assemblage composed of albite + quartz + Mg-rich chlorite ± sericite), silicification (quartz ± albite), and epidote-quartz alteration (epidote + quartz ± actinolite ± carbonate) (Morton and Franklin, 1987; Gibson et al., 1999; Santaguida et al.. 2002a, Santaguida et al., 2002b). Regional semiconformable alteration zones in felsic rocks in VMS producing camps such as Noranda (Quebec) or Sturgeon Lake (Ontario), typically comprise extensive zones of spilitization, silicification, and sericitization (sericite + quartz ± Mg-rich chlorite) (Morton and Franklin, 1987; Gibson et al., 1999). Both discordant and semiconformable alteration zones have been discovered in the Vermilion district and have been described by Hudak and Morton (1999), Peterson (2001), Odette et al. (2001), and Hudak et al. (2002a). Semiconformable alteration zones in the Lower Ely are dominated by mineral assemblages containing various proportions of quartz, epidote, zoisite/clinozoisite, Fe-chlorite, Mg-chlorite, actinolite, ferroactinolite, sericite/pyrophyllite, and albite. Odette et al. (2001a, 2001b) and Hudak et al. (2002a) have shown via mass balance analysis that semiconformable quartz + epidote ± actinolite ± albite ± chlorite alteration mineral assemblages in the Fivemile Lake area are metasomatically enriched in calcium 185 �and silica, and depleted in base metals (copper and zinc) by 50-90%. Pipe-like, northeast-trending disconformable alteration zones in the Lower Ely are largely composed of Fe-rich chlorite, sericite/ pyrophyllite, actinolite and/or ferroactinolite. Pipe-like alteration zones that have been mapped upsection have, to date, not led to the discovery of economically significant VMS deposits, but have been instrumental in locating potential base metal sulfide-bearing stratigraphic horizons and localized chemical exhalites. Figure 7-3. Simplified schematic model of a convective hydrothermal system associated with the formation of Noranda–type (Morton and Franklin, 1987) or lava-flow dominated-type (Gibson et al., 1999) VMS deposits (modified from Franklin , 1996). Lode Gold Ore Deposit Model and Gold in the Vermilion District The brief description of Archean lode-gold deposits that follows is presented as both a basic reference and also to highlight the important features of the model that will be seen during the field trip. Archean lodegold deposits are one category of ore deposit classified as mesothermal lode-gold deposits (Hodgson, 1993). This deposit type has also been called orogenic gold (Groves et al., 2000), greenstone gold (Robert et al., 1991), Archean lode-gold, mesothermal gold-quartz veins, shear-hosted gold, low-sulfide gold-quartz veins (Berger, 1986b), lode-gold, Mother Lode veins (Bohlke and Kistler, 1986), and iron formation-hosted gold deposits (Berger, 1986a; McMillan, 1996; Rye and Rye, 1974; Fripp, 1976; Kerswill, 1993; Thorpe and Franklin, 1984; and Vielreicher et al., 1994). Whatever the name, they are a widespread group of epigenetic ore deposits that have formed in similar settings throughout geologic time. In general, the deposits form during compressional or transpressional deformation at convergent plate margins in accretionary or collisional orogens (Fig.-7-4). They form over a large crustal-depth range (2 to 20 km) from deep-seated, low-salinity H2O-CO2 ± CH4 ± N2 ore fluids, with Au transported as reduced sulfur complexes. The ore fluids are generated during lower 186 �crustal metamorphism from dehydration reactions. Regional structures provide the main control on distribution of lode gold deposits and mining camps. In many terranes, first-order faults or shear zones appear to have controlled regional fluid flow, with greatest ore-fluid fluxes in, and adjacent to, subsidiary faults, shear zones and/or large folds. Highly competent and/or chemically reactive rocks are the most common hosts to the larger deposits. Gold deposition occurs late during the evolutionary history of the host terranes, normally within D3 or D4 in a D1-D4 deformation sequence. Absolute ages of mineralization support their late-kinematic timing, and, in general, suggest that deposits formed diachronously towards the end of the evolutionary history of hosting orogens. Figure 7-4. Generalized tectonic model for the formation of mesothermal gold deposits, after Groves et al., 2000. The late timing of lode-gold deposits is critical to geology-based exploration methods, and hence mineral potential evaluations for these deposits. The late timing is critical because of the present structural geometry of: (1) the deposits, (2) the mining camps, and (3) the enclosing geologic terranes are essentially all similar to the structural geometry during gold mineralization. Therefore the interpretation of bedrock geological maps and cross-sections can be used to discern the physical conditions that existed at the time of ore deposition. Exploration for mesothermal lode-gold deposits should incorporate various aspects of the ore deposit model into criteria that can vector into the most favorable areas for hosting such mineralization. The most fundamental characteristic of this class of deposit is the spatial association of the deposits to regional structures. Zones of widespread carbonate alteration (adjacent to regional structures) should be identified and used to focus subsequent exploration. Within carbonate alteration zones, gold is typically only in areas containing quartz veins, silicification, and/or sericite alteration (with or without sulfides). Two general structural controls on the orientation of lode gold ore shoots include deflections and curvatures of shear zones, and where high strain zones intersect favorable geological elements (Poulsen and Robert, 1989). A widespread area of gold mineralization occurs in numerous prospects east of Lake Vermilion, within the Vermilion greenstone belt of northeastern Minnesota. The mineralization occurs in rocks of the Neoarchean (~2.7 Ga) Bass Lake sequence (Peterson and Jirsa, 1999) of the Wawa subprovince of the Canadian Shield. This zone of abundant gold mineralization is bounded to the south by the Mud Creek shear zone and to the north by the Vermilion fault. The main access to these prospects is along the Mud Creek road (St. Louis County 38). A brief period of mineral exploration for lode-gold deposits in this immediate area of the Vermilion district occurred in the mid 1980s to early 1990s. These programs typically consisted of grid-based geologic mapping, bedrock sampling, ground geophysics, and the completion of soil geochemical surveys. Conversations with many of the people involved in gold exploration programs in the immediate field trip area (centered on Section 6, Township 62 North, Range 14 West), and compilation of all exploration data from the district as a whole (data from the terminated lease files of the Minnesota DNR), has led to the conclusion that interpretation of linear structural elements exposed in outcrops were not used in designing exploratory drilling plans. Therefore, many of the prospects discovered as a result of these exploration programs remain untested by drilling. 187 �FIELD TRIP All stop locations are given in Universal Transverse Mercator coordinates (UTM), Zone 15, using the North American Datum of 1983 (NAD83). Section subdivisions read from smallest to largest quarter; e.g., “NW, SE” should be read “NW quarter of the SE quarter.” The small map insets showing stop locations are taken from USGS 7.5-minute topographic quadrangles listed with each stop. The first day of the field trip will include stops located between the towns of Tower and Ely. A more detailed geological map with stop locations is given in Figure 7-5. Figure 7-5. Simplified geological map of the field trip stops in the Vermilion district. Geology modified from Peterson, 2001. FIELD TRIP STOPS From Ely, our route into the Vermilion district will include driving west along Minnesota Highway 169 to the Bear Head Lake road (County road 128). Turn left (south) and continue approximately 1.6 miles to an outcrop on the east side of the roadway. 188 �STOP 7-1. Silicified Fivemile Lake sequence pillow lavas / Regional semi-conformable alteration Location: T.62N., R.14W., sec.22 SE, SE; roadside outcrop along Highway 128 (Bear Head Lake State Park road) Eagles Nest 7.5-minute quadrangle UTM: 567,810E/5,297,800N DESCRIPTION: At this location we can observe part of the regionally extensive quartz-epidote alteration zone. The outcrop contains relatively undeformed bun- and mattress-shaped pillows. Interpillow hyaloclastite zones are generally pale to dark green in color, and are chlorite and/or actinolite-rich. Minor red-brown staining locally occurs in these zones, and is indicative of the presence of trace amounts of pyrite and/or chalcopyrite. Pillow selvedges commonly contain up to 10% round to oval, pipe-like quartz-epidote and/or actinolite chlorite amygdules. The cores of the pillows are typically pale green gray in color due to nearly wholesale replacement of the original igneous minerals by quartz and epidote. This quartz-epidote alteration is typical for much of the Lower Member of the Ely Greenstone (Lower Ely), and is one of the most important components of possible VMS exploration in the district (leaching of Cu & Zn) out of a large volume of rock due to hydrothermal alteration. NEXT: Continue on Highway 128 southeast for approximately 2.2 miles to the Purvis Forest Management Road. Turn left and continue about 0.4 mile to logging road on the right side (south). Follow logging road approximately 0.15 miles to a series of outcrops. STOP 7-2 Xenolithic hornblende diorite, Purvis pluton Location: T.62N., R.13W., sec.30 NE, SW. Eagles Nest 7.5-minute quadrangle UTM: 571,805E/5,296855N DESCRIPTION: The Purvis pluton is an east-west trending, moderate-sized (~3km3), sill-like multiphase dioritic to tonalitic intrusion with a strike length of 5.7 km and a thickness that ranges from 100-1200 meters 189 �(Peterson, 2001). This intrusion occurs in the lower stratigraphic section of the north limb of the TowerSoudan anticline (Peterson and Jirsa, 1999; Jirsa et al., 2001). Recent work by Drexler and Hudak (in press) indicate that the intrusion has several phases, including 1) xenolithic hornblende diorite; 2) xenolithic hornblende tonalite; 3) xenolithic leucotonalite; 4) leucotonalite and trondhjemite; and 5) leucotonalite dikes. Detailed field mapping by Peterson (2001), Hovis (2001) and Drexler et al. (2004) suggest that the Purvis pluton is a synvolcanic intrusion based on the following characteristics: 1) it lacks a contact metamorphic aureole; 2) its uppermost contact is proximally associated with intense, semiconformable quartz + epidote alteration zones; 3) D2 deformation fabrics occur in both the intrusion and the surrounding country rocks; and 4) early xenolithic diorite phases are cross-cut by thin, commonly D2-deformed dikes of younger tonalite and trondhjemite phases. Galley (2002) and Galley (2003) have indicated that these characteristics are key features of synvolcanic intrusions temporally associated with the genesis of many Precambrian VMS deposits. Peterson (2001) has suggested that the Purvis pluton may have been the heat source that drove hydrothermal systems that produced the Eagles Nest and Purvis Road VMS prospects. This locale offers an opportunity to investigate the xenolithic hornblende diorite phase of the Purvis pluton. The outcrop adjacent to the road predominately contains four types of xenoliths: 1) dark green xenoliths of amygdaloidal (5-8%) basalt-andesite pillow lavas which locally have preserved selvedges and interpillow hyaloclastite, and are locally contact metamorphosed along their margins; 2) pale green epidote + quartz-altered basalt-andesite lava xenoliths that are up to 15 cm in diameter; 3) rare coarsegrained gabbro/diorite xenoliths up to 3 cm in diameter; and 4) rare <1-2 cm diameter subangular chert xenoliths. Large iron-formation xenoliths up to several meters in diameter, and amphibolite xenoliths up to several centimeters in diameter, may be observed in xenolithic hornblende tonalite outcrops least of this location. Iron-formation xenoliths present in outcrops east of here were likely derived from iron-formation horizons that occur immediately southwest of Purvis Lake. Basalt and altered basalt fragments also were derived from the surrounding Lower Ely. The presence of epidote-quartz altered mafic xenoliths suggests that this phase of the Purvis pluton stoped its way upward into an earlier-formed proximal zone of quartzepidote alteration formed from high temperature seawater-rock interaction (e.g. Galley, 2003). Amphibolite xenoliths are believed to represent contact metamorphosed basalt fragments based on petrographic similarities (Drexler et al., 2004). Drexler et al. (2004) have shown that coarse-grained gabbro/diorite fragments likely represent xenoliths of the earliest phases of the pluton. Studies of ancient VMS deposits has documented that the deposits commonly occur in depressions on the paleo-seafloor (3rd-order basins) while modern deposits on the seafloor are found on high-standing structures, such as ridges. These differences are probably more apparent than real, in that the modern deposits are generally confined to the axial graben, or depression, of what are otherwise are high-standing structures. In addition, both ancient and modern deposits occur in areas of anomalously high heat flow, generally linked to synvolcanic intrusions beneath the hydrothermal systems. The recent mapping in the Purvis Road area has shown the presence of all of the attributes of typical VMS-forming hydrothermal systems. These attributes include a synvolcanic intrusive heat source (the Purvis pluton), a paleotopographic high-standing structure, VMS-style alteration mineral assemblages, and the presence of Cu and Zn-rich massive sulfide (recent logging in this area has exposed numerous angular boulders of massive sulfide in the basal till). NEXT: Return to Highway 169, turn left (west) and travel approximately 4.4 miles to the junction with the Murray Forest management road. Turn left (south) and travel approximately 0.2 miles to south verging bend in the road. Get out and walk along the old logging road to the west. 190 �Stop 7-3. Shallow water volcanic rocks of the Fivemile Lake sequence. Andesite, Rhyolite, and Scoria Location: T.62N., R.15W., sec.25 SW, NE Soudan 7.5-minute quadrangle UTM: 560,980E/5,297,025N DESCRIPTION: At this stop, we’ll be examining a series of outcrops that display the varied geology of the shallow-water Fivemile Lake sequence (Peterson and Patelke, 2003). The short traverse will include outcrops of highly vesicular/amygdaloidal basaltic andesite, rhyolite lava flows and breccias, and a unit of andesitic scoria. Rocks of similar texture occur throughout the central core of the Lower Ely greenstone. NEXT: Return to the bus and continue south along the Murray Forest Management road for approximately 0.6 miles to the junction of the old DM&IR rail line. Get out and walk west along the rail line for 50 meters to the outcrops on the north and south side of the rail line. Stop 7-4. Gold prospect along the Murray Shear Zone Location: T.62N., R.14W., sec. 30, SW, SW Soudan 7.5-minute quadrangle UTM: 561,490E/5,296,205N DESCRIPTION: This is one of the few gold prospects in the lower member of the Ely Greenstone. South of the old rail line, intense shearing associated with the north edge of the Murray shear zone culminated with the formation of chlorite-ankerite-sericite schists. In the mid-1980s, Newmont Exploration discovered lode gold mineralization along the northern margin of the Murray shear zone. Gold mineralization in this area, named the Murray prospect by Newmont, is associated with quartz-carbonate-pyrite-galena-tetrahedrite veins in strongly sheared and carbonatized rocks. Newmont reported values up to 12.5 g/t gold during the course of their exploration. 191 �An estimate of the amount of displacement within the panel of rocks bounded by the Murray shear zone is given in Table 7-2 (Peterson and Patelke, 2003). These values were calculated geometrically by using the average plunge of measured lineations (71°) and two measured lines of possible correlative stratigraphy offset by the bounding shear zones. The calculated total displacement values (net slip) are quite large (up to 13.8 km, or 43,000 feet of net slip), but the displaced rocks would still fall within the range of depth generally associated with greenschist facies metamorphism. Table 7-2. Calculated displacement along the Murray shear zone Calculated Displacement (Kilometers) Strike Slip Dip Slip Net Slip 71° 4.5 13.1 13.8 71° 3.0 8.7 9.2 Lineation Plunge . NEXT: Return to Highway 169 via the Murray Forest Management Road. Turn right on Highway 169 and travel approximately 0.7 miles to the junction of a logging/gravel road on the south side of the highway. Walk up the road to the southwest to the very large outcrop on the top of the hill. Stop 7-5. Central Basalt Sequence Sheet Flows, Pillow Lavas, and Perlitic Hyaloclastite Location: T.62N., R.14W., sec 19, SE, SW, Soudan 7.5-minute quadrangle UTM: 562,000E/5,297,800N DESCRIPTION: The Central Basalt sequence (Peterson and Patelke, 2003) comprises a steeply north-dipping (75°vertical), north-facing sequence of sparsely amygdaloidal pillowed and massive lava flows of basalt andesite to basalt composition that are believed to be correlative with the tholeiitic Armstrong Lake volcanic sequence mapped in the Eagles Nest quadrangle (Jirsa et al., 2001). Hudak et al. (2007) and Jansen et al. (in press) have shown that the lowermost sections of the Central Basalt Sequence is composed of submarine basaltic andesite to basalt lava flows that have rare earth element lithogeochemical patterns similar to mafic rocks in oceanic volcanic arcs. However, locally, submarine basalt lava flows that occur within 50-200m stratigraphically below the contact between the Central Basalt Sequence and the overlying Soudan Member of the Ely Greenstone Formation illustrate MORB-like lithogeochemical patterns, and have suggested that this may document a change from an oceanic arc to back-arc environment immediately prior to the deposition of the Soudan Member. Relative to massive and pillowed basalt and andesite flows in the Fivemile Lake sequence, Central Basalt sequence lavas flows are notably less amygdaloidal, and lack multiple pillow rind structures. In addition, the Central 192 �Basalt sequence lacks the thick sequences of scoriaceous basalt-andesite lapilli tuffs that are commonly interstratified with lava flows in the Fivemile Lake sequence. These characteristics of the Central Basalt sequence indicate eruption and deposition in a deeper submarine environment than the stratigraphically older Fivemile Lake sequence, and suggest overall increasing water depth during the temporal development of the Lower Ely. Deepening of the water column could be accommodated by extensional tectonics and normal faulting associated with the development of the proposed back-arc environment. The outcrop comprises two east-southeast striking massive basalt flows, ranging from at least five to nine meters in thickness, that are separated by a ten meter thick flow unit comprising pillows and pillow lobes (Fig.7-6). All three lava flows at this vicinity illustrate tholeiitic, MORB-like lithogeochemistries (Hudak et al., 2007). Figure 7-6. Detailed geological map of sheet flows, pillow lavas, in-situ hyaloclastite and associated “self-peperite”. Flow 1, at the southern part of the outcrop, is composed of a pale- to dark green, faintly feldspar-phyric (~10% 0.5-1 mm laths), sparsely amygdaloidal, basalt sheet flow that locally exhibits tortoise-shell jointing formed in response to contraction during cooling. The uppermost 10-40 cm of the coherent part 193 �of Flow 1 is generally silicified and epidotized. Petrographic observations indicate that this section of the flow also contains up to 70% <0.1 cm round spherulites. An irregular contact occurs between the coherent basalt flow and an overlying one- to two meter thick unit of dark green, exceptionally well-preserved perlitic in-situ hyaloclastite and associate self-peperite (c.f. Batiza and White, 2000). The hyaloclastite formed from non-explosive fracturing of the basalt glass developed on the flow top due to quenching by water, whereas the perlite formed following deposition by hydration of volcanic glass. An irregular contact occurs between the hyaloclastite and Flow 2, which is composed of north-facing mattress- to bunshaped pillow lavas and pillow lobes with numerous “neck and knob” structures. Individual pillows have well developed perlitic hyaloclastite margins that range from 1-4 cm in width. Pillow buds indicate propagation from east to west, suggesting the volcanic vent was located east of this location. The coherent pillows and lobes are overlain by up to 2.5 meters of hyaloclastite breccia that contains 20-40% subround to subangular pale gray green basalt lapilli in a jigsaw puzzle-fit dark green perlitic hyaloclastite matrix. The upper contact of Flow 2 and the overlying basalt sheet flow (Flow 3) is irregular, and is marked by thin (1-8 cm thick), sheet-like basalt fragments that are up to 1.6 meters in length. These fragments locally appear to be isoclinally folded about an east-west-trending fold hinge. Although the genesis of this structure is currently not well understood, it may be due to syneruptive deformation of either thin slabs of hot, basal flow margin crust from the overlying flow, or thin injections of basalt magma into the hyaloclastite from either the underlying pillows or the overlying sheet flow. Flow 3 comprises an at least ten-meter thick pale green-gray, slightly feldspar-phyric, sparsely amygdaloidal sheet flow. Steep, NNEtrending west dipping D3 joints are well developed in this unit, as are lens-shaped psuedo-pillows that are up to 50 cm in diameter. NEXT: Return to Highway 169, turn right and travel approximately 3.1 miles to the junction of the Mud Creek road (County 38). Turn left (north) on 38 and travel approximately 1.5 miles to a series of low-lying outcrops on the east side of the road. Stop 7-6. Fragmental rocks of the Gafvert Lake sequence. Location: T62N., R14W., sec. 10, NE,SW Eagles Nest 7.5-minute quadrangle UTM: 567,000E/5,301,490N DESCRIPTION: The informally named Gafvert Lake sequence (Peterson and Jirsa, 1999) is interpreted to represent an Archean stratovolcano of andesitic to dacitic composition that stratigraphically overlies rocks of the Ely Greenstone. The complex includes lava flows, fragmental rocks (tuff, lapilli tuff, tuff breccia, debris flow deposits) and porphyritic intrusions. The widespread nature of dacitic fragmental rocks of Gafvert affinity in the Vermilion district indicates that repeated episodes of explosive volcanism (Crater Lake type caldera formation) occurred in the area. Capping the central portion of the Gafvert Lake sequence are a number of thick massive pyrite horizons, that have metal signatures associated VMS, epithermal, 194 �and biologic affinity (Peterson, 2001). We will examine a series of outcrops of fragmental dacitic deposits located on the east side of the Mud Creek road. NEXT: Continue northwest on 38 for approximately 2.1 miles to the boat landing parking area along Mud Creek. Walk approximately 100 meters north along road to the outcrop along the east side of the roadway. STOP 7-7. Mud Creek shear zone Location: T.62N., R.14W., sec. 5, SE, SE; Outcrop just northwest of Mud Creek near road. Chad Lake 7.5-minute quadrangle. UTM: 564,230E/5,302,800N DESCRIPTION: The regional scale Mud Creek shear zone occupies the east-northeast trending valley of Mud Creek, which is clearly visible at this location. This shear zone separates rocks of the Newton Belt (here the Bass Lake sequence) to the north and rocks of the Soudan Belt (Gafvert Lake sequence and the Upper Ely Greenstone Formation) to the south. Development of this shear zone is a product of largely dextral transpressive deformation that has been partitioned into discrete zones, presumably late in D2 deformation. It is generally believed that gold-bearing mineralization was introduced during these later deformation events, and the Mud Creek shear zone and environs continue to attract considerable attention as a gold target. The Mud Creek shear zone is analogous with major faults (Destor-Porcupine fault) and “breaks” (Cadillac-Larder Lake break) of major lode-gold mining districts in Canada. Historic gold assays taken from rocks of the shear zone itself are essentially devoid of gold, as is the case for most major structures within Archean lode-gold mining camps. This series of outcrops are located within the northern margin of the internal highly strained zone of the shear, and include outcrops of: (1) ankeritesericite-quartz-green mica-pyrite schist with quartz and tourmaline knots, and (2) highly folded and compositionally banded phyllites with quartz veins. The protolith for these rocks are unknown, because of the intense deformation, but could be any of several rock types in the region, including quartzofeldspathic porphyry, metavolcanic rock, or graywacke. NEXT: Drive northwest along County 38 approximately 1.1 miles to widened portion of the road. 195 �Stop 7-8. Sheared quartz-feldspar porphyry, basal till, and detailed mapping interpretations Location: T62N., R14W., sec. 5, SW, NW. Chad Lake 7.5-minute quadrangle UTM: 563,190E/5,303,615N. DESCRIPTION: East of Lake Vermilion, the geology of the Bass Lake sequence is dominated by six basic rock types, which include: (1) Tholeiitic pillowed basalt flows interpreted to have formed in a deep-water setting based on volcanic textures; (2) Gabbro sills interpreted as synvolcanic in age due to their stratigraphic continuity and similar deformation as the enclosing pillowed basalts; (3) Felsic porphyries (feldspar porphyry and quartz-feldspar porphyry) interpreted to have intruded during late stages of D2 deformation based on field relationships and geochronology (quartz-feldspar porphyry from the Pac Man Pond prospect returned a 207Pb/206Pb age of 2683.0 +/- 1.4 Ma (Peterson et al., 2001)); (4) Algoma-type ironformation; (5) Thinly-bedded argillite and siltstone; and (6) Sheared rocks, which are dominated by chlorite-rich schist, phyllite, and phyllonite. In addition, localized areas of fragmental felsic volcanic rocks occur stratigraphically below distinct iron-formation horizons. In the last twenty years, numerous gold prospects have been discovered in the eastern portion of the sequence. These prospects generally fall into one of three categories; (1) auriferous quartz-carbonatepyrite veins and sulfidized zones in iron-formation; (2) auriferous quartz-sericite-ankerite-pyrite schists; and (3) felsic intrusive-hosted auriferous quartz veins and stockworks. All of the prospects are found within areas of moderate to strong iron-carbonate alteration, with the best mineralization commonly found within sericitic alteration zones. Numerous equigranular and porphyritic felsic intrusions occur within the areas of alteration and gold mineralization, and are a good guide for locating mineralized structures. The gold mineralization is generally related to deformation in subsidiary structures associated with movement along the D2 Mud Creek shear zone. Widening of the roadbed of County 38 in 2003 exposed a number of new outcrops and cuts into the basal till in this area. Detailed geologic mapping of gold prospects north of the Mud Creek shear zone by Peterson and Patelke (2004a) included mapping these new exposures of the Bass Lake sequence. For this stop, we will traverse along County 38 and look at these new exposures. NEXT: Continue northwest along County 38 approximately 0.55 miles to a small yellowish outcrop on the east side of the road. 196 �Stop 7-9. The Kerr McGee gold prospect Location: T63N., R14W., sec. 31, SE,SE Chad Lake 7.5-minute quadrangle UTM: 562,480E/5,304,610N DESCRIPTION: The Kerr McGee gold prospect is hosted within an extensive zone of highly strained rocks, interpreted to be a subsidiary structure associated with the Mud Creek shear zone. Moderate to high-grade gold mineralization at the Kerr McGee prospect occurs within multiple thin (0.2 – 2.0 meter) zones of quartzsericite-ankerite-pyrite ± green mica ± tourmaline schist hosted by an extensive zone of essentially goldbarren chlorite-rich schist. Thin and probably boudined iron-formation horizons occur locally in the chlorite-rich schist, and locally are strongly mineralized in this area. Mineralized zones locally contain extensive foliation and shear parallel quartz, ankerite, and/or quartz-ankerite veins, and may widen in zones of silicification. The style of gold mineralization exposed in the Kerr McGee prospect is similar to both the Clear Cut (~½ mile west) and Railroad Zone (1½ miles east) prospects. In fact, the sericitic zone that hosts the mineralization may have continuity to both of these other prospects. Three-dimensional visualization (Fig. 7-7) of the detailed lithological and structural mapping by Peterson and Patelke, (2004a) within the Kerr McGee prospect area reveals important information that can be used to design drilling plans that significantly increase the chance of intersecting gold mineralization exposed in outcrop at the surface. For example, drill hole RC-3, which is located 100 meters east of the main gold showing on the eastern side of this knob, was drilled due north (at a dip of 45º) and targeted to intersect the mineralization exposed in outcrop at the Kerr McGee showing. Chevron Resources drilled this hole in 1987, at the western boundary of their lease property (the Kerr McGee prospect was then held by Kerr McGee). Detailed structural mapping in these outcrops reveals that the rocks within the mineralized zone have moderate to strong elongation and intersection (foliation and shear planes) lineations trending 60º and dipping northeast at 72º. The best interpretation of the down-dip orientation of the mineralized zone is this lineation trend and plunge, and drill hole RC-3 never intersected the mineralized zone. 197 �Figure 7-7. Three-dimensional view of the relationship between structural boundaries, the mineralized zone exposed on the surface at the Kerr McGee prospect, and drill hole RC-3. Upward extension to the surface of the two anomalous zones (> 1,000 ppb gold) intersected in hole RC-3 would place these zones in the black spruce and cedar swamp located south-southeast of the prospect. NEXT: Return to Highway 169 via County 38. Turn right (west) and travel approximately 7.3 miles to the junction of Jasper road in the town of Soudan. Turn right on Jasper road and follow road to the Tjunction (~0.5 miles). Turn right, go up the hill, disembark at mine buildings and walk about 150 feet north and up-hill to outcrop on the right. STOP 7-10. Archean Soudan iron-formation member of Ely Greenstone No hammering please! Location: T.62N., R.15W., sec. 27, NE, NE; Soudan Mine State Park. Soudan 7.5-minute quadrangle UTM: 557,120E/5,296,660N DESCRIPTION: This classic exposure of the Soudan iron-formation member of the Ely Greenstone lies on the north limb of the Tower-Soudan anticline, and at the stratigraphic top of the volcanic sequences known collectively as the Lower member of the Ely Greenstone. The outcrop displays two generations of tight folding in 198 �delicate laminae of chert (creamy white), chert-hematite jasper (red), and magnetite-chert (black to silvercolored). The second generation of folds (F2) is tectonic in origin, having subvertical axial surfaces that trend east, and steeply plunging axes. Most display Z-asymmetry. The earlier folds (F0-1) appear to have been sharply refolded to produce complex interference patterns. Lundy (1985) studied folding at this locality and concluded that some of the apparent interference structures are the product of early-formed sheath folds that did not involve refolding by D2. The F1 structures are predominantly intrafolial, and exhibit a great variety of style and orientation; implying they formed by layer-parallel, soft-sediment slumping (Fig. 7-8). Lundy’s mapping of this outcrop is an interesting demonstration of unraveling details at a single outcrop that led to recognition that D1 deformation was not systematic here, but likely soft sediment. Furthermore, it is a microcosm of regional-scale deformation Figure 7-8. Outcrop map showing bedding trajectories and several generations of folds and faults (from Lundy, 1985). F1 folds are nonsystematic and include nappe and sheath fold geometries. It is interesting to observe the rhythmic microlaminae (1 mm or so thick) in various cherty beds exposed here and speculate about the paleoenvironment—that is, whether these represent daily heating/cooling, tidal, climatic, annual, or some other repetitive influence in the depositional environment. What is known about units of iron-formation in the Ely Greenstone, of which there are many, is that deposition occurred during periods of relative volcanic and tectonic quiescence by the slow subaqueous “rain” of chemical precipitates. The deep excavations in this area are the early workings of the Soudan iron mine, the first in Minnesota. The mine produced about 16 mt of high-grade hematite ore (60-63 percent iron content) from 1884 until 1962, when the land was deeded to the State of Minnesota and converted to a park. Most of the production came from underground workings that began here in 1900, and which now can be visited on guided tours. The mine also houses an underground physics research facility at 2340 feet below the surface. A massive expansion of that facility is under consideration to create a national underground laboratory at considerably greater depths (Peterson and Patelke, 2003). 199 �NEXT: Return to Highway 169 and turn right. Follow 169 through the town of Tower to the large outcrops immediately west of tons (approximately 3.1 miles). STOP 7-11 Archean fragmental volcanic rocks Location: T.62N., R.15W., sec. 32, SW, SW; Highway 169 road cut, west edge village of Tower. Tower 7.5-minute quadrangle. UTM: 553,380E/5,294,430N DESCRIPTION: This outcrop consists of fragmental, variably reworked volcanic conglomerate and tuffaceous rocks of the Gafvert Lake sequence of the Lake Vermilion Formation. The rock is composed of about 85-95 percent dacitic detritus, 3-5 percent gray clasts of graywacke, slate, and basaltic andesite, and a small percentage of magnetic and sulfidic fragments. Fragments range in size from a few millimeters to 20 cm. The generally poorly developed sorting and bedding, together with varied clast composition, implies a debrisflow origin. Compare these rocks with those of Stop 7-6. NEXT: Continue west on Highway 169 approximately 1.5 miles to road cut. STOP 7-12. Archean dacitic tuff/ Paleoproterozoic or Mesoproterozoic diabase dike Location: T.61N., R.16W., sec. 1, SW, NE; Highway 169 road cut. Tower 7.5-minute quadrangle. UTM: 551,160E/5,293,960N DESCRIPTION: These road cuts expose outcrops of white, dacitic tuffaceous sedimentary rock, a component of the Lake Vermilion Formation. Regionally, the formation consists of all compositional gradations between what 200 �appears to be first-cycle tuff, tuffaceous greywacke, and mixed-source greywacke, interbedded on all scales. In a general way, the tuffaceous component increases in proportion to the east toward the TowerSoudan anticline. The presumed source of the dacitic volcanic detritus exposed in this area is stratovolcanos of Gafvert Lake affinity, which overlie the composite volcanic shield complex of the Ely Greenstone. Ring plains and irregular basins composed of detritus shed from the high-standing volcanic complex are now represented by the Lake Vermilion Formation. The northeast-trending, steeply dipping, seven-meter wide diabase dike that cuts tuffaceous rocks has been the source of considerable debate. It’s petrographic (olivine-bearing) and geochemical (silica undersaturated) composition is similar to Mesoproterozoic dikes (Schmitz, 1994); yet it lies nearly along strike with, though east of, dikes of the Paleoproterozoic Kenora-Kabetogama dike swarm. NEXT: Continue west on Highway 169 approximately 0.7 mile to Stop 7-13. STOP 7-13. Multiply folded Archean greywacke Location: T.61N., R.16W., sec. 2, NE, NE; south side of Highway 169 just east of CR 526. Tower 7.5-minute quadrangle. UTM: 550,050E/5,294,000N DESCRIPTION: This outcrop at the road and several smaller ones in the bush nearby show the superposition of two generations of folds in thin-bedded, well-graded graywacke of the Lake Vermilion Formation. The second-generation folds (F2) are associated with a regional axial plane cleavage in which sedimentary clasts are flattened. The earlier F1 folds have no associated cleavage and tend to be erratic in form, trend, and distribution. Folds display “eye” and “mushroom” shapes that locally are interpreted to be sheath folds (Hudleston and others, 1987). These characteristics are consistent with deformation of poorly lithified sediment. The superposition of deformation events is manifest in the transection of F1 folds by cleavage related to D2. In this area and to the west, one can find anticlinal synclines and synclinal anticlines, indicating stratigraphic inversion prior to D2 folding. NEXT: Continue approximately 1.7 miles to the west to the junction of 77. Turn right on 77 and follow for approximately 0.6 miles to the outcrop on the left side of the road on the north side of the Pike River Dam. 201 �STOP 7-14. Archean greywacke at Pike River Dam Location: T.61N., R.16W., sec. 3, NW, SW; west side of County Road 77, on N side of river. [Note that Fortune Bay Casino—the overnight hotel—lies to the north off of CR 77]. Tower 7.5-minute quadrangle. UTM: 547,300E/5,293,340N DESCRIPTION One of the truly classic outcrops of greywacke of the Lake Vermilion Formation is beautifully exposed at this stop. Prior to about the 1950s, no depositional mechanism could satisfactorily explain the coincidence in graywacke of; 1) coarse sand derived from a source many kilometers distant and having an altered clayey matrix; 2) interbedded black slate; and 3) the lack of evidence for reworking in shallow water (indicative of deposition below wave base). This was changed when the concept of turbidity currents was introduced to the geological profession by Kuenen and Migliorini (1950). Despite widespread publication on turbidites in more modern geologic settings through the 1950s and 1960s, the facies model was not refined and applied to Archean and Proterozoic strata in the Lake Superior region until somewhat later (Morey, 1965; Ojakangas, 1966). The geology of field trip stops in the Virginia Horn area is presented in Figure 7-9. NEXT: Return to Highway 169 and turn west. Travel south on Highway 169 approximately 28 miles to the junction with Highway 53; follow 53 south approximately 0.7 miles to the Laurentian Divide wayside rest Stop. 202 �Figure 7-9. Generalized geologic map of the Virginia horn area (modified from Jirsa and others, 1998) showing details of field trip stops 9-15 to 9-19. 203 �STOP 7-15. Archean Giants Range batholith at “Confusion Hill,” Laurentian Divide Location: T.59N., R.17W., sec.19 SE, SE; wayside off Highway 53. Virginia 7.5-minute quadrangle. UTM: 534,337E/5,269,458N DESCRIPTION: Exposed near this wayside and in road cuts on both sides of the highway is an array of variably layered intrusions having both tonalitic (white) and dioritic (black) compositions. A cursory look shows intrusive relationships that conclusively demonstrate that diorite was emplaced into tonalite at one locality, and at another, tonalite was emplaced into diorite. In detail, all compositions intermediate between the two end members are also present locally. Although the dioritic component is abundant here, the bulk of the mapped unit is tonalitic. Emplacement of this unit, now known as the Lookout Mountain tonalite, probably involved some degree of magma mingling. Dikes of tonalite that cut the adjacent high-grade supracrustal rocks of the Minntac sequence contain metamorphic fabrics, yet little evidence of metamorphic origin can be seen in the interior of the body, implying it is syntectonic with respect to D2 deformation. U-Pb zircon dates (Boerboom and Zartman, 1993) of two components of the batholith exposed to the north bracket the age of D2 deformation between about 2674 and 2682 Ma. Exposures at Confusion Hill are a small part of the Giants Range batholith, which forms the core bedrock of the Laurentian (drainage) divide. The batholith is a 40-mile wide belt of intrusions that can be traced on geophysical maps and outcrop east to the Mesoproterozoic Duluth Complex, and west beyond the western border of Minnesota. It separates Archean supracrustal sequences in the Virginia horn from those of the Tower-Soudan area—making stratigraphic correlation between the two districts speculative. NEXT: Follow Highway 53 south through the town of Virginia. Take the exit for Highway 135 (east) approximately 0.5 miles to Bourgin Road. Turn right (south) on Bourgin Road and continue about 0.4 mile to large cut on left (east) side of road. 204 �STOP 7-16. Archean graywacke and slate, intruded by quartzofeldspathic porphyry. Location: T.58N., R.17W., sec.21 SW, SW; road cuts on east side of the Bourgin Road. Eveleth 7.5-minute quadrangle. UTM: 536,311E/5,260,659N DESCRIPTION: Outcrops along this side of the road expose quartzofeldspathic porphyry (QFP) intruded into variably deformed graywacke, siltstone, and slate of the Mud Lake sequence. The sedimentary rocks here are moderately deformed, but much of that deformation is inferred to predate the main cleavage-forming event D2, and some may be soft-sediment in origin. The QFP is large and continuous to the east, but at this locality it appears to be segmented into a zone of multiple dikes. Both graywacke and QFP are intensely altered to some combination of iron-carbonate minerals (ankerite, ferroan dolomite) and sericite. Regionally, this style of alteration is commonly, though not always associated with QFP intrusions— presumably because the QFP remained more structurally rigid than the enclosing sedimentary rocks during the shear-related deformation event that accompanied alteration late in D2. Most gold mineralization in the area is closely allied to this alteration, yet this outcrop is surprisingly barren. One of the earliest gold discoveries in Minnesota was made by J.W. Gruner (in Grout, 1937) in a railroad cut not far from stop 7-16. The cut exposes graywacke intruded by quartzofeldspathic porphyry, having visible gold associated with small quartz veins. Despite several episodes of mineral exploration in this area (most notably be Newmont Exploration in the 1980s) no economic gold deposits have been discovered. NEXT: Follow Bourgin road to the south and west to a frontage road on the east side of Hwy 53. Turn north (right) on the frontage road and travel about 0.2 miles to first road to right, turn up-hill and continue to #7 Mesabi Lane. 205 �STOP 7-17. Archean conglomerate Private driveway! Location: T.58N., R.17W., sec.20 SW, SE, No. 7 Mesabi Lane; village of Midway. Eveleth 7.5-minute quadrangle. UTM: 535,713E/5,259,459N DESCRIPTION: Archean conglomerate and lithic sandstone that form the driveway here are part of the northeast-trending Midway sequence, containing these strata types locally interbedded with subaerially deposited, calcalkalic (trachyandesitic) volcanic rocks. The sequence is inferred to have formed after earliest deformation (D1) of the enclosing graywacke and basaltic rocks of the Mud Lake sequence, but before the cleavage-forming D2 deformation that affected both sequences. The conglomerate contains clasts of basalt, graywacke, porphyritic trachyandesite, and quartzofeldspathic porphyry (QFP). This provenance indicates that the older Archean rocks of the Mud Lake sequence were intruded by QFP, deformed, and uplifted, to provide detritus to what was probably a successor or “pull-apart” basin developed along a major structure now occupied by the Pike River fault zone. Midway sequence conglomerate has previously been interpreted as a basal sediment (Sutton, 1963), and as a proximal turbidite fan deposit (Levy, 1991), depositionally transitional with graywacke and slate of the Mud Lake sequence. Subsequent work (Jirsa, 2000) indicates that the conglomerate is part of a Timiskaming-type clastic and volcanic sequence that unconformably overlies the older volcanic strata. Deposition of the Midway sequence required uplift, subaerial erosion, continental volcanism, and deposition in isolated basins along a major structural break. NEXT: Return to Highway 135. Turn right and follow 135 approximately 2.4 miles to a residential street on the northwest side of the town of Gilbert. Turn right and go 5 blocks and park by the Gilbert Junior High School. 206 �STOP 7-18. Archean pillowed and massive greenstone Location: T.58N., R.17W., sec.23 NW, SE, SW; north edge of athletic fields, Gilbert Junior High School. Gilbert 7.5-minute quadrangle DESCRIPTION: Outcrop of pillowed and massive basalt is part of the Archean Mud Lake sequence, metamorphosed to low greenschist-grade. Pillow shapes indicate stratigraphic facing is to the northwest, which places this outcrop on the south side of a major D1 structure known as the Mud Lake syncline. Note also the presence locally of fractures filled with reddish jasper, presumably deposited in depressions on the rock surface by overstepping of Paleoproterozoic seas during deposition of the Biwabik Iron Formation. NEXT: Follow residential roads to Highway 37 in the center of Gilbert. approximately 3.3 miles to the on/off ramps onto Highway 53. Turn right and travel on 37 STOP 7-19. Paleoproterozoic Pokegama Quartzite (A) and Biwabik Iron Formation (B) Location: T.58N., R.17W., sec.32 SE, SE, and adjacent, junction of Highways 37 and 53. Eveleth 7.5-minute quadrangle. UTM: scattered outcrops extend from 535,956E/5,256,913N on the north (stop 9-19A), to 536,263E/5,256,200N on the south (stop 9-19B). DESCRIPTION 7-19A: Unconformably overlying the Neoarchean rocks of the Virginia Horn area is the Animikie Group sediments of Paleoproterozoic age. Coarse grain size and massive beds as thick as 1.5 m characterize this outcrop of the sandy, upper member of the Pokegama Quartzite. Thin beds of shale and siltstone separate the massive beds. Ojakangas (1993) interpreted the deposition of this facies as within high-energy, lower tidal or subtidal environment. 207 �DESCRIPTION 7-19B: This exposure of gently southeast-dipping strata is part of the Lower Cherty member of the Biwabik Iron Formation. It overlies and is generally in transition with the Pokegama Quartzite at stop 5-5A. Notice that both formations have sandy textures and cross-bedding, implying a moderately high-energy depositional environment. The most significant difference between these two units is the abrupt change in sediment source from the extrabasinal quartz grains in the Pokegama, to recycled, chemically precipitated chert in the Biwabik. Measurements of cross-bedding in the iron-formation are bimodal, implying deposition in a tidally influenced marine environment (Ojakangas, 1993). OPTIONAL FIELD TRIP LOCATIONS Highlighting Neoarchean bedrock in the Vermilion District: 3 driving/hiking traverses within striking distance of Ely Figure 7-10. Geologic map of the Ely to Moose Lake area (clipped and modified from Jirsa and Miller, 2004) KAWISHIWI FALLS TRAVERSE (approximate round trip from Ely = 1-2 hours) Metabasalt and iron-formation of the Neoarchean Ely Greenstone. Stop KF-1—Pillowed metabasalt; UTM: 588,370E/5,307,100N (NAD 83) Directions: Drive east of Ely (set odometer at junction hwy 1 with hwy169 or County road 18Fernberg Trail) and drive east for about 1.3 miles. Outcrop on north (left) side of road is classic example of pillowed metabasalt; note radiating chlorite amygdules and concentric jointing in pillows. Stratigraphic younging is to the north. Stop KF-2—Iron-formation lens; UTM: 592,785E/5,309,288N (NAD 83) Directions: Continue east on hwy 169 (also County road 18) to crossing of Kawishiwi River at about 4.5 miles from starting mileage; park near bridge and walk farther east to first outcrop on south (right) side of road. Outcrop consists of gray, magnetite-hematite-chert iron-formation with minor pyrite. Stop KF-3—Kawishiwi Falls, metabasalt; UTM: 592,440E/5,309,755N (NAD 83) Directions: Cross bridge over Kawishiwi River; continue 0.3 mile farther east to driveway north of trail that leads to the falls parking lot. Follow walking trail northwest of parking lot for about 0.35 miles to falls. ECHO TRAIL TRAVERSE (approximate round trip from Ely = 1-2 hours) Neoarchean Newton Lake Formation and the Wawa/Quetico subprovince boundary. 208 �This traverse visits newly created roadside outcrops along the Echo Trail north of Ely. The Newton Lake Formation is a package of presumably thrust-stacked tholeiitic to komatiitic lava flows, mafic to ultramafic sills, and thin felsic tuff. It differs from the Ely Greenstone in the abundance of ultramafic rocks and general lack of iron-formation. Judging from geochronologic work in the adjacent Shebandowan Greenstone Belt in Ontario (Corfu and Stott, 1998), the two formations are approximately coeval at ca. 2720 Ma. Metasedimentary rocks of the Quetico subprovince to the north represent graywacke deposited as foreland-basin fill during D1 orogenesis and eventual accretion of the Wawa subprovince to the evolving craton at ca. 2699-2696 Ma. Directions to start of Echo Trail: Drive east of Ely along hwy 169 to County Road 88 about 0.9 mile east of junction of highways 1 and 169. Turn north (left) on 88 and follow it around the east end of Shagawa Lake for 2.2 miles to Echo Trail (County Road 116) turn-off on the right (north). Set odometer here and drive northward on Echo Trail to stops. Stop ET-1—Spherulitic pillowed basalt of the Newton Lake Formation; UTM: 585,745E/5,309,470N; 0.5 miles north of CR #88 on west (left) side of Echo Trail. Steeply dipping pillowed basalt, with stratigraphic younging to the south. Metamorphic grade is low greenschist. Note local presence of large vacuoles in pillows showing “drain-back” features. Stop ET-2—Metadiabase and felsic tuff of Newton Lake Formation UTM: 585,748E/5,310,655N; 1.25 miles north of 88 on west (left) side of Echo Trail. Roadcut exposes about 100 foot-thick unit of cherty-looking to granular felsic tuff displaying delicate pristine bedding features. This unit may be a block “floating” in metadiabase. Small outcrop on east side of road shows metadiabase cut by coarse grained hornblende-pyroxenebiotite lamprophyre containing inclusions of metadiabase. Stop ET-3—View of Burntside fault (a drive-by). UTM: 583,254E/5,311,035N; 3.25 miles north of 88 at junction with Somero Road (Tnsp #4651). Prominent NE-striking linear depression separates steep rock walls of low greenschist grade Newton Lake Formation of the Wawa subprovince on the south, from higher metamorphic grade rocks of the Quetico subprovince on the north. Stop ET-4—Quetico subprovince metasedimentary and intrusive rocks. UTM: 581,590E/5,312,295N; 4.0 miles north of 88 on east (right) side of Echo Trail about 0.1 mile north of junction with Passi Road (CR #803). Note that relict graded bedding and other sedimentary features are preserved locally, despite amphibolite grade metamorphism and local migmatization. MOOSE LAKE TRAVERSE (approximate round trip from Ely 2-4 hours) Neoarchean Knife Lake Group Directions: Drive about 16 miles east of Ely on hwy 169-Fernberg Road to left turn at Moose Lake Road (CR #183); go 2.5 miles northeast along Moose Lake Road to parking area on the right for the SecretBlackstone Lakes trail area. Hike eastward along a series of trails that cross outcrops, and veer off the trails onto some of the larger exposures. The Knife Lake Group contains the youngest supracrustal strata in the Vermilion district, exposed as thin layers on and tectonic wedges within older volcanic units (Ely and Newton Lake). Two major sequences are recognized; an older package of alkalic hornblende-phyric volcanic, volcaniclastic, and derived sedimentary rocks; and a younger sequence of polymictic conglomeratic strata known locally as the Ogishkemuncie conglomerate. It appears that parts of both sequences are exposed along the multitude of 209 �trails. Though both sequences are fragmental at this location; the Ogishkemuncie can be distinguished by the presence of clasts of red and black iron-formation, black and white chert, vein quartz, and quartzphyric granitoid derived from the Saganaga Tonalite exposed about 20 miles to the northeast. Lithology and sedimentary structures indicate alluvial fan, fluvial, and fluvial-lacustrine deposition; though some apparently deeper water turbiditic sediments occur locally. Moose Lake, just to the west, is one of a linear chain of lakes that parallels faults associated with the Shagawa Lake-Knife Lake trend. This fault system appears to have been long-lived, and may initially have controlled development of a successor basin into which at least some of the Knife Lake strata were deposited. These supracrustal rocks can be correlated on the basis of lithologic and relative temporal setting with the Shebandowan Assemblage in adjacent Ontario(Corfu and Stott, 1998). There, the alkalic volcanic rocks and Saganaga Tonalite were dated at about 2689 Ma, and the younger conglomerate units (like the Ogishkemuncie) have dates in the range of 2684-2682 Ma. The latter evolved during regional transpressive deformation (D2) at about 2685-2680 Ma. REFERENCES Batiza, R., and White, J. D. L., 2000, Submarine lavas and hyaloclastite: in Sigurdsson, H., Encyclopedia of Volcanoes: Academic Press, San Diego, CA, p. 361-381. 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Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2009 Description An account of the resource Institute on Lake Superior Geology. Ely, Minnesota. May 5-10, 2009. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2009 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English